Fluorescence lifetimes of Ave representative xanthene dye species‐the rhodamine B zwitterion (RB=), the rhoda‐mine B cation (RB+), the rhodamine 6G cation (R6G+), the rhodamine 101 zwitterion (R101) and the fluorescein dianion (F2‐)‐were measured in H2O, D2O and in a series of alcohol solvents ranging from methanol to octanol. The lifetimes of both RB= and RB+ increased markedly as the solvent was varied from water to octanol. In contrast, the lifetimes of R6G+ and R101± decreased slightly over the alcohol series and that of F2‐ increased only slightly in the same series. For all the dyes studied the fluorescence lifetimes observed in D2O were slightly longer than those in H2O. Possible causes for the variations observed are discussed.

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87assume the dimers have no internal energy [Eint(2) = 01, then the internal energy of the remaining monomers can be. calculated from eq 8 as 0.85 X J/molecule (427 cm-I). On the other hand, the maximum value for Ein,(2) is Do = 196 cm-I; otherwise, the dimers would dissociate. With this value, Eint(l) = 0.68 X J/molecule (340 cm-I). In both these calculations, E,(l) has been taken to be equal to AHvap10 These values for the internal energy are within experimental error of that previously reported, (0.97 f 0.34) X J/molecule (488 cm-I), and the value calculated from the isenthalpic expansion equation, 1.15 X J/molecule (578 cm-I). Since the "negative" signal observed in these experiments decreases by 90% upon heating the nozzle to 370 K, the cluster percentage clearly decreases at higher temperatures. At 400 torr, the percentage dimer limits are between 1% and 20% at 370 K instead of 9. 9%-39% as calculated for a 290 K, 400 torr source. This makes it unlikely that the difference between the measured value of Eint and that calculated from the isenthalpic expansion equation, which became significant at elevated temperatures,I0 could be accounted for by the presence of dimers.In conclusion, a sensitive laser technique has shown that dimers are present in neat beams of SF, under conditions corresponding to nozzle pressures ranging from 200 to 1000 torr and nozzle temperatures of 290 to 370 K. The percent of dimer found, however, is relatively small and does not appear to significantly affect the results of previous work.Acknowledgment. The authors appreciate many helpful discussions of the present work with Professor R. B. Bernstein.Flash photolysis and transient spectroscopy of six Cr(II1) compounds in liquid solution at room temperature have achieved a time resolution of 1 ps, about an order of magnitude better than in previous studies. Measurements were carried out in water and in five different organic solvent with no observable differences in the results. Excitation was at 314, 585, and 628 nm; transient spectra were monitored from 340 to 750 nm. In contrast to previous results (all using excitation at 530 nm), transient absorption appeared in less than 1 ps in all cases. For one particular set of excitation conditions, there was evidence for a transient with a lifetime of 1-3 ps. This could be the initially populated quartet or another species along the decay coordinate; or it may be artifactual. Except for that phenomenon, all transients may be explained by the known absorption of the lowest doublet in each complex. IntroductionThe photochemistry and photophysics of Cr(II1) coordination compounds have been the focus of considerable study.1-2 The electronic ground state is a quartet. Ligand field theory explains that the energy gap to the lowest excited doublet is approximately constant, while the energy of the lowest excited quartet state is variable, determined by the ligand field strength. Photochemical and photophysical behavior depend critically upon the relative positioning of the lowest state of each man...

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The role of the lowest excited doublet state in chromium(III) photochemistry has been investigated diligently for the past quarter of a century.1,2 Among numerous reviews, that by Kemp1 23