The excimer emission of 1o is recorded upon two photon excitations with the same solution (TPE: 700 nm < ex < 800 nm). One can notice that no TPE emission signal was observed for 2 in same conditions. Remarkably, the TPE spectra of 1 exhibits an emission band at the same energy as the 1-photon excimer emission, without any residual structured emission of monomeric pyrene (Figure ). In this system, the TPE measurement selectively probes the excimer, likely due to symmetry rules. The output intensity of two-photon excited fluorescence (TPEF) was linearly dependent on the square of the input laser intensity, confirming the nonlinearity of the absorption process. 1 shows a modest two-photon absorption cross section of 90 GM, typical for pyrene systems (Figure and Table ). Interestingly, while the emission intensity of one-photon excitation (OPE) decreases upon consecutive emission spectra acquisitions due to the competition with the ring-closure reaction (Figure ), the emission recorded on two-photon excitation remains stable (Figure ). The robustness of 1 toward TPE allowed us to perform an all-optical switching experiment where OPE was employed to trigger the photochromic reaction and TPE was used to record the "ON" or "OFF" signal, over 10 cycles without any loss of readout (Figure ).