Electron donor–acceptor compounds: exploiting the triptycene geometry for the synthesis of porphyrin quinone diads, triads, and a tetrad

Rigidly and covalently linked porphyrin quinones are well established as model compounds for studying photo-induced electron transfer (PET) reactions like those occurring during the primary processes of photosynthesis. In this context, the synthesis of a number of porphyrin quinones is reported in which one or two porphyrin electron donors are connected to either one or two quinone electron acceptors, resulting in diad, triad, and tetrad model systems, respectively. The porphyrin(s) and the quinone(s) are linked by triptycene, 1,4-phenylene, and cis- or trans-1,4-cyclohexylene bridges. The use of the 1,4-phenylene, and cis- or trans-1,4-cyclohexylene bridges results in donor–acceptor compounds with the same number of bonds between donor and acceptor(s), but differing in distance and orientation. Analysis of the 1H NMR spectra confirmed the chair conformation for the cis- and trans-cyclohexylene-linked diads and triads. NOE experiments gave information about the spatial arrangement of the target compounds. The key compounds in the syntheses of all these new PET model systems are the triptycene quinones, which are formed via a [πs4+πs2]-cycloaddition between an anthracene derivative and a suitable quinone. The triptycene system enforces a rigid orientation on the quinone acceptor(s) in the final model system. Evidence is given that the triptycene system has further potential for constructing tailor-made donor–acceptor compounds.