Energy Transfer Dynamics in Re^I−Based Polynuclear Assemblies: A Quantitative Application of Förster Theory

Abstract

The synthesis, structure, and photophysical properties of a new family of tetranuclear FeRe 3 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 CH 2Cl 2 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 Re (I)-based (3)MLCT excited state; the lifetimes are all significantly less than the corresponding AlRe 3 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 Re (I)-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 Re (I)-based excited state to the high-spin Fe (III) 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 Forster 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 Forster 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.

3-(4-Pyridyl)-2,4-pentanedione – a bridge between coordinative, halogen, and hydrogen bonds

“…3 Not surprisingly, the [Fe(acacPy) 3 ] unit in 2 is strongly distorted from its ideal trigonal donor geometry with N-Fe-N angles of 79.69(3)u, 127.62(3)u and 150.53(3)u. Similar geometries have been observed for the mononuclear iron complex itself, 8,11 in the central [Fe(acacPy) 3 ] moiety of tetranuclear aggregates 29 and in the same fragment in heterobimetallic networks. 8,11 The CSD 30,31 contains 11 examples for more general [M(acacPy) 3 ] residues; none of these can be considered a building block of trigonal symmetry.…”

Role of Spin–Orbit Coupling in Long Range Energy Transfer in Metal–Organic Frameworks

“…Third, the transition dipole moments of participating chromophores need to be appropriately aligned (described by κ 2 in eq ). , Finally, the spin selection rule, which predicts the overall spin angular momentum, needs to be preserved throughout the process …”

3-Substituted-2,4-pentanedionates: ligands for photoactive supramolecular assemblies

“…In octahedral metal ion complexes of 3-(4 0 -pyridyl)-2,4-pentanedione (3-pyacacH), this ligand being synthesised by diacetylation of 4-methylpyridine 64 (see Section 2.2.1), the Fo¨rster mechanism has been considered to be operative due to the closeness of the observed rate constant and that calculated with the Fo¨rster theory, using accurate dipole orientations and X-ray derived donor-acceptor distances. 86 The pendent pyridyl units of the complexes [M(3-pyacac) 3 ] (M = Al or Fe) can be used as sites to bind a photoactive [Re(bipy)(CO) 3 ] + unit (and some analogues), thus providing tetranuclear AlRe 3 or FeRe 3 species (Fig. 16).…”