The Singlet-Triplet Absorption Spectra of Heterocyclic Amine iV-Oxides. II. Pyridine N-Oxide Derivatives

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The singlet-triplet absorption spectra of various pyridine N-oxide derivatives with an electron-donating substituent have been observed by applying the oxygen-intensification technique. The nature of the lowest triplet (T1 states of these N-oxides and the substituent effect on the T1 state of pyridine N-oxide have been discussed experimentally and theoretically. The T1 states of the above substituted pyridine N-oxides were assigned to a π-π* transition of a 3La (CT) nature; 3A1 for the p-substituted pyridine N-oxides. All the singlet-triplet absorption bands as well as the singlet π-π* bands recorded here show a blue shift in polar solvents; this phenomenon is also discussed. Last, the mutual relation between the contact charge-transfer bands and the singlet-triplet absorption bands, both enhanced by compressed oxygen, has been discussed, since, for pyridine N-oxides with p-N(CH3)2, p-OCH3, and m-NH2, we could not record the singlet-triplet absorption band clearly.
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