Studies on a vinyl ruthenium-modified squaraine dye: Multiple visibleear-infrared absorbance switching through dye- and substituent-based redox processes

摘要

The bis(vinyl ruthenium)-modified squaraine dye 1 was synthesized by treatment of [RuHCl(CO)(PiPr_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. Switch hitter: Vinyl Ru attachment renders squaraine dye 1 a reversible polyelectrochromic switch, in which the strong visibleear-infrared (NIR) absorptions are either switched off or shifted deep into the NIR (see figure). Its radical cation 1~(. +) has a symmetrical, delocalized structure, whereas 1~(2+) has an unsymmetrical, valence-trapped, open-shell singlet ground state.