Crown ether functionalised dendrons—controlled binding and release of dopamine in both solution and gel-phases

Abstract

This paper reports the use of first (G1), second (G2) and third (G3) generation dendritic branches based on L-lysine building blocks with an [18]crown-6 unit at the focal point to bind protonated dopamine. The binding strength in CD₃OD is dependent on dendritic generation (G1 > G2 < G3). As expected, the presence of water (10%) disrupts the binding of dopamine; however, this effect is smaller for the G2 system than its G1 analogue, a result which may indicate a degree of dendritic protection from competitive solvents. The addition of K⁺ causes release of protonated dopamine, whilst the use of a base releases deprotonated dopamine. A key advantage of dendritic functionalisation in these crown ether derivatives is that it enables them to assemble via intermolecular hydrogen bond interactions and consequently form gels in non-hydrogen bonding solvents. Indeed, the G2 and G3 crown ethers (but not G1) form gels in toluene, as well as in oils used in pharmaceutical formulation (e.g. isopropyl palmitate). The thermal properties of these gels are reported, and a tape-like morphology imaged by scanning electron microscopy. Dopamine modifies the thermal stability of the gel-phase material and the subsequent addition of potassium ions breaks down the structure of the gel completely. This system therefore offers a demonstration of the way supramolecular gels can respond to specific ionic/molecular triggers, and indicates how controlled release applications may be developed for such materials in formulation science.