Energy Transfer Dynamics in ReI−Based Polynuclear Assemblies: A Quantitative Application of Förster Theory

The synthesis, structure, and photophysical properties of a new family of tetranuclear FeRe3 chromophore-quencher complexes having the general form [Fe(pyacac)3(Re(bpy′)(CO)3)3](OTf)3 (where pyacac = 3-(4-pyridyl)-acetylacetonate and bpy′ is 4,4′,5,5′-tetramethyl-2,2′-bipyridine (tmb, 1), 2,2′-bipyridine (bpy, 2), and 4,4′-diethylester-2,2′-bipyridine (deeb, 3)) are reported. Time-resolved emission data acquired in room-temperature CH2Cl2 solutions exhibited single exponential decay kinetics with observed lifetimes of 450 ± 30 ps, 755 ± 40 ps, and 2.5 ± 0.1 ns for complexes 1, 2, and 3, respectively. The emission in each case is assigned to the decay of the ReI-based 3MLCT excited state; the lifetimes are all significantly less than the corresponding AlRe3 analogues (2250 ± 100 ns, 560 ± 30 ns, and 235 ± 20 ns for 4, 5, and 6, respectively), which were also prepared and characterized. Electron transfer is found to be thermodynamically unfavorable for all three ReI-containing systems: this fact coupled with the absence of optical signatures for the expected charge-separated photoproducts in the time-resolved differential absorption spectra and favorable spectral overlap between the donor emission and the acceptor absorption profiles implicates dipolar energy transfer from the ReI-based excited state to the high-spin FeIII core as the dominant quenching pathway in these compounds. Details obtained from the X-ray structural data of complex 2 allowed for a quantitative application of Förster energy transfer theory by systematically calculating the separation and spatial orientation of the donor and acceptor transition moment dipoles. Deviations between the calculated and observed rate constants for energy transfer were less that a factor of 3 for all three complexes. This uncommonly high degree of precision testifies to both the appropriateness of the Förster model as applied to these systems, as well as the accuracy that can be achieved in quantifying energy transfer rates if relative dipole orientations can be accounted for explicitly.