Molecular transformations in biological systems benefit from the active species with strong redox power. Representative examples include photosystems which execute thermodynamically challenging redox reactions through a combination of two photoactive centers relayed through electron-transfer chains. This photoredox chemistry inspires chemists to devise useful catalytic systems for a variety of organic transformations that involve unusual photoredoxactive species as the key reaction intermediates. My group investigated photoinduced electron transfer of doublet molecules and low-valent metal complexes. We conducted mechanistic investigations for the transiently generated radical anions of organic molecules using electrochemical, transient absorption and photoluminescence spectroscopy and quantum chemical calculation techniques. The radical anions exhibit strong photoreducing power capable of generating synthetically important aryl radical intermediates from aryl halides which react with aryl boronate esters to produce C−C cross-coupled products. Our studies provided convincing evidence that the active catalyst species is the excited-state radical anion. As an alternative strategy to acquire the strong photoreducing power, we seek electron-rich, low-valent transition metal complexes as photoredoxcatalysts. Our exploration enables us to identify heteroleptic, linear Au(I) complexes as potent photoreducing catalysts with an excited-state oxidation potential as cathodic as −2.23 V vs standard calomel electrode. These Au(I) complexes can catalyze heteroaryl C−C cross-coupling reactions of redox-resistant aryl chlorides. Our investigations demonstrate that the Au(I) complexes offer several benefits, including strong visible-light absorption, a long excited-state lifetime, and the capacity of a 91% yield in the production of free-radical intermediates.