π-Aryl/heteroaryl conjugated coumarin-thiazoles: Synthesis, optical spectral and nonlinear optic properties

A series of π-aryl/heteroaryl conjugated coumarin-thiazole systems 8a-f has been synthesized by using Hantzsch thiazole protocol and Wittig olefination as the keys. In the UV-Visible spectra of 8a-f, a main absorption band associated with a dominant π-π* transition is observed in the region of 338 to 390 nm. Qualitatively, the values of λmax have been found to correlate satisfactory with the donor/acceptor characteristics of the π-attached chromophores. Marked changes observed in the absorption maxima of 8a under acidic conditions are rationalized on the basis of mono-or bis-protonation and modification of the donor/acceptor properties of chromophores undergoing protonation. The emission spectra of 8a-f, obtained by exciting the molecules at their main absorption bands showed emission maxima in the region of 429 nm to 537 nm, with relatively high Stokes shifts of 145 and 171 nm being observed for 8a and 8e, carrying a π-donor, dimethylaminophenyl and a π-acceptor, p-nitrophenyl chromophore, respectively. Although, the first hyperpolarisability β, measured by the hyper-Reileigh scattering (HRS) technique are modest (12 to 23 × 10−30 esu), all the compounds exhibited complete transparency in the frequency doubling region at 532 nm and showed high thermal stability (Td from 330 to 365 °C).