J. Phys. Chem. B 2009, 113, 14047–14051
14047
ARTICLES Intensive Chiroptical Properties of Chiral Polyfluorenes Associated with Fibril Formation Girish Lakhwani,† Jeroen Gielen,‡ Martijn Kemerink,† Peter C. M. Christianen,‡ Rene´ A. J. Janssen,† and Stefan C. J. Meskers* Molecular Materials and Nanosystems, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, and High Field Magnet Laboratory, Faculty of Science, Radboud UniVersity Nijmegen, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands ReceiVed: April 3, 2009; ReVised Manuscript ReceiVed: July 30, 2009
Thin films of chiral poly{9,9-bis[(3S)-3,7-dimethyloctyl]-2,7-fluorene} (1) were studied using circular dichroism (CD) spectroscopy. Films spin coated from chloroform solution, show CD with a degree of polarization gabs () +4 × 10-4 at 400 nm) that is independent of film thickness (50-290 nm). This implies that gabs is an intensive property of the material and related to the chiral organization of the molecules on a length scale less than 50 nm. Atomic force microscopy (AFM) on the films reveals fibrils. Addition of nonsolvent methanol to a solution of 1 in chloroform leads to fibril formation in solution and results in CD similar in band shape to that of the pristine spin coated films from chloroform solution and a gabs comparable in magnitude. Thus the chiral molecular arrangement leading to circular dichroism is part of the internal structure of these fibrils. Introduction
gabs )
The optical and electronic properties of thin films of π-conjugated polymers depend on the polymer organization within the film.1 The performance of polymer light emitting diodes, field effect transistors, or polymer photovoltaic cells is intimately related to the aggregation state of the polymer chains. Polyfluorenes (PF) are a class of π-conjugated polymers that has been studied intensively motivated from their excellent blue light-emitting properties.2 Polyfluorenes are interesting because they can occur in different liquid crystalline (LC) and (semi)crystalline states each with its own optoelectronic properties. Moreover the polyfluorene backbone can fold into a number of helical conformations. Processing conditions have a strong influence on which of the conformational and crystallization states would predominate in thin film deposited via, for example, spincoating. This makes PF very attractive for investigations on the relation between organization and optoelectronic properties.3 Circular dichroism (CD) spectroscopy4 has been used to study the molecular organization of chiral π-conjugated polymers,5 including polyfluorenes. Chirality here serves as a spectroscopic “label”. The magnitude and sign of the CD has been shown to be very sensitive to interchain electronic interaction6 but also to long-range helical ordering occurring in, for example, cholesteric phases. When analyzing CD, it is often convenient to express the results in terms of a degree of circular polarization or dissymmetry ratio. This ratio or g-value for absorption of light is usually defined as * To whom correspondence should be addressed. E-mail: s.c.j.meskers@ tue.nl. † Eindhoven University of Technology. ‡ Radboud University.
AL - AR 1 (A + AR) 2 L
(1)
where AL and AR denote the absorbance for left and right circular polarized light. For dilute solutions of chiral molecules, gabs is usually independent of concentration and can be considered an intensive property of the molecule. Solid films of chiral polythiophenes often show CD effects. For some types of polythiophene it has been shown that the magnitude of gabs is independent of the thickness of the film.7 This latter observation indicates that gabs is an intensive property of these films and that the difference in absorption of the two circular polarizations solely arises from the local molecular arrangement at the site of photon absorption. In other words gabs is an intrinsic property to the site where the excited state is created. In contrast, for thermally annealed films of chiral poly{9,9bis[(3S)-3,7-dimethyloctyl]-2,7-fluorene} (1, Figure 1), it has been shown that gabs varies strongly with the thickness of the film.8 In this case gabs is an extensive property and the origin of the chiroptical effect is extrinsic to the site of photon absorption.9 For annealed films of sufficient thickness, gabs can reach very high values (∼1). The extraordinary magnitude of these CD effects after annealing has been interpreted in terms of longrange order in the film induced by thermal annealing in the liquid crystalline state.8,10 For thermally annealed films of oligofluorenes also very large CD effects have been observed.10 These CD effects were explained quantitatively, starting from the chiral nematic ordering of these oligofluorenes in the film building upon models developed for explaining extensive circular dichroism.11 The occurrence of extensive CD effects for, for example, polyfluorenes, does not necessarily exclude a contribution to
10.1021/jp903083x CCC: $40.75 2009 American Chemical Society Published on Web 10/07/2009
14048
J. Phys. Chem. B, Vol. 113, No. 43, 2009
Lakhwani et al. in the liquid crystalline state results in extensive chiroptical properties. This illustrates a particular structure-property relation for thin films of π-conjugated polymers. Elucidation of structure-property relations in general is of relevance for optimization of the electro-optical response of polymer devices. Experimental Section
Figure 1. (a) CD spectra of film (thickness 2.3 × 102 nm) of 1 spincoated from chloroform solution (conc. 10 mg/mL) before (solid line) and after annealing (dashed line). (b) Degree of circular polarization in absorbance gabs as a function of film thickness for unannealed films of 1 spincoated from chloroform probed at 360 and 400 nm (peaks of maximum intensity CD), (c) same as panel b probed at 350 and 390 nm, but now after annealing at 120 °C for 10 min. Absorbance spectra of (d) unannealed and (e) annealed films of 1.
the CD of intensive nature. However, given the very high gabs values arising from extensive effects and the small magnitude of gabs that is typical for genuine intensive effects, an intensive effect can easily be overshadowed by an extensive effect. For films of chiral polyfluorene, intensive CD effects are not known. The existence of CD effects in dilute solution12 indicates that such intensive CD is likely to exist. In this paper, we describe intensive CD in films of chiral PF 1 spin coated from chloroform solution based on systematic variation of the film thickness. Atomic force microscopy (AFM) on these films shows the presence of fibrillar aggregates. Solvent-induced aggregation studies indicate that the intensive CD mainly results from interchain interactions between chains held in a chiral arrangement within the aggregate. These findings cumulate in a comprehensive view on the chiroptical properties of 1. Short range helical order present in the fibrillar structural subunits of the material results in intensive circular dichroism, whereas long-range helical order induced by thermal annealing
Material. Poly{9,9-bis[(3S)-3,7-dimethyloctyl]-2,7-fluorene (1) with amino end-groups having Mn ∼ 20 400 and Mw ∼ 37 600 (PDI 1.84) was synthesized by Suzuki polycondensation in close analogy to procedure reported in literature13 with a small percentage (1%) of p-bromonitrobenzene as the end-capper to control the molecular weight. Subsequent reduction by SnCl2 in EtOH/EtOAc led reduction of the nitro- to amino- end groups. On the basis of the molecular weight, the amino-terminated fluorene end groups make up 2-4% of the total number of monomeric units in the material. Preparation. Films of 1 were prepared by spincoating. Film thickness was varied by changing the concentration of the solution and/or the spin speed (in the range 500 to 4000 rpm). For films prepared from chloroform solution, the concentration ranged from 10 to 15 mg/mL. Films with different thickness from chloroform/methanol solvent mixture, were prepared by changing spin speed and keeping the polymer concentration constant (10 mg/mL). Thickness of the films was measured by Veeco surface profilometer. Thermal annealing of the films was done in air at 120 °C for 10 min. For CD experiments in solution, a stock solution was prepared in chloroform (conc. 7.6 × 10-3 mg/mL). Aliquots of this stock solution were taken and methanol was added dropwise until reaching the desired solvent composition. For dynamic and static light scattering, the same stock solution was used. Optical Spectroscopy. UV-vis measurements were done on a Perkin-Elmer Lambda 900 UV/vis/NIR spectrometer. CD spectra were measured on a Jasco J-815 spectropolarimeter where the sensitivity, and scan rate were chosen appropriately. Temperature dependent measurements were performed with a PFD-425S/15 Peltier-type temperature controller with a temperature range of 263-383 K and adjustable temperature slope. CD effects in films with ellipticity >2000 mdeg were measured using a Perkin-Elmer Lambda 900 UV/vis/NIR spectrometer and a circular sheet polarizer (Oriel). AFM. A Veeco MultiMode connected to a Nanoscope IV control unit was used for AFM. The scanning was done in tapping mode in air. The tips used were PPP-NCH-50 from Nanosensors and were typically operated at an amplitude set point of 1.5 V (free oscillation amplitude was 2.0 V) in order to obtain better phase images. Static (SLS) and Dynamic Light Scattering (DLS). The light output of an intensity-stabilized helium-neon laser (Spectra Physics, λo ) 632.8 nm, 4.5 mW) was focused in the center of a cylindrical glass cell (Helma). The scattered light was transferred to a single photon detector (ALV/SO-SIPD) through a single mode fiber to meet the spatial coherence conditions. SLS measurements were performed with a computerized homemade goniometer, using a step size of 1° in the angular range of 30 to 100°, corresponding to the scattering wavevector (q) of 6 to 20 µm-1
q)
4πn sin(θ/2) λo
where n ) refractive index of dispersant, and θ ) scattering angle.
Chiroptical Properties of Chiral Polyfluorenes
J. Phys. Chem. B, Vol. 113, No. 43, 2009 14049
TABLE 1: Relative Area Swept by the CD Trace for Unannealed and Annealed Films of 1 Spincoated from Chloroform and Chloroform Methanol (5:3 v/v) and in Solution (Chloroform/Methanol 5:3 v/v)a relative area solvent (film thickness)
104 × solution
chloroform (110 nm)
chloroform/MeOH (150 nm)
a
no CD
-5 (14% of + lobe) + -5 35 -35
104 × unannealed
annealed
-0.58 (6% of + lobe) + -0.14 +9.40 -9.84 -1.9 (46% of + lobe) + 0.0 +4.1 -6.0
+ 0.08 + +0.01
-0.06 (30% of - lobe) + -0.20 +0.06 0.003 (7% of - lobe) + -0.045 +0.038
The area is calculated using eq 2.
Integration time per angle was 5s. The intensity signal was processed with a 320-channel Multiple Tau Digital Correlator (ALV-5000/E) using a typical acquisition time of 300s. A temperature controller was used to stabilize the temperature at 293.0 ( 0.1 K. Evaluation of the autocorrelation function gave a value of the decay constant Γ. Here Γ ) Dq2, with D ) translational diffusion coefficient. Results and Discussion Solid films of 1 spincoated from chloroform solution show CD. This is illustrated in Figure 1a for a film of 2.3 × 102 nm thickness. The onset of the CD band at 415 nm wavelength coincides with the onset of the first absorption band of PF assigned to the allowed S0 f S1 optical transition. The band shape of the CD spectrum is bisignate with a maximum at a wavelength λ ) 400 nm and a minimum at λ ) 360 nm. From the CD and the absorption spectrum, gabs can be calculated. gabs reaches a maximum at 400 nm (+4 × 10-4) and a minimum at 360 nm (-2 × 10-4). In Figure 1b, we have plotted these gabs values for unannealed films as a function of the thickness of the films. In agreement with earlier reports, we find that upon annealing of the films at 120 °C, the band shape of the CD spectrum changes and the gabs values become much larger.4,10 For the annealed films, the gabs value strongly depends on film thickness (see Figure 1c) and reaches a value -0.25 at 390 nm for the 2.9 × 102 nm thick film. This strong variation of gabs with films thickness for the annealed films shows that in this case gabs is not an intensive property of the material. Absorbance spectra of the respective films with varying thickness before and after annealing are shown in Figure 1d,e. For the unannealed films with thickness ranging from 50 to 290 nm, we find that gabs is essentially constant. Therefore we can consider gabs to be an intensive property of the material, originating from a chiral molecular organization on a length scale
