Exploring the excited state multi-proton transfer path and the associated photophysical properties of P-TNS molecule by DFT and TDDFT theory

Abstract

Recently, Chen et al. designed and synthesized a new multi-purpose dye probe P-TNS with bifunctional groups, which has an ultra-fast excited state intramolecular proton transfer characteristic (ESIPT) (Spectrochim Acta A Mol Biomol Spectrosc., 2021, 262: 120084.). The P-TNS molecules are very sensitive to hydrazine (N2H4) and cyanide (CN-), which makes it have a good practical application prospect in the field of probe detection. However, the corresponding optical properties and the related mechanisms have not been systematically investigated in experiments. In the present work, the proton transfer pathways as well as the photophysical properties changes in the transfer path of P-TNS with three intramolecular hydrogen bonds (HB1, HB2 and HB3) are revealed for the first time. First, the analysis of potential energy surface and potential energy curve determined the stable conformation of P-TNS molecule and its isomer structure, and clarified the reaction mechanism of ESIPT process. Subsequently, the analysis of the frontier molecular orbitals reveal that the charge difference is the driving force required for the proton transfer process from the microscopic level, which led to the determination that the HB1 structure without the charge difference could not complete the ESIPT process. Finally, the variation of hydrogen bond strength and the path of proton transfer are demonstrated from several perspectives of hydrogen bond parameters, hydrogen bond energy, infrared vibrational frequency (IR) and reduced density gradient (RDG) surface. This study essentially explains the photophysical properties and proton transfer pathways of a new multipurpose dye probes for organic luminescence with bifunctional groups, which are important for the fields of spectroscopy and measurement technology, and also provides ideas for the design and synthesis of new fluorescent probes.