Ellipsometric Raman Spectroscopy - The Journal of Physical

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Ellipsometric Raman Spectroscopy Fernando Costa Basílio, Patricia Targon Campana, Eralci Moreira Therézio, Newton Martins Barbosa Neto, Françoise Serein-Spirau, Raigna Augusta Silva, Osvaldo Novais Oliveira, Jr., and Alexandre Marletta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08809 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 9, 2016

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The Journal of Physical Chemistry

Ellipsometric Raman Spectroscopy Fernando C. Basilio§, Patricia T. Campana*,†, Eralci M. Therézio‡, Newton M. Barbosa Neto#, Françoise Serein-Spirau§§, Raigna A. Silva§, Osvaldo N. Oliveira Jr.&, Alexandre Marletta§ §

Physics Institute, Federal University of Uberlândia, 38400-902, Uberlândia - MG, Brazil

†

School of Art, Sciences and Humanities, University of São Paulo, 03828-000, São Paulo-SP,

Brazil. ‡

Mathematic Department, Federal University of Mato Grosso, 78735-901, Rondonópolis - MT,

Brazil. #

Institute of Exact and Natural Sciences, Federal University of Pará, 66075-110, Belém - PA,

Brazil. §§

Institut Charles Gerhardt, Ecole Nationale Superieure de Chimie de Montpellier, 34296

Montpellier, France. &

Physics Institute of São Carlos, University of São Paulo, CP 369, 13560-970, São Carlos-SP,

Brazil.

ACS Paragon Plus Environment

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ABSTRACT

We introduce a new experimental technique referred to as Ellipsometric Raman Spectroscopy (ERS) to quantify the Raman Optical Activity (ROA) where ellipsometry is combined with Raman spectroscopy. The Stokes parameters were determined for Raman scattered light using Fourier decomposition in measurements with achromatic optical components in a 9-point method. We tested the methodology with ultrapure water, a well-known chiral alcohol and a chiral polymer, and show that ERS allows for studying not only the vicinities of chiral carbons but also molecular species coupled to form chiral structures. Furthermore, in ERS isotropic and anisotropic scattering contribute equally, thus making it possible to analyze the low-wavenumber region in samples such as the chiral polymer studied here, for which the ROA signal probably arises mainly from isotropic scattering.


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1. INTRODUCTION Chiral molecules are relevant for a number of technological applications. Their discovery has been central to the study of biological molecules related to life appearance and maintenance, which has sparked interest in scientists from various areas in the last century1. The structural details of chiral systems can be obtained by Optical Rotatory Dispersion (ORD) and Circular Dichroism (CD) for electronic transitions and vibrational and Raman optical activities for vibrational modes. The first experimental results on the difference in Raman intensities for left and right circularly polarized lights were reported in 19722, which were confirmed in the same year with the chiral molecules 1-phenylethylamine-(C6H5)CH(CH3)(NH2) and 1-phenylethanol (C6H5)CH(CH3)(OH)3. So far, no optical activity effect has been observed for Rayleigh scattering. The basic theory for Raman Optical Activity (ROA) was developed in 19694, with the effect being ascribed to the interference between scattered waves by the chiral centers of a molecule. In 2005, a theory was developed for the Raman modes at intermediate frequencies in which the optical heterodyne-detected Raman-induced Kerr-effect spectroscopy was combined with ROA (OHD-RIKOA).5 This OHD-RIKOA method allowed the several modes to be distinguished, thus increasing the structural information that could be extracted with enhancement in the signals associated with optical activity. Both optical activity and polarizability tensors from chiral centers depend on the polarization of the excitation light and on the circular components of scattered light. Barron and Buckingham introduced the dimensionless parameter (CID - circular intensity difference) to quantify the difference between the left circularly scattered light and the right circularly scattered light, which can be Rayleigh or Raman scattering, and is defined as1,6-7: ∆ =

(1)


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where IR and IL are the scattered intensities with circular polarized light right handed (R) and left handed (L), respectively. When there is no circularly polarized scattered light, IR and IL are both zero, thus generating a 0/0 indetermination, which will be discussed later on. In this work, we propose a new approach for ROA measurements through the direct determination of Stokes’ parameters for scattered light, referred to as Ellipsometric Raman Spectroscopy – ERS. To accomplish such task, we determined Stokes parameters by Fourier decomposition in a so-called 9-point method, for which ellipsometry and Raman spectroscopy apparatuses are combined. We tested the methodology with ultrapure water, a well-known chiral alcohol and a polymer.

2. EXPERIMENTAL AND THEORETICAL METHODS 2.1. Materials Chiral alcohol (S)-(-)-1-Phenylethanol (1-PhEtOH) was purchased from Sigma-Aldrich® (St. Louis, MO) and used as received8. The chiral -conjugated polymers based on a (1R,2R)diiminocyclohexane chiral unit, SW254,9 and dissolved in chloroform in the concentration of 104

mol/L. Ultrapure water from a Milli-Q® (Millipore, USA) system was used for the

measurements with water, and spectroscopic grade chloroform was purchased from SigmaAldrich® (St. Louis, MO).

2.2. Experimental apparatus for Ellipsometric Raman Spectroscopy The diagram in Figure 1 depicts the experimental setup of Ellipsometric Raman Spectroscopy (ERS), featuring the excitation source (argon ion laser Innova 70C from Coherent, at 514 nm), lenses, filters, linear polarizer, monochromator and a CCD detector. In the optical path were


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included an achromatic polarizer (2) at the vertical polarization direction (in the laboratory reference frame), a convergent lens with 200mm of focus and 50mm of diameter (3). The sample was kept in a quartz cuvette of 1cm optical path (4), mounted on a xyz motorized stage and a plane mirror (5) (optional) to increase the excitation power. The scattered light by the sample was collimated by means of two convergent lenses (6) with 200mm focus and 50mm diameter. The excitation beam was eliminated by using a notch filter (7) at 514.5nm. The scattered light was analyzed with achromatic optical elements: a fixed linear polarizer (8) with the polarization axis at the vertical direction (in the laboratory reference frame) and a quarter-wave plate (9) mounted on a goniometer that could be rotated in steps of 40. The signal was analyzed using an IHR550 (10) spectrometer with a CCD Synapse camera (11), both from Horiba Co (Japan).

Figure 1.Experimental apparatus for ERS: (1) Laser, (2) achromatic linear polarizer, (3) laser focus lens, (4) sample cell, (5) mirror, (6) set of focusing lenses, (7) notch filter, (8) achromatic linear polarizer, (9) achromatic quarter wave plate, (10) spectrometer, and (11) CCD detector. 2.3. Absorption measurements


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The UV-Vis spectra (from 190 to 350 nm for 1-PhEtOH and from 240 to 500nm for SW254 polymer) were measured in a Hitachi U2900 spectrophotometer using a rectangular 1cm path quartz cell.

2.4. Circular Dichroism (CD) measurements The far-UV (190-250nm) CD spectrum of SW254 was recorded in a Jasco J810 spectropolarimeter (Jasco Inc., Japan), at 25°C and in a 1cm path length quartz cell. For 1PhEtOH, the measurements were carried on a Jasco J710 spectropolarimeter (Jasco Inc., Japan), at 30°C, in a 0.2 mm path length quartz cell. For both experiments, the CD spectra were recorded after accumulation of 8 (at J815) and 16 runs (at J710) and smoothed using a FFT (Fast Fourier Transform) filter to minimize background effects.

2.5. Emission Ellipsometry (EE) measurements Emission ellipsometry measurements were performed by exciting the sample at 405 nm (laser diode LASER line IZI) and detecting the signal with an Ocean Optics spectrometer USB4000, an achromatic quarter wave plate, and a fixed linear polarizer (with vertical polarization in the laboratory

reference

frame)

placed

on

the

photoluminescence

optical

path.

The

photoluminescence intensity was recorded following the methodology described in references [10] through [15].

2.6. Ellipsometric Raman Spectroscopy (ERS) Several experimental methods can be used to calculate the Stokes parameters. In the setup exploiting Fourier’ Series Decomposition, the light beam propagates through a rotator quarter-


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wave plate and a fixed linear polarizer. Since there is no need to change optical components during the measurements, systematic errors and those arising from misalignment or corrections due to multiple reflections of optical components are minimized considerably. Using the Mueller matrices, given in the Supporting Information, for the rotator quarter-wave plate and a linear polarizer with vertical polarization axis (in the laboratory reference frame)16, the light intensity in the output may be given in terms of the Stokes vectors: ( )= [ −

(2 ) +

(4 ) −

(4 )]

(2)

where is the angle between the rapid axis of the quarter-wave plate and the vertical direction. The coefficients =

− ,

=− ,

=−

, and

= − will be unique since

they are parameters of a Truncated Fourier’Series. In practice, the quarter-wave plate is rotated by discrete angles j such that: = ∑

(3a)

= ∑

sin 2

(3b)

= ∑

cos 4

(3c)

= ∑

sin 4

(3d)

where N is the number of steps of the quarter-wave plate. Equation 2 can be solved considering the 8 possible combinations of the harmonic functions (sine and cosine) and the total intensity (parameter

). In other words, the minimum number of points for solving Eq. 2 is N = 9 for

Δ = 40 and, from the experimental point of view, one needs to ensure that the system is aligned, so that ( ) = ( + 2 ). This result is crucial to allow for the shortest measuring time possible, since some samples, particularly the biological ones, can be degraded due to the high intensity and long exposures to the excitation light. We refer to the new method as 9-point method. Additional information on the polarization state of the analyzed light using the


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parameters polarization degree (P), anisotropy (rEE) and asymmetry (g) is provided in the Supporting Information. The problem in Eq. 1 for a light intensity when  is not defined (ratio 0/0) remains to be solved. Let us consider two independent light beams (ESR beam as the measured one and BL as a baseline beam), arising from uncorrelated electromagnetic waves15. The Stokes parameter be written in terms of the Stokes parameters

for the analyzed light and

may

for a non-

polarized, isotropic light, as: =

+

(4)

where M is the Mueller matrix of the optical components along the light beam path. In the present case the components are the quarter-wave plate and the linear polarizer (see Eq.S2-S8 in the Supporting Information). Since

≪

,

will no longer present the inconsistency of

a 0/0 ratio in any of the spectral regions. Therefore, ellipsometry can be used, via Fourier decomposition, to study polarization of Rayleigh or Raman scattered light. Also important to note is that in the approach by Atkins and Barron1,4,6 with the experimental setup GUROAS (Glasgow University Raman Optical Activity Spectrometer)1 for calculating using expression 1, the total polarization of the light under analysis could not be obtained1. To obtain the chiral contribution of inelastic scattered light (Raman Optical Activity) one needs to calculate the tensor components of optical activity for Raman scattering. The transition dipole moment

, the magnetic dipole moment

,and the electrical quadrupole moment

,

are all subjected to an excitation electromagnetic field with electric and magnetic components , respectively. Using a multipole expansion and Einstein notation, one obtains17:

and (

)

=

+

(

)

=

…

+

E

+⋯

(5a) (5b)


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=

…

(5c)

where the gradient of the electric field E

with frequency is calculated at the molecule

coordinates as a function of the transition frequency in the Raman effect (

=

−

),

between the initial | ⟩ and final | ⟩ states. The electric field ⃗ of scattered light at a distance d from the scattering center and observed at a large distance R is given by: ⃗ =

( ⁄ − )

E

( )

(6)

where the tensor polarizability components

were replaced by

notation in Eq. 6. Let the polarization direction of excited light be direction of scattered light be

to simplify the and the polarization

. Using Eq. 6, the tensor components of the polarizability can be

calculated as4,16: = where

+ and

−

+

−

+⋯

(7)

are electric dipole-magnetic dipole and electric dipole-electric quadrupole

optical active tensors. The state of polarization of the scattered light should be obtained in terms of the observable Stokes parameters

,

,

,

of the scattered light7. In the method

proposed here, referred to as Ellipsometric Raman Spectroscopy (ERS), we measure directly the Stokes parameters of the scattered light using the Fourier decomposition setup, 9-point method, and the hypothesis of Eq. 4. The intensity of scattered light measured directly on the L-Format assembly in Figure 1 is, at least, four times less intense that the one measured from the backscattered light. However, with the improved signal-to-ratio noise of modern detectors, the rightangle scattering can be measured reliably. Consider the excitation light linearly polarized in the vertical direction (at the laboratory reference frame) described by the polarization vector ⃗( , ) = 1, where

= 0 and

= 0 are the azimuth and polar angles, respectively. For right-


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(90 ) of the scattered light in the

angle scattering configuration (90º) the Stokes parameters Raman effect are given by: (90 ) =

∗

12

(90 ) =

∗

−2

∗

+2 +3

∗

(90 ) =

where

=

−

∗

+ 12 ( )

⁄4

(8a)

∗

+3 −4

(90 ) = −

∗

+2

(8b) +6 ∗

∗

+6

+2

∗

∗

−2

(8c) ∗

(8d)

. For the cases of forward (0o) and backward (180º)

scattering, similar expressions are given in the Supporting Information. Note that the ⁄

⁄

and

ratios depend only on the tensors for optical activity, similarly to the circular intensity

difference (CID) calculated in references [1], [6], and [7], used in the GUROAS experiments1 or in the LP ROA (linear-polarization ROA) theoretical method proposed by Hecht and Nafie17.

3. RESULTS AND DISCUSSION The 9-point method to determine the Stokes parameters and the hypothesis in Eq. 4were tested firstly by analyzing the polarization of a 514.5nm linearly polarized line of an Ar+ laser added to the 633nm line of a completely depolarized He-Ne laser. The results show the expected polarization of the total electric field, described in detail in the Supporting Information. Figure 2a features the absorption bands in the UV-Vis spectrum for 1-PhEtOH (structure given in the inset) at ca. 250 nm, assigned to * transitions of the benzene ring, and the CD spectrum with positive values ([CD]>0), which means that the sample is mostly S-type, levorotatory (99.99% according to the manufacturer). Figure 2b shows the Raman spectrum with vibrational peaks assigned as in Table 1.


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Figure 2.(a) Optical absorption and Circular Dichroism spectra and(b) Raman spectrum of 1PhEtOH. The chemical formula of 1-PhEtOH is given in the inset of (a).

Table 1.Vibrational modes for 1-PhEtOH based on figure 2b and references18,19 Raman Shift (cm-1)

Vibrational Mode

506

Deformation of -ring (in-plane mode)

637

Deformation of -ring (off-plane mode)

758 862 920 1020 1096 1177 1046 1196 1225 1469

C-O stretching -C stretching -C stretching and COH bend CH3 asymmetric deformation

1620

-C-C stretching

CH bend (off-plane mode)

CH bend (in-plane mode)


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Figure 3a shows the nine measurements I(,) (Eq. 2 - i = 0, 40,..., 320º) for the Raman signal of 1-PhEtOH, while Figure 3b brings the Stokes parameters (Eq. 3), normalized by taking into account the total intensity ( ⁄

) and a baseline

(

)

= 100

≪

(

)

. The parameter

is related to the difference in the linearly polarized emission, being polarized in the same

direction as the incident beam, i.e. anisotropy parameter

⁄

< 0. This parameter is also associated with the

(Eq. S4, Supporting Information) of the sample, which may be used to

analyze depolarization of the incident light caused by scattering. The parameters ⁄

⁄

and

depend, essentially, on the vibrational mode of the chiral carbon, that is to say, on the -

Cc-, -Cc-CH3, and -Cc-OH vibrations.


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Figure3.(a) Raman spectra for various angles, (b) normalized Stokes parameters 1, 2 and 3, (c) parameters

(Eq. S1),

⁄

; =

(Eq. S4), (Eq. S5), (d) angles and (ERS

spectrum) for 1-PhEtOH. ⁄

The parameter

, in particular, conveys the same information of the ROA signal as in the

CID method16. Indeed, the bands are positioned at the same energies and with the same relative intensities as in reference [20]. The degree of polarization, Supporting Information) and asymmetry,

(Eq. S1), anisotropy,

(Eq. S4,

(Eq. S5, Supporting Information), obtained from the

Stokes parameters, are shown in Figure 3c. Note that the values of

are close to those for ,

and therefore most of the contribution with linear polarization has the same direction as the impinging light. As for the asymmetry g, it follows the trend of

⁄

(Fig. 3c), due to its

dependency only on the Stokes parameters. Analogously to the CD definition where the signal is quantified by the ellipticity of the absorbance of transmitted linear polarization light, we define the parameter rotation

, ellipticity angle, for the ERS spectrum. The elliptical parameters, angle of

and , are defined as functions of Stokes parameters, as follows:

tan(2 ) =

(10)

tan(2 ) =

(11)

Figure 3d shows

and

calculated for 1-PhEtOH. One should observe the similarity between

(Fig. 3d) and g (Fig. 3c) curves; both parameters that depend only on

(Eq. 11 and S5

respectively). This feature shows a correspondence between the ellipticity angle and the asymmetry of 1-PhEtOH molecule. In addition to 1-PhEtOH, which serves as a reference for being a well-known material, we studied a more complex molecule, the SW254 conjugated polymer, whose structure is given in


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the inset of Fig. 4a. The latter has an absorption spectrum with broad bands from ca. 240to 450nm. The transitions in the 250-300nm region can be related to benzene groups; from 300 to 350nm to stilbene groups, and from 350-450nm to conjugated* transitions. Marletta and Akcelrud observed aggregation of stilbene structures in poly(p-phenylenevinylene) (PPV), with aggregated stilbene displaying an absorption maximum around 360nm21. For the SW254 spectrum, the band at 340nm is broad because it contains the superimposition of the contribution from stilbene derivatives (max = 320nm) and aggregated stilbene (max = 360nm).

Figure 4.(a) Optical absorption and Circular Dichroism spectra and (b) Raman spectrum of SW254. The chemical formula of SW254 is given in the inset of (a). The CD spectrum in Figure 4a reflects the chirality of these electronic transitions. The spectrum is mainly characterized by two negative bands, at 415 and a small shoulder at 435nm, in addition to two small negative bands close to 250nm. Positive bands appear between 270 and 370nm. The positive band at 362nm and the negative bands at 415 and 435nmcould be assigned to chromophores' interactions due to their absorption. In principle, these bands should not be observed because there are no chiral structures. However, aggregation among stilbene groups and between stilbene and benzene groups in SW254, inferred from the absorption spectrum,


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makes the optical properties dependent on the conformation that the molecule assumes. Such interactions can occur throughout the conjugated polymer chain, thus constituting a chiral system with intermolecular exciton coupling. Figure S3 in the Supporting Information shows some proposed configurations for SW254 in which this coupling could occur. Hence, chirality resulting from the interactions between stilbene groups could generate the two positive peaks between 300nm and 350nm, while the positive and negative peaks at 250-300nm region could be due to benzene interactions. This phenomenon, already observed for several molecules, from acyclic polyols to carbohydrates, may occur between identical or different chromophores22. Figure 4b shows the Raman spectrum with vibrational peaks for SW254 assigned as in Table 2, for chiral and non-chiral groups. The peaks at 651, 1202 and 1496cm-1 are all related to the benzene and stilbene groups while the peak at 3009cm-1 is related to vibrational modes of chiral carbons. Table 2.Vibrational modes for SW254 based on Figure 4b and references [18] and [19]. Raman Shift (cm-1)

Vibrational Mode

651

1-4- in-plane quadrant bend

748

CH3 rock

1202 1496 3009

(-O)-C stretch C-C ring semi-circular stretch  C-H and Cc-H stretch

Figure 5a brings the nine measurements I(,) (Eq. 2 - i= 0, 40,..., 320º) for the Raman signal of SW254, with the normalized Stokes parameters shown in Figure 5b. Similarly to 1-PhEtOH, the difference in the linearly polarized emission is polarized in the same direction as the incident beam (

⁄

< 0). The parameters

⁄

and

⁄

depend, essentially, on the vibrational


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mode of the chiral carbon; for SW254 it is associated with the -Cc-H stretching mode (at ca. 3000cm-1). In figure 5b, there is one more band associated with

⁄

and

⁄

, at 650cm-1,

mainly related to the vibrational modes of benzene rings in SW254. Again, as benzene and stilbene structures themselves should not present chirality, this ROA signal at 650cm-1 was not expected. Nevertheless, as in the discussion regarding Figure 4, there is exciton coupling between stilbene and benzene groups in SW254 molecules, forming a chiral system at the electronic level. One may assume that this coupling also occurs for the vibrational levels, thus leading to a ROA signal for SW254 related to vibrational modes of benzene rings and stilbene groups.


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Figure 5.(a) Raman spectra for various angles, (b) normalized Stokes parameters 1, 2 and 3, (c) parameters

(Eq. S1),

(Eq. S4),

(Eq. S5), and angle

⁄

; =

(ERS spectrum) for

SW254 (inset). Figure 5c shows similar values for

and , suggesting that most of the contribution with

linear polarization has the same direction as the impinging light, for both chiral carbons (around 3000cm-1) and the contributions from coupled benzenes and stilbenes (around 650cm-1). The asymmetry g, also in Figure 5c, is similar to

parameter (Figure 5c inset) showing not only the

asymmetry of SW254 chiral carbons but also the asymmetry resulting from the exciton coupling between the stilbene and benzene groups. Taken together, these results point to a methodology that makes it possible to study not only the vicinities of chiral carbons but also molecular species coupled to form chiral systems. In addition, the methodology allowed measurements at the lowwavenumber region (700-500cm-1). One should mention that ROA spectral data at the low-wavenumber region have been reported by Barron and collaborators1,23-24. In monosaccharides, the ROA spectra in 200-700cm-1 can be associated with the ring conformation and distribution of hydroxyl groups24. For proteins, besides the contributions from chiral carbons between 870 and 1700 cm-1, vibrational modes of disulfide bridges may appear in the 500-550cm-1 region23. For both cases, the ROA signals are obtained from the inherent chirality of each group. Nucleic acids, on the other hand, possess another source of chirality since their achiral base rings are stacked into a chiral arrangement1,25, in a similar fashion to the interactions proposed here for the SW254 structure. A particular feature in the analysis of ROA data at the low-wavenumber region is that the approach is suitable for cases in which the signal arises from anisotropies, but may not be sensitive for samples in which the ROA signal is due to isotropic scattering. In contrast, in our


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approach both isotropic and anisotropic scattering contribute equally, being therefore advantageous for the analysis of low-wavenumber regions in samples such as SW254 in which the isotropic scattering is probably the main contributor for the ROA signal. Indeed, for SW254 the g value in Fig. 5c at 650cm-1, which is mainly assigned to isotropic scattering, is even more intense than the value for the -Cc-H stretching mode (at ca. 3000cm-1). One may now envisage further uses in the analysis of biomolecules. For example, existing ROA data for the proteins serum albumin, human immunoglobulin G, bovine ribonuclease A, and subtilin Carlsberg, indicate a signal in the 400-700cm-1 region23-27. The signal is weak, but it is correlated to the number of cysteins (the amino acid responsible for disulfide bridges) and to the bands between 500 and 550cm-1 assigned to disulfide bridges23. There is also a ROA signal at 550-650cm-1 range, which has not been assigned yet. The latter appears to be related to the same vibrational modes for benzene ring, in view of the correspondence between their intensity and the number of tyrosines and phenylalanines (both amino acids with side chain derived from benzene rings) in the protein structures. Furthermore, in these analyses it was not possible to measure either the absorption or the CD below 240nm, where absorption of chiral carbons takes place owing to the strong chloroform absorption. These issues may be overcome with our methodology since it allows for identifying the vibrational modes of chiral carbons and monitor changes in their vicinity. The anisotropy of the SW254 molecule was also tested by measuring the polarized emission and the Stokes parameters. In other words, by performing an emission ellipsometry experiment1015

and by applying the 9-point method developed in this work. Figures 6a and 6b show the results

for electroluminescence and the Stokes parameters for SW254, respectively. As expected, S1/S0 0 and

⁄

>= 0) overlapping

the spectral range of polarized absorption (Figure 4a, CD spectra, around 400-470nm). This effect can be due to the linearly polarized emitted light being reabsorbed (probably due to inner filter effects) alongside the optical path, and this absorption is polarized. This feature is particularly interesting for developing meta-materials27, as the polarization for absorption can be controlled by changing chemical groups near the chiral carbons in the polymer chain.

Figure 6.(a) Photoluminescence spectra for various angles, (b) normalized Stokes parameters ⁄

; = 1, 2 and 3, (c) parameters

(Eq. S1),

(Eq. S4),

(Eq. S5).

In order to confirm the usefulness and robustness of the ERS method, we studied ultrapure water in the spectral range 2700-4200cm-1 of O-H vibrational modes, whose results are shown in


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Figure 7. The bands at 3400cm-1 in Figure 7a are expected from the symmetric and asymmetric stretching of OH28. Figure 7b brings the normalized Stokes parameters, where largest and

⁄

and

⁄

⁄

is the

are practically zero. This latter result was expected because water

molecules possess no chirality. The dependence on the Raman shift for the parameters ,

and

is shown in Figure 7c. Note that the scattered light is most depolarized close to 3700cm-1, in the spectral region of OH vibration, showing that the system is entirely random. Analogously to the parameters

⁄

and

⁄

, the angles

(Eq. 10) and

(Eq. 11 – ERS spectrum),

respectively, are not altered, as seen in Figure 7d.


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Figure 7.(a) Raman spectra for various angles, (b) normalized Stokes parameters 1, 2 and 3, (c) parameters

(Eq. S1),

(Eq. S4), (Eq. S5), (d) angles

and

⁄

; =

(ERS spectrum)

for ultrapure water. Taken together, the results pointed to a very advantageous setup, due to the possibility of obtaining a Raman spectrum without the need to change the optical elements, in addition to requiring short times for acquiring the signal. These acquisition times are typically a few minutes for small molecules such as SW254, thus comparable to the available commercial instruments, such as ChiralRAMAN™, from BioTools Inc.29 In the system presented here, the use of an IHR550 spectrometer with a CCD Synapse camera provided a short acquisition time by adjusting the diffraction grating for 1000cm-1 spectral regions. The decrease in the required exposure time minimizes degradation effects, which is an important feature for polymeric and biological materials.

4. CONCLUSIONS We introduced a new methodology to acquire Raman Optical Activity spectra, based on the determination of Stokes parameters for the scattered light from Fourier series decomposition. Experimentally, this was achieved by slight modifications on an apparatus for emission ellipsometry15. An important result was the optimization in the number of experiments required to obtain the Stokes parameters

,

,

, and

. It was possible, using our method, to get

the Stokes parameters with only 9 measurements, which reduces exposure of the sample to the laser light. Furthermore, the methodology may be extended to the analysis of reflection, transmission, absorption and emission of light. The experimental setup proposed, referred to as Ellipsometric Raman Spectroscopy, is relatively simple with the only difficulty being associated


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with the need of a very precise alignment. The errors originating from misalignment are minimized by using achromatic optical components though. In fact, the use of achromatic lenses allows one to measure the full spectrum, in such a way that the drifts and errors associated with changing polarizers mechanically are minimized. Besides, the excitation light is linearly polarized with a fixed wavelength; during the analyses, a quarter wave plate and a polarizer are used which decrease the drifts in terms of spectral position. In this context, the setup proposed here is more precise than the commercially available equipment that uses a piezoelectric crystal to produce the circularly polarized light. This crystal may introduce, in addition to mechanicallyrelated errors, electronic noise. In addition, the definition of the parameter Δ in the regions where there are no vibrational bands could be obviated by adding a baseline. The method and experimental apparatus were tested with two laser lines, one of which was linearly polarized while the other was non-polarized. The resulting polarization degree confirmed the expected initial polarization state of both laser lines. For the ERS-ROA, we measured the chiral alcohol 1-PhEtOH, for which there is substantial amount of data to be used as benchmark. Even with a linearly polarized light as excitation source,

⁄

depends only on

the tensors related to the Raman optical activity, as in the case of the ROA signal in CID experiments (Eq. 1) and according to the theoretical prediction17. With ERS-ROA, it was also possible to estimate the degree of polarization, P, anisotropy, rEE, and asymmetry, g, for the scattered light. The most suitable parameter for ROA signals is the ellipticity , for its analogy to Circular Dichroism. The new methodology brings the possibility of obtaining Stokes parameters by analyzing linear or circular absorbance, reflectance, and emission of any kind of material. In particular, with the 9-point method results can be obtained with limited exposure of the samples to the laser


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light, which is promising for all materials that tend to photodegrade, such as some polymers and biological samples. Furthermore, the experimental setup permits acquisition of both isotropic and anisotropic scattered lights to the same extent, being a powerful tool for the low-wavenumber region, which is not fully exploited by existing approaches.

ASSOCIATED CONTENT Supporting Information. Degree of polarization, anisotropy, asymmetry, angle , ellipticity, Mueller Matrix for a polarizer and for a rotator, forward and backward scattering equations, testing the 9-point method, SW254 possible configurations. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Patricia T. Campana. Phone number: +55 11 30918883. Mobile/whatsapp: +55 11 982410290. E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work was supported by Brazilian agencies CNPq, CAPES, FAPEMIG and FAPESP (Grant number 2013/14262-7). The authors acknowledge Dr. Jean-Pierre Lère-Porte and Dr. Salem Wakim by the supervision and synthesis of the chiral polymer.

ABBREVIATIONS ERS, Ellipsometric Raman Spectroscopy; ROA, Raman Optical Activity; 1-PhEtOH, (S)-(−)-1phenylethanol; ORD, Optical Rotatory Dispersion; CD, Circular Dichroism; CID, circular intensity difference; FFT, Fast Fourier Transform; GUROAS, Glasgow University Raman Optical Activity Spectrometer; LP ROA, linear-polarization Raman Optical Activity.

REFERENCES [1] Barron, L. D.; Hecht, L.; Blanch, E. W., Bell, A. F. Solution structure and dynamics of biomolecules from Raman optical activity. Prog. Biophys. Mol. Biol. 2000, 73, 1-49. DOI: 10.1016/S0079-6107(99)00017-6. [2] Bosnich, B.; Moskovits, M.; Ozin, G. A. Raman circular dichroism. Its observation in alpha-phenylethylamine.

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[13] Therezio, E. M.; Rodrigues, P. C.; Tozoni, J. R.; Marletta, A.; Akcelrud, L. EnergyTransfer

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poly(1,4-phenylene-vinylene). Phys. Rev. B: Condens. Matter Mater. Phys. 1991, 43, 5109-5118. DOI: 10.1103/PhysRevB.43.5109. [20] Hug, W. Virtual Enantiomers as the Solution of Optical Activity's Deterministic Offset Problem. Appl. Spectrosc. 2003, 57(1), 1-13. DOI: 10.1366/000370203321165142. [21] Marletta, A.; Akcelrud, L. Interchain interactions effects on emission efficiency of poly(pphenylenevinylene) films. J. Lumin. 2009, 129(7), 672-678. DOI: 10.1016/j.jlumin.2009.01.012. [22] Berova, N.; Nakanishi, K.; Woody, R. W. Circular Dichroism: Principles and Applications; John Wiley and Sons Inc.: New York, U.S.A, 2000. [23] Zhu, F.; Isaacs, N. W.; Hecht, L.; Barron, L. D. Raman Optical Activity: A Tool for Protein Structure Analysis. Structure. 2005, 13(10), 1409-1419. DOI: 10.1016/j.str.2005.07.009. [24] Barron, L. D. The development of biomolecular Raman optical activity spectroscopy. Biomed. Spectrosc. Imaging. 2015, 4(3), 223-253. DOI: 10.3233/BSI-150113. [25] Barron, L. D.; Zhu, F.; Hecht, L.; Tranter, G. E.; Isaacs, N. W. Raman optical activity: An incisive probe of molecular chirality and biomolecular structure. J. Mol. Struct. 2006, 834, 7-16. DOI: 10.1016/j.molstruc.2006.10.033. [26] Barron, L. D. Structure and behavior of biomolecules from Raman optical activity. Curr. Opin. Struct. Biol. 2006, 16(5), 638-643. DOI: 10.1016/j.sbi.2006.08.004. [27] Liu, Y.; Zhang, X. Metamaterials: a new frontier of science and technology. Chem. Soc. Rev. 2011, 40, 2494-2507. DOI: 10.1039/C0CS00184H.


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[28] Carey, D.M.; Korenowski, G.M. Measurement of the Raman spectrum of liquid water. J. Chem. Phys. 1998, 108, 2669-2674. DOI: 10.1063/1.475659. [29] BioTools Home Page. http://www.btools.com/ (accessed Oct 2, 2016).

Table of Contents Graphic


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Figure 1.Experimental apparatus for ERS: (1) Laser, (2) achromatic linear polarizer, (3) laser focus lens, (4) sample cell, (5) mirror, (6) set of focusing lenses, (7) notch filter, (8) achromatic linear polarizer, (9) achromatic quarter wave plate, (10) spectrometer, and (11) CCD detector. Figure 1 274x120mm (299 x 299 DPI)



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Figure 2.(a) Optical absorption and Circular Dichroism spectra and (b) Raman spectrum of 1-PhEtOH. The chemical formula of 1-PhEtOH is given in the inset of (a). Figure 2 162x60mm (299 x 299 DPI)


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Figure 3.(a) Raman spectra for various angles, (b) normalized Stokes parameters (Sd i)⁄(Sd 0); i=1, 2 and 3, (c) parameters P (Eq. S1), rEE (Eq. S4), g (Eq. S5), (d) angles ψ and χ (ERS spectrum) for 1-PhEtOH. Figure 3 178x130mm (299 x 299 DPI)



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Figure 4.(a) Optical absorption and Circular Dichroism spectra and (b) Raman spectrum of SW254. The chemical formula of SW254 is given in the inset of (a). Figure 4 170x65mm (299 x 299 DPI)


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Figure 5.(a) Raman spectra for various angles, (b) normalized Stokes parameters (Sdi)⁄(Sd0); i = 1, 2 and 3, (c) parameters P (Eq. S1), rEE (Eq. S4), g (Eq. S5), and angle χ (ERS spectrum) for SW254 (inset). Figure 5 170x130mm (299 x 299 DPI)



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Figure 6.(a) Photoluminescence spectra for various angles, (b) normalized Stokes parameters (Sdi)/(Sd0); i=1, 2 and 3, (c) parameters P (Eq. S1), rEE (Eq. S4), g (Eq. S5). Figure 6 170x130mm (299 x 299 DPI)


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Figure 7.(a) Raman spectra for various angles, (b) normalized Stokes parameters (Sdi)/(Sd0); i= 1, 2 and 3, (c) parameters P (Eq. S1), rEE (Eq. S4), g (Eq. S5), (d) angles ψ and χ (ERS spectrum) for ultrapure water. Figure 7 170x130mm (299 x 299 DPI)


Ellipsometric Raman Spectroscopy - The Journal of Physical

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