The Photochemical Route to Octahedral Iron(V). Primary Processes and Quantum Yields from Ultrafast Mid-Infrared Spectroscopy

Abstract

Recently, the complex cation [(cyclam-ac)Fe III (N 3 )] + has been used in solid matrices under cryogenic conditions as a photochemical precursor for an octahedral iron nitride containing the metal at the remarkably high oxidation state +5. Here, we study the photochemical primary events of this complex cation in liquid solution at room temperature using femtosecond time-resolved mid-infrared (fs-MIR) spectroscopy as well as step-scan Fourier-transform infrared spectroscopy, both of which were carried out with variable-wavelength excitation. In stark contrast to the cryomatrix experiments, a photooxidized product cannot be detected in liquid solution when the complex is excited through its putative LMCT band in the visible region. Instead, only a redox-neutral dissociation of azide anions is seen under these conditions. However, clear evidence is found for the formation of the highly oxidized iron nitride product when the photolysis is carried out in liquid solution with UV light. Yet, the photooxidation must compete with photoreductive Fe−N bond cleavage leading to azide radicals and an iron(II) complex. Both, redox-neutral and photoreductive Fe−N bond breakage as well as photooxidative N−N bond breakage occur on a time scale well below a few hundred femtoseconds. The majority of fragments suffer from geminate recombination back to the parent complex on a time scale of 10 ps. Upper limits of the primary quantum yield for photooxidation are derived from the fs-MIR data, which increase with increasing energy of the photolysis photon.

Photoinduced Pedalo‐Type Motion in an Azodicarboxamide‐Based Molecular Switch

“…[40][41][42][43][44][45][46][47][48] Figure 3a displays transient IR spectra of tetrabenzylazodicarboxamide in chloroform (c = 6.6 mm)i nt he 1600-1775 cm À1 range associated with the carbonyl stretch region after excitation at 350 nm. [40][41][42][43][44][45][46][47][48] Figure 3a displays transient IR spectra of tetrabenzylazodicarboxamide in chloroform (c = 6.6 mm)i nt he 1600-1775 cm À1 range associated with the carbonyl stretch region after excitation at 350 nm.…”

“…To determine the light-triggered properties of the azodicarboxamide functional group,w eh ave performed picosecond time-resolved IR absorption spectroscopy,w hich has been shown to provide ah igh temporal resolution and structural sensitivity. [40][41][42][43][44][45][46][47][48] Figure 3a displays transient IR spectra of tetrabenzylazodicarboxamide in chloroform (c = 6.6 mm)i nt he 1600-1775 cm À1 range associated with the carbonyl stretch region after excitation at 350 nm. In agreement with our DFT calculations that show that the first electronic transition with nonzero oscillator strength is expected at around 340 nm, no transient signals could be observed upon excitation at longer wavelengths.T his transition corresponds to the S 3 !…”

Spin‐State‐Controlled Photodissociation of Iron(III) Azide to an Iron(V) Nitride Complex

“…[19,20] Later however, ultrafast experiments revealed the formation of iron nitrides at room temperature as well. [21][22][23] Here, we study photodissociation of iron(III) azides at molecular level with ion spectroscopy at variable temperature (3 K -310 K). [24][25][26][27] The experiments provide absorption spectra of mass-selected ions separated according to the individual dissociation pathways.…”

Spin‐State‐Controlled Photodissociation of Iron(III) Azide to an Iron(V) Nitride Complex

“…[19,20] However,u ltrafast experiments have recently shown the formation of iron nitrides at room temperature. [21][22][23] Herein, we report the photodissociation of iron(III) azides studied at the molecular level by ion spectroscopy using different temperatures (3-310 K). [24][25][26][27] Photo-dissociation of mass-selected iron azide complexes 1 [28] in the visible range resulted in photoreduction associated with N 3 C elimination or photooxidation, forming the desired iron-(V) nitride complex 3,concomitantly with the elimination of N 2 (Scheme 1).…”