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 multiple 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.