The Primary Photochemical Process of Isoquinoline N-Oxide in Hydroxylic Solvents

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Abstract
In order to deduce the nature of the reactive state of the photochemical isomerization of isoquinoline N-oxide in methanol or water, investigations by means of steady-light irradiation and flash spectroscopy were carried out under various conditions. Transient absorptions were observed in the spectral region between 360 nm and 600 nm; they were identified as being due to a T–T transition of isoquinoline N-oxide. Experiments on both the measurements of the quantum yields and the decay kinetics of the transient species under different conditions revealed that the excited singlet state of isoquinoline N-oxide was responsible for the isomerization. The photochemical behavior in aqueous solutions at various pH values was also examined; consequently, the photochemical isomerization was found to be independent of the hydrogen ions, although it did not occur in a strongly acidic solution (pH<3.0) because of the formation of the protonated N-oxide. The biacetyl-photosensitization for isoquinoline N-oxide led to the dissociation of the N–O bond instead of to isomerization.

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