Effects of π-expansion, an additional hydroxyl group, and substitution on the excited state single and double proton transfer of 2-hydroxybenzaldehyde and its relative compounds: TD-DFT static and dynamic study

Abstract

The effects of π-expansion, an extra hydroxyl group, and substituents (–CH₃ and –OCH₃) on the photophysical properties, excited state single proton transfer (SPT) and double proton transfer (DPT) of 2-hydroxybenzaldehyde (HBA) and its relatives have been theoretically investigated with static calculations and dynamics simulations using TD-DFT. For static calculations, bond distances involved in intramolecular hydrogen bond (IntraHB) reveal that IntraHB becomes strengthened in excited state (S₁), confirmed by the red shift of O–H stretching and topology analysis. Both absorption and emission spectra of the compounds having π-expansion and an extra hydroxyl group as di-PT-type are found to be longer compared with those of mono-PT-type (either with- or without-π-expansion), but shorter compared to their analogue compounds. The potential energy curves (PECs) along PT reaction in S₁ show that SPT of mono-PT-type without π-expansion occurs with a barrierless and exothermic reaction whereas that of mono-PT-type having π-expansion gives a high PT barrier with endothermic reaction. The combination of π-expansion and o-methoxy leads to a higher PT barrier and endothermic reaction, resulting in no PT process. Moreover, o-methoxy of di-PT-type induces the 1st PT barrier to be higher than the 2nd PT barrier, resulting in no DPT process. Furthermore, dynamic simulations on S₁ reveal that chances of SPT and DPT processes are directly controlled by PT barrier and reaction energy, in which PT process is always guaranteed for a low PT barrier with exothermic reaction but not for a high PT barrier with endothermic reaction.