Conjugation-Modulated Excitonic Coupling Brightens Multiple Triplet Excited States

The design and regulation of multiple room-temperature phosphorescence (RTP) processes are formidably challenging due to the restrictions imposed by Kasha's rule. Here, we report a general design principle for materials that show multiple RTP processes, which is informed by our study of four compounds where there is modulation of the linker hybridization between donor (D) and acceptor (A) groups. Theoretical modeling and photophysical experiments demonstrate that multi-ple RTP processes can be achieved in sp3 C-linked D-A compounds due to the arrest of intramolecular electronic communication between two triplet states (T1H and T1L) localized on the donor and acceptor or between two triplet states, one localized on the donor and one delocalized across aggregated acceptors. However, for the sp2 C-linked D-A counterparts, RTP from one locally excited T1 state is observed because of enhanced excitonic coupling between the two triplet states of molecular subunits. Single-crystal and reduced density gradient analyses reveal the influence of molecular packing on the coincident phosphorescence processes and the origin of the observed aggregate phosphorescence. These findings provide insights into higher-lying triplet excited-state dynamics and into a fundamental design principle for designing compounds that show multiple RTP.

Automated summary

We discovered a general design principle for materials that exhibit multiple room-temperature phosphorescence (RTP) processes. By modulating the hybridization between donor and acceptor groups, we achieved multiple RTP processes in sp3 C-linked donor-acceptor compounds. However, in sp2 C-linked counterparts, only one locally excited triplet state phosphorescence was observed due to enhanced excitonic coupling. Our findings provide insights into the dynamics of higher-lying triplet excited states and offer a fundamental design principle for compounds with multiple RTP.