Supramolecular fluorescence enhancement via coordination-driven self-assembly in bis-picolylcalixarene blue-emitting M2L2Xn macrocycles

Abstract

The lower-rim functionalised distal bis-(4-picolyl)-p-tert-butylcalix[4]arene (L) selectively and quantitatively self-assembles into a series of discrete [2 + 2] blue-emitting metallacycles of general formula M₂L₂X_n with diverse metal salts (M = Zn²⁺, Pd²⁺, Ag⁺, and Cd²⁺; X = Cl⁻, NO₃⁻, ClO₄⁻, BF₄⁻ , CF₃SO₃⁻, PF₆⁻, SbF₆⁻; n = 2, 4). Macrocycle assembly has been corroborated by 2D-DOSY NMR and ESI-MS analyses, which further indicate that the M₂L₂X_n entities are quite stable and persist as robust and discrete macrocyclic species in solution. While free L units display modest blue emission (λ_max = 307–405 nm), self-assembly of M₂L₂X_n results in amplified fluorescence (up to 13-fold). This remarkable enhancement may be primarily ascribed to the increase in conformational rigidity imposed on the L units by supramolecular assembly formation upon metal coordination to the pyridyl groups; in addition, subtle intensity-emission modulation may be provided by the different metal components. Titrations aimed at exploring the possibilities for ratiometric detection of metal cations or sensing of nitroaromatic species, revealed that the M₂L₂X_n platform may be a suitable “turn-on/off” system. Our results provide valuable insights into luminescence enhancement within the context of coordination-driven assemblies, which may be engineered to increase their fluorescence by imposing rigidity on the chromophores.