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Langmuir 1999, 15, 5676-5680
Notes Photophysical Consequences of Conformation and Aggregation in Dilute Solutions of π-Conjugated Oligomers Keith A. Walters, Kevin D. Ley, and Kirk S. Schanze* Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200 Received February 22, 1999. In Final Form: May 5, 1999
Introduction π-Conjugated polymers have been the focus of considerable recent interest due to their unique photophysical properties and the possibility that the materials may be used as the active medium in electronic, optical, and/or optoelectronic devices.1 In dilute solution alkyl- and alkoxy-substituted polyphenylenevinylene (PPV), polyphenyleneethynylene (PPE), and related polymers feature strong absorption and fluorescence bands arising from long-axis polarized π,π* transitions localized on a single polymer chain (i.e., intrachain excitations).2 By contrast, in the solid state (i.e., as amorphous films) the photophysics of the polymers originate from states that absorb and luminesce at a lower energy than the intrachain π,π* states.1a,2e,3 Although it is clear that the photophysical properties of the polymers in the solid state arise from “excimer-like” states created by interchain interactions, little evidence was available until recently for the involvement of interchain states in the photophysics of the polymers in dilute solution.4 We recently synthesized and examined the photophysics of a series of π-conjugated PPE-type oligomers that incorporate the metal chelating 2,2′-bipyridine-5,5′-diyl unit (Chart 1). These monodisperse oligomers were examined as models for structurally related π-conjugated polymers containing transition metal chromophores.5 During routine variable-temperature luminescence stud* To whom correspondence should be addressed. E-mail:
[email protected]. (1) (a) Bassler, H.; Rothberg, L. J. Chem. Phys. 1998, 227, 1. (b) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. Engl. 1998, 37, 402. (c) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R. Handbook of Conducting Polymers, 2nd ed.; Marcel Dekker: New York, 1998. (2) (a) Patil, A. O.; Heeger, A. J.; Wudl, F. Chem. Rev. 1988, 88, 183. (b) Swager, T. M.; Gil, C. J.; Wrighton, M. S. J. Phys. Chem. 1995, 99, 4886. (c) Yang, J. S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 5321. (d) Scurlock, R. D.; Wang, B.; Ogilby, P. R.; Sheats, J. R.; Clough, R. L. J. Am. Chem. Soc. 1995, 117, 10194. (e) Rothberg, L. J.; Yan, M.; Papadimitrakopoulos, F.; Galvin, M. E.; Kwock, E. W.; Miller, T. M. Synth. Met. 1996, 80, 41. (3) Blatchford, J. W.; Gustafson, T. L.; Epstein, A. J.; VandenBout, D. A.; Kerimo, J.; Higgins, D. A.; Barbara, P. F.; Fu, D. K.; Swager, T. M.; MacDiarmid, A. G. Phys. Rev. B 1996, 54, R3683. (4) (a) Rumbles, G.; Samuel, I. D. W.; Collison, C. J.; Moratti, S. C.; Holmes, A. B. Synth. Met. in press. (b) Samuel, I. D. W.; Rumbles, G.; Collison, C. J.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Synth. Met. 1997, 84, 497. (c) Samuel, I. D. W.; Rumbles, G.; Collison, C. J.; Moratti, S. C.; Holmes, A. B. Chem. Phys. 1998, 227, 75. (5) (a) Peng, Z.; Gharavi, A. R.; Yu, L. J. Am. Chem. Soc. 1997, 119, 4622. (b) Peng, Z.; Yu, L. J. Am. Chem. Soc. 1996, 118, 3777. (c) Ley, K. D.; Whittle, C. E.; Bartberger, M. D.; Schanze, K. S. J. Am. Chem. Soc. 1997, 119, 3423. (d) Ley, K. D.; Schanze, K. S. Coord. Chem. Rev. 1998, 171, 287.
ies of oligomers 1 and 2, we made observations which suggest that the oligomers aggregate in dilute solution at low temperatures. Specifically, the π,π* fluorescence observed from dilute solutions of the oligomers red-shifts, broadens, and decreases in intensity with decreasing temperature. Remarkably similar photophysical behavior was recently reported for a cyano-substituted alkoxyphenylenevinylene polymer (CN-PPV) (both in solution and in the solid state), and the phenomenon was attributed to competition between luminescence from intrachain and interchain (excimer) states arising from chromophore aggregates.4 Additionally, it is known that poly-3-alkylthiophenes exhibit thermochromism and solvatochromism which is due in part to aggregation.6 However, to our knowledge aggregate formation has not been previously reported for dilute solutions of medium molecular weight π-conjugated oligomers such as 1 and 2.7 Given the unusual nature of the initial observations made on 1 and 2,8 we initiated a detailed study of the oligomer variable-temperature photophysics in an effort to understand the origin of the luminescence temperature dependence. In the present note we describe the results of a series of absorption, fluorescence polarization, and resonance light scattering9 experiments which suggest that aggregation is the primary mechanism for the “excimer-like” fluorescence seen from oligomers 1 and 2 at low temperatures. Experimental Section Oligomers 1 and 2 and rhenium complexes Re-1 and Re-2 were synthesized as described elsewhere.10 2-Methyltetrahydrofuran (2-MTHF, Aldrich) was distilled over CaH2 prior to use. In all cases the sample concentrations were adjusted to produce “optically dilute” solutions (i.e., A < 0.20 at all wavelengths; typical final concentration is ca. 1.5 × 10-6 M). Variable-temperature studies were carried out with samples contained in an Oxford Instruments cryostat interfaced to an Omega CYC3200 automatic temperature controller. Measurements were performed at temperatures above the glass point of 2-MTHF (ca. 170 K) to eliminate the possibility that the observed (6) (a) Christiaans, M. P. T.; Langeveld-Vos, B. M. W.; Janssen, R. A. J. Synth. Metals in press. (b) Rughooputh, S. D. D. V.; Hotta, S.; Heeger, A. J.; Wudl, F. J. Polym. Sci., Part B: Polym. Phys. 1987, 25, 1071. (c) Rumbles, G.; Samuel, I. D. W.; Magnani, L.; Murray, K. A.; Demello, A. J.; Crystall, B.; Moratti, S. S.; Stone, B. M.; Holmes, A. B.; Friend, R. H. Synth. Met. 1996, 76, 47. (7) (a) Oelkrug, D.; Egelhaaf, H. J.; Haiber, J. Thin Solid Films 1996, 285, 267. (b) Gierschner, J.; Egelhaaf, H. J.; D., O. Synth. Met. 1997, 84, 529. (c) Oelkrug, D.; Tompert, A.; Gierschner, J.; Egelhaaf, H. J.; Hanack, M.; Hohloch, M.; Steinhuber, E. J. Phys. Chem. B 1998, 102, 1902. (8) The fluorescence decay time (τf) and quantum yield (φf) for lowto-medium molecular weight organic compounds typically increase with decreasing temperature because the nonradiative decay rate (knr) decreases with temperature. knr is proportional to the excited state/ ground-state vibrational overlap factors (i.e., the Franck-Condon factors) which decrease with temperature due to the decrease in Boltzmann population of excited vibronic levels and decreased electronphonon coupling to outer-sphere solvent modes. In the context of this “typical behavior” of low-to-medium molecular weight organic compounds, the temperature-dependent photophysics of 1 and 2 is unusual. (9) (a) Pasternack, R. F.; Collings, P. J. Science 1995, 269, 935. (b) Pasternack, R. F.; Schaefer, K. F.; Hambright, P. Inorg. Chem. 1994, 33, 2062. (c) Pasternack, R. F.; Bustamante, C.; Collings, P. J.; Giannetto, A.; Gibbs, E. J. J. Am. Chem. Soc. 1993, 115, 5393. (10) Ley, K. D.; Li, Y.; Schanze, K. S. Submitted.
10.1021/la990191n CCC: $18.00 © 1999 American Chemical Society Published on Web 06/19/1999
Notes
Langmuir, Vol. 15, No. 17, 1999 5677 Chart 1
photophysical behavior resulted from solvent glass formation. Absorption measurements were performed on a Hewlett-Packard 8452A diode-array spectrophotometer. Corrected emission, polarization, and resonance light scattering measurements were conducted on a SPEX F-112 fluorimeter. Time-resolved fluorescence decays were measured with time-correlated single photon counting (FLI, Photochemical Research Associates; excitation filter, Schott UG-11 (350 nm max); emission filter, 450, 500, or 550 nm interference filter). Lifetimes were determined from the observed decays with DECAN fluorescence lifetime deconvolution software.11 Wavelength-resolved polarization anisotropies, r(λ), were calculated with eq 112
r(λ) )
IVV - GIVH IVV + 2GIVH
(1)
where G is IHV/IHH and IXY is the emission intensity with the excitation and emission polarizers adjusted according to x and y, respectively (e.g., IHV is the emission intensity with horizontally polarized excitation light and vertically polarized emission detection).
Results A number of optical spectroscopic techniques were applied to 1 and 2 over the 170-298 K range in 2-MTHF solvent, where the solvent is fluid (the glass point is ca. 170 K). Moreover, all of the observed spectroscopic effects were completely reversible (i.e., the same changes were seen during the cooling and warming cycles). Fluorescence spectra for 1 and 2 at temperatures ranging from 170 to 298 K are shown in Figure 1. At room temperature the fluorescence spectra feature a strong band13 that exhibits a small Stokes shift from the absorption (see Figure 2), and on this basis the emission is assigned to the long-axis polarized 1π,π* state. This fluorescence is very similar in energy and band shape to that of related PPE and PPV π-conjugated polymers and oligomers.2,5d,14 The fluorescence of both oligomers redshifts with decreasing temperature, and at the lowest examined temperatures the emission is dominated by a broad band that lies to the red of the assigned “0-0” band. A corresponding thermochromism is seen in the lowestenergy 1π,π* absorption band for both oligomers as shown in Figure 2. The red-shift in the absorption of 1 and 2 is mirrored by corresponding thermally induced enhancement of the red side of the band observed in the fluorescence excitation spectra (data not shown). Excitation spectra measured at two emission wavelengths on the red side of the fluorescence band are nearly indistinguishable, which suggests that the states(s) involved in the observed photophysics are in strong electronic communication. (11) Boens, N.; De Roeck, T. DECAN; 1.0 ed.; Leuven, 1990. (12) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1983. (13) The fluorescence quantum yields of 1 and 2 are 0.70 and 0.80, respectively. Walters, K. A.; Ley, K. D.; Cavalheiro, C.; Schanze, K. S. Submitted.
Figure 1. Corrected emission spectra of (a) 1 at 298, 270, 240, 220, 200, and 180 K, and (b) 2 at 298, 270, 250, 230, 210, 190, and 170 K. Spectra are acquired from 2-MTHF solutions with an excitation wavelength of 380 nm and are corrected for thermochromism.
Decay times of the 1π,π* fluorescence from 1 and 2 at various emission wavelengths and temperatures are listed in Table 1. On the blue side of the fluorescence band (i.e., 450 nm), the fluorescence decay kinetics are always single exponential, and the decay time decreases only slightly with temperature (this effect is most pronounced for 2). By contrast, on the low-energy side of the fluorescence band (i.e., 500 and 550 nm), the decays are biexponential below 180 K for 1 and 220 K for 2. In both oligomers the low-temperature decays feature a long-lived component with a decay time that increases with decreasing emission energy. Although the amplitude of the long-lived component is small (R2 is typically 0.07-0.13 for 500 nm emission and 0.01-0.03 for 550 nm emission), the decay time is significant on the red-side of the band, which (14) Ley, K. D.; Walters, K. A.; Schanze, K. S. Synth. Met. in press.
5678 Langmuir, Vol. 15, No. 17, 1999
Notes
Discussion
Figure 2. Absorption spectra of (a) 1 at 298, 260, 230, 210, 190, and 170 K and (b) 2 at 298, 270, 250, 230, 210, and 190 K. Spectra are acquired from 2-MTHF solutions.
suggests that at low temperatures this component contributes substantially to the total emission yield.15 The wavelength-resolved polarization anisotropy, r(λ), for 1 and 2 was measured at various temperatures, and plots of r(λ) at 170 K are shown in Figure 3 along with their fluorescence spectra. The variation of r(525 nm) with temperature for both oligomers is also illustrated in the insets of Figure 3. At 298 K r(λ) is relatively constant across the fluorescence band (r ≈ 0.24) for both oligomers. With decreasing temperature r(λ) slightly increases on the blue side of the fluorescence band and decreases on the far red side of the band. An abrupt change in the plots is observed for 1 at T < 190 K and for 2 at T < 230 K. Note that the abrupt change in r(λ) for both 1 and 2 occurs at the temperature where the emission decay kinetics become biexponential as previously noted. Resonance light scattering (RLS) spectra for complexes Re-1 and Re-2 in 2-MTHF at 170 K are shown in Figure 4. Spectra are corrected for cryostat scattering by subtracting the “blank” signal (2-MTHF solvent alone in the cryostat) observed at room temperature from the obtained 170 K spectrum. Nevertheless, scattering bands caused by the cryostat windows and xenon arc lamp are observed in the Re-1 spectrum above 450 nm. Strong RLS is observed at 370 nm for Re-1 and 410 nm for Re-2, and these wavelengths closely correspond to the π,π* absorption bands of the metal complexes.14 (15) Birks, J. B. Organic Molecular Photophysics; J. Wiley: New York, 1973.
Although the fluorescence of oligomers 1 and 2 at room temperature is “typical”, the variable-temperature emission data is uncommon.8 Specifically, for both oligomers the fluorescence decreases in intensity and red-shifts with decreasing temperature. Additionally, at low temperatures (but above the glass point of the solvent) a new broad and red-shifted fluorescence band dominates. These effects are opposite to what is typically observed for aromatic fluorophores; lowering the temperature usually induces an increase in the fluorescence lifetime and quantum yield because the nonradiative decay rate (knr) decreases with decreasing temperature.15 The temperature-dependent fluorescence data for 1 and 2 strongly suggest that the oligomers exist in an aggregated state in low temperature solution. The existence of aggregates at the very lowest temperatures is clear. The fluorescence is dominated by a broad and red-shifted band, a feature that is typically associated with “excimer” emission from an excited-state dimer. Furthermore, the broad, red-shifted fluorescence band features a longer decay time than the blue “monomer” emission. This longer decay time is also consistent with an aggregated or “excimer-like” state. This is because an excimer state would have a lower radiative rate (and consequently a longer decay time) compared to a monomer excited state.16 Evidence for aggregation also comes from RLS studies of the metal complex oligomers Re-1 and Re-2. RLS is a sensitive technique that is able to detect aggregates of chromophores that have large oscillator strengths.9a,c Chromophore aggregates typically give rise to enhanced light scattering (i.e., RLS) in the wavelength region that corresponds to where the aggregate absorbs. RLS has been able to detect aggregates when their concentration is insufficient to allow detection by absorption.9a,b One shortcoming of RLS is its inability to discriminate between scattering and fluorescence.9a Consequently, we were forced to carry out RLS on the rhenium(I) complexes Re-1 and Re-2, which are nonfluorescent derivatives of 1 and 2.10,14 The RLS bands shown in Figure 4 for Re-1 and Re-2 suggest that at 170 K chromophore aggregates are present in dilute solutions of both oligomer complexes. As expected, the RLS bands occur in approximately the same region as the absorption of the ground-state oligomer complexes. RLS was also observed from dilute solutions of Re-1 and Re-2 at 298 K, but with a significantly reduced intensity relative to the scattering observed at 170 K (data not shown). This observation suggests that aggregates may be present in dilute solutions at ambient temperature but at a much lower concentration than at lower temperatures. The key to confirming aggregation as the probable mechanism for the fluorescence temperature-dependence of 1 and 2 lies in the polarization anisotropy data. Under ordinary conditions the polarization anisotropy of a fluorophore in dilute solution increases with decreasing temperature.12 The increase occurs because the primary mechanism for loss of polarization in dilute solutions is rotational diffusion, which slows with increasing solvent viscosity. An alternate mechanism for depolarization is long range (Fo¨rster) energy transfer or exchange energy (16) The rate constant for radiative decay of an excimer (krex) is proportional to the square of the transition moment for the groundstate monomer to dimer excited-state transition (Mex). Since the excimer state has some charge-transfer character, Mex is smaller compared to the transition moment of the isolated monomer. For a lucid discussion of the relationship between krex and excimer (and exciplex) structure and energetics, see: Gould, I. R.; Young, R. H.; Mueller, L. J.; Albrecht, A. C.; Farid, S. J. Am. Chem. Soc. 1994, 116, 8188.
Notes
Langmuir, Vol. 15, No. 17, 1999 5679 Table 1. Variable-Temperature Emission Decay Times of Oligomers 1 and 2a 450 nm emission
oligomer
T/K
R1
τ1/ps
1
298 270 230 200 180 298 270 230 220 210 190
1 1 1 1 1 1 1 1 1 1 1
952.2 969.4 1031.5 979.3 945.9 770.8 719.3 737.2 493.6 434.3 458.3
2
R2
500 nm emission τ2/ps
R1
τ1/ps
1 1 1 1 0.926 1 1 1 1 0.871 0.872
1006.3 1017.7 1102.3 1095.5 1087.1 724.1 773.3 831.5 1010.1 658.8 716.8
R2
550 nm emission τ2/ps
0.074
2951.5
0.129 0.128
1756.7 1853.2
R1
τ1/ps
1 1 1 1 0.974 1 1 1 0.992 0.991 0.992
1021.1 1059.3 1218.5 1234.5 1535.8 829.2 822.9 1054.0 1436.7 1401.1 1435.6
R2
τ2/ps
0.026
8049.9
0.008 0.009 0.008
9287.5 8785.6 9350.0
a 2-MTHF solutions; UG-11 excitation filter (350 nm maximum). Decay times (τ ) and normalized amplitudes (R ) are recovered from i i a biexponential fit of the oligomer π,π* fluorescence decay, I(t) ) R1 exp{-t/τ1} + R2 exp{-t/τ2}. Errors for the fit parameters are (5%.
Figure 4. Resonance light scattering spectra acquired from 2-MTHF solutions at 170 K for (solid line) Re-1 and (dashed line) Re-2. The intense scattering beyond 450 nm for Re-1 is due to ineffective correction of the excitation spectrum and scattering from the cryostat window in the Re-1 spectrum.
Figure 3. Emission polarization spectra acquired from 2-MTHF solutions at 170 K with an excitation wavelength of 380 nm for (a) 1 and (b) 2. Inset: Variation of emission anisotropy values observed at 525 nm at various temperatures for (a) 1 and (b) 2.
migration among aggregated chromophores. Thus, under conditions where rotational diffusion is slow (i.e., at cryogenic temperatures), observation of depolarization is a strong indication that energy transfer or energy migration within chromophore aggregates occurs. In view of these facts, we now consider the temperaturedependent polarization anisotropy data for 1 and 2. For 1 the trend is quite clear. As temperature decreases, the
anisotropy slightly increases as expected due to the decrease in the rate of rotational diffusion (Figure 3a inset). However, below 190 K there is an abrupt decrease in the anisotropy, which suggests that at this temperature aggregation occurs. For 2 the anisotropy also increases slightly with decreasing temperature and then drops precipitously between 230 and 210 K (Figure 3b inset), which again is likely due to the formation of chromophore aggregates. Note that the anisotropy data imply that oligomer 2 aggregates at a higher temperature than 1, consistent with the fact that 2 is a considerably larger “rod” than 1 (1, length ) 59.1 Å, four n-C18H37 alkyl sidechains; 2, length ) 95.3 Å, eight n-C18H37 alkyl chains). Although it is clear that the oligomers aggregate at low temperature, two subtle features demand a more complicated model to explain. (1) At intermediate temperatures (i.e., 298 > T > 220 K) the fluorescence is dominated by emission from the “monomeric” excited state, but the band red-shifts and decreases in intensity with decreasing temperature. (2) A rather abrupt change in the fluorescence band shape, decay kinetics, and wavelengthdependent polarization anisotropy occurs at T ≈ 220 K for 2 and T ≈ 180 K for 1. This latter feature implies that an abrupt change occurs in the system properties, which could arise from a change associated with the conformation of the backbone and/or side chains of the oligomers.
5680 Langmuir, Vol. 15, No. 17, 1999
Notes
Scheme 1. Proposed Oligomer Aggregation Mechanism
Because the polarization anisotropy remains high over the intermediate temperature range (i.e., 298 > T > 190 K for 1 and 298 > T > 230 K for 2), energy transfer resulting from aggregation does not occur at these temperatures. Therefore, the spectral changes must arise from intramolecular effects. We conclude that these changes arise from variation in the conformation of the oligomer backbone. Specifically, as temperature decreases the oligomers may adopt conformations in which the phenylene rings tend to lie more within the same plane. An increase in the planarity of the π-system would effectively increase the conjugation length, which in turn decreases the energy of the 1π,π* transition that is the basis for the absorption and fluorescence bands. This decrease may also increase knr, resulting in the observed decrease in fluorescence yield.17 An important consideration in formulating our conclusions is that the conformation of the oligomer backbone and the conformations and three-dimensional disposition of the n-C18H37 alkyl side chains must be correlated. Indeed, as the oligomer π-system becomes more planar, the n-C18H37 alkyl side chains probably tend to lie essentially in the same plane. It is possible that as temperature decreases the oligomers and side chains may adopt a planar “sheetlike” conformation that ultimately induces the formation of extended aggregates as suggested (17) Casper, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952.
in Scheme 1. Cooperation between side chain conformation and intermolecular (or interchain) interactions has been proposed to occur in “hairy-rod” π-conjugated polymers similar in structure to the oligomers,6b but to our knowledge this study may be the first example of such an effect in medium molecular weight “hairy-rod” oligomers. Conclusion Preliminary results are presented which indicate that oligomers 1 and 2 aggregate in dilute solution at low temperature. Although it is clear that aggregation contributes to the unusual photophysical behavior of the oligomers at low temperatures, effects arising from temperature-dependent changes in the backbone and side chain conformations also apparently modify the photophysics at intermediate temperatures and potentially provide a pathway to the formation of the observed aggregates. Future work is planned to explore the properties of oligomers that have shorter and/or branched alkyl side chains in an effort to examine the relationship between side chain structure and aggregation. Acknowledgment. We gratefully acknowledge the National Science Foundation for support of this work (Grant No. CHE 94-01620). LA990191N
