AbstractThe current work focuses on the investigation of two functionalized naphthyridine derivatives, namely ODIN‐EtPh and ODIN‐But, to gain insights into the hydrogen bond‐assisted H‐aggregate formation and its impact on the optical properties of ODIN molecules. By employing a combination of X‐ray and electron crystallography, absorption and emission spectroscopy, time resolved fluorescence and ultrafast pump‐probe spectroscopy (visible and infrared) we unravel the correlation between the structure and light‐matter response, with a particular emphasis on the influence of the polarity of the surrounding environment. Our experimental results and simulations confirm that in polar and good hydrogen‐bond acceptor solvents (DMSO), the formation of dimers for ODIN derivatives is strongly inhibited. The presence of a phenyl group linked to the ureidic unit favors the folding of ODIN derivatives (forming an intramolecular hydrogen bond) leading to the stabilization of a charge‐transfer excited state which almost completely quenches its fluorescence emission. In solvents with a poor aptitude for forming hydrogen bonds, the formation of dimers is favored and gives rise to H aggregates, with a consequent considerable reduction in the fluorescence emission. The urea‐bound phenyl group furtherly stabilizes the dimers in chloroform.