Spectroscopic and electrochemical sensing of lanthanides with π-extended chromophores incorporating ferrocenes and a coordinative end

In this study, we report the synthesis and characterization of three novel "push-pull" chromophores, in which multiple phenylenevinylene units are endcapped by ferrocene as donor units and malonate moieties as acceptor units. These chromophores have spectroscopic and electrochemical characteristics which consistently change according to the extension of the conjugated bridge, thus to the variation of the HOMO-LUMO band gap. The 1,3-dicarbonyl units, directly incorporated into the conjugated molecular structures, are able to coordinate Lewis acid-like cations, such as lanthanides, as confirmed by UV/Vis, H-1 NMR and cyclic voltammetry studies. The UV/Vis spectroscopic response upon complexation with Sc3+ or Eu3+ as the triflate salts is rather unselective and nonlinear in going from the least to the most pi-extended chromophore. Binding studies in MeCN, analyzed via equilibrium-restricted factor analysis, give values between log K-a = 1.21 and 3.07 and affirm a 1 : 1 stoichiometry of the host: guest complexes in all cases. On the other hand, cyclic voltammetry reveals a selectivity in the response to Sc3+ coordination over Ln(3+) (Eu3+, but also Lu3+ and Er3+ were tested) for the two shorter chromophores, whereas the ligand with the longest pi-bridge is able to sense Er3+ (Delta E-1/2 complexed/uncomplexed chromophore = 20 mV) selectively over the other lanthanides.