Control of the CT interaction between electron-donor and -acceptor moieties of a 1,4-dicyanonaphthalene-arene dyad for intermolecular exciplex or excimer formation in crystals

Article abstract of DOI:10.1016/j.tetlet.2010.08.086

Colorless crystals of 1,4-dicyano-2-(4′-methoxybenzyloxy) methylnaphthalene (2), which is a 1,4-dicyano-2-methylnaphthalene (DCMN)-4-methylanisole (MA) dyad linked by an ether unit, selectively form a fluorescent intermolecular DCMN-MA exciplex (greenish blue, λ _f^exciplex=456nm). In contrast, 1,4-dicyano-2-(4′- methylbenzyloxy)methylnaphthalene (3), which is a DCMN-p-xylene dyad, forms a fluorescent intermolecular DCMN-DCMN excimer in the crystalline state (blue, λ_f^excimer=404nm). These findings demonstrate that a moderate charge transfer interaction takes place between the DCMN moiety of 2 and MA moieties of the adjacent molecules of 2, which successfully facilitates the preparation of light-emissive organic crystals.

Full text of DOI:10.1016/j.tetlet.2010.08.086

Tetrahedron Letters 51 (2010) 5877–5880
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Control of the CT interaction between electron-donor and -acceptor moieties
of a 1,4-dicyanonaphthalene–arene dyad for intermolecular exciplex or
excimer formation in crystals
Mitsutaka Imoto a,b, Hiroshi Ikeda a,c, , Maki Ohashi a, Motonori Takeda b, Akihiro Tamaki b,
⇑
Hisaji Taniguchi a, Kazuhiko Mizuno a,c,
⇑
a Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
b Seika Corporation, 1-1-82 Kozaika, Wakayama 641-0007, Japan
c The Research Institute for Molecular Electronic Devices (RIMED), Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
a r t i c l e i n f o
a b s t r a c t
Colorless crystals of 1,4-dicyano-2-(40-methoxybenzyloxy)methylnaphthalene (2), which is a 1,4-dicy-
ano-2-methylnaphthalene (DCMN)–4-methylanisole (MA) dyad linked by an ether unit, selectively form
a fluorescent intermolecular DCMN–MA exciplex (greenish blue, kef xciplex ¼ 456 nm). In contrast, 1,4-dicy-
ano-2-(40-methylbenzyloxy)methylnaphthalene (3), which is a DCMN–p-xylene dyad, forms a fluores-
cent intermolecular DCMN–DCMN excimer in the crystalline state (blue, kef xcimer ¼ 404 nm). These
findings demonstrate that a moderate charge transfer interaction takes place between the DCMN moiety
of 2 and MA moieties of the adjacent molecules of 2, which successfully facilitates the preparation of
light-emissive organic crystals.
Article history:
Received 28 July 2010
Revised 26 August 2010
Accepted 27 August 2010
Keywords:
Charge transfer
CT complex
Ó 2010 Elsevier Ltd. All rights reserved.
Fluorescence
Light-emissive organic crystals
Crystal architecture
Inter-1 and intramolecular2 exciplexes have been studied in
solution from the viewpoint of the mechanism of their formation,
and their photophysical3,4 and photochemical properties.5 How-
ever, approaches to utilize the properties of exciplexes, especially
their fluorescence, for luminescent organic single crystals6 have
rarely been explored despite potential organic functional materials
in a variety of roles.7 Recently, we explored8 a new crystal architec-
ture concept9 in which charge transfer (CT) complexes in the crys-
talline state serve as the foundation of organic crystals that emit
fluorescence via exciplexes. In the first test of this concept, we ob-
served that the exciplex fluorescence of crystals of the 1,4-dicyano-
2-methylnaphthalene (DCMN)–N,N-dimethyl-p-toluidine (DMT)
dyad (1, Chart 1) was weak (Uf = 0.09) as a consequence of the con-
tribution of electron transfer quenching. This originated from the
strong CT interaction between the DCMN and DMT moieties in
the ground state. We believed that a potential solution to this
problem would be to allow only moderate CT interaction between
electron-donor (D) and -acceptor (A) moieties of a dyad in the crys-
tals, thus, avoiding electron transfer quenching and leading the D
and A moieties to preorient in a configuration suitable for exciplex
formation when photoexcited.
In this work, we synthesized two new dyads: 1,4-dicyano-2-(40-
methoxybenzyloxy)methylnaphthalene (2)10 and 1,4-dicyano-2-
(40-methylbenzyloxy)methylnaphthalene (3).11 The former is a
DCMN–4-methylanisole (MA) dyad linked by an ether unit, while
the latter is a DCMN–p-xylene (XY) dyad. Herein, we present con-
trasting results for 2, which forms a fluorescent intermolecular
DCMN–MA exciplex in the crystalline state, and 3, which forms a
fluorescent intermolecular DCMN–DCMN excimer (Fig. 1). It is
demonstrated that the moderate CT interaction between the
DCMN and MA moieties of 2 in the crystals is an important factor
in preparing light-emissive organic crystals.
Cyclohexane solutions of
2
and
3
(5.0 ꢀ 10ꢁ5 mol/L) are
colorless under natural light, as shown in Figure 2a and c. UV–vis
spectra of them (Fig. 3) exhibited intense absorption bands at
250–350 nm, which are nearly the same as those of DCMN,9
CN
CN
CH3
H3C
O
R
R
CN
CN
DCMN
1 R = N(CH3)2
2 R = OCH3
3 R = CH3
DMT R = N(CH3)2
MA R = OCH3
XY R = CH3
⇑
Corresponding authors. Tel.: +81 72 254 9289; fax: +81 72 254 9289.
E-mail addresses: ikeda@chem.osakafu-u.ac.jp (H. Ikeda), mizuno@chem.osaka
fu-u.ac.jp (K. Mizuno).
Chart 1. Structures of DCMN, 1–3, DMT, MA, and XY.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.08.086

5878
M. Imoto et al. / Tetrahedron Letters 51 (2010) 5877–5880
Table 1
(a) Intermolecular Exciplex Formation
CN
Fluorescence properties of 2 and 3 in cyclohexane and in the crystals
kmf onomer (nm)
kef xciplex (nm) kfexciplex (nm)
O
Dyad Solvent
kf
OCH3
2
3
Cyclohexanea 361
456
456
417
—
Ndb
—
0.22
0.39
0.25
0.29
NC
Ndb
CN
Crystals
Cyclohexanea 363
Ndb
404
O
Crystals
Ndb
CH3O
NC
a
5.0 ꢀ 10ꢁ5 mol/L.
2
b
Not detectable.
(b) Intermolecular Excimer Formation
CN
O
456 nm that is assignable to an intramolecular exciplex (Fig. 2a,
bottom, Fig. 3, blue bold, Uf = 0.22, fluorescence lifetime
sef xciplex ¼ 33:7 ns). This result contrasts with observations made
in studies with 3 where a cyclohexane solution emits blue fluores-
cence (Fig. 2c, bottom) when photoexcited at 320 nm. The fluores-
cence spectrum of 3 (Fig. 3, blue thin) shows a monomer emission
from the DCMN moiety at ca. 350 nm ðsmf onomer ¼ 1:4 nsÞ and a typ-
CH3
CN
CN
O
CH3
CN
3
ical intramolecular exciplex emission at 417 nm
12:1 nsÞ. The total Uf for these two emissions is 0.25 (Table 1).
ð ¼
sef xciplex
Figure 1. Schematic representation of intermolecular exciplex formation of 2 (a)
and intermolecular excimer formation of 3 (b) in the crystals.
Dyad 2 is colorless under natural light in the crystalline state as
well as in cyclohexane solution (Fig. 2b, top) and it displays green-
ish blue fluorescence when irradiated at 365 nm (Fig. 2b bottom).
As shown in Figure 4, fluorescence of 2 in the crystalline state
has a maximum at 456 nm with Uf = 0.39 that is higher than that
in cyclohexane (0.22) and matches that of the exciplex fluores-
cence band of 2 in cyclohexane (see Fig. 3 and Table 1). X-ray crys-
tallographic analysis shows that the DCMN and MA moieties of 2
are coplanar with each other and that 2 intermolecularly stacks
with MA and DCMN moieties of the adjacent molecules (Fig. 5).13
Thus, the intermolecular face-to-face overlap between DCMN and
MA moieties of 2 in the crystalline state results from CT interaction
in the ground state in which the CT transition dipole moments can-
cel each other, as shown in Figure 5.
Similarly to crystals of 2, crystals of 3 are colorless and emit
blue fluorescence (Fig. 2d, bottom) at 404 nm upon excitation at
320 nm (Uf = 0.29, Fig. 4, blue thin). However, an X-ray crystallo-
graphic analysis clearly shows that differences exist in the crystal-
line states of 2 and 3. Namely, effective
p-stacking takes place
between the DCMN moiety of one molecule 3 and the DCMN moi-
ety of an adjacent molecule 3 (Fig. 6).13 This observation suggests
that the blue fluorescence of crystals of 3 results from an intermo-
lecular DCMN–DCMN excimer.14 Note that the CT interaction be-
tween the DCMN and XY moieties of 3 is evidently inefficient
compared to the DCMN–DCMN interaction.
In conclusion, we have successfully prepared light-emissive or-
ganic crystals (Uf = 0.39) with the crystal architecture concept uti-
lizing the CT interaction of the DCMN–MA dyad of 2. We have
found that the moderate CT interaction between the D (MA) and
A (DCMN) moieties in the crystals is an important factor in the
Figure 2. Photographs of 2 (a and b) and 3 (c and d) in cyclohexane and in the
crystals under natural light (top) and under irradiation at 365 nm (bottom).
0.5
0.4
0.3
0.2
0.1
0
150
100
50
1
0.5
0
0
300
400
500
600
Wavelength / nm
Figure 3. UV–vis spectra of 2 (black, bold) and 3 (black, thin) in cyclohexane
(5.0 ꢀ 10ꢁ5 mol/L), and fluorescence spectra of 2 (blue, bold) and 3 (blue, thin) in
cyclohexane (excitation wavelength 320 nm).
together with week absorptions at 350–370 nm.12 These observa-
tions indicate that intramolecular CT complexes of 2 and 3 are
not formed in cyclohexane. When the cyclohexane solution of 2
is irradiated with 320 nm light, fluorescence from the DCMN moi-
ety is barely observed, but a greenish blue fluorescence is seen at
300
400
500
600
Wavelength / nm
Figure 4. Fluorescence spectra of crystals of
2
(blue bold) and
3
(blue thin)
(excitation wavelength 320 nm).

M. Imoto et al. / Tetrahedron Letters 51 (2010) 5877–5880
5879
Figure 5. Molecular geometry (top left) and crystal structure of 2.
Figure 6. Molecular geometry (top left) and crystal structure of 3.
preparation of light-emissive organic crystals. The moderate CT
interaction avoids electron transfer quenching on excitation and
leads the D and A moieties to preorient in a configuration suitable
for exciplex formation. In the case of 2, the emission of the organic
crystals is attributed to the exciplex fluorescence. In this sense, not
only are the electron-donating and electron-accepting properties
of the D and A moieties in the ground state important;17 but those
in the excited state are also crucial and controlling. Further studies,
aimed at generalization and quantification of the crystal architec-
ture concept, are now in progress.
Acknowledgments
This study was supported by the Cooperation for Innovative
Technology and Advanced Research in Evolutional Area (CITY
AREA) program by the Ministry of Education, Culture, Sports, Sci-

5880
M. Imoto et al. / Tetrahedron Letters 51 (2010) 5877–5880
9. (a) Greer, M. L.; McGee, B. J.; Rogers, R. D.; Blackstock, S. C. Angew. Chem., Int. Ed.
1997, 36, 1864; (b) Hasegawa, T.; Kagoshima, S.; Mochida, T.; Sugiura, S.; Iwasa,
Y. Solid State Commun. 1997, 103, 489; (c) Bis, J. A.; Zaworotko, M. J. Cryst.
Growth Des. 2005, 5, 1169; (d) Batsanov, A. S.; Bryce, M. R.; Chesney, A.;
Gershitenman, H.; Becker, J. Y.; Khodorkovsky, V. Y.; Perepichka, I. F.; Rotello,
V.; Gray, M. J. Mater. Chem. 2001, 11, 2181.
ence and Technology (MEXT), Japan. H.I. gratefully acknowledges
the financial support in the form of a Grant-in-Aid for Scientific Re-
search on Priority Areas ‘New Frontiers in Photochromism’ (Nos.
20044027 and 21021025 in the Area No. 471) and Innovative Areas
‘p-Space’ (Nos. 21108520 in the Area No. 2007), the Scientific Re-
10. Preparation
of
2:
A
solution
containing
1,4-dibromo-2-(40-
search (B) (No. 20044027), and the Challenging Exploratory Re-
search (No. 21655016) from the MEXT of Japan. We thank
Associate Professor Norimitsu Tohnai and Mr. Issei Tsuji (Osaka
University) for the X-ray crystallographic analysis and photograph-
ing the crystals.
methoxybenzyloxy)methylnaphthalene (2.2 g, 2.7 mmol), NaCN (0.53 g,
11 mmol), Pd(PPh3)4 (0.64 g, 0.55 mmol), and CuI (0.21 g, 1.1 mmol) in
propionitrile (7 mL) was stirred at reflux for 2 h and cooled to room
temperature. After addition of EtOAc (60 mL), the mixture was filtered
through a Celite pad, washed with water and brine, and the solvent was
concentrated in vacuo. The resulting dark-brownish residue was subjected to
silica-gel chromatography with hexane–EtOAc (9:1), giving 0.88 g of crude 2 as
pale yellow powder in 99% yield. Recrystallization from CH3OH gave a pure 2
as colorless columns. Mp 110–111 °C; 1H NMR (DMSO-d6, 400 MHz) dppm 3.74
(s, 3H, OCH3), 4.60 (s, 2H, CH2), 4.88 (s, 2H, CH2), 6.92 (AA0BB0, J = 8 Hz, 2H),
7.35 (AA0BB0, J = 8 Hz, 2H), 7.94–8.00 (m, 2H), 8.22–8.27 (m, 2H), 8.34 (s, 1H);
13C NMR (DMSO-d6, 100 MHz) dppm 55.1 (OCH3), 68.8 (CH2), 72.1 (CH2), 112.7,
113.7 (2C), 113.7, 114.8 (CN), 116.3 (CN), 125.3, 125.4, 129.5, 129.6 (2C),
130.4, 130.4, 130.8, 131.3, 132.4, 142.6, 158.9; UV–vis (cyclohexane) kmax
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a
similar procedure
described for 2: mp 124–125 °C (MeOH); 1H NMR (DMSO-d6, 400 MHz) dppm
2.29 (s, 3H, CH3), 4.63 (s, 2H, CH2), 4.89 (s, 2H, CH2), 7.17 (AA0BB0, J = 8 Hz, 2H),
7.30 (AA0BB0, J = 8 Hz, 2H), 7.94–8.00 (m, 2H), 8.21–8.27 (m, 2H), 8.35 (s, 1H);
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113.7, 114.8 (CN), 116.3 (CN), 125.3, 125.4, 128.0 (2C), 128.9 (2C), 130.4, 130.4,
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(
C
e
= 9038 molꢁ1 L cmꢁ1); IR (diamond ATR)
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in the ground state in
m
2218 cmꢁ1 (CN); Anal. Calcd for
a
interaction of the DCMN and DMT moieties of
cyclohexane.
2
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