Crystal structure, vibrational and theoretical studies of bis(1,4-H2-1,2,4-triazolium) hexachloridostannate(IV) monohydrate

Pressure-induced giant emission enhancement, large band gap narrowing and rich polymorphism in two-dimensional 1,2,4-triazolium lead bromide perovskite

Layered lead halide perovskites are attractive materials for optoelectronic applications. In this work, temperature-dependent photoluminescence (PL) as well as pressure-dependent Raman and PL studies of lead bromide comprising small disc shape 1,2,4-triazolium cations (Tz+) are reported. Temperature-dependent studies reveal that at room-temperature (RT) Tz2PbBr4 exhibits narrow emission at 2.89 eV related to a free exciton (FE). At low temperature, three new narrow and weakly red-shifted bands as well as a very broad and strongly red-shifted band near 2.2 eV appear. The narrow and broad bands are attributed to self-trapped excitons (STEs) trapped by shallow and deep donors (acceptors), respectively. Pressure-dependent Raman studies revealed the presence of three pressure-induced phase transitions near 2, 6 and 8 GPa to phases II, III and IV, associated with increased distortion of the inorganic subnetwork and apperance of a static disorder in phase IV. These structural changes affect the excitonic emission, which changes from a strong red shift on compression in the ambient pressure I to a weaker red shift in phase II, negligible shift in phase III and blue shift in phase IV. Moreover, a new narrow and weakly red-shifted band appears in phases II-IV. Most importantly, PL intensity increases 16.7 times when pressure changes from ambient to 7.77 GPa but decreases in phase IV. The increase in PL intensity can be attributed to the increase in STE formation energy and activation energy for non-radiative recombination, while the decrease in intensity may be related to the formation of point defects, which are the source of non-radiative recombination. Overall, high-pressure PL data show that application of external pressure allows band gap engineering and giant enhancement of the PL efficiency of Tz2PbBr4 perovskite.

Revisiting a (001)-oriented layered lead chloride templated by 1,2,4-triazolium: structural phase transitions, lattice dynamics and broadband photoluminescence

This study revisits a (001)-oriented layered lead chloride templated by 1,2,4-triazolium, Tz2PbCl4, which recently has been an object of intense research but still suffers from gaps in characterization. Indeed, the divergent reports on the crystal structures of Tz2PbCl4 at various temperatures, devoid of independent verification of chiral phases through second harmonic generation (SHG), have led to an unresolved debate regarding the existence of a low-temperature phase transition (PT) and the noncentrosymmetric nature of the low-temperature phase. Now, by combining differential scanning calorimetry, single-crystal X-ray diffraction, dielectric, as well as linear and nonlinear optical spectroscopies on Tz2PbCl4, we reveal a sequence of reversible PTs at T1 = 361 K (phase I-II), T2 = 339 K (phase II-III), and T3 = 280 K (phase III-IV). No SHG activity could be registered for any of the four crystal phases, as checked by wide-temperature range SHG screening, supporting their centrosymmetry. The dipole relaxation processes indicate a decrease in activation energy with increasing temperature, from 0.60, 0.38, to 0.24 eV observed for phase IV (space group P21/c), phase III (Pnma), and phase II (Cmcm), respectively. This change is interpreted as a result of the diminishing strength of H-bonds as the system transforms from phase IV to III and subsequently to II. The weaker H-bonds facilitate the reorientation of Tz+ cations in the presence of an external electric field. The photoluminescence spectra of Tz2PbCl4 reveal an intriguing interplay of narrow and broadband emission, linked respectively to free excitons and excitons trapped on defects. Notably, as the temperature decreases from 300 K to 16 K, both the emission bands exhibit distinctive blue and red shifts, indicative of increased in-plane octahedral distortion. This dynamic behaviour transforms the photoluminescence of Tz2PbCl4 from greenish-blue at 300 K to yellowish-green at 13 K, enriching our understanding of 2D lead halide perovskites and highlighting the optoelectronic potential of Tz2PbCl4.

Crystallographic study of the energetic salt 1,2,4-triazolium perchlorate

The molecular and crystal structure of 1H-1,2,4-triazolium perchlorate, C₂H₄N₃⁺·ClO₄^-, was determined as detailed crystallographic data had not been available previously. The structure has monoclinic (P2₁/m) symmetry. It is of interest in the field of energetic compounds because nitrogen-rich azoles are the backbone of high-density energetic compounds, and salt-based energetic materials can exhibit preferential energy-release behaviour. The bond angles of the 1,2,4-triazolium cation in this study were similar to those of a cationic triazole ring reported previously and were different from those of the neutral triazole ring. This study contributes to the available data that can be used to analyse the relationship between the structures and properties of energetic materials.

Phase transition in polar 2-nitroanilinium nitrate. Graph-set approach of hydrogen bonding patterns and analysis of vibrational spectra

A phase transition in new compound 2-nitroanilinium nitrate, (H2NA)NO₃, was found. A symmetry lowering from orthorhombic, Pmn2₁, to monoclinic one, P2₁, at 249 K is observed. During the phase transition, the H2NA⁺ and nitrate ions displace from the mirror plane in high-temperature phase. As a result, hydrogen bonding network constructed by the ammonio group and NO₃^- anion is changed. Especially, a bifurcated hydrogen bond disappears. Mathematical operations using both elementary and hydrogen bond graph-set descriptors were used for the first time to describe a mechanism of phase transition. Changes in hydrogen bonding network were also studied by means of vibrational spectroscopy. Band shift associated with stretching and bending vibrations of the ammonio group indicates that the energy of intermolecular interactions rises along with temperature decrease. SHG response for the studied compound is higher than KDP, I_(H2NA)NO3 = 1.1·I_KDP, although a decay of the signal was observed due to instability of the sample.

X-ray, vibrational and theoretical studies of weak hydrogen bonds in bis(4-nitroanilinium) hexachloridostannate

Crystal structure of bis(4-nitroanilinium) hexachloridostannate, (H4NA)2SnCl6, was determined by means of X-ray single crystal diffraction. In the crystal structure, weak N-H…Cl hydrogen bonds are present. These interactions were taken into consideration in calculations of vibrational spectra for the [(H4NA)Cl4O](5-) anion. Very good agreement between theoretical and experimental frequencies was achieved. The calculations were done at the B3LYP/6-311+G(2d,1p) level including Grimme's correction for dispersion. The relaxed potential energy surface indicated that the energy of H4NA(+) ion is approximately independent upon rotation of the ammonio group. The energy barrier is 0.01 kcal/mol. Therefore the position of the ammonio group can be easily modified the other interacting molecules during molecular self-assembly and crystal growth processes. The energy of the H4NA(+) ion significantly depends on the relative position of the nitro group towards the aromatic ring. The energy barrier of the nitro group is 4.35 kcal/mol.

Vibrational spectra of the ortho-nitroanilinium cation in torsional space. Theoretical studies vs. experimental data of ortho-nitroanilinium chloride

Crystal structure of the ortho-nitroanilinium chloride, (HoNA)Cl, was re-determined by means of X-ray single crystal diffraction. Hydrogen atoms of the ammonio form intra- and intermolecular hydrogen bonds which are arranged in chain and ring patterns. The patterns are described by the mathematical relations of the elementary graph-set descriptors. Since the interactions have a weak nature, the interpretation of the vibrational spectra was carried out with the help of theoretical calculations of the spectra for the HoNA+ ion. In order to properly assign experimental bands, theoretical spectra were calculated at the B3LYP/6-31G(d,p) level of theory for the geometry of global minimum of HoNA+ ion as a reference and for the other conformations, including in-crystal geometry of the ion, changing the relative position of the ammonio and nitro groups. Overall, the 89 spectra were analyzed as a two-dimensional dependence of each of 45 normal modes of the HoNA+ ion on two dihedral angles, dih(HNCC) and dih(ONCC). Additionally, calculations were done for the in-crystal conformation of the (HoNA)Cl3(2-) anion. Great increase of frequency is observed for the ν7 (641 cm(-1)), where the H1C atom is involved in, because the intramolecular N-H1C⋯O hydrogen bond weakens upon rotation of the NH3+ group. PED analysis shows that also the modes of vibrations changes upon rotation. The mode of vibrations for the (HoNA)Cl3(2-) anion differs from the HoNA+ ion, especially for the ν(N-H) vibrations. Besides, when three chloride anions where included in the calculations, only then the experimental spectra were well reproduced.

Weak hydrogen bonds in bis(3-nitroanilinium) hexachloridostannate monohydrate. X-ray, vibrational and theoretical studies

Crystal structures of bis(3-nitroanilinium) hexachloridostannate monohydrate, (H3NA)2SnCl6·H2O, was determined by means of X-ray single crystal diffraction. Relaxed potential energy surface of the H3NA(+) ion was calculated at the B3LYP/6-31(d,p) level. The energy of the H3NA(+) ion is approximately independent upon rotation of the ammonio group. It significantly depends on relative position of the nitro group towards aromatic ring. Theoretical spectra were calculated for the [(A-H3NA)Cl5·H2O](4-) and [(B-H3NA)Cl5](4-) anions, and thus hydrogen bonds of the ammonio group with the nearest neighboring atoms were included. PED results revealed that no coupling among all of the N-H oscillators exists. They vibrate separately because each hydrogen atom of the ammonio group of A- and B-H3NA(+) ions has different surroundings of the acceptors. Overall, very good agreement between theoretical and experimental frequencies was achieved.

A theoretical study on the molecular structure and vibrational (FT-IR and Raman) spectra of new organic-inorganic compound [N(C3H7)4]2SnCl6

Tetrapropylammoniumchloride was used as a ligand for the synthesis of the new organic-inorganic compound bis-tetrapropylammoniumhexachlorostannate. Vibrational study in the solid state was performed by FT-IR of the free Tetrapropylammoniumchloride ligand (TPACL) and by FT-IR and FT-Raman spectroscopies of the [N(C3H7)4]2SnCl6 compound. The comparative analysis of the Infrared spectra of the title compound with that of the free ligand was discussed. The structure of the [N(C3H7)4]2SnCl6 compound was optimized by density functional theory (DFT) using B3LYP method and shows that the calculated values obtained by B3LYP/LanL2MB basis are in much better agreement with the experimental data than those obtained by B3LYP/LanL2DZ. The vibrational frequencies were evaluated using density functional theory (DFT) with the standard B3LYP/LanL2MB basis, and were scaled using various scale factors. Root mean square (RMS) value was calculated and the small difference between experimental and calculated modes has been interpreted by intermolecular interactions in the crystal.