Photoinduced Dynamics of a Diazidocobalt(III) Complex Studied by Femtosecond UV-Pump/IR-to-Vis-Probe Spectroscopy

Abstract

The photochemistry of the cationic diazidocobalt­(III) complex, trans-[Co­(cyclam)­(N3)2]+, following its ligand-to-metal charge transfer (LMCT) excitation is studied in liquid dimethyl sulfoxide (DMSO) solution using femtosecond spectroscopy with detection in a very broad spectral region covering the near-ultraviolet (near-UV) all the way to the mid-infrared (MIR), thereby enabling a combined probing of electronic and vibrational degrees of freedom of the dynamically evolving system. The initially prepared singlet LMCT-state decays, via the metal-centered singlet excited state, 1MC­(1Eg), into the triplet ground state, 3MC (3Eg/3A1g), on a time scale shorter than 25 ps. During this time period, the vibrational spectrum demonstrates uniquely that the nature of the complex changes from a monoazidocobalt­(II) species bearing a neutral azide radical ligand immediately after photon absorption to a metal-centered open-shell diazidocobalt­(III) species. At the same time, the 3MC state is characterized by a very strong absorption band centered at 710 nm, which can be assigned to a transition to the triplet LMCT state. The 1LMCT lifetime is about 2 ps, whereas that of the excited state, 1MC, is defined by the primary intersystem crossing time of 6 ps. The ensuing intersystem recrossing from 3MC to the parent’s singlet ground state, 1A1g, occurs with a rate of 1/(110 ps). The mid-infrared pump–probe spectrum after 1 ns, gives evidence for a heterolytic Co–N bond fission with a quantum yield of ∼5%, leading to free azide anions and the monoazido species, trans-[Co­(cyclam)­(N3)­(OSMe2)]+, featuring an oxygen-bound DMSO ligand in its coordination sphere.

Cyanate Formation via Photolytic Splitting of Dinitrogen

“…The fast component hints at a nonstatistical energy distribution created by preferential population of those vibrational modes, which couple to the electronic transition. The time scale of τ 1 = 1.5 ps is consistent with intramolecular vibrational redistribution (IVR) to establish a quasi-equilibrium of the internal energy. − This assignment is supported by the observation that the amplitudes of the hot bands depend on the pump wavelength (see the SI). UV-pump (400 nm) mid-IR-probe spectroscopy using the strong CO stretching modes as spectroscopic probes also indicate fast internal conversion (IC) followed by cooling dynamics in the ground state (see the SI).…”

Ultrafast processes: coordination chemistry and quantum theory

“…time-resolved spectroscopies, sometimes supplemented by time-resolved X-ray absorption techniques, are mandatory to decipher the electronic excited state decays inherently linked to nuclear relaxation within ps-fs time-scales. 10 This is illustrated by recent studies dedicated to 1 st , 2 nd and 3 rd row transition metal complexes involved in chemical catalysis and biochemical processes, 11 in metal-based charge transfer chromophores, 12 phosphorescent materials for OLEDs, 13 photosensitizers and photocatalysts 14 or metallophilic oligomers designed for functional molecular assemblies and light-emitting devices. 15 Cu(I) complexes, seat of large photoinduced structural changes in the low-lying electronic excited states, have motivated a number of illuminating experimental and theoretical studies.…”

Vibrational Relaxation Dynamics of an Azido–Cobalt(II) Complex from Femtosecond UV-Pump/MIR-Probe Spectroscopy and Model Simulations with Ab Initio Anharmonic Couplings

“…A closer inspection of the data shows that although the peak position approaches its asymptotic long-time value, ν̃ 0 (∞), in an exponential fashion, its early time evolution starting from an initial value, ν̃ 0 (0), is reminiscent of inertial, Gaussian dynamics. Therefore, as we have done in previous studies, we use the Kubo relaxation function to represent the experimental data in a satisfactory manner. A nonlinear least-squares fit returns an inverse relaxation rate, 1/γ, of 3.5 ps together with a coupling strength, Δ, of 0.21 ps –1 .…”