Vibronic Absorption and Circular Dichroism Spectra of Chiral PDI Aggregates in Free Space and Optical Cavities

Manipulating molecular chirality via strong light–matter coupling holds promise for asymmetric photochemistry and chiral optoelectronics, but a microscopic understanding of how cavity confinement, excitonic delocalization, and vibronic coupling intertwine is still lacking. We address this by simulating the polaritonic absorption and CD spectra of helical perylenediimide (PDI) aggregates in optical cavities, using a generalized Holstein–Tavis–Cummings (HTC) model parametrized from ab initioelectronic-structure calculations and solved with the non-Markovian stochastic Schrödinger equation (NMSSE). Unlike conventional HTC approaches, our model fully incorporates local excitations (LEs), charge-transfer excitations (CTEs), and their mutual couplings. We show that cavity photons generate chiroptical responses beyond the reach of standard exciton theory, yielding strong CD signals even when direct inter-chromophore coupling is negligible. This non-local chiral effect originates from collective coupling to a shared cavity mode, which allows a local excitation to coherently address distant monomers through the quantized field. We further demonstrate that dark polaritonic states are highly sensitive to packing: symmetry breaking via disorder or dipole misalignment lifts their degeneracy into three bright bands. Vibronic coupling, however, broadens the central band and redistributes intensity into sidebands, partially masking the splitting. Together, these findings reveal the microscopic origins of cavity-mediated polaritonic chirality and offer a roadmap for engineering dark states in chiral assemblies.