Our results show that for an efficient triplet states population and solid-state phosphorescence in organic crystals, apart from the hindrance of the internal conversion, positioning of substituents in benzene dicarboxylic acids plays a significant role. In the case of para substitution, the electronic delocalisation over the entire molecule is stabilised in the lowlying singlet and triplet states, resulting in ππ * being the lowest in energy. With meta substitution electronic delocalisation is less favorable, leading to the stabilisation of mixed nπ * /ππ * or pure nπ * at their respective minima. This electronic effect significantly influences deactivation pathways. Our results indicate that the fluorescence, phosphorescence, and triplet-state population thought intersystem crossing processes are more efficient in IPA, which is in line with experimental observations. We believe this effect can be generalized to other substituents involved in electronic delocalization with the central ring, as well as to more complex conjugated systems. This result might be of interest for the design of highly efficient room-temperature phosphorescent materials.