Tunable and Switchable Solid State Fluorescence: Alkyl Chain Length‐Dependent Molecular Conformation and Self‐Reversible Thermochromism.

Abstract: Flexible alkyl chain has been used to control the molecular conformation, packing and intermolecular interactions of aggregation enhanced emissive (AEE) triphenylamine fluorophore, and tune the fluorescence maxima and mechano/thermochromism. 2‐(4‐(diphenylamino)‐2‐ethoxybenzylidene)malononitrile (DPABM‐2), 2‐(4‐(diphenylamino)‐2‐propoxybenzylidene)malononitrile (DPABM‐3), 2‐(4‐(diphenylamino)‐2‐butoxybenzylidene)malononitrile (DPA‐BM‐4), 2‐(4‐(diphenylamino)‐2‐pentyloxybenzylidene) malono‐nitrile (DPABM‐5), 2‐(4‐(diphenylamino)‐2‐hexyloxybenzylidene) malonon‐itrile (DPABM‐6) and 2‐(4‐(diphenylamino)‐2‐heptyloxybenzylidene) malononitrile (DPABM‐7) exhibited weak to strong solid state fluorescence (absolute quantum yield (Φf)=4 to 26%). DPABM‐2 did not show significant fluorescence changes to external stimuli, however, DPABM‐3 and DPABM‐4 exhibited off‐on fluorescence switching. DPABM‐3 also showed tunable fluorescence via polymorphism. Interestingly, DPABM‐5‐7 showed alkyl chain orientation dependent self‐reversible thermochromism. DPABM‐5 and DPABM‐7 exhibited decrease of fluorescence intensity with temperature whereas DPABM‐6 showed increase of fluorescence intensity. Solid state structural analysis revealed the influence of alkyl chain length on the molecular conformations and packing. Powder X‐ray diffraction (PXRD) studies indicate that phase change of materials from crystalline to amorphous and vice‐versa was responsible for fluorescence switching with external stimuli. Further, AEE of DPABM derivatives and the reactive ethylene‐malononitrile has also been exploited for fabricating hydrazine sensing polymer composite film. Thus simple change of alkyl chain lengths lead to the generation of polymorphism, mechanochromism and temperature dependent self‐reversible fluorescence switching materials. [ABSTRACT FROM AUTHOR]