2017. 58, 2 – UDC 541.6:541.65:548.737 STRUCTURAL AND SPECTRAL PROPERTIES OF 3-SUBSTITUTEDPHENYL-1,5-DIPHENYLFORMAZANS: A QUANTUM CHEMICAL STUDY H. Tezcan1, N. Tokay2 1Department of Chemistry, Faculty of Gazi Education, Gazi University, Ankara, Turkey 2Department of Chemistry, Faculty of Science, Hacettepe University, Ankara, Turkey E-mail: habibe@gazi.edu.tr Received April, 27, 2016 The structural and optical properties of 3-substitutedphenyl-1,5-diphenylformazans are studied by quantum chemical methods. <...> The density functional theory (DFT) is employed to optimize the ground state geometries of formazans substituted with different electron donating and withdrawing groups in both gas and solvent phases. <...> The absorption spectra of formazan derivatives are calculated using time dependent density functional theory (TD-DFT). <...> The polarizable continuum model (PCM) calculations of 3-substitutedphenyl-1,5-diphenylformazans are performed for bulk solvent effects. <...> The geometrical parameters, vibrational frequencies, and relative stabilities of isomers of 3-substitutedphenyl-1,5-diphenylformazans are studied. <...> The results obtained by TD-DFT calculations reveal that the substitution of electron withdrawing and donating substituents affects the absorption spectra of 3-substitutedphenyl-1,5-diphenylformazans. <...> The calculated maximum absorption wavelengths (ymax) are highly consistent with the experimental values as found from UV-vis spectra. <...> As a predictive tool for the structural properties, molecular modelization techniques allow a solution for the interpretation of experimental data. <...> In recent years, the time-dependent density functional theory (TD-DFT) has opened an efficient access to theoretical spectral absorption data in the visible and ultraviolet regions of the electromagnetic spectrum [ 8—10]. <...> In this study, the maximum absorption wavelengths (ymax) of 3-substitutedphenyl-1,5-diphenylformazans (Fig. 1) were studied with the help of TD-DFT quantum chemical calculations, and bulk © Tezcan H., Tokay N., 2017 . 269 – 277 270 H. TEZCAN, N. TOKAY Fig. 1. <...> Structures of 3-substitutedphenyl-1,5-diphenylformazans solvent effects on singlet vertical excited states were investigated using the PCM method [ 13—16 ]. <...> The calculated results were compared with the available experimental data. <...> Quantum chemical calculations of TPF (1) have been performed by DFT using B3LYP [18—20 ], and PBE0 [21, 22 ] functionals with several basis sets to determine the best appropriate functional-basis set combination. <...> A comparison of the experimental and calculated ymax values of TPF revealed that the combination of the PBE0 <...>