Efficient triplet formation can be achieved in RTP organic crystals in compounds where the presence of heteroatoms facilitates intersystem crossing rates between relevant singlet and triplet states. A second requirement for organic RTP is to establish a restrictive crystalline environment that suppresses large-amplitude molecular vibrations and reduces the rate of internal conversion to the ground state via conical intersections or vibrational wavefunction overlaps. Depending on the energy gaps and SOCs, RTP can compete with both prompt fluorescence (PF) and delayed fluorescence (DF). DF can occur through reverse intersystem crossing (rISC), which is crucial in thermally activated delayed fluorescence (TADF) chromophores. Two of the simplest organic crystals exhibiting RTP are the two isomers of benzenedicarboxylic acid in the crystalline phase: terephthalic acid (TPA) and isophthalic acid (IPA). These isomers differ in the relative positions of their substituents (Figure ). Transient photoluminescence spectroscopy of TPA and IPA in ethanol solution and disordered amorphous states reveals negligible luminescence quantum yields (below 0.6%). Upon crystallisation, both crystals exhibit dual emission, attributed to fluorescence and phosphorescence. The TPA crystal shows prompt deep blue emission at 388 nm with a lifetime of ⟨t⟩ = 0.53 ns and an emission efficiency of 8.4%. Additionally, a delayed fluorescence (DF) peak at 392 nm (slightly red-shifted compared to the prompt emission) and a shoulder at 511 nm, attributed to weak RTP, are observed. The experimental RTP lifetime is not determined on the millisecond timescale, suggesting longer lifetimes. The IPA crystal exhibits more efficient crystallisation-induced emission at 380 nm, with a quantum yield of 15.3%, attributed to prompt fluorescence. In addition to fluorescence, a long green afterglow lasting several seconds was observed, with distinct emission peaks at 384 nm and 506 nm, corresponding to DF and RTP, respectively. The measured lifetimes of prompt fluorescence, DF, and RTP in IPA are 1.81 ns, 9.6 ms, and 290 ms, respectively. Previous studies have investigated the mechanisms of fluorescence and phosphorescence in TPA and IPA. Shuai et al. found the crystalline phase induces a change of ordering of the lowest-lying singlet and triplet states of IPA and TPA in comparison with the vacuum where the S 1 state is a nπ * , the higher ππ * state becomes the lowest according to the CASSCF (8,8)/CASPT2/AMBER level of theory. This change induces higher luminescence yields and larger intersystem crossing rates in the crystal. Gong et al. performed a similar optimisation of TPA and IPA in both the vacuum and crystalline phases based on QM/MM (TD-M06-2x/UFF) computations. They found that the energy gap between the closestlying S 1 and T 2 states decreases while the spin-orbit coupling simultaneously increases in the crystal compared to the vacuum, leading to higher populations of the triplet state in the crystalline environment. Here, we evaluate the impact of using different levels of theory on predicting the ordering of states and consider the competition between various radiative and nonradiative mechanisms.