New cationic fluorescent dyes for optical sensing of chloride

Lucigenin (10,10′-dimethyl-[9,9′-biacridine]-10,10′-diium nitrate) is a fluorescent compound known for more than a century for its chloride sensitivity, but further research since then has not yielded definitive improvements or extensive structure–property relations. Chloride indicators that absorb and emit at longer wavelengths are highly desirable to minimize interference through scattering, autofluorescence of optical components and samples, particularly when measuring in biological probes. Herein, the synthesis and spectral properties of new π-extended lucigenin analogs are reported. N-Methyl benzo[b]acridinium (1) and two-fold-charged dyes incorporating the benzo[b]acridinium backbone (2, 3) are not emissive, and a fluorescent π-extended derivative incorporating a fluorene structure (4) does not show a sizeable response to chloride (K_SV NaCl = 0.8 M⁻¹). A novel dicationic perylene analog, 3,9-dimethylbenzo[1,2,3-de:4,5,6-d′e′]diquinoline-3,9-diium (6), shows typical spectral characteristics of perylene and fluorescence quantum yield close to unity but also a strong fluorescence response to chloride (K_SV NaCl = 160 M⁻¹). Compared to lucigenin (ε = 7.5 × 10³ M⁻¹ cm⁻¹ at 433 nm, Φ = 52%), 6 shows much higher (∼5-fold) brightness in the visible spectral range (ε = 2.2 × 10⁴ M⁻¹ cm⁻¹ at 455 nm, Φ = 97%). Finally, a hybrid of acridinium and quinolinium, 10-methyl-9-(1-methylquinolin-1-ium-5-yl)acridin-10-ium (5), is characterized by spectral properties, brightness (ε = 5.1 × 10³ M⁻¹ cm⁻¹ at 426 nm, Φ = 83%) and chloride response (K_SV NaCl = 145 M⁻¹) which are similar to lucigenin, but without detectable crosstalk to common anions such as nitrate and sulfate (K_SV NaNO3 ≈ 0, K_SV Na2SO4 ≈ 0). In addition, the indicator features extraordinarily long fluorescence lifetime (τ = 24.5 ns) that also decreases in presence of chloride, which suggests high potential of the new dye as a water-soluble probe for chloride mapping by fluorescence lifetime imaging methods. The dye can also be photoimmobilized into partly hydrolyzed poly(acrylonitrile) providing a sensor material with similar characteristics to lucigenin, but without crosstalk to nitrate and sulfate.