Polymorphism in the spin-crossover ferric complexes [(TPA)Fe^III(TCC)]PF₆

Abstract

We have identified two polymorphs of the molecular complex [(TPA)Fe((III))(TCC)]PF(6) [TPA = tris(2-pyridylmethyl)amine and TCC = 3,4,5,6-tetrachlorocatecholate dianion]: one is monoclinic and the other is orthorhombic. By lowering the temperature both undergo a thermal spin-crossover between a high-spin (S = 5/2) and a low-spin (S = 1/2) state, which we detected by magnetic, optical and X-ray diffraction measurements. The thermal crossover is only slightly shifted between the polymorphs. Their crystalline structures consist of similar cation layers alternating with PF(6) anion layers, packed differently in the two polymorphs. The magnetic and optical properties of the polymorphs are presented.

Time-Resolved Pump-Probe Diffraction Experiment at SOLEIL Synchrotron: Photoinduced Spin Transition in the Molecular Crystal [TPA Fe(III) TCC] PF6

“…The laser polarization was horizontal and nearly parallel to the crystal axis a, which maximizes the penetration depth into the sample [6]. The laser spot could be imaged on a paper screen placed at sample position, via backscattering toward a CCD camera.…”

“…In the monoclinic and orthorhombic polymorphs of this compound, a near infrared laser pulse can switch the spin state of a small portion of Fe atoms in less than 1 ps, from a low spin (LS, S = 1/2) to a high spin (HS, S = 5/2) state [4,5]. On the photoexcited molecular sites, the LS to HS transformation is accompanied by an increase of the length of the Fe-N bonds which link the Fe atoms to their ligand TPA [6]. In addition, the phonon population (or lattice temperature) locally increases due to non-radiative decays of electrons from the ligand-to--metal charge transfer states to the HS state.…”

“…At sub-ns and sub-ps resolutions, such experiments allow studying photoinduced phase transitions and out--of-equilibrium phenomena in condensed matter, which bring precious information on photon/matter interaction, as well as on couplings between lattice and other degrees of freedom. On CRISTAL, the sample will be excited by 800 nm, 25 fs FWHM laser pulses provided by a regenerative Ti:sapphire amplifier with tunable repetition rate [1][2][3][4][5][6][7][8][9][10]. The laser will be installed in a dedicated hutch close to the beamline.…”

White Light Snapshot of Non-Equilibrium High-Spin State in a Molecular Crystal

“…The experiment was performed on the monoclinic polymorph [5] of [(TPA)Fe(TCC)]PF 6 . The monocrystals used had plate shape and were selected for size, typically 200 × 50 µm 2 .…”

“…The vibrational contribution is gradually smeared out by rising temperature, and becomes hardly distinguishable at 330 K. In order to understand the spectral kinetics during spin crossover we tried a simple model in which we assume that the spectrum at any temperature is a sum of LS and HS spectra weighted by their respective molecular fractions. HS fraction can be extracted from temperature dependent magnetic measurements [5]. The LS spectrum is directly obtained at 90 K, whereas the HS spectrum is obtained from measurements at 330 K after correcting for a 0.1 contribution from LS state.…”