Hole acceptors

The dynamics of inter- vs intrastrand hole transport has also been the subject of several theoretical investigations. Bixon and Jortner [38] initially estimated a penalty factor of ca. 1/30 for interstrand vs intrastrand G to G hole transport via a single intervening A T base pair, based on the matrix elements computed by Voityuk et al. [56]. A more recent analysis by Jortner et al. [50] of strand cleavage results reported by Barton et al. [45] led to the proposal that the penalty factor depends on strand polarity, with a factor of 1/3 found for a 5 -GAC(G) sequence and 1/40 for a 3 -GAC(G) sequence (interstrand hole acceptor in parentheses). The origin of this penalty is the reduced electronic coupling between bases in complementary strands. 

Electronic states of DNA fragments where the electron hole is located on a pyrimidine base are of high energy because cytosine and thymine are relatively weak hole acceptors [7, 51]. Nevertheless, the electronic coupling between nucleobases within (GC) and (AT) pairs may be of interest when one compares different CT paths in DNA using the superexchange model [14]. In Table 5, we present results of the electronic coupling within the base pairs (GC) and (AT) [41]. 

That rate-limiting step in oxygen production may be (1) absorption of a facile hole-acceptor species, (2) hole transfer to an absorbed or electrolyte species, or (3) desorption of oxidized products. These steps must compete with bulk and surface recombination processes. Williams and Nozik (39) have shown that... 

It should be noted that while UPS could not detect an adsorbed hydroxide species with surface coverage less than 10% of a monolayer, such a species could be chemically active. The same hole-acceptor could be present at both the gas-solid and liquid-solid interface, but the rate of its formation may be inadequate to compete with oxygen diffusion into the bulk from the gas-solid interface. MUnuera (AO) has found that measurable rates of restoration of a hydroxyl species linked to the photoactivity of Ti02 powders for oxygen desorption require treatments harsher than immersion in liquid water. 

Using the S04-- radical to form G + with the help of the pulse radiolysis technique and tethered pyrene as hole acceptor, it has been shown that the apparent transfer rates follow the order G-+TG < G-+AC < G-+AG (Takada et al. 2003b). This means that A is a better medium for hole transfer than T. 

Broadly speaking, dynamic electron transfer involves two steps. The first step is the diffusion controlled formation of an encounter complex between the electron/hole acceptor molecule and the particle. The second step is the electrochemical interfacial charge-transfer, which may be characterized by a rate constant kct. Albery et al. [143] and Gratzel et al. [129] have independently arrived at the same result, relating the observed effective bimolecular rate constant for hole/electron acceptor oxidation/re-duction, /cobs (m3mor s I) to reactant diffusion and A ct. 

Using much smaller semiconductor particles (d dsc), the photogenerated electrons and holes can be easily transferred to the surface and react with the electron and hole acceptors, provided that the energetic requirements are fulfilled. The average transit time % within a particle of diameter d can be obtained by solving Fick s diffusion law as... 

An entirely different consequence of electron (or hole) transfer to addends is provided by semiconductor-sensitized decomposition of electron acceptors A-X (or electron donors, i.e., hole acceptors, D-Y) [17]. If organic molecules whose redox states (anion radical or cation radical) are unstable with respect to scission into a free radical and an ion [Eqs. (5,6)] are adsorbed onto a wide band gap semiconductor such as Ti02, etc., then back electron (hole) transfer can be inhibited, if the scission process is rapid. 

Fe(phen)3] to the solution, the standard potential of which is located very close to the valence band of WSc2, the energy bands remain pinned to their dark values [78], i.e. holes created by light excitation are efficiently transferred from the valence band to the hole acceptor in the solution. In the absence of a redox system, typical photocurrent transients have been observed in the range Ufb(dark) and Ufb(hv). Details of the charge transfer mechanism will be given in Sect. 4.3. 

In a series of papers, Cai and co-workers presented an explication of electron and hole transfer in /-irradiated DNA that clarified some confusion and some inconsistencies that existed in the literature at the time7 ° Using ESR at 77 K, Cai co-workers analyzed hole and excess electron transfer by observing, simultaneously, the concentration of both the electron/hole donor and the electron/hole acceptor as a function of time, thereby verifying that the underlying transfer occurs in the specific manner assumed for the mathematical analysis used. Because the analysis for excess electrons is identical to that for holes, we present both analyses here. 

The transfer distance with time, D, and the value of the tunneling decay constant, /3 were determined in the rate constant expression for tunneling, k = kge °t. Table 1 shows the results for these studies for both hole and electron transfer with MX as both the electron and the hole acceptor. 

Figure 7. Schematic energy level diagram showing the principle of the ionization method for detecting electron transfer in gas-phase adducts. Naphthalene cation (the hole donor) is formed by resonance-enhanced two-photon ionization of the neutral. A hole acceptor, whose ionization potential is lower than that of naphthalene, is not ionized, since its S level is not resonant with the UV photons used (vi). The vibrational levels of the ionic form of the acceptor are resonant with the naphthalene cation, and accept the hole easily. Detection is by photodissociation, using photons of different frequency (V2) that dissociate the naphthalene cation in a resonance-enhanced multiphoton absorption process. Charge transfer is detected by the diminution of the product ion signal in the presence of a suitable acceptor. Adapted from Ref. [32].

Fig. 9.11 Transient absorption spectrum of trapped electrons in 6.3 X 10 M TiOi colloids in solutions containing 5 x 10" M polyvinyl alcohol (hole acceptor) at pH 10 after a laser flash. Insert time profile of the absorption. (After ref. [44])...

Fig. 9.13 Transient absorption vs. time observed upon laser excitation (Aex = 355 nm) in Ti02 colloids loaded with Pt at 500 nm, in the presence of various concentrations of dichroroacetate (DCA) (hole acceptor) pH 2 aqueous air saturated solution, 1.0 M colloidal Ti02/Pt (1 %) particles absorbed photon concentration per pulse, 1.6 x 10" M. (After ref. [45])...

When solutions of CdS colloids containing no additional electron and hole acceptor in the solution, are exposed to a high intensity laser flash, a rather large absorption of an intermediate is observed around 700 nm, similarto that described for the laser excitation of Ti02 in the previous section. The absorption spectrum of the intermediate is given in Fig. 9.17 [52]. It is not due to trapped electrons and holes but it is identical with to the well-known spectrum of hydrated electrons as proved by radiolysis experiments [52]. The half-life of the hydrated electrons is a few microseconds. In the presence of typical hydrated electron scavengers, such as oxygen, acetone or cadmium ions, the decay of the intermediate became much faster. 

Fig. 9.17 Transient spectrum of hydrated electrons produced by a strong laser pulse in ZnS/CdS (3 1) co-colloids in the presence of Na2S as a hole acceptor. Concentrations ...

Hole-blocking Layers, Electron-acceptors and Hole-acceptors... 

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