D-A systems

Then F( ) = S(/ -t d )- 2( ), and the density of states D E) = dS/d/ . A system containing a large number of particles N, or an indefinite number of particles but with a macroscopic size volume V, normally has the number of states S, which approaches asymptotically to... 

A number of studies have focused on D-A systems in which D and A are either embedded in a rigid matrix [103-110] or separated by a rigid spacer with covalent bonds [111-118], Miller etal. [114, 115] gave the first experimental evidence for the bell-shape energy gap dependence in charge shift type ET reactions [114,115], Many studies have been reported on the photoinduced ET across the interfaces of some organized assemblies such as surfactant micelles [4] and vesicles [5], wherein some particular D and A species are expected to be separated by a phase boundary. However, owing to the dynamic nature of such interfacial systems, D and A are not always statically fixed at specific locations. 

In a general description of intramolecular electron-transfer (ET) processes one has to differentiate between charge separation in donor/acceptor (D/A) systems via the formation of photoexcited states and a charge-transfer or charge-shift reaction that is thermally activated (Cannon, 1980 Fox and Chanon, 1988 Meyer, 1978). 

It should be mentioned that data for energy-transfer-based probes simulated in this chapter are generic and can apply to any D-A system regardless of the decay time of the donor. The optimal modulation frequency will be determined by the decay time, but the magnitude of changes in phase and modulation will depend only on absolute energy transfer efficiency. 

The physical phenomenon of current generation in simple D/A systems can be thought of in terms of the six chemical steps depicted in Fig. 4. (1) The absorption of a photon leads to a localized exciton with energy oo on either the donor or... 

Within the two-state model, the d-a system can be described in terms of two adiabatic states ij/i and ij/i with energies Ei and E2. An orthogonal transformation... 

The electronic coupling of donor and acceptor sites, connected via a t-stack, can either be treated by carrying out a calculation on the complete system or by employing a divide-and-conquer (DC) strategy. With the Hartree-Fock (HF) method or a method based on density functional theory (DFT), full treatment of a d-a system is feasible for relatively small systems. Whereas such calculations can be performed for models consisting of up to about ten WCPs, they are essentially inaccessible even for dimers when one attempts to combine them with MD simulations. Semiempirical quantum chemical methods require considerably less effort than HF or DFT methods also, one can afford application to larger models. However, standard semiempirical methods, e.g., AMI or PM3, considerably underestimate the electronic couplings between r-stacked donor and acceptor sites and, therefore, a special parameterization has to be invoked (see below). 

Scheme 15.1 Principle of a switchable, redox-controlled fluorescence in D-A systems (D = TTF).

TTF-based D-A systems have been extensively used in recent years to play around photoinduced electron transfer processes. Typically, when an electron acceptor moiety that emits fluorescence intrinsically is linked to TTF (D), the fluorescence due to the A moiety may be quenched because of a photoinduced electron transfer process (Scheme 15.1). Accordingly, these molecular systems are potentially interesting for photovoltaic studies. For instance, efficient photoinduced electron transfer and charge separation were reported for TTF-fullerene dyads.6,7 An important added value provided by TTF relies on the redox behavior of this unit that can be reversibly oxidized according to two successive redox steps. Therefore, such TTF-A assemblies allow an efficient entry to redox fluorescence switches, for which the fluorescent state of the fluorophore A can be reversibly switched on upon oxidation of the TTF unit. 

If a third component (M), which can specifically stabilize one of the products of electron transfer, is introduced into the D -A system, the free energy change of photoinduced electron transfer is shifted to the negative direction, when the activation barrier of electron transfer is reduced to accelerate the rates of electron transfer, as shown in Figure 3, where M forms a complex with A ". It should be emphasized that there is no need to have an interaction of M with A and that the interaction with the reduced state (A ") is sufficient to accelerate the rate of photoinduced electron transfer. This contrasts sharply with the catalysis on conventional ionic or concerted reactions, in which the catalyst interacts with a reactant to accelerate the reactions. The initial interaction between M and A in the complex A-M, where charge is partially transferred from A to M, would also result in acceleration of the photoinduced electron transfer, since the reduction potential of A-M is shifted to the negative direction as compared to that of A. 

In these devices, the acceptor is joined to two donors which can in principle transfer electrons to it with comparable efficiencies. From the point of view of long-lived charge separation, molecules of this type offer no apparent advantages over simple D-A systems. However, as mentioned above, they may be useful in some other situations, especially as components of larger devices when A is a 2-electron acceptor. There are a few examples of porphyrin-quinone systems with this structure in the literature. 

Like the dmit complexes, the mnt complexes are also promising candidates for the synthesis of spin-ladder systems. As previously discussed (Section II.B.l.b), metal-like D-A conductors have been obtained when associating M(mnt)2 and Per (159). Spin-ladder D-A systems have been obtained when associating M(mnt)2 and the dithiopheno-tetrathiafulvalene (DT-TTF = C10H4S6) donor (Scheme 15). 

It is an interesting historical fact that the study of the photochemistry of conjugated dienes and trienes was initiated by the discovery of the photochemical transformation of ergosterol to vitamin D—a system... 

These steps can account for the I2 B X and A X emissions seen in l2/ 2(a) flames. The A H(2w) state does not radiate, but Nota et alJ used laser excitation of the D A system to show that A is populated. It was estimated that individual vibrational levels of A held populations comparable... 

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