Stilbene triplet excited states

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. 

Singlet as well as triplet excited states of oxiranes undergo C—C bond scission to produce carbonyl ylides which, upon cydoaddition with dipolarophiles, give tetra-hydrofuran (THF) derivatives. For example, trans- or ds-stilbene oxide on direct photolysis using 254 nm light in the presence of methyl acrylate gave diastereomeric... 

Two examples of 1,4-addition to the naphthalene ring have been reported. In one. the substituent effects upon the reaction between the triplet excited state of 1-acetyinaphthalene and variously substituted alkenes has been examined. while in the other, the 1.4-addition of a-methyl styrene and of 1,1-dlphenyl ethylene to the phthallmide (125) is shown to proceed by electron transfer from the alkene to the excited state of the phthallmide. The azaindone (126), which is Isoelectronic with naphthalene, also forms addition products when Irradiated with stilbene. The addition products are assigned the structures... 

The photoisomerization of all types of azobenzenes is a very fast reaction on either the singlet or triplet excited-state surfaces according to the preparation of the excited state, with nearly no intersystem crossing. Bottleneck states have lifetimes on the order of 10 ps. The molecules either isomerize or return to their respective ground states with high efficiency. So photoisomerization is the predominant reactive channel, and the azobenKnes are photochemically stable. Only aminoazobenzene-type molecules and pseudo-stilbenes have small quantum yields of photodegradation. 

On triplet sensitization the resulting cis ( c ) and trans triplet excited state (H ) convert to the perpendicular excited triplet state ( p ) p is equilibrated with by an equilibrium constant of = [ p ]/[H ] and deactivated to the ground state p, giving c and t [6,13-15,24,33-41]. Thus, the isomerization of stilbene takes place as a diabatic process. 

Table 1 compares the typical features of one- and two-way photoisomerization in the triplet excited state [1-4,18-65]. Briefly, stilbene undergoes typical two-way isomerization between cis and trans isomers in the triplet state the triplet state deactivates in a relatively short lifetime (60 ns) from p to the ground state to give c and t in almost the same ratio [15]. However, 2-styrylanthracene undergoes one-way cis - trans isomerization in the triplet excited state by an adiabatic conversion from the cis ( c ) to the trans conformer ( t ) the deactivation takes place only from t with a long lifetime on the order of 100/is to give t either by a unimolecular... 

These polynuclear gold(I) phosphine complexes have been found to possess rich photochemistry. The phosphorescence of 49 is found to be quenched by a series of energy acceptors such as trans- and cis-stilbene, styrene, hept-l-ene, and cyclohexene [129], The transient absorption difference spectrum of a degassed acetonitrile solution of 49 and irans-stilbene displays absorption bands at about 365 and 390 nm, typical of the triplet excited state absorption spectrum of trans-stilbene, indicating the energy transfer nature of the quenching mechanism, Be-... 

The triplet state has received rather less attention than the singlet in micellar photochemistry. Phosphorescent decay of solubilized polynuclear aromatic hydrocarbons from their Tj state may be observed in heavy-metal ion lauryl sulphate micelles. This involves a conventional intersystem crossing from Si- Ti promoted by spin-orbit coupling with the heavy atom. 1-Bromonaphthalene readily forms a triplet excited-state in micelles, which may be quenched by added sodium nitrite in water, the lifetime then being reduced from 2.8 x 10 s to 5 x 10 s. There is efficient triplet energy transfer from N-methylphenothiazine (Ti) to trans-stilbene (So), which is irreversible but reversible energy-transfer to naphthalene (So) occurs. ... 

Under optimum conditions electron transfer can produce excited states efficiently. Triplet fluoranthrene was reported to be formed in nearly quantitative yield from reaction of fluoranthrene radical anion with the 10-phenylphenothia2ine radical cation (171), and an 80% triplet yield was indicated for electrochemiluminescence of fluoranthrene by measuring triplet sensiti2ed isomeri2ation of trans- to i j -stilbene (172). 

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-... 

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. 

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. 

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 results described in this article establish that the stilbenes are among the most versatile of organic reactants in bimolecular photochemical reactions. Only triplet cyclo-alkenones can rival the ability of It to dimerize, form [2+2] adducts with both electron-rich and electron-poor alkenes, and form acyclic adducts with amines, heterocycles, and noncon-jugated dienes. All of the known bimolecular photochemical reactions of excited stilbenes involve It as the reactive excited state. The failure of - -c, and 3C to undergo bi-... 

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),... 

SCHEME 1. The effect of the triplet excitation energy of the sensitizer on the Z/E isomerization of stilbene at the photostationary state. Reprinted with permission from Ref. 7. Copyright (1964) American Chemical Society... 

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]. 

The linear correlation with slope AR/Ap = —0.36 shows that PPP bond orders can be used for excited-state relaxation. The comparable graph of AR/Ap for trans-stilbene has a linear correlation with slope —0.25. Similarly, it is seen in biphenyl[37] that the lowest triplet state has a bond order pattern corresponding to the quinonoidal structure, implying a planar geometry in the triplet state in conformity with the prediction of Lewis and Kasha[56]. 

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