Rational Design and Facile Synthesis of a Highly Tunable Quinoline-Based Fluorescent Small-Molecule Scaffold for Live Cell Imaging

Abstract

Small-molecule fluorescent probes are powerful tools for chemical biology; however, despite the large number of probes available, there is still a need for a simple fluorogenic scaffold, which allows for the rational design of molecules with predictable photophysical properties and is amenable to concise synthesis for high-throughput screening. Here, we introduce a highly modular quinoline-based probe containing three strategic domains that can be easily engineered and optimized for various applications. Such domains are allotted for (1) compound polarization, (2) tuning of photophysical properties, and (3) structural diversity. We successfully synthesized our probes in two steps from commercially available starting materials in overall yields of up to 95%. Facile probe synthesis was permitted by regioselective palladium-catalyzed cross-coupling, which enables combinatorial development of structurally diverse quinoline-based fluorophores. We have further applied our probes to live-cell imaging, utilizing their unique two-stage fluorescence response to intracellular pH. These studies provide a full demonstration of our strategy in rational design and stream-lined probe discovery to reveal the diverse potential of quinoline-based fluorescent compounds.

Devising Efficient Red‐Shifting Strategies for Bioimaging: A Generalizable Donor‐Acceptor Fluorophore Prototype

“…In Figure b, a relatively small fluorescent core skeleton via a concise and practical one‐pot synthetic route shows the emission redshift as the donor and acceptor get stronger at the electron‐rich sites in HOMO and LUMO, respectively, in accord with the destabilization of HOMO and stabilization of LUMO by the strategic substituents (Figure a) . In Figure c, a more compact dimethylamino quinoline “core” scaffold also demonstrates a clear red‐shifting trend of emission wavelength as the EWG gets stronger (i. e., larger values or the Hammett constants, see above) at the acceptor site as excited‐state ICT occurs from the EDG (dimethylamino group, highlighted in red) on one end to the C‐2 site on the other end (aryl R group as the acceptor, highlighted in blue) of the fluorophore . The underlying electron distribution and structural dynamics basis for the photophysics and/or photochemistry (ultimately leading to the fluorescence properties) can be elucidated by an integrated and powerful experimental platform of ultrafast electronic and vibrational spectroscopies, theoretical calculations, and organic synthesis as summarized in this review (see the frontispiece).…”

“…Our recent design strategy, driven by ab initio quantum calculations and ultrafast molecular spectroscopy (both in the electronic and vibrational domains), opens up a new and systematic direction to effectively red‐shift emission by taking advantage of the difference between electron density distributions in the ground and excited states. Therein, ground‐ and excited‐state energetics can be separately and synergistically tuned for a single‐core molecular framework with specific substituents, as validated by direct comparison between the predicted emission wavelengths and the observed spectral values (Figure ) as well as the newly synthesized fluorophores with various FQYs (Figure ) . This approach can therefore be viewed as a shortcut for achieving redder fluorophores with respect to the conventional ICT strategy.…”

“…Notably, electron density at the phenolate ring (P‐ring) decreases from HOMO to LUMO while that at the C‐2 position (as well as the methyl group) of the imidazolinone ring (I‐ring) increases. It indicates a dominant charge flow from the P‐ring to the C‐2 site of the I‐ring, which provides a basis for conventional ICT design, i. e., incorporating EWGs at the C‐2 site to enhance ICT and achieve a more stable ICT state for redder emission …”

“…Interestingly, other groups have designed and synthesized fluorescent small‐molecule scaffolds that lend further support to the color‐tuning scheme for D‐ ‐A systems (see Figure b for examples) . In Figure b, a relatively small fluorescent core skeleton via a concise and practical one‐pot synthetic route shows the emission redshift as the donor and acceptor get stronger at the electron‐rich sites in HOMO and LUMO, respectively, in accord with the destabilization of HOMO and stabilization of LUMO by the strategic substituents (Figure a) .…”

Design and Expeditious Synthesis of Quinoline‐Pyrene‐Based Ratiometric Fluorescent Probes for Targeting Lysosomal pH

“…Similarly, quinoline derivatives can also be used as effective probes for intracellular pH monitoring because of their highly accessible nitrogen but have remained largely unexplored because of difficulties associated with their synthesis . Typically, designing of the probe involves the synthesis of the core and then its modification to target specific functionality.…”