Electron Acceptors Based on Cyclopentannulated Tetracenes

Article abstract of DOI:10.1055/s-0037-1609949

New cyclopenta-fused polycyclic aromatic hydrocarbons (CP-PAHs) based on tetracene have been prepared by a palladium-catalyzed cyclopentannulation reaction. The new compounds have low-energy lowest unoccupied molecular orbitals (LUMOs) and relatively small band gaps. The photooxidative stability was intermediate to previously prepared CP-PAHs based on anthracene and pentacene as found in traditional acene stabilities. Scholl cyclodehydrogenation of pendant aryl groups led to materials that quickly formed endoperoxide products.

Full text of DOI:10.1055/s-0037-1609949

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Georg Thieme Verlag Stuttgart · New York
2018, 29, A–E
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G. Kulkarni et al.
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Electron Acceptors Based on Cyclopentannulated Tetracenes
Gajanan C. Kulkarni
Jean L. Morales-Cruz
Waseem A. Hussain
Ian J. Garvey
Longer 'Acenes'
Ar
Ar
Ar
Ar
Ar
Ar
Kyle N. Plunkett*
Department of Chemistry and Biochemistry, Southern Illinois
University, Carbondale, IL 62901, USA
kplunkett@chem.siu.edu
Ar
Ar
Ar
Ar
Ar
Ar
LUMOs
Stabilities
Optical Gaps
Published as part of the Cluster Synthesis of Materials
This Work
Intermediate:
Received: 01.08.2018
Accepted after revision: 19.08.2018
Published online: 11.09.2018
member ring inclusion in the PAHs allows for the formation
of cyclopentadienyl anion-like structures that are Hückel
aromatics in the reduced state.18 While we previously
demonstrated that the cyclopentannulation of a pentacene
core 2 led to more stable structures than the well-known,
and relatively stable, TIPS-pentacene, the stabilities were
found to be less than desirable.
In this work, we looked to address the relative instabili-
ty of the cyclopentannulated pentacenes by reducing the
acene portion of the molecule to a tetracene backbone. Fur-
thermore, the new tetracene structures provides a bridge in
structure from pentacenes 2 to shorter anthracene deriva-
tives 1 previously prepared. As traditional acene stabilities
decrease with increasing ring fusion,25 we believed that a
DOI: 10.1055/s-0037-1609949; Art ID: st-2018-v0489-c
Abstract New cyclopenta-fused polycyclic aromatic hydrocarbons
15
(CP-PAHs) based on tetracene have been prepared by a palladium-cata-
lyzed cyclopentannulation reaction. The new compounds have low-en-
ergy lowest unoccupied molecular orbitals (LUMOs) and relatively small
band gaps. The photooxidative stability was intermediate to previously
prepared CP-PAHs based on anthracene and pentacene as found in tra-
ditional acene stabilities. Scholl cyclodehydrogenation of pendant aryl
groups led to materials that quickly formed endoperoxide products.
Key words cyclopenta-fused polycyclic aromatic hydrocarbons, elec-
tron acceptors, acenes, palladium-catalyzed annulation
Cyclopenta-fused polycyclic aromatic hydrocarbons
CP-PAHs) based on extended acene backbones are a rela-
(
1–19
H3C
CH3
tively new class of electron-accepting molecules.
In
strict definition, acenes are linearly fused benzene rings
with no additional annulated rings. However, we have re-
cently suggested that the annulation of five-membered
rings to an acene backbone could provide a pseudo-acene
structure that is more representative of the reactivity of
acenes in comparison to six-membered ring annulated de-
C12H25O
OC12H25
15
rivatives. We proposed this trait by considering the main-
C12H25O
OC12H25
tained ability to possess a migrating π-sextet defined by
20,21
Klar.
In general, extended acenes are reactive and unsta-
H3C
CH3
ble in the presence of light and oxygen, sensitizing their
own destruction through the formation of dimers or en-
1
2
TMS
22,23
doperoxides.
In regard to cyclopentannulated struc-
tures, recent work from our lab has shown that the modifi-
cation of anthracene (1) and pentacene (2) cores (Figure 1)
can lead to low-energy lowest unoccupied molecular orbit-
15,16
3
TMS
als (LUMO) and small band-gap materials.
These prop-
erties are appealing for electron acceptors in organic photo-
voltaics where broad absorption would be beneficial in the
acceptor region. Central to the design criteria, the five-
Figure 1 Previously reported cyclopenta-fused acenes based on an-
thracene (1), pentacene (2), and tetracene (3) prepared by a palladium-
catalyzed cyclopentannulation
24
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Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

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G. Kulkarni et al.
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tetracene core could provide additional stability in relation
to the pentacene derivatives. Here, we have applied a palla-
dium-catalyzed cyclopentannulation3–5,16 between 5,11-di-
bromotetracene and two diarylethynylene compounds to
make two new CP-PAHs with modestly low band gaps and
reduced LUMO levels. In addition, we investigated the abili-
ty to form contorted aromatic structures from these com-
pounds through the Scholl cyclodehydrogenation reaction
of the pendant aryl groups. Interestingly, the resulting cy-
clodehydrogenated structure was found to be even more
photooxidatively reactive and was not successfully isolated.
The palladium-catalyzed cyclopentannulation reaction
between 5,11-dibromotetracene 4 and 1,2-bis(3-(dodecy-
loxy)phenyl)ethyne 5 was carried out with a catalyst sys-
tem composed of Pd dba , P(o-Tol) , LiCl, and KOAc in DMF
five-membered rings being pushed further apart in space.
We similarly found that the reaction of 5,11-dibromotetra-
cene with 1,2-bis(5-methylthiophen-3-yl)ethyne 7 formed
the single isomer 8 during a parallel reaction.
2
3
3
26
at 130 °C (Scheme 1). Although we previously showed
that 2,8-bis(trimethylsilyl)dicyclopenta[de,mn]tetracene17
3
could be made as a single isomer from 5,11-dibromo-
tetracene and trimethylsilylacetylene, it was presumed that
steric interactions between the acene rings and the TMS
group provided its selectivity. Therefore, we were initially
concerned that the current chemistry may lead to two iso-
mers owing to the symmetry of the diarylethynylene reac-
tant. However, we were happy to find that a single dicyclo-
penta[de,mn]tetracene isomer 6 was formed during the re-
action and none of the dicyclopenta[de,op]tetracene isomer
was formed. Of importance, the relative positions of the
five-membered rings of 6 differ from the previously syn-
Figure 2 Cyclic voltammetry of 6 and 8 in 0.1 M tetrabutylammonium
hexafluorophosphate in THF with glassy carbon working electrode,
platinum counter electrode, and an Ag/AgCl reference electrode. Scan
rate = 50 mV/sec. Ferrocene added as internal standard and referenced
to 0 V.
We investigated the electronic and optical properties of
these new CP-tetracene derivatives using cyclic voltamme-
try and UV/Vis spectroscopy. The cyclic voltammograms of
8,9
thesized 1 and 2, as well as other CP-tetracenes, with the
C12H25O
OC12H25
C12H25O
OC12H25
OC12H25
Pd2(dba)3, P(o-Tol)3
Br
KOAc, LiCl
+
DMF, 130 °C
25%
C12H25O
OC12H25
OC12H25
Br
OC12H25
C12H25O
4
5
(Not Observed)
6
H3C
S
S
CH3
3
H C
S
S
CH3
H3C
Pd2(dba)3, P(o-Tol)3
Br
KOAc, LiCl
S
DMF, 130 °C
+
S
28%
Br
CH3
H3C
S
S
CH3
H3C
S
S
CH3
4
7
8
(Not Observed)
Scheme 1 Synthesis of cyclopenta-fused tetracene derivatives 6 and 8. A single isomer was found for the palladium-catalyzed cyclopentannulation
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G. Kulkarni et al.
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6
and 8 are shown in Figure 2. As found in previous cyclo-
Table 1 Summary of Optoelectronic Properties of 1–3, 6, and 8a
15–17
pentannulated derivatives,
the new compounds gave
two reversible reduction waves. The two-electron reduc-
tion is imparted to the molecule because of the ability to
form cyclopentadienyl anion-like structures in the reduced
state (Figure 3).
Cmpd Eox/onset Ered/onset HOMO LUMO
E-chem Optical
gap (eV) gap (eV) (min)
τhalf–life
(
V)
(V)
(eV)
(eV)
1
2
3
6
0.66
0.55
0.56
0.49
0.41
–1.32
–5.46
–3.48
1.98
1.53
1.91
1.79
1.73
1.63
1.20
1.79
1.59
1.53
1119
192
–
–0.98
–1.35
–1.31
–1.32
–5.35
–5.36
–5.29
–5.21
–3.82
–3.45
–3.49
–3.48
RO
OR
RO
OR
502
753
8
2
e-
a
Measurements taken at sample concentration of 0.2 mM and potentials
measured relative to a ferrocenium/ferrocene redox couple used as an in-
ternal standard (Figure 2). Eox/onset is the onset of oxidation potential, Ered/on-
set is the onset of reduction potential. HOMO and LUMO values calculated
based on the oxidation of the ferrocene reference in vacuum (4.8 eV).
RO
OR
RO
OR
The absorption properties of 6 and 8 are shown in Fig-
ure 5. The high-energy absorptions dominate the spectra,
but a middle-energy absorption and a broad, charge-trans-
fer band centered at ca. 650 nm with an onset wavelength
of 780 nm and 810 nm, respectively, are also present. The
optical band gaps from these absorption profiles give val-
ues of 1.59 and 1.53 eV, respectively. These band gaps are
intermediary to those found for anthracene 1 (1.63 eV) and
pentacene 2 (1.20 eV) derivatives and are consistent with
the elongation of the acene backbone. We also investigated
the photostability of 6 and 8 in the presence of oxygenated
THF. The absorption intensity was monitored over time un-
der ambient light to give decomposition profiles (see the
Supporting Information) that were compared to the decom-
position rates of 1 and 2. Similar to the trends found for tra-
ditional acenes,25 the tetracene decomposition half-lives
(Table 1) were between the more stable anthracene and the
less stable pentacene derivatives.
6
Figure 3 Reduction of 6 leads to two new cyclopentadienyl anions
that are stabilized as Hückel aromatics leading to the electron-accept-
ing behavior
For 6, the oxidation was reversible, whereas for 8 the
oxidative process was pseudo-reversible. A ferrocene redox
couple was utilized as an internal standard and provided
the highest occupied molecular orbital (HOMO) and LUMO
energies (Table 1). The LUMO value was similar, albeit a lit-
tle higher than the previously synthesized anthracene de-
rivative and considerably higher in energy than previously
synthesized pentacene derivatives. The destabilization of
the tetracene derivative LUMOs could be due to the separa-
tion of the five-membered rings in relation to the previous
compounds or simple extension of the acene core. The DFT
minimized structure of 6 shows the perpendicular arrange-
ment of the appendage aryl rings and the HOMO and LUMO
contours do not reach onto these rings (Figure 4). The
HOMO contour is isolated along a trajectory involving the
two five-membered rings and the two inner six-membered
rings of the tetracene. Alternatively, the LUMO is distribut-
ed onto all rings of the CP-tetracene core.
Figure 5 UV/Vis spectra of 6, 8, and 10. The oxidized derivative 10
shows the loss of the long wavelength absorption of the tetracene back-
bone.
Figure 4 The B3LYP/6-311g(d,p) DFT calculated HOMO and LUMO
contours of compound 6
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Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E

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Fleetingly Observed
Isolated
C12H25O
OC12H25
C12H25O
C12H25O
OC12H25
OC12H25
FeCl3
O
O
CH3NO2
CH2Cl2
C12H25O
OC12H25
C12H25O
C12H25O
OC12H25
OC12H25
6
9
10
Scheme 2 Scholl cyclodehydrogenation reaction of 6. The oxidized product 10 was isolated instead of the desired compound 9.
The arrangement of the attached aryl groups of 6 pro-
vided opportunities to create extended π-networks through
the Scholl cyclodehydrogenation reaction, as previously
employed for anthracene and pyrene derivatives to make
Acknowledgment
G.C.K. thanks Dr. Akshita Patel for valuable discussions and support.
16,27
contorted aromatics.
While it became evident by NMR
Supporting Information
and mass spectrometry that the two ring closures were suc-
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1609949.
cessful upon addition of FeCl , we were unable to isolate
3
S
u
p
p
ortioIgnfmr oaitn
S
u
p
p
o
nrtogI
i
f
rm oaitn
compound 9 (Scheme 2). Instead, we isolated the endo-per-
oxide-containing derivative 10. The oxidation process was
fast, and the desired green color of the 9, which was ob-
served in the precipitated product, was quickly eroded to
the yellow color of the oxidized product 10 during filtra-
tion. Attempts to control the oxidation process were unsuc-
cessful and we propose the fully ring-closed 9 is even more
susceptible to oxidation than the precursor 6. The UV/Vis of
References and Notes
(1) Rudebusch, G. E.; Haley, M. M. Planar Cyclopenta-Fused Polycy-
clic Arenes, In Polycyclic Arenes and Heteroarenes; Miao, Q., Ed.;
Wiley-VCH Verlag GmbH & Co: Weinheim, 2015.
(2) Sangaiah, R.; Gold, A. J. Org. Chem. 1987, 52, 3205.
(3) Dang, H.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2001, 123, 355.
1
0 can also be found in Figure 5 and shows that the spectra
(4) Dang, H.; Levitus, M.; Garcia-Garibay, M. A. J. Am. Chem. Soc.
2002, 124, 136.
of the oxidized product are blueshifted with the loss of the
long-wave absorption band. The longest wavelength ab-
sorption was reduced to 460 nm, which clearly demon-
strates the loss of conjugation along the acene backbone.
The reduced photooxidative stability is consistent with our
previous work on pentacene derivatives in which we were
not able to isolate the Scholl cyclodehydrogenation product
(5) Eversloh, L. E.; Avlasevich, Y.; Li, C.; Müllen, K. Chem. Eur. J.
2011, 17, 12756.
(
(
6) Mohebbi, A. R.; Wudl, F. Chem. Eur. J. 2011, 17, 2642.
7) Mohebbi, A. R.; Yuen, J.; Fan, J.; Munoz, C.; Wang, M. f.; Shirazi,
R. S.; Seifter, J.; Wudl, F. Adv. Mater. 2011, 23, 4644.
8) Chaolumen; Murata, M.; Sugano, Y.; Wakamiya, A.; Murata, Y.
Angew. Chem. Int. Ed. 2015, 54, 9308.
(
28
because of rapid oxidation.
(9) Chaolumen; Murata, M.; Wakamiya, A.; Murata, Y. Org. Lett.
017, 19, 826.
2
In conclusion, we have prepared new CP-PAHs based on
a tetracene scaffold. The palladium-catalyzed cyclopentan-
nulation reaction was regioselective and provided a slightly
different bonding motif to previous CP-PAH based acenes
with the five-member rings being further separated in
space. The new compounds provide low LUMO levels and
broad absorption profiles. The stability of 6 and 8 are less
than anthracene derivatives but they are more stable than
pentacene derivatives, which mirrors traditional acene sta-
bility trends.
(
(
10) Xia, H.; Liu, D.; Xu, X.; Miao, Q. Chem. Commun. 2013, 4301.
11) Wegner, H. A.; Scott, L. T.; de Meijere, A. J. Org. Chem. 2003, 68,
883.
(
12) Lakshminarayana, A. N.; Chang, J.; Luo, J.; Zheng, B.; Huang, K.-
W.; Chi, C. Chem. Commun. 2015, 3604.
(13) Wombacher, T.; Gassmann, A.; Foro, S.; von Seggern, H.;
Schneider, J. J. Angew. Chem. Int. Ed. 2016, 55, 6041.
(
14) Wombacher, T.; Foro, S.; Schneider, J. J. Eur. J. Org. Chem. 2016,
69.
5
(
15) Bheemireddy, S. R.; Ubaldo, P. C.; Rose, P. W.; Finke, A. D.;
Zhuang, J.; Wang, L.; Plunkett, K. N. Angew. Chem. Int. Ed. 2015,
54, 15762.
(
(
(
16) Bheemireddy, S. R.; Ubaldo, P. C.; Finke, A. D.; Wang, L.;
Funding Information
Plunkett, K. N. J. Mater. Chem. C 2016, 4, 3963.
17) Wood, J. D.; Jellison, J. L.; Finke, A. D.; Wang, L.; Plunkett, K. N. J.
Am. Chem. Soc. 2012, 134, 15783.
We are grateful to the National Science Foundation (NSF CAREER
CHE-#1352431) for financial support. J.L.M.-C. was supported by NSF
18) Plunkett, K. N. Synlett 2013, 24, 898.
REU grant DMR-#1757954
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2
4
3
1)
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G. Kulkarni et al.
Cluster
(
19) Chaolumen; Murata, M.; Wakamiya, A.; Murata, Y. Angew.
Chem. Int. Ed. 2017, 56, 5082.
20) Clar, E. The Aromatic Sextet; Wiley: New York, NY, 1972.
129.60, 129.00, 128.89, 127.69, 126.93, 126.68, 125.64, 124.20,
122.93, 122.43, 116.58, 115.65, 114.27, 113.69, 68.15, 67.93,
31.93, 31.91, 29.71, 29.65, 29.60, 29.41, 29.38, 29.35, 29.22,
29.16, 26.01, 22.70, 22.68, 13.88. HRMS: m/z calcd for
(
(
(
(
(
21) Solà, M. Front. Chem. 2013, 1, 1–22.
22) Foote, C. S. Acc. Chem. Res. 1968, 1, 104.
23) Fudickar, W.; Linker, T. J. Am. Chem. Soc. 2012, 134, 15071.
24) Wadsworth, A.; Moser, M.; Marks, A.; Little, M. S.; Gasparini, N.;
Brabec, C. J.; Baran, D.; McCulloch, I. Chem. Soc. Rev. 2018, DOI:
C94H124O : 1316.9500; found: 1316.9111.
4
4,4′,4′′,4′′′-(Dicyclopenta[de,mn]tetracene-1,2,7,8-
tetrayl)tetrakis(2-methylthiophene) (8): In a glove box, 4
(41.3 mg, 0.107 mmol), 7 (56.0 mg, 0.257 mmol), Pd (dba)3
2
1
0.1039/c7cs00892a.
(9.80 mg, 0.0107 mmol), P(o-Tol)3 (4.92 mg, 0.0162 mmol),
KOAc (52.6 mg, 0.536 mmol), LiCl (9.12 mg, 0.215 mmol) and
DMF (10.7 mL) were combined in a sealed tube and stirred over-
night at 130 °C. The reaction mixture was cooled to room tem-
perature and poured dropwise into methanol (50 mL) and fil-
tered. The solid was washed with methanol and acetone to give
(
(
25) Zade, S. S.; Bendikov, M. J. Phys. Org. Chem. 2012, 25, 452.
26) 1,2,7,8-Tetrakis(3-(dodecyloxy)phenyl)dicyclo-
penta[de,mn]tetracene (6): In a glove box, 4 (0.127 g, 0.328
mmol), 5 (0.430 g, 0.786 mmol), Pd (dba) (30.0 mg, 0.0328
2
3
mmol), P(o-Tol)3 (15.1 mg, 0.0495 mmol), KOAc (0.161 g, 1.64
mmol), LiCl (27.8 mg, 0.657 mmol) and DMF (7.67 mL) were
combined in a sealed tube and stirred overnight at 130 °C. The
reaction mixture was cooled to room temperature and poured
dropwise into methanol (50 mL) and filtered. The solid was
washed with methanol and acetone to give 5 (0.164 g, 25.0%) as
1
7 (20.2 mg, 28.3%) as a green solid. H NMR (500 MHz, CD Cl ): δ =
2
2
8.47 (s, 2 H), 7.84 (d, J = 6.5 Hz, 2 H), 7.67 (d, J = 8.5 Hz, 2 H),
7.47 (dd, J = 8.4, 6.6 Hz, 2 H), 7.10 (d, J = 1.1 Hz, 2 H), 7.04 (d,
J = 1.1 Hz, 2 H), 6.91 (s, 2 H), 6.76 (s, 2 H), 2.59 (s, 6 H), 2.40 (s,
13
6 H). C NMR (126 MHz, CD Cl ): δ = 140.67, 139.63, 139.20,
2
2
1
a green solid. H NMR (500 MHz, CD Cl ): δ = 8.62 (s, 2 H), 7.87
137.89, 136.50, 135.69, 135.64, 132.68, 130.89, 128.97, 128.03,
127.55, 126.92, 126.90, 126.34, 125.12, 124.11, 122.34, 121.46,
15.27, 15.09. HRMS: m/z calcd for C42H28S4: 660.1074; found:
659.926.
2
2
(
6
(
6
d, J = 6.5 Hz, 2 H), 7.73 (d, J = 8.5 Hz, 2 H), 7.57 (dd, J = 8.4,
.6 Hz, 2 H), 7.50 (t, J = 7.7 Hz, 2 H), 7.29 (t, J = 7.9 Hz, 2 H), 7.24
d, J = 7.4 Hz, 2 H), 7.16 (s, 2 H), 7.13–7.09 (m, 4 H), 6.98 (s, 2 H),
.84 (dd, J = 8.1, 2.1 Hz, 2 H), 4.00 (t, J = 6.6 Hz, 4 H), 3.83 (t,
J = 6.6 Hz, 4 H), 1.80–1.71 (m, 8 H), 1.48–1.19 (m, 72 H), 0.91 (q,
(27) Bheemireddy, S. R.; Hautzinger, M. P.; Li, T.; Lee, B.; Plunkett, K.
N. J. Am. Chem. Soc. 2017, 139, 5801.
(28) Unpublished results.
13
J = 7.0 Hz, 12 H). C NMR (126 MHz, CD Cl ): δ = 159.38,
2
2
158.92, 141.12, 139.74, 139.39, 136.67, 136.33, 136.08, 131.06,
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 29, A–E