A DFT study on Electronic Excitations, Charge Transfer and NLO Properties of Visible Absorbing Squaraine and Thiosquaraine dyes

Density functional calculations have been performed to see the effect of sulfur substitution in place of oxygen at central four membered acceptor squarate ring on electronic excitations, charge transfer and second order non-linear optical properties in visible absorbing squaraines (SQ). Molecules oxy-thiosquaraines (OSQ) and thiosquaraines (SSQ) have shown red shift in absorption as compared to their corresponding SQ molecules. The lowest five electronic excitations for all the molecules have been calculated by using TDDFT method. Further, effect of electron donating and electron withdrawing groups on absorption maxima have been studied. The large red shifts in case of electron withdrawing group within the same series of molecules (SQ, OSQ and SSQ) are due to destabilization of HOMO and stabilization of LUMO levels. Charge transfers in these molecules are reported by using VMOdes. Second hyperpolarizabilities (  γ  ) for these molecules are calculated by SOS method. This study may be helpful in synthesizing new C-N bonding SQ, OSQ and SSQ dyes which are further useful in