Donor-bridge-acceptor dyads

Figure C 1.2.10. Representative examples of fullerene based donor-bridge-acceptor dyads and triads.

Guldi D M, Maggini M, Scorrano G and Prato M 1997 Intramolecular electron transfer in fullerene/ferrocene based donor-bridge-acceptor dyads J. Am. Chem. See. 119 974-80... 

More recently, pulse radiolysis started to play a major role in the characterization of photolytically generated (A —D ) radical pairs in a variety of fullerene containing donor-bridge-acceptor dyads (68,69). While the latter evolve from photoinduced intramolecular electron transfer reactions complementary employment of pulse radiolysis allowed to generate the reduced and oxidized entities in separate experiments and to superimpose the features of the two reactive moieties. In this context, it should be noted that conventional methods, such as cyclic voltammetry, due to their unfavorable time resolution, fail to contribute to the radical pair characterization. 

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]. 

The final example of the distance dependence of ET dynamics in norbomylogous bridged systems is photoinduced charge-shift ET in 22(4) and 22(6) which possess the A-methylpyridinium acceptor (Scheme 10).114 Local excitation of the DMN donor in the dyads results in charge-shift, from LD to LA, to form the distonic radical... 

A systematic fluorescence and flash photolytic investigation of a series of covalently linked Ceo-based ferrocene dyads (1, 2) is reported as a function of the nature of the bridge (i.e., flexible or rigid) and the separation (i.e., relative distance between the donor and acceptor site) [192, 193]. The fate of the fullerene singlet excited state was... 

Figure 40. A schematic illustrating the difference between the superexchange mechanism and molecular wire behavior in a D-B-A dyad. Superexchange the virtual bridge states lie well above the donor level (A is large) and, consequently, the electron is never localized within the bridge instead, the electron is transferred from donor to acceptor in one coherent jump. The distance dependence behavior is exponential decay. Molecular wire behavior The virtual bridge states are energetically comparable to the donor level (A is very small). In this case, the electron may be thermally injected into the bridge and becomes localized within the bridge, whereupon it moves from the donor to the acceptor incoherently as a defect, such as a polaron. The distance dependence behavior is Ohmic (varies inversely with distance).

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. 

An interesting redox switching of quadratic nonlinearities has been achieved on the open-shell donor-acceptor dyad 95. Such a compound is composed of two electroactive units linked by an CH=CH bridge, the acceptor unit being the polychlorinated triphenyl (PTM) radical and the... 

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... 

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

These noncovalent interactions introduce geometric restrictions that most probably affect the k values. Ground state absorption properties show that the chromophores are weakly coupled when compared to reference monomers comprised of the bridge-substituted donor or acceptor. Highly efficient ET rates, ranging from 70 to 13 ps have been observed for ET from the zinc donor to the free base acceptor. Almost no competition with hole transfer, due to excitation of the free base, was observed, except for dyad 35 in Figure 13.28. Excited state lifetimes for the individual chromophores and ET quantum yields are given in Table 13.3. 

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