Magnetic and thermal properties of 4f-3d ladder-type molecular compounds

We report on the low-temperature magnetic susceptibilities and specific heats of the isostructural spin-ladder molecular complexes L$_{2}$[M(opba)]_{3\cdot xDMSO$\cdot y$H$_{2}$O, hereafter abbreviated with L$_{2}$M$_{3}$ (where L = La, Gd, Tb, Dy, Ho and M = Cu, Zn). The results show that the Cu containing complexes (with the exception of La$_{2}$Cu$_{3}$) undergo long range magnetic order at temperatures below 2 K, and that for Gd$_{2}$Cu$_{3}$ this ordering is ferromagnetic, whereas for Tb$_{2}$Cu$_{3}$ and Dy$_{2}$Cu$_{3}$ it is probably antiferromagnetic. The susceptibilities and specific heats of Tb$_{2}$Cu$_{3}$ and Dy$_{2}$Cu$_{3}$ above $T_{C}$ have been explained by means of a model taking into account nearest as well as next-nearest neighbor magnetic interactions. We show that the intraladder L--Cu interaction is the predominant one and that it is ferromagnetic for L = Gd, Tb and Dy. For the cases of Tb, Dy and Ho containing complexes, strong crystal field effects on the magnetic and thermal properties have to be taken into account. The magnetic coupling between the (ferromagnetic) ladders is found to be very weak and is probably of dipolar origin.Comment: 13 pages, 15 figures, submitted to Phys. Rev.

Testing foundations of quantum mechanics with photons

The foundational ideas of quantum mechanics continue to give rise to counterintuitive theories and physical effects that are in conflict with a classical description of Nature. Experiments with light at the single photon level have historically been at the forefront of tests of fundamental quantum theory and new developments in photonics engineering continue to enable new experiments. Here we review recent photonic experiments to test two foundational themes in quantum mechanics: wave-particle duality, central to recent complementarity and delayed-choice experiments; and Bell nonlocality where recent theoretical and technological advances have allowed all controversial loopholes to be separately addressed in different photonics experiments.Comment: 10 pages, 5 figures, published as a Nature Physics Insight review articl

Theoretical investigation of the electronic structure of Fe(II) complexes at spin-state transitions

The electronic structure relevant to low spin (LS)high spin (HS) transitions in Fe(II) coordination compounds with a FeN6 core are studied. The selected [Fe(tz)6]2+(1) (tz=1H-tetrazole), [Fe(bipy)3]2+(2) (bipy=2,2’-bipyridine) and [Fe(terpy)2]2+ (3) (terpy=2,2’:6’,2’’-terpyridine) complexes have been actively studied experimentally, and with their respective mono-, bi-, and tridentate ligands, they constitute a comprehensive set for theoretical case studies. The methods in this work include density functional theory (DFT), time-dependent DFT (TD-DFT) and multiconfigurational second order perturbation theory (CASPT2). We determine the structural parameters as well as the energy splitting of the LS-HS states (ΔEHL) applying the above methods, and comparing their performance. We also determine the potential energy curves representing the ground and low-energy excited singlet, triplet, and quintet d6 states along the mode(s) that connect the LS and HS states. The results indicate that while DFT is well suited for the prediction of structural parameters, an accurate multiconfigurational approach is essential for the quantitative determination of ΔEHL. In addition, a good qualitative agreement is found between the TD-DFT and CASPT2 potential energy curves. Although the TD-DFT results might differ in some respect (in our case, we found a discrepancy at the triplet states), our results suggest that this approach, with due care, is very promising as an alternative for the very expensive CASPT2 method. Finally, the two dimensional (2D) potential energy surfaces above the plane spanned by the two relevant configuration coordinates in [Fe(terpy)2]2+ were computed both at the DFT and CASPT2 levels. These 2D surfaces indicate that the singlet-triplet and triplet-quintet states are separated along different coordinates, i.e. different vibration modes. Our results confirm that in contrast to the case of complexes with mono- and bidentate ligands, the singlet-quintet transitions in [Fe(terpy)2]2+ cannot be described using a single configuration coordinate

On Predicting Mössbauer Parameters of Iron-Containing Molecules with Density-Functional Theory

The performance of six frequently used density functional theory (DFT) methods (RPBE, OLYP, TPSS, B3LYP, B3LYP*, and TPSSh) in the prediction of Mössbauer isomer shifts(δ) and quadrupole splittings (ΔEQ) is studied for an extended and diverse set of Fe complexes. In addition to the influence of the applied density functional and the type of the basis set, the effect of the environment of the molecule, approximated with the conducting-like screening solvation model (COSMO) on the computed Mössbauer parameters, is also investigated. For the isomer shifts the COSMO-B3LYP method is found to provide accurate δ values for all 66 investigated complexes, with a mean absolute error (MAE) of 0.05 mm s–1 and a maximum deviation of 0.12 mm s–1. Obtaining accurate ΔEQ values presents a bigger challenge; however, with the selection of an appropriate DFT method, a reasonable agreement can be achieved between experiment and theory. Identifying the various chemical classes of compounds that need different treatment allowed us to construct a recipe for ΔEQ calculations; the application of this approach yields a MAE of 0.12 mm s–1 (7% error) and a maximum deviation of 0.55 mm s–1 (17% error). This accuracy should be sufficient for most chemical problems that concern Fe complexes. Furthermore, the reliability of the DFT approach is verified by extending the investigation to chemically relevant case studies which include geometric isomerism, phase transitions induced by variations of the electronic structure (e.g., spin crossover and inversion of the orbital ground state), and the description of electronically degenerate triplet and quintet states. Finally, the immense and often unexploited potential of utilizing the sign of the ΔEQ in characterizing distortions or in identifying the appropriate electronic state at the assignment of the spectral lines is also shown

Temperature Compensation for Radiometric Correction of Terrestrial LiDAR Intensity Data

Correction of terrestrial Light Detection and Ranging (LiDAR) intensity data has been increasingly studied in recent years. The purpose is to obtain additional insight into the scanned environment that is not available from the geometric information alone. Radiometric correction, as the name implies, corrects the received intensity to standard reflectance values in the range of ( 0 , 1 ) . This correction typically compensates for the dependence of angle and distance. This paper presents an additional compensation for temperature that may be necessary for some LiDAR instruments such as the Faro Focus 3 D X 330 laser scanner. It is also shown that temperature compensation is not necessary for the Riegl VZ–400. Another important contribution of this work is the verification of a previously published radiometric correction in different environments. The correction was applied to two different Terrestrial Laser Scanner (TLS) instruments: a Faro Focus 3 D X 330 and Riegl VZ-400. Overall, the VZ-400, without temperature compensation, produced better results with a Root Mean Square (RMS) of the standard deviation of error being 0.053 and a RMS of the mean error of 0.036 compared to 0.069 and 0.046 for the Faro Focus 3 D X 330. It was found, for the case of the Faro device, that the temperature of the instrument played an important role in the accuracy of the results. The proposed temperature compensation method improved the RMS standard deviation of the error by 1.4 times and the RMS of the error by 2.6 times, compared to the uncompensated results

Tetrathiafulvalene - Benzothiadiazoles as Redox-Tunable Donor - Acceptor Systems: Synthesis and Photophysical Study

Electrochemical and photophysical analysis of new donoracceptor systems 2 and 3, in which a benzothiadiazole (BTD) unit is covalently linked to a tetrathiafulvalene (TTF) core, have verified that the lowest excited state can be ascribed to an intramolecular-charge-transfer (ICT) (TTF)*(benzothiadiazole) transition. Owing to better overlap of the HOMO and LUMO in the fused scaffold of compound 3, the intensity of the 1ICT band is substantially higher compared to that in compound 2. The corresponding CT fluorescence is also observed in both cases. The radical cation TTF+. is easily observed through chemical and electrochemical oxidation by performing steady-state absorption experiments. Interestingly, compound 2 is photo-oxidized under aerobic conditions

Dynamic Jahn–Teller Effect in the Metastable High-Spin State of Solvated [Fe(terpy)₂]²⁺

Characterizing structural distortions in the metastable spin states of d4–d7 transition metal ion complexes is crucial to understand the nature of their bistability and eventually control their switching dynamics. In particular, the impact of the Jahn–Teller effect needs to be assessed for any electronic configuration that could be effectively degenerate, as in e.g. the high-spin (HS) manifold of highly symmetric homoleptic FeII complexes. However, capturing its manifestations remains challenging since crystallization generally alters the molecular conformations and their interconversion. With the rapid progress of ultrafast X-ray absorption spectroscopy, it is now possible to collect data with unprecedented signal-to-noise ratio, opening up for detailed structural characterization of transient species in the homogeneous solution phase. By combining the analysis of picosecond X-ray absorption spectra with DFT simulations, the structure of the photoinduced HS state is elucidated for solvated [Fe(terpy)2]2+ (terpy = 2,2′:6′,2″-terpyridine). This species can be viewed as the average 5B structure in D2 symmetry that originates from a dynamic Jahn–Teller effect in the HS manifold. These results evidence the active role played by this particular instance of vibronic coupling in the formation of the HS state for this benchmark molecule. Ultimately, correlating the interplay between intramolecular and intermolecular degrees of freedom to conformational strain and distortions in real time should contribute to the development of advanced functionalities in transition metal ion complexes