Ultrafast Water H-Bond Rearrangement in a Metal-Organic Framework Probed by Femtosecond Time-Resolved Infrared Spectroscopy.

We investigated the water H-bond network and its dynamics in Ni2Cl2BTDD, a prototypical MOF for atmospheric water harvesting, using linear and ultrafast IR spectroscopy. Utilizing isotopic labeling and infrared spectroscopy, we found that water forms an extensive H-bonding network in Ni2Cl2BTDD. Further investigation with ultrafast spectroscopy revealed that water can reorient in a confined cone up to ∼50° within 1.3 ps. This large angle reorientation indicates H-bond rearrangement, similar to bulk water. Thus, although the water H-bond network is confined in Ni2Cl2BTDD, different from other confined systems, H-bond rearrangement is not hindered. The picosecond H-bond rearrangement in Ni2Cl2BTDD corroborates its reversibility with minimal hysteresis in water sorption.

Zn doped MAPbBr3 single crystal with advanced structural and optical stability achieved by strain compensation.

A mechanistic understanding of perovskite degradation is one of the most urgent issues to push perovskite devices toward commercial applications. Surface coverings will lower the electrical injection and light extraction efficiency of perovskites. Therefore, structural modification of Zn doped perovskites has been proposed herein. The Zn doping will induce local lattice strain due to smaller ionic radius. It is interesting that the lattice structure at atomic resolution has been observed directly through cryo-TEM. Under light illumination, the photostriction will compensate for the local lattice strain, which leads to structural stability as evidence suggests no phase transition in temperature ranges of the temperature-dependent photoluminescence spectra. In addition, MPZB also shows less than 3% decrease in PL intensity after 60 days. This is because the Zn doping induced the lowest defect density in the MPZB SC (density of trap-states ntrp = 6.33 × 108 cm-3), which has been confirmed by the high performance of the photodetector. Such strain compensation is expected to fundamentally improve the stability of photoelectric devices.

Multiphotoluminescence from a Triphenylamine Derivative and Its Application in White Organic Light-Emitting Diodes Based on a Single Emissive Layer.

White organic light-emitting diode (WOLED) technology has attracted considerable attention because of its potential use as a next-generation solid-state lighting source. However, most of the reported WOLEDs that employ the combination of multi-emissive materials to generate white emission may suffer from color instability, high material cost, and a complex fabrication procedure which can be diminished by the single-emitter-based WOLED. Herein, a color-tunable material, tris(4-(phenylethynyl)phenyl)amine (TPEPA), is reported, whose photoluminescence (PL) spectrum is altered by adjusting the thermal annealing temperature nearly encompassing the entire visible spectra. Density functional theory calculations and transmission electron microscopy results offer mechanistic understanding of the PL redshift resulting from thermally activated rotation of benzene rings and rotation of 4-(phenylethynyl) phenyl)amine connected to the central nitrogen atom that lead to formation of ordered molecular packing which improves the π-π stacking degree and increases electronic coupling. Further, by precisely controlling the annealing time and temperature, a white-light OLED is fabricated with the maximum external quantum efficiency of 3.4% with TPEPA as the only emissive molecule. As far as it is known, thus far, this is the best performance achieved for single small organic molecule based WOLED devices.