Photophysical properties of PPP and PPV derivatives bearing polystyrene or polycaprolactone as side groups

European Polymer Journal 45 (2009) 940–945

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Photophysical properties of PPP and PPV derivatives bearing polystyrene or polycaprolactone as side groups Demet Goen Colak a, Daniel Ayuk Mbi Egbe b,*, Eckhard Birckner c, Seda Yurteri a, Ioan Cianga a, Emine Tekin d,e, Ulrich S. Schubert d,e, Yusuf Yagci a,* a

Istanbul Technical University, Department of Chemistry, Maslak 34469, Istanbul, Turkey Institute for Print and Media Technology, Chemnitz University of Technology, Strasse der Nationen 62/B004, D-09111 Chemnitz, Germany c Institute of Physical Chemistry, University of Jena, Lessingstr. 10, 07743 Jena, Germany d Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands e Laboratory of Organic Chemistry and Macromolecular Chemistry, Friedrich-Schiller University Jena, Humboldtstr. 10, 07743 Jena, Germany b

a r t i c l e

i n f o

Article history: Received 15 August 2008 Received in revised form 27 October 2008 Accepted 18 November 2008 Available online 6 December 2008

Keywords: Poly(p-phenylene-vinylene) Poly(p-phenylene) Polystyrene Poly(e-caprolactone) Photophysical properties

a b s t r a c t This contribution reports on detailed photophysical investigations of poly(p-phenylene) PPP and poly(p-phenylene-vinylene) (PPV) derivatives laterally decorated with polystyrene (PPV-PSt) or poly(e-caprolactone) (PPP-PCL, PPP-altPCL, PPV-PCL and PPV-PCL-Br). The polymers emit blue and exhibit very high relative and absolute photoluminescence quantum yield, Uf, in dilute solution, thin film (spin-coated and inkjet-printed) and bulk state. This is ascribed to the presence of the lateral macromolecules, which suppress the strong p–p interactions and consequently excimers formation. Lower Uf value was obtained for the bromine containing polymer and its corresponding model compound dibromodistyrylbenzene Br2-DSB, which was ascribed to heavy atom effect enabling intersystem crossing from S1 to T1. However, studies at 77 K did not reveal phosphorescence, in contrast an enhancement of the fluorescence intensity with respect to room temperature measurement was observed. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Conjugated polymers have been widely studied as materials due to their potential use in a wide range of practical applications such as information storage, optical signal processing, substitutes for batteries [1,2] and solar energy conversion [3]. Furthermore, electroluminescence [4–6] from conjugated polymers is a rapidly increasing research interest since the first report of polymeric lightemitting diodes (PLEDs) based on poly(p-phenylene-vinylene) (PPV) by Burroughes et al. [7]. Among a variety of conjugated polymers [8,9], derivatives of poly(p-phenylene) PPP and PPV have been thoroughly investigated in light* Corresponding authors. Fax: +49 0 371 531 836967 (D.A.M. Egbe), +90 212 285 63 86 (Y. Yagci). E-mail addresses: [email protected] (D.A.M. Egbe), [email protected] (Y. Yagci). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.11.040

emitting diodes (LEDs) [9,10] due to their relatively high photoluminescence (PL) and electroluminescence (EL) quantum efficiencies as well as good colour tunability through molecular structure designs. They also show high chemical and thermal stability and good mechanical properties. Parent PPP and PPV are intractable. This hinders their processability into thin films required for most applications. As a result, considerable efforts have been directed towards the preparation of well-defined conjugated polymers with improved solubility, processability and stability. The attachment of conformationally mobile, relatively long and flexible side chains to the polymer backbone is a common technique and has been important as it allows the controlled synthesis of fusible and soluble rigid-rod conjugated polymers. Besides enabling the solubility and the subsequent processability, the grafted side chains play an important role in the ordering of self assembly and they can also lead to changes in optical and electronic

D.G. Colak et al. / European Polymer Journal 45 (2009) 940–945

properties of such materials [11]. Simple linear alkyl or alkoxy groups as side chains for PPP and PPV derivatives have often been reported [12–14]. To obtain rigid macromolecules with targeted properties, one should consider the interdisciplinary connection between synthesis and material science which is to generate a defined molecular architecture. It is possible to form a new polymer with novel and interesting properties on combining a stiff, insoluble, rod-like polymer with a soft coil (such as polystyrene (PSt) or poly(e-caprolactone) (PCL). The macromonomer technique has proved to be useful for preparing graft copolymers and macromolecular designs [15–17]. By applying this strategy, well-defined macromonomers with desired functionalities required for the synthesis of PPPs by Suzuki or Yamamoto coupling and PPVs by Wittig reaction with bis(triphenylphosphonium) salts have been obtained and the details of the synthesis of the macromonomers and final polymers are given in our previous studies [18–27]. In this work, we report a detailed photophysical investigation of PPP and PPV derivatives laterally decorated with polystyrene PSt or PCL; PPP-PCL, PPP-altPCL, PPV-PSt, PPV-PCL and PPV-PCL-Br. For better understanding of the photophysical properties of the macromolecules, the properties of their basic chromophore systems were taken into consideration, namely p-terphenyl, distyrylbenzene, DSB, and dibromodistyrylbenzene, Br2-DSB. The polymers emit blue and exhibit very high relative and absolute photoluminescence quantum yield in dilute solution, thin film (spincoated and inkjet-printed) and bulk state.

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The kinetics of fluorescence was investigated with a CD900 time correlating single photon counting spectrometer (Edinburgh Instruments). The excitation source was a hydrogen filled nanosecond flash lamp which yielded an instrument response pulse of 1.3 ns FWHM. In order to calculate the fluorescence lifetime, the LEVEL 1 (up to 4 exponentials) package implemented in the Edinburgh Instruments software was used. (The analysis makes use of the iterative reconvolution technique and the Marquardt fitting algorithm.) Plots of weighted residuals and of the autocorrelation function and values of reduced residuals v2 were used to judge the quality of the fit. v2 values larger than 1.3 were not accepted. For solid state studies, thin films of the macromolecules were prepared by either spin coating or inkjet printing and their absolute photoluminescence quantum together with those of the solutions and bulk materials were measured on a Hamamatsu C9920-02 system. An UV–VIS/Fluorescence plate reader (Flash scan 530) from Analytik Jena (Jena, Germany) was used to measure the respective fluorescence spectra of spin coated and printed films. Inkjet printed films were made using a piezo based, single nozzle inkjet printer from Microdrop Technologies. Concentration of the solutions for inkjet printing and spin coating was 4 mg/mL in a solvent mixture of toluene/o-dichlorobenzene (90/10) and films of approximately 100 nm thickness were obtained. For good film formation, following inkjet printing parameters were necessary: pulse width 40– 70 V and pulse height 75–150 V. 3. Results and discussion

2. Experimental 2.1. Materials The synthesis and chemical characterization of PPVPSt, PPV-PCL and PPV-PCL-Br [27], as well as PPP-PCL and PPP-altPCL [21,23] and the model compounds DSB and Br2-DSB [28] have been described elsewhere. 2.2. Instrumentation Absorption spectra were recorded at room temperature on a LAMBDA 16 spectrophotometer (Perkin Elmer). Fluorescence emission and excitation spectra at room temperature and at 77 K were measured using a LS50B luminescence spectrometer (Perkin Elmer). Low temperature experiments recorded at 77 K used the low temperature accessory of the LS50B spectrometer and were performed with samples that had been placed in fused synthetic silica tubes of 2 mm inner diameter. The investigations in order to find phosphorescence were performed at 77 K in using the phosphorescence modus of the instrument with delay time and gate time of 1 ms and 10 ms, respectively. Relative fluorescencequantumyields were calculated relative to quinine sulphate (purum; FLUKA) in 0.1 N H2SO4 (pro analysis; Laborchemie Apolda) used as a standard (Uf = 0.55). The absorbance at the excitation wavelength was kept below 0.05 for the samples and the reference [11].

The structures of the macromolecules and their corresponding model compounds are depicted in Chart 1. Molecular weights of the starting macromonomers and the final polymers are given in Table 1. Synthetic details and general characteristics of the compounds have been reported elsewhere [21,23,27,28]. Photophysical properties of the polymers were investigated in dilute chloroform solution, in thin films (spincoated and inkjet-printed) and in bulk. Table 2 summarizes the dilute chloroform photophysical data of the studied compounds, namely the absorption maximum, ka, the maximum of the fluorescence excitation spectrum, kexc, the fluorescence maximum, kf, the Stokes shift, Dmaf, the relative fluorescence quantum yield, Uf, the fluorescence lifetime, s, the fluorescence radiative constant, kf, the fluorescence non-radiative constant, knr, and the fluorescence polarization, r. The absorption and emission of the macromolecules are essentially determined by the basic chromophore system p-terphenyl or distyrylbenzene and the steric influence due to the type and the number of laterally grafted macromolecules. For comparison reason the model compounds p-terphenyl and distyrylbenzene have been taken into consideration. The longest wavelength absorption of the PPP compounds clearly lies below 300 nm. The polymer with the strongest steric hindrance, PPP-PCL absorbs below 280 nm and fluoresces very weakly.

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D.G. Colak et al. / European Polymer Journal 45 (2009) 940–945

Chart 1. Chemical structures of the studied macromolecules and their model compounds.

Table 1 Molecular weights of the starting macromonomers and the final polymers. Starting Macromonomers Mn,HNMR

Mn,GPC

P.D

PSt-CHO PCL-CHO

2420 3314

2480 3720

1.34 1.36

PCL-Br

3264

4000

1.27

Final Polymers

PPV-PSt PPV-PCL PPV-PCL-Br PPP-PCL PPP-altPCL

Mn,GPC

P.D

18600 28320 32000 27150 32300

3.05 2.59 2.31 1.41 1.33

The polymer with unsubstituted phenylene unit within the backbone, PPP-altPCL, absorbs at 283 nm; its absorption spectrum is similar to that of p-terphenylene (Fig. 1) and both compounds similarly exhibit very high fluorescence quantum yield. The extended conjugation is responsible for the red shift of the emission of PPP-altPCL with respect to that

of the p-terphenyl. The non structuring of the emission spectrum of PPP-altPCL, which is evidence of non planarized S1 state, can be ascribed to the steric hindrance caused by the PCL side groups [12c,29]. A red shift of almost 100 nm is observed with the PPV derivatives as compared to their pure PPP counterparts. The position and form of the absorption bands differ very little. The basic chromophore of this series compounds is located around distyrylbenzene. The twist (caused by the polymeric substituents) in the adjacent phenylene units – Ph-Ph-Ph- hinders an extension of conjugation and leads to a segmentation of the conjugated polymeric chain. The small differences in the position of the absorption and fluorescence bands are related to steric and electronic effects (Fig. 2). For instance, less steric hindrance in the case of substitution with only one macro-side chain, as in PPV-PSt, leads to longer wavelength absorption and fluorescence as well as higher fluorescence quantum yield.

Table 2 Spectral and photophysical properties in dilute chloroform solution. a

Polymer

ka (nm)

kexc (nm)

kf

PPP-PCL

<280

PPP- altPCL

283

265 310 283

291 368 365

PPV-PSt

381

PPV-PCL PPV-PCL-Br DSB Br2-DSB

373 370 354 355

389 335 378 382

430/450 ca. 390 417/440 428/453 413 401/ 425

a

Underlined values are main emission bands.

(nm)

Dmaf (cm 1)

Uf

s (ns)

7940

0.56

3000

0.82

0.47; 95% 2.0; 5% = 0.5 0.71

0.88 1.2

0.90 0.25

0.26

2800 3660 4036 3230

0.67 0.26 0.85 0.23

0.65 0.34

1.0 0.76

0.51 2.2

0.25 0.25

kf (ns

1

)

knr (ns

1

)

r

0.25

D.G. Colak et al. / European Polymer Journal 45 (2009) 940–945

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Fig. 1. Absorption and emission spectra of PPP-altPCL and p-terphenyl in dilute chloroform solution.

Fig. 3. Absorption, fluorescence excitation and fluorescence spectra of Br2-DSB (a) and PPV-PCL-Br (b) at room temperature and at 77 K.

Fig. 2. Normalized absorption and emission spectra of the PPV derivatives and distyrylbenzene.

The donor effect of bromine in PPV-PCL-Br causes a bathochromic shift of the fluorescence as compared to the non bromine substituted compound PPV-PCL. The fluorescence spectra show comparable vibronic structures as in DSB. This suggests far better planarization of their S1 states as compared to that of PPP-altPCL. PPV compounds without Br-substitution fluoresce intensively. A comparison of the rate constants of the deactivation processes shows high kf values for all compounds, which is synonymous to strong allowed fluorescence transition. The low fluorescence quantum yield of the Br-substituted compound PPV-PCL-Br can be clearly ascribed to high radiationless deactivation, whereby intersystem process from S1 to T1 can be assumed, which is related to so called ‘‘heavy atom effect” due to the presence of bromine atoms [3c]. Similar decrease of Uf from 85% to 23% is observed when comparing the model compounds DSB and Br2-DSB. In order to prove the strong population of the T1 state, low temperature (77 K) measurements were carried out with PPV-PCL-Br and Br2-DSB. The highly structuring of the resulting fluorescence spectra leads to an enhancement of the fluorescence intensity as compared to room temper-

ature (Fig. 3). However no phosphorescence (radiative decay from T1 to S0) could be observed. As previously shown already, there is a red shift of the absorption and emission spectra going from solution to thin solid films [27]. Spin casted and inkjet printed films were obtained from a mixed toluene and o-chlorobenzene solution. The fluorescence spectra of the inkjet printed films of the polymers are depicted in Fig. 4, the shape and position of the bands are similar to those already reported [27]. Absolute photoluminescence quantum yield of the bulk materials, thin films and their starting solution were measured using an easy-to-handle Hamamatsu PL system. The results are summarized in Table 3. The trend observed above in solution (Table 2), i.e., higher Uf-values for non Br-substituted compounds than for Br-substituted compound, was confirmed in the solid state. The relatively high solid state Uf-values are due to the presence of macro-substituents, which effectively hinder very strong p–p interactions and concomitant excimer formations, which are a source of fluorescence quenching in the solid state [30]. Similar solution relative and absolute Uf values were obtained for the three PPV derivatives. In contrast PPPaltPCL displays a discrepancy between its relative (56%) and absolute (100%) Uf-values in solution. This reveals the limitation for measuring the exact absolute Uf of highly luminescent compounds absorbing at very short

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of the macro-substituents, an aspect which favours the use of the present macromolecules in the design of high efficient light-emitting diodes. Acknowledgement Prof. U. S. Schubert is grateful to the Dutch Polymer Institute (DPI) for financial support. References

Fig. 4. Fluorescence spectra of inkjet printed films.

Table 3 Absolute quantum yields in solution, thin films and bulk polymer materials. Compound

Bulk

Inkjet printed film

Spin coated film

Solution

PPV-PCL PPV-PCL-Br PPV-PSt PPP- altPCL

0.12 0.08 0.43 0.09

0.20 0.10 0.48 0.34

0.25 0.09 0.52 0.35

0.56 0.34 0.89 1.00

wavelength (<300 nm) using the intergrating sphere as found in the Hamamatsu PL system. 4. Conclusion Photophysical investigations on PPP and PPV derivatives bearing PSt or PCL as side groups reveal that their photophysical behaviour is governed by the basic chromophore units p-terphenylene and/or distyrylbenzene and steric effects caused by type and number of grafted macro-side chains. The presence of bromine at the same time leads to the red shift of the emission spectra and a decrease of the fluorescence quantum yield. Low temperature studies did not prove phosphorescence in the case of PPV-PCL-Br and its model compound Br2-DSB, instead an enhancement of fluorescence was observed as compared to room temperature. The relatively high solid state fluorescence quantum yields suggest minimized fluorescence deactivation channels due to the presence

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