Excited-State Properties of [M(COD)(µ-L)]2 Complexes (M = Rh, Ir; µ-L = Substituted Hydroxypyridinate). Relationship of Variable-Temperature Emission Lifetime and Quantum Yield Measurements to the Photoreduction of Halocarbons

Gary S. Rodman, Charles A. Daws, Kent R. Mann

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Abstract

We measured the electronic absorption and emission properties of [M(COD)µ-L)]2 (M = Rh, Ir; COD = 1,5-cyclooctadiene; H-L = hp (2-hydroxypyridinate), mhp (6-methyl-2-hydroxypyridinate), chp (6-chloro-2-hydroxypyridinate), 2hq (2-hydroxyquinolate), pz (pyrazolate)) complexes and the deactivation parameters of the 3B state of [Ir(COD)(µ-L)]2 in MTHF (MTHF = 2-methyltetrahydrofuran). Each Rh and Ir complex exhibits an intense absorption band around 420 and 490 nm, respectively, that is assigned to the 1A → 1B component of the dσ* → pσ transition. The Ir complexes exhibit temperature-dependent phosphorescence bands (λmax > 720 nm) and temperature-independent fluorescence bands (λmax = 610 nm). The temperature dependence of the phosphorescence intensity was fit to the following expression: 1 /Φobsd = 1 /Φ0 + A' exp(-Ea/kBT). For µ-L = hp, Ea, = 2030 cm-1, A' = 0.021 × 109, ΦO = 0.0189; for µ-L = mhp, Ea = 2070 cm-1, A' = 0.45 × 109, ΦO = 0.0162; for µ-L = pz, Ea = 4467 cm-1, A' = 280 × 109, ΦO = 0.093. Variable-temperature lifetime data was fit to an analogous equation: kobsd= k0 + A exp(-Ea/kbT). For µ-L = hp, Ea = 1620 cm-1, A = 3.2 × 1012 s-1, k0 = 1.04 × 106 s-1; for µ-L = mhp, Ea = 1950 cm-1, A = 5.6 × 1012 s-1, k0 = 1.11 × 10 s-1; for µ-L = chp, Ea = 1900 cm-1, A = 8.0 × 1012 s-1, k0 = 1.61 × 106 s-1; for µ-L = hq, Ea = 1950 cm-1, A = 3.4 × 1012 s-1, k0 = 1.74 × 106 s-1; for µ-L = pz, Ea = 3750 cm-1, A = 310 × 1012 s-1, k0 = 0.48 × 106 s-1. The hydroxypyridinate complexes have shorter intrinsic (temperature-independent) lifetimes and quantum yields than [Ir(COD)(µ-pz)]2, but no systematic differences in excited-state properties could be discerned within the hydroxypyridinate series. These results are rationalized on the basis of the “energy gap law” and the Strickler-Berg equation. The thermal deactivation barrier for the 3B excited state of [Ir(COD)(µ-pz)]2 is much larger in MTHF than in acetonitrile. Because of a lower energy ligand field (LF) state, the deactivation barrier in the hydroxypyridinate complexes is significantly smaller than in the pz complex. The differences observed in the photochemical reactivity of [Ir(COD)(µ-hp)]2 and [Ir(COD)(µ-pz)]2 with halocarbons are rationalized in terms of differences in the rates of reactions that follow electron transfer from the 3B excited state.

Original languageEnglish (US)
Pages (from-to)3347-3353
Number of pages7
JournalInorganic Chemistry
Volume27
Issue number19
DOIs
StatePublished - Sep 1 1988

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