In continuation of previous syntheses of benzene-, These donor-acceptor systems are of interest since, with regard to electron-transfer rates, they complete the series of naphthalene-, and anthracene-spacered porphyrin-quinone cyclophanes 1, 2, and 4, biphenylene-spacered porphyrin-1, 2, and 4. As expected the electron-transfer of 3 proceeds between that of 2 and 4 in an especially steep region of quinone cyclophanes of structure 3 as the "missing link" between 2 and 4 were prepared in multistep syntheses. distance dependences of electron-transfer rates.
To determine the distance dependence of photoinduced suitable system for studying the problem mentioned above.
In fact, the biphenylene-spacered porphyrin-quinone cyclo-electron-transfer from porphyrins to quinones, a series of new intramolecular porphyrin-quinone compounds of well-phane of structure 3 so far was a "missing link" in an especially interesting region with regard to the distance de-defined sandwich-type cyclophane structures was synthesized. The distance variation was achieved by perpendicu-pendence of electron-transfer rates in porphyrin-quinone compounds. larly oriented arene "spacers" with the intention to keep the porphyrin and quinone structures in transannular face-toface arrangements at well-defined distances. As such spacers 1,2-substituted benzene [2] , 1,7-substituted naphtha-Synthesis of Biphenylene-Spacered Porphyrin-Quinone lene [3] , and 1,7-substituted anthracene [1] so far have been Cyclophanes 3 and 19 and Related Cyclophanes used successfully for building up the porphyrin-quinone cyclophane structures 1, 2, and 4, in which, due to the As described in the preceding papers on the syntheses of spacered porphyrin-quinone cyclophanes 1, 2, and 4 [1] [2] [3] , graduated extension of the spacers, a stepwise increase of the porphyrin-quinone distances was realized.
the appropriate precursors for the preparation of these compounds were the corresponding dialdehydes consisting As discussed in the preceding papers [1] [2] [3] , time-resolved fluorescence spectroscopy as a measure of photoinduced of the tetramethyleneϪ2,5-dimethoxybenzeneϪtetramethylene bridges linked on both sides into the corresponding po-electron-transfer yielded approximated fluorescence lifetimes for 1 in the order of 1 picosecond (ps); for 2 the life-sitions of the respective arene spacers. These arene spacers must be substituted in the ortho-or 1-positions by the ter-time is only slightly increased by a factor of about 2. For the anthracene-spacered 4 with the longest porphyrin-quin-minal aldehyde groups which are essential for the cyclizing condensation with suitable pyrrole derivatives to form the one distance, however, a fluorescence lifetime was found in the order of 10 nanoseconds (e.g., 5.6 ns, in toluene) [4] [10] . porphyrin skeleton, thus completing the cyclophane structure. The demethylation of the two methoxy groups in the This remarkable difference of electron-transfer rates prompted us to try the synthesis of a cyclophane with a central aromatic ring of the bridge, and the subsequent oxidation to the quinones would then yield the desired arene-porphyrin-quinone distance in the critical region between the naphthalene-and the anthracene-spacered cyclophanes spacered porphyrin-quinone cyclophanes.
This general synthetic route, so far very successful and 2 and 4.
The biphenylene-spacered analogue 3 with an intermedi-well-established in all our syntheses of compounds of the 1-, 2-, and 4-series, had to be modified significantly, how-ate transannular distance between 2 and 4, with the same insertion angle to the arene spacers and an identical struc-ever, in the case of the synthesis of the biphenylene-spacered systems 3. The reasons are that the tetramethyleneϪbenze-ture of the quinone-containing bridge as well as the same substitution pattern on the porphyrin part seemed to be a neϪtetramethylene chains in the earlier syntheses were built up by twofold Wittig reactions with the aldehyde groups of [᭛] Part 16: Ref. [1] .
the arene spacers; these reactions lead to double bonds