Ultrafast dual-fluorescence dynamics of 2-(2'-hydroxyphenyl) benzimidazole by femtosecond transient absorption spectroscopy

2-(2'-Hydroxyphenyl) benzimidazole (HBI) and its derivatives are widely used in fluorescent probes, luminescent materials, and other sensing materials. In this study, the time-resolved fluorescence behaviour, and excited-state dynamics of HBI and 2-phenyl-1H-benzo[d]imidazole (BI) were studied using femtosecond transient absorption (fs-TA) spectroscopy. The stimulated emission (SE) and excited-state absorption signals of the BI molecule were located at 355 and 437 nm, respectively. For HBI, these signals were located at 475 and 384 nm, respectively. The SE signals of the BI and HBI molecules in their fs-TA spectra were consistent with the experimental short-wave fluorescence at 340 and 460 nm, respectively, in the steady-state spectra. The long-wave signal of HBI at 475 nm indicates its dual fluorescence, which may be caused by proton transfer or conformational torsion after photoexcitation. The large theoretical energy barrier of 40.97 kcal/mol in first excited (S1) state of HBI excludes the latter hypothesis. Additionally, the small energy barrier (1.88 kcal/mol) in the S1 state confirmed the excited-state intramolecular proton transfer (ESIPT). This indicates that ESIPT induces dual fluorescence behaviour of the HBI molecule, which was also demonstrated by the infrared spectra, theoretical fluorescence peaks, and molecular orbitals.