Controlling the Delayed Fluorescence-Phosphorescence Balance via Torsional Dynamics and Intramolecular Charge Transfer in Carbazole-Based Aminoboranes.

Carbazole-based aminoboranes are promising luminophores exhibiting both delayed fluorescence and room-temperature phosphorescence. However, the structural and electronic factors governing intersystem crossing (ISC) and reverse ISC (RISC) remain elusive. Using femtosecond transient absorption and stimulated Raman spectroscopy, we reveal that rapid evolution along the B-N stretching/torsional coordinates facilitates the formation of a twisted intramolecular charge-transfer (TICT) state, bringing the singlet and triplet states energetically closer. In a series of N-borylated carbazoles, bromine incorporation enhances ISC via heavy-atom effects, while "magic methyl" groups impact the torsional dynamics, which increase the nonradiative relaxation pathway and ISC rate. Solvent-dependent and computational studies found that both ISC and reverse ISC proceed through higher-lying triplet and hybrid charge-transfer states that enable spin flipping. This interplay between torsional control and spin-orbit coupling finely tunes the balance between delayed fluorescence and phosphorescence.