CH₃/CF₃ substituent dependent tunable fluorescence and thermofluorochromism triphenylamine-acetophenone fluorophore

Triphenylamine-acetophenone donor-acceptor fluorophore with methyl ((E)-3-(4-(diphenylamino)phenyl)-1-(p-tolyl)prop-2-en-1-one (1-CH₃)) and trifluoromethyl ((E)-3-(4-(diphenylamino)phenyl)-1-(4-(trifluoromethyl)phenyl)prop-2-en-1-one (1-CF₃)) substituents were synthesized and explored the substituent effect on the photophysical properties in solution as well as solid-state. 1-CH₃ and 1-CF₃ showed solvent polarity dependent tunable fluorescence between 489 and 564 nm (quantum yield (ϕ_F) = 0.258 (toluene) compared to quinine sulfate). 1-CH₃ and 1-CF₃ also exhibited strong green and yellow fluorescence in the solid-state at 535 and 570 nm, respectively. The change of substituent significantly altered the melting point, 1-CH₃ melted at 110 °C whereas 1-CF₃ melts at 55 °C. The low melting point of 1-CF₃ enabled to achieve thermoresponsive off-on reversible fluorescence switching in solids, aggregated state and polymer matrix at near room temperature. Both 1-CH₃ and 1-CF₃ exhibited strong reduction of fluorescence intensity upon crushing. But only 1-CF₃ showed recovering of fluorescence with time or by solvent exposure. Data encryption application was demonstrated using the thermoresponsive 1-CF₃ by integrating with polymer matrix (PMMA) and fabricating thin films. 1-CF₃-polmer exhibited complete disappearance of fluorescence upon heating and reversed to initial upon cooling at -15 °C for a minute. Thus, the present work elucidated the role of substituent group for developing functional fluorescent molecules for reversible temperature dependent fluorescence switching applications.