Premium
pH‐Dependent Optical Properties of Synthetic Fluorescent Imidazoles
Author(s) -
Berezin Mikhail Y.,
Kao Jeff,
Achilefu Samuel
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200801784
Subject(s) - imidazole , fluorescence , excited state , chemistry , deprotonation , protonation , fluorophore , photochemistry , ground state , stereochemistry , ion , organic chemistry , physics , quantum mechanics , nuclear physics
New pH‐sensitive probe design : A peek into the structure of fluorescent proteins led to the synthesis of fluorescent imidazoles. The prepared compounds demonstrated an array of remarkable pH‐dependent optical properties including at least two types of excited‐state charge transfers (see picture).An imidazole moiety is often found as an integral part of fluorophores in a variety of fluorescent proteins and many such proteins display pH‐dependent light emission. In contrast, synthetic fluorescent compounds with incorporated imidazoles are rare and have not been studied as pH probes. In this report, the richness of imidazole optical properties, including pH sensitivity, was demonstrated by means of a novel imidazole‐based fluorophore 1 H ‐imidazol‐5‐yl‐vinylbenz[ e ]indolium. Three species corresponding to protonated, neutral, and deprotonated imidazoles were identified in the broad range of pH 1–12. The absorption and emission bands of each species were assigned by comparative spectral analysis with synthesized mono‐ and di‐ N ‐methylated fluorescent imidazole analogues. p K a analysis in the ground and the excited states showed photoacidic properties of the fluorescent imidazoles due to the excited state proton transfer (ESPT). This effect was negligible for substituted imidazoles. The assessment of a pH‐sensitive center in the imidazole ring revealed the switching of the pH‐sensitive centers from 1‐N in the ground state to 3‐N in the excited state. The effect was attributed to the unique kind of the excited state charge transfer (ESCT) resulting in a positive charge swapping between two nitrogens.