Tuning Excited State Isomerization Dynamics through Ground State Structural Changes in Analogous Ruthenium and Osmium Sulfoxide Complexes

• Published in: Chemistry : a European journal Vol. 19; no. 35; pp. 11686 - 11695
• Main Authors: Garg, Komal, Engle, James T., Ziegler, Christopher J., Rack, Jeffrey J.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 26.08.2013; WILEY‐VCH Verlag; Wiley; Wiley Subscription Services, Inc
• Subjects: Analytical chemistry; Chemistry; Chemistry, Multidisciplinary; Electrodes; isomerization; photochromism; Physical Sciences; polypyridine; ruthenium; Science & Technology; sulfoxide; POLYMERS; polypyridine; ENERGY; MECHANISM; isomerization; photochromism; LINKAGE ISOMERIZATION; DIMETHYLSULFOXIDE; PHOTOISOMERIZATION; LIGHT; sulfoxide; ruthenium; EVOLUTION; SPECTROSCOPY; DERIVATIVES
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201301553

The complexes [Ru(bpy)2(pyESO)](PF6)2 and [Os(bpy)2(pyESO)](PF6)2, in which bpy is 2,2′‐bipyridine and pyESO is 2‐((isopropylsulfinyl)ethyl)pyridine, were prepared and studied by 1H NMR, UV–visible and ultrafast transient absorption spectroscopy, as well as by electrochemical methods. Crystals suitable for X‐ray structural analysis were grown for [Ru(bpy)2(pyESO)](PF6)2. Cyclic voltammograms of both complexes provide evidence for S→O and O→S isomerization as these voltammograms are described by an ECEC (electrochemical‐chemical electrochemical‐chemical) mechanism in which isomerization follows Ru2+ oxidation and Ru3+ reduction. The S‐ and O‐bonded Ru3+/2+ couples appear at 1.30 and 0.76 V versus Ag/AgCl in propylene carbonate. For [Os(bpy)2(pyESO)](PF6)2, these couples appear at 0.97 and 0.32 V versus Ag/AgCl in acetonitrile, respectively. Charge‐transfer excitation of [Ru(bpy)2(pyESO)](PF6)2 results in a significant change in the absorption spectrum. The S‐bonded isomer of [Ru(bpy)2(pyESO)]2+ features a lowest energy absorption maximum at 390 nm and the O‐bonded isomer absorbs at 480 nm. The quantum yield of isomerization in [Ru(bpy)2(pyESO)]2+ was found to be 0.58 in propylene carbonate and 0.86 in dichloroethane solution. Femtosecond transient absorption spectroscopic measurements were collected for both complexes, revealing time constants of isomerizations of 81 ps (propylene carbonate) and 47 ps (dichloroethane) in [Ru(bpy)2(pyESO)]2+. These data and a model for the isomerizing complex are presented. A striking conclusion from this analysis is that expansion of the chelate ring by a single methylene leads to an increase in the isomerization time constant by nearly two orders of magnitude. Faster than a speeding bullet: A six‐membered chelating sulfoxide ring leads to rapid excited isomerization (47 ps) in comparison to a five‐membered chelate (see graphic). This is the most rapid sulfoxide isomerization reported to date.