Methoxyethoxy-substituted coumarin-benzimidazole: Substituent group positions adjusted the self-assembly, optical properties and ions sensor

Two methoxyethoxy-substituted coumarin-benzimidazoles with two substituents in ortho (o-BIDC) or para (p-BIDC) positions have been synthesized and fully characterized. Interestingly, just with the different positions of methoxyethoxy groups, compounds p-BIDC and o-BIDC showed big differences in optical properties and self-assembled structures. In the solution of CH2Cl2, compound p-BIDC shows strong yellow fluorescence while compound o-BIDC nearly no fluorescence. And the fluorescent quantum yield of compound o-BIDC (Phi(f) = 0.007) is nearly 100 times smaller than that of p-BIDC (Phi(f) = 0.687). Surprisingly, the solid fluorescent quantum yield is opposite from that of solution. The solid state fluorescent quantum yield of compound o-BIDC is 0.76. while para-substituted p-BIDC is only 0.05, which is nearly 14 times smaller than that of compound o-BIDC. In addition, compound p-BIDC showed high selectivity and sensitivity to Hg2+ in the solution of HEPES (10 mM, pH = 7.21), and the fluorescence quenching efficiency reached 86% while the other similar metal ions had no significant quenching. Meanwhile, compound o-BIDC showed weak responses to metal ions in HEPES. More interestingly, self-assembly structures of p-BIDC and o-BIDC were different. With the same reprecipitation method and solvent system, compound p-BIDC was self-assembled into regular hexagons, while the organic gel with strong yellow fluorescence was formed with compound o-BIDC.