7024 J. Am. Chem. Soc., Vol. 121, No. 30, 1999
LoVell et al.
propellers on the high side is consistent with an interpretation
of the LD in terms of deviations of the dyes from the layers.
Furthermore, LD always decreased markedly with increasing
temperature indicating the prevalence of thermally excited
internal reorientations or librations of weakly bound dyes.
Conclusion
Are there crystals that, like polymers, can serve as general
hosts for guest molecules, yet provide the narrow orientational
distributions expected of crystals? The intriguing descriptions
of poppy acid prompted us to evaluate this possibility. We
carried out the first total synthesis of 1 while properly character-
izing the product (3) of earlier purported syntheses. We
determined the crystal structures for two polymorphs, 1a and
1b. The great majority of the dyes tested, 15 of 19, produced
crystals of 1a colored in particular growth sectors. 1b also
oriented dyes as did the hydrates of 3. The observations of
pronounced LD were consistent with mixed crystal growth
mechanisms in which the flat dyes substituted for molecules of
1 within the layers and were directed by hydrogen-bound rows
of molecules. Many of the details of the recognition process
still remain to be worked out in ongoing studies. Nevertheless,
the ability of 1 or molecules such as 1 to orient such a wide
variety of chromophores in their crystals indicates that the
application of single crystal matrix isolation may not be nearly
as restricted by constraints of isomorphism as is commonly
assumed.
Figure 4. Plot of Ia/Ib versus Ia/Ic for dyed crystals of 1a expressed as
angular deviations of the transition moments. Circles represent planar
dyes (AZ, MY, BY, BR, MO6, MO10, AY, AO, MB, NR) whereas
triangles represent propeller-shaped dyes (BF, CV, EV, TP).
the INDO/S-CI method.21 The vast majority of the visible
transitions were π-π* in nature and polarized in the mean
molecular planes of the flat or disk shaped dye molecules and
along the long molecular axes. The collected experimental
transition moments were plotted in the crystallographic space
of 1a (Figure 1, a and b) and are suggestive of a general
orientation for the dye molecules, which must span two or in
some cases three layers in the crystals of 1a depending on their
sizes.
Experimental Section
This most obvious interpretation of the experimental results
was unsettling for two reasons: (1) The characteristic feature
of the crystals, their lamellar structure, seemed to play no role
in the orientation of the dye molecules. The guests would be
protruding from the layers. Nevertheless, the maximum absorp-
tion in every case was observed when the incident light was
polarized in the layers of 1a. (2) If the layers are not responsible
for orienting dye molecules, why then would the crystals of 1a
be so effective at orienting in similar directions so many
molecules with very different sizes and shapes? In other words,
what could be the general recognition mechanism?
An alternative way to look at the LD data Ia/Ib and Ia/Ic is to
consider the projections of the transition moments as deviations
from 0° in the first case and deviations from 45° in the second
case. A transition moment calculated to lie between orthorhom-
bic crystallographic axes is not distinguished from a transition
that is disordered in the ac plane. If we assume that the lamellae
served to orient the dyes in the ac plane and that growth
anisotropy preferentially orientated the dyes in the lamellae
along a rather than c, then the data could be consistent with
such a family of structures so long as in every case a component
of the absorption came from molecules in other minor orienta-
tions, or from molecules in random orientations, as might occur
with dyes trapped in solution inclusions. Disorder would drive
the polarization directions away from the a axis.
Synthesis. 1H NMR and 13C NMR spectra were recorded with Bruker
AC-200 MHz and AC-300 MHz spectrometers, respectively. NMR
spectra of all compounds were obtained in DMSO-d6 except 2 (CDCl3).
The EI and high-resolution mass spectra were recorded with a JEOL
HX-110 double focusing magnetic sector mass spectrometer. IR and
UV-vis spectra were recorded with Perkin-Elmer 1640 FT-IR and
Hitachi U-2000 dual beam spectrophotometers, respectively.
3-Bromo-2,6-diethylcarboxylate-γ-pyrone (2). To a flask equipped
with a reflux condenser, stirrer, and dropping funnel was added
diethylacetonedioxalate (51.6 g) in dry CHCl3 (170 mL). Sodium sulfate
(20 g) and a solution of bromine (35.2 g) in CHCl3 (18 mL) were added
dropwise at ∼25 °C over a period of 12 h. The reaction mixture was
then allowed to stand overnight. The filtrate was washed in a separating
funnel with H2O, 2% NH3, and again with H2O. The CHCl3 solution
was dried over Na2SO4 and the solvent was evaporated under reduced
pressure until a pale yellow solid was obtained. The product was
recrystallized from absolute EtOH to give 2 (32.0 g, 50% yield), mp
68 °C; IR (Nujol) 1738, 1667, 1263, 1228, 1152, 1103, 1017, 990,
880, 857, 775 cm-1; 1H NMR (200 MHz, CDCl3) δ 7.14 (s, 1H), 4.40
(q, J ) 7.3 Hz, 2H), 4.36 (q, J ) 7.3 Hz, 2H), 1.41 (t, J ) 7.3 Hz,
3H), 1.34 (t, J ) 7.3 Hz, 3H); 13C{1H} NMR (CDCl3) δ 173.5, 159.0,
158.8, 152.2, 151.8, 118.4, 116.6, 63.7, 63.5, 14.0, 13.9; EI (direct
probe, 70 eV) 318, 290, 273, 262, 246, 234, 218, 200, 190, 177, 149,
147; high-resolution mass spectrum m/z 317.97500 (calcd for
C11H11O679Br 317.97500).
2-Oxalo-3-hydroxy-5-carboxyfuran, potassium salt (3). Diethyl-
bromochelidonate (17.1 g, 53.6 mmol) dissolved in dioxane (135 mL)
was heated to 70 °C in a 1 L flask to which was added a solution of
KOH (1 M, 267 mL). After the mixture was heated for 1 h, the dark
red solution obtained was cooled to room temperature, acidified with
5% HCl, and filtered over activated carbon. The dioxane was evaporated
under reduced pressure. The resulting precipitate was filtered, washed
with ether, and dissolved in hot water. The product was recrystallized
from dioxane to give 3 (4.8 g, 45% yield), mp 270 °C dec; IR (Nujol)
A simple test of this supposition is to plot deviations from
the a axis of the transition moment projected in (001) versus
the deviation from the a axis of the transition moment projected
in (010). The more disorder, the greater would be the deviation
from 0° for light incident on ab, and the smaller would be the
deviation from 45° for light incident on ac. Such a correlation
is shown in Figure 4, with a standard deviation of the residuals
of 2.1 with respect to the ordinate. In this figure we have
separated the planar dyes from the propeller-shaped dyes using
circles and triangles, respectively. The clustering of points for
3490, 1623, 1584, 1299, 1214, 1100, 1003, 952, 918, 809, 770 cm-1
;
1H NMR (200 MHz, DMSO-d6) δ 6.97 (s, 1H); 13C{1H} NMR (DMSO-
d6) δ 174.8, 163.1, 159.3, 157.5, 148.1, 139.7, 110.9; EI (70 eV) 200,
155, 128, 111, 69, 44; high-resolution mass spectrum m/z 199.99649
(calcd for C7H3O7 199.99684).
(21) Ridley, J.; Zerner, M. C. Theor. Chim. Acta 1973, 32, 111-134.