Donor - Acceptor Cyclophanes with Biradical - Ionic Ground States

In donor-acceptor cyclophanes DA one expects an electron transfer from the donor D to the acceptor A already in •the ground state, i.e. DA-r D+ A-, if the DA distance and the d'ifference between the ionization energy In of D and the electron affinity EA of A are small enough. We shall use the term biradicalionic•2 (bi) for a state which can be symbolized as D+qA-q with q > 1/2. Whereas bi ground states have been evidenced for many crystalline DA complexes or solutions of strong donors and acceptors in highly polar solvent;s,H ithis does not apply to DA cyclophanes. Even fm the cyclophanes la and lb (cf. Figure 1) wtth the D/ A paiir TMPD (N,N,N',N'-tetramethyl-p-phenylenediamin) /p-benzoquinone a bi ground state could be excluded due to spectroscopic investigations.