Harnessing the power of light for chemical transformation is a long-standing goal in organic synthesis, materials fabrication and engineering. Amongst all photochemical reactions, [2 + 2] photocycloadditions are inarguably the most important and most frequently used. These photoreactions have green characteristics by enabling new bond formation in a single step procedure under light irradiation, without the need for heat or chemical catalysis. More recently, substantial progress has been made in red-shifting the activation wavelength of photocycloadditions in response to research trends moving towards green and sustainable processes, and advanced applications in biological environments. In the past 5 years, our team has further expanded the toolbox of photocycloaddition reactions that can be triggered by visible light. In our exploration of photochemical reactivity, we found that reactivity is often red-shifted compared to the substrate’s absorption spectrum. Our efforts have resulted in red-shifted photochemical reactions, providing some of the lowest energy – and catalyst-free – photo-activated [2 + 2] cycloadditions (up to 550 nm). More recently, we introduced an additional level of control over such finely wavelength gated reactions by altering the pH of the reaction environment, thus exploiting halochromic effects to enhance or impede the photoreactivity of red-shifted [2 + 2] photocycloaddition reactions. In this account, we discuss the current state of halochromically regulated photochemical reactions and their potential in soft matter materials on selected examples
