The QM region was relaxed, while the MM region remained fixed at its optimised lattice positions. For TD-DFT optimisations in the solid state, the ONIOM(QM:MM)method was applied, with the QM region treated at the (TD-)ωB97XD/6-311G(d,p) level of theory. For MS-CASPT2/CASSCF(14,11)/aug-cc-pVDZ calculations in the solid state, the QM/MM simulations were performed using the Molcas code for the electrostatic embedding QM calculations and the Tinker code for the MM part considering CM5 point charges. In the (TD-)DFT calculations, the MM region was described using the OPLS-AA force field, with ESP and CM5 charges derived from HF/3-21G calculations on a single molecule. The RI-CC2/aug-cc-pVDZ calculations were done only using electrostatic embedding with CM5 charges. While the calculations with TDDFT and CC2 methods were done using the geometries optimised with the same method, MS-CASPT2 calculations considered the ωB97XD/6-311G(d,p) ground-state structures. All (TD-)DFT computations were performed with the Gaussian 16 program, CASSCF and CASPT2 computations with the Molcas code, and RI-CC2 computations were performed with the Turbomole v7.0 code. To investigate the excited state relaxation mechanisms in solution and crystal phase, we optimised the excited states minima (S 1 , T 1 , T 2 ) and S 1 -S 0 and T 1 -S 0 minimum energy crossing points in both environments. The excited states (S 1 , T 1 , and T 2 ) were optimised at the CASSCF(14,11)/aug-cc-pVDZ level of theory because the TD-ωB97XD/6-311G(d,p) does not not predict correctly the types and ordering of all excited states of TPA and IPA.