The UV-VIS absorptive and emissive properties of three laser dyes, viz. Rhodamine 6G, Rhodamine B and Rhodamine 101, have been studied in ethanolic and methanolic solutions. Time-resolved methods were used to study the decay of the first excited singlet state of each dye. The absorptive properties of the lowest excited singlet states of Rhodamine 6G, Rhodamine B and Rhodamine 101 were generated using picosecond laser pulses. Values for the molar absorption coefficient for the S, t S, absorption process were measured, for each dye, by both comparative and complete-depletion methods. Values for the S, t S, absorption process, in ethanol, were found to be 3.7 x lo4 (452 nm), 4.4 x lo4 (428 nm) and 4.3 x lo4 (430 nm) dm3 mol-' cm-' for Rhodamine 101, Rhodamine 6G and Rhodamine 6 respectively with the corresponding values for the S, t S, absorption process being measured as 7.7 x lo4 (576 nm), 6.4 x lo4 (532 nm) and 3.8 x lo4 (566 nm) dm3 mol-' cm-' . These results are compared and contrasted with available literature values.

Methylviologen quenches the fluorescence of DNA-intercalated ethidium bromide (EB) via electron transfer to yield reduced viologen. The majority of reduced viologen rapidly s) recombines with oxidized EB on the DNA helix; however, a small (ca. 2%) fraction escapes from the helix into bulk solution. Recombination of this fraction then occurs via a second-order process (k = (5.6 * 1.5) X lo9 M-' s-' ). The yield of reduced viologen which escapes the helix increases as the ionic strength of the solution rises. In the absence of methylviologen, excitation leads to small, long-lived absorptions in the visible region to yield oxidized EB.which are attributed to the first excited triplet state of EB. These are quenched by oxygen (k = (3.8 0.6) X lo7 M-' s-I ) IntroductionIn our previous work on the photophysics and photochemistry of DNA intercalated ethidium bromide (EB) we concentrated on the fluorescence quenching of excited EB by transition-metal ions.' V2 In particular we concluded that those ions which quenched EB fluorescence in aqueous solutions were capable of quenching the fluorescence of DNA-intercalated EB with greatly increased efficiency.' In the course of these studies we observed that DNA-intercalated EB fluorescence was also quenched by methylviologen (MV2+) and that reduced viologen was formed on the time scale of the EB fluorescence decay.2 Further, the reduced viologen was stable over hundreds of microseconds in nitrogensaturated solution. This indicates that EB fluorescence is quenched by MVZ+ via electron transfer from excited EB to MV2+ and also that some fraction of the reduced viologen is able to escape fast recombination with oxidized EB. A similar conclusion was reached recently by Fromherz and Rieger who used primarily steady-state methods to observe enhanced yields of electron transfer from EB excited singlet state to MV2+ in the presence of DNA.3In this work we use transient absorption spectroscopy to follow the reactions of the products of this fluorescence quenching. Of particular interest is the environment and quantum yield of the reduced viologen which escapes rapid recombination with oxidized EB. Since to our knowledge no previous work on time-resolved absorption spectroscopy in the EB-DNA system has been reported, with the exception of our own nanosecond study,4 we felt it necessary to investigate this system in addition to the system containing MV2+. This is an extension of our earlier study to longer time scales. We have shown that the singlet excited state

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Laser Ñash photolysis and pulse radiolysis have been used to study the triplet and the free radical states of Rhodamine 123. Using acridine and xanthone as sensitisers, the molar absorption of the triplet state at 395 nm was found to be 1.2 ] 104 dm3 mol~1 cm~1 in both aqueous and ethanolic solutions. No triplet state of Rhodamine 123 was observed on direct excitation in aqueous solution, indicating a triplet intersystem crossing yield of less than 0.002. The formation and decay of the excited singlet state at 420 nm was observed during the laser pulse. One-electron oxidised and reduced forms of Rhodamine 123, attributed to the radical cation and radical anion with absorption maxima at 450 and 390 nm, respectively, were produced. The radical cation formed at pH 2.5 (designated was shown to deprotonate to yield Both the cation RhH~2`) R h ~(pK a \ 5.7). and anion radicals of the dyes may be formed either following photoionisation of Rhodamine 123 or, in photoredox reactions involving Rhodamine 123. Photoionisation was observed and appeared to be neither solely mono-or biphotonic. The maximum quantum yield for the monophotonic process was estimated to be O0.005. Direct excitation of Rhodamine 123 to produce signiÐcant yields of the dye triplet state and, subsequently, singlet oxygen is an unlikely mechanism for photosensitised cell killing. Accumulation of Rhodamine 123 in mitochondria may account for the apparent e †ectiveness of the dye as a photosensitiser, even in inefficient photoprocesses.

We have studied the quenching of fluorescence from DNA-intercalated ethidium bromide excited states by the transition-metal ions Cu2+, Ni2+, and Co2+. Quenching by all three metal ions leads to biphasic fluorescence decay. We suggest that this results from an equilibrium between metal ions bound to the DNA phosphate groups and metal ions mobile around the DNA helix. A quantitative model is given and allows us to deduce that the metal ions may be arranged in order of increasing mobility as Cu2+ < Ni2+ < Co2+. The mobility of Co2+ is a factor of ca. 20 less than in water. We conclude that the metal ions move from base to base at a rate between 4 X 107 and 2 X 10® s"1 and have a residence time on the phosphate group between ca. 5 and 20 ns. Quenching of fluorescence may occur from metal ions bound to any of the nearest 6 phosphate groups to the intercalated excited state, a distance of ca. 1 nm. Quenching is suggested to occur via electron transfer from excited ethidium bromide to the metal ion.