Primary photoprocesses in isoquinoline N-oxides

Abstract

The photoisomerization of isoquinoline N-oxides (I) in a variety of solvents was investigated by use of conventional and laser flash photolysis techniques, combined with fluorescence and quantum yield measurements. The formation of isoquinolones (II) in polar solvents and 1,3-benzoxazepines (III) in non-polar solvents proceeds via singlet excited states. A triplet excited state of isoquinoline N-oxides was observed and characterized, but was shown not to contribute to the photochemical reactivity. A possible oxaziridine intermediate could not be observed. Experiments with a laser flash showed that the isoquinolone was formed within 20 ns. Since the singlet lifetime is 1 ns, there is little time left for the formation of an intermediate. Parameters for the various modes of decay of the first excited singlet state (e.g. fluorescence quantum yields, radiative lifetimes, and quantum yield for product formation) were measured for reactions in various solvents. From these the rate constants of product formation were calculated. The rate of formation of isoquinolone is nearly solvent-independent; the rate of formation of oxazepine in polar solvents is 20 times slower than that of isoquinolone and 30 times greater in non-polar solvents.