Substituent effects on photophysical properties of ESIPT-based fluorophores bearing the 4-diethylaminosalicylaldehyde core

Six novel molecules (EASA-PTZ, EASA-DAMC, EASA-PXZ, EASA-CNPPz, EASA-CNPQx, and EASA-CNBQx) were designed by replacing fluorene with electron-donating groups (PTZ, DAMC, and PXZ) and electron-withdrawing group (CNPPz, CNPQx, and CNBQx) within the structure of the 4-diethylaminosalicylaldazine-modified fluorene Schiff base (EASA-F). The influence of substituent effects on the excited-state intramolecular proton transfer (ESIPT) process and photophysical properties of the studied molecules were systematically explored utilizing the quantum chemistry method in acetonitrile (ACN). The variation of intramolecular hydrogen bond (IHB) intensity upon photoexcitation was revealed by computing geometric parameters, infrared (IR) spectra, and reduced density gradient (RDG) scatter plots. Potential energy curves (PECs) and corresponding transition states (TS) in S0 and S1 states were also acquired to accurately investigate the ESIPT mechanism. Besides, crucial parameters, including the ionization potential (IPa and IPv), the electron affinity (EAa and EAv), and the reorganization energy (λh and λe), were calculated to evaluate the potential application of these molecules in organic light-emitting devices.