Intramolecular Charge Transfer in 5-Methoxy-2-(2-pyridyl)thiazole-Derived Fluorescent Molecules with Different Acceptor or Donor Substituents

Mechanism of Color and Photoacidity Tuning for the Protonated Green Fluorescent Protein Chromophore

The neutral or A state of the green fluorescent protein (GFP) chromophore is a remarkable example of a photoacid naturally embedded in the protein environment and accounts for the large Stokes shift of GFP in response to near UV excitation. Its color tuning mechanism has been largely overlooked, as it is less preferred for imaging applications than the redder anionic or B state. Past studies, based on site-directed mutagenesis or solvatochromism of the isolated chromophore, have concluded that its color tuning range is much narrower than its anionic counterpart. However, as we performed extensive investigation on more GFP mutants, we found that the color of the neutral chromophore can be more sensitive to protein electrostatics than can the anionic counterpart. Electronic Stark spectroscopy reveals a fundamentally different electrostatic color tuning mechanism for the neutral state of the chromophore that demands a three-form model as compared to that of the anionic state, which requires only two forms ( J. Am. Chem. Soc. 2019, 141, 15250-15265). Specifically, an underlying zwitterionic charge-transfer state is required to explain its sensitivity to electrostatics. As the Stokes shift is tightly linked to excited-state proton transfer (ESPT) of the protonated chromophore, we infer design principles of the GFP chromophore as a photoacid through the color tuning mechanisms of both protonation states. The three-form model could also be applied to similar biological and nonbiological dyes and complements the failure of the two-form model for donor-acceptor systems with localized ground-state electronic distributions.

Multistimuli-Responsive Luminescence Switching of Pyrazine Derivative Based Donor-Acceptor-Donor Luminophores

Three symmetrical donor-acceptor-donor (D-A-D) luminophores (C1, C2, and C3) with pyrazine derivatives as electron-withdrawing groups have been developed for multistimuli-responsive luminescence switching. For comparison, reference compounds R1 and R2 without the pyrazine moiety have also been synthesized. Intramolecular charge transfer (ICT) interactions can be found for all D-A-D luminophores owing to the electron-withdrawing properties of the two imine nitrogen atoms in the pyrazine ring and the electron-donating properties of the other two amine nitrogen atoms in the two triphenylamine units. Moreover, luminophores C1, C2, and C3 exhibit "on-off-on" luminescence switching properties in mixtures of water/tetrahydrofuran with increasing water content, which is different from the "on-off" switching for typical aggregation-caused quenching (ACQ) materials and "off-on" switching for traditional aggregation-induced emission (AIE) materials. Additionally, upon grinding the pristine samples, luminophores C1, C2, and C3 display bathochromically shifted photoluminescence maxima that can be recovered by either solvent fuming or thermal annealing treatments. The piezofluorochromic (PFC) properties are more pronounced than those for reference compounds R1 and R2, which indicates that D-A molecules have the ability to amplify the PFC effect by tuning the ICT interactions upon tiny structural changes under pressure. Furthermore, the target luminophores demonstrate acid-responsive photoluminescence spectra that can be recovered in either basic or ambient environments. These results suggest that D-A complexes are potential candidates for multistimuli-responsive luminescence switching because their ICT profiles can be facilely tuned with tiny external stimuli.