Alkylation-, Heating-, and Doping-Induced Emission Enhancement of a Polyaromatic Tube in the Solid State

Article abstract of DOI:10.1002/asia.201800034

A polyaromatic tube with a subnanometer-sized cavity was efficiently prepared on a gram-scale through the stereo-controlled cyclotrimerization of a diphenylanthracene derivative as a key step. The facile exterior alkylation of the polyaromatic framework l

Full text of DOI:10.1002/asia.201800034

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Alkylation-, Heating-, and Doping-Induced Emission
Enhancement of a Polyaromatic Tube in the Solid State
Authors: Kiyonori Kuroda, Masafumi Otsuki, Kohei Yazaki, Yoshihisa
Sei, Munetaka Akita, and Michito Yoshizawa
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Alkylation-,
Heating-,
and
Doping-Induced
Emission
Enhancement of a Polyaromatic Tube in the Solid State
Kiyonori Kuroda,[a] Masafumi Otsuki,[a] Kohei Yazaki,[b] Yoshihisa Sei,[a] Munetaka Akita,[a] and Michito
Yoshizawa*[a]
Abstract: A polyaromatic tube with a subnanometer-sized cavity
was efficiently prepared on a gram-scale through the stereo-
controlled cyclotrimerization of a diphenylanthracene derivative as a
key step. The facile exterior alkylation of the polyaromatic framework
leads to a moderately fluorescent tube (R = -OC10H21; ΦF = 20%) in
the solid state. The emission intensity of the solid-state alkyl-
substituted tube is remarkably enhanced upon heating (up to 1.6
times, ΦF = 31%) as well as doping with fluorescent dyes (up to 4.2
times, ΦF = 83%) through efficient energy transfer.
enhanced upon heating and doping with fluorescent dyes (up to
1.6 and 4.2-fold, respectively). The doped solid with two different
dyes (i.e., coumarins and rubrene) exhibits strong emission (up
to ΦF = 83%) via an efficient energy transfer process.
Tubular nanostructures, providing well-defined one-dimensional
pores, have garnered much attention due to their characteristic
functions such as capturing, separating, and transporting
abilities toward molecules and metal ions. Successful synthesis
of molecular and supramolecular tubes has been widely
accomplished using covalent links[1] and non-covalent
interactions.[2-4] However, multiple-step and time-consuming
syntheses are often required for the previous tubes and their
gram-scale preparations are uncommon (except for e.g.,
cyclodextrins, cucurbiturils, pillarenes, and lipid nanotubes). In
addition, tubular (supra)molecules with strong florescent
properties in solution are quite limited[5-7] and those in the solid
state have not been reported so far. The main reason stems
from aggregation-caused emission quenching (ACEQ) effects
between fluorescent tubes (Figure 1a, left), as found for the solid
samples of common polyaromatic compounds (e.g., pyrene and
perylene).[8] We thus expected that the rational design of
molecular tubes with both high fluorescence and well-defined
cavities would develop novel organic porous materials with
flexible photofunctions, which are in contrast to typical inorganic
porous materials with rigid frameworks.[9]
Here we report a high-yielding, short-step, and gram-scale
synthesis of polyaromatic tube 1 (Figure 1b) with a well-defined
and subnanometer-sized cavity through a stereo-controlled
homocoupling reaction. The solid-state fluorescence of the tube
is greatly enhanced (up to 2.1-fold) by exterior alkylation due to
the suppression of intermolecular aggregation of the
polyaromatic frameworks (Figure 1a, right). Impact of the identity
of alkyl side chains attached on the tube is also explored by
using the derivatives. Moreover, we reveal that the solid-state
emission intensities of alkyl-substituted tube 2a are further
Figure 1. a) Cartoon representation of the solid-state morphology of
polyaromatic tubes without/with alkyl side chains and b) anthracene-based
molecular tubes
1 .
and 2a-c (synthesized in this work) and tube 3[11]
Stereochemical characters of c) meta-substituted 3’ and d) ortho-substituted 1’
and their optimized structures (R = -OCH3; DFT, B3LYP/6-31G* level).
We have recently focused on 9,10-diphenylanthracene
(DPA) as a simple and highly fluorescent building block[10] for the
preparation of new polyaromatic tubes. Prototypical tube 3
composed of three DPA frameworks formed by homocoupling
reaction of meta-substituted 3’ (Figure 1c).[11] However, DPA
derivative 3’ possesses two phenyl-anthyl bonds, which freely
rotate at room temperature, so that the main coupling products
were a mixture of undesired acyclic oligomers. The yield of tube
3 was only 15% under the optimized conditions. Thus we herein
[a]
K. Kuroda, M. Otsuki, Dr. Y. Sei, Prof. Dr. M. Akita, Dr. M.
Yoshizawa
Laboratory for Chemistry and Life Science
Institute of Innovative Research, Tokyo Institute of Technology
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
E-mail: yoshizawa.m.ac@m.titech.ac.jp
Dr. K. Yazaki
[b]
Faculty of Engineering, University of Yamanashi
4-3-11 Takeda, Kofu 400-8511, Japan
Supporting information for this article is given via a link at the end of
the document.
designed an ortho-substituted DPA derivative with
a cis
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conformation (cis-1’; Figure 1d)[12] to efficiently synthesize tube 1
through
a stereo-controlled homocoupling. The new parts
provides an ideal included angle (60º) for its cyclotrimerization
and does not isomerize to trans-1’ under the reaction conditions.
In addition, we envisioned that the subsequent deprotection and
alkylation of tube 1 would lead to alkyl-decorated tubes 2a-c
(Figure 1b), which afford emissive porous solids responding to
external stimuli.
ortho-Substituted molecular tube 1 (R = -OCH2CH2OCH3)
was synthesized in high overall yield (57% in 4-step reactions)
from commercially available 2,4-dibromophenol (Figure 2a).
After protecting the phenol, sequential lithiation, addition of 9,10-
anthraquinone, and reductive aromatization gave rise to a
mixture of DPAs cis-1’ and trans-1’ (in a 1:1 ratio) in 97% yield.
The two isomers were easily separated by silica-gel column
chromatography (Figure 2c). The isolated trans-1’ converted to a
1:1 mixture of the cis and trans isomers when heated at 180 ºC
for 1 h. When the separation and conversion protocols were
repeated for total three times, a white solid of pure cis-1’ was
obtained in 88% yield based on the anthraquinone. Finally,
milligram-scale homocoupling reaction of cis-1’ in the presence
of the Ni(cod)2/2,2’-bipyridine catalyst in DMF yielded tube 1
(16.0 mg, 11.9 µmol) in 65%.[13]
The tubular structure of 1 was fully confirmed by NMR, MS,
1
and X-ray crystallographic analyses. The H NMR spectrum of
tube 1 contained five aromatic signals (Ha-e) due to a virtual D3h
structure (Figure 2d). The upfield shift of signal Ha (Δδ = –0.17
ppm) is ascribed to shielding effects by the adjacent anthracene
panels. The MALDI-TOF MS spectrum exhibited a prominent
peak at m/z = 1430.0 assigned to a [1]+ species.[13] The single
crystals for X-ray crystallographic analysis were obtained by
slow diffusion of an acetone/acetonitrile solution of 1 at room
temperature. The molecular structure of 1 provides a cylindrical
cavity surrounded by the three anthracene panels linked by the
three biphenyl spacers (Figure 2b). Both of the inner diameter of
the cavity and the length of the tube are approximately 0.9 nm.
The conformation of the DPA moieties of tube 1 is identical to
that of cis-1’ (Figure 1d, top). The ortho-substituted, flexible
side-chains cover the outer surfaces of the anthracene panels.
The high-yielding and short-step synthetic procedure
facilitated the gram-scale preparation of tube 1.[14] We examined
a homocoupling reaction using cis-1’ (3.02 g, 4.74 mmol),
Ni(cod)2 (2.01 g, 7.32 mmol), 2,2’-bipyridyl (1.13 g, 7.22 mmol),
and dry DMF (800 mL) in a 1.0 L glass flask at 90 ºC for 24 h.
The crude product was washed with methanol and acetone to
give 1 as a white solid (1.43 g, 1.00 mmol; Figures 2f). Notably,
the isolated yield of the product (64%) is comparable to that from
the small-scale reaction described above.
Figure 2. a) Four-step synthesis of polyaromatic tube 1. Reagents and
catalysts: (i) 1-bromo-2-methoxyethane, K2CO3, CH3CN, 85 ºC; (ii) 9,10-
anthraquinone, n-BuLi, THF, –78 ºC to r.t. and then NaH2PO2•H2O, NaI, AcOH,
80 ºC; (iii) neat, 180 ºC; (iv) Ni(cod)2, 2,2’-bipyridine, DMF, 90 ºC. b) X-ray
crystal structure of tube 1 (top and side views). 1H NMR spectra (400 Hz,
CDCl3, r.t.) of c) cis-1’, d) tube 1, and e) tube 2a. f) A photograph of a sample
of tube 1 obtained by the gram-scale synthesis.
To suppress the self-aggregation of the polyaromatic
framework in the solid state, methoxyethoxy-substituted tube 1
converted to alkyl-substituted tubes 2a-c (Figure 1a,b) by
deprotection and etherification reactions (two steps). Tubes 2a
and 2b possess six linear and branched decyl side chains,
respectively, and tube 2c possesses six n-butyl side chains
whose lengths are comparable to that of the side chains on tube
1. NMR and MS analyses established full functionalization on
1
the outer surfaces of 2a-c (Figures S17-S28).[13] In the H NMR
spectrum of 2a (R = -OC10H21), for example, seven signals of the
pendant alkyl chains (HD-M) were clearly observed in the region
of 0.76 to 3.79 ppm (Figure 2e). The optimized structure of tube
2a indicates that the outer and core diameters are ~3 and ~1 nm,
respectively (Figure 3a). UV-visible spectrum of a CH2Cl2
solution of tube 1 showed typical absorption bands around 370
nm derived from the anthracene moieties and the similar
absorption bands were also observed for tubes 2a-c (Figures 3b
and S29). The solution-state emission property of 1 was also
comparable to that of 2a-c and meta-substituted tube 3.[13]
Fluorescence spectrum of 1 in CH2Cl2 exhibited an anthracene-
based emission band around 380-570 nm (λex = 375 nm, ΦF =
55%), which was similar to that of 2a in CH2Cl2 (Figure 3c).[16a]
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Figure 4. a) Solid-state fluorescent quantum yields (λex = 373 nm, r.t.) of 1 and
2a-d (i) before and (ii) after heating at 150 ºC for 0.5 h, and (iii) after doping
with fluorescent dye 4 (0.05 equiv.; λex = 350 nm). b) Differential scanning
calorimetry of 2a and 2b (heat flow: × 0.1 W/g). c) Powder X-ray diffraction
analysis of 2a at r.t. and 130 ºC.
Figure 3. a) An optimized structure of tube 2a (top and side views). b) UV-
visible (r.t.) and c) fluorescence spectra (λex = 375 nm, r.t.) of 1 and 2a in
solution (CH2Cl2, 0.025 mM) and in the solid-state.
The fluorescence properties of solid 2a changed
remarkably upon doping with small amounts of fluorescent dyes.
The solid of rod-shaped coumarin dye 4 (Figure 5b) showed
weak orange emission (ΦF = 9%, λex = 449 nm) due to the
ACEQ effect (ΦF = 90% in CH2Cl2), whereas mixed solid
2a•(4)0.05, prepared by evaporation of a CH2Cl2 solution of 2a
and 4 (0.05 equiv.), exhibited strong green emission (ΦF = 59%)
upon irradiation of the anthracene absorption band (λex = 350
nm; Figures 4a-iii). The emission intensity of the mixed solid is
3.0- and 6.6-fold higher than those of solids 2a and 4,
respectively.[19,20] In the fluorescence spectrum of 2a•(4)0.05
(Figure 5a), the observed prominent band at λex = 514 nm,
derived from the doped dye, indicates highly efficient energy
transfer (ET) from 2a to 4 in the solid state. The alkyl
substitutions on the tubular framework proved to be useful to
afford such a highly emissive solid. The emission intensities of
solids 2b•(4)0.05 and 2c•(4)0.05 (ΦF = 63 and 54%, respectively)
are also higher than that of solid 1•(4)0.05 (ΦF = 34%).[13]
Although pale yellow solids 1 and 2a, prepared by
evaporation of the corresponding CH2Cl2 solution, showed
similar broad absorption bands around 400 nm (Figure 3b), their
emission intensities are quite different. The original tube 1
emitted very weak blue fluorescence (ΦF = 10%) with an
emission maximum at 429 nm. In contrast, tube 2a bearing the
linear decyl groups displayed moderate blue fluorescence (ΦF =
20%) under the same conditions (Figure 3c). The emission
intensity is similar to that of tube 2b (ΦF = 21%) bearing the
branched decyl groups (Figure 4a-i). The length of the decorated
alkyl chains is of importance for the solid-state fluorescence: the
emission intensity of n-butyl-substituted tube 2c (ΦF = 15%) is
lower than that of 2a but higher than that of 1 and 2d (R = -OH;
ΦF = 2%). Thus, the alkylation induces polyaromatic tube 1 to
enhance its emission (up to 2.1 times) in the solid state.[17]
The solid-state emission intensity of tube 2a was further
enhanced by thermal stimulus. After heating the solid sample at
150 ºC for 30 min, the resultant pale yellow solid emitted blue
fluorescence (λmax = 432 nm) in 31% quantum yield at room
temperature (Figure 4a-ii), indicating heating-induced emission
enhancement (1.6-fold). In contrast, the emission intensity of
solid 1 decreased (0.6-fold) upon heating under the same
conditions.[18a] Differential scanning calorimetry (DSC) revealed
that only solid 2a shows two endotherm peaks at 130 and 161
ºC (Figure 4b), which are assignable to a phase transition and
melting temperature, respectively.[18b] Temperature-dependent
powder X-ray diffraction analysis of solid 2a elucidated the
transformation from a crystalline solid to an amorphous solid
around 130 ºC (Figure 4c). Efficient dispersion of the partially
stacked polyaromatic tube, through van der Waals attractive
forces between the long alkyl side-chains, triggered by thermal
stimuli most probably results in the observed emission
enhancement.
Furthermore, we found that mixed solid 2a•(5)0.05
,
containing a small amount (0.05 equiv.) of bulky rubrene (5),
emits a strong yellow fluorescence (ΦF = 71%) upon irradiation
with the same UV light (λex = 350 nm; Figures 5a,b). In sharp
contrast, solid 5 and a CH2Cl2 solution of 5 showed much lower
emissivities (ΦF = 12% and 52%, respectively) than 2a•(5)0.05
.
The additional doping of 4 (0.05 equiv.) further enhanced the
emission intensity of the mixed solid. The ternary solid
2a•(4•5)0.05 also showed an intense yellow fluorescence (ΦF
=
83%) under the same conditions (Figure 5a). Emission bands
from the anthracene moieties of 2a and 4 effectively quenched
in the fluorescence spectrum. The emission intensity of
2a•(4•5)0.05 is higher than that of 1:1 mixed solid 4•5 (ΦF = 43%)
and thereby the tubular framework most probably separates the
two different fluorescent dyes even in the solid state (Figure
5c).[16b] Eventually, the solid-state fluorescence of the original
tube 1 could be improved up to 8.3-times by combination of the
alkylation and doping.[21,22]
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[6]
Fluorescent aromatic macrocycles in solution: a) M. Iyoda, J.
Yamakawa, M. J. Rahman, Angew. Chem. Int. Ed., 2011, 50, 10522–
10553; b) Y. Segawa, A. Fukazawa, S. Matsuura, H. Omachi,
S.Yamaguchi, S. Irle, K. Itami, Org. Biomol. Chem., 2012, 10, 5979–
5984; c) T. Iwamoto, E. Kayahara, N. Yasuda, T. Suzuki, S. Yamago,
Angew. Chem. Int. Ed., 2014, 53, 6430–6434; d) K. Okada, A. Yagi, Y.
Segawa, K. Itami, Chemi. Sci., 2017, 8, 661–667; e) G. Povie, Y.
Segawa, T. Nishihara, Y. Miyauchi, K. Itami, Science 2017, 356, 172–
175.
[7]
In general, small aromatic macrocycles (<~1 nm in diameter) provide
low emissivities due to the ring strain (e.g., [7]CPP (d = 0.98 nm) in
CHCl3: ΦF = <1%,[6b] [4]CPY (d = 1.1 nm) in CHCl3: ΦF = 5%,[6c] and
carbon nanobelt (d = 0.83 nm) in CH2Cl2: ΦF = 3%[6e]).
[8]
[9]
B. Valeur, M. N. Berberan-Santos, Molecular Fluorescence: Principles
and Applications, 2nd ed., Wiley-VCH Verlag
Germany, 2013.
& Co., Weinheim,
Figure 5. a) Solid-state fluorescence spectra (λex = 350 nm, r.t.) of tube 2a,
2a•(4)0.05, 2a•(5)0.05, and 2a•(4•5)0.05. b) Fluorescent dyes 4 and 5. c) A
proposed mechanism of the efficient emission from ternary solid 2a•(4•5)0.05
upon irradiation of tube 2a.
Rigid porous materials: a) S. Kitagawa, R. Kitaura, S. Noro, Angew.
Chem. Int. Ed., 2004, 43, 2334–2375; b) Y. Li, J. Yu, Chem. Rev., 2014,
114, 7268-7316; c) S. Das, P. Heasman, T. Ben, S. Qiu, Chem. Rev.,
2017, 117, 1515–1563.
[10] G. Heinrich, S. Schoof, H. Gusten, J. Photochem., 1974, 3, 315–320.
[11] K. Hagiwara, M. Otsuki, M. Akita, M. Yoshizawa, Chem. Commun.,
2015, 51, 10451–10454.
In summary, a polyaromatic tube has been successfully
prepared in a gram-scale quantity through a key stereo-
controlled homocoupling reaction. The tube has a well-defined
[12] ortho-Substituted DPAs as molecular switches: a) D. Zehm, W.
Fudickar, T. Linker Angew. Chem. Int. Ed., 2007, 46, 7689–7692; b) D.
Zehm, W. Fudickar, M. Hans, U. Schilde, A. Kelling, T. Linker, Chem.
Eur. J., 2008, 14, 11429–11441.
[13] See the Supporting Information. 1H NMR and MALDI-TOF MS analyses
revealed the structures and cis-trans ratio of products cis-1’ and trans-1’
(Figure S5-8).
[14] The gram-scale synthesis of aromatic macrocycles is still uncommon[15]
and, to the best of our knowledge, that of a polyaromatic tube and belt
has not been reported so far.
subnanometer-sized
cavity
surrounded
by
fluorescent
anthracene panels. The solid-state emission of the polyaromatic
tube is greatly enhanced by its exterior alkylation (up to 2.1
times). That of the alkyl-substituted tube is further enhanced by
external stimuli such as heating and doping (up to 4.2 times).
The observed, multiple stimuli-responsive fluorescent properties,
which are rare for previously reported organic solids and
inorganic porous solids, prompt us to develop sensing materials
using the present organic solids with tubular subnanopores in a
future subject.
[15] a) J. Xia, J. W. Bacon, R. Jasti, Chem. Sci., 2012, 3, 3018–3021; b) V.
K. Patel, E. Kayahara, S. Yamago, Chem. Eur. J., 2015, 21, 5742–
5749; c) Commercially available CPPs: Tokyo Chemical Industry Co.,
Ltd. Catalog No. C2449 and C2931, and Kanto Chemical Co., Inc.
Catalog No. 08131-35, 08132-35, and 08137-65.
Acknowledgements
[16] a) Alkyl-substituted tube 2a is well-soluble in non-polar solvents (e.g.,
n-hexane)[13]; b) UV-visible study indicates the efficient binding of dye 4
in the subnanometer-sized cavity of tube 2a in n-hexane so that the
obtained ternary solid is most likely composed of slim dye 4 within 2a
and bulky dye 5 without 2a (Figures S30 and S40).
This work was supported by JSPS KAKENHI (Grant No.
JP25104011/JP26288033/JP17H05359) and “Support for
Tokyotech Advanced Researchers (STAR)”. We thank Dr. Keita
Hagiwara and Dr. Motoya Suzuki (Tokyo Institute of Technology)
for their support in this project.
[17] The solid-state emission quantum yield of 3 (ΦF = 10%) is comparable
to that of 1 (Figure S31).
[18] a) The emission quantum yields of solids 2b and 2c remained almost
unchanged after heating at 150 ºC for 0.5 h (Figure 4a); b) These peaks
were not observed for solids 1, 2b (Figure 4b), and 2c under the same
conditions (Figure S32).
Keywords: molecular tube, polyaromatic ring, fluorescence,
solid, host-guest
[1]
Reviews on covalent molecular tubes: a) J. W. Lee, S. Samal, N.
Selvapalam, H.-J. Kim, K. Kim, Acc. Chem. Res. 2003, 36, 621–630; b)
J. Lagona, P. Mukhopadhyay, S. Chakrabarti, L. Isaacs, Angew. Chem.
Int. Ed. 2005, 44, 4844–4870; c) G. Crini, Chem. Rev.,
2014, 114, 10940–10975; d) K. I. Assaf, W. M. Nau, Chem. Soc. Rev.,
2015, 44, 394–418; e) T. Ogoshi, Pillararenes, RSC Pub., Cambridge,
UK, 2015.
[19] The fluorescence lifetimes (r.t., λex = 355 nm) of solids 2a, 2a•(4)0.05
,
2a•(5)0.05, and 2a•(4•5)0.05, 4•5 were estimated to be 2.4, 2.3, 17.6, 16.2,
and 2.9 ns, respectively (Figure S38).
[20] Fluorescence properties of 4: A. S. Kristoffersen, S. R. Erga, B. Hamre,
O. Frette, J. Fluoresc., 2014, 24, 1015–1024.
[21] The solids of cis/trans-1’ and cis/trans-2a’, which are the partial
structures of polyaromatic tubes 1 and 2a, respectively, showed no
heating-induced emission enhancement. Although doping these solids
with 4 improved their emissivities, the emission quantum yields of
mixed solids cis/trans-1’•(4•5)0.017 and cis/trans-2a’•(4•5)0.017 were much
lower (≥ ~15%) than that of 2a•(4•5)0.05 under the same conditions
(Figures S41-45).[13,22]
[2]
[3]
A review on coordination tubes: M. Tominaga, M. Fujita, Bull. Chem.
Soc. Jpn., 2007, 80, 1473–1482.
Reviews on hydrogen-bonding tubes: a) N. Sakai, J. Mareda, S. Matile,
Acc. Chem. Res., 2005, 38, 79–87; b) T. Shimizu, M. Masuda, H.
Minamikawa, Chem. Rev., 2005, 105, 1401–1443; c) R. Chapman, M.
Danial, M. L. Koh, K. A. Jolliffe, S. Perrier, Chem. Soc. Rev., 2012, 41,
6023–6041.
[22] To the best of our knowledge, there has been no report on the emission
enhancement of tubular nanostuructures in the solid state by external
stimuli.
[4]
[5]
Reviews on π-stacking tubes (and capsules): a) W. Li, Y. Kim, M. Lee,
Nanoscale 2013, 5, 7711–7723; b) K. Kondo, J. K. Klosterman, M.
Yoshizawa, Chem. Eur. J. 2017, 23, in press (DOI:
10.1002/chem.201702519)
Fluorescent polyaromatic tubes in solution (ΦF = up to 70%): a) K.
Hagiwara, Y. Sei, M. Akita, M. Yoshizawa, Chem. Commun., 2012, 48,
7678–7680; b) K. Hagiwara, M. Akita, M. Yoshizawa, Chem. Sci., 2015,
6, 259–263.
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Entry for the Table of Contents
COMMUNICATION
K. Kuroda, M. Otsuki, K. Yazaki, Y. Sei,
M. Akita, and M. Yoshizawa*
Page No. – Page No.
Alkylation-, Heating-, and Doping-
Induced Emission Enhancement of a
Polyaromatic Tube in the Solid State
A new polyaromatic tube was efficiently prepared on a gram-scale through a stereo-
controlled homocoupling reaction as a key step. The facile exterior alkylation leads
to a moderately fluorescent tube in the solid state. The emission quantum yield of
the solid-state alkyl-substituted tube is remarkably enhanced upon heating (up to
1.6 times) as well as doping with fluorescent dyes (up to 4.2 times).
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