Unique photochemical behavior of novel tetracationic pyrene derivative on the clay surface

Article abstract of DOI:10.1016/j.tetlet.2012.08.079

Novel tetracationic pyrene derivative (1,3,6,8-tetrakis(N-methylpyridinium- 4-yl)pyrene, Py⁴⁺) was synthesized. Photochemical properties such as fluorescence quantum yield and fluorescence lifetime were observed for Py ⁴⁺ and Py

Full text of DOI:10.1016/j.tetlet.2012.08.079

Tetrahedron Letters 53 (2012) 5800–5802
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Tetrahedron Letters
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Unique photochemical behavior of novel tetracationic pyrene derivative
on the clay surface
Satomi Hagiwara a, Yohei Ishida a,b, Dai Masui c, Tetsuya Shimada a, Shinsuke Takagi a,
⇑
a Department of Applied Chemistry, Graduate Course of Urban Environmental Sciences, Tokyo Metropolitan University, Minami-ohsawa 1-1, Hachioji-shi, Tokyo 192-0397, Japan
b Japan Society for the Promotion of Science (DC1), Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan
c Department of Chemistry, Tokyo Medical University, Shinjuku 6-1-1, Shinjuku-ku, Tokyo 160-8402, Japan
a r t i c l e i n f o
a b s t r a c t
Novel tetracationic pyrene derivative (1,3,6,8-tetrakis(N-methylpyridinium-4-yl)pyrene, Py4+) was syn-
thesized. Photochemical properties such as fluorescence quantum yield and fluorescence lifetime were
observed for Py4+ and Py4+/clay complexes. Judging from Lambert–Beer plot analysis, Py4+ molecules
adsorb on the clay surface without aggregation up to 69% versus cation exchange capacity of the clay.
Py4+ molecule emits strong fluorescence from an excited state of monomer, while the emission from exci-
mer was not detected, in spite of high density adsorption condition on the solid surface. It is supposed
that strong interaction between host and guest by the ‘Size-Matching Effect’ inhibits the formation of
excimer on the clay surface.
Article history:
Received 9 July 2012
Revised 8 August 2012
Accepted 17 August 2012
Available online 23 August 2012
Keywords:
Pyrene
No excimer
Clay
Ó 2012 Elsevier Ltd. All rights reserved.
Organic/inorganic complex
Fluorescence
Clay minerals are inorganic materials that provide negatively
charged flat surface. Artificially synthesized clay aqueous solution
is colorless and transparent in the visible region. Several papers re-
ported the photochemical behavior of organic dyes/inorganic clay
complexes.1,2 We have successfully prepared the unique com-
plexes in which the porphyrin molecules adsorb on the clay sur-
faces without aggregation. By using such an adsorption structure,
an efficient energy transfer reaction was achieved in porphyrin/
clay complexes toward the establishment of artificial light-
harvesting system.3 The crucial factor for high density adsorption
of porphyrin with controlled intermolecular gap distance is match-
ing of the distance between negatively charged sites on the clay
sheet and that between positively charged sites in the porphyrin
molecules (‘Size-Matching Rule’).4–7 In the present Letter, we aim
to develop the artificial light-harvesting system by the addition
of pyrene as a dye, which can absorb shorter wavelength light in
the visible region. The absorption of porphyrin/clay complexes is
around 426–465 nm.4 It is expected that pyrene/clay complexes
absorb the light below 426 nm. Several researches reported a syn-
thesis of pyrene derivative and their photochemical behavior.8,9
However, as far as we know, 1,3,6,8-tetrakis(N-methylpyridini-
um-4-yl)pyrene (Py4+) has not been synthesized. In this work, we
synthesized Py4+ and observed its photochemical properties with
and without clay in aqueous solution.
Py4+ was synthesized as a tetracationic pyrene derivative as
shown in Scheme 1.
3.5 mL of tetrahydrofuran, 0.5 mL of toluene, and 3.0 mL of 2 M
aqueous potassium carbonate were added to a solid mixture of
1,3,6,8-tetrabromopyrene and 4-pyridinyl boronic acid (375.8 mg,
3.0 mmol) under N2 atmosphere. Finally, tetrakis(triphenylphos-
phine)palladium (Pd(PPh3)4) (3.25 mg, 0.0028 mmol) was added
to the mixture. The mixture was refluxed under nitrogen for
4 days.10 The crude product poured into 10 mL of methanol was fil-
tered with the PTFE membrane filter (
U = 0.1 lm) and washed by
water. The yellow powder was obtained (150.6 mg, 59%). Identity
HO
OH
N
N
N
B
N
Br
Br
Br
K2CO3
[Pd(PPh3)4]
In THF/Toluene/H2O
refluxed for 4day
Br
N
59%
0.95nm
+
N
+
+
+
+
N
N
N
CH3I
4I-
4PF-
NH4PF6
66%
InMeCN
6
1.1nm
refluxed for 6day
72%
N
+
+
+
N
N
N
⇑
Corresponding author. Tel.: +81 42 677 2839; fax: +81 42 677 2838.
E-mail addresses: takagi-shinsuke@tmu.ac.jp, hikari@tmu.ac.jp (S. Takagi).
Scheme 1. The synthetic routes of Py4+
.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.08.079

S. Hagiwara et al. / Tetrahedron Letters 53 (2012) 5800–5802
5801
of the product was confirmed using 1H NMR. Synthesized 1,3,6,8-
0.06
0.04
tetrakis(pyridine-4-yl)-pyrene (150.6 mg, 0.29 mmol) was added
to a solution of 3.6 mL of iodomethane and 90 mL of acetonitrile.
The mixture was refluxed under nitrogen for 6 days. After the reac-
tion mixture was evaporated and dried, 76 mL of water was added
and the formed precipitates were removed by filtration. Ammo-
nium hexafluorophosphate (219.4 mg) was added to the filtrate
under vigorous stirring. The precipitated yellow solid was collected
0.2
0.15
0.1
220 %
165 %
110 %
99 %
88 %
77 %
66 %
55 %
44 %
33 %
22 %
0.02
0
0
50 100 150 200 250
by filtration with the PTFE membrane filter (
U = 0.1 lm) (75.9 mg,
Loading level of Py4+ / % vs. CEC
48%). 1H NMR(CD3CN, 500 MHz) d (ppm): 8.85 (d, J = 9.2 Hz, 8H),
8.32 (d, J = 6.8 Hz, 4H), 8.31 (4H, s), 8.30 (s, 2H), 7.60 (s, 12H). Ele-
mental analysis: Found: C, 41.31; H, 2.76; N, 4.92; Calcd for
11 % versus CEC of the clay
0.05
0
C
40H34F24N4P4Á0.4H2O: C, 41.49; H, 3.03; N, 4.84. Sumecton SA
(SSA, Kunimine Ind. Co.) was used as clay minerals. Cation ex-
change capacity (CEC) of SSA is 0.997 meq gÀ1. According to the
PM3 calculation, estimated inter-cationic distances in Py4+ are
0.95 nm and 1.1 nm as indicated in Scheme 1, while inter-anionic
distance on the clay surface is 1.2 nm.4 This indicates that the
structure of Py4+ fulfills almost the ‘Size-Matching Rule’.
200
300
400
500
600
Wavelength / nm
Figure 2. Absorption spectra and Lambert–Beer plot of Py4+/clay complexes in
water. [SSA] = 4.0 Â 10À6 equiv LÀ1, [Py4+] = 1.1 Â 10À7–2.2 Â 10À6 M.
Photochemical properties such as absorption, fluorescence and
its lifetime were observed for Py4+ itself and Py4+/clay complexes
in water. Py4+/clay complex in water was prepared as follows.
Aqueous Py4+ solution was mixed with aqueous clay solution
under vigorous stirring. Absorption spectra were measured by
SHIMADZU UV-3150 ([SSA] = 4.0 Â 10À6 equiv LÀ1, [Py4+] = 1.1 Â
10À7–2.2 Â 10À6 M). Fluorescence spectra were measured by Jasco
all spectra were identical up to 66% versus CEC of the clay. As can
be seen in Figure 2 inset, Lambert–Beer plot was straight up to 69%
versus CEC of the clay. Absorption spectra of Py4+ in aqueous solu-
tion were superimposed above 69% versus CEC of the clay. These
indicate that Py4+ molecules adsorb on the clay surface up to 69%
versus CEC without aggregation. It should be noted that 69% is
extremely high value. In the case of methylene blue, that is a
typical dye, the aggregation was observed on the clay surface even
below 5% versus CEC of the clay.12 In the case of Py4+/clay com-
plexes, coulombic interaction between Py4+ molecule (guest) and
clay (host) is strong due to four points and effective interaction
between clay and dye due to ‘Size-Matching Effect’. Thus, the inter-
action between Py4+ molecules (guest–guest) that induces the
aggregation was suppressed.
FP-6600 ([SSA] = 6.7 Â 10À7–8.0 Â 10À4 equiv LÀ1
,
[Py4+] = 1.1 Â
10À7 M). Fluorescence quantum yield
(Uf) and fluorescence
lifetime (s
) were measured for Py4+ itself and Py4+/clay complexes
in water where the loading level is 0.06% versus CEC of the
clay ([SSA] = 8.0 Â 10À4 equiv LÀ1
,
[Py4+] = 1.1 Â 10À7 M). Time-
resolved fluorescence signals were measured by Hamamatsu
Photonics C4780 system based on a streak detector. A laser diode
(Hamamatsu Photonics C4725, 406 nm, FWHM 71 ps, 1 kHz) and
an Nd3+ YAG laser with an OPG (EKSPLA PL2210JE + PG-432,
410 nm, FWHM 25 ps, 1 kHz) were used for excitation.
Fluorescence kmax of Py4+ in aqueous solution and on the clay
surface were 485 nm and 498 nm, respectively (Fig. 1b). The shift
of fluorescence kmax by the complex formation with clay was
Absorption and fluorescence spectra are shown in Figure 1. kmax
of Py4+ in aqueous solution and on the clay surface were 422 nm
and 450 nm, respectively. The shift of kmax by the complex forma-
13 nm (
D
E = 5.4 Â 102 cmÀ1). The shapes of both fluorescence
tion with clay was 28 nm (
D
E = 1.5 Â 103 cmÀ1). In the case of por-
spectra were relatively sharp compared to excimer emission of
non-substituted pyrene (Py) as shown in Table 1.
phyrin/clay complexes where porphyrin molecule adsorbs on the
clay surface with a parallel orientation, kmax was shifted to longer
wavelength due to the flattening of the meso-substituents with re-
spect to the porphyrin ring.5,11 In the case of porphyrin molecule
with a standing orientation respect to the clay surface, such a large
shift of kmax was not observed. Thus, the kmax shift of Py4+ indicates
that Py4+ molecule adsorbs on the clay surface with a parallel
orientation as expected. It is due to the flattening of the 1,3,6,8-
substituents with respect to the pyrene ring.
Full width half maximum (FWHM) of monomer emission of Py
and that of excimer emission were 2774 cmÀ1 and 5137 cmÀ1
,
respectively. Full width half maximum of Py4+ on the clay surface
was 2319 cmÀ1. These indicate that Py4+ exists as monomer with-
out excimer formation on the clay surface, although Py forms exci-
mer easily on the clay surface.16 It should be noted that such
monomer adsorption on the solid surface is highly unique. In fact,
when Py4+ aqueous solution (700
l
L of 1.45 Â 10À5 M) was casted
Absorption spectra of Py4+/clay complex were measured at
various loading levels of Py4+. Judging from the normalized spectra,
on the quartz glass substrate, only excimer emission was observed
at 552 nm. It is supposed that Py4+ molecules can retain the long
(a)
(b)
without clay
0.04
485 nm 498 nm
1
0.5
0
without clay
with clay
λmax
with clay
0.03
0.02
450 nm
422 nm
0.01
0
200
400
600
440
540
640
Wavelength / nm
Wavelength / nm
Figure 1. Absorption (a) and fluorescence (b) spectra of Py4+ itself in water (dashed line) and Py4+/clay complexes in water (solid line). (a) [SSA] = 4.0 Â 10À6 equiv LÀ1
[Py4+] = 3.3 Â 10À7 M. (b) [SSA] = 8.0 Â 10À4 equiv LÀ1, [Py4+] = 1.1 Â 10À7 M.
,

5802
S. Hagiwara et al. / Tetrahedron Letters 53 (2012) 5800–5802
structural change of Py4+ on the clay surface would affect the non-
radiactive deactivation rate constant, and, thus, induce the change
of Uf
This Letter firstly reported the synthesis of 1,3,6,8-tetrakis(N-
methylpyridinium-4-yl)pyrene and its photochemical properties.
As a result, we found out that Py4+ adsorbs on the clay surface
without aggregation at high density condition. It is turned out that
Py4+ is a suitable dye for energy transfer reaction, especially as an
energy donor. In addition, interestingly, Py4+ molecules emit fluo-
rescence from its excited monomer state. These unique behaviors
were realized by the ‘Size-Matching Effect’.
Table 1
Photochemical properties of Py and Py4+
kfc (sÀ1
)
kisc + knr + kq[O2]
(sÀ1
c
.
FWHM Uf
s
(ns)
(cmÀ1
)
)
Py
Monomer
Excimer
Py4+
Without clay
With clay
2774a
5137a
0.04b
11.7b
3.42 Â 106
8.21 Â 107
2709d
2319e
0.70d,f
0.42e,f
3.4d
3.2e
2.06 Â 108
1.31 Â 108
8.81 Â 107
1.82 Â 108
a
b
c
Estimated from fluorescence spectra in Ref. 13.
Pyrene in aerated chloroform solution at concentration 10À5 M.14
Calculated using the equations, Uf = kfs and s = 1/(kf + kisc + knr + kq[O2]).
Acknowledgments
d
e
In aerated water ([Py4+] = 1.1 Â 10À7 M).
In aerated water where the loading level is 0.06% of Py4+ versus CEC of the clay
([SSA] = 8.0 Â 10À4 equiv LÀ1, [Py4+] = 1.1 Â 10À7 M).
The present work was supported partially by a Grant-in-Aid for
Precursory Research for Embryonic Science and Technology (PRE-
STO) from the Japan Science and Technology Agency (JST), and JSPS
Research Fellow DC1 from the Japan Society for the Promotion of
Science.
f
Uf values were determined using rhodamine 6G (Uf = 0.95) as a standard in
aerated condition.15
intermolecular distance and the formation of excimer was sup-
pressed due to the ‘Size-Matching Effect’4–7 that fixes the position
of dyes on the clay surface. When the adsorbed amount of Py4+ on
the clay surface increased, the shape of fluorescence spectra was
retained, although the fluorescence intensity decreased gradually.
This also supports that excimer does not form in the clay complex.
Uf of Py and Py4+ itself were 0.04 in aerated chloroform solution
and 0.70 in aerated aqueous solution, respectively. Uf of Py4+ is
much higher than that of Py. Uf of Py were 0.52 in de-aerated solu-
tion.14 Thus, fluorescence of Py is quenched by oxygen in aerated
condition.17 It is unique that high Uf of Py4+ was observed in spite
of the presence of oxygen. Uf of Py4+ in de-aerated aqueous solu-
References and notes
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tion was almost identical with that of Py4+ in aerated aqueous solu-
4+
tion, thus, the effect of oxygen was negligible for Py4+
. sPy was
3.4 ns in aerated aqueous solution. On the other hand, sPy was
150.1 ns in de-aerated chloroform solution.14 In the case of Py4+
,
´
it is supposed that the collision between Py4+ molecules and oxy-
13. Förster, Th. Angew. Chem., Int. Ed. Engl. 1969, 8, 333–343.
4+
ˇ
14. Kollár, J.; Hrdlovic, P.; Chmela, Š. J. Photochem. Photobiol., A 2010, 214, 33–39.
gen molecules is not effective in excited states, since sPy was
15. Kubin, R. F.; Fletcher, A. N. J. Lumin. 1982, 27, 455–462.
16. Cione, A. P. P.; Scaiano, J. C.; Neumann, M. G.; Gessner, F. J. Photochem.
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short. Generally,
s of substituted pyrene is shorter than that of
Py.8,18
Uf and
s
were measured for Py4+ itself in water and Py4+/clay
complexes in water at 0.06% loading level versus CEC. At such load-
ing level, fluorescence self-quenching does not occur. Uf of Py4+ it-
self and Py4+/clay complexes were 0.70 and 0.42, respectively. The