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

69 CHAPTER 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 lightemitting 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 investigated5,6 both for their potential applications in opto-electronics1(c) and for fundamental reasons7-10. 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 semiconductor1(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(pphenylenevinylene) (PPV)12 and its soluble derivatives.13 Recently, however, polyfluorenes14(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-dibromo9,9-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 9,9dihexylfluorene-2,7-bis(trimethylene boronate) and corresponding 1,4-dibromo-2,5bis(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 lH and 13 C NMR and elemental analysis. OCH3 OC10H21 [ ] C6H13 C6H13 n [ ] n [ ] C6H13 C6H13 OC10H21 PDHFDDOP C6H13 C6H13 OCH3 PDHFDMOP PDHF COOC2H5 ] [ C6H13 C6H13 n COOC2H5 CN [ ] C6H13 C6H13 PDHFDET Figure 4.1 PDHFCP 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 PerkinElmer LS 50B luminescence spectrometer, respectively. n n 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 PDHF PDHFDDOP PDHFDMOP PDHFDET PDHFCP Table 4.2 λmax, Em. (without methanol) λmax, Em. (with methanol) (nm) (nm) 418 426 415 396 412 382 450 465 410 418 UV-vis absorption spectral parameters of the polymers Polymers PDHF PDHFDET PDHFCP λmax, Abs. (nm) 382 355 369 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 bands. Both bands shift to blue, one of which shifts for 20 nms. a 1.8 1.6 Absorbance 1.4 0% 10% 10% (10 later) 90% 1.2 0.8 90% (10 later) 0.6 0.4 0.2 250 300 350 400 450 500 550 600 650 700 750 800 Wavelength (nm) b 250 Light Intensity 200 0% 10% 150 10% (10 later) 90% 100 90% (10 later) 50 350 400 450 500 550 600 650 700 750 Wavelength (nm) 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 Absorbance 2.5 0% 10% 10% (10 later) 90% 1.5 90% (10 later) 0.5 250 300 350 400 450 500 550 600 650 700 750 800 Wavelength (nm) b 250 Light Intensity 200 150 0% 10% 100 90% (10 later) 10% (10 later) 90% 50 350 400 450 500 550 600 650 700 Wavelength (nm) 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 bands. Both bands shift to blue, one of which shifts for 30 nms. 77 a 0.8 0.7 0% 10% 10% (10 later) 30% 50% Absorbance 0.6 0.5 0.4 70% 90% 90% (10 later) 0.3 0.2 0.1 250 300 350 400 450 500 550 600 650 700 750 800 Wavelength (nm) b 250 0% 10% Light Intensity 200 10% (10 later) 30% 150 50% 70% 90% 100 90% (10 later) 50 300 350 400 450 500 550 600 650 700 Wavelength (nm) 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. a 1.2 Absorbance 0% 10% 0.8 10% (10 later) 30% 50% 70% 90% 0.6 0.4 90% (10 later) 0.2 250 300 350 400 450 500 550 600 650 700 750 800 Wavelength (nm) b 70 Light Intensity 60 0% 50 10% 10% (10 later) 40 30% 50% 30 70% 20 90% 90% (10 later) 10 300 350 400 450 500 550 600 650 Wavelength (nm) 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 0.9 0.8 0% Absorbance 0.7 10% 10% (10 later) 30% 50% 0.6 0.5 0.4 70% 90% 0.3 90% (10 later) 0.2 0.1 250 300 350 400 450 500 550 600 650 700 750 800 Wavelength (nm) b 200 Light Intensity 180 160 0% 140 10% 10% (10 later) 120 30% 50% 100 80 70% 90% 60 90% (10 later) 40 20 350 400 450 500 550 600 650 700 750 Wavelength (nm) 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 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 electronwithdrawing 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 2and 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. [...]... (10 min later) 30% 50% 0.6 0.5 0 .4 70% 90% 0.3 90% (10 min later) 0.2 0.1 0 250 300 350 40 0 45 0 500 550 600 650 700 750 800 Wavelength (nm) b 200 180 Light Intensity 160 0% 140 10% 10% (10 min later) 120 30% 50% 100 80 70% 90% 60 90% (10 min later) 40 20 0 350 40 0 45 0 500 550 600 650 700 750 Wavelength (nm) Figure 4. 6 (a) UV-vis absorption spectra of PDHFCP in chloroform/methanol mixtures Inset: %... of PDHF This is attributed to the decrement of electron density in the backbone of polymer induced by the electronwithdrawing 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 2and 5-positions of phenylene ring can be substituted by different functional groups so that the electronic properties of. .. 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 0.9 0.8 0% Absorbance... 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,... 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... 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, 40 3 (c) Burroughes, J H.; Bradley,... 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... 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... 347 , 539 2 Hide, F.; Diaz-Garcia, M A.; Schwartz, B J.; Heeger, A J., Acc Chem Res., 1997, 30, 43 0 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. .. 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, 1 340 14 (a) Woo, E P.; Inbasekaran, M.; Shiang, W.; Roof, G R., Int Pat Appl WO 97/051 84, 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 . parameters of the polymers Polymers λ max, Em. (without methanol) (nm) λ max, Em. (with methanol) (nm) PDHF 41 8 42 6 PDHFDDOP 41 5 396 PDHFDMOP 41 2 382 PDHFDET 45 0 46 5 PDHFCP 41 0 41 8 . 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. 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