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Light-emitting properties of stereoregular cis-rich poly(phenylacetylene)s
25 June 1999
Chemical Physics Letters 307 Ž1999. 67–74
Light-emitting properties of stereoregular cis-rich poly žphenylacetylene/s C.W. Lee a , K.S. Wong
a,)
, W.Y. Lam b, B.Z. Tang
b
a b
Department of Physics, Hong Kong UniÕersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Department of Chemistry, Hong Kong UniÕersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Received 11 November 1998; in final form 19 April 1999
Abstract Absorption measurements of stereoregular polyŽphenylacetylene.s ŽPPA. with controlled cis contents indicate that the lowest energy p–pU absorption band is due to the cis segments in the polymer chains and that the trans segments in the cis-rich PPAs may contain only a few repeat units. Both the photoluminescence ŽPL. efficiency and the PL lifetime of the PPAs in chloroform decrease with increasing cis concentration, for concentrations ranging from 67 to 100%. The relatively strong PL for 100% cis PPA suggests that the emission originates from the cis segments in the polymer chains rather than from the trans segments. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction It is well known that trans polyacetylene ŽPA. does not emit, or emits only, weakly in the near IR and that cis PA emits reasonably well in the visible spectral region. The soliton model w1x or the energy position of the 2 1A g and 1 1 B u states w2,3x have been used to explain the light emission properties of the cis and trans PAs. On the other hand, PAs with substituents Žor substituted PAs. are usually highly luminescent w4–7x. However, the photoexcited states relaxation processes responsible for non-luminescent PA and highly luminescent substituted PAs are not fully understood.
) Corresponding author. Fax: q852 2358 1652; e-mail: [email protected]
The polymerizations of phenylacetylene and its derivatives initiated by the ‘classical’ metathesis catalysts based on the halides of molybdenum, tungsten, niobium, and tantalum cannot control the cis or trans contents of the resulting polyŽphenylacetylene.s ŽPPA.. The stereochemical structure of the PPAs is random, consisting of mixtures of cis and trans segments of various lengths. Although a number of papers have claimed that trans-rich substituted PAs such as monosubstituted polywo-Žtrimethylsilyl.phenylxacetylene4 ŽPTMSPA. w8x and disubstituted polyŽdiphenylacetylene. ŽPDPA. w6x have been used in their studies, the NMR spectra indicate that the geometric structure of the main chains is unsolved and that the cisrtrans ratio is undetermined w9x. The mixed nature of the stereostructure of the substituted PAs makes the interpretation of the PL results rather difficult, particularly on the origin of the light-emitting species in the polymers.
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 4 6 3 - 7
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C.W. Lee et al.r Chemical Physics Letters 307 (1999) 67–74
Recently, we have developed aqueous polymerization systems for the synthesis of substituted PAs w10–12x. These aqueous polymerization routes are environmentally benign, compared to the traditional techniques using organic solvents. These substituted PAs are highly luminescent and the emission color can be tuned throughout the visible spectral region by varying the types of substituents or the side groups attached to the main chains. Thus, the substituted PAs are useful for polymer light-emitting diode applications w4–7x. Here, we will concentrate on the study of the emission properties of the blue– green-emitting polymer, polyŽphenylacetylene. or PPA. The classical metathesis catalyst systems such as WCl 6 –Ph 4 Snrtoluene w13x only give random PPAs with mixed cis and trans segments. Our organorhodium complex-based aqueous polymerization system, however, can produce stereoregular PPAs with controlled amounts of cis and trans contents. This precise control of the cisrtrans ratio is a great advantage over the substituted PAs prepared by the metathesis polymerization routes. In this report, the optical and time-resolved photoluminescence ŽPL. properties of the PPAs in solution with various amounts of cis and trans contents were investigated and the origin and mechanism for the photoluminescence of the PPAs are discussed.
vacuum at room temperature to a constant weight. An orange powdery polymer was obtained in 82% yield. The molecular weight of the PPA was estimated by gel permeation chromatography in THF using a set of monodisperse polystyrenes as calibration standards: M w s 31200. 1 H NMR Ž300 MHz, CDCl 3 . d ŽTMS, ppm.: 6.94 Ž3 H, para and meta aromatic protons., 6.63 Ž2 H, ortho aromatic protons., 5.84 Ž1 H, cis olefinic proton.. 13 C NMR Ž75 MHz, CDCl3. d ŽTMS, ppm.: 142.87, 139.31, 131.82, 127.78, 127.55, 126.69.
2. Experimental technique The stereoregular PPAs were prepared according to our previously published procedure w10–12x. In a typical run, an aqueous catalyst solution Ž1 mM. was prepared by dissolving 2 mg of a water-soluble organorhodium complex, RhŽcod.Žtos.ŽH 2 O. Žcod s 1,5-cyclooctadiene, tos s p-toluene sulfonate., in 5 ml of distilled water, to which 0.5 ml of freshly distilled phenylacetylene was added with vigorous stirring at room temperature under nitrogen. After 1-h polymerization, the PPA product was separated from the aqueous catalyst solution by filtration and then washed with acetone several times. The PPA was dissolved in toluene and the resulting polymer solution was added dropwise to a large amount of methanol with stirring. The precipitated PPA was filtered off by a Gooch crucible and dried under
Fig. 1. 1 H NMR spectra of the PPAs prepared by RhŽcod.Žtos.ŽH 2 O. in water; cis content Ž%.: ŽA. 100, ŽB. 83.4. The spectra of the PPA prepared by WCl 6 –Ph 4 Sn in toluene ŽC. is shown for comparison.
C.W. Lee et al.r Chemical Physics Letters 307 (1999) 67–74
The PL experiment was performed using the frequency-doubled Ž l s 400 nm. output of the femtosecond Ti:Sapphire regenerative amplified laser. The time-resolved PL was measured by the frequency up-conversion technique with overall spectral and temporal resolutions of 2 nm and 300 fs, respectively. Details of this femtosecond time-resolved PL frequency up-conversion system can be found in Ref. w14x. All the experiments were done on PPAs in chloroform solution.
3. Results The stereostructure of the PPAs was elucidated by H NMR spectroscopy. Typical examples of the NMR spectra of the PPAs with different cis contents are given in Fig. 1. The peak at d 5.84 is due to the absorption of the cis proton, while the trans proton absorbs at d 6.78. The cis contents of the PPAs were calculated using the following equation: 1
%cis s A 5.84r Ž A totalr6 . = 100
Ž 1.
where A denotes the integrated absorption area and A total s A 5.84 q A 6.63 q A 6.78 q A 6.94 . From the inte-
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gration data, the cis contents of the PPAs shown in Fig. 1A,B are estimated to be 100 and 83.4%, respectively. The 300 MHz NMR spectrometer we used has high resolution and sensitivity. The spectrometer can readily detect the existence of chloroform with a 0.2% concentration, giving a clearly visible peak at d 7.24. The spectra of our polymers are of high quality and are well separated, which enables an accurate determination of the cis content. The reliability of the conformation determination is thus conservatively estimated at ca. 0.5%. As a comparison, the spectrum of the PPA synthesized from the classical metathesis catalyst of WCl 6 – Ph 4 Snrtoluene is also given in Fig. 1C. The tungsten-catalyzed polymerization proceeded in a stereostructurally irregular fashion and the resulting polymer possessed a random mixture of the cis and trans segments. The NMR spectrum is characterized by broad absorption ‘peaks’, and there is no defined absorption peak of the cis proton at d 5.84. It is difficult, if not impossible, to use Eq. Ž1. to evaluate the cis content of the random PPA. Fig. 2 shows the absorption spectra of chloroform solutions of the PPAs with various cis contents. For 67% cis PPA, there exists an absorption shoulder at ; 400 nm. As the cis content increases, the absorption band becomes more pronounced. This absorp-
Fig. 2. Absorption spectra of PPAs with different cis contents.
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C.W. Lee et al.r Chemical Physics Letters 307 (1999) 67–74
tion band is due to the p–pU interband transition of the polymer main chain w7x. It is well known that the p–pU interband transition for trans PA is at a lower energy than that for the cis PA. The enhanced lowest absorption band at 400 nm for 100% cis PPA ŽFig. 2. clearly indicates that the lowest energy absorption peak is due to the cis segment rather than the trans one. The stereoregular polymerizations of phenylacetylene initiated by organorhodium catalysts proceed via a cis-insertion mode w15x. The trans segments are the mismatching stereostructural ‘defects’, whose lengths thus should not be too long. Thus, the
trans segments may only consist of a few repeat units of the alternating single and double bonds, whose absorption may well be located in the deep UV region w16x. Fig. 3A shows the time-integrated photoluminescence spectrum of the PPA with 67% cis content. It has a broad emission peak at ; 490 nm. This broad PL spectrum shows the cis PPA has a wide distribution of conjugation length. All the other PPAs with different cis contents Ži.e., 70.5–100%. emit fluorescence with spectral shape and peak positions being essentially the same as that in Fig. 3A. This result indicates that the cis segment in the chain is responsible for the light emission in the PPAs. The relative PL intensity as a function of cis content is plotted in Fig. 3B. It is clearly shown that the PL intensity decreases linearly with increasing cis content. For example, the emission intensity of the 100% cis PPA is only about one third of that of the PPA with 67% cis content. Fig. 4 shows the time-resolved PL at the emission peak of 490 nm for the PPAs with various cis contents. The PL shows multi-component decays for all the cis-rich PPAs. This result indicates that the recombination dynamics in these materials is very complex. The PL has two fast and one slow decay components. Two fast components t 1 and t 2 have decay time constants of a couple of picoseconds Žps. and a few tens of picoseconds, respectively. These fast decay components are most likely associated with non-radiative recombination processes. The decay time for the slow component is of the order of a few hundred picoseconds and can be observed more clearly in a longer timescale ŽFig. 4 inset.. In general, the PL decays faster with increasing cis content. Furthermore, the PL rise time is very fast Ži.e. within 1 ps. for all the cis-rich PPAs studied.
4. Discussion Fig. 3. ŽA. PL spectrum of PPA with 67% cis content and ŽB. relative PL intensity as a function of cis content. The PL spectra for PPAs with different cis contents are measured in exactly the same optical configuration and using the same excitation laser power. The amount of excited laser absorption for different samples are measured. The relative intensity is then obtained by integrating the PL intensity in the emission curve and the difference in excited laser absorption for PPAs with different cis content is also corrected.
The absorption peaks for various mono- and disubstituted PAs span from near UV to the red spectral region w4,9x. For example, the absorption peak for PPA, which is the subject of this study, is at ; 400 nm Žcf. Fig. 2.. On the other hand, PTMSPA has an absorption peak at ; 540 nm with a strong
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Fig. 4. Time-resolved PL of PPAs with various cis contents. The time constants for the two fast decay components are also shown. The inset shows the time-resolved PL at large delay timescale for the 67% cis PPA. The dotted line is the theoretical fit to the fast decay component using a biexponential response function with time constants t 1 and t 2 .
absorption tail extends to 700 nm. However, all these absorption peaks for various kinds of substituted PAs are blue shifted relative to the unsubstituted trans PA. The current understanding of this result can be explained as the consequence of the introduction of bulky side groups in the substituted PAs. The side groups cause a local distortion of the polymer backbone due to steric hindrance. The backbone twisting
limits the effective conjugation length of the polymer chains, resulting in the blue shift of the absorption of the substituted PAs, compared to the parent PA. Furthermore, the steric effect generally increases as the substitution changes from mono to disubstitution. Consequently the absorption peak further blue shifts and the luminescence strengthens for disubstituted PAs. The observation of strong PL in disubstituted
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PAs was unusual because the polymers supposedly consist of a trans backbone which is non-luminescent in PA w6x. The explanation for this PL result was that the distortion of the polymer chains in substituted PAs due to the bulky sidegroups push the originally lower energy 2 1A g state above the 1 1 B u state which enhances the radiative recombination w5,6x. As we mentioned earlier, using classical metathesis catalysts to synthesize substituted PAs usually results in a mixture of cis and trans configurations, thus the exact location of the light-emitting species in the trans or cis segments is still uncertain. Another observation is that most of the substituted PAs are strongly luminescent in blue or green color w4–7x, but some are non-luminescent or only weakly luminescent particularly for those with absorption in the red spectral region w8,9x. Our results on the PPAs described above may provide an additional explanation for the interesting absorption and emission phenomena of the substituted PAs. In the present study, we demonstrated that the cis-rich PPAs can be synthesized using our rhodium-initiated aqueous polymerization technique. On the other hand, trans-rich PTMSPA is said to be preferentially formed using the WCl 6 catalyst w8x. This indicates that the formation of cis or trans configuration in the polymer backbones depends on the polymerization technique Žespecially the catalyst system. used andror the nature of the substituents or side groups. The combination of preferred cis or trans configuration and chain conformation due to steric hindrance may determine the absorption and emission behavior of the substituted PAs. Thus, the strongly luminescent substituted PAs with absorption in the short wavelength region may be dominantly cis-rich having a large backbone distortion or twisting, while the nonluminescent or weakly luminescent substituted PAs with absorption in orange–red wavelength region are probably trans-rich having a relatively long conjugation length. Our result shows that the luminescence efficiency decreases as the cis content of the PPA increases. This indicates that the non-radiative decay rate increases by increasing the cis content, which was confirmed by the time-resolved PL in Fig. 4 where rapid decay was observed. The origin of this ultrafast non-radiative recombination is uncertain, and experiments are currently under way to investigate the
rapid recombination processes. The soliton model was used to explain the luminescent properties of unsubstituted PAs w1x. Since soliton can only exist in the trans segment, one would therefore expect the PL efficiency to increase with increasing cis content. Our experimental results clearly conflict with this expectation, and thus the soliton model alone cannot explain the high PL efficiency observed in the PPAs. Since most of the substituted PAs consist of a cisrtrans mixture, solitons are still able to exist in the trans segments of the polymer chains even for cis-rich samples. Therefore, it is no surprise that there are evidences of rapid formation and decay of soliton–antisoliton pairs in PTMSPA w8,17,18x; and generation of spinless states upon doping in PDPA w19x. In fact, the substituted PAs with cisrtrans mixtures will support excitons, polarons, as well as solitons; indeed, all these elementary excitations have been observed in disubstituted PAs w20x. It is known that the stereoregularity of the PPA increases by increasing its cis content w10–12x, implying that there will be more short segments of cis PPA chains as the cis content decreases. This is further evidenced in the absorption spectrum of the PPAs. While the stereoregular PPA with 100% cis content exhibits a pronounced absorption peak at around 400 nm, the absorption spectrum of a random PPA in the same wavelength range is a structureless tail with low absorptivity Žcf. spectrum D in Fig. 6 in Ref. w21x.. Steric hindrance will affect the effective conjugation length and hence the emission efficiency w4x. The steric hindrance that causes the twisting of the polymer backbone reduces the effective conjugation length of the polymer chain; this in turn leads to better confinement of light-emitting excitons. These confined or trapped excitons will have fewer chances of encountering non-radiative sites, and a better overlap of the electron and hole wavefunctions of the excitons leads to more efficient PL. Thus, it seems that the high emission efficiency observed in the cis-rich PPAs is due to the confinement of the excitons within the short chain segments as a result of both steric hindrance and reduced stereoregularity. This stronger confinement of excitons by the short chains results in higher PL efficiency. It can be argued that the PL could have originated from the trans segment even for the 100% cis-rich PPA because the presence of a few trans units
C.W. Lee et al.r Chemical Physics Letters 307 (1999) 67–74
cannot be excluded, even in the 100% cis material due to the resolution limit of the NMR Ži.e. ; 0.5%.. The presence of a few trans units could have given rise to the observed luminescence, as an energy transfer mechanism such as Forster transfer is very ¨ efficient. However, we believe that this is unlikely to be the case for the following reasons. A relatively long PL rise time is expected for the energy transfer from the cis segments to the few trans units w22x, particularly for the 100% cis PPA sample according to the above model. Also the PPAs are in solution so that the interchain interaction cannot be effective, therefore the interchain energy transfer would be slow. As shown in Fig. 4, the observed PL rise time is less than 1 ps for all the cis-rich PPAs studied. Furthermore, the absorption results shown in Fig. 2 indicate that the absorption due to the short trans segments may be located in the deep UV, thus the PL from these trans units should also emit in a similar spectral region. The observed PL is in the blue–green spectral region which clearly conflicts with this expectation.
5. Conclusions We investigated absorption and emission properties of a series of PPAs with controlled amounts of cisrtrans ratio. The precise cisrtrans ratios in the stereoregular PPAs can be determined by NMR. The lowest energy absorption band arising from the p–pU interband transition is due to the cis segments in the PPA chains. The PL of the PPAs with cis contents ranging from 67 to 100% is relatively strong, and both the emission efficiency and the PL lifetime decrease with increasing cis content. This observation again suggests that the light-emitting species is located in the cis segments in the polymer chain. The time-resolved PL indicates complex recombination dynamics with fast non-radiative decay channels within the first few picoseconds. The enhancement in the emission efficiency and the PL lifetime for samples with a low cis content can be explained by the lower degree of stereoregularity, with decreasing cis content resulting in short segments of cis PPA giving rise to strong PL due to the confinement effect. Our results suggest that both the degree of the
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conformational twisting of the polymer chain Žwhich is controlled by the structure and size of the substituents or side groups. and the amount of cisrtrans content Žwhich is controlled by the polymerization process employed. determine the luminescence efficiency of the substituted PAs.
Acknowledgements This research was supported in part by the Research Grant Council of Hong Kong. The PL experiments were performed in the Joyce M. Kuok Laser and Photonics Laboratory and the Zheng Ge Ru Thin Film Physics Laboratory at HKUST.
References w1x L. Lauchlan, S. Etemand, T.C. Chung, A.J. Heeger, A.G. MacDiarmid, Phys. Rev. B 24 Ž1981. 3701. w2x K. Yoshino, S. Hayashi, Y. Inuishi, K. Hattori, Y. Watanabe, Solid State Commun. 46 Ž1983. 583. w3x D.B. Fitchen, Synth. Met. 9 Ž1984. 341. w4x R. Hidayat, M. Hirohata, T. Tada, M. Teraguchi, T. Masuda, K. Yoshino, Jpn. J. Appl. Phys. 36 Ž1997. 3740, Part 1. w5x M. Hirohata, T. Tada, R. Hidayat, T. Masuda, K. Yoshino, Jpn. J. Appl. Phys. 36 Ž1997. L302. w6x T. Tada, R. Hidayat, M. Hirohata, M. Teraguchi, T. Masuda, K. Yoshino, Jpn. J. Appl. Phys. 35 Ž1996. L1138. w7x R. Sun, T. Masuda, T. Kobayashi, Jpn. J. Appl. Phys. 35 Ž1996. L1673. w8x S. Takeuchi, T. Masuda, T. Kobayashi, Phys. Rev. B 52 Ž1995. 7166. w9x T. Masuda, T. Hamano, K. Tsuchihara, T. Higashimura, Macromolecules 23 Ž1990. 1374. w10x B.Z. Tang, W.H. Poon, S.M. Leung, W.H. Leung, H. Peng, Macromolecules 30 Ž1997. 2209. w11x K.L. Cheuk, W.Y. Lam, P.S. Lee, B.Z. Tang, Abst. 5th Symp. Chem. Res. HK, O-42, 1998. w12x W.Y. Lam, MPhil Thesis, Hong Kong University of Science and Technology, August 1998. w13x T. Masuda, T. Higashimura, Adv. Polym. Sci. 81 Ž1987. 121. w14x K.S. Wong, W.H. Peng, H. Wang, I.K. Sou, G.K.L. Wong, in: M. Scheffler, R. Zimmermann ŽEds.., Proc. 23rd Int. Conf. on the Phys. of Semicond., Vol. 1, World Scientific, 1996, p. 689. w15x H. Hirao, Y. Ishii, T. Terao, Y. Kishimoto, T. Miyatake, R. Noyori, Macromolecules 31 Ž1998. 3405.
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w16x R.M. Silverstein, G.C. Bassler, T.C. Morrill, Spectometric Identification of Organic Compounds, 5th edn., Chap. 7, Wiley, 1991. w17x S. Takeuchi, M. Yoshizawa, T. Masuda, T. Higashimura, T. Kobayashi, IEEE J. Quantum Electron. 28 Ž1992. 2508. w18x S. Takeuchi, T. Masuda, T. Higashimura, T. Kobayashi, Solid State Commun. 87 Ž1993. 655. w19x K. Tada, R. Hidayat, M. Hirohata, H. Kajii, S. Tatsuhara, A.
Fujii, M. Ozaki, M. Teraguchi, T. Masuda, K. Yoshino, SPIE Proceed. 3145 Ž1997. 171. w20x M. Liess, I. Gontia, T. Masuda, K. Yoshino, Z.V. Vardeny, SPIE Proceed. 3145 Ž1997. 179. w21x B.Z. Tang, X. Kong, X. Wan, X.-D. Feng, Macromolecules 30 Ž1997. 5620. w22x A. Watanabe, T. Kodaira, O. Ito, Chem. Phys. Lett. 273 Ž1997. 227.
Chemical Physics Letters 307 Ž1999. 67–74
Light-emitting properties of stereoregular cis-rich poly žphenylacetylene/s C.W. Lee a , K.S. Wong
a,)
, W.Y. Lam b, B.Z. Tang
b
a b
Department of Physics, Hong Kong UniÕersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Department of Chemistry, Hong Kong UniÕersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Received 11 November 1998; in final form 19 April 1999
Abstract Absorption measurements of stereoregular polyŽphenylacetylene.s ŽPPA. with controlled cis contents indicate that the lowest energy p–pU absorption band is due to the cis segments in the polymer chains and that the trans segments in the cis-rich PPAs may contain only a few repeat units. Both the photoluminescence ŽPL. efficiency and the PL lifetime of the PPAs in chloroform decrease with increasing cis concentration, for concentrations ranging from 67 to 100%. The relatively strong PL for 100% cis PPA suggests that the emission originates from the cis segments in the polymer chains rather than from the trans segments. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction It is well known that trans polyacetylene ŽPA. does not emit, or emits only, weakly in the near IR and that cis PA emits reasonably well in the visible spectral region. The soliton model w1x or the energy position of the 2 1A g and 1 1 B u states w2,3x have been used to explain the light emission properties of the cis and trans PAs. On the other hand, PAs with substituents Žor substituted PAs. are usually highly luminescent w4–7x. However, the photoexcited states relaxation processes responsible for non-luminescent PA and highly luminescent substituted PAs are not fully understood.
) Corresponding author. Fax: q852 2358 1652; e-mail: [email protected]
The polymerizations of phenylacetylene and its derivatives initiated by the ‘classical’ metathesis catalysts based on the halides of molybdenum, tungsten, niobium, and tantalum cannot control the cis or trans contents of the resulting polyŽphenylacetylene.s ŽPPA.. The stereochemical structure of the PPAs is random, consisting of mixtures of cis and trans segments of various lengths. Although a number of papers have claimed that trans-rich substituted PAs such as monosubstituted polywo-Žtrimethylsilyl.phenylxacetylene4 ŽPTMSPA. w8x and disubstituted polyŽdiphenylacetylene. ŽPDPA. w6x have been used in their studies, the NMR spectra indicate that the geometric structure of the main chains is unsolved and that the cisrtrans ratio is undetermined w9x. The mixed nature of the stereostructure of the substituted PAs makes the interpretation of the PL results rather difficult, particularly on the origin of the light-emitting species in the polymers.
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 4 6 3 - 7
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C.W. Lee et al.r Chemical Physics Letters 307 (1999) 67–74
Recently, we have developed aqueous polymerization systems for the synthesis of substituted PAs w10–12x. These aqueous polymerization routes are environmentally benign, compared to the traditional techniques using organic solvents. These substituted PAs are highly luminescent and the emission color can be tuned throughout the visible spectral region by varying the types of substituents or the side groups attached to the main chains. Thus, the substituted PAs are useful for polymer light-emitting diode applications w4–7x. Here, we will concentrate on the study of the emission properties of the blue– green-emitting polymer, polyŽphenylacetylene. or PPA. The classical metathesis catalyst systems such as WCl 6 –Ph 4 Snrtoluene w13x only give random PPAs with mixed cis and trans segments. Our organorhodium complex-based aqueous polymerization system, however, can produce stereoregular PPAs with controlled amounts of cis and trans contents. This precise control of the cisrtrans ratio is a great advantage over the substituted PAs prepared by the metathesis polymerization routes. In this report, the optical and time-resolved photoluminescence ŽPL. properties of the PPAs in solution with various amounts of cis and trans contents were investigated and the origin and mechanism for the photoluminescence of the PPAs are discussed.
vacuum at room temperature to a constant weight. An orange powdery polymer was obtained in 82% yield. The molecular weight of the PPA was estimated by gel permeation chromatography in THF using a set of monodisperse polystyrenes as calibration standards: M w s 31200. 1 H NMR Ž300 MHz, CDCl 3 . d ŽTMS, ppm.: 6.94 Ž3 H, para and meta aromatic protons., 6.63 Ž2 H, ortho aromatic protons., 5.84 Ž1 H, cis olefinic proton.. 13 C NMR Ž75 MHz, CDCl3. d ŽTMS, ppm.: 142.87, 139.31, 131.82, 127.78, 127.55, 126.69.
2. Experimental technique The stereoregular PPAs were prepared according to our previously published procedure w10–12x. In a typical run, an aqueous catalyst solution Ž1 mM. was prepared by dissolving 2 mg of a water-soluble organorhodium complex, RhŽcod.Žtos.ŽH 2 O. Žcod s 1,5-cyclooctadiene, tos s p-toluene sulfonate., in 5 ml of distilled water, to which 0.5 ml of freshly distilled phenylacetylene was added with vigorous stirring at room temperature under nitrogen. After 1-h polymerization, the PPA product was separated from the aqueous catalyst solution by filtration and then washed with acetone several times. The PPA was dissolved in toluene and the resulting polymer solution was added dropwise to a large amount of methanol with stirring. The precipitated PPA was filtered off by a Gooch crucible and dried under
Fig. 1. 1 H NMR spectra of the PPAs prepared by RhŽcod.Žtos.ŽH 2 O. in water; cis content Ž%.: ŽA. 100, ŽB. 83.4. The spectra of the PPA prepared by WCl 6 –Ph 4 Sn in toluene ŽC. is shown for comparison.
C.W. Lee et al.r Chemical Physics Letters 307 (1999) 67–74
The PL experiment was performed using the frequency-doubled Ž l s 400 nm. output of the femtosecond Ti:Sapphire regenerative amplified laser. The time-resolved PL was measured by the frequency up-conversion technique with overall spectral and temporal resolutions of 2 nm and 300 fs, respectively. Details of this femtosecond time-resolved PL frequency up-conversion system can be found in Ref. w14x. All the experiments were done on PPAs in chloroform solution.
3. Results The stereostructure of the PPAs was elucidated by H NMR spectroscopy. Typical examples of the NMR spectra of the PPAs with different cis contents are given in Fig. 1. The peak at d 5.84 is due to the absorption of the cis proton, while the trans proton absorbs at d 6.78. The cis contents of the PPAs were calculated using the following equation: 1
%cis s A 5.84r Ž A totalr6 . = 100
Ž 1.
where A denotes the integrated absorption area and A total s A 5.84 q A 6.63 q A 6.78 q A 6.94 . From the inte-
69
gration data, the cis contents of the PPAs shown in Fig. 1A,B are estimated to be 100 and 83.4%, respectively. The 300 MHz NMR spectrometer we used has high resolution and sensitivity. The spectrometer can readily detect the existence of chloroform with a 0.2% concentration, giving a clearly visible peak at d 7.24. The spectra of our polymers are of high quality and are well separated, which enables an accurate determination of the cis content. The reliability of the conformation determination is thus conservatively estimated at ca. 0.5%. As a comparison, the spectrum of the PPA synthesized from the classical metathesis catalyst of WCl 6 – Ph 4 Snrtoluene is also given in Fig. 1C. The tungsten-catalyzed polymerization proceeded in a stereostructurally irregular fashion and the resulting polymer possessed a random mixture of the cis and trans segments. The NMR spectrum is characterized by broad absorption ‘peaks’, and there is no defined absorption peak of the cis proton at d 5.84. It is difficult, if not impossible, to use Eq. Ž1. to evaluate the cis content of the random PPA. Fig. 2 shows the absorption spectra of chloroform solutions of the PPAs with various cis contents. For 67% cis PPA, there exists an absorption shoulder at ; 400 nm. As the cis content increases, the absorption band becomes more pronounced. This absorp-
Fig. 2. Absorption spectra of PPAs with different cis contents.
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tion band is due to the p–pU interband transition of the polymer main chain w7x. It is well known that the p–pU interband transition for trans PA is at a lower energy than that for the cis PA. The enhanced lowest absorption band at 400 nm for 100% cis PPA ŽFig. 2. clearly indicates that the lowest energy absorption peak is due to the cis segment rather than the trans one. The stereoregular polymerizations of phenylacetylene initiated by organorhodium catalysts proceed via a cis-insertion mode w15x. The trans segments are the mismatching stereostructural ‘defects’, whose lengths thus should not be too long. Thus, the
trans segments may only consist of a few repeat units of the alternating single and double bonds, whose absorption may well be located in the deep UV region w16x. Fig. 3A shows the time-integrated photoluminescence spectrum of the PPA with 67% cis content. It has a broad emission peak at ; 490 nm. This broad PL spectrum shows the cis PPA has a wide distribution of conjugation length. All the other PPAs with different cis contents Ži.e., 70.5–100%. emit fluorescence with spectral shape and peak positions being essentially the same as that in Fig. 3A. This result indicates that the cis segment in the chain is responsible for the light emission in the PPAs. The relative PL intensity as a function of cis content is plotted in Fig. 3B. It is clearly shown that the PL intensity decreases linearly with increasing cis content. For example, the emission intensity of the 100% cis PPA is only about one third of that of the PPA with 67% cis content. Fig. 4 shows the time-resolved PL at the emission peak of 490 nm for the PPAs with various cis contents. The PL shows multi-component decays for all the cis-rich PPAs. This result indicates that the recombination dynamics in these materials is very complex. The PL has two fast and one slow decay components. Two fast components t 1 and t 2 have decay time constants of a couple of picoseconds Žps. and a few tens of picoseconds, respectively. These fast decay components are most likely associated with non-radiative recombination processes. The decay time for the slow component is of the order of a few hundred picoseconds and can be observed more clearly in a longer timescale ŽFig. 4 inset.. In general, the PL decays faster with increasing cis content. Furthermore, the PL rise time is very fast Ži.e. within 1 ps. for all the cis-rich PPAs studied.
4. Discussion Fig. 3. ŽA. PL spectrum of PPA with 67% cis content and ŽB. relative PL intensity as a function of cis content. The PL spectra for PPAs with different cis contents are measured in exactly the same optical configuration and using the same excitation laser power. The amount of excited laser absorption for different samples are measured. The relative intensity is then obtained by integrating the PL intensity in the emission curve and the difference in excited laser absorption for PPAs with different cis content is also corrected.
The absorption peaks for various mono- and disubstituted PAs span from near UV to the red spectral region w4,9x. For example, the absorption peak for PPA, which is the subject of this study, is at ; 400 nm Žcf. Fig. 2.. On the other hand, PTMSPA has an absorption peak at ; 540 nm with a strong
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Fig. 4. Time-resolved PL of PPAs with various cis contents. The time constants for the two fast decay components are also shown. The inset shows the time-resolved PL at large delay timescale for the 67% cis PPA. The dotted line is the theoretical fit to the fast decay component using a biexponential response function with time constants t 1 and t 2 .
absorption tail extends to 700 nm. However, all these absorption peaks for various kinds of substituted PAs are blue shifted relative to the unsubstituted trans PA. The current understanding of this result can be explained as the consequence of the introduction of bulky side groups in the substituted PAs. The side groups cause a local distortion of the polymer backbone due to steric hindrance. The backbone twisting
limits the effective conjugation length of the polymer chains, resulting in the blue shift of the absorption of the substituted PAs, compared to the parent PA. Furthermore, the steric effect generally increases as the substitution changes from mono to disubstitution. Consequently the absorption peak further blue shifts and the luminescence strengthens for disubstituted PAs. The observation of strong PL in disubstituted
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PAs was unusual because the polymers supposedly consist of a trans backbone which is non-luminescent in PA w6x. The explanation for this PL result was that the distortion of the polymer chains in substituted PAs due to the bulky sidegroups push the originally lower energy 2 1A g state above the 1 1 B u state which enhances the radiative recombination w5,6x. As we mentioned earlier, using classical metathesis catalysts to synthesize substituted PAs usually results in a mixture of cis and trans configurations, thus the exact location of the light-emitting species in the trans or cis segments is still uncertain. Another observation is that most of the substituted PAs are strongly luminescent in blue or green color w4–7x, but some are non-luminescent or only weakly luminescent particularly for those with absorption in the red spectral region w8,9x. Our results on the PPAs described above may provide an additional explanation for the interesting absorption and emission phenomena of the substituted PAs. In the present study, we demonstrated that the cis-rich PPAs can be synthesized using our rhodium-initiated aqueous polymerization technique. On the other hand, trans-rich PTMSPA is said to be preferentially formed using the WCl 6 catalyst w8x. This indicates that the formation of cis or trans configuration in the polymer backbones depends on the polymerization technique Žespecially the catalyst system. used andror the nature of the substituents or side groups. The combination of preferred cis or trans configuration and chain conformation due to steric hindrance may determine the absorption and emission behavior of the substituted PAs. Thus, the strongly luminescent substituted PAs with absorption in the short wavelength region may be dominantly cis-rich having a large backbone distortion or twisting, while the nonluminescent or weakly luminescent substituted PAs with absorption in orange–red wavelength region are probably trans-rich having a relatively long conjugation length. Our result shows that the luminescence efficiency decreases as the cis content of the PPA increases. This indicates that the non-radiative decay rate increases by increasing the cis content, which was confirmed by the time-resolved PL in Fig. 4 where rapid decay was observed. The origin of this ultrafast non-radiative recombination is uncertain, and experiments are currently under way to investigate the
rapid recombination processes. The soliton model was used to explain the luminescent properties of unsubstituted PAs w1x. Since soliton can only exist in the trans segment, one would therefore expect the PL efficiency to increase with increasing cis content. Our experimental results clearly conflict with this expectation, and thus the soliton model alone cannot explain the high PL efficiency observed in the PPAs. Since most of the substituted PAs consist of a cisrtrans mixture, solitons are still able to exist in the trans segments of the polymer chains even for cis-rich samples. Therefore, it is no surprise that there are evidences of rapid formation and decay of soliton–antisoliton pairs in PTMSPA w8,17,18x; and generation of spinless states upon doping in PDPA w19x. In fact, the substituted PAs with cisrtrans mixtures will support excitons, polarons, as well as solitons; indeed, all these elementary excitations have been observed in disubstituted PAs w20x. It is known that the stereoregularity of the PPA increases by increasing its cis content w10–12x, implying that there will be more short segments of cis PPA chains as the cis content decreases. This is further evidenced in the absorption spectrum of the PPAs. While the stereoregular PPA with 100% cis content exhibits a pronounced absorption peak at around 400 nm, the absorption spectrum of a random PPA in the same wavelength range is a structureless tail with low absorptivity Žcf. spectrum D in Fig. 6 in Ref. w21x.. Steric hindrance will affect the effective conjugation length and hence the emission efficiency w4x. The steric hindrance that causes the twisting of the polymer backbone reduces the effective conjugation length of the polymer chain; this in turn leads to better confinement of light-emitting excitons. These confined or trapped excitons will have fewer chances of encountering non-radiative sites, and a better overlap of the electron and hole wavefunctions of the excitons leads to more efficient PL. Thus, it seems that the high emission efficiency observed in the cis-rich PPAs is due to the confinement of the excitons within the short chain segments as a result of both steric hindrance and reduced stereoregularity. This stronger confinement of excitons by the short chains results in higher PL efficiency. It can be argued that the PL could have originated from the trans segment even for the 100% cis-rich PPA because the presence of a few trans units
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cannot be excluded, even in the 100% cis material due to the resolution limit of the NMR Ži.e. ; 0.5%.. The presence of a few trans units could have given rise to the observed luminescence, as an energy transfer mechanism such as Forster transfer is very ¨ efficient. However, we believe that this is unlikely to be the case for the following reasons. A relatively long PL rise time is expected for the energy transfer from the cis segments to the few trans units w22x, particularly for the 100% cis PPA sample according to the above model. Also the PPAs are in solution so that the interchain interaction cannot be effective, therefore the interchain energy transfer would be slow. As shown in Fig. 4, the observed PL rise time is less than 1 ps for all the cis-rich PPAs studied. Furthermore, the absorption results shown in Fig. 2 indicate that the absorption due to the short trans segments may be located in the deep UV, thus the PL from these trans units should also emit in a similar spectral region. The observed PL is in the blue–green spectral region which clearly conflicts with this expectation.
5. Conclusions We investigated absorption and emission properties of a series of PPAs with controlled amounts of cisrtrans ratio. The precise cisrtrans ratios in the stereoregular PPAs can be determined by NMR. The lowest energy absorption band arising from the p–pU interband transition is due to the cis segments in the PPA chains. The PL of the PPAs with cis contents ranging from 67 to 100% is relatively strong, and both the emission efficiency and the PL lifetime decrease with increasing cis content. This observation again suggests that the light-emitting species is located in the cis segments in the polymer chain. The time-resolved PL indicates complex recombination dynamics with fast non-radiative decay channels within the first few picoseconds. The enhancement in the emission efficiency and the PL lifetime for samples with a low cis content can be explained by the lower degree of stereoregularity, with decreasing cis content resulting in short segments of cis PPA giving rise to strong PL due to the confinement effect. Our results suggest that both the degree of the
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conformational twisting of the polymer chain Žwhich is controlled by the structure and size of the substituents or side groups. and the amount of cisrtrans content Žwhich is controlled by the polymerization process employed. determine the luminescence efficiency of the substituted PAs.
Acknowledgements This research was supported in part by the Research Grant Council of Hong Kong. The PL experiments were performed in the Joyce M. Kuok Laser and Photonics Laboratory and the Zheng Ge Ru Thin Film Physics Laboratory at HKUST.
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