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.