Electron-rich pyrroloindacenodithiophenes: Synthesis, characterization, and spectroscopic studies

Article abstract of DOI:10.1021/jo3022154

N-Alkyl substituted pyrroloindacenodithiophene (PIDT) and their phenyl substituted derivatives were synthesized. Their single-crystal structures and electrochemical and spectroscopic properties were investigated. Experimental results showed PIDT displayed strong electron-donating properties, reversible redox behaviors, and strong fluorescence and could be controlled to oxidize to radical cation and dication with distinctive optical changes. These attractive properties demonstrated the potential applications of PIDT in the field of switches, molecular machines, and information memories.

Full text of DOI:10.1021/jo3022154

Note
pubs.acs.org/joc
Electron-Rich Pyrroloindacenodithiophenes: Synthesis,
Characterization, and Spectroscopic Studies
Yu Xiong, Qinghe Wu, Jie Li, Shitao Wang, Xike Gao, and Hongxiang Li*
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai, 200032, China
S
* Supporting Information
ABSTRACT: N-Alkyl substituted pyrroloindacenodithiophene (PIDT)
and their phenyl substituted derivatives were synthesized. Their single-
crystal structures and electrochemical and spectroscopic properties were
investigated. Experimental results showed PIDT displayed strong electron-
donating properties, reversible redox behaviors, and strong fluorescence
and could be controlled to oxidize to radical cation and dication with
distinctive optical changes. These attractive properties demonstrated the
potential applications of PIDT in the field of switches, molecular machines,
and information memories.
a
Scheme 1
lectron donors with reversible redox properties have
Eattracted a great deal of attention due to their wide
applications in the fields of switches,1 sensors,2 molecular
machines,3 and information memories.4 An ideal electron
donor for these fields should (i) have high HOMO energy level
(larger than −5.3 eV, so that it can be easily oxidized by
electrochemical technique or chemical species); (ii) exhibit
optical (absorption and/or emission) changes with the
oxidation; (iii) provide easy synthesis, purification and
modification; and (iv) have good solubility in organic solvents.
Though great effort has been taken, to date, the most common
electron donors used are tetrathiafulvalene (TTF)5 and
ferrocence (Fc),6 so there is a great need to develop new
type of strong electron donors with reversible redox property.
Pyrroloindacenodithiophene (PIDT), as a π-conjugated unit,
was synthesized by Donaghey and coauthors with the aim to
copolymerize with electron-deficient units and investigate the
applications of these donor−acceptor (D-A) type polymers in
transistors and organic solar cells.7 Devices characteristics
suggested PIDT was a good conjugated block for polymer
semiconductors. However, till now the physicochemical
properties of PIDT had not been reported. Herein, a series
of new type of N-alkyl substituted PIDTs (1) and their phenyl
substituted derivatives (2) were synthesized (Scheme 1). Their
physicochemical properties as well as single crystal structures
were thoroughly investigated. Experimental results showed that
1 and 2 (i) displayed two reversible redox peaks in cyclic
voltammetry with HOMO levels ranging from −4.9 to −5.1 eV;
(ii) with the titration of oxidant FeCl3, displayed new
absorption bands, with one of them located at the near-
infrared (NIR) region; and (iii) exhibited strong fluorescence in
solution. These results demonstrated the potential applications
a
(i) CuCl, LiCl, Pd(PPh3)4, DMF; (ii) 5 mol % Pd2dba3, 10 mol %
BINAP, NaOtBu, alkylamine, p-xylene, 150 °C, 2 h; (iii) NBS, THF/
DMF, −78°C; (iv) PhB(OH)2, Pd(PPh3)4, K2CO3, toluene, reflux for
6 h.
of PIDTs in molecular machines, switches, and information
memories.
The syntheses of compounds 1 and 2 are illustrated in
Scheme 1. The pyrroloindacenodithiophene (PIDT) core was
obtained following a recent reported method with a little
modification.7 Compound 3 was synthesized through Stille
coupling reaction and further reacted with alkylamine in the
presence of Pd catalyst (Buchwald amination reaction) to
Received: October 19, 2012
Published: December 3, 2012
© 2012 American Chemical Society
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afford compound 1. The Buchwald amination reaction was
carried out in refluxing p-xylene solvent instead of toluene,
which was used as solvent in Donaghey’s procedure. With this
modification, the PIDT core was prepared efficiently with great
yield improvement. Compound 1 reacted with NBS followed
by Suzuki coupling reaction with phenylboronic acid to give 2
in moderate yield. It should be noted, because of the strong
electron-donating property, the bromination of compound 1
needed to be carried out at low temperature. Compounds 1 had
good solubility in common organic solvents (such as CH2Cl2,
THF, and CHCl3), and compound 2 had low solubility in
organic solvents. Compounds 1 and 2 were fully characterized
by NMR, MS, and elemental analysis.
b = 5.770(11) Å, c = 17.973(4) Å, β = 102.020(4)°. In the
single crystal, the PIDT core exhibited planar structure and the
alkyl chains were positioned out of the PIDT plane. The C−
H···π interactions of the hydrogen atoms on the PIDT core and
alkyl chain to the adjacent PIDT core were observed. Similar to
1a, the conjugated backbone of 2a displayed planar structure
and the alkyl substituents were out of the core plane. Strong
C−H···π interactions of the hydrogen atoms on the PIDT core
and alkyl chain to the adjacent PIDT core were also observed.
However, the space group of 2a single crystals were different
with that of 1a and belonged to a monoclinic C2/C group with
unit cell parameters of a = 35.045(3) Å, b = 5.4519(2) Å, c =
24.299(2) Å, β = 117.063(2)°.
Figure 2a illustrated the absorption spectra of 1 and 2 in
dilute CH2Cl2 solutions. The absorption spectra of 1a and 2a
were nearly identical to those of 1b and 2b, respectively,
suggesting the changes of alkyl chains had no influence on the
HOMO−LUMO energy gaps. The onset absorptions of 1a,b
were 410 nm, ∼ 66 nm blue-shifted compared with those of
2a,b, indicating the enlarged conjugation length of 2a,b. The
optical energy band gaps calculated from solution were 3.02 eV
for 1 and 2.60 eV for 2. The fluorescence spectra of 1 and 2 are
shown in Figure 2b. Similar as the absorption spectra, 1a and 2a
exhibited identical emission spectra as 1b and 2b, respectively.
The fluorescence quantum yield estimated from solution (9,10-
diphenylanthracene as a reference standard)8 were 0.11 for 1a,
0.12 for 1b, 0.30 for 2a, and 0.26 for 2b.
Single crystals of 1a and 2a were grown by slow evaporation
of organic solvents at room temperature (Figure 1). XRD
The electronic properties of 1a,b and 2a,b were examined by
cyclic voltammetry. All compounds showed two reversible
redox waves (Figure 3). The redox potentials of 1a and 1b were
determined as E1/21 = 0.59 V, E1/22 = 1.22 V from the midpoint
of forward and backward scan. The first oxidation potential was
1
1
close to that of ferrocene (E1/2 = 0.43 V) and TTF (E1/2
=
0.37 V),9 which suggested PIDT was a strong electron-rich
compound and could be sequentially and reversibly oxidized to
radical cation and dication within an accessible potential
window like TTF.10 The HOMO energy levels of 1a,b
calculated from CV were −5.03 eV. It is well known TTF
derivatives with aromatic substituents usually display a redox
potential higher than that of TTF. Surprisingly, the redox
potentials of 2a and 2b were at E1/21 = 0.54 V and E1/21 = 1.05
V and slightly lowered than those of 1a and 1b, indicating that
the electrons on the phenyl rings were fully delocalized on the
whole backbone. The HOMO energy levels of 2a and 2b
calculated from CV were −4.98 eV.
Figure 1. Single crystal and packing of compound 1a (a,b) and 2a
(c,d).
results revealed that 1a single crystals belonged to a monoclinic
space group P21/n with unit cell parameters of a = 15.003(3) Å,
Figure 2. (a) UV−vis absorption spectra of 1a,b and 2a,b in CH2Cl2 solutions (1 × 10−4 mol/L). (b) Fluorescence spectra of 1a,b and 2a,b in
cyclohexane solution.
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Figure 5. Absorption spectra of radical cation and dication of 2a
measured by in situ spectroelectrochemistry.
Figure 3. Cyclic voltammograms of 1a,b and 2a,b in CH2Cl2 by using
0.1 M nBu4PF6 as electrolyte and SCE as reference under scan rate 100
mV/s. Fc/Fc+ was used as internal standard.
The reversible redox behavior of 1 and 2 suggested that they
could be controlled to oxidize to stable radical cation and
dication with distinctive optical properties. To confirm this, the
titration of 2a with FeCl3, as an example, was studied (Figure 4
ferric cation. To further confirm this assumption, the redox
potential of FeCl3 in CH2Cl2 solution was measured by cyclic
voltammetry. The reduce potential of FeCl3 was 0.88 V, which
is higher than the first redox potential and lower than the
second redox potential of compounds 1 and 2 (Figure S3 in
Supporting Information3). These results further proved that
compounds 1 and 2 can only be oxidized to radical cation by
ferric cation. Moreover, when a solution of SnCl2 was added to
the radical cation of 2a, the absorption of neutral 2a was
recovered, suggesting compounds 1 and 2 can be reversibly
tuned by oxidation and reduction (Figure S4 in Supporting
Information).
In summary, N-alkyl substituted PIDT (1a,b) and their
phenyl substituted derivatives (2a,b) were synthesized. These
compounds were strong electron donors with reversible redox
behaviors and displayed strong fluorescence. These compounds
can be controlled to oxidize to radical cation and dication with
distinctive optical changes. These attractive properties demon-
strated the potential applications of PIDT in the field of
switches, molecular machines, and information memories.
Figure 4. UV−vis absorption spectra of 2a in DCM (1.43 × 10−5 M)
with the titration of FeCl3.
EXPERIMENTAL SECTION
■
2,5-Dibromo-1,4-diiodobenzene (4).12 A solution of p-
dibromobenzene (36.0 g, 152.6 mmol) in concentrated sulfuric acid
(450 mL) was heated to 125−135 °C. Iodine (114.0 g, 449.2 mmol)
was added portion-wise over 2 h. The mixture was held at 125−135 °C
for 12 h and then cooled to room temperature. The reaction mixture
was quenched into ice, and the resulting blocky solid was collected and
washed with a saturated sodium bisulfate solution. The solid was
recrystallized from tetrahydrofuran and further from chloroform
and Figure S1 in Supporting Information). With the addition of
FeCl3, the intensity of the initial absorptions of 2a was
decreased, and new bands located at 586, 720, and 910 nm
emerged. No other new bands were observed upon the addition
of 6.0 equiv of FeCl3. Considering the redox potentials of 2a,
we inferred that 2a was oxidized to radical cation. To prove this
assumption, the absorption spectra of radical cation and
dication of 2a were measured by implementing the
potentiostatic electrochemical experiment with in situ spec-
troelectrochemistry measuring technique.11 According to cyclic
voltammetry results, oxidation potentials of 0.7 and 1.4 V were
applied separately to compound 2a for 200 s before the
absorption spectra were conducted (Figure 5). We found when
an oxidation voltage of 0.7 V was applied, 2a displayed an
identical spectrum as that titrated by FeCl3 (Figure S2 in
Supporting Information.) When an oxidation potential of 1.4 V
was used, besides the three new peaks at 586, 720, and 910 nm,
a shoulder peak located at 602 nm was also observed in the
absorption spectrum. These results evidenced that, unlike TTF
(which can be oxidized to radical cation and dication
sequentially),10 PIDT was only oxidized to radical cation by
1
affording 4 as white crystals:. 33.0 g (45%). H NMR (CDCl3) δ
8.05 (s, 2 H); MS (EI) (m/z) 487.
2-(Tri-n-butylstannyl)-3-bromo-thiophene (5). To a solution
of diisopropylamine (11.1 g, 110.0 mmol) in anhydrous tetrahy-
drofuran was added a 2.5 M solution of n-butyllithium (44.4 mL, 110.0
mmol) at −20 °C under nitrogen. The solution was stirred at −20 °C
for 5 min. The solution was warmed to room temperature and stirred
for another 20 min. The solution was transferred to a stirred solution
of 3-bromothiophene in anhydrous tetrahydrofuran at −20 °C via a
cannula under nitrogen. After 2 h, tributyltin chloride (35.8 g, 110
mmol) was added in one portion, and the reaction mixture was stirred
overnight. The reaction mixture was quenched into water (50 mL) and
extracted with n-hexane (50 mL). The aqueous layer was extracted
twice with n-hexane (2 × 50 mL). The combined organic layers were
washed with water (2 × 50 mL) and brine (50 mL) and dried over
MgSO4. Solvent was evaporated in vacuo to give compound 5 as a pale
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The Journal of Organic Chemistry
Note
brown oil. 1H NMR (CDCl3) δ 7.49 (d, J = 4.8 Hz, 1H), 7.12 (d, J =
4.8 Hz, 1H), 1.63 (m, 6H), 1.37 (m, 6H), 1.23 (m, 6H), 0.89 (m, 9H).
2,2′-(2,5-dibromo-1,4-phenylene)bis(3-bromothiophene)
(3).13 A Schlenk tube was charged with LiCl (2.7 g, 60 mmol),
compound 4 (4.8 g, 10 mmol), Pd(PPh3)4 (287.5 mg, 0.25 mmol),
and CuCl (5.0 g, 50 mmol). The tube was degassed under high
vacuum with a nitrogen purge. DMSO (60 mL) was added with
concomitant stirring followed by the addition of compound 5 (9.9 g,
11 mmol). The reaction mixture was stirred at room temperature for 1
h and then heated to 50 °C for 3 h. The reaction mixture was cooled,
and the precipitation was collected by filtration, affording compound 2.
Yield: 2.2 g (40%); 1H NMR (CDCl3) δ 7.72 (s, 2H), 7.42 (d, J = 5.1
Hz, 2H), 7.09 (d, J = 5.4 Hz, 2H).
General Procedure for the Synthesis of N-Alkyl Pyrroloin-
dacenodithiophenes (1a,b). A Schlenk tube was charged with
compound 3 (558.0 mg, 1 mmol), NaOtBu (768.0 mg, 8 mmol),
Pd2(dba)3 (92.0 mg, 0.1 mmol), and BINAP (124.0 mg, 0.2 mmol).
The tube was degassed under high vacuum with an nitrogen purge.
The corresponding amine (516 mg, 4 mmol) and p-xylene were
successively injected into the tube. The reaction mixture was heated to
reflux and stirred for 2 h. After cooling to room temperature, the solid
was filtered and washed with dichloromethane. The combined organic
phase was washed twice with water (2 × 50 mL) and then dried over
MgSO4. The solvents were removed under vacuum, and the residue
was purified by column chromatography (silica gel; eluent, hexane/
dichloromethane, 8/1, v/v) to afford compounds 1a,b.
4.34. Found: C 78.05, H 7.72, N 4.28. MS (MALDI-TOF): m/z 645.
+1
HR-MS (MALDI-MS): m/z 644.3249, calcd for (C42H48N2S2
)
644.3253. Because of the low solubility, the 13C NMR of 2a could not
be obtained.
Compound 2b Was Collected As a Yellow Solid. Yield: 185 mg
(58%); mp 213-215 °C; 1H NMR (CDCl3) δ 7.72 (m, 6H), 7.40 (m,
6H), 7.32 (s, 2H), 4.17 (m, 4H), 2.11 (m, 2H), 1.41−1.24 (m, 16H),
0.95−0.86 (m, 12H); MS (MALDI-TOF): m/z 645. Anal. Calcd for
C42H48N2S2: C 78.21, H 7.50, N 4.34. Found: C 78.30, H 7.62, N 4.23.
Because of the low solubility, the 13C NMR of 2b could not be
obtained.
ASSOCIATED CONTENT
* Supporting Information
General method for the experiment, absorption spectra, single
crystal structure, and CIF files. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*Tel: +86-21-54925024. Fax:+86-21-54925024. E-mail: lhx@
mail.sioc.ac.cn.
■
Notes
The authors declare no competing financial interest.
Compound 1a Was Collected As a Pale Yellow Solid. Yield: 344
mg (70%); mp 137−139 °C; H NMR (CDCl3) δ 7.63 (s, 2H), 7.35
1
ACKNOWLEDGMENTS
■
(d, J = 4.8 Hz, 2H), 7.08 (d, J = 5.1 Hz, 2H), 4.32 (t, J = 6.9 Hz, 4H),
1.91 (q, J = 7.2 Hz, 4H), 1.24 (m, 20H), 0.86 (t, 6H); 13C NMR
(CDCl3) δ 146.3, 137.7, 125.8, 119.4, 115.0, 110.6, 98.2, 45.5, 31.8,
29.6, 29.4, 29.2, 27.3, 22.6, 14.1; MS (MALDI-TOF) 492.3. Anal.
Calcd for C30H40N2S2: C 73.12, H 8.18, N 5.68. Found: C 73.43, H
8.22, N 5.66 .
This work was supported by National Natural Sciences
Foundation of China (21190031, 51273212) and National
Basic Research Program of China (2011CB808405)
REFERENCES
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General Procedure for the Synthesis of Diphenyl-pyrroloin-
dacenodithiophenes (2a,b). NBS (391.6 mg, 2.2 mmol) was added
portionwise into a solution of compound 1a,b (1.0 mmol) in
tetrahydrofuran (THF) and N,N-dimethyl formamide (DMF) at −78
°C under nitrogen. The reaction mixture was stirred for 0.5 h. Then
the reaction mixture was quenched with water and extracted twice with
dichloromethane (2 × 50 mL). The combined organic phase was dried
over MgSO4, and solvents were removed in vacuo. The obtained
dibromo-pyrroloindacenodithiophenes were purified by column
chromatography (silica gel; eluent, hexane).
A Schlenk tube was charged with dibromo-pyrroloindacenodithio-
phenes (324.0 mg, 0.5 mmol) and phenylboronic acid (134.1 mg, 1.1
mmol). The tube was degassed under high vacuum with an nitrogen
purge. A solution of 2 M K2CO3 (3 mL), Aliquat (catalytic), and
toluene (10 mL) were added successively. The reaction mixture was
stirred and purged with nitrogen for 15 min before tetrakis-
(triphenylphosphine) palladium(0) (28.7 mg, 0.025 mmol) was
added. The reaction mixture was heated at 110 °C for 6 h. After
cooling down to room temperature, dichloromehtane was added, and
the organic layer was collected. The organic layer was washed 3 times
with water (3 × 50 mL), and dried over anhydrous MgSO4. The
solvent was removed under vacuum, and the residue was purified by
column chromatography (silica gel; eluent, hexane/dichloromethane,
10/1, v/v) to afford compounds 2a,b.
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Compound 2a Was Collected As a Yellow Solid. Yield: 96 mg
(30%); mp 189−191 °C; 1H NMR (CDCl3) δ 7.72 (m, 6H), 7.40 (m,
6H), 7.32 (s, 2H), 4.32 (t, J = 1.2 Hz, 4H), 1.94 (q, 4H), 1.26 (m,
20H), 0.86 (m, 6H). Anal. Calcd for C42H48N2S2: C 78.21, H 7.50, N
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