Physicochemical Study on Leuco Triarylmethane Dyes : Comparisons of the Electronic Structures by CNDO/S-CI Calculation of Propensity to Photoionization of Phthalide and 3, 4-Dihydro-1 (2H)-phthalazinone

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It was shown that CNDO/S-CI calculation is capable of distinguishing the electronic structures of the excited singlet state of two ring systems ; phthalide (3) and 3, 4-dihydro-1 (2H)-phthalazinone (4). In the excited state calculations by the CNDO/S-CI method, the computed transition energies of 3 and 4 to the lowest π-π^* excited singlet state were found to be in reasonable agreement with the observed and reduced values for 3 and 4. The wavelengths of the maximum extinction coefficient of 3 and 4, which can be assigned to the lowest π-π^* singlet state, were deduced from the difference ultraviolet (UV) absorption spectrum of malachite green lactone (MGL) and malachite green hydrazide (MGH) against bis-(p-dimethylanilino)-methane, respectively. Inspection of the configuration mixing in the CI states disclosed that the lowest π-π^* excited singlet state is mainly composed of the HOMO-LUMO transition in both compounds, though the mixing ratios are different. However, the HOMO polarizations are quite different from each other ; the HOMO coefficient of the N₃-atom in the hydrazino moiety of 4 is much larger than that of the O₂-atom of 3. Thus, the excitation of 4 to the lowest excited singlet state results in a substantial intramolecular charge transfer of π-electrons from the N₃-C₄-group to the benzoyl moiety based on the variation of the atomic electron densities. The n-π^* excitation and the lowest π-π^* excited triplet state of 4 did not show similar propensities. These findings may be associated with the heterolytic cleavage of the C₄-N₃ bond in the lowest excited singlet state.