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Synthesis and Photophysical and Photovoltaic Properties of Porphyrin-Furan and -Thiophene Alternating Copolymers Tomokazu Umeyama,† Takeshi Takamatsu,† Noriyasu Tezuka,† Yoshihiro Matano,† Yasuyuki Araki,*,‡ Takehiko Wada,‡ Osamu Yoshikawa,§ Takashi Sagawa,§ Susumu Yoshikawa,§ and Hiroshi Imahori*,†,|,⊥ Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira, Aoba-ku, Sendai 980-8577, Japan, Institute of AdVanced Energy, Kyoto UniVersity, Uji, Kyoto 611-0011, Japan, Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, and Fukui Institute for Fundamental Chemistry, Kyoto UniVersity, 34-4, Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan ReceiVed: March 4, 2009; ReVised Manuscript ReceiVed: May 1, 2009
Conjugated polymers with alternating main chain structures of zinc porphyrin-furan (PZnPF) and zinc porphyrin-thiophene (PZnPT) have been synthesized by palladium(0)-catalyzed Stille coupling reaction. The optical, electrochemical, photophysical, and photovoltaic properties of PZnPF and PZnPT were investigated to elucidate the effects of the heterole bridges (i.e., furan vs. thiophene) in the porphyrin polymers. The optical bandgap of PZnPF (1.75 eV) is smaller than that of PZnPT (1.90 eV), implying the high delocalization of the π-electrons along the polymer main chain of PZnPF relative to PZnPT. The more extended π-conjugation in PZnPF results from the smaller steric repulsion of the meso-furan moiety with the porphyrin rings than that of the meso-thiophene. The time-resolved fluorescence spectrum of PZnPF showed a gradual Stokes shift to the longer wavelength in the subnanosecond time domain due to the relaxation from a twisted conformation with the large dihedral angles between the porphyrins and the furan rings to a coplanar conformation with the small dihedral angles, whereas the fluorescence spectrum of PZnPT did not exhibit the dynamic Stokes shift. Both PZnPF and PZnPT are electrochemically active in the oxidation and reduction regions and have suitable HOMO/LUMO levels that enable photoinduced electron transfer from the polymer to [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in the blend films. Indeed, the blend films displayed strong fluorescence quenching from the porphyrin moieties together with appearance of charge-transfer emission arising from the interaction between the porphyrin and the C60 moieties. This is the first observation on charge-transfer emission between conjugated porphyrin polymers and fullerenes. Bulk heterojunction solar cells were fabricated by using the blend films of PZnPF:PCBM and PZnPT:PCBM as a photoactive layer. The PZnPF:PCBM and PZnPT:PCBM devices revealed power conversion efficiencies of 0.048% and 0.027% under standard AM1.5 sunlight (100 mW cm-2). These results obtained here will provide fundamental information on the design of large chromophore-embedded conjugated polymers for solar energy conversion. Introduction Porphyrins and their derivatives have attracted considerable attention for many years, primarily because of their importance in biological systems.1 Porphyrins are more stable and synthetically accessible than chlorophylls. They absorb light strongly in the blue and moderately in the green regions. The optical bandgaps and the redox potentials of porphyrins can be tuned by metal insertion into the core and/or modification at the peripheral positions.2 Moreover, porphyrins contain an extensively conjugated π system that is suitable for efficient electron transfer (ET) reactions, because the uptake or release of electrons results in minimal structural and solvation change upon ET, * To whom correspondence should be addressed. E-mail: imahori@ scl.kyoto-u.ac.jp and
[email protected]. † Department of Molecular Engineering, Graduate School of Engineering, Kyoto University. ‡ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University. § Institute of Advanced Energy, Kyoto University. | Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University. ⊥ Fukui Institute for Fundamental Chemistry, Kyoto University.
leading to small reorganization energies of ET.3 Therefore, porphyrins have been frequently employed for organic electronics including organic solar cells.4–8 Recently, we and others have developed novel supramolecular photovoltaic cells using porphyrins as a donor (D) and fullerenes as an acceptor (A).9–12 Porphyrins and fullerenes are known to form supramolecular charge-transfer (CT) complexes involving closest contacts between one of the electron-rich 6:6 bonds of the fullerene and the geometric center of the porphyrin.4b,13 Preorganized porphyrin arrays using various nanoscaffolds including dendrimers,10 oligopeptides,11 and nanoparticles12 can be used for the bottomup organization of the porphyrins and fullerenes on nanostructured semiconducting electrodes. The modified electrodes exhibited remarkable photocurrent generation in the case that interdigitating D-A film structures are formed owing to the π-π interaction between the porphyrin and fullerene as well as the preorganized porphyrin arrays.4a–d,9a,b,10,12 With these results in mind, one can expect that a combination of novel linear porphyrin polymers with fullerenes will also yield
10.1021/jp902001z CCC: $40.75 2009 American Chemical Society Published on Web 05/21/2009
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CHART 1: Structure of PZnPF, PZnPT, and ZnP-ref
Experimental Section
similar supramolecular D-A structures exhibiting efficient photocurrent generation. Linear conjugated porphyrin arrays are highly promising for the strong complexation with fullerenes in addition to the improved light-harvesting and hole-transporting properties. So far considerable efforts have been directed toward the synthesis of linear conjugated porphyrin arrays.14–20 It is noteworthy that direct linkage between porphyrins at the meso positions leads to alternate orthogonal alignment of the porphyrin planes, precluding the extended π-conjugation.14 In contrast, a notable improvement in electronic communication between the porphyrins can be achieved by unhindered ethyne linkages attached at the meso- or β-carbon positions.15–18 Anderson et al. have extensively investigated the photophysical and electronic properties of conjugated porphyrin polymers with butadiyne bridges.17 The butadiyne-linked porphyrin polymers were found to exhibit large third-order nonlinear optical properties.17 On the other hand, detailed studies of linear conjugated porphyrin polymers, in which the porphyrins are linked by aromatic ringbased bridges, have been relatively limited.19 Thus, the effects of the bridge on photophysical and photovoltaic properties of heterole ring-linked porphyrin polymers have not been fully addressed. Very recently, Huang et al. reported the synthesis of porphyrin-dithienothiophene π-conjugated polymers and their applications to bulk heterojunction solar cells with [6,6]phenyl-C61-butyric acid methyl ester (PCBM) as an acceptor.20 However, the photophysical properties of the porphyrin polymer: PCBM composite films as well as those of the polymers in solutions have not been studied.20 Here we report the synthesis of novel conjugated polymers PZnPF and PZnPT (Chart 1) that possess a main chain structure with alternating units of porphyrin and a simple heterole ring, furan and thiophene, respectively. Both of the polymers are anticipated to reveal the excellent light-harvesting properties arising from the Soret and Q-bands of the porphyrin units.15 In addition, the elongation of the π-electron system through the less-sterically hindered heterole units may cause the broadening and red-shift of the Soret and Q-bands together with the splitting of the Soret band derived from the excition coupling of the porphyrin rings.15 Long alkoxy chains are introduced at the meta positions of meso-phenyl groups to improve the solubility of the polymers in organic solvents. Considering that the steric repulsion between the porphyrin ring and the heterole ring is smaller in PZnPF than in PZnPT, PZnPF is expected to show more conjugated electronic structure than PZnPT. Such difference would have a significant impact on the optical, electrochemical, photophysical, and photovoltaic properties. We also examined the effect of supramolecular CT complexes arising from the conjugated porphyrin polymer and fullerene on the photovoltaic properties in the hope that the cell performance would be enhanced by the CT formation.
General Methods. 1H NMR spectra were obtained with a JEOL JNM-EX400 NMR spectrometer. IR spectra were recorded on a JASCO FT/IR-470 Plus spectrometer, using KBr pellets. Gel permeation chromatography (GPC) measurements were carried out on a SHIMADZU Prominence equipped with JAIGEL-2.5HAF column, using chloroform as an eluent after calibration with standard polystyrene. Atomic force microscopy (AFM) images were obtained from a Digital Instruments Nanoscope III in the tapping mode. Thermogravimetric analysis (TGA) measurements were conducted with a SHIMADZU TG60 under flowing air at a scan rate of 10 deg min-1. Differential scanning calorimetry (DSC) analysis was made on a DSC 822e (Mettler) at a scan rate of 10 deg min-1. Matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectra were measured on a SHIMADZU Biotech AXIMA-CFR with 1,8dihydroxy-9(10H)-anthracenone (dithranol) as a matrix. UVvis-NIR absorption spectra were measured on a PerkinElmer Lambda 900 UV/vis/NIR spectrometer. Steady-state fluorescence spectra were recorded with a SPEX Fluoromax-3 spectrofluorometer (HORIBA) and FP-6600 (JASCO). Timeresolved fluorescence spectra and fluorescence decays were measured by a single-photon counting method, using the second harmonic generation (SHG, 400 nm) feature of a Ti:sapphire laser (Spectra-Physics, Tsunami 3950-L2S, 150 fs fwhm) and a streakscope (Hamamatsu Photonics, C4334-01) equipped with a polychromator (Acton Research, SpectraPro 150) as an excitation source and as a detector, respectively.4g,h,12c Photocurrent-voltage characteristics were measured by using a CEP2000 (Bunkoh-Keiki). Electrochemical measurements were made with an ALS 630a electrochemical analyzer. Redox potentials of the polymers were determined by cyclic voltammery (CV) in acetonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate as a supporting electrolyte. An ITO working electrode coated with polymer films, Ag/AgNO3 (0.01 M in acetonitrile) reference electrode, and Pt wire counter electrode were employed. Decamethylferrocene/decamethylferrocenium (+0.15 V vs. NHE) was used as an internal standard for all measurements. DFT calculations for geometry optimization were performed by using the B3LYP functional and 3-21G(*) basis set implemented in the Gaussian 03 program package. Materials. All solvents and chemicals were of reagent grade quality, purchased commercially and used without further purification unless otherwise noted. 5,15-Dibromo-10,20-bis(3,5dioctyloxyphenyl)porphyrinatozinc(II) (ZnP-Br2),21 2,5-bis(trimethylstannyl)furan (TMS-F),22 2,5-bis(tributylstannyl)thiophene (TBS-T),23 and 5,15-bis(3,5-dioctyloxyphenyl)porphyrinatozinc(II) (ZnP-ref)24 were synthesized according to the literatures. Synthesis of PZnPF and PZnPT. PZnPF and PZnPT were synthesized by Stille coupling reaction. A typical experimental procedure was conducted as follows. In a 50 mL round-bottomed flask containing 10 mL of dehydrated DMF were added ZnPBr2 (92 mg, 77 µmol), TMS-F (30 mg, 77 µmol), and Pd(PPh3)4 (44 mg, 39 µmol) under an argon atmosphere. Then the mixture was stirred for 72 h at 90 °C. The reaction mixture changed color from deep purple to dark green. The reaction was quenched by adding methanol after cooling to room temperature. The resulting dark green precipitate was obtained by filtration and dissolved in 10 mL of chloroform. The solution was added to methanol to precipitate the polymeric material. The polymer was further purified by reprecipitation from chloroform-hexane and dried under vacuum for 24 h to yield PZnPF as dark green powder (41 mg, 48%).
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SCHEME 1: Synthesis of PZnPF and PZnPT
PZnPF: IR (KBr) νmax/cm-1 2921, 2853, 1587, 1430, 1349, 1161, 1001, 944, 795; 1H NMR (400 MHz; CDCl3; Me4Si) δΗ 0.8 (12H, -CH3), 1.3 (40H, -CH2-), 1.8 (8H, -OCH2CH2-), 4.2 (8H, -OCH2-), 6.9 (2H, p-PhH), 7.4 (4H, o-PhH), 7.8 (2H, β-H of furan), 9.3 (4H, β-H of pyrrole), 9.8 (4H, β-H of pyrrole). PZnPT: IR (KBr) νmax/cm-1 2926, 2849, 1588, 1431, 1346, 1164, 983, 793; 1H NMR (400 MHz; CDCl3; Me4Si) δΗ 0.8 (12H, -CH3), 1.3 (40H, -CH2-), 1.8 (8H, -OCH2CH2-), 4.2 (8H, -OCH2-), 6.9 (2H, p-PhH), 7.4 (4H, o-PhH), 8.3 (2H, β-H of thiophene), 9.3 (4H, β-H of pyrrole), 9.8 (4H, β-H of pyrrole). Fabrication and Characterization of Polymer Solar Cells. For organic solar cell fabrication, indium tin oxide (ITO) on a glass substrate with a sheet resistance of 5 Ω/0 (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 (PEDOT:PSS, CLEVIOS P, H. C. Strack) and dried with a hot plate at 200 °C for 10 min. Under a nitrogen atmosphere, an active layer of polymer:PCBM (American Dye Source, Inc.) film was formed by spin coating a chlorobenzene solution of polymer:PCBM mixture at 1000 rpm on the ITO/ PEDOT:PSS. Then, a TiOx layer was prepared by spin coating an ethanol solution of titanium(IV) isopropoxide (Wako Pure Chemical Industries, Ltd.) at 2000 rpm then leaving for 30 min under ambient atmosphere. The titanium(IV) isopropoxide was hydrolyzed to yield the TiOx layer.25 Finally, an Al (The Nilaco Corporation) layer was deposited by thermal evaporation under vacuum (5 × 10-3 Pa) to yield the layered device structure (denoted as ITO/PEDOT:PSS/polymer:PCBM/TiOx/Al). Schematic illustrations of the top and side views of the ITO/PEDOT: PSS/polymer:PCBM/TiOx/Al device are depicted in Figure S1 (see the Supporting Information). Photocurrent-voltage characteristics were measured under ambient atmosphere and simulated solar light, air mass (AM) 1.5 (100 mW cm-2).25 Although a very thin layer of oxidized Al was formed on the surface of the electrode under the ambient conditions, such coatings were ignorable for the evaluation of polymer cell performances in terms of photocurrent-voltage characteristics such as short-circuit current, open-circuit voltage, fill factor, power conversion efficiency, and incident photon-to-current efficiency (IPCE). It took 10 min to measure the cell performances and there was no remarkable degradation (viz. the cell performances were stable) even when the cell was allowed to stand for several hours in contact with air.25 Results and Discussion Synthesis, Structural Characterization, and Thermal Properties of Porphyrin Polymers. The synthesis of PZnPF and PZnPT was conducted via a palladium-catalyzed Stille coupling starting from the equimolar amount of zinc porphyrin dibromide ZnP-Br2 (5,15-dibromo-10,20-bis(3,5-dioctyloxyphe-
nyl)porphyrinatozinc(II)) with bisstannyl heterole compounds TMS-F (2,5-bis(trimethylstannyl)furan) or TBS-T (2,5-bis(tributylstannyl)thiophene) as outlined in Scheme 1.15a,26 The relatively large amount of the catalyst was needed to obtain PZnPF and PZnPT (50 and 70 mol % of the catalyst, respectively) with number-average molecular weight (Mn) over 10 000 (Table S1, Supporting Information). The rate-determining step of the transmetalation in the coupling reaction26 may be slowed down by the large steric effect of the porphyrin units. In addition, the high sensitivity of the palladium catalyst to oxygen and light may be responsible for the quite low turnover number. The resulting polymers were then purified through dissolution and reprecipitation in chloroform-methanol and chloroform-hexane repeatedly to remove low molecular weight materials. The Mn values were estimated to be 10 500 (polydispersity index (PDI) ) 1.3) for PZnPF and 10 800 (PDI ) 1.3) for PZnPT by gel permeation chromatography (GPC) with chloroform as an eluent and polystyrene standards as calibrants (Table 1). The degrees of polymerizations are ca. 10 in both cases. The polymers were soluble in a wide range of common organic solvents including chloroform, toluene, and tetrahydrofuran and can yield visibly uniform thin films on glass substrates by a spin-coating method. Solid and solution samples of the polymers were stable in air for more than several months in the ambient atmosphere. The structures of the polymers were also characterized by 1 H NMR and IR spectroscopies and mass spectrometry (see the Experimental Section). The peak area ratio of furan β-protons and pyrrole β-protons at the porphyrin ring is 2:8 in the 1H NMR spectrum of PZnPF, supporting the high alternating nature of furan and porphyrin rings in the main chain (Figure S2a, Supporting Information). The 1H NMR spectrum of PZnPT also demonstrates similar highly alternating structure (Figure S2b, Supporting Information). The polymer structures were also characterized by matrix-assisted laser desorption/ionization timeof-flight mass (MALDI-TOF-MS) measurements (Figure S3, Supporting Information). Although detected peaks are mainly attributed to the polymers with only several repeating units arising from chain destruction during the ionization, all intervals between these peaks at high molecular weights are 1104.8 for PZnPF and 1120.9 for PZnPT, respectively. These values are consistent with the structures of the ZnP-furan and ZnPthiophene repeating units. These results also corroborate the highly alternating structures of the polymers. The thermogravimetric analysis (TGA) showed that PZnPF and PZnPT were thermally stable with 5% weight loss in air at temperatures of 304 and 360 °C, respectively. The differential scanning calorimetry (DSC) analysis revealed the glass transitions at 80 °C for PZnPF and 115 °C for PZnPT during heating, but no melting point was observed, suggesting that the polymers are amorphous. The lack of crystallinity in the polymers could serve as a drawback for the use as a donor in organic solar cells, taking into account that the formation of ordered nanostructure is essential for efficient hole transport of conjugated
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TABLE 1: Molecular Weights and Optical Properties solutionc
filmf
PCBM blend filmf
Mn (PDI)
λmax (abs)/ nm
λmax (em) /nm
λmax (abs)/ nm
λmax (em) /nm
λmax (abs)/ nm
λmax (em)e/nm
Egopt/eV
PZnPF
10500 (1.3)a
713
442 561 623
686
445 562 631
820
1.75h
PZnPT
10800 (1.3)a
626
446 561 615
641
451 563 616
820
1.90h
ZnP-ref
1043b
424 440d 555 615 424 451 555 603 412 536 571
577 630
-g
-g
-g
-g
2.16i
e
e
a Estimated by GPC analysis in chloroform on the basis of polystyrene standards. b Calculated from molecular structure. c Measured in toluene solution. d Observed as a shoulder. e Excited at the Soret band. f Spin-coated on glass plates. g Films were not formed. h Determined from the intersection of the absorption and emission spectra of film samples. i Determined from the intersection of the absorption and emission spectra in solution.
Figure 1. UV-visible absorption spectra of (a) PZnPF (solid line), (b) PZnPT (dotted line), and (c) ZnP-ref (dashed line) in toluene. The intensity of the molar absorption coefficient (ε) in ZnP-ref is reduced by 1/5 for comparison.
polymers.27 Further optimization of the conditions for polymerization reaction and chemical structure of polymers may be necessary to obtain polymers with higher molecular weight and crystallinity. Photophysical Properties of Porphyrin Polymers. Figure 1 displays the UV-visible absorption spectra of PZnPF, PZnPT, and ZnP-ref (Chart 1) in toluene. ZnP-ref shows a sharp Soret band at 412 nm and Q-bands at 536 and 571 nm (Table 1), which is a typical absorption characteristic of zinc porphyrins. As expected, in the spectra of PZnPF and PZnPT, narrow-width Soret bands of the porphyrin units are relatively broadened and red-shifted due to the extended conjugation of the π systems through the furan or thiophene bridge. The Soret band of PZnPT is obviously split, yielding the intense absorption peaks at about 424 and 451 nm (Table 1). PZnPF exhibits an intense Soret absorption peak at 424 nm and a shoulder at 440 nm (Table 1). The splitting of the Soret band for the polymers should be attributed to the presence of exciton coupling between the zinc porphyrin units.15,28 Bathochromic shift of absorption edges in PZnPF and PZnPT compared with that of ZnP-ref is evident. These absorption features also support the elongation of the conjugated π-electron system along the porphyrin polymer backbone. The UV-visible absorption spectra of PZnPF and PZnPT thin films are shown in Figure 2 (solid lines). The intermolecular interaction in an amorphous solid state obscures the splitting of the Soret band,15 whereas the absorption spectra become broadened and red-shifted significantly compared with those in toluene (Table 1). More broadened, red-shifted absorption bands of PZnPF than those of PZnPT in Figure 1 reveal the high delocalization of
Figure 2. (A) UV-visible absorption spectra of (a) PZnPF film (solid line) and (b) PZnPF:PCBM blend film (dashed line). (B) UV-vis absorption spectra of (c) PZnPT film (solid line) and (d) PZnPT:PCBM blend film (dashed line). The films were prepared by spin-coating a chlorobenzene solution (60 µL) of the polymer ([PZnPF or PZnPT] ) 10 g L-1) or the polymer:PCBM mixture ([PZnPF or PZnPT] ) 10 g L-1, [PCBM] ) 8 g L-1) on a glass substrate.
the π-electron system along the polymer main chain of PZnPF relative to PZnPT. The smaller steric repulsion between the porphyrin ring and furan due to the small atomic size of oxygen compared to that of sulfur in thiophene promotes coplanarity and extension of π-conjugation length along the polymer main chain. To predict the geometric structure of the polymer main chains, we performed B3LYP calculation with the 3-21G(*) basis set to optimize the structures of ZnP-F-ZnP and ZnP-T-ZnP without the meso-phenyl groups (Figure 3). As expected, the dihedral angles between the heteroles and the porphyrins are ca. 43° for ZnP-F-ZnP and ca. 70° for ZnP-T-ZnP. The smaller dihedral angles of ZnP-F-ZnP are in good agreement with the larger π extension in PZnPF obtained by the absorption measurements (vide supra). In addition, the low resonance energy of furan (22.2 kcal mol-1) compared to that of thiophene (27.7 kcal mol-1) may also contribute to the more red-shifted absorption bands of PZnPF than PZnPT.29
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Figure 3. Molecular structures and optimized molecular geometries of (a) ZnP-F-ZnP and (b) ZnP-T-ZnP calculated at the B3LYP/ 3-21G(*) level. The meso-phenyl groups were omitted for the calculations.
Figure 4. Steady-state fluorescence spectra of (a) PZnPF excited at 424 nm, (b) PZnPT excited at 424 nm, and (c) ZnP-ref excited at 412 nm in toluene. Spectra b and c are normalized to that of spectrum a for comparison.
The steady-state fluorescence spectra of the porphyrin polymers were measured in toluene (Figure 4) and the wavelengths for emission maxima are listed in Table 1. Upon excitation at the Soret band, PZnPF and PZnPT exhibit emissions with single peaks at 713 and 626 nm, respectively. The emissions become broadened and red-shifted in comparison with that of ZnP-ref. It should be noted here that the emission maximum of PZnPF is red-shifted by 87 nm compared with that of PZnPT. The large Stokes shift of PZnPF implies that photoexcitation of PZnPF induces considerable change in the conformation in the excited singlet state to lengthen the effective conjugation length (vide infra). To obtain further insight into the photodynamics of the polymers, time-resolved fluorescence spectra of PZnPF and PZnPT were measured in toluene by using the single-photon counting technique with an excitation wavelength of 405 nm and detection wavelengths of 600-770 nm (Figure 5). The fluorescence spectrum of PZnPF shows a gradual Stokes shift with the emission maximum from 660 nm at 0-100 ps to 700 nm at 700-800 ps (Figure 5A). The energy of the main emission peak decreases by ca. 0.11 eV. On the other hand, no spectral shift is detected for PZnPT in toluene (Figure 5B). The fluorescence time profiles of PZnPF and PZnPT in toluene are also recorded in Figure 6. The profile of PZnPF at 600-650 nm (Figure 6A(a)) reveals biexponential decays with time constants of 130 (71%) and 620 ps (29%). Similar biexponential decays with time constants of 66 (67%) and 750 ps (33%) are observed for PZnPT at 600-680 nm (Figure 6B(c)). These two components may be attributed to different conformers in the excited singlet state. Strikingly, the profile of PZnPF at 680-750 nm (Figure 6A(b)) discloses a rise component of 130 ps (18%) followed by a decay component of 760 ps (82%). The time
Figure 5. Time-resolved emission spectra of (A) PZnPF and (B) PZnPT in toluene excited at 405 nm.
constant of the rise at 680-750 nm matches well with that of the decay at 600-650 nm. This can be rationalized by a plausible dynamical process from the twisted conformer to the planar conformer at the picosecond time region. Namely, the excitation of PZnPF leads to formation of the major unstable conformer in which the porphyrins and furan moieties are rather twisted through the direct linkage. With increasing time at the picosecond time region the conformer undergoes bond rotation to yield the relaxed conformer in which the porphyrins and furan moieties are rather planar through the linkage. Similar dynamics have been implicated in the ultrafast photophysics of oligothiophenes30 and porphyrin oligomers linked by the butadiyne group.17h Consistent with the larger total number of porphyrin units, the dynamics associated with this process for PZnPF (130 ps) and butadiyne-linked porphyrin octamers ((yZnPy)8) (100 ps)17h occurs in a much longer time scale than that reported for hexamethylsexithiophene ((MeTh)6) (4 ps).30 Furthermore, PZnPF shows a notably large shift of the main emission peak (0.11 eV) compared with those of (yZnPy)8 (0.01 eV)17h and (MeTh)6 (0.05 eV).30 These results reflect that PZnPF has a larger difference in the conformation between the ground and excited singlet states in terms of planarity than that between (yZnPy)8 and (MeTh)6. It should also be noted here that such dynamics is absent in the spectra of PZnPT (Figure 5B). The larger steric repulsion between the porphyrin rings and the thiophenes due to the large atomic size of sulfur compared to that of oxygen in furan may inhibit coplanarity even after the photoexcitation. The thin films of the polymers spin-coated on glass plates also show emissions with peaks at 686 nm for PZnPF and 641
Porphyrin-Furan and -Thiophene Alternating Copolymers
Figure 6. Fluorescence time profiles of (A) PZnPF and (B) PZnPT in toluene excited at 405 nm. The fluorescence time profiles were monitored at (a) 600-650, (b) 680-750, and (c) 600-680 nm.
nm for PZnPT (solid lines in Figure 7). Compared with the fluorescence spectrum of PZnPF in toluene, the PZnPF film exhibits blue-shifted emission, presumably due to the suppression of conformational change from the less-conjugated twisted structure to the more-conjugated planar one. The slight redshifted emission of the PZnPT film relative to that of PZnPT in toluene can be ascribed to the intermolecular interaction between the polymers in a solid state. From the intersection of the normalized absorption and emission spectra of the films, the optical bandgap (Egopt) is determined to be 1.75 eV for PZnPF and 1.90 eV for PZnPT (Table 1).31 The Egopt value of ZnP-ref is also estimated to be 2.16 eV from the absorption and emission spectra in toluene solution. The fluorescence lifetimes for the films of PZnPF and PZnPT were measured by the single-photon counting technique at 500-760 nm with excitation at 405 nm. The decays reveal two components with 16 (95%) and 95 ps (5%) for the PZnPF film and 15 (95%) and 40 ps (5%) for the PZnPT film (Figure S4, Supporting Information). The time constants are much shorter than those in toluene, suggesting that the self-quenching of the porphyrin polymers by the interchain interaction is dominant in the films. Blend Films with PCBM. A soluble fullerene derivative, PCBM, is most commonly used as an electron acceptor in bulk heterojunction solar cells.27,32,33 Thus, at first blend films of PCBM with PZnPF and PZnPT were prepared to investigate the photophysical properties. A chlorobenzene solution of the polymer:PCBM (w/w, 5:4) was spin-coated on a glass plate at 1000 rpm. The absorption spectra of PZnPF:PCBM and PZnPT: PCBM films are depicted in Figure 2 (dashed lines). The absorption peaks of the Soret and Q-bands as well as absorption edges in the blend films are slightly red-shifted and broadened compared to those of PZnPF and PZnPT films, respectively
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Figure 7. (A) Steady-state fluorescence spectra of (a) PZnPF film (solid line) and (b) PZnPF:PCBM (1: 0.8, w/w) blend film (dashed line). For the excitation at 450 nm, absorbance of the PZnPF-PCBM blend film was adjusted to be identical with that of PZnPF film. (B) Steady-state fluorescence spectra of (c) PZnPT film (solid line) and (d) PZnPT: PCBM (1: 0.6, w/w) blend film (dashed line). For the excitation at 450 nm, absorbance of the PZnPT:PCBM blend film was adjusted to be identical with that of PZnPT film.
(Table 1). These results suggest that the porphyrin units in the polymer main chain interact with PCBM to form CT complexes in the ground state.13d–f Additionally, the compositions of the polymer and PCBM in the blend films were determined by dissolving each component into chlorobenzene from the spincoated blend films and measuring the absorbances due to the polymer and PCBM: [PZnPF]:[PCBM] ) 1:0.8 and [PZnPT]: [PCBM] ) 1:0.6 (w/w). Considering that PCBM itself does not form a thin film on the glass substrate by the spin-coating method, this result implies higher affinity of PZnPF with PCBM than PZnPT. The less twisted structure and slightly high electron-donating ability of PZnPF in comparison with that of PZnPT may be responsible for the stronger interaction of PZnPF with PCBM (vide infra). When the PZnPF:PCBM and PZnPT:PCBM blend films spincoated on the glass substrates were excited at 450 nm, the fluorescence intensities at 600-750 nm are decreased intensively relative to the PZnPF and PZnPT reference films (Figure 7, dashed lines). More importantly, in the fluorescence spectra of the blend films, weak emission bands due to CT interaction between the porphyrin and the C60 appear at 820 nm,13e–g which is consistent with the observation of CT absorption in the blend films (vide supra). To the best of our knowledge, this is the first observation on CT emission between conjugated porphyrin polymers and fullerenes. These results indicate the occurrence of photoinduced CT from the polymers to PCBM and subsequent formation of the CT state (i.e., exciplex) between the polymers and PCBM.34 To shed light on the photodynamics of the blend films, the fluorescence lifetimes of PZnPF:PCBM and PZnPT:PCBM films were measured by the single-photon counting technique at 500-770 nm with excitation at 405 nm (Figure S4, Supporting Information). The fluorescence decay curves of both blend films could be fitted as two components
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Umeyama et al. TABLE 2: Electrochemical Properties PZnPF PZnPT ZnP-ref
HOMO/eV
LUMO/eV
Eg/eV
-5.5 -5.6a -5.5b
-3.9 -3.9a -3.2b
1.6 1.7 2.3
a
a
a Determined from the onset of the first oxidation or reduction potential in films by using cyclic voltammetry. b Determined from the first oxidation or reduction peak potential in dichloromethane by using cyclic voltammetry.
TABLE 3: Photovoltaic Performance of the Cells
Figure 8. Cyclic voltammograms of the (A) PZnPF and (B) PZnPT measured in acetonitrile containing 0.1 M n-Bu4N+PF6- as a supporting electrolyte. ITO working electrodes coated with polymer films, Ag/ AgNO3 (0.01 M in acetonitrile) reference electrode, and Pt wire counter electrode were employed. Decamethylferrocene/decamethylferrocenium (+0.15 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 52 (88%) and 335 ps (12%) for the PZnPF:PCBM film and of 69 (75%) and 406 ps (25%) for the PZnPT:PCBM film. However, the fluorescence intensities of the polymer:PCBM films are much smaller than those of the respective reference films, suggesting ultrafast, efficient formation of the CT states consisting of the polymers and PCBM in the blend films. Indeed, ultrafast formation of the CT state (
