Intramolecular charge transfer in di-tert-butylaminobenzonitriles and 2,4,6-tricyanoanilines: A computational TDDFT study

Computational and Theoretical Chemistry 1036 (2014) 1–6

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Intramolecular charge transfer in di-tert-butylaminobenzonitriles and 2,4,6-tricyanoanilines: A computational TDDFT study Marek Z. Zgierski a, Edward C. Lim b, Takashige Fujiwara c,⇑ a

National Research Council of Canada, Ottawa K1A 0R6, Canada Department of Chemistry and The Center for Laser and Optical Spectroscopy, The University of Akron, Akron, OH 44325-3601, USA c Department of Chemistry and Biochemistry, The Ohio State University, Columbus, OH 43210-1117, USA b

a r t i c l e

i n f o

Article history: Received 30 November 2013 Received in revised form 25 February 2014 Accepted 25 February 2014 Available online 13 March 2014 Keywords: Intramolecular charge transfer TDDFT calculations Di-tert-butylaminobenzonitriles 2,4,6-Tricyanoanilines CC2-RI calculations

a b s t r a c t The time-dependent DFT calculations for the low-lying excited electronic states of 4- and 3-di-tert-butylaminobenzonitrile, and 2,4,6-tricyano-N,N-dimethylaniline and 2,4,6-tricyanoaniline have been performed to investigate the mechanism of photo-induced intramolecular charge transfer (ICT). In addition, CC2-RI calculations were performed for TCDMA. For di-tert-butylaminobenzonitriles, we found evidence for the pr -state mediated mechanism associated with the sequential state-switches: pp ! pr ! ICT. It is predicted that 2,4,6-tricyano-N,N-dimethylaniline (TCDMA), but not 2,4, 6-tricyanoaniline, possesses two ICT states, one of which shows the ICT-characterized quinoidal geometry and lies below the initially photo-excited S1 ðpp Þ state. The pp ! ICT formation in TCDMA feasibly occurs in accord with the observed time-resolved excited-state absorption spectra and the biexponential fluorescence decay from the mixed S1 ðpp Þ/ICT state of TCDMA. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction The intramolecular charge transfer (ICT) in the photoexcited 4(dimethylamino)-benzonitrile, 4-DMABN, and related electron donor–acceptor molecules, leading to the appearance of dual fluorescence [1], continues to be a topic of extensive experimental and theoretical studies [2]. Most studies have focused on the two fundamental issues associated with the ICT formation, namely the geometrical structure of the ICT state, and the reaction pathway that connects the initially photoexcited La -like pp state to the ICT state. The former issue has largely centered on the question of whether the ICT state of DMABN (which arises from the transfer of an electron from the dimethylamino group to the benzonitrile moiety) has a twisted amino group (TICT) [3], or a planar amino group (PICT) [4], relative to the plane of the phenyl ring. A large majority, if not all, of theoretical studies [5] indicate the amino-group twist as the reaction coordinate for the La ðpp Þ ! ICT formation. Very extensive time-resolved spectroscopic studies of DMABN and related aminobenzonitriles indicate the rise time of the ICT fluorescence is identical to the initial decay of the LE (locally excited) fluorescence [6]. Based on this observation, it has long been

⇑ Corresponding author. E-mail address: [email protected] (T. Fujiwara). http://dx.doi.org/10.1016/j.comptc.2014.02.029 2210-271X/Ó 2014 Elsevier B.V. All rights reserved.

assumed that the ICT state is formed directly from the LE state upon photo-excitation, i.e., the predecessor–successor relationship between the two relevant states. Recent femtosecond timeresolved excited-state absorption experiments on DMABN [7], however, indicate that a highly polar pr state, arising from the promotion of an electron from the aromatic p orbital to the r orbital localized on the cyano (CN) group, is the major precursor of the TICT state, as we have previously proposed [8,9]. More specifically, the rise time (4 ps for DMABN in acetonitrile) of the TICT-state transient at about 425 nm is identical to the decay time of the 700 nm picosecond transient, assigned to the pr -state absorption [7]. Moreover, there is evidence that the fluorescent ICT state differs from the TICT state that is observed in transient absorption [10,11]. Recent CASPT2/CASSCF calculations on DMABN by Coto et al. [12] support that the pr state plays a key role in a reaction pathway towards the ICT state in a polar environment and assign the excited-state absorption to the 90° twisted TICT state, and the ICT fluorescence to a partially twisted (54.4°) intramolecular charge transfer (pTICT) state. The summarized interpretations above have been questioned by Zachariasse et al. [13] who assign the 700 nm transient of DMABN to the absorption spectrum of the LE state. As the primary support, they provided the observation of around 700 nm pp pp absorptions from 2,6-dimethyl-4X (X = F, Br, CF3)N,N-dimethylanilines that do not involve a low-lying pr state. More recently, however, excited-state dynamics of 4-fluoro-N,

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Scheme 1.

N-dimethylaniline (FDMA) [14] has been investigated in closer analogy to DMABN derivatives. FDMA shows dual fluorescence from a pp state and a TICT state in both n-hexane and acetonitrile, and indicates that both states are effectively in thermal equilibrium at room temperature. Unlike 2,6-dimethyl-4-fluoro-N, N-dimethylanilines, no ultrafast excited-state absorption around 700 nm is observed for FDMA in either n-hexane or acetonitrile [14]. This fact suggests that there is not sufficient evidence to rule out a potential inclusion of the pr state for the ICT formation in photophysics of DMABN. Recently, Zachariasse and co-workers [15] reported the subpicosecond time-resolved excited-state absorption of 4-(di-tertbutylamino)benzonitrile (p-DTBABN), and 3-(di-tert(m-DTBABN), Scheme 1, which undergo efficient ICT formation in either polar/ non-polar solvent. The X-ray crystal analysis for m-DTBABN indicates that the amino-group twists 86:5 with respect to the phenyl plane. Interestingly, the excited-state absorption spectra of pDTBABN show a very rapid decay (70 fs in n-hexane and 60 fs in acetonitrile) from the LE state and a corresponding rise of the ICT absorption at 322 nm, indicating that the ICT state is formed from the LE state. The occurrence of such an efficient ICT in m-DTABN is very different from the absence of ICT in all other N,N-dialkylaminobenzonitriles. In another study by the same group [16], they have shown that 2,4,6-tricyano-N,N-dimethylaniline (TCDMA) and 2,4,6-tricyanoanilne (TCA) exhibit a single fluorescence band even in polar solvents (See Scheme 1), suggesting that the ICT formation from an initially photoexcited LE state does not occur in the two compounds. The excited-state absorption spectra of TCDMA, however, shows the decay of the 605 nm transient is accompanied by a corresponding rise of the 340 and 508 nm transients [Cf. Fig.4(b)], indicating that an excited-state dynamics other than the ICT formation exists. It may be subscribed to the existence of two conformational isomers (conformers) that lie nearby in the S1 ðpp Þ and rapidly interconnect with each other (<10 ps) [16]. In this paper, we present time-dependent DFT (TDDFT) calculations and, in part coupled-cluster (CC2) with auxiliary cc-pVDZ basis sets for TCDMA, for the energetics in the low-lying excited states of DTBABN and tricyanoanilines to interrogate the photophysics for the ICT formation. In comparison of the calculations with femtosecond excited-state absorption spectra, we discuss the possibility that the pr state plays a crucial role in the ultrafast dynamics of the ICT formation where an intermediate state transforms the pp to the final (twisted) ICT state.

basis sets [25,26] using the TURBOMOLE package. We particularly focused on the CCN angle (hCCN ) on the cyano moiety of the compounds and computed the adiabatic energy potentials as a function of hCCN . First, we optimized geometries of the pp or pr state. Subsequently, we used TD/BP86/cc-pVDZ or TD/B3LYP/cc-pVDZ level of theory [27–30] at the optimized CIS/cc-pVDZ geometries to calculate vertically electronic excitation energies at a fixed CCN angle with the optimized geometries. The TDDFT potential energy curves [31–35] were calculated as a function of hCCN . This allows deductions for the relative stability in various electronic states and barrier heights that must be overcome (or the minimum vibrational excess energy) to switch from one state to another. The TDDFT adiabatic energy potentials were mapped over the specific reaction coordinate of hCCN . Solvent effects were not included in this study. 3. Results and discussion 3.1. p-DTBABN and m-DTBABN Fig. 1 presents the adiabatic electronic energy potentials for pDTBABN obtained with the TD/BP86/cc-pVDZ level of theory. The blue dashed and dotted lines show the electronic energy curves calculated as a function of hCCN using the optimized geometry of the first excited 2A0 pp state. The potential energies are given relative to the minimum energy in the optimized ground-state geometry. The lowest pp state corresponds to the La state that is initially accessed upon photoexcitation (hCCN ¼ 180 ). Photoexcitation corresponding to 3.8 eV in experiment is indicated by a thick gray line in Fig. 1. As the CCN bond bends, the energy of S1 (lowest blue dashed line) increases. On the other hand, the energy of the S3 state (second lowest dashed blue line) decreases until it becomes

2. Computational methods The time-dependent density functional theory (TDDFT) calculations [17,18] were performed to obtain the excited-state geometries and transition energies using GAUSSIAN [19] (single-point calculations for DTBABN) and TURBOMOLE [20,21] (VMN5 version of the B3-LYP method [22]). In addition, the excited states of TCDMA were investigated by the second-order approximate coupled cluster (CC2-RI) method [23,24] with the auxiliary cc-pVDZ

Fig. 1. TD/BP86/cc-pVDZ adiabatic potential energies in p-DTBABN as a function of the CCN angle of the nitrile. The ordinate denotes energy in eV relative to the ground-state energy. The potential curves (blue dashed and dotted) are for a singlepoint TDDFT energy as a function of hCCN by using the 1 pp optimized structure, and the black and red solid curves are for a single-point TDDFT energy as a function of hCCN for the 1 pr structure. The gray horizontal band indicates an initially photoexcited level in experiment, and relevant conical intersections are depicted by arrows. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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involved in an avoided crossing with S2 , and it continues downward. The crossing occurs just below the energy used for excitation in the experiment. This downward in energy continuing line belongs to what is the pr state. The solid and red curves were obtained over hCCN using the optimized the 1 pr state geometry. At hCCN  125 we have the minimum of the pr state located at 0.5 eV below the minimum of the initial pp state. The pr state crosses the pp state at CCN near 152° with 0.24 eV above the energy minimum geometry at hCCN ¼ 180 in the La ðpp Þ state. Note that as hCCN increase, the pr state undergoes an avoided crossing with the S2 state encountering at hCCN  140 with 0.38 eV above the minimum of the pr state and 0.45 eV below the initial excitation energy. When hCCN increases above 140°, the pr character is taken over by the S2 state, whereas the S1 state becomes more of the charge-transfer character. This state reaches the minimum at hCCN ¼ 180 with about 0.57 eV below the initially excited pp state and 0.1 eV below the minimum of the pr state. The optimized structures of the low-lying pp ; pr , and ICT states are shown in Fig. 2. The ICT state of the p-DTBABN has been also optimized with the TD/B3LYP method in TURBOMOLE. Attempts with the unconstrained TD optimization of pr state resulted in the same ICT geometries. Fixing the CCN angle and running the TD optimization leads to the geometry and energy of the pr state. Table 1 lists the TD/BP86/cc-pVDZ energies for the pp ; pr , and ICT states along with the computed dipole moments. The dipole moments for the ICT states show reasonable agreement with the experimentally determined value of 17 D [15]. Fig. 3 presents the adiabatic energy potentials for m-DTBABN as a function of hCCN . Again, blue dashed and dotted lines present the excited-state potentials obtained by using the optimized S1 ðpp Þ

(a) ππ* state

Table 1 The relative energies and dipole moments of the low-lying excited states in pDTBABN obtained by TD/BP86/cc-pVDZ level of theory. State b

pr pr c pp b pp c ICTb ICTc a b c

Energya (eV)

Dipole moment (D)

0.08 0.32 0.57 0.05 0.00 0.73

15.1 19.7 12.1 13.8 15.7 15.7

Relative to the absolute energy of 694.214921696 au. Based on the CIS/cc-pVDZ optimized geometries. Based on the TD/BP86/cc-pVDZ optimized geometries.

62˚

Fig. 3. The same as Fig. 1, but for the m-DTABN isomer.

(b) πσ* state

(c) ICT state

geometry. While the S1 tends to rise more steeply than in the para isomer in energy with decreasing hCCN , the S4 state (the second lowest blue dotted line at hCCN ¼ 180 ) decreases in energy, eventually becoming the lowest pr state. The black and red solid lines, on the other hand, depict the TD/BP86 adiabatic potential energies calculated with the optimized geometries of the lowest pr state. The crossing of the lowest blue dashed line (pp ) with the lowest red solid line (pr ) sets the maximum barrier height of about 0.34 eV for pp ! pr transition at hCCN ¼ 148 . The barrier height is about 0.1 eV larger than in the para isomer. At hCCN  125 , the pr state has the minimum that lies at 0.12 eV below the minimum of the initial pp photoexcited state (hCCN ¼ 180 ). Increasing hCCN initially leads to cause an increase of energy in the lowest pr state until hCCN reaches 140°, where an avoided crossing occurs in the vicinity of ICT state. As a result of the avoided crossing, the maximum barrier for the pr ! ICT state

Table 2 The TD/BP86/cc-pVDZ geometrical parameters for the p-DTBABN.

Fig. 2. The optimized structures of p-DTBABN in the low-lying excited-states: (a) 1 pp , (b) 1 pr , and (c) ICT, obtained with TD/BP86/cc-pVDZ level of theory.

State

CNCC dihedral angle (°)

CN bond length (Å)

pp pr

62.1 78.3 93.9

1.148 1.284 1.178

ICT

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transition becomes about 0.24 eV. It lies, however, below the minimum of the initial pp state. With hCCN > 140 , the pr state energy increases until it crosses the photoexcited state (pp ) of 0.45 eV above its minimum, while the energy of ICT state decreases and the state reaches its minimum at hCCN  177 and 0.24 eV below the minimum of the pr state. The calculations for both DTBABN isomers show that the lowest pr state is naturally switched to the ICT state as follows: (a) straightening the CCN angle, (b) a slight torsion of the di-tertbutylamino group to its nearly perpendicular orientation with respect to the benzonitrile moiety, and (c) shortening of the CN bond length. The relevant geometrical parameters for the pp ; pr , and ICT states in p-DTBABN are collected in Table 2. The state switch from the pr to the ICT is accompanied by a small barrier which is substantially smaller than the initial excess energy. Such a sequential scheme for pp ðLa Þ ! pr ! ICT does render fairly large kinetic rate constants between the states involved, which is consistent with the observation of ultrafast ICT formation in the time scale of 60–70 fs in solutions of p-DTBABN [15]. It also rationalizes the observation of efficient ICT fluorescence in m-DTBABN, which is the only meta-substituted aminobenzonitriles known to exhibit ICT emission [15].

Table 4 TD/B3LYP/cc-pVDZ ICT- and state.

pr -state energies relative to the energy of the S1 ðpp Þ

Molecule

ICT state (eV)

pr State (eV)

TCA TCDMA

0.51 0.03

1.3 0.8

3.2. TCA and TCDMA Table 3 lists the TD/B3LYP/cc-pVDZ vertical excitation energies and oscillator strengths for the absorption in ground-state 2,4,6tricyanoaniline (TCA) and 2,4,6-tricyano-N,N-dimethylaniline (TCDMA), which are compared to the experimental absorption maxima [16]. The predicted absorption maxima are in good agreement with the experimental values. Among the low-lying excited states, the pr state is computed to be far above the S1 ðpp Þ state (>0.80 eV) in both molecules (Table 4), so that the pr state is not expected to play a significant role in the photophysics of the ICT formation when the molecule is photoexcited to the S1 ðpp Þ. The high energy of the pr state in the tricyano compounds is due to the presence of the extra electron-accepting CN groups in both TCDMA and TCA, which strongly blue-shifts the pr state relative to the S1 ðpp Þ state [36]. The excited-state absorption spectra of TCA, at the pump–probe delay times between 0.1 and 2.5 ps, have peak positions at about 640 nm and 520 nm in acetonitrile [16], as shown in Fig. 4(a). The observed peak wavelengths and their relative intensities are qualitatively in good agreement with the calculated vertical excitation energies from the S1 ðpp Þ state; the calculated spectra of 644 nm (oscillator strength, f = 0.236) and 550 nm (f = 0.178) are shown in Fig. 4(c), and collected in Table 5. The TD/B3LYP/cc-pVDZ calculation shows that the highly polar TICT state, with the –NH2 group perpendicular to the plane of the tricyanobenzene ring, is

Fig. 4. (a) The excited-state absorption spectra of TCA in acetonitrile, measured at pump–probe delay times between 0.1 and 2.5 ps, (b) The corresponding spectra of TCMDA in acetonitrile, measured at pump–probe delay times between 0.1 and 10.0 ps, following 356 nm excitation. Arrows indicate the time-evolutions of the transients, adapted from Ref. [16] with permission. (c) The electronic absorption spectrum (not scaled in intensity) of 1,3,5-tricyanobenzene radical anion (TCB ) in THF, adapted from Ref. [37] with permission. The stick spectra show the vertical excitation energies with its oscillator strengths in the S1 ðpp Þ, otherwise marked, calculated by TD/BP86/cc-pVDZ level of theory; the stick spectra (orange) correspond to TCA, whereas the ones (red) to TCDMA as listed in Table 5. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 3 The TD/B3LYP/cc-pVDZ vertical excitation energies and oscillator strengths (f) of the transitions from the ground states of TCA and TCDMA, and their comparison to the experimental absorption maxima in n-hexane.

a

Excitation

Energy (nm)

f

Electronic transition

State character

Experimenta (nm)

TCA S1 S2 S3 S4

S0 S0 S0 S0

324.4 255.9 222.6 219.5

0.104 0.139 0.416 0.209

H!L H !Lþ1 H1 !L H1 !Lþ1

pp pp pp pp

349.5 272.4

TCDMA S1 S0 S2 S0 S3 S0 S4 S0

356.6 296.7 242.8 230.1

0.060 0.298 0.036 0.214

H!L H !Lþ1 H  1 ! L þ 1, H  2 ! L H1 !L

pp pp pp pp

367.1 307.9

From Ref. [16].

M.Z. Zgierski et al. / Computational and Theoretical Chemistry 1036 (2014) 1–6 Table 5 Comparison of TD/B3LYP/cc-pVDZ excited-state absorption for TCA and TCDMA with experiment.

a b c

Transition

Energya (nm)

fb

Experimentc (nm)

TCA S5 S1 ðpp Þ S7 S1 ðpp Þ S1 ðpp Þ S9

644 550 527

0.236 0.178 0.192

640

TCDMA S3 S1 ðpp Þ S4 S1 (ICT) S5 S1 (ICT)

647 563 451

0.156 0.1229 0.2648

605

525

508

Calculated vertical excitation energy. Oscillator strength. The excited-state absorption maxima from Ref. [16].

about 0.51 eV above the S1 ðpp Þ state, Table 4. Consistent with the large TICT-pp electronic energy gap, the measured dipole moment of the S1 ðpp Þ state is only about 6.1 D in TCA [16]. For TCDMA, on the other hand, our TD/B3LYP/cc-pVDZ calculation predicts the ICT state very slightly below (0.03 eV) the S1 ðpp Þ state, Table 4. The near degeneracy of the S1 ðpp Þ state and the highly polar TICT state would lead to a larger dipole moment for the fluorescent S1 ðpp Þ state in TCDMA, consistent with a large dipole moment of 12 D, obtained from the solvatochromic analysis of the fluorescence [16]. The radiative decay constant of TCMDA is smaller than that of TCA, consistent with a larger ICT character of the S1 state of TCDMA relative to TCA. The picoseconds time-dependent excited-state absorption could also be observed due to the presence of the nearby excited states. The subpicosecond excited-state absorption spectra of TCDMA in acetonitrile have been reported by Zachariasse et al. [16], which are shown in Fig. 4(b). The transient of an initial maximum at 605 nm absorption decreases in intensity and shifts to 580 nm for delay time of 0.1–0.2 ps after excitation. The decrease in intensity of the 605 nm transient is accompanied by a corresponding rise in the 508 and 340 nm transients, both of which occur in about 2.2 ps. An isosbestic point appears around 530 nm, indicating that two kinetically interconnected excited-states are present. For the longer pump–probe delay times of 10 and 100 ps, the 508 and 340 nm transients do not exhibit significant spectral change (not shown). Their spectral features qualitatively resemble the electronic absorption spectrum of 1,3,5-tricyanobenzene radical anion (TCB ) in THF as shown in Fig. 4(c), which is evident that TCDMA reveals the ICT character upon photoexcitation. The stick spectra show that the vertical transition energies with their oscillator strengths, calculated with TD/BP86/cc-pVDZ, are also shown in Fig. 4(c), which support the occurrence of the ICT excited-state absorption of TCDMA.

(a) ICT-Q

(b) ICT-AQ

29.3˚

ΔE = –0.12 eV

5

The fluorescence of TCDMA in acetonitrile (not shown here) exhibits double-exponential decays with the low-energy part (541 nm) showing growth of the emission in about 2 ps, which is very similar to the short-time decay of the high-energy component (423 nm). These results have been interpreted by Zachariasse et al. [16] as evidence for the presence of two rapidly interconnecting S1 conformers as labeled with quinoidal (ICT-Q) and anti-quinoidal (ICT-AQ) structures. With the TDDFT calculations, we obtained the similar structures for the ICT-Q and the ICT-AQ, both of which have strong CT characters. We have then optimized both structures of ICT-Q/AQ states using the CC2/cc-pVDZ level of theory. Both optimized structures show the dimethylamino moiety, N(CH3)2, is perpendicular to the tricyanobenzene ring. The dimethylamino moiety partially twisted with respect to the phenyl ring plane by about 29.3° for the ICT-Q geometry, while the C–N bond in the dimethylamino moiety for ICT-AQ geometry lies in the phenyl ring plane as shown in Fig. 5. Both structures show strong CT characters with large dipole moments: 9.0 D for ICT-Q and 10.2 D for ICT-AQ. The CC2 calculations are qualitatively consistent with Zachariasse’s conclusion [16]; the ICT-Q structure is found to be a stable minimum, however, the ICT-AQ structure is in an unstable structure that has two imaginary frequencies of 227 i cm1 (out-of-plane motion of the dimethylamino moiety) and 30 i cm1 (pyramidalization of the dimethylamino moiety). It should be noted that Zachariasse et al. [16] carried out the high-level CASPT2/RASSCF calculations to obtain both quinoidal structures, and show no indication of force field calculations to verify the stability of those states obtained. Our CC2 result, however, predicts that the lower energy ICT-Q structure tends to be populated from the unstable ICT-AQ structure, which is responsible for the observed time-resolved fluorescence as well as the excited-state absorption from the mixed S1 ðpp Þ/ICT state of TCDMA.

4. Conclusion In summary, we have carried out TDDFT computational studies on the low-lying excited states of di-tert-butylaminobenzonitrile and 2,4,6-tricyanoaniline compounds that exhibit unusual photophysical behaviors leading to the ICT formation. In addition, CC2 calculations were employed for 2,4,6-tricyano-N,N-dimethylaniline (TCDMA). For 3-(di-tert-butylamino)benzonitrile (m-DTBABN) appears to be the only meta-substituted aminobenzonitrile that exhibits the ICT formation. On the other hand, TCDMA does not show clear evidence for the LE ? ICT formation from steady-state fluorescence studies, despite the greater electron acceptor strength of tricycanobenzene as compared to monocyanobenzene which is part of a DMABN compound. The calculations indicate that the ultrafast ICT formation in pDTBABN and m-DTBABN is due to the sequential mechanism of pp ðLa Þ ! pr ! ICT, involving conical intersections among the three closely-lying excited-states. In TCDMA, the TDDFT as well as the CC2 calculations predict the presence of a twisted ICT state that lies below the initially photoexcited S1 ðpp Þ state, which is responsible for the ultrafast dynamics observed in the excitedstate absorption in acetonitrile. In both cases for TCDMA and TCA, the pr state locates significantly higher in energy than the S1 ðpp Þ state (and the ICT state for TCA), thus precluding the pr ! ICT formation, which is believed to occur in a DMABN in polar environments.

ΔE = +0.002 eV

Fig. 5. The fully optimized structures of TCDMA in the low-lying ICT states: (a) quinoidal (ICT-Q) and (b) anti-quinoidal (ICT-AQ) structures calculated with CC2/ cc-pVDZ level of theory. The energy difference, DE, is relative to their LE (1 pp ) state.

Acknowledgment We are grateful to the Ohio Super Computer Center for the computing time to this work.

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