Publication date: 15th December 2025
The aggregation behavior of dipyrrolonaphthyridinedione (DPND) chromophores in the solid state critically determines their optoelectronic properties. Here, we investigate how systematic variation in the side-chain geometry-specifically the branching point and steric profile-governs molecular packing and excitonic coupling. Using crystal structure prediction (CSP) combined with experimental GIWAXS and solid-state NMR, we obtain the packing geometry and crystal structure for six DPND derivatives, three of them are only the cores structures (DPND-iPr, DPND-EtPr₂, and DPND-iBu) and three other extended structures with a cyano-furan groups in order to increase the absorption range of these systems . The results for the cores molecules reveal that side-chain branching at the first carbon atom promotes herringbone packing and J-type behavior, while non-branching at the first carbon lead to an brick-wall stacking and H-type behavior in the solid state. For the extended molecules, it reveals that side-chain branching at the first carbon atom promotes a brick-wall stacking and J-type behavior, while non-branching at the first carbon lead to an herringbone packing and H-type behavior in the solid state, which indicate a strong structure-properties relationship between the branching in the first carbon and the aggregate behavior. Optical simulations based on the Holstein exciton-vibrational Hamiltonian[1] reproduce experimental absorption and photoluminescence spectra, confirming the transition from J-like to H-like photophysics as the side-chain branching position shifts. This study demonstrates that fine-tuning alkyl side-chain geometry enables rational control of aggregation and excitonic behavior in cross-conjugated DPNDs[2], providing new design principles for functional organic semiconductors.
The authors acknowledge funding within the M-ERA-Net project STEEP UP (“Steep absorption with supramolecular self-assembled functional dyes for vacuum-deposited organic solar cells”) by the European Union and the Free State of Saxony. This work also benefits from the support of the Walloon Region as part of the funding of a FRIA grant.
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