Photoredox catalysis has become a powerful tool for chemists. However, the reliance on expensive Ir and Ru based photoredox catalysts (PC) motivates research for more sustainable options. Copper complexes based on favorable photophysical properties, high abundance and comparatively low prices offer a more sustainable alternative which is outlined in Chapter 1. This dissertation is dedicated to the development of new Cu(I) complexes and their investigation as PCs. Chapter 2 describes the synthesis of a new 2,2'-bipyridine (bpy) based ligand and the heteroleptic [Cu(N^N)(P^P)][PF6] complexes in combination with 4,5-Bis(diphenyl- phosphino)-9,9-dimethylxanthene (Xantphos, 1) and Bis[(2-diphenylphosphino)phenyl] ether (POP, 2). The complexes were thoroughly characterized and employed as PCs for the chlorotrifluoromethylation of activated alkenes with CF3SO2Cl. Complex 1 shows high activity with high yields and wide functional group tolerance in the chlorotrifluoromethylation of styrenes as well as alkenes adjacent to electron-withdrawing groups. The reaction with unactivated alkenes proved to be inefficient. Consequently, the design, synthesis and application of a new Cu(I) based photoredox catalyst is described. Chapter 3 details the use of complex 1 as PC for the chlorosulfonation of alkenes and alkynes with sulfonyl chlorides. The reactions occur under mild conditions and exhibit high functional group tolerance with broad scope in terms of both alkene and sulfonyl chloride. Notably, the reaction occurs with alkynes albeit in moderate to low yields to give exclusively the (E)‐product. Chapter 4 reports the synthesis of new bpy based ligands and their heteroleptic [Cu(N^N)(xantphos)][PF6] complexes to investigate possible structure-activity relationships. The synthesized Cu(I) complexes are characterized and their activities as photoredox catalysts compared to different Cu(I) based complexes are investigated. Consequently, it is shown that PC 1 is the most active catalyst in the chlorosulfonation of styrenes based on a variety of substrates due its long excited state lifetime in combination with a relatively high photoluminescence quantum yield and photo-stability.