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Whispering‐gallery‐mode (WGM) resonators composed of organic and/or inorganic materials have been used in highly sensitive sensors, logic gates, and miniaturized lasers because of the total internal reflection of photons. However, most fabrication methods have been limited to bottom‐up self‐assembly; whereas, a facile top‐down process is preferable. Here, a wet‐process fabrication of WGM resonators mixed with 5 µm‐diameter spherical silica‐gels and various π‐conjugated compounds is demonstrated. The materials are fabricated by dip‐coating photoluminescent molecules onto submicron‐scale silica‐gel spheres. The shapes and large specific surface areas of templates are maintained before and after coating, and the coated molecules are uniformly distributed. The hybrid spheres provided WGM photoluminescence spectra based on the total internal reflection of photoluminescence in the sphere. The hybrid spheres are applicable as WGM sensors because of their large specific surface area (>4 × 102 m2 g−1). In particular, gas sensing of toluene, which has been used as a standard for environmental contamination, and found that the WGM photoluminescence peaks are shifted by 5 nm is demonstrated. The detection resolution is calculated to be several hundred ppm level. The hybrid spheres can be placed or transferred onto various surfaces without micromanipulation, which suggested a wide range of possible applications. [ABSTRACT FROM AUTHOR]

A good solution for energy harvesting is to generate electricity using waste heat from our bodies or living environment. Therefore, the development of flexible and lightweight thermoelectric generators (TEGs) is urgently necessary, and studies on organic thermoelectric materials have become increasingly intensive. This article will present ongoing studies about a mysterious phenomenon in organic semiconductors, the giant Seebeck effect (GSE). The GSE was first discovered with pure C60 thin films and eventually confirmed to occur in various organic semiconductors. In the thin films or single crystals of organic small-molecule semiconductors with high purity, i.e., small carrier density, huge Seebeck coefficients, >0.1 V K−1, were reproducibly observed in the temperature range near 300–400 K. The facts revealed by the experiments to date will be presented, and unresolved mysteries will be discussed. [ABSTRACT FROM AUTHOR]

Flexible molecular crystal waveguides based on elastic molecular crystals (EMCs) are essential in flexible and compact optical materials. An increased loss coefficient α due to self‐absorption is often a problem in optical waveguides (OWGs) of fluorescent chemical materials waveguiding photons in active mode. Herein, the development of anthracene‐based elastic mixed molecular crystals (EMMCs) is reported for Förster Resonance Energy Transfer (FRET)‐assisted OWG. To yield a FRET crystal system based on elastic molecular crystals, 1%–5% acceptor doping for fluorescent molecular crystals of 9,10‐dibromoanthracene 1 was successful by selecting the same regioisomer having electron‐withdrawing group, 9,10‐diformylanthracene 2, as a dopant. In addition to conversion to the mixed system, there is a difference in the elastic modulus and hardness in EMC C1 and EMMC C2@1. However, the elastic behaviour was also shown in a few percent doping of the acceptor. The α value of this EMMC, composed of 1 including 1% of 2 (0.0077 dB/μm), is much lower than that of EMC composed of 1 (0.1258 dB/μm) because of reducing self‐absorption in the FRET system. An efficient and flexible OWG was successfully developed by selecting an appropriate acceptor molecule and its low doping rate for mixed crystal construction. This method is a practical approach in various functional and flexible crystal systems. [ABSTRACT FROM AUTHOR]

To realize organic integrated optoelectronic circuits, there is a need for anisotropic optical waveguides at the micro/nanoscale. Anisotropic alignment of one‐dimensional‐ordered supramolecular structures composed of light‐emissive π‐conjugated molecules in a crystal may meet the requirements of such waveguides. Here, a bipyridyl‐appended acrylonitrile‐based π‐conjugated molecule was designed, which produced a one‐dimensional supramolecular polymer constructed through non‐covalent bonding between a lone pair in bipyridyl and a σ‐hole in 1,4‐diiodo‐2,3,5,6‐tetrafluorobenzene. The one‐dimensional copolymer of bipyridyl and 1,4‐diiodo‐2,3,5,6‐tetrafluorobenzene is aligned horizontally with the two‐dimensional crystal surface because of the angle‐controlled supramolecular synthons. As a result of control over the non‐covalent bonding direction, anisotropic photoluminescence and photon transport (optical waveguiding) characteristics are realized by orienting the transition dipole moment horizontally with respect to the two‐dimensional surface. Compared with the loss coefficient αL=52 dB cm−1 for the long‐axis direction of the two‐dimensional platelet cocrystal, a very large difference of αS=2111 dB cm−1 is present in the crystal short‐axis direction. The anisotropic waveguiding ability, αL/αS, is estimated to be 41, which is more than an order of magnitude greater than previously reported two‐dimensional platelet crystal waveguides. This supramolecular synthon provides an approach to designing anisotropic photon transporters and highly regulated optical logic circuits. [ABSTRACT FROM AUTHOR]

Molecular crystals exhibiting optical waveguiding are strongly dependent on total internal reflection, molecular stacking arrangements, spatial distributions of transition dipole moments, and topologies. Fluoranthene derivatives are popular fluorescent dyes that have not yet been engineered into crystalline optical materials. Here, a naphthyl‐appended naphtho[k]fluoranthene that exhibits efficient fluorescence in solution and in the solid state is newly synthesized through a one‐pot Pd‐catalyzed reaction of 1,8‐dibromonapthalene. It selectively gives two crystal topologies derived from direction‐specific crystal growth. Using spatially resolved photoluminescence analyses, the direction‐oriented light that propagates along these topologies (each face of the crystal) is investigated. There are different optical‐loss coefficients along the various directions on each face. Therefore, face‐specific waveguiding depends on molecular arrangements. Anisotropic waveguiding in each face is observed. Overall, these results suggest potential applications of fluoranthene crystals and their topological 2D planes as optical waveguides and integrated optical circuits. [ABSTRACT FROM AUTHOR]

Emission‐switchable fluorophores often include stimuli‐responsive units in their molecular structures. This strategy works well, but the applicable compounds are limited to the derivatives of several kinds of photochromic molecules such as diarylethene, azobenzene, and spiropyran. Here, this work presents a simple methodology based on a photoresponsive precipitant for achieving color‐switchable photoluminescence. A luminescent dye, cyano‐substituted oligo(phenylenevinylene) (COPV), features both twisted intramolecular charge transfer and aggregation‐induced emission shift properties, leading to the change in the luminescence color from green to red upon precipitation. The COPV, together with photoisomerizable precipitant azobenzene (C6), is doped into spherical droplets of epoxy resin (ER) in a liquid state. The photoisomerization of C6 induced by UV irradiation and heating alternatively precipitates out and dissolves COPV in ER and changes the photoluminescence color, while maintaining the optical microresonator properties. This study will open a promising way for assembling/disassembling novel emission color‐switchable systems. [ABSTRACT FROM AUTHOR]