Visible Light Generation of Iodine Atoms and I−I Bonds: Sensitized I− Oxidation and I₃− Photodissociation

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Abstract

Direct 355 or 532 nm light excitation of TBAI(3), where TBA is tetrabutyl ammonium, in CH(3)CN at room temperature yields an iodine atom, I(*), and an iodine radical anion, I(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 x 10(10) M(-1) s(-1). The I(2)(-*) intermediates are unstable with respect to disproportionation and yield initial reactants, k = 3.3 +/- 0.1 x 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(2)H(5)CO(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 epsilon = 1.7 x 10(4) M(-1) cm(-1). Visible light excitation resulted in photoluminescence with a corrected maximum at 620 nm, a quantum yield phi = 0.14, and an excited state lifetime tau = 1.75 micros from which k(r) = 8.36 x 10(4) s(-1) and k(nr) = 5.01 x 10(5) s(-1) were abstracted. Arrhenius analysis of the temperature dependent excited state lifetime revealed an activation energy of approximately 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 x 10(-5). The MLCT excited state was dynamically quenched by iodide with K(sv) = 1.1 x 10(5) M(-1) and k(q) = 6.6 +/- 0.3 x 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, phi(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 = 2.4 +/- 0.2 x 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, DeltaG(o) = -1.64 eV, and well described by a second-order equal concentration kinetic model, k(cr) = 2.1 +/- 0.3 x 10(10) M(-1) s(-1).

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