http://www.chemistrymag.org/cji/2004/063019pe.htm |
Mar.21,
2004 Vol.6 No.3 P.19 Copyright |
Chen
Xudong, Wang Xinbo
(Institute
of Polymer Science, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China)
Abstract Kinetics of excimer formation of
sulfonated polystyrene (SPS) with various sulfonation degree has been investgated. The
fluorescence spectrum shows three emission bands at around 285nm, 335nm and 380nm,
corresponding to the monomer, excimer and "excimer-like" bands, respectively. The intramolecular "excimer-like"
emission enhances with the increase of sulfonation degree of SPS. The activation energy
and enthalpy for excimer formation of SPS in dilute solution increase with the increase of
sulfonation degree of SPS. The stability of the excimer increases by raising the
sulfonation degree of SPS.
Keywords Sulfonated polystyrene, Kinetics, Fluorescence, Activation energy, Excimer
1. INTRODUCTION
As a powerful tool for monitoring molecular
motions and ordered structure of macromolecules, excimer fluorescence is the object of
increasing interest. An excimer is a complex formed between an excited and a ground-state
aromatic molecule with a characteristic band in the emission spectrum. With arylvinyl
polymers, two types of emission are observed: the monomer and excimer emissions[1].
The extent of excimer emission is affected by physical condition, the ratio of excimer to
monomer fluorescence intensities(Ie/Im), is the quantity most often used to characterize
the fluorescence properties of vinyl aromatic polymer such as polystyrene. Sulfonated
polystyrene(SPS) has received wide spread academic and industrial interest as its
distinctive properties. The authors have reported that the"excimer-like" emission of sulfonated polystyrene in solution except
monomer and excimer emission is enhanced with the increase of concentration of polymer[2].
Up to now, the kinetics of excimer formation of sulfonated polystyrene has not been
investigated. In the present paper, The authors will give further in sight into the
fluorescence of SPS in solution using fluorescence spectra. The dependence of sulfonation
degree on the fluorescence spectra of SPS has been carefully studied. Detailed analysis of
the temperature dependence of fluorescence emission spectrum and kinetics of excimer
formation of SPS in dilute solution are carried out.
2. EXPERIMENTAL
Polystyrene samples(Mw=2.60〜104, Mw/Mn=3.32)
were obtain from Gaofu Limit. Co.(Zhejiang, China) and characterized by GPC. Sulfonated
polystyrene was prepared by the technique of the literature[3]. Unless
otherwise indicated, all solvents used were analytical grade.
Solution was prepared by weighing polymer into 25ml volumetric flasks
or burettes directly in the case of very high concentration and dilute with solvent. Over
48h it was allowed for dissolution with the sample of so high concentration, a magnetic
stirrer and gentle heating were employed to facilitate dissolution. The temperature
deviation in the measuring cells maintained within 0.1oC at all temperature ranging from 0
to 75oC. Fluorescence spectra were recorded on a Shimadzu RF-540
spectrofluorimeter. The excitation wavelength was 266nm. Spectral correction procedure was
taken according to the literature [4].
3. RESULTS AND DISCUSSION
3.1 Effect of sulfonation degree of SPS on fluorescence
spectra
Fig.1 shows the fluorescence spectra of SPS in
solution with various sulfonation degree in dichloroethane. The monomer and excimer
fluorescence peaks for SPS appear at 285 and 335nm, respectively. The significant red
shift of excimer peak has taken with the increasing of sulfonation degree. The previous
paper has described that the red shift of excimer emission of SPS with the increase of
concentration of SPS solution, is due to the "excimer-like"
emission in the region of 375-385 nm[2]. The "excimer-like" are
excited-state complexes formed by association of a dimer of chromophores and achromophore,
one of chromophores has been electronically excited as with excimer. The dimer is due to
the association of two aromatic groups in the ground states, and dimer formation is
facilitated for sulfo groups. We have found that the"excimer-like" emission increases, monomer and excimer emission decrease as the
sulfonation degree increases. We assume that the chromophores of SPS consist of a mixture
of monomers and dimers formed in the ground states. Excitation of monomers will yield
monomer and excimer emission, whereas excitation of dimers will yield "excimer-like" emission only. With the increase of sulfonation degree of SPS, the
adjacent phenyl groups with sulfonated groups can bind more readily and form more dimers.
In addition, the migration of excitation energy from the excited monomer and excimer to
the excited dimer sites can favor the "excimer-like" emissions, the migration of
excitation energy plays a more important role with the increase of content of sulfo
groups. Furthermore, the monomer emission appears in the highest energy region, the
"excimer-like" emission
appears in a considerably lower energy region than the excimer emission.
Fig.1 Fluorescence spectra of SPS in DCE. SPS solution concentration: 400g/L. Sulfonation degree of SPS(%, from 1 to 4) : 0;0.56;1.96;4.55.
3.2 Effect
of temperature on fluorescence spectra
A different temperature dependence is expected for dimers formed in the ground state
and excimers formed in the excited state. Fig.2 presents the effect of temperature on the
Ie/Im ratio for SPS in dichloroethane(DCE). The change in Ie/Im might be attributed to the
polymer chain contraction. Consequently, it may be concluded that it is due to the change
in polymer conformation. The ratio of excimer and monomer shows a pronounced maximum value
at 23
The "excimer-like" emission shows the similar results, the ratio of "excimer-like" emission to monomer emission(I380/I285) for SPS (sulfonation degree 4.55%) shows the maximum value at 45oC, whereas I380/I285 for SPS (sulfonation degree 1.96%) exists the maximum value at about 28oC. Because the interaction between asymmetric phenyl ring with sulfonated groups enhances with the increase of content of sulfonated groups, this would make it more difficult to take the internal rotations of the skeletal. In addition, there is a significant effect of hydrogen band formation between sulfonated groups of SPS and solvent molecules. This is consistent with the results reported by Himuro[6].
Fig.2 Temperature dependence of the ratio of excimer(or excimer-like) to monomer fluorescence intensity of SPS in DCE. SPS solution concentration(g/L): (a)40; (b)650; (c)650. Ie/Im: (a)I335/I285; (b)I380/I285; (c)I380/I285 . Sulfonation degree of SPS(mol%): (a)4.55;(b)1.96;(c)4.55.
3.3 Kinetic
treatment and thermodynamic data analysis
In general, the ratio of the emission intensities of excimer and monomer depends on
intrinsic and extrinsic characteristics of the system. Here, we only discuss the kinetic
of excimeric formation at low concentration, as the fluorescence spectra of SPS show, at
low concentration,two bands centered at 285nm and 335nm that can be assigned to monomeric
and excimeric emissions respectively.
The intramolecular excimer formation and dissociation are shown as
follows[6]:
Scheme 1
Where M and M* represent
the phenyl chromophore in the ground state and the excited monomeric state, respectively,
and D*《(MM)* is the intramolecular excimer. Rate
parameters are also indicated in the Scheme 1 for each of the
corresponding energy dissipation process. The ratio of the quantum yield of excimer
fluorescence to that of monomer fluorescence, FfD/FfM,
serves as a measurement of an apparent efficiency of excimer formation. Through analysis
with the steady-state approximation, the quantum yield of excimer and monomer can be
represented by the following equations:
FfD=[KDM/(KDM+KnM+KfM)]+[KfD/(KMD+KDM+KfD)]
(1)
FfM=KfM/(KDM+KfM)
(2)
FfD/FfM=(KfD/KfM)[KDM/(KfD+KnD+KMD)]
(3)
In the region of low temperature, where KMD and KnD
are negligibly small compared with KfD, FfD/FfM is almost solely determined by the
association rate constant KDM, since KfM and KfD are
generally independent of temperature. Then, the following equation can be deduced:
dln(Ie/Im)/d(1/T)=dln(FfD/FfM)/dln(1/T)= -EDM/R (4)
Where EDM is the activation energy for excimer formation, R
is the gas constant.
At high temperature, the dissociation of excimer becomes conspicuous
and, therefore the condition KDM>> KfD+KnD
is satisfied. Then, the enthalpy change of excimer formation can be represented by the
following form:
dln(Ie/Im)/d(1/T)=dln(FfD/FfM)/dln(1/T)= -(EDM-EMD)/R=-DH/R (5)
Where EMD is the activation energy for the aissociation of
excimer, DH is the enthalpy chang of excimer formation.
Based on the kinetic treatment described above, the plot of the
logarithm of Ie/Im versus the reciprocal of absolute temperature(1/T) was constructed for
the present system, as shown in Figure3.
Fig.3 Logarthm
plot of the ratio of excimer to monomer fluorescence intensity(Ie/Im) against the
reciprocal of temperature. SPS solution concentration: 400g/L, sulfonation degree of SPS:
4.55%.
From the slope in the low temperature side of the plot, the activation energy(EDM) for the excimer formation can be evaluated. While the slope in the higher temperature region provides the enthalpy change accompanied by the excimer formation. EMD for the dissociation of excimer is calculated by means of the formula EMD=EDM-DH. All the data of EMD, EDM, DH are listed inTab.1. It is well established that as ymmetrical paralleled sandwich arrangement of two aromatic rings with an interplanar spacing of 0.3-0.4nm is the most favorable geometry for a stable excimer, and the diffusion controlled encounter is, in general, the governing mechanism for the intramolecular formation[7]. For the intramolecular excimer formation in polystyrene and sulfonated polystyrene, it has been ascertained that the preferred excimer alignment specified above can be achieved when two aromatic groups are separated by three carbon atoms along the alkaline chain. Then, the rate of excimer formation is directly controlled by the internal rotation of the skeletal methylene chains. Tab.1 shows EDM of excimer formation of SPS increases with the increase of sulfonation degree. It is more plausible that the binding force between sulfonated groups enhances as the content of sulfo groups increases, it is more difficult to take internal rotation of the skeletal chains with the increase of sulfonation degree.
Tab.1 Value of EDM, EMD, DH estimated for sulfonated polystyrene with various sulfonation degrees.
Sulfonation degree(%) |
EDM(kJ·mol-1) |
EMD(kJ·mol-1) |
DH(kJ·mol-1) |
0.56 |
3.2 |
-25.1 |
28.3 |
1.96 |
3.9 |
-28.2 |
32.1 |
4.55 |
5.8 |
-32.1 |
37.9 |
From the data of association enthalpy(
DH) listed inTable1, it is found that the degree of stability of the excimer increases with the increasing of sulfonation degree. It is due to the increase of the binding energy between phenyl groups with sulfonated groups by raising the sulfonation degree of SPS, and the increase of stability for the excimer.4.CONCLUSIONS
The monomer and excimer emission bands of
sulfonated polystyrene appear at about 285 and 335nm, respectively. The significant red
shift of excimer peak with the increasing of sulfonation degree of SPS is due to the
"excimer-like" emission,
centered around 380nm. The "excimer-like"
are excited-state complexes formed by association of a dimer of chromophores and a
chromophore, one of chromophores has been electronically excited.
The"excimer-like" emission
enhances as the sulfonation degree is increased. The activation energy and enthalpy
changes for excimer formation of sulfonated polystyrene in dilution increase as the
sulfonation degree increases. From the EDM, it indicates that the stability of
excimer increases by raising the sulfonation degree of SPS.
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