Triplet state and semiquinone free radical of 5-methoxyquinizarin : A laser flash photolysis and pulse radiolysis study

Abstract

The triplet(T) state properties like T-T absorption spectra, quantum yield, energy level and decay kinetics of 5-methoxyquinizarin (5-methoxy-1,4-dihydroxy-9,10-anthraquinone; MQZ) have been investigated in cyclohexane, acetonitrile and isopropyl alcohol using nanosecond laser flash photolysis technique. In isopropyl alcohol, a neutral semiquinone radical is also formed which has been characterised by comparing the long lived transient spectra with the MQZ-semiquinone spectra obtained by pulse radiolysis of MQZ in the same solvent. A relatively small amount of a long lived transient formed in cyclohexane and acetonitrile, along with the triplet state of MQZ, could not be characterized unambiguously, but has been attributed to the semiquinone radical of MQZ, produced by the reaction of the excited states of the quinone with the solvent. The quantum yield of the semiquinone radical in isopropyl alcohol is considerably higher than the triplet quantum yield, showing that both the excited singlet and the triplet states of the quinone probably react with the solvent molecules to form the semiquinone radical. The photophysical properties of the triplet and the semiquinone radical of MQZ have been compared with those of simple 1,4-disubstituted anthraquinones.