Abstract
Triphenylamine (TPA) - acetophenone donor-pi-acceptor fluorophore with halogen (F, Cl and Br) substitution was synthesized and explored the influence of halogen atom on the solution as well as solid-state fluorescence properties. The unsubstituted 1 ((E)-3-(4-(diphenylamino) phenyl)-1-phenylprop-2-en-1-one) exhibited strong fluorescence in non-polar solvents but weak fluorescence in polar solvents. The halogen substitution (4F ((E)-3-(4-(diphenyl amino)phenyl)-1-(4-fluorophenyl)prop-2-en-1-one), 4Cl ((E)-3-(4-(diphenylamino)phenyl)-1-(4-chlorophenyl)prop-2-en-1-one) and 4Br ((E)-3-(4-(diphenylamino)phenyl)-1-(4-bromo phenyl)prop-2-en-1-one)) significantly influenced on the fluorescence both in solution as well as solid-state. 1 showed white light emission in polar methanol but 4F and 4Cl exhibited white light emission in polar as well as non-polar solvents. 4Br showed white light emission in CH3CN. Solid-state structural studies of 1 revealed the formation of inter-connected dimers via intermolecular H-bonding between carbonyl oxygen and TPA phenyl hydrogens. Though 4F also exhibited dimer via H-bonding of carbonyl oxygen, the dimers are interconnected by pi ... pi interaction of TPA phenyls. The Br ... pi interaction produced 1D structure in the crystal lattice of 4Br that was further interconnected by multiple intermolecular interactions. The molecular conformational and packing modification of 1 and halogen isomers produced tunable fluorescence between 522 and 593 nm. Interestingly, only 4Br showed clear external stimuli-induced reversible/self-reversible fluorescence switching between two fluores-cence states. The fluorescence was red shifted with slight reduction of intensity while crushing and heating/ solvent exposure caused recovering of fluorescence. Powder X-ray diffraction of 4Br indicated the reversible phase transition was responsible for stimuli-induced reversible fluorescence switching. Thus, the present studies utilized the halogen atom to tune the fluorescence and studied its role on the fluorescence switching and white light emission.