Given the paramagnetic nature of these species, we reasoned that a change in the EPR spectra of TPA .+ s in the presence of arene substrates (350 eq., mirroring the reaction conditions) would be more conclusive in corroborating precomplexation. The EPR signal of TpBPA .+ showed a triplet (a N = 8.9 G). Addition of mesitylene to TpBPA .+ caused its EPR signal to shift (B = 4.5 G) to lower G values (Figure ), but the signal shape was largely unchanged. This indicates that the spin density of the TpBPA .+ is largely unaffected when it undergoes precomplexation with mesitylene. A less pronounced shift in G value occurred in the presence of iodobenzene (B = 2.0 G) but flattening of the triplet shoulders was observed (see Supplementary Information for detailed discussion). Based on these spectroscopic differences, we hypothesize that a different type of precomplex must occur in this case that is unreactive, to rationalize the inability to engage this substrate under the reaction conditions despite its more accessible redox potential (E p ox = +2.07 vs. SCE) than toluene (E p ox = +2.28 V vs. SCE). ( ) In contrast to TpBPA .+ , simulated fitting of the EPR signal from TCBPA .+ revealed two radical species, one triplet and one superimposed singlet appearing as a large broad central feature. We presume that this particular TPA .+ exists as a mixture of rotamers in solution, one propeller-type as observed in the solid state, and one in which a biphenyl unit falls into conjugation with the N radical cation, consistent with reported behaviour for similar compounds. (78-80) Addition of PhCl caused a notable change in the signal shape, giving exclusively the triplet signal (Figure ), consistent with the change in UV-vis (Figure ) and indicating that the spin density of TCBPA .+ is notably affected by precomplexation with PhCl. A similar change was detected upon addition of 1,2-dichlorobenzene, but not for its 1,4-congenor (17% yield of 3jb vs. 6% yield of 3lb under TCPBA e-PRC) and not for PhBr (22% of 3fb from TCPBA e-PRC) which instead all gave spectra clearly favoring the unreactive complex (singlet signal). To shed further light on the spectroscopic observations, Density Functional Theory (DFT)