Extended CC bond chain controlled luminescence behavior of novel ESIPT-active benzothiazole-based fluorophore in near-infrared region: A TD-DFT insight

Abstract

Using density functional theory (DFT) and time-dependent DFT (TD-DFT), the excited state intramolecular proton transfer (ESIPT) mechanisms and luminescent properties of (E)-2-(3-(3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methylstyryl)-5,5-dimethylcyclohex-2-en-1-ylidene) malononitrile (BTCMN) and its derivatives were systematically investigated. Four BTCMN derivatives with varying intramolecular CC bond chain, namely BTCMN-1, BTCMN-2, BTCMN-3 and BTCMN-4 were designed to comprehensively explore the impact of CC bond chain length on BTCMN. The selected functional is very reasonable since the experimental absorption and fluorescence wavelengths are reproduced. Detailed analyses have shown that the intramolecular CC bond chain length obviously affects the ESIPT process and photophysical properties. With the elongation of the CC bond chain within the molecule, the excited state intramolecular hydrogen bond (IHB) weakens, the ESIPT barrier increases, and both absorption and fluorescence wavelengths shift red. Especially, the fluorescence wavelength achieves near-infrared region by extending the chain of intramolecular CC bond. Our results can provide more useful information to further design ESIPT-based fluorescent probes for living cell imaging.