Stilbene triplet

These reactions, while not very important for high-energy donors in the absence of azulene, do help define what the potential energy diagrams should look like, that is, the fram-stilbene triplet is a discrete intermediate while the cij-stilbene triplet is a high vibrational form of the phantom triplet. Herkstroeter and McClure(13> have been able to observe the fro/w-stilbene triplet via flash photolysis using low-temperature glasses however, no similar triplet was observed for cfr-stilbene. Only when they studied cfr-stilbene-like... 

On the other hand, reactions in which the return to So occurs from a "non-spectroscopic minimum (Fig. 3, path g) are probably the most common kind. The return is virtually always non-radiativef). This may be the very first minimum in Si (Ti) reached, e.g., the twisted triplet ethylene, or the molecule may have already landed in and again escaped out of a series of minima (Fig. 3, sequence c, e). For instance, triplet excitation of trans-stilbene 70,81-83) gives a relatively long-lived trans-stilbene triplet corresponding to a first spectroscopic minimum in Ti. This is followed by escape to the non-spectroscopic , short-lived phantom twisted stilbene triplet, corresponding to a second and last minimum in Ti. This escape is responsible for the still relatively short lifetime of the planar nn triplet compared to nn triplet of, say, naphthalene. A jump to nearby So and return to So minima at cis- and trans-stilbene geometries complete the photochemical process ). 

Many authors may object to our usage of the word intermediate for the spectroscopic excited state at the original geometry, such as trans-stilbene triplet, and there may even be some opposition to the usage of this term for the non-spectroscopic excited state, such as twisted stilbene triplet, and a tendency to reserve the term intermediate only for those species which have a minimum in the So hypersurface. 

A particularly interesting case of transfer from T2 has been noted238 in the 9,10-dichloroanthracene (DCA) sensitized isomerization of stilbene. As shown below, the DCA ground state which results from quenching of T2 by stilbene (St) can act as a quencher of stilbene triplets and thereby alter the stilbene cis. trans photostationary state. 

Malkin and Fischer181 report that the likelihood that the twisted stilbene triplet decay into the cis- or rrans-ground state is 0.5. Thus, once we find the probability that the excited trans-singlet decay into the twisted triplet, we will have completed our calculation for the overall isomerization. 

The proposition that locally excited triplet states can be formed from back electron transfer within a doublet-doublet radical ion pair has firm theoretical (88) and experimental support. For example, with time-resolved Resonance Raman spectroscopy, one can directly monitor the chemical fate of the exciplex, solvent separated ion pair, and doublet free radical ion pairs formed between stilbene and amines. As might be expected from the above discussion, adduct formation is observed from the exciplex or contact ion pairs, whereas enhanced intersystem crossing ensues from the solvent separated ion pairs, producing spectroscopically observable stilbene triplets. This back electron transfer process, eq. 30 (89),... 

Another extensively investigated system involves the interaction of two alkenes, each capable of geometric isomerization, viz., the system stilbene-dicyanoethylene, which also illustrates the involvement of ground-state charge-transfer complexes. Excitation of the ground-state complex results in efficient Z - E isomerization of the stilbene exclusively, because the stilbene triplet state lies below the radical ion pair, whereas the dicyanoethylene triplet state lies above it (Fig. 11) [163-166]. 

Quenching of stilbene triplet by 0 in benzene gives 0 with an efficiency of 18 57- so quenching is not exclusively one single process. Biphenyl enhances the rate of 9,10-dicyanoanthracene sensitized photoxidation reactions by increasing the 0 yield. Substituent effects in the quenching of acetophenone and... 

This sequence (Eq. 14) can be excluded since on direct excitation of fram-stilbene in fluid solutions, the generation of 3p has been observed neither by laser flash photolysis nor by quenching experiments with azulene [27]. Sensitized excitation of m-stilbene leads to the trans triplet state. This shows that 3p cannot be populated selectively without formation of 3t [412]. Actually, upon sensitized excitation of cis- or trans-stilbene one sees a common triplet state which may well be 3p. Saltiel et al. [194] have argued that 3p is the stable geometry of the stilbene triplet. Whatever the... 

Scheme 2. Transoid and Twisted Stilbene Triplets in Equilibrium...

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 ... 

A precedent to the spin-exchange quenching mechanism of olefin triplets by oxygen is provided by the interaction of stilbene triplets with the stable free radical di-rm-butyl nitroxide, (50). In this case triplet-sensitized ([t]/[c])j... 

Direct irradiation leads to isomerization via singlet state intermediates. " The isomerization presumably involves a twisted singlet state that can be achieved from either the cis or the trans isomer. The temperature dependence of the isomerization further reveals that the process of formation of the twisted state involves a small activation energy. This energy is required for conversion of the initial excited state to the perpendicular geometry associated with the state. Among the pieces of evidence indicating that a triplet intermediate is not involved in direct irradiation experiments is the fact that azulene, which is known to intercept stilbene triplets, has only a minor effect on the efficiency of the direct photoisomerization. 

Attempts to detect long-lived triplet state intermediates by means of phosphorescence or triplet-triplet absorption spectroscopy have to this time failed (Schulte-Frohlinde et al., 1962 Herkstroeter and Hammond unpublished). Study of the chemistry of stilbene triplet states has been successfully effected only by means of excitation transfer (Hammond and Saltiel, 1962, 1963 Saltiel and Hammond, 1963 Hammond et al., 1964a Saltiel, 1964). The behavior of the stilbenes upon irradiation iff the presence of several triplet excitation donors (sensitizers) not only suggests that stilbene triplets are intermediates in the direct excitation experiments but has led to certain important postulates regarding the types of triplet excitation transfer processes and the factors which determine their efficiency. 

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