To understand the C-H bond activation mechanism, we started with detecting in situ generated radical intermediates during the photocatalytic reactions. We used 2,2,6,6tetramethyl-1-piperidinyloxy (TEMPO) as the radical scavenger. We confirmed the formation of α-ethereal, α-amido, acyl, and benzylic radicals from THF, DMA, benzaldehyde, and toluene by detecting their TEMPO adducts using highresolution mass spectrometry (Figures and). Reactions were also completely inhibited due to the presence of TEMPO (Figure ). Similar reaction inhibition was observed after exposing the reaction to another common radical scavenger, oxygen molecules (entry 2 in Figure ). Both observations indicate that QD gel-catalyzed C-H bond activation led to neutral radical formation. The kinetic Figure . Photocatalytic coupling reactions (a) between amides and cyanoaromatics, (b) between amides and aromatic carbonyl compounds, and (c) three-compound coupling reaction. In all cases, 1 mL C-H partners 5 were used as solvent and 2x10 -3 mol% CdS QD gel was used as the catalyst. Isolated yields and regioisomeric ratios (rr, calculated from 1 H-NMR spectra) are shown under each product. # 6% of ketone product was also obtained (see Figure ). isotope effects (KIE) measured from two parallel reactions and intermolecular competition experiment show primary KIE values of 2.2 and 4.2, resulting from H/D substitution of THF (Figures and). Similar primary KIE values were observed for C-H activation of amide and benzaldehyde substrates (Figures and). These findings indicate that the C-H bond cleavage to form the radical intermediates occurred during the rate-determining step of the photocatalytic reactions. Next, we investigated how these neutral radicals were generated from their corresponding C-H substrates. We initially evaluated the possibility of electron transfer between photoexcited CdS QD gels and substrates, followed by deprotonation. To test it, we estimated the oxidation potentials of organic substrates from their onset potentials in the cyclic voltammograms (Figure ). We found that the electrode potentials required to oxidize the organic sub-strates were significantly higher (by > 380 mV) than the valence band maximum (VBM) of CdS QD gel (1.39 V vs NHE , Figure ), indicating the oxidizing power of excited QD gel is insufficient to perform electron transfer with the C-H substrates. Moreover, control experiments with inorganic or organic bases show that base had no positive effect on THF (entries 3-6 in Figure ) or DMA activation (Figure ), suggesting that deprotonation may not be required for producing these radicals, either. In addition, the photocatalytic reaction is insensitive to solvents (entries 7-11). Similar product yields were observed using acetone, acetonitrile (ACN), dichloromethane (DCM), and even H2O, indicating that carbocation intermediates are unlikely to be present because they would have been attacked by water. The slightly lower yield for dimethylformamide (DMF) is because DMF is also activated under the same reaction conditions (Figure ). The wide range of possible C-H bonds to be activated, along with the inefficient oxidizing power of CdS QD gels, the absence of the need for bases, and the insensitivity to solvents all suggest that there might be an alternative mechanism for C-H bond activation that differs from the sequential electron transfer and proton transfer pathway.