Europium amphiphilic naphthalene based complex for the enhancement of linearly polarized luminescence in Langmuir-Blodgett films

Article abstract of DOI:10.1039/c8nj03976c

Linearly polarized luminescence (LPL) in films of europium ions was induced by using naphthoic acid adopted alkyl chain (NaphC15), which plays the roles of both surfactant and photo-antenna. The synthesis of NaphC15 was newly developed, and the Eu complex composed in a 1:3 ratio of Eu³⁺ ion and NaphC15 was synthesized in the powder form for comparison. The structural aspects of LB films with a layered scheme were obtained from the measurements of synchrotron XRPD, X-ray photoelectron spectroscopy and waveguide polarized absorption spectra. Luminescence of europium in the film is more intense than that in the powdered complex. The luminescence efficiency calculated from the observed luminescence lifetimes and quantum yields also supported their structure-enhanced luminescence. Finally, we revealed that the LPL was sensitized by the electronic dipole moment localized on the NaphC15 moieties aligned independently in the Langmuir-Blodgett films.

Full text of DOI:10.1039/c8nj03976c

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Europium amphiphilic naphthalene based
complex for the enhancement of linearly
Cite this:DOI: 10.1039/c8nj03976c
polarized luminescence in Langmuir–Blodgett
films†‡
bc
Koushi Yoshihara,a Masamichi Yamanaka,
*
Shuhei Kanno,a Soichi Mizushima,a
Junko Tsuchiyagaito,a Kazuki Kondo,a Takahiro Kondo,a Daichi Iwasawa,a
Hiroaki Komiya,a Akira Saso,a Shogo Kawaguchi,
Shuhei Ogata,ac Hiromi Takahashi,f Ayumi Ishii
Kenta Goto,
d
e
acg
ac
*
and Miki Hasegawa
*
Linearly polarized luminescence (LPL) in films of europium ions was induced by using naphthoic acid
adopted alkyl chain (NaphC15), which plays the roles of both surfactant and photo-antenna. The
synthesis of NaphC15 was newly developed, and the Eu complex composed in a 1 : 3 ratio of Eu3+ ion
and NaphC15 was synthesized in the powder form for comparison. The structural aspects of LB films with
a layered scheme were obtained from the measurements of synchrotron XRPD, X-ray photoelectron
spectroscopy and waveguide polarized absorption spectra. Luminescence of europium in the film is more
intense than that in the powdered complex. The luminescence efficiency calculated from the observed
luminescence lifetimes and quantum yields also supported their structure-enhanced luminescence. Finally,
we revealed that the LPL was sensitized by the electronic dipole moment localized on the NaphC15
moieties aligned independently in the Langmuir–Blodgett films.
Received 6th August 2018,
Accepted 12th December 2018
DOI: 10.1039/c8nj03976c
rsc.li/njc
sensitized by photo-excitation of the p-electronic skeletons of
Introduction
the ligands through intramolecular energy transfer,3 since the
Polarized optical phenomena of europium (Eu) complexes have ff transitions of the trivalent lanthanide ions are Laporte
attracted continuous interest in the past few decades not only forbidden. Methods and molecular designs to improve lumi-
as a scientific curiosity,1 but also for the applications in security nescence intensities have been extensively researched.4
inks and display materials.2 The luminescence assigned to the
Recently, the reports of circularly and linearly polarized
4f–4f (abbreviated as ff) transitions localized on the Eu3+ ions is luminescence (abbreviated as CPL and LPL, respectively) for
lanthanide complexes increase, especially. CPL phenomena of
a College of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe,
Eu complexes have been frequently reported with ligands. The
principle to induce polarization phenomena in Eu complexes is
still unknown because it is generated after the energy transfer,
and the electron cloud of the Eu3+ ion should be spherical with
no polarization.
Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan.
E-mail: haesmiki@chem.aoyama.ac.jp; Fax: +81-42-759-6221
b Department of Chemistry, Faculty of Science, Shizuoka University, 836 Ohya,
Suruga-ku, Shizuoka 422-8529, Japan.
E-mail: yamanaka.masamichi@shizuoka.ac.jp
c Mirai Molecular Material Design Institute, Aoyama Gakuin University,
5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan
d Research & Utilization Division, Japan Synchrotron Radiation Research Institute
(JASRI/SPring-8), 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan
e Institute for Materials Chemistry and Engineering, Kyushu University,
744 Moto-oka, Fukuoka 819-0395, Japan
f System Instruments, Co. Ltd., 776-2 Komiya, Hachioji, Tokyo 192-0031, Japan
g JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan
† Dedicated to Professor Emeritus Dr Isao Ono on the occasion of his 77th
birthday.
‡ Electronic supplementary information (ESI) available: 1H NMR, attenuated total
reflection FT-IR, electronic absorption, excitation and luminescence spectra with
their decay profiles, synchrotron XRD patterns, XPS and thermal analyses. See
DOI: 10.1039/c8nj03976c
There are few papers on Eu complexes that generate LPL. For
instance, Eu complexes exist in liquid crystals5 or polymer
matrixes.6–8 Heegar et al. reported a LPL polymer of EuIII by
using polymethacryl acid, and suggested that the LPL of EuIII
induced the direction of the electronic dipole moment localized
on the organic photo-antenna.6 However, explanation from the
experimental evidences so far is insufficient. One of the keys to
enhance the LPL of a molecular system is the film formation in
polymer films or in self-assembly systems. Actually, the differ-
ences in the molecular orientations between the powdered and
the liquid crystal of Eu complexes enhance their polarized
photophenomena. There are only a few reports demonstrating
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Fig. 1 Schematic representation of the design of naphthoic acid deriva-
tive (abbreviated as NaphC15) with alkyl chains (top), schematic images of
this work. Ln, L and LB are lanthanide, Langmuir and Langmuir–Blodgett,
respectively. The present LB films form a Y-type film on the hydrophobic
substrate. d shows the interlayer distance of aligned Eu3+ ions in the Y-type
films.
Scheme 1 Synthesis of NaphC15. (a) Br2, CH3COOH, D, 64%; (b) CH3OH,
H2SO4, D, 73%; (c) 1-pentadecyne, Pd(PPh3)2Cl2, CuI, PPh3, Et3N, THF,
D, 40%; (d) H2, Pd/C, EtOAc, rt, 33%; (e) KOH, H2O, THF, CH3OH, rt, then
HCl, 72%.
the polarized luminescence of Eu3+ ions in the Langmuir–
Blodgett (LB) films.9 Furthermore, no studies were reported
that experimentally elucidated the principle of LPL of Eu com-
plexes with respect to molecular aggregation or intramolecular
energy transfer.
Scheme 2 Synthesis of Eu(NaphC15)3.
Herein, we examined a new molecular aggregation system to
induce bright LPL in Eu complexes in the LB films made with
an amphiphilic 1-naphthoic acid derivative (NaphC15; Fig. 1), and
we elucidated a possible explanation for the LPL. Naphthalene
and its derivatives have a high absorption coefficient,10 and the
polarized fluorescence of naphthalene derivatives was reported in
an imaging procedure of biomaterials, since their transition
dipole moment is high and their molecular rotational relaxa-
tion time on the protein is useful.11 Additionally, the excitation
state of the naphthalene moiety acts as an efficient energy
donor to the Eu3+ ion.12
The formation condition and structure of LB films
LB films of NaphC15 were obtained by the deposition from the
surface of neat water or an aqueous solution. Especially, since
the Ln complex, Ln(NaphC15)3, is not soluble in the solution,
the Eu/Gd-containing LB films of NaphC15 were deposited
from the surface of europium nitrate–water on the quartz sub-
strate. The isotherm curve of a liquid film of NaphC15 is shown
in Fig. 2, and has the characteristic shape for monolayer forma-
tion. The 10-layered LB films of NaphC15 were deposited on a
hydrophobic quartz substrate15 from the interface of a 1.0 ꢀ
10ꢁ5 M aqueous solution of europium nitrate at surface pres-
sures of 30, 35 and 40 mN mꢁ1 (abbreviated as LB30, LB35 and
LB40, respectively).
An X-ray photoelectron spectroscopy (XPS) of LB30, and the
existence of Eu3+ ions in the films were confirmed by Eu 3d XPS
bands (Fig. 3). The nitrate ion of europium nitrate was replaced
with the anionic 1-naphthyl ligand, which was determined by
the disappearance of the bands after complexation in the film.
Similar behaviour was shown by Eu 3d and N 1s bands in both
LB35 and LB40 (Fig. S4, ESI‡), but the intensity of the Eu 3d XPS
Results and discussion
Syntheses of the surfactant ligand, NaphC15, and its Eu
complex
NaphC15 was synthesized in five steps from commercial
1-naphthoic acid (1) (Scheme 1 and Fig. S1, ESI‡). Bromination
of 1 gave 5-bromo-1-naphthoic acid (2).13 The carboxyl group of
2 was converted into a methyl ester to afford 3.14 Sonogashira
coupling reaction between 3 and 1-pentadecyne afforded the
alkynylated product 4. Hydrogenation and subsequent hydroxyl-
ation of 5 yielded the desired NaphC15 as a white solid.
For the comparison of luminescence and structural pro-
perties between the film and the powder, the Eu complex with
NaphC15, abbreviated as Eu(NaphC15)3, was also synthesized
(Scheme 2 and Fig. S2, ESI‡). Eu(NaphC15)3 was characterized
as a 1 : 3 complex, which was reasonable because the same com-
ponent ratios were reported in a Eu complex with 1-naphthoic
acid.10e The thermal analyses also supported that Eu(NaphC15)3
has one coordinated water molecule and one crystalline water
molecule in the microcrystals (Fig. S3, ESI‡).
Fig. 2 Isothermal curve of NaphC15.
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Fig. 5 X-ray diffractions recorded on imaging plates for LB30 (left) and
LB35 (right).
Fig. 3 X-ray photoelectron spectra of Eu 3d (left) and N 1s (right) band in
Eu30. Binding energies were corrected by the Au 4f band (84.0 eV).
The peak position in LB30 was slightly shifted with an
increase in the surface pressure, and appeared at high a angles
according to the imaging plate-recording of all the diffractions
(Fig. 5). This suggests that the layered species in the film
aggregated randomly, when high surface pressure was applied
during the film formation. In fact, each peak in LB40 was split
into two, indicating existence of several molecular orientations.
Thus, homogenization at a surface pressure of 440 mN mꢁ1
must be achieved in order to eliminate the polarized lumines-
cence phenomenon. XRD patterns of the LB films using Gd are
similar to the above results.
band of LB40 was relatively broad compared with those of LB30
and LB35. It suggests that the complexation between NaphC15
and Eu in LB40 differed from those of the other LB films.
The synchrotron X-ray diffraction (XRD) patterns sharply
observed by using an attachment for small angles sharply
appeared LB30, LB35 and LB40 (Fig. 4, and Fig. S5, ESI‡)16
meaning the high ordering to an out-of-plane periodicity.
Furthermore, their patterns in LB films completely differed
from that in the powder-state Eu(NaphC15)3. It indicates that
Eu(NaphC15)3 LB films form a layered structure. The interlayer
distance d in LB30 was estimated to be ca. 49.8 Å (2y = 1.151),
as a bilayer formation, using Bragg’s equation. This distance is
shorter than the theoretical value of 52 Å, when the length of
NaphC15 was used as 26 Å with a perpendicular alignment.
It indicates the herring-born structure in the films will form as
our initial assumption in Fig. 1.
The temperature dependence of the synchrotron XRPD for
NaphC15 and Eu(NaphC15)3 in powder states was also observed
and showed no difference (Fig. S5(d), ESI‡) resulting of stable
molecular orientation.
Polarized electronic absorption and luminescence spectra
localized on NaphC15 in the LB films
The nature of the electronic structure of the ligand moiety in
the lanthanide complexes and the LB films supports to under-
stand the enhancement of the ff transitions through excitation
energy transfer from the ligand. NaphC15 in chloroform shows
the absorption bands at 317 and 327 (sh) nm, which correspond
to the 308 and 325 (sh) nm bands of naphthalene itself (Fig. S6,
ESI‡). It means that the electronic bands of the naphthalene
skeleton after the derivatization of the alkyl chain shift to a
longer-wavelength.
Linearly polarized absorption spectra localized on the ligand
in the mainly contributing MO’s for its first pp* electronic transi-
tion are shown in Fig. 6. The tilt angles of the naphthalene
skeletons in the film can be discussed by using polarized
absorption spectroscopy with the optical wave guide method
(see ESI‡).17 A bilayer of LB30 deposited on a waveguide quartz
plate exhibited a band at 315 nm, which was assigned to the
first 1pp* transition. This band was calculated at 324.7 nm
with an oscillator strength, f = 0.1413, which contributed the
HOMO–LUMO transition of 49% from the excited wavefunction,
C1 = 0.69j65,66 +. . .. The transition dipole moment m was also
calculated and is shown in Fig. 6b.
The optical density Ds is much higher than Dp at the first
1pp* transition position. The peak top in Ds is shifted more
towards the red than that of Dp. These anisotropic results indi-
cate that the naphthoic acid moieties are aligned in the films.
The tilt angles j of the naphthoic acid moieties in the LB30 will
be larger than that of LB35, since the Dp value of LB30 is lower
Fig. 4 Synchrotron XRD patterns of 10-layers in Eu-NaphC15 LB films (red
lines) formed under surface pressures of (a) 30, (b) 35 and (c) 40 mN mꢁ1
.
The XRD patterns were recorded by using imaging plates.18 Yellow and
black lines in (a) are the powder diffraction patterns of Eu(NaphC15)3 and its
ligand NaphC15, respectively. Exposure time was 600 s, source light power
was l = 1.000050 Å for powders and l = 0.999894 Å for films.
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Fig. 8 Luminescence and excitation spectra of Eu(NaphC15)3 in the
powder state. lex = 310 nm and lmon = 612 nm.
Eu3+ ion in the complex appear at 579, 593, 612.5, 649 and
Fig. 6 (a) Selected MO’s of 1-naphthoic acid calculated by TD-DFT.
(b) The transition dipole moment of the first singlet pp* transition. (c) Polarized
optical waveguide absorption spectrum of Eu-NaphC15 LB films deposited at
30 mN mꢁ1. Dp and Ds show the primary and secondary polarized light,
respectively. (d) Schematic representation of tilt angle j for the determination
by the polarized absorption spectrum (see ESI and Fig. S9‡).
7
7
7
698.3 nm assigned to the 5D0 - F0, 5D0 - F1, 5D0 - F2,
7
5
7
5D0 - F3 and D0 - F4 transitions, respectively. The excita-
tion spectrum monitored at 612.5 nm has a broad band around
340 nm and a sharp one at 393.1 nm. The former one corre-
sponds to the pp* electronic absorption band localized on the
naphthalene skeleton, and the latter one is the ff absorption
band of Eu3+ ion. The corresponding bands appear mostly at
the same position in LB30, LB35 and LB40 as shown in Fig. S7
(ESI‡). Thus, the ligand acts as an energy donor to enhance
the ff emission of Eu through the co-ordination bond in LB
films. Incidentally, there is no information from the spectra of
Eu(NaphC15)3 solution due to its low solubility.
The fluorescence and phosphorescence bands localized on
NaphC15 in the Gd-NaphC15 LB film were observed 385 and
510 nm, respectively, but the phosphorescence band disappeared
in the solid state Gd complex. It means that the film formation is
suitable to maintain the stability of the excited triplet state of the
ligand moiety. The phosphorescence band position and its broad
shape are effective to be an energy donor to Eu, and to induce the
ff emission in Eu (Fig. S10, ESI‡).
Fig. 7 Linearly polarized fluorescence spectra (lex = 310 nm) localized on
naphthalene moiety by using the 10 layer-LB film of NaphC15 deposited at
20 mN mꢁ1. The angles mean the rotation angles of the polarizer in front
of the detector.
The LPL of the Eu-containing LB film with NaphC15 was
successfully observed as shown in Fig. 9. The intensities were
than that of LB35 (Fig. S9, ESI‡). The film obtained at 40 mN mꢁ1 normalized at magnetic dipole transition, 5D0 - 7F1. The lumi-
5
7
showed no intensity differences between its Dp and Ds due to the nescence intensity of the D0 - F2 transition assigned to the
random molecular assembling. Thus, the surface pressure can be electric dipole transition in LB30 or LB35 drastically changed
tuned to refine the production of LPL devices by LB techniques. with different angles of the polarizer placed in front of the
It supports the results obtained from the synchrotron XRD analysis. detector. Neither LB40 showed any LPL behaviour nor did the
The surface pressure at 40 mN mꢁ1 is too high for the film complex in the solution as well as the powder of Eu(NaphC15)3.
formation and will collapse the monolayer at the water surface. It indicates that the structure as an assembly of LB40 is much
1
The linearly polarized fluorescence (the pp* transition) of more random than those of LB30 and LB35. This is supported
NaphC15 were observed in the LB film (Fig. 7). The fluores- by the XRD results as described above. The similarities of LB30
cence intensities of the LB film decreased with increasing the and LB35 in such LPL and the abovementioned photophysical
angle of the polarizer direction relative to the LB film in asso- properties are enhanced with the periodical stacking of the
ciation with the band shifting from 390 to 375 nm. This molecular layers compared to those in LB40.
indicates that the p-electronic system of NaphC15 will act as
It is worthwhile to note that the luminescence quantum
an energy donor with the refined transition dipole moment for yield jobs at the ff emission in LB30 is 5.8%, which is much
the ff luminescence after the complexation with Eu3+ ion.
greater than that in the powder (0.1%) (Table 1). The lumines-
cence decay profiles for Eu(NaphC15)3, LB30, LB35 and LB40
are shown in Fig. S8 (ESI‡). The luminescence lifetimes of the
Eu-NaphC15 LB films are also higher than that of the powder.
The ff-luminescence spectra of Eu complexes with NaphC15
and their polarized phenomena in LB films
Luminescence and its excitation spectra of Eu(NaphC15)3 in The radiative rate constants kR and efficiencies of energy transfer
the solid state are shown in Fig. 8. The luminescence bands of EnT were calculated. These values in the LB films correspond to
Z
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Experimental
Syntheses and characterization of NaphC1514,15 and the
Eu complex
5-Bromo-1-naphthoic acid (2). 1-Naphthoic acid (9.65 g
56.0 mmol; TCI Co. Ltd) and acetic acid (40 mL; TCI) were
placed in a three-necked flask. This mixture was heated at
110 1C and stirred during the addition of bromine (3.20 mL,
61 mmol; TCI), and then it was refluxed for 2 hours. A yellow
solid precipitated during heating, which was then stirred at
room temperature overnight. The mixture was poured into ice
water. The solid was filtered, washed with water, and crystallized
from acetic acid (250 mL; Kanto Chemical Co. Ltd) to afford
9.05 g (64%) of bromo acid as a white solid. 1H-NMR (CD3OD):
d 7.44 (dd, 1H), 7.65 (dd, 1H), 7.86 (dd, 1H), 8.23 (d, 1H), 8.47 (d, 1H)
and 8.90 (d, 1H).
Methyl 5-bromo-1-naphthoate (3). To a suspension of
5-bromo-1-naphthoic acid (9.05 g, 36 mmol) in anhydrous
methanol (360 mL) concentrated H2SO4 (9 mL) was added,
and the resulting mixture was refluxed for 6 hours. The mixture
was cooled and concentrated in vacuo to one-third the volume.
The organic phase was washed with water, dried (MgSO4), and
concentrated in vacuo. Silica gel column chromatography using
hexane/EtOAc (2 : 1) afforded 7.07 g (73%) of methyl 5-bromo-1-
naphthoate. 1H-NMR (CDCl3): d 4.01 (s, 3H), 7.43 (dd, 1H), 7.62
(dd, 1H), 7.83 (dd, 1H), 8.20 (dd, 1H), 8.49 (d, 1H), 8.87 (d, 1H).
Methyl 5-(pentadec-1-yn-1-yl)-1-naphthoate (4). Methyl 5-bromo-
1-naphthoate (1.00 g, 3.81 mmol), 1-pentadecyne (0.79 g,
3.81 mmol), bis(triphenylphosphane)palladium(II) dichloride
Fig. 9 Luminescence spectra of ff emission (red line) of Eu3+ ions in LB30
(a: lex = 310 nm). The excitation spectrum monitored at 615 nm are shown
in dotted line. Fluorescence (dashed line) and phosphorescence (dotted-
dash line; at 77 K) localized on p-electronic systems of the NaphC15
LB film with Gd ion. (b) Setup for the linearly polarized luminescence
(LPL) spectral measurement. LPL of the Eu-NaphC15 LB film deposited
at various surface pressures (c: 30; d: 35; and e: 40 mN mꢁ1).8 Inset
in (c) is a photograph of luminescence of LB30 under UV light (scale
bar: 20 mm).
Table 1 Photophysical data of the luminescence lifetimes tffa (ms; Fig. S8,
b
ESI), quantum yields jobs
,
radiative rate constant kR (sꢁ1), non-radiative
rate constant kNR (sꢁ1), net quantum yield jLn and energy transfer effi- (133 mg, 0.19 mmol), copper(I) iodide (33 mg, 0.19 mmol),
ciencies ZEnT of the Eu-NaphC15 LB films and the powder Eu(NaphC15)3
and triphenylphosphane (99 mg, 0.38 mmol) were placed in a
two-necked flask. After three successive vacuum/argon cycles,
THF (20 mL) and triethylamine (20 mL) were introduced, both
by syringe. The reaction mixture was stirred at 66 1C. The crude
tff1 tff2 tff3 jobs (%) kR kNR ꢀ 103 jLn (%) ZEnT (%)
Powder 0.07 0.03
62% 38%
—
0.1
450 15.8
490 2.5
410 1.2
440 1.7
2.7
16
25
20
33
49
19
30
product was concentrated onto silica. Purification by HPLC
LB30
LB35
LB40
0.26 0.08 0.56 5.8
48% 39% 13%
0.79 0.29 0.09 5.0
38% 37% 25%
0.29 0.65 0.07 6.2
49% 29% 22%
1
yielded the product as a white solid (409.9 mg, 40%). H NMR
(400 MHz, CDCl3): d = 8.84 (d, 1H), 8.60 (d, 1H), 8.19 (d, 1H),
7.66 (d, 1H), 7.49–7.57 (m, 2H), 4.01 (s, 3H), 2.56 (t, 2H), 1.71
(quint, 2H), 1.53 (quint, 2H), 1.39–1.25 (m, 18H), 0.88 (t, 3H).
Methyl 5-pentadecyl-1-naphthoate (5). To a solution of methyl
5-(pentadec-1-yn-1-yl)-1-naphthoate (409.2 mg, 1.04 mmol) in
ethyl acetate (60 mL), 10% palladium on activated carbon (42 mg,
10 wt%) was added. The mixture was stirred well under an atmo-
spheric pressure of hydrogen for 2 days. The catalyst was removed
by filtration. Purification by HPLC yielded the product as a
white solid (137 mg, 33%).
5-Pentadecyl-1-naphthoic acid (NaphC15) (5). 66.9 mg of
ester and 2.8 mL of THF : methanol = 1 : 1 were placed in a
round-bottom flask mounted over a magnetic stirrer. 0.56 mL
6 N KOH aqueous solution was added and the contents were
stirred for 72 hours. The aqueous solution was acidified to
pH = 2 with hydrochloric acid. The product precipitated on
standing, which was washed with water, filtered, and dried under
a
b
lex = 340 nm, lmon = 615 nm. lex = 310 nm, lmon = 550–720 nm.
those in the powder of Eu(NaphC15)3, while the non-radiative
rate constants kNR of the LB films drastically decreased com-
pared to that of the powder. The FT-IR, elemental and thermal
analyses of the Eu(NaphC15)3 powder show the existence of
coordinating water molecule in the powdered Eu(NaphC15)3
complex. This means that the differences of intermolecular
interaction between the powder and films affect the lumines-
cence ability of Eu with the enhancement of coordinating water
molecules resulting in the low kNR value in the films.
That is, the film formation of NaphC15 by the LB method
containing Eu3+ ions is suitable to obtain LPL of the ff emissions
according to the independently existing NaphC15 in the film, vacuum.
and to enhance the excitation energy transfer from NaphC15 to
Eu without any water interactions.
Eu(NaphC15)3. Eu(NO3)3ꢂ6H2O (45.4 mg, 102 mM) in ethanol
(7 mL) was slowly added to a solution of Naph-C15 (38 mg, 99.3 mM)
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in triethylamine (0.07 mL). After stirring for 24 h, the white spectrometer (Hamamatsu Photonics K. K.) and a Quantaurus-
precipitate was filtered and washed with ethanol (50.1 mg, 94.4%). Tau C11367-12 spectrometer (Hamamatsu Photonics K. K.),
Eu(NaphC15)3 was characterized as a 1 : 3 complex with a water respectively, with pulsed excitation light sources.
molecule. Anal. calcd for C78H113O7Eu as Eu(NaphC15)3ꢂH2O:
Time-dependent (TD) DFT calculation
C 71.3%, H 8.7%; found: C 71.9%, H 8.6%; yield 494%. The
existence of a coordination- and a crystalline-water molecule were
also found around 220 and 74 1C, respectively, as endothermic
peaks in the thermal gravimetric analyses (Fig. S3, ESI‡).
The singlet ground state of NaphC15 was optimized using DFT
with three-parameterized Becke–Lee–Yang–Parr (restricted B3LYP)
hybrid exchange–correlation functional, combined with the
6-31G(d,p) basis set for the non-metal atoms. The optimization
was performed using a single crystal X-ray structure of
1-naphthoic acid19 as a starting geometry. UV-vis spectrum was
studied using the TD-DFT method on the basis of the optimized
ground states with the same functional and basis set.
Deposition of LB films
A Milli-Q water purification system (Millipore Co.) was used to
purify water for the subphase. The quartz substrates (Eiko Co.) were
coated with octadecyl-trichlorsilan, 95% (ACROS ORGANICS Co.) to
provide the surface hydrophobicity. A mixed chloroform solu-
tion containing 1 : 7 molar ratios of NaphC15 and stearic acid
(abbreviated to SA) were used to form the monolayers on the
water/air interface.
Estimation of photophysical constants
The total emission quantum yield of Eu3+ sensitized by the
ligand (jobs) was determined by the triplet yield of the ligand
(jISC), the efficiency of energy transfer (ZEnT) and the efficiency
of the metal centred luminescence (jLn), as follows.20
Owing to the Ln containing deposition of the films, the
above solution of NaphC15 was spread on the water subphase
containing Ln(NO3)3ꢂ6H2O (1.0 ꢀ 10ꢁ5 M; Ln = Eu or Gd) in a
Langmuir trough (Filgen, Inc.) at 293 K. The monolayers were
transferred onto the hydrophobic quartz substrate16 as a Y-type
model (10 layers) at a surface pressure of 30, 35 or 40 mN mꢁ1 by the
jobs = jISC ꢀ ZEnT ꢀ jLn
(1)
Owing to the np* character of the ligand and the high spin–
orbit coupling constants of the lanthanide ion, it can be
assumed that jISC was approximately 1.21,22 The value of jLn
can be calculated from the observed emission lifetime (tff) and
the radiative rate constant (kR) of the lanthanide ion, as shown
below.
vertical depositing method with a transfer rate of 10 mm minꢁ1
.
At the same condition, the single monolayer was deposited on a
0.2 mm-thickness quartz plate for the ATR absorption spectral
measurement.
Apparatus
ZLn = kR ꢀ tff
(2)
1H-NMR spectra were recorded on a JEOL JMN-500II spectro-
meter in CDCl3 with tetramethylsilane. ATR FT-IR spectra were
measured using a Nicolet iS5 spectrometer (Thermo Scientific).
XPS measurements were performed on a KRATOS AXIS ULTRA
DLD (Shimadzu) equipped with a monochromatic Al-Ka X-ray
source (1253.6 eV); the binding energies were calibrated at the
Au 4f level (84.0 eV).
Synchrotron XRPD patterns were recorded by a large Debye–
Scherrer camera installed at SPring-8 BL02B2 (JASRI/SPring-8)
using an imaging plate as a detector.18a Powder samples were
prepared in capillaries with 0.3 mm radius and observed in an
automatically recording system.18b
The kR value of the emissive excited state, 5D0, is the sum of
7
the spontaneous emission probabilities, A(0,J), to the lower FJ
levels in Eu3+, and can in turn be calculated from the following
equation.
X
ITotal
kR
¼
A
¼ Að0;JÞ ꢀ
ꢀ n3
(3)
ð0;JÞ
I
ð0;1Þ
J
Here, ITotal/I(0,1) is the ratio of the total integrated intensity of
the corrected EuIII emission spectrum to the intensity of the
5D0 - 7F1 band. In this case, we obtained values of 9.0, 10, 8.3,
and 8.9 for ITotal/I(0,1) of powder, LB30, LB35, and LB40, respec-
tively. The spontaneous emission probability of the magnetic
Fast atom bombardment mass spectrometry (FAB-MS) was
performed using the MStation JMS-700A (JEOL). Thermal
analysis and differential scanning calorimetry were performed
with DTA50 and DSC60 (Shimadzu), respectively.
dipole 5D0
-
7F1 transition, A(0,1), is virtually independent
of the ligand field or the environment of the ions (14.65 sꢁ1
in vacuo).23
Electronic absorption spectra in the solid state and in the
solution were obtained using the diffused reflection method
with conversion of the y-axis by UV-3100 and UV-3600S
Conclusions
(Shimadzu), respectively. Polarized electronic absorption spectra The valuable ligand, NaphC15, was synthesized to induce the
using optical waveguide system was obtained using a modified LPL of the ff transition in Eu3+ ions in the LB films. The lumi-
SIS-5100 attached with a Glan–Taylor polarizer prism (System nescence of the Eu3+ ions incorporated into the films was
Instruments Co.).
induced through the excited triplet state of the coordinating
Luminescence and excitation spectra were recorded on a NaphC15, and was more intense than that in the powder. The
Fluorolog 3-22 (Horiba Jobin Yvon). Absolute luminescence conformation and transition dipole moment of the naphthalene
quantum yields and luminescence lifetimes were determined skeleton both play a role in inducing LPL in the Eu3+ ion through
using an absolute luminescence quantum yield C9920-02 energy transfer. Additionally, in our previous report8,9b we did not
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Paper
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Author contributions
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M. Hasegawa, M. Yamanaka, A. Ishii and K. Goto conceived and
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film formation, and most spectral and structural observations.
S. Ogata, S. Mizushima, S. Kanno, J. Tsuchiyagaito, K. Kondo,
T. Kondo, D. Iwasawa, H. Komiya and A. Saso, S. Kawaguchi
contributed to optimize the experiments at SPring-8. H. Takahashi
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Conflicts of interest
There are no conflicts to declare.
7 V. I. Srdanov, M. R. Robinson, M. H. Bartl, X. Bu and
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Acknowledgements
The authors acknowledge Prof. Mir Wais Hosseini (University of
Strasbourg) for helpful discussions. Professors Makoto Handa
and Michiko Egawa (Shimane University) also gave important
discussion. This work was partly supported by Grants-in-Aid for
Scientific Research on Innovative Areas of ‘‘Soft Crystals (Area
Number: 2903)’’ (No. 17H06374; MH and MY), MEXT Supported
Program for the Strategic Research Foundation at Private
Universities (2013-2017; MH), Network Joint Research Centre
for Materials and Devices (2018; MH) and a SOKEN Project
provided by Aoyama Gakuin University Research Institute
(2016–2017; MH). MH also thanks Izumi Science and Technol-
ogy Foundation for their financial support. AI acknowledges the
support of the FRONTIER project, JST PRESTO. The synchro-
tron radiation experiments were performed at the BL02B2 of
SPring-8 with the approval of the Japan Synchrotron Radiation
Research Institute (Proposal No. 2017A1648, 2017A1380, 2017B1268,
2016B1342, 2016B1706, 2016A1336, 2016A1333, 2014B1316,
2013B1776, 2013B1679, and 2013A1020).
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Anal., 2000, 43, 304; (b) A. Ishii, A. K. Jena and T. Miyasaka,
APL Mater., 2014, 2, 091102.
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