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.