Studies on a Vinyl Ruthenium-Modified Squaraine Dye: Multiple Visible/Near-Infrared Absorbance Switching through Dye- and Substituent-Based Redox Processes

The bis(vinyl ruthenium)-modified squaraine dye 1 was synthesized by treatment of [RuHCl(CO)(PiPr3)2] with bis(ethynyl)-substituted squaraine 8. Spectroscopic and electrochemical measurements on 1 and its organic precursors 68 were performed to study the effect of the vinyl ruthenium substituents, particularly with respect to (poly)electrochromism. Attachment of the vinyl ruthenium moieties endows metalorganic squaraine 1 with two additional oxidation waves and lowers the first two oxidation potentials by approximately 300 mV with respect to its organic precursors. Squaraines 6, 7, 8, and 1 strongly absorb at 648, 663, 656, or 709 nm. Although organic dyes 6, 7, and 8 fluoresce, no room-temperature emission is observed for 1. The radical cations and anions of 6, 7, 8, and 1 as well as the doubly oxidized dications have been studied by IR and UV/Vis/NIR spectroelectrochemistry, and the -/0/+/2+ redox sequences were found to be reversible in each case. Our results indicate that the 12-/-/0/+/2+ redox system constitutes a polyelectrochromic switch in which absorption in the visible or the near-infrared range is reversibly turned off or shifted deep into the NIR. They also show that radical cation 1.+ is an intrinsically delocalized system with only little contribution from the outer vinyl ruthenium tags to the oxidation process. Dication 12+ constitutes a class-II mixed-valent system with two electronically different vinyl ruthenium moieties and has an open-shell singlet electronic ground-state structure. ESR and NMR spectra of chemically prepared 1.+ and 12+ corroborate these results. It has also emerged that reduction involves an orbital that is strongly delocalized across the entire squaraine p system and strongly affects the peripheral vinyl ruthenium sites.