Acceptor-donor triad molecules

The construction of intramolecular molecular system whose photo active molecule linked with conducting molecular wire is an important subject in realization of. molecular electronic or photonic devices. For such objectives, systematization of donor-photosensitizer-acceptor triad molecules into large molecular systems is one of the feasible approaches because the exquisite incorporation of the photosensitizer and a suitable electron donor and/or acceptor into a conducting polymeric chain is useful for various molecular systems based on the photoinduced electron transfer. With this in mind, we synthesized symmetrical donor-acceptor-donor triad molecules which can be polymerized by the normal electrochemical oxidation. By the polymerization, one-dimensional donor-acceptor polymers with porphyrin moieties separated by ordered oligothienyl molecular wire which is considered as a proto-type molecular device was obtained. 

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. 

Transmembrane charge separation by sequential electron transfer between adjacent redox centers is epitomized by studies utilizing covalently linked donor -sensitizer acceptor triad molecules that are vectorially organized across planar bilayer mem-... 

C-P-Q Triad Molecules. As discussed above, the natural reaction center has solved the problem of energy loss due to rapid charge recombination by employing a multistep electron transfer strategy. The same strategy may be applied to the porphyrin-quinone type systems. As we pointed out in 1982 [28], this requires the addition of a secondary electron donor or acceptor moiety. This strategy came to fruition in 1983 when we reported the synthesis of carotenoid-porphyrin-quinone (C-P-Q) triad 2 [29, 30]. This molecule features porphyrin and quinone moieties similar to those found in 1, but a... 

Miura, A. et al.. Bias-dependent visualization of electron donor (D) and electron acceptor (A) moieties in a chiral DAD triad molecule, J. Am. Chem. Soc. 125, 14968-14969, 2003. 

For the attainment of marvelous electron transfer processes in the natural sequential potential fields, many noncovalentaly-bound donor-acceptor (DA) systems and covalently-bound DA systems " " have been previously reported. Most of them are artificial models of the photosynthesis comprising simple assemblies of the dyad (DA) or triad [donor-spacer-acceptor (DSA)] functional molecules with a chromophore such as a porphyrin. The quantum efficiency of such systems is lower (<25%) compared with the biological systems (=100%), and thus more efforts for constructing more efficient systems are necessary. Some of the covalen-taly-bound DA systems have been designed for the fabrication of molecule-scale devices based on a molecular electron-transport wire and/or highly ordered molecular arrays on the surface. " Most of such studies employed the DA nonconju-gated molecules. 

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. 

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

A relative of 12 has been prepared by Sanders, van der Plas, and coworkers [64], Triad 13 features an N,A-dimethylaniline-type donor and an anthraquinone acceptor. These moieties are linked to the ortho positions of the porphyrin aryl groups, and this leads to a folded conformation for the molecule, as determined from NMR studies. Both the free base and zinc derivatives of 13 were prepared. The folded conformation might be expected to facilitate electron transfer among the components of the triad, and while this could enhance the quantum yield of the initial charge separated state, it might unfavorably affect the yield of the final D+-P-QT state and its lifetime. Unfortunately, photochemical or spectroscopic studies were not reported. 

The year 1983 marked the appearance not only of the triad photosynthesis mimics with two donor moieties (C-P-Q), but also those with two acceptors (P-Q-Q). The first such molecule, 26, was reported by Sakata, Mataga and coworkers [45, 76]. Transient absorption studies on the picosecond time scale revealed that excitation of the porphyrin moiety resulted in rapid formation of the porphyrin radical cation. This result was interpreted in terms of the following series of events. 

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. 

The morphological problems associated with the BHJ solar cells, such as low concentration of percolating pathways which are needed in order to bring the separated charge carriers to their corresponding electrodes, have prompted the utilization of molecules in with the donor and the acceptor moieties were covalently linked. In this connection several examples of Pc-based polymers [161,162], Pc-C6o dyads [85,87,88] and triads [275] have been prepared and tested for photovoltaic applications, but the efficiencies of these systems have been proved to be still low. 

For the sake of consistency of terminology, triads of molecules in which the central unit acts simultaneously as both proton donor and acceptor will be termed sequential to distinguish such configurations from those in which the central molecule acts as double proton donor or double acceptor. A perhaps more quantitative expression of cooperativity is referred to in the literature as nonadditivity. The latter term is commonly taken as the difference between the total interaction energy of an aggregation of molecules on one hand and the sum of all the pairwise interactions on the other. 

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). 

Transfer of calcium cations (Ca2 + ) across membranes and against a thermodynamic gradient is important to biological processes, such as muscle contraction, release of neurotransmitters or biological signal transduction and immune response. The active transport can be artificially driven (switched) by photoinduced electron transfer processes (Section 6.4.4) between a photoactivatable molecule and a hydroquinone Ca2 + chelator (405) (Scheme 6.194).1210 In this example, oxidation of hydroquinone generates a quinone to release Ca2+ to the aqueous phase inside the bilayer of a liposome, followed by reduction of the quinone back to hydroquinone to complete the redox loop, which results in cyclic transport of Ca2 +. The electron donor/acceptor moiety is a carotenoid porphyrin naphthoquinone molecular triad (see Special Topic 6.26). 

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