Rational Tuning of AIEE Active Coumarin Based α-Cyanostilbenes toward Far-Red/NIR Region Using Different π-Spacer and Acceptor Units

A series of cyanostilbene based Dπ-A derivatives, comprising 6-(diethylamino)coumarin as a donor and benzene and benzothiazole as acceptors bridged by different Ï€-spacers (benzene, thiophene), have been synthesized and characterized. The influence of Ï€-spacer and acceptor units on the photophysical, electrochemical, and thermal properties of the dyes was investigated in detail. The incorporation of coumarin donor results in a significant increase of the fluorescence quantum yield. In the solution phase, the absorption and emission of the dye TB show a bathochromic shift (TB > BBT) indicating dominant dynamic intramolecular rotations (IMR) behavior of cyano group over the acceptor functionality. On the other hand, the emission is reversed (BBT > TB) in the case of aggregates. The synergistic combination of intramolecular planarization and torsional alleviation around cyanostilbenes induced by strong electron withdrawing benzothiazole unit in BBT enabled pronounced bathochromic shift upon aggregation. Upon varying the spacer/acceptor substitution, the emission wavelength of the aggregates shifts from visible to far red/NIR region with concomitant large Stokes shifts of 158-213 nm. The fluorescence intensity of the aggregates gradually decreases with increased planarity of the backbone. Various experimental techniques were employed to evaluate the structure-emission phenomena relationship. The aggregation induced enhanced emission (AIEE) behavior of the dyes was substantiated from fluorescence lifetime decay studies and scanning electron microscopy (SEM) analysis. Density functional theory (DFT) computed band gap follows a consistent trend with the values estimated from electrochemical-optical studies. All the dyes possess excellent thermal stability. Our study demonstrated far-red/NIR AIEE cyanostilbene derivatives exhibiting large Stokes shift having excellent thermal properties and fluorescence quantum yields could act as potential candidates for optoelectronic or imaging applications.