Molecular Phosphorescence in Polymer Matrix with Reversible Sensitivity

Ultralong
organic phosphorescence strongly depends on the formation
of aggregation, while it is difficult to obtain in dilute environments
on account of excessive internal and external molecular motions. Herein,
ultralong single-molecule phosphorescence (USMP) at room temperature
was achieved in the monomer state by coassembling biphenyl and naphthalene
derivatives at low density with poly­(vinyl alcohol) (PVA), where PVA
provides a confined environment to stabilize the triplet state. Various
factors that affect the USMP were studied, including aggregation,
conformation, temperature, and moisture. In these systems, the formation
of aggregates through intermolecular stacking and hydrogen bonding
interactions in the film or crystal phases completely suppresses the
USMP. However, the fluorescence is enhanced when coassembling these
compounds at high concentration with PVA and becomes stronger in their
powder state, indicating that the intersystem crossing process is
blocked by the aggregation. Theoretical calculations suggest that
the aggregation depresses spin–orbit coupling between the excited
singlet and triplet states and enhances the nonradiative quenching
process. Moreover, a relatively twisted conformation is more conducive
to the occurrence of intersystem crossing than planar conformation.
The USMP shows delicate and reversible sensitivity to the changes
of temperature and moisture, rendering them with the applicability
as smart organic optoelectronic materials.