Unsymmetrical aryl disulfides with excellent transparency in the visible region for second order nonlinear optics

Article abstract of DOI:10.1039/b204034d

Several unsymmetrically substituted aromatic donor-acceptor disulfides have been synthesized and analysed for their second order nonlinear optical properties. These molecules exhibit moderately high first hyperpolarizability (β) with excellent transparenc

Full text of DOI:10.1039/b204034d

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Unsymmetrical aryl disulfides with excellent transparency in the
visible region for second order nonlinear optics
Ramanathan Sudharsanam,a Srinivasan Chandrasekarana{ and Puspendu Kumar Das*b
aDepartment of Organic Chemistry, Indian Institute of Science, Bangalore 560012, India
bDepartment of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore
560012, India. E-mail: pkdas@ipc.iisc.ernet.in
Received 26th April 2002, Accepted 24th June 2002
First published as an Advance Article on the web 16th August 2002
Several unsymmetrically substituted aromatic donor–acceptor disulfides have been synthesized and analysed for
their second order nonlinear optical properties. These molecules exhibit moderately high first
hyperpolarizability (b) with excellent transparency in the visible region. Most of the unsymmetrical disulfides
have a cut-off wavelength below 420 nm. Calculations show that the molecules have an asymmetric charge
distribution around the disulfide bond which is responsible for their high b values. These results provide
motivation for the design and synthesis of nonlinear optical chromophores with multiple disulfide bonds for
large second order nonlinearity and excellent visible transparency.
aryl disulfides 2 and aryl monosulfides 3 were taken up for
comparison.
Introduction
The design and synthesis of new donor (D)–acceptor (A)
substituted organic molecules with asymmetric charge dis-
tribution is of current interest in nonlinear optics (NLO). This
class of molecules has potential application in optical data
processing and communications.1 It has been generally recog-
nized that molecules of the type D–p–A containing a p-backbone
with terminal D, A substitutions and exhibiting charge asym-
metry show large second order nonlinearity (b).2 However,
a large number of molecules designed on this basis absorb
light in the visible region which is detrimental for their
use in optoelectronics. Various molecular arrangements such
as orientationally correlated multichromophoric systems,3
chromophores with heteroatoms connecting the D- and
A-substituted p-fragments by s-bonds (thereby breaking the
conjugation),4 and collinearly oriented dipoles5 have been
prepared and some of them have shown excellent transparency
in the visible region but their b values are low.
To achieve high b values with excellent transparency in
the visible region the –S–S– linkage has been taken up for
consideration in the present study. The influence of an –S–S–
linkage has not been investigated in the context of NLO. The
–S–S– bridge is very much similar to the –CH2–CH2– bridge
except that the former is more polarizable than the latter. The
–S–S– bond is also desirable because its longest wavelength
absorption arises due to a p*– s* transition around 250 nm,
which is far from the visible region. This transition is less likely
to be shifted to the visible if we combine an aromatic acceptor
and donor through an –S–S– linkage. In fact, by changing
the twist around the –C–S–S–C– linkage, it is possible to shift
the p*–s* transition even deeper to the ultraviolet.6 Also, the
electron transfer ability across the bridge joining the aromatic
halves is expected to be extremely poor.7 On the other hand,
a p-linkage of the type –CLC– or –NLN– instead of –S–S–
will allow facile transfer of charge between the aromatic donor
and the acceptor but its longest wavelength charge transfer
transition will shift to the red making it unsuitable for use as a
NLO material.
Results and discussion
Synthesis
Aryl disulfides were synthesized, characterized and their
melting points were compared with literature data. Recently
in our laboratory, we have shown8 that benzyltriethylammo-
nium tetrathiomolybdate 5 is able to cleave the disufide bond.
It has also been shown that it converts organic thiocyanates to
corresponding disulfides. We have extended these routes to
prepare unsymmetrical disulfides 1a and 1b. Compound 1a was
obtained in the reaction of symmetrical disulfides 2a and 4 with
2 molar equivalents of benzyltriethylammonium tetrathio-
molybdate 5 in acetonitrile as shown in Scheme 1. Synthesis
of symmetrical aryl disulfide 2a was followed according to
the literature procedure.9 Reaction of 2b and 4 with 5 yielded
the corresponding unsymmetrical monosulfide. This could
be due to the high electron withdrawing ability of the nitro
group. Reaction of 4-thiocyanato-N,N-dimethylaniline 6 and
4-nitrophenylthiocyanate 7 with 2 equiv. of 5 yielded a mixture
of 4, 2b and 1b. Compound 1b was separated from the
mixture by chromatography. Compound 1f10 and the donor-
substituted symmetrical disulfide 4 11 were prepared according
to literature procedures. Compounds 1c–1e, 1g, 1h and 2b were
The general structure of the donor–acceptor-substituted
unsymmetrical aryl disulfides 1 is shown in Fig. 1. Symmetrical
{Honorary Professor, Jawaharlal Nehru Centre for Advanced
Scientific Research, Jakkur, Bangalore, India.
Fig. 1 General structures of disulfides 1, 2 and monosulfides 3.
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This journal is # The Royal Society of Chemistry 2002
DOI: 10.1039/b204034d

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below 400 nm and have virtually no absorption at 532 nm
as seen from their cut-off wavelengths (lcut-off) in Table 1. The
lcut-off is defined as the wavelength where the first derivative of
absorbance with respect to the wavelength at the lower energy
side of the spectrum becomes zero.
We note that the disulfides 1a,1b and 2a, 2b have lower lmax
values than those of the corresponding monosulfides 3a,3b.
Their lmax value is blue-shifted by ca. 10–30 nm relative to the
monosulfides. This indicates that the disulfide linkage hinders
to a large extent the charge transfer possible from the donor to
the acceptor. The peak position of this charge transfer band
varies as a function of the donor–acceptor strength.
Nonlinear optical properties
The b values of disulfides 1 and 2 and monosulfides 3 have been
measured using the hyper-Rayleigh scattering technique12,13 in
solution at 1064 nm. Chloroform and methanol were used as
solvents, and the b values were obtained using p-nitroaniline
(PNA) as an external reference (b ~ 17 6 10230 esu in CHCl3,
b ~ 22.4 6 10230 esu in MeOH).14 These molecules did not
exhibit any single photon fluorescence band near 532 nm when
they were excited at their respective lmaxs and, therefore, two/
multi-photon fluorescence contribution to b was not expected.
To ascertain that the SHG signal was free from any two/multi-
photon fluorescence, the signal from the HRS experiments was
dispersed through a monochromator. No broad band fluores-
cence was detected around the second harmonic wavelength.
The static hyperpolarizability (bo) was calculated using the
well known two-state model which is, generally, applicable far
from a resonant absorption band. The b values are listed in
Table 1. It is apparent from the table that all the mono- and
di-sulfides 1–3 have significantly higher b as well as bo values
compared with the reference compound PNA. In fact, bo of 1b
is seven times of that of PNA. These b and bo values are higher
than those of specially designed 1,8-di(hetero)aryl naphtha-
lenes reported by Bahl et al. for blue-transparency frequency
doublers.15 The b values of the symmetrically substituted
disulfides 2 were found to be lower than those of the corres-
ponding monosulfides 3. This is due to the well-known
nonlinearity-transparency trade-off and is quite expected.
What is interesting as well as surprising is that the unsym-
metrical disulfides 1, in spite of similar absorption character-
istics to those of symmetrical disulfides 2 and monosulfides 3,
exhibit much higher molecular hyperpolarizability. Of course,
the size increases by a factor of two in going from the mono-
sulfide to the unsymmetrical disulfide and one expects an
increase in hyperpolarizability. But if we compare symmetrical
disulfides with unsymmetrical disulfides of similar lengths, the
bo values of the unsymmetrical disulfides 1 are ca. 2.5 times
higher than those of the symmetrical disulfides 2. It is also
Scheme 1
prepared by oxidizing the respective donor- and acceptor-
substituted aryl thiols, and the products were separated by
column chromatography on neutral alumina.9
Linear optical properties
The UV-Vis absorption spectra were recorded in chloroform
for compounds 1–3a,b and for 1c–h in methanol using a
Hitachi U-3400 spectrometer. A representative absorption
spectrum is displayed in Fig. 2. All these compounds exhibit
an absorption maximum (lmax) in the ultraviolet region much
Fig. 2 UV-Vis absorption spectra of 1g, 2b and 3b in methanol.
Table 1 UV-Vis absorption maximum (lmax), cut-off wavelength (lcut-off), molar extinction coefficient (emax), molecular hyperpolarizabilities
(bcalAM1, b and bo) and calculated ground state dipole moment (mg) of compounds 1–3
S. no.
lmax/nm
e
max/M21 cm21
lcut-off/nm
mcalAM1/D
bcalAM1/10230 esu
ba/10230 esu
bo/10230 esu
1a
1b
1c
1
1e
304
319
321
319
322
319
317
269
304
317
316
346
348
32 000
17 000
30 000
11 000
11 000
11 000
7920
26 000
40 500
26 800
80 500
18 200
14 200
434
448
421
420
419.6
416.6
398
361.0
395
410
5.9
9.1
8.7
7.8
6.7
6.3
5.4
4.6
3.6
4.6
2.7
6.0
7.2
34.4
40.2
30.2
23.3
18.3
14.8
17.3
14.7
8.4
36.0
102
98
86
80
36
44
89
18.0
38.6
20.0
59.8
17.0
22.2
59.4
56.6
37.8
46.0
20.9
25.7
62.0
11.1
22.6
11.8
30.8
8.60
1f
1g
1h
2a
2b
3a
3b
PNA
8.4
371
450
428
10.0
10.3
4.6
aMeasured at 1064 nm.
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can influence the measured b values since the monosulfides,
symmetrical and unsymmetrical disulfides are expected to
exhibit different dispersion behaviour.
The theoretical ground state dipole moment (mg) and finite
field hyperpolarizability (bo) for 1–3 were calculated using the
semi-empirical AM1 procedure within the MOPAC package
and are listed in Table 1. The large ground state dipole moments
of the unsymmetrical disulfides 1 are well expected due to the
presence of the terminal donor and acceptor groups. Charge
density on each atom for 1 and 2 has also been calculated using
the GAUSSIAN 94 and MOLDEN programs using the AM1
optimized geometry as the input. One representative example
for the charge densities on 1 and 2 is given in Fig. 4. These
results show that an appreciable difference in electron densities
exists between the two sulfur atoms in the unsymmetrical
disulfides 1, whereas charges are symmetrically distributed in 2,
as expected. The charge separation in the dimethylamino-
phenylsulfide portion of the unsymmetrical disulfide 1b is
large. To check whether the corresponding monosulfide (donor
half of the unsymmetrical disulfide) would also exhibit such a
large charge separation, we calculated the ground state dipole
moment and first hyperpolarizability of p-methylthio-N,N-
dimethylaniline. We find that the ground state dipole moment
of p-methylthio-N,N-dimethylaniline is low (1.9 D) and
Fig. 3 Plot of experimental bo values vs. calculated b values for 1, 2, 3
and PNA (R factor 0.6).
noteworthy that 4-dimethylamino-4’-nitrostilbene, which is
analogous to 1b, has bo ~ 55.4 6 10230 esu16 which is lower
than that of 1b, while the lmax of this stilbene derivative
(427 nm) is higher than that of 1b by 110 nm. In spite of the
weak donor and acceptor in 1g, it shows the highest bo value.
The measured hyperpolarizability, bo, of the compounds has
been plotted against the calculated finite field hyperpolariz-
ability in Fig. 3. The correlation between the semi-empirical
values and measured values of hyperpolarizability is not great.
This is partly because the solvent effects are not included in
the finite field calculations, and both the solvents, chloroform
and methanol, that have been used for the measurements can
interact with the disulfides strongly. Also, dispersion effects
the charge distribution is shown in Fig. 4. The calculated
AM1
hyperpolarizability bcal
for this molecule is only 3.1 6
10230 esu. Thus, we infer that the charge asymmetry that
is present in unsymmetrical disulfides is responsible for their
large hyperpolarizability. Perhaps, a large change in dipole
moment upon excitation or an appreciable change in the
transition dipole moment explains the origin of the large b
values in them.
Fig. 4 Charge distribution in p-methylthio-N,N-dimethylaniline, 1b and 2b (for clarity, H atoms are not shown).
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added tetrathiomolybdate 5 17 (1.016 g, 2 mmol) in acetonitrile
(3 mL). The reaction was continued overnight and the solvent
was evaporated under vacuum. The residue was dissolved in
dichloromethane (2 mL) and diethyl ether (40 mL) was added.
The resulting solution was filtered through a Celite pad. The
organic solvent was evaporated and the crude product was
chromatographed on neutral alumina TLC using a petroleum
ether (60–80 uC)–CH2Cl2 mixture (7 : 3) as eluent which yielded
compound 1a (0.173g, 30%) as a pale yellow solid. LRMS (DI,
m/z): 289 (M1), 152. This sample decomposes slowly at room
temperature (25 uC) after a few hours (3 h).
Conclusions
In conclusion, we have demonstrated that unsymmetrical aryl
disulfides 1 show high second order optical nonlinearity and
excellent transparency in the visible region. Many natural
molecules such as peptides and proteins contain more than one
disulfide linkage. Modifying and designing NLO chromo-
phores with multiple disulfide bonds may provide an alternate
strategy for developing new optical materials.
Experimental
4-Methylthiobenzaldehyde(3a), 4-nitrothioanisole(3b), 4-methyl-
benzenethiol (8c), 4-aminobenzenethiol (8a), 4-methoxy-
benzenethiol (8b) and 4-nitrobenzenethiol (9a) were obtained
from commercial sources. 4-Cyanobenzenethiol (9b) was
obtained from 4-(methylthio)benzonitrile (3c), and bis(4-
formylphenyl) disulfide (2a) was from 3a following the
procedure by Young et al.9 Commercial grade solvents were
distilled prior to use. Analytical thin layer chromatography was
carried out on Merck precoated silica gel 60F-254, 0.25 mm
glass plates. Visualisation of spots was achieved by one or more
of the following techniques: (a) UV illumination, (b) exposure
to iodine, or (c) immersion of the plate in a 10% solution of
phosphomolybdic acid in ethanol followed by heating to ca.
200 uC. Column chromatography was carried out using 60–120
and 100–200 mesh Acme silica gel and neutral alumina. Thin
layer chromatography was performed using TLC grade neutral
alumina. Melting points were measured using a Buchi B-540
instrument. Spectroscopic measurements were made using the
following instruments: UV-Vis, Hitachi U-3400; FT IR, Perkin
Synthesis of 4-(N,N-dimethylamino)phenyl 4’-nitrophenyl
disulfide (1b)
Following the procedure described above, instead of symmetri-
cal disulfides, the reaction between 4-thiocyanato-N,N-dimethyl-
aniline (6) (0.151 g, 0.846 mmol) and 4-thiocyanatonitrobenzene
(7) (0.153 g, 0.846 mmol) was carried out using tetrathiomo-
lybdate 5 (1.028 g, 1.692 mmol). The product was purified
by chromatography on a neutral alumina column. Yield:
0.116 g (45%), m.p.: 100 uC, (lit.18 m.p.: 100–101 uC); nmax
/
cm21: 1509, 1337; dH (CDCl3, 300 MHz): 8.17 (d, J ~ 9.3 Hz,
J ~ 2.7 Hz, 2 H, Ar H), 7.71 (d, J ~ 9 Hz, 2 H, Ar H), 7.39 (d,
J ~ 7 Hz, 2 H, Ar H), 6.6 (d, J ~ 9.4 Hz, 2 H, Ar H), 2.98 (s,
6 H, 2 CH3); LRMS (DI, m/z): 306 (M1), 152.
General procedure for the synthesis of 1c–e, 1g and 1h
To a stirred solution of the donor-substituted aryl thiol 8
(1 equiv.) and acceptor substituted aryl thiol 9 (1 equiv.) in
dichloromethane (10 mL) was added an aqueous solution
(10 mL) of I2 (10 equiv.) and KI (10 equiv.). Stirring was
continued overnight (10 h) and the solution was washed with
a saturated solution of Na2SO3 until the dichloromethane
solution was free from iodine. The organic layer was finally
washed with water (10 mL) and dried over anhydrous Na2SO4.
The solvent was evaporated under vacuum at 25 uC and the
unsymmetrical disulfide 1 was separated from symmetrical
1
Elmer 781; H NMR, JEOL 300 MHz, BRUKER 200 MHz
(NMR chemical shifts are reported in d values in parts per
million downfield from the internal reference tetramethyl-
silane); LRMS, JEOL JMS-DX303 (only principal molecular
fragments are reported). Elemental analysis was performed
using a Carlo Erba elemental analyser model – 1106.
disulfides by chromatographic purification on
alumina column or by preparative TLC.
a neutral
Hyper-Rayleigh scattering (HRS) measurements
The hyper-Rayleigh technique was used to measure the first
hyperpolarizability of all the compounds at 1064 nm using a
Q-switched Nd:YAG laser (Spectra Physics, 10 Hz, 8 ns).
The experimental set-up used for the HRS is described in
detail elsewhere.13 All data were collected at laser powers
¡24 mJ pulse21 and the exciting beam was focussed by a
biconvex lens (f.l. 10 cm) to a spot 5 cm away after passing
through the glass cell containing the sample solution. The
second harmonic signal was collected at a perpendicular
direction thorough a set of collection optics which contained
a concave mirror, an aspherical lens, a collimating lens and
a couple of filters including a 4 nm bandwidth interference
filter at 532 nm. A high through-put monochromator (Czerny
Turner, 0.25 m) replaced the collection optics for wavelength
dispersion experiments. The resolution of the monochromator
was 0.4 nm and both the entrance and exit slit-widths were kept
at 1.25 mm. The monochromator was scanned at 2 nm intervals
and at each wavelength the signal output from the PMT
was averaged over 400 laser shots. The input power was
monitored using a power meter. The set-up was standardized
by measuring the b value of PNA in chloroform as well as
in methanol as external reference. All concentrations of the
solutions were kept at ¡1025 M.
Synthesis of 4-aminophenyl 4’-nitrophenyl disulfide (1c)
Following the general procedure described above, 4-amino-
benzenethiol (0.2 g, 1.597 mmol) (8a) and 4-nitrobenzenethiol
(0.247 g, 1.595 mmol) (9a) were reacted in dichloromethane
(10 mL) using I2 (4.05 g, 15.97 mmol) and KI (2.65 g,
15.97mmol) in aqueous solution (10 mL) for 10 h. The disulfide
1c was isolated in the pure form as a solid after chmatographic
purification. Yield: 0.066 g, (15%), m.p.: 118 uC (lit.19 m.p.:
118–120 uC); nmax/cm21: 3475, 3384, 1509, 1337; dH (CDCl3,
300 MHz): 8.17 (d, J ~ 10.2 Hz, 2 H, Ar H), 7.68 (d, J ~ 6 Hz,
2H, Ar H), 7.31 (d, J ~ 9.6 Hz, 2 H, Ar H), 6.59 (d, J ~ 5.7 Hz,
2 H, Ar H), 3.83 (br, 2 H, NH2); LRMS (DI, m/z): 278 (M1),
124.
Synthesis of 4-methoxyphenyl 4’-nitrophenyl disulfide (1d)
Following the general procedure described above, 4-methoxy-
benzenethiol (0.180 g, 1.288 mmol) (8b) and 4-nitrobenzene-
thiol (0.2 g, 1.288 mmol) (9a) were reacted in dichloromethane
(10 mL) using I2 (3.27 g, 12.88 mmol) and KI (2.13 g,
12.88mmol) in aqueous solution (10 mL) for 10 h. Yield:
0.0078 g (2%), m.p.: 71–71.5 uC (lit.20 m.p.: 72.5–73 uC);
n
max/cm21: 1508, 1336.
Synthesis of (4-formylphenyl)-4’-N,N-dimethylaminophenyl
disulfide (1a)
Synthesis of 4-methylphenyl 4’-nitrophenyl disulfide (1e)
To a stirred solution of bis(4-formylphenyl) disulfide (2a)
(0.152 g, 1 mmol) and bis[4-(N,N-dimethylamino)phenyl]
disulfide (4) (0.304 g, 1 mmol) in dry acetonitrile (3 mL) was
Following the general procedure described above, 4-methylben-
zenethiol (0.160 g, 1.288 mmol) (8c) and 4-nitrobenzenethiol
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(0.2 g, 1.288 mmol) (9a) were reacted in dichloromethane
(10 mL) using I2 (3.27 g, 12.88 mmol) and KI (2.13 g,
12.88 mmol) in aqueous solution (10 mL) for 10 h. Yield: 0.046
g (13%), m.p.: 62.5 uC (lit.18 m.p.: 62–63 uC).
Acknowledgement
This work was partially supported by the Council of Scientific
and Industrial Research, Govt. of India.
References
Synthesis of 4-chlorophenyl 4’-nitrophenyl disulfide (1g)
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To a stirred solution of bis(4-chlorophenyl) disulfide (0.7 g,
2.4 mmol) in a mixture of dioxane (8 mL) and water (2 mL) was
added triphenylphosphine (0.575 g, 2.19 mmol). The solution
was heated at 100 uC overnight and then the solution was
cooled and the solvent was evaporated. Following the general
procedure described above, the crude residue was reacted with
4-nitrobenzenethiol (0.377 g, 2.42 mmol) (9a) in dichloro-
methane (10 mL) using I2 (6.171 g, 24.2 mmol), KI (4.03g,
24.29 mmol) in aqueous solution (15 mL) for 10 h. The
unsymmetrical disulfide 1g was obtained as a pale yellow solid.
Yield: 0.0062 g, 1%, m.p.: 95–96 uC (lit.18 m.p.: 92–93 uC).
2
3
4
5
6
7
8
Synthesis of 4-cyanophenyl 4’-methylphenyl disulfide (1h)
Following the general procedure described above, 4-sulfanyl-
benzonitrile (0.2 g, 1.479 mmol) (9b) and 4-methylbenzenethiol
(0.183 g, 1.47 mmol) (8c) were reacted in dichloromethane
(10 mL) using I2 (3.756 g, 14.79 mmol), KI (2.45 g, 14.79 mmol)
in aqueous solution (15 mL) for 10 h. Yield: 0.043 g (17%),
m.p.: 88–89 uC; nmax/cm21: 2228; LRMS (DI, m/z): 257 (M1),
123. Anal. calc. for C14H11NS2: C, 65.35; H, 4.27; N, 5.44.
Found: C, 63.71; H, 4.32; N, 4.8%.
9
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A solution of of 4-nitrobenzenethiol (0.200 g, 1.28 mmol) (9a)
in DMSO (2 mL) was heated at 80 uC overnight and the
solution was cooled to room temperature (25 uC). The solution
was poured into cold water (50 mL). The precipitated pale
yellow solid was filtered and washed thoroughly with water.
The solid was recrystallised from acetic acid and compound 2b
was isolated in quantititave yield, m.p.: 182 uC (lit21 m.p.:
181 uC); nmax/cm21: 1515, 1343; dH(CDCl3, 200 MHz): 8.2 (d,
J ~ 12 Hz, 4 H, Ar H), 7.62 (d, J ~ 8 Hz, 4 H, Ar H); LRMS
(DI, m/z): 308 (M1).
2908
J. Mater. Chem., 2002, 12, 2904–2908