Carbon dots (CDs), nanoscale quasi-spherical carbon materials, have attracted considerable attention in recent years, owing to their unique properties such as superior photoluminescence (PL) properties as well as excellent chemical- and photo- stability. CDs could act as metal-free, green nano-catalysts and their tailorable surface groups such as hydroxyl, carboxylic acid and amino groups have been exploited for organic synthesis. Meanwhile, CDs have shown great potential as visible-light-absorbing photocatalysts due to their light-harvesting ability and electron-transfer efficiency. Still, their photcatalytic applications are limited to simple organic transformations. In this thesis, CDs were synthesized from abundant carbohydrate sources, following a microwave-based carbonization method. The use of different doping agents was explored to adjust the functional groups on the CD surface and tune the photophysical properties. The obtained CDs were implemented as nano-photocatalysts for complex organic transformations such as cross-coupling and isomerization reactions. In Chapter 2, CDs were immobilized on the surface of TiO2 to generate heterogeneous photocatalytic systems. The applicability of CD1/TiO2 nanocomposites as photocatalysts for nickel-catalyzed cross-couplings was demonstrated for a C-O arylation using visible-light. CD1/TiO2 nanocomposites also served as an active photocatalyst for the coupling of aryl halides with an alcohol, a thiol, a sodium sulfonate, and a sulfonamide. Next, the photostability and recyclability of the CD1/TiO2 nanocomposite were explored, showing superior performances in comparison to organic dyes. Having demonstrated the potential of CD1 as photosensitizer for dual photoredox/Ni catalysis, I assessed the effect of different carbon sources and doping agents on the photocatalytic reaction. In Chapter 3, CDs were used to construct a quasi-homogenous colloidal catalytic system in combination with a Ni complex. This approach bypassed the need of CD immobilization onto a semiconductor (TiO2) and expanded the scope of suitable Ni ligands. This quasi-homogeneous system was applied to catalyze a broad range of carbon–heteroatom cross-couplings. I further investigated the mechanism of a C-S cross-coupling, optimized the reaction conditions, and expanded the substrate scope. New CDs were synthesized and a series of characterizations were performed to explore their influence on the photocatalytic performance. In Chapter 4, CDs were tested as cheap and green visible-light-absorbing photocatalyst for E-to-Z isomerization, a reaction that follows an energy-transfer-based catalytic mechanism. The isomerization results of trans-stilbene proved the catalytic capabilities of CDs in energy-transfer reaction. Next, I showcased the potential of CDs for the photo-isomerization of ethyl (E)-3-(p-tolyl)but-2-enoate (4-3), an alkene exhibiting larger separation of the excited state energy with its Z-isomers. I further expanded the substrate scope of E-to-Z isomerization reactions and screened a collection of CDs as photocatalysts to shine light on the process of E-to-Z isomerization. Overall, I demonstrated that CDs could serve as valuable photocatalysts for complex organic transformations, enriching their applications in the field of photocatalytic organic synthesis. Their ease of preparation from abundant precursor, tunable PL properties, and superior photostability make CDs valuable alternatives for expensive metal-based photocatalysts.