Nitrostilbenes quantum yield

For nitrostilbenes (Table 5d) in nonpolar solvents at ambient temperature the quantum yields of isomerization are comparable to those of stilbene itself. However, 4-nitro-4 -R-stilbenes show a different dependence on solvent properties and temperature. On increasing the solvent polarity, decreases strongly for polar substituents for example, R = OCH3, NH2, and N(CH3)2, 4,4 -NMS, 4,4 -NAS, and 4,4 -NDS, respectively. nonpolar solvents (cyclohexane and benzene) are... 

TABLE 5d Quantum Yields of cis tram Photoisomerization for Nitrostilbenes"... 

Whitten and co-workers [230] have amply investigated stilbene isomerization induced by excited porphyrins. Electron-deficient nitrostilbenes quench excited Zn or Mg etioporphyrins through exciplex formation. Light absorbed by the porphyrin affects isomerization of 4-nitro- or 4,4 -dinitro-stilbene with low quantum yields ((%trans)sens = 99.5), possibly via a stilbene exchange chain mechanism [230]. Azastilbenes coordinate to me-talloporphyrins in both the ground and the excited state (but do not quench it), and isomerize efficiently upon selective excitation of the porphyrin [231]. Complexed 1,4 -NPE gives d , = 6.6, = 0.2, and (%trans)sens = 96,... 

Fluorescence Quantum Yields. For tram-stilbcne 4>f is independent of lexc and the concentration (typically < 10 4 M) and only slightly reduced by the presence of c/s-stilbene [13, 81, 99, 240, 241, 294]. The liquid solutions at room temperature f is about 0.04 for trons-stilbene and 4-bromostilbene, and larger for 4-chlorostilbene and several cyanostilbenes. For nitrostilbenes very small, but there are exceptions in the presence of an electron... 

The 4-nitrostilbenes represent an interesting class of compounds (37). For several of these, the optical density of the triplet-triplet absorption is independent of temperature and medium viscosity, indicating efficient triplet population under all conditions studied (76). For the same solvent the triplet-triplet absorption spectra are also temperature-independent, though the trans->cis quantum yield drops sharply as the temperature is decreased and the medium viscosity is increased (74). These results indicate a transoid equilibrium geometry for the triplets at all temperatures studied (76), consistent with large ferrocene (4.7 X 10 A/ s, cyclohexane) and azulene (4.4 x 10 s , cyclohexane)... 

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