Donor-acceptor triad

Fig. 9.27 Schematic representation of the ejcTTF-oMPE -C60 donor-acceptor triads and their appropriate reference compounds...

A spectacular example of a long-lived, charge-separated donor-acceptor triad was demonstrated by Gust and Moore. The Arizona researchers reported a molecular triad consisting of a diarylporphyrin (P) covalently linked to a carotenoid polyene (C) and a fullerene (C(,o) (Figure 42). In 2-methyltetrahydrofuran solution, the triad undergoes photoinduced electron transfer to yield C-P- + -Q,7, which evolves into C with an overall quantum... 

L. E. Polander, L. Pandey, S. Barlow, S. P. Tiwari, C. Risko, B. Kippelen, J.-L. Br6das, S. R. Marder, BenzothiadiazolesDithienopyrrole Donor-Acceptor-Donor and Acceptor-Donor-Acceptor Triads Synthesis and Optical, Electrochemical, and Charge-Transport Properties. J. Phys. Chem. C 2011, 115,23149-23163. 

In artificial photosynthetic models, porphyrin building blocks are used as sensitisers and as electron donors while fullerenes are used as electron acceptors. Triads, tetrads, pentads and hexads containing porphyrins and Qo have been reported in the literature (see the Further Reading section). 

Recently it has been reported (3 ) that in a triad molecule where a porphyrin is juxtaposed between a carotenoid and a quinone, a charge transfer donor-acceptor pair with a lifetime similar to that found experimentally in biological systems was produced on light irradiation. It was suggested that an electrical potential similar to the type developed in this donor-acceptor pair may be important in driving the chemical reactions in natural photosynthesis. 

Photoinduced intramolecular electron transfer in the donor-acceptor complex 87 (R = H) generates transient charge-separated open-shell species with the remarkably long lifetime of about 75 ps [89]. Dyads that contain Tt-extended tetrathiafulvalene units also form stable cationic species upon oxidation [90]. The dumbbell shaped triad 91 [91-93] (Scheme 4.13) was obtained by carrying out the reaction with the in situ generated bis-diene at room temperature, in the dark and in o-dichlorobenzene as a solvent in 50% yield. The product is thermally unstable and easily undergoes a retro-Diels-Alder reaction [91]. 

Reaction of 217 with Cjq leads to the amino-protected porphyrin-fulleropyrroli-dine, which can easily be deprotected to the corresponding amine [229, 277]. By further functionalization via amide coupling an easy access to extended donor-acceptor systems is possible. A carotene-porphyrin-fullerene triad was prepared by reaction of the amine with the appropriate carotene acid chloride. The motivation for the synthesis of all these donor-acceptor systems is the attempt to understand and imitate the photosynthetic process. On that score, a model for an artificial photosynthetic antenna-reaction center complex has been achieved by attaching five porphyrin cores in a dendrimer-like fashion to the fullerene [242]. 

Further donors, mainly based on aniline or ferrocene, were subjected to the sarcosin-based Prato reaction. The bisaldehydes 224 [260] or 227 [250] react with two CgQ-molecules, leading to dumbbell shaped dimers (Table 4.9). The Cgo dimer obtained from 224 could be considered as an A-D-A triad but the CV-data indicate that the dimer behaves electrochemically as two independent donor-acceptor units. 

Condensahon of N-tritylglycin with donor or acceptor subshtuted aldehydes results in fulleropyrrolidines whose amino functionality can be easily deprotected with triflic acid and further funchonalized with, e.g., another donor [236]. Thus, access to various triads is provided. An example for a donor-acceptor-donor triad is given in Scheme 4.38. 

Figure 4.32 Light driven proton pump using a donor-chromophore-acceptor triad and quinone proton transporter across a phospholipid bilayer membrane.

The construction of a myoglobin-based donor-sensitizer-acceptor triad (13), possessing a noncovalently linked cyclofcis(paraquat-p-phenytene), as the acceptor, with Ru(bpy)3, as the... 

As mentioned previously, photocycloaddition reactions are a useful method for obtaining fullerene derivatives fused directly to heterocycles. Such compounds have attracted much interest because they might have interesting properties, e.g., amino acid fullerene derivatives as biologically active compounds or pyrrolidino-fullerenes as key precursors for a great number of fullerenes donor-acceptor bridged dyads and triads. Fullerenes functionalized with silicon compounds are of great interest in materials science. The synthesis of these compounds will be described in this section. 

The following ten types of triad donor-acceptor molecules may be differentiated ... 

Moreover, the HOMO/LUMO orbital schemes of the triads (Fig. 9.10) manifest this donor-acceptor character. The HOMO is strongly localized on exTTF and reaches into the oPPE bridge, which facilitates electron injection into the bridge. However, in contrast to exTTF-oPPV -C6o, in which the HOMO is completely... 

Fig. 9.19 HOMO/LUMO orbital schemes as resulted from DFT optimizations displaying the donor-acceptor character of the triads. The HOMOs are represented in orange-red and LUMOs in green-blue...

Some novel C60-based assemblies were recently synthesized by [4+2] Diels-Alder cycloaddition reactions. Thus, fused tetrathiafulvalene-C60 dyads and C60-tetra-thiafulvalene-C60 dumbbell triads, in which the fullerene acceptor is doubly tethered to the donor tetrathiafulvalene through a rigidified cyclohexene ring [108], were prepared. With this novel approach, control of the relative orientation as well as the distance between the donor and acceptor units was achieved. Thereby, through-space interactions were expected to dominate because of the special topology of the constructed molecules. More examples of such donor-acceptor hybrid systems are discussed in the appropriate following sections, with their potential use in innovative technological applications. 

In addition to the previously described donor-acceptor hybrid dyad systems, a few triads have also been considered and constructed. The fundamentals and principles for designing such triad systems are more complex, as there exist diverse arrangement options for their construction. Thus, the multiple electroactive units of a triad can be arranged in different ways giving rise to different materials [245]. 

FIGURE 10.1 Donor-Chromophore-acceptor triad. X = a chalcogenide atom oxygen, sulfur, or selenium. 

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