Synthesis, Molecular and Photovoltaic Properties of Donor-Acceptor Conjugated Polymers Incorporating a New Heptacylic Indacenodithieno[3,2-b]thiophene Arene

Synthesis, Molecular and Photovoltaic Properties of Donor

−Acceptor

Conjugated Polymers Incorporating a New Heptacylic

Indacenodithieno[3,2

‑b]thiophene Arene

Huan-Hsuan Chang, Che-En Tsai, Yu-Ying Lai, De-Yang Chiou, So-Lin Hsu, Chain-Shu Hsu,

and Yen-Ju Cheng*

Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road, Hsin-Chu, 30010 Taiwan

*

S Supporting Information

ABSTRACT:

We have developed a new multifused

indacenodithieno[3,2-b]thiophene arene (IDTT) unit where

the central phenylene is covalently fastened with the two outer

thieno[3,2-b]thiophene (TT) rings, forming two

cyclo-pentadiene rings embedded in a heptacyclic structure. This

rigid and coplanar IDTT building block was copolymerized

with electron-de

ficient acceptors,

4,7-dibromo-2,1,3-benzothia-diazole (BT), 4,7-dibromo-5,6-di

fluoro-2,1,3-benzothiadiazole

(FBT) and 1,3-dibromo-thieno[3,4-c]pyrrole-4,6-dione

(TPD) via Stille polymerization, respectively. Because the

higher content of the thienothiophene moieties in the fully coplanar IDTT structure facilitates

π-electron delocalization, these

new polymers show much improved light-harvesting abilities and enhanced charge mobilities compared to PDITTBT copolymer

using hexacyclic diindenothieno[3,2-b]thiophene (DITT) as the donor moieties. The device using PIDTTBT:PC

₇₁

BM (1:4, w/

w) exhibited a decent power conversion e

fficiency (PCE) of 3.8%. Meanwhile, the solar cell using PIDTTFBT:PC

71

BM (1:4 in

wt %) blend exhibited a greater V

_oc

value of 0.9 V and a larger J

_sc

of 10.08 mA/cm

2

_{, improving the PCE to 4.2%. The enhanced}

V

oc

is attributed to the lower-lying of HOMO energy level of PIDTTFBT as a result of

fluorine withdrawing effect on the BT

unit. A highest PCE of 4.3% was achieved for the device incorporating PIDTTTPD:PC

₇₁

BM (1:4 in wt %) blend.

■

INTRODUCTION

Polymer solar cells (PSCs) using organic p-type (donor) and

n-type (acceptor) semiconductors have attracted tremendous

scienti

fic and industrial interest.

1

The most critical challenge at

molecular level is to develop p-type conjugated polymers that

can simultaneously possess su

fficient solubility for

process-ability and miscibility with an n-type material, low band gap

(LBG) for strong and broad absorption spectrum to capture

more solar photons, and high hole mobility for e

fficient charge

transport.

2

The most useful approach to make a LBG polymer

is to connect electron-rich donor and electron-de

ficient

acceptor segments along the conjugated polymer backbone.

Thiophene and benzene aromatic rings are the most important

ingredients to comprise p-type conjugated polymers.

Benzene-based derivatives such as tricyclic 2,7-

fluorene or 2,7-carbazole

units have shown to serve as useful building blocks to construct

donor

−acceptor polymers having deep-lying HOMO energy

levels that contribute to achieve high open-circuit voltage (V

_oc

)

(>0.8 V) for PSCs.

3

However, the intrinsic drawback is that

these polymers usually possess relatively large optical band gaps

(>2 eV) that limit their ability to harvest sunlight and thus

result in moderate short-circuit current (J

_sc

). On the other

hand, because of the lower aromaticity to adapt more quinoidal

resonance structure, thiophene-based D

−A polymers have

better light absorption ability to permit greater J

_sc

. However,

their V

oc

values are generally limited to ca. 0.6 V as a result of

the high-lying HOMO levels.

4

Thieno[3,2-b]thiophene (TT)

unit emerges as a superior thiophene-based building block to

achieve high mobility p-type semiconductors.

5

This compact

structure actually possesses higher aromatic stabilization energy

than a thiophene, which can potentially lower the HOMO level

for higher V

oc

.

6

Moreover, the C

2h

symmetry and coplanar

geometry may promote more ordered packing and stronger

interchain interactions to obtain exceptional hole mobility,

which is bene

ficial for J

_sc

.

7

Introducing the alkyl chains into the

two

β-positions of thieno[3,2-b]thiophene unit is usually

necessary to improve the solubility of the resulting polymers.

Unfortunately, these side chains inevitably impose a negative

e

ffect on the effective conjugation due to severe steric

hindrance-induced twisting between the neighboring aryls.

8

By implementing forced planarization via covalently fastening

adjacent aromatic units in the polymer backbone, advantageous

intrinsic properties such as reduced band gap and enhanced

charge mobility can be retained.

9

Therefore, it is of interest to

integrate benzene and thieno[3,2-b]thiophene units into a

molecular entity with forced rigidi

fication to simultaneously

Received: September 18, 2012

Revised: November 4, 2012

Published: November 27, 2012

extend the conjugation while maintaining the coplanarity.

However, development of ladder-type architectures requires

elegant molecular design and synthesis.

10

Recently, we

first

reported a multifused hexacyclic

diindenothieno[3,2-b]-thiophene (DITT) unit, where the central TT ring is connected

with two outer phenyl rings through two embedded

cyclo-pentadienyl (CP) rings (Scheme 1).

11

This electron-rich unit

was copolymerized with electron-de

ficient benzothiadiazole

acceptor to obtain a donor

−acceptor copolymer PDITTBT

(Scheme 1). Nevertheless, this polymer is short of absorption

coverage at the visible and near IR region due to the fact that

DITT

possesses high content of high-aromaticity benzene rings

(two benzene rings and one thieno[3,2-b]thiophene), leading

to relatively large optical band gap and thus limited

photocurrent. By reversing the arrangement of TT and benzene

units in the DITT framework, we present here a new

multifused heptacyclic structure,

indacenodithieno[3,2-b]-thiophene (IDTT), where the central phenylene ring is fused

with two outer TT rings by two carbon bridges. Compared to

hexacyclic DITT unit, this heptacyclic IDTT has extended

conjugation length with greatly increasing the content of the

thiophene moieties (one benzene and two

thieno[3,2-b]-thiophene units). Furthermore, the placement of the

thienothiophene units at the two terminal sides of IDTT is

advantageous for facile

α-bromination or stannylation for

subsequent polymerization. Four side chains introduced at the

bridging carbons in IDTT guarantee solubility without twisting

the coplanarity. Meanwhile, exploitation of suitable

electron-de

ficient acceptors in combination with IDTT donor in the

polymeric backbone is required. Benzothiadiazole (BT)

12

and

thieno[3,4-c]pyrrole-4,6-dione (TPD)

13

are the widely used

electron-de

ficient acceptors due to their suitable electron

affinity and easy availability. 5,6-Difluorobenzothiadiazole

(FBT) unit with two

fluorine atoms on BT unit also emerges

as a superior derivative for adjusting the molecular properties.

14

In this research, we report the detailed synthesis of the

distannyl-IDTT monomer which was copolymerized with BT,

FBT, TPD acceptor moieties to prepare a new class of D

−A

alternating IDTT-based copolymers (Scheme 1). Their

thermal, optical and electrochemical properties have been

carefully characterized. Preliminary results showed that the

IDTT-based polymers are promising for photovoltaic solar cell

applications.

Synthesis. The synthetic route for Sn-IDTT monomer is

depicted in Scheme 2. Stille coupling of diethyl

2,5-dibromoterephthate with

2-(tributylstannyl)thieno[3,2-b]-thiophene yielded compound 1. Double nucleophilic addition

of the ester groups in 1 by (4-octyloxy)phenyl magnesium

bromide led to the formation of benzylic alcohols in 2 which

was subjected to intramolecular Friedel

−Crafts cyclization

under acidic condition to furnish the heptacyclic IDTT arene in

a good yield of 81%. Bromination of IDTT by

N-bromosuccinimide generated Br-IDTT in a high yield of

87%. Finally, Br-IDTT was lithiated by n-butyllithium followed

by quenching with trimethyltin chloride to a

fford the distannyl

Scheme 1. Chemical Structures of Hexacyclic DITT and

Heptacyclic IDTT Arenes and Their Corresponding

Donor

−Acceptor Copolymers

Sn-IDTT

in a moderate yield of 54%. Sn-IDTT monomer was

copolymerized with 4,7-dibromo-2,1,3-benzothiadiazole (BT),

4,7-dibromo-5,6-di

fluoro-2,1,3-benzothiadiazole (FBT) and

1,3-dibromo-thieno[3,4-c]pyrrole-4,6-dione (TPD) acceptor

monomers via Stille coupling to a

fford three donor−acceptor

copolymers,

poly(indacenodithieno[3,2-b]thiophene-alt-benzo-thiadiazole) (PIDTTBT, M

_n

= 16.6 kDa, PDI = 1.7),

poly(indacenodithieno[3,2-b]thiophene-alt-di

fluorobenzothia-diazole) (PIDTTFBT, M

_n

= 24.0 kDa, PDI = 1.2) and

poly(indacenodithieno[3,2-b]thiophene-alt-thieno[3,4-c]-pyrrole-4,6-dione) (PIDTTTPD, M

_n

= 31.3 kDa, PDI = 2.0),

respectively (Scheme 3). These copolymers puri

fied by

successive Soxhlet extraction and precipitation showed

narrower molecular weight. The resulting copolymers

flanked

with four side chains on IDTT unit possess excellent

solubilities in common organic solvents, such as chloroform,

toluene, and THF.

Thermal and Optical Properties. The thermal stability of

the polymers was analyzed by thermogravimetric analysis

(TGA) (Figure S1, Supporting Information). PIDTTBT,

PIDTTFBT, and PIDTTTPD exhibited su

fficiently high

decomposition temperatures (T

d

) of 414, 391, and 384

°C,

respectively.

UV

−vis absorption spectra of the three polymers in THF

solutions and in thin

films are shown in Figure 1 and the

correlated optical parameters are summarized in Table 1.

PIDTTBT

and PIDTTFBT showed two obvious absorption

bands in the spectra. The longer wavelength absorbance is

attributed to the intramolecular charge transfer (ICT) between

the electron-rich and the electron-de

ficient segments. However,

the localized transition bands and ICT bands of PIDTTTPD

overlap into a broad band covering the whole visible region

from 400 to 700 nm, indicating that the accepting strength of

TPD

is weaker than that of BT unit. The absorption spectra of

the three polymers shift toward longer wavelengths from the

solution state to the solid state, indicating that the coplanar

structure of IDTT is capable of inducing strong interchain

interactions. The optical band gaps (E

gopt

) deduced from the

onset of absorption in the solid state are determined to be 1.69

eV for PIDTTBT, 1.77 eV for PIDTTFBT and 1.95 eV for

PIDTTTPD. Note that the optical band gap of PDITTBT is

2.15 eV, which is signi

ficantly larger than that of PIDTTBT,

suggesting that the heptacyclic IDTT unit with higher content

of thienothiophene moieties indeed facilitates the

π-electron

delocalization compared to the hexacyclic DITT unit.

Electrochemical Properties. Cyclic voltammetry (CV)

was employed to examine the electrochemical properties and

determine the highest occupied molecular orbital (HOMO)

and lowest unoccupied molecular orbital (LUMO) energies of

the polymers (Figure 2, Table 1). The three polymers showed

stable and reversible p-doping and n-doping processes, which

are important prerequisites for p-type semiconductor materials.

The LUMO energy levels of PIDTTBT, PIDTTFBT, and

PIDTTTPD

are determined to be

−3.40, −3.50, and −3.18 eV,

respectively. The LUMO energy levels are higher than that of

the PC

₇₁

BM acceptor (

−3.8 eV) to ensure energetically

favorable electron transfer. It should be noted that the

HOMO energy of PIDTTFBT (

−5.30 eV) is lower than that

of PIDTTBT (

−5.43 eV) due to the two electron-withdrawing

fluorine atoms on the BT units.

14

Furthermore, PIDTTTPD

shows the lowest HOMO energy level of

−5.45 eV, indicating

that TPD acceptor unit is also capable of lowering the HOMO

energy level of the resulting polymer. The HOMO energy

levels are within the ideal range to ensure better air-stability and

greater attainable V

oc

.

Theoretical Calculations. In order to gain more insight

into the molecular orbital properties of the polyaromatic

π-electron systems, quantum

−chemical calculations were

per-formed with the Gaussian09 suite employing the B3LYP and

TD-B3LYP density functionals in combination with the

6-311G(d,p) basis set. Considering an insigni

ficant effect on

Scheme 3. Synthesis of PIDTTBT, PIDTTFBT, and

PIDTTTPD Copolymers

electronic properties, all the side-chain substituents were

replaced with methyl groups for simplicity. Repeating units,

denoted as IDTTBT, IDTTFBT, and IDTTTPD, in their most

stable conformations were used as simplified model compounds

for PIDTTBT, PIDTTFBT, and PIDTTTPD, respectively.

The calculated HOMO/LUMO energy, excitation energy,

oscillator strength, and con

figurations of the excited states are

summarized in Table 2 and the frontier orbitals, HOMO (H),

LUMO (L) and the closeby LUMO+1 (L+1) are illustrated in

Table 3. The HOMO electron density distribution of

IDTTTPD

is analogous to that of IDTTFBT and IDTTBT,

where the electron density is not only distributed

homoge-neously along the molecular backbone of the IDTT moiety but

also on parts of the acceptor. Given that the contribution of the

IDTT

moiety to the HOMO energy level should be similar

among the three compounds, the di

fference in the HOMO

energy level therefore mainly depends on the nature of the

acceptors. Consistent with the electrochemical experiments, the

calculated HOMO energy levels of the three model compounds

follow the order: IDTTTPD (

−5.25 eV) < IDTTFBT (−5.23

eV) < IDTTBT (

−5.18 eV). On the contrary, the LUMO of

IDTTTPD

is higher in energy than that of IDTTFBT and

IDTTBT. IDTTFBT and IDTTBT have similar LUMO

electron density patterns of which the electron density is

mainly located on the acceptors (BT and FBT). Instead, the

electron density of LUMO in IDTTTPD is not only localized

on TPD unit but span from TPD to IDTT moieties.

Involvement of the IDTT orbitals in the LUMO of IDTTTPD

may result in a high-lying energy level of LUMO.

Table 1. Summary of the Intrinsic Properties of the Polymers

a

λmax(nm)

polymer M_n(kDa) PDI T_d(°C) E_gopt_{(eV) (Film) THF film HOMO (eV) LUMO(eV)}

PIDTTBT 16.6 1.7 414 1.69 420 430 −5.30 −3.40 622 645 PIDTTFBT 24.0 1.2 391 1.77 415 424 −5.43 −3.50 616 635 PIDTTTPD 31.3 2.0 384 1.95 588 591 −5.45 −3.18 a_E

goptfrom the onset of UV spectra in thinfilm.

Figure 2. Cyclic voltammograms of PIDTTBT, PIDTTFBT, and PIDTTTPDin the thinfilms at a scan rate of 50 mV/s.

Table 2. Calculated

a

HOMO/LUMO Energy, Excitation Energy, Oscillator Strength, and Configurations (with Large CI

Coe

fficients) of the Excited States

excitation energy

compound HOMO (eV) LUMO (eV) λmax,exp(nm)b λcalcd(nm) oscillator strength symmetry configurationc

IDTT −5.22 −1.87 393, 417 420 1.1262 singlet-A H→L 321 0.2295 singlet-A H−4→L H→L+2 IDTTBT −5.18 −2.78 622 597 0.6822 singlet-A H→L 420 433 0.7493 singlet-A H−1→L 430 0.315 singlet-A H→L+1 H−1→L H→L+1 H−2→L IDTTFBT −5.23 −2.89 616 607 0.6499 singlet-A H→L 415 435 0.2193 singlet-A H−3→L H−2→L H−1→L H→L+1 431 0.8399 singlet-A H→L+1 H−3→L H−2→L H−1→L IDTTTPD −5.25 −2.53 588 521 1.4519 singlet-A H→L

a_{TD−B3LYP/6-311G(d,p), PCM=THF.}b_{Experimental values were measured for nonsimplified IDTT, IDTTBT, IDTTFBT, IDTTTPD in THF}

As listed in Table 2, although there are variations in the

absolute values, the calculated absorptions are still in good

agreement with the experimental values. In order to have

further understanding of these electronic transitions, electron

density di

fference maps (EDDMs) were conducted (Figure

3).

15

The electronic transitions can therefore be visualized

through EDDMs. Red indicates a decrease in charge density,

while green indicates an increase. For IDTTBT, the lowest

energy singlet electronic transition (

λ

calc

= 597 nm;

λ

max,exp

=

622 nm) indicates a pronounced intramolecular charge transfer

(ICT) from IDTT to BT. The transitions at

λ

calc

= 433 and 430

nm are close in energy and may not be separable. In fact, in the

UV absorption spectrum, only one absorption peak (

λ

max,exp

=

420 nm) was observed. On the basis of the EDDMs, both

transitions belong to charge separations from the molecular

backbone of IDTT and one 4-methoxyphenyl side chain to the

BT

unit. The lowest energy singlet electronic transition (

λ

calc

=

607 nm;

λ

max,exp

= 616 nm) of IDTTFBT is a ICT band, which

is basically caused by the electron transfer from IDTT to FBT.

The transitions at

λ

calc

= 435 and 431 nm are also inseparable in

the experimental spectrum (

λ

max,exp

= 415 nm). One at

λ

calc

=

435 nm is assigned to the ICT from the

flanking

4-methoxyphenyl groups of IDTT to BT, and the other one at

λ

max

= 431 nm only occurs at localized regions. Lastly, the most

Table 3. Plots (Isovalue = 0.02 au) of Frontier Orbitals of IDTT, IDTTBT, IDTTFBT, and IDTTPD Calculated at the Level of

B3LYP/6-311G(d,P) in THF

Figure 3.Electron density difference maps (EDDMs) of selected singlet electronic transitions of IDTTBT (at 597, 433, and 430 nm), IDTTFBT (at 607, 435, and 431 nm), and IDTTTPD (at 521 nm). Red indicates a decrease in charge density, while green indicates an increase. All EDDMS were plotted with isovalue 0.0012 au.

probable vertical excitation of IDTTTPD is calculated to be

521 nm, which deviates slightly from the

λ

max,exp

of 589 nm. It

mainly results from the ICT from IDTT to TDP and the

π−π*

transition within the IDTT unit has a minor contribution.

Organic Field E

ffect Transistors. To investigate the

mobilities of the polymers, organic

field-effect transistors

(OFETs) were fabricated in the bottom-contact/top-contact

geometry as described in the Experimental Section (Table 4

and Figure 4). The hole mobilities were deduced from the

transfer characteristics of the devices in the saturation regime.

The polymers-based OFETs using SiO

2

as gate dielectric was

treated with octadecyltrichlorosilane (ODTS) to form a

self-assembly monolayer (SAM). With the modi

fication of a

ODTS-SAM layer along with annealing temperature at 150

Table 4. FET Characteristics of the Polymer Thin Films

polymer SAM layer annealing temperature (°C) mobility (cm2_V−1_s−1_{) on/off V}

t(V)

PIDTTBT ODTS 200 7× 10−3 1.9× 103 _−13.8

PIDTTFBT ODTS 150 1× 10−2 1.4× 107 _−21.7

PIDTTTPD ODTS 200 1× 10−3 3.2× 104 _−25.4

Figure 4.. Typical output curves (a, c, e) and transfer plots (b, d, f) of the OFET devices based on PIDTTBT, PIDTTFBT, and PIDTTTPD, respectively, with ODTS-SAM layer.

and 200

°C, the hole mobilities of the three polymers were

determined to be 7

× 10

−3

cm

2

V

−1

s

−1

, 1

× 10

−2

cm

2

V

−1

s

−1

,

and 1

× 10

−3

cm

2

_V

−1

_s

−1

_{with the on}

_{−off ratios of 1.9 × 10}

3

_,

1.4

× 10

7

, and 3.2

× 10

4

for PIDTTBT, PIDTTFBT, and

PIDTTTPD

devices, respectively. It should be mentioned that

PDITTBT

using hexacyclic DITT as the donor showed a much

lower mobility of 7

× 10

−5

cm

2

_V

−1

_s

−1

_.

11

This result again

indicates that higher content of thienothiophene unit can

improve the charge transporting properties.

Photovoltaic Characteristics. Bulk heterojunction

photvoltaic cells were fabricated by spin-coating the blends from

o-dichlorobenzene solutions at various polymer-to-PC

71

BM ratios

on the basis of ITO/PEDOT:PSS/polymer:PC

₇₁

BM/Ca/Al

con

figuration and their performances were measured under a

simulated AM 1.5 G illumination of 100 mW/cm

2

_{. The}

asymmetric PC

71

BM was used due to its stronger light

absorption in the visible region than that of PC

₆₁

BM. The

characterization data are summarized in Table 5 and the J

−V

curves of these polymers are shown in Figure 5. The blend ratio

of the active layers shown in the Table 5 is the result of the

optimized conditions for the devices. The device based on the

PIDTTBT:PC

71

BM (1:4 in wt %) blend exhibited a V

oc

of 0.84

V, a J

sc

of 8.32 mA/cm

2

, a FF of 55%, delivering a decent PCE

of 3.8%. It is noteworthy that the device based on PDITTBT

polymer using the hexacyclic DITT unit as the donor only

exhibited a lower V

oc

of 0.88 V, a J

sc

of 7.46 mA/cm

2

, resulting

in a lower PCE of 2.7%.

11

Meanwhile, the device using

PIDTTFBT:PC

71

BM (1:4 in wt %) blend exhibited a greater

V

_oc

value of 0.9 V and a larger J

sc

of 10.08 mA/cm

2

with an

improved PCE to 4.2%. The enhanced V

oc

is attributed to the

lower-lying of HOMO energy level of PIDTTFBT as a result of

fluorine withdrawing effect on the FBT unit. More

encourag-ingly, the device based on the PIDTTTPD:PC

71

BM (1:4 in wt

%) blend exhibited a high V

oc

of 0.90 V, a J

sc

of 7.99 mA/cm

2

, a

FF of 60%, leading to a highest PCE of 4.3%. Even though the

device derived from PIDTTFBT has the highest V

_oc

and J

_sc

, it

does not show a better PCE than the PIDTTTPD-based

device, which is due to the fact that the device of PIDTTTPD

has a much larger FF than that of PIDTTBT. To con

firm the

accuracy of the measurements of the devices, the corresponding

external quantum e

fficiency (EQE) spectra were measured

under illumination of monochromatic light (Figure 4). The J

sc

values calculated from integration of the EQE spectra agree well

with the J

sc

obtained from the J

−V measurements.

■

CONCLUSIONS

Compared to thiophene unit, C

_2h

-symmetry

thieno[3,2-b]-thiophene (TT) unit with higher aromatic stabilization energy

and coplanar geometry is a promising building block for

donor

−acceptor conjugated polymers to obtain higher V

oc

and

J

_sc

. However, introducing aliphatic side chains on the

β-positions of TT units leads to severe steric hindrance-induced

twisting between the neighboring aryls in the polymer

backbone. A straightforward approach to circumvent this

deficiency is to embed TT units in a multifused ladder-type

structure. A hexacyclic diindenothieno[3,2-b]thiophene

(DITT) unit has been

first developed. Nevertheless, DITT

possesses high content of high-aromaticity benzene rings

resulting in relatively large optical band gap and thus limited

J

sc

. By reversing the arrangement of TT and benzene units in

the DITT framework, we have successfully developed a new

multifused heptacyclic structure,

indacenodithieno[3,2-b]-thiophene (IDTT), where the central phenylene ring is fused

with two outer TT rings. The ladder-type IDTT framework can

be easily constructed by Friedel

−Crafts annulation. The optical

and electrochemical properties of the resulting polymers have

been characterized experimentally and theoretically. Compared

to the hexacyclic DITT unit, this heptacyclic IDTT has

extended conjugation length with signi

ficantly increasing the

content of the thiophene moieties. Because of the more

favorable

π-electron delocalization, IDTT-based polymers show

much improved light-harvesting abilities and enhanced charge

mobilities. The device using PIDTTBT:PC

71

BM (1:4, w/w)

blend exhibited an improved e

fficiency of 3.8%. Meanwhile, the

device using PIDTTFBT:PC

71

BM (1:4, w/w) blend exhibited

a greater V

oc

value of 0.9 V as a result of the

fluorine

withdrawing e

ffect on the BT unit, and a larger J

sc

of 10.08 mA/

cm

2

with a higher PCE of 4.2%. The device based on the

Table 5. PSCs Characteristics

polymer blend ratio polymer:PC71BM Voc (V) Jsc (mA/cm2_{) (%)}FF PCE_(%) PIDTTBT 1: 4 0.84 8.32 55 3.8 PIDTTFBT 1: 4 0.90 10.08 46 4.2 PIDTTTPD 1: 4 0.90 7.99 60 4.3

Figure 5.J−V characteristics of ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al under illumination of AM1.5, 100 mW/cm2, and corresponding EQE spectra.

PIDTTTPD:PC

₇₁

BM (1:4 in wt %) blend exhibited a high V

_oc

of 0.90 V and a highest PCE of 4.3%. We envisage that further

improvement of device performance is highly achievable by

optimizing the processing conditions which are underway in

our laboratories. This research demonstrated that the new

heptacyclic indacenodithieno[3,2-b]thiophene is one of the

most promising building blocks for constructing

high-perform-ance conjugated polymers.

■

EXPERIMENTAL SECTION

General Measurement and Characterization. All chemicals are purchased from Aldrich or Acros and used as received unless otherwise specified.1_{H and}13_{C NMR spectra were measured using Varian 300} and 400 MHz instrument spectrometers. Thermogravimetric analysis (TGA) was recorded on a Perkin-Elmer Pyris under nitrogen atmosphere at a heating rate of 10°C/min. Absorption spectra were recorded on a HP8453 UV−vis spectrophotometer. The molecular weight of polymers was measured on a Viscotek VE2001GPC, and polystyrene was used as the standard (THF as the eluent). The electrochemical cyclic voltammetry (CV) was conducted on a CH Instruments Model 611D. A carbon glass coated with a thin polymer film was used as the working electrode and Ag/Ag+_{electrode as the} reference electrode, while 0.1 M tetrabutylammonium hexafluoro-phosphate (Bu4NPF6) in acetonitrile was the electrolyte. CV curves were calibrated using ferrocence as the standard, whose oxidation potential is set at −4.8 eV with respect to zero vacuum level. The HOMO energy levels were obtained from the equation HOMO = −(Eoxonset − E(ferrocene)onset + 4.8) eV. The LUMO levels of polymer were obtained from the equation LUMO =−(Eredonset− E(ferrocene)onset + 4.8) eV.

OFET Fabrication. An n-type heavily doped Si wafer with a SiO2 layer of 300 nm and a capacitance of 11 nF/cm2_{was used as the gate} electrode and dielectric layer. Thinfilms (40−60 nm in thickness) of polymers were deposited on octadecyltrichlorosilane (ODTS)-treated SiO2/Si substrates by spin-coating their o-dichlorobenzene solutions (5 mg/mL). Then, the thin films were annealed at different temperatures (150 or 250 °C) for 30 min. Gold source and drain contacts (40 nm in thickness) were deposited by vacuum evaporation on the organic layer through a shadow mask, affording a bottom-gate, top-contact device configuration. Electrical measurements of OTFT devices were carried out at room temperature in air using a 4156C, Agilent Technologies. Thefield-effect mobility was calculated in the saturation regime by using the equation Ids= (μWCi/2L)(Vg− Vt)2, where Idsis the drain-source current,μ is the field-effect mobility, W is the channel width (1 mm), L is the channel length (100μm), Ciis the capacitance per unit area of the gate dielectric layer, Vg is the gate voltage, and Vtis threshold voltage.

PSCs Fabrication. ITO/Glass substrates were ultrasonically cleaned sequentially in detergent, water, acetone and iso-propanol (IPA). The cleaned substrates were covered by a 30 nm thick layer of PEDOT:PSS (Clevios P provided by Stark) by spin coating . After annealing in a glovebox at 150°C for 30 min, the samples were cooled to room temperature. Polymers were dissolved in o-dichlorobenzene (ODCB), and then PC71BM (purchased from Nano-C) was added. The solution was then heated at 80°C and stirred overnight at the same temperature. Prior to deposition, the solution werefiltered (0.45 μm filters). The solution of polymer:PC71BM was then spin coated to form the active layer. The cathode made of calcium (35 nm thick) and aluminum (100 nm thick) was sequentially evaporated through a shadow mask under high vacuum (<10−6Torr). Each sample consists of 4 independent pixels defined by an active area of 0.04 cm2_{. Finally,} the devices were encapsulated and characterized in air.

Synthesis of Compound 1. A mixture of diethyl 2,5-dibromoterephthalate (6.05 g, 15.9 mmol), 2-(tributylstannyl)thieno-[3,2-b]thiophene (15.71 g, 36.6 mmol), Pd(PPh3)4(0.74 g, 0.6 mmol), and degassed toluene (80 mL) was heated to 130°C under nitrogen atmosphere for 16 h. The reaction mixture was poured into water (150 mL) and extracted with ethyl acetate (300 mL× 3). The combined organic layer was dried over MgSO4and the solvent was removed in

vacuo. The residue was purified by column chromatography on silica gel (hexane/ethyl acetate, v/v, 20/1) and then recrystallized from hexane to give a light yellow solid 1 (6.9 g, 87%):1H NMR (CDCl3, 300 MHz, ppm)δ 1.13 (t, J = 7.1 Hz, 6 H), 4.21−4.28 (q, J = 7.1 Hz, 4 H), 7.26−7.29 (m, 4 H), 7.40 (d, J = 5.1 Hz, 2 H), 7.89 (s, 2 H);13_C NMR (CDCl3, 75 MHz, ppm)δ 13.8, 61.8, 119.3, 119.4, 127.4, 132.0, 133.8, 134.1, 139.3, 139.9, 142.0, 167.4; MS (EI, M+•, C24H18O4S4) calcd 498.01, found 498.

Synthesis of Compound 2. A Grignard reagent was prepared by the following procedure. To a suspension of magnesium turnings (3.2 g, 132.0 mmol) and 3−4 drops of 1,2-dibromoethane in dry THF (120 mL) was slowly added 1-bromo-4-(octyloxy)benzene (34.23 g, 120.0 mmol) dropwise and the mixture was then stirred for 1 h. To a THF (50 mL) solution of 1 (4.80 g, 9.6 mmol) under nitrogen atmosphere was added dropwise the freshly prepared 4-(octyloxy)phenyl-1-magnesium bromide (1 M, 76.8 mL, 76.8 mmol) at room temperature. The resulting mixture was heated at the refluxing temperature for 16 h, cooled to room temperature, poured into water (100 mL), and extracted with ethyl acetate (150 mL× 3). The collected organic layer was dried over MgSO4. After removal of the solvent under reduced pressure, the residue was purified by column chromatography on silica gel (hexane/ethyl acetate, v/v, 30/1) to furnish a yellow solid 2 (5.17 g, 43.7%):1_{H NMR (CDCl} 3, 300 MHz, ppm)δ 0.88, (t, J = 6.8 Hz, 12 H), 1.28−1.45 (m, 40 H), 1.75−1.80 (m, 8 H), 3.42 (s, 2 H), 3.94 (t, J = 6.6 Hz, 8 H), 6.27 (s, 2 H), 6.80 (d, J = 9 Hz, 8 H), 6.90 (s, 2 H), 7.08 (d, J = 9 Hz, 8 H), 7.13 (d, J = 5.1 Hz, 2 H), 7.30 (d, J = 5.1 Hz, 2 H);13C NMR (CDCl3, 75 MHz, ppm)δ 14.1, 22.7, 26.0, 29.2, 29.3, 29.4, 31.8, 68.0, 82.4, 113.8, 119.3, 120.3, 127.0, 129.1, 132.3, 136.0, 138.7, 139.5, 139.9, 143.9. 145.4. 158.3; MS (FAB, M+•, C76H94O6S4) calcd 1230.59, found 1230.

Synthesis of IDTT. To a solution of 2 (1.00 g, 0.81 mmol) in THF (100 mL) was added concentrated sulfuric acid (0.5 mL). The mixture was stirred for 2 h at 90°C, cooled to room temperature, poured into water (250 mL), and extracted with ethyl acetate (500 mL× 3). The combined organic layer was dried over MgSO4and the solvent was removed under reduced pressure. The residue was then purified by column chromatography on silica gel (hexane/ethyl acetate, v/v, 80/1) to afford an yellow solid IDTT (1.58 g, 81%):1_{H NMR (CDCl}

3, 400 MHz, ppm)δ 0.87 (t, J = 6.8 Hz, 12 H), 1.26−1.43 (m, 40 H), 1.70− 1.77 (m, 8 H), 3.89 (t, J = 6.6 Hz, 8 H), 6.79 (d, J = 8.8 Hz, 8 H), 7.19 (d, J = 8.8 Hz, 8 H), 7.25−7.29 (m, 4 H), 7.45 (s, 2 H) ;13_{C NMR} (CDCl3, 75 MHz, ppm)δ 14.1, 22.6, 26.1, 29.2, 29.3, 29.3, 31.8, 62.2, 67.9, 114.3, 116.7, 120.4, 126.3, 129.2, 133.6, 134.9, 136.0, 141.7, 143.0. 146.3. 153.5. 158.2; MS (FAB, M+•, C76H90O4S4) calcd 1195.79, found 1195.

Synthesis of Monomer Br-IDTT. To a solution of IDTT (1.1 g, 0.92 mmol) in THF (30 mL) was added N-bromosuccinimide (0.38 g, 2.12 mmol) at room temperature. The reaction mixture, kept away from light, was stirred for 12 h at room temperature, quenched by water (50 mL + 150 mL), and extracted with ethyl acetate (150 mL× 3). The collected organic layer was dried over MgSO4. After removal of the solvent under vacuum, the residue was purified by column chromatography on silica gel (hexane/ethyl acetate, v/v, 100/1) and then recrystallized from hexane to give a brown solid Br-IDTT(1.08 g, 87%):1_{H NMR (CDCl} 3, 300 MHz, ppm)δ 0.88 (t, J = 6.3 Hz, 12 H), 1.27−1.42 (m, 40 H), 1.72−1.77 (m, 8 H), 3.90 (t, J = 6.5 Hz, 8 H), 6.80 (d, J = 8.6 Hz, 8 H), 7.14 (d, J = 8.6 Hz, 8 H), 7.27 (s, 2 H), 7.44 (s, 2 H);13_{C NMR (CDCl} 3, 75 MHz, ppm)δ 14.1, 22.6, 26.0, 29.2, 29.3, 29.3, 31.8, 62.1, 67.9, 112.5, 114.4, 116.8, 123.1, 129.0, 133.9, 134.5, 135.8, 139.9, 142.1. 146.6. 153.6. 158.3; MS (FAB, M+·, C76H88 Br2O4S4) calcd 1353.58, found 1353.

Synthesis of Sn-IDTT. To a THF (30 mL) solution of Br-IDTT (0.85 g, 0.63 mmol) was added a hexane solution of n-BuLi (2.5M, 1.58 mmol) dropwise at −78 °C. The mixture was stirred at this temperature for 1 h and a THF solution of chlorotrimethylstannane (1.0 M, 1.89 mmol) was then introduced dropwise. It was quenched with water (50 mL) and extracted with ether (50 mL × 3). The collected organic layer was dried over MgSO4 and the solvent was removed in vacuo. The residue was recrystallized from hexane to give a brown solid Sn-IDTT(0.52 g, 54%):1_{H NMR (CDCl}

ppm)δ 0.37 (s, 18 H), 0.87 (t, J = 6.6 Hz, 12 H), 1.27−1.41 (m, 40 H), 1.69−1.78 (m, 8 H), 3.89(t, J = 6.6 Hz, 8 H), 6.79 (d, J = 8.7 Hz, 8 H), 7.19 (d, J = 8.7 Hz, 8 H), 7.30 (s, 2 H), 7.42 (s, 2 H);13C NMR (CDCl3, 100 MHz, ppm) δ −8.1, 14.1, 22.6, 26.1, 29.2, 29.3, 29.3, 31.8, 62.1, 67.9, 114.2, 116.6, 127.4, 129.3, 135.2, 136.1, 139.1, 140.3, 143.0, 143.8. 145.8. 153.3. 158.1.

Synthesis of PIDTTBT. To a 50 mL round-bottom flask were introduced Sn-IDTT (200 mg, 0.131 mmol), 4,7-dibromo-2,1,3-benzothiadiazole (38.5 mg, 0.131 mmol), Pd2(dba)3(4.8 mg, 0.005 mmol), tri(o-tolyl)phosphine (12.8 mg, 0.042 mmol), and dry chlorobenzene (7 mL). The mixture was bubbled with nitrogen for 10 min at room temperature. The reaction was then carried out in a microwave reactor under 270 W for 50 min. In order to end-cap the resultant polymer, tributyl(thiophen-2-yl)stannane (24.4 mg, 0.059 mmol) was added to the mixture, and the microwave reaction was continued for 10 min under 270 W. Subsequent to tributyl(thiophen-2-yl)stannane, another end-capping reagent, 2-bromothiophene (11.6 mg, 0.063 mmol), was added and the reaction was continued for another 10 min under otherwise identical conditions. The mixture was then added into methanol dropwise. The precipitate was collected by filtration and washed by Soxhlet extraction with acetone and hexane sequentially for 3 days. The crude polymer was dissolved in hot THF and the residual Pd catalyst and Sn metal in the THF solution was removed by Pd−thiol gel and Pd-TAAcOH (Silicycle Inc.). After filtration and removal of the solvent, the polymer was redissolved in THF and reprecipitated by methanol. The resultant polymer was collected byfiltration and dried under vacuum for 1 day to afford a dark-purplefiber-like solid (150 mg, 84%, Mn= 16600, PDI = 1.70): 1_{H NMR (CDCl}

3, 300 MHz)δ 0.87 (br, 12 H), 1.26−1.42 (br, 40 H), 1.74 (br, 8 H), 3.91 (br, 8 H), 6.84 (br, 8 H), 7.19 (br, 8 H), 7.35− 7.55 (m, 2 H), 7.75 (br, 2 H), 8.57 (br, 2 H).

Synthesis of PIDTTFBT. To a 50 mL round-bottom flask were introduced Sn-IDTT (200 mg, 0.131 mmol), 4,7-dibromo-5,6-difluoro-2,1,3-benzothiadiazole (43.2 mg, 0.131 mmol), Pd2(dba)3 (4.8 mg, 0.005 mmol), tri(o-tolyl)phosphine (12.8 mg, 0.042 mmol), and dry chlorobenzene (7 mL). The mixture was bubbled with nitrogen for 10 min at room temperature. The reaction was then carried out in a microwave reactor under 270 W for 50 min. In order to end-cap the resultant polymer, tributyl(thiophen-2-yl)stannane (24.4 mg, 0.059 mmol) was added to the mixture, and the microwave reaction was continued for 10 min under 270 W. Subsequent to tributyl(thiophen-yl)stannane, another end-capping reagent, 2-bromothiophene (11.6 mg, 0.063 mmol) was added and the reaction was continued for another 10 min under otherwise identical conditions. The mixture was then added into methanol dropwise. The precipitate was collected by filtration and washed by Soxhlet extraction with acetone and hexane sequentially for three days. The crude polymer was dissolved in hot THF and the residual Pd catalyst and Sn metal in the THF solution was removed by Pd−thiol gel and Pd−TAAcOH (Silicycle Inc.). After filtration and removal of the solvent, the polymer was redissolved in THF and reprecipitated by methanol. The resultant polymer was collected byfiltration and dried under vacuum for 1 day to afford a give a dark-purple fiber-like solid (160 mg, 86%, Mn= 24000, PDI = 1.18):δ1H NMR (CDCl3, 300 MHz)δ 0.86 (br, 12 H), 1.26−1.29 (br, 40 H), 1.74 (br, 8 H), 3.91 (br, 8 H), 6.84 (br, 8 H), 7.27 (br, 8 H), 7.54 (br, 2 H), 8.66 (br, 2 H). Synthesis of PIDTTTPD. To a 50 mL round-bottom flask were introduced Sn-IDTT (200 mg, 0.131 mmol), 1,3-dibromo-thieno[3,4-c]pyrrole-4,6-dione (55.6 mg, 0.131 mmol), Pd2(dba)3(4.8 mg, 0.005 mmol), tri(o-tolyl)phosphine (12.8 mg, 0.042 mmol), and dry chlorobenzene (7 mL). The mixture was bubbled with nitrogen for 10 min at room temperature. In order to end-cap the resultant polymer, tributyl(thiophen-2-yl)stannane (24.4 mg, 0.059 mmol) was added to the mixture, and the microwave reaction was continued for 10 min under 270 W. Subsequent to tributyl(thiophen-2-yl)stannane, another end-capping reagent, 2-bromothiophene (11.6 mg, 0.063 mmol) was added, and the reaction was continued for another 10 min under otherwise identical conditions. The mixture was then added into methanol dropwise. The precipitate was collected by filtration and washed by Soxhlet extraction with acetone and hexane sequentially for

three days. The crude polymer was dissolved in hot THF and the residual Pd catalyst and Sn metal in the THF solution was removed by Pd−thiol gel and Pd−TAAcOH (Silicycle Inc.). After filtration and removal of the solvent, the polymer was redissolved in THF and reprecipitated by methanol. The resultant polymer was collected by filtration and dried under vacuum for 1 day to give a dark-purple fiber-like solid (170 mg, 89%, Mn = 31300, PDI = 2.04): δ 1H NMR (CDCl3, 300 MHz)δ 0.87 (br, 15 H), 1.27−1.43 (m, 50 H), 1.75 (br, 10 H), 3.92 (br, 10 H), 6.84 (br, 8 H), 7.19 (br, 8 H), 7.50 (br, 2 H), 8.52 (br, 2 H).

■

ASSOCIATED CONTENT

*

S Supporting Information

Computational details, thermogravimetric analysis, and

1

H and

13

_{C NMR spectra of the new compounds and copolymers. This}

material is available free of charge via the Internet at http://

pubs.acs.org/.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: yjcheng@mail.nctu.edu.tw.

Notes

The authors declare no competing

financial interest.

■

ACKNOWLEDGMENTS

We thank the National Science Council and the

“ATU

Program

” of the Ministry of Education, Taiwan, for financial

support. We are also grateful to the National Center for

High-performance Computing (NCHC) in Taiwan for computer

time and facilities.

■

REFERENCES

(1) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789.

(2) (a) Thompson, B. C.; Fréchet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58. (b) Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Chem. Rev. 2009, 109, 5868. (c) Chen, J.; Cao, Y. Acc. Chem. Res. 2009, 42, 1709. (d) Li, Y.; Zou, Y. Adv. Mater. 2008, 20, 2952. (e) Li, Y. Acc. Chem. Res. 2012, 45, 723. (f) Huo, L.; Hou, J. Polym. Chem. 2011, 2, 2453. (g) Duan, C.; Huang, F.; Cao, Y. J. Mater. Chem. 2012, 22, 10416. (h) Zhou, H.; Yang, L.; You, W. Macromolecules 2012, 45, 607.

(3) (a) Svensson, M.; Zhang, F.; Veenstra, S. C.; Verhees, W. J. H.; Hummelen, J. C.; Kroon, J. M.; Inganäs, O.; Andersson, M. R. Adv. Mater. 2003, 15, 988. (b) Zhou, Q.; Hou, Q.; Zheng, L.; Deng, X.; Yu, G.; Cao, Y. Appl. Phys. Lett. 2004, 84, 1653. (c) Zhang, F.; Mammo, W.; Andersson, L. M.; Admassie, S.; Andersson, M. R.; Inganäs, O. Adv. Mater. 2006, 18, 2169. (d) Zhang, F.; Bijleveld, J.; Perzon, E.; Tvingstedt, K.; Barrau, S.; Inganäs, O.; Andersson, M. R. J. Mater. Chem. 2008, 18, 5468. (e) Schulz, G. L.; Chen, X.; Holdcroft, S. Appl. Phys. Lett. 2009, 94, 023302. (f) Huang, F.; Chen, K.-S.; Yip, H.-L.; Hau, S. K.; Acton, O.; Zhang, Y.; Luo, J.; Jen, A. K.-Y. J. Am. Chem. Soc. 2009, 131, 13886.

(4) (a) Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; Mühlbacher, D.; Scharber, M.; Brabec, C. Macromolecules 2007, 40, 1981. (b) Mühlbacher, D.; Scharber, M.; Zhengguo, M. M.; Zhu, M. M. Z.; Waller, D.; Gaudiana, R.; Brabec, C. Adv. Mater. 2006, 18, 2884. (c) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A. J. Science 2007, 317, 222. (d) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C. Nat. Mater. 2007, 6, 497. (e) Chen, C.-H.; Hsieh, C.-H.; Dubosc, M.; Cheng, Y.-J.; Hsu, C.-S. Macromolecules 2010, 43, 697.

(5) (a) Li, Y.; Wu, Y.; Liu, P.; Birau, M.; Pan, H.; Ong, B. S. Adv. Mater. 2006, 18, 3029. (b) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.; Mcgehee, M. D.; Toney, M. F. Nat. Mater. 2006, 5, 328.

(6) (a) Subramanian, G.; Schleyer, P. R.; Jiao, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 2638. (b) Hess, B. A., Jr.; Schaad, L. J. J. Am. Chem. Soc. 1973, 95, 3907.

(7) (a) Hayashi, N.; Mazaki, Y.; Kobayashi, K. J. Chem. Soc., Chem. Commun. 1994, 2351. (b) DeLongchamp, D. M.; Kline, R. J.; Lin, E. K.; Fischer, D. A.; Richter, L. J.; Lucas, L. A.; Heeney, M.; McCulloch, I.; Northrup, J. E. Adv. Mater. 2007, 19, 833. (c) Chabinyc, M. L.; Toney, M. F.; Kline, R. J.; McCulloch, I.; Heeney, M. J. Am. Chem. Soc. 2007, 129, 3226.

(8) (a) Zhang, X.; Kohler, M.; Matzger, A. J. Macromolecules 2004, 37, 6306. (b) Miguel, L. S.; Matzger, A. J. Macromolecules 2007, 40, 9233. (c) Miguel, S, L.; Matzger, A, J. J. Org. Chem. 2007, 72, 442. (d) Henssler, J, T.; Zhang, X.; Matzger, A, J. J. Org. Chem. 2009, 74, 9112.

(9) (a) Liang, Y.; Wu, Y.; Feng, D.; Tsai, S.-T.; Son, H.-J.; Li, G.; Yu, L. J. Am. Chem. Soc. 2009, 131, 56. (b) He, F.; Wang, W.; Chen, W.; Xu, T.; Darling, S. B.; Strzalka, J.; Liu, Y.; Yu, L. J. Am. Chem. Soc. 2011, 133, 3284. (c) Zhang, Y.; Zou, J.; Yip, H.-L.; Chen, K.-S.; Zeigler, D. F.; Sun, Y.; Jen, A. K.-Y. Chem. Mater. 2011, 23, 2289. (d) Cheng, Y.-J.; Wu, J.-S.; Shih, P.-I.; Chang, C.-Y.; Jwo, P.-C.; Kao, W.-S.; Hsu, C.-S. Chem. Mater. 2011, 23, 2361. (e) Wu, J.-S.; Cheng, Y.-J.; Dubosc, M.; Hsieh, C.-H.; Chang, C.-Y.; Hsu, C.-S. Chem. Commun. 2010, 46, 3259. (f) Wu, J.-S.; Cheng, Y.-J.; Lin, T.-Y.; Chang, C.-Y.; Shih, P.-I.; Hsu, C.-S. Adv. Funct. Mater. 2012, 22, 1711. (g) Wang, J.-Y.; Hau, S. K.; Yip, H.-L.; Davies, J. A.; Chen, K.-S.; Zhang, Y.; Sun, Y.; Jen, A. K.-Y. Chem. Mater. 2011, 23, 765. (h) Ashraf, R. S.; Chen, Z.; Leem, D. S.; Bronstein, H.; Zhang, W.; Schroeder, B.; Geerts, Y.; Smith, J.; Watkins, S.; Anthopoulos, T. D.; Sirringhaus, H.; Mello, J. C.; de; Heeney, M.; McCulloch, I. Chem. Mater. 2011, 23, 768. (i) Chen, C.-H.; Cheng, Y.-J.; Dubosc, M.; Hsieh, C.-H.; Chu, C.-C.; Hsu, C.-S. Chem. Asian. J. 2010, 5, 2480. (j) Zhang, M.; Guo, X.; Wang, X.; Wang, H.; Li, Y. Chem. Mater. 2011, 23, 4264. (k) Cheng, Y.-J.; Chen, C.-H.; Lin, Y.-S.; Chang, C.-Y.; Hsu, C.-S. Chem. Mater. 2011, 23, 5068. (l) Chen, C.-H.; Cheng, Y.-J.; Chang, C.-Y.; Hsu, C.-S. Macromolecules 2011, 44, 8415. (m) Cheng, Y.-J.; Ho, Y.-J.; Chen, C.-H.; Kao, W.-S.; Wu, C.-E.; Hsu, S.-L.; Hsu, C.-S. Macromolecules 2012, 45, 2690. (n) Chen, Y. -C.; Yu, C. -Y.; Fan, Y.-L.; Huang, L.-I.; Chen, C.-P.; Ting, C. Chem. Commun. 2010, 46, 6503. (o) Zhang, Y.; Zou, J.; Yip, H.-L.; Chen, K.-S.; Davies, J. A.; Sun, Y.; Jen, A. K. Y. Macromolecules 2011, 44, 4752.

(10) (a) Goldfinger, M. B.; Crawford, K. B.; Swager, T. M. J. Am. Chem. Soc. 1997, 119, 4578. (b) Goldfinger, M. B.; Swager, T. M. J. Am. Chem. Soc. 1994, 116, 7895. (c) Forster, M.; Annan, K. O.; Scherf, U. Macromolecules 1999, 32, 3159. (d) Scherf, U. J. Mater. Chem. 1999, 9, 1853. (e) Chmil, K.; Scherf, U. Acta Polym. 1997, 48, 208. (f) Jacob, J.; Sax, S.; Piok, T.; List, E. J. W.; Grimsdale, A. C.; Mullen, K. J. Am. Chem. Soc. 2004, 126, 6987. (g) Zheng, Q.; Jung, B. J.; Sun, J.; Katz, H. E. J. Am. Chem. Soc. 2010, 132, 5394. (h) Facchetti, A. Chem. Mater. 2011, 23, 733. (i) Guo, X.; Ortiz, R. P.; Zheng, Y.; Hu, Y.; Noh, Y.-Y.; Baeg, K.-J.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2011, 133, 1405. (j) Cheng, Y.-J.; Ho, Y.-J.; Chen, C.-H.; Kao, W.-S.; Wu, C.-E.; Hsu, S.-L.; Hsu, S. Macromolecules 2012, 45, 2690. (k) Chang, C.-Y.; Cheng, Y.-J.; Hung, S.-H.; Wu, J.-S.; Kao, W.-S.; Lee, C.-H.; Hsu, C.-S. Adv. Mater. 2012, 24, 549.

(11) Cheng, Y.-J.; Cheng, S.-W.; Chang, C.-Y.; Kao, W.-S.; Liao, M.-H.; Hsu, C.-S. Chem. Commun. 2012, 48, 3203.

(12) (a) Boudreault, P.-L. T.; Najari, A.; Leclerc, M. Chem. Mater. 2011, 23, 456. (b) Peng, Q.; Liu, X.; Su, D.; Fu, G.; Xu, J.; Dai, L. Adv. Mater. 2011, 23, 4554. (c) Hou, J. H.; Chen, H. Y.; Zhang, S. Q.; Yang, Y. J. Phys. Chem. C 2009, 113, 21202. (d) Wang, X. C.; Sun, Y. P.; Chen, S.; Guo, X.; Zhang, M. J.; Li, X. Y.; Li, Y.; Wang, H. Q. Macromolecules 2012, 45, 1208.

(13) (a) Nielsen, C. B.; Bjørnholm, T. Org. Lett. 2004, 6, 3381. (b) Chen, C. M.; Amb., S.; Graham, K. R.; Subbiah, J.; Small, C. E.; So, F.; Reynolds, J. R. J. Am. Chem. Soc. 2011, 133, 10062. (c) Chu, T. Y.; Lu, J.; Beaupré, S.; Zhang, Y.; Pouliot, J. R.; Wakim, S.; Zhou, J.; Leclerc, M.; Li, Z.; Ding, J.; Tao, Y. J. Am. Chem. Soc. 2011, 133, 4250. (d) Berrouard, P.; Najari, A.; Pron, A.; Gendron, D.; Morin, P.-O.; Pouliot, J.-R.; Veilleux, J.; Leclerc, M. Angew. Chem., Int. Ed. 2012, 51,

2068. (e) Zou, Y.; Najari, A.; Berrouard, P.; Beaupré, S.; Aïch, B. R.; Tao, Y.; Leclerc, M. J. Am. Chem. Soc. 2010, 132, 5330. (f) Zhang, Y.; Hau, S. K.; Yip, H. L.; Sun, Y.; Acton, O.; Jen, A. K. Y. Chem. Mater. 2010, 22, 2696. (g) Piliego, C.; Holcombe, T. W.; Douglas, J. D.; Woo, C. H.; Beaujuge, P. M.; Fréchet, J. M. J. J. Am. Chem. Soc. 2010, 132, 7595. (h) Najari, A.; Beaupré, S.; Berrouard, P.; Zou, Y.; Pouliot, J. R.; Pérusse, C. L.; Leclerc, M. Adv. Funct. Mater. 2011, 21, 718.

(14) (a) Zhang, Y.; Chien, S.-C.; Chen, K.-S.; Yip, H.-L.; Sun, Y.; Davies, J. A.; Chen, F.-C.; Jen, A. K. Y. Chem. Commun. 2011, 47, 11026. (b) Zhou, H.; Yang, L.; Stuart, A. C.; Price, S. C.; Liu, S.; You, W. Angew. Chem., Int. Ed. 2011, 50, 2995. (c) Schroeder, B. C.; Huang, Z.; Ashraf, R. S.; Smith, J.; D’Angelo, P.; Watkins, S. E.; Anthopoulos, T. D.; Durrant, J. R.; McCulloch, I. Adv. Funct. Mater. 2012, 22, 1663. (d) Li, Z.; Lu, J.; Tse, S.-C.; Zhou, J.; Du, X.; Tao, Y.; Ding, J. J. Mater. Chem. 2011, 21, 3226.

(15) O’Boyle, N. M.; Tenderholt, A. L.; Langner., K. M. J. Comput. Chem. 2008, 29, 839.