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Investigation into the molecular and electronic structures of a newly synthesized o-quinone derivative
Chemical Physics 524 (2019) 77–84
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Investigation into the molecular and electronic structures of a newly synthesized o-quinone derivative
T
Nobutsugu Hamamotoa, Sachie Araeb, Tetsuji Moriguchic, Ryo Irieb, Hitoshi Fujimotob,
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a
Department of Chemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan Department of Chemistry, Faculty of Advanced Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan c Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu 804-8550, Japan b
ARTICLE INFO
ABSTRACT
Keywords: Density functional theory Electronic structure O-Benzoquinone
We studied on the geometric and electronic structures of a newly synthesized compound, 3-(tert-butyl)dinaphtho [1,2-b;1′,2′-d]furan-12,13-dione (1), having a twisted molecular structure at an o-quinone region in a crystal. The density functional theory (DFT) has been applied to investigate the geometric and electronic structures using the model compound (1H), where a tertiary butyl group of the compound 1 is replaced by a hydrogen atom. The optimized geometry for the model dimer of 1H represented well the twisted molecular structure of 1. The timedependent DFT method explained the observed optical properties of 1 in a solution and a powdery solid. The DFT calculations also gave a good estimation of the ionization potential and the electron affinity of 1H, explaining a similarity in reduction and a difference in oxidation between 1H and o-benzoquinone. This similarity in the electron affinity was confirmed by measuring a reduction potential of the compound 1 with cyclic voltammetry.
1. Introduction It has taken over a half-century to be reported the crystal structure of o-benzoquinone [1] since the successful isolation [2] owing greatly to its instability in the atmosphere [3]. o-Quinones have been used as reagents of various reactions making good use of their reactivity. The reduction of quinones has been extensively studied, and it has been reported that o-benzoquinone shows an anomalous behavior in the second reduction [4]. Some reactions have been proposed for this anomality [5]. On the other hand, the polycyclic aromatic compounds have been expected and actually applied to numerous ways in the various fields, which would depend greatly on their functional flexibility through structural modifications. Recently, a variety of aromatic compounds have been energetically synthesized and applied to the electronic devices. Among them, an introduction of hetero atoms into the aromatic ring can provide high thermal and optical stability, and may lead to additional functionality. The compounds with a fused polycyclic thiophene structure have been investigated by many researchers especially in the past decade [6–10]; however, there exist few reports [11,12] on applications of fused polycyclic furans to the organic electronics because of their instability and synthetic difficulty. Nonetheless, the
⁎
Corresponding author. E-mail address: [email protected] (H. Fujimoto).
https://doi.org/10.1016/j.chemphys.2019.04.033 Received 21 January 2019; Accepted 30 April 2019 Available online 02 May 2019 0301-0104/ © 2019 Elsevier B.V. All rights reserved.
compounds containing the fused furan ring have been synthesized aiming to create the materials having novel physical properties [13–21]. The aromatic polycyclic compounds with a ketone group have been also interested in photochemical and electrochemical properties such as ability for the source of reactive oxygen species [22,23], the reversible two-electron reduction [24–27], the excited state intramolecular proton transfer using proton near the ketone group [28,29], and so on. The density functional theory (DFT) has been applied to investigate physicochemical properties of various functional materials through calculating the electronic structures of the ground and the excited states [30]. We also applied DFT methods to several compounds, and extended successfully to investigate physical properties of them in the solid state using small units such as dimers and trimers extracted from the crystal structure [31–33]. We recently succeeded to synthesize 3-(tert-butyl)dinaphtho[1,2b;1′,2′-d]furan-12,13-dione (1), a new derivative of o-benzoquinone with a fused furan ring, which is stable in the atmospheric circumstance (Scheme 1). We will report here the geometric and electronic structures of the monomeric and dimeric systems for this compound calculated by the DFT methods. The obtained results will be discussed in comparison with the experimentally observed data.
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Scheme 1. Molecular structure and two types of molecular arrangements of 1 in the crystal.
2. Experimental section
oxidation, purified by column chromatography, and crystallized from hexane and ethyl acetate (see Electronic Supplementary Data for the details). An X-ray analysis was performed on the crystal with approximate dimensions of 0.30 × 0.20 × 0.10 mm3. Crystal structure data were collected at 120 K with graphite-monochromatized Mo Kα radiation. The structural data have been deposited to the Cambridge Crystallographic Data Centre (CCDC No. 1817034). The electronic absorption and emission spectra of 1 were measured with a chloroform solution. A thin film was obtained with scrubbing the powdery sample on a quartz glass, and was also used for spectroscopic measurements. Cyclic voltammetry experiments were performed in acetonitrile using a conventional three-electrode cell with platinum plate working electrode, platinum wire counter electrode, and Ag/AgCl reference electrode. The potential was measured using ferrocene as the reference material.
2.1. Computational details Geometry optimization and the excitation energies of a monomer and two types of dimers (Types 1 and 2) in Scheme 1 were carried out using the DFT and time-dependent DFT (TD-DFT) methods. To investigate the solvent effect of n-hexane and acetonitrile, we used the CPCM method [34,35]. It has been shown [31–33] that the Minnesota functional may give reasonable results for the cases of weak interacting systems, where the long-range interaction plays an important role; therefore, we decided to use the MN15 functional [36] for the exchange correlation term. For all atoms, 6-311G(d) basis set was employed. All calculations were performed using the Gaussian16 program package [37], and the results were visualized by the GaussView program package [38]. In the calculations, a tertiary butyl group of 1 was replaced with a hydrogen atom for the sake of saving CPU times, which is denoted as the compound 1H in the following discussion. For the monomer of 1H, the geometry was optimized under two types of symmetry, C1 and Cs, whereas both dimers were calculated under Ci symmetry. Each moiety of the Type 1 dimer reproduced well the molecular structure of 1 in the crystal, but Type 2 did not (see Electronic Supplementary Data). In order to give insight into the electronic structures of 1 in the solid state, we optimized the geometry of the Type 2 model dimer under the following conditions: The distance and the tilt angle between the moieties were optimized under Ci symmetry, where the structural parameters for each moiety were fixed as those for the moiety of the Type 1 dimer. Analytical vibrational frequency computations at the optimized structure were then performed to confirm that the optimized structure was at an energy minimum.
3. Results and discussion 3.1. Molecular structure of 1 in the crystal Firstly, we will briefly report on the molecular structure of 1 observed in the crystal. The compound 1 crystallized into the triclinic system, space group P1: formula C24H18O3, M = 354.38, a = 8.505 (4) Å, b = 8.678 (4) Å, c = 11.950 (5) Å, V = 851.5 (6) Å3, α = 79.384 (4)°, β = 82.782 (4)°, γ = 80.781 (4)°, Z = 2. All molecules in the crystal had nonplanar structures with a dihedral angle of 9.2° in the obenzoquinone region as shown in Scheme 1. The short contact of 2.85 Å was found between carbon and oxygen atoms of neighbor molecules. 3.2. Optimized geometries and electronic structures of monomer and dimers We performed the geometry optimizations of the monomer 1H under the two kinds of symmetry, C1 and Cs, without and with constraint of a planar structure, respectively. The optimized structure with C1 symmetry was nearly planar and similar to one obtained for planar
2.2. Experimental details The compound 1 was synthesized by dehydrative condensation of 6tert-butylnaphthalen-2-ol and 1,4-naphthoquinone and the subsequent 78
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Fig. 1. Optimized geometry for the monomer of 1H.
Cs symmetry without any remarkable differences in the Gibbs free energy. The relative energy of the Cs geometry was estimated to be 0.02 kcal mol−1 lower than that of the C1 structure; therefore we would choose the higher symmetric Cs geometry for the discussion. The optimized geometry with Cs symmetry is shown in Fig. 1 and the main structural parameters are compiled in Table S1 of Electronic Supplementary Data. The geometries of both dimers were optimized under Ci symmetry. Each molecule in the Type 1 dimer reproduced well the observed structure of 1 in the crystal; however, the geometry for each moiety of the Type 2 dimer showed large difference from the observed structure (see Fig. S1 of Electronic Supplementary Data). This might imply that intermolecular interaction is weak in the Type 2 arrangement, which exists just for filling the space in the crystal. Then, the geometry of the Type 2 dimer was optimized again under the conditions, where each moiety of the dimer was fixed to that of the Type 1 dimer and the distance and tilt angle between moieties were optimized under Ci symmetry.
Fig. 3. Orbital energies near the frontier orbitals of 1H. The symmetry species of each orbital is labeled under the Cs and Ci point groups for the monomer and the two model dimers (Types 1 and 2), respectively.
Fig. 2 illustrates the optimized structures for the dimers of Types 1 and 2 using the MN15 method. The main structural parameters of the moiety for the Type 1 dimer are shown in Table S1 of Electronic Supplementary Data. The X-ray analysis showed that the distance between the carbonyl groups of neighbor molecules in the Type 1 arrangement is estimated to be 2.85 Å, which is shorter than the sum of van der Waals radii as shown in Scheme 1. The C…O distance of the optimized structure of the Type 1 dimer was evaluated to be 2.915 Å, which is similar value to the observed one. Moreover, the moieties in the optimized Type 1 dimer were twisted, where the dihedral angle of OCCO was estimated to be 3.2˚. It would be concluded that this compound takes a planar structure in an isolated condition such as a
Fig. 2. Optimized geometries for the two model dimers of 1H.
79
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calculated to have large contributions from carbon atoms of the aromatic backbone, but have little contributions from carbonyl groups. The orbitals of the dimers were generated from two orbitals of the moieties through in-phase or out-of-phase combinations. Two monomeric moieties in the Type 1 dimer overlapped each other closely in the region of carbonyl groups; therefore, the orbitals of the monomer with large contribution from carbonyl group may strongly affect these pair of orbitals created through in-phase and out-of-phase interactions. In fact, the LUMO and the LUMO + 1 of the Type 1 dimer were generated from the LUMOs of two monomeric moieties, and the energy difference between these orbitals was estimated to be 0.49 eV, which was bigger than the others. On the contrary, the HOMO−3 (65a’) of the monomer split into the HOMO−6 (74au) and HOMO−7 (74ag) of the dimer, and the energy difference between them was estimated to be a relatively small value of about 0.01 eV, which was similar to other orbitals. This may be attributed to the difference in the orbital characters between the LUMO and the HOMO−3 of the monomer, which were assigned as πand σ-type orbitals, respectively. In the case of the Type 2 dimer, two molecules overlapped each other widely in the whole molecular framework; however, the distance between two moieties were optimized to be about 3.3 Å, which was close to the X-ray analysis and was larger than twice of the van der Waals radius for carbon. This may imply that the intermolecular interactions, and also the energy splitting between the in-phase and out-of-phase combinations, are small for a pair of molecules in the Type 2 dimer. Actually, all of the energy differences between molecular orbitals generated by these two combinations near the frontier orbital were much smaller than the value between the LUMO and the LUMO + 1 of the Type 1 dimer.
Table 1 Orbital energy of the monomer and the dimers in eV. Monomer
Dimer Type 1
a
Orbital
Energy
Orbital
15a” 14a” 13a” 12a” 11a” 10a” 65a’
−0.521 −1.115 −2.527 −6.703 −7.661 −8.020 −8.147
79ag 79au 78ag 78au 77au 77ag 76ag 76au
Type 2 a
Energy
Orbitala
Energy
−0.868 −0.934 −2.086 −2.571 −6.494 −6.497 −7.459 −7.492
79ag 79au 78au 78ag 77au 77ag 76au 76ag
−1.083 −1.175 −2.485 −2.498 −6.686 −6.728 −7.600 −7.702
a The symmetry species of each orbital is labeled under the Cs and Ci point groups for the monomer and the dimers, respectively.
solution, but the twisted structure in the single crystal due to the intermolecular interaction. The optimized structure of the Type 2 dimer was slightly different from the experimental one as compared to that of the Type 1 dimer. The relative Gibbs free energies of the two optimized dimers were estimated to be lower than the sum of the independent monomers by 0.3 kcal mol−1 and 8.3 kcal mol−1 for the Types 1 and 2 dimers, respectively, at 298.15 K. Fig. 3 compares the energy diagrams of the monomer and the two types of dimers in the region of the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbitals. The orbital symmetries were labeled under Cs and Ci point groups for the monomer and the dimers, respectively. All orbitals below a dotted line are occupied. The orbital energies are also summarized in Table 1. The atomic contributions to the molecular orbitals are shown in Fig. 4 (see Figs. S2–S6 of Electronic Supplementary Data for the other orbitals between the HOMO − 3 and the LUMO + 3 of the monomer and between the HOMO − 7 and the LUMO + 4 of the dimers). The HOMO of the monomer was calculated to have a large contribution from carbon atoms except for carbons of carbonyl groups, whereas the LUMO had large contribution from carbon and oxygen atoms of the carbonyl groups. The molecular orbitals between the HOMO − 4 and the LUMO + 4 were all π-type orbitals except for the HOMO − 3 (65a’), which had a strong contribution from lone pair electrons of oxygen atoms in the carbonyl groups and was apart about 1.4 eV from the HOMO. All other orbitals of the monomer were
3.3. Electronic absorption spectra of monomer and dimers We calculated the excitation energies for the monomer and the two types of dimers of 1H using the TD-DFT method. The optically allowed excited states obtained by the TD-MN15/6-311G(d) method are shown as vertical lines in Fig. 5. The height of each vertical line represents the oscillator strength of excitation. The solid line of each panel shows the theoretical absorption spectrum obtained by convoluting each excited state with a Gaussian function, where the full width at half maximum was assumed to be 0.06 eV and the oscillator strength was used as the pre-exponential factor. The experimentally obtained absorption spectra of 1 for an n-hexane solution and a thin film obtained by scrubbing the powdery sample on a quarts substrate are also shown in the figure.
Fig. 4. Several molecular orbitals near the HOMO and LUMO of the monomer and the dimers of Types 1 and 2. The symmetry of each orbital is labeled under the Cs and Ci point groups for the monomer and the dimers, respectively. 80
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Information about the low-lying excited states is essential for the application of the functional molecules, especially for photoactive devices because the photochemical processes depend largely on these states. The n-hexane solution of 1 exhibited a week absorption features around 2.6 eV accompanying broad intense absorption bands in the ultraviolet region. Under the Cs point group, the ground state of the 1H monomer was assigned to be the totally symmetric 1A’ state, and all singlet excited states are symmetry allowed. The excited states with the A’ and A” irreducible representations were calculated to have transition dipoles in and out of the molecular plane, respectively. The first allowed excited state, 11A”, was calculated to have a transition from the HOMO−3 to the LUMO as the main contribution with negligibly small oscillator strength, which can be assigned to the n-π* transition. This may imply non-radiative decay from the excited states of 1H, and is consistent with a failure to observe the emission of light for the compound 1. The second excited state, 11A’, was calculated to have a transition from the HOMO to the LUMO as the main contribution with relatively large oscillator strength, which is able to be assigned to the ππ* transition. These two excited states, 11A” and 11A’, may correspond to the weak absorption peak around 2.6 eV appeared in the observed absorption spectrum of 1. It could be concluded, moreover, that the 11A’ state is more responsible to the observed low lying absorption band than 11A” one from the calculated oscillator strength and the nonradiative nature of 1. We also calculated the low-lying triplet excited states, and the excitation energies of the first two triplet states were estimated to be 2.02 and 2.19 eV for 1H, which are assigned to the ππ* and nπ* excited states, respectively. These low-lying triplet states would be also responsible to the non-radiative nature of the compounds 1. The observed spectrum for the thin film of 1 was similar to that obtained for the n-hexane solution, where the small red shifts and some broadenings of every feature were recognized. The ground states of both model dimers were calculated as the 1Ag state under the Ci point group; therefore, the optically allowed excited states should belong to the Au irreducible representation, which consist of one electron transitions between orbitals with different parity. As mentioned above, the optimized structure for the Type 2 dimer was slightly different from that for the corresponding molecular pair in the crystal of 1. We calculated the excited states for the pair of molecules in the crystal corresponding to the Type 2 dimer, and obtained similar results to those obtained for the Type 2 dimer (See Fig. S8 of Electronic Supplementary Data); therefore, the electronic structures of the optimized geometry for the Type 2 dimer were used in the following discussion. Simply speaking, the orbitals of the dimers, which are generated from those of two monomeric moieties through in-phase and out-ofphase combinations, belong to the different irreducible representations with different parity, ag and au, respectively, under the Ci point group. These orbitals with the different parity contribute individually to the different excited state with the different irreducible representation; therefore, the excited states of the dimers are also in pairs with the
Fig. 5. Excited states of the monomer and the two types of dimers obtained by the TD-MN15 calculations. The vertical line depicts the oscillator strength of each state. The theoretical absorption spectra were obtained by convoluting each excited state with a Gaussian function. The full width at half maximum was assumed to be 0.06 eV and the oscillator strength was used as the preexponential factor.
There exist two different molecular arrangements between two adjacent molecules in the crystal of 1, which are similar to those of the model dimers; therefore, we depict the sum of the absorption spectra of these dimers in the figure. The spectrum of each dimer is shown individually in Fig. S7 of Electronic Supplementary Data. The excitation energies of the monomer and the dimers are summarized in Tables 2 and 3 for the several optically-allowed excitations with oscillator strengths. The first state is only listed for the symmetry-forbidden excitation. The important components of the one electron transition, in which the coefficient is larger than 0.3, are also presented. The excitation energies until 6.0 eV and the important components of the monomer and dimers are shown in Tables S2–S4 of Electronic Supplementary Data.
Table 2 TD-MN15 excited states of the monomer (Solvent: n-hexane).
Cs
a b c
State
Main configuration (|C|≧0.30)
Ea
fb
pc
11A” 11A’ 21A” 21A’ 31A’ 13A’ 13A”
+0.67(65a′ → 13a″)[90%] +0.69(12a″ → 13a″)[95%] +0.61(64a′ → 13a″)[74%] −0.35(10a″ → 13a″)[25%] + 0.58(11a″ → 13a″)[67%] +0.53(10a″ → 13a″)[56%] + 0.32(11a″ → 13a″)[20%] +0.60(12a″ → 13a″)[72%] +0.67(65a′ → 13a″)[90%]
2.43 2.65 3.62 3.78 3.98 2.02 2.19
0.0000 0.115 0.0000 0.015 0.015 -
z x+y z x+y x+y -
Excitation energy in eV. Oscillator strength. Transition moment direction. 81
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Table 3 TD-MN15 excited states for the dimers of Types 1 and 2. Statea
Main configuration (|C| ≧ 0.30)
Eb
fc
pd
Type 1 Ci
1Ag 1Au 2Au 3Au 4Au
+0.34(74ag → 78ag)[23%] + 0.41(74au → 78au)[34%] +0.44(74ag → 78au)[39%] + 0.34(74au → 78ag)[23%] +0.64(77ag → 78au)[82%] +0.66(77au → 78ag)[87%] +0.31(71ag → 78au)[19%] + 0.42(76ag → 78au)[35%]
2.39 2.40 2.56 3.10 3.56
0.0000 0.005 0.073 0.061 0.029
x+y+z x+y+z x+y+z x+y+z
Type 2 Ci
1Ag 1Au 2Au 3Au
+0.46(74ag → 78ag)[42%] + 0.45(74au → 78au)[41%] +0.48(74ag → 78au)[46%] + 0.44(74au → 78ag)[39%] +0.47(77ag → 78au)[44%] + 0.50(77au → 78ag)[50%] +0.50(77ag → 78au)[50%]−0.48(77au → 78ag)[46%]
2.40 2.40 2.75 2.89
0.0000 0.0002 0.125 0.022
x+y+z x+y+z x+y+z
a The symmetry species of each state is labeled under the Ci point group, where the Ag and Au states are symmetry-forbidden and allowed, respectively. The first state is only listed for the symmetry-forbidden excitation. b Excitation energy in eV. c Oscillator strength. d Transition moment direction.
different parity, Ag and Au. In the case of the Type 1 dimer, the lowest excited state was calculated to be the symmetry-forbidden 1Ag state, where its counterpart was the optically-allowed 1Au one. These states consisted mainly of transitions from two occupied molecular orbitals, the HOMO−7 (74ag) and the HOMO−6 (74au), to two unoccupied molecular orbitals, the LUMO (78au) and the LUMO + 1 (78ag). The HOMO−7 and the HOMO-6 of the dimer were originated from the HOMO−3 (65a’) of the monomer, and the LUMO and the LUMO + 1 of the dimer were from the LUMO (13a”) of the monomer; therefore, both excited states of 1Ag and 1Au can be assigned to the n-π* transitions. The symmetry allowed excited states, 2Au and 3Au, consisted mainly of transitions between the orbitals of the dimer originated from the HOMOs and the LUMOs of the monomeric moieties, which can be specified to the π-π* transitions. As contrast with the case of 1Au excited state, one electron transitions between symmetry-allowed orbital pairs of 77ag-78au and 77au-78ag contribute independently to the different excited states of 2Au and 3Au, respectively. The excitation energies for the symmetry-forbidden counterparts, 2Ag and 3Ag were estimated to be 2.53 and 3.08 eV, respectively. In the case of the Type 2 dimer, the excited states were also calculated to become a pair of Ag and Au states, which are symmetry-forbidden and allowed states, respectively. The energies of the first three symmetry-allowed states, 1Au, 2Au and 3Au, were estimated to be 2.40, 2.75 and 2.89 eV, respectively. The lowest excited state was calculated to be the symmetry-forbidden Ag state for the Type 2 dimer, too. The X-ray study showed that inversion centers exist between molecules in the crystal of the compound 1, which are similar positions to those of the dimers of Types 1 and 2; therefore, it would be expected that the symmetry restrictions may persist for the optical transitions of this compound 1 in a solid state. The thin film of 1 showed a broad absorption feature as the low energy tail in the region from 2 to 3 eV, which could be assigned to correspond with the first three excited states similar to those calculated for the dimers of Types1 and 2. It should be noted that the first excited states of both dimers are calculated to be the symmetry-forbidden excited states. This is consistent with the experimental results that any light-emission was not able to be detected for the solid sample of the compound 1. In passing, it should be mentioned that the compound 1 is quite stable under atmospheric conditions in contrast to o-benzoquinone. Any change was not detected in an absorption spectrum for the sample kept in air for five months (see Fig. S9 of Electronic Supplementary Data).
Table 4 Ionization potential (Ip) and electron affinity (Ea) in eV of the monomer and the dimers for 1H. The values for o-benzoquinone are shown in parentheses. Monomer
Dimer Type 1
Calc. Ip
Vertical
Ea
Adiabatic Vertical Adiabatic
ΔG0red
Ip1 Ip2 Ip(σ)
7.61 – 9.06 7.48 1.56 1.80 3.71
(9.64a) (9.64a) (9.18a) (1.54a) (1.79a) (3.99a)
Obs.
Calc.
– – −(9.6b) – −(1.62c) – −0.981d
7.20 7.20 – – 1.89 – –
Type 2
7.41 7.44 – – 1.75 – –
a This work. In the first ionization process of o-benzoquinone, an electron is removed from the σ-type molecular orbital. b The value from Ref. [41]. c The value from Ref. [43]. d Reduction potential of 1 measured by cyclic voltammetry.
reaction (ΔGox and ΔGred) from the Gibbs free energy differences between the neutral and the corresponding ionic states. The obtained values are summarized in Table 4. It has been reported [39] that the calculated values of ΔGox and ΔGred for the optimized geometry in the gas phase are similar to those obtained for the geometry in the presence of the self-consistent reaction field; therefore, the solvent effect for calculations of ΔGox and ΔGred was taken into consideration by applying the CPCM method to the gas-phase geometries. The redox potentials were determined by using the free energy change for the half reactions represented by the thermodynamic cycle. The entropy was corrected for solvation with the method developed by Whitesides et al. [40]. The values of the first Ip (Ip1) for the compound 1H were estimated to be 7.61 and 7.48 eV for the vertical and adiabatic values, respectively, where those of Ea were 1.56 and 1.80 eV. The small differences between the vertical and adiabatic values would imply that there exist small differences between the stable structures of the neutral and the charged molecules. Koening et al. [41] reported the Ip1 value of o-benzoquinone to be 9.6 eV by ultraviolet photoelectron spectroscopy, whereas Honda et al. [42] estimated the value as 9.32 eV by symmetry adapted cluster-configuration interaction (SAC-CI) calculations. We also estimated the vertical and adiabatic Ip1 values of o-benzoquinone at the same calculation level with one used for the compound 1H to be 9.64 eV and 9.18 eV, respectively. On the other hand, Marks et al. [43] reported the Ea value of o-benzoquinone as 1.62 eV by electron photo-detachment spectroscopy, whereas Honda et al. [42] estimated the value to be 1.63 eV by SAC-CI calculations. We also evaluated the vertical and
3.4. Electronic property The vertical values of the ionization potential (Ip) and the electron affinity (Ea) were calculated by the ΔSCF method for the optimized geometries of the monomer and two types of dimers, and the adiabatic values of Ip and Ea were also estimated as the free energy of redox 82
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adiabatic Ea values for the o-benzoquinone to be 1.54 eV and 1.79 eV, respectively, with the same calculation level as the compound 1H. In comparison with the values of 1H, interestingly, the Ip1 value of obenzoquinone is lower about 2 eV than that of 1H, whereas both compounds show a similar Ea value to each other. This would be due to differences and similarities in nature of the electron donating and accepting levels between these two compounds. As pointed out previously [42], the present DFT calculations for obenzoquinone also showed that an electron is removed from the σ-type molecular orbital in the first oxidation process, which has strong contributions from carbonyl groups, and that an extra electron in the reduction process is accommodated in the LUMO, which is assigned to the π-type molecular orbital. In the case of 1H, the calculation results indicated that an electron is removed from or added to the π-type molecular orbitals, the HOMO or the LUMO, respectively. It should be noted that the HOMO of 1H spreads over the whole molecule, but the LUMO localizes only on the o-naphthoquinone region as shown in Fig. 4. This implies that a hole generated in oxidation can delocalize over the whole molecule, but an additional electron in reduction is confined to the o-naphthoquinone region. This would be confirmed by the calculated Ea values for several fragment molecules of 1H, all of which showed nearly similar values of 1.4 ± 0.1 eV (see Fig. S10 of Electronic Supplementary Data for the molecules and the values). The Ip and Ea values for the dimers were also calculated to be 7.20 and 1.89 eV for Type 1, and to be 7.41 and 1.75 eV for Type 2, respectively, with a small difference as compared to those of the monomer. These values were correlated well with the orbital energies of the HOMO and the LUMO. This would reveal that there are weak interactions between molecules in both types of the dimers, which is consistent with the results for the energy diagrams of the model dimers as discussed above. The second Ip (Ip2) values were calculated to be 7.20 and 7.44 eV for the dimers of Types 1 and 2, respectively. The Ip1 and Ip2 values were approximately equal, which was consistent with the pseudo-degeneration of the HOMO and HOMO − 1 for the dimers because of the weak intermolecular interactions. René et al. [5] reported that the o-benzoquinone derivatives take a type of Michael addition reaction subsequent to the second reduction, which corresponds to the extremely small size of the second reduction peak in a cyclic voltammogram. They also pointed out that a presence of water and a species of supporting electrolytes have a strong effect on the second reduction potential, but not much on the first one of these compounds. This shows that the careful investigations with rigorous experimental conditions should be needed for obtaining an accurate value of the second reduction potential. The compound 1 showed the first reduction peak at −1.08 V with a difference of 77 mV between anodic and cathodic potentials, which would reveal the process to be reversible, and was similar to the reported value of –0.954 V for the obenzoquinone derivative [5]. The second reduction peak was observed around −1.47 V without cathodic peaks corresponding to the first and second reduction processes (see Fig. S11 of Electronic Supplementary Data). The values of ΔGred were calculated to be 3.71 and 3.99 V for the compound 1H and o-benzoquinone, respectively. These values of ΔGred correspond to the observed first reduction potentials of these compounds, reflecting a similarity in the values of Ea for 1 and o-benzoquinone.
was used for calculations. The geometry optimizations using the MN15 functional showed that the monomer of 1H has a planar structure with Cs symmetry, whereas the dimer of 1H reproduced well the twisted structure of 1 in the crystal. This shows that the compound 1 takes a planar structure in an isolated condition such as a solution, but the twisted structure in the crystalline solid due to the intermolecular interaction especially at the carbonyl groups of neighbor molecules. The excitation energies were calculated for the monomer and the model dimers of 1H by the time-dependent DFT method using the optimized geometries. The excited states calculated for the monomer and the dimers corresponded well to the observed absorption spectra for a solution and a powdery solid of 1, respectively. Moreover, the results also explained the non-radiative nature of 1 both in solutions and in solids. The vertical values of the ionization potential (Ip) and electron affinity (Ea) were estimated by the ΔSCF method for the monomer and the two model dimers of 1H. The adiabatic values were also calculated from differences in the Gibbs free energy between the neutral and corresponding ionic states. The values of the first Ip (Ip1) for 1H are smaller than those for o-benzoquinone by 2.03 and 1.70 eV for the vertical and adiabatic values, respectively. This is due to a difference in character of the molecular orbital, from which an electron removal occurs in the ionization process. On the other hand, there was a small difference in the values of Ea between 1H and o-benzoquinone. In both compounds, an extra electron was accommodated in the lowest unoccupied molecular orbital (LUMO). This similarity in the Ea values would be explained by the LUMO of 1H, which will confine an extra electron to the o-benzoquinone region. This similarity in the Ea value of 1H to o-benzoquinone was also confirmed by measuring the reduction potential of the compound 1 with cyclic voltammetry. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.chemphys.2019.04.033. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
4. Conclusion
[19]
The density functional theory (DFT) method was applied to investigate the geometric and electronic structures of a newly synthesized o-benzoquinone derivative with a fused furan ring, 3-(tert-butyl)dinaphtho[1,2-b;1′,2′-d]furan-12,13-dione (1). The compound 1 crystallized into the triclinic system with the space group of P1, and all molecules had non-planar structures. The model compound 1H, where a tertiary butyl group of the compound 1 is replaced by a hydrogen atom,
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Chemical Physics journal homepage: www.elsevier.com/locate/chemphys
Investigation into the molecular and electronic structures of a newly synthesized o-quinone derivative
T
Nobutsugu Hamamotoa, Sachie Araeb, Tetsuji Moriguchic, Ryo Irieb, Hitoshi Fujimotob,
⁎
a
Department of Chemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan Department of Chemistry, Faculty of Advanced Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan c Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu 804-8550, Japan b
ARTICLE INFO
ABSTRACT
Keywords: Density functional theory Electronic structure O-Benzoquinone
We studied on the geometric and electronic structures of a newly synthesized compound, 3-(tert-butyl)dinaphtho [1,2-b;1′,2′-d]furan-12,13-dione (1), having a twisted molecular structure at an o-quinone region in a crystal. The density functional theory (DFT) has been applied to investigate the geometric and electronic structures using the model compound (1H), where a tertiary butyl group of the compound 1 is replaced by a hydrogen atom. The optimized geometry for the model dimer of 1H represented well the twisted molecular structure of 1. The timedependent DFT method explained the observed optical properties of 1 in a solution and a powdery solid. The DFT calculations also gave a good estimation of the ionization potential and the electron affinity of 1H, explaining a similarity in reduction and a difference in oxidation between 1H and o-benzoquinone. This similarity in the electron affinity was confirmed by measuring a reduction potential of the compound 1 with cyclic voltammetry.
1. Introduction It has taken over a half-century to be reported the crystal structure of o-benzoquinone [1] since the successful isolation [2] owing greatly to its instability in the atmosphere [3]. o-Quinones have been used as reagents of various reactions making good use of their reactivity. The reduction of quinones has been extensively studied, and it has been reported that o-benzoquinone shows an anomalous behavior in the second reduction [4]. Some reactions have been proposed for this anomality [5]. On the other hand, the polycyclic aromatic compounds have been expected and actually applied to numerous ways in the various fields, which would depend greatly on their functional flexibility through structural modifications. Recently, a variety of aromatic compounds have been energetically synthesized and applied to the electronic devices. Among them, an introduction of hetero atoms into the aromatic ring can provide high thermal and optical stability, and may lead to additional functionality. The compounds with a fused polycyclic thiophene structure have been investigated by many researchers especially in the past decade [6–10]; however, there exist few reports [11,12] on applications of fused polycyclic furans to the organic electronics because of their instability and synthetic difficulty. Nonetheless, the
⁎
Corresponding author. E-mail address: [email protected] (H. Fujimoto).
https://doi.org/10.1016/j.chemphys.2019.04.033 Received 21 January 2019; Accepted 30 April 2019 Available online 02 May 2019 0301-0104/ © 2019 Elsevier B.V. All rights reserved.
compounds containing the fused furan ring have been synthesized aiming to create the materials having novel physical properties [13–21]. The aromatic polycyclic compounds with a ketone group have been also interested in photochemical and electrochemical properties such as ability for the source of reactive oxygen species [22,23], the reversible two-electron reduction [24–27], the excited state intramolecular proton transfer using proton near the ketone group [28,29], and so on. The density functional theory (DFT) has been applied to investigate physicochemical properties of various functional materials through calculating the electronic structures of the ground and the excited states [30]. We also applied DFT methods to several compounds, and extended successfully to investigate physical properties of them in the solid state using small units such as dimers and trimers extracted from the crystal structure [31–33]. We recently succeeded to synthesize 3-(tert-butyl)dinaphtho[1,2b;1′,2′-d]furan-12,13-dione (1), a new derivative of o-benzoquinone with a fused furan ring, which is stable in the atmospheric circumstance (Scheme 1). We will report here the geometric and electronic structures of the monomeric and dimeric systems for this compound calculated by the DFT methods. The obtained results will be discussed in comparison with the experimentally observed data.
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Scheme 1. Molecular structure and two types of molecular arrangements of 1 in the crystal.
2. Experimental section
oxidation, purified by column chromatography, and crystallized from hexane and ethyl acetate (see Electronic Supplementary Data for the details). An X-ray analysis was performed on the crystal with approximate dimensions of 0.30 × 0.20 × 0.10 mm3. Crystal structure data were collected at 120 K with graphite-monochromatized Mo Kα radiation. The structural data have been deposited to the Cambridge Crystallographic Data Centre (CCDC No. 1817034). The electronic absorption and emission spectra of 1 were measured with a chloroform solution. A thin film was obtained with scrubbing the powdery sample on a quartz glass, and was also used for spectroscopic measurements. Cyclic voltammetry experiments were performed in acetonitrile using a conventional three-electrode cell with platinum plate working electrode, platinum wire counter electrode, and Ag/AgCl reference electrode. The potential was measured using ferrocene as the reference material.
2.1. Computational details Geometry optimization and the excitation energies of a monomer and two types of dimers (Types 1 and 2) in Scheme 1 were carried out using the DFT and time-dependent DFT (TD-DFT) methods. To investigate the solvent effect of n-hexane and acetonitrile, we used the CPCM method [34,35]. It has been shown [31–33] that the Minnesota functional may give reasonable results for the cases of weak interacting systems, where the long-range interaction plays an important role; therefore, we decided to use the MN15 functional [36] for the exchange correlation term. For all atoms, 6-311G(d) basis set was employed. All calculations were performed using the Gaussian16 program package [37], and the results were visualized by the GaussView program package [38]. In the calculations, a tertiary butyl group of 1 was replaced with a hydrogen atom for the sake of saving CPU times, which is denoted as the compound 1H in the following discussion. For the monomer of 1H, the geometry was optimized under two types of symmetry, C1 and Cs, whereas both dimers were calculated under Ci symmetry. Each moiety of the Type 1 dimer reproduced well the molecular structure of 1 in the crystal, but Type 2 did not (see Electronic Supplementary Data). In order to give insight into the electronic structures of 1 in the solid state, we optimized the geometry of the Type 2 model dimer under the following conditions: The distance and the tilt angle between the moieties were optimized under Ci symmetry, where the structural parameters for each moiety were fixed as those for the moiety of the Type 1 dimer. Analytical vibrational frequency computations at the optimized structure were then performed to confirm that the optimized structure was at an energy minimum.
3. Results and discussion 3.1. Molecular structure of 1 in the crystal Firstly, we will briefly report on the molecular structure of 1 observed in the crystal. The compound 1 crystallized into the triclinic system, space group P1: formula C24H18O3, M = 354.38, a = 8.505 (4) Å, b = 8.678 (4) Å, c = 11.950 (5) Å, V = 851.5 (6) Å3, α = 79.384 (4)°, β = 82.782 (4)°, γ = 80.781 (4)°, Z = 2. All molecules in the crystal had nonplanar structures with a dihedral angle of 9.2° in the obenzoquinone region as shown in Scheme 1. The short contact of 2.85 Å was found between carbon and oxygen atoms of neighbor molecules. 3.2. Optimized geometries and electronic structures of monomer and dimers We performed the geometry optimizations of the monomer 1H under the two kinds of symmetry, C1 and Cs, without and with constraint of a planar structure, respectively. The optimized structure with C1 symmetry was nearly planar and similar to one obtained for planar
2.2. Experimental details The compound 1 was synthesized by dehydrative condensation of 6tert-butylnaphthalen-2-ol and 1,4-naphthoquinone and the subsequent 78
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Fig. 1. Optimized geometry for the monomer of 1H.
Cs symmetry without any remarkable differences in the Gibbs free energy. The relative energy of the Cs geometry was estimated to be 0.02 kcal mol−1 lower than that of the C1 structure; therefore we would choose the higher symmetric Cs geometry for the discussion. The optimized geometry with Cs symmetry is shown in Fig. 1 and the main structural parameters are compiled in Table S1 of Electronic Supplementary Data. The geometries of both dimers were optimized under Ci symmetry. Each molecule in the Type 1 dimer reproduced well the observed structure of 1 in the crystal; however, the geometry for each moiety of the Type 2 dimer showed large difference from the observed structure (see Fig. S1 of Electronic Supplementary Data). This might imply that intermolecular interaction is weak in the Type 2 arrangement, which exists just for filling the space in the crystal. Then, the geometry of the Type 2 dimer was optimized again under the conditions, where each moiety of the dimer was fixed to that of the Type 1 dimer and the distance and tilt angle between moieties were optimized under Ci symmetry.
Fig. 3. Orbital energies near the frontier orbitals of 1H. The symmetry species of each orbital is labeled under the Cs and Ci point groups for the monomer and the two model dimers (Types 1 and 2), respectively.
Fig. 2 illustrates the optimized structures for the dimers of Types 1 and 2 using the MN15 method. The main structural parameters of the moiety for the Type 1 dimer are shown in Table S1 of Electronic Supplementary Data. The X-ray analysis showed that the distance between the carbonyl groups of neighbor molecules in the Type 1 arrangement is estimated to be 2.85 Å, which is shorter than the sum of van der Waals radii as shown in Scheme 1. The C…O distance of the optimized structure of the Type 1 dimer was evaluated to be 2.915 Å, which is similar value to the observed one. Moreover, the moieties in the optimized Type 1 dimer were twisted, where the dihedral angle of OCCO was estimated to be 3.2˚. It would be concluded that this compound takes a planar structure in an isolated condition such as a
Fig. 2. Optimized geometries for the two model dimers of 1H.
79
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calculated to have large contributions from carbon atoms of the aromatic backbone, but have little contributions from carbonyl groups. The orbitals of the dimers were generated from two orbitals of the moieties through in-phase or out-of-phase combinations. Two monomeric moieties in the Type 1 dimer overlapped each other closely in the region of carbonyl groups; therefore, the orbitals of the monomer with large contribution from carbonyl group may strongly affect these pair of orbitals created through in-phase and out-of-phase interactions. In fact, the LUMO and the LUMO + 1 of the Type 1 dimer were generated from the LUMOs of two monomeric moieties, and the energy difference between these orbitals was estimated to be 0.49 eV, which was bigger than the others. On the contrary, the HOMO−3 (65a’) of the monomer split into the HOMO−6 (74au) and HOMO−7 (74ag) of the dimer, and the energy difference between them was estimated to be a relatively small value of about 0.01 eV, which was similar to other orbitals. This may be attributed to the difference in the orbital characters between the LUMO and the HOMO−3 of the monomer, which were assigned as πand σ-type orbitals, respectively. In the case of the Type 2 dimer, two molecules overlapped each other widely in the whole molecular framework; however, the distance between two moieties were optimized to be about 3.3 Å, which was close to the X-ray analysis and was larger than twice of the van der Waals radius for carbon. This may imply that the intermolecular interactions, and also the energy splitting between the in-phase and out-of-phase combinations, are small for a pair of molecules in the Type 2 dimer. Actually, all of the energy differences between molecular orbitals generated by these two combinations near the frontier orbital were much smaller than the value between the LUMO and the LUMO + 1 of the Type 1 dimer.
Table 1 Orbital energy of the monomer and the dimers in eV. Monomer
Dimer Type 1
a
Orbital
Energy
Orbital
15a” 14a” 13a” 12a” 11a” 10a” 65a’
−0.521 −1.115 −2.527 −6.703 −7.661 −8.020 −8.147
79ag 79au 78ag 78au 77au 77ag 76ag 76au
Type 2 a
Energy
Orbitala
Energy
−0.868 −0.934 −2.086 −2.571 −6.494 −6.497 −7.459 −7.492
79ag 79au 78au 78ag 77au 77ag 76au 76ag
−1.083 −1.175 −2.485 −2.498 −6.686 −6.728 −7.600 −7.702
a The symmetry species of each orbital is labeled under the Cs and Ci point groups for the monomer and the dimers, respectively.
solution, but the twisted structure in the single crystal due to the intermolecular interaction. The optimized structure of the Type 2 dimer was slightly different from the experimental one as compared to that of the Type 1 dimer. The relative Gibbs free energies of the two optimized dimers were estimated to be lower than the sum of the independent monomers by 0.3 kcal mol−1 and 8.3 kcal mol−1 for the Types 1 and 2 dimers, respectively, at 298.15 K. Fig. 3 compares the energy diagrams of the monomer and the two types of dimers in the region of the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbitals. The orbital symmetries were labeled under Cs and Ci point groups for the monomer and the dimers, respectively. All orbitals below a dotted line are occupied. The orbital energies are also summarized in Table 1. The atomic contributions to the molecular orbitals are shown in Fig. 4 (see Figs. S2–S6 of Electronic Supplementary Data for the other orbitals between the HOMO − 3 and the LUMO + 3 of the monomer and between the HOMO − 7 and the LUMO + 4 of the dimers). The HOMO of the monomer was calculated to have a large contribution from carbon atoms except for carbons of carbonyl groups, whereas the LUMO had large contribution from carbon and oxygen atoms of the carbonyl groups. The molecular orbitals between the HOMO − 4 and the LUMO + 4 were all π-type orbitals except for the HOMO − 3 (65a’), which had a strong contribution from lone pair electrons of oxygen atoms in the carbonyl groups and was apart about 1.4 eV from the HOMO. All other orbitals of the monomer were
3.3. Electronic absorption spectra of monomer and dimers We calculated the excitation energies for the monomer and the two types of dimers of 1H using the TD-DFT method. The optically allowed excited states obtained by the TD-MN15/6-311G(d) method are shown as vertical lines in Fig. 5. The height of each vertical line represents the oscillator strength of excitation. The solid line of each panel shows the theoretical absorption spectrum obtained by convoluting each excited state with a Gaussian function, where the full width at half maximum was assumed to be 0.06 eV and the oscillator strength was used as the pre-exponential factor. The experimentally obtained absorption spectra of 1 for an n-hexane solution and a thin film obtained by scrubbing the powdery sample on a quarts substrate are also shown in the figure.
Fig. 4. Several molecular orbitals near the HOMO and LUMO of the monomer and the dimers of Types 1 and 2. The symmetry of each orbital is labeled under the Cs and Ci point groups for the monomer and the dimers, respectively. 80
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Information about the low-lying excited states is essential for the application of the functional molecules, especially for photoactive devices because the photochemical processes depend largely on these states. The n-hexane solution of 1 exhibited a week absorption features around 2.6 eV accompanying broad intense absorption bands in the ultraviolet region. Under the Cs point group, the ground state of the 1H monomer was assigned to be the totally symmetric 1A’ state, and all singlet excited states are symmetry allowed. The excited states with the A’ and A” irreducible representations were calculated to have transition dipoles in and out of the molecular plane, respectively. The first allowed excited state, 11A”, was calculated to have a transition from the HOMO−3 to the LUMO as the main contribution with negligibly small oscillator strength, which can be assigned to the n-π* transition. This may imply non-radiative decay from the excited states of 1H, and is consistent with a failure to observe the emission of light for the compound 1. The second excited state, 11A’, was calculated to have a transition from the HOMO to the LUMO as the main contribution with relatively large oscillator strength, which is able to be assigned to the ππ* transition. These two excited states, 11A” and 11A’, may correspond to the weak absorption peak around 2.6 eV appeared in the observed absorption spectrum of 1. It could be concluded, moreover, that the 11A’ state is more responsible to the observed low lying absorption band than 11A” one from the calculated oscillator strength and the nonradiative nature of 1. We also calculated the low-lying triplet excited states, and the excitation energies of the first two triplet states were estimated to be 2.02 and 2.19 eV for 1H, which are assigned to the ππ* and nπ* excited states, respectively. These low-lying triplet states would be also responsible to the non-radiative nature of the compounds 1. The observed spectrum for the thin film of 1 was similar to that obtained for the n-hexane solution, where the small red shifts and some broadenings of every feature were recognized. The ground states of both model dimers were calculated as the 1Ag state under the Ci point group; therefore, the optically allowed excited states should belong to the Au irreducible representation, which consist of one electron transitions between orbitals with different parity. As mentioned above, the optimized structure for the Type 2 dimer was slightly different from that for the corresponding molecular pair in the crystal of 1. We calculated the excited states for the pair of molecules in the crystal corresponding to the Type 2 dimer, and obtained similar results to those obtained for the Type 2 dimer (See Fig. S8 of Electronic Supplementary Data); therefore, the electronic structures of the optimized geometry for the Type 2 dimer were used in the following discussion. Simply speaking, the orbitals of the dimers, which are generated from those of two monomeric moieties through in-phase and out-ofphase combinations, belong to the different irreducible representations with different parity, ag and au, respectively, under the Ci point group. These orbitals with the different parity contribute individually to the different excited state with the different irreducible representation; therefore, the excited states of the dimers are also in pairs with the
Fig. 5. Excited states of the monomer and the two types of dimers obtained by the TD-MN15 calculations. The vertical line depicts the oscillator strength of each state. The theoretical absorption spectra were obtained by convoluting each excited state with a Gaussian function. The full width at half maximum was assumed to be 0.06 eV and the oscillator strength was used as the preexponential factor.
There exist two different molecular arrangements between two adjacent molecules in the crystal of 1, which are similar to those of the model dimers; therefore, we depict the sum of the absorption spectra of these dimers in the figure. The spectrum of each dimer is shown individually in Fig. S7 of Electronic Supplementary Data. The excitation energies of the monomer and the dimers are summarized in Tables 2 and 3 for the several optically-allowed excitations with oscillator strengths. The first state is only listed for the symmetry-forbidden excitation. The important components of the one electron transition, in which the coefficient is larger than 0.3, are also presented. The excitation energies until 6.0 eV and the important components of the monomer and dimers are shown in Tables S2–S4 of Electronic Supplementary Data.
Table 2 TD-MN15 excited states of the monomer (Solvent: n-hexane).
Cs
a b c
State
Main configuration (|C|≧0.30)
Ea
fb
pc
11A” 11A’ 21A” 21A’ 31A’ 13A’ 13A”
+0.67(65a′ → 13a″)[90%] +0.69(12a″ → 13a″)[95%] +0.61(64a′ → 13a″)[74%] −0.35(10a″ → 13a″)[25%] + 0.58(11a″ → 13a″)[67%] +0.53(10a″ → 13a″)[56%] + 0.32(11a″ → 13a″)[20%] +0.60(12a″ → 13a″)[72%] +0.67(65a′ → 13a″)[90%]
2.43 2.65 3.62 3.78 3.98 2.02 2.19
0.0000 0.115 0.0000 0.015 0.015 -
z x+y z x+y x+y -
Excitation energy in eV. Oscillator strength. Transition moment direction. 81
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Table 3 TD-MN15 excited states for the dimers of Types 1 and 2. Statea
Main configuration (|C| ≧ 0.30)
Eb
fc
pd
Type 1 Ci
1Ag 1Au 2Au 3Au 4Au
+0.34(74ag → 78ag)[23%] + 0.41(74au → 78au)[34%] +0.44(74ag → 78au)[39%] + 0.34(74au → 78ag)[23%] +0.64(77ag → 78au)[82%] +0.66(77au → 78ag)[87%] +0.31(71ag → 78au)[19%] + 0.42(76ag → 78au)[35%]
2.39 2.40 2.56 3.10 3.56
0.0000 0.005 0.073 0.061 0.029
x+y+z x+y+z x+y+z x+y+z
Type 2 Ci
1Ag 1Au 2Au 3Au
+0.46(74ag → 78ag)[42%] + 0.45(74au → 78au)[41%] +0.48(74ag → 78au)[46%] + 0.44(74au → 78ag)[39%] +0.47(77ag → 78au)[44%] + 0.50(77au → 78ag)[50%] +0.50(77ag → 78au)[50%]−0.48(77au → 78ag)[46%]
2.40 2.40 2.75 2.89
0.0000 0.0002 0.125 0.022
x+y+z x+y+z x+y+z
a The symmetry species of each state is labeled under the Ci point group, where the Ag and Au states are symmetry-forbidden and allowed, respectively. The first state is only listed for the symmetry-forbidden excitation. b Excitation energy in eV. c Oscillator strength. d Transition moment direction.
different parity, Ag and Au. In the case of the Type 1 dimer, the lowest excited state was calculated to be the symmetry-forbidden 1Ag state, where its counterpart was the optically-allowed 1Au one. These states consisted mainly of transitions from two occupied molecular orbitals, the HOMO−7 (74ag) and the HOMO−6 (74au), to two unoccupied molecular orbitals, the LUMO (78au) and the LUMO + 1 (78ag). The HOMO−7 and the HOMO-6 of the dimer were originated from the HOMO−3 (65a’) of the monomer, and the LUMO and the LUMO + 1 of the dimer were from the LUMO (13a”) of the monomer; therefore, both excited states of 1Ag and 1Au can be assigned to the n-π* transitions. The symmetry allowed excited states, 2Au and 3Au, consisted mainly of transitions between the orbitals of the dimer originated from the HOMOs and the LUMOs of the monomeric moieties, which can be specified to the π-π* transitions. As contrast with the case of 1Au excited state, one electron transitions between symmetry-allowed orbital pairs of 77ag-78au and 77au-78ag contribute independently to the different excited states of 2Au and 3Au, respectively. The excitation energies for the symmetry-forbidden counterparts, 2Ag and 3Ag were estimated to be 2.53 and 3.08 eV, respectively. In the case of the Type 2 dimer, the excited states were also calculated to become a pair of Ag and Au states, which are symmetry-forbidden and allowed states, respectively. The energies of the first three symmetry-allowed states, 1Au, 2Au and 3Au, were estimated to be 2.40, 2.75 and 2.89 eV, respectively. The lowest excited state was calculated to be the symmetry-forbidden Ag state for the Type 2 dimer, too. The X-ray study showed that inversion centers exist between molecules in the crystal of the compound 1, which are similar positions to those of the dimers of Types 1 and 2; therefore, it would be expected that the symmetry restrictions may persist for the optical transitions of this compound 1 in a solid state. The thin film of 1 showed a broad absorption feature as the low energy tail in the region from 2 to 3 eV, which could be assigned to correspond with the first three excited states similar to those calculated for the dimers of Types1 and 2. It should be noted that the first excited states of both dimers are calculated to be the symmetry-forbidden excited states. This is consistent with the experimental results that any light-emission was not able to be detected for the solid sample of the compound 1. In passing, it should be mentioned that the compound 1 is quite stable under atmospheric conditions in contrast to o-benzoquinone. Any change was not detected in an absorption spectrum for the sample kept in air for five months (see Fig. S9 of Electronic Supplementary Data).
Table 4 Ionization potential (Ip) and electron affinity (Ea) in eV of the monomer and the dimers for 1H. The values for o-benzoquinone are shown in parentheses. Monomer
Dimer Type 1
Calc. Ip
Vertical
Ea
Adiabatic Vertical Adiabatic
ΔG0red
Ip1 Ip2 Ip(σ)
7.61 – 9.06 7.48 1.56 1.80 3.71
(9.64a) (9.64a) (9.18a) (1.54a) (1.79a) (3.99a)
Obs.
Calc.
– – −(9.6b) – −(1.62c) – −0.981d
7.20 7.20 – – 1.89 – –
Type 2
7.41 7.44 – – 1.75 – –
a This work. In the first ionization process of o-benzoquinone, an electron is removed from the σ-type molecular orbital. b The value from Ref. [41]. c The value from Ref. [43]. d Reduction potential of 1 measured by cyclic voltammetry.
reaction (ΔGox and ΔGred) from the Gibbs free energy differences between the neutral and the corresponding ionic states. The obtained values are summarized in Table 4. It has been reported [39] that the calculated values of ΔGox and ΔGred for the optimized geometry in the gas phase are similar to those obtained for the geometry in the presence of the self-consistent reaction field; therefore, the solvent effect for calculations of ΔGox and ΔGred was taken into consideration by applying the CPCM method to the gas-phase geometries. The redox potentials were determined by using the free energy change for the half reactions represented by the thermodynamic cycle. The entropy was corrected for solvation with the method developed by Whitesides et al. [40]. The values of the first Ip (Ip1) for the compound 1H were estimated to be 7.61 and 7.48 eV for the vertical and adiabatic values, respectively, where those of Ea were 1.56 and 1.80 eV. The small differences between the vertical and adiabatic values would imply that there exist small differences between the stable structures of the neutral and the charged molecules. Koening et al. [41] reported the Ip1 value of o-benzoquinone to be 9.6 eV by ultraviolet photoelectron spectroscopy, whereas Honda et al. [42] estimated the value as 9.32 eV by symmetry adapted cluster-configuration interaction (SAC-CI) calculations. We also estimated the vertical and adiabatic Ip1 values of o-benzoquinone at the same calculation level with one used for the compound 1H to be 9.64 eV and 9.18 eV, respectively. On the other hand, Marks et al. [43] reported the Ea value of o-benzoquinone as 1.62 eV by electron photo-detachment spectroscopy, whereas Honda et al. [42] estimated the value to be 1.63 eV by SAC-CI calculations. We also evaluated the vertical and
3.4. Electronic property The vertical values of the ionization potential (Ip) and the electron affinity (Ea) were calculated by the ΔSCF method for the optimized geometries of the monomer and two types of dimers, and the adiabatic values of Ip and Ea were also estimated as the free energy of redox 82
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adiabatic Ea values for the o-benzoquinone to be 1.54 eV and 1.79 eV, respectively, with the same calculation level as the compound 1H. In comparison with the values of 1H, interestingly, the Ip1 value of obenzoquinone is lower about 2 eV than that of 1H, whereas both compounds show a similar Ea value to each other. This would be due to differences and similarities in nature of the electron donating and accepting levels between these two compounds. As pointed out previously [42], the present DFT calculations for obenzoquinone also showed that an electron is removed from the σ-type molecular orbital in the first oxidation process, which has strong contributions from carbonyl groups, and that an extra electron in the reduction process is accommodated in the LUMO, which is assigned to the π-type molecular orbital. In the case of 1H, the calculation results indicated that an electron is removed from or added to the π-type molecular orbitals, the HOMO or the LUMO, respectively. It should be noted that the HOMO of 1H spreads over the whole molecule, but the LUMO localizes only on the o-naphthoquinone region as shown in Fig. 4. This implies that a hole generated in oxidation can delocalize over the whole molecule, but an additional electron in reduction is confined to the o-naphthoquinone region. This would be confirmed by the calculated Ea values for several fragment molecules of 1H, all of which showed nearly similar values of 1.4 ± 0.1 eV (see Fig. S10 of Electronic Supplementary Data for the molecules and the values). The Ip and Ea values for the dimers were also calculated to be 7.20 and 1.89 eV for Type 1, and to be 7.41 and 1.75 eV for Type 2, respectively, with a small difference as compared to those of the monomer. These values were correlated well with the orbital energies of the HOMO and the LUMO. This would reveal that there are weak interactions between molecules in both types of the dimers, which is consistent with the results for the energy diagrams of the model dimers as discussed above. The second Ip (Ip2) values were calculated to be 7.20 and 7.44 eV for the dimers of Types 1 and 2, respectively. The Ip1 and Ip2 values were approximately equal, which was consistent with the pseudo-degeneration of the HOMO and HOMO − 1 for the dimers because of the weak intermolecular interactions. René et al. [5] reported that the o-benzoquinone derivatives take a type of Michael addition reaction subsequent to the second reduction, which corresponds to the extremely small size of the second reduction peak in a cyclic voltammogram. They also pointed out that a presence of water and a species of supporting electrolytes have a strong effect on the second reduction potential, but not much on the first one of these compounds. This shows that the careful investigations with rigorous experimental conditions should be needed for obtaining an accurate value of the second reduction potential. The compound 1 showed the first reduction peak at −1.08 V with a difference of 77 mV between anodic and cathodic potentials, which would reveal the process to be reversible, and was similar to the reported value of –0.954 V for the obenzoquinone derivative [5]. The second reduction peak was observed around −1.47 V without cathodic peaks corresponding to the first and second reduction processes (see Fig. S11 of Electronic Supplementary Data). The values of ΔGred were calculated to be 3.71 and 3.99 V for the compound 1H and o-benzoquinone, respectively. These values of ΔGred correspond to the observed first reduction potentials of these compounds, reflecting a similarity in the values of Ea for 1 and o-benzoquinone.
was used for calculations. The geometry optimizations using the MN15 functional showed that the monomer of 1H has a planar structure with Cs symmetry, whereas the dimer of 1H reproduced well the twisted structure of 1 in the crystal. This shows that the compound 1 takes a planar structure in an isolated condition such as a solution, but the twisted structure in the crystalline solid due to the intermolecular interaction especially at the carbonyl groups of neighbor molecules. The excitation energies were calculated for the monomer and the model dimers of 1H by the time-dependent DFT method using the optimized geometries. The excited states calculated for the monomer and the dimers corresponded well to the observed absorption spectra for a solution and a powdery solid of 1, respectively. Moreover, the results also explained the non-radiative nature of 1 both in solutions and in solids. The vertical values of the ionization potential (Ip) and electron affinity (Ea) were estimated by the ΔSCF method for the monomer and the two model dimers of 1H. The adiabatic values were also calculated from differences in the Gibbs free energy between the neutral and corresponding ionic states. The values of the first Ip (Ip1) for 1H are smaller than those for o-benzoquinone by 2.03 and 1.70 eV for the vertical and adiabatic values, respectively. This is due to a difference in character of the molecular orbital, from which an electron removal occurs in the ionization process. On the other hand, there was a small difference in the values of Ea between 1H and o-benzoquinone. In both compounds, an extra electron was accommodated in the lowest unoccupied molecular orbital (LUMO). This similarity in the Ea values would be explained by the LUMO of 1H, which will confine an extra electron to the o-benzoquinone region. This similarity in the Ea value of 1H to o-benzoquinone was also confirmed by measuring the reduction potential of the compound 1 with cyclic voltammetry. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.chemphys.2019.04.033. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
4. Conclusion
[19]
The density functional theory (DFT) method was applied to investigate the geometric and electronic structures of a newly synthesized o-benzoquinone derivative with a fused furan ring, 3-(tert-butyl)dinaphtho[1,2-b;1′,2′-d]furan-12,13-dione (1). The compound 1 crystallized into the triclinic system with the space group of P1, and all molecules had non-planar structures. The model compound 1H, where a tertiary butyl group of the compound 1 is replaced by a hydrogen atom,
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