Photoinduced electron transfer in iridium(spacer)pyridinium complexes

Photoinduced electron-transfer (ET) reactions in a series of iridium(spacer)pyridinium complexes. [Ir(μ-pz*)-(CO)PH₂POC₆H₄(CH₂)_n-A⁺] ₂ (pz* = 3,5-dimethylpyrazolyl; Ph = C₆H₅; C₆H₄ = phenylene: A⁺ pyridinium (py⁺) or substituted py⁺; n = 0-3), have been studied in acetonitrile solution at room temperature. The rates of singlet (¹Ir₂* → A⁺; ¹ET) reactions were determined for each complex, and for n = 1 and 2 species, the rates of thermal charge recombination (ET^b) also were measured. The ET rates for the n = 1 system display a Gaussian free-energy dependence (λ = 1.0 eV, H_AB = 5 cm^-1). With one exception, maximum ET rates exhibit an exponential dependence upon the number of carbon atoms (α_C) in the spacer. The exception is the n = 1 (α_C = 5) system; k_max is almost a factor of 100 slower than predicted by the exponential dependence on α_C, indicating that donor-acceptor electronic coupling through a single methylene link is unusually weak.