Article
pubs.acs.org/JPCC
Organic Compounds with Large and High-Contrast pH-Switchable
Nonlinear Optical Response
Stein van Bezouw,† Jochen Campo,† Seung-Heon Lee,‡ O-Pil Kwon,‡ and Wim Wenseleers*,†
†
Department of Physics, University of Antwerp (campus Drie Eiken), Universiteitsplein 1, B-2610 Antwerpen, Belgium
Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Korea
‡
ABSTRACT: Measurements of the nonlinear optical (NLO) response
(molecular first hyperpolarizability, β) of two pH-switchable dyes were
performed by means of hyper-Rayleigh scattering (HRS) over a wide range of
laser wavelengths and pH values. The extensive wavelength-dependent NLO
data combined with linear optical spectroscopy allow the results to be related
to the electronic structures of the push−pull systems and the NLO switching
contrast to be optimized. The results are modeled theoretically using a
vibronic β dispersion model including both homogeneous and inhomogeneous
broadening, yielding a full wavelength dependence of β, as well as of its
switching contrast. The hyperpolarizabilities measured for the basic “on”-states
are the highest ever reported for such compounds, reaching over 200 × 10−30
esu in the static limit and over 2700 × 10−30 esu at resonance. By combining a
high intrinsic contrast in the static hyperpolarizability β0 with a large shift in
resonance, an extremely large range in contrasts of the HRS signal is reached of more than 2 orders of magnitude at wavelengths
accessible with the widely used Ti:sapphire laser and moreover within a biologically compatible pH range.
■
INTRODUCTION
Organic compounds with large molecular second-order nonlinear optical (NLO) polarizability (first hyperpolarizability β)
are of interest for their use as active components in electrooptical devices, wavelength conversion of lasers,1 and as labels
for second harmonic imaging.2,3 A second-order NLO response
requires an asymmetrically polarizable system, typically
obtained by combining a long conjugated bridge with strong
electron donor and acceptor end groups to form a “push−pull”
chromophore. Larger hyperpolarizabilities can be achieved by
optimizing either the conjugated chain and/or the donor or
acceptor groups.1,4,5 For some compounds this response can be
reversibly switched, using redox-reactions,6 photoconversion,7−10 or pH changes,7−13 a property that has been broadly
investigated since the 1990s as it opens a new range of possible
applications for NLO materials.14,15
Because efficient NLO molecules exhibit intramolecular
charge-transfer transitions at long wavelengths, experimentally
measured β-values are very often enhanced by (two-photon)
resonance, which complicates direct comparison of different
compounds. This enhancement effect can be removed by
extrapolation of the experimental values to zero frequency,
yielding the static hyperpolarizability β0. To determine this
parameter reliably, one should measure the NLO response at
several wavelengths, preferably both on- and off-resonance
(requiring state-of-the-art tunable laser systems), and make use
of suitable dispersion models for extrapolation,5,16,17 which is
why detailed studies are scarce in the literature. If such data is
available, the practical vibronic β dispersion model that we
developed18 can be used, which takes into account both
© 2015 American Chemical Society
homogeneous and inhomogeneous broadening but restrains the
dispersion to be fully consistent with the experimental
absorption spectrum, such that an accurate dispersion can be
obtained while involving only a single shape-determining free
parameter.
Because of their noncentrosymmetric crystallization, both
compounds shown in Scheme 1 have previously been
investigated for their macroscopic second-order NLO properties,19−22 with crystals of 2 already finding their way to
commercialization for terahertz generation and detection. In
those previous studies, only the protonated forms were
considered, and also in a measurement of the molecular β of
2 (at 800 nm in methanol),21 a pH sensitivity of the compound
was not reported, and presumably only the response of the
protonated form was measured. Here we find that the
compounds show a pronounced pH dependence near neutral
pH, and for the first time for any pH-switchable compound, we
study the complete pH and wavelength dependence of their
linear and NLO response, including accurate modeling of the β
dispersion and consequently reliable extrapolation to β0. In this
way, we aim at understanding the origin of the NLO switching
behavior and at optimizing the switching contrast.
■
EXPERIMENTAL METHODS
Hyperpolarizabilities were determined using hyper-Rayleigh
scattering (HRS), i.e., incoherent second-harmonic light
Received: July 18, 2015
Revised: August 21, 2015
Published: August 24, 2015
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elongated push−pull compounds such as those considered
here, in which case βHRS = (6/35)1/2βzzz. For D2O, which was
used for all measurements above 1200 nm because of its
infrared (IR) transparency, the previously determined βdispersion of H2O26 was used, as the β of H2O and D2O
were found to be the same within experimental error, in
accordance with Kaatz et al.27
The obtained β values were accurately fitted using the
practical vibronic β dispersion model developed in ref 18,
taking into account both homogeneous and inhomogeneous
broadening, yet fully consistent with the experimental
absorption spectrum and involving only a single shapedetermining free parameter, the inhomogeneous line broadening Ginhom. This model is in fact an intermediate model
between two extreme cases that were developed before
involving only homogeneous or inhomogeneous broadening,16
and Ginhom determines the contribution of both classes of
broadening mechanisms. Ginhom values used in the dispersion
models of the compounds are as follows: in water, 1020 cm−1
(1b); in acetonitrile, 1500 cm−1 (1a), 570 cm−1 (1b), 1400
cm−1 (2a), and 1730 cm−1 (2b). For compound 1a (in water),
the best fit is obtained with the purely homogeneous vibronic
model16 (which corresponds to the homogeneous limit of the
more general model, i.e., Ginhom = 0). Uncertainties on the
Ginhom values are large (up to 500 cm−1), but in most cases the
improvement of the fits with this intermediate model compared
to either extreme case was significant, and the uncertainties on
Ginhom did not translate into large errors in calculated static
hyperpolarizabilities β0. Error margins on β0 reported below
include the effect of the uncertainty on the fitted Ginhom.
For the water-soluble compound 1, common buffer solutions
(phosphate (pH < 7.5) and TRIS (pH > 7.5)) could be used to
stabilize the pH for the pH-dependent absorption spectra
shown in Figure 1, as well as for the pH-sensitive HRS
measurements at 1550 nm in D2O depicted in Figure 2. All
given pH values were measured using a glass electrode pH
meter (Lab 850 from Schott Instruments). The used buffers
were found to exhibit negligible HRS signals (i.e., the HRS
signals of the buffer solutions were identical to those of pure
water). For the other HRS measurements (both in water and
acetonitrile), the pure acidic and basic forms of the dyes were
obtained by the addition of small amounts of 37% HCl/H2O
and 50% NaOH/H2O stock solutions. As NaOH does not
dissolve in pure acetonitrile, 5 vol % of H2O (D2O for λ > 1200
nm) was added to all solutions (including the acidic ones, as
Scheme 1. Investigated Compounds 1 and 2 in Acidic (a)
and Basic (b) Environments, the Latter with Their Two
Resonance Forms
scattering, from molecules in solution.23−25 The highly sensitive
and wavelength-tunable setup used for this study has been
described before. 26 Briefly, a Ti:sapphire chirped-pulse
regenerative laser amplifier pumps a dual-stage optical parametric amplifier to provide 2 ps pulses at a 1 kHz repetition rate
with a continuously tunable wavelength. The output beam is
focused into a cuvette containing the solution under
investigation, and the generated HRS light is detected using a
spectrograph coupled to a nitrogen-cooled, silicon CCD, which
ensures the parallel detection of a narrow spectral range around
the second harmonic wavelength. This spectral analysis is
essential for systematic and complete subtraction of any
multiphoton fluorescence contributions that often occur and
are easily distinguished as a broad background. All hyperpolarizabilities are determined by reliable, internal calibration
against the HRS signal of the pure solvent itself, which we
extensively calibrated before against CHCl3.26 Values are given
as the βzzz-component, which is the dominant component for
Figure 1. Left panel: Absorption spectra of 1 in H2O for a range of pH values. Isosbestic points are found at 448, 349, and 315 nm. Right panel:
Solvatochromism of 1b, where black dots are plotted with respect to the dielectric constant29 and red dots with respect to the empirical ENT (30)
scale.30
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measurements for its IR transparancy) a pKaD of 8.3 ± 0.1 was
observed, the difference of 0.4 resulting from the use of a glass
probe pH meter28 (this constant offset was corrected for in the
pH-dependent HRS measurements).
For the basic form of 1, both a neutral and zwitterionic
resonance structure might be important (see Scheme 1), where
the latter is stabilized by the gain in aromaticity. To verify
which resonance structure dominates in this compound, we
recorded absorption spectra in different polarity solvents to
reveal its solvatochromism, which is governed by the groundand excited-state dipole moments.31 While the solvatochromism data shows somewhat more scatter when plotted versus
dielectric constant, the shift is even more systematic when the
empirical ENT (30) polarity scale30 is used that runs from 0 for
gas-phase to 1 for water, and which is particularly suitable here,
as it was set up using a zwitterionic compound with rather
similar structural features as 1b. The negative solvatochromism
of the compound (Figure 1, right panel) indicates that indeed
its ground state is dominated by the zwitterionic structure, and
the excited state by the neutral one, and thus places this
compound in the right-hand side of the well-known bondlength alternation (BLA) diagram4 (or its generalization, the
“MIX” model,32,33 providing a very general description of the
NLO behavior of conjugated molecules), i.e., beyond the
cyanine limit. Such “right-hand side” molecules can have
hyperpolarizabilities of equally large magnitude (but opposite
sign) as the more common “left-hand side” molecules with the
same conjugation length and have the added benefit of very
large dipole moments, μ. This makes them also particularly
interesting for NLO devices based on poled polymer films (the
figure of merit for these being μ·β). Likewise, the ground state
of the basic form of 2 can also be expected to be dominated by
the aromatic resonance structure. This results, for the basic
forms of both compounds, in strong push−pull systems formed
by a strong electron acceptor (quinolinium or dicyanomethylidene) and a strong donor (−O−) for which a high optical
Figure 2. pH dependence of the effective (i.e., root-mean-square) first
hyperpolarizability of 1 at 1550 nm (measured in D2O, but pH values
were corrected for the isotope effect to their equivalent in H2O).
well as the reference samples), which did not influence the HRS
signal beyond experimental error. Ultraviolet−visible absorption spectra were measured using a Varian Cary 5E absorption
spectrometer.
■
RESULTS AND DISCUSSION
Compound 1 is water-soluble, which is an interesting feature
for biological purposes, and allowed us to measure its
hyperpolarizability over a wide range of pH values. To first
determine in which pH range its switching occurs, absorption
spectra were taken as a function of pH (Figure 1, left panel).
The spectra show a clear shift of intensity from the longest
wavelength absorption band maximum at λmax = 418 nm to one
at λmax = 520 nm upon increasing pH (with well-defined
isosbestic points at 448, 349, and 315 nm, confirming that only
two forms of the dye are involved, without any decomposition).
From these spectra a pKaH of 7.9 ± 0.1 in H2O was established.
From similar spectra in D2O (that was used in some HRS
Figure 3. a−c: Experimental wavelength-dependent HRS results (symbols) with β-dispersion models (solid curves) and normalized absorption
spectra (dashed curves) of the acidic (blue) and basic (red) form of 1 in water (a) and acetonitrile (b), and of 2 in acetonitrile (c). d−f: Dispersion
of the corresponding β contrast.
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Table 1. NLO Figures of Merit for the Compounds under Study, Corresponding to the Curves Plotted in Figure 3a
compound (solvent)
βmax,b
βmax,a
(βb/βa)max
β0,b
β0,a
β0,b/β0,a
1 (acetonitrile)
1 (water)
2 (acetonitrile)
2760 ± 140 (1185)
2425 ± 120 (1056)
2650 ± 130 (1154)
1430 ± 71 (895)
996 ± 51 (840)
1190 ± 59 (860)
13.9 ± 0.7 (1240)
9.4 ± 0.5 (1125)
10.7 ± 0.5 (1190)
207 ± 10
214 ± 15
163 ± 11.8
84.0 ± 5.8
88.4 ± 4.7
101 ± 6.8
2.4 ± 0.1
2.5 ± 0.1
1.6 ± 0.1
a
Hyperpolarizabilities are listed as βzzz components in units of 10−30 esu, and corresponding laser wavelengths of the maxima are listed in
parentheses (in nanometers).
sponding acidic forms, this result correlates well with the twolevel model.34 Also, the basic form of 1 shows static
hyperpolarizabilities larger than those of the already commercialized compound 2. In fact, the resonantly enhanced β-values
for the basic forms are significantly higher than for any pHswitchable compound reported before. Only the pH-sensitive
product measured on resonance by Asselberghs et al.9 is
comparable, with a βzzz,840 nm = 2203 × 10−30 esu (recalculated
using the more recent β value35 for their calibration
compound), and Mançois et al.36 also reported a very large
value (∼42% of the hyperpolarizability of compound 1b). [In
ref 36, βHRS,1064 nm = (⟨|βXZZ|2⟩ + ⟨|βZZZ|2⟩)1/2 = 475 × 10−30 esu
(rescaled for the different reference value used for acetonitrile
[βHRS,1064 nm = 0.284 × 10−30 esu (ref 37) instead of 0.258 ×
10−30 esu (ref 26)), to be compared with βHRS,1064 nm = 1143 ×
10−30 esu for our compound 1b.]
Also, the maximum β contrasts, occurring slightly red-shifted
from the λmax of the basic state, are very high (up to 13.9, see
Table 1) and correspond to huge contrasts in HRS signal
(proportional to β2), reaching almost 200 for 1 in acetonitrile at
1240 nm and almost 100 in water at 1125 nm, rendering the
signal coming from the acidic state virtually negligible. Only
one compound reported by Mançois et al.7 has a higher
maximum β contrast (∼26; albeit with lower absolute β), which
may be attributed to a larger separation of the longest
wavelength band of the two states, so that an increase in
contrast due to resonant enhancement in one form is not
opposed by resonant enhancement in the other.
nonlinearity can be anticipated. In contrast, the acidic forms
have a much weaker OH electron donor.
To examine the NLO switching of compound 1, its
hyperpolarizability was first measured over a range of pH
values at 1550 nm in D2O (Figure 2). A very good fit to the
experimental data is obtained using the well-known Henderson−Hasselbalchs equation for the concentrations of both
forms and calculating the root-mean-square of the hyperpolarizabilities of the components of a binary mixture, which is
the observable in HRS:
eff
=
βzzz
a 2
b 2
Na |βzzz
| + Nb |βzzz
|
Na + Nb
(1)
where Na (Nb) are the concentrations of the acidic (basic) form
and βa (βb) are their second-order polarizabilities; the usual
assumption is made that the hyperpolarizability is dominated by
a single diagonal tensor component along the long axis of the
conjugated chain. This fit yields a first hyperpolarizability of
(131 ± 6.6) × 10−30 esu for the pure acidic form (βa) and (431
± 22) × 10−30 esu for the pure basic form (βb), which amounts
to a β contrast (βb/βa) of 3.30 at 1550 nm.
To determine the dispersion of the hyperpolarizability of
both the protonated and deprotonated forms of compounds 1
and 2, and to allow for reliable extrapolation to the static limit,
the β-values of both compounds were determined over a wide
range of wavelengths, at both extremely acidic and basic pH
(Figure 3). Compound 2, not being water-soluble, was
measured in acetonitrile solution, while 1 was measured in
both H2O (D2O for λ > 1200 nm) and acetonitrile, to allow for
direct comparison with 2. These β values were then accurately
fitted using the aforementioned vibronic β dispersion model18
(Figure 3a−c). The large deviation of the shortest-wavelength
data-point of the basic form of compound 1 in water is likely to
be caused by contributions from higher-energy transitions
visible in the absorption spectrum that are not taken into
account in this model; therefore, this data point was omitted in
the fitting procedure. For the same reason, the two shortestwavelength data points of the basic form of both compounds in
acetonitrile were omitted. By dividing the β-dispersion curve
obtained for the basic form by the one obtained for the acidic
form, the wavelength dependence of the β switching contrast is
obtained (Figure 3d-f). Note that for our compounds, the
contrast drops below unity in some wavelength ranges; hence,
the terminology “on” and “off” sometimes used in the literature
for either state of a switchable compound does not strictly
apply at all wavelengths.
An overview of relevant figures of merit for the compounds
under study is given in Table 1. While for both 1 and 2 only the
acidic compounds have been crystallized (and used for
frequency doubling and terahertz pulse generation),20−22 here
we reveal much larger hyperpolarizabilities for the basic forms.
The λmax of the basic forms being significantly red-shifted and
the extinction coefficient increased compared to the corre-
■
CONCLUSION
■
AUTHOR INFORMATION
For the first time for pH-sensitive compounds, the first
hyperpolarizability β was measured over a wide range of
wavelengths and pH values, and the obtained experimental
results were used to model the β dispersion behavior, which not
only allows for understanding the results but also is a valuable
tool for practical purposes, e.g., in optimizing the β contrast.
The responses of the basic forms are the highest ever measured
for the “on”-state of pH-sensitive NLO compounds, with 1
showing higher responses than 2. Both compounds show very
high contrasts both at and off-resonance that are more than
suitable for practical applications. The fact that the switching
range of 1 is within biological pH limits (which is another
feature that has never been reported before), while a β contrast
of unity is reached at ∼800 nm and a high maximum contrast at
∼1100 nm, both within reach of the common Ti:sapphire laser,
makes compound 1 highly promising for pH sensing through
ratiometric second harmonic generation imaging.
Corresponding Author
*E-mail: Wim.Wenseleers@uantwerp.be.
Notes
The authors declare no competing financial interest.
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■
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ACKNOWLEDGMENTS
Financial support from the Fund for Scientific ResearchFlanders (FWO; Projects G020612 and G052213) and from
EU FP7 Marie-Curie action ITN Nano2Fun (REA #607721) is
gratefully acknowledged. J.C. is a postdoctoral fellow of the
FWO. O.-P.K. and S.-H.L. were supported by the National
Research Foundation (NRF) grants funded by the Korean
Government (MSIP) (2014R1A5A1009799 and
2013R1A2A2A01007232).
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