Triplet pathway

The photolysis of the diazobicyclo[2.2.2]heptene derivative (142) was studied at different temperatures and was found to give mixtures of syn (143) and anti (144) products. The experimental data support the homolytic (Xe2) pathway as the prevalent reaction channel at elevated temperatures for the generation of the sterically encumbered syn product, whereas at low temperatures the triplet pathway operates and loss of the syn selectivity is observed. The loss of syn selectivity at low temperatures is due to efficient intersystem crossing in the singlet-excited azoalkane to afford the planar, nitrogen-free triplet diradical which unselectively ring closes. 

Nitro-substitution especially at the 6-position of BIPS opens up a triplet pathway for photo-isomerization. This pathway runs in parallel to the singlet manifold. This increases the yield and, in turn, may lead to photo-aggregation that is observed for these compounds. Photochemical ring closure to the spiropy-ran form is more efficient for these 6-nitro-substituted compounds. The photochemistry of 6-nitro-BIPS merocyanine is similar to that of unsubstituted BIPS(s) however, the 6,8-dinitro compound efficiently cyclizes upon excitation to form the spiropyran closed form via a singlet manifold. 

The mixtures of diastereoisomers formed in the singlet and triplet pathway, respectively, are termed C and D (Sch. 55). 

Shim and coworkers have published several examples of the 2+2 between aryl-substituted alkynes and alkenes. Their data suggests that the singlet and triplet pathways give rise to the observed cyclobutenes. Exciplex emission was detected for the alkyne-alkene pairs. 

Photo-[4+2] reactions of the dienone steroid 105 illustrates interesting regio-, stereo-, and site selectivities (Sch. 24) [75-78]. Reaction with 1-acetoxy-1,3-diene 106 gives trans adduct 107 in good yield, epimeric at the acetate. The trans cycloaddition was attributed to a triplet pathway rather than a twisted enone intermediate [75]. Reaction with 2,3-dimethyl-1,3-butadiene 108 leads to four [4+2] adducts, with reaction at both alkenes groups of the dienone. Note that the products of reaction at the y,5-alkene are both cis. 

This may be due to a destabilization caused by twisting around the C32-C13 bond. Direct excitation isomerization yields (<)>jgo) are usually higher in systems characterized by high intersystem crossing. This suggests that a triplet isomerization mechanism plays an important role following direct excitation, a fact that was quantitatively confirmed for retinals in nonpolar solvents. With the exception of 11-cis PRSB, the lowest triplet state (T3) appears to possess a substantial isomerization barrier so that partition between cis and trans isomers takes place in higher vibronic states. An O2-induced mechanism, which is probably associated with a triplet pathway, characterizes the fluorescent derivatives ROH, RAc, and RBA. 

The oxa-di-TT-methane photochemical rearrangement of 3,Y-unsaturated systems normally occurs via a triplet pathway but Fuchs and co-workers have described an example in which direct irradiation (presumably singlet) gives the oxa-di-it-methane product whereas triplet sensitization follows a different path. 

As in the azobenzene type systems, the triplet pathway seems to be decoupled from the singlet route in pseudo-stilbenes. Little is known about the mechanism in the triplet state. The only information comes from calculations, and these show that the triplet surfaces are frequently similar in shape to the singlet surfaces. Thus, both mechanisms may be operative in the triplet state, too. 

Kaupp and Teufel. The adducts (88), (90), and (89) are formed in respective yields of 81%, 14%, and < 5% from the singlet exci-plex whereas triplet quenching and sensitization studies show that (90) is the major product from the triplet pathway which also yields an adduct of unknown structure. 

A similarly detailed study of the polymerization of methyl methacrylate by diphenyliodonium chloride in aq. AN using various aromatic ketone sensitizers has recently been carried out by Timpe s group [103a]. On the one hand the results were not complicated by monomer quenching of excited ketone sensitizer, but were complicated by operation of simultaneous singlet and triplet pathways for the electron-transfer sensitization. In this study the data were analyzed without reference to the possibility of triplet energy transfer sensitization of the iodonium salt photolysis [22], or the possible involvement of charge transfer complexes. 

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]. 

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. 

The five styrylphenanthrenes constitute a remarkable class of olefins for which several mechanisms are operative [278,435]. In addition, mechanisms can be switched by changing the solvent polarity. The irons - cis photoisomerization in n-hexane has been suggested to occur via the lowest triplet pathway for all except the 2-derivative, for which the upper excited triplet pathway operates. In acetonitrile, however, the lowest triplet pathway remains only for 4-styrylphenanthrene, the upper excited triplet pathway occurs for the 1-, 2-, and 3-derivatives, and a mixed Sj-T, mechanism accounts for the results with 9-styrylphenanthrene [436]. 

A mixed singlet triplet mechanism accounts for several cases, for example, 4,4 -NDS in nonpolar solvents, a quaternary salt analogue of 4,4 -NMS, and 1-StN. The last follows a pure singlet and triplet pathway at elevated and low temperatures, respectively, the change in mechanism taking place just below room temperature. The same holds true for Br-StN but, in contrast to 4-bromostilbene, 4>isc does not increase on cooling. This is one way of distinguishing a conventional triplet mechanism from an upper excited triplet mechanism. 

---