Dry THF was obtained from an MBraun SPS-800 solvent drying system and was subsequently stored over 4 Å molecular sieve. 1H NMR spectra were recorded using a Bruker Avance 500 spectrometer. Chemical shifts are calibrated to the residual solvent. ESI-MS spectra were recorded with a Waters Synapt G2 quadropole – Time of Flight spectrometer. Iron(II) bromide (99.8 %) was purchased from Sigma Aldrich, tetrakis(dimethylamino)zirconium from abcr. 1,1’-(1,3-Phenylene)bis(3-methyl-1-imidazolium) diiodide was synthesized based on literature methods.1 The synthesis of bis(2,6-bis(3-methylimidazol-1-ylidene)phenyl)iron(III) hexafluorophosphate ([Fe(ImP)2][PF6] is based on a modified literature synthesis.2,3 1,1’-(1,3-Phenylene)bis(3-methyl-1-imidazolium) diiodide (1976 mg, 4 mmol) and tetrakis(dimethylamino)zirconium (1124 mg, 4.2 mmol) were suspended in dry THF (20 mL) in a glovebox. The yellow suspension was stirred for 2 h, then iron(II) bromide (432 mg, 2 mmol) was added. The mixture stirred for another 16 h. The red/orange mixture was worked up under laboratory atmosphere and methanol (2 mL) was added. The now blue suspension was stirred under atmosphere for 1 h, until no further precipitation of a pale solid was observed. The suspension was filtered through a cotton ball and afterwards through a porous glass frit. The respective filter cakes were washed with acetonitrile until the filtrate turned colorless. The solvent of the filtrate was evaporated using a rotary evaporator. The blue solid was dissolved in dichloromethane and filtered over a silica column. The column was washed thoroughly with dichloromethane. The blue band was eluted with acetonitrile. The solvent of the blue fraction was evaporated. The solid was dissolved in methanol and KPF6 (2 eq., 736 mg, 4 mmol) were added. The desired compound 1 was precipitated by the addition of water and filtered off. It was redissolved in methanol and treated again with KPF6 (2 eq) and precipitated again with water to ensure full replacement of the counterion. The suspension was filtered, and the blue solid was dried thoroughly under vacuum. It was then dissolved in a minimal amount of dichloromethane and pentane was allowed to diffuse into the solution. Dark blue long needles of 1[PF6] were obtained (415 mg, 0.62 mmol, 31 %) after crystallization overnight which were dried in a vacuum (10-3 mbar) for 6 h prior to elemental analysis and spectroscopic measurements. 1H NMR (500 MHz, CD3CN) δ(ppm) = 24.70 (4H), 9.68 (12 H), 2.90 (4H), -2.39 (4H), -35.79 (2H; ESI-MS: Calculated for [1-PF6]+ (C28H26FeN8) 530.1625, found 530.1621; Infrared (ATR): 3166 cm-1, 3141 cm-1, 2926 cm-1, 1587 cm-1, 1470 cm-1, 1455 cm-1, 1405 cm-1, 1344 cm-1, 1263 cm-1, 1232 cm-1, 1075 cm-1, 874 cm-1 ,824 cm-1, 769 cm-1, 714 cm-1, 682 cm-1, 555 cm-1, 394 cm-1, 354 cm-1, 258 cm-1 Elemental Analysis for C28H26F6FeN8P (Calculated, Found (%)): C( 49.80, 49.76), H (3.88, 4.25), N (16.59, 16.30).
X-ray diffraction analysis.
The single crystal data were recorded using a Bruker SMART CCD area-detector diffractometer equipped with a graphite monochromator. The measurements were carried out using Mo Kα radiation (λ = 1.54178 Å) at T = 200(2) K, since at lower temperatures a phase transition occurred, which caused a vaguer diffraction pattern. Structure solution was carried out by direct methods4 and structure refinement was conducted using full-matrix least squares refinement based on F².4 All non-H-atoms were refined anisotropically and the hydrogen atom positions were derived from geometrical reasons – except hydrogens of methyl groups. They were located from Fourier map using HFIX 137 by SHELX.4 All hydrogen atoms were refined at idealized positions riding on the carbon atoms with isotropic displacement parameters Uiso(H) = 1.2Ueq(C) resp. 1.5Ueq (-CH3) and C-H bond lengths of 0.93-0.96 Å. All CH3 hydrogen atoms were allowed to rotate but not to tip. One dichloromethane solvent molecule could not be modelled during refinement and was treated using SQUEEZE from the platon software package.5–7
(C28H26N8Fe)(PF6), Mr = 675.40 Da, purple block, size 0.42 x 0.37 x 0.25 mm³, monoclinic space group P21/c with Z=4, a = 13.6367(12) Å, b = 8.7801(8) Å, c = 27.099(3) Å, β= 96.285(2)°, V=3225.2(5) Å³, Dc=1.391 mg/m³, µ=0.583 mm-1, F(000)=1380, θmax=26.462°, reflections collected: 28802, independent reflections: 6622, Rint=0.0455, refinement converged at R1=0.0571 [I>2σ(I)], wR2=0.1683 [all data], min./max. ΔF: -0.578 eÅ³ (0.61 Å from F123) / 0.804 eÅ³ (0.62 Å from F123)
Crystallographic data deposition.
Crystallographic data have been deposited at the Cambridge Crystallographic Data Centre assigned to the deposition number CCDC 2002774. Copies are available free of charge via www.ccdc.cam.ac.uk.
Mößbauer spectroscopy. Mößbauer spectra were recorded with a 57Co source in a Rh matrix using an alternating constant acceleration Wissel Mössbauer spectrometer operated in the transmission mode and equipped with a Janis closed-cycle helium cryostat. Isomer shifts are given relative to iron metal at ambient temperature. Simulation of the experimental data was performed with the Mfit program using Lorentzian line doublets: E. Bill, Max-Planck Institute for Chemical Energy Conversion, Mülheim/Ruhr, Germany.
Magnetic susceptibility measurements. Temperature-dependent magnetic susceptibility measurements were carried out with a Quantum-Design MPMS3 SQUID magnetometer in the range from 300 to 2.0 K at a magnetic field of 0.5 T. The powdered sample was contained in a polycarbonate capsule and fixed in a non-magnetic sample holder. Each raw data file for the measured magnetic moment was corrected for the diamagnetic contribution of the sample holder and the capsule. The molar susceptibility data were corrected for the diamagnetic contribution.
Cyclic voltammetry and square wave voltammetry.
Dry acetonitrile was obtained by passing HPLC grade acetonitrile (fisher) over a column of MP Biomedicals MP Alumina N-Super I which was activated in an oven at 150 °C for multiple days. The cyclic and square wave voltammetry measurements were performed at room temperature in 0.1 M [(nBu4N][PF6] dry acetonitrile solution with an analyte concentration of 0.001 M under a solvent-saturated Argon atmosphere. A three electrode arrangement with a 1 mm Pt working electrode and Pt-wire counter electrode (both Metrohm) and an Ag/AgCl reference electrode (custom built) was used, with the PGSTAT101 potentiostat from Metrohm. Ferrocene was added after the measurements as an internal standard, all potentials were referenced against the FcH0/+ couple. The voltammograms were analyzed using the NOVA software (version 2.1.3), diagnostic criteria for reversibility are based on those proposed by Nicholson8,9 and the Randles-Sevcik 10,11 equation.
UV/Vis/NIR spectroelectrochemical experiments were performed using a BioLogic SP-50 voltammetric analyzer and a Specac omni-cell liquid transmission cell with CaF2 windows equipped with a Pt-gauze working electrode, a Pt-gauze counter electrode and a Ag wire as pseudo reference electrode, melt-sealed in a polyethylene spacer (approximate path length 1mm) in CH3CN containing 0.1 M [nBu4N][PF6].12
Absorption spectroscopy (steady state).
Acetonitrile of spectroscopic grade (Spectronorm VWR Acetonitrile) was used as solvent for steady-state absorption spectroscopy.
Steady state absorption spectra were recorded using solutions with concentrations of about 10‑5 M in quartz-cuvettes (pathlength 10 mm) by a Cary 50 spectrometer.
Room temperature emission spectroscopy.
For steady-state emission spectroscopy acetonitrile of spectroscopic grade was used as solvent.
Steady-state emission spectra were recorded in 10 mm quartz cuvettes on a Jasco FP8300 or a Horiba Scientific FluoroMax-4 spectrometer. The solutions for the measurements under argon were degassed via the freeze-pump-thaw technique.
Variable temperature emission spectroscopy.
Variable-Temperature Emission spectra were recorded on a Varian Cary Eclipse spectrometer. For low temperature photoluminescence measurements, a solution of the complex in butyronitrile (refluxed over Na2CO3 and KMnO4, distilled and stored over aluminium oxide) was filled into a quartz cuvette in an argon-filled glovebox and the cuvette was sealed and transferred to an Oxford cryostate (Oxford instruments Opti-statDN). Measurements were conducted between 295 K and 77 K.
Time Correlated Single Photon Counting and Time Resolved Emission Spectroscopy.
Time Resolved Emission Spectroscopy (TRES) was performed by time correlated single photon counting (TCSPC) using a Horiba Ultima-01-DD system (Horiba Jobin Yvon GmbH). The degassed sample solution was excited at 346 nm using a Horiba DD350 pulsed LED with a maximum repetition rate of 20 MHz. The emission was recorded in 5 nm steps between 380 and 490 nm for 1 h per wavelength in reverse mode with a slit of 6 nm of the emission monochromator. Excitation at 374 nm was performed with a Horiba DD375L laser diode with a maximum repetition rate of 100 MHz, the emission was recorded in 5 nm steps between 390 and 455 nm in reverse mode with a slit of 6 nm of the emission monochromator. Repetitive start-stop signals were recorded by a multi-channel analyzer over the course of 1 h per wavelength. A histogram of photons was recorded as a function of 16383 channels on a time-range of 100 ns (0.012 ns per channel). To evaluate the TRES data, a global fitting procedure was applied to the data, similar to the analysis of the transient absorption data.
Femtosecond Transient Absorption Spectroscopy.
Femtosecond transient absorption spectra were recorded using excitation wavelengths in three different optical regions and thereby somewhat different pump-probe setups. In all cases they are based on regenerative Ti:sapphire laser systems operating at a frequency of 1 kHz and at a center wavelength of 775 nm (CPA 2001, Clark MXR, Inc.) respectively 800 nm (Spitfire Pro, Spectra-Physics). For probing, a white light continuum generated by focusing a small fraction of the Ti:sapphire output into a CaF2 crystal was used. Pump and probe beam were focused onto the sample to overlapping spots with diameters in the range of 200 to 400 µm for the pump and of 100 µm for the probe. The polarizations of the pump and probe pulses were set to magic angle with respect to each other. After the sample, the probe was dispersed by a prism and transient absorption changes were spectrally resolved recorded by an array detector.
For pumping the sample with an excitation wavelength of 400 nm the output of the Ti:sapphire system (Spitfire Pro) was frequency doubled by a BBO crystal. The resulting time resolution was about 150 fs.
To obtain ultrashort excitation pulses in the visible with a center wavelength of 600 nm a non-collinear optical parametric amplifier (NOPA) pumped by the Ti:sapphire system (CPA 2001) was applied. The dispersion of the NOPA pulses was minimized by a compressor based on fused silica prisms resulting in an overall time resolution of better than 100 fs.
For excitation in the UV, i. e., at a center wavelength of 330 nm, the NOPA was tuned to 660 nm and its output was frequency doubled by 100 µm thick BBO crystal cut for type I phase matching.
For all measurements, the iron complex was dissolved in acetonitrile under argon and the sample solution was filled into a fused silica cuvette with a thickness of 1 mm.
The obtained data was fitted using a global fit. In the global fit, the multi-exponential model function , convoluted with the temporal response of the pump-probe setup, is fitted to the complete set of time dependent transient absorption spectra. In the present case three exponential decay components were necessary to reproduce the data with satisfying accuracy, i. e. N = 3
Streak Camera Measurements.
In addition to the TCSPC measurements, the time resolved luminescence was also investigated applying a streak camera (Streakscope C10627, Hamamatsu Photonics). The samples were prepared and measured under argon in 1 cm cuvettes. For excitation at 330 nm, a NOPA pumped by a Ti:sapphire laser system (CPA 2001, Clark MXR, Inc.) was set to a center wavelength of 660 nm and its output pulses were frequency doubled by a BBO crystal. To ensure that only radiation at 330 nm reaches the sample, a fused silica prism was applied to separate the UV pulses from the fundamental.
The luminescence lifetimes were determined by fitting a monoexponential decay to the data in the spectral region 640 to 840 nm and a double exponential decay to the data of the region 390 to 600 nm.
Quantum-chemical calculations were performed at D2d symmetry with DFT and linear response TDDFT using the optimally-tuned long-range separation functional LC-BLYP together with combined basis set: def2TZVP (Fe) and 6-311G(d,p) (all other atoms). Tuning of the functional was done with the so-called ΔSCF method13–15, the details can be found in the work of Bokarev et al.16 The following parameters were obtained for the present complex: α=0.0 (percentage of exact exchange in short-range) and 0.15 bohr-1 (long-range separation parameter). Solvent effects (acetonitrile) were taken into account within the polarized continuum model (PCM) approach.17 Calculations were done with G1618 and the Q-Chem 5.319 packages. Excited state analysis was performed using the TheoDORE package.20 Analysis of Huang-Rhys factors, tuning of functional, and generation of geometries along normal modes were done with in-house codes.
The data generated and analyzed during the current study are available from the corresponding author on reasonable request. X-ray source data for fig. 1b are deposited under CCDC identifier CCDC 2002774 and are available in the supporting information.
The code used for analysis of Huang-Rhys factors, tuning of functional, and generation of geometries along normal modes is available from O.K. upon reasonable request.