Models for photochemical electron transfer at fixed distances. Porphyrin-bicyclo[2.2.2]octane-quinone and porphyrin-bisbicyclo[2.2.2]octane-quinone

We report the syntheses of three rigidly linked side-by-side porphyrinquinone molecules 2-4 whose edge-to-edge distances are fixed at 6, 10, and 14 Å (Figure 1). The spacer units bicyclo[2.2.2]octane and bibicyclo[2.2.2]octane prevent translational displacement and only allow rotational freedom. Our preliminary studies of the fluorescence yields of both the free base PLQ 2a-4a and Zn PLQ 2b-4b systems, relative to 1, a similar porphyrin not equipped with a quinone, reveal a distance dependence. This suggests that 2-4 will be useful systems for studying the incremental effect of distance on photochemical electron transfer from excited-state porphyrins to quinones.