Engineering Intersystem Crossing in π-Conjugated Molecules through Heavy-Atom Effects: Computational Design Insights

Efficient intersystem crossing (ISC) is crucial for dictating the photophysical properties of organic systems, enabling their utility in optoelectronic applications such as phosphorescence, and photosensitizers for delayed fluorescence via triplet-triplet annihilation. In this work, we employed time-dependent density functional theory to investigate the excited state properties of azabenzanthracene (1-AzBA). We designed a new structural motif, 12-Br-1-AzBA, and proposed strategies to enhance spin-orbit coupling (SOC) for achieving efficient ISC. Our state-of-the-art theoretical analysis established that incorporating a bromine atom near the pyridinic nitrogen at the bay site strengthens the heavy-atom perturbation and promotes orbital angular momentum change. Consequently, a strong SOC of 108.39 cm-1 between the 1ππ* and 3nπ* states is realized, facilitating ultrafast ISC (1012 s-1) and effectively quenching the fluorescence, whereas 1-AzBA itself shows pronounced fluorescence. Among the possible bromine substitution sites in 1-AzBA, the 12-position uniquely delivers ultrafast ISC efficiency. We demonstrated that the combined effects of orbital angular momentum change, the heavy-atom effect, and favorable excited state energetics constitute an effective strategy for achieving efficient ISC in organic systems. Accordingly, 12-Br-1-AzBA emerges as a promising framework for designing metal-free π-conjugated organic materials for organic light-emitting devices.