Room temperature phosphorescent crystals consisting of cyclized guests and their uncyclized mother host molecules

Article abstract of DOI:10.1246/cl.200278

A unique strategy for the fabrication of room temperature phosphorescent guest/host crystals, which consist of π-extended cyclized guests and their mother uncyclized flexible brominated hosts, is reported. Combinations of bisbenzofuropyrazines/bis-(2-bromophenoxy)pyrazines are the examples. Judicious choice of the guest/host pair allows to create highly phosphorescent crystals.

Full text of DOI:10.1246/cl.200278

Advance Publication Cover Page
Room Temperature Phosphorescent Crystals Consisting of Cyclized Guests and Their
Uncyclized Mother Host Molecules
Shotaro Nakamura, Madoka Tsuboi, Taisei Taniguchi, Yuji Nishii,
Norimitsu Tohnai, and Masahiro Miura*
Advance Publication on the web May 13, 2020
doi:10.1246/cl.200278
©
2020 The Chemical Society of Japan
Advance Publication is a service for online publication of manuscripts prior to releasing fully edited, printed versions. Entire
manuscripts and a portion of the graphical abstract can be released on the web as soon as the submission is accepted. Note that the
Chemical Society of Japan bears no responsibility for issues resulting from the use of information taken from unedited, Advance
Publication manuscripts.

1
Room Temperature Phosphorescent Crystals Consisting of Cyclized Guests and Their
Uncyclized Mother Host Molecules
1
1
1
2
1
1
Shotaro Nakamura, Madoka Tsuboi, Taisei Taniguchi, Yuji Nishii, Norimitsu Tohnai, and Masahiro Miura*
1
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871
2
Frontier Research Base for Global Young Researchers, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871
E-mail: miura@chem.eng.osaka-u.ac.jp
1
2
3
4
5
6
7
8
A unique strategy for the fabrication of room
48 consisting of ‘uncyclized host’ and ‘cyclized guest’. The host
49 and guest molecules employed are given in Figure 1.
temperature phosphorescent guest/host crystals, which
consist of π-extended cyclized guests and their mother
uncyclized flexible brominated hosts, is reported. The
combinations
of
bisbenzofuropyrazines/bis(2-
bromophenoxy)pyrazines are the examples. Judicious choice
of the guest/host pair allows to create highly phosphorescent
crystals.
9
0
Keywords: Bisbenzofuropyrazine, Guest/Host Crystal,
Phosphorescence
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
Metal-free pure organic compounds that show room
temperature phosphorescence (RTP) as well as fluorescence
have attracted significant attention in recent years from both
the fundamental and practical points of view. Thus, such
organic phosphors displaying large Stokes shifts and long
emission lifetimes have been extensively studied in the last
decade, as they can find valuable applications such as light
5
0
1
storage, bioimaging, and optoelectronic devices. To realize
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
6
6
6
6
7
7
7
7
7
7
7
7
7
7
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
Figure 1. Structures of host and guest molecules.
the RTP under ambient conditions, it is crucial to facilitate
the intersystem crossing from the exited singlet state to the
triplet state and to suppress non-radiative decay from the
triplet state to the singlet ground state through molecular
vibration and rotation and by air. For tackling the former
issue, heteroatoms involving N, O, S, and halogen have been
introduced to certain π-conjugated systems to appropriately
enhance spin-orbit coupling. Either crystallization of
We have recently reported that 3,9-di-tert-butyl-
substituted bisbenzofuro[2,3-b:2’,3’-e]pyrazine (2a), which
is thermally stable and soluble in common organic solvents,
exhibits intense blue fluorescence in both solution and the
solid state. Interestingly, it forms two crystalline polymorphs,
and both of them show mechanochormic fluorescence, while
1
3
they have different self-recovering profiles. The compound
is readily prepared from 2,5-bis[2-bromo-4-(tert-
butyl)phenoxy]pyrazine (1a) by palladium-catalyzed
2
phosphors or fixing phosphors in certain host matrixes such
3
4
5
as cyclodextrin, hydroxysteroid, zeolite, and polymer film
such as poly(methyl methacrylate)6 is known to be an
effective strategy for solving the latter issue. It has also been
reported that the inclusion of a given phosphor molecule as
guest in an appropriate organic crystal matrix results in the
boost of RTP. This would be an attractive approach as there
are a large number of guest/host combinations that allow fine-
tuning of luminescent color and lifetime. However, this
concept has been proven effective in only a limited number
1
3
intramolecular double direct arylation.
During the
successive investigation into the heterocyclic compounds, we
serendipitously discovered that the starting diether 1a has
good crystallinity and acts as the host crystalline matrix for
the cyclized compound 2a and the resulting inclusion crystal
shows RTP together with fluorescence.
In an initial experiment, after dissolving the compounds
1a and 2a in a ratio of 10:1 wt/wt in dichloromethane
(DCM)/n-heptane, slow evaporation of the solvents at room
1
, 7-9
of examples.
guest/host
In 2011, Kim and coworkers reported a
combination of 2,5-dialkoxy-4-
temperature
gave
the
corresponding
bromobenzaldehyde/2,5-dialkoxy-1,4-dibromobenzene
fluorescent/phosphorescent crystals. Figure 2 shows the
sample tube used for the experiment. Upon irradiation of UV
light (365 nm), the granular crystals formed at the bottom and
the fine crystals deposited at the side wall showed pale and
blue luminescence, respectively. By turning off the light, the
yellow afterglow was observed from the bottom crystals. GC
analysis of one of the bottom crystals confirmed that it was
the host 1a including the guest 2a in ca. 1 wt% (2a@1a). The
gives rise to crystals that emit green RTP as an early elegant
7
example. Zhou, Yi, Wang, and coworkers demonstrated that
an
N-phenyl-2-naphthylamine/4,4’-diboromobiphenyl
8
system effectively shows yellowish green RTP. Besides, co-
crystallization of phosphors with small molecules such as
iodobenzene, 2,3,4,5-tetrafuluoro-1,4-diodobenzene, and
melamine is known to induce RTP. Herein we report a
unique strategy for the fabrication of phosphorescent crystals
1
0
11
1
2

2
1
2
blue luminescence from the wall crystals was attributable
mainly to the unincluded 2a.
46 and Figure 5, respectively. The photophysical data are
47 summarized in Table 2. While the crystal 2a@3 did show
4
8
RTP, it was much weaker than 2a@1a (Entry 2).
3
4
5
6
Figure 2. Photographs of steady state luminescence upon irradiation of
UV light (365 nm) and afterglow of crystals formed from 10:1 wt/wt
mixture of compounds 1a and 2a in DCM/n-heptane.
4
9
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
Figure 3 (top) shows the steady state photoluminescence
(PL) and delayed photoluminescence (DL) spectra of crystals
of compounds 1a and 2a (form 1)13 and of the inclusion
crystal 2a@1a upon excitation at 350 nm under ambient
conditions. Table 1 summarizes the photophysical data. Each
of these crystals show DL. The DL of the crystal 2a itself was
very weak. In contrast, even in the steady state spectrum, the
crystal 2a@1a exhibited a small but obvious DL peak at 547
nm which is assignable to phosphorescence (see below)
together with a large fluorescence peak at 405 nm (dotted
green line in Figure 3). The measured DL lifetime of 2a@1a
was 874 ms, which is significantly longer than those of 1a
(10 ms) and 2a (63 ms) (Figure S1). It should be noted that
the fluorescence peak wavelength of 2a@1a is almost
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
5
5
0
1
Figure 3. Steady state PL and DL spectra (delay time: 1 ms) of 1a, 2a
1
3
52 (top) and 2a@1a upon excitation at 350 nm, and DL excitation spectra
3
identical with that of 2a in CHCl solution, suggesting that
-
5
5
5
3
4
and DL spectra of 1a (at room temperature), 2a in MeCy (1.0 x 10 M,
at 77 K), and 2a@1a (at room temperature) (bottom).
the molecules of 2a are dispersed in the crystalline matrix of
1a and thereby electronic interactions between 2a and the
adjacent molecules are very small. This appears to be the key
to understanding RTP of 2a@1a that shows a relatively long
lifetime. At least, the guest 2a is predominantly responsible
for the afterglow of 2a@1a.
Table 1. PL and DL maxima and DL lifetimes of 1a, 2a, and 2a@1a.
PL
DL
Crystal

max /nm ()

max /nm
DL/ms
1
2
a
a
403 (0.11)
436 (0.52)
405 (0.18)
525
546
10
63
2
a@1a
547, 566(sh), 590(sh)
874
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
5
5
Figure 3 (bottom) shows the DL spectrum of a
methylcyclohexane (MeCy) solution of 2a recorded at 77 K
(assignable to phosphorescence) together with those of
crystals 1a and 2a@1a at room temperature and their
excitation spectra. The fact that the DL spectrum of 2a in the
solution at 77 K is similar to that of inclusion crystal 2a@1a
indicates that the RTP of 2a@1a is predominantly derived
from the dispersed guest molecule 2a as deduced above.
t
We then fabricated the host crystals of 1a (R = Bu), 1b
(R = Me), and non-bromine-containing 3 including guest
t
molecules 2a-c (R = Bu, Me, H) and the non-nitrogen-
containing 4 (the guests were prepared by the palladium-
catalyzed double cyclization of 1a-c and 1,4-bis(4-tert-
butylphenoxy)benzene, respectively, see the SI for details).
Their steady state and delayed photoluminescence spectra
and photographs of the crystals are shown in Figure 4 (top)
5
5
6
7
Figure 4. Steady state PL and DL spectra of crystals 2a@1a, 2a@3,
58 4@1a, and 2b@1b (top), and 6a@5a, 6b@5a, 6c@5a, and 6c@5b
59 (bottom) (excitation wavelength = 350 nm).

3
1
28 version 6b@5b (R = Me) were weaker than those of 6a@5a
29 (Entry 7). The inclusion crystal 6c@5a using the
30 unsubstituted guest 6c and tert-butylated host 5a showed
31 relatively stronger fluorescence, but the phosphorescence
32 was weak (Entry 8). Surprisingly, the unsubstituted guest 6c
33 in the methylated host 5b (6c@5b) exhibited the very strong
Table 2. Fluorescence (F) and phosphorescence (P) wavelengths and
quantum yields and phosphorescence lifetimes of various guest-
included crystals.
a
F
P
c
Entry Crystal
 /nm
 /nm

F

P
p
 /ms
a
1
2a@1a
405
547,
0.15
0.03
874
3
3
36
4
5
phosphorescence together with the relatively weaker
5
66(sh),
fluorescence (
F
= 0.09 and 
P
= 0.38, Entry 9).
5
90(sh)
553
a
2
3
2a@3
368,
0.13 0.001
970
750
4
24(sh)
387
Entry
1
Crystal
Room light
Under UV Afterglow
a
4@1a
496,
0.28
0.16
0.09
0.04
0.02
5
5
14(sh),
2a@1a
2a@3
4@1a
5
37(sh)
549,
a
4
5
2b@1b
406
417
291
370
68(sh),
5
93(sh)
485(sh), 0.20
47
2
3
b
2c@1b
5
b
b
6
7
6a@5a
408
399
499
497,
12(sh)
516
0.16
0.07
0.10
0.04
740
280
6b@5b
5
b
8
9
6c@5a
401 (sh),
0.23
0.09
n.d.
n.d.
290
4
25,
35(sh)
403, 495
4
4
2b@1b
b
6c@5b
482,
0.38
4
95
a
The crystals were obtained by mixing guest/ host in a ratio of 1:10
5
6
7
2c@1b
6a@5a
6b@5b
(
wt/wt) in DCM/n-heptane. The approximate guest contents (X) of
entries 1-4 determined by GC (x/100, guest/host peak ratio) were 1,
2
guest and host in a ratio of 0.1:10 (wt/wt) in DCM/n-heptane.
Determined at the maximum wavelengths (Figure S1).
b
, 3 and 2, respectively. The crystals were obtained by mixing the
c
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
In contrast, the crystal 4@1a fabricated with the di-
tert-butylated bisbenzofurobenzene 5 displayed substantial
fluorescence and phosphorescence in the steady state
spectrum (
F
P
= 0.28 and  = 0.09, Entry 3). The blue-shift
of fluorescence and phosphorescence peaks of 4@1a
compared with those of 2a@1a indicates that the PL derives
predominantly from the guest molecule 4. Attempted
fabrication of crystal of 1a containing the dimethylated
bisbenzofropyradine 2b (2b@1a) was failed to induce visible
RTP (not shown), and it was confirmed that no inclusion
crystal was formed by the fabrication method. However, the
dibromide 1b was found to be an effective host for 2b (Entry
4). Thus, 2b@1b exhibited RTP as efficient as 2a@1a, while
the lifetime of 2b@1b was shorter than that of 2a@1a (291
ms vs 874 ms). The unsubstituted guest 2c could show in the
methylated host 2b (Entry 5 and Figure S2).
8
9
6c@5a
6c@5b
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
Figure 5. Photographs of various guest@host crystals.
3
3
3
4
4
4
4
4
4
4
4
4
7
8
9
0
1
2
3
4
5
6
7
8
Comparison of the phosphorescence of 2a@1a (Entry
1) with that of 2a@3 (Entry 2) suggests that the external
heavy atom effect of the bromine atoms in 1a to enhance
We
also
examined
2,6-bis(2-
2c,7,8,14
spin-orbit coupling promotes the RTP property.
It was
bromophenoxy)pyrazines 5 as hosts and their cyclized
compounds 6 as guests to investigate the structural isomeric
effect on the RTP properties (see the ESI for the synthesis of
5 and 6). For the fabrication of a series of crystals, 10:0.1
wt/wt mixtures of 5 and 6 in the mixed solvent system were
used. The guest/host crystal 6a@5a (R = Bu) showed
substantial PL and PH (
Table 2 and Figure 5). The PL and PH of the methylated
15
confirmed by X-ray crystallography that the crystal-packing
mode of 1a including 2a (2a@1a) is essentially the same with
that of 1a itself (Figure 6, top). It was found that there is an
intermolecular short contact between one of the bromine
atoms and one of the t-butyl hydrogens (2.97 Å), which may
be one of the major fixing forces in the crystal, while
t
F
= 0.16 and 
P
= 0.10, Entry 6 in
2b
flexibility of the ether linkage in 1a would allow to replace

4
4
0
References and Notes
1
1
2
3
4
5
6
a 1a host molecule to a 2a guest molecule. Although the
interaction between 1a and 2a in the crystal cannot be
observed by the X-ray analysis due to the small partial ratio
of 2a, there would be a certain contact to induce the external
For reviews, see: a) S. Mukherjee, P. Thilagar, Chem. Commun.
2015, 51, 10988-11003. b) S. Xu, R. Chen, C. Zheng, W. Huang,
Adv. Mater. 2016, 28, 9920-9940. c) S. Hirata, Adv. Opt. Mater.
2017, 5, 1700116. d) M. Baroncini, G. Bergamini, P. Ceroni,
Chem. Commun. 2017, 53, 2081-2093. e) A. Forni, E. Lucenti,
C. Botta, E. Cariati, J. Mater. Chem. C 2018, 6, 4603-4626. f) H.
Yuasa, S. Kuno, Bull. Chem. Soc. Jpn., 2018, 91, 223-229. g) X.
Ma, J. Wang, H. Tian, Acc. Chem. Res. 2019, 52, 738-748. h) L.
Xiao, H. Fu, Chem. Eur. J. 2019, 25, 714-723. i) P. Data, Y.
Takeda, Chem. Asian J. 2019, 14, 1613-1636.
8
,14
heavy atom effect.
2
Recent examples, see: a) A. Sakai, E. Ohta, Y. Matsui, S.
Tsuzuki, H. Ikeda, ChemPhysChem 2016, 17, 4033. b) M.
Shimizu, T. Kinoshita, R. Shigitani, Y. Miyake, K. Tajima,
Mater. Chem. Front. 2018, 2, 347-354. c) J. Wang, C. Wang, Y.
Gong, Q. Liao, M. Han, T. Jiang, Q. Dang, Y. Li, Q. Li, Z. Li,
Angew. Chem. Int. Ed. 2018, 57, 16821-16826. d) J. Wang, Z.
Chai, J. Wang, C. Wang, M. Han, Q. Liao, A. Huang, P. Lin, C.
Li, Q. Li, Z. Li, Angew. Chem. Int. Ed. 2019, 58, 17297-17302.
e) K. Narushima, Y. Kiyota, T. Mori, S. Hirata, M. Vacha, Adv.
Mater. 2019, 31, 1807268. f) S. Hirata, Adv. Sci. 2019, 6,
1
900410. g) B. Chen, X. Zhang, Y. Wang, H. Miyao, G. Zhang,
Chem. Asian J. 2019, 14, 751-754. h) P. Data, M. Okazaki, S.
Minakata, Y. Takeda, J. Mater. Chem. C, 2019, 7, 6616-6621. i)
Y. Tani, M. Terasaki, M. Kouma, T. Ogawa, J. Mater. Chem. C,
2
019, 7, 11926-11931. j) Y. Wen, H. Liu, S. Zhang, Y. Gao, Y.
Yan, B. Yang, J. Mater. Chem. C, 2019, 7, 12502-12508.
S. Scypinski, L. J. C. Love, Anal. Chem. 1984, 56, 322-327.
a) S. Hirata, K. Totani, J. Zhang, T. Yamashita, H. Kaji, S. R.
Marder, T. Watanabe, C. Adachi, Adv. Funct. Mater. 2013, 23,
7
3
4
8
9
Figure 6. Crystal-packing modes of host compound 1a (top) and 5b
(bottom).
3
386-3397. b) S. Hirata, J. Phys. Chem. Lett. 2018, 9, 4251-4259
5
6
H. Mieno, R. Kabe, N. Notsuka, M. D. Allendorf, C. Adachi,
Adv. Opt. Mater. 2016, 4, 1015-1021.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
The X-ray analyses of 5b and 6c@5b also confirmed the
inclusion of 6c did not change the crystal packing mode of
5b. In the case of 5b, there were found to be bromine-bromine
a) L. Kropp, W. R. Dawson, J. Phys. Chem. 1967, 71, 4499-4506
b) D. Lee, O. Bolton, B. C. Kim, J. H. Youk, S. Takayama, J.
Kim, J. Am. Chem. Soc. 2013, 135, 6325-6329. c) J. S. Ward, R.
S. Nobuyasu, M. A. Fox, A. S. Batsanov, J. Santos, F. B. Dias,
M. R. Bryce, J. Org. Chem. 2018, 83, 14431-14442.
2
(3.64 Å) and bromine-π (η type on the pyrazine ring) (ca.
3.49 Å) short contact interactions (Figure 6, bottom).
Although details of the significant enhancement of the RTP
of 6c by the host 5b are not definitive at this stage, the guest
molecules may be suitably dispersed and fixed in the host
matrix due to their good compatibility, so that vibration and
rotation of the guest are suppressed, the guest is protected
from the air oxygen, and the heavy atom effect works
effectively.
In summary, the present work demonstrates that the
structurally compatible and flexible uncyclized dibromo-
compounds can act as the effective hosts for the
corresponding or related cyclized, -extended guests,
allowing to show RTP. This strategy for the fabrication of
RTP crystals appears to be rather general, and we will further
seek related effective RTP host/guest systems and the results
will be reported in due course.
7
8
9
1
1
O. Bolton, K. Lee, H. J. Kim, K. Y. Lin, J. Kim, Nat. Chem. 2011
3
, 205-210.
J. Wei, B. Liang, R. Duan, Z. Cheng, C. Li, T. Zhou, Y. Yi, Y.
Wang, Angew. Chem., Int. Ed. 2016, 55, 15589-15593.
Y. Lei, W. Dai, Y. Tian, J. Yang, P. Li, J. Shi, B. Tong, Z. Cai,
Y. Dong, J. Phys. Chem. Lett. 2019, 10, 6019-6025.
0
1
T. Ono, A. Taema, A. Goto, Y. Hisaeda, Chem. Eur. J. 2018, 24,
1
7487-17496.
H. Y. Gao, X. R. Zhao, H. Wang, X. Pang, W. J. Jin, Cryst.
Growth Des. 2012, 12, 4377-4387.
B. Zhou, D. Yan, Adv. Funct. Mater. 2019, 29, 1807599.
S. Nakamura, N. Tohnai, Y. Nishii, T. Hinoue, M. Miura,
ChemPhotoChem, 2019, 3, 46-53.
1
1
2
3
1
4
G.G. Giachino, D. R. Kearns, J. Chem. Phys. 1970, 52, 2964-
2
974.
15 The CCDC numbers are 1993108 (1a), 1993109 (5b), and
993110 (6c). The crystal structures of 2a and 2c were
1
previously reported (ref. 17). These data are available free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
This research was supported by a Grant-in-Aid for
Scientific Research from JSPS (Specially Promoted Research,
Grant No. JP 17H06092). We thank Prof. Y. Takeda (Osaka
University) for helpful discussion.
The authors declare no competing financial interest.
Supporting Information (detailed experimental
procedures, spectra, and X-ray crystallographic data)
available on http://dx.doi.org/10.1246/cl.******.