Distinguishing between Dexter and rapid sequential electron transfer in covalently linked donor-acceptor assemblies

Soler, Monica; McCusker, James K.


The syntheses, physical, and photophysical properties of a family of complexes having the general formula [M-2(L)(mcb)(Ru(4,4'-(X)(2)-bPY)(2))](PF6)(3) (where M = Mn-II or Zn-II, X = CH3 or CF3, mcb is 4'-methyl-4-carboxy-2,2'-bipyridine, and L is a Schiff base macrocycle derived from 2,6-diformyl-4-methylphenol and bis(2-aminoethyl)-N-methylamine) are described. The isostructural molecules all consist of dinuclear metal cores covalently linked to a Ru-II polypyridyl complex. Photoexcitation of [Mn-2(L)(mcb)(Ru((CF3)(2)-bPY)(2))]-(PF6)(3) (4) in deoxygenated CH2Cl2 solution results in emission characteristic of the (MLCT)-M-3 excited state of the Ru-II chromophore but with a lifetime (tau(obs) = 5.0 +/- 0.1 ns) and radiative quantum yield (Phi(r) approximate to 7 x 10(-4)) that are significantly attenuated relative to the Zn-II model complex [Zn-2(L)(mcb)(Ru((CF3)(2)-bP)(2))](PF6)(3) (6) (tau(obs) = 730 +/- 30 ns and Phi(r) = 0.024, respectively). Quenching of the (MLCT)-M-3 excited state is even more extensive in the case of [Mn-2(L)(mcb)(Ru((CH3)(2)-bpy)(2))](PF6)(3) (3), whose measured lifetime (tau(obs) = 45 +/- 5 ps) is > 10(4) shorter than the corresponding model complex [Zn-2(L)(mcb)(Ru((CH3)(2)-bPY)(2))](PF6)(3) (5) (tau(obs) = 1.31 +/- 0.05 mu s). Time-resolved absorption measurements on both Mn-containing complexes at room-temperature revealed kinetics that were independent of probe wavelength; no spectroscopic signatures for electron-transfer photoproducts were observed. Time-resolved emission data for complex 4 acquired in CH2Cl2 solution over a range of 200-300 K could be fit to an expression of the form k(nr) = k(0) + A.exp{-Delta E/k(B)T} with k(0) = 1.065 +/- 0.05 x 10(7) s(-1), A = 3.7 +/- 0.5 x 10(10) s(-1), and Delta E = 1230 +/- 30 cm(-1). Assuming an electron-transfer mechanism, the variable-temperature data on complex 4 would require a reorganization energy of lambda - 0.4-0.5 eV which is too small to be associated with charge separation in this system. This result coupled with the lack of enhanced emission at temperatures below the glass-to-fluid transition of the solvent and the absence of visible absorption features associated with the Mn-2(II) core allows for a definitive assignment of Dexter transfer as the dominant excited-state reaction pathway. A similar conclusion was reached for complex 3 based in part on the smaller driving force for electron transfer (Delta G(0)(ET) = -0.1 eV), the increase in probability of Dexter transfer due to the closer proximity of the donor excited state to the dimanganese acceptor, and a lack of emission from the compound upon formation of an optical glass at 80 K. Electronic coupling constants for Dexter transfer were determined to be similar to 10 cm(-1) and similar to 0. 15 cm(-1) in complexes 3 and 4, respectively, indicating that the change in spatial localization of the excited state from the bridge (complex 3) to the periphery of the chromophore (complex 4) resuts in a decrease in electronic coupling to the dimanganese core of nearly 2 orders of magnitude. In addition to providing insight into the influence of donor/acceptor proximity on exchange energy transfer, this study underscores the utility of variable-temperature measurements in cases where Dexter and electron-transfer mechanisms can lead to indistinguishable spectroscopic observables.

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Título según WOS: Distinguishing between Dexter and rapid sequential electron transfer in covalently linked donor-acceptor assemblies
Volumen: 130
Número: 14
Fecha de publicación: 2008
Página de inicio: 4708
Página final: 4724


Notas: ISI