Excited-state dynamics of metal string complex Ni3(dpa)4x2** from femtosecond transient absorption spectra

Abstract

The excited-state dynamics of Ni3(dpa)4X2, in which dpa is the ligand di(2-pyridyl)amido and X=NCS or Cl, are investigated by transient absorption spectroscopy. The ππ* and dd states are excited upon irradiation at wavelengths of 330 and 600 nm, respectively. Similar transient spectra are observed under the experimental temporal resolution. The transient species also show weak absorption. It is proposed that a low-lying metal-centered dd state is accessed immediately after excitation. Analyses of the experimental kinetic traces reveal a rapid conversion from a ligand-centered ππ* state to a metal-centered dd state in 0.1-0.4 ps. Vibrational cooling occurs with a time coefficient of 3.0-15.9 ps. From the spectral shift observed in the transient spectra relative to the steady-state spectra, the dd state is assigned as B1/B2(Nit). This dd state eventually converts to the electronic ground state, in about 100 ps for the isothiocyanate complex and 200 ps for the chloride. In this trimetal string complex, relaxation to the ground electronic surface after excitation is therefore rapid. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.