Multi-stimuli responsive responsive fluorescence switching of D-A tetraphenylethylene functionalized cyanopyridine isomers

Three new positional isomers (Z)-2-(pyridin-2-yl)-3-(4'-(1,2,2-triphenylvinyl)-[1,1'-biphenyl]-4-ypacry- Ion itrile (o-PyTBA), (Z)-2-( pyridin-3-yl)-3 ,2,2-triphenylvi nyl)-[ 1,1 '-bipheny1]-4-yl)acrylonitrile (m-PyTBA) and (Z)-2-(pyridin-4-yl)-3-(4'-(1,2,2-triphenylvinyl)41,1'-biphenyl]-4-yl)acrylonitrile (p-PyTBA) incorporating tetraphenylethylene (TPE) as donor and pyridine unit and cyano-group as acceptor were designed and synthesized. The positional change exploited in these isomers influenced the acceptor strength and molecular packing. These three isomers displayed distinct solvatochromic behaviour and different aggregation induced emission (A1E) properties. The density functional theory (DFT) studies revealed good separation of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) supporting the distinct intramolecular-charge transfer (ICT) transition. Moreover, the three isomers exhibited reversible mechanochromism in the order o-PyTBA (56 nm) > m-PyTBA (49 nm) > p-PyTBA (43 nm). The powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC) and field emission scanning electron microscopy (FESEM) studies indicated that the morphological conversion from crystalline to amorphous state should be responsible for the mechanochromism. Meanwhile, the extension in molecular conjugation caused by planarization of molecular conformation and subsequent planar intramolecular charge transfer (PICT) process by external force were responsible for the red-shifts in the fluorescence spectra. Interestingly, protonation-deprotonation of the pyridine moiety in o-PyTBA, m-PyTBA and p-PyTBA had a significant effect on the frontier molecular orbitals as well as very distinctive emission characteristics upon trifluoroacetic acid (TFA) and trietylamine (TEA) stimuli. The film of o-PyTBA, m-PyTBA and p-PyTBA achieved a low detection limit for trifluoroacetic acid which was estimated to be 13.2 ppm, 17 ppm and 7.2 ppm, respectively. (C) 2019 Elsevier Ltd. All rights reserved.