An improved efficiency/transparency trade-off for second-harmonic generation by extending the π-electron bridge of an optically nonlinear dye

Article abstract of DOI:10.1039/b009911m

The second-harmonic intensity from Langmuir-Blodgett films of 5-{4-[2-(4-dimethylaminophenyl)vinyl]benzylidene}-2-octadecyl-5,6,7,8- tetrahydroisoquinolinium octadecyl sulfate(1), interleaved with poly(tert-butyl methacrylate), increases quadratically with thickness to more than 100 bilayers. The susceptibility, molecular hyperpolarisability, thickness, refractive indices and chromophore tilt angle, relative to the substrate normal, are X_zzz⁽²⁾=45 ± 5 pm V^-1 at 1064 nm, β = 8 × 10^-38 m⁴ V^-1, l=3.65 ± 0.12 nm bilayer ^-1, n^ω = 1.56 ± 0.01 and ψ = 44 ± 2 °. The moderately high susceptibility arises from an optimised transparency/efficiency trade-off, the alternate-layer LB film being transparent at 1064 nm and having a very slight absorbance of 3.5 × 10^-4 bilayer ^-1 at 532 nm. In contrast, alternate-layer films of the correseponding 7-diethylamino-2-oxo-2H-chromen-3-ylmethylene (2) and 3-(dimethylaminophenyl)prop-2-enylidene (3) derivatives, also reported, have absorbances of 1.1 × 10^-3 and 1.5 × 10^-3 bilayer^-1 at 532 nm and resonantly enhanced susceptibilities of 60 and 95 pm V^-1 respectively.

Full text of DOI:10.1039/b009911m

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An improved efficiency/transparency trade-off for second-harmonic
generation by extending the p-electron bridge of an optically
nonlinear dye
Geoffrey J. Ashwell,*a Anne J. Whittam,a Mukhtar A. Amiri,a Richard Hamilton,a
Andrew Greena and Ulrich-Walter Grummtb
aThe Nanomaterials Group, Centre for Photonics and Optical Engineering, The Whittle
Building, Cranfield University, Cranfield, UK MK43 0AL.
E-mail: g.j.ashwell@cranfield.ac.uk
bInstitut fu¨r Physikalische Chemie, Friedrich Schiller Universita¨t, Lessingstrasse 10, 07743
Jena, Germany
Received 12th December 2000, Accepted 7th February 2001
First published as an Advance Article on the web 19th March 2001
The second-harmonic intensity from Langmuir–Blodgett films of 5-{4-[2-(4-
dimethylaminophenyl)vinyl]benzylidene}-2-octadecyl-5,6,7,8-tetrahydroisoquinolinium octadecyl sulfate(1),
interleaved with poly(tert-butyl methacrylate), increases quadratically with thickness to more than 100 bilayers.
The susceptibility, molecular hyperpolarisability, thickness, refractive indices and chromophore tilt angle,
relative to the substrate normal, are xzzz(2)~45¡5 pm V21 at 1064 nm, b~8610238 m4 V21, l~3.65¡0.12 nm
bilayer21, nv~1.53¡0.01, n2v~1.56¡0.01 and Q~44¡2‡. The moderately high susceptibility arises from an
optimised transparency/efficiency trade-off, the alternate-layer LB film being transparent at 1064 nm and having
a very slight absorbance of 3.561024 bilayer21 at 532 nm. In contrast, alternate-layer films of the
corresponding 7-diethylamino-2-oxo-2H-chromen-3-ylmethylene (2) and 3-(dimethylaminophenyl)prop-2-
enylidene (3) derivatives, also reported, have absorbances of 1.161023 and 1.561023 bilayer21 at 532 nm and
resonantly enhanced susceptibilities of 60 and 95 pm V21 respectively.
ments at successive interfaces.1–3 This may be overcome by
Introduction
tailoring the molecular structure to suppress realignment: for
example, (i) by utilising terminally charged CnH2nz1–D–p–A2
dyes and thereby relying on Coulomb repulsion to suppress
head-to-head packing;4,5 (ii) by alkylating both ends of the
hydrophilic chromophore, CnH2nz1–D–p–A–CmH2mz1, and
thereby enforcing a hydrophobic film surface.6–9 Alternatively,
the LB layers may be simply interleaved to give non-
centrosymmetric structures for second-harmonic generation
(SHG), the spacers being complementary dyes with the alkyl
tail attached at the opposite end to effect the required
alignment,10–13 amphiphilic donors which alternate with
amphiphilic acceptors14 or, in fact, any amphiphile15–22
including polymeric materials15,16 and those with compatible
interlocking geometries.17,18
Molecules with a hydrophilic head and a hydrophobic tail
spontaneously align at the air–water interface but, when
deposited to form a multilayer structure, the amphiphilic layers
tend to adopt centric head-to-head and tail-to-tail arrange-
Among the extensively studied dyes, 4-[2-(4-dimethylamino-
phenyl)vinyl]-1-docosylpyridinium bromide23,24 and its deriva-
tives,17,18,25 including several polymeric analogues,16,26,27 have
attracted much attention for LB alignment. They have an
absorbance of ca. 261023 layer21 at 532 nm and are
unfavourable for waveguiding applications but slight mod-
ification of the molecular structure can cause a shift of the
overlapping charge-transfer band. For example, films of 5-(4-
diethylaminobenzylidene)-2-octadecyl-5,6,7,8-tetrahydroiso-
quinolinium octadecyl sulfate(4) have a reduced absorbance of
561024 layer21 at the harmonic wavelength and, when
interleaved with poly(tert-butyl methacrylate), exhibit a high
second-order susceptibility of 67 pm V21 at 1064 nm.8 This
may be compared with an optimum value of 50–70 pm V21 for
the hemicyanine derivatives above.17,18
In this study, we report the linear and nonlinear optical
properties of three analogues of dye 4, each with an extended p-
electron bridge. Of these, 1 has the weakest absorbance of
3.561024 layer21 at 532 nm and the LB absorption maximum
DOI: 10.1039/b009911m
J. Mater. Chem., 2001, 11, 1345–1350
This journal is # The Royal Society of Chemistry 2001
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is shifted to ca. 350 nm. Its alternate-layer films with poly(tert-
butyl methacrylate) exhibit the theoretically predicted quad-
ratic dependence of the second-harmonic intensity with the
number of active layers, i.e. I(N)2v~ I(1)2vN2 and a moderately
high susceptibility of 45¡5 pm V21. The compatible spacer
readily deposits on the downstroke, the preferred direction
when interleaving with optically nonlinear dyes,3 and is
transparent throughout the visible and near infrared regions.
It has a thickness of 1.00¡0.03 nm layer21 compared with
3.65¡0.12 nm bilayer21 for the alternate-layer structure and,
therefore, reduces the apparent second-order susceptibility of
the dye in the interleaved film by only ca. 30%.
carbaldehyde (0.77g, 3.2 mmol) in methanol (70 cm3) was
added piperidine (0.1 cm3) and the resultant mixture heated
under reflux for 48 h. Upon cooling, the iodide salt of 2 was
filtered and recrystallised from methanol: 1.42 g, 60%; mp 170–
171 ‡C. lmax (CHCl3): 374, 493 nm. dH (200 MHz, CDCl3, J/
Hz): 0.88 (t, J 5, 3H, CH3), 1.24 (br s, 32H, CH2), 1.73 (br s,
6H, N(CH2CH3)2), 1.98–1.90 (m, 2H, CH2), 3.08 (t, J 5, 2H,
CLC–CH2), 3.11 (t, J 5, 2H, CLC–CH2), 3.42–3.48 (m, 4H,
N(CH2CH3)2), 4.71 (t, J 5, 2H, CH2Nz), 6.49 (s, 1H, Ar–H),
6.64 (d, J 7, 1H, Ar–H), 7.39 (d, J 7, 1H, Ar–H), 7.57 (s, 1H,
Ar–H), 7.83 (s, 1H, Ar–H), 8.06 (d, J 5, 1H, Qn–H), 8.43 (d, J
5, 1H, Qn–H), 9.36 (s, 1H, Qn–H). m/z (FAB): 613 (Mz2I2,
100%).
Experimental
5-[3-(4-Dimethylaminophenyl)prop-2-enylidene]-2-octadecyl-
5,6,7,8-tetrahydroisoquinolinium iodide. To a solution of 2-
octadecyl-5,6,7,8-tetrahydroisoquinolinium iodide (1.0 g,
2.0 mmol) and 4-dimethylaminocinnamaldehyde (0.35 g,
2 mmol) in methanol (60 cm3) was added piperidine (0.1 cm3)
and the resultant mixture heated under reflux for 12 h. Upon
cooling, the iodide salt of 3 was filtered and recrystallised from
methanol: 0.8 g, 60%; mp 63–64 ‡C. lmax (CHCl3): 385, 495 nm.
Synthesis
4-[2-(4-Dimethylaminophenyl)vinyl]benzonitrile. To a solu-
tion of 4-(dimethylamino)benzaldehyde (3 g, 20 mmol) in
methanol (100 cm3) was added (4-formylbenzyl)triphenylphos-
phonium bromide (8.3 g, 20 mmol) and potassium tert-
butoxide (2.45 g, 20 mmol). The mixture was heated under
reflux for 24 h and then cooled. The resultant yellow precipitate
was purified by column chromatography on silica gel (eluent:
CHCl3) to afford 4-[2-(4-dimethylaminophenyl)vinyl]benzoni-
trile: yield 4.9 g, 100%; mp 197–198 ‡C (dec). dH (CDCl3,
200 MHz, J/Hz): 3.03 (s, 6H, N(CH3)2), 6.35 (d, J 10, 2H, Ar–
H), 6.64 (d, J 10, 2H, Ar–H), 6.74 (d, J 7, 2H, Ar–H), 6.89 (d, J
13, 1H, CLC–H), 7.12 (d, J 7, 2H, Ar–H), 7.15 (d, J 13, 1H,
CLC–H). m/z (FAB): 249 (Mz, 100%).
dH (200 MHz, CDCl3, J/Hz): 0.87 (t, J 5, 3H, CH3), 1.23 (br s,
32H, CH2), 1.93 (br s, 2H, CH2), 2.76 (t, J 5, 2H, CLC–CH2),
2.97 (t, J 5, 2H, CLC–CH2), 3.04 (s, 6H, N(CH3)2), 4.60 (t, J 6,
2H, CH2Nz), 6.67 (d, J 7, 2H, Ar–H), 6.97 (d, J 6, 2H, Ar–H),
7.34–7.47 (m, 3H, CLC–H), 8.08 (d, J 6, 1H, Qn–H), 8.65 (d, J
7, 1H, Qn–H), 8.85 (s, 1H, Qn–H). m/z (FAB): 543 (Mz2I2,
100%).
4-[2-(4-Dimethylaminophenyl)vinyl]benzaldehyde. To
a
LB deposition
cooled solution of 4-[2-(4-dimethylaminophenyl)vinyl]benzo-
nitrile (0.5 g, 2 mmol) in chloroform (100 cm3) was added
diisobutylaluminium hydride (1.5 M in toluene: 4 cm3,
6 mmol). The resultant solution was maintained at 278 ‡C
for 1 h and then, after attaining room temperature, was poured
into acidified water (150 cm3). The organic layer was extracted
and the aqueous phase repeatedly washed with chloroform
(3650 cm3). The combined organic layers were then washed
with water (50 cm3), extracted and dried (MgSO4), and the
solvent removed in vacuo yielding 4-[2-(4-dimethylaminophe-
nyl)vinyl]benzaldehyde as a yellow solid: 0.35 g, 70%. dH
(CDCl3, 200 MHz, J/Hz): 3.03 (s, 6H, NCH3), 6.75 (d, J 7, 2H,
Ar–H), 6.95 (d, J 13, 1H, CLC–H), 7.22 (d, J 13, 1H, CLC–H),
7.46 (d, J 7, 2H, Ar–H), 7.61 (d, J 7, 2H, Ar–H), 7.84 (d, J 7,
2H, Ar–H), 9.96 (s, 1H, CHO). m/z (FAB): 251 (Mz, 100%).
Dilute solutions of sodium octadecyl sulfate in methanol and
the iodide salts of the dyes in chloroform were co-spread, in a
1 : 1 mole ratio, onto the pure water subphase of an LB trough
(Nima Technology, model 622), left for 10 min and compressed
at 0.5 cm2 s21 (ca. 0.1% s21 of compartment area). The water-
soluble ions, Naz and I2, dissolve into the subphase and,
therefore, leave the amphiphilic cation and amphiphilic anion
as the floating monolayer. The dyes were deposited on the
upstroke at 35 mN m21
.
The inactive spacer, poly(tert-butyl methacrylate), used in
the fabrication of the alternate-layer structures, was spread
from dilute chloroform solution onto the water subphase of the
second compartment of the trough. Interleaved films were
obtained by cycling hydrophilically treated glass substrates (for
SHG and reflectance studies) and gold coated quartz crystal
(for QCM studies) from below the surface, to deposit the
optically nonlinear dye on the first upstroke at 35 mN m21 and
poly(tert-butyl methacrylate) on the subsequent downstroke at
5-{4-[2-(4-Dimethylaminophenyl)vinyl]benzylidene}-2-octade-
cyl-5,6,7,8-tetrahydroisoquinolinium iodide. To a solution of 2-
octadecyl-5,6,7,8-tetrahydroisoquinolinium iodide (1.0 g,
2.0 mmol) and 4-[2-(4-dimethylaminophenyl)vinyl]benzalde-
hyde (0.5 g, 2 mmol) in methanol (100 cm3) was added
piperidine (0.1 cm3) and the resultant mixture heated under
reflux for 72 h. Upon cooling, the iodide salt of 1 was obtained
as dark microcrystals. The product was recrystallised from
methanol: 0.42 g, 28%; mp 190 ‡C (dec.). lmax (CHCl3): 343,
455 nm. dH (200 MHz, CDCl3, J/Hz): 0.88 (t, J 5, 3H, CH3),
1.25 (br s, 32H, CH2), 1.90–2.01 (m, 2H, CH2), 2.43–2.51 (m,
4H, CLC–CH2), 3.04 (s, 6H, N(CH3)2), 4.72 (t, J 5, 2H,
CH2Nz), 6.72 (d, J 7, 2H, Ar–H), 6.90 (d, J 13, 1H, CLC–H),
7.13 (d, J 13, 1H, CLC–H), 7.45–7.57 (m, 7H, Ar–H and CLC–
H), 8.16 (d, J 5, 1H, Qn–H), 8.73 (d, J 5, 1H, Qn–H), 8.98 (s,
1H, Qn–H). m/z (FAB): 604 (Mz22CH32I2, 6%), 619
(Mz2CH32I2, 100%), 634 (Mz2I2, 6%).
5-(7-Diethylamino-2-oxo-2H-chromen-3-ylmethylidene)-2-
octadecyl-5,6,7,8-tetrahydroisoquinolinium iodide. To a solu-
tion of 2-octadecyl-5,6,7,8-tetrahydroisoquinolinium iodide
(1.61 g, 3.2 mmol) and 7-diethylamino-2-oxo-2H-chromene-3-
Fig. 1 Surface pressure versus area isotherm of dye 1.
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12 mN m21. Multilayer films were fabricated by repeating the
process with a deposition rate of 80 mm s21 in both directions.
Table 1 Thickness and relative permittivity data obtained from 105
bilayer film of dye 1
Wavelength/nm
Thickness/nm bilayer21
er
ei
n
Results and discussion
532.0
594.1
604.0
611.9
632.8
1064
3.57
3.68
3.74
3.68
3.69
3.53
2.43
2.36
2.37
2.37
2.37
2.33
0.03
0.00
0.00
0.00
0.00
0.00
1.56
1.54
1.54
1.54
1.54
1.53
Dye 1
The pressure-area (p–A) isotherm shows a limiting area of ca.
1.6 nm2 molecule21 at p~0 decreasing to 0.45 nm2 molecule21
just prior to collapse, the latter corresponding to the combined
cross-sectional areas of the octadecyl groups of the amphiphilic
cation and amphiphilic anion (Fig. 1). When transferred to a
10 MHz AT-cut quartz plate, at the deposition pressure of
35 mN m21, the frequency change (DF) gives an area of
0.54¡0.02 nm2 molecule21 in contact with the central gold
electrodes. This coincides with the corresponding value at the
air–water interface and is obtained from the Sauerbrey
equation:28
aMean thickness: l~3.65¡0.12 nm bilayer21. Standard deviations:
er, ¡0.04; n, ¡0.01.
polarised light (Fig. 2), the film being investigated at six
different wavelengths: the fundamental (1064 nm) and second-
harmonic (532 nm) of a Nd:YAG and at four wavelengths
(632.8, 611.9, 604.0 and 594.1 nm) of a multilane HeNe laser.
The thickness and optical constants of the 105 bilayer film of 1
were derived using
a three-layer (air–LB–glass) model.
A~{4F02M=DF(rm)1=2L
(1)
Reflections from the multiple interfaces were applied to the
absorbing surfaces and analysed, by fitting to theoretical
equations,29 using a Simplex curve fitting routine to minimise
the sum of the least squares fit. The data are summarised in
Table 1, the mean thickness being 3.65¡0.12 nm bilayer21
with the refractive index increasing from 1.53 at 1064 nm to
1.56 at 532 nm, the slight change being consistent with the fact
that the peak absorbance is at ca. 350 nm with very little
absorption at the second-harmonic wavelength. The difference
(n2v2nv~0.03) is significantly smaller than reported30 for
LiNbO3 and, from the following equation, the data yield a
coherence length for SHG at 1064 nm of ca. 8 mm at normal
incidence:
Fo is the resonance frequency (10 MHz) of the quartz crystal, r
the density of quartz (2.648 Mg m23), m the shear modulus
(2.9476107 Mg m21 s22), and M and L are the molecular mass
(969.55) and Avogadro constant (6.02261023 mol21) respec-
tively. The product of area and thickness provides a volume of
1.43¡0.12 nm3 molecule21 and, thus,
1.1¡0.1 Mg m23
a
density of
.
The layer thickness, referred to above, was determined by
analysis of the reflectance data from an alternate-layer film on
glass and corrected for the thickness of the poly(tert-butyl
methacrylate) spacer, its film characteristics being determined
separately by an identical method. The reflectance was
determined as the angle of incidence of the laser beam, relative
to the LB film, was rotated from 10 to 83‡ for both p and s
L
LB~l=4(n2v cos h2v{nv cos hv)
(2)
The second-harmonic intensity, as for the majority films, is
negligible at normal incidence and, therefore, was investigated
with the p-polarised laser beam (Nd : YAG, l~1.064 mm) at an
angle of 45‡ relative to the LB film. The intensity increases
quadratically with the film thickness, in this case to 105 bilayers
(Fig. 3), indicating long-range structural order when poly(tert-
butyl methacrylate) is alternated with the dye layers. The films
exhibit moderately strong SHG with a normalised intensity,
I
2v/N2 where N is the number of bilayers, of 1.161024 versus
the optimum intensity from the Maker fringe envelope of a Y-
cut quartz reference (d11~0.5 pm V21). The polarisation of the
fundamental beam was rotated using a half-wave plate and,
using the method of Kajikawa et al.,31 the polarisation
Fig. 2 Reflectance from a 105 bilayer film of dye 1 and poly(tert-butyl
methacrylate) on a glass substrate: (a) l~1064 nm and (b) l~532 nm,
p and s polarised in each case.
Fig. 3 Variation of the square root of the second-harmonic intensity
with the number of bilayers of dye 1 interleaved by poly(tert-butyl
methacrylate).
J. Mater. Chem., 2001, 11, 1345–1350
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Fig. 5 Surface pressure versus area isotherms of dyes 2 (solid line) and 3
(broken line).
Table 2 Thickness and relative permittivity data at l~632.8 nm for
alternate-layer films of the dyes and poly(tert-butyl methacrylate)
(ptbm) and homomolecular films of the spacer
Film structure
Thickness/ nm repeat21
er
ei
Alternate-layer
1zptbm
2zptbm
3.65¡0.12
3.90¡0.12
3.60¡0.12
2.37¡0.04
2.31¡0.04
2.23¡0.04
0.00
0.00
0.00
3zptbm
Homomolecular
ptbm
1.00¡0.03
2.18¡0.04a
0.00
aPublished values of er: 2.13¡0.03 (spun-coated films); 2.17¡0.07
(LB films).32
Fig. 4 Interleaved film of dye 1 and poly(tert-butyl methacrylate): (a)
spectrum showing the gradual increase in film absorbance following the
deposition of 1 to 105 bilayers and (b) the linear dependence of the
absorbance at 350 nm with thickness. The peak absorbance
(lmax~350 nm) is close to the lower limit of the transparency
window of the glass substrate and any asymmetry, as found for dyes
2 and 3, cannot be observed.
inorganic anion with octadecyl sulfate and interleaving with
poly(tert-butyl methacrylate).13 However, the films have an
absorbance of 361023 bilayer21 at the harmonic wavelength
whereas, in this work, the corresponding value is an order of
magnitude weaker (Fig. 4). The absorbance peaks at ca.
350 nm (cf. 343 and 455 nm in CHCl3) and then decreases to
3.561024 bilayer21 at 532 nm with the films being transparent
at 1064 nm. Thus, together with alternate-layer films of dye 4,8
which has a susceptibility of 67 pm V21 and an absorbance of
only 561024 bilayer21, the susceptibility is among the highest
obtained to date for an effectively transparent alternate-layer
dependence, I2v(pAp)/I2v(sAp), indicates that the chromo-
phore is tilted by 44¡2‡ from the normal to the substrate.
The films are almost transparent at the harmonic wave-
length. Thus, it may be assumed that Kleinman’s symmetry is
valid and that the susceptibility components are limited to
xzzz(2) and xzxx(2) which, from the data above, may be estimated
as 45¡5 pm V21 and 21¡2 pm V21 respectively. Further-
more, as the hyperpolarisability is probably dominated by the
component along the molecular charge-transfer axis, the
relation between b and x(2) is as follows:
xz(2z)z~Nf 2v(f v)2b cos3 w
(3)
x(z2x)x~1=2Nf 2v(f v)2b cos w sin2 w
(4)
N is the number of molecules per unit volume, f2v and fv are
local field correction factors at 2v and v respectively where
f~(n2z2)/3 and n is the refractive index at the corresponding
wavelength, and w is the chromophore tilt angle. The molecular
hyperpolarisability is ca. 8610238 m4 V21
.
Interestingly, the second-harmonic intensity is similar to (or
higher than) resonantly enhanced values reported for alternate-
layer films of the many different hemicyanine derivatives.13,16–18
For example, Han et al.18 have reported a susceptibility of
50 pm V21 for electrically poled LB films of (E)-N-octadecyl-4-
{2-[4-(N,N-diethylamino)phenyl]ethenyl}pyridinium bromide
interleaved with docosanoic acid (cf. 16 pm V21 when not
poled). This may be increased to 70 pm V21 by substituting the
Fig. 6 Visible spectrum of the interleaved film of dye 2 and poly(tert-
butyl methacrylate) comprising 95 bilayers. Analysis has indicated that
the combined absorbance of separate symmetrical bands at 360, 420
and 470 nm can give rise to the unusual asymmetric spectrum and such
values may be compared with lmax~364, 450 (sh) and 495 nm when the
dye is dissolved in chloroform.
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J. Mater. Chem., 2001, 11, 1345–1350

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the dye layer is ca. 1.1 Mg m23 and consistent with the value
obtained for 1.
Their films are less transparent in the visible region (Figs. 6
and 7), having absorbances of 1.161023 (2) and 1.561023
bilayer21 (3) at the second-harmonic wavelength and showing
maxima at 422 and 465 nm respectively (cf. lmax~350 nm for
1). The transitions are strangely asymmetric and this is
particularly pronounced for dye 2. Its absorption band may
be resolved as separate peaks at 470 and 420 nm and albeit
shifted, as is usual for LB film spectra, the maxima relate to
those of the solution (lmax~493, sh 450, 374 nm in CHCl3).
When interleaved with poly(tert-butyl methacrylate), the two
dyes exhibit quadratic SHG enhancement with the number of
layers (Fig. 8), the second-order susceptibility, thickness and
chromophore tilt angle, relative to the substrate normal, being
ca. 60 pm V21, 3.9 nm bilayer21 and 37‡ respectively for the 7-
diethylamino-2-oxo-2H-chromen-3-ylmethylene analogue (2)
and ca. 95 pm V21, 3.6 nm bilayer21 and 32‡ for the 3-
(diethylaminophenyl)prop-2-enylidene derivative (3). The
higher susceptibilities, relative to 45 pm V21 for dye 1
(Table 3), arise from increased resonance enhancement but
are far less interesting because the dyes are more absorbing at
532 nm. Nonetheless, judicious choice of the counterion,
amphiphilic rather than spherical, and the polymeric spacer
has resulted in improved deposition characteristics and
quadratic enhancement of the second-harmonic intensity
with thickness to more than 100 bilayers. This is noteworthy,
because relatively few alternate-layer films.13–22 have exhibited
long-range order to such thicknesses.
Fig. 7 Visible spectrum of the interleaved film of dye 3 and poly(tert-
butyl methacrylate) comprising 105 bilayers.
Conclusion
Alternate-layer films of the three optically nonlinear dyes
exhibit second-order susceptibilities of 45 to 100 pm V21 at
l~1064 nm, the most interesting of these having the weakest
susceptibility but a very slight absorbance of 3.561024
bilayer21 at the harmonic wavelength. The interleaving
layers readily deposit with the dye on the upstroke and the
poly(tert-butyl methacrylate) on the subsequent downstroke,
this arrangement resulting in long-range order as indicated by
the quadratic SHG dependence. This study is now being
extended at Cranfield to include similar dyes but where further
extension of the p-electron bridge results in this being the
hydrophobic substituent for alignment at the air–water inter-
face. Such molecules then do not require the aliphatic tail and,
consequently, the second-order susceptibility may be further
improved.
Fig. 8 Variation of the square root of the second-harmonic intensity
with the number of bilayers of the dyes interleaved by poly(tert-butyl
methacrylate): dye 2 ($); dye 3 (6).
film. The enhancement is attributed to the proximity of the
charge-transfer band, which is barely overlapping but opti-
mises the trade-off in efficiency and absorbance.
Related dyes
The p–A isotherm of dye 2 (Fig. 5) is similar to that of dye 1 but
Acknowledgements
the steep high-pressure regime extends from ca.
8 to
52 mN m21 with a limiting area of ca. 0.50 nm2 molecule21
just prior to collapse. In contrast, that of dye 3 is generally
featureless and has a similar limiting area of 0.43 nm2
molecule21 (cf. 0.45 nm2 for 1), this common value corre-
sponding to the combined cross-sectional areas of the octadecyl
groups of the amphiphilic cation and amphiphilic anion. The
areas in contact with the substrate, obtained by the quartz
crystal microbalance (QCM) technique at the deposition
pressure of 35 mN m21, are 0.50 nm2 molecule21. Thus,
when related to the thickness data of Table 2, the density of
We acknowledge the EPSRC (UK) for funding the nonlinear
optics programme at Cranfield and for providing PhD
studentships to A. J. W., M. A. A. and R. H. We are also
grateful to previous students, Rakesh Ranjan and Karl
Skjonnemand, for technical assistance.
References
1
A. Ulmann, An Introduction to Ultrathin Films: from Langmuir–
Blodgett to Self-Assembly, Academic Press, San Diego, 1991.
M. C. Petty, Langmuir–Blodgett Films: an Introduction, Cam-
bridge University Press, Cambridge, 1996.
2
3
4
G. J. Ashwell, J. Mater. Chem., 1999, 9, 1991.
Table 3 Alternate-layer films: spectroscopic data, susceptibilities and
chromophore tilt angles
G. J. Ashwell, E. J. C. Dawnay, A. P. Kuczynski, M. Szablewski,
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Dye
lmax/nm
A
2v/bilayer21
xzzz(2)/pm V21
Q/‡
1
2
3
350
422
465
3.561024
1.161023
1.561023
45¡5
60¡5
95¡5
44¡2
37¡2
32¡2
5
J. Mater. Chem., 2001, 11, 1345–1350
1349

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