The exciton couplings are much smaller than the classical barrier for exciton hopping (λ/4), indicating that the S 1 excitons mostly remain localised on a single site during exciton diffusion in crystals. The exciton diffusion mechanism can be represented as a series of hopping events between isolated molecules. In this context, the exciton hopping rates for this mechanism can be evaluated based on the Marcus theory (Eq. 2). Applying the computed exciton couplings and reorganisation energies for IPA and TPA, the estimated exciton hopping rates for these molecules are 2.7×10 10 and 8.3×10 9 s -1 , respectively. This will compete with the vibrational relaxation to S 1 in the picosecond scale (rates around 10 12 s -1 ) followed by other processes such as fluorescence and ISC. Due to a smaller reorganisation energy, the exciton hopping rate in IPA are ∼4 times larger than in TPA. Upon the photoexcitation to the bright state, the exciton diffusion competes with the intramolecular relaxation processes, in first place with vibrations relaxation to the S 1 state minimum, usually occurring at the ps timescale. We note that the thermal effects, which can significantly affect the exciton couplings and exciton diffusion rates and mechanism, are neglected here.