Yoon's research lab at the University of Wisconsin-Madison focuses on developing new reaction methods for organic synthesis, especially those involving transition metal photochemistry, stereocontrolling, and dual catalysis.
Overview
In particular, Yoons group aims to leverage the ability of visible light–absorbing transition metal complexes to catalyze synthetic reactions. They investigate various mechanisms of photocatalytic activation, which differ from complex to complex depending on reactivity patterns of intermediates and their ability to be activated by sources of white light, including sunlight. Traditionally, chemists have used high-energy UV light to activate simple organic molecules, but Yoons group focuses instead on expanding the application of visible light sources to synthesize increasingly complex target molecules. By providing strategies for activation of organic substrates that do not require specialized high-pressure UV photolysis apparatuses, these procedures are rendered more environmentally-friendly and widely available to synthetic and organic chemists.
Significant Developments
One notable process explored by Yoon's research is the generation of photoreductants by irradiation of [Ru(bpy)] that can initiate desired cycloaddition. The group proved [Ru(bpy)]Cl to be an efficient photocatalyst for the formal [2+2] cycloaddition of enones and yields potential for development of new reaction protocols with reduced environmental impact. 
Yoon's group has also researched into crossed intermolecular [2+2] heterodimerizations, proving the possibility of using two dissimilar enone substrates to successfully produce these dimers. This method bypasses some synthetic limitations of cycloadditions conducted under standard UV photolysis conditions.
Yoon reviews the ways how cocatalyst strategies can be applied to synthesis, ranging from developments in organic photochemistry and the precedents that brought interest in photocatalytic synthesis. The interaction between an excited photocatalyst and organic molecule can show a diverse sample of reactive intermediates that can be manipulated to form a synthetic bond construction. This impacts the photocatalyst and the photoactivation steps such as the interaction with the excited state of the photocatalyst or controlling the rate and selectivity of the photoactivation steps.
Additionally, Yoon takes a dual approach to the asymmetric of enantioselective [2+2] photocycloadditions by using visible light that can absorb transition metal and a Lewis acid cocatalyst. Yoon was able to see that each catalyst can be enabled to be independent resulting in a broader scope and greater flexibility and efficiency in enantioselective photochemical cycloadditions. Along with the metal photocatalyst being compatible with several types of Lewis acid catalyst.  
Yoon developed the first highly enantioselective intermolecular reaction of α-amino radicals by using the dual-catalyst protocol to combine the transition metal photoredox catalysis with the chiral Lewis acid catalysis. The combination of these catalysts provided an approach to control the stereochemistry of a wide variety of photoinitiated organic reactions.
Furthermore, Yoon was able to perform quantum yield measurements to showcase that three distinct photoredox processes and involvement in the formation of chain reactions. In the combination of doing quantum yield and luminescence quenching experiments, it displays a method to estimate the length of these chains, to determine a lower limit for these chains and to diagnose inefficient initiation steps in photoredox reactions. Yoon demonstrated that the chain processes dominated the product formation of the three photoredox transformations.