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

Abstract

Femtosecond spectroscopy was applied to study the ultrafast dynamics for the excited states of dithiolate Cu(ii) and Ni(ii) complexes. The detailed information on the initial steps after the absorption of a photon by the metal complexes is of fundamental importance to understand the mechanism of photochemical reactions. The fast processes for the dithiolate complexes have hardly been studied. In this review the spectra of transients and their lifetimes will be presented. For example, the xanthogenate Ni(S2COEt)2 complex in acetonitrile and CCl4 after the pulse of the second harmonic (100 fs, 400 nm) of a Ti:S laser moves to the excited (1)LMCT state which decays in 0.76 ps to the excited (3)LF state. In 6.8 ps the (3)LF state undergoes vibrational cooling and then it slowly decays in 550 ps to the ground state. However, for many dithiolate complexes the kinetic curves can be well treated in a two-exponential approximation. A short time (less than 1 ps) may include several processes (relaxation of the Franck-Condon state, redistribution of vibrational energy (IVR), internal conversion (IC) and intersystem crossing (ISC)). A long time (a few picoseconds) usually reflects the vibrational cooling of the ground state. The quantum yields of the dithiolate and dithiolene complex disappearance in halogen containing solvents have a strong dependence on the wavelength of irradiation. It is very likely that electron transfer to the acceptor becomes effective when the electron in the excited complex moves to antibonding ligand orbitals localized at the periphery of the complex close to the acceptor molecule (halogenated solvent).

Molecular Design Principles to Elongate the Metal-to-Ligand Charge Transfer Excited-State Lifetimes of Square-Planar Nickel(II) Complexes

“…A very recent study reported ligand-based fluorescence from a Ni II complex in solution without direct metal contribution . Weakly luminescent Ni II compounds emitting from triplet states were found in two other recent works, albeit only in the solid state, in which the processes illustrated in Figure a/b are largely suppressed. , Studies in fluid solution at room temperature by transient absorption and transient X-ray absorption spectroscopy were initially limited to porphyrins, phthalocyanines, , and dithiolato complexes of Ni II and revealed very fast decays to relaxed 3 d–d (MC) excited states. , A recent study aimed to install an emissive (metal-centered) spin-flip excited state but was not successful in that regard . Another recent study claimed that emission is observable for two octahedrally coordinated Ni II complexes in the solid state at 150 K, but the emission was unusually broad and overlapped substantially with the lowest-energy absorption bands .…”

“…77,80 Studies in fluid solution at room temperature by transient absorption and transient X-ray absorption spectroscopy were initially limited to porphyrins, phthalocyanines, 81,82 and dithiolato complexes of Ni II and revealed very fast decays to relaxed 3 d−d (MC) excited states. 83,84 A recent study aimed to install an emissive (metal-centered) spin-flip excited state but was not successful in that regard. 85 Another recent study claimed that emission is observable for two octahedrally coordinated Ni II complexes in the solid state at 150 K, but the emission was unusually broad and overlapped substantially with the lowest-energy absorption bands.…”

Ultrafast Excited-State Dynamics of Ligand-Field and Ligand-to-Metal Charge-Transfer States of CuCl42– in Solution: A Detailed Transient Absorption Study

“…In aqueous solutions, the coordinated ligand can be photochemically substituted by the water molecule (the photoaquation reaction). − For example, Fehlner and Turner proposed that aqueous NiLCl 2 , where L = NH 2 (CH 2 ) 2 NH­(CH 2 ) 3 NH­(CH 2 )­NH 2 , upon LMCT excitation forms NiL­(H 2 O) 2 . The femtosecond studies of metal complexes with organic ligands have revealed another relaxation pathway of LMCT excited states, such as highly efficient internal conversion and intersystem crossing into low-lying d–d states followed by their relaxation into the ground state (GS) on femto- and picosecond time scales. − The femtosecond studies of aqueous ferric hexacyanide, [Fe­(CN) 6 ] 3− , revealed two main relaxation channels following LMCT excitation: CN – elimination followed by aquation to form [Fe­(CN) 5 H 2 O] 2– and internal conversion into the vibrationally hot ground electronic state. , …”

Static and Dynamic Electron Correlation in the Ligand Noninnocent Oxidation of Nickel Dithiolates

“…31 Along with a wide range of DFT studies on this class of molecules, several ab initio studies have largely focused on predicting the singlet− triplet gap of the neutral complex. 32−36 Finally, photoelectron 37−39 and femtosecond 40 spectroscopy have recently been used to study the ionization and ultrafast dynamics of these compounds.…”

“…DFT combined with X-ray absorption spectroscopy (XAS) data has shown that the ligands in these species act as the source or sink of electrons during the redox process, and the ligands are described as redox noninnocent. − Ligand noninnocence is not limited to metal dithiolates, however, and generally allows the metal ions to assume unexpected oxidation states. − In the case of nickel dithiolates, the metal remains in a Ni­(II) oxidation state throughout the oxidative series . Along with a wide range of DFT studies on this class of molecules, several ab initio studies have largely focused on predicting the singlet–triplet gap of the neutral complex. − Finally, photoelectron − and femtosecond spectroscopy have recently been used to study the ionization and ultrafast dynamics of these compounds.…”