A new rod-shaped BODIPY-acetylene molecule for solution-processed semiconducting microribbons in n-channel organic field-effect transistors

Article abstract of DOI:10.1039/c7nj00266a

BODIPY-based π-conjugated small molecules have been extensively studied in various fields of sensing and biochemical labelling; however, their use in organic optoelectronic applications is very limited. A new solution-processable acceptor-donor-acceptor (

Full text of DOI:10.1039/c7nj00266a

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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A New Rod-Shaped BODIPY-Acetylene Molecule for Solution-
Processed Semiconducting Microribbons in N-Channel Organic
Field-Effect Transistors
Mehmet Ozdemir,1 Donghee Choi,2 Yunus Zorlu,3 Bunyemin Cosut,3 Hyungsug Kim,2
Choongik Kim*,2 and Hakan Usta*1
1
Department of Materials Science and Nanotechnology Engineering, Abdullah Gül
University, Kayseri, Turkey
2
Department of Chemical and Biomolecular Engineering, Sogang University, Mapoꢀgu,
Seoul, Korea
3 Department of Chemistry, Gebze Technical University, Gebze, Kocaeli, Turkey
*Correspondence to: Prof. Hakan Usta (Eꢀmail:hakan.usta@agu.edu.tr), Prof. Choongik Kim (Eꢀmail:
choongik@sogang.ac.kr).
1

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Abstract
BODIPYꢀbased πꢀconjugated small molecules have been extensively studied in
various fields of sensing and biochemical labelling; however, their use in organic
optoelectronic applications is very limited. A new solutionꢀprocessable, acceptorꢀdonorꢀ
acceptor (AꢀDꢀA) type small molecule, BDY-PhAc-BDY, consisting of BODIPY πꢀacceptors
and rodꢀshaped 1,4ꢀbisꢀ(thienylethynyl)2,5ꢀdialkoxybenzene πꢀdonor, has been synthesized
and fully characterized as a novel nꢀchannel semiconductor in bottomꢀgate/topꢀcontact
organic fieldꢀeffect transistors (OFETs). The new semiconductor exhibits an electrochemical
band gap of 2.12 eV with highly stabilized HOMO/LUMO energy levels of ꢀ5.68 eV/ꢀ3.56
eV. Singleꢀcrystal Xꢀray diffraction (XRD) analysis of BDY-PhAc-BDY reveals a relatively
low “BODIPYꢀmesoꢀthiophene” dihedral angle (θ = 44.94°), antiparallel πꢀstacked BODIPY
dimers with an interplanar distance of 3.93 Å, and strong “CꢀH···π (2.85Å)” interactions. The
OFET devices fabricated by solution processing shows the formation of highlyꢀcrystalline,
oneꢀdimensional (1ꢀD) microribbons, which results in clear nꢀchannel semiconductivity with
electron mobility of 0.004 cm2/V·s and the on/off current ratio of 105ꢀ106. This is the highest
reported to date for BODIPYꢀbased small molecular semiconductors having alkyne linkages.
Our results clearly demonstrate that BODIPY is an effective πꢀacceptor unit for the design of
solutionꢀprocessable, electronꢀtransporting organic semiconductors and easily fabricable 1ꢀD
semiconductor microꢀ/nanoꢀstructures for fundamental/applied research in organic
optoelectronics.
2

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Introduction
Solutionꢀprocessable πꢀconjugated small molecules allow for the fabrication of thin, lowꢀ
weight, and flexible organic optoelectronic devices such as fieldꢀeffect transistors (OFETs)1,
lightꢀemitting transistors (OLETs)2, and photovoltaics (OPVs)3. They offer significant
physical/chemical advantages over their inorganic counterparts including lowꢀcost,
compatibility with flexible substrates and rollꢀtoꢀroll printing process, and structural
versatility.4–7 In the past several decades, the major advances in the performances of organic
optoelectronic devices have been mainly due to the development of novel semiconducting
structures employing properly designed πꢀarchitectures and the detailed understanding of their
optoelectronic properties.8–14 Therefore, the continued research efforts for the search of new
semiconductor πꢀstructures and elucidating structureꢀpropertyꢀdevice performance
relationships is critical to developing novel optoelectronic platforms with varied
functionalities. To this end, 4,4ꢀdifluoroꢀ4ꢀboraꢀ3a,4aꢀdiazaꢀsꢀindacene (BODIPY) stays as a
relatively unexplored πꢀcore for use in semiconductor structures, despite it has been heavily
studied in the past decades as highly fluorescent functional dyes for chemosensors,
fluorescent switches and biochemical labels.15 BODIPY is a coplanar and highly electronꢀ
deficient πꢀcore with a high dipole moment (ꢁ = 3.4ꢀ4.2 D), which makes it an ideal πꢀ
acceptor unit to build donorꢀacceptor type semiconductor architectures.16,17 Furthermore,
considering its excellent photophysical and photochemical properties, BODIPYꢀbased
semiconductors would be ideal functional materials for lightꢀbased optoelectronic applications
such as OLETs and OPVs.18 However, to date there has been very few reports on BODIPYꢀ
based semiconductor small molecules for optoelectronics with typical charge carrier
mobilities of 10ꢀ5ꢀ10ꢀ3 cm2/V·s.19 To this end, our recent study revealed that properly designed
AꢀDꢀA (acceptorꢀdonorꢀacceptor) molecular πꢀarchitectures employing BODIPY acceptor
unit can induce nꢀchannel semiconductivity with electron mobilities of up to ~0.01 cm2/V·s
3

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and Ion/Ioff ratios of 107ꢀ108, which has been the highest performance reported to date for a
BODIPYꢀbased semiconductor molecule.20 Motivated with our initial promising findings and
to elucidate the full potential of BODIPY πꢀcore in electronꢀtransporting molecules, we are
interested in pursuing new BODIPYꢀbased AꢀDꢀA πꢀarchitectures and exploring their
semiconducting characteristics in nꢀchannel OFETs.
Figure 1. The chemical structure of BDY-PhAc-BDY developed in this study indicating
donor, acceptor, and solubilizing parts.
In this paper, we report the synthesis, characterization, and fieldꢀeffect response of a novel
nꢀchannel semiconductor molecule, BDY-PhAc-BDY (Figure 1). In this structure, rodꢀshaped
1,4ꢀbisꢀ(thienylethynyl)2,5ꢀdialkoxybenzene πꢀarchitecture employs 2ꢀethylhexyloxy
electronꢀdonating groups on the central phenyl ring and terminal highly electronꢀdeficient
BODIPY πꢀunits. Alkyne linkages are employed between stericallyꢀbulky central phenyl core
and thienyl units as a spacer to result in shapeꢀpersistent rodꢀlike structure with further
extended effective πꢀconjugation length.21 Note that, due to its quasiꢀcylindrical electronic
symmetry, alkyne linkages are advantageous to accommodate steric and conformational
constraints.22 The rationale of having BODIPYs linked to fiveꢀmembered thiophene units
from their meso positions is to minimize interꢀring torsions between donor and acceptor units,
and to extend πꢀconjugation along the molecular backbone. Furthermore, 2ꢀethylhexyloxy
lipophilic substituents provide the required solubility in common organic solvents for
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convenient purification and thinꢀfilm fabrication. Donorꢀacceptor backbone provides a low
optical band gap of 2.2 eV with a relatively stabilized LUMO/HOMO energy level of ꢀ3.56/ꢀ
5.68 eV for unipolar nꢀchannel semiconductivity. Singleꢀcrystal Xꢀray diffraction (XRD)
analysis shows important structural features about the new semiconductor. The solutionꢀ
processed BDY-PhAc-BDYꢀbased devices exhibit ꢁe = 0.004 cm2/V·s with Ion/Ioff ratios of
105ꢀ106, which is the highest OFET performance reported so far for BODIPYꢀbased small
molecules having alkyne linkages. Detailed morphological and microstructural thinꢀfilm
analysis reveals the formation of highlyꢀcrystalline oneꢀdimensional (1ꢀD) microribbons
([010] in the outꢀofꢀplane direction) as a result of favorable “ꢀCH···π” and “π···π”
intermolecular interactions. Our results clearly offer crucial guidance for the molecular design
of BODIPYꢀbased semiconductor molecules and easily fabricable microribbonꢀbased thinꢀ
films for nꢀchannel OFETs.
EXPERIMENTAL
Materials and Methods
Schlenk techniques were used in the reactions, and the reactions were carried out under N2
unless otherwise noted. All reagents were obtained from commercial sources and used
without any purification unless otherwise noted. 1H/13C NMR characterizations were
13
performed on a Bruker 400 spectrometer (1H, 400 MHz; C, 100 MHz). Elemental analyses
were done on a LecoTruspec Micro model instrument. MALDIꢀTOF was performed on a
Bruker Microflex LT MALDIꢀTOFꢀMS Instrument. Thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) measurements were performed on Perkin Elmer
Diamond model instruments at a heating rate of 10 °C/min under nitrogen. UVꢀVis absorption
and fluorescence emission measurements were performed on a Shimadzu, UVꢀ1800 UVꢀVis
Spectrophotometer and a Varian Eclipse spectrofluorometer, respectively. The fluorescence
quantum yield was determined in dichloromethane as compared to the fluorescence of
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Rhodamine 6G standart (ΦF = 0.76 in water). Electrochemistry was performed on a C3 cell
stand electrochemical station equipped with BASꢀEpsilon software (Bioanalytical Systems,
Inc. Lafayette, IN). The molecular geometry optimizations and total energy calculations were
carried out using density functional theory (DFT) at the B3LYP/6ꢀ31G** level by using
Gaussian 09.23
Synthesis and Characterization
The synthesis of 5ꢀbromoꢀ2ꢀthiophenecarbaldehyde and 2ꢀmethylpyrrole reagents were
performed in accordance with our previously reported procedures.20
Synthesis of 1,4-dibromo-2,5-bis((2-ethylhexyl)oxy)-2,5-diethynylbenzene (1). A mixture
of potassium carbonate (4.0 g, 28.97 mmol) and 2ꢀethylhexyl bromide (8.24 g, 40.55 mmol)
was dissolved in 40 mL DMF under nitrogen. 2,5ꢀdibromohydroquinone (3.88 g, 14.48
mmol) was added slowly to this mixture, and the resulting reaction solution was stirred at 100
°C for 48 h. The mixture was then cooled to room temperature and quenched with water. The
reaction mixture was extracted with dichloromethane, and the organic phase was washed with
water, dried over Na2SO4, filtered, and evaporated to dryness to give the crude product. The
crude was then purified by column chromatography on silica gel using hexane as the eluent to
1
give compound 1 as a colorless oil (6.9 g, 97%). H NMR (400 MHz, CDCl3) δ 7.09 (s, 2H),
3.83 (d, 4H, J=8.0 Hz), 1.76 (m, 2H, J=12.0 Hz), 1.43ꢀ1.57 (m, 8H), 1.32ꢀ1.35 (m, 8H), 0.91ꢀ
0.96 (m, 12H). 13C NMR (100 MHz, CDCl3): 150.1, 118.1, 111.0, 72.5, 39.4, 30.4, 29.0, 23.8,
23.0, 14.1, 11.1.
Synthesis
of
((2,5-bis((2-ethylhexyl)oxy)-1,4-phenylene)bis(ethyne-2,1-diyl))
mixture of 1,4ꢀdibromoꢀ2,5ꢀbis((2ꢀethylhexyl)oxy)ꢀ2,5ꢀ
bis(trimethylsilane) (2).
A
diethynylbenzene (1) (2.0 g, 4.06 mmol), Pd(PPh3)2Cl2 (0.1.71 g, 0.243 mmol), and CuI
(0.039 g, 0.203 mmol) in Et3N (40 mL) was stirred 5 minutes. Next, ethynyltrimethylsilane
(1.0 g, 10.16 mmol) was added, and the reaction was heated at 90 °C under nitrogen for 48 h.
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Then, the reaction mixture was cooled down to room temperature and filtered; the filtrate was
evaporated to dryness to yield a crude. The crude was then purified by column
chromatography on silica gel using hexane:ethylacetate (30:1) as the eluent to compound 2 as
a yellow oil (1.69 g, 79%). 1H NMR (400 MHz, CDCl3) δ 6.89 (s, 2H), 3.81ꢀ3.86 (dd, 4H),
1.72ꢀ1.75 (m, 2H, J=12.0 Hz), 1.44ꢀ1.58 (m, 8H), 1.32ꢀ1.34 (m, 8H), 0.91ꢀ0.96 (m, 12H),
0.26 (s, 18H).
Synthesis of 1,4-bis((2-ethylhexyl)oxy)-2,5-diethynylbenzene (3). The suspension of ((2,5ꢀ
bis((2ꢀethylhexyl)oxy)ꢀ1,4ꢀphenylene)bis(ethyneꢀ2,1ꢀdiyl))bis(trimethylsilane) (2) (0.29 g,
0.55 mmol) and KOH ( 0.93 g, 16.51 mmol) in THF:methanol (9:1) (30 mL) was stirred at
room temperature for 1 h. Then, the reaction was quenched with water, and the resulting
mixture was extracted with dichloromethane. The organic phase was washed with water, dried
over Na2SO4, filtered, and evaporated to dryness to compound 3 as a yellow oilꢀsolid (0.21 g,
1
100% yield). H NMR (400 MHz, CDCl3): δ 6.96 (s, 2H), 3.85ꢀ3.86 (d, 4H, J=4.0 Hz), 3.33
(s, 2H), 1.75ꢀ1.78 (m, 2H), 1.46ꢀ1.57 (m, 8H), 1.27ꢀ1.34 (m, 8H), 0.91ꢀ0.96 (m, 12H).
Synthesis
of
8-(2-bromothien-5-yl)-3,5-dimethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-
indacene (BDY-Th-Br). Trifluoroacetic acid (TFA) (4 drops) was added to a solution of 5ꢀ
bromoꢀ2ꢀthiophenecarbaldehyde (0.90 g, 4.70 mmol) and 2ꢀmethylpyrrole (0.86 g, 10.64
mmol) in degassed CH2Cl2 (300 mL), and the resulting mixture was stirred at room
temperature overnight. Next, 2,3ꢀdichloroꢀ5,6ꢀdicyanoꢀ1,4ꢀbenzoquinone (DDQ) (1.06 g, 4.70
mmol) was added, and the reaction mixture was stirred for additional 2.5 h. Finally, N,Nꢀ
diisopropylethylamine (iꢀPr)2EtN (1.33 g, 10.31 mmol) and boron trifluoride diethyl etherate
(BF3·Et2O) (2.33 g, 16.44 mmol) were added, and the reaction mixture was stirred for
additional 2 h. The reaction mixture was poured into water and extracted with CH2Cl2. The
organic phase was dried over Na2SO4, filtered, and evaporated to dryness to give a crude
product, which was further purified by column chromatography on silica gel using
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CH2Cl2:Hexanes (2:1) as the eluent. The pure product was obtained as a crystalline red solid
1
(0.78 g, 44% yield). m.p. 132ꢀ133 °C. H NMR (400 MHz, CDCl3): δ 7.18 (m, 2H), 7.05 (d,
2H, J = 4.0 Hz), 6.31 (d, 2H, J=4.0 Hz), 2.66 (s, 6H). 13C NMR (100 MHz, CDCl3): 15.0,
116.9, 119.7, 130.1, 130.6, 131.7, 133.2, 133.9, 136.1, 158.2.
Synthesis of 10-(5-((4-((5-(5,5-difluoro-3,7-dimethyl-5H-4l4,5l4-dipyrrolo[1,2-c:2',1'-
f][1,3,2]diazaborinin-10-yl)thiophen-2-yl)ethynyl)-2,5-bis((2-
ethylhexyl)oxy)phenyl)ethynyl)thiophen-2-yl)-5,5-difluoro-3,7-dimethyl-5H-4l4,5l4-
dipyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinine (BDY-PhAc-BDY). The reagents 8ꢀ(2ꢀ
bromothienꢀ5ꢀyl)ꢀ3,5ꢀdimethylꢀ4,4ꢀdifluoroꢀ4ꢀboraꢀ3a,4aꢀdiazaꢀsꢀindacene
(BDY-Th-Br)
(0.675 g, 1.77 mmol), CuI (0.0076 g, 0.040 mmol) and Pd(PPh3)2Cl2 (0.056 g 0.08 mmol) in
Et3N:THF (2:1) (21 mL) were stirred for 5 minutes. Then, 1,4ꢀbis((2ꢀethylhexyl)oxy)ꢀ2,5ꢀ
diethynylbenzene (3) (0.308 g, 0.8 mmol) in 5 mL of THF was added, and the resulting
mixture was heated at 80 °C under nitrogen for 24 h. Then, the reaction mixture was cooled to
room temperature and evaporated to dryness. The crude was then purified by column
chromatography on silica gel using dichloromethane as the eluent to give the pure product
as a dark red solid (0.228 g, 29%). 1H NMR (400 MHz, CDCl3): δ 7.34 (m, 4H), 7.09 (d, 4H,
J= 4.0 Hz), 7.02 (s, 2H), 6.33 (d, 4H, J= 4.0 Hz), 3.94 (m, 4H), 2.67 (s, 12.0 H), 1.80 (m, 2H),
1.52ꢀ1.58 (m, 8H), 1.27ꢀ1.36 (m, 8H), 0.98 (t, 6H), 0.90 (t, 6H). 13C NMR (100 MHz,
CDCl3):δ 158.0, 153.9, 136.0, 134.0, 133.6, 132.0, 131.6, 130.2, 127.9, 119.6, 116.0, 113.7,
93.1, 87.5, 72.0, 39.6, 30.7, 29.1, 24.1, 23.1, 15.0, 14.1, 11.4. m.p. = 196ꢀ197 °C.
MS(MALDIꢀTOF) m/z (M+): calcd. for C56H60B2F4N4O2S2: 982.43, found: 983.302 [M+H]+,
963.534 [MꢀF]+, 870.685 [Mꢀ1×(2ꢀEH)]+, 758.961 [Mꢀ2×(2ꢀEH)]+. Anal.calcd. for
C56H60B2F4N4O2S2: C, 68.43; H, 6.15; N, 5.70, Found: C, 68.78; H, 6.25; N, 5.74.
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Device Fabrication and Characterization
The OTFTs were fabricated by the bottomꢀgate/topꢀcontact (BG/TC) structure.
Highly nꢀdoped silicon wafers (capacitance per unit area Ci =11.4 nF·cmꢀ2) with thermally
oxidized 300 nm SiO2 layer were cleaned via acetone sonication for 10 min and oxygen
plasma cleaning for 5 min (Harrick plasma, PDCꢀ32G, 18 W). The organic layers of BDYꢀ
PhAcꢀBDY were deposited via solutionꢀshearing on PSꢀbrushꢀtreated substrates. The PSꢀ
brush (Mw = 10 kg/mol) treatments were implemented by the general recipe24 and the
solutionꢀshearing process was accomplished as the reported procedure.25 The solutionꢀsheared
substrates were annealed in a vacuum oven at 100 °C for 24 h to remove the residual solvent.
The concentration of BDYꢀPhAcꢀBDY solution (1 – 2 mg/mL), solvent type, substrate
temperature (50 – 80 % of the solvent boiling point), and shearing speed (0.5 – 8 mm/min)
were optimized. The thickness of organic films (40 – 62 nm) were measured by the
profilometer (DEKTAKꢀXT, Brucker). The Au layers (40 nm) with various channel widths
(W; 1000 and 500 ꢁm) and lengths (L; 100 and 50 ꢁm) were thermally evaporated to define
the source and drain electrodes. The electronic performances of OTFTs were characterized in
vacuum condition (<10ꢀ2 Torr) at room temperature with the probe station (Keithely 4200ꢀ
SCS). The microstructure and surface morphology of thinꢀfilms were measured by atomic
force microscope (AFM, NX10, Park systems), scanning electron microscope (SEM, JSMꢀ
6010LA, JEOL), and Xꢀray diffraction (XRD, Smartlab, Rigaku).
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RESULTS AND DISCUSSIONS
Synthesis, Single-Crystal Structure and Thermal Characterizations
Scheme 1. Synthetic route to BDY-PhAc-BDY.
The synthesis of BDY-PhAc-BDY is shown in Scheme 1. 2,5ꢀDibromohydroquinone was
first reacted with 2ꢀethylhexylbromide (2EHꢀBr) in the presence of K2CO3 base in DMF to
yield double alkylated product 1 in 97% yield. Double alkynylation reaction of 1 was
performed via Sonogashira crossꢀcoupling reaction with ethynyltrimethylsilane (HC≡Cꢀ
SiMe3) by using Pd(PPh3)2Cl2 catalyst in the presence of CuI/Et3N. This reaction yielded 2 in
79% yield, which then underwent quantitative (100% yield) desilylation with KOH to form
hydrogenꢀended ethynyl groups in compound 3. In the final Sonogashira crossꢀcoupling
reaction, 3 was reacted with BDY-Th-Br in the presence of Pd(PPh3)2Cl2 catalyst and
CuI/Et3N cocatalyst/base mixture. Note that BDY-Th-Br was prepared in accordance with
our reported procedure.20 Thanks to the good solubility of the target small molecule in
common organic solvents, the purification was carried out via column chromatography using
CHCl3 as eluent (29% yield). The chemical structure and purity of the intermediate
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compounds and the resulting small molecule, BDY-PhAc-BDY, were characterized by 1H/13C
NMR (Figures S1ꢀS6), MALDIꢀTOF (Figure S7), and Elemental Analysis. Diffractionꢀquality
singleꢀcrystals of BDY-PhAc-BDY were obtained by diffusion of methanol into a chloroform
solution at room temperature, and the corresponding solidꢀstate structure was confirmed by
singleꢀcrystal Xꢀray analysis (Figure 2). BDY-PhAc-BDY is crystallized in the triclinic space
group Pꢀ1 and it is located on a crystallographic inversion center. BODIPY frame (C9BN2)
adopts a nearly coplanar πꢀstructure with the boron atom locating slightly out of this plane by
0.159(10) Å. As shown in Figure 2B, BODIPY πꢀcore shows a dihedral angle (θdihedral) of
44.94° with the mesoꢀthiophene ring, which is slightly lower than our previously reported
“BODIPYꢀThiophene” dihedral angle (48.80°).20 Moreover, this is much smaller than those
measured between the dipyrrin framework and the mesoꢀphenyl and thienyl units in
previously reported BODIPY small molecules (θdihedral > 54−90°).19,26–28 This coꢀplanarization
reflects the structural advantages of the current πꢀsystem including absence of βꢀpyrrole
substitutions and stericallyꢀconfined nature of the fiveꢀmembered thiophene ring.29 The B1–
F1 (1.385(11) Å) and B1–F2 (1.378(10) Å) bond distances and F1–B1–F2 (110.1(7)°), N1–
B1–N2 (106.8(7)°), and N–B–F (av. 109.975°) bond angles are similar to those found in the
literature.30 As shown in Figure 2C, the detailed crystal packing analysis of BDY-PhAc-BDY
indicates that the presence of 2ꢀethylhexyloxy moieties on the central benzene ring prevents
the πꢀπ stacking interactions to occur between the complete molecular πꢀbackbones. However,
BODIPY πꢀcores are still found to employ antiparallel arranged, πꢀstacked dimers with an
interplanar distance of 3.93 Å in the unit cell (Figure 2D). This arrangement is most likely a
result of the energetically favorable antiparallel dipoleꢀdipole interactions between the
BODIPY cores’ strong molecular dipole moments (ꢁ = 3.38 Debye)17 oriented toward the
4,4′ꢀfluorine substituents. In addition, relatively weaker “π···π” (~4.51 Å) interactions were
observed between ethynyl (ꢀC≡Cꢀ) groups which, in combination with the strong edgeꢀtoꢀface
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“CꢀH···π (dialkoxyphenylic) (~2.85Å)” interactions between πꢀelectronꢀdeficient BODIPYꢀ
pyrrole hydrogens and the central πꢀelectronꢀrich dialkoxybenzene ring or ethynyl (ꢀC≡Cꢀ)
group, is found to play a critical role not only in solidꢀstate formation, but also in thinꢀfilm
crystallization to form highly crystalline microribbons (Figure 5C, vide infra).
Figure 2. A. Xꢀray crystal structure of BDY-PhAc-BDY with 20% ellipsoids. The grey, blue,
red, pink, yellow, and white coloured atoms represent C, N, O, B, F, and H, respectively, B.
the perspective view of the interꢀring dihedral angle between boronꢀdipyrromethene and
mesoꢀaromatic unit planes, C. Crystal packing of BDY-PhAc-BDY, and D. The perspective
view of antiparallel arranged, πꢀstacked dimers (3.928 Å) occurred between two BODIPY
units. The local dipole moments (ꢀ = 3.38 Debye) of BODIPY cores are shown in blue.
As shown in Figure 3, BDY-PhAc-BDY was found to be highly thermally stable with the
thermolysis onset temperature (5% mass loss) located at 300 °C. A twoꢀstep decomposition
behavior was observed with a small step at ~87% of the original weight, corresponding to the
mass loss of one of the alkyl substituents (ꢀCH2CH(C2H5)(C4H9)). Differential scanning
calorimetry (DSC) measurement shows a sharp endothermic peak at 198 °C with an enthalpy
of 62.65 J/g. This peak indicates the thermal transition of the crystalline BDY-PhAc-BDY
solid into an isotropic liquid, which was further confirmed by conventional meltingꢀ
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temperature measurement (Tm.p. = 196ꢀ197 °C). This is very different from our previously
reported nonꢀsubstituted BODIPYꢀsmall molecules, BDY-3T-BDY/BDY-4T-BDY, which
have showed thermal decompositions with no observable melting points.20 This reflects the
critical role of flexible swallowꢀtailed 2ꢀethylhexyloxy substituents to tune intermolecular
interactions, which eventually induces melting process. Note that once the crystalline solid
obtained from chromatographic purification melts to an isotropic liquid, no corresponding
crystallization peak was observed in the cooling cycle. This indicates that BDY-PhAc-BDY
adopts an amorphous solidꢀstate upon cooling and it does not exhibit further melting peak in
the secondꢀheating cycle (Figure 3B).
Figure 3. A. Thermogravimetric analysis (TGA), and B. Differential scanning calorimetry
(DSC) measurement curves of BDY-PhAc-BDY at a temperature ramp of 10 °C/min under
N2.
Optical and Electrochemical Properties
The electrochemical and optical properties of BDY-PhAc-BDY were evaluated by UVꢀvis
absorption and photoluminescence spectroscopies, and cyclic voltammetry. BDY-PhAc-BDY
exhibits two absorption peaks in dichloromethane solution with λmax located at 355 nm and
527 nm. The large molar extinction coefficient (ε = 1 × 105 M−1 cm−1) for the absorption peak
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at 527 nm and the outꢀofꢀplane vibronic feature at 495 nm (∼
1200 cm−1 shift from λmax) are
unique features of the BODIPYꢀbased π−π* (S0 → S1) transition.15 The broad peak with lower
intensity at 355 nm is ascribed to a combination of BODIPYꢀbased S0 → S2 and 1,4ꢀbisꢀ
(thienylethynyl)2,5ꢀdialkoxybenzeneꢀbased π−π* transitions. The optical band gap is
estimated as 2.22 eV from the lowꢀenergy absorption edge. In the thinꢀfilm state (spinꢀcoated
on glass substrate), the absorption maxima show bathochromic shifts (λmax = 365/543 nm) and
the optical band gap is lowered to 2.06 eV (solidꢀstate) with respect to those in the solution.
This is indicative of molecular πꢀbackbone planarization and solidꢀstate ordering as compared
to the solution phase. The fluorescence spectrum of BDY-PhAc-BDY in dichloromethane
solution (Figure S8) exhibits a broad emission peak with a largely shifted maxima of 646 nm
(Stokes shift = 119 nm) and a very low quantum yield (ΦF) of ~0.02%. This is very different
from typical BODIPY emission features such as high quantum yield and extremely low
Stokes shifts, and it reflects most likely the presence of nonradiative pathways in the excited
state via intraꢀ/interꢀmolecular charge transfer processes.31 When the solvent dielectric
constant is lowered (THF (ε = 8.9) → toluene (ε = 2.3)), a highly blueꢀshifted PL spectra was
measured in toluene (Figure S8, λem = 578 nm) indicating positive solvachromatism. This
further confirms that the final relaxed excited state employs a large dipole moment as a result
of charge transfer (CT) between subchromophoric units. Based on the DFT calculations
(B3LYP/6ꢀ31G** level of theory), lowest unoccupied molecular orbital (LUMO) of BDY-
PhAc-BDY is found to be symmetrically localized on the outer BODIPY πꢀacceptor units,
while highest occupied molecular orbital (HOMO) is delocalized only on the central πꢀdonor
part with noticeable contributions from the oxygens on the alkoxy groups (Figure 4ꢀinset).
This further supports the emission characteristics of BDY-PhAc-BDY, that the
photoexcitation is probably accompanied with an intramolecular chargeꢀtransfer (CT) in the
excited state from donor to acceptor unit (HOMO → LUMO). Cyclic voltammetry
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measurements reveal two reversible reductions and one quasiꢀreversible oxidation for BDY-
PhAc-BDY with the first halfꢀwave reductionꢀpotential (E1/2redꢀ1) located at ꢀ0.84 V and the
first onset oxidationꢀpotential (Eonsetoxꢀ1) located at 1.28 V. The electrochemical band gap
(2.12 eV) shows excellent agreement with the optical band gap (2.22 eV) measured in
solution. The HOMO and LUMO energy levels are estimated as −5.68 eV and −3.56 eV,
respectively. Note that, as a result of πꢀelectron deficiency of the mesoꢀsubstituted BODIPY’s
at the molecular termini, LUMO energy level of the new molecule is sufficiently stabilized to
be in the energetic range of previously reported nꢀchannel semiconductors (−2.9 to −4.3
eV).32 Furthermore, when compared with the observed pꢀdoping characteristics, highly
reversible and favorable reductive properties of BDY-PhAc-BDY are indicative of its
electronꢀtransporting semiconductor potential.
Figure 4. A. Optical absorption in dichloromethane solution (black line) and as thinꢀfilm (red
line) of BDY-PhAc-BDY, and B. Cyclic voltammogram of BDY-PhAc-BDY in
ꢀ
dichloromethane (0.1M Bu4N+PF6 , scan rate = 50 mV·sꢀ1). Inset shows the calculated (DFT,
B3LYP/6ꢀ31G**) topographical orbital representations and experimentally estimated HOMO
and LUMO energy levels.
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Thin-Film Microstructure/Morphology and Field-Effect Transistor Characterization
The semiconductor thinꢀfilms (40ꢀ62 nm) were fabricated by solutionꢀshearing BDY-
PhAc-BDY solution in 1,2,4ꢀtrichlorobenzene (1.0 mg/mL) on PS (polystyrene)ꢀbrush treated
n++ꢀSi/SiO2 substrates. Bottomꢀgate/topꢀcontact (BG/TC) OFETs were completed by thermal
evaporation of source and drain Au electrodes (40 nm) via thermal evaporation (deposition
rate = 0.2 Å/s) to yield various channel lengths (L: 100 and 50 ꢁm) and widths (W: 1000 and
500 ꢁm). The microstructure of solutionꢀsheared BDY-PhAc-BDY thinꢀfilm was investigated
by outꢀofꢀplane Xꢀray diffraction measurement, which displays a major diffraction peak at 2θ
= 8.94° (Figure 5A). The second and third order diffractions of the same crystalline phase
were also observed revealing a high degree of ordering across the thinꢀfilm thickness. The
simulation of the powder pattern based on the singleꢀcrystal structure of BDY-PhAc-BDY
(Figure S9) shows that all these peaks are well indexed along the (010) lattice plane, and
second and third order diffractions correspond to (020) and (030), respectively. The
periodicity (dꢀspacing) of the (010) plane in thinꢀfilm phase is measured as 9.88 Å, which
corresponds to “bꢀaxis (10.928 Å) × sin α (66.26˚)” and suggests that the molecules are
adopting a highly tilted molecular orientation on the substrate (Figure 5C). Atomic force
microscopy (AFM) and scanning electron microscopy (SEM) characterizations show that the
morphology of the solutionꢀsheared thinꢀfilm of BDY-PhAc-BDY consists of oneꢀ
dimensional (1ꢀD) highly crystalline micronꢀsized ribbons, which are perfectly aligned along
the shearing direction (Figure 5B). The lengths of the microribbons can reach to ∼0.1 mm and
their widths were ~0.5ꢀ2.0 ꢁm. Detailed analysis of the AFM image also shows the formation
of wellꢀconnected smaller grains having ~200ꢀ500 nm lengths along the ribbon longꢀaxis. In
order to elucidate the structural packing in these microꢀribbons, the BFDH (Bravais, Friedel,
Donnay, and Harker) theoretical crystal morphology was used, which predicts a high aspect
ratio crystal growth perpendicular to the [010] outꢀofꢀplane direction. Since the selfꢀassembly
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process during thinꢀfilm crystallization may be similar to singleꢀcrystal formation, the
thermodynamically/kinetically more favored primary crystal growth along the [100]
crystallographic direction most likely reflects the anisotropic ribbon growth due to similar
directional intermolecular forces.33–35 This shows that the major intermolecular interactions
governing the thinꢀfilm crystallization are: (i) strong “ꢀCH···π” contacts (~2.85Å) between πꢀ
deficient BODIPYꢀpyrrole hydrogens and the central πꢀelectron rich dialkoxybenzene
ring/ethynyl (ꢀC≡Cꢀ) groups, (ii) “π···π” (~4.5Å) interactions between ethynyl (ꢀC≡Cꢀ)
groups, and (iii) “π···π” (~3.9 Å) and dipoleꢀdipole interactions between antiparallel arranged
BODIPY frameworks (vide infra). It appears to us that also the directional solutionꢀshearing
process used for the current thinꢀfilm fabrications might play a key role to facilitate BDY-
PhAc-BDY’s molecular selfꢀassembly into anisotropic ribbons along the shearing direction.25
The formation of ribbonꢀlike morphology with the solutionꢀshearing method is consistent with
our previous rodꢀlike BODIPY small molecules (BDY-3T/4T-BDY) which has also yielded
highꢀaspect ratio microfibers in thinꢀfilm.
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Figure 5. θꢀ2θ Xꢀray diffraction (XRD) scans (A) and SEM/AFMꢀtopographic images (B)
of solutionꢀsheared BDY-PhAc-BDY thinꢀfilm. (C) The molecular arragement in the outꢀofꢀ
plane [010] direction and the BFDH (Bravais, Friedel, Donnay and Harker) theoretical crystal
morphology showing the primary crystal growth direction ([100]). The arrow shows the
shearing direction. “ꢀCH···π” contacts (~2.85Å) and “π···π” (~4.5Å) interactions are shown
between two representative molecules.
OFET device characteristics were measured wih a Keithley 4200ꢀSCS semiconductor
characterization system at room temperature. The transistor characteristics in the saturation
regime such as charge carrier mobilities (ꢁ) and threshold voltages (VT) were extracted from
the equation:
ꢁsat= (2IDSL)/[WCi(VG –VT)2]
where IDS is the drain current, L and W are the channel length and width, respectively, Ci is
the areal capacitance of the gate dielectric, VG is the gate voltage, and VT is the threshold
voltage. Typical transfer and output curves are shown in Figures 6 and S10. Consistent with
the theoretical and experimental optoelectronic characterizations on the small molecule (vide
supra), these devices exhibited nꢀchannel behavior with ꢁe = 0.004 cm2/V·s and Ion/Ioff = 105ꢀ
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106. This clearly shows that BODIPY is an effective πꢀacceptor unit to afford a substantial
lowering of the LUMO level and to induce nꢀchannel semiconductivity in πꢀconjugated small
molecules for use in optoelectronic devices. Although the current electron mobility for BDY-
PhAc-BDY thinꢀfilm is much lower than the stateꢀofꢀtheꢀart performances achieved
previously with other molecular πꢀsystems, it is still remarkable from a molecular design
perspective since it is the highest reported to date for a BODIPYꢀbased small molecular
semiconductor having alkyne linkages. Besides, the growth of wellꢀoriented microꢀ and nanoꢀ
sized organic semiconductor structures from solution phase has always been challenging in
the literature and it offers great advantages in fabricating transistor arrays for circuit
design.34,36 Although πꢀπ stacking between whole molecular skeletons are not evident along
the chargeꢀtransport (inꢀplane) direction, the localized πꢀπ stackings between individual
acceptor or donor units in the twisted arrangement of AꢀDꢀA πꢀarchitecture,37 and strong “ꢀ
CH···π” interactions in the faceꢀtoꢀedge herringbone packing may still contribute to 3ꢀD
charge transport, which, along with the highlyꢀcrystalline microstructure, explains the
observed mobility. Since the thinꢀfilm crystallinity and morphology of the current
semiconductor remains similar to our previously reported nonꢀsubstituted semiconductor,
BDY-4T-BDY, the slightly reduced (2.5×) OFET performance is most likely related to the
presence of σꢀinsulating alkyl substituents on the central benzene ring. Based on our findings,
note that BODIPY πꢀcore offers unique advantage by providing good solubility to the
molecular πꢀsystem even in the absence of alkyl substituent. Therefore, for the future
development of solutionꢀprocessable BODIPYꢀbased semiconductors, we rationalize that nonꢀ
substituted systems should be preferred to yield higher mobilities.
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Figure 6. Representative transfer plot in the nꢀchannel region for bottomꢀgate/topꢀcontact
(BG/TC) OFET devices fabricated with solutionꢀsheared BDY-PhAc-BDY thinꢀfilm.
Conclusions
A new solutionꢀprocessable BODIPYꢀAcetylene small molecule, BDY-PhAc-BDY,
based on AꢀDꢀA πꢀarchitecture was designed, synthesized and fully characterized. The new
semiconductor exhibited a low solidꢀstate optical band gap of 2.06 eV with stabilized HOMO
and LUMO energy levels of ꢀ5.68 eV and ꢀ3.56 eV. Singleꢀcrystal Xꢀray diffraction (XRD)
analysis reveals crucial structural features and intermolecular interactions such as relatively
low “BODIPYꢀmesoꢀthiophene” dihedral angle (θdihedral = 44.94°) and antiparallel πꢀstacked
BODIPY dimers with an interplanar distance of 3.93 Å. Highly crystalline oneꢀdimensional
(1ꢀD) microribbons of BDY-PhAc-BDY were grown from the chloroform solution on PS
(polystyrene)ꢀbrush treated substrates via solutionꢀshearing method. Strong edgeꢀtoꢀface “Cꢀ
H(pyrrolic)···π(dialkoxyphenylic) (~2.85Å)” and relatively weaker “π(ethynyl)···π(ethynyl)”
(~4.51 Å) directional interactions are found to be effective in the formation of current highly
crystalline microribbons along the [100] direction. The bottomꢀgate/topꢀcontact OFET
devices based on these microribbons exhibited clear nꢀchannel operation and afforded
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electron mobility of 0.004 cm2/V·s and the on/off current ratio of 105ꢀ106, which is the highest
reported till now for BODIPYꢀbased small molecular semiconductors with alkyne linkages.
Our findings clearly demonstrate that BODIPY is an effective πꢀacceptor unit to realize
solutionꢀprocessable donorꢀacceptor type small molecules for electronꢀtransport. Here, we
also offer crucial guidance for the design of future BODIPYꢀbased semiconductor molecules
with further improved electrical performances and also easily fabricable 1ꢀD semiconductor
thinꢀfilm morphologies for fundamental/applied research in organic optoelectronics.
Acknowledgement
H. U. acknowledges support from TUBITAK 114M226. H. U. acknowledges support
from Turkish Academy of Sciences, The Young Scientists Award Program (TUBAꢀ
GEBĐP 2015). C. K. acknowledges support from Basic Science Research Program through
the National Research Foundation of Korea (NRF) (NRFꢀ2014R1A1A1A05002158).
Supporting Information
SI includes Figures S1ꢀS7 (1H/13C NMR spectra and MALDIꢀTOF spectra for the
intermediate compounds and the final small molecule), Figure S8 (PL spectra of BDYꢀPhAcꢀ
BDY), Figure S9 (Simulated XRD powder pattern of BDYꢀPhAcꢀBDY), and Figure S10
(output plot for the OFET devices). Table S1 and CCDC 1482958 (for BDY-PhAc-BDY)
contains the supplementary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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