Photoexcited triplet states

Electron Paramagnetic Resonance Study of Dimerization Effects on Porphyrins in the Photoexcited Triplet State... 

An EPR study is made of the effect of dimerization on the photoexcited triplet state of a series of porphyrins. The systems of interest are tetra(benzo-15crown-5)porphyrin (TCP), tetra(4 sulfonatophenyl)porphyrin (TPPS), tetra(4-trimethyl-ammoniumphenyl)porphyrin (TTAP), and Zn-substituted derivatives of these porphyrins. Dimerization of TCP and ZnTCP can be induced by addition of cations. 

The study of these face-to-face dimers is concerned with the question of whether or not dimerization effects on zfs and kinetic parameters of the photoexcited triplet state can be related to dimer structure. 

The results of EPR studies of photoezcited triplets of model systems show that it is not possible to give generally applicable rules for the interpretation of the spectroscopic data. In a number of cases there appears to be a well-understood relationship between dimerization effects and dimer geometry. In most of the systems considered here that is not the case. It is not clear to what difference in make-up of the dimers this discrepancy must be attributed and this is an interesting point of further investigation. Evidently, as long as the data on fairly well characterized model systems are not fully understood it will be impossible to derive definitive conclusions concerning the structure of the special pair from data on its photoexcited triplet state. 

The addition of the azepine to the photoexcited triplet state of C6o is suggested to be involved in the reaction. A diradical intermediate as shown in Fig. 33... 

The photogeneration of OH by DOM is most likely a consequence of water oxidation by photoexcited triplet states (Equations 17.20 and 17.37) [8]. 

Fig. 5 Spectrum of photoexcited triplet states in C60 in glassy toluene...

The photochemist is rather familiar with the photoexcited triplet states and the associated intersystem crossing processes. It is well documented that the photoexcited triplet state plays an important role in organic photochemistry. It is thus conceivable that the electron spin polarization of the photoexcited triplet can be further transferred to a radical pair formed by the reactions of the triplet with a suitable substrate. Such a photoexcited triplet mechanism was first proposed by Wong and Wan in 1972 (135) to account for the "initial polarization" observed in the naphthosemiquinone radical formed in the photoreduction of the parent quinone in isopropanol. It was further considered that the triplet mechanism might also lead to CIDNP if such initially polarized radicals react rapidly to give products with nuclear spin polarization induced via the Overhauser mechanism. 

We are now in a position to consider the lifetime of a RP. For simplicity, we will consider a RP born in a triplet state. Figure 8.5 shows the formation and subsequent reaction processes of a RP born from a photoexcited triplet state. 

FIGURE 8.5 The life cycle of a geminate RP formed from a photoexcited triplet state. 

Although quantitative measurements of spin concentrations are very important in kinetic studies, the determination of this absolute parameter by ESR remains most difficult. Nevertheless, this should not discourage a photochemist who is accustomed to similar problems, such as measurements of photoexcited triplet-state concentrations in flash photolysis. 

Fine-Structure Constants (D ) and Degree of Charge-Transfer Characters (% CT) Obtained with Photoexcited Triplet States of Molecular Complexes3... 

The role of the triplet state in biological and biochemical systems continues to receive wide attention. Photoexcited triplet states of prophins and derivatives (389), prophyrins (390), aromatic amino acids (390), aromatic amines (391), and monoanionic thymine (392) have been observed at 77°K. Shiga and Piette (393) confirmed the interpretation of ESR data on triplet-state excitation in proteins by a simultaneous phosphorescence study. 

Wagnert L, Rubin R, Berg A, Mahammed A, Gross Z, Levanon H (2010) Photoexcited triplet state properties of brominated and non-brominated Ga(III)-corroles as studied by time-resolved electron paramagnetic resonance. J Phys Chem B 114 14303-14308... 

Bennati M, Grupp A, Mehring M and Bauerle P (1996a) Pulsed EPR spectroscopy of the photoexcited triplet states of thiophene oligomers in frozen solution. J Phys Chem 100 2849-2853... 

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