tunability of their electronic structures by chemical design. [1] Additionally, their flexibility and processability allow their implementation in photoelectric devices through conventional techniques such as drop casting or spin coating. Therefore, organic semiconductors with a suitable energy band position and solution-processability show substantial advantages in comparison with commonly employed polymeric or inorganic photo-electrocatalysts in scenarios such as water splitting or water purification. [2] Such materials often require harsher protocols for the formation of thin films on substrates, [3] have wide bandgaps, or do not display adequate energy band positions to achieve the oxidation and reduction of water. [4] While the development of organic semiconductors for photo-electrochemical water splitting has emerged strongly within the last decade, most of the reports in literature focused on the design of linear polymers through chemical design [5] or on their role as materials in heterojunctions. [6] Remarkably, the relationship between the dimensionality of organic semiconductors and their photoelectric performance has barely been shown so far.Indeed, the organic backbone of these semiconductors as well as their purity and their supramolecular organization in solution or solid state play a crucial role in their photo-electric 1,2-diketone-based naphthalimide has emerged as an important electronwithdrawing building block in the synthesis of organic semiconductor assemblies for a wide range of applications. The reaction with different diamine derivatives enables the formation of pyrazine linkers which promotes planarity and rigidity of the naphthalimide-based semiconductors, therefore tunning their electronic structure and processability. Despite the significant use of this versatile building block to generate flat and rigid assemblies, the correlation between rigidity and dimensionality in these systems and their photocatalytic activity remains unexplored so far. Herein, with the aim to modulate the photocatalytic activity of these materials, a new family of assemblies with different dimensionality end-capped with naphthalimide moieties are developed in which one (NIPB), three (3NIPT), and four (4NIPTM) units have been selectively introduced. The good processability of these novel semiconductors combined with their suitable energy levels allows their application as photo-electrocatalysts for the degradation of organic dyes and for water splitting. The derivative with the highest dimensionality, 4NIPTM, shows higher photo-electrocatalytic activity and lower charge transfer resistance than that of conventional semiconductors (such as TiO 2 and C 3 N 4 ), reaching photocurrents up to 20 mA cm −2 .

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