Indian Journal of Chemistry Vol. 31A, November 1992, pp. 811-816 Triplet state and semiquinone free radical of 5-methoxyquinizarin : A laser flash photolysis and pulse radiolysis study H Pal, D K Palit, T Mukherjee· & J P Mittal Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay Received 24 February 1992; accepted 20 July 1992 The triplet(T)state properties likeT-T absorption spectra, quantum yield,energyleveland decay kinetics of 5-methoxyquinizarin (5-methoxy-I,4-dihydroxy-9,IO-anthraquinone; MQZ) have been investigated in cycIohexane, 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 ofMQZ in the samesolvent.A relativelysmall amount of a long livedtransient formed in cycIohexaneand acetonitrile, along with the triplet state ofMQZ, could not be characterisedunambiguously,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,showingthat both the excitedsingletand the triplet states ofthe quinone probably react with the solvent molecules to form the semiquinone radical. The photophysical properties of the triplet and the semiquinone radical ofMQZ have been compared with those of simple I,4-disubstituted anthraquinones. Amino and hydroxy substituted 9,IO-anthraquinones have long been used as dispersed dyes for colouring synthetic polymer materialsI. It has been observed that the light fastness of these anthraquinonoid dyes is often hampered by catalytic fading when applied as mixtures. Among several mechanisms suggested2 for photobleaching, the most popular is the formation of intermediate singlet molecular oxygen by reactions of the triplet states of the anthraquinonoid dyes with ground state triplet oxygen. Hence the ability of bleaching is related to the quantum efficiencies of formation of the triplet states «PT), which depends upon the nature and position ofthe substituents in the basic anthraquinone moiety3 - s. Triplet states of Quinones and their derivatives are well known to form semiquinone radicals by abstracting hydrogen atoms from hydrogen donating solvents6•7• The anthrasemiquinone radicals are also involved in the photosensitized degradation of textile materials dyed with anthraquinonoid dyes8. The photochemistry of Quinones also plays an important role in a number of biological processes9, e.g., the initiation of biological electron transport through light absorption by the qumones. The triplet state and the semlqumone characteristics of 1,4-amino and hydroxy disubstituted anthraquinones are already reporteds. In the present work the triplet state and the semiquinone characteristics of MQZ, a 5-methoxy derivative of I ,4-dihydroxy-9, IO-anthraquinone, were investigated in different solvents like cycIohexane, acetonitrile and isopropyl alcohol to understand the effect of the additional methoxy group on the excited state characteristics of the substituted Quinones. Materials and Methods Pure MQZ was kindly donated by Dr J M Bruce of Manchester University, U.K., and was used without further purification. All solvents were of spectroscopic grade from Spectrochem (India) and used without further purification. Anthracene, biphenyl and fluorenone (scintillation grade from J. T. Baker Chemical Co., USA) and perylene (Aldrich) were used as received. All solutions were deoxygenated by bubbling oxygen-free nitrogen (IOLAR grade from Indian Oxygen Ltd.) for about 15 min, using a proper solvent trap prior to the solution. Ground state absorption spectra were recorded with a Shimadzu UV-160 A spectrophotometer. Nanosecond flash photolysis experiments were carried out using the laser kinetic spectrometer model K 347 (Applied Photophysics, U.K.), the details of which have been described earlierIo. Pulse radiolysis experiments were conducted with a 7 MeV linear electron accelerator (Ray Technology, U.K.). The details of the pulse radiolysis set up have been described e1sewhereII.