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 (áγñ) 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 NLO applications.

中文翻译：

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DFT研究可见光吸收方酸和方酸染料的电子激发，电荷转移和NLO性质

已经进行了密度泛函计算，以查看在中心四元受体方形环上硫取代氧取代对可见光吸收方晶（SQ）中的电子激发，电荷转移和二阶非线性光学性质的影响。与它们相应的SQ分子相比，氧基-硫代夸脱分子（OSQ）和硫代夸脱（SSQ）分子显示出吸收的红移。使用TDDFT方法已计算出所有分子的最低五次电子激发。此外，已经研究了给电子基团和吸电子基团对最大吸收的影响。在相同系列的分子（SQ，OSQ和SSQ）中，吸电子基团发生大的红移是由于HOMO不稳定和LUMO水平稳定所致。这些分子中的电荷转移是通过使用VMOdes报告的。这些分子的第二超极化率（áγñ）通过SOS方法计算。这项研究可能有助于合成新的CN键合SQ，OSQ和SSQ染料，这些染料在NLO应用中进一步有用。