The synthesis and non-linear optical properties of (N-alkylpyrrol-2-yl)squaraine derivatives. Part 1

Article abstract of DOI:10.1039/a604111f

The synthesis of a soluble poly(pyrrolylsquaraine) was achieved by N-alkylation of the pyrrole nucleus with 1-bromooctadecane and subsequent condensation with squaric acid. Bis(1-octadecylpyrrol-2-yl)-squaraine was also produced and found to exist in a variety of inseparable isomeric forms. When deposited as Langmuir-Blodgett monolayers on hydrophilic substrates, these dyes exhibit second harmonic generation (SHG) with d_eff = 0.5 ± 0.1 pm V^-1 for both. For comparison, bis(3,5-dimethyl-1-octadecylpyrrol-2-yl)squaraine was synthesised and revealed a higher SHG activity of d_eff = 1.8 ± 0.2 pm V^-1. Poly(1-octadecylpyrrolylsquaraine) is the first alternating donor-acceptor polymer to give SHG.

Full text of DOI:10.1039/a604111f

View Article Online / Journal Homepage / Table of Contents for this issue
The synthesis and non-linear optical properties of (N-alkylpyrrol-2-
yl)squaraine derivatives. Part 1
,
a
a
a
b
Daniel E. Lynch,* Uwe Geissler, Ian R. Peterson, Mathias Floersheimer,
b
b
b
c,d
Roland Terbrack, Li Feng Chi, Harald Fuchs, Nicolas J. Calos,
d
d
e
Barry Wood, Colin H. L. Kennard and G. John Langley
a
School of Natural and Environmental Sciences, Coventry University, Coventry, UK CV1 5FB
Physikalisches Institut, Universität Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster,
b
Germany
c
Centre for Microscopy and Microanalysis, The University of Queensland, Brisbane Q4072,
Australia
d
Department of Chemistry, The University of Queensland, Brisbane Q4072, Australia
Department of Chemistry, University of Southampton, Southampton, UK SO17 1BJ
e
The synthesis of a soluble poly(pyrrolylsquaraine) was achieved by N-alkylation of the pyrrole nucleus
with 1-bromooctadecane and subsequent condensation with squaric acid. Bis(1-octadecylpyrrol-2-yl)-
squaraine was also produced and found to exist in a variety of inseparable isomeric forms. When
deposited as Langmuir–Blodgett monolayers on hydrophilic substrates, these dyes exhibit second
Ϫ1
harmonic generation (SH G ) with deff = 0.5 ± 0.1 pm V for both. For comparison, bis(3,5-dimethyl-1-
octadecylpyrrol-2-yl)squaraine was synthesised and revealed a higher SH G activity of d = 1.8 ± 0.2 pm
eff
Ϫ1
V
. Poly(1-octadecylpyrrolylsquaraine) is the first alternating donor–acceptor polymer to give SH G.
Introduction
made as 10% dispersions in polycarbonate. Apart from this and
fluorescence studies on bis(4-acetyl-3,5-dimethylpyrrol-2-
3
,4-Dihydroxycyclobut-3-ene-1,2-dione, commonly known as
squaric acid, condenses with two molar equivalents of an elec-
tron rich aromatic compound, such as an aniline or phenol, to
form intensely coloured squaraine dyes. Bis(anilino)squaraines
and bis(benzothiazolium)squaraines, in particular, have been
studied for a whole range of photophysical applications. The
most recent discovery has been that Langmuir–Blodgett mono-
layers of the bis(anilino)squaraines deposited on hydrophilic
glass substrates exhibit a strong second-order non-linear optical
NLO) effect when irradiated with near-infrared light. When
such electron rich compounds reveal two active sites, poly-
condensation can take place and polysquaraines are formed.
These alternating donor–acceptor polymers have been studied
as small-band-gap semiconducting polymers with band gaps
8,9
yl)squaraine, no other physical properties, excluding the
common chemical analyses supporting the synthetic work, have
been reported on either bis(pyrrolyl)squaraine or poly(pyr-
rolylsquaraines). Insolubility of the polymer system has been a
large deterrent for such studies. However, introducing a long
alkyl chain to a polyarylene backbone is a general route to
1
10
enhance the solubility of this material. N-Substitution of the
pyrrole with an octadecyl chain not only aids solubility but it
also allows possible deposition onto solid substrates using the
Langmuir–Blodgett (LB) technique. For this, chloroform or the
like are the preferred solvents.
In this current work we report the synthesis and second-order
NLO intensity of chloroform soluble poly(1-octadecylpyrrol-2-
ylsquaraine) 3 as well as the separation, identification and sub-
sequent second-order properties of bis(1-octadecylpyrrol-2-yl)-
squaraine 4, and its comparison with bis(3,5-dimethyl-1-
octadecylpyrrole-2-yl)squaraine 5. Poly(1-octadecylpyrrol-2-
2
,3
(
4
,5
Ϫ5
Ϫ9
reported down to 0.5 eV and conductivity levels of 10 –10
S
Ϫ1
cm
.
Squaric acid is also known to condense with pyrroles where-
by the reaction occurs over the highly reactive α-position to
give violet dyes. If both α-positions are free then blue to blue–
green insoluble polymers are formed, 1, for which first reports
6
6
gave n = 3 repeating units. More recent synthetic methods in
7
butanol, as opposed to originally ethanol, give a compound
with the chemical formula 2, where n > 5. The product was also
found to have one associated water molecule per squaraine unit.
This dark blue polymer was partially soluble in dimethyl-
3
formamide (DMF) and m-cresol while χ measurements were
J. Chem. Soc., Perkin Trans. 2, 1997
827

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Fig. 1 Visible absorption spectrum of poly(1-octadecylpyrrol-2-yl-
squaraine) 3a in chloroform (1.8 × 10 )
Fig. 2 π–A isotherms for poly(1-octadecylpyrrol-2-ylsquaraine) 3a
(—–—), bis(1-octadecylpyrrol-2-yl)squaraine 4ϩ (——) and bis(3,5-
dimethyl-1-octadecylpyrrol-2-yl)squaraine 5 (–––)
Ϫ4
ylsquaraine) represents the first example of second harmonic
generation (SHG) from a centrosymmetric, alternating donor–
acceptor polymeric chain.
Results and discussion
Poly(1-octadecylpyrrol-2-ylsquaraine)
Fig. 1 shows the visible absorption spectrum of poly(1-octa-
decylpyrrol-2-ylsquaraine) in chloroform. Interestingly, two
maxima are observed at 562 and 574 nm. Infrared analysis of
this compound indicated that the polymer was primarily a 1,3-
substituted squaraine system (as shown for 3) from a strong
Ϫ1
Ϫ1
νC᎐O peak at 1620 cm ; but a weak ν
peak at ca. 1750 cm
C᎐O
᎐
also indicated that a percentage of 1,2-squarate substitutions
were present, hence the chemical formula for this compound
becomes 3a. Generally, 1,2-squarates are produced by using
Fig.
3
Visible absorption spectra of poly(1-octadecylpyrrol-2-
ylsquaraine) 3a, bis(1-octadecylpyrrol-2-yl)squaraine 4ϩ and bis(3,5-
Ϫ5
dimethyl-1-octadecylpyrrol-2-yl)squaraine 5 in chloroform (1 × 10 )
than expected for LB monolayers. Atomic force microscopy
(AFM) studies of Langmuir films of 3a deposited on hydro-
philic substrates showed that islands of squaraine bilayers were
formed on top of the first monolayer. The film thicknesses were
determined by AFM to be 16 ± 3 Å for the monolayer and
3
9 ± 2 Å for the bilayer islands. The islands showed no sharp
edges yet distinct steps could clearly be recognised. Such evi-
dence suggests that 3a does not form a typical ordered LB
monolayer even with the added octadecyl chains. For this
reason the N-alkyl chains are only required to enhance solu-
bility and there may be a relationship between the alkyl chain
length and inherent solubility. This will be the subject of further
studies.
acid-catalysed squaryl dichloride instead of squaric acid
although the polycondensation of squaric acid with N-alkyl-
7
carbazole is another example where 1,2-squarate units (36–
4
3%) were formed. Gel permeation chromatography (GPC)
results for 3a showed a broad distribution of polymer chain
lengths with repeat unit lengths of ca. 5–20. As is typical for a
1
polymeric system, the solution H NMR spectrum for 3a
showed a broad multiplet of peaks where the expected proton
shifts usually occur but a comparison of the integration value
Bis(1-octadecylpyrrol-2-yl)squaraine and bis(3,5-dimethyl-1-
octadecylpyrrol-2-yl)squaraine
for the end pyrrole protons (δ = 7.5–8.1 ppm) with the inte-
In our initial attempts to synthesise compound 3 it was noticed
that the reaction colour started as red–violet, slowly darkening
to the required blue. This reaction was then repeated in ethanol,
which as expected increased the reaction time and prolonged
the presence of the red–violet phase. For this reason a reaction
time of 2 h in ethanol was used to produce the dye responsible
for this phase. Subsequently, this dye was identified as bis(1-
octadecylpyrrol-2-yl)squaraine whose absorption spectrum
is shown in Fig. 3. Bis(3,5-dimethyl-1-octadecylpyrrol-2-yl)-
squaraine 5 was chosen for comparison with bis(1-octadecyl-
H
gration values of both the chain pyrrole protons (δ = 6.0–7.2
H
ppm) and the NCH protons (δ = 3.0–5.0 ppm) suggested an
2
H
average repeat length of ca. 35. Thus the average polymer chain
lengths are only in the order of 10s of units as opposed to 1000s
or 10 000s for other more common polymers. Differential scan-
ning calorimetry (DSC) results for the bulk polymer gave a
melting range of 29–89 ЊC.
A representative surface–area isotherm (π–A curve) for 3a is
shown in Fig. 2, where the molecular areas are distinctly smaller
8
28
J. Chem. Soc., Perkin Trans. 2, 1997

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1
Fig. 5 Solution H NMR of bis(1-octadecylpyrrol-2-yl)squaraine 4ϩ
Fig. 4 IR spectrum of bis(1-octadecylpyrrol-2-yl)squaraine 4ϩ
be that CH protons are being both shielded and deshielded due
to ring current effects, as in the case of [1.8]paracyclophane.
pyrrol-2-yl)squaraine because it is one of the simplest known
bis(pyrrolyl)squaraines. However, as well as the apparent
absorption intensity differences (Fig. 3) the two dyes have very
different aggregational states. Certain squaraine dyes are
known to exist both in solution and solid-state as isomers. Yet,
even though 5 has two possible conformations (cis and trans), it
2
1
2
This could certainly be the case if the alkyl chains are
assembling as micelles. However, this theory is still only specu-
lative and requires further study. Not knowing the exact form in
which 4 and 4a exist does not detract from the major purpose
of this work, hence, this mixture of materials will for the rest of
this report be referred to as 4ϩ.
1
1
Differential scanning calorimetry results for 4ϩ and 5 are
also interesting and indicate very different thermal properties.
When first heated, 4ϩ underwent a solid–solid endothermic
phase change from 31–42 ЊC during which the sample reduced
in size. The sample then remained stable until 57 ЊC where it
started shrinking again, then melting ca. 67–75 ЊC. Upon cool-
ing and reheating the sample melted at the first phase change
(31–42 ЊC). Compound 5 upon heating released a small amount
of energy (exothermic) from 46–61 ЊC with no visible change.
The sample then shrank and melted from 67–90 ЊC. This pro-
cess was repeatable upon cooling and reheating. It has already
been noted that 4ϩ exists in a variety of states. What is not
currently known is how these states change or even undergo
polymerisation upon heating, which may be the reason for dif-
fering thermal characteristics during the second scan.
The surface pressure–area isotherms (π–A curves) for 4ϩ and
5
are shown in Fig. 2. For bis[4-(N-methyl-N-octadecylamino)-
most likely exists only in the trans form due to steric hindrance
of the 4-methyl groups and N-alkyl chains around the squarate
oxygens. Both the H and C NMR spectra for 5 indicate only
one conformation. Compound 4 however has no such restraints
and can freely exist in either form. Additionally, the IR spec-
trum of 4 (Fig. 4) indicates the presence of the 1,2-squarate 4a
phenyl]squaraine both the experimental and calculated area per
2
Ϫ1
13
1
13
molecule are ca. 60 and 59.5 Å mol respectively, indicating
that the area per mol characteristics are dependent on the area
of the chromophores. Compounds 4ϩ and 5, however, com-
press to cross-sectional molecular areas which are closer to
those of long alkyl chains (ca. 20 Å mol ). A possible reason
for this is the fact that pyrrole squaraines have such an affinity
for water, commonly precipitating with one water molecule per
2
Ϫ1
6
,7
Ϫ1
᎐
C O ᎐ central ring. Attempts to spread a 0.1 mg ml solu-
4 2
tion of bis(3,5-dimethylpyrrol-2-yl)squaraine in chloroform
onto a water subphase resulted in the chromophore being dis-
persed into the subphase. This would further indicate that the
chromophores of 4ϩ and 5 could sink into the subphase allow-
ing the surface pressure to be dependent on the compressed
alkyl chains. Preliminary observations suggest that the sub-
sequently deposited monolayers are drawn out of the subphase
containing a significant amount of water. Thus, any attempts to
lower the substrate back into the subphase immediately after
deposition result in the monolayer pealing off the substrate
back onto the subphase. Drying of the films can be seen visually
on monolayers of 5 which initially appear red–violet but change
to purple–blue within a few minutes of deposition. Table 1
shows that 4ϩ and 5 are slightly solvatochromic with wave-
length shifts of only a few nanometres.
Ϫ1
with a ν
peak at 1750 cm , adjacent to the expected νC᎐O
Ϫ1
C᎐O
᎐
stretch at 1625 cm . This concurs with the presence of the 1,2-
squarate units in the polymer system 3a. Unfortunately, neither
the different isomers nor the 1,2-squarate form could be iso-
lated. This, coupled with the presence of a single symmetrical
absorption peak, indicates that this red–violet fraction is a
complex, or possibly aggregated micelles of both 4 and 4a. An
inspection of the H NMR spectrum (Fig. 5) of the fraction
reveals that the alkyl chain peak areas are not what would usu-
ally be expected. A possible reason for this phenomenon could
1
Grazing incidence X-ray diffraction (GIXD) experiments on
monolayers of 4ϩ and 5 showed that 4ϩ is present as randomly
orientated, crystalline stacks, with no apparent orientation rela-
J. Chem. Soc., Perkin Trans. 2, 1997
829

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Table 1 Absorption maxima, ADXPS determined thicknesses and second-order non-linear optical susceptibilities for poly(1-octadecylpyrrol-2-yl-
squaraine) 3a, bis(1-octadecylpyrrol-2-yl)squaraine 4ϩ and bis(3,5-dimethyl-1-octadecylpyrrol-2-yl) squaraine 5
λmax/nm
Thickness data/Å
Ϫ1
Ϫ1 a
Dye
EtOH
CHCl3
C
N
O
dyzz/pm V
deff/pm V
3
4
5
a
ϩ
0.15 ± 0.04
0.7 ± 0.2
2.1 ± 0.4
0.5 ± 0.1
0.6 ± 0.1
2.0 ± 0.2
542.5
573.0
548.5
579.0
0.6 ± 0.6
19 ± 1
21 ± 1
4 ± 1
16 ± 2
16 ± 2
a
For s polarised laser light and p polarised second-harmonic signal the experiment is sensitive to the susceptibility dyzz. If both beams are p polarised
then various non-linear optical coefficients contribute to an effective susceptibility. The absolute value of the effective susceptibility is given here for
an angle of incidence of 45Њ.
Fig. 6 Experimental geometry used for the measurement of optical nonlinearities
tionships to either the silicon surface or to the other stacks. In
contrast to this, 5 forms a smooth, amorphous monolayer with
average structure from the alkene groups observed at a d-
spacing of 5.4 Å. Both films were then non-destructively depth
profiled using angle dependent X-ray photoelectron spectro-
scopy (ADXPS) with a view to structural analysis of the mono-
layers. For both films XPS survey spectra showed only the
presence of C, N, O and Si. The Si sweep showed two distinct
geometry indicated in Fig. 6) was exposed to a laser beam
(frequency ω) with defined polarisation (p or s polarisation). In
contrast to the bare substrate, films of 3a, 4ϩ and 5 generated
frequency-doubled light (2ω). The signals from each mono-
layer, either side of the substrate, interfered both constructively
and destructively with each other as the sample was rotated (as
indicated). The optical non-linearities of each sample were cal-
culated from the second-harmonic intensities in dependence on
the rotational angle and on the polariser orientations. The sam-
ple symmetry was proved to be C∞ν with the polar axis normal
to the surface. The non-linearities of the samples were calibrated
against a quartz standard with an optical susceptibility of
peaks due to an elemental Si and a SiO surface component.
2
The isolation of the elemental and oxide components of the Si
signal provided a useful check on the fitting algorithms in
ensuring that sensible SiO thicknesses were derived.
2
Ϫ1 18
For the layered systems dealt with in this study, there were
four layers considered. These were given nominal compositions
for the purposes of calculating average electron densities and
photoelectron penetration depths. These layers are described as
d11 = 0.4 pm V . Additionally, relative permittivities for films
of 4ϩ and 5 were not determined. For our calculations we used
1
9
the interface model of Mizrahi and Sipe which neglects the
refractive indices of the monolayer for the calculation of the
electric fields at the surface. The non-linearities of the three
dyes are remarkably high for centrosymmetric species, such as
the symmetrically substituted squaraines, with the second-
harmonic light intensity increasing quadratically with increas-
ing susceptibility.
(
a) the alkane group, represented as (CH ) , (b) the squaraine
2 n
head-group, represented as C H N O , (c) the oxidised Si
1
2
4
2
2
surface—the true substrate for the monolayer—and (d) an
infinitely thick elemental Si layer. Thicknesses found for dyes
4
ϩ and 5 are listed in Table 1. In each case there was 100%
coverage of the surface by the monolayer. The larger thickness
for the nitrogenous layer found in 4ϩ, and the very thin carbon-
aceous overlayer implies that the head-group is isotropically
distributed throughout the organic layer, indicating that the
surface is uneven. AFM studies on monolayers of 4ϩ confirm
that the film is layered (as in 3a), with the total film thickness
being determined by this technique as 28 ± 2 Å. Again the
layering is in the form of regular terraces with the most fre-
quent step height being two but both one and three layer steps
are also observed. AFM additionally confirmed that there was
Efficient SHG can usually be attributed to π-electron rich
polar molecules that act as electric–dipole oscillators. The
polymer backbone in 3a, however, possesses no overall polar-
isation, i.e. it is centrosymmetric, although we have been unable
to determine the contribution, if any, from the minor amounts
2
0
of the 1,2-squarate units. A recent paper has demonstrated
that unsymmetrically substituted 1,2-squarates possess second-
order susceptibilities but in the case of 3a, and 4ϩ, all the 1,2-
squarates are symmetrical. Hence, the SHG in this case could
be due to a polarisation of the aromatic backbone by the
underlying hydrophilic substrate. For highly extended charge
distributions, however, the electric–dipole approximation may
be invalid and the optical non-linearities would be due to
magnetic–dipole or electric–quadrupole interactions. In such
a scenario noncentrosymmetry would not be required for
SHG. Closer investigation of the origin of the optical non-
linearities for these systems looks highly interesting and promis-
ing. If the SHG is due to electric dipoles, the non-linearities
of a monolayer could be influenced by the selection of the
underlying material. LB film devices could be developed, for
example, with alternating layers of squaraines and polar
materials. If, on the other hand, the optical non-linearity is
due to magnetic–dipole or electric–quadrupole interactions,
devices could be developed that do not require a polar order
of the molecules.
1
00% coverage of the surface by the organic. The observed
layering would certainly explain the smaller than expected
molecular areas displayed in the π–A isotherm for this dye. In
contrast, dye 5 has discrete C and N containing layers with the
nitrogenous layer beneath the surface C layer. The thickness of
the total organic monolayer for 5 is consistent with that
1
4
of octadecane-type tail groups, e.g. 21.93 Å for stearic acid.
Furthermore, a thickness of 16 Å was found in both cases for
the SiO layer, which agrees with other published values for
2
1
5,16
surface oxide layers on silicon.
Second harmonic generation
The optical nonlinearities of the three films on hydrophilic sub-
1
7
strates were measured using the Maker-fringe technique and
are listed in Table 1. By this technique the sample (experimental
8
30
J. Chem. Soc., Perkin Trans. 2, 1997

View Article Online
ϩ
ϩ
Conclusions
m/z 717 (MH , 100%) and 739 (MNa , 50). GPC results showed
this material to have a single molecular mass ca. 700.
2
Through a variety of chemical and instrumental techniques we
have shown in this work that the previously reported poly-
pyrrolylsquaraines) first form as bis(pyrrolyl)squaraine dimers
and that these dimers can subsequently be synthesised. The fact
that pyrrolylsquaraines have a sustained dimeric form, when
synthesised in a low boiling point solvent, may be due to the
characteristic resonant nature of these dyes. The α-position in
pyrrole is the favoured site for electrophilic substitution due
to the generation of a negative charge which is stabilised by
,4-Dimethyl-1-octadecylpyrrole was made using the same
procedure (and molar amounts) as for the production of 1-
octadecylpyrrole (yield 8.8 g, 85%). δ (250 MHz; CDCl ;
SiMe ) 0.87 (3 H, t, CH ), 1.25 (30 H, br s, CH ), 1.66 (2 H,
(
H
3
4
3
2
quintet, NCH CH ), 2.05 (3 H, s, 2-CH ), 2.16 (3 H, s, 4-CH ),
2
2
3
3
3
.68 (2 H, t, NCH ), 5.67 (1 H, s, 3-H), 6.33 (1 H, s, 5-H).
Bis(3,5-dimethyl-1-octadecylpyrrol-2-yl)squaraine was syn-
thesised according to the literature procedure for the synthesis
of bis(3,5-dimethylpyrrol-2-yl)squaraine by refluxing squaric
2
6
2
1
induction from the electronegative nitrogen atom. After two
pyrroles have condensed with squaric acid the resonance of
the dye affords both nitrogen atoms to be partially electro-
positive, balancing the resultant zwitterionic state of the dye,
and hence disrupting the possibility of a sustained negative
charge on the free α-positions. Therefore, higher temperatures
or prolonged reaction times are required for successful poly-
merisation. Furthermore, we have also shown that poly-
acid (200 mg, 1.75 mmol) and 2,4-dimethyl-1-octadecylpyrrole
3
(1.2 g, 3.5 mmol) in ethanol (50 cm ) for 2 h and was purified by
column chromatography (Al O ) using incremented amounts of
2
3
chloroform in light petroleum (bp 40–60 ЊC) (yield 582 mg,
4
3%) (Found: C, 80.6; H, 11.6; N, 3.6%. C H N O requires
5
2
88
Ϫ1
2
2
C, 80.7; H, 11.5; N, 3.6%; M, 772); νmax/cm 1610 (C᎐O);
δ (250 MHz; CDCl ; SiMe ) 0.87 (6 H, t, CH ), 1.22 (60 H, br s,
H
3
4
3
CH ), 1.55 (4 H, br m, NCH CH ), 2.30 (6 H, s, CH ), 2.63 (6 H,
2
2
2
3
(pyrrolylsquaraines) can be made soluble in chloroform by the
s, CH ), 4.63 (4 H, t, NCH ), 6.05 (2 H, s, H); δ (62.9 MHz;
3
2
C
N-alkylation of the pyrrole nucleus with octadecane. This there-
fore provides a suitable solvent for LB purposes as well as
other thin film techniques such as dip-coating and spin-
coating. Comparisons between bis(pyrrolyl)squaraine and the
known bis(3,5-dimethylpyrrol-2-yl)squaraine suggest that in
their N-octadecyl alkylated forms these two chemically simi-
lar dyes have very different solid-state and solution properties
which, in turn, are directly related to their aggregational
states.
CDCl ) 176.75 (0), 175.86 (0), 146.50 (0), 138.88 (0), 127.14 (0),
3
1
2
1
17.81 (1), 47.07 (2), 32.13 (2), 31.95 (2), 29.92 (2), 29.87 (2),
9.84 (2), 29.79 (2), 29.66 (2), 29.57 (2), 26.85 (2), 22.89 (2),
ϩ
5.31 (3), 14.32 (3), 13.62 (3); m/z 773 (MH , 100%) and 795
ϩ
(MNa , 20).
IR spectra were recorded as pressed KBr disks on a Nicolet
2
05 FTIR spectrometer. UV–VIS measurements were made on
1 13
a Shimadzu UV-2100 spectrometer. H and C NMR spectra
were recorded on a Bruker AC250 NMR spectrometer. Electro-
spray mass spectra were recorded in positive ion mode on a
Micromass Platform mass spectrometer. Samples for gel per-
meation chromatography were run on a Spectra Physics Model
SP 8100 attached to a Kronwald UV-detector and a Waters
Model R401 differential refractometer and were calibrated
versus polystyrene. Samples for differential scanning calor-
Experimental
1
-Octadecylpyrrole was synthesised using the following pro-
cedure: a solution of pyrrole (2.0 g, 30 mmol) in dry toluene (25
3
cm ) was added under nitrogen to a stirred suspension of potas-
Ϫ1
sium tert-butoxide (4.0 g, 36 mmol) and dibenzo-18-crown-6
imetry were run on a Perkin-Elmer DSC 1B heated at 8 ЊC min
3
(
(
300 mg) in dry toluene (75 cm ). After 1 h, 1-bromooctadecane
from 20–300 ЊC. AFM measurements were made on a Digital
Instruments Nanoscope III. Images were obtained using both
the tapping and contact modes while thicknesses were obtained
3
8.0 g, 24 mmol) in dry toluene (25 cm ) was added and the
resultant mixture refluxed for 12 h. Upon cooling, water (50
3
Ϫ8
cm ) was added and the organic phase separated after which the
using the contact mode with applied forces of ca. 10 N.
3
aqueous phase was extracted with toluene (3 × 25 cm ). The
GIXD experiments were carried out using the BIGDIFF
facility at BL20B at the Photon Factory, Tsukuba, Japan. The
patterns were recorded on image plates at a wavelength of
1.116 34 Å with an incident angle of 0.1375Њ, as described by
combined organic phases were then washed three times with
water and then dried using anhydrous magnesium sulfate. After
removal of toluene the crude 1-octadecylpyrrole was purified
by column chromatography (Al O ) using light petroleum (bp
2
2
Foran et al. The electron spectrometer employed for ADXPS
studies was a Physical Electronics Industries PHI Model 560,
using a model 25–270 AR Cylindrical Mirror Analyser
2
3
4
0–60 ЊC) (yield 8.6 g, 90%). δ (250 MHz; CDCl ; SiMe ) 0.90
H
3
4
(
3 H, t, CH ), 1.25 (30 H, br s, CH ), 1.75 (2 H, quintet,
3
2
2
3
NCH CH ), 3.86 (2 H, t, NCH ), 6.13 (2 H, dd, 3-H), 6.65 (2 H,
(CMA). This double-pass CMA with rotatable drum device is
advantageous because the sample–analyser–source geometry
remains fixed, thus avoiding errors due to misalignment which
2
2
2
dd, 2-H).
Poly(1-octadecylpyrrol-2-ylsquaraine)
was
synthesised
2
4
according to the literature procedure for the synthesis of
may occur by tilting the sample. Multiple sweeps of the 1s
peaks of C, N and O with binding energies of 285.0, 398.0 and
532.0 eV respectively from the monolayers, and the Si 2p peaks
at 101.8 and 105.4 eV from the substrates were used for the
analyses. Spectra relative to the sample surface were collected at
87.7, 69.9, 51.5 and 34.5Њ for dye 4ϩ and 87.7, 34.5 and 7.7Њ for
dye 5. Integrated intensities were normalised for instrumental
sensitivity factors. Mean free paths for the photoelectrons
through each of the media were calculated according to the
7
poly(1-methylpyrrol-2-ylsquaraine) by refluxing equimolar
amounts of squaric acid (200 mg, 1.75 mmol) and 1-octadecyl-
3
pyrrole (560 mg, 1.75 mmol) in butan-1-ol (50 cm ) for 2 h. The
crude product was collected by filtration upon cooling of the
reaction solution and was purified by column chromatography
(
Al O ) first by using incremented amounts of chloroform in
2 3
light petroleum (bp 40–60 ЊC) to separate unreacted 1-octa-
decylpyrrole as well as short chain oligomeric materials, fol-
lowed by THF to collect the dark blue product (yield 334 mg,
2
5
formulae given by Ashley. Using functions of the form given
1
5
4
8%).
Bis(1-octadecylpyrrol-2-yl)squaraine was synthesised by
by Marshbanks et al., a set of functions describing the angle-
dependent relative intensities of each of the C, N, O and Si
signals were derived for dye 4ϩ and dye 5-type monolayers. A
trial-and-error least-squares fitting of the observed data was
undertaken, using free variables of elemental scale factors, per-
cent coverage of the Si surface and thickness of each layer.
Surface pressure–area isotherms for compounds 3a, 4ϩ and 5
were run on a Nima Technology, series 2000 Langmuir-
Blodgett trough. Compounds 3a, 4ϩ and 5 were spread from
refluxing squaric acid (200 mg, 1.75 mmol) and 1-octadecyl-
3
pyrrole (560 mg, 1.75 mmol) in ethanol (50 cm ) for 2 h. The
crude product was collected by filtration upon cooling of the
reaction solution and was purified by column chromatography
(
Al O ) using incremented amounts of chloroform in light pet-
2 3
roleum (bp 40–60 ЊC) with the major red–violet fraction being
collected (yield 340 mg, 43%) (Found: C, 80.8; H, 11.4; N,
Ϫ3
3
.9%. C H N O requires C, 80.5; H, 11.1; N, 3.9%; M, 716);
chloroform solutions (ca. 0.1 mg cm ) onto a pure water sub-
4
8
79
2
2
J. Chem. Soc., Perkin Trans. 2, 1997
831

View Article Online
phase of a custom-built Nima ‘Münster’ LB trough. LB mono-
layers were deposited on (a) mica for AFM studies, (b) both
sides of a hydrophilic fused silica substrate and (c) hydrophilic
7 L. P. Yu, M. Chen, L. R. Dalton, X. F. Cao, J. P. Jiang and
R. W. Hellwarth, Mater. Res. Soc. Symp. Proc., 1990, 173, 607.
B. Patrick, M. V. George, P. V. Kamat, S. Das and K. G. Thomas,
J. Chem. Soc., Faraday Trans., 1992, 88, 671.
S. Das, K. G. Thomas, P. V. Kamat and M. V. George, Proc. Ind.
Acad. Sci., Chem. Sci., 1993, 105, 513.
8
9
(
111)-cut silicon wafers for GIXD and ADXPS studies at a
Ϫ1
compressed surface pressure of 30 mN m for compound 3
and 35 mN m for compounds 4ϩ and 5 on the upstroke by
Ϫ1
10 C. P. Brock, W. P. Schweizer and J. D. Dunitz, J. Am. Chem. Soc.,
1985, 107, 6964.
11 C. W. Dirk, W. C. Herndon, F. Cervantes-Lee, H. Selnau,
S. Martinez, P. Kalmegham, A. Tan, G. Campos, M. Velez, J. Zyss,
I. Ledoux and L. Cheng, J. Am. Chem. Soc., 1995, 117, 2214.
2 J. R. Dyer, in Applications of Absorption Spectroscopy of Organic
Compounds, Prentice-Hall, Englewood Cliffs, USA, 1965, p. 82.
3 K.-Y. Law and C. C. Chen, J. Phys. Chem., 1989, 93, 2533.
4 M. Goto and E. Asada, Bull. Chem. Soc. Jpn., 1978, 51, 2456.
5 T. L. Marshbanks, H. K. Jugduth, W. N. Deglass and E. I. Franses,
Thin Solid Films, 1993, 232, 126.
passing the substrate through the floating monolayer at 25 mm
Ϫ1
min . Frequency-doubling experiments were run using a
Spectra Physics GCR 190/30 laser at a wavelength of 1064 nm
Ϫ2
and intensity of 1.5 MW cm
.
1
1
1
1
Acknowledgements
The authors acknowledge financial support from the School of
Natural and Environmental Sciences (Coventry University,
UK) and would like to thank A. Pearse (Coventry University,
UK) for running DSC, K. Janze (Institut für Makromolekulare
Chemie, Universität Hannover, Germany) for GPC data and
1
1
6 T. J. Sarapatka, Thin Solid Films, 1993, 226, 219.
7 C. Bosshard, K. Sutter, P. Pretre, J. Hullinger, M. Floersheimer,
P. Kaatz and P. Guenter, in Organic Nonlinear Optical Materials,
vol. 1 of Advances in Nonlinear Optics, eds. F. Garito and F. Kajzar,
Gordon and Breach, Basel, Switzerland, 1995.
8 K.-H. Hellwege and A. M. Hellwege, Landolt-Boernstein, Elastische,
piezoelektrische, pyroelektrische, piezooptische Konstanten und
Nichtlineare dielektrische Suszeptibilitaeten von Kristallen, Springer
Verlag, Berlin, 1979, vol. III/11.
1
13
J. Bickerton (Coventry University, UK) for H and C NMR.
GIXD work was made possible by financial support from
ANSTO and the use of the Australian National Beamline Facil-
ity, KEK with the help of G. Foran and D. Cookson, Tsukuba,
Japan. U. G. is at Coventry University because of a European
Community, Human Capital and Mobility Scheme. Dr Darren
G. Hamilton (Cambridge, UK) is also thanked for helpful
discussion.
1
1
9 V. Mizrahi and J. E. Sipe, J. Opt. Soc. Am., 1988, 660, B5.
20 B. R. Cho, J. T. Je, S. J. Lee, S. H. Lee, H. S. Kim, S. J. Jeon,
O.-K. Song and C. H. Wang, J. Chem. Soc., Perkin Trans. 2, 1996,
2
141.
2
1 J. A. Joule and G. F. Smith, Heterocyclic Chemistry, Chapman &
Hall, London, 2nd edn., 1990, ch. 20.
2
2 G. J. Foran, J. B. Peng, R. Steitz, G. T. Barnes and I. R. Gentle,
Langmuir, 1996, 12, 774.
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