Intensity-Dependent Photoresponse of Biphotochromic Molecule Composed of a Negative and a Positive Photochromic Unit

Light-selective multiple photochromic systems are important for advanced photoswitching of chemical reactions and biological activities. While UV light has been frequently utilized to induce photochromic reactions, visible light is energetically acceptable to avoid undesired reactions. However, many of the reported multiphotochromic systems still rely on UV light to induce at least a part of photochromic reactions. In this work, we designed a biphotochromic molecule showing intensity-dependent multiple coloration with a visible-light source by incorporating two T-type photochromic units; a colorless positive photochromophore and a colored negative photochromophore in a molecule. The negative photochromophore acts as a visible-light sensitizer for the positive photochromic reaction. The compound shows an intensity-dependent color change under visible-light irradiation. The weak visible-light excitation leads to gradual decoloration from orange to yellow, whereas intense laser excitation clearly changes the color to green. This characteristic photochromism can be achieved by control of the photochromic reaction rates of the negative and positive photochromic reactions. The combination of negative and positive photochromic reactions gives attractive important insight into the development of multiresponsive optical materials.