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  English.dicp.cas.cn    Posted:2017-07-26
Lecture: Optimizing Fe2+ molecular switches by Mössbauer

Time: July 26, 2017, 10:30-11:30 am
Venue: Room 201, No.5 Basic Energy Building
Lecturer:Prof. Dénes Lajos Nagy, Wigner Research Centre for Physics, Hungarian Academy of Sciences


Switchable molecular compounds are in the forefront of research as potential candidates in the design of new, quick switching devices and high-density data storage systems. One of the most important family of these materials is formed by transition metal, especially Fe2+ centered organometallic complexes of medium ligand-field strength, the so called spin-crossover systems which - under certain circumstances - can undergo spin-state switching between low-spin (LS) and high-spin (HS) states. Regarding their room-temperature functionality, they became even more hopeful since the discovery of the light-induced excited-spin-state trapping (LIESST) phenomenon  which allows for switching the molecules back and forth using light pulses of different wavelengths. 

One of the most promising candidate for practical usage is the tridentate polypyridne ligand containing [Fe(tpy)2]2+ (tpy: 2,2':6',2''-terpyridine) complex which has, in spite of its strong ligand field, – at low temperatures and in certain matrices – a 10 orders of magnitude longer lived light-induced HS state than other similar strong-field complexes like [Fe(bipy)3]2+ (bipy: 2’,2”-bipyridine). In this complex the iron(II) center is surrounded by two pairs of 3 N donor atoms in D2d symmetry as, due to geometrical constrains, the 6 N atoms cannot form an ideal octahedron around it. However, as proposed by McCusker and coworkers , releasing this geometric constrain may lead to a more relaxed and close-to-ideal local structure with enhanced ligand-field strength, a key requirement to get longer-lived excited states thus a real potential for room-temperature applications. 

The tpy ligand can be modified based on both chemical intuition and quantum chemical calculations to design new, even more efficient molecular switches. The iron(II) complex of 2,6-di(quinolin-8-yl)pyridine (dqp) where an extra (condensed) side ring shifts the donor atoms further out thereby allowing for a less strained coordination is a probable candidate. The new molecules can be tested by various methods including Mössbauer spectroscopy. We present detailed comparison of the local symmetry, electronic structure and vibrational behavior of the tpy, tpy-OH and dqp complexes of iron(II) with SO42– and PF6counter ions based on the analysis of hyperfine interactions (HFI), second-order Doppler shift δSOD and the temperature dependence of the Lamb–Mössbauer factor fLM as taken from 57Fe transmission Mössbauer spectra in the 77 K ≤ T ≤ 300 K temperature range. 

Density-functional theory (DFT) calculations using the COSMO-B3LYP functional  show, in case of all complexes and counter ions investigated, excellent and fair correlation between experiment and theory for the isomer shift (IS) and the quadrupole splitting (QS), respectively. 

The Debye model of lattice vibrations describes well the experimental temperature dependence of fLM and δSOD. Nevertheless, with the fitted values of the Debye temperatures ΘD, the temperature dependence of δSOD cannot be explained. This is the consequence of the fact that the temperature dependence of fLM is related to <x2> and determined by acoustic phonons while that of δSOD is related to <v2> and determined by optical vibration modes. 

DFT is also capable of predicting vibration modes of a molecule. Nuclear inelastic scattering (NIS) active modes were calculated for all complexes so that a) the dispersion (intra-molecular van der Waals) interaction was taken into account, b) the effect of the counterion was neglected and c) no effect of the crystal beyond the molecule was accounted for. A ‘multi-Einstein’ model based on 12–19 modes identified below 700 cm-1 for the three complexes describes well the temperature dependence of δSOD, however it fails to explain that of fLM. Indeed, the true NIS-active phonon density of state (DOS) consists of an additional low-ΘD Debye-like part corresponding to the ‘acoustic’ lattice modes and a multi-Einstein part corresponding to the ‘optical’ molecular modes broadened by the lattice modes. Evaluation of the experimental data in terms of this combined model is in progress. In conclusion, by combining the HFI data and the temperature dependence of fLM and δSOD from laboratory Mössbauer experiments with DFT calculations, a better understanding of switching phenomena, a basis for ligand design and further development of molecular switches can be reached. 

Contact: GE RILE, DNL2005



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