Raman Anisotropy Measurements: An Effective Probe of Molecular ...

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Raman Anisotropy Measurements: An Effective Probe of Molecular 

Orientation in Conjugated Polymer Thin Films** 

By Hai-Ming Liem, Pablo Etchegoin,* Katherine S. Whitehead, and Donal D. C. Bradley* 

Understanding the molecular alignment of conjugated polymers within thin-film samples is essential for a complete picture of 

their optical and transport properties, and hence for the continued development of optoelectronic device applications. We 

report here on the efficacy of Raman anisotropy measurements as a probe of molecular orientation, presenting results for 

aligned polyfluorene nematic glass films. Comparison is made with the results of optical dichroism measurements performed 

on the same samples. We show that in many cases molecular orientation can be more directly characterized by Raman anisotropy, 

and that it can have a greater sensitivity to the degree of molecular orientation than conventional optical dichroism. The fact 

that the Raman measurements can be readily performed on the same thin films (~ 100 nm thickness) that are required for 

optical dichroism means that there is no ambiguity in a direct comparison of results. This situation differs from that for standard 

X-ray diffraction measurements (these require film thicknesses of several lm) and electron diffraction or electron energy loss 

spectroscopy measurements (these require film thicknesses of 10 nm or less). The Raman data allow the angle (relative to the 

chain axis) for the optical dipole transition moment to be deduced from the dichroic ratio, and confirm the role that its off-axis 

component plays in limiting this ratio. The added fact that Raman anisotropy data can be collected in situ, in reflection geometry 

for standard device structures, and with microscopic resolution and chemical specificity makes the technique even more 

attractive as a non-invasive device probe. 

1. Introduction 

The intrinsic anisotropy associated with strong intra- and 

weak inter-chain p-bonding within conjugated polymer films 

opens up the possibility of constructing semiconductor devices 

with novel electrical and optical properties, including a highly 

polarized emission [1±6] that is of interest for liquid-crystal display 

backlights. Orientation can also facilitate enhanced charge 

transport [7,8] and optical confinement, [9±11] leading to improved 

optoelectronic and photonic devices. Enhanced mobility offers 

advantages for light emitting diode (LED) displays, since it allows 

higher current densities and thus offers access to the high 

peak brightness needed for passive matrix addressed displays. [7] 

Thin-film transistors benefit from higher mobility through faster 

switching. [8] In photovoltaic diodes, mobility limits efficiency 

by controlling the film thickness from which carriers can be 

efficiently extracted. For typical organic materials this is below 

the optimum for full light absorption. [12] Mobility is also expected 

to be important for achieving electrically pumped 

amplifiers and lasers in which high current densities are 

required. Optical confinement is assisted both by the orientation 

of the emission dipoles [9] and by the large accompanying 

± 

[*] Prof. D. D. C. Bradley, Dr. P. Etchegoin, Mr. H. M. Liem, 

Dr. K. S. Whitehead 

Imperial College of Science, Technology, and Medicine 

Prince Consort Road, SW7 2BZ London (UK) 

E-mail: d.bradley@imperial.ac.uk, p.etchegoin@imperial.ac.uk 

[**] This work was supported by the United Kingdom Engineering and Physical 

Sciences Research Council (GR/M21201) and the Dow Chemical Company 

(ªPolymer Semiconductorsº). We thank Mark Bernius, Mike Inbasekaran 

and Jim O'Brien of the Dow Chemical Company for providing the polymers 

used in this study. 

birefringence, [10] and leads to reduced gain thresholds as a 

result of reduced mode volumes. [11] In order to optimize these 

benefits for device application, relatively high degrees of 

molecular orientation are required, somewhat limiting the 

choice of materials systems and orientation methods. Grell and 

Bradley [4] have recently reviewed the methods for alignment of 

molecular electronic materials. Uniaxial alignment is relatively 

easy to achieve via tensile drawing, [13,14] mechanical rubbing, 

[15±17] and via lyotropic [3] or thermotropic [1,18] mesophase 

formation. In particular, the availability of a thermotropic liquid-crystal 

phase allows a straightforward approach to the fabrication 

of highly oriented thin films on device substrates. [3,5,8,10] 

Spin coating a film onto a rubbed [1,3,5] or photoactivated [19] 

alignment layer is followed by a heat-treatment protocol to 

initiate the orientation process. The film is then either 

quenched to room temperature to freeze in the alignment within 

a liquid crystalline glass, or slowly cooled to generate a crystalline 

film. Mono-domain alignment has been demonstrated 

for a range of fluorene homopolymers [1,3,20] and copolymers. [18] 

Photoluminescence (PL) and electroluminescence (EL) anisotropy 

and optical dichroism measurements have been used 

to directly characterize the emission properties and to probe 

the underlying degree of molecular alignment [1,2,5,7,14,17,18,21] . 

Typically, the molecular order parameters are estimated from 

the ratio at the long wavelength absorption peak for light 

polarized parallel and perpendicular to the alignment direction, 

as defined for example by the rubbing direction of the 

alignment layer. PL and EL measurements use the intensity 

ratio at the peak of the emission spectrum, or the ratio of the 

integrated areas of the emission spectra for parallel and perpendicular 

polarizations. Each of the resulting anisotropies is 

expected to be influenced by the details of the polymer elec- 

66 Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1616-301X/03/0101-0066 $ 17.50+.50/0 Adv. Funct. Mater. 2003, 13, No. 1, January

H.-M. Liem et al./Molecular Orientation in Conjugated Polymer Films 

tronic structure, by the nature and effectiveness of the interactions 

with the alignment substrate, by the alignment protocols 

used, and by the sample's thermal history. The largest anisotropies 

obtained so far for polyfluorene homo- and co-polymers 

aligned on rubbed substrates are ~ 25 in EL, [5] ~ 10 in PL [5] and 

~ 11 in UV-vis absorption. [3] Two questions that arise are: 

i) How does the anisotropy relate to the structural order 

parameter, S, describing the average microscopic orientation of 

molecular chain axes relative to the macroscopic alignment 

direction? 

ii) Why is the anisotropy found to be different for different 

experimental techniques? 

The optical dichroism, D, is expected to be related to S by: [22] 

D £ (1 + 2S)/(1 ± S) (1) 

Note that the equality applies only for a transition dipole 

moment that is aligned along the molecular chain axis, and for 

a mono-domain sample. Mono-domain formation has been 

clearly demonstrated for the thermotropic polyfluorene samples 

that we have studied to date. [1,18] The alignment of the 

transition dipole is, however, unknown. Polyphenylenes and 

polyarylene ethynylenes are expected to have axial transition 

dipole moments on account of their extended linear structure. 

[2] Polyfluorenes are not. Theoretical considerations of the 

polyfluorene geometry and experimental light scattering studies 

[23] point to an angle of some 19 between the r-bonds at 

opposite ends of each fluorene moiety. This distortion of shape 

compared with a linear biphenyl moiety is forced by the C9 carbon 

link, and can be expected to induce a non-axial transition 

dipole. As a consequence, even for perfect chain alignment 

(i.e., S = 1), the optical dichroism would be limited to a finite 

value. This situation can be described by rewriting Equation 1 

with an effective order parameter S¢ = S ” S op with: 

S op = 1/2 (3 cos 2 b ±1) (2) 

Here b is the angle that the transition dipole makes with the 

polymer chain axis. Interestingly, the value of b for a single 

molecular repeat unit does not necessarily dictate the value for 

the effective chromophore as a whole (the polymer chain segment 

involved in the transition). The chain conformation/configuration 

may lead to cancellation of off-axis transition dipole 

contributions from neighboring repeat units within an alternating 

higher order structure. This is the case for the 2 1 helical 

conformation of poly(9,9-dioctylfluorene) (PFO), for which 

the optical dichroism is substantially enhanced. [23] A trans±cisoid 

configuration for polyarylene vinylenes [14] similarly leads to 

an enhancement of the photoluminescence anisotropy. 

We note also that the effects of heterogeneity need to be 

considered when comparing EL and PL anisotropy and optical 

dichroism measurements. In the EL experiments the location 

of the recombination zone, carrier trapping effects, and energy 

migration processes lead to selection of a subset of chain segments 

from which the emission occurs. In PL it is simply energy 

migration that matters but the effect is the same. For absorption 

dichroism, however, the whole of the heterogeneous ensemble 

of chain segments is probed simultaneously. This measurement-dependent 

selectivity of emission sites has been cited 

as an explanation for the differences seen between absorption, 

PL and EL anisotropy measurements in PFO. [5] 

There have been attempts to compare anisotropies obtained 

by optical methods with truly structural probes like X-ray diffraction. 

However, as noted above, these measurements cannot 

be readily taken from the same films. X-ray experiments are 

typically conducted on fibers or thick films, and the degree of 

molecular alignment may not then be directly comparable with 

that for the thin films needed for optical dichroism and used in 

EL devices. Indeed, materials such as PFO are known to show 

strong morphology dependence as a function of processing 

conditions. [24] Access to a structural probe that would allow 

measurements to be undertaken on the same thin film samples 

used for optical anisotropy measurements, would ensure direct 

comparability of results. Removing the ambiguity would then 

allow determination of b, thus facilitating a greater understanding 

of the differences between absorption, PL and EL anisotropy. 

We report here on the use of Raman scattering as just such a 

thin-film structural probe, and show that it is very effective in 

this regard. The use of Raman scattering data requires knowledge 

of the Raman tensor for one or more specific vibrations 

so that the angular variation of the Raman intensity can be 

mapped onto a molecular-orientation distribution function. 

Such information is in general either unavailable or difficult to 

infer, but we have previously investigated the tensor properties 

of the ~ 1600 cm ±1 in-plane stretching mode characteristic of 

phenyl/phenylene rings. [25] This mode has a highly uniaxial 

Raman tensor that is little dependent on the details of chemical 

structure. It is therefore suitable for use as a monitor of molecular 

structure. We note further that this mode is expected to be 

found in the Raman spectra of a wide range of conjugated 

polymers that are currently under development for device 

applications, and hence that the following discussions have reasonable 

generality. 

2. Results and Discussion 

2.1. Theoretical Description of Optical Dichroism and Raman 

Anisotropy and Their Dependence on Molecular Orientation 

We consider a distribution of rod-shaped molecules (representing 

individual chain segments) in the coordinate system 

defined in Figure 1. We restrict ourselves to the case of uniaxial 

polymer-chain orientation, i.e., to systems in which only the 

angles h and b (see Fig. 1) take non-random values. This is expected 

to be entirely appropriate for the oriented nematic-glass 

samples that we have studied here. We also introduce the simplest 

description of the orientation distribution function due to 

Fraser. [26] 

We do this to gain insight into the problem, and for lack of a 

justifiable alternative. The assumption is that a fraction, f, of 

the molecules are perfectly oriented along z, whilst the remaining 

(1 ± f) are isotropically (randomly) oriented. The perfectly 

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θ 

φ 

β 

µ 

ψ 

H 17 C 8 C 8 H 17 

H 17 C 8 C 8 H 17 

N N 

S 

Fig. 1. The general coordinate system used to describe the molecular orientation 

of a polymer chain axis with respect to the macroscopic sample axes x, y, andz. 

The angles h and u define the orientation of the chain axis with respect to the 

axes x, y, andz. The dipole transition moment l lies at an angle b with respect to 

this axis, with an azimuthal angle w. The chemical structures of PFO (upper) and 

F8BT (lower) are also shown. 

oriented fraction, f, corresponds to the Hermans' orientation 

function that can be derived from X-ray diffraction data. [27] 

Note also that if the Fraser distribution function is expanded in 

terms of spherical harmonics, f can be shown to be equal to the 

average of the second-order term in the expansion, namely 

áP 2 cos bñ. [28] We next assume that we are considering molecular 

vibrations with a highly uniaxial Raman tensor (as is the 

case for the ~ 1600 cm ±1 in-plane stretching mode [25] ) and that 

their principal axis is coincident with the polymer-chain axis. 

This might seem an inadvisable restriction, but the experimental 

evidence indicates that there are many families of polymers 

where this condition applies (and sometimes for more than one 

mode (see below)). In accordance with a highly uniaxial Raman 

tensor, we assume that only the a zz component of the 

polarizability makes a significant contribution to the scattering 

efficiency for optical polarizations that are accessible in backscattering 

from the plane of a thin film in the experimental 

geometry shown in Figure 2. 

Backscattered 

light 

e → 

S 

Aligned film is 

rotated to measure 

Incoming 

laser 

e → L 

Alignment 

direction 

Fig. 2. Schematic scattering geometry: Measurements were made with the polarization 

direction of the laser e L initially perpendicular to the alignment direction 

of the film. The scattered polarization e S was kept parallel to e L for all measurements 

and the film was rotated. 

A word must be said at this stage about the nature of the 

Raman tensor because this is crucial for the applicability of the 

technique. In previous studies, we have found different depolarization 

ratios for different experimental conditions, a situation 

that can only be rationalized if a certain degree of alignment 

of the chains occurs. When the polymer was dissolved in 

a solvent (i.e., the orientation of the rings is completely random), 

the depolarization ratio of the strongest in-plane breathing 

(stretching) mode of the phenylene rings was 0.74. For the 

same reason, this value was also found in films that were spin 

coated under conditions where solvent evaporation occurs 

rapidly, leading to a freezing-in of the isotropic structure found 

in solution. [25] This suggests a traceless tensor as proposed by 

Liem et al. [25] The tensor must have two diagonal components 

different from zero: Otherwise it could not explain both the 

isotropic depolarization ratio seen in solution and the depolarization 

ratio of one seen, under special circumstances, at the 

polymer surface. [25] This is in good agreement with the known 

properties of the Raman tensor for the equivalent mode in 

benzene, and with the fact that benzene-related modes do not 

drastically change their symmetry even when the environment 

of the benzene ring is altered. There is plenty of experimental 

evidence to the latter effect from detailed Raman studies on 

mono- and di-substituted benzene molecules. In the case of oriented 

films, our previous work [29] suggests that the phenylene 

rings of the backbone lie perpendicular to the surface. Accordingly, 

the polarized Raman measurements have access to a projection 

of the Raman tensor where only one of the components 

is different from zero. This is what produces the large anisotropy 

that we observe experimentally (see below). 

In this paper we consider how a departure from perfect 

alignment along z will influence the observed anisotropy and 

therefore how we can use Raman anisotropy to probe the chain 

alignment. It is interesting, and perhaps somewhat surprising, 

to note that straightforward Raman anisotropy measurements 

seem not to have previously been used to study the extent of 

molecular alignment in conjugated polymer films. This is despite 

the very large number of publications in the literature 

that have been published concerning the optical and structural 

properties of these materials. The only related studies concern 

Raman measurements on oriented films of trans- [30] and cispolyacetylene. 

[31] All of these studies were performed under 

resonance-excitation conditions, with the primary interest 

focused on the role that the electronic excitation plays in determining 

the observed features of the Raman spectra. A particular 

focus was on understanding the nature of the vibrational± 

electronic mode coupling effects that arise under resonance 

excitation, especially for trans-polyacetylene. The variation in 

depolarization across the carbon±carbon Raman modes and 

the variations in lineshape, parametric in resonant-excitation 

wavelength, were interpreted within a conjugation-length distribution 

model on the assumption that axial alignment of the 

Raman tensor occurs along the molecular chain. [30] Experiments 

on cis-polyacetylene had a similar focus, with an additional 

interest in understanding the changes that occur during 

the isomerization process from cis- totrans-polyacetylene. [31] 

We emphasize that resonance excited Raman depolarization 

measurements were used in all of these studies. Furthermore, 

the measurements were performed on stretch-aligned films 

several micrometers thick, for which X-ray diffraction was used 

to characterize the state of order, and for which significant, 

model based, corrections of the scattering intensities were 

required to account for the effect of the anisotropic optical 

constants. In contrast, our measurements are performed with 

non-resonant excitation and, because of the experimental ar- 



rangement that we use (see below for further details), we do 

not need to calculate correction coefficients for the scattering 

intensities. Our focus is on using Raman anisotropy to extract 

information on the state of order, not on using resonance enhancements 

to understand the electronic±vibrational mode 

coupling. Moreover, we study thin film samples, as required for 

device structures, and for which X-ray scattering measurements 

of the state of order are difficult to perform. 

In our experimental scattering geometry (see Fig. 2), when 

the polymer is highly oriented, one of the components of the 

tensor (in the direction of the beam wave vector k) cannot be 

measured because the polarization is always perpendicular to 

k. We define the direction perpendicular to z within the plane 

of the film to be x. The light beam then impinges onto the sample 

at normal incidence along y. Under these conditions, the 

Raman intensity due to the perfectly oriented fraction I(perf) 

for incoming and outgoing beams both polarized along z, and 

for a normalized incident laser intensity, I = 1, is given by: 

I zz (perf) = fa 2 zz (3) 

and components I zx (perf) and I xx (perf) are clearly zero. 

For the random (1 ± f) fraction we have to evaluate the average 

of the Raman tensor. After Margulies and Stockburger, [32] 

we can write this average using tensor invariants: 

I zz (rand) = I xx (rand) = 1 45 (1 ± f)(45a2 +4c 2 ±5d 2 ) (4) 

Here a 2 , c 2 , and d 2 are the usual isotropic, anisotropic, and 

asymmetric invariants of the Raman tensor. Since a zz is the 

only non-zero component in the plane, these invariants take 

simple forms, namely a 2 = 1/9a 2 zz, c 2 = a 2 zz, and d 2 = 0. The expected 

Raman anisotropy, defined as R = I zz /I xx , then becomes: 

R = I zz 

ˆ Izz …perf†‡I zz …rand† ˆ …1‡4 f † 

I xx I xx …rand† …1 f † 

and hence, 

…R 1† 

f = (6) 

…R‡4† 

We therefore have a simple relationship linking the perfectly 

oriented fraction f of Fraser's model with the observed Raman 

anisotropy. Note that, as expected for random (isotropic) alignment, 

R = 1 and f = 0, whereas for perfect uniaxial alignment 

R = ¥ and f =1. 

We can now calculate the corresponding relationship expected 

between the optical absorption dichroism and the perfectly 

oriented fraction f of Fraser's model. The dichroism, D, 

is defined as D = A z /A x , where A z and A x are the absorption 

intensities at the long wavelength absorption peak of the first 

dipole-allowed transition for polarization parallel to z and x, 

respectively. Assuming that there are N absorption dipoles per 

unit volume in the sample, each with a transition moment l, 

and that this transition moment is, like the Raman tensor, 

aligned with the polymer-chain axis, the observed intensities 

would be given by: 

(5) 

A z =kNl 2 f+ 1 3 kNl2 (1 ± f) (7) 

A x = 1 3 kNl2 (1 ± f) (8) 

with k a constant. The kNl 2 f term in Equation 7 is the absorbance 

due to oscillating dipoles within the perfectly oriented 

fraction, whereas the 1/3kNl 2 (1 ± f) term is the projected contribution 

along z from the isotropically distributed fraction. 

The x component has only contributions from the latter. The 

dichroic ratio would then be given by: 

D = 1‡2 f 

1 f 

and hence, 

f = D 1 

D‡2 

(9) 

(10) 

It would then be possible to determine a value for f by measuring 

D. As expected, an absence of dichroism (D = 1) corresponds 

to an isotropic sample (f = 0) and infinite dichroism corresponds 

to a perfectly aligned sample (f = 1). 

In general, however, the transition dipole will not lie along 

the polymer-chain axis, but will rather be oriented at an angle 

b, as already shown in Figure 1 and discussed above. The effect 

that the angle b has on the expected relationship between f and 

D is, from Fraser, [28] as follows: 

…D 1† …D 

f = 0 

‡2† 

…D‡2† …D 0 

1† 

(11) 

Here D is the dichroic ratio that would be observed if the all 

chains were parallel to z. It can be shown [28] that D 0 = 2 cot 2 b, 

such that: 

 

2 …D 1† 

f = 

3 cos 2 b 1 …D‡2† 

(12) 

Equation 12 can then be rearranged to yield an expression 

for D in terms of f: 

D = 1‡2 f hP cos bi 

2 

1 f hP 2 

cos bi 

(13) 

Here áP 2 cos bñ is the second order Legendre polynomial in 

cos b. Its value varies from ±1/2 for a transition dipole oriented 

perpendicular to the polymer chain axis, through zero for random 

orientation (where ácos 2 bñ = 1/3), to unity for parallel orientation. 

Correspondingly, D varies from (2 ±2f)/(2 + f) for a 

perpendicularly oriented transition dipole to (1 + 2f)/(1 ± f) for 

a parallel dipole. A comparison with Equation 5 immediately 

reveals that R should always be greater than D for a given value 

of f. Alternatively, R and D can be related to the second and 

fourth order parameters, P 2 and P 4 , as discussed in detail in 

LagugnØ Labarthet et al. [33] 

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2.2. Experimental Studies on Oriented PFO and F8BT 

Nematic Glass Films 

Raman scattering and UV-vis absorption dichroism measurements 

were performed on oriented nematic-glass films of PFO 

and poly[(9,9-dioctylfluorene)-co-benzothiadiazole] (F8BT), 

and the results are now directly compared within the theoretical 

framework discussed above. Raman spectra were measured 

with the incident laser light polarized parallel (i.e., I zz ) and perpendicular 

(i.e., I xx ) to the alignment direction of the film. The 

resulting spectra are plotted in Figure 3 across the spectral 

region of the dominant 1605 cm ±1 phenyl/phenylene in-plane 

stretching mode in PFO and F8BT, and BT stretching mode at 

a) 

270 

b) 

300 

240 

300 

330 

210 

330 

0 

180 

0 

30 

150 

30 

60 

90 

120 

60 

a) 

270 

90 

PF0 Intensity [arb. units] 

⊥ 

1550 1580 1610 1640 

-1 

c) 

240 

210 

330 

180 

0 

150 

30 

120 

b) 

300 

60 

F8BT Intensity [arb. units] 

⊥ 

1510 1540 1570 1600 1630 

-1 

Fig. 3. Raman spectra of a) an aligned PFO film and b) an aligned F8BT film, 

collected using 633 nm laser excitation. The spectra were taken for incoming and 

outgoing polarizations either both parallel (i), or both perpendicular (^) tothe 

alignment direction. Raman anisotropies of 16.67 and 43.33 were obtained for 

PFO and F8BT, respectively, from the ratio of these intensities. 

Fig. 4. Polar plots of parallel Raman intensities as a function of angle for the 

a) PFO 1605 cm ±1 (open diamonds), b) the F8BT 1544.9 cm ±1 (open triangles), 

and c) the F8BT 1608.5 cm ±1 (open squares) modes. 

~ 1545 cm ±1 . As already discussed, this mode has the highly uniaxial 

Raman tensor that is assumed in the above derivations. 

We also note that the Raman intensities for polarization precisely 

parallel and perpendicular to the orientation direction 

are expected to be independent of the strong birefringence that 

arises in aligned samples. [10,34] Thus by selecting data from measurements 

along the main optical axes of the film we can minimize 

the polarization scrambling effects expected to accompany 

the film birefringence. Differences in reflectivity arising 

from the birefringence along the main axes are negligible for 

the Raman measurements. We obtain Raman anisotropy values 

of 16.67 for the 1605 cm ±1 Raman active mode of PFO, and 

43.33 for both the 1544.9 cm ±1 and 1608.5 cm ±1 modes of F8BT. 

The anisotropy for the 1544.9 cm ±1 mode of F8BT implies that 

it too is characterized by a highly uniaxial Raman tensor with 

its principal axis along the chain direction. For completeness, 

the parallel Raman intensities as a function of angle are plotted 

in Figure 4 for the 1605 cm ±1 mode of PFO and the two modes 

of F8BT. The Raman intensity reaches a maximum (minimum) 

when the polarization direction of the laser (e L ) is parallel (perpendicular) 

to the alignment direction. At intermediate angles, 

the polarized Raman signal is affected by both the relative orientation 

of the Raman tensor with respect to e L and e S , and the 

intrinsic birefringence of the film. If e L is not parallel to the 

main dielectric axes of the sample, the polarized Raman intensities 

will be altered by polarization scrambling due to birefringence. 

Figure 5 shows the polarized absorption (dichroism) spectra 

in the region of the first dipole-allowed UV absorption of 

a) PFO and b) F8BT. The observed dichroic ratios obtained as 

the absorbance ratio at the maxima of the absorption spectra 

for PFO and F8BT are 4.4 and 8.2, respectively. The Raman anisotropies 

are therefore four (in PFO) and five (in F8BT) times 

larger than the dichroic ratios obtained from polarized absorption. 

Note that larger dichroism values have been observed for 

more highly aligned PFO samples than studied here. Even 

then, however, the maximum dichroic ratio measured was only 

~ 7. That is a factor of more than two less than the results presented 

here for Raman scattering from a more poorly aligned 

sample. It is thus clear that, as anticipated theoretically above, 

higher anisotropy can be measured via Raman scattering than 

via dichroism. 

270 

240 

210 

180 

150 

120 

90 



a) 

b) 

⊥ 

340 390 440 

and dD/df for different values of f. It is straightforward to show 

that dR/df is smaller than dD/df at small f values, but that the 

opposite applies above a threshold value of f. The off-axis dipole 

moment angle b plays a significant role in determining this 

threshold. Above the threshold, the sensitivity dR/df overtakes 

dD/df. As an example, the threshold f value is 0.07 for 

b = 26.5, but increases to 0.46 for b = 19. These thresholds are 

relatively low compared with the required orientations for 

practical application and it is therefore anticipated that, in 

many cases, Raman anisotropy measurements will be more 

sensitive than polarized absorption measurements as a probe 

of small changes in molecular alignment. 

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⊥ 

400 450 500 550 

Fig. 5. Polarized optical absorption of a) PFO aligned film and b) F8BT aligned 

film, with light polarized parallel (//) and perpendicular (^) to the alignment direction. 

In the simplest model of molecular orientation treated here, 

and for a uniaxial Raman tensor (entirely consistent with the 

data in Fig. 4), the only parameter that determines the Raman 

anisotropy is the fraction, f. For the PFO sample studied here, 

the value of f calculated from R using Equation 6 is 0.76. If we 

combine this with the measured value of D, we obtain b = 26.5 

(using Eq. 12). Similarly, for the F8BT aligned film, f is 0.89 

from the Raman measurement, and when combined with the 

optical dichroism measurement a value b = 22.0 can be deduced. 

It would clearly be interesting to compare these values 

with quantum chemical calculations of the transition dipole 

moment orientation with respect to the repeat unit geometry. 

Such a comparison would assist in assessing the relative importance 

of any polymer-chain superstructure on the effective orientation 

of the transition dipole in these materials. A study of 

the angle b as a function of the degree of alignment, characterized 

for instance by the value of f deduced from Raman anisotropy 

data, would also be interesting in this respect, as would a 

study of the influence of the film structure (crystalline vs glassy 

vs the extended-chain phase [24] ). 

The specific b values deduced for PFO and F8BT seem reasonable. 

If, as discussed above, the transition dipole aligns with 

the para-para axis of a phenylene subunit of the fluorene moiety 

for these polymers, then b should be ~ 19. Note also that 

irrespective of the precise value of b, as previously discussed, 

its finite value is the main limitation on the magnitude of the 

dichroic ratio. Even for f = 1 (perfect orientation), with the 

deduced values of b, the largest possible dichroic ratios would 

be D = 8 and D = 12.3 for PFO and F8BT, respectively. 

We can further discuss the relative merits of dichroism and 

Raman anisotropy, this time in terms of their relative sensitivity 

to increasing degrees of orientation. This sensitivity is 

important in the context of the optimization of devices: The 

ability to reliably monitor small changes in orientation is critical 

to generate the largest anisotropy. We assess the different 

sensitivities of the techniques by considering the changes dR/df 

3. Conclusion 

In closing, we have reported polarized Raman and absorption 

measurements on oriented nematic-glass films of the two 

conjugated polyfluorenes PFO and F8BT. These data were 

used to demonstrate the efficacy of Raman anisotropy measurements 

as a probe of molecular orientation, and comparison 

was made with the results of more conventional optical dichroism 

measurements performed on the same samples. We demonstrated 

that the molecular orientation can be more directly 

characterized by Raman anisotropy, and that Raman anisotropy 

can also have a greater sensitivity to the degree of molecular 

orientation for highly oriented films. The relation between 

Raman anisotropy and optical dichroism was analyzed on the 

assumption that a highly uniaxial Raman tensor exists in the 

plane of the film. This condition is met for the in-plane ring 

stretching modes at ~ 1605 cm ±1 in PFO and F8BT, and BT 

stretching mode at ~ 1545 cm ±1 in F8BT. Using Raman anisotropy 

to determine the Hermans' orientation function, f, it was 

then possible to analyze the optical dichroism data to yield a 

value for b, the average angle that the optical transition dipole 

moment makes with the polymer chain axis. Values of b = 26.5 

and 22 were deduced for PFO and F8BT, respectively. The 

off-axis orientation of the transition dipole moment is in accord 

with previous inferences [1,5,23] and limits the maximum dichroic 

ratio that can be expected. This result confirms the need for 

alternative approaches to maximize emission anisotropy. [5,9,23] 

Finally, the fact that Raman anisotropy data can be collected 

in-situ, in reflection geometry for standard device structures, 

and with microscopic resolution and chemical specificity makes 

the technique very attractive as a non-invasive device probe. 

4. Experimental 

The films of PFO and F8BT were prepared on rubbed polyimide alignment 

layers on Spectrosil substrates as previously described [1,20]. The conjugated 

polymers were spin coated onto the polyimide, and then annealed at a temperature 

in their nematic liquid-crystal (LC) phases to induce orientation. The polyimide 

layers were prepared by spin coating a polyamic acid precursor (Merck 

Liquicoat PI ZLI 2650) from a 30 g L ±1 solution. The precursor was baked to 

form an insoluble polyimide layer, and then rubbed using a machine with a rotating 

velvet-coated drum. This mechanical rubbing induces the requisite preferential 

orientation direction. PFO and F8BT were subsequently spin-coated on top 

of the alignment layer and heated into their nematic phase on a Linkham Microscope 

Hotstage purged with nitrogen (to avoid oxidation and degradation). PFO 



FULL PAPER 

was heated to 200 C and slowly cooled to 170 C at a rate of 1 C min ±1 . F8BT 

was heated to 265 C and slowly cooled to 235 C at a rate of 1 C min ±1 . Both 

were then quenched to room temperature to produce aligned nematic-glass films. 

Raman microscopy was performed using a Renishaw 2000 CCD spectrometer 

coupled to an Olympus BH-2 confocal microscope. The 633 nm line of a He±Ne 

laser was used as the excitation source to avoid absorption and resonance effects. 

A ”50 objective was used with a laser power of approximately 1 mW on the sample. 

Integration times of 60 s were used for each measurement. Polarized absorption 

spectra were obtained using a Unicam UV-vis spectrophotometer equipped 

with a Glan-Thompson polarizer oriented either parallel or perpendicular to the 

alignment direction. 

Received: May 17, 2002 

Final version: September 2, 2002 

± 

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