Photoinduced electron transfer in ruthenium(II) trisbipyridine complexes connected to a naphthalenebisimide via an oligo(phenyleneethynylene) spacer

The preparation and the characterization of three new dyads composed of a ruthenium trisbipyridine complex linked to a naphthalene bisimide electron acceptor via a phenyleneethynylene spacer of different length (one or two units) are reported. The dyads also differ by the anchoring position of the spacer on the bipyridine, which is appended either at the 4-position or the 5-position. Cyclic voltammetry and the UV-Vis absorption spectroscopy suggested that the spacer linked at the 5-position ensures a longer pi-conjugation length but the electron transfer rates indicate a lower electronic coupling, than in 4-position. Photoinduced emission yields indicate a significant quenching of the MLCT excited-state of the ruthenium complex in these dyads. Except for the dyad linked in 5 position with one phenyleneethynylene unit, the transient absorption spectroscopy of all the other dyads evidences that the MLCT excited-state decays almost exclusively by electron transfer to form the charge-separated state Ru-III-NBI-. This state could not be observed, presumably because the subsequent recombination to the ground state was much faster than its formation. In the dyad linked in 5 position with only one phenyleneethynylene unit, at room temperature, the (MLCT)-M-3* state is in equilibrium with the (NBI)-N-3* state, and it also decays via electron transfer. The notable feature of these dyads is first the occurrence of a relatively long-range electron transfer reaction via a bis(phenylethynylene) linking unit anchored at the 5 position. Secondly, we show within these series of compounds that subtle variations in the structure of the dyads (length of the spacer and anchoring position on bipy) have a strong impact on the rates and in the mechanism of decay of the (MLCT)-M-3* state. The photophysical properties of the dyads can be explained in terms of energy proximity of different excited states and magnitude of the electronic coupling according to the anchoring position