Photophysical Rate Constants and Oxygen Dependence for Si and Ge Rhodamine Zwitterions

Area of Science:

• Photochemistry
• Biophysical Chemistry
• Materials Science

Background:

• Group XIV rhodamine zwitterions (Si, Ge) are advanced fluorescence probes.
• They offer near-infrared fluorescence, photostability, and high quantum yields in aqueous solutions.
• These properties make them valuable for labeling proteins and biological molecules.

Purpose of the Study:

• To fully characterize the photophysical rates of exemplar Si and Ge rhodamine zwitterions.
• To investigate the influence of oxygen concentration on radiative and non-radiative decay pathways.
• To compare the reactive oxygen production and photostability with traditional rhodamine fluorophores.

Main Methods:

• Utilized a Jablonski diagram model to analyze singlet and triplet excited states.
• Measured radiative and non-radiative rates, including intersystem crossing and triplet depopulation.
• Employed fluorescence correlation spectroscopy (FCS) with a novel optical saturation extrapolation method.

Main Results:

• Si and Ge rhodamine zwitterions show intrinsically fast triplet depopulation rates, attributed to enhanced spin-orbit coupling.
• Dissolved oxygen accelerates intersystem crossing and triplet depopulation, with Stern-Volmer analysis quantifying oxygen quenching.
• These fluorophores exhibit significantly slower initial reactive oxygen production compared to rhodamine 6G, explaining their superior photostability.

Conclusions:

• The enhanced photophysical properties of Si and Ge rhodamine zwitterions stem from their unique group XIV substitution.
• Their reduced reactive oxygen species generation is key to their photostability in biological imaging.
• These findings support their utility as robust fluorescent labels in aqueous environments.