Band-Gap Narrowing and Electric Transport Regulation of Hybrid Perovskites via Pressure Engineering.

Lead-free organic-inorganic hybrid perovskites are one class of promising optoelectronic materials that have attracted much attention due to their outstanding stability and environmentally friendly nature. However, the intrinsic band gap far from the Shockley-Queisser limit and the inferior electrical properties largely limit their applicability. Here, a considerable band-gap narrowing from 2.43 to 1.64 eV with the compression rate up to 32.5% is achieved via high-pressure engineering in the lead-free hybrid perovskite MA3Sb2I9. Meanwhile, the electric transport process changes from the initial interaction of both ions and electrons to only the contribution of electrons upon compression. The alteration in electrical characteristics is ascribed to the vibration limitation of organic ions and the enhanced orbital overlap, resulting from the reduction of the Sb-I bond length through pressure-induced phase transitions. This work not only systematically investigates the correlation between the structural and optoelectronic properties of MA3Sb2I9 but also provides a potential pathway for optimizing electrical properties in lead-free hybrid perovskites.

Chiral Hybrid Germanium(II) Halide with Strong Nonlinear Chiroptical Properties.

Due to the pronounced anisotropic response to circularly polarized light, chiral hybrid organic-inorganic metal halides have been regarded as promising candidates for the application in nonlinear chiroptics, especially for the second-harmonic generation circular dichroism (SHG-CD) effect. However, designing novel lead-free chiral hybrid metal halides with large anisotropy factors and high laser-induced damage thresholds (LDT) of SHG-CD remains challenging. Herein, we develop the first chiral hybrid germanium halide, (R/S-NEA)3 Ge2 I7 ⋅H2 O (R/S-NGI), and systematically investigated its linear and nonlinear chiroptical properties. S-NGI and R-NGI exhibit large anisotropy factors (gSHG-CD ) of 0.45 and 0.48, respectively, along with a high LDT of 38.46 GW/cm2 ; these anisotropy factors were the highest values among the reported lead-free chiral hybrid metal halides. Moreover, the effective second-order nonlinear optical coefficient of S-NGI could reach up to 0.86 pm/V, which was 2.9 times higher than that of commercial Y-cut quartz. Our findings facilitate a new avenue toward lead-free chiral hybrid metal halides, and their implementation in nonlinear chiroptical applications.

Pressure Tuning of Optical Properties and Structures in All-Inorganic Halide Perovskite Rb7Sb3Cl16.

All lead-free inorganic halide perovskites, as efficient solid-state light emission materials, have become ideal green optoelectronic materials to replace lead halide perovskites for diversified lighting and display applications with their excellent stability. Here, we investigated the pressure-derived optical and structural response of a zero-dimensional lead-free perovskite Rb7Sb3Cl16 through applying controllable pressure. A pressure-induced blue shift of the broadband emission was achieved, and it was followed by the emission color transformation from yellow to green, which was ascribed to the electron-phonon coupling weakening and the suppression of structural deformation upon lattice contraction. In parallel, the band gap was narrowed by about 0.5 eV as a result of enhanced metal halide orbital overlap under high pressure. This work provides a fundamental understanding for modulating the optical properties of the low-dimensional metal halide perovskites.

Ligand-Triggered Platinum(II) Metallacycle with Mechanochromic and Vapochromic Responses.

Supramolecular coordination complexes with solid-state stimuli-responsive characteristics are highly desirable but are rarely reported. Herein, we describe two coordination-driven self-assembled monoanthracene or dianthracene-based hexagonal metallacycles by subtle structure modification. Notably, the dianthracene-containing hexagon 1 exhibits tricolor mechanochromic and vapochromic characteristics, while the monoanthracene-containing hexagon 4 does not show obvious changes toward mechanical force. Further studies have indicated that changes in hexagon 1, especially the ulterior anthracene of hexagon 1 in the molecular stacking through intermolecular interactions toward external stimuli, are responsible for the above behavioral differences. Furthermore, the present work also demonstrates a novel light-harvesting strategy for achieving high-contrast mechanochromic fluorescence involving solid-state energy transfer from hexagon 1 to an organic carbazole derivant 6 without mechanofluorochromism or tetraphenylethylene derivant 7 exhibiting inconspicuous mechanofluorochromism.

Tuning the circularly polarized phosphorescence of platinum(II) complexes through a chiral cation strategy.

Circularly polarized phosphorescent (CPP) materials, especially chiral platinum(II) complexes, which combine the advantages of both circularly polarized luminescence (CPL) and phosphorescence, show broad potential applications in chiral optoelectronic devices. Developing CPP emitters with both excellent chiroptical properties and high yield is urgently needed. Here, a chiral cation strategy is employed to construct the CPP Pt(II) complexes R/S-ABA·[Pt(ppy)Cl2] and R/S-MBA·[Pt(ppy)Cl2] through a simple one-step reaction with almost 100% yield. The circular dichroism and CPL spectra confirm that the chirality was successfully transferred to the [Pt(ppy)Cl2]- anion. The luminescence asymmetry factors (glum) are +1.4/-1.8 × 10-3 for R/S-ABA·[Pt(ppy)Cl2] and +4.4/-2.8 × 10-3 for R/S-MBA·[Pt(ppy)Cl2]. The stronger chiroptical property of R/S-MBA·[Pt(ppy)Cl2] is attributed to the enhanced chiral structural deformation and better matched electric and magnetic transition dipole moments. This chiral cation strategy is confirmed to efficiently construct CPP Pt(II) complexes, which will accelerate the development of CPP emitters towards commercialization.

Harvesting High-Quality White-Light Emitting and Remarkable Emission Enhancement in One-Dimensional Halide Perovskites Upon Compression.

The pressure induced emission (PIE) behavior of halide perovskites has attracted extensive interest due to its potential application in pressure sensors and trademark security. However, the PIE phenomenon of white-light-emitting hybrid perovskites (WHPs) is rare, and that at pressures above 10.0 GPa has never been reported. Here, we effectively adjusted the perovskite to emit high-quality "cold" or "warm" white light and successfully realized pressure-induced emission (PIE) upon even higher pressure up to 35.1 GPa in one-dimensional halide perovskite C4N2H14PbCl4. We reveal that the degree of structural distortion and the rearrangement of the multiple self-trapped states position are consistent with the intriguing photoluminescence variation, which is further supported by in situ high-pressure synchrotron X-ray diffraction experiments and time-resolved photoluminescence decay dynamics data. The underlying relationship between octahedron behavior and emission plays a key role to obtain high-quality white emission perovskites. We anticipate that this work enhances our understanding of structure-dependent self-trapped exciton (STE) emission characteristics and stimulates the design of high-performance WHPs for next generation white LED lighting devices.