Triplet-State Isomerization in Retinal

The picture of triplet-excited region was slightly modified after it was determined spectroscopically in terms of stretching force constants that include the contributions of both the a and n bonds (vide infra). 

Cis-Trans Isomerization Examined by Electronic Absorption and Raman Spectroscopies and by High-Performance Liquid Chromatography Analysis 

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Scheme 3.2 A schematic presentation of triplet-state isomerization in retinal. The pathways of isomerization are shown by arrows, whose lengths are proportional to the values of quantum yield [8].

Dynamic Triplet-Excited Region in Retinal As Revealed by Deuteration Effects on the Quantum Yields of Isomerization via the T State (Okumura, Koyama, unpublished results)... 

Pulse radiolysis is used also for preparation of excited states of dienes and polyenes. This is done by irradiation of the diene/polyene in toluene solution. The radiolysis of toluene yield high concentration of molecules in the triplet excited state of the solute. Wilbrandt and coworkers61 pulse-radiolysed 1 mM solution of al I -lrans-1,3,5-heptatriene in toluene solution and observed the absorption spectra of the triplet state of the heptatriene with a maximum at 315 nm. The same group62 produced and measured the absorption spectra of several isomeric retinals in their lowest excited triplet state by pulse irradiation of their dilute solution in Ar-saturated benzene containing 10 2 M naphthalene. Nakabayashi and coworkers63 prepared the lowest triplet states of 1,3-cyclohexadiene,... 

Since so (trans) < 0.005], it is possible to rule out a photosensitized isomerization mechanism involving the establishment of thermal equilibrium between 11-cis and all-trans forms during the lifetime of the triplet state. Also excluded is the quantitative population of a common minimum along the C q-C 2 torsional coordinate. This conclusion is consistent with recent studies (169,175) which, in variance with the previous investigation (171), have reported different T-T spectra upon excitation of all-trans and 11-cis retinal. 

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. 

Hie isomerization of 31 proceeds by a quantum chain process for several directions [125,126,130]. For example, all-trans retinal undergoes isomerization giving the 9-cis and 13-ds isomers in the excited triplet state the quantum yield of isomerization of the all-trans isomer increased from 0.13 at 1.1 X 10 M to... 

Triplet quantum yields for all-trans- and 11- -retinal have been established. The fact that triplet-energy transfer to - -retinal leads to geometric isomerization with 15—17% efficiency, whereas triplet sensitization of the all-lw/ts-isomer does not lead to isomerization, may be the result of an isomerization process which occurs only from vibrationally excited triplets also, in the case of 11- -isomer, relatively more energy is accumulated in the isomerization active torsional modes of the 11—12 bond.214 Related studies of the triplet states of retinals and their Schiff bases have been reported.216... 

Ramamurthy and Liu [85] reported on the effect of chain length on the potential energy surfaces of triplet-sensitized isomerization of polyenes of retinal homologs. Thus dienes and trienes undergo unimolecular deactivation from the twisted triplet because the planar trans triplet and the twisted triplet are in equilibrium and are very close in energy. On the other hand, in tetraenes and higher polyenes, deactivation proceeds from the planar triplet states, which are much more stable than the twisted triplet states. 

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