Spectral stability study and molecular modeling of fluorence based conjugated polymers 4

69
CHAPTER 4 THE STUDY OF AGGREGATION IN
SOLUTIONS FOR SOME POLYFLUORENE DERIVATIVES



70
4-1 Introduction

Conjugated organic polymers play an increasingly important role as organic
semiconductors, due to their superb optical and emissive properties. In the past decade,
they have found applications as emitting layers in LED's,
1
"plastic" lasers,
2
light-
emitting electrochemical cells,
3
and polarizers for LC displays, just to name a few.
Conjugated polymers have uniquely useful characteristics such as ease of
processability and the ability to form large, flexible films. Additionally, facile
manipulation of substituents and backbones enables tuning of their (molecular) band
gap. These properties make them competitive with their inorganic counterparts.
1,4


The optical and electronic properties of
π
-conjugated polymers have been
intensively investigated

5,6
both for their potential applications in opto-electronics
1(c)

and for fundamental reasons
7-10
. So far, mainly the properties of single polymer chains
have attracted attention, thereby neglecting the possibility of interaction between
different polymer chains. This interaction can result in the formation of aggregates.

The discovery of electroluminescence (EL) in a polymer semiconductor
1(c)
has
led to intensive research in this field, and progress is documented in several recent
reviews.
1(b),11
Most attention to date has focused on precursor-route poly(p-
phenylenevinylene) (PPV)
12
and its soluble derivatives.
13
Recently, however,
polyfluorenes
14(a)
have emerged as attractive alternatives, and they have become the
subject of considerable attention.
15-31
The physicochemical properties of polyfluorenes
can be tailored via side chain substitution without substantially changing the electronic

71
properties of the backbone.
17
Band gap and energy level engineering, on the other
hand, can be readily achieved via copolymerization.
14(b),18,19


We now have access to high molecular weight polyfluorene derivatives to
study their aggregation behavior in solutions
. Five representative polyfluorene
derivatives were examined with respect to their absorption and emission in
chloroform/methanol mixtures. The correlation between the backbone structural
modifications for polyfluorene and the spectral stability and the electronic properties
of the resulting polymers will be discussed in this chapter.

4-2 Experimental section

4-2-1 Materials

The chemical structures of five polyfluorene derivatives are shown in Figure
4.1. Poly(9,9-dihexylfluorene-2,7-diyl) (PDHF) was synthesized from 2,7-dibromo-
9,9-dihexylfluorene by nickel(0)-mediated polymerization.
32
Poly[(9,9-
dihexylfluorene)-alt-co-(2,5-dimethoxy-1,4-phenylene)] (PDHFDMOP), poly[(9,9-
dihexylfluorene)-alt-co-(2,5-didecyloxy-l,4-phenylene)] (PDHFDDOP), poly[(9,9-
dihexylfluorene)-alt-co-(2,5-diethylterephthalate)] (PDHFDET), poly[(9,9-

dihexylfluorene)-alt-co-(2-cyano-l,4-phenylene)] (PDHFCP) were prepared from 9,9-
dihexylfluorene-2,7-bis(trimethylene boronate) and corresponding 1,4-dibromo-2,5-
bis(alkoxy)benzenes, 1,4-dibromo-2,5-diethylterephthalate or 1,4-dibromobenzonitrile
through Suzuki coupling reaction as described in previous publication.
33,34
The



72
structures and purity of the polymers were confirmed by
l
H and
13
C NMR and
elemental analysis.

C
6
H
13
C
6
H
13
[]
n
C
6
H

13
C
6
H
13
[
OC
10
H
21
OC
10
H
21
]
n
C
6
H
13
C
6
H
13
[
C
N
]
n
PDHF

PDHFDDOP
PDHFCP
C
6
H
13
C
6
H
13
[
COOC
2
H
5
COOC
2
H
5
]
n
PDHFDET
C
6
H
13
C
6
H
13

[
OCH
3
OCH
3
]
n
PDHFDMOP


Figure 4.1 Chemical structures of five polyfluorene derivatives

4-2-2 Measurement

For mixed solutions preparation, PDHFDDOP, PDHF, PDHFDMOP,
PDHFDET and PDHFCP were dissolved in chloroform/methanol mixtures with the
concentration of 10 mg/L. The UV-visible absorption and photoluminescence (PL)
spectra were recorded on a Shimadzu UV 3101 spectrophotometer and on a Perkin-
Elmer LS 50B luminescence spectrometer, respectively.







73
4-3 Results and discussion

UV-vis absorption and photoluminescence spectra of PDHFDDOP, PDHF,

PDHFDMOP, PDHFDET and PDHFCP in chloroform/methanol mixtures are shown
in Figures 4.2 through 4.6, respectively. Photoluminescence and UV-vis absorption
spectral parameters of the polymers are listed in Table 4.1 and Table 4.2, respectively.

Table 4.1 Photoluminescence spectral parameters of the polymers

Polymers
λ
max, Em.
(without methanol)
(nm)
λ
max, Em.
(with methanol)

(nm)
PDHF 418 426
PDHFDDOP 415 396
PDHFDMOP 412 382
PDHFDET 450 465
PDHFCP 410 418


Table 4.2 UV-vis absorption spectral parameters of the polymers

Polymers
λ
max, Abs.
(nm)
PDHF 382

PDHFDET 355
PDHFCP 369












74
For PDHFDDOP, with the concentration of methanol increasing, the original
peak of UV spectrum shifts to red, and the spectrum become broaden. The peak of PL
spectrum splits into 2 bands. Both bands shift to blue, one of which shifts for 20 nms.

a
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8

250 300 350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
Absorbance
0%
10%
10% (10 min later)
90%
90% (10 min later)









b
0
50
100
150
200
250
350 400 450 500 550 600 650 700 750
Wavelength (nm)
Light Intensit
y
0%
10%

10% (10 min later)
90%
90% (10 min later)

Figure 4.2 (a) UV-vis absorption spectra of PDHFDDOP in chloroform/methanol
mixtures. Inset: % methanol. (b) Photoluminescence spectra of
PDHFDDOP in chloroform/methanol mixtures. Inset: % methanol.




75
For PDHF, with the concentration of methanol increasing, the original peak of
UV spectrum shifts to red slightly, and the spectrum become broaden. The peaks of PL
spectrum shift to red, with a new peak emerging between 380 to 400 nm.

a
0
0.5
1
1.5
2
2.5
3
250 300 350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
Absorbance
0%
10%
10% (10 min later)

90%
90% (10 min later)









b
0
50
100
150
200
250
350 400 450 500 550 600 650 700
Wavelength (nm)
Light Intensity
0%
10%
10% (10 min later)
90%
90% (10 min later)

Figure 4.3 (a) UV-vis absorption spectra of PDHF in chloroform/methanol
mixtures. Inset: % methanol. (b) Photoluminescence spectra of PDHF
in chloroform/methanol mixtures. Inset: % methanol.




76

For PDHFDMOP, with the concentration of methanol increasing, the original
peak of UV spectrum shifts to red, and the spectrum become broaden. When the
concentration of methanol increases to 50 %, a new band emerges at 362 nm in PL
spectrum. When the concentration of methanol increases to 90 %, the peak of PL
spectrum splits into 2 bands. Both bands shift to blue, one of which shifts for 30 nms.

77

a
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
250 300 350 400 450 500 550 600 650 700 750 800
Wavelength (nm)

Absorbance
0%
10%
10% (10 min later)
30%
50%
70%
90%
90% (10 min later)


















b
0
50

100
150
200
250
300 350 400 450 500 550 600 650 700
Wavelength (nm)
Light Intensity
0%
10%
10% (10 min later)
30%
50%
70%
90%
90% (10 min later)

Figure 4.4 (a) UV-vis absorption spectra of PDHFDMOP in chloroform/methanol
mixtures. Inset: % methanol. (b) Photoluminescence spectra of
PDHFDMOP in chloroform/methanol mixtures. Inset: % methanol.


For PDHFDET, with the concentration of methanol increasing, the original
peak of UV spectrum shifts to blue slightly, and the spectrum become broaden. The



78

original peak of PL spectrum shifts to red. When the concentration of methanol
increases to 70 %, a new band starts to emerge at 350 nm in PL spectrum.









250

a

0
0.2
0.4
0.6
0.8
1
1.2
300 350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
Absorbance
0%
10%
10% (10 min later)
30%
50%
70%
90%
90% (10 min later)







b
0
10
20
30

40
50
60
70
300 350 400 450 500 550 600 650
Wavelength (nm)
Light Intensity
0%
10%
10% (10 min later)
30%
50%
70%
90%
90% (10 min later)



Figure 4.5 (a) UV-vis absorption spectra of PDHFDET in chloroform/methanol
mixtures. Inset: % methanol. (b) Photoluminescence spectra of
PDHFDET in chloroform/methanol mixtures. Inset: % methanol.





79
For PDHFCP, with the concentration of methanol increasing, the original peak
of UV spectrum shifts to red, and the spectrum become broaden. The original peak of
PL spectrum shifts to red. When the concentration of methanol increases to 50 %, a

new band starts to emerge at 365.5 nm in PL spectrum. The new peak shifts to red with
further increase of the concentration of methanol.
a
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
250 300 350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
Absorbance
0%
10%
10% (10 min later)
30%
50%
70%
90%
90% (10 min later)

b
0
20
40
60
80
100
120
140
160
180
200
350 400 450 500 550 600 650 700 750
Wavelength (nm)
Light Intensity
0%
10%
10% (10 min later)
30%
50%
70%
90%
90% (10 min later)

Figure 4.6 (a) UV-vis absorption spectra of PDHFCP in chloroform/methanol
mixtures. Inset: % methanol. (b) Photoluminescence spectra of
PDHFCP in chloroform/methanol mixtures. Inset: % methanol.



80

The results mentioned above show that firstly it is evident that methanol
induces aggregation behavior of PDHF solution. It is because the relatively high
planarity of polyfluorene backbone favors the formation of aggregate states between
subunits. Secondly, the extent of aggregation for PDHFDDOP and PDHFDMOP are
less than that for PDHF. This is attributed to the poor planar configuration of the
backbones and the efficient separation of the side chains on phenylene units for
backbones for the 2 polymers. In addition, the extent of aggregation for PDHFDMOP
is less than that for PDHFDDOP. This is probably a consequence of a higher order in
the latter polymer. It is reported that the polymer with the greatest liquid crystalline
order produces the greatest excited state interchain communication. And our previous
experimental results show that PDHFDDOP has higher crystallization tendency than
PDHFDMOP.

For the electron-withdrawing group substituted polymers, the extent of
aggregation for PDHFDET is larger than that for PDHFCP. This may be due to the
strong electron-withdrawing capacity of PDHFDET than that of PDHFCP. When

electron-withdrawing groups, such as cyano (PDHFCP) and ester groups (PDHFDET),
are introduced into the polymer main chain, an obvious blue shift could be observed in
the absorption spectra as compared with that of PDHF. This is attributed to the
decrement of electron density in the backbone of polymer induced by the electron-
withdrawing effect of the substituted groups. The advantage of the backbone of poly
(fluorene-phenylene) over the backbone of polyfluorene is that the positions at the 2-
and 5-positions of phenylene ring can be substituted by different functional groups so
that the electronic properties of the resulting polymers can be tuned. When strong
electron-withdrawing groups, such as cyano (PDHFCP) and ester (PDHFDET) groups,



81
are attached to the phenylene ring, the electronic properties of the resulting polymers
with the new backbone structure can be tuned in a wide range while the polymers are
still blue emissive.

4-4 Conclusions

In conclusion, methanol induces aggregation behavior of PDHF, PDHFDDOP,
PDHFDMOP, PDHFDET and PDHFCP solutions. The backbone structural
modifications for PDHF benefit for the prevention of aggregation behavior, and are
capable of tuning electronic properties of the resulting blue light-emitting polymers.

82
References

1. (a) Neher, D., Adv. Mater., 1995, 7, 691. (b) Kraft, A.; Grimsdale, A. C.;
Holmes, A. B., Angew. Chem., 1998, 37, 403. (c) Burroughes, J. H.; Bradley,
D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.;
Holmes, A. B., Nature, 1990, 347, 539.
2. Hide, F.; Diaz-Garcia, M. A.; Schwartz, B. J.; Heeger, A. J., Acc. Chem. Res.,
1997, 30, 430.
3. Pei, Q. B.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J., Science, 1995, 269,
1086.
4. Ball, P., Made to Measure; Princeton University Press: Princeton, NJ, 1997; p
17-62.
5. Kobayashi, T., ed., Nonlinear Optics of Organics and Semiconductors,
Springer, Berlin, 1989.
6. Etemad, S.; Soos, Z.G., Spectroscopy of Advanced Materials, eds. Clark, R. J.
and Hester, R. E., Wiley, New York, 1991.
7. Su, W. P.; Schrieffer, J. R.; Heeger, A. J., Phys. Rev. Letters, 1979, 42, 1698.

8. Heeger, A. J.; Kivelson, S.; Schrieffer, J. R.; Su, W. P., Rev. Mod. Phys., 1988,
60, 781.
9. Rauscher, U.; Bässler, H.; Bradley, D. D. C.; Hennecke, M., Phys. Rev. B,
1990, 42, 9830.
10. Mukamel, S.; Wang, H. X., Phys. Rev. Letters, 1992, 69, 65.
11. (a) Bradley, D. D. C., Curr. Opin. Solid State Mater. Sci., 1996, 1, 789. (b)
Salbeck, J., Ber. Bunsen-Ges. Phys. Chem., 1996, 100, 1666. (c) Sixl, H.;
Schenk, H.; Yu, N., Phys. Bl., 1998, 54, 225. (d) Greiner, A., Polym. Adv.



83
Technol., 1998, 9, 371. (e) Friend, R. H.; Gymer, R. W.; Holmes, A. B.;
Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D.
A.; Brédas, J. L.; Löglund, M.; Salaneck, W. R., Nature, 1999, 397, 121.
12. Hörhold, H. H.; Helbig, M.; Raabe, D.; Opfermann, J.; Scherf, U.; Stockmann,
R.; Weiss, D. Z., Chem., 1987, 27, 126.
13. Spreitzer, H.; Becker, H.; Kluge, E.; Kreuder, W.; Schenk, H.; Demandt, R.;
Schoo, H., Adv. Mater., 1998, 10, 1340.
14. (a) Woo, E. P.; Inbasekaran, M.; Shiang, W.; Roof, G. R., Int. Pat. Appl. WO
97/05184, 1995. (b) Inbasekaran, M.; Wu, W.; Woo, E. P., US Pat. 5,777,070,
1998.
15. Redecker, M.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P., Appl. Phys.
Lett., 1998, 73, 1565.
16. Grice, A.; Bradley, D. D. C.; Bernius, M. T.; Inbasekaran, M.; Wu, W. W.;
Woo, E. P., Appl. Phys. Lett., 1998, 73, 629.
17. Yang, Y.; Pei, Q., Polym. Prepr., 1997, 38, 335.
18. Ranger, M.; Leclerc, M., Can. J. Chem., 1999, 76, 1571.
19. Grell, M.; Redecker, M.; Whitehead, K. S.; Bradley, D. D. C.; Inbasekaran, M.;
Woo, E. P.; Wu, W., Liq. Cryst., 1999, 26, 1403.

20. Grell, M.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P., Adv. Mater., 1997,
9, 798.
21. Schartel, B.; Wachtendorf, V.; Grell, M.; Bradley, D. D. C.; Hennecke, M.,
Phys. Rev. B, 1999, 60, 277.
22. Grell, M.; Knoll, W.; Lupo, D.; Meisel, A.; Miteva, T.; Neher, D.; Nothofer, H.
G.; Scherf, U.; Yasuda, A., Adv. Mater., 1999, 11, 671.
23. Grell, M.; Bradley, D. D. C., Adv. Mater., 1999, 11, 895.



84
24. Bradley, D. D. C.; Grell, M.; Long, X.; Mellor, H.; Grice, A., Proc. SPIE,
1997, 3145, 254.
25. Grell, M.; Bradley, D. D. C.; Long, X.; Chamberlain, T.; Inbasekaran, M.;
Woo, E. P.; Soliman, M., Acta Polym., 1998, 49, 439.
26. Klärner, G.; Miller, R. D., Macromolecules, 1998, 31, 2007.
27. Janietz, S.; Bradley, D. D. C.; Grell, M.; Giebeler, C.; Inbasekaran, M.; Woo,
E. P., Appl. Phys. Lett., 1998, 73, 2453.
28. Kreyenschmidt, M.; Klärner, G.; Fuhrer, T.; Ashenhurst, J.; Karg, S.; Chen, W.
D.; Lee, V. Y.; Scott, J. C.; Miller, R. D., Macromolecules, 1998, 31, 1099.
29. Klärner, G.; Davey, M. H.; Chen, W. D.; Scott, J. C.; Miller, R. D., Adv.
Mater., 1998, 10, 993.
30. Ranger, M.; Rondeau, D.; Leclerc, M., Macromolecules, 1997, 30, 7686.
31. Ranger, M.; Leclerc, M., Synth. Met., 1999, 101, 48.
32. Pei, Q.; Yang, Y., J. Am. Chem. Soc., 1996, 118, 7416.
33. Yu, W. L.; Pei, J.; Cao, Y.; Huang, W.; Heeger, A. J., Chem. Commun., 1999,
1837.
34. Liu, B.; Yu, W. L.; Lai, Y. H.; Huang, W., Chem. Mater., 2001, 13, 1984.