Fluorescent Supramolecular Polymers: Metal Directed Self-Assembly of Perylene Bisimide Building Blocks

Abstract

Complexation properties of 2,2‘:6‘,2‘ ‘-terpyridine (tpy) have been studied with a series of first row transition metal ions by UV−vis, 1H NMR, and isothermal titration calorimetry, and ΔH values for the tpy complexation processes have been determined. These studies reveal that terpyridine−Zn2+ complex constitutes an ideal supramolecular building block characterized by thermodynamically stable and kinetically labile coordination bonding. Thus, at room-temperature, perylene bisimide fluorophores equipped with one or two terpyridine functionalities formed coordination dimers and polymers, respectively, upon addition of Zn2+ metal ions. The reversible formation of coordination dimers and polymers has been established by 1H NMR and additionally by DOSY NMR and fluorescence anisotropy measurements. The optical properties of dimeric and polymeric complexes have been investigated by UV−vis and fluorescence spectroscopy which prove that the Zn2+ coordination to the terpyridine unit does not effect the advantageous fluorescence properties of perylene bisimide moieties. Coordination polymer strands can be visualized by atomic force microscopy (AFM), which also reveals the formation of a monolayer film at higher concentration. The average polymer length has been determined by AFM to 15 repeat units, which correlates well with the value estimated by 1H NMR to >10 repeat units.