Unilaterally Fluorinated Acenes: Synthesis and Solid-State Properties

Article abstract of DOI:10.1002/ange.202006489

The rapid development of organic electronics is closely related to the availability of molecular materials with specific electronic properties. Here, we introduce a novel synthetic route enabling a unilateral functionalization of acenes along their long side, which is demonstrated by the synthesis of 1,2,10,11,12,14-hexafluoropentacene (1) and the related 1,2,9,10,11-pentafluorotetracene (2). Quantum chemical DFT calculations in combination with optical and X-ray absorption spectroscopy data indicate that the single-molecule properties of 1 are a connecting link between the organic semiconductor model systems pentacene (PEN) and perfluoropentacene (PFP). In contrast, the crystal structure analysis reveals a different packing motif than for the parent molecules. This can be related to distinct F???H interactions identified in the corresponding Hirshfeld surface analysis and also affects solid-state properties such as the exciton binding energy and the sublimation enthalpy.

Full text of DOI:10.1002/ange.202006489

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Akzeptierter Artikel
Titel: Unilaterally Fluorinated Acenes: Synthesis and Solid State
Properties
Autoren: Ulrich Koert, Gregor Witte, Matthias Tripp, Philipp Hofmann,
Daniel Bischof, Yvonne Grell, Anna Schiller, Tobias Breuer,
and Sergei Ivlev
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Zitierweise: Angew. Chem. Int. Ed. 10.1002/anie.202006489
Link zur VoR: https://doi.org/10.1002/anie.202006489

10.1002/ange.202006489
Angewandte Chemie
COMMUNICATION
Unilaterally Fluorinated Acenes: Synthesis and Solid State
Properties
Philipp E. Hofmann[a,§], Matthias W. Tripp[a,§], Daniel Bischof[b,§], Yvonne Grell[a,§], Anna L. C. Schiller[a],
Tobias Breuer[b], Sergei I. Ivlev[a], Gregor Witte*[b] and Ulrich Koert*[a]
Abstract: The rapid development of organic electronics is closely
related to the availability of molecular materials with specific elec-
tronic properties. Here, we introduce a novel synthetic route enabling
a unilateral functionalization of acenes along their long side, which is
functionalization.[7,10] In particular, pentacene and its derivatives
have become prototypical model systems because they form
crystalline films and are sufficiently thermally stable.[11] Fluori-
nation is a common strategy to modify the electronic properties of
organic semiconductors.[12] While functionalization by fluorinated
side groups such as trifluoromethyl leads to a significant change
in the molecular shape and thereby affects the packing motif due
to steric hindrance,[13] the molecular shape is not significantly
changed by direct fluorination. In that case, the polar C-F bonds
directly modify the energy levels of the π-system and, in the case
of perfluorination, cause an inversion of the charge density
distribution, as depicted in Fig. 1 for the case of pentacene.[14] So
far, only a small number of partially fluorosubstituted pentacenes
have been synthesized, mainly with symmetrical fluorination at
the outer rings,[15] or in the case of asymmetric substitution only
at the short molecular sides.[13,16] The latter molecules have a
permanent dipole moment and geometrically combine two smaller
aromatic subsystems (one being fluorinated) to one π-system
with a push-pull character. More challenging, however, is the case
where all aromatic rings are partially fluorinated by introducing a
unilateral substitution pattern, as this would cause a true mixing
of the electronic states of the parental acene and its perfluorinated
analogue. However, common synthetic methods based on
symmetric cycloadditions cannot be applied here as they do not
allow controlling the regioselectivity of unilateral substitution
patterns.
demonstrated
by
the
synthesis
and the
of
1,2,10,11,12,14-
1,2,9,10,11-
hexafluoropentacene
(1)
related
pentafluorotetracene (2). Quantum chemical DFT calculations in
combination with optical and X-ray absorption spectroscopy data
indicate that the single molecular properties of 1 are a connecting link
between the organic semiconductor model systems pentacene (PEN)
and perfluoropentacene (PFP). In contrast, the crystal structure
analysis reveals a different packing motif than for the parental
molecules, which can be related to distinct F…H interactions identified
in the corresponding Hirshfeld surface analysis and also affects solid
state properties such as the exciton binding energy and the
sublimation enthalpy.
Molecular-based organic electronics is a rapidly growing field of
technology that promises the fabrication of flexible and cost-
effective optoelectronic devices or sensors[1] and has expressed
the need to explore new molecular materials with specific
characteristics.[2] While electronic properties of molecules can be
variably tailored through appropriate design, the optoelectronic
properties of molecular solids also depend crucially on the
molecular packing and intermolecular coupling.[3-4] Nowadays, the
electronic properties of proposed molecules can be calculated
precisely,[5] whereas their concrete synthesis often remains a big
challenge. Predicting crystal structure and packing motifs of such
van der Waals bound molecular solids is also difficult since small
molecular modifications can induce drastic changes of the
packing motifs, leading to significantly altered optoelectronic solid
state properties.[4,6-9] Among the organic semiconductors (OSC),
acenes are frequently studied because their aromatic frame
enables versatile control of the electronic structure through their
topology (e.g. length, branching, etc.) as well as chemical
[a]
P. E. Hofmann§, M. W. Tripp§, Y. Grell§, A. L. C. Schiller, Dr. S. I.
Ivlev, Prof. Dr. U. Koert*
Fachbereich Chemie
Philipps-Universität Marburg
Hans-Meerwein-Straße 4, 35043 Marburg, Germany
E-mail: koert@chemie.uni-marburg.de
D. Bischof§, Dr. T. Breuer, Prof. Dr. G. Witte*
Fachbereich Physik
[b]
[§]
Philipps-Universität Marburg
Figure 1. (a) Synthetic targets in this study and electrostatic contour plots of (b)
PEN, (c) the newly synthesized unilaterally fluorinated pentacene (1) and (d)
PFP obtained by DFT. Packing motifs in the crystalline phase of (e) PEN and (f)
1, together with molecular charge distribution.
Renthof 7, 35032 Marburg, Germany
Email: gregor.witte@physik.uni-marburg.de
These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/….
1
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10.1002/ange.202006489
Angewandte Chemie
COMMUNICATION
Here, we introduce a novel synthetic strategy which can be
applied to realize nearly any kind of substitution. Our new route
provides access to a class of unilaterally substituted acenes,
which is demonstrated for the case of the unilaterally fluorinated
1,2,10,11,12,14-hexafluoropentacene (1) and the related
1,2,9,10,11-pentafluorotetracene (2). Using DFT-based electro-
nic structure calculations, X-ray absorption spectroscopy
(NEXAFS) and UV/Vis measurements, we show that 1 exhibits a
truly bivalent behavior between PEN and PFP and has a distinct
dipole moment of 6.6D along the molecular M-axis. Due to the
changed electrostatic potential at the molecular rim, a different
packing motif stabilized by F…H bonds can now be expected, as
shown in Fig. 1f, which leads to a significant modification of the
solid state properties, in particular the exciton binding energy and
sublimation enthalpy.
to aldehyde 13. Reduction to the corresponding alcohol and Appel
reaction resulted in benzylic bromide 14.
a) Tf2O, pyridine
b) CO, Pd(OAc)2,
dppf, Et3N, MeOH
c) [Ir(OMe)(cod)]2,
F
F
O
F
F
O
L1, B2pin2
F
F
OMe
OMe
89%
93%
Bpin
11
12
SiHiPr2
F
F
F
OH
L1 =
Ph
P
Ph
10
F
F
f) NaBH4
g) NBS, PPh3
d) MOMCl
e) nBuLi; DMF
F
F
F
OMOM
F
OMOM
Br
H
O
65%
87%
13
14
Most known syntheses of symmetrically substituted pentacenes
of type 3 use symmetrical intermediates such as 4 and 5 which
are accessible by [4+2] cycloadditions (cf. Scheme 1).[13,15,16] Due
to regioselectivity problems within the [4+2] cycloadditions
unilaterally substituted pentacenes 6 need a different approach
with two bonds being formed in subsequent steps. Closure of the
central ring via the red marked bond could be possible from a key
intermediate 7, which should be accessible in a convergent
manner from two naphthalene building blocks 8 and 9 by
formation of the blue marked bond.
F
F
O
F
F
O
H
h) Pd(PPh3)4,
Cs2CO3
i) TFA
F
F
OMe
k) DIBAH
l) (COCl)2, DMSO,
Et3N
j) Tf2O, pyridine
OTf
OTf
F
12
14
+
54%
71%
16
15
F
F
F
F
F
F
F
F
F
F
F
m) Ni(cod)2, dppp,
quinuclidine
OH
n) MsCl, DBU
85%
F
F
59%
F
F
O
previous works:
symmetric approach
R
R
R
R
17
1
F
F
O
Scheme 2. Synthesis of F6-PEN 1. Reagents and conditions: a) Tf2O (1.2 eq),
pyridine (2.0 eq), CH2Cl2, 0 °C, 50 min; b) CO (1 atm), Pd(OAc)2 (5 mol%), dppf
(10 mol%), Et3N (2.0 eq), DMF/MeOH 9:5, 65 °C, 4.5 h; c) [Ir(OMe)(cod)]2
(2.5 mol%), L1 (5.0 mol%), B2pin2 (1.0 eq), THF, 75 °C, 26 h; d) NaH (1.5 eq),
MOMCl (1.5 eq), DMF, rt, 1 h; e) n-BuLi (1.0 eq), THF/n-pentane 20:9, –78 °C,
6 h; DMF (1.0 eq), –78 °C to rt, 45 min; f) NaBH4 (6.0 eq), THF, rt, 30 min;
g) NBS (2.0 eq), PPh3 (2.0 eq), CH2Cl2, 0 °C, 2 h; h) 14 (1.0 eq.), 12 (1.1 eq),
Pd(PPh3)4 (3.0 mol%), Cs2CO3 (3.0 eq), THF/H2O 10:1, 75 °C, 18 h; i) TFA
(5.0 eq), CH2Cl2, 0 °C to rt, 5.5 h; j) Tf2O (1.2 eq), pyridine (2.8 eq), CH2Cl2, 0 °C,
30 min; k) DIBAH (2.5 eq), THF, 0 °C to rt, 18 h; l) (COCl)2 (1.5 eq), DMSO
(3.0 eq), CH2Cl2, 78 °C, 30 min; Et3N (5.0 eq), –78 °C, 30 min; rt, 1 h;
m) Ni(cod)2 (1.0 eq), dppp (1.2 eq), quinuclidine (1.0 eq), toluene, 70 °C, 43 h;
n) MsCl (3.0 eq), DBU (5.0 eq), CH2Cl2, 0 °C to rt, 1 h; 40 °C, 2 h.
R
R
R
R
4
R
R
R
R
3
5
this work:
unilateral approach
A
B
R
R
R
R
R
R
R
R
R
R
R
R
6
7
A Suzuki coupling of both naphthalenes 12 and 14 provided the
desired linkage of both building blocks. Subsequent cleavage of
MOM ether with TFA and installation of a triflate gave methylene
bridged bisnaphthalene 15. Reduction of methyl ester and Swern
oxidation led to aldehyde 16. With 16 in hand, the closure of the
central ring was investigated next. Using conditions for a Ni(0)-
catalyzed intramolecular carbonyl Heck reaction[21] resulted in
complete decomposition of the starting material. Decreasing the
reaction temperature to 70 °C gave alcohol 17 in low yield,
probably as a product of a Barbier-type reaction instead of a
carbonyl Heck reaction (see Supp. Inf.). Stoichiometric addition of
Ni(0) reagent in combination with dppp and quinuclidine in toluene
A
B
R
C
R
D
+
R
R
R
R
8
9
Scheme 1. Different synthetic routes to symmetrical pentacenes
unilaterally substituted pentacenes 6.
3 and
The synthesis of unilaterally substituted hexafluoropentacene 1
started from trifluoronaphthol 10 as common precursor for both
building blocks (see Scheme 2).[17] Triflation of naphthol 10 fol-
lowed by carbonylative cross coupling[18] gave the methyl ester 11,
which was subjected to an iridium catalyzed directed ortho C-H
borylation to deliver the boronate 12.[19] Alternatively, MOM-
protection of naphthol 10 allowed for an ortho-lithiation[20] leading
at 70 °C provided 17 in good yield.
A
subsequent
mesylation/elimination resulted in the formation of the
hexafluoropentacene 1. 1H- and 19F-NMR analysis of 1 was
performed in naphthalene-d8 at 368 K (95 °C) which showed all
the significant proton- and the expected fluorine signals,
respectively.
2
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No fluorine substituent was chosen at C13 in the central ring of 1
because of the known chemical instability of this position which is
causing a partial defluorination of PFP upon contact with metal
substrates.[22] Following the above described route, related
1,2,9,10,11-pentafluorotetracene (2) was synthesized too (see
Supp. Inf.).
As for device applications the solid state properties are more rele-
vant, we also carried out UV/Vis absorption measurements on
vapor-deposited molecular films. As depicted in Fig. 2a (dark
lines), the absorption spectra reveal a new band below the
HOMO-LUMO transition, due to excitonic excitations in the
molecular solid. The exciton binding energies (ΔE), which can be
approximated by the difference between the lowest energy
excitations in solution (Egap) and solid, is significantly smaller in 1
(150 meV) compared to PEN (320 meV) and PFP (210 meV),
hence reflecting significant difference in the solid state electronic
properties. To rationalize this effect a crystal structure analysis is
required. Since the low solubility of 1 hampers conventional
crystallization from solution, we have instead employed liquid
assisted crystallization techniques using ionic liquids,[24] which
yields distinct mesoscopic single crystals (cf. Fig. 3b) and thus
enabled a crystal structure analysis by X-ray diffraction (for details
see Supp. Inf.). While PEN and PFP adopt a herringbone-
arrangement in their bulk structure,[8] the novel molecules 1 as
well as 2 crystallize in a criss-cross packing motif with a dipole
parallel packing, as depicted in Fig. 3a (see also Supp. Inf.). This
contradicts the expectation of a compensation of the dipoles by
antiparallel stacking, as observed for acenes partially fluorinated
along the short side.[13]
Next, we report on the physico-chemical characterization of 1,
considering first the electronic molecular properties. UV/Vis
solution spectra yield a HOMO-LUMO gap of 2.11eV, which is
very similar to that of the parental molecules (PEN: 2.13eV, PFP:
1.99eV, cf. Fig. 2a). This may appear surprising at first glance,
since fluorination is expected to have notable impact on the
molecular energy levels. Accompanying DFT-calculations reveal
indeed a distinct energetic shift of the frontier orbitals of 1 which
are located between those of PEN and PFP (cf. Supp. Inf.).
However, as this affects both the HOMO as well as the LUMO,
the optical gap is not significantly altered upon partial fluorination,
an effect which was found before also for other aromatic
molecules.[23] Complementary information on the unoccupied
states were obtained from NEXAFS-spectroscopy. The C1s-
NEXAFS spectrum (cf. Fig. 2b) exhibits characteristic sharp π*-
resonances (corresponding to transitions from core levels into
unoccupied π-orbitals, associated with the LUMO, LUMO +1, …
levels) and broad resonances due to transitions into unoccupied
σ-orbitals. Comparison of the π*-region of the differently
fluorinated pentacenes (cf. Fig. 2c) shows that the π*-resonances
of 1 are well described as a superposition of the respective
signatures of PEN and PFP, thus demonstrating that it exhibits
final states with mixed character of both parental acenes. This
conclusion is corroborated by DFT calculations of the frontier
orbitals (see Supp. Inf.), unveiling 1 as a connecting link between
PEN and PFP in terms of single molecular electronic structure.
Figure 3. (a) Crystal packing of 1, (b) optical micrograph of crystals of 1,
(c) Hirshfeld surface and (d) corresponding fingerprint plot of 1.
A more detailed insight into the intermolecular interactions leading
to this packing motif is provided by Hirshfeld surface plots and
two-dimensional fingerprint spectra of 1 (for computational details
and comparison with other acenes see Supp. Inf.).[25] Fig. 3c
depicts the Hirshfeld surface of 1, where the red dots visualize
regions of strong intermolecular interactions, which can be
associated to F…H hydrogen bridge bonds. This is further
evidenced by the statistical analysis of all atomic contact points
between neighbouring molecules in the crystal, in form of a corre-
sponding 2D fingerprint plot as shown in Fig. 3d. Contrary to
fingerprint plots of PEN and PFP (see ref. 9 and Supp. Inf.) in case
of 1 conspicuous spikes appear, which reflect the F…H
interactions and provide approximately 45% of all intermolecular
contacts. This strongly indicates that the fluorine-hydrogen
interactions govern this packing motif, acting as mediators for the
alignment of molecules in their solid state.[26] Interestingly, also
strong C...C interactions are observed, which can be ascribed to
the π-stacked like packing along the b-axis and might indicate
larger intermolecular electronic coupling than in pentacene.[7,27]
To quantify the overall strength of intermolecular van der Waals
Figure 2. (a) UV/Vis spectra of PEN, 1 and PFP in solution (saturated solution
in CH2Cl2) and for solid films evaporated onto glass substrates with labelled
maximum of lowest absorption band (Egap) and exciton binding energies (ΔE).
(b) C1s-NEXAFS spectrum of a 1 thin film prepared on SiO2. (c) Comparison of
the leading C1s-NEXAFS resonances of PEN, 1 and PFP thin films.
3
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COMMUNICATION
S. C. B. Mannsfeld, A. P. Zoombelt, Z. Bao, A. Aspuru-Guzik, Nat.
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interactions, the sublimation enthalpy of 1 was determined using
the Knudsen method (for details see Supp. Inf.). Surprisingly, the
determined value ∆Hsub = 121.3±7.5 kJ/mol is considerably
smaller than the value of PEN (∆Hsub = 156.9±13.6 kJ/mol).[28]
This effect can be rationalized by the different quadrupole
moments of these molecules (see Supp. Inf.) and demonstrates
that additional electrostatic and F…H interactions influence the
molecular packing motif but at the same time can reduce the
dispersion interaction. A similar situation was found before for
oxo-species of PEN,[29] which reveal similar packing motifs as 1.
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In conclusion, we have introduced a novel synthetic route
enabling the realization of
a new class of unilaterally
functionalized acenes, which was demonstrated by the synthesis
of pentacene and tetracene derivates with unilateral fluoro-
substitution patterns. While these compounds reveal single
molecule properties which can be considered as intermediates
between the non and perfluorinated parental acenes, they also
show distinctly different solid state properties, emphasizing the
necessity to characterize not only single molecular electronic but
also solid state electronic properties of newly synthesized
materials. Especially when employing such new materials in solid
thin film devices. The novel synthetic strategy not only provides
access to unilateral substitution patterns but is also applicable to
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Acknowledgements
We acknowledge support by the German Science Foundation
(Grant SFB 1083, TP A2 and A8) and thank the Helmholtz Center
Berlin (electron storage ring BESSY II) for provision of synchro-
tron radiation at the beamline HE-SGM. The contribution of Dr.
Klaus Harms and Dr. Matthias Conrad to the X-ray structural
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Conflict of interest
The authors declare no conflict of interest.
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Keywords: acene synthesis • unilateral substitution • electronic
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10.1002/ange.202006489
Angewandte Chemie
COMMUNICATION
Entry for the Table of Contents
Regioselective functionalization of acenes: A novel synthetic strategy is introduced that enables a regioselective functionalization
of acenes, which is demonstrated using the example of unilaterally substituted fluoroacenes. The packing motif and optoelectronic
solid state properties of these new materials are compared with the properties of their parental acenes.
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This article is protected by copyright. All rights reserved.