Remarkably Intense Emission from Ruthenium(II) Complexes with Multiple Borane Centers

The electrochemical, spectroscopic, and phophysical properties of a series of Ru(II) complexes having a triarylborane-appended 2,2'-bipyridine (bpy) ligand(s) (RuBbpys: [Ru(Bbpy)(n)(bpy)(3-n)](2+) (B1n) and [Ru(B(2)bpy)(n)(bpy)(3-n)](2+) (B2n), B = (dimesityl)boryldurylethynyl group(s) at the 4- or 4,4'-position(s) in bpy, n = 1-3) are described. In the excited states of the complexes, the intramolecular charge transfer transitions between the p-orbital of the aryl group and the vacant pi-orbital on the boron atom (pi(aryl)-p(B) CT) synergistically interact with the metal-to-ligand charge transfer (MLCT) transitions. The molar absorption coefficient of the MLCT band (e(MLCT)) of the complex increased with increasing n, and B23 showed extremely intense absorption with e(MLCT) = 5.6 x 10(4) M-1 cm(-1) at 488 nm. Furthermore, B23 showed the highest emission quantum yield (0.43) among those of the polypyridine Ru(II) complexes hitherto reported. As one of the interesting results, we report that the radiative rate constant of B2n shows the correlation with epsilon(MLCT). The effects of the synergistic MLCT/pi(aryl)-p(B) CT interactions on the spectroscopic and photophysical characteristics of RuBbpys are discussed in detail.