Nanostructured oligo(p-phenylene vinylene)/silicate hybrid films: One-step fabrication and energy transfer studies

Article abstract of DOI:10.1021/ja058610s

Novel hybrid materials containing silicate and charged oligo(p-phenylene vinylene) (OPV) amphiphiles were fabricated in one step by spin casting using evaporation-induced self assembly. The conjugated segments were substituted with trimethylammonium bromide groups at both termini, and tetraethyl orthosilicate served as the silicate precursor. X-ray diffraction scans of the hybrid films revealed Bragg diffraction peaks with d-spacings of 2.76 and 1.37 nm, indicating the presence of order in the hybrid structure. Optical properties of the hybrid films were characterized by UV-vis absorption and fluorescence spectra, and molecular orientation was characterized by IR spectroscopy. A rhodamine B derivative containing a triethoxysilane group was covalently incorporated into the silicate network of the films during the sol-gel reaction. Relative to disordered polymer films with identical organic composition, the ordered hybrid films revealed significantly enhanced emission from rhodamine B and also fluorescence quenching from OPV segments. These results indicate that the ordered and nanostructured environment leads to highly efficient energy transfer among organic components in these hybrid films.

Full text of DOI:10.1021/ja058610s

Published on Web 04/04/2006
Nanostructured Oligo(p-phenylene Vinylene)/Silicate Hybrid
Films: One-Step Fabrication and Energy Transfer Studies
Keisuke Tajima,† Liang-shi Li, and Samuel I. Stupp*
Contribution from the Department of Chemistry, Department of Materials Science and
Engineering, Feinberg School of Medicine, Northwestern UniVersity, EVanston, Illinois 60208
Received December 20, 2005; E-mail: s-stupp@northwestern.edu
Abstract: Novel hybrid materials containing silicate and charged oligo(p-phenylene vinylene) (OPV)
amphiphiles were fabricated in one step by spin casting using evaporation-induced self assembly. The
conjugated segments were substituted with trimethylammonium bromide groups at both termini, and
tetraethyl orthosilicate served as the silicate precursor. X-ray diffraction scans of the hybrid films revealed
Bragg diffraction peaks with d-spacings of 2.76 and 1.37 nm, indicating the presence of order in the hybrid
structure. Optical properties of the hybrid films were characterized by UV-vis absorption and fluorescence
spectra, and molecular orientation was characterized by IR spectroscopy. A rhodamine B derivative
containing a triethoxysilane group was covalently incorporated into the silicate network of the films during
the sol-gel reaction. Relative to disordered polymer films with identical organic composition, the ordered
hybrid films revealed significantly enhanced emission from rhodamine B and also fluorescence quenching
from OPV segments. These results indicate that the ordered and nanostructured environment leads to
highly efficient energy transfer among organic components in these hybrid films.
Introduction
fabrication techniques provide ways to achieve structural order
at the nanometer scale, they involve multistep processes that
Light-harvesting complexes in green plants and some bacteria
are good examples for showing the importance of spatial
molecular arrangements in controlling the direction and ef-
ficiency of electron or energy transfer.1 Inspired by these
systems, many studies have been carried out to introduce
structural order in semiconducting organic materials to control
charge transport and energy transfer processes, especially for
the goal of developing high performance devices such as field
effect transistors (FETs)2 or organic solar cells.3 Fabrication
techniques such as Langmuir-Blodgett (LB) methodology4 or
layer-by-layer deposition5 offer the possibility of controlling
nanoscale structure and molecular orientation in organic materi-
als. For example, Swager et al. reported previously on multilayer
LB films of conjugated polymers with different band gaps to
achieve directional energy transfer in thin films.4b While these
are not easily scalable and may be difficult to implement in the
context of device fabrication. It is therefore highly desirable to
investigate what self assembly may offer to create materials with
interesting optical and electronic properties. Meijer and co-
workers reported that chiral oligo(p-phenylene vinylene) (OPV)
molecules self assembled to form helical stacking structures in
solution.6 Very recently, they reported that structural order in
the self-assembled helix significantly affects the efficiency of
energy transfer between OPVs.7
We report here on a strategy for the one-step fabrication of
a nanostructured hybrid material containing OPV segments and
silicate. Our strategy takes advantage of “evaporation-induced
self assembly” developed recently by Brinker et al. to synthesize
mesoporous silicate films.8,9 In this method, the hydrophilic
segments of amphiphilic molecules interact with precursors for
inorganic materials and self assemble into ordered phases such
† Current address: Department of Applied Chemistry, School of Engi-
neering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-
8656, Japan.
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Schulten, K. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 5935-5941.
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Nanostructured Oligo(p-phenylene Vinylene)/Silicate
A R T I C L E S
as cubic, hexagonal, or lamellar structures during solvent
evaporation. We designed the amphiphilic molecules with OPV
segments that have good chemical stability as well as high
electroluminescence and hole conductivity.10 OPV is therefore
an interesting target for both organic light emitting diodes
(OLEDs) and organic solar cells. The synthesis of a PPV/silicate
composite material has been previously reported using soluble
PPV precursor polymers and silicon alkoxides for the sol-gel
process;11 however, structural order is not expected in these
polymer-silicate mixtures. Nanostructured PPV/clay films have
also been fabricated using a layer-by-layer deposition technique,
and in this case, enhanced stability of the films toward UV
irradiation was reported.12 Our work here investigates the use
of self-assembling molecules to create highly ordered and
nanostructured films based on OPV and silicate in one step.
OPV segments are especially useful for the design of self-
assembling molecules given their tendency to aggregate through
π-π stacking interactions.13 We use here an amphiphilic
compound containing OPV segments as opposed to polymeric
components to generate highly ordered structures. An am-
phiphile was used given the well-known tendency for surfactants
to form ordered phases. Furthermore, a “strongly amphiphilic”
charged compound was selected to raise the probability that
ordered phases would not be disrupted by the introduction of
precursors for inorganic materials. This would then offer the
possibility of one-step formation of a highly ordered hybrid
material. Hybrid films formed through self assembly are
extremely important model structures to understand how nano-
scale order parameters influence electronic properties. In this
paper, we address this question by modifying the system with
a rhodamine B derivative and probing energy transfer in the
hybrid films.
Figure 1. Chemical structures of oligo(p-phenylene vinylene) amphiphiles
(1) and of the rhodamine B derivative used in the formation of the silicate
domain (2).
Figure 2. Plot of film thickness as a function of absorbance at 350 nm by
oligo(p-phenylene vinylene) segments in 1a/silicate films.
Results and Discussion
Synthesis. We were able to synthesize OPV amphiphiles 1
(Figure 1) using the Horner-Emmons reaction for the formation
of trans-vinyl bonds and subsequent methylation of the dim-
ethylamines to the tetramethylammonium salts. 1a (n ) 2) was
found to be readily soluble in MeOH, whereas 1b-d, with
longer alkyl linkers, had lower solubilities relative to 1a.
Because MeOH is a good solvent for the silicate precursor
tetraethyl orthosilicate (TEOS) and its hydrolyzing agent HCl,
we were able to create hybrid films of 1a and silicate by simply
spin casting MeOH solutions on a variety of substrates such as
silicon, quartz, and glass. The films obtained by spin casting
solutions were macroscopically homogeneous and transparent
with thicknesses on the order of 40-80 nm.
Characterization of Hybrid Films. The thickness of hybrid
films was found to be controlled by rotation speed used during
spin casting. We also found a linear relationship between optical
density measured by UV-vis absorption and film thickness
determined by ellipsometry (Figure 2), suggesting a uniform
distribution of the amphiphile in the hybrid films. Small-angle
X-ray diffraction (XRD) scans of 1a/silicate films spin cast at
2000 rpm revealed peaks at 2.76 and 1.37 nm (Figure 3),
indicating the presence of a periodic structure in the hybrid film.
The d-spacing of 2.76 nm is slightly larger than the length of a
fully extended molecule of 1a (calculated distance between two
nitrogens in 1a is 2.5 nm). When we used OPV amphiphiles
with longer alkyl spacers of n ) 3 (1b) and n ) 4 (1c), the
d-spacing of 1/silicate films increased to 3.0 and 3.3 nm,
respectively. However, the OPV amphiphile with n ) 6 (1d)
yielded an inhomogeneous, opaque film because of its low
solubility in MeOH. The results obtained with 1b and 1c indicate
that the periodicity observed in the film is directly controlled
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Mater. 1999, 11, 579-585. For synthesis of mesoporous silicate films,
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C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.;
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9866.
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A R T I C L E S
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films deposited on undoped silicon substrates (Figure 5A). We
used the IR spectrum of an isotropic sample (Figure 5B), which
is prepared by scraping off the hybrid film from the substrate
and dispersing it into a KBr pellet, as a standard for a random
orientation. By comparing these two spectra, we can estimate
the orientation of the OPV segments in the hybrid films. Peaks
at 1602 cm-1 and 962 cm-1 are assigned to vibrational modes
of the OPV segments corresponding to phenyl ring quadrant
stretching (parallel to the axis of 1,4-substitution) and trans-
vinyl C-H out-of-plane wagging (perpendicular to the phenyl
plane), respectively.15 We assume that the films are azimuthally
isotropic and use the “partial symmetrical planar orientation”
model.16 The orientation of OPV segments in the film is
calculated with the formula
Figure 3. X-ray diffraction pattern of the hybrid oligo(p-phenylene
vinylene) amphiphile (1a)/silicate film.
Asample,962/Asample,1602
2 - F
)
Arandom,962/Arandom,1602
2F
where Asample and Arandom are peak areas of absorptions in the
hybrid film and the isotropic sample, respectively, and
F ) 2π sin2 γ ‚ f(γ)dγ ) sin2 θ
∫
0
where θ is the average molecular angle of the OPV segments
with respect to the film normal. From the IR spectra shown in
Figure 5, we can obtain a peak ratio of 1.5; thus, F ) 0.5 and
θ ) 45°. This result leads to an order parameter of 0.25.
Therefore, we can conclude that the OPV segments are not
oriented with high-order parameter normal to the substrate, but
at the same time, the segments do not have isotropic orientation
in the hybrid films. The actual orientation of the molecules in
these obviously periodic hybrid films will require further
experiments in the future.
UV-vis absorption spectra of the 1a/silicate films showed a
broad peak with a maximum at 350 nm, showing a blue-shift
relative to that of the peak obtained in a MeOH solution of 1a.
(362 nm) (Figure 6A). The corresponding fluorescence spectra
are shown in Figure 6B. The maximum in the excitation spectra
of 1a/silicate films (356 nm) reveals a slight blue-shift relative
to 1a in MeOH (362 nm), while a red-shift from 400 to 450
nm is observed in emission spectra. Based on these spectroscopic
measurements, we conclude that OPV segments of 1a in the
films stack into H aggregates held together by π-π interactions
within the organic domains (Figure 4A).
Energy Transfer in Hybrid Films. The periodic nanostruc-
ture in hybrid films offers the opportunity to investigate how
its order parameters influence energy transfer. For this purpose,
rhodamine B derivative 2 (Figure 1) was synthesized and
introduced into the 1a/silicate films as an energy acceptor.17
The ethoxysilane groups of 2 participate in the formation of
the silicate network during the film deposition, resulting in the
formation of a periodic structure with organic domains of donor
1a and silicate domains doped with acceptor 2 (schematically
shown in Figure 4B).
Figure 4. (A) Schematic representation of possible structure in oligo(p-
phenylene vinylene) amphiphile (1a)/silicate films and (B) similar films
when doped with the rhodamine B derivative (2). Organic OPV domains
may also contain some silica.
by the dimensions of the amphiphilic molecule. When the molar
ratio of 1a to TEOS in the precursor solution was lowered from
0.15 to 0.08, the intensity of the first peak was decreased with
little change in d-spacing (2.70 nm). At a lower ratio of 0.05,
X-ray diffraction peaks are no longer observed. We conclude
from these results that OPV domains in the hybrid films are
separated by layers of silicate which do not grow significantly
as the concentration of TEOS is increased. This implies that
the silicate layers in the hybrid films are thin and have a self-
limiting thickness. Therefore, increasing TEOS concentration
simply generates silicate that is not part of the periodic hybrid
structure, explaining why the diffraction peaks disappear at high
TEOS concentration. The d-spacings of 2.76 and 1.37 nm can
be assigned to (001) and (002) diffractions of a layered structure,
respectively. However, these d-spacings could correspond also
to (100) and (200) diffractions of a hexagonal structure aligned
parallel to the substrate. This has been observed in mesoporous
silica films in which orientation of the hexagonal structure
parallel to the substrate precludes observation of the (110)
diffraction.14 Detailed structural analysis of these films is
currently underway using synchrotron X-ray experiments. The
structure of the hybridized film is proposed in Figure 4A
schematically.
(14) Hillhouse, H. W.; Egmond, J. W. v.; Tsapatsis, M.; Hanson, J. C.; Larese,
J. Z. Microporous Mesoporous Mater. 2001, 44-45, 639-643.
(15) (a) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared
and Raman Spectroscopy; Academic Press: New York, 1975. (b) Bradley,
D. D. C.; Friend, R. H. Polymer 1986, 27, 1709-1713.
(16) R. Zbinden, Infrared Spectroscopy of High Polymers; Academic Press: New
York and London, 1964; p 216.
(17) Verhaegh, N. A. M.; van Blaaderen, A. Langmuir 1994, 10, 1427-1438.
To determine molecular orientation in the hybrid films, we
obtained infrared (IR) transmittance spectra of the 1a/silicate
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A R T I C L E S
Figure 5. Infrared absorption spectra of (A) an oligo(p-phenylene vinylene) amphiphile (1a)/silicate film on a silicon substrate (blue line) and (B) the film
scraped off from the substrate and dispersed in a KBr pellet (red line). The orientation of OPV segments in the film was estimated by comparing the ratio
of the areas under the peaks at 1602 and 962 cm-1 (see main text).
presence of the periodic structure observed in undoped hybrid
films. UV-vis absorption spectra of the films showed a peak
around 560 nm corresponding to absorption by 2 in addition to
the absorption by OPV segments (Figure 7A). The peak maxima
red-shifted from 556 nm (0.5 mol %) to 572 nm (10 mol %) as
the amount of 2 in the films was increased. This red-shift is
assigned to the formation of dimeric species in the films,
commonly observed at this concentration of the dye in silicate
films.18 The absorbance of 2 in the films was linear with the
amount of 2 in the precursor solution as shown in Figure 7C,
indicating that the rhodamine B derivative is incorporated
homogeneously into the films up to a concentration of 10 mol
%. Considering the XRD data and the absorption spectra, we
conclude rhodamine B derivative 2 was successfully incorpo-
rated into the silicate network without disrupting the ordered
structure.
Fluorescence spectra of the hybrid films doped with 2 are
shown in Figure 8A. When the amount of 2 in the films was
increased, fluorescence at 480 nm from OPV segments was
quenched. The fluorescence intensity at 480 nm was plotted as
a function of absorbance from 2 in Figure 8C.19 This result
clearly demonstrates that the introduction of 2 in the films
caused efficient quenching of OPV fluorescence. At the same
time, fluorescence from the rhodamine B derivative can be
observed as films are doped with 2. The fluorescence from 2
showed both a decrease in intensity and a red-shift of the peak
maximum from 582 nm (0.5 mol %) to 605 nm (10 mol %) as
Figure 6. (A) UV-vis absorption spectra of oligo(p-phenylene vinylene)
the amount of 2 was increased. These observations can be
explained by concentration quenching, which is an increase of
the nonfluorescent dimeric species of 2 in the films.18 Because
2 is not fluorescent with excitation at 350 nm, emission from 2
clearly indicates energy transfer occurs from 1a to 2 in the
hybrid films. This is further supported by the excitation spectrum
of the hybrid film doped with 2 (Figure 9, red line). Emission
amphiphile (1a) in MeOH (solid line) and 1a/silicate film (dashed line).
(B) Excitation (left) and emission (right) fluorescence spectra of 1a in MeOH
(solid line, λem ) 418 nm and λex ) 350 nm) and a 1a/silicate film (dashed
line, λem ) 452 nm and λex ) 350 nm).
Precursor solutions for the hybrid films are prepared by
adding the proper amounts of 2 while the concentrations of 1a
and silicate precursors in the solution are kept constant. The
amount of 2 in precursor solutions was changed from 0.5 to 10
mol % with respect to the amount of 1a.
(18) (a) Valdes-Aguilera, O.; Neckers, D. C. Acc. Chem. Res. 1989, 22, 171-
177. (b) Nishikiori, H.; Fujii, T. J. Phys. Chem. B 1997, 101, 3680-3687.
(c) Fujii, T.; Nishikiori, H.; Tamura, T. Chem. Phys. Lett. 1995, 233.
(19) The fluorescence intensities were corrected with the absorbance at 350 nm
in the UV spectra, i.e., the amount of 1a in the films.
XRD scans of the spin-cast films doped with up to 10 mol
% of 2 showed a diffraction peak at 2.7 nm, indicating the
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Tajima et al.
Figure 7. (A) Absorption spectra of oligo(p-phenylene vinylene) am-
phiphile (1a)/silicate films and (B) 1a/poly(2-hydroxyethyl methacrylate)
(PHEMA) films doped with rhodamine B derivative (2). The amounts of 2
in the precursor solutions were changed from 0 to 10 mol % with respect
to the amount of 1a. (C) Absorption peak maximum of 2 (∼560 nm) plotted
as a function of concentration of 2 in precursor solutions; [: 1a/silicate
film; 9: 1a/PHEMA film.
Figure 8. (A) Fluorescence spectra of oligo(p-phenylene vinylene) am-
phiphile (1a)/silicate films and (B) 1a/poly(2-hydroxyethyl methacrylate)
(PHEMA) films doped with rhodamine B derivative (2). The amounts of 2
in the precursor solutions were changed from 0 to 10 mol % with respect
to 1a. (C) Fluorescence peak intensities of 1a at 480 nm corrected with the
absorbance of 1a at 350 nm plotted against the absorbance at the peak
maximum of 2; [: 1a/silicate film; 9: 1a/PHEMA film.
from rhodamine B was monitored at 590 nm, showing the
excitation contribution is mostly from OPV absorption around
350 nm and less from direct excitation of the dye around 560
nm.
It has been recently reported that cross-linking in OPV-based
materials enhances energy transfer from OPV to rhodamine B
compared to mixtures of the two in solution.20 In this previous
work, 50 mol % dye molecules were introduced with respect
to OPV in fibrous xerogel films, and quenching of OPV
fluorescence and fluorescence from the dye were observed.
Although the material investigated is very different from that
reported here, it is interesting to point out that in the ordered
hybrid films only 0.5 mol % acceptor is necessary to observe
quenching of OPV and emission from the rhodamine B
derivative. This is 100 times lower than the mol % dye used in
the xerogel reported previously.
(20) (a) Ajayaghosh, A.; George, S. J.; Praveen, V. K. Angew. Chem., Int. Ed.
2003, 42, 332-335. (b) George, S. J.; Ajayaghosh, A. Chem.-Eur. J. 2005,
11, 3217-3227.
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Figure 9. Excitation fluorescence spectra of an oligo(p-phenylene vinylene)
amphiphile (1a)/silicate film (red line) and of a 1a/poly(2-hydroxyethyl
methacrylate) film (blue line) from the precursor solutions containing 2
mol % of 2 (emission wavelength: 590 nm).
Figure 10. Photographs of oligo(p-phenylene vinylene) amphiphile (1a)/
poly(2-hydroxyethyl methacrylate) film (left) and 1a/silicate film (right)
from precursor solutions containing 2 mol % of rhodamine B derivative
(2) under UV irradiation (λ ) 365 nm).
enhanced energy transfer in the ordered environment of the
hybrid films.
Energy Transfer in Disordered Films. To investigate the
role of the ordered structure on energy transfer, we prepared
amorphous control films with the same OPV amphiphile using
a polymer matrix. We used poly(2-hydroxyethyl methacrylate)
(PHEMA) instead of silicate because of its good solubility in
MeOH. The concentrations of 1a and 2 in the films were
adjusted to make them comparable to the corresponding silicate
films. The films obtained were uniform and optically transparent.
XRD scans of these 1a/PHEMA control films did not reveal
any peaks, confirming the absence of the periodic structure
observed in 1a/silicate films. When the amount of 2 was
increased, the absorption peak corresponding to 2 increased as
well, and a red-shift from 556 nm (0.5 mol %) to 566 nm (10
mol %) was observed (Figure 7B). As in the case of silicate
films, this can be attributed to the formation of dye dimeric
species. The absorbance maximum of 2 scaled linearly with the
amount of 2 in precursor solutions (Figure 7C). The greater
slope of this curve relative to that of silicate films indicates
that 2 was more effectively incorporated into the polymer films.
The amount of 2 in the disordered films was estimated using
the absorbance at the peak maximum around 560 nm.
Interestingly, the red fluorescence from 2 in the ordered
hybrid films was found to be significantly stronger than that in
the disordered control films. The difference is visibly striking
to the naked eye even when the amount of 2 doped into the
films was low. As shown in Figure 10, the 1a/silicate films
deposited on a substrate from a precursor solution with 2 mol
% of 2 appears red under irradiation with 365 nm light, whereas
the polymer films appear blue under the same irradiation. This
could be attributed to both the stronger quenching of the
emission from OPV segments and the enhancement of emission
from the rhodamine B derivative in the ordered silicate films.
The difference in quenching behavior of 480 nm emission in
ordered vs disordered films is compared quantitatively in Figure
8C. Excitation spectra of the films with 2 mol % doping of 2
were measured at the emission wavelength of 590 nm (Figure
9). The 1a/PHEMA films showed a smaller contribution from
OPV absorption (around 350 nm) compared with the ordered
1a/silicate films. Also, the contribution from 2 absorption
(around 560 nm) is smaller in the silicate films than in the
disordered polymer films. These results suggest a much
The significant enhancement of energy transfer in 1a/silicate
films with nanoscale periodicity could be attributed to several
factors. One possible factor is that 2 is dispersed in silicate
domains of hybrid films, and thus, the average distance between
1a and 2 could be shorter than in the PHEMA films. Another
possibility is a larger spectral overlap between emission of 1a
and excitation of 2 in the 1a/silicate films. However, this seems
unlikely because the difference in shapes of emission peaks from
OPV segments is small between the two types of films (Figure
8A,B). Another factor that may play a role in the enhancement
of energy transfer in the ordered hybrid films at a low
concentration of dye is efficient energy migration within ordered
domains of 1a. When OPV segments in the hybrid films are
excited by irradiation, both energy transfer from 1a (donor) to
2 (acceptor) and energy migration within 1a domains could
occur. At the low concentration of 2 (0.5 mol % relative to
1a), the probability of direct energy transfer from 1a to 2 is
expected to be low. However, efficient energy migration in OPV
domains of the ordered structure would raise the probability of
an acceptor being adjacent to an excited donor, thus significantly
enhancing the energy transfer process from 1a to 2. In the
control polymer films, energy migration within 1a could be
suppressed by the disordered environment, resulting in less
efficient energy transfer. This intradomain energy migration and
interdomain energy transfer could be features of the efficient
energy collecting systems of nature. Future dynamic fluores-
cence studies on the films would give us more detailed
information on the relative importance of various factors in the
significant energy transfer observed in the self-assembling
hybrid systems studied.
Conclusions
Using self-assembling amphiphiles with electronically active
segments and charged groups, we were able to create in a single
deposition step an organic-inorganic hybrid film that is periodic
at the nanoscale. Extremely efficient energy transfer occurs in
these materials from the electronically active segments to dyes
grafted covalently to the inorganic silicate domains of the
periodic structure. The physical basis of the efficient energy
9
J. AM. CHEM. SOC. VOL. 128, NO. 16, 2006 5493

A R T I C L E S
Tajima et al.
ω-((Dimethylamino)alkyl)-methanesulfonate Hydrochloride (3a,b).
N,N-Dimethylpropanolamine (4.13 g, 40 mmol, 1 equiv) was dissolved
in 100 mL of CH2Cl2, and the solution was cooled to 0 °C.
Methanesulfonyl chloride (3.7 mL, 48 mmol, 1.2 equiv) was added
slowly, and the mixture was stirred for 24 h at r.t. White precipitate
was filtered out and dried in vacuo to afford the product 3b (8.42 g,
97% yield). H NMR (DMSO-d6): δ 10.78 (s, 1H), 4.30 (t, 2H, J )
5.0 Hz), 3.22 (s, 3H), 3.11 (t, 2H, J ) 6.0 Hz), 2.73 (s, 6H), 2.08 (m,
2H).
transfer is possibly related to short distances between donors
and acceptors and also efficient energy migration within organic
domains. These findings are useful to the design of easily
fabricated materials for light emission devices or light harvesting
devices in photovoltaic devices.
1
Experimental Section
General. Unless otherwise noted, all starting materials were obtained
from commercial suppliers and used without further purification. H
4-(ω-(Dimethylamino)alkoxy)-benzaldehyde (4a,b). 4-Hydroxy-
benzaldehyde (4.73 g, 38.7 mmol, 1 equiv), 3b (8.42 g, 38.7 mmol, 1
equiv), potassium carbonate (21.4 g, 155 mmol, 4 equiv), and 18-crown-
6-ether (0.95 g, 3.87 mmol, 0.1 equiv) were placed in a flask with a
magnetic stirring bar and a cooling column and dissolved in 200 mL
of acetone. The mixture was refluxed for 24 h. After cooling, the
mixture was filtrated and concentrated in vacuo. The crude product
was subjected to a column chromatography using 5% MeOH/CH2Cl2
to afford the product 4b as clear oil (4.47 g, 56%). 1H NMR (CDCl3):
δ 9.87 (s, 1H), 7.82 (d, 2H, J ) 8.8 Hz), 7.00 (d, 2H, J ) 8.2 Hz),
4.10 (t, 2H, J ) 6.4 Hz), 2.46 (t, 2H, J ) 7.1 Hz), 2.25 (s, 6H), 1.99
(m, 2H).
1
NMR spectra were recorded on a Varian Unity 400 (400 MHz)
spectrometer using the solvent proton signal as standard. UV-vis
spectra were recorded using an HP 8452 spectrometer. Fluorescence
spectra were recorded on ISS PC1 Photon Counting Fluorometer using
reflection geometry. At least three different points of one sample were
measured and averaged to determine the fluorescence intensities from
the films. Film thickness of the samples was determined by applying
Cauthy model to ellipsometric data in the transparent region recorded
on a SOPRA MOSS ES4G spectroscopic ellipsometer. XRD patterns
were recorded on a Rigaku RINT 2400 X-ray diffractometer, and
infrared spectra were recorded using a Thermo Nicolet Nexus 870 FT-
IR spectrometer.
4-(ω-Bromoalkoxy)-benzaldehyde (5c,d). 4-Hydroxybenzaldehyde
(1.22 g, 10 mmol, 1 equiv), 1,6-dibromohexane (3.7 g, 15 mmol, 1.5
equiv), potassium carbonate (2.8 g, 20 mmol, 2 equiv), and 18-crown-
6-ether (0.25 g, 1 mmol, 0.1 equiv) were placed in a flask with a
magnetic stirring bar and a cooling column and dissolved in 200 mL
of acetone. The mixture was refluxed for 24 h. After cooling, the
mixture was filtrated and concentrated in vacuo. The crude product
was subjected to a column chromatography using CH2Cl2 to afford the
Synthesis. Preparation of OPV/Silicate Hybrid Films. Precursor
solutions for the OPV amphiphile/silicate hybrid films were prepared
by first dissolving 6.0 mg of 1a (9.0 × 10-6 mol) in 0.75 mL of MeOH
and then adding 15 µL of 35 wt % HCl aqueous solution and 14 µL of
tetraethyl orthosilicate (TEOS, 6.25 × 10-5 mol) to the OPV solution.
The solutions were stirred for 30 min at room temperature. The final
reactant molar ratios were 1 TEOS:0.15 OPV (1a): 300 MeOH:8.3 H2O:
2.5 HCl. The solution was membrane filtered (pore size: 0.45 µm)
and deposited on a quartz, glass, or silicon substrate by spin casting at
500-3000 rpm. The films were left overnight at ambient atmosphere
and subsequently dried in vacuo for 3 h.
1
product 5d as clear oil (1.7 g, 60%). H NMR (CDCl3): δ 9.88 (s,
1H), 7.83 (d, 2H, J ) 8.6 Hz), 6.98 (d, 2H, J ) 8.5 Hz), 4.05 (t, 2H,
J ) 6.5 Hz), 3.43 (t, 2H, J ) 6.8 Hz), 1.90 (m, 2H), 1.83 (m, 2H),
1.53 (m, 4H).
4-(ω-(Dimethylamino)alkoxy)-benzaldehyde (4c,d). Dimethy-
lamine solution (18 mL, 34.5 mmol, 2.0 M in THF, 5 equiv) was added
to 5c (1.78 g, 6.9 mmol, 1 equiv) at r.t. and stirred for 24 h to give a
white suspension. The reaction mixture was washed with H2O/CH2-
Cl2, and the organic layer was dried and evaporated. The crude product
was subjected to a column chromatography using 5% MeOH/CH2Cl2
to afford the product 4c as yellow solid (1.93 g, 61% yield). 1H NMR
(CDCl3): δ 9.87 (s, 1H), 7.82 (d, 2H, J ) 8.6 Hz), 6.98 (d, 2H, J )
8.6 Hz), 4.06 (t, 2H, J ) 6.4 Hz), 2.36 (m, 2H), 2.26 (m, 6H), 1.84
(m, 2H), 1.67 (m, 2H).
Preparation of Hybrid Films Doped with the Rhodamine B
Derivative. The rhodamine B derivative was introduced in the hybrid
films by adding 2 (2 mM solution in MeOH) into the precursor solution.
The final reactant molar ratios in the precursor solution for the hybrid
films with 2 mol % of 2 were 1 TEOS:0.15 OPV 1a: 0.003 rhodamine
B derivative 2:300 MeOH:8.3 H2O:2.5 HCl. Films were deposited in
the same manner as the undoped hybrid films.
Preparation of the OPV/Polymer Films. OPV amphiphile/polymer
films were prepared by using PHEMA instead of TEOS. To the solution
of 6.0 mg of 1a (9.0 × 10-6 mol), PHEMA (5 mg), and the appropriate
amount of 2 in 0.75 mL of MeOH was added 15 µL of 35 wt % HCl
aqueous solution. The solution was stirred for 30 min at room
temperature. 1a/PHEMA films were prepared by spin casting the
precursor solutions at 2000 rpm.
Tetraethyl p-Xylylenediphosphonate (5). R,R′-Dibromo-p-xylene
(5.28 g, 20 mmol, 1 equiv) and triethyl phosphite (10.3 mL, 60 mmol,
3 equiv) were placed in a flask with a magnetic stirring bar. A
distillation apparatus was attached to collect ethyl bromide formed along
with the reaction. The mixture was immersed in an oil bath and heated
to 130 °C for 2 h. After cooling, the white crystal was crushed out and
Rhodamine B Derivative Attached Silicate Precursor (2).
Rhodamine B derivative 2 was synthesized following the procedure
described in ref 17. Rhodamine B isothiocyanate (5.36 mg, 1 × 10-5
mol, 1 equiv) was dissolved in anhydrous MeOH in a dried 5 mL
volume flask. 3-aminopropyltriethoxysilane (2.34 µL, 1.1 × 10-5 mol,
1.1 equiv) was added to the solution. The solution was stirred at r.t.
for 24 h to afford 2 mM MeOH solution of 2 and used for the sol-gel
reaction without further purification. 1H NMR of the material was
obtained after the solvent and the excess 3-aminopropyltriethoxysilane
were removed in vacuo and the residue was redissolved in DMSO-d6:
δ 8.2-6.3 (m, Ar-H, 9H), 3.75 (m, SiOCH2CH3), 3.34 (m, 8H), 2.74
(br, 2H), 1.59 (br, 2H), 1.14 (m, SiOCH2CH3), 1.04 (m, 12H), 0.60
(br, 2H). Broadening of the propylene bridge peaks and a smaller
intensity of the triethoxysilane groups were observed, possibly because
of partial hydrolysis and condensation of the triethoxysilane groups
during the process. Completion of the addition reaction was also
confirmed by the disappearance of the characteristic peak for -Nd
CdS stretching in IR spectra (Nujol, 2050 cm-1).
1
recrystalized from hexane to give the product (6.74 g, 89% yield). H
NMR (CDCl3): δ 7.24 (s, 4H), 4.00 (m, 8H), 3.12 (d, 4H, J ) 20.2
Hz), 1.23 (t, 12H, J ) 7.0 Hz).
1,4-bis(2-(4-(ω-(Dimethylamino)alkoxyphenyl))-(E)-1-ethenyl) Ben-
zene (6). Compounds 5 (0.72 g, 1.91 mmol, 1 equiv) and 4c (0.93 g,
4.2 mmol, 2.2 equiv) were dissolved in 100 mL of THF and cooled to
0 °C. t-BuOK solution (10 mL, 10 mmol, 1.0 M in t-BuOH) was slowly
added to the solution with stirring. The reaction mixture was stirred
overnight at room temperature and quenched by adding an excess
amount of water. White solid precipitated out and was collected by
filtration and recrystalized from CHCl3/hexane to afford the product
1
6c as a pale-yellow solid (0.72 g, 74% yield). H NMR (CDCl3): δ
7.47 (m, 8H), 7.07 (d, 2H, J ) 16.1 Hz), 6.96 (d, 2H, J ) 16.3 Hz),
6.89 (d, 4H, J ) 8.2 Hz), 4.00 (t, 4H, J ) 6.0 Hz), 2.33 (t, 4H, J ) 7.3
Hz), 2.24 (s, 12H), 1.82 (m, 4H), 1.65 (m, 4H).
1,4-bis(2-(4-(ω-(Trimethylammonium)alkoxyphenyl))-(E)-1-eth-
enyl) Benzene Dibromide (1). To a suspension of 6c (0.5 g, 0.98 mmol,
9
5494 J. AM. CHEM. SOC. VOL. 128, NO. 16, 2006

Nanostructured Oligo(p-phenylene Vinylene)/Silicate
A R T I C L E S
1 equiv) in 100 mL of THF, MeBr solution (5 mL, 10 mmol, 2.0 M in
t-BuOMe) was added at room temperature. The mixture was stirred
overnight, and the white solid was collected by filtration to afford the
product 1c as a pale-yellow solid (0.69 g, 100% yield). 1H NMR
(DMSO-d6): δ 7.54 (m, 8H), 7.21 (d, 2H, J ) 16.4 Hz), 7.08 (d, 2H,
J ) 16.4 Hz), 6.95 (d, 2H, J ) 8.47 Hz), 4.04 (t, 4H, J ) 5.5 Hz),
3.35 (m, 4H), 3.06 (s, 9H), 1.81 (m, 4H), 1.73 (m, 4H).
Acknowledgment. This work was supported by the U.S.
Department of Energy under Award DE-FG02-00ER54810. We
are grateful for the use of equipment at the Keck Biophysics
Facility, the Jerome B. Cohen X-ray Diffraction Facility, and
the Analytical Services Laboratory at Northwestern University.
JA058610S
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J. AM. CHEM. SOC. VOL. 128, NO. 16, 2006 5495