Disentangling Vibronic Coupling and Conformational Disorder in Flexible NDI–T2 Donor–Acceptor Co-Oligomers

The unusually broad and intense visible (Vis) absorption band observed in naphthalenediimide–bithiophene (NDI-T2) co-polymers reflects complex electronic and vibronic interactions between donor and acceptor units. While the charge-transfer character arising from electronic coupling across the moieties likely contributes to the red-shifted absorption, the pronounced spectral broadness may also originate from strong coupling to vibrations or from conformational diversity within the polymer. Distinguishing the relative roles of these effects—electronic, vibronic, and environmental—requires a quantitative theory that can treat multiple electronic and excitonic states and their coupling to many vibrational modes without resorting to simplified models. Here, we address this challenge by focusing on well-defined NDI-T2 co-oligomers and employing a first-principles–parameterized linear vibronic coupling model combined with a matrix product state (MPS) approach, which allows us to treat dozens of excitons and strongly coupled vibrational modes in full quantum detail. This enables a direct, quantitative link between molecular geometry, vibrational structure, and the experimentally observed ultraviolet-visible (UV–vis) absorption and fluorescence anisotropy spectra, going beyond the capabilities of standard excited-state modeling approaches and providing a microscopic framework for understanding optical line shapes in flexible donor–acceptor materials.