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The time-resolved fluorescence spectroscopy of seven strongly fluorescent phenazine derivatives (two which can be associated approximately to the C2h point group, two to the C2v point group and three to the Cs point group) in organic solvents of varying polarity have been reported for the first time. An increase in excited singlet lifetime and fluorescence quantum yield was observed as the polarity of the solvent increases for the C2h and the Cs compounds, however the C2v compounds presented no appreciable solvent effect. In all five cases where there is a change in lifetime, solvent polarity has almost no effect on the radiative rate constants, whereas the rate constants for non-radiative decay can vary by an order of magnitude, or more. These results are explained on the basis of decreased intersystem crossing as solvent polarity is increased. This model is supported by the results of transient absorption spectroscopy and semi-empirical molecular orbital calculations on the excited states of some of the compounds studied. The large solvent effect on the time-resolved fluorescence observed for the compounds belonging to the C2h and Cs point groups, coupled with their extremely high fluorescence quantum yields, indicate these compounds as potentially very important molecular polarity probes. The strong correlation between the experimentally observed solvent effects and the theoretically calculated excited state energy levels for each point group, as well as an obvious symmetry dependence of the observed behavior is discussed. |
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