Previous
work on intramolecular electron-transfer Physical properties related to structure and electron-transfer processes of 1 are reported. To determine the distance compounds of the quinone-porphyrin cyclophane type with gradually increasing electron-acceptor strength was now dependence of the interaction between the electron-acceptor TCNQ and the porphyrin, first results on the synthesis of the extended to acceptor-porphyrin cyclophanes with 7,7,8,8tetracyanoquinodimethane (TCNQ) as an especially strong corresponding naphthalene-spacered TCNQ-porphyrin cyclophane 15 are presented. electron-acceptor. The vertically stacked TCNQ-porphyrin cyclophane 1 was prepared in a multi-step synthesis.
Donor-acceptor cyclophanes are of increasing interest for stituents covering already a wide range of electron affinities.
In the present paper, however, this program is extended investigating charge-and electron-transfer interactions. In contrast to the flexible intermolecular interactions of indi-further by using the quinone-related 7,7,8,8-tetracyanoquinodimethane (TCNQ) as a very strong electron-acceptor in vidual donor-and acceptor-molecules, the cyclophane structures provide an intramolecular fixation of electron-the cyclophane bridge opposite to the porphyrin. Porphyrin-tetracyanoquinodimethane cyclophanes like 1 provide donating and electron-accepting units which are linked together in well-defined and adjustable orientations and dis-the further advantage that in the longitudinal axis of the electron-acceptor both quinone oxygens in the 2-family are tances. Thus, the structural variability of donor-acceptor cyclophanes is certainly the most suitable way of studying now replaced by the planar dicyanomethylene groups with the four linear CϪCϵN bonds, leading to a considerably charge-transfer interactions and electron-transfer reactions in their dependence on electronic and steric effects.
longer spatial extension as compared to the quinones. Thus, this elongated structure of the electron-acceptor should pre-In preceding papers [1] [2] [3] [4] we reported the synthesis and properties of porphyrin-quinone cyclophanes of which vent the remarkable deviations from a parallel orientation of the quinones to the porphyrin plane as have been ob-the photoinduced electron-transfer from porphyrins to the quinones as models for the first step of biological photosyn-served for the porphyrin-quinone cyclophanes of type 2 by X-ray structure analysis [2] and conformational studies by thesis was of special interest. In this context, the reduction potentials of the acceptors and the oxidation potentials of low-temperature 1 H-NMR spectra [5] . the porphyrins were varied systematically [2] , and the porphyrin-acceptor distances were increased step-by-step by porphyrin-acceptor cyclophanes with rigid spacers of differ-Syntheses of Porphyrin-TCNQ Cyclophanes ent size, keeping porphyrin and acceptors in gradually fixed
The 7,7,8,8-tetracyanoquinodimethane (TCNQ) itself distances [3] . The rates of intramolecular electron-transfer can be prepared easily from p-benzoquinone by a modified reactions from donor to acceptor units were measured with Knoevenagel reaction with dicyanomethane [malononitrile 'ultrafast spectroscopy' by the Munich group of Michel-CH 2 (CN) 2 ; TiCl 4 , pyridine] [6] . In preliminary experiments, Beyerle, Heitele, Pöllinger, and their coworkers in an 2,5-dimethyl-1,4-benzoquinone with an analogous alkylexemplary cooperation [4] . substitution pattern as in 2 was reacted to 2,5-dimethyl-So far, as electron-acceptors we used quinones with a va-TCNQ in the presence of tetraphenylporphyrin to prove riety of electron-withdrawing and electron-donating subthat the porphyrin under the reaction conditions remains