We also optimised the S 1 -S 0 MECI using less computationally expensive single-reference methods, TD-DFT and ADC (2). These methods predicted the minimal energy crossing structures involving benzene ring puckering, which are much higher in energy compared to The S 1 -S 0 MECI for IPA lies at ∼5.2 eV, ∼1.2 eV above the vertical excitation, and for TPA at ∼6.0 eV, ∼2 eV above the vertical excitation, and they are not classically accessible after the excitation to S 1 state. The excitation the second bright state of TPA at 4.74 eV (S 2 ) would also not enable the internal conversion to the ground state. The excitation of high-energy bright states of IPA and TPA with energies close to 6 eV (∼200 nm), would enable internal conversion, but that would require the application of vacuum UV radiation, not used in the experiment. The crystal environment increases the energy barriers to the S 1 -S 0 MECI with respect to solution in 0.4 eV for IPA and 0.8 eV for TPA. From the transition state theory, we can obtain that the fractions of nonradiative rates between the solid state and solution are in the order 10 -7 for IPA and 10 -14 TPA respectively. This is in line with the restricted access to the conical intersection (RACI) model, where the crystal environment blocks the access to the conical intersections associated with the S 1 -S 0 nonradiative transitions explaining the enhancement of solid-state luminescence for these systems. We should notice that for both S 1 -S 0 MECI, T 1 gets almost degenerate with S 0 and S 1 providing an additional nonradiative pathway to the ground state, which can be activated by providing larger excitation energies.