Optical properties of disubstituted polyacetylene

ELSEVIER

SyntheticMetals

Optical 1.1. Gontiaa,

properties

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of disubstituted

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polyacetylene

S. Frolovb, M. Liess ‘, K. Yoshinod,

Z.V. Vardenya

a Physics Department, University of Utah, Salt Lake City,Utah,8QII2,USA b Bell Laboratories, Lucent Technologies, 700 Mountain Ave., Murray Hill, NJ 07974, USA ’ I. U. T. GmbH, Rudower ChausseeS, D-12489, Berlin, Germany d Electronic Engineering Department, Osaka University, Yamada-Oka 2-l Suita, Japan Abstract We have investigated the classof disubstituted polyacatylene (PDPA) such as pristine and doped with C6s, using both transient and steady state spectroscopies. These polymers show high yield photoluminescence which causes spectral narrowing at high excitation intensities, despite of their ground state degeneracy. In PDPAnBu/Cec mixture we found photoinduced charge transfer from the degenerate ground state polymer chains to the Cse molecules. Keywords: Photoluminescence, Spectral narrowing, Lasing , Photoinduced absorption spectroscopy, Charge transfer

1. Introduction Trans-polyacetylene (t-PA ) is the most generic degenerate ground state (DGS) polymer having a simple backbone structure; in the past two decades this polymer has played an important role in understanding the physics of r-conjugated polymers. It has been known, however that t-PA has a very weak photoluminescence (PL) band in the infrared spectral range[l], with a quantum efficiency v=10P5. Similarly, monosubst.ituted t-PA exhibits weak PL in the near infrared[2]. This weak PL from degenerate ground state polymers has been explained by the relative positions of the lowest excited electronic levels: l’B, and 2lA,. In t-PA 2A, is located below lB, [3],where the transition lA, - 2A, is dipole forbidden. However, intense PL has been observed in soluble disubst.ituted polyacetylcnes such as poly(l-phenyl-2-pn-butylacetylene (PDPA-nBu) (Fig.1) at 520 nm and poly(lhexyl-2phenyl acetylene)(PHxPA) at 455nm. Futhermore , in some PDPA films it is possible to achieve laser action, or emission spectral narrowing, at sufficiently high excitation intensities[4]. Apparently the disubstitution of t-PA drastically changes the optical emission properties. This might be due to the fact that the substituted side group molecules may influence the electronic energy states of the poly-

mer backbone structure 141. Our studies focus on PDPA-nBu (Fig.1 the right inset), which is a degenerate ground state (DGS) polymer, as recently shown by doping st,udies[5].We have investigated spontaneous and stimulated emission in thin films of PDPA-nBu and showed that the strong PL is accompanied by high optical gain and emission spectral narrowing at high intensities [4]. We also report the results of steady state photoinduced absorption (PA) spectroscopy studies of C&c doped PDPA-nBu at various doping concentrations. We obtained spectral evidence of photoinduced charge transfer from the excited state of this polymer onto adjacent C&c molecule through PA measurements. This is in contrast to the common belief that in DGS polymers the photoinduced charge transfer is suppressed by soliton generation, being faster than the charge transfer process[6]. In addition, polaron and soliton PA bands were also identified. 2. Experimental Thin films of pristine PDPA-nBu were prepared from chloroform solution by spin-casting on quartz substrates. Absorption and PL spectra were measured using a Perkin Elmer spectrophotometer and a fluorescence Hitachi F-2000 spectrophotometer, re-

0379-6779/99/$ - seefrontmatter0 1999ElsevierScience S.A. All rightsreserved. PII: SO379-6779(98)00347-6

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spectively. For the spontaneous and stimulated emission measurements we used a laser beam with 100 ps pulses at 1OOHz repetition rate, produced by a Nd:YAG regenerative laser amplifier, which was frequency-tripled at 355nm. The emission spectra were measured using a scanning l/4 meter spectrometer with 2nm resolution. The solution of PDPA-nBu/Csa mixture was prepared by dissolving in xylenes both PDPA-nBu and Cse of appropriate weight ratio. Thin films were obtained by drop casting the solution onto KBr substrates and evaporating off the solvent in vacuum. The PA spectra were obtained at 80K by measuring the change (AT) in transmission (T) in response to an external photopumping source, which in our case was an Ar laser beam at 458nm and 360nm, respectively. The laser beam was modulated with a mechanical chopper at a typical frequency of 100Hz. The probe beam was derived from a tungsten-halogen lamp in the spectral range 500nm-2.4mm, and a glow-bar for the range 2.4mm -15mm. A parabolic mirror directed the light transmitted through the sample onto the entrance slit of a l/4 meter monochromator. At the exit slit of the monochromator the light was detected by a solid state photodetector. Depending on the spectral range, the following detectors were used: Si, Ge, InSb, and MCT. The signal from the detector was preamplified and detected by means of a lock-in amplifier referenced to the pump beam modulation frequency. Several long pass filters and gratings combinations were used to cover the entire spectral range of our measurements, which was 0.1 to 2.5 eV. 3. Results

and

Discussion

Fig.l(a) shows the absorption and PL spectra of PDPA-nBu, of which repeat unit is given in the inset. The lack of an absorption tail that extends in to the optical gap suggests a relatively defect-free film, the conjugation length distribution being narrow. PDPA-nBu has strong PL with a quantum efficiency higher than 60% as measured with an integrated sphere. The PL spectrum consists of a broad band peaked at 2.4 eV with no vibronic structure. In Fig.1 , the left inset, it is seenthat the PL excitation spectrum follows the absorption spectrum which is an indication that the PL comesfrom the polymer chains rather than the side group molecules. So far, all DGS polymers such as unsubstituted t-PA and monosubstituted t-PA were found to belong to the group of non luminescent polymers, since their infrared PL is extremely weak. The weak PL in these DGS polymers has been traditionally explained by the soliton model, in which the primary excitations are solitonantisoliton (S - 3) pairs [7].Then upon photoexcitation the photogenerated e-h pair, which immediately

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(within 1OOfs)transforms into S+-S- [8], relaxes very quickly (within 4OOfs)into a SD-Sopair with similar even parity structure as the 2A, state[g].We believe that the strong PL in PDPA-nBu polymer originates from strongly bound S+-S-~-pairs, namely solitonic exciton, which do not immediately transform into the So - so manifold.

0.8

h ? 2% 2 0.4

0 1

5

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Photon krgy

(et)

Fig. 1 Absorption and PL(dashed and solid line, respectively) spectra of PDPA-nBu film. The right inset showsthe repeat unit of this polymer and the left one the PL excitation spectrum. Fig.2 illustrates the changesoccurring in the emission spectrum of a PDPA-nBu film, as the excitation intensity is increased. The emission linewidth of PDPA-nBu narrows from 100 nm to below 10 nm.

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Fig. 2 Optical emissionspectra at different excitation pulse energies of PDPA-nBu film. The inset is the emissionpeak intensity dependence on the excitation intensity.

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et al. I Synthetic

The peak intensity of the emission, &eak at 510 nm, was measured at different excitation intensities Ipump, as shown in the insert of Fig.2. It is seen that l&k below the threshold excitation intensity, Ith is sublinear with Ipump , whereas for Ipump> Ith the dependence is superlinear. This change of slopes defines a threshold excitation intensity, Ith at 1MW/cm2 or lmJ/cm2 per pulse. This is comparable to our best DOO-PPV films [lo]. In Fig.3(a) we show the PA spectrum of a pristine PDPA-nBu film. The spectrum contains only one PA band peaked at 1.7eV which is at much higher energy than the S+ absorption band induced upon iodine doping[5]. This PA band was identified as due to photogenerated neutral soliton-antisoliton So -3’ pairs, using photoinduced absorption detected magnetic resonance measurements[5]. Fig.3(b) illustrates the PA spectrum of a photooxidized PADA-nBu film. This PA spectrum shows not only the characteristics of S+ and S”,but in addition two PA band at 0.37eV and 2.35 eV, respectively , which we assigned to Pr and Pa transitions of polarons (P+). The photoinduced infrared active vibrations are the signature of photoinduced charged excitations in the polymer chains. Its huge oscillator strength is caused by the soliton small kinetic mass[ll].

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of electron transfer from the polymer chain onto C&c molecule. It has been shown that Ccc itself does not have any absorption features or PA bands between 0.05-1.4 eV[12]; the peak at 1.15eV and its vibronic structures are therefore due to C;,,. These transitions were also observed in the absorption spectrum of CL0 generated by y ray irradiation[l3]. The photoinduced charge transfer between the conjugated polymer chains and intercalated dopant is governed by energetics i.e. by the relative position of the polymer energy levels with respect to those of the dopant. C&e is a weak dopant for PDPA-nBu having its LUMO level within the gap of the polymer. It has been shown that there are two different channels for photoinduced charge transfer: either e- transfer from the photoexcited polymer chain, or hole transfer from the photoexcited C&e molecule [14].

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Fig. 4 (a), (b) PA spectra of various PDPA-nBu/ 7% Csc showing sample dependence spectra.

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Fig. 3 (a) PA spectrum 80K, (b) PA spectrum film at 80K.

1.5 Energy

2

2.5

(eV)

of pristine PDPA-nBu film at of photooxidized PDPA-nBu

In C&c doped PDPA-nBu we observed (Fig.4) photoinduced charge transfer; namely we found a PA band at 1.15 eV due to the generated C&s as a result

mol

The PA spectrum of PDPA-nBu/Csc 7% mixture shown in I’ig.4also exhibits PA bands due to charged solitons, polaron and neutral solitons superimposed on the PA band of the C&c triple-triplet transition. We noticed that charge transfer is strongly sample dependent due to Csc segragation in the parent solution. In the two samples prepared from the same solution (7% C&c) we got qantitatively different PA spectra (Fig.4(a) and Fig.4(b), respectively) where the strength of the various PA bands are different; the more C&c clusters are form in the films, the less photoinduced charge transfer occurs. The amount of Ccc clusters formed in the film are reflected in the strength of the Csetriplet-triplet transition in the PA

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spectrum, whereas the efficiency of the charge transfer is given by the strength of the charged soliton transition and C$ absorption. From the quantum efficiency action spectra of polarons and charged solitons PA bands (Fig.5 (a) and Fig.5 (b), respectively ) in PDPA-nBu/Cso, it can be seen that the polarons and charged solitons generation are not correlated; the processby which two positively charged polarons of opposite spins can recombine to form a positive chargedsoliton-antisoliton pair cannot explain these spectra. A possible scenario by which positive charged soliton antisolitori pairs are produced upon photoinduced charge transfer, is the dissociation of two excitons onto two adjacent Cse molecules on the same polymer chain. As a result of the two exciton dissociation, a solitonantisoliton pair in the polymer chain is produced and two c, monoions are also generated. This scenario is supported by the dynamics (frequency, intensity and temperature dependences)of the- corresponding PA bands showing that the processby which charged so&on and C& are photogenerated is correlated.

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Photon

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Energy (eV)

Fig. 5 The action spectra of (a) charged soliton, (b) polaron photogeneration. When the exciton dissociates only onto one C, molecule near the polymer chain, the c,& molecule becomesa negative monoion while a positive polaron remains on the polymer chain. The photogenerated polaron forms a bound state with cc0 reducing its mobility, This is consistentwith the lack of correlation between the photoinducedpolaron and chargedsoliton formation. 4. suqmary

We found that films of disubstituted polyacetylene such as PDPA-nBu show strong PL and associ-

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ated with it high gain and spectral narrowing at high excitation intensity. The emissionspectrum changes from a broad PL band of 1OOnmat low excitation intensities to a narrow, stimulated emission band of loam width at high intensities. We also showed that although PDPA-nBu has a degenerate ground state, when doped with C60, a photoinduced electron transfer from the excited states of the polymer to Cse molecule still occurs. The evidence of this process is the presence of a PA band at 1.15eV and the associated vibronic structures assignedto C&absorption.

Acknowledgments

This work was supported in part by DOE grant No FG-03-96-ER 45490 and NSF grant No 32820 DMR 97.

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Orenstein and G.L. Baker, Phys. Rev. Lett. 50, 2032 (‘1983); P2J N.S. Sariciftci and -4.J. Heeger, Int. J. Mod. Phys. B ,8, 237 (1994); [I31 Kato et al, Chem. Phys. Lett. 180, 446 (1991); [I41 A. A. Zakhidov , H. Araki, K. Tada and K. Yoshino, Synth. Met. 77, 127 (1996);