Effectively Controlling NIR Emissive Property and the ESIPT Behavior of Modified Styryl Dyes by Atomic Substituent:...

Recent literature on biosensing and bioimaging has explored excited state intramolecular proton transfer (ESIPT) cyanide dyes. These classes of fluorescence dyes generally use the classical pyridinium or indolium cations acceptor units' styrene with the ESIPT core. This work studied the photophysical and ESIPT kinetics of novel flavylium cation as an acceptor unit styrene with an ESIPT core using DFT/TD-DFT calculations. Two new ESIPT cyanine dyes, namely (E)-4-(3-(benzo[d]thiazol-2-yl)-2-hydroxystyryl)-7-(dimethylamino)-2-phenyl chromenylium (PSS) and (E)-4-(3-(benzo[d]oxazol-2-yl)-2-hydroxystyryl)-7-(dimethylamino)-2-phenylchromenylium (PSO) were designed and fully studies. This is concerned with studying changes in intramolecular hydrogen bonds, molecular orbitals at the frontier of the ESIPT process, absorption and fluorescence spectra, and excited state energy barriers. As a result, both the systems considered here can undergo an ultrafast ESIPT reaction with PSS and then PSO. Furthermore, ESIPT is more accessible in the normal enol-form first excited singlet (S₁) state, with shorter hydrogen bonds. The intersystem crossing between the S₁ state and the triplet (T₁) state greatly influences the fluorescence efficiency of PSO and PSS. The potential energy curve and transition state energy profiles of PSS and PSO show that ultrafast ESIPT occurs in the state. Furthermore, the PSS shows less energy barriers, which leads to faster proton transfer than PSO. The current study will advance knowledge of the mechanism behind the ESIPT process and help enhance the qualities of the cyanine dye used in ESIPT.