SUPRAMOLECULAR SENSITIZERS FOR PHOTOINDUCED ELECTRON-TRANSFER

The application of crownether units as building blocks for the design and the synthesis of supramolecular ruthenium complexes for the photoinduced water cleavage permits the optimization and the fine-tuning of the photophysical properties such as the photostability and the photoelectron-transfer properties. The ruthenium pedate (1) provides an excellent example for the remarkable increase of the photostability (Kphotoanation << 1 x 10(-4), compared to [Ru(bpy)(3)]Cl-2) of a water-soluble complex featuring a very high luminescence lifetime (tau(L) = 1986 [ns]) and a high luminescence quantum yield (phi(L) = 0.052). Ruthenium sensitizers possessing an external crownether cavity for the reversible binding of electron-acceptor-substrates such as viologens have been employed for the design of supramolecular photodonor-acceptor systems for fast photoelectron transfer. Fast electron transfer can be regarded as another efficient strategy to more photostable photochemical devices, because the fast deactivation of the excited (3)MC and (3)MLCT states of ruthenium photosensitizers by electron transfer suppresses decomposition reactions, which occur from these states (especially (3)MC). In this report special aspects of the binding mechanisms of viologens to crown ether ruthenium complexes are discussed on the basis of a Rehm-Weller-plot of a series of 12 bis-heteroleptic ruthenium complexes and the electron relays MV(2+) and OV2+, the comparison of the binding constants (by H-1-NMR, absorption/luminescence titrations) and the comparison of the time resolved and steady state quenching behavior of one particular crown ether complex. As conclusion from these experiments we found experimental evidence for the fast equilibrium between different binding modes of the viologens inside and outside the crownether moiety.