PHOTOINDUCED ELECTRON-TRANSFER IN IRIDIUM(SPACER)PYRIDINIUM COMPLEXES

Photoinduced electron-transfer (ET) reactions in a series of iridium(spacer)pyridinium complexes, [Ir(mu-pz*)(Co)ph(2)POC(6)H(4)(CH2)(n)-A(+)](2) 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 (Ir-1(2)* --> A(+): (1)ET) 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 (lambda = 1.0 eV, H-AB = 5 Cm-1). With one exception, maximum ET rates exhibit an exponential dependence upon the number of carbon atoms (alpha(C)) in the spacer. The exception is the n = 1 (alpha(C) = 5) system; k(max) is almost a factor of 100 slower than predicted by the exponential dependence on alpha C, indicating that donor-acceptor electronic coupling through a single methylene link is unusually weak.