4-Nitrostilbene triplet

Nitro substituted stilbenes have been used as models in stUbene cis trans-isomerization studies. It had been reported previously, that cis -v benzene solution, and a photostationary state of 99.5% trans isomer had been reached 132) ... 

Identical spectra and lifetimes were observed for the first three compounds listed above. It is assumed, that the observed transients are triplet states of the nitrostilbenes on the basis of the following results ... 

Zn(MPIXDME) IL, 3IL Nitrostilbenes Benzene Formation of both singlet and triplet exciplexes (A - Q) suggested [35,36]... 

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]. 

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]. 

Photostationary cis/trans ratios [2] and triplet transfer rate constants [211] as a function of ET obtained with a-methylstilbene [cf. 219] are analogous to those observed with stilbene. For several nitrostilbenes it has been shown that the position of the photostationary state depends only slightly on substituent and medium [201]. A solvent dependence however, was found for two polar nitrostilbenes 4,4 -NMS and 4,4 -NDS. The shift of the photostationary state to the tram isomer with increasing solvent polarity accounts partly for the decrease of d> c. A small decrease of d>f-.c with increasing polarity has also been found for several azastilbenes (Table 9c). 

With ferrocene as quencher and triphenylene as sensitizer the cis trans isomerization of several 4-nitrostilbenes has been examined [201], From linear plots of ([t]/[c])sens versus [Q], larger slope/intercept ratios were found as compared to direct excitation conditions (Table 8). A cis -> trans pathway partly bypassing the triplet state accounts for this difference (Section VI). Plots of ([t]/[c])scns as a function of the azulene or ferrocene concentrations are shown in Figure 10 for three stilbenes. 

In liquid solutions in the absence of a sensitizer, T-T absorption has hitherto been observed predominantly with nitro-substituted frans-arylethy-lenes (Tables 16c-e) such as nitrostilbenes [31,200], nitroazastilbenes [172,416], and nitrostyrenes [417], Apparently, the nitro group enhances strongly thioindigo dyes [418,419]. No triplet could be measured for stilbene under direct excitation conditions at room temperature [96,200] because the triplet yield is too small (see Section 1V.B.6). 

Identical T-T absorption spectra have been recorded with cis and trims isomers of nitrostilbenes in fluid solutions [188,200]. The reason is that the same triplet is generated from both isomers, from the cis side either directly... 

NDS), the solvent polarity influences the form and position of the triplet absorption spectrum significantly [180]. Virtually the same T-T absorption spectra have been recorded in liquid solution and rigid media as illustrated for 4-nitrostilbene in GT (Figure 15). Small shifts of the T-T absorption... 

Figure 16. Temperature dependence of the reciprocal triplet lifetime (log scale) of trans isomers of 4-bromostilbene in ethanol (O), 4-benzoylstilbene in GT ( ) and ethanol ( ), 4-nitrostilbene in ethanol ( ), and 4,4 -NMS in glycerol (A) [31, 114, 121, 180],...

The rate constant for the nonradiative deactivation (knj) of the trans triplet configuration is much larger than that for the radiative transition (kr). An estimation for 4-nitrostilbene in ethanol shows that kt is at least 2000 times smaller than fcnr, using 4>jsc = 0.7 [407] and 4>p = 3 x 10-4. This places kr at 0.05s 1 since knr(zs 1/tt) is about 102s. The more than 30-fold decrease in d>p for stilbenes not carrying carbonyl or nitrogen substituents (n, n states) can be explained by a comparable kt value and the corresponding decrease in [Pg.65]

Wismontski-Knittel and Das have shown that for the two StNs and the three DNEs tT ranges between 0.1 and 1 /as [410,426]. Elisei et al. [122] have studied the temperature dependence of r, for 1-StN and the chloro and bromo derivatives, examples are shown in Figure 6. The triplet decay is essentially the same as for nitrostilbenes. For example, tt of 1-StN in MTHF ranges from 5 ms at — 196°C to 0.3/as at 25"C. Longer triplet lifetimes at low temperatures were also measured for 2-styrylanthracene and several derivatives [427],... 

Figure 17. Temperature dependence of the triplet yield of traws-4-bromostilbene in GT ( ) and ethanol (O) and rrtms-4-nitrostilbene in GT (A) [114,119].

Since the lowest triplet state of nitrostilbenes at room temperature can be probed by laser flash photolysis, quenching rate constants could be measured directly [188,200], Values for kq are close to the diffusion-controlled limit (Table 17). Comparable results have been obtained for naphthyl-[410] and 2-anthrylethylenes [433] and for ADBs [33, 143, 145-147, 232, 411], For substituted 1-NPEs, conclusions about the position of the triplet equilibrium were drawn on the basis of rate constants for quenching by ferrocene and oxygen [441]. 

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). 

The reactivity toward singlet oxygen [238,454] should be taken into consideration if triplet states are involved (either as intermediate or as sensitizer) on irradiation of air- or oxygen-saturated solutions. Benzaldehyde, hydroperoxides, and dioxetanes have been found as products from the reaction of stilbenes with reactive oxygen species [464,465]. A rearrangement reaction has been reported for nitrostilbenes formation of a-oximinoketones, typical for / -nitrostyrenes, which may be accompanied by cleavage products [466],... 

Radical anions of frans-stilbene and stiff stilbenes, generated by y-radi-ation in MTHF at — 196°C, exhibit maxima at 500-560nm, e = (1 — 2) x 104M 1cm 1 [509], A similar spectrum and e50o = 6.1 x 104M 1 cm 1 was recorded by pulse radiolysis at room temperature [510]. The anion and cation radicals of cis-stilbene lead efficiently to the trans isomer, while an analogous conversion to the cis isomer from the radical anion and radical cation of frans-stilbene was not found [491]. The cis-stilbene radical cation isomerizes to the more stable frans-stilbene radical cation [511]. The radical anion of cis-4-nitrostilbene has been observed by EPR [512], In the cases of 4-nitro-,4,4-dinitrostilbene and 4,4 -NMS the electron transfer occurs in the triplet state, for example, from DABCO toward the 3t state [513]. 

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. 

For nitrostilbenes, in contrast to a number of other stilbenes, photocycliz-ation (Table 18) and even formation of the triplet state of DHP [118] play practically no role. 

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 ... 

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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