Balancing Brightness and Photobasicity: Modulating Excited-State Proton Transfer Pathways in Push-Pull Fluorophores...

Area of Science:

• * Photophysics and photochemistry
• * Supramolecular chemistry
• * Bioimaging probe development

Background:

• * Push-pull fluorophores with donor-π-acceptor architectures are crucial for two-photon microscopy due to their large two-photon absorption cross-section and brightness.
• * Excited-state proton transfer (ESPT) can occur in polarized excited states, potentially interfering with probe response.
• * Understanding pH-dependent responses is vital for designing reliable fluorescent probes.

Purpose of the Study:

• * To investigate whether ESPT is responsible for the pH-dependent emission of the Zn(II)-selective fluorescent probe chromis-1.
• * To elucidate the mechanism underlying the pH-dependent response of chromis-1.
• * To guide the design of fluorescent probes with minimized pH sensitivity.

Main Methods:

• * Steady-state and time-resolved spectroscopic studies.
• * Investigation of the photobasicity of the pyridine acceptor in chromis-1.
• * Analysis of the role of the pendant bis-isonicotinic acid chelating group.

Main Results:

• * The pH-dependent emission of chromis-1 is primarily due to the pendant bis-isonicotinic acid chelating group, not the core's photobasicity.
• * Protonation of the chelating group facilitates excited-state intramolecular proton transfer to the pyridine acceptor.
• * A structural modification (ortho- vs. para-position of pyridine nitrogen) significantly reduced excited-state basicity without affecting two-photon brightness.

Conclusions:

• * The chelating moiety, rather than the core fluorophore, dictates the pH-dependent emission response in chromis-1.
• * Careful selection of both the fluorophore core and chelating group is essential for developing robust fluorescent probes.
• * Strategies to minimize ESPT-related pH sensitivity can improve the reliability of probes in biological imaging.