Theoretical Insights into Branched and Fused Expanded Pyridiniums

Jul 22, 2010 - Designing Multifunctional Expanded Pyridiniums: Properties of Branched and Fused Head-to-Tail Bipyridiniums. Jérôme Fortage , Cyril P...
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Theoretical Insights into Branched and Fused Expanded Pyridiniums by the Means of Density Functional Theory Cyril Peltier,† Carlo Adamo,† Philippe P. Laine´,*,‡ Sebastiano Campagna,§ Fausto Puntoriero,§ and Ilaria Ciofini*,† LECIME, Laboratoire d′E´lectrochimie, Chimie des Interfaces et Mode´lisation pour l′E´nergie (CNRS UMR-7575), E´cole Nationale Supe´rieure de Chimie de ParissChimie ParisTech, 11, rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France, Laboratoire de Chimie et Biochimie Pharmacologiques et Toxicologiques (CNRS UMR-8601), UniVersite´ Paris Descartes, 45, rue des Saints Pe`res, F-75270 Paris Cedex 06, France, and Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, UniVersita` di Messina, Via Sperone 31, I-98166 Messina, Italy ReceiVed: May 15, 2010; ReVised Manuscript ReceiVed: June 23, 2010

With the aim of getting insights into the peculiar electronic, structural, and photophysical properties of four expanded pyridinium systems of potential use as electron acceptors in supramolecular architectures, their electronic and geometrical structures, at both the ground and the excited states, were investigated by the means of density functional theory (DFT) and time-dependent DFT (TD-DFT). Solvent effects were included by the means of a polarizable continuum model (PCM) at both the ground and the excited states. In particular, the computed photophysical behaviors (absorption and emission) of the fused architectures were compared to those of the respective branched precursors in order to clarify the origin(s) of (i) the extension of their electronic absorption toward the visible region and (ii) the increase of their luminescence quantum yields and redshifted emission wavelengths experimentally observed. The theoretical insights gained allow for a clear-cut explanation of the different behavior of these systems of interest as electron acceptors and luminophores for more complex supramolecular architectures and opens the route for a joint experimental and theoretical design of new pyridinium-based acceptors. 1. Introduction Pyridiniums play a crucial role in several biological processes,1 as in the instance of the NAD+/NADH cofactor, an oxidoreductase coenzyme present in living cells, whose redoxactive core is a pyridinium fragment. They are also well-known for their electron-withdrawing (“pull” effect) and electronaccepting (redox) properties and are widely used, including for their mere cationic charge, in several domains of application in chemistry.2 Typically, as cationic species, they have been used as polar heads in amphiphilic molecules, components for liquid crystals,3 either fully organic4 or mixed organic/inorganic.5 Nevertheless, it is for their electrochemical properties that pyridiniums have mostly been exploited.6 These species have been largely used not only as main components of zwitterionic solvatochromic dyes7 and other biologically active mesomeric betaines8 but also as subunits of push-pull dyads showing NLO properties.9–14 Focusing on the field of artificial photosynthesis, pyridiniums have been recently employed as full building blocks of multicomponent functional assemblies, either purely organic15,16 and inorganic,17–25 designed to undergo photoinduced electron transfers (PET) to form charge-separated states (CSS). In order to tune both the redox and the photophysical properties of expanded pyridiniums, in analogy to the strategy used to enhance the properties of simple aromatic molecules (such as benzene),26,27 a possible approach relies on an extension * Corresponding authors. E-mail: [email protected] (I.C.); [email protected] (P.P.L.). † ´ Ecole Nationale Supe´rieure de Chimie de ParissChimie ParisTech. ‡ Universite´ Paris Descartes. § Universita` di Messina.

Figure 1. Branched (1 and 2) and fused (1f and 2f) expanded pyridiniums.

of their π-systems. This strategy was recently experimentally applied to tetra- and hexa-phenylpyridinium derivatives (Figure 1, 1 and 2, respectively) in order to test whether or not their electrochemical and photophysical (here absorption and emission) properties were changed upon pericondensation into their fused polycyclic derivatives (1f and 2f in Figure 1).28 To this end, fused pyridinium derivatives (1f and 2f, Figure 1) were fully characterized, including from structural, electrochemical, and photophysical viewpoints, and their properties compared to those of parent branched architectures (1 and 2).28 More specifically, concerning photophysics, it was experimentally found that, when going from branched to fused pyridinium derivatives, not only the emission is red-shifted while

10.1021/jp104439q  2010 American Chemical Society Published on Web 07/22/2010

Theoretical Insights into Pyridiniums via DFT absorption is extended toward the visible but also related quantum yields and absorptivities are significantly increased.28 The aim of the present paper is to define, from a fully theoretical point of view, the prominent electronic features giving rise to this important change in photophysical behavior upon condensation and, at the same time, to set up a computational protocol able to predict the behavior at the ground and excited state of related, i.e., functionalized, extended pyridinium cores. These latter are indeed envisaged as potential building blocks for supramolecular architectures design for PET processes. To this end, density functional theory (DFT29) and timedependent DFT (TD-DFT30) were applied to fully characterize the ground and excited states of both branched and fused architectures. Nowadays, a combined use of DFT and TD-DFT has become a reliablesand extensively benchmarkedstool for the analysis and prediction of ground state properties as well as of vertical absorption spectra of organic chromophores.21–41 In particular, this computational protocol is able to provide valence excitations of organic dyes with an average error of a few tenths of an electronvolt,33 when solvent effects are also properly taken into account (for instance, using a simple, yet efficient, polarizable continuum model, PCM).33,36–40 More recently, a similar computational protocol has been validated in the case of emission energies36–40 The paper will be organized as follows. After the discussion of the ground and excited state structural features (sections 3.1 and 3.2), the absorption (section 3.3) and emission (section 3.4) properties of both fused and branched architectures will be analyzed. Finally, in section 4, some general conclusions on the electronic origin of the different behavior of branched and fused architecture will be drawn and some perspectives on future work in design and modeling of pyridinium-based acceptors for photochemical molecular devices will be given.

J. Phys. Chem. A, Vol. 114, No. 32, 2010 8435 TABLE 1: Main Structural Parameters (distance in Å, angles in deg) Computed for the Relaxed Geometries of 1 and 2 in Their Relevant S0, S1, and T1 Electronic Statesa 1 N 1C 2 C 2 C3 C 3 C4 C 4 C5 C 5 C6 C 6 N1 N1C11 C2C21 C3C31 C4C41 C5C51 C6C61 θ1 θ2 θ3 θ4 θ5 θ6

2

S0 (exp.)

S1

T1

S0

S1

T1

1.381 (1.369) 1.391 (1.371) 1.408 (1.399) 1.408 (1.394) 1.391 (1.368) 1.381 (1.374) 1.462 (1.466) 1.484 (1.488) 1.475 (1.475) 1.484 (1.479) 70.4 (78.6) 60.7 (55.9) 28.0 (25.1) 57.2(71.8)

1.429 1.380 1.417 1.427 1.377 1.419 1.415 1.468 1.465 1.481 43.9 46.9 20.4 44.0

1.407 1.376 1.442 1.442 1.376 1.407 1.449 1.485 1.415 1.485 65.0 59.0 2.5 57.8

1.376 1.403 1.417 1.417 1.403 1.376 1.466 1.489 1.492 1.487 1.492 1.489 73.9 68.6 63.2 67.3 70.7 77.6

1.419 1.383 1.429 1.428 1.383 1.419 1.444 1.486 1.488 1.481 1.487 1.485 67.1 62.4 64.9 58.9 63.9 70.3

1.415 1.381 1.452 1.456 1.387 1.404 1.451 1.491 1.488 1.443 1.481 1.490 76.5 63.6 43.4 51.3 68.0 76.9

a Available X-ray data18 are reported in parentheses. Refer to Figure 1 for labeling scheme.

2. Computational Methods

strains making use of TD-DFT derivatives, as implemented in the Gaussian code.47 Vibrationally resolved spectra within the harmonic approximation were computed using the FCClasses program.48–51 The simulated spectrumsat 300 K for the absorption and 77 K for the emissionswas convoluted using Gaussian functions with a full-width at half-maximum of 320 cm-1 (0.04 eV). A total of 40 initial vibrational states, a maximal number of 24 overtones for each mode, and 19 combination bands on each pair of modes were considered. The maximum number of integrals to be computed for each class was set to 106.

All calculations were carried out using a development version of the Gaussian code.42 When not differently specified, a hybrid Hartree-Fock/density functional model, referred to as PBE0, was used.43 The PBE0 was obtained by casting the PBE exchange and correlation functional44 in a hybrid DFT/HF scheme, where the HF:DFT exchange ratio is fixed a priori to 1:3. The molecular structure of each compound was fully optimized and the nature of each stationary point was defined by a subsequent frequency calculation. If not explicitly specified, both for the structural optimizations and for the calculation of the electronic properties, all atoms were described by a double-ζ quality basis set (LANL2DZ).45 Vertical electronic transitions were computed using the TDDFT approach at the same level of theory. For clarity, only computed transitions with non-negligible oscillator strength (f g 0.06) are reported in the tables. These transitions were computed at least down to 200 nm for all systems studied. Solvent effects were taken into account in all calculations by applying an implicit solvation model that is the polarizable continuum model (PCM) of Tomasi and co-workers.46 More specifically, we used the conductor-like PCM model as implemented in the Gaussian Development version Rev G.01.42 Acetonitrile was considered as solvent. Fluorescence was computed from the first excited state (S1) at the same level of theory (TD-DFT, PBE0) and the first five excited states were computed in order to optimize the first one (S1). Also at the excited state, the molecular structure of each compound was fully optimized in absence of symmetry con-

3. Results and Discussion 3.1. Ground and Excited States Structural Features. In the case of flexible or semirigid molecular systems, determining and predicting the changes in structure when going from the ground to the excited state or from the native to a reduced/ oxidized form can be of great importance to interpret and predict their photochemical and electrochemical behavior. Beside the possibility of understanding the poor ability of such systems to self-assemble, intervening large structural changes upon oxidation/reduction are also likely to affect the electrochemical behavior of a system, in particular the reversibility of the redox process(es), and impact the rate of electron transfers via the reorganization energy. Analogously, significant structural relaxation upon excitation will determine large Stokes shifts and/ or change in emission quantum yields. Furthermore, the presence of soft vibrational modes for both the ground and the excited states could imply the presence of sizable vibrational progressions in the absorption and emission spectra. For all these reasons, particular care was taken in analyzing the structural features of the semirigid pyridinium-based systems considered in this paper, at both the ground and first excited state. The main geometrical parameters, computed for ground (S0) and triplet (T1) states of molecules 1 and 2 (Table 1) and 1f and 2f (Table 2) are reported in comparison with the available experimental data.

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TABLE 2: Main Computed Structural Parameters (distance in Å, angles in deg) for 1f and 2fa 1f N1 C2 C 2 C3 C 3 C4 C 4 C5 C 5 C6 C 6 N1 N1C11 C2C21 C3C31 C4C41 C5C51 C6C61 θ1 θ2 θ4 θ6 Ψaxb Ψlnc

2f

S0 (exp.)

S1

T1

S0 (exp.)

T1

1.404 (1.399) 1.393 (1.381) 1.398 (1.386) 1.398 (1.386) 1.393 (1.381) 1.404 (1.399) 1.428 (1.433) 1.460 (1.456) 1.476 (1.480) 1.460 (1.455) 30.0 (34.4) 179.8 (178.9) 179.8 (178.6)

1.429 1.383 1.410 1.410 1.383 1.429 1.410 1.451 1.463 1.451 24.7 179.1 179.6

1.431 1.393 1.412 1.412 1.393 1.431 1.402 1.444 1.468 1.444 22.9 179.6 179.8

1.380 (1.366) 1.407 (1.390) 1.430 (1.418) 1.430 (1.422) 1.407 (1.405) 1.380 (1.369) 1.468 (1.470) 1.491 (1.492) 1.471 (1.468) 1.440 (1.441) 1.471 (1.483) 1.491 (1.491) 87.7 (89.0) 74.6 (70.7) 74.6 (80.2) 0.3 (1.6) 25.1 (26.7)

1.421 1.404 1.440 1.440 1.404 1.421 1.461 1.479 1.460 1.419 1.460 1.479 79.6 62.7 58.2 0.93 24.0

a Available X-ray data are reported in parentheses. Refer to Figure 1 for labeling scheme. b Ψax corresponds to the C6N1C11C16 and C3C4C41C42 dihedral angles for 1f and 2f respectively. c Ψln corresponds to the N1C2C21C26 and C2C3C31C32 dihedral angles for 1f and 2f respectively. X-ray data from reference 28

Figure 2. Optimized ground state (S0) structures obtained for 1, 1f, 2, and 2f.

In the same tables, the geometrical parameters computed for all systems but 2f in their first singlet excited state (S1) are also reported. The labeling scheme used is given in Figure 1, while the optimized ground state structures of each system investigated are collected in Figure 2 First of all, a good agreement between the available experimental data28,52 and the computed ones can be noted in the case of ground-state structures of 1, 2, 1f, and 2f. The largest deviation, with respect to the available X-ray data,28 is, in fact, only ca. 0.02 Å in bond lengths and ca. 6° for the most significant dihedral angles. Therefore, we can reasonably assume that the level of theory used is able to correctly describe structural features at least at the ground state, thus being suitable to provide reasonable geometrical parameters also in the case of monoreduced systems. In the case of branched systems 1 and 2, photoexcitation and subsequent relaxation (both in the singlet and the triplet states)

mainly induces a planarization of phenyl substituents, as revealed by the decrease in the computed torsional angles (θ, Table 1). This effect is of larger amplitude in the case of 1 due, most probably, to the smaller steric hindrance at the periphery of the pyridinium core. Upon excitation to S1, a planarization of ca. 30° around θ1 is indeed computed for 1, while only 7° of planarization is expected for 2. As a matter of fact, all the phenyl rings are computed to be less conjugated (coplanar) with the pyridinium core in the case of 2 than for 1, both at the ground and the excited states, thus substantiating the hypothesis of a larger intramolecular (interbranch) steric effect for the former compound (that is, 2). Furthermore, the fact that the computed bond lengths of the pyridinium core of the two branched compounds are very similar (both at the ground and the excited states) rules out the possibility of differences in the electronic structures as main causes of the different planarization patterns. If this would be the case, a strong variation of the pyridinium core structure should also be computed. Due to the quite large geometrical relaxation upon excitation observed for 1 and, to a lesser extent, for 2, a large Stokes shift is expected at least for compound 1. Furthermore, these torsional motions (planarization) are expected to be associated with soft (i.e., low energy) vibrational modes both at the ground and excited state. In particular, the computed vibrational frequencies associated with the rotation of the phenyl rings leading to planarization are computed to be 38 and 21 cm-1 for 1 and 41 and 23 cm-1 for 2 at the ground and excited states, respectively. The larger vibrational frequencies computed both at the ground and the excited state in the case of system 2 further substantiate the larger energy penalty associated with planarization for this compound. Furthermore, since these structural rearrangements are expected to be efficient nonradiative deactivation pathways, they are anticipated to significantly alter (reduce) the emission quantum yield of the branched systems with respect to their fused analogues. Considering the fused compounds (Table 2), the overall structural rearrangement upon excitation, computed only in the case of 1f,53 is less marked than in the corresponding branched system, the planarization of the phenyl translating into a decreased torsional angle (θ) of only a few degrees. Analogously to the corresponding branched architectures, these planarization motions are related to low-frequency vibrations of ca. 43 and 44 cm-1 at the ground state and the excited state for 1f. Concerning more specifically the structure of fused species, it is worth noting that the calculations predict a nonplanar molecular scaffold both at the ground and excited state, as revealed by the analysis of the axial and longitudinal dihedral angles, Ψax and Ψln, reported in Table 2. While for 2f this finding very nicely correlates with X-ray structures obtained with different counterions (such as BF4-, Cl-, or FeCl4-) and various solvents,28 the situation is less clear for 1f. In this case, a perfectly planar fused scaffold is indeed experimentally observed for [1f](BF4)•CH3CN28 while with more bulky counterions (such as PhSO3-) a curved backbone is also reported for another solid-state structure,49 in agreement with the computational outcomes.28 Therefore, we can reasonably ascribe the planarity of the fused core observed for [1f](BF4)•CH3CN not to intrinsic properties of the systems but to crystal packing,24 while in presence of sufficient room for bending, this perfect planarity is lost.49

Theoretical Insights into Pyridiniums via DFT

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Figure 3. Optimized ground state (S0) structure of systems 3f and 4f.

Figure 4. Tail-to-tail (left52) and head-to-head (right28) 1f dimers observed in X-ray structures with BF4- as counterion.

On the other hand, the pronounced distortion computed and observed for 2f is necessarily related to intramolecular constrains and not to the crystal packing. To get insights into the nature of prevalent factors governing this distortion, a structural optimization was also performed for molecule 2f deprived of the phenyl rings at positions 2 and 6 (hereafter 3f, Figure 3 and Supporting Information, Table SI.1 and Figure SI.1). 3f displays a perfect planarity of the fused core, proving that the distortion computed for molecule 2f is mainly related to the steric hindrance generated by the two phenyls ortho to the N-pyridinio phenyl group. Finally, in order to understand the selective formation of 2f during the photochemical cyclization of 2,28 the other possible hemifused isoelectronic compound corresponding to the photocyclized product about the N-pyridinio phenyl-ring side (4f, Figure 3) was computed. As for 2f, also in the case of 4f, a bent molecular scaffold is computed, with the aryl substituents linked at positions 3, 5, and 4 of the pyridinium ring lying practically orthogonal to the pyridinium ring. Complete structural parameters of 4f are reported in the Supporting Information (Table SI.1) Indeed, 4f is computed to be ca. 35 kJ/mol higher in energy than the isoelectronic 2f, in line with the selective formation of this latter by photocyclization of 2 experimentally observed.28 3.2. Ground State Features: Intermolecular Interactions for Fused Compounds. Up to now we have analyzed the structural features of isolated branched and fused pyridinium systems. By virtue of their large conjugated π systems, the hemifused compounds 1f and 2f are also natural candidates for displaying extended self-assembled supramolecular architectures relying on π-π stacking interactions. From solid-state single-crystal X-ray analyses, different crystal packings are reported in the literature for 1f, depending on the solvent used for crystallization. In particular, in presence of the same counterion (BF4-), a layered tail-to-tail structure was reported for a crystal of 1f obtained from methanolic solution by Mullen and collaborators,52 while a columnar headto-head structure was found for a crystal of the solvate [1f](BF4)•CH3CN obtained from acetonitrile by us (Figure 4).28 On the other hand, the counterion plays a major role in determining the intramolecular parameters of 1f and, in particular, in forcing a planar or bent conformation of its molecular structure, as previously discussed.

It is worth noting that in all cases the interplanar distances between the stacked 1f molecules are in the range of those measured for neutral polycyclic aromatic hydrocarbons (PAHs), that is, ca. 3.4-3.5 Å.54–56 The internal structural parameters and the packing of 2f seem to be less sensitive to both the counterion and the nature of the crystallization solvent used. In fact, this system always presents a curved scaffold and a sandwich-herringbone packing with formation of head-to-head pseudodimers without significant 3D stacking, regardless of the crystallization solvent and counterion.28 In order to explain the different packing observed for 1f and to give a semiquantitative evaluation of the stacking energies, both for a head-to-head and for a tail-to-tail arrangement, a model containing two 1f molecules fixed at their X-ray structures, but in absence of cocrystallized solvent molecules and counterions, was considered (Figure 4). Interaction energies of the so-obtained dimers were then computed at the DFT level using a recently proposed exchange correlation functional (the B97-D57) purposely tailored to correctly evaluate dispersion interactions, normally not quantitativelyreproducedbystandardsincludingstandardhybridsapproaches. To this end, a larger basis set [the Pople 6-31+G(d) basis58] was also applied. At this level of theory, the computed interactions energies (corrected for the basis set superposition error, BSSE59,60) are expected to give a rather quantitative estimate of the stacking energy for these model dimers. In such a way, interaction energies corresponding to a stabilization with respect to their corresponding monomers (also frozen at the X-ray geometry) of 1.0 and 0.7 eV were computed for the head-to-head and tailto-tail dimers, respectively. Even though our model is quite rough, these similar interaction energies reveal that there is no preferential stacking mode. Therefore, it is not surprising that intervening single cocrystallized acetonitrile solvent molecule can result in sharply different crystal packings, highlighting the determining role of secondary interactions beyond the nature of the counteranion, which can even be the same (namely BF4-). This lack of preferential interaction is clearly in line with abovementioned experimentally observed pseudopolymorphism. Finally, to rule out the eventual formation of aggregates in solution, two 1f dimers in different conformations (head-tohead and T-stacked) were fully optimized at the same level of

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Figure 5. Experimental absorption spectra [black full line, ε (M-1 cm-1) on left axis] together with the corresponding computed vertical transition energies (red vertical bars, f in au on right axis). From top left to right bottom: 1, 1f, 2, 2f.

TABLE 3: Principal Computed Electronic Transitions (in nm) with Associated Oscillator Strength (f in au) in Parentheses λmax (f) 1 1f 2 2f

205 213 213 219

(0.14) (0.52) (0.24) (0.10)

230 226 215 222

(0.09) (0.43) (0.14) (0.75)

231 279 238 224

(0.06) (0.21) (0.35) (0.37)

308 285 270 261

(0.61) (0.28) (0.15) (0.14)

320 316 335 265

(0.22) (0.55) (0.10) (0.51)

theory. In all cases, the dimers are computed to be unstable with the corresponding monomers, in agreement NMR experimental outcomes.28 Both the intermolecular electrostatic repulsion and the intrinsic sizable curvature of the fused molecular scaffold are likely to explain the poor aggregation propensity evidenced for naked 1f molecules in solution.28 3.3. The Pyridinium-Based Systems as Chromophores: Insights into Absorption Properties. The most striking differences between branched and fused pyridinium-based architectures are expected to reside in their spectral (absorption and emission) properties, since embedding a pyridinium fragment within an extended π system is anticipated to sharply vary the overall molecular orbital pattern with respect to the corresponding branched architectures. In particular, for fused species a densification of the excited states and their shift to lower energies are expected, giving rise to molecular systems absorbing more strongly in the visible part of the spectra and to overall more complex spectral patterns. Inspection of both experimental and computed absorption spectra of 1, 2, 1f, and 2f (Figure 5) fully confirms this expectation. If the electronic spectra of branched pyridiniums (in particular tetrabranched ones of type 1) are characterized by a simple spectral pattern (typically a single band at ca. 310 nm),21,28 fused species 1f and 2f present complex spectral features, displaying up to 10 distinct bands in the UV-visible region.28,61

328 (0.06) 368 (0.11) 283 (0.11)

340 (0.16)

396 (0.11)

313 (0.14)

319 (0.13)

329 (0.57)

396 (0.19)

TD-DFT calculations, performed for all branched and fused derivatives, allow one to define clearly the nature of these absorption patterns. It is worth reminding the reader that gaining theoretical insights into the absorption properties of both the native and the reduced forms of these pyridinium-based systems could help in predicting whether these molecules could be used as efficient intramolecular charge transfer probes. To this end, the electrochromic behavior of the most promising fused electrophore (1f) was also assessed at the same level of theory. For all systems, the computed transitions are reported in Table 3 and in Figure 5. Let us first discuss in more detail the electronic transitions computed for branched systems 1 and 2. System 1 shows two main (i.e., with largest oscillator strength, f) electronic transitions at 320 and 308 nm giving rise to the absorption band experimentally measured ca. 310 nm. Both transitions show an intramolecular charge transfer (ICT) character (Figure 6) from phenyl rings at positions 2, 6 and 4 to the central pyridinium ring, where the lowest unoccupied molecular orbital (LUMO) is centered (Supporting Information, Figure SI.2). On the other hand, for system 2 no transition with large oscillator strength is computed in the near-UV spectral domain, in line with the experimental issue showing no dominant band but only a series of closely lying weak absorptions (Figure 5).

Theoretical Insights into Pyridiniums via DFT

Figure 6. Computed change in electron density associated with the S0fS1 transition (contour value 0.0025 au). Green (positive) and red (negative) lobes are related to an increase/depletion of electron density upon excitation.

Analysis of the near HOMO-LUMO molecular orbitals of 2 shows indeed that the LUMO is still centered on the pyridinium core (Supporting Information, Figure SI.2) and that the lowest energy transitions, computed at 368 and 335 nm, still have a partial ICT character from the peripheral phenyl rings to the central pyridinium core (Figure 6). Nevertheless, due to its enhanced intramolecular (interbranch) steric hindrance, with all the peripheral aryl rings lying almost perpendicularly to the central pyridinium ring in the case of 2, the intensity of the low-lying transitions is drastically reduced. Moreover, in the case of 2, the ICT is mainly occurring from the phenyl ring linked at position 4 to the pyridinium core, while for 1 the ICT mainly involves a transition from the phenyls at positions 2 and 6 (Figure 6). To understand if the difference between 1 and 2 could partially be ascribed to inductive donor effects, due to the presence, in the case of 2, of a methyl substituent on the phenyl at position 4 or related only to the different connectivity around the pyridinium core, a modified system 2 where the methyl group is replaced with a hydrogen atom (2H, Supporting Information, Table SI.2) was also investigated. Inspection of the ground state structural parameters of 2H (Supporting Information, Table SI.2) shows no relevant difference with respect to the parent molecule, 2. Furthermore, both the computed LUMO and change in density associated with the first electronic transition (Supporting Information, Figure SI.3) displays a practically identical pattern to those computed for molecule 2. Therefore, we can attribute the different ICT patterns observed for 1 and 2 to the different connectivity at the pyridinium ring (4 substituent vs 2, 6 substituents, respectively) and not to the donor inductive effect related to the presence of the peripheral methyl group. The analysis of the difference in total electron density computed when going from the ground to the excited state (that is of the ICT patterns) can also be related to the different photocyclization sites observed for 1 and 2. In particular, since photobiscyclization requires the breaking of CH bonds at the excited state for rearomatization,28 one can reasonably assume that when the electron density of adjacent phenyl rings is depleted (at the excited state), their H atoms become more acidic (or reactive) accordingly, that is, more prone to be abstracted. By inspection of Figure 6, it is clear that upon excitation in the

J. Phys. Chem. A, Vol. 114, No. 32, 2010 8439 case of 1, the phenyl rings at positions 2 and 6 are substantially deprived of electron density as a result of the ICT, thereby weakening their C-H bonds and making their H atoms more acidic. Cyclization is thus expected to involve these two rings. On the other hand, in the case of 2, the phenyl ring at position 4 is definitely the one that is the most deprived of electron density upon excitation and it rules the side (and site) of photobiscyclization. The analysis of the vertical transitions computed for the fused systems (1f and 2f) provides one with a completely different picture of the electronic landscape. Overall, for both 1f and 2f the computed transitions’ patterns closely fit the experimental one (Figure 5). A slight, but systematic bathochromic shift of the computed values with respect to the experimental ones without band inversion, appearance, or disappearance can be noticed. For 1f in water, this shift was attributed mainly to the limited basis set used.62 Hence, one may consider that transition energies reported in various tables (absorption, emission), which were all calculated using the double-ζ quality basis, are all slightly underestimated.62 All transitions occurring in the UV-vis region for both 1f and 2f are of π-π* type with no significant ICT character, as previously reported in a detailed analysis of the spectral properties for 1f computed in water.62 Indeed, for both fused compounds, the first transition (at 396 nm) corresponds to an HOMO-LUMO excitation, both orbitals being centered on the fused polycyclic moiety with the LUMO presenting a larger contribution on the pyridinium ring, especially in the case of 2f (Figure 6). Nevertheless, for both 1f and 2f a sizable contribution of the pendant phenyl ring(s) to the LUMO is also observed (Supporting Information, Figure SI.2).63 This contribution is essential to understand and to explain the spectral behavior around 400 nm. In fact, if in this spectral region only one electronic transition is computed for both compounds, experimentally two closely lying bands are recorded, their absorption maxima being at 407 and 429 nm for 1f and 402 and 423 nm for 2f (in MeCN). On the bases of both the relatively small (and constant, for affiliated molecules)61 energetic separation between the two measured bands (ca. 0.152 eV/1230 cm-1 for both 1f and 2f) and the presence of fused polycyclic cores, the question about the vibronic nature of these bands was experimentally raised. From the computed vibrational spectra, at the ground state these quite rigid systems display 10 and 19 normal modes with a frequency lower than 207.4 cm-1 (that is the thermal quantum at room temperature, 298.5 K) for 1f and 2f, respectively. As previously mentioned, one of the lowest of them (computed at 43 and 25 cm-1 for 1f and 2f, respectively) corresponds to the torsion of the pendant phenyl ring(s) with respect to the heterocyclic planar core of the molecule. These observations further substantiate the possible vibronic nature of the two bands observed around 400 nm. To assess this point, the smallest system (1f) was fully optimized in its first excited state (S1) at the TD-DFT level using the same level of theory applied for the ground state (PBE0 in acetonitrile) and its vibrational frequencies at the excited state were subsequently computed. Harmonic vibrational frequencies computed at the excited state show 10 vibrations below the thermal quantum, one of the smallest (at 44 cm-1 for 1f) corresponding to the torsion of terminal phenyl ring as in the case of the ground state. It is worth noting that due to the very low frequencies associated with the phenyl torsion in both the ground and the excited states, we expect the thermal effect to

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Figure 7. First band of the absorption spectra of 1f computed in acetonitrile at T ) 300 K including vibronic coupling effects. Intensity is given in arbitrary units.

be significant and the harmonic approximation to be only qualitatively correct in describing the thermally excited states associated with these low-frequency oscillators. Nevertheless, beside this approximation, the level of theory used is still sufficient to validate the assignment of the electronic absorption spectra and to confirm the vibronic nature of the first two observed bands. The 0-0 transition energy for 1f is computed at 2.93 eV/422 nm in contrast to the vertical excitation energy computed at 3.13 eV/396 nm and in better agreement with the experimental data (429 nm). The spectrum computed for 1f at 300 K, when considering the Boltzmann population of vibrationally excited oscillators both for the ground and excited states, is given in Figure 7. Two absorption maxima (at 422 and 399 nm) are computed: the splitting of the two bands (1370 cm-1) is in very good agreement with the experimental one (1230 cm-1), the absolute shift in absorption maxima energies being related to basis set effects. Furthermore, the relative intensities of the two bands are also in qualitative agreement with the experimental data. Finally,sincepyridinium-containingsystemssuchasviologens64,65 are widely recognized for their suitable redox and electrochromic features, it is important to assess whether or not expanding the molecular scaffold of a pyridinium also contributes to improve its poor electrochromic properties. In particular, if the reduced 1f species (hereafter 1fred) retains a well-structured spectral signature in the UV/vis domain moreover markedly different from the native compound, this species could be used as a valuable probe for photoinduced electron transfer processes. 1f was selected since, experimentally, two well-separated monoelectronic reduction waves are measured.28 As a consequence, it is experimentally possible to obtain the spectral signature of 1fred in a pure form. From the theoretical side, the absorption spectra of both 1fred and reduced N-methylpyridinium (MethPy) (hereafter MethPyred) were computed. Both the experimental one-electron reduction (performed at -1.2 V vs SCE in acetonitrile solution) and the calculations of 1fred provide with an ill-defined spectrum without dominant pattern as derived from Table 3 and shown in Figure 8. Furthermore, the computed spectral pattern of 1fred somehow includes the characteristic bands of the bare MethPyred (MethPy ) 1-methylpyridine) core (Figure 8). Therefore, it is clear that expanding the π system improves the absorption properties of

Peltier et al.

Figure 8. Experimental absorption spectra (full line66) of 1fred together with the computed vertical transition energies (vertical bars) for 1fred (black dashed) and MethPyred (red solid).

Figure 9. Computed spin density of 1fred (left) and MethPyred (right). Isocontour value 0.0025 au.

pyridinium-based systems but does not alter their electrochemical behavior, which very closely resembles that of the parent pyridinium. A possible explanation is related to the large contribution of the pyridinium core to the LUMO even in the fused systems and, consequently, to the very similar pattern of spin density expected for 1fred and MethPyred. The computed spin density for the reduced systems (reported in Figure 9) confirms that beside their completely different molecular backbones 1f and MethPy show very similarsand poorselectrochromic behavior upon reduction: based on a Mulliken density partition scheme, even in the case of 1fred a fraction of 0.7 |e-| is still computed to be localized on the pyridinium ring, in spite of the extensive π conjugation. 3.4. The Pyridinium-Based Systems as Luminophores: Insights into Emission Properties. The quantitative estimation of both phosphorescence and fluorescence energies for relatively complexseven though purely organicssystems, is still a demanding task, even for TD-DFT-based approaches. Proper inclusion of solvent effects further increases the computational burden while known failures of DFT approaches in the description of through-space charge transfer (CT) excited states67 can dramatically develop upon excited state relaxation, thus particularly affecting the luminescent properties (both in terms of structure and energetics). Since all transitions involved in the emission are of π-π* character, with relevant ICT character in the case of branched architectures, these compounds are expected to provide a solid benchmark for our computational approach. Computed and

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TABLE 6: Computed Emission Energies (in nm) for Fluorescence (λfl) and for Phosphorescence (λph) with Respect to Experimental Emission Data Recorded at Low Temperature,28 in Parentheses λfl 1 1f 2 2f a

381 (390) 427 (439) 432 (395) NCa (430)

λph 443 473 456 506

(450) (494) (455) (498)

NC ) not computed.

If the 0-0 transition (computed at 427 nm) is practically coincident with respect to the computed vertical emission, a significant vibronic progression shows up in the simulated spectra (Figure 10). In particular, the calculated pattern closely resembles those of the absorption spectra, featuring the characteristic mirror image attached to fluorescence emission, with two bands separated by ca. 1370 cm-1 clearly visible, in very good agreement with the low-temperature experiments showing the bands, with the same lifetimes (that is, related to the same emitting state) with a separation of 1150 cm-1.28 4. Conclusions and Perspectives

Figure 10. Emission spectra of 1f computed in acetonitrile at T ) 77 K including vibronic coupling effects. Intensity is given in arbitrary units.

experimentally observed low-temperature emission energies are reported in Table 6. First, it is worth noting the quantitative agreement between the computed and experimental energies in terms of both absolute transition energie values and relative shifts when going from branched to fused architectures. Analyzing the absolute values of fluorescence and phosphorescence energies, it is worth noting a significant red shift upon pericondensation, thus enhancing the photophysical properties in the visible region. In agreement with the experimental data, significantly larger Stokes shifts (SS) for fluorescence are computed for branched architectures with respect to the corresponding fused analogues. In particular, a SS of ca. 5000 and 4026 cm-1 are computed for 1 and 2, while only a SS of 1833 cm-1 is predicted for 1f. This large difference can be ascribed to the previously discussed larger structural relaxation (related to the adiabatic planarization of the phenyl rings) taking place within the branched systems. Furthermore, the smaller SS obtained for 2 with respect to 1 can be rationalized by taking into account the greater stiffness of the planarization motion computed for 2, as discussed in the structural section. In particular, the presence of six aryl substituents in the direct surrounding of the pyridinium core contributes to markedly rigidifying the overall system 2, thereby affecting the computed SS and blue shifting its fluorescence with respect to the less burdened tetrasubstituted compound 1. Finally, since the inclusion of vibronic effects was of primary importance for the accurate description of the absorption spectra of the fused system 1f, its relevance for emission properties was also tested. In particular, the emission spectra computed in acetonitrile at 77 K when including vibrational effects for 1f is reported in Figure 10.

Let us first focus on the technical evaluation of the computational approach here applied. Even if relatively simple and computationally cheap (due to the implicit solvent model and to the relatively small basis set used), this level of theory is able to reproduce fine structural and electronic effects both at the ground and the excited state, confirming the good performances of the hybrid PBE0 exchange correlation functional already reported in literature.33 In particular, the computed absorption and emission energies as well as the band shape profiles calculated when including vibronic effects confirm the good description provided at this level of theory of both the ground and excited states’ potential energy surfaces. Indeed, it is worth noting that in the case of leading π-π staking interactions the explicit inclusion of dispersion forces, and thus the use of purposely developed functionals (such as B97-D), is mandatory. On the basis of the results obtained in this work, from a purely methodological point of view we can conclude that this level of theory is able to qualitatively and quantitatively predict the ground and excited state behavior of relatively complex organic molecules, when fine effects, such as vibronic coupling, are properly included. More interestingly, from a chemical point of view, the results obtained allowed one to assess several points concerning the structural and photophysical behavior of these expanded pyridinium systems: (a) From a structural point of view, we demonstrate that the planarity observed for the fused architecture of type 1f in the solid state is related to stacking and crystal packing but does not reflect an intrinsic feature of its backbone, which is computed to display a sizable curvature, instead. Furthermore, stacking forces are computed to be relatively weak, so that different 1-3D arrangements are experimentally obtained only in the solid state (no aggregation in solution for 1f) by tuning the crystallization conditions. With regard to the novel hemifused compound 2f, the noticeable structural distortion of the whole backbone is well-reproduced and is demonstrated to originate from intramolecular steric constraints. (b) The enhancement of the photophysical properties of the fused systems is related to a densification of states accompanied with a raise of the energy of the HOMO upon pericondensation, thereby inducing a red-shift of the lower energies transitions and a complexification of the absorption spectra in the visible region upon ring fusion. The vibrational progression responsible for the peculiar shape of the first absorption band and the emission band of the fused compounds has been related to the torsion of the pendant phenyl ring with respect to the fused part of the molecular backbone. (c) In the case of the branched compounds, a partial intermolecular charge transfer character of the first transition

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can be derived from the analysis of the change in electron density associated with the S0f S1 transition. (d) The different photocyclization sites experimentally observed for 1 and 2 could be rationalized by inspection of the change in electron density associated with the first electronic excitation. In particular, the phenyl rings being the most involved in the ICT transitions as donor groups (that is partly deprived of electron density at the excited state) are more likely to photocyclize due to an enhancement of the weakening (that is, activation) of their C-H bonds. This qualitative analysis can be extended to othersmore complexsbranched pyridiniums systems providing a simple rule to evaluate the reactivity of these systems toward photocyclization. (e) The LUMO of various expanded pyridinium studied is computed to remain rather insensitive to the degree of extension of the π-conjugated system, since it is found to remain essentially localized on the pyridinium core in all cases, moreover lying at roughly the same energy within branched and fused species. This localization of the LUMO on the pyridinium regardless of the branched or fused nature of the species explains the experimental observation that the potential attached to the first reduction process is roughly the same for all species.28 (f) If fused compounds display a clear enhancement of their photophysical properties (as compared to branched parents), their electrochromic behavior remain poor. Indeed the corresponding reduced species retain the spectral features of the parents branchedspyridinium systems, including mere reference Nmethylpyridinium. This observation, in line with computational issues, is explained by the fact that the first reduction is a localized phenomenon centered on the pyridinium core. Thus, the possibility of using the fused architectures as internal probe for intramolecular photoinduced electron transfer processes can, unfortunately, be ruled out. On these bases, the computational approach here applied is currently envisaged for the joint theoretical and experimental design of new expanded pyridiniums displaying not only enhanced photophysical features but also appealing electrochemical properties. The final aim is to obtain truly multifunctional pyridiniums of renewed interest in the various fields of chemistry. Acknowledgment. This work is dedicated to the memory of Dr. Gaston Berthier, a brilliant scientist and a cultivated human being with whom we had the chance to work and discuss. I.C., C.A., C.P. and P.P.L. are grateful to the French National Agency for Research (ANR) “programme blanc” (NEXUS project; No. BLAN07-1_196405). Tangui Le Bahers and Ciro A. Guido are thanked for helpful discussions. Supporting Information Available: Additional figures and tables as descussed in the text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gebicki, J.; Marcinek, A.; Zielonka, J. Acc. Chem. Res. 2004, 37, 379–386. (2) Sliwa, W. Curr. Org. Chem. 2003, 7, 995–1048. (3) Binnemans, K. Chem. ReV. 2005, 105, 4148–4204. (4) Kumar, S.; Kumar Pal, S. Tetrahedron Lett. 2005, 46, 4127–4130. (5) Neve, F.; Crispini, A.; Francescangeli, O. Inorg. Chem. 2000, 39, 1187–1194. (6) One of the first use of pyridiniums was as herbicide, in the form of the benchmark 1,1′-dimethyl-4,4′-bipyridinium, known as “paraquat” or “viologen”. It is worth noting that this activity actually relies on the electronaccepting properties of the molecule. See Thummel, R. P.; Lefoulon, F.; Chirayil, S.; Goulle, V. J. Org. Chem. 1988, 53, 4745–4747.

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J. Phys. Chem. A, Vol. 114, No. 32, 2010 8443 (60) Simon, S.; Duran, M.; Dannenberg, J. J. J. Chem. Phys. 1996, 105, 11024–11031. (61) Katritzky, A. R.; Zakaria, Z.; Lunt, E. J. Chem. Soc., Perkin Trans. 1 1980, 1879–1887. (62) Peltier, C.; Laine´, P. P.; Scalmani, G.; Frisch, M. J.; Adamo, C.; Ciofini, I. J. Mol. Struct. (THEOCHEM) 2009, 914, 94–99. (63) At this point it is worth noticing the large contribution of the atom C44 (see Figure 1) to the LUMO of 2f, reminiscent of that of atom C4 for the pyridinium core. This peculiar contribution is likely to explain the lack of reversibility of the first reduction process experimentally observed, since position C44 is not sterically hindered, that is, protected from nucleophilic attack (i.e., reductive dimerization and pimerization)..28 (64) Wardman, P. J. Phys. Chem. Ref. Data 1989, 18, 1637–1755. (65) Takahashi, K.; Nihira, T.; Akiyama, K.; Ikegami, Y.; Fukuyo, E. J. Chem. Soc., Chem. Commun. 1992, 620–622. (66) Spectroelectrochemical measurements were performed at-1.2 V vs SCE in acetonitrile solution (0.5 mL; [1f] ) 5 × 10-4 M) using TBAPF6 as support electrolyte. The experiments were recorded with a JASCO V570 spectrophotometer using an Omni-cell with CaF2 windows, connected with an EG&G 273A potentiostat. (67) Dreuw, A.; Head-Gordon, M. J. Am. Chem. Soc. 2004, 126, 4007–4016.

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