Excited State Aromaticity and Antiaromaticity - ACS Publications

Review pubs.acs.org/CR

Excited State Aromaticity and Antiaromaticity: Opportunities for Photophysical and Photochemical Rationalizations Martin Rosenberg,† Christian Dahlstrand,‡ Kristine Kilså,*,†,§ and Henrik Ottosson*,‡ †

Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark Department of Chemistry - BMC, Uppsala University, Box 576, 751 23 Uppsala, Sweden

‡

5.4. Photochemical Formation of ortho-Xylylenes and Analogs 6. Usage of Excited State Aromaticity for Synthesis of 4nπ-Electron Cycles 6.1. Photochemical Routes to 4π-Electron (Hetero)Annulenic Species 6.2. Photochemical Routes to 8π-Electron (Hetero)Annulenic Species 6.3. Photochemical Syntheses of Larger and Polycyclic Compounds with 4nπ-Electron Perimeters 7. Conclusions and Outlook Author Information Corresponding Authors Present Address Notes Biographies Acknowledgments References Note Added in Proof

CONTENTS 1. Introduction 2. Brief Historical Perspective 3. Theoretical and Computational Studies of Excited State (Anti)Aromaticity 3.1. Qualitative Molecular Orbital Theoretical Description 3.2. Qualitative Valence Bond Theoretical Descriptions 3.3. Computational Evaluations of (Anti)Aromaticity of Annulenes in Their Lowest Excited States 3.3.1. Energy-Based Aromaticity Indices 3.3.2. Geometry-Based Aromaticity Indices 3.3.3. Magnetically Based Aromaticity Indices 3.3.4. Electronic Structure Based Aromaticity Indices 3.4. Synopsis of Theoretical and Computational Studies 4. Impact of Aromaticity on Excited State Properties 4.1. Excited State Energies 4.2. Excited State Geometries 4.3. Excited State Magnetic Properties 4.4. Excited State Polarities 4.4.1. Manipulation of Excited State Energies of Fulvenic Molecules 4.5. Photoacidity and Photobasicity 5. Influence of Excited State Aromaticity and Antiaromaticity on Photochemical Reactions 5.1. T1 State Z/E-Isomerizations of Annulenyl Substituted Olefins 5.2. Photosolvolyses, Photodecarboxylations, and Related Reactions 5.3. Applications of Excited State Acid−Base Chemistry

© 2014 American Chemical Society

5379 5380 5382 5382 5386

5407 5410 5411 5413

5417 5418 5419 5419 5419 5419 5419 5420 5420 5425

1. INTRODUCTION Aromaticity is one of the most widely applied concepts within chemistry. It plays a major role in the rationalization of a range of chemical properties in the electronic ground state, despite the fact that it cannot be uniquely defined and only indirectly be measured and quantified.1−6 However, in contrast to its wide use for rationalization of ground state properties and reactions, the application of the aromaticity concept to explain excited state properties and reactions is scarce. Research reports concerned with excited state aromaticity, in one way or the other, have hitherto been dispersed through time as well as through the various chemical subdisciplines. Therefore, a summary of the aromaticity concept applied to the lowest electronically ππ* excited states is needed. In addition to explaining how the concept of excited state (anti)aromaticity can be used to rationalize earlier observations of photophysical and photochemical behavior, we also anticipate that the concept will offer new insights of use for molecular design. For the electronic ground state (the S0 state), Hückel’s “4n + 2” rule states that annulenes with 4n + 2 π-electrons (n = 0, 1, 2, ...), such as benzene, are stabilized by cyclic delocalization of the π-electrons relative to their open-chain polyene analogues, whereas ground state annulenes with 4n π-electrons are correspondingly destabilized by cyclic π-electron delocaliza-

5388 5388 5389 5391 5394 5395 5396 5396 5396 5398 5399 5400 5401 5403 5404 5406 5406

Received: November 24, 2012 Published: April 8, 2014 5379

dx.doi.org/10.1021/cr300471v | Chem. Rev. 2014, 114, 5379−5425

Chemical Reviews

Review

tion.7,8 These stabilizing and destabilizing features are often referred to as aromatic and antiaromatic effects, respectively. In general, aromatic stabilization results in structures with a high degree of symmetry and delocalized π-electron distributions, while antiaromatic destabilization leads to distorted structures with distinctly localized π-bonds. Additionally, (4n + 2)π- and 4nπ-electron annulenes exhibit a range of characteristic physical and chemical properties, which in combination with the (de)stabilization span the concepts of (anti)aromaticity. Although the concepts of aromaticity and antiaromaticity traditionally are considered only for the singlet ground state of molecules, these concepts can also be shown to influence electronically excited states of both singlet and triplet multiplicities. They allow for rationalization of a number of different photophysical and photochemical observations, observations that are usually not set in relation to an excited state (anti)aromatic influence. The theoretical foundation of excited state aromaticity was laid in the late 1960s and during the 1970s, and it expanded from the extensive investigations of structures, reactivities, and spectroscopic properties of πconjugated hydrocarbons carried out during the 1950s and 1960s. The field then progressed into a more dormant phase which lasted from the 1980s until the late 1990s, despite several highly important experimental findings being made during those decades. However, it has seen a revival in the past few years, primarily within the theoretical and computational chemistry community. Herein, our ambition is to summarize the theoretical evidence for excited state aromaticity and antiaromaticity, and thereafter, to apply these concepts to the rationalizations of earlier reported experimental results. Our intention is to convince the reader that excited state (anti)aromaticity can have a similarly important impact on photophysics and photochemistry as ground state (anti)aromaticity has had on regular ground state chemistry. A unified view of ground and excited state aromaticity, including compounds with Hückel as well as Möbius orbital topologies, could also be a key to a deeper understanding of aromaticity.

Figure 1. (A) Single cycle with the 2pπ atomic orbitals arranged in a πconjugated ring with Hückel topology, and (B) a Möbius band with the 2pπ atomic orbitals arranged in a π-conjugated ring with Möbius topology. Note in particular the phase shift between two adjacent AOs at the upper rim of the Möbius band.

energies of electronic ground state vs lowest excited state and did so for reactions that progressed with Hückel or Möbius orbital topologies at their transition states. Both Dewar and Zimmerman in their first reports analyzed electrocyclic ring closure reactions in terms of transition state aromaticity and antiaromaticity, and it was shown that photochemically induced electrocyclic ring closures preferentially lead to products that would have to be formed via antiaromatic transition states if they were to be formed in the ground state.8,11,12,14,16 Later, Dougherty extended this theoretical analysis to pericyclic reactions in general, and concluded that “pericyclic reactions take place via aromatic transition states” regardless of whether they occur thermally or photochemically.17 This formed the basis of the analysis of both thermal and photochemical pericyclic reactions in terms of transition state aromaticity. In short, the feasibility of a pericyclic reaction depends on a combination of properties at the transition state: the number of electrons (4n + 2 vs 4n), the orbital topology (Hückel vs Möbius), and the electronic state (S0 vs S1/T1) in the sense that a change in each of these factors lead to a reversal in the allowedness and forbiddenness. In particular, the results pointed to a reversal of Hückel’s (4n + 2)π- and 4nπ-electron counting rules for aromaticity and antiaromaticity when going from the ground state to the lowest ππ* excited state. The analysis of thermal and photochemical pericyclic reactions in terms of transition state aromaticity and antiaromaticity complemented analyses using state correlation diagrams and frontier orbitals as basis for the Woodward− Hoffmann selection rules.18,19 In 1972, Baird expanded the aromaticity and antiaromaticity concepts to molecular structures that correspond to minima on the excited state potential energy surface (PES).20 Baird used PMO theory, similar to Dewar, to conclude that ππ* triplet excited states (T1 states) of 4nπ-electron annulenes are aromatic while (4n + 2)π-electron annulenes are antiaromatic, a rule which now gradually has become known as Baird’s rule, the photochemistry analogue of Hückel’s rule. In his 1972

2. BRIEF HISTORICAL PERSPECTIVE The quantum chemical basis for the understanding of πconjugation, aromaticity, and antiaromaticity was laid by Erich Hückel in the early 1930s through his LCAO-MO theoretical formulation which today is known as Hückel MO theory (HMO theory).7,9 However, the first explanation as to why the (4n + 2)π- and 4nπ-electron annulenes are energetically stabilized (aromatic) and destabilized (antiaromatic), respectively, was given by Dewar in the mid-1960s when he used perturbation MO (PMO) theory to qualitatively derive and compare the formation energies of analogous open-chain polyenes and cyclic annulenes.8 In his report he also briefly examined aromatic stabilization and antiaromatic destabilization in the lowest ππ* excited state, and concluded on the reversal of Hückel’s rules in this state as compared to the S0 state. Simultaneously, Zimmerman regarded the circle mnemonic earlier deduced by Frost and Musulin for facile calculation of HMO energies of ordinary annulenes with Hückel topology,10 and he expanded it to annulenes with Möbius topology (Figure 1).11−14 Two years earlier, in 1964, Heilbronner had shown that the aromaticity and antiaromaticity rules were reversed for Möbius as compared to Hückel topology annulenes.15 With the circle mnemonics for both Hückel and Möbius topology annulenes in hand Zimmerman could compare the HMO 5380


Chemical Reviews

Review

now essentially an odd parity system, with an odd number of electron pairs. Such a system has a characteristic of an aromatic stabilization so that the lowest lying triplet state is expected to have an aromatic character”. Thus, a 4nπ-electron annulene in its lowest triplet biradical state can be regarded as a (2 + (4n − 2))π-electron cycle composed of two nonbonding same-spin πelectrons plus a closed-shell Hückel aromatic cycle with 4n − 2 π-electrons.32 With regard to the triplet state Rzepa and coworkers a few years later also showed based on computed geometries and NICS values that perfluorinated Möbius annulenes with 4n + 2 π-electrons are aromatic in the T1 state,33 in line with Aihara’s earlier finding on reversal in resonance stabilization/destabilization in the ππ* excited state when the orbital topology is changed. In 2004, the magnetic susceptibilities and aromatic characters of several singlet and triplet excited states of benzene were calculated by Kataoka at the Pariser−Parr−Pople (PPP) semiempirical level, and he found that the 11B2u state (the S1 state), in addition to the 13B1u state (the T1 state), is strongly antiaromatic.34 In 2008, the extension of Baird’s rule to the lowest singlet excited state was further confirmed by Karadakov through NICS values of CBD, benzene, and COT calculated at CASSCF level, and he wrote that “the results demonstrate that the well-known “triplet aromaticity” of cyclic conjugated hydrocarbons represents a particular case of a broader concept of excited-state aromaticity and antiaromaticity”.35,36 Recently, Poater and co-workers also investigated the π-delocalization in a series of neutral and ionic π-electron systems using the πdelocalization index δπ, and concluded that 4nπ- and (4n + 2)πelectron species can be described as aromatic and antiaromatic, respectively, when in their S1 and T1 states,37,38 and this was extended to an analysis using a series of other electron delocalization measures.39 Also, through an analysis of the occupied-unoccupied π-MO transitions in terms of the angular momentum quantum numbers Λ of the π-MOs, involved in the generation of diatropic or paratropic ring currents, Soncini and Fowler were able to generalize Baird’s rule.40 Combined with ring current calculations they concluded that annulenes with 4n + 2 π-electrons are aromatic (antiaromatic) in states with even (odd) total spins, while the opposite applies for 4nπ-electron annulenes. In another notable and recent theoretical study based on simulations by optimal control theory, reported by Ulusoy and Nest, it was shown that the aromaticity of benzene in the S0 state can be switched off through an ultrashort laser pulse which mixes either the S0 and S1 states or the S0 and S2 states in equal portions.41 The aromaticity concept can be extended to three-dimensional spherical compounds in their S0 states as shown by Hirsch and co-workers,42 and the corresponding rule is now denoted as Hirsch’s 2(n+1)2 rule. Recently, Poater and Solà revealed the analogue of Baird’s rule for such spherical compounds as they could manifest by computations of NICS and electronic multicenter indices that there is an open-shell spherical aromaticity for cage compounds with 2n2 + 2n + 1 electrons and total spins of S = n + 1/2.43 In the past decade there has also been a growing attention of aromaticity in allmetal compounds, initiated by the identification of the Al42‑ cluster by Boldyrev and co-workers as the first all-metal aromatic compound.44 Indeed, also open-shell aromaticity in such species should exist as the Ta3− and Hf3 clusters have been concluded by Solà and co-workers to be aromatic in their first quintet and triplet states, respectively, when based on

paper Baird used semiempirical calculations at the NNDO level to confirm the qualitative results.20 A few years later, in 1978, Aihara showed that as one goes from the ground state to the lowest ππ* excited state, the resonance energies for Hückel as well as Möbius topology annulenes reversed from positive to negative, or the opposite depending on whether the annulene has 4n + 2 or 4n π-electrons.21 He particularly wrote that “either conformation of any annulene in the excited state can be predicted to have an aromatic character opposite to that in the ground state”. On the experimental side, Breslow and co-workers in 1967 and 1973 succeeded in the formation and spectral characterization of the cyclopentadienyl cation and the perchloro derivative, and these species were shown to possess triplet ground states.22−25 Also, zero E-values were observed which indicated planar pentagonally symmetric structures for both cations. These findings should support the triplet state aromatic character of these two 4π-electron species as their lowest triplet states are found at even lower energies than the Jahn−Teller distorted closed-shell singlet states. Baird’s and Aihara’s conclusions on aromaticity and antiaromaticity in the lowest ππ* excited states were based on qualitative theory and semiempirical calculations, and the first ab initio study that explicitly addressed aromaticity effects in excited states was performed by Janoschek and co-workers in 1982.26 They carried out calculations of the S1 and T1 states of cyclobutadiene (CBD) using a limited configuration interaction (CI) treatment and, based on the calculated bond length alternations, concluded that cyclobutadiene indeed possesses some aromatic character in these two excited states. A few years earlier it had been observed by Chapman et al. that CBD, formed photochemically at cryogenic temperatures by the matrix isolation technique, decomposed over time into two acetylene molecules upon continuous irradiation.27 Janoschek argued that the reason for the slow photodegradation, as compared to the rapid dimerization in the S0 state, was likely due to the aromatic stabilization of CBD in the excited state.26 It was further suggested that the aromaticity of benzene was decreased upon excitation, which also agrees with Aihara’s earlier conclusion that the aptitude of benzene to photorearrange stems from its large negative resonance energy in the excited state.21 In 1998 two important computational and theoretical contributions were made. First, Schleyer and co-workers verified Baird’s rule when the aromatic character of a number of neutral and charged 4nπ-electron annulenes in their lowest triplet states was confirmed through high-level quantum chemical computations at coupled cluster and hybrid density functional theory levels.28 They particularly computed nucleus independent chemical shifts (NICS), magnetic susceptibility exaltations, 1H NMR chemical shifts, and aromatic stabilization energies (ASE) to assess the reversal in aromaticity and antiaromaticity upon excitation from the S0 to the T1 state. Bond length equalization and high Dnh symmetry also indicate aromaticity, and later, Wörner et al. showed by photoelectron spectroscopy that the triplet ground state of the cyclopentadienyl cation indeed possesses D5h symmetry, in support of an aromatic character.29−31 In 1998, Zilberg and Haas also reported on a valence bond theoretical analysis of the triplet biradical states of CBD and cyclooctatetraene (COT).32 They wrote that “in the triplet state, the most symmetric form can be maintained, since these two [unpaired] electrons cannot form a bond, losing their distortive power. Therefore, the system is 5381


Chemical Reviews

Review

primarily interested in the applications of the excited state (anti)aromaticity concept, there is a synopsis of the theoretical and computational studies in section 3.4.

computations of two-center and multicenter electron delocalization indices.45 The relationship between aromaticity and photochemical properties and processes has also been noted. Photochemical pericyclic reactions are examples that may be rationalized in terms of aromaticity and antiaromaticity at the transition states, however, the use of the excited state aromaticity concept to explain observations in other photochemical processes is rare. In addition to the excited state aromaticity explanation given by Janoschek and co-workers for the slow decomposition of CBD,26 Wan and Krogh wrote in 1985 that the driving force for photochemical solvolysis of fluoren-9-ol is “believed to be the formation of an aromatic 4π cationic system in the excitedstate”.46 In 1999, Jursic used quantum chemical calculations to examine if triplet excited triafulvene through various reaction channels could be converted into triplet aromatic cyclobutadiene, however, concluded that this would not be a feasible approach to generate CBD.47 In 2001, Ottosson and co-workers observed in a computational study of T1 state Z/Ephotoisomerizations of aryl-substituted olefins that “the structure with the highest substituent aromaticity in T1 corresponds to the minimum on the T1 PES”,48 and for the singlet state photochemistry of COT Garavelli et al. concluded in 2002 that excited COT at its D8h symmetric structure “represents the collecting point on S1” which can be considered as “stabilized by a kind of aromatic effect”.49 In this context it is noteworthy that Paquette and co-workers already in 1974 reported that irradiation of acetone solutions of 1,2dialkylcyclooctatetraenes for 100 h lead to 87−92% recovery of the starting material.50 They wrote that “this photochemical stability contrasts in an interesting way the ready thermal rearrangements of these molecules”, a photostability which should be due to excited state aromaticity, in this case T1 state aromaticity, of cyclooctatetraenes. The conclusion on the “collecting point” feature of D8h symmetric COT in S1 by Garavelli et al. is also supported by the fact that Maier in his 1988 review on the smaller 4nπ-electron annulene CBD, and the isomeric tetrahedrane, lists as many as ten different photochemical routes to CBD starting from various precursors.51 Without doubt, CBD and COT are both located in energetic sinks on their excited state potential energy surfaces. In summary, the application of aromaticity and antiaromaticity for the rationalization of a range of photochemical properties and processes could be very rewarding and provide a handy tool both for enhanced mechanistic understanding and for molecular design.

3.1. Qualitative Molecular Orbital Theoretical Description

The first MO theoretical description of the electronically excited states of CBD and COT was given by Snyder in 1962 when he analyzed Jahn−Teller distortions in the three lowest singlet states and the lowest triplet state.53 Later on, Voter and Goddard provided a link between the MO and VB descriptions of these states for CBD (vide infra, sections 3.1 and 3.2).54 For CBD and COT the three lowest singlet states and the lowest triplet state can be constructed from the four electron configurations obtained by population of the two nonbonded molecular orbitals (NBMOs) with two electrons (Figure 2). At

3. THEORETICAL AND COMPUTATIONAL STUDIES OF EXCITED STATE (ANTI)AROMATICITY Early theoretical investigations focused on the characteristics of ground state wave functions of cyclic 4nπ-electron species as a tool to study their instabilities as opposed to cyclic (4n + 2)πelectron species. Although the lowest excited state wave functions were constructed, excited states were not the focus.52,53 In general, aromaticity and antiaromaticity in the electronic ground state can qualitatively be rationalized by either molecular orbital (MO) or valence bond (VB) theory, and these two theoretical procedures can also be applied to deduce the rules for aromatic stabilization and antiaromatic destabilization in the lowest electronically excited states. The qualitative MO- and VB-theoretical formulations are given in sections 3.1 and 3.2, respectively, whereas section 3.3 contains computational verifications. However, for the reader

Figure 2. Schematic representations of (A) the π-orbitals of CBD and the orbital energies at HMO level, and (B), the four electron configurations (three singlet multiplicity and one triplet multiplicity) which are degenerate at the HMO level.

HMO level these four configurations are degenerate, and it is first when electron repulsion is considered that the degeneracy is lifted. Based on early configuration interaction (CI) calculations Snyder showed that CBD in the first triplet state and the two singlet excited states should adopt D4h symmetric structures (Figure 3), whereas this symmetry on the PES of the lowest singlet state corresponds to the transition state structure for the Jahn−Teller distortion between two bond length alternant D2h symmetric structures. Here, it should be remarked 5382


Chemical Reviews

Review

degenerate eu orbitals doubly occupied. With regard to COT, Snyder found that it possesses D8h symmetry in the lowest triplet as well as the two excited singlet states,53 opposite to the situation for the singlet ground state where it adopts a D2d symmetric tub-shaped structure as shown experimentally some years prior to Snyder’s paper.55,56 Even though the early MO theoretical analysis of Snyder was not given in terms of aromaticity, it suggests that highly symmetric structures with CC bond length equalization and πelectron delocalization are favored in the T1 and S1 states of 4nπ-electron annulenes. Indeed, based on Hund’s rule one would expect the 13A2g triplet state to be the ground state, and this is also the situation according to MO calculations that neglect electron correlation effects. For example, Voter and Goddard reported relative energies from HF calculations on each of the four states, giving the 13A2g state as the ground state.54 However, inclusion of electron correlation at either the full π-CI level, or more recently, at the multireference average-quadratic coupled cluster (MR-AQCC) level as reported by Eckert-Maksić et al. (Figure 3B) reverses this order so that the 11B1g singlet state becomes the ground state.54,57 This is in line with the analysis and classification of biradicals as disjoint vs nondisjoint biradicals put forth by Borden and Davidson.58 The D4h symmetric CBD is a disjoint biradical, and as such, the degenerate NBMO pair (eu,x and eu,y) can be completely localized to two different sets of C atoms. As a consequence thereof, the electron correlation in the 11B1g state, involving the two paired electrons in either of the two NBMOs and the two paired electrons in the lowest πMO (a1g) of the 1Φxx and 1Φyy electron configurations, turns this state into the ground state at the square geometry.59 Independently, Dewar and Zimmerman were the first to note the reversal in the rules for aromaticity and antiaromaticity in the lowest excited state as compared to the ground state, and they were the first to use this terminology for the rationalization of photochemical processes.8,11,12 Earlier, Frost and Musulin had derived an easy-to-use mnemonic for computation of the MO energies of planar (Hückel orbital topology) annulenes according to HMO theory,10 and Zimmerman expanded this mnemonic to annulenes with Möbius orbital topology.11,12 He regarded the disrotatory vs conrotatory ring closures of polyenes, reactions which progress over transition states with Hückel vs Möbius topologies, respectively. By comparison of the HMO energies of the linear polyene in either the ground state or the lowest excited state with the analogous Hückel and Möbius annulenes in the corresponding states it was deduced whether the process is thermally or photochemically allowed. For example, the HMO energies of 1,3-butadiene in the ground and excited states are 4α + 4.48β and 4α + 3.14β, respectively (Figure 4). By comparing these state energies with those of ground and excited state Möbius CBD (4α + 5.64β and 4α + 2.82β, respectively) one finds that Möbius CBD is more stable than 1,3-butadiene in the S0 state but less stable in the S1/T1 states. The fact that Möbius CBD is more stable than the linear 1,3-butadiene thus exemplifies Heilbronner’s formulation on S0 state aromaticity of 4nπ-electron Möbius annulenes.15 The opposite is found for the regular Hückel-topology CBD because for this species both the ground state and lowest excited state of 1,3-butadiene correlate with the states of Hückel-topology CBD which have state energies of 4α + 4β at the HMO level. Thus, the opposing stabilities of Hückel vs Möbius CBD in the ground state as well as the lowest excited state, when compared

Figure 3. (A) Three lowest singlet states and the lowest triplet state of D4h symmetric CBD described by the four electron configurations which are degenerate at HMO level, and the relative energies (eV) obtained through multireference average-quadratic coupled cluster (MR-AQCC) calculations using the aug’-cc-pVTZ basis set as reported in ref 57, and (B) a qualitative representation of the potential energy curves of the four states as a function of the D2h → D4h → D2h distortion coordinate ξ. Dashed lines represent electron configurations 1 Φxx and 1Φyy, respectively, which approximately describe either of the two minima on the singlet ground state surface.

that Snyder, in contrast to Voter and Goddard, described the lowest singlet state as the in-phase combination of the two closed-shell electron configurations having either of the two 5383


Chemical Reviews

Review

Figure 4. Illustration of the changes in orbital energies at HMO level upon electrocyclic ring closures of 1,3-butadiene following either a disrotatory or conrotatory path. Orbital energies deduced through the suitable circle mnemonics. The ground state electron configurations are displayed by red filled circles and the lowest excited state configurations are displayed by green bars.

to 1,3-butadiene, become clear through this qualitative theoretical analysis. Dewar analyzed Hückel’s rule by use of PMO theory, and in his analysis compared the energies gained or lost when merging two polyenyl monoradicals into a cyclic annulene as compared to an open-chain polyene. Noteworthy, in these formal merges it is exclusively the π-bond framework which is considered. Both the open-chain polyenes and the cyclic annulenes are even alternant hydrocarbons (AHs) which fulfill the pairing theorem, and at HMO theory level they have bonding and antibonding π-MOs placed symmetrically above and below the energy of the NBMOs (E = α at HMO level).8 In the approach by Dewar, an annulene is said to be aromatic if the resonance energy of the cyclic annulene, i.e., the deviation in energy of the conjugated AH from a sum of bond energies for localized bonds, is higher than the resonance energy of the open-chain analog, and it is antiaromatic if the resonance energy is lower. The two radical fragments (A and B), that both consist of an odd number of C atoms, are each described by a set of orbitals constructed by linear combination of the 2pπ atomic orbitals of the C atoms. The NBMOs at each of the two radical fragments are combined to form a pair of molecular orbitals, one in-phase (bonding) π-MO and one out-of-phase (antibonding) π-MO, of either an annulene or a linear polyene. For convenience one of the radicals is chosen as the methyl radical whereas the other polyenyl radical is composed of 2k − 1 C atoms, where k is either an odd or an even number. If k is odd, then the resulting polyene/annulene possesses 4n + 2 π-electrons, while if k is even the resulting polyene/annulene possesses 4n π-electrons. The schematic construction of the π-bond framework of either a 4nπ- or a (4n + 2)π-electron compound from the two radical fragments, used in the PMO theoretical approach of Dewar, is illustrated in Figure 5. The first-order perturbation energy for the combination of the two radical fragments A and B into either a cyclic or a linear polyene (the π-stabilization energy) is expressed in terms of the resonance integral β as

Figure 5. Schematic illustration of construction of the π-bond frameworks of (i) either 1,3,5-hexatriene or benzene from the πbond framework of a pentadienyl (A) and a methyl (B) radical and (ii) either 1,3-butadiene or CBD from the π-bond framework of an allyl (A) and a methyl (B) radical.

The coefficient a0x is the coefficient of the singly occupied NBMO at carbon atom x of fragment A, and coefficient b0y is the coefficient of the singly occupied NBMO at carbon atom y of fragment B.8 The atoms x and y are the C atoms in the radical fragments which will be connected by C−C bonds in the annulene/polyene. If k of a radical fragment with 2k − 1 C atoms is even, then the lobes at the two terminal carbon atoms (C1 and C2k−1) of the singly occupied NBMO will have opposite phases while if k is odd then the lobes at these atoms will have equal phase. The general principle of phase changes is illustrated in Figure 6. Due to the phase changes of the NBMO of the 2k − 1 C atom fragment the π-stabilization energy, Eπ, of an S0 state annulene is different depending on whether k is odd or even, and can be related to the Eπ of an analogous polyene through eqs 2−4. Polyene: Eπ = 2βb0(a0,1) Annulene: Eπ = 2βb0(a0,1 − a0,2k − 1)

(k even)

(3)

Eπ = 2βb0(a0,1 + a0,2k − 1)

(k odd)

(4)

Thus, a (4n + 2)π-electron annulene in the S0 state can be classified as aromatic because Eπ (k odd, eq 4) is larger (more stabilizing) than that of the polyene analog (eq 2). Conversely, as Eπ of a 4nπ-electron annulene (k even, eq 3) is lower (less

Eπ = 2β ∑ a0xb0y x ,y

(2)

(1) 5384


Chemical Reviews

Review

reactions. In short, allowed pericyclic reactions, regardless if thermal or photochemical, proceed via aromatic transition states (Table 1), whereas forbidden pericyclic reactions, when forced to progress in concerted manners, proceed over antiaromatic transition states. Table 1. Requirements for Pericyclic Reactions to Proceed via Aromatic Transition States in Dependence of Orbital Topology, Number of Electrons Included in the Cyclic Array, and Electronic State ground state

Figure 6. Illustration of the phases of the π-orbital lobes a0,1 and a0,2k‑1 (red and blue representing positive and negative phase, respectively) at the two terminal C atoms of the singly occupied NBMOs in polyenyl radical fragments with 2k − 1 C atoms, with k odd (top) and k even (bottom), and the phase of the 2pπ orbital (b0) of an interacting methyl radical.

no. of electrons

excited state

Hückel topology

Möbius topology

Hückel topology

Möbius topology

4n + 2

4n

4n

4n + 2

Baird’s work, on the other hand, focused on the qualitative understanding of aromaticity and antiaromaticity of 3ππ* (T1) state equilibrium geometries of 4nπ- and (4n + 2)π-electron annulenes, respectively.20 Similar to Dewar, Baird used the PMO theoretical approach but applied it slightly differently as his analysis of the orbital interaction in formation of the triplet biradical annulene from two radical fragments involved all πMOs matched according to their symmetries. The orbital interactions in Baird’s approach are divided into two classes, denoted as types I and II interactions (Figure 8), where the

stabilizing) than that of the polyene analog, such an annulene can be termed antiaromatic in the S0 state. Using this model, the rules can be extended to the lowest ππ* excited singlet state. In a first approximation, the ππ* excited state of the annulene has one electron in the bonding MO, which result from the interaction between the two NBMOs of the two polyenyl radicals, and another electron in the antibonding MO (Figure 7). At HMO level the antibonding

Figure 7. Orbital energy levels of the highest π and π* orbitals of an annulene obtained by combination of the singly occupied NBMOs at (A) HMO level and (B) PMO level of two polyenyl radical fragments, and the population of the ππ* state.

MO is destabilized to the same extent as the bonding MO is stabilized. This level of approximation leads to no change in energy of the excited state annulene when compared to the two radical fragments (Figure 7A), and it should be categorized as nonaromatic. However, the crude HMO picture, in which overlap between different 2pπ AOs is neglected, is erroneous and as remarked by Dewar it “grossly underestimates the antibondingness of the antibonding MOs”.8 When overlap is included one can show that the antibonding MO is destabilized to a greater extent than the corresponding bonding MO is stabilized (Figure 7B).60 With more appropriate relative orbital energies of the π and π* MOs taken into account the resulting ππ* excited state of the annulenes becomes destabilized relative to the two radicals. This led Dewar to conclude that “[if] then one uses the terms aromatic and antiaromatic in an extended sense, to denote the energy of any cyclic system relative to that of an analogous open-chain structure, one must conclude that the first π−π* excited state of an even aromatic AH is antiaromatic, and that of an antiaromatic AH is aromatic.”8 Rigorous discussions of the PMO description of aromaticity and antiaromaticity in ππ* excited states were subsequently presented independently by Dougherty and Baird. 17,20 Dougherty focused his analysis to transition state (anti)aromaticity and generalized the analysis to all pericyclic

Figure 8. Schematic drawing that displays the types I and II interactions (ΔEI and ΔEII, respectively) between the π-orbitals of suitable polyenyl radical fragments which when combined yield triplet biradical benzene and triplet biradical COT. The nonzero types I and II interactions are marked with dashed lines in red and blue, respectively. The symmetry or antisymmetry of a π-MO with respect to a bisecting mirror plane is labeled with an S and an A, respectively. A total interaction ΔEI + ΔEII > 0 corresponds to a stabilization upon cyclization and a triplet state aromatic ring, whereas a total interaction ΔEI + ΔEII < 0 corresponds to a destabilization upon cyclization and a triplet state antiaromatic ring.

total interaction is the combination of types I and II interactions. The type I interaction represents the interaction between the singly occupied NBMOs of the two radical fragments, while type II interaction represents the symmetrymatched interaction between the NBMO of one radical with the doubly occupied as well as vacant π-MOs of the other polyenyl radical fragment. 5385


Chemical Reviews

Review

π-electrons.11,12,14 Thus, if benzene with Möbius topology could be formed it would be aromatic in its T1 and S1 states, while Möbius CBD in these states would be antiaromatic. It should, however, be stressed that the rules represented by the aromaticity/antiaromaticity cube of Figure 9 are qualitative and that exceptions and complications can occur, particularly for the S1 state as several different electron configurations can be important for this state. The rules should therefore primarily be seen as guidelines providing qualitative tools for rationalization of experimental observations as well as tools for molecular design.

Following this approach, the most convenient formal split of triplet state benzene is into two allyl radicals, an odd AH with an NBMO which is antisymmetric with respect to a bisecting mirror plane.20 The suitable split of COT is into an allyl radical and a pentadienyl radical, where the NBMO of the pentadienyl radical is symmetric with respect to a bisecting mirror plane. For triplet state benzene the type I interaction leads to a large destabilization because the out-of-phase antibonding MO combination is raised in energy more than the in-phase bonding MO combination is lowered in energy, and consequently, with one electron in each MO the type I interaction for triplet state benzene is destabilizing when compared to two allyl radicals (ΔE < 0, in the sign convention used by Baird). For COT the type I interaction leads to no change in energy because of the mismatch in the symmetries of the NBMOs of the allyl and pentadienyl radicals. With regard to the type II interaction the situation is opposite; for benzene this interaction is zero due to orbital symmetry mismatch while for COT it is nonzero. Indeed, for COT the type II interaction leads to a small stabilization when combining an allyl radical with a pentadienyl radical into triplet COT. Taken together, the overall energy change upon fusion of the two polyenyl radicals to triplet annulenes will be a net (aromatic) stabilization for COT and a net (antiaromatic) destabilization for benzene. Recently, annulenes with Möbius topology and 4n πelectrons, which are therefore aromatic in the S0 state, have received growing attention due to significant advancement in the design and synthesis of increasingly more Möbius aromatic molecules.61−63 The experimental studies have thus proven Heilbronner’s postulation from 1964 on Möbius aromaticity.15 However, one may also deduce rules for excited state (anti)aromaticity of species with Möbius orbital topology. As the uniqueness of Möbius annulenes stems from the odd number of nodes between the 2pπ atomic orbitals that constitutes the π-orbital framework, a reversal in the electron counting rule for excited state Möbius aromaticity as compared to ground state Möbius aromaticity can be derived through a Baird-type analysis similar to that of Figure 8. Consequently, when varying the number of π-electrons, the orbital topology, and the electronic state, there are four combinations that will lead to aromatic stabilization (Figure 9). In the lowest ππ* excited states, these occur for Hückel-topology annulenes with 4n π-electrons and for Möbius-topology annulenes with 4n + 2

3.2. Qualitative Valence Bond Theoretical Descriptions

Valence bond (VB) theory can also provide means for understanding the aromatic and antiaromatic characters of ground and excited states of 4nπ- and (4n + 2)π-electron annulenes, although it is less straightforwardly seen than from MO theory. For D4h symmetric CBD, Craig reported in 1951 that two singlet states result from the equal combination of VB structures A and B shown below into two singlet states; the (A + B) and (A − B) combinations of which he concluded that the first corresponds to the ground state.52 Although the discussion did not address aromaticity in electronically excited states Craig was the first to apply VB theory for the description of the lowest few singlet states of CBD. Later on, van der Hart and coworkers addressed aromaticity through an extended VB theory in which the Heitler-London bond functions are exchanged to general two-electron functions.64 Kuwajima addressed aromaticity and antiaromaticity in terms of ring-permutations and showed that the contribution from permutations which permute electrons circularly around the ring accounts for the difference between even and odd-parity annulenes in terms of destabilizing and stabilizing effects, respectively.65

Voter and Goddard explicitly examined the lowest few πelectronic states of CBD by usage of VB theory and also made a connection to the MO theoretical description.54 In contrast to Craig, they found that the ground state at D4h symmetry is the singlet multiplicity out-of-phase combination of the VB structures A and B (11B1g), whereas the in-phase combination is the antiresonant destabilized excited state (11A1g). Moreover, three triplet states, the lowest being the resonant 13A2g state (T1 + T2 + T3 + T4) and the two higher being the doubly degenerate antiresonant 13Eu states (T1 − T3 and T2 − T4, respectively), were constructed from the four equivalent triplet structures T1, T2, T3, and T4 displayed below.

The excited state (anti)aromaticity in VB terminology is most readily seen for the triplet state, and it was first deduced by Zilberg and Haas.32 Since the two unpaired electrons with equal spin in VB structures T1 to T4 do not form a bond, the 4nπ-electron annulenes in their lowest triplet biradical state, the resonant 13A2g state, can be viewed as a pseudoclosed-shell (4n − 2)π-electron cycle which possesses Hückel-aromatic stability and, additionally, two nonbonded same-spin π-electrons (Figure 10). By a similar interpretation of the T1 states of

Figure 9. Mnemonic illustration of the π-electron counting rules for aromaticity and antiaromaticity. The cube displays the 3-fold variation with (i) number of π-electrons (4n or 4n + 2), (ii) orbital topology (Hückel or Möbius), and (iii) electronic state (S0 or S1/T1). 5386


Chemical Reviews

Review

Figure 11. Illustration of the twin-state model. The black solid lines represent the potential energy surfaces for the two individual resonance structures L and R described by the wave functions |ψL⟩ and |ψR⟩, respectively. A linear combination of these resonance structures generates two new electronic states, a ground (S0) state, and an excited (S1) state, with corresponding potential energy surfaces represented by the blue dashed lines and separated by an avoided crossing.

Figure 12. Illustration of the two resonance structures L and R for a planar C2kH2k annulene constituted of k spin pairings.

Figure 10. Pictorial description of (A) triplet state cyclooctatetraene as a combination of a closed-shell Hückel aromatity COT dication plus two π-electrons of the same spin which are unable to form a bond, and (B) triplet state benzene as a combination of a pseudoclosed-shell Hückel antiaromatity benzene dication which distorts so as to place the two additional π-electrons of the same spin as far from each other as possible.

|ψL⟩ =

∑ εpPφ̂ 1(1)φ2(2)...φ2k(2k)·[α(1)β(2) p

− β(1)α(2)][α(3)β(4) − β(3)α(4)]... [α(2k − 1)β(2k) − β(2k − 1)α(2k)]

|ψR ⟩ =

(4n + 2)π-electron annulenes it follows that these compounds tend to distort away from the most symmetric structures as such structures are antiaromatic having pseudoclosed-shell 4nπelectron cycles.32 However, unlike S0 state antiaromatic annulenes the structural distortion of T1 state (4n + 2)πelectron annulenes is not along the regular ξ-coordinate (vide infra, Figure 11) but it is instead directed toward a structure represented by a resonance structure with the two unpaired same-spin π-electrons moved as far apart as possible. Zilberg and Haas also used VB theory to tackle aromaticity in the lowest singlet excited state, and made use of the twin-state model of Shaik and Hiberty (Figure 11).66−70 Planar annulenes composed of an even number (2k, k = 1, 2, ...) of C atoms were considered as linear combinations of two resonance structures L and R, with alternating CC double and single bonds (Figure 12). The two resonance structures represent 2k pπ-electron spin pairings either counter-clockwise (L) or clockwise (R). The wave functions representing the two resonance structures |ψL⟩ and |ψR⟩ are written as

(5)

∑ εpPφ̂ 1(1)φ2(2)...φ2k(2k)·[α(1)β(2k) p

− β(1)α(2k)][α(2k − 1)β(2k − 2) − β(2k − 1)α(2k − 2)]...[α(3)β(2) − β(3)α(2)] (6)

Here ϕj(i) represents the 2pπ-atomic orbital on the jth C atom occupied by the ith electron (i, j = 1, 2, ..., 2k) and α and β are electron spin functions. The summation is over 2k! permutations, each with parity εp. The total wave function of a planar annulene is described by a linear combination of |ψL⟩ and |ψR⟩, and the two possible combinations are the in-phase |Ψ+⟩ = cL(ξ)|ψL⟩ + cR(ξ)|ψR⟩ and the out-of-phase |Ψ−⟩ = cL(ξ)|ψL⟩ − cR(ξ)|ψR⟩ combinations. The coefficients cL(ξ) and cR(ξ) are dependent on the “interchange” coordinate, ξ, between the two resonance structure, and it should be interpreted as illustrated in Figure 11. Which one of the two states |Ψ+⟩ and |Ψ−⟩ is the S0 state and which one is the S1 state depends on the number of pπ-electron spin pairings, the parity, in the annulene. The energies for the two states are given by 5387


Chemical Reviews Eπ =

Review

for the change in Kcyclic upon distortion along the ξ coordinate (ΔKcyclic) was also derived, however, it was argued that ΔKcyclic is rather small and cannot constitute the main reason for the geometric destabilization of 4nπ-electron annulenes in their S0 states. The profiles of the excited state PES, as reflected in the vibrational frequencies for the Kekulé mode (Figure 14), are

⟨ψL|Ĥ |ψL⟩ + ⟨ψR |Ĥ |ψR ⟩ ± 2⟨ψL|Ĥ |ψR ⟩ 2 ± 2⟨ψL|ψR ⟩

(7)

From evaluation of the matrix elements in eq 7 for |Ψ+⟩ and |Ψ−⟩ for the fully symmetric annulene (ξ = 0, and cL(ξ) = cR(ξ)) Zilberg and Haas concluded that |Ψ−⟩ is the ground state for annulenes that have an even number pπ-electron spin pairings (4nπ-electron annulenes) and |Ψ+⟩ is an excited state,67 although this view was later criticized by Dufey,71 vide infra. For annulenes with an odd number of pπ-electron spin pairings ((4n + 2)π-electron annulenes) they conclude that |Ψ+⟩ is the ground state and |Ψ−⟩ an excited state. The potential energy curves shown in Figure 11 illustrate the basic idea of the twinstate model. Yet, to explain the differences in aromaticity and antiaromaticity between 4nπ- and (4n + 2)π-electron systems, respectively, Zilberg and Haas invoked the different exchange contributions in the cross term ⟨ψL|Ĥ |ψR⟩ in eq 7.67 In addition to the regular Coulomb and transposition-type exchange integrals this cross-term also contains cyclic exchange integrals Kcyclic. If the cross-term is negative, the ground state will be the in-phase combination and the out-of-phase combination the excited state. The opposite is the case when ⟨ψL|Ĥ |ψR⟩ is positive. They conclude that these terms for the fully symmetric structure of 4nπ-electron annulenes result in a destabilization in their S0 states and stabilization in the S1 states. Conversely, for (4n + 2)π-electron annulenes, the S0 state can be stabilized enough to generate a minimum at the symmetric structure, whereas this structure is elevated in energy in the S1 state. This leads to perturbations of the potential energy curves of Figure 11, and those perturbations are illustrated in Figure 13. Yet, the

Figure 14. Kekulé mode motions for benzene and COT which are important in the two twin-states of each of the two molecules.

interesting in the context of aromatic stabilization and antiaromatic destabilization. First, the frequency exaltation observed for benzene upon excitation is well-known and noteworthy.70,72,73 More specifically, a frequency exaltation from 1300 to 1570 cm−1 was observed for the b2u mode upon excitation of benzene from 11A1g (S0) to the 11B2u (S1) state. Importantly though, the calculated CASSCF frequencies for the corresponding Kekulé modes, scaled by 0.87, are significantly higher for excited state CBD and COT (1825 and 2491 cm−1, respectively) than for benzene, reflecting that the PESs of the excited twin-states of the former two molecules have steeper profiles, which should be a result of their stabilization through excited state aromaticity. These observations lead us naturally over to computational results that confirm excited state aromaticity and antiaromaticity. 3.3. Computational Evaluations of (Anti)Aromaticity of Annulenes in Their Lowest Excited States

The correct assessment of aromaticity has been much debated as aromaticity is a property that cannot be directly observed and measured.74,75 Despite this, several indirect measures and indices have been developed in order to quantify the degree of aromaticity of a specific compound, and these have been applied to a range of different compound classes. Only some of these indices have been used to evaluate aromaticity in electronically excited states. Below, we show how the application of computed aromaticity indices applied to simple annulenes and annulenyl anions and cations verify the qualitative theories of excited state aromaticity described in the previous sections. 3.3.1. Energy-Based Aromaticity Indices. The first aromaticity index to be applied to electronically excited states was the aromatic stabilization energy (ASE), or more specifically, the Dewar resonance energy (DRE),76,77 as calculated by Baird based on energies from semiempirical computations using the NNDO method (Table 2).20 By comparing the energies of triplet state annulenes with the energies of the corresponding most stable triplet state polyene it was shown that annulenes with 4n π-electrons in general are stabilized in their triplet states. Positive DRE values, as found for triplet state CBD and COT (14.1 and 17.7 kcal/mol, respectively), were indicative of aromaticity. On the other hand, the two triplet state benzene isomers shown in Figure 15 were found by Baird to have DRE values of −16.4 and −12.3 kcal/ mol, clearly destabilized and representative of antiaromatic species. Later on, Schleyer and co-workers deduced ASEs (Table 2) at the (U)CCSD(T)/cc-pVDZ level through isogyric

Figure 13. Potential energy surfaces for motion along the interchange coordinate, ξ illustrated for (4n + 2)π-electron and 4nπ-electron annulenes in the S0 (black) and S1 states (blue). The different perturbations of the potential energy surfaces arise from the differences in the cross terms in eq 7. The (un)perturbed curves are presented as (dashed) solid lines.

actual degree of (de)stabilization has to be determined numerically for each compound in question.71 It can also be noted that according to this model the excited state (4n + 2)πelectron systems are not distorted in the interchange coordinate but sustains high symmetry upon excitation. Yet, the conclusion by Zilberg and Haas that the S0 state is the in-phase combination for (4n + 2)π-electron annulenes and the out-of-phase combination for 4nπ-electron annulenes was challanged by Dufey.71 Even though he did not explicitly address the excited states he concluded that the two VB functions enter with like sign in the S0 state of both aromatic and antiaromatic annulenes, and that the opposite should apply to the corresponding excited twin state. An alternative formula 5388


Chemical Reviews

Review

Table 2. Values of Energy Based Aromaticity Indices in kcal/ mol for CBD, Cyclopentadienyl Cation, Benzene, and COT species C4H4 C5H5+ C6H6 C8H8

state S0 T1 S0 T1 T1 T1

point group D2h D4h C2v D5h D2h D8h

state symmetry 1

Ag A2g 1 A1 3 A2′ 3 B1u 3 A2u 3

ASEa −34.1 7.0 −28.5 23.2

DREb 14.1

−16.4/−12.3c 17.7

a

Aromatic stabilization energies (ASE) using isogyric equations based on (U)CCSD(T)/cc-pVDZ energies. Note that the same sign convention as used for the DRE presented in ref 17 is used, which is opposite to the convention of ref 28. bDewar resonance energies (DRE) calculated at the semiempirical NNDO level (ref 20). cValues are for the diallylic and biquinoidal forms of benzene, respectively, as shown in Figure 15 (ref 20).

equations for CBD and the cyclopentadienyl cation in both the S0 (−34.1 and −28.5 kcal/mol, respectively, when following the sign convention as used earlier for the DRE) and T1 states (7.0 and 23.2 kcal/mol, respectively).28 These high-level quantum chemical calculations thus confirm significant degrees of stabilization of 4nπ-electron annulenes in their lowest ππ* excited triplet states, in line with Baird’s results from earlier semiempirical calculations.

Figure 16. Isomerization stabilization energies (ISE) in kcal/mol for the triplet states of 2-methylcyclopentadiene, methylcyclobutadiene, toluene, and methylcyclooctatetraene at UB3LYP/6-311++G(d,p) level and in the S0 state of toluene at B3LYP/6-311+G(d,p) + ZPE level.78,79 Note that aromaticity (antiaromaticity) corresponds to negative (positive) values, in contrast to the DRE and ASE values as reported above. The ISE(S0)corr and ISE(T1)corr values are ISE values corrected for anti−syn diene mismatches.

Table 3. Resonance Energies of Annulenes Calculated by Means of HMO Theorya resonance energy (β)

Figure 15. Two triplet state isomers calculated by Baird to have Dewar resonance energies of −16.4 (I) and −12.3 (II) kcal/mol at the NNDO semiempirical level.20

More recently, Zhu et al. used the approach of isomerization stabilization energies (ISEs),78 introduced by Schleyer and Pühlhofer and in which aromaticity is evaluated by the energy difference between methyl derivatives of an annulene and an isomeric species with acyclic conjugation and an exocyclic methylene group.79 The calculated ISEs of eight monocyclic 4nπ-electron species as well as pentalene in their lowest triplet states were negative in line with aromatic character while triplet state benzene displays a positive ISE typical of antiaromaticity (Figure 16). In 1978, Aihara calculated the resonance energies based on HMO theory of both Hückel and Möbius annulenes in their ground states as well as lowest excited states (Table 3).21 For calculations of the resonance energies, i.e., the difference between the HMO energy of a conjugated compound and that of a corresponding idealized structure with localized CC double bonds, he made use of a characteristic polynomial termed the reference polynomial to describe the localized structure. These calculated resonance energies, given in terms of the resonance integral β, were in accordance with the four aromatic and the four antiaromatic combinations shown in Figure 9 because positive values were found for the aromatic states and negative for the antiaromatic ones. A particularly negative resonance energy was calculated for benzene in the first excited state, and this destabilization was concluded to be one cause for the complex photochemistry of benzene, which forms benzvalene, Dewar benzene, and prismane upon irradiation. Aihara also

a

species (π-orbital topology)

ground state (S0)

excited state (T1/S1)

C4H4 (Hückel) C4H4 (Möbius) C6H6 (Hückel) C6H6 (Möbius) C8H8 (Hückel) C8H8 (Möbius) C10H10 (Hückel) C10H10 (Möbius)

−1.226 0.431 0.273 −0.799 −0.595 0.201 0.159 −0.474

0.305 −0.867 −0.692 0.236 0.186 −0.550 −0.451 0.152

Values given in resonance integral β as reported in ref 21.

found stabilizing resonance energies for (4n + 2)π-electron annulenes with Möbius topology in their lowest excited states (e.g., Möbius benzene in the T1/S1 states). Two years later Ilić, Sinković and Trinajstić reported topological resonance energies (TREs) for a large series of Hückel as well as Möbius annulenes in the ground states and lowest ππ* excited states.80 The same observation as that of Aihara was made, yet, it was also found that the TREs for large annulenes of both Hückel and Möbius topologies go to gradually smaller values (e.g., the TREs for CBD and [20]annulene in their ππ* states are 0.304 and 0.080, respectively). More recently the finding on ππ* excited state Möbius aromaticity was confirmed computationally for the T1 state of 4n + 2 π-electron perfluorinated annulenes by Rzepa and co-workers;33 however, the validity of this theory remains to be verified experimentally. 3.3.2. Geometry-Based Aromaticity Indices. A small (or no) variation in the CC bond lengths is often taken as a rough indicator of aromaticity. Indeed, all of the 4nπ-electron annulenes, except the cyclononatetraenyl cation (C9H9+) studied at (U)B3LYP/6-311+G(d,p) level by Schleyer and co-workers have T1 state equilibrium geometries of the highest 5389


Chemical Reviews

Review

Figure 17. Optimal (U)B3LYP/6-311G+(d,p) geometries showing the bond lengths (Å), NICS(0) values (ppm, given in the ring centers), and 1H NMR chemical shifts (ppm) of CBD, cyclopentadienyl cation, benzene dication, tropylium anion, COT, and cyclononatetraenyl cation in their their lowest singlet and triplet states. Values as given in ref 28.

with antiaromaticity. 81,82 However, an analysis of the correlation between HOMA and the magnetically based indices earlier calculated by Gogonea et al. (vide infra)28 showed that the two indices described the degree of aromaticity similarly for only a few species. Still, from the study of the HOMA index it was concluded that the 4nπ-electron annulenes in their triplet states exhibit partial aromatic character which increases as the size of the ring increases (Table 4).81 Noteworthy though, assessment of aromaticity and antiaromaticity in the excited states using models based on structural criteria such as HOMA should be done with caution as these models have been developed for compounds in their singlet ground states. Using a limited CI with singly excited configurations, Janoschek and co-workers found CBD to be D4h symmetric with bond lengths in the S1 and T1 states of 1.436 and 1.440 Å, respectively, and it was concluded that both states display a certain degree of aromaticity.83 More recent high-level ab initio studies revealed, however, that CBD in the S1 state has a slightly distorted C2h symmetric structure with two hydrogen atoms pointing out of the plane (e.g., at MR-CISD level the

possible Dnh symmetry, and the CC bond lengths for the gradually larger CnHn(±) (n = 4−8) are 1.440, 1.424, 1.427, 1.414, and 1.403 Å, respectively (Figure 17).28 Although C9H9+ does not possess D9h symmetry the calculated structure in the T1 state is highly planarized with all CC bond lengths in the range 1.404−1.409 Å. In general, (4n + 2)π-electron annulenes, which are highly symmetric in the S0 state, undergo distortions to lower symmetry upon excitation, while the opposite is true for 4nπ-electron systems (Table 4). Still, some (4n + 2)πelectron annulenes prefer planarity and bond length equalization in the S1 state despite their antiaromatic character, as recently pointed out by Karadakov.35 This last finding is in line with the analysis of the S1 state within the twin-state model (Figure 13).67 The geometries of Gogonea et al. were subsequently used by Krygowski and Cyranski to evaluate the aromaticity using the HOMA index (harmonic oscillator model of aromaticity), a geometry-based aromaticity index where a value close to 1.0 suggests high degree of aromaticity, a value around zero corresponds to nonaromaticity, and negative values comply 5390


Chemical Reviews

Review

state is the CC bond length which is significantly longer than in the aromatic S0 state (1.435 vs 1.398 Å).84,85 A more general discussion on the excited state geometries is given in section 4.2. Compounds with Möbius topology and 4n + 2 π-electrons also display properties in their lowest ππ* excited triplet states that suggest substantial aromatic character, as predicted qualitatively (section 3.1). Rzepa and co-workers studied such species at ROB3LYP/6-31G(d) level, and the most notable examples in this category are the triplet state C9F9− and C14F142‑, two species with 10 and 14 π-electrons, respectively.33 In these Möbius annulenes, the mixing between the σ- and πorbitals is minimized due to the perfluoro substitution whereby the π-electronic effect is distinguished. Indeed, rather small CC bond length variations were found (in 3C9F9− 1.377−1.411 Å and in 3C14F142‑ 1.377−1.404 Å), in support of the T1 state aromatic character of (4n + 2)π-electron annulenes with Möbius topology. 3.3.3. Magnetically Based Aromaticity Indices. Through computation of the absolute magnetic shielding tensor at the geometric center of a potentially aromatic (antiaromatic) cycle the diatropic (paratropic) ring current can be evaluated. In 1996, Schleyer and co-workers introduced the nucleus independent chemical shift (NICS) as an aromaticity index,87 and in its original form it was taken as the negative of the calculated isotropic shift at the ring center (NICS values with negative signs correspond to aromatic character and those with positive sign to antiaromatic character). However, the NICS method has been improved since its introduction, and it has been concluded through a comparison of different NICS

Table 4. Values from Geometry Based Aromaticity Indices species C4H4 C5H5+ C5H5− C6H62+ C6H6 C7H7− C8H8 C9H9+

state S0 T1 S0 T1 S0 T1 S0 T1 S0 S0 T1 S0 T1 S0 T1

point group D2h D4h C2v D5h D5h C2v Cs D3d D6h C2 D7h D2d D8h Cs Cs

state symmetry 1

Ag A2g 1 A1 3 A1′ 1 A1′ 3 A1 1 A′ 3 Bg 1 A1g 1 A 3 A1′ 1 A1 3 A2u 1 A′ 3 A′ 3

HOMA

Δrcca

−3.99 0.30b −1.34b 0.67b 0.812c 0.308c 0.29b 0.61b 0.99b 0.18b 0.83b −0.21b 0.94b 0.30b 0.91b

0.244b 0b 0.228b 0b 0c 0.110c 0.070b 0b 0b 0.139b 0b 0.132b 0b 0.125b 0.005b

b

a

Bond length alternations in Å. bData as reported in ref 81 based on geometries from ref 28 at the B3LYP/6-311+G(d,p) level. cData based on B3LYP/6-311+G(d,p) calculations reported in ref 86.

out-of-plane bending is 30.5°, and the CC bond length is 1.455 Å).57 An interesting observation in the context of bond length variation is the finding that benzene also prefers D6h symmetry in the S1 state, despite that it is strongly antiaromatic according to other indices (vide infra).35 As noted above the D6h symmetry is, however, in line with the conclusions from the twin-state model. Yet, an interesting feature of benzene in its S1

Figure 18. (A) Benzene in the S0 state, (B) CBD in the S0 state, (C) CBD in the T1 state, and (D) COT in the T1 state.89 NICS values in ppm are given as a function of distance (Å) above the molecular plane (black square = out-of-plane component, red circle = in-plane component, and blue triangle = isotropic shift). The NICS values were calculated at GIAO-B3LYP/6-311+G(d)//B3LYP/6-311G(d) level of theory, and are analogous to the GIAO-HF/6-311+G(d)//B3LYP/6-311G(d) values reported in ref 89, yet, include electron correlation as recommended by Soncini et al. (ref 91). 5391


Chemical Reviews

Review

Table 5. Values from Magnetically Based Aromaticity Indices species C4H4

C5H5+ C5H5− C6H62+ C6H6

C7H7− C8H8

C9H9+

state S0 S0 S1 S1 T1 T1 S0 T1 S0 S0 T1 S0 S1 T1 S0 T1 S0 S0 S0 S1 T1 S0 T1

point group D2h D4h D2h D4h D2h D4h C2v D5h D5h Cs D3d D6h D6h D6h C2 D7h D2d D4h D8h D8h D8h Cs Cs

state symmetry 1

Ag B1g 1 B1g 1 A1g 3 B1g 3 A2g 1 A1 3 A1′ 1 A′ 1 A′ 3 Bg 1 A1g 1 B2u 3 B1u 1 A 3 A1′ 1 A1 1 A1g 1 B1g 1 A1g 3 A2g 1 A′ 3 A′ 1

NICS(0)a a

c

18.4 /27.6 36.4 17.3 3.4 −1.2 −3.7a/−5.3c 49.2c/54.1e −4.5c −19.4e 11.0c −1.5c −8.2a/−9.7c 45.8 39.6 42.9c −11.9c 1.16a/3.0c 16.1a/30.1c 40.7 −8.9 −8.9, −12.4c 9.1c −9.7c

NICS(1)a 12.3 28.2 7.4 −4.3 −5.4 −6.5

−9.5 34.7 30.1

−1.6 12.0 32.2 −9.0 −9.0

NICS(0)zza NICS(1)zza σiso(1H)a 91.6 145.9 59.5 24.6 31.1 24.3

−12.2 145.9 130.5

14.1 55.4 128.7 −21.5 −20.6

39.3 88.1 12.9 −16.4 −13.3 −16.5

−27.8 102.8 90.6

4.2 37.6 98.2 −26.3 −25.6

26.70 27.60 21.59 23.96 24.87 25.15

24.90 29.54 29.31

27.26 28.91 31.70 25.43 25.57

χisoa −21.76 −12.20 −17.26 −28.78 −30.32 −32.16

−59.33 2.43 −6.16

−61.51 −33.10 20.87 −88.38 −88.55

χtotb,c −7.7

c

−22.8c −1.6/ 4.8c −28.4c −67.7b −13.7c −28.2c −51.3c

24.7c −64.5c −46.2c 4.1c

Λc

δ(1H)d

12.5

5.9

−3.5 32.5 −3.3 −17.2e

7.4 5.2 8.0

−13.4

7.0 8.5 7.8

60.4

3.1 7.7 6.0 3.1

−81.6c 1.0 8.6

a

CASSCF-GIAO calculations with 6-311++G(2d,2p) basis set for C4H4 and C6H6 with ground state geometries, while for C8H8 the magnetic properties were calculated for optimized excited state geometries at the CASSCF-GIAO/6-311G(d)//CASSCF(8,8)/6-31G(d,p) level.35,36 b Magnetic susceptibilities and anisotropic magnetic susceptibilities (in ppm cgs).99 cValues at GIAO-HF/6-31+G(d)//B3LYP/6-311+G(d,p) level.28 dThe 1H chemical shifts have been corrected for charge.28 eGIAO-HF/6-31G(d)//B3LYP/6-31G(d).99

indices against the ASEs of five-membered (hetero)annulenes that the NICS(0)πzz index, which is based on the π-contribution to the out-of-plane zz tensor component, is the most precise.88 Yet, the more readily available NICS(1)zz index provides values of almost equal quality. Soon after the introduction of the NICS method, Schleyer and co-workers calculated the NICSs, magnetic susceptibilities, and magnetic susceptibility exaltations of 4nπ-electron annulenes ranging from CBD to C9H9+ in their lowest singlet and triplet states.28 Negative NICS values were observed for the triplet states of each of the 4nπ-electron annulenes, revealing that these species display diatropic ring-currents, thus supporting aromatic character. In contrast, positive (antiaromatic) NICS values were obtained for the singlet states, revealing paratropic ring-currents. Moreover, the calculated 1H NMR chemical shifts for the 4nπ-electron annulenes in their lowest triplet states were found in the narrow interval 7.4−8.6 ppm, thereby closely resembling the corresponding shift value of benzene in its aromatic S0 state (7.8 ppm). The NICS method has not only been used for planar annulenes in their lowest ππ* excited states but also for compounds with Möbius topology in such states.33 The triplet states of the C9F9− and C14F142‑ anions, with Möbius topology and 10 and 14 π-electrons, respectively, have NICS(0) values of −13.0 and −12.1 ppm, revealing diatropic ring currents and aromatic character at UB3LYP/6-31G(d)//ROB3LYP/631G(d) level. Hence, this confirms the double reversal in the electron counting rule; first when going from Hückel to Möbius topology, and thereafter when going from S0 to T1.

As an improvement of the NICS method Stanger as well as Solà and co-workers independently deduced a method where the NICS values were plotted as a function of distance from the molecular plane as NICS values determined at the ring center and 1.0 Å above the ring plane may give contradictory aromaticity assignments.89,90 The plots include the NICS values dissected into their in-plane and out-of-plane components as well as the isotropic chemical shifts (Figure 18). In this way one obtained results that are more consistent with other aromaticity indices than those provided by a single (isotropic) NICS value, and a typical NICS plot of an aromatic compound is characterized by a minimum for the out-of-plane component, as seen for the S0 state of benzene in Figure 18A.89 The NICS plot of an antiaromatic annulene, in contrast, does not display such a minimum, but now the out-of-plane component instead goes steadily from paratropic values toward zero as the distance is increased. In Figure 18B this feature is displayed for CBD in the S0 state. In the NICS plots of Figure 18C and D, the T1 states of both CBD and COT show typical aromatic behavior with minimum values in their curves of the out-of-plane component of −17.5 (1.1 Å above the ring plane) and −32.4 ppm (0.9 Å above the ring plane), respectively. These values can be compared to that found for benzene in the S0 state (−29.1 ppm, 1.0 Å above the ring plane). In Figure 18C one sees that the T1 state of CBD displays a somewhat smaller minimum value in the out-of-plane component curve (−17.5 ppm), which is due to the close proximity of the NICS probe to the bonds and atoms that enhances the paramagnetic contribution. This explanation is supported by the fact that the cyclopentadienyl cation in the T1 5392


Chemical Reviews

Review

calculated isotropic proton shieldings in these states (25.4 and 25.6 ppm, respectively) to that calculated for benzene in the S0 state (24.9 ppm). Other magnetic properties than NICS have also been computed and analyzed. The degrees of aromaticity and antiaromaticity in singlet excited as well as as higher excited triplet states of benzene were evaluated by Kataoka through magnetic susceptibilities given in the form of the relative magnetic susceptibility Λrel = Λexcited state/Λground state and calculated at the PPP semiempirical level of theory.34 For the S1 (11B2u) and T1 (13B1u) states he found Λrel to be −4.79 and −2.00, respectively; that is, they display clear antiaromatic character when compared to the aromatic S0 state. With regard to the higher excited S2 (B1u), S3 (E1u), T2 (E1u), and T3 (B3u) states he calculated the magnitudes of Λrel as 6.29, 0.75, 0.75, and 3.39, respectively. Explicit current-density maps, calculated at coupled-ROHF/ CTOCD-DZ/6-31G(d,p)//ROHF/6-31G(d,p) level using the ipsocentric approach, i.e., an approach in which a variable point-of-origin is used for computation of the magnetic response, have been reported by Fowler and co-workers (Figure 19).40,94 These maps clearly show that 4nπ-electron annulenes with triplet multiplicity display diamagnetic ring

state has a clearly aromatic NICS plot with a minimum value in the out-of-plane component of approximately −24 ppm at 1.1 Å above the plane.89 By partitioning the NICS(0)πzz aromaticity index into its α and β contributions, Mandado et al. found that triplet state annulenes are aromatic both with regard to their α- and their βcontributions in the same way as closed-shell singlets because both the former and the latter type of species have 2n + 1 παelectrons and 2m+1 πβ-electrons (n, m = 0, 1, 2, ...).92 Thus, for S0 state aromatic species n = m, while for T1 state aromatic species n = m + 1. At the (U)B3LYP/6-311++G(d,p) level, benzene in the S0 state has NICS(0)πzz contributions of −17.9 ppm from the πα-electrons as well as from the πβ-electrons, and the partitioned NICS(0)πzz values in the T1 states of C4H4 (−15.1 (α), −6.7 (β)), C5H5+ (−17.6 (α), −6.6 (β)), C7H7− (−24.6 (α), −16.8 (β)), and C8H8 (−24.7 (α), −16.3 (β)) all indicate that these species are both α- and β-aromatic. To examine the aromaticity in singlet excited states and other more highly excited states, Karadakov utilized a CASSCFGIAO protocol to calculate NICS values in the S1, S2, and T2 states, and to compare these to the calculated values in the S0 and T1 states (Table 5).35,36 The annulenes and states calulated were; benzene in the S0, S1, and T1 states, CBD in the S0, S1, S2, and T2 states, as well as COT in the S0, S1, T1, and the first septet states. Both the square and the rectangular structures of CBD were studied as well as the D8h, D4h, and D2d symmetric structures of COT. The geometry that was utilized in the calculations of benzene was taken from the gas-phase geometry of the S0 state established based on the ν4 vibration−rotation bands in the rotational Raman spectra of C6H6 and C6D6.93 Finally, the COT geometries were obtained through quantum chemical calculations at the CASSCF(8,8)/6-31G(d,p) level. A comparison of the NICS(1)zz values of benzene in the S1 (1 1B2u ) and T1 (13B1u) states (102.8 and 90.6 ppm, respectively) with those of CBD in its square singlet ground state (11B1g) structure (88.1 ppm) revealed clear similarities, supporting the view that the two lowest ππ* excited states of benzene are antiaromatic, in line with Baird’s rule.35,36 With regard to the excited states of CBD, the S1 (11A1g) state of the square structure is slightly less aromatic than the T1 state (Table 5). When investigating the values of the four NICS indices (NICS(0), NICS(1), NICS(0)zz, and NICS(1)zz) for the S1 state of square CDB, the NICS(0)zz and the NICS(1)zz values are similar to the corresponding values found in the T1 state, the NICS(1) value amounts to approximately 65% of that of the T1 state, while the NICS(0) value (3.4 ppm) agrees poorly with that of an aromatic molecule, although it is closer to the NICS(0) of benzene in the S0 state (−8.2 ppm) than to the NICS(0) value of square CBD in the S0 state (36.4 ppm). With regard to the S2 state of CBD (11B2g (square)/11B1g (rectangular)) it is noteworthy that the square structure is preferred despite that the NICS values indicate that CBD in this structure is more antiaromatic than rectangular CBD.35 Finally, from the NICS(0), NICS(1), and NICS(1)zz values of the S1 and the T1 states of D8h symmetric COT it becomes clear that these two states are aromatic as they are similar to the corresponding values found for benzene in the S0 state.36 Noteworthy, the NICS(0)zz value is substantially more negative for the S1 (−21.5 ppm) and T1 (−20.6 ppm) states of octagonal COT than for benzene in S0 (−11.2 ppm), thus, revealing highly aromatic characters of those states. It could also be noted that the aromatic character of COT in both the S1 and T1 states is further supported by the close similarity of the

Figure 19. π-Current-density maps deduced from coupled-ROHF/ CTOCD-DZ/6-31G(d,p)//ROHF/6-31G(d,p) calculations for the triplet states of (a) CBD, (b) C5H5+, (c) C6H62+, (d) C6H62‑ (both the benzene dication and the dianion were calculated at the RHF geometry of neutral benzene), (e) C7H7−, and (f) COT. The currentdensity is induced by a magnetic field perpendicular to the plane of the molecules. The maps are plotted at 1a0 (a0 = atomic unit for length = 0.529 Å) above the molecular plane with contours representing magnitude and with arrows representing in-plane projections. Diatropic ring-currents are displayed as anticlockwise circulations. Adapted with permission from ref 40. Copyright 2008 Elsevier. 5393


Chemical Reviews

Review

Table 6. Values from Electron Density Based Aromaticity Indices species C4H4 C5H5+ C5H5− C6H62+ C6H6 C7H7+ C7H7− C8H8 C9H9+

state S0 T1 S0 T1 S0 T1 S0 T1 S0 T1 S0 T1 S0 T1 S0 T1 S0

point group D2h D4h C2v D5h D5h C2v Cs D3d D6h D2h D7h C2v C2 D7h D4h D8h Cs

state symmetry 1

Ag A1g 1 A1 3 A1 1 A1′ 3 A1 1 A′ 3 Bg 1 A1g 3 B1u 1 A1 3 A1 1 A 3 A1′ 1 Ag 3 A2g 1 A′ 3

RCBV (ELFπ)a

ΔBV (ELFπ)a

0.092 0.648 0.040/0.148f 0.816 0.832 0.073

0.907 0.144 0.941 0.134 0.119f/0.094f 0.768 0.359 0.845

0.908 0.000 0.960/0.852f 0.000 0 0.707

0 0.745 0 0.831 0.837f/0.810f 0 0.637 0

Δπα b

Δπβ b

S

−0.299 0.302 0.348 0.335

−0.299 0.383 0.371 0.335

0.361

0.361

0.367

0.367

0.286

0.362

0.325

MCIc,d

S e

0.361

e

c

FLUc,d d

0.009 /0.009 0.063 −0.021c/−0.021d

0.211c/0.104d 0.012 0.054d

0.072c/0.072d 0.014 −0.020 0.157d 0.073c/0.073d −0.002 0.058 −0.009 −0.004d

0.000c/0.000d 0.019 0.011d 0.000c/0.000d 0.025 0.000 0.025 0.036d

−0.001c/−0.001d 0.028c/0.055d 0.016d

0.105c/0.052d 0.002c/0.001d 0.000d

a Geometries optimized with (U)OLYP/6-311G(d,p) (ref 100). bThe α (SΔπα) and β (SΔπβ) contributions to the scaled n-center delocalization index (ref 92). cMulticenter index (MCI) and aromatic fluctuation index (FLU) from geometries at B3LYP/6-311G(d,p) level (ref 38). dMulticenter index (MCI) and aromatic fluctuation index (FLU) from geometries at B3LYP/6-311G(d,p) level (ref 37). eReported as calculated at a C2v symmetric structure (ref 92). fConstrained to C2v symmetric structure which is near the C2 symmetric minimum structure (ref 100).

Benzene as an example of a [4n+2]π-electron annulene is both α-[2n + 1]-aromatic and β-[2n + 1]-aromatic in its S0 state (S = 0), both α-[2n + 2]-antiaromatic and β-[2n]-antiaromatic in its lowest triplet state (S = 1), and both α-[2n + 3]-aromatic and β-[2n − 1]-aromatic in its lowest quintet state (S = 2). 3.3.4. Electronic Structure Based Aromaticity Indices. Aromaticity indices directly linked to electron density properties have also been applied. For example, the aromaticity of annulenes in the lowest singlet and triplet states has been explored by usage of the π-component of the electron localization function (ELFπ).100 This function describes the excess local kinetic energy due to Pauli repulsion between same-spin electrons.101,102 The ELFπ(r) takes values in the range [0,1] and the analysis is based on ELFπ(r) isosurface plots. In regions where the ELFπ value is close to 0 there is a large degree of Pauli repulsion, while in regions where the ELFπ has values close to 1 there is localization of either a lone-pair or of a single electron. The value of the ELFπ(r) at which two basins merge, or one basin splits, is called a bifurcation value (BV). In particular, the ring-closure bifurcation value (RCBV(ELFπ)) is the ELFπ(r) value at which all basins merge to form one continuous cyclic basin and together with the maximal difference in bifurcation values (ΔBV(ELFπ)), it can be used to confirm aromaticity. Two criteria for aromaticity of all-carbon annulenes were set up; (i) the ΔBV(ELFπ) should be small, ideally zero, and (ii) the RCBV(ELFπ) should be above a certain threshold (0.64−0.70).100,103 Both (4n + 2)π-electron annulenes in their S0 states and 4nπelectron annulenes in their lowest 3ππ* state display zero ΔBV(ELFπ), and the RCBV(ELFπ) of these compounds calculated with (U)OLYP/6-311G(d,p) range from 0.648 in T1 state CBD to 0.940 in the S0 state of the tropylium cation (C7H7+) (Table 6). The ELFπ properties thus support Baird’s rule on triplet state aromaticity. On the other hand, for the annulenes with 4nπ-electrons in the lowest singlet state and (4n + 2)π-electrons in triplet states the ΔBV(ELFπ) were in the range 0.637−0.960 and the RCBVs were found in the interval

currents. Moreover, double Baird-type aromaticity was observed in the quintet state of cyclic C16 as diatropic ring currents are displayed in both the out-of-plane (πout) and inplane (πin) 16π-electron circuits.95 In the triplet state the current sense in πout is diatropic and in πin it is paratropic, and the stepwise change in aromaticity when going from the S0 state over the T1 to the Qu1 state which was observed is in line with earlier observed NICS(0) values for the C12 ring reported as +50.0, −8.8, and −31.5 ppm, respectively.96 Yet, the proper description of electron correlation in the current densities of triplet state aromatic species is important and calculations with the gauge-including magnetically induced current method at the MP2 and CCSD levels by Taubert, Sundholm and Jusélius showed that the diamagnetic ring-current strength of cyclobutadiene in its T1 state is about one-third that of benzene in its singlet ground state.97 Based on current densities Soncini and Fowler further observed that (4n + 2)π-electron annulenes are aromatic in states with even total spins (e.g., S = 0 (singlet), 2 (quintet)), while 4nπ-electron annulenes will be aromatic in states with odd total spins (e.g., S = 1 (triplet), 3 (septet)), providing a generalized form of Baird’s rule.40 This generalization to higher spin states was, however, challenged by Karadakov who computed the NICS value of the 17A2g state of COT at both CASSCF-GIAO and UHF-GIAO levels.36 It was found that UHF assigns this state an aromaticity which is slightly lower than that of benzene in the S0 state, but CASSCF, which gives better-quality results, showed the 17A2g state of COT to be only marginally aromatic. Still, the generalized Baird’s rule was recently analyzed further by Soncini and Fowler, and it was applied to singly charged benzene and COT ions as oddelectron cycles with counter-rotating spin-polarized ring currents.98 The formulation of the electron counting-rule for diatropic and paratropic ring currents were given in terms of the angular momentum quantum number Λ of the frontier πMOs. E.g., a ΔΛ = 1 for the HOMO-to-LUMO transition implies a strong diatropic HOMO ring current and aromaticity. 5394


Chemical Reviews

Review

based aromaticity indices also confirm that Baird’s rule is extendable to the lowest singlet excited state of prototypical annulenes (Table 6).

0.040−0.359. These values are in clear support of antiaromatic character. Moreover, when regarding bond length distorted aromatic annulenes it was recently shown that the π-electrons, when judged from the ELFπ properties, are less localized than would be expected for that particular bond length.103 For example, it was noted that when using the same bond-length alternant D4h symmetric geometry of COT in the S0 state, very different values for ΔBV(ELFπ) were obtained for the S0 (0.637) and T1 (0.116) states, reflecting distinct differences in π-electron delocalization of these two states even when identical geometries were used. At this point it should, however, be remarked that the ELF (and especially the usage of ELFπ and ELFσ) has recently received critique for the arbitrariness of the σ/π dissection and its inability to describe subtle contrasting electronic properties.104 In a similar way as with the NICS(0)πzz index, Mandado et al. calculated the α and β contributions (SΔπα and SΔπβ, respectively) to the scaled n-center delocalization index (nDI).92 Positive values of SΔπ were found for aromatic species while negative values were found for antiaromatic ones. The S πα Δ and SΔπβ results for the triplet state annulenes (CBD (0.302 (α), 0.383 (β)), C5H5+ (0.348 (α), 0.371 (β)), C7H7− (0.286 (α), 0.362 (β)), and COT (0.325 (α), 0.361 (β))) reveal aromatic character, and they are in line with the partitioned NICS results described in section 3.3.3. It was, however, noted that the relative magnitudes of the n-DIs of the investigated triplet state annulenes are different from those of the corresponding partitioned NICS values because the n-DIs reflect electron delocalization while NICS reflects a response property. Yet, it could be concluded that singlet and triplet annulenes are restricted to the same condition for aromaticity since the numbers of α- as well as β-electrons independently should be odd numbers. Furthermore, Feixas et al. explored the aromaticities of the lowest singlet and triplet states of aromatic and antiaromatic annulenes, heterocycles, and polyaromatic hydrocarbons through the patterns of π-delocalization upon addition or removal of two electrons calculated at B3LYP/6-311G(d,p) level.37,38 They also calculated the multicenter index (MCI), aromatic fluctuation index (FLU), para-delocalization index (PDI), as well as NICS and HOMA, at the same level of theory. The lower the FLU, the higher the PDI, and the more positive the MCI, the more aromatic is the system in question. Even though they observed similar degrees of total π-electron delocalization in both the singlet ground state and lowest triplet state of annulenes with 4n π-electrons, the three calculated aromaticity indices make a clear distinction between the two states. However, by decomposition of the π-electron delocalization into cross-terms, specific patterns were found when either adding or removing two electrons, and this allowed for unique assignment of aromaticity because the cross-terms showed opposite trends for the S0 and T1 states. Recently, Feixas et al. also examined several additional excited states including the lowest singlet excited states using the electronic aromaticity indices of their earlier study,37,38 now computed at CASSCF level.105 For benzene it was found that all indices except the FLU index classify benzene in the S1 state as antiaromatic. The calculations of CBD and COT were performed at the D2h and D4h symmetric structures and it was observed that the ππ* singly excited state, which is the S2 state for both molecules at these structures, is aromatic in character. It was also noted that upon relaxation of the COT geometry the S2 state should become the S1 state. Thus, the electronically

3.4. Synopsis of Theoretical and Computational Studies

Earlier reported analyses of excited state aromaticity and antiaromaticity have been carried out with both qualitative theoretical tools and quantum chemical calculations. Geometries and relative energies are two properties that can be related to aromaticity and antiaromaticity, and that can be analyzed qualitatively by MO- and VB-theory. With regard to geometries, it was early predicted that 4nπ-electron annulenes in their S1 and T1 states are stable at their highest possible Dnh symmetries,53 and later investigations with, for example, the qualitative twin-state model support these results.67 Moreover, high-level quantum chemical calculations confirm the qualitative results revealing that highly symmetric structures of 4nπelectron annulenes are preferred in the T1 and S1 states (see Figure 17 for results in the T1 state),28,49 although there are exceptions such as CBD which adopts a rhomboid structure in its S1 state.57 To qualitatively deduce the geometries of (4n + 2)π-electron annulenes in the T1 and/or S1 states is, however, more complex, and these species have not been analyzed to the same extent as excited 4nπ-electron annulenes. A relative stabilization of cyclic 4nπ-electron molecular structures (equilibrium geometries or transition states) in their lowest electronically excited states when compared to openchain π-conjugated analogs, and a corresponding destabilization of cyclic (4n+2)π-electron structures, was first addressed independently by Dewar and Zimmerman in their analyses of thermal and photochemical pericyclic reactions in terms of transition state (anti)aromaticity.8,11,12 Subsequent qualitative analysis of the π-orbital interactions when combining the πorbitals of two polyenyl monoradicals to those of a triplet biradical annulene allowed Baird to account for the stabilization of 4nπ-annulenes and the destabilization of (4n + 2)π-electron annulenes when in their lowest triplet states.20 Through symmetry matching and mismatching between the two nonbonded MOs (NBMOs) of the two polyenyl radicals, as well as between the NBMO of one radical and the doubly occupied or vacant π-MOs of the other, he could account for aromatic stabilization and antiaromatic destabilization in various triplet state annulenes. Using an approach based on Hückel MO theory Aihara deduced resonance energies of the four smallest neutral annulenes (C4H4 until C10H10) in either Hückel or Möbius orbital topology and in either the S0 or lowest ππ* excited state (multiplicity independent) as listed in Table 3.21 From these energies it became apparent that an annulene upon excitation from its ground state to the lowest ππ* excited state, regardless if it is a 4nπ- or (4n + 2)π-electron annulene and regardless if it has Hückel or Möbius orbital topology, always changes from aromatic to antiaromatic, or vice versa. An interesting aspect, clarified through VB-theory, is the fact that triplet biradical 4nπ-electron annulenes can be regarded as a (2 + (4n − 2))π-electron species (see Figure 10), i.e., cyclic biradicals with two nonbonding same-spin electrons and a closed-shell Hückel-aromatic (4n − 2)π-electron ring.32 It has also been shown that one can consider the aromaticity/ antiaromaticity contributions of α- and β-electrons separately, and that the rules for aromaticity and antiaromaticity can be generalized to states of different multiplicities.40,92 In short, an annulene with Hückel orbital topology is aromatic when the 5395


Chemical Reviews

Review

number of α-electrons and the number of β-electrons are both odd numbers. For Möbius topology annulenes the opposite should apply; that is, they are aromatic in the lowest closedshell singlet state and lowest triplet state when the numbers of α-electrons and β-electrons both are even numbers. In recent years, there has been a steady increase in the number of different computational approaches applied for the assessment of excited state (anti)aromaticity. As described in section 3.3, Schleyer and co-workers were the first to explicitly examine triplet state (anti)aromaticity computationally for a number of neutral and charged 4nπ-electron annulenes, and they made use of aromatic stabilization energies (ASEs), magnetic susceptibility exaltations, and the NICS index in their study (see Figure 17).28 Recently, this latter index was also applied to the S1 state of small annulenes (see Table 5).35,36 In brief, the qualitative theory on aromaticity of Hückel topology 4nπ-electron annulenes and antiaromaticity of (4n + 2)πelectron annulenes in the ππ* excited T1 and S1 states, now termed Baird’s rule, is solidly supported by results from highlevel quantum chemical computations of a range of wellestablished aromaticity indices based on energetic, geometric, magnetic, or electron density criteria. For example, see Figure 16 for isomerization stabilization energies (ISE) which support triplet state aromaticity of 4nπ-electron annulenes. Taken together, the theoretical and computational data published confirm that the change in aromatic and antiaromatic character when changing the number of electrons, orbital topology, and electronic state can be summarized as in Figure 9. In the next, we will discuss the impact of excited state (anti)aromaticity on various excited state properties and reveal how it influences photochemical processes.

than CBD, its lowest excited states are higher in energy. For COT the vertical S0 → T1 and S0 → S1 transition energies are found at 3.05 and 4.43 eV, respectively.113 For larger 4nπelectron cycles the lowest excitation energies are even smaller, e.g., for cycl[3.3.3]azine (1) which has a 12π-electron perimeter the S1 state is found at 0.96 eV.114 Here, it should be noted that these differences in the transition energies between 4nπ- and (4n + 2)π-electron annulenes are in accord with the energy gaps deduced by Zilberg and Haas using qualitative VB theory (see Figure 13).67 In particular, the avoided crossing situation at the high-symmetry structures of 4nπ-electron cycles, which implies a low-energy S1 state, is in particular in line with the distinctly lower fluorescence quantum yields of 4nπ-electron species when compared to benzenoid hydrocarbons as observed by Wirz.115 At this point it can be noted that the S0 → S1 transition energy of pentafulvene (2), a nonaromatic cross-conjugated isomer of benzene, is found at a significantly lower energy than the S0 → S1 transition energy of benzene (3.45 vs 4.90 eV).116,117 An interesting feature of fulvenes is that they are polar in the S0, T1, and S1 states (vide infra, section 4.4). However, the dipole moments in the S1 and T1 states are reversed as compared to the S0 state, a finding which is explained by Baird’s vs Hückel’s rule.118,119 These reversed polarities enabled us earlier to vary the energy difference between the S0 and T1 states of pentafulvenes by 1.55 eV (35.7 kcal/mol) through the choice of substitution pattern.120 For example, a (de)stabilization of the (ground) excited state of 2 is achieved by electron withdrawing substituents at the exocyclic position which (dis)favor the (ground) excited state aromatic resonance structure (vide infra, section 4.4.1).

4. IMPACT OF AROMATICITY ON EXCITED STATE PROPERTIES The gain or loss of aromaticity in 4nπ- or (4n + 2)π-electron systems upon excitation to the S1 and T1 states is reflected in a series of excited state properties. For example, a change in (anti)aromaticity upon excitation results in geometry changes, as discussed in the previous sections for simple annulenes. The electronic and magnetic properties as well as the reactivities of compounds with aromatic or antiaromatic segments are also influenced differently in the lowest excited states when compared to the ground state. In this section we expand on section 3 and discuss changes in the properties of photophysical character based on experimental as well as computational data.

Indeed, the triplet can even be the ground state as for the parent and pentachloro substituted cyclopentadienyl cations as well as the hexachlorobenzene dication which are nondisjoint biradicals at their highest symmetric structures.22,23,121 This fact is also emphasized by the recent finding that the #6094C68 fullerene has a triplet multiplicity ground state which is 8.2 kcal/mol below the closed-shell singlet state and which is influenced by aromaticity according to NICS calculations.122 An older observation in similar vein is the finding that the triplet cyclopentadienyl cation at (U)MP2/6-31G(d)// (U)HF/6-31G(d) level is 2.6 kcal/mol more stable than the singlet closed-shell vinylcyclopropenium cation (3), a Hückelaromatic compound.123 Yet, at the more advanced G2 model chemistry level of computation, the relative stability order is reversed so that 3 becomes 2.8 kcal/mol more stable than the triplet cyclopentadienyl cation.124 Still, these calculations reveal that a Baird-aromatic compound can be essentially isoenergetic with a Hückel-aromatic isomer.

4.1. Excited State Energies

The increase or decrease in aromaticity upon excitation of an annulene is reflected in the excitation energies of the S1 and T1 states. In general, 4nπ-electron annulenes have low HOMO− LUMO energy gaps relative to (4n + 2)π-electron species,106,107 reflected in lower excitation energies of the former than of the latter class of compounds. For the 4nπ-electron annulenes the small HOMO−LUMO gaps and excitation energies parallel unfavorable antiaromatic character in their S0 states and favorable aromatic character in their S1/T1 states. The opposite applies for (4n + 2)π-electron annulenes which have large HOMO−LUMO gaps and excitation energies. This situation is illustrated by the vertical S0 → T1 and S0 → S1 transition energies for CBD and benzene, which for CBD have been estimated experimentally to 1.24 and 4.13 eV,108−110 and for benzene measured to 3.95 and 4.90 eV, respectively.111,112 Thus, despite benzene having a longer π-conjugated cyclic path

4.2. Excited State Geometries

The two prototypes of 4nπ- and (4n + 2)π-electron annulenes are cyclobutadiene and benzene, respectively. Benzene in the S0 state is D6h symmetric whereas matrix isolation IR spectroscopy at cryogenic temperatures unambiguously has shown that CBD is rectangular in this state and not square as first concluded.125−127 Recent computational studies of CBD 5396


Chemical Reviews

Review

structures which are merely C 2h symmetric (Figure 21).66,67,148−154

estimate CC bond lengths of 1.562 and 1.349 Å, while a D4h symmetric (square) structure with CC bond lengths of 1.447 Å corresponds to minima on both the S1 state and T1 state PESs.35,57 Yet, with full-optimized reaction space multiconfiguration self-consistent field and multireference coupled cluster methods, the S1 state minimum geometry was calculated to have a rhombic rather than square structure as a result of pseudo-Jahn−Teller distortions.57,128,129 Computational results at the CASSCF level as well as experiments with highresolution two-photon spectroscopy show that benzene upon excitation to the S1 (11B2u) state retains the D6h symmetry although with significantly extended CC bonds (comp.: 1.430− 1.445 Å, exp.: 1.432 Å130), while it has D2h symmetry in T1 (computed CC bond lengths of 1.466 and 1.370 Å, respectively).131−135 In the S0 state, COT has a tub-shaped D2d symmetric structure.136 A theoretical investigation of excited state reaction dynamics of COT performed by Garavelli et al. suggests that the most favorable pathway away from the Franck−Condon region on the S1 state PES is the planarization to D8h symmetry.49 It was concluded that this structure represents a collecting point on the S1 state PES stabilized by “a kind of aromatic effect”. The same high symmetry has been shown to be optimal also in the T1 state.28,36,100,103,137−142 Additionally, results from photoelectron spectroscopic experiments performed on the cyclooctatetraene radical anion have been used to establish that the T1 structure of COT indeed possesses D8h symmetry.143 Interestingly, COT has shown to have a very large nonvertical triplet energy transfer,144−146 and the fact that it goes from a markedly puckered structure in the S0 state to a planar D8h symmetric structure in the T1 state should contribute to this. Here, the photochemical interconversion between two bond-shift isomers of 1,2-disubstituted COTs, such as the methyl-2-methylcyclooctatetraenecarboxylate isomers 4a and 4b (Figure 20), is also interesting.147 The

Figure 21. General structures and symmetries of pentalene (5) and sindacene (6) in the S0 and S1 states, respectively, according to results of CASSCF/6-31+G(d) and CNDO/SDCI calculations.149,154

In the S0 state of biphenyl (7), the two connected phenyl rings are not coplanar.155 In contrast, early quantum chemical calculations by Imamura and Hoffmann of 7 in the S1 and T1 states, suggested a planar structure, large bond length alternations in the two phenyl groups, and an increased double bond character of the central CC bond (Figure 22).156 Recent

Figure 22. Important resonance structures of biphenyl (7) and biphenylene (8) in the S0 (black), T1 (red), and S1 (blue) states, respectively.

quantum chemical SAC−CI and TD-PBE0 calculations of several excited states of 7 reveal that the relaxed geometry of the T1 (13B1) state is planar with a short inter-ring C−C bond (1.411 Å with SAC−CI) and quinoid structure of the two phenyl groups (CC bond lengths 1.465, 1.382, and 1.427 Å, respectively).157 The S1 state, which is the 11B3 state, on the other hand, has a relaxed planar geometry with benzenoid rings and longer inter-ring C−C bond (1.457 Å). However, the S3 state state (11B1), which is the first allowed transition, is calculated to have a planar quinoid structure when relaxed geometrically. Due to the large geometric distortion this state shows a large computed Stokes’ shift of 1.1 eV. This structural change has experimental support because a Stokes’ shift of 3310 cm−1 has been observed, and it has been argued to be caused by very substantial structural changes in the S1 state.158 Similarly, computational investigations at CASSCF level by Beck et al. reveal that biphenylene (8) also undergoes interesting geometrical changes after excitation to the S1 and T1 states (Figure 22).159−161 In the S1 state the CC bonds connecting the two benzene rings contract from 1.524 Å in the S0 state to 1.412 Å, and the CC bonds between the central fourmembered ring and the two flanking benzene rings are elongated from 1.432 to 1.472 Å.162,163 Similar geometry changes have been calculated to occur when going from the S0 to the T1 state,164,165 and these results indicate that the

Figure 20. Bond-shift isomerization in methyl-2-methylcyclooctatetraenecarboxylate (4) which can be achieved thermally, giving a 4a:4b ratio of 17:1, or photochemically, giving isomer 4b in slightly greater amount than 4a.147

irradiation of a 17:1 mixture of 4a and 4b, which represents the S0 state equilibrium, in CDCl3 for 6 h at −30 to −50 °C gave a solution with a slight excess of 4b as compared to 4a. However, increase of the temperature to −12 °C returned the ratio between the isomers to its S0 state equilibrium value. The nearly equal proportions of 4a and 4b after irradiation should suggest a planar and nearly octagonal excited state minimum structure from which decay to the S0 state occurs. Geometry changes in the excited states of larger and/or fused compounds can also be rationalized by an increase or a decrease in aromaticity upon excitation. Pentalene (5) and sindacene (6) have been shown through computational studies to be D2h symmetric in the S1 state in contrast to their S0 state 5397


Chemical Reviews

Review

11a. As supported by quantum chemical calculations on the smaller model 11b it was conluded that also this compound is strongly pucked in the S0 state (the COT bent angle θ = 40.6°) but that it strives toward planarity in the S1 state. Yet, there is also a shallow local minimum in the excited state at a slightly Vshaped geometry (θ = 22.8°). The flexibility of 11a is restricted in a polymer matrix while it is not in solution leading to dual fluorescence (green in solution and blue in the polymer matrix, respectively). In the crystalline state 11a forms a 2-fold πstacked array and it was concluded that the red emission observed for the crystal is due to excimer-like emission. In general, results of calculated excited state structures for several 4nπ- and (4n+2)π-electron compounds in their T1 and S1 states show that there are clear analogies between the geometrical effects of aromaticity and antiaromaticity in the S1 and T1 states and the geometry effects seen in the S0 state as a result of ground state aromaticity and antiaromaticity, respectively.81,86,100,138,166,169−172

resonance structures for 8 in the upper right part of Figure 22 have significant importances in the T1 and S1 states. Thus, 8 is influenced by aromaticity in all three states; in the S0 state the two benzene rings are isolated Hückel-aromatic 6π-electron cycles, while in the T1 and S1 states a Baird-aromatic 12πelectron cycle at the perimeter of the compound is important. Other examples of very large geometrical changes in the excited state as compared to the S0 state have been observed for dibenzo[b,f ]oxepine (9) and dibenzo[b,f ]thiopine (10) (Figure 23).166 The anomalously large Stokes’ shift of 4720

4.3. Excited State Magnetic Properties

In excited states, magnetic properties such as ring currents which are characteristic for S0 state aromatic and antiaromatic molecules mentioned in section 3.3 are not easily investigated experimentally due to the short lifetimes of the excited states. Obviously, no experimental NMR studies on excited state aromaticity have been reported. However, EPR spectroscopic techniques have been used to verify the triplet multiplicity of the ground states of both the cyclopentadienyl cation (C5H5+) and pentachlorocyclopentadienyl cation (C5Cl5+),22,23 species which are nondisjoint biradicals at the D5h symmetry, and for which the lowest triplet state therefore can be lower in energy than the lowest (biradicaloid) singlet state.58 Furthermore, recent experimental results have shown that the cyclopentadienyl cation indeed possesses D5h symmetry.29−31 Significantly, the pentaphenylcyclopentadienyl cation was found to have a singlet instead of a triplet ground state as this compound cannot obtain the high D5h symmetry.22,24,173 Soncini and Fowler used calculated ring currents to analyze aromaticity in various open-shell compounds in addition to the triplet state annulenes discussed above.40,94 Most interestingly, they found that azulene (12, Figure 24) in the lowest excited quintet state (Qu1 state) possesses a strong diatropic ring current in the perimeter, similar as in the S0 state. This supports the view that this compound in Qu1 is influenced by both α-[2n + 1] aromaticity and β-[2m + 1] aromaticity with n = 3 and m = 1, respectively, in line with the general formulation of Baird’s

Figure 23. (A) Illustration of the formation of the cyclic conjugated 8π-electron arrays in the S1 states of dibenzo[b,f ]oxepine (X = O, 9) and dibenzo[b,f ]thiopine (X = S, 10) that lead to planarization of the molecules in their S1 states, with the π-electrons delocalized mainly in the seven-membered ring.166 (B) Compound 11 which undergoes a change from puckered to planar structure upon excitation to the S1 state, and which exhibits red, green, and blue emission depending on the environment.168

cm−1 for 9 has, similar as for 7, been suggested to be the consequence of extensive structural changes on the S1 state PES. The structure of 9 is bent out of the molecular plane in the S0 state,167 but was concluded to become planar in the S1 state. Compound 10 showed a similarly large Stokes’ shift which was also concluded to be a consequence of extensive structural changes. In addition, the structure of 9 was optimized using semiempirical π-SCF PPP calculations in the planar conformation in the S0 and S1 states, and the results showed that the electron density is evenly distributed over the entire central seven-membered ring in the S1 state, while the opposite is the situation in the S0 state. It was therefore concluded that the planarization of the molecule in S1 is driven by the formation of the internal cyclic conjugated 8π-electron system, which should be S1 state aromatic, rather than the formation of a peripheral cyclic conjugated 16π-electron system (Figure 23A).166 In this context, the compound 11 (Figure 23B) which also is composed of a central 8π-electron cycle with flanking wings (antraceneimide wings) has interesting emission properties, as reported by Saito, Irle, Yamaguchi and co-workers.168 Strong environment-dependent luminescence was found for

Figure 24. Resonance structures of azulene (12) in the lowest quintet state (Qu1 state) with resonance structure 12-I, being a resonance hybrid of structures of the type in square bracket, reflecting the 7αaromatic/3β-aromatic quintet state, while the dipolar resonance structures 12-II and 12-III have triplet aromatic cycles and a polarity which is opposite to that of azulene in the S0 state. 5398


Chemical Reviews

Review

rule. Yet, calculations at the UOLYP DFT as well as the CASSCF levels also reveal a reversal of the dipole in Qu1 as compared to S0 (with CASSCF (and UOLYP): μ(S0) = 0.8 (0.9) D and μ (Qu1) = −1.3 (−1.7) D) which should imply that dipolar resonance structures 12-II and 12-III with triplet state aromatic rings also contribute to the electronic structure of the Qu1 state (see next section).118 4.4. Excited State Polarities

Compounds that are dipolar in the S0 state because of the influence of zwitterionic aromatic resonance structures can be dipolar in the lowest excited states as well. It has for long been known that the π-electrons in the ground states of fulvenes and [n,m]fulvalenes (n ≠ m) are not evenly distributed, and that dipolar aromatic resonance structures play important roles in the ground states of these compounds.174−183 For example, despite that the S0 state of 2 is best represented by a crossconjugated resonance structure of localized double and single bonds (Figure 25), the magnitude of the dipole moment (0.42

Figure 26. Dipole moments (D) in the S0 state (red arrows) and in the T1/Qu1 state (black arrows) calculated at the (U)OLYP (normal print) and CASSCF (italics) levels. Values from ref 118.

for aromaticity, Figure 26.118,189,190 Natural population analysis reveals that similar amounts of π-electron density are pushed between the rings in the S0 state as in the T1/Qu1 states but in opposing directions. These results indicate that the compounds are influenced by oppositely polarized aromatic resonance structures in the S0 versus the T1/Qu1 states (Figure 27). We therefore coined the term “aromatic chameleon” to categorize these and similar compounds which can adjust their electron distributions so as to achieve an influence of aromaticity in several electronic states.

Figure 25. Different resonance structure representations of the S0 state structure of pentafulvene (2). The cross-conjugated resonance structure makes a much larger contribution than the 6π-electron aromatic resonance structure.

D) and its direction reveal that a zwitterionic resonance structure with a negatively charged ring and a positively charged exocyclic C atom influences the S 0 state. 184,185 The contribution of this dipolar aromatic resonance structure can be varied through choice of substituents, and it has also been shown that the thermal stability of (penta)fulvenes is enhanced through introduction of electron donor substituents at the exocyclic carbon as these provide for a higher aromatic character of the ring.186 Subsequent CI/SINDO semiempirical calculations also suggested increased dipole moments in the S1 and T1 states of fulvene when compared to the S0 state, which at that time was suggested to indicate a moderate increase in aromaticity.187 Yet, these early interpretations of the lowest excited states were carried out in terms of S0 state ((4n + 2)π-electron) aromaticity rather than in terms of the excited state aromaticity. Later, the lowest energy structure of 2 on the S1 state PES have shown through computations to possess a dipole moment of 0.67 D with a direction opposite to that of the S0 state.133 This reversal is supported by recent results of absorption spectroscopic solvatochromic investigations on a small series of differently substituted fulvenes which reveal experimentally that fulvenes indeed reverse the direction of their dipole moments when excited to the S1 state.119 Indeed, reversals in the dipole moments of triafulvene (13, Figure 26) and heptafulvene (14) in addition to 2 upon excitation from the S0 state to the S1 and T1 states were observed based on semiempirical PPP calculations by Tyutyulkov et al. already in 1971.188 Later calculations at the CASSCF and (U)OLYP levels also showed that upon excitation to the T1 states of series of fulvenes and the Qu1 states of fulvalenes and azulene (12), the dipole moments are reversed relative to their ground state dipole moments due to the reversal of the electron count-rule

Figure 27. Oppositely polarized aromatic and antiaromatic resonance structures of pentafulvene (2) and heptafulvene (14) in their S0 (black) and T1 (red) states. The antiaromatic resonance structures are placed in parentheses to indicate that these play a negligible role for the electronic structure of a particular state. 5399


Chemical Reviews

Review

The reversal of the dipole moment in ground and excited states of fulvenes implies that the relative energies of S0 and T1 states can be tuned by introducing suitable substituents at the exocyclic carbon atom or at the ring. Ottosson and co-workers showed this experimentally for a small selection of substituted pentafulvenes and computationally for a larger set of derivatives of 2, heptafulvene (14), benzofulvene, and dibenzofulvene.120 Interestingly, the effects of a particular substituent positioned at the exocyclic carbon atom of a pentafulvene were reversed when placed on the analogous carbon of a heptafulvene. Furthermore, the reversal in the polarity was exploited also for the tuning of the S1 state excitation energies of pentafulvenes through substitution (Figure 28).119,120

Figure 29. Zwitterionic aromatic resonance structures in the S0 and S1 states of 12 that rationalize the observed polarity reversal upon excitation from S0 to S1.

Scheme 1. Nucleophilic Substitution of Azulene and 1Nitroazulene upon Irradiation Yielding 1-Cyanoazulene and 1-Methoxyazulene, respectively194−197

Figure 28. Illustration of how strategic substitution by either electron withdrawing groups (EWG) or electron donating groups (EDG) at the exocyclic position of penta- and heptafulvenes tune the energy gap between the S0 and T1 states (ΔEST).

distributions in the S1 and T1 states as compared to the S0 state is a property that can be exploited as a convenient tool for tuning the energy gap between S0 and T1 states of fulvenic molecules. This has been shown experimentally for a limited set of substituted (penta)fulvenes and computationally for larger sets of pentafulvenes, heptafulvenes, benzofulvenes, and dibenzofulvenes.119,120 For pentafulvenes, substitution by electron donating groups in the exocyclic positions increases the energy gap when compared to the parent fulvene, while electron withdrawing groups have the opposite effect, and early computations using the PPP semiempirical method revealed that the first transition has strong intramolecular charge-transfer character.198 Noteworthy, a reversed substituent effect was calculated for a particular functional group when positioned as a substituent at the exocyclic carbon atom of a heptafulvene as compared to when the same substituent was placed at this position in pentafulvene (Figure 28). This feature of fulvenes can also be exploited for tuning their S0 → S1 transition energies through substitution.119,120 Systematic manipulations of excitation energies through substitution of other fulvenic molecules such as cyclopentadienone have also been investigated earlier.199−202 In

It was further found that the polarity of azulene (12) is reversed in the Qu1 state when compared to the S0 state, agreeing with resonance structures 12-II and 12-III above (Figure 24). Experimental and computational results also show that 12 displays such a polarity reversal in the S 1 state,175,191−193 in line with the favoring of 4nπ-electron cycles in the S1 state vs (4n+2)π-electron cycles in the S0 state (Figure 29). Similar reversal of the S1 state dipole moment has been measured in the 3,5-dimethylcyclopenta[e,f ]heptalene (18), for which analogous arguments can be made.193 As pointed out by Yamaguchi et al. this characteristic feature of excited state azulene could be expected to lead to a reversal in its reactivity toward electrophilic and nucleophilic reagents in S1 as compared to the S0 state.193 This has indeed shown to be the case for the reaction between excited state 12 and nucleophiles, as illustrated in Scheme 1.194−197 Here, nucleophilic substitution in the excited state takes place at the positions that are reactive toward electrophilic substitution in the ground state. 4.4.1. Manipulation of Excited State Energies of Fulvenic Molecules. The oppositely polarized electron 5400


Chemical Reviews

Review

form the dibenzocycloheptatrienyl anion. At this point it should be noted that an opposite excited state acidity of fluorene and fluorene derivatives had been invoked earlier by Donckt et al.214 They estimated the pKa* of fluorene to be −8.5 using the Förster cycle method. However, the prediction based on this methodology is not valid as the chromophore systems of the fluorene and fluorenyl anion are different. The S0 → S1 transition of the fluorenyl anion is a π → π* transition in a 14π-electron molecule,215 while the S0 → S1 transition of fluorene is a π → π* transition in a 12π-electron molecule.216 Whereas 4nπ-electron anions are stabilized in their lowest excited states, (4n+2)π-electron anions are instead destabilized and should, as a consequence, have higher proton affinities in their S1 and T1 states than in their ground states. This is indeed the case because the 10π-electron S0 aromatic cyclooctatetraenyl dianion (24) and the cyclononatetraenyl anion (26) are protonated by 1-hexyne when irradiated (Scheme 3), indicating

contrast to fulvene, the calculated dipole moment of cyclopentadienone in the S0 state has a magnitude of 3.169 D and it is directed from a negative charge on the exocyclic O atom to a positive charge in the ring moiety.203 The instability of this molecule at ambient temperatures supports that this polarization of the electron density is unfavorable in the S0 state molecule. However, the S0 → S1 transition energy for the stable 2,3,4,5-tetraphenyl derivative of cyclopentadienone (tetracyclone) has been shown through experimental and computational investigations to be sensitive to substitution, especially in the 4-positions of the phenyl rings.199−202 Here, it should be emphazised that although the S0 → S1 transition of cyclopentadienone is calculated to be an n → π* transition,203 the S0 → S1 transition for tetracyclone is calculated to be a π → π*,202 similar as for fulvene, and rationalizations in terms of excited state (anti)aromaticity should thus be justified. In general, electron donating groups in these positions of the phenyl rings tend to decrease the lowest transition energy when compared to tetracyclone, while electron withdrawing groups have the opposite effect. Thus, this substituent sensitivity of the S0 → S1 transition energy of tetracyclone can be rationalized in terms of the different dipolar characteristics of the cyclopentadienone moiety in the S0 vs the S1 state structures.200−202 Conclusively, there seems to be a clear systematic method to tune the lowest transition energies of fulvenic molecules, and this method utilizes the reversals in the electron counting rules for ground and excited state aromaticity.

Scheme 3. Cyclooctatetraenyl Dianion (24) and the Cyclononatetraenyl Anion (26) Protonated by 1-Hexyne Only When Irradiated217−219

4.5. Photoacidity and Photobasicity

It is well established that cyclopentadiene and 9H-fluorene (22) are particularly acidic π-conjugated hydrocarbons in S0 as they form aromatic (4n + 2)π-electron annulenyl anions when deprotonated. However, experiments by Wan and Krogh showed that the opposing ground state acidities of 22 and 5H-suberene (23), as well as related compounds, are reversed upon excitation to the S1 state because 22 becomes nonacidic and 23 becomes acidic (Scheme 2).204−213 More specifically,

that these species become destabilized, potentially antiaromatic, in their S1 states.217−219 Yet, the 6π-electron cyclopentadiene anion is not protonated upon irradiation in tert-butanol, but instead hydrogen abstraction and subsequent proton transfer lead to formation of dimeric products.220 Thus, reaction channels that compete with the acid−base reaction exist on the excited state potential energy surfaces, as has been pointed out in a theoretical work by Steuhl and Klessinger.221 In a recent computational study of proton as well as hydride affinities of a series of (benz)annulenyl anions and cations, respectively, we confirmed the reversals in the relative values of these properties when going from the S0 to the T1 states (Figure 30).86 For example, a species with a low proton affinity in S0 will have a high value in the T1 state. Results at G3MP2// (U)B3LYP/6-311+G(d,p) level verified that 4nπ-electron anions/cations in the T1 state in general have lower proton/ hydride affinities than (4n + 2)π-electron anions/cations.

Scheme 2. Irradiation of 9H-Fluorene (22) and 5H-Suberene (23) in D2O/CH3CN Mixtures, Leading to Formation of the Deuterium Exchanged Product in the Second Systema

a

Opposite behaviors are found in experiments performed in the dark.204

irradiation of 22 in D2O/CH3CN mixture did not lead to deuterium exchange in the 9-position; however, this was observed when an identical solution was kept in the dark. In contrast, the irradiation of 23 under similar conditions led to deuterium exchange in the 5-position, but to no deuterium exchange when an identical solution was kept in the dark. Wan, Krogh and Chak concluded that these findings “suggest that 8π (4n) conjugated cyclic carbanions have aromatic character in S1”;204 that is, 23 easily deprotonates in the excited state to

Figure 30. Relationship between the relative magnitudes of the proton and hydride affinities of (benz)annulenyl anions and cations and the electronic state as well as the number of π-electrons. 5401


Chemical Reviews

Review

The classic example of a compound which enhances its acidity upon excitation is 2-naphthol (33), as first described by Förster in 1950,224 and more recently in several reviews.208,225 In short, the pKa of 33 changes from 9.5 in the S0 state to 2.8 when excited to the S1 state, i.e., it becomes markedly acidic in the S1 state and is appropriately named a photoacid. The major change in the acidity of 33 when excited can be interpreted as a consequence of a change in the electron density distribution as represented by the resonance structures in Figure 32.226

Moreover, (4n + 2)π-electron anions/cations have higher proton/hydride affinities in their T1 states than in their S0 states, whereas 4nπ-electron anions/cations have higher proton/hydride affinities in their S0 states than in their T1 states. Thus, T1 state 4nπ-electron aromaticity ((4n + 2)πelectron antiaromaticity) influences the acid−base properties in the T1 state similarly as S0 state (4n + 2)π-electron aromaticity (4nπ-electron antiaromaticity) influences these properties in

Figure 32. Illustration of the change in the electron density distribution of 2-naphthol (33) upon excitation to the S1 state.

Increased influence of quinoidal resonance structures unquestionably makes the excited state of 33 a stronger acid as a protonated carbonyl group is more acidic than a hydroxyl group. This difference in the electron density distribution between the S0 and the S1 state can be interpreted as a means for the molecule to diminish its influence of unfavorable S1 state antiaromatic 10π- and 6π-electron circuits of the naphthalene moiety. In support of this interpretation, semiempirical computational data for the 2-naphtolate ion (34) in the S1 state suggest a quinoide structure in which the negative charge is distributed into the rings, resulting in a shortened CO bond and alternating short/long CC bonds (Figure 33).227 In the S0

Figure 31. Reactions A and B: azulene (12) and benz[a]azulene (29) in equilibria with their respective protonated forms. The two equilibria are shifted toward the neutral species when excited to their S1 state. Reaction C: the equilibrium between indozoline (31) and its protonated form does not show any detectable shift toward the neutral form when irradiated.222,223

the S0 state. The corresponding magnitude reversal has so far not been examined by computational means for the S1 state, but the experimental observations above support the existence of such a reversal. So in what way can the reversal in excited state acid−base behavior be applied? Results of flash-photolysis experiments reveal for example that the basicities of azulene (12) and benzo[a]azulene (29) increase in their S1 states.222 Thus, the equilibria displayed in Figure 31 (reactions A and B) are shifted to the left when the compounds are excited to their S1 states. Interestingly, similar experiments performed on indolizine (31; Figure 31, reaction C) show no detectable change in the position of the corresponding equilibrium upon excitation.223 The authors explain the findings by a slow deprotonation rate of the latter compound when compared to deactivation of the excited state. An additional reason for the different experimental outcomes for the three compounds could be the different degrees of (anti)aromaticity in the S0 vs S1 states. Azulene is dipolar both in the S0 and S1 states, but in opposite directions due to its aromatic chameleon nature (Figure 29). Upon protonation to 28 a S0 state aromatic tropylium cationic ring forms, a moiety that, however, is antiaromatic in the S1 state. By shifting the excited state equilibrium to the left when irradiated, 12 is thus able to avoid formation of this excited state antiaromatic species, and the same applies to 29. Indozoline (31) is unable to act as an aromatic chameleon and therefore no such shift of the equilibrium between the S0 and S1 states will occur.

Figure 33. Various resonance structures of 2-naphtolate (34). Results of semiempirical calculations show that the quinoid resonance structures are the best representations of the S1 state structure.227

state, ion 34 is best described by a resonance structure with negative charge at the O atom. Similar explanation holds for phenol and many substituted phenols where the change in pKavalue between the S0 and the S1 state are of the same magnitude as for 2-naphtol.228 The excited state acidity of protonated 2-naphtylamine is also greatly enhanced upon excitation (pKa(S0) = 4.1 vs. pKa(S1) = −2), which could be rationalized by a similar model as that of 33. In contrast, the acidity of 2-napthoic acid (35) in the S1 state decreases significantly (pKa(S0) = 4.2 vs pKa(S1) = 10− 12),226 an observation which is explainable by the fact that the carboxylate group, formed upon dissociation of the acid, is 5402


Chemical Reviews

Review

toward the electron withdrawing substituent, and this leads to an enhanced basicity of the compounds. These results are in line with the conclusion of Jug and Malar that mono- and disubstituted benzene rings loose their aromaticity in the S1 and T1 states, associated with geometrical changes.135,236 Similar changes in the electron densities between the S0 and S1 states are important for excited state intramolecular proton transfer in salicylic acid derivatives (36−38, Figure 35).237 It

neither prone to share negative charge with the naphtyl group due to its electron withdrawing character nor participate in quinoide-type bonding with the ring as such resonance structures would imply highly unfavorable charge splitting (Figure 34). Similar arguments can be used for phenylic or

Figure 34. In contrast to 2-naphtol (33), the acidity of 2-napthoic acid (35) is decreased in the S1 state as compared to the S0 state. This can be rationalized to be a consequence of an enforced impact of the charge-separated quinoide resonance structure in the S1 state and shown to the right. Figure 35. Keto−enol tautomerism in salicylic acid derivatives in the S0 state (black) and in the S1 state (blue).

naphtylic systems bearing sulfonic, phosphonic, or arsonic acids instead of carboxylic acid.228 Taken together, results of the acid−base behavior of 33, 35, and 2-napthylamine reveals that substituents which are able to help alleviate the S1 antiaromatic character of the naphtyl group lower the pKa in S1 as compared to the S0 state, and those that instead counteract this lead to higher pKa values. Further increased (photo)acidity is observed in certain cyano substituted derivatives of 33.229,230 In accordance with Figure 32 the acidity of 33 in the ground state is enhanced when cyano groups are placed in the 6- and 8-positions, allowing the negative charge to be more delocalized. Conversely, the photoacidity of the S1 state is increased for 5-cyano- and 8cyano-2-naphtol, which agrees with calculated changes in electron density between the S0 and S1 states.225 In the T1 state, the pKa of 33 has been determined as 8.1, thus, the acidity is not increased to the same extent as in the S1 state.226 Jackson and Porter related this to the stronger biradicaloid character of 33 in T1 than in S1, where instead the zwitterionic structure is more influential (Figure 32). However, recent computational results suggest that the 6-bromo-2naphtol is significantly more acidic in the T1 state (pKa = 4.2)231 than in the S0 state (pKa = 9.23),232,233 indicating that substituents may influence the acidity of 33 in the T1 state as well. Based on a series of experiments, Wehry and Rogers also concluded that the acidity of excited state phenol is affected by substituents.234 Most interestingly, when compared to the ground state they concluded that conjugative effects are much more important than inductive effects in the excited state. This finding is explainable since the antiaromaticity of the phenyl group in S1 only can be reduced through π-conjugative interactions between the ring and the substituents, whereas in S0 π-conjugative interaction through influence of quinoid-type resonance structures will weaken the aromaticity of the phenyl group and such interactions are therefore less pronounced. Furthermore, experimental results on a series of monosubstituted benzene derivatives with π-electron withdrawing groups reveal that their proton affinities are enhanced by 15− 39 kcal/mol in their S1 states relative to the ground state.235 Thus, excited state benzene can compensate for the antiaromatic destabilization by shifting the electron density

was concluded that the observed shift in the keto−enol tautomerism is due to changes in aromaticity in the S1 state relative to the ground state; that is, 36−38 avoid the six πelectrons in the benzene moiety by favoring the enol form in the S1 state.238,239 Similar conclusions have been drawn for related compounds.240 Taken together, the acid−base behavior described above is related to the general photoreactivity of (4n + 2)π-electron annulene derivatives. In brief, these reactions can be considered as ways to reduce the S1/T1 antiaromatic character of a central cyclic (4n + 2)π-electron moiety through involvement of the substituents. Further examples of this kind are discussed next in section 5.

5. INFLUENCE OF EXCITED STATE AROMATICITY AND ANTIAROMATICITY ON PHOTOCHEMICAL REACTIONS The concept of excited state aromaticity has already found some usage within mechanistic organic photochemistry, with the most widely recognized applications being in the rationalization of photochemical pericyclic reactions. The DewarZimmerman approach tells that a photochemical pericyclic reaction for which the transition state can be described by a cyclic orbital array of Hückel-topology is allowed if the transition state involves 4n π-electrons and forbidden if it involves 4n + 2 π-electrons. The classical photochemical pericyclic reactions are photocycloadditions and electrocyclizations. However, Dewar-Zimmerman analyses of these and similar reactions are now covered thoroughly in textbooks in physical organic chemistry241 and will not be discussed herein. The various photoreactions of benzene in the S1 state are interesting (Scheme 4) and reflect the antiaromatic character of benzene in the S1 state which makes it prone to photorearrange, as concluded by Aihara,21 to pentafulvene (2) and benzvalene (39). The S1 (11B2u) state is also the singlet excited state which was found through NICS CASSCF calculations by Karadakov to display even more antiaromatic (positive) NICS values than cyclobutadiene in its D2h symmetric ground state structure.35 In light of this, the photorearrangement aptitude of benzene is obvious, and it can be noted that 6π-electron 5403


Chemical Reviews

Review

Z/E-photoisomerization as well, but to a different extent as the S1 state lifetimes are shorter than for the T1 state. In a simplified way the triplet excitation can be viewed as being either mainly localized to the CC olefin bond, described as an olefin-excitation, or to the aryl (annulenyl) substituent, described as a ring-excitation.255,256 Obviously, these two types of excitation energy localizations represent the two extreme points on a continuum which describes the distribution of the excitation energy between the olefin bond and the aryl group. If the planar olefin structure on the T1 state PES should be characterized as an olefin-excited structure, then it has a significantly lowered CC π-bond order. As a result, the CC bond twist will be facile and the Z/E-photoisomerization proceeds via the nonadiabatic mechanism. This is the case for (4n + 2)π-electron annulenyl substituted olefins as the substituents in such olefins will avoid to host the triplet biradical due to their T1 state antiaromatic character. Styrene (40) is an example of this type of olefin (Figure 36A).

Scheme 4. Summary of the Benzene Photochemistry That Takes Place upon Excitation to the Antiaromatic S1 State

heteroannulenes, such as pyrrole, display similar aptitudes for photorearrangements.242 Yet, benzene photochemistry has also been covered in earlier reviews and textbooks,243−249 and is not considered extensively here. However, substituents and different structural features in derivatives of benzene and other (hetero)annulenes can help alleviate the antiaromatic character of the excited (4n + 2)πelectron cycle and thereby open up reaction channels that are not available to the annulene itself. These reaction channels allow the annulene derivatives to display a photochemistry which is different from that of the parent annulene. In a similar vein, substituents and other structural features may also help open up reaction channels for formation of excited state aromatic 4nπ-electron cycles, for instance, through photorearrangements or split-off of small molecules such as CO2 or N2. The processes described in this section are either photochemical processes that involve the deantiaromatization of excited state (4n + 2)π-electron species, i.e., rearrangement processes that reduce the antiaromatic character, or processes in which excited state aromatic 4nπ-electron species are formed. Photoinduced formation of targeted 4nπ-electron (hetero)annulenes is also considered further in section 6. We begin this section with a well-established photochemical reaction, the triplet state Z/E-photoisomerization of aryl olefins, for which we earlier revealed that (anti)aromaticity plays a crucial role for the shape of the T1 state PES. Subsequently, we review how excited state (anti)aromaticity affects photosolvolysis, photodecarboxylation, and photoreactions in which ortho-xylylenes and similar compounds are generated. 5.1. T1 State Z/E-Isomerizations of Annulenyl Substituted Olefins

The Z/E-photoisomerization of aryl olefins is a photochemical reaction that can be influenced by aromaticity. It is one of the most fundamental photoreactions and found in a range of areas from nanotechnology to biochemistry. For general aspects of the olefin Z/E-photoisomerizations, we refer to previous reviews and book chapters.247,250−252 The shape of the excited state PES for rotation about an olefinic CC bond is one of the important factors that determine the efficiency of the Z/E-photoisomerization, and it also influences the mechanism by which the isomerization proceeds (nonadiabatic vs adiabatic). The computational studies which explicitly deal with aromaticity effects on the PES for olefin Z/ E-photoisomerizations concern the T1 state isomerization and they were performed with DFT methods.48,253,254 As discussed in previous sections, the excited state aromaticity concept also applies to the S1 state, which suggests that the aromaticity arguments given in this section should be valid for singlet state

Figure 36. Schematic drawings of the S0 state and T1 state PESs along the twist angle θ for rotation about the olefinic CC bonds of (A) styrene (40) and (B) vinylcyclobutadiene (41).

However, when going from planar to perpendicularly twisted T1 state structure of 40, some portion of closed-shell (4n + 2)πelectron (S0 state) aromaticity is regained, manifested by CC bond lengths of the phenyl groups which become more equal (the CC bond length variation, Δrcc(Ph), decreases from 0.086 to 0.037 Å at UBLYP/6-31G(d) level).48 As a consequence, 40 5404


Chemical Reviews

Review

at the perpendicularly twisted T1 structure, 3p*, can largely be described by a 1,2-biradical resonance structure with the biradical at the olefin bond and a closed-shell 6π-electron (S0) aromatic phenyl group, in accord with an earlier description of Caldwell and Zhou.257 This interpretation is supported by calculations of NICS, HOMA, and properties based on the ELFπ which reveal distinct increases in aromaticity along the isomerization paths from planar to perpendicularly twisted T1 structures for a set of five different olefins with T1 state antiaromatic (4n + 2)π-electron annulenyl substituents (set A olefins, Figure 37A).253,254

substituents which are T1 state aromatic and S0 state antiaromatic (set B olefins, Figure 37A). The prototype of such an olefin is vinylcyclobutadiene (41, Figure 36B), a species which only has been studied computationally.253,254 When progressing along the reaction coordinate for the CC bond twist the aromaticity of the ring is reduced while the (bi)radical character of the olefinic CC bond increases. This reduction in aromaticity leads to an increase in energy, i.e., a barrier on the T1 state PES, a feature which was observed for all olefins in set B. As a results of the differences in (anti)aromatic character of the substituents along the T1 state PESs of set A olefins as compared to set B olefins, the variations in the values of a series of aromaticity indices and other properties in general display a zigzag shape when the number of π-electrons of the substituents gradually increases in steps of two (Figure 37B). It has been concluded that a fully adiabatic mechanism is followed for T1 state Z/E-photoisomerizations when the 3p* structure is at least 7 kcal/mol above the planar T1 state structure.252 In fact, a large loss in T1 state aromaticity upon CC bond twist should explain the exceptionally poor isomerization quantum yields (ΦZZ−EZ = 0.0075 and ΦEZ−EE = 0.0040) for the T1 state isomerization of bis(styryl)cyclooctatetraene 42 (Figure 38A).258 For this compound, the triplet excitation energy is completely localized to the central COT ring (Figure 38B), leading only to a negligible reduction in the π-bond order of the CC olefin bond. Thus, the barrier on the T1 state PES for CC bond rotation remains very high (similar as in the S0 state), and the photoisomerization is hampered.

Figure 37. (A) Model olefins with (4n+2)π-electron annulenyl substituents (set A) and 4nπ-electron annulenyl substituents (set B), and (B) a schematic graph displaying the general variation in differences in the values of aromaticity indices and other properties determined for planar and perpendicularly twisted olefin structures as a function of the number of π-electrons of the annulenyl substituents. The aromaticity indices and properties determined were NICS, HOMA, ΔrCC(Ar), spin densities, and relative (U)OLYP and HMO energies.253

On the other hand, if the olefin in the planar T1 state structure is better characterized by a ring-excitation, then the olefin bond will keep substantial CC double bond character and the Z/E-photoisomerization will proceed by the adiabatic mechanism on a T1 state PES which has a barrier for rotation about the CC bond. One end point on the “ring-excitation to olefin-excitation” scale is realized with olefins that have annulenyl substituent with triplet state energies that are extremely low. This is the situation for 4nπ-electron

Figure 38. (A) Isomerization quantum yields (Φ) for the T1 state Z/ E-photoisomerization of bis(styryl)cyclooctatetraene (42) from the Z,Z- to the E,Z- and E,E-isomers, as well as (B) the most important resonance structure of the Z,Z-isomer in the T1 state.258 5405


Chemical Reviews

Review

Interestingly, when examining a series of ethylenes with fivemembered ring -C4H3X heteroannulenyl substituents, and for which the (anti)aromaticity varies between strongly aromatic and strongly antiaromatic, we recently found good correlations between the energy and aromaticity changes along the T1 state PES when going from planar to perpendicularly twisted structures (Figure 39).259 The olefins were split into sets

(43) to 9-methoxy-9H-fluorene (44) in the electronic ground state, Wan and Krogh found that only very harsh conditions would yield a reaction, while a reaction quantum yield of 20% was observed under mild photosolvolytic conditions (Scheme 5, reaction A).46,260 Interestingly, neither diphenylmethanol Scheme 5. Photosolvolysis of 9H-Fluoren-9-ol (43), Diphenylmethanol (45), and 5H-Suberen-5-ol (46)260

(45), which lacks the central five-membered ring, nor 5Hsuberen-5-ol (46), which has a larger seven-membered central ring, underwent photosolvolysis. This led to the conclusion that formation of a 4nπ-electron cation in the excited state was a prerequisite for the photoreaction.260,262 In the excited state such a cationic structure is stated to possess aromatic stabilization, and is thus easily generated, while in the ground state it is destabilized by antiaromaticity making the reaction route highly unfavorable. The results of time-resolved spectroscopic experiments performed by Mecklenburg and Hilinski,261 and later by McClelland et al.,264 suggest that upon photosolvolysis of 9H-fluoren-9-ol the ground state fluorenium cation is generated in less than 20 ps and the existence of the excited state fluorenium cation has not been verified experimentally. However, the presence of the ground state fluorenium cation within less than 20 ps suggests that the heterolysis may proceed via an ultrafast nonadiabatic reaction pathway toward the ground states of the products. Photodecarboxylation of a series of benzannelated acetic acids in aqueous solution shows a different, yet similar, trend to that of the photosolvolysis reactions (Scheme 6).265,266 Photodecarboxylation of 5H-suberene-5-carboxylic acid (47) afforded 50% product 23, while photolysis of 5H-fluorene-9carboxylic acid (48) and diphenylacetic acid (49) resulted in low yields (≤3% and 6%, respectively) of the decarboxylated products 22 and 50. The formation of deuterated product in D2O solution revealed that the photodecarboxylation involves the formation of anionic species, and the yields showed that such anions are favorably formed if they possess 4nπ-electrons. To summarize, both the photosolvolysis and the photodecarboxylation studies clarify that formation of cyclic 4nπelectron species are favored in the S1 state over formation of both cyclic (4n + 2)π-electron and acyclic species.

Figure 39. Energy changes along the T1 PESs (ΔE(T1) = E(T1;twisted) − E(T1;planar)) (kcal/mol) calculated at the M062X/6-311++G(d,p) level and plotted against the T1 aromaticity changes (ΔNICS(T 1 ;1) zz = NICS(T 1 ;twisted,1) zz - NICS(T1;planar,1)zz) (ppm) for olefins CH2CH(C4H3X) with X = CH+, SiH+, BH, AlH, CH2, SiH2, O, S, NH, and CH−.259 Blue markers correspond to data for olefins with the vinyl group in the 2-position of the five-membered ring (set A), and red markers to olefins with the vinyl group in the 3-position of the five-membered ring (set B).

(sets A and B) depending on which position of the ring was attached to the olefin. Sigmoidal functions were fitted to the two data sets as the ΔE(T1) has an upper bound and a lower bound representing idealized olefins with the triplet excitation fully localized to the ring and fully localized to the olefin bond, respectively. Noteworthy, the fit to the data of the set A olefins was significantly better than the fit to the set B olefins. This shows that the change in aromaticity of the substituents along the excited state Z/E-photoisomerization pathway for annulenyl substituted olefins is a determining factor for the shape of the T1 state potential energy landscape, in line with our earlier conclusion based on DFT calculations of a series of experimentally examined aryl substituted olefins.48 5.2. Photosolvolyses, Photodecarboxylations, and Related Reactions

The rates and yields of photosolvolysis reactions of a selection of cyclic π-conjugated hydrocarbons have been shown to depend markedly on the formation of excited state 4nπ-electron cycles.46,260−264 In the case of conversion of 9H-fluoren-9-ol

5.3. Applications of Excited State Acid−Base Chemistry

During the last years, excited state acidities of phenol and 2naphthol have seen interesting applications in organic syn5406


Chemical Reviews

Review

reaction was considered to progress by excitation to the ππ* excited S1 state, a state which should have significant antiaromatic character. It was reasoned that upon C−F bond elongation a higher excited singlet state of πσ* character is lowered in energy and crosses the ππ* (S1) state.283 Progression along this reaction channel leads to difluoroquinomethide 64 and a fluoride ion (or hydrogen fluoride) as products. This reaction is particularly interesting as it reveals that a C−F bond, the strongest single bond to carbon, can be cleaved photochemically. It thus illustrates the extent of destabilization that results from the antiaromaticity of benzene in its S1 state. Similar reactions were observed for ortho- and meta-trifluoromethylaniline (Scheme 8).283 From an applications point of view the photoreaction should be a viable route for transformation of trifluoromethyl substituted aryl alcohols and amines into the corresponding carboxylic acid derivatives.

Scheme 6. Photodecarboxylation of 5H-Suberene-5carboxylic Acid (47), 9H-Fluorene-9-carboxylic Acid (48), and Diphenylacetic Acid (49)265,266

5.4. Photochemical Formation of ortho-Xylylenes and Analogs

The reactions described in section 5.3 reveal active participation of the benzene ring in the photochemical process. For example, (4n + 2)π-electron annulenyl moieties are not photochemically inert spectators but participate in photoreactions as triggers of structural rearrangements in their vicinities. The findings support the view that the aromaticity in the S0 state is exchanged to an antiaromatic destabilization in the lowest excited state, a feature which initializes structural rearrangements. Further examples of reactions which lead to ortho-xylylenes or ortho-heteroxylylenes, but which in contrast to those described in section 5.3 do not progress by elimination of HX, are given next. The photoreaction to form 2,2-dimethylisoindene 71 (Scheme 9, reaction A), an ortho-xylylene derivative, can begin either by irradiation of 70 at λ = 254 nm, which is absorbed locally by the benzene ring, or by irradiation at λ > 285 nm which excites the azoxy chromophore.284 Isoindene 71 can further be transformed photochemically to 5,5dimethylbenzobicyclo[2.1.0]pent-2-ene 72 (Scheme 9, reaction B); however, this latter species also photorearranged back to

thesis.267−281 Irradiation of 2- or 4-substituted phenols or 3- or 6-substituted 2-naphthols, in which a variety of different leaving groups are placed in the benzylic position, leads to formation of short-lived quinonon methides and naphtoquinonon methides, respectively. The photochemical pathways to these compounds correspond to reaction channels that help decrease the antiaromatic character of the benzene and naphtalene moieties in their S1 states. Subsequently, the quinonon methides and naphtoquinonon methide intermediates undergo further reactions with suitable nucleofiles (Scheme 7). A further example of photoreactions of substituted phenols, and which was reported already in the early 1970s, is the photochemical transformation of a trifluoromethyl group of a substituted phenol into a carboxylate group (Scheme 8), a reaction carried out in aqueous solution with the hydroxide ion as nucleophile.282,283 Irradiation of the three different isomers of trifluoromethylphenols and the eight different isomers of trifluoromethylnaphthols yield the corresponding acids. The

Scheme 7. Illustration of the Photoactivation of 2- or 4-Substituted Phenols and 3- or 6-Substituted 2-Naphtoles (X = Leaving Group) upon Generation of Quinonon Methides and Naphtoquinonon Methides, Which Further Can React with a Nucleophile (Nu)a

a

For details concerning the variety of X and Nu, see refs 267−281. 5407


Chemical Reviews

Review

Scheme 8. Photochemical Transformation of Trifluoromethyl Substituted Phenols and Anilines to the Corresponding Carboxylate Substituted Phenols and Anilines282,283

excited state antiaromatic character of the naphthalene moiety. Interestingly, 74 was isolable; however, similar ortho-xylylenes formed photochemically and reported in the same study were intermediates that either rearranged through thermal sigmatropic hydrogen shifts to vinyl substituted arenes or ring-closed to benzo- and naphtocyclobutanes. The photochemical back-reaction from 72 to 71 goes from a strained benzocyclobutene to an ortho-xylylene derivative. The parent benzocyclobutene, however, does not open photochemically in a similar manner, tentatively due to a too strong CC single bond. Yet, the exchange of one sp3 hybridized C atom in the saturated segment to an sp3 hybridized Si atom changes this situation because 75 readily opens upon irradiation to yield the transient 76 (Scheme 10, reaction A).286 These transient species, which are highly destabilized in the S0 state due to both the ortho-xylylene framework and the SiC double bond,287 are trapped with alcohols leading to silylethers 77 in 44 − 80% yield. The reactions of 75 with alcohols in the dark were sluggish, and presence of oxygen did not affect the outcome. Thus, it was concluded that the formation of 76 from 75 progresses in the singlet excited state. Indeed, the photochemical cleavage of a bond between two saturated C atoms in a pure hydrocarbon is observed in dihydronapthalene 78 (Scheme 10, reaction B).288 Experiments with deuterated samples clarified that the formation of 80 does not involve hydrogen migration, but instead that the transients s-cis and strans 79 are formed. Another example of photoinduced formation of an orthoquinoid analog is found in the photolysis of 1H-benzotriazole 81, a compound which displays a desceptively high photostability.289 In fact, upon irradiation the N(H)−N bond of this compound cleaves within a few picoseconds to afford a 6-diazo2,4-cyclohexadiene-1-imine 82 (Scheme 11); yet, its recycliza-

Scheme 9. Photochemical Expulsion of N2O and CO from Benzene and Naphthalene Derivatives Leading to Stable ortho-Xylylene Derivatives 71 and 74284,285

71. It is noteworthy that 71 seemed situated in a sizable well on the S1 surface as the rearrangement to 72 needs thermal activation. Moreover, isoindene 71 is very stable toward UV irradiation when kept in a rigid glass at 77 K, and it displays a strong fluorescence. Conclusively, these reactions show that reactant 70 as well as product 72, which when excited both contain benzene rings that are influenced by S1 state antiaromaticity, are prone to photorearrange to 2,2-dimethylisoindene 71. Another ortho-xylylene derivative, 74, is formed through (Norrish Type-I) photodecarbonylation of 73 by irradiation at 300 nm for 8 h, and it is formed in 80% yield (Scheme 9, reaction C).285 The double α-cleavage of the two C−C bonds flanking the carbonyl group should be facilitated by relief of the

Scheme 10. Photochemical Formation of Transient ortho-Xylylene Derivatives from Compounds 75 and 78 Which Lack Heteroatoms with Lone-Pair Electrons286,288

5408


Chemical Reviews

Review

[4 + 2] cycloadduct 93 in 85% yield. It has been shown that photoenolization is a general reaction which works for a variety of different substrates;296,297 yet, the influence of excited state (anti)aromaticity is not apparent due to the nπ* character of the excitation. In contrast to the photoenolizations, the excited state intramolecular proton transfer (ESIPT) reactions in which a proton is transferred from a hydroxyl group to a carbon atom should be a process that progresses by lowering in the antiaromaticity of the ππ* excited S1 state of a cyclic (4n + 2)πelectron moiety (Scheme 15). However, the ESIPT to a carbon is often an intrinsically inefficient process, even though it was recently shown that unfavorable conformations in the ground state lead to the low reaction quantum yields.302 For 2-phenyl1-naphtol (95), which instead has a suitable ground state conformation, a highly efficient ESIPT (overall Φ = 0.73 ± 0.07) could be observed.303 Interestingly, the ESIPT reaction with proton transfer to a carbon atom has been found to be the cause for photoracemization and photocyclization of optically active 1,1′-bi-2-naphtol (97, BINOL).304 It could be noted that the ESIPT reaction for proton transfer to heteroatoms has been studied extensively,305 and it has been concluded that the S1 state PES of the reaction is adiabatic and that it does not have a significant energy barrier.306

Scheme 11. Photolytic N−N Bond Cleavage of 1HBenzotriazole Leading to 6-Diazo-2,4-cyclohexadiene-1imine 82 and Its Further Transformations289

tion in the ground state is very rapid. When the photoreaction is carried out in a rigid glassy solution at low temperature, the initial photoproduct 82 further decomposes to iminocarbene 83 and 6-iminofulvene 84.290−292 The expulsion of a small molecule and the opening of a ring adjacent to a (4n + 2)π-electron cycle are two ways for reduction of the S1 state antiaromatic destabilization of such a cycle. A third way is photochemically induced hydrogen shifts that lead to breakage of the through-conjugated nature of the cyclic (4n + 2)π-electron path. The [1,5]-sigmatropic hydrogen shift can progress either in a suprafacial manner which is thermally allowed or in an antarafacial manner which is photochemically allowed. The antarafacial shift is facilitated in a nonplanar diene moiety having an adjacent alkyl group. Compounds 85 and 88 (Scheme 12) will adopt nonplanar conformations due to steric congestion when planar, and they rearrange through photochemical antarafacial [1,5]-hydrogen shifts to short-lived ortho-xylylene and ortho-heteroxylylene derivatives which are trapped in Diels−Alder reactions with suitable dienophiles.293,294 A deceivingly similar reaction to those of Scheme 12 is the light-induced intramolecular [1,5]-hydrogen shifts observed for ortho-methyl substituted benzophenones 91 and similar arylketones (Scheme 13).295−301 This reaction, first reported already in 1961,295 can be described as a photochemical enolization as it leads to α-hydroxy-o-xylylenes such as 92. However, it progresses via an nπ* excitation of the carbonyl functionality and is therefore not truly analogous to photoreactions that progress upon ππ* excitations. The photoenolization has been shown to involve a triplet 1,4-biradical 94 which intersystem crosses to the singlet ground state to form the α-hydroxy-o-xylylenes (Scheme 14),300,301 a species which can be trapped with dimethyl acetylenedicarboxylate to give the

At this point it is noteworthy that not only hydrogen atoms but also phenyl groups have been shown to migrate because irradiation of phenyl substituted indene 98 results in isoindene derivative 99 (Scheme 16).307,308 The final product constitutes an exchange in the positions of a hydrogen and a phenyl group. The migration of variously substituted phenyl groups was also examined, and the formation of the short-lived isoindenes upon flash photolysis could be confirmed through NMR spectroscopy at −70 °C and trapping of the products. Finally, benzenoid compounds which lack the excited state reaction channel to ortho-xylylene derivatives as a route for alleviation of the S1/T1 state antiaromaticity can display other photorearrangements. For example, 8,16-methano[2.2]metacyclophane-1,9-diene (101) was found to rearrange to 8b,9a-dihydro-9H-cyclopropa[e]pyrene (103) with formation of *102 as an excited state intermediate (Scheme 17, reaction A).309 This intermediate reveals an excited state C−C bond

Scheme 12. Photochemically Induced Antarafacial [1,5]-Sigmatropic Hydrogen Shifts Leading to ortho-Xylylene Intermediates (86 and 89) and Their Subsequent Trapping in Thermal Diels−Alder Reactions293,294

5409


Chemical Reviews

Review

Scheme 13. Photoenolization of ortho-Methyl Substituted Benzophenones and the Subsequent Trapping by a Dienophile295,297

Scheme 14. Photoenolization of an ortho-Alkyl Arylketone Progressing via a 1,4-Biradical 95 (Triplet Photoenol) Generated upon Intersystem Crossing of an nπ* Excited State300,301

Scheme 17. Photoisomerizations Leading from (A) 8,16Methano[2.2]metacyclophane-1,9-diene (101) to 8b,9aDihydro-9H-cyclopropa[e]pyrene (103) Irreversibly,309 and (B) between the Dimethyl Substituted Metacyclophane 104 and trans-15,16-Dimethyldihydropyrene 105 Reversibly310−312

Scheme 15. Excited State Intramolecular Proton Transfer (ESIPT) from 2-Phenyl-1-naphtol (95)303

antiaromaticity of (4n + 2)π-electron rings in their S1 states is eliminated. To summarize, the notable aptitude of (4n + 2)π-electron annulene derivatives to undergo structural rearrangements upon excitation is obvious. It falls in line with Aihara’s early conclusion that the photoreactivity of benzene stems from its extensive destabilization in the S1 state.21 In a sense, both benzene and naphthalene moieties, and likely also larger (4n + 2)π-electron annulenes, act as molecular versions of “Dr. Jekyll and Mr. Hyde”. In their electronic ground states they are exceptionally stable due to aromaticity, but when electronically excited they change character; molecular segments adjacent to the S0 state aromatic ring may be expelled, or alternatively, the structural neighborhood around the ring is rearranged so as to diminish its S1/T1 state antiaromaticity. Conversely, 4nπelectron cycles which have low general stability in the S0 state are formed very favorably in the lowest excited states as will be seen next. Indeed, the gathered findings reported in sections 5 and 6 should be useful both for the development of new reactions in synthetic organic photochemistry and for deepened understanding of the reasons for photostability of compounds used in, e.g., organic solar cell applications.

Scheme 16. Photochemically Induced Phenyl Group Migration Leading to the ortho-Xylylene Intermediate (99) and a Subsequent Thermally Allowed Suprafacial [1,5]Sigmatropic Hydrogen Shift307,308

formation between two benzene rings, and should examplify another mode for the alleviation of S1 state antiaromaticity. This last type of photoreaction is also found in the metacyclophanediene-dihydropyrene photochromic molecular systems which can be switched from isomers of type 104 with two benzene rings to the isomeric 14π-electron annulene 105 by UV light and then back by visible light (Scheme 17, reaction B).310−312 Thus, in terms of excited state antiaromaticity one could describe this reversible photoisomerization as an isomerization that in both directions works so that the

6. USAGE OF EXCITED STATE AROMATICITY FOR SYNTHESIS OF 4nπ-ELECTRON CYCLES Gain of aromaticity in the S0 state is one of the driving forces exploited in organic synthesis for formation of a certain product in a reaction. Antiaromatic compounds, on the other hand, are highly reactive and they often either oligomerize, rearrange, become oxidized/reduced, or decompose in other ways. Based 5410


Chemical Reviews

Review

Scheme 18. Photochemical Formation of Cyclobutadiene Using Various Precursors Such as Compounds 106,127,320−323 108,326 109,318 110,327 111,319,324,325 112,328 113,326,327 114,332 115,331 116,330 117,334 and 118329 and the Weakly Bound T-Shaped Acetylene Dimer Complex 119333

patterns become more apparent in light of the gains in excited state aromaticity that are accomplished through the reactions.

on the reversal of the electron counting rules for aromaticity and antiaromaticity in the ground state vs the lowest excited states one could expect that gain of excited state aromaticity can be exploited for synthesis of cyclic compounds with 4nπelectrons. Compounds with cyclic (4n + 2)π-electron arrays, on the other hand, should be more difficult to form photochemically, and as seen in section 5, they sometimes even tend to photorearrange. As we show below, rational usage of excited state aromaticity as a conceptual tool can lead to interesting photochemical reactions with potential applications in targetdirected synthesis. The concept should be particularly useful for synthesis of compounds with cyclic 4nπ-electron moieties, a compound class with interesting optical features for potential applications in organic electronics and photovoltaics.142,313 Heteroannulenes with 4n π-electrons could also be subjected to further synthetic transformations, and efficient routes to these heteroannulenes may thus represent interesting and useful entries to a wide variety of medium-sized heterocycles. Several of the reactions discussed in this section are photochemically allowed pericyclic reactions according to the Woodward−Hoffmann rules. The interesting item to note, though, is that these reactions progress either so that an S1/T1 state aromatic ring is formed. From a quick glance without consideration of excited state aromaticity/antiaromaticity the reactions may seem contradictory as, e.g., strong σ-bonds and nonaromatic molecular fragments are traded into weaker πbonds and 4nπ-electron cycles. Yet, the photochemical reaction

6.1. Photochemical Routes to 4π-Electron (Hetero)Annulenic Species

The first attempts to form CBD were performed through thermal reactions,314 but it was only when this compound was generated photochemically and trapped in an argon matrix at cryogenic temperatures that the first spectral data of this otherwise short-lived species were recorded through IR spectroscopy.27,127 The first unambigously monomeric CBD was formed through a photochemical cyclization of α-pyrone (106) to an isomeric β-lactone (107), which subsequently was photolyzed to CBD and CO2 (Scheme 18). Nowadays the photolytic formation of CBD from α-pyrone is classical,315 and the route has also been used to form CBD and 1,3dimethylcyclobutadiene encapsulated in large host molecules and matrices.316,317 In fact, a large variety of precursors (106− 119) have been shown to afford CBD upon photolysis (Scheme 18),315,318−334 and various derivatives of the precursors have been used in photochemical syntheses of substituted cyclobutadienes.315,335−337 Among these, 107−114 each contain a cyclic C4H4 segment, in addition to a segment that splits off upon photolysis. In 107−113 these second segments correspond to stable well-known molecules, and each of these reactions are photochemical [2 + 2] cycloeliminations in which two C−C σ-bonds are traded into two π-bonds. It is 5411


Chemical Reviews

Review

particularly noteworthy that one of the new π-bonds formed is part of the CBD ring. Precursor 114, on the other hand, is interesting as the Fe(CO)3 fragment formed upon decomposition is not a stable molecule, and both the CBD and Fe(CO) 3 products were only detected as short-lived intermediates by kinetic mass spectrometry.332 The photodecomposition of 114 thus seems to be driven forward by formation of the excited state aromatic CBD, a dissociation which is highly unfavorable in the S0 state because of the strong antiaromaticity of CBD in this state. Indeed, photodecomposition of 114 can be used for in situ formation of a highly reactive CBD which is trapped to give, e.g., benzeneoid compounds in reactions with acetylenes.338,339 In contrast to the photocycloeliminations of 106−113, the degree of photochemical rearrangements upon excitations of 115−118 are more complex. This is particularly true for 118 which photodecarbonylates upon irradiation (λ = 254 nm) to give CBD via cyclopropenylmethylene as intermediate.329 It is also highly interesting that two acetylene molecules, weakly bound into a T-shaped dimer (119) as structurally verified by IR spectroscopy, forms CBD, vinylacetylene, and 1,3-butadiyne as main products upon indirect photolysis at λ = 248 nm in a xenon (but not in an argon) matrix.333 In many of the reactions of Scheme 18 the generation of CBD is highly selective. For instance, irradiation of 111 leads exclusively to the [2 + 2] cycloaddition dimer of CBD and 1,3dihydro-2-benzofuran (phthalan),335 whereas photolysis of 117 gives an approximate 1:1 product distribution between the CBD dimer and pyridazine.334 For photolysis of 115 it is concluded that CBD is the major product.331 On the other hand, irradiation of 113 for eight hours in a matrix at cryogenic temperatures yields the CBD dimer as the exclusive product in a yield of just 9%.327 At this point it should be emphasized that substituted CBDs also can be synthesized by methods that exclusively progress in the S0 state315 and that there exist several CBD derivatives stabilized by steric effects of the substituents to the extent that they allow for crystal structure determination and long-term stability at ambient temperatures.340−343 The photochemical properties of such stable CBDs are interesting in the context of excited state aromaticity. Upon direct irradiation, the cyclobutadienes with four sterically congestive tert-butyl or trimethylsilyl substituents dissociated to the corresponding acetylenes and/or rearranged to the isomeric tetrahedranes,342,344−346 species where steric congestion within the molecule is lower. Yet, tetrakis(trimethylsilyl)cyclobutadiene photorearranged only slowly as it took 70 h for it to convert fully into almost equivalent proportions of bis(trimethylsilyl)acetylene and tetrakis(trimethylsilyl)tetrahedrane.346 Recently, it was also clarified that the photochemical relationship between tetrahedranes and cyclobutadienes varies by substitution because perfluoroaryltetrahedranes (120) isomerize photochemically to perfluoroarylcyclobutadienes (121) when irradiated with λ > 300 nm (Scheme 19).347 Clearly, there is an intriguing photochemical relationship between tetrahedranes and cyclobutadienes which depends on substitution pattern. A few heterocyclobutadienes, which are interesting in the context of excited state aromaticity, have also been reported. The recent tetrasilacyclobutadiene with bulky 1,1,7,7-tetraethyl3,3,5,5-tetramethyl-s-hydrindacen-4-yl (EMind) substituents reported by Tamao and co-workers, and which is stable at ambient temperature, is noteworthy.348 Although this tetrasilacyclobutadiene has a singlet ground state with rhomboid

Scheme 19. Photochemically Induced Conversion of tris(Trimethylsilyl)pentafluorophenyl-tetrahedrane (120) to tris(Trimethylsilyl)pentafluorophenylcyclobutadiene (121)347

geometry according to X-ray crystallography, its lowest energy triplet state, which has a square structure, is calculated to be merely 2.1 kcal/mol higher in energy than the S0 state at the (U)B3LYP/6-31G(d,p) level. Maier and Schäf er also prepared different transient azacyclobutadienes, such as 124, by irradiation of 1,3-oxazin6-ones (122) in argon matrices at cryogenic temperatures (Scheme 20). This photochemical route is analogous to that for Scheme 20. Photochemical Generation of 2-tert-Butyl-4methyl-1-azocyclobutadiene (124)349

formation of CBD from 106,349,350 but in contrast to CBD the actual formation of azacyclobutadienes could only be inferred through analysis of the decomposition products. Still, quantum chemical calculations of the reaction mechanisms for the photochemical formations of cyclobutadiene and azacyclobutadiene, starting from α-pyrone and 1,3-oxazin-6-one, respectively, supported that both 4π-electron species are formed via the S1 state.351,352 Taken together, cyclobutadienes and azacyclobutadienes clearly seem located at energetic sinks (collection points) on their S1 state PESs as a result of their excited state aromatic character. This is experimentally verified by, for example, the sheer number of (at least) thirteen different photochemical routes all leading to the parent CBD. Furthermore, as photolysis is an efficient route to azacyclobutadienes one may also postulate that photolyses of appropariate precursors could be efficient routes to other 4π-electron heteroannulenes, e.g., borole and aluminole derivatives. However, to the best of our knowledge no such photochemical routes have so far been reported. 5412


Chemical Reviews

Review

6.2. Photochemical Routes to 8π-Electron (Hetero)Annulenic Species

Among the photochemical routes to the parent COT, the photorearrangement of tricyclo[3.3.0.02,6]octa-3,7-diene 125 (Scheme 21, reaction A), which requires breaking of two C−C single bonds, was found to occur at −60 °C upon direct UV irradiation and it gave semibullvalene 126 as side-product.367 After irradiation of 125 in toluene for 80 min the product distribution was 26% COT, 12% 126, and 62% starting material, and when carried out in isopentane the product distribution was 29% COT, 19% 126, and 52% starting material. The reaction was also attempted in dimethylether at −60 °C, but the conversion was then very small. However, the most efficient photochemical route to the unsubstituted COT is the decarbonylation of bicyclo[4.2.1]nona-2,4,7-trien-9-one (127) displayed in reaction B of Scheme 21 as it afforded COT in 82% yield upon direct irradiation at 300 nm in tetrahydrofuran (THF) and 70% when irradiated at 254 nm in diethylether.371,372 The photolysis of tetracyclo[4.2.0.02,4.03,5]octane (128) as shown in reaction C (Scheme 21), also produced COT in reasonable yield (appr. 50%) after exposure to 254 nm direct irradiation for 5.5 h, the other portion being starting material.373 Similarly, compound 129 gave COT upon direct irradiation (reaction D), but with a yield which was slightly lower than that of reaction C.374 Moreover, reactions A, C, D, and F are interesting as they clarify that C8H8 isomers, which structurally are very different from COT, transform into COT upon photolysis even though the yield for COT formation upon sensitized irradiation in reaction F is only minute and instead semibullvalene is the major product. A route to the parent COT of fundamental importance is the photocycloaddition of acetylene to benzene (Scheme 21, reaction E), a reaction which is accomplished through irradiation (λ = 254 nm) of an acetylene-saturated benzene solution at room temperature.375 Yet, the quantum yield of this particular reaction was very low (Φ = 0.001), but similar photocycloaddition reactions between arenes and longer alkynes progress in high yields (vide infra) and are of obvious synthetic value. As noted above, the parent COT can also be formed by irradiation of barrelene 130 in presence of acetone as photosensitizer, although in a yield of merely 1−2% (Scheme 21, reaction F).376 The primary photoproduct was instead semibullvalene 126 formed through a di-π-methane rearrangement. Yet, direct photolysis of 126 produced COT but no barrelene, and significantly improved yields of COT derivatives in barrelene-to-COT photorearrangements are obtained when benzo- and dibenzobarrelenes are irradiated in absence of sensitizer (Scheme 22). Still, the existence of six different photochemical routes to COT suggests, similar to the thirteen different photochemical routes to CBD, that gain in excited state aromatic character is a driver for photochemical reactivity. A crucial parameter for the yield of COTs is the electronic state (the S1 or the T1 state) in which the photoreaction occurs, and this is particularly apparent for the barrelene photochemistry. The differences in singlet and triplet state photochemistries should therefore be discussed further. As seen in reactions A and B, Scheme 22, the major products upon irradiation of benzo- or dibenzobarrelenes in the presence of a triplet sensitizer (acetone or acetophenone) are either benzoand dibenzosemibullvalenes (up to 99% yield).363,364,377 In contrast, the corresponding COT derivatives 133 and 136 dominate as products when the reaction is carried out without sensitizer (up to 95% yield). Here it should be pointed out that the reactions often involve intermediates, and the connection between the electronic state

While the various routes for photochemical formation of cyclobutadienes primarily are of fundamental interest, synthetic routes to COT derivatives could find more apparent usage in the tailoring of active compounds for photovoltaics, organic electronics, and similar applications.142,353 8π-Electron heteroannulenes may also represent useful intermediates in the syntheses of a plethora of different medium-sized heterocycles. They can be generated by routes that progress completely in the S0 state,354 but there are also a series of photochemical alternatives.355−362 The photochemical syntheses of these species progress via, for example, photorearrangements or photoeliminations using suitable precursors, or via photocycloadditions of CC or CN triple bonded compounds to 6π-electron (hetero)annulenes followed by ring-openings to COT and COT derivatives (Schemes 21−24).363−370 It is Scheme 21. Photochemical Reactions Leading to the Parent Cyclooctatetraenea

a

For reaction A see ref 367, for reaction B see refs 371 and 372, for reaction C see ref 373, for reaction D see ref 375, and for reaction E see ref 376.

particularly noteworthy that the photochemical routes in many cases progress in high yields. However, rather than providing a full treatise covering all COT derivatives that have been generated photochemically this section highlights examples that show different aspects of photochemical formation of COTs that can be related to excited state aromaticity, and we illustrate with examples of the different reaction types. 5413


Chemical Reviews

Review

Scheme 22. Photochemical Formation of Benzocyclooctatetraenes (133a and 133b), Dibenzocyclooctatetraenes (136 and 140), and Octafluorocyclooctatetraene (138), Together with the Corresponding Semibullvalenes, under Direct vs Sensitized Irraditationa

a

For reaction A see refs 377 (X = H) and 369 (X = F), for reaction B see refs 377 (direct irradiation) and 370 (sensitized irradiation), for reaction C see ref 379, for reaction D see ref 381, and for reaction E see ref 384.

(139) in presence of MBF4 salts (M+ = Na+, K+, Cs+) gave the dibenzocyclooctatetraene 140 in acetonitrile or benzene solution but the dibenzosemibullvalene 141 in the solid state, the latter a result of a strong cation effect (Scheme 22, reaction D).381 Interestingly, the lighter sodium cation provided a stronger cation effect than the potassium ion and it was considered to potentially be a result of the stronger cation-π interaction of Na+ than of K+,382 and consequently, a more efficient intersystem crossing to the triplet state giving photoproduct 141.381 Finally, whereas 137 yields the corresponding COT upon both direct and sensitized irradiation the opposite is found to be the case for several dibenzobarrelenes with ester substituents attached to the vinyl bond,360 a result which likely stems from rapid intersystem crossing from

and formation of the COT vs the semibullvalene isomer is not always clear-cut. For example, octafluorobarrelene 137 photorearranges exclusively to octafluorocyclooctatetraene 138 both upon direct and sensitized irradiation (Scheme 22, reaction C),378,379 suggesting that also gain in T1 state aromaticity can function as a driver for the rearrangement. On the other hand, the napthobarrelene photochemistry was found to be dependent on the connectivity because 1,2-napthobarrelene gave the corresponding 1,2-napthocyclooctatetraene as major product (76%) upon direct irradiation while the 2,3-napthobarrelene lead to 2,3-napthosemibullvalene upon both direct and sensitized irradiation.380 The product distribution can also be tuned by an external agent because direct irradiation of dibenzobarrelene derivatives with crown ether functionalities 5414


Chemical Reviews

Review

rearrangement, was accomplished through a ground state Diels−Alder reaction between [24](1,2,4,5)cyclophane (148) and dicyanoacetylene (Scheme 24, reaction A).386 The subsequent irradiation of 149 in THF solution led to cyclophane 150 (yield 71%), which instead of two benzene rings was composed of one COT ring and one benzene ring. This cyclophane, however, proved to be thermally unstable as it rearranged at room temperature to bicyclo[4.2.0]octatriene 151 over an activation barrier of 26.0 kcal/mol. As mentioned above (Scheme 21, reaction D) and as seen in reactions B and C of Scheme 24, the 1,2-photocycloaddition between a 6π-electron (hetero)annulene and an alkyne, followed by a cyclobutene ring-opening, can be a highly efficient route to COT derivatives. This reaction was first reported independently by the groups of Grovenstein and Bryce-Smith, 387,388 and it has proven valuable in the preparation of several 1,2-disubstituted cyclooctatetraenes.358,389 The reaction leads to bicyclo[4.2.0]octa-2,4,7triene intermediates, e.g., 153, which can yield the corresponding COT derivatives in a thermally allowed disrotatory cyclohexadiene ring-opening and/or a photochemically allowed disrotatory cyclobutene ring-opening, with the importance of the latter process being highly substituent dependent.390,391 Indeed, reports on formation of COTs starting from an arene and an alkyne can be applied to a range of different arenes and a range of different acetylenes. In the first experiments acetylenes with π-electron withdrawing substituents were used. However, also acetylenes with σ-electron withdrawing substituents as in bis(trifluoromethyl)acetylene as well as acetylenes with σ-electron donating groups (Me3Si−CC−) all provide cyclooctatetraenes in photocycloadditions reactions with benzenes, although sometimes in only modest yields.392,393 The products formed may also be suitable for further transformations. For example, the intramolecular arene/ alkynylsilane photoaddition which led to a COT in good yield (Scheme 24, reaction C) has been considered to have potential for synthesis of natural products based on the bicyclo[6.3.0]undecane skeleton.393 Similar to cyclobutadienes, cyclooctatetraenes can be formed by photoextrusion of a small molecule from a suitable precursor. Photoextrusion of CO from 127 (Scheme 21, reaction B) generates both the parent COT and dialkyl substituted COTs in very good yields,368,372 and 1,2disubstituted cyclooctatetraenes are easily formed in excellent yields (85 − 95%) through cheletropic photoextrusion of SO2 from suitable precursors such as 158 (Scheme 24, reaction D).394 Finally, it was found that irradiation of 4,15bis(phenylethynyl)-[2.2]paracyclophane 160 by a medium pressure mercury lamp for 4.5 h led to two dimeric products, 161 and 162, where the first is a bis([2.2]paracyclophane)cyclooctatetraene derivative (Scheme 24, reaction E).395 The mechanism for formation of 161 was considered to progress via dimerization of the CBD formed through photocycloaddition between the two CC triple bonds of 160. The facile photochemical formation of COT and various COT derivatives by a range of different routes, including from C8 isomers that are structurally very dissimilar to COTs, demonstrate that the excited state D8h symmetric minimum of COT in S1 acts as a collection point in this state, similarly as concluded by Garavelli et al. when based on quantum chemical calculations.49 With this as background, the remarkable photostability of COT, commented upon in several early studies, is noteworthy. For example, Zimmerman and Iwamura

S1 to T1 promoted by the carbonyl functionalities. Still, there are also diester substituted dibenzobarrelenes that give dibenzocyclooctatetraenes upon direct irradiation.383 The difference in COT vs semibullvalene product distribution as a function of electronic state (S1 vs T1) is also observed when using other reactants than barrelenes. Direct irradiation of 2,3-benzobicyclo[4.2.0]octa-2,4,7-triene 142 led to benzocyclooctatetraene 133a in a yield of 39% and Φ = 0.11 (Scheme 22, reaction E), while sensitized irradiation with p-dimethylaminobenzophenone gave the benzosemibullvalene 132a as the main product in a 54% yield (Φ = 0.082).384 Taken together, the generally preferred photochemical route for formation of COTs should be, despite the exceptions given above, by direct irradiation regardless of reactant. The NICS(0)zz of COT in the S1 and T1 states (−21.5 and −20.6 ppm, respectively) indicate that the S1 state is slightly more aromatic in character than the T1 state,36 thus potentially providing a stronger driving force for COT formation in the first of these two excited states. Another interesting variation in the barrelene photochemistry is the influence of the position of a single methyl substituent on the reaction outcome. Whereas direct irradiation of unsubstituted dibenzobarrelene 134 (Scheme 22, reaction B) in acetonitrile at λ = 254 nm gave both dibenzocyclooctatetraene (Φ = 0.24) and dibenzosemibullvalene (Φ = 0.29), direct irradiation of 7-methyldibenzobarrelene 143 afforded 3methyldibenzocyclooctatetraene 145 with a nearly doubled quantum yield (Φ = 0.44; Scheme 23, reaction A).385 Scheme 23. Photochemical Rearrangements of Substituted Dibenzobarrelenes to Substituted Dibenzocyclooctatetraenes upon Direct Irradiationa

a

For reaction A see ref 385, and for reaction B see ref 377.

Moreover, 145 could be formed in a crude yield of 89% upon irradiation of 1-methyldibenzobarrelene 144 at 254 nm in benzene. Cyclooctatetraenes with larger alkyl substituents at the ring, such as 147, are also produced smoothly upon direct irradiation (Scheme 23, reaction B).377 A series of more elaborately substituted COT derivatives have also been synthesized photochemically. A hexasubstituted barrelene (149) with four alkyl tethers and two cyano substituents, and which undergo a barrelene-to-COT photo5415


Chemical Reviews

Review

Scheme 24. Photochemical Reactions Leading to COT Derivativesa

a

For reaction A see ref 386, for reaction B see refs 387 and 396, for reaction C see ref 393, for reaction D see ref 394, for reaction E see ref 395.

alkyne as phenyl-tert-butylacetylene gave no azocine product in contrast to 1-phenylacetylene. Azocines can also be attained through photocycloadditions between conjugated nitriles and benzenes with donor and acceptor substituents in para arrangements (Scheme 25, reaction B).399 A remarkable photorearrangement leading to an excited state aromatic 8πelectron cycle is the formation of 1,2-diazacyclooctatetraene 170 accomplished through irradiation of 7,8-diazatetracyclo[3.3.0.02,4.03,5]oct-7-ene 169 into its triplet state by four different methods (Scheme 25, reaction C).400 Noteworthy, the overall process of this reaction is analogous to the photochemical formation of COT according to reaction D, Scheme 21.374 Seven-membered heterocycles can also be formed, e.g., irradiation at 254 nm converts the 3-aza-7oxatricyclo[4.1.0.02,5]hept-3-enes 171a-c into the isomeric 1,4oxazepins 172a-c in yields of 90−95% (Scheme 25, reaction D).401 Finally, a method of preparative value for the formation of 1,2-diazepine derivatives is the irradiation of 1-iminopyridinium ylide 173 which gives the diazepin 174 in yields of 41− 97% (Scheme 25, reaction E).402−406 This rearrangement is potentially also interesting from a fundamental perspective as it turns a 6π-electron S0 state aromatic species (173) into an 8πelectron excited state aromatic species (174). However, the process has been considered, based on semiempirical AM1 calculations, to involve a bicyclic 1,7-diazanorcaradiene

found that photochemical conversion of COT into semibullvalene (126) proceeded only slowly and that extended photolysis gave a stationary state composed of 88% COT and 12% 126.397 Paquette and co-workers reported that acetone solutions of 1,2- and 1,4-dimethylcyclooctatetraenes could be irradiated for up to 100 h with 87−92% recovery and wrote that “this photochemical stability contrasts in an interesting way with the ready thermal rearrangements of these molecules”.50 The remarkable photostability (“sink” feature) of COT should be tractable to its S1 and T1 state aromatic character. Indeed, this photostability should be highly suitable for compounds designed for photovoltaics and similar applications. Analogous to the photochemical generation of COTs there are also many examples of photochemical generation of 8πelectron heteroannulenes. Photocycloadditions between a 6πelectron annulene and an acetylene followed by a ring-opening to an 8π-electron ring can be carried out with pentafluoropyridine and 1-phenylpropyne, leading to a substituted azacyclooctatetraene (an azocine, 166), as displayed in reaction A, Scheme 25.398 For this reaction it has, however, to be noted that pentafluoro substitution at the pyridine is required so as to change the order in energy between the nπ* and ππ* states of pyridine as the latter state is the active one in the [6 + 2] photocycloaddition reaction. Moreover, the product formation varied strongly with the bulk of the alkyl substituent at the 5416


Chemical Reviews

Review

6.3. Photochemical Syntheses of Larger and Polycyclic Compounds with 4nπ-Electron Perimeters

Scheme 25. Photochemical Reactions Leading to 8πElectron Heterocyclesa

Larger 4nπ-electron annulenes as well as more complex polycyclic conjugated compounds with 4nπ-electron cyclic pathways are also possible to generate photochemically, once again reflecting the gain in excited state aromaticity as a driver for the reactions. The number of studies of photochemical formation of such compounds is, however, much more limited. Pentalene (2) and pentalene derivatives with 8π-electron perimeters can be formed through photolytic cleavages considered to proceed in a two-step sequence (Scheme 26), where both steps are photoinduced.409,410 Scheme 26. Photolysis of Pentalene Dimers for Generation of the Pentalene Monomer 5410

Neumann and Jug also proposed, based on semiempirical SINDO1 calculations, to form pentalene through irradiation of diazoazulene 178 (Scheme 27).411 However, the experimental

a

Scheme 27. Proposal for a Photochemical Route to Pentalene 5, Based on Results from Quantum Chemical Calculations As Reported by Neumann and Jug411

intermediate with the second step leading to the diazepin progressing as a ground state rearrangement.407 A range of additional examples on photochemical formation of 8π-electron (hetero)annulenes also exist, and these or similar reactions can likely be used to form new types of heteroannulenes. However, from the publication dates of the articles upon which this section is based it becomes clear that the area of photochemical synthesis of 8π-electron (hetero)annulenes flourished on the 60s, 70s, and 80s. After this period of blooming most COT derivatives have been synthesized by metal-mediated routes.408 Yet, photochemical routes for formation of substituted COTs and 8π-electron heteroannulenes should represent inexpensive, efficient, and environmentally benign but underexploited synthetic alternatives that deserve more attention from the synthetic method development community. Such routes could provide, for example, access to a series of novel bioactive medium-sized heterocycles.

validation of this method has still to be reported. If successful experimentally a synthetic validation would reveal that a compound with a 10π-electron perimeter can be rearranged photochemically into a compound with an 8π-electron perimeter and which has distinct antiaromatic character in S0. It would be a 10π → 8π-electron analogue of the photorearrangement from the 6π-electron cycle 173 to the 8πelectron cycle 174 displayed in reaction E, Scheme 25. Photochemistry can also be used to form the next larger 4nπelectron annulene, [12]annulene (181), as this compound can

For reaction A see ref 398, for reaction B see ref 399, for reaction C see ref 400, for reaction D see ref 401, and for reaction E see ref 402.

5417


Chemical Reviews

Review

be formed photochemically at a low temperature from syntricyclo[8.2.0.02,9]dodeca-3,5,7,11-tetraene 180 in a good yield (Scheme 28).412,413 However, at temperatures above −40 °C

In summary, it is evident that the [4]-, [8]-, [12]-, and [16]annulenes all can be formed photochemically, in many cases with high yields. The reaction pathways toward cyclic conjugated 4nπ-electronic annulenic compounds tends to be favorable on the lowest excited state PESs, in contrast to the corresponding pathways on the ground state PESs. The general situation is illustrated in Figure 40. Thus, the concept of excited

Scheme 28. Photochemical Formation of [12]Annulene 181 from syn-Tricyclo[8.2.0.02,9]dodeca-3,5,7,11-tetraene (180), Its Further Thermal and Photochemical Rearrangement to 182 and 183, and Its Photochemical Backrearrangement to 181412

Figure 40. Illustration of the photochemical formation of a 4nπelectron annulenic compound exploiting its relative energetic stabilization in the S1 or T1 state due to ππ* excited state aromaticity.

state aromaticity could in future serve as a convenient tool when planning target-directed syntheses of 4nπ-electron cycles or, in general, medium-sized heterocycles. Moreover, in the search of improved or novel photochemical pathways to 4nπelectron annulenes modern quantum chemical calculations could nowadays likely provide precise and detailed guidance to experimental mechanistic studies, in contrast to the situation in the 60s through the 80s.

181 rearranges thermally to 182, or when irradiated at temperatures above −70 °C it rearranges to 183. Yet, 181 can be regenerated from both 182 and 183 when these compounds are irradiated at −100 °C. Thus, 181 can be reached photochemically from three different reactants, and the formation progresses in high or very high yields, indicating that this rather complex and large 12π-electron annulene is located at an aromatic collecting point on the S1 state PES. Interestingly though, the photochemical ring-opening of 180 initially yields another isomer of [12]annulene than the E,Z,E,Z,E,Z-cyclododeca-1,3,5,7,9,11-hexaene displayed in Scheme 28.412 However, high structural flexibility and conversion between various [12]annulene isomers is achieved through a twist-coupled bond-shifting that progress over lowenergy activation barriers stabilized by Möbius aromaticity.414 Accordingly, the photochemical and thermal processes that interconvert between 180, 182, 183 and the E,Z,E,Z,E,Z-isomer of [12]annulene exploit the possibilities for excited state aromaticity as well as Möbius aromaticity of 4nπ-electron annulenes, thus exemplifying the impact of two of the nonstandard aromaticity forms displayed in Figure 9. Hence, it is not surprising that the next larger 4nπ-electron annulene, [16]annulene (185), also can be formed photochemically, in this case through photochemical electrocyclic ring-opening of 184 (Scheme 29), i.e., a dimer of COT.415 Yet, the yield is now significantly reduced when compared to that for photochemical formation of [12]annulenes.

7. CONCLUSIONS AND OUTLOOK In this review we have described the development of the excited state aromaticity concept over time, a development which has progressed slowly over several decades and in different branches of chemistry. As a result, the concept has not been as visible to the chemical community as desirable. This review provides a cumulative collection of studies that strongly support the existence of aromaticity and antiaromaticity effects in the lowest excited states of cyclic π-conjugated molecules, and our hope is that it provides the full picture as of today. The excited state (anti)aromaticity effects could be equally useful for the understanding of excited state properties and processes as ground state aromaticity is useful for rationalization of ground state properties and processes. With the unambiguous existence of aromaticity effects in the lowest electronically excited states the continued search for a unifying phenomenological background to the aromaticity concept should also be encouraged. In the 60s and 70s the theory for excited state aromaticity and antiaromaticity was formulated, and recent high-level quantum chemical studies confirm that the qualitative concept is correct. The time should be ripe to exploit this concept as a tool for rational design of optically interesting and useful compounds as well as for improved understanding of photochemical and photophysical properties and processes in a range of different areas. Still, there are fundamental issues that remain to be explored, e.g., one may ask how and to what extent the excited state aromaticity concept can be used for understanding the excited state properties and the photochemistry of metallaaromatic compounds and cage compounds.

Scheme 29. Photochemical Formation of [16]Annulene 185 from Cyclooctatetraene Dimer 184415

5418


Chemical Reviews

Review

Present Address

Also, is there an excited state homoaromaticity similar as displayed by some nonclassical carbocations in the ground state? It has been confirmed that excited state aromaticity has an impact on geometries, dipole moments, excitation energies, and reactivities in the S1 and T1 states. For instance, photoswitches often rely on Z/E-isomerizations and electrocyclizations, and since many photoswitches occur by a photoreaction (excited state potential energy surface) in one direction and a thermal reaction (ground state potential energy surface) in the other, control of switching ability by exploiting excited state (anti)aromaticity should be viable. Similarly, in molecular assemblies for photoinduced electron transfer processes, systems may be designed where the forward reaction exploit excited state aromaticity in such a way that photoinduced electron transfer can be channeled to go from a markedly excited state antiaromatic moiety of the molecular assembly to an excited state nonaromatic, or even aromatic, molecular segment. On a further note, the schematic potential energy surfaces with relative energies for reactants and products in ground and excited states depicted in Figure 40 resembles markedly the required profiles of the S0 state and S1 state potential energy surfaces of the solar thermal fuel recently designed by Kolpak and Grossman.416 Although their solar thermal fuel is composed of azobenzene-functionalized carbon nanotubes (CNTs) and therefore does not rely on aromaticity effects, it shows that considerations of both ground and excited state potential surfaces is a prerequisite to exploit the full potential of such devices. Thus, in further developments of functional materials, excited state (anti)aromaticity should be important to take into consideration. The predictable control of the energetic position of the excited states of fulvenes using a combined look at ground and excited state aromaticity hints at how to control material band gaps. It is likely that molecular materials with very small as well as very large band gaps can be designed in similar ways. Furthermore, excited state aromaticity and antiaromaticity are underexploited tools for steering the selectivity in organic photochemical reactions. In fact, it should be a highly useful feature for synthesis of compounds with interesting (designed) optical and electronic properties, compounds which may be cumbersome to access by conventional synthesis in the electronic ground state. To end, the future will hopefully see a range of new interesting and unforeseeable developments and applications in the area of excited state aromaticity. One might speculate that excited state aromaticity could become a cornerstone for (organic) photochemistry similar as regular Hückel-type aromaticity has developed for (organic) chemistry in the electronic ground state. Four decades after the publication of Baird’s landmark theory paper on excited state aromaticity,20 it is time this research area receives more attention and that it is also exploited in applications-oriented chemistry, materials, and energy research.417

§

Faculty of Landscape Architecture, Horticulture and Crop Production Science, Swedish University of Agricultural Sciences, Box 55, SE-23053 Alnarp, Sweden. Notes

The authors declare no competing financial interest. Biographies

Martin Rosenberg was born in 1984, in Kalundborg, Denmark. He obtained his M.Sc. (Chemistry) from University of Copenhagen, Denmark in 2008 in the field of ultrafast femtosecond spectroscopy under the supervision of Associate Professor Theis I. Sølling. He began his doctorial studies in the end of 2008 in the group of Associate Professor Kristine Kilså. His doctoral work focused on the application of excited state π-electron delocalization as a tool for the manipulation of excited state energies and other properties. In 2012, he graduated from the University of Copenhagen and continued working as a scientific associate in the optical spectroscopy laboratory of Kristine Kilså. Currently, Martin is working as a postdoc in the group of Professor Bo W. Laursen at the University of Copenhagen and he was in 2013 awarded the Research Talent prize from The Danish Council of Independent Research (DFF). His current work and research focus on design, synthesis, and characterization of red- and NIR-emitting dyes for use in optical sensor materials.

Christian Dahlstrand was born 1979, in Västerås, Sweden. After working several years as a chef he decided to pursue a career in chemistry and received his M.Sc. in Chemical Engineering in 2008 from Mälardalen University in Eskilstuna. He attained his Ph.D. in 2012 at Uppsala University under the supervision of Associate Professor Henrik Ottosson. The Ph.D. project was mainly directed towards the use of the ground and excited state aromaticity concept to tune the properties of fulvenes and other fulvenoid molecules. As he both performs synthesis and computations his future hope is to bridge

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 5419


Chemical Reviews

Review

Colorado at Boulder, USA. In autumn 2000 he started as Assistant Professor at Uppsala University, and in 2003 he was promoted to Associate Professor in physical organic chemistry. He leads a research group that performs both experiments and computations, and which besides research on excited state (anti)aromaticity effects is active in studies on the structure and reactivity of unsaturated silicon compounds with a focus on selective reactions of potential usage in organic synthesis (in collaboration with Dr. Patrick Steel, University of Durham, U.K.), as well as design of silicon-containing compounds with novel conjugation topologies for possible use in molecular electronics.

the gap between theoretical and synthetic chemists. After a short postdoc under Associate Professor Joseph Samec at Uppsala University where he studied metal catalyzed reaction mechanisms using DFT calculations, he joined Associate Professor Joseph Samec’s industrial venture which focuses on the conversion of lignin to a renewable liquid fuel feedstock.

ACKNOWLEDGMENTS First, H.O. thanks earlier group members who have contributed to the studies from his group presented herein, several of whom have been supported through postdoctoral fellowships from the Carl Trygger Foundation and Wenner-Gren Foundations, as well as Prof. Josef Michl for many stimulating discussions on the topic of excited state (anti)aromatcity. We are grateful to the Department of Chemistry, University of Copenhagen for financial support of the Ph.D. fellowship of M.R., and Uppsala University for support of C.D. Finally M.R. and K.K. are grateful to the Niels Bohr Foundation, the Oticon Foundation, and the Torkil Holm Foundation, and C.D. is grateful to the Ingegerd and Viking Olov Björks Foundation, for funds that enabled the present collaboration.

Kristine Kilså was born in Denmark in 1972, and obtained her M.Sc. (Chemistry) from University of Copenhagen, Denmark in 1995. She then moved to Chalmers University of Technology in Gothenburg, Sweden to obtain her Ph.D. in 2000 with Professor Bo Albinsson in optical spectroscopy. After her postdoc with Professor Harry Gray at California Institute of Technology, she started her independent career at Uppsala University, Sweden, which in 2004 was followed by an Associate Professorship at University of Copenhagen and in 2012 by a Professorship with special duties within teaching and education, also in Copenhagen. Her chemical research focuses on light-induced energy and electron transfer, with the purpose of gaining molecular control over excited state properties and behavior, including spin state influence. However, she has recently moved to the Swedish University of Agricultural Sciences, where she is the coordinating director of studies and the head of the administrative educational center at the Faculty of Landscape Architecture, Horticulture and Crop Production Science.

REFERENCES (1) Minkin, V. I.; Glukhovtsev, M. N.; Simkin, B. Y. Aromaticity and Antiaromaticity. Electronic and Structural Aspects; John Wiley and Sons: New York, 1994. (2) Schleyer, P. v. R. Ed.; Chem. Rev. 2001, 101, complete issue 5. (3) Schleyer, P. v. R. Ed.; Chem. Rev. 2005, 105, complete issue 10. (4) Stanger, A. Chem. Commun. 2009, 1939. (5) Martin, N., Haley, M. M., Tykwinski, R. Eds: Aromaticity: A Web Themed Issue, Chem. Commun. 2012, 48, 10471. (6) Gleiter, R.; Haberhauer, G. Aromaticity and Other Conjugation Effects; Wiley-VCH: Weinheim, Germany, 2012. (7) Hückel, E. Grundzüge der Theorie Ungesättiger und Aromatische Verbindungen; Verlag Chemie: Berlin, 1938. (8) Dewar, M. J. S. Tetrahedron Suppl. 1966, 22 (Suppl. 8), 75. (9) Hückel, E. Z. Phys. 1931, 70, 204. (10) Frost, A.; Musulin, B. J. Chem. Phys. 1953, 21, 572. (11) Zimmerman, H. E. J. Am. Chem. Soc. 1966, 88, 1564. (12) Zimmerman, H. E. J. Am. Chem. Soc. 1966, 88, 1566. (13) Zimmerman, H. E. Angew. Chem., Int. Ed. 1969, 8, 1. (14) Zimmerman, H. E. Acc. Chem. Res. 1971, 4, 272. (15) Heilbronner, E. Tetrahedron Lett. 1964, 1923. (16) Dewar, M. J. S. Angew. Chem., Int. Ed. 1971, 10, 761. (17) Dougherty, R. C. J. Am. Chem. Soc. 1971, 93, 7187. (18) Woodward, R. B.; Hoffmann, R. Angew. Chem., Int. Ed. 1969, 8, 781. (19) Woodward, R. B.; Hoffmann, R. The Conservation of Orbital Symmetry; Verlag Chemie GmbH/Academic Press Inc.: Weinheim, Germany, 1970. (20) Baird, N. C. J. Am. Chem. Soc. 1972, 94, 4941. (21) Aihara, J.-i. Bull. Chem. Soc. Jpn. 1978, 51, 1788. (22) Breslow, R.; Chang, H. W.; Hill, R.; Wasserman, E. J. Am. Chem. Soc. 1967, 89, 1112. (23) Saunders, M.; Berger, R.; Jaffe, A.; McBride, J. M.; O’Neill, J.; Breslow, R.; Hoffman, J. M., Jr.; Perchonock, C.; Wasserman, E.; Hutton, R. S.; Kuck, V. J. J. Am. Chem. Soc. 1973, 95, 3017. (24) Breslow, R. Pure Appl. Chem. 1982, 54, 927. (25) Breslow, R.; Jaun, B.; Kluttz, R. Q.; Xia, C.-Z. Tetrahedron 1982, 38, 863. (26) Fratev, F.; Monev, V.; Janoschek, R. Tetrahedron 1982, 38, 2929.

Henrik Ottosson was born in 1966 in the western parts of Sweden. In 1992 he obtained his M.Sc. (Chemical Engineering with branch of studies Engineering Mathematics) at Chalmers University of Technology, Gothenburg, Sweden. This was followed by doctoral studies in computational quantum chemistry carried out under supervision of Professor Dieter Cremer, University of Gothenburg. After having received the Ph.D. degree in 1996 he worked one year as research associate the Department of Organic Chemistry at Chalmers University of Technology, before he embarked on postdoctoral research in the group of Professor Josef Michl at the University of 5420


Chemical Reviews

Review

(27) Chapman, O. L.; McIntosh, C. L.; Pacansky, J. J. Am. Chem. Soc. 1973, 95, 614. (28) Gogonea, V.; Schleyer, P. v. R.; Schreiner, P. R. Angew. Chem., Int. Ed. 1998, 37, 1945. (29) Wörner, H. J.; Merkt, F. Angew. Chem., Int. Ed. 2006, 45, 293. (30) Wörner, H. J.; Merkt, F. J. Chem. Phys. 2007, 127, 34303. (31) Wörner, H. J.; Merkt, F. Angew. Chem., Int. Ed. 2009, 48, 6404. (32) Zilberg, S.; Haas, Y. J. Phys. Chem. A 1998, 102, 10851. (33) Kastrup, C. J.; Oldfield, S. P.; Rzepa, H. S. Chem. Commun. 2002, 6, 642. (34) Kataoka, M. J. Chem. Res. 2004, 573. (35) Karadakov, P. B. J. Phys. Chem. A 2008, 112, 7303. (36) Karadakov, P. B. J. Phys. Chem. A 2008, 112, 12707. (37) Feixas, F.; Matito, E.; Solà, M.; Poater, J. J. Phys. Chem. A 2008, 112, 13231. (38) Feixas, F.; Matito, E.; Solà, M.; Poater, J. Phys. Chem. Chem. Phys. 2010, 12, 7126. (39) Feixas, F.; Vandenbussche, J.; Bultinck, P.; Matito, E.; Solà, M. Phys. Chem. Chem. Phys. 2011, 13, 20690. (40) Soncini, A.; Fowler, P. W. Chem. Phys. Lett. 2008, 450, 431. (41) Ulusoy, I. S.; Nest, M. J. Am. Chem. Soc. 2011, 133, 20230. (42) (a) Hirsch, A.; Chen, Z.; Jiao, H. Angew. Chem., Int. Ed. 2000, 39, 3915. (b) Chen, Z.; Jiao, H.; Hirsch, A.; Thiel, W. J. Mol. Model. 2001, 7, 161. (43) Poater, J.; Solà, M. Chem. Commun. 2011, 47, 11647. (44) Li, X.; Kuznetsov, A. E.; Zhang, H. F.; Boldyrev, A.; Wang, L. S. Science 2001, 291, 859. (45) Feixas, F.; Matito, E.; Duran, M.; Poater, J.; Solà, M. Theor. Chem. Acc. 2011, 128, 419. (46) Wan, P.; Krogh, E. J. Chem. Soc., Chem. Commun. 1985, 1207. (47) Jursic, B. S. J. Mol. Struct. (THEOCHEM) 1999, 490, 133. (48) Brink, M.; Möllerstedt, H.; Ottosson, C.-H. J. Phys. Chem. A 2001, 105, 4071. (49) Garavelli, M.; Bernardi, F.; Cembran, A.; Castaño, O.; Frutos, L. M.; Merchán, M.; Olivucci, M. J. Am. Chem. Soc. 2002, 124, 13770. (50) Paquette, L. A.; Ley, S. V.; Meisinger, R. H.; Russell, R. K.; Oku, M. J. Am. Chem. Soc. 1974, 96, 5806. (51) Maier, H. Angew. Chem., Int. Ed. Engl. 1988, 27, 309. (52) Craig, D. P. J. Chem. Soc. 1951, 3175. (53) Snyder, L. C. J. Phys. Chem. 1962, 66, 2299. (54) Voter, A. F.; Goddard, W. A. J. Am. Chem. Soc. 1986, 108, 2830. (55) Flett, M. S. C.; Cave, W. T.; Vago, E. E.; Thompson, H. W. Nature 1947, 159, 739. (56) Kaufman, H. S.; Fankuchen, I.; Mark, H. Nature 1948, 161, 165. (57) Eckert-Maksić, M.; Vazdar, M.; Barbatti, M.; Lischka, H.; Maksić, Z. B. J. Chem. Phys. 2006, 125, 064310. (58) Borden, W. T.; Davidson, E. R. J. Am. Chem. Soc. 1977, 99, 4587. (59) Borden, W. T. J. Am. Chem. Soc. 1975, 97, 5968. (60) Baird, N. C.; West, R. M. J. Am. Chem. Soc. 1971, 93, 4427. (61) Ajami, D.; Oeckler, O.; Simon, A.; Herges, R. Nature 2003, 426, 819. (62) Herges, R. Chem. Rev. 2006, 106, 4820. (63) Yoon, Z. S.; Osuka, A.; Kim, D. Nat. Chem. 2009, 1, 113. (64) van der Hart, W. J.; Mulder, J. J. C.; Oosterhoff, L. J. J. Am. Chem. Soc. 1972, 94, 5724. (65) Kuwajima, S. J. Am. Chem. Soc. 1984, 106, 6496. (66) Zilberg, S.; Haas, Y. J. Phys. Chem. A 1998, 102, 10843. (67) Zilberg, S.; Haas, Y. Int. J. Quantum Chem. 1999, 71, 133. (68) Hiberty, P. C. J. Mol. Struct. (THEOCHEM) 1998, 451, 237. (69) Shaik, S.; Shurki, A.; Danovich, D.; Hiberty, P. C. Chem. Rev. 2001, 101, 1501. (70) Shaik, S.; Zilberg, S.; Haas, Y. Acc. Chem. Res. 1996, 29, 211. (71) Dufey, F. Int. J. Quantum Chem. 2006, 107, 764. (72) Haas, Y.; Zilberg, S. J. Am. Chem. Soc. 1995, 117, 5387. (73) Shaik, S.; Shurki, A.; Danovich, D.; Hiberty, P. C. J. Am. Chem. Soc. 1996, 118, 666. (74) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. v R. Chem. Rev. 2005, 105, 3842.

(75) Feixas, F.; Matito, E.; Poater, J.; Solà, M. J. Comput. Chem. 2008, 29, 1543. (76) Dewar, M. J. S. The Molecular Orbital Theory of Organic Chemistry; McGraw-Hill: New York, 1969; Chapters 5 and 6. (77) Cyranski, M. K. Chem. Rev. 2005, 105, 3773. (78) Zhu, J.; An, K.; Schleyer, P. v. R. Org. Lett. 2013, 15, 2442. (79) Schleyer, P. v. R.; Pühlhofer. Org. Lett. 2002, 4, 2873. (80) Ilić, P.; Sinković, B.; Trinajstić, N. Isr. J. Chem. 1980, 20, 258. (81) Krygowski, T. M.; Cyranski, M. K. Tetrahedron 1999, 55, 11143. (82) Krygowski, T. M. J. Chem. Inf. Comput. Sci. 1993, 33, 70. (83) Janoschek, R.; Fratev, F.; Monev, V. Tetrahedron 1982, 38, 2929. (84) Callomon, J. H.; Dunn, T. M.; Mills, I. M. Phil. Trans. R. Soc. London A 1966, A259, 499. (85) Langseth, A.; Stoicheff, B. P. Can. J. Phys. 1956, 34, 350. (86) Rosenberg, M.; Ottosson, H.; Kilså, K. J. Org. Chem. 2010, 75, 2189. (87) Schleyer, P. v R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes, N. J. R. v. E. J. Am. Chem. Soc. 1996, 118, 6317. (88) Fallah-Bagher-Shaidaei, H.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. v. R. Org. Lett. 2006, 8, 863. (89) Stanger, A. J. Org. Chem. 2006, 71, 883. (90) Jiminéz-Halla, J. O. C.; Matito, E.; Robles, J.; Solà, M. J. Organomet. Chem. 2006, 691, 4359. (91) Soncini, A.; Teale, A. M.; Helgaker, T.; De Proft, F.; Tozer, D. J. J. Chem. Phys. 2008, 129, 074101. (92) Mandado, M.; Garña, A. M.; Pérez-Juste, I. J. Chem. Phys. 2008, 129, 164114. (93) Cabana, A.; Bachand, J.; Giguère, J. Can. J. Phys. 1974, 52, 1949. (94) Fowler, P. W.; Steiner, E.; Jenneskens, L. W. Chem. Phys. Lett. 2003, 371, 719. (95) Bean, D. E.; Fowler, P. W.; Soncini, A. Chem. Phys. Lett. 2009, 483, 193. (96) Martín-Santamaría, S.; Rzepa, H. Chem. Commun. 2000, 1503. (97) Taubert, S.; Sundholm, D.; Jusélius, J. J. Chem. Phys. 2011, 134, 054123. (98) Soncini, A.; Fowler, P. W. Chem. − Eur. J. 2013, 19, 1740. (99) Jiao, H.; Schleyer, P. v R.; Mo, Y.; McAllister, M. A.; Tidwell, T. T. J. Am. Chem. Soc. 1997, 119, 7075. (100) Villaume, S.; Fogarty, H. A.; Ottosson, H. ChemPhysChem 2008, 9, 257. (101) Becke, A. D.; Edgecombe, K. E. J. Chem. Phys. 1990, 92, 5397. (102) Savin, A.; Nesper, R.; Wengert, S.; Fässler, T. F. Angew. Chem., Int. Ed. 1997, 36, 1808. (103) Zhu, J.; Dahlstrand, C.; Smith, J. R.; Villaume, S.; Ottosson, H. Symmetry 2010, 2, 1653. (104) Steinmann, S. N.; Mo, Y.; Corminboeuf, C. Phys. Chem. Chem. Phys. 2011, 13, 20584. (105) Feixas, F.; Vandenbuschke, J.; Bultinck, P.; Matito, E.; Solà, M. Phys. Chem. Chem. Phys. 2011, 13, 20690. (106) Dietz, F.; Tyutyulkov, N.; Rabinovitz, M. J. Chem. Soc. Perkin Trans. 2 1993, 157. (107) Dietz, F.; Rabinovitz, M.; Tadjer, A.; Tyutyulkov, N. J. Chem. Soc. Perkin Trans. 2 1995, 735. (108) Wirz, J.; Krebs, A.; Schmalstieg, H.; Angliker, H. Angew. Chem., Int. Ed. 1981, 20, 192. (109) Masamune, S.; Suda, M.; Ona, H.; Leichter, L. M. J. Chem. Soc., Chem. Commun. 1972, 1268. (110) Bally, T.; Masamune, S. Tetrahedron 1980, 36, 343. (111) Lassettre, E. N.; Skerbele, A.; Dillon, M. A.; Ross, K. J. J. Chem. Phys. 1968, 48, 5066. (112) Doering, J. P. J. Chem. Phys. 1969, 51, 2866. (113) Frueholz, R. P.; Kuppermann, A. J. Chem. Phys. 1978, 69, 3614. (114) Leupin, W.; Wirz, J. J. Am. Chem. Soc. 1980, 102, 6068. (115) Wirz, J. Electronic Structure and Photophysical Properties of Planar Conjugated Hydrocarbons with a 4n-Membered Ring, Part II; Jerusalem Symposia on Quantum Chemistry and Biochemistry 1977; Vol. X, p 283. (116) Brown, R. D.; Domaille, P. J.; Kent, J. E. Aust. J. Chem. 1970, 23, 1707. 5421


Chemical Reviews

Review

(117) Lassettre, E. N.; Skerbele, A.; Dillon, M. A.; Ross, K. J. J. Chem. Phys. 1968, 48, 5066. (118) Möllerstedt, H.; Piqueras, M. C.; Crespo, R.; Ottosson, H. J. Am. Chem. Soc. 2004, 126, 13938. (119) Rosenberg, M.; Ottosson, H.; Kilså, K. Phys. Chem. Chem. Phys. 2011, 13, 12912. (120) Ottosson, H.; Kilså, K.; Chajara, K.; Piqueras, M. C.; Crespo, R.; Kato, H.; Muthas, D. Chem.Eur. J. 2007, 13, 6998. (121) Wasserman, E.; Hutton, R. S.; Kuck, V. J.; Chandross, E. A. J. Am. Chem. Soc. 1974, 96, 1965. (122) Dang, J.-S.; Zheng, J.-J.; Wang, W.-W.; Zhao, X. Inorg. Chem. 2013, 52, 4762. (123) Glukhovtsev, M. N.; Reindl, B.; Schleyer, P. v. R. Mendeleev Commun. 1993, 3, 100. (124) Glukhovtsev, M. N.; Bach, R. D.; Laiter, S. J. Phys. Chem. 1996, 100, 10952. (125) Masamune, S.; Souto-Bachiller, F. A.; Machiguchi, T.; Bertie, J. E. J. Am. Chem. Soc. 1978, 100, 4889. (126) (a) Orendt, A. M.; Arnold, B. R.; Radziszewski, J. G.; Facelli, J. C.; Malsch, K. D.; Strub, H.; Grant, D. M.; Michl, J. J. Am. Chem. Soc. 1988, 110, 2648. (b) Arnold, B. R.; Radziszewski, J. G.; Campion, A.; Perry, S. S.; Michl, J. J. Am. Chem. Soc. 1991, 113, 692. (c) Arnold, B. R.; Michl, J. J. Phys. Chem. 1993, 97, 13348. (127) Chapman, O. L.; De La Cruz, D.; Roth, R.; Pacansky, J. J. Am. Chem. Soc. 1973, 95, 1337. (128) Balková, A.; Bartlett, R. J. J. Chem. Phys. 1994, 101, 8972. (129) Koseki, S.; Toyota, A. J. Phys. Chem. A 1997, 101, 5712. (130) Lombardi, J. R.; Wallenstein, R.; Hänsch, T. W.; Friedich, D. M. J. Chem. Phys. 1976, 65, 2357. (131) Palmer, I. J.; Ragazos, I. N.; Bernardi, F.; Olivucci, M.; Robb, M. A. J. Am. Chem. Soc. 1993, 115, 673. (132) Osamura, Y. Chem. Phys. Lett. 1988, 145, 541. (133) Dreyer, J.; Klessinger, M. Chem.Eur. J. 1996, 2, 335. (134) Buma, W. J.; van der Waals, J. H.; van Hemert, M. C. J. Chem. Phys. 1990, 93, 3733. (135) Malar, E. J. P.; Jug, K. J. Phys. Chem. 1984, 88, 3508. (136) Bastiansen, O.; Hedberg, L.; Hedberg, K. J. Chem. Phys. 1957, 27, 1311. (137) Dewar, M. J. S.; Merz, K. M., Jr. J. Phys. Chem. 1985, 89, 4739. (138) Jug, K.; Malar, E. J. P. J. Mol. Struct. (THEOCHEM) 1987, 153, 221. (139) Hrovat, D. A.; Borden, W. T. J. Am. Chem. Soc. 1992, 114, 5879. (140) Zilberg, S.; Haas, Y. J. Phys. Chem. A 1998, 102, 10851. (141) Klärner, F.-G. Angew. Chem., Int. Ed. 2001, 40, 3977. (142) Nishinaga, T.; Ohmae, T.; Iyoda, M. Symmetry 2010, 2, 76. (143) Wenthold, P. G.; Hrovat, D. A.; Borden, W. T.; Lineberger, W. C. Science 1996, 272, 1456. (144) Forward, P. J.; Gorman, A. A.; Hamblett, I. J. Chem. Soc., Chem. Commun. 1993, 250. (145) Das, T. N.; Priyadarsini, I. J. Chem. Soc. Faraday Trans. 1994, 90, 963. (146) Frutos, L.-M.; Castao, O.; Merchn, M. J. Phys. Chem. A 2003, 107, 5472. (147) Anet, F. A. L.; Bock, L. A. J. Am. Chem. Soc. 1968, 90, 7130. (148) Bearpark, M. J.; Bernardi, F.; Olivucci, M.; Robb, M. A. Int. J. Quantum Chem. 1996, 60, 505. (149) Klann, R.; Bäuerle, R. J.; Laermer, F.; Elsaesser, T.; Niemeyer, M.; Lüttke. Chem. Phys. Lett. 1990, 169, 172. (150) Falchi, A.; Gellini, C.; Salvi, P. R.; Hafner, K. J. Phys. Chem. A 1998, 105, 5006. (151) Bearpark, M. J.; Robb, M. A. J. Phys. Chem. A 2000, 104, 1075. (152) Bussotti, L.; Foggi, P.; Gellini, C.; Moroni, L.; Salvi, P. R. Phys. Chem. Chem. Phys. 2001, 3, 3027. (153) Fujimura, Y.; Yamaguchi, H.; Nakayama, T. Bull. Chem. Soc. Jpn. 1972, 45, 384. (154) Falchi, A.; Gellini, C.; Salvi, P. R.; Hafner, K. J. Phys. Chem. 1995, 99, 14659. (155) Hargreaves, A.; Rizvi, S. H. Acta Crystallogr. 1962, 15, 365.

(156) Imamura, A.; Hoffmann, R. J. Am. Chem. Soc. 1968, 90, 5379. (157) Fukuda, R.; Ehara, M. Phys. Chem. Chem. Phys. 2013, 15, 17426. (158) Berlman, I. B. J. Chem. Phys. 1970, 52, 5616. (159) Zimmermann, R. J. Mol. Struct. 1996, 377, 35. (160) Rayez, J. C.; Dannenberg, J. J.; Kassab, E.; Evleth, E. M. J. Mol. Struct. 1980, 68, 235. (161) Beck, M. E.; Rebentisch, R.; Hohlneicher, G.; Fülscher, M. P.; Serrano-Andrés, L.; Roos, B. O. J. Chem. Phys. 1997, 107, 9464. (162) Beck, M. E.; Rebentisch, R.; Hohlneicher, G.; Fülscher, M. P.; Serrano-Andrés, L.; Roos, B. O. J. Chem. Phys. 1997, 107, 9464. (163) Yokozeki, A.; Wilcox, C. F.; Bauer, S. H. J. Am. Chem. Soc. 1974, 96, 1026. (164) Zimmermann, R. J. Mol. Struct. 1996, 377, 35. (165) Rayez, J. C.; Dannenberg, J. J.; Kassab, E.; Evleth, E. M. J. Mol. Struct. 1980, 68, 235. (166) Shukla, D.; Wan, P. J. Am. Chem. Soc. 1993, 115, 2990. (167) Drake, J. A. G.; Jones, D. W. Acta Crystallogr. 1982, 838, 200. (168) Yuan, C.; Saito, S.; Camacho, C.; Irle, S.; Hisaki, I.; Yamaguchi, S. J. Am. Chem. Soc. 2013, 135, 8842. (169) Malar, E. J. P. Tetrahedron 1996, 52, 4709. (170) Jug, K. J. Org. Chem. 1983, 48, 1344. (171) Jug, K.; Hahn, G. J. Comput. Chem. 1983, 4, 410. (172) Malar, E. J. P.; Jug, K. Tetrahedron 1986, 42, 417. (173) Breslow, R.; Chang, H. W.; Yager, W. A. J. Am. Chem. Soc. 1963, 85, 2033. (174) Anderson, A. G.; Steckler, B. M. J. Am. Chem. Soc. 1959, 81, 4941. (175) Tobler, H. J.; Bauder, A.; Günthard, H. H. J. Mol. Spectrosc. 1965, 18, 239. (176) Kitahara, Y.; Murata, I.; Ueno, M.; Sato, K.; Watanabe, H. Chem. Commun. 1966, 180. (177) Baron, P. A.; Brown, R. D.; Burden, F. R.; Domaille, P. J.; Kent, J. E. J. Mol. Spectrosc. 1972, 43, 401. (178) Norden, T. D.; Staley, S. W.; Taylor, W. H.; Harmony, M. D. J. Am. Chem. Soc. 1986, 108, 7912. (179) Bachrach, S. M.; Liu, M. J. Phys. Org. Chem. 1991, 4, 242. (180) Grimme, S. Chem. Phys. Lett. 1993, 201, 67. (181) Burk, P.; Abboud, J.-L. M.; Koppel, I. A. J. Phys. Chem. 1996, 100, 6992. (182) Scott, A. P.; Agranat, I.; Biedermann, P. U.; Riggs, N. V.; Radom, L. J. Org. Chem. 1997, 62, 2026. (183) Dahlstrand, C.; Yamazaki, K.; Kilså, K.; Ottosson, H. J. Org. Chem. 2010, 75, 8060. (184) Hafner, K.; Häfner, K. H.; König, C.; Kreuder, M.; Ploss, G.; Schultz, G.; Strum, E.; Vöpel, K. H. Angew. Chem., Int. Ed. Engl. 1963, 2, 123. (185) Brown, R. D.; Burden, F. R.; Kent, J. E. J. Chem. Phys. 1968, 49, 5542. (186) Hafner, K. Angew. Chem., Int. Ed. 1964, 3, 165. (187) Malar, E. J. P.; Neumann, F.; Jug, K. J. Mol. Struct. (THEOCHEM) 1995, 336, 81. (188) Tyutyulkov, N.; Fratev, F.; Ivanova, M. Theor. Chim. Acta 1971, 20, 385. (189) Ghigo, G.; Shahi, A. R. M.; Gagliardi, L.; Solstad, L. M.; Cramer, C. J. J. Org. Chem. 2007, 72, 2823. (190) Dahlstrand, C.; Rosenberg, M.; Kilså, K.; Ottosson, H. J. Phys. Chem. A 2012, 116, 5008. (191) Baumann, W. Chem. Phys. 1977, 20, 17. (192) Dietz, F.; Vogel, H.; Schleitzer, A.; Tyutyulkov, N.; Rabinovitz, M. Z. Naturforsch. 1997, 52b, 1072. (193) Yamaguchi, H.; Ikeda, T.; Mametsuka, H. Bull. Chem. Soc. Jpn. 1976, 49, 1762. (194) Lok, C. M.; Lughtenburg, J.; Havinga, E. Tetrahedron Lett. 1970, 11, 4701. (195) Vink, J. A. J.; Lok, C. M.; Cornelisse, J.; Havinga, E. J. Chem. Soc., Chem. Commun. 1972, 710, 710. (196) Lok, C. M.; Boer, M. E. d.; Cornelisse, J.; Havinga, E. Tetrahedron 1973, 29, 867. 5422


Chemical Reviews

Review

(197) Cornelisse, J.; Havinga, E. Chem. Rev. 1975, 75, 353. (198) Fabian, J.; Junek, H. Dyes Pigm. 1985, 6, 251. (199) Thaller, F. J.; Trucker, D. E.; Becker, E. I. J. Am. Chem. Soc. 1951, 73, 228. (200) Coan, S. B.; Trucker, D. E.; Becker, E. I. J. Am. Chem. Soc. 1953, 75, 900. (201) Coan, S. B.; Trucker, D. E.; Becker, E. I. J. Am. Chem. Soc. 1955, 77, 984. (202) Potter, R. G.; Hughes, T. S. J. Org. Chem. 2007, 73, 2995. (203) Serrano-Andrés, L.; Pou-AméRigo, R.; Fülscher, M. P.; Borin, M. A. J. Chem. Phys. 2002, 117, 1649. (204) Wan, P.; Krogh, E.; Chak, B. J. Am. Chem. Soc. 1988, 110, 4073. (205) Wan, P.; Budac, D.; Krogh, E. J. Chem. Soc., Chem. Commun. 1990, 255. (206) Wan, P.; Budac, D.; Earle, M.; Shukla, D. J. Am. Chem. Soc. 1990, 112, 8048. (207) Budac, D.; Wan, P. J. Org. Chem. 1992, 57, 887. (208) Wan, P.; Shukla, D. Chem. Rev. 1993, 93, 571. (209) Budac, D.; Wan, P. Can. J. Chem. 1996, 74, 1447. (210) Budac, D.; Wan, P. J. Photochem. Photobiol. A: Chem. 1996, 98, 27. (211) Brousmiche, D.; Shukla, D.; Wan, P. Chem. Commun. 1997, 709. (212) Shukla, D.; Wan, P. J. Photochem. Photobiol. A: Chem. 1998, 113, 53. (213) Shukla, D.; Lukeman, M.; Shi, Y.; Wan, P. J. Photochem. Photobiol. A: Chem. 2002, 154, 93. (214) Donckt, E. V.; Nasielski, J.; Thiry, P. Chem. Commun. 1969, 1249. (215) Tolbert, L. M. Acc. Chem. Res. 1986, 19, 268. (216) Yoshinaga, T.; Hiratsuka, H.; Tanizaki, Y. Bull. Chem. Soc. Jpn. 1978, 51, 996. (217) Brauman, J. I.; Schwartz, J.; Tamelen, E. E. v. J. Am. Chem. Soc. 1968, 90, 5328. (218) Schwartz, J. Chem. Commun. 1969, 833. (219) Fox, M. A. Chem. Rev. 1979, 79, 253. (220) Tamelen, E. E. v.; Brauman, J. I.; Ellis, L. E. J. Am. Chem. Soc. 1967, 89, 5073. (221) Steuhl, H.-M.; Klessinger, M. Angew. Chem., Int. Ed. 1994, 33, 2431. (222) Grellmann, K. H.; Heilbronner, E.; Seiler, P.; Weller, A. J. Am. Chem. Soc. 1968, 90, 4238. (223) Mason, S. F.; Smith, B. E. J. Chem. Soc. A 1969, 325. (224) Förster, T. Z. Elektrochem. 1950, 54, 531. (225) Agmon, N. J. Phys. Chem. A 2005, 109, 13. (226) Jackson, G.; Porter, G. Proc. Royal Soc. Lond. A 1961, 260, 13. (227) Agmon, N.; Rettig, W.; Groth, C. J. Am. Chem. Soc. 2002, 124, 1089. (228) Ireland, J. F.; Wyatt, P. A. H. Adv. Phys. Org. Chem. 1976, 12, 131. (229) Tolbert, L. M.; Haubrich, J. E. J. Am. Chem. Soc. 1990, 112, 8163. (230) Huppert, D.; Tolbert, L. M.; Linares-Samaniego, S. J. Phys. Chem. A 1997, 101, 4602. (231) Pretali, L.; Doria, F.; Verga, D.; Profumo, A.; Freccero, M. J. Am. Chem. Soc. 2009, 74, 1034. (232) Rosenberg, J. L.; Brinn, I. M. J. Chem. Soc. Faraday Trans. 1 1976, 72, 448. (233) Rosenberg, J. L.; Brinn, I. M. J. Phys. Chem. 1972, 76, 3558. (234) Wehry, E. L.; Rogers, L. B. J. Am. Chem. Soc. 1965, 87, 4234. (235) Freiser, B. S.; Beauchamp, J. L. J. Am. Chem. Soc. 1977, 99, 3214. (236) Malar, E. J. P.; Jug, K. J. Phys. Chem. 1985, 89, 5235. (237) Formosinho, S. J.; Arnaut, L. G. J. Photochem. Photobiol. A: Chem. 1993, 75, 21. (238) Cuma, M.; Scheiner, S.; Kar, T. J. Mol. Struct. (THEOCHEM) 1999, 467, 37. (239) Premvardhan, L. L.; Peteanu, L. A. J. Photochem. Photobiol. A: Chem. 2002, 154, 69.

(240) Forés, M.; Duran, M.; Solà, M.; Adamowicz, L. J. Phys. Chem. A 1999, 103, 4413. (241) See, for example, (a) Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic Chemistry; University Science Books: Sausalito, CA, 2004; p 889. (b) Carroll, F. A. Perspectives on Structure and Mechanism in Organic Chemistry, 2nd ed.; John Wiley & Sons: New York, 2010; p 763. (242) D’Auria, M. J. Org. Chem. 2000, 65, 2494. (243) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: Chichester, U.K., 1970. (244) Bryce-Smith, D.; Gilbert, A. In Rearrangements in Ground and Excited States; De Mayo, P., Ed.; Academic Press: New York, 1980; Vol. 3, p 349. (245) Gilbert, A.; Baggott, J. Essentials of Molecular Photochemistry; Blackwell Scientific Publications: Oxford, U.K., 1991. (246) Cornelisse, J. Chem. Rev. 1993, 93, 615. (247) Klessinger, M.; Michl, J. Excited States and Photochemistry of Organic Molecules; VCH: Weinheim, Germany, 1995. (248) Klan, P.; Wirz, J. Photochemistry of Organic Compounds; Wiley: Chichester, U.K., 2009. (249) Steit, U.; Bochet, C. G. Beilstein J. Org. Chem. 2011, 7, 525. (250) Michl, J.; Bonačić-Koutecký, V. Electronic Aspects of Organic Photochemistry; Wiley-Interscience: New York, 1990. (251) Waldeck, D. H. Chem. Rev. 1991, 91, 415. (252) Arai, T.; Tokumaru, K. Chem. Rev. 1993, 93, 23. (253) Kato, H.; Brink, M.; Möllerstedt, H.; Piqueras, M. C.; Crespo, R.; Ottosson, H. J. Org. Chem. 2005, 70, 9495. (254) Villaume, S.; Ottosson, H. J. Phys. Chem. A 2009, 113, 12304. (255) Kikuchi, O.; Segawa, K.; Takahashi, O.; Arai, T.; Tokumaru, K. Bull. Chem. Soc. Jpn. 1992, 65, 1463. (256) Segawa, K.; Takahashi, O.; Kikuchi, O.; Arai, T.; Tokumaru, K. Bull. Chem. Soc. Jpn. 1993, 66, 2754. (257) Caldwell, R. A.; Zhou, L. J. Am. Chem. Soc. 1994, 116, 2271. (258) Anger, I.; Sundahl, M.; Wennerström, O.; Auchter-Krummel, P.; Müllen, K. J. Phys. Chem. 1995, 99, 650. (259) Zhu, J.; Fogarty, H. A.; Brink, M.; Möllerstedt, H.; Ottosson, H. Chem.Eur. J. 2013, 19, 10698. (260) Wan, P.; Krogh, E. J. Am. Chem. Soc. 1989, 111, 4887. (261) Mecklenburg, S. L.; Hilinski, E. F. J. Am. Chem. Soc. 1989, 111, 5471. (262) Gaillard, E.; Fox, M. A.; Wan, P. J. Am. Chem. Soc. 1989, 111, 2180. (263) Krogh, E.; Wan, P. Can. J. Chem. 1990, 68, 1725. (264) McClelland, R. A.; Mathivanan, N.; Steenken, S. J. Am. Chem. Soc. 1990, 112, 4857. (265) McAuley, I.; Krogh, E.; Wan, P. J. Am. Chem. Soc. 1988, 110, 600. (266) Krogh, E.; Wan, P. J. Am. Chem. Soc. 1992, 114, 705. (267) Nakatani, K.; Higashida, N.; Saito, I. Tetrahedron Lett. 1997, 38, 5005. (268) Modica, E.; Zanaletti, R.; Freccero, M.; Mella, M. J. Org. Chem. 2001, 66, 41. (269) Chiang, Y.; Kresge, A. J.; Zhu, Y. J. Am. Chem. Soc. 2002, 124, 6349. (270) Brousmiche, D. W.; Xu, M.; Lukeman, M.; Wan, P. J. Am. Chem. Soc. 2003, 125, 12961. (271) Wang, P.; Liu, R.; Wu, X.; Ma, H.; Cao, X.; Zhou, P.; Zhang, J.; Weng, X.; Zhang, X.-L.; Qi, J.; Zhou, X.; Weng, L. J. Am. Chem. Soc. 2003, 125, 1116. (272) Chang, J. A.; Kresge, A. J.; Zhan, H.-Q.; Zhu, Y. J. Phys. Org. Chem. 2004, 17, 579. (273) Flegel, M.; Lukeman, M.; Huck, L.; Wan, P. J. Am. Chem. Soc. 2004, 126, 7890. (274) Richter, S. N.; Maggi, S.; Colloredo-Mels, S.; Palumbo, M.; Freccero, M. J. Am. Chem. Soc. 2004, 126, 13973. (275) Colloredo-Mels, S.; Doria, F.; Verga, D.; Freccero, M. J. Org. Chem. 2006, 71, 3889. (276) Verga, D.; Richter, S. N.; Palumbo, M.; Gandolfia, R.; Freccero, M. Org. Biomol. Chem. 2007, 5, 233. 5423


Chemical Reviews

Review

(277) Kostikov, A. P.; Malashikhina, N.; Popik, V. V. J. Org. Chem. 2008, 74, 1802. (278) Arumugam, S.; Popik, V. V. J. Am. Chem. Soc. 2009, 131, 11892. (279) Di Antonio, M.; Doria, F.; Richter, S. N.; Bertipaglia, C.; Mella, M.; Sissi, C.; Palumbo, M.; Freccero, M. J. Am. Chem. Soc. 2009, 131, 13132. (280) Verga, D.; Nadai, M.; Doria, F.; Percivalle, C.; Di Antonio, M.; Palumbo, M.; Richter, S. N.; Freccero, M. J. Am. Chem. Soc. 2010, 132, 14625. (281) Basarić, N.; Ž abčić, I.; Mlinarić-Majerski, K.; Wan, P. J. Org. Chem. 2010, 75, 102. (282) Seiler, P.; Wirz, J. Tetrahedron Lett. 1971, 20, 1683. (283) Seiler, P.; Wirz, J. Helv. Chim. Acta 1972, 55, 2693. (284) Dolbier, W. R., Jr.; Matsui, K.; Dewey, H. J.; Horák, D. V.; Michl, J. J. Am. Chem. Soc. 1979, 101, 2136. (285) de Fonseka, K. K.; McCullough, J. J.; Yarwood, A. J. J. Am. Chem. Soc. 1979, 101, 3277. (286) Kang, K. T.; Yoon, U. C.; Seo, H. C.; Kim, K. N.; Song, H. Y.; Lee, J. C. Bull. Korean Chem. Soc. 1991, 12, 57. (287) For a recent review on SiC double bonded compounds see, for example, Ottosson, H.; Eklöf, A. M. Coord. Chem. Rev. 2008, 252, 1287. (288) Cookson, R. C.; De B. Costa, M. S. M.; Hudec, J. Chem. Commun. 1969, 1272. (289) Wang, H.; Burda, C.; Persy, G.; Wirz, J. J. Am. Chem. Soc. 2000, 122, 5849. (290) Murai, H.; Torres, M.; Strausz, O. P. J. Am. Chem. Soc. 1980, 102, 1421. (291) Shizuka, H.; Hiratsuka, H.; Jinguji, M.; Hiraoka, H. J. Phys. Chem. 1987, 91, 1793. (292) Tomioka, H.; Ichikawa, N.; Komatsu, K. J. Am. Chem. Soc. 1992, 114, 8045. (293) Pratt, A. C. J. Chem. Soc., Chem. Commun. 1974, 183. (294) Kessar, S. V.; Singh, T.; Mankotia, A. K. S. J. Chem. Soc., Chem. Commun. 1989, 1692. (295) Yang, N. C.; Rivas, C. J. Am. Chem. Soc. 1961, 83, 2213. (296) Sammes, P. G. Tetrahedron 1976, 32, 405. (297) Hart, H. Chem. Rev. 1979, 79, 515. (298) Scaiano, J. C. Acc. Chem. Res. 1982, 15, 252. (299) Gabicki, J.; Krantz, A. J. Chem. Soc. Perkin Trans. 2 1984, 1623. (300) Garcia-Garibay, M. A.; Gamarnik, A.; Bise, R.; Pang, L.; Jenks, W. S. J. Am. Chem. Soc. 1995, 117, 10264. (301) Leonenko, Z. V.; Gritsan, N. P. J. Struct. Chem. 1997, 38, 536. (302) Lukeman, M.; Wan, P. J. Am. Chem. Soc. 2002, 124, 9458. (303) Basarić, N.; Došlić, D.; Ivković, J.; Wang, Y.-H.; Malis, M.; Wan, P. Chem.Eur. J. 2012, 18, 10617. (304) Flegel, M.; Lukeman, M.; Wan, P. Can. J. Chem. 2008, 86, 161. (305) For a recent review see Kwon, J. E.; Park, S. Y. Adv. Mater. 2011, 23, 3615. (306) Lochbrunner, S.; Stock, K.; Riedle, E. J. Mol. Stuct. 2004, 700, 13. (307) Griffin, G. W.; Marcantonio, A. F.; Kristinsson, H. H.; Petterson, R. C.; Irving, C. S. Tetrahedron Lett. 1965, 6, 2951. (308) de Fonseka, K. K.; Manning, C.; McCullough, J. J.; Yarwood, A. J. J. Am. Chem. Soc. 1977, 99, 8257. (309) Wirz, J.; Persy, G.; Rommel, E.; Murata, I.; Nakasuji, K. Helv. Chim. Acta 1984, 67, 305. (310) Blattmann, H.-R.; Meuche, D.; E. Heilbronner, E.; Molyneux, R. J.; Boekelheide, V. J. Am. Chem. Soc. 1965, 87, 130. (311) Blattmann, H.-R.; Schmidt, W. Tetrahedron 1970, 26, 5885. (312) Mitchell, R. H. Eur. J. Org. Chem. 1999, 2695. (313) Ohmae, T.; Nishinaga, T.; Wu, M.; Iyoda, M. J. Am. Chem. Soc. 2009, 132, 1066. (314) Criegee, R. Angew. Chem., Int. Ed. Engl. 1962, 1, 519. (315) For reveiws on cyclobutadiene see, for example, (a) Maier, G. Angew. Chem., Int. Ed. Engl. 1974, 13, 425. (b) Bally, T.; Masamune, S. Tetrahedron 1980, 36, 343. (c) Maier, G. Angew. Chem., Int. Ed. Engl. 1988, 27, 309.

(316) Cram, D. J.; Tanner, M. E.; Thomas, R. Angew. Chem., Int. Ed. Engl. 1991, 30, 1024. (317) (a) Legrand, Y.-M.; van der Lee, A.; Barboiu, M. Science 2010, 329, 299. (b) Scheschkewitz, D. Science 2010, 330, 1047. (c) Alabugin, I. V.; Gold, B.; Shatruk, M.; Kovnir, K. Science 2010, 330, 1047. (d) Legrand, Y.-M.; van der Lee, A.; Barboiu, M. Science 2010, 330, 1047. (318) Chapman, O. L.; McIntosh, C. L.; Pacansky, J. J. Am. Chem. Soc. 1973, 95, 614. (319) Masamune, S.; Suda, M.; Ona, H.; Leichter, L. M. J. Chem. Soc., Chem. Commun. 1972, 1268. (320) Lin, C. Y.; Krantz, A. J. Chem. Soc., Chem. Commun. 1972, 1111. (321) Krantz, A.; Lin, C. Y.; Newton, M. D. J. Am. Chem. Soc. 1973, 95, 2744. (322) Pong, R. G. S.; Huang, B. S.; Laureni, J.; Krantz, A. J. Am. Chem. Soc. 1977, 99, 4153. (323) Huang, B. S.; Pong, R. G. S.; Laureni, J.; Krantz, A. J. Am. Chem. Soc. 1977, 99, 4154. (324) Masamune, S.; Souto-Bachiller, F. A.; Machiguchi, T.; Bertie, J. E. J. Am. Chem. Soc. 1978, 100, 4889. (325) Masamune, S.; Sugihara, Y.; Morio, K.; Bertie, J. E. Can. J. Chem. 1976, 54, 2679. (326) Maier, G.; Hartan, H.-G.; Sayrac, T. Angew. Chem., Int. Ed. 1976, 15, 226. (327) Maier, G.; Hoppe, B. Tetrahedron Lett. 1973, 861. (328) Maier, G.; Reisenauer, H. P. Tetrahedron Lett. 1976, 3591. (329) Maier, G.; Hoppe, M.; Lanz, K.; Reisenauer, H. P. Tetrahedron Lett. 1984, 25, 5645. (330) Lage, H. W.; Reisenauer, H. P.; Maier, G. Tetrahedron Lett. 1982, 23, 3893. (331) Maier, G.; Hoppe, M.; Reisenauer, H. P. Angew. Chem., Int. Ed. 1983, 22, 990. (332) Tyerman, W. J. R.; Kato, M.; Kebarle, P.; Masamune, S.; Strausz, O. P.; Gunning, H. E. Chem. Commun. 1967, 497. (333) Maier, G.; Lautz, C. Eur. J. Org. Chem. 1998, 769. (334) Kaisaki, D. A.; Dougherty, D. A. Tetrahedron Lett. 1987, 28, 5263. (335) Masamune, S.; Suda, M.; Ona, H.; Leichter, L. M. J. Chem. Soc., Chem. Commun. 1972, 1268. (336) Maier, G. Angew. Chem., Int. Ed. 1988, 27, 309. (337) Maier, G.; Alzérreca, A. Angew. Chem., Int. Ed. 1973, 12, 1015. (338) Pruitt, P. L.; Bielh, E. R.; Reeves, P. C. J. Organomet. Chem. 1977, 134, 37. (339) Gist, A. V.; Reeves, P. C. J. Organomet. Chem. 1981, 215, 221. (340) Kimling, H.; Krebs, A. Angew. Chem., Int. Ed. Engl. 1972, 11, 932. (341) Delbaere, L. T. J.; James, M. N. G.; Nakamura, N.; Masamune, S. J. Am. Chem. Soc. 1975, 97, 1973. (342) (a) Maier, G.; Pfriem, S.; Schäfer, U.; Mutsch, R. Angew. Chem., Int. Ed. 1978, 17, 520. (b) Maier, G.; Pfriem, S.; Schäfer, U.; Malsch, K.-D.; Matusch, R. Chem. Ber. 1981, 114, 3965. (343) Sekiguchi, A.; Tanaka, M.; Matsuo, T.; Watanabe, H. Angew. Chem., Int. Ed. 2001, 40, 1675. (344) Maier, G.; Fleischer, F. Tetrahedron Lett. 1991, 32, 57. (345) Maier, G.; Born, G. Angew. Chem., Int. Ed. Engl. 1989, 28, 1050. (346) (a) Maier, G.; Neudert, J.; Wolf, O. Angew. Chem., Int. Ed. 2001, 40, 1674. (b) Maier, G.; Neudert, J.; Wolf, O.; Pappusch, D.; Sekiguchi, A.; Tanaka, M.; Matsuo, T. J. Am. Chem. Soc. 2002, 124, 13819. (347) Inagaki, Y.; Nakamoto, M.; Sekiguchi, A. J. Am. Chem. Soc. 2011, 133, 16436. (348) Suzuki, K.; Matsuo, T.; Hashizume, D.; Fueno, H.; Tanaka, K.; Tamao, K. Science 2011, 331, 1306. (349) Maier, G.; Schäfer, U. Tetrahedron Lett. 1977, 18, 1053. (350) Maier, G.; Schäfer, U. Liebigs. Ann. Chem. 1980, 798. (351) Neumann, F.; Jug, K. J. Org. Chem. 1994, 59, 6437. (352) Neumann, F.; Jug, K. J. Org. Chem. 1994, 59, 6442. 5424


Chemical Reviews

Review

(398) Šket, B.; Zupan, M. Tetrahedron 1989, 45, 1755. (399) Al-Jalal, N.; Gilbert, A.; Heath, P. Tetrahedron 1988, 44, 1449. (400) Trost, B. M.; Scudder, P. H.; Cory, R. M.; Turro, N. J.; Ramamurthy, V.; Katz, T. J. J. Org. Chem. 1979, 44, 1264. (401) Kurita, J.; Iwata, K.; Tsuchiya, T. J. Chem. Soc., Chem. Commun. 1986, 1188. (402) Streith, J.; Cassal, J. M. Tetrahedron Lett. 1968, 43, 4541. (403) Sasaki, T.; Kanematsu, K.; Kakahi, A. Chem. Commun. 1969, 432. (404) Balasubramanian, A.; McIntosh, J. M.; Snieckus, V. J. Org. Chem. 1970, 35, 433. (405) Streith, J. Pure Appl. Chem. 1977, 49, 305. (406) Snieckus, V.; Streith, J. Acc. Chem. Res. 1981, 14, 348. (407) Ames, A. E.; Halpern, A. M.; Ruggles, C. J. J. Phys. Chem. 1990, 94, 4464. (408) Wang, C.; Xi, Z. Chem. Commun. 2007, 5119. (409) Hafner, K.; Dönges, R.; Goedecke, E.; Kaiser, R. Angew. Chem., Int. Ed. 1973, 12, 337. (410) Bally, T.; Chai, S.; Neuenschwander, M.; Zhu, Z. J. Am. Chem. Soc. 1997, 119, 1869. (411) Neumann, F.; Jug, K. J. Phys. Chem. 1995, 99, 5834. (412) Oth, J. F. M.; Röttle, H.; Schröder, G. Tetrahedron Lett. 1970, 61. (413) Röttle, H.; Martin, W.; Oth, J. F. M.; Schröder, G. Chem. Ber. 1969, 102, 3985. (414) Castro, C.; Karney, W. L.; Valencia, M. A.; Vu, C. M. H.; Pemberton, R. P. J. Am. Chem. Soc. 2005, 127, 9704. (415) Schröder, G.; Oth, J. F. M. Tetrahedron Lett. 1966, 4083. (416) Kolpak, A. M.; Grossman, J. C. Nano Lett. 2011, 11, 3156. (417) Ottosson, H. Nat. Chem. 2012, 4, 969. (418) Firouzi, R. Chem. Phys. Lett. 2014, 595−596, 48. (419) An, K.; Zhu, J. Eur. J. Org. Chem. 2014, DOI: 10.1002/ ejoc.201301810.

(353) Ohmae, T.; Nishinaga, T.; Wu, M.; Iyoda, M. J. Am. Chem. Soc. 2009, 132, 1066. (354) Wang, C.; Xi, Z. Chem. Commun. 2007, 5119. (355) Huisgen, R.; Mietzsch, F. Angew. Chem., Int. Ed. Engl. 1964, 3, 83. (356) Scott, L. T.; Jones, M., Jr. Chem. Rev. 1972, 72, 181. (357) Paquette, L. A. Tetrahedron 1975, 31, 2855. (358) Bryce-Smith, D.; Gilbert, A. Tetrahedron 1977, 33, 2459. (359) Smith, L. S. J. Chem. Educ. 1978, 55, 569. (360) Chen, J.; Scheffer, J. R.; Trotter, J. Tetrahedron 1992, 48, 3251. (361) Paquette, L. A. Acc. Chem. Res. 1993, 26, 57. (362) Ramaiah, D.; Sajomin, M. C.; Joseph, J.; George, M. V. Chem. Soc. Rev. 2005, 34, 48. (363) Zimmerman, H. E.; Givens, R. S.; Pagni, R. M. J. Am. Chem. Soc. 1968, 90, 6096. (364) Zimmerman, H. E.; Givens, R. S.; Pagni, R. M. J. Am. Chem. Soc. 1968, 90, 4191. (365) Al-Jalal, N.; Gilbert, A.; Heath, P. Tetrahedron 1988, 44, 1449. (366) Streith, J. Pure Appl. Chem. 1977, 49, 305. (367) Meinwald, J.; Tsuruta, H. J. Am. Chem. Soc. 1970, 92, 2579. (368) Paquette, L. A.; Meisinger, R. H.; Wingard, R. E., Jr. J. Am. Chem. Soc. 1973, 95, 2230. (369) Brewer, J.; Heany, H. Chem. Commun. 1967, 811. (370) Ciganek, E. J. Am. Chem. Soc. 1966, 88, 2882. (371) Kurabayashi, K.; Mukai, T. Tetrahedron Lett. 1972, 11, 1049. (372) Antkowiak, T. A.; Sanders, D. C.; Trimitsis, G. B.; Press, J. B.; Shechter, H. J. Am. Chem. Soc. 1972, 94, 5366. (373) Smith, L. R.; Gream, G. E.; Meinwald, J. J. Org. Chem. 1977, 42, 927. (374) Stapersma, J.; Klumpp, G. W. J. R. Netherlands Chem. Soc. 1982, 101, 274. (375) Bryce-Smith, D.; Gilbert, A.; Grzonka, J. Chem. Commun. 1970, 498. (376) Zimmerman, H. E.; Grunewald, G. L. J. Am. Chem. Soc. 1966, 88, 183. (377) Rabideau, P. W.; Hamilton, J. B.; Friedman, L. J. Am. Chem. Soc. 1968, 90, 4465. (378) Lemal, D. M. Acc. Chem. Res. 2001, 34, 662. (379) Ralli, P.; Zhang, Y.; Lemal, D. M. Tetrahedron Lett. 2008, 49, 7349. (380) Zimmerman, H. E.; Bender, C. O. J. Am. Chem. Soc. 1970, 92, 4366. (381) Ihmels, H.; Schneider, M.; Waidelich, M. Org. Lett. 2002, 4, 3247. (382) Ma, C.; Dougherty, D. A. Chem. Rev. 1998, 98, 1303. (383) Pokkuluri, P. R.; Scheffer, J. R.; Trotter, J. J. Am. Chem. Soc. 1990, 112, 3676. (384) Asplund, C. L.; Bender, C. O.; Dolman, D. Can. J. Chem. 1994, 72, 1999. (385) Cristol, S. J.; Kaufmann, R. L.; Opitz, S. M.; Szalecki, W.; Bindel, T. H. J. Am. Chem. Soc. 1983, 105, 3226. (386) Garbe, J. E.; Boekelheide, V. J. Am. Chem. Soc. 1983, 105, 7384. (387) Grovenstein, E., Jr.; Rao, D. V. Tetrahedron Lett. 1961, 2, 148. (388) Bryce-Smith, D.; Lodge, J. E. Proc. Chem. Soc. London 1961, 333. (389) Bryce-Smith, D.; Lodge, J. E. J. Chem. Soc. 1963, 695. (390) Warrener, R. N.; Nunn, E. E.; Paddon-Row, M. N. Tetrahedron Lett. 1976, 27, 2355. (391) Warrener, R. N.; McCay, I. W.; Tan, R. Y. S.; Russell, R. A. Tetrahedron Lett. 1979, 34, 3183. (392) Liu, R. S. H.; Krespan, C. G. J. Am. Chem. Soc. 1969, 34, 1271. (393) Pirrung, M. C. J. Org. Chem. 1987, 52, 1635. (394) Rigby, J. H.; Warshakoon, N. C. Tetrahedron Lett. 1997, 38, 2049. (395) Kubitschke, J.; Hopf, H.; Jones, P. G.; Dix, I.; Ernst, L. Eur. J. Org. Chem. 2008, 548. (396) Bryce-Smith, D.; Gilbert, A. Tetrahedron 1976, 32, 1309. (397) Zimmerman, H. E.; Iwamura, H. J. Am. Chem. Soc. 1968, 90, 4763.

NOTE ADDED IN PROOF During the final editing of this review two additional articles have been published that report on computational studies on aromaticity in the lowest triplet state. Firouzi analyzed the anisotropy of the π-electron density and found that it can be used to distinguish between aromatic and antiaromatic annulenes also in the lowest triplet state, thus supporting Baird’s rule.418 An and Zhu have applied the isomerization stabilization energy (ISE) index in a refined form to assess also the aromaticity of larger annulenes (until [20]annulene) in their triplet states.419 Baird’s rule is still valid for the larger rings, yet, steric factors also have impacts on the ISE values.

5425


Excited State Aromaticity and Antiaromaticity - ACS Publications

Recommend Documents