Pressure has long been recognized as a fundamental thermodynamic variable but its application was previously limited by the available pressure vessels and probes. The development of megabar diamond anvil cells and a battery of associated in-laboratory and synchrotron techniques at the turn of the century have opened a vast new window of opportunities. With the addition of the pressure dimension, we are facing a new world with an order of magnitude more materials to be discovered than all that have been explored at ambient pressure. Pressure drastically and categorically alters all elastic, electronic, magnetic, structural, and chemical properties, and pushes materials across conventional barriers between insulators and superconductors, amorphous and crystalline solids, ionic and covalent compounds, vigorously reactive and inert chemicals, etc. In the process, it reveals surprising high-pressure physics and chemistry and creates novel materials. This review describes the principles and methodology used to reach ultrahigh static pressure: the in situ probes, the physical phenomena to be investigated, the long-pursued goals, the surprising discoveries, and the vast potential opportunities. Exciting examples include the quest for metallic hydrogen, the record-breaking superconducting temperature of 203 K in ${\mathrm{H}}_n\mathrm{S}$, the complication of “free-electron gas” alkali metals, the magnetic collapse in $3d$ transition elements, the pressure-induced superconductivity from topological insulators, the novel stoichiometry in simple compounds, the interaction of nanoscience, the accomplishment of 750 GPa pressure, etc. These highlights are the integral results of technological achievements, specific measurements, and theoretical advancement; therefore, the same highlights will appear in different sections corresponding to these different aspects. Overall, this review demonstrates that high-pressure research is a new dimension in condensed-matter physics.

While viologen derivatives have long been known for electrochromism and photochromism, here we demonstrated that a viologen-carboxylate zwitterionic molecule in the crystalline state exhibits piezochromic and hydrochromic behaviors. The yellow crystal undergoes a reversible color change to red under high pressure, to green after decompression, and finally back to yellow upon standing at ambient pressure. Ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance X-ray diffraction and DFT calculations suggested that the piezochromism is due to the formation of radicals via pressure-induced electron transfer from carboxylate to pyridinium, without a crystallographic phase transition. It was proposed that electron transfer is induced by pressure-forced reduction of intermolecular donor–acceptor contacts. The electron transfer can also be induced by dehydration, which gives a stable green anhydrous radical phase. The color change is reversible upon reabsorption of water, which triggers reverse electron transfer. The compound not only demonstrates new chromic phenomena for viologen compounds, but also represents the first example of organic mechanochromism and hydrochromism associated with radical formation via electron transfer.

While stimuli-responsive chromic phenomena are well known for various viologen-containing organic and metal–organic materials, viologen-based piezochromism is a very recent discovery in organic compounds. Here we present the first piezochromic metal–viologen material and the modulation of the pressure-responsive behavior through coordination structures. By means of ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance, in situ/ex situ X-ray diffraction and DFT calculations, we demonstrated that a zigzag-chain Cd^II coordination polymer (1) with a viologen-dicarboxylate zwitterionic ligand shows reversible piezochromism, with modulations in threshold pressure and visible-light absorption (color) compared with the free ligand. We also illustrated that piezochromism can be suppressed upon coordination of the same ligand in a rigid 3D framework with the same metal ion. Two basic requirements were proposed from viologen-based piezochromism: appropriate donor–acceptor contacts providing electron transfer pathways, and structural flexibility allowing pressure to further reduce the contacts. We expect great prospects in tuning piezochromism and designing new pressure-responsive materials through diverse metal–viologen combinations. The very fast photochromic response of compound 1 at ambient pressure was also studied, which was attributed to the rather short donor–acceptor contacts in the structure.

Stimuli-induced solid-state electron transfer (ET) underlies the use of viologen compounds as responsive materials, but unequivocal structure–property correlations for solid-state ET are still lacking. With different pseudopolymorphic solids derived from N,N′-bis(4-carboxylphenyl)viologen ([H₂bcpV]²⁺), here we report a systematic study on photo- and piezochromic properties associated with ET. We show that the higher the water content in the lattice, the less sensitively the compounds respond to light and pressure. It is proposed that the lattice water does not act as an electron donor but serves to change the ET energetics through its unique polarity and hydrogen bonding capability. The impedimental impact of water on solid-state ET of viologen compounds has not yet been recognized and elucidated prior to this work. The study also suggests that pressure is more powerful than light in inducing ET.