Abstract
Abstract The steric and electronic structure and absorption spectra of the fluorophore Hoechst 33258 (I) and a number of model compounds in various protonated forms were calculated by the semi-empirical quantum chemical methods PM3 and CNDO/S. The low-frequency transition was shown to provide an increase in electron affinity of the imidazole rings. The fluorescence quenching of I in neutral aqueous solutions was suggested to result from the keto form (or bipolar structure), derived via proton transfer in the excited state of the dimer associate of I. The polar (approximately 10 D) fluorescence state of I due to intramolecular charge transfer is markedly sensitive to the polarity of the environment.
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