The effects of electron-withdrawing and electron-donating groups on the photophysical properties and ESIPT of salicylideneaniline

Abstract

The photophysical properties and excited state intramolecular proton transfer (ESIPT) of salicylideneaniline (1a) and its derivatives (1b-1e) with different substituents have been investigated using the DFT and TD-DFT methods. The calculated results indicate that the introduction of electron-withdrawing group CN weakens the intramolecular hydrogen bond (H···N). However, the introduction of electron-donating group N(CH3)2 strengthens it. When the CN and N(CH3)2 groups are introduced simultaneously, the intramolecular hydrogen bond (H···N) is weakened. In addition, swapping the CN and N(CH3)2 group positions can enhance the intramolecular hydrogen bond (H···N). Compared to 1a, the absorption and emission spectra of compounds 1b-1e are red-shifted. Frontier molecular orbital analyses prove that the more intense intramolecular charge transfer characters caused by CN and N(CH3)2 substituents is responsible for the red shift of spectra. Potential energy curves indicate that ESIPT in salicylideneaniline (1a) and the CN substituted derivative (1b) is a non-barrier process, while in the N(CH3)2 substituted derivative (1c) and the CN and N(CH3)2 co-substituted derivative (1d), ESIPT needs to overcome the energy barriers of 2.32kcal/mol and 3.38kcal/mol, respectively. Exchanging the positions of CN and N(CH3)2 groups (1e) makes the ESIPT into a barrier-free process. Therefore, the substitution and position of CN and N(CH3)2 groups can affect the ESIPT process and induce different photophysical properties.