Investigation of Excited-State Intramolecular Proton Transfer and Structural Dynamics in Bis-Benzimidazole Derivative (BBM)

The bis-benzimidazole derivative (BBM) molecule, consisting of two 2-(2 '-hydroxyphenyl) benzimidazole (HBI) halves, has been synthesized and successfully utilized as a ratiometric fluorescence sensor for the sensitive detection of Cu2+ based on enol-keto excited-state intramolecular proton transfer (ESIPT). In this study, we strategically implement femtosecond stimulated Raman spectroscopy and several time-resolved electronic spectroscopies, aided by quantum chemical calculations to investigate the detailed primary photodynamics of the BBM molecule. The results demonstrate that the ESIPT from BBM-enol* to BBM-keto* was observed in only one of the HBI halves with a time constant of 300 fs; after that, the rotation of the dihedral angle between the two HBI halves generated a planarized BBM-keto* isomer in 3 ps, leading to a dynamic redshift of BBM-keto* emission.

Automated summary

In this study, a bis-benzimidazole derivative (BBM) molecule consisting of two 2-(2 '-hydroxyphenyl) benzimidazole (HBI) halves was synthesized and used as a ratiometric fluorescence sensor for sensitive detection of Cu2+ based on enol-keto excited-state intramolecular proton transfer (ESIPT). The primary photodynamics of the BBM molecule were investigated using femtosecond stimulated Raman spectroscopy, time-resolved electronic spectroscopies, and quantum chemical calculations. The results showed that ESIPT occurred only in one of the HBI halves with a time constant of 300 fs, followed by rotation of the dihedral angle between the two HBI halves and generation of a planarized BBM-keto* isomer in 3 ps, resulting in a dynamic redshift of BBM-keto* emission.