Then, we measured the UV/vis absorption spectra of various combinations of copper salts, phenylboronic acid 3a, and Box ligands (L10, L19, and L25) (Supporting Information, Figures ). It was found that the equimolar mixtures of Cu(I)/L10, Cu(I)/L19, Cu(I)/L25 showed absorption band around the visible region. Notably, among them, the UV/vis absorption of Cu(I)/L19 is the strongest, and can be greatly enhanced upon addition of phenylboronic acid 3a. This observation is also in accordance with the optimization study, wherein the reaction efficiency of Cu(I)/L19-catalzyed arylative radical cross-coupling of 4-aryl 1,3-enynes can be obviously improved upon purple LED irradiation.