Halochromic luminescent quinoxalinones as a basis for pH-sensing in organic and aqueous solutions

Abstract

The protonation of a series of quinoxalin-2-ones dyes with N,N-dialkylaminostyryl substituent in different positions of the quinoxalinone (Qon) moiety has been experimentally and computationally studied. Both absorption and emission spectra are sensitive to protonation, an evidence that opens perspectives for their applications as luminescent pH-sensors. Combined NMR, UV/Vis and quantum chemical modeling revealed that the protonation undergoes different paths depending both on the alkyl group in the N,N-dialkylaminostyryl substituent, as well as on the position of the donor substituent relative to the Qon ring. Furthermore, to allow for pH-sensing in aqueous solutions the dyes have been encapsulated into l-α-phosphatidylcholine(PC)-based bilayers through the modified thin film hydration technique, and the spectral properties of the assemblies have been characterized. The pH-sensitive emission in the PC-dye bilayers presents a noticeable structural dependence, which can be traced back to electronic effects of the substituents and to the partition of the dyes between the polar head region and the hydrophobic core. Our results confirm that the proposed dyes can be used as pH-sensors with a maximum sensitivity in the 3.0–6.0 pH range.