Anthraquinone derivatives 2AQA2(HEt2) and 2AQC2(HEt2) were examined as light-activated agents that initiate DNA cleavage. The substituents control the electronic configuration of the lowest excited state of the anthraquinone. 2AQC2(HEt2) has a lowest nπ* excited state and can react by electron transfer or hydrogen atom abstraction. 2AQC2(HEt2) has a lowest excited state of ππ* or intramolecular charge-transfer character and reacts only by electron transfer. Spectroscopic evidence indicates that both quinones bind to double-stranded DNA by intercalation with essentially the same affinity. Picosecond time-resolved laser spectroscopy shows that single electron transfer from the DNA bases to either bound quinone occurs rapidly and to the same extent. Irradiation of either intercalated 2AQA2(HEt2) or 2AQC2(HEt2) followed by treatment with hot piperdine leads to equally effective cleavage of DNA at the 5‘-G of GG steps. These findings indicate that electron transfer from a DNA base to the excited quinone is the dominant path for the GG-selective DNA cleavage. At high concentrations, where some quinone is free in solution, irradiation of 2AQC2(HEt2), but not 2AQA2(HEt2), leads to nonselective spontaneous cleavage of DNA. This second path to DNA cleavage is identified as direct hydrogen atom abstraction from the deoxyribose backbone by excited, nonintercalated 2AQC2(HEt2).

The excited states of several families of thermally stable radical ions in solution are surveyed by transient absorption and steady-state fluorescence spectroscopy to determine prevalent deactivation mode(s). Of the species investigated, weak fluorescence can be observed only for substituted triarylamine radical cations, presumably from their lowest excited doublet states. The primary excited-state deactivation pathway for radical anions of the quinones, aryl ketones, and cyanoarene hydrocarbons examined here is internal conversion from the lowest excited doublet state to the ground-state doublet. The efficiency of this deactivation mode arises typically from a low D r D 1 energy gap, as demonstrated by near-infrared absorbance in each species. Contrary to a prior literature report, the 9,lO-anthraquinone radical anion and the 9,lO-dicyanoanthracene radical anion are nonluminescent in solution. Luminescent side products generated in ground-state reactions of these radical anions are identified as 9,9-bianthrone dianion (from the dimerization and deoxygenation of the anthraquinone radical anion) and 10-cyanoanthrolate (from the reaction of dicyanoanthracene radical anion with molecular oxygen).

The results of AMI molecular orbital calculations on two series of arylmethylsulfonium salt derivatives have indicated that the nature of the lowest unoccupied molecular orbital (LUMO) switches from * when aryl = phenyl, 1-naphthyl, and 9-anthryl to * when aryl = 5-naphthacenyl. Electrochemical, photochemical, fluorescence quantum yield, and singlet-lifetime data were found to support the conclusions from the calculations.

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