Engaging Aldehydes in CuH-Catalyzed Reductive Coupling Reactions: Stereoselective Allylation with Unactivated 1,3-Diene Pronucleophiles

Article abstract of DOI:10.1002/anie.201911008

Recently, CuH-catalyzed reductive coupling processes involving carbonyl compounds and imines have become attractive alternatives to traditional methods for stereoselective addition because of their ability to use readily accessible and stable olefins as surrogates for organometallic nucleophiles. However, the inability to use aldehydes, which usually reduce too rapidly in the presence of copper hydride complexes to be viable substrates, has been a major limitation. Shown here is that by exploiting relative concentration effects through kinetic control, this intrinsic reactivity can be inverted and the reductive coupling of 1,3-dienes with aldehydes achieved. Using this method, both aromatic and aliphatic aldehydes can be transformed into synthetically valuable homoallylic alcohols with high levels of diastereo- and enantioselectivities, and in the presence of many useful functional groups. Furthermore, using a combination of theoretical (DFT) and experimental methods, important mechanistic features of this reaction related to stereo- and chemoselectivities were uncovered.

Full text of DOI:10.1002/anie.201911008

Article
pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C 2019, 123, 2565−2572
Selective Protonation of Unbounded Sodium Cholates for
Reversible Blue-Shift Absorption Spectra of Single-Chirality Single-
Walled Carbon Nanotube in Solution
Huaping Li,* Lili Zhou, and Nathan K. Smolinski
Atom Optoelectronics 440 Hindry Avenue, Unit E, Inglewood California 90301, United States
S
* Supporting Information
ABSTRACT: In contradiction to the long-standing optical depletion of single-walled carbon
nanotubes (SWCNT) by acid doping, this work reports the blue-shifted and optical density-
increased S11 (the first Van Hove singularity transition) peak absorption spectra of single-
chirality (6,5) SWCNT dispersed in specific surfactant compositions such as 1% SDS (sodium
dodecyl sulfate) /1% SC (sodium cholate) and 2% SDS/0.5% SC, which were caused by
addition of acid. These anomalous optical changes can be reversed by adding base, and they are
discussed on the basis of surfactant arrangements on SWCNT sidewalls, which arose from the
selective protonation of unbounded SC to shift the thermodynamic equilibrium of SC adsorbed
SWCNT to the desorption of SC from SWCNT surface. The spectroscopic interpretation
obeys the dipole moment transition related energy level variation and oscillator strength
change.
interactions between photoexcited SWCNT polarization and
polarized solvents.32
INTRODUCTION
■
With sharp spectrum features near the infrared region, single-
chirality single-walled carbon nanotubes (SWCNTs) have
been utilized as optical probes for bioimaging and biosensing
applications.1,2 The band gap optical properties of SWCNTs
are very sensitive to the adsorbed molecules (including
dispersing agents and O2),3−15 solution media (including pH
and ionic strength),16−23 and environment effects such as
temperature and pressure.23−28 Understanding photophysical
properties of SWCNT is critical to precisely interpret the
spectroscopic results.
SWCNTs are highly hydrophobic macromolecules that need
to be dispersed in solvents using surfactants or polymers for
manipulation and practical applications.29 The measured
optical spectral shifts of SWCNTs dispersed in different
solvents are reported to scale with their dielectric constants.30
The effective dielectric constant surrounding SWCNTs
depends on their unknown surface coverages of the
surfactants.31 The local dielectric constant is a dielectric
superposition of the adsorbed surfactants and incorporated
solvents. In this theoretical interpretation, SWCNTs are
commonly considered highly polarizable macromolecules
with zero net dipole moment. In polar solvents, the dipole
induced polarization difference before and after optical
excitation was quantified using Onsager polarity function,
f(ε,η) = 2(ε −1)/(2 × ε + 1) − 2(η −1)/(2 × η + 1), where ε
and η are the solvent dielectric constant and the reflective
index, respectively.30 In nonpolar solvents, photoexcited
SWCNT polarization can induce dynamic polarization of a
solvent. The spectral shift was correlated to solvent-induction
polarization function f(η2) = (η2 − 1)/(2η2 + 1) for
Recently, we revealed that the optical spectral shifts of
single-chirality (6,5) SWCNT in aqueous solution are related
to the geometric morphologies and compositions of surfactants
(sodium dodecyl sulfate, SDS and sodium cholate, SC) on
SWCNT sidewalls.33 With different geometric morphologies
and compositions, the surface areas of (6,5) SWCNT exposed
to polar solvents varied and their absorption spectra shifted
accordingly. With full coverage of different sole surfactants,
different hydration layers were formed around sidewalls of
SWCNTs, leading to the differentiated spectral shifts. These
results are well consistent with the classic solvatochromic
theory in polar solvents.34,35 Generally, photoexcited SWCNT
is polarized to carry photoinduced dipole moments that can be
correlated to the optical density of SWCNT absorbance
(oscillator strength of exciton, f ∞ |D|2, D is the induced dipole
vector). The energy state of polarized SWCNT is more stable
when more sidewall surfaces are exposed to a polar solvent,
which results in a more red-shifted absorption spectrum.25
Because of the slow movement of a solvent nuclear part, the
solvent electron cloud is also polarized by polarized SWCNT
(Franck−Condon effect). The polarized solvent electron cloud
exerts back to the polarized SWCNT. These interpolarization
processes continue until an equilibrium state is reached. The
degree of dipole of initially photoexcited SWCNT reduces
during these interpolarization processes. Accordingly, the
Received: November 5, 2018
Revised: January 3, 2019
Published: January 9, 2019
© 2019 American Chemical Society
2565
DOI: 10.1021/acs.jpcc.8b10785
J. Phys. Chem. C 2019, 123, 2565−2572

The Journal of Physical Chemistry C
Article
oscillator strength f (optical density in absorption spectrum) of
SWCNT decreases. Additionally, the symmetry of surfactant
morphology plays certain roles in spectral changes. For
instance, the polarized SWCNT is preferred in surfactant
arrangements with low symmetry rather than high symmetry
because high symmetrical ground state SWCNT transits to low
symmetric photoexcited SWCNT. Thus, the polarized
SWCNT in low symmetric surfactant coverage should have
lower free Gibbs energy, with peak absorption appearing at
longer wavelength than that in high symmetric surfactant
encapsulation. The dipole moment transition of photoexcited
SWCNT (oscillator strength of absorption) in low symmetric
surfactant coverage should be smaller than that in high
symmetric surfactant encapsulation.
Since Smalley’s research group reported the diminished
absorption features of SDS dispersed HiPCO SWCNTs in acid
solution,21 those bleached absorbances were demonstrated to
be reversed either by degassing solution21,36 or using
dispersing agents (for examples, SC37 and dodecyl tethered
flavin mononucleotide38) that can isolate oxygen from the
surfaces of SWCNTs. This acid caused decrease of SWCNTs
absorption is generally considered as acid doping, which
ignores the changes of SWCNT surface dielectric constant,
morphology and symmetry. In this contribution, we report that
the diverse absorption spectral changes of (6,5) SWCNT
aqueous solution upon addition of acid were observed in
various surfactant compositions. For (6,5) SWCNT with more
sidewall surface areas exposed to water molecules, their
absorption spectra generally shifted to the blue with increasing
optical densities by adding acid, and saturated at the points
where the surfactant arrangements were finalized. These
spectral changes were reversibly recovered by adding base to
neutralize the solution. We ascribe these observed spectral
changes to reversibly and competitively thermodynamic
exchanges of SDS and SC moieties on SWCNT sidewalls,
which were tailored by the acidity of aqueous solutions. For
the SWCNT with less sidewall surface areas exposed to water
molecules or fully covered with sole surfactants, their
absorption spectra exhibited surfactant-dependent optical
density decreases, similar to those reported protonations of
SWCNT by acid doping.21−23
temperature. The optical path length is 1 cm. The starting
solution volumes are 1 mL.
RESULTS AND DISCUSSION
■
With the sharp absorbance of the first Van Hove singularity
transition (S11), the single-chirality (6,5) SWCNT extracted
from high pressure CO conversion carbon nanotubes can be
employed as the optical probe without the need of spectral
deconvolution.39,40 In this contribution, the diverse S11
absorbance changes of (6,5) SWCNT dispersed in aqueous
solutions with various surfactant compositions (sodium
dodecyl sulfate (SDS), sodium cholate (SC)) were inves-
tigated. Their chemical structures are illustrated in Supporting
Information, Figure S1. The visible near-Infrared absorption
spectra of (6,5) SWCNT dispersed in 5% SDS (pH = 7.6), 2%
SDS (pH = 7.4), 2% SDS/0.05% SC (pH = 8.0), 2% SDS/
0.5% SC (pH = 8.5), 1% SDS/1% SC (pH = 8.7), and 5% SC
(pH = 9.0) were measured using NS3 Applied Nano
Spectralyzer (Supporting Information, Figure S2). To elucidate
the spectral shift, the measured absorption spectra were
normalized at S11 peak absorbances and presented in Figure
1A. Their S11 peak wavelengths were plotted against different
Figure 1. (A) Normalized S11 absorption spectra of (6,5) SWCNT
dispersed in 5% SDS (blue curve), 2% SDS (dark blue curve), 2%
SDS/0.05 SC (light blue curve), 2% SDS/0.5% SC (dark red curve),
1% SDS/1% SC (red curve), and 5% SC (pink curve). (B) Plot of S11
peak wavelengths of (6,5) SWCNT against different surfactant
compositions in which (6,5) SWCNT was dispersed.
surfactant compositions (Figure 1B). Clearly, the S11 peak
absorption of (6,5) SWCNT dispersed in 5% SDS and 2% SDS
appears at 977 nm, 2% SDS/0.05% SC at 979 nm, 2% SDS/
0.5% SC at 989 nm, 1% SDS/1% SC at 991 nm, and 5% SC at
982 nm, respectively. A minor peak located at 1030 nm could
be due to other SWCNT with different chirality, such as
(10,2). The S11 peak absorption of (6,5) SWCNT dispersed in
high concentration SDS (2% and 5%) occurred at the bluest
region. When SC was introduced to (6,5) SWCNT dispersed
in high concentration SDS solution, the S11 peak absorption of
(6,5) SWCNT started shifting to the red. A larger shift was
observed with more SC, i.e., the S11 peak absorption of (6,5)
SWCNT dispersed in 2% SDS/0.5% SC is at the red site of
that of (6,5) SWCNT dispersed in 2% SDS/0.05% SC. The
redshift of (6,5) SWCNT dispersed in 1% SDS/1% SC
reached an upper limit. The S11 peak absorption of (6,5)
SWCNT dispersed in high concentration SC (5%) shifted back
to blue, between those of (6,5) SWCNT dispersed in 2% SDS/
0.5% SC and 2% SDS/0.05% SC. These results are consistent
with our reported spectral changes of (6,5) SWCNT dispersed
in 2% SDS titrated with SC.33 These spectral variations thus
can be interpreted using our previous hypothesis.33 In high
concentration SDS solutions (5% SDS and 2% SDS), rod-like
micelles of SDS around (6,5) SWCNT were formed. One SC
EXPERIMENTAL SECTION
■
Materials. Sodium dodecyl sulfate (SDS) and sodium
cholate (SC) were purchased from Sigma-Aldrich and used as
received. Sephacryl gel S200 was purchased from GE health.
Single-walled carbon nanotubes (SWCNTs) raw powder was
produced by Rice University Mark III high pressure carbon
monoxide reactor using less catalyst in a yield of 1 g per hour.
Acetic acid (5%, 0.833 mol/L), HCl (3.65%, 1.0 mol/L) and
NaOH (3.32%, 0.83 mol/L) were used as acids and base,
respectively. The raw SWCNTs were used for the extraction of
(6,5) SWCNT using gel permeation chromatography. The
(6,5) SWCNT stock solutions dispersed in 5% SDS, 2% SDS,
2% SDS/0.05% SC, 2% SDS/0.5% SC, 1% SDS/1% SC, and
5% SC were obtained by eluting the trapped (6,5) SWCNT in
Sephacryl gels with according surfactant compositions. All
concentration units are weight percentage unless there are
specific descriptions.
Measurements. All visible near-infrared absorption spectra
of (6,5) SWCNT dispersed in different surfactant systems were
measured on a NS3 Applied Nano Spectralyzer at ambient
2566
DOI: 10.1021/acs.jpcc.8b10785
J. Phys. Chem. C 2019, 123, 2565−2572

The Journal of Physical Chemistry C
Article
moiety replaced the corresponding SDS rod-like segment to
open SWCNT surface to water molecules, leading to the red-
shifted S11 absorbance of (6,5) SWCNT. More SC molecules
can supersede more segments of rod-like SDS micelles to
expose more SWCNT surfaces to water solvent with the more
red-shifted S11 absorption spectra. Likely, (6,5) SWCNT
dispersed in 1% SDS/1% SC has maximal surfaces exposure to
water molecules, showing the maximally red-shifted S11 peak
absorption. With high concentrated SC, the SC micelles with
rigid helix structures were formed around SWCNT sidewalls,
having limited surface areas open to polar water media, less
than the opened surface areas of SWCNTs dispersed in 2%
SDS/0.5% SC, but more than those of SWCNTs dispersed in
2% SDS/0.05% SC.
The absorption spectra of (6,5) SWCNT dispersed in 1%
SDS/1% SC (1 mL solution) were measured upon addition of
0 μL (pH = 8.7), 1 μL (pH = 3.9), 2 μL (pH = 3.8), 3 μL (pH
= 3.8), 4 μL (pH = 3.6), 5 μL (pH = 3.6), 7 μL (pH = 3.5), 10
μL (pH = 3.4), 15 μL (pH = 3.4), 20 μL (pH = 3.3) of 5%
HAc (Supporting Information, Figure S3). Their S11
absorbance spectra are exhibited in Figure 2A. Their S11
5 μL (pH = 3.4), 10 μL (pH = 3.4), 15 μL (pH = 3.6), 20 μL
(pH = 8.7), 25 μL (pH = 11.6), and 30 μL (pH = 11.9)
(Supporting Information, Figure S4). Their S11 absorption
spectra are shown in Figure 2C and the plots of their S11 peak
wavelengths (dark red curve) and S11 peak optical densities
(red curve) against the concentrations of NaOH are presented
in Figure 2D. The results unambiguously display that the S11
peak absorption wavelength of (6,5) SWCNT shifted back
from 981 to 995 nm, and their S11 peak optical densities
decreased from 1.21 to 0.93.
The similar spectral changes were observed for (6,5)
SWCNT dispersed in 2% SDS/0.5% SC (pH = 8.5), but
with reduced amplitudes. The measured absorption spectra of
(6,5) SWCNT dispersed in 2% SDS/0.5% SC upon the
addition of 5% HAc are shown in Supporting Information,
Figures S5 and S6. Their S11 absorption spectra are exhibited
in Supporting Information, Figures S7A and S8A respectively.
The plots of their S11 peak wavelengths and optical densities
against the added volume of 5% HAc were plotted in
Supporting Information, Figures S7B and S8B. The reprodu-
cible results show the S11 peak wavelength of (6,5) SWCNT
dispersed in 2% SDS/0.5% SC (pH = 8.5) shifted from 989 to
977.5 nm upon addition of 15 μL (pH = 3.4) of 5% HAc. The
S
11 peak wavelength of (6,5) SWCNT remained constant after
addition of 5 μL (pH = 3.6) of 5% HAc. The maximal optical
density of S11 peak absorbance was reached upon addition of
10 μL (pH = 3.4) of 5% HAc and then slightly decreased upon
further addition of 5% HAc. The reversed absorption spectra
of (6,5) SWCNT dispersed in 2% SDS/0.5% SC were also
observed upon addition of 3.3% NaOH in 2 μL (pH = 3.4), 4
μL (pH = 3.4), 6 μL (pH = 3.5), 8 μL (pH = 3.5), 10 μL (pH
= 3.6), and 12 μL (pH = 3.8) (Supporting Information, Figure
S9). Their S11 absorption spectra are shown in Supporting
Information, Figure S10A and the plots of their S11 peak
wavelengths and S11 peak optical densities against the added
volumes of 3.3% NaOH are presented in Figure S10B. The S11
peak wavelength of (6,5) SWCNT shifted from 978 nm back
to 989 nm when 12 μL of 3.3% NaOH was added.
Simultaneously, their peak optical densities decreased back to
their original peak optical density (slightly lower than the
starting absorption spectrum).
HAc is a weak organic acid. To rule out organic solvent
effect, 3.65% HCl was used as the substitute for 5% HAc.15
The absorption spectra of (6,5) SWCNT dispersed in 2%
SDS/0.5 SC (pH = 8.5) were measured upon addition of 1 μL
(pH = 3.0), 2 μL (pH = 2.7), 3 μL (pH = 2.5), 4 μL (pH =
2.4), and 5 μL (pH = 2.3) of 3.65% HCl and then 2 μL (pH =
2.4), 4 μL (pH = 2.9) and 6 μL (pH = 3.2) of 3.3% NaOH,
sequentially (Supporting Information, Figure S11). Their S11
absorption spectra are presented in Figure 3A, blue curves for
addition of HCl and red curves for addition of NaOH. The
plots of S11 peak wavelengths and optical densities against the
concentrations of 3.65% HCl (bottom coordinate) and of
NaOH (top coordinate) are shown in Figure 3B. The plots
show S11 peak wavelength of (6,5) SWCNT dispersed in 2%
SDS/0.5% SC shifted from 988.5 to 978 nm after addition of
15 μL of 3.65% HCl, and then reversed back to 989 nm after
addition of 12 μL (in total) of 3.32% NaOH. The S11 peak
optical densities of (6,5) SWCNT dispersed in 2% SDS/0.5%
SC increased from 0.77 to 0.89 after addition of 15 μL of
3.65% HCl and reversed back to 0.77 after addition of 12 μL
(in total) of 3.32% NaOH.
Figure 2. (A) S11 absorption spectra of (6,5) SWCNT dispersed in
1% SDS/1% SC upon addition of 5% HAc in volumes of 0 (black
curve), 1, 2, 3, 4, 5, 7, 10, 15, and 20 μL (blue curves). (B) Plots of S11
peak wavelengths (black curve) and S11 peak optical density (blue
curve) against the concentrations of HAc. (C) S11 absorption spectra
of (6,5) SWCNT dispersed in 1% SDS/1% SC (black curve), after
addition of 20 μL of 5% HAc (blue curve), and after addition of
3.32% NaOH in volumes of 5, 10, 15, 20, 25, and 30 μL (red curves).
(D) Plots of S11 peak wavelengths (dark red curve) and S11 peak
optical density (red curve) against the concentrations of NaOH.
peak wavelengths (black curve) and optical densities (blue
curve) are plotted against the concentrations of HAc as shown
in Figure 2B. The results show the S11 peak absorption
wavelength shifted from 994 to 981 nm and the S11 peak
optical density increased from 1.00 to 1.21 when 5% HAc was
added from 1 to 20 μL. The S11 peak wavelength of (6,5)
SWCNT remained constant at 981 nm after the addition of 10
μL or more of 5% HAc. The optical density of S11 peak
absorption reached a maximum after the addition of 15 μL of
5% HAc, then slightly decreased with further addition of 5%
HAc. These spectral changes were reversed by adding various
volumes of 3.3% NaOH. The absorption spectra of (6,5)
SWCNT dispersed in 1% SDS and 1% SC added with 20 μL of
5% HAc were measured by further addition of 3.3% NaOH in
2567
DOI: 10.1021/acs.jpcc.8b10785
J. Phys. Chem. C 2019, 123, 2565−2572

The Journal of Physical Chemistry C
Article
Scheme 1. Cartoon Diagrams To Show the Acid Effect on
Surfactant Arrangements of (6,5) SWCNT Dispersed in
a
Different Surfactant Compositions
Figure 3. (A) S11 absorption spectra of (6,5) SWCNT dispersed in
2% SDS/0.5% SC upon addition of 3.65% HCl in volumes of 0 (black
curve), 1, 2, 3, 4, and 5 μL (blue curves) and addition of 3.32%
NaOH in volumes of 2, 4, and 6 μL (red curves). (B) Plots of S11
peak wavelengths (black curve) and S11 peak optical density (blue
curve) against the concentrations of HCl (bottom coordinate) and
plots of S11 peak wavelengths (dark red curve) and S11 peak optical
density (red curve) against the concentration of NaOH (top
coordinate).
These results show these blue-shifted and optical density
increased S11 absorption spectra of (6,5) SWCNT dispersed in
1% SDS/1% SC and 2% SDS/0.5% SC upon addition of acids
(either HAc or HCl) are reproducible and consistent
phenomenon, and can be reversed back after addition of a
base (NaOH). Interestingly, the S11 absorption spectra of (6,5)
SWCNT dispersed in 1% SDS/1% SC and 2% SDS/0.5% SC
implied that SWCNT surface areas exposure to water
environment approach to their maxima. On the basis of our
previous spectral dilution and titration studies of (6,5)
SWCNTs in different surfactants33 and well-known solvato-
chromic effects on SWCNTs absorption spectra,30−32 we
rationalize these blue-shifted S11 absorption spectra of (6,5)
SWCNT to the surfactant reorganization induced by addition
of acid that mitigated SWCNT sidewall surface areas exposure
to water molecules. The pKa values of HAc, dodecyl sulfuric
acid, cholic acid are 4.8,41 1.942 and 4.9843 respectively. With
pH > 1.9, SDS will not be protonated. Likely, SC will be
protonated when pH value reaches 5 or less.44 For example,
the cloudy solution appeared when 10 μL of 5% HAc was
injected into (6,5) SWCNT dispersed in 5% SC solution,
implying the protonation of SC moieties (Supporting
Information, Figure S12). SDS adsorbed on SWCNT should
have the pKa value similar to that of the free SDS molecule
based on their similar ζ potential values.45 SC adsorbed on
SWCNT could have pKa value much smaller than that of free
SC molecule because SC dispersed SWCNT solution has
about 28 mV more negative ζ potential in comparison to SC
without SWCNT.46 On the basis of these analyses, the
addition of enough acid (either of HAc or HCl), free SC
molecules would selectively be protonated and form aggregates
in aqueous solution (Scheme 1). Additionally, the critical
micelle concentration of SC would decrease with the
protonation of SC by adding acid. The reduced free SC
moieties then shifted the thermodynamic equilibrium of SC
bounded SWCNTs to SC desorption from the SWCNT
sidewall. Consequently, SDS molecules from free SDS moieties
and SDS micelles would occupy these surface areas and
diminish the surface areas exposure to water solvent via
molecular exchanges.33 In this situation, the blue-shifted
absorption spectra were observed because of the upraised
energy level of polarized SWCNTs in less polar environment.
Simultaneously, the dipole moment of a polarized SWCNT
was weakly suppressed by a less polar environment in
a
Key: (A) Highly concentrated SDS (2% and 5%), no protonation for
individual SDS or rod-like SDS on SWCNT surface by adding acid
with pH from 4.98 to 1.9. (B) Highly concentrated SC (5%), with
protonation of individual SC but no protonation of SC micelles on
SWCNT surfaces by adding acid with pH from 4.98 to 1.9. (C)
Highly concentrated SDS with SC (2% SDS/0.5% SC and 1% SDS/
1% SC), selective protonation of individual SC but no protonation of
individual SDS, rod-like SDS on SWCNT surfaces and SC micelles on
SWCNT surfaces by adding acid with pH from 4.98 to 1.9.
comparison to the strong dipole moment suppression of
polarized SWCNT by polar media, leading to the increase of
oscillator strength f of S11 peak absorption. Reversely,
deprotonation of cholic acid by adding NaOH base led to
the increased concentration of free SC, which in turn
competitively substitute SDS adsorbed on SWCNTs through
molecular exchanges simply because free SC has stronger
binding strength on the SWCNT sidewall than SDS
moieties.33,47,48 Thus, the sidewall surface of the SWCNT
was reopened and exposed to polar water solvent, leading to
the reversed changes in S11 absorption spectra.
The absorption spectra of (6,5) SWCNT dispersed in 2%
SDS/0.05% SC (pH = 8.0), 2% SDS (pH = 7.4), 5% SDS (pH
= 7.6) and 5% SC (pH = 9.0) after addition of 5% HAc were
measured (Supporting Information, Figures S13−S16). Their
2568
DOI: 10.1021/acs.jpcc.8b10785
J. Phys. Chem. C 2019, 123, 2565−2572

The Journal of Physical Chemistry C
Article
S11 absorption spectra are illustrated in parts A, C, E, and G of
Figure 4, and the plots of their S11 peak wavelengths and peak
initially when 1 μL (pH = 3.9) of 5% HAc added. The
decreases in amplitudes are significantly different with different
surfactant compositions.37 Here, (6,5) SWCNT dispersed in
5% SDS and 2% SDS showed large decreases in amplitudes of
S
11 peak optical densities, from 0.98 to 0.80 and from 1.035 to
0.915, respectively. The decreases in amplitudes of the S11 peak
optical densities of (6,5) SWCNT dispersed in 5% SC (from
1.316 to 1.276) and 2%SDS/0.05% SC (from 0.842 to 0.822)
are relatively small. Consistent with other literature re-
ports,23,36,37 these declined S11 peak optical densities were
restored by adding 3.3% NaOH solution.
For SWCNTs dispersed in 5% SDS (pH = 7.6), 2% SDS
(7.4), 2% SDS/0.05% SC (pH = 8.0) and 5% SC (pH = 9.0),
their sidewall surface areas exposure to water solvent were
constrained and hardly disturbed by addition of acid. No S11
peak wavelength shift was thus observed upon addition of
acids. This indicates no perturbation on the equilibrium of
SDS and SDS encapsulated SWCNT by adding acids with the
eventual pH ranging from 4.98 to 1.9 (Scheme 1A). Although
the protonation of unbounded SC moieties theoretically
occurred, sufficient amounts of SC molecules still remained
in solution, and the conformation of SWCNT wrapped with
SC helix structure remained the same (Scheme 1B). These
unchanged SWCNT surfaces displayed their nonshifted S11
peak absorption. These observed S11 peak optical density
decreases have been interpreted by the protonation of
SWCNT sidewall surfaces due to acid doping. The differential
decreases in amplitude of the S11 peak absorption of (6,5)
SWCNT dispersed in sole SDS or SC may be interpreted by
their ζ potentials,45,46 where SC adsorbed SWCNT has more
negative ζ potential than SDS adsorbed SWCNT to limit the
penetration of the added acid. The slightly red-shifted S11 peak
wavelengths of (6,5) SWCNT dispersed in 5% SDS could be
due to attractive depletion (high concentrated electrolytes
induced aggregations leading to red-shifted wavelength and
optical density decrease) of nanoscale particles.37 This kind of
S11 peak optical density decrease can be expected to occur for
(6,5) SWCNT dispersed in 1% SDS/1% SC and 2% SDS/0.5%
SC solutions (actually, the slightly decreased S11 peak optical
densities were observed after the maximum S11 peak optical
densities were reached upon addition of acids). The obtained
results in Figure 2 and Figure 3 imply the spectral changes
were dominated by S11 peak optical density increases due to
diminished surface areas exposure to water polar media.
With the above-discussed absorption of spectral diversities
of (6,5) SWCNT dispersed in various surfactant compositions
upon addition of acids, their variabilities to elute SWCNTs
trapped in gels were tested. Though the obvious optical
changes of (6,5) SWCNT dispersed in 1% SDS/1% SC and 2%
SDS/0.5% SC solutions were observed, the eluting strengths of
1% SDS/1% SC and 2% SDS/0.5% SC in gel chromatography
using Sephacryl gel 200 as the media were negligibly different
upon addition of acids. These eluants might already have
strong eluting strengths because of the presence of a significant
amount of SC molecules. While no apparent S11 peak
wavelength shift was observed for (6,5) SWCNT dispersed
in 2% SDS/0.05% SC solution, the strength of 2% SDS/0.05%
SC to elute SWCNT was significantly enhanced by addition of
acids in gel chromatography using Sephacryl gel 200 as the
media.49 For 2% SDS elution, the maximal eluting strength was
observed when 4 μL of 5% HAc (pH = 3.6) was added. More
interestingly, loading (6,5) SWCNT dispersed in 0.5% SDS at
pH ∼ 5 in Saphacryl gel 200, no SWCNT could be trapped in
Figure 4. (A) S11 absorption spectra of (6,5) SWCNT dispersed in
2% SDS/0.05% SC upon addition of 5% HAc in volumes of 0, 1, 2, 3,
5, 10, and 15 μL. (B) Plots of S11 peak wavelengths (black curve) and
S11 peak optical density (blue curve) of (6,5) SWCNT dispersed in
2% SDS/0.05% SC against the concentrations of HAc. C) The S11
absorption spectra of (6,5) SWCNT dispersed in 2% SDS upon
addition of 5% HAc in volumes of 0, 1, 2, 4, 6, 10, 15, 20, and 25 μL.
(D) Plots of S11 peak wavelengths (black curve) and S11 peak optical
density (blue curve) of (6,5) SWCNT dispersed in 2% SDS against
the concentrations of HAc. (E) S11 absorption spectra of (6,5)
SWCNT dispersed in 5% SDS upon addition of 5% HAc in volumes
of 0, 1, 2, 3, 4, 5, 10, 15, and 20. (F) Plots of S11 peak wavelengths
(black curve) and S11 peak optical density (blue curve) of (6,5)
SWCNT dispersed in 5% SDS against the concentrations of HAc. (G)
S11 absorption spectra of (6,5) SWCNT dispersed in 5% SC upon
addition of 5% HAc in volumes of 0, 1, 3, and 7 μL. (H) Plots of S11
peak wavelengths (black curve) and S11 peak optical density (blue
curve) of (6,5) SWCNT dispersed in 5% SC against the
concentrations of HAc.
optical densities against the concentrations of HAc are
presented in parts B, D, F, and H of Figure 4, respectively.
Different from these spectral changes of (6,5) SWCNT
dispersed in 1% SDS/1% SC and 2% SDS/0.5% SC, their
S11 peak absorbances remained constant except for (6,5)
SWCNT dispersed in 5% SDS with slightly red-shift upon
addition of 5% HAc. Their S11 peak optical densities decreased
upon addition of 5% HAc, except for (6,5) SWCNT dispersed
in 2% SDS/0.05% SC with a slightly increase in optical density
2569
DOI: 10.1021/acs.jpcc.8b10785
J. Phys. Chem. C 2019, 123, 2565−2572

The Journal of Physical Chemistry C
Article
the gel. Neutralizing (6,5) SWCNT dispersed in 0.5% SDS to
pH ∼ 7.8 by adding NaOH, the retention of (6,5) SWCNT in
Sephacryl gel was obtained. These varied eluting strength
changes by adding acids may not seem relevant to the
surfactant arrangement on the SWCNT sidewall, which likely
arose from the ζ potential disparity of different surfactant
compositions by addition of acid and base, which led to various
electrostatic screening effects that determined the retention of
SWCNT in gels.33,45,46
transition and symmetry selection, which seems to be missing
in the literature.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpcc.8b10785.
■
S
Chemical structures of studied materials, all visible-near-
infrared absorption spectra, and a photographic image of
cloudy solution of (6,5) SWCNT after addition of 10 μL
of 5% HAc (PDF)
CONCLUSION
■
In summary, the absorption spectra of (6,5) SWCNT
dispersed in a series of surfactant compositions like 5% SDS,
2% SDS, 2% SDS/0.05% SC, 2% SDS/0.5% SC, 1% SDS/1%
SC, and 5% SC were measured upon addition of varied
amount of acids (either 5% HAc or 3.65% HCl). For these
(6,5) SWCNT dispersed in surfactant compositions such as
1% SDS/1% SC and 2% SDS/0.5% SC exhibiting S11 peak
absorption in red-shift region (more SWCNT surface areas
exposure to water microenvironment), their S11 peak
absorption wavelengths were significantly blue-shifted and
their S11 peak optical density increased upon addition of acid.
These spectral changes were reversed by adding 3.32% NaOH.
We conceivably ascribed these optical changes to surfactant
rearrangements around a SWCNT sidewall that mitigated the
SWCNT surface areas exposure to water molecules triggered
by the protonation of free SC moieties. The decreased SC
concentration in aqueous solution shifted the thermodynamic
equilibrium of SC adsorbed SWCNT to SC desorption from
SWCNT sidewall surface, and the desorbed SC moieties were
subsequently substituted by SDS moieties. The blue-shifted S11
peak absorption can be explained by the increased energy level
of polarized (6,5) SWCNT in a less polar environment, and
the increased S11 peak optical density can be credited to the
release of suppressed dipole moment of polarized (6,5)
SWCNT in polar medium. For these (6,5) SWCNT dispersed
in surfactant compositions such as 5% SDS, 2% SDS, 2% SDS/
0.05% SC, and 5% SC with S11 peak absorptions in a relatively
blue wavelength region (well encapsulated with surfactants to
have less SWCNT surface exposure to water media), their S11
peak absorbances generally remained the same but with varied
optical density decrease amplitudes because of their differential
protonation of SWCNT surfaces is considered as acid doping.
It seems that the more negative ζ potential of SWCNTs
dispersed with such surfactant as 5% SC causes a smaller
decrease in amplitude of S11 peak optical density. This implies
that the more negative SWCNT surface ζ potential prevented
the attack of H+ to SWCNT, which has been considered as
proton-mediated oxidation in literature. Additionally, the
eluting strengths of different surfactant compositions for
SWCNT separation using gel chromatography were examined
by adding acids. Generally, the results show that SWCNTs
dispersed in surfactants that are in an acidic condition could
not be trapped in Sephacryl gels. In order to retain SWCNTs
in gels, SWCNT solutions dispersed in surfactants need to be
adjusted to basic conditions with high pH values.50 Moreover,
we expect that this contribution can cast light on the use of
single-chirality SWCNT as a near-infrared probe for
bioimaging and biosensing, for instance, as pH indicator in
in vitro and in vivo experiments. Also, we expect this
contribution can stimulate more fundamental researches on
SWCNT spectroscopies, especially involving dipole moment
AUTHOR INFORMATION
Corresponding Author
*(H.L.) E-mail: huaping.li@atomoe.com.
ORCID
Huaping Li: 0000-0003-1285-4727
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the generous financial support from Marine
Corp System Command Office (Contract # M67854-17-C-
6559). We also thank the reviewers for their invaluable
suggestions and comments, and appreciate Professor Zhaohua
Dai from Pace University for his diligent editions.
ABBREVIATIONS
■
SWCNT,single-walled carbon nanotube; SDS,sodium dodecyl
sulfate; SC,sodium cholate; S11,the first Van Hove singularity
transition; HAc,acetic acid; HCl,hydrochloric acid; NaOH,so-
dium hydroxide.
REFERENCES
■
(1) Yomogida, Y.; Tanaka, T.; Zhang, M.; Yudasaka, M.; Wei, W.;
Kataura, H. Industrial-Scale Separation of High-Purity Single-Chirality
Single-Wall Carbon Nanotubes for Biological Imaging. Nat. Commun.
2016, 7, 12056.
(2) Heller, D. A.; Jin, H.; Martinez, B. M.; Patel, D.; Miller, B. M.;
̈
Yeung, T.-K.; Jena, P. V.; Hobartner, C.; Ha, T.; Silverman, S. K.;
et al. Multimodal Optical Sensing and Analyte Specificity Using
Single-Walled Carbon Nanotubes. Nat. Nanotechnol. 2009, 4, 114−
120.
(3) Zheng, Y.; Bachilo, S. M.; Weisman, R. B. Quenching of Single-
Walled Carbon Nanotube Fluorescence by Dissolved Oxygen Reveals
Selective Single-Stranded DNA Affinities. J. Phys. Chem. Lett. 2017, 8,
1952−1955.
(4) Harvey, J. D.; Baker, H. A.; Mercer, E.; Budhathoki-Uprety, J.;
Heller, D. A. Control of Carbon Nanotube Solvatochromic Response
to Chemotherapeutic Agents. ACS Appl. Mater. Interfaces 2017, 9,
37947−37953.
(5) Harvey, J. D.; Zerze, G. H.; Tully, K. M.; Mittal, J.; Heller, D. A.
Electrostatic Screening Modulates Analyte Binding and Emission of
Carbon nanotubes. J. Phys. Chem. C 2018, 122, 10592−10599.
(6) Roxbury, D.; Jena, P. V.; Shamay, Y.; Horoszko, C. P.; Heller, D.
A. Cell Membrane Proteins Modulate the Carbon Nanotue Optical
Bandgap via Surface Charge Accumulation. ACS Nano 2016, 10,
499−506.
2570
DOI: 10.1021/acs.jpcc.8b10785
J. Phys. Chem. C 2019, 123, 2565−2572

The Journal of Physical Chemistry C
Article
(7) Zheng, M.; Diner, B. A. Solution Redox Chemistry of Carbon
Nanotubes. J. Am. Chem. Soc. 2004, 126, 15490−1494.
(8) Cathcart, H.; Nicolosi, V.; Hughes, J. M.; Blau, W. J.; Kelly, J.
M.; Quinn, S. J.; Coleman, J. N. Ordered DNS Wrapping Switches on
Luminescence in Single-Walled Nanotube Dispersions. J. Am. Chem.
Soc. 2008, 130, 12734−12744.
(9) Dukovic, G.; White, B. E.; Zhou, Z.; Wang, F.; Jockusch, S.;
Steigerwald, M. L.; Heinz, T. F.; Friesner, R. A.; Turro, N. J.; Brus, L.
E. Reversible Surface Oxidation and Efficient Luminescence
Quenching in Semiconductor Single-Wall Carbon Nanotubes. J.
Am. Chem. Soc. 2004, 126, 15269−15276.
(10) Duque, J. G.; Densmore, C. G.; Doorn, S. K. Saturation of
Surfactant Structure at the Single-Walled Carbon Nanotube Surface. J.
Am. Chem. Soc. 2010, 132, 16165−16175.
(11) Duque, J. G.; Oudjedi, L.; Crochet, J. J.; Tretiak, S.; Lounis, B.;
Doorn, S. K.; Cognet, L. Mechanism of Electrolyte-Induced
Brightening in Single-Wall Carbon Nanotubes. J. Am. Chem. Soc.
2013, 135, 3379−3382.
(12) Niyogi, S.; Densmore, C. G.; Doorn, S. K. Electrolyte Tuning of
Surfactant Interfacial Behavior for Enhanced Density-Based Separa-
tions of Single-Walled Carbon Nanotubes. J. Am. Chem. Soc. 2009,
131, 1144−1153.
(13) Hirana, Y.; Tanaka, Y.; Niidome, Y.; Nakashima, N. Strong
Micro-Dielectric Environment Effect on the Band Gaps of (n,m)
Single-Walled Carbon Nanotubes. J. Am. Chem. Soc. 2010, 132,
13072−13077.
(14) Larsen, B. A.; Deria, P.; Holt, J. M.; Stanton, I. N.; Heben, M.
J.; Therien, M. J.; Blackburn, J. L. Effect of Solvent Polarity and
Electrophilicity on Quantum Yields and Solvatochromic Shifts of
Single-Walled Carbon Nanotube Photoluminescence. J. Am. Chem.
Soc. 2012, 134, 12485−12491.
(15) Wang, R. K.; Chen, W.-C.; Campos, D. K.; Ziegler, K. J.
Swelling the Micelle Core Surrounding Single-Walled Carbon
Nnaotubes with Water-Immiscible Organic Solvents. J. Am. Chem.
Soc. 2008, 130, 16330−16337.
(16) Hirano, A.; Tanaka, T.; Urabe, Y.; Kataura, H. pH- and Solute-
Dependent Adsorption of Single-Wall Carbon Nanotubes onto
Hydrogels: Mechanistic Insights into the Metal/Semiconductor
Separation. ACS Nano 2013, 7, 10285−10295.
(17) Wang, L.; Li, Y. Selective Band Structure Modulation of Single-
Walled Carbon Nanotubes in Ionic Liquids. J. Am. Chem. Soc. 2009,
131, 5364−5365.
(18) Salem, D. P.; Gong, X.; Liu, A. T.; Koman, V. B.; Dong, J.;
Strano, M. S. Ionic Strength-Mediated Phase Transitions of Surface-
Adsorbed DNA on Single-Walled Carbon Nanotubes. J. Am. Chem.
Soc. 2017, 139, 16791−16802.
(19) Ding, J.; Li, Z.; Lefebvre, J.; Du, X.; Malenfant, R. L.
Mechanistic Consideration of pH Effect on the Enrichment of
Semiconducting SWCNTs by Conjugated Polymer Extraction. J. Phys.
Chem. C 2016, 120, 21946−21954.
(20) Nakayama-Ratchford, N.; Bangsaruntip, S.; Sun, X.; Welsher,
K.; Dai, H. Noncovalent Functionalization of Carbon Nanotubes by
Fluorescein-Polyethylene Glycol: Supramolecular Conjugates with
pH-Dependent Absorbance and Fluorescence. J. Am. Chem. Soc. 2007,
129, 2448−2449.
(21) Strano, M. S.; Huffman, C. B.; Moore, V. C.; O’Connell, M. J.;
Haroz, E. H.; Hubbard, J.; Miller, M.; Rialon, K.; Kittrell, C.; Ramesh,
S.; et al. Reversible, band-Gap-Selective Protonation of Single-Walled
Carbon Nanotubes in Solution. J. Phys. Chem. B 2003, 107, 6979−
6985.
(24) Moonoosawmy, K. R.; Kruse, P. Cause and Consequence of
Carbon Nanotube Doping in Water and Aqueous Media. J. Am. Chem.
Soc. 2010, 132, 1572−1577.
(25) Rohlfing, M. Redshift of Excitons in Carbon Nanotubes Caused
by the Environment Polarizability. Phys. Rev. Lett. 2012, 108, 087402.
(26) Berger, S.; Iglesias, F.; Bonnet, P.; Voisin, C.; Cassabois, G.;
Lauret, J.-S.; Delalande, C.; Roussignol, P. Optical Properties of
Carbon Nanotubes in a Composite Material: the Role of Dielectric
Screening and Thermal Expansion. J. Appl. Phys. 2009, 105, 094323.
(27) Hirano, A.; Tanaka, T.; Kataura, H. Thermodynamic
Determination of the Metal/Semiconductor Separation of Carbon
Nanotubes Using Hydrogels. ACS Nano 2012, 6, 10195−10205.
(28) Liu, H.; Tanaka, T.; Urabe, Y.; Kataura, H. High-Efficiency
Single-Chirality Separation of Carbon Nanotubes Using Temper-
ature-Controlled Gel Chromatography. Nano Lett. 2013, 13, 1996−
2003.
(29) Moore, V. C.; Strano, M. S.; Haroz, E. H.; Hauge, R. H.;
Smalley, R. E.; Schmidt, J.; Talmon, Y. Individually Suspended Single-
Walled Carbon Nanotubes in Various Surfactants. Nano Lett. 2003, 3,
1379−1382.
(30) Choi, J. H.; Strano, M. S. Solvatochromism in Single-Walled
Carbon Nanotubes. Appl. Phys. Lett. 2007, 90, 223114.
(31) Strano, M. S.; Moore, V. C.; Miller, M. K.; Allen, M. J.; Haroz,
E. H.; Kittrell, C.; Hauge, R. H.; Smalley, R. E. The Role of Surfactant
Adsorption During Ultrasonication in the Dispersion of Single-Walled
Carbon Nanotubes. J. Nanosci. Nanotechnol. 2003, 3, 81−86.
(32) Silvera-Batista, C. A.; Wang, R. K.; Weinberg, P.; Ziegler, K. J.
Solvatochromic Shifts of Single-Walled Carbon Nanotubes in
Nonpolar Microenvironments. Phys. Chem. Chem. Phys. 2010, 12,
6990−6998.
(33) Zhou, L. L.; Liu, X.; Li, H. P. The Release of Retained Single-
Walled Carbon Nanotubes in Gels. Langmuir 2018, 34, 12224−
12232.
(34) McRae, E. G. Theory of Solvent Effects on Molecular
Electronic Spectra. Frequency Shifts. J. Phys. Chem. 1957, 61, 562−
572.
(35) Nagae, H. Theory of Solvent Effects on Electronic Absorption
Spectra of Rodlike or Disklike Solute Molecules: Frequency Shifts. J.
Chem. Phys. 1997, 106, 5159−5170.
(36) Blanch, A. J.; Quinton, J. S.; Shapter, J. G. The Role of Sodium
Dodecyl Sulfate Concentration in the Separation of Carbon
Nanotubes Using Gel Chromatography. Carbon 2013, 60, 471−480.
(37) Blanch, A. J.; Shapter, J. G. Surfactant Concentration
Dependent Spectral Effects of Oxygen and Depletion Interactions
in Sodium Dodecyl Sulfate Dispersions of Carbon Nanotubes. J. Phys.
Chem. B 2014, 118, 6288−6296.
(38) Ju, S.-Y.; Kopcha, W. P.; Papadimitrakopoulos, F. Brightly
Fluorescent Single-Walled Carbon nanotubes via An Oxygen-
Excluding Surfactant Organization. Science 2009, 323, 1319−1323.
(39) Li, H. P.; Liu, H. Y.; Tang, Y. F.; Guo, W. M.; Zhou, L. L.;
Smolinski, N. Electronically Pure Semiconducting Single-Walled
Carbon Nanotube for Large Scale Electronic Devices. ACS Appl.
Mater. Interfaces 2016, 8, 20527−20533.
(40) Li, H. P.; Tang, Y. F.; Guo, W. M.; Liu, H. Y.; Zhou, L. L.;
Smolinski, N. Polyfluorinated Electrolyte for Fully Printed Carbon
Nanotube Electronics. Adv. Funct. Mater. 2016, 26, 6914−6920.
(41) Acetic Acid in Wikipedia homepage. https://en.wikipedia.org/
wiki/Acetic_acid/ (accessed December 8, 2018).
(42) Chakraborty, S.; Shukla, D.; Jain, A.; Mishra, B.; Singh, S.
Assessment of Solubilization Characteristics of Different Surfactants
for Carvedilol Phosphate As A Function of pH. J. Colloid Interface Sci.
2009, 335, 242−249.
(43) Cholic Acid in Drugbank homepage. https://www.drugbank.
ca/drugs/DB02659/ ( accessed December 8, 2018).
(22) Blackburn, J. L.; McDonald, T. J.; Metzger, W. K.; Engtrakul,
C.; Rumbles, G.; Heben, M. J. Protonation Effects on the Branching
Ratio in Photoexcited Single-Walled Carbon Nanotube Dispersions.
Nano Lett. 2008, 8, 1047−1054.
(23) Wang, D.; Chen, L. Temperature and pH-Responsive Single-
Walled Carbon Nanotube Dispersions. Nano Lett. 2007, 7, 1480−
1484.
(44) Li, H. P.; Zhou, L. L. Visualizing Helical Wrapping of
Semiconducting Single-Walled Carbon Nanotubes by Surfactants and
Their Impacts on Electronic Properties. Chem. Select 2016, 1, 3569−
3572.
2571
DOI: 10.1021/acs.jpcc.8b10785
J. Phys. Chem. C 2019, 123, 2565−2572

The Journal of Physical Chemistry C
Article
(45) White, B.; Banerjee, S.; O’Brien, S.; Turro, N. J.; Herman, I. P.
Zeta-Potential Measurements of Surfactant-Wrapped Individual
Single-Walled Carbon Nanotubes. J. Phys. Chem. C 2007, 111,
13684−13690.
(46) Sun, Z.; Nicolosi, V.; Rickard, D.; Bergin, S. D.; Aherne, D.;
Coleman, J. N. Quantitative Evaluation of Surfactant-stabilized Single-
walled Carbon Nanotubes: Dispersion Quality and Its Correlation
with Zeta Potential. J. Phys. Chem. C 2008, 112, 10692−10699.
(47) Shastry, T. A.; Morris-Cohen, A. J.; Weiss, E. A.; Hersam, M. C.
Probing Carbon Nanotube−Surfactant Interactions with Two-
Dimensional DOSY NMR. J. Am. Chem. Soc. 2013, 135, 6750−6753.
(48) Oh, H.; Sim, J.; Ju, S.-Y. Binding Affinities and Thermody-
namics of Noncovalent Functionalization of Carbon Nanotubes with
Surfactants. Langmuir 2013, 29, 11154−11162.
(49) Flavel, B. S.; Kappes, M. M.; Krupke, R.; Hennrich, F.
Separation of Single-Walled Carbon Nanotubes by 1-Dodecanol-
Mediated Size-Exclusion Chromatography. ACS Nano 2013, 7, 3557−
3564.
(50) Zeng, X.; Yang, D.; Liu, H.; Zhou, N.; Wang, Y.; Zhou, W.; Xie,
S.; Kataura, H. Detecting and Tuning the Interactions Between
Surfactants and Carbon Nanotubes for Their High Efficiency
Structure Separation. Adv. Mater. Interfaces 2018, 5, 1700727.
2572
DOI: 10.1021/acs.jpcc.8b10785
J. Phys. Chem. C 2019, 123, 2565−2572