The Lowest Triplet of Tetracyanoquinodimethane via UV–vis

Article pubs.acs.org/JPCA

The Lowest Triplet of Tetracyanoquinodimethane via UV−vis Absorption Spectroscopy with Br-Containing Solvents Olga G. Khvostenko,* Renat R. Kinzyabulatov, Laysan Z. Khatymova, and Evgeniy E. Tseplin Institute of Molecule and Crystal Physics, Ufa Research Center of Russian Academy of Sciences, Prospekt Oktyabrya 151, Ufa 450075, Russia S Supporting Information *

ABSTRACT: This study was undertaken to find the previously unknown lowest triplet of the isolated molecule of tetracyanoquinodimethane (TCNQ), which is a widely used organic semiconductor. The problem is topical because the triplet excitation of this compound is involved in some processes which occur in electronic devices incorporating TCNQ and its derivatives, and information on the TCNQ triplet is needed for better understanding of these processes. The lowest triplet of TCNQ was obtained at 1.96 eV using UV−vis absorption spectroscopy with Br-containing solvents. Production of the triplet band with sufficient intensity in the spectra was provided by the capacity of the Br atom to augment the triplet excitation and through using a 100 mm cuvette. The assignment of the corresponding spectral band to the triplet transition was made by observation that this band appeared only in the spectra recorded in Brcontaining solvents but not in spectra recorded in other solvents. Additional support for the triplet assignment came from the overall UV−vis absorption spectra of TCNQ recorded in various solvents, using a 10 mm cuvette, in the 1.38−6.5 eV energy range. Singlet transitions of the neutral TCNQo molecule and doublet transitions of the TCNQ¯ negative ion were identified in these overall spectra and were assigned with TD B3LYP/6-31G calculations. Determination of the lowest triplet of TCNQ attained in this work may be useful for theoretical studies and practical applications of this important compound.

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INTRODUCTION Tetracyanoquinodimethane (TCNQ) is a well-known organic semiconductor, which has been investigated intensively from the early sixties to the present day.1,2

recombination in photoactive systems that contain this acceptor.10 It should be noted that the first triplet for thin solid film of TCNQ was obtained previously at 2 eV by means of the electron energy-loss spectroscopy.11 However, this value in principle can differ from T1 of the TCNQ isolated molecule because of interactions between the electron shells of the closely spaced molecules in the film. These interactions can change the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) involved in T1, and it can also influence the Coulomb (Jiv) and the exchange (kiv) integrals, on which the triplet transition energy also depends. Thus, determination of the T1 energy for the isolated TCNQ molecule is necessary in spite of the fact that the T1 energy for the film of TCNQ is known. Data on T1 (TCNQ) are also needed for a better insight into the properties of the TCNQ¯ ion itself. This follows from theoretical work where the existence of the low-energy negative ion of TCNQ¯ in the quartet state (4TCNQ¯) was suggested.12 In the same study, the condition for this was defined: the energy of 4TCNQ¯ should be lower than the energy of T1. It should be noted that such long-lived ion-quartets have been determined previously in many other chemical systems, beginning from the He atom13 and ending with a series of organic compounds.14−17 (see page 317 in ref 14) However,

Considerable interest in TCNQ results from its unique property as a strong electron acceptor, caused by its large electron affinity of 2.8 eV.3 The features of a strong acceptor allow TCNQ to form the stable negative molecular ion (TCNQ¯) that, in turn, provides the basis for many applications of this compound in photonics, magnetism, and other fields.4−6 However, the lowest (first) triplet (T1) of the isolated TCNQ molecule remains unknown to the present day due to the lack of phosphorescence in this compound.7 Meanwhile, information on T1 (TCNQ) is necessary for a better understanding of photophysical processes in which TCNQ and its derivatives participate, since the molecular triplets can be involved in these processes.8 For instance, T1 of Chloranil, a compound related to TCNQ, facilitates electron-transfer from a donor when a long-lived triplet radical-ion pair is formed, providing effective charge separation.9 The necessity and deficiency of the information on T1 (TCNQ) were noted in a study of charge © 2017 American Chemical Society

Received: June 8, 2017 Revised: September 3, 2017 Published: September 13, 2017 7349

DOI: 10.1021/acs.jpca.7b05623 J. Phys. Chem. A 2017, 121, 7349−7355

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The Journal of Physical Chemistry A the existence of the 4TCNQ¯ ion can be determined with reasonable confidence only if T1 (TCNQ) is found.

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EXPERIMENTAL AND THEORETICAL METHODS The present work was undertaken to search for T1 (TCNQ) using UV−vis absorption spectroscopy performed with a Shimadzu UV-2401 spectrometer. Br-propane and 1,2-di-Brethane were employed as the solvents, because a heavy atom (Br in this case) enhances (via the spin−orbit interaction) the probability of the triplet excitation occurring directly under light radiation.18 Simultaneously, a 100 mm cuvette was used to achieve sufficient intensity when searching for the TCNQ triplet band in the spectra. UV−vis absorption spectra of TCNQ in a 100 mm cuvette were also obtained in hexane, tetrahydrofuran, CCl4, ethanol, and isopropanol for comparison with those obtained in the Br-containing solvents. Additionally spectra were recorded in a 10 mm cuvette in all of the abovelisted solvents in order to determine the overall absorbance picture of TCNQ. This was an important control experiment for this study. B3LYP/6-31G calculations were used everywhere. More details of experimental and calculated results are given in the Supporting Information.

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RESULTS AND DISSCUSSION To achieve the main objective of the study, determination of the lowest triplet of TCNQ, the above-mentioned overall spectra of TCNQ were first obtained in all used solvents, in 10 mm cuvettes, with appropriate concentrations of TCNQ. These spectra divided themselves into two groups, depending on the polarity of the solvent. The first group consists of the spectra recorded in nonpolar solventshexane, CCl4, and 1,2-di-Brethane, as well as in the weakly polar solventsin Br-propane and tetrahydrofuran. The spectra of this first group present the singlet (S) transitions of the TCNQo neutral molecule. The second group consists of spectra obtained in the strongly polar solventsisopropanol and ethanol. They show formation of TCNQ¯ negative ions and exhibit its several intense transitions of TCNQ¯ from the ground state to various electronically excited doublet (D) states. The spectra recorded in hexane and ethanol are typical representatives of these two groups. Therefore, they were selected and shown in Figure 1A and in Figure 2A, respectively. The overall spectra recorded in other solvents are shown in Figure S1 of the Supporting Information. The spectra of both types were assigned on the basis of TD B3LYP/6-31G calculations of the electronic spectra of the TCNQo neutral molecule and the TCNQ¯ negative ion, which are also shown in Figure 1B and Figure 2B, respectively. As evident from Figure 1A,B, as well from Figure 2A,B, there is good agreement between the calculations and the experiments. This lends support to the validity of the calculation method used, indicates a weak influence of the solvents (like some of the previous observations19), and lays the foundation for a reliable assignment of the overall TCNQ spectra. This assignment is described briefly below. The first excited singlet (S1) of TCNQo is clearly seen at 3.17 eV in the spectrum recorded in hexane and shown in Figure 1A. It is the well-known transition previously observed close to this energy value, for instance, at 3.1 eV.20 Other singlets (S2 − S8) of TCNQo were determined in this work. Among them, the S2 − S5 transitions are characterized by low intensities. Therefore, their positions on the experimental energy scale were

Figure 1. Electronically excited singlet states of TCNQo molecule from UV−vis absorption spectra in hexane in 10 mm and 100 mm (fragment) cuvettes (A); TD B3LYP/6-31G (NStates = 100) calculation (B); details in Table 1; TD B3LYP/6-31G (NStates = 100) with polarizable continuum model (PCM) in n-hexane (red).

Figure 2. Doublet states of the TCNQ¯ ion from UV−vis absorption spectra in ethanol in a 10 mm cuvette and in isopropanol in a 100 mm one (fragment) (A); TD B3LYP/6-31G (NStates = 100) calculation (B); details in Table 2; asterisk denotes negative ion of dimer. TD B3LYP/6-31G (NStates = 80) with polarizable continuum model (PCM) in ethanol (red).

approximated using the spectrum recorded in hexane in a 100 mm cuvette (fragment in Figure 1A) and from positions on 7350

DOI: 10.1021/acs.jpca.7b05623 J. Phys. Chem. A 2017, 121, 7349−7355

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The Journal of Physical Chemistry A the energy scale of the calculated S2 − S5 singlets (Figure 1B). The S6−S8 singlets were defined with certainty. Symmetries, oscillator strengths, and the calculated and experimental energies of all S1−S8 singlets are given in Table 1.

Table 2. Electronically Excited D1−D9 Doublet Transitions of the TCNQ¯ Negative Ion from UV−vis Spectra Recorded in Ethanol and Isopropanol (expE), and Obtained from a TD B3LYP/6-31G Calculation (Oscillator Strengths f > 0.01) No

Table 1. Electronically Excited S1−S8 Singlet Transitions of the TCNQo Molecule from UV−vis Spectra Recorded in Hexane (expE), and Obtained from TD B3LYP/6-31G Calculation (Oscillator Strengths f > 0) No S1 S2 S3 S4 S5 S6 S7 S8

symmetry 1

B1U B3U 1 B2U 1 B3U 1 B1U 1 B2U 1 B2U 1 B1U 1

f 1.0742 0.0006 0.0707 0.0117 0.0073 0.0097 0.0967 0.4059

exp

E (eV)

3.17 ∼4.34 ∼4.55 ∼4.80 ∼4.97 5.61 6.20 6.32

calc

D1 D2 D3 D4 D5 D6 D7 D8 D9

E (eV) 3.02 4.72 5.08 5.42 5.70 6.42 6.81 6.98

symmetry 2

B3U 2 B3U 2 B3U 2 B1U 2 Au 2 Au 2 Au 2 B3U 2 B3U

f 0.3065 0.5739 0.0812 0.0818 0.0300 0.0369 0.1077 0.1805 0.1950

exp

E (eV)

1.47 3.59 ∼4.41 ∼4.75 ∼4.90 ∼5.11 ∼5.3 5.53 6.09

calc

E (eV) 1.78 3.60 4.51 5.28 5.79 6.03 6.61 6.65 6.88

UMO, and this configuration is presented in Figure S6 of the Supporting Information along with the electron configurations of the rest of the D2−D9 ion states. The overall picture of TCNQ excitation described above indicates that T1 of TCNQ should be searched for below E(S1) = 3.14 eV, because T1 lies at obviously lower energy than S1. Besides, it shows that the first D1 transition of the TCNQ¯ negative ion can appear in this region of interest at 1.47 eV. Considering this, the UV−vis spectra of TCNQ were recorded in the various solvents listed above, including B-propane and 1,2-di-Br-ethane, in a 100 mm cuvette. The 100 mm cuvette provides a strong enhancement of the spectral band intensity, as a result of which the absorbance reading was “off-scale” in the region of S1, i.e., at an energy E > 3 eV. At the same time, the region where E < 3 eV remains accessible to observation. This E < 3 eV region is shown in Figure 3 as a fragment of each spectrum. The left column of Figure 3 shows the fragments of the spectra recorded in the nonpolar solvents, i.e., in 1,2-di-Brethane, hexane, and CCl4 (Figure 3A, B, and C, respectively), and the right column shows the analogous fragments of the spectra recorded in the polar solvents, i.e., in Br-propane, tetrahydrofuran, and isopropanol (Figure 3D, E, and F, respectively). The spectrum recorded in ethanol in a 100 mm cuvette is not shown since it is very similar to the spectrum obtained in isopropanol. One can see from Figures 3D−F that in the spectra recorded in polar solvents, the well-known, described above D1 band of the TCNQ¯ negative ion is seen at 1.45 eV, and it is absent in the spectra recorded in nonpolar solvents (Figure 3A−C). In addition, the spectra recorded in both Br-containing solvents exhibit a new, clear, pronounced band, with a maximum at 1.96 eV in 1,2-di-Br-ethane (Figure 3A), and at 1.85 eV in Br-propane (Figure 3D); this does not appear in the spectra recorded in other solvents. We assign this new band to the first triplet of TCNQ. It should be noted that the energy of T1 (TCNQ) of 1.96 eV observed in 1,2-di-Brethane is probably more preferential than 1.85 eV observed in Br-propane because the peak at 1.85 eV in Br-propane may become higher due to a contribution from the TCNQ¯ ion transition assigned at 1.47 eV; it is possible that a vibronic band associated with the ion transition interferes with the T1 ← S0 transition of the neutral molecule assigned at 1.96 eV. Therefore, we think that the real maximum of the TCNQ triplet most likely lies at 1.96 eV. One can see that the value of 1.96 eV is extremely close to 2.0 eV obtained previously by other authors through energy-loss spectroscopy of a solid film of TCNQ.11 This means that the multiple effects on the triplet

Characteristics of the molecular orbitals (OMOs, and UMOs) involved in the S1−S8 transitions are shown in Figure S5 of the Supporting Information, along with the electronic configurations of the S1−S8 singlet states. These characteristics of the molecular orbitals are the images of them and the sequence numbers, which they have in B3LYP/6-31G calculation of TCNQo molecule ground state. The general data on all OMOs and UMOs of TCNQo molecule that were obtained with B3LYP/6-31G calculations, are shown in Figure S3 of the Supporting Information, together with the literature photoelectron spectrum (PES).21 Here, results from PES validate the our computational approach, as it was developed previously in a series of studies.22−25 The overall spectrum recorded in ethanol and shown in Figure 2A (the spectrum recorded in isopropanol is shown in Figure S1 of the Supporting Information), exhibits the formation of TCNQ¯ negative ions in this solvent. The ions give the well-known first transition to the electronically excited doublet state (D1) at 1.47 eV, just as it does in many other previously published spectra of TCNQ recorded in the polar solvents.26−31 Other transitions of the TCNQ¯ ion are recorded here and labeled in Figure 2 as D2−D9. Like several abovedescribed singlets, some of them (D3−D7) have low intensities. Their energies were also found approximately using the spectrum recorded in isopropanol in a 100 mm cuvette (the top of Figure 2A) and taking into account their calculated energies. The intense bands of the D8 and D9 transitions are identified beyond a great doubt. It should be noted that D8 has higher intensity than D9 in the calculation, and vice versa in the experimental spectrum. This is caused by the presence of some amount of TCNQo neutral molecules (that contribute the S1 and S8 singlets to the spectrum (red in Figure 2A)) in the ethanol solution, along with TCNQ¯ negative ions. The S8 singlet adds to the intensity of the D9 band, which is then observed in the measured spectrum. Thus, the electronically excited states of the TCNQ¯ negative ion, observed in the 1.38−6.5 eV energy range, were assigned. Their symmetries, as well as oscillator strengths, and the energies of the corresponding D1 − D9 transitions are given in Table 2. Table 2 indicates, in particular, that the D1 state has 2B3u symmetry of the D2h group, exactly coinciding with data from the literature.26,27 This ion possesses the electron vacancy on the highest OMO and two paired electrons on the lowest 7351

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Figure 3. UV−vis absorption spectra of TCNQ in a 100 mm cuvette in 1,2-di-Br-ethane (A), in hexane (B), CCl4 (C), Br-propane (D), tetrahydrofuran (E), and isopropanol (F); asterisk represents electronically excited transition of the negative ion of the cyclic dimer (2.5 eV). TCNQ was fully dissolved in 1,2-di-Br-ethane that allowed determination of concentration and molar absorptivity (ε) of the triplet band in this solvent (A); ε (S1) = 87679.3 L/mol-cm is given in Figure S1 (B) of the Supporting Information. TCNQ is poorly dissolved in hexane, CCl4, Br-propane, and isopropanol that hindered determination there of TCNQ concentration and ε. In tetrahydrofuran, TCNQ is quite soluble, but ε cannot be determined there because the portion of TCNQ molecules, which transform themselves into ions in this solution, is unknown.

energy in the condensed phase evidently cancel each other out, which results in the similarity of two values. In order to be sure that the assignment of the new band observed at 1.96 eV to the triplet transition of TCNQ is correct, alternative sources of this band should be considered. We see two such possible sources. The first could be a neutral or negatively charged dimer (or dimers) of TCNQ. The second may be a charge-transfer complex which is a well-known structure capable of transition into an electronically excited state via light absorbance in the visible region. The analysis of the spectra presented in Figure 3 allows immediate elimination of some of the suggested alternatives. Figure 3A shows that the new band at 1.96 eV appears in the spectrum in 1,2-di-Brethane, although TCNQ¯ negative ions are not formed in this solvent. This is clearly seen from the absence of the ion band at 1.47 eV. This means: (i) the charge-transfer complex is absent in this solvent, and the band at 1.96 eV observed in this spectrum cannot be as a result of this complex; (ii) similarly, the negative ion of any dimer cannot be the source of this band either. Thus, the sole alternative source of the band at 1.96 eV must be a neutral dimer (or dimers) of TCNQ. In the present work, the dimer structure of TCNQ was searched for via B3LYP/6-31G calculations. The calculations give three dimers: planar cyclic stacked (face-to-face coupled), and bonded dimer (“butterfly”coupled via the centers of two TCNQ o molecules) (Figure 4).

Figure 4. Structures of TCNQ dimers found with B3LYP/6-31G calculations.

Cartesian coordinates, total energies, and frequencies of the dimers found are given in the Supporting Information. Among them, the cyclic dimer is the best option in terms of total energy; therefore, this structure is the most probable. The stacked dimer is doubtful because it gives a negative frequency, and the bonded dimer “butterfly” is the worst option for total energy; however, the analogous “butterfly” structure is known in the condensed aromatics.32 In spite of the problems with the stacked and “butterfly” structures, the electronic spectra of the neutral states of all three dimers were calculated and presented in Figure 5. Figure 5 shows that the lowest transition of any neutral dimer is placed at relatively high energy: at 3.01 eV (cyclic), 3.16 eV (stacked), and 4.04 eV (“butterfly”). These energies are significantly higher than the discussed experimental band at 1.96 eV. Therefore, it is unlikely that one of these dimers relates to the band at 1.96 eV, which supports the assignment of this band to the triplet transition of TCNQ. It should be noted that geometries, frequencies (see the Supporting Information), and the electronic spectra of the 7352

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The obtained value of the energy of T1(TCNQ) can be useful from theoretical and practical points of view for insight into some molecular processes. In particular, this value is significant in the field of photophysical and photochemical studies, as well as in the field of the efficient development of photoconducting materials. For instance, it can provide the estimation of the energy balance of the electron-transfer reactions including the ion-pair recombination in the photoactive molecular systems incorporating TCNQ, as it was noted in the previous study.10 Besides, the value of the energy of T1(TCNQ) provides a new theoretical knowledge of the mechanisms governing by the formation of the long-lived negative molecular ions, and not only of TCNQ¯. As to TCNQ itself, the value of E(T1) of this compound gives the answer to the above-mentioned question of whether TCNQ forms the long-lived negative ions in the quartet state. There are the previous data on the resonance electron capture by TCNQ molecules, which were first obtained by Compton and Cooper as early as 1976.35 They revealed formation of the anomalously long-lived TCNQ¯ ions in three resonance states at the captured electrons energies of ∼0; 0.7 and 1.3 eV, as well as the forth resonant state at ∼3.2 eV, where the less long-lived TCNQ¯ ions are formed. The value of E(T1) = 1.96 eV indicates convincingly that the first three ones correspond to the quartet mechanism, because they lie below the triplet in energy (as it was suggested by the authors of the previous work12). However, the fourth state at ∼3.2 eV is formed, obviously by another mechanism, because this state lies above the triplet being unstable relative to it. This unknown mechanism is the important one, since it operates, possibly, in fullerenes, where, in addition to the anomalously long-lived negative ions, which are stable relative to T1 and are formed apparently in the quartet electron configurations, the resonant maxima with the long-lived negative ions are observed above T1.36 Both types of the compounds TCNQ and fullerenes are the well-known electron acceptors, which are widely used in electronics. Therefore, the knowledge of the mechanisms of the formation of their negative ions is an essential question, which should be investigated further with the use of the corresponding triplet data.

Figure 5. Electronic spectra of TCNQ neutral dimers obtained by means of TD B3LYP/6-31G calculations; f is oscillator strength.

negative ions of all above-listed dimers were also calculated in the present paper using TD B3LYP/6-31G. The spectrum of the negative ion of the most probable planar cyclic dimer is shown in Figure 2B by the dashed line. It has the lowest transition at 2.56 eV, which allowed an explanation of the weak “shoulder” observed at 2.5 eV (asterisk in Figures 2 and 3) in the experimental spectra, which only occurs in those spectra characterized by negative ion formation. One can see that there is very good agreement between the calculated (2.56 eV) and the experimental (2.5 eV) values for the case of the dimer ion. This is more than likely due to the neutral and negative ionic TCNQ cases just described, as well as a series of previously published observations.33,34,24 It is of note that one reasonable calculated value of the lowest triplet of the isolated TCNQ molecule was obtained in the present work using CIS(D) method. It gives E(T1) = 1.57 eV that is rather close to the experimental 1.96 eV strongly supporting it in this way. At the same time, there are many other calculations which give significantly lower value of the energy of T1 of TCNQ: < 1 eV from the time-dependent method (TD), and ≈1 eV obtained as the difference between the total energies of the molecule in the ground state and of the molecule in the triplet state computed in the same equilibrium molecular geometry (Table S3 of the Supporting Information). Therefore, in order to check the probability of such a low energy of T1 of TCNQ, the spectrum of TCNQ was also recorded in Br-propane in a 100 mm cuvette with a Shimadzu UV-3600, which has a lower recording bound of 3600 nm (0.35 eV). As a result of that measurement, no electronically excited transitions were revealed in the range between 3600 nm (0.35 eV) and 900 nm (1.38 eV), the latter being the lower recording bound of the Shimadzu UV-2401 used throughout this work. Thus, the value of 1.96 eV was verified as the lowest triplet of TCNQ. Moreover, the value of 1.96 eV obtained in the present work as due to T1 (TCNQ) is significantly more reasonable than the value of 1 eV, as it agrees better with the common energy gap (ΔE) between S1 and T1. In the review,8 it was remarked: “For conjugated polymers where the first excited state is of a ππ* character, ... the S1 − T1 gap is experimentally found to be about 0.7 eV, virtually independent of chemical structure”. In the case of TCNQ, ΔE1 = 3.14−1 = 2.14 eV if E(T1) = 1 eV, and ΔE2 = 3.14−1.96 = 1.18 eV if E(T1) = 1.96 eV. The value of ΔE2 = 1.18 eV is closer to 0.7 eV than ΔE1 = 2.14 eV, and consequently, the value of E(T1) = 1.96 eV is more reasonable.

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CONCLUSIONS The main objective of the present study was determination of the lowest triplet of the isolated molecule of TCNQ. For this purpose, the UV−vis spectra of TCNQ were recorded in various solvents in a100 mm cuvette, where the latter provided sufficient intensity of the triplet band, and only in Br-containing solvents the band at ∼1.96 eV (633 nm) was observed, which was assigned to the sought triplet. Other possible sources of this band were considered, such as neutral and negatively charged dimers of TCNQ, and a charge-transfer complex. All of these possibilities were rejected after the analysis of the spectra obtained and of the quantum chemical calculations (B3LYP/631G) of the isolated TCNQ molecule, of its negative ion and dimers. In summary, it may be said that there is good reason to believe that the previously unknown energy of the first triplet of a TCNQ has been established. This information is needed for a fundamental understanding of the mechanisms governing by formation of the long-lived molecular negative ions, as well as for further development of practical application of this important compound. 7353

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(10) Mandal, P.; Sahu, T.; Misra, T.; Pal, S. K.; Ganguly, T. J. Experimental investigations by using electrochemical, steady state and time resolved spectroscopic tools on the photoreactions of disubstituted indole in presence of tetracyanoquinodimethane (TCNQ) and a theoretical approach by using time-dependent density functional theory. J. Photochem. Photobiol., A 2007, 188, 235−244. (11) Ritsko, J. J.; Brillson, L. J.; Sandman, D. J. Electron energy-loss spectroscopy of tetracyanoquinodimethane, TCNQ, tetrathiafulvalene, TTF, and the salt TTF-TCNQ. Solid State Commun. 1977, 24, 109− 112. (12) Skurski, P.; Gutowski, M. Excited electronic states of the anion of 7,7,8,8- tetracyanoquinodimethane (TCNQ). J. Mol. Struct.: THEOCHEM 2000, 531, 339−348. (13) Blau, L. M.; Novick, R.; Weinflash, D. Lifetimes and fine structure of the metastable autoionising (1s2s2p)4pJ states of the negative helium ion. Phys. Rev. Lett. 1970, 24 (23), 1268−1272. (14) Allan, M. Vibrational and electronic excitation in pbenzoquinone by electron impact. Chem. Phys. 1984, 84, 311−319. (15) Khvostenko, O. G.; Tuimedov, G. M. Doublet - quartet conversion in negative ions as a possible mechanism of the electron autodetachment delay. Rapid Commun. Mass Spectrom. 2006, 20, 3699−3708. (16) Khvostenko, O. G.; Shchukin, P. V.; Tuimedov, G. M.; Muftakhov, M. V.; Tseplin, E. E.; Tseplina, S. N.; Mazunov, V. A. Negative ion mass-spectrum of resonance electron capture by molecules of p-benzoquinone. Int. J. Mass Spectrom. 2008, 273, 69−77. (17) Khvostenko, O. G.; Lukin, V. G.; Tseplin, E. E. Anomalously long-lived molecular negative ions of duroquinone. Rapid Commun. Mass Spectrom. 2012, 26, 2535−2547. (18) Karatay, A.; Miser, M. C.; Cui, X.; Kucukoz, B.; Yilmaz, H.; Sevinc, G.; Akhuseyin, E.; Wu, X.; Hayvali, M.; Yaglioglu, H. G.; Zhao, J.; Elmali, A. The effect of heavy atom to two photon absorption properties and intersystem crossing mechanism in aza-borondipyrromethene compounds. Dyes Pigm. 2015, 122, 286−294. (19) Tseplin, E. E.; Tseplina, S. N.; Khvostenko, O. G. Specific effects of a polar solvent in optical absorption spectra of 1,2naphthoquinone. Opt. Spectrosc. 2016, 120, 274−279. (20) Jonkman, H. T.; Kommandeur, J. The UV spectra and their calculations of TCNQ and its mono- and di-valent anion. Chem. Phys. Lett. 1972, 15, 496−499. (21) Herman, F.; Batra, I. P. Electronic structure of the tetracyanoquinodimethane (TCNQ) molecule. Phys. Rev. Lett. 1974, 33, 94−97. (22) Millefiori, S.; Alparone, A. Electronic properties of neuroleptics: ionization energies of benzodiazepines. J. Mol. Model. 2011, 17, 281− 287. (23) Khvostenko, O. G.; Tzeplin, E. E.; Lomakin, G. S. Assignment of benzodiazepine UV absorption spectra by the use of photoelectron spectroscopy. Chem. Phys. Lett. 2002, 355, 457−464. (24) Khvostenko, O. G. Electronically excited states of chloroethylenes: experiment and DFT calculations in comparison. J. Electron Spectrosc. Relat. Phenom. 2014, 195, 220−229. (25) Khvostenko, O. G.; Lukin, V. G.; Tuimedov, G. M.; Khatymova, L. Z.; Kinzyabulatov, R. R.; Tseplin, E. E. Electronically excited negative ion resonant states in chloroethylenes. J. Electron Spectrosc. Relat. Phenom. 2015, 199, 1−9. (26) Haller, I.; Kaufman, F. B. Spectra of tetracyanoquinodimethane monovalent anion: vibrational structure and polarization of electronic transitions. J. Am. Chem. Soc. 1976, 98, 1464−1468. (27) Brinkman, E. A.; Gunther, E.; Brauman, J. I. Bound excited electronic states of anions studied by electron photodetachment spectroscopy. J. Chem. Phys. 1991, 95, 6185−6187. (28) Brinkman, E. A.; Gunther, E.; Schafer, O.; Brauman, J. I. Bound excited electronic states of anions. J. Chem. Phys. 1994, 100, 1840− 1848. (29) Zakrzewski, V. G.; Dolgounitcheva, O.; Ortiz, J. V. Electron binding energies of TCNQ and TCNE. J. Chem. Phys. 1996, 105, 5872−5877.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b05623. Description of experiments and calculations, overall UV− vis spectra of TCNQ in various solvents, images of molecular orbitals of TCNQ, electronic configurations of excited singlets of TCNQo molecule and doublets of the TCNQ¯ negative ion, optimized structures of the neutral and negative ionic forms of isolated TCNQ molecule, of dimers, and vibrational frequencies (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Olga G. Khvostenko: 0000-0003-2014-9462 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the Federal Agency of Science Organizations (FASO) through Basic Research Program Project No 0248-2014-005:14.5. The authors are grateful to Professor Sergey L. Khursan for discussion of a possible role of charge-transfer complexes, and to Professor Valeriy V. Kuznetsov for his help in the question concerning with TCNQ dimers. All quantum-chemical calculations were carried out using the equipment installed in the Center for collective use “Khimiya” (Chemistry) at the Ufa Institute of Chemistry of the Russian Academy of Sciences.

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REFERENCES

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DOI: 10.1021/acs.jpca.7b05623 J. Phys. Chem. A 2017, 121, 7349−7355