J. Phys. Chem. B 2010, 114, 1277–1286
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Unraveling Excited-State Dynamics in a Polyfluorene-Perylenediimide Copolymer Eduard Fron,† Ania Deres,† Susana Rocha,† Gang Zhou,‡ Klaus Mu¨llen,‡ Frans C. De Schryver,† Michel Sliwa,§ Hiroshi Uji-i,† Johan Hofkens,† and Tom Vosch*,† Department of Chemistry and Institute for Nanoscale Physics and Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200F, 3001 HeVerlee, Belgium, Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany, and Laboratoire de Spectrochimie Infrarouge et Raman (UMR 8516 du CNRS), Centre d’Etudes et de Recherches Lasers et Applications (FR 2416 du CNRS), UniVersite´ Lille 1 Sciences et Technologies de Lille, Baˆt C5, 59655 VilleneuVe d’Ascq Cedex, France ReceiVed: September 29, 2009; ReVised Manuscript ReceiVed: December 10, 2009
Insight into the exciton dynamics occurring in a polyfluorene-perylenediimide (PF-PDI) copolymer with a reaction mixture ratio of 100 fluorene units to 1 N,N′-bis(phenyl)-1,6,7,12-tetra(p-tert-octylphenoxy)-perylene3,4,9,10-tetracarboxylic acid diimide (PDI) is presented here. Time-correlated single photon counting and femtosecond transient absorption spectroscopy measurements on the PF-PDI copolymer have been employed to investigate the excited-state properties of the polyfluorene subunit where the exciton is localized (PF) and the incorporated PDI chromophore. The experimental results were compared with those obtained from a polyfluorene polymer (model PF) and a N,N′-bis(2,6-diisopropylphenyl)-1,6,7,12-tetra(p-tert-octylphenoxy)perylene-3,4,9,10-tetracarboxylic acid diimide (model PDI) which were used as reference compounds. Because of the high polydispersity of the PF-PDI copolymer, there is a polymer fraction present that contains no PDI chromophores (polyfluorene polymer fraction (PF polymer fraction)), and wide-field imaging of single polymers chains of the synthesized PF-PDI copolymer was used to estimate this PF polymer fraction. Following the primary excitation of the PF in the PF-PDI copolymer, energy hopping between PF’s can occur. A fraction of the energy of the absorbed photons will be transferred to a PDI chromophore via energy transfer from a PF. In a polar solvent, a charge transfer state having the S1 of the PDI moiety as a precursor state is found to form with high efficiency on a nanosecond time scale. The data suggest that a fraction of the absorbed energy is directed, transferred, and used in charge separation, providing a clear view of a multistep mechanism of exciton dissociation into charges. Introduction The properties and dynamics of the excited states provide important information for the detailed understanding of complex processes often occurring in conjugated polymers. Investigation and tuning of these photophysical processes led to the development of new functional polymer materials for optoelectronic applications.1,2 One approach of tuning the spectroscopic properties is by controlling the effective conjugation length.3 Extending the conjugation length in a polymer chain leads to a bathochromic shift of the absorption and emission characteristics.4 However, the presence of sites of different conjugation lengths can contribute to the formation of energy traps as the sites with the largest conjugation act as local minima. While application-tailored spectroscopic properties were achieved, the increased complexity of these multichromophoric systems involving either chromophore-chromophore interactions or intrinsic conformations motivates the study of specific systems containing well-known donor/acceptor units. However, the study of the excited-state dynamics of polymer chains with incorporated chromophores or donor/acceptor units remains a challenge, mainly for two reasons. First, the large polydispersity and the difficulty in controlling the ratio of the constituents per chain in the synthesis processes can result in a distribution of kinetic * To whom correspondence should be addressed. E-mail: Tom.Vosch@ chem.kuleuven.be. † Katholieke Universiteit Leuven. ‡ Max-Planck-Institut fu¨r Polymerforschung. § Universite´ Lille 1 Sciences et Technologies de Lille.
components in time-resolved ensemble experiments. Also, small fractions of impurities or uncoupled subunits can show up in the observations, which could lead to erroneous conclusions. Second, competing excited-state processes such as singlet exciton migration, triplet formation, or charge transfer are more likely to occur, possibly affecting the fluorescence emission and making, for example, single molecule investigations complex and challenging.5-11 Among the various types of conjugated systems, polyfluorene copolymers have shown remarkable luminescence and optoelectronic properties12-15 that can be tuned toward the development of promising alternatives to inorganic light-emitting diodes (LEDs),16,17 and polymer-based solar cells.18-21 Attaching molecular moieties with energy-accepting and highly luminescent properties (rylene dyes, phenylene vinylene units, etc.)15,22,23 to the polyfluorene π-system also offers the prospect of tuning the emission wavelength for polymer-based LED applications.16 Polyfluorene-based polymers with fluorene segments and molecular units with electron accepting or donating properties were shown to improve the overlap with the solar spectrum in polymer-based solar cells.24,25 Also, for instance, in a polyfluorene/fullerene blend, a high conversion efficiency of the excitation energy into charges has been observed at low acceptor concentration.26 While the efficiency of light absorption is basically improved, the charge generation and recombination, which is a key factor in their performance, remains poorly investigated, and their understanding requires further assessment. Hence, it is important to study and understand conformational
10.1021/jp909295h 2010 American Chemical Society Published on Web 01/05/2010
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J. Phys. Chem. B, Vol. 114, No. 3, 2010
Figure 1. Chemical structures of model PF, model PDI, and PFPDI. The reaction mixture ratio of n to m is 100 to 1.
aspects, the energy transfer pathways, exciton dynamics, and charge conversion in conjugated polymers and more specifically in polyfluorene systems. For example, the degree of planarity between the adjacent fluorene units was shown to substantially change the electronic and optical properties. By monitoring the energy of the 0-0 transition, Feldmann et al. classified single molecules belonging to three different phases, depending on the degree of planarization.27 Besides local polymer conformations and the presence of chromophores with longer conjugation lengths (emissive traps), random chemical defects28 could modify the exciton diffusion significantly, resulting in very different steady-state emission properties. Therefore, it is relevant to understand the energy transfer pathways, exciton dynamics, and charge conversion of the polyfluorene systems in different environments. In this study, we explore the photophysical properties and dynamics of a polyfluorene-perylenediimide copolymer (PFPDI) upon excitation of the PF (the polyfluorene subunit where the exciton is localized) and PDI (the N,N′-bis(phenyl)-1,6,7,12tetra(p-tert-octylphenoxy)-perylene-3,4,9,10-tetracarboxylic acid diimide chromophore) parts by both ensemble and singlemolecule techniques. The results of the ensemble measurements were then compared with those obtained from two reference compounds: a polyfluorene polymer (model PF) and N,N′bis(2,6-diisopropylphenyl)-1,6,7,12-tetra(p-tert-octylphenoxy)perylene-3,4,9,10-tetracarboxylic acid diimide (model PDI). Time-correlated single-photon counting (TC-SPC) and femtosecond transient absorption techniques were employed to unveil and identify the excited-state processes. We explore various aspects such as the localization of the primary exciton, its migration along the polymer chain, and the properties of the electronic excited states in solvents of different polarity. Because of the high polydispersity of the PF-PDI copolymer, there is a polymer fraction present that contains no PDI chromophores (polyfluorene polymer fraction (PF polymer fraction)). Widefield imaging29 of single polymers chains of the synthesized PFPDI copolymer was used to estimate this PF polymer fraction in order to obtain a quantitative estimation of the energy transfer occurring in the PF-PDI copolymer fraction. Experimental Section Synthesis of Materials. The syntheses of model PDI, model PF, and PF-PDI were described previously,16 and their chemical structures are shown in Figure 1. Briefly, for the synthesis of the PF-PDI copolymer, a mixture of N,N′-bis(4-bromophenyl)1,6,7,12-tetra(p-tert-octylphenoxy)-perylene-3,4,9,10-tetracar-
Fron et al. boxylic acid diimide (1%) and 2,7-dibromo-9,9-dioctylfluorene was polymerized with nickel(0) in dimethylformamide (DMF)toluene for 24 h, followed by a reaction with an excess of bromobenzene to remove any residual terminal bromine functionality (end-capping). The PF-PDI has an average molecular weight of about 130500 g · mol-1, a polydispersity index of 3.83 (see Ego et al. for further details),16 and, based on the starting reaction mixture ratio, could contain on average of 1% of PDI units. The large polydispersity index together with Mn and the percentage of incorporated PDI’s result in a very broad chain length distribution of polymeric macromolecules as well as a broad distribution of the number of PDI units present in the individual chains (as a result of this, there is also a fraction containing no PDI units). Wide-field imaging of individual polymers chains was used to determine the size of the fraction of polymer chains that do not contain any PDI chromophores, the PF polymer fraction (vide infra). Before measuring the photophysical properties of the PF-PDI copolymer, an extra purification step was performed to remove nonincorporated PDIs (a fraction of 0.18 was removed; see Supporting Information). These free PDIs were removed by passing the PF-PDI copolymer through a recycling high-performance liquid chromatography/gel permeation chromatography (HPLC/GPC) system (LC-9101 from Japan Analytical Industry Co. Ltd.). Steady State Measurements. The stationary measurements were recorded in cyclohexane (dielectric constant ) 2, refractive index ) 1.42), toluene (dielectric constant ) 2.43, refractive index ) 1.49), chloroform (dielectric constant ) 4.8, refractive index ) 1.44), tetrahydrofuran (THF, dielectric constant ) 7.6, refractive index ) 1.4) and benzonitrile (PhCN, dielectric constant ) 25, refractive index ) 1.52) as solvents of different polarity using a spectrophotometer (Lambda 40, Perkin-Elmer) and a fluorimeter (Fluorolog FL3-11, Perkin-Elmer) corrected for the wavelength dependence of the detection system.30 All solvents were spectroscopic grade. The optical density at the absorption maximum of all solutions was kept below 0.1 in a 1 cm cuvette, and for the emission spectra the excitation wavelengths were set to 375 and 543 nm for the PF and PDI moieties, respectively. The fluorescence quantum yields of the compounds were determined using perylene in toluene (for λex ) 390 nm, Φf ) 0.71) and cresyl violet in ethanol (for λex ) 543 nm, Φf ) 0.5) as references.31,32 Single Molecule Wide-Field Imaging and Analysis. Widefield imaging of single PF-PDI molecules was performed using an inverted optical microscope (IX71, Olympus) equipped with a 1.3 N.A., 100× oil immersion objective (Plan Fluorite, Olympus) and a highly sensitive cooled charge-coupled device (CCD) camera with 512 × 512 pixels (Hamamatsu EM-CCD ImagEM Model C9100-13) with a pixel size of 16 × 16 µm2. For excitation, 532 nm light from a diode-pumped solid state laser (CDPS532M-50, JDS Uniphase Co.) and 375 nm light from a CW diode laser (Excelsior Diode CW Laser, SpectraPhysics) were used. Excitation intensities were set to 250W/ cm2 for 532 nm and 10W/cm2 for 375 nm. The wide-field illumination for excitation was achieved by focusing the expanded and collimated laser beam onto the back-focal plane of the objective. The polarization of the excitation light in the sample plane was carefully tuned to be circular using zero-order λ/4 waveplates to compensate for polarization shifts of the dichroic mirror. Emission was collected by the same objective and imaged by the CCD after passing through a dichroic mirror (z532rdc, Chroma Technology Co.) and an additional spectral filter (HQ545LP or HQ400LP, Chroma Technology Co.), removing the excitation light. The image was further magnified
Excited-State Dynamics in a PF-PDI Copolymer 3.3 times with a camera lens (Olympus) before the CCD camera, resulting in a maximum field of view of 24.6 × 24.6 µm2 (48 × 48 nm2 per pixel). All measurements were done at room temperature under ambient atmosphere. The obtained images were analyzed using a routine written in MatLab software, in order to reconstruct the intensity trajectory of a single spot in the recorded movies (0.5 s integration time per frame).33 Samples were prepared by spin-casting ∼10-10 M solutions of the PFPDI copolymer dissolved in toluene containing 5 mg/mL poly(methyl methacrylate) polymer (PMMA, Aldrich, Mw ≈ 93 000) onto thoroughly cleaned glass coverslips. Picosecond Fluorescence TC-SPC Experiments. The fluorescence decay times have been determined by TC-SPC measurements described in detail previously.34 A time-correlated single photon timing PC module (SPC 830, Becker & Hickl) was used to obtain the fluorescence decay histogram in 4096 channels. The decays were recorded with 10 000 counts in the peak channel in time windows of 25 and 15 ns corresponding to 6 ps/ch and 3.7 ps/ch and analyzed globally with timeresolved fluorescence analysis (TRFA) software.35,36 The full width at half-maximum (fwhm) of the instrument response function (IRF) was typically on the order of 40 ps. The quality of the fits has been judged by the fit parameters χ2 (
