transition metal - catalyzed cross - coupling reactions have become one of the most utilized carbon  carbon and carbon  heteroatom bond - forming reactions in chemical synthesis . 
 more recently , nickel catalysis has been shown to participate in a wide variety of c  c bond forming reactions , most notably negishi , suzuki 
 miyaura , stille , kumada , and hiyama couplings1,2 . despite the tremendous advances in c 
 c fragment couplings , the ability to forge c  o bonds in a general fashion via nickel catalysis has been largely unsuccessful . the challenge for nickel - mediated alcohol couplings has been the mechanistic requirement for the critical c  o bond forming step ( formally known as the reductive elimination step ) to occur via a ni(iii ) alkoxide intermediate . in this manuscript , we demonstrate that visible light - excited photoredox catalysts can modulate the preferred oxidation states of nickel alkoxides in an operative catalytic cycle , thereby providing transient access to ni(iii ) species that readily participate in reductive elimination . using this synergistic merger of photoredox and nickel catalysis , 
 we have developed a highly efficient and general carbon 
 oxygen coupling reaction using abundant alcohols and aryl bromides . 
 more significantly , we have developed a general strategy to  switch on  important yet elusive organometallic mechanisms via oxidation state modulations using only weak light and single - electron transfer ( set ) catalysts .