Charge-separated radical pairs

The total quantum yield [4>cs(total)] for CS is decreased to 0.17 in dimethyl-formamide (DMF) due to the competition of the CSH from Fc-ZnP-H2F+-C6o (1.63 eV) to Fc-ZnP- -HzP-Cso (1.34 eV) versus the decay of Fc-ZnP-Fl2P -C6o to the triplet states of the freebase porphyrin (1.40 eV) and the Ceo (1.50 eV) [47]. In contrast to the case of most donor-acceptor-linked systems, the decay dynamics of the charge-separated radical pair (Fc -ZnP-H2P-C6o ) does not obey first-order kinetics, but, instead, obeys second-order kinetics [47]. This indicates that the mframolecular electron transfer in Fc -ZnP-H2P-C6o" is too slow to compete with the diffusion-limited inter-molecular electron transfer in solution. 

Steady-state and time-resolved photolysis reveal that the fullerene singlet excited states in Ceo-TTF dyads are subject to rapid intramolecular electron-transfer events yielding a charge-separated radical pair, namely, C6o -TTF + [306, 314, 315], The gain of aromaticity, as a result of the formation of the TTF radical cation, is a promising concept to increase the stabilization of the charge-separated state. In fact, the lifetime of the charge-separated state in Ceo-TTF dyads was increased by a factor of relative to comparable systems that do not contain TTF (e.g., ferrocene, aniline, etc.) [306, 314]. A radical pair lifetime of ca 2 ns was observed for the closely spaced Ceo-TTF dyad (3), while much shorter lifetimes of 0.526 ns, 0.045 ns and 0.294 ns are reported for similarly spaced Carotene-Ceo, ZnP-Ceo (22) and H2P-C60 dyads (23) (see Section 5.5.3), respectively. 

As a leading example for short-spaced dyads, a n-n stacked porphyrin-fullerene dyad (ZnP-Ceo) 21 should be mentioned, which was probed in light of their electron transfer and back electron transfer dynamics [361, 362], The close van der Waals contact ( 3.0A) is responsible for pronounced electronic interactions in the ground state between the two 7t-chromophores. For example, the ZnP Soret-and Q-bands in the n-n stacked dyad 21 show a bathochromic shift and lower extinction coefficients compared to free ZnP [361], In the n-n stacked dyad 21 the linkage of the two bridging units occurs in the trans-2 position at the fullerene. A charge-separated radical pair evolves from a rapid intramolecular electron transfer k 35 ps) between the photoexcited metalloporphyrin and the fullerene core in a variety of solvents (i.e., ranging from toluene to benzonitrile). Remarkably, the lifetimes in tetrahydrofuran (t = 385 ps) and DCM (t = 122 ps) are markedly increased relative to the more polar solvents dichloromethane (r = 61 ps) and benzonitrile (t = 38 ps) [362]. This dependency prompts to an important conclusion ... 

The following summarizes recent studies, whose aim is to retard the fast back electron transfer, commonly observed in supermolecular systems [386-389]. In this context, a reversible coordination of the acceptor moiety (ligand or substrate) to the donor (coordination center or receptor) rather than a covalent linkage was pursued [386-389]. This was expected to enable the diffusional splitting of the charge-separated radical pair after the initial electron transfer takes place. 

The spin properties of charge-separated ion pairs can also be exploited for the purposes of all optical switching. Radical pair intersystem crossing (RP-ISC) of the form [D+ -A- ] D+ -A- ] to yield the spin-correlated triplet state is observed in... 

The inhomogeneity of the micellar aggregate also affords assisted spin trapping and the exploitation of magnetic field effects on the charge separated ion pairs [48]. Optical modulation spectroscopy can be used, for example, to follow the decay of radicals formed in homogeneous solution and in SDS micelles. Enhancements of a factor of about 50 in the lifetimes and the steady state concentrations of the radical were observed in the micelle, and a kinetic analysis led to a value of 2 x 103 s 1 for the exit rate constant from the micelle [49]. 

In the present case both radical centers are C atoms, so the dot-dot states are clearly lower in energy than the charge-separated hole-pair ones. Since the electron count gives 3cr, 3 r for the dot-dot and 4o, In for the hole-pair states, correlation lines can be drawn immediately as shown in Figure 6.27. Thus, a diagram is obtained that is basically identical to the results represented in Figures 4.18 and 6.21 except for the barriers revealed by natural orbital correlations. 

The dynamics of photoinduced charge separation, kcs, and charge recombination, kcr (Fig. 2a), have been studied in several families of hairpins containing an Sa linker and a single G C base pair by means of femtosecond time-resolved transient absorption spectroscopy [27, 28]. Both the singlet state and anion radical of Sa have strong transient absorption centered at 575 nm. The difference in the independently determined band shapes for Sa ... 

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

DPB as well as other DPP molecules (t-stilbene, diphenyl-hexatriene) with relatively low ionization potential (7.4-7.8 eV) and low vapor pressure was successfully incorporated in the straight channel of acidic ZSM-5 zeolite. DPP lies in the intersection of straight channel and zigzag channel in the vicinity of proton in close proximity of Al framework atom. The mere exposure of DPP powder to Bronsted acidic ZSM-5 crystallites under dry and inert atmosphere induced a sequence of reactions that takes place during more than 1 year to reach a stable system which is characterized by the molecule in its neutral form adsorbed in the channel zeolite. Spontaneous ionization that is first observed is followed by the radical cation recombination according to two paths. The characterization of this phenomenon shows that the ejected electron is localized near the Al framework atom. The reversibility of the spontaneous ionization is highlighted by the recombination of the radical cation or the electron-hole pair. The availability of the ejected electron shows that ionization does not proceed as a simple oxidation but stands for a real charge separated state. 

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