A Theoretical Study On Photoinduced Excited State Intramolecular Proton Transfer Of Schiff Base Derivatives With Extended Conjugated Chromophores

The photoinduced excited state intramolecular proton transfer (ESIPT) is a significantly fundamental photochemical process. Of its particular interest of this paper is to deal with the occurrence of ESIPT of some new Schiff base derivatives with extended conjugated chromophores with various substituent groups, including (2-((4'-N,N-dimethylamino-stilbene-4-ylimino)methyl)phenol(C1), 4-((4'-N,N- dimeth- ylamino-stilbene-4-ylimino)methyl)phenol(C2), 2-((4'-nitro- stilbene-4-ylimino)methyl) phenol(C3), 2-((4'- cyano -stilbene- 4- ylimino)methyl) phenol(C4), 2((4'- methoxyl -stilbene-4-ylimino)methyl)phenol(C5), 2-((4'-N,N-diethylamino -stilbene-4-ylimino)) methyl)phenol(C6), 4-((4'-nitro-stilbene-4-ylimino)methyl)phenol(C7), 2-((4'-N,N-di- ethylamino-stilbene-4-ylimino))methyl)phenol(C8), 4-amino-3',5'-(bisimino)stilbene- di(2-hydroxylbenzaldehyde)(C9). The properties of ground and excited state of these derivatives, which include structural parameters, energy, rate constant, frontier orbits, Mulliken charge, dipole moment and so on, were studies by quantum chemistry calculation method at HF, CIS, DFT and TDDFT level, and the ultraviolet/vision absorption, normal fluorescence and ESIPT fluorescence spectra of the compounds were predicted in theory also.The theoretical results show that when enol form molecules at excited state could undergo proton transfer more easily than that at ground state, but the chromophore part increases some difficulty to undergo ESIPT for the derivatives, and this is an endothermic reaction which need overcome an energy barrier, but the possibility of occurrence of ESIPT is quite high also.On the other hand, electron-donating and electron-withdraw substituent group shows different effects on the ESIPT properties of the derivatives, and this is in turn suggestes that the former have more ability to undergo ESIPT than the latter. Because the former can drive the ESIPT process to become an exothermic reaction and the derivatives with them only need overcome very low energy batteries, while the latter is unfavorable for proton transfer due to the enhancement of higher energy barrier.In addition, this paper also had examined ESIPT of the molecule, which has two Schiff base fragments and a chromophore part. The potential energy curves showed that the proton transfer process was neither a step by step transfer mode nor coordinated transfer mode, but only for a single proton transfer, but it has low efficiency.