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Positional Dependence of the Heavy Atom Effect in the Cis-Trans Photoisomerization of Bromostilbenes Sir:Comparison of the temperature dependence of fluorescence, &, and isomerization, +t+c and quantum yields for the stilbenes and the p-halostilbenes has revealed a substantial bromine substituent effect. 3-5 Increasing the temperature in the parent hydrocarbon from -180 to 25" increases C$t-c while decreasing &, these two processes being coupled throughout the temperature range.

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