Tuning UV Absorption in Imine-Linked Covalent Organic Frameworks via Methylation.

Covalent organic frameworks (COFs) are porous materials with high surface areas, making them interesting for a large variety of applications including energy storage, gas separation, photocatalysis, and chemical sensing. Structural variation plays an important role in tuning COF properties. Next to the type of the building block core, bonding directionality, and linking chemistry, substitution of building blocks provides another level of synthetic control. Thorough characterization and comparison of various substitution patterns is relevant for the molecular engineering of COFs via rational design. To this end, we have systematically synthesized and characterized multiple combinations of several methylated and non-methylated building blocks to obtain a series of imine-based COFs. This includes the experimental assignment of the COF structure by solid-state NMR. By comparing the properties of all COFs, the following trends were found: (1) upon methylation of the aldehyde nodes, COFs show increased Brunauer-Emmett-Teller surface areas, reduced pore collapse, blue-shifted absorbance spectra, and ∼0.2 eV increases in their optical band gaps. (2) COFs with dimethylated amine linkers show a lower porosity. (3) In tetramethylated amine linkers, the COF porosity even further decreases, the absorbance spectra are clearly red-shifted, and smaller optical band gaps are obtained. Our study shows that methyl substitution patterns on COF building blocks are a handle to control the UV absorbance of the resulting frameworks.

Model-free phasor image analysis of quantitative myocardial T1 mapping.

Model-free phasor image analysis, well established in fluorescence lifetime imaging and only recently applied to qMRI [Formula: see text] data processing, is here adapted and validated for myocardial qMRI [Formula: see text] mapping. Contrarily to routine mono-exponential fitting procedures, phasor enables mapping the lifetime information from all image voxels to a single plot, without resorting to any regression fitting analysis, and describing multi-exponential qMRI decays without biases due to violated modelling assumptions. In this feasibility study, we test the performance of our recently developed full-harmonics phasor method for unravelling partial-volume effects, motion or pathological tissue alteration, respectively on a numerically-simulated dataset, a healthy subject scan, and two pilot patient datasets. Our results show that phasor analysis can be used, as alternative method to fitting analysis or other model-free approaches, to identify motion artifacts or partial-volume effects at the myocardium-blood interface as characteristic deviations, or delineations of scar and remote myocardial tissue in patient data.

Full-Harmonics Phasor Analysis: Unravelling Multiexponential Trends in Magnetic Resonance Imaging Data.

Phasor analysis is a robust, nonfitting, method for the study of multiexponential decays in lifetime imaging data, routinely used in Fluorescence Lifetime Imaging Microscopy (FLIM) and only recently validated for Magnetic Resonance Imaging (MRI). In the established phasor approach, typically only the first Fourier harmonic is used to unravel time-domain exponential trends and their intercorrelations across image voxels. Here, we demonstrate the potential of full-harmonics (FH) phasor analysis by using all frequency-domain data points in simulations and quantitative MRI (qMRI) T2 measurements of phantoms with bulk liquids or liquid-filled porous particles and of a human brain. We show that FH analysis, while of limited advantage in FLIM due to the correlated nature of shot noise, in MRI outperforms single-harmonic phasor in unravelling multiple physical environments and partial-volume effects otherwise undiscernible. We foresee application of FH phasor to, e.g., big-data analysis in qMRI of biological or other multiphase systems, where multiparameter fitting is unfeasible.

Structural Investigations of MA1-xDMAxPbI3 Mixed-Cation Perovskites.

Recently, a number of variations to the hybrid perovskite structure have been suggested in order to improve on the properties of methylammonium lead iodide, the archetypical hybrid halide perovskite material. In particular, with respect to the chemical stability of the material, steps should be taken. We performed an in-depth analysis of the structure of MAPbI3 upon incorporation of dimethylammonium (DMA) in order to probe the integrity of the perovskite lattice in relation to changes in the organic cation. This material, with formula MA1-xDMAxPbI3, adopts a 3D perovskite structure for 0 < x < 0.2, while a nonperovskite yellow phase is formed for 0.72 < x < 1. In the perovskite phase, the methylammonium and dimethylammonium ions are distributed randomly throughout the lattice. For 0.05 < x < 0.2, the phase-transition temperature of the material is lowered when compared to that of pure MAPbI3 (x = 0). The material, although disordered, has apparent cubic symmetry at room temperature. This leads to a small increase in the band gap of the material of about 20 meV. Using 14N NMR relaxation experiments, the reorientation times of the MA and DMA cations in MA0.8DMA0.2PbI3 were established to be 1.6 and 2.6 ps, respectively, indicating that both ions are very mobile in this material, on par with the MA ions in MAPbI3. All of the produced MA1-xDMAxPbI3 materials were richer in DMA than the precursor solution from which they were crystallized, indicating that DMA incorporation is energetically favorable and suggesting a higher thermodynamic stability of these materials when compared to that of pure MAPbI3.

Solid-state NMR of hybrid halide perovskites.

Recent advances in the development of perovskite based solar cells have increased the demand for in-depth characterisation of the perovskite structures and the dynamics of their various constituents in relation to the potential impact on the photovoltaic performance. NMR can play an important role in this respect; NMR has been used to study the incorporation of different ionic species, characterize their internal dynamics and diffusion, and monitor the chemical stability of these technologically relevant materials, including upcoming lower dimensional perovskite materials. Furthermore, the flexibility of NMR allows the study of the materials under relevant conditions e.g. under illumination. Here we present an overview of the recent literature on NMR of (hybrid) halide perovskites, focusing on the insights that NMR can provide.

Dimethylammonium Incorporation in Lead Acetate Based MAPbI3 Perovskite Solar Cells.

Over the last years, several different pathways have been suggested for producing perovskite thin films for solar cell applications. While the merit of these methods with respect to the solar cell efficiency have been shown, the actual composition of the resulting thin films is often not investigated. Here, we show that methylammonium lead iodide films produced using lead acetate as a lead source can have up to 15 % dimethylammonium incorporated into their crystal structure, even though this ion is often consider to be too large for incorporation. The origin of this ion lies in the precursor solution, where it is formed in a reaction that is facilitated by the basic character of the acetate ions. We further show that these dimethylammonium ions are incorporated in a random fashion throughout the crystal structure, owing to the lack of observable ordered domains.

Quadrupolar nutation NMR to discriminate central and satellite transitions: Spectral assignments for a Ziegler-Natta catalyst.

In this contribution we used solid state 35Cl (I=3/2) quadrupolar NMR to study a MgCl2/2,2-dimethyl-1,3-dimethoxypropane (DMDOMe) adduct that serves as a model system for Ziegler-Natta catalysis. Employing large Radio-Frequency (RF) field strengths we observe three spectral features with strongly varying line widths. The assignment of the spectra is complicated because of the large difference in quadrupolar interactions experienced by the different sites in the system. The satellite transitions (ST) of relatively well-defined bulk Cl sites are partially excited and may overlap with the central transition (CT) resonances of more distorted surface sites. We show that nutation NMR of the ST of I=3/2 spins yields a unique pattern that makes a clear distinction between an extensively broadened central transition and the satellite transitions of a component with a smaller quadrupolar interaction. This allows us to unambiguously unravel the spectra of the MgCl2 adduct showing that we observe CT and ST of the bulk phase of MgCl2-nanoparticles with a CQ of 4.6MHz together with the CT of surface sites displaying an average CQ of ∼10MHz.

Symmetry, Dynamics, and Defects in Methylammonium Lead Halide Perovskites.

In order to better understand the structure and dynamics of methylammonium lead halide perovskites, we performed NMR, NQR, and DFT studies of CH3NH3PbI3 in the tetragonal and cubic phase. Our results indicate that the space group of the tetragonal phase is the nonpolar I4/mcm. The highly dynamic methylammonium moiety shows no indication of the occurrence of additional orientations of the C-N bond close to the c-axis at temperatures approaching the cubic phase. Crystal quality effects are shown to influence the 14N NMR and 127I NQR spectra, and the effects of high-temperature annealing on defects can be observed. A strong increase in T2 relaxation time of the 207Pb NMR signal on cooling is found, and is an indication of slow motions in the PbI6 octahedra at room temperature. These results aid in the understanding of the structure of methylammonium lead halides and enable further studies of defects in these materials.