4-Nitrostilbene lifetime

Laser flash photolysis (30 ns, 50 mj, 347 nm) of 4-nitrostilbene, 4,4 -dinitro-stilbene, 4-nitro-4 -methoxystilbene and 4-dimethyl-amino-4 -nitrostilbene permits the observation of transients 133) por the latter two compounds, the lifetime and the absorption spectra of the transients vary strongly with the polarity of the solvent used. First order decay rate constants are given in Table 4. 

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 ... 

Here, /lfexp —(E(/RT) and k0 are rate constants for activated and nonac-tivated decay steps of t, respectively. For stilbene, halogenated, and some other stilbenes (4-R CN, CH3, OCH3) the activated step has been assigned to twisting in the excited singlet state [27, 105, 114, 357, 358], whereas for nitrostilbenes intersystem crossing is involved in the activated step [31]. At low temperatures lifetimes of 1-2 ns have been reported for several trans-stilbenes [314, 337, 342], Below — 150°C an enhanced frans-stilbene was excited by a 532-nm picosecond laser pulse, in addition to a 266-nm pulse [317]. 

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],...

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],... 

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