Developments have also been made in methods that transfer energy and activate labeling reagents (Scheme 6A). Such activation could proceed through Dexter energy transfer between an excited Ir-photocatalyst and a diazirine-based labeling reagent, for instance (Scheme 6B, 6C). This activation of diazirine in the proximity of the catalyst generates carbene, a highly reactive chemical species with a half-life of 2 ns. The photoactivation allows for precise control of protein modification reactions within a tightly restricted area less than 4 nm around the catalyst. The first-generation Ir photocatalyst based on Ir[dF(CF3)ppy]2(dtbbpy) (Ir-G1 cat) not only produces carbene but also activates arylazide, generating nitrene. This catalytic activity has been applied to analyze protein-protein interactions (PPI) on cell membrane surfaces, ligand binding site mapping, control labeling radius by altering labeling reagents, study binding proteins of sialylated glycoproteins, and dynamic analysis of phagocytic surfaces. Additionally, the second-generation Ir photocatalyst (Ir-G2 cat), which addresses cell membrane permeability issues of the first generation catalysts, has been employed for small molecular compound target identification, analysis of PPI in chromatin proteins, and application to stress granule components in cells. Activation of arylazide has been further explored using organic dyes such as Acridine Orange, fuorescein, rhodamine 123, a red light-activated osmium photocatalyst, and Sn IV chlorin e6 catalyst. These catalysts function through a SET mechanism rather than energy transfer, involving the reduction of arylazide in the presence of NADH through a stepwise reductiondissociation-oxidation pathway.