Bimolecular Electron and Energy Transfer Reactivity of Exchange-Coupled Dinuclear Iron(III) Complexes

Abstract

Bimolecular quenching between photosensitizers and exchange-coupled transition metal complexes has been studied in an effort to experimentally establish a link between Heisenberg spin exchange and chemical reactivity. The acceptors are members of the oxo/hydroxo-biscarboxylato class of dinuclear Fe(III) compounds, where protonation of the oxo bridge provides a means for modulating the magnitude of spin exchange within the cluster. Photoexcitation of solutions containing Ru(II) polypyridyl sensitizers and the Fe(III) complexes results in quenching of emission from the (3)MLCT excited state of the Ru(II) chromophores; nanosecond time-resolved absorption measurements demonstrate that quenching occurs, in part, by electron transfer. Decoupling electron transfer driving force (DeltaG(0)(ET)) from changes in the magnitude of spin exchange was achieved by varying the bridging carboxylate to afford a series of complexes of the form [Fe(2)O(H)(O(2)CR)(2)(Tp)(2)](n)(+) (n = 0, 1, 2). Electrochemical measurements reveal a greater than 500 mV shift in cluster reduction potential across the series (i.e., R = CH(3) to CF(3)), whereas variable-temperature magnetic susceptibility measurements demonstrate a corresponding invariance in spin exchange between the metal centers (J(oxo) = -119 +/- 4 cm(-1) and J(hydroxo) = -18 +/- 2 cm(-1) for H = -2JS(1).S(2)). Structural analyses suggest that reorganization energies (lambda) associated with electron transfer should be identical for all molecules within a given series (i.e., oxo or hydroxo bridged); likewise Deltalambda between the series is expected to be small. A comparison of quenching rates for the two extended series firmly establishes that neither reorganization energy nor electron transfer driving force considerations can account for differences in reactivity between oxo-bridged (large spin exchange) and hydroxo-bridged (small spin exchange) quenchers. Upon consideration of energy transfer contributions, it is determined that reactivity differences between the oxo- and hydroxo-bridged quenchers must lie in the relative rates of Dexter energy transfer and/or electron transfer, with the origin of the latter linked to something other than DeltaG(0)(ET) or lambda. Finally, the extent to which spin exchange within the dinuclear Fe(III) quenchers can be identified as the key variable influencing these reactivity patterns is discussed.

Trends in Exchange Coupling for Trimethylenemethane-Type Bis(semiquinone) Biradicals and Correlation of Magnetic Exchange with Mixed Valency for Cross-Conjugated Systems

“…Correlation of Exchange Coupling in Biradical Complexes, and Mixed Valency in Quinone/Semiquinone Anions. The correlation of exchange coupling parameters with electron-transfer parameters is an area of intense, recent interest. − We feel that biradicals having exchange parameters that can be measured by magnetometry, and stability that permits X-ray crystallographic analysis are perhaps the optimal molecules to study the correlation of electron/energy transfer and exchange coupling.…”

Ultrafast Time-Resolved Spectroscopy of Self-Assembled Cyclic Fe(II)−Bisterpyridine Complexes

“…McCusker and co-workers have studied the time-resolved excited-state dynamics in various Fe(II)-ligand complexes in solution. [16][17][18][19][20] From those reports, ultrafast deactivation of the MLCT (metal-to-ligand charge transfer) process in monomeric Fe(II)-polypyridyl compounds has been suggested. Studying both structural aspects as well as excited-state energy relaxation processes, Chergui and coworkers have recently reported the structural change of a Fe(II)-polypyridyl compound in terms of SCO process by pump-probe X-ray spectroscopy.…”

“…In this context, the ultrafast energy relaxation dynamics on the transition metal−ligand complexes also have been studied, focusing on SCO phenomena. McCusker and co-workers have studied the time-resolved excited-state dynamics in various Fe(II)−ligand complexes in solution. − From those reports, ultrafast deactivation of the MLCT (metal-to-ligand charge transfer) process in monomeric Fe(II)−polypyridyl compounds has been suggested. Studying both structural aspects as well as excited-state energy relaxation processes, Chergui and co-workers have recently reported the structural change of a Fe(II)−polypyridyl compound in terms of SCO process by pump−probe X-ray spectroscopy. − One of the main results in these studies is the direct observation of the Fe−N bond length change (∼0.2 Å) in the SCO process, which has been found in Fe(II)−ligand compounds.…”

Intramolecular Excimer Formation in a Naphthalene-Appended Dinuclear Iron−Oxo Complex

“…These compounds provide a convenient platform due in large part to the ease with which spin coupling within the dinuclear core can be modulated. For example, protonation of Fe 2 O(O 2 CCH 3 ) 2 (Tp) 2 (where Tp is hydrotris(pyrazolyl)borate) affords the corresponding OH-bridged complex in which the exchange interaction between the two high-spin Fe III centers is attenuated by nearly a factor of 10. − We have used this system as the basis for a detailed study of the bimolecular reactivity of exchange-coupled complexes . This work was successful in demonstrating that changes in driving force and reorganization energy could not account for the substantial difference in the reactivity observed between the oxo- (strong coupling) and hydroxo- (weak coupling) bridged complexes.…”