Chalcogenated biazulenediimides: A promising class of triplet harvesters

Visible-light absorbing planar perylenediimide and its chalcogenated congeners (X-PDIs; X = O, S, Se) showed huge promise in triplet harvesting. Analogous biazulenediimides (X-BAzDIs) may also hold great opportunity owing to the expected large spin-orbit coupling (SOC) via internal structural-twist and the heavy-atom effect due to chalcogen substituents. Herein, the rich photophysics of X-BAzDIs are explored and well understood by implementing time-dependent optimally tuned range separated hybrid density functional theory, achieving a reliable and accurate description of the excited electronic states. Increased SOC down the chalcogen group in X-BAzDIs due to the heavy-atom effect results in greater intersystem crossing (ISC) rates, ranging from ∼108 to 1013 s-1. Furthermore, ∼6-7 orders smaller fluorescence rates than the ISC of S/Se-BAzDIs suggest their excellent triplet harvesting efficacy. Unlike the planar O-PDI, an appreciable ISC rate of ∼108 s-1 is found for O-BAzDI, resulting from the inherent structural-twist. However, comparable ISC rates of ∼1011-1013 s-1 are found for both S/Se-BAzDIs and S/Se-PDIs. Inherently twisted-structures of X-BAzDIs are expected to surpass the self-aggregation. In addition, the high energy lowest excited triplet, and a modest lifetime, together with large ISC rates, position them as better triplet photosensitizers than X-PDIs. These findings will be helpful for understanding the molecular photophysics and also designing analogous azulene-based twisted organic systems to efficiently harvest high energy and long-lived triplets.