Tailoring intersystem crossing of perylenediimide through chalcogen-substitution at bay-position: A theoretical...

Molecular-scale design strategies for promoting intersystem crossing (ISC) in small organic molecules are ubiquitous in developing efficient metal-free triplet photosensitizers with high triplet quantum yield (Φ_T). Air-stable and highly fluorescent perylenediimide (PDI) in its pristine form displays very small ISC compared to the fluorescence due to the large singlet-triplet gap (ΔE_S-T) and negligibly small spin-orbit coupling (SOC) between the lowest singlet (S₁) and triplet state (T₁). However, its Φ_T can be tuned by different chemical and mechanical means that are capable of either directly lowering the ΔE_S-T and increasing SOC or introducing intermediate low-lying triplet states (T_n, n = 2, 3, …) between S₁ and T₁. To this end, herein, a few chalcogen (X = O, S, Se) bay-substituted PDIs (PDI-X₂) are computationally modeled aiming at introducing geometrical-strain at the PDI core and also mixing nπ* orbital character to ππ* in the lowest singlet and triplet excited states, which altogether may reduce ΔE_S-T and also improve the SOC. Our quantum-chemical calculations based on optimally tuned range-separated hybrid reveal the presence of intermediate triplet states (T_n, n = 2, 3) in between S₁ and T₁ for all three PDI-X₂ studied in dichloromethane. More importantly, PDI-X₂ shows a significantly improved ISC rate than the pristine PDI due to the combined effects stemming from the smaller ΔE_S-T and the larger SOC. The calculated ISC rates follow the order as PDI-O₂ < PDI-S₂ < PDI-Se₂. These research findings will be helpful in designing PDI based triplet photosensitizers for biomedical, sensing, and photonic applications.