Crystallization-induced reversible fluorescence switching of alkyl chain length dependent thermally stable supercooled organic fluorescent liquids

Highly stable supercooled fluorescent liquids and crystallization-induced reversible fluorescence switching have been achieved via varying the alkyl chain length of the alkoxy group in the triphenylamine aldehyde, 4-(diphenylamino)-2-methoxybenzaldehyde (DPAMB). DPAMB and different alkyl chain substituted compounds, namely, ethyl (DPAEB), propyl (DPAPrB), butyl (DPABB), pentyl (DPAPeB), hexyl (DPAHB) and cyanohexyl (DPAH-CNB), exhibited weak fluorescence (λ between 429 and 490 nm) in the solid state except DPAEB which did not show any measurable fluorescence. Structural analysis was performed to obtain an insight into the alkyl chain length effect on molecular packing in the solid state. Hard crushing of all the solids led to a drastic enhancement of the fluorescence intensity without altering λ. Interestingly, the melts of all the compounds showed strong fluorescence with a red shift of the fluorescence λ (460-489 nm). Slight rubbing of the melts produced reversible fluorescence switching. Powder X-ray diffraction (PXRD) studies suggested that the conversion from crystalline into amorphous and vice versa while melting the solids and rubbing the melts was responsible for the fluorescence switching. Importantly, the melt showed an alkyl chain length dependent stable supercooled fluorescent liquid at room temperature. Differential scanning calorimetry (DSC) indicated crystallization of the melts while heating/cooling the DPAMB and DPAEB melts, whereas DPAPrB, DPABB, DPAPeB, DPAHB and DPAH-CNB did not show any crystallization in the subsequent cooling/heating cycles. Thus, the alkyl chain length and propeller molecular shape of triphenylamine have been utilized for developing strongly fluorescent supercooled fluorescent liquids at room temperature.