Bis-alkynylphosphine Oxide Pt(II) Complexes: Aggregation-Induced Phosphorescence Enhancement and Mechanochromic Luminescence Properties

Four bis-alkynyl Pt(II) complexes [Pt(dtbpy)(C2-L-P(O)Ph2)2] with dtbpy = 4,4'-ditertbutyl-2,2'-bipyridine and alkynylphosphine oxide ligands (L = no linker, Pt0; phenyl, Pt1; biphenyl, Pt2; naphthyl, Pt3) have been synthesized and fully characterized by spectroscopic methods and single crystal XRD analysis. It has been found that the nature of the π-conjugated linker is a key factor in fine-tuning the emission energy of the complexes in solution and in achieving the aggregation-induced phosphorescence enhancement (AIPE) effect for complex Pt0 with the most compact linker. Phosphine oxide fragment, which can be involved in weak intermolecular interactions, promotes the existence of two solid forms with different luminescence properties. These two forms can be switched from one to another upon grinding, thus featuring distinct mechanochromic luminescence properties. TDDFT calculations are consistent with the experimental results and assign mixed 3MLCT and 3LL'CT solution emission character and 3MMLCT and 3LL'CT emission nature in supramolecular dimeric structures.

Critical Evaluation of Childs Method for the NMR Spectroscopic Scaling of Effective Lewis Acidity: Limitations and Resolution of Earlier Discrepancies

Quantifying Lewis acidity is essential for understanding and optimizing the performance of Lewis acids in diverse applications. Next to the widely accepted Gutmann-Beckett (GB) method, using triethyl phosphine oxide (TEPO) as a probe, the Childs method-employing trans-crotonaldehyde (TCA)-gained attention as an NMR-based technique for measuring effective Lewis acidity (eLA). Despite its steady use, the robustness of Childs method and its correlation with other measures remain underexplored. Previous comparisons between the GB and Childs scales revealed significant discrepancies, suggesting that hard and soft acid/base (HSAB) characteristics may be operative. In this study, we compare thermodynamic data for TCA binding to 117 Lewis acids (representing global Lewis acidity, gLA) with their corresponding NMR-induced chemical shifts in TCA. Our findings showcase notable deviations that reinforce key distinctions between eLA and gLA perspectives. Moreover, we identify significant limitations in the Childs method. First, the weak donor strength of TCA limits its applicability to only the strongest Lewis acids. Second, the exposed protons of TCA are prone to secondary interactions, obscuring the measurement of true Lewis acidity. Finally, our analysis reconciles discrepancies, refuting earlier assumptions that these arise from HSAB effects.

Cyclometalated Au(III) complexes with alkynylphosphine oxide ligands: synthesis and photophysical properties

A series of cyclometalated Au(III) complexes [Au(C^N^C)(C2-L-P(O)Ph2)] with C^N^C = 2,6-diphenylpyridine and alkynylphosphine oxide ligands (L = no linker, Au1; phenyl, Au2; biphenyl, Au3; naphthyl, Au4; anthracenyl, Au5) were synthesized and fully characterized by spectroscopic methods and single crystal XRD analysis. The complexes obtained exhibit triplet (Au1-Au3) and dual (Au4, Au5) emissions in solution, in the solid phase and in the PMMA film, whose characteristics depend on the linker's nature of the alkynylphosphine oxide ligand. The description of electronic transitions responsible for energy absorption and emission in Au(III) complexes was made on the basis of a detailed analysis of the results of DFT calculations and has shown to involve ILCT, LLCT and MLCT transitions of singlet and triplet nature. It was demonstrated that packing in the crystal affects the solid-state emission of Au(III) complexes, which differs from that in solution. Based on DFT calculations for the supramolecular dimer for Au1, it was shown that this phenomenon is the result of packing of the complex molecules in the crystal.

Twin hydrogen bonds with phosphine oxide: anticooperativity effects caused by competing proton donors

In this computational work we study complexes with two equivalent intermolecular hydrogen bonds formed between trimethyl phosphine oxide and two identical proton donors ("twin" hydrogen bonds) for a set of 70 proton donor molecules. The changes in the phosphorus chemical shift and stretching frequency of the PO group upon complexation correlate quite well with the total strength of two hydrogen bonds. A set of explicit numerical dependences is proposed for assessing interatomic distances and hydrogen bond strengths from spectral data. Comparison with the results obtained for analogous previously studied 1 : 1 complexes allowed us to analyze in detail anticooperativity effects on the geometry, energy and spectral parameters. Two hydrogen bonds compete for the PO acceptor group and their mutual weakening increases nonlinearly with the strengthening of the complex, reaching approximately 25% in energy (which corresponds to 0.1 Å lengthening for short strong H-bonds), which is clearly seen in NMR and IR spectra and correlates well with the changes in the spectral parameters.

Phosphine selenides: versatile NMR probes for analyzing hydrogen OH⋯Se and halogen I⋯Se bonds

Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for studying the structure and dynamics of various non-covalent interactions. However, often spectral parameters that are applicable for estimation of parameters of one type of non-covalent interaction will be inapplicable for another. Therefore, researchers are compelled to use spectral parameters that are specifically tailored to the type of non-covalent interaction being studied. This complexity makes it difficult to compare different types of non-covalent interactions with each other and, consequently, to establish a strict unified classification for them. This pioneering study proposes to use phosphine selenides as universal probes for investigating hydrogen and halogen bonding in solution. The study was carried out using the example of triethylphosphine selenide Et3PSe complexes with hydrogen bonds of Se⋯HO type and R3PSe (where R: Me, Et, n-Bu, t-Bu and Ph) with halogen bonds of Se⋯X type (where X: I and Br) in solution. The presence of non-covalent interactions was confirmed experimentally by means of 1H, 31P and 77Se NMR, as well as by quantum chemical calculation methods (optimization: PW6B95-D3/def2-QZVP; NMR: B97-2/pcsSeg-2).

Unveiling the electronic structure peculiarities of phosphine selenides as NMR probes for non-covalent interactions: an experimental and theoretical study

In this work, R3PSe (R = Me, Et, n-Bu, t-Bu and Ph) were studied experimentally using NMR spectroscopy in solution and the solid-state in combinaton with quantum chemical methods. The study shows that the NMR parameters of these phosphine selenides, such as δP, δSe, and 1JPSe, are sensitive to subtle changes in the electronic environment of the P and Se atoms. Consequently, phosphine selenides R3PSe can serve as promising spectral probes for the detection and quantitative investigation of various non-covalent interactions. Additionally, the variations of R in phosphine selenides influence the observed NMR spectral parameters, primarily through effects such as π-backdonation and hyperconjugation, which have been observed experimentally and confirmed theoretically.

Complexes of phosphine oxides with substituted phenols: hydrogen bond characterization based on shifts of PO stretching bands

In this work IR spectral characteristics of PO groups are used to evaluate the strength of OHO hydrogen bonds. Three phosphine oxides: triphenylphosphine oxide, tributylphosphine oxide and hexamethylphosphoramide are investigated as proton acceptors. The results of the experimental IR study and DFT calculation of 30 complexes formed by phosphine oxides with various substituted phenols or CF3CH2OH in CCl4 solution at room temperature are reported. We show that the PO vibrational frequency changes non-linearly upon hydrogen bond formation and strengthening and that the shift of the PO band could be used for the estimation of hydrogen bond strength in complexes with phosphine oxides. The accuracy of these estimations and the influence of solvation effects on the main characteristics of complexes are discussed.

Molecular dynamics and NMR reveal the coexistence of H-bond-assisted and through-space JFH coupling in fluorinated amino alcohols

The JFH coupling constants in fluorinated amino alcohols were investigated through experimental and theoretical approaches. The experimental JFH couplings were only reproduced theoretically when explicit solvation through molecular dynamics (MD) simulations was conducted in DMSO as the solvent. The combination of MD conformation sampling and DFT NMR spin-spin coupling calculations for these compounds reveals the simultaneous presence of through-space (TS) and hydrogen bond (H-bond) assisted JFH coupling between fluorine and hydrogen of the NH group. Furthermore, MD simulations indicate that the hydrogen in the amino group participates in both an intermolecular bifurcated H-bond with DMSO and in transmitting the observed JFH coupling. The contribution of TS to the JFH coupling is due to the spatial proximity of the fluorine and the NH group, aided by a combination of the non-bonding transmission pathway and the hydrogen bonding pathway. The experimental JFH coupling observed for the molecules studied should be represented as 4TS/1hJFH coupling.

The Distance between Minima of Electron Density and Electrostatic Potential as a Measure of Halogen Bond Strength

In this study, we present results of a detailed topological analysis of electron density (ED) of 145 halogen-bonded complexes formed by various fluorine-, chlorine-, bromine-, and iodine-containing compounds with trimethylphosphine oxide, Me3PO. To characterize the halogen bond (XB) strength, we used the complexation enthalpy, the interatomic distance between oxygen and halogen, as well as the typical set of electron density properties at the bond critical points calculated at B3LYP/jorge-ATZP level of theory. We show for the first time that it is possible to predict the XB strength based on the distance between the minima of ED and molecular electrostatic potential (ESP) along the XB path. The gap between ED and ESP minima exponentially depends on local electronic kinetic energy density at the bond critical point and tends to be a common limiting value for the strongest halogen bond.