Intramolecular Energy Transfer Involving Heisen berg Spin-Coupled Dinuclear Iron—Oxo Complexes.

The synthesis, structure, and physical properties of a series of oxo-bridged dinuclear Fe(III) complexes containing pendant naphthalene groups are described. The compounds [Fe2O(O2CCH2-C10H7)(tren)2](BPh4)(NO3)2 (8), [Fe2O(O2-CCH2-C10H7)(TPA)2](ClO4)3 (9), Fe2O(O2CCH2-C10H7)2(Tp)2 (10), and Fe2O((O2CCH2CH2)2-C10H6)(Tp)2 (11) (where tren is tris(2-aminoethyl)amine, TPA is tris(2-pyridyl)amine, and Tp is hydrotrispyrazolylborate) have been characterized in terms of their structural, spectroscopic, magnetic, and photophysical properties. All four complexes exhibit moderately strong intramolecular antiferromagnetic exchange between the high-spin ferric ions (ca. -130 cm-1 for H = -2JS1·S2). Room-temperature steady-state emission spectra for compounds 8-11 in deoxygenated CH3CN solution reveal spectral profiles similar to methyl-2-naphthyl acetate and [Zn2(OH)(O2CCH2-C10H7)2(TACN-Me3)2](ClO4) (13, where TACN-Me3 is N,N,N-1,4,7-trimethyltriazacyclononane) but are significantly weaker in intensity relative to these latter two compounds. Time-resolved emission data for the iron complexes following excitation at 280 nm can be fit to simple exponential decay models with τobsS1 = 36 ± 2, 32 ± 4, 30 ± 5, and 39 ± 3 ns for compounds 8-11, respectively. The decays are assigned to the S1 → S0 fluorescence of naphthalene; all of the lifetimes are less than that of the zinc model complex (τobsS1 = 45 ± 2 ns), indicating quenching of the S1 state by the iron-oxo core. Nanosecond time-resolved absorption data on [Zn2(OH)(O2CCH2-C10H7)2(TACN-Me3)2](ClO4) reveal a feature at λmax = 420 nm that can be assigned as the T1 → Tn absorption of the naphthalene triplet; the rise time of 50 ± 10 ns corresponds to an intersystem crossing rate of 2 × 107 s-1. A similar feature (though much weaker in intensity) is also observed for compound 8. The order-of-magnitude reduction in the T1 lifetime of the pendant naphthalene for all of the iron-oxo complexes (τobsT1 = 5 ± 2 µs vs 90 ± 10 µs for [Zn2(OH)(O2CCH2-C10H7)2-(TACN-Me3)2](ClO4)) indicates quenching of the naphthalene triplet with an efficiency of >90%. Neither the naphthalene radical cation nor the reduced FeIIFeIII species were observed by transient absorption spectroscopy, implying that energy transfer is the most likely origin for the quenching of both the S1 and T1 states. Spectral overlap considerations strongly support a Förster (i.e., dipolar) mechanism for energy transfer from the S1 state, whereas the lack of phosphorescence from either the free naphthyl ester or the Zn model complex suggests Dexter transfer to the diiron(III) core as the principal mechanism of triplet quenching. The notion of whether spin exchange within the diiron(III) core is in part responsible for the unusual ability of the iron-oxo core to engage in energy transfer from both the singlet and triplet manifolds of naphthalene is discussed. [ABSTRACT FROM AUTHOR]