Organic donor-acceptor dyads

Organic Solar Cells Using Simple Donor-Acceptor Dyads... 

Hybrid systems have been constructed in which a metal complex is covalently linked to an organic species so as to produce a donor-acceptor dyad, with either subunit functioning as the chromophore. Thus, ruthenium(II) tris(2,2 -bipyridyl) complexes have been synthesized bearing appended anthraquinone or tyrosine functions. Both systems enter into intramolecular electron-transfer reactions. With an appended anthraquinone moiety, direct electron transfer occurs from the triplet excited state of the metal complex to the quinoid acceptor. This is not the case with tyrosine, which is an electron donor, but the metal complex can be photooxidized by illumination in the presence of an added acceptor. The bound tyrosine residue reduces the resultant ruthenium(III) tris(2,2 -bipyridyl) complex... 

The study of photoinduced ET in covalently linked donor-acceptor assemblies began with comparatively simple dyad systems which contain a transition metal center covalently linked to a single electron donor or acceptor unit [26]. However, work in this area has naturally progressed and in recent years complex supramolecular assemblies comprised of one or more metal complexes that are covalently linked to one or more organic electron donors or acceptors have been synthesized and studied [27-36]. Furthermore, several groups have utilized the useful photoredox properties of transition metal complexes to probe electron and energy transfer across spacers comprised of biological macromolecules such as peptides [37,38], proteins [39,40], and polynucleic acids [41]. 

The MLCT basis for the reactive excited state leads to interesting consequences with respect to the orbitals involved in photoinduced forward and back ET in metal complex dyads. In order to categorize this difference we define two categories of dyad systems Type 1 dyads contain an electron acceptor covalently attached to the d6 metal chromophore and type 2 dyads contain an electron donor covalently attached to the d6 metal chromophore (see Fig. 1). In the type 1 dyads, photoinduced forward ET involves transfer of an electron from a tt orbital localized on the acceptor ligand, L, to a ir orbital on the organic electron acceptor, A. Back ET involves transfer of an electron from a it orbital of the organic electron acceptor, A, to the -shell of the transition metal center. By contrast, in the type 2 dyads photoinduced forward ET involves transfer of an electron from a tt orbital on the organic donor, D, into the hole in the d-shell of the... 

Figure 1 Photoinduced electron transfer schemes for type 1 and type 2 metal-organic dyads. Key L is a diimine ligand such as 2,2 -bipyridine M is a transition metal dn indicates the electron count in the valence shell d-orbitals of M A is an organic electron acceptor D is an organic electron donor FET is forward ET BET is back ET.

This chapter provides a comprehensive overview of photoinduced ET in metal-organic dyads. The focus is on systems in which intramolecular ET occurs between a metal center and an organic donor (or acceptor) that are held in proximity by an organic spacer. Emphasis is placed on systems in which the thermodynamic driving force for ET is well defined. 

Wasielewski and coworkers have studied a series of molecules in which porphyrins are linked to quinones and similar electron acceptors, and determined that when a solvent such as 2-methyltetrahydrofuran freezes, about 0.8 eV of driving force is lost [116]. These researchers have prepared dyads with a driving force for photoinduced electron transfer greater than this value in polar solvents, and found that they are capable of displaying photoinduced electron transfer even in frozen organic glasses. This, in turn, has opened the door to low-temperature EPR studies of the charge-separated states in various donor-acceptor systems and their decay pathways [66, 99, 117]. 

A large number of covalently linked systems are currently being synthesized and investigated, differing in the nature of A, B, and L, as well as in the number of functional units in the supramolecular system (nuclearity). It is common to call simple two-component donor-acceptor systems such as that of Eq. 2 dyads , and progressively more complex systems triads , tetrads , pentads , etc.. Systems where all the A and B units are organic molecules are dealt with in Chapter 1 of this section. The present chapter deals with systems where at least one of the A/B functional units is a transition metal coordination compound. From this definition, however, are excluded (a) systems where A and/or B are porphyrins or related species (dealt with in Chapter 2) and (b) systems of high nuclearity with dendritic structures (dealt with in Chapter 9). 

The group of Meijer and Schenning has constructed ambipolar field-efifect transistors from imides-diaminotriazines H-bonded p-n dyad complexes 29 based on OPV4T in combination with PBI-2 [89]. The transistors show two independent pathways for charge transport. In contrast, processing of OPV and PBl that are not connected by H-bonds formed charge transfer donor-acceptor complexes. They showed no mobility in field-effect transistors, presumably due to an unfavorable supramolecular organization. 

Fig. 2 Examples of covalently linked donor-acceptor systems (dyads) used for the study of photoinduced electron transfer across organic spacers. Boxes are drawn to identify the photoexcitable chromophore (left), the bridge (center), and the acceptor unit (right).

Much simpler is the organization of C6o-[Ru(bpy)3] dyads by a reaction of suitable Ceo-bipyridyl precursors, bipyridyl ligands i.e., in a 1 2 stochiometry) and ruthenium (II) chloride. Spacers such as androstane, polyglycol, crown ester and hexapeptide were employed as molecular rulers to separate a Cso acceptor unit from the bipyridyl ligand, yielding innovative donor-acceptor ensembles with diverse topographies -Figure 21. - ... 

A detailed discussion of the energetics of photoinduced intramolecular ET in organic dyads is available in the literature [44]. Therefore, only a brief discussion of the energetic features which are unique to transition metal dyads are presented herein. The free energy for photoinduced forward and back ET (AG and AGbet, respectively) between an electron donor and an electron acceptor (D and A, respectively) is given by... 

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