ARTICLE pubs.acs.org/JPCC
Thermal Conversion of Precursor Polymer to Low Bandgap Conjugated Polymer Containing Isothianaphthene Dimer Subunits Tomokazu Umeyama,†,‡ Kohei Hirose,† Kei Noda,§ Kazumi Matsushige,*,§ Tetsuya Shishido,† Hironobu Hayashi,† Yoshihiro Matano,† Noboru Ono,|| and Hiroshi Imahori*,†,||,^ †
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § Department of Electronic Science and Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ^ Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4, Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan
)
‡
bS Supporting Information ABSTRACT: Thermal conversion strategy has been utilized in the synthesis of a novel low bandgap polymer containing isothianaphthene (ITN) dimer structure and benzodithiophene (BDT) unit in the backbone (PBIBDT). First, a highly soluble precursor polymer with an alternating main chain structure of bicyclo[2.2.2]octadiene-fused thiophene dimer and BDT (PPBIBDT) was synthesized by a palladium(0)-catalyzed Stille coupling reaction. Then, heating of the yellow PPBIBDT film spin-coated on a glass plate yielded a dark blue film of PBIBDT that was insoluble in any organic solvents. Thermogravimetric analysis of PPBIBDT showed 14% weight loss with an onset at 230 C, corroborating the occurrence of the thermally induced retroDielsAlder reaction. The PBIBDT film showed red-shifted, broad absorption in the visible and near-infrared regions with a maximum at 706 nm compared to the precursor polymer PPBIBDT with an absorption peak at 445 nm. The introduction of an ITN dimer unit in the backbone lowered the bandgap owing to the stabilized quinoid resonance structure. The field-effect hole mobility of PBIBDT was determined to be 1.1 104 cm2 V1 s1 with an onoff ratio of 2.5 102, while the PPBIBDT-based device revealed no p- and n-type responses. Organic photovoltaic devices were fabricated based on the planar heterojunction structure of PBIBDT and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and showed a power conversion efficiency of 0.07% under standard AM1.5 sunlight (100 mW cm2). These results obtained here will provide fundamental information on the design of thermally induced low bandgap polymers for device applications.
’ INTRODUCTION π-Conjugated polymers have been the subjects of intense research, especially for device applications including organic field effect transistors (OFETs)1 and organic photovoltaics (OPVs)2 because they can be processed with cost-effective solution techniques compared to small organic molecules that typically require vacuum deposition to fabricate the devices. Another advantage of polymeric materials in these applications is the ability to tailor the physical, optical, and electronic properties by altering the chemical structure of the polymer backbone and side chain. Significant progress in this field has been achieved through the development of regioregular polythiophenes such as poly(3-hexylthiophene) (P3HT), which forms polycrystalline structure and thus gives rise to a high charge carrier mobility of up to ∼0.3 cm2 V1 s1 and a power conversion efficiency (η) of up to 5%.3,4 Recently, other thiophene-containing polymers with various fused aromatic rings in the main chain have been reported to improve the device performance.510 The fused rings in the backbone can lead to a more rigid and planar structure, thereby extending effective π conjugation r 2011 American Chemical Society
length, enhancing intermolecular stacking, and preventing chain folding. In addition, the rigid fused-ring structure can lower the reorganization energy of the polymers, facilitating intermolecular charge hopping, and increasing charge carrier mobility.11 Conjugated polymers containing thieno[3,2-b]thiophene,5 cyclopenta[2,1-b:3,4-b0 ]dithiophene,6 dithieno[3,2-b:20 ,30 -d]-silole,7 benzo-[1,2-b:4,5-b0 ]dithiophene,8 thieno[3,4-b]thiophene,9 and thieno[3,4-c]pyrrole-4,6-dione10 units are representative examples that exhibit high field-effect mobility as high as 0.5 cm2 V1 s1 and/or excellent η values of up to 78%. Isothianaphthene (ITN) is also a class of promising building blocks for such functional conjugated polymers in which a thiophene ring is fused to a benzene ring at the β-positions. In analogy with the thieno[3,4-b]thiophene unit,9 conjugated polymers containing ITN units in the main chain tend to have low Received: September 12, 2011 Revised: November 21, 2011 Published: December 07, 2011 1256
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Scheme 1. Synthetic Route to PBIBDT
bandgaps because their main chains favor quinoid resonance forms to preserve the benzene aromaticity at the expense of the thiophene aromaticity.12 Poly(isothianaphthene) (PITN), homopolymer of ITN unit, is one of the early developed low bandgap polymers, which was synthesized by electrochemical polymerization and chemical oxidation methods.13 Irrespective of the quite low bandgap (≈ 1.0 eV), the poor solubility of PITN in common organic solvents resulted in low processability and thus hindered the device applications. Another approach to ITN-based polymers is the incorporation of ITN units into copolymer structures in an alternating fashion. Recently, metal-catalyzed condensation polymerization with difunctionalized ITN monomers has been conducted, and the resulting low bandgap polymers were applied to OFET and OPV devices.1416 Conjugated polymers consisting of isothianaphthene, thiophene, and p-phenylene units with solubilizing alkoxy groups (PITPT) were reported to have a strong tendency to self-assemble in an interdigitated packing mode as a result of the strong ππ stacking of the polymer main chain.14 The resulting polymer film showed a p-type conducting nature with an OFET hole mobility of 1.0 103 cm2 V1 s1.14 On the other hand, donoracceptor alternating copolymers with the combination of ITN and benzothiadiazole were synthesized, and the photovoltaic behavior of the blend film with [6,6]-phenylC61-butyric acid methyl ester (PCBM) was investigated.15 The η values of the OPV devices in the optimized condition were in the range of 0.80.9%.15 Despite the high potential of ITN, the device performances of ITN-based polymers still remain moderate compared to conjugated polymers containing other thiophene-fused aromatic units.510 One of the plausible reasons for the moderate performances is the instability of ITN monomers owing to its high reactivity at the 1- and 3-positions, resulting in the low molecular weights (number-average molecular weights (Mn) < 10000). In addition, the monomer instability may cause considerable defect structures in the resulting polymers. Alternative routes for ITNbased polymers to circumvent the labile ITN monomers are highly desirable. A potential strategy to solve the instability problem of the ITN monomers is the synthesis of precursor polymers17 from stable monomers and the conversion of the precursors to the target ITN-containing polymers by external stimuli such as heat and light. Ono et al. reported the preliminary synthesis of ITN oligomers (i.e., monomer to trimer) via bicyclo[2.2.2]octadiene (BCOD)-fused thiophene, which can be converted to ITN structure by thermally induced retro-DielsAlder reaction.18,19
Although this method enabled us to avoid the synthesis and purification of unstable and insoluble ITN monomers, such strategy has yet to be applied to the synthesis of ITN-based conjugated polymers that would further extend π conjugation relative to the short oligomers. Herein, we report the first application of thermal conversion strategy to ITN-based polymers, that is, the synthesis of novel conjugated polymers containing ITN dimer structure and benzodithiophene (BDT) unit in the backbone (Scheme 1). The target polymer PBIBDT is expected to have a relatively narrow bandgap due to the highly planar structure of the ITN dimer that induces the stabilized quinoid resonance structure. An additional benefit of this precursor route17,19 is high processability of the precursor polymer PPBIBDT, which can be anticipated to exhibit higher solubility in various organic solvents than PBIBDT because of the more sterically hindered, regio-irregular structure of PPBIBDT. In this report, we present the thermal, optical, and electrochemical properties of the ITN-based polymer in comparison with the precursor polymer. Fabrication and evaluation of OFET and OPV devices using the precursor and ITN-based polymers are also presented for discussion.
’ EXPERIMENTAL SECTION Instruments. 1H NMR spectra were measured with a JEOL
JNM-EX400 NMR spectrometer. FT-IR spectra were recorded on a JASCO FT/IR-470 Plus spectrometer (KBr pellets) or a SHIMADZU IRPrestige-21 with DuraScope system (attenuated total reflectance (ATR)). Gel permeation chromatography (GPC) measurements were carried out on a SHIMADZU Prominence equipped with JAIGEL-3HAF (Japan Analytical Industry) column using chloroform as an eluent after calibration with standard polystyrene. UVvisnear-infrared (NIR) spectra of solutions and films were measured with a PerkinElmer Lambda 900 UV/vis/NIR spectrometer. Atomic force microscopy (AFM) analyses were carried out with an Asylum Technology MFP-3DSA in the AC mode. The samples were coated on Si wafers unless otherwise noted. Thermogravimetric analysis (TGA) measurements were conducted with a SHIMADZU TG-60 under flowing nitrogen at a scan rate of 10 C min1. Differential scanning calorimetry (DSC) analysis was made on a DSC 822e (Mettler) at a scan rate of 10 C min1. Cyclic voltammetry (CV) measurements were performed using an ALS 630A electrochemical analyzer in benzonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate as a supporting electrolyte. An ITO (Geomatec) 1257
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The Journal of Physical Chemistry C working electrode, Ag/AgNO3 (0.01 M in acetonitrile) reference electrode, and Pt wire counter electrode were employed. X-ray diffraction (XRD) analyses were conducted by a Rigaku A2 diffractometer using Cu Kα radiation. The samples were coated on SiO2 substrates. A time-correlated single photon counting (TCSPC) method was used for the fluorescence lifetime measurements in the nanosecond and subnanosecond time scale, and the time resolution was ∼60 ps (fwhm) for an excitation wavelength of 405 nm.20 Currentvoltage characteristics for space charge limited current (SCLC) measurements were measured using a Keithley 4200-SCS characterization system under vacuum (1.5 102 Pa).21 Photocurrentvoltage characteristics were measured by PECK2400-N with PEC-L11 solar simulator (Peccell Technologies) under standard two-electrode conditions (100 mW cm2, AM1.5). Photocurrent action spectra were recorded with PEC-S20DC (Peccell Technologies). Materials. All solvents and chemicals were of reagent grade quality, purchased commercially, and used without further purification unless otherwise noted. Details for the synthesis of compound 1 are described in the Supporting Information. 1,5Bistrimethyltin-4,8-dioctylbenzo[1,2-b:4,5-b0 ]dithiophene (4) was prepared according to the literature.9a Monomer Synthesis. Bi(4,7-dihydro-10,10- or 11,11-dimethyl4,7-ethanobenzo[c]thiophene) (2). Compound 1 (1.14 g, 6.00 mmol) was dissolved in 30 mL of anhydrous THF under an atmosphere of argon. The solution was cooled down to 78 C, and 4.00 mL of n-butyllithium (1.63 M in n-hexane, 6.52 mmol) was added via syringe. After being stirred at 10 C for 30 min, the mixture was cooled down to 78 C again, and copper(II) chloride (1.61 g, 12.0 mmol) was added. After the addition, the mixture was kept at 78 C for 2 h and room temperature for 2 h. Then, it was poured into 50 mL of 1 M HCl and extracted with dichloromethane. The organic layer was washed with 1 M HCl and saturated NaHCO3 solution, water, and saturated NaCl solution and dried over anhydrous Na2SO4 and evaporated. Purification by column chromatography on silica gel using hexane as eluent yielded 2 (776 mg, 2.04 mmol, 68%) as pale yellow oil. Compound 2 was used for the next step without separation of the regioisomers. 1H NMR (CDCl3, 400 MHz): δ 6.70, 6.68 (2H, ArH); 6.51 (2H, CHdCH); 3.99, 3.95, 3.70, 3.56, 3.25 (4H, bridge head); 1.36 (4H, CH2); 1.05, 1.02 (3H, CH3); 0.80, 0.77, 0.73 (3H, CH3). IR (ATR): νmax/cm1 3048, 2949, 2926, 2862, 1607, 1466, 1449, 1381, 1362, 1342, 1227, 1130, 947, 899, 820, 723, 681. HRMS (APCI): calcd for C24H27S2 [M + H] 379.1554, found 379.1539. 1,10 -Bi(4,7-dihydro-3-iodo-10,10- or 11,11-dimethyl-4,7ethanobenzo[c]thiophene) (3). Iodine (181 mg, 0.71 mmol) was dissolved in 2.0 mL of anhydrous THF in a 10 mL flask under an atmosphere of argon. Compound 2 (108 mg, 0.28 mmol) was dissolved in 3.0 mL of anhydrous THF in a 50 mL flask under an atmosphere of argon, and the solution was cooled down to 78 C. Then, 0.50 mL of lithium diisopropylamide (2.0 M in THF/heptane/ethylbenzene, 1.0 mmol) was added dropwise. After being stirred at 0 C for 30 min, the mixture was cooled down to 78 C again, and iodine solution was transferred to the 50 mL flask via transfer tube. After the addition, the mixture was stirred at room temperature for 1 h. Then, the mixture was quenched with 10% Na2S2O3 solution (5 mL) and extracted with diethyl ether. The organic layer was washed with water, saturated NaCl solution and dried over anhydrous Na2SO4 and evaporated. Purification by column chromatography on silica gel using hexane as eluent yielded 3 (48.7 mg, 0.076 mmol, 27%)
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as pale yellow solid. Compound 3 was used for the next step without separation of the regioisomers. 1H NMR (CDCl3, 400 MHz): δ 6.50 (2H, CHdCH); 3.95, 3.89, 3.60, 3.55, 3.17 (4H, bridge head); 1.33 (4H, CH2); 1.06, 1.02 (3H, CH3); 0.79, 0.77, 0.75 (3H, CH3). IR (ATR): νmax/cm1 3049, 2951, 2926, 2862, 1602, 1447, 1362, 1341, 1223, 1165, 1132, 1099, 1016, 951, 905, 860, 824, 781, 729, 684, 663. HRMS (APCI): calcd for C24H25I2S2[M + H] 630.9487, found 630.9477. Polymer Synthesis. PPBIBDT. A dried Schlenk tube was charged with 3 (157 mg, 0.24 mmol), 4 (185 mg, 0.24 mmol), tetrakis(triphenylphosphine)palladium(0) (13 mg, 0.012 mmol), and 4 mL of distilled toluene under an argon atmosphere. The mixture was stirred at 90 C for 12 h. The cooled mixture was subsequently poured into methanol, and the orange precipitate was collected on a membrane filter (ADVANTEC). The product was washed with methanol and hexane. Purification by reprecipitation with CHCl3/methanol and CHCl3/hexane yielded the polymer as a yellowish-orange solid (118 mg, 60%). The BCODfused thiophene dimer units in the obtained polymer still have regioisomers. GPC (polystyrene standards, chloroform): Mn = 35 000, PDI = 2.1. 1H NMR (CDCl3, 400 MHz) δ 7.52, 7.49 (2H, ArH); 6.61 (2H, CHdCH); 4.34 (4H CH2O); 4.34, 4.13, 3.99, 3.77 (8H, bridge head); 1.95 (4H, CH2); 1.63, 1.42, 1.32, 1.18, 1.12, 0.99, 0.89 (42H). IR (KBr): νmax/cm1 3049, 2924, 2858, 1577, 1559, 1523, 1437, 1362, 1262, 1169, 1129, 1030, 818, 732, 683. PBIBDT. The film of PPBIBDT was deposited on glass surface by spin coating of chlorobenzene solution (10 mg mL1) at 1000 rpm for 1 min. The substrate was heated on a hot plate at 220 C for 30 min under an atmosphere of argon to yield PBIBDT film. IR (KBr): νmax/cm1 3069, 2925, 2855, 1560, 1450, 1363, 1187, 1113, 1044, 874, 823, 731, 692, 671. Fabrication and Characterization of OFET Devices. Topcontact, bottom-gate FET devices with PPBIBDT or PBIBDT as the semiconductor were prepared on silicon wafer. A heavily n-doped silicon wafer with a 300 nm thermal silicon dioxide (SiO2) was used as the substrate/gate electrode, with the top SiO2 layer serving as the gate dielectric. The SiO2 surface of the wafer substrate was cleaned by ultrasonication in ethanol and vapor degreasing by ethanol. The semiconductor layer was deposited on top of SiO2 surface by spin coating using a polymer solution of PPBIBDT in chlorobenzene (10 mg mL1) at 1000 rpm for 1 min. Then, ∼25 nm thick gold thin films were deposited as source and drain contacts using a shadow mask. The FET devices had a channel length (L) of 50 μm and a channel width (W) of 1 mm (Figure S1, see Supporting Information). In the case of the device with PBIBDT, the substrate was heated on a hot plate at 220 C before fabrication of source and drain contacts. The currentvoltage characteristics of the FET devices were measured using a Keithley 4200-SCS characterization system under vacuum (2 102 Pa). The carrier mobility (μ) was calculated from the data in the saturation regime according to the equation ID = μεoxε0(W/2Ld) (VG VT)2, where ID is the drain current in the saturation regime, μ is the field-effect mobility, εox is permittivity of gate dielectric, ε0 is permittivity of vacuum, d is thickness of gate dielectric, and VG and VT are gate voltage and threshold voltage, respectively. VT was derived from the relationship between the square root of |ID| at the saturation regime and VG by extrapolating the measured data to ID = 0. Fabrication and Characterization of OPV Devices. The organic solar cells were fabricated as follows.10d,22 Indium tin oxide 1258
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The Journal of Physical Chemistry C (ITO) on a glass substrate with a sheet resistance of 5 Ω/sq. (Geomatec) was used. The substrates were sonicated consecutively with acetone and ethanol for 10 min. After blow-drying and UV-ozone treatment, the substrates were spin-coated at 4000 rpm with poly(ethylene dioxythiophene) doped with polystyrene sulfonic acid (PEDOTPSS, Clevios P) and dried with a hot plate at 200 C for 10 min. Under an argon atmosphere, a layer of PBIBDT was formed by spin coating a chlorobenzene solution of PPBIBDT (15 mg mL1) at 1000 rpm for 1 min on the ITO/ PEDOTPSS and heated on a hot plate at 220 C for 30 min to cause the thermal conversion. Then, a layer of PCBM (American Dye Source, Inc.) was formed by spin-coating a chlorobenzene solution of PCBM (15 mg mL1). The thicknesses of the PBIBDT and PCBM layers estimated by AFM were 55 and 15 nm, respectively. Finally, a layer of Al (The Nilaco Corporation) was deposited by thermal evaporation under vacuum (6 106 Pa) to yield the layered device structure (denoted as ITO/PEDOTPSS/PBIBDT/ PCBM/Al). Schematic illustrations of the top and side views of the ITO/PEDOTPSS/PBIBDT/PCBM/Al device are depicted in Figure S2 (see Supporting Information). Photocurrent voltage characteristics were measured under ambient atmosphere and simulated solar light, air mass (AM) 1.5 conditions (100 mW cm2). For the fabrication of the active layer with bulk heterojunction structure, a mixed solution of PPBIBDT (15 mg mL1) and PCBM (15 mg mL1) in chlorobenzene was spin-coated at 1000 rpm for 1 min onto the ITO/PEDOTPSS. Then, Al was deposited to obtain the device with the composite film of PPBIBDT and PCBM (denoted as ITO/PEDOTPSS/ PPBIBDTPCBM/Al). Thermal conversion of PPBIBDT to PBIBDT at 220 C for 30 min was conducted prior to the Al evaporation to construct the device with PBIBDT and PCBM (denoted as ITO/PEDOTPSS/PBIBDTPCBM/Al). Both of the PPBIBDTPCBM and PBIBDTPCBM films had the thicknesses of 200 nm. Schematic illustrations of the device with bulk heterojunction films are also depicted in Figure S2 of the Supporting Information.
’ RESULTS AND DISCUSSION Precursor Polymer Synthesis and Characterization. The synthesis is outlined in Scheme 1. The stable precursor monomer, BCOD-fused thiophene 1, was prepared following the reported procedure.18a It should be emphasized here that the introduction of dimethyl substituents at the fused ring was essential to increase the solubility of the precursor polymer. The oxidative dimerization of monolithiated 1 with CuCl2 gave the dimer 2, which contained regioisomers of head-to-tail, headto-head, tail-to-tail as well as diastereomers, depending on the position of dimethyl units. We did not separate the isomers, since the structural irregularity of the BCOD-fused thiophene dimer unit could reduce the crystallinity, thereby improving the solubility of the monomer and the resulting polymer. Furthermore, the irregularity could be eliminated by the thermal conversion to the ITN dimer unit in the target polymer. Thus, we used the dimer as a mixture for the following reaction to yield α,ω-diiodo BCOD-fused thiophene dimer 3. The compound 3 was copolymerized with distannylated BDT comonomer 49a through Stille coupling reactions to afford the precursor polymer PPBIBDT. The precursor polymer PPBIBDT showed yellowish-orange color in a powder state and excellent solubility in common organic solvents such as toluene, o-dichlorobenzene, and
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Figure 1. TGA of PPBIBDT. The analysis was performed under nitrogen with a heating rate of 10 C min1.
Figure 2. DSC curves of PPBIBDT. The analyses were performed under nitrogen with a heating rate of 10 C min1.
chlorobenzene, as expected. This high solubility permitted the polymer to be readily processed by solution technique for the film formation. The chemical structure of the polymer was identified by 1H NMR and FT-IR spectra. The number-average molecular weight (Mn) and polydispersity index (PDI) were estimated to be 35 000 and 2.1, respectively, by gel permeation chromatography (GPC) with polystyrene standards in chloroform. The molecular weight of PPBIBDT is much higher than those of previously reported ITN-containing copolymers prepared by metal-catalyzed condensation polymerizations.1416 Thermal Conversion of PPBIBDT to PBIBDT. Thermogravimetric analysis (TGA) of PPBIBDT exhibited two clear steps of weight loss with the onsets at 230 and 320 C (Figure 1). The first step with an onset at 230 C suggests the occurrence of the thermal conversion (Scheme 1), since the 14% mass loss corresponds to the removal of CH2dCMe2 by the retro-Diels Alder reaction (14% calcd). The second weight loss may correspond to the thermal decomposition of the converted polymer, indicating that PBIBDT was thermally stable up to 320 C in an inert atmosphere. Differential scanning calorimetry (DSC) measurement of PPBIBDT in the temperature range from 20 to 290 C exhibited an endothermic peak with an onset at 235 C (Figure 2). This signal can be attributed to the occurrence of the retro-DielsAlder reaction, as suggested by TGA. Moreover, concomitant exothermic phase transition such as crystallization may be obscured by this intense endothermic peak. The first cooling and the second heating, which correspond to the thermal property of PBIBDT, showed no phase transitions. The absence of the endothermic signals over 235 C suggests the completion of the thermally induced detachment during the first heating. The resulting PBIBDT powder after the thermal conversion was 1259
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completely insoluble in any organic solvents, and thus, it was unable to characterize PBIBDT in solution by NMR and GPC measurements. It is known that the thermal conversion of BCODfused thiophene dimer and trimer yields pure ITN dimer and trimer quantitatively without purification procedures.18a Thus, incidence of cross linking during the thermal conversion of PPBIBDT to PBIBDT would be low. Nevertheless, the occurrence of cross linking in the polymer is undeniable, considering the complete insolubility and limited ππ stacking tendency of PBIBDT (vide infra). Overall, the complete insolubility of PBIBDT may result from the occurrence of unexpected crosslinking reactions during the thermal treatment in addition to the removal of sterically hindered BCOD units. Optical and Electrochemical Properties. The UVvis absorption spectrum of PPBIBDT (Figure 3) in a diluted chloroform solution displays an absorption maximum at 447 nm (Table 1). This is blue-shifted by 34 nm from that of 4,40 -dimethyl-2,20 bithienyleneBDT alternating copolymer (PBTBDT),8b indicating the shorter effective conjugation length of PPBIBDT than PBTBDT. The bulky BCOD group with dimethyl substituents may cause significant torsion in the backbone and thereby hinder effective through-bond electronic communication. A cast film of PPBIBDT from chloroform solution revealed absorption bands similar to those in the solution state (Figure 3), which is in sharp contrast to PBTBDT and regioregular P3HT.3,4,8b This result is indicative of no effective intermolecular interaction in the ground state of PPBIBDT film due to the bulky fused ring. The emission spectrum of PPBIBDT in chloroform excited at the absorption peak exhibited two peaks at 522 and 559 nm (2.38 and 2.22 eV, respectively) (Figure 4 and Table 1). The energy difference between the emission peaks is ∼0.16 eV and can be ascribed to a CdC bond stretching frequency in the polymer, as observed in the spectrum of P3HT.23 The PPBIBDT
film also showed the similar fluorescence, while the peaks were slightly red-shifted (535 and 571 nm) compared to those in solution. The fluorescence lifetime of PPBIBDT was measured by the time-correlated single-photon counting (TCSPC) technique detected at the emission peak with an excitation wavelength of 405 nm. The fluorescence decay of PPBIBDT in chloroform was analyzed by a single component with a lifetime (τ) of 0.48 ns, which is comparable to that of P3HT in solution (∼0.5 ns).10d On the other hand, the fluorescence decay curve of PPBIBDT film was fitted by a fast major component (τ < 60 ps, 99%) and a slow minor component (τ = 0.83 ns, 1%). This result implies the occurrence of ultrafast nonradiative quenching of the singlet excited state of PPBIBDT in the film state, which is beyond the time resolution of the present TCSPC system (ca. 60 ps). After the thermal treatment of PPBIBDT film for 30 min at 220 C under an argon atmosphere, the polymer film became dark blue. The absorption spectrum of the PBIBDT film depicted a red-shifted, broad band in the visible and near-infrared (NIR) region with a maximum at 706 nm (Figure 3). As expected, the introduction of highly planar ITN dimer units by thermal conversion increased the effective intramolecular conjugation length and thereby lowered the bandgap. In addition, more effective interaction between the polymer main chains in the PBIBDT film than in the PPBIBDT film may narrow the bandgap significantly. From the onset of the absorption spectra, the optical bandgaps (Egopt) of the polymer films are determined to be 2.4 eV for PPBIBDT and 1.3 eV for PBIBDT (Table 1).24 On the other hand, steady-state emission measurement of the PBIBDT film did not reveal any detectable emission in the visible and NIR region. Besides, the fluorescence lifetime measurement in the same region did not show any decay component. These indicate the occurrence of the ultrafast nonradiative vibrational relaxation
Figure 3. UVvis absorption spectra of (a) PPBIBDT in chloroform (8.7 μM with regard to the repeat unit) (dashed line), (b) PPBIBDT film on glass plate spin-cast from chloroform solution (12 mM) at a rotation speed of 1000 rpm (solid line), and (c) PBIBDT film thermally converted from PPBIBDT film in (b) (dotted line).
Figure 4. Photoluminescence spectra of PPBIBDT (a) in chloroform (0.87 μM with regard to the repeat unit) (dashed line) and (b) in film state on glass plate (solid line). The samples were excited at the absorption maxima. The spectrum of the PPBIBDT film was normalized to the fluorescence intensity of PPBIBDT solution at the maxima for comparison.
Table 1. Optical and Electrochemical Properties Abs λmax/nm
Em λmax/nm
Abs λonset/nm
solution
film
solution
film
film
Egopt/eV
HOMOa/eV
LUMOb/eV
PPBIBDT
447
445
522, 559
535, 571
520
2.4
5.4
3.0
PBIBDT
c
706
c
c
950
1.3
5.1
3.8
Determined from the onset of the oxidation potential in films by using cyclic voltammetry. b Determined from HOMO levels and optical band gaps. c Not determined. a
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Figure 5. Cyclic voltammograms of (a) PPBIBDT and (b) PBIBDT films measured in acetonitrile containing 0.1 M n-Bu4NPF6 as a supporting electrolyte at a scan rate of 50 mV s1. ITO working electrodes coated with polymer films, Ag/AgNO3 (0.01 M in acetonitrile) reference electrode, and a Pt wire counter electrode were employed. Ferrocene/ferrocenium (+0.64 V vs NHE) was used as a standard for all measurements. The vertical dashed lines show the onsets of the first reduction and oxidation potentials.
of the PBIBDT excited state, which is beyond the time resolution of the measurement system (
