Enabling Peculiar Photophysics and Mechanochromic Luminescence by Introducing Bromine in Push-Pull Pyridine Derivatives

Pyridine-based derivatives Py-1, Py-2, and Py-3 were synthesized by the Suzuki cross-coupling reaction, and the effect of different substituents at the ortho position of the pyridine ring was investigated. In the molecular framework of push-pull compounds, electron acceptor (A) pyridine was substituted with an electron donor (D) dimethoxy triphenylamine unit. The effect of passing from a dipolar (D-A, Py-1) to a quadrupolar (D-A-D, Py-3) structure as well as the impact of functionalizing the pyridine with a bromine in the dipolar structure (D-A-Br, Py-2) was investigated. All the pyridine derivatives were found to be highly emissive both in solution and in the solid state, with fluorescence properties markedly sensitive to the environment and responsive to external stimuli. The spectral and photophysical properties in solution were investigated through conventional steady-state and advanced time-resolved spectroscopies with nanosecond and femtosecond temporal resolution. The expected efficient intersystem crossing for the heavy-atom-containing molecule was surprisingly not highlighted by our experiments, but an increased push-pull character was enabled by functionalizing the pyridine acceptor with the bromine. The ultrafast spectroscopic study revealed a peculiar excited state deactivation mechanism for the Br derivative among the others, involving photoinduced intramolecular charge transfer accompanied by twisting of the molecular structure (population of a TICT excited state). Similarly, a significant color change in the solid-state emission was observed for Py-2 upon mechanical grinding, suggesting a mechanochromic luminescence behavior in this case. Our findings suggest introducing heavy atoms in organic fluorophores as a new design strategy to obtain mechanoluminescent materials.