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 1 H‐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(XOMe) and 6 , as representative examples of single‐bridged quinone‐porphyrin cyclophanes, and of the corresponding tetramethoxybenzene‐bridged octamethylporphyrin cyclophane 4(XOMe) are reported and discussed. In contrast to the previously solved crystal structure of the quinone‐porphyrin‐quinone cyclophane 1(XH) , all three X‐ray structure analyses of the single‐bridged porphyrin cyclophanes proved the existence of unsymmetrical, tilted‐plane structures similar to those discussed as the stable conformations in the “swinging bridge” process.