-O-4 (C-O-C) bonds are the most abundant linkages in lignin (~60%). Consequently, cleavage of these bonds can significantly enhance the yield of valuable aromatic monomers. Especially in the last two decades, many efforts have been devoted to the depolymerization of lignin by taking advantage of -O-4 bond cleavage reactions. Nevertheless, due to the complexity of lignin's structure, scientists have mostly investigated the cleavage reactions on lignin model compounds containing -O-4 bonds such as 2-phenoxyacetophenone (2-PAP) (Scheme 2). Most of the current methods of lignin/model compounds depolymerization processes involve chemical oxidation/reduction reactions performed under harsh conditions using expensive or toxic chemicals and/or elevated temperatures. Electrochemical reactions, despite their efficiency, suffer mostly from the necessity of sophisticated equipment which limit their large-scale applications. Given all these challenges, it is obvious that lignin valorization still remains as a difficult task if one aims to substitute current platform chemicals in the industry. In the context of Green Chemistry, photocatalytic depolymerization reactions of lignin, have recently garnered significant interest due their milder reaction conditions and higher efficiency. The photocatalytic conversion of lignin into value-added aromatic compounds has been lately achieved by several groups using different semiconductors mostly under inert atmosphere. However, only few studies have reported the depolymerization of lignin/model compounds using metal-free photocatalysts under ambient conditions. It has been previously illustrated that phenacyl and ketyl radicals formed by sunlight exposure in UV region (>300nm) can initiate the depolymerization of wood materials like lignin through -O-4 cleavage, resulting in the phenomenon called as lignin yellowing. Moreover, electron spin resonance (ESR) investigations have revealed the sensitivity of lignin model compounds to Norrish type I cleavage and the crucial role of phenacyl and ketyl radicals in these cleavage reactions, especially under air atmosphere rather than nitrogen atmospthere. Yet, lignin's dark color blocks UV light due to the presence of highly absorbing chromophore groups such as conjugated phenolics, ketones and quinoids. Indeed, structurally modified lignin has been used as UV-protective material in sun screens. Low penetration of UV light into lignin causes a severe limitation to the formation of further ketyl/phenacyl radicals rendering the uncatalyzed photo-depolymerization of lignin ineffective.