A Solution-Processable meso-Phenyl-BODIPY-Based n-Channel Semiconductor with Enhanced Fluorescence Emission

Article abstract of DOI:10.1002/cplu.201900317

The molecular design, synthesis, and characterization of an acceptor-donor-acceptor (A-D-A) semiconductor BDY-Ph-2T-Ph-BDY comprising a central phenyl-bithiophene-phenyl π-donor and BODIPY π-acceptor end-units is reported. The semiconductor shows an optic

Full text of DOI:10.1002/cplu.201900317

Accepted Article
Title: A Solution-Processable meso-Phenyl-BODIPY-Based n-Channel
Semiconductor with Enhanced Fluorescence Emission
Authors: Hakan Usta, Emrah Ozcan, Mehmet Ozdemir, Dongil Ho,
Yunus Zorlu, Resul Ozdemir, Choongik Kim, and Bunyemin
Cosut
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10.1002/cplu.201900317
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A Solution-Processable meso-Phenyl-BODIPY-Based
n-Channel Semiconductor with Enhanced Fluorescence Emission
Emrah Ozcan,1 Dr. Mehmet Ozdemir,2 Dongil Ho,3 Prof. Yunus Zorlu,1 Resul Ozdemir,2
Prof. Choongik Kim,*3 Prof. Hakan Usta,*2 Prof. Bunyemin Cosut*1
1 Department of Chemistry, Gebze Technical University, Gebze, Kocaeli, Turkey
2
Department of Materials Science and Nanotechnology Engineering, Abdullah Gül
University, Kayseri, Turkey
3
Department of Chemical and Biomolecular Engineering, Sogang University, Mapo-gu,
Seoul, Korea
Correspondence:
bc@gtu.edu.tr, hakan.usta@agu.edu.tr, choongik@sogang.ac.kr
1
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Abstract
The molecular design, synthesis, and characterization of a new acceptor-donor-acceptor (A-
D-A) semiconductor BDY-Ph-2T-Ph-BDY comprising a central phenyl-bithiophene-phenyl
π-donor and BODIPY π-acceptor end-units is reported. The new semiconductor shows an
optical band gap of 2.32 eV with a highly stabilized HOMO/LUMO (-5.74 eV/-3.42 eV).
Single-crystal X-ray diffraction (XRD) reveals D-A dihedral angle of ~66° and strong
intermolecular “C-H···π (3.31 Å)” interactions. Reduced π-donor strength, increased D-A
dihedral angle, and restricted intramolecular D-A rotations allows for both good fluorescence
efficiency (ΦF = 0.30) and n-channel OFET transport (μe = 0.005 cm2/V·s; Ion/Ioff = 104-105).
This indicates a much improved (~6-7 fold) fluorescence quantum yield compared to meso-
thienyl BODIPY semiconductor BDY-4T-BDY. Photophysical studies reveal important
transitions between locally excited (LE) and twisted intramolecular charge-transfer (TICT)
states in solution and solid-state, which could be controlled by solvent polarity and nano-
aggregation. This is the first time that such high emissive characteristics are reported for a
BODIPY-based n-channel semiconductor.
2
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Introduction
π-Conjugated small molecules with photon absorption/emission properties have attracted
great scientific and technological interest as semiconducting materials in organic
photovoltaics (OPVs), field-effect transistors (OFETs), and light-emitting transistors
(OLETs).[1–4] The delocalization of π-electron density over the entire molecular system is very
critical to the electronic/photonic properties of molecular semiconductors.[5–9] In addition to
their promising charge-transport and photophysical properties, the most favorable physical
and chemical properties of π-conjugated small molecules are structural versatility, ease of
synthesis-purification and fabrication, and synthetic (batch-to-batch) reproducibility.[10–13]
Among various π-building blocks, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) has
recently attracted attention in the design of molecular semiconductors for optoelectronics.[14]
In the past few decades, BODIPY-based small molecules have been studied as fluorescent
[15–18]
switches/sensors, chemosensors, biochemical labels, and photodynamic therapy agents,
and they have shown remarkable photophysical properties such as high fluorescence quantum
yield, large extinction coefficient, small Stokes shifts, high photostability, and tunable
absorption/fluorescence profiles.[19] The recent optoelectronic applications of BODIPY π-
systems include organic photovoltaics (OPVs),[20] dye-sensitized solar cells (DSSCs),[21]
organic thin film transistors (OTFTs),[22] light harvesting,[23] and near-infrared absorption.[24]
Owing to its highly π-electron deficient electronic structure resulting in an energetically
stabilized frontier orbitals, BODIPY is one of the unique π-acceptor building blocks that
could be synthesized and functionalized in a few steps. Thanks to its excellent structural
properties including high π-core planarity, large dipole moment (μ ~ 3-4 D), good π-
delocalization, and high solubility, BODIPY is an ideal building block to form solution-
processable donor-acceptor (D-A) type n-channel semiconductor architectures.[25–28] In
addition to these structural features, the electronic properties of BODIPY-based
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semiconductors are unique because the majority charge carrier is highly dependent on its π-
architecture. While aromatic substitution on BODIPY’s meso-position yields n-channel
(electron-transporting) semiconductivity,[29] π-extension through 2,6-positions leads to hole-
transport (p-channel) characteristics.[26] Facile synthetic modifications on BODIPY have been
demonstrated to finely tune chemical/photophysical and charge-transport characteristics,
which then allows the development of optimized BODIPY π-structures for specific
optoelectronic applications such as OLETs and OPVs.[29,30] To this end, we have recently
demonstrated that when meso-thienyl BODIPYs are used as π-acceptor end-units in acceptor-
donor-acceptor (A-D-A) type π-architectures (e.g., BDY-4T-BDY, Figure 1), highly efficient
electron-transport (μe ~ 0.001-0.01 cm2/V·s) characteristics in n-channel OFETs could be
achieved.[31] In this design, we took advantage of the negative inductive (-I) and mesomeric (-
M) effects provided by the BODIPY π-core, which enabled electron
injection/stabilization/transport across the corresponding molecular solid-state. Despite all
these recent efforts, there is still only a handful example of BODIPY-based n-channel
semiconducting molecules reported for use in optoelectronics.[32] Furthermore, for the known
BODIPY-based n-channel molecular semiconductors, emissive properties have either not
been studied or reported to be very low as a result of strong D-A electronic structures. For
instance, our previously reported high-performance small molecule BDY-4T-BDY showed
meager fluorescence quantum yields (ΦF ~ 0.04-0.05).[31] Therefore, from a materials
development/functionality standpoint, it is still precious to design and develop novel
BODIPY-based semiconducting π-systems with improved photophysical properties. This
way, good electron-transport characteristics and emissive properties could be simultaneously
achieved to open new avenues for next-generation solution-processed optoelectronics.
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Figure 1. The chemical structures of BDY-Ph-2T-Ph-BDY developed in this study and the
[31]
reference compound, BDY-4T-BDY, developed in our earlier study
red) and π-acceptor (in black) building blocks.
showing π-donor (in
In this study, we demonstrate the design, synthesis, and characterization of a new solution-
processable A-D-A π-conjugated small molecule, BDY-Ph-2T-Ph-BDY (Figure 1),
employing phenyl-bithiophene-phenyl (Ph-2T-Ph) central π-donor unit and BODIPY π-
acceptor end units. The molecular design rationales used here are: (i) attaching BODIPY end
units to the π-donor unit through meso-positions to facilitate electron-transport, (ii) decreasing
π-donor strength (Ph-2T-Ph vs. 4T in BDY-4T-BDY) and (iii) increasing D-A dihedral
angle/rotational barrier (BODIPY-Ph vs. BODIPY-T in BDY-4T-BDY) to minimize non-
[33–36]
radiative decays and the formation of twisted intramolecular charge-transfer (TICT).
The new semiconductor shows a large dihedral angle (~66°) between BODIPY and meso-
phenyl units and large local dipoles (μ = 3-4 D); both provide good solubility in common
organic solvents. This solution processibility enables convenient purification and thin-film
fabrication for the new semiconductor. Also, the absence of long lipophilic substitution and
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the presence of significant local dipoles could lead to efficient intermolecular interactions
1
favoring solid-state ordering. The new molecule was characterized by H/13C NMR, mass
spectrometry, elemental analysis, and thermogravimetric analysis. The single-crystal structure
of the key intermediate compound BDY-Ph-Br reveals crucial structural properties in the
design of the new semiconductor. BDY-Ph-2T-Ph-BDY shows an optical band gap of 2.32
eV with highly stabilized HOMO/LUMO energies of -5.74 eV/-3.42 eV. BDY-Ph-2T-Ph-
BDY was studied in solution-processed OFETs; it shows n-channel semiconductor behavior
with electron mobility of 0.005 cm2/V·s and Ion/Ioff ratio of 104-105. The detailed
photophysical characterizations show significant transitions between locally excited (LE) and
twisted intramolecular charge-transfer (TICT) states via solvent polarity change and nano-
aggregation. Significantly enhanced fluorescence characteristics (~6-7 fold increase in
quantum yield) were achieved compared to the previously developed n-channel BODIPY
semiconductors. To the best of our knowledge, this is the first time that such high emissive
characteristics are reported for a BODIPY-based n-channel semiconductor. This is
undoubtedly the result of rational molecular design employed here, which demonstrates an
essential step towards the development of solution-processable highly fluorescent BODIPY-
based n-channel semiconductors for next-generation optoelectronics.
Experimental Section
Materials and Methods
The reactions were carried out under N2 unless otherwise noted by using Schlenk
techniques. All reagents were obtained from commercial sources and used without any further
1
purification unless otherwise noted. H/13C NMR characterizations were performed by a
13
Bruker 400 spectrometer (1H, 400 MHz; C, 100 MHz). Elemental analyses were done by a
LecoTruspec Micro model instrument. MALDI-TOF was performed by a Bruker Microflex
LT MALDI-TOF-MS Instrument. Thermal properties of compounds were investigated by a
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Mettler Toledo TGA/SDTA 851 Thermogravimetric Analysis (TGA) Instrument and
Differential Scanning Calorimeter DSC 821 equipped with METTLER TOLEDO STAR
software at a heating rate of 10 °C/min under nitrogen. UV-Vis absorption and fluorescence
emission measurements were performed by
a
Shimadzu, UV-1800 UV-Vis
Spectrophotometer and Varian Eclipse spectrofluorometer, respectively. The fluorescence
quantum yield was determined in dichloromethane as compared to the fluorescence of
Rhodamine 6G standard (ΦF = 0.76 in water). Electrochemistry was performed by a C3 cell
stand electrochemical station equipped with BAS-Epsilon software (Bioanalytical Systems,
Inc. Lafayette, IN). Dynamic light-scattering (DLS) measurements were performed on a
Malvern, Nano ZS Zetasizer (Abdullah Gül University-Central Research Facility (AGU-
CRF)). SEM images were taken using the ZEISS GeminiSEM 300 – Field Emission Scanning
Electron Microscope at a 3 kV accelerating voltage (Abdullah Gül University-Central
Research Facility (AGU-CRF)).
The parameters for fluorescence quantum yields
The fluorescence quantum yield values for BDY-4T-BDY and BDY-Ph-2T-Ph-BDY were
determined in benzene by comparing to the fluorescence of Rhodamine 6G as a standard.
Fluorescence quantum yields (ΦF) were calculated by the comparative method (Eq. 1).[37]
F.AStd.n2
(1)
ΦF
 Φ (Std)
F
F
Std.A .nS2td
where ΦF(Std) is the fluorescence quantum yield of standard. Rhodamine 6G is employed as
the standard (ΦF = 0.76 in water)[38]. F and FStd are the areas under the fluorescence emission
curves of samples (BDY-4T-BDY and BDY-Ph-2T-Ph-BDY) and the standard, respectively.
A and AStd are the respective absorbance of the samples and standard at the excitation
η2
wavelengths.
and ηS2td are the refractive indices of solvents used for the sample and
standard, respectively. The concentration of the solutions at the excitation wavelength was
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fixed at 1x10-5 M.
Synthesis and Characterization
The synthesis of BDY-4T-BDY was performed following our previously reported
procedure. [31]
Synthesis of 8-(4-bromo-1-phenyl)-3,5-diethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene
(BDY-Ph-Br): To a flask containing CH2Cl2 (300 ml), 4-bromobenzaldehyde (1150 mg, 6.25
mmol) and N-ethylpyrrole (1.3 mL, 12.5 mmol) were added, respectively, under nitrogen,
followed by the addition of 3 drops of trifluoroacetic acid. The reaction mixture was stirred at
room temperature for 12 h. Next, p-chloranil (1500 mg, 6.25 mmol) was added to the reaction
medium, and the mixture was stirred at room temperature for an additional 30 min. Then,
triethyl amine (5 mL) and boron trifluoride diethyl etherate (BF3.OEt2) (5 mL) were added,
sequentially. The reaction mixture was stirred at room temperature for an additional 3 h. The
reaction mixture was extracted with CH2Cl2 and water. Organic layer was dried with Na2SO4
and evaporated to dryness to give a crude product. The crude product was purified by silica
gel column chromatography using CH2Cl2:Hexanes (3:1) as the eluent to give BDY-Ph-Br as
1
a red solid (495 mg, 20% yield). H NMR (400 MHz, CDCl3):  (ppm) 7.63-7.61 (d, J = 8.4
Hz, 2H), 7.37-7.35 (d, J = 8.4 Hz, 2H), 6.71-6.70 (d, J = 4.2 Hz, 2H), 6.36-6.35 (d, J = 4.2
13
Hz, 2H), 3.10-3.05 (q, J = 7.6 Hz, 4H), 1.36-1.33 (t, J = 7.6 Hz, 6H). C NMR (100 MHz,
CDCl3):  (ppm) 164.2, 141.4, 134.1, 133.2, 131.9, 131.6, 130.3, 124.6, 117.7, 22.2, 12.9. MS
(MALDI-TOF) m/z [M]+: calcd for C19H18BBrF2N2: 402.07; found: 402.280 [M]+, 383.357
[M-F]+.
Synthesis of 8,8'-([2,2'-bithiophene]-5,5'-diylbis(4,1-phenylene))bis(3,5-diethyl-4,4-difluoro-
4-bora-3a,4a-diaza-s-indacene) (BDY-Ph-2T-Ph-BDY): The solution of BDY-Ph-Br (150
mg, 0.372 mmol), 5,5'-bis(trimethylstannyl)-2,2'-bithiophene (89.2 mg, 0.181 mmol), and
Pd(PPh3)4 (10.5 mg, 9.1 µmol) in anhydrous toluene (15 mL) was stirred at 110 °C for 24
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hours. Then, the reaction mixture was allowed to warm to room temperature and evaporated
to dryness to give a crude solid. The dark colored crude solid was isolated and washed with
methanol; then, it was purified by silica gel column chromatography using CH2Cl2:Hexanes
(2:1) as the eluent to afford BDY-Ph-2T-Ph-BDY as a dark red solid (112 mg, 73% yield).
1H NMR (400 MHz, CDCl3):  (ppm) 7.73 (d, 2H, J = 8.0 Hz), 7.55 (d, 2H, J = 8.0 Hz), 7.39
(d, 1H, J = 4.0 Hz), 7.28 (d, 1H, J = 4.0 Hz), 6.83 (d, 2H, J = 4.0 Hz), 6.39 (d, 2H, J = 4.0 Hz),
13
3.08 (q, 4H, J = 22.8 Hz), 1.35 (t, 6H, J = 15.2 Hz). C NMR (100 MHz, CDCl3):  (ppm)
163.6, 142.1, 137.5, 135.6, 134.1, 133.5, 131.2, 130.2, 125.2, 125.1, 124.9, 117.4, 22.1, 12.8.
MS (MALDI-TOF) m/z [M]+: calcd for C46H40B2F4N4S2: 810.28; found: 810.11[M]+, 791.085
[M-F]+. Elemental analysis calcd (%) for C46H40B2F4N4S2: C, 68.16; H, 4.97; N, 6.91; found:
C, 67.89; H, 5.03; N, 7.01.
Field-Effect Transistor Fabrication and Characterization
Top contact/bottom gate (TC/BG) OFETs were fabricated on highly n-doped silicon
wafers having thermally grown 300 nm oxide layer (areal capacitance; Ci =11.4 nF cm-2). The
substrates were sonicated in acetone for 10 min and then treated with air plasma for 5 min
(Harrick plasma, PDC-32G, 18 W). PS-brush (Mw = 1.7 – 28 kg mol-1) treatments were
implemented on the cleaned substrates following the conventional procedure.[39] The solution-
shearing method was employed to form organic semiconductor layers on PS-brush-treated
substrates where the concentration of the organic solution, solvent type, substrate temperature,
and shearing speed were optimized. The substrates were placed in a vacuum oven to remove
residual solvent. Finally, Au source and drain electrodes (40 nm) were thermally evaporated
to define the semiconductor channel width (W) and length (L) of 1000 and 50 μm,
respectively. Electrical characteristics of OFETs were measured in vacuum conditions at
room temperature with a semiconductor parameter analyzer (Keithley 4200-SCS). The
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saturation charge-carrier mobility (μsat) for an OFET device was calculated in the saturation
regime as the average value of at least 10 OFET devices using the formula:
μsat = (2IDSL)/[WCi (VG - Vth)2]
where IDS is the source-drain current, L is the channel length, W is the channel width, Ci is the
areal capacitance (11.4 nF·cm-2) of the gate dielectric, VG is the gate voltage, and Vth is the
threshold voltage. The voltage ranges used in charge carrier mobility calculations are between
50-80 V. The thin-film microstructure and surface morphology were analyzed by atomic force
microscopy (AFM, NX10, Park systems) and wide-angle X-ray diffraction (XRD, Ultima IV,
Rigaku).
Results and Discussion
Computational Modeling, Synthesis, Characterization, and Single Crystal X-ray
Analysis
The density functional theory (DFT) calculations performed at the B3LYP/6-
31G**[40] level of theory indicates that the central Ph-2T-Ph π-donor unit in BDY-Ph-2T-Ph-
BDY is coplanar with small inter-ring dihedral angles of ~15-19° (Figure 2A). This angle is
below the threshold value of torsional angle (~30-35°) at which π-conjugation breaks,[41] and
it provides a very efficient π-orbital delocalization through the donor unit. Indeed, the HOMO
completely delocalizes on this π-donor framework with no contribution from BODIPY
acceptor π-system. On the other hand, sizeable inter-ring dihedral angle of ~61° is calculated
between “boron-dipyrromethene” and “meso-phenyl” π-units, which matches well with the
single-crystal structure of the BDY-Ph-Br intermediate compound (Figure 2B). As a result of
this mostly broken π-conjugation between donor and acceptor units, the LUMO has
significantly larger electron densities on the terminal BODIPY π-acceptors with minimal
contributions from phenyl/thienyl units. The HOMO and LUMO energies for BDY-Ph-2T-
Ph-BDY were calculated to be -5.50 eV and -2.71 eV, respectively, which are relatively
higher than those calculated for the reference compound BDY-4T-BDY (EHOMO/ELUMO = -
5.58/-3.17 eV). The nonsymmetric increase in frontier orbital energies results in increased
HOMO-LUMO energy gap for the new compound (EHOMO-LUMO for BDY-Ph-2T-Ph-BDY =
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2.79 eV) as compared with BDY-4T-BDY (EHOMO-LUMO = 2.41 eV), most likely a result of
lowered π-donor strength (Ph-2T-Ph vs. 4T) and increased dihedral twist (Ph-BDY vs. T-
BDY) between donor and acceptor units.
The synthetic route to BDY-Ph-2T-Ph-BDY is outlined in Scheme 1. In order to
prevent potential instability of the dipyrromethene π-core, the BODIPY structure employed in
this study is synthesized bearing two ethyl substituents at α-positions. BDY-Ph-Br was
synthesized in 20% yield by first reacting 4-bromobenzaldehyde with 2-ethyl pyrrole in the
presence of catalytic amount of trifluoroacetic acid, followed by oxidization with tetrachloro-
p-benzoquinone and coordination with trifluoroborane-dietherate (BF3·OEt2) in the presence
of N,N-diisopropylethylamine. BDY-Ph-2T-Ph-BDY was synthesized via Stille cross-
coupling reaction between BDY-Ph-Br and 5,5′-bis(trimethylstannyl)-2-2′-bithiophene in the
presence of tetrakis(triphenylphosphine)palladium(0) catalyts in toluene at 110 °C. The good
solubility of the target compound, BDY-Ph-2T-Ph-BDY, enabled convenient purification via
column chromatography (silica gel/CH2Cl2:Hexanes (2:1)), and the pure product was obtained
as a dark red solid in 73% yield. Consistent with the previously developed BODIPY
semiconductors, BDY-Ph-2T-Ph-BDY’s A-D-A π-system is found to be highly soluble in
common organic solvents despite the absence of long lipophilic alkyl substituents. This points
to the combined effects of large molecular dipoles of BODIPY units and increased inter-ring
torsions between D and A units enhancing interactions with the solvent molecules. The
chemical structures and purities of BDY-Ph-Br and BDY-Ph-2T-Ph-BDY were established
by 1H/13C NMR (Figures S1, S2, S4, S5), MALDI-TOF mass spectrometry (MS) (Figures S3
and S6), and elemental analysis. The thermogravimetric analysis (TGA) (10 °C min-1 heating
rate, under nitrogen) of the new molecule indicates excellent thermal stability with the
decomposition onset (5% mass loss) temperature at 367 °C (Figure S7). However, standard
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melting temperature measurement and differential scanning calorimetry (DSC) analysis did
not show any thermal transition prior to the molecular decomposition.
Scheme 1. Synthetic route to BDY-Ph-2T-Ph-BDY.
Although single-crystals of the final molecular structure, BDY-Ph-2T-Ph-BDY,
could not be obtained, red crystals of the intermediate compound BDY-Ph-Br (Scheme 1)
suitable for single-crystal analysis were grown by diffusion of hexane into a chloroform
solution at room temperature (Figure 2). BDY-Ph-Br crystallizes in the monoclinic P21/n
space group, and the solid-state structure reveals highly coplanar “boron-dipyrromethene”
core (C9BN2) subunit with two ethyls (-C2H5) lying nearly in the same plane with tilt angles
of 1.57° and 9.16°. The “B-F” bond distances and “F-B-F”, “N-B-N”, and “N-N-F” bond
angles match closely with those of previously reported BODIPY-based π-conjugated small
[31,42]
molecules
As shown in Figure 2B, the dihedral angle between the meso-phenyl group
and the boron-dipyrromethene π-core was found to 65.97°, which matches well with the
computationally optimized geometries. This angle is large as compared to that of meso-
thiophene substituted BODIPY (θdihedral = 48.8°) in BDY-4T-BDY, which doubtless reflects
sterically encumbered nature of the six-membered phenyl ring having “C-H” bonds pointing
towards the BODIPY’s 1,7-positions. Note that this observed dihedral angle is in the range of
[43,44]
those previously reported meso-phenyl substituted BODIPY small molecules
Although
π-π stacking interactions in the solid-state are not effective showing relatively large centroid-
to-centroid distances of > 4.6 Å, strong “C-H···π” (3.31 Å) interactions and short “C-H···Br”
(2.55 Å << rvdw(Br) + rvdw(H) = 3.09 Å) and “C-H···F” (2.51-2.55 Å << rvdw(F) + rvdw(H) =
12
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2.67 Å) contacts are found to play key roles in forming three-dimensional crystal packing
(Figure 2C). Specifically, “C-H···π” interactions (herringbone-type packing) between
aromatic moieties of the final π-structure could be quite advantageous for three-dimensional
charge transport in BDY-Ph-2T-Ph-BDY thin-film.
Figure 2. A. Optimized molecular geometry for BDY-Ph-2T-Ph-BDY showing inter-ring
torsional angles (θ), computed HOMO/LUMO energies, and frontier molecular orbital
topographies. B. X-ray single-crystal structure of BDY-Ph-Br with 50% ellipsoids and the
perspective view of the inter-ring dihedral angle between “boron-dipyrromethene” and “meso-
phenyl” π-units. (The grey, blue, pink, yellow, and white coloured atoms represent C, N, B, F,
and H, respectively) C. The perspective views of the short “C-H···π”, “C-H···Br”, and “C-
H···F” contacts. (CCDC 1876266 (BDY-Ph-Br) contains the supplementary crystallographic
data for this paper, and these data can be obtained free of charge from The Cambridge
Crystallographic Data Centre)
Optical and electrochemical properties
The optical properties and electronic structure of the new small molecule were studied
by optical absorption/fluorescence spectroscopies and cyclic voltammetry. As shown in
Figure 3A, BDY-Ph-2T-Ph-BDY shows an absorption maximum at 512 nm with the out-of-
plane vibronic features at 484/451 nm (1129/2641 cm-1 from the maximum). This is the
unique absorption profile of meso-aromatic BODIPY’s π-π* transition (S0 → S1).[45] The
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optical band gap is estimated as 2.32 eV from the low energy absorption onset. When
opt
compared with the reference compound BDY-4T-BDY (λmax = 526 nm, Eg = 2.19 eV), the
absorption maximum for BDY-Ph-2T-Ph-BDY shifts to higher energy and the optical band
gap increases. This reflects the combined effects of lowered π-donor strength (Ph-2T-Ph vs.
4T) and increased dihedral angle (lowered π-conjugation) between meso-aromatic and
BODIPY moieties (~66° for phenyl vs. ~49° for thienyl) in the new molecule. UV-Vis
absorption spectra of the corresponding solution-processed BDY-Ph-2T-Ph-BDY thin-film
exhibits a significantly red-shifted absorption profile with the maximum located at 542 nm.
The solid-state optical band gap is estimated as 2.11 eV. This result points to intramolecular
π-core planarization and enhanced intermolecular interactions in BDY-Ph-2T-Ph-BDY’s
solid-state, which is consistent with the observed thin-film crystallinity (vide infra). Cyclic
voltammetry measurement in THF solution shows a reversible reduction for BDY-Ph-2T-Ph-
BDY with the half-wave potential (E1/2red) located at -0.98 V (vs Ag/AgCl). This indicates a
stable n-doping/undoping characteristics with a low LUMO energy level of -3.42 eV for
BDY-Ph-2T-Ph-BDY. The HOMO energy level is estimated as low as -5.74 eV. Based on
the frontier orbital energetics and electrochemical behavior, it is evident that BDY-Ph-2T-Ph-
BDY is a potential n-channel semiconductor in field-effect transistors.
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Figure 3. A. Optical absorption spectra of BDY-Ph-2T-Ph-BDY in THF solution (1x10-5 M)
and as thin-film, and the corresponding optical band gaps (Eg). B. Cyclic voltammogram of
-
BDY-Ph-2T-Ph-BDY in THF (0.1 M Bu4N+PF6 , scan rate = 50 mV·s-1) and experimental
HOMO-LUMO energy levels.
The fluorescence properties of the new molecule were explored by both steady-state
and time-resolved fluorescence spectroscopy techniques. Although the absorption profile of
the new molecule remains nearly the same (Δλmax ~ 3-4 nm) in several solvents of different
polarity (Figure 4A), the corresponding fluorescence spectra show dramatic changes upon
polarity increase (Figure 4B). This indicates that the ground-state of BDY-Ph-2T-Ph-BDY
has much less polarity than the excited state. The fluorescence spectrum of BDY-Ph-2T-Ph-
BDY (1x10-5 M in solution, λexc = 510 nm) shows a single sharp emission peak λmax located at
536-538 nm (Stokes shift ~ 25 nm) in a low polarity solvent (e.g., 1,4-dioxane and toluene).
When the fluorescence spectrum is taken in a more polar medium (e.g., chloroform,
dichloromethane, THF), the intensity of the emission peak at higher energy significantly
decreases, and a new broad highly red-shifted (Stokes shift ~ 103-138 nm) emission peak
appears with λmax located at 615-660 nm. Based on these fluorescence characteristics, while
the higher energy peak is attributed to a locally excited (LE) state, lower energy peak
originates from a twisted intramolecular charge transfer (TICT) state.[33,34,46] The higher
wavelength peak shows a pronounced solvatochromism indicating energetic stabilization of
polar TICT state in polar solvents. To further explore the effect of medium polarity on
fluorescence characteristics of BDY-Ph-2T-Ph-BDY, the emission measurements were
carried out by admixing nonpolar toluene and polar THF at different ratios. As shown in
Figure 4C, reduction of solvent polarity by gradual addition of toluene into THF solution
(volume fraction: from 0 to 100%) progressively increases the intensity of the emission peak
at higher energy (~536 nm) and decreases the lower energy emission peak intensity along
with a hypsochromic shift (λem = 635 nm → 613 nm). In solvent mixtures of >30% toluene,
the higher energy peak exhibits higher intensities relative to the lower energy peak and
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eventually becomes the only peak observed in the emission spectra in pure toluene. All these
observations corroborate the assignments of LE and TICT states for BDY-Ph-2T-Ph-BDY.
Figure 4. (A, B) Optical absorption and fluorescence emission spectra of BDY-Ph-2T-Ph-
BDY in solvents with varied polarities. (C) Fluorescence emission spectra of BDY-Ph-2T-
Ph-BDY in Toluene/THF mixtures (solution concentration = 1x10-5 M; excitation wavelength
= 510 nm). (D) Fluorescence decay plots along with fitted data for BDY-4T-BDY (top curve)
and BDY-Ph-2T-Ph-BDY (bottom curve) solutions in THF and Toluene upon excitation at
390 nm.
These fluorescence characteristics are in sharp contrast to those of BDY-4T-BDY that
exhibits only broad emission peaks with highly red-shifted (Stokes shift ~ 104-152 nm)
maxima at 630/678 nm (in Toluene/THF) with no observable high-energy emission (Figure
S8). Therefore, BDY-4T-BDY fluorescence seems to originate only from a twisted
intramolecular charge transfer (TICT) state. These observations clearly indicate that highly
emissive locally excited (LE) state becomes more effective in the new molecule while non-
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radiative decay pathways are minimal as a result of increasing D-A dihedral angle (~66° for
meso-phenyl vs. ~49° for meso-thienyl), reducing π-donor strength (Ph-2T-Ph vs. 4T), and
restricting intramolecular D-A rotations (higher rotational barrier for meso-phenyl vs. meso-
thienyl).[35,36] However, note that higher polarity medium is still found to facilitate LE-state →
TICT-state transition in the new molecular structure, and concurrent emissions from both
emissive states could be observed in relatively polar solvents (Figures 4B and 4C).[34] The
fluorescence quantum yield (ΦF) and lifetime (τF) measurements point to improved
fluorescence characteristics in the new molecular design, and they align well with the
proposed fluorescence mechanisms. Much larger quantum yields of 0.30/0.25 (in
THF/Toluene) are measured for BDY-Ph-2T-Ph-BDY indicating ~6-7 fold increase
compared to the reference compound BDY-4T-BDY (ΦF = 0.053/0.055 in THF/Toluene).
Also, fluorescence lifetimes of 1.72 ns (λem = 535nm) and 1.84 ns (λem = 635nm) are measured
in THF for BDY-Ph-2T-Ph-BDY, which are much larger than those of BDY-4T-BDY (0.4-
0.5 ns) (Figure 4D and Table S3). Furthermore, BDY-Ph-2T-Ph-BDY exhibits much shorter
lifetime of 0.40 ns (λem = 535nm) in toluene since the emission originates only from LE state
with no contribution of charge-transfer.
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Figure 5. (A) Fluorescence emission spectra of BDY-Ph-2T-Ph-BDY in water/THF mixtures
(solution concentration = 1x10-5 M; excitation wavelength = 510 nm). Arrows indicate the
spectral changes upon nonsolvent water addition. (B) The size distribution of the nano-
aggregates in water/THF (70:30, v/v) solution determined by dynamic light-scattering (DLS)
and the SEM image of the BDY-Ph-2T-Ph-BDY nanoparticles/aggregates as film drop-
casted on Si(100) from the same solution. Scale bar denotes 1 μm.
BDY-Ph-2T-Ph-BDY also demonstrates aggregation induced emission (AIE)
behavior, which – when combined with charge-transport properties – has been very crucial to
design novel photonic applications.[47,48] The AIE behavior was studied by adding a
nonsolvent (water) to a well-dissolved solution of the new molecule in THF. As shown in
Figure 5A, upon increasing water volume fraction from 0% to 25%, the lower energy
emission peak initially shows a bathochromic shift of ~40-50 nm as a result of the energetic
stabilization of the TICT state due to the effect of polarity increase. The fluorescence intensity
is observed to be severely quenched at the same time. When nonsolvent water volume fraction
further increases to 50-70% yielding a low-solubility environment for BDY-Ph-2T-Ph-BDY,
hypsochromic shift (~30-40 nm) of the lower energy maximum was observed along with a
significant fluorescence intensity increase in the higher energy peak at 536 nm. This
obviously reveals that BDY-Ph-2T-Ph-BDY molecules transform into solid-state forming
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nano-aggregates, which leads to restricted intramolecular D-A rotations and reduced medium
polarity around the molecules.[33,49] As a result, LE emission becomes dominant with minimal
contribution from the TICT state. Dynamic Light-Scattering (DLS) measurement in
water/THF (70:30, v/v) solution confirms the formation of BDY-Ph-2T-Ph-BDY nano-
aggregates in sizes of ~200-900 nm (davg ~ 460 nm) (Figure 5B). The scanning electron
microscope (SEM) image (Figure 5B) of the corresponding drop-casted film prepared from
this solution on Si(100) shows a 3D network morphology of micron-sized aggregates that
consist of nearly spherical BDY-Ph-2T-Ph-BDY nanoparticles (~200-600 nm), which is
consistent with the DLS particle size distribution.
Thin-Film Microstructure/Morphology and Field-Effect Transistor Characterization
The charge transport characteristics of the new BODIPY small molecule was studied
in top-contact/bottom-gate (TC/BG) OFET devices by solution-shearing BDY-Ph-2T-Ph-
BDY solution (1 mg/mL in chlorobenzene) on PS-brush treated n++-Si/SiO2 substrates. The
microstructure and surface morphology of the solution-sheared BDY-Ph-2T-Ph-BDY thin-
film were characterized by θ-2θ X-ray diffraction (XRD) measurement and atomic force
microscopy (AFM). XRD scans for the semiconductor thin-film showed multiple peaks for
the same diffraction family up to the sixth-order suggesting a reasonably crystalline
microstructure (Figure 6C). The primary diffraction peak (001) was observed at 2θ = 4.26°
corresponding to d-spacing of 20.7 Å, which is smaller than the computed molecular length
(28.8 Å) of BDY-Ph-2T-Ph-BDY. Therefore, it is very probable that the semiconductor
molecules adopt tilted edge-on orientation on the surface with a ~44° tilting angle from the
substrate normal. As shown in Figure 6D, AFM characterization reveals the presence of
micrometer-sized crystalline domains aligned along the shearing direction. The OFET devices
were completed by thermally evaporation of Au source-drain electrodes on the semiconductor
layer. OFETs demonstrated typical n-channel transistor characteristics under vacuum with
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appreciable electron mobility of 0.0054 cm2/V·s and Ion/Ioff ratio of 3.7 × 104 (Figures 6A and
B), which correlate well with BDY-Ph-2T-Ph-BDY’s computed/measured electronic
structure and the observed solid-state microstructure/morphology. The semiconducting
performance of BDY-Ph-2T-Ph-BDY is among the highest of previously reported n-channel
BODIPY semiconductors (μe ~10-4-0.01 cm2/V·s).[14] The small decrease in electron mobility,
as compared with the best results of BDY-4T-BDY (μe = 0.01 cm2/V·s), could be attributed to
increased LUMO energy level, narrower π-delocalization, and increased D-A backbone twist
in the new molecular structure. Increasing D-A backbone twists lead to a decrease in effective
π-conjugation and charge carrier π-delocalization across the molecular backbone, which
results in slightly lowered charge-transport characteristics for the new molecule.[41] Increased
emissive properties of the new molecule could be combined with its appreciable electron
mobility to enable novel photonic applications.
Figure 6. Representative transfer (A) and output (B) curves for n-channel OFETs based on
solution-sheared BDY-Ph-2T-Ph-BDY semiconductor layer. θ-2θ XRD scan (C) and AFM
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topographic image (5 × 5 μm2) of solution-sheared BDY-Ph-2T-Ph-BDY thin-film. The
arrow shows the shearing direction.
Conclusions
We have described the molecular design, synthesis, and characterization of a new
acceptor-donor-acceptor (A-D-A) semiconductor BDY-Ph-2T-Ph-BDY comprising a central
phenyl-bithiophene-phenyl π-donor and BODIPY π-acceptor end-units. Single-crystal X-ray
diffraction (XRD) analysis of the key intermediate compound BDY-Ph-Br shows an
increased D-A dihedral angle of ~66° and strong intermolecular “C-H···π (3.31 Å)”
interactions. BDY-Ph-2T-Ph-BDY exhibits an optical band gap of 2.32 eV with highly
stabilized HOMO and LUMO energies of -5.74 eV and -3.42 eV, respectively. Detailed
photophysical studies reveal important transitions between locally excited (LE) and twisted
intramolecular charge-transfer (TICT) states in solution and solid-state, which could be
controlled by solvent polarity and nano-aggregation. Restricted intramolecular D-A rotations,
reduced π-donor strength, and increased D-A dihedral angle allows for both good
fluorescence efficiency (ΦF = 0.30) and n-channel OFET transport (μe = 0.005 cm2/V·s; Ion/Ioff
= 104-105) for BDY-Ph-2T-Ph-BDY, which indicates a much improved (~6-7 fold)
fluorescence quantum yield compared to meso-thienyl BODIPY semiconductor BDY-4T-
BDY. To the best of our knowledge, considering its electron mobility and emission
characteristics, BDY-Ph-2T-Ph-BDY is among the highest performing BODIPY-based
emissive n-type small molecules. Undoubtedly, our results provide important guidelines
towards designing solution-processable highly fluorescent BODIPY-based molecular
semiconductors for next-generation organic optoelectronics.
Acknowledgement
H.U. acknowledges support from the Scientific and Technological Research Council of
Turkey (TUBITAK) grant number of 114M226.
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Keywords
BODIPY, fluorescence, organic field-effect transistor, organic semiconductor, small molecule
Supporting Information
Chemical characterizations (1H NMR/13C NMR/MALDI TOF-MS spectra) for BDY-Ph-Br
and BDY-Ph-2T-Ph-BDY; thermogravimetric analysis (TGA) of BDY-Ph-2T-Ph-BDY;
single crystal structure data and structure refinement details for BDY-Ph-Br; fluorescence
emission spectra of BDY-4T-BDY in toluene and THF solutions.
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