Energy and electron transfer from fluorescent mesostructured organosilica framework to guest dyes

Article abstract of DOI:10.1021/la204645k

Energy and electron transfer from frameworks of nanoporous or mesostructured materials to guest species in the nanochannels have been attracting much attention because of their increasing availability for the design and construction of solid photofunction

Full text of DOI:10.1021/la204645k

Article
pubs.acs.org/Langmuir
Energy and Electron Transfer from Fluorescent Mesostructured
Organosilica Framework to Guest Dyes
Norihiro Mizoshita,†,‡ Ken-ichi Yamanaka,†,‡ Satoru Hiroto,§ Hiroshi Shinokubo,§ Takao Tani,†,‡
and Shinji Inagaki*,†,‡
†Toyota Central Research and Development Laboratories, Incorporated, Nagakute, Aichi 480-1192, Japan
‡Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), Kawaguchi,
Saitama 332-0012, Japan
§Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan
S
* Supporting Information
ABSTRACT: Energy and electron transfer from frameworks of nanoporous
or mesostructured materials to guest species in the nanochannels have been
attracting much attention because of their increasing availability for the design
and construction of solid photofunctional systems, such as luminescent
materials, photovoltaic devices, and photocatalysts. In the present study,
energy and electron-transfer behavior of dye-doped periodic mesostructured
organosilica films with different host−guest arrangements were systematically
examined. Fluorescent tetraphenylpyrene (TPPy)−silica mesostructured films
were used as a host donor. The location of guest perylene bisimide (PBI) dye
molecules, acting as an acceptor, could be controlled on the basis of the molecular design of the PBI substituent groups. PBI dyes
with bulky substituents and polar anchoring groups were located at the pore surface with low self-aggregation, which induced
efficient energy or electron transfer because of the close host−guest arrangement. However, PBI dye with bulky and hydrophobic
substituents was located in the center of template surfactant micelles; the fluorescence emission from the host TPPy groups was
hardly quenched when the host−guest distance was longer than the critical Forster radius (ca. 4.5 nm). The relationship between
̈
the energy or electron-transfer efficiency and the location of guest species in the channels of mesostructured organosilica was first
revealed by molecular design of the PBI substituents.
doped mesoporous biphenyl−silica hybrids.8 The energy-
INTRODUCTION
■
transfer efficiency for this host−guest system was almost
100%, even at a low coumarin concentration of 0.8 mol % to
the biphenyl units. The coumarin dye was thought to be
located in the proximity of the hydrophilic pore wall to result in
such a high FRET efficiency. Periodic mesostructured films
doped with fluorescent dyes have also exhibited highly efficient
and color-tunable photoluminescence because of FRET from
the organosilica pore walls to the guest dyes.10,11 Appropriate
dispersion of the fluorescent dye dopants in the mesochannels
enabled effective tuning of the emission colors and high
fluorescence quantum yields. On the other hand, photo-
induced electron transfer has been observed for hydrogen
evolution systems, where charge-transfer sites are formed by
covalently attaching an electron-receptive viologen onto the
pore wall surface of biphenyl−silica mesoporous materials.13
For potential applications, the distance and arrangement of the
energy/electron donors and acceptors are of great importance
to induce significant energy/electron transfer between the pore
walls (hosts) and guests. Teramae and colleagues examined the
Periodic mesostructured and mesoporous organosilicas pre-
pared by surfactant-directed polycondensation of bridged
organosilane precursors {R[Si(OR′)3]n, where n ≥ 2, R =
organic bridging group, and R′ = Me, Et, etc.} are a new class of
inorganic−organic hybrid materials with organic-functionalized
frameworks.1−7 A variety of organic bridging groups (R),
ranging from aliphatic and aromatic groups to heterocyclic
compounds, have been used to tailor the functionalities of the
silica-based pore walls.1−7 This type of mesostructured material
allows for different organic chromophores to be located in two
spatially separated regions, i.e., in the framework and in the
mesochannels. Appropriate selection of host framework organic
groups (R) and guest materials induces energy and electron
transfer between the pore walls and the guest molecules located
in the mesochannels upon photoexcitation.
Recently, the use of energy and electron transfer between
pore walls and mesochannels has broadened potential
applications of periodic mesostructured and mesoporous
organosilicas, such as light-harvesting antenna properties,8,9
color-tunable fluorescence emission,10−12 photocatalytic hydro-
gen evolution,13 and enhanced photocatalytic CO2 reduction.14
Received: November 24, 2011
Revised: January 16, 2012
Published: January 17, 2012
Highly efficient Forster resonance energy transfer (FRET) from
̈
the pore walls to guest dyes has been reported for coumarin-
© 2012 American Chemical Society
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local environments of coumarin dyes in mesostructured silica−
surfactant nanocomposites by measuring the time-dependent
fluorescence spectral shift.15 Zink and colleagues studied the
precise energy and electron-transfer behavior in multi-doped
mesostructured thin films.16 However, there has been no
systematic study on the energy and electron-transfer behavior
of dye-doped periodic mesostructured organosilicas with
different donor−acceptor arrangements.
In the present study, we have examined the influence of the
dispersion state and location of guest acceptor molecules on the
energy or electron-transfer behavior in periodic mesostructured
organosilica films by systematically changing the donor−
acceptor arrangement based on molecular design of the
acceptor substituent groups. Blue-fluorescent tetraphenylpyr-
ene (TPPy)−silica mesostructured films11 were selected as host
(energy/electron donor) materials, and perylene bisimide
(PBI) dyes were selected as guest (acceptor) molecules (Figure
1). Figure 2 shows a schematic illustration of the possible
photochemical processes that occur in the PBI-doped
mesostructured films. PBI derivatives exhibit strong absorption
in the visible light wavelength region and also have an electron-
deficient core; therefore, efficient FRET occurs when the guest
PBI dyes are located within the critical Forster radius of the
̈
TPPy−PBI (donor−acceptor) pairs (Figure 2C); photo-
induced electron transfer can also occur when a close host−
guest arrangement is realized at the pore wall surface (Figure
2B). For induction of such a host−guest interaction, four PBI
dyes 1−4 were synthesized with substituents (R1 and R2) that
possess different functionality and bulkiness on the imide
groups and 2, 5, 8, and 11 positions (Figure 1). Although
chemical modification of PBIs had relied on N substitution of
imide groups or transformation of the bay area (1, 6, 7, and 12
positions),17 selective functionalization of the 2, 5, 8, and 11
positions of PBIs has recently been exploited by Shinokubo and
co-workers.18 This new synthetic method has enabled the
systematic synthesis of PBI dyes with various substituents. The
obtained PBI dyes were incorporated into TPPy−silica
mesostructured films containing non-ionic template surfactants.
Energetic and electronic host−guest communications in PBI-
doped TPPy−silica mesostructured films were examined on the
basis of their fluorescence emission and quenching behavior.
EXPERIMENTAL SECTION
■
Materials and Methods. All reagents and solvents were
purchased from Aldrich, Gelest, and Tokyo Chemical Industry and
used without further purification. Poly(ethylene oxide)-block-poly-
(propylene oxide)-block-poly(ethylene oxide) (PEO−PPO−PEO)
triblock co-polymer (EO20−PO70−EO20, P123, Aldrich) and poly-
(ethylene oxide) octadecyl ether (EO10−C18H37, Brij76, Aldrich) were
used as a non-ionic template surfactant. H and 13C nuclear magnetic
1
resonance (NMR) spectra were measured using a Jeol JNM-ECX400P
spectrometer. Mass spectra were recorded on a Bruker Daltonics
Autoflex mass spectrometer with matrix-assisted laser desorption/
ionization (MALDI). X-ray diffraction (XRD) measurements were
performed using Cu Kα radiation (Rigaku RINT-TTR, 50 kV, 300
mA). Ultraviolet−visible (UV−vis) absorption spectra were measured
using a Jasco V-670 spectrometer. Fluorescence spectra were obtained
with a Jasco FP-6500 spectrometer. Femtosecond time-resolved
absorption spectra were measured using a pump−probe technique
(CDP, ExciPro). The output of a mode-locked Ti:Sapphire oscillator
(Coherent, Vitesse) was pumped by the second harmonic generation
(SHG) of a continuous wave Nd3+:YVO4 laser (Coherent, Verdi) and
was amplified with a regenerative amplifier (Coherent, Legend)
pumped by a Nd3+:YLF laser (Coherent, Evolution). The output of
the amplifier [2.4 W, 100 fs full width at half maximum (fwhm), 1
kHz] was divided into two parts. One part was converted to the Signal
(1570 nm) using an optical parametric amplifier (OPA) system
(Coherent, OPerA). The Signal light was coincident with the
fundamental light in a 0.5 mm thick β-BaB2O4 (BBO) crystal for
sum frequency generation (SFG), where the SFG process converts the
Signal and fundamental light into SFG. The SFG pulse (530 nm) was
directed through a half-wave plate and then used as a pump light after
the repetition rate was modulated to 0.5 kHz using a chopper. The
relative polarization between the excitation and probe pulses was fixed
at the magic angle (54.7°). The other part of the amplified Ti:Sapphire
output was directed through an optical delay circuit with a computer-
controlled stepping motor and then focused into a 5 mm H2O cell to
generate a white-light continuum. The white light was collimated with
an achromatic lens and sent through a filter (Hoya, C5000) for
elimination of the fundamental light. A small portion of the collimated
white light was used as the reference light, and the residual light was
focused on the sample. A rotating sample holder was used. The
transmitted probe light and the reference light were detected with a
polychromator (Solartii, MS3501I) and a multichannel photodiode
array detector. The spectral data at every fixed delay were averaged
Figure 1. Chemical structures of TPPy-derived organosilane precursor
TPPy−Si and PBI dyes 1−4.
Figure 2. Possible photochemical processes in mesostructured TPPy−
silica films containing PBI dyes upon photoexcitation of the TPPy
moieties: (A) fluorescence emission from the TPPy moiety, (B)
photo-induced electron transfer from TPPy to PBI, and (C) energy
transfer from TPPy to PBI, followed by fluorescence emission of PBI.
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over 500 pulse shots. The chirp structure of the probe light was
corrected by measuring the optical Kerr signal.19 The time resolution
of this system was less than 300 fs. The excitation wavelength (530
nm) was adjusted to the absorption band of PBI acceptor molecules to
detect the direct electron-transfer process from the TPPy moiety to
the excited PBI chromophore.
Synthesis of Organic Components. Organosilane precursor
TPPy−Si and dye 2 (Figure 1) were synthesized using previously
reported procedures.11,18a PBI dyes 1, 3, and 4 with different
substituents were synthesized according to the route shown in Scheme
1.
h. The reaction mixture was diluted with chloroform (5 mL) and
separated on silica gel using a chloroform/ethanol (10:1, v/v) eluent
to afford the product (1.18 g, 88%). H NMR (CDCl3, ppm) δ: 0.91
1
(t, J = 7.1 Hz, 12H), 1.43 (m, 2H), 1.51 (m, 2H), 1.61 (d, J = 7.0 Hz,
6H), 1.95 (m, 10H), 2.23 (m, 2H), 2.39−2.47 (m, 12H), 3.64 [t(br), J
= 8.1 Hz, 8H], 5.29 (m, 2H), 7.28−7.35 (m, 36H), 7.63 (m, 24H),
7.85 (s, 4H). 13C NMR (CDCl3, ppm) δ: 11.6, 14.8, 18.5, 24.9, 31.6,
32.0, 46.9, 49.4, 52.9, 120.1, 124.2, 126.6, 127.8, 129.4, 131.7, 132.7,
135.1, 135.7, 152.0, 163.7. MS (MALDI, m/z): calcd for
C122H120N4O4Si4 1817.85; [M + H]+, found 1817.85.
Preparation of Organosilica Films. The TPPy-based precursor
TPPy−Si (150 mg), tetraethoxysilane (150 mg), and non-ionic
template surfactant (200 mg) were dissolved in 8.0 mL of a 1:1 (w/w)
mixture of THF and ethanol, followed by the addition of 2 M
hydrochloric acid (40 μL) and deionized water (200 μL). The total
amount of solution was adjusted to 10.0 mL using a volumetric flask
([TPPy−Si] = 9.6 mM). The sol mixtures were stirred at room
temperature for 24 h. Appropriate amounts of dyes (0−10.0 mol %
ratios to the amount of TPPy units) were added to the sol solution
using a 9.6 mM solution of dyes in THF, and then dye-doped
organosilica thin films were prepared by spin-coating the solutions
onto quartz substrates (4000 rpm for 30 s) and drying under reduced
pressure at room temperature for 24 h. The organosilica films were
stored in the dark prior to optical measurements.
Scheme 1. Synthetic Route for PBI Dyes 1−4
N,N′-Didodecyl-2,5,8,11-tetrakis[2-(trimethoxysilyl)ethyl]-
perylene-3,4:9,10-tetracarboxylic Acid Bisimide (1). A mixture of
N,N′-didodecylperylene-3,4:9,10-tetracarboxylic acid bisimide (72.3
mg, 0.10 mmol), trimethoxyvinylsilane (220 mg, 1.5 mmol),
mesitylene (0.3 mL), and RuH2(CO)(PPh3)3 (5.5 mg, 0.006 mmol)
was stirred at 160 °C for 48 h. The reaction mixture was passed
through a pad of gel permeation chromatography (GPC) gel (Bio-Rad
Bio-Beads S-X1) with tetrahydrofuran (THF). The concentration of
the solution under reduced pressure provided the product (125.2 mg,
RESULTS AND DISCUSSION
■
The PBI dyes 1−4 were successfully synthesized by imidation
of perylene tetracarboxylic dianhydride with aliphatic amines
and subsequent Ru-catalyzed 2, 5, 8, and 11 alkylation using
vinylsilane compounds (Scheme 1). R1 and R2 substituents
were selected to tune the polarity and bulkiness of the PBI dyes
(Figure 1). PBI dyes 1 and 2 have trimethoxysilyl moieties as
R2 to anchor on the pore wall surface. Linear and branched
alkyl chains were appended for dyes 1 and 2, respectively, to
examine the steric effect of R1. Dye 3 possesses the same
branched R1 as dye 2, but the R2 is a bulky and hydrophobic
tris(trimethylsiloxy)silyl group; therefore, dye 3 is an entirely
hydrophobic molecule designed to suppress self-aggregation.
Dye 4 has polar amino groups as R1 and bulky triphenylsilyl
groups as R2. In this dye, the R1 amino group, which is
protonated in an acidic sol−gel process, is a polar anchoring
group for the silica-based pore walls. A rigid aromatic R2
substituent was introduced to suppress self-aggregation of the
PBI moieties and to promote segregation of the dye molecules
from micellar cores consisting of aliphatic chains of the
template surfactants.
Solutions of the PBI dyes 1−4 in THF exhibited similar
UV−vis absorption and fluorescence spectra, with absorption
maximum wavelengths (λmax) of 520−524 nm and fluorescence
wavelengths of 536−544 nm, because of the same chemical
structure of the PBI chromophore (see Figure S1 of the
Supporting Information). Figure 3 shows absorption and
fluorescence spectra of a host TPPy−silica mesostructured
film containing non-ionic surfactant P123 as a template
(denoted as P123−TPPy)11 and a solution of PBI dye 2 in
THF. The large overlap of the fluorescence spectrum of P123−
TPPy (curve B) and the absorption spectrum of the PBI dye
(curve C) is preferable for efficient FRET from the TPPy
1
95%). H NMR (CDCl3, ppm) δ: 0.88 (t, J = 7.0 Hz, 6H), 1.23 (m,
8H), 1.26 (m, 32H), 1.40 (m, 4H), 1.73 (m, 4H), 3.63 (m, 8H), 3.68
(s, 36H), 4.19 (t, J = 7.5 Hz, 4H), 8.43 (s, 4H). 13C NMR (CDCl3,
ppm) δ: 10.8, 14.2, 22.7, 27.3, 28.1, 29.4, 29.7, 30.4, 31.9, 40.4, 50.7,
119.5, 124.3, 126.8, 131.8, 133.2, 152.2, 163.2. MS (MALDI, m/z):
calcd for C68H106N2O16Si4 1318.66; [M]+, found 1318.66.
N,N′-Bis(3-pentyl)-2,5,8,11-tetrakis{2-[tris(trimethylsiloxy)silyl]-
ethyl}perylene-3,4:9,10-tetracarboxylic Acid Bisimide (3). A mixture
of N,N′-bis(3-pentyl)perylene-3,4:9,10-tetracarboxylic acid bisimide
(0.265 g, 0.50 mmol), tris(trimethylsiloxy)vinylsilane (2.42 g, 7.50
mmol), mesitylene (5.0 mL), and RuH2(CO)(PPh3)3 (46.0 mg, 0.05
mmol) was stirred at 160 °C for 48 h. The reaction mixture was
diluted with hexane (5 mL) and separated on silica gel using a hexane/
1
chloroform (3:1, v/v) eluent to afford the product (0.34 g, 37%). H
NMR (CDCl3, ppm) δ: 0.15 (m, 108H), 0.91 (t, J = 7.1 Hz, 12H),
1.01 (m, 8H), 1.94 (m, 4H), 2.32 (m, 4H), 3.51 (m, 8H), 5.11 (m,
2H), 8.32 (s, 4H). 13C NMR (CDCl3, ppm) δ: 1.9, 11.4, 16.2, 25.1,
31.1, 57.2, 120.1, 124.2, 126.2, 132.0, 132.9, 152.7, 164.1. MS
(MALDI, m/z): calcd for C78H150N2O16Si16 1818.73; [M]+, found
1818.73.
N,N′-Bis(4-diethylamino-1-methylbutyl)perylene-3,4:9,10-tetra-
carboxylic Acid Bisimide. 2-Amino-5-diethylaminopentane (3.17 g,
20.0 mmol) was added to a mixture of perylene-3,4:9,10-
tetracarboxylic dianhydride (1.57 g, 4.00 mmol) and imidazole (10.0
g) and stirred at 160 °C for 16 h. After cooling to room temperature,
ethanol (100 mL) was added, and the mixture was stirred for 0.5 h.
The resultant precipitate was collected by suction filtration, washed
with ethanol, and dried under reduced pressure to afford the product
as a red solid (2.60 g, 97%). 1H NMR (CDCl3, ppm) δ: 0.95 (t, J = 7.1
Hz, 12H), 1.40 (m, 2H), 1.51 (m, 2H), 1.61 (d, J = 7.0 Hz, 6H), 1.90
(m, 2H), 2.23 (m, 2H), 2.39−2.50 (m, 12H), 5.30 (m, 2H), 8.53 (d, J
= 8.1 Hz, 4H), 8.60 (d, J = 8.1 Hz, 4H).
(donor) to PBI (acceptor) units. The critical Forster radius for
̈
the donor−acceptor pair was estimated to be ca. 4.5 nm from
the overlap function (see the Supporting Information).8,20
Mesostructured P123−TPPy films containing the PBI dyes
were prepared on quartz substrates by a spin-coating method.
The mesoscale periodicity of the P123−TPPy films observed
by XRD measurements was ca. 8.7 nm, as reported in our
N,N′-Bis(4-diethylamino-1-methylbutyl)-2,5,8,11-tetrakis[2-
(triphenylsilyl)ethyl]perylene-3,4:9,10-tetracarboxylic Acid Bisimide
(4). A mixture of N,N′-bis(4-diethylamino-1-methylbutyl)perylene-
3,4:9,10-tetracarboxylic acid bisimide (0.50 g, 0.74 mmol), triphe-
nylvinylsilane (3.18 g, 11.1 mmol), mesitylene (10.0 mL), and
RuH2(CO)(PPh3)3 (64.4 mg, 0.07 mmol) was stirred at 160 °C for 22
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aggregation and rather facilitates the self-aggregation by
hydrophobic interaction and van der Waals forces. On the
other hand, λmax for the PBI dyes 2−4 doped in the films were
524, 523, and 534 nm, respectively (Figure 4). The larger red
shift of λmax for dye 4 in the P123−TPPy film implies that dye 4
is located in a more polar environment, e.g., in close proximity
to organosilica walls, compared to dyes 2 and 3.
The fluorescence emission and quenching behavior of the
P123−TPPy films containing PBI dyes was examined upon
excitation at λ = 400 nm (excitation of the host TPPy units)
and 490 nm (direct excitation of the PBI dyes). Figure 5 shows
Figure 3. UV−vis absorption and fluorescence spectra of the
mesostructured P123−TPPy film (curves A and B) and a dilute
solution of PBI dye 2 in THF (1.0 × 10−5 M) (curves C and D). The
excitation wavelengths in curves B and D are 400 and 490 nm,
respectively.
previous work (Figure S3b of the Supporting Information).11
The XRD pattern of the film also showed broad diffractions at
d = 5.0 and 4.3 nm. These diffractions suggest the formation of
a two-dimensional (2D) mesochannel array close to hexagonal
packing, because the reciprocal of d-spacing values is in the
ratio of ca. 1:√3:2. UV−vis absorption spectra of the dye-
doped P123−TPPy films were measured to examine the
dispersion states of the PBI dyes in the films (Figure 4). The
Figure 5. Fluorescence spectra of PBI-doped P123−TPPy mesostruc-
tured films containing dyes (a) 1, (b) 2, (c) 3, and (d) 4 upon
excitation at λ = 400 nm (left) and 490 nm (right). Gray solid line,
non-doped P123−TPPy; black solid line, 1 mol % dye doping; black
broken line, 5 mol % dye doping.
Figure 4. UV−vis absorption spectra of P123−TPPy films containing
5 mol % of dyes (A) 1, (B) 2, (C) 3, and (D) 4. The inset shows an
optical microscope image of the P123−TPPy/1 film containing
micrometer-scale dye aggregates.
fluorescence spectra of the mesostructured films containing 0,
1.0, and 5.0 mol % of PBI dyes. Figure 6 shows the quenching
efficiency (1 − I/I0, where I is the fluorescence intensity at λ =
450 nm and I0 is the fluorescence intensity of the non-doped
host film at λ = 450 nm)8,20 of the TPPy chromophore as a
function of the doped PBI dye concentration. The fluorescence
intensity I was normalized using the ratio of absorbance of the
mesostructured films at λ = 400 nm. Strong fluorescence
quenching of the host TPPy units by PBI doping was observed
for the P123−TPPy/2 and P123−TPPy/4 films (Figure 6). On
the other hand, the fluorescence behaviors of the PBI dyes are
quite different depending upon the molecular structures of the
dyes and the excitation wavelength (Figure 5).
P123−TPPy films containing dyes 2−4 (5.0 mol % to the
TPPy units) had weak but pronounced absorption bands
attributable to the PBI dyes at λ = 450−560 nm (curves B, C,
and D of Figure 4), and their spectral shapes were similar to
those of the dyes in THF solution. In contrast, no absorption
band of dye 1 was apparent for the P123−TPPy/1 film (curve
A of Figure 4). These results indicate that dyes 2−4 are well-
dispersed in the mesostructured films, whereas dye 1 forms
large aggregates in the films. Optical microscopy observation of
the P123−TPPy/1 film showed the growth of micrometer-scale
aggregates of dye 1 (inset of Figure 4). The long linear alkyl
chain (R1) of dye 1 seems to exhibit no steric hindrance for dye
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the mesocylinder is calculated to be 5.0 nm, which is longer
than the critical Forster radius. Moreover, the molecular radius
̈
of dye 3 with the large R2 substituent is ca. 1.1 nm; the distance
between the PBI core of dye 3 and the TPPy units embedded
within the pore walls could be further elongated by the large
substituents (R2) of dye 3.
To examine the effect of the host−guest distance on the
FRET from the TPPy bridging groups to dye 3, a TPPy−silica
mesostructured film with smaller mesoscale periodicity was
prepared using the non-ionic surfactant Brij76 [HO-
(CH2CH2O)10C18H37] as a template. The host mesostructured
film (Brij76−TPPy) had XRD peaks at d = 5.6, 3.2, 2.7, 2.1, and
1.9 nm (Figure S3c of the Supporting Information). The
reciprocal of the d-spacing values was in the ratio of
1:√3:2:√7:3, which is typical of a 2D hexagonal arrangement
of mesochannels. In comparison to the P123−TPPy film, the
Brij76−TPPy film templated by nonpolar hydrocarbon chains
seems to have a more ordered periodic mesostructure. The
radius of the cylindrical structure was calculated to be 3.3 nm,
which was 1.7 nm shorter than that of P123−TPPy. Figure 7a
Figure 6. Quenching efficiency of the P123−TPPy films doped with
PBI dyes.
The P123−TPPy/1 films containing aggregates of dye 1
exhibited slight fluorescence quenching of the TPPy units
(Figure 5a). Although micrometer-scale aggregates of dye 1 are
contained in the film, a portion of the dye aggregates seems to
be finely dispersed and chemically adsorbed on the TPPy−silica
framework with the silyl groups of R2, which leads to a
quenching efficiency of ca. 0.3 (Figure 6). However, no
fluorescence emission was observed from dye 1 upon excitation
at λ = 400 nm, which suggests energy transfer from the TPPy
moieties to less emissive dye aggregates. Direct excitation of the
dye at λ = 490 nm resulted in a broad emission at λ = 650 nm,
which is attributable to an excimer-band emission from the dye
aggregates.
The P123−TPPy/2 films exhibited efficient FRET from the
host TPPy to the PBI dye. The fluorescence emission of the
TPPy groups upon excitation at λ = 400 nm was strongly
quenched by doping with dye 2 (Figures 5b and 6), and the
fluorescence emission of dye 2 was observed at λ = 534 nm
(Figure 5b), which is a monomer-band-like emission. In
comparison to dye 1, the introduction of a branched alkyl
substituent as R1 had a significant effect on the suppression of
dye aggregation, which was supported by the different
fluorescence behavior of the PBI dyes upon direct excitation
at λ = 490 nm (panels a and b of Figure 5). The efficiency of
fluorescence quenching of the TPPy groups reached more than
0.9 for 5 mol % dye doping (Figure 6). Efficient fluorescence
quenching and energy transfer are realized by chemical
adsorption of the PBI dye 2 onto the TPPy−silica framework
with the silyl groups of R2.
The P123−TPPy/3 films did not exhibit significant
fluorescence quenching of the TPPy units (Figure 5c). Upon
direct excitation of dye 3 at λ = 490 nm, fluorescence emission
from PBI was observed with higher emission intensity at λ =
575 nm than that at λ = 540 nm. The spectral shape indicates
that the PBI dye molecules are locally concentrated in the films,
exhibiting mixed fluorescence emission from monomeric
species and aggregates, while self-quenching by face-to-face
association of the PBI cores is suppressed because of the bulky
R1 and R2 substituents. The strong fluorescence emission from
the TPPy units in the P123−TPPy/3 films upon excitation at λ
= 400 nm suggests that the distance between the host TPPy
and the guest PBI chromophores is too far to cause FRET. It is
probable that the hydrophobic dye 3 is located at the
hydrophobic center of the template surfactant micelles. The
Figure 7. (a) Fluorescence spectra of the Brij76−TPPy/3 films upon
excitation at λ = 400 nm and (b) comparison to fluorescence
quenching for mesostructured TPPy−silica films by doping with dye 3.
shows fluorescence spectra for the Brij76−TPPy/3 films upon
excitation at λ = 400 nm. The fluorescence emission from TPPy
was adequately quenched, and the monomer band emission of
dye 3 was observed at λ = 531 nm, which indicates that FRET
occurred in the Brij76−TPPy/3 films. In this case, the
fluorescence spectral shape of dye 3 was similar to that of a
solution of dye 3 in THF, which differs from the P123−TPPy/
3 films (Figure 5c). This is probably because dye 3 with
lipophilic trimethylsilyl and 3-pentyl groups is more readily
dispersed in the hydrophobic region consisting of Brij76 alkyl
chains rather than in the region consisting of polar poly-
critical Forster radius estimated for the TPPy−PBI pair is ca.
̈
4.5 nm; however, the mesoscale periodicity of the host film
obtained by XRD measurements is 8.7 nm. Assuming the
formation of a 2D hexagonal-like mesostructure, the radius of
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(propylene oxide) chains of P123. Figure 7b compares the
quenching efficiency, which is equal to the FRET efficiency in
this case, of the Brij76−TPPy/3 and P123−TPPy/3 films; the
FRET efficiency reached 0.7 for the Brij76−TPPy/3 films. The
use of a host mesostructured film with smaller mesoscale
periodicity was effective to achieve a large enhancement of the
FRET efficiency from the host organosilica framework to guest
dye molecules. It should be noted that the FRET efficiency is
saturated at ca. 0.7 for Brij76−TPPy/3 and ca. 0.1 for P123−
TPPy/3 (Figure 7b), which suggests that hydrophobic dye 3 is
selectively incorporated into the center of the surfactant
micelles and is not located in the vicinity of the pore walls.
The P123−TPPy/4 films exhibited the most efficient
fluorescence quenching of the host TPPy units (Figures 5d
and 6). For the PBI dye 4 incorporated into the film, the
protonated amino groups functioned as an anchor for the
TPPy−silica pore walls and the bulky triphenylsilyl substituents
suppressed self-aggregation of the dye molecules. The efficient
fluorescence quenching (Figure 6) and the clear UV−vis
spectrum of dye 4 with a slight red shift of λmax (curve D of
Figure 4) verified the dispersion of dye 4 in the polar region of
the film. However, no fluorescence emission was observed from
dye 4 upon excitation at λ = 400 or 490 nm (Figure 5d);
therefore, the fluorescence quenching most likely corresponds
to photo-induced electron transfer from TPPy to PBI.21 To
clarify the formation of electron donor (TPPy)−acceptor (PBI)
pairs, dye 4 was incorporated into a P123-templated
mesostructured silica film (P123−silica) with no TPPy bridging
group (Figure 8).22 The λmax of dye 4 in the P123−silica film
Figure 9. Transient absorption spectra of the P123−TPPy/4 film
containing 10 mol % dye 4 after laser excitation at λ = 530 nm. The
inset shows time profiles of Δabsorbance at λ = 580 and 740 nm.
PBI.23 An absorption band around λ = 770 nm is attributable to
excited species of partially formed dye aggregates.24 On the
other hand, the transient absorption band at λ = 580 nm is
characteristic of the cationic radical of TPPy (TPPy• +), which
was confirmed by UV−vis spectroscopy of chemically oxidized
TPPy with SbCl5 (Figure S6 of the Supporting Information).
The time profiles of the transient absorption intensities at λ =
580 and 740 nm exhibited a rise and a decay, respectively, with
a similar time constant (inset of Figure 9). These results
indicate that the photo-induced electron transfer from the
TPPy bridging groups to excited dye 4 occurred in the P123−
TPPy/4 film, which suggests that a close host−guest
arrangement is realized in the film. Generation of the radicals
occurred within ca. 100 ps, and the resultant charge-separated
state was retained over a nanosecond time scale.
The dispersion states of the PBI dyes 1−4 in the
mesochannels are summarized in Figure 10a. The appropriate
Figure 8. (a) UV−vis absorption and (b) fluorescence (excited at λ =
490 nm) spectra of dye 4 incorporated into the P123−silica film
containing no TPPy units (P123/4 = 100:0.2, w/w).
was 530 nm, which is slightly longer than that in THF solution
(λmax = 524 nm). Upon excitation at λ = 490 nm, the film
exhibited strong fluorescence emission at λ = 545 nm
attributable to the monomer band emission of dye 4, which
confirms that the dye molecules incorporated into the P123−
silica film are well-dispersed and highly fluorescent without
being quenched by the silica framework. This result conversely
verifies the strong electronic interaction of the TPPy units and
dye 4 in the P123−TPPy/4 films.
Photo-induced electron transfer in the P123−TPPy/4 films
was revealed by time-resolved absorption spectroscopy. Figure
9 shows transient absorption spectra of the P123−TPPy/4 film
containing 10 mol % dye 4 after selective excitation of dye 4 at
λ = 530 nm. The spectra show transient absorption bands
around λ = 700 and 580 nm, with intense bleaching observed at
λ = 492 and 531 nm (S0 → S1 transition of the PBI dye). The
broad absorption band around λ = 700 nm can be attributed to
both the singlet state (1PBI*) and anionic radical (PBI• −) of
Figure 10. (a) Schematic illustration of the distribution of PBI dyes in
the mesochannel. (b) Plausible scheme for the adsorption of dyes 2
and 4 onto the TPPy−silica framework.
selection of substituents (R1 and R2) enabled control of the
location and dispersion of the dye molecules. Dye 1 with a
linear R1 group is not easily dispersed in the mesostructured
films because of strong self-aggregation. Dye 3, which is a bulky
and entirely hydrophobic dye, is located in the center of the
template surfactant micelles. In this case, the FRET efficiency is
varied according to the dimension of the mesostructure that
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Notes
determines the host−guest distance. This host−guest arrange-
ment is useful in certain cases (e.g., photoluminescent
materials), because undesired electron transfer is completely
suppressed as a result of the isolation of donors and acceptors.
Dyes 2 and 4, which have both polar anchoring groups for
organosilica pore walls and bulky substituents to suppress self-
aggregation, are appropriate for molecular-level dispersion of
the dyes and inducing efficient host−guest interaction. Efficient
FRET and fluorescence emission from the PBI dye were
observed in the mesostructured TPPy−silica films containing
dye 2. In contrast, the TPPy−silica films containing dye 4
exhibited photo-induced electron transfer from the host TPPy
to the guest PBI. The different energy and electron-transfer
behavior between dyes 2 and 4 is attributable to the difference
in the functionality of the polar substituents. The four
trimethoxysilyl moieties of dye 2 not only function as an
anchor to organosilica pore walls but also connect the PBI
molecules to each other. In contrast, the protonated cationic
amino groups of dye 4 strongly interact with the organosilica
framework and also suppress dye aggregation by electrostatic
repulsion, which results in monolayer adsorption of the dyes
onto the surface of the TPPy−silica framework. The lengths of
the polar substituents [−(CH2)2Si(OH)3 for dye 2 and
−CH(CH3)(CH2)3NH+Et2 for dye 4] can also affect the
TPPy−PBI interaction (Figure 10b). When the terminal polar
groups anchor onto the silica moieties of the TPPy−silica pore
wall, the PBI core of dye 4 can become closer to the TPPy unit
than that of dye 2, because the length of the polar substituent of
dye 4 is similar to that of the −OCONH(CH2)3− linker of
TPPy−Si. The close proximity of the TPPy moiety and PBI
core of dye 4 is conducive to photo-induced electron transfer.
The authors declare no competing financial interest.
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CONCLUSION
■
Systematic design of PBI dye-doped mesostructured TPPy−
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ASSOCIATED CONTENT
■
S
* Supporting Information
Supplementary spectroscopic data, XRD patterns, and calcu-
(22) The P123−silica film was prepared from 200 mg of P123 and
300 mg of tetraethoxysilane, using the similar procedure to that used
for the preparation of the organosilica films.
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lation of the critical Forster radius. This material is available free
̈
of charge via the Internet at http://pubs.acs.org.
̈
(24) In the P123−TPPy/4 (10 mol %) film, a part of the dye
molecules can form dimers or aggregates because of the excessive
doping. The aggregates of dye 4 turned out to exhibit a transient
absorption band at λ = 765 nm from the time-resolved absoprtion
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: inagaki@mosk.tytlabs.co.jp.
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spectra of a P123−silica/4 film (P123/4 = 100:8.7, w/w). See Figures
S7 and S8 of the Supporting Information.
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