Towards an efficient delayed fluorescence mechanism from charge transfer and local transitions. The role of heavy halide atoms†

Finding organic nanostructures exhibiting luminescent behavior is a relevant issue in the development of functional molecules. In this context, the possibility of incorporating them into the architecture of organic light-emitting diode (OLED) devices has been investigated to improve their efficiency and photophysical stability for predicting the suitability of the emissive material. Herein, we unraveled the excitation mechanism and the role of the coupling constant in an uncommon planar structure. Intersystem crossing (ISC) represents a necessary process, so understanding the rate constant allows assessment of the suitability of a material. We calculated the ISC and reverse rates for hexamethylazatriangulene-triazine HMAT-TRZ (1a) and its functionalization with an iodide-atoms counterpart HMAT-TRZ-I₂ (1b), thereby uncovering the role of the heavy atoms. Our results account for moderate singlet–triplet gaps, ΔE_ST (0.1–0.2 eV). Moreover, HMAT-TRZ-I₂ enhanced the spin–orbit parameter, which accounted for ISC rates at the μs scale, and thus improved their luminescent characteristics. Calculations were conducted using time-dependent density functional theory, with the PBE0-D3 level providing an accurate response for the large molecules studied herein.