Styrylpyridines 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... 

Styrylpyridine photochemistry has been important in one other system. The photoprocesses in the complexes W(CO)sX (X = pyridine, 2-styrylpyridine, and 4-styrylpyridine) have been investigated.129 Unlike the XRe(CO)3(taz s-styryl-pyridine)2 and Ru(II)-styrylpyridine complexes having lowest IL and CT states, respectively, the W(CO)5X complexes have lowest LF excited states with only a small contribution from W - pyridyl CT. The one-electron diagram for low-spin d6, Cnv complexes shown in Scheme 7 is appropriate here. Both photosubstitution and photoisomerization reactions are found for the W(CO)5X complexes and some quantum yield data are found in Table 23.129 The data show that substitution efficiencies for... 

However, while 313 and 366 nm direct irradiation yields a cis- trans quantum yield of-0.005, the benzophenone triplet sensitized and the low energy direct irradiation yield is only —0.001. These results support the notion that two reactive excited states are involved one associated with the styrylferrocene which leads to the more efficient isomerization and a ferrocene-type excited state at lower energy which has a smaller tendency to undergo cis -trans isomerization. It is interesting to note that the 100% trans photostationary state is likely very close to the amount expected at thermal equilibrium. Such a result would be consistent with an intramolecular CT transition to yield a radical-anion ferricinium complex as in reaction (47). The radical anion could yield cis trans isomerization just as in the Ru(II)-styrylpyridine complexes. The wavelength dependence of the isomerization of the styrylferrocene is consistent with (47) in that upper excited states are required for the photooxidation of ferrocene to ferricinium in CC14.135 ... 

If no direct measurement of the fluorescence lifetime is available the relations between the radiative lifetime and the fluorescence and absorption spectra can be used in conjunction with the quantum yield to obtain an indication of the fluorescence lifetime. Birks and Munro (1967) have reviewed the methods of calculating the radiative lifetime. In general these methods are limited to specific groups of compounds. For example, Favaro et al. (1973) applied Stickler and Berg s (1962) formula to the spectral data obtained from an excited state acid-base study of some styrylpyridines and found a lack of quantitative agreement between the measured and calculated lifetimes. 

Photolysis of XRe(CO)3L2 (X = Cl or Br L = trans-3- or mwis-4-styryl-pyridine) causes trans -> cis isomerization of co-ordinated L with quantum yields and resulting photostationary states similar to those of the free ligand (Table 3).222 As the lowest excited state is assigned to an intra-ligand excited state- localized on the styrylpyridine, these results suggest that the perturbing effect upon co-ordination is small. 

The only photoreaction which the 4-styrylpyridine (4-stilbazole) complexes [Ru(bipy)(cw-4-stilbazole)] + and [Ru(bipy)(/ ra 5-4-stilbazole)] + undergo is that of cis trans isomerization of the stilbazole ligands there is no significant photodissociation. The anion [Pt(NH3)4(NH2)(N02)] " photoisomerizes to [Pt(NH3)4(NHa)(ONO)]. The quantum yield has been determined for this process, but no kinetic results are available. ... 

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