TD-DFT investigation on the electronic spectra of novel N-methylmaleimides linked with indolizine ring system

The major electronic absorption features in visible region were computationally investigated for a series of novel N-methylmaleimide derivatives. The newly synthesized compounds through the reaction of indolizines with N-methylmaleimides exhibited their maxima ranging 587-634 nm in solution at room temperature. Time dependent density functional theory (TD-DFT) calculations were performed in order to elucidate their structure-color relationship, as well as to assess its performance using a variety of exchange-correlation (XC) functionals and basis sets. 4-(Indolizin-3-yl)-1-methyl-2,5-dioxo-1H-pyrrole-3-carbonitrile (referred to 3a hereafter), a representative of the new compounds, was analyzed in detail on its intramolecular push-pull electronic structure. As a consequence of systematic evaluation of computational strategies for 3a, we found that the theoretical λmax obtained by PCM-TD-DFT/6-31+G(d, p)//DFT/6-311G(2d, 2p) level of theory, which was consistently employed throughout this study, has converged within 2 nm deviation from λmax of the most elaborated level considered. Among some of the molecules with near-planarity structure, unusual discrepancies between the theoretical and the experimental λmax were observed, which should be attributed to the well-known TD-DFT limitation to describe multi-configurational or charge transfer (CT) excitations.