NLOphoric donor-rigidified ESIPT dyes - Synthesis, pH study, solvatochromism and DFT insights

Four newly synthesized julolidine based ESIPT dyes were studied for their solvatochromic and solvatofluoric behavior using polarity plots. The dyes exhibit ESIPT as revealed by TD-DFT and natural bond orbital second order perturbation theory. The dyes showed enhancement in emission intensity at pH 10 and 12. In alkaline pH the dyes showed additional absorption peak at the longer wavelength region 430480 nm which is assigned to the keto form. They showed charge transfer phenomenon in local as well as charge transfer excitation which was confirmed by Weller's plot, Generalized Mulliken-Hush analysis and bond length alternation studies. The dyes showed red shifted absorption and emission wavelengths as compared to their previously reported congeners. Vertical excitation and emission values obtained from TD-DFT were in close agreement with the experimental values. The non-linear optical properties of ESIPT dyes were studied using solvatochromic method (alpha(CT) = 5.6 x 10(23) e.s.u; beta(CT) = 0.788 x 10(-28) e.s.u. and gamma(exp) = 0.37 x 10(-34) e.s.u.) and computational (highest values of alpha(0) = 6.32 x 10(-23); beta(0) = 2.7 x 10(-28) and gamma = 10.7 x 10(-34)) approach and compared with urea (JNNO(2) show 10 times more alpha(0), 814 times more beta(0) and 239 times more gamma value than urea).