Project Summary Photon-driven processes have emerged as a powerful tool for achieving challenging bond cleavage and bond formation. Photocatalysis offers the benefit of temporal and spatial control with low energy light, which has been widely advantageous for efficiently building molecular complexity from simple starting materials. The judicious choice of photocatalysts enables the precision of reactivity that is rarely achieved with other forms of catalysis and heating. An underused area of photocatalysis is photothermal conversion. Irradiation of specific nanoparticles or dyes with visible light creates intense thermal gradients in a photothermal conversion process. In contrast to bulk heating, where the temperature remains uniform across a reaction medium, substrates would only experience thermal energy within a few nanometers of excitation under temporal heating. Consequently, this process would use irradiation to drive chemical processes at high temperatures with temporal and spatial control. Spatial control enables the selective formation of highly reactive species without competing bimolecular side reactions. The proposed research comprises three fundamental projects exploring the use of photothermal catalysis to enable the synthesis of complex molecules using visible light irradiation. First, high-temperature thermal rearrangements will be achieved using carbon-based nanoparticles and visible light irradiation under mild conditions. This strategy will enable the synthesis of complex products without thermal decomposition generally associated with bulk thermolysis. Additionally, the identification of various photothermal agents and synthetic elaborations will generate more efficient catalysts. In the second project, photothermal heating will generate carbon-centered radicals through C–C bond homolysis. Intercepting these highly reactive intermediates will forge new C–C bonds in ring expansion reactions, in intramolecular cyclizations. Specific photothermal agent design will enable intermolecular ring expansions to build molecular complexity. In a third project, the C–C bond homolysis reactions identified in project two will be used in dynamic kinetic resolutions for atom economical synthesis of enantioenriched pharmaceutical compounds. Emulsions will confine thermal gradients to within micelles enabling the coupling of thermal epimerization reactions with highly selective enzymatic reactions. Combined, these three projects will develop an understanding of photothermal catalysis and the effect of light and intensity on generation of thermal gradients. A thorough understanding of how these thermal gradients can be leveraged for high temperature reactions with the spatial and temporal control of visible light will enable new synthetic bond disconnections previously unrealized.