Transforming delayed fluorescence into blue-shifted phosphorescence in aminoboranes via oxygen-to-sulfur substitution on the donor amine

Thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) materials are found in diverse applications, from optoelectronic devices to time-gated bioimaging. Recently, aminoboranes with donor-acceptor structures have been identified as promising candidates due to their inherent capacity to harvest the triplet excitons by their unique orbital configurations (El Sayed rule). This work reports the delayed luminescence behaviors of two aminoboranes, BNO and BNS, featuring phenoxazine (PXZ) or phenothiazine (PTZ) donors coupled with a dixylylborane acceptor. BNO exhibits efficient TADF emission in aggregates, thin films, and solid states. In contrast, BNS shows delayed fluorescence (DF) in aggregate states and RTP in solid and thin-film states. Notably, BNS shows a rare blue-shifted phosphorescence relative to its prompt fluorescence, which has not been reported for aminoboranes. Photoluminescence studies and computational calculations reveal that ISC and rISC processes in these systems involve higher triplet states. The unprecedented blue-shifted phosphorescence in BNS is attributed to perturbations in energy levels, which are driven by unique quasi-axial and quasi-equatorial conformations and the stronger spin-orbit coupling of heavier S over O.