Structure-property investigations of conjugated thiophenes fused onto a dehydro[14]annulene scaffold

Article abstract of DOI:10.1021/jo800225u

(Chemical Equation Presented) A series of 12 thieno-fused macrocycles based on the dehydro[14]annulene framework have been prepared. Studies have focused on the optical and electronic properties of the dehydrobenzothieno[14]annulenes (DBTAs) and dehydrothieno[14]annulenes (DTAs) utilizing NMR spectroscopy, UV-vis spectrophotometry, electrochemistry, and DFT computations. X-ray crystal structures were also obtained for two of the macrocycles. The structure-property relationships were found to vary significantly based on the relative orientation of the thiophenes. The stability, properties, and reactivity of these macrocycles were found to be more typical of dehydroannulenes rather than oligothiophenes.

Full text of DOI:10.1021/jo800225u

Structure-Property Investigations of Conjugated Thiophenes Fused
onto a Dehydro[14]annulene Scaffold
Matthew J. O’Connor, Robert B. Yelle,† Lev N. Zakharov, and Michael M. Haley*
Department of Chemistry and Materials Science Institute, UniVersity of Oregon, Eugene, Oregon 97403,
and Computational Science Institute, UniVersity of Oregon, 5294 UniVersity of Oregon, 1600 Millrace
DriVe, Suite 105, Eugene, Oregon 97403,
haley@uoregon.edu
ReceiVed January 28, 2008
A series of 12 thieno-fused macrocycles based on the dehydro[14]annulene framework have been prepared.
Studies have focused on the optical and electronic properties of the dehydrobenzothieno[14]annulenes
(DBTAs) and dehydrothieno[14]annulenes (DTAs) utilizing NMR spectroscopy, UV-vis spectropho-
tometry, electrochemistry, and DFT computations. X-ray crystal structures were also obtained for two of
the macrocycles. The structure-property relationships were found to vary significantly based on the
relative orientation of the thiophenes. The stability, properties, and reactivity of these macrocycles were
found to be more typical of dehydroannulenes rather than oligothiophenes.
Introduction
organometallic cross-coupling reactions3 has provided access
to a vast array of compounds whose preparation was not possible
Highly unsaturated conjugated compounds have attracted
considerable attention from both synthetic and theoretical
chemists in recent years.1 In particular, alkyne-containing
systems2 have garnered the bulk of this interest, which can be
attributed to two main factors. First, the advent of new synthetic
methodologies for the preparation of such molecules based on
(2) Inter alia: (a) Modern Acetylene Chemistry, Stang, P. J., Diederich, F.,
Eds.; VCH: Weinheim, Germany, 1995. (b) Bunz, U. H. F.; Rubin, Y.; Tobe,
Y. Chem. Soc. ReV. 1999, 28, 107–119. (c) Bunz, U. H. F. Chem. ReV. 2000,
100, 1605–1644. (d) Nielsen, M. B.; Diederich, F. In Modern Arene Chemistry;
Astruc D., Ed.; Wiley-VCH: Weinheim, Germany, 2002; pp 196-216. (e)
Acetylene Chemistry: Chemistry, Biology, and Material Science; Diederich, F.,
Stang, P. J., Tykwinski, R. R., Eds.; Wiley-VCH: Weinheim, Germany, 2005.
(3) (a) Metal Catalyzed Cross-Coupling Reactions, 2nd ed.; de Meijere, A.,
Diederich, F., Eds.; Wiley-VCH: Weinheim, Germany, 2004. (b) Transition Metal
Catalyzed ReactionssIUPAC Monographs Chemistry for 21st Century; Davies,
S. G., Murahashi, S., Eds.; Blackwell Science: Oxford, UK, 1998.
* Corresponding author. Fax: 541-346-0487. Phone: 541-346-0456.
† Computational Science Institute, University of Oregon.
(1) Inter alia: (a) Topics in Current Chemistry (Carbon Rich Compounds I);
de Meijere, A., Ed.; Springer-Verlag: Berlin, Germany, 1998; Vol. 196. (b) Topics
in Current Chemistry (Carbon Rich Compounds II); de Meijere, A., Ed.; Springer-
Verlag: Berlin, Germany, 1999; Vol. 201. (c) Watson, M. D.; Fechtenko¨tter,
A.; Mu¨llen, K. Chem. ReV. 2001, 101, 1267–1300. (d) Grimsdale, A. C.; Mu¨llen,
K. Angew. Chem. Int. Ed. 2005, 44, 5592–5629. (e) Carbon-Rich Compounds:
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VCH: Weinheim, Germany, 2006.
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M. M. J. Mater. Chem. 2001, 11, 2943–2945. (d) Slepkov, A.; Hegmann, F. A.;
Tykwinski, R. R.; Kamada, K.; Ohta, K.; Marsden, J. A.; Spitler, E. L.; Miller,
J. J.; Haley, M. M. Opt. Lett. 2006, 31, 3315–3317. (e) Bhaskar, A.; Guda, R.;
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4424 J. Org. Chem. 2008, 73, 4424–4432
10.1021/jo800225u CCC: $40.75 2008 American Chemical Society
Published on Web 05/29/2008

Structure-Property InVestigations of Conjugated Thiophenes
before. Second, many of these carbon-rich molecules and
macrocycles of higher dimensionality have been shown to
exhibit interesting materials properties such as nonlinear optical
(NLO) activity,4 liquid crystalline behavior,5 and molecular
switching.6 Furthermore, it has recently been demonstrated that
dehydrobenzoannulenes7 and related phenyl-acetylene macro-
cycles8 are useful precursors to a number of carbon-rich
polymeric systems, such as molecular tubes,9 ladder polymers,10
and novel allotropes of carbon.11 It is therefore imperative to
have ready access to a wide variety of these high carbon content
molecules in sufficient quantities via easy synthetic processes
if their technological potential is to be harnessed.
We have been investigating alkyne-rich dehydrobenzoannu-
lenes (DBAs)7,12 with the aim of exploring their diverse
chemical and physical properties. Over the past decade, our
group has developed new or improved existing synthetic
techniques for the preparation of such macrocycles. As one
particular consequence, it is now possible to introduce donor
and/or acceptor functional groups on the phenyl rings of the
DBA in a discrete manner, thus effectively “tuning” the
electronic and optical properties of the annulene.12a In our
continuing endeavor to introduce more variations in the DBA
structure for detailed structure-property relationship studies,
we elected to incorporate thiophene moieties onto the dehy-
droannulene skeleton.13 The choice of thiophene as the fused
aromatic ring was inspired by factors such as the chemical and
electrochemical polymerizability of thiophene, the ability to form
two-dimensional π-systems useful for electronics and photonics,
the easier polarizability of thiophene, and the interaction among
the individual macrocycles due to the lone pairs on the sulfur
in each thiophene ring.14 Compared to other heteroaromatic
FIGURE 1. Examples of known dehydrothienoannulenes.
molecules, thiophenes are typically easier to handle and to
functionalize than furans and pyrroles, allowing ready access
to tailored thiophene derivatives. By locking the conjugated unit
into planarity, thiophene-containing macrocycles15 and cyclic
thiophene-acetylene hybrids16–18 have the potential to be more
efficient materials due to enforced π-orbital overlap, increasing
the quinoidal character of the delocalized system and thus
lowering the HOMO-LUMO energy gap.19
Previous work on dehydrothienoannulenes (DTAs, Figure 1) by
the groups of Youngs (e.g., 1)17 and Marsella (e.g., 2)18 relied on
metal-mediated intermolecular couplings for molecule assembly.
By virtue of the structure of the starting materials, only Cnh- and
Dnh-symmetric macrocycles were produced. To probe the structure/
property relationships among the various structural isomers, it is
necessary to introduce the thiophene moieties in a systematic,
stepwise manner to produce DTAs possessing lower symmetries
(C2V, Cs). Our initial communication focused on the [18]DTA
skeleton;13 however, stability problems with the thienyldiyne
intermediates forced us to examine other topologies. This article
describes the design and stepwise preparation of benzo/thieno-fused
[14]DBTA hybrids 3-9 and the corresponding all-thiophene
containing [14]DTAs 10-14 starting with a few common synthons
(Figure 2). The solid state properties of 8 and 13 are investigated
by X-ray crystallography. We also report the effect of structural
variations on the overall effective conjugation of the macrocycles
as determined by electronic absorption spectroscopy, electrochem-
istry, and DFT computations. Finally, we discuss the thermal
properties of these macrocycles.
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Results and Discussion
Macrocycle Synthesis. The use of building blocks 15-24
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J. Org. Chem. Vol. 73, No. 12, 2008 4425

O’Connor et al.
SCHEME 1. Synthesis of “Symmetrical” [14]DBTA 6
were either commercially available (19) or prepared by literature
methods (23, 24).21
[14]DBTAs 3, 6, 8, and 9 and [14]DTAs 10, 13, and 14
possess two fused benzenes or two fused thiophenes in a
symmetrical orientation on the lower portion of the annulene,
and thus were assembled by using a 2-fold coupling of the
respective diyne to a central dihaloarene core. Scheme 1
illustrates a representative synthesis starting from 16. Sequential
Sonogashira reactions with TMSA and TIPSA (the latter
requiring elevated heating over several days for completion)
afforded diyne 21 in 91% yield for the two steps. Selective
removal of the TMS group with K2CO3 in MeOH followed by
cross-coupling to 1,2-diiodobenzene (19) furnished tetrayne 25
in 86% yield. Protiodesilylation with TBAF and subsequent
acetylenic homocoupling utilizing PdCl2(PPh3)2 catalyst, CuI
cocatalyst, and iodine as an oxidant22 gave hybrid 6 in 53%
yield.
FIGURE 2. Target thieno-fused dehydro[14]annulenes 3-14.
[14]DBTAs 4, 5, and 7 and [14]DTAs 11 and 12 possess two
different fused arenes or two fused thiophenes in unsymmetrical
orientations on the lower portion of the annulene, and are thus
formed by sequential cross-couplings to the dihaloarene “crown”.
[14]DBTAs 4 and 5 were prepared by cross-coupling 21 and 22
to previously reported halodiyne 24.21 Scheme 2 shows a repre-
sentative synthesis of an “unsymmetrical” macrocycle, DTA 12.
Starting diyne 21 was desilylated with mild base and then
immediately cross-coupled to 15. The resulting halodiyne was
cross-coupled to monodesilylated 20 to give 26 in a combined 80%
yield. The macrocyclization step was performed as before utilizing
the Pd(II)/Cu(I) catalyst system under oxidizing conditions, af-
fording 12 in 64% yield. Use of more traditional CuCl/Cu(OAc)2
in pyridine for the homocoupling reactions22b afforded the DTAs
in comparable yields. A summary of the syntheses off all 12
macrocycles is given in Table 2.
FIGURE 3. Building blocks 15-24.
TABLE 1. Yields for Preparation of Differentially Protected
Diynes 20-22
yield of TMSA
cross-coupling
yield of TIPSA
cross-coupling (pdt)
dihalothiophene
15
16
17
96%
94%
95%
95%, 20
92%, 21
93%, 22
assembly of thiophene-based dehydro[14]annulenes relatively
convenient and facile. All macrocycles reported herein were
constructed starting from the appropriate dihalothiophenes
15-18, which were in turn prepared from commercially
available thiophenes following known procedures.20 Halides
15-17 were subsequently converted into differentially protected
diynes 20-22 by sequential Sonogashira cross-coupling with
(trimethylsilyl)acetylene (TMSA) and (triisopropylsilyl)acety-
lene (TIPSA) in excellent yields (Table 1). The remaining pieces
(21) (a) Haley, M. M.; Bell, M. L.; English, J. J.; Johnson, C. A.; Weakley,
T. J. R. J. Am. Chem. Soc. 1997, 119, 2956–2957. (b) Bell, M. L.; Chiechi,
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4426 J. Org. Chem. Vol. 73, No. 12, 2008

Structure-Property InVestigations of Conjugated Thiophenes
SCHEME 2. Synthesis of “Unsymmetrical” [14]DTA 12
S· · ·S contacts with divalent sulfur, which could be explained based
on its orbital orientation.23
The packing of the molecules in the crystal structure of
[14]DTA 13 is different than that found in 8. The two nearest
molecules have opposite orientation to form pairs (Figure 5a)
and these pairs are stacked in the crystals of 13 in columns
(Figure 5b). Most of the S · · ·S intermolecular contacts are in
the range 3.60-3.82 Å and thus are comparable with typical
intermolecular S· · ·S contacts.23 The four-molecule packing
arrangement found in 8 is not present in 13. The S(3) atoms in
both symmetrically independent molecules are disordered over
two positions, which indicates that there are no directional
preferences of nonbonded atomic S · · ·S contacts in 13. Interest-
ingly, neither crystal structure contains the packing motif
conducive for topochemical polymerization,24 which is found
in the parent [14]DBA 27,25 nor are the R-positions of the
thiophenes in reasonable proximity for subsequent reactivity.
This likely explains the broad featureless exotherms observed
in the DSC scans.
The molecular structures of 8 and 13 are essentially planar
and show the typical bowed diacetylenic linkage of the strained
dehydro[14]annulene core.25,26 The sp-carbon bond angles of
thediynebridgesin8(166.3(3)-169.0(3)°)and13(166.9(4)-170.9(4)°)
are nearly the same as those in 27 (168.8-171.4°). For the
monoyne linkages less strain is contained in the sp-carbon nearer
the diyne unit (175.2(3)-178.9(3)°) than the sp-carbon proximal
to the arene “crown” (168.5(3)-174.2(3)°).
NMR Analysis. Previous work on the octadehydro[14]annulene
framework, common to molecules 3-14 as well, examined the
effects of thienoannelation and successive benzannelation (e.g.,
27-29, Figure 6) upon the diatropicity of the 14-membered
ring.27 Experimentally these alterations were studied by analyz-
ing the changes of the NMR chemical shifts of the alkene
protons within the series of macrocycles and with their acyclic
precursors. Such effects can also be observed on the fused-
benzene and thiophene protons due to ring-current competition,
though to a smaller degree.28 Although they do not possess
sensitive alkene protons, annulenes 3-14 exhibit distinct
downfield shifts of their arene protons upon cyclization,
indicative of a diatropic current in the central 14-membered core.
As opposed to an exhaustive discussion of all 12 annulenes,
we will focus on [14]DBTAs 6-9 due to the two different aryl
groups and C2V-symmetry (or near symmetry), which makes
interpretation of the data easier. The arene proton chemical shifts
of 6-9 and of their acyclic precursors along with the corre-
sponding data for 27-29 are given in Table 3.29 The proton
labeling scheme is shown in Figure 6.
Unlike related DBA structures, the thieno-fused annulenes
exhibit moderate to poor stability during workup and are
sensitive to acid, light, and silica, which accounts for the lower
than normal cyclization yields. Workups were more successful
when avoiding halogenated solvents and utilizing fast chro-
matographic methods (chromatotron or flash chromatography).
NMR characterization of the annulenes was done primarily in
deuterated benzene, dichloromethane, or tetrahydrofuran. In
general, those molecules with UV-vis absorptions greater than
400 nm showed the most sensitivity and/or where both sulfurs
are oriented “down” (sulfur R to the diacetylene linkage) as is
the case with 8 and 13. Most macrocycles could be stored in
solution or in the solid state for a few days if kept cold and
dark under argon.
DSC analyses of 3-14 show that the macrocycles polymerize/
decompose prior to melting to afford black solids. Hybrids 3-8
exhibit somewhat sharp exotherms, with 6 where the sulfurs
are oriented “up” reacting at a higher temperature (190 °C) than
isomer 8 where the sulfurs are oriented “down” (120 °C). In
the case of the [14]DTAs 10-14, the thermograms show two
broad transitions occurring near 150 °C. All of the data are
indicative of fairly disordered polymerizations. Complete de-
composition of 3-14 proceeds rapidly after 200 °C.
X-Ray Structural Data. Single crystals of 8 and 13 suitable
for X-ray diffraction were obtained by slow vapor diffusion of
pentane into THF solutions of the macrocycle. The X-ray structure
and the crystal packing of hybrid 8 are shown in Figure 4. The
main structural unit of 8 consists of layers of four molecules joined
by intermolecular π· · ·π and S · · ·S interactions. Two molecules
lie in the same plane (coplanar within 0.12 Å) and are additionally
connected by two S · · ·S interactions (S · · ·S distances of 3.61 Å).
The remaining two molecules are above and below the average
plane of the pair in such a fashion that the four S atoms in these
molecules form a rectangle with S · · ·S distances in the range
3.75-3.88 Å. In the crystal these structural units are connected
by much weaker S · · ·S contacts (4.10 Å), forming infinite chains.
The arrangement of the molecules clearly indicates that intermo-
lecular S · · ·S interactions play an important role in the association
of the molecules in the crystal structure of 8, and are in good
agreement with known directional preferences of nonbonded atomic
Examination of the 1H NMR spectra of all seven precyclized
molecules shows that the range of chemical shifts of the distal
and proximal benzene protons (A and B, respectively) is
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J. Org. Chem. Vol. 73, No. 12, 2008 4427

O’Connor et al.
annulene, yield
TABLE 2. Synthesis and Yields of DBTAs and DTAs 3-14
“equivalents” of monodesilylated diyne
haloarene
20
21
22
23
precursor, yield
18
24
24
19
19
19
19
18
15
15
18
18
2
pre-3, 90%
pre-4, 89%
pre-5, 64%
pre-6 (25), 86%
pre-7, 15%
pre-8, 90%
pre-9, 61%
pre-10, 67%
pre-11, 28%
pre-12 (26), 81%
pre-13, 52%
pre-14, 47%
3, 62%
4, 29%
5, 33%
6, 53%
7, 33%
8, 32%
9, 37%
10, 30%
11, 30%
12, 64%
13, 45%
14, 38%
1
2
1
1 (second)
2
1 (first)
2
2
2
1 (first)
1 (second)
2
1 (second)
1 (first)
extremely small: 7.30-7.36 ppm for A and 7.50-7.56 ppm for
B. These values are essentially the same as in 1,2-diethynyl-
benzene (7.29 and 7.50 ppm),28b suggesting very little or no
influence from the group(s) appended at the end of the 1,2-
diethynyl linkages. Upon macrocyclization, protons A and B
in 6-8 move downfield to 7.58-7.61 and 8.09-8.12 ppm,
respectively. Although 6-8 differ in the orientation of the
thiophene rings, the results show that this structural variation
has no effect on the remotely located benzene protons. The
changes in shifts, ∆δ ≈ 0.25 ppm for A and 0.55 ppm for B,
are similar in magnitude to those in 28, which suggests that
two 2,3-fused thiophene rings have approximately the same
bond-fixing ability as one fused benzene ring.30
Interestingly, 3,4-fusion of the two thiophenes in 9 results in
downfield shifts of protons A and B about half as large as those
in 6-8, and slightly smaller than in 27. The 3,4-bond in a
FIGURE 4. (a) X-ray structure of [14]DBTA 8; ellipsoids at the 30% probability level. (b) Fragment of the crystal packing of 8 showing S· · ·S
interactions.
FIGURE 5. (a) X-ray structure of [14]DTA 13; ellipsoids drawn at the 30% probability level. Only one position for the disordered atoms is shown.
(b) Crystal packing of 13.
4428 J. Org. Chem. Vol. 73, No. 12, 2008

Structure-Property InVestigations of Conjugated Thiophenes
absorption at 365 nm.25 In comparison, the analogous band in the
spectra of monothiophenes [14]DBTAs 3-5 (Figure 7a) displays
a gradual bathochromic shift indicative of a narrowing of the
HOMO-LUMO energy gap. Replacing the benzene “crown” with
a 2,3-fused thiophene as in hybrid 3 shifts this absorption to 378
nm. Macrocycles 4 and 5, where a 2,3-fused thiophene has replaced
one of the side benzene rings, afford absorption bands at 386 and
391 nm, respectively. Incorporating a thiophene into the macrocycle
slightly increases the dipole moment, while thiophene orientation
affects the net dipole moment as the sulfur orientation changes
from “up” in 4 to “down” in 5.
The UV-vis spectra of the macrocycles incorporating two
thiophenes are given in Figure 7b. As noted above, changing the
sulfur atom orientation from “up” to “down” progressively shifts
the low-energy absorption further into the red, from 373 nm for 6
(both “up”) to 398 nm for 7 (“alternating”), then to 408 nm for 8
(both “down”). The extinction coefficients for similar bands also
increase correspondingly. In the case of [14]DTA 9, however, this
band blue shifts to 339 nm, reflecting the weaker conjugation of
the 3,4-fusion of both thiophenes and analogous to behavior
previously observed for the larger [18]DTAs.13
FIGURE 6. Proton labeling scheme in Table 3 for hybrids 6-9 and
known [14]annulenes 27-29.
TABLE 3. 1H NMR Chemical Shifts (δ, ppm) of [14]DBTAs 6-9
and Their Precursor Tetraynes in CD2Cl2, and Comparison with
Known [14]DBAs 28-30
benzene
thiophene
compd
proton A
proton B
proton C
proton D
Interestingly, the absorption spectra show little difference
between 6-8 and 10-13, where in the latter series three
thiophenes are 2,3-fused onto the dehydro[14]annulene backbone
(Figure 7c). The most red-shifted is DTA 13 at 408 nm (sulfurs
“down”), the same wavelength as 8, while the most blue-shifted
annulene is 12 at 395 nm (sulfurs “alternating”). DTAs 10 and
11 share approximately the same peak at 400 nm and have very
similar UV-vis absorption spectra. As before, switching to 3,4-
fusion as in 14 results in weaker overall conjugation and thus
the lowest energy band appears at 368 nm. Changing solvents
from CH2Cl2 to toluene results in nearly identical absorption
spectra for 3-14. The observed electronic transitions for this
class of macrocycles are most likely πfπ* with small variations
in vibronic structure.
25 (pre-6)
6
pre-7
7
pre-8
8
7.36
7.61
7.34
7.61, 7.58
7.33
7.60
7.30
7.39
7.34
7.43
7.33
7.64, 7.56
7.32
7.72
7.55
8.10
7.53, 7.51
8.12, 8.09
7.52
8.12
7.52
7.75
7.56
7.89
7.54
8.11, 7.99
7.50
8.28
7.24
7.53
7.22, 7.15
7.60, 7.53
7.19
7.60
7.49
7.75
7.07
7.26
7.13, 7.07
7.50, 7.25
7.04
7.51
7.45
7.47
pre-9
9
pre-27a
27b
pre-28c
28c
pre-29c
29c
a Reference 29; in C6D6. b Reference 25; in CDCl3. c Reference 27b.
Electrochemistry. Cyclic voltammograms (CV) were taken
of macrocycles 6-14 with use of a Pt disk working electrode,
Ag wire reference, and Pt swirl auxiliary in CH2Cl2 solutions
of TBAHFP and referenced to ferrocene. Table 4 lists the
energies for the first oxidation and first reduction waves for
6-14; representative CVs are given in the Supporting Informa-
tion. Unlike a majority of thiophenes, which produce well-
behaved CVs, the CVs of the DBTAs and DTAs were complex.
Scanning in the positive potential direction generated one and
sometimes two irreversible oxidations, while scanning negatively
produced three (sometimes more) quasireversible reductions.
The oxidation peak became more difficult to find when the
oxidation peak occurred either at or concurrently with the
CH2Cl2 solvent window. Unlike previous studies of thiophene-
acetylenehybrids,16,17 inclusionaspartofthedehydro[14]annulene
framework clearly alters the typical redox behavior of a
thiophene moiety in a deleterious manner.
thiophene ring is well-known to possess less double bond
character than the 2,3-bond.31 This results in less efficient
delocalization in the 14-membered nucleus of 9 and thus poorer
bond-fixing ability compared to that of 6-8. Alternatively, the
difference in character between 6-8 and 9 can also be explained
by viewing 9 as a dehydrobisthia[20]annulene with two zero-
bridges, in which 22 π-electrons are delocalized over the whole
periphery,while6-8arebisthieno-annelateddehydro[14]annulenes.
Following the usual trend, the larger annulene 9 is less aromatic
than the effectively smaller annulenes 6-8.7b
Thiophene protons C and D resonate in the range of
7.04-7.13 and 7.15-7.24 ppm for the precyclized molecules,
and move ca. 0.3-0.4 ppm downfield upon forming 6-8. In
general the thiophene doublets are farther downfield when the
thiophenes are oriented “down” (e.g., 8) compared to the isomers
with the thiophenes oriented “up” (e.g., 6). As with the benzene
proton shifts above, asymmetric 7 displays features that are a
combination of both 6 and 8 (Table 3). In the case of 9, the
thiophene proton shifts and change in shifts are comparable to
those for 6-8. While puzzling, we will table further discussion
to avoid the possibility of overinterpretation.
Computations. DFT calculations (B3LYP/6-311G**)32 were
performed on [14]DBTAs 6-9 and [14]DTAs 10-14 to help
elucidate the observed trends. The calculated HOMO-LUMO
energy gaps from the optimized structures, along with the
HOMO-LUMO gaps experimentally determined from optical
and electrochemical data, are given in Table 4. Although the
computed values overestimate the experimental HOMO-LUMO
Electronic Absorption Spectra. The electronic absorption
spectrum of parent [14]DBA 27 shows a weak, low-energy
(32) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098–3100. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789. (c) Becke, A. D. J. Chem.
Phys. 1993, 98, 5648–5652. (d) Becke, A. D. J. Chem. Phys. 1993, 98, 1372–
1377.
(30) Mitchell, R. H. Chem. ReV. 2001, 101, 1301–1316.
(31) Fringuelli, F.; Marino, G.; Taticchi, A. J. Chem. Soc., Perkin Trans.
1974, 2, 332–337.
J. Org. Chem. Vol. 73, No. 12, 2008 4429

O’Connor et al.
FIGURE 7. UV-vis absorption spectra of (a) 3-5, (b) 6-9, and (c) 10-14. All spectra recorded in CH2Cl2 at analyte concentrations of 15-25
µM.
TABLE 4. HOMO-LUMO Energy Level Correlations of Selected [14]DBTAs and [14]DTAs
lowest energy abs.
λmax [nm] (cutoff)
optical energy
gap [eV]
electrochemical
energy gap [eV]
DFT calcd
energy gap [eV]
Epa (ox)
Epc (red)
(vs Fc) [V]
calcd
Dipole [D]
compd
(vs Fc) (calcd Epa)a [V]
6
7
8
9
10
11
12
13
14
374 (380)
398 (410)
408 (420)
339 (360)
399 (410)
399 (410)
395 sh (410)
404 (414)
368 (370)
3.26
3.12
3.04
3.66
3.10
3.10
3.16
3.06
3.37
3.26
3.12
2.93
3.39
2.87
2.72
3.09
2.82
3.39
3.53
3.39
3.26
3.97
3.37
3.41
3.48
3.34
3.63
1.39 (1.10)
1.38 (1.04)
1.12 (0.97)
1.60 (1.24)
1.13 (1.01)
1.19 (0.97)
1.32 (1.06)
1.18 (1.05)
1.62 (1.13)
-1.87
-1.74
-1.81
-1.79
-1.74
-1.54
-1.77
-1.64
-1.81
1.46
2.91
3.99
2.33
1.02
2.58
2.08
3.53
1.66
a The second number is the DFT calculated Epa value [V].
gaps, the numbers are in good overall agreement with the
observed trends. For hybrids 6-9, the ordering of the energy
gaps is in excellent agreement among all three methods. The
narrowest energy gap (2.93-3.26 eV) corresponds to 8 with
the sulfurs “down”, while the widest energy gap (3.39-3.97
eV) corresponds to 9, attributable to the 3,4-fusion of the
thiophenes. Macrocycles 6 (sulfurs “up”) and 7 (sulfurs
“alternating”) lie in between these bookend values (Table 4).
The pattern of relative sulfur orientation (“up”, “down”,
“alternating”) and the resulting effect on optoelectronic proper-
ties, while not observed for the [18]DTAs,13 is an unmistakable
feature in the [14] series. One possible correlation is the increase
in dipole moment (Table 4): as the sulfur orientation switches
from “up” to “alternating” then to “down”, there is a small
increase in the dipole moments and thus diminishing energy
gaps. The dipole moments, however, are sufficiently small to
begin with, which accounts for the apparent lack of solvato-
chromism. Another consideration is the repulsion between the
sulfur electrons adjacent to the bowed alkynes. When examining
thiophene cyclooligomers, the sulfur-sulfur repulsion of a bent
chain, as in the case of Ba¨uerle’s cyclic thiophenes,15 blue-
shifts the absorption spectra of the molecule with respect to a
linear oligomer containing the same number of repeat units. A
similar interaction could be operational between the sulfur lone
pair of an “up” thiophene and the alkyne π-orbital of the bowed
monoyne, thus causing the hypsochromic shifts. An alternative
For all-thiophene DTAs 10-14, there is excellent agreement
of energy gap ordering between the calculated and optically
determined numbers. Given the aforementioned difficulties in
obtaining the CVs, it is not surprising that ordering in the
electrochemical data is slightly off for the 2,3-fused systems.
Nonetheless, 3,4-fused DTA 14 has the widest energy gap
(3.37-3.63 eV) analogous to 9. As with 8, the sulfurs “down”
orientation of 13 is the smallest value of the all-thiophene series,
in excellent agreement with the optical energy gap trend, and
in close agreement with the electrochemical energy gap trend.
4430 J. Org. Chem. Vol. 73, No. 12, 2008

Structure-Property InVestigations of Conjugated Thiophenes
FIGURE 8. Calculated HOMO/LUMO plots of [14]DBTAs 6-9.
view would be a decreased interaction between the sulfur lone
pair of a “down” thiophene and the alkyne π-orbital of the
bowed diyne linkage, resulting in bathochromic shifts. We are
pursuing further experimental and computation studies to
ascertain the exact cause of this effect.
of previously reported thiophene macrocycles, attributable to
the rigid structure and aromatic nature of the [14]DBA ring
further enhanced by the electron richness of the thiophenes
1
themselves. The downfield chemical shifts in the H NMR
spectra show evidence of increased delocalization and enhanced
ring current. Furthermore, the arrangement and number of
thiophenesinthemacrocycledramaticallyaffectstheHOMO-LUMO
energy gaps of the molecules, with the “down” orientation of
the sulfurs in 2,3-fused systems (8, 13) affording the lowest
energies, as confirmed by UV-vis, electrochemical, and
computational data. These data also show that the 3,4-fused
macrocycles (9, 14) possess the highest HOMO-LUMO energy
gaps. X-ray crystal structures of 8 and 13 exhibit solid-state
packing behavior unfavorable toward topochemical polymeri-
zation, thus explaining the broad exotherms found from DSC
scans. The electronic delocalization in the macrocycles most
likely creates molecules more similar to dehydroannulenes than
thiophene oligomers, accounting for the limited stability of the
compounds and poorly behaved electrochemical studies. Future
studies are focusing on alkyl-stabilized [14]DTAs and on closely
related [15]DTAs; this work will be reported in due course.
The computed molecular orbital plots for 6-14 (Figure 8
for 6-9, Figure S1 in the Supporting Information for 10-14)
show that there is little charge separation in the HOMOs and
LUMOs across the entire series of molecules, thus confirming
the πfπ* nature of the transitions in the electronic absorption
spectra. For example, macrocycles 6-8 have HOMOs and
LUMOs that topologically look very similar; thus, it is difficult
to glean any meaningful information regarding the effects of
sulfur orientation. The same can be said for DTAs 10-13. The
orbital plots for 9 and 14, however, illustrate the dramatic
difference between 2,3-fused and 3,4-fused macrocycles. The
order of the HOMOs and LUMOs is swapped, i.e., the HOMO
and LUMO of 6-8 and 10-13 correspond to the HOMO-1
and LUMO+1, respectively, of macrocycles 9 and 14. In
addition, HOMO-LUMO electron density of 9/14 is concen-
trated on the 1,2-bis(thienylethynyl)benzene portion of the
molecule, not the diacetylene linkage. The electron density shifts
to the diacetylene in the HOMO-1 and LUMO+1. The 3,4-
fusion, and thus reduced double bond character, significantly
disrupts the net conjugation of the entire macrocycle.
Experimental Section
General Methods. These are described in ref 12a. Dihal-
othiophenes 15-1720 and diynes 21,12c 23,21 and 2421 were
prepared according to known procedures.
Conclusions
2-(Triisopropylsilylethynyl)-3-(trimethylsilylethynyl)th-
iophene (20). 2-Bromo-3-iodothiophene (15, 3.0 g, 10 mmol), CuI
(125 mg, 0.7 mmol), and PdCl2(PPh3)2 (251 mg, 0.033 mmol) were
dissolved in DIPA (100 mL) and THF (100 mL). The solution was
degassed under Ar for 30 min. Through a syringe, TMSA (1.5 mL,
10.6 mmol) was added and the mixture was stirred at room
temperature in a sealed vessel overnight. The solvent was removed
under reduced pressure and the crude mixture was run through a
A series of 12 dehydro[14]annulene macrocycles successively
fused with one (3-5), two (6-9), or three thiophene rings
(10-14) have been prepared. Locking the entire π-system into
planarity helps to promote orbital overlap, optical tunability,
and increased ring currents in the macrocycles. The net
electronic delocalization of these annulenes is greater than that
J. Org. Chem. Vol. 73, No. 12, 2008 4431

O’Connor et al.
silica plug (5:1 hexanes/CH2Cl2, 300 mL) and then concentrated.
The residue was dissolved in DIPA (100 mL) and THF (100 mL),
CuI (125 mg, 0.7 mmol), and PdCl2(PPh3)2 (251 mg, 0.033 mmol)
were added, then the mixture was degassed under Ar for 30 min.
Through a syringe, TIPSA (10 mL, 47 mmol) was added and the
mixture was stirred at room temperature in a sealed vessel overnight.
The solvent was removed under reduced pressure and the residue
was chromatographed on silica (hexanes) to afford 20 (3.2 g, 91%)
as an orange oil. 1H NMR (300 MHz, CDCl3) δ 0.24 (s, 9H), 1.15
(s, 21H), 6.98 (d, J ) 5.3 Hz, 1H), 7.08 (d, J ) 5.3 Hz, 1H); 13C
NMR (75 MHz, CDCl3) δ -0.1, 18.8, 98.0, 98.3, 98.7, 100.0, 125.5,
halodiyne precursor. This material was redissolved in THF/DIPA
(1:1, 0.1 M) and the second diyne (1.5 equiv), desilylated in the
manner described above, was added. The solution was degassed
under Ar for 30 min and then CuI (6 mol %) and Pd(PPh3)4 (3 mol
%) were added. The sealed vessel was stirred at room temperature
until TLC showed the reaction complete (12 h to 2 d). The solvent
was removed in vacuo and the crude product was chromatographed
on silica to afford the annulene precursor.
Tetrayne 26. Chromatography on silica (10:1 hexanes:CH2Cl2)
furnished 27 (702 mg, 81%) as an orange oil. 1H NMR (300 MHz,
CDCl3) δ 1.11 (s, 21H), 1.12 (s, 21H), 7.03 (d, J ) 5.3 Hz, 1H),
7.04 (d, J ) 5.3 Hz, 1H), 7.08 (d, J ) 5.3 Hz, 1H), 7.14 (d, J )
5.3 Hz, 1H), 7.17 (d, J ) 5.3 Hz, 1H), 7.28 (d, J ) 5.3 Hz, 1H);
13C NMR (75 MHz, CDCl3) δ 11.3, 18.66, 18.73, 85.3, 85.8, 91.7,
92.4, 95.4, 98.0, 100.6, 100.9, 125.7, 126.2, 126.27, 126.30, 126.58,
126.60, 126.76, 126.82, 129.2, 129.6, 129.9, 130.1; IR (NaCl) V
2146, 2192, 2864, 2890, 2942 cm-1; MS (ACPI) m/z (%) 657.2
(100, MH+), 729.3 (90, MH+ + THF); HRMS (MALDI)
C38H48S3Si2 [M]+ calcd 656.2457, found 656.2455.
General Procedure for Annulene Formation. To a solution
of annulene precursor (1 equiv) in THF (0.1 M) was added TBAF
(20 equiv, 1.0 M THF). The mixture was stirred for 5 min and
then Et2O was added. The solution was washed 4-6 times with
brine and then water, and the organic layer was dried (MgSO4).
The solution was concentrated, rediluted with 1:1 THF/DIPA to
ca. 50 mL, and then injected over 8 h via syringe pump into a
THF/DIPA solution (1:1, 0.05 M) containing PdCl2(dppe) (3 mol
%), CuI (6 mol %), and I2 (0.5 equiv). Once TLC showed
completion of the reaction, the solvent was removed under reduced
pressure and the residue was run through a silica plug (1:1 hexanes:
CH2Cl2). The product was further purified on a chromatotron (8:1
hexanes:EtOAc) to afford the annulene as an unstable tan solid,
which should be stored cold, in the dark, and under an Ar
atmosphere to minimize decomposition.
127.0, 127.2, 129.8; IR (NaCl) V 2153, 2865, 2993, 2943 cm-1
;
MS (APCI) m/z (%) 431.2 (100, M+ + THF), 359.1 (30); HRMS
(EI) for C20H23SSi2 [M]+ calcd 360.1770, found 360.1763.
3-(Triisopropylsilylethynyl)-4-(trimethylsilylethynyl)thio-
phene (22). 3-Bromo-4-iodothiophene (17, 4.7 g, 15.4 mmol),
CuI (29 mg, 0.15 mmol), PdCl2(PPh3)2 (61 mg, 0.075 mmol), and
TMSA (2.40 mL, 17.0 mmol) were reacted as described above for
20. The solvent was removed under reduced pressure and the crude
mixture was run through a silica plug (hexanes) and then concen-
trated. The residue was dissolved in THF (20 mL) and DIPA (20
mL), CuI (29 mg, 0.15 mmol), and Pd(PPh3)Cl2 (41 mg, 0.05 mmol)
were added, then the mixture was degassed under Ar for 30 min.
Pd(PPh3)4 (26 mg, 0.025 mmol) and TIPSA (4.48 mL, 20.0 mmol)
were added next. The reaction mixture was stirred in a sealed
pressurized vessel at 115 °C for 4 d. Upon cooling, the solvent
was removed in vacuo and the residue was chromatographed
1
(hexanes) to afford 22 (4.2 g, 88%) as an orange oil. H NMR
(300 MHz, CDCl3) δ 0.23 (s, 9H), 1.14 (s, 21H), 7.38-7.42 (m,
2H); 13C NMR (75 MHz, CDCl3) δ -0.1, 18.7, 92.8, 96.4, 98.1,
99.8, 125.0, 125.2, 129.0, 129.3; IR (NaCl) V 2158, 2865, 2893,
2944, 3110 cm-1; MS (APCI) m/z (%) 431.2 (40, M+ + THF);
HRMS (EI) for C20H23SSi2 [M]+ calcd 360.1763, found 360.1763.
General Procedure for Preparation of “Symmetrical”
Annulene Precursors. To a solution of diyne (2.5 equiv) dissolved
in MeOH/THF (10:1, 0.05 M) was added K2CO3 (10 equiv) and
the mixture was stirred for 30 min. The solution was filtered and
then Et2O was added. The solution was washed successively with
H2O, 10% NaCl solution, and water, then dried (MgSO4). Solvent
was removed in vacuo and the residue was added to a solution of
THF/DIPA (1:1, 0.1 M) and dihaloarene (1 equiv). The mixture
was degassed under Ar for 30 min after which CuI (6 mol %) and
Pd(PPh3)4 (3 mol %) were added. The sealed vessel was stirred at
room temperature until TLC showed the reaction complete (12 h
to 2 d). The solvent was removed in vacuo and the crude product
was chromatographed on silica to afford the annulene precursor.
Tetrayne 25. Chromatography on silica (10:1 hexanes:CH2Cl2)
furnished 25 (251 mg, 86%) as an orange oil. 1H NMR (300 MHz,
CDCl3) δ 1.13 (s, 42H), 7.04 (d, J ) 5.3 Hz, 2H), 7.17 (d, J ) 5.3
Hz, 2H), 7.30 (AA′BB′m, 2H), 7.51 (AA′BB′m, 2H); 13C NMR (75
MHz, CDCl3) δ 11.3, 18.7, 86.3, 95.2, 95.8, 100.7, 125.4, 126.3,
126.90, 126.92, 128.1, 129.9, 131.5; IR (NaCl) V 2149, 2213, 2863,
2941 cm-1; MS (ACPI) m/z (%) 723.3 (20, M+ + THF), 651.2
(100, MH+); HRMS (EI) for C40H50S2Si2 [M]+ calcd 650.2892,
found 650.2901.
General Procedure for Preparation of “Asymmetrical”
Annulene Precursors. To a solution of diyne (1.0 equiv) dissolved
in MeOH/THF (10:1, 0.05 M) was added K2CO3 (2.5 equiv) and
the mixture was stirred for 30 min. The solution was filtered and
then Et2O was added. The solution was washed successively with
H2O, 10% NaCl solution, and water, then dried (MgSO4). Solvent
was removed in vacuo and the residue was added to a solution of
THF/DIPA (1:1, 0.1 M) and dihaloarene (1 equiv). The mixture
was degassed under Ar for 30 min after which CuI (6 mol %) and
Pd(PPh3)4 (3 mol %) were added. The sealed vessel was stirred at
room temperature until TLC showed the reaction complete (12 h
to 2 d). The solvent was removed in vacuo and the crude product
was chromatographed on a short plug of silica to afford the
[14]DBTA 6. Yield 9.7 mg, 53%. 1H NMR (300 MHz, C6D6) δ
6.50 (d, J ) 5.3 Hz, 2H), 6.68 (d, J ) 5.3 Hz, 2H), 6.95 (AA′BB′m,
2H), 7.71 (AA′BB′m, 2H); 13C NMR (75 MHz, CD2Cl2) δ 81.9,
85.3, 87.5, 99.2, 122.3, 124.6, 126.4, 128.3, 128.6, 132.6, 135.4;
IR (KBr) V 2102, 2924, 3102 cm-1; UV (CH2Cl2) λmax (log ε) 309
(4.44), 316 (4.46), 329.0 (4.74), 363 (4.74), 373 (3.90) nm; MS
(APCI) m/z (%) 409.0 (100, MH+ + THF); HRMS (EI) C22H8S2
[M]+ calcd 336.0067, found 336.0067.
1
[14]DTA 12. Yield 34 mg, 64%. H NMR (300 MHz, C6D6) δ
6.46 (d, J ) 5.3 Hz, 1H), 6.53 (d, J ) 5.3 Hz, 1H), 6.61 (d, J )
5.3 Hz, 1H), 6.77 (d, J ) 5.3 Hz, 1H), 7.02 (d, J ) 5.3 Hz, 1H),
7.18 (d, J ) 5.3 Hz, 1H); 13C NMR (75 MHz, C6D6) δ 82.9, 83.3,
86.5, 87.0, 87.9, 89.9, 95.0, 96.0, 123.2, 123.9, 124.3, 125.5, 126.4,
127.0, 127.3, 129.8, 132.2, 133.4; IR (KBr) V 2140, 2183, 2871,
2959, 2982, 3109 cm-1; UV (CH2Cl2) λmax (log ε) 335 (4.99), 319
(4.74), 379 (4.28), 385 (4.26) nm; MS (APCI) m/z (%) 415.0 (100,
MH+ + THF); HRMS (MALDI) C20H6S3 [M]+ calcd 341.9632,
found 341.9636.
Acknowledgement. We thank the ACS Petroleum Research
Fund and the National Science Foundation (CHE-0718242) for
financial support. We thank Prof. J. E. Hutchison for use of his
group’s potentiostat. We also thank the UO Neuroinformatics
Center for use of the ICONIC Grid, a high-performance
computational infrastructure constructed with support from NSF-
MRI grant BCS-0321388.
Supporting Information Available: Experimental details for
all new compounds; calculated HOMO/LUMO plots of 10-14;
CVs of selected annulenes; copies of 1H and 13C NMR spectra;
computational details; CIF files for 8 and 13. This material is
available free of charge via the Internet at http://pubs.acs.org.
JO800225U
4432 J. Org. Chem. Vol. 73, No. 12, 2008