Photooxidative coupling of thiophenol derivatives to disulfides

Article abstract of DOI:10.1021/jp1077483

Disulfide bonds play an important role in determining the structure and stability of proteins and nanoparticles. Despite extensive studies on the oxidation of thiols for the synthesis of disulfides, little is known about the photooxidation of thiols, whic

Full text of DOI:10.1021/jp1077483

1
2010
J. Phys. Chem. A 2010, 114, 12010–12015
Photooxidative Coupling of Thiophenol Derivatives to Disulfides
Hyung Jun Kim, Jun Hee Yoon, and Sangwoon Yoon*
Department of Chemistry, Dankook UniVersity, 126 Jukjeon-dong, Suji-gu, Yongin, Gyeonggi 448-701, Korea
ReceiVed: August 16, 2010; ReVised Manuscript ReceiVed: September 29, 2010
Disulfide bonds play an important role in determining the structure and stability of proteins and nanoparticles.
Despite extensive studies on the oxidation of thiols for the synthesis of disulfides, little is known about the
photooxidation of thiols, which may be a clean, safe, and economical alternative to the use of harmful and
expensive metal-containing oxidants and catalysts. In this paper, we report the photooxidative coupling of
thiophenol derivatives to disulfides. Para-substituted thiophenol derivatives, p-SHC
Cl, and OCH ), are irradiated, and disulfides, X (C , are identified as the major photoproducts using
Raman, UV-vis, IR, and NMR spectroscopies. For p-nitrothiophenol (pNTP), 4,4′-dinitrodiphenyldisulfide
6
4 2
H X (X ) NO , COOH,
3
2
6 4 2 2
H ) S
(
DNDPDS) is produced in 81% yield. The product yield changes with pH, being the highest at pH ≈ 5,
suggesting that both neutral thiol and anionic thiolate forms of pNTP are required for the photoreaction to
occur. Excitation at 455 nm, at which the thiolate form of pNTP absorbs strongly, leads to the largest yield
of DNDPDS, whereas very little DNDPDS is formed by excitation of the thiol form of pNTP at 325 nm. Our
observations suggest that the photooxidation occurs via collisions of the electronically excited thiolate
form of pNTP with the surrounding neutral thiol forms of pNTP. The photooxidation reaction happens regardless
of the electron-withdrawing or electron-donating properties of the substituents if the pH and excitation
wavelengths are properly chosen. The versatility of light and generality of the photooxidative coupling reaction
of thiophenol derivatives may open new possibilities for selective and site-specific photocontrol of disulfide
bond formation in biology and nanomaterial science as well as in synthetic chemistry.
1
. Introduction
The formation of disulfide bonds is an important reaction
SCHEME 1: Photooxidation of Thiophenol Derivatives
because these bonds play a key role in determining the structure
and stability of proteins and nanoparticles. Reversible disulfide
linkages between cysteine residues determine the structure and
1
biological functions of many proteins. Thiol is also one of the
most commonly used surface-stabilizing agents for nanopar-
ticles, and the formation or cleavage of disulfide bonds on their
surfaces significantly influences nanoparticle stability.2 The
formation of new disulfide bridges by breaking original disulfide
bonds with thiols is the basis of the important thiol-disulfide
exchange reactions used to detect thiols in solution and in
substituted by -NO
mercaptobenzoic acid, pMBA), -Cl (p-chlorothiophenol, pCTP),
and -OCH (p-methoxythiophenol, pMTP), are irradiated, and
2
(p-nitrothiophenol, pNTP), -COOH (p-
,3
3
the photoproducts are identified by Raman, IR, UV-vis, and
NMR spectroscopies. Raman spectroscopy is particularly ad-
vantageous in many respects. As a vibrational spectroscopy, it
provides fingerprints of the products. It also allows for the
monitoring of the formation of photoproducts in situ and in real
time as the reaction progresses. Further investigation into the
effects of pH, excitation wavelength, and substituents provides
clues to the reaction mechanism.
4
–7
biological samples such as blood.
The most common approach for the synthesis of disulfides
is the oxidation of thiols. Various oxidants with catalysts have
8
9–12
13
been used, including K
2 2
3
PO
4
,
halogens,
4
NH OH, and
14
H O . Although these conventional methods produce disulfide
products in high yields, they often involve complicated purifica-
tion processes, hazardous chemicals, and expensive metal-
containing reagents or catalysts. Therefore, developing new
strategies to prepare disulfides in a cost-effective and an
environmentally-friendly fashion is a quest worth pursuing.
One clean, safe, and economical way of driving chemical
reactions is through the use of light. In addition to its many
other useful properties, light has the unique capability of spatial
control. By delivering light to a specific area of interest, one
2
. Experimental Section
Samples. All reagents were purchased and used without
further purification. For the photooxidation of pNTP, 10 mM
pNTP was prepared in 10 mL of tetrahydrofuran (THF)/water.
To dissolve both the reactant and the possible disulfide
photoproduct, a 1:1 mixture of THF and water was used as the
solvent. The concentrations of the other thiophenol derivatives
were 50 mM for pMBA and pCTP and 100 mM for pMTP in
ethanol/water. A 4 mL volume of solution was transferred to a
quartz cuvette (10 mm ×10 mm) for irradiation. The pH of the
solution was adjusted by adding HCl or NaOH.
can induce reactions locally, an ability that has applications in
biomedicine, surface sciences, and nanomaterial sciences.15–17
In this article, we report the photooxidative coupling of thiols
to disulfides (Scheme 1). Thiophenols, with their para positions
Irradiation. A wide range of wavelengths from various light
sources was used to drive the photoreaction. Hg-Xe lamps
*
To whom correspondence should be addressed. Phone: +82-31-8005-
3
152. Fax: +82-31-8005-3148. E-mail: sangwoon@dankook.ac.kr.
1
0.1021/jp1077483  2010 American Chemical Society
Published on Web 10/22/2010

Photooxidative Coupling of Thiophenol Derivatives to Disulfides
J. Phys. Chem. A, Vol. 114, No. 45, 2010 12011
Figure 1. (a) UV-vis absorption spectra of pNTP in THF/H
nm and 150 mW for 1 h. The UV-vis absorption spectrum of DNDPDS is included for comparison. The inset shows the visual color change of
pNTP upon irradiation. (b) Raman spectra of pNTP in THF/H O (10 mM) and photoproduct. Raman spectra of DNDPDS (5 mM) and pATP (100
mM) are included to identify the photoproduct. Spectra are displayed with offset for clarity.
2
O (80 µM) and the photoproduct formed by irradiation of the pNTP solution at 455
2
SCHEME 2: Two Possible Photoreaction Pathways for
pNTP
(
Vilber Lourmat and Hamamatsu) provided 254 and 365 nm
wavelengths. Light-emitting diodes (Thorlabs) were used to
irradiate the samples at 455, 530, and 660 nm. A He-Cd laser
(
Kimmon) provided continuous wave (cw) light with a wave-
length of 325 nm. An Nd:YAG laser with KDP crystals (Spectra
Physics) provided pulsed light (8 ns, 10 Hz) at 532, 355, and
2
66 nm.
Spectroscopy. Raman spectra were acquired using a Raman
microscope system (Kaiser). A diode laser at 785 nm was
focused onto the sample by a 10× objective. Raman scattering
from the sample was collected at 180° by the same objective
and directed to a spectrometer equipped with a holographic
grating (f/1.8, focal length 85 mm) and a Peltier-cooled CCD.
All presented Raman spectra were acquired for a total exposure
-
1
time of 60 s with a spectral resolution of 4 cm . The
photoproducts were further characterized using UV-vis absorp-
tion spectroscopy (Perkin-Elmer, Lambda 25), NMR spectros-
copy (Bruker, Avance 600, 600 MHz), and IR spectroscopy
photoreduction of the nitro group to an amine group, forming
p-aminothiophenol (pATP). The other is the photooxidation of
the thiol group, yielding 4,4′-dinitrodiphenyldisulfide (DND-
PDS). To identify the actual reaction product, we acquired
Raman spectra of the commercially available pATP and
DNDPDS as references. Figure 1b shows that the Raman
spectrum of the photoproduct produced by irradiation of pNTP
at 455 nm is exactly identical to that of the reference DNDPDS.
The UV-vis absorption spectrum of DNDPDS also matches
that of the photoproduct (Figure 1a).
(
Perkin-Elmer, Spectrum 100).
3
. Results and Discussion
.1. Observation of the Photoreaction of pNTP and
3
Characterization of the Photoproduct. We first investigated
the photoreaction of pNTP. A 10 mM solution of pNTP in a
mixture of THF and H O was irradiated at 455 nm and 150
2
mW for 1 h. Upon irradiation, the color of the solution changed
from orange to yellow, as shown in the inset of Figure 1a. The
UV-vis absorption spectrum of the pNTP solution, also
presented in Figure 1a, exhibits two absorption bands at 326
and 441 nm, attributed to absorption by a thiol form and a
thiolate form of pNTP, respectively, as discussed later. Irradia-
tion results in a new absorption spectrum with a single
absorption band at 320 nm.
Because the vibrational spectra of DNDPDS are not well
known,1
8,19
we assigned the Raman spectrum using density
2
0
functional theory calculations (B3LYP/6-31(d,p)). Figure 2
presents the optimized structures of pNTP and DNDPDS, along
with the calculated Raman spectra, in which the Raman
intensities at scaled harmonic frequencies are convoluted with
the Lorentzian function with a full-width at half-maximum
-
1
(
fwhm) of 4 cm . The optimized structure of DNDPDS shows
The changes observed in the Raman spectra are more
pronounced. Figure 1b shows that upon irradiation, the Raman
peaks at 1101 and 1310 cm- of pNTP disappear and new peaks
that the length of the S-S bond is 2.07 Å and the dihedral angle
90°. The vibrational mode analysis shows that the C-S
stretching mode is sensitively influenced by the formation of a
disulfide linkage. The C-S stretching vibration of pNTP at 1101
1
-1
arise at 1078 and 1110 cm . Furthermore, these changes are
induced by light only, strongly indicating that a photoreaction
does take place. When we left the sample in the dark or under
fluorescent light bulbs for the same period of time as irradiation
or when we heated the sample to 55 °C to see if the reaction
was possibly driven by the heat generated by irradiation, the
Raman spectrum of the pNTP solution remained unchanged
-
1
-1
cm red shifts to 1078 cm as the adjacent S-H bond is
replaced by the S-S bond, indicating that the C-S bond is
-1
slightly weakened by the disulfide bond. The peak at 1110 cm
in the DNDPDS spectrum is assigned to the C-N stretch, which
is invisible in the spectrum of pNTP. Because of the clear
distinction in the peak position and the peak intensity between
(
Supporting Information, Figure S1).
-
1
Because pNTP has two functional groups, two photoreaction
pathways are possible, as shown in Scheme 2. One is the
pNTP and DNDPDS, these two peaks at 1078 and 1110 cm
are used to monitor the formation of DNDPDS. The 1310 cm
-
1

1
2012 J. Phys. Chem. A, Vol. 114, No. 45, 2010
Kim et al.
Figure 4. Relative populations of the neutral thiol and anionic thiolate
form of pNTP as a function of pH. The relative yield of photooxidation
of pNTP at each pH was measured by the Raman intensity of the
-
1
photoproduct DNDPDS at 1110 cm
.
in the solvent. The Raman spectrum of the precipitate matches
that of solid DNDPDS (Supporting Information, Figure S4).
Figure 2. Assignment of the vibrational Raman spectra (left) and
optimized structures (right) of pNTP and DNDPDS as calculated by
density functional theory.
3
.2. Effect of pH. For a better understanding of the photo-
oxidative coupling reaction of thiophenol derivatives and to
unravel the reaction mechanism, we explored the effects of three
experimental parameters: pH, photoexcitation wavelength, and
substituents of reactants. First, we examined the effect of pH
on the reactivity. We found that pNTP, analogously to p-
nitrophenol,21 exists either in the neutral or the anionic thiol
form, depending on pH. These two forms exhibit distinctively
different spectroscopic features. Figure 3 shows the UV-vis
absorption spectra and Raman spectra of pNTP over a wide
range of pH. In acidic conditions, pNTP in the thiol form has
a maximum absorbance at 326 nm. As pH increases, the
absorption band at 326 nm decreases and is replaced by a new
band at 441 nm. The UV-vis absorption spectrum is dominated
by the 441 nm band at high pH. Figure 3b shows the color
peak, present only in pNTP, is attributed to the N-O stretch of
pNTP in the anionic thiolate form.
In addition to the UV-vis and Raman spectroscopic mea-
surements, we further confirmed the formation of DNDPDS as
a major photoproduct from pNTP using IR and NMR spec-
troscopies (Supporting Information, Figures S2 and S3). The
prominent S-H stretching band at 2545 cm- in the IR spectrum
of pNTP is not observed in the IR spectrum of the photoproduct,
1
1
indicating the formation of a disulfide product. The H NMR
spectrum of the photoproduct is in excellent agreement with
that of the reference DNDPDS. The NMR peak originating from
the proton in S-H (δ 3.77) is absent in the NMR spectrum of
DNDPDS. Protons on the benzene ring are deshielded by the
nearby disulfide bond.
Quantitative analysis of the Raman and NMR spectra provides
the yield of the disulfide photoproduct. Comparison of the
Raman peak intensity of the photoproduct from 10 mM of pNTP
with that of the 5 mM reference DNDPDS results in a yield of
2
changes of pNTP solution in THF/H O in response to pH
changes. The thiolate form of pNTP is red, reflecting the
absorption at 441 nm. The neutral pNTP species is colorless
because it absorbs in the UV region. A main feature of the
Raman spectra of the neutral thiol forms of pNTP is the N-O
-1
stretching mode at 1340 cm . As pNTP becomes deprotonated
-
1
8
1%, which is also consistent with the value obtained from the
at higher pH, the N-O stretching mode shifts to 1310 cm ,
peak areas of the NMR spectrum.
and the overall Raman intensity increases, presumably because
21
The different solubilities of pNTP and DNDPDS allow easy
separation of the photoproduct. Disulfides are generally strongly
hydrophobic, whereas thiols form hydrogen bonds with water.
As we irradiated pNTP dissolved in ethanol/water, the photo-
products were recovered as solids because of their poor solubility
of the preresonance effect (Figure 3c). Therefore, the ratio
-
1
between the 1310 and the 1340 cm peak intensities gives the
relative population of the two forms of pNTP in the solution.
In Figure 4, we plotted the relative population of the thiol
and the thiolate forms of pNTP as a function of pH. The fraction
Figure 3. (a) UV-vis absorption spectra, (b) color, and (c) Raman spectra of pNTP at pH levels indicated in the figure.

Photooxidative Coupling of Thiophenol Derivatives to Disulfides
J. Phys. Chem. A, Vol. 114, No. 45, 2010 12013
It is not clear yet how electronic excitation of thiolate leads
to the formation of disulfide. We performed time-dependent
density functional theory calculations (TD DFT) to find the
nature of the electronic excited state of the thiolate form of pNTP
and to gain insights into the reaction mechanism. The calcula-
tions revealed that the only transition with a nonzero oscillator
strength (f) is to the excited singlet state at 435 nm (f ) 0.54),
in good agreement with the measured absorption at 441 nm
(
Supporting Information, Figure S6). The molecular orbitals
involved in the transition show that the electron density moves
from the anionic sulfur atom to the nitro group upon transition
from the ground state to the excited singlet state. We also note
Figure 5. Dependence of the photooxidation reaction yield of pNTP
on excitation wavelength. The pNTP solution at pH 4.3 was irradiated
at 325, 365, 455, 530, and 660 nm at 25 mW for 2 h. The yield was
measured by the Raman intensity of the photoproduct DNDPDS at 1078
·
that thiyl radicals (RS ) have been proposed as intermediates
in the formation of disulfide in previous catalytic oxidation
8,13
studies.
Therefore, it is possible that photoexcitation of
-
1
cm . The absorption spectrum of the solution used in the experiment
solid line) is included to indicate which species, the thiol or the thiolate
thiolate may facilitate the production of the thiyl radicals by
removing the electron density from the anionic sulfur atom,
leading to the formation of disulfide. Further experimental and
theoretical investigations are required to unravel the reaction
mechanism.
(
form of pNTP, is excited to yield the product. Absorption spectra of
the thiol (dotted line) and thiolate (dashed line) forms of pNTP are
adopted from Figure 3 to show the absorption region of each species.
3
.4. Effect of the Substituents. Because the electron-
of the thiol form of pNTP is calculated from the Raman peak
intensity at 1340 cm- at each pH divided by the Raman
intensity at pH 1.7, where pNTP exists exclusively in the thiol
form. The 1310 cm- Raman peak intensity was used for
calculation of the fraction of the thiolate form of pNTP.
Calculation of the fraction using the UV-vis absorption bands
at 326 and 441 nm produced the same results. Plotted together
is the relative amount of the photoproduct by irradiation of pNTP
at each pH, measured by the Raman intensity of DNDPDS at
withdrawing/donating properties of substituents often influences
the reactivity of aryl compounds, it is possible that the
photooxidative coupling reaction we observed is limited to the
1
1
2
strong electron-withdrawing group, namely, -NO . Thus, we
explored the photooxidation of other functional-group-substi-
tuted thiophenols. We selected pMBA for its moderate electron-
withdrawing functional group (-COOH), pCTP for its weak
electron-withdrawing functional group (-Cl), and pMTP for
3
its strong electron-donating functional group (-OCH ). The
-
1
1
110 cm . Figure 4 shows that the formation of DNDPDS
samples were prepared in ethanol/water and irradiated at selected
wavelengths. Because the excitation of the thiolate form is key
to driving the photooxidation reaction as discussed above, we
measured the UV-vis absorption spectra of the thiolate form
of each molecule and selected the excitation wavelengths
accordingly. Figure 6 shows the UV-vis absorption spectra of
pMBA, pCTP, and pMTP in ethanol/water (red lines) and of
the corresponding thiolate forms in the basic condition (gray
lines) to identify the absorption band of the thiolate. The
wavelengths used for photoexcitation are marked by the arrows
in the figure. Upon irradiation, photoproducts were formed and
measured by Raman spectroscopy.
reaches a maximum at pH ≈ 5. In other words, photooxidation
occurs most efficiently in the pH range between 4 and 5 and
does not occur at pH lower than 2 or higher than 6. This suggests
that the reaction requires both thiol and thiolate forms of pNTP.
The requirement for the presence of both forms of pNTP is
further corroborated by the observation that DNDPDS was not
produced in pure organic solvents such as THF and dichlo-
romethane (DCM), where only the thiol form of pNTP exists
(
Supporting Information, Figure S5). Quantitative analysis of
the fraction of molecules in Figure 4 indicates that photooxi-
dation occurs best when the neutral form is about three times
more abundant than the anion form. As discussed in the
following section, photooxidation of pNTP occurs via electronic
excitation of the anionic thiolate form of pNTP. Therefore, a
higher population of the thiol form of pNTP surrounding the
electronically excited thiolate form may increase the chance of
reactive collisions between the two species, leading to the best
yield.
Figure 6d-f presents Raman spectra of pMBA, pCTP, pMTP,
and their photoproducts. The Raman spectra of the photoprod-
ucts are characterized by the absence of the S-H stretching
-1
peak near 2560 cm and the appearance of a new peak at ∼500
-
1
22
cm , assigned to the S-S stretching mode. This observation
strongly suggests that disulfide bonds are formed by irradiation.
In addition, the red shift of the C-S stretching mode from
∼
3
.3. Effect of Excitation Wavelength. To find which form
-
1
1100 to ∼1080 cm is observed, as is the case for pNTP.
of pNTP is excited by irradiation, leading to the photoreaction,
we varied the excitation wavelengths and measured the product
yield using Raman spectroscopy. Figure 5 presents the Raman
Furthermore, comparison between the Raman spectrum of the
product from irradiation of pCTP and that of the reference, 4,4′-
dichlorodiphenyldisulfide (DCDPDS), confirms that the photo-
oxidative coupling reaction has indeed occurred (Supporting
Information, Figure S7). Therefore, the photooxidative coupling
of thiophenol derivatives occurs regardless of the electron-
donating/withdrawing properties of the substituents.
-1
intensity of the DNDPDS photoproduct at 1078 cm , acquired
after irradiation of pNTP at 325, 365, 455, 530, and 660 nm
(
25 mW, 2 h). Overlaid are the absorption spectra of pNTP at
pH 4.3 (solid line), where the experiments were performed,
together with the absorption spectra of the thiol (dotted line)
and the thiolate (dashed line) forms of pNTP, with λmax at 326
and 441 nm, respectively. DNDPDS is best produced by
irradiation at 455 nm, which is where the thiolate form of pNTP
absorbs strongly. Excitation of neutral pNTP at 325 nm does
not yield significant amounts of the photoproduct. Therefore,
the photooxidation of pNTP occurs via electronic excitation of
the thiolate form rather than the thiol form of pNTP.
Our observations imply that photooxidative coupling may be
generally applicable to a variety of thiophenol molecules if the
excitation wavelength and pH conditions are properly selected.
The synthesis of disulfides via photooxidation is easy, safe,
clean, and thus well suited to green chemistry. The separation
of the products is also straightforward. Furthermore, the photo-
oxidative coupling reaction enables the synthesis of asymmetric

1
2014 J. Phys. Chem. A, Vol. 114, No. 45, 2010
Kim et al.
Figure 6. (a-c) UV-vis absorption spectra of (a) pMBA, (b) pCTP, and (c) pMTP solutions in ethanol/water (red lines). The absorption spectra
of the thiolate form of each molecule obtained in basic conditions are also included to identify the absorption band of the thiolate (gray lines). The
arrows mark the excitation wavelength used in the photooxidation experiments. (d-f) Raman spectra of (d) pMBA, (e) pCTP, and (f) pMTP before
and after irradiation at the wavelengths indicated by the arrows.
disulfides by choosing appropriate wavelengths and pH levels,
which is difficult to achieve in a conventional synthetic
approach. Photooxidation also opens the possibilities of the
modification of the local structures of proteins and nanomaterials
by forming disulfide bonds in specific areas. Therefore, the
observation of the photooxidative coupling of thiophenol
derivatives to disulfides and the understanding of the reaction
mechanism that this study provides will contribute to advancing
biology and nanomaterial sciences as well as synthetic chemistry.
much less product. The effects of pH and excitation wavelengths
on the reactivity suggest that the photooxidative coupling of
thiophenol derivatives occurs via reactive collisions of the
electronically excited thiolate form of pNTP with the neutral
thiol form of pNTP. Replacement of -NO with less electron-
2
withdrawing groups such as -COOH and -Cl or a strong
electron-donating group such as -OCH leads to the same
3
results, yielding the disulfides by excitation of the thiolate forms.
Although more molecules should be investigated, it seems that
the photooxidative coupling reaction is universal for thiophenol
derivatives if pH and excitation wavelengths are properly
chosen. The generality of the reaction and versatility of light
may open new possibilities for selective and site-specific
photocontrol of disulfide bond formation in biology and
nanomaterial science as well as synthetic chemistry.
4
. Conclusions
We observed photooxidative coupling of thiophenol deriva-
tives to disulfides. Irradiation of pNTP at 455 nm produces the
disulfide product, DNDPDS, in 81% yield, as identified by
UV-vis, Raman, IR, and NMR spectroscopies. Density func-
tional theory calculations revealed that the characteristic vibra-
tional frequencies of DNDPDS at 1078 and 1110 cm-1 can be
assigned to the C-S and C-N stretching modes, respectively.
The yield of the product changed with pH, being the highest at
pH ≈ 5, at which the thiol and thiolate forms of pNTP exist in
a 3:1 ratio. The disulfides are not formed at very low or high
pH, suggesting that the photooxidative coupling reaction requires
the presence of both thiol and thiolate forms. In the presence
of both forms of pNTP, irradiation at 455 nm, at which the
thiolate form strongly absorbs, leads to the best yield of
DNDPDS, whereas excitation of the thiol form at 325 nm yields
Acknowledgment. This work was supported by the Dankook
University research fund (2009).
Supporting Information Available: Control experiments
confirming the photoreaction of pNTP; IR, NMR, and solid
Raman data for identification of photoproducts; photooxidation
of pNTP in various organic solvents; time-dependent density
functional theory calculations of the excited thiolate state. This
material is available free of charge via the Internet at http://
pubs.acs.org.

Photooxidative Coupling of Thiophenol Derivatives to Disulfides
J. Phys. Chem. A, Vol. 114, No. 45, 2010 12015
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