Structure–Property Studies Toward the Stimuli-Responsive Behavior of Benzyl–Phospholium Acenes

A series of new phospholium acenes, quaternized with
benzyl groups,
was synthesized. Both different π-conjugated backbones and electron-donating/-withdrawing
benzyl groups were systematically studied to reveal details on the
nature of their structural dynamics. Extensive NMR studies (including
variable concentration/temperature and 2D) suggested that the systems
undergo intramolecular conformation changes in solution
that are strongly affected by the electronic nature of the benzyl
group, and thereby significantly affecting the phosphole-typical σ*−π*
interaction. This class of “smart” phosphole system
exhibits enhanced emission in the solid state and at low temperature
in solution, due to aggregation-induced enhanced emission (AIEE).
The dynamic features of these smart phospholes also endow the systems
with external-stimuli (thermal and mechanical force) responsive photophysical
properties. Crystallographic studies and theoretical calculations
confirmed that the thermal response of the phospholium system is mainly
due to the conformation changes in solution, while the mechanical
response of the system can be attributed to both the intramolecular conformation and the intermolecular organization
changes in the solid state.