3C11 Broadband ESFG Study of Coumarins at Air-water Interface: New Insight into
Interfacial Polarity and Molecular Property(Molecular Spectroscopy Lab., RIKEN) { Sobhan SEN, Shoichi YAMAGUCHI, Tahei TAHARA
Introduction:
Polarity expresses the ability of solvents to stabilize a charge or a charge-dipole near its
vicinity. The concept of polarity and solvatochromism at interfaces has profound importance inunderstanding the nature of interfacial solute-solvent interactions. Numerous experimental and theoreticalstudies have been performed regarding solvent polarity and dynamics at various interfaces.1,2 These studies
showed that the properties of solvents and solute molecules at interfaces are substantially different fromthat of their bulk counterpart.1,2 Among the various experiments, surface-specific nonlinear Second
Harmonic Generation (SHG) spectroscopy has been employed extensively to study the polarity and solvent
response at the interface by measuring resonantly enhanced electronic spectra of interfacial molecules andtheir time-delay dependence.1 However, this technique relies on extensive as well as time-consuming
procedure of registering output SHG (2ω) intensity by scanning the input fundamental wavelength (ω).1,2Thus, the resulting interfacial electronic spectra consist of sparse data points with limited signal-to-noise
ratio.1,2
A long standing challenge was to obtain precise electronic spectra of interfacial molecules with higher
signal-to-noise ratio, which could be compared directly to their linear absorption spectra. Towards this end,
the advancements of femtosecond laser technology leads us to design surface-specific multiplex Electronic
Sum Frequency Generation (ESFG) technique based on a narrow band near-IR laser pulse and a broadband
white light continuum, together with a detection capability of the entire ESFG spectra in a single-shot,using CCD.3 This technique enables one to register subtle electronic spectra (χ(2)2) of interfacial
molecules with ultra-high signal-to-noise ratio and very dense data points. It is worth noting that the precise
shape and structure of the electronic spectra obtained with this technique contain wealth of information
regarding polarity and solute-solvent interaction at the interface. The uniqueness of this technique has
already been explored by our group to unravel the precise electronic spectra of nile blue.3
The purpose of the present study is two fold: (1) to measure the polarity and solute-solvent interaction(spectral shape) sensed by the three structurally different coumarin dyes, Coumarin-314 (C-314),
Coumarin-110 (C-110) and Coumarin-6H (C-6H) at the air-water interface and (2) to establish therelationship between the molecular orientation of coumarins and their local polarity.
Experimental: The energy diagram and broadband ESFG setup are shown in fig.1.3 Briefly, aTi:sapphire regenerative amplifier (Spitfire, Spectra Physics) together with seeds from a mode-locked
oscillator (Tsunami, Spectra Physics) was used to generate 1 mJ pulses (1 kHz, 100 fs) at 800 nm. A part ofthe output was focused into flowing water cell to generate white light continuum (ω2) (fig. 1B). The otherpart of the 800 nm was attenuated and used as ω1 pulse (bandwidth 160 cm-1). The ω1 (16 µJ, p-polarized)and ω2 (7 µJ, p-polarized) pulses were focused onto them (excited state)
800 nm ω1 ω1+ω2same spot at air-water interface of the coumarin solutionn (excited state)
540 nm ω~ 1.1 µm 2with incidence angles 48° and 38° respectively (see fig.
g (ground state)(A)
1B). The delay between ω1 and ω2 was controlled by a ω2MLM
M
CCD
translation stage. Thus, temporal overlap of ω1 and ω2 ω1
Filterω+ωL
12pulses ensures the generation of ESFG signal in reflection
p-polarization
48p-polarization
mode. The ESFG signal was collected through an iris, a38Iris
pair of identical lenses and dielectric filters and wasSample
focused into a polychromator (HR-320, Jobin Yvon). The
Stirrer
entire, spectrally-dispersed ESFG signal (p-polarized) was(B)registered onto a nitrogen-cooled CCD (Spec-10:2KBUV,2048 × 512 pixels). The resonant χ(2)2 spectra of45
coumarins were then obtained by normalizing coumarin’s
SFG spectra by the standard non-resonant ESFG spectra ofSample
a quartz plate, collected at same condition as that ofStirrer(C)coumarin sample.3
The polarization dependent SHG and subsequent angle
Fig. 1: Energy level diagram (A) andof orientation of coumarins were measured using
experimental setup (B) of multiplex ESFG.
conventional SHG setup shown in fig. 1C. In brief, a part(C) SHG Experimental setup for measuringmolecular orientation. L, M and F denote(10 µJ) of the amplifier output was used as input
lenses, mirrors and filters respectively.
fundamental pulse (ω). The ω-pulse was tuned near theοPolychromator

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Absorption and χ(2) 2 spectra (Norm.)

Absorption and χ(2) 2 spectra (Norm.)

resonant frequencies of the coumarins and the SHG signal (2ω, 45°-polarized) was collected with a PMT
after passing through analyzer and depolarizer, by varying the input polarization of ω. This ensures that theangle of orientation, calculated from the polarization dependent SHG signal, is essentially the angle of theS1←S0 transition dipole moment of coumarin relative to the surface normal of the air-water interface.
Aqueous solutions of C-314, C-110 and C-6H (all Aldrich)
Coumarin-314 in:
(A)
Water
were made by dissolving in HPLC grade water (Wako, Japan).
Ethanol
THF
Experiments were done with constantly stirred solutions in a glass
HexaneAir-waterinterface
petri-dish (see fig. 1).
(2) 2
The ESFG (χ  ) spectra of C-314
Results and Discussions:and C-110 adsorbed at air-water interface are shown in fig. 2. It isreadily seen from fig. 2 that the broadband ESFG technique is
unique to obtain precise two-photon single-resonant χ(2)2 spectra
Coumarin-110 in:of molecules (coumarin in this case) with ultra-high signal-toWater(B)Ethanol
THF
noise ratio. Unlike earlier studies on electronic-SHG of coumarins
HexaneAir-water
at air-water interface, the ESFG spectra obtained in the presentinterface
work show significantly broadened spectral shape.2a The spectral
shapes of ESFG for all the coumarins are nearly identical to theirlinear absorption spectra in highly polar solvents. The FWHM (53nm) of C-314 ESFG spectrum is just a little less than that of its
Wavelength (nm)bulk absorption spectrum in water of 55 nm. Similarly, for C-110,Fig.
2:
Bulk
absorptionand χ(2)2
FWHM is 54 nm (air-water) compared to 61 nm in bulk water andspectra of C-314 (A) and C-110 (B)for C-6H (data not shown), 60 nm (air-water) compared to 75 nm
in various environments.
(bulk water). These results clearly show that even at molecularly
sharp air-water interface, where molecules experience an anisotropic dielectric environment, there exist a
broad distribution of solvation sites for coumarins.
Another interesting finding is the difference in static local solvation of the coumarins. From fig. 2A, itis clearly seen that C-314 experience polarity in between hexane and THF, which is substantially nonpolar(peak at 420 nm) than that of the bulk water (peak at 448 nm). Interestingly however, C-110 (fig. 2B) andC-6H experience a very polar environment similar to ethanol or propanol at the air-water interface.
Surprisingly, the ESFG spectrum of C-110 is nearly identical to its linear absorption in ethanol. Thus, thestatic local solvation of coumarins depends largely on their1.0molecular structure and not the average dielectric environment of0.8
the two bulk phases, separated by the interface.
0.6
An important property of interfacial molecules, towards their
0.4
preferential dielectric environment, could come from their angle
0.2
of orientation at the interface. Owing to the sharpness and0.0
roughness of the interface, a small change in molecular-50
050
100
150
200250
Input polarization angle (degree)orientation could affect their local solvation appreciably. Fig. 3
shows the variation of 45°-polarized output SHG-intensity ofFig. 3: Polarization dependent SHG ofinterfacial C-314 with input polarization. The polarization
C-314. Output polarization set at 45°.
dependent SHG intensity can be expressed in terms ofmacroscopic second-order nonlinear susceptibility tensor elements as:(1)It(2ω) ∝ c1 χXXZ cosγ sinγ sinξ + [(c2 χXXZ + c3 χZXX + c4 χZZZ) cos2γ + c5 χZXX sin2γ] cosξ2where, γ is input polarization angle, ξ (45° in the present study) is the analyzer angle for output SHG andci’s are the constants calculated from the relative dielectric constants of the two media and angle ofincidence. Note that, γ = 90° corresponds to s-polarized and γ = 0° corresponds to p-polarized fundamental
ω-pulse. The solid line in fig. 3 shows the fit to data points using eq. (1). Finally, the macroscopic
susceptibility tensor elements are related to the molecular hyperpolarizabilities (β) to calculate themolecular orientation at the interface. Preliminary calculations of molecular orientation show that C-314
orient in a more vertical fashion than that of C-110 and C-6H at the air-water interface. Eventually, a large
portion of C-314 molecule dives more into air-like environment than that of C-110 and C-6H. Thus, ourresults show local polarity of coumarins at air-water interface is sensitive to their molecular structure and,subsequently, the coumarin orientation at the interface.
References:(1) (a) Benjamin, I. Chem. Rev. 2006, 106, 1212. (b) Eisenthal, K. B. Chem. Rev. 1996, 96, 1343.
(2) (a) Nguyen, K. T.; Shang, X.; Eisenthal, K. B. J. Phys. Chem. B 2006, ASAP article. (b) Steel, W. H.;
Walker, R. A. J. Am. Chem. Soc. 2003, 125, 1132.
(3) Yamaguchi, S; Tahara, T. J. Phys. Chem. B 2004, 108, 19079.
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SHG intensity (a.u.)

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