Pushpendra Singh, Menaka Chauhan, Abha Jha, Kalyan K Sadhu
Modification of boranils with dual state emissions in solution and solid states have been targeted by introducing free amine functional group at the terminal position. Herein, we designed and synthesized four new primary amine-substituted benzothiazole-based boranils from substituted salicylaldehyde. Their photophysical properties were investigated through steady-state absorption and emission measurements in both solution- and solid-states. X-ray structure analysis of the boranils revealed that the benzothiazole rings bearing the amine functional group are highly twisted, thereby avoiding the detrimental exciton interaction between the dimers and enabling emission in the solid state. In the solution-state, the naphthalene and methoxy substituted boranils exhibited significant intramolecular charge transfer and a large Stokes shift of > 150 nm. Julolidine or diethylamine substituted boranils exhibited reversible acid-base responsiveness via fluorescence ON-OFF properties. On the other hand, protonation and deprotonation of naphthalene and methoxy substituted boranils showed reversible blue and red shift emission. Computational studies revealed that the photo-induced electron transfer (PET) and photo-induced charge transfer (PICT) process were responsible for fluorescence enhancement and wavelength shift upon trifluoroacetic acid (TFA) treatment. Based on the solvent dependent wide emissive range, methoxy substituted boranil was further utilized for the generation of white light emission with anthracene as an ancillary component.