PHOTOINDUCED ELECTRON TRANSFER BETWEEN PYRENE/HYDROXYPYRENE AND AROMATIC AMINO ACIDS

Abstract

Photoinduced electron transfer systems have optical applications such as sensory photoreceptors and light-emitting diodes. The photonic energy which is harvested by an antenna is transformed into ground state chemistry utilizing photoinduced electron transfer or photochemical bond reorganization [1]. Amino acids often possess particular properties, such as weak van der Waals and hydrogenbonds, wide transparency ranges in the visible regions and zwitterionic nature of the molecules which are very important in materials science [2]. In order to be able to optimize the efficiency of organic optical devices, it is an advantage to understand completely the physical behaviour of compounds in gas phase and in solution. We have studied geometrical structure, electronic structure and optical excitation of pyrene(Py)/1-hydroxypyrene (PyOH) with aromatic amino acids in vacuum and in water using DFT and TD-DFT calculations. The density functional theory (DFT) computations have been performed at ωB97XD/6-31G(d,p) level using Gaussian 09 program [3]. Pyrene and hydroxypyrene were chosen as molecular antenna and aromatic amino acids were chosen as electron donors. Our results show that more stable complexes are formed in vacuum. Tryptophan (Trp) complexes are the most stable among all optimized complexes. Significant geometrical changes have been observed for the optimized complexes between gas phase and water. Pyrene/hydroxypyrene-aminoacid interactions via π-π stacking promote the photoinduced transfer reactions. Calculated HOMO-LUMO energies confirm that charge-transfer occurs between the molecules. The most significant charge transfer is observed for Trp complexes between HOMO-1 and LUMO. It is concluded that the lowering of highest occupied orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy gap appears to be cause of its enhanced charge transfer.