Nonvertical triplet excitation transfer

Nonvertical triplet excitation transfer was proposed by Hammond et al. based on their observation that several sensitizers ( S ) with triplet energies lower than that of ds-stilbene (57 kcalmoP ) underwent energy transfer to ds-stilbene with much higher rate constants than expected from the spectroscopic triplet energies [5,6]. Thus the endothermic exdtation transfer from S to c was supposed to produce not c but p accompanying torsion of the C=C double bond. 

The role of the central bond torsion and of the double bond and phenyl-vinyl torsions in nonvertical triplet excitation transfer to stilbenes was stressed [83]. The... 

Synthesized nonvertical triplet excitation transfer (NVET), attributed to large structural changes between ground and triplet states. 

Results of calculation of potential energy curves for twisting about the central bond in So and Ti of stilbene were reported (Figure 4.5) [22]. The origin of nonvertical triplet excitation transfer and the relative role of double-bond and phenyl-vinyl torsions in stilbenes were described. 

The role of central bond torsion in nonvertical triplet excitation transfer to stilbenes, an example of stilbene analogues, biindanylidenes, was recently discussed... 

An argument in favor of the intermediacy of triplet states in at least some of these reactions < an be based on the observation that cycloadditions do not appear to occur when vertical excitation transfer from the triplet state of the ketone to the olefin is energetically feasible. It has been found, for example, that ketones with triplet excitation energies sufficiently greater than those of the stilbenes or the 1,3-pentadienes, among several systems, are quenched with unit efficiency by the olefins and that no adducts are formed (Hammond el at., 1964a). The occurrence of Reaction (64) implies that cycloaddition can compete with nonvertical triplet excitation transfer. 

According to recent time-resolved triplet energy transfer experiments [224,225] with c/s-stilbene and 1,2-diphenylnorbornene (a model compound with inhibited double bond twisting), both compounds exhibit virtually the same nonvertical triplet excitation and relaxation behavior the respective relaxation energies are 63 and 42kJmol 1 [224], Single-bond (phenyl-... 

Figure 4.11 demonstrates a difference between experimental data for the energy transfer rate constants from donors to ds-stilbene and theoretically predicted vertical excitation of the stilbene acceptor. This difference appears to be a clear indication in favor of the nonvertical triplet-triplet energy transfer mechanism [46]. 

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-... 

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 ... 

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. 

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]. 

When Ej is in excess of 60 kcal/mol, -/ran -butadiene Ej = 60 kcal/ mol), which strongly predominates in the thermal equilibrium, is excited and produces mainly divinylcyclobutanes. When the of the sensitizer is not high enough to excite 5-tra/t5-butadiene, energy transfer to 5-m-butadiene (Ef = 54 kcal/mol) occurs instead, yielding vinylcyclohexene. If Ej < 50 kcal/mol, nonvertical energy transfer to a twisted diene triplet is believed to occur (Liu etal., I%5). 

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