Synthesis and structure-property relationships of donor/acceptor- functionalized bis(dehydrobenzo[18]annuleno)benzenes

Article abstract of DOI:10.1039/b800875b

Seven new bis(dehydrobenzo[18]annuleno)benzenes (bis[18]DBAs) 1-7 functionalized with electron-donating dibutylamino groups and/or accepting nitro groups at various positions along the peripheries of the chromophores have been prepared. The effects of var

Full text of DOI:10.1039/b800875b

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Synthesis and structure–property relationships of
donor/acceptor-functionalized bis(dehydrobenzo[18]annuleno)benzenes†
Eric L. Spitler and Michael M. Haley*
Received 17th January 2008, Accepted 22nd February 2008
First published as an Advance Article on the web 17th March 2008
DOI: 10.1039/b800875b
Seven new bis(dehydrobenzo[18]annuleno)benzenes (bis[18]DBAs) 1–7 functionalized with
electron-donating dibutylamino groups and/or accepting nitro groups at various positions along the
peripheries of the chromophores have been prepared. The effects of varying the donor/acceptor charge
transfer pathways, chromophore lengths and molecular symmetries upon the optical band gaps are
studied using UV–visible spectroscopy, and structure–property correlations are identified. It is found
that bis[18]DBAs possessing donor–p–donor and acceptor–p–acceptor pathways exhibit the smallest
band gaps, especially when an acceptor–p–acceptor pathway is situated along the longest chromophore
length in the molecule. The all-donor species 1 is also found to exhibit efficient fluorescence with
dramatic solvatochromism. The results may have value to the rational design of future NLO/TPA
device components.
Introduction
The last decade has seen a dramatic expansion in the study of a
special class of carbon-rich conjugated macrocycles, dehydroben-
zoannulenes (DBAs),1 due in part to their demonstrated unique
optical, electronic and materials properties.2 The increasing syn-
thetic accessibility of desired molecular topologies and their site-
specific functionalization allows these planar phenyl–acetylene
macrocycles to display a host of intriguing characteristics, includ-
ing enhanced p-orbital overlap, intermolecular p-stacking, and
highly tunable dipoles and symmetries.3 They thus possess poten-
tial in a wide array of applications such as fluorimetric sensing,
organic electronics, organopolymers, and nonlinear optical (NLO)
Fig. 1 Previously studied mono[18]DBAs with various conjugated path-
ways a–d and target bis[18]DBAs with conjugated pathways a–e.
device components.4 Large DBA assemblies have been shown
to approximate the expected properties of theoretical all-carbon
networks graphyne and graphdiyne,5 and to serve as precursors
for our purposes is here taken simply as the energy corresponding
to the longest wavelength kmax in the absorption spectra), the
for ordered nanomaterials.6
We have previously reported that DBA macrocycles and
closely related phenyl–acetylene model systems functionalized
with electron-donating and/or electron-accepting groups in a
site-specific manner (e.g. Fig. 1) induce varying degrees of
intramolecular charge transfer (ICT).2a–d,7 We have also shown
that the maximal low-energy two-photon absorption (TPA) cross-
section, a key parameter in third-order NLO materials, of a
particular DBA structure is often closely correlated to its ICT
optical band gap in the steady-state one-photon absorption
spectrum.8 Thus, we have sought to elucidate structure–property
relationships in donor/acceptor-functionalized DBAs for the
design of customized NLO materials that also possess high molar
extinction coefficients. We have demonstrated that while chro-
mophore length affects optical band gap for these systems (which
number of chromophores (defined here as a linear conjugated
pathway) affects the extinction coefficient.2a,5 To that end, we
now present a series of two-dimensional conjugated quadrupolar
bis(dehydrobenzo[18]annuleno)benzenes (1–7, Fig. 2) functional-
ized with donor and acceptor groups at their peripheries. It is
hoped that probing the photophysical effects of small variations
in functional group orientation and conjugated charge transfer
pathway topology will lead to an improved understanding of
structure–property relationships, with ultimate application toward
the rational design of the next generation of organic electronics.
BisDBAs 1–7 represent an expansion of our previous work on
donor/acceptor-functionalized mono[18]DBAs.2a,b The fusion of
two chromophore units into one molecule is expected to lead to
similar optical band gaps in the absorption spectrum, but with
higher molar extinction coefficients. Of the six target molecules, 2
and 6 are the only two belonging to an acentric point group and
posses a net dipole. Furthermore, 1 is an all-donor analogue, and is
believed to primarily exhibit non-ICT p–p* excitations (although a
certain degree of weak charge transport from the donor nitrogens
into the alkyne network is likely).2c These molecules thus represent
Department of Chemistry and Materials Science Institute, 1253 University
of Oregon, Eugene, Oregon 97403-1253, USA. E-mail: haley@uoregon.edu;
Fax: +1-541-346-0487; Tel: +1-541-346-0456
† Electronic supplementary information (ESI) available: Experimental
details for synthesis of 2–6 and 1H NMR spectra for all new compounds.
See DOI: 10.1039/b800875b
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Fig. 2 Bis(dehydrobenzo[18]annuleno)benzenes 1–7, the synthetic targets of this study.
good targets for comparison to quadrupolar bis[18]DBAs 3–5
and 7.
Results and discussion
Synthesis
Synthesis of 1–7 is accomplished using a highly modular approach
based on successive Pd-catalyzed Sonogashira cross-coupling
reactions. Known donor/acceptor segments 8–112a,b (Fig. 3) can
be appended to 1,2,4,5-tetraiodobenzene (e.g. Scheme 1) or an
appropriate isomer of dibromodiiodobenzene (e.g. Scheme 2)
using our selective in situ desilylation/cross-coupling protocol.
Undesired homocoupling of the alkyne segments can be mini-
mized to varying degrees of success by slowly injecting a solution
of the triyne into the reaction mixture over a period of 8–
12 h. Further desilylation followed by an oxidative intramolecular
Scheme 1 Representative synthesis of 1.
homocoupling under Pd-catalyzed oxidative conditions furnishes
the corresponding bis[18]DBA in a wide range of yields (Table 1).
The variation in isolated yield is likely due to a significant
degree of intermolecular oligomerization. Certain topologies (2
and 3) have proven unstable, and rapidly decompose in solution.
Consequently, only small amounts of these isomers could be
prepared. This behaviour was also observed in several bis[14]-
and bis[15]DBAs possessing similar donor/acceptor topologies
(to such an extent that the 14-membered analogue of 3 could not be
isolated at all), and is attributed to an uncontrolled topochemical
1,3-diacetylene polymerization promoted by strong intermolec-
ular self-association and p-stacking (vide infra). The extreme
Fig. 3 Differentially silylated donor/acceptor alkyne segments 8–11.
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Table 1 Yields for alkyne segment couplings and cyclization to afford
DBAs 1–7
Electronic absorption spectra
The UV–visible absorption spectra of 1–7 are given in Fig. 4.
All compounds display longest-wavelength absorption maxima
between 450 and 520 nm (Table 2). Except for tetra-donor 1, all
contain strong donor/acceptor functionalization.
1st coupled
segment (product,
yield)
2nd coupled
segment
(product, yield)
DBA
(yield)
Entry Arene
1
2
3
4
5
8 (12, 94%)
10 (15, 70%)
11 (16, 92%)
9 (17, 86%)
11 (19, 78%)
—
—
—
1 (82%)
4 (60%)
5 (49%)
2 (10%)
6 (19%)
8 (18, 18%)
10 (20, 21%)
6
7
9 (21, 71%)
11 (13, 87%)
8 (22, 24%)
10 (14, 63%)
3 (9%)
7 (66%)
Fig. 4 Electronic absorption spectra of DBAs 1–3 (top) and 4–7 (bottom)
in CH2Cl2 at ca. 10–25 lM concentrations.
Frontier molecular orbital (FMO) plots of these types of
systems consistently predict strong localization of the HOMO on
the donor-functionalized alkyne chromophore segments and the
LUMO on the acceptor segments, implying intramolecular charge
transfer as the lowest energy transition.2c,d,7a The low intensity,
broadened peaks in this region for 2–7, often accompanied
by bathochromic shifts relative to 1, are indicative of ICT
behaviour and are consistent with our previous DBA studies.
Table 2 Lowest energy absorptions in 1–7
Scheme 2 Representative synthesis of 7.
Longest wavelength
Approximate
DBA
kmax/nm (e/M−1 cm−1
)
kcutoff/nma
insolubility of 2 and 3 also precludes extensive chromatographic
purification; trituration of the crude product with THF was
found to be the only viable alternative. With these limitations in
mind, 4–7 were assembled specifically to increase the number of
donor/acceptor pathways and to avoid the strong polarization
that leads to this degradative polymerization. The additional
dibutylamino groups also greatly increase solubility. Due to an-
ticipated synthetic and solubility issues, an all-acceptor analogue
was not pursued. DBAs 4–7 are both soluble and stable in CD2Cl2
and CDCl3, and do not undergo decomposition even when left in
solution for several days.
1
2
3
4
5
6
7
481 (94 000)
471 (66 200)
517b (12 500)
451c (117 100)
490 (12 700)
456b (34 600)
480 (22 900)
557
604
614
578
575
568
564
a Cutoffs defined as the wavelength after the lowest energy kmax where there
is less than a 200 M−1 cm−1 variation over a 5 nm range. b Approximate kmax
,
as band is only a shoulder. c There is likely a low-intensity ICT transition
manifested at ca. 500 nm, but the overlap with the next highest band makes
it difficult to determine its kmax
.
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In all compounds, conversion from the acyclic precursors to the
corresponding bisDBA results in a bathochromic shift in the
absorption spectra (example spectra, Fig. 5). This is likely due to
an enhanced conjugation effect from the enforced planarization.
It is notable that for DBAs 1 and 4, with donor groups situated
along the longest linear chromophore path, cyclization causes an
increase in molar extinction coefficient in the longest wavelength
band in addition to a bathochromic shift, whereas in the other
systems a corresponding decrease is seen. This is presumably due
to enhanced ICT and delocalization in the planarized macrocycles.
high TPA cross-sections.2f ,8 The DBAs presented here, which
possess both qualities, are expected to follow suit.
The absorption spectra of the four-donor/four-acceptor DBAs
4–7 provide further insight into structural effects on band gap.
Each of these compounds contains four donor groups and four
acceptor groups; only their orientation with respect to the DBA
skeleton varies. Here only 6 possesses a net dipole, while all the
others are quadrupolar, thus allowing us to investigate only the
effects of conjugated pathway topology on the band gap. It is
interesting to note that 4–7 display absorption cutoffs at very
similar wavelengths. The most striking feature, however, is the
high molar extinction coefficient in 4, whose spectrum somewhat
resembles tetradonor 1. This is not surprising, since the four donor
groups lie along the longest conjugated chromophores as in 1, with
the acceptor groups along the bridging diacetylenes. A close look
at the spectrum of 4 reveals a possible lower energy ICT transition
around 500 nm, similar to those of 5–7, but the next highest
band overlaps too greatly for even speculative deconvolution. 5–7
display more characteristic low-intensity ICT bands red-shifted
from 4, with 5 possessing the smallest band gap. DBA 5 most
resembles a ‘tetraacceptor’ system, with acceptor groups along
the longest conjugated pathway, and donors along the diacetylene
bridges. DBAs 6 and 7 lie in between these two extremes. In these
cases, the dipolar DBA 6, which contains only linear donor–p–
acceptor pathways, is only slightly red-shifted from 4, while 7
(with only two linear ICT pathways) and 5 (with no linear ICT
pathways) are more red-shifted. Thus, we are led to the conclusion
that for these systems, a supposedly ‘efficient’ linear donor–p–
acceptor conjugated chromophore does not lead to the lowest
energy charge transfer transition.
Comparing all the spectra of 1–7, we can note that placing
like groups along a longest chromophore (donor–p–donor or
acceptor–p–acceptor) results in the smallest band gaps, with
acceptor–p–acceptor arrangements being particularly effective.
This makes sense when considering the likely FMO localizations
(which were not computed for these systems here, but for which
there is extensive precedent from our earlier studies2c–e,7a): rather
than exhibiting the behaviour of a donor atom transferring
electron density to an acceptor atom, the spectra imply a
transition from an electron-rich chromophore to an electron-
poor chromophore, wherein two directly conjugated donor groups
contribute to generate the former, and two acceptor groups
contribute to the latter. The band gap is thus more dependent
on the length of the linear chromophores, and hence is smaller
for the quadrupolar systems than for the dipolar ones. This
‘synergistic cooperation’ was also observed for our previously-
reported mono[18]DBAs:2a the smallest band gaps occurred when
a donor was conjugated to a donor and an acceptor to an acceptor.
Linear donor/acceptor pathways exhibited larger band gaps. In
those cases, however, the necessary asymmetry of most of the
DBAs could lead to potential skewing from dissimilar net dipole
effects which are not present in most of the bis[18]DBAs presented
here. Finally, our most bathochromic systems here, 3 and 5, display
charge transfer bands with molar extinction coefficients roughly
double those of analogous mono[18]DBAs, but in the same 490–
530 nm region. In donor-functionalized systems, the bisDBAs
display both higher extinction coefficients from fusion of a second
annulenic unit and longer wavelength absorptions from extension
of the chromophore lengths.
Fig. 5 Electronic absorption spectra of acyclic precursors 12 and 14 and
their respective DBAs 1 and 7.
Comparing two-donor/two-acceptor DBAs
2 and 3 to
tetradonor 1, we see a significant broadening of the corresponding
bands, with absorption cutoffs extending past 600 nm. In the case
of 3, the lowest discernible band exists as a shoulder beginning
near 517 nm, indicating a narrowed optical band gap relative to
1. Interestingly, the ICT band in 2 is slightly blue-shifted from
1. This was initially a surprising result, since 2 contains two long,
linear conjugated pathways from the donor groups to the acceptor
groups; one might intuitively expect such a topology to exhibit a
significantly narrowed band gap. This result is consistent, however,
with previous observations by Meier et al.9 who also noted
hypsochromic shifts in the absorption spectra of certain extended
chromophores upon strong donor/acceptor functionalization.
In those cases, the nearly complete lack of calculated Franck–
Condon overlap actually resulted in predominantly non-HOMO–
LUMO transitions as the dominant absorptions. Furthermore,
Moonen and Diederich have observed a lack of correlation
between optical band gap and conjugated pathway efficiency
for certain donor/acceptor cyanoethynylethene systems,10 finding
instead that quadrupolar compounds with bent donor/acceptor
pathways (analogous to pathway c–d in Fig. 1) often dis-
play smaller band gaps than corresponding dipolar analogues
with linear pathways (path e in Fig. 1). Our own systematic
structure–property studies involving isomeric permutations of
particular acyclic donor/acceptor motifs have confirmed this
phenomenon.2c,7 Note that 3 is quadrupolar rather than dipolar,
and exhibits the smallest band gap of the DBAs presented here.
This property could potentially have consequences for the future
design of organic NLO materials, particularly those that rely on
good two-photon absorption (TPA) parameters. We have shown
that systems with either symmetric multi-chromophore units or
strong donor/acceptor-functionalization can display particularly
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Electronic emission spectra
Many of the DBAs and donor/acceptor systems we have presented
in the past exhibit efficient fluorescence.2c,d,5,7 Small structural
variations of the conjugated pathways (e.g. chromophore topology,
polarization, planarity, donor or acceptor strength, protonation
or metal ion complexation ability) can fine-tune both emission
wavelength and quantum yield. In nitro-functionalized systems,
however, the high internal conversion rate caused by mixing of
vibronic states as well as solvent interaction effectively quenches
fluorescence.2c,7b The only highly emissive DBA presented here
is therefore tetradonor 1 (Uf = 0.34 in CH2Cl2). The long,
electron-rich conjugated pathways and the lack of a net dipole
likely result in an excited state that is highly polarized relative
to the ground state. The photophysical manifestation of this
is a dramatic degree of fluorescence solvatochromism (Fig. 6),
with emission maxima ranging from 486 nm in hexane to
587 nm in acetone. The absorption spectra, by contrast (Ta-
ble 3), display only slight solvent dependence, implying that
the effect is enhanced by relaxation to increasingly low-lying
vibrational levels of the S1 state after initial excitation (note
the gradual loss of vibronic fine structure in Fig. 6 as solvent
polarity increases). In contrast, DBA 4, which also has its donor
groups along the longest chromophores but also acceptors along
the diacetylene bridges, is only slightly fluorescent (quantum
yield < 0.05), with much less solvent dependence (Fig. 7). DBAs
2, 3, and 5–7 do not fluoresce at all. No concentration dependence
was noted in either absorption or emission spectra for any
compound.
Fig. 7 Normalized electronic emission spectra of DBA 4 in hexanes (Hex)
and dichloromethane (DCM). Excitation at 381 and 386 nm, respectively.
Self-association
Our previously reported bis[14]- and bis[15]DBAs (e.g. 23 and 24,
Fig. 8),2c also functionalized with dibutylamino and nitro groups,
displayed a significant degree of intermolecular self-association in
solution, presumably due to p-stacking enhanced by attraction
between donor and acceptor groups between molecules. The
aggregation manifested upfield shifts of the 1H-NMR signals of the
aromatic protons with increasing concentration. It was confirmed
that the monomer–dimer equilibrium is the dominant process
in these systems, and hence dimerization constants could be
calculated from the concentration-dependent chemical shifts. This
effect was not observed in DBAs 1 or 4–7, or in equimolar mixtures
of 4 and 5 (which could theoretically form dimer aggregates when
respective donor and acceptor groups from each molecule are
p-stacked). It is believed that the ortho relationship between the
alkylamino and nitro groups on each arene ring can prevent strong
polarization (dipolar or quadrupolar) and effectively ‘cancel out’
any long-range intermolecular attraction. Also, aggregation may
be prevented in these cases by the fact that ortho substitution will
result in one or both of the substituents not being co-planar with
the arene ring. This can create ‘thickness’ in the molecule that
inhibits stacking interactions. Tetradonor 1 possesses no acceptor
groups to enhance p-stacking via intermolecular attraction.
Fig. 6 Normalized electronic emission spectra of 1 in hexanes (Hex),
toluene (PhMe), chloroform (CHCl3), dichloromethane (DCM), acetoni-
trile (MeCN), and acetone (Me2CO) at ca. 10–25 lM concentrations.
Excitation at 365 nm. Inset: photographs of solutions of 1 in corresponding
solvents (left to right) under illumination by high-intensity 365 nm lamp.
Table 3 Lowest energy absorption and emission wavelengths of 1 in
various solvents
Fig. 8 Examples of previously reported bis[14]- and bis[15]DBAs that
display self-association in solution.
Solvent
Absorption kmax/nm
Emission kmax/nm
DBA 2 underwent rapid degradation in CDCl3 solution, so
concentration-dependent experiments could not be conducted.
DBA 3, however, remained stable long enough for several
concentration-dependence data points to be collected (Fig. 9).
The protons residing on the donor and acceptor arene rings
experienced the greatest degree of upfield shifts, and the central
Hexanes
Toluene
Chloroform
Dichloromethane
Acetonitrile
Acetone
466
471
478
481
482
479
486
502
522
551
575
587
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Experimental
General methods
1H- and 13C-NMR spectra were recorded in CDCl3 using a
Varian Inova 300 (1H: 299.94 MHz, 13C: 75.43 MHz) or 500
(1H: 500.10 MHz, 13C: 125.75 MHz) spectrometer. Chemical
shifts (d) are expressed in ppm relative to the residual chloroform
(1H: 7.26 ppm, 13C: 77.0 ppm) reference. Coupling constants are
expressed in Hertz. Melting points were recorded on a Melt-Temp
II melting point apparatus in open-end capillary tubes or on a
TA Instruments 2920 DSC and are uncorrected. IR spectra were
recorded using a Nicolet Magna FTIR 550 spectrometer. UV–vis
spectra were recorded on an HP 8453 UV–vis spectrophotometer.
Fluorescence data were recorded on a Hitachi F-4500 fluorescence
spectrophotometer. Mass spectra were recorded on an Agilent
1100 SL Series LC/MSD spectrometer. THF and Et2O were
distilled from Na–benzophenone under N2. All other chemicals
were of reagent quality and used as obtained from manufacturers.
Column flash chromatography was performed using N2 or air
pressure on Sorbent Technologies silica gel (230–450 mesh). Pre-
coated silica gel plates (Sorbent Technologies, 200 × 200 × 0.50
mm) were used for analytical thin layer chromatography. Eluting
solvents were reagent quality and used as obtained from the
manufacturers. Reactions were carried out in an inert atmosphere
(dry Ar) when necessary. All deprotected terminal alkynes were
used directly without further purification. Syntheses of segments
8–11 have been described previously.2a,b
Fig. 9 Concentration-dependent chemical shifts of the aromatic protons
of bis[18]DBA 3 in CDCl3 at 20 ◦C.
arene ring protons experienced the least, indicating the greatest
degree of change in the shielding environment where the donors
of one molecule interact with the acceptors of another, and vice
versa. The gradual upfield shifts were fitted to curves using the
HypNMR program11 from which a dimerization constant of 515
31 M−1 was extracted. This value is only slightly larger than for
the smaller analogous DBA 23 (326 51 M−1), and may be due
to more efficient stacking due to increased p-orbital overlap in
the strain-free bis[18]DBA, as well as due to the increased total
number of p–p interactions.
General alkyne coupling procedure A. Alkyne segment 8, 10,
or 11 (5.0 equiv.) and 1,2,4,5-tetraiodobenzene (1 equiv.) were
i
dissolved in Pr2NH–THF (1 : 1, 0.003 M) and the solution was
purged for 30 min with bubbling Ar. PdCl2(PPh3)2 (0.03 equiv.
per transformation), CuI (0.06 equiv. per transformation) and
aqueous KOH (5 equiv. per transformation) were added and the
solution was purged for another 20 min. The reaction mixture
was stirred at 55 ◦C for 24–48 h under an Ar atmosphere until
complete by TLC. The mixture was then concentrated, rediluted
with hexanes, and filtered through a pad of silica gel. The solvent
was removed in vacuo and the crude material was used without
further purification.
Conclusions
Bis[18]DBAs 1–7 represent an interesting systematic investigation
into the effects of conjugation pathway topology, chromophore
length, and symmetry on the optical band gaps of carbon-
rich intramolecular charge transfer systems. We have demon-
strated that quadrupolar chromophores with linear donor–p–
donor and acceptor–p–acceptor pathways can exhibit narrowed
band gaps versus donor–p–acceptor analogues, and comparison
to our earlier mono[18]DBAs shows that, while extending the
length of a donor-functionalized system results in absorption
red shifts, increasing the number of chromophore units in the
system causes an increase in molar extinction coefficient. Our
studies also support observations in other systems that strong
donor/acceptor functionalization of an extended chromophore
can actually lead to increased band gaps due to lack of Franck–
Condon overlap of segment-isolated FMOs.9,10 It has been
shown in the past that highly conjugated organic molecules
often exhibit high TPA cross-sections, especially when possessing
either high symmetry or donor/acceptor functionalization.2e,f ,8
The systems presented here satisfy both conditions, so it is
expected that they will display particularly favorable results, and
we hope to report the outcome of these experiments in the near
future.
General alkyne coupling procedure B. Acceptor segment 9 or 11
(2.5 equiv.) and the appropriate isomer of dibromodiiodobenzene
(1 equiv.) were dissolved in iPr2NH–THF (1 : 1, 0.003 M)
and the solution was purged for 30 min with bubbling Ar.
PdCl2(PPh3)2 (0.03 equiv. per transformation), CuI (0.06 equiv. per
transformation) and aqueous K2CO3 (5 equiv. per transformation)
were added and the solution was purged for another 20 min. The
reaction mixture was stirred at room temperature for 24–48 h
under an Ar atmosphere until complete by TLC. The mixture was
then concentrated, rediluted with hexanes, and filtered through a
pad of silica gel. The solvent was removed in vacuo and the crude
material was used without further purification.
General alkyne coupling procedure C. Acceptor-functionalized
tetrayne 13, 17, 19 or 21, and aqueous KOH (5 equiv. per transfor-
mation) were dissolved in iPr2NH–THF (3 : 2, 0.004 M) and the
solution was purged for 30 min with bubbling Ar. Pd(PhCN)2Cl2
(0.03 equiv. per transformation), PtBu3·HBF4 (0.07 equiv. per
transformation) and CuI (0.06 equiv. per transformation) were
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added and the solution was purged for another 20 min under
bubbling Ar. Donor segment 8 or 10 (2.5 equiv.) was dissolved
in THF (0.04 M), the solution was purged for 30 min under
bubbling Ar, and then injected via syringe pump into the stirred
polyyne solution over 8 h at 55 ◦C under an Ar atmosphere. The
reaction was stirred until complete by TLC. The mixture was then
concentrated, rediluted with hexanes–CH2Cl2 (4 : 1), and filtered
through a pad of silica gel. The solvent was removed in vacuo, and
the crude material was purified by column chromatography.
(72 mg, 0.148 mmol) at rt for 16 h using general alkyne coupling
procedure B. The crude material was chromatographed on silica
gel (4 : 1 hexanes–CH2Cl2) to give 13 (153 mg, 87%) as a red
oil. mmax (KBr)/cm−1 2958, 2937, 2860, 2150, 1652, 1591, 1540,
1506, 1472, 1457, 1338, 1277. dH (300 MHz; CDCl3; Me4Si) 7.82
(2 H, s), 7.72 (2 H, s), 7.20 (2 H, s), 3.12 (8 H, t, J 6.9 Hz),
1.53 (8 H, quin, J 7.5), 1.29 (8 H, sext, J 7.5), 1.15 (42 H, s),
0.88 (12 H, t, J 7.2). dC (75 MHz; CDCl3; Me4Si) 144.25, 140.92,
137.61, 130.59, 128.52, 126.35, 125.21, 124.54, 116.94, 102.96,
96.13, 81.84, 81.51, 80.07, 78.98, 51.96, 29.71, 20.29, 18.98, 14.06,
11.54. m/z (APCI) 1187 (M+(279Br), 42%), 1189 (MH+(79Br81Br),
100), 1190 (MH+(13C281Br), 65).
DBA precursor 14. Segment 10 (106 mg, 0.192 mmol) was
cross-coupled to acceptor-functionalized arene 13 (106 mg, 0.089
mmol) at 55 ◦C using general alkyne coupling procedure C (10
h). The crude material was chromatographed on silica gel (4 :
1 hexanes–CH2Cl2) to give 14 (111 mg, 63%) as a dark red oil.
kmax (CH2Cl2)/nm 294 (log e/dm3 mol−1 cm−1, 4.97), 322 (4.98),
353 (4.94), 445 (4.59). mmax (NaCl)/cm−1 2953, 2932, 2891, 2856,
2218, 2147, 1595, 1536, 1505, 1487, 1466, 1428, 1337, 1274. dH
(300 MHz; CDCl3; Me4Si) 7.90 (2 H, s), 7.82 (2 H, s), 7.60 (2 H,
s), 7.22 (2 H, s), 7.10 (2 H, s), 3.13 (16 H, m), 1.52 (16 H, m),
1.29 (16 H, m), 1.16 (42 H, s), 1.14 (42 H, s), 0.88 (24 H, m). dC
(75 MHz; CDCl3; Me4Si) 145.32, 144.30, 140.91, 139.50, 138.01,
131.68, 131.62, 130.59, 128.79, 125.81, 125.55, 125.33, 124.11,
117.03, 113.25, 103.56, 102.98, 100.25, 99.87, 96.09, 82.71, 82.44,
81.63, 80.76, 80.29, 79.60, 78.86, 51.96, 51.76, 29.71, 20.26, 18.94,
14.02, 11.53.
Bis[18]DBA 7. Precursor 14 (65 mg, 0.033 mmol) was sub-
jected to general Pd-catalyzed cyclization procedure D (40 h).
The crude material was purified by filtration through a pad of
silica gel followed by concentration in vacuo and trituration with
hexanes to afford 7 (29 mg, 66%) as a dark orange powder. Mp:
160–180 ◦C (dec). kmax (CH2Cl2)/nm 403 (log e/dm3 mol−1 cm−1,
4.89), 479 (4.36). mmax (NaCl)/cm−1 2955, 2924, 2868, 2213, 1651,
1592, 1535, 1479, 1431, 1370. dH (300 MHz; CD2Cl2; Me4Si) 8.01
(2 H, s), 8.00 (2 H, s), 7.88 (2 H, s), 7.36 (2 H, s), 7.32 (2 H,
s), 3.20 (16 H, m), 1.55 (16 H, m), 1.29 (16 H, m), 0.91 (24 H,
m). dC (75 MHz; CDCl3; Me4Si) 145.09, 139.86, 139.75, 139.50,
137.46, 136.69, 131.72, 129.77, 129.41, 125.77, 125.25, 124.25,
124.06, 113.15, 113.03, 82.14, 81.92, 81.35, 81.27, 80.94, 80.78,
80.71, 80.19, 79.83, 79.41, 77.46, 77.41, 51.70, 29.71, 20.33, 14.05.
General Pd-catalyzed cyclization procedure D. Annulene pre-
cycle was dissolved in THF, Et2O and MeOH (2 : 1 : 0.01, 0.005
M) and Bu4NF (TBAF, 1.0 M soln in THF, 10 equiv.) was added.
The solution was stirred at room temperature until complete by
TLC (typically <1 h). The reaction mixture was then concentrated,
dissolved in Et2O, and washed with H2O (3 × 50 mL). The organic
phase was collected, dried over MgSO4, and filtered through a
pad of silica gel eluting with hexanes. The solvent was removed in
vacuo and the deprotected precycle was dissolved in THF (0.005
M). PdCl2(dppe) (0.1 equiv. per transformation), CuI (0.2 equiv.
per transformation), and I2 (0.25 equiv. per transformation) were
i
dissolved in Pr2NH–THF (3 : 2, ∼0.5 L per mmol precycle). To
this mixture, the deprotected precycle solution was injected via
syringe pump over 40 h at 65 ◦C. The reaction mixture was stirred
until complete by TLC, then concentrated, rediluted in hexanes–
CH2Cl2 (3 : 1) and filtered through a pad of silica gel. The solvent
was removed in vacuo and the product was triturated with hexanes.
DBA precursor 12. Segment 8 (306 mg, 0.605 mmol) was cross-
coupled to 1,2,4,5-tetraiodobenzene (78 mg, 0.134 mmol) at 55 ◦C
using general alkyne coupling procedure A (12 h). The crude
material was chromatographed on silica gel (4 : 1 hexanes–CH2Cl2)
to give 12 (227 mg, 94%) as a dark red oil. kmax (CH2Cl2)/nm 400
(log e/dm3 mol−1 cm−1, 4.73), 466 (4.81). Em. kmax/nm 551. Uf
12
0.18.
m
max
(NaCl)/cm−1 2959, 2935, 2856, 2191, 2155, 1589, 1530,
1479, 1365, 1223, 1113. dH (300 MHz; CDCl3; Me4Si) 7.51 (2 H,
s), 7.35 (4 H, d, J 9.0), 6.66 (4 H, d, J 2.7), 6.49 (4 H, dd, J 9.0,
2.7), 3.27 (16 H, t, J 7.5), 1.54 (16 H, quin, J 7.5), 1.34 (16 H,
sext, J 7.2), 1.18 (84 H, s), 0.96 (24 H, t, J 7.2). dC (75 MHz;
CDCl3; Me4Si) 148.30, 137.66, 134.58, 128.57, 125.49, 114.89,
111.72, 110.40, 105.84, 94.59, 85.37, 81.84, 78.87, 75.99, 50.78,
29.50, 20.48, 18.97, 14.16, 11.60. m/z (APCI) 1719 (M+ − TIPS +
THF, 66%), 1720 (M+(13C) − TIPS + THF, 100).
Bis[18]DBA 1. Precursor 12 (136 mg, 0.075 mmol) was sub-
jected to general Pd-catalyzed cyclization procedure D (40 h). The
crude material was purified by filtration through a pad of silica gel
followed by concentration in vacuo and trituration with hexanes
to afford 1 (72 mg, 82%) as a brick red powder. Once purified, the
product exhibited very poor solubility, thus precluding acquisition
of 13C-NMR data. Mp: 200–220 ◦C (dec). kmax (CH2Cl2)/nm 445
(log e/dm3 mol−1 cm−1, 4.96), 481 (4.98). Em. kmax/nm 551. Uf
0.34.12 mmax (KBr)/cm−1 2956, 2929, 2871, 2136, 1591, 1590, 1482,
1368, 1218, 1110. dH (300 MHz; CDCl3; Me4Si) 7.78 (2 H, s), 7.49
(4 H, d, J 8.7), 6.84 (4 H, s), 6.66 (4 H, d, J 11.4), 3.30 (16 H, t, J
6.9), 1.59 (16 H, m), 1.40 (16 H, sext, J 7.8), 0.99 (24 H, t, J 7.2).
dH (300 MHz; THF-d8; Me4Si) 7.84 (2 H, s), 7.50 (4 H, d, J 9.0),
6.92 (4 H, s), 6.80 (4 H, d, J 9.0), 3.39 (16 H, t, J 7.8), 1.63 (16 H,
m), 1.40 (16 H, sext, J 7.5), 0.99 (24 H, t, J 7.5).
Acknowledgements
We thank the National Science Foundation (CHE-0718242) for
financial support. E.L.S. acknowledges the NSF for an IGERT
fellowship (DGE-0549503). We thank the laboratory of Prof.
D. W. Johnson for use of the HypNMR program software.
Notes and references
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