Aggregation Suppresses Internal Conversion and Enhances Triplet Yield in π-Rich Natural Dyes by Vibronic Coupling Control: A Case Study of Hypericin Supramolecular Assembly

Hypericin, a π-rich anthraquinone natural product, exhibits anomalously high triplet yields despite inherently weak spin-orbit coupling. Using long-time-scale molecular dynamics integrated with time-dependent density functional theory calculations, it is demonstrated that early stage supramolecular aggregation modulates hypericin's excited-state dynamics through vibronic control. Dimerization constrains low-frequency nuclear motions, reducing nonadiabatic couplings and thereby suppressing internal conversion. Concurrently, excitonic redistribution compresses singlet-triplet energy gaps, enabling efficient intersystem crossing in the absence of heavy atoms. Quantitative coupling analysis reveals mixed vibronic-excitonic regimes that account for the experimentally observed enhancement of triplet formation under self-assembly conditions. These findings establish a mechanistic basis for hypericin's photodynamic activity.