Optical properties of a quinoid molecule

Article abstract of DOI:10.1016/S0009-2614(03)01152-7

We report on linear and third order nonlinear optical properties of the quinoid molecule 5,5′ -bis(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadiene-1-ylidene)-5,5′-dihydro-2, 2′-bithiophene.Variable angle spectroscopic ellipsometry has been used to determine th

Full text of DOI:10.1016/S0009-2614(03)01152-7

Chemical Physics Letters 377 (2003) 243–248
www.elsevier.com/locate/cplett
Optical properties of a quinoid molecule
a,
b
b
b
Franco DÕAmore *, Marta Lanata , Maria C. Gallazzi , Giuseppe Zerbi
a
CoreCom, Via Colombo 81, 20133 Milano, Italy
b
Dipartimento di Chimica, Materiali e Ingegneria Chimica ‘‘G.Natta’’, Politecnico di Milano,
Piazza Leonardo da Vinci 32, 20133 Milano, Italy
Received 12 May 2003; in final form 10 June 2003
Published online: 24 July 2003
Abstract
We report on linear and third order nonlinear optical properties of the quinoid molecule 5,50-bis(3,5-di-tert-butyl-4-
oxo-2,5-cyclohexadiene-1-ylidene)-5,50-dihydro-2, 20-bithiophene.Variable angle spectroscopic ellipsometry has been
used to determine the dispersion of the dielectric in a spectral range from the near infrared to the near ultraviolet. We
have measured a high value of the index of refraction in the transparency region (i.e., n ¼ 1:886 at k ¼ 1:5 lm), in-
dicating the high polarisability of the molecule under study. The nonlinear optical properties have been measured by
third harmonic generation in the near infrared spectral range. At k ¼ 1:5 lm the nonlinear coefficient of the molecule
has been estimated to be 4.2 Â 10À19 m2/V2 (3.0 Â 10À11 esu).
Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction
which is connected to the real part of the third
order NLO susceptibility vð3Þðx; Àx; x; Àx).
The extent of p-electron delocalisation in poly-
conjugated organic materials is the basic molecular
structural property required for attaining large
vð3Þ. The interest of many researchers has focused
on polymeric materials (with a one-dimensional
p-electron system) because of the large variety of
their chemical structures, low cost, fast nonlinear
optical response and the possibilities to process
these materials into the form of films.
Often a high degree of polymerization is not
necessary to achieve large vð3Þ values; conjugated
oligomers with a chain length of the order of
several tens of nanometers should have similarly
large vð3Þ values [3]. Further, it has been shown
that third order NLO properties are related to the
Since the discovery of large and fast nonreso-
nant third order nonlinear optical (NLO) proper-
ties of organic materials in the 1980s, polymers
have been subject to intense experimental and
theoretical studies [1,2]. Considerable attention
has been devoted to the possibility of implement-
ing NLO waveguides for photonic applications,
such as all-optical switches and all-optical data
processing devices, by exploiting the intensity de-
pendent index of refraction (n2) of such materials,
* Corresponding author. Fax: +39-02-2399-8922.
E-mail address: damore@corecom.it (F. DÕAmore).
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0009-2614(03)01152-7

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F. DÕAmore et al. / Chemical Physics Letters 377 (2003) 243–248
)78 °C and quenched with (CH3)3SiCl at )50 °C
to give 1. 5,50-bis(tri-ethylidene)-2,20-bithiophene
(2) was synthesized following the procedures re-
ported in literature [6].
The catalyst Pd(0) was prepared under inert
atmosphere adding a toluene solution of isoBut2
AlH 0.5 mmol/ml (1.72 ml, 0.86 mmol) to PdCl2
(PPh3)2 (0.302 g, 0.43 mmol) in dry THF (15 ml).
To the catalyst suspension 3.82 g (10.7 mmol) of 1
and a THF solution of 2 (3.08 g, 5.35 mmol) were
successively added. The reaction mixture was ref-
luxed for two days, then cooled to room temper-
ature and quenched with water. The aqueous
phase was extracted with ether and the resulting
organic phase was dried and then purified by silica
gel flash chromatography using hexane as eluent,
affording the compound 3 as yellow crystals (40%
yield).
As reported [5] compound 3 was deprotected
with tetrabutylammonium fluoride (TBAF) to give
5,50-bis(3,5-di-t-butyl-4-hydroxyphenyl)-2,20-bithi-
ophene (4) in quantitative yield and the oxidation
of 4 was performed to obtain 5,50-bis(3,5-di-t-bu-
tyl-4-oxo-2,5-cyclohexadiene-1-ylidene)-5,50-dihy-
dro-2,20-bithiophene (5) (HQPQ) (90% yield).
Thin films of the oligomer were obtained by
spin coating from a chloroform solution having a
concentration of 10 mg/ml. Film thickness has
been measured with a Tencor P15 step profiler and
Fig. 1. Heteroquaterphenoquinones chemical structure.
wavelength of absorption: molecules having a low
energy gap show a high vð3Þ coefficient [3,4].
Oligomers with a quinoid structure seem to be
very attractive because they posses the following
fundamental properties: planarity, low band gap,
and high degree of electron delocalisation. For
these reasons we have started a systematic study of
highly p-conjugated heteroquinoid molecules (see
Fig. 1). In this Letter we report for the first time on
linear and third order nonlinear optical properties
of 5,50-bis(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadi-
ene-1-ylidene)-5,50-dihydro-2,20-bithiophene (het-
eroquaterphenoquinone, n ¼ 2 in Fig. 1).
2. Experimental
For the synthesis, sketched in Scheme 1, of the
molecule we modified the pathway reported in
literature [5]. The commercial 4-bromo-2,6-di-t-
butylphenol (Aldrich) was reacted with BuLi at
Scheme 1. Synthesis pathway.

F. DÕAmore et al. / Chemical Physics Letters 377 (2003) 243–248
245
by ellipsometry: results determined with this two
independent method agree within 2%.
For ellipsometric measurements we have used a
commercial J.A. Wollam Co. VASE ellipsometer.
The instrument is a rotating analyser one, equip-
ped with an autoretarder input unit, to improve
sensitivity. Data have been acquired in the spectral
range from 4.13 eV (300 nm) to 0.73 eV (1700 nm)
in steps of 30 meV at three different angle of in-
cidence (from 55° to 65° in step of 5°). Data has
been fitted by a J.A. Woollam Co. WVASE soft-
ware.
Third order NLO coefficient of the quinoid
molecule has been measured by third harmonic
generation (THG) in the spectral range from 1200
to 1900 nm using Maker fringes technique. It
consists in measuring the intensity of the third
harmonic beam (3x) as a function of the angle of
incidence of the beam at fundamental frequency
(x). The main advantage of this technique is that it
allows the measurements of NLO coefficient of
very thin films (few tens of nm) at moderate pump
intensity. It is also sensitive only to ultrafast elec-
tronic contribution, and can be used to estimate
other NLO effects, as the intensity dependent in-
dex of refraction [7].
Fig. 2. HQPQ ellipsometric data. Empty symbols (broken
lines): experimental (fitted) D; full symbols (continuous lines):
experimental (fitted) w. jðÃ), 50°; rð}), 55°; dðs), 60°. Only
50% of the collected data have been shown for clarity.
constant of the sample to fit the experimental data.
As a first step we have fitted the transparency re-
gion (k ¼ 0), between 900 and 1700 nm, using for n
a Cauchy model and leaving the film thickness as a
free parameter. We have than directly inverted the
ellipsometric equation to fit the absorption region,
allowing n and k to vary and keeping fixed the
previously determined film thickness [10]. The
agreement of the fitting with the experimental data
is very good. The complex refractive index for the
heteroquaterphenoquinone is reported in Fig. 3. It
is worth noting the high value of the index of re-
fraction in the transparency region (i.e.,
n ¼ 1:886 Æ 0:006 at k ¼ 1:5 lm).
The Maker fringes measurements have been
performed using an optical parametric oscillator
laser source which delivers 5 ns pulses with optical
power of tens of MW/cm2 at 10 Hz repetition rate.
For details on the experimental set up see [8]. The
THG data analysis algorithm has been imple-
mented on MATLAB software and is based on [9].
A Kramers–Kronig consistent oscillator model
has also been tried to fit the entire spectral range;
for details on this fitting procedure see [11].
3. Results and discussion
In Fig. 2 we report the ellipsometric data w and
D (being, respectively, the real and imaginary part
of the complex ratio Rp/Rs, where Rp (Rs) indicates
the complex reflection coefficient for light polar-
ised parallel (perpendicular) to the plane of inci-
dence [10]) for a 20 nm thick film deposited on a
1 mm fused silica substrate. The back surface of
the substrate was roughened in order to avoid in-
coherent backside reflection. To extract the infor-
mation on the optical properties of the film we had
to build an appropriate model for the optical
Fig. 3. Real (n) and imaginary (k) part of HQPQ complex re-
fraction index.

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F. DÕAmore et al. / Chemical Physics Letters 377 (2003) 243–248
The functional behaviour of e2 (being connected
intense Raman line at 1301 cmÀ1 [12], and the
difference between E3 and E2 coincide with another
strong Raman line at 1076 cmÀ1 [12]. The first of
these Raman modes is associated with a collective
stretching vibration which involves the inter-ring
bonds, while the second one is a collective
stretching vibration which involves the single
bonds within the thiophene and phenyl rings. Such
Raman modes have a high vibronic coupling with
the dipole allowed electronic transition responsible
for the UV–Vis absorption peak [12]. The position
of E1 and E5, due to their small oscillator strength,
is affected by a considerable uncertainty, so it is
difficult to correlate their relative differences to the
Raman signals.
to the real and imaginary part of the refractive
index by the relation e2 ¼ 2n  k) could be fitted
accurately by a collection of Gaussian oscillators
given by the following expression:
X
À
Á
n
n
n
n
e2ðxÞ ¼
An eÀðEÀE =Br Þ þ eÀðEþE =Br Þ
;
E ¼ ꢀhx;
ð1Þ
n
where En is the characteristic energy of the nth
oscillator, An and Brn are its amplitude and
broadening parameters, respectively.
The real part of the dielectric function
(e1 ¼ n2 À k2) is then found by the Kramers–Kro-
nig transformation of Eq. (1)
Z
Four more Gaussian oscillators were necessary
to fit the higher energy spectral range.
In Fig. 4 we report the Maker fringes of a 15 nm
thick film of the oligomer deposited on a 200 lm
silica substrate, together with the Maker fringes of
the substrate alone, at pump wavelength of 1.5 lm.
2
1 x0e2ðx0Þ
e1ðxÞ ¼ e1ð0Þ þ
P
dx0:
ð2Þ
p
x02 À x2
0
The result of the fitting is in excellent agreement
with the previously described one, indicating that
the data presented in Fig. 3 are Kramers–Kronig
consistent. To fit the main absorption peak due to
the HOMO–LUMO transition a collection of five
oscillators were necessary, and the fitting param-
eters are listed in Table 1.
The minimum number (five) of oscillators we
have used to fit the main absorption peak is in
agreement with the UV–Vis absorption spectrum
of HQPQ in CHCl3 solution [12], whose structures
are less broad than the corresponding solid state
one, and the presence of four shoulders are visible
together with the main absorption peak.
It is possible to correlate the differences in en-
ergies of the vibronic peaks in the UV–Vis, to the
Raman lines of the compound under study [13,14].
We notice that the difference in energy between E4
and E3 (related to the stronger peaks), coincide,
within the experimental error, with the most
Fig. 4. Maker fringes of a 200 lm fused silica (s) and of a
15 nm HQPQ film on a 200 lm fused silica substrate (Ã)
(continuous lines are fittings). Pump wavelength: 1.5 lm.
Table 1
Gaussian oscillator model for the main absorption peak of HQPQ
E (eV)
E (nm)
E (cmÀ1
)
A
Br (eV)
1
2
3
4
5
1.52 Æ 0.02
1.620 Æ 0.004
1.756 Æ 0.003
1.91 Æ 0.01
215 Æ 0.02
817 Æ 9
766 Æ 2
705 Æ 1
648 Æ 4
577 Æ 5
12 240 Æ 135
13 055 Æ 34
14 184 Æ 20
15 432 Æ 95
17 331 Æ 150
1.0 Æ 0.1
1.6 Æ 0.3
4.5 Æ 0.5
3.5 Æ 0.2
0.6 Æ 0.1
0.21 Æ 0.02
0.115 Æ 0.009
0.176 Æ 0.008
0.27 Æ 0.03
0.21 Æ 0.02

F. DÕAmore et al. / Chemical Physics Letters 377 (2003) 243–248
247
By comparing, whit an appropriate fitting proce-
dure, the intensity of the fringes of the silica sub-
strate (whose nonlinear coefficient has been
accurately determined [15]) to those of the film
plus substrate system, collected in the same ex-
perimental conditions, one is able to extract the
NLO coefficient of the film.
by the samples under THG experimental condi-
tions (illumination with ns-pulsed infrared light at
tens of MW/cm2).
In view of possible applications of this molecule
as a material for optical communication devices, it
is very important to evaluate its absorption coef-
ficient around 1.5 lm. We have estimated the ab-
sorption coefficient by measuring the attenuation
of 0.53% in weight CS2 solution of the polymer in
a 9 cm long silica cell. For experimental details on
the absorption measurements see [8]. We have
obtained an absorption coefficient for our material
equal to 1.5 cmÀ1, which is still an acceptable one.
A direct measure of the attenuation on a guiding
structure would be necessary to asses with no
doubts the role of absorption in the present com-
pound.
In Fig. 5 we report the dispersion of the third
order NLO coefficient vð3Þð3x; x; x; x) of hetero-
quaterphenoquinone. The enhancement of the
nonlinear coefficient at long wavelength is due to a
tree photon resonance with the HOMO–LUMO
transition, as it can be seen by comparing the
frequency dependence of the NLO coefficient with
the dispersion of the imaginary part of the re-
fractive index. The nonlinear coefficient of the
heteroquaterphenoquinone at k ¼ 1:5 lm has been
estimated to be 4.2 Â 10À19 m2/V2 (3.0 Â 10À11 esu,
with an estimated error of about 10%), and is al-
most out of the main resonance which occurs at
longer wavelength, and would lead to n2 ¼ 3:3 Â
10À13 cm2/W. This value is comparable with the
one characteristic of other organic nonlinear op-
tical media such as poly(p-phenylenevinylene) [16]
(2.0 Â 10À11 esu at k ¼ 1:5 lm), and polydiacetyl-
enes [8] (1.0 Â 10À11 esu at k ¼ 1:5 lm), which are
considered as promising material for photonic
applications.
4. Conclusions
In conclusion we have reported on the linear
and nonlinear optical properties of a quinoid
molecule, namely HQPQ. Regarding the linear
optical properties, we have found a high value of
the index of refraction, in the transparency region
(n ¼ 1:886 at k ¼ 1:5 lm), which, among other
things, is an indication of the high nonlinearity of
the compound. This is directly confirmed by the
measurements of the nonlinear optical coefficient
of the molecule (vð3Þð3x; x; x; xÞ ¼ 4:2 Â 10À19
m2/V2 at k ¼ 1:5 lm).
An important issue for applications is the ma-
terial stability. This molecule is stable at room
temperature on exposure to natural light in air and
no evidence of optical damaging has been shown
As a final remark the high nonlinear optical
coefficient of HQPQ, in conjunction with its good
transparency at telecommunication wavelength,
points toward the interest of this material as a
promising medium for photonic applications [17].
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Fig. 5. Dispersion of the third order NLO coefficient of HQPQ.
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