Synthesis and optical properties of phenylene-containing oligoacenes

Article abstract of DOI:10.1021/ja3043883

Synthesis of a new class of fully unsaturated ladder structures, phenylene-containing oligoacenes (POAs), using 3,4-bis(methylene)cyclobutene as a building block for sequential Diels-Alder reactions is described. The geometric effects of strain and energetic cost of antiaromaticity can be observed via the optical and electrochemical properties of the reported compounds. The resulting shape-persistant ladder structures contain neighboring chromophores that are partially electronically isolated from one another while still undergoing a reduction in the band gap of the material.

Full text of DOI:10.1021/ja3043883

Article
pubs.acs.org/JACS
Synthesis and Optical Properties of Phenylene-Containing
Oligoacenes
Rebecca R. Parkhurst and Timothy M. Swager*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139,
United States
S
* Supporting Information
ABSTRACT: Synthesis of a new class of fully unsaturated
ladder structures, phenylene-containing oligoacenes (POAs),
using 3,4-bis(methylene)cyclobutene as a building block for
sequential Diels−Alder reactions is described. The geometric
effects of strain and energetic cost of antiaromaticity can be
observed via the optical and electrochemical properties of the
reported compounds. The resulting shape-persistant ladder
structures contain neighboring chromophores that are partially
electronically isolated from one another while still undergoing
a reduction in the band gap of the material.
as a method of creating two dissociated charge carriers from a
single photon.3
INTRODUCTION
■
The promise of organic electronics to enable creation of
inexpensive, flexible, and multifunctional devices1 is presently
limited by access to stable high-performance materials. One
strategy to develop such materials is to create extended rigid
aromatic systems that promote delocalization and minimize
energetic disorder in the electronic states. Among the polycyclic
aromatic hydrocarbons (PAHs), acenes, defined as linearly
annelated arenes with the fewest number of rings containing
Clar sextets,2 are considered one of the most important
candidates for high-performance organic semiconductors
(Figure 1a).
Close relatives to the acenes are the [N]phenylenes, a family
of PAHs wherein benzene rings are fused together by four-
membered rings, resulting in a ladder structure with alternating
aromatic and antiaromatic character (Figure 1b).4 Molecules of
this family have classically been synthesized via the [2 + 2 + 2]
cycloaddition methodology developed largely by Vollhardt et al.
It has been demonstrated both experimentally and theoretically
that the π bonds in these systems tend to localize within the six-
membered rings so as to minimize the 4π antiaromatic
character of the cyclobutadiene linkage.5 Recently, a
dibenzannelated biphenylene framework served as key
intermediate in the synthesis of the first example of
[4]circulene.6
Analogously to the acenes, linear annulation in the
[N]phenylenes correlates to a more rapidly decreasing band
gap than does angular.7 This behavior is consistent with the
theoretical prediction that, as with polyacene, [N]phenylenes of
infinite length could serve as molecular wires.8 We report
herein that antiaromatic phenylene linkages can be used in
conjunction with oligoacene units to create high-stability
extended, shape-persistent phenylene-containing oligoacenes
(POAs) with interesting optical properties.
Figure 1. General structure of (a) acenes, (b) [N]phenylenes, and (c)
phenylene-containing oligoacenes (POAs).
A natural strategy to create higher charge mobilities in an
organic material is to increase the spatial electronic
delocalization in the constituent molecular building blocks.
However, as the length of the acene increases, the reduction in
aromaticity also causes a rapid decrease in the HOMO−LUMO
gap and the chemical stability of these materials becomes
problematic. The higher acene homologues do appear to have
improved electronic properties, such as increasing charge
mobility, decreasing reorganization energy, and a predicted
decrease in exciton binding energy. This final property is
appealing for solar cell applications, where recently the unique
ability of acenes to undergo singlet fission has been of interest
RESULTS AND DISCUSSION
■
Diels−Alder Reactivity of 3,4-Bis(methylene)-
cylobutene. The synthesis reported herein relies on the
unique Diels−Alder reactivity of 3,4-bis(methylene)-
cyclobutene (1), generated by flash vacuum pyrolysis of 1,5-
hexadiyne according to the published procedure (Scheme 1).9
Received: May 6, 2012
© XXXX American Chemical Society
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Scheme 1. Synthesis of 3,4-Bis(methylene)cyclobutene (1)
a
by Flash Vacuum Pyrolysis (FVP)
a
Compound 1 is not reactive as a Diels−Alder diene.
Although the exocyclic methylene groups of 1 are locked in the
desired s-cis conformation, it is not reactive as a Diels−Alder
diene as the resulting product would contain cyclobutadiene.
However, we have shown that initial reaction of the endocyclic
double bond with a diene such as 1,3-diphenylisobenzofuran
(Scheme 2) is facile and selective. This reaction, in turn,
generates a single isomer of 2a, which is reactive as a Diels−
Alder diene. A second Diels−Alder reaction with 1,4-
naphthoquinone (NQ) yields a single isomer of polycyclic 3.
The stereoselectivity of each Diels−Alder reaction has been
confirmed by both 2D NMR of 2 (Figure 2) and X-ray
crystallography of 3 (Figure 3a). Although Diels−Alder
reactions of isobenzofurans tend to result in a mixture of
isomers,10 the initial reaction in this case is selective for the exo
product, consistent with previous reports by Kaupp et al.11
Long-range couplings (J3)12 between Ha and carbons C1 and
C2 are expected in the endo product with corresponding
torsion angles of 31° and 167°, respectively, in the energy-
minimized structure. These cross-peaks are not present in the
heteronuclear multiple bond correlation (HMBC-NMR)
spectrum, indicating formation of exo product, in which the
torsion angles are 77° and 57°, respectively.
Figure 2. Determination of the stereochemistry of 2a. (a)
Heteronuclear single quantum coherence-NMR (HSQC-NMR), (b)
HMBC-NMR, (c) exo product, and (d) endo product.
Examining the calculated LUMO of 1 (B3LYP/6-31g**,
Figure 2b), secondary orbital interactions (SOI) of the orbitals
on C3/4 with the diene HOMO would favor the endo
transition state; however, the coefficients at this position are
significantly smaller than those at C5/C6.13 Additionally, the
geometry of the highly strained ring brings the C5/C6
molecular orbitals closer to the reaction site, causing instead
an antibonding interaction in the endo transition state. In the
second Diels−Alder reaction, the product results from
approach of the dienophile to the less hindered side of diene 2.
Synthesis of POAs. With 3 in hand, its fully unsaturated
counterpart, compound 6, is readily available through a series of
synthetic transformations (Scheme 2). Addition of a strong
Figure 3. (a) X-ray crystal structure of 3. (b) LUMO of 1 (B3LYP/6-
31g**). (c) Endo and (d) exo approaches of 1 to a furan diene.
base such as lithium hexamethyldisilazide (LiHMDS) generates
the high-energy bis-enolate, which then oxidizes to the 1,4-
quinone, presumably by action of air introduced during the
workup. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is
then used to aromatize the neighboring ring, giving compound
4 in an 88% yield over two steps (for X-ray crystal structure of
4 see Figure S5 and S6, Supporting Information). Nucleophilic
Scheme 2. Sequential Diels−Alder Reactions to Yield 3 and Synthesis of POA 6
B
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addition of TIPS-protected lithium acetylide followed by
reductive deoxygenation yields the TIPS-anthracene-containing
5. Finally, 5 is dehydrated by pyridinium p-toluenesulfonate
(PPTS) in acetic anhydride (Ac2O) to give 6. Analysis by
differential scanning calorimetry (DSC) indicates the high
thermal stability of POA 6 (Figure S1, Supporting Informa-
tion).
On the basis of the synthetic methodology demonstrated
above, molecules of this family can be extended into even larger
structures (Scheme 3). 1,3-Diphenylisonaphthofuran14 can be
absorption due to the diphenylnaphthalene chromophore and a
lower energy absorption (λmax = 464 nm) due to the diethynyl-
anthracene chromophore. The 21 nm bathochromic shift
between TIPS-anthracene and compound 6 demonstrates that
although the phenylene linkage limits delocalization between
the two chromophores there is still sufficient communication to
cause a reduction in the band gap of about 0.17 eV. A similar
pattern is evident in the solution absorbance curves of 9a/b,
with a reduction in the band gap with each increase in the
length of the POA.
There is a corresponding bathochromic shift in the solution
emission curve of each POA. Emission quantum yields decrease
Scheme 3. Bisaryne Diels−Alder Reaction and
Aromatization To Yield POAs 9a/b
within the series from TIPS-Anth (ϕem = 0.94) to 9b (ϕem
=
0.26). In each case the Stokes shifts are small, ranging from 2 to
5 nm, consistent with the rigid, shape-persistent nature of these
materials.
In each case, the solid-state absorbance curves are
characterized by only very small bathochromic shifts (ranging
from 1 to 11 nm) as compared to the solution state
measurements, and there is virtually no change in the vibronic
structure. The small changes may indicate that electronic
coupling between neighboring molecules in the solid state is
not as efficient as in unsubstituted acenes.17 This feature is
likely the result of the steric isolation from the pendant phenyl
and TIPS groups that also create large intermolecular spacings
in the crystal packing of 6 and 9a (vide infra). The apparent
weak electronic coupling between POAs and the small red
shifts in the optical band gap with increasing length will likely
limit the utility of these specific materials in organic
photovoltaic materials.
The change in the emission spectrum of TIPS-Anth in going
from solution to solid state is likewise very small. However, the
case is different for POAs 6 and 9a/b, where new large
bathochromic emission features are observed. In films of POAs
6 and 9a there appear to be two emission processes, with one
resembling the unimolecular solution S1−S0 transitions at 477
and 514 nm, respectively. Extension of the outer acenes appears
to create intermolecular actions that produce a new longer
wavelength emission, and in the case of POA 9b only the low-
energy emission band (630 nm) is evident. These new features
are relatively sharp and have hints of vibrational fine structure
on the bathochromic side of the major peaks. As a result of this
structure they do not appear to be excimer emissions. The
minimal spectral changes in the absorption spectra upon going
from solution to solid state are not suggestive of bulk formation
of J-aggregates; however, it is possible that a small amount of a
J-aggregated material is present in the film and that excitonic
migration to this state produces this large Stokes-shifted
material. A small band bathochromic of the absorbance
spectrum of 9b (∼580 nm) supports this claim.
The ionization potential and stability of the oxidized forms of
the POAs were analyzed by cyclic voltammetry (CV) (Table 1
and Figure S3, Supporting Information). The difference in the
potential of the first oxidation peak (Eox vs Fc/Fc+) of TIPS-
Anth, 6, and 9a is quite small, decreasing from 710 mV for
TIPS-Anth to 678 mV for 9a. Therefore, the reduction in the
band gap for these three species can primarily be attributed to a
decreasing LUMO level, confirming oxidative stability.
Although TIPS-Anth and 6 both undergo only one reversible
oxidation, compound 9a displays a second oxidation peak
within the window studied. Compound 9b, alternatively,
exhibits three reversible oxidation peaks, with the first two
shifted to significantly lower potentials than those previously
used to generate diene 2b. Reacting a bis-functionalized
dienophile with 2 results in symmetrical structures. As a
bisaryne precursor, we chose to use 1,2,4,5-tetrabromo-1,6-
bis(triisopropylsilyl)ethnynylbenzene (7), synthesized via the
procedure previously reported by our laboratory.15 Products
from the bisaryne Diels−Alder addition of 2 with 7 were taken
on to the aromatization step without further purification to
yield isomeric mixtures of precursors 8a/b. Dehydration was
then carried out with p-toluene sulfonic acid (pTsOH) in acetic
anhydride to give the desired POAs 9a/b. POA 9a also exhibits
high thermal stability, and no phase changes are evident up to
400 °C via DSC. POA 9b appears to be moderately stable in
the solid state in the absence of light.
Optical and Electrochemical Properties. Analysis of the
optical properties of 6 and 9a/b as compared to those of TIPS-
anthracene (TIPS-Anth)16 is shown in Figure 4. In a solution
of chloroform (CHCl3), the absorbance curve of 5 is almost
identical to that of TIPS-Anth with the exception of a slightly
lower energy absorption around 260 nm corresponding to the
phenyl substituents (Figure S2, Supporting Information). In the
case of 6, there are two distinct absorbances: a higher energy
C
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Figure 4. (a) Overlay of the absorbance curves of TIPS-Anth, 6, and 9a/b. (b) Solution and (c) solid state emission. (d) Solution state (solid line)
absorbance and emission: TIPS-Anth λmax = 442 nm, λem = 444 nm, ϕem = 0.97, τ = 5.04 ns; 6 λmax = 464 nm, λem = 469 nm, ϕem = 0.69, τ = 7.72 ns;
9a λmax = 500 nm, λem = 502 nm, ϕem = 0.45, τ = 5.60 ns; 9b λmax = 513 nm, λem = 517 nm, ϕem = 0.26, τ = 6.71 ns. Solution state measurements were
performed in CHCl3. Solid state (dotted line) absorbance and emission: TIPS-Anth λmax = 443 nm, λem = 452, 473, 504 nm; 6 λmax = 467 nm, λem
=
477, 543 nm; 9a λmax = 506 nm, λem = 514, 594, 623 nm; 9b λmax = 524 nm, λem = 630 nm. Thin films were spin cast from solutions of CHCl3.
membered ring C. Bonds linking the acenoid segments have
even greater single-bond character with a bond length of 1.498
Å in each case.
Table 1. Electrochemical Data for POAs
a
b
b
c
Eox (mV) vs Fc/
HOMO
(eV)
LUMO
(eV)
Eg (optical)
compound
Fc+
710
(eV)
As mentioned above, although theoretical studies have
shown [N]phenylenes to be planar in the lowest energy
comformation, crystal structures of this family of hydrocarbons
have often proved to be bent, likely due to the effects of crystal
packing forces on the increased flexibility of the backbone.18 In
the case of 6 and 9a there are small torsion angles around the
phenylene linkage (5.60°/1.31° and 1.58°/0.05°, respectively);
however, the molecules are very close to planar.
For both crystals of 6 and 9a, the molecules assemble into
two cofacial one-dimensional stacks with different orientations.
The intermolecular distances in the crystal packing are
relatively large for PAHs, 7.50 Å for 6 and 7.80 Å for 9a,
likely due to the steric bulk of the side groups. There is also a
shorter edge-to-face interaction between the phenyl substitu-
ents of opposing stacks of 4.89 Å for 6 and 4.19 Å for 9a.
TIPS-
Anth
−5.42
−2.68
2.74
6
702
678
527
−5.41
−5.39
−5.24
−2.84
−2.96
−2.87
2.57
2.43
2.37
9a
9b
a
Performed in a 0.1 M solution of TBAPF6 in CH2Cl2, Pt button
electrode, scan rate 150 mV/s, ferrocene as an external standard.
b
HOMO levels were determined from Eox and used in conjunction
c
with the optical band gap to determine LUMO levels. Determined
from λonset in CHCl3.
discussed. Consistent with these findings, 9b appears to be
susceptible to slow oxidative degradation in the solution state,
whereas the same was not evident in solutions of 6 and 9a.
Structural Analysis. Compounds 6 and 9a were both
analyzed by X-ray crystallography (Figure 5). Examination of
the bond lengths around the phenylene linkage confirms the
increased localization of π bonds, with bond lengths alternating
between ∼1.44 Å exocyclic and ∼1.36 Å endocyclic to the four-
SUMMARY
■
In conclusion, we reported the design and synthesis of a new
class of nonbenzenoid PAHs, phenylene-containing oligoa-
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Figure 5. X-ray crystal structures of (a) 6 and (b) 9a. Selected bond lengths (Angstroms) in (c) 6 and (d) 9a. Crystal packing of (e) 6 and (f) 9a.
cenes. The unique reactivity of 3,4-bismethylene-cyclobutene
was exploited to build ladder structures via sequential Diels−
Alder reactions. The resulting POAs represent stable, shape-
persistent chromophores, with band gap and quantum
efficiencies that decrease as length increases. Studies on the
ability of this class of molecules to participate in singlet fission
are underway.
Energy Sciences under Award Number DE-SC0001088. We
thank the National Science Foundation for departmental X-ray
diffraction instrumentation (CHE-0946721).
The authors thank Dr. Peter Muller and Dr. Michael Takase
̈
for collecting and solving X-ray structure data. The authors also
thank Dr. Brett VanVeller for providing compound 7 for initial
studies.
ASSOCIATED CONTENT
* Supporting Information
Experimental data including synthetic procedures, character-
ization data for all compounds, and additional figures. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
E-mail: tswager@mit.edu
Notes
■
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