Visible Light Generation of Iodine Atoms and I-I Bonds: Sensitized I~- Oxidation and l_3~- Photodissociation

摘要

Direct 355 or 532 nm light excitation of TBAI_3, where TBA is tetrabutyl ammonium, in CH_3CN at room temperature yields an iodine atom, I~*, and an iodine radical anion, l_2~-. In the presence of excess iodide, the iodine atom reacts quantitatively to yield a second equivalent of I_2~- with a rate constant of k = 2.5 ± 0.4 × 10~(10) M~(-1) s~(-1). The I_2~- intermediates are unstable with respect to disproportionation and yield initial reactants, κ= 3.3 ± 0.1 × 10~9 M~(-1) s~(-1). The coordination compound Ru(bpz)_2(deeb)(PF_6)_2, where bpz is 2,2'-bipyrazine and deeb is 4,4'-(C_2H_5CO_2)_2-2,2'-bipyridine, was prepared and characterized for mechanistic studies of iodide photo-oxidation in acetonitrile at room temperature. Ru(bpz)_2(deeb)~(2+) displayed a broad metal-to-ligand charge transfer (MLCT) absorption band at 450 nm with ε = 1.7 × 10~4 M~(-1) cm~(-1). Visible light excitation resulted in photoluminescence with a corrected maximum at 620 nm, a quantum yield ø = 0.14, and an excited state lifetime τ = 1.75 μs from which k_r = 8.36 × 10~4 s~(-1) and k_(nr) = 5.01 × 10~5 s~(-1) were abstracted. Arrhenius analysis of the temperature dependent excited state lifetime revealed an activation energy of ~2500 cm~(-1) and a pre-exponential factor of 10~(10) s~(-1), assigned to activated surface crossing to a ligand field or MLCT excited state. Steady state light excitation of Ru(bpz)_2(deeb)~(2+) in a 20 mM TBAI acetonitrile solution resulted in ligand loss photochemistry with a quantum yield of 5 × 10~(-5). The MLCT excited state was dynamically quenched by iodide with K_(sv) = 1.1 × 10~5 M~(-1) and k_q = 6.6 ± 0.3 × 10~(10) M~(-1) s~(-1) a value consistent with diffusion-limited electron transfer. Excited state hole transfer to iodide was quantitative but the product yield was low due to poor cage escape yields, ø_(ce) = 0.042 ± 0.001. Nanosecond transient absorption was used to quantify the appearance of two photoproducts [Ru(bpz~-)(bpz)(deeb)]~+ and I_2~-. The coincidence of the rate constants for [Ru(bpz~-)(bpz)(deeb)]~+ formation and for excited state decay indicated reductive quenching by iodide. The rate constant for the appearance of I_2~- was about a factor of 3 slower than excited state decay, k=2A± 0.2 × 10~(10) M~(-1) s~(-1) indicating that I_2~- was not a primary photoproduct of excited state electron transfer. A mechanism was proposed where an iodine atom was the primary photoproduct that subsequently reacted with iodide, I~* + I~- → I_2~-. Charge recombination Ru(bpz~-)(bpz)(deeb)~+ + I_2~-→ Ru(bpz)_2(deeb)~(2+) + 2I~- was highly favored, ΔG° = -1.64 eV, and well described by a second-order equal concentration kinetic model, k_(cr) = 2.1 ± 0.3 × 10~(10) M~(-1) s~(-1).