Color-tunable lanthanide metal–organic framework gels

Abstract

Fluorescence significantly improves the performance of gels. Various strategies, such as embedment and crosslinking, have been used to integrate extrinsic luminophores into gel systems, but the procedures are usually complex. Herein, for the first time, we report gels with intrinsic and tunable emission color prepared with 5-boronoisophthalic acid (5-bop) and Eu³⁺, Tb³⁺, and/or Dy³⁺ similar to the procedure for the preparation of metal–organic frameworks (MOFs). The single-metal gels exhibit intrinsic trichromatic fluorescence, due to which full-color emissions are readily obtained by tuning the type and/or ratio of Ln³⁺ ions to prepare mixed-metal gels. The emission is governed by an antenna effect and is thus excited with single-wavelength at 275 nm. The nucleation-growth mechanism reveals that the Ln³⁺ ions and 5-bop produce separated layers, which then grow anisotropically to form nanoribbons by high coordinated valence of Ln³⁺ ions and biased carboxyl distribution as well as steric hindrance and hydrogen bonds of the boric acid group in 5-bop. The nanoribbons entangle together to generate chemical-physical hybrid gels. To the best of our knowledge, this is the first example of gels with inherent and tunable emission color. Due to their optical and viscoelastic properties, the gels have numerous potential applications such as tunable emission and multi-target detection.