Intramolecular electron density redistribution upon hydrogen bond formation in the anion Methyl Orange at the water/1,2-dichloroethane interface probed by phase interference second harmonic generation

Article abstract of DOI:10.1002/1521-3765(20000915)6:18<3434::AID-CHEM3434>3.0.CO;2-L

Surface second harmonic generation (SSHG) studies of the azobenzene derivative p-dimethylaminoazobenzene sulfonate, often referred as Methyl Orange (MO), at the neat water/1,2-dichloroethane (DCE) interface is reported. The two forms of the anionic MO dye

Full text of DOI:10.1002/1521-3765(20000915)6:18<3434::AID-CHEM3434>3.0.CO;2-L

FULL PAPER
Intramolecular Electron Density Redistribution upon Hydrogen Bond
Formation in the Anion Methyl Orange at the Water/1,2-Dichloroethane
Interface Probed by Phase Interference Second Harmonic Generation
Juliette Rinuy,[a] Alexis Piron,[a] Pierre FrancÎois Brevet,[a] Mireille Blanchard-Desce,[b] and
Hubert H. Girault*[a]
Abstract: Surface second harmonic
generation (SSHG) studies of the azo-
benzene derivative p-dimethylaminoa-
zobenzene sulfonate, often referred as
Methyl Orange (MO), at the neat water/
1,2-dichloroethane (DCE) interface is
reported. The two forms of the anionic
MO dye, which are usually observed in
bulk solution, with one form being
hydrogen bonded to a water molecule
through the azo nitrogens (MO/H2O)
and the other form not being hydrogen
bonded (MO) have also been observed
at the water/DCE interface. Their equi-
librium constant has been compared
with the corresponding bulk solution
and found to be identical. The adsorp-
tion equilibrium of the two forms has
been determined and the Gibbs energy
of adsorption measured to be
À30 kJmolÀ1 for both forms. From a
light polarisation analysis of the SH
signal, the angle of orientation of the
MO transition dipole moment was
found to be 34 Æ 28 for MO and 43 Æ 28
for MO/H2O under the assumption of a
Dirac delta function for the angle dis-
tribution, a difference explained by the
different solvation properties of the two
forms. Furthermore, the wavelength de-
pendence analysis of these data revealed
an interference pattern resulting from
the electronic density redistribution
within the hydrated anionic form occur-
ring upon the formation of the hydrogen
bond with a water molecule. This inter-
ference pattern was clearly evidenced
with the use of another dye at the
interface in order to define a phase
reference to both forms of Methyl
Orange.
Keywords: electronic structure ´ hy-
drogen bonds ´ liquid/liquid interfa-
ces ´ Methyl Orange ´ nonlinear
optics
total internal reflection (TIR) was used, whereby the incident
wave impinges onto the interface from the medium with the
highest refractive index. The transmitted evanescent wave
intensity then decreases with a typical value for the character-
istic penetration depth in the nanometer range, of about
150 nm for example at the water-DCE interface with an angle
of incidence of 678. In many cases, this interfacial sensitivity is
not sufficient enough to dwarf the response from the two
adjacent bulk phases; furthermore dedicated properties of the
fluorescent probes, such as surfactant behavior, have to be
met in order to record true interfacial processes.[2, 3] Despite
these drawbacks, interfacial phenomena such as photoin-
duced electron transfer reactions have been reported.[4] In this
context, the use of nonlinear optics as a tool to probe surfaces
and interfaces has rapidly been recognized as an efficient
technique. Indeed, nonlinear processes of even order are
forbidden in the electric dipole approximation in centrosym-
metric media like liquids and thus signals only arise from the
surface or the interface where the centrosymmetry is bro-
ken.[5, 6] Surface second harmonic generation (SSHG) is a first
order nonlinear process whereby two photons at the funda-
mental frequency w are converted into one photon at the
harmonic frequency 2w. Despite the weakness of the SH
Introduction
Many fundamental biological reactions involve species ad-
sorption at membrane surfaces, for instance before the
transfer of the species across the membrane into or out of
the cell. In this respect, liquid/liquid interfaces can be used as
simple models to study fundamental processes in biological
membranes. In the past adsorption processes at these
interfaces have been extensively studied, notably by surface
tension measurements.[1] More recently, the interfacial envi-
ronment of adsorbed species has been investigated by optical
means in order to get a molecular picture of the solute ± sol-
vent interactions at the liquid/liquid interface. To achieve
interfacial sensitivity with the optical signal, the geometry of
[a] Prof. H. H. Girault, J. Rinuy, A. Piron, Dr. P. F. Brevet
Laboratoire dꢀElectrochimie
Â
Â
Ecole Polytechnique Federale de Lausanne
1015 Lausanne (Switzerland)
Fax : (‡41) 21 693 36 67
E-mail: Hubert.Girault@epfl.ch
[b] Dr. M. Blanchard-Desce
Â
Departement de Chimie
Ecole Normale Superieure de Paris, URA CNRS 1679
24 Rue Lhomond, 75231Paris Cedex 05 (France)
Â
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Chem. Eur. J. 2000, 6, No. 1 8

3434±3441
signals, the technique has allowed to probe liquid interfaces
with a detailed insight into molecular properties.[7±9] At liquid/
liquid interfaces, orientation, solvation, and chemical equi-
libria have been studied[10, 11] and polarised interfaces have
also been investigated.[12, 13] Furthermore, an interfacial polar-
ity scale has been established for a wide range of liquid
interfaces with solvatochromic probes.[14, 15]
Results and Discussion
Linear and nonlinear spectroscopy of MO: p-Dimethylami-
noazobenzenesulfonate (MO), see structure in Scheme 2, was
first studied by UV/Visible spectrophotometry in bulk phases
in order to fully resolve its absorption spectrum. In aqueous
We present in this study a nonlinear spectroscopic inves-
tigation of the anion Methyl Orange (MO) at the water/DCE
interface in order to understand the solvation environment of
the dye and to consequently get an insight into the subsequent
electronic reorganisation within the moiety. MO belongs to
the family of the p-aminoazobenzene derivatives, a family of
compounds which have been used in the past to study
hydration at protein surfaces as a model to understand the
nature of the interaction prevailing in enzyme ± substrate
associations.[16] Second harmonic generation from these com-
pounds has already been reported to have a good efficiency.[17]
Scheme 2. Hydration equilibrium between the hydrogen bonded and the
non-hydrogen bonded forms of Methyl Orange.
Experimental Section
solutions, the UV/Vis absorption spectrum consists of two
bands, see Figure 1, corresponding to two p ± p* transitions.
The band on the blue side of the spectrum lies at 418 nm and is
attributed to the p ± p* transition of the nonhydrated moiety,
hereafter labelled MO. The band on the red side lies at 467 nm
and is attributed to the p ± p* transition of the hydrated form
MO/H2O, which is hydrogen bonded to a water molecule. For
the latter form, the presence of the dimethylamino group
substituted in para position on the end aromatic ring enhances
the basicity of the azo group allowing the formation of a
hydrogen bond between a water molecule and one of the two
nitrogen atoms, see Scheme 2. This bond stabilizes the moiety
in polar solvents owing to the resulting charge transfer,
yielding a shift to the red of the p ± p* transition wavelength.
The equilibrium constant Khyd ˆ a(MO/H2O)/a(MO) be-
tween the hydrated and the nonhydrated forms has been
evaluated in the past to be about 100 for 4,4'-diaminoazo-
benzene, another member of this family of dyes, and closer to
unity for MO.[19, 20] From the UV/Vis spectra in bulk water for
MO concentrations ranging from 5mm to 10mm, we deter-
mined a value of 1.2Æ 0.1for the equilibrium constant with a
fit to a double Gaussian function. The extinction coefficient of
the hydrogen bonded MO/H2O form at 467 nm was measured
to be 29700mÀ1 cmÀ1. In the bulk DCE phase, the UV/Vis
spectrum is markedly different from the bulk aqueous
spectrum with the principal band indicative of the non-
hydrogen bonded form. The extinction coefficient for the
nonhydrated form MO at 418 nm was measured to be
16500mÀ1 cmÀ1. The shoulder at longer wavelengths in the
DCE phase, as depicted in Figure 1, shows that the hydrogen-
bonded form of the anion MO/H2O still coexists in the organic
phase, however, with a much lower intensity. This is not
surprising since a non-negligible amount of water is present in
the DCE phase when the water/DCE interface is at equili-
brium. Due to these differences between the UV/Vis absorp-
tion spectra in the two media, Methyl Orange is a sensitive
probe to study interfacial hydrogen bonding between solutes
and water molecules.
Nonlinear optical measurements were performed with a nanosecond Nd3‡
/
YAG laser pumped optical parametric oscillator (OPO). The idler output
of the OPO was used at wavelengths ranging from 800 to 1020 nm. Pulses at
a repetition rate of 10 Hz with 5 ns duration and energy of 18 mJ were
delivered by the laser source. The laser beam was attenuated down to 1mJ,
selectively polarised and focussed onto the water/DCE interface from the
DCE phase with an angle of 69.58 to obtain the total internal reflection
(TIR) geometry. The TIR configuration indeed greatly enhances the
intensity of the SH signal arising from the interface. The SH output from
the cell was then separated from the fundamental beam with a filter,
collected with a lens, selectively polarised and eventually detected by a
photomultiplier tube after wavelength selection through a monochromator.
Data were finally acquired with a boxcar integrator. All data were
corrected for the wavelength dependence of the laser source power and the
acquisition system and the experiments were performed well below the
threshold of damages to the surface, ensuring the stability of the interface
during the course of the data collection.
Water was purified by reverse osmosis followed by ion exchange (Millipore,
Milli-Q SP reagent system). 1,2-Dichloroethane (DCE) (Merck, extra
pure) and sodium Methyl Orange (NaMO) (Aldrich, 95% pure) were used
as received. Tetrabutylammonium Methyl Orange (TBAMO) was made by
extraction into DCE from an equimolar aqueous mixture of NaMO and
tetrabutylammonium chloride (TBACl) (Aldrich) with subsequent solvent
evaporation and recrystallisation in acetone. The phase reference dye
VA348 was synthesized from N-dibutylaniline following
sequence shown in Scheme 1, and characterized by elemental analysis,
and mass and NMR spectra.[18]
a four-step
Scheme 1. Synthesis of VA348: a) POCl3, DMF; b) H2O; c) NaH, THF;
d) H3O , THF; e) NaH, THF; f) MeI.
‡
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H. Girault et al.
FULL PAPER
monolayer of a mixture of different species, the susceptibility
$
tensor csꢀ2† is the linear combination of each species suscept-
ibility tensor weighted by its interfacial molar fraction:
N
$
$
ˆ
s (ahbMO/H 0i ‡ (1 À a)hbi)
(3)
$
csꢀ2†
2
e0
$
where the brackets around the b tensors mean ensemble
average over all possible orientation configurations of the
adsorbed molecules. In Equation (3), a is the interfacial mole
fraction of the hydrated form MO/H2O. The interfacial SH
spectrum of the Methyl Orange dye monolayer is shown in
Figure 1. It has been measured with both the fundamental and
the harmonic waves p-polarised, that is with g ˆ G ˆ 08. This
polarisation configuration yields the largest SH signals, see
below and Figure 3. All experiments were conducted in a non-
buffered aqueous solution with a pH measured to be about
6.5. Since the pKa of Methyl Orange is 3.4, the dye is in its
anionic form in water at all times. It is interesting to add that
the bulk DCE UV/Vis spectrum of the protonated form of
Methyl Orange observed after partitioning from a pH 2
aqueous solution into DCE is similar to the one observed for
the anionic form in DCE obtained from the dissolution of the
salt TBAMO. This suggests that the protonated form trans-
ferred in DCE is the neutral form and not the zwitterionic
form. However, at the interface the neutral form is unlikely to
exist, as the sulfonate group is likely to reside in the aqueous
phase. The MO surface spectrum is closely related to that of
the anionic form, since the zwitterionic protonated form has
its resonance band located at longer wavelengths. Similar to
the UV/Vis spectra, two bands can clearly be seen located at
430 nm and 470 nm; this indicates that both the hydrated MO/
H2O and the nonhydrated MO forms are present at the water/
DCE interface. The band at 430 nm is attributed to the MO
form and appears red shifted as compared to the bulk
solution. This shift cannot be attributed to an interference
effect between the resonant and the non-resonant part of the
hyperpolarisability of the MO form. This particular case has
been discussed for other compounds and will be discussed in
detail below.[15] This red shift has already been observed for
Methyl Orange in water± organic solvent mixtures and has
been discussed in terms of a long range ordering of the
aqueous medium with the introduction of a small amount of
the organic solvent.[20] This can be related to the modifications
of the solvent ± solvent interactions on the aqueous side of the
interface as discussed in molecular dynamics calculations at
the water/DCE interface in terms of the strengthening of the
hydrogen bonds between water molecules at the interface.[22]
Hence, the change in the solvent ± solvent interaction is
reflected in the solute ± solvent interaction at the interface
provided a sensitive probe is used. Note finally that the low
frequency band also shifts towards the red side of the
spectrum, however, with a much more reduced shift of only
3 nm, close to the experimental resolution. The overall width
of the SH spectrum appears much reduced compared with the
linear spectrum, that is about 70 nm against 120 nm for the
aqueous linear spectrum, which is a direct result of the
nonlinear process. Furthermore, the band located at 470 nm
on the SH spectrum is also much larger than its counterpart at
430 nm, a possible result of the rather broad distribution of
Figure 1. UV/Vis spectra of the aqueous salt NaMO in bulk water (dashed
line) and the organic salt TBAMO in bulk DCE (solid line) and surface SH
spectrum at the water/DCE interface (open circles).
The surface SH spectrum of MO adsorbed at the water/
DCE interface can then be obtained from the wavelength
dependence of the SH intensity, with the expression given in
Equation (1):[6]
p
w 2
e1W
IW
ˆ
j c j (Iw)2
(1)
2
8 e0 c 3 e1wꢀemW À e1wsin2q1w†
where IW is the SH intensity in W cmÀ2, Iw is the fundamental
intensity in the same units, e1w and e1W are the relative optical
dielectric constants of the medium through which the light
impinges onto the interface, in this case the organic phase, at
the fundamental and the harmonic frequency, respectively, emW
is the relative optical dielectric constant of the interface at the
harmonic frequency and q1w is the angle of incidence of the
fundamental beam. The dielectric constant eWm was taken as
the average of the relative optical dielectric constants of the
two bulk phases in line with the recent measurements of the
polarity of interfaces.[14] Furthermore, all optical dielectric
constants were taken as real quantities and without any
dispersion relationships. The use of complex dielectric func-
tions to account for the absorptive nature of the aqueous
phase was deemed not necessary since the imaginary part was
too small to make any significant effect. The same argument
holds for the dispersion effect. The quantity c in Equation (1)
is defined as:
c ˆ a1 csꢀ;2X†ZX sin2gsinG ‡ ꢀa2 csꢀ;2X†ZX ‡ a3 csꢀ;2Z†XX ‡ a4 csꢀ;2Z†ZZ† cos2gcosG
(2)
‡ a5 csꢀ;2Z†XXsin2gcosG
where ai, i ˆ 1± 5 are coefficients depending on the relative
optical dielectric constants and the angle of incidence q1w. The
angles of polarisation g and G correspond to the fundamental
and the harmonic wave, respectively. The tensor elements
ꢀ2†
ꢀ2†
ꢀ2†
c
, c
, and c
are the three nonzero independent
s;XZX
s;ZXX
s;ZZZ
elements of the surface susceptibility tensor characterising the
nonlinear response of the interface. In the presence of a SHG
active monolayer, the contribution to the SH signal of the
nearby solvent molecules is negligible, as in the case with the
MO dye monolayer, so that the only susceptibility tensor is
left for the dye monolayer.[21] This macroscopic susceptibility
$
tensor csꢀ2† may then be related to molecular quantities. It is
indeed the product of the total number of optically nonlinear
active molecules adsorbed at the interface Ns and the
molecular hyperpolarisability b of a single moiety. For a
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Methyl Orange
3434±3441
the bond distances between the azo group of MO/H2O and the
hydrogen bonded water molecule. With these results, how-
ever, it is not possible to fully determine the hydration
equilibrium constant of Methyl Orange at the interface, that is
without the ratio of the hyperpolarisability tensors magnitude
of the two forms. This question will be addressed below. One
conclusion of this first section is that the Methyl Orange
principally resides on the aqueous side of the interface since
its surface SH spectrum closely resembles the one observed in
the bulk aqueous phase by UV/Vis absorption spectroscopy.
polarisability of the dye. The fit of the experimental data to
Equation (1) to (3) and a Langmuir isotherm expression is
also displayed on Figure 2 and yielded a value of À30 Æ
3 kJmolÀ1 for the Gibbs energy of adsorption for both the
MO/H2O and the MO forms. The SH spectrum was also
recorded at different surface coverage. However, no change in
its general shape was observed, just a mere change in the
overall intensity following the isotherm data. Since the bulk
aqueous solution equilibrium constant and the adsorption
Gibbs energies for both species is known, the surface
equilibrium constant can be deduced from a simple thermo-
dynamic cycle between the bulk and the surface; this suggests
that the partitioning in the organic phase is negligible. Since
both forms have the same Gibbs energy of adsorption, the
surface equilibrium constant is about 1.2 and therefore
identical to the bulk solution. From this result and the SH
spectrum, the ratio of the hyperpolarisability magnitude is
close to unity.
Adsorption isotherm of MO at the water/DCE interface: The
nonlinear optical response of the Methyl Orange monolayer
adsorbed at the neat water/DCE interface was monitored as a
function of the Methyl Orange aqueous bulk concentration, in
order to determine the Methyl Orange adsorption properties.
As seen from Equations (1) to (3), the SH response is
proportional to the square of the total number of adsorbed
molecules, Ns. Hence, a plot of the square root of the SH
intensity against the bulk aqueous concentration of Methyl
Orange follows an adsorption isotherm, see Figure 2. In order
to probe both Methyl Orange forms, the measurements were
conducted for the SH wavelength resonant with the main
absorption band of the MO/H2O form at 470 nm and the MO
form at 430 nm. The bulk aqueous dye concentration was
varied from 0 up to 4mm, although aggregation into dimers
and eventually oligomers do occur at higher bulk concen-
trations. The SH signal was recorded for the fundamental and
the harmonic waves both p-polarized. The isotherm observed
on Figure 2 exhibits a simple Langmuir behavior, which
indicates that there are no strong interactions between the
species present at the interface. Furthermore, from the
polarisation dependence at different bulk aqueous dye con-
centrations it was deduced that local fields could be neglected.
At very low concentrations, both isotherms exhibit a linear
dependence with the bulk aqueous concentration, as shown in
the inset of Figure 2 for the MO/H2O form. The flattening of
Finally, it is worth mentioning that a surface coverage as low
as 1% was clearly observed with Methyl Orange. This supports
the use of Methyl Orange as an interfacial molecular probe
with minimal influence on the surface properties themselves
at low coverage. With molecularly engineered compounds
with an extremely high nonlinear activity, such as the VA348
compound, coverage as low as 0.01% can easily be detected.
Orientation of MO at the water/DCE interface: The orienta-
tion of the two forms of MO at the water/DCE interface can
be obtained from a light polarisation analysis of the SH
intensity. Indeed, and as suggested in Equations (1) and (2),
the three susceptibility tensor elements characterizing the
Methyl Orange monolayer are determined through measure-
ments as a function of the fundamental wave polarisation
angle g for the SH wave S- (G ˆ 908) and P- (G ˆ 08) polarized.
In a simple one component monolayer experiment, only one
single wavelength measurement would be necessary. How-
ever, because the MO monolayer is composed of the two
forms of Methyl Orange, see Equation (3), each susceptibility
tensor component is the sum of the MO and the MO/H2O
contribution and therefore the problem requires measure-
ments at several wavelengths. The two forms contribute to the
susceptibility tensor elements through their angle of orienta-
tion as well as their surface mole fraction and their nonlinear
optical activity. The latter quantity is given by the hyper-
polarisability tensor that can be reduced to a single non-
vanishing element bzzz, that is the component of the tensor
along the MO transition moment direction, since the experi-
ments are performed in the vicinity of the p ± p* electronic
resonance for both forms. Azobenzene dyes are known to
have a n ± p* transition in the vicinity of the p ± p* one.
However, its contribution to the hyperpolarisability tensor is
much weaker than the contribution of the p ± p* band owing
to both a reduced oscillator strength and a reduced charge
transfer. The angle of orientation deduced from this analysis
refers to the orientation of this transition moment direction
with the surface normal. The orientation of the molecule itself
at the interface is obtained from the knowledge of the
orientation of the transition moment within the molecule but
this internal angle is rather small and the transition moment
Figure 2. Adsorption isotherm of the non-hydrogen bonded MO form
(open circles) and hydrogen bonded MO/H2O form (filled circles). The
solid line is a fit to a Langmuir adsorption isotherm. The inset shows the
adsorption isotherm of the hydrated MO/H2O form at very low concen-
trations.
the isotherm at bulk aqueous concentrations above 1m m is
characteristic of the full monolayer coverage. Unfortunately,
no absolute surface density values can be extracted from these
measurements by lack of a separate knowledge of the
absolute value of the SH intensity and the nonlinear hyper-
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H. Girault et al.
FULL PAPER
direction may be taken as identical to the long axis of the
molecule. Equation (3) can then be developed for the three
non-vanishing susceptibility tensors, yielding:
1
cXZX ˆ cZXX
ˆ
Nsbzzz,MO(1 À aapp)hcosqMOsin2qMO
i
2 e0
1
‡
Nsbzzz,MOaapp eiDfhcosqMO/H 0sin2qMO/H 0
i
(4)
2
2
2 e0
1
cZZZ
ˆ
Nsbzzz,MO[(1 À aapp)hcos3qMOi ‡ aappeiDqhcos3qMO/H 0
]
2
e0
In Equation (4), the expression is derived without any
assumption on the ratio of the bzzz element of the two species
MO/H2O and MO. This implies in particular that the mole
fraction aapp is only an apparent mole fraction given by:
ab
zzz;MO=H2
O
aapp
ˆ
(5)
ꢀ1 À a†bzzz;MO ‡ ab
zzz;MO=H2
O
where a is the true interfacial mole fraction. As mentioned
above, since the ratio bzzz,MO/H O/bzzz,MO is close to unity, the
2
apparent mole fraction is nearly equal to the true mole
fraction. Furthermore, a phase angle between the hyper-
polarisability tensor elements of the MO and the MO/H2O
forms Df has been introduced. Indeed, since the electron
density is redistributed within the molecule upon the hydro-
gen bond formation with the water molecule in the MO/H2O
form, a phase shift is expected between the two forms.
Figure 3 gives the P-curves obtained when the SH intensity is
recorded as a function of the fundamental beam polarisation
angle at three different wavelengths and different Methyl
Orange bulk aqueous concentrations. Only the P-polarised
SH curve is given since the S-polarised one does not exhibit
any significant change, just a mere overall intensity change.
The general form of the S-polarised curve follows a sin22g
function, see Equations (1) and (2).
Figure 3. Polarisation curves for different MO concentrations and har-
monic wavelengths: a) 3mm, 507 nm; b) 30mm, 470 nm; c) 1m m, 387 nm.
The harmonic beam is P-polarised. The solid lines are fits obtained using
Equations (1) to (5).
The three curves of Figure 3 are normalised with respect to
the isotherms presented in Figure 2 and to the Methyl Orange
surface spectrum in Figure 1. The molecular parameters
Table 1. Molecular parameters obtained for the hydrated MO/H2O and
the nonhydrated form MO of Methyl Orange at the water/DCE interface.
obtained from these curves, namely, qMO/H O, qMO, and aapp
2
are given in Table 1. In Figure 3a) is reported the curve
obtained with the SH wavelength set at 507 nm. This wave-
length corresponds to a fundamental wavelength of 1014 nm
and the Methyl Orange bulk aqueous concentration was set at
3mm. This low bulk concentration ensured that the curve was
recorded at a very low surface coverage where interactions
between neighboring Methyl Orange species at the interface
are minimized. From the isotherm, the corresponding surface
coverage is estimated to be 2%. The SH wavelength of
507 nm was chosen to yield measurements with the hydrated
MO/H2O form only, as the MO form transition band is located
far away to the blue at 430 nm. Hence, the polarisation curve
can be taken as the one of pure MO/H2O and Equation (4)
allows the determination the MO/H2O transition moment
angle of orientation unambiguously. This angle of orientation
of the transition dipole moment was found to be 43 Æ 28 with
respect to the surface normal, assuming an infinitely sharp
distribution. This latter assumption underlines the character
of this angle which defines the angle distribution but does not
give the real tilt angle of the adsorbed dye molecules at the
MO/H2O
MO
q [8]
bNR
43
34
À 0.44
À 5.9 Â 10À7
470
0.0054
A
3.1 Â 10À7
w [nm]
aapp
430
0.5
0.5
interface. The absolute orientation can not be measured with
this experiment, but it is reasonable to assume that the
sulfonate polar head points towards the aqueous side of the
interface. The polarisation curve in Figure 3b) was then
measured at a Methyl Orange bulk aqueous concentration of
30mm corresponding to a still rather low surface coverage of
about 15% where solute ± solute interfacial interactions are
rather weak, and a SH wavelength of 470 nm. At this
wavelength, the measured SH signal contains an overwhelm-
ing contribution from the MO/H2O form, but a non-negligible
contribution from the MO form leads to the observed
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Chem. Eur. J. 2000, 6, No. 1 8

Methyl Orange
3434±3441
distorted shape of the polarisation curve. It is to be pointed
out that these measurements where performed with an
achromatic half-wave plate in order to avoid any improper
polarisation rotation. Keeping the MO/H2O angle of orienta-
tion deduced above as a constant parameter, the MO angle of
orientation of the transition moment can then be determined,
which is found to be 34 Æ 28 with respect to the surface
normal. This procedure was repeated at several other wave-
lengths, for example at 387 nm, see Figure 3c), where the
dominant contribution arises from the MO form and yielded a
similar value. The transition dipole moment of the MO/H2O
has thus the tendency to lie flatter at the water DCE interface
than that of the MO form. The apparent mole fraction of the
MO/H2O form, aapp, also determined from the polarisation
curve, was found to be 0.5. This value is in agreement with the
value of 0.55 corresponding to the equilibrium constant of
about 1.2. The surface hydration equilibrium constant be-
tween the two Methyl Orange forms at the neat water/DCE
interface is therefore the same as in bulk water. The last
parameter determined from the fit of the polarisation curves
is the phase shift between the two hyperpolarisability tensor
elements of the MO and of the MO/H2O forms. This phase
shift Df was found to be weakly dependent with the
wavelength and equal to 2068. It was, however, noted that
rather good fitted curves could be obtained with a wide range
of values for this phase shift Df, but that these values always
fell between 1208 and 2408. This observation of a constant
phase shift is rather surprising as this angle should have been
evolving along the spectrum in this wavelength region that
corresponds to the neighborhood of the resonance. This is the
normal expectation for a single adsorbed compound, the phase
evolving by 1808 across the resonance, its value being exactly
908 at the resonance. The weak wavelength dependence of the
Df parameter is, however, explained by the interference pattern
between the two forms of Methyl Orange and the presence of
a significant non-resonant contribution to the hyperpolaris-
ability tensor. Indeed, the value of 2068 suggests that the two
forms of MO have an opposite direction, one pointing up and
the other down. Furthermore, the non-resonant contribution
to the hyperpolarisability tensor is rather independent of the
wavelength. Thus, the overall phase shift Df is the difference
in the two phase shifts which partially compensate along the
spectrum yielding this independent behavior as a function of
wavelength, provided the sulfonate hydrophilic group con-
stantly points into the aqueous phase.
where the angle q, taken be-
tween 0 and p, defines the
weighted contributions of the
neutral and the zwitterionic
forms in the ground and the
excited state. In this frame-
work, the neutral form is the
basic
electronic
structure
where no charge transfer has
taken place whereas the zwit-
terionic one is the one ob-
tained after that full charge
transfer has occurred. In the
case of the dye Methyl Orange,
the two forms N and Z are
shown on Figure 4 for the
hydrated and the nonhydrated
forms. The nonhydrated MO
form is a classical push ± pull
type compound with a donor
group, the dimethylamino moi-
Figure 4. Neutral (N) and zwit-
terionic (Z) forms of nonhy-
drated
a) and
hydrated
b) Methyl Orange within the
two-state two-level model.
ety, and an acceptor group, the sulfonate group, well
separated by an extensive p-electron system.
The neutral form is therefore the basic electronic structure,
whereas the zwitterionic form ZMO is obtained with a charge
transfer from the donor amino group to the acceptor sulfonate
group, along the full length of the molecular long axis. In the
case of MO, the neutral form has a dominant contribution to
the description of the ground state structure (i.e., 0 < q < p/2)
due to the large aromatic stabilisation brought by the two
phenyl rings and the high energetic cost associated with the
charge separation over the total length of the molecule. The
context is different for the hydrated form for which different
N and Z forms can be proposed due to the presence of the
hydrogen bond. The neutral form is the basic structure with
the hydrogen bond formed. The charge transfer from the
dimethylamino donor to the now azo group acceptor yields a
different zwitterionic form ZMO/H O, which is strongly stabi-
lized by hydrogen bonding and involves less aromatic
stabilisation loss and shorter charge separation than ZMO
2
.
Due to the relative stabilisation of the Z form with respect to
the N form brought by the hydrogen bonding, one could
expect a dominant contribution of the Z form in the case of
the hydrated form MO/H2O (i.e., p/2 < q < p). This is
expected to lead to opposite sign of b.[23] The distance along
which the charge transfer occurs being different and the
mixing element between the neutral and the zwitterionic
forms being not strictly opposite, it is possible that the phase
shift between the two MO forms does not exactly equal p.
Hence, an electron density redistribution upon the hydrogen
bond formation is experimentally observed through the
interference of the two hyperpolarisability elements as
exhibited by the phase angle Df of about 2068 between the
two forms MO and MO/H2O in the light polarisation data.
The phase shift of 1808 between the two tensor elements for
the two Methyl Orange forms implies an electronic reorgan-
isation within the molecule subsequent to the hydrogen bond
formation. A simple picture can be proposed within the
framework of the two-level two-state model.[23] For com-
pounds undergoing a rather large charge transfer upon
excitation, the fundamental jygi and the excited jyei states
can be taken as a superposition of a neutral jNi and a
zwitterionic jZi form. This is usually written as:[23]
q
q
Phase interference between MO and MO/H2O: In order to
further evidence the phase inversion of the hyperpolarisabil-
ity tensor element between MO and MO/H2O, the SH
spectrum of these two species was recorded in the presence
j ygi ˆ cos j Ni ‡ sin j Zi
2
2
(6)
q
q
j yei ˆÀ sin j Ni ‡ cos j Zi
2
2
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3439

H. Girault et al.
FULL PAPER
of a third compound, see Figure 5. This VA348 compound is
another push ± pull type molecule with a very high nonlinear
optical activity whose main electronic transition lies around
579 nm in dichloromethane. This compound is thus used as a
reference phase for the two forms of Methyl Orange since it is
expected that its phase evolution with the wavelength is
VA348 resonance. The general phase interference pattern is
therefore inverted for the two MO and MO/H2O forms, as
schematically shown on Figure 6, but similar for the MO and
the VA348 compounds. This inversion of phase clearly
emphasizes the electron density redistribution that occurred
in the Methyl Orange moiety upon the hydrogen bond
formation.
Figure 5. Surface SHG spectra of the hydrated MO/H2O and dehydrated
MO forms of Methyl Orange adsorbed at the water/DCE interface (open
circles) and of a mixed monolayer of Methyl Orange and VA348 (filled
circles). The solid lines are simulated curves obtained with the data given in
Table 1. Arrows give the region of constructive (arrows up) and destructive
(arrows down) interference.
Figure 6. Schematic of the orientation of the two forms of Methyl Orange
and the absolute orientation of VA348 at the water/DCE interface. Donor
groups are underlined by rectangles and acceptor groups by ellipses.
The SH spectra with and without the VA348 phase
reference were also simulated using Equation (1) to (5)
extended to take into account the VA348 contribution. The
wavelength dependence of the bzzz elements was described
within the two-level model as:
smooth enough in the range 400 to 500 nm. To perform this
measurement, the Methyl Orange dye was first dissolved in
the organic phase as a tetrabutylammonium salt since the
VA348 is not soluble in the aqueous phase.
AMO
NR
bMO ˆ b
‡
It is to be noted that the observed surface SH spectrum of
the Methyl Orange organic salt was identical to the one
reported on Figure 1; this indicates that the surface solvation
properties of Methyl Orange were independent of the phase
from where it was adsorbing. The spectrum obtained in the
presence of a small amount of VA348 clearly shows that on
the red side of the resonance of MO/H2O a constructive
interference occurs, as exhibited by the sharp increase in
intensity. On this side of the spectrum, MO/H2O and VA348
have thus similar phases. This is exactly what is expected since
VA348 is a classical push ± pull molecule, with a behavior
similar to the nonhydrated form MO. In particular it is
expected that its donor group, the dibutylamino end will point
into the organic phase just as the dimethylamino end group of
MO points into the organic phase. On the red side of the MO/
H2O resonance, and therefore the blue side of the VA348
resonance, these two compounds have constructive phases,
see Figure 5. On the blue side of the 470 nm resonance, a clear
drop in intensity is observed indicating a destructive interfer-
ence in accordance with the flip of the MO/H2O phase across
the resonance. A similar destructive pattern occurs on the red
side of the 430 nm resonance. This implies as expected that
the MO form is phase shifted as compared to VA348 in this
region since this corresponds to the blue side of the VA348
resonance and the red side of the MO resonance. On the blue
side of the 430 nm band, however, the interference is
constructive again but the effect tends to vanish as the
wavelength range is now more than 100 nm away from the
MO
‰w02;MO À 4w2 À 4igMOwŠ
(7)
A
MO=H2
O
bMO/H O ˆ bNR
‡
2
MO=H O
2
‰w02;MO=H À 4w2 À 4igMO=H OwŠ
O
2
2
NR
NR
MO=H
where b
and b
are the non-resonant part of the
MO
O
2
molecular hyperpolarisability tensor, AMO and AMO/H O are
2
two constants, w0,MO and w0,MO/H O are the transition frequen-
2
cies and gMO and gMO/H O the damping constants. To account
2
for the opposition of the transition moments, bMO/H O was set
2
to a negative value, whereas bMO and bVA348 were set positive.
In these calculations, the orientation angles found in the light
polarisation analysis were used and the spectrum without the
VA348 contribution, similar to the one shown on Figure 1, was
simply obtained by setting the bVA348 tensor to zero, keeping
all the two MO forms parameters untouched. All the micro-
scopic data used are reported in Table 1. It is important to
note though that the width of the resonance deduced from the
fitting procedure accounts for the inhomogeneous broadening
as a result of the use of the macroscopic susceptibility tensor.
This is not a microscopic parameter and therefore does not
appear in Table 1. As can be seen in Figure 5, the simulated
curves are in good agreement with the experimental data.
One interesting feature obtained from the data is the rather
strong contribution arising from the non-resonant back-
ground of the hydrated form of Methyl Orange as compared
to the nonhydrated form. Figure 6 gives a schematic of the
different orientation of the compounds at the interface.
3440
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Chem. Eur. J. 2000, 6, No. 1 8

Methyl Orange
3434±3441
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Conclusion
The interfacial properties of hydration of the dye Methyl
Orange have been studied at the neat water/DCE interface.
We found that a simple Langmuir isotherm with a Gibbs
energy of adsorption of about À30 kJmolÀ1 could describe
correctly the interfacial adsorption of both the hydrated MO/
H2O and the nonhydrated MO forms at the neat water DCE
interface. This suggests in turn a surface hydration equili-
brium identical to the bulk aqueous phase one. Furthermore,
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flatter at the interface. From the light polarisation analysis,
and more interestingly, from a phase interference with a third
compound used as an internal phase reference, an intra-
molecular electron density redistribution is clearly observed.
Upon the hydrogen bond formation with a water molecule, a
complete flip of the phase of the hyperpolarisability tensor
has occurred within the dye compound. The interference
pattern underlines the sensitivity of surface nonlinear spec-
troscopy to the direct investigation of intramolecular elec-
tronic rearrangements within molecules at interfaces resulting
from the interactions with the solvent.
Â
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Acknowledgements
The authors would like to thank fruitful discussions with Prof. R. M. Corn
at the beginning of this work. The support of the Fonds National Suisse de
la Recherche Scientifique under grant No. 2000-043381.95/1 is kindly
Â
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Â
Â
acknowledged. J.R. acknowledges the Ecole Polytechnique Federale de
Lausanne for a research fellowship. The Laboratoire dꢀElectrochimie is
part of the European Training and Mobility of Researchers network
ªOrganisation, Dynamics and Reactivity at Electrified Liquid/Liquid
Interfaces (ODRELLI)º.
Received: October 11, 1999
Revised version: December 23, 1999 [F2079]
Chem. Eur. J. 2000, 6, No. 18
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3441