Spin-transition behaviour of transition metal complexes with 2,6-bis-(benzimidazol-2′-yl)-pyridine induced by deprotonation of the complex

Summary 2,6-bis-(Benzimidazol-2-yl)-pyridine (bzimpy = H₂ L) acts as a bidentate ligand when combining with transition metal ions. The complexes [M(bzimpy)₂](ClO₄)₂ (M = Fe²⁺, Mn²⁺, Zn²⁺, Co²⁺, and Ni²⁺) were obtained as solids. The protonation constants (logK) for the ligand and the complexes were evaluated in 30:70 (v/v) H₂O:EtOH at 293 K and at constant ionic strength of 0.12M KCl. Coordination of the ligand to the metal ions leads to an increase of acidity of the imino-hydrogen of the benzimidazole group of the ligand as a function of the complex stability. Deprotonation leads to a spin-state transition (intermediate spin-state low-spin) of the iron(II)-complex, followed by a shift of the metal-to-ligandcharge transfer band (MLCT) to lower energies (_max=563 to 580 nm). The d-d absorption bands are found to shift to higher energies and the low-spin isomer is favoured at room temperature. An opposite shift of theMLCT band (_max=563 to 557 nm) is observed when HClO₄ is added to the complex solution, rendering the high-spin state of the complex more favourable.On leave from the Chemistry Department, Jahangirnagar University, Dhaka, Bangladesh     

Band structure effects on the nonlinear optical response of bilayer graphene

Recently, Rabi-like oscillations that occur far from resonance were predicted in monolayer graphene. In bilayer graphene, when the trigonal warping effect is taken into account, this new Rabi frequency shows a zero non-trivial minimum as a function of the strength of the applied electric field in addition to the trivial minimum at zero field. The zero non-trivial minimum occurs where the ‘leg pocket’ of the Fermi surface develops, described in the pioneering work of McCann et al. [Eur. Phys. J. Special Topics 148, 91 (2007)]. Thereafter, the anomalous Rabi frequency varies linearly with the square of the intensity of the applied field consistent with a bilayer system without trigonal warping. It is seen that this anomalous Rabi frequency is affected much more by trigonal warping than the conventional Rabi frequency. The induced current is also significantly affected by the trigonal warping. A fully numerical solution of the optical Bloch equations completely corroborates the analytical findings and provides a basis for the approximation schemes employed.     

Characterization of Diastereomeric Equilibria of Pseudotetrahedral Bis[(R or S)‐N‐1‐(Ar)Ethylsalicylaldiminato‐κ2N,O]zinc(II) with Λ/Δ‐Chirality‐At‐Metal Induction

A family of bis[(R or S)‐N‐1‐(Ar)ethylsalicylaldiminato‐κ²N,O]‐Δ/Λ‐zinc(II) {Ar=C₆H₅ (ZnRL¹ or ZnSL¹), p‐CH₃OC₆H₄ (ZnRL² or ZnSL²) and p‐ClC₆H₄ (ZnRL³ or ZnSL³)} compounds was synthesized and investigated by multiple methods. They feature Λ/Δ‐chirality‐at‐metal induction along the pseudo‐C ₂ axis of the molecules. The chirality induction is quantitative in the solid state, explored by X‐ray crystallography and powder X‐ray diffraction (PXRD), where R or S‐ligated complexes diastereoselectively yield Λ or Δ‐configuration at the metal. On the other hand, Λ and Δ‐diastereomers co‐exist in solution. The Λ⇆Δ equilibrium is solvent‐ and temperature‐dependent. Electronic circular dichroism (ECD) spectra confirm the existence of a diastereomeric excess of Λ‐ZnRL¹⁻³ or Δ‐ZnSL¹⁻³ in solution. DSC analysis reveals thermally induced irreversible phase transformation from a crystalline solid to an isotropic liquid phase. ECD spectra were reproduced by DFT geometry optimizations and time‐dependent DFT (TD‐DFT) calculations, providing ultimate proof of the dominant chirality atmetal in solution.     

Signatures of bulk topology in the non-linear optical spectra of Dirac-Weyl materials

Graphene, topological insulators (TI) and the Weyl semimetal are shown to be well-characterized using the phenomenon of anomalous Rabi oscillation (ARO). These oscillations occur far from conventional resonance and Floquet theory shows them to be unique to these systems. Of particular interest is the bulk topological insulator (TI) where the wave-vector dependent frequency of the ARO is seen to be gapped in topologically trivial situations and gapless when there is a non-vanishing Chern number. It is shown that the Chern number may be directly inferred by performing a pump-probe experiment in the bulk without referring to surface states. A simpler alternative to the Lindblad method is invoked in order to incorporate dephasing effects that despite leading to a non-unitary time evolution of the wave-function, is nevertheless, probability conserving. The differential transmission coefficient versus the pump pulse duration (when all else is held fixed) has the form of a sinusoidal function with an amplitude that decays as a power law in the pump duration (alternatively, the “area” of the pump pulse). The exponent of this power law decay is indicative of the Chern number of the bulk in case of TI and more generally indicative of the particular member of the family of materials that may be collectively referred to as – Dirac-Weyl materials.     

Characterization of Diastereomeric Equilibria of Pseudotetrahedral Bis[(R or S)-N-1-(Ar)Ethylsalicylaldiminato-κ2N,O]zinc(II) with Λ/Δ-Chirality-At-Metal Induction

A family of bis[(R or S)-N-1-(Ar)ethylsalicylaldiminato-κ²N,O]-Δ/Λ-zinc(II) {Ar=C₆H₅ (ZnRL¹ or ZnSL¹), p-CH₃OC₆H₄ (ZnRL² or ZnSL²) and p-ClC₆H₄ (ZnRL³ or ZnSL³)} compounds was synthesized and investigated by multiple methods. They feature Λ/Δ-chirality-at-metal induction along the pseudo-C₂ axis of the molecules. The chirality induction is quantitative in the solid state, explored by X-ray crystallography and powder X-ray diffraction (PXRD), where R or S-ligated complexes diastereoselectively yield Λ or Δ-configuration at the metal. On the other hand, Λ and Δ-diastereomers co-exist in solution. The Λ⇆Δ equilibrium is solvent- and temperature-dependent. Electronic circular dichroism (ECD) spectra confirm the existence of a diastereomeric excess of Λ-ZnRL¹⁻³ or Δ-ZnSL¹⁻³ in solution. DSC analysis reveals thermally induced irreversible phase transformation from a crystalline solid to an isotropic liquid phase. ECD spectra were reproduced by DFT geometry optimizations and time-dependent DFT (TD-DFT) calculations, providing ultimate proof of the dominant chirality atmetal in solution.     

Spin-crossover complexes in solution. III. Substituent- and solvent effects on the spin-equilibrium of 4-substituted iron(II)-2,6-bis-(benzimidazol-2′-yl)-pyridine systems

Summary Transition metal complexes of the composition [Fe(4-X-bzimpy)₂](ClO₄)₂ [bzimpy = 2,6-bis-(benzimidazol-2-yl)pyridine and X=H, OH, Cl] show thermally accessible spin-crossover behaviour in solution that depends on both the ligand and the solvent.¹H-NMR spectroscopy and UV-visible spectroscopy measurements suggest that ligand substituent effects, solvent donor-acceptor properties and hydrogen-bonding may be employed to fine-tune the ligand field strength and hence to affect the spin-crossover behaviour. The ligand substitution changes in solution are reflected by the magnetic data (X=H:_exp=2.50 _B; X=OH:_exp=4.20 _B and X=Cl:_exp=4.30 _B at 294 K in MeOH), and by the shift of metal-to-ligandcharge-transfer band (X=H, =557 nm; X=OH, =520 nm; X=Cl, =500 nm). [Fe(bzimpy)₂](ClO₄)₂ exhibits a pronounced spin-crossover equilibrium (¹A₁ ⁵T₂) in solution (K _sc=0.26 at 293 K; _exp=1.30 3.40 _B for 213 328 K in MeOH). A small variation of magnetic moments of [Fe(4-OH-bzimpy)₂](ClO₄)₂ (_exp=3.77 4.73 _B at 220 332 K) might indicate either the existence of (temperature dependent) hydrogen bonding between the ligand and solvent molecules or a temperature dependent variation in the population of the⁵E_g sublevel. The presence of strong donor solvents (DN 30) shifts the spin-state of the complexes.In course of absence from the Chemistry Department, Jahangirnagar University, Dhaka, Bangladesh