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)(P i Pr 3 ) 2 ] with bis(ethynyl)‐substituted squaraine 8 . Spectroscopic and electrochemical measurements on 1 and its organic precursors 6 – 8 were performed to study the effect of the vinyl ruthenium “substituents,” particularly with respect to (poly)electrochromism. Attachment of the vinyl ruthenium moieties endows metal–organic 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 1 2−/−/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 1 2+ 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 1 2+ corroborate these results. It has also emerged that reduction involves an orbital that is strongly delocalized across the entire squaraine π system and strongly affects the peripheral vinyl ruthenium sites.