Aminoquinoxaline-Based Dual Colorimetric and Fluorescent Sensors for pH Measurement in Aqueous Media

Abstract

This research is focused on the development of pH indicators based on the quinoxaline signaling group for acidic aqueous solutions (pH 1–5). A push–pull quinoxaline QC1 in which two electron-donating (3-aminopropyl)amino substituents are attached to positions 6 and 7 of the electron-deficient quinoxaline moiety was prepared using the palladium-catalyzed C–N cross-coupling reaction. The 3-aminopropyl residues are mostly protonated in aqueous solutions below pH 8, thus serving as hydrophilizing substituents that render quinoxaline derivative QC1 water-soluble in this range of acidities and useful for measurements in the pH range of 1–5. This chromophore is a dual optical chemosensor that exhibits shifts of both absorption and emission bands in response to external stimuli. The presence of naturally relevant metal cations (13 ions) does not interfere with spectrophotometric and fluorescence measurements of the optical response of aminoquinoxaline in the visible region. Moreover, these spectral changes are easily observed by the naked eye, allowing for rapid semi-quantitative analyses under “in-field” conditions.

Push-Pull Structures Based on 2-Aryl/thienyl Substituted Quinazolin-4(3H)-ones and 4-Cyanoquinazolines

“…Over the last two decades, there has been a remarkable increase in the number of fluorescent diaza-heterocycles [ 1 , 2 , 3 , 4 ]. Among them quinoxaline and quinazoline derivatives are considered as pH sensors [ 5 , 6 , 7 ], luminescent detectors [ 8 , 9 , 10 ], imaging agents [ 11 ], components for organic light-emitting diodes (OLED) materials [ 12 , 13 ], solar cells and organic photovoltaic [ 14 , 15 ], and so on. The fundamental work has been done for the establishment of detailed structure–property relationships (SPRs) providing beneficial information for fine-tuning of the key properties and the rational design and synthesis of fluorophores.…”

Polyoxa- and Polyazamacrocycles Incorporating 6,7-Diaminoquinoxaline Moiety: Synthesis and Application as Tunable Optical pH-Indicators in Aqueous Solution

“…Only products of C-Br bond reduction and oligomers were observed in the reaction mixture. The application of the Pd(dba) 2 /DPPF (8/9 mol%, DPPF—1,1′-bis(diphenylphosphino)ferrocene), which previously was shown to be optimal for the diamination of 1 with 1,3-diaminopropane [ 36 ], provided the desired macrocycle 5e in 48% yield. Earlier JosiPhos ligand (( R )-1-[( SP )-2-(diphenylphosphino)ferrocenyl]ethyldi- tert -butylphosphine) and ligands of this type were noted for its good performance in the Pd-catalyzed amination of halogen heteroarenes [ 63 , 64 ], and its application in our case increased the yield of 5e to 77%.…”

“…Initially, the spectral properties of the obtained macrocycles were studied in acetonitrile solutions, and the main spectral data are given in Table 1 . The compounds 3 and 5a – h possess well-pronounced photoluminescent properties, which are characteristic of the electron push-pull systems of quinoxaline [ 36 , 65 , 66 ]. The absorption spectra possess strong bands at 260–280 nm corresponding to π–π* transitions and broad bands in the 404–410 nm region corresponding to ICT [ 67 ].…”

“…For A 408 < 0.1 ( Figures S8 and S10 ). Polyazamacrocycle 5e is sufficiently soluble in water without methanol additive due to partial protonation of the dialkylamino groups, and the Beer–Lambert law is observed for a wider range of concentrations, likely it was the case with a non-cyclic analog [ 36 ]. Compounds 3 , 5c and 5e possess fluorescence in the green region, their emission maxima being 507, 510 and 526 nm, respectively ( Table S1 ).…”

“…Quinoxalines and their derivatives are important structural fragments of various chemosensors due to their optical properties. They are applied as signaling groups for detecting metal cations [ 25 , 26 , 27 , 28 ], anions [ 29 , 30 , 31 , 32 , 33 ] and as pH detectors [ 34 , 35 , 36 ]; quinoxalines are also used in the fabrication of novel organic light-emitting diodes (OLEDs) [ 27 , 37 , 38 ]. The ability of quinoxalines to sustain fluorescent properties in the solid phase allows the manufacturing of fluorescent materials using this fluorophore which also can be used for detection needs [ 39 , 40 ].…”