Abstract
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.
