Ultrafast
excited-state dynamics of CuCl4
2– 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 200 fs with the
primary deactivation mode consistent to be Cu–Cl stretch. A
fraction of the CuCl4
2– complexes excited
into the LMCT states undergoes ionic dissociation to form products
that survive longer than 1 ns. The remaining fraction undergoes internal
conversion, which can be viewed as back electron transfer, populating
the lower vibrationally hot LF states. The LF states populated from
the LMCT states exhibit the same lifetimes as the Franck–Condon
LF states and likewise decay directly into the ground state.