J. Phys. Chem. 1993,97, 1383-1388
1383
Circular Dichroism Spectroscopy at Interfaces: A Surface Second Harmonic Generation Study T. Petralli-Mallow, T. M. Wong, J. D. Byers, H. I. Yee,and J. M. Hicks’ Department of Chemistry, Georgetown University, Washington, D.C. 20057 Received: August 6, 1992; In Final Form: October 20, 1992
A new technique which links the phenomenon of surface second harmonic generation (SHG) with circular dichroism (CD) spectroscopy is introduced. SHG spectra of chiral molecules (2,2’-dihydroxy- 1,l’-binaphthyl) adsorbed at the air/water interface are dependent on the helicity of the incident circularly polarized beam. Moreover, the magnitude of the circular dichroic effect is lo3times greater than that observed in ordinary CD spectroscopy. Several factors including molecular orientation at the surface are considered as possible explanations for this enhancement.
1. Introduction
Since the pioneering work of Rosenfeldl, Moffitt2, Tinoco3, Nakani~hi,~ and others5, it has become possible to routinely examine secondary structural features of molecules in solution using circular dichroism (CD) spectroscopy. By studying CD spectra, the “handedness”of compounds can be determined, and characteristic molecular architecturessuch as a-helix and @-sheet can be identified. It would be useful to achieve a spectroscopy sensitiveto the chirality of molecules localized at interfaces. Linear optical techniques such as absorption spectroscopy and CD spectroscopy are not particularly surface sensitive. The nonlinear optical process of second harmonic generation (SHG) has an intrinsic sensitivity to the asymmetry at the interface between two centrosymmetric media.6 This property allows SHG to be applied in a variety of surface studies.’-’ Frequency-doubled SHG, like absorptionand circular dichroism, originates from the electronic response of materials to light. In this paper, we show that circular dichroic effects observable in ordinary absorption spectroscopy also appear in the frequency-doubled radiation produced at an interface. Thus, the possibility of a surfacecircular dichroism spectroscopy, a tool which would be of considerable value in chemistry and biochemistry, is put forth. We begin by briefly sketching the microscopic descriptionsof surface SHG and CD, as a starting point for the theory of how they may be linked. The emphasis of this paper is on experimental results obtained on model systems, ( R ) -and (S)-2,2’-dihydroxy1,l’-binaphthyl, adsorbed at the air/water interface. 1.1. Surface Second Harmonic Generation. Surface SHG is generally performed as a reflection experiment in which plane polarized light at fundamental frequency v is converted to light at 2v via a second-order nonlinear interaction with a surface6 (Figure 1). Photons at 2v are observable only when intense laser pulses (of MW to GW per square centimeter) are utilized. SHG is forbidden, in the electric dipole approximation, from centrosymmetric media such as bulk gases and liquids. SHG can originate, however, from the thin layer between two centrosymmetric media where symmetry is broken. SHG signals are described as the result of an effective secondorder polarizability of the medium, FLi;.”
B and
are the effective electric and magnetic fields of the light at the fundamental frequency in the medium, respectively. The first term represents SHG in the electric dipole approximation, where x:2) is the second-order nonlinear susceptibility of the medium. The second and third terms are due to quadrupolar 0022-3654/93/2097- 1383$04.00/0
Figure 1. Method of generating second harmonic reflection from a surface and separating it from the fundamental beam using filters.
effects, and the last term represents magnetic dipole effects. Quadrupolar and magnetic contributions can arise from surface and bulk material: in most cases these terms are thought to be small compared to the electric dipole term. Thus, in the electric dipole approximation, the SHG intensity is proportional to the square of x:’). It has been shown in many cases7J1that
where N,is the number density of molecules on the surface, d2) is the microscopic nonlinear polarizability of the individual molecules, and the brackets denotean average over all orientations of the molecules. The perturbation expansion for d2)contains terms such asll
The;ij,k terms are the electric dipole transition moments between ground state a and excited states band c. The r terms aredamping coefficients. An enhancement of theSHG signal results whenever v or 2v approaches a natural frequency of the molecule. Thus, by tuning the laser fundamental frequency, the resulting SHG intensities refer directly to resonance of the clean surfaceloor of the surface molecule^.^ 1.2. Circular Dichroism Spectroscopy. In ordinary CD spectroscopy, one measures a difference in absorbance of left- and right-circularly polarized light by optically active molecules. This can be expressed as a difference in molar extinction coefficients Ae (Ae = el - er) for dilute samples following Beer’s law. The optical activity for each molecule is characterized by its chiral polarizability, 8. @ is a parameter defined by the following two microscopicequationsresponsible for the interaction of light with 0 1993 American Chemical Society
1384 The Journal of Physical Chemistry, Vol. 97, No. 7 , 1993
Petralli-Mallow et al.
mat ter:5
+ smaller terms
(4)
+ Ec + smaller terms at
(5)
= a r -~ = KB
c at
200
aE
Here, GI and f i l are the induced electric dipole and the magnetic dipole moments, respectively. a is the electric polarizability, and K is the magnetic susceptibility. It can be seen that the induced electric (or magnetic) moment is due to a term that goes as the electric (or magnetic) field, and to a term that goes as the time derivative of the magnetic (or electric) field. @/c is the proportionality factor for this latter contribution and can be obtained from either equation. The first quantum treatments of the chiral polarizability were written by Rosenfeld’ and later reviewed by Condon.12 The perturbation energy operator for CD is as follows:
100
300
250
350
Wavelength (nm)
a “ O
- 1 00
HGD = -Gl*&&l-g + quadrupole terms
(6) By using eq 6 to calculate the induced electric and magnetic dipoles and then by relating the results to eqs 4 and 5 , one can obtain an expression for 8. For an isotropic sample, it can be shown that the quadrupole terms vanish, and then 8 is given by the following:
-200
I
I
I
I
200
250
300
350
Wavelength Here, and f i are the electric dipole and the magnetic dipole transition moments between ground state a and excited state b. The damping term is not shown. Ac is proportional to the complex part of 8.13 The rotational strength-of an electronic transition, R, is the numerator in eq 7: R = Im pfi. R can be obtained experimentally by measuring the band area of the CD spectrum. Various mechanisms for generating ordinary CD have been summarized by T i n ~ c o .There ~ are two classes of optically active chromophores: those that are intrinsically dissymmetric(so that pfi # 0) and those that are symmetric but are in an asymmetric environment (for example, a chiral center). 1.3. SHG-CD. There is a fundamental link between the phenomena of SHG and CD, both arising from electronic and magnetic responses of molecular systems. There is relatively little previous work linking nonlinear optical effects with optically active molecules or circularly polarized light, even in bulk phases. Giordomaine et al. observed fairly strong sum frequency generation (SFG) (another second-order nonlinear optical effect) at 231 nm in transmission through a 2.46 M aqueous solution of arabin0se.1~ They noted that SFG is electric dipole-allowed in an isotropic solution if the solute is chiral, but SHG is not. Simon et al. observed circularly polarized SHG from a noncentrosymmetric, optically activecrystal at 532 nm.I5 More recently, Meijer et al. observed SHG in a centrosymmetric crystal of a racemic mixture of (R)and (a-N-acetylvaline using elliptically polarized light at 1064 nm.I6 The relatively strong signal (1% that of a quartz crystal) was attributed to the magnetic term in eq 1. A theory linking electric field-induced SHG with CD was developed by Lam et al.; electric, magnetic, and quadrupolar contributions to SHG were included.” At present there is no microscopic theory of the combination of SHG and CD that adequately explains the data presented here. This work is in progress in our laboratory.’* In order to experimentally test the link between surface SHG and CD, a chiral model compound, 2,2’-dihydroxy- 1,l’-binaphthyl (BN), is studied at the air/water interface. BN was selected because it has absorbanceand CD spectra in the second harmonic
(nm)
Figure 2. Circular dichroism spectrum of (R)-2,2’-dihydroxy-l, 1’binaphthyl in water (1 mm pathlengthcell). Inset: CDspectrum including the region of our doubled laser frequencies (29S315 nm) (10 mm pathlength cell).
Figure 3. Configuration of (R)-2,2’-dihydroxy-l, 1’-binaphthyl.
range of our laser system (see Figure 2), it is slightly soluble in water, and it can be obtained in a high degree of optical purity. The optical activity of a solution of BN is a result of the skewed conformation of the molecule19(Figure 3). The electric dipole transition moments of the two naphthalenesubunits in BN interact to form an excitonic splitting in the first excited states. The relevant expression for rotational strength in the exciton case is3
c
uu jitibfc
h(v: -):Y
.,
Kabjac is the inteection potential between distinct chromophores i and j , and the Ri,j values are position vectors.20 Thz damping term is not shown. The rotational strength depends on the cross product of the two electricdipoletransition moments and therefore is proportional to the sine of a,the inter-ring twist angle. The twisted molecular conformation (a* 1O O O ) permits the interaction between the rings and thus the observed chiral effects. For a left
Circular Dichroism Spectroscopy at Interfaces
The Journal of Physical Chemistry, Vol. 97, No. 7 , 1993 1385
dye laser
compressor
"1 I
YAG laser
A
Len CP Light 0 Right CP Light
1 .o
autocorrelator single photon counting
PMT
mc
PMT
mc
290
300
295
305
310
Second Harmonic Wavelength (nm)
Figure 4. Layout of laser system used to generate second harmonic from surfaces, including monochromator (mc), achromatic quarter wave plate lenses (L,,), and filters (F,,). FI is used to eliminate spurious SHG from optical elements.
(+a)versus right (-a)twist, the sign of R changes, thus the two enantiomers of BN give opposite CD effects. The large CD bands of BN near 220-230 nm ( A ~ 2 4= 200) originate in the zero order exciton associated with a very strong long-axis polarized IBb transition (€224 = 130 OOO).4 The resonances and CD bands in the doubled frequency region of the laser near 280 nm are associated with the short-axis polarized 'La state and are much weaker (e280 = 9500 and Ac280 = 7).
Figure 5. Relative SHG efficiencies as the laser frequency is varied, plotted as a function of the second harmonic wavelength for the R enantiomer of BN using left-circularly polarized (A)and right-circularly polarized (0) light. Intensity from neat water: 0.01 au. A
Left CP Light 0 Right CP Light
T
I
=I Q
II I.
0.8
U
T
2. Experimental Section
(R)-and (S)-2,2'-dihydroxy- 1,l'-binaphthyl (R-BN and S-BN) and a nonchiral analog, 1-naphthol, were obtained from Sigma. The purity of the samples was greater than 99% as verified by thin layer chromatography and high pressure liquid chromatography (HPLC). HPLC grade water (filtered through 0.1 pm filters) was obtained from Fisher ScientificCo. Saturated aqueous solutions of each enantiomer and 1-naphthol are prepared by stirring at 25 OC under N2 for several days. Using the extinction coefficient for BN in ethanol ( ~ =29460),21 ~ ~ the concentrations of the binaphthol solutionsareestimated to be 25 p M . 1-Naphthol concentrations are 1.6 mM (based on €311 = 4350, measured in ethanolZ2). The solutions are stored in the dark and under N2 prior to and during the experiments. CD spectra are obtained using a Jasco 710 spectropolarimeter. Surface tensions of the solutions are measured in temperature controlled cells by the method of capillary rise.23 The surface tensions of the BN solutions at 10 OC are the same (within experimental error) as for the pure water (72.0 dyn/cm); the surface coverage is thus estimated to be less than a monolayer. For 1-naphthol, the surface tension is 67.5 dyn/cm, indicating substantial surface concentration. The laser system (Figure 4) consists of a mode locked Nd3+: YAG laser (Spectra Physics 3800s) with a pulse compressor and a cavity-dumped dye laser. The dye laser uses rhodamine-6G and has a 4-MHz tunable output consisting of 0.8-ps pulses with approximately 25 nJ each. The beam is converted from plane polarized to circularly polarized utilizing an achromatic quarter waveplate (MeadowlarkOptics). The beam impinges the sample at 61' from the normal and is focused to approximately 20 pm using a 50 mm focal length achromatic lens. A Plexiglashousing, purged with nitrogen, covers the samplecell to protect the interface from oxygen and dust. A circulating bath is used to maintain thesampleat 10f 1 "C.The reflected second harmonic radiation is elliptically polarized, and it is separated from the fundamental
0.2
0.0
4 290
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1
Second Harmonic Wavelength (nm)
Figure 6. Relative SHG efficiencies as the laser frequency is varied, plotted as a function of the second harmonic wavelength for the S enantiomer of BN using left-circularly polarized (A)and right-circularly polarized (0)light. Intensity from neat water: 0.05 au.
frequency by a Corning 7-54 filter and a monochromator and then imaged onto the detector. Because the monochromator grating has a known polarization bias, we searched for possible artifacts in our data by replacing the monochromator with a stack of Corning 7-54 filters. Aside from changing the efficiency of the collection system,no significant effect on the data was observed. Detection is achieved using photomultiplier tubes (Hamamatsu R585) and a two channelgated photon counter (Stanford Research SR400). The sample signal is normalized to the laser intensity by a reference SHG signal which is simultaneously generated from a gallium arsenide surface and collected by an identical optical detection system. We correct for the slight frequency dependence of the GaAs signal. SHG spectra are obtained by measuring the SHG counts over 100 s at each laser wavelength and polarization. The counts are normalized and plotted versus the second harmonic wavelength (290-3 15nm). The SHG background from the neat water surface is small compared to the signalsobserved from the solutionsstudied here, and it is constant over the wavelength range.
3. Results SHG spectra of BN and 1-naphthol were first obtained using a plane polarized input beam, with the polarization axis oriented
Petralli-Mallow et al.
1386 The Journal of Physical Chemistry, Vol. 97, No. 7, 1993
"1
0
12-
MCPLigbt 0 Right CPLigbt
- 1.2
A
T
10-
1.01
- i o
-
08
h
- 0 6
-04
0.04 290
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00
'I
0
Second Harmonic Wavelength (nm)
Figure 7. Relative S H G efficiencies as the laser frequency is varied, plotted as a function of the second harmonic wavelength for a 5050 mixture of Rand Senantiomersof BN using left-circularly polarized (A) and right-circularly polarized (0)light. Intensity from neat water: 0.07 au.
45O from the plane of incidence. These spectra are similar to the
UV absorption spectra over the wavelength region sampled, as is expected from the relationship between SHG and resonance, as represented in eq 3, for example. When circularly polarized light is used as excitation, the SHG spectra depend strikingly on the helicity of the light. Figures 5 and 6 show the SHG spectra for R-BN and S-BN using left- and right-circularly polarized light. For each enantiomer, there is a substantial intensity asymmetry upon reversal of photon helicity. Left-circularly polarized light produces a large SHG signal for the R enantiomer. Furthermore, the spectral shape of the signal follows the ordinary CD bandshape. Right-circularly polarized light gives a weaker SHG response. Comparable and opposite behavior is observed for the S enantiomer. In bulk binaphthol solutions, the ordinary CD effect is small, with absorption coefficients for left- versus right-circularly polarized light varying by a factor of Ac/e = 0.1% (see Figure 2). A measure of the SHG-CD intensity asymmetry can be defined similarly:
The SHG-CD intensity asymmetry reaches high values of up to 100% near 290 nm. A number of control surfaces were tested for SHG-CD effects. No photon helicity-dependent spectra were observed for the optically inactive (but resonant) 1-naphthol solution or for water itself. For a 5050 R-BN:S-BN racemic mixture (Figure 7),the differences in SHG intensities for left- and right-circularly polarized excitations were smaller than the error bars. A slight systematic shift in the spectra for the two cases is being studied further. The SHG-CD spectra were studied as a function of concentration of BN in solution. Figure 8 shows the relative SHG intensitiesat 294 nm for plane polarized, right- and left-circularly polarized light. For electric dipole-allowed (surface) SHG, the SHG intensity is related to the square of the number of molecules on the surface (eq 2). Figure 8 shows the fits of the data to a Langmuir adsorption isotherm, expected for surface adsorption in which the species act independently of each other:23
0 is the fractional coverage (for a monolayer, 6 = l), k is a constant, and c is molar concentration. The observed dependence of the
I
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1
2
3
'
00
[RBN] moleslliter x ~ O ' ~
Figure 8. Relative S H G efficiencies as the concentration of R-BN is varied in aqueous solution (left scale), generating light at 294 nm using right-circular (0),left-circular (A), and plane ( 0 )polarizations. The data are weighted according to their standard deviations and fit to the equation: ISHG= IO + A ( k c / l + kc)2, where IO is the water background. k and I,-, were fit to all data simultaneously and then fixed at 2.6 X los and 0.08,respectively. For right-circular, left-circular, and plane polarized light, A = 0.06(1), 0.63(5), and 1.1(6), respectively. Also shown is the ( 0 )(right scale). SHG-CD intensity asymmetry ISHG-CD
SHG intensity on concentration strongly suggests that the signal indeed originates from thesurface. Note that monolayer coverage is not achieved at thesaturatedconcentration, a finding consistent with our surface tension results. For right- and left-circularlypolarized light, theSHG intensitics grow in smoothly with concentration, i.e., there is no sudden onset of signal that might indicate aggregation of adsorbates. ISHG-CD is also plotted in Figure 8. The asymmetry is constant within experimental error over the concentration range. Finally, the time dependenceof theSHG signal from the surface of the saturated BN solution was studied in order to probe for possible fluctuations due to diffusion of aggregates. Zhao et al. have shown that aggregates comparable in size to the laser spot can be detected in this manner.* Data were taken every second for 30 min. An autocorrelation analysis revealed only a rapidly decaying peak expected for random laser noise. There was no evidence of aggregation of the BN molecules adsorbed at the water surface. 4. Discussion
The nonlinear optical CD effect observed here implies that binaphthol retains its low symmetry when adsorbed at the water surface. Binaphthol cannot lose its optical activity without bond breakage, t h u s it is not surprising to see that the BN molecule at the surface is optically active. What is significant, however, is the magnitude of the SHG-CD signal. In our laser range, we are resonant only in the weak CD bands of BN (Figure 2). These weak bands correspond to very small differences in the extinction coefficients for left- and right-circularly polarized light (Ae/c = 0.001). However, the asymmetry observed in the SHG signal is large, on the order of 1 at 290 nm. This raises an interesting question-why would a circular dichroic effect, which is exceedingly weak in the linear measurement of a bulk system, show such a strong response when measured via a nonlinear optical response at a surface? To address this question, we first consider the second harmonic generation process.
Circular Dichroism Spectroscopy at Interfaces As indicated in eq 1, the nature of contributions to SHG can vary among electric dipole, quadrupole, and magnetic dipole effects. Which term or terms are most important for SHG-CD observed here? Ordinarily, magnetic dipole-allowed and electric quadrupole-allowed SHG are thought to be small. Sometimes it is pointed out that because the magnetic or quadrupole signal can originate from the bulk solution (i.e., a longer pathlength of approximately A/27r),11 it may become as strong as the electric dipole-allowed term from the interface. In our case of very dilute BN solutions, this argument does not hold. Furthermore, in preliminary studies for submonolayer BN a t the air/quartz interface, we observe the SHG-CD effect.24 For the solid substrate, of course, there is no bulk BN contribution possible. Thus SHG-CD cannot be attributed to magnetic dipole-allowed or electric quadrupole-allowed SHG originating from the bulk solution. The magnitudes of our signals are comparable to those from other molecular monolayers (such as phenol) whose SHG is interpreted as electric dipole-allowed.25 For these reasons, it is concluded that the SHG signal from BN arises from the electric dipole-allowed terms in eq 1. In Lam's theory,I7the electric dipole term should not give rise to SHG-CD; the other terms can contribute. Lam's theory based on magnetic dipole and quadrupole effects predicts a very small SHG-CD intensity asymmetry, much smaller than we observe. Therefore,we suggest that there must be an additional mechanism connected with the electric dipole terms that permit SHG-CD effects to occur. There are several other factors to consider in connection with SHG. For SHG intensity, the second-order nonlinear polarizability terms are squared, and this can magnify small differences among them. Also, to calculate the absolute efficiencyfor surface SHG, one must include simple Fresnel factors to account for the strength of the light fields in the surface layer." Thus, indices of refraction q of the media at Y and 2v are involved. For chiral media, it is generally true that q~ # q ~ The . differences between 9~ and q~ for the saturated binaphthol solution at our range of v and 2v are estimated to be
