Photoinduced hydrogen production in dilute solutions and organized assemblies

Abstract

A new class of covalently linked cyanine dye-electron relay compounds have been investigated. As well as 2-[1-ethyl-1,2-dihydro-chinolinyliden-(2)-methyl]-1-ethyl-chinolinium chloride (1), 1-[3-[2-[(1-ethyl-2(1H)-chinolinyliden)methyl]chinolinium-1-yl]propyl]-1′-methyl-4,4′-bi-pyridinium trichloride (2) and 1-[3-[2-[(1-ethyl-2(1H)-chinolinyliden)methyl]chinolinium-1-yl]pentyl]-1′-methyl-4,4′-bipyridinium triiodide (3) were able to form huge molecular associates with a well-defined three-dimensional structure in dilute, aqueous solutions (critical gel concentration ccrit > 2 × 10−3mol 1−1). Photochemical properties of compounds (1)-(3) were different in gels and in solutions (c < ccrit) without organized dye associates. Owing to antenna effects, organized assemblies provide an intense light harvesting effect. Covalently linked viologen groups quench the excited dye intramolecularly with a higher efficiency. Furthermore, the increased quenching efficiency was found to be dependent on the degree of organization. In dilute aqueous solutions (c < ccrit, λEX > 280 nm), compounds (1)-(3) could act as photosensitizers for light-induced hydrogen production (via intermolecular charge separation) with quantum yields of φ12H2 = 2%. In organized assemblies (c >ccrit), intramolecular charge separation [in compounds (2) and (3)] could be observed and hydrogen production experiments were performed successfully.