Ultra­pH­Sensitive Small Molecule Probe Showing a Ratiometric Fluorescence Color Change

Abstract

We synthesized an amphiphilic small molecule 1 by attaching oligo(ethylene glycol) (OEG) and N,N­diethylamine (DEA) units to a fluorescent dicyanodistyrylbenzene (DCS) core. The molecule is fully soluble in aqueous solution at low pH (pH<4.3) by protonation of the DEA groups

but, above pH？4.3, it self­assembles to form nanoparticles due to increased hydrophobicity upon deprotonation, leading to a ratiometric fluorescence color change from green emission in the monomeric form (λem,max=538？nm) to red emission in the aggregated form (λem,max=646？nm). It should be noted that the ratio of emission intensities at 538？nm and 646？nm (R=I538/I646) changes drastically about 42­fold within a very narrow pH？range of 4.3？4.6.