Substituted stilbenes isomerization quantum yield

The direct photoisomerization of substituted stilbenes has also received attention. Several 4,4 -disubstituted stilbenes in which one substituent is electron withdrawing and the other electron donating, such as 14, have quantum yields for cis -> trans isomerization similar to that of m-stilbene, but exhibit very low quantum yields for trans -> cis isomerization in hydrocarbon solvents and zero quantum yields in ethanol.250 Likewise, certain salts of 4 -amino-2-styrylpyridine, such as 15, do not undergo direct trans -> cis photoisomerization.251 The strong interactions between the ring systems in the ground states of 14 and 15 are probably increased in the excited states. Consequently the planar... 

The fluorescence of Irons-stilbene and four methoxy-substituted stUbene derivatives has been detected in a variety of solvents [37]. Compared to other stUbene derivatives, trons-3,5-dimethoxystilbene displayed a large quantum yield of fluorescence and a low quantum yield of trans-cis isomerization in polar organic solvents. 

The effect of temperature on the quantum yields of isomerization and fluorescence was determined for frons-stilbene and for certain halogen-substituted frans-stilbenes (Dyck and McClure, 1962 Malkin and Fischer, 1962). It was found that as the temperature is decreased (below about 200°K) the trans cis quantum yield decreases but that the fluorescence quantum yield increases. These results suggest that an activated process (i.e., internal conversion or intersystem crossing) competes with fluorescence from S - and that this process is an essential step in the mechanism for isomerization. The results for intersystem crossing into T. ... 

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