Stilbenes triplet states

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]. 

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absorption spectroscopy have to this time failed (Schulte-Frohlinde et al., 1962 Herkstroeter and Hammond unpublished).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. 

Herkstroeter and Hammond found support for this postulate from a flash photolysis study. They were able to measure directly the rate of sensitizer quenching (energy transfer) by cis- and fra/w-stilbene. When a sensitizer triplet had insufficient excitation energy to promote fims-stilbene to its triplet state, the energy deficiency could be supplied as an activation energy. The decrease in transfer rate as a function of excitation energy of the sensitizer is given by... 

If a common intermediate were not involved in the triplet state isomerization of stilbene, the sum of d>c T and T-.0 could have any value from zero to two. 

Before we cite another experiment that supports the idea that isomerization occurs from the phantom triplet state, it will be helpful to complete our set of stilbene reactions and construct an experimental potential energy diagram. 

The previous sections have shown that cis-trans isomerization of stilbene can take place via the lowest triplet state of stilbene. The question to be considered now is whether the isomerization upon direct photolysis takes place via the singlet state, the triplet state, or a vibrationally excited ground state.a 7 81 50)... 

A recently popular mechanism involves the intersystem crossing of the cis- or trons-stilbene singlet state, produced upon direct photolysis, to its corresponding triplet states, which would then undergo the type of reactions given in Eqs. (9.8M9.10) and (9.17M9.19) ... 

The photochemistry of a-methylstilbene (5) resembles stilbene photochemistry in many ways. However, as pointed out earlier, both the cis and trcms isomers are nonclassical acceptors of triplet excitation. This suggests that both the cis and trcms triplet states correspond to high-energy vibrational levels of the twisted or phantom triplet. Azulene does not alter the photo-... 

Stilbene is known to undergo cis-trans isomerization in its triplet state to yield a photostationary state which is 60% cis and 40% irons,... 

The initial quantum yields for cis- to tam-stilbene isomerization (O0 T) and for trans to cis isomerization (4>T-.C) are consistent with Hammond s postulate that isomerization takes place from a common state, most likely the twisted or phantom triplet state ... 

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. 

Chemically inert triplet quenchers e.g. trans-stilbene, anthracene, or pyrene, suppress the characteristic chemiluminescence of radical-ion recombination. When these quenchers are capable of fluorescence, as are anthracene and pyrene, the energy of the radical-ion recombination reaction is used for the excitation of the quencher fluorescence 15°). Trans-stilbene is a chemically inert 162> triplet quencher which is especially efficient where the energy of the first excited triplet state of a primary product is about 0.2 eV above that of trans-stilbene 163>. This condition is realized, for example, in the energy-deficient chemiluminescent system 10-methyl-phenothiazian radical cation and fluoranthene radical anion 164>. 

The effects of nitro substituents on the cis-trans isomerization of stilbenes has been reviewed70 (equation 63). The trans-to-cis isomerization occurs from a triplet excited state, whereas the reverse cis-to-trans isomerization occurs through a main route which bypasses the triplet state. A nitro substituent usually causes a significant enhancement of the quantum yield of the intersystem crossing. Nitro substituent effects on the photoisomerization of trans-styrylnaphthalene71 (equation 64), trans-azobenzenes72 and 4-nitrodiphenylazomethines73 (equation 65) have been studied for their mechanisms. 

Anthronylidene 64 forms no cycloaddition products with cis- or trans-4-methyl-2-pentene, but [1 -f-2]-cycloaddition is observed with stilbene derivatives. This is also attributed to the reacting triplet state I02,i30-i32) ... 

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. 

The photosensitized isomerization of ethyler.es and stilbenes has been studied in detail by Hammond and his group and it has been found that in each case both the cis-trans and the trans-cis isomerizations occur with equal efficiency. A common (tc, jt ) triplet state is assumed to be the intermediate in each case. This triplet state is referred to as the perpendicular triplet state and was termed a phantom triplet state by Hammond. The energy of the perpendicular triplet state is expected to be low since in this configuration the overlap between the re and rc orbitals is minimized. The potential energy of S0, Tx and Sx states as a function of twist angle is given in Figure 7.7... 

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. 

A quantitative variant of this interception approach is useful for evaluating the yields of triplet states from certain redox processes [86,87]. Consider, for example, the FA(-)/10-MP(+) reaction in the presence of trans-stilbene. No emission is seen because the following quenching process occurs ... 

FIGURE 4. Energy level diagram for the trans-stilbene locally excited singlet and triplet states, the trans-stilbene-fumaronitrile (FN) exciplex, and triplet fumaronitrile. 

The observation of triplet sensitized cycloaddition reactions of 10 but not t-1 with vinyl ethers reflects the requirement of a planar stilbene excited state for effective interaction with ground state electron-rich or electron-poor alkenes. While triplet sensitized reactions of other cyclic stilbene analogues (e.g., 5-9) have not been reported, it appears quite likely that they should occur. 

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),... 

Norrish Type II processes from the triplet state involve a biradical even when concerted reaction would be exothermic. Photolysis of 60 gives the products in Equation 13.65. Concerted reaction with the formation of triplet stilbene (a process corresponding to Equation 13.29) would be exothermic, but triplet stilbene is not formed The 7 state of stilbene decays to a 60 40 cis-trans mixture, but photolysis of 60 gives 98.6 percent tra/u-stilbene.94... 

The excited fraws-stilbene/fumaronitrile complex produces a locally excited triplet state, which is considered to be responsible for isomerization of substituted stilbenes. Similarly, the CT complexes between aromatic hydrocarbons and fumaronitrile produce the isomerization of fumaronitrile to malonitrile34. 

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