Pyrene functionalized cyclotriphosphazene-based dyes: Synthesis, intramolecular excimer formation, and fluorescence receptor for the detection of nitro-aromatic compounds

Abstract

A series of pyrene functionalized cyclotriphosphazene-based dyes, mono- (1), cis- and trans-non-geminal-bis (2a, 2b), geminal-bis-(4), geminal-tetrakis-(6), and hexalcis-(7) derivatives, has been synthesized and, their photo physical properties such as molar absorption coefficients (epsilon), fluorescence emission, fluorescence quantum yields (phi F) and fluorescence lifetimes (tau F) have been investigated. On the relationship between molecular geometry and emission properties of dyes has been also discussed. It was found that bis-pyrene substituted compounds (2a, 2b and 4) exhibited three different fluorescence spectra which consisted of an excimer, a monomer and both monomer and excimer emissions in the same solvents, respectively. The emission difference between 2a and 2b clearly revealed that intramolecular excimer formation, which permitted to characterize unambiguously as a cis isomer, observed for compound 2a whereas 2b showed only monomer emission and consequently as a trans-isomer. These formation behaviors were supported by DFT calculation with wB97XD/6-31G (d, p) and P-31 NMR spectroscopy on addition of chiral solvating agent (CSA) experimental studies. Additionality, sensing behaviors of 2a, 4 and 6 were investigated for the detection of nitro aromatic compounds (NACs). It was found that the fluorescence emissions of the compounds were significantly quenched in presence of TNT, nitrobenzene or nitrophenol, while other tested metal ions induced no spectral changes.