Probing Excited State Structural Control and Intramolecular Charge Transfer in Aminoboranes by Ultrafast Transient Absorption and Raman Loss Spectroscopy

Aminoboranes are an emerging class of materials that exhibit versatile emission properties, such as delayed fluorescence (DF) and room temperature phosphorescence (RTP), yet their excited-state dynamics remain poorly understood. Here, we employ femtosecond transient absorption and stimulated Raman spectroscopy for the real-time tracking of ultrafast electronic relaxation and structural dynamics during charge transfer in carbazole-based aminoboranes. Following excitation of the S₁ state, transient absorption in polar solvents reveals the ultrafast onset of a red-shifted stimulated emission band, indicating evolution of a locally excited (LE) state into an intramolecular charge-transfer (ICT) configuration. Density functional theory calculations and numerical simulations using the response-function formalism and the multimode Brownian oscillator model suggest that swift evolution along the B-N stretching and torsional coordinates guides the development of ICT character, the time scale of which depends on the polar solvation time. Such excited-state structural evolution is accompanied by a blue-shifted B-C stretching frequency in transient Raman loss signals, reporting electron density localization around the boryl acceptor upon charge transfer and forming a distorted ICT state. The formation of this state can be structurally controlled and thus is important for controlling the balance between the RTP and delayed fluorescence efficiency. Our findings highlight the importance of excited-state structural control and engineering in designing aminoborane-based emitters with tailored luminescence properties.