Donor-acceptor dyads, hydrogen-bonded

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]... 

Fig. 2 (a) Hydrogen-bonded and covalently linked ZnP donor and FeP electron-acceptor dyads (Ref. [15]). (b) The self-assembled... 

Although energetically possible, photoinduced electron transfer does not occur in these hydrogen-bonded dyads, even in polar solvents. The absence of charge separation has been accounted for by the low electronic coupling between the donor and acceptor in the excited state as result of the long distance between the chromophores. 

The ET behavior of supramolecular assembled dyads was compared to similar donor-acceptor systems where the donors were covalently bonded to the fullerenes. Surprisingly, in similar covalently linked systems, the lifetime of the radical-ion pairs is lower than those observed in supramolecular assemblies. As an example, the lifetime of the CSS of the fullerene-(zinc phthalocyanine) dyad (Figure 2.29b) assembled through hydrogen bonds was found to be 1.5 p,s, when a similar but covalently linked dyad ° exhibited lifetimes for the radical-ion pair of only 3 ns. 

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