Aggregation-induced photocatalytic activity and efficient photocatalytic hydrogen evolution of amphiphilic rhodamines in water

Article abstract of DOI:10.1039/d0sc04285d

The development of photocatalysts is an essential task for clean energy generation and establishing a sustainable society. This paper describes the aggregation-induced photocatalytic activity (AI-PCA) of amphiphilic rhodamines and photocatalytic functions of the supramolecular assemblies. The supramolecular assemblies consisting of amphiphilic rhodamines with octadecyl alkyl chains exhibited significant photocatalytic activity under visible light irradiation in water, while the corresponding monomeric rhodamines did not exhibit photocatalytic activity. The studies on the photocatalytic mechanism by spectroscopic and microscopic analyses clearly demonstrated the AI-PCA of the rhodamines. Moreover, the supramolecular assemblies of the rhodamines exhibited excellent photocatalytic hydrogen evolution rates (up to 5.9 mmol g-1 h-1).

Full text of DOI:10.1039/d0sc04285d

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DOI: 10.1039/D0SC04285D
ARTICLE
Aggregation-induced photocatalytic activity and efficient
photocatalytic hydrogen evolution of amphiphilic rhodamines in
water
Received 00th January 20xx,
Accepted 00th January 20xx
Hajime Shigemitsu,*a,b,c Youhei Tani,a Tomoe Tamemoto,a Tadashi Mori,a Xinxi Li,d Yasuko
Osakada,d,e Mamoru Fujitsukad and Toshiyuki Kida*a
DOI: 10.1039/x0xx00000x
The development of photocatalysts is an essential task for clean energy generation and establishing a sustainable society.
This paper describes aggregation-induced photocatalytic activity (AI-PCA) of amphiphilic rhodamines and photocatalytic
functions of the supramolecular assemblies. The supramolecular assemblies consisting of amphiphilic rhodamines with
octadecyl alkyl chains, exhibited significant photocatalytic activity under visible light irradiation in water, while the
corresponding monomeric rhodamines did not exhibit photocatalytic activity. The studies on the photocatalysis mechanism
by spectroscopic and microscopic analyses clearly demonstrated AI-PCA of the rhodamines. Moreover, the supramolecular
assemblies of the rhodamines exhibited excellent photocatalytic hydrogen evolution rates (up to 5.9 mmol·g1·h1).
Introduction
Photocatalysts are promising materials for the conversion of
solar energy into storable chemical energy and are expected to
contribute significantly to clean and renewable energy
generation.1 In 1974, Fujishima and Honda reported
photocatalytic water-splitting using
a titanium dioxide
electrode, demonstrating the possibility of artificial
photosynthesis.2 Since then, a wide range of photocatalysts,
based on inorganic,3 molecular,4 and polymeric5 compounds,
have been actively developed. Besides their application in
artificial photosynthesis, the redox reactivity of photocatalysts
has been utilized for environmental remediation,6 organic
synthesis,7 and photodynamic therapy.8 The emergence and
development of new photocatalysts have contributed to the
progress in artificial photosynthesis and generated new
opportunities in the related fields.9
Based on these backgrounds, we explored a new class of
photocatalysts and focused on supramolecular assemblies. The
photophysical properties of supramolecular assemblies are
different from those of the constituting monomers because of
the interaction between the adjacent molecules.10 Various
Figure 1. (a) Schematic representation of aggregation-induced
photocatalytic activity (AI-PCA). (b) Chemical structures of
rhodamine derivatives (left: rhodamine B (RhB), right: rhodamine
19 (Rh19)).
a.Department of Applied Chemistry, Graduate School of Engineering, Osaka
University, Suita 565-0871, Japan.
b.Frontier Research Base for Global Young Researchers, Graduate School of
Engineering, Osaka University, Suita 565-0871, Japan.
characteristic aggregation-induced photophysical phenomena,
such as aggregation-caused quenching (ACQ),11 aggregation-
induced enhanced emission,12 light-harvesting,13 and nonlinear
optical phenomena (e.g., photo-upconversion14 and singlet
fission15) have been intensively studied and applied to solar
energy collection,16 molecular sensing,17 and biological
applications (e.g., bioimaging,18 optogenetics,19 and
c. Global Center for Medical Engineering and Informatics, Osaka University, Suita
565-0871, Japan.
d.The Institute of Scientific and Industrial Research (SANKEN), Osaka University,
Mihigaoka 8-1, Ibaraki, Osaka 567-0047, Japan.
e. Institute for Advanced Co-creation Studies, Osaka University, 1-1 Yamadagaoka,
Suita, Osaka, 565-0871, Japan
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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phototherapy20). However, aggregation-induced photocatalytic
activity (AI-PCA) has never been reported despite of the high
potential as a novel photocatalytic material. Taking into account
previous reports on aggregation-induced triplet excited state
generation21 and charge carrier migration22 in self-assembled
nanostructures of organic dyes, we considered that various
organic dyes may cause AI-PCA. These phenomena cause
elongation of excited state lifetime23 and increasing collision
frequency with substrates,24 which are important for
proceeding photocatalytic reactions. AI-PCA would lead to
expansion of molecular design of photocatalysts that enables
adjustment of absorption wavelength and redox potential. In
addition, self-assembled supramolecular photocatalysts (SA-
SPCs) possessing AI-PCA are expected to produce unprecedent
photocatalytic soft-materials25 (gel, liquid crystal, and
membrane etc.) taking advantages of unique properties of
supramolecular assembly26 (e.g. reversibility and stimuli-
responsiveness).
View Article Online
DOI: 10.1039/D0SC04285D
Herein, we demonstrate AI-PCA of amphiphilic rhodamines
(Figure 1a). Rhodamines are very common hydrophilic organic
dyes with excellent photophysical properties, such as high light
absorption and quantum yield, which can be tuned through
chemical modification.27 The two SA-SPCs composed of
amphiphilic rhodamines (Rhodamine B (RhB) and Rhodamine
19 (Rh19) (Figure 1b)) exhibited photocatalytic activity under
visible light irradiation in water, while the monomeric
rhodamines did not exhibit photocatalytic activity. In particular,
the SA-SPC exhibited an excellent hydrogen evolution rates.
Figure 2. (a,c) UV-vis absorption (UV-vis) and (b,d)
photoluminescence (PL) spectra of RhB-C2 and RhB-C18 in
DMSO (a,b) and water (c,d). Experimental conditions: [RhB-C2]
= [RhB-C18] = 5.0 M, rt, water, excitation wavelength: 540 nm.
Insets: Optical photos of RhB-C2 and RhB-C18 in DMSO and
water under (a,c) daylight and (b,d) UV light (365 nm).
photophysical and photocatalytic properties. Further, Rh19-C2
and Rh19-C18 also exhibited similar UV-vis and PL spectra
(Figure S1a,b) with maxima at λabs = 539 nm and λem = 565 nm,
Results and discussion
Molecular design of the rhodamine derivatives
respectively, which indicate that the Rh19-C2/Rh19-C18 pair
has similar electronic states regardless of the alkyl chain length.
Four rhodamine derivatives with short and long alkyl chains
were used in this study (Figure 1b). Two common rhodamines
with different absorption bands, RhB and Rh19, were selected
as hydrophilic organic dyes to examine the concept of AI-PCA.
The amphiphilic rhodamines with octadecyl alkyl chains (RhB-
C18 and Rh19-C18) were expected to form supramolecular
assembles in water through hydrophobic interaction between
the alkyl chains. More hydrophilic rhodamine derivatives with
an ethyl ester group (RhB-C2 and Rh19-C2) compared to those
with octadecyl alkyl chains were prepared as control
compounds to evaluate the effect of self-assembly on
photocatalytic activity.
The photophysical properties of monomeric RhB-C2 and
RhB-C18 were evaluated from their UV-vis absorption (UV-vis)
and photoluminescence (PL) spectra measured in dimethyl
sulfoxide (DMSO), which is a good solvent for these compounds
(Figure 2a,b). The absorption spectra of RhB-C2 and RhB-C18
corresponded well with each other (Figure 2a) and both the
compounds exhibited absorption maxima at 566 nm. RhB-C2
and RhB-C18 also exhibited similar PL spectra with an emission
peak at λem = 592 nm (Figure 2b). These results indicate that the
electronic states of RhB-C2 and RhB-C18 are quite similar
despite the different alkyl chain lengths. We thus conclude that
the pair is suitable for evaluating the effect of self-assembly on
Self-assembling properties of the rhodamines in water
To examine the self-assembly properties, the UV-vis and PL
spectra of RhB-C2 and RhB-C18 were recorded in water
(concentration: 5.0 M) (Figure 2c,d). The UV-vis spectrum of
RhB-C2 in water is similar to that in DMSO with a slightly red-
shifted absorption maximum (λabs: 559 nm (DMSO), 562 nm
(water)) (Figure 2c). In contrast, RhB-C18 exhibited a broad
spectrum with split peaks at 530 and 559 nm that can be
assigned to aggregation states of RhB-C18 28
, which suggests the
formation of a supramolecular assembly of RhB-C18 in water.
The PL spectra of RhB-C2 and RhB-C18 were significantly
different (Figure 2d). RhB-C2 exhibited an intense emission in
water, whereas RhB-C18 exhibited a very weak emission. This
suggests ACQ in RhB-C18 11
Furthermore, the addition of
.
nonionic surfactant (Triton X-100, 0.3 vol%) to the RhB-C18
aqueous solution drastically increased the PL intensity (Figure
S2b), which suggests the dissociation of the RhB-C18
supramolecular assembly by Triton X-100. The photophysical
properties of the Rh19-C2/Rh19-C18 pair exhibited the same
trend as that exhibited by those of the RhB-C2/RhB-C18 pair
(Figure S1c,d and S2c,d). Rh19-C18 formed supramolecular
2 | J. Name., 2012, 00, 1-3
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The rhodamines and HITCI were mixed in water, and the UV-vis
View Article Online
DOI: 10.1039/D0SC04285D
spectra were recorded after photoirradiation of the mixture (λ
= 560 nm (FWHM: 10 nm, Xe lamp, 300 W)). In the case of RhB-
C2, the absorption band at around 735 nm arising from HITCI
gradually decreased (Figure 3a). Considering the self-
photooxidation of HITCI upon photoirradiation (Figure S6b,c),
the photocatalytic activity of RhB-C2 in HITCI oxidation is
considered negligible. In contrast, RhB-C18 remarkably oxidized
HITCI within a short period of photoirradiation (Figure 3b,c). In
addition, similar to RhB-C2, Rh19-C2 caused slow degradation
of HITCI (Figure S7a), whereas the supramolecular assemblies
of Rh19-C18 rapidly degraded HITCI (Figure S7b,c). The rate of
HITCI oxidation by the rhodamines were estimated as 8.2 × 10–
3, 3.1 × 10–1, 2.9 × 10–3, and 1.0 × 10–1 min–1 for RhB-C2
C18 Rh19-C2, and Rh19-C18, respectively, by linear regression
fitting (Figure 3d and S7d). Compared with RhB-C2 and Rh19-
C2 RhB-C18 and Rh19-C18 accelerated HITCI oxidation 39-fold
and 34-fold, respectively. Additionally, in the presence of Triton
X-100, the rates of HITCI photooxidation by the RhB-C2 RhB-
C18 and Rh19-C2 Rh19-C18 pairs were almost the same (Figure
S8). These results indicate that the formation of supramolecular
assemblies of rhodamines enhanced the oxidation of HITCI
, RhB-
,
,
/
/
.
Figure 3. (a,b) UV-vis absorption spectra of the mixture of HITCI
with (a) RhB-C2 and (b) RhB-C18 after photoirradiation. (c) Time-
course of absorbance at 735 nm after photoirradiation using RhB-
C2 (blue circle) and RhB-C18 (red circle). (d) Linear regression plots
of HITCT absorbance with RhB-C2 (black circle) and RhB-C18 (red
circle). Experimental conditions: [RhB-C2] = [RhB-C18] = 5.0 μM,
[HITCI] = 2.5 μM solvent: water, rt, irradiation wavelength: 560 nm
(FWHM: 10 nm, Xe lamp, 300 W).
Mechanistic study on photocatalytic activity of the rhodamine
SA-SPCs
The photoreaction of SA-SPCs is considered to occur via two
types of mechanisms involving electron and/or energy transfer
processes.30 Under aerobic conditions, in the energy transfer
process, singlet oxygen (1O2) is commonly involved in the
photocatalytic reaction, whereas oxygen radicals such as
superoxide anion radicals (O2•–) and hydroxyl radicals (OH•–) are
involved in the electron transfer mechanism (Figure S9).31 To
explain the mechanism of the photocatalytic reaction, electron
spin resonance (ESR) spectroscopy was performed using 4-
hydroxy-2,2,6,6-tetramethylpiperidine (4-OH-TEMP)32 and 5,5-
dimethyl-1-pyrroline N-oxide (DMPO)33 as spin trap reagents to
assemblies, whereas Rh19-C2 did not form supramolecular
assemblies in water. With increasing concentration, the
absorbances of the rhodamine derivatives measured in water
increased linearly (Figure S3, 4); this indicates that RhB-C2 and
Rh19-C2 did not form supramolecular assemblies until 50
(Figure S3), and the excessive aggregation of RhB-C18 and
Rh19-C18 did not occur at least until 100 M (Figure S4).
M
1
detect O2 and oxygen radicals (e.g. O2•–, OH•–), respectively

(Figure S10a,b). The ESR experimental conditions were first
determined using Rose Bengal (Figure S10c) as a standard.34 The
ESR spectra of RhB-C2 and RhB-C18 exhibited a characteristic
1:1:1 triplet corresponding to the TEMPOL radical (Figure 4a).
RhB generates a triplet state despite the low quantum yield of
intersystem crossing (quantum yield (ΦT): 0.006).35 Therefore,
it is reasonable that RhB-C2 exhibited an ESR signal for the
TEMPOL radical. No significant differences were observed in the
signal intensities of the TEMPOL radicals of RhB-C2 and RhB-
C18, which implies that the rate of energy transfer to oxygen did
not drastically change after self-assembly. These results suggest
that the photocatalytic reaction of RhB-C18 does not occur
through the energy transfer mechanism. Subsequently, we
evaluated the generation of oxygen radial species via the
electron transfer mechanism. The ESR spectrum for RhB-C2 in
DMPO did not exhibit clear signal for a DMPO adduct (Figure
4b), whereas the ESR spectrum of RhB-C18 exhibited a signal for
a
Dynamic light scattering (DLS) measurements and
transmission electron microscopy (TEM) observations were
performed to confirm the formation of supramolecular
assemblies. DLS measurements indicated the presence of
supramolecular assemblies of RhB-C18 and Rh19-C18 having
average sizes of 200 and 82 nm, respectively (Figure S5a,b). TEM
revealed the formation of spherical supramolecular assemblies
of RhB-C18 and Rh19-C18 (Figure S5c,d). The selected area
electron diffraction (SAED) patterns obtained by TEM exhibited
diffused rings and no diffraction spots, which indicate the
amorphous nature of the rhodamine supramolecular
assemblies (Figure S5e,f).
Photocatalytic activities of the rhodamines in water
Since it became clear that the rhodamine pairs were suitable for
evaluation of AI-PCA, we initially examined their photocatalytic
activities
in
water
using
1,1',3,3,3',3'-
hexamethylindotricarbocyanine iodide (HITCI) (Figure S6a).29
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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The RhB-C18 SA-SPC functioned for 80 min without any
View Article Online
DOI: 10.1039/D0SC04285D
Figure 4. ESR signals of (a) 4-OH-TEMP and (b) DMPO adducts observed for RhB-C2 and RhB-C18 after light irradiation. (c) schematic
representation of generation of hydroxy radical through electron transfer and chain reactions. Experimental conditions: [RhB-C2] = [RhB-
C18] = 50 μM, [4-OH-TEMP] = [DMPO] = 100 mM, solvent: water, rt, irradiation wavelength: 560 nm (FWHM: 10 nm, Xe lamp, 300 W, 3
min).
DMPO hydroxy radical adduct (DMPO-OH). Since superoxide
anion (O2•–) is unstable in aqueous media, it reacts with protons
immediately upon addition to an aqueous medium (Figure
S9(a)) and did not react with DMPO to form DMPO-OOH. Hence,
no DMPO-OOH peak appeared in the ESR spectrum. However,
we confirmed the generation of hydroxy radical (OH•–) that was
produced by the chain reaction starting from O2•– ions through
electron transfer in water (Figure 4b, c, S9(a)). The experimental
results showed that RhB-C18 exhibits photocatalytic activity
mainly through an electron transfer mechanism. Further, the
ESR spectra of the Rh19-C2 Rh19-C18 pair were similar to that
of the RhB-C2 RhB-C18 pair (Figure S11).
Figure 5. (a) Schematic representation of photocatalytic
hydrogen evolution by SA-SPC composed of rhodamine
derivatives. (b) Time-course of hydrogen evolution by RhB-C18
and RhB-C18 under visible light. Experimental conditions:
[Rhodamines] = 50 μM, [Pt] = 100 μM, [Asc] = 500 mM, solvent:
water, rt, irradiation light wavelength: > 360 nm. Asc: Ascorbic
acid, Dha: Dehydroascorbic acid.
/
/
Photocatalytic hydrogen evolution using the rhodamine SA-
SPCs
Photocatalytic hydrogen evolution is one of the important
reactions toward realizing artificial photosynthesis.2 The
hydrogen evolution abilities of various photocatalysts have
been actively examined to date. The electron transfer
mechanism of the SA-SPC encouraged us to investigate the
photocatalytic hydrogen evolution by the rhodamine SA-SPCs
using Pt nanoparticle as a co-catalyst and ascorbic acid (Asc) as
a sacrificial reagent (Figure 5a). The HOMO levels of the
rhodamine derivatives (Table S1, RhB-C2: –5.4 eV, RhB-C18: –
5.5 eV Rh19-C2: –5.4 eV, and Rh19-C18: –5.5 eV) determined by
cyclic voltammetry or square wave voltammetry (Figure S12 and
S13) were sufficiently lower than that of Asc (–4.6 eV),36 while
the LUMO levels of the rhodamine derivatives (Table S1, RhB-
decrease in photocatalytic activity, and the average hydrogen
evolution rate (HER) was determined as 3.7 mmol·g ·h
(Figure 5b), which is comparable to that of other excellent

1

1


1 36
1
organic systems such as g-C3N4 (0.67 mmol·g ·h ) and a


1
covalent organic framework (10.1 mmol·g ·h ).37 80 min after
light irradiation, the photocatalytic activities of the SA-SPCs
were decrease due to the decomposition of rhodamines. Rh19-
C18 exhibited photocatalytic hydrogen evolution (HER: 2.9
1

1

1
mmol·g ·h ), while Rh19-C2 did not (Figure S15a). One of the
reasons for the high HER would be intermolecular electron
migration among the rhodamines.38 The photoirradiation of
rhodamines generated intermolecular charge separation states,
and the migration between the rhodamines may have
facilitated efficient electron transfer to the Pt nanoparticle.39
The apparent quantum efficiencies of RhB-C18 and Rh19-C18
were 0.059 and 0.039% under the condition, respectively.
To examine the effects of SA-SPC concentration on the
hydrogen evolution reaction, the SA-SPCs concentrations were
C2: –3.2 eV, RhB-C18: –3.5 eV Rh19-C2: –3.1 eV, and Rh19-C18
:
–3.4 eV) were enough for proton reduction reaction. The
addition of Asc and H2PtCl6 as a precursor of Pt nanoparticles
hardly affected the size of SA-SPCs (Figure S14a, b).
The time-courses of photocatalytic hydrogen evolution by
RhB-C2 and RhB-C18 are shown in Figure 5b. After light
irradiation (λ > 360 nm, 300 W (Xe lamp)), Pt nanoparticles were
formed (Figure S14c-f) and RhB-C18 exhibited hydrogen
generation, while RhB-C2 did not display hydrogen evolution.
increased from 50 to 100 M. The amorphous self-assembled
spherical structures were almost unchanged (Figure S16,
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2
3
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average particle diameter: RhB-C18: 223 nm, Rh19-C18: 122
Hashimoto, H. Irie and A. Fujishima, Jpn. J. AppVl.iePwhAyrtsic.le2O0n0lin5e,
DOI: 10.1039/D0SC04285D
nm) after the increasing. The hydrogen evolution rates
44, 8269.


1
1
decreased from 3.7 to 2.4 mmol·g ·h for RhB-C18 (Figure
S17(a)). On the other hand, in the case of Rh19-C18, the
hydrogen evolution rate significantly increased from 2.9 to 5.9
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
1

1
mmol·g ·h (Figure S17(b)). These results indicate that the
hydrogen evolution rates are significantly affected by the
concentration of SA-SPC. The versatile factors including size,
morphology, surface area, fluidity, electric state of SA-SPC, and
interactions between SA-SPC and Pt nanoparticles or ascorbic
acid, would sensitively effect on the hydrogen evolution. The
detail understanding of the changes of RhB-C18 and Rh19-C18
for photocatalytic hydrogen evolution rates is currently difficult.
We will study how each factor effects on photocatalytic activity
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Conclusions
In summary, we demonstrated AI-PCA of two rhodamine
derivatives. The rhodamine SA-SPCs showed an excellent
photocatalytic hydrogen evolution rates (up to 5.9

1

1
mmol·g ·h ). ESR spectroscopic analysis revealed that the
photocatalytic reaction proceeded via an electron transfer
mechanism. We think that the concept of AI-PCA might be
applicable to a wide range of photoactive molecules. Further
investigations on the effects of organic dyes, morphologies, and
the molecular arrangements of supramolecular assemblies on
the photocatalytic activity of SA-SPCs, and the detailed
mechanism of AI-PCA are currently underway.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
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,
This work was performed at Osaka University and financially
supported by a Grant-in-Aid for Young Scientists (JSPS KAKENHI
Grant No. JP18K14189), AMED (Grant No. JP19lm0203007 and
JP19lm0203014), Takeda Science Foundation, Kowa Life Science
Foundation, Senri Life Science Foundation, and Kato Memorial
Bioscience Foundation. We appreciate Mr. Y. Murakami for
technical support for TEM observation. We also thank Dr. Y.
Morimoto and Dr. K. Oohora for technical support for ESR
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DOI: 10.1039/D0SC04285D
TOC
Simple amphiphilic rhodamines formed supramolecular assemblies and exhibited aggregation-induced photocatalytic activity
and hydrogen evolution in water.