Photochemical detection of intramolecular energy transfer between nonconjugated chromophores in rigid model compounds

Abstract

Irradiation of the 4,5-benzotetracyclo[6,3,1,0^2,7,0^9,11]dodecane-10-spiro-9′-fluorene-3,6-dione (III) with light absorbed only by the tetralindione chromophore led to rearrangement of the fluorenespirocyclopropane system to dibenzofulvene. The photorearrangement of the ‘isolated’ acceptor chromophore, the tricyclo[3,2,1,0^2,4]-octane-3-spiro-9′-fluorene (V), to 2-(fluornen-9-ylidene)bicyclo[2,2,1]heptane was studied separately by direct and sensitized excitation. The photochemical quantum yield of the intramoleculary sensitized reaction in (III) was comparable to that of the intermolecularly sensitized photoisomerization of (V). This is consistent with an efficient energy transfer from the n,π* triplet of the ketone to the lowest π,π* triplet of fluorene in model compound (III). The fluorenespirocyclopropane system is well suited for use as acceptor in model compounds, having nonconjugated donor and acceptor chromophores inflexibly connected to the same molecular frame. The energy transfer efficiency in such compounds can be evaluated quantitatively from the extent of acceptor rearrangement on selective excitation of the donor. The mechanism of the photoisomerization is discussed.