Fluorescent coumarin derivatives with viscosity sensitive emission--synthesis, photophysical properties and computational studies

New derivatives of (benzo[d]azolyl)-benzo[f]chromenone were synthesized from the intermediate 3-(1,3-benzazol-2-yl)naphthalen-2-ol, obtained from 3-hydroxynaphthalene-2-carboxylic acid and 2-amino phenol in the presence of PCl3 in chlorobenzene at 130-135 °C. The compounds were characterized by FT-IR, (1) H NMR, mass spectroscopy and elemental analysis. The synthesized compounds are fluorescent which absorb in the range of 296 to 332 nm while emit in the range of 368 to 404 nm. The experimental absorption and emission wavelengths for the compounds 5 and 6 are in good agreement with those predicted using the Time-Dependent Density Functional Theory (TD-DFT) [B3LYP/6-31G(d)]. The largest wavelength difference between the experimental and computed absorption maxima was 29 nm (tetrahedrofuran) for compound 5 while for emission it was 61 nm (dichloromethane) for compound 7. The emission intensities of all the compounds decrease continuously as the viscosity of the microenvironment increases. The compounds are thermally stable up to a temperature range of 300 to 350 °C.