AUTHOR=Tamaki Yusuke , Tokuda Kazuma , Yamazaki Yasuomi , Saito Daiki , Ueda Yutaro , Ishitani Osamu 

TITLE=Ruthenium Picolinate Complex as a Redox Photosensitizer With Wide-Band Absorption

JOURNAL=Frontiers in Chemistry

VOLUME=7

YEAR=2019

URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2019.00327

DOI=10.3389/fchem.2019.00327

ISSN=2296-2646

ABSTRACT=<p>Ruthenium(II) picolinate complex, [Ru(dmb)<sub>2</sub>(pic)]<sup>+</sup> (<bold>Ru(pic)</bold>; dmb = 4,4′-dimethyl-2,2′-bipyridine; Hpic = picolinic acid) was newly synthesized as a potential redox photosensitizer with a wider wavelength range of visible-light absorption compared with [Ru(N<sup>∧</sup>N)<sub>3</sub>]<sup>2+</sup> (N<sup>∧</sup>N = diimine ligand), which is the most widely used redox photosensitizer. Based on our investigation of its photophysical and electrochemical properties, <bold>Ru(pic)</bold> was found to display certain advantageous characteristics of wide-band absorption of visible light (λ<sub>abs</sub> &lt; 670 nm) and stronger reduction ability in a one-electron reduced state (<inline-formula><mml:math id="M1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>E</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mo>/</mml:mo><mml:mn>2</mml:mn></mml:mrow><mml:mrow><mml:mtext>red</mml:mtext></mml:mrow></mml:msubsup></mml:math></inline-formula> = −1.86 V vs. Ag/AgNO<sub>3</sub>), which should function favorably in photon-absorption and electron transfer to the catalyst, respectively. Performing photocatalysis using <bold>Ru(pic)</bold> as a redox photosensitizer combined with a Re(I) catalyst reduced CO<sub>2</sub> to CO under red-light irradiation (λ<sub>ex</sub> &gt; 600 nm). TON<sub>CO</sub> reached 235 and Φ<sub>CO</sub> was 8.0%. Under these conditions, [Ru(dmb)<sub>3</sub>]<sup>2+</sup> (<bold>Ru(dmb)</bold>) is not capable of working as a redox photosensitizer because it does not absorb light at λ &gt; 560 nm. Even in irradiation conditions where both <bold>Ru(pic)</bold> and <bold>Ru(dmb)</bold> absorb light (λ<sub>ex</sub> &gt; 500 nm), using <bold>Ru(pic)</bold> demonstrated faster CO formation (TOF<sub>CO</sub> = 6.7 min<sup>−1</sup>) and larger TON<sub>CO</sub> (2347) than <bold>Ru(dmb)</bold> (TOF<sub>CO</sub> = 3.6 min<sup>−1</sup>; TON<sub>CO</sub> = 2100). These results indicate that <bold>Ru(pic)</bold> is a superior redox photosensitizer over a wider wavelength range of visible-light absorption.</p>