2772
J. Am. Chem. Soc. 1996, 118, 2772-2773
Ground and Excited State Aromatic-Aromatic
Interactions with Distance Control by Hydrogen
Bonding†
Frederick, D. Lewis,* Jye-Shane Yang, and
Charlotte L. Stern
Department of Chemistry, Northwestern UniVersity
2145 Sheridan Road, EVanston, Illinois 60208
ReceiVed NoVember 6, 1995
The structures of the dimers and aggregates formed between
aromatic molecules in the ground and excited states are a subject
of continuing interest.1-5 Theoretical modeling of the benzene
dimer suggests that the face-to-face dimer has an optimum
center-to-center distance of ca. 3.8 Å, whereas the edge-to-face
dimer has an optimum center-to-center distance of 5 Å.2,3
Ground state dimer structure has a profound effect on the
photochemical behavior of excited dimers.4,5 Face-to-face
dimers typically display strongly perturbed excimer-like fluo-
rescence and may undergo photodimerization, whereas edge-
to-face dimers display weakly perturbed exciton-like fluores-
cence and do not photodimerize. Schmidt and co-workers6
found that chlorination of stilbene and several other arylolefins
can result in face-to-face crystal packing with a short axis of 4
Å, as a consequence of attractive intrastack Cl-Cl interactions.7
Thus 2,4-dichlorostilbene displays excimer fluorescence at low
temperature and photodimerizes at higher temperature.6b In
contrast, stilbene-fuctionalized phospholipids are proposed to
adopt edge-to-face geometries in Langmuir-Blodgett assemblies
or vesicles.5 These assemblies display exciton-like fluorescence.
Secondary amide derivatives of both alkane- and arenedi-
carboxylic acids are frequently found to crystallize with 5 Å
translation-related hydrogen-bonded secondary structures (Figure
1a).8,9 The up-down translational arrangement of amide-amide
hydrogen bonding provides a molecular scaffold which permits
investigations of arene-arene interactions at a fixed distance
of 5 Å. We report here the preliminary results of our
investigation of the crystal packing and solid state photochemical
behavior of secondary diamides 1a-3a and of the association
Figure 1. Schematic representations of (a) the 5 Å translational axis
created by optimum amide-amide hydrogen bonding, (b) a hydrogen-
bonded tape of stilbene or biphenyl diamides 1a and 2a, (c) a hydrogen-
bonded tape of diamide 3a, and (d) a hypothetical face-to-face
hydrogen-bonded tape for 2a or 3a.
The arenedicarboxamides were synthesized from the corre-
sponding dicarboxylic acids by standard procedures and recrys-
tallized from mixed solvents.10,11 The crystal structures of 1a-
3a conform to the translational-packing motif of Figure 1a.12
The individual stilbenedicarboxamide molecules in 1a are
nonplanar and noncentrosymmetric with a phenyl-phenyl
dihedral angle of 28.1° and phenyl-amide dihedral angles of
37.6° and 24.8°. Hydrogen-bonded pairs of molecules have the
same 28.1° dihedral angle between phenyl planes separated by
4.95 Å. This packing arrangement is shown schematically in
Figure 1b. The crystal structure of the biphenyldicarboxamide
2a has inter- and intramolecular phenyl-phenyl dihedral angles
of 35.5° with phenyl planes separated by 4.92 Å. The crystal
structure of 3a has a dihedral angle of 67.9° between adjacent
planar diphenylacetylenediamides separated by 5.05 Å, as shown
schematically in Figure 1c.
The separation between hydrogen-bonded pairs of diamides
in 1a-3a corresponds to the optimum amide hydrogen-bonded
structure.8,9 The interplane dihedral angle of 67.9° in 3a is near
the maximum in the distribution of phenylalanine-phenylala-
nine dihedral angles observed for globular proteins.13 Since
(5) (a) Song, X.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1995,
117, 7816. (b) Song, X.; Geiger, C.; Leinhos, U.; Perlstein, J.; Whitten, D.
G. J. Am. Chem. Soc. 1994, 116, 10340.
(6) Schmidt, G. M. J. J. Chem. Soc. 1964, 2014. (b) Cohen, M. D.; Green,
B. S.; Ludmer, Z.; Schmidt, G. M. J. Chem. Phys. Lett. 1970, 7, 486.
(7) For a review see: Venkatesan, K.; Ramamurthy, V. In Photochemistry
in Organized and Constrained Media; Ramamurthy, V., Ed.; VCH: New
Yrok, 1991.
(8) Leiserowitz, L.; Tuval, M. Acta Crystallogr. 1978, B34, 1230.
(9) Garcia-Tellado, F.; Geib, S. J.; Goswami, S.; Hamilton, A. D. J. Am.
Chem. Soc. 1991, 113, 9265.
(10) (a) Lee, B. H.; Marvel, C. S. J. Polym. Sci. Chem. Ed. 1982, 20,
393. (b) Burdett, K. A. Synthesis 1991, 441. (c) Salunkhe, M.; Wu, T.;
Letsinger, R. L. J. Am. Chem. Soc. 1992, 114, 8768.
(11) Reaction of the stilbenedicarboxylic acid with a 1:1 mixture of
methyl- and dimethylamine yielded a ca. 1:2:1 mixture of 1a, 1b, and 1c,
from which 1b was obtained by fractional crystallization. 2b was prepared
analogously.
(12) Crystallographic data were obtained at -120 ( 1 °C using an Enraf-
Nonius CAD4 diffractometer with graphite monochromated Mo KR
radiation. The following are data for the crystal system, space group, color,
unit cell parameters (Å and deg), Z, R, Rw, and GOF: (1a) monoclinic,
P21/c, colorless, a ) 9.889(2), b ) 22.248(3), c ) 6.970(2), â ) 102.85(2),
4, 0.041, 0.035, 1.64. (2a) monoclinic, P21/c, colorless, a ) 9.831(4), b )
19.812 (4), c ) 7.049(2), â ) 101.40(2), 4, 0.050, 0.039, 1.82. (3a) triclinic,
P1h, colorless, a ) 5.853(2), b ) 8.417(4), c ) 17.354(5), R ) 118.14(3),
â ) 94.51(3), γ ) 98.26(4), 2, 0.036, 0.037, 2.02.
and fluorescence of the secondary-tertiary diamides 1b and
2b in organic solvents. These results indicate that amide-amide
hydrogen bonding favors an edge-to-face geometry for neigh-
boring arenes. Comparison of the fluorescence spectra of 1a-
3a in the solid state with those of 1b and 2b in organic solvents
further suggests that these arenedicarboxamides adopt similar
structures in solution and the solid state.
† Dedicated to Marshall Gates, scientist, teacher, editor, on the occasion
of his 80th birthday.
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0002-7863/96/1518-2772$12.00/0 © 1996 American Chemical Society