The photonic responses of densely-packed dye molecule assemblies are strongly dependent on their organization and environment. The precise control of molecular orientations and distances relative to the substrate and to each other, is thus a key-point in the design of photonic molecular materials. Herein, we report the preparation of a homogeneous and well-organized single monolayer of perylenediimide derivative by means of the Langmuir-Blodgett technique. Its optical properties disclose an intense charge-transfer excitonic absorption band related to an important intermolecular coupling. Furthermore, an important immunity to photobleaching is observed for such a molecular assembly. The dipolar orientations of the molecules along the substrate have been unambiguously determined by angle of incidence resolved polarized absorption and by back focal plane fluorescence mapping. In addition, time-resolved spectroscopy reveals a fast two-dimensional diffusion of excitons consistent with a strong $\pi$-stacking of adjacent PDI molecules.