Internally diketopyrrolopyrrole-bridged bis-anthracene macrocycle: a multifunctional fluorescent platform

A covalently bridged macrocycle (5) comprising two anthracene strands connected at the lactam positions of a diketopyrrolopyrrole (DPP) chromophore has been constructed. The crystal structure reveals that the central DPP chromophore is wrapped with the externally twisted bis-anthracene macrocycle. The internally bridged macrocycle architecture endows 5 with multifunctional properties. Due to shielding by the double anthracene straps, 5a and a polymer derived from it, DPP-Cycle, display strong fluorescence emission features in both organic media and the solid state. Moreover, the emission colors of these macrocyclic materials can be effectively tuned through external stimuli such as mechanical and thermal treatments, as well as solvent fuming. Compound 5a is stable in the presence of most metal cations but degrades rapidly when it comes in contact with Cu²⁺ in acetonitrile. This decomposition, which is thought to involve a reaction at the central DPP <i>via</i> a radical-mediated mechanism, was found to be accelerated in 5a compared to the non-cyclic analogue 2a. This leads us to suggest that internally bridged macrocycles, such as those described here, may have a role to play as fluorescent Cu²⁺ sensors. Finally, the high fluorescence of 5a in the solid state enables its use in the area of latent fingerprint (LFP) imaging.