Picosecond conformational relaxation of singlet excited polyfluorene ...

JOURNAL OF CHEMICAL PHYSICS

VOLUME 118, NUMBER 15

15 APRIL 2003

Picosecond conformational relaxation of singlet excited polyfluorene in solution Fernando B. Dias Instituto de Tecnologia Quı´mica e Biolo´gica (ITQB), Ap. 127, P2781-901 Oeiras, Portugal

Antońio L. Maçanita Instituto de Tecnologia Quı´mica e Biolo´gica (ITQB), Ap. 127, P2781-901 Oeiras, Portugal and Instituto Superior Tećnico (IST), Departamento de Quı´mica, Lisboa, Portugal

J. Seixas de Melo and Hugh D. Burrowsa) Departamento de Quı´mica, Universidade de Coimbra, P3004-535 Coimbra, Portugal

Roland Gu¨ntner and Ulli Scherf Polymerchemie, Institut fu¨r Physikalische und Theoretische Chemie, Universita¨t Potsdam, D-14476 Golm, Germany

Andrew P. Monkman Department of Physics, University of Durham, Durham DH1 3LE, United Kingdom

共Received 16 October 2002; accepted 23 January 2003兲 Poly关9,9-di共ethylhexyl兲fluorene兴 was studied by steady-state and time-resolved fluorescence techniques in solution in cyclohexane, methylcyclohexane, tetrahydrofuran, and decalin over the temperature range from 343 to 77 K. A decrease in temperature leads to a decrease in the inhomogeneous broadening of the emission band. Fluorescence decays were biexponential, consistent with a two-state model involving two different polymer conformers. Global analysis of the time profiles of luminescence collected at different emission wavelengths shows a long decay-time of 371.5⫾1.5 ps, which is temperature and solvent independent. The second shorter time (29⫾3 ps at 313 K and 100⫾3 ps at 233 K in methylcyclohexane兲 appears as a decay-time at the onset of the emission spectrum and as a risetime at longer wavelengths. Whilst the slow process was independent of temperature, the fast process showed Arrhenius type behavior, with an activation energy value of 0.10 eV found in both methylcyclohexane and decalin solutions. However, the risetime in the more viscous decalin was longer than that in methylcyclohexane. The observed behavior is interpreted in terms of fast conformational relaxation of the initially excited polymer, leading to a more planar conjugation segment. © 2003 American Institute of Physics. 关DOI: 10.1063/1.1560939兴

I. INTRODUCTION

ation processes of the electronic excited states, which has lead to a considerable effort in characterizing their behavior in both solution and films.17,18 In the molecular exciton description, the excited state properties of conjugated organic polymers are best treated within a molecular framework, corresponding to selflocalization of electron–hole pairs,19 such that the optical properties of these materials are directly influenced by an effective conjugation length, involving interaction between various monomer units in the polymer chain. The absorption and emission maxima become red shifted with increasing conjugation length. It is currently accepted that the conjugation length results from chain segments of different size interrupted by twists or chemical defects, where each segment contains 7 to 15 conjugated monomer units.17 It should be noted, however, that differences of opinion exist as to whether these conjugation lengths involve straight segments 共stick model兲20 or a more wormlike model,21 where breaks arise as a consequence of fast fluctuations.22 Of particular interest in the present study is what happens to the polymer following initial excitation. It has been reported that optical excitations in conjugated polymers can

Poly共alkylfluorene兲s have become amongst the most important candidates for efficient, blue light emitting conjugated polymers for use in light emitting diodes 共LEDs兲 for display and lighting applications.1– 8 They combine high photoluminescence quantum yields, good thermal stability and good solubility in a variety of solvents. By copolymerisation9–12 or doping13,14 with appropriate long wavelength fluorophores it is possible to tune the emission over the whole visible spectrum. Further, liquid crystalline phases of poly共alkylfluorene兲s are available, leading to the possibility of preparing aligned ‘‘polarized’’ light emitting polymer films.15,16 However, a number of important questions remain on both their spectral and photophysical properties, which are relevant to applications in light emitting devices. In particular, the efficiency of optical devices based on these conjugated polymers strongly depends on the properties and relaxa兲

Author to whom correspondence should be addressed. Electronic mail: [email protected]

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migrate along the polymer chain, or between different chains, executing a random walk inside the broadened distribution of states assigned to segments with different conjugation length and therefore with different excitation energy.18,23,24 Within this picture the Stokes shift, commonly observed between the absorption and emission bands, essentially results from energy relaxation to the lowest energy segment.23 This description does not, however, show how energy relaxation occurs, or what other processes are involved. Further, in recent studies the view that energy migration between the conjugation segments in these polymers is responsible for the narrowed emission spectra has been questioned.25 It is well known that it is possible to tune the ␲-electron density, and hence photoluminescence properties, of conjugated polymers by conformational changes. Recent studies of the photophysics of both polymers26 and oligomers25,27 have shown that conformational disorder is an important factor in the deactivation of excited isolated polymer chains. Sluch et al.25 presented experimental evidence to suggest that conformational relaxation and increases in the planar coupling between rings, should be considered in interpreting the observed time-resolved emission data. In this work we will concentrate on the behavior of the lowest excited singlet state of dilute solutions of poly共alkylfluorene兲s under conditions where interchain and aggregation effects can be ignored.28 The derivative chosen is the well characterized branched side chain poly关9,9di共ethylhexyl兲fluorene兴共PF2/6兲, whose physical properties have been described elsewhere.29 Steady state and time resolved fluorescence data are reported in various solvents as functions of temperature to understand the dynamics of excited singlet state decay in isolated chains. Results are compared with those on the same polymer in films and on the related rigid rod ladder-type methyl-substituted poly共pphenylene兲, MeLPPP. II. EXPERIMENTAL DETAILS

Poly关9,9-di共ethylhexyl兲fluorene兴共molecular weight from gel permeation chromatography: M n ⫽1.3⫾0.1⫻105 , M w ⫽2.5⫾0.1⫻105 , structure inset in Fig. 1兲 was prepared by Yamamoto coupling,30 as described elsewhere.31,32 Synthesis by Yamamoto coupling produces polymers with low levels of impurities. Studies on the emission spectrum of a dilute solution (A 380 nm⫽0.08) of the polymer in cyclohexane with various excitation wavelengths show very similar spectra, confirming that there are no significant impurities, which can be detected by fluorescence. The most likely impurity of this polymer results from photo-oxidation of one of the dialkylfluorene rings 共most probably at ‘‘defect’’ monosubstituted sites兲 to give a fluorenone.7 However, this leads to a broad emission above 500 nm, which was not observed in pristine films of the polymer sample used. The synthesis, purification and properties of MeLPPP have previously been reported.33–35 The polymer had a molecular weight M n ⬇6.9⫻104 and a polydispersity ⬇2.8. Methylcyclohexane 共MCH兲 was purchased from BDH and purified as previously described.36 Decalin was purchased from Riedel-de Hae¨n and purified by passing through a column of activated silica gel. For photophysical measure-

FIG. 1. Excitation and emission spectra of PF2/6 in MCH at 293 K 共a兲 and 77 K 共b兲. For fluorescence spectra, excitation wavelength is 380 nm 共maximum of RT absorption兲.

ments, solutions were prepared with absorbance less than 0.1 at the excitation wavelength 共386 and 440 nm, respectively, for PF2/6 and MeLPPP兲. The low absorbance of these samples prevents errors in fluorescence spectra due to selfabsorption or the inner filter effect. Ultraviolet absorption and fluorescence spectra were recorded with an Olis-15 spectrophotometer and a SPEX Fluorolog 2 fluorimeter, respectively. Fluorescence decays were measured using picosecond time correlated single photon counting 共TCSPC兲, as described previously.37 The excitation source consists of a frequency-doubled Ti:sapphire picosecond laser system 共Spectra Physics, Inc.兲, pumped by a Ar⫹ or a Millennia X 共Spectra Physics, Inc.兲, laser repetition rate 4 MHz, FWHM ⫽38 ps. Measurements on the ultrafast excited state proton transfer reactions of 7-hydroxy-4-methylflavylium cation give an instrument response time ⭐6.4 ps.37 In the same paper, as well as in Refs. 38 and 39 a detailed description of the deconvolution procedure is given. Temperature control was achieved using a homebuilt system as described elsewhere.40 Alternate collection of pulse and sample was performed at the magic angle (103 counts at the maximum per cycle兲 until 5⫻103 counts at the maximum were acquired. The fluorescence decays were deconvoluted in a PC, using the UNIX version of George Striker’s program.39

III. RESULTS A. Absorption and steady state fluorescence spectra

In Fig. 1 we show the excitation and the emission spectra of PF2/6 in methylcyclohexane solution at 293 K 共a兲 and 77 K 共b兲. The absorption spectrum measured at room temperature was identical to the excitation spectrum and no significant differences were observed in the excitation spectrum


FIG. 2. Normalized emission spectra for PF2/6 in MCH with excitation at 360, 380, 398, and 405 nm. Shown as an inset is a magnified view of the spectrum in the 390– 450 nm range.

with emission observed at different wavelengths between 392 and 470 nm, showing that fluorescence originates from a single electronic level. The room temperature spectra are in good agreement with those observed for this and related polymers.41– 44 The observation of a nonstructured excitation 共or absorption兲 spectrum at room temperature (␭ max⫽381 nm, 3.25 eV兲, is attributed to inhomogeneous broadening,44 and can be explained by the presence both of different local environments for chromophores, and the presence of chain segments of different conjugation length in the same polymer chain.23 The emission showed a structured fluorescence spectrum between ⬇390 and 530 nm 共3.18 –2.34 eV兲, attributed to at least three vibronic components. In studies on films of the same polymer, each of these vibronic bands has been shown to comprise at least three different vibrational modes.44 The fluorescence spectrum, however, shows small but significant differences in both the relative intensities and widths of the vibronic components upon excitation with different wavelengths. These are shown in Fig. 2. In contrast to the behavior at room temperature, at 77 K, the fluorescence excitation spectrum becomes structured. In addition, it shows a redshift in the maximum of 10 nm 共80 meV兲关Fig. 1共b兲兴. Similar spectral narrowing has previously been observed with an oligo共p-phenyleneethynylene兲 at low temperatures.25 The structureless absorption 共or excitation兲 spectrum at room temperature is interpreted as resulting from contributions from different conjugation lengths as a consequence of a relatively low energy barrier for interconversion between these forms. The observation of a structured emission spectrum at both room and low temperatures 共Fig. 1兲, suggests that emission is coming from a single conjugation length 共or small subset thereof兲, while the marked Stokes shift observed at room temperature suggests that either the average length of the conjugation segment increases in the excited state by some activated relaxation process, or that energy migration to more relaxed states is occurring, possibly through some dispersive process.23,44 In the frozen matrix 共77 K兲, the excitation and emission spectra present a mirror image relationship, which suggests very similar geometrical parameters for the conjugation segments involved in ground and excited states. This can be explained by a freezing of molecular motion in the ground state at low temperatures,

Relaxation of singlet excited polyfluorene

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FIG. 3. Emission spectra of PF2/6 in MCH at 293 K 共full line兲 and 77 K 共dashed line兲 with excitation at 380 nm.

and a subsequent reduction in the possible range of conjugation lengths. As will be discussed later, this may be associated with a reduction in the twist angle between monomer units. An alternative explanation for the increase in structure of the excitation spectrum at low temperatures is that specific sites are introduced into the matrix due to partial crystallization of the solvent. However, although this cannot be completely ruled out, it is rendered unlikely, since the samples appeared fairly isotropic, the spectra were reproducible, which is not usually true when microcrystallites form, and MCH is known to be a good glass forming solvent. It is worth noting that although the excitation spectrum band onset is identical at 293 and 77 K, the maximum shows a redshift of 10 nm 共381 nm at room temperature to 391 nm at 77 K兲, which supports the idea that at low temperature a greater percentage of more planar conformers exists.45,46 In Fig. 3, the emission spectra at 293 and 77 K are normalized at their maxima. It can be seen that in addition to a very slight increase in the vibrational resolution of the 77 K spectrum, a clear narrowing of the emission band is observed at low temperatures. This again is in agreement with the room temperature emission spectrum involving a mixture of non-relaxed and relaxed segments, while at 77 K the conformer distribution is greatly reduced. Although there is a slight blueshift of 3 nm 共30 meV兲 in the emission maximum 共first vibronic transition兲 at 77 K relative to 293 K, suggesting a slight increase in the energy separation between ground-state and excited state potential energy curves, more important is the increase in the intensity of the 0–0 relative to the 0–1 band on decreasing temperature corresponding to a decrease in the Huang–Rhys factor.47 This strongly suggests47 that at low temperatures little configurational relaxation is occurring between the emitting level and the ground state. B. Time-resolved fluorescence of PF2Õ6 in solution

Time resolved fluorescence decays of PF2/6 in MCH solution at 293 K (␭ exc⫽386 nm) observed at 398 and 416 nm showed decays that are well fitted with sums of two exponentials 共Fig. 4 and Table I兲. Similar behavior was observed in cyclohexane and tetrahydrofuran solutions 共not shown兲. The fluorescence kinet-

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FIG. 4. Global analysis 共simultaneous analysis of the decays兲 for fluorescence decays of PF2/6 at 293 K, in MCH with excitation wavelength at 386 nm.

ics were found to be independent of excitation intensity, while from studies of the emission decays at a number of emission wavelengths over the range 396 – 446 nm, the relative amplitudes appear to reflect the superposition of two emitting species 共relaxed and nonrelaxed兲. In fact when the emission decay is collected at 396 nm 共blue edge of the emission spectra兲 the pre-exponential factor associated with the fast component has a positive amplitude, i.e., it corresponds to an emission decay 共see Table I兲. This is valid until ⬇406 nm. For lower energy values the fast component is associated with a negative amplitude, i.e., a risetime 共see Figs. 4 and 5兲. The longer component always has a positive amplitude, giving rise to the main contribution for the overall emission decay. For convenience, for showing the relative weights of the two components at wavelengths ␭⭐404 nm, the amplitudes have been normalized, such that A 1 ⫹A 2 ⫽1. Such a normalization procedure is no longer valid at longer wavelengths, where there both a risetime and decay are present. In this case, we have chosen to fix A 1 ⫽1 to show the increasing importance of the grow-in of the second component. Further analysis of Table I also shows a gradual increase in the absolute value of the negative pre-exponential with the

TABLE I. Fluorescence decay times and normalized amplitudes obtained for PF2/6 in MCH at 293 K (␭ exc⫽386 nm). ␭ em 共nm兲

␶ 2 ⫾3 共ps兲

␶ 1 ⫾1.5 共ps兲

A2

A1

396 398 401 404 406 409 411 416 421 436 446

41 41 40 38 40 38 40 41 40 40 41

373 370 371 372 373 372 371 370 373 373 373

0.35 0.28 0.19 0.11 ⫺0.01 ⫺0.11 ⫺0.14 ⫺0.25 ⫺0.30 ⫺0.27 ⫺0.29

0.65 0.72 0.81 0.89 1 1 1 1 1 1 1

FIG. 5. Temperature dependence of the fluorescence decay times 共A兲 and pre-exponential factors at ␭ em ⫽398 共B兲 and 416 nm 共C兲, for PF2/6 in methylcyclohexane. Data are obtained by global analysis of the decays.

increase in the emission wavelength. This is totally consistent with the emission of two species overlapping more at the blue region of the emission spectra than at the red. Although there is always a major contribution of the slower component, at lower energies the fast emitting species gradually appears in the S 1 state at the expense of the former. Further data were obtained in MCH solution as a function of temperature. The slow component was found to be temperature independent, over the range ca. ⫺40 to ⫹40 °C, but the fast component showed a clear temperature dependence 共Fig. 5兲, while from the experimental amplitude values at 398 nm, emission at this wavelength becomes more important at low temperatures. The above data can be interpreted as a two-state process 共see scheme 1兲 involving an instantaneous formation of a first species (A * ), which can decay to the ground-state with 1/␶ A or give rise (k 1 ) to a second species (B * ) which can decay to the ground state with 1/␶ B . As will be discussed later, we believe this behavior is due to formation of an initial nonrelaxed conformer, which can decay or give rise to the more stable 共presumably more planar兲 relaxed excited state conformer 共40 ps兲, which then decays to the ground state 共370 ps兲. It is worth noting in these plots 关Fig. 5共b兲兴 that extrapolation to the melting point of MCH 关⫺127 °C 共Ref. 48兲兴 leads to a single exponential decay (A 2 ⬇1) with decay time close to 300 ps, corresponding to decay of the initially formed species at this temperature. As mentioned above, in scheme 1 the reciprocals of ␶ A and ␶ B are the unquenched rate constants for the deactivation of the species A * and B * . The specific value for ␶ A should be obtained for a model compound where no additional B * species is formed. We will later test if the more rigid MeLPPP fulfills this criterion. However since it is a more rigid polymer, we may predict that it could be a good model compound for the B species. Anticipating this discussion we can state that the major component in the decay of the polymer MeLPPP in cyclohexane solution is found to be around 310 ps, and is solvent and temperature independent. This gives strong support for the idea that the longer decay time



FIG. 6. Arrhenius-type plot for the fast decay time of PF2/6 in methylcyclohexane 共䊊兲 and decalin 共䊉兲.

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FIG. 7. Temperature dependence of the fluorescence decay times 共A兲 and pre-exponential factors at ␭ em ⫽398 共B兲 and 416 nm 共C兲, obtained from global analysis of fluorescence decays of PF2/6 in MCH 共䊉,䊊兲 and decalin 共夡, 䊐兲 as a function of temperature.

should be associated with a more planar conformation in PF2/6.

Strictly, for such a two state model the kinetic scheme should allow for reversibility between A * and B * . This is the most complete kinetic model. However, in the present system such reversibility is unlikely if we are going from a twisted to a more relaxed conformation. In a two state model where no reversibility exists the rate constant for the B species formation at the expense of the A species can be given according to Eq. 共1兲, where ␶ 2 represents the fast component in the fluorescence decay, k 1⫽

1 1 ⫺ . ␶2 ␶A

共1兲

However, if we assume that the effect of 1/␶ A on k 1 关Eq. 共1兲兴 is small, we can obtain an approximate value of the activation energy for this ‘‘relaxation to a more planar conformer’’ by plotting the logarithm of the reciprocal of ␶ 2 versus 1/T 共Arrhenius plot兲, as in Fig. 6. Good linear behavior is observed, and from the plot an activation energy E a ⫽2.38 kcal mol⫺1 共0.10 eV兲, and preexponential factor k 01 ⫽1.5⫻1012 s⫺1 were determined. At room temperature 共293 K兲 the rate constant for formation of B * was k 1 ⫽2.5⫻1010 s⫺1 . The observed activation energy, while being relatively modest, does suggest some structural relaxation in going from A * to B * . Studies of the fluorescence decay of PF2/6 were also made as a function of temperature in decalin 共DC, decahydronaphthalene兲. This has fairly similar polarity to MCH 关di␧ (DC at 25 °C) electric constants ␧ (MCH at 20 °C) ⫽2.020, ⫽2.1542 共Ref. 48兲兴, but is 3– 4 times more viscous 关viscosities ␩ (MCH at 30 °C) ⫽0.627, ␩ (DC at 25 °C) ⫽2.41 共Ref. 49兲兴. Studies using global analysis again suggested two components in the kinetics, with a fast grow-in and a slow decay.

However, whilst there were no differences in the lifetime for the slow component in the two solvents 共typically ⬇373 ps兲, the grow-in was slower in the more viscous decalin than in MCH 关Fig. 7共a兲兴. With the amplitudes of the fast and slow components, that for the fast component at 398 nm was slightly higher in decalin 共⬇60%–70%兲关Fig. 7共b兲兴, whilst at 416 nm no significant differences were observed 关Fig. 7共c兲兴. The temperature behavior of the fast decay in decalin was also fitted to an Arrhenius plot 共Fig. 6兲, and gave an activation energy 2.32 kcal mol⫺1 共0.10 eV兲 identical to that in MCH. However, the pre-exponential factor (k 01 ⫽0.8 ⫻1012 s⫺1 ) and the rate constant for formation of B * at room temperature 共293 K兲 are smaller (k 1 ⫽1.5⫻1010 s⫺1 ) in the more viscous solvent. C. Time-resolved fluorescence of MeLPPP in solution

The steady state and time resolved fluorescence results on PF2/6 in solution strongly suggest that the observed dynamics and spectral shifts are associated with conformational relaxation involving the bond between the fluorene rings. However, whereas the bond angles between chemical repeat units in PF2/6 in the ground state are about 23°,31 and may relax on excitation, those in MeLPPP are zero. The excitation 共absorption兲 and emission spectra of MeLPPP are mirrorsymmetric 共Fig. 8兲, i.e., the absence of twisted conformations removes the higher energy absorptions that are observed with PF2/6. Upon excitation of a solution of MeLPPP in cyclohexane at 386 nm, and observation of emission in the maximum 共460 nm兲, the decays are essentially single exponential 共300 ps, see Fig. 9兲. Although the emissions at both the onset 共450 nm兲 and tail 共490 nm兲 of the band are better fitted with sums of two exponentials, no risetime of emission was observed over the whole wavelength region studied. We note that with both PF2/6 and MeLPPP, the five membered rings are probably not completely planar, and it is possible to have changes in the degree of ‘‘buckling’’ of these upon excitation. However, the presence of a rise time in PF2/6 and its absence in MeLPPP suggests that this is not important for the observed photophysical behavior.

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FIG. 8. Excitation 共full line兲 and emission 共dash line兲 spectra of MeLPPP in cyclohexane at 293 K. For fluorescence spectrum excitation wavelength is 380 nm. Shown as inset is the structure of the polymer; R 1 ⫽n-C6 H13 , R 2 ⫽1, 4-(C6 H4 )-n-C10H21 .

IV. DISCUSSION

Before discussing the observed photophysical behavior of PF2/6, which under the conditions studied can be considered as an isolated polymer during the measured fluorescence decay times, it is important to review what we know about this system. X-ray structural studies on fluorene show that it is very slightly nonplanar, with a V-shape formed from two assymetric units.50,51 When the alkyl fluorene units combine they form nonplanar conformations, with equilibrium twists as seen by electron diffraction of 36/144° in the ground state.52 From molecular modelling of PF2/6, the individual monomer units only deviate slightly from the main backbone structure, while the two 2-ethylhexyl side chains can adopt three arrangements.31 From small angle neutron diffraction and dynamic light scattering studies in toluene solution,29 the polymer is present in solution as a wormlike chain. In toluene, a cross-section diameter of 1.8 nm and persistent length 7 nm have been given. Differences may be expected in the

FIG. 9. Single exponential fit for fluorescence decay of MeLPPP at 293 K, in cyclohexane with excitation wavelength at 386 nm and emission wavelength at 460 nm.

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degree of coiling that is observed in the behavior of polyfluorenes in good solvents, such as toluene and THF and relatively poor solvents such as cyclohexane or MCH.15 However, no significant differences were observed in the photophysical properties, suggesting that only intrachain segments are responsible for fluorescence. Studies on oligo共alkylfluorenes兲53,54 suggest an effective conjugation length of 11–12 bonded fluorene units in poly共alkylfluorene兲s. Although care must be exercised when comparing data on films and dilute solutions, it is worth noting for the present results that photophysical studies of poly共9,9dioctylfluorene兲共PFO兲 in liquid crystalline49 and thin film41 forms show spectral changes that have been interpreted in terms of changes in intrachain ordering. The most important result of the present study is the observation of a thermally activated risetime observed at the high-energy end of the PF2/6 fluorescence. Various explanations have been invoked for fast relaxation processes observed in many conjugated organic polymers, including intrachain vibrational relaxation/redistribution, within the polymer,42,44 charge separation on photoexcitation followed by charge recombination and fluorescence,43,55 excitation energy migration in the polymer to the longest 共lowest energy兲 conjugation segments,23,42,44,56 and torsional motions leading to conformational reorganization of the excited state.25,27,57 The fact that similar behavior is observed in good solvents such as THF and in the poorer solvents cyclohexane and MCH strongly indicates that the observed photophysical processes in PF2/6 fluorescence decay involve intrachain phenomena. Vibrational relaxation processes can be ruled out since these are not expected to be thermally activated, and in addition are expected to occur on much shorter time scales 共100 fs兲共Refs. 48, 56兲 than the observed risetime 共40 ps兲. Similarly, whilst formation of geminate polaron pairs, followed by recombination to form singlet or triplet excited states is feasible in films, in solutions of isolated polymer chains at low excitation energies this pathway is extremely unlikely. In distinguishing between the other two mechanisms for the fast process in PF2/6 in solution, various results are more consistent with the risetime being due to conformational relaxation and torsional motion in the lowest excited state than to any energy migration process. In particular, the risetime is markedly slower and its associated amplitude higher in the viscous solvent decalin than in MCH, while no risetime is observed with the rigid rod polymer MeLPPP. Energy relaxation in the lowest singlet state of conjugated poly-mers 共in the solid state兲 is commonly interpreted usinga Fo¨rster mechanism involving dipole–dipole interaction.13,26,42,44,56,58,59 Although it is difficult to see how this should be affected by either solvent viscosity or chain rigidity, energy migration cannot, at present, be ruled out. Time resolved fluorescence anisotropy has been shown to be an excellent method to distinguish energy migration from other mechanisms.26 We hope in the near future to carry out related studies on this system. Based on time-resolved anisotropy studies Sundstro¨m and co-workers found that the decay of fluorescence anisotropy within 150 ps gives a direct measure of the conforma-


tional disorder of the polymer.26 They also found that on average, electronic excitation transfers over about six spectroscopic units before being trapped. Torsional dynamics are implicated in the singlet state relaxation processes of 1,4-bis共p-phenylene-ethynyl兲benzene27 and oligo共p-phenylenethynylene兲.25 In these oligomeric systems, an energy migration mechanism is excluded. Given the fact that the results support conformational relaxation as the dominant mechanism for the observed fast fluorescence dynamics, two questions remain. What conformational process is involved, and why do we not observe energy migration? Two conformational changes can be considered, differences in the angle between the fluorene rings, and changes in the conformations of the 2-ethylhexyl side chains. Whilst the latter process is feasible, it is difficult to see why this should depend upon excitation of the polyfluorene moieties. We favor a mechanism for the risetime involving bonds between adjacent fluorene rings possessing more double bond character, and the rings assuming a more planar conformation. The observed activation energies are in the range commonly observed for rotations about C–C bonds 共typically 1–3 kcal mol⫺1兲.60 Further, molecular modelling and molecular orbital calculations on PF2/6, using a torsional angle of about 144° obtained by electron diffraction, suggest that it probably exists in the ground state as a 5/2 helix.31 It is well known with biphenyl that, while the two rings are nonplanar in the ground state, in the lowest excited state planarisation occurs.61– 64 As previously suggested for terfluorenes,52 it is reasonable that similar torsional angle changes are occurring in the lowest excited state of PF2/6 in solution. As with reports on oligo共p-phenyleneethynylene兲,25 conformational relaxation is likely to be accompanied by solvent relaxation. However, considering the rather open wormlike structure that PF2/6 adopts in solution, it is difficult to separate conformational changes from any solvent reorientation, since both will occur on similar time scales, and have fairly similar activation energies. As the observed thermally activated risetime is suggested to result from conformational relaxation in the excited state, it is relevant now to address the question of the importance of electronic energy migration in the photophysics of conjugated polymers. Time-resolved fluorescence anisotropy measurements on polythiophenes26 show that excitation energy migration along conjugated polymer chains is an important process. The observation of a broad absorption 共or excitation兲 spectrum, and a vibronically structured, redshifted fluorescence spectrum with PF2/6 in solution can be explained in terms of such migration occurring from higher energy to the lowest energy segments. However, the fact that we do not observe this in our time resolved emission measurements with PF2/6 may suggest that the initial energy migration to the lowest energy segments is faster than the time resolution of our equipment, such that subsequent energy migration is to isoenergetic conjugation segments, and, hence, does not lead to any change in the fluorescence spectra. However, an important alternative explanation comes from the work of Scholes et al.,22 who suggest conjugation fluctuations and light absorption occur on a fast time scale, such that we see the blurring effect of the fluctuations,


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whereas emission is a slow process giving time for intrachain excitation migration 共hopping or Fo¨rster transfer兲 to the longest conjugation segments. We see here that the behavior of the PF2/6 in solution is very different from that in solid state. This is especially clear in the behavior of the excitation profiles. At low temperature 共frozen solution兲 we do now observe a structured absorption band, whereas in the solid state this is never the case even at very low temperatures. This we understand to be consistent with very low energy vibrational modes65 playing a key role in the fluctuation of the length of each conjugated segment, as in the model of Scholes et al.22 In frozen solution the well solvated chains when frozen must be more strongly locked-in by the more closely spaced small solvent molecules which hinder such low energy modes. In the solid state there is always enough free space to allow for these backbone motions. V. CONCLUDING REMARKS

We have followed the decay kinetics in the whole range of the emission spectra of two related polymers 共PF2/6 and MeLPPP兲. With the ‘‘less-rigid’’ polymer 共PF2/6兲, a fast risetime component is found to exist in addition to a slower decay component, whilst with the ‘‘more-rigid’’ polymer 共MeLPPP兲 only a slow decay component of lifetime approximately 300 ps is important in the observed emission region and no rise were obtained. The fast component observed with PF2/6 is temperature dependent with activation energy of ⬇2.38 kcal mol⫺1, compatible with values for rotation around C–C between bonded fluorene units. The possibility of this behavior being due to energy transfer between different conjugation segments within the polymer is therefore excluded and it is suggested that the relaxation process is best interpreted in terms of planarisation of the adjacent fluorene rings. ACKNOWLEDGMENTS

The authors are grateful to Sapiens/FCT and the Royal Society for financial support. A.P.M. acknowledges the Leverhulme Trust for a Fellowship. F.B.D. acknowledges FCT 共Portugal兲 for a postdoctoral grant. 1

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