Multicolor Fluorescence Photoswitching: Color-Correlated versus Color-Specific Switching

Fluorescence photoswitching systems using photochromic molecules, which turn on and off their fluorescence upon light irradiation, have emerged as highly promising material systems during the past two decades related to their optoelectronic applications such as high-density optical memory, bioimaging, and super-resolution microscopy. Single-color fluorescence photoswitching, which provides only two different states (on/off), is limited in terms of its practical applications such as interference from autofluorescence in biological applications and limited switching states in logic gate and optical memory applications. To address such issues, studies on multicolor fluorescence photoswitching systems incorporating photochromic molecules have witnessed an explosive growth in the past decade in terms of the academic principles and technological applications. In the earlier part, this review briefly introduces the principle of fluorescence photoswitching based on the representative single-color fluorescence photoswitching systems. Then, the review turns into the main topic of multicolor fluorescence photoswitching systems which are organized in two different subcategories of 1) color-correlated photoswitching and 2) color-specific photoswitching. Not only the material systems and principles of the multicolor fluorescence photoswitching, but also their important applications are described and discussed here. In the last section of this review, a brief summary and outlook on the future development are provided.