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

Ultrafast excited-state dynamics of CuCl42– in acetonitrile is studied by femtosecond broadband transient absorption spectroscopy following excitation of the complex into all ligand-field (LF or d–d) states and into the two ligand-to-metal charge transfer (LMCT) states corresponding to the most intense steady-state absorption bands. The LF excited states are found to be nonreactive. The lowest-lying 2E LF excited state has a lifetime less than 150 fs, and the lifetimes of the second (2B1) and the third (2A1) LF excited states are 1 and 5 ps, respectively. All three LF states decay directly into the ground 2B2 state. Such significant differences in excited-state decay time constants were rationalized computationally through time-dependent density functional theory (TD-DFT) computations. TD-DFT mapping of the relaxation pathway along the symmetric Cl–Cu–Cl umbrella bending vibration gives evidence for a conical intersection between the 2E excited state and the ground 2B2 state. The LMCT states decay within ...