Absorption and phosphorescence spectra of 4-nitropyridine N-oxides and 4- and 3-nitroquinoline N-oxides

Abstract

The absorption, phosphorescence and phosphorescence excitation spectra of 4-nitropyridine N-oxide (4NPO) and its methyl derivatives, 4-nitroquinoline N-oxide (4NQO), and 3-nitroquinoline N-oxide (3NQO) were recorded. For 4NPO, the lifetime of phosphorescence, degree of polarization of the phosphorescence and excitation spectra, and the singlet—triplet separation energy were analyzed in detail. The experimental results were also compared with those of SCFMOCI calculation. The following results are reported. (i) The electronic spectra of 4NPO, 4NQO and 3NQO, and the effect of steric hindrance on the spectra are interpreted by the aid of theoretical calculation. At the longest wavelength absorption band the intramolecular charge transfer from the N-oxide group oxygen atom to the nitro group is the main configuration, which may play an important role in the photochemical reaction of these substances. (ii) A blue shift was always observed in the phosphorescence spectra on changing the solvent from ether to isopropyl alcohol, the importance of hydrogen bonding effect on this phenomenon being stressed. (iii) The characteristics of the lowest triplet state of the N-oxides are discussed in detail on the basis of the spectroscopic information. It is concluded that the lowest triplet state is of the nature of a π—π* transition, for 4NPO this being 3 A 1.