A New Conjugated Polymer Chemosensor Functionalised with 2,6-Bis(1,3,4-oxadiazole-2-yl)pyridine for Metal Ion Recognition

Nam Choul Yang, Jae Kyun Jeong, Dong Hack Suh
2003 Chemistry Letters  
A new conjugated alternative copolymer based on 2,6bis(1,3,4-oxadiazol-2-yl)pyridine and 9,9-dialkylfluorene in a main chain showed strong blue fluorescence with band maxima at 389 nm and 405 nm in the 1,2-dichloroethane solution and posessed sensitivity to various metal ions such as Ni 2þ and Fe 3þ as a fluorescence-mode chemosensor. Chemosensors based on conjugated polymers recently have attracted considerable interests due to their some merits over the sensor systems based on small molecules
more » ... on small molecules in the enhanced sensitivity, many transduction methods and facile processibility for condensed phase applications. 1;2 The semiconductive molecular structure formed along the conjugated polymer chain provides transductional amplification and various transduction signals such as optical absorption, fluorescence and electrical conductivity. 1;2 The conjugated polymers functionalized with electron-donor groups such as crown ethers, aza crown ethers and calixarenes as side chains for sensing metal ions have been the most dominantlystudied sensory systems. However, these conjugated polymers are adequate to recognize small size alkali metal ions such as Li þ , Na þ , and K þ . 1 To be sensitive to various metal ions including transition metal ions, a 2,2 0 -bipyridyl group, one of well-known bidentate ligands, has been employed in the main chain of conjugated polymers. 3{6 Recently, oligomeric poly(p-phenylenevinylene) with a terpyridyl side chain sensitive to transition metal ions has been reported. 7 To our knowledge, these type of conjugated polymers sensitive to metal ions are still rare. In this letter, we describe the synthesis and characterization of a new conjugated polymer alternatively having 9,9-didodecylfluorene and 2,6-bis(1,3,4-oxadiazol-2-yl)pyridine designed as a terpyridine analogue appropriate to metal ion recognition. The conjugated polymer PFOxPy is prepared via a polyhydrazide precursor PrePFOxPy. This synthetic route was chosen to exclude the possibility of introduction of metal ions during preparation of the polymer to the utmost. The white fibrous polyhydrazide PrePFOxPy was polymerized from 9,9 0 -didodecylfluorene-2,7-dicarboxylic acid and pyridine-2,6-dicarboxylic acid dihydrazide by a Yamazaki-Higashi phosphorylation polyamidation. 8 Dehydrocyclization of PrePFOxPy using POCl 3 as both a dehydrating agent and a reaction solvent resulted in the polyoxadiazole PFOxPy as light gray fibrous powder. 9 Determined by IR and NMR spectra, the conversion from PrePFOxPy 11 to PFOxPy 12 was found to be complete. The yield of PrePFOxPy and PFOxPy was 99% and 95%. Most polyoxadiazoles showed poor solubility in common organic solvents due to the chain rigidity from a heterocyclic group. However, PFOxPy was very soluble in some common organic solvents such as tetrahydrofuran, 1,4-dioxane, chlorobenzene and chlorinated hydrocarbons such as chloroform and 1,2-dichloroethane (DCE) due to the increase of solubility by introducing long alkyl chains in a fluorene group and the mlinkage in a pyridine group to kink the rigid main chain. 6 Measured by gel permeation chromatography (GPC) with tetrahydrofuran as an eluent, the weight-average and numberaverage molecular weights of PFOxPy were 3:3 Â 10 4 and 1:6 Â 10 4 , respectively. PFOxPy in the DCE solution showed symmetrical UVvisible absorption and fluorescence spectra with fine vibrational structures. UV-visible absorption and fluorescence maxima of PFOxPy were 352 and 372 nm and 389 and 405 nm, respectively, so this polymer showed deep blue fluorescence to human eyes. Determined by comparison with 9,10-diphenylanthracene (fluroescence quantum efficiency = 0.90 in cyclohexane excited at 350 nm), the fluorescence quantum efficiency of PFOxPy was relatively high (0.46), which provided good applicability to a fluorescent chemosensor. The metal ion recognition property of PFOxPy was monitored in 1 Â 10 À5 M (based on a repeating unit) DCE by UVvisible absorption and fluorescence spectroscopies. Metal ions (Hg 2þ , Al 3þ , Fe 3þ , Ni 2þ , Mn 2þ , Co 2þ , Cu 2þ , Pb 2þ , and Zn 2þ ) Scheme 1.
doi:10.1246/cl.2003.40 fatcat:34auiitelbegxnfirftmksuxde