Switchable Intersystem Crossing (sISC) in Organic Dyes: Unlocking Dynamic Photonic Properties

Switchable intersystem crossing (sISC) is a novel concept in the design of functional dyes that enables dynamic modulation of fluorescence and triplet state formation in response to changes in the dye's environment. Unlike conventional dyes, where intersystem crossing and fluorescence rates are entirely determined by molecular structure, sISC dyes allow reversible switching between these functions based on external factors, such as solvent polarity. sISC can arise from various photophysical mechanisms involving charge–transfer states, whose energies are sensitive to the surrounding environment. Mechanisms such as thermally activated delayed fluorescence and spin–orbit charge transfer intersystem crossing can be employed to design sISC dyes. The ability to fine-tune photophysical processes without chemical modification of the dye's structure opens new possibilities for biomedical applications, including bioimaging, photodynamic therapy, and optogenetic control. This paper introduces the concept of sISC, provides key examples of dyes exhibiting this behavior, and discusses approaches for their design.