Unraveling the Transient State of Photo-Induced Bond Cleavage Intramolecular Charge Transfer in Organoboron

Abstract

The cleavage of chemical bonds is typically characterized by a transient intermediate state prior to complete dissociation. Deciphering the transient interaction and mechanisms of the intermediate state is crucial for molecular function and reaction design. In this study, a bond-cleavage-induced intramolecular charge transfer (BICT) molecule BF-thio, containing B←O coordination bond, is investigated in detail on the photo-induced B←O bond cleavage process. The intrinsic instability of the B ← O bond in the excited state results in the temperature-dependent dual emission. Femtosecond transient absorption (TA) spectroscopy results reveal that bond cleavage and charge transfer can be considered to occur in a concerted manner. Based on the temperature-variable TA measurements, the activation energy of B ← O bond dissociation in the excited-state is determined to be 0.18 eV. In addition, by integrating one-dimensional potential energy curve model with spectroscopic data, the potential energy profiles as a function of the B←O bond length and the critical bond length of 2.6 Å at transition-state [B•••O] ‡ is estimated. This work provides a direct experimental insight into the photo-induced BICT process and demonstrates an application example of dative B←O bond cleavage materials in photoresponsive temperature sensing.