Energy transfer in molecular dyads comprising metalloporphyrin and ruthenium(II) tris(2,2 '-bipyridyl) terminals. Competition between internal conversion and energy transfer in the upper excited singlet state of the porphyrin

The photophysical properties of a tripartite supermolecule comprising zinc porphyrin and ruthenium(II) tris(2,2'-bipyridyl) terminals separated by a t rans Pt-II bis-sigma-acetylide fragment bearing tri-n-butylphosphine residu es have been recorded in solution. Thus, excitation into the ruthenium(II) tris(2,2'-bipyridyl) fragment is followed by fast intramolecular energy tra nsfer to the triplet state of the porphyrin with only a minor contribution from competing (spin-forbidden) triplet-to-singlet energy transfer. Deactiv ation of the first excited singlet state localized on the porphyrin involve s singlet-to-triplet energy transfer to populate the triplet state of the r uthenium(II) tris(2,2'-bipyridyl) complex, which rapidly transfers excitati on energy to the triplet state of the porphyrin. There is no experimental e vidence in support of intramolecular electron transfer between the terminal s, such processes being inhibited by poor thermodynamics and by the barrier imposed by the central Pt-II bis-sigma-acetylide fragment. Following excit ation into the second excited singlet state of the porphyrin moiety, which has an inherent lifetime of ca. 3 ps, ultrafast singlet-to-singlet energy t ransfer to the ruthenium(II) tris(2,2'-bipyridyl) complex competes with int ernal conversion. Rapid intersystem crossing within the excited-state manif old of the ruthenium(II) tris(2,2'-bipyridyl) fragment is then followed by slower triplet energy transfer to the porphyrin.