2 General computational details TURBOMOLE 7.3 was used for geometry optimisations of the lowest-lying singlet excited states of each dimer at the spin-component-scaled 127,128 approximate coupled cluster singles doubles (SCS-CC2 129 ) level with the def2-TZVP 130 tripleζ (TZ) atomic-orbital (AO) basis set and a geometry convergence criterion of 10 -7 E h . Use of the RICC2 module in TURBO-MOLE limits the symmetry consideration to Abelian point groups such that all excimers are calculated with D 2h symmetry. The excited-state optimised structures were used to generate nonrelaxed dissociation curves as follows: the internal coordinates of each monomer were frozen, then total energies of the lowestlying singlet excited states were calculated across a range of intermonomer separations up to 16 Å, and then each energy was taken relative to 16 Å, which represents the asymptote for the given state. The geometry optimised excimer structures were not comprised of perfectly planar monomers, which has been previously noted during both TD-DFT and CC2 131 optimisations of these excimers and exciplexes comprised of the same monomers. Therefore, we defined the inter-monomer separation for our dis-sociation curves as the distance between two central carbon atoms, opposite each other on each monomer; this parameter is visually defined for each excimer structure in Fig. ). D e , is defined as the difference between the total energy of the first excited state at the 16 Å point and the total energy of the same state at the minimum point. The effect of geometric relaxation falls outside the scope of this study. However, frozen-monomer structures at the SOS-CIS(D 0 ) 132 level have been shown to give good estimates of the equilibrium inter-monomer distance, with excimer dissociation energies that are generally overestimated by ca. 2 kcal/mol in the case of the benzene excimer. As this study uses the same structures throughout, internal comparison of the dissociation energies is reasonably justified.