Localized and Delocalized Nonadiabatic Vibronic Models Unravel Chiroptical Trends and Design Principles in Symmetric π Conjugated Macrocycles

The chiroptical response of π-conjugated macrocycles assembled from repeating chromophores can be remarkably intense, yet how it scales with ring size, and what controls it, remains only partly understood. Here we investigate a series of axially chiral diethynylspirobifluorene macrocycles, (Pn)-DESn (n = 2, 3, 4; D2, D3 and D4 symmetry), combining accurate nonadiabatic quantum dynamics with two complementary linear vibronic coupling Hamiltonians: a delocalized model, whose diabatic states span the whole macrocycle, and a localized, exciton-like model built from the excitations of the individual fragments. Both reproduce the experimental one-photon absorption and electronic circular dichroism (ECD) spectra and reveal that nonadiabatic interactions, conical intersections and (pseudo-)Jahn–Teller couplings govern their shapes; a hierarchical representation of the Hamiltonian keeps the simulations tractable, even for (P4)-DES4, with more than 500 vibrational modes. A simple symmetry analysis bridges the two pictures, tracing every coupling back to the single excited-state gradient of the monomer and partitions the magnetic transition dipole into an intrinsic contribution, from the strained monomer and an extrinsic, excitonic one. This decomposition rationalizes the observed trends explaining why the ECD grows super-linearly with ring size. As a result, the response crosses over from strain-dominated chirality in (P2)-DES2 to excitonic chirality in (P4)-DES4 at a nearly constant dissymmetry factor. Crucially, the excitonic rotatory strengths form a nearly conservative couplet whose oppositely signed branches partly cancel, and their splitting (0.12–0.16 eV) is comparable to the width of the vibronic progressions. The intrinsic band shape of the monomer, not the splitting alone, therefore sets how much ECD survives. We translate these findings into geometric, excitonic and vibronic design principles for g-factor improved symmetric chiral macrocycles, suggesting that our proposed computational approach can be a fast screening tool for new designed macrocycles before synthesis.