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

Photoinduced processes of tetrahexylammonium tetrachlorocuprate [(C6H13)(4)N](2)(CuCl4)-Cl-II 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 CuCl32- 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)(4)N](2)CuCl4. A global fitting SVD analysis of the femtosecond transient spectra suggested three relaxation times, similar to 400 fs, similar to 1.4 ps and similar to 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 similar to 600 fs) and 181 cm(-1) (damping time similar to 400 fs). These peaks can be tentatively attributed to v(s)(Cu-Cl) symmetric stretching frequency A(1) and T-2 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-.