Dipyrrolonaphthyridinedione Aggregation in the Solid State─The Importance of Side-Chain Geometry

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 three DPND derivatives (DPND-iPr, DPND-EtPr, and DPND-iBu). The results reveal that side-chain branching at the first carbon atom promotes herringbone packing and J-type behavior, while branching at the second carbon induces brick-wall stacking and H-type behavior in the solid state. Optical simulations based on the Holstein exciton-vibrational Hamiltonian 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, providing new design principles for functional organic semiconductors.