LETTER pubs.acs.org/JPCL
Circular Selective Reflection of Light Proving Cholesteric Ordering in Thin Layers of Chiral Fluorene Polymers Girish Lakhwani† and Stefan C.J. Meskers* Molecular Materials and Nanosystems, Eindhoven University of Technology, P.O. Box 513, NL 5600 MB Eindhoven, The Netherlands ABSTRACT: Thermally annealed and aligned thin films of polyfluorene and two other chiral fluorene copolymers preferentially reflect left-circular-polarized light with wavelengths just below the onset of the lowest-allowed ππ* optical transition. This provides evidence for a left-handed cholesteric arrangement of the polymer chains in the film. SECTION: Macromolecules, Soft Matter
luorene and alternating fluorene copolymers are used as active materials in polymer optoelectronic devices such as light-emitting diodes1 and solar cells.2,3 Polymers of this type often show a liquid-crystalline state at elevated temperature. In this state, the polymers can be aligned using, for example, an alignment layer. The induced molecular order can be vitrified upon rapid cooling, and polymer layers with highly anisotropic optoelectronic properties can be obtained.47 Chiral fluorene polymers8 and copolymers911 often show a large difference in optical and optoelectronic properties for left (L)- and right (R)-circular-polarized light after thermal annealing in the liquidcrystalline state. In this way, diodes emitting circular-polarized electroluminescence12 could be realized, and also photovoltaic cells showing different short circuit photocurrents for L- and R-polarized illumination have been shown.13 The nature of the molecular arrangement induced by the annealing and responsible for the large chiroptical properties has remained unclear. Detailed studies on fluorene nonamers have shown that in thick layers (4 μm), the molecules adopt a cholesteric molecular ordering, as proven by the observation of a selective reflection band.14,15 Here, we report on circular selective reflection of light by aligned and thermally annealed thin layers of two fluorene alternating copolymers,10 namely, poly(9,9-bis((3S)-3,7-dimethyloctyl)-2,7-fluorene-alt-dithienylbenzothiadiazole) (Mn ≈ 9 kg/mol, PDI 1.2) 1, poly(9,9-bis((3S)-3,7-dimethyloctyl)-2,7fluorene-alt-benzothiadiazole) (Mn ≈ 9 kg/mol, PDI 1.4) 2, and the homopolymer poly [9,9-bis((3S)-3,7-dimethyloctyl)-2,7fluorene] (Mn ≈ 20.4 kg/mol, PDI 1.84) 3; see Figure 1. Dedicated reflection experiments show that the circular differential reflectance derives from a reflection process in which the light keeps its circular polarization upon reflection. In contrast to ordinary dielectrics, which reflect light with inversion of circular
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r 2011 American Chemical Society
polarization, cholesteric liquid crystals reflect light with conservation of the circular polarization.16 Hence, we conclude that a cholesteric molecular organization exists in thin, annealed films of chiral fluorene (co)polymer. Films of 1, 2, and 3 were deposited by spin coating from solution onto a glass substrate with a rubbed polyimide alignment layer. The samples were annealed for 15 min at 150 °C in a N2 atmosphere, which is above the clearing temperature.10 Optical properties of these films were first investigated using a variable angle ellipsometer (Woollam). Measurement of the transmission of light through a ∼70 nm thick film of 1 at normal incidence and with the rubbing direction oriented vertically shows a sharp drop in transmission at 2.0 eV corresponding to the onset of the allowed S0S1 transition, with ππ* orbital nature; see Figure 1a. The S0S1 transition is polarized in a direction along the polymer backbone. The preferential absorption of vertically (V) polarized light over horizontally (H) polarized light shows that the polymer chains align parallel to the rubbing direction of the substrate.17 An annealed film of 1 selectively reflects L-circular-polarized light. The solid line in Figure 1b shows the degree of circular polarization in reflection as expressed by the dissymmetry ratio or g value grefl ¼
IL IR ðIL þ IR Þ=2
ð1Þ
Here, IL denotes the total intensity of reflected light upon L incident light, and IR is the corresponding intensity for R incident Received: April 13, 2011 Accepted: May 24, 2011 Published: May 24, 2011 1497
dx.doi.org/10.1021/jz200500a | J. Phys. Chem. Lett. 2011, 2, 1497–1501
The Journal of Physical Chemistry Letters
LETTER
Figure 1. Transmission (a,e,i), g values for reflection (solid line) and transmission (dashed line) of light (c,g,k), optical rotation (b,f,j), and reflectance (d,h,l) for annealed films of polymers 1, 2, and 3, respectively. Spectra sets (ad) belong to thin films of polymer 1, (eh) to thin films of polymer 2, and (il) to thin films of polymer 3. Here, V stands for vertical and H for horizontal; p and s stand for parallel and perpendicular components of incident light to the plane of incidence. In reflection measurements, the angle of incidence θi is 15°; for transmission and optical rotation, θi = 0° The reflectance shown in d, h, and i is obtained after rotating the sample in a counterclockwise direction over, respectively, 15, 70, and 22° when viewing along the propagation direction of the incident light beam and 0° rotation corresponding to the rubbing direction oriented vertically. This rotation maximized the linear dichroism in reflectance.
light. grefl reaches a first maximum (þ0.07) at 2.0 eV and a second maximum (þ0.10) at 2.5 eV. The reflected light is measured with an angle of incidence θi of 15°. The magnitude of grefl varies with the orientation of the film (by (50%), yet sign and band shape do not change upon rotation around the surface normal. The selective reflection of L-polarized light provides direct evidence for a left-handed cholesteric arrangement of the polymer films with a helical pitch length comparable to the wavelength of visible light.16,18 The left-handed cholesteric arrangement also gives rise to other pronounced chiroptical effects. For instance, the dashed line in Figure 1b shows the degree of circular polarization in the transmission of L- and R-polarized light (measured at θi = 0°) as expressed by the g value gtrans ¼
IL IR ðIL þ IR Þ=2
ð2Þ
Here, IL (IR) denotes the total intensity of transmitted light upon L- (R)-polarized incident light. At a photon energy of 2.1 eV, slightly above the onset of the S0S1 absorption, gtrans reaches a maximum of þ0.1. gtrans varies little (
