Solvent Dependence of the Fast Photooxidation of Transition-Metal Maleonitriledithiolate Complexes, [M(S2C2(CN)2)2]2- (M = Ni, Pt), in Acetonitrile-Chloroform Mixtures

Abstract

The photochemistry of M(mnt)22-(M = Ni, Pt) in CHCI3-CH3CN solvent mixtures was investigated upon excitation in the near-UV region of the spectrum. In both complexes, the quantum yield increased with increasing concentration of CHCI3 up to a limiting value of 0.014 and 0.024 for Ni(mnt)22~a nd Pt(mnt)22-, respectively, under 365-nm excitation. Similar behavior was observed upon 313-nm photolysis, but in this case, the limiting quantum yields were 0.25 and 0.14 for Ni(mnt)22™ and Pt(mnt)22-, respectively. Normalization of the quantum yields with respect to the limiting quantum yield for each wavelength yields identical solvent dependence curves for both complexes indicating that the solvent dependence is independent of the wavelength dependence. Picosecond laser flash photolysis was used to probe the fundamental processes occurring in the photooxidation of M(mnt)22-(M = Ni, Pt). The picosecond spectra are identical in both halogenated and nonhalogenated solvents. No evidence for primary radicals which could limit yields by geminate recombination is found. It is proposed that the main factor which controls the reaction must be the electronic factor of mixing of the initially excited state with a CTTS state.

Femtosecond spectroscopy of the dithiolate Cu(ii) and Ni(ii) complexes

“…71,72 For the [Ni-(mnt) 2 ] 2− complex, the quantum yield increases with increasing mole fraction of chloroform in a CH 3 CN-CHCl 3 mixture. 46 When the lifetime of the excited state is about 2-3 ps, a diffusion can deliver an electron acceptor (for example a CCl 4 molecule) to the excited complex over a distance less than 1 Å. Thus, electron transfer is possible if the molecule of the acceptor is in the first (along the axial axis of the flat complex with the MS 4 unit) coordination sphere of the excited complex.…”

“…Among them, studies devoted to the complexes with a sulfur containing ligand can be mentioned 46 46 The intermediate absorption is independent of the solvent (CH 3 CN or CHCl 3 ) and almost vanishes in 100 ps. In this paper the results of the study of ultrafast processes for the dithiolate Cu(II) and Ni(II) complexes will be presented.…”

“…In the literature, work on the photophysics of metal complexes mainly with nitrogen-containing ligands [42][43][44][45] is present. Among them, studies devoted to the complexes with a sulfur containing ligand can be mentioned 46 which consider the photophysics and photochemistry of the Ni(mnt) gives rise to a broad absorption band over the range 426-675 nm with a dip at 475 nm caused by the disappearance of absorption from the ground state of the complex. 46 The intermediate absorption is independent of the solvent (CH 3 CN or CHCl 3 ) and almost vanishes in 100 ps.…”

“…Among them, studies devoted to the complexes with a sulfur containing ligand can be mentioned 46 which consider the photophysics and photochemistry of the Ni(mnt) gives rise to a broad absorption band over the range 426-675 nm with a dip at 475 nm caused by the disappearance of absorption from the ground state of the complex. 46 The intermediate absorption is independent of the solvent (CH 3 CN or CHCl 3 ) and almost vanishes in 100 ps. In this paper the results of the study of ultrafast processes for the dithiolate Cu(II) and Ni(II) complexes will be presented.…”

Photochemical studies as a function of solvent viscosity. A new photochemical pathway in the reaction of (η5-C5H4Me)2Mo2(CO)6 with CCl4

“…One remaining possibility that can explain the significant quantum yield at high viscosity and the absence of any observed products other than CpЈMo(CO) 3 Cl is electron transfer from an excited state of CpЈ 2 Mo 2 (CO) 6 to CCl 4 , possibly mediated by a charge-transfer-to-solvent (CTTS) state. In support of this suggestion, it is noted that photoinduced electron transfer to halogenated solvents is well established [25][26][27][28][29][30] and can be as fast as 10 Ϫ10 -10 Ϫ11 s. 27 In particular, oxidative quenching of the excited state of Cp 2 Mo 2 (CO) 6 by CHBr 3 was proposed by Meyer and Caspar: 8, 31 The degree of reaction that occurs by the alternative pathway is significant in all solvents: the quantum yield for the reaction with CCl 4 by this route ( x ) was usually between 0.10 and 0.24. The only exception was in the ethanol-propylene glycol system, where x was 0.05.…”