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

Abstract

Vibrational energy relaxation is of critical importance for the lightcontrolled reactivity of transition-metal complexes. In time-resolved optical spectroscopies, it gives rise to pronounced spectral redistributions with complex band shifts and thus to nonexponential kinetics, all of which are very difficult to quantify. Here we study the vibrational relaxation dynamics of a pentacoordinated azido−cobalt(II) complex in liquid solution following its ultrafast charge-transfer excitation in the near-ultraviolet (UV). The complex is photochemically remarkably stable and returns within the experimental time resolution back to its quartet electronic ground state via internal conversion. The nonadiabatic transition effectively instantaneously converts the entire photon energy into kinetic energy of the vibrational degrees of freedom. The ensuing relaxation dynamics of the vibrationally highly excited complex are monitored as a function of time using femtosecond mid-infrared (MIR) spectroscopy in the antisymmetric stretching region of the azido ligand and occur on a time scale of a few tens of picoseconds. The dynamic evolution of the MIR spectrum due to vibrational cooling of the complex can be understood quantitatively within the framework of an anharmonic coupling model, which relies on an ab initio intramolecular cubic/quartic force field from density functional theory combined with second-order vibrational perturbation theory. The simulations suggest that the primary internal conversion preferentially dumps the excess energy into the low-frequency bending modes of the azido ligand, whereas its high-frequency stretching modes are barely affected by the initial nonadiabatic transition. Surprisingly, the two bending vibrations appear to relax independently of one another, each with its own characteristic cooling time.

Cyanate Formation via Photolytic Splitting of Dinitrogen

“…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. 70 − 75 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 “end-on”-to-“side-on” binding-mode isomerization of an iron–carbon dioxide complex

“…However, immediately after electronic relaxation, the parent's ground state is dressed with substantial vibrational excess energy and the vibrational cooling manifests itself in the complex dynamic spectral redistribution. 42 Finally, the upshifted absorption arising from the secondary product exhibiting the ''side-on''-bound CO 2 -ligand is also evident as a pronounced shoulder building up on the high-frequency edge of the equilibrated in-phase CQO stretching band of the thermalized parent.…”

“…Both degrees of freedom can couple to a fictitious bath mode with a harmonic frequency, ñ0c , by virtue of the off-diagonal anharmonicities, x ac and x bc . The absorption spectrum of the vibrationally excited This journal is © the Owner Societies 2021 electronic ground state is then written as 38,42…”

Photodissociation Dynamics of Nitric Oxide from N-Acetylcysteine- or N-Acetylpenicillamine-bound Roussin’s Red Ester

“…TRIR spectroscopy uses an IR probe pulse to measure a series of transient vibrational spectra of the photoexcited sample. 31,32 The vibrational spectrum is sensitive to the structure of the compound, enabling the assignment of reaction intermediates, thereby helping determine the detailed dynamics of the photoexcited molecule. TRIR spectra in broad time spanning from 0.3 ps to 10 μs (time order of 10 8 ) encompass most spectral features related to the primary and secondary reactions.…”

“…Time-resolved (TR) spectroscopy, in particular, pump-probe spectroscopy, has been widely applied in the study of the dynamics of photoexcited molecules. − In femtosecond TR pump-probe spectroscopy, an equilibrium sample is excited by an intense femtosecond laser pulse (pump), and the subsequent reaction is monitored by collecting transient spectra sequentially with another femtosecond laser pulse (probe). TRIR spectroscopy uses an IR probe pulse to measure a series of transient vibrational spectra of the photoexcited sample. , The vibrational spectrum is sensitive to the structure of the compound, enabling the assignment of reaction intermediates, thereby helping determine the detailed dynamics of the photoexcited molecule. TRIR spectra in broad time spanning from 0.3 ps to 10 μs (time order of 10) encompass most spectral features related to the primary and secondary reactions.…”