Electron-transporting PAHs with dual perylenediimides: Syntheses and semiconductive characterizations

Article abstract of DOI:10.1039/c3cc40704g

An effective method was developed to prepare triphenyleno[1,2,3,4-ghi] perylenediimide derivatives, via ICl-induced annulation, dehalogenation, followed by photocyclization. A perylenediimide (PDI) dimer featuring a terphenyl bisethynylene linker was thereby transformed into a benzo[k]tetraphene fused with two benzoperylenediimides. These PDI derivatives exhibited electron mobility up to 0.079 cm² V^-1 s^-1 in solution-processed thin film transistors. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc40704g

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Electron-transporting PAHs with dual
perylenediimides: syntheses and semiconductive
Cite this: DOI: 10.1039/c3cc40704g
characterizations†
Received 28th January 2013,
Accepted 18th February 2013
Zhuoran Zhang, Ting Lei, Qifan Yan, Jian Pei* and Dahui Zhao*
DOI: 10.1039/c3cc40704g
www.rsc.org/chemcomm
An effective method was developed to prepare triphenyleno[1,2,3,4-ghi]- Various PDI derivatives were synthesized exhibiting attractive optical
perylenediimide derivatives, via ICl-induced annulation, dehalogenation, and electronic properties.5–8 PAH derivatives comprising multiple
followed by photocyclization. A perylenediimide (PDI) dimer featur- interconnected PDI units with a low-lying LUMO are promising
ing a terphenyl bisethynylene linker was thereby transformed into a high-performance n-type semiconductors.9,10 The preparation of rele-
benzo[k]tetraphene fused with two benzoperylenediimides. These vant molecules generally involves coupling PDI units with various
PDI derivatives exhibited electron mobility up to 0.079 cm2 VÀ1 sÀ1 in conjugative linkages.5–7 To obtain polycyclic skeletons, efficient ligation
solution-processed thin film transistors.
and fusion protocols, particularly those applicable to electron-deficient
aromatic substrates, are necessary. Iodonium-induced cyclization,
Solution-processed organic field-effect transistors (OFETs) have developed by Swager et al. and Larock et al., is an effective annulation
attracted tremendous interest due to the appealing merits of low cost, protocol, producing iodo-substituted polycyclic molecules.11,12
large flexible devices.1 In addition to charge transporting ability, the Recently, the scope of the ICl-annulation protocol was further
film-forming property of active materials is another critical factor expanded to electron-deficient substrates in our laboratory, establish-
governing the device performance of solution-processed OFETs. ing a facile synthetic approach to achieving larger PAHs from PDI.13
Amorphous films with optimal homogeneity are more often attained
We now report the syntheses of a series of new electron-deficient
from conjugated polymers.2 Compared with the polymeric materials, PAHs 1–3, which are all obtained using PDI as the main modular
small organic molecules bestow superior charge carrier mobility by unit (Scheme 1). Compound 1 is a PDI dimer featuring terphenyl
virtue of the more ordered molecular packing structures. Additionally, bisethynylene as the linker to join two PDI units. PAH 2, a different
small molecules are easier to purify, thus offering more stable device PDI dimer having a benzo[k]tetraphene linker, is obtained when ICl-
performance and reproducible production quality. However, the induced annulation is carried out on 1, followed by dehalogenation.
evident crystallization propensity of small molecules typically curtails Subsequently, photocyclization of 2 accomplishes a large PAH 3,
the homogeneity of solution-processed films, which greatly limits which comprises a central benzo[k]tetraphene structure fused with
their performance in OFETs.3 Thus, developing organic molecules two benzo[ghi]perylenediimide moieties. The good solubility and
possessing properly balanced tendencies to form ordered molecular film-forming property displayed by all three molecules allow the
packing structures and to achieve thin films with long-range homo- fabrication of OFETs through solution processing. Molecules 1 and 3
geneity upon solution processing is a pivotal issue for acquiring high- afford electron mobility up to 0.079 and 0.014 cm2 VÀ1 sÀ1
,
performance OFET materials. Large polycyclic aromatic hydrocarbons respectively. The results demonstrate the great potential of synthe-
(PAHs) with flexible side chains at the periphery are promising sized molecules to serve as n-type semiconductors. Moreover, the
candidates to manifest such properties.
synthetic methodologies developed herein provide effective tools for
A low LUMO level is an indispensable requisite for air-stable making new, various electron-deficient PAHs.
n-channel semiconductive materials. Due to its electron-
Specifically, molecule 1 is a PAH containing a terphenyl group
deficient characteristic, PDI emerged as a useful structure in bearing two PDI units tethered to the central phenyl ring through
designing new electron-transporting organic semiconductors.4 acetylene spacers. The molecule was synthesized via double Sonoga-
shira cross-coupling of bromo-substituted PDI 4 with di(4-tert-butyl-
Beijing National Laboratory for Molecular Sciences, the Key Laboratories of Polymer phenyl)diethynylbenzene 514 (Scheme 1). Compound 1 was designed
Chemistry and Physics and of Bioorganic Chemistry and Molecular Engineering of
to undergo ICl-induced annulation subsequently. According to pre-
the Ministry of Education, College of Chemistry, Peking University, Beijing 100871,
vious studies, addition of ICl to the triple bonds in 1 could possibly
China. E-mail: dhzhao@pku.edu.cn, jianpei@pku.edu.cn; Fax: +8610 62751708;
proceed along two different cyclization pathways.12,13 That is, the
Tel: +8610 62753973
annulation could take place with either the terphenyl or PDI groups.
Since tert-butylphenyl was much more electron-rich and reactive
† Electronic supplementary information (ESI) available: Syntheses, characteriza-
tion data, and DFT calculations. See DOI: 10.1039/c3cc40704g
c
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Fig. 1 UV-Vis absorption (in CH2Cl2) and emission spectra (inset, in CHCl3) of 1
and 3, in comparison with those of PDI.
steric congestion near the bay region of the PDI groups
(see Fig. S11 (ESI†) for DFT calculated structures).
After screening a set of different conditions, compound 3 was
eventually attained by exposing a solution of 2a to UV-light irradiation
at room temperature. The conversion of this photocyclization process
was about 60%. Nonetheless, no such cyclization reaction was observed
for 2b under the same conditions. In light of the fact that isomer 2b
could be converted to 2a at elevated temperature under oxygen-free
conditions, we designed an experiment to optimize the overall reaction
yield by combining the isomerization and photocyclization steps in one
pot. Specifically, a mixture of 2a and 2b was irradiated by UV light in
refluxed toluene under an N2 atmosphere. This strategy effectively
Scheme 1 Syntheses of studied compounds.
toward electrophilic addition than electron-deficient PDI, we expected improved the total yield from 18% to 68% for the two steps.
that double cyclization on the terphenyl moiety could be achieved Compounds 1 and 3 both showed broad absorptions spanning
selectively by controlling the reaction temperature. After several trials nearly the entire visible region. The absorption onsets of 1 and 3
at different temperatures, molecule 6 was found to be most efficiently shifted bathochromically by ca. 50 nm compared to that of PDI (Fig. 1),
generated at about À40 1C, by treating 1 with two equivalents of ICl, resulting from their extended p-skeletons. Molecule 3 manifested a
with an isolated yield of 89%. No significant by-product was observed band shape similar to that of coronenediimide,13,21 consistent with its
under such conditions. In the literature, the ICl annulation could be cyclized structure. The absorption maximum lmax of 2a did not differ
triggered at a much lower temperature of À78 1C for alternative significantly from that of PDI (Fig. S1, ESI†), but the band shape
substrates.15 A higher temperature was required for the reaction of became much broadened and featureless. This reflects the sterically
compound 1, most likely because the CRC triple bonds in 1 were encumbered nature of 2a. Compounds 1 and 3 displayed relatively
deactivated by the electron-withdrawing PDI units. Whereas when high emission quantum yields of 0.66 and 0.72, with emission maxima
this reaction was carried out at higher temperatures, e.g. near room
lmax of 614 and 591 nm, respectively. Molecule 3 displayed a smaller
temperature, evident side reactions were found to occur.
Stokes shift and more pronounced vibrational structures, in agreement
At first we attempted to obtain the final product 3 directly with its more rigid p-scaffold. The cyclic voltammograms (CV) of 1 and
from 6 via intramolecular Ullmann-type C–H transformation or 3 both exhibited two reversible reduction waves (Fig. S3 and S4, ESI†).
Heck coupling reaction.7,16–19 Unfortunately, no direct cycliza- The half-wave reduction potentials were determined to be À1.08/
tion product was observed. Instead, reductive dehalogenation À1.30 V for 1 and À1.26/À1.48 V for 3 (Table 1).
occurred under the applied conditions and afforded product 2
OTFT devices were then fabricated using these newly synthesized
(Scheme 1).20 The H NMR spectrum of the resultant reaction molecules, with a top-gate/bottom-contact configuration. The semi-
mixture revealed that two major products with nearly identical conductive layer was deposited by spin-casting dichlorobenzene
chemical structures were generated with similar quantities. solutions of respective compounds (at 8 mg mLÀ1) on patterned
Nonetheless, only one of them (2a) was isolatable, the structure
1
of which was fully characterized and confirmed. The other
product, 2b, which was of slightly lower polarity as shown by thin
layer chromatography, was unstable under ambient conditions.
The two molecules, 2a and 2b, were suspected to be stereoisomers
of each other according to their very similar 1H NMR spectra. This
hypothesis was further corroborated by the observation that the
unstable product 2b was partially converted to the stable product
2a upon heating at 120 1C in DMF under a nitrogen atmosphere.
Table 1 Photophysical, electrochemical, and FMO data of 1 and 3
max/nm Ega/eV Er1b/V Er2b/V LUMOc/eV HOMOd/eV
l
PDI 525
2.30
2.13
2.10
À1.05
—
À3.74
À6.04
À5.85
À5.64
1
3
557
569
À1.08 À1.30 À3.72
À1.26 À1.48 À3.54
a
b
Band gap calculated from the absorption onset in optical spectra.
The 1st and 2nd half-wave reductive potentials (vs. Fc/Fc+) measured by
c
d
CV in CH2Cl2. LUMO energy levels calculated from Er1. HOMO energy
The isomerism of 2a and 2b is speculated to result from the severe levels estimated based on CV-determined LUMO and optical band gaps.
c
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crystallization), along with a lower LUMO, exhibited the most
desirable film morphology and semiconductive capacity.
In conclusion, a new synthetic protocol was developed for
preparing triphenyleno[1,2,3,4-ghi]perylenediimide derivatives,
involving a sequence of ICl-induced cyclization, dehalogenation,
and photocyclization reactions. Using this method, PAH 3 featuring
a benzo[k]tetraphene moiety fused with two benzoperylenediimides
was prepared from terphenyl bisethynylene PDI dimer 1. The
electron-deficient characteristics of these PDI derivatives, in combi-
nation with the optimal film-forming ability, entailed desirable
electron-transporting performance in solution-processed OFET
devices. Electron mobilities up to 0.079 and 0.014 cm2 VÀ1 sÀ1
were achieved for 1 and 3, respectively. The work presented herein
has offered both new structures and potent synthetic tools for
developing large, electron-deficient PAH-based electron-transport-
ing semiconductors.
Fig. 2 The transfer (a, VSD = 100 V) and output characteristics (b) of 1 (red) and
3 (blue), measured from OFET devices (L = 5 mm, W = 100 mm) fabricated with
CYTOP on the top (capacitance Ci = 3.7 nF cmÀ2).
This work was supported by the National Natural Science
Foundation of China (no. 21174004, 51073002, and 21222403).
Au(source/drain electrodes)/SiO2/Si substrates in the glove box. After
thermal annealing the obtained organic films at 160 1C for about
10 min, a poly(perfluorobutenylvinylether) CYTOP solution was spin-
coated on top of the organic films to serve as the dielectric layer
(ca. 500 nm thick). Then an aluminum layer was thermally evapo-
rated to act as the gate electrode. All devices were tested under
ambient conditions. The highest electron mobility detected for
molecule 1 reached 0.079 cm2 VÀ1 sÀ1 (Fig. 2). Compound 3
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c
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Chem. Commun.