Structure-property relationships of donor/acceptor-functionalized tetrakis(phenylethynyl)benzenes and bis(dehydrobenzoannuleno)benzenes

Article abstract of DOI:10.1021/ja044175a

A series of tetrakis(phenylethynyl)benzenes and bis(dehydrobenzoannuleno) benzenes have been synthesized containing tetra-substitutions of neutral, donor, and mixed donor/acceptor groups. To ascertain the importance of substitutional and structural differ

Full text of DOI:10.1021/ja044175a

Published on Web 02/04/2005
Structure-Property Relationships of Donor/
Acceptor-Functionalized Tetrakis(phenylethynyl)benzenes and
Bis(dehydrobenzoannuleno)benzenes
Jeremiah A. Marsden, Jeremie J. Miller, Laura D. Shirtcliff, and Michael M. Haley*
Contribution from the Department of Chemistry and Materials Science Institute,
UniVersity of Oregon, Eugene, Oregon 97403-1253
Received September 24, 2004; E-mail: haley@uoregon.edu
Abstract: A series of tetrakis(phenylethynyl)benzenes and bis(dehydrobenzoannuleno)benzenes have been
synthesized containing tetra-substitutions of neutral, donor, and mixed donor/acceptor groups. To ascertain
the importance of substitutional and structural differences of the phenylacetylenes, the optical absorption
and emission properties of each series were examined. Conjugation effectiveness, electron density, planarity,
and geometry of charge-transfer pathways were found to have a pronounced effect on the overall optical
and material properties. Considerable self-association behavior due to face-to-face stacking in solution
was observed for donor/acceptor-functionalized macrocycles and was quantified by concentration-dependent
1H NMR measurements. A solvent-dependent polymerization of one macrocycle regioisomer was observed
and characterized. To provide further insight into the energy levels and electronic transitions present,
computational studies of each system were performed.
Introduction
with a specific application in mind, knowledge of the material’s
structure-property relationship is necessary. With this under-
standing, the ability to “tune” a class of materials to enhance
an explicit property can be achieved.
Organic compounds possessing a high degree of conjugation
have long been recognized as ideal materials for advanced
electronic and photonic applications including organic LEDs,
liquid crystal displays, thin film transistors, solar cells, and
optical storage devices.1-3 Changes in the substitutional groups
and substitution pattern, conjugation, and molecular electronic
structure can bring about very different optical and physical
properties for such materials.2-7 To design an organic molecule
Functionalized cruciform-conjugated phenylacetylene and
phenylvinylene structures are known to exhibit remarkable
optical and electronic properties because of their multiple
conjugated pathways.2,6-9 Possessing this cruciform-type struc-
ture, 1,2,4,5-tetrakis(phenylethynyl)benzenes (TPEBs) provide
an ideal carbon framework for studying the intrinsic effect of
functional group substitution on each conjugated pathway and
on the overall material properties of each system. Substitution
of electron donor and/or acceptor groups at the 4′-positions gives
rise to discreet two-dimensional π-chromophores. By varying
the substitution pattern of the donor and acceptor groups, each
individual charge-transfer pathway can be enhanced. We10-12
(1) (a) Chen, J.; Reed, M. A.; Dirk, S. M.; Price, D. W.; Rawlett, A. M.; Tour,
J. M.; Grubisha, D. S.; Bennett, D. W. NATO Sci. Ser. II: Math., Phys.,
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Haldi, A.; Barlow, S.; Marder, S. R.; Kippelen, B. J. Polym. Sci., Part B:
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9
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10.1021/ja044175a CCC: $30.25 © 2005 American Chemical Society

Donor/Acceptor-Functionalized TPEB and bisDBA Systems
A R T I C L E S
substituted compounds have also been prepared and analyzed.
The optical absorption and emission characteristics of these
systems were examined for structure-function relationships. We
herein also report a computational study of the conjugation
effectiveness of each system through geometry optimization and
molecular orbital plots analysis. Last, the aggregation properties
of the donor/acceptor-functionalized bisDBAs are described.
Results and Discussion
Figure 1. Conjugated pathways present in the TPEB and DBA systems.
Synthesis. The general synthetic strategy for the phenylacety-
lene series relied heavily on the Sonogashira cross-coupling
protocol, for which the reactivity difference between aryl iodides
and aryl bromides allowed specific placement of the donor and
acceptor units.16 At the heart of each molecule was a central
tetrahaloarene to which both triisopropylsilyl (TIPS)- and
trimethylsilyl (TMS)-protected alkynes were attached for the
TPEB syntheses. Likewise, donor- and/or acceptor-functional-
ized diethynylarenes were selectively cross-coupled to the
central tetrahaloarenes for the DBA syntheses. For the ring
closure of the annulene systems, we employed both Cu-
mediated17 as well as oxidative Pd-catalyzed18 homocoupling
to control selectivity of the ring geometry.19
The target TPEBs are shown in Figure 2, which consist of
the known parent compound (1),14 tetra-donor (2), and each of
three regioisomers containing two donor and two acceptor
groups with like substituents fused either ortho (3), meta (4),
or para (5) to each other.20 The assembly of TPEB 2 is shown
in Scheme 1. Phenylacetylene 6 was made by Sonogashira cross-
coupling of trimethylsilylacetylene (TMSA) with N,N-dibutyl-
4-iodoaniline.12 The silyl protecting group was next removed
with TBAF, and after a 4-fold cross-coupling with 1,2,4,5-
tetraiodobenzene, tetra-donor 2 was isolated.
The syntheses of the donor- and acceptor-functionalized
TPEBs are exemplified in Scheme 2 with the synthesis of
phenylacetylene 3. Direct cross-coupling of the phenylacetylenes
as in Scheme 1 was initially attempted, but proved inconsistent
and low yielding. Improved results were attained through
sequential installation of silyl-protected acetylenes to the central
ring followed by attachment of the donor- and acceptor-
substituted arenes. Starting from 1,2-dibromo-4,5-diiodobenzene
7,21 TMSA was first cross-coupled to the more reactive iodo
positions to give 8, followed by a second cross-coupling of
TIPSA to the remaining bromo positions at an elevated
temperature, providing 9 in 75% for the two steps. The
selectivity of the silane protective groups came into play next,
as the TMS group was preferentially removed with K2CO3/
MeOH and the resulting alkyne was cross-coupled with N,N-
dibutyl-4-iodoaniline to give 10. Finally, deprotection of the
and others5-9,13,14 have previously identified linear conjugation
between donor and acceptor groups as ideal for lowering the
HOMO-LUMO gap for charge-transfer, thereby enhancing the
optical and nonlinear optical (NLO) properties of the material.
The TPEB framework provided us with an ideal system for
studying the differences between the linear-conjugated (Figure
1, path a), cross-conjugated (path b), and “bent” conjugated (path
c) pathways to gain a better understanding of the geometrical
aspects of the charge-transfer pathways in phenylacetylene
networks.
We have also long been interested in the effect of planarity
on the conjugation of donor/acceptor-functionalized systems.
Because of the nearly free rotation about the phenyl-ethynyl
bonds in the TPEB systems, conjugation may be disrupted.
Therefore, we have designed a series of bis(dehydrobenzoan-
nuleno)benzenes (bisDBAs), which possess the same basic
carbon backbone as the TPEBs, yet are locked into planarity
by a diacetylene bridge, providing an ideal system for direct
comparison of conjugation effectiveness (Figure 1).8d,9,10 We
have previously studied the consequences of donor and acceptor
substitution patterns in dodecadehydro[18]annulenes on the
optical and NLO properties of DBAs.12,15 Because of the
symmetry constraints of these monoannulene systems, however,
placement of functional groups was somewhat limited, and
isolation of the exact effects due to each unique chromophore
proved difficult. The new bisannulene and TPEB systems
described in this article, on the other hand, provide a more
rational approach to functional group placement at each of four
equivalent positions on the periphery of the conjugated cruci-
form backbone. With functionalization at each of these positions,
the entire phenylacetylene framework contributes to the overall
conjugation in each system (with only minimal contribution from
the diacetylene bridge of the DBAs), allowing discrete identi-
fication of property changes due only to differences in substitu-
tion pattern. By studying analogous substitution patterns between
the TPEB and DBA systems, we hope to determine the
consequence of planarity on conjugation effectiveness. Since
the DBA systems should demonstrate better overall conjugation,
their optical and nonlinear optical properties should also be
enhanced.
(16) (a) Marsden, J. A.; Haley, M. M. In Metal-Catalyzed Cross-Coupling
Reactions, 2nd ed.; de Meijere, A., Diederich, F., Eds.; Wiley-VCH:
Weinheim, Germany, 2004; pp 317-394. (b) Sonogashira, K. In Metal-
Catalyzed Cross-Coupling Reactions; Diederich, F., Stang, P. J., Eds.;
Wiley-VCH: Weinheim, Germany, 1998; pp 203-230.
(17) Siemsen, P.; Livingston, R. C.; Diederich, F. Angew. Chem., Int. Ed. 2000,
39, 2632-2657.
(18) (a) Marsden, J. A.; O’Connor, M. J.; Haley, M. M. Org. Lett. 2004, 6,
2385-2388. (b) Lei, A.; Srivastava, M.; Zhang, X. J. Org. Chem. 2002,
67, 1969-1971. (c) Liu, Q.; Burton, D. J. Tetrahedron Lett. 1997, 38,
4371-4374. (d) Takano, S.; Sugihara, T.; Ogasawara, K. Synlett 1990,
453-454. (e) Rossi, R.; Carpita, A.; Bigelli, C. Tetrahedron Lett. 1985,
26, 523-526.
(19) Marsden, J. A.; Miller, J. J.; Haley, M. M. Angew. Chem., Int. Ed. 2004,
43, 1694-1697.
(20) Miller, J. J.; Marsden, J. A.; Haley, M. M. Synlett 2004, 165-168.
(21) Miljanic, O. S.; Vollhardt, K. P. C.; Whitener, G. D. Synlett 2003, 29-30.
In this article, we describe the synthesis and full characteriza-
tion of a series of donor- and acceptor-functionalized nonplanar
tetrakis(phenylethynyl)benzenes and their planar bis(dehy-
drobenzo[14]- and [15]annuleno)benzene counterparts. For
control comparisons, the tetra-donor and neutral tetra-decyl-
(13) Hilger, A.; Gisselbrecht, J. P.; Tykwinski, R. R.; Boudon, C.; Schreiber,
M.; Martin, R. E.; Lu¨thi, H. P.; Gross, M.; Diederich, F. J. Am. Chem.
Soc. 1997, 119, 2069-2078.
(14) Kondo, K.; Yasuda, S.; Tohoru, S.; Miya, M. J. Chem. Soc., Chem.
Commun. 1995, 55-56.
(15) Sarkar, A.; Pak, J. J.; Rayfield, G. W.; Haley, M. M. J. Mater. Chem.
2001, 11, 2943-2945.
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A R T I C L E S
Marsden et al.
Figure 2. Target TPEBs 1-5.
Scheme 1. Preparation of TPEB 2a
Table 1. Yields for Syntheses of TPEBs 3-5
a Conditions: (a) TMSA, Pd(PPh3)2Cl2, CuI, DIPA, THF; (b) K2CO3,
THF, MeOH; (c) 1,2,4,5-tetraiodobenzene, Pd(PPh3)4, CuI, DIPA, THF,
40 °C.
Scheme 2. Synthesis of TPEB 3a
This selectivity between bisDBAs is controlled through the
conditions used for cyclization: Cu-mediated vs Pd-catalyzed
homocoupling of the alkynes, vide infra. Precursor C is made
by a convergent synthesis consisting of sequential cross-
couplings of the decyl, donor, and/or acceptor diynes (D) to
the central tetrahaloarene (E). The target series of bis(dehy-
drobenzo[14]- and [15]annulene)benzenes is shown in Figure
3. Analogous to the TPEB systems, the DBAs consist of neutral
pure hydrocarbons 11 and 12 substituted with decyl groups to
improve solubility, tetra-donors 13 and 14, and the series of
donor- and acceptor-functionalized bis[14]- and [15]annulenes
15-20.
The diyne units (21a-c) were made from known iodoarenes
22a-c11,12 through a simple cross-coupling with TMSA fol-
lowed by deprotection of the TMS group (Scheme 4). The
precursors to cyclization (23a,b) for the tetra-decyl and tetra-
donor systems were assembled by 4-fold Sonogashira coupling
of diynes 21a or 21b with 1,2,4,5-tetraiodobenzene. Because
of anticipated solubility issues, the tetra-acceptor systems were
not attempted.
a Conditions: (a) TMSA, Pd(PPh3)2Cl2, CuI, DIPA, THF; (b) TIPSA,
Pd(PPh3)2Cl2, CuI, DIPA, THF, 60 °C; (c) K2CO3, THF, MeOH; (d) N,N-
dibutyl-4-iodoaniline, Pd(PPh3)4, CuI, DIPA, THF; (e) TBAF, THF; (f)
4-iodo-1-nitrobenzene, Pd(PPh3)4, CuI, DIPA, THF.
The syntheses of the donor/acceptor-substituted cyclization
precursors start with three regioisomers of dibromodiiodoben-
zene as exemplified by the assembly of 24 (Scheme 5). Cross-
coupling of 2 equiv of nitro-diyne 21c to the iodo positions of
7 gave the electron-deficient dibromoarene 25. Since alkyne
cross-couplings are known to favor electron-poor haloarenes
and electron-rich alkynes,16 this set up an ideal system for the
second coupling of donor diyne 21b to the bromo positions.
As a result, a moderate temperature of only 60 °C was required
to provide precursor 24 in a very good 76% yield. Using this
synthetic methodology, we also constructed the precursors to
annulenes 16, 17, 19, and 20 with the yields reported in Table
2.22
TIPS groups with TBAF and coupling of the resultant terminal
alkyne with 4-iodo-1-nitrobenzene afforded TPEB 3 as a red
powder. The remaining isomers 4 and 5 followed analogous
synthetic schemes with yields as reported in Table 1.22
The synthetic strategy for construction of the DBA systems
is outlined in Scheme 3. As we have previously reported for
similar annulenic systems,19 we can control ring size and
geometry of the DBAs through the cyclization technique,
thereby making either bis[15]annulenes (A) with cyclization
across the meta-fused alkynes or bis[14]annulenes (B) cyclized
across the ortho-fused alkynes from the same precursor (C).
(22) See Supporting Information for details.
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Donor/Acceptor-Functionalized TPEB and bisDBA Systems
A R T I C L E S
Scheme 3
of the acetylides, showed preferential formation of the bis[15]-
annulene, as expected, giving a 4:1 mixture of 13:14.
We subjected all of the precursor polyynes to both the Cu-
pyridine and Pd(dppe)Cl2 methods in order to assemble the full
series of bisannulenes shown in Figure 3. As outlined in Table
2, yields for the electron-rich tetra-donor systems were the
highest, followed by moderate yields for the neutral DBAs, and
poorer yields for the donor/acceptor systems. Even with very
dilute conditions and slow injection of the deprotected polyyne,
cyclization of the donor/acceptor-functionalized DBAs produced
copious amounts of oligomeric byproduct because of competing
intermolecular homocoupling. Therefore, despite numerous
attempts at cyclization and modifications to the reaction
conditions, construction of DBA 20 remained elusive, while all
other DBA syntheses were successful.
As structural isomers, the bis[14]- and [15]annulenes have
very similar spectral characteristics, making identification
difficult. There was, however, one key difference in the 1H NMR
spectra between each pair of compounds. For example, the
singlet from the two protons on the central ring of 13 and 14
displayed a marked difference in chemical shift, whether it was
intraannular (bis[15]annulene 13, δ ) 8.42 ppm) or between
rings (bis[14]annulene 14, δ ) 8.24 ppm).25,26 On the basis of
molecular modeling calculations,24 the central protons of the
bis[15]annulene are much closer to the internal alkyne units
than the analogous protons on the bis[14]annulene because of
the internal bowing of the phenyl-ethynyl bonds of 13.
Therefore, a larger anisotropic deshielding effect due to the
proximitiy of the triple bonds gave the central protons of 13 a
much more downfield shift than those of 14. This trend
continued for each substitution of annulene.22 Proof that our
structural assignments were correct was provided through
definitive construction of 13 by cyclization of one ring at a
time.19 This route, which allowed no possibility for formation
of 14-membered rings, afforded material possessing identical
spectral characteristics.
Electronic Absorption Spectra. The UV-vis spectra of the
TPEBs and DBAs provide a great deal of insight into the
electronic structure of the systems. All of the TPEB systems
display a characteristic pattern of two broad bands as seen in
the pure hydrocarbon parent 1 (Figure 4, Table 3). With the
added electron density of four NBu2 groups, a large red-shift
(∼60 nm) of both bands was observed for phenylacetylene 2
with a dramatic increase in the absorption of the lower energy
peak. The corresponding bands of the donor/acceptor-substituted
TPEB spectra are remarkably broadened and show further
bathochromic shifts with absorption cutoffs extending beyond
550 nm, indicative of intramolecular charge-transfer transitions.
An obvious difference is seen between each substitution pattern
of the donor and acceptor groups. TPEBs 3 and 4, which both
possess linear-conjugated pathways, have a much more distinct
low-energy band. Lacking this linear charge-transfer conduit,
the analogous low-energy band of TPEB 5 is greatly weakened.
This further indicates that linear-conjugated pathways are more
efficient than the bent-conjugated or cross-conjugated donor to
acceptor pathways present in 5. A distinct correlation is also
observed between the net dipole moments of the compounds
As we have recently reported and noted above, upon
cyclization an interesting selectivity for ring formation based
upon the cyclization conditions employed was discovered.19 This
selectivity is illustrated for the cyclization of the tetra-donor
precursor 23b (Scheme 6). After deprotection of the silanes,
intramolecular homocoupling under Glaser or Eglinton condi-
tions with Cu-pyridine gave exclusively the bis[15]annulene
13 (70%), which is cyclized across the meta fusion of alkyne
units. An often overlooked consideration when performing this
type of reaction is the geometry of the Cu-acetylide prior to
reductive elimination. The most commonly accepted mechanistic
picture for this sequence involves a pseudo-trans configuration
involving a dimeric Cu(I)-acetylide (A, Scheme 7).23 Not
surprisingly, on the basis of simple molecular modeling calcula-
tions,24 formation of the bis[15]annulene adopts a trans-like
configuration of the alkynes for ideal ring closure. Conversely,
for cyclization across the ortho-fused alkynes, as in bis[14]-
annulene (14), a cis-like geometry is favored. To enforce this
cis geometry in the system, we turned to oxidative palladium
chemistry.18 A key benefit of a Pd-homocoupling procedure is
the ability to fine-tune the catalytic system by the judicious
choice of ligand. A cis addition of the acetylides to the Pd center
could be enforced with cis-bidentate ligated Pd(dppe)Cl2 (dppe
) diphenylphosphinoethane) (B, Scheme 7), resulting in
exclusively bis[14]annulene 14 in a respectable 84% yield
(Scheme 6). On the other hand, intramolecular homocoupling
using monodentate PPh3 ligands, which allows trans addition
(23) Bohlmann, F.; Scho¨nowsky, H.; Inhoffen, E.; Grau, G. Chem. Ber. 1963,
97, 794-800.
(24) PM3(tm) calculations performed on an SGI Octane workstation using
Spartan SGI version 5.1.3 by Wavefunction, Inc.: Irvine, CA, 1998.
(25) Tobe, Y.; Kishi, J.; Ohki, I.; Sonoda, M. J. Org. Chem. 2003, 68, 3330-
3332.
(26) Baldwin, K. P.; Matzger, A. J.; Scheiman, D. A.; Tessier, C. A.; Vollhardt,
K. P. C.; Youngs, W. J. Synlett 1995, 1215-1218.
9
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A R T I C L E S
Marsden et al.
Figure 3. Target bisDBAs 11-20.
Scheme 4
Scheme 5
bis[14]- and [15]annulene topologies. The larger extinction
coefficient and greater number of sharp peaks present in bis-
[14]annulene 12 suggest that more complex vibronic transitions
are at work. A reasonable explanation for this difference is due
to the presence of a weak ring current in the aromatic
14-membered rings.27 A similar degree of red-shift was detected
for the tetra-donor annulenes 13 and 14 from TPEB 2, although
less fine structure is present in these systems than in the tetra-
decyl-substituted DBAs (Figure 6). A noticeable difference
between the bis[15]annulenes (11 and 13) and bis[14]annulenes
calculated using the B3LYP/6-31G* basis set and the extent of
bathochromic shift (Table 3). The meta-substituted TPEB 4
possesses the largest dipole as well as the longest wavelength
λmax
.
The effect of planarity on the cruciform framework was
distinctly seen by UV-vis spectroscopy. The free rotation of
the central phenyl-ethynyl bonds in 1 was eliminated in
annulenes 11 and 12, effectively locking the structures into
planarity by the outer diacetylene bridge. Because of the
resultant enhancement of conjugation in the DBAs, a perceptible
bathochromic shift of the low-energy bands of ∼65 nm was
observed (Figure 5, Table 3). Interestingly, there was a great
difference in the fine structure of the absorption bands between
(27) (a) Boydston, A. J.; Haley, M. M.; Williams, R. V.; Armantrout, J. R. J.
Org. Chem. 2002, 67, 8812-8819. (b) Bell, M. L.; Chiechi, R. C.; Johnson,
C. A.; Kimball, D. B.; Matzger, A. J.; Wan, W. B.; Weakley, T. J. R.;
Haley, M. M. Tetrahedron 2001, 57, 3507-3520.
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Donor/Acceptor-Functionalized TPEB and bisDBA Systems
A R T I C L E S
Table 2. Yields for Syntheses of DBAs 11-20
Scheme 7
a No detectable product formation. b Yield for tetra-decyl coupling. c Not
applicable.
changes in donor/acceptor substitution or planarity. Similar to
the TPEB series, meta fusion of donor and acceptor groups on
the DBA’s central phenyl ring (17 and 19) gave rise to the
largest, most red-shifted charge-transfer band with a similar
correlation in the calculated net dipole moments.
Scheme 6
Protonation of the NBu2 substituents by concentrated aqueous
HCl showed a marked affect on the UV-vis spectra of the tetra-
donor and donor/acceptor TPEBs and DBAs. Upon protonation,
Figure 4. UV-vis spectra of TPEBs 1-5.
Table 3. UV-Vis Data for TPEBs and DBAs
peak 1
λ
max (nm)
peak 2
λ
max (nm)
cutoff
(nm)
net dipole
(D)a
compound
(ꢀ
[L mol-1 cm-1])
(ꢀ
[L mol-1 cm-1])
(12 and 14) was the higher absorption of the longest-wavelength
1
2
316 (136 900)
377 (119 500)
318 (128 200)
326 (61 800)
328 (72 800)
316 (53 300)
317 (44 800)
307 (105 900)
343 (129 700)
330 (100 000)
307 (134 300)
323 (107 700)
319 (58 400)
318 (99 200)
308 (113 600)
330 (91 300)
326 (71 200)
325 (68 700)
350 (47 800)
414 (94 300)
349 (46 600)
417 (45 200)
426 (50 100)
346 (61 100)
383 (90 100)
417 (51 900)
419 (11 000)
472 (101 500)
422 (28 000)
463 (94 400)
456 (30 100)
491 (57 300)
430 (46 400)
473 (29 400)
453 (34 400)
466 (40 600)
395
481
395
565
565
426
567
438
434
507
447
499
600
600
470
620
600
600
0.00
0.00
b
18.51
13.24
b
0.00
0.00
0.00
0.00
b
0.00
15.06
18.34
b
λ
max in the bis[15]annulene systems. This rise in intensity may
2‚HCl
3
4
4‚HCl
5
11
12
13
13‚HCl
14
15
16
16‚HCl
17
be due to the greater linearity of conjugated phenylacetylenic
pathways present in 11 and 13, while the alkynyl bonds in the
bis[14]annulene topologies show a higher degree of bending
based on molecular modeling calculations.
The energy of the charge-transfer optical band gap is further
minimized in donor/acceptor-substituted DBAs 15-19. Batho-
chromic shifts of ∼60 nm are observed for the longest
wavelength absorptions of the planar annulenes over the
nonplanar donor/acceptor-functionalized TPEBs with absorption
cutoffs extending past 600 nm (Figure 7). The DBAs with linear-
conjugated pathways all display two large, broad peaks, while
the low-energy absorption of annulene 17, lacking linear-
conjugated pathways, is distinctly reduced. Little change in the
wavelengths of the higher energy bands was noticed with
0.00
15.26
18.33
18
19
a Calculated using the B3LYP/6-31G* basis set. b Not calculated.
9
J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005 2469

A R T I C L E S
Marsden et al.
Figure 5. UV-vis spectra of neutral TPEB 1 and DBAs 11 and 12.
Figure 8. Protonation/deprotonation of TPEB 2.
Table 4. Fluorescence Data for TPEBs and DBAs
a
compound
solvent
λ
em (nm)
Stokes shift (nm)
ΦF
1
CH2Cl2
PhMe
CH2Cl2
PhMe
CH2Cl2
PhMe
PhMe
CH2Cl2
PhMe
CH2Cl2
PhMe
CH2Cl2
PhMe
CH2Cl2
PhMe
505
506
512
507
562
508
547
513
513
504
508
541
528
513
511
513
513
565
560
517
580
560
560
154
157
98
0.02
0.02
0.71
0.22
0.09
0.01
0.30
0.25
0.23
0.14
0.18
0.21
0.38
0.18
0.02
0.57
0.43
0.11
0.17
0.31
0.06
0.10
0.09
2
98
2‚HCl
213
164
79
97
97
85
89
72
63
91
94
63
63
113
75
88
109
114
97
3
11
12
13
13‚HCl
14
CH2Cl2
PhMe
PhMe
PhMe
CH2Cl2
PhMe
Figure 6. UV-vis spectra of tetra-donor TPEB 2 and DBAs 13 and 14.
15
16
16‚HCl
17
18
19
PhMe
PhMe
a Measured with reference to fluorescein or 3-aminofluoranthene.
ance of the charge-transfer bands of the donor/acceptor-
functionalized DBAs by protonation was also readily observable.
Although red-shifted by ∼15 nm, the absorption spectrum of
16 after acidic treatment closely resembled that of neutral 11,
suggesting that the influence of the nitro acceptor groups alone
is minor.
Fluorescence Spectroscopy of the TPEBs and DBAs. A
strong fluorescence emission was observed for the majority of
the TPEB and DBA systems. The degree of red-shift in their
emission spectra closely matched the shift patterns observed in
the electronic absorption spectra, with relatively large Stokes
shifts and λem values ranging from 505 to 580 nm (Table 4).
Discrete differences in emission energy and quantum efficiency
based on the electronics and planarity of each system were
observed. A dramatic solvent dependency was also observed,
due to the highly polarized excited states. Through control of
the structural elements of the molecules as well as the solvent
systems used, a broad range of emissions across the visible
spectrum was achieved (Figure 9). Indeed, a mixture of tetra-
decyl 11, TPEB 3, and DBA 17 in solution gave a bright “white”
emission, demonstrating the ability to “tune” the systems for
desired optical properties.
Figure 7. UV-vis spectra of donor/acceptor TPEB 4 and DBAs 15-19.
the low-energy charge-transfer bands were effectively quenched.
As can be seen in Figure 8, protonation completely disrupted
any electron-donating capability of the donor groups. The
absorption bands of the protonated form of TPEB 2 closely
match that of the unsubstituted parent TPEB 1, with hypso-
chromic shifts of the low-energy bands of more than 60 nm.
Treatment of the protonated species of 2 with aqueous NaOH
regenerated its neutral form, demonstrating the reversibility of
the process.
Similar shifts due to protonation were also observed in the
electronic absorption spectra of the DBA systems. Protonation
of tetra-donor 13 eliminated the low-energy peaks and matched
the spectrum of the neutral tetra-decyl DBA 11. The disappear-
9
2470 J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005

Donor/Acceptor-Functionalized TPEB and bisDBA Systems
A R T I C L E S
Figure 10. Emission consequences of protonation of DBA 13.
Figure 9. Fluorescence emission of selected TPEBs and DBAs.
The emission λmax was very similar for the neutral systems
1, 11, and 12 in both CH2Cl2 and toluene, and little difference
in quantum efficiency was noticed between solvents (Table 4).
The increased conjugation and planarity of the annulene systems,
on the other hand, allowed much higher quantum yields (ΦF )
0.20-0.32) than the surprisingly low value for the nonplanar
TPEB 1 (ΦF ≈ 0.02) and was highest for the bis[15]annulene
11. However, this trend was completely different for the tetra-
donor-substituted species 2, 13, and 14, which showed dramatic
solvent and structural dependence. The highest quantum yield
measured in this study belonged to TPEB 2 in CH2Cl2 (ΦF )
0.71), while its efficiency was nearly 3.5 times lower in toluene.
Interestingly, the tetra-donor bis[14]annulene 14 (ΦF ) 0.57,
0.43) demonstrated a much higher quantum efficiency than bis-
[15]DBA 13 (ΦF ) 0.21, 0.38) with a reversal in solvent effect.
For the donor/acceptor-substituted systems, solvent effects
dictated the quantum efficiencies over structural differences.
Very little fluorescence occurred for these systems in polar
solvents because of stabilization of the charge-transfer excited
state, which is minimized in more nonpolar solvents, allowing
higher emission efficiencies through other pathways. Hence,
very low quantum yields (<0.01) were observed for the donor/
acceptor TPEBs apart from the ortho-substituted TPEB 3 in
toluene (ΦF ) 0.30). Similarly, in CH2Cl2 low efficiencies
(<0.01) for the donor/acceptor DBAs were measured, while in
the less polar solvent toluene, an increase in the quantum yield
to ΦF ) 0.06-0.17 was seen.
Protonation of the NBu2 groups also resulted in distinct
changes in the emission spectra of the molecules, although the
effects were remarkably different for each system. Treatment
of the tetra-donor TPEB 2 with concentrated aqueous HCl in
both CH2Cl2 and toluene effectively diminished and red-shifted
the emission (Table 4). However, more complex, solvent-
dependent energy levels were present in the annulene systems.
The emission peak of protonated tetra-donor DBA 13 in CH2Cl2
remained intense (ΦF ) 0.18) with a blue-shift to identically
match the emission spectra of the neutral DBA 11, while in
toluene a larger quenching of the fluorescence emission was
observed (Figure 10, Table 4). Donor/acceptor DBA 16 proved
to be an interesting system for study. In its neutral form, a
modest emission was observed in toluene (ΦF ) 0.17), while
very little fluorescence occurred in CH2Cl2 (ΦF > 0.01).
Conversely, the protonated form of 16 gave an intense emission
in CH2Cl2 (ΦF ) 0.31) with minute fluorescence in toluene
Figure 11. ORTEP representation of 3 at the 50% probability level.
(ΦF > 0.01). These results demonstrate the importance of
solvent parameters for fluorescence spectroscopy.
Computational Studies. Further insight into the electronic
nature of the TPEB and DBA systems can be attained through
computational studies. Geometry optimization provided us with
additional information regarding the conjugation effectiveness
in order to better understand the differing charge-transfer
pathways (Figure 1) and structural topologies present in each
system. Examination of the molecular orbital diagrams provided
further insight into the charge transfer and π-π* transitions
and the relationship between the HOMO-LUMO gaps and the
absorption energies present in the UV-vis spectra of each
system.
Geometry Optimization. Geometry optimization calculations
were performed with the B3LYP/6-31G* basis set.22 All NBu2
groups were replaced with NMe2 groups, and all decyl groups
were replaced with protons to reduce calculation time and should
have little effect on overall electronics and geometry. Slightly
shorter bond lengths, with good overall correlation, were seen
between the calculated bond lengths of 2 and 3 and those
experimentally obtained from their X-ray crystal structures
(Figure 11), validating the accuracy of the calculations.22
The calculated bond lengths and their alternation can provide
a great deal of information about the degree of charge transfer
present between the donor and acceptor groups in the ground
state. The bond length alternation can be expressed in terms of
9
J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005 2471

A R T I C L E S
Marsden et al.
the quinoid character (δr) of each aryl ring and is an indicator
of the extent of charge transfer present along the conjugated
backbones (eq 1; see Figure 11 for explanation of a, b, and
c).13,28 Fully aromatic with no quinoid character, the δr value
of benzene is 0, while values of 0.08-0.10 are typical for fully
quinoidal rings.
is opposing, thus creating an overall centrosymmetric packing
arrangement. The stacking distance between planes of 3.71 Å
is similar to the distance observed for a slipped parallel
arrangement of a benzene dimer.29
Molecular Orbital Plots. The molecular orbital plots dis-
played a marked difference between the electronic transitions
of the neutral and tetra-donor systems versus the donor/acceptor-
functionalized phenylacetylenes. Analysis of the calculated
absorption maxima showed the first excited state for the TPEBs
and DBAs to comprise mainly an electronic transition between
the HOMO and the LUMO.
((a + a′)/2 - (b + b′)/2) + ((c + c′)/2 - (b + b′)/2)
δr )
2
(1)
Interesting trends were observed for the δr values obtained
from the calculated bond lengths of the TPEBs and DBAs
(tabulated in Supporting Information). Values for the donor/
acceptor-functionalized TPEB systems (3 and 4) displayed the
highest quiniodal character (δr ) 0.028-0.029 for the donor
rings), followed by the bis[14]annulenes (18-20, δr ) 0.025-
0.026 for the donor rings), and finally the bis[15]annulenes (15-
17, δr ) 0.024 for the donor rings). While these numbers are
comparable to δr values obtained for tetraethynylethylenes
(TEEs) with NMe2 donor groups containing similar pathways,6
the trend for each series is in disagreement with the data obtained
from UV-vis measurements and would indicate a reversal of
the charge-transfer efficiency. This anomaly most likely arises
from an oversimplification of the defined quinoid pathways
existing in the DBA systems. Because of the large number of
conjugated pathways other than donor to acceptor and the
structural differences between each macrocycle, a competition
arises for conjugation between the charge-transfer pathways and
the fused 14- or 15-membered rings. Bond length distortion due
to ring strain may also have an appreciable effect on bond length
alternation. Nevertheless, good agreement between UV-vis data
and quinoid values was observed between the neutral (1, 11,
and 12), tetra-donor (2, 13, and 14), and donor/acceptor-
substituted TPEBs and DBAs (3-5 and 15-20) with an increase
in the δr value going from neutral to tetra-donor to donor/
acceptor system. A correlation between the degree of red-shift
from the electronic absorption spectra and the largest quinoid
value for each series was also seen, with the meta-substituted
isomers (4, 16, and 19) displaying the largest degree of charge
transfer.
Crystals of 2 and 3 suitable for X-ray structure analysis were
grown by slow diffusion of hexanes into CH2Cl2 solutions.
Although bond lengths were slightly shorter overall, quinoid
values of 2 and 3 measured from the X-ray crystal data matched
those obtained from calculations quite well (Supporting Infor-
mation). However, the average δr value for 3 was much smaller
than calculated, because of the considerable out-of-plane twisting
of one phenyl ring in the solid state (up to ca. 70°, Figure 11).
This twisting greatly reduced the conjugation effectiveness and
charge-transfer ability of the donor/acceptor conduit, resulting
in a negative quinoid value (δr ) -0.0055) for that pathway,
while the planar pathway displayed a quinoid value close to
that calculated in the gas-state assuming total planarity (δr )
0.0234). This indicated again the importance of planarity for
effective charge transfer in these systems.
Strong delocalization was present in the HOMOs of both the
neutral (1) and tetra-donor (2) TPEB systems (Figure 12).
However, the LUMOs show more localization of the isosurfaces
of the wave functions around the central benzene, with weaker
delocalization moving outward toward the peripheral phenyl
rings illustrating significant π*-antibonding character. In contrast
to the neutral and tetra-donor species, the donor/acceptor TPEBs
(3-5) exhibit strong charge-transfer transitions from the HOMO
to the LUMO. The orbitals in the HOMOs reside wholly at the
donor-substituted phenyls and weaken down the ethynyl bonds
to the central benzene ring. In the LUMOs it is transferred to
the opposing ends of the charge-transfer pathways and is
localized at p-nitrophenyl acceptors. Interestingly, the para-
substituted species 5, lacking linear-conjugated pathways, shows
the most complete localization of the isosurface between the
donor and acceptor units of the molecule, which may explain
the longer absorption cutoff in its UV-vis spectrum.
Characteristics similar to the TPEBs were observed for the
HOMO and LUMO plots of the DBA systems (Figure 12).
Tetra-decyl 11 showed complete delocalization of the orbitals
around the macrocycle and phenyl rings in the HOMO, which
becomes more localized to the π*-antibonding orbitals in the
LUMO. Tetra-donor DBA 13 displayed slightly more localiza-
tion on the central cruciform backbone in the HOMO and more
localization on the outer diacetylene bridges of the LUMO.
Analogous to the TPEB systems, the donor/acceptor DBAs (15-
17) show strong charge transfer from the donor to the acceptor
for the HOMO-LUMO transition. Relatively little isosurface
is present on the diacetylene bridges in any of the orbital plots
of the donor/acceptor DBAs, indicating that this linker has little
contribution to conjugation and exists solely for planarization
of the system. Molecular orbital plots of the analogous bis[14]-
annulenes (not shown) displayed similar characteristics to the
bis[15]annulenes, also highlighting the distinct differences
between the π-π* transitions of the neutral and tetra-donor
systems and the charge-transfer transitions of the donor/acceptor
DBAs.
There existed some discrepancy between the HOMO-LUMO
transitions from ab initio calculations and the experimentally
obtained optical gaps acquired from UV-vis spectroscopy in
CH2Cl2 that must be rationalized. The calculated absorption
maxima for all neutral and tetra-donor systems showed a
correlation of (30 nm. The calculated wavelengths for the
nonplanar species were longer than the experimental, while the
DBA systems were shorter (Table 5). The higher energy of the
calculated optical energy gap in the neutral and tetra-donor
DBAs could suggest that the lowest energy transition may not
Also of interest was the crystal packing of 3, which shows
alternating slipped stacks with unidirectional alignment of the
dipole in each layer.22 The dipolar alignment in alternating layers
(28) Dehu, C.; Meyers, F.; Bre´das, J. L. J. Am. Chem. Soc. 1993, 115, 6198-
(29) Tsuzuki, S.; Honda, K.; Uchimaru, T.; Mikami, M.; Tanabe, K. J. Am.
Chem. Soc. 2002, 124, 104-112.
6206.
9
2472 J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005

Donor/Acceptor-Functionalized TPEB and bisDBA Systems
A R T I C L E S
Figure 12. Molecular orbital plots of simplified structures of TPEBs 1-5′ and DBAs 11′, 13′, and 15′-17′. The upper plots represent the LUMOs, and the
lower plots represent the HOMOs.
be due to a HOMO-LUMO transition. The larger amount of
fine structure present in the UV-vis spectra of these annulenes
suggests more complex energy transitions (Figures 5 and 6).
The calculated energy levels of the HOMOs for the tetra-donor
systems were ca. 1.0 eV higher than the neutral species. Also
of interest were the lower energy HOMOs of the bis[14]-
annulenes over the bis[15]annulenes (0.1-0.2 eV), suggesting
that more strain or other energy may be present in the
15-membered rings and that the aromaticity of the 14-membered
rings may also help stabilize the systems.
experimentally obtained absorption maxima (Table 5). The
lowest energy absorptions for these systems, however, appear
very broad and do extend well beyond the calculated values
(Figures 4 and 7). The trends between isomers for the calculated
optical gaps are also dissimilar from the experimental data.
Calculations show the highest energy transitions belonging to
the meta isomers (4, 16, and 19), which demonstrate the lowest
energy UV-vis absorptions. The lowest energy absorptions
from calculations belong to the para isomers, which do possess
red-shifted shoulders extending beyond the other isomers (Figure
7). Although the variance between calculated and experimental
data was large in some cases, overall the B3LYP/6-31G*
The calculated HOMO-LUMO gaps for the donor/acceptor-
functionalized species were on average 100-nm longer than the
9
J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005 2473

A R T I C L E S
Marsden et al.
dimers or larger aggregates. Aggregation was also detectable
in the UV-vis spectra of concentrated samples of donor/
acceptor DBAs as a broadening and red-shift of the absorption
bands.22 This self-association phenomenon is well-known for
functionalized, ethynyl-bridged metacyclophanes30-35 and hexa-
(arylethynyl)benzene derivatives,36 but has not been observed
for dehydrobenzoannulene systems. Self-association such as
described by weak noncovalent interactions including π-stacking
and donor/acceptor interactions are an important aspect of
supramolecular chemistry.30-37 Therefore, we decided to further
investigate the aggregation properties of the donor/acceptor
DBAs to gain insight into their self-association characteristics
and to identify trends in aggregation propensity, if any, for each
unique isomer and ring topology.
Figure 13. Concentration-dependent chemical shifts of the aromatic protons
of DBA 15 in CD2Cl2 at 20 °C.
The self-association properties of the DBAs and TPEBs were
investigated quantitatively by analysis of the concentration-
dependent 1H NMR chemical shifts of the aromatic protons and
1
further verified by vapor phase osmometry (VPO). H NMR
spectroscopy is the most widely used technique for analyzing
aggregation behavior because of its ease of measurement, good
precision, and for the secondary benefit of providing structural
information pertaining to the aggregates.38 The 1H NMR method
cannot, however, distinguish between dimer and higher ag-
gregate formation, necessitating VPO measurements. Significant
concentration-dependent chemical shifts were observed for each
of the donor/acceptor-functionalized annulenes in a variety of
solvents. For example, upfield shifts of the aromatic protons of
more than δ 0.7 ppm were observed for DBA 15 in CD2Cl2 at
20 °C as the concentration was increased from 0.04 to 10 mM
(Figure 13). The strong propensity for aggregation even at very
low concentration can be seen, suggesting a large association
constant. The upfield character of the protons and little chemical
shift change at concentrations above ∼2.0 mM are indicative
of this propensity, while below ∼2.0 mM concentration the
chemical shifts move sharply downfield. No concentration-
dependent chemical shifts were observed for the neutral or tetra-
donor DBAs or the TPEBs with or without donor and acceptor
groups. This difference indicates that both planarity and donor
and acceptor substitution are necessary for aggregation in these
materials.
Figure 14. Differences in concentration-dependent chemical shifts of
selected aromatic protons of DBA 18 in CD2Cl2 at 20 °C.
Table 5. Calculated (B3LYP/6-31G*) and Experimental
HOMO-LUMO Band Gaps for TPEBs 1-5 and DBAs 11-20
calcda
exptl
(nm)
E(HOMO)
E(LUMO)
∆E(L−H)
λ
(nm)
λ
1
2
3
4
0.00
+1.03
+0.30
+0.19
+0.32
+0.11
-0.22
+1.01
+0.82
+0.25
+0.16
+0.27
+0.11
-0.03
+3.21
+3.97
+2.53
+2.61
+2.50
+3.13
+3.02
+3.78
+3.78
+2.45
+2.53
+2.40
+2.40
+2.50
+2.36
3.21
2.94
2.23
2.42
2.18
3.02
3.24
2.78
2.97
2.20
2.37
2.12
2.29
2.53
2.23
386
422
555
512
570
411
383
447
418
563
524
584
542
490
556
350
414
417
426
383b
417
419
472
463
456
491
473c
453
466
NA
To calculate the association constants (K2), we assumed that
the monomer-dimer equilibrium was the predominant process
and verified this assumption by VPO, as will be described later.
To determine K2, the chemical shifts at various concentrations
were curve-fitted to eq 2,39 where δ is the chemical shift at
substrate concentration Ct, and δm and δd are the chemical shifts
5
11
12
13
14
15
16
17
18
19
20
(30) 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.
+0.14
(31) Nakamura, K.; Okuba, H.; Yamaguchi, M. Org. Lett. 2001, 3, 1097-1099.
(32) Ho¨ger, S.; Bonrad, K.; Mourran, A.; Beginn, U.; Mo¨ller, M. J. Am. Chem.
Soc. 2001, 123, 5651-5659.
a Values in electronvolts relative to 1. b Shoulder at 480 nm. c Shoulder
at 550 nm.
(33) Lahiri, S.; Thompson, J. L.; Moore, J. S. J. Am. Chem. Soc. 2000, 122,
11315-11319.
calculations provided insight into trends in the strength and type
of transitions.
(34) Shetty, A. S.; Zhang, J.; Moore, J. S. J. Am. Chem. Soc. 1996, 118, 1019-
1027.
(35) Zhang, J.; Moore, J. S. J. Am. Chem. Soc. 1992, 114, 9701-9702.
(36) Kobayashi, K.; Kobayashi, N. J. Org. Chem. 2004, 69, 2487-2497.
(37) (a) Roesky, H. W.; Andruh, M. Coord. Chem. ReV. 2003, 236, 91-119.
(b) Claessens, C. G.; Stoddart, J. F. J. Phys. Org. Chem. 1997, 10, 254-
272. (c) Philp, D.; Gramlich, V.; Seiler, P.; Diederich, F. J. Chem. Soc.,
Perkin Trans. 2 1995, 875-886. (d) Hunter, C. A. Chem. Soc. ReV. 1994,
101-109. (e) Desiraju, G. R. Crystal Engineering: The Design of Organic
Solids; Elsevier: New York, 1989. (f) Foster, R. Organic Charge-Transfer
Complexes; Academic Press: London, 1969.
Self-Association of Donor/Acceptor Macrocycles in Solu-
tion. During routine examination and characterization of donor/
acceptor-functionalized macrocycles 15-19 by 1H NMR spec-
troscopy, very large concentration-dependent chemical shifts
were observed for the aromatic protons (over ca. δ 1 ppm for
the range of 0.1-10 mM). This observation suggested the
occurrence of self-association in solution by π-stacking to form
(38) Martin, R. B. Chem. ReV. 1996, 96, 3043-3064.
(39) Horman, I.; Dreux, B. HelV. Chim. Acta 1984, 67, 754-764.
9
2474 J. AM. CHEM. SOC. VOL. 127, NO. 8, 2005

Donor/Acceptor-Functionalized TPEB and bisDBA Systems
A R T I C L E S
Table 6. Association Constants (K2) and Solubilities of Donor/Acceptor-Functionalized DBAs at 20 °C
CD2Cl2
THF-d8
CDCl3
PhMe-d8
1
1
1
1
K
2 (M-
)
solubility
K
2 (M-
)
solubility
K
2 (M-
)
solubility
K
2 (M-
)
solubility
15
16
17a
18
19
3020 ( 360
1940 ( 290
b
129 ( 10
326 ( 51
10 mM
20 mM
0.8 mM
25 mM
25 mM
1090 ( 310
294 ( 53
b
29.3 ( 8.4
52.0 ( 7.8
3 mM
6 mM
1.2 mM
15 mM
15 mM
1540 ( 660
12 mM
1.5 mM
10 mM
15 mM
15 mM
605 ( 240
b
748 ( 348
186 ( 66
300 ( 48
2 mM
5000 ( 3000
2600 ( 860
121 ( 6
>0.1 mM
20 mM
5 mM
172 ( 35
10 mM
a Calculated using the infinite association model (see below). b Solubility too low for calculation of K2.
Figure 15. Associations constants (K2) of aromatic protons of DBA 16 in
CDCl3 at 20 °C.
of the monomer and dimer, respectively.
1 - 8K C + 1
x
2
t
δ ) δm + (δd - δm) 1 +
(2)
(
)
4K2Ct
1
Figure 16. Polymerization of DBA 17 in CDCl3 observed by H NMR.
The association constants were calculated for each of the
aromatic protons present in DBAs 15-19 in CDCl3, CD2Cl2,
THF-d8, and PhMe-d8. A large variance of the concentration-
dependent chemical shift was seen for several DBAs depending
on the local environment of each proton as can be seen in the
dilution curve of DBA 18 (Figure 14). Typically exo-annular
protons exhibited the largest association constants, while in-
traannular protons, or protons residing in the “pocket” between
rings showed a much smaller K2, as illustrated for DBA 16
(Figure 15). The association constants of all aromatic protons
were averaged for each annulene, giving values summarized in
Table 6 along with the solubility data for each system. In
general, the bis[15]annulenes (15-17) showed an association
constant about 1 order of magnitude larger than that of the bis-
[14]annulenes (18 and 19) and were also much more soluble in
the solvents used for the measurements. Solubility had a
significant effect on the K2 values and the error associated with
the data because of the smaller range of solute concentrations
achievable for measurement (Table 6). Solvent polarities have
previously been observed to have a crucial effect on self-
association.30,32,33 Not surprisingly, the DBAs with larger net
dipoles were generally much more soluble in polar solvents.
For example, macrocyle 16 with the largest calculated dipole
of 18.34 D demonstrated increasing solubility as solvent polarity
increased, while DBA 17 with no net dipole had much better
solubility in less polar solvents. We also found that more polar
solvents, in general, saw enhancement of K2. Interestingly,
association constants were also generally higher in halogenated
solvents (CDCl3 and CD2Cl2) than nonhalogenated (THF-d8 and
PhMe-d8). The bis[14]annulenes 18 and 19, moreover, saw an
increase of K2 in PhMe-d8. Similar observations have also been
made for m-phenyleneethynylene macrocycles in aromatic
solvents.30,31,33
measurements were performed in CHCl3 at 40 °C using benzil
and TPEB 2 as standards. The stoichiometric molal concentra-
tions were calculated from the solute weights, and the colligative
molal concentrations were obtained by comparing their VPO
readings against those of benzil and TPEB 2.34,40 The colligative
molal concentrations of 15, 16, 18, and 19 with solute
concentrations between 1.0 and 5.0 mM were consistent with
aggregate sizes of dimers, while DBA 17 strongly indicated
aggregates much larger than dimer. Thus, the association
constants for 17 were calculated using the infinite association
model assuming K2 ) K3 ) K4 ) ... ) Kn.30,38
During the investigations into the self-association properties
of the donor/acceptor-functionalized macrocycles, an interesting
phenomenon was observed for the para-substituted DBA 17.
Although stable in the solid state, in CDCl3 or CHCl3 solutions
greater than ∼1.0 mM at room temperature a facile polymer-
1
ization occurred that could be monitored by H NMR (Figure
16). The aromatic chemical shifts progressed downfield with
time, indicating a decrease in concentration until after about
20 min, no annulene was left in solution, and an insoluble red-
brown polymer precipitated out. We were unable to find an
organic solvent in which the resulting precipitate was soluble,
making analysis difficult. SIMS-TOF mass spectrometry of the
material showed broad repeating humps, indicating polymeric
material. Powder X-ray diffraction of this polymer suggested
an ordered polymerization, showing very sharp peaks with the
largest representing a repeat distance of ∼21.5 Å, roughly the
distance across the macrocycle. Therefore, what is most likely
occurring is a topochemical 1,3-diacteylene polymerization26,41
(40) Schrier, E. E. J. Chem. Educ. 1968, 45, 176-180.
(41) (a) Shirakawa, H.; Masuda, T.; Takeda, K. In The Chemistry of the Triple-
Bonded Functional Groups; Patai, S., Rappoport, Z., Eds.; Wiley: New
York, 1994; pp 704-728. (b) Enkelmann, V. AdV. Polym. Sci. 1984, 63,
91-136. (c) Wegner, G. Pure Appl. Chem. 1977, 49, 443-454. (d) Wegner,
G. Z. Naturforsch 1969, 24b, 824-832.
To verify the usage of the dimer model for self-association,
average aggregate size was determined by VPO. Osmotic
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A R T I C L E S
Marsden et al.
driven by preorganization from the self-association. This is a
well-known phenomenon involving cyclic and other 1,3-
diacetylenes, although it is typically observed in the solid state.
Strangely, this polymerization occurs only with the para isomer
(17) and, of the solvents tested, only in CDCl3 or CHCl3. Acid-
catalyzed polymerization was ruled out because no difference
was observed between CHCl3 pretreated with K2CO3 and
untreated CHCl3 containing trace amounts of acid. Although
we have not been able to explain the selectivity for DBA 17,
polymerization may explain the difficulties associated with
synthesis of the elusive DBA 20 containing the same para
substitution of donor and acceptor groups. Because of the added
strain energy on the diacetylenes of the bis[14]annulenes,
polymerization of 20 may be even more facile than 17 and occur
before isolation of the DBA is possible.
Through computational studies, further insight into the
electronic transitions was gained. A distinct difference was found
between the π-π* transitions of the neutral and tetra-donor
systems and the charge-transfer transitions of the donor/acceptor
compounds as seen by the molecular orbital plots. The degree
of quinoid character of the system was calculated and observed
to correlate well with experimental measurements of charge-
transfer effectiveness.
Self-association properties for the donor/acceptor DBAs to
form dimers based on face-to-face π-π stacking in solution
were also observed, and the association constants were mea-
sured. Both donor and acceptor substitution as well as planarity
were necessary for self-association. The geometry of the ring
was found to be a large factor in aggregation propensity, as the
15-membered rings showed around 1 order of magnitude larger
association constants. An interesting ordered polymerization
triggered by self-association was observed for only one isomer
(17) in only CDCl3, suggesting that the reaction is very sensitive
to geometrical and solvent effects. We are continuing our studies
on donor/acceptor pathways through experiments involving
macrocycles of different sizes and saturations and degree of
donor and acceptor substitution, and we will report these results
in due course.
Conclusions
We have synthesized a series of tetrakis(phenylethynyl)-
benzenes, bis(dehydrobenzo[14]annuleno)benzenes, and bis-
(dehydrobenzo[15]annuleno)benzenes functionalized with all
neutral, all donor, or mixed donor/acceptor groups to compare
the electronic and optical properties between each species to
develop a structure-function relationship. During this synthesis,
an interesting selectivity between ring sizes was found based
on reagents used in the cyclization. We noticed an enhancement
of the optical activity transitioning from neutral to tetra-donor
to donor/acceptor systems as seen by large bathochromic shifts
of the UV-vis λmax and the development of strong charge-
transfer bands. Further enhancement was achieved by locking
the nonplanar TPEBs into planarity as DBAs, thereby increasing
overall conjugation. As observed for previous donor/acceptor
systems, we found that larger net dipoles and linear-conjugated
pathways provided the largest bathochromic shifts. We were
able to tune the optical fluorescence emission of the series over
a broad range of the visible spectra, and we established that the
tetra-donor compounds provided the best efficiency for fluo-
rescence. In both absorbance and emission, protonation of the
donor groups quenched or altered the optical enhancements.
Acknowledgment. This work was supported by the National
Science Foundation (CHE-0414175). J.A.M. and L.D.S. ac-
knowledge the NSF for IGERT fellowships. We thank Elliott
Hinds for help acquiring some characterization data and Dr.
Grant Palmer for preliminary synthetic studies. We thank Drs.
Fusen Han and Elizabeth Rather for the X-ray crystallography
of 2 and 3.
Supporting Information Available: Experimental section.
Detailed experimental procedures and spectral data for all new
compounds and their intermediates, computational details,
1
quinoid calculations, and self-association H NMR and UV-
vis data (CIF, PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
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