Carbazole-Based Eu³⁺ Complexes for Two-Photon Microscopy Imaging of Live Cells

Area of Science:

• Coordination Chemistry
• Bioinorganic Chemistry
• Photophysics

Background:

• Lanthanide(III) complexes with two-photon absorbing antennas are promising for live-cell microscopy due to near-infrared (NIR) excitation.
• Europium(III) complexes conjugated to cell-penetrating peptides (CPPs) offer potential for intracellular imaging.

Purpose of the Study:

• To synthesize and characterize two novel Europium(III) complexes functionalized with specific antennas and conjugated to TAT peptides.
• To investigate the luminescence properties, particularly quantum yield and its dependence on medium polarity.
• To evaluate the two-photon absorption characteristics and live-cell imaging capabilities of these complexes.

Main Methods:

• Synthesis of two Europium(III) complexes: mTAT[Eu·L-CC-Ar-Cz] and mTAT[Eu·L-Ar-Cz].
• Spectroscopic characterization including luminescence measurements and quantum yield determination in various solvents.
• Two-photon cross-section measurements.
• Live-cell imaging experiments using confocal microscopy.

Main Results:

• Both complexes exhibited very low emission quantum yields (< 0.002) in aqueous buffers, contrasting with previous observations in mixed solvents.
• Spectroscopic studies revealed that the emission quantum yield of mTAT[Eu·L-Ar-Cz] is highly dependent on solvent polarity, increasing in less polar media.
• The quenching in water is attributed to photoinduced electron transfer between the carbazole antenna and Eu3+, competing with energy transfer.
• mTAT[Eu·L-Ar-Cz] demonstrated a significant two-photon cross-section (100 GM at 750 nm).
• Live-cell imaging confirmed cytosolic delivery of mTAT[Eu·L-Ar-Cz] when co-incubated with a dimeric TAT derivative.

Conclusions:

• The studied Europium(III) complexes show limited utility as luminescent probes in aqueous environments due to efficient quenching mechanisms.
• The polarity-dependent emission and efficient photoinduced electron transfer are key factors influencing their photophysical behavior in water.
• Despite low aqueous luminescence, the significant two-photon absorption cross-section of mTAT[Eu·L-Ar-Cz] makes it a candidate for two-photon microscopy.
• Effective cytosolic delivery was achieved, highlighting the potential of these peptide-conjugated complexes for cellular applications, albeit with limitations in luminescence efficiency in aqueous media.