# Electron transfer in rigid and semi-rigid iridium d8-d8 donor-spacer-acceptor complexes

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## Abstract

A series of rigid and semi-rigid donor-spacer-acceptor complexes, [Ir(µ-pz*)(CO)(Ph2P-O-C6H4-(CH2)n-py[superscript +]-R)]2(pz* = 3,5-dimethylpyrazolyl; C6H4 = phenylene; py[superscript +] = pyridinium; R = H, 4-tert-butyl, and 4-amide; and n = 0,1,2, and 3, has been synthesized for the purpose of studying photoinduced electron-transfer (ET) reactions. The spacers separating the iridium center (electron donor, Ir2) and pyridinium cation (electron acceptor, py[superscript +]) are based on terminal phosphinite ligands, consisting of a phenylene group and a number of methylene groups ranging from 0 to 3. Three distinct ET reactions can be studied in each complex: singles excited-state electron transfer ([superscript 1]ET), triplet excited-state electron transfer ([superscript 3]ET), and thermal back electron transfer (ET[superscript b]). Atomic positions, obtained from the X-ray crystal structure of [Ir(µ-pz*)(CO)(Ph2P-O-C6H4-CH3]2 were used as a basis for molecular mechanics calculations, furnishing solution structures for the series of Ir2-py[superscript +] donor-acceptor complexes. These results revealed that the spacers in complexes where n = 0 and n =1 are rigid, and that in complexes where n = 2 and n = 3, the spacers are semi-rigid, taking on either folded or stretched conformations in fluid solution. Steady-state and time-resolved emission and absorption experiments were employed to determine [superscript 1]ET, [superscript 3]ET, and ET[superscript b] rates in these complexes. The [superscript 1]ET and [superscript 3]ET rates for the n = 2 and n = 3 complexes exhibit Gaussian free-energy dependence, in excellent agreement with classical ET theory (n = 2: [lambda] = 1.10 eV, H[subscript DA] = 26 cm[superscript -1]; n = 3: [lambda] = 1.05 eV and H[subscript DA] = 7 cm[superscript -1]). However, the [superscript 1]ET and [superscript 3]ET rates in n = 0 and n = 1 complexes exhibit dramatically different behavior: the [superscript 3]ET rates in these rigid complexes are on the order of 10,000 times slower than the corresponding [superscript 1]ET rates. H[subscript DA]s for the ET[superscript b] reactions (n = 1,2) are similar to those of the corresponding [superscript 1]ET reactions. These results are discussed in terms of the solution structure parameters obtained for the series of donor-acceptor complexes. Evidence that through-bond and through-space couplings play different roles in singles and triplet electron transfer is presented for the first time.

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