Département de chimie moléculaire

Photosensitizers used in homogeneous photocatalytic systems for artificial photosynthesis, such as hydrogen prodn., are typically based on expensive transition metal complexes such as d6 ruthenium(II) or iridium(III). In this work, we demonstrate efficient H2 prodn. in acidic water by using an org. dye derived from the triazatriangulenium (TATA+) family as a visible-light-absorbing photosensitizer (PS). By assocg. the hydrosol. tris(ethoxyethanol)triazatriangulenium with an efficient H2-evolving cobalt catalyst and ascorbic acid as sacrificial electron donor (SD), remarkable photocatalytic performances were reached in aq. soln. at pH 4.5, under visible-light irradn., with up to 8950 catalytic cycles vs. catalyst. The performances of this dye largely exceed those of the benchmark Ru tris-bipyridine in the same exptl. conditions when low concns. of catalyst are used. This higher efficiency has been clearly ascribed to the remarkable robustness of the reduced form of the org. dye, TATA.bul.. Indeed, the combination of the planar structure of TATA+ together with the presence of the three electron-donating nitrogen atoms promotes the stabilization of TATA.bul. by delocalization of the radical, thereby preventing its degrdn. in the course of photocatalysis. By contrast, the reduced form of the Ru photosensitizer, [RuII(bpy)2(bpy.bul.-)]+ ("Ru-"), is much less stable. Nanosecond transient absorption expts. confirm the formation of TATA.bul. in the course of the photocatalytic process in accordance with the mechanism initiated by the reductive quenching of the singlet excited state of TATA+ by ascorbate. The second electron transfer from TATA.bul. to the catalyst has also been evidenced by this technique with the detection of the signature of the reduced Co(I) form of the catalyst. The present study establishes that certain org. dyes are to be considered as relevant alternatives to expensive metal-based PSs insofar as they can exhibit a high stability under prolonged irradn., even in acidic water, thereby providing valuable insights for the development of robust mol. systems only based on earth-abundant elements for solar fuel generation. [on SciFinder(R)]