Modular Fluorescent Benzobis(imidazolium) Salts: Syntheses, Photophysical Analyses, and Applications

摘要

A series of benzobis(imidazolium) (BBI) salts has been prepared and studied as a new class of versatile fluorescent materials. Using a high yielding, modular synthetic strategy, BBI salts with a range of functionality poised for investigating fundamental and applications-oriented characteristics, including emission wavelength tunability, solvatochromism, red-edge excitation, chemical stability, multiphoton excitation, and protein conjugation, were prepared in overall yields of 40-97%. Through structural variation, the BBIs exhibited λ_(em) ranging between 329 and 561 nm while displaying Φ_fS up to 0.91. In addition, the emission characteristics of these salts were found to exhibit strong solvent dependencies with Stokes shifts ranging from 4570 to 13 793 cm~(-1), depending on the nature of the BBI core. Although red-edge effects for BBI salts with Br and BF_4 counterions were found to be similar, unique characteristics were displayed by an analogue with MeSO_4 anions. The stability of an amphiphilic BBI was quantified in aqueous solutions of varying pH, and > 85% of the emission intensity was retained after 2 h at pH 3-9. Through multiphoton excitation experiments in aqueous solutions, a BBI salt was found to exhibit three-photon fluorescence action cross sections similar to serotonin. The application of BBI salts as fluorescent protein tags was demonstrated by conjugating bovine serum albumin to a maleimide-functionalized derivative.