Control and Switching of Aromaticity in Various All-Aza-Expanded

Review pubs.acs.org/CR

Control and Switching of Aromaticity in Various All-Aza-Expanded Porphyrins: Spectroscopic and Theoretical Analyses Young Mo Sung,† Juwon Oh,† Won-Young Cha,† Woojae Kim,† Jong Min Lim,†,‡ Min-Chul Yoon,†,§ and Dongho Kim*,† †

Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University, Seoul 120-749, South Korea ‡ Physical and Theoretical Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom § Manufacturing Engineering Team, Memory Manufacturing Operation Center, Samsung Electronics, Samsungjeonja-ro 1, Hwasung-si, Gyeonggi-do 18448, South Korea ABSTRACT: Modification of aromaticity is regarded as one of the most interesting and important research topics in the field of physical organic chemistry. Particularly, porphyrins and their analogues (porphyrinoids) are attractive molecules for exploring various types of aromaticity because most porphyrinoids exhibit circular conjugation pathways in their macrocyclic rings with various molecular structures. Aromaticity in porphyrinoids is significantly affected by structural modification, redox chemistry, NH tautomerization, and electronic states (singlet and triplet excited states). Conversely, aromaticity significantly affects the spectroscopic properties and chemical reactivities of porphyrinoids. In this context, considerable efforts have been devoted to understanding and controlling the aromaticity and antiaromaticity of porphyrinoids. Thus, a series of porphyrinoids are in the limelight, being expected to shed light on this field because they have some advantages to demonstrate the switching of aromaticity; it is possible to control the aromaticity by lowering the temperature, adding and removing the protons of expanded porphyrins, changing the chemical environment, and switching the electronic states (triplet and singlet excited states) by photoexcitation. In this regard, this Review describes the control of aromaticity in various expanded porphyrins from the spectroscopic point of view with assistance from theoretical calculations.

CONTENTS 1. Introduction 2. Control of Aromaticity by Regulating the Temperature 2.1. Change of Aromaticity in meso-Hexakis(pentafluorophenyl) Hexaphyrin at Low Temperatures 2.2. Change of Aromaticity in meso-Hexakis(trifluoromethyl) Hexaphyrins at Low Temperatures 3. Control of Aromaticity by Protonation and Deprotonation 3.1. Control of Aromaticity by Protonation 3.1.1. Effects of Protonation in [4n+2]π Expanded Porphyrins 3.1.2. Effects of Protonation in [4n]π Expanded Porphyrins 3.2. Control of Aromaticity by Deprotonation 3.2.1. Effects of Deprotonation in [4n+2]π Expanded Porphyrins 3.2.2. Effects of Deprotonation in [4n]π Expanded Porphyrins

3.2.3. Deprotonation Effects in Larger Expanded Porphyrins 4. Control of Aromaticity by Various Solvents 4.1. Control of Aromaticity in Heptaphyrin by Solvent 5. Reversal of Aromaticity in the Excited State 5.1. Reversal of Aromaticity in the Lowest Triplet State 5.2. Reversal of Aromaticity in the Singlet Excited State 5.3. Reversal of Aromaticity for Mö bius Compounds in the Triplet Excited State 6. Summary and Outlook Author Information Corresponding Author ORCID Notes Biographies Acknowledgments

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2283 Special Issue: Expanded, Contracted, and Isomeric Porphyrins

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Received: May 16, 2016 Published: December 16, 2016 © 2016 American Chemical Society

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DOI: 10.1021/acs.chemrev.6b00313 Chem. Rev. 2017, 117, 2257−2312

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theory proposed by Hückel, various concepts of aromaticity have been proposed, including half-twisted Möbius, doubly twisted (figure-eight structures), and three-dimensional (spherical) conjugation compounds.41,42 In 1964, Heilbronner reported that stable molecular systems with [4n]π electrons, exhibiting closed-shell configuration, can be achieved by distorting the molecular structures to a Möbius strip topology.45 In his report, compounds with Möbius structures exhibit aromatic and antiaromatic character in [4n] and [4n+2]π electronic systems, respectively, which is in direct contrast to the prediction made by the Hückel [4n+2] rule. Typically, nonplanar steric multiconjugated annulenes can be divided into Hückel or Möbius molecules on the basis of their structural topologies by theoretical methods employing a coiled ribbon model proposed by Rzepa.42 In their expanded definition, twisted molecules with an even integer for the linking number are aromatic when they have [4n+2]π electrons that satisfy the Hückel [4n+2] rule. By contrast, entangled molecules with an odd integer for the linking number are aromatic when they have [4n]π electrons, referred to as Möbius aromaticity. In company with the concepts of aromaticity based on molecular topology, in 1972, Baird proposed the modulation of electronic features of the lowest triplet state.46 More specifically, Baird predicted that the aromaticity of planar conjugated ring systems in the ground state is reversed in their lowest triplet state, which was developed by utilizing the perturbation molecular orbital theory. Similar to the concept of aromaticity in Möbius topology, the electron-counting rule for defining aromaticity in the lowest triplet state is contrary to the Hückel [4n+2] rule; annulenes having [4n]π electrons would be aromatic, while annulenes with [4n+2]π electrons would be antiaromatic. Baird demonstrated the reversal of aromaticity in the lowest triplet state, while a number of theoretical studies have been conducted to extend Baird’s rule to the first singlet excited state.47,48 In 2008, Karadakov suggested that the aromaticity of annulene in the lowest singlet excited state is also possibly reversed as compared with that in its ground state.47,48 For confirming these various concepts of aromaticity, a significant number of methods for evaluating the degree of aromaticity have been investigated, especially for quantum mechanical calculations. The calculated indices for aromaticity are divided into three groups: energetic aromaticity indices, topological aromaticity indices, and magnetic-based aromaticity indices. Energetic aromaticity indices are represented as aromatic stabilization energy (ASE), indicative of the energy difference between aromatic (antiaromatic) and nonaromatic isomers.49,50 Topological aromaticity indices, such as bond length alternation (BLA) and harmonic oscillator model of aromaticity (HOMA), are considerably utilized and also have been regarded as approximate indicators of aromaticity.41,51 Magnetic aromaticity indices, represented by nucleus-independent chemical shift (NICS), are considered to be precise and powerful for estimating the degree of aromaticity.52 With these theoretical aromaticity indices, experimental aromaticity indices have also been in focus. As antiaromatic annulenes are energetically unstable, it has been difficult to synthesize stable antiaromatic annulenes, thereby impeding studies on the experimental aromaticity indices. Nevertheless, since stable aromatic and antiaromatic expanded porphyrins having more than four pyrroles with a large cavity size have been successfully synthesized,11,38,53 various physical properties, such as energetic, magnetic, structural, and photophysical properties, have been suggested to be utilized for confirming whether the

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1. INTRODUCTION Expanded porphyrins, which are macrocyclic compounds containing more than four pyrrole or pyrrole-related rings (e.g., furan, thiophene, and/or other heterocyclic subunits), have attracted significant attention over the past few decades.1−8 Since the first report of expanded porphyrin named “sapphyrin” in 1966 by Woodward and co-workers,9,10 numerous studies about expanded porphyrins have been reported.11 In 1975, Day, Marks, and Wachter reported a uranyl−superphthalocyanine complex, representing one of the most important characterization studies in the synthesis of expanded porphyrin.12 After that, in 1975, Berger and LeGoff reported other significant examples of expanded porphyrins, such as [22]platyrin; in 1985, Rexhausen and Gossauer reported [22]pentaphyrin.13,14 In the 1990s, Sessler et al. reported the synthesis of expanded porphyrins, such as [22]sapphyrin, [24]amethyrin, [24]rosarin, [26]rubyrin, [28]heptaphyrin, and [40]turcasarin.15−17 In addition, they reviewed various synthetic procedures and demonstrated the potential applications of expanded porphyrins for applications, such as photodynamic therapy (PDT) and anion recognition, as well as their use in functional near-infrared (NIR) dyes for nonlinear optical (NLO) applications.15,16,18−29 Furthermore, Chandrashekar and co-workers have synthesized several coremodified expanded porphyrins by substituting pyrroles with furans, thiophenes, and selenophenes and investigated their structural diversity and degree of aromaticity.30−34 In particular, in 2007, the Latos-Grażyński group successfully synthesized di-pbenzi[28]hexaphyrin(1.1.1.1.1.1), which exhibits an interesting phenomenon: Hückel−Möbius aromaticity switching depending on temperature.35 In addition, the Latos-Grażyński group has revealed the relationship between molecular conformation and aromaticity, which was mainly characterized by NMR spectroscopy and quantum chemical calculations.36 Since this intriguing finding of Hückel−Möbius aromaticity switching, the groups of Kim and Osuka have extended the field of Möbius expanded porphyrins by synthesizing and characterizing various Möbius (anti)aromatic expanded porphyrins37−39 and, by extension, Hückel (anti)aromatic expanded porphyrins. Although all expanded porphyrins are tremendously interesting, this Review focuses on all-aza-expanded porphyrins. Among various subjects in the study of expanded porphyrins, aromaticity is the topic of interest, which has been explored from both theoretical and experimental perspectives.11,37−39 Aromaticity, which is a measure of the unique stability of conjugated ring systems, has been regarded as one of the most intriguing and significant concepts in chemistry, dating back to the discovery of benzene by Faraday in 1825 and its proposed resonance structure by Kekulé in 1865.40 Numerous studies have been reported on aromaticity as the degree of aromaticity in conjugated ring systems determines their chemical reactivity and properties.41−43 Furthermore, aromaticity can be utilized for various applications, such as the synthesis of novel π-conjugated systems and development of synthetic routes via the prediction of reactant stability. In 1931, Hückel reported the [4n+2] rule for conjugated ring systems.44 In his theory, annulenes having [4n+2]π electrons, such as benzene, exhibit closed electronic configuration, demonstrating aromatic character, while annulenes having [4n]π electrons, such as cyclobutadiene, exhibit openshell configuration with electrons in nonbonding molecular orbitals (MOs), demonstrating antiaromatic character. Since the 2258


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molecule is aromatic, antiaromatic, or nonaromatic.40 Accordingly, expanded porphyrins are now regarded as the optimum test subjects for investigating the correlation between molecular properties and (anti)aromaticity and exploring novel aromatic concepts for the experimental evaluation of molecular (anti)aromaticity. As part of our efforts toward experimental aromaticity indices, in-depth analyses have demonstrated that various photophysical properties, such as excited-state lifetime, absorption spectral features, and fluorescence, can be utilized for confirming whether expanded porphyrins are aromatic, antiaromatic, or nonaromatic.38,39,54 In particular, expanded porphyrins exhibit unique absorption and emission spectral features depending on their aromaticity. For aromatic expanded porphyrins, intense and sharp B-like (also called Soret-like band) and weak but structured Q-like bands are characteristically observed around visible and NIR regions, respectively, in their absorption spectra; these spectral features can be explained on the basis of Gouterman’s four-orbital model.55 Aromatic expanded porphyrins exhibit two pairs of frontier molecular orbitals (FMOs), almost degenerate, where the configuration interaction between these four FMOs results in intense B-like and weak Q-like absorption bands. In addition, in aromatic expanded porphyrins, fluorescence in the NIR region is characterized by vibronic bands.38 On the other hand, antiaromatic expanded porphyrins exhibit different features. As compared to aromatic ones, antiaromatic expanded porphyrins exhibit significantly attenuated absorption bands in the visible region with characteristic absorption tailing over the NIR region.54 Moreover, antiaromatic expanded porphyrins do not exhibit fluorescence. These absorption and emission spectral features are understood by their MO structures and energy diagram. In contrast to the four degenerate FMOs of aromatic expanded porphyrins, antiaromatic ones exhibit nondegenerate highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) with a small energy gap (Figure 1). Because these HOMOs and LUMOs have gerade symmetry, the lowest electronic transition between HOMO and LUMO is optically forbidden by the selection rule, resulting in the tailing absorption in the NIR region.39 In addition, the energy gaps between HOMO−1/HOMO−2 and LUMO+1/LUMO+2 are decreased as compared with those of aromatic expanded porphyrins, in which HOMO → LUMO+1/LUMO+2 and HOMO−1/HOMO−2 → LUMO transitions mainly result in relatively weak absorption bands in the visible region. Interestingly, these different features of expanded porphyrins, which are dependent on their aromaticity, are extended to the results obtained from transient absorption (TA) measurements. TA spectroscopy measures a difference absorption (ΔA) spectrum (absorption spectrum of the species in the excited state minus absorption spectrum of the species in the ground state) via the promotion of molecules to an electronically excited state for typically investigating their excited-state dynamics (Figure 2).56 As molecules (maximum up to 10%) can be excited by a pump pulse, the TA spectra comprise the following three main features: (1) ground-state bleaching (GSB) signals, representing negative signals in ΔA attributed to the decrease of ground-state absorption, caused by vacancies in the groundstate population by the excitation of molecules; (2) stimulated emission (SE) signals, observed when the excited population returns to the ground state after the passage of the probe pulse; and (3) excited-state absorption (ESA) signals, attributed to absorption by excited molecules to high energy levels. For aromatic expanded porphyrins, their intense B-like bands exhibit

Figure 1. Schematic diagrams for molecular orbitals and electronic structures of aromatic and antiaromatic expanded porphyrins.

intense GSB signals with relatively weak ESA signals on both sides of the GSB signal in the visible region; they also exhibit long excited-state lifetimes, which is in good agreement with their fluorescent property.38 Notably, the excited-state lifetime is significantly shorter than those of H2TPP (10 ns) and ZnTPP (2 ns), attributed to the flexible structures of expanded porphyrins in addition to the reduced HOMO−LUMO energy gap. Expanded porphyrins with rigid structures formed by internal bridges and fused constituents exhibit a large increase in the S1 state lifetime.54,57,58 On the other hand, significantly different TA spectral features and excited-state dynamics are observed in antiaromatic expanded porphyrins. Their TA spectra exhibit lowintensity GSB signals and strong ESA signals in the visible region, which is in sharp contrast to the high-intensity GSB and weaker ESA signals observed in the TA spectra of aromatic expanded porphyrins.39 Furthermore, although antiaromatic expanded porphyrins are rapidly relaxed from the excited state to the ground state, they exhibit distinctive excited-state dynamics, in which two-stepwise relaxation processes (one in the order of hundreds of femtoseconds and the other in the order of tens of picoseconds) are observed.38 Excited-state dynamics is distinguished from typical structural relaxation in the lowest excited state because structurally locked expanded porphyrins also exhibit similar relaxation dynamics without significant changes.54,57,58 This unique excited-state dynamics is in accordance with the presence of the optically dark state, as indicated by absorption tailing in the NIR region; the optically dark state facilitates and accelerates two-stepwise internal conversion (relaxation) processes, which also indicate that the excited-state dynamics of antiaromatic expanded porphyrins are less sensitive to the conformational effect. In addition, aromaticity-dependent behaviors could be observed in a two-photon absorption (TPA) phenomenon, 2259


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Figure 2. Schematic illustration of TA spectroscopy (right) and spectral components to TA spectrum (left).

Chart 1. Schematic Molecular Structures of PF26H and PF28H (Reprinted with Permission from Ref 63; Copyright 2009 American Chemical Society)

involved change of molecular properties for experimentally determining aromaticity. In this Review, the control and switching of aromaticity in expanded porphyrins via their photophysical characterization were discussed and summarized. The discussion was divided into four sections by the methods for controlling aromaticity. In the first section, the change of aromaticity by decreasing the temperature was illustrated. Next, the issue of aromaticity on protonation and deprotonation was addressed. In the following section, the alternation of aromaticity by regulating solvent polarity was discussed. Finally, the reversal of aromaticity in the excited state of expanded porphyrins was described by analyzing the absorption spectra between the ground and lowest singlet/ triplet excited states.

which depends on second-order hyperpolarizability. The TPA cross section value can be regarded as another important experimental factor to determine the extent of aromaticity for expanded porphyrinoids. Notably, previous studies on a series of octupolar systems possessing electron donor−acceptor moieties conclude that the TPA cross section value is linearly proportional to the first hyperpolarizability (β).59,60 Actually, these researchers did not directly state the relationship between hyperpolarizabilities or between hyperpolarizability and BLA. However, from their results and logics, the imaginary part of γ, which is the second-order hyperpolarizability, is concluded to determine the TPA cross section. Furthermore, in an earlier study, Brédas et al. have reported that BLA is an essential parameter to determine the NLO properties shown in conjugated organic systems.61 Moreover, experimental results reveal that there is a close relationship between the aromaticity and TPA cross section values.62−70 The aromaticity and TPA cross section value are closely related with regard to both ring current and molecular hyperpolarizability. Thus, irrespective of the baseline criterion chosen for defining the aromaticity, the TPA cross section values can be used as experimental criteria for molecular aromaticity. Consequently, these photophysical features of expanded porphyrins in the absorption, emission, TA spectra, and TPA cross section values, which are dependent on their aromaticity, originate from the characteristic electronic structures of (anti)aromatic nature. In this context, these photophysical features can serve as reliable indices for determining the aromaticity, antiaromaticity, and nonaromaticity of expanded porphyrins. The molecular aromaticity of expanded porphyrins can be easily controlled by changing the surrounding environment, such as temperature,62−64 viscosity and polarity of solvents,64,65 and protonation and deprotonation with acids and bases.66−70 Furthermore, recently, the aromaticity of expanded porphyrins in the ground state has been reported to be reversed in the excited singlet and triplet states.57,71−73 That is, the aromaticity of expanded porphyrins can be easily modulated by changing the conditions of molecules, indicating that expanded porphyrins are an ideal platform for investigating aromaticity switching and

2. CONTROL OF AROMATICITY BY REGULATING THE TEMPERATURE The aromaticity or antiaromaticity of expanded porphyrins can be switched by changing the surrounding environment; temperature variation is one of the methods best known to change the aromaticity of expanded porphyrins. Expanded porphyrins exhibit a change in aromaticity, in addition to conformational changes, at low temperature. Experimental results and theoretical calculations completely support the change of aromaticity at low temperature. In this section, the change of aromaticity by decreasing the temperature was discussed. 2.1. Change of Aromaticity in meso-Hexakis(pentafluorophenyl) Hexaphyrin at Low Temperatures

In 2008, Kim, Osuka, and co-workers reported temperaturedependent photophysics of meso-hexakis(pentafluorophenyl) [26]- (PF26H) and [28]hexaphyrins(1.1.1.1.1.1) (PF28H) (Chart 1).63 In tetrahydrofuran (THF) at room temperature, the absorption spectrum of PF26H show an intense B-like band at 571 nm and Q-like bands at 712, 768, 892, and 1018 nm with low oscillator strength. On the other hand, that of PF28H shows 2260


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an asymmetric B-like band centered at 596 nm with relatively broad Q-like bands at 764, 847, 892, and 997 nm (Figure 3). The absorption spectrum of PF28H at 173 K exhibits a bathochromic-shifted B-like band at 605 nm accompanied by the

Figure 4. Synchronous (top) and asynchronous (bottom) 2D absorption correlation spectrum generated from temperature-dependent spectral variation of PF28H. Solid and dashed lines represent positive and negative cross peaks, respectively. Reprinted with permission from ref 63. Copyright 2009 American Chemical Society.

Figure 3. Temperature-dependent absorption spectra of PF26H (top) and PF28H (bottom) from 293 to 173 K in THF. Reprinted with permission from ref 63. Copyright 2009 American Chemical Society.

growth of a shoulder at 650 nm. PF26H also shows a red-shift in the B-like band, but the extent of spectral shift is significantly small as compared to PF28H. The authors have performed a consistent 2D correlation spectroscopy to find the origin of distinct spectral evolution in the absorption spectra of PF28H. In this technique, two independent variables, providing information on structural changes according to the temperature change, were used to plot spectral intensity.74,75 At low temperature, the band at 605 nm in the synchronous spectrum is divided into two distinct peaks at 593 and 608 nm in the asynchronous spectrum (Figure 4), which supports that at least two structurally different species of PF28H coexist in the ground state. Especially, the change of relative band intensity between two separated peaks depending on temperature represents ground-state equilibrium shift between the two conformers: one corresponding to the peak at 593 nm is dominant at high temperature, while the other corresponding to the peak at 608 nm gradually becomes dominant upon lowering the temperature. The change of absorption spectra and the corresponding population change of individual PF28H conformers with changing temperature were evaluated by a self-modeling curve resolution (SMCR) method (Figure 5) in more detail.63 By using this method, two ground-state absorption spectra, which are responsible for the two different conformers, were extracted; one conformer exhibits a B-like band at 604 nm with Q-like bands at 764 and 890 nm, while the other one exhibits a blue-shifted B-like

Figure 5. ALS (alternating least-squares)-based SMCR results for temperature-dependent absorption spectra of PF28H in THF. Red and black solid lines indicate the ground-state absorption spectra of the individual chemical components, and inset shows relative population ratio changes from 293 to 173 K. Reprinted with permission from ref 63. Copyright 2009 American Chemical Society.

band at 589 nm and broad and featureless Q-like bands. As described in the Introduction, expanded porphyrins that have aromatic character generally exhibit distinct Q-like transitions in 2261


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the absorption spectra. In contrast, featureless and smeared Qlike bands are observed in the case of antiaromatic expanded porphyrins.76,77 In this regard, the two differentiated absorption spectra revealed by the SMCR analysis finally tell us that Möbius aromatic and Hückel antiaromatic PF28H conformers, which have the [4n]π conjugation pathway in common but form different structures (the former is half-twisted structure while the latter is planar structure), coexist in equilibrium. Further, by the temperature-dependent SMCR analysis, population shift between the two conformers could be quantitatively analyzed. With decreasing temperature, the relative portion of Möbius aromatic PF28H increases from 55% to 75%, whereas that of planar Hückel antiaromatic congener decreases from 45% to 25%, implying that twisted Möbius aromatic PF28H is energetically more stable than planar Hückel antiaromatic PF28H in the ground state. NMR measurements with varying temperature can give detailed information on the activation energy barrier associated with the structural interconversion between planar Hückel and twisted Möbius PF28H in the ground state. In particular, the interconversion rate constants (kint) between the two different conformers can be obtained by analyzing the NMR spectral changes. At low temperature, structural changes take place more slowly than in the NMR timescale. Meanwhile, the NMR spectrum reveals two different sets of signals originating from two respective nuclei. With increasing temperature, these processes occur on the NMR timescale, resulting in peak broadening followed by a change into coalescent spectral shapes. At high temperature where these processes occur more quickly than the NMR timescale, only a single narrow line is observed at the central position between the two chemical shifts. In reality, the 13C NMR spectra of PF28H at 173 K exhibit two signal sets, which are observed at 98 and 106 ppm, but these two peaks gradually broaden and coalesce with increasing temperature. Eventually, at 293 K, only a single peak is observed at 102 ppm. On the basis of these results, the Arrhenius plot of ln kint vs 1/T can be drawn, and the slope of the plot represents the activation energy barrier of which the value is evaluated to be ∼8 kcal/mol. The same result was also observed by analyzing the 1H NMR spectra. Next, the authors carried out single-point energy calculation based on the optimized geometry of Hückel antiaromatic and Möbius aromatic PF28H in order to evaluate the energy difference between the two conformers. Consequently, it was revealed that the total energy of twisted Möbius aromatic PF28H is lower than that of planar Hückel antiaromatic PF28H by ∼3.7 kcal/mol. The fluorescence spectra of PF26H and PF28H by varying the temperature from 293 to 173 K are shown in Figure 6. Intense peaks are observed at 1032 nm for PF26H and 1034 nm for PF28H, with weak vibronic side bands observed around 1250 nm for both compounds. The full-widths at half-maximum (FWHMs) of the intense fluorescence bands of PF26H and PF28H are presumed to be ∼330 and ∼700 cm−1, respectively. With decreasing temperature, the fluorescence spectra commonly become sharper, blue-shifted, and enhanced for both compounds. On the other hand, the degree of change in relative fluorescence quantum yields and Stokes shifts is rather different between PF26H and PF28H. On the basis of the lowest Q-like transitions in absorption, the Stokes shifts of PF26H and PF28H are evaluated to be around 133 and 358 cm−1 at room temperature, respectively. The Stokes shifts of PF26H become smaller, decreasing from 133 to 108 cm−1 (∼25 cm−1) with decreasing temperature, while those of PF28H change more

Figure 6. Temperature-dependent absorption and fluorescence spectra of PF26H (top) and PF28H (bottom) excited by the 442 nm line of a He−Cd laser at 293, 263, 233, 203, and 173 K in THF. Reprinted with permission from ref 63. Copyright 2009 American Chemical Society.

distinctly from 358 to 254 cm−1 (∼104 cm−1) (Figure 7). These results imply that the conformational relaxations from the Franck−Condon to relaxed geometry in the S1 state potential energy surface are more prominent for PF28H than those for PF26H. The relative fluorescence quantum yields (Φf) between PF26H and PF28H exhibit interesting features. Assuming that the fluorescence quantum yields of PF26H and PF28H at 173 K are unity, the decreasing ratio of relative Φf for PF28H with temperature is more prominent than that for PF26H [−3.6 × 10−3 K−1 vs −1.75 × 10−3 K−1], which also supports the coexistence of various conformers at high temperature in the case of PF28H. Femtosecond and nanosecond TA experiments gave fruitful information on the excited-state dynamics of PF26H and PF28H depending on their temperature-dependent structural changes. (Table 1, Figures 6 and 7).78 In the femtosecond TA spectra of PF26H and PF28H, strong GSB signals are observed at 565 and 605 nm, corresponding to their B-like bands, respectively, with weak ESA bands on both sides of the GSB bands (Figure 8). The decay profiles of PF26H at all wavelength regions were well-fitted to a single exponential decay function. Further, their temperature dependences were nearly negligible, which again indicates that PF26H exists as a well-defined Hückel aromatic conformer. On the other hand, PF28H shows a drastic change in S1-state lifetime and its probe wavelength as well as temperature dependence. First, with decreasing temperature, the kinetic profiles at 616 nm show a single exponential decay, but the decay component considerably increases from 186 (293 K) to 260 (173 K) ps. Especially, the kinetic profiles at 638 nm show double-exponential behavior with time constants of ∼17 (30%) and ∼210 (70%) ps at 293 K. 2262


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Figure 8. Femtosecond TA spectra of PF26H (top) and PF28H (bottom) in THF at 173 K. The inset shows the temperature-dependent S1-state kinetic profiles from 293 to 173 K in THF. Reprinted with permission from ref 63. Copyright 2009 American Chemical Society.

Figure 7. Temperature-dependent changes in Stokes shifts (top) and relative fluorescence quantum yields (bottom) of PF26H and PF28H. Reprinted with permission from ref 63. Copyright 2009 American Chemical Society.

Table 1. Singlet and Triplet (π, π*) Excited-State Lifetimes of PF26H and PF28H in Temperatures from 293 to 173 K63 PF26H

of ∼17 and ∼200 ps, is observed at 580 nm. Notably, the contribution of the shorter decay component (44%) when using 580 nm as a pump is greater than that when using 620 nm (22%). Similar to that observed in the femtosecond TA spectra, strong GSB and ESA signals are also shown for both PF26H and PF28H in the nanosecond TA experiments.82 At 293 K, the lifetime of the T1 state of PF26H is determined to be ∼4.8 μs, whereas that of PF28H cannot be determined, supposedly due to a short T1-state lifetime within the instrument response time of 100 ns. In contrast, at 173 K, the T1-state lifetime of PF28H greatly increases up to 90 μs, which is longer than that of PF26H, 11.9 μs (Figure 9). This result is analogous to the trend of excited-state dynamics observed by femtosecond TA measurements and also consistent with the temperature-dependent structural interconversion dynamics between Möbius aromatic and Hückel antiaromatic PF28H. On the basis of the time-resolved and steady-state spectroscopic results with varying temperature, the energy relaxation pathways for PF26H and PF28H at 293 and 173 K can be depicted as shown in Figure 10. The excited-state dynamics in both S1 and T1 states of PF26H are insensitive to the temperature change because of its structurally rigid and planar [4n+2]π Hückel aromatic nature. On the other hand, PF28H is more sensitive to the temperature, resulting from the conformational interconversion processes between the two structures, [4n]π planar Hückel antiaromatic and [4n]π Möbius aromatic conformers. Nonplanar porphyrins typically reveal larger Stokes shifts, broader optical spectra, shorter S1-state lifetimes, and more

PF28H

temp (K)

τS (ps)

τT (μs)

τS (ps)

τS (ps)

τT (μs)

293 273 253 233 213 193 173

113 121 126 137 138 139 140

4.8 5.8 6.5 8.7 10.5 10.9 11.9

183 179 203 223 250 250 260

17 (30%), 210 (70%) 17 (17%), 246 (83%) 17 (19%), 269 (81%) 17 (15%), 232 (85%) 17 (10%), 243 (90%) 250 260

5000 ps are extracted in the decay-associated spectra (DAS) of TFM28H obtained by global analysis (Figure 17c). Notably, temporal evolution and spectral shapes of the TA spectra for the faster two

time constants, 1 and 10 ps, are quite similar in comparison to those observed for vinylene-bridged [28]hexaphyrins (0.53 and 8.6 ps in toluene), which is the Hückel antiaromatic compound.54 This result indicates that the TA spectra after photoexcitation at 500 nm are mainly governed by the excited state of the Hückel antiaromatic conformer. In contrast, the longer transient species having lifetime constants of 255 ps and >5 ns are responsible for the S1- and T1-state lifetimes of the Möbius conformer of TFM28H, respectively, because both time constants and spectral shapes are analogous to those of the Möbius aromatic hexaphyrins, PF28H.60 When the pump wavelength was changed to 560 nm corresponding to the B-like band for the Möbius conformer of TFM28H, some different excited-state dynamics were observed (Figure 17b, d, f). Not only the decay time constants but also the spectral shapes of DAS are similar, while the magnitude of the GSB bands for the Hückel and Möbius conformers, particularly their DAS of ∼1 and 250 ps, are dissimilar (Figure 17c, d). These differences distinctly indicate that the relative population of the excited Hückel conformer is greater than that of the Möbius conformer, which can be regulated by selective excitation. These changes in the excited population controlled by the excitation wavelength again identify the coexistence of two different conformers in TFM28H depending on their aromaticity. Because the optically dark state of the antiaromatic conformer 2267


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Figure 16. TA spectra (top) of TFM26H in toluene (a, b) and CH2Cl2 (c, d) with the photoexcitation at 580 nm and temporal profiles (bottom) probed at 570 (black circles) and 660 nm (red circles) retrieved from (a) and (c), respectively, with least-squared fitting curves. Reprinted with permission from ref 64. Copyright 2011 American Chemical Society.

Figure 17. TA spectra (left), decay-associated spectra (middle), and temporal profiles (right) of TFM28H in toluene with photoexcitation at 500 (a, c, and e) and 560 nm (b, d, and f). Reprinted with permission from ref 64. Copyright 2011 American Chemical Society.

can serve as a ladder state during the relaxation processes, the time constant of 10 ps for the Hückel antiaromatic conformer in TFM28H stems from the faster internal conversion process to

the ground state, which is governed by the energy-gap law (Figure 18).95 2268


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Figure 18. Schematic diagram for energy relaxation dynamics of (a) TFM26H and (b) TFM28H in toluene. The values in parentheses were determined in CH2Cl2 solution. VR and IC represent the vibrational-relaxation and internal-conversion processes, respectively. Reprinted with permission from ref 64. Copyright 2011 American Chemical Society.

As mentioned above, 1H NMR spectroscopy is one of the optimum methods for investigating molecular aromaticity.53,96 Temperature-dependent 1H NMR spectra can give a deeper insight into the structural information and equilibrium dynamics of the two structures in accordance with their aromaticity (Figure 19).

mixture experiencing conformational interconversion splits into two structurally different conformers. At high temperature, >300 K, a strong paratropic ring current is observed in the simulated 1 H NMR spectrum, verifying that the Hückel conformer is antiaromatic. On the other hand, at a temperature below 150 K, the diatropic ring current becomes prominent, which indicates that the Möbius conformer is aromatic. In the case of TFM26H, temperature dependence is not observed. On the basis of the temperature-dependent 1H NMR spectra of TFM28H, a thermodynamic model can be utilized by using the van’t Hoff equation.27 Suppose the following conformational equilibrium between two different compounds: H↔M

Here, species H and M designate the Hückel and Möbius conformers, respectively, in TFM28H. For a faster conformational interconversion process than the 1H NMR timescale, the observed chemical shift is an average chemical shift of the two isomers during the NMR timescale.97,98 For these reasons, the observed chemical shift δi(T) of proton i at a given temperature T can be described by the following equation: δ i(T ) = x H(T )δ Hi + xM(T )δMi

(1)

Here, xH(T) and xM(T) represent the mole fractions of the Hückel and Möbius conformers at temperature T, and δiH and δiM represent the chemical shifts of the ith protons for the Hückel and Möbius conformers, respectively. Moreover, on the basis of the van’t Hoff equation, the equilibrium constant K(T) for this reversible process can be estimated by

1

Figure 19. Temperature-dependent H NMR spectra of TFM28H in THF-d8. Reprinted with permission from ref 64. Copyright 2011 American Chemical Society.

With decreasing temperature from 298 to 173 K, dramatic changes are observed in the 1H NMR spectra of TFM28H as follows: (i) the most deshielded signal (14.43 ppm) of the NH proton is significantly upfield-shifted (to 7.08 ppm) and also the intersection points between two NH protons emerge between 213 and 233 K; (ii) the complicated signal, which is originated from the six β-CH protons (7.22 ppm), is divided into four deshielded peaks (from 7.98 to 8.33 ppm) and two shielded peaks (4.11 and 4.42 ppm); and (iii) the signal intensities of all peaks increase. These spectral evolutions can be explained with regard to repressed structural dynamics at low temperature; a

K (T ) =

[M]eq [H]eq

=

⎛ ΔS x M(T ) ΔH ⎞⎟ = exp⎜ − ⎝ R x H(T ) RT ⎠

(2)

Using eq 2 and xH(T) + xM(T) = 1 under any condition, eq 1 can be rewritten as δ i(T ) = 2269

K (T ) (δMi − δ Hi ) + δ Hi 1 + K (T )

(3)


Chemical Reviews

Review

Finally, ΔH, ΔS, δiH, and δiM can be determined by unweighted least-squares fitting by minimizing quantity R as follows:99 R=

isomers is rather sensitive to the polarity of the solvent, the temperature-dependent 1H NMR spectra of TFM28H were also performed in toluene-d8. The overall change in the 1H NMR spectra of TFM28H in toluene-d8 resembles that of the 1H NMR spectra in THF-d8. However, slightly smaller ΔS (−35.5 J·K−1· mol−1) and ΔH (−9.9 kJ·mol−1) were observed.61 Interestingly, the two NH proton peaks were not intersected in toluene-d8 from 183 to 298 K. The extrapolated chemical shifts for the Hückel and Möbius conformers could also be estimated based on the temperature-dependent NMR spectra of TFM28H. Because the number of single peaks used in the analyses of all temperature-dependent NMR data is half of the total number of protons in hexaphyrin, the Hückel and Möbius conformers should have C2 symmetry (Table 2). This requirement is fulfilled in the case of Hückel conformer, while the Möbius conformer has a low symmetry affording complicated calculations for the 1H NMR spectra. However, this is different from the structure predicted from temperature-dependent NMR data (Table 3 and Figure 21).61 This discrepancy implies that even at 173 K dynamic interconversion processes occur as rapidly as ever. An analogous dynamic equilibrium between the Hückel antiaromatic and Möbius aromatic conformers was also observed in di-p-benzi-[28]hexaphyrins reported by Latos-Grażyński et al.23 In their case, it is important to note that the Möbius aromatic conformer exhibits C2 symmetry based on both quantum mechanical calculation results and X-ray crystallographic data. Surprisingly, the spectral evolution in the 1H NMR spectra of TFM28H with varying temperature is analogous to those of PF28H in 1,1,2,2-tetrachloroethane-d2 (198−413 K).59 In the case of PF28H, the crossing temperature (Tcross) is estimated to be ∼400 K, affording an estimated critical temperature (Tunity) of ∼380 K, which is significantly greater than that of TFM28H. Notably, the 1H NMR peaks are divided and become more complex with the loss of C2 symmetry at a temperature below 200 K. Nevertheless, at room temperature, it can be said that the Möbius aromatic conformer is dominant at equilibrium and rapid

∑ (δ i(Tj)calc + δ i(Tj)expt )2 (4)

i,j

Here, i and j are the index peaks and temperature values, respectively. The results of nonlinear curve-fitting analysis with eqs 3 and 4 by using the temperature-dependent 1H NMR spectra of TFM28H in THF-d8 are illustrated in Figure 20, and several parameters are displayed in Table 2.

Figure 20. Experimental (closed circles) and fitted (solid lines) 1H NMR chemical shifts of TFM28H in THF-d8 as a function of temperature. Reprinted with permission from ref 64. Copyright 2011 American Chemical Society.

The determined thermodynamic parameters of ΔS (−57.2 J· K−1·mol−1) and ΔH (−12.4 kJ·mol−1) suggest that the proposed mechanism is not only exothermic but also spontaneous. That is, the Möbius conformation is more stable than the Hückel conformation. Furthermore, as the equilibrium between the two

Table 2. Experimental and Simulated 1H NMR Band Positions of TFM28H in THF-d8 and Toluene-d864 chemical shifts solvent THF-d8

toluened8

temp (K)

2(NH)

2(NH)

2(β-CH)

2(β-CH)

2(β-CH)

2(β-CH)

2(β-CH)

2(β-CH)

K (T)

298 273 253 233 213 193 173 Hückel conformer Möbius conformer 298 273 253 233 213 193 183 Hückel conformer Möbius conformer

14.43 (14.54) 13.91 (13.9) 13.17 (13.11) 11.97 (12.02) 10.47 (10.44) 8.66 (8.68) 7.08 (7.11) 15.90

12.42 (12.34) 12.23 (12.22) 12.05 (12.08) 11.82 (11.88) 11.55 (11.6) 11.27 (11.29) 11.08 (11.01) 12.58

7.22 (7.33) 7.42 (7.42) 7.6 (7.53) 7.75 (7.69) 7.93 (7.9) 8.14 (8.15) 8.33 (8.37) 7.14

7.22 (7.17) 7.25 (7.26) 7.36 (7.39) 7.54 (7.56) 7.78 (7.8) 8.07 (8.07) 8.33 (8.31) 6.95

7.22 (7.32) 7.42 (7.38) 7.49 (7.45) 7.6 (7.56) 7.74 (7.7) 7.78 (7.86) 8.02 (8.01) 7.19

7.22 (7.18) 7.25 (7.25) 7.3 (7.33) 7.43 (7.45) 7.61 (7.61) 7.8 (7.8) 7.98 (7.97) 7.04

7.22 (6.96) 6.63 (6.73) 6.34 (6.46) 5.97 (6.05) 5.48 (5.52) 4.9 (4.9) 4.42 (4.34) 7.44

7.22 (7.2) 6.92 (6.94) 6.6 (6.61) 6.14 (6.13) 5.51 (5.5) 4.76 (4.77) 4.11 (4.11) 7.77

0.15 0.24 0.37 0.61 1.12 2.31 5.64

5.55

10.73

8.59

8.55

8.15

8.13

3.79

3.46

11.3 (11.32) 10.27 (10.28) 9.32 (9.31) 8.29 (8.27) 7.27 (7.23) 6.29 (6.31) 5.89 (5.92) 16.25

11.63 (11.62) 11.36 (11.36) 11.12 (11.12) 10.86 (10.86) 10.6 (10.6) 10.37 (10.37) 10.28 (10.27) 12.86

7.29 (7.31) 7.37 (7.36) 7.42 (7.41) 7.47 (7.47) 7.53 (7.52) 7.57 (7.57) 7.58 (7.59) 7.04

7.15 (7.16) 7.26 (7.26) 7.36 (7.35) 7.48 (7.45) 7.56 (7.55) 7.63 (7.64) 7.67 (7.68) 6.69

6.91 (6.92) 7 (6.99) 7.06 (7.06) 7.14 (7.13) 7.21 (7.21) 7.27 (7.27) 7.29 (7.3) 6.57

6.91 (6.94) 6.94 (6.94) 6.97 (6.97) 7 (6.99) 7.03 (7.02) 7.04 (7.05) 7.05 (7.06) 6.79

5.51 (5.48) 5.17 (5.16) 4.85 (4.87) 4.52 (4.55) 4.19 (4.24) 3.88 (3.96) 3.97 (3.84) 6.97

5.24 (5.22) 4.83 (4.83) 4.46 (4.47) 4.07 (4.09) 3.69 (3.7) 3.34 (3.36) 3.25 (3.21) 7.05

4.81

10.00

7.65

7.78

7.38

7.08

3.50

2.80

2270

0.76 1.09 1.54 2.31 3.74 6.67 9.34


Chemical Reviews

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Table 3. Measured and Calculated Chemical Shifts in 1H NMR Spectra of Two Conformers in TFM28H64 δ (Hückel conformer) [ppm]

a

δ (Möbius conformer) [ppm]

signal

THF-d8

toluene-d8

GIAOa

THF-d8

toluene-d8

GIAOa

2(NH) 2(NH) 2(β-CH) 2(β-CH) 2(β-CH) 2(β-CH) 2(β-CH) 2(β-CH)

15.90 12.58 7.14 6.95 7.19 7.04 7.44 7.77

16.25 12.86 7.04 6.69 6.57 6.79 6.97 7.05

19.61, 19.64 15.34, 15.36 7.08, 7.04 5.84, 5.90 6.95, 6.97 6.38, 6.43 8.77, 8.77 8.30, 8.35

5.52 10.73 8.59 8.55 8.15 8.13 3.79 3.46

4.81 10.00 7.65 7.78 7.38 7.08 3.50 2.80

−0.14, 0.17 (inner 2(NH)) 3.13(NH), 13.34 (NH) 7.90−9.50 (outer 8(β-CH))

4.51, 5.14 (2(β-CH)) −3.68, −4.15 (2(β-CH))

GIAO = gauge-including atomic orbital calculation.

omatic and aromatic species can be influenced by the movement of a p-benzene component.53 Even though a detailed mechanism of the interconversion processes could not be decided yet, it is believed that a large conformational change, triggered by bond rotation occurring through two Cα−Cmeso bonds, is necessary for the structural change between the Hückel antiaromatic and Möbius aromatic conformers (Figure 22).

Figure 21. Optimized structures and molecular orbital densities of the lowest π-MOs for Hückel (left side with 161st MO) and Möbius conformers (right side with 159th and 162nd MOs) at B3LYP/631G(d) level. Reprinted with permission from ref 64. Copyright 2011 American Chemical Society.

interconversion results in the C2 symmetry in the 1H NMR spectra. This observation indicates that the intrinsic 1H NMR spectra of the Möbius aromatic conformer can be observed at a temperature significantly lower than 173 K where the solvent is completely frozen. According to all temperature-dependent NMR data of three types of [28]hexaphyrins, their conformational equilibrium enormously relies on external factors, e.g., solvent and temperature. The Tunity value increases in ascending order of TFM28H (216 K in THF-d8), di-p-benzi[28]hexaphyrins (330 K in CDCl3), and PF28H (380 K in 1,1,2,2tetrachloroethane-d2), which identifies that the activation energy barrier for the conformational interconversion processes between the Möbius aromatic and Hückel antiaromatic conformers increases in the order stated above.53 The authors suggested that a nearly barrierless activation energy for the structural changes of TFM28H originates from the fact that the figure-eight structure of TFM28H with stretically less-hindered trifluoromethyl substituents is structurally more flexible as compared to di-p-benzi[28]hexaphyrins and PF28H.53 Similarly, the activation energy barrier declines with increasing solvent polarity in TFM28H on the basis of its solvent-dependent 1H NMR and absorption spectra. For di-p-benzi[28]hexaphyrins, the conformational interconversion process between antiar-

Figure 22. Schematic picture for structural change from Hückel conformer to Möbius conformer of TFM28H. Blue- and red-colored pyrrole moieties represent different sides of the ring. Orange-colored pyrroles are vertically arranged with respect to the paper plane. It is noted that two green arrows meaning main rotational axes for isomerization have the same direction of rotation. Reprinted with permission from ref 64. Copyright 2011 American Chemical Society.

In summary, Hückel aromatic TFM26H exists as a figure-eight conformer in solution, while TFM28H exists as the two structural isomers under dynamic equilibrium, and their equilibrium is rather sensitive to temperature. For TFM28H, one isomer showing a short S1-state lifetime around 10 ps is believed to correspond to the Hückel antiaromatic species having a flexible figure-eight structure, while the other conformer is believed to correspond to the Möbius aromatic hexaphyrin with twisted topology, exhibiting a long S1-state lifetime (250 ps). From these results, it was delineated that the control of conformational interconversion between aromatic and antiaromatic mixtures by regulating temperature can afford significant insight into the relationship between the molecular structure and the chemical properties in aromatic and antiaromatic expanded porphyrinoids. 2271


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two expanded porphyrins possess 30 and 38 π electrons satisfying the Hückel [4n+2] counting rule, they have distorted molecular structures. In this regard, a comparative analysis between PF30h and PF38N in neutral and protonated forms (PF30hp and PF38Np) leads to in-depth comprehension for the photophysical properties and aromaticity triggered by protonation-induced structural changes.67 Although PF30h and PF38N in their neutral forms possess [4n+2]π electrons in the core cyclic π-delocalization for satisfying the Hückel [4n+2] rule, their 1H NMR spectra indicate weak aromatic character,100,101 which indicates that their distorted figure-eight conformations give rise to a weak aromatic nature. For freebase PF38N, its X-ray structure clearly supports the weak aromaticity arising from distorted sturctures. Here, a notable behavior of PF30h is a spectral change in the 1H NMR spectrum upon the addition of trifluoroacetic acid (TFA). As compared to the 1H NMR spectrum of its neutral form, protonated PF30h (PF30hp) displays a strong diatropic ring current, reflecting the strong aromatic character. Because the Xray crystallography of PF30hp indicates a planar structure with three inverted pyrrole rings, the intensified aromatic character seems to arise from a conformational change from the twisted to planar conformations.100 The 1H NMR spectrum of protonated PF38N (PF38Np) shows a large diatropic ring current, which is also attributed to a conformational change upon the addition of methanesulfonic acid (MSA) (Figure 23). On the other hand, the β-CH proton peaks are observed at 4−8 ppm in the 1H NMR spectrum of PF38N. With the addition of MSA, the 1H NMR spectrum of PF38Np displays a series of peaks in the range of −7 to 18 ppm.101 Furthermore, the 1H−1H-COSY spectroscopic results of PF38Np indicate that the counteranion of MSA is restricted in the cavity of the core nonaphyrin macrocycle under the paratropic ring-current effect of PF38Np.67 Although it was not possible to further confirm its structure by X-ray crystallography analysis, these NMR data provide distinct conformational information that PF38Np is more planar and has intensified aromatic character compared to its neutral

3. CONTROL OF AROMATICITY BY PROTONATION AND DEPROTONATION The NH protons in expanded porphyrins significantly affect the determination of their molecular structures. By modifying the number of protons of expanded porphyrins by protonation or deprotonation, molecular structures would change from a twisted form to an unfolded form, which plays a key role in the formation of π-conjugation pathway and change of aromaticity. In this regard, the protonation and deprotonation of expanded porphyrins are considered to be easy and efficient for modifying the aromaticity of expanded porphyrins. 3.1. Control of Aromaticity by Protonation

3.1.1. Effects of Protonation in [4n+2]π Expanded Porphyrins. In this section, the effect of protonation on photophysical properties of meso-aryl-substituted expanded porphyrins are described with two expanded porphyrins, mesohexakis(pentafluorophenyl) [30]heptaphyrin (1.1.1.1.1.1.0) and meso-hexakis(pentafluorophenyl) [38]nonaphyrin (1.1.0.1.1.0.1.1.0) (PF30h and PF38N, respectively; Chart 3),100,101 focusing on revealing the relationship between the Chart 3. Molecular Structures of PF30h and PF38N (Reprinted with Permission from Ref 67; Copyright 2009 American Chemical Society)

photophysical properties and the structural planarity in connection with their aromaticity. These two porphyrins contain the identical meso-peripheral substituents and are composed of seven and nine pyrroles, respectively. Moreover, even though the

Figure 23. 1H NMR spectrum of PF38N with 2 equiv of MSA in THF-d8: downfield region (top) and upfield region (down). Reprinted with permission from ref 67. Copyright 2009 American Chemical Society. 2272


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Figure 24. Spectrophotometric titration of PF30h (a, b) with TFA in CHCl3 and (c, d) with MSA in CH2Cl2. Reprinted with permission from ref 67. Copyright 2009 American Chemical Society.

congener PF38N. On the basis of these conformational changes by the 1H NMR spectroscopic data, the twisted and relatively planar structures for PF30h and protonated PF38Np are suggested. The stepwise spectral changes in their absorption spectra by the addition of acid also delineate the protonation effect in detail. The acid titration absorption spectra of PF30h with TFA exhibit two-step spectral changes with distinct isosbestic points (Figure 24). Upon increasing TFA concentration ranging from 0 to 2.1 × 10−2 M, the B- and Q-like bands are red-shifted with an appearance of additional longer wavelength bands (Figure 24a). With increasing TFA concentration up to 2.3 × 10−1 M, the Blike band is more intensified and blue-shifted (Figure 24b). Even though the Q-like bands display entangled spectral changes, the four absorption bands in the NIR region become well-defined and intense. This change of absorption spectral features during titration with TFA illustrates the conformational changes of PF30h triggered by protonation. Also, the spectrophotometric titration of PF30h with MSA in CH2Cl2 shows similar spectral changes with two-stepwise processes (Figure 24c, d). Thus, the addition of MSA to the CH2Cl2 solution of PF30h results in the same conformational changes as those observed by the addition of TFA, where the only difference is that the required amount of MSA for protonation (6.6 × 10−5 M) is significantly less than that of TFA, representing the higher acidity and stronger binding ability of MSA. The acid titration for PF38N with the addition of MSA also displays the absorption spectral changes. In spectrophotometric titration, intensified B- and Q-like bands with simultaneous small red-shift are observed in the absorption spectrum of PF38N (Figure 25). Notably, compared to PF30h, the spectral change of PF38N occurs at the addition of a large amount of MSA (2.9 × 10−1 M). This demonstrates that the highly twisted conformation of PF38N restricts appending protons to pyrrolic nitrogen atoms, owing to steric hindrance and more effective intramolecular hydrogen bonds, thereby decreasing the proton affinity of imine nitrogen atoms.

Figure 25. Spectrophotometric titration of PF38N with MSA in CH2Cl2. Reprinted with permission from ref 67. Copyright 2009 American Chemical Society.

These neutral and protonated expanded porphyrins display significant different features in their absorption spectra (Figures 24 and 26). Compared to the absorption spectrum of PF38N, that of PF38Np shows no significant absorption bands in the range of 300−500 nm. The similar spectral change is also observed in the absorption spectrum of PF30hp (by the addition of TFA). In addition, by protonation, PF30h and PF38N show narrower and enhanced B-like bands with negligible spectral 2273


Chemical Reviews

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(Figure 27). The reduced Stokes shifts by protonation illustrate that reorganization energy under solvent condition changes by

Figure 26. Steady-state absorption (black) and fluorescence (red) spectra of PF30h, PF38N, and their protonated forms (PF30hp and PF38Np, respectively) upon addition of MSA in CH2Cl2. Reprinted with permission from ref 67. Copyright 2009 American Chemical Society.

Figure 27. Steady-state absorption (black) and fluorescence spectra (red) of neutral PF30h (top) and its protonated form PF30hp (bottom) with TFA in CHCl3. Reprinted with permission from ref 67. Copyright 2009 American Chemical Society.

protonation. According to the reported study for the protonation of porphyrins,86 this change in Stokes shift indicates the protonation-induced conformational changes of PF30h and PF38N, especially from twisted to planar conformation. Because the protonation triggers the change of conformation, electronic structure, and aromaticity, PF30h and PF38N show different excited-state dynamics upon the addition of acid. A set of TA spectra for both neutral and protonated speceis welldescribe such change in the excited-state dynamics (Figure 28). In femtosecond TA measurement, PF30h and PF30hp with MSA show single exponential decay, and the S1-state lifetimes are estimated to be ∼47 and 400 ps (average values from the GSB and ESA signals), respectively (Table 5). Similarly, PF30hp with TFA exhibits a singlet excited-state lifetime but significantly elongated compared to that of its neutral form (∼167 ps). A similar change of singlet excited-state lifetime by protonation is obtained in the TA decay profiles of PF38N (Figure 29), where the singlet excited-state lifetimes are evaluated as ∼18 ps for PF38N and ∼44 ps for PF38Np (average values of GSB and ESA signals). In addition, their triplet excited-state lifetimes by nanosecond TA spectroscopy exhibit similar changes to their S1-state lifetimes. Upon protonation with MSA, the triplet excited-state lifetimes of 3.1 and 0.2 μs for PF30h and PF38N are increased to 6.1 and 6.2 μs, respectively (Tables 4 and 6). PF30hp by TFA addition shows 2.0 μs as a triplet excited-state lifetime, whereas neither GSB nor ESA signal is observed for PF30h in CHCl3, probably due to less-efficient intersystem crossing and rapid triplet state decay of less than a few tens of nanoseconds (Table 6). On the basis of these time-resolved spectroscopic measurements, the S1- and T1-state lifetimes of PF30hp and PF38Np can be argued to be significantly longer than those of their neutral

shifts (Figure 26). By the addition of acid, the molar absorptivities of PF30h and PF38N increase more than three times. Simultaneously, their FWHMs of the B-like bands decrease. Because expanded porphyrins having symmetrical structures generally display narrow absorption bands, these absorption spectral features reflect that protonation induces conformational changes of PF30h and PF38N from twisted to substantially planar structures. Notably, PF30hp and PF38Np exhibit strong Q-like bands, while weak and ill-defined Q-like bands are observed for PF30h and PF38N. As described in the Introduction, in-depth analyses for the relationship between the photophysical properties and aromaticity of expanded porphyrins allow that their optical spectral features provide indices for determining their aromatic nature. On the basis of these absorption spectral features of expanded porphyrins, weak aromatic character of PF30h and PF38N is assigned by their broad, smeared, and weak Q-like bands, which is in accordance with their 1H NMR spectroscopic results, even though they have π electrons meeting the [4n+2] rule for Hückel aromaticity. In other words, reduced aromaticity is attributed to the distorted conformations of PF30h and PF38N. Their fluorescence spectra in the NIR region also display spectral changes upon acid titration (Figure 26). PF30h and PF38N show a blue-shift of fluorescence emission bands, which is well-matched with their blue-shift of lowest-energy Q-like bands. Even though the magnitude of blue-shifts for the lowest-energy Q-like absorption bands of PF30h and PF38N is not the same as that of their fluorescence, they exhibit a decrease in Stokes shifts from ∼280 to ∼80 cm−1 and from ∼280 to ∼120 cm−1, respectively, by the addition of MSA (Table 4). For PF30hp with TFA, a similar decrease of Stokes shift from ∼330 to ∼100 cm−1 is observed

Table 4. Steady-State Absorption and Fluorescence Bands, Stokes Shifts, Singlet and Triplet Excited-State Lifetimes, TPA Cross Sections σ(2), and NICS Values of Neutral and Protonated PF30h and PF38N in CH2Cl267

PF30h PF30hp PF38N PF38Np

λabs (nm)

λflu (nm)

EStokes (cm−1)

τS (ps)

τT (μs)

σ(2)(GM)(λex/nm)

NICS (ppm)

634, 1106 633, 1037 725, 1280 746, 1260

1141 1046 1328 1280

280 80 280 120

47 400 18 44

3.1 6.1 0.2 6.2

1350 ± 200 (1290) 6300 ± 500 (1300) 1300 ± 200 (1390) 6040 ± 500 (1450)

−8.7 −14.3 −8.7 −11.5

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Figure 28. Femtosecond TA spectra and decay profiles (inset) of (a) neutral PF30h and (b) its protonated form PF30hp with MSA in CH2Cl2. For all cases, the pump excitation is at 630 nm. Reprinted with permission from ref 67. Copyright 2009 American Chemical Society.

Figure 29. Femtosecond TA spectra and decay profiles (inset) of (a) neutral PF38N and (b) its protonated form PF38 Np with MSA in CH2Cl2. For all cases, the pump excitation is at 730 nm. Reprinted with permission from ref 67. Copyright 2009 American Chemical Society.

Table 5. Fitted Time Constants in the TA Decay Profiles of Heptaphyrins and Nonaphyrins67

Table 6. Steady-State Absorption and Fluorescence Bands, Stokes Shifts, Singlet and Triplet Excited-State Lifetimes, and TPA Cross Sections σ(2) of PF30h and Its Protonated Form PF30hp with TFA in CHCl367

λpump (nm) PF30h

630

PF30hp

630

PF38N

730

PF38Np

730

PF30h

630

PF30hp

630

λprobe (nm) 633, 680 630, 710 726, 780 726, 780 638, 680 630, 675

τ1 (ps) (fraction)

τ2 (ps) (fraction)

43, 50 403, 396

PF30h

16, 220

PF30hp

λabs (nm)

λflu (nm)

ΔEStokes (cm−1)

τS (ps)

634, 1101 633, 1037

1142

330

51

1050

100

167

τT (μs)

2.0

σ(2) (GM) (λex/nm) 1290 ± 200 (1290) 5680 ± 500 (1300)

46, 41 48 (97%), 54 (86%) 169 (69%), 165 (62%)

motions in the flexible structure of neutral forms, resulting in suppression of nonradiative decay channels and deceleration of energy-relaxation processes of the excited states for protonated forms. Chirvony et al. provided a similar description for the change of photophysical properties of nonplanar porphyrin dication (diprotonated forms of H2TPP and H2OEP).86 In their study, porphyrin dications show larger Stokes shifts, broadened optical bands, and a decrease in fluorescence lifetimes compared to those observed in their planar neutral forms; the same spectral features are observed for twisted PF30h and PF38N. Chirvony et al.86 described that the altered properties of porphyrin dications are attributed to their conformational flexibility and nonplanarity. Because structural flexibility can lead to various conformations in the excited electronic state, Chirvony et al.86 suggested that the funnel point is enhanced at which internal conversion occurs, because of the small energy gap between the ground and excited states, thereby decreasing the S1-state lifetimes in porphyrin dications. Therefore, these results support that the conformational changes in PF30hp and PF38N by protonation trigger a change of photophysical properties. As described in the Introduction, the conformational change from a distorted to a planar structure by the effect of protonation is supported by the TPA cross section values, reflecting the π

>1 ns (3%), >1 ns (14%) ∼1.12 ns (31%), ∼1.28 ns (38%)

counterparts (Table 4). Considering a great change in the S1/T1state lifetimes and Stokes shift between neutral and protonated forms of PF30h and PF38N, these significant changes delineate structural rigidification by protonation. Because the connection of pyrrole rings by meso-methine carbons of expanded porphyrins provides flexible structures inherently, their excited state dynamics is governed by effective nonradiative decay processes through several low-frequency vibrational modes, and they show relatively short singlet excited-state lifetimes compared to typical porphyrins. Here, the protonation of expanded porphyrins induces intermolecular interactions with the counteranion of acid via hydrogen-bonding interactions and Coulombic repulsion between pyrrolic protons, which leads to rigidification of their structures. In reality, the X-ray structure of PF30hp under TFA condition apparently illustrates its hydrogen-bonding interactions with counteranions of TFA.100 Namely, by protonation, structural rigidification, accompanied by the planarization of PF30hp and PF38Np, restricts molecular 2275


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electron delocalization. For obtaining the maximum TPA cross section values without any linear absorption artifacts, the twophoton excitation wavelengths were examined in the doubled wavelength region of B-like absorption bands for neutral and protonated species to exclude the possibility of one-photon absorption. PF30h and PF38N show the TPA cross section values of 1350 and 1300 GM, respectively. On the other hand, significantly higher TPA cross section values are measured for the protonated forms of PF30hp and PF38Np with MSA (6300 and 6040 GM, respectively). Moreover, by the addition of TFA, the TPA cross section value of PF30h increases from 1290 to 5680 GM (Table 6). These increased TPA cross section values of the protonated forms qualitatively illustrate that π electrons are more effectively delocalized with the enhancement of structural planarity induced by protonation. Nevertheless, further indepth studies are required for deep understanding of the increased TPA cross section values of PF30hp and PF38Np. On the basis of the second-order perturbation theory,102 if the incident photon frequency is adjusted close to the energy of the imaginary state or the transition dipole moment increases, the TPA cross section values increase.103−106 When the TPA values are measured near B-like bands, the Q-like bands of expanded porphyrins play an essential part because they can act as an intermediate state in the TPA process.107 The Q-like bands of the lowest energy in PF30h and PF38N become enhanced by the addition of acid, exhibiting blue-shifts (Figure 26). Therefore, strong Q-like bands lead to increased transition dipole moment and TPA cross section values. This increase in the TPA cross section values arises from the structural planarity and electronic structure of the molecule. The NICS calculation provides a quantitative analysis for the aromaticity change by protonation. The optimized geometries of PF30hp and PF38N for NICS calculation are relatively planar and figure-eight distorted conformation, respectively, being consistent with their X-ray structures (Figures 30 and 31). On the other hand, owing to the absence of X-ray crystallographic results, the optimized structures of PF30h and PF38Np based on their 1H NMR spectra display figure-eight distorted and relatively planar conformations, respectively. In particular, five pyrrole rings are oriented inward and four pyrrole rings are oriented outward in the optimized structure of PF38Np, which is consistent with 1H−1H-COSY and 1H NMR spectroscopy.67 As the calculated chemical shifts of the inner or outer NH and β-CH protons for these optimized structures are in agreement with the experimental chemical shifts,67 these optimized structures are likely to be close to the actual molecular structures, although the optimized structures were not obtained from X-ray structures. Moreover, the optimized geometries of PF30hp and PF38N are in agreement with the planar and figureeight structures, respectively, as confirmed by X-ray crystallography.67 In these optimized geometries, although π electrons on the π-conjugation pathway of PF30h and PF38N follow the [4n +2] rule of Hückel aromaticity, the calculated NICS(0) values are in the range of small negative values (−3.1 to −7.8 ppm for PF30h and −0.4 to −7.1 ppm for PF38N), describing their weak aromaticity, attributed to the largely twisted conformations. On the other hand, along with the protonation effect giving rise to the planar structures and intensified ring currents of PF30h and PF38N, the NICS(0) values estimated in the protonated structures are highly negative, −25.5 to −26.7 ppm (−14.3 ppm in the center of macrocycle) for PF30hp and −18.8 to −21.9 ppm (−11.5 ppm in the cavity center of macrocycle) for PF38Np. Although NICS values show a small increase at the

Figure 30. Calculated (a) oscillator strength and (b) NICS(0) values (in ppm) of neutral (top in panel a, left in panel b) and protonated (bottom in panel a, right in panel b) forms of PF30h based on the optimized structures (B3LYP/6-31G* level). To show clearly, meso-pentafluorophenyl rings are substituted for hydrogen atoms after the optimization procedures. Reprinted with permission from ref 67. Copyright 2009 American Chemical Society.

Figure 31. Calculated (a) oscillator strength and (b) NICS(0) values (in ppm) of neutral (top in panel a, left in panel b) and protonated (bottom in panel a, right in panel b) forms of PF38N based on the optimized structures (B3LYP/6-31G* level). To show clearly, meso-pentafluorophenyl rings are substituted for hydrogen atoms after the optimization procedures. Reprinted with permission from ref 67. Copyright 2009 American Chemical Society.

molecular center, it is sufficient for verifying the reinforced ring current by protonation, because the different distances between the center and π-conjugation pathway can provide a significant effect on the NICS values. In particular, the molecular centers of PF30h and PF38N are slightly close to the molecular frame because of their figure-eight structures, compared to those in their planar protonated structures of PF30hp and PF38Np. This 2276


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Chart 4. Molecular Structure of PF36O (Left) and PF38O (Right) (Reprinted with Permission from Ref 68; Copyright 2010 American Chemical Society)

observation is in accordance with the NICS values around mesopositions; because the distances between the probed near-mesopositions and molecular frame are constant regardless of the conformational change, the increased NICS values at those positions obviously indicate ring-current enhancement. In accordance with the 1H NMR spectra, the NICS values estimated at various positions of neutral and protonated forms of PF30h and PF38N are well-matched with the fact that the aromaticity increases through planar structures by protonation; on the other hand, they display weak aromatic character in the neutral state because of their figure-eight twisted conformations. As mentioned above, the photophysical properties of PF30h and PF38N clearly indicate the protonation-induced conformation changes from twisted to planar structures. Upon protonation, the spectroscopic features are remarkably changed, and all experimental data are in agreement with the increased planarity and rigidity; for example, relatively narrow absorption bands, decreased Stokes shift, significantly higher TPA cross section values, and longer excited-state lifetimes of PF30hp and PF38Np are observed as compared with their figure-eight twisted neutral forms. Consequently, more planar structures give rise to enhanced aromatic character, resulting in highly negative NICS values. These results indicate that the conformational distortion of expanded porphyrins, which is inevitable particularly with the increasing number of pyrroles, can be modified and controlled by protonation, as well as anion binding leading to planar structures to enhance the Hückel aromaticity. 3.1.2. Effects of Protonation in [4n]π Expanded Porphyrins. Under neutral conditions, the intramolecular hydrogen bonding of expanded porphyrins leads to a large difference in their conformation and the number of π electrons participating in core cyclic π-conjugation. For example, in nonpolar solvents, PF30h and PF38N exhibit figure-eight structures, despites their weak [4n+2]π aromaticity, because the more effective intramolecular hydrogen bondings are formed in the figure-eight conformation.66 On the other hand, under acidic conditions, protonation breaks the intramolecular hydrogen bonds between pyrroles in expanded porphyrins. Instead, the released aminic pyrrole units in expanded porphyrins interact with counteranions of acid molecules and form intermolecular hydrogen bonds, often resulting in drastic transformation of their conformation, in addition to the incresed aromaticity. Generally, the above-mentioned PF30h and PF38N achieve the [4n+2]π Hückel planar structures under acidic conditions, resulting from the aromatic stabilization of planar topology. Nevertheless, it is not totally excluded by this result that the planar form is not the most stable conformation for [4n]π expanded porphyrins because [4n]π electronic systems can undergo topological changes through aromatic stabilization resulting in twisted

Mö bius topology. For elucidating this critical point, the protonation effect on a couple of redox [4n] and [4n+2]π congeners, [36] and [38]octaphyrins, is comparatively discussed here.68 For clarity, octaphyrins having the same meso-aryl substituents, being named as meso-pentafluorophenyl-substituted [36]- (PF36O) and [38]octaphyrins(1.1.1.1.1.1.1.1) (PF38O), are discussed.108,109 These compounds differ only in their arrangement of intramolecular hydrogen bondings between pyrroles and the number of π electrons. As the octaphyrin macrocycle is sufficiently large for disregarding conformational restraint, the π electrons on the core cyclic conjugation pathway play the most important role in determining its conformation under acidic conditions. By considering all factors, such large redox congeners in their protonated forms are most suitable for addressing the issue of [4n+2] or [4n]π electron aromatic stabilization with respect to the geometry of expanded porphyrins. Here, the discussion about the relationship between aromaticity and photophysical properties can offer the deep comprehension of Hückel or Möbius aromatic switching by the protonation effect. Through the X-ray crystallography, a figure-eight conformation of PF36O being composed of two porphyrin-like tetrapyrrolic “hemimacrocycles” is clearly revealed (Chart 4).68 All pyrrolic nitrogen atoms are oriented inward, constructing an intramolecular hydrogen-bonding network between the pyrrole constituents. PF36O shows proton peaks in the 6−8 ppm region in the 1H NMR spectrum. Moreover, the NICS values in the range from −2.1 to 2.1 ppm are estimated at several positions around the macrocycle.68 These results illustrate that PF36O is nonaromatic.110 In the steady-state absorption spectrum of PF36O, two broad bands in the visible region without any Q-like bands (Figure 32) are also characteristic spectral features of nonaromatic expanded porphyrins. Even though there is no crystal structure of PF38O, its 1H NMR spectrum illustrates the diatropic ring current. The wellresolved β-proton signals in the moderately shielded region (δ = 2.51 and 2.78 ppm) arise from the inner pyrrolic protons Ha and Hb in Chart 4, and other signals in the deshielded region result from the outer pyrrolic protons, which are indicative of its obvious diatropic ring current.68 Moreover, as the low-temperature 1H NMR analysis of PF38O at 183 K did not exhibit any broadening of signal, these proton signals represent the lessplanar Hückel aromatic conformation,62 in which the protons are not entirely lying in the shielding region of the diatropic ring current. In addition, the total eight signals for 16 protons reflect substantially symmetric structure in solution on the 1H NMR timescale. In the steady-state absorption spectrum of PF38O, a B-like band in the visible region and Q-like bands in the NIR region are observed, which are typical spectral features of 2277


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analysis, the protonated species is assigned to be a dication of PF36O (PF36O-TFA2),68 where a large, flexible core macrocycle of [36]octaphyrin permits smooth π-conjugation on an overall half-twisted Möbius topology, where torsional angles along the π-conjugation circuit are well-settled within 30°.30,33 The temperature-dependent 1H NMR spectra of [36]octaphyrins with controlling the addition of acid amount provide the change of aromaticity and conformation in detail.68 In the TFA titration NMR spectra in CD3CN at 233 K, the signal intensities from protonated species, such as PF36O-TFA1 and PF36O-TFA2, are increased at the expense of the decrease in the signal intensities from PF36O. Upon the addition of 3 equiv of TFA, the signals of PF36O disappear and new signals from PF36O-TFA1 and PF36O-TFA2 simultaneously appear. Several proton signals in the region of −1 to −5 ppm indicate the large shielding effect on inner protons, which reflects their aromaticity. With the further addition of 10 equiv of TFA, a simple 1H NMR spectrum consisting of signals of only PF36O-TFA2 is obtained. Furthermore, with even an excess amount of TFA or its deutrated form, there is no spectral change, indicating no further protonated species, such as PF36O-TFAn (n = 3 or 4). Thus, TFA only allows the protonation of PF36O up to a dication species. The titration with MSA exhibits similar changes in the absorption spectra, which is indicative of the production of PF36O-MSA1 and PF36O-MSA2 with a mild counteranion effect in the protonation of PF36O. The highly shielded inner protons represent that these protonated species exhibit strong aromaticity and that subtle enhancement is observed from PF36O-MSA1 to PF36O-MSA2. Because of the stronger acidity of MSA, the proton signals of PF36O almost disappear by the addition of equivalent MSA. In addition, 4 equiv of MSA results in new signals, in addition to those of PF36O-MSA2, which are probably induced by PF36O-MSAn (n = 3 or 4). The moderate chemical shifts of the shielded inner protons propose that, as compared to PF36OMSA1 and PF36O-MSA2, PF36O-MSAn exhibits relatively weak aromaticity.68 The acid titration to PF36O with MSA gives rise to the twostepwise absorption spectral changes.68 Up to the amount of 2.72 × 10−4 M MSA addition, a distinct spectral change occurs with two isosbestic points at 426 and 667 nm, respectively. With increasing intensity, a broad band in the visible region is redshifted, and additional broad and weak bands appear in the NIR

Figure 32. Steady-state absorption (black) and fluorescence spectra (red) of PF36O, PF38O, and their protonated forms (PF36O-MSA2, PF36O-MSAn, and PF38O-MSA2, respectively) upon addition of MSA in CH2Cl2. Reprinted with permission from ref 68. Copyright 2010 American Chemical Society.

aromatic expanded porphyrins (Figure 32).77,79,94 These spectral features of neutral PF38O illustrate a [38]π electronic Hückel aromatic conformation with two inverted pyrrole rings, as shown in Chart 4. Figure 33 shows the crystal structure of protonated PF36O.68 By protonation, the figure-eight geometry of neutral PF36O is changed to an unfolded conformation. All intramolecular hydrogen bondings in PF36O are broken and changed to intermolecular hydrogen bondings between pyrrolic protons and TFA molecules by protonation, in which five nitrogens atoms (located at the pyrrole rings marked as A, C, D, F, and G in Figure 33) are oriented outward for forming hydrogen bonds with surrounding TFA molecules. From X-ray crystallographic

Figure 33. X-ray crystal structure of diprotonated PF36O-TFA2. meso-Aryl substituents are omitted for clarity. Reprinted with permission from ref 68. Copyright 2010 American Chemical Society. 2278


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range. These spectral features, sharp B-like and distinct Q-like bands, are typically characteristic of aromatic porphyrinoids.77,79,94 Hence, this spectral change illustrates the enhancement of aromaticity by protonation. With increasing MSA concentration up to 3.79 × 10−3 M, the B-like band becomes more intensified and the Q-like bands become well-resolved and intense. Because of the complicated titration process of PF36O by protons, it is difficult to obviously distinguish each species from various protonated forms. Nevertheless, the titration results analyzed by Hill plots describe that two protons are attached in the first titration process using an MSA concentration of 2.72 × 10−4 M (Figure 34).

Figure 35. Analyses of the protonation process of PF36O: (a) analyzed absorption spectra of three protonated forms and (b) progress of protonation from neutral to mono-, di-, and tetraprotonated forms (inset describes concentration range from 0 to 0.0008 M). Reprinted with permission from ref 68. Copyright 2010 American Chemical Society.

protonated species of PF36O in spectrophotometric titration up to MSA concentrations of 2.72 × 10−4 and 3.79 × 10−3 M can be assigned to diprotonated PF36O-MSA2 and multiprotonated PF36O-MSAn, respectively (Figures 32 and 36). Figure 37 displays the crystal structure of protonated PF38O.68 The structure of protonated PF38O is surprisingly similar to that of protonated PF36O (PF36O-TFA2). The octaphyrin macrocycle exhibits a mirror symmetry with a plane being vertical to A and E pyrrole rings. Five nitrogen atoms (located at the pyrrole rings marked as A, C, D, F, and G in Figure 37) are oriented outward as those of PF36O-TFA2. The octaphyrin molecules form intermolecular hydrogen bonds with surrounding TFA, ethanol, isopropyl alcohol, and water molecules. The protonated species is considered again to be the dication form of PF38O (PF38O-TFA2) based on their number of counteranions. Despite the fact that PF36O-TFA2 and PF38O-TFA2 are structurally similar, the π-conjugation circuit of PF38O-TFA2 exhibits Hückel planar geometry. Notably, the distinction of Möbius−Hückel geometry between PF36O-TFA2 and PF38O-TFA2 is attributed to the small difference in the dihedral angles between F and G pyrrole rings. In the crystal structure of PF38O-TFA2, all pyrrole planes including F and G faces are directed to almost the same direction, affording an approximately Hückel planar structure, while F and G rings in PF36O-TFA2 are substantially distorted to the plane composed of the other pyrrole rings and show overall Möbius conformation in the X-ray structure of PF36O-TFA2. With the addition of TFA or MSA, a one-stepwise simple spectral change, from PF38O to the protonated forms, occurs in the 1H NMR titration analysis. Even though the protonated form

Figure 34. Hill plots for the titration of PF36O in an MSA concentration range of (a) 0 to 1.09 × 10−5 M and (b) 1.09 × 10−5 to 2.72 × 10−4 M. Reprinted with permission from ref 68. Copyright 2010 American Chemical Society.

The singular-value decomposition (SVD) analysis for the spectrophotometric titration spectra provides three linearly independent optical spectra assigned to arise from monoprotonated, diprotonated, and tetraprotonated species, respectively (Figure 35).68 At an MSA concentration of

Control and Switching of Aromaticity in Various All-Aza-Expanded

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