Distinguishing between Dexter and Rapid Sequential Electron Transfer in Covalently Linked Donor−Acceptor Assemblies
journal contribution
posted on 2008-04-09, 00:00 authored by Monica Soler, James K. McCuskerThe syntheses, physical, and photophysical properties of a family of complexes having the general
formula [M2(L)(mcb)(Ru(4,4‘-(X)2-bpy)2)](PF6)3 (where M = MnII or ZnII, 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 RuII polypyridyl complex. Photoexcitation of [Mn2(L)(mcb)(Ru((CF3)2-bpy)2)](PF6)3 (4) in deoxygenated CH2Cl2 solution results in emission characteristic of the 3MLCT excited state of
the RuII chromophore but with a lifetime (τobs = 5.0 ± 0.1 ns) and radiative quantum yield (Φr ≈ 7 × 10-4)
that are significantly attenuated relative to the ZnII model complex [Zn2(L)(mcb)(Ru((CF3)2-bpy)2)](PF6)3 (6)
(τobs = 730 ± 30 ns and Φr = 0.024, respectively). Quenching of the 3MLCT excited state is even more
extensive in the case of [Mn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (3), whose measured lifetime (τobs = 45 ±
5 ps) is >104 shorter than the corresponding model complex [Zn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (5) (τobs
= 1.31 ± 0.05 μs). 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 knr = k0 + A·
exp{−ΔE/kBT} with k0 = 1.065 ± 0.05 × 107 s-1, A = 3.7 ± 0.5 × 1010 s-1, and ΔE = 1230 ± 30 cm-1.
Assuming an electron-transfer mechanism, the variable-temperature data on complex 4 would require a
reorganization energy of λ ∼ 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 MnII2 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 (ΔG0ET
= −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 ∼10 cm-1 and
∼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) results 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.
