Revealing the sensing mechanism of a fluorescent pH probe based on a bichromophore approach

Fluorescence sensing plays an increasingly important role in biology and biomedicine. For many practical applications of fluorescent probes, an "off-on" response is preferred. The question of how fluorescence quenching/enhancement occurs is fundamental and of high importance for application and design of new fluorescent probes. The sensing mechanism of an aminorhodamine (TMARh) pH probe is investigated using femtosecond transient absorption spectroscopy and quantum chemical calculations, showing that this probe is best understood using the bichromophore model rather than the more common models such as photoinduced electron transfer or intramolecular charge transfer. Under excitation in the main absorption band at 530 nm, a fast internal conversion to the first excited state (S₁) is observed for TMARh; meanwhile, no new transient components are obtained when TMARh is excited directly to S₁ in the weakly absorbing red tail at 630 nm. It is confirmed that the S₁ of TMARh is a dark "off" state. Theoretical calculations show that the S₁ "off" state is an intramolecular charge transfer state from an aminophenyl group to a rhodamine chromophore. After protonation of the aminophenyl group, to yield HTMARh, the transient S₂/S₁ internal conversion process that occurs in TMARh under 530 nm excitation is absent, suggesting that the charge transfer state becomes highly unfavorable. All calculations and spectral data confirm that HTMARh has localized transition in the rhodamine chromophore, in agreement with this being the bright "on" state of the pH probe. The current work not only provides a photophysical insight into the sensing mechanism of this specific probe, but also shows that the bichromophore model is useful and may be relevant for analyzing other probes or in the designing of new ones.