PHOTOINDUCED ELECTRON-TRANSFER IN PORPHYRIN-QUINONE CYCLOPHANES .8. CONFORMATIONAL MOBILITY AND CRYSTAL-STRUCTURES OF PORPHYRIN-QUINONE CYCLOPHANES

In order to correlate photoinduced electron-transfer in porphyrin-quinone cyclophanes to specific molecular conformations, the internal mobility of single- and double-bridged porphyrin-quinone cyclophanes was studied by low-temperature H-1-NMR measurements. The dynamic processes observed are discussed in terms of N-H/N tautomerisation, of the rotation of the rings in the bridges, and of a specific ''swinging bridge'' process back and forth between two equivalent unsymmetrical conformations with a tilted orientation of the porphyrin and quinone ring planes. For comparison with the conformational situation in solution the results of X-ray structure analyses of 3(X = OMe) and 6, as representative examples of single-bridged quinone-porphyrin cyclophanes, and of the corresponding tetramethoxybenzene-bridged octamethylporphyrin cyclophane 4(X = OMe) are reported and discussed. In contrast to the previously solved crystal structure of the quinone-porphyrin-quinone cyclophane 1(X = H), all three X-ray structure analyses of the single-bridged porphyrin cyclophanes proved the existence of unsymmetrical, tilted-plane structures similar lo those discussed as the stable conformations in the ''swinging bridge'' process.