The effects of cyclic conjugation and bending on the optoelectronic properties of paraphenylenes

Article abstract of DOI:10.1021/ol403168x

Cycloparaphenylenes (CPPs) have optoelectronic properties that are unique when compared to their acyclic oligoparaphenylene counterparts. The synthesis and characterization of two bent heptaphenyl-containing macrocycles has been achieved in order to probe

Full text of DOI:10.1021/ol403168x

Letter
pubs.acs.org/OrgLett
The Effects of Cyclic Conjugation and Bending on the Optoelectronic
Properties of Paraphenylenes
Penghao Li, Thomas J. Sisto, Evan R. Darzi, and Ramesh Jasti*
Department of Chemistry and Division of Materials Science and Engineering, Boston University, Boston, Massachusetts 02215,
United States
S
* Supporting Information
ABSTRACT: Cycloparaphenylenes (CPPs) have optoelectronic
properties that are unique when compared to their acyclic
oligoparaphenylene counterparts. The synthesis and characterization
of two bent heptaphenyl-containing macrocycles has been achieved in
order to probe the effects of bending and cyclic conjugation on the
properties of the CPPs. The study suggests that both bending and
cyclic conjugation play a role in the novel properties of the CPPs.
he [n]cycloparaphenylenes (CPPs) are structurally unique
macrocycles consisting of n para-linked benzene units
(Figure 1). Since first synthesized and characterized in 2008,
most obvious structural differences are the bent phenylene
units, the cyclic conjugation, and the smaller torsional angles of
the CPPs.8 A fundamental understanding of the effects that
structure has on optoelectronic properties is paramount when
designing better-performing conjugated organic materials.8,9 To
probe the effects these structural characteristics have on the
properties of the CPPs, we have synthesized alkyl-tethered
heptaphenyl-containing macrocycles 1 and 2 (Figure 1).
Drawing inspiration from the related studies by Bodwell and
co-workers,10 we recognized that we can systematically vary the
degree of bending of the heptaphenyl moiety through the
insertion of various length alkyl chains into the backbone of a
macrocycle. This alkyl tether also serves to break the
conjugation of the macrocycle. Comparison of [7]CPP to
macrocycle 1 by cyclic voltammetry, absorption, and
fluorescence experiments allows for an approximation of the
effect of cyclic conjugation due to their similar degrees of
bending and torsional angles (vide infra). Comparison of 1, 2,
and linear heptaphenyl 3 allows for insight into the effects of
bending. Herein, we report the syntheses and characterization
of 1 and 2 in order to gain insight into these phenomena.
Our strategy for the synthesis of 1 and 2 relies upon the 3,6-
syn-dimethoxycyclohexa-1,4-diene moieties of fragment 6 as
masked arene units to provide the curvature necessary for a
macrocyclization reaction with tethers 9 and 10 (Schemes 1
and 2). Subsequent aromatization of the resultant macrocycles
would lead to 1 and 2. To synthesize 6, we began with a lithium
halogen exchange of 41b followed by addition of monoketal
protected benzoquinone (Scheme 1). Subsequent deprotection
of the crude reaction mixture at room temperature with 10%
aqueous acetic acid yields hydroxy ketone 5 in 90% yield over
two steps. Deprotonation of 5 with NaH at −78 °C leads to a
charged sodium alkoxide that directs the addition of lithiated
1,4-bromochlorobenzene in a diastereoselective manner.1b This
T
Figure 1. DFT optimized geometries of [7]CPP, 1, and 2, along with
previously reported linear heptaphenyl 3.6b
numerous routes to a variety of [n]CPPs have been reported by
the Jasti,1 Itami,2 and Yamago3 laboratories. Although often
cited for their potential application in the bottom-up growth of
uniform carbon nanotubes,4 the CPPs have unique optoelec-
tronic properties in their own right. As new conjugated organic
materials, the CPPs are attractive due to their size-dependent
optoelectronic properties,1a,b,3b as well as their guest−host
properties1d,5 and highly porous solid state structures.1e,2b,c,3c
The CPPs have increasing HOMO energies and decreasing
LUMO energies (narrowing band gap) with decreasing
molecular size.3b This behavior is exactly opposite to that
observed in the case of acyclic paraphenylenes.6 In addition, the
solubility of the CPPs is vastly improved as compared to the
linear oligoparaphenylenes, presumably due to differences in
the number of intermolecular π−π contacts accessible when
comparing nonplanar compounds to flat linear compounds
(vide infra). All known CPPs (sizes [6]−[18]CPP) are soluble,
while unsubstituted paraphenylenes larger than sexiphenyl are
completely insoluble.7
When rationalizing the origin of the unique optoelectronic
properties of the CPPs versus acyclic oligoparaphenylenes, the
Received: November 2, 2013
Published: December 6, 2013
© 2013 American Chemical Society
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Letter
bisboronate 9 or 10 formed macrocycles 11 and 12 in 14% and
21% yield, respectively. The yields are predictably low due to
the high strain of the macrocycles and the indiscriminate nature
of the coupling reaction, which leads to high amounts of
oligomerization. Subjecting macrocycles 11 and 12 to single
electron reductant sodium napthalenide at −78 °C gives bent
heptaphenyls 1 and 2 in 39% and 47% yield, respectively.
Characterization by NMR (1H and 13C), IR, and MALDI-TOF
confirmed the structural assignment.14
Scheme 1. Synthetic Route to Coupling Partners 6 and 8
With the syntheses complete, we next probed the electronic
differences between each molecule utilizing cyclic voltammetry
(Table 1).14 Similar to all CPPs, [7]CPP shows a reversible
a
Table 1. Cyclic Voltammetry Data for [7]CPP, 1, 2, and 3
entry
Ered (V) (half-wave)
Eox (V) (onset)
[7]CPP
−2.57
−2.74
−2.75
N/A
0.47
0.63
0.71
1.0
1
2
36b
a
V versus ferrocene/ferrocenium couple.
peak in the negative potential range corresponding to a
reduction wave with a half-wave potential of −2.57 V (vs Fc/
Fc+), along with a reversible peak in the possitive potential
range, corresponding to an oxidation wave with a half-wave
potential of 0.53 V (vs Fc/Fc+). Compound 1 exhibits a quasi-
reversible peak corresponding to a reduction wave; however,
the half-wave potential is significantly higher at −2.74 V (vs Fc/
Fc+). Additionally, the oxidation of 1 is irreversible with an
onset potential at 0.63 V (vs Fc/Fc+). Since macrocycle 1
contains phenylene units that are bent to a similar extent as
[7]CPP and the torsional angles between phenyl units are also
similar (vide infra), one can conclude that cyclic conjugation
plays a substantial role in raising the HOMO and lowering the
LUMO in the CPPs. Similar to macrocycle 1, structure 2 has a
quasi-reversible peak corresponding to a reduction wave with a
half-wave potential at −2.75 V (vs Fc/Fc+) and an irreversible
peak corresponding to an oxidation wave with an onset
potential of 0.71 V (vs Fc/Fc+). Macrocycles 1 and 2 are very
similar with only a slight increase in potentials as compared to
the difference between [7]CPP and 1. The oxidation potential
of 3, as reported by Rathore,6a shows a first half-wave of 1.4 V
versus SCE, which correlates to a half-wave potential of 1 V
versus Fc/Fc+. This large difference in oxidation potentials
between 2 and 3 provides experimental evidence for bending in
oligophenylenes also leading to a narrowing of band gaps (i.e.,
raising of the HOMO and lowering of the LUMO).
Scheme 2. Macrocyclization and Aromatization Reactions to
Prepare Bent Heptaphenyls 1 and 2
a
Compounds 1 and 2 are represented by DFT optimized geometries. b
Next we investigated the UV−vis and fluorescence spectra of
the new bent heptaphenyl-containing macrocycles for compar-
ison with [7]CPP. Macrocycle 1 displayed an absorption
maximum of 321 nm along with a small shoulder peak at 390
nm (Figure 2 and Table S1). The extinction coeffecients for
these features are 5.15 × 104 and 0.814 × 104 M−1 cm−1,
respectively. Macrocycle 2 has a similar absorption pattern,
displaying one large absorption maximum and one smaller
shoulder peak (λabs1 = 319 nm, ε1 = 4.45 × 104 M−1 cm−1; λabs2
= 375 nm, ε2 = 1.43 × 104 M−1 cm−1). For comparison,
[7]CPP displays an absorption maximum at 340 nm (ε = 6.58
× 104 M−1 cm−1) along with a very weak shoulder peak at 408
nm (ε = 0.316 × 104 M−1 cm−1). Linear heptaphenyl 3 displays
a single absorption at 326 nm with an extinction coefficient of
9.16 × 104 M−1 cm−1.6b Notably, the HOMO−LUMO
Conditions: Pd(OAc)2 (16 mol %), S-Phos (40 mol %), K3PO4 (2
equiv.), DMF/H2O, 100 °C.
addition is followed by a simple iodomethane quench to lead
directly to dichloride 6 in 90% yield. Notably, this synthesis can
readily produce tens of grams of 6 from commercially available
quionone monoketal using standard 2-L glassware. Synthesis of
alkyl tethered coupling partners 9 and 10 began with
dibromides 711 and 812 and is achieved in 90% and 99%
yield, respectively, through simple lithium halogen exchange
and subsequent quench with isopropoxyboronic acid pinacol
ester (Scheme 1).
With the necessary fragments in hand, we turned our
attention to preparing the macrocyclic precursors to structures
1 and 2. A Suzuki coupling13 of dichloride 6 and either
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Letter
anomalies provide small fluctuations within the larger overall
trend. In addition, we observe a trend of slightly increasing
average torsional angles when moving from [7]CPP to
macrocycle 1 to macrocycle 2 to acyclic analogue heptaphenyl
(25.5°, 29.9°, 30.3°, and 36.3°, respectively). When the data
was evaluated, the most important finding was the significant
difference between the band gap of [7]CPP and that of any
tethered macrocycle, presumably due to cyclic conjugation.
Furthermore, another large difference exists between the
HOMO energy levels of the largest tethered macrocycle and
linear heptaphenyl.
In the past five years, the cycloparaphenylenes have become
synthetically accessible in various sizes and on gram scale.
Throughout the development of these syntheses, character-
ization of the optoelectronic properties of the cycloparapheny-
lenes has revealed striking differences when compared to acyclic
paraphenylenes. We have demonstrated experimentally that
there are two structural features, bending and cyclic
conjugation, that play a prominent role in the optoelectronic
differences between the CPPs and OPPs. By breaking the cyclic
conjugation of [7]CPP with one methylene, we have shown
that this similarly bent heptaphenyl 1 is significantly harder to
oxidize and reduce. Decreasing the bending of the heptaphenyl
unit by increasing the macrocycle tether length results in a
minor increase in the oxidation and reduction potentials.
Similarly, we have shown that bent heptaphenyl has
substantially different optical properties compared to acyclic
heptaphenyl, while being much easier to oxidize and reduce.
This series of compounds has allowed for the experimental
demonstration that the bending and cyclic conjugation of the
CPPs provide optoelectronic characteristics and solubility
rendering them novel building blocks to new conjugated
organic materials.
Figure 2. UV−vis absorption and fluorescence data for [7]CPP, 1, and
2.
transition that is forbidden in the CPPs (the minor red-shifted
absorption) becomes more prominent as symmetry is broken
by the alkyl tether.8,15 A large difference also exists between the
wavelengths of the HOMO−LUMO transitions of the bent
molecules when compared to the linear paraphenylene,
consistent with cyclic voltammetry data. TD-DFT calculations
confirm this trend by showing that the λHOMO−LUMO blue shifts
while the oscillator strength increases moving from [7]CPP
through the methylene spaced heptaphenyl macrocycles (1−20
methylenes).16 Fluorescence data were also collected for each
of the final compounds (Figure 2 and Table S1). [7]CPP
displays a weak fluorescence emission at 588 nm (ϕf = 0.006),
while 1 and 2 emit strongly at blue-shifted values of 502 nm (ϕf
= 0.23) and 469 nm (ϕf = 0.25). Acyclic analogue 3 is reported
to emit at 408 nm, interestingly with a much higher quantum
yield of 1.0.6b
In order to further understand the molecular geometry and
electronic structures of this class of molecules, we performed
theoretical calculations for bent heptaphenyls with methylene
tether lengths from 1 to 20 carbons (Figure 3).17 [7]CPP and
linear p-heptaphenyl were also calculated for comparison.
Predictably, as the tether is increased in size the heptaphenyl
becomes less bent. As this occurs, the HOMO energies
decrease while the LUMO energies increase, though structural
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental procedures and characterizations for all
the compounds, cyclic voltammetry spectra, optical data, and
calculation details. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: jasti@bu.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the Alfred P. Sloan Foundation for financial support.
■
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■
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