Synthesis, photophysical properties, and photoinduced luminescence switching of trinuclear diimine rhenium(I) tricarbonyl complexes linked by an isomerizable stilbene-like ligand

Two series of trinuclear diimine rhenium(l) tricarbonyl complexes bridged by 1,3,5-tris(4-ethenylpyridyl)benzene (L1) or 1,3,5-tris(4-ethynylpyridyl)benzene (L2) have been synthesized and characterized. The L2-bridged complexes (group 13) exhibit strong luminescence and long emission lifetimes in room-temperature solution which are typical of decay from (MLCT)-M-3 excited states, while the L1-bridged complexes (group A) show only very weak luminescence and short lifetimes under the same experimental conditions, although the emission positions and profiles do not vary that much. The low emission quantum yields and short lifetimes in group A complexes are attributed to intramolecular sensitization of the (3)pipi* excited state localized on the olefin link of the bridging ligand accompanied by a subsequent trans-cis isomerization process. The effects of different diimine ligand substitutents on the triplet-triplet energy transfer mechanisms are explored. The excited-state decay of group B complexes follows the energy gap law on the basis of an observed linear relationship between ln k(nr) and emission energy.