Ultrafast Photochemistry of Copper(II) Monochlorocomplexes in Methanol and Acetonitrile by Broadband Deep-UV-to-Near-IR Femtosecond Transient Absorption Spectroscopy

Abstract

Photochemistry of copper(II) monochlorocomplexes in methanol and acetonitrile solutions is studied by UV-pump/broadband deep-UV-to-near-IR probe femtosecond transient absorption spectroscopy. Upon 255 and 266 nm excitation, the complexes in acetonitrile and methanol, respectively, are promoted to the excited ligand-to-metal charge transfer (LMCT) state, which has a short (sub-250 fs) lifetime. From the LMCT state, the complexes decay via internal conversion to lower-lying ligand field (LF) d−d excited states or the vibrationally hot ground electronic state. A minor fraction of the excited complexes relaxes to the LF electronic excited states, which are relatively long-lived with lifetimes >1 ns. Also, in methanol solutions, about 3% of the LMCT-excited copper(II) monochlorocomplexes dissociate forming copper(I) solvatocomplexes and chlorine atoms, which then further react forming long-lived photoproducts. In acetonitrile, about 50% of the LMCT-excited copper(II) monochlorocomplexes dissociate forming radical and ionic products in a ratio of 3:2. Another minor process observed following excitation only in methanol solutions is the re-equilibration between several forms of the copper(II) ground-state complexes present in solutions. This re-equilibration occurs on a time scale from sub-nanoseconds to nanoseconds.

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

“…The electronic absorption spectrum of CuCl 4 2– (Figure ) displays intense UV bands corresponding to LMCT transitions and much weaker near-IR (NIR) bands because of symmetry-forbidden LF transitions between the d orbitals of a copper ion. − As shown in the previous work, CuCl 4 2– in acetonitrile has D 2 d symmetry, which is an intermediate geometry between the square planar and the tetrahedral ones. For solid Cs 2 CuCl 4 , where the copper­(II) ion also has D 2 d local symmetry and a similar absorption spectrum, Fergusson assigned the absorption bands to the individual transitions on the basis of spectral polarized absorption measurements on a single crystal at low temperatures .…”

Comparison of structural dynamics and coherence of d–d and MLCT light-induced spin state trapping

“… 2 – 6 Photophysics was deeply investigated in systems based on iron, ruthenium, cobalt, copper, chromium or manganese. 7 – 11 LIESST has been intensely studied in Fe II N 6 spin-crossover materials in which the Fe ion experiences an almost octahedral ligand-field. At low temperature, the ground state is low spin 1 A 1 (LS, S = 0, t62ge0gL 0 ), where L refers to unoccupied ligand orbitals.…”

Copper Chloro-Complexes Concentrated Solutions: An Electrochemical Study

“…Differences in absorbance are justified by the different molar extinction coefficients of the complexes involved. On the other hand, phosphate anions do not enter the inner coordination sphere of the copper [44] as demonstrated by the spectra in Figure 8b of a diluted solution (200 times) with the same composition of solutions 9 and 11. These spectra show the LMCT bands with an unvaried presence of CuCl 3 − .…”