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Synthesis and optical properties of a novel polyfluorene derivative
Synthetic Metals 139 (2003) 491–495
Synthesis and optical properties of a novel polyfluorene derivative C. Vijila a,1 , H.-K. Kyyrönen b , M. Westerling a , R. Österbacka a,∗ , T. Ääritalo b , J. Kankare b , H. Stubb a a
Department of Physics, Åbo Akademi University, Turku, Finland Department of Chemistry, University of Turku, Turku, Finland
b
Received 30 December 2002; received in revised form 15 April 2003; accepted 22 April 2003
Abstract We report the synthesis and optical properties of a novel polyfluorene derivative: poly(3,5-dimethylphenyl ester of 4-(9-methyl-9H-fluorene-9-yl)-butane-1-sulfonic acid) (PFS). Optical spectroscopies such as absorption, photoluminescence, cw and transient photoinduced absorption (PA) techniques were used to study the optical properties. The absorption spectrum shows a weaker band at 3.5 eV and also contains strong bands peaked at 3.95, 4.1 and 4.3 eV. The PL spectrum shows a structureless band centered at 2.8 eV. The PA spectrum at 80 K shows a broad high energy (HE) band peaked at 2.1 eV and three low energy (LE) bands located at 0.5, 0.71 and 1.05 eV. The recombination dynamics of the photoexcitations at 2.1 eV was studied by measuring the intensity and frequency dependence of the PA signal and by fitting the results using a bimolecular recombination (BR) process. We have also studied the temperature dependence of the transient decay of the HE PA band from 80 to 290 K. The BR coefficient was approximately constant from 80 to 220 K and showed an Arrhenius temperature dependence with an activation energy of 0.24 eV for T > 240 K. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Polyfluorene derivative; Photoinduced absorption spectroscopy; Bimolecular recombination
1. Introduction In recent years, polyfluorenes and their derivatives have emerged as an attractive class of materials for device applications such as LEDs [1] and solid-state lasers [2,3] due to their efficient electroluminescence (EL) and photoluminescence (PL), good charge-transport properties, and thermal stability [4]. In addition, these polymers show different film morphologies upon thermal or solvent vapor treatment, which have interesting photophysical properties [5,6]. In particular, poly(9,9-dioctylfluorene) (PFO) films show a liquid crystalline (LC) state at ∼170 ◦ C, and by slowly cooling the sample to room temperature one can obtain a crystalline film where the periodicity is governed by the alkyl side chain length [7,8]. These phenomena enrich the device physics with additional properties such as increased carrier mobility, polarized EL, laser action at ∗ Corresponding author. Tel.: +9-358-2215-4923; fax: +9-358-2215-4776. E-mail address: [email protected] (R. Österbacka). 1 Present address: Institute of Materials Research and Engineering, Singapore.
reduced excitation intensities, and spectral narrowing at different wavelengths. It has also been shown that modifications of the side chains strongly influence the solid-state packing of the polymer and also the optical emission of the electroluminescent polymers [9,10]. These considerations have led to the synthesis of new polymers with different side groups in order to improve the device characteristics. In this paper, we report the synthesis and optical properties of a novel polyfluorene derivative: poly(3,5-dimethylphenyl ester of 4-(9-methyl-9H-fluorene-9-yl)-butane-1-sulfonic acid) (PFS). In order to study the optical properties, we have used absorption, photoluminescence, cw and transient PA spectroscopic techniques. We measured the intensity and frequency dependence of PA in order to study the recombination dynamics of the photoexcitations, and found that the excited state dynamics can be explained with a bimolecular recombination (BR) process. The decay of the transient PA signal has also been studied by varying the temperature from 80 to 290 K and fitting the data with a BR process. We found that the BR coefficient is approximately constant for temperatures from 80 to 220 K and shows an Arrhenius dependence for T > 240 K with an activation energy of E = 0.24 eV.
0379-6779/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(03)00334-5
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2. Materials and methods 2.1. Synthesis The polymer PFS was synthesized according to the scheme shown in Fig. 1 and the synthesizing procedures are given below. 2.1.1. 2,7-Dibromo-9-methylfluorene (I) The compound 9-methylfluorene was prepared from fluorene and methanol by using the procedure described by Schoen and Becker [11]. 9-Methylfluorene (1.53 g; 8.5 mmol) and anhydrous iron(III) chloride (75 mg) were dissolved in chloroform (15 ml). The solution was cooled in ice and bromine (0.9 ml; 17 mmol) was added dropwise. After addition, the solution was stirred at room temperature for 1 day and poured into water (20 ml). Sodium thiosulfate was added until the red color of the solution disappeared. The organic phase was extracted, dried over MgSO4 and evaporated. After recrystallization from ethanol–THF, 1.8 g of pure product was obtained. The yield was 63%, mp 141–142 ◦ C which is consistent with the literature value [12]. 2.1.2. Lithium 4-(2,7-dibromo-9-methyl-9H-fluorene-9yl)-butane-1-sulfonate (II) Diisopropylamine (0.7 ml; 5 mmol) was dissolved in THF (2 ml) under nitrogen atmosphere. The solution was
cooled to −78 ◦ C and butyl lithium (2 ml; 2.5 M solution) was added dropwise. After 30 min, the solution of 2,7-dibromo-9-methylfluorene (1.69 g; 5 mmol) in THF (5 ml) was added and the reaction mixture turned bright red. Stirring was continued for 1 h while the temperature was allowed to rise gradually. The reaction mixture was again cooled down to −78 ◦ C and the solution of 1,4-butane sultone (0.68 g; 5 mmol) in THF (5 ml) was added. Stirring was continued under cooling for a few minutes and the reaction mixture was allowed to warm to room temperature. Stirring was continued overnight, the solution was poured into water and washed with diethyl ether. Evaporation of the water solution and drying in vacuum at 50 ◦ C for 2 days gave 1.65 g of the crude product. 1 H NMR (D2 O): 7.34 (2H, s), 7.15 (4H, s), 2.54 (2H, m), 1.64 (2H, m), 1.42 (2H, m), 0.99 (3H, s), 0.49 (2H, m). 2.1.3. 4-(2,7-Dibromo-9-methyl-9H-fluorene-9-yl)butane-1-sulfonylchloride (IIa) Triphenylphosphine (2.1 g; 8 mmol) was dissolved in dichloromethane (10 ml) and the solution was cooled to 0 ◦ C. Sulfurylchloride (0.65 ml; 8 mmol) was slowly added and the resulting yellowish solution was allowed to warm to room temperature. A solution of compound (II) (1.97 g; 4 mmol) in dichloromethane (30 ml) was added and the mixture was stirred at room temperature overnight. The solution was poured into a 1:1 mixture of n-hexane and dichloromethane (400 ml), filtered and evaporated. The title
Fig. 1. Synthetic route for PFS.
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product (1.3 g) was obtained by filtration through a short silica gel column with ethyl acetate/hexane (1:3). The product was used without further characterization in the next step. 2.1.4. 4-(2,7-Dibromo-9-methyl-9H-fluorene-9-yl)-butane1-sulfonic acid 3,5-di-tert-butylphenol ester (III) To a solution of compound (IIa) (1.2 g; 2.4 mmol) and 3,5-di-tert-butylphenol (0.5 g; 2.4 mmol) in dichloromethane (2.5 ml) triethylamine (0.33 ml; 2.4 mmol) was slowly added. The reaction mixture was stirred for 10 min during which time a precipitate was formed. The mixture was poured into a mixture of 9 ml of diethyl ether and 1 ml of dichloromethane. The solution was filtered and the filtrate washed with two 10 ml portions of aqueous Na2 HPO4 (0.44 g/100 ml). The organic phase was dried over MgSO4 and evaporated. Recrystallization from hexane/ethyl acetate (3:1) gave 960 mg of the title product. 1 H NMR (CDCl3 ): 7.52–7.47 (6H, m), 7.31 (1H, t), 6.98 (2H, d), 2.99 (2H, m), 2.02 (2H, m), 1.46 (3H, s), 1.29 (18H, s), 0.75 (2H, m). 2.1.5. Polymerization The polymerization of compound (III) was done by using the nickel coupling reaction. Nickel(II) chloride (28 mg; 0.15 mmol) and zinc dust (323 mg; 4.6 mmol) were dried in an oven. After drying, they were mixed with triphenylphosphine (200 mg; 1.14 mmol) and DMF (20 ml). The mixture was warmed to 80 ◦ C and a solution of compound (III) (2 g; 3 mmol) in DMF (20 ml) was added. Heating was continued overnight and the polymer (IV) (PFS) was separated by filtration and washed with methanol. 2.2. Methods The PFS film was prepared by solution casting from 5 mg/ml chloroform solution on a sapphire substrate and was annealed at 100 ◦ C for 8 h under vacuum. The thickness of the film was obtained by scratching the film surface and measured using AFM (Park Scientific Instruments, Autoprobe CP). The absorption and photoluminescence spectrum of the film (thickness ∼590 nm) was measured using a Hitachi U-3200 spectrophotometer and Perkin-Elmer LS50B Luminescence spectrometer, respectively. The sample was placed inside a continuous gas flow cryostat (Oxford, Optistat Bath 20 mm) and the temperature of the sample was maintained at 80 K. The photoinduced absorption (PA) spectrum was recorded using standard photomodulation techniques, where a modulated pump photoexcites the sample and the modulated change, T, in the optical transmission T is probed [13]. The cw Ar+ laser (multi-line UV with λmax ∼ 360 nm) was used as an excitation source and the laser beam was modulated by an acousto-optic modulator (Neos, N35085-3), and focused on a 2 mm × 8 mm area of the sample using a cylindrical lens. The probe light from a 250 W incandescent tungsten lamp was focused on the same spot as the laser beam using a spherical mirror and collected into a monochromator (Acton, SpectroPro-300i)
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with proper filters, and detected with Si, Ge and liquid nitrogen cooled InSb detectors, depending on the wavelength range. The signal from the detector was amplified using a preamplifier (EG&G Judson, PA-7) and measured using a lock-in (Stanford, SR830) amplifier. The PL signal was measured without the probe beam on the sample and subtracted from the PA signal at each wavelength. The time resolved decay of the transient PA was measured by replacing the Ar+ laser with a pulsed N2 laser (337 nm, pulse width ∼7 ns with ∼250 J/pulse) and using a digital oscilloscope (Tektronix, TDS 680 B) instead of the lock-in amplifier. The digital oscilloscope was used in ac mode, which meant that the internal time constant of the oscilloscope τosc = 22.3 ms, had to be compensated in order to extract the true PA-decay from the signal. The time resolution of the experimental system was 20 s, which was limited by the time constant of the preamplifier Si-detector combination.
3. Results and discussion The absorption and photoluminescence spectra of PFS are shown in Fig. 2. The absorption spectrum shows a weaker band at 3.5 eV and also contains strong bands peaked at 3.95, 4.1 and 4.4 eV. The energy of the –∗ transition depends upon the conjugation length; the absorption energy redshifts with increasing the conjugation length. Therefore, we assign the weak absorption band at 3.5 eV, to the absorption of long conjugated segments and the absorption from 3.8 to 4.8 eV, to the absorption of the shorter conjugated segments, i.e. oligomers. The PL spectrum shows a structureless band centered at 2.8 eV, which again indicates the oligomeric nature of the material with different conjugation lengths. The PA spectrum of PFS is shown in Fig. 3. The PA spectrum shows a broad high energy (HE) band peaked at 2.1 eV and three bands located in the low energy (LE) region at 1.05, 0.71 and 0.5 eV. In the earlier reports on polyfluorenes, the PA bands near 0.5 and 2 eV have been assigned to the P1 and P2 transitions of polarons, respectively [14,15]. We accordingly assign the HE PA band to the P2 transition and the
Fig. 2. Absorption and photoluminescence spectra of PFS.
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Fig. 3. PA spectrum of PFS measured at 80 K (film thickness d = 0.8 × 10−4 cm, modulation frequency f = 133 Hz, excitation wavelength ∼360 nm, and laser intensity 222 mW/cm2 ).
LE PA bands to the P1 transition. It has been demonstrated that the polaron (P1 ) absorption depends on the conjugation length of the polymer irrespective of the conjugated backbone system; n is inversely proportional to the energetic position of P1 , where n is the number of the conjugation rings in the polymer [16]. From this relation, we calculated the conjugation length from the LE PA bands, obtaining n = 5, 6 and 10 units, which is in good agreement with the absorption and PL spectral results, indicating the presence of polymer segments with different conjugation lengths. The pump intensity and frequency dependence of the PA signal at 2.1 eV is shown in Fig. 4(a) and (b), respectively. The intensity dependence shows a crossover from linear to square-root behavior at higher intensities. This is the characteristic feature of BR and hence we fitted the intensity and frequency dependence with a BR process using Eqs. (1) and (2) [17]: T GτB σd (1) = T RAD 2 (ω0 τB )2 + (ωτB )2 ω0 τB ≡ − 21 ((ωτB )2 − 2) + 21 ((ωτB )2 + 2)2 − 2
(2)
The term τ B is defined as τB = (βG)−1/2 , where β is the BR coefficient, G is the generation which is directly proportional to the laser intensity, σ is the absorption cross-section for the excited states, and d is the thickness of the sample. Eq. (1) is the approximate solution for the case αd < 1, i.e. a thin film. For a thick film (αd > 1) one has to integrate over the sample because the film is now non-uniformly excited. For our film αd ≤ 1 (λ ∼ 360 nm, d = 0.8 m) so the condition αd < 1 is fulfilled. From the fit to Eq. (1), we have calculated the BR coefficient using the values d = 0.8 × 10−4 cm and σ = 1.0 × 10−16 cm2 [13] yielding βI = 0.9 × 10−16 cm3 /s for the intensity dependence and βω = 0.7 × 10−16 cm3 /s for the frequency dependence. The values of βI and βω are in reasonably good agreement with each other. We have also studied the transient decay of the PA band at 2.1 eV for different temperatures ranging from 80 to 290 K. Fig. 5 shows the transient decay of PA for three different
Fig. 4. (a) Intensity dependence of the PA signal at 2.1 eV measured at 80 K using excitation wavelength ∼360 nm and modulation frequency f = 8 Hz. (b) The frequency dependence using the excitation intensity: 833 mW/cm2 . The solid lines are fits of Eq. (1).
temperatures 80, 120 and 260 K. We assume that the initial density of photoexcitations follows Lambert–Beer’s law, n(0, x) = n(0)e−αx , where x is measured from the surface and α is the absorption coefficient of the laser light. Solving the bimolecular rate equation dn(t)/dt = −βn(t)2 and integrating over the sample we get: T(t) σ 1 + n(0)βt − = ln (3) T αβt 1 + n(0)βte−αd The solid lines in Fig. 5 are the fits using Eq. (3) and from the fit we obtained the BR coefficient for all the temperatures. We also fitted all the data without integrating over
Fig. 5. Transient decay of the PA signal at 2.1 eV for three different temperatures. The solid lines are the fits using Eq. (3).
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from the PA spectrum. The recombination dynamics of the HE PA band at 2.1 eV was found to obey BR with the BR coefficient being approximately constant for temperatures from 80 to 220 K, while showing an Arrhenius dependence for T > 240 K with an activation energy of ∆E = 0.24 eV.
Acknowledgements This work was sponsored in part by the Academy of Finland through grants no. 48853 and 50575 and Technology Development Centre in Finland (TEKES), CV acknowledges CIMO for a research grant. Fig. 6. Temperature dependence of the bimolecular recombination. The solid line is a fit using β = β0 exp[−E/kT], yielding E = 0.24 eV.
References the sample, and found that the differences in the BR coefficients were negligible. The BR coefficient obtained for 80 K is βtr = 1.3 × 10−16 cm3 /s which is in good agreement with the values obtained from the frequency and intensity dependence. The variation of the BR coefficient with temperature is shown in Fig. 6. The BR coefficient is almost independent of temperature up to 220 K and is thermally activated for T > 240 K. The solid line is a fit using β = β0 exp[−E/kT], yielding an activation energy E = 0.24 eV. If we assume that we have Langevin type recombination, the BR coefficient can be related to the mobility of the charge carriers through the relation β = eµ/εε0 , where β is BR coefficient, e is the electronic charge (1.602 × 10−19 C), µ is the mobility of charge carriers, ε is the permittivity in free space (8.85 × 10−14 F/cm) and ε0 is dielectric constant of the polymer which is ∼3 taken from the literature [18]. By substituting the value of bimolecular coefficient at 290 K (2.2 × 10−15 cm3 /s), the mobility (µ) at 290 K has been estimated to 4 × 10−9 cm2 /V s. 4. Conclusion We have synthesized and studied the optical properties of a novel polyfluorene derivative using absorption, photoluminescence, cw and transient PA spectroscopic techniques. The absorption and PL spectra indicate the oligomeric nature of the material. We assigned the PA bands to transitions of charged photoexcitations, namely polarons and estimated the effective conjugation lengths to 5, 6 and 10 repeat units
[1] A.W. Grice, D.D.C. Bradley, M.T. Bernius, M. Inbasekaran, W.W. Wu, E.P. Woo, Appl. Phys. Lett. 73 (1998) 629. [2] G. Heliotis, D.D.C. Bradley, G.A. Turnbull, D.W. Samuel, Appl. Phys. Lett. 81 (2002) 415. [3] M.N. Shkunov, R. Österbacka, A. Fuji, K. Yoshino, Z.V. Vardeny, Appl. Phys. Lett. 74 (1999) 1648. [4] M. Redecker, D.D.C. Bradley, M. Inbasekaran, E.P. Woo, Appl. Phys. Lett. 73 (1998) 1565. [5] A.J. Cadby, P.A. Lane, H. Mellor, S.J. Martin, M. Grell, C. Giebeler, D.D.C. Bradley, Phys. Rev. B 62 (2000) 15604. [6] M. Ariu, D.G. Lidzey, D.D.C. Bradley, Synth. Met. 111–112 (2000) 607. [7] S. Kawana, M. Durrell, J. Lu, J.E. Macdonald, M. Grell, D.D.C. Bradley, P.C. Jukes, R.A.L. Jones, S.L. Bennett, Polymer 43 (2002) 1907. [8] D.D.C. Bradley, M. Grell, A. Grice, A.R. Tajbakhsh, D.F. O’Brien, A. Bleyer, Opt. Mater. 9 (1998) 1. [9] Y.K. Nakazawa, S. A Carter, H.-G. Nothofer, U. Scherf, V.Y. Lee, R.D. Miller, J.C. Scott, Appl. Phys. Lett. 80 (2002) 3832. [10] U. Scherf, E.J.W. List, Adv. Mater. 14 (2002) 477. [11] K. Shoen, E. Becker, Org. Synth. Coll. IV, Wiley, New York, 1963, p. 623. [12] H. Wieland, O. Probst, Liebigs Ann. Chem. 530 (1937) 274. [13] Z.V. Vardeny, X. Wei, Handbook of Conducting Polymers, Dekker, New York, 1997. [14] A.J. Cadby, P.A. Lane, M. Wohlgenannt, C. An, Z.V. Vardeny, D.D.C. Bradley, Synth. Met. 111–112 (2000) 515. [15] O.J. Korovyanko, Z.V. Vardeny, Chem. Phys. Lett. 356 (2002) 361. [16] M. Wohlgenannt, X.M. Jiang, Z.V. Vardeny, R.A.J. Janssen, Phys. Rev. Lett. 88 (2002) 197401. [17] M. Westerling, C. Vijila, R. Österbacka, H. Stubb, Chem. Phys. 286 (2003) 315. [18] M. Pope, C.E. Swenberg, Electronic Processes in Organic Crystals and Polymers, Oxford University Press, New York, 1999.
Synthesis and optical properties of a novel polyfluorene derivative C. Vijila a,1 , H.-K. Kyyrönen b , M. Westerling a , R. Österbacka a,∗ , T. Ääritalo b , J. Kankare b , H. Stubb a a
Department of Physics, Åbo Akademi University, Turku, Finland Department of Chemistry, University of Turku, Turku, Finland
b
Received 30 December 2002; received in revised form 15 April 2003; accepted 22 April 2003
Abstract We report the synthesis and optical properties of a novel polyfluorene derivative: poly(3,5-dimethylphenyl ester of 4-(9-methyl-9H-fluorene-9-yl)-butane-1-sulfonic acid) (PFS). Optical spectroscopies such as absorption, photoluminescence, cw and transient photoinduced absorption (PA) techniques were used to study the optical properties. The absorption spectrum shows a weaker band at 3.5 eV and also contains strong bands peaked at 3.95, 4.1 and 4.3 eV. The PL spectrum shows a structureless band centered at 2.8 eV. The PA spectrum at 80 K shows a broad high energy (HE) band peaked at 2.1 eV and three low energy (LE) bands located at 0.5, 0.71 and 1.05 eV. The recombination dynamics of the photoexcitations at 2.1 eV was studied by measuring the intensity and frequency dependence of the PA signal and by fitting the results using a bimolecular recombination (BR) process. We have also studied the temperature dependence of the transient decay of the HE PA band from 80 to 290 K. The BR coefficient was approximately constant from 80 to 220 K and showed an Arrhenius temperature dependence with an activation energy of 0.24 eV for T > 240 K. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Polyfluorene derivative; Photoinduced absorption spectroscopy; Bimolecular recombination
1. Introduction In recent years, polyfluorenes and their derivatives have emerged as an attractive class of materials for device applications such as LEDs [1] and solid-state lasers [2,3] due to their efficient electroluminescence (EL) and photoluminescence (PL), good charge-transport properties, and thermal stability [4]. In addition, these polymers show different film morphologies upon thermal or solvent vapor treatment, which have interesting photophysical properties [5,6]. In particular, poly(9,9-dioctylfluorene) (PFO) films show a liquid crystalline (LC) state at ∼170 ◦ C, and by slowly cooling the sample to room temperature one can obtain a crystalline film where the periodicity is governed by the alkyl side chain length [7,8]. These phenomena enrich the device physics with additional properties such as increased carrier mobility, polarized EL, laser action at ∗ Corresponding author. Tel.: +9-358-2215-4923; fax: +9-358-2215-4776. E-mail address: [email protected] (R. Österbacka). 1 Present address: Institute of Materials Research and Engineering, Singapore.
reduced excitation intensities, and spectral narrowing at different wavelengths. It has also been shown that modifications of the side chains strongly influence the solid-state packing of the polymer and also the optical emission of the electroluminescent polymers [9,10]. These considerations have led to the synthesis of new polymers with different side groups in order to improve the device characteristics. In this paper, we report the synthesis and optical properties of a novel polyfluorene derivative: poly(3,5-dimethylphenyl ester of 4-(9-methyl-9H-fluorene-9-yl)-butane-1-sulfonic acid) (PFS). In order to study the optical properties, we have used absorption, photoluminescence, cw and transient PA spectroscopic techniques. We measured the intensity and frequency dependence of PA in order to study the recombination dynamics of the photoexcitations, and found that the excited state dynamics can be explained with a bimolecular recombination (BR) process. The decay of the transient PA signal has also been studied by varying the temperature from 80 to 290 K and fitting the data with a BR process. We found that the BR coefficient is approximately constant for temperatures from 80 to 220 K and shows an Arrhenius dependence for T > 240 K with an activation energy of E = 0.24 eV.
0379-6779/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(03)00334-5
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2. Materials and methods 2.1. Synthesis The polymer PFS was synthesized according to the scheme shown in Fig. 1 and the synthesizing procedures are given below. 2.1.1. 2,7-Dibromo-9-methylfluorene (I) The compound 9-methylfluorene was prepared from fluorene and methanol by using the procedure described by Schoen and Becker [11]. 9-Methylfluorene (1.53 g; 8.5 mmol) and anhydrous iron(III) chloride (75 mg) were dissolved in chloroform (15 ml). The solution was cooled in ice and bromine (0.9 ml; 17 mmol) was added dropwise. After addition, the solution was stirred at room temperature for 1 day and poured into water (20 ml). Sodium thiosulfate was added until the red color of the solution disappeared. The organic phase was extracted, dried over MgSO4 and evaporated. After recrystallization from ethanol–THF, 1.8 g of pure product was obtained. The yield was 63%, mp 141–142 ◦ C which is consistent with the literature value [12]. 2.1.2. Lithium 4-(2,7-dibromo-9-methyl-9H-fluorene-9yl)-butane-1-sulfonate (II) Diisopropylamine (0.7 ml; 5 mmol) was dissolved in THF (2 ml) under nitrogen atmosphere. The solution was
cooled to −78 ◦ C and butyl lithium (2 ml; 2.5 M solution) was added dropwise. After 30 min, the solution of 2,7-dibromo-9-methylfluorene (1.69 g; 5 mmol) in THF (5 ml) was added and the reaction mixture turned bright red. Stirring was continued for 1 h while the temperature was allowed to rise gradually. The reaction mixture was again cooled down to −78 ◦ C and the solution of 1,4-butane sultone (0.68 g; 5 mmol) in THF (5 ml) was added. Stirring was continued under cooling for a few minutes and the reaction mixture was allowed to warm to room temperature. Stirring was continued overnight, the solution was poured into water and washed with diethyl ether. Evaporation of the water solution and drying in vacuum at 50 ◦ C for 2 days gave 1.65 g of the crude product. 1 H NMR (D2 O): 7.34 (2H, s), 7.15 (4H, s), 2.54 (2H, m), 1.64 (2H, m), 1.42 (2H, m), 0.99 (3H, s), 0.49 (2H, m). 2.1.3. 4-(2,7-Dibromo-9-methyl-9H-fluorene-9-yl)butane-1-sulfonylchloride (IIa) Triphenylphosphine (2.1 g; 8 mmol) was dissolved in dichloromethane (10 ml) and the solution was cooled to 0 ◦ C. Sulfurylchloride (0.65 ml; 8 mmol) was slowly added and the resulting yellowish solution was allowed to warm to room temperature. A solution of compound (II) (1.97 g; 4 mmol) in dichloromethane (30 ml) was added and the mixture was stirred at room temperature overnight. The solution was poured into a 1:1 mixture of n-hexane and dichloromethane (400 ml), filtered and evaporated. The title
Fig. 1. Synthetic route for PFS.
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product (1.3 g) was obtained by filtration through a short silica gel column with ethyl acetate/hexane (1:3). The product was used without further characterization in the next step. 2.1.4. 4-(2,7-Dibromo-9-methyl-9H-fluorene-9-yl)-butane1-sulfonic acid 3,5-di-tert-butylphenol ester (III) To a solution of compound (IIa) (1.2 g; 2.4 mmol) and 3,5-di-tert-butylphenol (0.5 g; 2.4 mmol) in dichloromethane (2.5 ml) triethylamine (0.33 ml; 2.4 mmol) was slowly added. The reaction mixture was stirred for 10 min during which time a precipitate was formed. The mixture was poured into a mixture of 9 ml of diethyl ether and 1 ml of dichloromethane. The solution was filtered and the filtrate washed with two 10 ml portions of aqueous Na2 HPO4 (0.44 g/100 ml). The organic phase was dried over MgSO4 and evaporated. Recrystallization from hexane/ethyl acetate (3:1) gave 960 mg of the title product. 1 H NMR (CDCl3 ): 7.52–7.47 (6H, m), 7.31 (1H, t), 6.98 (2H, d), 2.99 (2H, m), 2.02 (2H, m), 1.46 (3H, s), 1.29 (18H, s), 0.75 (2H, m). 2.1.5. Polymerization The polymerization of compound (III) was done by using the nickel coupling reaction. Nickel(II) chloride (28 mg; 0.15 mmol) and zinc dust (323 mg; 4.6 mmol) were dried in an oven. After drying, they were mixed with triphenylphosphine (200 mg; 1.14 mmol) and DMF (20 ml). The mixture was warmed to 80 ◦ C and a solution of compound (III) (2 g; 3 mmol) in DMF (20 ml) was added. Heating was continued overnight and the polymer (IV) (PFS) was separated by filtration and washed with methanol. 2.2. Methods The PFS film was prepared by solution casting from 5 mg/ml chloroform solution on a sapphire substrate and was annealed at 100 ◦ C for 8 h under vacuum. The thickness of the film was obtained by scratching the film surface and measured using AFM (Park Scientific Instruments, Autoprobe CP). The absorption and photoluminescence spectrum of the film (thickness ∼590 nm) was measured using a Hitachi U-3200 spectrophotometer and Perkin-Elmer LS50B Luminescence spectrometer, respectively. The sample was placed inside a continuous gas flow cryostat (Oxford, Optistat Bath 20 mm) and the temperature of the sample was maintained at 80 K. The photoinduced absorption (PA) spectrum was recorded using standard photomodulation techniques, where a modulated pump photoexcites the sample and the modulated change, T, in the optical transmission T is probed [13]. The cw Ar+ laser (multi-line UV with λmax ∼ 360 nm) was used as an excitation source and the laser beam was modulated by an acousto-optic modulator (Neos, N35085-3), and focused on a 2 mm × 8 mm area of the sample using a cylindrical lens. The probe light from a 250 W incandescent tungsten lamp was focused on the same spot as the laser beam using a spherical mirror and collected into a monochromator (Acton, SpectroPro-300i)
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with proper filters, and detected with Si, Ge and liquid nitrogen cooled InSb detectors, depending on the wavelength range. The signal from the detector was amplified using a preamplifier (EG&G Judson, PA-7) and measured using a lock-in (Stanford, SR830) amplifier. The PL signal was measured without the probe beam on the sample and subtracted from the PA signal at each wavelength. The time resolved decay of the transient PA was measured by replacing the Ar+ laser with a pulsed N2 laser (337 nm, pulse width ∼7 ns with ∼250 J/pulse) and using a digital oscilloscope (Tektronix, TDS 680 B) instead of the lock-in amplifier. The digital oscilloscope was used in ac mode, which meant that the internal time constant of the oscilloscope τosc = 22.3 ms, had to be compensated in order to extract the true PA-decay from the signal. The time resolution of the experimental system was 20 s, which was limited by the time constant of the preamplifier Si-detector combination.
3. Results and discussion The absorption and photoluminescence spectra of PFS are shown in Fig. 2. The absorption spectrum shows a weaker band at 3.5 eV and also contains strong bands peaked at 3.95, 4.1 and 4.4 eV. The energy of the –∗ transition depends upon the conjugation length; the absorption energy redshifts with increasing the conjugation length. Therefore, we assign the weak absorption band at 3.5 eV, to the absorption of long conjugated segments and the absorption from 3.8 to 4.8 eV, to the absorption of the shorter conjugated segments, i.e. oligomers. The PL spectrum shows a structureless band centered at 2.8 eV, which again indicates the oligomeric nature of the material with different conjugation lengths. The PA spectrum of PFS is shown in Fig. 3. The PA spectrum shows a broad high energy (HE) band peaked at 2.1 eV and three bands located in the low energy (LE) region at 1.05, 0.71 and 0.5 eV. In the earlier reports on polyfluorenes, the PA bands near 0.5 and 2 eV have been assigned to the P1 and P2 transitions of polarons, respectively [14,15]. We accordingly assign the HE PA band to the P2 transition and the
Fig. 2. Absorption and photoluminescence spectra of PFS.
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Fig. 3. PA spectrum of PFS measured at 80 K (film thickness d = 0.8 × 10−4 cm, modulation frequency f = 133 Hz, excitation wavelength ∼360 nm, and laser intensity 222 mW/cm2 ).
LE PA bands to the P1 transition. It has been demonstrated that the polaron (P1 ) absorption depends on the conjugation length of the polymer irrespective of the conjugated backbone system; n is inversely proportional to the energetic position of P1 , where n is the number of the conjugation rings in the polymer [16]. From this relation, we calculated the conjugation length from the LE PA bands, obtaining n = 5, 6 and 10 units, which is in good agreement with the absorption and PL spectral results, indicating the presence of polymer segments with different conjugation lengths. The pump intensity and frequency dependence of the PA signal at 2.1 eV is shown in Fig. 4(a) and (b), respectively. The intensity dependence shows a crossover from linear to square-root behavior at higher intensities. This is the characteristic feature of BR and hence we fitted the intensity and frequency dependence with a BR process using Eqs. (1) and (2) [17]: T GτB σd (1) = T RAD 2 (ω0 τB )2 + (ωτB )2 ω0 τB ≡ − 21 ((ωτB )2 − 2) + 21 ((ωτB )2 + 2)2 − 2
(2)
The term τ B is defined as τB = (βG)−1/2 , where β is the BR coefficient, G is the generation which is directly proportional to the laser intensity, σ is the absorption cross-section for the excited states, and d is the thickness of the sample. Eq. (1) is the approximate solution for the case αd < 1, i.e. a thin film. For a thick film (αd > 1) one has to integrate over the sample because the film is now non-uniformly excited. For our film αd ≤ 1 (λ ∼ 360 nm, d = 0.8 m) so the condition αd < 1 is fulfilled. From the fit to Eq. (1), we have calculated the BR coefficient using the values d = 0.8 × 10−4 cm and σ = 1.0 × 10−16 cm2 [13] yielding βI = 0.9 × 10−16 cm3 /s for the intensity dependence and βω = 0.7 × 10−16 cm3 /s for the frequency dependence. The values of βI and βω are in reasonably good agreement with each other. We have also studied the transient decay of the PA band at 2.1 eV for different temperatures ranging from 80 to 290 K. Fig. 5 shows the transient decay of PA for three different
Fig. 4. (a) Intensity dependence of the PA signal at 2.1 eV measured at 80 K using excitation wavelength ∼360 nm and modulation frequency f = 8 Hz. (b) The frequency dependence using the excitation intensity: 833 mW/cm2 . The solid lines are fits of Eq. (1).
temperatures 80, 120 and 260 K. We assume that the initial density of photoexcitations follows Lambert–Beer’s law, n(0, x) = n(0)e−αx , where x is measured from the surface and α is the absorption coefficient of the laser light. Solving the bimolecular rate equation dn(t)/dt = −βn(t)2 and integrating over the sample we get: T(t) σ 1 + n(0)βt − = ln (3) T αβt 1 + n(0)βte−αd The solid lines in Fig. 5 are the fits using Eq. (3) and from the fit we obtained the BR coefficient for all the temperatures. We also fitted all the data without integrating over
Fig. 5. Transient decay of the PA signal at 2.1 eV for three different temperatures. The solid lines are the fits using Eq. (3).
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from the PA spectrum. The recombination dynamics of the HE PA band at 2.1 eV was found to obey BR with the BR coefficient being approximately constant for temperatures from 80 to 220 K, while showing an Arrhenius dependence for T > 240 K with an activation energy of ∆E = 0.24 eV.
Acknowledgements This work was sponsored in part by the Academy of Finland through grants no. 48853 and 50575 and Technology Development Centre in Finland (TEKES), CV acknowledges CIMO for a research grant. Fig. 6. Temperature dependence of the bimolecular recombination. The solid line is a fit using β = β0 exp[−E/kT], yielding E = 0.24 eV.
References the sample, and found that the differences in the BR coefficients were negligible. The BR coefficient obtained for 80 K is βtr = 1.3 × 10−16 cm3 /s which is in good agreement with the values obtained from the frequency and intensity dependence. The variation of the BR coefficient with temperature is shown in Fig. 6. The BR coefficient is almost independent of temperature up to 220 K and is thermally activated for T > 240 K. The solid line is a fit using β = β0 exp[−E/kT], yielding an activation energy E = 0.24 eV. If we assume that we have Langevin type recombination, the BR coefficient can be related to the mobility of the charge carriers through the relation β = eµ/εε0 , where β is BR coefficient, e is the electronic charge (1.602 × 10−19 C), µ is the mobility of charge carriers, ε is the permittivity in free space (8.85 × 10−14 F/cm) and ε0 is dielectric constant of the polymer which is ∼3 taken from the literature [18]. By substituting the value of bimolecular coefficient at 290 K (2.2 × 10−15 cm3 /s), the mobility (µ) at 290 K has been estimated to 4 × 10−9 cm2 /V s. 4. Conclusion We have synthesized and studied the optical properties of a novel polyfluorene derivative using absorption, photoluminescence, cw and transient PA spectroscopic techniques. The absorption and PL spectra indicate the oligomeric nature of the material. We assigned the PA bands to transitions of charged photoexcitations, namely polarons and estimated the effective conjugation lengths to 5, 6 and 10 repeat units
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