Contrasting intermolecular and intramolecular exciplex formation of a 1,4-dicyano-2-methylnaphthalene- N,N -dimethyl- p -toluidine Dyad

Article abstract of DOI:10.1021/ol100324b

Figure presented An intramolecular exciplex is formed upon excitation of the cyclohexane solution of the 1,4-dicyano-2-methylnaphthalene-N,N-dimethyl-p- toluidine dyad, but little if any intramolecular CT complex exists in the ground state of this substance in solution. In contrast, in the crystalline state, the dyad forms an intermolecular mixed-stack CT complex in the ground state and an intermolecular exciplex when it is photoexcited.

Full text of DOI:10.1021/ol100324b

ORGANIC
LETTERS
2010
Vol. 12, No. 9
1940-1943
Contrasting Intermolecular and
Intramolecular Exciplex Formation of a
1,4-Dicyano-2-methylnaphthalene-N,N-
Dimethyl-p-toluidine Dyad
Mitsutaka Imoto,†,‡ Hiroshi Ikeda,*,†,§ Takayuki Fujii,† Hisaji Taniguchi,†
Akihiro Tamaki,‡ Motonori Takeda,‡ and Kazuhiko Mizuno*,†,§
Department of Applied Chemistry, Graduate School of Engineering, and The Research
Institute for Molecular Electronic DeVices (RIMED), Osaka Prefecture UniVersity,
1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan, and Seika Corporation,
1-1-82 Kozaika, Wakayama 641-0007, Japan
ikeda@chem.osakafu-u.ac.jp; mizuno@chem.osakafu-u.ac.jp
Received February 7, 2010
ABSTRACT
An intramolecular exciplex is formed upon excitation of the cyclohexane solution of the 1,4-dicyano-2-methylnaphthalene-N,N-dimethyl-p-
toluidine dyad, but little if any intramolecular CT complex exists in the ground state of this substance in solution. In contrast, in the crystalline
state, the dyad forms an intermolecular mixed-stack CT complex in the ground state and an intermolecular exciplex when it is photoexcited.
Inter-1 and intramolecular2 exciplexes are often observed in
solution. Their formation mechanism and photophysical and
-chemical properties3 have been studied in terms of molecular
structure4 and solvent effects.5 Very recently, exciplex
emission resulting from charge recombination of the electron-
hole in organic elctroluminescence devices is becoming the
focus of attention.6 However, approaches to the utilizing of
exciplex emission rarely have been tried on luminescent
organic single crystals, although luminescent organic crystals
attract much attention due to their potential applications to
a variety of organic functional materials.7
Hirayama found that linking a donor (D) and an acceptor
(A) with a three-carbon methylene chain, as represented by
D-(CH2)3-A, results in effective formation of an intramo-
lecular exciplex through conformational change leading to
charge-transfer (CT) processes (Hirayama rule).8 Although
most exciplexes are observed in the solution phases, some
exciplexes are observed in the crystalline or solid states.9
For instance, Becker et al. found an intramolecular
anthracene-ethylene exciplex generated by photoexcitation
of a lepidopterene in the crystals.10 To the best of our
knowledge, however, few studies have been conducted on
molecules which form inter- and intramolecular exciplexes
† Graduate School of Engineering, Osaka Prefecture University.
‡ The Research Institute for Molecular Electronic Devices (RIMED),
Osaka Prefecture University.
§ Seika Corporation.
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10.1021/ol100324b  2010 American Chemical Society
Published on Web 03/30/2010

in both the crystalline and solution states,11 though the
formation of excimers in the crystalline state is relatively
common.12
Crystal engineering of CT complex is a very interesting
research field, because an exquisite packing of D and A can
be readily achieved in the crystalline state.13 We think of
idea that we apply the crystal engineering of CT complex to
preparing luminescent organic crystals, in which a face-to-
face alignment of D and A forms emissive exciplex.
Therefore, in this work, we designed and synthesized a new
dyad, 1,4-dicyano-2-[4′-(N,N-dimethylamino)benzyloxy]-
methylnaphthalene (1, Figure 1), and studied its optical
Figure 3. Photographs of DCMN (a and c) and 1 (b, d, and e) in
cyclohexane and in crystals under natural light (top) and under light
at 365 nm (bottom).
spectrum of 1 in cyclohexane (Figure 4, black and bold) is
virtually the sum of the spectra of DCMN (black and solid)
and DMT (black broken line) and exhibits intense absorptions
at 250-350 nm and weak absorptions at 350-400 nm.14
These findings indicate that an intramolecular CT complex
of 1 is hardly formed in cyclohexane, even though the strong
acceptor DCMN is interlinked with the strong donor DMT.
When a cyclohexane solution of DCMN (5.0 × 10-5 mol/
L) is excited with 320-nm light, blue fluorescence is observed
at ca. 350 nm (Figure 3a, bottom, and Figure 4, blue) with
a quantum yield (Φf) of 0.29 and a fluorescence lifetime
(τfMONO) of 3.7 ns. When DMT is added to the DCMN
solution ([DMT] ) 5.0 × 10-3 mol/L), a weak emission band
corresponding to an intermolecular exciplex between DCMN
and DMT is observed at 526 nm along with an intense
monomer emission from the singlet excited DCMN at ca.
350 nm. On the other hand, excitation of a dilute cyclohexane
solution of dyad 1 (5.0 × 10-5 mol/L) at 320 nm leads to
Figure 1. Chemical structures of DCMN, 1, and DMT.
properties in solution and the crystalline or solid states. In
this paper, we present the results of studies that show that
an intermolecular exciplex is formed in crystals of 1 while,
as expected, dyad 1 forms an intramolecular exciplex in
cyclohexane solution (Figure 2). In addition, observations
significantly decreased emission from the DCMN moiety
MONO
(λem
) 354 nm, τfMONO ) 3.7 ns) and orange fluores-
EX
cence (λem ) 550 nm, Φf ) 0.05, τfEX ) 13.7 ns; Figure
3b, bottom, and Figure 4, red). The fluorescence, which is
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Figure 2. Schematic for the formation of CT complexes and
intermolecular and intramolecular exciplexes of 1.
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made in this effort clearly demonstrate that the intermolecular
exciplex of 1 in the crystalline state results directly from
excitation of a ground-state intermolecular CT.
As shown in Figure 3a,b, cyclohexane solutions of 1 and
DCMN are colorless under natural light. The absorption
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Org. Lett., Vol. 12, No. 9, 2010
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similar to that of the intermolecular exciplex of DCMN and
DMT in cyclohexane, is assigned to an intramolecular
exciplex involving the DCMN and DMT moieties of 1.
Although a cyclohexane solution of 1 is colorless under
natural light (see Figure 3b), needle crystals of 1 (from 1:3
CHCl3-CH3OH, mp 160-161 °C) are yellow under natural
light (Figure 3d). Interestingly, prisms of 1 (evaporatively
recrystallized from CH3OH solution of the yellow needles,
mp 165-166 °C) are red (Figure 3e). For comparison
purposes, columnar crystals of DCMN are essentially color-
less (Figure 3c). These observations suggest that an interac-
tion takes place between the DCMN and DMT moieties in
both yellow needles and red prisms of 1. Actually, a diffuse
reflection spectrum of the red prisms of 1 in a KBr pellet,
obtained after Kubelka-Munk transformation, contains a
broad and structureless band in the 350-550 nm region
(Figure 5, black bold line). A similar band is not observed
5, blue), which is referred to aggregation of DCMN in the
crystals. In contrast, yellow needles of 1 emit yellow
fluorescence (Φf ) 0.10; Figure 3d bottom) at 550 nm, being
in good agreement with the exciplex fluorescence in cyclo-
hexane (see the Supporting Information). The corresponding
red prisms show orange fluorescence (Φf ) 0.09; Figure 3e,
bottom) upon excitation at 365 nm.
The red prisms of 1 fluoresce in the 500-750 nm region
EX
(λem ) 567 nm; Figure 5, red), a band that is associated
with exciplex fluorescence. In fact, X-ray crystallographic
analysis of the red prisms15,16 shows that the DCMN moiety
in 1 is twisted by 106° from the plane of the DMT moiety
Figure 4. Absorption spectra of 1 (black, bold), DCMN (black,
solid), and DMT (black, broken) in cyclohexane (5.0 × 10-5 mol/
L) and fluorescence (λex ) 320 nm) spectra of 1 (red) and DCMN
(blue) in cyclohexane.
Figure 5. Diffuse reflection and fluorescence (λex ) 320 nm) spectra
of colorless columnar crystals of DCMN (black thin and blue) and
red prism crystals of 1 (black bold and red) in KBr.
Figure 6. Molecular geometry (top, left) and crystal structure of
the red prisms of 1.
for DCMN (Figure 5, black thin line). This finding indicates
that CT interaction between the DCMN and DMT moieties
occurs in the red prisms of 1.
Columnar crystals of DCMN display typical blue fluo-
rescence upon excitation at 365 nm (Figure 3c, bottom). Also,
a broad emission band at ca. 450 nm is produced by
excitation of columnar crystals of DCMN (Φf ) 0.49; Figure
(Figure 6),17 and it is successfully stacked with the DMT
moiety of an adjacent molecule of 1. The DCMN moiety of
this “adjacent molecule” is further stacked with the DMT
moiety of another molecule of 1, as shown in Figure 6. Thus,
intermolecular face-to-face overlap between the DCMN and
DMT moieties of 1 in the red prisms results in formation of
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Org. Lett., Vol. 12, No. 9, 2010

an intermolecular one-to-one mixed-stack ground state CT
complex that has a transition dipole moment continuously
oriented in a zigzag manner. The results clearly demonstrate
that emission from the red prisms of 1 is a consequence of
an intermolecular exciplex occurring between the DCMN
and DMT moieties aligned in a face-to-face orientation
through CT interactions.
through a pathway involving a conformational change and
CT. In contrast, in the crystalline state 1 exists in an infinite
intermolecular CT ground-state complex that is capable of
directly forming an intermolecular exciplex when photoex-
cited.
In conclusion, we have found that a dyad 1 hardly forms
a CT complex in the ground state but that it produces an
intramolecular exciplex between the DCMN and DMT
moieties in cyclohexane in accord with the Hirayama rule.
In contrast, crystals of the dyad 1 form an intermolecular
mixed-stack, one-to-one CT complex in the ground state.
The observations made in this effort clearly demonstrate that
the intermolecular exciplex, generated by excitation of
crystals of 1, arises directly from the ground state CT
complex, in which the DCMN and DMT moieties are
immobilized in a face-to-face manner by intermolecular CT
interactions. In addition, the studies have shown that the
difference in color between the yellow needles and the red
prisms would be due to a difference in their crystal structures.
We are now in the process of investigating the effects of
crystal strucure on the formation of the intermolecular
exciplex of 1 and the mechanism for the formation of inter-
and intramolecular exciplexes of 1 in crystals and various
solutions.
Scheme 1. Simplified Energy Diagrams of Intra- and
Intermolecular Exciplex Formation of 1 in Cyclohexane and in
the Crystals, Respectivelya
a Key: A, DCMN moiety; D, DMT moiety.
Acknowledgment. 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, Science and
Technology (MEXT), Japan. We thank Associate Professor
Norimitsu Tonai (Osaka University) for photographing the
crystals and Dr. Yasuhito Miyake (Industrial Technology
Center of Wakayama Prefecture) for X-ray crystallographic
analysis. H.I. acknowledges financial support in the form of
a Grant-in-Aid for Scientific Research on Priority Areas
“New Frontiers in Photochromism” (Nos. 20044027 and
21021025 in the Area No. 471) and Innovative Areas “π-
Space” (Nos. 21108520 in the Area No. 2007), the Scientific
Research (B) (No. 20044027), and the Challenging Explor-
atory Research (No. 21655016) from the MEXT of Japan.
Scheme 1 shows simplified energy diagrams for intra- and
intermolecular exciplex formation for 1 in cyclohexane
solution and the crystalline state. Photoexcitation of a
cyclohexane solution of dyad 1, in which its ground state
does not participate in strong CT interactions, results in
production of an exciton that, prior to monomer emission
from the DCMN moiety, forms an intramolecular exciplex
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S.; Mochida, T.; Sugiura, S.; Iwasa, Y. Solid State Commun. 1997, 103,
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H.; Becker, J. Y.; Khodorkovsky, V. Y.; Perepichka, I. F.; Rotello, V.;
Gray, M. J. Mater. Chem. 2001, 11, 2181–2191.
Supporting Information Available: Synthetic details and
optical and electrochemical properties of 1. This material is
available free of charge via the Internet at http://pubs.acs.org.
(14) The weak absorption between 350-400 nm may indicate a feeble
CT interaction of the DCMN and DMT moieties of 1 in the ground state.
However, solvent effects on absorption spectrum are not significant. See
the Supporting Information.
(15) Crystallographic data of the red prisms (mp 165-166 °C) of 1 have
been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC 752019.
(16) Many efforts to obtain a satisfactory single crystal for X-ray
crystallographic analysis of the yellow needles (mp 160-161 °C) of 1 were
unsuccessful, but are still continued and will be published elsewhere.
OL100324B
(17) The molecular geometry determined with X-ray crystallographic
analysis was also supported by density function theory calculation B3LYP/
6-31G(d,p).
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