New photoactive catalysts have been synthesized from Europium diphthalocyanine molecules dissolved in dimethylformamide and transferred to aqueous dispersion of detonation nanodiamonds (~4.5 nm in size, positive ζ-potential ~30–40 mV), forming diphthalocyanine–diamond complexes. The weight ratio of diphthalocyanine to nanodiamonds was varied in the range CR = 0.1–1.0%. The formation of complexes was detected by optical absorption, dynamic light scattering and Raman spectroscopy. The complexes remained stable in water and demonstrated fractal-type self-assembly into chain-like structures at low fraction of molecular component (CR = 0.1–0.2%). However, at critical concentration CR * ~0.3% it was detected a transition to branched structures of diamond particles linked via hydrophobic molecules of diphthalocyanine. Dried complexes with Eu atoms settled on diamond carriers showed an intensive characteristic luminescence at wavelengths ~600 and 700 nm under X-ray irradiation (0.154 nm). The developed hybrid structures may serve as catalysts for singlet oxygen generation in surrounding media (air, water, biological tissues) for their cleaning and disinfection. It is assumed the advanced usage in photodynamic therapy and in processes with chemical photoreactions exploiting the activity of molecular oxygen by using light of long wavelength ~600–700 nm.