Quantification of Synergistic Two-Color Covalent Bond Formation

The emergence of highly wavelength resolved reactivity information for complex photochemical reaction processes allows the establishment of multi-color reaction modes. One particularly powerful mode is the synergistic two-color reaction, where two colors of light have to be present in the same volume element to either enable or enhance photochemical reactivity that leads to a specific photoproduct. Herein, we introduce a two-color synergistic photochemical reaction system based on a diaryl indenone epoxide (DIO) photoswitch and the cis-to-trans isomerization of a bridged ring-strained azobenzene (SA), which respond to ultraviolet (365 nm) and visible light (430 nm), respectively, with different rates, forming a well-defined heterocyclic photoadduct, DIOSA, that we structurally confirm via single crystal x-ray diffraction (SXRD). To quantitatively capture the effectiveness of the dual-color irradiation as a function of the reaction conditions such as light intensity and starting material ratio as a function of product yield, we introduce a parameter, the photochemical synergistic ratio ${\phi }_{syn}$ . A reduced ${\phi }_{syn}$ termed ${\phi }_{syn}^0$ -that extrapolates to conditions of infinitesimal conversions-allows to compare the efficiency of the synergistic photochemistry at varying reaction conditions.