Linear and Nonlinear Optical Properties of Mesoionic Oxyallyl Derivatives: Enhanced Non-Resonant Third Order Optical Nonlinearity in Croconate Dyes

Role of the donor groups connected to the oxyallyl central moiety (suggested to be a powerful acceptor) has been emphasized in the literature for tuning the absorption maxima and obtaining large NLO activity in croconates and squaraine dyes which are oxyallyl derivatives. Here we have prepared a series of new croconate dye model molecules with aniline and substituted anilines as strong donors to the oxyallyl ring. Using experimental and theoretical techniques like UV−visible spectra, degenerate four wave mixing (DFWM), density functional theory, time-dependent density functional theory, and symmetry adopted cluster-configuration interaction (SAC-CI), we have characterized linear absorption, nonresonant third order optical nonlinearity, charge transfer, and excited states of these molecules. We find that these molecules have smaller singlet diradical character and the absorption is not in the near-infrared (NIR), as other reported croconates, but in the 440−480 nm range. There is a larger charge transfer (CT) from the side groups to the central ring in these croconates evident from SAC-CI calculations compared to CT in NIR croconates. This clearly supports the model that the NIR absorption in oxyallyl derivatives is correlated to the singlet diradical character and not to the donor capacity. The γ values determined by DFWM experiments show large nonresonant values of −2.4 to −5.3 × 10<sup>−32</sup> esu, which is larger than that of similar squaraines suggesting that the larger oxyallyl ring size and diradical character in croconates play the major role and not the donor groups. We conclude that with a noncentrosymmetric structure, tunable absorption (visible-NIR), and larger γ values, these less studied croconate dyes are more interesting and will have a major role to play than the widely reported centrosymmetric squaraines as molecular materials.