Other Donor-Acceptor Systems

Extensive studies on the third-order NLO properties of arylated TEEs have been carried out, and one important result from these studies is that the arrangement of an anilino donor and / -nitrophenyl acceptor in linear conjugation (as in 41) provides a much higher value of the second hyperpolarizability than when placed in cross-conjugation (as in 40) [35]. Strong two-photon absorption properties of sublimable donor-substituted cyanoethylenes indicate good potential for optoelectronic applications [33]. 

---

The complementary hydrogen bonding motif formed by the amidinium-carboxylate ion pair has been used to construct a number of other donor-acceptor systems. For example, Nocera and co-workers showed that compound 23a, 24a and 24b formed stable dimers (Fig. 6.15) [60]. Martin and co-workers reported the formation of complex 23b-22a (Fig. 6.15) [61]. The electron transfer behavior of both hydrogen bonded complexes was investigated in THF. 

The aim of this chapter is to give a state-of-the-art report on the plastic solar cells based on conjugated polymers. Results from other organic solar cells like pristine fullerene cells [7, 8], dye-sensitized liquid electrolyte [9], or solid state polymer electrolyte cells [10], pure dye cells [11, 12], or small molecule cells [13], mostly based on heterojunctions between phthaocyanines and perylenes [14], will not be discussed. Extensive literature exists on the fabrication of solar cells based on small molecular dyes with donor-acceptor systems (see for example [2, 3] and references therein). 

However, these observations are not proof of the role of a donor-acceptor complex in the copolymcrization mechanism. Even with the availability of sequence information it is often not possible to discriminate between the complex model, the penultimate model (Section 7.3.1.2) and other, higher order, models.28 A further problem in analyzing the kinetics of these copolyincrizations is that many donor-acceptor systems also give spontaneous initiation (Section 3.3.6.3). 

In this context it is useful to remember that the concept of the possible recombination of triplet radical ion pairs is not an ad hoc assumption to rationalize certain Z - E isomerizations, although the CIDNP effects observed during an isomerization reaction played a key role in understanding this mechanism. Triplet recombination has been accepted in several donor-acceptor systems as the mechanism for the generation of fast (optically detected) triplets [169-171], and invoked for several other reaction types [172]. The CIDNP technique is a sensitive tool for the identification of this mechanism, for example, in the geometric isomerization of Z- and E-1,2-diphenylcyclopropane and in the valence isomerization of norbornadiene (vide infra). Most of these systems have in common that the triplet state can decay to more than one minimum on the potential surface of the parent molecule. 

However, the efficiency of the latter strategy decreases when other potent electron donors are present in the donor-acceptor system. An illustrative example is the mixed ether/thiotether-linked phthalimide 69, which yielded the branched macrocyclic compound 71 arising from competing sulfur oxidation as main product (Sch. 32) [55]. 

Radical-anion and radical-cation intermediates, for example, reaet with each other after PET in donor-acceptor systems. After proton reorganization they undergo cyclization to provide a direct synthetic route to macrocycles and A-heterocycles with a variety of ring sizes [230]. Cycloadditions via radical ion pairs [231] and the C -C bond formation between Ceo and A,0-ketene acetals [232] also fit this eategory. 

Thus, within the context of Marcus-Hush theory, the three important variables that determine the ET rate constant are V i, A, and AG, . It therefore follows that an understanding of ET processes entails an understanding of how these three variables are determined by factors such as the electronic properties of the donor and acceptor chromophores, the nature of the intervening medium, and interchromo-phore separation and orientation. The distance dependence of ET dynamics is a particularly important problem and, in order to investigate it experimentally, it is obviously advisable to employ donor acceptor systems that are rigid in the sense that the donor and acceptor groups are held in well-defined distances and orientations with respect to each other. 

Porphyrin-containing donor acceptor systems have been an active area of research for over 20 years, and there is an extremely large body of literature in the area [10]. In this chapter, we exemplify the major approaches to this research, but the main focus areas are illustrated with one or two examples, and references to additional work are provided. In many cases, we illustrate concepts with molecules studied in our own laboratories simply for reasons of convenience, although a number of other studies may have been reported. 

Both through-bond and through-space transfer mechanisms can operate in donor-acceptor systems. Very often these mechanisms occur competitively and in some cases they can practically add to each other their relative contributions are difficult to evaluate. 

In general, for the triads and other types of linked donor-acceptor systems to be discussed, electron transfer is assumed to occur in the nonadiabatic regime. That is, the mixing between the electronic state of the donor and acceptor before electron transfer occurs and the corresponding state after electron transfer is weak ( IcaT) [33]. The electron transfer event is assumed to be fast compared to the time scale of nuclear motions. The electron transfer theory proposed by Marcus [35,38] states that the electron transfer rate constant is given by Eq. (2). 

---