Femtosecond spectroscopy and TD-DFT calculations of CuCl42− excited states

Abstract

Photoinduced processes of tetrahexylammonium tetrachlorocuprate [(C₆H₁₃)₄N]₂Cu^IICl₄ in chloro-organic solvents were investigated by steady state photolysis and femtosecond transient absorption spectroscopy. The quantum yield of photoreduction of CuCl₄²⁻ was estimated to be about 1%; the process resulted in the formation of the copper(I) chlorocomplex Cu^ICl₃²⁻ and a chlorine atom. Femtosecond laser photolysis with a 422 nm, 40 fs pulse revealed a three-exponential decay of the LMCT excited state of [(C₆H₁₃)₄N]₂CuCl₄. A global fitting SVD analysis of the femtosecond transient spectra suggested three relaxation times, ∼400 fs, ∼1.4 ps and ∼5.8 ps. Oscillations in transient absorption kinetic traces were documented for CuCl₄²⁻ solutions in 2-chlorobutane. The oscillation Fourier transform analysis of the oscillations and linear predictive singular value decomposition revealed peaks at 283 cm⁻¹ (damping time ∼600 fs) and 181 cm⁻¹ (damping time ∼400 fs). These peaks can be tentatively attributed to ν_s(Cu–Cl) symmetric stretching frequency A₁ and T₂ reflecting excited state vibrational coherence. Quantum chemical calculations suggest a possible scheme for relaxation pathways in CuCl₄²⁻. The observed transient excited state absorption bands agree semiquantitatively with the calculated transition bands of CuCl₄²⁻.