Ultrafast photoinduced electron transfer within a self-assembled donor-acceptor system

A photoactive supramolecular assembly that is based on the hydrogen-bonded system H1·G2, consisting of a methyl viologen-functionalized barbiturate host (H1) (1-(N-(3,5-bis[[(6-tert-butylacetylamino-2-pyridyl)-amino]carbonyl])- phenylacetamide)-1′-methyl-4,4′-bipyridium) and a [Re(Br)(CO) 3(barbi-bpy)] (barbi-bpy = 5-[4-(4′-methyl)-2,2′- bipyridyl]methyl-2,4,6-(1H3H,5H)-pyrimidinetrione) complex as the guest (G2) is described. The host molecule contains a well-known electron accepting group (methyl viologen), whereas the guest system can act as an efficient electron donor in the excited state. Upon self-assembly, the resulting adduct (H1·G2) represents an interesting noncovalently linked donor-acceptor system. The H1·G2 complex has been characterized in acetonitrile-d 3 using 1H NMR and diffusion-ordered NMR spectroscopy (DOSY). The photophysical properties of the components and of the assembly have been studied in dichloromethane, in which the assembly has a high binding constant (Kass ≥ 2 × 105 M-1), using time-resolved fluorescence and transient absorption spectroscopy. A detailed investigation of the hydrogen-bonded complex H1·G2 revealed that, upon excitation of the rhenium compound G2, an ultrafast electron-transfer process occurs from the metal-based component to the acceptor unit. The kinetics of the forward and back electron-transfer processes have been determined.