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

Abstract

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 (Δ E 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 Δ E 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 Δ E 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 Δ E S− T and the larger SOC. The calculated ISC rates follow the order as PDI-O2 [Formula: see text] PDI-S2 [Formula: see text] PDI-Se2. These research findings will be helpful in designing PDI based triplet photosensitizers for biomedical, sensing, and photonic applications.