PL Quenching of Poly(3-hexylthiophene) by 2,2′,7,7′-Tetradiphenylamino-9,9′-Bifluorenylidene

Article abstract of DOI:10.1080/15421406.2014.936803

2,2′,7,7′-Tetradiphenylamino-9,9′-bifluorenylidene, TDPABF, was synthesized by reductive dimerization using Lawesson's reagent. It is soluble in common organic solvents such as dichloromethane, chloroform, and 1,2-dichlorobenzene. Its synthesis was confir

Full text of DOI:10.1080/15421406.2014.936803

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PL Quenching of Poly(3-hexylthiophene)
by 2,2′,7,7′-Tetradiphenylamino-9,9′-
Bifluorenylidene
a
a
a
On You Park , Hee Un Kim & Do-Hoon Hwang
a
Department of Chemistry and Chemistry Institute for Functional
Materials, Pusan National University, Busan, Republic of Korea
Published online: 17 Nov 2014.
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To cite this article: On You Park, Hee Un Kim & Do-Hoon Hwang (2014) PL Quenching of Poly(3-
hexylthiophene) by 2,2′,7,7′-Tetradiphenylamino-9,9′-Bifluorenylidene, Molecular Crystals and Liquid
Crystals, 600:1, 129-137, DOI: 10.1080/15421406.2014.936803
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Mol. Cryst. Liq. Cryst., Vol. 600: pp. 129–137, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.936803
PL Quenching of Poly(3-hexylthiophene) by
,2 ,7,7 -Tetradiphenylamino-9,9 -Bifluorenylidene
ꢀ
ꢀ
ꢀ
2
ON YOU PARK, HEE UN KIM,
∗
AND DO-HOON HWANG
Department of Chemistry and Chemistry Institute for Functional Materials,
Pusan National University, Busan, Republic of Korea
ꢀ ꢀ ꢀ
,2 ,7,7 -Tetradiphenylamino-9,9 -bifluorenylidene, TDPABF, was synthesized by reduc-
2
tive dimerization using Lawesson’s reagent. It is soluble in common organic solvents
such as dichloromethane, chloroform, and 1,2-dichlorobenzene. Its synthesis was con-
1
13
firmed by H-, C- NMR, MALDI-TOF, and FT-IR spectroscopy. Its HOMO and LUMO
energy levels were –5.33 eV and –2.98 eV, respectively, using photoelectron spectroscopy
and UV-Vis absorption spectra. Its fluorescence quenching properties were investigated
through a Stern-Volmer quenching experiment. The PL intensity significantly decreased
on increasing the concentration of TDPABF in a P3HT chloroform solution. The calcu-
4
−1
lated Stern-Volmer quenching constant was 9.1 × 10 M , which is higher than that
reported for PC61BM.
ꢀ
Keywords 9,9 -bifluorenylidene derivative; electron acceptor; photoluminescence
quenching
1. Introduction
Recently, organic photovoltaic devices (OPVs) that use organic p-type (donor) and n-
type (acceptor) semiconductors have been extensively studied as alternate energy sources
because of their many advantages over silicon-based inorganic solar cells, such as simple
device structure, easy fabrication, lightweight, and low production cost [1–3]. Fullerene
derivatives have been widely used as electron-accepting materials for OPVs because they
have high electron mobility and suitable lowest unoccupied molecular orbital (LUMO)
energy levels that can be tuned through molecular engineering [4–7].
However, fullerene derivatives also have some drawbacks as electron acceptors in
OPVs, such as weak light absorption in the UV-visible region, difficult synthetic and
purification processes, and high production cost [8]. Moreover, chemical modification of
fullerene derivatives is limited to several addition reactions. Therefore, the development
of new electron-accepting materials is very important in order to overcome the drawbacks
of fullerene derivatives.
∗
Address correspondence to Prof. Do-Hoon Hwang, Department of Chemistry, Pusan
National University, Busan 609-735, Korea. Tel.: +82-51-510-2232; Fax: +82-51-516-7421. E-mail:
dohoonhwang@pusan.ac.kr
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
[641]/129

130/[642]
O. Y. Park et al.
Several non-fullerene–type electron acceptors have been reported for OPVs, such as
molecules and polymers containing benzothiadiazole [8], diketopyrrolopyrrole (DPP) [9],
ꢀ
ꢀ
ꢀ
or 9,9 -bifluorenylidene (99 BF) [10] derivatives. Among these, 99 BF derivatives have
attracted the most scientific interest because of their unique electron-accepting properties.
ꢀ
Wudl et al. suggested that, in the ground state, 99 BF is forced to adopt a coplanar structure
because of steric repulsion between H1-H1 and H8-H8 . The addition of one electron could
break the double bond, enabling the two fluorenes to easily tilt. Also, one of the fluorene
ꢀ
ꢀ
units could have a 14-π-electron system, which fulfills H u¨ ckel’s requirement for aromacity
ꢀ
[
10]. Accordingly, 99 BF has a high electron affinity, and therefore, easily accepts electrons
from donor polymers in OPVs.
In this paper, we describe the design and synthesis of a new 99 BF derivative, i.e.,
ꢀ
ꢀ
ꢀ
ꢀ
2
,2 ,7,7 -tetradiphenylamino-9,9 -bifluorenylidene (TDPABF), as an electron-accepting
ꢀ
molecule. Four diphenylamine groups were introduced to 99 BF to raise the LUMO energy
level. We also characterized the optical and electrochemical properties and performed a
Stern-Volmer quenching experiment with P3HT to evaluate the effectiveness of TDPABF
as an electron acceptor for OPVs. The synthetic route to TDPABF is shown in Scheme 1.
ꢀ
ꢀ
ꢀ
Scheme 1. Synthetic route for 2,2 ,7,7 -tetra(diphenylamino)-9,9 -bifluorenylidene (TDPABF).
2. Experimental
2
.1 Materials
Fluorene and Lawesson’s reagent were purchased from Alfa Aesar, and chromium oxide,
diphenylamine, sodium (tert-butoxide), and tris(tert-butyl) phosphine were purchased from
Aldrich. Tris(dibenzylidene acetone) dipalladium(0) was purchased from Strem Chemicals,
Inc. All chemicals were used as received without further purification.

PL Quenching of Poly(3-hexylthiophene)
[643]/131
2.2 Material Synthesis
2,7-Dibromo-9H-fluorene (1) and 2,7-dibromo-9H-fluoren-9-one (2) were prepared ac-
cording to previously reported syntheses [11].
2.2.1 Synthesis of 2,7-bis(diphenylamino)-9H-fluoren-9-one (3). A prepared catalyst so-
lution of tris(dibenzylidene acetone)dipalladium(0) (0.2 g, 0.2 mmol) and tris(tert-butyl)
phosphine (0.04 g, 0.2 mmol) in toluene was added to a solution of 2 (3.0 g, 8.9 mmol),
diphenylamine (3.3 g, 19.5 mmol), and sodium (tert-butoxide) (2.0 g, 20.4 mmol) in toluene
◦
(
150 mL); the mixture was refluxed at ∼110 C under a nitrogen atmosphere for one day.
Crude product was obtained after cooling, extraction with ethyl acetate, and the usual work-
up. The crude product was purified by column chromatography (dichloromethane/hexane
1
=
1:3) to give the desired product (3.9 g, 85%). H NMR (300 MHz, CDCl3; δ [ppm]): 7.34
13
(
s, 2H), 7.24–7.30 (m, 10H), 7.03–7.14 (m, 14H). C NMR (75 MHz, CDCl3; δ [ppm]):
1
93.68, 148.54, 147.41, 138.36, 136.06, 129.71, 128.89, 124.84, 123.70, 120.64, 119.72.
−1
FT-IR (KBr, cm ): 3036 (multi, aromatic C H stretches), 1711 (single, C O), 1589,
1493 (single, aromatic C C).
ꢀ ꢀ ꢀ
.2.2 Synthesis of 2,2 ,7,7 -tetra(diphenylamino)-9,9 -bifluorenylidene (TDPABF). A mix-
2
ture of compound 3 (3.0 g, 5.8 mmol) and Lawesson’s reagent (1.7 g, 4.1 mmol) in toluene
70 mL) was heated at reflux for 12 h. Crude product was obtained after cooling, extrac-
(
tion with ethyl acetate, and the usual work-up. The crude product was purified by column
chromatography (dichloromethane/hexane = 1:6) to give the desired product (1.2 g, 21%).
1
H NMR (300 MHz, CDCl3; δ [ppm]): 8.07 (s, 4H), 7.40 (d, 4H), 7.17 (t, 16H), 6.91–6.96
13
(m, 28H). C NMR (75 MHz, CDCl3; δ [ppm]): 147.92, 146.56, 141.82, 139.70, 136.74,
−1
1
29.46, 127.75, 124.15, 123.75, 122.50, 120.35. FT-IR (KBr, cm ): 3032 (multi, aromatic
C–H stretches), 1589, 1493 (single, aromatic C C). MALDI-TOF (m/z = 996.49 (M+)).
Elemental analysis for C74H52N4: calculated, C: 89.13, H: 5.26, N: 5.62; found, C: 88.58,
H: 5.16, N: 6.02.
2.3 Measurements
1H and 13C NMR spectra were recorded using a Varian 300 spectrometer, and Fourier-
transform infrared (FT-IR) spectra were recorded using a Nicolet 380 spectrometer. Ther-
mals analysis was performed on a Q600 under a nitrogen atmosphere at a heating and
◦
cooling rate of 10 C/min. The MALDI-TOF mass spectra were recorded using a Voyager
DE-STR mass spectrometer (Applied Biosystems). The absorption and photoluminescence
spectra were recorded using a Shimadzu UV-3600 spectrometer and Shimadzu RF 5301 PC
fluorometer, respectively. The photoelectron spectra were measured using an AC-2 (Hitachi
High Tech) spectrometer.
3. Result and Discussion
3.1 Characterization
TDPABF was synthesized via reductive coupling of 2,7-bis(diphenylamino)-9H-fluoren-9-
1
one using Lawesson’s reagent, as shown in Scheme 1. Figures 1 show the H NMR spectra
of TDPABF. The aromatic protons in TDPABF appear between 8.07 and 6.91 ppm. The
signal for the four protons that are nearest to the core double bond (Hc) is shifted furthest
downfield and appears at 8.07 ppm as a singlet. The four protons in the fluorene ring (Ha)

132/[644]
O. Y. Park et al.
Figure 1. 1H NMR spectra of TDPABF.
correspond with the doublet at 7.40 ppm, while the sixteen protons in the diphenylamine
group (He) are evident at 7.17 ppm as a triplet. The remaining aromatic proton signals
overlap between 6.91 and 6.96 ppm.
The FT-IR spectra of 3 and TDPABF are shown in Figure 2. The spectra feature
−
1
aromatic C–H stretching at 3036 and 3032 cm , respectively, and aromatic C C stretching
−1
at 1589 and 1493 cm , respectively. The spectrum of 3 shows a strong carbonyl absorption
−1
peak at 1711 cm , whereas the carbonyl peak is completely absent in the spectrum of
TDPABF. The absorption peak corresponding to the core double bond was not observed
because the symmetrically substituted double bond is inactive in FT-IR spectroscopy.
The thermal stability of TDPABF was evaluated by thermogravimetric analysis (TGA)
under a nitrogen atmosphere. TDPABF showed good thermal stability, losing <5% of its
◦
weight upon heating up to 400 C. This value is similar to that for PC61BM and other
fullerene derivatives [12]. The melting point of TDPABF is 162.3 C, as determined by
◦
differential scanning calorimetry (DSC). The TGA and DSC thermograms of TDPABF are
shown in Figure 3.
3.2 Simulation of Molecular Structure
To elucidate the three-dimensional shapes of the molecules and the electron distributions
in the highest occupied molecular orbital (HOMO) and LUMO energy levels of TDPABF,
density functional theory (DFT) calculations were performed on a model compound using
DMol 3 software [13, 14]. Figure 4 shows the three-dimensional shapes of TDPABF and
the electron distribution of the HOMO and LUMO energy levels. The dihedral angle (θ)

PL Quenching of Poly(3-hexylthiophene)
[645]/133
Figure 2. FT-IR spectra of 3 and TDPABF.
◦
between the two core fluorene units that are attached by a double bond is 40.4 because
of the steric interaction of the bulky diphenylamine groups. The electrons in the HOMO
energy level are distributed on one side of the fluorene and diphenylamine units, whereas
they are mainly distributed on the core double bond and fluorene units in the LUMO energy
level.
3.3 Optical and Electrochemical Properties
Figure 5 shows the UV-visible absorption spectra of TDPABF in chloroform solution
and as a solid thin film. The absorption spectrum of the chloroform solution shows four
main absorption bands at approximately 306, 319, 375, and 462 nm; the absorption peaks
Figure 3. TGA and DSC thermograms of TDPABF.

134/[646]
O. Y. Park et al.
Figure 4. Three-dimensional shape of TDPABF and electron distribution of the HOMO and LUMO
energy levels.
∗
in the ultraviolet region correspond to the π-π transitions of the diphenylamine and
∗
fluorene units, and the absorption band at 462 nm corresponds to the π-π transition of the
9 BF structure. The latter absorption band of TDPABF is slightly red-shifted from that
previously reported for 99 BF (458 nm) [10] because of the longer effective conjugation
ꢀ
9
ꢀ
Figure 5. UV-vis spectra of TDPABF in chloroform solution and as a solid thin film.

PL Quenching of Poly(3-hexylthiophene)
[647]/135
Figure 6. Photoelectron spectra of TDPABF.
of TDPABF due to the incorporation of the diphenylamine groups. Interestingly, the UV-
visible absorption spectra of TDPABF in solution and as a solid thin film state were very
similar, which is consistent with the simulated structure of TDPABF, because the tilted-core
structure likely impairs effective packing of the molecule in the solid state. The maximum
absorption (λmax) and edge wavelengths (λonset) of the TDPABF solid thin film state were
4
66 and 528 nm, respectively. The optical band gap energy (Egopt) of the TDPABF solid
thin film was determined to be 2.35 eV from its onset absorption wavelength (λonset). The
ꢀ
measured optical band gap of TDPABF is slightly smaller than that reported for 99 BF
(
∼2.42 eV) because of the introduction of electron-donating diphenylamine moieties.
The HOMO energy level of the TDPABF solid thin film was determined using pho-
toelectron spectroscopy. The measured ionization potential of TDPABF was −5.33 eV, as
shown in Figure 6. The LUMO energy level of TDPABF was determined to be −2.98 eV
by adding the measured optical band gap and HOMO energy level (i.e., ELUMO = EHOMO
opt
ꢀ
+
Eg ). The LUMO energy level of TDPABF is much higher than that reported for 99 BF
(
−3.37 eV), which was measured by cyclic voltammetry, possibly because of the increased
electron density in TDPABF owing to the incorporation of the electron-donating dipheny-
lamine groups. Therefore, the electron-donating diphenylamine groups raise the LUMO
ꢀ
energy level of TDPABF relative to that of 99 BF.
To evaluate the electron-accepting ability of TDPABF, we investigated the fluorescence
quenching of poly(3-hexylthiophene) (P3HT) by TDPABF. The photoluminescence (PL)
intensities of P3HT solutions were measured with different mole ratios of TDPABF. At low
concentrations of fluorescence quencher, relationship between the fluorescence intensity
and concentration of quencher can be expressed by Stern-Volmer equation as follows:
ꢀ
◦
ꢀ
ꢀ = 1 + Ksv[quencher],
o
where ꢀ and ꢀ represent the PL intensity in the absence and presence of the quencher,
respectively, and Ksv is the Stern-Volmer quenching constant. The obtained Stern-Volmer
plot for TDPABF is shown in Figure 7. The plot is linear in the quencher concentration

136/[648]
O. Y. Park et al.
Figure 7. PL-quenching spectra of P3HT:TDPABF in chloroform solution with increasing concen-
−
5
−5
tration of TDPABF are shown in the inset: (1) 0.0 M, (2) 2.0 × 10 M, (3) 4.0 × 10 M, (4) 6.0 ×
−
5
−5
10
M, (5) 8.0 × 10 M.
range 0–8.0 × 10− M. The PL intensity of P3HT dramatically decreased after the addition
5
4
−1
of TDPABF. The obtained Ksv value was 9.1 × 10 M , which is higher than that reported
3
−1
for PC61BM (2.4 × 10 M ) with P3HT as the fluorophore [15]. The high Ksv value of
TDPABF indicates that it has a higher binding affinity for P3HT than PC61BM. The PL
quenching experiment and high Ksv value suggest that TDPABF efficiently accepts electrons
from the P3HT donor polymer. TDPABF is expected to be used as a new electron acceptor
in organic photovoltaic cells. The fabrication of OPVs using TDPABF is currently being
investigated.
4. Conclusions
We designed and synthesized TDPABF as a non-fullerene–type electron acceptor. The
1
13
synthesis of TDPABF was confirmed using H NMR, C NMR, MALDI-TOF, and FT-IR
spectroscopy. TDPABF showed a broader absorption spectrum than fullerene derivatives
and exhibited a good fluorescence-quenching ability with a relatively high Stern-Volmer
quenching constant. We expect that TDPABF will be applied as a new n-type electron
acceptor in organic photovoltaic cells.
Funding
This work was supported by the industrial strategic technology development program
(No. 10039141, Development of core technologies for organic materials applicable to
OLED lightings with high color rendering index) funded by the Ministry of Knowledge
Economy (MKE, Korea).

PL Quenching of Poly(3-hexylthiophene)
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