Charge triplet state

Liddell P A, Kuciauskas D, Sumida J P, Nash B, Nguyen D, Moore A L, Moore T A and Gust D 1997 Photoinduced charge separation and charge recombination to a triplet state in a carotene-porphyrin-fullerene triad J. Am. Chem. Soc. 119 1400-5... 

The Stale Correlation Diagram (SCD) approach introduced by Shaik and Pross96 appears similar in some respects. However, the LUMO, HOMO and the first two exciled stales are considered, (refer Figure 1.5)4,53 Thus, if we consider the interaction of the radical with the olefin in its ground (singlet) state (R + C=C ) and excited (triplet) state (R + C=CJ) and two charge transfer... 

A large red shift observed in polar solvents was indicative of the intramolecular charge transfer character of the triplet state. The change of dipole moment accompanying the transition Tj - Tn, as well as rate constants for electron and proton transfer reactions involving the T state of a-nitronaphthalene, were determined. The lower reactivity in polar solvents was attributed to a reduced n-n and increased charge transfer character of the triplet state... 

To obtain more information on this point, let us examine the data given in Table 3.6<42-47> for some substituted benzophenones. The data in Table 3.6 indicate that benzophenone derivatives having lowest triplet states of n->TT character undergo very efficient photoreduction in isopropyl alcohol. Those derivatives having a lowest it- -it triplet, on the other hand, are only poorly photoreduced, while those having lowest triplets of the charge-transfer type are the least reactive toward photoreduction. In additon, in some cases photoreduction is more efficient in the nonpolar solvent cyclohexane than in isopropanol. This arises from the solvent effect on the transition energies for -> , ir- , and CT transitions discussed in Chapter 1 (see also Table 3.7). 

Thus the change in the direction of the spin angular momentum of the electron effectively imparts some singlet character to a triplet state and, conversely, triplet character to a singlet state. This relaxes the spin selection rule since J S St dr is no longer strictly zero. The greater the nuclear charge,... 

Comparison of these experimental results with the calculated charge densities (S0 and Si) at the 2 and 3 positions (Table 11.5) shows that this is the expected result. Except for those compounds discussed below, the failure to observe quenching with triplet quenchers and reaction in the presence of a photosensitizer indicated singlet reactions. Compound (89) was found to also undergo benzophenone-photosensitized substitution, indicating that the triplet state of this compound is also reactive. The reaction, however, was less clean than that observed in the direct photolysis. Similarly, 1,6-dinitro-naphthalene was found to undergo both direct and benzophenone-photosensitized substitution ... 

The time course of the charge-separated intermediate I can be measured in a flash photolysis experiment that monitors the (I — A) transient absorbance difference at a ground state/triplet state isosbestic point (e.g., 432 nm for Mg, and 435 nm for Zn). We have observed this intermediate for the [M, Fe] hybrids with M = Mg, Zn representative kinetic progress curves are shown in Fig. 3 [7a]. In a kinetic scheme that includes Eqs. (1) and (2) as the only electron-transfer processes, when the I A step is slow (kb < kp) the intermediate builds up (exponentially) during the lifetime of A and exponentially disappears with rate constant kb (Fig. 4A). This behavior is not observed for the hybrids, where the I - A process is more rapid than A - I, with kb > kp. In this case, I appears exponentially at early times with rate-constant kb and is expected to disappear completely in synchrony with A in an exponential fall with rate-constant kp (Fig. 4B). 

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