A metal-lustrous porphyrin foil

Color Change of Indole Squaraine between Blue and Yellow with Golden Luster

Color change of materials by external stimulus has attracted much attention for various applications. We synthesized 1,3-bis(3,3-dimethyl-2-methylene indolenine)squaraine (ISQ), which was a yellow powder with golden luster (ISQ(gold)) and whose color changed to blue (ISQ(blue)) by being ground using mortar and pestle. Scanning electron microscopy indicated that ISQ(gold) consisted of laminated plate-like crystals with a large flat surface, which was attributable to (2 0-2) facets of monoclinic crystals. Measurements of the diffuse reflectance spectra revealed that J- and H-type aggregates of ISQ were formed by increasing the weight percent of ISQ(blue) in the mixture with BaSO4. A thin film, which was prepared by a spin-coating of ethanol with dissolved ISQ(blue) on glass substrates, looked blue on a white paper, whereas it looked yellowish on a black paper. By repeating the coating process, the film thickness increased, and the thick film exhibited yellow with golden luster regardless of the background color. These results indicate that the color depends on whether the transmitted light through ISQ or the reflected light on the ISQ surface mainly contributes to color vision. To neglect the effect of the transmitted light and intensify the reflection on the surface, a pellet with a smooth surface was prepared by pressing ISQ(gold) or ISQ(blue) at 20 kg/cm2. The ISQ(blue) pellet consisted of the laminated sheets and was dark blue-green. However, under light irradiation, the ISQ(blue) pellet exhibited a yellow color with golden luster, whose specular reflection spectrum was similar to that of the ISQ(gold) pellet. Our results indicate that the color of ISQ is tunable by controlling the transmission and the reflection of the light, including the specular and diffuse reflections.

Silver to Gold Metallic Luster Changes in Stimuli-Responsive Diacetylene Derivatives Uniquely Arranged within Crystals

Eye-catching metallic luster materials, especially those whose colors can be controlled by external stimuli, have many potential applications. Here, we present a silver luster material that changes color to gold upon UV irradiation. Diacetylene (DA) derivatives with stilbenes introduced via linkers at both ends (DS-DAn (n = 1-6)) exhibited significantly different metallic luster and color change behaviors depending on the linker carbon number (n). The results revealed that the stacked structure of platelet crystals consisting of DS-DA1 with the shortest linker carbon chain exhibited a silver luster and changed its appearance to gold upon UV irradiation; this was an exceptional property of this material. More importantly, we found a unique crystal structure formed by DS-DA1, where the two assembled states coexisted. Partial topochemical polymerization of DA within this unique crystal structure dramatically changed its color from silver to gold. The findings of this study not only contribute to the development of the basic science of DA polymerization but also facilitate the development of new applications of metallic luster materials due to their attractive features that are adaptable to photomask patterning and UV laser lithography.

Fully Conjugated Porphyrin Glass: Collective Light-Harvesting Antenna for Near-Infrared Fluorescence beyond 1 μm

Expanded π-systems with a narrow highest occupied molecular orbital-lowest unoccupied molecular orbital band gap encounter deactivation of excitons due to the "energy gap law" and undesired aggregation. This dilemma generally thwarts the near-infrared (NIR) luminescence of organic π-systems. A sophisticated cofacially stacked π-system is known to involve exponentially tailed disorder, which displays exceptionally red-shifted fluorescence even as only a marginal emission component. Enhancement of the tail-state fluorescence might be advantageous to achieve NIR photoluminescence with an expected collective light-harvesting antenna effect as follows: (i) efficient light-harvesting capacity due to intense electronic absorption, (ii) a long-distance exciton migration into the tail state based on a high spatial density of the chromophore site, and (iii) substantial transmission of NIR emission to circumvent the inner filter effect. Suppression of aggregation-induced quenching of fluorescence could realize collective light-harvesting antenna for NIR-luminescence materials. This study discloses an enhanced tail-state NIR fluorescence of a self-standing porphyrin film at 1138 nm with a moderate quantum efficiency based on a fully π-conjugated porphyrin that adopts an amorphous form, called "porphyrin glass".