A three-dimensional assembly of pyrene dye on a tetraphenylethane scaffold enhances fluorescence quantum yield

Article abstract of DOI:10.1039/c3cc41312h

A three-dimensional pyrene assembly on a tetraphenylethane skeleton enhanced the fluorescence quantum yield compared to the yield obtained when using monomeric species, without changing the shape of the emission spectrum. The unique spacing of the pyrene units may prevent intramolecular fluorescence quenching or non-radiative decay.

Full text of DOI:10.1039/c3cc41312h

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A three-dimensional assembly of pyrene dye on a
tetraphenylethane scaffold enhances fluorescence
quantum yield†
Cite this: Chem. Commun., 2013,
49, 3893
Received 20th February 2013,
Accepted 20th March 2013
a
Kentaro Sumi,a Yosuke Niko,a Katsumi Tokumaruz and Gen-ichi Konishi*ab
DOI: 10.1039/c3cc41312h
www.rsc.org/chemcomm
A three-dimensional pyrene assembly on a tetraphenylethane that has received attention is the assembly of dyes in three
skeleton enhanced the fluorescence quantum yield compared dimensions.9 For example, spirobifluorene skeletons distribute
to the yield obtained when using monomeric species, without chromophores in three dimensions and in high density10
changing the shape of the emission spectrum. The unique spacing without causing ACQ and unnecessary changes in the emission
of the pyrene units may prevent intramolecular fluorescence spectra. However, this method is also insufficient, because the
quenching or non-radiative decay.
obtained quantum yields are usually unchanged compared to
the model compounds. To the best of our knowledge, there are
Highly luminescent dyes have recently attracted considerable few reports that describe the realization of both an increase in
attention and are being studied for their applicability in various the quantum yield and the accumulation of the dyes.
fields, such as organic light-emitting diodes (OLEDs)1,2 and
In this communication, we report a new strategy for obtaining
bioimaging,3 where they are finding increasing importance. To high luminous efficiency, i.e, both high absorption and quantum
obtain an ideal dye with high luminous efficiency, it is neces- yield, without changing the shape of the emission spectrum.
sary to achieve both high absorbance and high quantum yield. Here, to arrange the chromophores in three dimensions aniso-
There have been many reports concerned with improving the tropically with high density, we focused on a tetraphenylethane
luminous efficiency of dyes.2 For example, the expansion of skeleton into which four target dyes could be introduced per
p-conjugation in target dyes in one or two dimensions4 or their molecule. We synthesized an assembled dye, TPPE, which
introduction to scaffolds such as cryptates,5 polymers,6 and contained four phenylpyrene units on the tetraphenylethane
dendrimers7 represents typical methods. The former realizes scaffold (Fig. 1), and compared the fluorescence properties of
an increased oscillator strength, leading to the enhancement of the dye with those of appropriate model compounds. The
absorption and emission efficiency in the target dyes. The latter chemical structure of TPPE was highly unique. Although the
enables the target dye to strongly emit with little contribution shape of the obtained emission spectrum was similar to that of
from non-radiative processes. However, these methods also the model dye itself, the quantum yield for TPPE was higher
suffer from drawbacks: a drastic enhancement of absorption than that of the model dye.
efficiency is not expected and/or the emission spectra are
A pyrene dye, 1,1,2,2-tetrakis(4-(7-tert-butyl-1-pyrenyl)phenyl)-
often changed. One simple method to effectively enhance the ethane (TPPE), and two model compounds, 7-tert-butyl-1-
absorption efficiency is to assemble the dyes on some type of phenylpyrene (PP) and 7-tert-butyl-1-tolylpyrene (TP), were
structural template or skeleton. However, by this method, synthesized in good yields via the palladium-catalyzed
aggregation-caused quenching (ACQ)8 often occurs, and, as a
result, the emission efficiency is decreased. Yet another method
a Department of Organic and Polymeric Materials, Tokyo Institute of Technology,
2-12-1-H-134 O-okayama, Meguro-ku, Tokyo 152-8552, Japan.
E-mail: konishi.g.aa@m.titech.ac.jp; Fax: +81-3-5734-2888; Tel: +81-3-5734-2321
b PRESTO, Japan Science and Technology Agency (JST), Japan
† Electronic supplementary information (ESI) available: Experimental section:
synthetic procedures and 1H NMR, 13C NMR, FT-IR spectra of TPPE, PP, and PE.
UV-vis absorption and fluorescence spectra of TPPE, PP, and TP in deaerated THF
((a) c = 1.0 ꢀ 10ꢁ6 M, (b) abs = 0.1, lex = lmax = 342 nm). See DOI: 10.1039/
Fig. 1 Optimized structure of TPPE’s core (1,1,2,2-tetrakis(4-pyrenylphenyl)-
c3cc41312h
ethane) by MM2 calculation.
‡ Professor Emeritus of Tsukuba University.
c
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Scheme 1 Synthesis of TPPE and chemical structures of model compounds
(PP and TP).
Suzuki–Miyaura coupling reaction11 according to Scheme 1 and
Scheme S1 (see ESI†). The chemical structures of these com-
1
pounds were confirmed using H NMR, 13C NMR, FT-IR, mass
spectrometry, and elemental analysis (see Fig. S1–S12, ESI†).
Each dye was found to be soluble in various organic solvents
such as tetrahydrofuran (THF), chloroform, dichloromethane
(CH2Cl2), and toluene. We recrystallized each compound sev-
eral times and confirmed their fluorescence properties at the
steady state, i.e., where the quantum yield of each compound
reached a saturated value. The fluorescence quantum yields of
the model compounds were identical to the literature values.12
Fig. 2 (a) UV-vis absorption and (b) fluorescence spectra of TPPE, PP, and TP in
deaerated THF ((a) c = 1.0 ꢀ 10ꢁ5 M, (b) abs = 0.1, lex = lmax = 342 nm).
The UV-vis spectra, fluorescence spectra, absolute quantum emissions were observed in the spectra of any of the com-
yields (Ff), and lifetimes (tf) of TPPE, PP, and TP were obtained pounds at these concentrations. The observed fluorescence
in deaerated THF or CH2Cl2. The spectroscopic parameters spectra showed that the fluorescence wavelengths of TPPE
are listed in Table 1. The UV-vis spectra revealed absorption and TP were independent of solvent polarity (Table 1). Further-
maxima (lmax) of TPPE, PP, and TP (c = 1.0 ꢀ 10ꢁ5 M) at 343, more, in the case of THF, the fluorescence quantum yield of
342, and 343 nm, respectively, with molar absorption (lmax
)
TPPE was about 1.45 times larger than those of the model
coefficients (e) of 130 000, 33 000, and 32 000 Mꢁ1 cmꢁ1, respec- compounds (PP and TP), although TPPE had the same chromo-
tively (Fig. 2a). The e value of TPPE was approximately four phores with model compounds, that is, 1-phenylpyrene
times higher than that of PP or TP, in accordance with the fact (Table 1). Further, as a solvent effect, the rate of increase in
that TPPE had four chromophores per molecule compared to the fluorescence quantum yield between TPPE and the model
one each in PP and TP. Moreover, there were no clear differ- compounds changed. In CH2Cl2 solution, the quantum yield of
ences between any of the dyes in terms of the UV-vis spectra in TPPE was 1.23 times higher than those of the model com-
dilute THF solutions of 1.0 ꢀ 10ꢁ5 M (Fig. 2a) or 1.0 ꢀ 10ꢁ6 M pounds. In addition, every compound had a similar fluores-
(Fig. S13, ESI†).
cence lifetime.
The observed fluorescence spectra under the same absorp-
The fluorescence quantum yield can be expressed as Ff =
tion conditions (abs = 0.1; lex = lmax = 342 nm; Fig. 2b and kfts, or Ff = kf/(kf + kisc + knr) under degassed conditions, in
Fig. S14, ESI†) showed that all of the compounds exhibited which kf, kisc, and knr represent the rate constants for fluores-
intense purple-blue emissions (lem), at 385 and 403 nm for cence radiation, intersystem crossing, and nonradiative decay,
TPPE, 385 and 403 nm for PP, and 384 and 403 nm for TP, in respectively. Assuming that the emission in these compounds
both THF and CH2Cl2 solutions, respectively. No excimer involves a unimolecular process, there are two potential
Table 1 Spectroscopic parameters of TPPE, PP, and TP at room temperature in THF and dichloromethane
THF
CH2Cl2
eb
lem
ts
kfg
kisc + knr
labs
lem
ts
kfg
kisc + knr
a
c
d,f
g
a
c
d,f
g
labs
Entry
[nm]
[105 Mꢁ1 cmꢁ1
]
[nm]
Ffd,e
[ns]
[107 sꢁ1
]
[107 sꢁ1
]
[nm]
[nm]
Ffd,e
[ns]
[107 sꢁ1
]
[107 sꢁ1
]
TPPE
PP
TP
342
343
342
1.30
0.33
0.32
385, 403
385, 403
384, 403
0.58
0.40
0.41
35.2
33.4
33.8
1.6
1.2
1.2
1.2
1.8
1.7
342
343
342
385, 403
385, 403
384, 403
0.47
0.38
0.38
20.6
23.5
23.3
2.3
1.6
1.6
2.6
2.6
2.6
a
b
c
d
e
Absorption maximum. Molar absorption coefficient.
l
ex = lmax = 342 nm. Measurement at Abs = 0.1 condition. Absolute quantum yields.
f
g
Fluorescence lifetime. Radiative rate constant is calculated by the following equations: Ff = kfts, or Ff = kf/(kf + kisc + knr).
c
3894 Chem. Commun., 2013, 49, 3893--3895
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a
Table 2 Values of knr + kisc and kdif of TPPE, PP, and TP in THF and CH2Cl2
to gain further insight into this new fluorescence phenomenon.
In order to reveal the effect of the internal conversion process,14
we intend to synthesize additional soluble TPPE analogs and
investigate their photophysical properties in detail. In addition,
to confirm exciton behaviors, we try to evaluate the photo-
physical properties of TPPE in the crystal or aggregate state.13
THF
CH2Cl2
b
b
kdif
kisc + knr cc
kdif
,
kisc + knr cc,
[104
s
ꢁ1] [107 ꢁ1] [10ꢁ6 mol Lꢁ1] [104 sꢁ1] [107 sꢁ1] [10ꢁ6 mol Lꢁ1
s ]
TPPE 1.0
PP 3.8
TP 3.9
1.2
1.8
1.7
0.74
2.9
3.0
1.0
4.8
4.5
2.6
2.6
2.6
0.66
3.2
3.0
a
b
Notes and references
Measurement at Abs = 0.1 condition. Calculated by the equation:
kdif = 8RT/3Z m3 molꢁ1 sꢁ1
. Concentration at Abs = 0.1 condition.
c
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value of kisc + knr decreased in THF only, or the value of the
radiative rate constant kf increased. We considered the follow-
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fixation of the dyes. In the model compounds, intermolecular
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rate constants (kf) of TPPE were higher than those of the model
compounds. Furthermore, as solvatochromic effects, the
fluorescence inactivation rate constant (kisc + knr) of TPPE was
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from the others because the shapes of the spectra were quite
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changed. This may be because the rate of increase in the
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quantum yield of TPPE was about 1.45 times higher than those
of the model compounds in THF solution, although all
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 3893--3895 3895