Aryl-Decorated Ru^II Polypyridyl-type Photosensitizer Approaching NIR Emission with Microsecond Excited State Lifetimes

Bis-tridentate Ru(II) complexes based on the dqp scaffold (dqp is 2,6-di(quinolin-8-yl)pyridine) with multiple aryl substituents were explored to tailor the absorption and emission properties. A synthetic methodology was developed for the facile synthesis and purification of homo- and heteroleptic bis-tridentate Ru complexes. The effect of the aryl substituents in the para positions of the pyridine and quinoline subunits was detailed by X-ray crystallography, steady state and time-resolved spectroscopy, electrochemistry, and computational methods. The attachment of the aryl groups results in enhanced molar extinction coefficients with the largest effect in the pyridine position, whereas the quinoline substituent leads to red-shifted emission tailing into the NIR region (up to 800 nm). Notably, the excited state lifetimes remain in the microsecond time scale even in the presence of O2, whereas the emission quantum yields are slightly increased with respect to the parental complex [Ru(dqp)2](2+). The peripheral functional groups (Br, Me, OMe) have only a minor impact on the optical properties and are attractive to utilize such complexes as functional building blocks.