The 0-0 transition energies of 2-, 4-, and 5-MP were computed at the CASPT2 level with and without IPEA shift, similar to a previous study. The character of the S 1 transition of these MP derivatives is n → π * transition. To obtain 0-0 transition energies, we first optimized the system at both the ground and excited states; the respective energies are denoted here by E GS and E ES . Then, at each minimum, vibrational frequency calculations were performed to obtain the zero-point vibrational energy (ZPVE; within harmonic approximations): E GS ZPVE and E ES ZPVE . The 0-0 transition energy E 0-0 is defined as the difference between ZPVE-corrected energies: