Reaction Optimization. The reaction development was initiated by using aryl dibenzothiophenium salt 4a readily accessed from the corresponding arene as the model substrate for 1,2-arylazidation of alkenes with TMSN3 under photoredox/iron dual-catalyzed conditions. Through extensive investigation of various bases, photocatalysts, iron catalysts, and solvents, the optimal conditions for the reaction were identified as follow: the combination of the photocatalyst fac-Ir(ppy)3 (2 mol%), Fe(OTf)2 (20 mol%), 4-picoline (2.5 equiv) in MeCN (0.1 M) at room temperature under light irradiation with 456 nm blue light-emitting diode (LED) afforded arylethylazide product 4 in 84% yield and complete regioselectivity (Table , entry 1). Notably, the residue dibenzo [b,d]thiophene (DBT) was recovered in 94% yield and can be reused. When 4-picoline was replaced with other bases, the product yields decreased (entry 2). The choice of solvent was vital to this transformation, as evidenced by the low yield (0-60%) obtained when using other solvents (entry 3, see Table for details). Switching Fe(OTf)2 to other iron catalysts also resulted in decreased yield of azidoarylation product 4 (Table , entry 4). Substituting the photocatalyst from fac-Ir(ppy)3 led to no product formation, possibly because that their reduction potentials are not enough to reduce sulfonium salt 4a (E red = -1.1 V vs SCE) 12f (entries 5 and 6). When NaN3 was used as the nitrogen source instead of TMSN3 and 4-picoline, the yield of desired product 4 was obtained in 69% yield (entry 7). In addition, control experiments showed that the absence of fac-Ir(ppy)3, iron catalyst, 4-picoline or light resulted in no product formation, highlighting the necessity of these components in this reaction (entries 8-10). a Yields were determined by analysis of the crude 1 H NMR spectra using CH2Br2 as an internal standard. b Isolated yields. See Tables S1-S4 for details.