Twist triplet

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

Irradiation of the cis isomer results in the isomerization with quantum yield, equal to 0.56. The fact that the quantum yields of cis and trans formation are similar and add to unity suggests the presence of a common intermediate, thought to be a twisted triplet state. Gas-phase photolysis of trans-... 

A possible kinetic scheme for this system is shown below, where A-T and A-C are the trans and cis enones, superscripts s and t refer to singlet and triplet states, and A-P corresponds to the common triplet intermediate (twisted triplet) produced after triplet energy transfer ... 

The value of the pss calculated from Eq. (9.24) is 2.74 and the measured value<7> is 2.64. While this agreement could be taken as evidence in favor of a triplet mechanism, it could also mean that the twist singlet and twist triplet have similar geometries (see Figure 9.2), and therefore similar decay ratios. In this mechanism we have neglected fluorescence from fraws-stilbene and dihydrophenanthrene formation from cw-stilbene. (See Problem 4.)... 

On the other hand, reactions in which the return to So occurs from a "non-spectroscopic minimum (Fig. 3, path g) are probably the most common kind. The return is virtually always non-radiativef). This may be the very first minimum in Si (Ti) reached, e.g., the twisted triplet ethylene, or the molecule may have already landed in and again escaped out of a series of minima (Fig. 3, sequence c, e). For instance, triplet excitation of trans-stilbene 70,81-83) gives a relatively long-lived trans-stilbene triplet corresponding to a first spectroscopic minimum in Ti. This is followed by escape to the non-spectroscopic , short-lived phantom twisted stilbene triplet, corresponding to a second and last minimum in Ti. This escape is responsible for the still relatively short lifetime of the planar nn triplet compared to nn triplet of, say, naphthalene. A jump to nearby So and return to So minima at cis- and trans-stilbene geometries complete the photochemical process ). 

The above considerations lead us naturally to the question of the nature of the one or more intermediate states involved directly in the photochemical reaction. We have displayed in Figure 27 the simplest scheme consistent with the above, admittedly preliminary, experimental results. The prime superscript refers to the frans-isomer, while the processes denoted 1, 2, 3,4, and 5 describe fluorescence, Sx S0 internal conversion, Si -> X intersystem crossing, isomerization, and deactivation to the ground state, respectively. Again, X and X may represent a common state, as, for example, is the case with the twisted triplet in ethylene (cf. Mulliken and Roothaan190 and Kaldor and Shavitt191). As late as 1962, investigators were still unable to determine the source of the temperature... 

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. 

All other matrix elements of the perturbations, Vso and Fvlb, between the zero-order states, are taken to be zero. That is, we neglect the direct coupling of the singlet with the manifold of twisted triplets, and assume that all the zero-order states have been chosen such that there are no nonvanishing diagonal matrix elements of Fao or Fvlb, nor any vibronic interactions within the set of closely-spaced states, />. ... 

Now, in the statistical limit for which we can treat our final manifold of closely spaced twisted triplets as an effective continuum, the sum in eq. (12-24) can be replaced by a definite integral whose weight function is taken to be the density of vibronic states, p(E). Thus we write... 

Thus eq. (12-40) reduces to eq. (12-39) in the particular limiting case described by (A/rr)2 1 as, of course, it must. In the same appendix, an approximate solution is obtained for the time development of a model system in which the initial state is coupled to the quasicontinuum of the twisted triplet through more than one discrete level in the higher-lying triplet. 

The stilbenes absorb light strongly, and their excited singlets cross over to triplets fairly efficiently, from which state photoisomerization occurs. The lowest energy T - S0 absorption bands measured for cis-and rnm-stilbene are at 57 and 48-50 kcal, respectively. However, it is quite likely again that both spectroscopically formed triplets relax very rapidly to a common twisted triplet which decays to both geometric isomers. 

The energetics and dynamics of the triplet surface remain subject to controversy. Saltiel et al. (21) have recently proposed that triplet energy transfer to either t-1 or c-1 yields a common twisted triplet 3p which is more stable than 3t by 2.1 kcal/mole and thus predominates at equilibrium. Other workers have favored a more balanced 3p 3p equilibrium (22,... 

Dimethyl-l-butenyl)fluoranethene 94a undergoes a Z-to-E one-way isomerization [231], but 8-styrylfuruoranthene 94b affords a Z/E mixture upon camphorquinone sensitization in benzene (Sch. 38) [229], Interestingly, the (Z/E)pss ratio in the latter case is concentration dependent, varying from 57 43 at 0.5mM to 18 82 at 8.5mM, for which the equilibrium between ( )-triplet and twisted triplet species is thought to be responsible. 

For M2H4 the given data are for the trans-bent C2h dimetallenes (46) for M2F4 the data are for the twisted triplet ( lit M2F4 in C2 symmetry. Trans-beat C2h M2F4 are not minima for all M. 

The same factors stabilize the twisted triplet which can be visualized as having transferred one of the p electrons on Cl in 13 c, with spin inversion, to a ct orbital which bonds the two lithium atoms. 

Schuster et al. have isolated four products (78)-(81) from irradiation in acetonitrile at ambient temperatures. In a detailed study the mechanism for the addition was discussed in terms of intervention of planar and twisted triplet states of the... 

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. 

Flexible acyclic di-ji-methane systems tend to undergo rearrangement from the lowest singlet state, whereas the triplet state undergoes radiationless decay, because the twisted triplet is efficiently converted to the ground state, often accompanied by E Z isomerization (Scheme 6.30)654... 

Intermediate for deactivation Twisted triplet ( p ) Trans triplet (H )... 

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]. 

Other quenchers that have been used in the benzophenone-sensitized cis trans isomerization of stilbene are /9-carotene, oxygen, and di-tert-butyl nitroxide. /9-Carotene shifts the photostationary state to the trans side similar to the azulene effect [237]. In the presence of di-tert-butyl nitroxide, a radical quencher, the photostationary state is slightly shifted to the cis side Caldwell and Schwerzel [226] have suggested the involvement of the twisted triplet state and a quenching mechanism other than energy transfer, probably vibrational relaxation to the ground state caused by spin exchange. 

T tt tp Ks Ky a p Quantum yield of population of the observed triplet Singlet lifetime Triplet lifetime (from T-T absorption) Phosphorescence lifetime Stern-Volmer constant for fluorescence quenching Stern-Volmer constant for triplet quenching Fraction of twisted triplet decaying to the trans form Fraction of twisted excited singlet decaying to the tram form... 

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