Self-assembly of C3-symmetrical hexaaryltriindoles driven by solvophobic and CH-π interactions

Article abstract of DOI:10.1021/jo902013v

(Graph Presented) The synthesis and aggregation properties of a series of differently substituted star-shaped hexaaryltriindoles both in solution and in the solid state are being reported.While thesemolecules do not show any significant intermolecular aggregation inCDCl₃, it has been possible to induce aggregation by increasing the polarity of the solvent and therefore facilitating the occurrence of solvophobic forces. A study of the influence of the electronic character of peripheral substituents on the self-association behavior in solution has shown that increasing the electron-donor character of the substituents facilitates self-association while derivatives substituted with electron-acceptor substituents do not self-assemble. The electronic nature of the substituents also has an influence in the geometry of the stacking of these derivatives observed in the solid state. While unsubstituted hexaphenyl triindole self-assemble in a staggered face-toface arrangement, attaching six cyano functional groups results in an offset stacking. The influence of the substituents in the strength and geometry of the stacking tendency contrasts with the trend expected for an aggregation induced solely by π-π interactions, but can be explained considering an important contribution of multiple cooperative CH-π interactions.

Full text of DOI:10.1021/jo902013v

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Self-Assembly of C3-Symmetrical Hexaaryltriindoles Driven by
Solvophobic and CH-π Interactions
Eva M. Garcıa-Frutos,† Gunther Hennrich,‡ Enrique Gutierrez,† Angeles Monge,† and
,†
ꢀ
Berta Gomez-Lor*
†Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain and
‡
´
ꢀ
Departamento de Quımica Organica, Universidad Autonoma de Madrid, 28049 Madrid, Spain
ꢀ
bgl@icmm.csic.es
Received September 17, 2009
The synthesis and aggregation properties of a series of differently substituted star-shaped hexaaryl-
triindoles both in solution and in the solid state are being reported. While these molecules do not show any
significant intermolecular aggregation in CDCl3, it has been possible to induce aggregation by increasing
the polarity of the solvent and therefore facilitating the occurrence of solvophobic forces. A study of the
influence of the electronic character of peripheral substituents on the self-association behavior in solution
has shown that increasing the electron-donor character of the substituents facilitates self-association
while derivatives substituted with electron-acceptor substituents do not self-assemble. The electronic
nature of the substituents also has an influence in the geometry of the stacking of these derivatives
observed in the solid state. While unsubstituted hexaphenyl triindole self-assemble in a staggered face-to-
face arrangement, attaching six cyano functional groups results in an offset stacking. The influence of the
substituents in the strength and geometry of the stacking tendency contrasts with the trend expected for an
aggregation induced solely by π-π interactions, but can be explained considering an important
contribution of multiple cooperative CH-π interactions.
Introduction
complicate their individual study. The influence of the elec-
tronic nature of substituents attached to the aromatic units on
the association processes has been systematically analyzed in
order to shed light on the intrinsic nature of these interactions,
but sometimes the effect exerted by the substituents in the
strength of the different interactions follows opposed trends
and therefore conclusions have to be drawn with caution.
For example, π-π stacking is usually favored by attach-
ment of electron-withdrawing substituents to the aromatic
surfaces3 as a consequence of the decrease of a repulsive
Intermolecular interactions involving aromatic rings are
of primary importance in biochemistry and supramolecular
and materials chemistry.1 Important processes such as
protein-ligand recognition, host-guest binding, or the self-
assembly of organized nanostructures are the result of the
interplay of different weak interactions2 such as π-π stack-
ing and XH-π (X = O, N, C) or cation-π interactions.
In spite of their importance both in synthetic and natural
events, the study of these interactions is usually difficult due to
their weak nature and their cooperative occurrence that
(3) Several models have been designed to quantify the influence of the
substituents on the stacking: (a) Cockroft, S. L.; Perkins, J.; Zonta, C.; Adams,
H.; Spey, S. E.; Low, C. M. R.; Vinter, J. G.; Lawson, K. R.; Urch, C. J.;
Hunter, C. A. Org. Biomol. Chem. 2007, 5, 1062–1080. (b) Cozzi, F.; Annun-
ziata, R.; Benaglia, M.; Cinquini, M.; Raimondi, L.; Baldridge, K. K.; Siegel,
J. S. Org. Biomol. Chem. 2003, 1, 157–162. (c) Cozzi, F.; Cinquini, M.;
Annunciata, R.; Dwyer, T.; Siegel, J. S. J. Am. Chem. Soc. 1992, 114, 5729–
5733.(d)Rashkin,M. J.;Waters, M. L. J. Am. Chem. Soc. 2002, 124, 1860–1861.
*To whom correspondence should be addressed. Phone: (þ34) 91-
3349031. Fax: (þ34) 91-3720623.
(1) Meyer, E. A.; Castellano, R. K.; Diederich, F. Angew. Chem., Int. Ed.
2003, 42, 1210–1250.
(2) Steed, J. W.; Atwood, J. L. Supramolecular Chemistry; John Wiley and
Sons: Chichester, UK, 2009.
1070 J. Org. Chem. 2010, 75, 1070–1076
Published on Web 01/14/2010
DOI: 10.1021/jo902013v
r
2010 American Chemical Society

Garcıa-Frutos et al.
JOCArticle
component resulting from two approaching π electron
clouds.4 In contrast, the effect of the substituents on
cation-π5 or CH-π6 interactions follows an opposite trend
and the magnitude of these interactions usually increases as
attaching electron-donating substituents. The substituents
also have an influence in the geometry of the stacking motifs.
Thus, electron-withdrawing groups decrease the π electron
density and facilitate a face-to-face geometry as a result of
the diminution of the π electron repulsion while increasing
the π electron density favors an offset or T-shaped staking,
hence maximizing attractive π-σ interactions.
The recent growth of the field of organic electronics has
aroused a renewed interest on the understanding of weak
intermolecular interactions involving π-conjugated mole-
cules, a prerequisite to induce their self-assembly into one-
dimensional nanostructures. Face-to-face π-stacks of ex-
tended aromatics with the π-surfaces located within van der
Waals distances have emerged as promising nanowires and
are among the best performing organic semiconductors.7 In
addition, self-assembled arrays formed in solution have been
successfully transferred onto solid supports,8 resulting in
highly oriented films with important implications in the
development of molecule-based electronic devices.
precursors for the construction of triazafulleres,10 C3-sym-
metrical tripods,11 or optically active materials in electronic
devices.12 In addition, we have recently reported on the
stacking properties in solution of a number of hexaarylalk-
ynyl triindoles.13
In this paper we describe the synthesis and self-assembly of a
series of N-alkyl-substituted hexaaryltriindoles functionalized
with peripheral substituents of different electronic nature. The
electronic communication of the terminal groups with the
central electron-rich triindole core has been confirmed by
optical spectroscopy and cyclic voltammetry. While these
molecules exist as monomers in CDCl3 solutions, we have
induced aggregation by adding polar solvents and therefore
facilitating the appearance of solvophobic forces in some of
these derivatives. These molecules pack in the solid state as
dimeric assemblies as has been determined by X-ray analysis of
single crystals of two different derivatives showing opposed
aggregation behavior in solution. In fact, crystallographic
packing evidences the cooperative contribution of a number
of CH-π interactions between the R-methylenic group of the
N-alkyl chains and the external rings of the triindole core.
Results and Discussion
In this context we became interested on the electron-rich
10,15-dihydro-5H-diindolo[3,2-a:30,20-c]carbazole (triindole).
This molecule can be formally considered as an extended
π-system in which three carbazole units share an aromatic
ring. Triindole-based single crystalline or liquid crystalline
materials have been found to exhibit high charge mobilities
(up to μ = 0.4 cm2 V-1 s-1) since they combine the good
hole transport properties characteristic of carbazoles with
highly ordered columnar supramolecular arrangements.9
Triindole derivatives have also been recognized as promising
Synthesis of Hexaaryltriindoles. The synthesis of the new
hexasubstituted triindoles starts from known13 symmetrical
N-dodecyl hexabromotriindole 1. The presence of six bro-
mine atoms at strategic positions in this derivative offers
varied opportunities for a versatile functionalization apply-
ing different cross-coupling methodologies.
Six-fold Suzuki coupling of 1 with a variety of phenyl-
boronic acids in the presence of Pd(PPh3)4 and 2 M aqueous
K2CO3, using THF as solvent, readily gave 2a-e (61-89%),
displaying six aryl groups surrounding a triindole nucleus in
good yield. The presence of the long alkyl chain on the
nitrogen functionalities confers to the molecule enhanced
solubility and stability and therefore facilitates the Suzuki
coupling reactions (Scheme 1).
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series of triindoles have been investigated by UV-vis ab-
sorption and fluorescence spectroscopy (Table 1, Figure 1)
and cyclic voltammetry (Figure 2 and in the Supporting
Information Figures S1-S5).
With the exception of 2e whose fluorescence is efficiently
quenched due to the presence of the nitro substituents,14 the
triindoles are well fluorescent with compound 2a displaying
an especially high fluorescence quantum yield of 0.81 in
solution.15 The large Stokes shifts between 78 and 127 nm
observed for all triindole derivatives are the consequence of
both the nuclear reorganization taking place after irradiation
and prior to emission as a result of electronic redistribution
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FIGURE 1. (a) UV-vis absorption spectra of 2a-e in CH2Cl2, c = 5 ꢀ 10-6 mol L-1; (b) normalized fluorescence spectra of 2a, 2b, and 2d,
c = 5 ꢀ 10-6 mol L-1; excitation at 350 nm; 2c omitted for clarity.
SCHEME 1. Synthesis of Hexaaryltriindoles 2a-e
TABLE 1. UV-Vis and Fluorescence Spectroscopic Properties of 2a-e
in CH2Cl2
compd
λmax,abs/ nm
log (ε)
λmax,em/nm
Φf
2a
2b
2c
2d
2e
341
339
338
351
386
5.055
5.133
5.074
4.989
4.777
419
419
422
478
0.81
0.73
0.31
0.12
and the conformational changes in the biphenyl fragments
(planarization) upon excitation.16
The influence of the peripheral groups on the electronic
properties of the differently substituted hexaaryltriindoles is
clearly reflected in their spectroscopic behavior. While ab-
sorption and emission spectra of 2a-c are almost identical,
absorption and emission spectra of 2d-e show distinc-
tive features of charge transfer: high extinction coefficients
FIGURE 2. Cyclic voltammogram of 2b, 2c, and 2e at c=10-3 mol
L-1 in CH2Cl2 and 0.1 M tetra-n-butylammonium hexafluorophos-
phate (TBAPF6) at a scan rate of 100 mV/s, using a Pt working
electrode.
(16) (a) Mank, D.; Raytchev, M.; Amthor, S.; Lambert, C.; Fiebig, T.
Chem. Phys. Lett. 2003, 376, 201–206. (b) Hennrich, G.; Cavero, E.; Hanes,
R. E.; Serrano, J. L. Eur. J. Org. Chem. 2008, 4575–4579.
(17) The spectroscopic study of compounds 2a-d was conducted at
different concentrations (5 ꢀ 10-5 to 10-7 mol/L) and different solvents
(MeOH and CH2Cl2) to determine if some aggregation behavior can be
observed by optical spectroscopy. No aggregation is observed in the sub-
millimolar concentration range required for spectroscopy.
in the order of 105 M-1 cm-1, bathochromic shifts of the
absorption and emission maxima (Δλmax,abs (2b-e) = 47 nm,
Δλmax,em (2b-e) = 59 nm) accompanied by a continuous loss
of the vibronic fine structure of the respective band.17
1072 J. Org. Chem. Vol. 75, No. 4, 2010

Garcıa-Frutos et al.
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Compounds 2d-e represent π systems with a marked octo-
polar (nondipolar) character,18 in which the charge-transfer
process is predominantly occurring between the electron-
rich triindole core and the peripheral substituents.
To determine the average size of the aggregates, we have
performed DOSY (diffusion-ordered spectroscopy) experi-
ments in CDCl3/CD3OD (7:3) on two different derivatives,
one that aggregates in this solvent mixture (2a) and another
that does not associate (2d) as indicated by the insensitivity of
its NMR signals to concentration changes.
This NMR technique provides information of the transla-
tional diffusion coefficient of the molecular species in solu-
tion and is being increasingly employed to gain insight into
the nature of supramolecular structures in solution.22 The
diffusion coefficients (D) can be related to the hydrodynamic
radius (R) of the species in solution by the Stokes-Einstein
equation: D = kBT/(6πηR), where T is the temperature, η is
the viscosity of the solvent, R is the hydrodynamic radius of
the molecule or assembly, and kB is the Boltzmann constant.
Thus, the size of the corresponding species can be compared
based on their diffusion coefficients when the other para-
meters are identical.
The increase of the electron density in the π system upon
attaching electron-donating (2a,b) substituents has been de-
monstrated by cyclic voltammetry. Compounds 2a-e can be
easily oxidized to stable radical cations19 and higher cationic
charged species in the accessible potential window of the
solvent. The oxidizing potentials are markedly influenced by
the electronic nature of the substituents reflecting the electronic
communication of the external substituents with the central
electron-rich triindole core. Thus, attaching electron-donating
groups (2a,b) results in an increase of the electron density in the
π system and in consequence in a shift of the oxidation
potentials to lower values. In contrast, upon attaching periphe-
ral electron-withdrawing groups (2d,e), the oxidation potential
shifts anodically with increasing the acceptor character of the
substituents (Figure 2; also see the Supporting Information).
Self-Assembly Behavior in Solution. Disk-shaped mole-
cules and related shape-persistent macrocycles have a strong
tendency to exhibit aggregation in solution forming either
dimeric20 structures or extended stacks.21 The aggregation
behavior of this family of molecules was studied by concentra-
tion-dependent 1H NMR spectroscopy. Chemical upfield shifts
of the aromatic signals upon increasing the concentration in
nuclear magnetic resonance spectroscopy have been well docu-
mented as a signature of aromatic interactions. When two or
more aromatic units come into close vicinity of each other, the
nuclei of one molecule are affected by the ring-current magnetic
anisotropy of the other, resulting in resonance shifting.
The DOSY spectra of 2a and 2d were recorded in CDCl3/
CD3OD (7:3) at a 1.5 ꢀ 10-2 M concentration. The two-
1
dimensional spectra shown in Figure 3 correlate H NMR
chemical shifts with the diffusion coefficient for 2a and 2d,
respectively. For 2a only a single apparent diffusion coefficient
was observed since there is a fast exchange between the mono-
mer and the aggregated species. The apparent diffusion coeffi-
cient found for compound 2a is very similar to that of mono-
meric 2d (approximately 2.9 ꢀ 10-10 m2 s-1) used as reference.
This observation suggests the preference toward dimerization
over extended aggregation. Due to the extended shape of these
molecules dimers are expected to have only a slightly larger
hydrodynamic volume than monomers.22a,23 In contrast highly
aggregated species would have much larger R values than
monomers and consequently when comparing DOSY NMR
spectra of 2a with 2d an important decrease of the D value
would be expected.
Further evidence of the tendency of these molecules to
dimerize was obtained from the MALDI-TOF MS spec-
tra of 2a in which a major peak for the monomer and a
peak of lower intensity for the dimer can be detected, and no
higher aggregates are observed (Supporting Information,
Figure S6).
1
The H NMR spectra of 2a-e show no concentration-
dependent chemical shifting indicating that these molecules
only exist as monomers in chloroform solutions. However, we
attempted to drive self-assembly by adding certain amounts of
polar solvents. Since the compounds are not soluble in neat
CD3OD and CD3COCD3 at the NMR concentration level,
the NMR samples were prepared by adding the maximum
volume of polar solvents to chloroform solutions (prior to
1
precipitation). The H NMR spectra of 2a-c recorded in
mixed solvent systems CDCl3/CD3COCD3 (7:3) and CDCl3/
CD3OD (7:3) are notably sensitive to concentration changes
with signals of the aromatic protons moving upfield as the
concentration increased from 10-3 to 10-2 M . A pronounced
upfield effect can also be observed for the R-CH2 proton
signals of the N-dodecyl chains. In contrast, 1H NMR spectra
of 2d,e are not sensitive to concentration changes (Figure S13,
Supporting Information).
The relative tendency of compounds 2a-c to aggregate in
solution was therefore quantitatively investigated assuming a
predominant monomer-dimer equilibrium.24 The association
constants (Ka) were determined by least-squares curve fitting25
(22) (a) Chen, Z.; Stepanenko, V.; Dehm, V.; Prins, P.; Siebbeles, L. D.
€
A.; Seibt, J.; Marquetand, P.; Engel, V.; Wurthner, F. Chem.;Eur. J. 2007,
13, 436–449. (b) Giuseppone, N.; Schmitt, J.-L.; Allouche, L.; Lehn, J.-M.
Angew. Chem., Int. Ed. 2008, 120, 2267–2271. (c) For a review on DOSY
experiments on supramolecular chemistry see: Cohen, Y.; Avram, L.; Frish,
L. Angew. Chem., Int. Ed. 2005, 44, 520–554.
(18) (a) Zyss, J.; Ledoux, I. Chem. Rev. 1994, 94, 77–105. (b) Beljonne, D.;
Wensleers, W.; Zojer, E.; Shuai, Z.; Vogel, H.; Pond, S. J. K.; Perry, J. W.;
Marder, S. R.; Bredas, J.-L. Adv. Funct. Mater. 2002, 12, 631–641.
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(19) Gomez-Lor, B.; Hennrich, G.; Alonso, B.; Monge, A.; Gutierrez-
Puebla, E.; Echavarren, A. M. Angew. Chem., Int. Ed. 2006, 45, 4491–4494.
€
€
(23) (a) Dobrawa, M.; Lysetska, M.; Ballester, P.; Grune, F.; Wurthner,
F. Macromolecules 2005, 38, 1315–1325. (b) Viel, S.; Mannina, L.; Segre, A.
Tetraheron Lett. 2002, 43, 2515–2519.
(24) Although definitive correlations between the behavior in solution
and in solid state cannot be driven the aggregation motif observed in the solid
state further supports the tendence of these molecules to dimerize.
(25) The least-squares curve fitting to the theoretical equation:
€
(20) (a) Wu, J.; Fechtenkotter, A.; Gauss, J.; Watson, M. D.; Kastler, M.;
Fechtenkotter, C.; Wagner, M.; Mullen, K. J. Am. Chem. Soc. 2004, 126, 11311–
€
€
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11321. (b) de Frutos, O.; Granier, T.; Gomez-Lor, B.; Jimenez-Barbero, J.;
Monge, A.; Gutierrez-Puebla, E.; Echavarren, A. M. Chem.;Eur. J. 2002, 8,
2879–2890. (c) Shetty, A. S.; Zhang, J.; Moore, J. S. J. Am. Chem. Soc. 1996, 118,
!
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1 - 8KCt þ 1
€
1019–1027. (d) Klyatskaya, S.; Dingenouts, N.; Rosenauer, C.; Muller, B.;
€
Hoger, S. J. Am. Chem. Soc. 1996, 128, 3150–3151.
€
(21) (a) Kastler, M.; Pisula, W.; Wasserfallen, D.; Pakula, T.; Mullen, K.
J. Am. Chem. Soc. 2005, 127, 4286–4296. (b) Klyatskaya, S.; Dingenouts, N.;
δ ¼ δm þ ðδa -δmÞ 1 þ
4KCt
where Ct is the total substrate concentration and δ the observed 1H NMR
chemical shifts, was carried out in order to determine the association
constant Ka, and the chemical shifts of the monomer δm and the dimer δd.
Martin, R. B. Chem. Rev. 1996, 96, 3043–3064.
€
€
Rosenauer, C.; Muller, B.; Hoger, S. J. Am. Chem. Soc. 1996, 128, 3150–3151.
(c) Tobe, Y.; Utsumi, N.; Kawabata, K.; Nagano, A.; Adachi, K.; Araki, S.;
Sonoda, M.; Hirose, K.; Naemura, K. J. Am. Chem. Soc. 2002, 124, 5350–5364.
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FIGURE 4. Concentration dependence of 1H NMR chemical shifts
for the lowest field singlets of 2a-c in CDCl3/CD3COCD3 (7:3).
TABLE 2. Association Constants and Chemical Shifts Calculated for
the Lowest Field Resonance for 2a-c
CDCl3/CD3COCD3 (7:3)
CDCl3/CD3OD (7:3)
a
b
c
a
b
δd
c
δm
compd
Ka (M-1
)
δd
δm
Ka (M-1
)
2a
2b
2c
223 ( 26
161 ( 20
68 ( 9
7.57
7.64
7.64
7.97
8.01
8.02
421 ( 140
286 ( 120
247 ( 81
7.69
7.73
7.77
8.21
8.15
8.23
aDetermined at room temperature on the basis of a dimerization
model. bCalculated chemical shifts for the dimer. cCalculated chemical
shifts for the monomer.
surrounding solvent and therefore decreases their stacking
propensity.
However, the involvement of other additional forces in the
aggregation process cannot be discarded. In particular, the
clear upfield shift of the R-CH2 proton signals of the N-
dodecyl chains upon increasing the concentration would be
in agreement with the contribution of CH-π interactions in
these aggregation processes. In this platform, the occurrence
of CH-π interactions is facilitated by the electron-with-
drawing effect of the nitrogen on the CH component and is
expected to be favored upon increasing the electron density
of the aromatic partner explaining the observed stacking
trends.27
Crystal Structure. To obtain deeper understandings of the
intermolecular interactions taking place between these deri-
vatives we have grown crystals of two triindole derivatives
(2c and 2d) showing opposed aggregation trends, mimicking
the conditions found in solution to induce self-assembly.28
Compound 2c forms stable crystals by slow evaporation of a
CH2Cl2:acetone (7:3) solvent mixture. Crystals of compound
2d were obtained from slow evaporation of a CH2Cl2:EtOAc
(7:3) solvent mixture, but they readily decompose out of the
crystallization mixture presumably due to the loss of the
cocrystallized solvent molecules. Therefore the crystals were
soaked in oil immediately after extraction from the solvent
FIGURE 3. DOSY NMR spectra of 2a and 2d at c = 1.5 ꢀ 10-2
mol L-1 in CDCl3/CD3OD (7:3). The signals of chloroform and
methanol are marked with an asterisk. Chloroform and methanol
molecules appear at significantly larger diffusion coefficient as
expected from their much smaller size.
of the concentration-dependent NMR chemical shifts for
triindole hydrogens in positions meta to the nitrogens26 since
these are the lowest field resonance and are clearly differen-
tiated from the rest of the signals (Figure 4).
The association constants calculated were found to range
from 68 to 223 M-1 in CDCl3/ CD3COCD3 (7:3) and
from 247 to 421 M-1 in CDCl3/CD3OD (7:3) (Table 2, see
Supporting Information).
From the value of the association constants it can be
concluded that increasing the electron-donating character
of the terminal p-phenyl substituents leads to self-associa-
tion while attaching electron-withdrawing groups inhibits
aggregation as previously observed in related phenylethy-
nyl derivatives.13 Derivatives functionalized with elec-
tron-withdrawing groups have
a marked octopolar
character as a result of a push-pull effect occurring
between the electron-rich core and the peripheral substi-
tuents. Presumably, increasing the polarity of the stacking
surfaces diminishes their mutual repulsion from the
(27) Nishio, M.; Hirota, M.; Umezawa, Y. The CH/π interaction. Evi-
dence, Nature and Consequences; Wiley-VCH: New York, 1998.
(28) It has been found that the study of intermolecular aggregates by
solution NMR can provide some insights into the structure of crystalline
materials, offering useful hints about the crystal nucleation. Spitaleri, A.;
Hunter, C. A.; McCabe, J. F.; Packer, M. J.; Cockroft, S. L. CrystEngComm
2004, 6, 489–493.
(26) Unequivocal 1H assignments were made on the basis of HSQC and
HMBC and experiments performed on compound 2a.
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Garcıa-Frutos et al.
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FIGURE 5. View of the dimeric packing perpendicular to the molecular mean plane of 2c (left) and 2d (right).
FIGURE 6. CH-π interactions stabilizing the dimeric motif of 2c. The dimer is stabilized by six additional interactions (not shown) related by
an inversion center to those depicted.
crystallization mixture, and cooled in a nitrogen stream
where data collection was performed at -100 °C.29
attractive π-σ interactions, while decreasing π electron
density facilitates the face-to-face geometry as a result of
the diminution of π electron repulsion. The stacking motifs
observed cannot be rationalized just in terms of solvophobic
forces and point to the contribution of an additional com-
ponent inherent to the molecular structure in the aggrega-
tion. In fact close inspection of both dimeric structures shows
that a number of cooperative CH/π interactions are playing
an important role in the formation of these aggregates. Short
contacts between the hydrogens on the methylenic group R
to the nitrogens and the external aromatic rings of the
triindole platform are observed in the dimeric structure of
2c and 2d (12 interactions: Figure 6; four interactions: Figure
S15, Supporting Information).
Although correlations between the behavior in solution
and in solid state must be considered with caution, compari-
sons of the molecular packing in these two structures allow
us to see additional interesting features. Molecules of 2c pack
in the crystal forming dimeric aggregates (Figure 5a), with
˚
the triindole units situated at 3.65(2) A (average distance
between centroids of the central aromatic rings of the
platforms). Within the dimers, the molecules are oriented
face-to-face in an alternate arrangement, one molecule ro-
tated 180° with respect to the next molecular unit, being the
central aromatic rings are perfectly superimposable. In con-
trast, the closest molecules of 2d in the crystal lattice are
found slipped in parallel planes (distance between mean
˚
˚
planes: 3.82(2) A), shifted 6.61(2) A (distance between the
centroids of the central aromatic rings) (Figure 5b).
Conclusion
In conclusion, we have described the self-assembly proper-
ties in solution and in solid state of a series of differently
substituted N-alkyl hexaaryltriindoles. Self-assembly in so-
lution could be induced by adding polar solvents. A study of
the electronic character of the triindole π-systems on their
self-association behavior indicates that in solution increasing
the electron-donating character of the terminal p-phenyl
substituents facilitates self-association, while electron-with-
drawing groups inhibit aggregation. X-ray crystal structure
analysis reveals further details on the nature of the aggre-
gates in the solid state. While molecules of hexaphenyltriin-
dole 2c pack in the crystals as dimeric aggregates, with a
The effect of the substituents on the geometry of the
stacking in the solid follows again an opposite behavior to
that expected for an aggregation ruled exclusively by π
interactions, predicting that an increase of the π-electron
density favors an offset or T-shaped staking by maximizing
(29) Both crystals diffract poorly and a large portion of measured
reflexions had negative intensities and were eliminated. Because of this the
completeness percentages are low in both crystals. In addition the long
alkylic chains are highly disordered and had to be refined with restriction.
This is a common feature of crystals of organic compounds with long alkyl
chains. In spite of these problems, the central core of the molecules could be
perfectly determined with accuracy.
J. Org. Chem. Vol. 75, No. 4, 2010 1075

Garcıa-Frutos et al.
JOCArticle
perfect face-to-face stacking arrangement, withdrawing elec-
tron density of the system by attaching six peripheral cyano
functional groups (2d) results in an offset geometry of the
aggregates. The effect of the substituents in the strength and
geometry of this interaction cannot be explained in terms of
the decrease of a repulsive component resulting from inter-
action of the π-clouds as usually invoked in π-π interac-
tions. However, it can be rationalized considering an
important contribution of several cooperative CH-π inter-
actions between CH moieties of the alkylic chains and the
external rings on the triindole platform.
was precipitated with CH2Cl2 and acetonitrile and filtered to
give a yellow solid 2a (101 mg, 89%): mp 136-138 °C; 1H NMR
(200 MHz, CDCl3) δ 8.27 (s, 3H), 7.57 (s, 3H), 7.25-7.19 (m,
12H), 6.84 (d, J = 8.7 Hz, 6H), 6.83 (d, J = 8.5 Hz, 6H), 4.92 (m,
6H), 3.83 (s, 18H), 2.07 (m, 6H), 1.21 (m, 54H), 0.86 (t, J = 6.7
Hz, 9H); 13C NMR (50 MHz, CDCl3) δ 158.1, 157.9, 140.3,
139.4, 135.5, 135.4, 135.0, 132.6, 131.3, 123.4, 122.3, 113.3,
111.5, 102.8, 55.1, 47.0, 31.9, 30.4, 29.6, 29.3, 26.8, 22.7, 14.1;
UV (CH2Cl2, 25 °C) λmax (log ε) 341 (5.05); MALDI-TOF MS
m/z 1487 (Mþ þ 1); HRMS (MALDI-TOF) calcd for
C102H123N3O6 1485.94064, found 1485.94543.
The results presented here highlight the importance of
evaluating the delicate balance between all the possible
different interactions when designing new superstructures
involving aromatics.
Acknowledgment. Financial support from the MEC
(CTQ2007-65683/BQU, is gratefully acknowledged). E.M.
G.-F. thanks the Consejo Superior de Investigaciones Cien-
tıficas (Program I3P) for a posdoctoral contract. We are
grateful to Dr. Manuel Martin (Universidad de Santiago de
Compostela) for assistance with DOSY NMR experiments.
Experimental Section
Typical Coupling Procedure: Preparation of 2,3,7,8,12,13-
Hexakis(p-methoxyphenyl)-5,10,15-tris(dodecyl)-10,15-dihydro-
5H-diindolo[3,2-a:30,20-c]carbazole (2a). A mixture of 1 (100 mg,
0.075 mmol), Pd(PPh3)4 (43 mg, 0.037 mmol), and 4-methoxy-
phenylboronic acid (130 mg, 0.85 mmol) was degassed. Then
0.5 mL of 2 M aqueous K2CO3 and 4 mL of THF was added.
The mixture was heated at 90 °C for 4 days under nitrogen. The
orange suspension was partitioned between H2O and CH2Cl2,
then dried (MgSO4). The solvent was evaporated and the residue
Supporting Information Available: Experimental proce-
dures, synthesis, characterization, and copies of 1H NMR and
13C NMR spectra of all new compounds, MALDI-TOF MS
spectrum of 2a, DOSY-NMR experiment details, tabulated 1H
NMR data, calculated association constants, copies of 1H NMR
experiments at variable concentrations of 2a and 2d in CDCl3/
CD3COCD3 (7/3), and X-ray data of 2c and 2d. This material is
available free of charge via the Internet at http://pubs.acs.org.
1076 J. Org. Chem. Vol. 75, No. 4, 2010