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Quantum Chemical Studies on Electron-Accepting Overcrowded Ethylene with a Polarizable Skeleton Tsukasa Tada,*,† Takanori Fukushima,‡ Masahiko Hada,† and Yutaka Majima§ †

Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Minami-Ohsawa 1-1, Hachioji, Tokyo 192-0397, Japan ‡ Laboratory for Chemistry and Life Science, Institute of Innovative Research and §Laboratory for Materials and Structures, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan S Supporting Information *

ABSTRACT: We report the quantum chemical studies on the neutral and radical anion forms of an electron-accepting overcrowded ethylene (OCE1) featuring a highly polarizable skeleton based on the density functional theory (DFT) approach using the M06-2X hybrid functional. Calculated results indicate that OCE1 (bis{4H,8H-4-(dicyanomethylene)benzo[1,2-c:4,5-c′]bis[1,2,5]thiadiazol-8-ylidene}) shows conformational behaviors and energetics similar to those of bianthrone (OCE2), a typical thermochromic overcrowded ethylene. Neutral OCE1 and its radical anion have antifolded (afOCE1) and twisted (tOCE1) isomers on their potential energy surfaces. The calculated isomerization barrier heights of OCE1 and its radical anion are considerably low, indicating that its conformation is susceptible to interactions with surrounding molecules. While two afOCE1 molecules can form a simple π-stacked dimer, tOCE1 tends to be converted to afOCE1 when the two tOCE1 molecules come close together, indicating the instability of tOCE1 in the homogeneous OCE1 solid state. The thermochromic behavior difference between OCE1 and OCE2 in solution is closely associated with the considerably small energy difference between the afOCE1 and the tOCE1 as compared with OCE2. The properties of OCE1 are also compared with other typical electron-accepting overcrowded ethylenes in terms of electronic structure, energetics, and conformational behaviors.

1. INTRODUCTION

This molecule has a unique design featuring electron-deficient thiadiazole rings and electron-withdrawing CN groups. The S− N bond of the thiadiazole ring is highly polarized because the sulfur and nitrogen atoms can accommodate positive and negative charges, respectively. Furthermore, since the thiadiazole rings and the dicyanomethylene groups can be involved in the planar framework of OCE1, the π-electrons are expected to be effectively delocalized over the entire molecular skeleton (Figure 2). These structural and electronic features could affect the conformational behavior of OCE1 in solution and in the solid state.17 In fact, OCE1 has two interconvertible conformers, i.e., antifolded and twisted isomers, similar to those of conventional overcrowded ethylenes such as bianthrone

Overcrowded ethylenes are a class of compounds that show thermochromic1−5 and photochromic behaviors6−8 and have been extensively studied as prototype models of molecular motors and electro- or photoswitchable units.9−13 Recently, some overcrowded ethylenes based on the 9,9′-bisfluorenylidene (BF) backbone have been reported as effective electronaccepting components in bulk heterojunction solar cells.14−16 In 1997, bis{4H,8H-4-(dicyanomethylene)benzo[1,2-c:4,5-c′]bis[1,2,5]thiadiazol-8-ylidene} (denoted as OCE1, Figure 1) was reported to be a strong electron acceptor, comparable to tetracyanoquinodimethane (TCNQ), and displays a conformational behavior analogous to existing overcrowded ethylenes.17

Figure 2. Typical resonance structures of OCE1. Figure 1. Chemical structures of overcrowded ethylenes (OCE1− OCE4) studied in this work. © XXXX American Chemical Society

Received: September 14, 2017

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DOI: 10.1021/acs.jpca.7b09172 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

3. RESULTS AND DISCUSSION 3.1. Structures and Energetic Properties of the OCE1 Isomers. Similar to the well-known overcrowded ethylene bianthrone (OCE2),1,2,25−27 OCE1 is reported to have the following two isomers: antifolded (afOCE1) and twisted (tOCE1) isomers.17 The optimized minimum energy geometries of OCE1 and their frontier orbitals are shown in Figures 3 and 4, respectively. Figure 3 indicates that OCE1 has

(OCE2, Figure 1), but there exist distinct differences in the conformational and thermochromic behaviors between OCE1 and overcrowded ethylenes composed of benzene rings.17 Here we report the first theoretical study on the electronic structures, relative stability of isomers, and its energetics based on quantum chemical calculations to get insights into the origins of the molecular properties characteristic of this overcrowded ethylene. We also compare the molecular properties of OCE1 with those of other electron-accepting overcrowded ethylenes shown in Figure 1 (OCE2−OCE4) on the basis of the unified level of molecular orbital (MO) calculations using the hybrid meta-generalized gradient approximation (meta-GGA) exchange-correlation (Exc) functional.

2. METHOD OF CALCULATION In our MO calculations on the overcrowded ethylenes the Kohn−Sham density functional theory (DFT) approach with the M06-2X hybrid meta-GGA functional18−20 was employed throughout the present work. Although the B3LYP functional has been widely employed so far, this functional is reported to systematically underestimate reaction barrier heights and to give inaccurate results in estimating noncovalent interactions such as aromatic−aromatic stacking and van der Waals attraction energies.19 On the other hand, the M06-2X functional is reported to have improved performance for main-group thermochemistry, barrier heights, and noncovalent interactions.18−20 To calculate overcrowded ethylenes including anionic systems, the 6-311G(d,p) and diffuse-augmented 6-311+G(d,p) split valence basis sets21 were employed. To avoid convergence problems, full geometry optimizations were carried out by using the 6-311G(d,p) basis set. To estimate energetic quantities such as the adiabatic electron affinity and reaction barrier heights, we carried out single-point calculations with the 6-311+G(d,p) basis set at the 6-311G(d,p)-optimized geometries throughout this work. Vibrational frequency analyses with the 6-311G(d,p) basis set were carried out to confirm true minimum and transition state geometries. To confirm transition states on the isomerization reaction path, intrinsic reaction coordinate (IRC) calculations were also carried out. Considerably twisted overcrowded ethylenes are reported to have a biradical character.22 However, we confirmed that the twisted OCE1 has a not so serious biradical character due to its relatively small dihedral angle (36.46°) and the following reasons. First, a more stable broken symmetry solution has not been found in our KS-DFT calculations on OCE1. Second, we also confirmed that the main configuration, which corresponds to the Hartree−Fock determinant, still dominates in our calculated CASSCF wave functions of OCE1 isomers. For the estimation of solvent effects on the relative stability of OCE1 isomers in solution, self-consistent reaction field (SCRF) calculations with IEFPCM (integral equation formalism polarizable continuum model)23 were carried out. In addition to the KS-DFT calculations with the M06-2X Exc functional, DFT calculations with various Exc functionals and Møller−Plesset second-order perturbation (MP2) calculations were also carried out in our SCRF calculations. The Cartesian coordinates of the optimized OCE1 structures are reported in the Supporting Information (Tables S1−S8). All MO calculations were performed using the Gaussian 09 MO program package.24

Figure 3. Optimized structures of afOCE1 and tOCE1 (M06-2X/6311G(d,p)): (a) afOCE1 and (b) tOCE1.

the above-mentioned two isomers, in agreement with the experimental result.17 The afOCE1 and tOCE1 structures have C2h and C2 symmetries, respectively. For the center CC bond of the two isomers, the optimized bond lengths of afOCE1 and tOCE1 are calculated to be 1.351 (1.362 Å (exp.)) and 1.374 Å

Figure 4. Calculated frontier orbitals of afOCE1 and tOCE1 (M062X/6-311+G(d,p)//M06-2X/6-311G(d)). B


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The Journal of Physical Chemistry A (1.388 Å (exp.)), respectively.17 Thus, a fairy good agreement is found between the calculated and the experimental CC center bond length. In the case of tOCE1, the dihedral angle between the mutually twisted two planar units is calculated to be 36.46° for in vacuo level calculations, which is a somewhat small value compared to the X-ray analysis value (48.1°)17 on the benzonitrile solvated crystal. Concerning OCE1, there exists only a 3.17 kcal/mol energy difference between the two different isomers in our M06-2X/6311+G(d,p)//M06-2X/6-311G(d,p) level of calculations. afOCE1 is only slightly more stable than tOCE1. In the case of thermochromic overcrowded ethylenes, the enthalpy difference between the two isomers is reported to be 4−30 kcal/ mol.4 From this point of view, OCE1 is slightly out of the family of thermochromic overcrowded ethylenes. In fact, thermochromic behavior is hardly spectroscopically observed for OCE1 unlike other thermochromic overcrowded ethylenes such as OCE2.1,2,4 OCE1 shows a deep violet color in benzonitrile solvent, and the color appears to be unchanged even at the boiling temperature of benzonitrile.17 In accordance with this, cyclic voltammetry of OCE1 displays a clear and reversible two-step two-electron redox profile, indicating the generation of populated tOCE1 conformers in solution.17 Levy et al. reported that in some overcrowded ethylenes (10(9′H-fluoren-9′-ylidene)-9(10H)-anthracenone, 10-(11′Hbenzo[b]fluoren-11′-ylidene)-9(10H)-anthracenone, and 10(1′,8′-diaza-9′H-fluoren-9′-ylidene)-9(10H)-anthracenone or 1.8-diazafluorenylidene-anthrone), the energies of the folded isomers are fairly close to those of the twisted ones.4 In particular, the last of these three overcrowded ethylenes (denoted as OCE5 in our work) shows conformational and thermochromic behaviors similar to OCE1. It is reported that the folded isomer is slightly more stable than the twisted isomer in OCE5 (ΔE = 2.68 kcal/mol (B3LYP/6-311G(d,p)). OCE5 (1,8-diazafluorenylidene-anthrone) shows a yellow color in the solid state and turns dark red in solution like OCE1. For the above-mentioned overcrowded ethylenes, it is also reported that the folded and twisted conformations rapidly interconvert in solution-based NMR measurements.4 Considering the small energy difference (3.17 kcal/mol) between afOCE1 and tOCE1, there is a high possibility that the antifolded and twisted isomers of OCE1 are also in equilibrium with a rapid isomerization in solution. While the solvated tOCE1 crystal is obtained from a benzonitrile solution, it should be noted that afOCE1 can crystallize from a dichloroethane solution on standing for a long period of time.17 To compare OCE1 with OCE5 at the unified level of theory, we also calculated OCE5 by the M06-2X/6-311+G(d,p)//M06-2X/6-311G(d,p) level of theory. As is already reported, OCE5 has folded (fOCE5) and twisted (tOCE5) isomers. Optimized structures of fOCE5 and tOCE5 are shown in the Supporting Information (Figure S1). In the case of fOCE5, the 1,8-diazafluorenylidene unit is not folded while the anthrone unit is fairly folded. This peculiar structure of fOCE5 may destabilize the folded isomer in comparison with OCE2 (bianthrone). Like OCE1, fOCE5 is slightly more stable than tOCE5. The energy difference between the two isomers of OCE5 is calculated to be 4.31 kcal/mol, which is slightly larger than that of OCE1 (3.17 kcal/mol). Considering the experimental results on OCE5 and the energy differences between the two isomers on OCE1 and OCE5, the calculated results are consistent with the experimental result that OCE1 shows a deep violet color upon dissolution into organic solvents

and does not show a color change upon heating in solution. From the similarity in conformational and thermochromic behaviors between OCE1 and OCE5, OCE1 seems to be in the same overcrowded ethylene family including OCE5 reported by Levy et al.4 In the case of the homogeneous solid state of OCE1, the antifolded structure (afOCE1) dominates, showing a yellow color. Different from OCE2, the homogeneous OCE1 solid state does not show thermochromic behavior. However, upon grinding, OCE1 in the solid state shows a color change from yellow to violet on the grinded surface region. The color returns to the original yellow color upon heating. The violet color generation indicates the generation of populated tOCE1. At present, the exact mechanism of the solid-state thermochromic behavior in overcrowded ethylenes still remains an open question. Recently, Naumov et al. reported experimental and theoretical studies5 on the origins of the solid-state thermochromism of overcrowded ethylenes including bianthrone (OCE2). They insist that the color change is probably due to a dynamic process instead of a phase transition or permanent molecular distortion caused by folding and twisting. As shown in Figure 4, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of afOCE1 and tOCE1 are delocalized throughout the molecular framework. While the HOMO and LUMO energy levels of afOCE1 are calculated to be −8.71 and −3.40 eV, respectively, the same HOMO and LUMO energy levels of tOCE1 have energies of −8.16 and −3.90 eV, respectively. From these calculated results the two isomers of OCE1 are expected to have a large electron affinity (EA) compared to other overcrowded ethylenes. In the case of tOCE1, the HOMO−LUMO gap is decreased compared with that of afOCE1, resulting in a considerably low LUMO energy level value. Considering this characteristic on the LUMO energy level, the twisted isomer is expected to be energetically advantageous over the antifolded isomer when it receives an electron. We also found that there exist the same antifolded and twisted isomers in the radical anion of OCE1. The optimized geometries of these isomers are shown in Figure 5. Like neutral OCE1, the structures of afOCE1 and tOCE1 radical anions have C2h and C2 symmetries, respectively. The shape of the singly occupied molecular orbital (SOMO) in the OCE1 radical anion is similar to that of the LUMO in OCE1 as shown in the Supporting Information (Figure S2). The SOMO levels of the afOCE1 and tOCE1 radical anions are calculated to be −2.93 and −3.49 eV, respectively. In contrast to neutral OCE1, the tOCE1 radical anion has lower energy than the afOCE1 radical anion. The energy difference between two isomers of the OCE1 radical anion is increased to 9.99 kcal/mol compared with that of neutral OCE1. This difference between neutral OCE1 and its radical anion can be attributed to the relatively low LUMO energy levels of tOCE1 compared with afOCE1. The calculated result indicates that it is easier to observe the twisted type radical anion than to observe the antifolded type one as the isolated molecule level. This is consistent with the experimental result that the twisted-type conformer is only involved in the electrochemical reduction process.17 In the case of tOCE1 and its radical anion, one side of a twisted molecule is free from steric effects and can take a sufficiently planar structure, different from the case of 29,29,30,30-tetracyanobianthraquinodimetane (TBAQ) (deC


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Figure 6. Optimized transition state structure (M06-2X/6-311G(d,p)) on the isomerization reaction path between afOCE1 and tOCE1 and its energy level diagram (M06-2X/6-311+G(d,p)//M06-2X/6-311G(d,p)).

Figure 5. Optimized structures of afOCE1 and tOCE1 radical anions (M06-2X/6-311G(d,p)): (a) afOCE1 radical anion and (b) tOCE1 radical anion.

noted as OCE4).28,29 From this point of view, the occupied π orbitals of the tOCE1 radical anion in particular are expected to be fairly stabilized, providing high EA. The EA values of tOCE1 and afOCE1 are calculated to be 3.70 and 3.13 eV, respectively. The calculated EA value of tOCE1 is comparable to that of TCNQ (3.81 eV). These calculated results are consistent with the first reduction potential determined by cyclic voltammetry of OCE1.17 3.2. Energy Barrier Heights in the Isomerization between the Two Isomers of OCE1 and Its Radical Anion. As described in the previous section, both OCE1 and its radical anion have the antifolded and twisted isomer structures on the potential energy surfaces as the isolated molecule level. However, the stability of each isomer strongly depends on their potential energy surfaces, especially on the barrier heights of isomerization. From this point of view we calculated the reaction barrier heights as shown in Figures 6 and 7. Figure 6 shows the calculated transition state structure between afOCE1 and tOEC1 and its isomerization energy diagram. As shown in this figure we find that afOEC1 is easily changed to tOEC1 by twisting around the central CC bond. The reaction barrier height from afOEC1 to tOEC1 is calculated to be 4.50 kcal/mol, while the barrier height from tOEC1 to afOEC1 is calculated to be 1.33 kcal/mol. These low reaction barrier heights indicate that the two isomers are in an equilibrium state at room temperature at least as an isolated molecule level. In particular, in the case of tOEC1, the potential energy depth is as low as 1.33 kcal/mol. This calculated result indicates that the stability of tOCE1 is susceptible to interactions with surrounding molecules and that tOCE1 is not so stable unless it is stabilized by the surrounding molecules. Thus, tOCE1 can be observed in the presence of solvent molecules in solution and suitable guest molecules in the solid state. Our calculated results indicate that the Mulliken charge of the sulfur atom in the thiadiazole ring is increased

Figure 7. Optimized transition state structure (M06-2X/6-311G(d,p)) on the isomerization reaction path between afOCE1 and tOCE1 radical anions and its energy level diagram (M06-2X/6-311+G(d,p)// M06-2X/6-311G(d,p)).

from 0.698 to 0.760 (M06-2X/6-311+G(d,p)//M06-2X/6311G(d,p)) when afOCE1 is converted to tOCE1, meaning that the thiadiazole ring of tOCE1 is more polarized than that of afOCE1. Thus, tOCE1 is expected to be more stabilized than afOCE1 by a dielectric polarization effect caused by polar solvent molecules in solution. The calculated transition state structure between the two isomers of the OCE1 radical anion and its energy diagram are shown in Figure 7. The transition state has a structure which is close to that of afOCE1 radical anion. The reaction barrier height from tOCE1 radical anion to afOCE1 radical anion is calculated to be 10.06 kcal/mol. On the other hand, the reaction barrier height from afOCE1 radical anion to tOCE1 radical anion is calculated to be as low as 0.07 kcal/mol. Although the afOCE1 radical anion has a minimum energy point on the potential energy surface, this considerably low potential energy depth indicates that the afOCE1 radical anion is difficult to be experimentally observed. When OCE1 receives an electron it is expected that the tOCE1 radical anion dominates. In fact, only the tOCE1 radical anion has been observed so far. 3.3. Molecular Structures in a π-Stacked Dimer and Its Intermolecular Interaction. Considering the considerably small energy difference between the two OCE1 isomers and their low barrier heights in the isomerization reaction, it is expected that afOCE1 and tOCE1 isomers rapidly interconvert in the case of gas phase at room temperature. However, D


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in-phase overlap with the HOMO of the neighboring molecule. The binding energy between the two afOCE1 isomers is calculated to be 24.37 kcal/mol. Thus, the stabilizing effect from this type of stacking arrangement is advantageous for generating molecular aggregates, where molecules are compactly and regularly located. This is consistent with the fact the afOCE1 structure dominates in the homogeneous OCE1 solid state. On the other hand, the calculated results indicate that the tOCE1 isomers cannot form a simple π-stacked dimer. If two tOCE1 molecules come to be close to each other to form a πstacked dimer, one tOCE1 isomer is converted to an afOCE1 isomer. The optimized molecular structure of the resulting dimer is shown in Figure 9. This dimer is a π-stacked dimer between a twisted-type isomer and an antifolded-type isomer. The two isomers are located in crossed form different from the case of the afOCE1 π-stacked dimer. This sensitive structural change of tOCE1 isomer can be attributed to its shallow potential energy depth of 1.33 kcal/mol. The binding energy of this type of dimer is calculated to be 28.56 kcal/mol. These calculated results indicate that tOCE1 tends to be converted to afOCE1 in the homogeneous OCE1 solid state. Thus, the molecular interactions among homogeneous OCE1 molecules are considered to advantageously act on the stability of afOCE1 instead of tOCE1. We suggest that the molecular interactions among OCE1 molecules play an important role in realizing the OCE1 solid state where the afOCE1 structure dominates. 3.4. Self-Consistent Reaction Field (SCRF) Approach to the Relative Stability of the Two Different OCE1 Isomers in Solution. As discussed in the previous section (section 3.2), the two OCE1 isomers are expected to be in equilibrium with a rapid isomerization in solution like the case of OCE5. As molecular interactions in the homogeneous OCE1 solid state play an important role in determining the dominant OCE1 molecular structure, molecular interactions between OCE1 and solvent molecules are also expected to play an important role in determining the relative stability of the two isomers in solution. Because the bond polarization in the thiadiazole ring of tOCE1 is somewhat larger than that of afOCE1, solvent molecules are expected to more stabilize tOCE1 than afOCE1 in solution as discussed in the previous section (section 3.2). This suggests that the energy difference between afOCE1 and tOCE1 is decreased in solution, particularly in the case of polar solvents. From this point of view, self-consistent reaction field (SCRF) calculations using the IEFPCM23 model at the 6-311+G(d,p)//6-311G(d,p) level

afOCE1 is reported to dominate in the solid state at least at room temperature, unless solvated molecules are present or shared stress is applied.17 This conformational preference of afOCE1 in the solid state is possibly be due to intermolecular interactions. In the previous sections we discussed minimum energy and transition state structures as an isolated molecule level. However, in condensed phase, the molecular structure may be largely affected by surrounding molecules through their intermolecular interactions. To investigate the effect of intermolecular interactions on the molecular structure of OCE1 in the homogeneous solid state, we calculated OCE1 dimers in the case of both antifolded- and twisted-type isomers. Figure 8 shows the minimum energy geometry of the afOCE1

Figure 8. Optimized structure of an antifolded dimer of OCE1 (M062X/6-311G(d,p)) and its HOMO (M06-2X/6-311+G(d,p)//M062X/6-311G(d,p)): (a) structure and (b) HOMO.

dimer and its HOMO. Two afOCE1 molecules easily form a simple π-stacked dimer, where the HOMO of one molecule is

Figure 9. Optimized dimer structure consisting of tOCE1 and afOCE1 (M06-2X/6-311G(d,p)): (a) side view, (b) top view. E


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Table 1. Relative Stability of afOCE1 and tOCE1 in Benzonitrile Solution and Stabilization Energies Caused by the Solvent Molecules Estimated by the 6-311+G(d,p)//6-311(d,p) Level of SCRF Calculationsa calcd energies (kcal/mol) methods

E1a(wo/SCRF)

E1b(w/SCRF)

E2(afOCE1)

E2(tOCE1)

ΔE2

MP2 PBE0 B3LYP M06 M06-2X ωB97X

1.71 0.71 0.95 1.50 3.17 4.76

1.01 −0.01 0.29 0.73 2.44 4.25

−15.955 −17.687 −18.705 −18.906 −17.326 −18.079

−16.656 −18.402 −19.363 −19.672 −18.052 −18.591

0.701 0.715 0.658 0.766 0.726 0.512

a E1a: Energy difference between afOCE1 and tOCE1 without SCRF calculations. E1b: Energy difference between afOCE1 and tOCE1 based on SCRF calculations. E1 = E(tOCE1) − E(afOCE1). E2(afOCE1): Stabilization energy by the solvent molecules for afOCE1. E2(tOCE1): Stabilization energy by the solvent molecules for tOCE1. ΔE2: Stabilization energy difference between afOCE1 and tOCE1 caused by the solvent molecules. ΔE2 = E2(afOCE1) − E2(tOCE1).

have been carried out on afOCE1 and tOCE1 isomers to investigate the solvent effects on the two isomers. Table 1 summarizes the relative stability of afOCE1 and tOCE1 estimated by the SCRF calculations in benzonitrile solution using the MP2 wave function approach and the KSDFT approach with several Exc functionals. As shown in Table 1, the KS-DFT results are fairly consistent with the MP2 result. Except for the PBE0 result, the calculated results indicate that afOCE1 is still more stable than tOCE1 even in benzonitrile solution. However, it should be noted that the energy difference between afOCE1 and tOCE1 is unexceptionally decreased in benzonitrile solution in comparison with the in vacuo level calculations, regardless of the calculational approaches. This is because tOCE1 is around 0.7 kcal/mol more stabilized than afOCE1 by the solvent molecules, as is expected. Thus, the calculated results clearly indicate that the polar benzonitrile solvent provides the populated tOCE1 in solution. In order to further estimate the dielectric stabilization effect on OCE1 caused by the surrounding solvent molecules, we also carried out SCRF calculations on OCE1 in fictitious solvents having various static dielectric constant with a fixed highfrequency optical dielectric constant (2.337535 of benzonitrile). By these SCRF calculations, we can estimate the trend of stabilization effect caused by the reorientation of polar solvent molecules in solution. As indicated in Table 2, the stabilization effect by solvent molecules is increased with an increase in the dielectric constant of solvents. The stabilization effect difference between afOCE1and tOCE1 is also increased with an increase in the dielectric constant of solvents, resulting in the smaller energy difference between the two different isomers. Thus, the calculated results clearly indicate that the dielectric constant of solvents plays an important role in determining the relative stability of afOCE1 and tOCE1 and their population ratio in solution. Polar solvents, in particular, are expected to increase the tOCE1 population in solution, as indicated in Table 2. Thus, our calculated results are consistent with the experimental results that the solvated tOCE1 crystal is obtained from a benzonitrile solution, while the solvated afOCE1 can crystallize from a dichloroethane solution on standing for a long period of time. 3.5. Comparison with Other Electron-Accepting Overcrowded Ethylenes. Various electron-accepting overcrowded ethylenes have been reported so far. To compare OCE1 with other electron-accepting overcrowded ethylenes, the M06-2X/ 6-311+G(d,p)//M06-2X//6-311G(d,p) level of calculations have also been carried out on the following three typical electron-accepting overcrowded ethylenes: bianthrone

Table 2. Variation of afOCE1 and tOCE1 Stabilization Energies by Solvent Molecules with an Increase in Static Dielectric Constant of the Solvent Based on the M06-2X/6311+G(d,p)//M06-2X/6-311G(d,p) Level of SCRF Calculationsa calcd energies (kcal/mol) ε (s)

E1

E2(afOCE1)

E2(tOCE1)

ΔE2

wo/SCRF 10 20 25.592 30 40 50 60 70 80

3.170 2.506 2.455 2.444 2.438 2.430 2.425 2.422 2.419 2.418

0 −15.486 −16.981 −17.326 −17.509 −17.779 −17.942 −18.052 −18.131 −18.191

0 −16.151 −17.697 −18.052 −18.241 −18.519 −18.687 −18.801 −18.882 −18.943

0 0.665 0.716 0.726 0.732 0.740 0.745 0.749 0.751 0.752

a ε(s): static dielectric constant of the solvents. E1: Energy difference between afOCE1 and tOCE1 based on SCRF calculations. E1 = E(tOCE1) − E(afOCE1). E2(afOCE1): Stabilization energy by the solvent molecules for afOCE1. E2(tOCE1): Stabilization energy by the solvent molecules for tOCE1. ΔE2: Stabilization energy difference between afOCE1 and tOCE1 caused by the solvent molecules. ΔE2 = E2(afOCE1) − E2(tOCE1)

(OCE2), 9,9′-bisfluorenylidene (OCE3), and TBAQ (OCE4)28,29 (Figure 1). The calculated minimum energy structures of these overcrowded ethylenes and their radical anions are shown in Figures 10 and 11, respectively, together with the structures of OCE1 and its radical anions. Table 3 summarizes calculated molecular properties of the four overcrowded ethylenes and their radical anions. These overcrowded ethylenes have similar antifolded and twisted isomers except for OCE4. In the case of radical anions, it should be noted that these overcrowded ethylenes, except for OCE3, also have similar antifolded and twisted isomers. The conformational and energetic behaviors of OCE2 are similar to those of OCE1. As was already reported, OCE2 has antifolded and twisted isomers. Our calculated results indicate that the radical anion of OCE2 also has the same two isomers, as shown in Figure 11. Similar to OCE1, afOCE2 is more stable than tOCE2 in the neutral state. The energy difference (9.72 kcal/mol) between afOCE2 and tOCE2 is larger than that calculated for OCE1. In the case of radical anion, tOCE2 isomer is more stable than the afOCE2 isomer as in the case of OCE1. The energy difference between the afOCE2 radical F


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Figure 10. Optimized structures of OCE1, OCE2, OCE3 and OCE4 electron-accepting overcrowded ethylenes (M06-2X/6-311G(d,p)).

be 7.73 kcal/mol, which is close to the reported energy difference value.4 As shown in Figure 10, the degree of folding in afOCE3 seems to be smaller than that for the antifolded isomers in other overcrowded ethylenes. The lower stability of afOCE3 may be closely related to this small degree of folding. Our calculated results on OCE3 are consistent with the experimental observation that the twisted structure dominates in the condensed phase.31−33 In the case of OCE3 radical anion, on the other hand, a minimum energy point with an antifolded structure cannot be found on the potential energy surface. Only the twisted structure exists on the OCE3 radical anion as shown in Figure 11. This calculated result is consistent with the fact that no radical anion with the antifolded structure has been observed so far. The EA value of tOCE3 is calculated to be 1.83 eV. It is interesting to compare OCE1 with OCE4 because of the structural similarity between these two overcrowded ethylenes. OCE4 shows a peculiar conformational behavior which is different from the other overcrowded ethylenes studied in this work. Although OCE4 has two different isomers, the isomer with the twisted-type structure cannot be found on its potential energy surface. Instead of the twisted isomer, there exists a synfolded isomer (sfOCE4) together with an antifolded isomer

anion and the tOCE2 radical anion is calculated to be 16.42 kcal/mol, which is larger than that of OCE1 radical anions. The EA values of afOCE2 and tOCE2 are calculated to be 1.68 and 2.81 eV, respectively. Thus, tOCE2 has a fairly high EA value. Our calculated energy difference between afOCE2 and tOCE2 is fairly large in comparison with previously reported calculated results (2.88 kcal/mol by B3LYP/6-31G(d) and 4.10 kcal/mol by B3LYP/6-311G(d,p)).4 According to our MP2/6311G(d,p) and CCSD(T)/6-311G(d,p)//MP2/6-311G(d,p) level of calculations, however, the energy difference is estimated to be 7.28 and 9.90 kcal/mol, respectively. Despite a good agreement between the calculated result by the B3LYP/631G(d) approach and the experimentally observed results in solution,30 it seems that DFT calculations using the B3LYP functional tend to underestimate the energy difference in comparison with more rigorous approaches in the case of OCE2. On the other hand, OCE3 shows somewhat different behaviors. Similar to OCE1 and OCE2, OCE3 has antifolded (afOCE3) and twisted (tOCE3) isomers on its potential energy surface. However, the tOCE3 isomer is more stable than afOCE3, in contrast to the cases of OCE1 and OCE2. The energy difference between afOCE3 and tOCE3 is calculated to G


Article


Figure 11. Optimized structures of OCE1, OCE2, OCE3, and OCE4 radical anions (M06-2X/6-311G(d,p)).

mol, which is larger than the case of OCE1. Thus, our calculated results are consistent with the experimental results that afOCE4 has been observed in solution, and the color of OCE4 does not change upon heating.29 Theoretical studies on OCE4 were already reported by Pogodin et al.29 The calculated energy difference (11.20 kcal/ mol)29 between the two isomers is close to our result (12.02 kcal/mol). However, our optimized geometry of sfOCE4 (C2 symmetry) is somewhat different from their reported geometry (C2v symmetry). Similarly, the sfOCE4 minimum energy structure with C2 symmetry has also been obtained by our MP2/6-311G(d,p) level of calculations. In the case of radical anion, the syn-folded minimum energy structure cannot be found on its potential energy surface. The OCE4 radical anion has antifolded- and twisted-type isomers as shown in Figure 11. However, the energy difference between the two isomers is calculated to be only 0.50 kcal/mol. The tOCE4 radical anion is only slightly more stable than the afOCE4 radical anion. In the case of OCE1 radical anion, the twisted structure is 9.99 kcal/mol more stable than the antifolded one. The EA value of afOCE4 is calculated to be 2.47 eV. If the EA of OCE4 is defined as the energy difference between neutral afOCE4 and the tOCE4 radical anion, the EA value is calculated to be 2.50 eV. These relatively small EA

Table 3. Calculated Molecular Properties of ElectronAccepting Overcrowded Ethylenes (M06-2X/6311+G(d,p)//M06-2X/6-311G(d,p))a neutral molecules

afOCE1 tOCE1 afOCE2 tOCE2 afOCE3 tOCE3 afOCE4 sfOCE4 tOCE4

ΔE (kcal/mol)

EA (eV)

0 +3.17 0 +9.72 0 −7.73 0 +12.02

3.13 3.70 1.68 2.81 1.83 2.47

radical anions

symmetry C2h C2 C2h D2 C2h D2 C2h C2

ΔE (kcal/mol)

symmetry

0 −9.99 0 −16.42

C2h C2 C2h D2

0 0

D2 C2h

−0.50

C2

ΔE: Relative energy of the two different isomers of electron-accepting overcrowded ethylenes studied in this work. EA: Electron affinity. a

(afOCE4) as shown in Figure 10. The twisted-type structure can only be found as a stationary point having two imaginary frequencies on the potential energy surface. As expected, afOCE4 is more stable than sfOCE4. The energy difference between afOCE4 and sfOCE4 is calculated to be 12.02 kcal/ H


Article

The Journal of Physical Chemistry A values in comparison with those of OCE1 can be attributed to the fact that four CN groups cannot be located on each πsystem plane due to its steric effect. In terms of the electronaccepting capability, OCE1 shows the highest EA value among the four electron-accepting overcrowded ethylenes studied in this work. As is discussed, OCE1 shows conformational behaviors similar to OCE2 both for the neutrally charged molecule and for the radical anion. However, OCE1 shows different thermochromic behaviors in comparison with OCE2. While OCE2 shows thermochromic behaviors, OCE1 does not show thermochromic behaviors as discussed in the previous section (section 3.1). This difference can be attributed to the different energy differences between the antifolded and the twisted isomers (ΔE = 3.17 kcal/mol for OCE1 and ΔE = 9.72 kcal/ mol for OCE2). Considering the case of an isolated molecule level, the calculated results indicate that the antifolded and twisted isomers of OCE1 and OCE2 are in equilibrium with a rapid isomerization. This rapid isomerization is considered to proceed also in solution as in the case of OCE5.4 OCE2 solution shows a yellow color of afOCE2 at room temperature due to a too small tOCE2 population to show a deep green color. With an increase in solution temperature, populated tOCE2 causes a color change from yellow to deep green. Although the afOCE2 population is still higher than the tOCE2 population even at the elevated temperature in solution, the yellow color of afOCE2 is considered to be masked with the dark green color of populated tOCE2. The same thing can be said for OCE1 solution at room temperature. In the case of OCE1 we suggest that sufficient tOCE1 population in showing a deep violet color is already generated at room temperature in solution like OCE5, showing no thermochromic behavior for OCE1 in solution. On the other hand, the rapid isomerization between the antifolded and the twisted isomers is considered to be suppressed by a cage effect caused by neighboring molecules in the homogeneous solid state of OCE1, showing a yellow color of afOCE1. However, if this cage effect in the OCE1 solid state would be partly destroyed by some causes, the rapid isomerization between afOCE1 and tOCE1 is expected to proceed, generating the partly violet-colored region. OCE1 in homogeneous solid state shows a color change from yellow to violet upon grinding. We suggest that the rapid isomerization between the two isomers begins to proceed in the grinded region caused by the cage effect reduction by grinding. This grinding effect is expected to partly generate populated tOCE1, providing the mechanochromical behavior of OCE1.

(2)

(3)

(4)

(5)

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this respect, OCE1 may belong to the OCE5 family reported by Levy et al.4 We suggest that molecular interactions among OCE1 molecules tend to destabilize tOCE1, resulting in the dominance of the afOCE1 structure in homogeneous OCE1 solid state. OCE1 is considered to be in an equilibrium state with a rapid interconversion between the antifolded and the twisted conformers in solution due to the considerably low isomerization barrier height and the considerably small energy difference between the two isomers, like the case of OCE5. Polar solvents, in particular, more stabilize tOCE1 than afOCE1, giving populated tOCE1 in solution. The thermochromic behavior difference between OCE1 and OCE2 is closely associated with the considerably small energy difference between the antifolded and the twisted isomers of OCE1. We suggest that generation of the rapid interconversion state between the two different conformers and the populated tOCE1 in solution gives a violet color at room temperature, like the case of OCE5. In the case of OCE2, it is considered that there only exists too small tOCE2 populations to show a dark green color in solution at room temperature due to the relatively large energy difference between afOCE2 and tOCE2, resulting in a yellow color of afOCE2 in solution at room temperature. tOCE4 radical anion is not so stabilized by the delocalization of the π-system due to the fact that four CN groups cannot be located on each π-system plane due to its steric effect, resulting in the relatively small EA value. OCE1 has the highest electron affinity among the calculated four electron-accepting overcrowded ethylenes due to its electron-deficient thiadiazole and electronwithdrawing CN groups which are free from steric effects.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b09172. Tables reportong optimized geometries of afOCE1 and tOCE1, optimized geometries of afOCE1 and tOCE1 radical anions, transition state geometries between the two different isomers of neutral OCE1 and OCE1 radical anion, and optimized geometries of the afOCE dimer and the dimer consisting of afOCE1 and tOCE1 (PDF)

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4. CONCLUDING REMARKS We theoretically investigated an electron-accepting overcrowded ethylene with a polarizable skeleton (OCE1), putting emphasis on its electronic structure and molecular properties. Neutral OCE1 and its radical anion have also been compared with other typical electron-accepting overcrowded ethylenes in terms of electronic structure, energetics, and conformational behavior. The following principal conclusions are derived from the present work. (1) OCE1 shows conformational behaviors similar to OCE2 (bianthrone), a typical electron-accepting overcrowded ethylene. OCE1 and its radical anion have antifolded and twisted isomers. Although afOCE1 is more stable than tOCE1, their energy difference is considerably small. In

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Takanori Fukushima: 0000-0001-5586-9238 Masahiko Hada: 0000-0003-2752-2442 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Research Center for Computational Science, Institute for Molecular Science, for the molecular orbital calculations. This study was partially I


Article


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supported by the Elements Strategy Initiative to Form a Core Research Center, funded by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), and by the BK Plus program, Basic Science Research program (NRF2014R1A1030419).

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