J. Phys. Chem. B 1999, 103, 7853-7859
7853
Fluorescence of Poly[2-methoxy-5-(2′-ethylhexoxy)-p-phenylene vinylene]. Effect of Pressure in a Variety of Solid Polymeric Media G. Yang, Y. Li, J. O. White, and H. G. Drickamer* School of Chemical Sciences, Department of Physics and The Frederick Seitz Materials Research Laboratory, UniVersity of Illinois, 600 S. Mathews AVenue, Urbana, Illinois 61801-3792 ReceiVed: July 16, 1999
The effect of pressure to 65 kbar has been measured on the steady-state and time-dependent emission of MEH-PPV in seven polymers. Both the shape and intensity of the emission spectra vary strongly with pressure. In all blends there is a drop in intensity over the first 20-25 kbar probably in part associated with long-range intramolecular electron transfer increased by compression. Above 30 kbar some blends show a considerable increase in intensity in the higher energy region of the spectrum. The time-dependent emission can, in general, be fit using single or multiple exponentials. However, in all but one blend the emission at high energy can be equally well fit using the Fo¨rster energy-transfer theory. Those blends that show a significant increase in intensity associated with the higher energy emission at high pressure also show an increase in the efficiency of energy transfer. Those blends that do not show an increase in energy transfer efficiency show no increase in intensity.
Introduction A variety of polymers that fluoresce and conduct electricity have been widely studied in recent years.1-3 Derivatives of poly(p-phenylene vinylene) (PPV) are prominent among these.4-9 The fluorescence properties have been largely discussed qualitatively in terms of polarons and bipolarons.10-12 In more chemical terminology the strongest peak is discussed only as S1 f S0 with a vibrational sideband,13 but the nature of the emission is not established nor the path from the absorbing state, although some form of charge transfer is implied. For some emitting polymers the absorbing and emitting centers are suggested to be as much as five to six monomer units apart, but no real evidence is offered and no such speculations have been applied to PPV derivatives explicitly. Most of the studies refer to their useful electrooptical behavior. The motivations for this set of experiments is to provide a better basis for a satisfactory treatment by theorists and to introduce the possibility of using these materials as probes of the interior conformation and interactions of polymer hosts. In a previous paper14 we presented the effect of pressure on the steady-state and time-dependent emission for MEH-PPV as a neat polymer and in blends with polystyrene (PS) and poly(methyl methacrylate) (PMMA). The steady-state spectra were structured such that to obtain the intensity as a function of pressure for the neat polymer, it was necessary to fit the data with three Gaussian bands, while to fit the blends that displayed significant emission at higher energy than the neat polymer, additional bands were used. We emphasized that these were empirical fits and that each band might represent combinations of emissions. The three lower energy bands in the blend clearly resembled those in the neat polymer, so we arbitrarily grouped the total area under these three bands as a LEB and labeled the sum of the areas under the additional higher energy bands as HEB. * To whom correspondence should be sent.
The main results were the following. The neat polymer was much less efficient in fluorescence than the blends, and the emission intensity in the neat polymer decreased rapidly (but reversibly) with increasing pressure. This result was assigned to interpolymer electron transfer and quenching, which increased with compression. Second, the emission from the PS blend was significantly more intense per molecule excited than for PMMA and there were differences in the time-dependent behavior. We associated these effects in large part to weaker van der Waals (polarizability) attraction between MEH-PPV and PMMA and consequent differences in the geometry of MEH-PPV, which leads to long-range intrapolymer quenching. The intensity of the LEB decreased with pressure and leveled above ∼30 kbar. The HEB exhibited an initial decrease in intensity and then an increase above 20-30 kbar. Lifetimes taken at different energies could be fit well by multiple exponential decays, but the highest energy time dependence could be fit equally well by the equation describing Fo¨rster energy transfer. It was postulated that conceivably the increase in HEB intensity at higher pressures was due to an increase in effectiveness of the energy transfer. In this paper we present the results of high-pressure luminescence measurements involving very dilute blends of MEHPPV in seven polymers. Experiment Poly[2-methoxy-5-(2′-ethylhexoxy)-p-phenylene vinylene] (MEH-PPV) was obtained from Prof. F. Wudl and Dr. R. Helgeson and used without further purification. Poly(2-vinylnaphthalene) (P2VN, molecular weight 100 000), poly(4-methylstyrene) (P4MS, molecular weight 72 000), poly(tert-butyl methacrylate) (PtBMA), and poly(4-vinylpyridine) (P4VP, molecular weight 160 000) were purchased from Aldrich. Poly(vinyl chloride) (PVCl, medium molecular weight), poly(vinyl acetate) (PVA, medium molecular weight), and poly(4-chlorostyrene) (P4CS) were purchased from Polysciences, Inc. The polymers were used without further purification because none
10.1021/jp992449o CCC: $18.00 © 1999 American Chemical Society Published on Web 08/31/1999
7854 J. Phys. Chem. B, Vol. 103, No. 37, 1999
Figure 1. Emission spectra of MEH-PPV in poly(4-methylstyrene) (P4MS).
of them gave any emission when irradiated at the excitation laser wavelengths. MEH-PPV and the polymer were dissolved in spectral grade chloroform or tetrahydrofuran, and the solution was then poured in a glass dish to form a blend film after solvent evaporation at room temperature. The films were then put in a vacuum oven for a few days at 45 °C. We used red light in processing the materials, and transparent films were obtained. The concentration of MEH-PPV in the polymers was very low to avoid phase separation (0.025 wt % in PVA; 0.05% in PVCl, P4MS, P4CS, P4VP, PMMA, and PtBMA; 0.1% in P2VN). The blends are studied in a Merrill-Bassett diamond anvil cell (DAC). The hole diameter of the gasket to hold the sample is about 300 µm. The experimental setups for the luminescence, absorption, and the time dependence of the emission experiments for the DAC have been described elsewhere.14-17 For the steady-state emission measurements the excitation is by means of the 441.6 nm line of a He-Cd CW laser. For the measurements of timedependent decay of the emission the samples are excited by 100 fs pulses from a 76 MHz mode-locked Ti:sapphire laser after frequency doubling to 405 nm with a BBO crystal. The detection system involves a Spex 0.27 m monochromator, a Tennelec 863 TAC, an Ortec 582 discriminator, and Hamamatsu R1564 MCP-PMT. The system has a basic exponential decay with a time constant of ∼15 ps. However, at an intensity of about one-100th of the initial value there is a further complex component of about 100 ps lifetime. Results Steady State. In Figures 1-7 we present the spectra at a number of pressures for each of the seven blends involved in this study. In addition to the differences in the 1 atm spectra the remarkable feature is the very large differences in the perturbation introduced by compression. In an earlier paper14 we showed that the neat polymer emission could be fit well with three bands. All of these bands exhibit, in addition to features analogous to the spectrum of the neat polymer, significant emission at higher energy. As discussed in that paper, we fit these spectra by introducing additional bands. For these spectra three additional bands give
Yang et al.
Figure 2. Emission spectra of MEH-PPV in poly(4-chlorostyrene) (P4CS).
Figure 3. Emission spectra of MEH-PPV in poly(2-vinylnaphthalene) (P2VN).
a good fit. An example is shown in Figure 8. Since the fit is arbitrary, we divide the area into a low-energy band (LEB) consisting of the three peaks universally present and a highenergy band (HEB) consisting of the peaks present only in the blends. It must be pointed out that the fits are accurate but empirical and that each band may consist of combinations of emissions. There is a degree of arbitrariness in the split between LEB and HEB, but the trends are unmistakable. Table 1 presents the relative emission efficiency at 1 atm for the seven blends, normalized for the concentration and for the absorption efficiency at 441.6 nm. Also shown is the relative amounts of LEB and HEB at 1 atm. Figure 9 exhibits the change of total intensity with pressure. All bands show a drop by a factor of 0.4-0.6 in the first 2030 kbar, but the behavior at higher pressure varies strongly. In PVCl there is a very large increase by a factor of over 2 by 65 kbar. P2VN and P4CS show very significant increases, and P4MS has a more modest increase. The other bands show no
Fluorescence of MEH-PPV
J. Phys. Chem. B, Vol. 103, No. 37, 1999 7855
Figure 4. Emission spectra of MEH-PPV in poly(4-vinylpyridine) (P4VP).
Figure 6. Emission spectra of MEH-PPV in poly(vinyl acetate) (PVA).
Figure 5. Emission spectra of MEH-PPV in poly(vinyl chloride) (PVCl).
Figure 7. Emission spectra of MEH-PPV in poly(tert-butyl methacrylate) (PtBMA).
increase in intensity or a small continuing decrease. A possible explanation for this result will appear when the time dependence is discussed below. The fraction of LEB (Figure 10) drops for all blends at pressures greater than 30 kbar. The increase in total intensity from 30 to 65 kbar is very largely if not entirely due to an increase in intensity of the HEB. The bands are, in general, 700-1000 cm-1 wide and do not change in width significantly with pressure. There is a shift to lower energy with increasing pressure of 300-800 cm-1, but above ∼30 kbar there is for some bands a small (100-300 cm-1) blue shift. In view of the empirical nature of the bands one cannot emphasize any interpretation of blue shifts. In any case, they are much too small to account for the large intensity changes via the energy gap law as applied, for example, to (π* f p) or (π* f n) emissions. The data presented in Figures 9 and 10 are the average of two or more runs that were in close agreement. Upon release
of pressure the spectrum immediately returned to the same shape (distribution of emission) as the original 1 atm spectrum. The total intensity sometimes showed deviations of (10-12%. On a couple of occasions the released samples sat for 24 h or more in the cell and the intensity approached more closely the original value. Time Dependence. The time dependence of the emission was measured at two or three different wavelengths and, in general, at five different pressures for each blend. Except for the decays obtained for PVCl and P2VN at the longer wavelength (lower energy), it was necessary to fit the emission with two or three exponentials. The results are summarized in Table 2. Where the progression with pressure is regular we present only the fitting at 1 atm and the highest pressure. The decays at the shortest wavelength (highest energy) were the most complex and required three exponentials, one in the range 1-2 ns, one in the range 0.3-0.6 ns, and one in the range 0.04-0.08 ns. It
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Figure 9. Change of total emission intensity with pressure for the seven blends. Figure 8. Typical fit for emission spectra of MEH-PPV in poly(vinyl chloride) (PVCl) at 1 atm and 65 kbar.
TABLE 1: Relative Fluorescence Emission Ability of MEH-PPV in Polymers polymer
total
LEB
HEB
R(LEB/HEB)
PVCl P2VN P4CS PVA P4VP P4MS PTBMA neat MEH-PPV
9.51 1.77 5.31 2.20 1.32 1.07 2.57 1.00
8.92 1.57 4.47 1.27 0.61 0.62 1.38 1.00
0.59 0.20 0.84 0.93 0.71 0.45 1.19
15.1 7.9 5.3 1.4 0.9 1.4 1.2
must be pointed out that in view of the limitations presented by the instrument decay (detailed in ref 14), lifetimes shorter than 0.06-0.08 ns must be considered as upper limits to the relaxation time. Certain generalizations are possible. At all wavelengths all relaxation times get longer with increasing pressure, even in the regions where the intensity drops strongly. This tends to emphasize the decoupling of the emission from the energy gap law, which relates a decrease in emission energy to an increase of the nonradiation decay rate from the emission state and consequent loss of intensity and faster decay times. In a previous paper on PMMA and PS we pointed out that the decay at high energy could be fit at least equally well using the Fo¨rster energy-transfer theory18,19 in the form
ln
[
()]
I t t )+ 2γ I0 τD τD
1/2
(1)
where τD is the donor lifetime. (We treat the highest energy peak as the donor.) γ is a coupling coefficient given by:
2 γ ) Π3/2nAR03 3
(2)
Figure 10. Fraction of LEB vs pressure for the seven blends.
where nA is the concentration of acceptors and R0 is the “Fo¨rster distance” i.e., the D-A distance such that there is a 50% chance of radiationless energy transfer. Figure 11 displays typical decay curves for PVA and P2VN. The line through the data represents the fit using Fo¨rster energytransfer theory. The fit using multiple decays is equally good. Fits of this high quality were found for PVCl, P2VN, PtBMA, P4MS, and PVA. For P4MS the fit to the 1 atm data was equally good, but the higher pressure data gave poorer fits. No satisfactory fits were obtained for P4VP.
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J. Phys. Chem. B, Vol. 103, No. 37, 1999 7857
TABLE 2: Fluorescence Decay of MEH-PPV in Polymersa polymer P2VN
P4CS
PVCl
P4MS
P4VP
PtBMA
PVA
b
energy-transfer fitting
measured position/nm
pressure
565 595 640 685 530 530 565 590 640 680 525 530 580 600 600 650 670 570 540 580 570 665 660 525 530 510 585 580 675 665 515 500 585 550 645 645 515 500 585 545 665 615 505 490
1 atm 48 kbar 1 atm 48 kbar 1 atm 48 kbar 1 atm 47 kbar 1 atm 47 kbar 1 atm 47 kbar 1 atm 15 kbar 62 kbar 1 atm 62 kbar 15 kbar 62 kbar 1 atm 61 kbar 1 atm 61 kbar 1 atm 34 kbar 61 kbar 1 atm 63 kbar 1 atm 63 kbar 1 atm 63 kbar 1 atm 61 kbar 1 atm 61 kbar 1 atm 61 kbar 1 atm 68 kbar 1 atm 68 kbar 1 atm 68 kbar
τ/ns
γ
1.07 2.04
0.52 0.94
1.76 3.69b
0.82 1.44b
0.92 1.14
0.41 0.66
1.75 1.53 2.22
1.45 1.52
2.86 2.05
0.69 0.49 0.84
0.34 0.35
0.96 0.45
exponential fitting a1
τ1/ns
0.23 0.14 0.11 0.11 0.12 0.24 0.19 0.10
0.95 1.57 1.20 1.67 1.22 1.39 1.27 2.03
0.45 0.13 0.07 0.62 0.10 0.16 0.24 0.31 0.10 0.18 0.15 0.18 0.13 0.29 0.17 0.08 0.25 0.06 0.35 0.44 0.41 0.07 0.58 0.05 0.61 0.38 0.39
0.76 1.02 1.00 1.10 1.15 1.44 1.36 1.32 2.08 1.25 1.90 1.14 1.60 1.50 2.47 1.16 1.51 1.18 1.48 1.30 1.36 1.31 1.44 1.30 1.36 1.48 1.74
a2
τ2/ns
1.00 1.00 1.00 1.00 0.57 0.60 0.89 0.89 0.88 0.76 0.55 0.54 1.00 1.00 1.00 1.00 1.00 0.41 0.58 0.31 0.38 0.30 0.51 0.54 0.49 0.58 0.42 0.50 0.33 0.32 0.40 0.29 0.29 0.75 0.28 0.65 0.48 0.46 0.19 0.42 0.20 0.39 0.30 0.44
0.58 0.99 0.59 1.02 0.47 0.54 0.62 0.81 0.61 0.83 0.56 0.69 0.57 0.70 0.92 0.58 0.96 0.33 0.49 0.39 0.56 0.41 0.75 0.56 0.62 0.75 0.51 0.65 0.39 0.59 0.58 0.68 0.45 0.93 0.43 0.92 0.59 0.67 0.48 0.70 0.49 0.69 0.45 0.76
a3
τ3/ns
0.20 0.26
0.03 0.05
0.26 0.36
0.09 0.17
0.14 0.29 0.62
0.05 0.05 0.05
0.60 0.33 0.22 0.20 0.32 0.40 0.35 0.49 0.55 0.31 0.54 0.63
0.05 0.03 0.04 0.06 0.06 0.06 0.07 0.06 0.07 0.07 0.12 0.04
0.66
0.04
0.08 0.13 0.74
0.02 0.04 0.07
0.75
0.07
0.32 0.17
0.05 0.03
a a , a , a : relative amount of the decay curve associated with the corresponding τ. τ: lifetime. γ: coupling coefficient, defined in the text. 1 2 3 The fit at high pressure for P4CS is poorer than the rest. The value is included to indicate the trend.
If there is no change in the character of the acceptor with pressure, nA should increase by 30-35% owing to the compression of the medium. From eq 2 this would imply increases in R0 of ∼10% in PVCl, P2VN, and (probably) P4CS, virtually no change in R0 for PtBMA, and a decrease of ∼30% in PVA. Discussion It would be highly desirable to discuss these results in terms of the nature of the absorbing and emitting states. There exist no such assignments, as mentioned in the Introduction, and our results are not of the nature to permit us to make them. The new bands that appear at higher energy in the blends (the HEB) and that grow relative to the LEB have not previously been noted in any paper we can find. It is possible that these represent shorter-range electron transfer where a new emitting state arises nearer the absorbing center, but this is pure speculation. It is hoped that theorists will be encouraged by these results to attack the assignment problem. A question may arise as to the degree of clustering of the MEH-PPV in the host polymers. In the first place the blends appear perfectly clear and uniform even under the microscope.
In the second place, we have made measurements at different further dilutions (e.g., PVCl at 0.025% and 0.0063%) for several of the blends and find that both the 1 atm spectrum and the pressure effect are identical with those shown for the blends presented here. In all blends the intensity drops in the first ∼20-25 kbar, primarily owing to effects in the LEB. One argument is that this is due to clustering and intermolecular charge transfer, which increases with compression. It seems to us more likely that the MEH-PPV assumes configurations that permit long-range intramolecular charge transfer that would also be an effective quenching mechanism and would become more effective with compression. If the rate-controlling step is an electron-transfer process along the chain from the absorbing to the emitting site, the increase in lifetime τ in the first 20-25 kbar could allow for more thermal dissipation of the excitation and account for the observed decrease in emission intensity. The behavior above 25-30 kbar is more diverse. For PVCl and P2VN there is a strong increase in intensity. A significant
7858 J. Phys. Chem. B, Vol. 103, No. 37, 1999
Yang et al. efficiency of Fo¨rster energy transfer. (It should be noted that the quantitative value for P4CS at 47 kbar is quite suspect.) For P4MS there is at first a decrease in efficiency and then a significant increase that mirrors the behavior of the intensity. For PtBMA there is no change in efficiency, while PVA shows a decrease. These blends exhibit no change in intensity at high pressure. The high-energy emission for P4VP could not be fit to the energy-transfer equation, and the intensity shows a continuous decrease with increasing pressure. A really quantitative application of FET would, of course, involve isolating and studying the emission characteristics of the donor with no acceptor present. Such a study has been made in polymers under pressure using well-defined donor and acceptor molecules.20 In the experiments presented here neither the donor nor the acceptor is defined. The application of FET is a first-order attempt to explain the surprising increase in intensity of emission at high pressure in some media and not others. The existence of energy transfer has not been proven. However the very good qualitative agreement between the changes in HEB intensity and the change in calculated FET efficiency appears to be too consistent to be coincidental. Conclusions
Figure 11. Typical time-dependent decays: (top) decays of blend in PVA at 1 atm (505 nm) and 68 kbar (490 nm); (bottom) decays of blend in P2VN (at 530 nm) at 1 atm and 48 kbar. (O, 4) experimental data; (-) fitting with Fo¨rster energy transfer. The multiple exponential fit is essentially identical.
TABLE 3: Relative Fo1 rster Efficiency (E) Normalized to 1 atm for MEH-PPV in Polymers at Higher Energy Side of the Emission polymer
pressure
E
P2VN
1 atm 48 kbar 15 kbar 62 kbar 1 atm 34 kbar 61 kbar
1.00 1.32 1.00 1.32a 1.00 0.84 0.88
PVCl P4MS
a
polymer
pressure
E
P4CS
1 atm 47 kbar 1 atm 68 kbar 1 atm 61 kbar
1.00 1.20 1.00 0.68 1.00 1.02
PVA PtBMA
Normalized to 15 kbar value.
increase is also observed for P4CS and a modest increase for P4MS. There is little change of intensity above 30 kbar in PVA and PtBMA and a distinct decrease for P4VP. The increases in intensity are almost totally associated with the HEB. A plausible explanation can be based on the energy-transfer concept. Fo¨rster’s analysis gives three different methods of establishing the efficiency of energy transfer, which have been shown to give equivalent values. The one that can be applied to our data is
E ) π1/2γ exp(γ2)[1 - erf(γ)]
(3)
where the error function erf(γ) is given by:
∫0γ exp(-x2) dx
erf(γ) ) 2π1/2
(4)
In Table 3 we list the efficiencies at high pressure relative to that at 1 atm, or in the case of PVCl, to 15 kbar where a significant amount of HEB is first apparent. For PVCl, P2VN, and P4CS in which blends the largest increase in intensity is observed at high pressure, there is a significant increase in the
The emission characteristics of MEH-PPV in solid polymeric media are strongly perturbed by compression. These pressure effects are very different in different polymeric media. The complex emission spectra can be divided into a LEB, which is also seen in the neat polymer, and a HEB, which consists of a group of emissions that appear at higher energy in the blends. The decrease in emission intensity up to ∼20-25 kbar is probably associated with long-range intramolecular electron transfer. In some polymeric media there is an increase in emission above 25-30 kbar associated with the HEB. In these blends there is an increase in the efficiency of the Fo¨rster energy transfer calculated for the highest energy emission. For blends that show no increase of Fo¨rster emission efficiency with pressure there is no increase in emission intensity. This would indicate that energy transfer may be a significant component in establishing the efficiency of HEB emission. Acknowledgment. The authors express our very profound gratitude to Professor Fred Wudl and Dr. Roger Helgeson of UCLA for furnishing us with the sample of MEH-PPV. We are also very pleased to acknowledge the continuing support from the U.S. Department of Energy, Division of Materials Science, Grant DEFG 02-96ER45439, through the University of Illinois at Urbana Champaign Frederick Seitz Materials Research Laboratory. The lifetime studies were performed in the MRL laser laboratory. References and Notes (1) Berggren, M.; Granstrom, M.; Inganas, O.; Anderson, M. AdV. Mater. 1995, 7, 900. (2) Grem, G.; Leditzky, G.; Ullrich, B.; Leising, G. AdV. Mater. 1992, 4, 36. (3) Tessler, N.; Denton, J.; Friend, R. H. Nature (London) 1996, 382, 695. (4) Wudl, F.; Alleman, P. M.; Srdanov, G.; Ni, Z.; McBranch, D. ACS Symp. Ser. 1991, 455. (5) Buroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature (London) 1990, 347, 539. (6) Nguyen, T.-Q.; Doan, V.; Schwartz, B. J. J. Chem. Phys. 1999, 110, 4068. (7) Tang, C. W.; Vanslyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (8) Dogariu, A.; Vacar, D.; Heeger, A. J. Phys. ReV., B 1998, 58, 10218.
Fluorescence of MEH-PPV (9) Frolov, S. V.; Gellermann, W.; Ozaki, M.; Yoshino, K.; Vardeny, Z. V. Phys. ReV. Lett. 1997, 78, 729. (10) Blatchford, J. W.; Jessen, S. W.; Lin, L. B.; Lih, J. J.; Gustafson, T. L.; Epstein, A. J.; Fu, D. K.; Marsella, M. J.; Swager, T. M.; MacDiarmid, A. G.; Yamaguchi, S.; Hamaguchi, H. Phys. ReV. Lett. 1996, 76, 1513. (11) Yan, M.; Rothberg, L. J.; Papadimitrakopoulos, F.; Galvin, M. E.; Miller, T. M. Phys. ReV. Lett. 1994, 72, 1104. (12) Mizes, H. A.; Conwell, E. W. Phys. ReV. B 1994, 50, 11243. (13) Gelinck, G. H.; Warman, J. M.; Staring, E. G. J. J. Phys. Chem. 1996, 100, 5485.
J. Phys. Chem. B, Vol. 103, No. 37, 1999 7859 (14) Yang, G.; Li, Y.; White, J. O.; Drickamer, H. G. J. Phys. Chem. B 1999, 103, 5181. (15) Jurgensen, C. W.; Drickamer, H. G. Phys. ReV. B 1984, 30, 7202. (16) Dreger, Z. A.; Lang, J. M.; Drickamer, H. G. Chem. Phys. Lett. 1991, 185, 184. (17) Dreger, Z. A.; Yang, G.; White, J. O.; Li, Y.; Drickamer, H. G. J. Phys. Chem. A 1997, 101, 9511. (18) Fo¨rster, T. Z. Ann. Phys. (Leipzig) 1948, 55, 6. (19) Fo¨rster, T. Z. Faraday Soc. Discussion, Chem. Soc. 1959, 27, 7. (20) Lang, J. M.; Drickamer, H. G. J. Phys Chem. 1993, 97, 5058.
