Synthesis and photophysics of a series of lipophilic phosphazene‐based fluorescent indicators

Abstract

Abstract This work explores the 7‐amino‐4‐(trifluoromethyl)coumarin backbone for designing lipophilic fluorescent pH indicators. Four novel fluorescent phosphazene bases with an advantageous combination of properties—high lipophilicity of both neutral and charged forms, lack of localized charges in the cations, and prominent spectral changes upon protonation/deprotonation—were synthesized using the Staudinger reaction. The molecular structures of the synthesized compounds were confirmed using attenuated total reflectance Fourier‐transform infrared spectroscopy (ATR‐FT‐IR), nuclear magnetic resonance (NMR), and Fourier‐transform ion‐cyclotron resonance (FT‐ICR) high‐resolution mass spectrometry. The compounds were further characterized by ultraviolet‐visible (UV‐Vis) absorption and fluorescence emission spectra, lipophilicities (log P values) and p K a values in water and acetonitrile, as well as “biphasic” p K a values in the water/1‐octanol solvent system. The bases could potentially be useful as working agents in optical pH sensors and acid‐base indicators for lipophilic membranes.