Self-Reversible Mechanochromism and Thermochromism of a Triphenylamine-Based Molecule: Tunable Fluorescence and Nanofabrication Studies

A triphenylamine-based fluorophore,
4-((4-methoxyphenyl)­(phenyl)­amino)­benzaldehyde
(1), exhibits external-stimuli-responsive self-reversible
solid-state fluorescence switching, tunable fluorescence, and a rare
phenomenon of temperature-dependent fluorescence. Mechanically grinding
a crystalline powder of 1 converts the blue fluorescence
(λmax = 457 nm) to green (λmax =
502 nm), but blue fluorescence robustly self-recovers within 8 min.
X-ray analysis and theoretical studies suggest that the change from
a highly twisted molecular conformation and crystalline form into
an amorphous phase with more planar conformation is responsible for
the fluorescence switching. Self-reversible fluorescence switching
did not show a significant change in fluorescence for several cycles
of measurement. Interestingly, 1 in toluene showed a
rare phenomenon of fluorescence enhancement with increasing temperature
via activating more vibrational bands that lead to stronger twisted
intramolecular charge-transfer (TICT) emissions. Morphological-change-mediated
fluorescence tuning has also been demonstrated by fabricating nanoparticles
of 1. The conversion of highly polydispersed, featureless,
different-shaped nanoparticles into nearly uniformly sized spherical
nanoparticles (20–25 nm) converts green (λmax = 502 nm) to blue fluorescence (λmax = 478 nm).
The self-reversible multi-stimuli-responsive fluorescence switching
and polymorphism and nanofabrication-mediated fluorescence tuning
suggest its potential application in sensors, particularly for fluorescent
thermometers.