Studies On Synthesis, Characterization, And Photophysical Properties Of Cationic Water-Soluble Poly(Aryleneethynylene)s

Water-soluble conjugated polymers (WSCPs) are under intense investigation due to their great potential as highly sensitive fluorescent sensory materials for a variety of chemical analytes and biomolecules. However, this area is relatively new and in need of further studies on the relationship between the structure of WSCPs and their fluorescence quenching effects in detection. Previous studies indicate that WSCPs exhibit various aggregation with the alteration of structure and environmental conditions due to their amphiphilic nature. This is one of the most important factors which influence their photophysical property and results of fluorescence detection. This dissertation is focused on modifying the aggregation of WSCPs by means of structure design and changing environmental conditions, and studying the structure-property relationship. The contents are as follows.1. A highly fluorescent chromophore—fluorene was introduced into the structure of water-soluble poly(p-phenyleneethynylene) and a series of cationic water-soluble poly(aryleneethynylene)s (PAEs) P1'-P3' were synthesized. Through Sonogashira reaction and post-quarternization treatment, three polymers with close molecular weight were obtained and their quaternization degrees were 100%. Water solubility was gradually improved from P3' to P1' by increasing the content of hydrophilic side chains, but they all readily dissolve in methanol.2. By means of investigating the UV-Vis absorption and photoluminescence (PL) spectra, fluorescence lifetime and dynamic light scattering data, a speculation on the respective aggregation of P1'-P3' in methanol and water was given. With the decrease of water solubility from P1' to P3', they exhibited a gradually increased extent of aggregation in water. Specifically, P1' presented minimal aggregation while P2' and P3' exhibited relatively disordered interchain aggregation instead of cofacial Ï€-aggregation. The effects of fluorenyl unit and the content of hydrophilic side chains on the formation of aggregation were analyzed.3. The relationship between the PL quantum yields of P1'-P3' and their respective extent of aggregation in water was studied. Their PL quantum yields in water were 0.26, 0.22 and 0.08, respectively, which displayed a decreased tendency consistent with their increased extent of aggregation, while their PL quantum yields in methanol were predominantly determined by the structure of conjugated backbones. This implies that aggregation may be an important reason for the low PL quantum yields of WSCPs in water.4. Two water-soluble trimers T1' and T2' with analogous structure to P2' and P3' were synthesized. The amplified fluorescence quenching of these WSCPs by Fe(CN)₆^4- in water was studied by comparing with corresponding analogous trimer. Stern-Voimer (SV) constants (K_SV) of P1' and P3' were 2.4 × 10⁸ M^-1 and 7.3 ×10⁸ M^-1 , respectively, which implies their great potential in sensory application. The results of quenching experiments indicated that K_SV in water was higher than that in methanol in each case, which reflect that the association constants for Fe(CN)₆^4- binding to these compounds are most likely larger in water than in methanol.5. Studies on the UV-Vis absorption and PL spectra and SV curves of P1'-P3' suggest that the effects of aggregation on the fluorescence quenching may be two-edged in these cases. P1' exhibited minimal aggregation in water and still displayed a very high K_SV, which indicates that its excellent fluorescence quenching effects is determined by its structure.6. The effects of multivalent inorganic counterions (non-quencher) and surfactant on the aggregation of P1'-P3' were studied in water. Because P1' exhibited minimal aggregation in water, the disturbance from aggregation in its quenching experiments could be avoided to a large degree. Thus, interactions between P1' and several organic quenchers were investigated and the structure-property relationship was analyzed.7. A meta-linked cationic water-soluble poly(phenyleneethynylene) (m-PPE-NEt₂Me⁺) was synthesized. Its solvent- and pH-influenced self-assembly behavior and conformation changes were studied by means of analyzing the changes of UV-Vis absorption and PL spectra and fluorescence lifetime. In good solvent methanol it adopted random-coil conformation while it folded into helix conformation in water. Moreover, it exhibited the tendency of folding into helix conformation with the pH increase in these two solvents mixture at the volumecomposition of MeOH/H₂O = 4/1. The reason for self-assembly and specific features of the helix conformation was analyzed.8. Through studies on the fluorescence quenching of m-PPE-NEt₂Me⁺ by Fe(CN)₆^4-, sodium anthraquinone-2,7-disulfonate (AQS) and an iron-sulfur protein (Rubredoxin) with electron transfer center, the extent of quenching was found to be stronger in helix conformation than in random-coil conformation, especially in the case of AQS. The effects may arise due to the rapid migration of exciton between the Ï€-stacked segments of helix conformation or possibly due to the fact that the quencher can interact with a larger fraction of the chain when it was intercalated into the helix conformation.9. Through the introduction of fluorenyl unit into m-PPE-NEt₂Me⁺ backbone and tuning the meta phenylene contents and hydrophilic side chains, a series of meta-linked cationic water-soluble PAEs P4'-P6' were designed and synthesized. From P3' to P6', with gradually increased meta phenylene contents in their backbones, transformation of aggregation from relatively disordered structure (intermolecular aggregation) to helix structure (intramolecular fold) was found. The specific features of the helix conformation of P5' and P6' were proposed and the reason for their formation was interpreted.10. Through studies on the fluorescence quenching of P3'-P6' by Rubredoxin, the extent of quenching was found to be stronger in the polymer with more typical features of helix conformation. By comparing with the fluorescence quenching effects of m-PPE-NEt₂Me⁺, the influences of conformation on fluorescence quenching were investigated.