Bz-8HQ: a novel supramolecular fluorochrome exhibiting multiple stimuli-responsiveness

Abstract

A novel multifunctional fluorescent molecule (Bz-8HQ) has been designed and synthesized by the linkage of benzimidazole (Bz) and 8-hydroxyquinoline (8HQ). The synthesis was straightforward and involved reacting (piperazin-1-yl)-1H-benzo[d]imidazole with 5-(chloromethyl)quinolin-8-ol, which gives Bz-8HQ in a high yield. The structure of Bz-8HQ was subsequently characterized through different spectroscopic techniques such as ¹H-NMR, ¹³C-NMR and high-resolution mass spectrometry. Due to the two distinct binding sites, Bz-8HQ exhibits responsiveness to multiple stimuli, including pH, cucurbit[7]uril (CB7) and cadaverine (CAD). The response of Bz-8HQ toward these perturbations was investigated by ¹H-NMR spectroscopy and UV-visible absorption titration experiments. At pH 2.9 both the Bz and 8HQ moieties are protonated while at pH 6.5 both become deprotonated. The pK_a values of the Bz and 8HQ groups were determined to be 6.2 and 2.5, respectively. The ¹H-NMR titrations of Bz-8HQ with CB7 (0–4.4 equivalents) showed a preferential encapsulation of the Bz group at up to 2.9 equivalents. At 4.4 equivalents of CB7, both Bz and 8HQ sites were encapsulated. Upon the competitive titration with 0.5 equivalents of CAD, the Bz part was partially displaced by CAD but the 8HQ part remained fully encapsulated by CB7. The fluorescence titration of Bz-8HQ with HCl and CB7 showed that the energy transfer increases on decreasing the pH or increasing the concentration of CB7. The multicolor photoluminescence revealed the sensitivity and selectivity of the fluorescent probe. Bz-8HQ exhibits photoinduced electron transfer (PET) and fluorescence resonance energy transfer (FRET). The ability to modulate PET and FRET in fluorescent probes, such as Bz-8HQ, will be a valuable tool in future biological and medical imaging applications.