Nonvertical excitation

As the triplet energy of the sensitizer becomes less than that necessary to excite either form of the butadiene (Et < 50 kcal/mole), it is proposed that energy is transferred via nonvertical excitation to lower energy twist forms of the diene triplets.(11> The product distribution again reflects the ground state population of s-cis and s-trans forms ... 

Describe the structure of the vertical and nonvertical excited states of alkenes and show how the interconversion and deactivation of these states leads to stereochemical isomerisation. 

The relaxed nonvertical excited state is referred to as the p-state as it has adjacent p-orbitals which are orthogonal due to a 90° angle of twist from the geometry of the vertical excited state. Figure 8.1 shows that the same p-state is produced from both geometrical isomers, and similarly, rapid radiationless decay of this p-state can produce either the cis or the trans isomer. 

Figure 8.1 Energy diagram showing the vertical and nonvertical excited singlet states of an alkene...

This ratio is called the photostationary state composition. In the photostationary state, the rate of formation of each isomer from the nonvertical excited state is equal to its rate of removal by absorption of light. There is a roughly equal probability of the relaxation of the nonvertical excited state forming either the cis or the trans isomer and so the main factor influencing the photostationary state composition is the competition for absorbing light. This will, of course, depend on the relative values of the molar absorption coefficients of the two isomers at the particular wavelength chosen. 

The early discussion of the role of nuclear reorganization in energy transfer, especially among classical photochemists, centered on the concept of nonvertical excitation transfer . While many of the conclusions fully agree with the spirit of the Marcus-Jortner approach, the term itself is rather unfortunate as it suggests that the process may be violating the Franck-Condon principle and the Bom-Oppenheimer approximation. 

The above discussion is applied to most of the arylethenes. The importance of the single-bond rotation was argued from the view point of nonvertical excitation transfer from the low-energy sensitizer to the cw-stilbene [33-36,39, 112,113]. In the case of 8a, even for the trans Isomer the dihedral angle between the pyrenyl plane and the double bond is estimated to be about 30° due to the steric effect of the hydrogen atom at the peri position. In contrast, the dihedral... 

The increased trans content in the photostationary state below 209kJmol 1 may be due to an almost unchanged rate of transfer to cis-stilbene and/or to an unusually rapid decrease of the rate of transfer to trans-stilbene. To account for this observation Hammond has put forward the concept of nonvertical excitation of c/s-stilbene by low-energy sensitizers directly into the nonplanar twisted triplet state [2, 25, 211], However, certain objections to this interpretation have been raised [212,213] From the expected different overlap of the long-wavelength tails of the S0 -> T transition bands in the stilbenes with the bands of low-energy sensitizers, Yamauchi and Azumi [213] have inferred that the transfer rate indeed decreases more rapidly for the trans isomer than for the cis. A modified model has been given by Saltiel et al. [11]. 

The fact that Reactions (26) and (29) can be induced by excitation transfer from donors with relatively low triplet excitation energies probably implies that nonvertical excitation transfer is involved. A similar excita-... 

Absorption of a photon by an alkene produces a (tt,Jt ) vertical (Franck-Condon) excited state in which the geometry of the ground state from which it was formed is retained. Since the (it,it ) state has no net n bonding, there is little barrier to free rotation about the former double bond. Thus, relaxation takes place rapidly, giving a nonvertical (it,it ) state with a lower energy and different geometry to the vertical excited state. 

Photostationary cis/trans ratios vary with sensitizer structure in a manner not yet perfectly understood, and both the time required for attainment of a photostationary state and actual flash spectroscopic quenching rates indicate that energy transfer to diphenylcyclopropane is quite inefficient. Such should be the case if nonvertical energy transfer with the production of a biradical is occurring. When care is taken so that only the sensitizer absorbs light, only cis-trans isomerization is observed. Direct excitation of the cyclopropane produces 1,3-diphenyl-propene and 1-phenylindane as well.298... 

Hammond, Saltiel and co-workers subsequently observed that the rate constant of energy transfer becomes slower than that predicted by Equation 2.47 when the structure of the relaxed excited triplet state of the acceptor differs substantially from that in ground state, as is the case for stilbene.112 These findings initiated extensive investigations on the twisted triplet state geometry of stilbene and related compounds (Section 5.5). The expression nonvertical energy transfer (NVET) was coined to describe this situation. 

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