Triphenylethylene- and Tetraphenylethylene-Functionalized 1,3-Bis(pyrrol-2-yl)squaraine Dyes: Synthesis, Aggregation-Caused Quenching to Aggregation-Induced Emission, and Thiol Detection

Abstract

Three novel pairs of 1,3-bis(pyrrol-2-yl)squaraine dyes, N-alkylated SQ1a-1b, and N-phenylated SQ2a-2b in which triphenylethylene moieties functionalized at 5-position of pyrrole, as well as SQ3a-3b with tetraphenylethylene (TPE) moieties attached at N-position of pyrrole, were synthesized. All six dyes were found not to exhibit aggregation-induced emission (AIE) properties. Spectrophotometric studies showed that N-TPE-functionalized SQ3a-3b exhibited much larger molar extinction coefficients (?: 1.36-2.14 × 105 M-1 cm-1) than 5,5?-triphenylethylene-functionalized SQ1a-2b (?: 2.17-8.22 × 104 M-1 cm-1). Surprisingly, SQ2b showed a remarkable red-shifted maximum absorption (max: 723 vs 631-652 nm) compared to that of other squaraine dyes. All six squaraine dyes selectively responded to the addition of thiol-containing biomolecules, such as cysteine and gluthatione, with the disappearance of max in the near-infrared region in their respective absorption spectra. Interestingly, the thiolated species of SQ3a-3b were AIE active, with the characteristic AIE emission of TPE at max = 484-490 nm upon addition of water. Further thiol sensing on solid supports was examined, indicating the potential applications of TPE-functionalized squaraine dyes as bioprobes for the detection of important thiol-containing biomolecules, with a clear change from aggregation-caused quenching to AIE. © Copyright 2018 American Chemical Society.