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.