Effect of refractive index on molecular radiative lifetimes

Abstract The effect of the refractive index of the medium (n) on the fundamental photophysical parameters τR = 1 k f has been investigated, using the highly-fluorescent compound 9,10-diphenylanthracene (DPA). Experiments show that for DPA in isopentane solution, reducing the temperature from room temperature to −160°C (whereupon n increases from 1.351 to 1.439) causes the experimental fluorescence lifetime to decrease from 8.8 to 7.5 ns. These data and behavior of the integrated extinction coefficient are consistent with an n−2 to n−3 dependence of the radiative lifetime. Radiative lifetimes for the thermalized gas phase are compared with those in solution for naphthalene and pyrene and are also consistent with an n−2 to n−3 dependence of the radiative lifetime. It is concluded that extrapolations of lifetimes data from one set of conditions to another should not be made unless absorption and/or quantum yield data are known under both conditions. When these data are not available, an n−2 correction factor applied to τR probably is accurate to within ten percent.

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