A3-Type star stilbene and cyanostilbene molecules: synthesis, fluorescence properties and bio-imaging application

Stilbene and cyanostilbene based new A(3)-type star molecules containing blocked isocyanate functionalities were synthesized and structures of the compounds were confirmed. The photophysical properties of the molecules were studied using absorption and fluorescence spectroscopic techniques. The intramolecular charge transfer absorption of star stilbene and star cyanostilbene was found at 370 nm and 430 nm, respectively. Upon excitation at ICT absorption, the stilbene and cyanostilbene star molecules showed fluorescence with a maximum in the region of 443-470 nm and 498-540 nm, respectively. Upon increasing the solvent polarity, the fluorescence maximum of both the star molecules showed a bathochromic shift with an increase in the Stokes shift. The change in the dipole moment upon excitation was calculated from the Lippert-Mataga plot and it was found that the molecules were more polar in their excited state. The fluorescence quantum yields and fluorescence lifetimes of these molecules were determined; the average lifetime value for the stilbene based molecule was found to be 2.05 ns and this value for the cyanostilbene based molecule was found to be 0.66 ns. Star cyanostilbene showed aggregation-induced emission; it displayed lower wavelength emission in pure THF but it exhibited emission with a red shift in a water/THF binary mixture. These molecules also showed considerably low band gap energies between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO); the values were in the range of 2.55-2.87 eV. Based on the photophysical studies, the molecule containing the cyanostilbene unit was used as a fluorophore for bio-imaging application in zebrafish eggs as the model and the results obtained were found to be excellent.