Chirality-Selective Functionalization of Semiconducting Carbon Nanotubes with a Reactivity-Switchable Molecule

Article abstract of DOI:10.1021/jacs.7b05906

Chirality-selective functionalization of semiconducting single-walled carbon nanotubes (SWCNTs) has been a difficult synthetic goal for more than a decade. Here we describe an on-demand covalent chemistry to address this intriguing challenge. Our approach

Full text of DOI:10.1021/jacs.7b05906

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Article
Chirality-Selective Functionalization of Semiconducting
Carbon Nanotubes with a Reactivity-Switchable Molecule
Lyndsey R Powell, Mijin Kim, and YuHuang Wang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b05906 • Publication Date (Web): 27 Aug 2017
Downloaded from http://pubs.acs.org on August 27, 2017
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Chirality-Selective Functionalization of Semiconducting Carbon Nanotubes
with a Reactivity-Switchable Molecule
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Lyndsey R. Powell#, Mijin Kim#, YuHuang Wang#†*
# Department of Chemistry and Biochemistry, University of Maryland, 8051 Regents Drive,
College Park, Maryland 20742, United States
† Maryland NanoCenter, University of Maryland, College Park, Maryland 20742, United States
* yhw@umd.edu
ABSTRACT: Chirality-selective functionalization of semiconducting single-walled carbon
nanotubes (SWCNTs) has been a difficult synthetic goal for more than a decade. Here we describe
an on-demand covalent chemistry to address this intriguing challenge. Our approach involves the
synthesis and isolation of a chemically inert diazoether isomer that can be switched to its reactive
form in situ by modulation of the thermodynamic barrier to isomerization with pH and visible light
that resonates with the optical frequency of the nanotube. We found that it is possible to completely
inhibit the reaction in the absence of light, as determined by the limit of sensitive defect
photoluminescence (less than 0.01% of the carbon atoms are bonded to a functional group). This
optically-driven diazoether chemistry makes it possible to selectively functionalize a specific
SWCNT chirality within a mixture. Even for two chiralities that are nearly identical in diameter
and electronic structure, (6,5)- and (7,3)-SWCNTs, we are able to activate the diazoether
compound to functionalize the less reactive (7,3)-SWCNTs, driving the chemical reaction to near
exclusion of the (6,5)-SWCNTs. This work opens opportunities to chemically tailor SWCNTs at
the single chirality level for nanotube sorting, on-chip passivation, and nanoscale lithography.
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INTRODUCTION
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Single-walled carbon nanotubes (SWCNTs) are molecular cylinders of sp2 carbon that can
take the form of many different structures known as chiralities.1 In a typical synthetic mixture,
approximately two-thirds of the chiralities are semiconductors and the remainder is made up of
metals. Chirality-selective chemistry has long been sought after as the key to sorting by chirality,2
on-chip passivation,3 and nanotube lithography.2 For semiconducting SWCNT-based
optoelectronic and biomedical devices, even a small amount of heterogeneity can dramatically
hinder their performance.4-5 Chirality selective chemistry may provide a chemical route to
selectively block conductive channels in nanotube arrays directly on-chip.3 The selective
chemistry would also add a new layer of chemical control that is orthogonal to existing methods
for improving resolution between difficult-to-separate SWCNT chiralities.6-9
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Strano and co-workers first reported that metallic SWCNTs can react with aryldiazonium
salts to near exclusion of the semiconducting chiralities.2 This selectivity for metallics arises from
the abundant available electron density of metallic SWCNTs as opposed to the semiconductors.
The metal-semiconductor selectivity can be improved by tuning the relative redox potential of the
bonding moiety against that of the SWCNT,10 or by tuning the reaction conditions.11-12 For
example, the addition of electron-donating groups to the aryldiazonium bonding moiety lowers the
electron affinity of the diazonium salts, suppressing their reaction with semiconducting SWCNTs
and consequently improves type selectivity.13 Recently, Darchy et al.10 achieved a record 50-fold
improvement in metal-semiconductor selectivity by using a diazoether compound, which has a
significantly higher oxidation potential than aryldiazonium salts. These advances highlight the
significant efforts in developing selective covalent chemistry for SWCNTs; however, the
selectivity has generally been limited to metals vs. semiconductors. Achieving chemical selectivity
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at the chirality level has remained a significant challenge. Among semiconductors, smaller
diameter chiralities with larger bandgaps are more reactive toward aryldiazonium salts.2, 14-15
Although extensive covalent functionalization can significantly disrupt the optical16 and
transport characteristics of SWCNTs, controlled chemical modification of SWCNTs can be
minimally disruptive to transport properties17 and impart new functionality by creating molecularly
tunable fluorescent quantum defects.18-19 These chemically implanted quantum defects trap mobile
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-
excitons, allowing them to relax radiatively via a new emission state, E11 , significantly brightening
the near-infrared (NIR) photoluminescence (PL) of SWCNTs.18 The defect PL originating from
this state is highly tunable, in both intensity and energy, by varying the chemical nature of the
defect group.18-19 This allows the defect PL to be used as a sensitive quantitative indicator of
functional degree that can be used to monitor the surface chemistry of SWCNTs in situ.20
Herein, we show that it is possible to isolate covalent bonding within a mixture of chiralities
to that of a chirality which is otherwise less reactive, achieving unprecedented chiral selectivity.
We overcome the inherent trends in SWCNT reactivity by using a stereoisomer of a diazoether
compound and directing the reaction to the target SWCNT in two ways. First, we control the
thermodynamic barrier to isomerization of the diazoether by tuning the pH of the environment.
Second, we select the target SWCNT chirality for functionalization by exciting the nanostructure
at one of its van Hove transitions.20-21 As an illustration of this concept, we synthesized a pair of
diazoether isomers, (E)- and (Z)-3-O-p-nitrobenzenediazoascorbic acid (p-NO2-DZE), and
exploited their distinct reactivity toward semiconducting SWCNTs (Figure 1). Ultimately, we
show that this diazoether chemistry can be used in conjunction with our optical selection technique
to isolate chemistry to a single SWCNT chirality within a mixture of semiconducting species.
Particularly, we demonstrated preferential functionalization of (7,3)-SWCNT over the more
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reactive (6,5)-SWCNT. This on-demand reactivity switching has not been previously reported, for
SWCNTs in particular, and is further exploited here to achieve chirality-selective functionalization
within a mixture of structurally similar semiconducting SWCNTs.
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Figure 1. On-demand switching of diazoether reactivity toward a SWCNT generates a
covalently bonded aryl functional group. Localized isomerization of the inert E-isomer of 3-O-
p-nitrobenzenediazoascorbic acid to the reactive Z-isomer is achieved by tuning of the
thermodynamic barrier to isomerization with pH and driven by resonant optical excitation of the
SWCNT substrate. Isomerization from E to Z results in covalent attachment of a nitrobenzene
functional group to the sp2-hybridized carbon lattice, creating a fluorescent quantum defect that
fluoresces in the NIR.
MATERIALS AND METHODS
Preparation of chirality-enriched SWCNT suspensions. (6,5)-SWCNT enriched samples were
separated from HiPco materials (batch no. 194.3, Rice University) by gel chromatography8 using
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a Sephacryl S-200 HR column (GE Healthcare). The sorted SWCNTs were individually suspended
by 1% wt./v sodium dodecyl sulfate (SDS) in 99.8% deuterium oxide (D2O, Cambridge Isotopes,
Inc.). The absorbance of the (6,5)-SWCNT solution was adjusted to 0.12 (1 cm optical path length)
at E11, corresponding to a carbon concentration of approximately 0.66 mg L-1.
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(7,3)-SWCNT enriched samples were isolated from CoMoCAT SWCNTs (SG65i, lot no.
SG65i-L39, Southwest Nanotechnologies, Inc.) by aqueous two-phase extraction7 using single-
stranded DNA oligomers with sequence TCT(CTC)2TCT (purified by standard desalting,
Integrated DNA Technologies). After extraction of the (7,3)-SWCNT, the DNA was precipitated
by the addition of sodium thiocyanate (98%, Sigma Aldrich) and then removed after purification.
The remaining SWCNTs were suspended in 1% wt./v SDS in D2O.
Synthesis of 3-O-p-nitrobenzenediazoascorbic acid. 3-O-p-nitrobenzenediazoascorbic acid was
isolated using a modified procedure from Doyle et al.22 First p-nitrobenzenediazonium
tetrafluoroborate was synthesized as we previously described20 from p-nitroaniline (99%, Sigma
Aldrich) and nitrous acid (48 wt.% tetrafluoroboric acid solution in water, Sigma Aldrich; sodium
nitrite, 97%, Sigma Aldrich). The diazonium salt was coupled to L-ascorbic acid (99%, Sigma
Aldrich) at room temperature, the solvent was removed, and the yellow solid precipitate was dried
and recrystallized from 2-butanone (99%, Sigma Aldrich) with dichloromethane (99.5%, Sigma
Aldrich) at room temperature. See Supporting Information for details. Samples were stored under
argon at 0 °C. The structure was confirmed by 1H-nuclear magnetic resonance spectroscopy (1H-
NMR, Figure S1 and Table S1). The diazoether was prepared for experiments by dissolving ~1
mg of a solid in 0.5 mL of acetonitrile-D3 (99.8%, Cambridge Isotope Laboratories) in a 5 mm
diameter NMR tube (NORELL, 507HP). The analysis was performed on a Bruker DRX-400 high
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resolution spectrometer. The spectrum was analyzed and coupling constants were calculated with
MestReNova software (v9).
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Optically-activated functionalization of SWCNTs with 3-O-p-nitrobenzenediazoascorbic
acid. An aliquot of 50 µM 3-O-p-nitrobenzenediazoascorbic acid in D2O was added to a
suspension of chirality-enriched SWCNT in a 1 cm quartz cuvette, which was stirred and protected
from light at 21 °C ([p-NO2-DZE] : [SWCNT carbon] = 1:550). The temperature of the solution
was maintained throughout the experiment using a circulating water bath (FL-1027, Horiba Jobin
Yvon and Fisher Scientific Isotemp 3016D). The addition of the ascorbate diazoether solution
amounted to less than a 0.3% increase in volume, which avoided significantly changing the
SWCNT concentration. Any pH adjustment was completed before addition of the diazoether by
the addition of aliquots of 50 μM H3PO4 or 50 μM NaOH in D2O. [H+] was monitored with a pH
meter (Accumet AB15 pH/mV/temperature meter) and corresponding electrode (Accumet gel-
filled pencil-thin pH electrode) which was calibrated against buffer solutions. The capped solution
was stirred for 20 min with protection from light before being continuously irradiated with the
monochromator-selected (10 nm bandpass; 565 nm or 505 nm as noted in the text) light from a
450 W Xenon arc lamp (Ushio) for 5 h. The power density was measured with an optical power
meter (Newport 1916-C) and silicon detector (Newport 918-SL-OD3). The reactions were
protected from ambient light throughout the extent of the experiment and the reaction progress
was monitored spectroscopically by following the evolution of defect PL.
Thermal effects. The pH of a suspension of (6,5)-SWCNT was adjusted by the addition of aliquots
of 50 μM NaOH or 50 μM H3PO4 and measured with a pH meter and electrode (Accumet pH/ATC
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electrode). Next, a circulating water bath was used to control the temperature of the solution within
a capped 1 cm quartz cuvette throughout the reaction (either 21 °C or 70 °C). The solution
temperature was measured with a LabQuest 2 module and temperature sensor (Vernier). Then, an
aliquot of 50 µM 3-O-p-nitrobenzenediazoascorbic acid in D2O was added to the SWCNT
suspension ([p-NO2-DZE] : [SWCNT carbon] = 1:550) with stirring and protection from light.
After 5 h, the reaction was characterized spectroscopically (pH adjusted back to 7.5, see Methods).
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Spectroscopic characterization of covalent functionalization. Fluorescence spectra and
excitation-emission maps were collected on a HORIBA Jobin Yvon Nanolog Spectrofluorometer
equipped with a 450 W Xenon arc lamp and a calibrated multichannel InGaAs array detector in a
1 cm quartz bi-mirrored cuvette. All PL measurements reported were taken at neutral pH (~7.5)
after adjusting the pH of solutions with small volumes of 50 μM NaOH or H3PO4 solutions in D2O.
Measurements were taken after 5 h of reaction. The reported PL intensities are peak intensities
(not integrated areas) unless otherwise noted. UV-vis-NIR absorption spectra were measured in
the same way using a PerkinElmer Lambda 1050 UV-vis-NIR spectrophotometer equipped with a
photomultiplier tube and a broadband InGaAs detector.
We recognize that PL intensity is pH dependent and took steps to characterize and mitigate
its effects. Upon the addition of H3PO4, SWCNT PL features were rapidly quenched (Figure S2),
but were recovered upon the addition of NaOH. The minimal loss of PL intensity after titrations
may be due to the addition of water and salts from the acid-base reactions in the otherwise pure
D2O solutions.
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Chirality-selective functionalization within a mixture of (6,5)- and (7,3)-SWCNTs. To create
a well-defined model system, we prepared chirality-enriched solutions of (6,5)- and (7,3)-
SWCNTs suspended in SDS and dissolved in D2O, as previously described. We then mixed equal
amounts of each solution together, using their absorbance at E11 as an indicator (Figure S3a).
The pH of the SWCNT suspension was adjusted to 3.8 with H3PO4 in D2O. From this
solution, we made three 1 mL samples to which an aliquot of 3-O-p-nitrobenzenediazoascorbic
acid in D2O was added to achieve a molar ratio of 1:500 [p-NO2-DZE]:[SWCNT carbon]). One of
the samples was protected from light for 4 h, while the other two were either irradiated with 505
nm (E22 of (7,3)-SWCNT) or 565 nm (E22 of (6,5)-SWCNT) light for the same time period. The
intensity of the light was approximately the same (9.9 mW cm-2 vs. 9.5 mW cm-2 for 505 nm and
565 nm, respectively) for each irradiated sample. Then the samples were titrated with NaOH to
pH 7.4 for the PL measurements.
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Computational methodology. Geometry optimization and frequency calculations were
performed for the E- and Z-isomers of 3-O-p-nitrobenzenediazoascorbic acid at either protonation
state of the O-2 atom (where the total charge of the molecule, q, is 0 and -1), using density
functional theory (DFT) at the B3LYP/ 6-31G+(d,p) level of theory as implemented in the
Gaussian 09 software package.23 Solvent effects were included by creating a solute cavity via a set
of overlapping spheres in the framework of the polarizable continuum model using the integral
equation formalism variant (IEFPCM).24 Water (ε = 78.3553) was chosen as the solvent media.
By normal mode analysis, only real (positive) vibrational frequency values were obtained for all
isomers (E and Z) in both protonation states (q = 0 and q = -1), demonstrating that the optimized
geometries (Figure S4) correspond to the minima on the potential energy surface. The E-isomers
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had N-O bond lengths of 1.39 and 1.41 Å in the cases of protonation and deprotonation,
respectively. However, we found that the optimized geometries of the Z-isomer have extended N-
O bond lengths (2.45 Å and 2.37 Å in the case of the protonated and deprotonated structures,
respectively) corresponding to two non-covalently bonded species, ascorbate and p-
nitrobenzenediazonium, as opposed to the covalently bonded Z-isomer.
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In order to find the saddle point, we fixed the N-O bond distance, decreasing its length
from the initially determined value to 1.4 Å (0.100 Å and 0.050 Å step size in the cases of
protonation and deprotonation, respectively), while allowing all the internally redundant
coordinates to relax, using the same functional and basis set (Figure S5). We found that at ~1.7 Å,
the total energy of the molecule reached a maximum, beyond which the geometry minimized to
the predicted sandwich-like Z-isomer. A constrained optimization of the geometry at this critical
bond length was performed by applying tight convergence criteria to the calculation, resulting in
N-O bonds lengths of 1.73 and 1.70 in the cases of protonation and deprotonation, respectively for
the Z-isomers. The frequency analysis revealed one imaginary (negative) vibrational frequency.
The imaginary frequency is attributed to the stretching of the N-O bond, suggesting that this bond
is only weakly covalent resulting in the instability of the Z-isomer geometry. We calculated the
thermodynamic values for the E-to-Z isomerization using the geometries of the Z-isomers
calculated in this manner (Table 1). The equilibrium constants at 21 °C and 70 °C were calculated
using these calculated thermochemical values and the van ‘t Hoff equation.
The thermodynamic barrier to isomerization for the diazoether isomers, ∆ꢀꢁꢃꢂꢃ, at either
protonation state was computed following eq. (10):
∆ꢀꢁꢃꢂꢃ ꢄ ꢅꢆꢇ ꢈ ꢉꢊꢁꢋꢋꢂꢌꢍ ꢎ ꢅꢆꢇ ꢈ ꢉꢊꢁꢋꢋꢂ ꢏ
ꢌ
(1)
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in which ꢆꢇ is the electronic energy and ꢉꢊꢁꢋꢋꢂ is a correction factor for Gibbs energy due to the
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ꢅ
ꢌ
internal energy. The ꢆꢇ ꢈ ꢉꢊꢁꢋꢋꢂ values are directly obtained from ground state frequency
calculations. Similarly, the enthalpy of the isomerization was derived using the corresponding
correction factor. These values are tabulated in Table 1.
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RESULTS AND DISCUSSION
We synthesized a pair of 3-O-p-nitrobenzenediazoascorbic acid (p-NO2-DZE) isomers by
coupling an aryldiazonium salt to ascorbate (Figure 2a). First isolated by Doyle et al. in 1989,22
the E-isomer of this ascorbate diazoether compound is stabilized in comparison to other O-
coupling derivatives of aryldiazonium by its pseudo-ring structure.25-27 In order to determine the
reactivity of the two p-NO2-DZE isomers with semiconducting SWCNTs, we added an aliquot of
a solution containing either the Z- or E-diazoether to a suspension of HiPco SWCNTs that had
been enriched in the (6,5) chirality by gel chromatography and monitored the reaction progress via
PL spectroscopy. The chirality enrichment of the SWCNT suspension results in a well-defined
-
system for clear observation of the PL from both the intrinsic (E11) and defect (E11 ) excitonic
emission states that allowed us to monitor the reaction progress. The results from spectroscopic
monitoring of the reaction show that the Z-isomer readily reacts with (6,5)-SWCNT to introduce
covalently bonded p-nitroaryl functional groups to the carbon surface (Figure 2b), which is in
contrast to a previous report where the reactivity of the Z-isomer was not observed with
semiconducting SWCNTs.10 Upon reaction with the Z-isomer, defect PL from the E11- state (1141
nm) was observed, redshifted from that of the E11 exciton emission (980 nm). Conversely, when a
similar experiment is performed using the E-isomer, defect PL was not observed (Figure 2c).
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Figure 2. Diazoether stereoisomers exhibit distinct pH-dependent reactivity toward
semiconducting SWCNTs. (a) Schematic showing the O-coupling reaction between ascorbate
and p-nitrobenzenediazonium tetrafluoroborate (p-NO2-BDT) to synthesize the diazoether
isomers. The atoms on ascorbate are numbered for clarity. (b) The Z-isomer reacts readily with
-
(6,5)-SWCNTs (pH 6.4, green), as determined by monitoring the evolution of the defect PL (E11 )
at 1141 nm (λex = 565 nm). (c) In contrast, the E-isomer is inert with SWCNT (blue) unless
activated by SWCNT-resonant irradiation (λex = 565 nm, pH 3.8, red). (d) The optical reactivity
of the E-isomer can be tuned with pH (red filled circles) as determined by the PL intensity of the
-
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E11 state. The PL intensities of E11 for each experiment were normalized to that of the highest
value for comparison. All PL measurements were taken at pH 7.5, regardless of the pH condition
during irradiation. The kinetic barrier of Z-to-E isomerization (activation energy, Ea, of
isomerization, open circles) does not vary significantly with pH as determined by pH- and
temperature-dependent kinetic experiments.
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Despite the E-isomer’s apparent inactivity within the limits of our sensitive spectral
technique, we found that we can isomerize it to the reactive Z-isomer by optically exciting the
SWCNT within a particular pH range (Figure 2c). After the reactants were irradiated with 565 nm
light for 5 h (pH 3.8, 21 °C), we observed strong defect PL. Notably, while the 565 nm light
resonates strongly with the E22 excitonic transition of (6,5)-SWCNT, it does not resonate with the
diazoether compound as is evident from the optical absorption spectra of the reactants (Figure
S6a). We also note that care was taken to avoid pH-dependent PL effects in this work by taking
PL measurements at the same and approximately neutral pH (see Experimental Methods). The
optical selection technique suggests reaction specificity can be obtained, localized to a certain
chirality via the resonant excitation, as discussed in our previous work with diazonium salts.20
Monitoring the temporal evolution of the defect PL was complicated by the quenching effect of
the acid-base reaction products necessary for consecutive PL measurements. For this reason,
extensive kinetic studies were not completed, but we do show that defect PL evolves with time in
Figure S7 in the case of the photo-activated reaction of (6,5)-SWCNT with the E-isomer of the
diazoether at pH 3.69.
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We explored the reaction’s dependence on pH and found that the reaction maximizes at pH
3.8 about a narrow range (Figure 2d). In order to understand the origin of this pH-dependence, we
investigated the energetic barriers associated with Z-to-E isomerization. We experimentally
determined the kinetic barrier to isomerization (Ea) by monitoring the coupling reaction in aqueous
solution in situ by UV-visible absorption spectroscopy. These measurements revealed the
isomerization of the Z-isomer of p-NO2-DZE (λmax = 290 nm) to the thermodynamically stable E-
isomer (λmax = 360 nm) (Figure S6). We monitored spectra changes associated with the
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isomerization at temperatures ranging from 12 °C to 27 °C and pHs spanning the E-isomer’s
reactivity window, from 1.70 to 4.26.
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We used a first-order kinetic model for a series reaction to describe the Z-to-E
isomerization and found that the kinetic barrier to isomerization did not vary significantly with pH
within the reactivity range of the E-isomer (Figure 2d). While the reaction proceeds via two steps,
the rate constants of which are given by ꢐꢑ, and ꢐꢁ, we observed that the formation of the Z-isomer
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(first coupling step) is much faster than the isomerization step, such that ꢐꢑ ꢒꢒ ꢐꢁ, suggesting that
isomerization is the rate-limiting step so that it can be described independently. We note that while
Darchy et al.10 also models the isomerization step using a similar first order model, we derived a
different equation which takes into account the spectral overlap between the two isomers and
expanded the study to uncover the pH dependence of the kinetic barrier as described here. The
related rate expression for the isomerization step is given by eq. (2).
ꢓꢔꢍꢕ
ꢔ ꢕ
ꢄ ꢎꢐꢁ ꢗ
(2)
ꢓꢖ
ꢔ ꢕ
in which ꢗ is the concentration of the Z-isomer and ꢘ is time. The concentration of the Z-isomer
at time ꢘ is given by eq. (3), which accounts for the spectral overlap of the two isomer’s absorbance
peaks (see Supporting Information).
ꢙꢚꢔꢗꢕꢖ ꢄ ꢙꢚꢅ3.259ꢛꢖꢅꢗꢌ ꢎ ꢛꢖꢅꢜꢌꢌ ꢈ ꢉ
(3)
in which ꢉ is a constant. By plotting ꢙꢚꢔꢗꢕꢖ versus time, we obtained highly linear relationships
(Figure S2). The rate constants for Z-to-E isomerization were inferred from the linear regressions
of these plots. We applied Arrhenius theory, eq. (4), to determine the kinetic barrier to
isomerization, activation energy at various pH, ꢜꢝꢅꢞꢟꢌ.
ln ꢐꢁ ꢄ ꢎ ꢏ ꢅꢡꢢꢌ ꢤꢦꢥꢧ ꢈ ln ꢛ
(4)
ꢠ
ꢣ
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in which ꢨ is the gas constant, ꢩ is temperature and ꢛ is the pre-exponential factor. By plotting
ln ꢐꢁ versus ꢩꢪꢥ, we were able to extract ꢜꢝꢅꢞꢟꢌ from the slopes of the linear regressions. We
experimentally determined this barrier to be a deep energetic trap for the E-isomer, namely (80 ±
3) kJ mol-1 at pH 1.70, which is in accord with a previous report.10 We studied the pH dependence
of this barrier and found that while the isomerization rate constants varied considerably with pH
and temperature, the kinetic barrier itself was stable despite the change in reactivity of the E-isomer
within this range (Figure S8). We did not expand our kinetic study beyond pH 4.26 because
isomerization occurred too quickly to reliably monitor at elevated temperatures, which resulted in
significant uncertainty in the calculated energy values. Isomerization from the E-to-Z isomer was
not observed experimentally, possibly reflecting a significantly larger kinetic barrier for this
process.
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The photo-induced reaction could be switched on and off by alternating the pH of the
solution. In Figures 3 and S9, we demonstrate this high level of control by switching the pH of the
reaction between 3.8 and 7.4 (i.e., the reactive and inactive reaction regimes, respectively) and
observed a corresponding pattern in the evolution of defect PL when the sample was irradiated,
revealing the responsive nature of the thermodynamic barrier to isomerization. Interestingly, even
as we changed the [H+], we did not detect any defect PL when the sample was protected from light,
which indicated the importance of the resonant optical excitation of the nanostructure for the
reaction to occur.
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Figure 3. The stable E-diazoether can be activated via light and pH to functionalize
SWCNTs. Step-wise reactivity of the E-isomer with (6,5)-SWCNTs was achieved with irradiation
(565 nm, red) and pH switching (grey) as compared to the dark control (blue). All PL
measurements were obtained at pH 7.5 regardless of the pH condition during each reaction interval.
In order to better understand whether the role of irradiation on the reaction of SWCNT with
the E-isomer of diazoether is to simply induce a bulk heating effect rather than a localized, chirality
specific one, we explored the pH dependence upon heating (Figures 4 and S10). Solutions of (6,5)-
SWCNT and the E-isomer at different [H+] were reacted at either 21 °C or 70 °C with protection
from light throughout reaction. Their PL was measured after adjusting the pH to 7.5 and cooling
the solution down to 21 °C. Interestingly, no defect PL was observed when the reaction was
maintained at 21 °C. Upon heating to 70 °C, the pH dependence broadened significantly, in
comparison to the photo-controlled reaction, and maximized at a pH of ~7.5.
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Figure 4. pH dependence of the thermal reaction of (6,5)-SWCNTs with the E-diazoether.
The dependence of defect PL on solution pH for suspensions of (6,5)-SWCNT reacted with the E-
diazoether and were protected from light at either 21 °C (filled circles) or 70 °C (open circles).
The pH-dependence is significantly broadened compared to that of the photo-controlled reaction
maximum.
The observed pH dependence of the E-isomer’s reactivity may reflect the pH-dependence
of isomerization energy from the inactive E-isomer to the reactive Z-isomer. The E-isomer of p-
NO2-DZE is stabilized in a pseudo-ring structure via hydrogen bonding between the Nα and the
proton at the O-2 atom,25 the pKa of which is 10.2, as shown in Figure S11a.22 The Z-isomer of
this particular diazoether consists of a sandwich-like structure with an extended N-O bond.10 This
thermodynamically unstable structure is prone to cleavage (as evidenced by its reactivity with
SWCNT shown here) or quickly isomerizes to the more stable E-isomer. While the kinetic barrier
to isomerization does not vary significantly with pH as previously discussed, the stability of the
E-isomer, versus that of Z-isomer, may be reflected in the thermodynamic barrier to isomerization.
Furthermore, because the stabilizing pseudo ring structure of the E-isomer is dependent on
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hydrogen bonding, it is hypothesized that its stability may be tuned with H+, which is utilized to
control its reactivity as explored in this work.
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Density functional theory (DFT) calculations reveal that the thermodynamic barrier to
isomerization, Gibbs free energy (ꢫꢀꢁ°ꢂꢃ), from E to Z is strongly dependent on the protonation
state of the O-2 atom (Figure 5). We performed geometry optimization and frequency calculations
of the isomers to obtain the isomers’ ground state energies under different protonation states (Table
1, see Materials and Methods). We found that protonation of the O-2 atom reduced the ꢫꢀꢁ°ꢂꢃ by
58%, from 50 kJ mol-1 to 21 kJ mol-1. This explains the strong pH dependence observed from our
optical excitation experiments performed at 21 °C, where the reaction was most efficient at low
pH, maximizing at pH 3.8, where the O-2 atom is protonated.
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Figure 5. Calculated Gibbs free energies of isomerization. The magnitude of the
thermodynamic barrier to isomerization (∆ꢀꢁꢂꢃ) is strongly dependent on whether the O-2 atom of
the diazoether is (a) deprotonated or (b) protonated. Energies were calculated using DFT at the
B3LYP/6-31G+(d,p) level of theory. The y-axes are the sum of the electronic energy (ꢆꢃ) and the
correction factor for Gibbs free energy (ꢉꢊꢁꢋꢋꢂ) and have the same magnitude (see Materials and
Methods). The pink arrows in (b) denote the protons. Calculated values are indicated by solid lines,
and dashed lines show putative isomerization through the transition state, whose peak height was
obtained from experiment, see Supporting Information).
We also calculated the enthalpy (ΔH°) and entropy (ΔS°) of isomerization under the two
protonation states to better understand the effect of heating on the reaction’s pH dependence.
Experimentally, we observed significant broadening of the reaction’s pH dependence at 70 °C (vs.
21 °C). The van ‘t Hoff plots (Figure S11b) show negative slopes, confirming that the E-to-Z
isomerization is thermodynamically unfavorable for both high and low pHs. However, in their
deprotonated states, the isomerization of E-to-Z is more sensitively dependent on temperature and
the calculated equilibrium constant of isomerization, ꢬꢭꢮ, increases more significantly with
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temperature, which is consistent with a 2.4-fold increase in the slopes of the van’t Hoff plot upon
deprotonation. These experimental observations are in good agreement with the values derived
from the computation (Table 1). The strong temperature dependence suggests that optically-
induced local heating28 plays an important role in triggering the reaction by thermally switching
physically absorbed E-diazoether to its reactive Z-form.
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Table 1. Calculated thermochemical values of E-to-Z isomerization of 3-O-p-
nitrobenzenediazoascorbic acid at either protonation state of O-2. Gibbs free energies,
enthalpies, and entropies of E-to-Z isomerization were obtained from thermochemical calculations
using the DFT method at the B3LYP/6-31G+(d,p) level of theory. ꢬꢭꢮ is the equilibrium constant
corresponding to [Z-isomer]:[E-isomer].
Protonated (q = 0)
Deprotonated (q = -1)
ΔHiso (kJ mol-1)
ΔGiso (kJ mol-1)
ΔSiso (J mol-1·K-1)
Keq at 21 °C
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-25
2.3 x 10-4
5.9 x 10-4
1.6 x 10-9
2.0 x 10-8
Keq at 70 °C
We further exploited the observed photo-activated and pH-dependent reactivity for
chirality selective covalent functionalization of semiconducting SWCNTs (Figures 6 and S12). As
a demonstration, we irradiated suspensions enriched in the (6,5)- and (7,3)-SWCNT chiralities
using light that resonated with only one of the structures. We aimed to drive covalent bonding of
the E-isomer on the surface of this target chirality and to locally switch its reactivity via chirality-
specific photoexcitation. We chose a system consisting of the (6,5)- and (7,3)-SWCNT chiralities
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for this experiment because their E22 optical transitions are well-resolved (505 nm vs. 565 nm for
(7,3) and (6,5), respectively), allowing for unambiguous illustration of the high optical selectivity
of the reaction. We note that relative SWCNT reactivity is correlated with diameter (0.706 nm vs.
0.757 nm, for (7,3) and (6,5), respectively),29 E11 optical frequency (990 nm vs. 976 nm, for (7,3)
and (6,5), respectively),15 and redox potentials. Specifically, (7,3)-SWCNT is less reactive with
aryldiazonium salts than (6,5)-SWCNT.
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Figure 6. Chirality selective functionalization of SWCNTs within a mixture using the E-
diazoether. (a) Redox potentials (black lines) of the (6,5)-SWCNT,31 (7-3)-SWCNT, and the E-
diazoether.10 The redox potential of the (7,3)-SWCNT was deduced based on the relative band gap
energy. (b) Absorption spectra of SDS-suspended (6,5)- and (7,3)-SWCNTs in D2O. The E22 peaks
of these two chiralities are sufficiently separated so that they can be optically excited exclusive of
each other and the E-diazoether (blue line, 50 μM aqueous solution, x 1/12 for clarity). Resonant
PL emission spectra resulting from excitation at 565 nm (top) and 505 nm (bottom) of each sample,
which is either (c) the unfunctionalized mixture or irradiated with (d) 565 nm or (e) 505 nm light.
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Surprisingly, we observed that the less reactive (7,3)-SWCNT is selectively functionalized
vs. the more reactive (6,5)-SWCNT using this chemistry and (7,3)-resonate photoexcitation
(Tables S2 and S3). When irradiated with light that resonates with the more reactive (6,5) chirality
(565 nm), the reaction was limited exclusively to the (6,5) chirality. In fact, no emission from the
(7,3) defect state (1165 nm) was detected, indicating that covalent functionalization of the (7,3)-
SWCNT did not occur (within the detection limit of our spectrofluorometer, which corresponds to
less than 0.01% of the carbon atoms bonded to a functional group). Conversely, when irradiated
with light that resonates the (7,3) species, we successfully drove functionalization of this less
reactive chirality. In this case, the defect PL from the (6,5) chirality was only ~7.2% of that
detected in the 565 nm irradiation experiment. It is also important to note that when the binary
mixture was protected from light, no defect PL was detected (Figure S13). This indicates that the
diazoether chemistry is much more controllable than the light-driven diazonium reaction
previously reported by us20 and oxidation by Star et al.21
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We note that our experimental method, namely the measurement of the defect PL, is
associated with some uncertainty in absolute quantification of chirality selectivity. While the
relative PL quantum yield (QY) of the first optical transition (E11) for (6,5)-SWCNT is higher than
that of (7,3)-SWCNT,30 the PL QY of defect PL is not yet quantitatively correlated with the degree
of functionalization. Characterization by other spectral methods was impeded by their potential to
induce functionalization from the intense incident light (e.g., Raman scattering) and the extremely
low functional degree of the SWCNT, which lies below the limit of detection of readily available
methods.18 The PL excitation spectra (Figure S3b,c) may serve as a guide on the extent of
selectivity possible when resonantly exciting a specific chirality in a mixture. However, the
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selectivity of the reaction is so high that it can be driven to promote functionalization of the less
reactive chirality over the more reactive species.
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The high selectivity of the optically-driven diazoether chemistry arises from the fact that
the E-diazoether, is trapped deeply in a non-reactive molecular conformation until destabilized by
H+ and triggered optical excitation of the SWCNT substrate to switch its molecular conformation
to the reactive Z-diazoether. This triggered reactivity is significantly more selective than the widely
used diazonium salts either with20 or without optical excitation,2 as well as the reactive Z-
diazoether.10 Our experimental results show that without light and H+, the reaction can be
completely suppressed at room temperature within the limits of our sensitive spectral technique.
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CONCLUSIONS
We show that the E-diazoether, a molecule that is inert with SWCNT, can be controllably
switched to the reactive form by resonant optical excitation of the nanotube substrate and
controlling pH. Kinetics studies and DFT calculations reveal a deep energetic trap for the E-
diazoether whose thermodynamic barrier to isomerization into the reactive Z-form can be
modulated by pH. Optical excitation of the SWCNTs then drives the reaction selectively to those
specific nanotube structures that resonate with the light, by thermally switching the physically
absorbed E-diazoether to the reactive isomer due to optically-induced local heating, as suggested
by the temperature dependence of the reaction. This triggered reactivity serves as the foundation
for covalent modification of semiconducting SWCNTs at the chirality-selective level. We
experimentally illustrate this chiral selectivity by driving the covalent functionalization of (7,3)-
SWCNT in the presence of (6,5)-SWCNT, a structurally similar semiconductor that is intrinsically
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more reactive. This type of on-demand selectivity may find applications in nanotube sorting, on-
chip passivation, and nanoscale lithography.
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Associated Content
Supporting Information.
The Supporting Information is available free of charge on the ACS Publications website.
Supporting materials and methods, Figures S1-S13 and Tables S1-S3 as referred to in the text.
Author Information
Corresponding Author
*yhw@umd.edu
Notes
The authors declare no competing financial interest.
Acknowledgements
This work was supported in part by AFOSR through MURI grant FA9550-16-1-0150, by NSF
through grant CHE-1507974, and by NIH/NIGMS through grant R01GM114167.
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