White light emissive organic fluorophores and halogen atom-controlled tunable and stimuli-induced reversible fluorescence switching.

Triphenylamine (TPA) – acetophenone donor-π-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 CH 3 CN. Solid-state structural studies of 1 revealed the formation of interconnected 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 π ... π interaction of TPA phenyls. The Br ... π 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 fluorescence 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. • Halogen atom controlled solid-state fluorescence efficiency. • Single molecule white light emission in polar to non-polar solvents by halogen substitution. • Tunable solid-state fluorescence and molecular packing in the crystal lattice. • Halogen atom dependent mechano-responsive reversible fluorescence switching. [ABSTRACT FROM AUTHOR]