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Abstract

The advent of precision photochemistry is a logical consequence from advances in light‐generation technologies that enable access to almost any desired wavelength, coupled with wavelength‐resolved reactivity insights into complex photochemical processes. Beyond optimizing individual photoreactions, the integration of multiple wavelengths into a single system unlocks entirely new reaction modes. The synergistic two-color reaction represents a particularly powerful mode, in which an irreversible transformation (i.e., bond formation) is enabled – or significantly enhanced – only when two distinct wavelengths of light coincide in the same volume element. Herein, a synergistic two-color reaction system is introduced, based on the interplay of two photoswitches activated at different wavelengths. A diaryl indenone epoxide (DIO) undergoes ring expansion under ultraviolet light (365 nm), whereas a bridged ring-strained azobenzene (SA) performs cis-to-trans isomerization under visible light (430 nm) irradiation. When triggered simultaneously, the photoactivated species readily undergo cycloaddition to form the DIOSA cycloadduct, which was structurally elucidated by single-crystal X-ray diffraction (SXRD). The photochemical synergistic ratio ϕ_syn is introduced as a function of product yield to quantitatively assess the effectiveness of dual-color irradiation under defined reaction conditions such as photon flux and starting material ratio. A reduced form, ϕ_syn^0 – extrapolated to infinitesimal small conversion – serves as a conversion-independent metric for comparing the efficiency of synergistic photochemistries under varied reaction conditions and across different systems. Building on the above model study, the reaction was next transferred from the small-molecule to the macromolecular scale to enable synergistic two-color polymer network formation and lithography as a wavelength-gated strategy with enhanced spatiotemporal control over crosslinking. For photoresist preparation, DIO and SA moieties were incorporated into different multifunctional macromolecular scaffolds and dissolved in acetophenone. The successful translation of the synergistic model reaction into crosslinking for network formation was first confirmed via size-exclusion chromatography (SEC) kinetics under one- and two-color light-emitting diode (LED) exposure. Finally, the reaction was implemented in a dual-laser lithography setup, enabling the synergistic fabrication of intricate geometries such as ring segments and butterfly architectures under specifically optimized printing conditions.