Jan K. Zaręba

Beyond Single-Wavelength SHG Measurements: Spectrally-Resolved SHG Studies of Tetraphosphonate Ester Coordination Polymers

Powder second-harmonic generation (SHG) efficiencies are usually measured at single wavelengths. In the present work, we provide a proof of concept of spectrally resolved powder SHG measured for a newly obtained series of three non-centrosymmetric coordination polymers (CPs). CPs are constructed from tetrahedral linker-tetraphenylmethane-based tetraphosphonate octaethyl ester and cobalt(II) ions of mixed, octahedral (Oh), and tetrahedral (Td), geometries and different sets of donors (CoO6 vs CoX3O). Isostructurality of the obtained materials allowed for the determination of anion-dependent tunability of SHG optical spectra and their relationship with solid-state absorption spectra.

Recurrent supramolecular motifs in discrete complexes and coordination polymers based on mercury halides: prevalence of chelate ring stacking and substituent effects

In recent years, the crystal engineering library has been enriched with a number of previously unrecognized or unnoticed intermolecular interactions, such as agostic, tetrel, chalcogen, pnicogen bonding and chelate ring stacking – collectively referred to as “unconventional interactions”. Many open questions remain unaddressed regarding their ability to form synthon interactions, specificity, and cooperativity, for example with π–π stacking interactions. In this work, we throw light on the formation of chelate ring stacking in metal–organic assemblies of nicotinohydrazide ligands (N′-(1-(2-pyridyl)ethylidene)nicotinohydrazide (HL) and N′-(phenyl(pyridin-2-yl)methylene)nicotinohydrazide (HL1)) with mercury(II) halide (HgBr2, HgI2) salts. Their reaction produced five compounds, namely [Hg(μ-L)BrHgBr2]n (1), [Hg(μ-L1)Br]n (2), [Hg(L)I2] (3), [Hg(HL1)I2]·(CH3OH) (4), and [Hg(μ-L1)I]n (5). Crystal structure analysis reveals that chelate ring stackings are formed in four of the reported metal–organic compounds, and are common also in the literature precedents. The energies of chelate ring stackings and π–π heterocycle stackings have been computed and analyzed by means of DFT calculations, and the results were verified using Baders theory of “atoms in molecules”. These results provide a rationale for preferential formation of both unconventional and conventional stackings and allow us to conclude that chelate ring interaction may be considered as a synthon interaction for nicotinohydrazide metal complexes. Interpretations for packing differences imposed by the substituent effect (substitution of methyl group in HL for phenyl group in HL1) were provided based on the Hirshfeld surface analysis and 2D fingerprint plots of the crystal structures reported here.

Utilizing formation of dye aggregates with aggregation-induced emission characteristics for enhancement of two-photon absorption

Femtosecond Z-scan measurements of aggregates of a quadrupolar D-π-D dye, 9,10-bis(N-hexylcarbazol-2-yl-vinyl-2)anthracene (BHCVA), provide direct evidence that aggregation of this AIE (aggregation-induced emission) chromophore leads to a sizeable enhancement of the intrinsic two-photon absorption cross section. Aggregation-induced improvement of nonlinear response is explained by theoretical analysis of electronic structure and geometrical parameters of aggregates.

Spectrally resolved two-photon absorption properties and switching of the multi-modal luminescence of NaYF4:Yb,Er/CdSe hybrid nanostructures

Spectrally resolved femtosecond nonlinear optical study in the range of 950–1350 nm has been conducted for three colloidal NaYF4:Yb,Er/CdSe hybrid nanostructures, which differ in the size of the surface decorating CdSe quantum dots (6.6, 6.8 and 7.0 nm). The two-photon absorption cross sections σ2 of the hybrid nanoconstructs are as large as 2.55 × 105 GM in the near-infrared region (∼1150 nm); their very good nonlinear absorption properties being imparted by the CdSe nanoparticles. Further, we demonstrate that by merging NaYF4:Yb,Er and the CdSe nanoparticles into hybrid materials, it is possible to combine their mechanistically distinct two-photon-induced luminescence properties. Indeed, we report the switching of multi-modal luminescence of the colloidal NaYF4:Yb,Er/CdSe hybrid nanomaterials, in which both constituents can be excited separately or simultaneously, depending on the selection of the excitation source. The emissive modes include visible up-conversion and near-infrared emission of erbium(III) ions doping the NaYF4 core, as well as one- or two-photon induced band-gap luminescence of the outer layer, comprising the CdSe quantum dots. Toggling between the emission modes is achieved by single or dual beam excitation of the respective components, using a continuous wave 980 nm laser diode for NaYF4:Yb,Er nanoparticle emission, and femtosecond laser near-infrared pumping for CdSe quantum dot band-gap emission. The Forster resonance energy transfer between both optically active components of the synthesized hybrid nanostructures, i.e. up-converting nanoparticles (donor) → quantum dots (acceptor), has been determined to have 2.5% to 6% efficiency, depending on the size of the participating CdSe nanoparticles. Finally, we show that the multimodal luminescence properties of these hybrid nanomaterials can be potentially employed for 2D anti-counterfeiting applications.

On the origin of ferroelectric structural phases in perovskite-like metal–organic formate

Metal–organic frameworks (MOFs), formed of metal centers coupled by organic molecules, exhibit inherent porosity and crystallinity. Although these systems have been examined for many potential applications, their multiferroic properties remain poorly understood. One of the approaches to get an insight into the ferroic features of these materials is the study of frameworks templated by protonated organic molecules possessing rotational and conformational freedom. The embedded organic cation with an internal electric dipole moment may be ordered on cooling, leading to the appearance of ferroelectric-like properties. Herein, we report a study of two structural phase transitions in one such material, the methylhydrazinium zinc formate [CH3NH2NH2][Zn(HCOO)3] framework (MHyZn), using various experimental techniques. High-temperature (HT) improper ferroelectric phase transition and low-temperature (LT) biferroic phase transition in MHyZn single crystals have been investigated in a broad temperature range using X-ray diffraction, longitudinal ultrasonic velocity and attenuation measurements, pyrocurrent measurements, and broadband dielectric spectroscopy. In addition, noncentrosymmetric to centrosymmetric structural transformation was confirmed for the HT phase transition with the use of temperature-resolved second harmonic generation on powder samples, while polarizing microscopy observation of a large deformation of ferroelastic-like domains attested to the noncontinuous character of the crystallographic system transformation (trigonal to triclinic) in the LT biferroic phase transition. Finally, we proved that the spontaneous electric polarization occurs due to freezing of motion around the 2- and 3-axes of the methylhydrazinium cation.