Recombination donor/acceptor mediated

Acceptor and donor related processes involve interfacial electron transfer from free/trapped charge carries, including back reactions 3b and 3d [22, 27]. These latter processes are essentially donor/acceptor mediated recombination reactions. 

For example, in 1963 the photochemistry of magnesium phthalocyanine with coordinated uranium cations was studied in pyridine and ethanol and indicated the occurrence of PET to the uranium complex . A rapid photoinduced electron transfer (2-20 ps) followed by an ultrafast charge recombination was shown for various zinc and magnesium porphyrins linked to a platinum terpyridine acetylide complex . The results indicated the electronic interactions between the porphyrin subunit and the platinum complex, and underscored the potential of the linking para-phenylene bisacetylene bridge to mediate a rapid electron transfer over a long donor-acceptor distance. 

Paddon-Row MN (2003) Superexchange-mediated charge separation and charge recombination in covalently linked donor-bridge-acceptor systems. Aust J Chem 56 729-748... 

Surface related properties are carrier trapping on intrinsic (due to surface dangling bonds) and extrinsic (related to adsorbates, including donor and acceptor) surface states, carrier recombination mediated by surface states [26], and mass transfer of acceptor and donor and products from/to bulk solution. 

The overall process performance, as measured by photon efficiency (number of incident photon per molecule reacted, like the incident photon to current conversion efficiency, or IPCE, for PV cells), depends on the chain from the light absorption to acceptor/donor reduction/oxidation, and results from the relative kinetic of the recombination processes and interfacial electron transfer [23, 28]. Essentially, control over the rate of carrier crossing the interface, relative to the rates at which carriers recombine, is fundamental in obtaining the control over the efficiency of a photocatalyst. To suppress bulk- and surface-mediated recombination processes an efficient separation mechanism of the photogenerated carrier should be active. 

In a second paper, Sanders and coworkers presented a simple model mediated by pyridine ligation to zinc porphyrin. " In the donor-spacer-acceptor system, photoinduced ET occurs with a rate con.stant of 2.13 x I0 s for zinc mesoporphyrin II dimethyl ester 141 and 53.3 X lO s" for zinc tetraphenylporphyrin, while the recombination rate constants were 6.35 x 10 s and 3.81 X 10 s , respectively. Although the two porphyrins have similar redox potentials, the forward ET rates were unambiguously different. The authors speculated that the two photodonor porphyrins have different solvent reorganization energies. 

Transient absorption spectroscopy has been used to study isolated Photosystem 2 (PS2) reaction centres stabilised by the use of anaerobic conditions. In the absence of added artificial electron donors and acceptors, the light induced electron transfer properties of the reaction centre are restricted to the formation of the radical pair P680+Pheophytin and charge recombination pathways from this state [1]. This charge recombination has been observed to produce a 23% yield of a chlorophyll triplet state [1]. Attempts to reconstitute these particles with quinone have until now been limited to the observation of a steady state, quinone-mediated photoreduction of the cytochrome b-559 [2]. 

The research groups of Lewis and Wasielewski estimated the rate constants of the charge separation (kcs) and recombination (kcR) between the nucleobase, which acts as the electron donor, and the electron acceptor at the loop position of the DNA hairpins, and also investigated the free energy dependence of the electron transfer rate. " It was found that the single-step electron transfer in DNA mediated by nucleobases can be described by the Marcus theory (4) developed for nonadiabatic electron transfer system. 

Osuka and co-workers (Osuka et al, 1995) have compared polyynes (9) and polyenes (10) as mediators for electronic interaction between porphyrin electron donors and acceptors. Transient absorption spectra were recorded to confirm ET by the observation of the charge-separated state (ZnP ). However, the charge-separated state was difficult to detect and fast charge recombination was discussed as one possible explanation. The ET is approximately two times more efficiently mediated by polyene-bridges than polyyne-bridges. The attenuation factor P (Eq. 12) was found to be 0.08 and 0.1 for the polyene and polyyne series, respectively. The conclusion drawn from the small attenuation factor is that the electronic coupling between the donor and acceptor is efficiently... 

The efficiency of a DSSC in the process for energy conversion depends on the relative energy levels and the kinetics of electron transfer processes at the sensitized semiconductorlelectrolyte interface. For efficient operation, the rate of electron injection (Fig. 10.1, equation 10.2) must be faster than the decay of the dye excited state. Also, the rate of re-reduction of the oxidized sensitizer (or dye cation) by the electron donor in the electrolyte (equation 10.4) must be faster than the rate of back reaction (recombination) of the injected electrons with the dye cation (equation 10.3), as well as the rate of reaction of injected electrons with the electron acceptor in the electrolyte (equation 10.6). This reaction, also called dark current , is the main loss mechanism for the DSSC. Finally, the kinetics of the reaction at the counter-electrode must also guarantee the fast regeneration of the charge mediator (equation 10.5), or this reaction could also become rate limiting in the overall cell performance. ... 

Figure 3.7 Schematic illustration of the relationship between structure and interfacial electron transfer properties in dye-sensitized nanostructured semiconductor materials. Control of the balance between electron injection, recombination and transport depends crucially on the physical separation and electronic coupling capabilities between the molecular donor and semiconductor acceptor states mediated by designated anchor and spacer groups.

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