where the dimensionless oscillator strength is defined as 𝑓 = 2 3 ∆𝐸 𝑆 1 𝑆 0 |𝜇 𝑆 1 𝑆 0 | 2 and 𝑐 is the speed of light. We note that experimental fluorescence lifetimes display only little variations depending on the solvent. Another relevant quantity is the excimer binding energy, evaluated as: where 𝐸 𝑆 1 (𝑃𝑦 2 ) is the total energy of a given relaxed conformation in the S1 state, while 𝐸 𝑆 0 (𝑃𝑦) and 𝐸 𝑆 1 (𝑃𝑦) are the energy of an optimized pyrene molecule in its ground and first excited states. Finally, CT provides the charge transfer character of the S0-S1 transition, calculated with the TheoDORE toolbox. Noteworthily, the excited monomer is considered equilibrated in the S1 state, as thermal relaxation processes that dissipate the excess excitation energy and solvent reorientation occur on a much shorter time scale than the diffusion-controlled processes underlying excimer formation. The geometrical counterpoise correction was applied to mitigate the well-known basis set superposition error (BSSE), which is expected to affect the values calculated with the double- basis set employed here.