Nitrostilbenes photoisomerization

Photoisomerization of 4-methoxy-4/-nitrostilbene 91 suffers from a strong solvent effect (Sch. 35). The (Z/E)pss ratio upon excitation at 366 nm is 91 9 in nonpolar petroleum ether, but the Z/E preference is dramatically switched to give the Z/E ratio of 17 83 in polar dimethylformamide [218]. Laser flash photolysis studies revealed the multiplicity of the excited state involved. The singlet mechanism is operative in the photoisomerization of ( )-4-cyano-4/-methoxystilbene and ( )-4-methoxy-4/-nitrostilbene [219,220], while the photoisomerization of nitrostilbenes involves the triplet state [221]. 

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

Quenching measurements with ferrocene or azulene have confirmed the triplet route as the major pathway in tram - cis photoisomerization of several nitrostilbenes [118, 188, 192]. As a typical example, , c and 4>c, of 4-nitrostilbene in benzene are shown as a function of the ferrocene concentration (Figure 7). A related case is NOz-StN where the triplet pathway also operates [199]. 

From a mechanistic point of view Fischer s classification may be modified in the following manner For stilbenes of group 1, trans - cis photoisomerization occurs mainly via the singlet mechanism (Section VI.A.2). Contribution of the upper excited triplet mechanism has been determined in cases where < isc is substantial (e.g., 4-chloro- and 4-bromostilbene). Nitrostilbenes constitute a class of stilbenes in which trans -> cis photoisomerization occurs mainly via the lowest triplet state (Section VI.B.2). For polar nitrostilbenes a radiationless deactivation pathway not leading to the cis isomer is operative (Section VI.C.3). 

Based on similar results (but less intense investigations), Gorner recently concluded a close mechanistic relationship between 4-acetyl- or 4-benzoyl-stilbene and 4-nitrostilbene [121]. Bong et al. [163] and Shim et al. [228,525] have examined a series of diarylethylenes containing heterocyclic groups and found also essentially the triplet pathway for the direct trans - cis photoisomerization. 

The rate constant for the individual transitions may depend on a variety of features, such as substitution, medium and temperature. One important parameter is the height of the activation barrier between t and p. If the rate constant for the step 11 — 1 p is of the size comparable to that for the step t -> 3t, both processes may contribute to tram-> cis photoisomerization. For most nitrostilbenes examined, is substantial and for 4-nitro-, 4,4 -dinitrostilbene, and 4,4 -NMS a singlet pathway has been ruled out. However, for 4,4 -NDS in nonpolar solvents [160] and for quaternary salts of 4-nitrostyrylquinolinium [229], a contribution of the singlet mechanism has been concluded. 

In view of the well-documented ability of Oj to function generally as an acceptor of triplet energy, it had been expected that its presence should also increase ([t]/[c])3 ratios for the triplet sensitized photoisomerization of stilbene-like olefins. However, as was first noted for nitrostilbenes, the quenching of the olefin triplets by O2 does not alter ([t]/[c])s (42,43). For the parent stilbene it was shown that when azulene is used as a quencher of stilbene triplets the slope of the ([t]/[c])s-vs. -[Az] plot is strongly attenuated by O2 see Figure 5 (41,44). Since the intercept of the azulene plot was not influenced by O2, it was reasoned that O2 deactivates p without changing the decay firaction 8 as predicted by Schemes 1 and 2 (44). To account for stilbene triplet deactivation by O2 without change in 8 two possibilities were considered ... 

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