Part III: Study on Optical Properties and Fluorescence Quenching of Cationic Water-Soluble Poly(p-phenyleneethynylene) under
Scheme 4.3.1 Chemical structures of ionic polymers used in our investigation
4.3.3.1 UV-vis Absorption and Emission of Polyelectrolyte Complex
Figure 4.3.1 showed the UV-vis absorption and emission spectra of a dilute aqueous solution (5 àM) of PPE-NEt3Br in the presence of PAANa with different concentrations. In Figure 4.3.1, the absorption peak of PPE-NEt3Br broadened and red shifted firstly according to the increase of [PAANa] and the corresponding maximum
reached the reddest at 387 nm when [PAANa]/[PPE-NEt3Br] = 1.2. After further increasing the amount of PAANa, the absorption maximum blue shifted and recovered to the initial position at 381 nm. Such a variation of absorption peaks may be highly related to the variation of conjugation in the complexation. As Figure 4.3.1 displayed, with the addition of PAANa into the aqueous solution of PPE-NEt3Br, the pristine emission peak at 446 nm slightly red shifted to 453 nm and its fluorescence intensity steadily decreased with the [PAANa]/[PPE-NEt3Br] ratio ongoing from 0 to 1.2. It is interesting to find that further increase of the [PAANa]/[PPE-NEt3Br] ratio caused the emission maximum to return back to its original position and the fluorescence intensity to increase over the original one, which is well accorded with the corresponding change appeared in absorption spectra. Close association of π-systems of a conjugated polymer may induce a red-shifted aggregation-dominated emission spectrum and often cause a substantial decrease in PL quantum yield relative to isolated polymer chains.
Thus, the spectra change shown in Figure 4.3.1 is believed to originate from the aggregation of PPE-NEt3Br main chains. The results suggested that the complex formation between oppositely charged polymer chains through Coulombic attraction is not a simple bichain combination but a cluster involving more than one PPE-NEt3Br and PAANa chains, which is according to the [PAANa]/[PPE-NEt3Br] ratio. After adding a small amount of PAANa, PAANa chain played a role as an anionic center to make PPE-NEt3Br chains surround them and induce these conjugated chains to form interchain aggregation. When further increasing the amount of PAANa and finally PAANa was much more than PPE-NEt3Br, the complex structure was changed. On the contrary, it could be anticipated that the aggregation of PPE-NEt3Br chains was destroyed and PPE-NEt3Br chain, instead of PAANa, served as the cationic center and was besieged by several PAANa chains. As a result, PPE-NEt3Br chains were
separated with each other and existed in isolated state and recovered their optical properties as single chains. It is noteworthy that when the amount of PAANa continued enhancing, its fluorescence intensity increased obviously and exceeded the pristine one.
Chen reported that adding surfactant into aqueous solution of PPV-SO3- significantly increased its fluorescence intensity because the surfactant could reduce the number of kink defects and frustrate the tendency of polymer chains to self-association.8 Thus, we proposed that the enhancement of fluorescence intensity after adding enough amount of PAANa result from the same reason appeared in the interaction between conjugated polymer and appropriate surfactant.
Figure 4.3.1 UV-vis absorption and emission spectra of PPE-NEt3Br in the presence of PAANa with different concentration
However, further investigation showed that the structure of conjugated polymer-coil polymer complex was drastically influenced by the structure change of side group of the anionic saturated polymers. When PMAANa, instead of PAANa, was chosen to
250 300 350 400 450 500
0.0 0.2 0.4 0.6 0.8 1.0
PL Intensity
PAA = 0 àM PAA = 2 àM PAA = 5 àM PAA = 7.5 àM PAA = 15 àM
Absorption (a. u.)
Wavenumber (nm)
400 450 500 550 600
0 20 40 60 80 100
form the complex with PPE-NEt3Br, the results showed obvious difference with that of PAANa/PPE-NEt3Br complex. In Figure 4.3.2, the absorption peak of PPE-NEt3Br blue shifted gradually and became much narrower accompanied with the increase of the amount of PMAANa and the absorption maximum reached the bluest at 363 nm.
Such an obvious blue-shift of PMAANa/PPE-NEt3Br complex was contrast with the slight red-shift of PAANa/PPE-NEt3Br complex, indicating the existence of different structure between the two complexes. The only structure difference between PAANa and PMAANa is the existence of methyl group in side chain of PMAANa and therefore it was suggested that the existence of methyl group in PMAANa result in a more twisted structure of PPE-NEt3Br main chain and a correspondingly less effective conjugated length and a blue-shift absorption spectra when PMAANa chain entangled with PPE-NEt3Br chain. Meanwhile, the obtained more twisted structure of PPE-NEt3Br main chain and the methyl group itself could efficiently block the formation of interchain aggregation for PPE-NEt3Br through steric effect. While for PAANa, the non-existence of methyl group in its side chain seemed to reduce the steric effect and be beneficial with interchain aggregation of PPE-NEt3Br. It was also found that the absorption peak became narrow after adding PMAANa and we attribute this phenomenon to the reduced conformational disorder through the interaction between PMAANa and PPE-NEt3Br.8 The difference of the emission spectra between PMAANa/PPE-NEt3Br and PAANa/PPE-NEt3Br further demonstrated that the existence of bulky group in side chain significantly influence the structure of complex through electrostatic attraction. In Figure 4.3.2, the emission maxima obviously blue shifted from 446 nm to 419 nm and the obvious vibronic structure was disappeared after adding enough PMAANa, indicating the formation of shorter efficient conjugated length. Meanwhile, the emission spectra of PPE-NEt3Br also showed gradually
enhanced fluorescence intensity after adding PMAANa without suffering the procedure of decreased fluorescence intensity, which is apparent after adding PAANa.
Such an enhanced fluorescence intensity can also be explained by the reduced number of kink defects and the decreased self-association after adding PMAANa, just as what we discussed aon its absorption spectra.
Figure 4.3.2 UV-vis absorption and emission spectra of PPE-NEt3Br in the presence of PMAANa with different concentration
To further clarify the side-group-effect on the complex structure, curves of relative fluorescence intensity (the intensity ratio of PAANa/PPE-NEt3Br or PMAA-PPE-NEt3Br complexes to pure PPE-NEt3Br in aqueous solution) vs.
PAANa:PPE-NEt3Br or PMAANa:PPE-NEt3Br molar ratio ([PAANa]/[PPE-NEt3Br]
or [PMAANa]/[PPE-NEt3Br]) were shown in Figure 4.3.3. When the PAANa:PPE-NEt3Br reached 1.2, the fluorescence intensity decreased to the minimum at 0.2 (compared with the fluorescence intensity of the pristine PPE-NEt3Br in aqueous
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0.0 0.2 0.4 0.6 0.8
1.0 PMAA = 0 àM
PMAA = 3 àM PMAA = 5 àM PMAA = 10 àM PMAA = 12 àM
Absorption (a. u.)
Wavenumber (nm)
400 450 500 550 600
PL Intensity
0 20 40 60 80 100
solution), indicating the formation of strongest aggregation. When the PAANa:PPE-NEt3Br reached above 1.2, the fluorescence intensity increased gradually and reached the highest at [PAANa]:[PPE-NEt3Br] ≈ 3.5, which means that the PAANa (low concentration)-induced aggregation of PPE-NEt3Br was destroyed and nearly three PAANa chains is enough to obtain the saturation of the fluorescence intensity of PPE-NEt3Br through entangling one PPE-NEt3Br chain.
Figure 4.3.3 Curves of relative fluorescence intensity (the intensity ratio of PAANa/PPE-NEt3Br complexes to pure PPE-NEt3Br in aqueous solution) vs.
PAANa:PPE-NEt3Br or PMAANa:PPE-NEt3Br molar ratio
For PMAANa/PPE-NEt3Br complex, no decreased fluorescence intensity was found and the saturated fluorescence intensity was obtained at PMAANa:PPE-NEt3Br ≈ 3.
Meanwhile, the effect of PMAANa on fluorescence enhancement of PPE-NEt3Br (FL/FL0 = 2.4) was higher than that of PAANa (FL/FL0 = 2.0), indicating that such an anionic polymer-induced fluorescence enhancement of conjugated polymers was
0 1 2 3 4 5
0.0 0.5 1.0 1.5 2.0 2.5 3.0
PAANa PMAANa
PL/PL 0
[Anionic polymer]/[PPE-NEt
3Br]
Figure 4.3.4 The Stern-Volmer plot of PAANa/PPE-NEt3Br (5 àM) complex quenched by Fe(CN)64- in aqueous solution with different concentrations of PAANa