We also explored the impact of increasing the photosensitizer concentrations (0.1 -0.5 mol%) using a hexane solution of isoprene (15% in hexane). An increased sensitizer concentration should favor the bimolecular triplet energy transfer, and as seen in Fig. , the yield of isoprene dimers increased by up to a value of 54% with 0.4 mol% of 1,1dinaphthylmethanone. This is almost three times higher than in the reaction using the solution with 0.1 mol% photosensitizer. However, with 0.5 mol% photosensitizer the dimer yields dropped, a result of either self-quenching between two photosensitizer molecules or quenching of 3 2 by a photosensitizer molecule. Although endergonic, the latter may occur given that the E(T1) of isoprene is close to that of the sensitizer. Thus, a loading of 0.4 mol% is the optimal photosensitizer loading in the diluted condition of 15 wt.% isoprene in hexane. We also tested the neat isoprene (99%) with a higher concentration of photosensitizer (0.4 mol%), but in this case, the yield of dimers did not increase when compared to the 0.1 mol% case (Table , ESI).