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

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 (S 1 ) and triplet state (T 1 ). 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 S 1 and T 1 . To this end, herein, a few chalcogen (X = O, S, Se) bay-substituted PDIs (PDI-X 2 ) 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 S 1 and T 1 for all three PDI-X 2 studied in dichloromethane. More importantly, PDI-X 2 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-O 2 < PDI-S 2 < PDI-Se 2 . These research findings will be helpful in designing PDI based triplet photosensitizers for biomedical, sensing, and photonic applications.