Phenazine Fused Benzo Coumarins with Negative Solvatochromism and Positive Solvatochromic Emission - Synthesis, Photo Physical Properties, DFT and TDDFT Studies

5-Hydroxybenzo[a]phenazine-6-carbaldehyde was synthesized and condensed with substituted active methylene compounds to obtain four novel phenazine fused coumarin dyes. Solutions of imidazole containing dyes in various solvents exhibited yellow to orange fluorescence while benzothiazole containing dyes showed brilliant bluish green fluorescence. The dyes showed pronounced negative solvatochromism. However, the emission experienced progressive red shift with increasing polarity. The excited states of these dyes are proved to be more polar than the ground state. The dyes showed fairly good quantum yield in the range 0.1-0.7, and displayed high thermal stability, as determined using thermo gravimetric analysis. The density functional theory computations showed that intramolecular charge transfer occurs from the 3 position to the 7 position. The excited state computation using time dependent functional theory predicted the emission well.