Synthesis, Optoelectronic, and Transistor Properties of BODIPY- and

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Synthesis, Optoelectronic and Transistor Properties of BODIPY and Cyclopenta[c]thiophene Containing #-Conjugated Copolymers Sashi Debnath, Saumya Singh, Anjan Bedi, Kothandam Krishnamoorthy, and Sanjio S. Zade J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b02743 • Publication Date (Web): 16 Jun 2015 Downloaded from http://pubs.acs.org on July 1, 2015

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The Journal of Physical Chemistry

Synthesis, Optoelectronic and Transistor Properties of BODIPY and Cyclopenta[c]thiophene Containing π-Conjugated Copolymers

Sashi Debnatha#, Saumya Singhb#, Anjan Bedia, Kothandam Krishnamoorthyb, Sanjio S. Zadea*

a

Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741252, India.

b

Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, CSIRNetwork of Institutes for Solar Energy, Dr HomiBhabha Road, Pune 411008, India

[*] email: [email protected]

Abstract Three new low band gap copolymers were synthesized by fusing dipyrromethene difluoroborane (BODIPY) as acceptor (A) and thiophene capped 5,5-bis(hexyloxymethyl)5,6-dihydro-4H-cyclopenta[c]-thiophene (CPT) as donor (D). The BODIPY unit has been copolymerized through the ῾α᾿ positions (1,7-positions) in P1 and through ῾β᾿ positions (2,6positions) in P2 and P3. Additional acetylene between the D and A in P3 enhanced the conjugation by minimizing the possible steric hindrance compared to P2. Whereas, P1 showed more red shifted absorption than that of P2 and P3 due to the more effective conjugaion through ῾α᾿ positions of BODIPY. Importantly, the optical band gaps ( ) obtained from onset of absorption spectra are 1.28, 1.71 and 1.57 eV for P1, P2 and P3, 1 ACS Paragon Plus Environment


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respectively. P1 has the lowest band gap for any CPT containing polymer. In the best transistor devices, a mobility improvement by four orders from 3.22 × 10−6 cm2 V−1 s−1 for P2 to 0.01 cm2 V−1 s−1 for P1 was achieved. DFT calculations alongside charge transport properties indicated the appreciable alterations in the optoelectronic properties with change in minor structural features of the polymers. The polymers were further characterized by thin film XRD, AFM and spectroelectrochemistry to investigate their material and electrochemical aspects.

Introduction All-conjugated polymers (CPs) and small molecules have attained tremendous progress in the last two decades due to their applications in organic electronic devices such as organic photovoltaics (OPVs), organic field effect transistors (OFETs), organic light emitting diodes (OLEDs) and organic lasers.1−10 Recently, dipyrromethene difluoroborane (BODIPY) based conjugated materials have drawn a great attention due to their excellent absorption/emission profile (molar extinction coefficient ~ 106 cm−1 mol−1), high fluorescence quantum yields, structural rigidity, ability to π-stack in solid state and remarkable photostability.11−18,19,20 The fine tuning of optoelectronic properties was obtained by major structural modification on the acceptor units in a series of BODIPY containing copolymers.21 BODIPY containing polymers and small molecules were rarely explored as active materials for FET devices and generally showed low mobilities (~10−7–10−3 cm2 V−1 s−1)21,22−26 in FET devices except a report by Facchetti et al.19 BODIPY-based electroactive small molecules25 or donor (D)-acceptor (A) copolymers27,28 were applied as electron donors in bulk heterojunction (BHJ) solar cells. Although there are few reports on BODIPY based D-A copolymers, the copolymerization of other electron-rich conjugated system through α

2 ACS Paragon Plus Environment

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and β-position of the pyrrole rings of the substituted BODIPY has not been systematically studied so far. Chalcogenophene capped cyclopenta[c]thiophene (CPT) based oligomers exhibit planarity in solid state and their polymers were theoretically investigated to maintain that

with optical band gaps ( ) around 1.9 eV.29 Compared to 3,4-ethylenedioxythiophene, the cyclopentane ring on the 3,4-position of thiophene imparts better oxidative stability30,31 while, it provides essential planarity to the resulting systems in comparison to 3,4-dialkyl substitutions. In extended conjugation with a weak acceptor 2,2′-bithiazole component, CPT based copolymer30 shows hole mobility ( ) of 0.052 cm2 V−1 s−1 whereas, in a pure D-A copolymer the lowest in CPT based polymer of 1.3 eV. So, the copolymer consisted of electron rich CPT and electron deficient BODIPY unit could lead further improvement in terms of properties for the application in organic electronics. The substantial effect of a methyl group in BODIPY based polymers were described by Facchetti et al.19 where polymerization goes through β-position of the BODIPY unit, but in case of polymerization through its α position and their spectroelectrochemistry are sparsely reported.32,33 Here we report three new CPs having BODIPY as acceptor (A) and CPT based terthiophene unit as donor (D). In P1 and P2, D was connected to through α and β positions of A, whereas in P3 additional acetylene spacer was added to the structure of P2 to observe changes in the optical, electrochemical and transistor properties of all three copolymers. OFET devices showed an average hole mobility of 4.5 × 10−3 cm2 V−1 s−1 for P1, which stands only after the hole mobility value reported by Facchetti et al.19



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Results and discussion Compound 2 was synthesized by condensation of 4-(2-ethyl)hexyloxybenzaldehyde with excess pyrrole using catalytic amount of trifluoroacetic acid at rt (Scheme 1). It was then brominated by N-bromosuccinimide at −78 °C and oxidized by DDQ at the same temperature to give dibromodipyrromethene, which was then coordinated with trifluroborane dietherate to give monomer 3. Compound 5 and 7 were synthesized from tetramethyl-BODIPY derivative (See SI ). Compound 8 and 9 were synthesized according to the previously reported procedures.34,35

Scheme 1. Synthesis of BODIPY-based monomer. P1 and P2 were synthesized by Stille coupling reaction36 (Scheme 2) in presence of Pd2(dba)3 and P(o-tolyl)3. To synthesize P3 with an additional acetylene spacer between the donor and acceptor part Sonogashira coupling reaction in presence of Pd(PPh3)4 and CuI was performed. In both the polymerization processes the crude reaction mixtures were poured in excess of methanol to get precipitates, which were then subjected to Soxhlet extraction with methanol, acetone and hexane till all unwanted side products and residual catalysts were went off. A final chloroform extraction followed by concentration on rotary evaporator and drying under high vacuum afforded the pure polymers. The molecular weights of the polymers were determined by GPC using chloroform as the eluent and summarized in Table 1.


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OEH OC 6H13

C 6H 13O S

S N N B F F OEH

S

P3

n

Pd(PPh3 )4 , CuI, i Pr NH, PhMe, 2 60 oC, 24 h R= Br OEH

C 6H13O

OEH C6 H 13O

OC 6 H13

S

S N N B F F

S

OEH

S R S 8 R= Br N N Br N N B 9 R= SnMe3 B Br F F Br F F Pd2 (dba) 3 Pd2 (dba) 3 3 5 P(o-tolyl) 3 P(o-tolyl)3 C6H13O PhMe PhMe 110 oC, 48 h 110 o C, 48 h S R= SnMe 3 R= SnM e3 EH = 2-ethylhexyl R

Br

N N B F F OC 6H 13 7

S

OC 6H 13

N F B F N

n

P2

OEH

S S

P1

n

Scheme 2. Synthesis of copolymers by Stille and Sonogashira coupling reaction.

Table 1. Physical properties of the polymers Polymer

Mn

Mw

PDI

Td (°C)

P1

3200

4600

1.44

238

P2

3050

6090

1.99

160

P3

3440

5520

1.60

182

Thermal stability Thermal stability of the polymers, which is essential for their practical application as electronic material, was studied by thermogravimetic analysis (TGA) at the heating rate of 10 °C min−1 under a nitrogen atmosphere (Figure 1a). The decomposition temperature (Td) is defined as the temperature at which a polymer loses its 5% mass. TGA of P1 and P3 showed the weight loss in two steps, however, major changes in the thermograms were observed at 238 and 182 °C, respectively (Table1). P2 exhibited a Td of 160 °C featured by a single mass 5 ACS Paragon Plus Environment


decay. Two steps in mass loss in P1 and P3 with targeted planarity of the backbone (vide infra) can be due to their differencial defragmentations of the side chains and polymer main chains whereas, the lowest Td and single step mass decay in P2 may be attributed to the steric hindrance by the methyl groups that played a crucial role to ease the rupture of the polymer backbone. The Td values are high enough for annealing during fabrication of FET devices. Differential scanning calorimetry (DSC) of polymers did not show any clear phase transition (Figure 1b) indicating the amorphous nature of the polymers, which is also in agreement with their thin film X-ray diffraction patterns (Figure S1).34,37 100

(b)

P1 P2 P3

P1 P2 P3

4

Heat Flow (mW)

(a)

80

Weight %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

40

2 0 -2 -4

20 100

200

300

400

500

600

40

Temperature (°C)

60

80

100 120

140 160

180

200

Temperature (°C)

Figure 1. (a) TGA thermograms and (b) DSC plots of the polymers at heating rate of 10 °C min−1 under a N2 atmosphere.

Optical Properties Absorption spectra of the polymers in UV-vis-NIR region were studied in dilute chloroform solution (10−5 M) and also as thin films on ITO coated glass. All the three polymers showed two strong bands in blue and red regions (Figure 2a). The higher energy bands (360-490 nm) could result from the π-π* transitions and the longer wavelength (500-920 nm) bands are attributed to the intramolecular charge transfer (ICT). Polymer P1 with α-connectivity of BODIPY showed much broader and red shifted absorption than that of P2 and P3 with β6 ACS Paragon Plus Environment

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connectivity. The absorption maxima (λmax) of P1, P2 and P3 in chloroform solutions are 721, 559 and 631 nm, respectively. Significant broadening and red shift (50 nm) of the absorption spectra were observed in the thin films compared to solution-state spectra due to enhanced interchain interactions in solid state (Figure 2b). The vibronic feature in the low energy band of P1 is due to the presence of inter-chain aggregates in the solution and thin film, which becomes more pronounced in the thin film. Interdigitation of the alkyl side chains may help to form aggregates in solution and solid state of P1 compared to the other polymers.

P1 P2 P3

0.8

0.6

0.4

0.2

(b) 1.0 Normalized absorbance (a.u)

(a) 1.0 Normalized absorbance (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


P1 P2 P3

0.8

0.6

0.4

0.2

0.0

0.0 400

500

600

700

800

900

400

1000

600

800

1000

Wavelength(nm)

Wavelength(nm)

Figure 2. UV-vis-NIR spectra of the polymers (a) in 10−5 M chloroform solution and (b) as thin films on ITO coated glass. Absorption spectrum of P1 in thin film covers almost all visible region of the solar spectrum and is extended in near infra-red (NIR) region, which is important for the light harvesting applications. obtained from the absorption onset are 1.28, 1.71 and 1.57 eV for P1, P2 and P3, respectively. Due to a steric hindrance of methyl substituents on BODIPY, P2 adopted non-planar structure;19 whereas acetylene spacer between the terthiophene unit and the BODIPY core in P3 assists the polymer backbone to achieved planarity. Therefore, higher λmax of P3 compared to P2 is attributed to the incorporation of acetylene spacer, which is further supported by DFT calculation (vide infra, Figure S9). P1 possesses smallest band



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gap ( ) among these three polymers. The α-connectivity of BODIPY with terthiophene unit and absence of methyl substituents on BODIPY in P1 resulted in more effective conjugation compared to those of P2 and P3. This is the first evidence of the direct band gap tuning of BODIPY-containing donor-acceptor polymers by undergoing subtle changes in the BODIPY core. We note that the planarization through acetylene linkage improve the absorption nearly by ~50 nm, however, the α-connectivity of BODIPY with terthiophene unit red shifted the absorption by large extent (~200 nm).

Electrochemical Properties Redox properties of the polymers were determined by cyclic voltammetry (CV) in a threeelectrode cell using 0.1 M tetrabutylammonium perchlorate (TBAPC) as the supporting electrolyte in acetonitrile (Figure 3). The electrochemical potentials were calculated from onsets of the oxidation and reduction peaks (Table 2). All voltammograms were calibrated against the redox potential for the ferrocene/ferrocenium couple as standard vs Ag/AgCl reference electrode. The onset oxidation and reduction potentials and corresponding HOMO and LUMO energy levels are summarized in Table 2. All polymers show quasi-reversible oxidation and irreversible reduction in CV. Oxidation of the polymers could result from CPT based terthiophene unit, whereas the reduction may be the characteristic of BODIPY core. Thus polymers show the ability of ambipolar doping. The HOMO energy levels of P1, P2 and P3 are −5.23, −5.31 and −5.33 eV, respectively; the corresponding LUMO energy levels are −3.74, −3.54 and −3.62 eV, respectively. The electrochemical band gaps ( ) were tuned from 1.49 to 1.77 eV by changing the different connectivities of structurally different BODIPY unit with terthiophene unit. The electrochemical band gaps ( ) of the polymers


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are higher than their probably due to the interface barrier for charge injection.38 P2 and P3 showed similar HOMO energy levels, whereas P1 showed elevated HOMO level due to better π-conjugation in the polymer backbones through the α-positions of BODIPY unit. LUMO energy levels of the polymers were stabilized consistently from P2 to P3 to P1, which could be attributed to better ICT through the increase in effective conjugation in the polymer core. Table 2. Optical and electrochemical properties of the polymers. Polymer

a

b

c

Solution

Film

(V)

(V)

(eV)

(eV)

(eV)

(eV)

P1

434, 721

442, 774

0.75

−0.74

−5.23

−3.74

1.49

1.28

P2

394, 458, 396, 462,

0.83

−0.94

−5.31

−3.54

1.77

1.71

0.85

−0.86

−5.33

−3.62

1.71

1.57

P3 a

λ (nm)

559

586

401, 631

431, 681

= − + 4.48#. b = − + 4.48#. c = 1240/λ

(a)

(b)

P1 P2 P3

Current

P1 P2 P3

Current

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

0.4

0.8

-1.2

1.2

Voltage(V)

-0.8

-0.4

0.0

Voltage(V)

Figure 3. Cyclic voltammograms for copolymers on thin films coated on platinum disk electrode: (a) oxidation only scans (b) reduction only scans.



Before approaching to characterize the polymers with intrinsic doping it is important and informative to check their electrochemical stability. So, spectroelectrochemical changes of polymer film on ITO coated glass slide were investigated using a UV-vis-NIR spectrophotometer in a comonomer free, 0.1 M TBAPC/ ACN solution at various applied potential from −0.1 to +1.4 V. As P1 oxidized beyond 0.7 V (Figure. 4) a gradual diminution of absorption bands (arises due to the π-π* transition in the neutral polymer) was observed and new absorption band generated at 1210 nm due to the formation of singly charged species (polaron) and formation of doubly charged species (bipolaron) was not clear may be due to the strong electron-accepting nature of BODIPY unit that restricted to withdrawal of further electron from the resulting polaronic structure. Similarly, P2 was anodically doped beyond 0.8 V (Figure. S2) and polaron formation was observed at 790nm. Here bipolaron formation was also not clear upon further oxidation. Unfortunately, P3 did not respond to extrinsic doping under chronoamperometric study (Figure S3).

'dedoped'

1.0

-0.1V 0.1V 0.2V 0.3V 0.4V 0.5V 0.6V 0.7V 0.8V 0.85V 0.9V 0.95V 1.0V 1.05V 1.1V 1.15V 1.2V 1.3V 1.4V

0.8

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.6 0.4 0.2 0.0

'doped'

400

600

800

1000

1200

1400

1600

Wavelength(nm)

Figure 4. Spectroelectrochemistry of P1 on ITO coated glass as a function of applied potential between −0.1 and 1.4 V in acetonitrile.

Theoretical calculations


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To get more insights into the structural and optoelectronic properties of polymers P1 – P3, density functional theory (DFT) calculations were carried out at PBC/B3LYP/6-31G(d) on model polymers (by replacing O-alkyl by O-methyl groups) comprising of the repeat unit of each parent copolymer (Figure 5). Optimized structure of P1 is nearly planar with significant curvature in the conjugated backbone. Whereas optimized model structure of P2 shows deviation from the planarity by 43° with linear backbone. Nevertheless, optimized model structure of P3 possesses very planar structure with slight curvature in the backbone (Figure S9).possibly because alkyne spacer between the donor and acceptor parts minimizes the steric and conformational constraints by its quasi-cylindrical electronic symmetry.39 The theoretical HOMO/LUMO energies were found to be −4.37/−2.82 eV for P1, −4.71/−2.50 eV for P2 and −4.60/−2.68 eV for P3. The increasing band gaps in the order of P1 < P3 < P2 are in agreement with the electrochemical/optical characterizations. For all optimized polymer structure, a significant portion of the respective LUMO electron density is localized on the BODIPY acceptor units, while the HOMO electron density varies from being delocalized on the entire aromatic system in P1 to being localized on the donor unit in P2 and P3.

Figure 5. DFT calculated frontier molecular orbitals of the polymers.



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Polymer field-effect transistors (PFETs) To measure the charge carrier mobility, PFETs were fabricated using prefabricated bottom contact FET substrates. Gold electrodes were used as source and drain contacts with heavily doped silicon as gate electrode. SiO2 (210 nm) was used as gate dielectric. Thin films of the polymers were spin coated from a chloroform solution on top of the FET substrates. The drain voltage (VD) was varied between 0 and −100 V while holding the gate potential at various constant voltages. Upon increase in VD, the drain current (ID) started increasing, which then saturated at higher VD. The ID was found to increase as a function of gate voltage (VG) as well indicating typical FET characteristics (Figure 6a). Irrespective of the VG, a clearly linear and saturation regimes were observed. The hole mobility of P2 was found to be 1.4 × 10−6 cm2/Vs. From the transfer characteristic curves, the threshold voltage was found to be −18 V (Figure S5b). In case of P3, the mobility was found to increase by two orders (1.51

× 10−4 cm2/Vs). The highest mobility was found for P1 (1.4 × 10−3 cm2/Vs). Considering the presence of alkyl chain in all these polymers, we envisioned an increase in device performance while using OTS modified gate dielectric. OTS modified FET substrates were prepared by following the reported procedure.40 The hole mobility of P2 was found to be 1.5

× 10−6 cm2/Vs, which is comparable to the mobility for P2 in unmodified FET substrates. Contrary to this, the hole mobility increased by two and three time for P3 and P1, respectively (Table 3). In case of P2, the β position of BODIPY was directly connected to the thiophene ring, which imparted distortion between the two repeat units leading to poor conjugation. This distortion and poor conjugation result in poor charge carrier mobility. On the other hand, the additional ethynyl spacer in P3, decrease the distortion between the BODIPY and the thiophene rings, hence the mobility increased by two orders. Maximum mobility of 0.01 cm2/Vs was found for P1. The highest mobility observed for P1 is due to 12 ACS Paragon Plus Environment

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nearly planar structure (less steric hindrance as a result of absence of any substituents on BODIPY moiety compared to P2) and better conjugation (through α-connectivity as compared to the β-connectivity in P2 and P3).

(b)

-80

-4

I (A) D

2x10

-40

-2

1x10

-4

1x10

-3

5x10

-5

5x10

-20

0

0 0

-20

-40 -60 V (V) D

-80

Before exposure 2 min exposure 10 min exposure 20 min exposure 30 min exposure 40 min exposure 50 min exposure 60 min exposure

-6

-8x10

-6

-6x10

-6

-4x10

-6

-2x10

0 0

0 0

-100

(c)

I D ( A)

2x10

I (A) D 1/2 1/2 I (A) D

2x10

I

(µ A) D

-60

-2

-4

0V -20 V -40 V -60 V -80 V -100 V

-20

-40 -60 V (V) G

(d)

-6

-6x10

-6

-4x10

-6

-2x10

0 0

-20 -40 -60 -80 -100 VD (V)

-80

-100

Before exposure 60 min exposure 2 min after removal 4 min after removal 10 min after removal 20 min after removal 30 min after removal 1h after removal

-6

-8x10

I D ( A)

(a)

ID1/2(A)1/2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-20 -40 -60 -80 -100 VD (V)

Figure 6. Output (a) and transfer (b) characteristics of polymer P1 on OTS modified FET substrate at W/L= 1000. ID vs VD plot as a function of time for polymer P1 at VG = -80 V on unmodified FET substrate for W/L= 500 (c) after the FET substrate was exposed to NH3 gas. (d) after removal of NH3 gas from the FET substrate. We were interested in using the OFET to detect gases. Towards this objective, OFET was fabricated using P1, which was then placed in a plastic enclosure. Then the source, drain and gate contacts were made. The ID was recorded at VG = -80 V, while sweeping the VD between 0 to -100 V. The substrate was exposed to ammonia gas (NH4OH solution), and then ID was recorded as a function of time. We found that the ID decreased as a



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function of time (Figure 6c). After one hour, the NH4OH solution was removed from the plastic enclosure, and the IV curves were recorded again as a function of time. The ID was found to increase as a function of time (Figure 6d). The IV curves show the reversible nature of gas sensing by polymer P1. Hole mobility values on exposure to ammonia and after ammonia removal was plotted as a function of time(figure S5c, d). This behavior indicates the polymer based FET could be a reversible gas sensor. Table 3. Summary of OFETs obtained under different conditions Polymers

µ (cm /Vs)a,b

2

VT (V) a

Ion/off a

µ (cm /Vs )b,c

2

VT (V) c

Ion/off c

P1

1.41 × 10−03

−8

2.1 × 103

4.49 × 10−03

−9

4.0 × 102

3

8.3 × 103

−25

7.4 × 103

(2.92 × 10−03) P2

(1.01 × 10−02)

1.40 × 10−06

−18

3.25 × 102

(3.89 × 10−06) P3

(3.22× 10−06)

1.51 × 10−04

−30

8.2 × 103

(4.41 × 10−04) a

Unmodified SiO2 substrate,

1.53 × 10−06

3.70 × 10−04 (7.94 × 10−04)

b

)* values are given in parenthesis,

c

OTS modified SiO2

substrates.

Morphology analysis In order to understand the influence of active layer morphology on the device performance of copolymer films, tapping mode height images (5 × 5 µm2, Figure.7) and phase images (5 × 5 µm2, Figure S8) were investigated by atomic force microscopy (AFM). P1 showed a clustered like network with root mean square roughness (RMS) 5.12 nm which may be caused by strong intermolecular interactions. Such type of uniformly compact grains leads to high field eﬀect performance in transistors. P2 showed a comparably smoother surface with


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low RMS roughness of 0.313 nm. P3 showed larger grain size and the boundary is deeper than those of P1, (Figure 7f, RMS roughness of 4.155 nm) which is not beneficial for charge transport between the grains. As a consequence, FET performance of P3 device was lower than the P1 device, which agrees with the results from transistor property.

Figure 7. AFM topographic images (a), (b), (c) of thin films of P1, P2, P3 (5 × 5 µm2) and 3D surface images (d), (e) (f) of P1, P2, P3 (5 × 5 µm2) at room temperature.

Conclusion In summary, we have successfully synthesized three new low band gap donor-acceptor copolymers containing CPT based terthiophene and BODIPY units via different connectivities. Different BODIPY acceptor unit in polymer backbones significantly altered their structures and optoelectronic properties. Spectroelecctrochemical characterization confirmed electrochemical stability of the polymers. Solution-processed OTFTs of P1 signify a p-channel mobility of 0.01 cm2 V−1 s−1 in the best devices, which is higher than P2 (βconnected and with methyl groups by an order of 103) and highest till now for any polymers 15 ACS Paragon Plus Environment


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with α-connecting BODIPY unit. The superiority in transitor characteristics for P1 may be attributed to the absence of the methyl groups which imply planarity (from DFT) by reducing steric hindrance as well as the α-connection between D and A units that alters the HOMO/LUMO levels and gaps. So the copolymerization of BODIPY unit with CPT based terthiophene with subtle structural variation on the former unit proved to be efficacious in tuning the optoelectronic properties of BODIPY based copolymers, whereas acetylene spacer improvement hole mobility by the order of 102 and stabilized the LUMO level compared to those for P2. P1 with the lowest optical band gap among any CPT containing polymer and a broad spectral response will provide additional motivation for future broad-band light sensing and harvesting applications.

Experimental Section Materials. All reagents were purchased from commercial sources and used as received without further purification, unless otherwise specified. Toluene and tetrahydrofuran (THF) were dried over sodium/benzophenone, while N,N-diisopropylamine and dichloromethane were dried over anhydrous calcium hydride. Air and moisture-sensitive reactions were performed in oven-dried glassware using a standard Schlenk line under an inert atmosphere of dry nitrogen. Characterization. 1H NMR and

13

C NMR spectra were recorded at room temperature on a

Jeol JNM-ECS400 spectrometer, with tetramethylsilane (TMS) as the internal reference; chemical shifts (δ) values are given in parts per million (ppm) and all J values are in Hertz (Hz). The number average molecular weight and polydispersity of the polymers were obtained by gel permeation chromatography analysis with polystyrene as the standard using CHCl3 as eluent at a flow rate of 0.3 mL/min at rt. Thermogravimetric analysis (TGA) was performed under nitrogen with a Mettler Toledo TGA/SDTA 851 thermogravimetric analyzer 16 ACS Paragon Plus Environment

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at a heating rate of 10 °C min−1. Differential scanning calorimetry (DSC) was conducted on Mettler Toledo DSC 1 under nitrogen at a heating/cooling rate of 10 °C/min. UV-vis-NIR absorption spectra were recorded on a Agilent Cary-60 UV-vis spectrophotometer. For solid state characterization, polymer films were prepared by drop-casting the polymer from chloroform solution on ITO coated glass plates and then drying in vacuum Cyclic voltammetry was performed on a computer-controlled Princeton Applied Research 263A electrochemical workstation using a three-electrode arrangement. Polymer films were drop cast from chloroform solution on platinum (Pt) disk working electrode. A Pt wire was used as the counter electrode, and Ag/AgCl reference electrode. Ferrocene was used as an internal standard. 0.1 M TBAPC dissolved in dry acetonitrile (ACN) was used as supporting electrolyte. Half-wave potential of ferrocene/ferrocenium couple was calculated to be 0.32 V. All electrochemical experiments were performed under nitrogen atmosphere. Topographic images and phase images were taken using AFM with an NT-MDT instrument, model no. AP-0100 in tapping-mode. OFET: Prefabricated bottom contact FET substrates were cleaned with acetone followed by isopropanol. Surface modification of SiO2 surface was done using OTS (octyltrichlorosilane) solution in chloroform. OFET measurements were performed on Keithley 4200-SCS semiconductor characterization system. The device fabrication and measurements were carried out inside argon filled glove box. Polymer thin films were spin coated (concentration ≅ 5 mg/mL in chloroform, 1500 rpm for 60 seconds) on both unmodified and OTS modified FET substrates and post annealed at 60 °C for 20 min. Charge carrier mobilities were calculated using the standard linear regime quadratic model equation µ = IDS/(VGS-Vth)VDS × L/WCOX . Synthesis



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Synthesis of P1: 3,5-dibromoBODIPY, 3 (73 mg, 0.13 mmol) and compound 9 (110 mg,0.13 mmol) were added to 15 mL dry toluene kept in a two necked round-bottomed flask under nitrogen and the solution was purged with nitrogen for 15 min. Then Pd2(dba)3 (13.45 mg,10 mol %) and P(o-tolyl)3 (19.78 mg, 50 mol %) were also added into the solution at rt followed by a reflux for 24 h. The crude reaction mixture was then precipitated in excess methanol and collected via gravity filtration which was then subjected to Soxhlet extraction with methanol, acetone and hexane as washing solvent to remove the cooligomers with smaller chain size and catalyst residues. A final chloroform extraction resulted in a deeply colored solution which was then concentrated under reduced pressure and dried in vacuum to yield a black polymer P1 (80 mg, yield 67%).1H NMR (400 MHz, CDCl3): δ8.24 (br, 2H) 7.46 (br, 2H), 7.21−6.91 (br, 4H), 6.82 (br, 4H), 3.93 (br, 2H), 3.51−3.40 (br, 8H), 2.92−2.65 (br, 4H), 1.77 (br, 1H), 1.52−1.39 (br, 8H), 1.38−1.21 (br, 16H), 1.01−0.81 (br, 12H). Synthesis of P2: 2,6-dibromoBODIPY, 5 (67 mg, 0.11 mmol) and compound 9 ( 93 mg, 0.11 mmol) were added to 15 mL dry toluene, kept in a two necked round-bottomed flask under nitrogen and the solution was purged with nitrogen for 15 min. Pd2(dba)3 (12 mg, 10 mol %) and P(o-tolyl)3 (17 mg, 50 mol %) were also added into the solution at rt followed by a reflux for 24 h. The crude reaction mixture was then precipitated in excess methanol and collected via gravity filtration which was then subjected to Soxhlet extraction with methanol, acetone and hexane as washing solvent to subsequently remove the cooligomers, unwanted side products and residual catalyst. Then, a final chloroform extraction resulted in a deeply colored solution, which was then concentrated under reduced pressure and further dried in vacuum to yield a deep violet polymer P2 (48 mg, yield 45%). 1H NMR (400 MHz, CDCl3): δ 7.23−7.12 (br, 2H), 7.11−6.98 (br, 4H), 6.77 (br, 2H), 3.90 (br, 2H), 3.41 (br, 8H), 2.78−2.68 (br, 4H), 2.67−2.60 (br, 6H), 1.76 (br, 1H), 1.59−1.42 (br, 14H), 1.36−1.30 (br, 16H), 0.97−0.80 (br, 12H). 18 ACS Paragon Plus Environment

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Synthesis of P3: 2,6-diethynylBODIPY, 7 (70 mg, 0.14 mmol) and compound 8 (95 mg, 0.14 mmol) were added to a 10 mL Schlenk flask. Pd(PPh3)4 (8.08 mg, 6.9 µmol ,5 mol %) and CuI (2.6 mg, 13.7 µmol, 10 mol %) were added to the flask under nitrogen. A degassed mixture of anhydrous diisopropylamine (2.5 mL) and anhydrous toluene (2.5 mL) were also added to the flask, then the polymerization was carried out for 24 h at 60 °C. The crude reaction mixture was then precipitated in excess methanol and collected via gravity filtration which was then subjected to Soxhlet extraction with methanol, acetone and hexane as washing solvent for further purification. A final chloroform charging in the Soxhlet aparatus resulted in a deeply colored solution, which was then concentrated under reduced pressure and dried in vacuum for an hour to yield a deep violet polymer P3 (70 mg, yield 50%).1H NMR (400 MHz, CDCl3): δ 7.18−7.00 (br, 5H), 6.95 (br, 2H), 6.83 (br, 1H), 3.91 (br, 2H), 3.41 (br, 8H), 2.75−2.61 (br, 10H), 1.77 (br, 1H), 1.57−1.39 (br, 14H), 1.39−1.21 (br, 16H), 0.99−0.78 (br, 12H).

Associated Content Supporting Information Detailed synthetic procedures, 1H NMR and 13C NMR of all new compounds; AFM phase images, optimized geometries of P1-P3, their coordinates and output and transfer characteristic curves of FET devices. This information is available free of charge via the Internet at http://pubs.acs.org Author Information Corresponding Author * email: [email protected] # The authors contributed equally to the work.



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Acknowledgement The financial support from CSIR, India, is gratefully acknowledged. S.D. acknowledges UGC-India for research fellowship, A.B. acknowledges research fellowships from CSIR, India.

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Synthesis, Optoelectronic, and Transistor Properties of BODIPY- and

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