Triplet energy dissipation

Conformational Changes and the Inversion of Spin-Polarization Identified by Low-Temperature Electron Paramagnetic Resonance Spectroscopy of the Reaction Center-Bound 15-cis-Spheroidene A Hypothetical Mechanism of Triplet-Energy Dissipation [19]... 

We found something totally unexpected, but it turned out that the observation was along the line of our hypothetical mechanism of triplet-energy dissipation that accompanies the conformational changes of the triplet carotenoid. Figure 3.22 shows the time-dependent changes in spin polarization, which can be characterized as follows (1) The time profile of a sum of the three components shows... 

The Structure of the RC-Bound Spheroidene in the T, State and a Possible Mechanism of Triplet-Energy Dissipation... 

Figure 6. Jablonski diagram for the excited-state proton transfer and energy dissipation in TIN kSo s0> ks,s,-, kT,Tl- rate constants of proton-transfer processes in the ground state, first excited singlet state, and triplet state, respectively, and k,j rate constants of radiationless deactivations and k,- rate constants of intersyslem...

Recently, it was shown that in deuterated tetraphenylporphin the fluorescence quantum yield and lifetime, and the triplet yield increase in a similar manner relative to the undeuterated one (24). Such a deuterium isotope effects on the state suggested the participation of Sq internal conversion in the energy dissipating... 

High quantum and chemical yields of ODPM rearrangements are obtained with substrates in which the ly-enone chromophore is part of a conformationally rigid molecular assembly which at the same time guarantees adequate orbital overlap of the C=C and rr-bonds. Whereas in bicyclic and bridged 3, y-unsaturated ketones these prerequisites are widely met, acyclic p,7-unsaturated ketones usually rearrange inefficiently since other channels of energy dissipation from the triplet state predominate. Exceptions to this rule are substrates in which the C= ir-bond is part of a styrene or an a, -enone moiety (Section 2.6.3.1). In this context it should also be noted that p,7-unsaturated aldehydes, except for one case, are ODPM unreactive (Section 2.6.3.2). 

The requirement that a molecule possess an appropriate triplet energy level implies the presence of a chromophore. All three of the compounds presented in Figure 12 are UV-B absorbers and have the possibility to dissipate excitation energy via an E/Z isomerization. This means they are capable of performing this highly efficient process and are not consumed in the process. Figure 12 illustrates the... 

It is well known from both early resonance Raman data (Lutz et al., 1976, 1982) as well as the crystal structures (Allen et al., 1988 Yeates et al., 1988 Amoux et al., 1989, 1995 Ermler et al., 1994a,b) that spheroidene occurs in the 15,15 -c/5 conformation in bacterial RCs. Since the l5-cis isomer has a strong tendency for triplet-sensitized all-trans conversion (Hashimoto and Koyama, 1988 Hashi-moto et al., 1989 Kuki et al., 1991 Koyama and Mukai, 1993) and because there appeared to be indications of cis-to-trans isomerization in the RC (Boucher and Gingras, 1984), a mechanism of energy dissipation was proposed that involved the twisting of the 15,15 -cis double bond into an all-trans position (Koyama et al., 1990 Koyama, 1991). To test this... 

It has been suggested (Koyama et al. 1990, Kuki et al. 1995) that cis carotenoid isomers, and specifically the l5-cis isomer, exists in the RC to enhance the ability of the carotenoid to dissipate triplet energy via cis-to-trans isomerization. Bautista et al. (1998) tested this mechanism using locked-15,15"-cw-spheroidene. By a comparison of the behavior of the locked-15,15"-c/x-spheroidene with the unlocked spheroidene incorporated into RCs, and also comparing these samples with wild type RCs, it was possible to determine whether cis-to-trans isomeri-... 

The problem of the free-rotor effect and the efficient dissipation of triplet energy is still of interest. Hixson and Tausta42 have found that the excited singlet state of the naphthyl olefin (53a), a typical substrate for a di-7r-methane reaction, undergoes cw-Zra/w-isomerization to (53b) and conversion into the cyclopropane (54). Continued irradiation leads to the formation of cyclopropane at the expense of as-olefin (53b). Sensitized irradiation gave very efficient... 

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