Evaluations were performed in writing speed and subjective impression of overwriting task to documents displayed on paper, LCD tablet, and e-Paper (electronic paper). Tasks for subjects were to write Japanese characters, to write Arabic numerals, and to write the circle marking. Three kinds of task documents with different font sizes were prepared for the writing tasks. The e-Paper showed similar performance with that of the paper and better performance than that of the LCD tablet, regardless of tasks and font sizes. e-Paper is thus expected as a promising medium which can replace paper medium.

A new material concept of Soft Crystals is proposed. Soft Crystals respond to gentle stimuli such as vapor exposure and rubbing but maintain their structural order exhibiting remarkable visual changes in their shape, color, and luminescence. These crystals can be organic crystals, inorganic crystals, or inorganic-organic hybrid crystals (metal complex crystals) etc. In this article, several interesting examples of Soft Crystals, which exhibit vapochromic, mechanochromic, or superelastic phenomena, are introduced. These phenomena draw increasing attention for the scientific viewpoints of response systems with high structural order. In addition, Soft Crystals are expected to have potential application to visual sensing and finely controllable electronic devices. The newest topics can be seen in this special issue of Soft Crystals.

Superelasticity is a kind of diffusion-less plastic deformation showing spontaneous shape recovery. The unique mechanical property enables so-called shape-memory alloys to be used in practical applications such as a catheter, clothes, and sensors. Superelasticity had been considered as a mechanical property of a special kind of alloys since its discovery in Au-Cd alloy in 1932. Despite the conventional thinking, we discovered superelasticity in an organic crystal, named “organosuperelasticity”, in 2014. Following studies on deformability of organic crystals revealed that superelasticity and ferroelasticity-diffusion-less plastic deformation showing spontaneous strain-are more common than previously thought. Herein we show examples of organosuperelastic and organoferroelastic materials. Another kind of plastic deformability, superplasticity, which enables organic crystals deform over hundreds of percent in retaining crystal integrity will be also described.

We investigate new type of soft crystals constructed from pillar-shaped macrocyclic compounds pillar[n]arenes. Pillar[n]arenes are able to capture guests based on their host-guest interactions depending on their cavity sizes. Therefore, the guest-shape selective vapor uptake by pillar[n]arene crystals are very clear. In addition, pillar[n]arene crystals are able to capture alkane vapors because of multiple CH/κ interactions between pillar[n]arene and alkanes. Furthermore, pillar[n]arenes have high functionality. We selectively install functional groups into pillar[n]arene platforms by various organic reactions. Using the selective complexation ability with alkanes and high functionality, we successfully visualize shape-selective alkane-vapor uptake by pillar[n]arene crystals by their color and state changes.

Our computer simulation techniques and their applications for analyzing the phase transition process of “Soft Crystals” are introduced. Potential energy that depends on the crystal structure parameters (molecular structure in asymmetric unit, space group, lattice constants) is evaluated by a thermodynamic state quantity called “crystal energy”, which is calculated from the sum of intra and intermolecular interaction energies described by the functions of a crystal force field based on molecular mechanics. When an organometallic complex exhibiting a soft crystal phenomenon is calculated using a crystal force field, it is often necessary to develop new force field parameters. Here, we introduce some cases in which we have developed new parameters and succeeded in reproducing the crystal structures. We also introduce a new method that succeeded in predicting an unknown crystal structure and its physical properties by implementing the diffraction pattern similarity as an evaluation index in the crystal structure prediction method. It is expected to contribute to elucidation of the soft crystal phenomena by finding many crystal polymorphs on the potential energy surface and reproducing their kinetics and thermodynamics.

Long-persistent luminescent (LPL) materials can absorb photon energy and emit light over a long period of time. While all existing LPL materials are composed of inorganic materials, we demonstrated an organic LPL (OLPL) system composed only of organic materials. The OLPL system consists of an electron donor material and an acceptor material that stores energy in a charge-separated state rather than an excited state. The OLPL systems do not require rare metals, can be fabricated from a solution, and has characteristics that are difficult to achieve with conventional inorganic materials such as flexibility and transparency. The emission color of OLPL system is controlled by the energy gap between the HOMO level of the donor and the LUMO level of the acceptor. In addition, the triplet excited states of each materials affect the LPL process. The extra fluorescent dopants enable to control the emission color and extend the emission duration.

Rare earth ions exhibit very high color purity from their own electronic configuration localized on f-orbitals. In general, it is known that the ff-emission of rare-earth ions by photoexcitation is promoted by the antenna effect due to the molecular energy transfer using organic ligands having high absorption coefficients. Recently, rare earth complexes exhibiting triboluminescence have been frequently discovered. However, there are several hypotheses for this principle, and there are many unclear points in triboluminescence compared to photoluminescence by the ligand-excitation. In order to understand easily and qualitatively the degree of stimulation to the emission intensity of the system which shows the phenomenon of light emission from rare earth when the crystal is crushed, an evaluation method using a Drop-Tower system is explained. A europium complex EuL^val coordinating a chiral moiety introduced into the ligand was used as the rare earth complex.

DNA has ability to incorporate various functional materials and to form highly ordered structure of DNA/functional materials, so to speak, “DNA softcrystal”. We aim to fabricate DNA-based functional complex by associating DNA and ruthenium (Ru(II)) complexes. In this review, we introduce unique optical properties of DNA/Ru(II) complex and their application for electrochemical devices. Among various functionality of Ru(II) complexes, we are focusing on electrochemiluminescence (ECL). Electrochemical and ECL properties of DNA/Ru(bpy)₃²⁺ hybrid film on ITO electrodes were investigated. The hybrid film fabricated by electrophoretic migration contained unique micrometre-scale aggregates of Ru(bpy)₃²⁺ in DNA matrix. The Ru(bpy)₃²⁺ in the DNA/Ru(bpy)₃²⁺ hybrid film exhibited significant electrochemical reactivity. As a result, orange-coloured ECL was observed from only the aggregated structures in the hybrid film at the high AC frequency of 10kHz, which corresponds to a response time shorter than 100μs.