Designing Copper(I) Photosensitizers for the Norbornadiene−Quadricyclane Transformation Using Visible Light: An Improved Solar Energy Storage System

Four copper(I) complexes have been designed as photosensitizers for assisting the norbornadiene (NBD) → quadricyclene (Q) isomerization with the purpose of carrying out the reaction using visible light and achieving a high NBD/photocatalyst turnover. All complexes contain π-delocalized bidentate N⌒O monoanionic ligands. Their syntheses have been performed either by the metathesis reaction from CuCl or by reaction of the monoprotic ligands with Cu 5 Mes 5 (Mes ≡ 2,4,6-Me 3 C 6 H 2 ) in the presence of norbornadiene. The dinuclear complexes [Cu 2 L 2 (μ-NBD)], 3, [Cu 2 L‘ 2 (μ-NBD)], 4, [Cu 2 L‘‘ 2 (μ-NBD)], 5, and [Cu 2 L‘‘‘ 2 (μ-NBD)], 6 [where L = 2-methyl-8-oxoquinolinato, L‘ = 2-methyl-5,7-dichloro-8-oxoquinolinato, L‘‘ = 4-oxoacridinato, and L‘‘‘ = 2-(2-oxo-3,5-di- tert -butyl phenyl)benzotriazole], have been isolated and characterized in the solid state, including the X-ray analysis on 3 and 6 . All compounds show absorption in the visible spectrum, due to the presence of metal-to-ligand charge-transfer (MLCT) bands. Most were active in assisting the NBD → Q conversion, although the best results were obtained in the case of 3, for which we give a detailed report. Complex 3 is the precursor, in the presence of NBD, of the photoactive monomeric species [CuL(NBD)], 9, where a single C C binds copper(I), of which complex 7, [CuL(NBN)] [NBN ≡ norbornene], can be regarded as a model compound. The NBD photoisomerization assisted by 3 (see 9 ) has been carried out in methanol by visible light irradiation (λ = 405 nm) with a photoisomerization quantum yield of 0.029. Turnover numbers (moles of Q produced/mole of photocatalyst) of ca. 5000 were obtained in this solvent. An increase of the temperature gave rise to a significative quantum yield enhancement. The other compounds were less efficient than 3 in assisting the NBD → Q photoisomerization under visible light irradiation, most probably due to the low energy of their MLCT excited states.