Combining electronic and steric effects for highly stable unsymmetric pentacenes

Article abstract of DOI:10.1002/chem.201402003

This paper describes the reactivity of unsymmetrically substituted pentacenes to photochemical oxidation. Acenes in general, and pentacenes in particular, are a key family of compounds for a variety of organic electronics applications. The instability of many pentacene derivatives, particularly to oxidation, is an important restriction in their applicability. Several substitution strategies for decreasing the reactivity of pentacene exist, but these almost always involve symmetrically substituted derivatives, restricting the chemical space of structures from which to choose. In this paper, we demonstrate that combining electronic and steric effects yields highly stable unsymmetrically substituted pentacenes. High fives! Highly stable pentacene derivatives are described, which combine an electronically deactivating alkyne substituent with a sterically-encumbering dialkylaryl substituent on the reactive 6- and 13-positions (see scheme). Resistance to photooxidation of these unsymmetrically substituted pentacenes is comparable to diethynylpentacenes, opening new chemical space for the discovery of stable organic semiconductors.

Full text of DOI:10.1002/chem.201402003

DOI: 10.1002/chem.201402003
Communication
&
Acenes
Combining Electronic and Steric Effects for Highly Stable
Unsymmetric Pentacenes
Jingjing Zhang, Robert H. Pawle, Terry E. Haas, and Samuel W. Thomas, III*[a]
have developed alkyl- and arylthiol-substituted acenes that
Abstract: This paper describes the reactivity of unsym-
metrically substituted pentacenes to photochemical oxida-
tion. Acenes in general, and pentacenes in particular, are
a key family of compounds for a variety of organic elec-
tronics applications. The instability of many pentacene de-
rivatives, particularly to oxidation, is an important restric-
tion in their applicability. Several substitution strategies
for decreasing the reactivity of pentacene exist, but these
almost always involve symmetrically substituted deriva-
tives, restricting the chemical space of structures from
which to choose. In this paper, we demonstrate that com-
bining electronic and steric effects yields highly stable un-
symmetrically substituted pentacenes.
show remarkable stability.[7,21] Electron-withdrawing fluorine
substituents have also been shown to improve acene stabili-
ty.[22,23]
Nearly all reports of acenes with backbones longer than an-
thracene are substituted symmetrically with respect to the
long axis of the molecule. Therefore, although these ap-
proaches to acene design have yielded groundbreaking results,
such a limitation on structure restricts the chemical space for
discovering new molecules with improved properties. The prin-
cipal exceptions to this have come from Tykwinski and co-
workers, who described several years ago the preparation of
unsymmetrically substituted diethynylpentacenes by control-
ling stoichiometry and rate of addition of ethynyllithium re-
agents to pentacenequinone.[24–26] Earlier this year, the same
group reported three 6-anthryl-13-ethynylpentacene deriva-
tives, one of which showed ambipolar behavior in thin film
transistors.[27]
Acenes are one of the most successful classes of organic semi-
conductors, with pentacene among the leaders in per-
formance.[1–4] Unsubstituted pentacene, however, presents
a number of challenges, such as photooxidative instability and
low solubility in organic solvents, which limit its promise in
emerging technologies, such as field-effect transistors. There
are a number of successful strategies that use substituent ef-
fects to ameliorate these drawbacks of pentacene, while still
retaining functional properties of the acene. One of the earliest
examples was substitution with phenyl groups or other aro-
matic substituents;[5,6] these substituents improve the solubility
of pentacene, but unless specifically designed to do so,[7] gen-
erally do not improve acene stability.[8] 6,13-Diarylpentacene
derivatives have found use as emissive species in organic light
emitting devices (OLEDs).[9,10]
1
Our group has developed a general approach to O2-respon-
sive luminescent materials that rely on energy transfer from
a light-harvesting chromophore to acene; subsequent acene–
1O2 cycloaddition reactions yield a robust, ratiometric response
in luminescence.[28,29] Designing next generations of these ma-
terials for new applications requires fine control over acene ab-
1
sorbance, luminescence, reactivity with O2, and selectivity for
this reaction over other acene decomposition pathways, such
as alkyne–acene cycloaddition and [4+4] “butterfly” dimeriza-
tion.[30–33] Our overall goal in this project is to determine how
1
substitution could control band gap and reactivity with O2 of
substituted pentacene derivatives, including unsymmetric aryl-
ethynylpentacenes. Herein we highlight an approach to new,
highly stable unsymmetric pentacene derivatives through ra-
tional design that incorporates both electronic and steric ef-
fects.
Anthony and co-workers popularized the use of diethynyl
acenes, including pentacenes, which show superior optoelec-
tronic properties and are both highly soluble and stable.[11–16]
The stability of diethynylpentacenes is at least in part due to
physical quenching of singlet oxygen (1O2), cycloaddition with
which is a major decomposition pathway of most penta-
cenes,[17] by these materials.[18] The strategy of combining aryl
groups and electronically deactivating ethynyl substituents has
yielded stable heptacenes and nonacenes that have been char-
acterized by X-ray crystallography.[19,20] Miller and co-workers
Scheme 1 shows the structures of the substituted penta-
cenes described in this study: symmetric 6,13-diphenylpenta-
cene (1)[10] and 6,13-diphenylethynylpentacene (2)[13] are
known in the literature, while the unsymmetric 6-aryl-13-phe-
nylethynylpentacenes 3–5 are new compounds. Synthesis of
3–5 first involved slow addition of lithiated phenylacetylene
followed by protonation and purification of the intermediate
hydroxyketone. Subsequent addition of an excess of the ap-
propriate aryllithium to the hydroxyketone followed by reduc-
tion of the resulting diol with SnCl2 in aqueous HCl yielded the
target pentacene derivatives, each of which was purified by
column chromatography followed by recrystallization. Com-
pound 3 simply combines the two types of substituents pres-
[a] J. Zhang, R. H. Pawle, Prof. T. E. Haas, Prof. S. W. Thomas, III
Department of Chemistry, Tufts University
62 Talbot Avenue, Medford, MA 02155 (USA)
E-mail: sam.thomas@tufts.edu
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402003.
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Scheme 1. Structures of 6,13-disubstituted pentacenes 1–5.
ent in symmetric 1 and 2. To examine the influence of sterically
demanding ortho-alkyl substituents of the aryl groups on both
the photophysics and reactivity of the pentacenes, compounds
4 and 5 have methyl or ethyl groups, respectively, in ortho po-
sitions.
X-ray crystallography of compound 5 (Figure 1) revealed
that the pentacene core in this molecule is highly planar (twist
angle ca. 38). Consistent with previous reports on aryl-substi-
tuted pentacenes, the plane defined by the aryl substituent is
Figure 2. Normalized UV/Vis a) absorbance and b) fluorescence emission
spectra of pentacenes 1–5.
Table 1. Steady-state absorbance and fluorescence parameters of penta-
cenes 1–5. Molar extinction coefficients are reported at the longest wave-
length maximum for each compound.
lonset,abs
lmax,abs
e
lmax,fl
Ff
[nm]
[nm]
[mÀ1
]
[nm]
1
2
3
4
5
630
695
680
680
680
516, 554, 599
311, 562, 605, 660
309, 588, 630
309, 587, 630
309, 588, 631
12700
27300
9400
14000
13000
610
675
688
672
675
0.09
0.13
0.02
0.03
0.04
Figure 1. X-ray crystal structure of compound 5. Thermal ellipsoids shown at
50% probability. Hydrogen atoms and a disordered CH2Cl2 molecule have
been omitted for clarity.
sorbance spectra of the new pentacene derivatives in CH2Cl2
showed no alteration in shape between 30 and 0.3 mm, indicat-
ing the absence of aggregation of the chromophores in solu-
tion at concentrations suitable for measurement of absorbance
and fluorescence spectra of these compounds. All absorbance
spectra showed vibronic structure, as is characteristic for linear
acenes. An immediately noticeable trend is that the magnitude
of the band gaps of these pentacenes, as determined by the
onset of absorbance, increases in the order 2 (1.78 eV)<3–5
(1.82 eV)<1 (1.94 eV). This trend correlates with the degree of
p-conjugation present in these pentacene derivatives. The aryl
substituents attached directly to the pentacene core are highly
nearly perpendicular (868) to the plane defined by the penta-
cene core. The phenyl ring of the phenylethynyl substituent is
also twisted (torsional angle of 588), which we attribute to
a combination of the low barrier of rotation of this group and
crystal packing forces.
Figure 2 shows the UV/Vis absorbance and fluorescence
emission spectra of pentacenes 1–5 in CH2Cl2, while Table 1
summarizes their steady-state photophysical properties. Ab-
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twisted and contribute less additional delocalization to the
pentacene core than arylethynyl groups that can adopt a more
coplanar conformation, resulting in the observed decrease in
band gap with increasing phenylethynyl substitution.[13,34] Alkyl
substitution on the aryl groups caused virtually no change in
the absorbance or fluorescence spectra of compounds 4 and 5
relative to 3. Consistent with larger conformational reorganiza-
tion in the excited state for aryl-substituted acenes, com-
pounds 1 and 3–5 showed larger Stokes shifts (42–58 nm)
than 2 (15 nm). The ethynyl–arylpentacenes also showed low
quantum yields of fluorescence, between 2–4%, which we at-
tribute to these compounds having a combination of features
from both 1—vibrational modes from twisting around the
aryl–acene bond—and 2—a narrow HOMO–LUMO gap—that
leads to increased rates of nonradiative decay.
sorbance spectra that overlap significantly with the absorbance
spectrum of MB, we used a 665 nm long-pass filter to ensure
the majority of incident light was absorbed by the MB. We also
adjusted the decomposition kinetics to reflect only the decom-
position due to absorbance of light by MB with the following
corrections: 1) correcting the observed rate by the fraction of
light that was absorbed directly by MB, and 2) subtracting the
absorbance of MB from all UV/vis spectra before determining
the remaining concentration of pentacene. To correct for ex-
perimental variability, we compared all kinetics to the MB-
mediated photooxidation of 9,10-diphenylanthracene (DPA),
which has a bimolecular rate constant of 3ꢁ106 mÀ1 sÀ1, under
identical conditions (concentrations of acene and MB, light
flux, etc.). Kinetic data of the pentacene derivatives deviated
from pseudo-first-order behavior by reacting faster than pre-
dicted by initial kinetics as the concentration of pentacene de-
creased, which suggests that the steady-state concentration of
1O2 increased during the reaction; this scenario is possible if
acene–1O2 reactions (a combination of physical and chemical
Photochemical decomposition of pentacene derivatives, par-
ticularly in the presence of O2, can limit their utility. Among
the most prevalent degradation pathways is a photoexcited
1
pentacene derivative donating energy to O2 to generate O2,
1
which then undergoes fast [4+2] cycloaddition with the cen-
quenching) compete with unimolecular decay of O2.[35] In all
tral ring of the pentacene to yield the corresponding endoper-
s
oxide with bimolecular rate constants 108–109 mÀ1 À1. To char-
cases, we fit the initial data to a pseudo-first-order kinetics
model (Figure 3); Table 2 summarizes the relative rates of
acterize the photochemical reactions of these new unsymmet-
rically substituted pentacenes, we first determined the prod-
ucts of photooxidation of 3 in CDCl3 (Scheme 2). Either 1)
Table 2. Kinetics of photooxidations of 1–5 in CHCl3. Reported relative
rates are from fits of initial rate data to first-order kinetics and were deter-
mined relative to that for DPA (k=3ꢁ106 mÀ1 À1) and are the means of
s
2–3 independent experiments; each error is one standard deviation from
the mean. Maximum measured deviations in half-lives upon direct irradia-
tion are ꢀ10% of the means.
Relative rate
(sensitized)[a]
t1/2 [s]
(l>400 nm)
1
2
3
4
5
ca. 30
<4
1040
30
810
1100
(0.27Æ0.17)[b]
(10Æ1)[c]
(0.33Æ0.13)[c]
(0.18Æ0.01)[c]
Scheme 2. Photooxidation of 3 to endoperoxide 3-O2.
[a] Relative rate of DPA decomposition is set at 1. [b] Rate of decomposi-
tion was 25% of this value when MB was not present. [c] Rate of decom-
position was 5–7% of this value when MB was not present.
direct irradiation of 3 using visible light (> 400 nm), or 2) irra-
diation of a mixture of 3 and the photosensitizing dye methyl-
ene blue (MB) with a 665 nm long-pass filter, using a 200 W
Hg/Xe lamp proceeded to completion within 15 min, yielding
a major product with 1H NMR and 13C NMR data consistent
with endoperoxide formation across the central 6 and 13 posi-
tions (see Supporting Information). Neither [4+4] butterfly di-
merization nor alkyne–acene cycloaddition was evident from
acene decomposition under these conditions. In addition, we
also monitored the disappearance of acene upon irradiation
under identical conditions (l> 665 nm), in the absence of MB.
In these control experiments, only compound 2, because of its
red-shifted absorbance that overlaps strongly with that of MB,
showed significant decomposition—a rate of 25% of that ob-
served in the presence of MB. In the absence of MB, 3–5
showed only 5–7% of the rate of decomposition upon irradia-
tion at l>665 nm.
1
the H NMR spectrum of the irradiated samples. Diphenylpen-
tacene 1 is also known to give a structurally analogous endo-
peroxide across the 6- and 13-positions upon irradiation in
air.[5]
To understand the effect of substitution on the photochemi-
cal stability of these pentacene derivatives, we measured the
One critical trend in structure–property relationships is that
unsymmetrically substituted arylethynyl compound 3 shows
a rate of reaction with photosensitized O2 that is intermediate
1
1
kinetics of their disappearance upon exposure to O2 generat-
ed by irradiation of MB in an air-equilibrated CHCl3 solution by
UV/vis spectrophotometry. In these experiments, the initial
concentrations of the pentacene derivative and methylene
blue were 50 and 80 mm, respectively. Because 1–5 have ab-
between diphenylpentacene 1 and diphenylethynylpentacene
2: the rate of reaction of 3 is approximately threefold smaller
than 1, with a rate constant with O2 similar to that of diaryl-
1
tetracenes, indicating that one ethynyl group stabilizes the
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optical spectra of 3, 4, and 5 are nearly identical, there is not
a significant difference in conjugation, and therefore it is un-
likely that there is significant difference in stabilization of radi-
cal or ionic character at the 6- and 13-positions in the transi-
tions states for oxidation of compounds 3–5. The nearly identi-
cal energies of HOMOs (À4.78 to À4.80 eV) and LUMOs (À2.81
to À2.83 eV) of these three compounds as determined by DFT
calculations (tabulated in the Supporting Information) using
the B3LYP functional with the 6-311G(d,p) basis set further sup-
port this conclusion. We therefore interpret this striking differ-
ence in rate of oxidation to steric shielding of the reactive cen-
tral positions of pentacene by the alkyl groups above and
1
below the pentacene core from reacting with O2. Steric hin-
drance is known to direct cycloaddition reactions of 6,13-diary-
lpentacenes with dieneophiles larger than O2 to the 5,14- or
7,12-positions,[36–38] and Miller and co-workers have noted an
analogous increase in half-life (up to 25-fold) for diarylpenta-
cenes with o-alkyl substituents.[7] The relative rate of decompo-
sition of 4 and 5 is similar to that previously reported for 6,13-
bis[(triisopropylsilyl)ethynyl]pentacene under photosensitized
oxidation conditions with MB, which has a bimolecular rate
constant of about 1ꢁ106 mÀ1 sÀ1 [18]
.
Although it is complicated by differences in absorbance
spectra of the acenes, and how those spectra overlap with the
spectral irradiance curves of light sources, assessing the photo-
stability of acenes upon direct irradiation in the absence of ex-
ternal sensitizers is important for optoelectronic applications.
We therefore followed the decomposition of pentacenes 1–5,
using UV/Vis spectrophotometry, upon irradiation with l>
400 nm at a power density of 10 mWcmÀ2—Table 2 shows the
half-lives of the pentacenes under these conditions. In agree-
ment with the results of earlier MB-mediated photooxygena-
tion reactions, the half-lives of these compounds span three
orders of magnitude, with the order of persistence 5ꢁ2>4>
3>1. Direct comparison of 3, 4, and 5, which have nearly iden-
tical absorbance spectra and extinction coefficients, agrees
with the results from external photosensitization, with sterically
hindered 4 and 5 having half-lives about 30-fold longer than
less-hindered 3. As evidence for the efficacy of combining
steric and electronic factors in our acene designs, diethyl deriv-
ative 5 has a half-life nearly identical to that of 2.
Figure 3. a) Pseudo-first-order initial kinetics of disappearance of com-
pounds 1–5 and DPA upon exposure to MB (80 mm) and irradiation with
l>665 nm in air-equilibrated CHCl3. Absorbance spectra of b) 3 and c) 5 as
a function of irradiation time at l>665 nm in the presence of MB. The ab-
sorbance of MB has been subtracted from each spectrum to reflect the
actual concentration of acene.
pentacene towards photooxidiation significantly. This result is
consistent with the electronic stabilizing effect that ethynyl
substituents supply to the oxidation of acenes, which Fudikar
and Linker have ascribed to 1) destabilization of biradical or
zwitterionic intermediates for stepwise pathways that are
faster than concerted reactions, and 2) physical quenching of
1O2 by ethynylpentacenes.[18]
This study demonstrates that combining steric and electron-
ic effects can yield unsymmetrically substituted pentacene de-
rivatives that are highly persistent under photooxidative condi-
tions. Each of the substituent effects: 1) the steric hindrance of
the central 6- and 13-positions through o-alkyl groups on one
aryl substituent, and 2) the electronic deactivation of penta-
cenes through substitution with one ethynyl substituent by in-
1
Combining this electronic stabilizing effect of ethynyl sub-
stituents with steric hindrance due to ortho-alkyl substituents
on aryl substituents led to pentacene derivatives with persis-
tence under photooxidative conditions that rivals the highly
stabilized diethynylpentacenes. Compounds 4 and 5, which
substitute methyl or ethyl groups on the ortho-positions of the
aryl substituent bound directly to the acene, react with singlet
oxygen 30–50 times more slowly than 3, which has hydrogen
atoms as ortho-substituents on the aryl substituent. Since the
ducing physical quenching of O2, slows the rate of pentacene
photoinduced decomposition. This work therefore highlights
that new, unsymmetric arylethynylpentacene derivatives can
show photostability akin to the more established, symmetrical-
ly substituted diethynyl derivatives, significantly broadening
the structural space of acene derivatives that may find practical
use as organic semiconductors.
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Acknowledgements
This work was supported by the National Science Foundation
(Awards CHE-1305832 and CHE-1229426). We also thank Pro-
fessor Yu-Shan Lin (Tufts University) for assistance with DFT cal-
culations.
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Keywords: acenes · cycloaddition reactions
photochemical reactions · singet oxygen
· oxidation ·
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Received: February 12, 2014
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Published online on March 13, 2014
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