Benzocoumarin-Styryl Hybrids: Aggregation and Viscosity Induced Emission Enhancement.

Two benzo[h]chromen-3-yl)ethylidene) malononitrile styryl hybrid dyes are synthesized and characterized by NMR and elemental analysis. One is based on nitrogen donor and other on oxygen (3b and 3b respectively). Dyes are low emissive in the solution but dramatically showed increase in emission intensity in aggregates form in the THF (tetrahydrofuran) /water system. Dyes are also sensitive to viscosity and showed increased emission intensity in the DCM:PEG 400 system and DMF:PEG 400 system respectively. Dyes 3a and 3b showed higher viscosity sensitivity constant (0.67 and 0.39 respectively) in DMF:PEG 400 system compared to DCM:PEG 400 (0.47 and 0.21 respectively) system which is contrary to the traditional concept of FMRs. Results shows that lowering of twisted intramolecular charge transfer (TICT) and increase in intramolecular charge transfer (ICT) in the excited state could be the reason for such behavior in the aggregate and highly viscous state. This study may provide the new insights into the field of AIEE and FMR research of such hybrid molecules.