Theoretical study of excited state intramolecular proton transfer (ESIPT) mechanism for a fluorophore in the polar and nonpolar solvents

Abstract

2, 4-dibenzothiazolyl-phenol (2, 4-DBTP), a fluorophore developing induced emission properties, undergoes excited state intramolecular proton transfer (ESIPT) taking place along its strong hydrogen bond. 2,4-DBTP has been investigated in the polar and nonpolar solvents by means of time-dependent density functional theory (TDDFT). The TDDFT absorption and emission wavelengths obtained through the vertical approximation are in good agreement with their experimental counterparts, confirming the existence of an ESIPT process and indicating that the selected level of theory is reasonable. Hydrogen bond strengthening has been testified in the S1 state based on comparing primary bond lengths and bond angles involved in the intramolecular hydrogen bond between the S0 and S1 state. Furthermore, the frontier molecular orbitals (MOs) analysis method manifest the intramolecular charge transfer, which reveals the tendency of ESIPT process. To further characterize the ESIPT process, we determined the potential-energy curves of the S0 and S1 states.Compared with 2,4-DBTP monomer, the S1 state potential energy curve of 2,4-DBTP-MeOH complex has a bigger slope with no potential barrier. Therefore, we can indicate that the ESIPT process for 2,4-DBTP is much easier in methanol solvent.