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)Ph_2POC_6H_4(CH_2)_n-A^+]_2 (pz* = 3,5-dimethylpyrazolyI; Ph = C_6H_5; C_6H_4 = phenylene; A^+ = pyridinium (py^+) or substituted py^+; n = 0-3), have been studied in acetonitrile solution at room temperature. The rates singlet (^1Irz* → 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.