A. Cuppoletti et al., Bond-coupled electron transfer reactions: Photoisomerization of norbornadiene to quadricyclane, J PHYS CH A, 103(51), 1999, pp. 11253-11256
In this paper, we report on the bond coupled electron transfer (BCET) isome
rization reaction of norbornadiene (NB) to quadricyclane (Q). We have used
several tripler sensitizers to examine electron and energy quenching by NE.
In the case of acetophenone, energy transfer generates the detectable (NB)
-N-3, which rearranges to give (3)Q which relaxes to Q. Using chloranil, be
nzoquinone, and 2,5-dichlorobenzoquinone as triplet sensitizers, the triple
t ion pairs are generated, but cannot access either (NB)-N-3 or (3)Q, and s
o the ion pair undergoes return electron transfer to the ground state. Howe
ver, with 3,3',4,4'-benzophenonetetracarboxylic dianhydride as the triplet
sensitizer, the triplet ion pair is generated, but can undergo BCET to (3)Q
efficiently, which then relaxes to Q. To our knowledge, this is the first
example of an efficient electron-transfer mechanism to photoisomerize NE to
Q without the intermediacy of (NB)-N-3. Consequently, in the valence isome
rization of NE to Q, it is possible to directly access the NB-Q triplet sur
face by controlling the energy of the triplet ion pair photogenerated, and
in effect, the quantum yield can be modulated.