European Polymer Journal 52 (2014) 181–192

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European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

Star-shaped polyfluorene: Design, synthesis, characterizationand application towards solar cells

0014-3057/$ - see front matter � 2014 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.eurpolymj.2014.01.008

⇑ Corresponding author. Tel.: +91 3324734971.E-mail address: psusm2@iacs.res.in (S. Malik).

Chanchal Chakraborty a, Animesh Layek b, Partha Pratim Ray b, Sudip Malik a,⇑a Polymer Science Unit, Indian Association for the Cultivation of Science, 2A & 2B Raja S.C. Mullick Road, Jadavpur, Kolkata 700032, Indiab Department of Physics, Jadavpur University, Kolkata 700032, India

a r t i c l e i n f o

Article history:Received 20 December 2013Received in revised form 7 January 2014Accepted 8 January 2014Available online 15 January 2014

Keywords:Star polymersDonor–acceptor copolymersBand gapPolymer solar cellsPolyfluorenes

a b s t r a c t

The present manuscript reports the design, synthesis and characterization of three star-shaped polymers consisting of three different arylimides such as perylene (PR)-, naphtha-lene (NT)- and benzene (BZ) tetracarboxylicdiimide as core and polyfluorene (PF) as arms.Chemical structure of star-shaped polymers was aimed at broadening as much as possibletheir absorption profile. Arylimide cored star polymers (PF-BZ, PF-NT and PF-PR) were pre-pared through palladium catalyzed Suzuki polycondensation to tune the band gap of thepolymers. The prepared polymers were characterized by elemental analysis, NMR, GPC,UV–Vis, photoluminescence and cyclic voltammetry studies. Electrochemical and opticalresponses of three polymers revealed the lowering of band gap from linear PF to star-shaped polymers. TCSPC study confirmed the partial energy transfer from PF arms to aryli-mide cores. The unexpected keto defect in linear PF was also reduced by preparation of starpolymer with large arylimide cores. TGA exhibited the enhancement of thermal stability ofstar polymer than linear PF. By using star polyfluorenes as the donor and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the acceptor, bulk heterojunction (BHJ) solar cells ofthe structure ITO/PEDOT:PSS/star polymer: PCBM/Al were fabricated and studied with asolar simulator under AM1.5G (100 mW/cm2) irradiation intensity. Those cells showedthe open circuit voltage (Voc) �0.52–0.55 V, the short circuit current density (Jsc) �0.84–1.13 mA cm�2, the fill factor �0.39–0.44 and the power conversion efficiency (PCE)�0.18–0.26%.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Star polymers, which are multibranched polymers com-prising one functional central core and several linearchains emerging from the central cores, have been of greatinterest in the past decade [1–7]. Recently, star shapedconjugated polymers and oligomers are very attractive inmacromolecular research because of their unusual molec-ular structure and enhanced optoelectronic properties,which make them promising candidate in applications aslight emitting diodes [1–9], solar cells [10,11], and thinfilm transistors [12]. The molecular structure, the chemical

and physical properties of the designed star polymers canalso be well controlled by judicious choice of synthesismethodology and proper selection of core as well as arms[1,2,13–17]. Proper design of star polymers having donor(D) arms and electron acceptor(A) cores like dyes, may pro-duce smart DA type polymers/copolymers which have re-ceived tremendous attention now-a-days [18,19].

Polyfluorenes (PFs) based copolymers are one of themost promising blue light emitting materials for electronicand optoelectronic application such as light emitting de-vices (LEDs) [20–25], photovoltaics [26–28], and field ef-fect transistors [29,30] owing to its high quantum yieldand good thermal stability [31]. PFs are also used as hostpolymer matrix for fluorescent and phosphorescent dyesto produce effective host–guest blend for triplet energy

182 C. Chakraborty et al. / European Polymer Journal 52 (2014) 181–192

transfer [32]. As PF has high band gap, it has the tendencyto act as a donor in presence of an electron deficientmoiety attached with it. Importantly, different electrondeficient arylimides like functionalized perylene tetracarb-oxylicdiimide (PRTBI), naphthalene tetracarboxylicdiimide(NTTBI) and benzene tetracarboxylicdiimide (BZTBI) havebeen picked up as the low band gap core materials forthe star polymers. Particularly, PRTBI is a low band gapchromophore dye with excellent light fastness, high chem-ical stability and high photoluminescence quantum yield[33,34].

In last decade, considerable attention has been focusedto develop as well as fabricate the efficient organic or poly-meric photovoltaics, and particularly the development ofbulk hetero junction (BHJ) solar cell as it provides severaladvantages like large scale production feasibility, low fab-rication cost, easy handlings and light weight over its inor-ganic counterpart [35–38]. Polymeric BHJ have beensubject of great interest due to its uniform depositionand electronic tunability [35–37]. Efforts have been direc-ted on the design and synthesis of novel semiconductingpolymer to increase the device efficiency along with thedevelopment of processing techniques as well as devicearchitectures. To achieve good performance in polymericBHJ solar cell, key factors should be consider as: (a) a broadabsorption bands to harvest solar spectrum, (b) efficientenergy transfer, (c) efficient hole transport and (d) rela-tively deep HOMO energy level [39–41]. All these factorshave influenced the efficiencies of solar devices, particu-larly the short circuit current density (Jsc) open circuit volt-age (Voc) and the fill factor (FF).

In this circumstance, we have designed and synthesizedthree star shaped polymers (Scheme 1–3) with four blueemissive arms and three different electron deficient aryli-mide cores to produce a partial energy transfer that willlower the band gap of star polyfluorene. Lowering of bandgap in star polymers will make these polymers promisingfor photovoltaic application compared to linear PF8.

NH2

NO2

NH2

NO2

BrBr Br

OO

O

O

O

O H2N

BrBr

+

Br2

ACOHNaNO2

HCl, EtOH

Zn(OAC)2, 180

Quinoline

OO

O

O

O

O+

H2N

BrBr Reflux

DMF

OO

O

O

O

O H2N

BrBr Reflux

AcOH+

98%

Scheme 1. Synthesis of arylimide core

2. Experimental

2.1. Materials

Commercial grade reagents (fluorene, CuBr2, alumina,1-bromooctane, n-BuLi, perylene-3,4,9,10-tetracarboxylicdianhydride, naphthalene-3,4,9,10-tetracarboxylic dianhy-dride, benzene-1,2,4,5-tetracarboxylic dianhydride andPd(PPh3)4 were purchased from Sigma–Aldrich Co. Ltd.Remaining chemicals were from Merck India Pvt. Ltd. andwere used without further purification unless otherwisestated. Solvent dimethylformamide (DMF) and quinolineand tetrahydrofuran (THF) were distilled in N2 atmosphereto use in further reaction. 1H and 13C NMR spectra were re-corded using a 300 and 500 MHz Bruker DPX spectrometer,using CDCl3 and DMSO-d6 as solvent and TMS as standardreference with chemical shift given in parts per million.

2.2. Measurements

Gel-permeation chromatography (GPC) was performedon a Shimadzu size-exclusion chromatographer (SEC)equipped with a guard column and a styragel HT-6E(7.8 � 300 mm, Waters) column with differential refractiveindex and UV/Vis detection by using THF as an eluent(1 mL/min at 35 �C) and polystyrene standard. The FTIRspectra were recorded in an FTIR-8400S instrument(Shimadzu) using the KBr pellets of the samples.Matrix-assisted laser desorption ionization time-of-flight(MALDI-TOF) mass spectrometry was carried out with Bru-ker Ultra flextreme (Bruker Daltonics Pvt. Ltd.) usingdiathranol as a matrix. Thermogravimetric analysis (TGA)was done with TA thermal analysis system at heating rate10 �C/min under N2 environment. The UV–Vis spectra of allsamples as solid films were studied with Hewlett–PackardUV–Vis spectrophotometer (model 8453). Photolumines-cence studies in solid thin film were recorded with a Hor-iba Jobin Yvon Fluoromax 3 spectrometer at an excitation

NO2

Br

NH2

BrBr

NN

O

O

O

O

Br

Br

Br

Br1

SnCl2THF, EtOH

oC

NN

Br

Br

Br

Br

O

O

O

O

NN

O

OO

O

Br

Br

Br

Br

3

2

88% 80%

89%

92%

96%

s for different star polyfluorenes.

Scheme 2. Synthesis of fluorene monomers.

Scheme 3. Synthesis of the different star polymers: PF-PR, PF-NT, PF-BZ and linear PF8.

C. Chakraborty et al. / European Polymer Journal 52 (2014) 181–192 183

wavelength 360 nm. UV–Vis and the photoluminescencespectroscopy of chloroform solution of PF and three starpolymers (2 mg/10 mL) were also done by above men-tioned instruments. Fluorescence lifetimes were measuredby using a time-correlated single photon counting (TCSPC)fluorometer (Fluorecule, Horiba Jobin Yvon). The systemwas excited with a 375 nm NanoLED from Horiba JobinYvon having kmax at 375 nm with a pulse duration of<200 ps. The electrochemical properties of the polymerswere investigated by cyclic voltammetry. The CV curveswere recorded with reference to an Ag/Ag+ electrode,which was calibrated by the ferrocene–ferrocenium (Fc/Fc+) redox couple (4.8 eV below the vacuum level). Theelectrochemical properties of the polymers were investi-gated by using cyclic voltammetry (Metrohm AutolabElectrochemical Analyzer) with a standard three-electrodeelectrochemical cell in a 0.1 M tetrabutylammonium hexa-fluorophosphate (TBAPF6) solution in CH2Cl2 at room tem-perature under nitrogen atmosphere with a scanning rateof 100 mVs�1. A glassy carbon working electrode, a Pt wirecounter electrode, and an Ag/AgNO3 (0.01 M in CH2Cl2)reference electrode were used. The concentration ofpolymer was 2 mg/5 mL in every set of the experiments.

All Suzuki coupling reactions were conducted under anitrogen (N2) environment without light exposure.

2.3. Fabrication and characterization of solar cell

BHJ polymer solar cell was fabricated on indium tin oxide(ITO) coated glass substrate modified by poly(3,4-ethylene-dioxythiophene):polystyrene-sulfonic acid (PEDOT:PSS).Two types of BHJ solar cells have been prepared by usingstar shaped polymers – one with PCBM ([6,6]-phenyl C61-butyric acid methyl ester) and other without PCBM. TheBHJ solar cell having structure ITO/PEDOT:PSS/star poly-mer:PCBM/Al was fabricated in the following way. Atfirst, the ITO coated glass was cleaned ultrasonically inethanol, methanol and acetone sequentially. PEDOT:PSSwas spin-coated onto the ITO substrate and dried at110 �C for 1 h. The active layer of the solar cell was depos-ited from the THF solution of star polymer or star poly-mer:PCBM blend onto the PEDOT:PSS layer by spincoating. Total solute concentration (star polymer + PCBM)was kept constant at 10 mg/mL and their weight ratiowas 1:1. The active layer of the solar cell was characterizedin dark and under white light. Aluminum electrode was

184 C. Chakraborty et al. / European Polymer Journal 52 (2014) 181–192

deposited by thermal evaporation to complete solar cellfabrication. All the solar cells were made under identicalcondition and area of each solar cell was 7 mm2. The cur-rent density–voltage (J–V) characteristics of solar cell weremeasured with a solar simulator under AM1.5G (100 mW/cm2) irradiation intensity.

2.4. Synthesis part

2.4.1. Synthesis of 2,6-dibromo-4-nitroanilineTo a vigorously stirred solution of p-nitroaniline (10 g,

72.4 mmol) in glacial acetic acid (90 mL) at 65 �C, bromine(7.42 mL, 144.8 mmol) in glacial acetic acid (55 mL) wasadded within 5 h. A heavy precipitation was formed afterabout one third of bromine had been added and the precip-itate was re-dissolved by adding hot water (20 mL) andthen remaining bromine was added. After complete addi-tion the reaction mixture was poured with stirring intoslurry of water and ice. After thorough washing with waterand air-drying the compound was re-crystallized withchloroform. Then the niddle shaped crystals were driedover night into vacuum oven to get 22.81 g (98% yields)of the product compound as a yellow-green solid. 1HNMR (300 MHz, CDCl3): d (ppm) 8.34 (2H, s), 5.28 (2H, brs); 13C NMR (300 MHz, CDCl3): d (ppm) 159.56, 142.52,126.39, 122.81 HRMS: C6H4Br2N2O2: 295.92 (calculated)and 295.82 (found).

2.4.2. Synthesis of 3,5-dibromonitrobenzeneTo a stirred, boiling (90 �C) mixture of 2,6-dibromo-4-

nitroaniline(10 g, 34.03 mmol), ethanol (110 mL) and con-centrated sulfuric acid (11.5 mL), sodium nitrite (7.57 g,109.71 mmol) was added in portions as rapidly as foamingwould permit. After refluxed at 90 �C for 36 h, the mixturewas allowed to cool, poured into ice water and the solidswere collected by filtration. The residue was thoroughlywashed with water and the 3,5-dibromo nitrobenzenewas separated from remaining inorganic salts by dissolvingit in boiling ethanol and filtering in hot condition. It pro-duced brown crystalline product and after drying in vac-uum oven 8.31 g (88.4%) of the desired product wasobtained. 1H NMR (300 MHz, CDCl3): d (ppm) 8.32 (2H,d), 7.99 (1H, t); 13C NMR (300 MHz, CDCl3): d (ppm)152.76, 141.32, 126.19, 125.79. HRMS: C6H3Br2NO2:280.9 (calculated) and 281.02 (found).

2.4.3. Synthesis of 3,5-dibromoanilineTo a solution of 3,5-dibromonitrobenzene (4.89 g,

17.43 mmol) in ethanol (42 mL) and THF (40 mL) stirredunder air was added slowly in portion tin(II) chloridedehydrate (19.46 g, 86.23 mmol). The mixture was allowedto stir at room temperature for 20 h. The solvent was thenevaporated in vacuum and an aqueous NaOH solution wasadded to neutralize it. After stirring the mixture, it was ex-tracted with diethyl ether. The combined organic phasewas dried over Na2SO4, filtered and dried in reduced pres-sure vacuum to get a solid product that was purified bycolumn chromatography (silica gel, chloroform as eluent)to afford 3.45 g (80%) pale brown solid. 1H NMR(300 MHz, CDCl3): d 7.01 (1H, t), 6.74 (2H, d), 3.76 (2H,br); 13C NMR (300 MHz, CDCl3): d (ppm) 148.73, 123.45,

123.70, 116.60. HRMS: C6H5Br2N: 250.92 (calculated) and250.72 (found).

2.4.4. Synthesis of N,N0-Bis(3,5-dibromophenyl)-perylene-3,4,9,10-tetracarboxdiimide (PRTBI)(1)

Into a three necked round bottom flask equipped withnitrogen inlet and outlet, perylene-3,4,9,10-tetracarboxylicdianhydride (0.47 g, 1.2 mmol), zinc acetate (0.35 g) andquinoline (12 ml) were added. The mixture was heated to100 �C and maintained the same for half an hour. To thatmixture, solution of 3,5-dibromoaniline (1.51 g,6.02 mmol)in 5 mL quinoline was added. The reaction mixture wasstirred at 200 �C for 24 h. It was cooled to room tempera-ture and poured into ethanol (200 ml). After filtration,the red solid was washed successively with potassium car-bonate solution (5 � 500 ml) and methanol (3 � 50 ml).The resultant product was dried at 80 �C under vacuumfor 24 h to get 0.92 g (89%) of title product as red solid.1H NMR (500 MHz, CDCl3): d 8.02 (4H, d), 7.84 (4H, d),7.34–7.52 (6H, m); MALDI-TOF. C36H14Br4N2O4: 858.12(calculated) and 857.16 (found).

2.4.5. Synthesis of N,N0-Bis(3,5-dibromophenyl)-naphthalene-1,4,5,8-tetracarboxdiimide (NTTBI)(2)

About 1 g of naphthalene-1,4,5,8-tetracarboxylic-dian-hydride (3.73 mmol) and 1.87 g of 3,5-dibromoaniline(7.5 mmol) were taken in a two neck round bottom flaskand the total mixture was refluxed with dry DMF in argonatmosphere for 6 h. The reaction mixture was cooled andthe product was precipitated out. Product was filteredand recrystallised in DMF to give 2.52 g of product 2 as yel-low crystal (92%). 1H NMR (500 MHz, CDCl3): d 7.95 (4H, d),7.74 (4H, s), 7.50 (s, 2H). MALDI-TOF. C26H10Br4N2O4 (cal-culated) and 733.98 and 733.49 (found).

2.4.6. Synthesis of N,N0-Bis(3,5-dibromophenyl)-benzene-1,2,4,5-tetracarboxdiimide (BZTBI)(3)

500 mg of benzene-1,2,4,5-tetracarboxylic-dianhydride(2.29 mmol) and 1.26 g of 3,5-dibromoaniline (5.01 mmol)were taken in a two neck round bottom flask and the totalmixture was refluxed with dry acetic acid in argon atmo-sphere for overnight. The reaction mixture was cooled toget the crystalline product that was filtered and recrystal-lised in DMF to get 1.5 g of product 3 as pale yellow crystal.(Yield 96%) 1H NMR (500 MHz, CDCl3): d 8.52 (2H, s), 7.67(4H, s), 7.53 (2H, s). MALDI-TOF, C22H8Br4N2O4: 682.93(cal-culated) and 682.07 (found).

2.4.7. Synthesis of 2,7 Dibromofluorene (4)2.4.7.1. Preparation of CuBr2 on Alumina. To a solution ofcopper (II) bromide (10 g, 44.8 mmol) in distilled water(100 mL) was added 20 g of neutral alumina (150 mesh).The water was removed under reduced pressure in freezedryer to get a brown powder which was dried overnightat 90 �C in vacuum.

2.4.7.2. Synthesis of 2,7-dibromofluorene. To a solution offluorene (1.5 g, 9.0 mmol) in carbon tetrachloride (CCl4,80 mL), 30 g of copper-(II) bromide on alumina was added.The mixture was stirred at reflux for 5 h, the solution wascooled to room temperature, the solid material was filtered

C. Chakraborty et al. / European Polymer Journal 52 (2014) 181–192 185

and washed with CCl4 (50 mL). The organic solution wasdried over magnesium sulfate. Removal of solvent pro-duced 2.87 g (98%) of the title product as yellow solids.Recrystallization was made in a mixture of ethyl acetate/hexane (5:95 v/v) to get pale yellow crystals as pure prod-uct. 1H NMR (300 MHz, CDCl3): d (ppm) 7.65 (2H, s) 7.60(2H, d, J = 8 Hz) 7.51 (2H, dd, J = 8 Hz), 3.85 (2H, s). 13CNMR (300 MHz, CDCl3): d (ppm) 144.92, 139.81, 130.27,128.43, 121.31, 121.07, 36.68. HRMS, C13H8Br2: 321.90(calculated) and 321.77 (found).

2.4.8. Synthesis of 9,9-dioctyl-2,7-dibromofluorene (5)2,7-Dibromofluorene (1.23 g, 5 mmol) was added to a

mixture of aqueous potassium hydroxide (100 mL,50 wt%), tetrabutylammonium bromide (0.33 g, 1 mmol)and 1-bromooctane (9.65 g, 50 mmol) at 75 �C. After15 min, the mixture was cooled to room temperature andextracted with CH2Cl2. The combined organic layers werewashed successively with water, aqueous HCl (1 M), againwater, and brine. It was finally dried over Na2SO4. After re-moval of the solvent and the excess 1-bromooctane by dis-tillation, the residue was purified by silica-gel columnchromatography using hexane and dichloromethane (1:2)as the eluent to give 1.5 g (72%) of pale yellow title prod-uct. 1H NMR (300 MHz, CDCl3): d (ppm) 7.61 (2H, s)7.43–7.55 (4H, m), 1.88–1.93 (4H, m), 1.09–1.25 (m, 20H), 0.82–0.88 (m, 6H), 0.57 (m, 4H). 13C NMR (300 MHz,CDCl3): d (ppm) 148.82, 140.31, 131.07, 129.13, 121.45,121.23, 56.71, 40.23, 30.22, 29.94, 29.76, 29.22, 23.26,22.83, 14.12. HRMS, C29H40Br2: 548.44 (calculated) and548.24(found).

2.4.9. Synthesis of 2,7-bis(trimethyleneborate)-9,9-dioctylfluorene (6)

To a solution of 2,7-dibromo-9,9-dioctylfluorene (1 g,1.82 mmol) in dry THF (30 mL) in a two necked round bot-tom flux in nitrogen atmosphere at -78 �C, n-BuLi (2.5 mL1.6 M in hexane, 4.00 mmol) was slowly added. After stir-ring of 1 h, trimethylborate (0.6 mL, 5.2 mmol) was addedto the mixture at a time. It was kept stirred for another24 h at room temperature. 2 M of HCl (15 mL) was addedand stirred for 1 h. The mixture was extracted with diethylether and organic extract was evaporated to give a yellow-ish liquid (9,9-dioctylfluoren-2,7-yl boronic acid). It wasmixed with 1,3-propanediol (0.4 mL, 5.4 mmol), toluene(15 mL) and refluxed overnight at 130 �C. Evaporation ofthe solvent under reduced pressure produced the crudeproduct that was purified by column chromatographyusing 1:4 ethylacetate/hexane as eluent to get yellowishviscous product (712 mg, 68%). 1H NMR (300 MHz, CDCl3):d (ppm) 7.22 (2H, s) 7.40–7.62 (4H, m), 4.01 (8H, t), 1.90(4H, t), 1.59 (m, 4 H), 0.96–1.42 (m, 20H), 0.74 (t, 6H),0.53 (m, 4H). 13C NMR (300 MHz, CDCl3): d (ppm) 152.6,139.71, 132.17, 129.82, 121.65, 121.13, 66.02, 56.12,39.57, 40.34, 30.23, 29.87, 29.65, 23.21, 22.81, 14.11. HRMS,C35H52B2O4: 558.41 (calculated) and 558.12 (found).

2.4.10. Synthesis of Perylene cored polymer PF-PR9,9-Dioctyl-2,7-dibromofluorene (5) (274 mg, 0.5 mmol)

and compound 6 (279 mg, 0.5 mmol) were dissolved in dryTHF (15 mL) and 2 M aqueous solution of K2CO3 (10 ml).

The reaction mixture was degassed and purged with N2 for15 min and Pd(PPh3)4 (12 mg, 2 mol%) was added to one por-tion under N2. After refluxing for 48 h, DMF dispersion ofPRTBI (1, 43 mg, 0.05 mmol) was added to the mixture andagain refluxed for 6 h. Bromine ending groups were convertedto boronic acid ending groups by refluxing for several hourswith compound 6 (56 mg, 0.1 mmol). At last, bromobenzene(8 mg, 0.05 mmol) was added to the mixture and refluxedfor another 6 h to endcap the polymer. Upon completion,the mixture was cooled, and poured into methanol. The pre-cipitates were collected by filtration, solubilized in chloroformand reprecipitated in methanol. The precipitation was washedwith excess methanol and water successively for five times.The solid precipitation was extracted with acetone for 48 husing soxhlet apparatus to remove the unreacted monomers,oligomers, and catalyst residues. Finally, a reddish brownproduct was obtained after being dried at 60 �C under vacuumfor 24 h (340 mg, yield 60%) 1H NMR (300 MHz, CDCl3): d(ppm) 8.04 (d, perylene H), 7.76–7.87 (m, Perylene and fluo-rene H), 7.65–7.72 (m, fluorene H), 7.61 (s, fluorene H),7.41–7.56 (m, benzene H), 2.10–1.94 (m, aliphatic H), 1.31–1.04 (m, aliphatic H), 0.87–0.83 (m, aliphatic H). 13C NMR(300 MHz, CDCl3): d (ppm) 14.21, 22.62, 24.38, 29.56, 29.82,30.53, 31.32, 40.54, 55.37, 120.82, 121.91, 126.75, 140.54,141.73, 152.47. Elemental analysis: C, 88.83; H, 10.27; N,0.27 (found). GPC results: Mw – 10440 Da, PDI – 2.17 (usingpolystyrene reference and THF as eluent). MALDI-TOF (dithra-nol as matrix): 5695.37.

2.4.11. Synthesis of naphthalene cored star polymer PF-NTPF-NT was prepared by following the synthesis proce-

dure of PF-PR using NTTBI (2, 37 mg, 0.05 mmol) and com-pound 6. The yellowish solid (385 mg, yield 70%). 1HNMR (300 MHz, CDCl3) d (ppm) 8.01 (d, naphthalene H),7.76–7.84 (m, fluorene H), 7.58–7.73 (m, fluorene H), 7.37–7.50 (m, benzene H), 2.10–1.98 (m, aliphatic H), 1.26–1.01(m, aliphatic H), 0.88–0.68 (m, aliphatic H). 13C NMR(300 MHz, CDCl3): d (ppm) 14.17, 22.78, 24.52, 29.60,29.87, 30.24, 31.89, 40.37, 55.49, 119.96, 121.23, 126.26,139.71, 140.27, 153.11. Elemental analysis: C, 88.61; H,10.22; N, 0.36 (found). GPC results: Mw – 7707 Da, PDI –1.75 (using polystyrene reference and THF as eluent).

2.4.12. Synthesis of benzene cored star polymer PF-BZPF-BZ was prepared by following the synthesis proce-

dure of PF-PR using BZTBI (3, 34 mg, 0.05 mmol) and com-pound 6. A brownish solid (375 mg, yield 68.5%) 1H NMR(300 MHz, CDCl3): d (ppm) 8.08 (d, benzene core H),7.71–7.76 (m, fluorene H), 7.44–7.50 (m, fluorene H),7.29–7.31 (m, benzene H), 1.82–2.15 (m, aliphatic H),1.01–1.29 (m, aliphatic H), 0.82 (m, aliphatic H). 13C NMR(300 MHz, CDCl3): d (ppm) 14.08, 22.70, 24.66, 29.28,29.81, 30.11, 31.90, 40.47, 55.42, 119.35, 122.97, 126.89,138.97, 141.41, 150.86. Elemental analysis: C, 88.55; H,10.26; N, 0.36 (found). GPC results: Mw – 7771 Da,PDI – 1.72 (using polystyrene reference and THF as eluent).

2.4.13. Synthesis of linear polymer (PF8)Linear PF8 was synthesized with the same procedure as

PF-PR, without using any tetrabromo core and end cappingonly by bromobenzene. About 353 mg gray powder was

PF-PR

(a)

186 C. Chakraborty et al. / European Polymer Journal 52 (2014) 181–192

obtained with a yield of 74%. 1H NMR (300 MHz, CDCl3): d(ppm) 7.67–7.32 (m, aromatic fluorene and benzene H),2.17–1.91 (m, aliphatic), 1.28–1.07 (m, aliphatic), 0.82–0.68 (m, aliphatic). 13C NMR (300 MHz, CDCl3): d (ppm)14.12, 22.73, 23.86, 29.32, 30.16, 31.93, 34.13, 40.50,55.52, 119.40, 122.91, 126.55, 138.84, 141.41, 150.80. Ele-mental analysis: C, 89.63; H, 10.37 (found). GPC results:Mw – 7769 Da, PDI – 1.72 (using polystyrene referenceand THF as eluent).

δ ppm

8 6 4 2 0δ (ppm)

PRTBI

8 6 4 2 0

8 6 4 2 0

PF-NT

δ (ppm)

NTTBI

(b)

(c)

Fig. 1. 1H NMR of (a) PF-PR star polymer and perylene core PRTBI and (b)PF-NT star polymer and NTTBI (c) PF-BZ star polymer and BZTBI core inCDCl3. Corresponding values of integration are given there.

3. Results and discussion

Starting from p-nitroaniline, 3,5-dibromoaniline wassynthesized via bromination and deamination by diazoreaction and followed by reduction with SnCl2 accordingto Scheme 1. Three arylimide cores PRTBI, NTTBI and BZTBIwere synthesized using the respective dianhydride and3,5-dibromoaniline. Two fluorene based monomers: prod-uct 5 and 6 were synthesized with moderate yield as persynthetic Scheme 2. Purities of monomers and three aryli-mide cores were checked by 1H NMR and HRMS results.The three star polymers PF-PR, PF-NT, PF-BZ and linearpolyfluorene (PF8) were synthesized by Suzuki couplingpolymerization as depicted in Scheme 3 with excellentyield. Initially, a trial was taken to synthesize PF-PR usingPRTBI core in polymerization step along with fluorene togrow the PF arms. The yield was low due to the poor solu-bility of PRTBI moieties in THF medium. Those experiencesdirected us to use PRTBI as an end capping agent in thepolymerization process of PF-PR to produce the satisfac-tory results. The molecular weight and PDI of linear PFand three star polymers determined by GPC and are sum-marized in Table 1. The presence of characteristic down-field peak above 8 ppm in 1H NMR of star polymer PF-PR,PF-NT and PF-BZ in Fig. 1 is attributed to the proton of per-ylene/naphthalene/benzene core and these signals confirmthe incorporation of arylimide moiety in the respective starpolymer. GPC results and the elemental analysis data re-veal that five flourene units are present in each arm of fourarmed star polymer of PF-PR and PF-NT whereas six fluo-rene units are present in each arms of PF-BZ. Linear PF8contains �20 units of fluorene. MALDI-TOF measurementof PF-PR (Fig. S1) gives peak at 5696, indicating the pres-ence of a perylene based core and 12 fluorene unit in PF-PR, (i.e. 3 fluorene units in each arm with one phenyl groupto end-cap). From the deeper insight of 1H NMR of starpolymers, number of proton is available from the integra-tion value of whole aromatic region and the peak around8 ppm. Excluding the number of proton in end capped phe-nyl group as well as core proton in the 7.8–7.3 ppm, the

Table 1Molecular weight, PDI and TGA analysis of polymers.

Polymers Mw from GPC (g mol�1) PDI No. of fluorene unit in each arm

PF8 7769 1.72 –PF-BZ 7771 1.72 6PF-NT 7707 1.75 5PF-PR 10440 2.17 5

a Td(10%) calculated from 10% degradation of each from TGA.

number of proton in only fluorene unit is easily counted.Using this calculation, the arm length of individual starpolymer is estimated and we have found that 4 fluoreneunits are in each arms of star polymer (Table 1). The

From GPC No. of fluorene unit in each arm From NMR Td (10%)a (�C)

– 2174 2304 2974 345

C. Chakraborty et al. / European Polymer Journal 52 (2014) 181–192 187

number of fluorene unit in star polymer estimated fromGPC method as well as from NMR technique slightly differsand it is possibly due to the overestimation of GPCtechnique.

The absorption spectra of star polymers in solution andsolid film are shown in Fig. 2a and b. In solution spectra ofliner PF8 (Fig. 2a), the characteristic peak for p–p⁄ transi-tion of fluorene is at 348 nm. In PF-BZ, the major absorp-tion is around 348 nm and in PF-NT, it is slightly redshifted to 352 nm. Absorption for naphthalene core inPF-NT falls in absorption region of polyfluorene, so thepeak for naphthalene may merge with polyfluorene absorp-tion at 352 nm. PF-PR exhibits the maximum absorption at360 nm due to the PF arms and a relatively small butprominent absorption for perylene core at above 500 nmare also seen. Solid state UV–Vis spectra of thin film ofthe polymers (Fig. 2b) show that the linear polyfluorenePF8 (without any core) produces the UV–Vis absorptionmaxima at 366 nm. Star polymers with different cores gen-erate a brilliant red shift of the absorption maxima from366 to 377 nm for PF-BZ, to 393 nm for PF-NT and to394 nm for PF-PR, respectively. When the four electron richfluorene polymer is attached with an electron deficientcore, a partial electron donation will take place. As a resultof the effective exciton confinement by electron deficientcore, a red shift is occurred along with the broadening ofthe absorption peak for polyfluorene segment [42–44].The extent of shift is maximum (�28 nm) for PF-PR starpolymer.

In emission spectra in solution (Fig. 3a), linear PF8 andpolyfluorene with benzene core i.e. PF-BZ exhibit only onesegment for emission around 412 nm, characteristic emis-sion of PF chains (excited wavelength 350 nm). However,in case of PF-NT and PF-PR, spectra show two segmentsof emission maxima, one for four polyfluorene arms�412 nm and another low energy emission for emissivecores at the same excitation wavelength. For PF-NT, thislow energy emission is �530 nm and for PF-PR �571 nm.The core PRTBI gives emission maxima at 550 nm whereasperylene core of PF-PR shows the red shift of emission peakthat is due to the partial charge transfer from electron richPF chain to electron acceptor perylene moiety. In solid film,

0

0

0

0

0

0

Abs

orba

nce

300 400 500 600 700

PF-BZ

Abs

orba

nce

(a. u

.)

Wavelength (nm)

PF-PR

PF

PF-NT

300 375 450 525 600 675Wavelength (nm)

PRTBINTTBI

(a)

Fig. 2. UV–Vis absorption spectra in chloroform solution (a) and thin solid film (perylene and naphthalene cores are given in inset of (a).

PL spectra of linear PF8 and PF-star polymers (Fig. 3b)show the luminescence maxima at around 417 nm that ischaracteristic emission of polyfluorene segment when itis excited at 370 nm. In case of PF-PR, a low emission at590 nm is clearly observed even though the polymer is ex-cited in 370 nm. The presence of this characteristic emis-sion of perylene in the emission spectra of PF-PR clearlyproves the partial energy transfer from arm polyfluoreneto central perylene core.

Again, it is very much pronounced in Fig. 3b that linearPF8 generates an undesirable green emission around550 nm when thin film of PF8 is exposed in UV light irradi-ation which is completely disappeared in all three starpolymers. This green emission band of PF8 film after UVexposed in air is due to the synergic effect of on-chainketo defects which are exclusively attributed to excimerformation in excited state followed by interchain excitondiffusion due to the aggregation of conjugated PF chains[45–48]. For the electron deficient arylimide cored starpolyfluorene, the arylimide core effectively traps the exci-ton. As a result no effective interchain exciton transfertakes place [49] within PF arms. It does mean that theinterchain exciton diffusion is prohibited to suppress thedetrimental green emission of PF.

To ensure the partial energy transfer in star polymerswhere arylimide cores are covalently attached with fluo-rene arms, we have performed UV–vis and emission stud-ies of the blends of PF8 and corresponding arylimide core(PRTBI/NTTBI/BZTBI) in CHCl3 solution normalizing to coreconcentration of the blends with covalently linked star-polymer. In the UV–vis spectra at Fig. 4a, there are thepresence of characteristic absorption of polyfluorene andabsorption of perylene core in blend. In emission study ofblends in Fig 4b, there is absence of emission peak fromeither perylene or naphthalene core in the respectiveblend, when the solution of blend has been excited atabsorption wavelength of PF8. Results reveal that covalentattachment of perylene core with the polyfluorene arm isrequired for the partial electron transfer from polyfluorenearm to perylene core.

In order to obtain better insight into the excited-stateproperties of the prepared star polymers and linear PF8, a

350 400 450 500.0

.1

.2

.3

.4

.5

PF-BZ

PF-PR PF-NT

Wavelength (nm)

PF8(b)

b) of linear PF8 and three star polymers. Corresponding UV–Vis. spectra of

PF-BZ PF-NT

Wavelength (nm)

PF-PR

400 450 500 550 600 650 700 750Wavelength (nm)

Inte

nsity

(a. u

.)

NTTBI PRTBI

Inte

nsity

(nor

m)

PF8

400 450 500 550 600 650 700 400 450 500 550 600 650 700

PF-NT PF-PR

PF-BZ

Inte

nsity

(a. u

.)

Wavelength (nm)

PF8(a) (b)

Fig. 3. Emission spectra in chloroform solution (a) and thin solid film (b) of linear PF8 and three star polymers excited at 370 nm and pathlength 10 mm.Corresponding emission spectra of perylene and naphthalene cores are shown in inset of (a).

PF8+BZTBI PF8+NTTBI PF8+PRTBI

Inte

nsity

(A.U

.)

Wavelength (nm)400 450 500 550 600 650 700300 400 500 600 700

0.0

0.3

0.5

0.8

1.0

Abs

orba

nce

(A.U

.)

Wavelength (nm)

PF8+BZTBI PF8+NTTBI PF8+PRTBI

(a) (b)

Fig. 4. (a) UV–Vis spectra, and (b) emission spectra of blends of PF8 and corresponding arylimide cores.

188 C. Chakraborty et al. / European Polymer Journal 52 (2014) 181–192

series of time-resolved fluorescence experiments wereperformed in chloroform using 9,10-diphenylanthracene(U = 1.0) as the standard. The fluorescence decay curvesare shown in Fig. 5 and fluorescence decay times of thelinear PF8, PF-BZ, PF-NT and PF-PR obtained by TCSPCexperiments of 1 lM solutions with an excitation laserpulse of 375 nm are summarized in Table 2. PF8 decaysin chloroform biexponentially and can be best fitted with

5000 6000 7000 80000

1000

2000

3000

4000

5000

Time (ps)

Coun

ts

PF8 PF-BZ PF-NT PF-PR

Fig. 5. Time resolved fluorescence decay plot of linear PF8 and starpolymers in chloroform at an excited wavelength 375 nm.

two time constants of 339 and 616 ps with an average lifetime of 469 ps. The average lifetime of the PF-BZ, PF-NTand PF-PR polymers are gradually increased to 811, 813and 833 ps respectively and decay is best fitted by threetime constant. In PF-PR, the positive amplitude of the lagertime constant of 3.226 ns is clearly attributed to the fluo-rescence of perylene (PRTBI) subunit in that co-polymer.Appearing of new positive amplitude of 3.226 ns timeconstant, lowering of characterized amplitude of polyfluo-rene at 415 ps and the decrease of quantum yield in PF-PRhave indicated the energy trapping by PRTBI unit in starpolymer PF-PR [50].

FTIR studies of the arylimide cores and correspondingstar polymers (Fig. 6) have revealed that the characteristicstretching vibration of imide C@O at 1701 and 1662 cm�1

in all arylimide cores like PRTBI, NTTBI and BZTBI. For thePF-PR polymer, the stretching frequency of imide C@O isfairly present at 1700 and 1660 cm�1. This small but recog-nizable stretching for imide C@O in polymer is due to thepresent of the small amount of perylene core in the poly-mer. Presence of little imide C@O stretching frequencyin FTIR spectra of PF-NT and PF-BZ star polymers alsoconfirms the incorporation of corresponding arylimidecore in corresponding star polymers.

Thermal stabilities of the linear PF8 and all star polymerswere investigated by thermogravimetric analysis (Fig. 7).

Table 2TCSPC results and quantum efficiency yield of polymers.

Polymer a1 s1 (ns) a2 s2 (ns) a3 s3 (ns) hsi (ns) UF

PF8 53 0.339 – – 47 0.616 0.469 0.96PF-BZ �147 0.385 1.24 1.494 245.77 0.553 0.811 0.93PF-NT �113.3 0.416 1.39 1.621 211.93 0.596 0.813 0.91PF-PR �224.3 0.415 0.44 3.226 323.87 0.541 0.833 0.88

500 1000 1500 2000 2500 3000 3500 4000

Tran

smitt

ance

(%)

PF-BZ

PF-NT

Wave no. (cm-1)

BZTBI

NTTBI

PF-PR

PRTBI

Fig. 6. FTIR spectroscopy of three arylimide cores and corresponding starpolymers diluted with KBr.

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-1x10-5

0

1x10-5

2x10-5

3x10-5

Potential (V)

Curr

ent (

A)

PF8 PF-BZ PF-NT PF-PR

(a)

C. Chakraborty et al. / European Polymer Journal 52 (2014) 181–192 189

The linear PF8 polymer exhibits two step of degradation,one in lower region of 250–400 �C for degradation of thealkyl group and at 550–700 �C for main chain degradation.In case of the star polymers, thermal stabilities of thepolymers are increased especially in PF-PR and PF-NT.Incorporation of large arylimide group to produce anordered structure (Fig. S2) which increases the thermalstability of both steps of the degradation. Degradationtemperature for 10% weight loss of liner PF8 and all threestar polymers are presented in Table 1 which indicates

100 200 300 400 500 600 700 800

0

25

50

75

100

Wei

ght l

oss

%

Temperature (0C)

PF-PR PF-NT PF-BZ PF8

Fig. 7. Thermogravimetric plot of linear PF and three star polymers underN2 atmosphere.

the enhance of thermal stability. Star polymer PF-PRachieves the highest thermal stability as it contains a largecore group like perylene diimide.

The electrochemical properties of the four polymerswere investigated by cyclic voltammetry. The CV voltam-mogram was referenced to an Ag/Ag+ electrode, whichwas calibrated by the ferrocene–ferrocenium (Fc/Fc+)redox couple (4.8 eV below the vacuum level). Cyclicvoltammograms of polymers PF-PR, PF-NT and PF-BZ(Fig. 8a) show the same redox behavior as polyfluorene,presenting a relatively broad quasireversible wave withan onset oxidation potential at around +0.6 V (vs. Fc/Fc+)assigned to the reversible p-doping processes of the poly-fluorene conjugated chain [51]. No reduction processes

(b)

Fig. 8. (a) Cyclic-voltammograms of linear PF8 and all star polymers inCH2Cl2 using Ag/Ag+ electrode calibrated with Fc/Fc+ and TBAPF6 aselectrolyte. (b) Schematic of exciton generation and charge separation ofPF-PR and PCBM.

0.0 0.1 0.2 0.3 0.4 0.5 0.6

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Cur

rent

den

sity

(mA

/cm

2 )

Voltage (V)

PF-PR PF-BZ PF-NT

Fig. 9. Current density–voltage characteristics of star polymer basedsolar cell and structure of polymer solar cell (inset).

190 C. Chakraborty et al. / European Polymer Journal 52 (2014) 181–192

on the polymeric backbone of linear PF8 are seen up to�2.0 V under our experimental conditions. It is very inter-esting that in three star polymers, i.e. PF-PR, PF-NT andPF-BZ where electron acceptor arylimide central core ispresent, a detectable reduction wave is present below�1 V, evidencing a moderate electron transfer from armedpolyfluorene chain to central arylimide cores. The HOMO,LUMO and the band gap data of the different polymersare presented in Table 3 and corresponding energy leveldiagram of PF-PR/PCBM is shown in Fig. 8b.

The BHJ solar cells were also fabricated with a configu-ration of ITO/PEDOT: PSS/star polymer: PCBM/Al [52]. Theactive layers of all devices were spin-coated from amixture solution of star polymer/PCBM (1:1, w/w) in THFwithout thermal annealing. The thickness of the activelayer was controlled by the spin-coating rate. To checkthe device applicability of a photosensitive material, pho-tosensitivity (ratio of photoconductivity and dark conduc-tivity) was also measured. Photocurrent and dark currentof three star polymers and star polymers/PCBM compositesare given in Table 4. Photosensitivities of three star poly-mers are in satisfactory level. It concludes that these mate-rials have potential to act as solar cell device. PF-PR:PCBMshows the highest photosensitivity �92 followed by 80 forPF-BZ:PCBM and 68 for PF-NT:PCBM. The J–V characteristicof devices has been measured and plotted in Fig. 9. Wehave also checked the device property of star shaped poly-fluorene based BHJ solar cell without using PCBM. How-ever, the performance is very poor in comparison to starpolymer:PCBM solar cell. The HOMO levels of star poly-mers have been fixed to maximize open circuit voltage(Voc). Table 4 summarizes the photovoltaic performanceof star polymer:PCBM devices. PF-PR: PCBM (1:1) basedsolar cell produces the lowest Voc value of 0.52 V whereasPF-NT: PCBM (1:1) and PF-BZ: PCBM (1:1) based solar cellgive Voc of 0.55 and 0.53 V, respectively. According to thedesign rule for the donor polymer in the BHJ solar cell,

Table 3Electrochemical potentials and energy levels of linear PF and star polymers.

Polymers Ox. Onset(V vs. FOC)

HOMO(eV)

k onset(nm)

LUMOa

(eV)Band gapb

(eV)

PF8 0.55 �5.35 420 �2.40 2.95PF-BZ 0.43 �5.23 421 �2.29 2.94PF-NT 0.51 �5.33 434 �2.47 2.86PF-PR 0.38 �5.18 442 �2.38 2.80

a Calculated according to the equation: ELUMO = EHOMO + EBandgap.b Estimated from the onset wavelength of optical absorption of the thin

solid film.

Table 4Photosensitivity and solar cell parameters for three star polymer:PCBM(1:1) devices.

Samples Photosensitivity Voc

(V)Jsc

(mAcm�2)FF PCE

(%)

PF-BZ:PCBM 80 0.53 1.0 0.40 0.22PF-NT:PCBM 68 0.55 0.84 0.39 0.18PF-PR:PCBM 92 0.52 1.13 0.44 0.26PF-PR 15 0.36 0.28 0.39 0.04

Voc of the cell can be estimated by the HOMO energy levelof the conjugated polymer and the LUMO energy level ofPCBM [51]. By taking into account a LUMO level for PCBMat �4.3 eV, Voc of our three devices with PF-PR: PCBM (1:1),PF-NT: PCBM (1:1) and PF-BZ: PCBM (1:1) are estimated to0.58 V, 0.73 V and 0.63 V respectively, which are slightlyhigher than our experimental values, indicating that theperformance of our cells can be tuned further with higherVoc.

The short-circuit current density (Jsc) of three deviceswith PF-PR: PCBM (1:1), PF-NT: PCBM (1:1) and PF-BZ:PCBM (1:1) are estimated from Fig. 9 to 1.13, 0.84 and1 mA/cm2, respectively. Device with PF-PR:PCBM producesthe highest value of Jsc compared to other two and it hasthe highest photosensitivity among the three. The solarcells with three star polymers produce the promising Voc

and fill factor (FF) value and moderate power conversionefficiency (PCE%). These factors get highest value in PF-PR:PCBM (1:1) device (FF = 0.44 and PCE = 0.26%) among thethree as it has the highest photosensitivity, lowest bandgap and broad absorption maxima in solid state absorptionspectra.

4. Conclusion

Three polyfluorene based arylimide cored star polymers(PF-BZ, PF-NT and PF-PR) and one linear polyfluorene havebeen successfully designed and synthesized by Suzuki cou-pling reaction using core compound at end cap stage. Allthese monomers and polymers are successfully character-ized. Emission from perylene based core moiety in PF-PRupon excitation at polyfluorene absorption and TCSPCanalysis confirm the partial energy transfer from blueemissive polyfluorene arm to electron deficient dye core.Electrochemical analysis exhibits the electronic state ofthe different polymers and their band gap is gradually de-creased with the enhancement of electron deficiency ofcore arylimide, i.e., from perylene based core to benzenecore. Thermogravimetric analysis demonstrates theenhancement of thermal stability of the star polymer with

C. Chakraborty et al. / European Polymer Journal 52 (2014) 181–192 191

increasing the core size. Undesired keto defects in PFchains can easily be prevented by preparation of arylimidecored star polyfluorene as it selectively controls the poly-fluorene chain aggregation and exciton diffusion. The sim-ple solar cells prepared by blending separately of threepolymer and PCBM exhibit a moderate PCE of 0.26%, a highopen-circuit voltage (0.52 V), short circuit current(�1.13 mA/cm2) and a relatively high FF of around 44%for perylene cored polyfluorene star polymer. Overall, thiswork provides a concept for developing star shaped conju-gated polymers for photovoltaic applications.

Acknowledgements

C.C. is indebted to CSIR, New Delhi, India for his SeniorResearch Fellowship and S.M. acknowledges the unit ofnanoscience (DST, India) as well as MALDI-ToF facility ofIACS.

Appendix A. Supplementary material

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.eurpolymj.2014.01.008.

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