Ion pair states

Another important factor to determine the charge separation efficiency is the distance between and the mutual orientation of the donor and the acceptor in the geminate ion-pair state. The rate of charge recombination depends on whether... 

The reaction exothermicities ( —AG°) for forward and back ET in polar media were approximately estimated to be 1.39 and 2.18 eV, respectively [120], Since the back ET is highly exothermic, the relatively small kb-1 values for the compartmentalized system may be ascribed to the combined effect of the inverted region [97-99] and the loose ion-pair state. 

The participation of such ionic states would load one to expect disproportionation rates to increase in the series CH8, C2H5J iso-C8H7, and ferf-butyl, since the ionization potentials of these radicals form a descending series 10.0 e.v., 8.8 e.v., 7.9 e.v., and 6.9 e.v., respectively. This effect of decreasing ionization potential, which might be expected to push the ion-pair state down in energy, considerably below the radical pair state, is considerably off-set by the increasing value of rt (eqs. 5 and 6). ... 

H. J. Neusser In reply to Prof. Woste let me mention that a situation similar to the high Rydberg states I discussed has been proposed for a dipolar bound electron in molecular anions. Here large distances of the electron and small binding energies are expected. First experimental indications were found. Ion pair states represent another interesting example. 

T. P. Softley With regard to the question by Prof. Woste, I would like to add that ion pair states of many small molecules are known experimentally. For example H2 has been excited to H+ H states very close to the dissociation limit by S. Pratt and co-workers [1], and then these states are dissociated by small pulsed electric fields. The difficulty in all such experiments is in accessing the very large inter-nuclear distances necessary to get close to the transition state. ... 

By means of time-resolved fluorescence studies we were able to determine the C60 fluorescence deactivation rates, as 2.1 x 1010 s-1 in 9a, 6.6 x 109 s 1 in 9b and 1.3 x 109 s-1 in 9c. Importantly, the indulging trend resembles the relationship between the quantum yields of the conjugates (9a-d) and reference (1). In short, an intensified excited-state deactivation emerges with decreasing bridge length. However, no measurable decay rates were found for the trimer 9d. Conclusively, the indirect or direct population of Cgo possibly leads to an exothermic electron-transfer reaction, resulting in the radical-ion-pair state ... 

Conclusively, in the presence of a strong electron donor, namely exTTl, the resulting product of the photoexcitation is not the triplet excited state of C60 but the energetically lower-lying radical-ion-pair state, exTTF +-oPPE -C6o Here, it should be stressed that the spectral identification of the radical ion pair holds for... 

Fig. 9.7 Time-absorption profile of the spectra shown above at 1010 nm to monitor the formation of the radical-ion-pair state (blue = monomer 9b, light blue = dimer 9c)...

Indisputably, photoexcitation is followed by a rapid deactivation of the singlet-excited state of the oPPE moiety resulting in the generation of a charge-separated species, i.e. the radical-ion-pair state exTTF +-oPPE -C o, which is apparently lower in energy than the corresponding triplet state of C o- The radical ion pairs decay on the ps time-scale with charge-recombination rates that prove wire-like... 

In fact, it was possible to prove the formation of the radical ion pair state by transient absorption spectroscopy. Particularly, at the expense of the vanishing H2P/ZnP singlet absorption new features with maxima in the 600-700 nm range as well as at 480 nm grow in. These maxima correspond to the one-electron oxidized 7t-radical cations of H2P (H2P +) and ZnP (ZnP +). Additionally, in the near-infrared region the spectral signatures of the one-electron reduced anion of Ceo are discernible at 1000 nm (Fig. 9.23). 

Kinetic analyses of the formation of the radical ion pair state—formed through bond in 17a and through space in 17b, 17c, and 17d—revealed that the latter are meta-stable on the femto-/picosecond time scale. Hence, charge recombination... 

Fig. 9.31 a Differential absorption spectra (visible and near-infrared) obtained upon nanosecond flash photolysis (355 nm) of 17c (2.0 x 10 6 M) in nitrogen-saturated oDCB solutions with a time delay of 100 ns at room temperature, indicating the radical ion pair state features at 680 and 1010 nm. b Time-absorption profiles of the spectra shown above at 1010 nm to monitor the decay of the radical ion pair state... 

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