(1-(4-(5-Phenyl-1,3,4-oxadiazol-2-yl)phenyl)-1 H-1,2,3-triazol-4-yl)-methylenyls α,ω-Bisfunctionalized 3- and 4-PEG: Synthesis and Photophysical Studies

Abstract

Two new azaheterocycle-based bolas, such as (1-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)-1H-1,2,3-triazol-4-yl)-methylenyls α,ω-bisfunctionalized PEGs, were prepared via Cu-catalyzed click reaction between 2-(4-azidophenyl)-5-(aryl)-oxadiazole-1,3,4 and terminal ethynyls derived from PEG-3 and PEG-4. Due to the presence of two heteroaromatic cores and a PEG linker, these bola molecules are considered as promising fluorescent chemosensors for electron-deficient species. As a result of a well-pronounced "turn-off" fluorescence response towards common nitro-explosive components, such as 2,4-dinitrotoluene (DNT) and 2,4,6-trinitrotoluene (TNT), hard-to-detect pentaerythritol tetranitrate (PETN), as well as Hg²⁺ cation was observed.

Keywords: Hg2+; PEGs; bola molecules; fluorescence; fluorescence quenching; nitro-explosive components; oxadiazole; pentaerythritol tetranitrate; sensor; “click” reactions.

Scheme 1

Synthesis of α,ω-bisfunctionalized by the (1-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)-1H-1,2,3-triazol-4-yl)-methylenyls PEGs 3a-b.

Figure 1

Absorption (a) and emission (b) spectra of the probe 3a (10⁻⁵ M) in different solvents.

Figure 2

Absorption (a) and emission (b) spectra of the probe 3b (10⁻⁵ M) in different solvents.

Figure 3

Electron density difference patterns between S₀ and S₁ state for 3b.

Figure 4

Optimized configurations of 3a-PETN and 3b-PETN adducts.

Figure 5

Molecular orbitals and energy levels of 3b, DNT, and TNT calculated at the B3LYP_631pGs-level of theory.

Figure 6

DFT Calculations for the HOMO and LUMO energies for the sensor 3b and PETN.