Ultrafast dynamics of dual fluorescence of 2-(2′-hydroxyphenyl) benzothiazole and its derivatives by femtosecond transient absorption spectroscopy

The dual fluorescence behaviors of 2-(2′-Hydroxyphenyl) benzothiazole (HBT) and 2-(2′-hydroxyphenyl) benzoxazole (HBO) molecules are studied by femtosecond transient absorption (fs-TA) spectroscopy. The steady-state fluorescence (515 nm) of HBT appears in the stimulated emission region in fs-TA spectroscopy, and the other fluorescence (363 nm) is covered by the strong excited state absorption signal. The large energy barrier (25.06 kcal/mol) of conformational torsion from O–H⋯N to O–H⋯S form show that the torsion is hardly to occur in the S1 state, which is also supported by the inconsistent spectra behaviors with the experimental results. Meanwhile, the nonadiabatic dynamics of HBT shows that the proton transfer (PT) from Enol to Keto occurs in 115 fs, which is consistent with the result of fs-TA in 117.2 fs. The fluorescence (493 nm) of the Keto form of HBT is consistent with experimental result (515 nm), and the other fluorescence (363 nm) comes from the Enol form of HBT. Together with favorable energy barrier (1.68 kcal/mol), these results demonstrate that the PT occurred in fs timescale is the main factor for dual fluorescence of HBT. Compared to HBT, HBO has higher energy barrier (3.74 kcal/mol) and longer PT time (164.7 fs). The combination of fs-TA spectroscopy and theoretical simulation provides insight to understand the fluorescent dynamics of HBT and HBO. •Time-resolved ESIPT is used to explore the dual-fluorescence of HBT.•The fs-TA spectroscopy and nonadiabatic dynamics confirms the ESIPT in 115 fs.•Attribute the long-wavelength fluorescence of HBT in fs-TA to ESIPT.