In this work, the mechanism of the CO ligand dissociation of [Re(CO)3(Pyta)(TPP)] + complex has been investigated with state-of-the-art computation methods as the dominant process of the PLS reaction and formation of the photoproduct [Re(CO)2(CH3CN)(Pyta)(TPP))] + . The topology of the excited states have been modeled by the UDFT method with the B3LYP functional. After the photoexcitation, the system is able to decay into the MLCT state through ISC and the molecular geometry does not considerably change.