SPECTROSCOPY OF ATOMS AND MOLECULES. Bogdan Khmelnitsky National University, Cherkasy, Ukraine b

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1 ISS 3-4X, Optics and Spectroscopy, 215, Vol. 118, o. 5, pp Pleiades Publishing, Ltd., 215. Original Russian Text E.V. Stromylo, G.V. Baryshnikov, B.F. Minaev, M. Grigoras, 215, published in Optika i Spektroskopiya, 215, Vol. 118, o. 5, pp SPECTROSCOPY OF ATOMS AD MOLECULES Quantum-Chemical Investigation of the Structure and Electronic Absorption Spectra of Symmetric Triphenylamine Oligomers Conjugated to Vinylene, Imine, Azine, and Ethynylene Groups E. V. Stromylo a, G. V. Baryshnikov a, B. F. Minaev a, b, and M. Grigoras c a Bogdan Khmelnitsky ational University, Cherkasy, 1831 Ukraine b Royal Institute of Technology (KTH), SE Stockholm, Sweden c Petru Poni Institute of Macromolecular Chemistry, 7487 Jaşi, Romania е-mail: stromchem@rambler.ru; bfmin@rambler.ru; boris@theochem.kth.se Received October 3, 214 Abstract Based on the density functional theory (DFT) and using the B3LYP and BMK hybrid exchangecorrelation functionals, we have studied the structure and electronic spectral properties of some triphenylamine oligomers that contain various π-electron spacers of end groups and that are of interest as electro- and photoactive materials. Good agreement between calculation results and experimental data on absorption spectra in the visible and UV ranges has been obtained. The nature of visible spectral bands has been elucidated based on interrelations with structural changes of oligomer molecules. DOI: /S34X15427 ITRODUCTIO Modern optoelectronics and nanoengineering have been developing rapidly in recent years, interpenetrating and supplementing each other. One of the branches of this progress is the development of new organic light-emitting diodes (OLEDs) [1, 2], which are now widely used in industry and everyday life. OLEDs have a layered structure, in which each of the layers performs a certain function and frequently consists of polymer or oligomer material [3 7]. The advantage of oligomers is that they possess properties of corresponding polymers to a large extent but, in T1 T T T T5 Fig. 1. Structure of molecules of oligomers T1 T5 and numeration of their atoms. 73

2 74 STROMYLO et al. Table 1. Bond lengths (in Å) of the Т1 Т5 molecules in singlet and triplet states Bond Т1 Т2 Т3 Т4 Т5 S T 1 S T 1 S T 1 S T 1 S T C 1 C C 2 C C C 3 C C 3 C C C 4 C C 4 C C 5 C C C 6 C C 7 C C 7 C C 19 C C 2 C C C C 23 C C 24 C C 24 C contrast to the latter, oligomers can be more easily chemically modified. This circumstance makes it possible to synthesize new electro- and photoactive oligomers that possess a more extended spectrum of photophysical properties compared to existing polymers. Examples of such oligomers include symmetric conjugated triphenylamines that contain vinylene (Т1), imine (T2, T3), azine (T4), and ethynylene (T5) bridge groups [7] (Fig. 1). It has been noted that these molecules are promising for application in OLED as a hole transport layer [7, 8]. Furthermore, vinylene oligomer T1 dissolved in chloroform shows good fluorescence quantum yield (Φ =.). One can assume that the value of quantum yield Φ for compound T1 in the condensed state (i.e., under OLED conditions) will be comparatively higher because there is no fluorescence quenching by the solvent, which, in turn, gives grounds to speak of promising electroluminescent properties of oligomer T1. This study is devoted to the theoretical investigation of the structure and spectral properties of oligomers T1 T5. The main objective of this work is to quantum-chemically interpret the nature of absorption bands in the visible range of the spectrum of the Т1 Т5 molecules in conjunction with the analysis of the structure of the oligomers under study both in the ground singlet and in an excited triplet electronic state, which can be populated under conditions of operation of OLEDs. Results that we obtained considerably supplement existing data on the structure and spectra of triphenylamine oligomers and can serve as a reliable theoretical basis for perfection of OLEDs based on these oligomers. CALCULATIO METHOD The structure of the Т1 Т5 molecules was optimized based on the density functional theory (DFT) using the B3LYP hybrid exchange-correlation functional [9, 1] and the 6-31G(d) basis set of atomic orbitals [11], which are frequently used in calculations of the molecular structure and excited states of organic compounds and polymers [12 16]. The absence of imaginary vibrational modes in calculated IR absorption spectra of equilibrium structures of compounds Т1 Т5 indicates that the true energy minima of these OPTICS AD SPECTROSCOPY Vol. 118 o

3 QUATUM-CHEMICAL IVESTIGATIO 75 molecules were found. For the molecules under study, using the TD DFT method [17] with the BMK (Boese Martin hybrid functional [18]), we calculated 3 singlet singlet electronic transitions in the vacuum approximation and using the PCM continual solvation model [19] (with chloroform as a solvent, which was used in the experimental investigation of absorption spectra [7]). The profiles of curves of calculated electronic absorption spectra were simulated with the SWizard program [2, 21] using the Gauss line shape (with a line halfwidth of 25 cm 1 ). All calculations were done in terms of the Gaussian 9 software package [22] on a PDC supercomputer at the Royal Institute of Technology (Stockholm, Sweden). Table 2. Dihedral angles (in deg) of investigated molecules in the ground singlet and first triplet states Compound Angle S T 1 T1 C 2 C 3 C 5 C C 4 C 3 C 7 C C 11 C 1 13 C C 9 C 1 13 C T2 C 4 5 C 6 C C 11 C 1 13 C C 9 C 1 13 C T3 C 1 C 16 C 17 C C 3 5 C 6 C C 11 C 1 13 C C 9 C 1 13 C C 2 22 C 23 C C 28 C 27 3 C C 26 C 27 3 C T4 C 6 C 7 1 C C 8 C 7 1 C C 9 C 4 C T5 C 2 C 3 C 7 C C 11 C 1 13 C C 9 C 1 13 C RESULTS AD DISCUSSIO Analysis of the Structure of the Т1 Т5 Molecules In the ground singlet state, oligomers Т1 Т5 have a symmetric structure with respect to the center of the molecule (Tables 1, 2). By optimizing the structure of all the molecules under consideration in their first excited triplet state, we checked whether the geometry of the molecule can be changed as a result of its excitation. Figure 2 presents spatial wave functions characterizing the first excited state; the highest occupied molecular orbital (HOMO) is shown at the top of the figure, while the lowest unoccupied molecular orbital (LUMO) is presented at the bottom. Since the first excited singlet state is mainly represented by the HOMO LUMO excitation, as well as the triplet excited state (Table 3), we assume that deformations of the structure of molecules upon their excitations into the first singlet and the first triplet excited states should be similar. Considering each case individually, one can note that, upon optimization of the first excited triplet state, the geometry of the molecules under investigation changes considerably. For example, in the case of the Т1 molecule, changes in bond lengths are observed that lead to deformations of entire molecular fragments inside of the oligomer. amely, a clearly pronounced double bond between the С 5 and С 6 atoms is elongated (see Table 1); in this case, the lengths of neighboring single bonds decrease and a system of three conjugated bonds with no clearly pronounced multiplicity is formed. That is, it is observed that the alternation of lengths of С С bonds in chains С С=С С on either side of the central benzene ring is suppressed, and, in its turn, the ring in the excited state acquires a quinoid structure (see Table 1). Similar deformation effects are also observed upon excitation of the Т2 molecule into its first triplet state (see Table 1). In the Т3 molecule, the occurrence of the С= double bond between two conjugated phenyl rings (see Fig. 1) leads to a spontaneous violation of the symmetry in the electronically excited state (these facts were observed previously based on DFT calculations of platinum acetylides [23, 24]). For a vertical HOMO LUMO transition in the Т3 molecule (see Fig. 2), we can see that the С= bond for the wave function of the LUMO is strongly antibonding, with the HOMO LUMO transition involving a considerable fraction of the electronic excitation in the functional nucleus of the triphenylamine group. To reduce the degree of this antibonding, the dihedral angle C 3 5 C 6 C 7 changes upon excitation (see Table 2) and the orientation of the triphenylamine ring changes sharply. That is, upon passage of an electron to the LUMO level, the excited state is stabilized because of a weakening of the antibonding character of the wave function of the LUMO, which is achieved by a sharp change in the geometry of the molecule. The spontaneous violation of the symmetry (of the left part of the Т3 molecule with respect to its right part; Table 1) is explained by the fact that considerable changes in the geometry of the two parts of the Т3 molecule are attended with too large energy losses, which are not compensated by the advantage in the stabilization OPTICS AD SPECTROSCOPY Vol. 118 o

4 76 STROMYLO et al. T1 T2 T3 T4 T5 Fig. 2. Frontier molecular orbitals of oligomers T1 T5: (top) highest occupied molecular orbital (HOMO); (bottom) lowest unoccupied molecular orbital (LUMO). energy due to a simultaneous weakening of the antibonding for the two С= bonds. Since the molecule of compound Т4 does not have any central phenyl ring, a change in the alternation in it is observed only upon conjugation of five central bonds, which link two end triphenylamine groups, with the initial multiplicity of the molecule being lost (Table 1). In the case of the Т5 molecule, the occurrence of the triple bond С 5 С 6 upon excitation into the first triplet state leads to the formation of conjugated systems the multiplicity of bonds in which is close to that of the double bond, and the central phenyl ring, as in the case of the Т1 and Т2 molecules, acquires a quinoid structure. Such changes in structural fragments of molecules that take place upon optimization of the first triplet state by the B3LYP method were also observed previously in works dealing with molecules with triple bonds [23, 24]. Electronic and Spectral Properties of the Т1 Т5 Molecules Electronic absorption spectra of compounds under investigation calculated using the BMK functional have the first absorption band in the range of nm (see Table 3; Fig. 3), which corresponds to previously obtained experimental data [7]. The first transition, which corresponds to the first absorption band in all electronic spectra, has the configuration OPTICS AD SPECTROSCOPY Vol. 118 o

5 QUATUM-CHEMICAL IVESTIGATIO 77 Table 3. Wavelengths of electronic transitions (λ), their assignment, and oscillator strengths (f) in absorption spectra of oligomers Т1 Т5 (calculation by the BMK/6-31G(d) method using the PCM model with CHCl 3 as a solvent) Compound State λ, nm λ exp, nm f Assignment T1 T HOMO LUMO (67%) HOMO 2 LUMO (17%) T HOMO 1 LUMO (39%) HOMO LUMO+1 (35%) S HOMO LUMO (89%) HOMO 1 LUMO+1 (7%) S HOMO 1 LUMO (81%) HOMO LUMO+1 (14%) S HOMO LUMO+5 (5%) HOMO 1 LUMO+4 (41%) S HOMO LUMO+4 (5%) HOMO 1 LUMO+5 (41%) S HOMO 1 LUMO+1 (43%) HOMO LUMO+6 (19%) T2 T HOMO LUMO (61%) HOMO 2 LUMO (15%) T HOMO LUMO+1 (41%) HOMO 1 LUMO (38%) S HOMO LUMO (8%) HOMO 1 LUMO+1 (1%) S HOMO 1 LUMO (66%) HOMO LUMO+1 (29%) S HOMO LUMO+3 (48%) HOMO 1 LUMO+2 (4%) S HOMO LUMO+4 (49%) HOMO 1 LUMO+5 (44%) S HOMO LUMO+5 (49%) HOMO 1 LUMO+4 (44%) T3 T HOMO LUMO (51%) HOMO 1 LUMO+1 (21%) T HOMO LUMO+1 (42%) HOMO 1 LUMO (34%) S HOMO LUMO (69%) HOMO 1 LUMO+1 (2%) S HOMO 1 LUMO (51%) HOMO LUMO+1 (42%) S HOMO 1 LUMO+5 (46%) HOMO LUMO+4 (45%) S HOMO 1 LUMO+4 (46%) HOMO LUMO+5 (45%) S HOMO 1 LUMO+1 (47%) HOMO LUMO (26%) OPTICS AD SPECTROSCOPY Vol. 118 o

6 78 STROMYLO et al. Table 3. (Contd.) Compound State λ, nm λ exp, nm f Assignment T4 T HOMO LUMO (7%) HOMO 2 LUMO (16%) T HOMO 1 LUMO (51%) HOMO LUMO+1 (31%) S HOMO LUMO (91%) HOMO 1 LUMO+1 (6%) S HOMO 1 LUMO (86%) S HOMO LUMO+4 (57%) HOMO 1 LUMO+5 (39%) S HOMO LUMO+5 (57%) HOMO 1 LUMO+4 (39%) T5 T HOMO LUMO (66%) HOMO 2 LUMO (17%) T HOMO 1 LUMO (44%) HOMO LUMO+1 (34%) S HOMO LUMO (87%) HOMO 1 LUMO+1 (8%) S HOMO 1 LUMO (79%) HOMO LUMO+1 (15%) S HOMO 2 LUMO (67%) HOMO 1 LUMO+1 (19%) S HOMO LUMO+4 (26%) HOMO LUMO+5 (24%) HOMO LUMO with a small admixture of HOMO 1 LUMO+1, which is characteristic of compounds used to fabricate OLEDs [6, 13 15]. As a result of this transition, the electron density is shifted from end triphenylamine groups to conjugated phenyl fragments, thereby concentrating in the internal part of the molecule (see Fig. 2). Therefore, although the contribution of the charge transfer is considerable, the S S 1 excitation is characterized by a high transition dipole moment and a high intensity (see Table 3). It is well known that OLEDs are fabricated from fluorescent compounds the first singlet singlet transition of which is well-resolved in the electric-dipole approximation, due to which intense luminescence in the visible spectral range is achieved. Precisely this property is characteristic of compounds Т1 Т5 that we examine. Thus, the oscillator strength of the S S 1 transition in compound Т1 is ~2.8, while that for compounds Т3 and Т5 exceeds 3 (see Table 3); i.e., compounds Т1 Т5 should have an intense band in the fluorescence spectrum, which is observed in experiment [7]. The second electronic transition in the spectra of the Т1 Т5 oligomers is adjacent to S S 1 and is almost forbidden, being overlapped completely by the main (first) absorption band (see Table 3), broadening it in the form of a weak inflection on the left (see Fig. 3). The majority of high-lying electronic transitions to states S 5 S 1 are well-resolved in the electric-dipole approximation and, in fact, specify the second absorption band in the calculated spectrum, which is located at 285 nm, is of medium intensity, and is well reproduced (with a slight bathochromic shift) in the experimental spectrum (see Fig. 3; Table 3). COCLUSIOS Our quantum-chemical investigation of the electronic structure and absorption spectra of symmetric triphenylamine oligomers conjugated to vinylene, imine, azine, and ethynylene bridge groups based on the density functional theory and using two hybrid exchange-correlation functionals, B3LYP and BMK, have shown good detailed agreement with all the available experimental data and existing notions on the OPTICS AD SPECTROSCOPY Vol. 118 o

7 QUATUM-CHEMICAL IVESTIGATIO 79 I, rel. units I, rel. units T1 T2 T3 T4 I, rel. units T5 Wavelength, nm Fig. 3. Absorption spectra of oligomers T1 T5: (dashed curves) calculated spectra and (solid curves) experimental spectra. structure and spectral peculiarities of examined oligomers. From the data of calculations by the TD DFT method using the PCM continual model, we have found that intense absorption of oligomers under investigation in the visible spectral range is caused by a wellresolved singlet singlet electronic transition to the S 1 state, the orbital nature of which should resemble to a large extent the T 1 state (the two of them correspond to the same HOMO LUMO electronic configuration). The two states are of the charge-transfer nature: upon excitation, the electron density is shifted from end triphenylamine groups to the central part of molecules Т1 Т5. In this case, the concentration of the electronic excitation in the central part, which links triphenylamine groups, leads to the formation of a large S S 1 electronic transition dipole moment. ACKOWLEDGMETS We are deeply grateful to Prof. Hans Agren, head of the Department of Theoretical Chemistry and Biol- OPTICS AD SPECTROSCOPY Vol. 118 o

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