ORIGINAL PAPER TDDFT Calculations of Electronic Spectra of Benzoxazoles Undergoing Excited State Proton Transfer Y. Syetov Received: 17 September 2012 / Accepted: 24 February 2013 / Published online: 9 March 2013 # Springer Science+Business Media New York 2013 Abstract Energies and oscillator strengths of vertical transi- tions for various rotameric and tautomeric species of 2-(2′- hydroxyphenyl)benzoxazole (HBO), 2,5-bis(2-benzoxazolyl) phenol (DBP) and 2,5-bis(2-benzoxazolyl)hydroquinone (BBHQ) have been calculated in the ground and first excited states with the use of TDDFT methods. The TDDFT results demonstrate good correspondence to the frequencies of ab- sorption and fluorescence bands of the benzoxazoles reported for measurements in supersonic jets and solution, but fail to predict relative energies of the enol and keto tautomers of DBP and BBHQ in the excited state. Low intensity of the fluores- cence bands attributed to the conformations of HBO and DBP that do not undergo excited state proton transfer is shown to be caused by low concentrations of the conformations in the ground state. For the three compounds large-amplitude twist- ing of the keto tautomer is found to be one of radiationless processes resulting in decrease of the fluorescence with a large Stokes shift. Keywords Density functional theory . Excited state proton transfer . Benzoxazole derivatives . Fluorescence . Absorption . Radiationless transitions Introduction Excited state proton transfer (ESIPT) has been extensively studied both experimentally and theoretically regarding its importance as a photochemical reaction as well as its rele- vance for various applications [1–7]. 2-(2′-hydroxyphenyl) benzoxazole (HBO), 2,5-bis(2-benzoxazolyl)phenol (DBP) and 2,5-bis(2-benzoxazolyl)hydroquinone (BBHQ) are benzoxazole derivatives that exhibit ESIPT from the pheno- lic hydroxy group to nitrogen atom, the tautomers are enol and keto structures [ 1 , 8 ]. In the ground state the benzoxazoles possess intramolecular hydrogen bonds and can form rotamers with OH…N and OH…O H-bonds. Only the conformations with OH…N hydrogen bonds undergo ESIPT and show fluorescence with an anomalously large Stokes shift, the OH…O conformers of HBO and DBP have been supposed to be responsible for emission with a normal Stokes shift [9–13]. BBHQ has been found to have enol and keto structures that are in equilibrium in the excited state, the enol tautomer exhibits fluorescence with a normal Stokes shift [14–17]. Despite the presence of two identical hydrogen bonding sites in the molecule, only transfer of one proton has been identified to occur in the excited state of BBHQ [18, 19]. Some tautomeric and rotameric structures of molecules of HBO, DBP and BBHQ are shown in Fig. 1. Quantum yield of tautomeric fluorescence of HBO in solution is strongly dependent on the viscosity of the solvent and temperature. The radiationless decay of the excited state has been attributed to large-amplitude twisting around the bond linking the phenol and benzoxazole moieties of the keto tautomer [20, 21]. A radiationless process with activa- tion energy similar to the value found in HBO (about 0.15 eV) has been revealed for BBHQ in solution [14] and supersonic jets [16] but decay of the excited enol state has been proposed for explanation of the observations [14]. DBP demonstrates a high quantum yield (0.48) in solution [8] while a threshold of about 0.05 eV that triggers decrease of intensity of the keto fluorescence has been determined in supersonic jet conditions [13]. Quantum chemical description of ESIPT requires multireference methods involving dynamical electron correla- tion such as CASPT2 and MRCI because the excited states are expected to have crossing of potential energy surfaces (PES) Y. Syetov (*) Department of Solid State Physics and Optoelectronics, Oles Gonchar Dnipropetrovsk National University, Prosp. Gagarina, 72, 49010 Dnipropetrovsk, Ukraine e-mail: setov2003@yahoo.com J Fluoresc (2013) 23:689–696 DOI 10.1007/s10895-013-1196-8