Photoredox Chemistry of Chloro and Bromo Complexes of Copper(II) in Methanolic Medium

Abstract

Investigation of the photoreduction of copper(II) to copper(I) in methanolic solutions of copper(II) chloride and copper(II) bromide indicates the halide-to-copper(II) charge-transfer (CT) responsible for the photoreduction. Quantum yields for the formation of copper(I) as a function of irradiating wavelength and copper(II) halide concentration show that [Cu(CH3OH)5Cl]+ and [Cu(CH3OH)4Cl2] have identical photoreaction efficiency, while [Cu(CH3OH)4Br2] is more photoreactive than [Cu(CH3OH)5Br]+. In the copper(II) chloride system the chloride-to-copper(II) CT band centered around 272 nm appears to be responsible for the photoreaction, while the bromide-to-copper(II) CT band centered around 245 nm effects the photoreaction in copper(II) bromide system.

Photochemistry of copper(II) chlorocomplexes in acetonitrile: Trapping the ligand-to-metal charge transfer excited state relaxations pathways

“…Therefore, the quantum yields of the primary ionic dissociation reaction cannot be reliably determined using steady-state methods alone. A fraction of the radical products may reform the parent complex on a long (>1 ns) timescale, whereas the remaining fraction may react with other compounds present in solution [21][22][23][24][25][26]. Possible recombination of the radical products after 1 ns and assumptions made when estimating the ground-state bleach amplitudes in the 500-fs and 1-ns A spectra allows us to compare the transient absorption and steady-state photolysis results only qualitatively.…”

“…Much less is known regarding photochemical reactions of these complexes on much longer time scales. On the other side, for a great number of simple copper complexes, steady-state photochemical studies were performed revealing quite interesting photochemistry [19][20][21][22][23][24][25][26], but almost no ultrafast data have been reported to date.…”

“…Therefore, Cl → Cu(II) LMCT excitation is expected to result in redox Cu Cl bond breaking with the formation of copper(I) complex and chlorine atom fragments, i.e., the copper(II)-to-copper(I) photoreduction and ligand oxidation [19][20][21][22][23][24][25][26]. In the most steady-state photolysis studies [21][22][23][24][25][26], copper(II) halide complexes, [CuX n ] 2−n upon LMCT excitation are thought to be dissociative, forming a copper(I) halide [CuX n−1 ] 2−n complex and a halogen atom X • , which subsequently undergo oxidation or halogenation reactions. However, copper(II)-to-copper(I) quantum yields are small, especially for copper(II) chloride complexes, where it do not exceed 13% for acetonitrile and methanol solutions [21,25].…”

“…In the most steady-state photolysis studies [21][22][23][24][25][26], copper(II) halide complexes, [CuX n ] 2−n upon LMCT excitation are thought to be dissociative, forming a copper(I) halide [CuX n−1 ] 2−n complex and a halogen atom X • , which subsequently undergo oxidation or halogenation reactions. However, copper(II)-to-copper(I) quantum yields are small, especially for copper(II) chloride complexes, where it do not exceed 13% for acetonitrile and methanol solutions [21,25].…”

Ultrafast Photochemistry of Copper(II) Monochlorocomplexes in Methanol and Acetonitrile by Broadband Deep-UV-to-Near-IR Femtosecond Transient Absorption Spectroscopy

“…Halogen atoms then can react with other components of the reaction mixture, for example, with the solvent molecules. 34,37 In our previous work by means of femtosecond pump−probe spectroscopy, we showed that upon LMCT excitation of the copper(II) monochlorocomplex in methanol, the major relaxation pathway is internal conversion into the vibrationally hot ground electronic state. 31 However, a minor portion of the excited molecules undergoes photodissociation with the formation of copper(I) solvatocomplexes and chlorine atoms.…”