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Fluorescence Quenchers in Mixed Phase Polyfluorene Films Ashu K. Bansal, Arvydas Ruseckas, Paul E. Shaw, and Ifor D. W. Samuel* Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, UniVersity of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, United Kingdom ReceiVed: June 16, 2010; ReVised Manuscript ReceiVed: August 27, 2010
Fluorescence quenching dynamics were studied in poly(9,9-dioctylfluorene) films with a wide range of β-phase concentrations. Measurements of time-resolved fluorescence and fluorescence quantum yield showed that the radiative decay rate was independent of the β phase content, whereas the nonradiative decay rate was found to be higher in β-phase rich films. The trend was analyzed in terms of diffusion-mediated excitation energy transfer to quenchers. The concentration of fluorescence quenchers is found to be less than 0.01% of the number of fluorene repeat units and independent of the fraction of the film in the β phase. The results suggest that fluorescence quenching is predominantly by chemical defects rather than by excimer formation. 1. Introduction Conjugated polymers combine favorable semiconducting optoelectronic properties with simple processing, making them attractive materials for applications in light-emitting diodes, lasers, and photovoltaic devices. Poly(9,9-dioctylfluorene) (PFO) has a distinctive ability to form a more planar chain conformation, the so-called β phase, which shows a red-shifted fluorescence with a much smaller line width and a lower Huang-Rhys factor than the glassy phase.1-5 Light emitting diodes made of PFO films containing β-phase chains were shown to give a more efficient and stable performance as compared to the pure glassy phase.6 Films containing the β phase can also be used to make plastic lasers with low lasing threshold7-9 and spatial control of the lasing wavelength can be achieved via masked solvent vapor exposure.10 The photoluminescence quantum yield (PLQY) has been reported to be about 0.5 in glassy PFO films and in films with a low concentration of β phase at room temperature,2 which indicates that about half of photoexcitations decay nonradiatively. The fluorescence quenching mechanism is not fully understood in PFO films, there is still an ongoing debate about whether it is mainly caused by on-chain fluorenone defects11-14 or whether the formation of interchain excimers and cross-linked chains may also play a significant role.15-17 It is desirable to identify the dominant quenchers in order to develop more efficient materials and devices. In this work we studied the dynamics of fluorescence quenching in PFO films containing varying proportions of β phase. The β phase concentration was varied from 0 to 42% by changing the spin-coating solvent and/or temperature, as detailed in the Experimental Section. We found that the nonradiative decay rate increases with the proportion of β phase in the film and that this can be explained by increased exciton diffusivity. The concentration of the fluorescence quenching sites is found to be independent of the concentration of β phase, which suggests quenching is by chemical defects.
Solutions containing 10 mg/mL of PFO were prepared by stirring overnight at a temperature of 35 °C and thin films were spin-coated onto quartz substrates at 1500 rpm in a nitrogen atmosphere. The glassy phase film (0% β phase) was spin-coated from toluene solution, which was put on a hot plate at 100 °C for 2 min just before spin-coating. Two different approaches were used to introduce the β phase in PFO films. Spin-coating from chloroform, toluene, and o-xylene solutions at 20 °C gave films with 5%, 10%, and 30% of β phase, respectively. In the other approach, a low concentration of 1,8-diiodooctane (DIO) was added to the PFO solutions in toluene before spin-coating.18,19 Toluene solutions with 0.5%, 1%, 2%, 8%, and 16% of DIO by volume gave films with 15%, 21%, 27%, 39%, and 42% of β phase, respectively. Absorption spectra of the films were measured with a Varian Cary 300 UV-vis spectrophotometer. Fluorescence spectra were obtained with a Jobin-Yvon Fluoromax 2 fluorimeter upon excitation with 380 nm light. The thicknesses of the films were measured with a Dektak surface profilometer and were in the range of 70-100 nm. The photoluminescence quantum yields of the films were determined with a He-Cd laser operating at 325 nm to excite the sample and an integrating sphere to collect the resulting emission following the method of Greenham et al.20 For time-resolved fluorescence measurements samples were excited with 100 fs light pulses at 400 nm of the second harmonic of a Ti:sapphire oscillator (Spectra-Physics Mai-Tai) output at 80 MHz. The emission from the samples was passed through a long-pass filter to remove the remaining excitation light and focused onto a slit of a spectrograph (Chromex 250i). The output from the spectrograph was time-resolved with a Hamamatsu streak camera operating in synchroscan mode. The fluorescence decays were captured at shorter and longer time ranges to ensure that any initial decay was resolved to a resolution of 2 ps. The excitation density used was 5 nJ/cm2 or less to make sure all measurements were within the linear regime of the response.
2. Experimental Section
3. Results
PFO was bought from American Dye Sources, Inc. (product name ADS329BE) and was used without further purification.
Figure 1 shows the absorption and the fluorescence spectra of PFO films having different percentages of the β phase. The glassy phase PFO has a broad absorption with maximum at 385 nm and a PL peak at 425 nm. The films containing β phase
* To whom correspondence should be addressed.
10.1021/jp105545r 2010 American Chemical Society Published on Web 09/30/2010
Fluorescence Quenchers in Polyfluorene Films
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Figure 1. Absorption (solid line) and fluorescence spectra (dotted blue line) of PFO films. Each panel is labeled with the percentage of β phase chromophores in the film and the measured fluorescence quantum yield values. Dashed red lines show absorption spectra of the β phase, which were obtained by subtracting the glassy phase spectrum after normalization at 354 nm.
show absorption peaks at 385, 405, and 435 nm. The peak at 435 nm is assigned to the 0 f 0 vibronic transition from the ground S0 to the first excited electronic state S1 of the β phase chains, the 405 nm peak to the 0 f 1 transition, and the 385 nm peak to the 0 f 2 transition. The absorption of the β phase has been evaluated by subtracting the glassy film absorption spectrum from that of the mixed phase after normalization at 354 nm and is shown by the dashed line. The amount of each phase in the film is then estimated in proportion to the area under the corresponding absorption curve. This procedure assumes that the β phase absorption is negligible at 354 nm and thus gives a lower estimate of its amount.2,3 The fluorescence spectrum of the glassy film shows a series of peaks and shoulders located at 422, 446, 480, and around 520 nm, which are spaced by about 1460 cm-1 and are assigned to a vibronic progression of the lowest energy electronic transition. The fluorescence spectra of the mixed-phase films show vibronic peaks at 440, 466, 500, and around 540 nm, which agree well with the previous reports1-10 and are attributed to the β phase chromophores. The β phase emission dominates the fluorescence spectra even in films with a small proportion amount of the β phase chromophores, which indicates efficient energy transfer from glassy to the β phase. Figure 2 shows the fluorescence decays of thin films having different percentages of the β phase. The glassy PFO film has a monoexponential decay with a lifetime of 250 ps. The fluorescence decays in the films with β phase are dominated by one exponential component, as can be seen from the solid lines in Figure 2. We used the time constant of this dominant component in subsequent analysis. To confirm the validity of this approach we have performed a two-exponential fit and found that the longer component had an amplitude of 5% or less in all films. Figure 3a shows PLQY and fluorescence decay time versus the percentage of β phase in the films. A good correlation is observed between these two quantities. Glassy phase films and films with up to 21% of β phase chromophores show PLQY between 0.5 and 0.55, which agrees well with previously reported values of 0.53 and 0.55.2 Further increases in the concentration of the β phase lead to a decrease in the PLQY (down to 0.25 for films with 42% of β phase), which correlates with a decrease of the fluorescence decay time. We are not aware of any previous reports of PLQY and fluorescence lifetime in PFO films with high proportions (>20%) of β phase chromophores.
Figure 2. Fluorescence decays detected in the spectral windows of 415-460 nm in a glassy PFO film (0% β) and 430-470 nm in the mixed-phase films. The concentration of β phase and the fluorescence lifetime determined from fitting to an exponential decay (solid lines) are also given. Fluorescence intensities are scaled for clarity. The inset shows the chemical structure of PFO.
Figure 3. (a) The photoluminescence quantum yield (solid shapes) and lifetime (open shapes) (b) and radiative (solid shapes) and nonradiative decay (open shapes) rates as a function of the percentage of β phase chromophore concentration in films. All data are shown as a function of β phase fraction, which was controlled by choice of solvent, temperature, and using DIO additive as shown in the legend. Where no temperature is shown, the films were spin-coated at room temperature.
Using the measured photoluminescence quantum yield Φ and decay time τ and assuming that photon absorption generates emissive excitons only, we can determine the radiative kr and nonradiative knr decay rates of singlet excitons using
Φ ) kr /(kr + knr)
(1)
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1 ) kr + knr τ
Bansal et al.
(2)
The results are shown in Figure 3b. The radiative decay rate is found to be independent of the β phase content in films within the 10% accuracy of the measurements. This validates the assumption that photon absorption predominantly generates emissive excitons. The average value is kr ) (2.4 ( 0.3) × 109 s-1 and the errors are mainly caused by uncertainty in PLQY values. We can use our value for kr to determine the transition dipole moment of fluorescence |df| via the relation21-23
|df | 2 )
3πε0p4c3kr〈E-3〉 n0
(3)
where 〈E-3〉 ) ∫E-3I(E) dE/∫I(E) dE is obtained from the fluorescence intensity I(E) in units of the relative number of quanta at the photon energy E, ε0 is the vacuum dielectric constant, p ) h/2π is Planck’s constant, c is the speed of light and n0 is the refractive index of the medium (in this case of the glassy film as β phase chromophores are dispersed in the glassy phase). Using the reported value n0 ) 1.8 from ref 24 and the PL spectra in Figure 2 we get |df|/e ) (4.3 ( 0.5) Å, where e is the elementary charge. This value is in good agreement with the value found in PFO solution23 indicating the similar nature of the light emitting state in both solution and film. The nonradiative decay rate in the film increases gradually with the concentration of β phase chromophores and a steeper increase is observed when the β phase concentration reaches 40%. Figure 3 clearly shows that the behavior is the same regardless of whether the concentration of β phase is changed by solvent, temperature, or the use of a high boiling point additive. This indicates that the photophysics is defined by the concentration of β phase rather than by the preparation route. 4. Discussion Several mechanisms can contribute to the nonradiative decay, including internal conversion to the ground state, intersystem crossing to the triplet state, exciton-exciton annihilation, quenching by photogenerated charges and triplet excitations, and energy transfer to quenchers. Exciton-exciton annihilation as well as quenching by photogenerated charges and triplets would be dependent on excitation density. In our time-resolved fluorescence measurements the excitation intensity was attenuated to a level much lower than the onset of intensity-dependent kinetics, which allows us to discard exciton-exciton annihilation and quenching by photogenerated charges and triplet excitations. Then the nonradiative decay rate is given by
knr ) kintra + kq
(4)
where kintra is the intramolecular decay rate due to internal conversion and intersystem crossing and kq is the energy transfer rate to quenchers. In a solution of PFO, where energy transfer to quenchers can be neglected and the nonradiative decay is by intramolecular processes only, the PLQY of the glassy and β phase emission has been found to be about 0.8 and the time constants of the radiative decay were 0.54 and 0.45 ns, respectively.23 Then, from eq 1 we determine kintra ) (5 ( 0.5) × 108 s-1. Fluorescence decays for films in Figure 2 can be reasonably well approximated by an exponential decay function,
Figure 4. Density of quenching sites in films having different percentage of β phase.
therefore, the kq rate can be considered to be time independent, which indicates that this process is controlled by threedimensional exciton diffusion and can be described by the Smoluchowski equation
kq ) 4πDRqNq
(5)
where D is the diffusion coefficient, Rq is the quenching radius, and Nq is the quencher concentration. Previous study showed that singlet-singlet exciton annihilation in mixed phase films is also controlled by exciton diffusion.18 Annihilation arises at high excitation densities when excitons are able to interact with each other, and a pair of annihilating excitons fuses to form a higher energy exciton. The loss of excitons to annihilation is dependent on how fast they can diffuse and in the long time limit the annihilation rate constant, which is
γ ) 4πDRa
(6)
where Ra is the annihilation radius. It is assumed that one exciton is lost per encounter. From eqs 5 and 6 we get,
Nq )
kq Ra γ Rq
(7)
The ratio Ra/Rq depends on the Fo¨rster overlap integral of the fluorescence spectrum with the excited state absorption of the main emitter and with the absorption spectrum of the quencher and therefore is independent of the β phase concentration in films. We assume Rq ) Ra and use the published values of the annihilation rate18 to obtain the dependence of Nq on the amount of β phase (Figure 4). The result shows that the concentration of quenchers is independent of the concentration of β phase in the films. The error bars are determined by the accuracy of the measurement of the annihilation rate constant in ref 18. The expected value of the Ra/Rq ratio is between 0.5 and 2 because all transitions are dipole-allowed, thus, the absolute concentration of quenchers is likely to be accurate within a factor of 2. In any case it is independent of the amount of β phase, which is an important result. If quenching was by excimer formation, then we would expect the quencher concentration to depend on the amount of β phase because interchain interaction would be different between twisted chromophores of the glassy phase and planar chromophores of the β phase. The result indicates that fluorescence quenching is not by excimer formation but is rather
Fluorescence Quenchers in Polyfluorene Films caused by on-chain defects, such as impurities from synthesis or fluorenone defects, which are not dependent on chain conformation. Even though the concentration of the quenchers is independent of the amount of β phase, the quenching rate is higher in β phase rich films, due to faster diffusivity of excitons. The best efficiency of light emitting devices is therefore expected to be achieved in materials with a low amount of β phase (
