Non-linear absorption of 2,5-dialkynyl thiophenes

Article abstract of DOI:10.1002/poc.881

Three diarylalkynyl-substituted thiophenes were synthesized and the optical power limiting (OPL) effect at 532 nm was investigated for solutions of these compounds. The relationship between the OPL and parameters obtained from molecular-orbital-based calc

Full text of DOI:10.1002/poc.881

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2005; 18: 426–433
Published online 8 November 2004 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.881
Non-linear absorption of 2,5-dialkynyl thiophenes
Per Lind,1 Anders Eriksson,2 Cesar Lopes2 and Bertil Eliasson1*
1
˚
˚
Department of Chemistry, Organic Chemistry, Umea University, SE-901 87 Umea, Sweden
2Division of Sensor Technology, Swedish Defence Research Agency (FOI), SE-581 11 Linko¨ ping, Sweden
Received 14 June 2004; revised 23 August 2004; accepted 13 September 2004
ABSTRACT: Three diarylalkynyl-substituted thiophenes were synthesized and the optical power limiting (OPL)
effect at 532 nm was investigated for solutions of these compounds. The relationship between the OPL and parameters
obtained from molecular-orbital-based calculations is discussed. Semi-empirical calculations indicate that the
compounds can have a broad distribution of conformations due to inter-ring twisting, but that the second
hyperpolarizability (ꢀ) can be significant despite a ring twist. The calculations imply that substitution by alkyl
groups can lead to enhanced ꢀ values. The measurements and calculations show a greater increase of the OPL and ꢀ
effects from compound 5 to 6 than from 6 to 7. For these compounds, which differ mainly by the length of the ꢁ-
electron system, the magnitude of non-linear absorption seems to be well correlated with several properties of the
electronic ground state estimated by standard ab initio molecular orbital calculations, as well as with ꢀ values from
the semi-empirical calculations. Copyright # 2004 John Wiley & Sons, Ltd.
KEYWORDS: optical limiting; non-linear absorption; hyperpolarizability; thiophenes; thiophenyl; phenylacetylenes
INTRODUCTION
that the third-order optical non-linearity can be increased
by incorporation of a sulfur atom in the organic struc-
ture.20 The increased non-linearity was seen as the result
of strong ꢁ-overlap and involvement of the sulfur atom
orbitals in the molecular ꢁ-orbitals.
Over the last few decades there has been a considerable
optimism concerning the use of non-linear optical (NLO)
organic materials for all-optical switching and optical
computing.1–5 Both a light-induced refractive index
change as well as a transmittance change of a material
should be useful for all-optical signal manipulation. The
non-linear refraction of a bulk substance has its equiva-
lence at the molecular level in the real part of the second
hyperpolarizability tensor ꢀ.6,7 Within the same theore-
tical framework, the imaginary part of ꢀ describes non-
linear absorption, of which two-photon absorption (TPA)
is an important mechanism.6 The TPA event is being
applied already in two-photon fluorescence micro-
scopy8,9 and is promising for other applications such as
data storage10 and optical power limiting (OPL).11–17
Another mechanism that can contribute to the OPL
performance of a material is excited state absorption
(ESA), where one or several excited states (singlets
and/or triplets) reveal a stronger absorption of light
than that of the electronic ground state.12–15,18,19
In comparison with benzenoid rings, the heterocyclic
rings in the series furan, thiophene and selenophene are
less aromatic and can have stronger interaction with
attached groups. By the same token, the thiophene ring
is a slightly better ꢁ-electron donor in comparison with
the benzene ring. The ability of thiophenes to take part in
electron delocalization while still showing only weak
absorption of visible light is one reason for our choice of
thiophenes for this study. A fair number of compounds
having a thiophenyl group have been studied with respect
to third-order NLO effects,21 and a few will be mentioned
briefly here. Experimental22 and theoretical23,24 investi-
gations have shown that thiophene displays significant
instantaneous second hyperpolarizability, i.e. a purely
electronic process, must likely originating from localized
electrons on the sulfur atom. It has been shown also that
thiophenes with unsaturated substituents in the 2,5-posi-
tions can display TPA, e.g. 2,5-bis-(benzothiazol-2-yl)-
3,4-(dimethoxy)-thiophene at ꢀ602 nm.25,26 Significant
values of ꢂ(3) and the two-photon absorption coefficient
(ꢃ) of dithienylethylenes have been found by degenerate
four-wave mixing (DFWM) experiments with laser
pulses in the picosecond regime at 532 nm and by OPL
experiments in the visible region, respectively.27
This work focuses on the optical limiting behavior of
dialkynyl-substituted thiophenes 5–7 (Scheme 1) at the
wavelength of 532 nm in tetrahydrofuran solution. Earlier
investigations on ꢁ-conjugated molecules have shown
*Correspondence to: B. Eliasson, Department of Chemistry, Organic
˚
Chemistry, Umea University, SE-901 87, Umea, Sweden.
E-mail: bertil.eliasson@chem.umu.se
Contract/grant sponsor: Swedish Defence Material Administration
(FMV).
˚
Copolymers of ethynyl-bridged carbazole and thio-
phene units, being virtually transparent above 475 nm,
Copyright # 2004 John Wiley & Sons, Ltd.
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ARYLETHYNYL-SUBSTITUTED THIOPHENES FOR OPTICAL POWER LIMITING
427
detailed set-up has been reported elsewhere.32,33 The
OPL measurements were performed on tetrahydrofuran
(THF) solutions of compounds 5–7 in 2 mm quartz
cuvets. The focus of the laser beam was positioned
carefully at the center of the cell to avoid damage
of quartz surfaces and chemical reactions of the com-
pounds at the surfaces. The OPL of neat THF in a quartz
cuvet was found to be insignificant compared with that
of solutions of the investigated compounds.
Scheme 1
have been studied using picosecond pulses at 1064 nm
with the DFWM technique.28 Large optical non-linear-
ities, originating from instantaneous electronic polariza-
tion, were determined for both a solution and a thin film
of the polymer. Interestingly, investigations on thiophene
and bithiophene using 400 nm sub-picosecond or femto-
second pulses have shown ultrafast triplet state formation
from a TPA-populated singlet state for the com-
pounds.19,29 At least bithiophene was found to have
excited singlet and triplet state absorption. In another
study, triplet thiophene was probed by weak phosphores-
cence, with a maximum at 430 nm.30 Hence, we found the
thiophene building block to be interesting in the search
for efficient OPL compounds because both TPA and ESA
may take place in thiophene derivatives.
Quantum chemistry calculations
Semi-empirical AM134 calculations of non-resonant (sta-
tic) ꢀ were performed with MOPAC 200035 using a time-
dependent Hartree–Fock (TDHF) method.36 ZINDO cal-
culations37,38 were carried out with the Gaussian 98W39
package and AM1-optimized geometries. Molecular
geometry optimizations were also performed with the
B3LYP40 quantum chemistry method and the 6–31G(d)41
basis set, using Gaussian 98W software.
Synthesis
The compounds 4-pentylphenylacetylene, 2,5-diiodothio-
phene, 3-n-dodecyl-2,5-dibromothiophene and PdCl2
(PPh3)2 were obtained from Aldrich and were used as
received. The solvents used in the reactions were of p.a.
quality. 4-(4-Iodophenyl)-2-methyl-3-butyn-2-ol was pre-
pared as described earlier.42,43 Thin-layer chromatography
(TLC) was performed on silica gel 60 F254 (Merck) and
flash column chromatography was performed on silia gel
EXPERIMENTAL
General
Infrared measurements were performed with a Mattson
Fourier-transform 60AR instrument. The transmission
spectra were recorded with Shimadzu UV-3101PC and
Cary 5G UV-VIS-NIR spectrophotometers. The former
was used also for optical absorbance measurements.
The 1H and 13C NMR spectra were obtained on a Bruker
DRX-400 instrument. Tetramethylsilane (TMS) was
used as an internal chemical shift standard for all
CDCl3 samples except for the 13C NMR spectrum of
compound 7, where the tetrahydrofuran-d8 ([2H8]THF)
13C—O peak at ꢄ 67.15 was used as a reference. Mass
spectrometry for different compounds was carried out
on either of two instruments: a JMS-SX/SX102A dou-
ble-focusing magnetic sector mass spectrometer (Jeol,
Tokyo), using direct inlet and electron impact ioniza-
tion (EIþ), with ionizing voltage 70 eV, acceleration
voltage 10 kV and resolution 1000; or a Waters Micro-
mass ZQ quadrupole mass spectrometer for compounds
6 and 7, using direct inlet, electrospray ionization (ESI)
and a sampling cone voltage of 40 V. The CH3OH–
CH2Cl2 (1:2) solutions were 0.1 mM with respect to
both the thiophene and AgNO3.31 An f/5 optical ar-
rangement and a frequency-doubled Nd:YAG laser
delivering 5 ns pulses at 532 nm with a repetition rate
of 10 Hz was used in the OPL investigations. The
˚
(Matrex 60 A, 35–70 mm, Grace Amicon).
2-Methyl-4-[4-(4-pentylphenylethynyl)phenyl]-3-
butyn-2-ol (1). To a solution of 4-(4-iodophenyl)-2-
methyl-3-butyn-2-ol (4.1 g, 14 mmol) in 80 ml of pyri-
dine and 80 ml of triethylamine was added PdCl2(PPh3)2
(20 mg, 0.29 mmol), PPh3 (60 mg, 0.11 mmol) and CuI
(55 mg, 0.29 mmol). 4-Pentylphenylacetylene (4.8 g,
28 mmol) was added dropwise to the mixture and the
reaction was stirred for 20 h at room temperature.
The solvent was removed under reduced pressure
and the resultant pale brown solid was collected using
ether. The organic phase was washed twice with 50 ml of
1 M HCl, twice with 50 ml of water and dried with
MgSO4. Recrystallization from heptane of the brownish
solid gave 4.1 g (89%) of white fluffy product. Rf ¼ 0.25
in heptane–EtOAc (10:1); m.p. ¼ 99–102 ꢁC. IR:
ꢅ(cmꢂ1) ¼ 3471 br m, 2925 s, 2854 m, 2163 w, 1516 s,
1452 m, 1360 s, 1271 m, 1157 s; 1H NMR (CDCl3):
ꢄ ¼ 0.87 (t, 3H), 1.29 (m, 4H), 1.60 (m, 8H), 2.00 (s,
1H), 2.59 (t, 2H), 7.15 (d, 2H), 7.37 (d, 2H), 7.42 (t, 4H);
13C NMR (CDCl3): ꢄ ¼ 14.0, 22.5, 30.9, 31.4, 35.9, 65.6,
81.9, 88.3, 91.4, 95.4, 120.1, 122.3, 128.5, 131.4, 131.5,
131.6, 143.7.
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428
P. LIND ET AL.
1-Ethynyl-4-(4-pentylphenylethynyl)benzene (2).
The protected alkyne 1 (1.39 g, 4.2 mmol) was dissolved
in 100 ml of benzene, NaH (0.5 g, 50% dispersion in oil)
was added and the mixture was slowly distilled until
50 ml of the distillate had been collected. After being
allowed to cool, the solution was poured into cold water.
The organic phase was dried with MgSO4, filtered and
evaporated. The brown–yellow product was passed
through a silica column (hexane–EtOAc, 10:1). The
resultant yellow product was passed through a short
alumina column using hexane as eluent. This gave 1.0 g
(88%) of white product. Rf ¼ 0.9 in heptane–EtOAc
(10:1); m.p. ¼ 65–67 ꢁC. IR: ꢅ(cmꢂ1) ¼ 3270 s, 2923 s,
2213 w, 1598 w, 1514 m, 1464 m; 1H NMR (CDCl3):
ꢄ ¼ 0.87 (t, 3H), 1.30 (m, 4H), 1.59 (m, 2H), 2.59 (t,
2H), 3.15 (s, 1H), 7.16 (d, 2H), 7.40–7.44 (m, 6H); 13C
NMR (CDCl3): ꢄ ¼ 13.8, 22.3, 30.7, 31.2, 35.7, 78.5,
83.1, 88.0, 91.4, 119.8, 121.4, 123.8, 128.3, 131.2, 131.3,
131.8, 143.6; EI þ MS: m/z (int %) ¼ 215 (100), 272 (67).
temperature. The solvent was removed and the residue
dissolved in CH2Cl2 and washed with 1 M HCl and water.
The organic phase was dried with MgSO4, filtered and
evaporated, and the product was purified on a silica
column using heptane as eluent. This gave 0.78 g (41%)
of a pinkish product; m.p. 62–64 ꢁC. IR: ꢅ(cmꢂ1) ¼
3033 w, 2919 m, 2194 w, 1747 m, 1649s, 1479s, 1203 m;
1H NMR (CDCl3): ꢄ ¼ 0.88 (t, 6H), 1.32 (m, 8H), 1.62
(m, 4H), 2.61 (t, 4H), 7.12 (s, 2H), 7.15 (d, 4H), 7.42 (d,
4H); 13C NMR (CDCl3): ꢄ ¼ 14.0, 22.5, 30.8, 31.6, 35.9,
81.7, 94.2, 119.7, 124.7, 128.5, 131.4, 131.5, 143.9; EI þ
MS: m/z (int %) ¼ 424 (100).
3-Dodecyl-2,5-di-[4-(4-pentylphenylethynyl)-phe-
nylethynyl]thiophene (6). To a solution of 3-n-dode-
cyl-2,5-dibromothiophene (0.38 g, 0.9 mmol) in 10 ml of
pyridine and 10 ml of triethylamine under Ar atmosphere
was added PdCl2(PPh3)2 (50 mg, 0.074 mmol), PPh3
(40 mg, 0.15 mmol) and CuI (20 mg, 0.11 mmol). Alkyne
2 was dissolved in 20 ml of pyridine and added dropwise
over a period of 2 h. The solution was stirred for 48 h at
room temperature and for 2 h at 50 ꢁC. The solvent was
removed and the residue dissolved in CH2Cl2 and washed
with 1 M HCl and water. The organic phase was dried with
MgSO4, filtered and evaporated. The solid was dissolved
in hot EtOAc–hexane (5:95) and filtered through a short
silica column (Rf ¼ 0.25 in hexane). Repeated recrystal-
lization from hot hexane yielded 0.33 g (46%) of com-
pound 6 as yellow crystals; m.p. ¼ 83–85 ꢁC. IR:
ꢅ(cmꢂ1) ¼ 3029 w, 2918 s, 2852 s, 2192 w, 1916 w,
2-Methyl-4-{4-[4-(4-pentylphenylethynyl)pheny-
lethynyl]phenyl}-3-butyn-2-ol (3). The coupling bet-
ween compound 2 (0.60 g, 2.2 mmol) and 4-(4-
iodophenyl)-2-methyl-3-butyn-2-ol (0.40 g, 1.4 mmol)
was performed as described for the synthesis of com-
pound 1. The yield of compound 3 was 0.50 g (83%);
m.p. ¼ 218–220 ꢁC. IR: ꢅ(cmꢂ1) ¼ 3465 br m, 2925 s,
2856 m, 1924 w, 1518 s, 1454 s, 1373 m, 1270 m,
1
1157 s; H NMR (CDCl3): ꢄ ¼ 0.83 (t, 3H), 1.26 (m,
4H), 1.56 (m, 8H), 1.94 (s, 1H), 2.55 (t, 2H), 7.09 (d, 2H),
7.31–7.42 (m, 10H); 13C NMR (CDCl3): ꢄ ¼ 14.0, 22.5,
30.9, 31.4, 35.8, 64.5, 81.8, 88.4, 90.7, 91.7, 95.4, 95.6,
120.0, 122.5, 122.7, 122.9, 123.6, 128.5, 131.4, 131.5,
131.6, 132.4, 143.7.
1
1511 m, 1454 m, 1375 m, 1307 w; H NMR (CDCl3):
ꢄ ¼ 0.88 (m, 9H), 1.25–1.33 (m, 26H), 1.61 (m, 6H),
2.61 (t, 4H), 2.72 (t, 2H), 7.06, (s, 1H), 7.15 (d, 4H),
7.43–7.50 (m, 12H); 13C NMR (CDCl3): ꢄ ¼ 14.0, 14.1,
22.5, 22.7, 29.2, 29.4, 29.4, 29.5, 29.6, 29.7, 29.7, 30.1,
30.9, 31.5, 31.9, 35.9, 83.8, 84.5, 88.5, 88.5, 91.8, 91.9,
93.8, 96.2, 120.0, 120.0, 120.1, 122.2, 122.5, 123.1,
123.5, 123.7, 128.5, 131.2, 131.3, 131.5, 131.5, 133.2,
143.8, 148.1; ESI þ MS: m/z ¼ 899 ([6 þ Ag]þ; signifi-
cant peaks at 899, 900, 901, 902, as expected from the
calculated major isotope distribution at 899.4, 900.4,
901.4, 902.4), 1692 ([6 þ 6 þ Ag]þ; significant peaks at
1692, 1693, 1694, 1695, 1696, as expected from the
calculated major isotope distribution at 1691.9, 1692.9,
1693.9, 1694.9, 1695.9).
1-Ethynyl-4-(4-(4-pentylphenylethynyl)phenyle-
thynyl)benzene (4). The same procedure as described
for synthesis of alkyne 2 from compound 1 was applied
for the preparation of compound 4 from compound 3. The
yield of compound 4 was 89%; m.p. ¼ 176–181 ꢁC. IR:
ꢅ(cmꢂ1) ¼ 3268 s, 2915 s, 1926 m, 1517 s, 1407 m,
1
1267 m; H NMR (CDCl3): ꢄ ¼ 0.87 (t, 3H), 1.30 (m,
4H), 1.60 (m, 2H), 2.59 (t, 2H), 3.16 (s, 1H), 7.16 (d, 2H),
7.41–7.48 (m, 10H); 13C NMR (CDCl3): ꢄ ¼ 14.0, 22.5,
30.9, 31.4, 35.9, 79.0, 83.2, 88.4, 90.5, 91.1, 91.7, 120.0,
122.0, 122.4, 123.5, 123.6, 128.5, 131.5, 131.5, 131.5,
132.1, 132.4, 143.7; EI þ MS: m/z (int %) ¼ 315 (82), 372
(100).
3-Dodecyl-2,5-di-{4-[4-(4-pentylphenylethynyl)-
phenylethynyl]phenylethynyl}thiophene (7). To a
solution of 3-n-dodecyl-2,5-dibromothiophene (0.11 g,
0.27 mmol) in 5 ml of toluene and 5 ml of triethylamine
under Ar atmosphere was added PdCl2(PPh3)2 (15 mg,
0.022 mmol), PPh3 (14 mg, 0.054 mmol) and CuI (6 mg,
0.032 mmol). Alkyne 4 was dissolved in warm toluene
and added dropwise over a period of 2 h. After reaction at
90 ꢁC for 48 h, the solvent was removed and the residue
dissolved in CH2Cl2 and washed with 1 M HCl and water.
The organic phase was dried with MgSO4, filtered and
2,5-Di-(4-pentylphenylethynyl)thiophene (5). 2,5-
Diiodothiophene (1.5 g, 4.5 mmol) was dissolved in a
mixture of 20 ml of pyridine and 20 ml of triethy-
lamine under Ar atmosphere. PdCl2(PPh3)2 (60 mg,
0.085 mmol), PPh3 (70 mg, 0.27 mmol) and CuI (25 mg,
0.13 mmol) were added to the solution, followed by
dropwise addition of 4-pentylphenylacetylene (2 g,
12 mmol). The reaction was stirred for 48 h at room
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ARYLETHYNYL-SUBSTITUTED THIOPHENES FOR OPTICAL POWER LIMITING
429
evaporated. The solid was dissolved in hot toluene–
heptane (50:50) and filtered through a short silica column
using heptane–EtOAc (50:50) as eluent (Rf ¼ 0.4; hep-
tane–EtOAc, 10:1). The volume of the filtrate was re-
duced and a yellow precipitate was collected. The solid
was recrystallized by dissolving it in hot CH2Cl2 and
adding hexane until a precipitate was formed. The yield
of the yellow product was 75 mg (28%); m.p. ¼ 175–
178 ꢁC. IR: ꢅ(cmꢂ1) ¼ 3037 w, 2921 s, 2851 s, 2188 w,
1922 w, 1517 s, 1454 m, 1407 m, 1307 w; 1H NMR
(CDCl3): ꢄ ¼ 0.88 (m, 9H), 1.25–1.33 (m, 26H), 1.63
(m, 6H), 2.62 (t, 4H), 2.73 (t, 2H), 7.08 (s, 1H), 7.15 (d,
4H), 7.43–7.50 (m, 20H). 13C NMR (THF-d8): ꢄ ¼ 14.1,
14.2, 23.1, 23.3, 29.8, 29.9, 30.0, 30.1, 30.3–30.4, 30.7,
31.6, 32.1, 32.6, 36.4, 78.6, 79.0, 79.3, 84.2, 84.8, 88.8,
91.2, 91.2, 91.7, 92.2, 94.2, 96.8, 120.6, 120.9, 123.3,
123.5, 123.8, 124.0, 124.1, 124.5, 129.1, 131.9, 132.0–
132.2, 134.1, 144.3, 149.0; ESI þ MS: m/z ¼ 1099
([7 þ Ag]þ; significant peaks at 1099, 1100, 1101,
1102, 1103, as expected from the calculated major iso-
tope distribution at 1099.4, 1100.4, 1101.4, 1102.4,
1103.4).
thiophenes instead of diiodo analogs can be used for the
preparation of larger quantities of this type of compound.
The coupling reaction is a convenient and reliable method
to build and lengthen arylalkynyl systems, but is likely to
result in low overall yields in linear syntheses of extended
systems.
Early work in this project showed that the parent
compound of 6 lacking alkyl groups, here denoted 60,
has rather low solubility in THF. Therefore, the continued
work utilized pentyl groups for compound 5 and both
pentyl and dodecyl groups for compounds 6 and 7 to
increase their solubility. In spite of the alkyl groups,
compound 6 and especially compound 7 still have quite
low solubility in THF. Even though the unsubstituted
compounds were not studied experimentally, the struc-
tures 50–70 were included in the theoretical work reported
below.
Structural considerations, calculated properties
and absorption spectroscopy of OPL compounds
In this section we wish to give a background for the
choice of compounds in the study and report on some
structural features and properties of the compounds as
derived from quantum chemistry calculations. Some of
our guiding points in the search for efficient OPL systems
were as follows:
RESULTS AND DISCUSSION
Synthesis
A general Pd- and Cu-catalyzed reaction was employed
repeatedly to achieve coupling between a haloaryl and an
ethynylaryl unit in the preparation of compounds 5–7
(Scheme 2).44 The synthesis of the alkynes also involved
protection–deprotection of one terminal alkyne.42,43 It
may be worth mentioning that the coupling with the 2,5-
diiodothiophene in the preparation of compound 5 did not
result in a better yield than the reaction with a corre-
sponding dibromothiophene for the synthesis of com-
pound 6. Hence, it appears that more available dibromo
(i) The OPL compounds should be assembled from not
more than a few smaller OPL-active molecular
entities (A), having significant non-linear absorption
in relation to their number of atoms and ꢁ electrons.
The assembled compounds should, of course, have
substantially better OPL capacity than that expected
from adding together the OPL response of the
subunits.
(ii) The A groups should be connected by bridging
groups (B) that allow only a weak electronic
Scheme 2. Reagents and solvents: (a) PdCl2(PPh3)2, CuI, PPh3, pyridine, triethylamine; (b) NaH, C6H6
Copyright # 2004 John Wiley & Sons, Ltd. J. Phys. Org. Chem. 2005; 18: 426–433

430
P. LIND ET AL.
Table 1. Experimental and calculated absorption para-
interaction between adjacent A groups and only a
moderately efficient interaction between A and B, so
that for instance the absorption spectrum of the
molecule resembles a superimposition of the spectra
of different A and B groups.
meters and ꢀ values of thiophenes 5–7 and model thio-
phenes 50–70
Theoretical
Experimentala
Compound
ꢆ
f b
ꢀ/105c
(a.u.)
ꢆmax " (ꢆmax)/104
(iii) The OPL compounds should have good transpar-
ency in the visible range.
ꢂ1
)
(nm)b
(nm)
(cmꢂ1
M
(iv) The OPL compounds should be rigid to avoid
having the OPL response being averaged by mole-
cular conformations of unknown distribution, which
would complicate interpretation of the results. A
high molecular symmetry (inversion centre or C2v
symmetry) should be an advantage45 because it may
facilitate a theoretical description of various excited
states.
5
381
389
393
378
388
392
1.60
2.95
3.79
1.50
3.01
3.91
6.0
18.8
35.5
4.3
18.0
35.1
2.1
352
380
385
3.5
8
13
6d
7d
50
60
70
BMTe
a
Tetrahydrofuran solution.
Both ꢆ and f (oscillator strength) for the dominant excitation from ZINDO
b
(v) The compounds should be stable both chemically
and thermally.
calculations.
c
Orientationally averaged second hyperpolarizability from AM1 calcula-
tions.
d
Methyl instead of longer alkyl groups in compounds 6 and 7 were used to
simplify the calculations.
Reasons for considering the thiophenyl group as an A
component were mentioned in the Introduction. The
ethynyl group appeared to be a suitable B component,
because its interaction with aromatic rings such as phenyl
and thiophenyl can be described as a weak coupling due
to ꢁ-orbital mismatch. The mismatch arises from a low-
lying HOMO and a high-lying LUMO in the ethynyl
group. An example of this rather weak interaction is that
several para-disubstituted diphenylacetylenes (tolanes)
show a blue-shift of ꢆmax in the absorption spectrum
compared with the corresponding ethylenes (stilbenes).46
A possible drawback of using ethynyl instead of ethenyl
groups may be that somewhat lower ꢀ values can be
expected. Such an outcome was demonstrated by THG
experiments using 636 nm output wavelength on the
tolanes and stilbenes.46 Nevertheless, a DFWM study at
620 nm on thin films of phenyl-ethynyl compounds hav-
ing alkoxy substituents showed a ꢂ(3) value of a phenyl—
CꢃC—(p-phenylene)—CꢃC—phenyl compound that
was only 46% lower than that of the corresponding
polymer,47 suggesting that rather small units may have
significant NLO properties if electron-donating groups
are attached to the aromatic rings. Hence the CꢃC bridge
e
2,5-Bis(benzothiazol-2-yl)-3,4-(dimethoxy)thiophene; see text.
compound 5 and of somewhat simplified structures of 6
and 7 having methyl instead of the pentyl and dodecyl
groups suggested that the compounds should give trans-
parent solutions because the only absorptions near the
visible region would be at a shorter wavelength than
400 nm (Table 1). This was corroborated from the UV–
VIS absorption spectra of THF solutions of 5–7 (Fig. 1).
The position of the absorption peak is, as expected,
shifted towards the red from compound 5 to 6 and 7,
with absorption maxima of 352, ꢀ380 and 385 nm,
respectively. Along with the shift in peak position
there is a marked increase in extinction coefficient
demonstrating increased oscillator strength for the
more conjugated structures. However, although the
absorptions increase the yellowish appearance of com-
pounds 6 and 7 compared with 5, there is virtually
no absorption at wavelengths longer than 450 nm.
Solutions of all three compounds therefore have good
transparency at the highest possible concentrations in
THF (see below).
—
can, to some extent, in comparison with the C C spacer,
—
reduce the electronic coupling between ꢁ-electron sys-
tems and thus provide compounds with low absorbance in
the visible region. Obviously, ethynyl vs. ethenyl spacers
will result in stiffer and more rod-like compounds.
We used phenylene rings to extend the ꢁ-system from
compound 5 to 6 and 7 in order to investigate the effect of
the conjugation length on OPL. In addition to the con-
tribution of ꢁ-electrons, the use of phenylene rings is
mainly due to synthetic convenience and thermal stability
reasons.
Semi-empirical AM1 molecular orbital calculations
showed the lowest energy conformations of compounds
5–7 to have coplanar rings, as expected. This was also the
result from B3LYP/6–31G(d) calculations of compounds
50–70 (lacking alkyl groups). ZINDO calculations of
Figure 1. Absorption spectra for thiophenes 5–7 in THF
J. Phys. Org. Chem. 2005; 18: 426–433
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ARYLETHYNYL-SUBSTITUTED THIOPHENES FOR OPTICAL POWER LIMITING
431
The hyperpolarizability calculations show substantial
increase of non-resonant ꢀ from the smaller to the larger
compounds: almost by a factor of six from compound 5 to
7 and a factor of eight from 50 to 70 (Table 1). The ꢀ values
increase more when going from 50 to 60 than from 60 to 70.
This is also found for the series of compounds 5–7 but
here the trend is less pronounced. In addition, the calcu-
lations show that alkyl substitution gives an increase of ꢀ;
this is especially apparent when 5 is compared with 50.
Because alkyl groups function as weak electron donors in
normal organic molecules, other electron-donating sub-
stituents can be of interest for modification of the OPL
effect in this type of compound.
For a comparison with compounds 5–7, ꢀ of the related
compound 2,5-bis(benzothiazol-2-yl)-3,4-(dimethoxy) thi-
ophene (see Introduction) was calculated. This com-
pound, abbreviated to BMT in Table 1, has a markedly
lower ꢀ value than the other structures, which suggests
that compounds 5–7 can be better choices for OPL
studies than BMT. However, one should bear in mind
that calculations of ꢀ may bear little information on the
magnitude of OPL, because several factors contribute
that are not taken into account in the calculations.
Examples of these factors are the lifetime of excited
singlet and triplet states in conjunction with laser pulse
duration and quality, and concentration/medium ef-
fects,48 which may involve intermolecular interactions
between chromophores.
Other common aryl groups instead of thiophene also
were considered in the design of OPL systems, with 1,9-
substituted anthracene being one such example. Although
1,9-di(phenylethynyl)anthracene has a calculated ꢀ value
of 6.0 ꢄ 105 a.u., which is slightly greater than that of 50,
this compound has considerable optical absorption in the
region of 400–500 nm,49 which made it less interesting
for our OPL study.
AM1 semi-empirical calculations of a series of push–
pull diaryl acetylenes have shown that twisting from the
favored planar geometry to the conformation with ortho-
gonal rings requires an energy of only 0.3 kcal mol .
ꢂ1 50 It
was found also from MNDO-based calculations of the
molecular first hyperpolarizability that the donor–accep-
tor interaction is still a dominant contribution even when
the two phenyl rings are set orthogonally to each other.50
We performed similar calculations of non-resonant ꢀ
using the TDHF/AM1 method to estimate the effect of
inter-ring torsion in compound 5. The geometry optimi-
zation of the conformation with the thiophene ring con-
strained orthogonal to both phenylene rings shows that
this conformer has only 0.5 kcal molꢂ1 higher energy
(ÁHf) than that of the lowest energy, in which all
three rings are coplanar. The former conformation has
a value of ꢀ that is 47% of the maximum. A third
conformation of compound 5, having one phenylene
ring orthogonal to the two other rings, has ÁHf and ꢀ
values intermediate to the other values. Further-
more, compound 70 with the middle phenylene ring
constrained in the calculations to be perpendicular to
the adjacent rings, in both of the thiophene alkynyl
substituents, has a ꢀ value of 20.3 ꢄ 105 a.u., which is
58% of the value for the conformer having all rings
coplanar. Although calculations at this level of theory
are unreliable with respect to both energy and hyperpo-
larizability, it seems likely that compounds 5–7 will have
a fairly wide distribution of conformations around the
energy minimum at normal temperature, with different
inter-ring torsion angles, but that ꢀ will not be reduced
severely because of that.
In recent work, we applied a chemometrics (partial
least squares, PLS) approach for investigating relation-
ships between non-linear absorption at 532 nm and read-
ily accessible molecular ground-state parameters from
DFT calculations.51 For a set of 23 organic compounds
and 41 initial variables, a model was found where six
molecular properties were important for describing the
optical limiting ability of the compounds. The parameters
found to be relevant were: the number of electrons; the
number of occupied molecular orbitals above ꢂ10 eV;
the mean polarizability; the mean quadrupole moment;
the total energy of the five highest occupied molecular
orbitals; and the difference in energy between the five
lowest unoccupied and five highest occupied molecular
orbitals. The values of these parameters for compounds
5–70 are, respectively: 148, 252, 356; 15, 25, 35; 321,
709, 1122; 111, 186, 259; 32.7, 31.1, 29.5 eV; and 30.0,
26.0, 22.4 eV, respectively. When going from 50 !
60 ! 70, all six parameters change in the order predicted
by the model to give better OPL.
Non-linear absorption measurements
The non-linear absorption of compounds 5–7 was re-
corded using 5 ns laser pulses at 532 nm (see Experi-
mental). Values of the transmitted energy (Iout) as a
function of the input energy (Iin) from the laser for
THF solutions of compounds 5–7 are given in Table 2.
A typical OPL curve of compound 5 is shown in Fig. 2.
Generally, the OPL curves display a leveling of Iout at
Table 2. Experimental linear (T ) and non-linear transmis-
sion data of 2,5-dialkynyl-substituted thiophenes 5–7 at
532 nm in THF solution
Compound
Conc. (M) T (%) Iout (mJ) at Iin ¼ 150 mJ
5
1.2
0.50
0.10
0.050
0.010
0.061
0.010
0.018
0.010
76
94
98
99
>99
92
99
97
4.2
7
12
15
23
11.5
16
6
7
13
14
99
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2005; 18: 426–433

432
P. LIND ET AL.
SUMMARY
The non-linear absorption at 532 nm of 0.01 M solutions
of the arylalkynyl-substituted thiophenes increases from
compound 5 to 6 and 7, which shows that the extent of ꢁ-
conjugation is important for the magnitude of the re-
sponse, as expected. This is also indicated by ꢀ values
and other electronic parameters obtained from quantum
chemistry calculations, but the magnitude of the OPL
enhancement from compound 6 to 7 appears to be less
than suggested by the calculations. The values of ꢀ from
AM1 TDHF calculations are somewhat greater for com-
pounds 5–7 having alkyl groups than for the model
compounds 50–70 that do not have alkyl groups. The ꢀ
values for 50–70 suggest that a larger increase of NLO
effects is to be expected when going from 50 to 60 rather
than from 60 to 70. The same trend is noticed for
compounds 5–7, but the trend is less pronounced.
Although the conformational space of the molecules
was not fully explored, calculations on compounds 5
and 70 reveal that twisting of one or two aryl rings relative
to the other rings will not drastically decrease the ꢀ
values.
Figure 2. Optical limiting at 532 nm of 0.5 M thiophene 5 in
THF
values of Iin in the range 100–150 mJ, but this leveling
is not always complete to give a true clamping level.
At even higher values of Iin, damage of the cuvet or
the solute molecules occurs. For simplicity, the values
of Iout are reported at Iin ¼ 150 mJ in Table 2. The best
OPL is found for the highest concentration (1.2 M)
of compound 5. Although a clamping level of ꢅ 1 mJ
for the transmitted light is preferred for good laser
protection of eyes, the value of ꢀ4 mJ is interesting
in view of the simplicity of structure 5. Compounds
6 and 7 could not be measured at such high concent-
ration because of their limited solubility in THF. The
maximum concentration of those two compounds was
ꢀ0.06 and ꢀ0.02 M, respectively. However, a compara-
tive study of 0.010 M solutions of compounds 5–7 showed
increased optical limiting in the expected order 5 < 6 < 7
(Table 2).
Values of the linear transmission at 532 nm are also
given in Table 2, showing good transparency of normal
light at that wavelength even for the most concentrated
sample of compound 5. A transmission spectrum of
compound 5 is shown in Fig. 3.
Additional work aimed at a better understanding of the
mechanisms for OPL in this series of compounds is in
progress.
Acknowledgment
This work was part of a Photonics in Defense Applica-
tions program supported by the Swedish Defence Mate-
rial Administration (FMV).
REFERENCES
1. Kuzyk MG, Garvey DW, Vigil SR, Welker DJ. Chem. Phys. 1999;
245: 533.
2. Frazier DO, Penn BG, Smith DD, Witherow WK, Paley MS,
Abdeldayem HA. Plast. Eng. (NY) 1998; 49: 693.
3. Stegeman GI. In Nonlinear Optical Organic Molecular and
Polymer, Nalwa HS, Miyata S (eds). CRC Press: Boca Raton,
FL, 1997; 799–812.
4. Zyss J. Molecular Nonlinear Optics: Materials, Physics, and
Devices. Academic Press: Boston, 1994.
5. Shi S. Contemp. Phys. 1994; 35: 21.
6. Kershaw S. Opt. Eng. (NY) 1998; 60: 515.
7. Luther-Davies B, Samoc M. Curr. Opin. Solid State Mater. Sci.
1997; 2: 213.
¨
8. Kohler RH, Cao J, Zipfel WR, Webb WW, Hanson MR. Science
1997; 276: 2039.
9. Birch DJS. Spectrochim. Acta, Part A 2001; 57: 2313.
10. Cumpston BH, Ananthavel SP, Barlow S, Dyer DL, Ehrlich JE,
Erskine LL, Heikal AA, Kuebler SM, Lee IYS, McCord-
Maughon D, Qin JQ, Rockel H, Rumi M, Wu XL, Marder SR,
Perry JW. Nature 1999; 398: 51.
11. Van Stryland EW, Hagan DJ, Xia T, Said AA. In Nonlinear
Optical Organic Molecular and Polymer, Nalwa HS, Miyata S
(eds). CRC Press: Boca Raton, FL, 1997; 841–860.
12. Tutt LW, Boggess TF. Prog. Quant. Electron. 1993; 17: 299.
13. Sun YP, Riggs JE. Int. Rev. Phys. Chem. 1999; 18: 43.
14. Dini D, Barthel M, Hanack M. Eur. J. Org. Chem. 2001;
20: 3759.
15. Reinhardt BA, Brott LL, Clarson SJ, Dillard AG, Bhatt JC,
Kannan R, Yuan LX, He GS, Prasad PN. Chem. Mater. 1998;
10: 1863.
Figure 3. Linear transmission spectrum of 0.94 M thiophene
5 in THF
Copyright # 2004 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2005; 18: 426–433

ARYLETHYNYL-SUBSTITUTED THIOPHENES FOR OPTICAL POWER LIMITING
433
16. Kannan R, He GS, Yuan LX, Xu FM, Prasad PN, Dombroskie
AG, Reinhardt BA, Baur JW, Vaia RA, Tan LS. Chem. Mater.
2001; 13: 1896.
17. Baur JW, Alexander MD Jr., Banach M, Denny LR, Reinhardt
BA, Vaia RA, Fleitz PA, Kirkpatrick SM. Chem. Mater. 1999; 11:
2899.
18. Perry JW. In Nonlinear Optical Organic Molecular and Polymer,
Nalwa HS, Miyata S (eds). CRC Press: Boca Raton, FL, 1997;
813–840.
19. Paa W, Yang JP, Rentsch S. Synth. Met. 2001; 119: 525.
20. Zhao M, Samoc M, Prasad PN, Reinhardt BA, Unroe MR, Prazak
M, Evers RC, Kane JJ, Jariwala C, Sinsky M. Chem. Mater. 1990;
2: 670.
38. Zerner MC. In Reviews in Computational Chemistry, vol. 2, Boyd
DB(ed). VCH: New York, 1991; 313–365.
39. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA,
Cheeseman JR, Zakrzewski VG, Montgomery JA, Stratmann RE,
Burant JC, Dapprich S, Millam JM, Daniels AD, Kudin KN,
Strain MC, Farkas O, Tomasi J, Barone V, Cossi M, Cammi R,
Mennucci B, Pomelli C, Adamo C, Clifford S, Ochterski J,
Petersson GA, Ayala PY, Cui Q, Morokuma K, Malick DK,
Rabuck AD, Raghavachari K, Foresman JB, Cioslowski J, Ortiz
JV, Stefanov BB, Liu G, Liashenko A, Piskorz P, Komaromi I,
Gomperts R, Martin RL, Fox DJ, Keith T, Al-Laham MA, Peng
CY, Nanayakkara A, Gonzalez C, Challacombe M, Gill PMW,
Johnson BG, Chen W, Wong MW, Andres JL, Head-Gordon M,
Replogle ES, Pople JA, Gaussian 98, Revision A. 4. Gaussian:
Pittsburgh, PA, 1998.
21. Nalwa HS. In Nonlinear Optical Organic Molecular and Polymer,
Nalwa HS, Miyata S (eds). CRC Press: Boca Raton, FL, 1997;
611–797.
22. Kamada K, Ueda M, Sakaguchi T, Ohta K, Fukumi T. Chem.
Phys. Lett. 1996; 263: 215.
40. Becke AD. J. Chem. Phys. 1993; 98: 5648.
41. Hehre WJ, Ditchfield R, Pople JA. J. Chem. Phys. 1972; 56:
2257.
¨
23. Kamada K. J. Phys. Chem. A 2000; 104: 4723.
24. Millefiori S, Alparone A. Chem. Phys. Lett. 2000; 332: 175.
25. He GS, Xu GC, Prasad PN, Reinhardt BA, Bhatt JC, Dillard AG.
Opt. Lett. 1995; 20: 435.
26. Das GP, Dudis DS. Chem. Phys. Lett. 1999; 312: 57.
27. Sahraoui B, Phu XN, Baldeck P, Nunzi J-M, Frere P, Roncali J.
Synth. Met. 2000; 109: 315.
42. Hanninen E, Takalo H, Kankare J. Acta Chem. Scand., Ser. B
1988; B42: 614.
43. Mongin O, Gossauer A. Tetrahedron 1997; 53: 6835.
44. Takahashi S, Kuroyama Y, Sonogashira K, Hagihara N. Synthesis
1980; 8: 627.
45. Albota M, Beljonne D, Bredas JL, Ehrlich JE, Fu JY, Heikal AA,
Hess SE, Kogej T, Levin MD, Marder SR, McCord-Maughon D,
Perry JW, Rockel H, Rumi M, Subramaniam C, Webb WW, Wu
XL, Xu C. Science 1998; 281: 1653.
28. Zhan X, Liu Y, Zhu D, Liu X, Xu G, Ye P. Chem. Phys. Lett. 2001;
343: 493.
29. Yang JP, Paa W, Rentsch S. Synth. Met. 1999; 101: 624.
30. Evans CH, Scaiano JC. J. Am. Chem. Soc. 1990; 112: 2694.
31. Roussis SG, Proulx R. Anal. Chem. 2002; 74: 1408.
32. Vincent D. Nonlin. Opt. 1999; 21: 413.
46. Cheng L-T, Wilson T, Marder SR, Stiegman AE, Rikken G,
Spangler CW. J. Phys. Chem. 1991; 95: 10643.
47. Wautelet P, Moroni M, Oswald L, Le Moigne J, Pham A, Bigot
JY, Luzzati S. Macromolecules 1996; 29: 446.
33. Vincent D, Cruickshank J. Appl. Opt. 1997; 36: 7794.
34. Dewar MJS, Zoebisch EG, Healy EF, Stewart JJP. J. Am. Chem.
Soc. 1985; 107: 3902.
35. MOPAC-2000, Chem3D Pro 7.0. Cambridge Soft Corp.,
Cambridge, MA.
48. Riggs JE, Martin RB, Walker DB, Guo Z, Sun Y-P. Phys. Chem.
Chem. Phys. 2004; 6: 703.
49. Maulding DR, Roberts BG. J. Org. Chem. 1969; 34: 1734.
50. Barzoukas M, Fort A, Klein G, Boeglin A, Serbutoviez C, Oswald
L, Nicoud JF. Chem. Phys. 1991; 153: 457.
¨
36. Karna S, Dupuis M. J. Comp. Chem. 1991; 12: 487.
37. Ridley JE, Zerner MC. Theo. Chim. Acta 1976; 42: 223.
51. Lind P, Lopes C, Oberg K, Eliasson B. Chem. Phys. Lett. 2004;
387: 238.
Copyright # 2004 John Wiley & Sons, Ltd.
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