Donor-acceptor dyads, electronic

Figure 14.19 Schematic representation of a photoinduced electron transfer in a fullerene-based donor-acceptor dyad.

The synthesis of the triazolino[4, 5 l,2][60]fullerene 204, a novel donor-acceptor dyad exhibiting efficient electron-transfer dynamics, was reported by Guldi et al. (44) (Scheme 9.44). The azido tetrathiafulvalene 203, on heating with [60]fullerene in o-dichlorobenzene at 60 °C, gave the triazoline 204 in 24%... 

The feasibility of intramolecular electron- and energy-transfer depends on distance and is usually studied in covalently linked systems. However, donor-acceptor dyads can be also arranged by self-assembly what resembles the situation of electron transfer in biological systems. Artificial dyads tethered by a small number of hydrogen bonds immediately dissociate in methanol or water. To improve the binding while keeping the reversibility, a photoinducible electron donor-acceptor dyad linked by a kinetically labile bond was designed. [19]... 

Ferrocene is composed of a pair of 6-7r-electron carbon arrays and a 6-d-electron iron(II) atom. Ferrocene-fullerene donor-acceptor dyads carry all the requisites for electron-transfer phenomena. However, data for the formation of ferrocene-fullerene hybrids are not abundant. Some such dyads have already been synthesized following the methodology of 1,3-dipolar cycloaddition of the appropriate azome-thine ylides to C60, with either variable-spacing building blocks or a rigid-bridge all-cj-bonded framework, in order to tune the redox properties of the system [40,234, 248-251]. Another novel dyad that contained two covalently bound ferrocene units was recently synthesized via cyclopropanation of the fullerene core [252]. 

Fig. 13.14 Electron donor-acceptor dyads affording long-lived CS states [41-44].

Molecular electron transfer is the basis for many important natural and commercial processes. During the past decade photochemists have relied upon supramolecular arrays of molecules to facilitate their understanding of the chemical and physical basis for this fundamentally important process. It therefore seems appropriate that several chapters in this volume examine thermally and photo-chemically induced electron transfer in supramolecular assemblies consisting of inorganic molecular building blocks such as covalently linked donor-acceptor dyads, transition metal clusters, and nanocrystalline semiconductor particles. 

Electron depopulation of the donor and concomitant population of the acceptor in the complex results in a lowering of the vibrational frequencies in the IR spectra of the donor and acceptor moieties. Additionally, complex formation can decrease the symmetry of the donor/ acceptor dyad and can lead to increased IR intensity or the appearance of new bands. For example, in halogen/alkylbenzene complexes, the stretching frequencies of the halogens are lowered, as seen in the shift of chlorine band from 557 cm-1 in free chlorine to 530 cm-1 in the benzene complex, to 527 cm-1 in the toluene complex, and to 524 cm-1 in the p-xylene complex. Increases in the intensity of some of the arene bands are also clearly seen [23b]. 

Quantitative Evaluation of Arenes as Electron Donors 437 Spectral (UV/vis) Probe for the Formation of CT Complexes 438 IR Spectroscopic Studies of Charge-Transfer Complexation 442 Thermodynamics of Charge-Transfer Complexation 443 Structural Features of Arene Charge-Transfer Complexes 445 Bonding Distance of the Donor/Acceptor Dyad in Arene Complexes 446 Relationship Between Hapticity and Charge Transfer in Arene Complexes 447 Effect of Charge Transfer on the Structural Features of Coordinated Arenes 448... 

When six coordination sites around octahedral metal ion are occupied by only bidentate ligands, stereoisomers around the metal ion are formed. However, the coordination of symmetric tridentate ligands to a six-coordinate metal ion leads to only one isomer. Furthermore, tridentate bridging ligands connect metals in a linear fashion, resulting in the formation of stereochemically well-defined supramolecular systems. The rigid structure of these systems is suitable for studies of electron or energy transfer events between the donor-acceptor dyads. 

Fullerenes, especially Qo, have been used as electron acceptors in covalently-linked donor-acceptor dyads and triads (see Part IV on Artificial Photosynthesis). A critical comparison has been made between the rates of... 

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... 

Otsuki et al. [27] have demonstrated that amidinium-carboxylate salt bridges, which have been used earlier to construct electron donor-acceptor dyads or a donor-spacer-acceptor triad, can also be used to assemble energy donor-acceptor dyad 13 and pentad 14. The salt bridge consists of complementary double hyi-ogen bonds and electrostatic interactions and, therefore, offers... 

The p>orphyiins have attracted considerable attention because are ubiquitous in natural systems and have prosp>ective applications in mimicking enzymes, catalytic reactions, photodynamic therapy, molecular electronic devices and conversion of solar energy. In particular, munerous p>orphyrins based artificial light-harvesting antennae, and donor acceptor dyads and triads have been prepared and tested to improve our understanding of the photochemical aspect of natural photosynthesis. 

Some examples of donor-acceptor dyads for studies of photoinduced electron transfer across molecular bridges are shown in Fig. These systems are usually... 

P. Samori, X. Yin, N. Tchebotareva, Z. Wang, T. Pakula, F. Jackel, M.D. Watson, A. Venturini, K. MuUert, J.P. Rabe, Self-assembly of electron donor-acceptor dyads into ordered architectures in two and three dimensions surface patterning and columnar double cables . J. Am. Chem. Soc. 126, 3567-3575 (2004)... 

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