Alkoxy-Substituted Quadrupolar Fluorescent Dyes

Polar and polarizable π-conjugated organic molecules
containing
push–pull chromophores have been investigated extensively in
the past. Identifying unique backbones and building blocks for fluorescent
dyes is a timely exercise. Here, we report the synthesis and characterization
of a series of fluorescent dyes containing quadrupolar A–D–A
constitutions (where A = acceptor and D = donor), which exhibit fluorescence
emission at a variety of different wavelengths. We have investigated
the effects of different electron-withdrawing groups, located at both
termini of a para-terphenylene backbone, by steady-state
UV/vis and fluorescence spectroscopy. Pyridine and substituted pyridinium
units are also introduced during the construction of the quadrupolar
backbones. Depending on the quadrupolarity, fluorescence emission
wavelengths cover from 380 to 557 nm. Time-resolved absorption and
emission spectroscopy reveal that the photophysical properties of
those quadrupolar dyes result from intramolecular charge transfer.
One of the dyes we have investigated is a symmetrical box-like tetracationic
cyclophane. Its water-soluble tetrachloride, which is non-cytotoxic
to cells up to a loading concentration of 1 μM, has been employed
in live-cell imaging. When taken up by cells, the tetrachloride emits
a green fluorescence emission without any hint of photobleaching or
disruption of normal cell behavior. We envision that our design strategy
of modifying molecules through the functionalization of the quadrupolar
building blocks as chromophores will lead to future generations of
fluorescent dyes in which these A–D–A constitutional
fragments are incorporated into more complex molecules and polymers
for broader photophysical and biological applications.

过往针对含推拉型生色团（push–pull chromophores）的极性与可极化π共轭（π-conjugated）有机分子的研究已十分广泛。开发具有独特骨架与构筑单元的荧光染料，是一项兼具时效性的重要课题。本研究报道了一系列含四极型A-D-A结构（quadrupolar A–D–A，其中A代表受体（acceptor），D代表给体（donor））的荧光染料的合成与表征工作，这类染料可在多种不同波长下发射荧光。我们通过稳态紫外-可见吸收光谱（steady-state UV/vis）与荧光光谱（fluorescence spectroscopy），研究了位于对三联苯骨架两端的不同吸电子基团对染料性能的影响。在构筑四极型骨架的过程中，我们还引入了吡啶单元与取代吡啶鎓（pyridinium）单元。受四极极性程度影响，该系列染料的荧光发射波长覆盖380至557纳米区间。时间分辨吸收与发射光谱研究表明，这类四极型染料的光物理特性源于分子内电荷转移（intramolecular charge transfer）过程。本研究涉及的一款染料为对称箱型四阳离子环蕃（tetracationic cyclophane）。其水溶性四氯盐在浓度高达1 μM时仍对细胞无细胞毒性，已被应用于活细胞成像（live-cell imaging）实验。该四氯盐被细胞摄取后可发射绿色荧光，未表现出任何光漂白（photobleaching）现象，也不会干扰细胞的正常生理行为。我们认为，通过将四极型构筑单元功能化为生色团来修饰分子的设计策略，将助力新一代荧光染料的开发：将A-D-A结构片段引入更复杂的分子与聚合物中，以拓展其在光物理与生物领域的应用范围。