Synthesis and photophysical properties of 2,2′-bis(oligothiophene)-9,9′-bifluorenylidene derivatives

Article abstract of DOI:10.1039/c7nj03926c

Herein, three novel 9,9′-bifluorenylidene derivatives containing thiophenyl, terthiophenyl, and tetrathiophenyl groups at the 2- and 2′-positions of the 9,9′-bifluorenylidene skeleton were synthesized and characterized. Their photophysical and electrochemical properties were investigated systematically. The ground state geometries and electronic distributions of the HOMO and LUMO energy levels of the 9,9′-bifluorenylidene derivatives were calculated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) at the B3LYP/6-31G(d) level.

Full text of DOI:10.1039/c7nj03926c

View Article Online
View Journal
NJC
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: T. Yu, W. Guan, X.
Wang, Y. Zhao, Q. Yang, Y. Li and H. Zhang, New J. Chem., 2018, DOI: 10.1039/C7NJ03926C.
This is an Accepted Manuscript, which has been through the
Volume 40 Number
1 January 2016 Pages 1–846
Royal Society of Chemistry peer review process and has been
accepted for publication.
NJC
Accepted Manuscripts are published online shortly after
New Journal of Chemistry
www.rsc.org/njc
A journal for new directions in chemistry
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

Page 1 of 29
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ03926C
Synthesis and photophysical properties of
2,2'-Bis(oligothiophene)-9,9'-bifluorenylidene derivatives
Tianzhi Yu1,*, Weijia Guan1, Xin Wang1, Yuling Zhao2, Qianguang Yang1, Yanmei Li1, Hui
Zhang1
1Key Laboratory of OptoꢀElectronic Technology and Intelligent Control (Ministry of Education),
Lanzhou Jiaotong University, Lanzhou 730070, China
2School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070,
China
Abstract: Three novel 9,9'ꢀbifluorenylidene derivatives containing thiophenyl, terthiophenyl and
tetrathiophenyl groups at 2ꢀ and 2'ꢀposition of 9,9'ꢀbifluorenylidene skeleton, respectively, were
synthesized and characterized. Their photophysical and electrochemical properties were
investigated systematically. The ground state geometries and electronic distributions in HOMO
and LUMO energy levels of the 9,9'ꢀbifluorenylidene derivatives were calculated by density
functional theory (DFT) and timeꢀdependent density functional theory (TDꢀDFT) at
B3LYP/6ꢀ31G(d) level.
Keywords: 9,9'ꢀBifluorenylidene derivative; Oligothiophene; Electron acceptor; DFT study;
HOMO and LUMO energy levels
1. Introduction
Nowadays, with concerns about environmental pollution and depletion of fossil energy, the
*Corresponding Author. Tel. +86ꢀ931ꢀ4956935; Fax: +86ꢀ931ꢀ4938756; eꢀmail: yutzh@mail.lzjtu.cn (T.Z. Yu).
1

New Journal of Chemistry
Page 2 of 29
View Article Online
DOI: 10.1039/C7NJ03926C
renewable energies and all kinds of green energy have attracted more and more attentions. The
renewable energies mainly involve natural resources such as sunlight, wind, water, biomass, tides
and geothermal energy. Among the possible renewable energy resources, solar energy is one kind
of clean, renewable, abundant and the most promising energy resource, which is an ideal
alternative to the traditional energy resources. Over the past few decades, siliconꢀbased inorganic
solar cells have got great development and achievement, their power conversion efficiency (PCE)
has closely reached 40% [1]. However, high material and manufacturing costs and high
environmental pollution limit its wide applications. Organic photovoltaics (OPVs) have drawn
great attention over the last two decades as alternative photovoltaic cells. Compared with the
inorganic solar cells, the organic photovoltaics have some advantages of easy fabrication, low cost,
light weight and the possibility to fabricate flexible devices [2ꢀ4]. At present, the field of organic
photovoltaics has progressed quite significantly, particularly through the development of
solutionꢀprocessed bulk heterojunction (BHJ) OPVs, and the highest efficiency is over 12% [5ꢀ7]
.
Contemporary organic photovoltaic cells are based on a bulk heterojunction that results from
the contact between electron donor and electron acceptor materials. In a typical BHJ OPVs, the
photoactive blend layer consists of a conjugated polymer donor and a soluble electron acceptor,
which is sandwiched between a PEDOT:PSS coated ITO positive electrode and a low work
function metal negative electrode. The conjugated polymer donor serves as the main solar light
absorber and as the hole transporting phase, whereas the electron acceptor transports electrons.
Broad visible absorption, higher charge carrier mobility, and suitable electronic energy levels of
both the donor and acceptor materials are crucial and momentous for highly efficient polymer
solar cells (PSCs) [8]. In order to improve the power conversion efficiency of polymer solar cells,
2

Page 3 of 29
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ03926C
various new donor and acceptor photovoltaic materials have been investigated in recent years
[3,5ꢀ7,9ꢀ17]. Among the donor materials, the polythiophene derivatives are the most representative
donor materials, such as polythiophene derivatives with conjugated side chains for broad
absorption [18,19], crossꢀlinking polythiophene derivatives with conjugated bridges for higher hole
mobility [18,20], poly(thienylene vinylene) derivatives with broad visible absorption [21], and the
copolymers of benzodithiophene and thieno[3,4ꢀb]thiophene with lower HOMO energy levels and
broad absorption band [22ꢀ24]. In addition to the polymer donor materials, there are various
smallꢀmolecular donor materials. The smallꢀmolecular donor materials for OPVs have attracted
increasing attention due to their advantages such as wellꢀdefined molecular structures, definite
molecular weights and high purities [25]. Some representative smallꢀmolecular donors, such as dyes
[28,29]
[26,27], oligothiophenes
and triphenylamineꢀbased molecules [30,31], have been applied
successfully in OPV devices. In the OPVs, although the acceptor materials play an equally
important role as the donor materials for highꢀperformance OPV devices, the research efforts
devoted to the acceptor materials have been much less than those on the donor materials. The
fullerene derivatives, such as phenylꢀC61/71ꢀbutyric acid esters (PC60/70BM) and indeneꢀC60/70
bisadducts (IC60/70BA), have been the most successful acceptors in OPV devices for higher power
conversion efficiency so far, which have unique πꢀelectron system, excited state electronic
properties, higher electron affinity and good electron transporting ability [3,32]. Despite fullerene
derivatives have many excellent optoelectronic properties and even dominated the field of
acceptors of BHJ solar cells, they still have some shortcomings, such as weak absorption in the
visible region, poor solubility in common solvents, lengthy synthetic routes, low yields and
difficult purification processes. Therefore, it is very important and urgently required for higher
3

New Journal of Chemistry
Page 4 of 29
View Article Online
DOI: 10.1039/C7NJ03926C
PCE to synthesize nonꢀfullerene acceptors with higher absorption in the visible spectrum, lowꢀcost
synthesis and controllable frontier orbital energy levels. Recently, researchers have reported some
representative nonfullerene small molecule acceptors with adjustable energy level, excellent
solubility and wide absorption in the visible spectrum for highꢀperformance OPV devices [5ꢀ7,33,34]
,
including diketopyrrolopyrrole[35,36], rylene diimide [37,38], indacenodithieno[3,2ꢀb]thiophene (ITIC)
[5ꢀ7], benzothiadiazole[39] derivatives and so on.
Oligothiophenes consist of πꢀconjugated units with highly delocalized πꢀsystems, imbuing
them with unique electrical and optical properties that can be easily tuned via rational structural
modifications [40]. Some functionalized oligothiophenes have now been rationally designed,
synthesized and validated as efficient materials for organic electronic devices [41,42]. The
electronꢀwithdrawing substituents, such as dicyanovinyl, perfluoroalkyl and perfluoroarene,
introduction into πꢀelectron frameworks has proven to be a promising approach to enhance
electron injection and electron transport properties of these materials. In comparison to polymeric
materials, πꢀconjugated small molecules and oligomers have the distinct advantages of
straightforward synthesis and purification leading to defined molecular structures, which greatly
improve the fabrication reproducibility and avoid batchꢀtoꢀbatch variations.
Recently, 9,9′ꢀbifluorenylidene (99'BF) derivatives as nonꢀfullerene type electron acceptors
have attracted much attentions because 99'BF skeleton could easily accept one electron to form
14ꢀ πꢀelectron system and thus it could have aromaticity [43ꢀ48]. Brunetti et al. fabricated the first
BHJ
solar
cell
with
P3HT/D99'BF
(12ꢀ(3,6ꢀdimethoxyꢀfluorenꢀ9ꢀylidene)ꢀ12Hꢀdibenzo[b,h]fluorene), which showed a reasonable
photovoltaic effect (Voc = 1.10 V, PCE = 1.7%) [43]. Most noticeably, when blended with P3HT,
4

Page 5 of 29
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ03926C
D99'BF increases the optical density and allows the ultrafast charge transfer and the charge
separation, and an electron transfer between the LUMO energies offset lower than the empirical
value of 0.3–0.5 eV. Though several 99'BF derivatives have been synthesized as small molecule
electronꢀacceptors for PSCs [46ꢀ48], their BHJ solar cells have lower PCE due to their lower
electron mobility.
In this work, we synthesized three novel 9,9'ꢀbifluorenylidene derivatives containing
thiophenyl, terthiophenyl and tetrathiophenyl groups at 2ꢀ and 2'ꢀposition of 9,9'ꢀbifluorenylidene
skeleton, respectively, and investigated their photophysical and electrochemical properties. The
synthetic routes and chemical structures of the 9,9'ꢀbifluorenylidene derivatives are shown in
Scheme1.
2. Experimental
2.1. Materials and methods
2ꢀBromoꢀ3ꢀoctylthiophene, 2ꢀbromineꢀ9ꢀfluorenone, 5,5'ꢀdibromoꢀ2,2'ꢀbithiophene were
purchased from Puyang Huicheng Electronic Material Co., Ltd (China). nꢀBuLi,
tetrakis(triphenylphosphine)palladium (Pd(PPh3)4), Lawesson’s reagent, bis(pinacolato)diboron,
[1,1'ꢀbis(diphenylphosphino)ferrocene]dichloropalladium(II) (PdCl2(dppf)) and tetrabutyl
ammonium bromide (TBAB) were bought from Energy Chemical (China).
2ꢀIsopropoxyꢀ4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolane was purchased from Yurui (Shanghai)
Chemical CO., LTD (China). Potassium acetate was bought from Tianjin Kaixin Chemical
Industry CO., LTD (China), and Nꢀbromobutanimide (NBS) was purchased from Shanghai Crystal
Pure Reagent CO., LTD (China). THF, toluene and 1,4ꢀdioxane were purified and freshly distilled
prior in the related reaction. All the other chemicals were analytical grade reagent.
5

New Journal of Chemistry
Page 6 of 29
View Article Online
DOI: 10.1039/C7NJ03926C
1
H and 13C NMR spectra were recorded on Varian Mercury Plus 400 MHz and Agilent
Technologies DDZ 600 MHz. IR spectra (400–4000 cmꢀ1) were measured on a Shimadzu
IRPrestigeꢀ21 FTꢀIR spectrophotometer with KBr pellets. Mass spectrum was recorded using a
Thermo Scientific Orbitrap Elite mass spectrometer. UVꢀvis absorption was recorded on a
Shimadzu UVꢀ2550 spectrometer. CV (CH Instruments 760 B) was performed with a 0.10 mol/L
solution of tetrabutylammonium hexafluorophosphate (Bu4NPF6) in dichloromethane, with the
analyte present in a concentration of 10ꢀ3 mol/L and employing a scan rate of 100 mV/s at room
temperature. A Pt electrode was used as the working electrode, while a Pt wire and an Ag/Ag+
electrode were used as the counter electrode and reference electrode, respectively.
2.2 Synthesis and characterization of the 9,9'ꢀbifluorenylidene derivatives (STBF, TTBF and
FTBF)
2.2.1. Synthesis and characterization of STBF
4,4,5,5-Tetramethyl-2-(3-octylthiophen-2-yl)-1,3,2-dioxaborolane: Under N2 atmosphere,
2ꢀbromoꢀ3ꢀoctylthiophene (2 g, 7.27 mmol) and 50 mL anhydrous tetrahydrofuran were placed in
a round bottom flask (250 mL), then nꢀBuLi (3 mL, 2.5 M in hexane) was added the reaction
mixture
under
ꢀ
78
oC
and
stirred
for
2
h.
Then
2ꢀisopropoxyꢀ4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolane (1.66 mL, 8.0 mmol) was added into the
o
mixture and the mixture was stirred at ꢀ78 C for 5 h. Afterwards, saturated salt water was added,
and the mixture was extracted with dichloromethane (3 × 100 mL). The combined organic layer
was dried by anhydrous MgSO4 and concentrated under reduced pressure. The residue was
purified by column chromatography on silica gel using dichloromethane/petroleum (1:6, v/v) as
eluent to yield brownish yellow oily compound (1.5 g, 64%). 1H NMR (400 MHz, CDCl3, δ, ppm):
6

Page 7 of 29
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ03926C
7.48 (d, J = 8.0 Hz, 1H), 7.01 (d, J = 8.0 Hz, 1H), 2.89 (t, J = 8.0 Hz, 2H), 1.57 (t, J = 8.0 Hz, 2H),
1.33 (s, 12H), 1.30–1.26 (m, 10H), 0.87 (t, J = 4.0 Hz, 3H).
2-(3-octylthiophen-2-yl)-9H-fluoren-9-one:
Under
N2
atmosphere,
4,4,5,5ꢀTetramethylꢀ2ꢀ(3ꢀoctylthiophenꢀ2ꢀyl)ꢀ1,3,2ꢀdioxaborolane (2.25 g, 6.98 mmol),
2ꢀbromineꢀ9ꢀfluorenone (2 g, 7.72 mmol), Pd(PPh3)4 (0.8 g, 0.7 mmol) and TBAB (0.112 g, 0.35
mmol) was dissolved in 50 mL freshed toluene, then 10 mL of 2M Na2CO3 was added in the
mixture solution. The mixture was refluxed for 30 h. After cooling, saturated salt water was added
and the product was extracted with dichloromethane (3 × 100 mL), then the combined organic
layer was dried by anhydrous MgSO4 and concentrated under reduced pressure. The residue was
purified by column chromatography on silica gel using ethyl acetate/petroleum (1:20, v/v) as
eluent to yield brown oily compound (2.02 g, 77.3%). 1H NMR (400 MHz, CDCl3, δ, ppm): 7.73–
7.70 (m, 1H), 7.70–7.65 (m, 1H), 7.56–7.47 (m, 4H), 7.30 (t, J = 8.0 Hz, 1H), 7.25 (t, J = 6.0 Hz,
1H), 6.99 (d, J = 8.0 Hz, 1H), 2.66 (t, J = 8.0 Hz, 2H), 1.65–1.54 (m, 2H), 1.35–1.14 (m, 10H),
0.84 (t, J = 8.0 Hz, 3H). 13C NMR (100 MHz, CDCl3, δ, ppm): 193.55, 144.21, 143.04, 139.45,
136.48, 135.98, 135.30, 134.79, 134.44, 129.66, 129.07, 125.07, 124.39, 124.09, 120.35, 31.84,
30.88, 29.40, 29.33, 29.21, 28.73, 22.64, 14.07.
(E)-2,2'-Bis(3-octylthiophen-2-yl)-9,9'-bifluorenylidene (STBF): To a toluene solution (20
mL) of 2ꢀ(3ꢀoctylthiophenꢀ2ꢀyl)ꢀ9Hꢀfluorenꢀ9ꢀone (1g, 2.67 mmol) was added Lawesson’s reagent
(0.62 g, 1.53 mmol) under N2 atmosphere. The mixture was stirred and refluxed overnight. The
cooled solution was concentrated under reduced pressure. The residue was purified by column
chromatography on silica gel using dichloromethane/petroleum (1:10, v/v) as eluent to yield dark
brown sticky compound, then it was freezeꢀdried under reduced pressure to give dark brown solid
7

New Journal of Chemistry
Page 8 of 29
View Article Online
DOI: 10.1039/C7NJ03926C
1
STBF (0.5 g, 52.2%). H NMR (400 MHz, CDCl3, δ, ppm): 8.50–8.33 (m, 4H), 7.73–7.60 (m,
4H), 7.43–7.06 (m, 8H), 6.89 (dd, J = 8.0, 8.0 Hz, 2H), 2.69–2.46 (m, 4H), 1.58–1.37 (m, 4H),
1.28–0.97 (m, 20H), 0.93–0.70 (m, 6H). 13C NMR (100 MHz, CDCl3, δ, ppm): 141.13, 140.24,
138.75, 138.69, 138.60, 138.49, 138.43, 138.38, 137.98, 133.63, 130.38, 129.55, 129.43, 129.40,
127.91, 127.45, 127.07, 126.98, 126.74, 123.57, 120.02, 119.90, 119.83, 31.91, 31.09, 31.00,
29.53, 29.47, 29.43, 29.36, 29.32, 28.79, 28.59, 22.74, 14.18, 14.15. HRMS: Calcd. for C50H52S2,
716.35; Found: 717.3572.
2.2.2. Synthesis and characterization of TTBF
2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-fluoren-9-one: A stirred mixture of
2ꢀbromineꢀ9ꢀfluorenone (2 g, 7.72 mmol), bis(pinacolato)diboron (3.8 g, 14.96 mmol),
Pd(dppf)Cl2 (0.28 g, 0.386 mmol), potassium acetate (2.9 g, 29.55 mmol) and 1,4ꢀdioxane (60 mL)
was refluxed for 24 h. After cooling, saturated salt water was added and the product was extracted
with dichloromethane (3 × 100 mL), then the combined organic layer was dried by anhydrous
MgSO4 and concentrated under reduced pressure. The residue was purified by column
chromatography on silica gel using ethyl acetate/petroleum (1:6, v/v) as eluent to yield yellow
solid compound (2.0g, 84.6%). 1H NMR (400 MHz, CDCl3, δ, ppm): 8.12 (s, 1H), 7.94 (d, J = 8.0
Hz, 1H), 7.66 (d, J = 8.0 Hz, 1H), 7.56–7.46 (m, 3H), 7.31 (t, J = 8.0 Hz, 1H), 1.36 (s, 12H).
5''-bromo-3-octyl-2,2':5',2''-terthiophene (1): The preparation of the compound 1 was
similar to that described for the compound 2ꢀ(3ꢀoctylthiophenꢀ2ꢀyl)ꢀ9Hꢀfluorenꢀ9ꢀone, which was
obtained
from
the
reaction
between
4,4,5,5ꢀTetramethylꢀ2ꢀ(3ꢀoctylthiophenꢀ2ꢀyl)ꢀ1,3,2ꢀdioxaborolane (1.5 g, 4.654 mmol) and
5,5'ꢀdibromoꢀ2,2'ꢀbithiophene (1.961 g, 6.05 mmol) in the presence of Na2CO3 (1.48 g, 13.96
8

Page 9 of 29
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ03926C
mmol), TBAB (0.075 g, 0.233 mmol) and Pd(PPh3)4 (0.269 g, 0.233 mmol). The crude product
was purified by column chromatography on silica gel using petroleum as eluent to yield yellow
oily compound (1.0 g, 49 %). 1H NMR (400 MHz, CDCl3, δ, ppm): 7.17 (d, J = 7.6 Hz, 1H), 7.04
(s, 1H), 6.98 (dd, J = 8.0, 7.4 Hz, 2H), 6.92 (dd, J = 8.0, 7.6 Hz, 2H), 2.75 (t, J = 8.0 Hz, 2H),
1.69–1.58 (m, 2H), 1.39–1.22 (m, 10H), 0.87 (t, J = 8.0 Hz, 3H). 13C NMR (100 MHz, CDCl3, δ,
ppm): 139.98, 138.63, 136.88, 136.72, 130.61, 130.06, 130.00, 126.42, 124.16, 123.97, 123.60,
110.91, 31.87, 30.63, 29.50, 29.39, 29.24, 29.23, 22.66, 14.11.
2-(3''-octyl-[2,2':5',2''-terthiophen]-5-yl)-9H-fluoren-9-one (4): The preparation of the
compound
4
was
similar
to
that
described
for
the
compound
2ꢀ(3ꢀoctylthiophenꢀ2ꢀyl)ꢀ9Hꢀfluorenꢀ9ꢀone, which was obtained from the reaction between
2ꢀ(4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolanꢀ2ꢀyl)ꢀ9Hꢀfluorenꢀ9ꢀone (0.669 g, 2.184 mmol) and
5''ꢀbromoꢀ3ꢀoctylꢀ2,2':5',2''ꢀterthiophene (0.80 g, 1.82 mmol) in the presence of Na2CO3 (0.695 g,
6.552 mmol), TBAB (0.029 g, 0.090 mmol) and Pd(PPh3)4 (0.105 g, 0.091 mmol). The crude
product was purified by column chromatography on silica gel using dichloromethane/petroleum
1
(1:6, v/v) as eluent to yield orange powdery compound 4 (0.64 g, 65.3%). H NMR (600 MHz,
CDCl3, δ, ppm): 7.88 (d, J = 7.2 Hz, 1H), 7.67 (d, J = 7.6 Hz, 2H), 7.50 (d, J = 7.6 Hz, 3H), 7.29
(d, J = 7.8 Hz, 2H), 7.21–7.11 (m, 3H), 7.03 (d, J = 7.8 Hz, 1H), 6.94 (d, J = 7.2 Hz, 1H), 2.79 (d,
J = 6.0 Hz, 2H), 1.66 (d, J = 12.0 Hz, 2H), 1.43–1.23 (m, 10H), 0.88 (d, J = 6.0 Hz, 3H). 13C
NMR (150 MHz, CDCl3, δ, ppm): 193.45, 144.20, 142.94, 141.64, 139.92, 137.11, 136.62, 135.64,
135.00, 134.92, 134.85, 134.32, 131.06, 130.23, 130.11, 128.99, 126.50, 124.49, 124.41, 124.09,
123.90, 121.02, 120.78, 120.33, 31.89, 30.65, 29.55, 29.43, 29.30, 29.27, 22.68, 14.12.
(E)-2,2'-bis(3''-octyl-[2,2':5',2''-terthiophen]-5-yl)-9,9'-bifluorenylidene (TTBF): The
9

New Journal of Chemistry
Page 10 of 29
View Article Online
DOI: 10.1039/C7NJ03926C
preparation of the compound TTBF was similar to that described for the compound STBF, which
was obtained from the compound 4 in the presence of Lawesson’s reagent. The crude product was
purified by column chromatography on silica gel using dichloromethane/petroleum (1:8, v/v) as
eluent to yield dark brown oily compound, then it was cured from cold methanol to give brown
powdery solid TTBF (45% yield). 1H NMR (600 MHz, CDCl3, δ, ppm): 8.60 (s, 1H), 8.56 (s, 1H),
8.34 (d, J = 7.8 Hz, 2H), 7.61–7.49 (m, 4H), 7.43 (d, J = 8.0 Hz, 2H), 7.31–7.24 (m, 2H), 7.15 (m,
4H), 7.07–6.80 (m, 9H), 2.80–2.68 (m, 4H), 1.70–1.57 (m, 4H), 1.43–1.23 (m, 20H), 0.95–0.85
(m, 6H). 13C NMR (150 MHz, CDCl3, δ, ppm): 143.28, 143.00, 141.13, 141.02, 141.00, 140.44,
140.34, 139.73, 139.55, 138.63, 138.56, 138.27, 137.00, 136.85, 136.38, 136.19, 135.20, 135.12,
132.76, 132.49, 130.55, 130.46, 130.09, 129.99, 129.52, 129.45, 126.87, 126.71, 126.69, 126.42,
126.33, 124.50, 124.40, 123.86, 123.81, 123.76, 123.75, 123.70, 123.59, 120.30, 120.23, 120.08,
119.96, 31.96, 30.69, 30.61, 29.63, 29.59, 29.51, 29.35, 29.34, 29.32, 22.75, 14.20. HRMS: Calcd.
for C66H60S6, 1044.30; Found: 1045.3076.
2.2.3. Synthesis and characterization of FTBF
3,3'''-dioctyl-2,2':5',2'':5'',2'''-quaterthiophene (2): The preparation of the compound 2
was similar to that described for the compound 2ꢀ(3ꢀoctylthiophenꢀ2ꢀyl)ꢀ9Hꢀfluorenꢀ9ꢀone, which
was
obtained
from
the
reaction
between
4,4,5,5ꢀTetramethylꢀ2ꢀ(3ꢀoctylthiophenꢀ2ꢀyl)ꢀ1,3,2ꢀdioxaborolane (1.8 g, 5.585 mmol) and
5,5'ꢀdibromoꢀ2,2'ꢀbithiophene (0.778 g, 2.40 mmol) in the presence of Na2CO3 (1.776 g, 16.755
mmol), TBAB (0.0387 g, 0.12 mmol) and Pd(PPh3)4 (0.139 g, 0.12 mmol). The crude product was
purified by column chromatography on silica gel using petroleum as eluent to yield yellow oily
compound 2 (57% yield). 1H NMR (400 MHz, CDCl3, δ, ppm): 7.18 (d, J = 8.0 Hz, 2H), 7.12 (d, J
10

Page 11 of 29
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ03926C
= 7.6 Hz, 2H), 7.02 (d, J = 7.8 Hz, 2H), 6.94 (d, J = 8.0 Hz, 2H), 2.78 (t, J = 8.0 Hz, 4H), 1.70–
1.60 (m, 4H), 1.41–1.23 (m, 20H), 0.87 (t, J = 8.0 Hz, 6H). 13C NMR (100 MHz, CDCl3, δ, ppm):
139.87, 136.78, 135.29, 130.28, 130.07, 126.51, 123.84, 123.80, 31.88, 30.67, 29.52, 29.41,
29.26, 22.68, 14.12.
5-bromo-3,3'''-dioctyl-2,2':5',2'':5'',2'''-quaterthiophene (3): The compound 2 (1.8 g, 3.24
mmol) was dissolved in 30 mL chloroform. NBS (0.534 g, 3.0 mmol) was dissolved in a mixed
solvent of acetic acid (20 mL) and chloroform (20 mL), and then the resulting solution was added
dropwise to the compound 2 solution. The mixture was stirred overnight at room temperature.
Afterwards, the mixture was neutralized with KOH solution, and the product was extracted with
chloroform (3 × 100 mL), then the combined organic layer was dried by anhydrous MgSO4 and
concentrated under reduced pressure. The residue was purified by column chromatography on
1
silica gel using petroleum as eluent to yield yellow oily compound 3 (1.06 g, 51.6%). H NMR
(600 MHz, CDCl3, δ, ppm): 7.19 (t, J = 8.0 Hz, 1H), 7.16–7.10 (m, 2H), 7.05 (t, J = 8.0 Hz, 1H),
7.00–6.94 (m, 2H), 6.92 (s, 1H), 2.82 (t, J = 6.0 Hz, 2H), 2.74 (t, J = 6.0 Hz, 2H), 1.75–1.61 (m,
4H), 1.46–1.26 (m, 20H), 1.00–0.87 (m, 6H). 13C NMR (150 MHz, CDCl3, δ, ppm): 140.39,
139.88, 137.37, 136.47, 135.63, 133.82, 132.71, 131.90, 130.31, 130.11, 126.90, 126.48, 124.01,
123.89, 123.76, 110.61, 31.98, 31.96, 30.72, 30.58, 29.62, 29.51, 29.46, 29.36, 29.33, 29.29, 22.77,
14.21.
2-(3''',4-dioctyl-[2,2':5',2'':5'',2'''-quaterthiophen]-5-yl)-9H-fluoren-9-one
(5):
The
preparation of the compound 5 was similar to that described for the compound
2ꢀ(3ꢀoctylthiophenꢀ2ꢀyl)ꢀ9Hꢀfluorenꢀ9ꢀone, which was obtained from the reaction between
2ꢀ(4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolanꢀ2ꢀyl)ꢀ9Hꢀfluorenꢀ9ꢀone (0.348 g, 1.137 mmol) and the
11

New Journal of Chemistry
Page 12 of 29
View Article Online
DOI: 10.1039/C7NJ03926C
compound 3 (0.60 g, 0.947 mmol) in the presence of Na2CO3 (0.361 g, 3.406 mmol), TBAB
(0.015 g, 0.047 mmol) and Pd(PPh3)4 (0.055 g, 0.047 mmol). The residue was purified by column
chromatography on silica gel using dichloromethane/petroleum (1:2, v/v) as eluent to yield
1
reddishꢀbrown oily compound 5 (60% yield). H NMR (400 MHz, CDCl3, δ, ppm): 7.87 (s, 1H),
7.66 (d, J = 8.0 Hz, 2H), 7.48 (d, J = 8.0 Hz, 3H), 7.29 (d, J = 8.0 Hz, 1H), 7.22 (s, 1H), 7.18 (d, J
= 7.8 Hz, 1H), 7.13 (d, J = 7.8 Hz, 2H), 7.06 (t, J = 8.0 Hz, 1H), 7.02 (d, J = 8.0 Hz, 1H), 6.94 (d,
J = 8.0 Hz, 1H), 2.78 (t, J = 8.0 Hz, 4H), 1.73–1.62 (m, 4H), 1.42–1.25 (m, 20H), 0.88 (t, J = 7.2
Hz, 6H). 13C NMR (100 MHz, CDCl3, δ, ppm): 193.54, 144.26, 142.91, 140.85, 140.48, 139.90,
136.99, 136.61, 135.48, 135.03, 134.90, 134.86, 134.35, 131.09, 130.66, 130.26, 130.08, 128.97,
126.78, 126.51, 126.49, 124.42, 123.92, 123.89, 123.86, 120.98, 120.77, 120.32, 31.89, 31.88,
30.65, 30.50, 29.59, 29.58, 29.53, 29.43, 29.41, 29.27, 29.26, 22.67, 14.11.
(E)-2,2'-bis(3''',4-dioctyl-[2,2':5',2'':5'',2'''-quaterthiophen]-5-yl)-9,9'-bifluorenylidene
(FTBF): The preparation of the compound FTBF was similar to that described for the compound
STBF, which was obtained from the compound 5 in the presence of Lawesson’s reagent. The
residue was purified by column chromatography on silica gel using dichloromethane/petroleum
(1:4, v/v) as eluent to yield black oily compound, then it was freezeꢀdried under reduced pressure
to give black powdery solid FTBF (45% yield). 1H NMR (400 MHz, CDCl3, δ, ppm): 8.76 (s, 1H),
8.64 (s, 1H), 8.42 (t, J = 8.0 Hz, 2H), 7.67 (t, J = 8.0 Hz, 4H), 7.56 (t, J = 8.0 Hz, 2H), 7.35 (dd, J
= 8.0, 8.0 Hz, 2H), 7.28–7.05 (m, 8H), 7.04–6.83 (m, 8H), 2.83–2.59 (m, 8H), 1.71–1.58 (m, 6H),
1.49 ((t, J = 6.0 Hz, 2H), 1.44–1.07 (m, 40H), 1.00–0.64 (m, 12H). 13C NMR (100 MHz, CDCl3, δ,
ppm): 142.09, 141.55, 141.17, 141.08, 140.59, 140.40, 139.85, 139.69, 138.65, 138.27, 136.75,
136.44, 135.22, 132.76, 130.41, 130.01, 129.80, 129.50, 126.92, 126.75, 126.49, 126.38, 126.29,
12

Page 13 of 29
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ03926C
126.14, 126.01, 123.80, 123.69, 120.26, 119.93, 31.92, 31.89, 30.66, 30.63, 30.48, 30.33, 29.65,
29.54, 29.44, 29.38, 29.31, 29.27, 22.71, 22.69, 14.14, 14.12. HRMS: Calcd. for C90H96S8,
1432.52; Found, 1433.5321.
Scheme 1. The synthetic routes and chemical structures of the 9,9'ꢀbifluorenylidene derivatives
(STBF, TTBF and FTBF).
13

New Journal of Chemistry
Page 14 of 29
View Article Online
DOI: 10.1039/C7NJ03926C
3. Results and discussion
STBF, TTBF and FTBF had been synthesized by two key synthetic approaches, namely,
[49]
SuzukiꢀMiyaura cross coupling reaction
and oneꢀpot conversion of fluorenone derivatives to
9,9'ꢀbifluorenylidene derivatives in the presence of Lawesson’s reagent in boiling toluene [50]. The
synthetic routes were outlined in Scheme 1. Firstly, using SuzukiꢀMiyaura cross coupling reaction,
the appropriate fluorenone intermediates were synthesized by stirring bromofluorenone and
thiopheneboronic ester or bromooligothiophenes and fluorenoneboronic ester under nitrogen
atmosphere in a mixture of H2O and toluene overnight at 110 °C together with K2CO3 and
catalytic amounts of Pd(PPh3)4. Next, the target 9,9'ꢀbifluorenylidene derivatives (STBF, TTBF
and FTBF) were obtained from a oneꢀpot reaction by treating the appropriate fluorenone
intermediates in the presence of Lawesson’s reagent in boiling toluene. For the step of conversion
of fluorenone derivatives to 9,9'ꢀbifluorenylidene derivatives, Agranat et al. [50] proposed that the
process undergoes the formation of fluoreneꢀthione, and then desulfuration and dimerization via
the
carbene
fluorenylidene
and
2,4ꢀbis(fluorenylidene)ꢀ1,3ꢀdithiane
leading
to
[51]
9,9'ꢀbifluorenylidene derivative, while Steliou et al.
suggested a possible pathway for the
conversion of fluoreneꢀthione to 9,9'ꢀbifluorenylidene, which involves the 1,2ꢀdithietane
intermediate generated from the fluoreneꢀthione and the formation of a carbonꢀcarbon double
bond upon extrusion of the diatomic sulfur.
Three final compounds (STBF, TTBF and FTBF) were characterized by 1H NMR, 13C NMR,
1
MS and FTꢀIR spectra. In the H NMR spectra of the STBF, TTBF and FTBF, the aromatic
moieties (H1 and H8) could be differentiated by the low field 1H NMR signals (> 8 ppm), while in
their corresponding precursors the chemical shifts of the aromatic moieties appeared in higher
14

Page 15 of 29
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ03926C
field (˂ 8 ppm). In the 13C NMR spectra, the characteristic chemical shifts of C=O for the
corresponding precursors of STBF, TTBF and FTBF separately appeared at 193.55 ppm, 193.54
ppm and 193.54 ppm. However, these characteristic chemical shifts disappeared in the 13C NMR
spectra of the three target compounds. In the FTꢀIR spectra of the corresponding precursors of
STBF, TTBF and FTBF, the characteristic absorption bands of C=O appeared at 1722, 1719 and
1714 cmꢀ1, respectively. In the FTꢀIR spectra of STBF, TTBF and FTBF, the characteristic
stretching vibration of C=O disappeared completely. STBF, TTBF and FTBF were further
identified by their high resolution mass spectra, their molar masses were detected clearly.
Figure 1 shows the UVꢀvis absorption spectra measured for PCBM and the bifluorenylidene
derivatives STBF, TTBF and FTBF in CH2Cl2 solutions and solid thin films. The corresponding
photophysical data are listed in Table 1. As shown in Fig. 1a, the absorption spectrum of STBF in
solution exhibits two intense absorption bands at 247 and 298 nm, and one weak absorption band
at 466 nm. The band below 350 nm is attributed to the correspond π–π* transition of thiophene
units, whereas the band of longer wavelength at 466 nm is assigned to π–π* transition of the
bifluorenylidene structure [48]. From the structure of STBF, the presence of two octyl chains at
3ꢀposition of the thiophene rings reduces the coplanarity between thiophene units and
bifluorenylidene due to the steric hindrance of the two octyl groups. Thus, the π–π* transition of
thiophene units and the π–π* transition of the bifluorenylidene structure emerged clearly.
Moreover, the presence of two octyl chains increases the conjugation of the thiophene units and
the molar absorptivity of the thiophene units due to the hyperconjugation effects between
thiophene rings and aliphatic chains. In the absorption spectrum of TTBF in solution, there are
two intense absorption bands at 249 and 380 nm, and one weak absorption band at 475 nm. The
15

New Journal of Chemistry
Page 16 of 29
View Article Online
DOI: 10.1039/C7NJ03926C
former two absorption bands correspond to the π–π* transition of terthienyl units, and the last
absorption band is assigned to the π–π* transition of the bifluorenylidene skeleton. Because the
thiophene rings adjacent to the bifluorenylidene skeleton were not grafted by octyl chains, the
coplanarity between terthienyl units and bifluorenylidene skeleton is larger, which leads to partial
overlapping of the absorption bands of terthienyl units and bifluorenylidene skeleton, and causes
redꢀshifts of the absorption bands in contrast with STBF. The absorption spectrum of FTBF in
solution only shows two intense absorption bands at 249 and 407 nm, corresponding to the π–π*
transition of the quaterthiophene units. The absorption band of the bifluorenylidene skeleton was
not observed in its absorption spectrum. The presence of two octyl chains at 3ꢀposition of the
thiophene rings adjacent to the bifluorenylidene skeleton reduces the coplanarity between the
quaterthiophene units and the bifluorenylidene skeleton due to the steric hindrance of the two
alkyl groups. Besides, there are stronger hyperconjugation effects between two quaterthiophene
rings and four octyl chains, which increases the molar absorptivity of the quaterthiophene rings.
Thus, the absorption band of the bifluorenylidene skeleton is probably packaged in the absorption
bands of the quaterthiophene units. Compared with TTBF, the absorption band at longer
wavelength of FTBF is redꢀshifted by about 27 nm because the conjugation of the
quaterthiophene unit is larger than that of terthienyl unit. The absorption spectrum of PCBM in
solution exhibits four characteristic absorption peaks at 261, 329, 430 and 493 nm [52]. The former
two absorption bands are relatively strong, but the latter two bands are very weak. Compared with
PCBM, the absorption of STBF, TTBF and FTBF is stronger than that of PCBM in 250–600 nm
regions.
As shown in Fig. 1b, the absorption spectra of STBF, TTBF, FTBF and PCBM in solid thin
16

Page 17 of 29
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ03926C
films were similar to that of them in CH2Cl2 solutions. Due to more intermolecular interaction in
the solid state, their absorption spectra were much broader than that in CH2Cl2 solutions, and the
maximum absorptions and edge wavelengths of solid thin films moved slightly to longer
wavelength regions than those of solutions. The maximum absorption bands of STBF, TTBF and
FTBF in solid thin films were located at 472, 388 and 413 nm, respectively.
Solution
1.0
STBF
TTBF
0.8
0.6
0.4
0.2
0.0
FTBF
PCBM
a
300
400
500
600
700
800
Wavelength(nm)
Thin film
1.0
0.8
0.6
0.4
0.2
0.0
STBF
TTBF
FTBF
PCBM
b
300
400
500
600
700
800
Wavelength (nm)
Fig. 1. UVꢀvis absorption spectra of STBF, TTBF, FTBF and PCBM in CH2Cl2 solutions (a) and
in thin films (b).
In order to investigate the frontier orbitals of STBF, TTBF and FTBF, cyclic voltammetry
(CV) was carried out to investigate their electrochemical behavior. Their highest occupied
molecular orbital (HOMO) energy levels were calculated from the onset of the oxidation
potentials of their cyclic voltammetry measurements, while their lowest unoccupied molecular
17

New Journal of Chemistry
Page 18 of 29
View Article Online
DOI: 10.1039/C7NJ03926C
orbital (LUMO) energy levels were deduced from their HOMO energy levels and optical band
gaps determined from their extrapolated UVꢀVis absorption edges [53]. The cyclic voltammograms
of STBF, TTBF and FTBF were measured in degassed dichloromethane solution with Bu4NPF6
(0.1 mol/L) as the electrolyte (Fig. 2). A platinum working electrode and a saturated Ag/AgCl
reference electrode were used. Ferrocene was used for potential calibration. As shown in Fig. 2,
the onset oxidation potentials (Eox) for STBF, TTBF and FTBF are 1.36, 0.94 and 0.73 V versus
Ag/Ag+, respectively. At the same condition, the Fc/Fc+ potential was measured to be 0.52 V.
Thus, the HOMO energy levels of STBF, TTBF and FTBF were calculated to be ꢀ5.64, ꢀ5.22 and
ꢀ5.01 eV regarding the energy level of ferrocene/ferrocenium as ꢀ4.8 eV [54]. The optical band
edges of STBF, TTBF and FTBF were estimated to be at ca. 521, 523 and 512 nm, which
correspond to 2.38, 2.37 and 2.42 eV. Therefore, the LUMO energy levels of STBF, TTBF and
FTBF were calculated to be ꢀ3.26, ꢀ2.85 and ꢀ2.59 eV, respectively.
2.0
Ferrocene
STBF
TTBF
1.5
1.0
FTBF
0.5
0.0
-0.5
-1.0
-1.5
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
+
Potential (vs Ag/Ag ,V)
Fig. 2. Cyclic voltammograms of ferrocene and the compounds STBF, TTBF and FTBF in
dichloromethane solutions with 0.1 mol/L Bu4NPF6 at a scan rate of 100 mV/s.
18

Page 19 of 29
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ03926C
Table 1. Optical and electrochemical properties of STBF, TTBF and FTBF
Compound
UVꢀvis
(nm)a
247, 298, 466
249, 380, 475
λmax
(nm)b
472
λonset
(nm)
521
EOX
(V)c
1.36
0.94
Eopt
(eV)d
2.38
2.37
HOMO LUMO
(eV)e
ꢀ5.64
ꢀ5.22
(eV)f
ꢀ3.26
ꢀ2.85
STBF
TTBF
388
523
249, 407
413
512
0.73
2.42
ꢀ5.01
ꢀ2.59
FTBF
aAbsorption bands in CH2Cl2 solution at room temperature;
bMaximum absorption band in thin film at room temperature;
cAll the values are referred to Ag/AgCl and calibrated by Fc/Fc+;
f
dEopt ≈ 1241/λonset
;
eEHOMO = ꢀe(EOX ꢀ 0.52 + 4.8) (eV); ELUMO = Eopt + EHOMO
For comparing the energy levels of the synthesized compounds and the traditional donor and
receptor materials P3HT and PCBM, Figure 3 depicts the corresponding HOMO/LUMO energy
diagram of STBF, TTBF, FTBF, P3HT and PCBM. The HOMO and LUMO energy levels of
P3HT and PCBM are from the data reported by Koster et al. [55]
.
Fig. 3. Electronic energy levels of STBF, TTBF, FTBF, P3HT and PCBM
The bulk heterojunction polymer solar cells based on P3HT/PCBM are the most
representative and attractive OPVs due to their reproducible high efficiency (> 4%) and low cost
of the photovoltaic materials. However, too low LUMO energy level of PCBM results in low open
circuit voltage (Voc, ca. 0.6 V) of the devices, which limits further improvement of the device
19

New Journal of Chemistry
Page 20 of 29
View Article Online
DOI: 10.1039/C7NJ03926C
efficiency. For increasing efficiency of the bulk heterojunction polymer solar cells, various new
donor and acceptor photovoltaic materials have been developed in recent years. With respect to the
P3HTꢀbased organic photovoltaic cells, the development of new acceptors with upꢀshifted LUMO
energy levels is very important for higher open circuit voltage and improved photovoltaic
efficiency. As shown in Fig. 3, it is found that the LUMO energy levels of the synthesized
9,9'ꢀbifluorenylidene derivatives were gradually upꢀshifted to be ꢀ3.26–ꢀ2.59 eV with the number
of thiophene ring in oligothiophene units increased. The LUMO energy level of STBF is higher
than that of PCBM by 0.54 eV. The LUMO energy offꢀset of P3HT and STBF is 0.56 eV, which is
roughly meeting the empirical offꢀset energy of 0.3–0.5 eV [44] as the driving force for the exciton
dissociation at the donor/acceptor interface, suggesting that STBF could be used as an electron
acceptor in the P3HTꢀbased organic photovoltaic cells. The LUMO energy level of TTBF (ꢀ2.85
eV) is close to that of P3HT (ꢀ2.7 eV), whereas the LUMO energy level of FTBF (ꢀ2.59 eV) is
higher than that of P3HT. Considering from the LUMO energy levels, TTBF and FTBF could be
more suitable for use as electron donor materials.
Frontier molecular orbitals play an important role in electric and optical properties for
materials. The groundꢀstate geometries of STBF, TTBF and FTBF were fully optimized by
density functional theory (DFT) at B3LYP/6ꢀ31G(d) basis set and an ethyl group was used instead
of the octyl group here to make the calculation easier. The electronic distributions in HOMO and
LUMO energy levels of the synthesized 9,9'ꢀbifluorenylidene derivatives obtained from
timeꢀdependent DFT (TDꢀDFT) calculation are shown in Figure 4. For STBF, the electron
densities in the HOMO and LUMO orbitals reside on the 9,9'ꢀbifluorenylidene cores. For TTBF
and FTBF, the electrons in HOMO energy levels are mainly distributed on one side of
20

Page 21 of 29
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ03926C
oligothiophene units and their adjacent fluorene rings, while the electrons are mainly distributed
on the core 9,9'ꢀbifluorenylidene units in the LUMO energy levels.
Fig. 4. The spatial electron distributions of HOMO and LUMO energy densities of STBF, TTBF
and FTBF by DFT B3LYP/6ꢀ31G (d) calculation.
4. Conclusions
We have designed and synthesized three new 9,9'ꢀbifluorenylidene derivatives STBF, TTBF
and FTBF. Their UVꢀvis absorption bands were much broader and stronger than that of fullerene
derivative PCBM. The LUMO energy levels of the synthesized 9,9'ꢀbifluorenylidene derivatives
were gradually upꢀshifted with the number of thiophene ring in oligothiophene units increased.
Considering from the LUMO energy level, STBF could be used as an electron acceptor in the
P3HTꢀbased organic photovoltaic cells, whereas TTBF and FTBF could be more suitable for use
as electron donor materials.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant
51563014).
21

New Journal of Chemistry
Page 22 of 29
View Article Online
DOI: 10.1039/C7NJ03926C
References
[1] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar cell efficiency tables
(Version 45), Prog. Photovolt: Res. Appl., 2015, 23, 1ꢀ9.
[2] G. Grancini, M. Maiuri, D. Fazzi, A. Petrozza, HꢀJ. Egelhaaf, D. Brida, G. Cerullo, G. Lanzani,
Hot exciton dissociation in polymer solar cells, Nat. Mater., 2013, 12, 29.
[3] Y.J. He, Y.F. Li, Fullerene derivative acceptors for high performance polymer solar cells, Phys.
Chem. Chem. Phys., 2011, 13, 1970ꢀ1983.
[4] G. Li, R. Zhu, Y. Yang, Polymer solar cells, Nat. Photonics, 2012, 6, 153ꢀ161.
[5] W.C. Zhao, D.P. Qian, S.Q. Zhang, S.S. Li, O. Inganäs, F. Gao, J.H. Hou, Fullereneꢀfree
polymer solar cells with over 11% efficiency and excellent thermal stability, Adv. Mater.,
2016, 28, 4734ꢀ4739.
[6] S.S. Li, L. Ye, W.C. Zhao, S.Q. Zhang, S. Mukherjee, H. Ade, J.H. Hou, Energyꢀlevel
modulation of smallꢀmolecule electron acceptors to achieve over 12% efficiency in polymer
solar cells, Adv. Mater., 2016, 28, 9423ꢀ9429.
[7] W.C. Zhao, S.S. Li, S.Q. Zhang, X.Y. Liu, J.H. Hou, Ternary polymer solar cells based on two
acceptors and one donor for achieving 12.2% efficiency, Adv. Mater., 2017, 29, 1604059.
[8] J.A. Mikroyannidis, A.N. Kabanakis, S.S. Sharma, G.D. Sharma, A simple and effective
modification of PCBM for use as an electron acceptor in efficient bulk heterojunction solar
cells, Adv. Funct. Mater., 2011, 21, 746ꢀ755.
[9] Q. Zhang, B. Kan, F. Liu, G.K. Long, X.J. Wan, X.Q. Chen, Y. Zuo, W. Ni, H.J. Zhang, M.M.
Li, Z.C. Hu, F. Huang, Y. Cao, Z.Q. Liang, M.T. Zhang, T.P. Russell, Y.S. Chen,
Smallꢀmolecule solar cells with efficiency over 9%, Nat. Photonics, 2015, 9, 35ꢀ41.
[10] N.L. Qiu, X. Yang, H.J. Zhang, X.J. Wan, C.X. Li, F. Liu, H.T. Zhang, T.P. Russell, Y.S. Chen,
Nonfullerene small molecular acceptors with a threeꢀdimensional (3D) structure for organic
22

Page 23 of 29
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ03926C
solar cells, Chem. Mater., 2016, 28, 6770ꢀ6778.
[11] D. Deng, Y.J. Zhang, J.Q. Zhang, Z.Y. Wang, L.Y. Zhu, J. Fang, B.Z. Xia, Z. Wang, K. Lu, W.
Ma, Z.X. Wei, Fluorinationꢀenabled optimal morphology leads to over 11% efficiency for
inverted smallꢀmolecule organic solar cells, Nat. Commun., 2016, 7, 13740.
[12] J.J. Intemann, K. Yao, H.ꢀL. Yip, Y.ꢀX. Xu, Y.ꢀX. Li, P.ꢀW. Liang, F.ꢀZ. Ding, X.S. Li, A.K.ꢀY.
Jen, Molecular weight effect on the absorption, charge carrier mobility, and photovoltaic
performance of an indacenodiselenopheneꢀbased ladderꢀtype polymer, Chem. Mater., 2013, 25,
3188ꢀ3195.
[13] H.F. Yao, Y. Cui, R.N. Yu, B.W. Gao, H. Zhang, J.H. Hou, Design, synthesis, and
photovoltaic characterization of a small molecular acceptor with an ultraꢀnarrow band gap,
Angew. Chem. Int. Ed., 2017, 56, 3045ꢀ3049.
[14] J.Y. Wang, S.X. Dai, Y.H. Yao, P. Cheng, Y.Z. Lin, X.W. Zhan, Spirobifluoreneꢀbased
acceptors for polymer solar cells: Effect of isomers, Dyes and Pigments, 2015, 123, 16ꢀ25.
[15] J.B. Zhao, Y.K. Li, H.R. Lin, Y.H. Liu, K. Jiang, C. Mu, T.X. Ma, J.Y.L. Lai, H.W. Hu, D.M.
Yu, H. Yan, Highꢀefficiency nonꢀfullerene organic solar cells enabled by
a
difluorobenzothiadiazoleꢀbased donor polymer combined with a properly matched small
molecule acceptor, Energy Environ. Sci., 2015, 8, 520ꢀ525.
[16] L.Y. Yang, S.Q. Zhang, C. He, J.Q. Zhang, H.F. Yao, Y. Yang, Y. Zhang, W.H. Zhao, J.H. Hou,
New wide band gap donor for efficient fullereneꢀfree allꢀsmallꢀmolecule organic solar cells, J.
Am. Chem. Soc., 2017, 139, 1958ꢀ1966.
[17] M. Zhang, J. Wang, F.J. Zhang, Y. Mic, Q.S. An, W.B. Wang, X.L. Ma, J. Zhang, X.F. Liu,
Ternary small molecule solar cells exhibiting power conversion efficiency of 10.3%, Nano
Energy, 2017, 39, 571ꢀ581.
[18] Y.F. Li, Y.P. Zou, Conjugated polymer photovoltaic materials with broad absorption band and
high charge carrier mobility, Adv. Mater., 2008, 20, 2952ꢀ2958.
23

New Journal of Chemistry
Page 24 of 29
View Article Online
DOI: 10.1039/C7NJ03926C
[19] J.H. Hou, Z.A. Tan, Y. Yan, Y.J. He, C.H. Yang, Y.F. Li, Synthesis and photovoltaic properties
of twoꢀdimensional conjugated polythiophenes with bi(thienylenevinylene) side chains, J. Am.
Chem. Soc., 2006, 128, 4911ꢀ4916.
[20] Y. Wang, E.J. Zhou, Y.Q. Liu, H.X. Xi, S.H. Ye, W.P. Wu, Y.L. Guo, C.A. Di, Y.M. Sun, G. Yu,
Y.F. Li, Solutionꢀprocessed organic fieldꢀeffect transistors based on polythiophene derivatives
with conjugated bridges as linking chains, Chem. Mater., 2007, 19, 3361ꢀ3363.
[21] L.J. Huo, T.L. Chen, Y. Zhou, J.H. Hou, Y. Yang, Y.F. Li, Improvement of photoluminescent
and photovoltaic properties of poly(thienylene vinylene) by carboxylate substitution,
Macromolecules, 2009, 42, 4377ꢀ4380.
[22] H.ꢀY. Chen, J.H. Hou, S.Q. Zhang, Y.Y. Liang, G.W. Yang, Y. Yang, L.P. Yu, Y. Wu, G. Li,
Polymer solar cells with enhanced openꢀcircuit voltage and efficiency, Nat. Photonics, 2009, 3,
649ꢀ653.
[23] Y.Y. Liang, Z. Xu, J.B. Xia, S.ꢀT. Tsai, Y. Wu, G. Li, C. Ray, L.P. Yu, For the bright
future—bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%,
Adv. Mater., 2010, 22, E135ꢀE138.
[24] L.J. Huo, J.H. Hou, S.Q. Zhang, H.Y. Chen, Y. Yang, A polybenzo[1,2ꢀb:4,5ꢀb']dithiophene
derivative with deep HOMO level and its application in highꢀperformance polymer solar cells,
Angew. Chem. Int. Ed., 2010, 49, 1500ꢀ1503.
[25] X.H. Zhu, J.B. Peng, Y. Cao, J. Roncali, Solutionꢀprocessable singleꢀmaterial molecular
emitters for organic lightꢀemitting devices, Chem. Soc. Rev., 2011, 40, 3509ꢀ3524.
[26] J. Xue, B.P. Rand, S. Uchida, S.R. Forrest, A hybrid planar–mixed molecular heterojunction
photovoltaic cell, Adv. Mater., 2005, 17, 66ꢀ71.
[27] G.D. Wei, X. Xiao, S.Y. Wang, J.D. Zimmerman, K. Sun, V.V. Diev, M.E. Thompson, S.R.
Forrest, Arylamineꢀbased squaraine donors for use in organic solar cells, Nano Lett., 2011, 11,
4261ꢀ4264.
24

Page 25 of 29
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ03926C
[28] F. Zhang, D.Q. Wu, Y.Y. Xu, X.L. Feng, Thiopheneꢀbased conjugated oligomers for organic
solar cells, J. Mater. Chem., 2011, 21, 17590ꢀ17600.
[29] J.Y. Zhou, X.J. Wan, Y.S. Liu, G.K. Long, F. Wang, Z. Li, Y. Zuo, C.X. Li, Y.S. Chen, A
planar small molecule with dithienosilole core for high efficiency solutionꢀprocessed organic
photovoltaic cells, Chem. Mater., 2011, 23, 4666ꢀ4668.
[30] Z.J. Ning, H. Tian, Triarylamine: a promising core unit for efficient photovoltaic materials,
Chem. Commun., 2009, 5483ꢀ5495.
[31] L.ꢀY. Lin, Y.ꢀH. Chen, Z.ꢀY. Huang, H.ꢀW. Lin, S.ꢀH. Chou, F. Lin, C.ꢀW. Chen, Y.ꢀH. Liu,
K.ꢀT. Wong, A lowꢀenergyꢀgap organic dye for highꢀperformance smallꢀmolecule organic solar
cells, J. Am. Chem. Soc., 2011, 133, 15822ꢀ15825.
[32] J.A. Mikroyannidis, A.N. Kabanakis, S.S. Sharma, G.D. Sharma, A simple and effective
modification of PCBM for use as an electron acceptor in efficient bulk heterojunction solar
cells, Adv. Funct. Mater., 2011, 21, 746ꢀ755.
[33] J.E. Anthony, A. Facchetti, M. Heeney, S.R. Marder, X.W. Zhan, nꢀType organic
semiconductors in organic electronics, Adv. Mater., 2010, 22, 3876ꢀ3892.
[34] A. Mishra, P. Bäuerle, Small molecule organic semiconductors on the move: Promises for
future solar energy technology, Angew. Chem. Int. Ed., 2012, 51, 2020ꢀ2067.
[35] P. Sonar, G.M. Ng, T.T. Lin, A. Dodabalapur, Z.ꢀK. Chen, Solution processable low bandgap
diketopyrrolopyrrole (DPP) based derivatives: novel acceptors for organic solar cells, J. Mater.
Chem., 2010, 20, 3626ꢀ3636.
[36] A.L. Tang, C.L. Zhan, J.N. Yao, E.J. Zhou, Design of diketopyrrolopyrrole (DPP)ꢀbased small
molecules for organicꢀsolarꢀcell applications, Adv. Mater., 2017, 29, 1600013.
[37] G.D. Sharma, P. Suresh, J.A. Mikroyannidis, M.M. Stylianakis, Efficient bulk heterojunction
devices based on phenylenevinylene small molecule and perylene–pyrene bisimide, J. Mater.
Chem., 2010, 20, 561ꢀ567.
25

New Journal of Chemistry
Page 26 of 29
View Article Online
DOI: 10.1039/C7NJ03926C
[38] J.A. Mikroyannidis, P. Suresh, G.D. Sharma, Synthesis of a perylene bisimide with
acetonaphthopyrazine dicarbonitrile terminal moieties for photovoltaic applications, Synth.
Met., 2010, 160, 932ꢀ938.
[39] J.T. Bloking, X. Han, A.T. Higgs, J.P. Kastrop, L. Pandey, J.E. Norton, C. Risko, C.E. Chen,
J.ꢀL. Brédas, M.D McGehee, A Sellinger, Solutionꢀprocessed organic solar cells with power
conversion efficiencies of 2.5% using benzothiadiazole/imideꢀbased acceptors, Chem. Mater.,
2011, 23, 5484ꢀ5490.
[40] A.R. Murphy, J.M.J. Fréchet, Organic Semiconducting Oligomers for Use in Thin Film
Transistors. Chem. Rev. 2007, 107, 1066ꢀ1096.
[41] S.H. Dong, H.T. Zhang, L. Yang, M.L. Bai, Y. Yao, H.L. Chen, L. Gan, T.Y. Yang, H. Jiang,
S.M. Hou, L.J. Wan, X.F. Guo, SolutionꢀCrystallized Organic Semiconductors with High
Carrier Mobility and Air Stability. Adv. Mater., 2012, 24, 5576ꢀ5580.
[42] P. Bäuerle, Oligothiophenes. In Electronic Materials: The Oligomer Approach; Müllen, K.,
Egner, G., Eds.; WileyꢀVCH: Weinheim, 1998; pp 105ꢀ197.
[43] F.G. Brunetti, X. Gong, M. Tong, A.J. Heeger, F. Wudl, Strain and Hückel aromaticity:
Driving forces for a promising new generation of electron acceptors in organic electronics,
Angew. Chem. Int. Ed., 2010, 49, 532ꢀ536.
[44] X. Gong, M.H. Tong, F.G. Brunetti, J. Seo, Y.M. Sun, D. Moses, F. Wudl, A.J. Heeger, Bulk
heterojunction solar cells with large openꢀcircuit voltage: Electron transfer with small
donorꢀacceptor energy offset, Adv. Mater., 2011, 23, 2272ꢀ2277.
[45] F.G. Brunetti, A. Varotto, N.A. Batara, F. Wudl, “Deconvoluted fullerene” derivatives:
Synthesis and characterization, Chem. Eur. J., 2011, 17, 8604ꢀ8608.
[46] H.U. Kim, J.ꢀH. Kim, H. Suh, J. Kwak, D. Kim, A.C. Grimsdale, S.C. Yoon, D.ꢀH. Hwang,
High open circuit voltage organic photovoltaic cells fabricated using 9,9'ꢀbifluorenylidene as a
nonꢀfullerene type electron acceptor, Chem. Commun., 2013, 49, 10950ꢀ10952.
26

Page 27 of 29
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ03926C
[47] S. Kawata, J. Furudate, T. Kimura, H. Minaki, A. Saito, H. Katagiri, Y.ꢀJ. Pu, Controlling the
excitedꢀstate energy levels of 9,9'ꢀbifluorenylidene derivatives by twisting their structure to
attaining singlet fission character in organic photovoltaics, J. Mater. Chem. C, 2017, 5,
4909ꢀ4914.
[48] O.Y. Park, H.U. Kim, J.ꢀH. Kim, J.B. Park, J. Kwak, W.S. Shin, S.C. Yoon, D.ꢀH. Hwang,
Tetrafluoreneꢀ9,9'ꢀbifluorenylidene as a nonꢀfullerene type electron acceptor for P3HTꢀbased
bulkꢀheterojunction polymer solar cells, Sol. Energy Mater. Sol. Cells, 2013, 116, 275ꢀ282.
[49] (a) N. Miyaura, A. Suzuki, PalladiumꢀCatalyzed CrossꢀCoupling Reactions of Organoboron
Compounds, Chem. Rev., 1995, 95,2457ꢀ2483; (b) N. Miyaura, T. Yanagi, A. Suzuki, The
palladiumꢀcatalyzed crossꢀcoupling reaction of phenylboronic acid with haloarenes in the
presence of bases, Synth. Commun., 1981, 11, 513ꢀ519.
[50] A. Levy, M. Goldschmidt, I. Agranat, A simple facile synthesis of bifluorenylidenes, Lett. Org.
Chem., 2006, 3, 579ꢀ584.
[51] (a) K. Steliou, P. Salama, X.P. Yu, Does diatomic sulfur (S2) react as a free species? J. Am.
Chem. Soc., 1992, 114, 1456ꢀ1462; (b) K. Steliou, Diatomic sulfur, Acc. Chem. Res., 1991, 24,
341ꢀ350.
[52] Y.L. Zhao, X. Wang, T.Z. Yu, H. Zhang, C.J. Wei, X.X. Liu, Synthesis and photovoltaic
properties of carbazoleꢀsubstituted fullerene derivatives, New J. Chem., 2017, 41, 4702ꢀ4706.
[53] T.Z. Yu, Z.Y. Zhu, Y.J. Bao, Y.L. Zhao, X.X. Liu, H. Zhang, Investigation of novel
carbazoleꢀfunctionalized coumarin derivatives as organic luminescent materials, Dyes
Pigments, 2017, 147, 260ꢀ269.
[54] C.M. Cardona, W. Li, A.E. Kaifer, D. Stockdale, G.C. Bazan, Electrochemical considerations
for determining absolute frontier orbital energy levels of conjugated polymers for solar cell
applications. Adv. Mater., 2011, 23, 2367ꢀ71.
[55] L.J.A. Koster, V.D. Mihailetchi, P.W.M. Blom, Ultimate efficiency of polymer/fullerene bulk
27

New Journal of Chemistry
Page 28 of 29
View Article Online
DOI: 10.1039/C7NJ03926C
heterojunction solar cells, Appl. Phys. Lett., 2006, 88, 093511.
28

Page 29 of 29
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ03926C
Graphical abstract
Three 9,9'-bifluorenylidene derivatives were synthesized, in which STBF could
be as an electron acceptor in the P3HT-based organic photovoltaic cells.
C8H17
S
S
C8H17
STBF