Jahn–Teller Couplings and Exciton Models Reveal Origins of Chiroptical Activity in Symmetric Cyclic π-Conjugated Systems

Understanding the origin of chiroptical responses in symmetric cyclic π-conjugated systems is key to advancing their application in optoelectronic and spintronic technologies. Here, we document that such a response is intrinsically nonadiabatic and arises from the entangled contributions of electronic states and molecular vibrations. We exploit a comprehensive theoretical framework that unifies localized exciton coupling and delocalized Jahn-Teller (JT) vibronic models─two traditionally distinct approaches─to accurately predict the ECD spectra of chiral spirobifluorene macrocycles. We show that using a flexible method to parametrize linear vibronic coupling (LVC) models in both localized and delocalized representations enables a direct comparison of excitonic and nonadiabatic effects. We demonstrate that quantum dynamical simulations on both models reproduce the experimental ECD spectrum of the <i>D</i>₃-symmetric macrocycle (<i>P</i>,<i>P</i>,<i>P</i>)-<b>1</b> with high accuracy and reveal that the coupling between nuclear and electronic motions plays a dominant role in shaping its chiroptical response. This analysis highlights the critical influence of conical intersections, exciton-charge transfer interactions, and symmetry-guided combinations of local transition dipoles. Importantly, the same approach captures the spectral features of the less symmetric <i>C</i>₂ stereoisomer (<i>P</i>,<i>P</i>,<i>M</i>)-<b>1</b>, underscoring the generality of the method. This dual-framework strategy provides deep mechanistic insight into the interplay of symmetry, vibronic coupling, and excitonic interactions and offers a powerful design tool for next-generation chiral materials.