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Azidoiron complexes serve as valuable photochemical precursors for catalytically active species containing high-valent iron. In bioinorganic chemistry, azido(tetraphenylporphinato)iron(III), i.e., [FeIII(tpp)(N3)] with tpp = 5, 10, 15, 20-tetraphenylporphyrin-21, 23-diido, constitutes the archetypal model system that was used to access for the first time the terminal nitridoiron core, FeV ≡ N, in the biomimetic redox-non-innocent ligand environment. So far, the light-induced dynamics leading to the oxidation of the metal and the release of dinitrogen from the N3-ligand have only been studied for precursors featuring redox-innocent auxiliary ligands that simplify the electronic structure change accompanying the photo-transformation. Here, we monitored the primary events of the above paradigmatic complex, following its optical excitation in the ultraviolet-to-visible spectral range using femtosecond spectroscopy with probing in both the UV–vis and mid-infrared regions. Following ultrafast Soret-excitation at 400 nm, the complex relaxes to the lowest excited sextet state by a first internal conversion in less than 200 fs. The excited state then undergoes vibrational relaxation on a time scale of roughly 2 ps before internally converting yet again to recover the sextet electronic ground state within 19.5 ps. Spectroscopic evidence is obtained neither for a transient occupation of the energetically lowest metal-centered state, 41A1, nor for vibrational relaxation in the ground-state. The primary processes seen here are thus in contrast to those previously derived from ultrafast UV-pump/vis-probe and UV-pump/XANES-probe spectroscopies for the halide congener [FeIII(tpp)(Cl)]. Any photochemical transformation of the complex arises from two-photon-induced dynamics. [ABSTRACT FROM AUTHOR]

The dynamics of vibrational relaxation of carbon dioxide in water has been studied using femtosecond mid-infrared pump–probe spectroscopy with excitation of the anti-symmetric stretching (ν3) fundamental state of the solute. The relaxation dynamics were recorded at a constant pressure of 500 bars and in the temperature range between 300 and 600 K, thereby covering the liquid-to-near-critical region of the solvent. The excited state of the ν3-mode is deactivated in two competing pathways: (i) direct relaxation to the ground state with resonant transfer of the excess vibrational energy into the bending–librational continuum of the water solvent and (ii) relaxation to the bending fundamental state with transfer into the intramolecular bending mode of H2O. The rate of pathway (i) decreases with increasing temperature, from ∼1/(9 ps) at 300 K to ∼(1/16 ps) at 600 K and obeys Fermi's golden rule strictly, provided that the spectral density of energy-accepting solvent states is derived from the stationary infrared absorption profile of H2O. The rate of pathway (ii) is 1/(23 ps) and assumed to be temperature-independent within our data analysis. Finally, the bending fundamental of CO2 can also relax to the ground state by resonantly transferring the remaining excess energy to the librational fundamentals of the solvent. [ABSTRACT FROM AUTHOR]

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Transition‐metal nitrides/nitrenes are highly promising reagents for catalytic nitrogen‐atom‐transfer reactivity. They are typically prepared in situ upon optically induced N2 elimination from azido precursors. A full exploitation of their catalytic potential, however, requires in‐depth knowledge of the primary photo‐induced processes and the structural/electronic factors mediating the N2 loss with birth of the terminal metal‐nitrogen core. Using femtosecond infrared spectroscopy, we elucidate here the primary molecular‐level mechanisms responsible for the formation of a unique platinum(II) nitrene with a triplet ground state from a closed‐shell platinum(II) azide precursor. The spectroscopic data in combination with quantum‐chemical calculations provide compelling evidence that product formation requires the initial occupation of a singlet excited state with an anionic azide diradical ligand that is bound to a low‐spin d8‐configured PtII ion. Subsequent intersystem crossing generates the Pt‐bound triplet azide diradical, which smoothly evolves into the triplet nitrene via N2 loss in a near barrierless adiabatic dissociation. Our data highlight the importance of the productive, N2‐releasing state possessing azide ππ* character as a design principle for accessing efficient N‐atom‐transfer catalysts. [ABSTRACT FROM AUTHOR]

The dynamics of intramolecular hydrogen-bonding involving sulfur atoms as acceptors is studied using two-dimensional infrared (2DIR) spectroscopy. The molecular system is a tertiary alcohol whose donating hydroxy group is embedded in a hydrogen-bond potential with torsional C3-symmetry about the carbon–oxygen bond. The linear and 2DIR-spectra recorded in the OH-stretching region of the alcohol can be simulated very well using Kubo's line shape theory based on the cumulant expansion for evaluating the linear and nonlinear optical response functions. The correlation function for OH-stretching frequency fluctuations reveals an ultrafast component decaying with a time constant of 700 fs, which is in line with the apparent decay of the center line slopes averaged over absorption and bleach/emission signals. In addition, a quasi-static inhomogeneity is detected, which prevents the 2DIR line shape to fully homogenize within the observation window of 4 ps. The experimental data were then analyzed in more detail using a full ab initio approach that merges time-dependent structural information from classical molecular dynamics (MD) simulations with an OH-stretching frequency map derived from density functional theory (DFT). The latter method was also used to obtain a complementary transition dipole map to account for non-Condon effects. The 2DIR-spectra obtained from the MD/DFT method are in good agreement with the experimental data at early waiting delays, thereby corroborating an assignment of the fast decay of the correlation function to the dynamics of hydrogen-bond breakage and formation. [ABSTRACT FROM AUTHOR]

The ferrioxalate actinometer is widely used as an analytical standard to determine the photon flux of light sources in photochemical reactors. Yet, the underlying mechanistic functioning of the actinometer at the molecular level is entirely unknown. Here, we present results from femtosecond UV-pump/mid-infrared-probe spectroscopy to reveal the elementary events including the primary C–C and Fe–O-bond breakages that follow an initial photon absorption by the ferrioxalate complex.

The elementary light-triggered dynamics of the transition metal complex, trans-[CoIII(acac)2(N3)(py)] (trans-[1]), was investigated using femtosecond mid-infrared (mIR) and UV-Vis spectroscopies following ultrafast acetylacetonate-to-cobalt charge-transfer excitation at 266 nm. The spectro-temporal response is indicative of an ultrafast return to the electronic ground state via a sequential intramolecular electron transfer. The locally excited ligand-to-metal charge transfer state, 1LMCTacac, decays within 200 fs via azide-to-acac electron transfer to another charge-transfer state, 1LMCTazide, that features an azidyl radical ligand attached to a cobalt(II) center. A subsequent cobalt-to azidyl electron transfer occurring within 1.8 ps generates an excited metal centered state, 1MC, thereby re-establishing the fully closed-shell character of the anionic azido and acac ligands. The fully relaxed electronic ground state of trans-[1] is then recovered via internal conversion and vibrational energy relaxation on a 10 ps-time scale. [ABSTRACT FROM AUTHOR]

Femtosecond two-dimensional infrared (2DIR) spectroscopy was carried out on stereo-selectively synthesized poly-alcohols featuring a quasi-linear array of hydrogen-bonds. From the 2DIR spectra pump-frequency-dependent vibrational lifetimes are extracted, which in turn reflect the strength of the coupling between the hydroxyls constituting the H-bonded chain. The line-broadening dynamics reflect uniquely the conformational control of the scaffold supporting the H-bond wire and the resulting structural flexibility of the hydrogen-bond network.

Liquid and supercritical ammonia (NH3) is photo-ionized at an energy of 9.3 eV with 100-fs duration pulses at a wavelength of 266 nm. The ionization involves two photons and generates fully solvated electrons via the conduction band of the solvent within the time resolution of the experiment. The dynamics of their ensuing geminate recombination is followed in real time with femtosecond near-infrared (IR) probe pulses. The recombination mechanism can be understood as an ion-pair mediated reaction. The electron survival probability is found to be in quantitative agreement with the classical Onsager theory for the initial recombination of ions.

The ultrafast photolysis of the cation complex [(cyclam-ac)FeN3]+ is studied by femtosecond spectroscopy with ultraviolet excitation and mid-infrared probing. Immediately after UV absorption, the excited complex undergoes internal conversion and azide dissociation within 2 ps. The subsequent vibrational relaxation in the electronic ground state and geminate recombination of the fragments take place on time scales of 13 and 20 ps, respectively.

Femtosecond mid-infrared pump-probe and two-dimensional mid-infrared spectroscopy have been used to investigate the dynamics of vibrational relaxation and vibrational spectral diffusion of the asymmetric N3-stretching vibration of pseudo-octahedral azidoiron(III) complexes, [L6-nFe(N3)n]+ with n = 1 or 2 and L being an auxiliary ligand of denticity 6-n, in acetonitrile at room temperature. Compared to the free azide anion in acetonitrile solution, the vibrational relaxation dynamics are considerably accelerated. Vibrational energy transfer to the solvent is accelerated by virtue of a resonance with an overtone transition of the solvent. Intramolecular vibrational redistribution is found to be accelerated by virtue of a coupling between the initial azide stretching vibration and the torsional modes involving the axial ligands. Vibrational spectral diffusion within the asymmetric N3-stretching resonance was found to be insensitive to solvent fluctuations because the axial azide ligands are only partially accessible to the solvent. The particular role of intramolecular structural relaxations of the complex for shaping the linear and nonlinear two-dimensional infrared spectra is discussed in terms of ultrafast symmetry-breaking torsional fluctuations and on the basis of density functional theory calculations. [ABSTRACT FROM AUTHOR]