In this section, we analyse different mechanisms associated with the excited state relaxation of TPA and IPA. Our main focus is understanding the competition of different pathways in the solid state. In the S 1 -S 0 MECI structures optimised in the solution, the ∠HOCO is close to 90 • , while the rest of the molecule is planar. On the other hand, in the solid state, the more favourable CI structures involve the bending of the COO-plane with respect to the rest of the molecule, while the H-atom dihedral motion is less activated (∠HOCO is 60 • in IPA and 53.5 • in TPA). A similar type of MECI structure was found in a phenyl-derivative dicarboxylic acid. The XMS-CASPT2 method is an appropriate choice for the optimisation of conical intersections, due to the multireference character of the wave function near these intersections.