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.