Preparation of lamellar inorganic-organic hybrids from tetraethoxysilane and a coumarin derivative containing a triethoxysilyl group and photodimerization of the interlayer coumarin groups

Article abstract of DOI:10.1039/c0jm00407c

Lamellar inorganic-organic hybrids were prepared as powders and films via hydrolysis and cocondensation of tetraethoxysilane (TEOS) and a coumarin derivative containing a triethoxysilyl group. The formation of lamellar structures consisting of siloxane layers and interlayer coumarin groups with ca. 4 nm periodicities, revealed by XRD and TEM, is induced by self-assembly of the oligomeric species derived from TEOS and the derivative. The UV-vis absorption spectrum of the as-synthesized hybrid film showed an absorption maximum blue-shifted from that observed for a CHCl₃ solution of the monomeric coumarin derivative, which supports the formation of a face-to-face structure of the interlayer coumarin groups. Dimerization of the coumarin groups occurred by irradiation with visible light (>310 nm). The groups in the lamellar hybrid film showed faster dimerization and the yield of dimers was higher, if compared with those in a disordered hybrid film and the neat coumarin derivative, indicating the efficient dimerization of the chromophores with a controlled arrangement. The molecular design of a coumarin derivative strongly influences the property of hybrids as well as the mesostructural formation.

Full text of DOI:10.1039/c0jm00407c

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Preparation of lamellar inorganic–organic hybrids from tetraethoxysilane and
a coumarin derivative containing a triethoxysilyl group and photodimerization
of the interlayer coumarin groups†
a
Shintaro Nasu, Ayako Tsuchiya and Kazuyuki Kuroda
a
ab
*
Received 13th February 2010, Accepted 12th May 2010
DOI: 10.1039/c0jm00407c
Lamellar inorganic–organic hybrids were prepared as powders and films via hydrolysis and
cocondensation of tetraethoxysilane (TEOS) and a coumarin derivative containing a triethoxysilyl
group. The formation of lamellar structures consisting of siloxane layers and interlayer coumarin
groups with ca. 4 nm periodicities, revealed by XRD and TEM, is induced by self-assembly of the
oligomeric species derived from TEOS and the derivative. The UV–vis absorption spectrum of the as-
synthesized hybrid film showed an absorption maximum blue-shifted from that observed for a CHCl3
solution of the monomeric coumarin derivative, which supports the formation of a face-to-face
structure of the interlayer coumarin groups. Dimerization of the coumarin groups occurred by
irradiation with visible light (>310 nm). The groups in the lamellar hybrid film showed faster
dimerization and the yield of dimers was higher, if compared with those in a disordered hybrid film and
the neat coumarin derivative, indicating the efficient dimerization of the chromophores with
a controlled arrangement. The molecular design of a coumarin derivative strongly influences the
property of hybrids as well as the mesostructural formation.
Photopatterning due to the control of fluorescence emission in
Introduction
a hybrid film is realized by selective photocleavage of dimer to
monomer.9
Coumarin and its derivatives have been investigated as lasers1
and antenna models of artificial photosynthesis2 because of their
high quantum yield. The derivatives have also been investigated
as photoresponsive materials because they dimerize on irradia-
tion with light longer than 310 nm and reversibly transform into
their monomeric states (photocleavage) by UV (ꢀ254 nm)
irradiation. The reversible dimerization induces UV–vis and
fluorescence spectral changes.2d Photodimerization has been
applied for cross-linking of polymers,2d,3 photoalignment of
liquid crystals,4 and increasing the stability of nanotubes, formed
by self-assembly of a hexabenzocoronene derivative, against
solvents.5
These reports demonstrate the potential of hybrid materials
containing coumarin groups as novel photoresponsive materials.
However, coumarin groups are not deliberately arranged in the
cases described above. The arrangement and amount of
coumarin groups in hybrids are important for photoresponsive
efficiency, because dimerization of coumarin derivatives occurs
when the C3–C4 bonds (shown in Scheme 1) of the chromophores
are arranged parallel to each other within a distance shorter than
approximately 0.42 nm.10
These specific features can be introduced into hybrid materials.
Doping of coumarin derivatives into amorphous matrices by sol–
gel processes,6 and cross-linking of inorganic–organic hybrid
polymers have been reported.7 A unique application is the use of
dimerization for a gate control of drug release by modifying
coumarins onto the entrance of channels of mesoporous silica.8
aDepartment of Applied Chemistry, Faculty of Science and Engineering,
Waseda University, Ohkubo 3-4-1, Shinjuku-ku, Tokyo, 169-8555, Japan
bKagami Memorial Research Institute for Materials Science & Technology,
Waseda University, Nishiwaseda-2, Shinjuku-ku, Tokyo, 169-0051, Japan.
E-mail: kuroda@waseda.jp; Fax: +81 3-5286-3199; Tel: +81 3-5286-3199
† Electronic supplementary information (ESI) available: Synthetic
scheme of 1; 1H NMR spectrum of 7-(9-decenoxy)coumarin; 1H, 13C,
1
and 29Si NMR spectra and FAB-MS spectrum of 1; H and 29Si NMR
spectra of the precursor solution during the hydrolysis; 29Si MAS
NMR spectrum of the hybrid obtained by the self-condensation of 1;
2D XRD pattern of F1; Fluorescence emission spectra of F1 and D1
before and during photoirradiation (>310 nm); UV–vis absorption
spectra of neat 1 before and during photoirradiation (>310 nm). See
DOI: 10.1039/c0jm00407c
Scheme 1 Structure of 1 and preparation of the hybrids.
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The controlled arrangement of coumarin groups is reported
for self-assembled monolayers (SAMs) prepared from coumarin
derivatives containing long alkyl chains.4a,11 However, photo-
patterning on SAMs is not practically successful because of low
contrast between irradiated and unirradiated areas due to a small
amount of immobilized coumarin groups.11 Consequently, the
design of hybrid materials, realizing both immobilization of
ample coumarin groups and their controlled arrangement, is
a challenge in the development of new photoresponsible
materials.
Co., >99.0%), triphenylphosphine (Tokyo Kasei Co., >95.0%),
potassium carbonate (Wako Chemical Co, >99.5%), plat-
inum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solu-
tion in xylene (ꢀ2% Pt, Aldrich) and triethoxysilane (Tokyo
Kasei Co., >97.0%) were used as received for the preparation of 1
(see below). Other chemicals, including acetone, dichloro-
methane, chloroform, and toluene (dried) were purchased from
Wako Chemical Co., and used as received.
Synthesis of 7-(9-decenoxy)coumarin
The basic idea reported here is the utilization of silica-based
lamellar mesostructures. They are formed by self-assembly of
surfactants and various silica species, which can contribute to
controlled arrangements of organic groups, because alkyl chains
of surfactants are packed parallel to each other in the interlayer
spaces of inorganic layers. However, the introduction of a large
amount of organic molecules into mesostructured materials by
solubilization or co-condensation, is unsuccessful because the
amount of introduced organic groups is largely limited.12
We have already reported the formation of ordered silica-
based inorganic–organic mesostructured materials by self-
assembly of organoalkoxysilanes with long alkyl chains.13
Functional organic groups can be introduced to the organic parts
of the starting materials. Organoalkoxysilanes containing
alkenyl groups are used to form layered thin films containing well
arranged double bonds between the layers.13acg UV irradiation
induces polymerization of the double bonds, which results in an
increase in the hardness of the films and scratch resistance.13g Hsu
and co-workers reported the formation of mesostructured
materials by using surfactants containing chromophores.14 If we
can prepare a lamellar hybrid by self-assembly of an organo-
alkoxysilane containing a coumarin group, a large number of
coumarin groups can be immobilized, and the arrangement of
the chromophores should be suitable for photodimerization,
which hopefully would result in the preparation of new photo-
responsible materials. Morphological control by using well
designed precursors13a is also advantageous. In particular, films
are adequate media because of the transparency in a visible light
region, coverage of large area, and homogeneous surface
obtained by simple spin- or dip-coating. The inorganic–organic
interface with Si–C bonds is more stable than those of conven-
tional mesostructured materials formed by electrostatic interac-
tion and/or hydrogen bonding.
The synthetic routes of 7-(9-decenoxy)coumarin and 1 are shown
in Scheme S1, ESI.† 10-Bromo-1-decene was synthesized from
9-decene-1-ol, carbontetrabromide, and triphenylphosphine as
reported previously.15 A mixture of 4.99 g of 7-hydrox-
ycoumarin, 7.56 mL of 10-bromo-1-decene and 4.80 g of K2CO3
in 200 mL of acetone (the molar ratio of 7-hydroxycoumarin/
10-bromo-1-decene/K2CO3 was 1: 1.5: 1.5) was refluxed for 12 h
and concentrated under a reduced pressure. The thick residue
was poured into chloroform, neutralized with aqueous NaHSO4
(1 M), and washed with brine. The organic layer was dried over
anhydrous Na2SO4. After the evaporation of the solvents, the
crude product was purified on a silica gel column chromatog-
raphy using CHCl3 as an eluent to give pale yellow solid. dH (500
MHz; CDCl3; Me4Si, Fig. S1, ESI†) 1.25–1.47 (10H, m, –CH2–),
1.81 (2H, quintet, –OCH2CH2CH2–), 2.05 (2H, quartet,
–CH2CH ] CH2), 4.01 (2H, t, –OCH2CH2–), 4.97 (2H, m, –CH
] CH2), 5.81 (1H, m, –CH ] CH2), 6.24 (1H, d, H-3), 6.80 (1H,
d, H-8), 6.83 (1H, dd, H-6), 7.36 (1H, d, H-5), and 7.63 (1H, d,
H-4).
Synthesis of 7-(10-triethoxysilyl)decoxycoumarin (1)
To a solution of 3.00 g of 7-(9-decenoxy)coumarin in 35 mL of
toluene was added 2.77 mL of triethoxysilane, followed by
100mL of a platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane
complex solution in xylene (ꢀ2% Pt) as catalyst. The mixture was
ꢁ
stirred under N2 at 50 C for 24 h, followed by the removal of
volatile components under a reduced pressure to give viscous
yellow liquid. Pure 1 was isolated by gel permeation chroma-
tography (LC-9105, Japan Analytical Industry Co., Ltd) using
chloroform as an eluent. dH (500 MHz; CDCl3; Me4Si, Fig. S2,
ESI†) 0.63 (2H, t, –CH2Si–), 1.22–1.80 (25H, m, –CH2CH2CH2–
and OCH2CH3), 3.82 (6H, q, OCH2CH3), 4.00 (2H, t,
–OCH2CH2–), 6.21 (1H, d, H-3), 6.77 (1H, d, H-8), 6.82 (1H, dd,
H-6), 7.35 (1H, d, H-5), and 7.63 (1H, d, H-4); dC (125.65 MHz;
CDCl3; Me4Si, Fig. S3, ESI†) 10.4 (-CSi–), 18.3 (–SiOCH2CH3),
22.8 (–CH2CH2CH2Si–), 26.0 (–CH2CH2CH2Si–), 29.0–29.6
(–CH2CH2CH2–), 33.2 (–OCH2CH2CH2-), 58.3 (–SiOCH2CH3),
68.7 (–OCH2CH2–), 101.3 (C-8), 112.4 (C-4a), 112.9 (C-3), 113.0
(C-6), 128.8 (C-5), 143.5 (C-4), 155.9 (C-8a), 161.2 (C-7), and
162.5 (C-2); dSi (99.25 MHz; CDCl3; Me4Si, Fig. S4, ESI†) ꢂ44.6
(–CH2Si(OCH2CH3)3); FAB-MS m/z 621 ([M + NBA]+), 487
([M + Na]+), and 419 ([M – OEt + H]+, Fig. S5, ESI†); lmax
(CHCl3)/nm 326.
In this study, we synthesized a new coumarin derivative
7-(10-triethoxysilyl)decoxycoumarin (1 in Scheme 1), containing
a long-chain alkyl group with a triethoxysilyl group, and
prepared a lamellar mesostructured hybrid with tetraethox-
ysilane (TEOS) by hydrolysis and cocondensation. Subsequently,
photodimerization of the coumarin groups in the hybrid film (F1)
was examined. Furthermore, in order to investigate the effect of
the mesostructure on the dimerization, a disordered hybrid film
(D1) with the same composition as F1 was also prepared.
Experimental
Materials
Preparation of mesostructured hybrids
7-Hydroxycoumarin (Tokyo Kasei Co., >98.0%), 9-decene-1-ol
(Tokyo Kasei Co., >95.0%), carbontetrabromide (Tokyo Kasei
1 and TEOS were hydrolyzed and cocondensed as follows. Dil.
HCl was added to a THF solution containing 1 and TEOS for the
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hydrolysis and cocondensation of 1 and TEOS. The molar ratio
was 1/TEOS/THF/H2O/HCl ¼ 1: 8: 92: 35: 0.055. The emulsion
mixture became homogeneous within 5 min. The solution after
stirring for 4 h at room temperature in the dark was drop-coated
on glass substrates and air-dried in the dark for 2 days. The
resulting thick films were scraped off from the substrates and
pulverized to afford a powdery sample (P1). In order to inves-
tigate the mesostructure, the powdery sample after calcination at
550 ꢁC for 6 h in an air atmosphere was prepared. A hybrid thin
film (F1) was also obtained by spin-coating (3000 rpm, 10 s) of
the precursor solution on a quartz substrate. After drying for 2
days in the dark, F1 was obtained.
diffractometer (Mn filtered Fe Ka radiation). The two-dimen-
sional (2D) XRD patterns were recorded under a reflection
geometry using RINT Rapid-S (RIGAKU). The X-ray beam
was collimated with a 0.3 mm pinhole. The incident angle was set
to 1ꢁ. Transmission electron microscopic (TEM) images were
obtained on a JEOL JEM-2010 microscope at an accelerating
voltage of 200 kV. Samples for cross-sectional TEM were
prepared by a focused ion beam (FIB) technique (SEIKO EG&G
SMI2050). Fourier transform infrared (FT-IR) spectra were
obtained by the KBr disc technique using a JASCO FT/IR-6100
Spectrometer. UV–vis spectra were recorded on a Shimadzu
spectrophotometer (UV-3101PC). Fluorescence spectra were
recorded on a Hitachi spectrofluorophotometer (F-4500). The
spectra were corrected using an attachment of a standard tung-
sten lamp (Hitachi). Fluorescent microscopic images were
obtained on a Olympus BX60 microscope.
Preparation of a disordered hybrid film (D1)
In order to investigate the effect of the mesostructure on the
arrangement and photodimerization of coumarin groups,
a disordered hybrid film (D1) was prepared under the same
composition of 1/TEOS and an excess amount of HCl to accel-
erate cocondensation before self-assembly (the molar ratio was
1/TEOS/THF/H2O/HCl ¼ 1: 8: 92: 35: 5.5). The precursor
solution was stirred for 2 weeks and spin-coated on a quartz
substrate, and a disordered film D1 was obtained.
Results and discussion
Hydrolysis and cocondensation of 1 and TEOS
1
The hydrolysis process was traced by H NMR. After reaction
for 15 min, the signals due to ethanol (HOCH2CH3) at 3.6 ppm
appeared (Fig. S6a, ESI†). The intensity of the signals increased
and that of the signals due to ethoxy groups (Si–OCH2CH3) at
3.8 ppm decreased with the reaction time. After 60 min, the
signals due to the ethoxy groups almost disappeared (Fig. S6d,
ESI†), indicating complete hydrolysis. The hydrolysis was also
confirmed by the 29Si NMR data. After reaction for 20 min
(Fig. S7a, ESI†), the signal due to T0 (Si(OEt)3) site of 1 dis-
appeared and the signal at ꢂ71.6 ppm due to Q0 (Si(OH)4)
site was observed. The observed signals at ꢂ81.0, ꢂ82.1, and
ꢂ83.0 ppm can be assigned to Q1 (Si(OEt)3-x(OH)x(OSi)) sites,
respectively. After 50 min (Fig. S7b, ESI†), these peaks almost
completely disappeared and broadened signals at around
ꢂ90 ppm due to Q2 (Si(OEt)2-x(OH)x(OSi)2) sites were observed,
suggesting the formation of oligomeric siloxane species. The
solution was stirred for another 3 h for the promotion of the
reaction.
Photodimerization and photocleavage of coumarin groups in the
hybrid films
The dimerization of coumarin groups in F1 and D1 was per-
formed by photoirradiation with a wavelength longer than
310 nm by using a 200-W mercury xenon lamp (San-Ei Electric
Co., SUPERCURE-203S) from a distance of 1.5 cm under a N2
flow. A film after the photoirradiation of F1 for 60 min is
designated as F2.
The photocleavage of coumarin dimer moieties in F2 was
performed by photoirradiation with UV (ꢀ254 nm) by using
a low pressure mercury lamp (Ushio Inc., UL0-6DQ) from
a distance of 1.0 cm under a N2 flow. A film after the UV pho-
toirradiation of F2 for 30 min is designated as F3.
Obvious evidences for the cocondensation between 1 and
TEOS were not obtained from the 29Si NMR data, because it is
difficult to identify siloxane species formed by the cocondensa-
tion due to the low intensity and the broadness of the signals.
However, the reaction is plausible, because self-condensation of
organotrialkoxysilanes with long alkyl chains is normally steri-
cally hindered to some extent13de and precipitations as a result of
self-condensation of alkyltrialkoxysilane13e were not formed in
the present study. The 29Si MAS NMR data of P1 also support
the cocondensation (see below).
Characterization
Liquid-state 1H NMR spectra were recorded on a JEOL
Lambda-500 spectrometer. Sample solutions were put in 5-mm
glass tubes, and tetramethylsilane (TMS) was added as an
internal reference, and CDCl3 was used to obtain lock signals.
The liquid-state 13C and 29Si NMR spectra were obtained on the
same spectrometer with resonance frequencies of 125.65 MHz
and 99.25 MHz, respectively. Solid-state 29Si MAS NMR spectra
of hybrid powders were recorded on a JEOL JNM-CMX-400
spectrometer at a resonance frequency of 79.42 MHz with a 45ꢁ
pulse and a recycle delay of 200 s. Solid-state 13C cross-polari-
zation (CP)/MAS NMR spectra were recorded on the same
spectrometer at a resonance frequency of 100.53 MHz with
a contact time of 5 ms and a recycle delay of 5 s. The chemical
shifts for solid-state 29Si and 13C NMR were referenced to TMS
at 0 ppm. FAB-MS spectra were recorded on a JEOL GC-Mate
II spectrometer using 3-nitrobenzyl alcohol as a matrix. NaI was
added to the samples in order to promote cation attachment and
to give intense molecular ion peaks. X-ray powder diffraction
(XRD) patterns were obtained with a Mac Science M03XHF22
The coumarin group of 1 was retained without undesired
reaction (e.g. hydrolysis of lactone) during the hydrolysis of
ethoxy groups, which is proven by the 1H NMR signals
observed at around 6–8 ppm due to coumarin groups (Fig. S6,
ESI†).
Characterization of the powdery sample (P1)
The q-2q XRD pattern of P1 shows the peaks at d ¼ 4.3, 2.2, and
1.4 nm (Fig. 1a) and no peaks were observed in the high angle
region (data not shown). The TEM image of P1 shows stripe
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Fig. 1 q-2q XRD patterns of (a) P1 and (b) powders after calcination of
P1. Inset: TEM image of P1.
patterns with a ca. 4 nm periodicity (Fig. 1, inset). The ordered
structure disappeared after calcination (Fig. 1b). These results
indicate the formation of a lamellar mesostructure by the self-
assembly as well as the multilayered films from tetraalkox-
ysilane and alkyltrialkoxysilanes,13abe even though coumarin
groups are attached to the end of the alkyl groups. The behavior
is similar to those of mesostructured materials formed from
surfactants containing chromophores.14 The arrangement of the
organic groups in the lamellar mesostructure is considered to be
a bilayer arrangement from the XRD data and molecular size of
1.16 The results mean that the coumarin groups act as a hydro-
phobic part of the amphiphile during the self-assembly and
the chromophores are located at the center of the interlayer
spaces as shown in Scheme 1. On the other hand, Guo et al.
reported the formation of Langmuir-Schaefer (LS) films from
4-octadecyloxylcoumarin or 7-octadecyloxylcoumarin and
they suggested that the coumarin group acts as a hydrophilic
part.17 On the basis of the report as well as our results, we
can regard coumarin groups as hydrophobic parts of amphi-
philes by the introduction of hydrophilic substituents into
derivatives.
Fig. 2 (A) 29Si MAS and (B) 13C CP/MAS NMR spectra of P1.
The 29Si MAS NMR spectrum of P1 is shown in Fig. 2A. A
small shoulder observed at ꢂ49 ppm can be assigned to the T1
(Tx: CSi(OH)3-x(OSi)x) unit and two signals assigned to the T2,
and T3 units are observed at ꢂ56 and ꢂ64 ppm, respectively. The
signals observed at ꢂ91, ꢂ100, and ꢂ109 ppm are assigned to the
Q2, Q3, Q4 units (Qx: Si(OH)4-x(OSi)x), respectively. The presence
of the T2 and T3 signals as main components of the T units is
indicative of cocondensation of hydrolyzed 1 and TEOS, because
the 29Si MAS NMR spectra of self-condensed alkyltrialkox-
ysilane normally show T1 and T2 signals dominantly as a result of
suppression of further condensation by the steric hindrance.13e In
fact, the 29Si MAS NMR spectrum of the hybrid obtained by the
self-condensation of 1 showed a T2 signal as the main component
and the intensity of the T3 signal was relatively low (Fig. S8,
ESI†), which is in clear contrast to that observed for P1.
The introduction of the coumarin groups into P1 without
decomposition is proven by 13C CP/MAS NMR and IR. The
signals observed at 100–170 ppm in the 13C CP/MAS NMR
spectrum of P1 (Fig. 2B) can be assigned to the groups. In the IR
Fig. 3 FT-IR spectra of (a) P1, (b) F1, and (c) F2.
spectrum of P1 (Fig. 3a), the bands due to C ] O stretching and
C ] C stretching are observed at 1736 cmꢂ1 and 1619 cmꢂ1
respectively.11,17
,
The diffuse reflectance UV–vis spectrum of P1 shows an
absorption maximum at 330 nm and a shoulder at 349 nm
(Fig. 4b). The absorption band is shifted to longer wavelength
from that observed for 1 in CHCl3 (lmax ¼ 326 nm, Fig. 4a).
According to the excitation theory,18 the red shift indicates the
formation of a J-aggregate.17,19 However, the broadness of the
band (the full width at half-maximum (FWHM): ca. 10000 cmꢂ1
in P1; 5000 cmꢂ1 in CHCl3 solution of 1) means that the aggre-
gate of coumarin groups in P1 contains some disordering.
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observed bands at 1736 cmꢂ1 due to C ] O stretching and at
1619 cmꢂ1 due to ring C ] C stretching indicate the introduction
of coumarin groups in F1 as well as P1.
Although the mesostructures of F1 and P1 are comparable, the
arrangement of interlayer coumarin groups in the film showed
a significant difference. The UV–vis absorption spectrum of F1
(Fig. 4c) showed the band with the maximum at l ¼ 321 nm, and
the peak is blue shifted from that observed for 1 in CHCl3
(Fig. 4a) by 5 nm. A blue shift of the UV–vis absorption band
generally indicates a face-to-face arrangement of chromo-
phores.17,18 The FWHM of the band is 6100 cmꢂ1, and the width
is narrower than that observed for P1 and is closer to that of
a CHCl3 solution of 1, which means a more controlled
arrangement of chromophores in F1 than that in P1. The results
indicate a face-to-face arrangement of the coumarin groups in
F1, as shown in Fig. 4c, inset.
The controlled arrangement is explained by the effect of an
interface between the precursor solution and the substrate. In the
case of mesostructured materials prepared through evaporation-
induced self-assembly (EISA) process, mesostructures are
generally formed from both liquid/solid interface and air/liquid
interface, and the former affects the alignment of mesostructures
in the range of some hundreds nanometres.20 In this study, the
effect also induces the controlled arrangement of coumarin
groups at the liquid/solid interface, because the mesostructure is
formed by the self-assembly of oligomeric species derived from 1
and TEOS. The effect is larger in F1 than that in a cast thick film
during the preparation of P1, because F1 is thinner and the
influence of the liquid/solid interface reaches the entire film. The
result indicates the importance of the formation of mesostruc-
tured materials as films in order to arrange the chromophores.
The observed value of the blue shift (5 nm, Fig. 4c) of F1 is
smaller than that observed for a LS film, which is prepared from
7-octadecyloxylcoumarin (24 nm), as reported by Guo et al.17
The fluorescence emission spectrum of F1 (Fig. S10A, ESI†)
showed a strong emission with a peak at 399 nm, although H-
aggregates of the chromophores generally quench the fluores-
cence emission.18 These result indicates that the coumarin groups
in F1 are arranged in a face-to-face manner, though, the inter-
actions between the chromophores are weak. The weak interac-
tions are due to loose packing of the organic groups in F1, which
is supported by the trans-gauche conformation of the alkyl
groups and the absence of periodicity between the chromophores
proven by XRD (data not shown), whereas 7-octadecylox-
ylcoumarin molecules are assembled with all-trans conformation
of alkyl groups.17 In fact, the authors also reported that the UV–
vis absorption spectrum of a LS film composed of 4-octadecy-
loxylcoumarin with trans-gauche packing showed a band shifted
by only 3 nm,17 which is in clear contrast to the case of 7-octa-
decyloxycoumarin.
Fig. 4 UV–vis absorption spectrum of (a) CHCl3 solution of 1, b)
diffuse reflectance UV–vis absorption spectrum of P1, c) UV–vis
absorption spectra of F1, and d) D1. Inset: proposed structure of the
arrangement of the coumarin groups in F1.
Characterization of the hybrid films (F1 and D1)
The hybrid film (F1) is transparent and colorless. The film
thickness was ca. 300 nm, and the thickness can be controlled by
the concentration of the precursor solution and the rotation
speed of spin-coating. The q-2q XRD (Fig. 5a) and 2D XRD
(Fig. S9, ESI†) patterns of F1 showed the periodicity of 4.2 nm
and 4.1 nm, respectively. The cross-sectional TEM image of F1
(Fig. 5, inset) showed striped patterns with a periodicity of 4 nm
oriented parallel to the substrate. These results demonstrate the
formation of a lamellar mesostructure, which is consistent with
the structure proposed for P1. In the IR spectrum of F1, the
In order to investigate the effect of self-assembly and forma-
tion of the lamellar mesostructure on the control of the
arrangement of coumarin groups, a hybrid film (D1) with the
same composition as F1 but without an ordered mesostructure
was prepared. The disordered structure, confirmed by q-2q XRD
(data not shown), means the suppression of the self-assembly.
The UV–vis absorption spectrum of D1 showed the absorption
maximum at 326 nm (Fig. 4d), which is comparable to that
observed for a CHCl3 solution of 1 (Fig. 4a). The result indicates
Fig. 5 q-2q XRD patterns of (a) F1, (b) F2, and (c) F3. Inset: cross-
sectional TEM image of F1.
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that the coumarin groups are introduced into D1 as a monomeric
state without electronic interaction between the chromophores.
The narrow absorption band (FWHM: 6900 cmꢂ1) also supports
the monomeric state of the coumarin groups. The results indicate
that the self-assembly and the formation of the ordered lamellar
mesostructure are effective to organize chromophores with
a controlled manner in the interlayer spaces.
the formation of the syn head-to-head dimer because the dimer
has the smallest angle between the C7 positions in four isomers.2d
In general, the band due to the C ] O stretching of the syn head-
to-tail dimer shifts to lower wavenumbers by a few tens cmꢂ1
from that observed for the syn head-to-head dimer and is
observed at around 1730 cmꢂ1, which is overlapped with the band
due to coumarin monomers.11 Therefore, even if the syn head-to-
tail dimer is formed in F2, it is difficult to prove it by FT-IR. The
formation of anti head-to-head dimer in F2 is thought to be
difficult because of the large angles between the C7 positions. The
formation of anti head-to-tail dimer is generally difficult by
simple photoirradiation2d because it needs a precise control of the
arrangement of coumarin groups, as reported by Brett et al.21 On
the basis of these results and the findings reported previously, the
possibility of the formation of other dimers is not completely
excluded, though the formation of the syn head-to-head dimer by
photodimerization between adjacent coumarin groups on the
same siloxane layer is thought to be predominant in F2.
Photodimerization and photocleavage of the interlayer coumarin
groups in the hybrid films and the effect of the self-assembly
(i) Photodimerization of the coumarin groups in F1. The
interlayer coumarin groups were dimerized by photoirradiation
(>310 nm) of F1 and the irradiated hybrid film was designated as
F2. The q-2q XRD pattern of F2 showed a peak at d ¼ 4.0 nm
(Fig. 5b), indicating the retention of the lamellar mesostructure.
It is plausible that the shrinkage by 0.2 nm is due to that of the
organic layers due to photodimerization of the coumarin groups,
because the photoreaction of coumarin groups is accompanied
by changes in packing of the organic groups17 and we have
already reported that shrinkage of siloxane layers is negligible for
multilayered hybrid films derived from organoalkoxysilane and
TEOS by heating and UV irradiation.13e
(ii) Effect of the mesostructure on the photodimerization. In
order to investigate the effect of the ordering between coumarin
groups on the photodimerization, the dimerization behavior of
the chromophores in F1 and D1 was investigated. The conversion
ratio of the coumarin groups from the monomeric state to the
dimeric one in F1 was calculated from the decrease of the
absorbance at 321 nm in the UV–vis absorption spectra shown in
Fig. 6A. More than 60% of coumarin groups are dimerized
within 10 min and 85% within 60 min and the conversion ratio
was saturated (Fig. 7).
Even though the mesostructure was basically retained after the
photoirradiation, the UV–vis and fluorescence spectra of F2 are
drastically changed. The absorption band at 321 nm due to
monomeric coumarin groups gradually decreased with the pho-
toirradiation time as shown in Fig. 6A. The fluorescence emission
of F1 (lem ¼ 399 nm) due to the coumarin groups almost dis-
appeared within 10 min (Fig. S10A, ESI†). These spectral changes
are characteristic of dimerization of coumarin groups.2d,8a,9,10
The photodimerization is also supported by FT-IR. In the FT-
IR spectrum of F2 (Fig. 3c), the absorption band at 1619 cmꢂ1
due to ring C ] C stretching of monomeric coumarin groups
decreased. The band due to C ] O stretching of monomeric
coumarin groups observed for F1 at 1736 cmꢂ1 also decreased,
and a new broad band due to C ] O stretching appeared at 1764
cmꢂ1. These spectral changes indicate dimerization of coumarin
groups.11,17
The photoirradiation (>310 nm) of D1 also induced the
decrease of the absorbance (Fig. 6B) and quenching of the
fluorescence (Fig. S10B, ESI†) due to monomeric coumarin
groups. However, the groups showed slower dimerization and
lower yield of dimers than those in F1. In 10 min, only 45% of the
coumarin groups are dimerized and the reaction was not satu-
rated even after 100 min (Fig. 7). The difference in the reaction
efficiency between F1 and D1 can be attributed to the arrange-
ment of coumarin groups. For photoreactions in inorganic–
organic hybrids, the short-range order between organic groups is
important.13e,22 In the case of coumarin derivatives, dimerization
reaction occurs effectively when C3–C4 bonds are arranged
The band due to C ] O stretching at 1764 cmꢂ1 indicates the
formation of the syn head-to-head dimer in F2 because the
formation of syn head-to-head dimers is normally accompanied
by shifts of the bands to higher wavenumbers.11,17 The arrange-
ment of the organic groups in F1 is considered to be favorable for
Fig. 7 Conversion from monomer to dimer of coumarin groups vs.
irradiation time for F1 (filled circle), D1 (open circle), and neat 1 on
a quartz substrate (triangle).
Fig. 6 UV–vis absorption spectra of (A) F1 and (B) D1 before and
during the photoirradiation (>310 nm).
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parallel to each other within the distance shorter than approxi-
mately 0.42 nm.10 We consider that the controlled arrangement
of the coumarin groups in F1, which is proven by UV–vis
absorption, accelerates the dimerization. Even though coumarin
groups are introduced into D1 as a monomer, the dimerization
occurs. The dimerization is allowed because of the exceptionally
large acceptable range of the angle between the C3–C4 bonds, as
7-substituted coumarin derivatives can be dimerized even if the
angle between the bonds is rotated by 65 .10
ꢁ
It is worth noting that hybridization of 1 and silica species also
contributes to the effective dimerization by suppressing photo-
decomposition. When neat 1 on a quartz substrate was irradiated
by visible light (>310 nm), the absorbance at 322 nm decreased
(Fig. S11, ESI†). Only 55% of the chromophores were dimerized
after 90 min (Fig. 7), even though coumarin derivatives con-
taining a long alkyl chain on C7 position are generally favorable
to photodimerization.11,17 The low yield of the dimerization is
probably due to partial photodecomposition of the derivative as
reported by Li et al.11
Fig. 9 UV–vis absorption spectra of F2; (a) before photoirradiation and
(b – d) after photoirradiation (ꢀ254 nm) for (b) 2.5 min, (c) 10 min, and
(d) 30 min (F3).
minutes.7b,8,9 In this study, when F2 was irradiated with a light of
ꢀ254 nm for 2.5 min, the absorbance at around 320 nm scarcely
increased (Fig. 9b), indicating the retention of the dimeric state.
After irradiation for 10 min (Fig. 9c), the absorbance increased,
however, the recovery of the absorbance was only 33% of that of
F1, meaning partial photocleavage (Scheme 1, F3). The yield of
the photocleavage is much lower than those of coumarin groups
in polymers (55—80%)3,7a and LS films (ca. 100%).17 After irra-
diation for 30 min, the absorbance slightly decreases (Fig. 9d),
suggesting the partial decomposition of the coumarin groups. The
result indicates that the photocleavage in F2 is much slower than
that observed for other materials. The q-2q XRD pattern of F3
showed a peak at d ¼ 4.0 nm (Fig. 5c), meaning the retention of the
d spacings during the photocleavage. Because the layers lying
parallel to the substrate shrink during the photodimerization, the
photodissociation returning to the monomeric state may be
kinetically suppressed due to too much energy being required to
expand the layers. The details of the effect of ordered nano-spaces
on the photocleavage reaction are worthy of further investigation.
The multilayer mesostructure also contributes to the photo-
responsive efficiency by the introduction of larger amounts of
chromophores than those found for conventional SAMs. The
absorbance of F1 at l ¼ 321 nm was 0.68. This value is ca. 100
times larger than those of coumarin SAMs.4a,23 In the case of LS
films, repeating steps more than 20 times are necessary to fabricate
samples with absorbance comparable to that of F1.17 Addition-
ally, the absorbance of F1 is adjustable due to the film thickness as
described above, which is advantageous for obtaining photo-
functional hybrids with desired photoresponsive efficiency.
The photoresponsive on–off fluorescence property of
coumarin groups is useful for the photopatterning.9,24 The fluo-
rescent microscopic images of the hybrid films obtained by
photoirradiation to F1 (>310 nm) and F2 (ꢀ254 nm) through
copper TEM grids are shown in Fig. 8A and 8B, respectively. The
films showed definite patterns of contrast between the unirradi-
ated region covered by a TEM grid and the irradiated region.25
The narrowest part of the pattern is ca. 20 mm in width. Such
a photopatterning could not be obtained on SAM prepared from
a coumarin derivative,11 because the amount of coumarin groups
is not enough to make a clear contrast between the irradiated and
the unirradiated regions.
Conclusions
Lamellar mesostructured hybrids were prepared from tetrae-
thoxysilane and a coumarin derivative containing a triethoxysilyl
group through self-assembly. The self-assembly and the forma-
tion of lamellar mesostructure induce a face-to-face arrangement
of coumarin groups in F1. The face-to-face arrangement of the
chromophores is favorable to photodimerization. The coumarin
groups in the lamellar film show faster photodimerization reac-
tion than those in D1 and neat coumarin derivatives. The
lamellar mesostructure also contributes to the immobilization of
a large amount of coumarin groups, which is effective for pho-
topatterning. These findings are quite useful for the sophisticated
design of functional organic–inorganic hybrid materials by
adequate molecular design of general photofunctional deriva-
tives. Such a strategy will provide promising ways of using hybrid
materials for practical photofunctional materials.
Photocleavage of dimerized coumarin groups in F2
When dimerized coumarin derivatives are irradiated with a light
shorter than 254 nm, the derivatives are normally dissociated to
the monomeric states.2d,11 The photocleavage of the coumarin
dimers in silica–coumarin hybrids also completely occurs in a few
Acknowledgements
Fig. 8 Fluorescent microscopic images; (A) After photoirradiation
(>310 nm) to F1 and (B) after photoirradiation (ꢀ254 nm) to F2 through
copper TEM grids. Scale bar: 100 mm.
We thank Professor A. Shimojima (The University of Tokyo)
and Mr H. Watanabe (Waseda University) for helpful
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14 (a) C. J. Bhongale and C.-S. Hsu, Angew. Chem., Int. Ed., 2006, 45,
1404; (b) C. J. Bhongale, C.-H. Yang and C.-S. Hsu, Chem.
Commun., 2006, 2274; (c) C.-H. Yang, C. J. Bhongale, Y.-M. Liao
and C.-S. Hsu, J. Mater. Chem., 2007, 17, 243; (d) L.-L. Li,
C.-J. Fang, H. Sun and C.-H. Yan, Chem. Mater., 2008, 20, 5977;
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discussion. This work was partially supported by the Global
COE program ‘‘Practical Chemical Wisdom’’ and Elements
Science and Technology Project from MEXT, Japan. The A3
Foresight Program ‘‘Synthesis and Structural Resolution of
Novel Mesoporous Materials’’ supported by the Japan Society
for Promotion of Science (JSPS) is also acknowledged.
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16 The observed d values of the hybrids (4.3 nm for P1 and 4.2 nm for F1)
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