Pyrazine‐Functionalized Ru(II)‐Complexes as Visible‐Light Activated Photobases

Excited‐state proton transfer (ESPT) roots in significantly increased acidities or basicities of up to ca. 10 p K a units in the excited state compared to the ground state. While organic photobases are either of “single use” in the case of photobase generators or are limited functionally by their UV absorption, metal complex‐based photobases offer intriguing properties. By exciting the characteristic metal‐to‐ligand charge transfer (MLCT) transitions in the visible range, multiple and reversible ESPT processes can be triggered. In this contribution we present the synthesis of two novel Ruthenium(II)‐complexes with pyrazine‐functionalized polypyridyl ligands and study their photobasic properties by ultrafast spectroscopy. We find that MLCT excitation in aqueous solution leads to a Δp K a of 9 units and that the involved ESPT process takes place within ca. 300 picoseconds. Our investigations combine experimental spectroscopy with theoretical calculations.

Visible light excitation leads to the accumulation of electron density in pyrazine‐containing Ru(II) complexes via metal‐to‐ligand charge transfer (MLCT). In aqueous solutions of appropriate pH, excited‐state proton transfer (ESPT) takes place on a timescale of hundreds of ps. We resolve the underlying kinetics via transient absorption (TA) spectroscopy and corroborate our studies by photometric titrations and DFT calculations.