Design and Synthesis of Coumarin-Imidazole Hybrid Chromophores: Solvatochromism, Acidochromism and Nonlinear Optical Properties.

A set of linear and asymmetric coumarin-imidazole hybrid compounds having a N,N-diethylamine at 7-position and imidazole at 3-position on the coumarin were synthesized. Insertion of thiophene π-spacer between coumarin and imidazole moieties (5b, 5d) leads to redshifted absorption and emission compared to 5a and 5c. All the compounds show a noticeable response to trifluoroacetic acid with a redshifted absorption and an increase in emission intensity by twofold. The ratio of ground and excited state acidity constant was calculated using Förster energy cycle, and the ratios were found to be 0.25, 0.96, 0.52 and 1.87, respectively, for 5a-5d. Due to the thiophene π-spacer, elongation of π-conjugation in 5b and 5d leads to high values of polarizability (α), first-order hyperpolarizability (β) and second-order hyperpolarizability (γ). Compound 5b exhibits a high value (895 GM) of two-photon absorption cross section (σ2PA), measured using two-level model.