Amphiphilic polyelectrolytes containing carbazolyl chromophores (a-CzCa) were prepared by hydrolysis of alternating copolymers of iV-viny lcarbazole (VCz) and citraconic anhydride and their fluorescence properties were compared with those for alternating copolymers of VCz and maleic acid (a-CzMa) and random copolymers of VCz and methacrylic acid (r-CzMA(x)) in aqueous solution. The fluorescence spectrum of a-CzCa, derived from the anhydride copolymer with the equimolar monomer feed ratio, showed a predominant monomer emission in both organic and aqueous solutions, while the fluorescence intensity changed drastically upon dissociation of carboxyl groups in aqueous solution and had a minimum value at acidic pH which was rather smaller than that for the corresponding random copolymer. The solution behavior showed that a-CzCa had a less hydrophobic character and thus retained a loose form even at low pH. Therefore, the remarkable self-quenching occurring in a-CzCa may be caused primarily by efficient energy migration to very few quenching sites. Quenching experiments gave additional evidence for hydrophilicity of a-CzCa. The fluorescence of a-CzCa at pH 3 was found to be quenched less efficiently by an amphiphilic quencher, bis(2-hydroxyethyl) terephthalate (BHET), compared with the corresponding random copolymer.
Alternating copolymers of N-vinylcarbazole (VCz) with diethyl fumarate (DEF), maleic anhydride (MAn), and citraconic anhydride (CAn) (a-CzEF, a-CzMAn, and a-CzCAn, respectively) were synthesized. The fluorescence spectra of a-CzEF and a-CzCAn changed as a function of the monomer feed ratios although all the copolymers had nearly 50 mol 9% of VCz content Cfcz), indicating the presence of excimer-forming sites in the polymer chain. The copolymer obtained at fCz = 0.5 in the monomer feed was found to be the most desirable "alternating" copolymer with the highest fluorescence quantum yield. Comparison of the spectral features for a-CzMAn and a-CzCAn showed that hindered groups on comonomers effectively restricted the excimeric interaction in copolymers. A small number of anthryl energy traps were covalently incorporated into a-CzEF and a random copolymer of VCz and methyl methacrylate with fCz = 0.5 (r-CzMMA(50)) by terpolymerization with 9-anthrylmethyl methacrylate. Energy transfer to anthracene was more favorable in a-CzEF than in r-CzMMA(50). This result strongly suggests that longer-range energy transfer assisted by efficient energy migration is occurring in the alternating copolymer, which may be due in part to the large Forster radius for carbazole-carbazole for self-transfer (-2.1 nm). IntroductionSinglet energy migration and excimer formation in aromatic polymers have been studied extensively.' Inhibition of excimer formation and other quenching processes is crucial not only for the design of "photonharvesting" polymers but also for increasing our understanding of the complicated physical behavior of polymer systems. One can expect that in the former case energy transfer to traps would be facilitated2 and that in the latter case the kinetic scheme in the excited states would be simplified.3 In this context, a lot of effort has been directed to avoiding excimer formation in various types of polymers;2 e.g., introduction of bulky groups on chrom o p h o r e~,~~~ fixing of chromophores to the rigid polymer chain: separation of chromophores from the main chain,6J and alternation of chromophores.a10 Recently, Ito et al. demonstrated that poly[2-(9-carbazolyl)ethyl methacrylate] does not form an excimer even in a neat film, leading to efficient energy migrati~n.~ A successful alternative photon-harvesting polymer system was designed very recently by Morishima et al. in which hydrophobic association of naphthyl chromophores in aqueous solution, in spite of favorable excimer formation, enables the most rapid and efficient energy transfer in synthetic polymers reported so far."We have been investigating the photophysical properties of a series of amphiphilic alternating copolymers as promising candidates for photon-harvesting polymer~.~*9JO In a previous paper, we reported that poly(9-vinylphenanthrene-alt-methacrylic acid) showed no self-quenching in organic solution, implying efficient energy migration.10 Sensitization of the intramolecular energy traps (anthryl groups), however, was only slightly enhanced compared...
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