Layered donor-acceptor-acceptor arrangement in complex of molecular tweezer based on dioxa[2.2]orthocyclophane

Article abstract of DOI:10.1016/S0040-4039(99)02095-X

Flexible molecular receptors having three aromatic chromophores can bind electron deficient π-system such as TCNQ and TCNB with both face-to-face and face-to-edge interactions. The receptors have tweezer-type conformations when a guest was trapped within their cavities. Sandwich-type donor-acceptor- acceptor (DAA) arrangement was realized in the crystalline supramolecular complex.

Full text of DOI:10.1016/S0040-4039(99)02095-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 477–480
Layered donor–acceptor–acceptor arrangement in complex of
molecular tweezer based on dioxa[2.2]orthocyclophane
Hirotaka Kurebayashi, Takeharu Haino and Yoshimasa Fukazawa ∗
Department of Chemistry, Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
Received 30 August 1999; revised 25 October 1999; accepted 2 November 1999
Abstract
Flexible molecular receptors having three aromatic chromophores can bind electron deficient π-system such
as TCNQ and TCNB with both face-to-face and face-to-edge interactions. The receptors have tweezer-type
conformations when a guest was trapped within their cavities. Sandwich-type donor–acceptor–acceptor (DAA)
arrangement was realized in the crystalline supramolecular complex. © 2000 Elsevier Science Ltd. All rights
reserved.
Noncovalent interaction between aromatic rings and neutral planar molecules plays an important
role both in the structure and property of supramolecular complexes. Molecular tweezers,1 containing
two aromatic chromophores connected by a spacer are suitable receptors for planar, π-electron guests
since they can hold the guest with the two aromatic arms through the π-stacking interactions.2 Efficient
binding of a π-electron acceptor can be attained by the two aromatic binding arms of the host forming
a charge transfer type interaction.3 Rigid tweezers,4 having a preorganized face-to-face arrangement
of the two aromatic chromophores are known to be favorable for strong binding of a guest. Flexible
tweezer,5 on the other hand, has the adaptability for the variation of guests, though there should be an
entropic cost for the face-to-face orientation of the two arms. In a previous paper,6 we reported that
the flexible tweezer (1) based on dioxa[2.2]orthocyclophane7 can bind π-electron deficient guests such
as tetracyanoquinodimethane (TCNQ), pyromellitic dianhydride (PDA), and tetracyanobenzene (TCNB)
effectively to form a 1: 1 complex both in solution and in the crystalline state (1·TCNQ). We also reported
that if one of the two binding chromophores was as small as a benzene ring, a 1:1 complex formation was
not observed in the crystalline state (2·TCNQ), hence, the cavity size of the tweezer plays an important
role in effective binding.
∗
Corresponding author.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02095-X
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In a continued effort to understand the detailed mechanism of guest binding, we have found that the
replacement of the benzene ring of the binding arm in 2 by benzoic acid ester resulted in the effective
binding of the π-electron deficient guest with the tweezer-type conformation in the crystalline state. In
this paper, we report on the detailed molecular structures of these supramolecular complexes.
The molecular receptors 3 and 4 were synthesized by a similar route as those of 1 and 2 (Scheme 1).
Successive coupling of 1,2,4,5-tetrakis(bromomethyl)benzene 5 with 9,10-dihydroxyphenanthrene 6 and
the corresponding aromatic diols in the presence of Cs2CO3 in acetone gave the corresponding receptors
3 or 4.
Scheme 1. (a) 1,2,4,5-Tetrakis(bromomethyl)benzene, Cs2CO3, acetone, 20%; (b) for 3: methyl-3,4-dihydroxybenzoate,
Cs2CO3, acetone, 89% for 4: dimethyl-4,5-dihydroxyphthalate, Cs2CO3, acetone, 81%
The structures of the supramolecular complexes were examined by X-ray crystallography8 and ORTEP
drawings of these complexes are shown in Fig. 1. As can be seen clearly, host 3 has a tweezer-type
conformation with face-to-face arrangement of the two terminal aromatic rings. They lie parallel to each
other and the distance between them is 6.78 Å. The guest (TCNQ) resides in the cavity of the host and
has both the stacking interaction between the two terminal aromatic chromophores and the edge-to-face
interaction9 with the central aromatic ring of the host. The guest is parallel to each of the two aromatic
chromophores of the host. The distances between the two chromophores and the guest are 3.56 (from
methyl benzoate side) and 3.37 Å (from phenanthrene side), respectively. The two hydrogens of the guest
are pointing towards the central aromatic ring of the host at distances of 2.78 and 3.03 Å, suggesting a
favorable edge-to-face interaction. The ester group of 3 lies within the plane of the benzene ring to which
it attached. It might have the role of expanding the cavity size in which the guest resides in order to
increase the contact area between the host and guest.
Due to the heavy repulsion between them, the two ester groups in 4 cannot lie within the aromatic
plane to which they are attached. In spite of this nonplanar aromatic chromophore, 4 can grasp an electron
deficient guest (TCNB) in the crystalline state. The host has also a tweezer-type conformation with a face-
to-face arrangement of the two terminal aromatic rings. These aromatic rings lie parallel to each other and
the distance between them is 7.10 Å, slightly longer than the corresponding value for 3·TCNQ, reflecting
the nonplanar aromatic chromophore. The structural characteristics of 4·TCNB are very similar to those

479
Fig. 1. ORTEP drawings of (a) 3·TCNQ and (b) 4·TCNB
in 3·TCNQ except for the longer interplanar distances between the nonplanar chromophore and the guest
and between the above mentioned terminal chromophore. The structural details of these complexes are
summarized together with those of 1·TCNQ for comparison (Table 1).
Table 1
Interplanar distances (Å) and angles (Å) of supramolecular complexes
Apart from their role in expanding the contact area between the host and guest, the electron with-
drawing ester groups change the electronic character of the terminal aromatic ring from donating to
accepting. The change of the electronic character can be clearly shown with the semi-empirical molecular
orbital calculation (PM3) for 7 (Table 2). Thus, the sandwich arrangement of the three components,
phenanthrene, TCNQ, and methyl benzoate produces donor–acceptor–acceptor (DAA) stacking.10
Table 2
HOMO and LUMO energy (eV) of 7
A simple consideration based on the perturbation theory11 suggested the decrease of the LUMO energy
level of acceptor guest by the interaction between the terminal aromatic ester moiety, since the LUMO
of the latter is higher in energy (Table 2) than the former (TCNQ (−3.07 eV), TCNB (−2.33 eV)).

480
The decrease of the guest LUMO facilitates the charge transfer interaction between the other terminal
chromophore (phenanthrene). Thus, the aromatic ester terminal plays an important role not only in the
increase of the contact area with the guest but also in facilitating the charge transfer interaction.
In conclusion, we demonstrated sandwich-type arrangement of donor–acceptor–acceptor (DAA) both
in 3·TCNQ and 4·TCNB in the crystalline state. It is a clear demonstration that not only the layered
arrangement of the donor–acceptor–donor (DAD) but also DAA sandwich-type arrangement gives stable
intermolecular complexes in the crystalline state.
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prisms, space group Pb21a, orthorhombic, a=15.557(1), b=17.218(1), c=29.998(1) Å, V=8035.3(1) Å3, Z=4, 8729 measured,
8000 were observed (F>3σ (F)). R=0.089, wR2=0.082; Complex 42·TCNB3: orange prisms, space group P21/c, monoclinic,
a=16.341(1), b=13.104(1), c=18.190(1) Å, b=93.30(2)°, V=3904(1) Å3, Z=2, 9897 measured, 7482 were observed (F>3σ
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