Unraveling the transient state of bond-cleavage-induced intramolecular charge transfer in organoboron compounds

Abstract

The cleavage of chemical bonds is typically characterized by a transient intermediate state prior to their 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 a B←O coordination bond, is investigated in detail, focusing on the photoinduced B←O bond cleavage process. The intrinsic instability of the B←O bond in the excited state results in temperature-dependent dual emission. Femtosecond transient absorption (TA) spectroscopy results reveal that the 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 the B←O bond dissociation in the excited state is determined to be 0.18 eV. In addition, by integrating a one-dimensional potential energy curve model with the spectroscopic data, the potential energy profiles as a function of the B←O bond length and a critical bond length of 2.6 Å at the transition-state [B⋯O]^‡ are estimated. This work provides direct experimental insights into the photoinduced BICT process and demonstrates an example of the application of dative B←O bond-cleavage materials in photoresponsive temperature sensing.