Poly(5-tert-butyl)benzothiophene: A soluble form of polyisothianaphthene with a large nonlinear optical response

Article abstract of DOI:10.1039/a804209h

A soluble form of polybenzothiophene has been produced and chemically characterised. NMR spectroscopy indicates that there is a large quinoidal character contribution to the ground state of the polymer. The third order nonlinear optical properties of the

Full text of DOI:10.1039/a804209h

J O U R N A L O F
Poly(5-tert-butyl)benzothiophene: a soluble form of
polyisothianaphthene with a large nonlinear optical response
Anna Drury, Simon Burbridge, Andrew P. Davey and Werner J. Blau*
Physics Department, Trinity College, Dublin 2, Ireland
C H E M I S T R Y
Received (in Cambridge) 4th June 1998, Accepted 6th August 1998
A soluble form of polybenzothiophene has been produced and chemically characterised. NMR spectroscopy
indicates that there is a large quinoidal character contribution to the ground state of the polymer. The third order
nonlinear optical properties of the polymer have been probed and it is found that the response is relatively large in
the near-infrared region.
The third order nonlinear optical properties of conjugated
organic semiconducting polymers have been the subject of
much investigation for the past 15 years. It is expected that
such materials will find use in optical switching devices based
on planar waveguides.1
Many different classes of such polymers have been
investigated2 in the search for a material which exhibits a
sufficiently large third order nonlinear optical response for use
in all optical switching devices. To date, a material which
meets all device requirements has still not been identified. For
certain applications within telecommunications, materials
which exhibit a large and fast third order nonlinearity in the
region of 1.3–1.5 mm are required.
More recently, conjugated molecules with absorption in the
near-infrared region have attracted attention since it is hoped
that the nonlinear optical response will be relatively large.
Polyisothianaphthene (PITN) (or polybenzothiophene) is just
such a material. It was first produced some years ago in an
insoluble form3 and has been shown to possess a small
bandgap4 (ca. 1 eV ).
The absence of soluble forms of this polymer has however
precluded any study of its nonlinear optical properties in
solution or high quality thin film forms. Recently, Pomerantz
et al. reported the synthesis of a soluble form of a similar
polymer in a preliminary communication.5
method of Hanack6 and the product isolated by careful low
pressure distillation. The dihydro precursor 2 was synthesised
using the method previously reported for the unsubstituted
derivative.7 The polymerisation is catalysed by 1 equiv. of
FeCl 8.
3
NMR analysis of 3 is in good agreement with values
reported for PITN9 and suggests that there is a large degree
of quinoidal character in the ground state. The 1H NMR
analysis serves to confirm this observation, the broad signal
at d 8.2–8.6 being assigned to the deshielded environment of
the proton attached to carbon 6 (see Scheme 1). The broadness
of the signals may also be explained by this observation since
a large contribution of quinoidal structure serves to ‘rigidise’
the polymer backbone, decreasing freedom of rotation about
the bonds connecting repeat units.
The product polymer is soluble in polar organic solvents
such as chloroform, THF or toluene. Fig. 1 shows the visible/
near-infrared absorption spectrum of a 1 g l−l solution of the
polymer in chloroform.
The broad absorption shows several vaguely defined
shoulders probably indicating a large degree of coupling
between electronic transitions and vibrational levels.
Temperature dependent absorption studies currently in pro-
gress will yield further information regarding this point. An
estimate of the optical gap from this spectrum gives E =ca.
g
We report here on the synthesis and optical (both linear
and nonlinear) characterisation of another soluble form of
this polymer, namely, poly(5-tert-butyl)benzothiophene.
The route employed to synthesise this polymer is depicted
in Scheme 1. The key intermediate 1 was produced by the
1.1 eV. This value is in close agreement with that reported by
Pomerantz et al.5 and is marginally greater than the value
originally reported for the insoluble form.3
The experimental method used to determine the microscopic
third order nonlinear optical coefficient (c) of a compound at
1064 nm in solution is described in detail elsewhere.10 It is
based on self-diffraction from transient laser-induced diffrac-
tion gratings. Under thin grating conditions,11 an expression
relating the diffraction efficiency, g into the first order, to the
H3C
CH3
BrH2C
CH2Br
i
But
But
1
ii
S
S
1
3
iii
7a
7
3a
n
4
5
6
But
But
3
2
Scheme 1 Reagents and conditions: i, NBS, CCl , reflux, 1 h, then
4
room temp., 18 h; ii, Na S, EtOH, reflux, 1 h; iii, FeCl , CHCl , 50 °C,
2
3
3
dry air, 72 h.
Fig. 1 Electronic absorption spectrum of 3.
J. Mater. Chem., 1998, 8(11), 2353–2355
2353

been synthesised and characterised. The optical properties of
the polymer show that the material possesses a small gap,
slightly larger than that of the insoluble form. It is also likely
from these spectra that there is a strong degree of coupling
between electronic transitions and vibrational states. This last
point is the subject of further investigations.
The microscopic nonlinear optical response is larger than
that of comparable molecules and polymers. This is most
likely due to the closer position of the absorption band to the
measurement wavelength.
Experimental
Fig. 2 Concentration dependence of the laser induced grating
diffraction efficiency for 3.
All solvents were dried prior to use using standard methods.
All reactions were carried out in an argon atmosphere unless
otherwise stated. NMR spectra were recorded on a Bruker
MSL 300 spectrometer and TMS was used as an internal
reference. IR spectra were recorded on a Nicolet 510-P FTIR
spectrometer. GPC analysis was performed using a Waters
600t system.
third order material nonlinearity x(3) is valid [eqn. (1)],
|x(3)|=4 e c n2 l √g/(3 p d I )
(1)
o
o
where c is the speed of light, e is the permittivity of free
o
space, n is the refractive index of the sample, d is the sample
thickness and I is the input pulse intensity. In the experiments
reported here, d=1 mm and n is taken to be the refractive
o
4-tert-Butyl-1,2-dimethylbenzene
index of the solvent, because of the low fractional volume
of solute.
The third order nonlinearity x(3) for a solvent/solute mixture
may be expressed as shown in eqn. (2),
106.2 g (1 mol) o-Xylene and 92.1 g (1 mol) tert-butyl chloride
were well mixed (magnetic stirrer). 1.l g Anhydrous ferric
chloride was added slowly (30 min) at room temperature.
When the evolution of hydrogen chloride had ceased, excess
tert-butyl chloride (20.5 g) was added and the mixture stirred
for a further 1 h. It was then heated in a water bath for 15 min
(turning brown at approx. 65 °C) and filtered through charcoal
(125 g). The resulting yellow solution was distilled and various
fractions of colourless liquid were collected (bp 155–175,
185–200 and 205–210 °C). The highest boiling fraction was
found to be 4-tert-butyl-1,2-dimethylbenzene. Yield: 90.6 g
(55.8%); d (300 MHz, CDC1 ) 1.3 (s, 9H), 2.2 (s, 3H), 2.25
|x(3)|=[(x(3) +Re x(3) )2+(Im x(3) )2]1/2
solv sol sol
(2)
where Re x(3) and Im x(3) are the real and imaginary
sol
sol
components of the material nonlinearity. By determining the
concentration dependence of |x(3)|, the contribution from x(3)
solv
sol
may be extracted and the magnitude of Re x(3) and Im x(3)
sol
may be determined. Furthermore, the sign of Re x(3) may
sol
be determined from the concentration dependence of the real
H
3
part of |x(3)|.
(s, 3H), 7.1 (m, 3H).
The c values of a solute may then be derived [eqn. (3)],
1,2-Bis(bromomethyl)-4-tert-butylbenzene 1
c=|x(3)|/(N C L 4)
(3)
A
L
8.125 g 4-tert-Butyl-o-xylene (0.05 mol), 17.8 g N-bromosuc-
cinimide (0.1 mol), 0.2 g benzoyl peroxide and 50 ml dry
carbon tetrachloride were placed in a 250 ml round-bottomed
flask and refluxed with magnetic stirring in the dark under
argon for 3 h. The mixture was left overnight at room tempera-
ture (under argon), then it was filtered (to remove succinimide
salts) and concentrated in vacuo. The product was collected
by vacuum distillation (bp 116–118 °C at 0.12 mmHg). Yield:
4.66 g (29%); d (300 MHz, CDCl 1.31 (s, 9H), 4.79 (s, 2H),
where C is the molecular concentration (for polymer samples,
the repeat unit concentration), N is Avogadro’s constant and
A
L
is the Lorentz local field factor, which is taken to be that
L
of a linear molecule (i.e. L =1).12.
Fig. 2 shows the concentration dependence of the diffraction
L
efficiency. As already described, theory predicts a parabolic
dependence. It is clear however that there is a deviation from
such a dependence in this case. At low concentration, the
dependence on diffraction efficiency is parabolic. At a certain
limit, however, this dependence begins to deviate before
returning at higher concentrations to a second parabolic
dependence. Such behaviour is not well understood but is
clearly due to some form of electronic interaction between
polymer chains.
H
3
4.81 (s, 2H), 7.40 (m, 2H), 7.55 (d, 1H); d (CDCl ) 30.53
C
3
and 30.56 (CH Br), 31.03 (CH ), 34.86 (Me C), 126.83 (CH),
2
3
3
128.66 (CH), 131.53 (CH), 134.39 (quaternary C), 136.88
(quaternary C), 152.95 (quaternary C).
The values of c measured for 3 from fitting data in the low
concentration region are given in Table 1. The c values
obtained for 3 are remarkably high in comparison to other
polymers. In comparison to a polythiophene (a structural
relative), for example, the values are one order of magnitude
greater. This is thought to be due to relatively closer pos-
itioning of the electronic absorption band to the wavelength
of measurement.
1,3-Dihydro-5-tert-butylisothianaphthene 2
l.05 g (0.013 mol) Anhydrous sodium sulfide was dissolved in
75 ml dry ethanol in a 250 ml round-bottomed two-necked
flask fitted with a magnetic stirrer and condenser. 3.98 g
(0.012 mol) 1,2-Bis(bromomethyl)-4-tert-butylbenzene was
added dropwise during 30 min. The solution went from pale
blue to bright yellow. It was refluxed for 1 h and the ethanol
removed in vacuo. The remaining brown–black oil was dis-
solved in CH Cl and filtered to remove sodium bromide. The
In summary, a soluble form of polyisothianaphthene has
2
2
CH Cl was removed in vacuo and the final product obtained
Table 1 Values of c for 3 compared to those of a polythiophene
(ref. 13).
2
2
by vacuum distillation (bp 88 °C at 6×10−2 mmHg). Yield:
1.53 g (64%); d (300 MHz, CDCl ) 1.33 (s, 9H), 4.25 (s, 2H),
H
3
Compound
c
(esu)
|c| (esu)
Im
|c| (esu)
4.28 (s, 2H), 7.18 (dd, 1H), 7.26 (dd, 1H), 7.29 (s, 1H);
Re
d (CDCl ) 31.30 (CH ), 34.49 (Me C), 37.67 (CH ), 38.16
C
3
3
3
2
3
−7.2×10−32 21.5×10−32 22.7×10−32
(CH ), 121.41 (CH), 123.97 (CH), 129.32 (CH), 137.38 (quat-
Poly(3-butyl)thiophene
−4.3×10−33
7.2×10−33
8.4×10−33
2
ernary C), 140.28 (quaternary C), 150.02 (quaternary C)(Calc.
2354
J. Mater. Chem., 1998, 8(11), 2353–2355

for C H S: Theory: C, 74.94; H, 8.39; S, 16.28. Found: C,
74.71; H, 8.52; S, 16.46%).
This work has been carried out as part of the E.C.’s RACE
2015 ARTEMIS & ACTS 053 MIDAS projects. We would
also like to thank Professor M. Hanack for helpful discussions.
12 16
Poly (5-tert-butyl-1,3-dihydroisothianaphthene) 3
References
0.7 g (3.6 mmol) 5-tert-Butyl-1,3-dihydroisothianaphthene
was placed into a three-necked flask equipped with condenser
and drying tube, dropping funnel and inlet for dry air. 0.5 g
Anhydrous ferric chloride dissolved in 50 ml chloroform was
added (20 min). The solution was warmed to 50 °C and stirred
for 24 h, with air bubbling through. The resulting black
1
2
3
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4
5
solution was then washed with water to remove FeCl .
3
6
7
Concentrated ammonia (20 ml) was added and the solution
stirred for 30 min at room temperature. It was washed several
times with water and dried over magnesium sulfate. The
solvent was removed in vacuo and the low molecular weight
fractions removed by Soxhlet extraction with methanol. Yield:
0.25g (36.5%). The experiment was repeated using monomer
recovered from the methanol wash giving a total yield of
53.1%. GPC measurement indicated a high molecular weight
for the product: M =49 362; M =10 842; d (CDCl ) 0.9–1.7
8
9
10 H. J. Byrne, W. Blau and K. Y. Jen, Synth. Met., 1989, 32, 229.
11 H. J. Eichler, P. Gunter and D. W. Pohl, in Laser Induced
¨
Gratings, Springer Series in Optical Sciences, Springer Verlag,
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w
n
H
3
3
(9H), 7.1–7.6 (2H), 8.2–8.6 (1H); d (CDCl ) 30–33, 36–38,
C
12 Y. R. Shen, The Principles of Nonlinear Optics, Wiley Interscience,
New York, 1984.
13 H. J. Byrne, Ph.D. thesis, University of Dublin, 1989.
120–127, 137–143; n (KBr)/cm−1 1686, 1595, 1545, 1480,
max
1458, 1410, 1362, 1254, 1140, 1003, 943 and 843 (Calc. for
C H S: C, 76.57; H, 6.42; S, 17.01. Found: C, 76.06; H, 6.66;
12 12
S, 17.11%).
Paper 8/04209H
J. Mater. Chem., 1998, 8(11), 2353–2355
2355