calculations were employed to model precomplexation of various TPA .+ /arene combinations (Table ). For unsymmetrical (halo)arene substrates, orientations of the complex with halogen facing both 'in' to the N radical cation and 'out' were explored (see Supplementary Information for full investigations). We assumed that -stacking interactions (81-82) at the TPA .+ 's biphenyl unit could be responsible for precomplexation. Attempts to position PhCl or mesitylene substrates in a sandwich or parallel-displaced - stacking interaction ("-" complex) around the inner N-bearing ring of their respective TPA .+ s led predominantly to dissociation, whereas positioning of the substrates around the terminal aromatic ring identified a local minima for the complexes resembling a T-type stacking interaction ("T-" complex). For this complex, minimal change in the spin density was detected for TpBPA .+ + mesitylene (Figure ), whereas a large shift in spin density occurred for TCBPA .+ + PhCl where the Cl atom was facing inwards (Figure ). This is consistent with the changes in EPR and UV-vis spectra, and so we assigned this "T-" complex as the one responsible for the triplet EPR signal and successful reactivity, since the oxidizing power of the N radical cation remains localized on its N atom. On the other hand, for less successful substrate PhBr and unsuccessful PhI (no product, 60% 2b recovered), a "-" complex was presumed to be responsible for the broad singlet EPR signal. Delocalization of the N radical cation over the biphenyl aromatic system would lead to stabilization, presumably decreasing E p ox of the photoexcited TPA .+ . Table . Free energies and intermolecular distances for T- or - precomplexes. Free energies of precomplexation were all found to be endergonic at the level of theory employed, intermolecular distances for T- stacking and - stacking were close to previously-reported distances for simpler complexes/dimers. (83-89) Mesitylene, chlorobenzene, 1,2-dichlorobenzene and their respective TPA .+ s; for which the 'reactive' T- complex was predicted by EPR studies; all had accessible G values (+3.5-7.2 kcal mol -1 ). An accessible G value was also found for 1,3-dichlorobenzene (+4.9-5.2 kcal mol -1 ) consistent with its reactivity (20% 3kb under the TCBPA e-PRC of Table ). Iodo-and bromobenzene as substrates gave very high G values for T- complexes and their - complexes were more accessible albeit still highly endergonic. Attempts to obtain a T- complex for TCBPA .+ with 1,4-dichlorobenzene led to dissociation, while its- complex was found to be accessible. Time-dependent density functional theory (TD-DFT) calculations were then used to investigate the energies higher order excited Dn states for TpBPA .+ , TCBPA .+ and TdCBPA .+ , using both CAM-B3LYP/6-31G(d,p) and wB97XD/6-31+G(d,p) (a solvent of MeCN or DCM was modelled implicitly, counterions were omitted). The calculated UV-visible spectra of the radical cations of TpBPA .+ and TCBPA .+ in MeCN were in reasonable agreement with their experimental spectra (Figure ). Since the broad visible band (ca. A mechanism is proposed consistent with spectroscopic and computational studies herein (Figure ). Anodic SET oxidation generates the TPA .+ from its TPA. Photoexcitation of the TPA .+ to its D1/D2 or higher Dn states followed by bimolecular SET reductive quenching is prohibited by the TPA .+ 's picosecond lifetime. Instead, preassociation occurs to give a reactive T- or an unreactive - precomplex, depending on the sterics of the arene substrate. In the latter case, conjugative stabilization of the N radical cation decreases E p ox of the *TPA .+ below the threshold for productive unimolecular SET such that photoexcitation leads simply to non-radiative photophysical relaxation processes (such as internal conversion). In the former case, photoexcitation yields unimolecular SET reductive quenching of the *TPA .+ , regenerating the TPA and generating the arene radical cation to be intercepted by the N-heterocyclic nucleophile 2b followed by loss of protons and further SET (anodic or by the TPA .+ ) to yield product 3. The prerequisite for precomplexation rationalizes the typical requirement for an excess of arene (3.5 eq. up to 1 mL, ~40 eq. herein) to drive precomplexation equilibrium in arene amination reactions mediated by radical cations .