Nonradiative Decay Channels for a Structurally-Distorted, Monostrapped BODIPY Derivative.

A boron dipyrromethene (BODIPY) derivative has been synthesized whereby a phenoxyl ring attached at the 3-position is bound through the oxygen atom to the boron center. This compound is structurally distorted, with the molecular surface being curved, and undergoes further geometrical perturbation at the excited singlet state level. Fluorescence is readily observed in solution at ambient temperature, with the quantum yield rising with increasing viscosity of the surrounding solvent. Dual-exponential decay kinetics are observed, corresponding to E-type delayed fluorescence. In solution, the emission yield falls with increasing temperature, but the opposite situation is found for the same compound dispersed in an amorphous sugar. These results are considered in terms of two radiationless decay channels. A viscosity-dependent avenue allows the fluorophore to function as a conventional fluorescent rotor for tracking changes in local rheology. A temperature-dependent channel leads to trapping within a new conformation, which is weakly coupled to the ground state but is able to repopulate the emitting state on a relatively slow time scale. Analysis of the experimental data allows estimation of some of the key kinetic parameters as a function of temperature.