21) This observation shows that the cis keto isomer Q1 does not have the same conformation as the excited cis keto isomer produced by excitation of the enol. As shown in reference 17, a large number of keto and enol conformations of 1 are actually possible.

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4909emission. In the latter case every initially occupied cluster is finally left with one emitter so that the extent of slow emission will be A C P , = A ( l -e-*) which in this case is a fraction (1 -e -b ) / b of the total emission.The term b, the average number of excited states per cluster, is clearly a linear function of the laser flash intensity and also varies with the cluster concentration at constant emitter concentration, which, as we have seen above, depends on the SDS concentration. The data in Figure 4 and similar data obtained by using 3 mM SDS have been analyzed in terms of these equations.The details for 6 mM SDS are obtained as follows: the total initial excited emitter concentration at a particular laser intensity was obtained by measuring the initial absorption change at 453 nm following the flash in a 40 mM SDS solution containing 50-pM emitter. Using the known absorption coefficients for ground and excited states and correcting for the lower absorption in 6 mM SDS at the laser wavelength of 532 nm, we calculated the total initial excited-state concentration in 6 mM SDS. At the higher m 1 light intensity used this was 20.0 pM. The steady-state experiments at 6 mM SDS and 50 MM R~( b p y ) ,~+ show that there are 6.4-pM clusters, Le., b = 20/6.4 = 3.13 excited states per cluster. Hence from the two equations above the calculated fraction of slow emission are 0.16 and 0.30. When the maximum and slow emission measurements made as described above are used the observed fraction is 0.25. Data for a range of light intensities are presented in Table I1 for 6 and 3 mM SDS. It will be seen that all are in reasonable agreement with the calculated quenching assuming that one excited emitter is left after the annihilation process, Le., that both energy quanta are pooled onto a single R~( b p y ) ,~+ ion which very rapidly undergoes internal conversion to the emitting CT state. Acknowledgment. We are grateful for the expert assistance of Dr. S. J. Atherton and Mr. D. Harper in performing some of the experiments and Dr. D. C. Foyt and Mr. J. Lynch for computational guidance.A new photochromic system, exhibiting decisive advantages compared to others, is presented. This system works on the basis of photoreversible photooxidation. It consists of the hydrocarbon dimethylhomoocoerdianthrone (HOCD), its corresponding endoperoxide, and a singlet oxygen sensitizer. In comparison with the photochromic systems known so far, in which endoperoxides are used as active components, the novel system exhibits a better reversibility, a larger wavelength shift for the colorless/colored changeover, and an especially high thermal stability. HOCD can only be photooxidized in the presence of sensitizers. This peculiarity enables for the first time the composition of a system which can be used 89 either a photoreversible photochromic system (decolorization by irradiation of the sensitizer at X = 405 nm, colorization by irradiation of the endoperoxide at X = 313 nm) or a photostable system (reading by irradiation of the hydrocarbo...

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