Carbon-Bridged Phenylene-Vinylenes: On the Common Diradicaloid

Article pubs.acs.org/JPCC

Carbon-Bridged Phenylene-Vinylenes: On the Common Diradicaloid Origin of Their Photonic and Chemical Properties Rafael C. González-Cano,† Simone Di Motta,‡,§ Xiaozhang Zhu,∥ Juan. T. López Navarrete,† Hayato Tsuji,∥,⊥ Eiichi Nakamura,∥ Fabrizia Negri,*,‡,§ and Juan Casado*,† †

Department of Physical Chemistry, University of Málaga, Campus de Teatinos s/n, Málaga 29071, Spain Dipartimento di Chimica ‘G. Ciamician’, Università di Bologna, Via F. Selmi, 2, 40126 Bologna, Italy § INSTM, UdR Bologna, Bologna, Italy ∥ Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan ⊥ Department of Chemistry, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka 259-1293, Japan ‡

S Supporting Information *

ABSTRACT: In this paper, experimental and computational investigations are combined to analyze the impact of the diradical character in the photonic and chemical properties of two tetracyano quinodimethane derivatives of phenylene-vinylenes. The photonic properties are evaluated with high-level quantum-chemical calculations and are rationalized in the context of the fourlevel model typical of diradical molecules, including one triplet and three singlet states, whereas the diradical extent in the ground electronic state is shown to be the driving force for the dimerization reactions. To this end DFT, CASSCF, and CASPT2// CASSCF calculations are carried out and complemented with experimental UV−vis−NIR absorption and emission measurements, as a function of the temperature along with resonance Raman spectroscopy investigations. The study of diradical molecules with enhanced chemical stability in the framework of organic electronics is an important field of research in organic π-conjugated molecules with direct applications in organic electronics.

I. INTRODUCTION The term diradical alludes to molecules with two unpaired electrons where their interelectron (interspin) interaction is sufficient to define a manifold of singlet and triplet electronic states (Scheme 1).1 The ground electronic state can be either triplet or singlet depending if the bonding interaction between the two odd electrons is greater or smaller than the exchange coupling.2 In the case that the bonding fraction is significant, as it is for para-substituted benzenes (or in general in para-substituted aromatic rings), the so-called Kekulé diradicals,3,4 the ground electronic state is a singlet while if exchange dominates (for instance in meta-substituted benzene rings) it results in a high spin triplet. 5,6 In the former case of homosymmetric diradicaloids, the three singlet states correspond to (i) the ground electronic state (S0 in Scheme 1), (ii) the singly excited or single exciton (H → L) state (S1 for closed-shell and intermediate diradical character and S2 for large diradical character in Scheme 1), and (iii) a doubly excited or double exciton (H,H → L,L) state (S2 for closed-shell and intermediate diradical character and S1 for large diradical character in Scheme 1). Together with the triplet, the interplay of these four © XXXX American Chemical Society

Scheme 1. Four Levels (One Triplet and Three Singlet States) Description as a Function of the Diradical Character in Benzo-quinodimethane Moleculesa

a

n, number of repeating units.

Received: August 11, 2017 Revised: September 26, 2017 Published: September 26, 2017 A

DOI: 10.1021/acs.jpcc.7b08011 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

oligomer21,22 units at both ends with two and three pquinodimethane units (see Scheme 3). These compounds are characterized by the formation of a quinoidal structure in the benzene ring which extends from one extreme to the other: whereas QM1CN displays a well-defined quinoidal arrangement with a singlet closed-shell (CS) ground electronic state, QM2CN has a stable ground electronic state with singlet openshell (OS) character due to the combined effect of aromaticity gaining and strain release in the five membered rings upon recovery of the stilbenoid vinylene bridge. In this paper, we explore the excited state properties of these compounds focusing on the photonic properties which, in turn, are related to the energy order and distribution of the excited states, their nature, and optical activity. Particular emphasis is given to the nature and energy of the singlet excited states and their connection with the singlet open-shell ground state as all of them constitute the intrinsic fingerprint of the individual diradical species. Beside assessing the diradical character and excited state pattern, the second objective of this study concerns the chemical reactivity of the diradicals by probing the propensity to form stair-chain oligomers by bonding between the unpaired electrons. While the quinoidal form is still persistent in the isolated monomer, this disappears in the reaction event leading to a full aromatic product. Nonetheless, these processes, from a chemical point of view, are reversible and are associated with a strong change in the color of the samples, a color exchange that is tunable with soft external stimuli such as temperature. To this end, we report a thorough characterization in terms of a set of spectroscopic properties (UV−vis−NIR absorption, Raman, and resonance Raman) in combination with variable temperature measurements and 1H NMR. The experimental study is fully complemented with computational investigations carried out at DFT, CASSCF, and CASPT2//CASSCF levels of theory. In particular, CASSCF was required to afford the analysis of excited states with multiexcitation character and to investigate at multiconfigurational level the diradical character of the ground state. We show in this study how the subtle interplay between quinoidal and aromatic forms provide systems with multifunctional properties, highly desirable in organic materials applications.23

states dictates the photonic properties of these Kekulé singlet diradicals.7 At the intermolecular level, bonding between two unpaired electrons of different π-conjugated diradicals often operates through π-electron fixation as it is the case of dimerization and polymerizations of para-xylylene diradicals (see Scheme 2 for Scheme 2. Thermal Dimerization and Polymerization of para-Xylenea

a

p-DQM is the para-quinodimethane intermediate.

the well-known polymerization of xylylene8−11). In this case, coupling between the unpaired electrons competes with intermolecular inter-radical interactions. Hence, when intermolecular chemical bonding is significant, the above-mentioned photonic properties are completely erased and new ones appear. In this regard, dimerization reactions forming rather long CC bonds, between the carbons bearing the radical centers, have been described mostly in monoradical12−14 species and more recently in diradicals15−17 as well, both causing notable chromic effects. Diradicals also have been described to form stair-chain polymers15,16 by bonding between one radical center and another of the vicinal monomer nucleating the elongation step of the reaction polymerization mechanism. Quinoidal structures built on pro-aromatic rings are very suitable to stabilize Kekulé diradicals by playing with the number of rings.18−20 The gaining of aromaticity (Clar’s sextets) of various pro-aromatic rings is the driving force to compensate the energy required to break a double bond and to form a diradical. In contrast with the general situation where diradicals are regarded as very reactive species, thus with short lifetimes and hard to characterize, the aromatic Kelulé diradicals are rather stable given their fractional bonding, what allows us to consider their chemical properties in terms of controllable reactions that will take place in a small regime of free energy variations. Recently some of us have reported the synthesis and characterization of carbon-bridged phenylene-vinylene

II. RESULTS AND DISCUSSION II.1. Photonic Properties. QM1CN and QM2CN show absorption and emission spectra with peculiar characteristics.21 In particular, these rigid systems display numerous absorption peaks which is reminiscent of their particular electronic structure such as outlined in Scheme 1. Most notably, the appearance of an emission signal is an unusual property also because of the characteristic small optical gap of quinodimethane quinoidal molecules.24,25 No exhaustive elucidations

Scheme 3. Chemical Structures of the Studied Compounds, QM1CN (Left) and QM2CN (Right)

B


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indeed, from the intrinsic ionic character arising from the excitation of one of the unpaired electrons (i.e., SOMO-α) of the singlet diradical into the other (i.e., SOMO-β). Indeed, in a localized orbital approach, if we consider ϕΑ(1) and ϕΒ(2), the localized orbitals containing each unpaired electron, the ground state is described as ϕΑ(1)ϕΒ(2) − ϕΒ(1)ϕΑ(2) combination while the ionic singlet excited state can be described as the ϕΑ(1)ϕΑ(2) − ϕΒ(1)ϕΒ(2) combination. In terms of delocalized orbitals ψ+ and ψ− (linear combinations of the localized orbitals) this ionic state is described as 1ψ+ ψ−.7 This excitation is the HOMO → LUMO one-electron promotion: the HOMO is of quinoidal-like character within the benzene rings, while the LUMO transforms this pattern to somehow aromatic-like implying a significant electronic reorganization within the excitation in order to transmit the promotion from one side to the other. Another very important feature of the S0 → S2 excitation is the associated vibronic structure. We have calculated the Franck−Condon (FC) vibronic structure of the electronic excitation (see also the spectra depicted in Figure 1 with dotdashed lines) at the B3LYP/6-31G** level and obtained two main peaks, the 0−0 components and a vibronic replica. The first transition is assigned to the most intense bands as discussed in the previous paragraph (to facilitate the comparison with experiment, the 0−0 bands of the S0 → S2 simulated spectra in Figure 1 were shifted to the experimental 0−0 bands). The vibronic replica following the most intense band for these excitations can be associated with the experimental bands measured at 578 nm in QM1CN and at 769 nm in QM2CN (see Figure 1). These vibronic bands are spaced by 1374 cm−1 in QM1CN and 1338 cm−1 in QM2CN, in agreement with the largest predicted Franck−Condon activity (see the Supporting Information, SI) for these two vibrations. Interestingly, the FC active vibrations correspond to the intense bands of the Raman spectra of the two compounds (see Figure 2) measured at 1300 and 1367/1151 cm−1 when these are taken in resonance with the absorption bands corresponding to the S0 → S2 excitations: λexc = 532 nm for QM1CN in resonance with the vibronic bands at 578/627 nm and λexc = 785 nm for QM2CN in resonance with the vibronic bands at 769/850 nm in Figure 1 (see Figure S1 for the theoretical Raman spectrum of QM1CN). Conversely, if the Raman spectrum is taken with the λexc = 1064 nm in the case of QM1CN, the spectrum displays the strongest Raman band at higher energies, 1600 cm−1, revealing that the nature of the excitation has changed (this will be discussed in the next section). All these strong Raman bands correspond to C = C/C−C stretching vibrational modes delocalized over the whole molecule, in consonance with these modes being also FC active in the S0 → S2 excitation and according to the typical vibronic activity in related polyenic systems.26−28 In summary, the TD-DFT vertical excitation energies, in line with previous calculations for quinoid systems,25−28 reveal a low lying strongly allowed excited state for all the systems investigated, accounting for the strong feature observed in the experimental absorption spectra of QM1CN and QM2CN (see Figure 1). TD-DFT theory, given its single-excitation nature, is able to predict with accuracy the intensity and wavelength of the first one-photon allowed excitation that in both molecules corresponds to the S0 → S2 excitation. For the model molecules featuring hydrogens instead of phenyl or aryl groups, this

of the structure and shape of the absorption and emission spectra of these types of diradicaloid molecules in terms of the relevant low energy lying excited states exist in the literature, and we will take advantage of QM1CN and QM2CN to get insights on these structure−property relationships. II.1.A. Absorption Properties: The S0 → S2 Excitation from TD-DFT Calculations. Figure 1 displays the experimental absorption spectra of QM1CN and QM2CN together with the theoretical spectra predicted by TD-B3LYP/6-31G** calculations.

Figure 1. (a) Top: experimental absorption spectra in CH2Cl2 of QM1CN (blue) and QM2CN (red). Bottom: the corresponding computed vibronic structures for the S0 → S2 (dot-dashed line) and S0 → S1 (dotted line) transitions from TD-DFT and CASSCF calculations, respectively (blue lines for QM1CN and red lines for QM2CN). (b) Top: experimental absorption and emission spectra in CH2Cl2 of QM1CN (blue) and QM2CN (red). Bottom: computed vibronic structures (Franck−Condon (FC) activity) for the S1 → S0 transitions (solid lines) from CASSCF calculations together with the theoretical absorption spectra in panel a of this figure.

According to this, the first one-photon active excitation is due to the S0 → S2 for both compounds which is predicted at 595 nm (603 nm at the TD-DFT/OB3LYP level) with oscillator strength f = 1.80 for QM1CN and at 793 nm (799 nm at the TD-DFT/OB3LYP level) with oscillator strength f = 1.23 for QM2CN. These calculations compare very well with the experimental absorptions at 627 nm (log ε = 5.12) and 850 nm (log ε = 5.37), respectively, for QM1CN and QM2CN. From the perspective of the diradical structure of these molecules, the rather large experimental molar absorptivity and theoretical oscillator strength for these bands are originated, C


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Figure 2. Left: Raman spectra of QM1CN in resonance with the highest (532 nm) and lowest (1064 nm) energy parts of the absorption spectrum. Right: Raman spectrum in resonance with the highest energy component of the absorption band. Spectra are recorded in solid state at room temperature. Shaded bands are discussed.

transition is assigned to the 1 1Ag → 11Bu one electron promotion. II.1.B. Absorption Properties: The S0 → S1 Transition from CASPT2//CASSCF Calculations. Despite the correct extension of the simulated vibronic progression associated with the strongest transition in absorption S0 → S2, the TD-DFT theoretical description is insufficient to account for the experimental absorption spectra of QM1CN and QM2CN, in particular for the bands observed at lower energies (675 and 744 nm for QM1CN and 980 and 1102 nm for QM2CN). The S0 → S1 excitation is at the origin of these low energy features26 which constitute a relevant portion of the photonic properties of these molecules. Therefore, according to the excited state pattern typical for diradicaloids (Scheme 1), we explored, at the CASPT2//CASSCF level, the location of the singlet excited state dominated by the (H,H → L,L) double excitation.7 Based on optimized CASSCF geometries of the ground and double excitonic state, we simulated the FC vibronic structure associated with the S0 → double exciton (S1) transition in QM1CN and QM2CN. This double exciton state is wellknown to be two-photon active for polyenes.29,30 Note however that, because of the flexibility of phenyl and phenyl-C8H17 substituents in QM1CN and QM2CN, the S0-S1 transition is not strictly forbidden by symmetry (as it is for polyenes) in one-photon spectroscopy, and therefore it can show a weak 0− 0 band. Accordingly, we assume that the lowest energy band in the experimental absorption spectra is the true origin of the transition. This assumption is supported by the CASPT2// CASSCF estimate of adiabatic transition energy, see below, and by the fact that the experimental onset of absorption and emission are substantially coincident. The results of the vibronic structure simulations, carried out (see the SI) by determining the Huang−Rhys parameters for each vibrational mode (see also Figures S2−S3), are depicted in Figure 1 by dotted lines. These calculations show that the 744 and 675 nm bands in the absorption spectrum of QM1CN and the 1102 and 980 nm bands in that of QM2CN can be assigned to the origin and to the 0−1 vibronic transition associated with the S0 → S 1 double excitation. The higher energy vibronic components predicted by theory are masked by the strong one-photon allowed transition, as clearly shown in Figure 1. The FC active vibrations responsible for the major vibronic progressions are, as expected,29 the same as those active in the S0 → S2 transitions, corresponding to out of phase oscillations of adjacent CC bonds. The 1064 nm Raman spectrum of QM1CN in Figure 2 shows the spectrum with the λexc = 1064 nm excitation which is in resonance with the S0 → S1 band due to the double

excitation. The strongest Raman band of this spectrum is at 1606 cm−1 which is in consonance with the appearance of vibronic activity for the normal modes at 1667 and 1607 cm−1 (Figures S2/S3, FC = 0.21 and 0.20, respectively). No 1064 nm Raman spectrum in resonance with the double excitation band of QM2CN could be obtained likely due to the interference of fluorescence. II.1.C. Ground and Double Exciton States from CASPT2// CAS-SCF Calculations. The singlet ground electronic state and the double exciton state S1, together with the single exciton state S2 form the low-energy lying set of singlet states for the QM1CN and QM2CN systems.7 While the CASSCF ground state structure of QM1CN compares well with the B3LYP result (see Figure S4), the CASSCF structure of QM2CN is more markedly quinoid than the B3LYP or UB3LYP structures (see Figure S5). Furthermore, at the CASSCF level, QM1CN and QM2CN have unrealistically similar quinoid characters as confirmed by the similar CASSCF diradical character (ca. 9− 10%) predicted for both compounds. This seemingly wrong description results from the unrealistically large energy difference between the ground and the double exciton states at the CASSCF level. The inclusion of the CASPT2 correction drives the energy differences between the two states to values that compare very well with the experimentally determined low energy features in the absorption spectra (see Table S1). Concomitantly, the CASPT2 energy correction further implies a geometry change of the ground state of QM2CN toward a more diradical structure. Unfortunately, however, a CASPT2 geometry optimization was not available in the software package we used, and we were forced to follow an alternative strategy to account for the effect of dynamical correlation on the ground electronic state geometry of QM2CN. From the analysis in section II.1.B, we learn that the bond length changes upon excitation to S1 follow a pattern very similar to the bond length differences between the quinoidal CS and the diradicaloid benzenoidal aromatic OS structures, or equivalently, there is a vibrational mode, that at 1338 cm−1, consisting on the out-of-phase oscillations of adjacent CC bonds (see Figure S3 and S6) that mimics the quinoidal → aromatic transformation (see Figure S4). Therefore, we can generate increasingly diradicaloid ground-state geometries by displacing the initial CASSCF optimized ground-state structure of QM2CN along (i) the mode dominated by the out-of-phase CC stretching character or (ii) the S0-S1 geometry difference (see Figure S6). This is done in Figure 3, left, where the potential energy profiles (PEPs) of both the ground and S1 states are computed at CASSCF and CASPT2 levels for displacements along the S1−S0 geometry change, starting from D


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is the result of an avoided crossing between the ground and first excited state produced by the coupling of both through the 1 b2u Kékulé CC stretching mode. This situation is feasible in QM2CN since (i) the aromatic character of the benzenes in the diradical ground electronic state and (ii) the displacement coordinate in Figure 3 (out-of-phase oscillations of adjacent CC bonds mimicking the quinoidal → aromatic transformation) is intimately connected to the mode 8a of benzene and this is of the same nature than the 1 b2u Kékulé mode in benzene (mode 14).31 II.1.D. Emission Properties in the NIR Region: The Radiative S1 → S0 Transition. The detection of emission in organic π-conjugated molecules in the near-infrared region is uncommon as the reduction of the optical gap accelerates the incidence of nonradiative deactivation. QM1CN and QM2CN thus represent rare examples of true NIR emission, although with small quantum yields, as it is shown in Figure 1. The two observed emissions clearly originate from the double exciton state since, independently of the excitation wavelength used, the same emission is recorded thus ruling out the likely emission from the dipole-allowed state (i.e., against the Kasha’s rule). Given the close energy and vicinity of the two singlet excited states in both molecules (overlapping S0 → S2 and S0 → S1 absorptions), efficient S2 → S1 internal conversion takes place followed by S1 → S0 emission.32 Figure 1 reports the simulation of the emission spectra from the double-exciton state of QM1CN and QM2CN. The agreement with the experiment is good, and the slight differences in the intensity pattern can be justified by the fact that optimized geometries were obtained at the CASSCF level, namely without including dynamical correlation effects such as discussed in the previous section. The frequencies of the normal modes displaying the largest HR factors in the S1 → S0 emission now correspond to values lower than the equivalent in the S0 → S1 absorption which is justified by the larger reorganization energies in S1 compared with S0 for the relevant out-of-phase stretching vibrational modes. The larger frequency values in the excited state discussed in section II.1.C for the particular out-of-phase CC stretch vibronic coupling mode might justify the observation of emission as deduced in our case by the reversal of the reorganization energy values. II.1.F. Singlet Triplet Energy Gap and the Diradical Character. The above calculations estimate a degree of diradical character of 9−10% for QM1CN and of 44% for QM2CN implying, in particular for the latter, a small singlet− triplet gap. These gaps have been calculated at the B3LYP/631G** level and, in particular for QM2CN with intermediate diradical character, imply a small singlet−triplet gap which is the reason for the appearance of the ESR signal in solution of QM2CN at room temperature due to the population of the triplet excited state. Recently we have shown17 that some particular quinoidal molecules are able to dimerize and polymerize through the formation of σ-bonds and that this particular behavior is associated with the existence of small diradical character. This might seem somehow counterintuitive as the larger molecular diradical character is, the better the two radical centers are liberated from intramolecular restrictions to be involved in intermolecular bonding. This opens the scenario of the discussion to analyze the chemical properties of QM1CN and QM2CN, which respectively have small and intermediate

Figure 3. For QM2CN: (left) Potential energy profiles of S0 and S1 at the CASSCF level and including CASPT2 corrections, along a displacement which matches closely the normal coordinate dominated by the out of phase stretching of adjacent CC bonds and (right) reorganization energies (λgr and λexc) associated with the S0 → S1 excitation from CASSCF(12,12)/6-31G* + CASPT2 calculations. The largest value of λexc (0.135 eV) compared to λgr (0.083 eV) suggests a frequency increase in the S1 state of the mode dominated by the out of phase stretching of adjacent CC bonds, due to the coupling between the two states, as shown on the right part of the figure.

the CASSCF optimized ground-state structure (see Table S2 in the SI for details). We place the minimum on the CASSCF S0 PEP at the origin of the graph in Figure 3 which thus shows that a displacement of the S0 state CASSCF minimum toward the minimum of the CASPT2 level produces a remarkably more diradicaloid structure which amounts to a diradical character of ca. 44% as a result of the S1−S0 energy reduction at this level of theory and the concomitant configurational mixture of the singlet states. The CASPT2 results in Figure 3, left, for QM2CN reconcile, on one hand, the computed (1.16 eV) with the observed (1.13 eV) S1−S0 energy difference and, on the other, the expected increase of diradical character resulting from the extension of conjugation in longer members of phenylenevinylene oligomers with tetracyanoquinodimethane units. As discussed above, and shown in Figure S6, the projection of the computed S0−S1 geometry change on ground state normal coordinates shows the largest contribution for the mode dominated by the out of phase stretching of adjacent CC bonds (see Figures S2 and S3, 1338 cm−1 frequency mode for QM2CN). The overlap between the S1−S0 geometry change and this nuclear motion indicates the presence of a strong electron−phonon coupling between the two electronic states and the vibrational mode. A second signature of this large coupling is the magnitude of reorganization energies associated with the S0 → S1 excitation (see Figure 3, right) computed at the CASPT2 level. For QM2CN the excited state reorganization energy is larger than for the ground state. We note that this might result in an unusual excited state frequency increase compared to the ground state, as it has been shown for other quinoidal chromophores26 and as it is well-known for polyenes and carotenoids.29,30 This theoretical feature is interesting since a similar exalted frequency effect in benzene has been reported for the 1 b2u Kékulé vibrational mode of the 1 B2u electronic excited state and explained in terms of the π-distortive character of the π-energy in the ground electronic state of benzene. This E


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The Journal of Physical Chemistry C diradical characters, with regard to the propensity to intervene in σ-dimerization and polymerizations reactions. II.2. Chemical Properties. Dimerization, oligomerization, and polymerization reactions are well documented chemical processes that affect monoradicals and diradicals of πconjugated molecules.12−17 These take place through the formation of long weak CC bonds by cofacial coupling of the π-conjugated platforms, and these reactions impart sudden changes in the color of the samples or chromism. II.2.A. Dimerization/Polymerization on Cooling and in Solid State. Figure 4 displays the UV−vis−NIR absorption

where M represents the monomer and D (M2) the dimer product. According to this stoichiometric reaction we are able to calculate the equilibrium constant and its dependence with the temperature (Figure S7), from which we obtain the standard dimerization enthalpy which amounts to −1.43 kcal/ mol revealing exothermic character for the dimerization, which is thus favored by cooling. The same UV−vis spectral evolution of the monomer at room temperature by cooling was detected at room temperature by forming an as spun thin solid film (see Figure 4b), suggesting that very similar aggregated species to that formed in the frozen solution are formed in the solid. II.2.B. DFT Insight on the Dimerization Reaction. In order to gain insight into the molecular and electronic structure of the dimerization product, DFT calculations were performed for QM1CN in the reaction formation of σ-bonded dimers in Figure 5. Note that phenyl groups replacing the aryl groups have been considered due to the significant effect (i.e., steric hindrance) in the intermolecular couplings. This strong steric repulsion already rules out the formation of double σ-bond formation in a cyclic dimer (two intermolecular CC bonds between two monomers). Thus, we have scanned at the UM052X/6-31G** level of theory the potential energy curve for the formation of singly bonded open σ-dimers in which one intermolecular CC bond might be established in a face-to-face configuration in the terminal parts of the molecules (the molecular region liberated from steric crowding, see Figure 5). For the formation of this singly bonded open σ-dimer, a welldefined minimum in the dimerization potential energy curve at a distance of 1.6−1.7 Å is found which is characteristic of the formation of a long strained σ-bond. The formation of this long CC single bond displaces the dicyano groups from the molecular plane as a result of the pyramidalization of these carbon atoms which evolve from a sp2 π character to a sp3 hybridization. Energetically, at the UM05-2X/6-31G** level, a value of the variation of the free energy of dimerization (at 298 K) of −8.22 kcal/mol is estimated for the reaction yielding the open singly bonded σ-dimer, which is in agreement with the spontaneous dimerization reaction observed experimentally. Further experimental validation of the formation of this class of open σ-dimer is the detection of indirect paramagnetic activity upon its formation. We have followed the cooling process by 1 H NMR spectroscopy. At room temperature a multiplet of sharp bands at the aromatic H region (7.0−7.5 ppm) can be observed that broadens its structure when cooling to 190 K (see Figure S8) when the formation of the open σ-dimer is completed. This 1H NMR broadening is the consequence of the spin-electron coupling with the spin-nuclei that takes place due to the presence of a magnetically active double radical or biradical species (i.e., the singly bonded open σ-dimer). Figure 4a shows the TD-DFT calculations carried out on the open singly bonded σ-dimer. The main absorption band upon cooling of QM1CN is displaced at lower wavelengths, at 386 nm from the individual spectrum at room temperature, 627 nm, which is nicely reproduced by the TD-DFT/UM05-2X/631G** level calculations in the open dimer at 416 nm from 609 nm in the monomer (i.e., at 470 nm in the open dimer and 642 nm in the monomer at the TD-DFT/B3LYP-D/6-31G** level). II.2.C. Driving Force for the Dimerization/Polymerization in QM1CN. From the point of view of the molecular structures, the main change upon dimerization of QM1CN is the full recovery of the aromaticity in the benzene rings of the

Figure 4. (a) Comparison between the UV−vis absorption spectra of QMCN1 in methylcyclohexane as a function of the temperature on cooling (top) and TD-DFT UM05-2X-calculated vertical transition energies for the QM1CN monomer (green) and σ-dimer (blue). (b) Comparison between the UV−vis−NIR absorption spectra of QMCN1 in methylcyclohexane at 140 K (blue) and as solid dropcast thin film at room temperature (red).

spectra of compound QM1CN in methylcyclohexane as a function of the temperature on cooling. We observe the progressive disappearance of the intense absorption band (S0 → S1 + S0 → S2 excitations) with the strongest peak at 627 nm, largely predominant at 300 K, up to 140 K where this band is fully bleached giving way to the growing of a structured feature at 361/382 nm. This conversion progresses through a welldefined isosbestic point changing the solution color from intense reddish to colorless. The thermal cycle is fully reversible and can be repeated several times. Given that both bands are well separated in the spectrum, it allows to accurately estimate their integrated absorbance at every temperature. Interestingly, these UV−vis thermal spectral changes do not happen for compound QM2CN. Given that cooling fuels the spectral transformation, our starting hypothesis is that an aggregation/ dimerization reaction might take place, such as 2M ⇆ D F


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Figure 5. Top: Potential energy curve, computed at the UM05-2X/6-31G** level, for QM1CN and QM2CN σ-dimers. Here rCC is the distance of the CC σ-bond formed between two QM1CN/QM2CN. Bottom: Recovery of aromaticity in the two compounds after σ-dimerization either from quinoidal QM1CN or from partially aromatic QM2CN monomers.

which the vibronic structure is due to the same vibrational modes. The nature of these vibrations has been recognized in the resonance Raman spectra. The diradical character which drives these photonic properties is also the origin of the chemical reactivity of these molecules in dimerization/polymerization reactions. In particular, the small diradical character of QM1CN produces a large energy gaining by aromatization of four benzene rings in the singly bonded open σ-dimer from quinodal rings in the monomer reactants. We have studied the dimerization reaction by low temperature UV−vis−NIR absorption spectroscopy and DFT calculations. However, QM2CN, which already expresses a significant diradical character and subsequently their aromatic rings are partially aromatized in the monomer state, does not give rise to a sufficient driving force for the dimerization reactions.

conjugated core which is mostly quinoidal in the monomer state. In this case, the recovery of the aromaticity from reactants to products is of four benzene rings (the formation of four Clar’s sextets in Figure 5) which thus becomes the driving force for the formation of the dimers and of the exothermic character of the reaction upon cooling. However, the pronounced diradical character of QM2CN in the monomer state (discussed in previous sections) imparts a significant aromaticity recovery in the starting reactants. In the open singly bonded σ-dimer, the formation of six Claŕs sextets does not represent enough driving force for its formation since these six aromatic rings were already almost established in the two monomers. In this case, in the absence of enthalpy fuelling, the entropic cost of the dimerization energy is sufficient to impede the progression of the aggregation reaction.

III. CONCLUSIONS In conclusion, we have shown that a correct picture of the lowlying excited states, obtained by combining appropriate levels of theory, helps to understanding the interesting photonic properties of these important organic π-conjugated molecules. From a more conceptual point of view, we have disclosed a further example (beside quinoidal oligothiophens26) of typical polyenic character in the pattern of excited states of these derivatives of phenylene-vinylene oligomers. The electronic states of diradical molecules can be suitably represented by a four-state model, three singlet and one triplet, whose energy order has a role in tuning the photonic and chemical properties of these Kekulé molecules. The presence of incipient or well developed diradical character governs the relative energy sequence of these states and this has been shown here for QM1CN and QM2CN. In both cases, the absorption properties are explained by the presence of a one-photon dipole−dipole allowed and a two-photon allowed excitations in

IV. EXPERIMENTAL SECTION IV.1. Spectroscopic Measurements. UV−vis absorption and fluorescence spectra are taken from ref 13 and were recorded on JASCO V-570 spectrometers. The measurements were carried out in degassed, anhydrous dichloromethane. All solvent used were of spectroscopic grade purchased from Aldrich. No fluorescent contaminants were detected upon excitation in the wavelength region of experimental interest. For the FT-Raman spectra a module of a VERTEX 70 FT-IR spectrometer with a continuous-wave Nd−YAG laser working at 1064 nm was employed for excitation, at a laser power in the sample not exceeding 30 mW. Raman scattering radiation was collected in a back-scattering configuration with a standard spectral resolution of 4 cm−1. 2000 scans were averaged for each spectrum. The resonant Raman spectra were recorded by using the 532, 633, and 785 nm excitations and collected by using the 1 × 1 camera of a Bruker Senterra Raman microscope G


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by averaging spectra during 50 min with a resolution of 3−5 cm−1. A CCD camera operating at −50 °C was used. IV.2. Computational Details. All geometries have been optimized at the DFT level (using B3LYP, B3LYP-D, O3LYP, and M05-2X functionals) and a CASSCF level using CASPT2 corrections. TDDFT calculations were done with the same functionals. See the SI for further details.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b08011. Computational details and details on the simulation of vibronic structures. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Xiaozhang Zhu: 0000-0002-6812-0856 Eiichi Nakamura: 0000-0002-4192-1741 Juan Casado: 0000-0003-0373-1303 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the financial support from MINECO, Spain project reference CTQ2015-69391-P. H.T. thanks the financial support from MEXT (16H04106).

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REFERENCES

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Carbon-Bridged Phenylene-Vinylenes: On the Common Diradicaloid

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