Substituent enhanced fluorescence properties of star α-cyanostilbenes and their application in bioimaging

A series of star α-cyanostilbenes with D-π-A structures comprising triphenylamine as the donor, the cyanovinyl group as the acceptor and different substituents on the terminal phenyl rings were synthesized and characterized. The influence of the substituents on the photophysical, electrochemical and thermal properties of the star molecules was investigated in detail. A strongly electron withdrawing nitro substituent on the phenyl ring (NTBTNPA) increased the absorption and emission maxima, the Stokes shift, and the difference between the ground and excited state dipole moments and decreased the fluorescence quantum yield, fluorescence lifetime and band gap energy between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). This molecule (i.e., NTBTNPA) also showed aggregation-induced enhanced emission; it showed emission at 608 nm in pure DMF and displayed red-shifted emission at 625 nm and new emission at 706 nm in a DMF/water (50 : 50) binary mixture. Aggregation of this molecule in different DMF/water mixtures was confirmed by the dynamic light scattering method. The HOMO, LUMO and band gap energy values of all the star molecules calculated theoretically using density functional theory (DFT) were in good agreement with the experimentally determined values. The biocompatibility of NTBTNPA was tested with two Gram positive and two Gram negative bacteria and also with a fungus. Based on the photophysical properties, NTBTNPA was used as a fluorophore for bio-imaging application of a fungus, Rhizoctonia solani, as a model and the results obtained were excellent.