We report the experimental and theoretical characterization of neutral Si(6)X(12) (X = Cl, Br) molecules that contain D(3d) distorted six-member silicon rings due to a pseudo-Jahn-Teller (PJT) effect. Calculations show that filling the intervenient molecular orbitals with electron pairs of adduct suppresses the PJT effect in Si(6)X(12), with the Si(6) ring becoming planar (D(6h)) upon complex formation. The stabilizing role of electrostatic and covalent interactions between positively charged silicon atoms and chlorine atoms of the subject [Si(6)Cl(14)](2-) dianionic complexes is discussed. The reaction of Si(6)Cl(12) with a Lewis base (e.g., Cl(-)) to give planar [Si(6)Cl(14)](2-) dianionic complexes presents an experimental proof that suppression of the PJT effect is an effective strategy in restoring high Si(6) ring symmetry. Additionally, the proposed pathway for the PJT suppression has been proved by the synthesis and characterization of novel compounds containing planar Si(6) ring, namely, [(n)Bu(4)N](2)[Si(6)Cl(12)I(2)], [(n)Bu(4)N](2)[Si(6)Br(14)], and [(n)Bu(4)N](2)[Si(6)Br(12)I(2)]. This work represents the first demonstration that PJT effect suppression is useful in the rational design of materials with novel properties.
We discuss fundamental differences in electronic structure as reflected in one- and two-photon absorption spectra of semiconductor quantum dots and organic molecules by performing systematic experimental and theoretical studies of the size-dependent spectra of colloidal quantum dots. Quantum-chemical and effective-mass calculations are used to model the one- and two-photon absorption spectra and compare them with the experimental results. Currently, quantum-chemical calculations are limited to only small-sized quantum dots (nanoclusters) but allow one to study various environmental effects on the optical spectra such as solvation and various surface functionalizations. The effective-mass calculations, on the other hand, are applicable to the larger-sized quantum dots and can, in general, explain the observed trends but are insensitive to solvent and ligand effects. Careful comparison of the experimental and theoretical results allows for quantifying the range of applicability of theoretical methods used in this work. Our study shows that the small clusters can be in principle described in a manner similar to that used for organic molecules. In addition, there are several important factors (quality of passivation, nature of the ligands, and intraband/interband transitions) affecting optical properties of the nanoclusters. The larger-size quantum dots, on the other hand, behave similarly to bulk semiconductors, and can be well described in terms of the effective-mass models.
The M-[TCNE] (M = 3d metal; TCNE = tetracyanoethylene) system is one of the most interesting classes of molecule-based magnets, exhibiting a plethora of compositions and structures (inorganic polymer chains, 2D layers, 3D networks, and amorphous solids) with a wide range of magnetic ordering temperatures (up to 400 K). A systematic study of vibrational (both infrared and, for the first time, Raman) properties of the family of new TCNE-based magnets of M(II)(TCNE) (NCMe)(2)[SbF(6)] [M = Mn, Fe, Ni] composition is discussed in conjunction with their magnetic behavior and newly reso-lved crystal structures. The vibrational properties of the isolated TCNE(●-) anion in the paramagnetic Bu(4)N [TCNE(●-)] salt and recently characterized 2D layered magnet Fe(II)(TCNE)(NCMe)(2)[FeCl(4)] are also reported for comparison. Additionally, a linear correlation between ν(C=C) (a(g)) frequency of the TCNE ligand and its formal charge Z (the spin density on the π* orbital), Z = [1571 - ν(C=C) (a(g))]/154.5 [e], is presented. It is shown that monitoring Z by Raman spectroscopy is of great use in providing information that allows understanding the peculiarity of the superexchange interaction in M-[TCNE] magnets and establishing the structure-magnetic properties correlations in this class of magnetic material.
The structural, spectroscopic and magnetic properties of the two-dimensional (2D) molecule-based magnets of [Mn(II)(TCNE)(NCMe)2]X (X = PF6, AsF6, SbF6; TCNE = tetracyanoethylene, NCMe = acetonitrile) composition are reported. It is shown that the alteration of the interlayer distance by increasing the anion size has little effect on the critical magnetic ordering temperature, Tc, suggesting that it depends predominantly on the intra-plane magnetic exchange. The observed field-induced irreversibility in static magnetization, a slow decay of isothermal remanence below Tc, and the dynamic susceptibility data are in accord with a re-entrant spin-glass nature of the ground state of all materials. In contrast to the isostructural Fe-based magnets, in which strong magnetocrystalline anisotropy facilitates the finite temperature magnetic ordering with the magnetization easy axis perpendicular to the μ4-TCNE(•-) plane, in the studied Mn-based magnets the easy axis is canted away from the normal direction, due to a small magnetocrystalline anisotropy. The two magnetic transitions observed on cooling are assigned to the ferrimagnetic long-range ordering of the normal magnetization component followed by the re-entrant spin-glass type transition resulting from a random freezing of the in-plane magnetization component.
The chemical bond and its role as a mediator of magnetic exchange interaction remains a crucial aspect in the study of molecular magnetism. Within the M[TCNE] (M = 3d metal; TCNE = tetracyanoethylene) class of organic-based magnets, only V [TCNE] x (x ∼ 2) orders magnetically above room temperature (T c ∼ 400 K), while structural factors underlying this exceptional behavior remain elusive. Conversely, Mn-[TCNE] complexes of diverse crystal structure have recently become available, e.g., 2D-layer [Mn(TCNE)(NCMe) 2 ][SbF 6 ] (T c ∼ 75 K), and 3D-network [Mn(TCNE) 1.5 ](I 3 ) 0.5 (T c ∼ 170 K). Using this experimental structural data, DFT simulations have been performed and the spin-polarized electronic structures resolved. The nature of orbital interactions crucial for understanding magnetic behavior was revealed. Magnetic coupling, spin−orbital hybridization, as well as formation of exchange/superexchange pathways have been identified and interpreted in terms of the dimensionality of magnetic interaction. These results illustrate the complex nature of the electron exchange landscape in M[TCNE] molecule-based magnets. ■ INTRODUCTIONFor several decades, magnetism in the solids containing 3d electrons has remained one of the main focuses of modern materials science targeting applications in spintronics. In contrast to the itinerant ferromagnetic exchange between almost free electrons in metals (direct exchange), the main mechanism of exchange interaction in magnetic insulators like simple transition metal oxides is a virtual hopping of electrons between almost isolated ions (metal and oxygen) leading either to anti-or ferromagnetic Heisenberg exchange interaction between unpaired spins of metals, traditionally defined as indirect-or superexchange. 1−3Molecule-based magnets (MBMs) are a relatively new class of magnetic materials, in which molecular moieties bearing unpaired spin density interact electronically and magnetically. 4,5 Compared to conventional metallurgic and ceramic magnets, the main benefits of MBMs are usually associated with their lightweight, mechanical flexibility, tunable color or transparency, low-temperature processing, solubility, and compatibility with polymers and other classes of molecular materials. 6 Furthermore, the use of MBMs in the area of spintronics has the potential to become a disruptive technology, since organic materials can enhance preservation of electron spin orientation lifetime relative to inorganic conductors due to their inherently weak spin−orbit coupling. The M[TCNE] (M = 3d metal; TCNE = tetracyanoethylene) complexes represent one of the most interesting classes of MBMs, possessing numerous compositions and structures with varying dimensionalities of magnetic coupling from one-dimensional (1D) inorganic polymer chains 7 and two-dimensional (2D) layers 8−10 to three-dimensional (3D) networks 11 and amorphous solids. 12 M[TCNE] MBMs exhibit a wide range of magnetic ordering temperatures (T c ), with the highest of 400 K observed in V[TCNE] x (x ∼ 2). 12 Recently, interest...
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