Triplet states and semiquinone free radicals of 1,5- and 1,8-disubstituted anthroquinones: a laser flash photolysis and pulse radiolysis study

Abstract

The triplet state properties of 1,5-diamino-9,10-anthroquinone and 1,5- and 1,8-dihydroxy-9,10-anthraquinone in cyclohexane, acetonitrile and isopropyl alcohol were investigated using a nanosecond laser flash photolysis technique. The T-T absorption spectra, the triplet quantum yield, the triplet energy level and the triplet decay kinetics were estimated. In isopropyl alcohol, along with the triplet, the neutral semiquinone radicals are also formed and were characterized by comparing the long-lived transient spectra with the semiquinone spectra obtained by pulse radiolysis of these quinones in isopropyl alcohol. Higher quantum yields of the semiquinone radicals isopropyl alcohol, in comparison with the triplet quantum yield, are explained by assuming that both the excited singlet and the triplet states abstract hydrogen atoms from the solvent molecules to form the semiquinone radicals. A comparison of the photophysical properties of 1,4-disubstituted anthraquinones with the present series is discussed.