Synthesis and fluorescent propertyies of phosphaphenanthrene-containing polystyrene

The introduction of phosphorus can effectively improve the flame retardancy, thermal stability, biocompatibility and other properties of polymers. The novel styrene derivant (MED) with phosphaphenanthrene side-group was first synthesized by means of two-step esterification of 2-(6-oxide-6H-dibenz <c, e > < 1, 2 > oxaphos-phorin-6-yl) - 1 , 4-dihydroxyphenylenes (ODOPB) as the center unit with 4- pentyloxybenzoic acid and 4-ethenylbenzoic acid, respectively. The free-radical polymerization of MED initiated by AIBN was carried out for 12 h in THF solution. The yield of product (PED) is over 80% . Comparing with that of MED, the 1H-NMR spectrum of PED demonstrated that the polymerization of MED is realized successfully. Due to the strong π-π stacking and polar interaction between phosphaphenanthrene- containing pendent groups, as well as the rigid main-chain of polystyrene, PED can easily form aggregation state in polar solvents, such as DMF and THF, as a result the molecular weights of PED are beyond the detection range of GPC. At the same time, the TGA results reveal the excellent thermal stability of MED and PED because the temperatures are 339°C for MED and 345°C] for PED at the 5% weight loss, respectively. For the induction of main-chain on the ordered arrangement and the strong interaction between the side-groups, the intramolecular rotation of phosphaphenanthrene groups in PED is more difficult than that in MED. So that, the non-radiative deactivation is decreased and the fluorescence intensity of PED in the pure THF solution is remarkably enhanced. With the addition of water or n-hexane, the fluorescence intensity of PED is induced to decrease because of the disordered aggregation between chains. When the water content was over 70% , the complexes based on the hydrogen bond bridge among a water molecule and two phosphaphenanthrene groups enhanced the intramolecular rotation restriction of some phosphaphenanthrene groups. Therefore, new shoulder peak at 450 nm is shown and the PL intensity increases. Clearly, the light emission of PED is induced by aggregating formation; in other word, PED is aggregation-induced emission (AIE) -active. When n-hexane as precipitant instead of water, the above-mentioned phenomenon is not shown in the absence of the formation of hydrogen- bonded complex structure. The fluorescence intensity continuously reduced with increasing of the content of n-hexane in the THF/n-hexane mixtures. The results allow us to design new materials with higher performance for chemo- and bio-detection applications.