The S 1 -S 0 minimum energy conical intersections (MECIs) optimisations were first carried out in solution and crystal phase at the SA-2-CASSCF(14,11)/6-31G(d) level of theory, using the branching plane update method as implemented in Molcas code. The obtained S 1 -S 0 MECI geometries are reoptimised at the extended multistate CASPT2 level (XMS-CASPT2). The minimum energy crossing point between the ground and T 1 states was optimised based on T 1 states optimisation and S 0 -T 1 energy gap criterion. In the solid state, the optimisations were done at the QM/MM level as described above. The active spaces comprised of twelve π orbitals and 2 p-orbitals of carboxyl group oxygen atoms for IPA and TPA (Figure , Supporting Information), were used in all optimisations and single point computations. The single-point CASPT2 computations were based on configuration state functions obtained at the SA-6-CASSCF(14,11)/aug-cc-pVDZ level. They were done without an IPEA shift and applying an imaginary shift of 0.1 au, which improves the convergence in the case of possible intruder states. Linearly interpolated pathways (LIIC) between S 0 and S 1 -S 0 MECI geometries in the solution and crystal were created and six lowest-lying singlet and triplet excited states were computed at the MS-6-CASPT2/CASSCF(14,11)/aug-cc-pVDZ level at the obtained geometries. The spin-orbit couplings (SOCs) between relevant singlet and triplet states were computed at the excited states minima in solution and crystal and along the LIIC pathway in the crystal, as implemented in the Molcas code. The method relies on the computation of matrix elements of one-electron spin-orbit part of the Hamiltonian in the atomic mean field approximation in the basis of the CASSCF wave functions. The SOCs between singlet and triplet pairs are obtained from the computed components corresponding to transitions between a singlet and three spin triplet components defined with quantum numbers