Chargetransfer Processes

Due to the 1,3,5-trisubstituted benzene rings in the stil-benoid dendrimers, the electronic interactions between the stilbene units are small. Nevertheless, the cross-conjugated dendrons are highly efficient for a singlet energy transfer to a suitable core or to a special chromophore fixed on the focal point of the core.

The compounds 5, 6, and 8 are typical examples in which an exothermic energy transfer to the core, which has a red-shifted absorption, is possible. In 5, 6, and 8, an emission of the core can be observed irrespective of the excitation wavelengths [43, 44]. A quantified estimation of the energy-transfer efficiency revealed in these series percentages between 65 and 100% [44].

The compounds 12c [46] and 12k [47,48] permit an energy transfer from the excited stilbene units to the fullerene moiety. Interestingly, compound 12f, the higher generation of 12c, does not exhibit a fullerene emission on excitation in the range of the absorption of the stilbene building blocks. Electron transfer was supposed to play a key role in this case [46]. Transient absorption spectroscopy of 12k revealed that, despite the excitation of the dendrons at A = 355 nm, no singlet-singlet absorption of the dendrons could be detected after the 18 ps laser pulse. Instead of that, an absorption at 880 nm was observed, which can be unequivocally attributed to the fullerene singlet-singlet absorption [47, 48]. On a longer time scale, a triplet-triplet absorption at A = 700 nm could be identified. The strong spin-orbit coupling in C60 can provoke an intersystem crossing

Moreover, the triplet formation competes with an activated electron transfer, which generates a dipolar radical pair (dendron)+^-C-0V The charge separation of the donor-acceptor system 12k was established by transient absorption measurements. The radical pair has, in THF and ben-zonitrile, average lifetimes of 350 and 725 ns, respectively. The fullerene radical anion has a typical NIR absorption at 1000 nm, and the stilbenoid radical cation an absorption at ca. 480 nm in the visible region [47, 48].

Figure 6 summarizes the important photophysical (and photochemical) processes in stilbenoid dendrimers, which have a chromophore (electron acceptor) with a low excitation energy in the core or attached to the focal point. The efficiency of competitive processes, expressed in quantum yields, depends on the special system (core-dendron), but for a consistent rationalization, further experiments are desirable—for example, to the relation of triplet states and charge-transfer states or for irreversible photoreactions of the excited singlet and triplet states.

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