Intersystem-crossing, facilitation

High stereospecificity is observed when the rotation of the diradical intermediate is slow in comparison with cyclization to cycloadduct or reversion to reactants. With the presence of external heavy atoms, it could facilitate the intersystem crossing (ISC) of the first-formed singlet diradical to the longer-lived triplet counterpart. The triplet diradical will have a chance to undergo rotation before it reverts back to singlet and cyclizes or cleaves to reactants. This then accounts for the reduced stereospecificity. The alternative possibility of a zwitterionic intermediate is considered unlikely because there is no interception of zwitterions by water. 

Laser flash photolysis of 46 showed results similar to those obtained for 45. The lifetimes and yields of Z and E photoenols from 46 are comparable to those obtained for 56. Similarly, laser flash photolysis of 47 reveals that the major reactivity pattern of 47 is intramolecular H-atom abstraction to form Z-58 and E-58 even though no products were observed that can be attributed to the formation of photoenol 58. Laser flash photolysis of 47 in methanol showed formation of biradical 57 ( max 330 nm, r = 22ns), which was efficiently quenched with oxygen (Scheme 32). Biradical 57 intersystem crosses to form Z-58 and E-58, which have maximum absorption at 400 nm. Enols Z-58 to E-58 were formed in the approximate ratio of 1 4. Enol Z-58 had a lifetime of 6.5)0,s in methanol, but its lifetime in dichloro-methane was only 110 ns. The measured lifetime of E-58 in methanol was 162)0,s, while it was 44 ms in 2-propanol. Thus, E-58 is considerably shorter-lived than E-56. Furthermore, E-58 is also shorter-lived than the analogous E-59 (Scheme 33), which cannot decay by intramolecular lactonization and has a lifetime of 3.6 ms in methanol. Thus, we proposed that E-58 undergoes solvent-assisted reketonization that is facilitated by the intramolecular H-atom bonding, as shown in Scheme 34. 

A small value of AEst facilitates intersystem crossing. We expect singlet state to be fast depleted along this pathway if the lowest excited state is of (n, it ) type. This pathway is further promoted due to the fact that t y by a factor of ten, due to the forbidden character of... 

Lee and coworkers (17) have also investigated the solution-phase photochemistry of cyclobutanone and have found (X - 310 nm) to be l/20th of that obtained in the vapor phase. This reduction has been attributed to enhanced predissociation by the interaction of vibrationally excited with neighboring solvent molecules. If this solvent interaction in addition to ring strain already present in the cyclic ketone both facilitate a-cleavage from the vibrationally excited state, relative to intersystem crossing to T, then it is not unreasonable to propose the cyclobutanone state as the precursor to some, if not all, of the observed photoproducts. 

From this state, ring strain facilitated predissociation to a "biradical-like" transition state [135] or vibrational relaxation (k ) to S may occur. It is also conceivable that transition state [135] could be produced directly from S °. Alternatively, molecules in the S ° state could intersystem cross (kST) to the triplet manifold (T ). For 2-alkylidenecyclobutanones, reactivity is manifested in isomerization about the exocyclic carbon-carbon double bond, while for the saturated cyclobutanone derivatives studied, definitive evidence for solution-phase reactivity is not available. If analogy is again made to the vapor-phase photochemistry of cyclobutanone [21], reactivity could conceivably result in decarbonylated products. Indeed, preliminary evidence has been obtained from sensitization experiments employing m-xylene as triplet sensitizer that decarbonylation of a saturated cyclobutanone is enhanced by selective population of its state (35). ... 

As can be seen from Table 11 all carbena-cyclopentadienes (2) add stereospecifically as singlets (S). The only exceptions are tetrachloro-cyclopentadienylidene (2g) and the fluorenyhdenes 2c, 2d). The nonstereospecific addition — which is of the same order as for diphenyl-carbene — in 2g is most probably due to a heavy-atom effect of chlorine favouring intersystem crossing to the triplet (T). An aromatic ring as in 2c and 2d (as well as Br substituents) also facilitates the S—T inter con version step. 

The synthesis, optical, and structural properties of another Pt(II) polyyne-containing biphenyl moiety, P12, were reported recently.44 The system has also been extended to the Au(I) and Hg(II) congeners (see Sections IV and V). The influence of the metal center on the spatial extent of S and T excited states was characterized in detail. The ligand-based phosphorescence emissions can be harvested by the heavy-atom effect of these transition metals, which facilitates efficient intersystem crossing from the S] state to the T] state. 

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