Schiff base route to stackable Pseudo-triphenylenes: Stereoelectronic control of assembly and luminescence

Article abstract of DOI:10.1021/ja061307m

Tandem tautomerization-trapping of isomeric mixtures of tris(N-salicylideneamine)s facilitated access to a new class of discotic molecules that are geometrically analogous to 2,6,10-trisubstituted triphenylenes. The shape and electrostatic complementarity associated with the molecular C₃ symmetry assists cofacial stacking of π-faces to afford either infinite one-dimensional columns or discrete dimeric capsules. Structural reinforcement achieved by such interlocked geometry resulted in significant fluorescence enhancement upon aggregation in solution, as determined by dynamic light scattering and fluorescence spectroscopy. Copyright

Full text of DOI:10.1021/ja061307m

Published on Web 08/04/2006
Schiff Base Route to Stackable Pseudo-Triphenylenes: Stereoelectronic
Control of Assembly and Luminescence
Justin A. Riddle, Stephen P. Lathrop, John C. Bollinger, and Dongwhan Lee*
Department of Chemistry, Indiana UniVersity, 800 East Kirkwood AVenue, Bloomington, Indiana 47405
Received February 23, 2006; E-mail: dongwhan@indiana.edu
π-Conjugated discotic molecules constitute an important class
of materials for electronic and photonic applications.1 One-
dimensional (1-D) columnar stacks of planar aromatics, for example,
display high charge carrier mobility along the columnar axis.2
Considerable research efforts have been devoted to prepare co-
facially stacked supramolecular structures by overcoming intrinsic
repulsive interactions between electron-rich π-faces.3-5 In this
communication, we disclose an expedient two-step synthesis of a
new class of discotic molecules that are geometrically analogous
to triphenylenes. The shape and electrostatic complementarity
associated with their three-fold symmetric molecular core (a)
effectively suppress herringbone-type interactions and assist co-
lumnar organization in the solid state and (b) significantly enhance
their emission efficiency through aggregation in solution.
We have recently shown that C3h-symmetric tris(N-salicyli-
deneamine) (2) can serve as a viable structural surrogate of
triphenylene in constructing biconcave three-dimensional structures
2a-c (Scheme 1).6 In the key triple Schiff base condensation
reaction between 1,3,5-triformylphloroglucinol (1) and ArNH2,
steric constraints imposed by the m-terphenyl fragments facilitate
the exclusive formation of the C3h-symmetric isomer 2. Attempts
to prepare less hindered derivatives of 2, however, have typically
furnished mixtures of inseparable isomers 2 + 3,7 significantly
compromising the utility of these compounds in establishing
structure-property relationships. In parallel with exploring steric
controller groups to meet this synthetic challenge,6 we have been
pursuing a fundamental solution that is less restricted by the
substrate structure.
bonds. Subsequent trapping by suitable {MLx}+ (M ) metal; L )
ligand) fragments should furnish the C3h-symmetric adduct 5 as
the single product (Scheme 1).9,10 This chelation-driVen stereo-
selection was successfully implemented.
The reaction between 1 and p-toluidine (5 equiv) in refluxing
1
EtOH afforded a yellow solid (yield ) 90%). H NMR spectra of
this material revealed the presence of both C3h (2f) and Cs (3f)
isomers in ∼1:2 ratio (Figure S1).11 Upon treating with > 3 equiv
of LDA at 0 °C, a heterogeneous mixture of 2f + 3f in BF3‚Et2O
turned to a clear orange-yellow solution. Quenching with H2O and
subsequent filtration afforded analytically pure 5f in 88% yield as
1
a benchtop stable fluorescent solid. A single sharp H resonance
associated with the three vinyl protons indicated a symmetric core
geometry (Figure S1), which was unambiguously confirmed by
X-ray crystallography (Figure S2). This tandem tautomerization-
trapping protocol was applicable to other aniline derivatives to
afford 5d-i in 70-96% purification yields (Scheme 1).11
The peripheral aryl groups of these pseudo-triphenylenes ef-
fectively assist 1-D organization in the solid state. As shown in
Figure 1, cofacially stacked molecules of 5e are rotated 60° with
respect to each other to minimize steric crowding. Notably, this
gearlike molecular arrangement maximizes electrostatic comple-
mentarity between adjacent stacks by positioning electronegative
fluorine atoms (red, Figure 1) in direct van der Waals contact with
electropositive imine carbon atoms (blue, Figure 1).12 The molecular
3-fold symmetry reinforces this noncovalent interaction by provid-
ing six pairs of intermolecular Fdifluoroboronyl(δ-)‚‚‚Cimine(δ+) contacts
between each stacking pair. Alternating center-to-center distances
of 3.549 and 4.196 Å were measured between the perfectly eclipsed
benzene rings that constitute the 1-D column of 5e (Figure 1c).
Compound 5f affords similar structures in the solid state with
alternating center-to-center distances of 3.877 and 4.030 Å (Figure
S2).
As shown in Scheme 1, the keto-enamine (2 and 3) versus enol-
imine (2′ and 3′) tautomerism8 of the tris(N-salicylideneamine) core
is a concerted shuttling of three protons, which exclusively affords
keto-enamine both in solution and solid states.6,7 Restricted rotations
about the enamine C-C double bonds suppress interconversion
between 2 and 3. We envisioned that deprotonation of a mixture
of 2 + 3 would afford enolates 4 with freely rotating C-C single
The substitution pattern on the peripheral aryl groups has a
profound effect on the assembly behavior of 5. With bulky groups
Scheme 1. Synthetic Route to an Isomeric Mixture of Tris(N-salicylideneamine) 2 + 3 and Their Stereoselective Conversion to
Tris(difluoroboronyl) Adduct 5
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J. AM. CHEM. SOC. 2006, 128, 10986-10987
10.1021/ja061307m CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. Emission spectra (T ) 25 °C, λex ) 402 nm) of 5f (0.50 mM)
in CHCl3-hexanes (a) and in CHCl3-acetone (b) with increasing volume
fractions (indicated by arrows) of 0, 15, 30, 45, 60, 75, and 90% (v/v)
hexanes (a) and acetone (b), respectively. Measurements were made
immediately after mixing CHCl3 stock solutions of 5f with appropriate
solvents.
4.3%) of sterically hindered 5i was also essentially invariant to the
addition of hexanes or acetone.15 Details of these intriguing
structure-property relationships are currently under investigation.
In summary, cooperative steric and electrostatic interactions
enforce cofacial stacking of discotic molecules, the fluorescence
efficiency of which increases upon aggregation in solution. Such
structural and photophysical properties promise the synthetic utility
of this new class of materials that are modularly accessible from
simple precursors.
Figure 1. Capped-stick representation of the X-ray structure of 5e viewed
along the columnar axis (a) and its side view (b). Alternating layers of
molecule A (top) and B (bottom) constitute an infinite 1-D columnar stack
shown in panel c as space-filling model and its schematic representation.
Shown next to A and B are electrostatic potential mapped onto the electron
density surface of the {C6O6(CHN)3(BF2)3} core fragment (dotted circle):
red indicates negative electrostatic potential and blue indicates positive
potential.
installed at the 2- and 6-positions, discrete dimeric structures were
obtained instead of infinite cofacial stacks. As shown in Figures 2
and S3, six interdigitating isopropyl groups of dimeric 5i serve as
a molecular “seam” to completely enclose a cavity (∼140 Å3)
inside, which was occupied by one benzene (∼100 Å3) molecule.
The solid-state structure of this inclusion complex reveals the
π-stacking propensity of the molecular core.
Shape-dependent assembly of pseudo-triphenylenes was further
probed by fluorescence spectroscopy and dynamic light scattering
(DLS) studies of solution samples. In neat CHCl3, 5f is weakly
fluorescent (ΦF ) 2.9%). With increasing volume fraction of
hexanes in CHCl3-hexanes mixed solvents, however, its fluores-
cence efficiency increases by as much as 4-fold without shifts in
Acknowledgment. This work was supported by Indiana Uni-
versity, the National Science Foundation (CAREER CHE 0547251),
and the American Chemical Society Petroleum Research Fund
(Grant 42791-G3).
Supporting Information Available: Experimental details and
crystallographic data in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
References
(1) (a) Mu¨llen, K., Wegner, G., Eds. Electronic Materials: The Oligomer
Approach; Wiley-VCH: New York, 1998. (b) Guillon, D. Struct. Bonding
1999, 95, 41-82. (c) Watson, M. D.; Fechtenko¨tter, A.; Mu¨llen, K. Chem.
ReV. 2001, 101, 1267-1300.
(2) Wu, J.; Grimsdale, A. C.; Mu¨llen, K. J. Mater. Chem. 2005, 15, 41-52.
(3) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525-
5534.
(4) For a recent review on the incorporation of noncovalent interactions to
direct cofacial π-stacking of discotic molecules, see: Bushey, M. L.;
Nguyen, T.-Q.; Zhang, W.; Horoszewski, D.; Nuckolls, C. Angew. Chem.,
Int. Ed. 2004, 43, 5446-5453 and references therein.
(5) Supramolecular Dye Chemistry; Wu¨rthner, F., Ed.; Springer-Verlag:
Berlin, Germany, 2005.
λmax,em (Figure 3a). Preliminary DLS studies revealed the formation
of submicrometer-sized solution aggregates of 5f. This self-
association process was optimized using a 1:9 (v/v) mixture of
CHCl3-hexanes, in which 5f (0.50 mM) reproducibly afforded
particles with hydrodynamic diameters (DH) approaching 400 nm
within 1 h (Figure S4a).13 The time-dependent evolution of the
system involved a rapid buildup of narrowly dispersed aggregates
with a concomitant increase in fluorescence intensity (Figure S4).
We speculate that 5f confined within such aggregates has limited
torsional motions, and this structural rigidification presumably
suppresses nonradiative decay of the excited states.14 The addition
of polar solvents such as acetone induced neither aggregation of
5f nor change in emission (Figure 3b).15 The quantum yield (ΦF )
(6) Riddle, J. A.; Bollinger, J. C.; Lee, D. Angew. Chem., Int. Ed. 2005, 44,
6689-6693.
(7) (a) Yelamaggad, C. V.; Achalkumar, A. S.; Rao, D. S. S.; Prasad, S. K.
J. Am. Chem. Soc. 2004, 126, 6506-6507. (b) Sauer, M.; Yeung, C.;
Chong, J. H.; Patrick, B. O.; MacLachlan, M. J. J. Org. Chem. 2006, 71,
775-788.
(8) Sobczyk, L.; Grabowski, S. J.; Krygowski, T. M. Chem. ReV. 2005, 105,
3513-3560.
(9) Atwood, D. A.; Harvey, M. J. Chem. ReV. 2001, 101, 37-52.
(10) For transition metal complexes supported by phloroglucinol-derived
“triplesalen” ligands, see: (a) Glaser, T.; Gerenkamp, M.; Fro¨hlich, R.
Angew. Chem., Int. Ed. 2002, 41, 3823-3825. (b) Glaser, T.; Heidemeier,
M.; Grimme, S.; Bill, E. Inorg. Chem. 2004, 43, 5192-5194.
(11) See Supporting Information.
(12) A 1:1 cocrystal of triphenylene-perfluorotriphenylene has stacked
columnar arrangements through intermolecular electrostatic interactions:
Weck, M.; Dunn, A. R.; Matsumoto, K.; Coates, G. W.; Lobkovsky, E.
B.; Grubbs, R. H. Angew. Chem., Int. Ed. 1999, 38, 2741-2745.
(13) Upon standing over an extended period of time at 298 K, yellow needles
of 5f began to deposit on the surface.
(14) (a) Sandanayake, K. R. A. S.; Nakashima, K.; Shinkai, S. J. Chem. Soc.
Chem. Commun. 1994, 1621-1622. (b) McFarland, S. A.; Finney, N. S.
J. Am. Chem. Soc. 2001, 123, 1260-1261.
Figure 2. (a) Space-filling representation of the X-ray structure of (5i)2 ⊃
C6H6: red, top molecule; blue, bottom molecule. (b) Capped stick
representation of (5i)2 and encapsulate benzene molecule in space-filling
representation. (c) Top view of (5i)2 ⊃ C6H6 showing only the bottom 5i
(blue) and benzene (gray).
(15) Only random autocorrelation function was obtained in DLS.
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