The unprecedented J-aggregate formation of rhodamine moieties induced by 9-phenylanthracenyl substitution

Article abstract of DOI:10.1039/c5cc03969j

We report a substitution of 9-phenylanthracenyl group into rhodamine derivatives that can induce the J-aggregate formation of rhodamine moieties in the aqueous solution upon the addition of a halide ion. From X-ray crystallographic analysis, the dramatic red-shift in the absorption band (i.e. app. 100 nm) originates from the cooperative slipped-stacking of rhodamine and anthracene molecules.

Full text of DOI:10.1039/c5cc03969j

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Unprecedented J-Aggregate Formation of Rhodamine
Induced by 9-Phenylanthracenyl Substitution†
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
a,c
b
Sooyeon Kim, Mamoru Fujitsuka, Norimitsu Tohnai, Takashi Tachikawa, Ichiro Hisaki, Mikiji
a
a
Miyata and Tetsuro Majima*
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report a substitution of 9-phenylanthracenyl group to
rhodamine derivatives can induce rhodamine J-aggregate in the
aqueous solution upon the addition of halide ion. From X-ray
crystallographic analysis, the dramatic red-shift in absorption
band (i.e. app. 100 nm) originates from cooperative slipped-
stacking of rhodamine and anthracene molecules.
Fluorophore aggregation is an important phenomenon for the
1
TMRꢀAn
SiRꢀAn
-7
design of bio-related fluorescence probes. In general, it is Fig. 1 Chemical structure of TMR-An and SiR-An.
considered to be problematic because of fluorescence
quenching and reduced water solubility. Thus, many
1
1-14
researchers have tried to introduce hydrophilic groups to the at the high concentration.
Meanwhile, slipped-stacking-
probe in order to stay them as a monomer in the aqueous induced J-aggregate of the widely-used fluorophores has been
solution. In recent years, aggregate formations are rather mostly achieved by adsorption on the surface such as glass or
1
5-19
intended to develop intriguing traits generated by fluorophore zeolite.
assemblies. For instance, aggregation-induced emission (AIE) cyanine derivatives that many rational molecular designs have
Compared to porphyrin, perylene diimide, and
2
0-25
gives higher signal-to-noise (S/N) ratio as dye concentration been introduced to induce various aggregation,
there have
been limited reports to control self-assembly of fluorophores
formation of aggregates occasionally in solution without a help of external templates.
accompanies a vivid color change, which can be easily
In this study, we aimed to develop a method to facilitate a J-
detected by the naked eye and advantageous for a practical aggregate formation of fluorophore by substituting a simple
4
, 6
increases and hence is especially suitable for in vivo imaging.
Furthermore,
a
1
, 2, 8
sensory material.
chemical group. For this purpose, we focused on a
According to the exciton theory, two interacting molecules phenylanthracenyl group that can construct diverse crystal
can form either blue-shifted and non-fluorescent dimer or red- structures from the slipped-stacked column to herringbone
9, 10
shifted and fluorescent dimer.
The former is called H- structures upon the addition of solvent, organic and inorganic
2
6-28
aggregate, whereas the latter is assigned to J-aggregate, salts.
It was hypothesized that a combination between
depending on the values of the angle (θ) between the phenylanthracene and charged fluorophore moieties can
longitudinal axis of the monomer transition moments (54.7° < trigger unusual fluorophore assemblies apart from the H-
θ < 90° is H-aggregate, whereas 0° < θ < 54.7° is J-aggregate). dimer, typical aggregates in the aqueous solution. Among
As mentioned above, it is the H-aggregate formation that various fluorophores, rhodamine derivatives are chosen
causes fluorescence quenching of fluorophores in poor solvent because of i) their size and planarity similar to anthracene and
ii) electron-deficient characteristic as compared to electron-
rich anthracene moiety, which further assists the
intermolecular interaction. As a consequence, we have found
spontaneous J-aggregate formations of tetramethylrhodamine
(TMR) and silicon-containing tetramethylrhodamine (Si-TMR)
substituted by 9-phenylanthracenyl group, namely TMR-An
and SiR-An, respectively (Fig. 1).
In the aqueous solution, rhodamine derivatives typically
1
1, 12, 14
form H-aggregate.
To our surprise, TMR-An and SiR-An
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COMMUNICATION
Journal Name
a)
a)
0
.15
.10
.05
.00
J
M
(
Nor.)
0
0
0
J
ꢀ
H
[
Cl ] ≈ 0 mM
1
00 mM
4
00
600
800
Wavelength / nm
b)
b)
ꢀ
[
Cl ] ≈ 0 mM
100 mM
*
750
Fig. 2 (a) Normalized absorption spectra of TMR-An monomer in methanol
(
black) and changes in absorption spectra of TMR-An in MilliQ water upon the
6
50
700
800
addition of NaCl from 0 to 100 mM (green to red). M and J indicate the
absorption from the monomer and J-aggregates, respectively. [TMR-An] = 5 µM,
pathlength: 0.2 cm. Black and red arrows show spectral changes reflecting
arrangements of TMR and anthracene moieties in a slipped-stacked manner,
respectively. (b) Pink to blue colorimetric changes during aggregation of TMR-An
from a monomer (in methanol) to J-aggregate (MilliQ: [NaCl] ≈ 0 mM; PBS buffer:
Wavelength / nm
Fig. 3 (a) Normalized absorption spectra of SiR-An monomer in methanol (black)
and changes in absorption spectra (dashed black line to red) and (b) fluorescence
spectra of SiR-An in MilliQ water upon the addition of NaCl from 0 to 100 mM
(
black to red). M, H, and J indicate the absorption from monomer, H- and J-
aggregate, respectively. [SiR-An] = 5 µM, pathlength: 0.2 cm, λex = 640 nm. Black
and red arrows show spectral changes reflecting arrangements of Si-TMR and
anthracene moieties in a slipped-stacked manner, respectively. A red asterisk
indicates emission from Si-TMR J-aggregates.
[
NaCl] ≈ 110 mM).
are found to change their aggregation to J-type upon the
addition of NaCl (Fig. 2a and 3a). Especially, λabs of TMR-An is
red-shifted approximately 55 nm as TMR-An monomers form both exhibited 11 nm and 16 nm red-shifts, respectively, as
J-aggregate, accompanying a vivid color change from pink to [NaCl] increased (Fig. 2a and 3a, respectively). In other words,
blue (Fig. 2b). Meanwhile, color of SiR-An solutions showed anthracene moieties in the electronically ground state have
merely slight decoloration (Fig. S4) because λabs of SiR-An is interaction with rhodamine moieties, which are making J-
red-shifted approximately 100 nm, leading to the J-band coupling at the same time (this is in accordance with the
formation in the near-infrared region (Fig. 3a). After 7 days obtained X-ray crystal structure, see Fig. 4). Thus, the
from the sample preparation, blue precipitates were only fluorescence quenching of TMR-An and SiR-An J-aggregate
found for TMR-An and SiR-An in the PBS solution ([NaCl] ≈ 110 presumably originates from ultrafast relaxation of exciton in
mM) (Fig. S5), indicating the size of aggregates increases rhodamine J-aggregates, associated with the intermolecular
spontaneously in the presence of halide ion. Considering interaction with anthracene moieties.
together, J-aggregate formations of both compounds are
In order to reveal a role of a phenylanthracenyl group in the
accelerated depending on the concentration of halide ion, induction of rhodamine J-aggregate, detailed molecular
while TMR-An and SiR-An are present as monomer-J- arrangements of rhodamine and anthracene parts in J-
aggregate mixture and H-aggregates, respectively, under the aggregates should be clarified. After slow evaporation of
infinitesimal existence of halide ion (green line and black methanol solution of SiR-An, we could obtain nanocrystal of
dashed line in Fig. 2a and 3a, respectively).
SiR-An and perform X-ray crystallographic analysis. As shown
Unexpectedly, the number of photons emitted (cf. integral in Fig. 4, Si-TMR and anthracene moieties are slipped-stacked
of fluorescence intensities at each wavelength) of TMR-An and each other, and SiR-An stacks are faced to the water layers
SiR-An decreases as more J-aggregate forms (Fig. S6 and 3b). including chloride (Fig. S7a). In particular, θ between the
Even though J-aggregate is known to be fluorescent based on longitudinal axes of two Si-TMR molecules (face-to-face) is
9
,
10
◦
the exciton theory,
fluorescence of both J-aggregates is determined to be 20.9 , while that between Si-TMR and
◦
negligible (a red asterisk of Fig. 3b). To explain this anthracene moiety (edge-to-face) is 38.9 (Fig. 4b). The X-ray
disagreement, not only the intermolecular interaction among crystallographic and our experimental results are in good
rhodamine moieties, but that between rhodamine and agreements with the characteristics of J-aggregate: i) the angle
◦
anthracene moieties should be also considered. Indeed, the of coplanar inclined transition dipoles (θ) is in a range of 0 < θ
◦
absorption band of anthracene moieties in TMR-An and SiR-An < 54.7 ; and ii) a sharp red-shifted absorption band with 2−3
nm Stokes shift (Fig. 3).
2
| J. Name., 2012, 00, 1-3
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It is surprising how SiR-An can form J-aggregate in the buffer
solution, whereas many other Si-TMR derivatives including Si-
TMR and dimethylanthracene dyad (cf. Si-DMA that we
a)
1
9
2
previously reported as singlet oxygen ( O ) probe, Fig. S3)
undergo only H-aggregate formations under the same
condition. We consider such a difference originates from the
presence of a central phenyl ring connecting Si-TMR and
anthracene moieties which allows cooperative face-to-face
and edge-to-face interactions among SiR-An molecules (Fig.
4b). As a matter of fact, π-π stacking between two Si-TMR
molecules is rather smaller than that between Si-TMR and
anthracene moieties (Fig. 4b). The driving force to maintain a
slipped-stacked arrangement among Si-TMR moieties is
considered to be the π-π interaction between two C-N bonds
at the edge of Si-TMR moieties (a blue box in Fig. 4c). Also, CH-
b)
π
interaction may partly assist this slipped-stacked
arrangement (Fig. S7b). Both interactions require that two Si-
TMR moieties should be close enough to make π-π interaction.
In other words, if a counterion of SiR-An is too bulky, such an
alignment of SiR-An in crystal will be interrupted (Fig. 4).
Indeed, bromide and iodide ion could induce J-aggregate of
SiR-An (Fig. S8), whereas exchanging counteranion to bulkier
-
one such as hexafluorophosphate (PF ) and tetraphenylborate
6
-
BPh ) disabled any aggregate formation although
(
4
hydrophobicity increased by coupling with lipophilic
counterions (Fig. S9). On the other hand, J-aggregate
formation of TMR-An evidences that a dimethyl group in Si-
TMR is not the most important factor to induce and maintain
slipped-stacking of rhodamine moieties. However, J-aggregate
of TMR-An probably exhibits less slipped-stacking
◦
◦
(
presumably, 20.9 < θ < 54.7 ) because of the absence of a
dimethyl group in the TMR moiety, resulting in more π-π
interaction among TMR moieties than that of SiR-An
.
From dynamic light scattering (DLS) measurements, we have
found there are at least two populations in the size of SiR-An J-
aggregate regardless of the concentration of SiR-An: one in a
range of 200−800 nm and another growing up to tens of µm
during 2.5 hr incubation (Fig. S10). Nonetheless, in the absence
of NaCl or just after the addition of NaCl, the size of SiR-An
aggregate was undetectable, indicating that the size of H-
aggregate is smaller than the detection limit of DLS (i.e. 1−3
nm). Concerning the previous studies on rhodamine
aggregates, rhodamine derivatives tend to form antiparallel H-
c)
-5
29, 30
dimer (i.e. directing an opposite side each other) at 10 M.
Particularly, in the case of Si-TMR, a dimethylsilyl group of Si-
TMR may interrupt the parallel stacking of Si-TMR molecules
Fig. 4 (a) Extended views of the crystal structure of SiR-An aggregate. (b)
Geometry of three neighboring SiR-An molecules depicted in a black square in
(Fig. S11). Overall, in the aqueous solution, SiR-An presumably Fig. 4a. (c) Plausible π-π interactions between two neighboring Si-TMR moieties
of SiR-An shown in a blue box. Carbon, silicon, nitrogen atoms, water molecule,
and chloride ion are represented in gray, yellow, blue, red, and green,
respectively.
forms antiparallel H-dimer, whereas SiR-An J-aggregate
(200−800 nm) are formed in the presence of halide ion, and
spontaneously gathered together, leading to the secondary
3
aggregates with a loss of J-aggregate characteristics.
1
Furthermore, SiR-An is found to be cell-permeable and
reside in mitochondria selectively (Fig. 5). This phenomenon is
mostly due to +1 net charge of Si-TMR and appropriate
lipophilicity of SiR-An, which shares the same mechanism with
≈
120 mM), but there was no difference in cell permeability
and mitochondria localization between SiR-An J-aggregate and
Si-DMA H-aggregate. This result strongly implies the possibility
of the dyad system composed of fluorophore and
phenylanthracene moieties that can be exploited in in vivo
1
1
other Si-TMR derivatives. One interesting aspect is that SiR-
-
An must have formed J-aggregate in the culture medium ([Cl ]
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We are grateful to Prof. A. Sugimoto for helpful advice on
chemical synthesis, P. Zhang for SEM images, A. Kuroda for the
preparation of Si-Me, and T. Miyano for the measurement of
X-ray crystallography (all belonged to Osaka University). X-ray
diffraction data were collected at the BL38B1 in the SPring-8
with approval of JASRI (2014A1252). This work has been partly
supported by the Innovative Project for Advanced Instruments,
Renovation Center of Instruments for Science Education and
Technology, Osaka University, and a Grant-in-Aid for Scientific
Research (Projects 25220806, 25288035, and others) from the
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