A room-Temperature phosphorescent polymer film containing a molecular web based on one-dimensional chiral stacking of a simple luminophore

Article abstract of DOI:10.1039/c7cc00395a

We propose a new approach for a totally organic, room-Temperature phosphorescent system with a lifetime on the order of milliseconds, which is achieved by promotion of the one-dimensional chiral stacking of a simple, small fluorophore in a solution state. The versatility of this system is highlighted by its good applicability to the fabrication of a phosphorescent polymer film. This paper demonstrates the first example of a room-Temperature phosphorescent polymer film prepared with our methodology.

Full text of DOI:10.1039/c7cc00395a

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A room-temperature phosphorescent polymer film containing a
molecular web based on one-dimensional chiral stacking of a
simple luminophore†
Received 00th January 20xx,
Accepted 00th January 20xx
Kyohei Yoshida,a Yutaka Kuwahara,a Koji Miyamoto,a Seiya Nakashima,a Hirokuni Jintoku,a Makoto
Takafujiab and Hirotaka Ihara*ab
DOI: 10.1039/x0xx00000x
www.rsc.org/
We propose
a new approach for a totally organic, room-
temperature phosphorescent system with a lifetime on the order
of milliseconds, which is achieved by promotion of the one-
dimensional chiral stacking of a simple, small fluorophore in a
solution state. The versatility of this system is highlighted by its
good applicability to the fabrication of a phosphorescent polymer
film. This paper demonstrates the first example of a room-
temperature phosphorescent polymer film prepared with our
methodology.
Room-temperature phosphorescence (RTP) is an attractive
technique in various research fields, and has been used for
optical applications1, solar cells2, oxygen probes in cells3, and
luminescent sensors for identification of chemical substances4.
Most reported RTP materials are organometallic compounds
that contain rare-metals5, which result in a heavy metal effect,
and complicated organic materials with sterically bulky
substituents. Moreover, there are few solution-based RTP
Fig.
1 Schematic illustration on our conceptual approach for the phosphorescent
molecular web in solution and polymer. In our approach, the molecular web can be
produced through one-dimensional stacking of a benzothiophene derivative as a simple,
planar luminophore for solution and polymer systems. PL: Phosphorescence. The
chemical structures are the benzothiophene derivatives used in this study.
systems.6 Therefore, a new RTP system composed of a simple method has several advantages: (1) no sterically bulky
and purely organic compound in solution is desired. This new substituents are needed in the molecular design of the
type of RTP system would have the distinct advantages of high luminophore, which makes the molecular design more flexible.
applicability in a normal-pressure wet process, easy synthesis, (2) Enhanced phosphorescence intensity is achieved through
easy handling and low cost.
molecular stacking as a result of effective suppression of
This paper proposes a new approach to solution-state RTP that molecular vibration. This is essential for RTP in solution
is applicable to a phosphorescent polymer system. Our because it reduces the consumption of the excitation energy
proposed RTP system is based on one-dimensional chiral through thermal processes.6a (3) The large dipole moment in a
stacking of a simple luminophore in solution and the formation planar luminophore suppresses H-aggregation, which causes
of a phosphorescent molecular web in a polymer (Fig. 1). For luminescence quenching, and induces J-aggregation, which
this purpose, a planar benzothiophene (BT) derivative with a enhances the luminescence.7 (4) Our solution-state RTP system
large dipole moment was selected as
a
candidate supports
a normal-pressure wet fabrication process for
luminophore. The BT derivative was functionalised with a transparent optical materials by promoting one-dimensional
chiral molecular tool (G) to promote three-dimensional aggregation of the luminophore over three-dimensional
network formation of its one-dimensional aggregates. This growth, which causes strong light scattering.
In this paper, we demonstrate a new solution-state RTP
system using a BT derivative functionalised with a tool for
promoting one-dimensional aggregation (G, Fig. 2). We also
describe the fabrication of a functional transparent polymer
film containing a webbed phosphorescent network in its RTP
system.
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Fig. 3 (a) Fluorescence confocal microscopy image of a methylcyclohexane solution of
G-BT (0.15 mM, excitation at 488 nm, observation at 520–600 nm, and modification
with noise reduction). (b) TEM image obtained from the G-BT solution (0.15 mM) after
staining with uranyl acetate. [As complementary results of the confocal microscopy,
luminescence of G-BT in methylcyclohexane was observed under UV and blue light
excitation, as shown for details in Fig. S2 and the experimental section of ESI†.]
observed in chloroform, which is a good solvent for
G-BT. From
these observations, we conclude that G-BT is dispersed in non-
polar solvents such as methylcyclohexane as fibrillar
aggregates with nano-sized diameters. The resultant solution
appears to be transparent because of the low light-scattering
ability of these aggregates.
The fluorescence lifetime (
summarised in Table 1, -free BT exhibited a nanosecond-
= 7.29 ns at 334 nm), which is almost equal to
in BT. These values correspond to those of
in BT can be
attributed to the monomeric dispersion state. In contrast,
reached 1.22 ms for species . A similarly long was observed
in the non-polar solvent cyclohexane. However, when
BT were dispersed at trifluoroacetic acid (TFA) was added to solutions of BT in
non-polar solvents to inhibit molecular aggregation, species
disappeared (Fig. S32) (ESI†) and this was accompanied by a
remarkable decrease in to a value similar to that of the
monomer (Table 1). A similar effect was observed when the
concentration was reduced to 10 M. These results indicate
that the long lifetime is only achieved on aggregation of
In agreement with this hypothesis, the crystalline solid of
τ) was further investigated. As
G
Fig. 2 Luminescence decay curves and fluorescence spectra of G-free BT and G-BT
(0.15 mM) in methylcyclohexane at 298 K. The fluorescence spectra and the
luminescence decay curves were obtained by irradiation at 300 and 280 nm,
respectively. The solid dark green line shows the fitting curve.
order lifetime (
that of species
τ
A
G-
molecular luminophores.8 Therefore, species
A
G-
τ
G
-Free BT (ESI†) and
good solubility in various organic solvents. Clear solutions
were obtained when -free BT and
G-functionalised BT (G-BT) (ESI†) showed
B
τ
G
G-
G-
B
0.15 mM in methanol, ethanol, THF, chloroformtoluene, and
methylcyclohexane. However, the luminescent colours of the
τ
G
-free BT and G-BT solutions in methylcyclohexane at room
temperature (298 K) were completely different from each
other when they were excited at 300 nm (Fig. 2). This
wavelength corresponds to the maximum of the absorption
band of a benzothiophene moiety.
ꢀ
G-
BT
.
G
-
free BT obtained by recrystallisation from toluene solution
τ =
The difference in the luminescence colours can be clearly
understood from the fluorescence spectra of the two
exhibited a large Stokes shift of 215 nm and a long lifetime (
1.38 ms at 515 nm), as shown in Fig. S43 (ESI†).
compounds, shown in Fig. 2. The spectrum of
two peaks at 330 (species ) and 537 nm (species
that of -free BT contains a single peak at 334 nm (species A′).
The Stokes shift was as large as 237 nm in species . In
addition, we found that the longest-lived component of the
fluorescence lifetime ( was of the millisecond order ( = 1.22
ms) for species , but remained on the order of nanoseconds
= 7.12 ns) for species (Fig. S1) (ESI†). (The decay curves in
G
-
BT exhibits
Table 1. Solvent and concentration dependence of τ in G-free BT and G-BT at 297 K.
A
B), whereas
Concentration
(mM)
τ b at peak
wavelength
G
Solvent
Luminophore
Name
εa
B
7.72 ns
(330 nm) (540 nm)
7.12 ns 1.22 ms
1.27 ms
Cyclohexane 2.02
0.15
0.15
0.15c
0.01
0.15
0.15
τ)
τ
B
(330 nm) (537 nm)
(τ
A
Methyl-
2.07
< 1 ns
none
Figs. 2 and S1 include decays based on the excitation light-
G
-BT
cyclohexane
(372 nm)
scattering.) To understand this considerable difference, the
7.14 ns
none
dispersion state of
The aggregation morphology was investigated. The confocal
microscopy image (Fig. 3a) shows that BT was dispersed as
fluorescent fibrils with lengths of more than 10 m. The
G-BT was further investigated.
(330 nm)
1.25 ns
none
G
-
Chloroform
4.80
2.07
(342 nm)
ꢀ
Methyl-
7.29 ns
none
G-free BT
diameters (thicknesses) of the fibrils were determined using
transmission electron microscopy (TEM), and were a minimum
of 10 nm, which corresponds to several times larger than the
cyclohexane
(334 nm)
a Dielectric constant. b The measurement was carried out by excitation at 280 nm.
The peak-top wavelength in the observed luminescent band is noted in a bracket.
c Trifluoroacetic acid (0.05 v/v%) was added to the methylcyclohexane solution.
molecular length of
G-BT. In contrast, no aggregates were
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Fig. 5 Luminescence decay curves at 525 nm of a transparent poly(ethylene-co-vinyl
acetate) (EVA) film containing 0.64 wt% G-BT prepared by a casting method on a
quartz glass. The film thickness was 125 ꢀm. The dark and light green lines indicate
the observed and fitted decay curves, respectively. Photographs (a) and (b) are of the
G-BT-containing EVA film under normal and UV light, respectively. The confocal
microscopic image (c) was obtained with excitation at 488 nm and observation at 520–
600 nm.
around 330 nm increased but also the phosphorescence newly
appeared around 537 nm at 298 K (Fig. 4b) and a slight red
shift was observed (Fig. 4c), whereas H-aggregation promote
only fluorescence quenching. These observations allow us to
conclude that the G-moiety plays an essential role in the
phosphorescence-promoting aggregation of BT moieties in a
non-polar solvent system.
In this paper, we also demonstrate the first example of the
incorporation of our RTP system into a polymer film, which
would expand its potential applicability in optical applications.
For this purpose, a composite film was prepared by simple
mixing of
component) in
G
-
BT (0.64 wt% with respect to the polymer
poly(ethylene-co-vinyl acetate) (EVA)-
a
containing methylcyclohexane solution, followed by drop-
casting on a quartz plate. The obtained film had a thickness of
Fig. 4 Temperature dependence of (a) CD, (b) fluorescence, and (c) UV-visible spectra
of G-BT solutions. Concentration, 0.15 mM; solvent, methylcyclohexane. The red and
black lines were obtained at 298 and 363 K, respectively.
125
To investigate the optical properties and aggregation
morphology of BT in the EVA film, fluorescence spectroscopy,
ꢀm and high transparency as shown in Fig. 5a.
UV-visible and circular dichroism (CD) spectra were measured
to obtain further information on the molecular orientation of
G
-
fluorescence lifetime, and confocal microscopy experiments
were conducted. We observed two luminescence peaks at 331
and 525 nm (Fig. S54) (ESI†), which are similar to those in the
solution system. In addition, these experiments confirmed that
the luminescence of the peak at 525 nm was phosphorescence
with a lifetime of 1.28 ms (Fig. 5). Furthermore, we observed
luminescent aggregates in the polymer system using confocal
microscopy. The observed aggregation morphology was similar
to that in the solution system. These results show that the new
RTP system cannot only be realised in solution, but that it is
also applicable for wet processing.
G
-BT. As shown in Fig. 4a, the CD spectrum at 298 K exhibited a
strong Cotton effect with positive signal around the
a
absorption band of the BT moiety. Because the BT moiety does
not contain a chiral centre and almost no CD signal was
detected when the solution was heated to 363 K to promote
the aggregate-to-monomer phase transition, it is likely that the
CD signal observed at 298 K is due to the induction of chirality
in the G-BT aggregates.
The CD pattern provides us with considerable information on
the chiral orientation of the BT moiety. Only a positive Cotton
effect was observed, which indicates R-chirality, but this effect
is completely different from that observed in Davydov
splitting.9 Therefore, the chiral orientation around the BT
moiety is not based on H-like aggregation.10,11 As support for
this assumption, in our system, the fluorescence intensity
In conclusion, we have established a new solution RTP system
through one-dimensional aggregation of
a
simple
benzothiophene derivative functionalised with a chiral self-
assembling tool. We have also demonstrated the first example
of a polymer film exhibiting RTP. These achievements were
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Jintoku, T. Sagawa, M. Takafujia and H. Ihara, Org. Biomol.
Chem., 2009, , 2430.
11 Y. Kira, Y. Okazaki, T. Sawada, M. Takafuji and H. Ihara,
brought about by the ability of the G-derivative to assist in the
formation of nano-fibrillar aggregates of the luminophores
with chiral stacking, in both solution and polymer systems. Our
new RTP system has the potential for use in various
transparent optical applications.
7
Amino Acids, 2010, 39, 587.
Acknowledgements
We are grateful to Dr. Kenji Kato, who helped us in this study
as local collaboration coordinator of the PHOENICS project
supported by the Ministry of Education, Culture, Sports,
Science, and Technology of Japan (MEXT), although he is no
longer of this world. This work was supported by a Grant-in-
Aid for Scientific Research from MEXT, and a Grant-in-Aid from
the Strategic International Collaborative Research Program
between the Japan Science and Technology Agency (JST) in
Japan and the Agence Nationale de la Recherche (ANR) in
France.
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