THE POLYMERIC THERMAL RELAXATION OF CONJUGATED-GROUPS IN POLYFLUORENES-

DERIVATIVES

Rafael F. Cossiello 1, Gregório C. Faria 2, Eduardo R. deAzevedo 2, Tito J. Bonagamba 2, Teresa D. Z. Atvars 1*

1 Instituto de Química, UNICAMP, CP 6154, CEP 13083-970, Campinas, SP, Brasil – [email protected] [email protected] * 2 Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil – [email protected] , [email protected] , [email protected] .

The thermal relaxation processes in polyfluorene and derivatives is reported. Bulk methods such as DMTA and temperature dependent steady-state spectroscopy were employed to analyze the molecular motions. From the two main transitions observed (glass transition process between 5 and 60 oC and β-relaxation between -90 and -60 oC), it was demonstrated that the first is strongly correlated with the dissociation of a fluorescent emissive interchain complex and deactivation non-radiative pathway and the second relaxation involves movements of the lateral substituents and/or conjugated backbone of the main chain. A kinetic model involving the dissociation of interchains and the decrease of conjugation length can be proposed to explain the spectra profiles in different temperatures.

Introduction

Conjugated have useful electronic, optoelectronic, and photonic properties which are currently being successfully used in various device applications, including emitting diodes, photovoltaic cells, thin film transistors, and electrochromic cells. Derivatization of polyfluorenes with long groups and/or alkoxy ramifications as the copolymerization was the first approach to obtain conjugated soluble electroluminescent that combine the wide spectral emission range common to the organic molecular compounds, with easy processability and good mechanical properties. Solubility after derivatization is due to the lowering of the interchain interactions 1-3.An understanding of the photoluminescence (PL) properties in the solid state is a matter of particular interest due to its correlation with the efficiency of electroluminescence devices. In this work we present a study of the polymeric relaxations using dynamic-mechanical thermo-analysis (DMTA) and temperature variable steady-state fluorescence of polyfluorenes derivatives. Polyfluorenes-derivatives present a strong morphology dependent of processing conditions. In that way, the interaction -solute rules the formation of crystalline cells (nematic and mesophases) arrangements.

Experimental

Materials

Polyfluorenes derivatives were purchased from ADS Company (American Dyes Source, Quebec, Canada) and were used as received according chemical structures inside in figure 1. Poly[(9,9-dioctyl-2,7-divinylenefluorenylene)] (Blue Emitter 329)

MM: 40.000 – 80.000 g/mol n

BE329 C H C H 8 17 8 17 Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4- vinylenephenylene)] (Green Emitter 128)

MM: 60.000 – 200.000 g/mol n

GE128 C H C H 8 17 8 17 Poly[{9,9-dioctyl-2,7-divinylene- fluorenylene}-alt-co-{2-methoxy-5-(2- ethylhexyloxy)-1,4-phenylene}] (Green Emitter 108) O

MM: 40.000 – 80.000 g/mol n

GE108 C H C H 8 17 8 17 MeO Poly[9,9-dioctylfluorenyl-2,7-diyl)-co-1,4- S N N benzo-{2,1’-3}-thiadiazole)] (Yellow Emitter

133) MM: 10.000 – 30.000 g/mol (can not n

C H C H prepare auto-sustained films); YE133 8 17 8 17 Poly[(9,9-dioctyl-2,7-divinylenefluorenylene)- alt-co-(9,10-)] (Red Emitter 106)

MM: 50.000 – 200.000 g/mol n

C8H17 C8H17 RE106 Figure 1: Chemical structure, names and molecular mass of polifluorenes derivatives.

Chloroform was supplied from Vetec 99.8% of purity, Brazil. The were dried using molecular sieves (4 Å). Fresh solutions were used for each film preparation. Films were cast in a

Anais do 9 o Congresso Brasileiro de Polímeros saturated atmosphere of chloroform during 1 hour and dried in dynamic-vacuum oven during 12 hours at 60 oC.

Methods

Steady-state fluorescence spectroscopy was performed using a system described elsewhere. 4 Samples were excited by a Cd-He laser whose excitation wavelength of 22 624 cm -1 ( 442 nm line emission). Films (1x1 cm) were inserted between two quartz windows held in an optical support. The optical holder within the cryo-system (APD Cryogenics) was positioned to minimize the scattering of light from the excitation beam in a way to stay immobile. The samples were maintained under a dynamic vacuum of 10 -4 Torr during the measurements. Temperature was changed from -260 to 140 oC in steps of 10 oC by a digital temperature controller (Scientific Instruments model 9650) and allowed to attain equilibrium for 5 min between each measurement. One spectrum was recorded at each temperature, and because all of the experimental conditions were maintained constant, the emission intensities were comparable over the entire experiment. Dynamic mechanical thermal analysis DMTA 242-Netzsch Thermisch Analyse equipment was used to perform the thermal analyses. The dynamic mechanical characterization (DMTA) was performed in the tension mode, under a steady load of 1.78 MPa and a dynamic stress of 1.98 MPa, under a nitrogen flow of 50 mL/min. The scans were made over the temperature interval of -150 to 170 oC at a heating rate of 2 oC/min at frequency of 1.0 Hz. Each sample was cut from chloroform cast films to a small rectangular sample, 10 mm long, 4.5-5.2 mm wide, and about 0.02 mm thick.

Results and Discussion

Temperature-dependence Fluorescence Spectroscopy

The heating process above glass transition involves the dissociation of interchain species. Preceding the Tg process, these polymers undergo a secondary relaxation process around -60 oC that we called β-relaxation 5. This is the temperature where some unexplained behavior of several polymers electrical and optical properties can be noted. Table 1 depicts the photoluminescence (PL) spectra (column A) in different temperatures, from -260 (blue curve) to 140 oC (red curve) showing the fluorescence intensity as function of wavelength ( λ) in cast films prepared by chloroform solutions. The column B shows the integrated spectral area to evidence the fluorescence thermal relaxation.

Anais do 9 o Congresso Brasileiro de Polímeros Table 1: Steady-state fluorescence spectra and its normalized integrated spectral area analysis of polyfluorene derivatives.

Fluorescence Spectra (column A) Integrated Spectral Area (column B) Polymer

150000 1,0 15K 415K 0,8

100000 T = 85 oC g 0,6

0,4 o Tβ = -75 C 50000 Fluorescence / a.u. Fluorescence BE329 0,2 Normalizedintegral spectra

0,0 0 420 440 460 480 500 520 -250 -200 -150 -100 -50 0 50 100 150 ο λ / nm Temperature / C

140000 1,0 o 15K T = 130 C 415K m 120000

0,8 100000

0,6 80000

60000 0,4 T = -60 oC β Fluorescencea.u. /

128GE 40000 0,2 Normalizedspectra integral

20000 o Tg = 70 ± 30 C 0,0 0 -250 -200 -150 -100 -50 0 50 100 150 460 480 500 520 540 560 580 o Temperature / C λ / nm

1,0 120000 15K 415K 0,8 100000

o 80000 T = -90 ± 30 C 0,6 β

60000 0,4

o Fluorescence / a.u. Fluorescence / 40000 Tm = 90 C GE 108

0,2 o Normalizedintegral spectra 20000 Tg = 60 C

0 0,0 500 520 540 560 580 600 620 -250 -200 -150 -100 -50 0 50 100 150 λ / nm Temperature / oC

Anais do 9 o Congresso Brasileiro de Polímeros 140000 1,0

120000 15K 415K 0,8 100000

0,6 80000

60000 0,4 Fluorescence / a.u. Fluorescence/

YE 133 40000 o Normalizedintegral spectra T = -80 C 0,2 β

20000

0,0 0 -250 -200 -150 -100 -50 0 50 100 150 520 540 560 580 600 620 Temperature / oC λ / nm

15K 1,0 100000 415K

0,8 80000

0,6 60000

0,4 o 40000 o T = 130 C T = -90 C m β RE106 Fluorescence / a.u. Fluorescence / o Normalized spectra integral Normalizedspectra 0,2 T = 70 C 20000 g

0,0 0 -250 -200 -150 -100 -50 0 50 100 150 520 540 560 580 600 620 640 ο λ / nm Temperature / C

The fluorescence spectra of BE329 show three characteristic picks at -250 oC: 446 nm, 470 nm and 477 nm and a shoulder at 458 nm. These three characteristic picks are narrow in low temperatures, but when the temperature is increased, the intensity at 446 nm decreases and this peak is blueshifted to 443 nm, while the picks in 470 nm, 477 nm and the “shoulder” at 458 nm become an only band centered at 464 nm. The transition at -75 oC evidences the movements of lateral chain (octyl side-group), while the transition at 85 oC evidences the glass transition of the polymer. The BE329 polymer adopts a number of distinct nearplanar type conformational . 6 One of these conformational sequences is an unusual low-energy absorption and emission band known as the β- phase (metastable crystalline) at 430 nm and can be originate by choosing solvent characteristics. This means that segments of highly extended conjugated conformation in the crystalline state contribute insignificantly to optical emission due to longer lifetime and poorer efficiency. 7-9 The fluorescence spectra of GE128 show two characteristic picks at -250 oC: 488 nm and 530 nm. When the temperature is increased, the intensity of the peak at 488 nm decreases and is blueshifted to 477 nm as the same with the peak in 530 nm to 512 nm. The transition at -60 oC evidences movements of the lateral chain, while the transition at 130 oC evidences the process of

Anais do 9 o Congresso Brasileiro de Polímeros glass transition. A more extended and conjugated conformation was found in this film comparing to the BE329 . When the temperature is increased, the intensity of the peak at 530 nm decreases and is blueshifted to 524 nm and the intensity of the peak at 570 decreases turning a “shoulder” of main peak. The blueshift effect and the icrease of fluorescence intensity can be explained not only for the kT Boltzmann population (inhomogeneous broadening) but the homogeneous broadening that are responsible to break the transition dipole . Futhermore, the temperature influences in fluorescence spectra reveals the rupture of aggregate structure when the temperature is increased.5 Differently that occurs in other polyfluorene derivatives, the fluorescence intensity of YE133 spectra decrease when the temperature increased, the peak intensity at 540 nm decreases and is redshifted to 540 nm and the “shoulder” at 575 nm disappears. The transition at -80 oC evidences movements of the lateral chain of the octyl side-group. The falling down of fluorescence intensity means that the aggregate could not break to increase the fluorescence quantum yield with increasing of the temperature. The molecular weight influences the spectral properties and selects the exciton emission (blue or red edge spectra) of isolated chains in a bimodal emission of polyfluorene derivatives. The red forms have been assigned to polymer chains that have intra- or interchain contacts.6 The fluorescence spectra of RE106 show a peak in -250 oC at 573 nm. When the temperature is increased, the peak intensity decreases and is blueshifted to 570 nm with the rupture of aggregate structure. The transition at -90 oC evidences movements of the lateral chain, while the transition at 70 oC evidences the process of glass transition. The process at 130 oC can be characterized as melting transition of this polymer. The polyfluorene forms have disordered organization in the as-coated film, and ordered phase in the crystallized film. The strength of electron-phonon interaction is enhanced with increasing temperature.10, 11 The annealing process can improve irreversibly the excitons mobility, reduce the energetic disorder and polaron binding energy, both of which may result from crystallization of the material. 12 Exciton migration and conformational relaxation (twisting of part of the chain), occur within polyfluorene polymers. The first process is dominant and temperature independent, but as the chain length is decreased, exciton migration is eliminated and the conformational relaxation becomes the principal mechanism by which excited state energy relaxation to the emission site can occur. 13

Dynamic-Mechanical Thermo-Analysis (DMTA)

Anais do 9 o Congresso Brasileiro de Polímeros Table 2 depicts the Dynamic-Mechanical Thermo-Analysis (DMTA) showing the Young modulus ( loss module (red curve) (E’’), storage module (blue curve) (E’) and tan δ (black curve)) in different temperatures, from -130 to 110 oC in cast films prepared by chloroform solutions. The column “Polymeric transitions” shows the temperature of thermal relaxation obtained in DMTA.

Table 2: Normalized Dynamic-Mechanical Thermo-Analysis (DMTA) of polyfluorene derivatives.

DMTA Polymeric transitions Polymer

DMTA BE329

o 1,0 Tg = 85 C 1,0

0,8 0,8 o T = -75 oC Tβ = -75 C 0,6 β 0,6 tan

δ 0,4 0,4

BE329 o

Normalized Modulus Storage Modulus Tg = 85 C Loss Modulus 0,2 0,2 Tan δ

0,0 0,0 -100 -50 0 50 100 150 200 Temperature / oC

DMTA A128GE

100 1 T =130 oC c

T =70 oC o T = - 60 oC g 10 T = -60 C β β

0,1 tan

Storage Modulus δ Loss Modulus 1 128GE Tan δ o

Normalizedmodulus Tg = 70 C

0,01

0,1

-100 -50 0 50 100 150 Temperature / oC

Anais do 9 o Congresso Brasileiro de Polímeros DMTA 108GE

T = 90 oC m 1 o Tg= 60 ± 20 C

o T = - 90 ± 20 C 10 β o Tβ = -90 C δ

0,1 tan 1 Storage modulus

GE 108 o Normlizedmodulus Loss modulus Tg = 60 C Tan δ

0,01 0,1 -100 -50 0 50 100 Temperature / oC

Can not prepare auto-sustained films Can not prepare auto-sustained films YE 133

DMTA A106RE

T = 150 oC 1 m 1

Storage Modulus Loss Modulus o Tβ = -70 C Tan δ T = 70 oC g 0,1 δ T = -70 ± 30 oC

β tan

RE106 o 0,1 Tg = 70 C Normalized Modulus 0,01

-100 -50 0 50 100 150 Temperature / oC

The Dynamic-Mechanical Thermo-Analysis is the one of most sensible technique to analyze the thermal relaxations in polymers. 14 The Yang loss module show a plate in temperature range of - 70 oC. All the polyfluorene derivatives showed a polymeric relaxation between -60 and -90 oC, which can be characterized as Tβββ side-groups relaxations as the alkyl groups movements (octyl side- group). The loss module in the range between 60 and 85 oC is very pronounced and can be characterized as glass transition of this polymer. Do not was possible to prepare auto-sustainable films with the polymer YE133 because the is small if comparing with the other polymers that difficult to acquire results using the Dynamic-Mechanical Thermo-Analysis. The table 3 show the polymeric thermal transition of polyfluorene derivatives.

Anais do 9 o Congresso Brasileiro de Polímeros Table 3: Polymeric thermal transition of polyfluorene derivatives ( oC). Thermal Transition of Polymers / oC Transition Characterization BE329 GE128 GE108 YE133 RE106 Tβββ Side-group -75 -60 -90 -80 -90 Tg Transition glass 85 70 60 - 70 Tm Melting - 130 90 - 130

Conclusions

Two relaxation processes were identified in solid-state polyfluorene derivatives. DMA and fluorescence spectroscopy showed that the glass transition takes place in the 5 – 60 oC involving the dissociation of interchain species while the secondary relaxation process around -60 oC, called β-relaxation, involves the side-groups movements. These results suggest that this is a very complex process involving the conjugated substituents of the polymeric chain.

Acknowledgments T.D.Z.A. and R.F.C. thank FAPESP, CNPq and MCT/PADCT/IMMP (Instituto do Milênio de Materiais Poliméricos) for the financial support and a fellowship.

References 1. Chen, Z. K.; Lee, N. H. S.; Huang, W.; Xu, Y. S.; Cao, Y., New phenyl-substituted PPV derivatives for polymer light-emitting diodes - Synthesis, characterization and structure-property relationship study. Macromolecules 2003, 36, (4), 1009-1020. 2. Nguyen, T. Q.; Yee, R. Y.; Schwartz, B. J., Solution processing of conjugated polymers: the effects of polymer solubility on the morphology and electronic properties of semiconducting polymer films. Journal of Photochemistry and Photobiology a-Chemistry 2001, 144, (1), 21-30. 3. Cossiello, R. F.; Akcelrud, L.; Atvars, D. Z., Solvent and molecular weight effects on fluorescence emission of MEH-PPV. Journal of the Brazilian Chemical Society 2005, 16, (1), 74-86. 4. Talhavini, M.; Atvars, T. D. Z., Modificações em um espectrofluorímetro para o uso dedicado ao estudo de relaxações em polímeros. Quimica Nova 1995, 18, 298-300. 5. Cossiello, R. F.; Kowalski, E.; Rodrigues, P. C.; Akcelrud, L.; Bloise, A. C.; deAzevedo, E. R.; Bonagamba, T. J.; Atvars, T. D. Z., Photoluminescence and relaxation processes in MEH-PPV. Macromolecules 2005, 38, (3), 925-932. 6. Grey, J. K.; Kim, D. Y.; Donley, C. L.; Miller, W. L.; Kim, J. S.; Silva, C.; Friend, R. H.; Barbara, P. F., Effect of temperature and chain length on the bimodal emission properties of single polyfluorene copolymer molecules. Journal of Physical Chemistry B 2006, 110, (38), 18898-18903. 7. Ariu, M.; Sims, M.; Rahn, M. D.; Hill, J.; Fox, A. M.; Lidzey, D. G.; Oda, M.; Cabanillas-Gonzalez, J.; Bradley, D. D. C., Exciton migration in beta-phase poly(9,9-dioctylfluorene). Physical Review B 2003, 67, (19). 8. Cheun, H.; Tanto, B.; Chunwaschirasiri, W.; Larson, B.; Winokur, M. J., Near-term aging and thermal behavior of polyfluorene in various aggregation states. Applied Physics Letters 2004, 84, (1), 22-24. 9. Grell, M.; Bradley, D. D. C.; Ungar, G.; Hill, J.; Whitehead, K. S., Interplay of physical structure and photophysics for a liquid crystalline polyfluorene. Macromolecules 1999, 32, (18), 5810-5817. 10. Asada, K.; Kobayashi, T.; Naito, H., Temperature dependence of photoluminescence in polyfluorene thin films - Huang-Rhys factors of as-coated, annealed and crystallized thin films. Thin Solid Films 2006, 499, (1-2), 192-195. 11. Anni, M.; Caruso, M. E.; Lattante, S.; Cingolani, R., The role of excitons' quasiequilibrium in the temperature dependence of the poly(9,9-dioctylfluorene) beta phase photoluminescence. Journal of Chemical Physics 2006, 124, (13). 12. Kreouzis, T.; Poplavskyy, D.; Tuladhar, S. M.; Campoy-Quiles, M.; Nelson, J.; Campbell, A. J.; Bradley, D. D. C., Temperature and field dependence of hole mobility in poly(9,9-dioctylfluorene). Physical Review B 2006, 73, (23). 13. Vaughan, H. L.; Dias, F. M. B.; Monkman, A. P., An investigation into the excitation migration in polyfluorene solutions via temperature dependent fluorescence anisotropy. Journal of Chemical Physics 2005, 122, (1). 14. Vegt, A. K. v. d., From Polymers to Plastics . Delfit University Press: Delft, Netherlands, 2005; p 279.

Anais do 9 o Congresso Brasileiro de Polímeros