Donor charge-transfer assemblies

Recent work focuses on non-classical mesogenes which are built up by self-assembly. One example is a family of polymers containing disk-like groups which form no liquid crystalline phase, but ean act as an electron acceptor or donor. Charge transfer complexation with a complementary low molecular mass compound induces nematic or columnar discotic liquid crystalline order [153,154]. Figure 13 demonstrates this with the example of a polyester, bearing electron-rich tetra(alkoxy)tri-phenylene-units in the main chain, mixed with the electron deficient aromatic 2,4,7-trinitro-9-fluorenone (TNF). While the pure polymer shows a non-ordered isotropic melt, a columnar phase appears on addition of TNF. 

The first example of a donor-acceptor molecular complex was noted in 1949 by Bensei and Hildebrand [137] in their studies involving charge transfer complexes between benzene and molecular iodine. Subsequently such complexes were studied by Mulliken [138] and now more recently have been used by Stoddart et al. [16,139] in designing novel self-assembling systems. 

There is no evidence for ground state charge-transfer complex formation between stilbenes and neutral amines. Amine cations and dications are powerful electron acceptors and can form ground state complexes in which t-1 serves as the electron donor. Complex formation between t-1 and the organic dication methyl viologen is responsible for quenching of the fluorescence of surfactant stilbenes in organized assemblies (112). 

Control of self-assembly by switchable redox processes in complexes between dendrimers with re-donor and re-acceptor units permitted specific prediction of the space occupied by dendritic aggregates. Use of cucurbit[8]uril (see Fig. 6.9) as donor-host compound for viologen guests (dendronised 4,4 -bipyridines as acceptors) promotes formation of such charge-transfer complexes [24]. 

Figure 2.5 Immobilization of one-dimensional columnar charge-transfer (CT) assemblies of an amphiphilic triphenylene donor and various acceptors confined within mesoporous silica nanochannels.

It is reasonable to assume that any molecule that has an asymmetry in its electronic structure, such as carbon monoxide, is potentially a molecular rectifier. An Aviram and Ratner rectifier is merely an extension of this principle in that it is a modular assembly of parts which are well understood from charge transfer studies. The electronic asymmetry of the rectifier is obtained by placing an electron donor at one end of the molecule and an electron acceptor at the other. Some common... 

Among the systems proposed as models for the photosynthetic reaction center, supramolecular assemblies in which Ru(II)-polypyridine complexes and 4,4 -bipyridinium units are held together noncovalently in threaded and interlocked structures have been extensively studied [43, 82-88]. In such assemblies, connections between the molecular components rely on charge transfer interactions between the electron acceptor bipyridinium units and aromatic electron donor groups (Fig. 3). For instance, in the various pseudorotaxanes formed in acetonitrile solution at 298 K by the threading of cyclophane 4 + by the dioxybenzene-containing tethers of 192+ (Fig. 17) [84], an efficient photoinduced electron... 

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