The two-photon absorption (TPA) cross section of the dihydroazulene-spiropyran (DHA-SP) multistate multifunctional dyad has been investigated using quantum chemistry methods to assess its potential as a write nonlinear optical (NLO) switch for 3D data storage. TPA-induced switching is found to trigger reactions from DHA-SP to DHA-E-MCH, with a TPA cross section value of 2500 GM. Conversely to the one-photon absorption, no simple additivity is observed in the dyad TPA spectra but cooperative effects are highlighted in DHA-E-MCH, the most active form. Excitation channel analyses, based on the sum-over-states formalism and the missing state analysis, show that the TPA cross section of the DHA-E-MCH form mostly results from local excitations on the E-MCH parent unit with important charge transfer contributions from DHA to E-MCH. More generally, it also highlights that the first singlet excited state is the most important excitation channel in all forms presenting large TPA cross sections. This is further confirmed by a simplified three-state model. Overall, the results of this paper demonstrate the promising potential of the DHA-SP multistate multifunctional molecular switch for NLO read/write applications from the viewpoint of memory devices
