Cation Recognition by Self-Assembled Layers of Novel Crown-Annelated Tetrathiafulvalenes

Abstract

The combination of molecular recognition [1] and selfassembly processes [2] offers a powerful route to the development of nanoscale systems that have technological applications as sensors, devices, and switches. [3] Macrocyclic ligands continue to attract widespread attention, [4] especially when complexation of a neutral or ionic guest at one site in the molecule induces a change in the optical [5] or redox properties [6] of the system.The recent incorporation of the electroactive tetrathiafulvalene (TTF) unit into metal-binding macrocyclic structures has demonstrated that they can function as metal-cation sensors in organic media. [7] The presence of a metal cation imposes an inductive effect on the polarizable TTF system, resulting in an anodic shift of the first oxidation potential as indicated by cyclic voltammetry experiments (e.g., for compound 1, a maximum shift of DE 1,1/2 = 80 mV for Na + complexation has been observed); [7] the second oxidation potential is essentially unchanged, consistent with the expulsion of the metal cation after the first oxidation. There is current interest in electrochemically active self-assembled monolayers (SAMs), [8] and the recent observation of stable redox chemistry of TTF derivatives 2 in self-assembled monolayers [8b] prompted us to investigate thin-film electrochemical sensors based on the cation-sensitive redox-active molecules 6 and 9 immobilized on a metal surface. SAMs represent an attractive method for device fabrication, having the advantages of straightforward preparation and being generally very robust (stable to solvents, acids, and bases). The key starting reagent in our syntheses of compounds 6 and 9 (Scheme 1) is compound 3, [9] the hydrolysis of which under basic conditions (1 M sodium hydroxide in dioxane) yielded, after acidic work-up, the acid derivative 4 (75 % yield). Treatment of acid 4 with either 12-bromo-1-dodecanol or 6-bromo-1-hexanol in the presence of dicyclohexylcarbodiimide (DCC) and N,N-dimethylaminopyridine (DMAP) afforded ester derivatives 5a and 5b, respectively (75 and 85 % yield, respectively) with subsequent conversion of the bromide into a thiol being achieved by treatment with thiourea followed by basic hydrolysis of the intermediate isothiouronium salt to afford the target derivatives 6a and 6b (42 and 33 % yield, respectively). In both cases, this last step gave a complex reaction mixture; consequently, the purified yields for compounds 6a and 6b were reduced as a result of repeated chromatography. The byproducts could not be obtained pure and their structures are unknown. Reduction of compound 3 to afford the methanol derivative 7 occurred in dichloromethane at ±78 C using diisobutylaluminum hydride (DIBAL-H) [10] (85 % yield). Using a modification [11] of the Mitsunobu reaction [12] we successfully converted alcohol 7 into sulfanylmethyl derivative 9:[13] a mixture of compound 7 and thioacetic acid in tetrahydrofuran was added to a stirred mixture of diisopropyl azodicarboxylate (DIAD) and triphenylphosphine in tetrahydrofur...

Metal ion recognition and molecular templating in self-assembled monolayers of cyclic and acyclic polyethers

“…Relatively recently, Reinhoudt et al (19)(20)(21)(22) and Bryce et al (23,24) reported, almost simultaneously, the incorporation of crownether groups into SAMs and their use as potential metal ion sensors. The Reinhoudt group used electrochemical impedance spectroscopy (EIS) for SAM characterization and to study the ion recognition processes (19)(20)(21)(22).…”

“…25 and 26). The report by Bryce et al (23,24) described SAMs of crown-annelated tetrathiafulvalene (TTF) derivatives, which showed, by cyclic voltammetry (CV), a redox potential shift of the electroactive, surface-confined, crown-TTF group on ion complexation (23,24). The first TTF-crown SAMs reported by this group were apparently unstable after several electrochemical scans (23).…”

Electrochemistry and time dependent DFT study of a (vinylenedithio)-TTF derivative in different oxidation states

“…This proposed organisation within the monolayer is not only a function of the behaviour of, and interaction between the molecules themselves, but also the interaction between the molecules and the surface [26]. Stable redox chemistry in self-assembled monolayers of TTF derivatives have been reported by several groups including those of Bryce, Ward, Cooke, Sallé, Echegoyen and Stoddart, where the core TTF moiety is anchored to the surface using a surface active functional group such as an alkyl chain with a thiol end group on Au [13,14,[27][28][29][30], thioctic acid disulfide linkers on Au [31][32][33] and oxide-free hydrogen-terminated Si(1 0 0) surfaces [34]. Non-covalent binding of TTF derivatives on graphite has also been reported where the molecule's core TTF unit has a strong interaction with the -system of the graphite surface; this allows the molecule to orientate parallel to the graphite surface through -interactions [35,36].…”

“…The non-aromatic 14 -electron system undergoes aromatisation when going from the neutral species to the dithiolium form in the singly and doubly oxidised states. TTF and its derivatives have featured in a wide variety of applications from crown-ether annelated electrochemical sensors [13][14][15][16], to molecular electronics [17][18][19][20][21] including organic field-effect transistors [22] and other advanced molecular assemblies [23][24][25].…”

Triethylene Glycol Ether End-grafted Carbosilane Dendrimer: A Potential Ionophore for Potassium Ion Recognition

“…9,10 We recently prepared TEG (triethylene glycol monomethyl ether)-and PEG (polyethylene glycol monomethyl ether)-end-grafted carbosiloxane and carbosilane dendrimers and studied their ability to detect alkali metal-ion binding. 11 The general idea was to use the glycols to form ion binding domains at the periphery (similar to crown ethers) via self assembly.…”