Synthesis and Characterization of Polysubstituted Dibenzopyrenes as Charge-Transporting Materials

Article abstract of DOI:10.1021/acs.orglett.6b02354

A new class of benzopyrene-based semiconducting molecules is prepared and characterized. A four-step protocol involving Suzuki coupling and aromatic dehydrogenation reactions renders the new unsymmetrical framework. Introduction of various substituents at the dibenzopyrene framework modulates mainly the optoelectronic properties rather than the packing motif. Single-crystal field-effect transistors fabricated from these materials show a mobility ranging from 0.7 to 3.2 cm²/(V s). The highest mobility, 3.2 cm²/(V s), with an on/off ratio of 10⁴-10⁵ was achieved for 11-methoxy-8-(4-methoxyphenyl)dibenzo[a,e]pyrene.

Full text of DOI:10.1021/acs.orglett.6b02354

Letter
pubs.acs.org/OrgLett
Synthesis and Characterization of Polysubstituted Dibenzopyrenes
as Charge-Transporting Materials
Sushil Kumar, Man-Tzu Ho, and Yu-Tai Tao*
†Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan
S
* Supporting Information
ABSTRACT: A new class of benzopyrene-based semiconducting molecules is prepared and characterized. A four-step protocol
involving Suzuki coupling and aromatic dehydrogenation reactions renders the new unsymmetrical framework. Introduction of
various substituents at the dibenzopyrene framework modulates mainly the optoelectronic properties rather than the packing
motif. Single-crystal field-effect transistors fabricated from these materials show a mobility ranging from 0.7 to 3.2 cm2/(V s). The
highest mobility, 3.2 cm2/(V s), with an on/off ratio of 104−105 was achieved for 11-methoxy-8-(4-methoxyphenyl)dibenzo-
[a,e]pyrene.
he new era of flexible electronics is right around the
with high π−π stacking interaction is difficult.7 A post analysis
Tcorner, presumably due to the fast development of various of the single-crystal structure may sometimes provide a clue to
the property observed.
organic electronic materials and devices. In these advance-
ments, organic field-effect transistors (OFETs) are a critical
component in most of the organic electronic devices.1
Tremendous effort has been devoted to the search for high-
performance field-effect transistor materials.2 Much progress
has been made in the quest for suitable organic materials for
transistor applications, and general guidelines for molecular
design have been proposed.3 That is, a higher electronic
coupling between neighboring molecules in the conducting
channel as well as a lower reorganization energy for the
molecule involved will be favored for charge transport in
molecular materials. Toward that aim, extended conjugation
and rigid aromatics are being explored. However, a clear
structure−property correlation is elusive due to the compli-
cated interplay of film morphology, molecular packing, as well
as electron distribution in the frontier molecular orbitals, all of
which affect the electronic coupling between neighboring
molecules.4 More systematic studies with new materials are
needed to unravel the correlation between structure and
properties.5
After the seminal reports of Clar and Schiedt on unsym-
metrical dibenzopyrene,8 its derivatives or even the parent
framework was seldom studied for general transistor
applications. Recently, thin-film OFETs of pyrene-based
materials were examined, giving some promising results.9
These facts prompt us to design and synthesize a new series
of benzopyrene-embedded derivatives carrying various func-
tional groups and examine their potential in OFET application.
The electronic properties and field-effect performances of
selected derivatives show good potential for these molecules to
be used in transistor applications.
The synthetic plan of the derivatives is illustrated in Schemes
1 and S1 (SI). The methodology used for the selected
benzopyrenes is a simple and four-step chemical conversion of
anthracene10 to the target polycylic aromatic hydrocarbons.
The aryl olefins (4−10) were obtained from dichloro olefin (3)
and different aryl boronic acids via a Suzuki coupling reaction in
excellent yields Schemes 1 and S1. To synthesize the final
benzopyrene derivatives (DBPs), the aryl olefins were
subjected to iodine-mediated (1.2 equiv of iodine) aromatic
dehydrogenation reaction by exposing the reaction mixture to
UV light for 6−24 h.
Polyfused aromatic hydrocarbons are widely used as
transistor materials.6 An extensive conjugate π-system and the
π−π stacking interactions impart semiconducting properties to
these molecules. For the same framework of molecules, the
crystal packing as well as the molecular orbital levels may be
perturbed by substitution with electron-donating or accepting
functional groups of different sizes. As compared to sym-
metrical polyaromatics, the crystal packing for unsymmetrical
ones are less predictable, so that a rational design of molecules
The iodine-mediated aromatic coupling exhibited high
selectivity for the dehydrogenation reaction with respect to
the ring that was annulated, presumably due to the more stable
Received: August 7, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02354
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Letter
Scheme 1. Synthesis of Benzopyrene (DBP) Derivatives
intermediate involved (Schemes S2 and S3). These coupling
reactions are widely used to construct the featured product
frameworks in excellent yields with or without the use of
methyloxyrane, depending on the reactivity of aryl olefin
chosen for the coupling reaction.11 Besides iodine, a variety of
reagents, such as iron chloride, aluminum chloride, DDQ, and
others, have also been explored for aromatic dehydrogenation
reactions.12 Except for our report on the Scholl reaction,10
previous works describing oxidative aromatic coupling proto-
cols focused more on highly symmetrical molecules, while
those used for highly selective coupling could not avoid the
formation of undesired products.13 Indeed, the chosen reaction
protocol here furnished single-fused benzopyrene products in
higher isolated yields, irrespective of the functional group it
carried. Nevertheless, a shorter reaction time (6 h) was needed
for derivatives carrying electron donors such as methoxy groups
with enhanced reaction yields (85−92%). The chemical
structures proposed for these benzopyrene derivatives were
established by NMR, high-resolution mass, and single-crystal X-
ray analyses (Figure S36a−e).
The physical vapor transport (PVT) method was used to
grow single crystals that are amenable to X-ray analyses and
single-crystal FET device fabrication. Among the compounds,
Met-DBP2 and TMet-DBP exhibited typical melting behavior,
but their vaporization and crystallization were not successful.
The compound DBP and others crystallized as yellow, needle-
shaped crystals. The X-ray crystal analyses show that the DBP
molecule exhibits a near-planar polyfused aromatic core,
whereas the unfused phenyl group twists away from the central
core plane due to steric interactions (Figure 1a, and Figure
S37c). The torsional angle for the unfused aryl unit varies in a
small range from ∼67° to ∼71° for the DBP derivatives,
depending on the substituent. All of the DBP derivatives show
slipped cofacial packing between the fused aromatic blocks with
an interplanar distance of ∼3.3−3.7 Å (Figure 1a−j). This
packing direction coincides with the long axis of the needle, as
indexed from X-ray diffraction (Figure S38). Introducing
fluorine and methoxy groups brings about additional van der
Waals interactions and helps create stronger packing arrange-
ments in the crystal. In Flu-DBP and HFlu-DBP molecules, the
CH···F interaction results in both electrostatic interactions
between fluorinated phenyl rings and a short contact for C···F
Figure 1. (a−j) Molecular packing showing π−π stacking distance in
DBP (3.6 Å), Flu-DBP (3.3 Å), Met-DBP1 (3.6 Å), TFlu-DBP (3.7
Å), and Me-DBP (3.4 Å).
(2.6 Å) (Figures S36 and 37). The methoxy group has a short
contact with the polyaromatic segment of the neighboring
molecule at a distance of 2.7 Å due to a shift in the polyfused
cores. The small differences in stacking in polyaromatic
molecules will also influence their photophysical, electro-
chemical, and charge-transporting properties.14
The benzopyrene derivatives give light yellow to yellow
colors in their solid states. They are very soluble in solvents like
dichloromethane, chloroform, and benzene, except for HFlu-
DBP, which is sparingly soluble in these solvents. The
absorption spectra of the derivatives as measured in their
dilute dichloromethane solutions are given in Figures 2 and
S39a with the related data presented in Table S1. The parent
derivative DBP absorbs light at 385, 366, 307 (max), 296, and
278 nm, respectively. As compared with that of the smaller
parent compound pyrene,15 the two sets of absorption bands in
the region of 250−330 and 330−420 nm are assigned as from
the excitations occurring in longer and shorter axes of the DBP
Figure 2. Normalized absorption/emission spectra of benzopyrenes as
recorded in dichloromethane.
B
DOI: 10.1021/acs.orglett.6b02354
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Letter
molecule (Figure 3a). The HOMO orbitals are primarily
localized over benzopyrene, and the absorption pattern of the
Table 1. Electro-optical and Thermal Properties of
Benzopyrene Derivatives
c
b
EHOMO /ELUMO
a
d
e
compd
DBP
λabs/ λem (nm)
(eV)
Eg (eV) Td (°C)
307/422
307/416
309/422
311/445
309/431
5.58/2.46
5.65/2.57
5.53/2.45
5.40/2.40
3.12
3.08
3.08
3.00
3.06
378
405
395
336
431
Flu-DBP
Me-DBP
Met-DBP1
HFlu-DBP
5.82/2.76
a
b
Maxima of absorption and emission. EHOMO = 4.8 + Eox (oxidation
potential of compound with respect to ferrocene, Eox = Ecompound
−
c
d
EFc). ELUMO = EHOMO − Eg. Optical bandgap, Eg, was obtained from
e
the intersection of absorption and emission spectra. Thermal
decomposition temperature measured at 10% weight loss.
substitution from two to six fluorine atoms in Flu-DBP and
HFlu-DBP, respectively. Band gaps of the benzopyrenes were
determined from their absorption edges, and the LUMO
energies were obtained by subtracting the optical band-gaps
from the HOMO values. Low-lying LUMO levels were
obtained for HFlu-DBP having multiple fluorine atoms
attached to the polyaromatic framework. Like redox stability,
thermal stability of polyaromatics is also vital for OFET device
stability and lifetime.18 The benzopyrene derivatives displayed
good thermal stability, and their thermal decomposition
temperatures were >300 °C (Table 1 and Figure S40).
Embedding fluorine atoms at the polyfused framework
successively enhances the thermal stability of HFlu-DBP
among the DBP series.
To explore the potential of these benzopyrene-based
aromatics as channel material, single-crystal field-effect
transistors (SCFETs) were fabricated from these derivatives.
Thus, a top-contact, top-gate SCFET was fabricated by
laminating the needle-like crystal on a glass substrate. Painted
colloidal graphite was used as the source and drain. Parylene
film was grown on the crystal as the dielectric, while colloidal
graphite on the top was the gate electrode. The channel length,
width, and parylene thickness were 1.0−0.5 mm, 0.25−0.20
mm, and 1.8−2.5 μm, respectively. The derivatives demon-
strated p-carrier behavior as judged from transfer curves (Figure
3c,d, Figures S42 and S43, and Table S1).
Four to eight devices were fabricated for these molecules. An
average mobility of 2.1 cm2/(V s) with a maximum mobility of
3.2 cm2/(V s) was observed for Met-DBP1-based devices. The
Flu-DBP gave a lower average mobility of 0.54 cm2/(V s), with
a maximum mobility of 0.7 cm2/(V s). HFlu-DBP, while
providing nice crystals, gave no measurable mobility. Even
though similar packing motifs were adopted, the derivatives
gave widely different mobilities, which may attest to the high
sensitivity of the mobility to the perturbation in the packing
arrangement and thus electronic coupling.
In conclusion, we have successfully synthesized a new series
of unsymmetrical and polyfused aromatics comprising pyrene
as the central core. These derivatives were characterized by
spectroscopic techniques such as NMR and MALDI-TOF mass
techniques, while crystal XRD analyses were obtained for
selected samples. The derivatives showed a high p-type SCFET
performance of 0.7−3.2 cm2/(V s) in the single-crystal OFET
configuration. The derivative with para-linked methoxy groups
gave the highest field-effect mobility of 3.2 cm2/(V s) due to a
facile charge injection to the channel material.
Figure 3. (a, b) HOMO plots of DBP and Met-DBP1, (c, d) current−
voltage (IDS − VGS) curves for benzopyrenes.
DBPs is similar to the known benzo[e]pyrene framework;16
thus, the absorption features in these derivatives are likely to
originate from the benzo[e]pyrene portion. A marginal
bathochromic shift in the absorption maxima was observed
by embedding electron donors such as methoxy at the
dibenzopyrene framework. The derivatives show blue to blue-
green emission in their dilute solution. The DBP shows two
emission bands at 539 and 422 nm, while its completely fused
analogue gives only one emission band at 493 nm.10 These two
emission bands are possibly originated from the excited state
with a twisted aryl unit attached to the main aromatic core and
its relaxed form to a planar and low-energy form. This is a
structural and intramolecular feature rather than the
intermolecular excimer formation among DBP molecules
(Figure S40a,b). It is noticed that the methoxy-substituted
derivative gives single emission band. We suggest that a good
donor restricts the relaxation to the planar excited state. The
intramolecular origin of both emission bands in DBP was
further supported by excitation of DBP at 430 and 550 nm
wavelengths (Figure S40c), producing an excitation spectrum
similar to that of the absorption spectrum. There was a slight
bathochromic shift in the emission maxima with methoxy- and
fluorine-substituted DBPs, possibly due to a polar effect on the
excited state involved (Figures 2 and S39b).
The oxidation potentials together with HOMO−LUMO
energies of these compounds are compiled in Tables 1 and S1,
with their oxidation waves sketched in Figure S39c,d. Most
compounds displayed single quasi-reversible oxidation waves,
whereas TMet-DBP having multiple electron donors gave
multiple oxidation waves. The oxidation potential values (Eox)
for these derivatives range from 0.59 to 1.02 V. The oxidation
waves were possibly generated from oxidation of the parent
fused core, and Eox values were influenced by both the character
and number of substituents at the polyaromatic frame-
work.12a,17 Thus, the oxidation potential was lowered by
introducing electron donors such as methoxy groups in Met-
DBPs and TMet-DBP, while it was raised by changing the
C
DOI: 10.1021/acs.orglett.6b02354
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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Detailed experimental procedures and complete spectro-
scopic analyses (PDF)
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(b) Grzybowski, M.; Skonieczny, K.; Butenschon, H.; Gryko, D. T.
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AUTHOR INFORMATION
Corresponding Author
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2744. (b) Zhou, Y.; Liu, W.-J.; Zhang, W.; Cao, X.-Y.; Zhou, Q.-F.; Ma,
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*E-mail: ytt@chem.sinica.edu.tw. Fax: +886-2-27831237. Tel:
+886-2-27898580.
Notes
The authors declare no competing financial interest.
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Denmark, S. E.; Stoddart, J. F. J. Am. Chem. Soc. 2014, 136, 10669−
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ACKNOWLEDGMENTS
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Financial support from Ministry of Science and Technology,
Taiwan (Grant No. 103-2120-M-009-003-CC1) is gratefully
acknowledged.
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