Mechanisms and salt effects in photoredox and quenching process involving cobalt (III) complexes
Date
2018-07-13
Authors
Cai, Lezhen
Journal Title
Journal ISSN
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Abstract
The novel complexes [special characters omitted] and
[special characters omitted] were prepared and characterized. Photoredox quantum
yields for the formation of [special characters omitted] from the above compounds were measured on
irradiation at 360 nm to be 0.065, 0.082, 0.0088 and 0.0040 respectively. With added
thiocyanate a significant increase in ΦCo2+ occurred. This can be modeled in two ways;
(i) scavenging of thiocyanate radical from an initial caged radical pair giving 6-25 ps
estimates for the lifetime of the latter species; (ii) photolysis of a thiocyanate/complex
ion pair, giving formation constants of 0.19, 0.09, 0.08 and 0.05 for the complexes
[special characters omitted] and [special characters omitted] respectively. Subnanosecond
laser flash photolysis studies showed evidence for the formation of [special characters omitted].
The effects of added electrolytes and of viscosity on the formation and decay of [special characters omitted]
were also investigated.
To help to distinguish between the above two mechanisms, the zero-charged
novel complex [special characters omitted] (tacn = 1,4,7-triazacyclononane) was synthesized and
characterized. It is thermally stable in aqueous/DMSO solution, but on irradiation at 360
nm undergoes parallel photosubstitution to form DMSO and aqua-substituted products
with an overall quantum yield of 0.012. The product yields increase linearly with added
thiocyanate. For a 1 M thiocyanate solution, the quantum yield for disappearance of the
starting complex rose to 0.022 and a small redox yield of 0.0008 was found. Under these
same conditions, ns laser flash photolysis at 355 nm revealed a transient absorption
owing to [special characters omitted], which was produced with a quantum yield of 0.036. These results are
interpreted in terms of scavenging of radical pair species by thiocyanate ion followed by
back electron transfer to give a photosubstituted product, and a radical pair quantum yield
of 0.29 and lifetime of 12 ps was derived.
The emission of [special characters omitted] (where pop = μ-pyrophosphite-P,P’) can be
quenched by the complexes [special characters omitted] (where X = [special characters omitted]) only
in the presence of electrolytes. The salt effects have been studied using the salts MCl,
M'Cl2, or [special characters omitted] (where M, M’ and R represent alkali, alkaline earth metals, and
alkyl respectively, with n = 0-3), and [special characters omitted]. For 0.5 M cation concentration, second-order quenching rate constants kq lie in the
range [special characters omitted]. For the different quencher complexes used, kq decreases in the
order [special characters omitted]. The oxidative
quenching products [special characters omitted] are observed, and their quantum
yields are 0.083 and 0.027 respectively for the reaction of [special characters omitted] with
[special characters omitted] and [special characters omitted] in 0.5 M KCl / pH2 solution. The quenching occurred
by atom transfer (dominant) and electron transfer (minor) for quencher [special characters omitted] or
[special characters omitted], while only electron transfer was observed for [special characters omitted] and
[special characters omitted] quenchers. The quenching efficiency of the cobalt complexes increases
with electrolyte concentration and specific cation effects are observed in the kq with the
following trends Li+ < Na+ < K+ < Cs+: Mg2+ < Ca2+ < Sr2+ < Ba2+;
NH4+ < MeNH3+ < Me2NH2+ < Me3NH+: Et3NH+ < Et2NH2+ < EtNH3+:
n-PrNH3+ < EtNH3+ < MeNH3+.
Description
Keywords
Cobalt compounds, Synthesis, Complex compounds