When the reaction was repeated with 2.0 eq. of H-TEDA(BF 4 ) 2 , the yield increased from 38% to 65% (Figure ). In this method, the auxiliary (4-fluorobenzoate) acts mainly as a photosensitizer and H-TEDA(BF 4 ) 2 as a superior activating reagent of SF ® via changing the aggregation state. To manifest aggregation trends of SF ® and H-TEDA(BF 4 ) 2 , we performed further diffusion ordered spectroscopy (DOSY) measurements for the pure components and for the reaction mixture and calculated the related volumes to determine aggregation trends (Table ). At synthetic reaction concentrations ( that different ion pair formations should be irrelevant (entries 12-14). Even though the overall ion concentration of the homocomplex situation is higher at 90 mM (entries 2 and 6), the volumes of SF ® and H-TEDA(BF 4 ) 2 increase at lower ion concentrations of 40 mM in their 1:1 component mixture (entry 8). Again, this corroborates heterocomplex formation of SF ® and H-TEDA(BF 4 ) 2 . Therefore, we suggest the additional + N-H---F-N + interaction within the complex as a driving force for the preferred heterocomplex formation similar to the reported structures of Toste and co-workers (see Figure , vide infra). In our previous study, we demonstrated methyl 4-fluorobenzoate as a non-ketone photosensitization catalyst for C(sp 3 )-H fluorinations. Although a variety of small molecules with different functional groups were tolerated, yields were variable (31 -94%) and some substrates required a 4-fluorobenzoyl auxiliary to achieve satisfactory (>50%) yields. One of the poorly reactive substrates under photocatalytic conditions was 4-phenylbutyl benzoate (1c), which afforded only 10% of 2c when treated with 1 mol% photocatalyst (MFB) under 400 nm irradiation for 24 h (Table , entry 1) (see the SI file for the details). In the absence of any photocatalyst, the reaction proceeds in a similar yield showing that the benzoyl group can serve as a photosensitizing auxiliary, however it is very inefficient compared to 4-fluorobenzoyl as we previously reported. Under the standard reaction conditions (with 1 mol% MFB) but with 2.0 eq. of H-TEDA(BF 4 ) 2 present at the start of the reaction, the yield of 2c increased dramatically to 68% (Table , entry 2). Using 6.0 eq. H-TEDA(BF 4 ) 2 provided an even higher yield of 2c (81%) (Table , entry 3), while 10.0 eq. H-TEDA(BF 4 ) 2 provided only 57%, presumably due to substantial light occlusion under these low solubility conditions. TEA-H•BF 4