Crystal-Structures of Three Long, Rigid, Norbornylogous Compounds of Relevance to Distance-Dependence Studies of Long-Range Intramolecular Electron-Transfer Processes

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Results of previous studies revealed that photoinduced intramolecular electron transfer between the dimethoxynaphthalene donor and dicyanomethylene acceptor groups in the series of rigid alicyclic compounds (2a)-(6a) is extremely rapid. As the first step towards determining the distance dependence of the rate of photoinduced intramolecular electron transfer in these molecules, crystal structures of (3a), (4b) and (5b), have been measured. The edge-to-edge distance between the two chromophores in (3a), (4b) and (5b) is 6.8, 9.4, and 11.5 � respectively. The estimated centre-to-centre separation between the chromophores in (2a)-(6a) ranges from c. 7.1 � in (2a) to c. 14.9 � in (6a). From these results, dipole moments of the charge-separated states of (2a)-(6a), following photoinduced intramolecular electron transfer, could be estimated. These predicted values were in excellent agreement with those determined experimentally by using the time-resolved microwave conductivity technique. An interesting structural feature of (3a), (4b) and (5b) is the pronounced curvature of the molecular backbone in these molecules. It is predicted that approximately 24 linearly fused norbornyl and bicyclo[2.2.0] units would complete a circle, forming a member of a novel class of macrocycles, tentatively called dogcollaranes.