Ultrafast Excited-State Dynamics of Ligand-Field and Ligand-to-Metal Charge-Transfer States of CuCl₄^2– in Solution: A Detailed Transient Absorption Study

Abstract

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.

Ultrafast Excited-State Dynamics of CuBr3–Complex Studied with Sub-20 fs Resolution

“…Previous studies demonstrated that the LMCT states of copper(II) complexes are usually repulsive and short-lived. 17,18,24,40 Indeed, we observed that the LMCT state of the CuBr 3 − complex is short-lived and undergoes internal conversion into the vibrationally hot LF excited state with time constants of 30 and 40 fs in acetonitrile and dichloromethane, respectively. This LF state undergoes internal conversion into the ground state.…”

“…The LMCT states of copper­(II) complexes usually have a repulsive nature along a copper–ligand coordinate, , and the LMCT excitation of such species often results in the metal–ligand bond-breaking either with or without a change of the metal oxidation state. Previous studies have indicated that upon LMCT excitation, copper­(II) halide complexes undergo either ionic dissociation with formation of the halide ion or radical dissociation with formation of a halogen atom accompanied by metal ion photoreduction (radical dissociation channel). − However, the photodissociation quantum yield of copper­(II) complexes is relatively small, rarely exceeding 5%. In turn, the main relaxation channel of the LMCT state is an internal conversion into lower-lying excited states through either ballistic or coherent potential energy surface crossing, which is extremely fast (<300 fs) .…”

“…In turn, the main relaxation channel of the LMCT state is an internal conversion into lower-lying excited states through either ballistic or coherent potential energy surface crossing, which is extremely fast (<300 fs) . Some authors proposed that the LMCT states relax directly into the ground state, − and others concluded that relaxation proceeds stepwise through the set of low-lying LF excited states. ,,,− …”

Giant Optical Activity and Second Harmonic Generation in 2D Hybrid Copper Halides

“…(R-/S-MBA) 2 CuCl 4 exhibited mirrored CD signals peaking at 378, 266, and 226 nm, which are consistent with their absorption bands, as a result of the Cotton effect. [15] In addition, the peak at 378 nm corresponds to the characteristic ligand-to-metal charge transfer (LMCT) transition band of [CuCl 4 ] 2À , [16] as a consequence of charge transfer from fully occupied chloride ion localized orbitals or s Cu-Cl and p Cu-Cl orbitals to copper ion semioccupied d xy orbital. [17] These results demonstrate that the chirality of the organic component was successfully…”

Virtual Issue on Ultrafast Spectroscopy

“…18 Ultrafast transient absorption (TA) of CuCl 4 2− revealed state dependent photochemistry, with a low lying ligand field energy level being photoinactive, while a higher energy LMCT state was dissociative. 19 Studies of the ultrafast to millisecond excited state evolution in [Cr(NCS) 6 ] 3− showed multiple pathways for relaxation and photochemistry. 20 simulations of ultrafast decay rate constants for Ru complexes were presented.…”