Synthesis, Electrochemistry, and Photophysical Studies of Ruthenium(II) Polypyridine Complexes with D-π-A-π-D Type Ligands and Their Application Studies as Organic Memories.

A new class of ruthenium(II) polypyridine complexes with a series of D-π-A-π-D type (D=donor, A=acceptor) ligands was synthesized and characterized by H NMR spectroscopy, mass spectrometry, and elemental analysis. The photophysical and electrochemical properties of the complexes were also investigated. The newly synthesized ruthenium(II) polypyridine complexes were found to exhibit two intense absorption bands at both high-energy (λ=333-369 nm) and low-energy (λ=520-535 nm) regions. They are assigned as intraligand (IL) π→π* transitions of the bipyridine (bpy) and π-conjugated bpy ligands, and IL charge-transfer (CT) transitions from the donor to the acceptor moiety with mixing of dπ(Ru )→π*(bpy) and dπ(Ru )→π*(L) MLCT characters, respectively. In addition, all complexes were demonstrated to exhibit intense red emissions at approximately λ=727-744 nm in degassed dichloromethane at 298 K or in n-butyronitrile glass at 77 K. Nanosecond transient absorption (TA) spectroscopy has also been carried out, establishing the presence of the charge-separated state. In order to understand the electrochemical properties of the complexes, cyclic voltammetry has also been performed. Two quasi-reversible oxidation couples and three quasi-reversible reduction couples were observed. One of the ruthenium(II) complexes has been utilized in the fabrication of memory devices, in which an ON/OFF current ratio of over 10 was obtained.