Reducing Regioisomers of Fullerene-Bisadducts by Tether-Directed
Remote Functionalization: Investigation of Electronically and
Sterically Isomeric E
ffects on Bulk-Heterojunction Solar Cells
Ming-Hung Liao, Yin-Yu Lai, Yu-Ying Lai, Yen-Ting Chen, Che-En Tsai, Wei-Wei Liang,
and Yen-Ju Cheng*
Department of Applied Chemistry, National Chiao Tung University 1001 Ta Hsueh Road, Hsin-Chu, 30010 Taiwan
*
S Supporting InformationABSTRACT: C60 bis-adduct containing a mixture of
regio-isomers with different LUMO energy levels and steric
geometries could greatly affect the morphological and bulk properties. To investigate the regio-isomer effect on solar cell performance, we have successfully designed and synthesized a regio-selective 4-acetatephenyl-4′-methylphenylmethano C60 bis-adduct (S-APM-CBA) by“tether-directed remote function-alization” strategy and a random
4-acetatephenyl-4′-methyl-phenylmethano C60 bis-adduct denoted as R-APM-CBA by
traditional cyclopropanation. The dramatic reduction in the
number of regio-isomers in S-APM-CBA is confirmed by the
1H NMR and HPLC measurements and theoretical
calcu-lation. Compared to the R-APM-CBA-based device with a Jscof 6.63 mA/cm2, an FF of 44.3% and a PCE of 2.46%, the device using S-APM-CBA yielded a much lower Jscof 1.48 mA/cm2, an FF of 32.2%, and a PCE of 0.38%. Consistently, the electron-only device using S-AMP-CBA exhibited lower electron mobility than the R-AMP-CBA-based device. These results imply that the electronic shallow-trap effect ascribed to the LUMO energy variations turned out to be insignificant in the AMP-CBA system. The lower efficiency and mobility of AMP-CBA might due to the assumption that the most probable trans-4-III isomer in S-AMP-CBA prevents the intermolecular facial contact of fullerenes, thereby hindering the electron transporting. Furthermore, the nanomorphology of S-AMP-CBA and R-AMP-CBA active layers could be different because of their different three-dimensional structures. This research demonstrated that steric effect of regio-isomers in a given C60bis-adduct is more crucial than electronic shallow-trap effect.
KEYWORDS: regioisomers, bis-adducts, tether, bulk-heterojunction, solar cells, shallow-trap effect
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INTRODUCTIONResearch on polymer solar cells (PSCs) has attracted tremendous scientific and industrial interest and attention in recent years.1−9The power conversion efficiency (PCE) of a photovoltaic device is determined by open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF).
Increasing Voc is one of the approaches to improve PCEs.
The magnitude of Voc value is generally proportional to the
energy difference between the highest occupied molecular
orbital (HOMO) energy level of the donor and the lowest unoccupied molecular orbital (LUMO) level of the accept-or.10−12 Therefore, either raising the LUMO level of the fullerene material or lowering the HOMO level of the polymer can obtain higher Voc. A straightforward strategy is to design new fullerene-based materials that possess intrinsically high-lying LUMO energy levels. Introduction of electron-donating groups on the addend of the fullerenes has been attempted to modulate the LUMO energy levels.13−16However, the changes in LUMO energy level of fullerene derivatives are too small to affect the Voc because of the fact that the through-space
interaction without directπ-conjugation is negligible. Develop-ment of bis-adduct C60materials has become a more practical approach.17−32 The double functionalization on the C60 core structure to reduce the electron delocalization in the C60makes bis-adduct C60derivatives have larger electrochemical reduction potentials and thus higher-lying LUMO levels. For example, the bisPC61BM with 56π-electrons exhibits higher LUMO level by ca. 0.1 eV than the PC61BM with 58π-electrons. Consequently, the Vocof the P3HT:bisPC61BM-based device improves to 0.73 V compared to the P3HT:PC61BM-based device with Vocof 0.6 V. Based on the strategy, many fullerene bis-adduct derivatives, such as bisPC61BM, IC60BA, IC70BA, and DMPCBA, have been developed and applied in the PSCs.17,18,20,28
Despite that saturating fullerene is beneficial for obtaining high Voc, double functionalization on the core of fullerene in a random fashion inevitably generates a mixture of regio-isomers
Received: October 11, 2013 Accepted: December 17, 2013 Published: December 17, 2013
that are difficult to isolate by typical chromatography. The synthesis of mixtures of fullerene bis-adduct suffers from batch-to-batch reproducibility, because the isomer ratio and distribution are highly dependent on synthetic conditions such as temperature, reaction time, and concentration of the reactants.
For two symmetrical A2C-addends (two A substitutents on the carbon of cyclopropane shown in Figure 1a) functionalized on C60 through cyclopropanation reaction, there are eight possible regio-isomers with different substituent locations.33
When the two A2C-addends are located on the same
hemisphere, there are three possible isomers defined as cis-1, cis-2, and cis-3. Similarly, if the two A2C-addends are attached on opposite hemispheres, four possible isomers, denoted as trans-1, trans-2, trans-3, and trans-4, are generated. When one A2C-addend is functionalized on the equator of the fullerene sphere, the only one possible isomer is defined as E. If addend is unsymmetrical (substitutents are A and B), more possibilities
are generated; for instance, 4 different stereoisomers are possible in the trans-4 configuration (4-I, 4-II, trans-4-III, trans-4-IV in Figure 1b). Overall, there will be at most 27 probable regio-isomers and stereoisomers (see the Supporting Information for 27 detailed structures). Because the structures of C60bis-adduct are not well-defined, the structural change of isomers could lead to different steric and electronic properties, which in turn affects device performance.
First, the different isomeric structure of bis-adducts may alter the molecular packing and thereby the resultant morphology.34 Moreover, each regio-isomer is expected to possess different reduction potentials. The existence of variations in the LUMO energy levels of isomers may cause energetic traps (i.e., shallow traps) which are supposed to have negative impact on Jsc and FF. Although the shallow-trap effect has been speculated to be detrimental, the in-depth and detailed studies are still limited.35−39 To investigate whether the shallow-trap effect, resulting from a bunch of regioisomers, is important in affecting Figure 1.(a) Eight possible regio-isomers of a C60adduct substituted with a symmetrical addend, (b) four possible stereoisomers of trans-4
device performance or not, a straightforward method is to reduce the number of regioisomers of a given C60-bisadduct by chemical control. However, it is synthetically very challenging. To regulate specific regio-isomers within C60 bis-adduct,
Diederich et al. introduced a “tether-directed remote
functionalization” (TDRF) based on the idea that two addend groups are covalently attached on a tether moiety.40−46 After the tether-induced regio-selective reaction with C60, the tether
group can be chemically removed to resume the specific
fullerene bis-adduct. Herein, we attempt to investigate the isomeric effect of a new C60bis-adduct, APM-CBA, where two (4-acetatephenyl)-4-methylphenyl methano (APM) moieties are functionalized on a C60 through cyclopropanation (Figure 1c). We deliberately synthesized APM-CBA by means of two independent approaches. Random APM-CBA (R-APM-CBA) was prepared by the standard cyclopropanation, whereas regio-selective APM-CBA (S-APM-CBA) was synthesized by the TDFR strategy. The difference in numbers of isomers between R-APM-CBA and S-APM-CBA provides a model to investigate the isomeric (i.e., shallow-trap and steric) effect in bulk heterojunction solar cells.
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RESULTS AND DISCUSSIONThe molecular design is shown in Figure 2. Two diphenylmethano-based anchoring groups are linked with a
tether group. The tosylhydrazone functional groups are used to undergo cyclopropanation reaction with fullerene. The length and rigidity of a tether moiety play an important role in governing regio-selectivity of the two anchoring groups on C60. By careful manipulation, the two anchoring groups can be located in the same hemisphere (cis form) or opposite hemisphere (trans form) of C60. 1,6-Hexylene and 1,4-xylyl moieties are selected as a soft tether and an rigid tether, respectively, to connect two 4-methylbenzophenone tosylhy-drazone addends together (Scheme 1 and Figure 2) for preliminary study.
Synthesis. The synthesis of S-APM-CBA is shown in
Scheme 1. Friedel−Craft acylation of 4-methoxy benzoyl
chloride with toluene afforded
4-methoxy-4′-methylbenzophe-none (1).47 Demethylation of compound 1 by boron
tribromide led to 4-hydroxy-4′-methylbenzophenone (2).
Under basic conditions, compound 2 was reacted with a tether molecule, 1,4-bis(bromomethyl) benzene or 1,6-dibromohex-ane, to give 3a or 3b, respectively. Condensation of compound
3a and 3b with p-toluenesulfonhydrazide furnished
bis-tosylhydrazone 4a and 4b in good yield, respectively. In the
presence of sodium hydride at 130 °C, cyclopropanation of
tosylhydrazone groups in compound 4a and 4b with C60led to the formation of tether-directed C60bis-adduct intermediate 5.
Dealkylation of compound 5a by BBr3 successfully yielded
compound 6 with two hydroxy groups that were allowed to react with acetyl chloride to yield the desired S-APM-CBA.
On the other hand, the synthesis of R-APM-CBA is shown in
Scheme 2. Compound 7 reacted with C60 to yield C60
bis-adduct 8. In a similar manner, demethylation of compound 8 followed by acylation generated thefinal R-APM-CBA.
Proton NMR and HPLC Measurements. Wefirst studied
the effect of the different tether linkers on the regio-selectivity by measuring NMR spectra of compound 5a and 5b shown in a and b in Figure S1 in the Supporting Information, respectively. Compared to 5b, 5a with p-xylene as the tether exhibited less complicated peaks in the NMR spectrum, implying that the rigid 1,4-xylyl tether results in less regio-isomers than the soft n-hexyl tether. As a result of the better regio-selectivity, we selected only compound 5a for the following synthetic steps to
make thefinal S-APM-CBA. The NMR spectra of S-APM-CBA
and R-APM-CBA were measured to evaluate the effectiveness
of tether-directed approach, as shown in panels a and b in Figure 3, respectively. Again, R-APM-CBA showed very complicated peaks from 2.0 to 2.6 ppm, which ascribed to the methyl groups of toluene as well as the methyl groups of ester. In contrast, S-APM-CBA showed much simpler and clearer peaks. Similar phenomena were observed in the aromatic region from 7 to 8.5 ppm, indicating that S-APM-CBA synthesized by the TDRF approach indeed contains many fewer regio-isomers than R-APM-CBA made by the typical method.
The stereochemistry of two 4-acetatephenyl-4 ′-methylphe-nylmethano groups in S-APM-CBA is determined by its precursor 5a. S-APM-CBA should inherit the stereochemistry of 5a and the number of stereoisomers should be identical for S-APM-CBA and 5a. 5a was then used to estimate the possible isomers of S-APM-CBA. With the help of a molecular-structure visualization program-GaussView 5.0, we found that 5a has at most 7 stereoisomers on account of the fact that the presence of the 1,4-xylyl tether forbids sterically the formation of the other isomers. The optimized structures of the 7 isomers are depicted in Figure 4, which includes 2-I, 2-III, 3-I, cis-3-II, E-II, E-III, and trans-4-III. Roman numerals are used to symbolize the stereoisomers resulting from the same class of regio-isomer illustrated in Figure 1 (also see the Supporting Information, Figure S1, for nomenclature). Since the interconversion between these isomers is nearly impossible, the distribution of isomers for 5a should be govern primarily by the activation barrier for the reaction from 4a to 5a among isomers. For this reaction, we reason that 4a wouldfirst react with C60to form a monoadduct with exclusive stereochemistry. Subsequently, the monoadduct would undergo another cyclo-addition to give 5a. The second cyclocyclo-addition is expected to be the rate-determining step for this reaction and therefore the
energy difference in the transition state for the second
cycloaddition among the isomers would determine the relative product distribution. Furthermore, it can be envisaged that the energy difference between either the transition states or the resultant isomers would highly correlate with the strain resulting from the cyclic tether-addend. In light of this assumption, the energy difference between the isomers could Figure 2.Rigid and soft tether moieties in this research for
be used to roughly approximate the energy difference between the transition states and thus the distribution of isomers. Theoretical calculations on the PM3 level of theory were employed to calculate the electronic energy and Gibbs free energy (25°C in kcal mol−1) for the 7 isomers. Trans-4-III has
the lowest energy which is set as zero for calibration. The electronic energy and Gibbs free energy relative to trans-4-III for the rest of 5a isomers (i.e.,ΔE and ΔG) are shown in Table 1. It was found that there is no significant difference between the electronic energy and Gibbs free energy. We therefore Scheme 1. Synthesis of S-APM-CBA by the Tether-Directed Remote Functionalization
choose the electronic energy for the following discussion. Comparison of the optimized geometries with the correspond-ing electronic energies revealed that an isomer with highly strained conformation also possesses high energy. The energy
of an isomer appears to be determined by the strain of the cyclic addend. The electronic energy of the isomers follows the order: trans-4-III < cis-2-I < cis-2-III < cis-3-II < E-III < E-II < cis-3-I. On the basis of the calculated energies, trans-4-III, cis-2-Figure 3.1H NMR spectrum of (a) S-APM-CBA and (b) R-APM-CBA.
I, and cis-2-III are the more likely isomers for 5a. The computational results support that the tether-directed remote functionalization can significantly reduce the number of regio-isomers.
High-performance liquid chromatography (HPLC) was also employed to estimate the regio-isomers of bis-adduct C60. The HPLC chromatogram of R-APM-CBA showed very broad distribution containing at least 12 distinct peaks ranging from 20 to 60 min retention time (Figure 5). To roughly identify the
composition of the isomers in the normal phase of the HPLC, it is expected that the short-time signals belong to the trans-based isomers with lower polarity, whereas the long-time signals are assigned to the cis-based isomers with higher polarity. In contrast, the S-APM-CBA chromatogram showed only two major signals peaked at 38 and 47 min without observation of signals around shorter time (22−33 min) and longer time (50−60 min). These results are fairly consistent with the theoretical calculations suggesting that trans-4-III, cis-2-III, cis-3-I, and cis-3-II are the most probable isomers whose polarities are relatively moderate among the full isomers. HPLC data again indicated that a bis-adduct C60 with significantly reduced regio-isomers can be achieved by applying the tether-directed approach.
Optical Properties. The optical absorption of
R-APM-CBA, S-APM-CBA and PC61BM was characterized in THF
with a concentration of 1× 10−5M (Figure 6). The absorption
profiles of R-APM-CBA and S-APM-CBA without shoulders
around 260 and 330 nm are dissimilar to that of PC61BM.
Interestingly, the absorption intensity of R-APM-CBA is much higher than that of S-APM-CBA under the same concentration, indicating that different regio-isomers can possess very distinct absorption coefficients.34,42,48
Electrochemical Properties. The electrochemical proper-ties of the R-APM-CBA and S-APM-CBA were investigated by cyclic voltammetry (CV) in Figure 7. The cyclic voltammetry showed three well-defined and reversible redox waves in the potential ranging from 0 to−2.5 V. In the negative potential
region, R-APM-CBA and S-APM-CBA showed similar reduction potentials which shift more negatively by 0.15 eV
compared to PC61BM. The LUMO energy levels of
S-APM-CBA, R-APM-S-APM-CBA, and PC61BM are estimated to be−3.77,
−3.76, and −3.91 eV, respectively. The higher LUMO energy level of the bis-adducts would be advantageous for improving Voc. Notably, the energetic difference of 0.01 eV between S-APM-CBA and R-S-APM-CBA might cause the shallow-trap effect. Besides, we found that the CV profile of R-APM-CBA is slightly broader than that of S-APM-CBA, which is consistent with the fact that R-APM-CBA contains more regio-isomers with small variations of energetic energy levels.30
Photovoltaic Characteristics. Bulk heterojunction solar
cells with configuration (ITO/PEDOT:PSS/P3HT:fullerene
derivatives/Ca/Al) were fabricated and characterized under
simulated 100 mW cm−2 AM 1.5 G illumination (Figure 8).
The device characteristics with the optimal blending ratio are shown in Table 3. The device using P3HT:R-APM-CBA blend (1:1 in wt %) showed a Vocof 0.84 V, FF of 44.3%, and Jscof
Table 1. Electronic Energy Difference (ΔE) and Gibbs Free
Energy Difference (25 °C) (ΔG) in kcal mol−1Relative to trans-4-III for the 5a Isomers
trans-4-III cis-2-I cis-2-III cis-3-I cis-3-II E-II E-III
ΔE 0 1.85 4.02 16.09 7.29 12.74 9.33
ΔG 0 1.30 4.50 17.90 7.40 12.50 11.40
Figure 5. High-performance liquid chromatography (HPLC) chromatogram of R-APM-CBA and S-APM-CBA using toluene and ethyl acetate (4:1, v/v) as eluents at aflow rate of 1 mL/s.
Figure 6. Absorption spectra of S-APM-CBA, R-APM-CBA and PC61BM in THF solution under the concentration of 1× 10−5 M. Inset: Enlarged absorption spectra from 400 to 800 nm.
Figure 7. Cyclic voltammetry of R-APM-CBA, S-APM-CBA, and PC61BM at a scan rate of 100 mV/S.
Table 2. Onset Reduction Potentials and LUMO Energy Levels of Fullerene Derivatives
fullerene derivatives E1a(V) E2(V) E3(V) LUMO (eV)
S-APM-CBA −0.92 −1.33 −1.83 −3.77
R-APM-CBA −0.94 −1.33 −1.85 −3.76
PC61BM −0.81 −1.22 −1.73 −3.91
6.63 mA/cm2, resulting in a PCE of 2.46%. Surprisingly, under similar conditions, the device with S-APM-CBA containing less regioisomers turned out to exhibit a much lower PCE of 0.38% with a Vocof 0.80 V, an FF of 32.16%, and a Jscof 1.48 mA/cm2. Considering that the only difference between R-APM-CBA and S-APM-CBA is the number of isomers, the inferior device performance of S-APM-CBA implies that the electronic
shallow-trap effect in R-APM-CBA is not pronounced. The
discrepancy is thus ascribed to the steric effect of the regio-isomers. The different structural geometry of the regio-isomers causes different intermolecular interaction and morphology of the composite. We speculated that the trans-4-III isomer where the two addends locate on different hemispheres of C60 might sterically suppress the facial contact between the fullerene cages.36Therefore, the electron transportation in R-APM-CBA might be significantly hindered within the fullerene phase. On the basis of the speculation, the electron-only devices (ITO/ ZnO/P3HT:bisadducts (1:1 in wt%)/Al) were fabricated to study the charge transporting capability of S-APM-CBA and R-APM-CBA-based blends (Figure 9). The R-R-APM-CBA-based device yielded a mobility of 8.01× 10−6cm2V−1s−1, which is higher than the corresponding S-APM-CBA-based device by 4-folds (1.8× 10−6cm2V−1s−1). The smaller electron mobility is responsible for the inferior solar cell performance of S-APM-CBA. As a result, the shallow-trap effect is insignificant in the BHJ solar cells in comparison of the steric effect.
Morphological Study. The morphology of the P3HT:R-APM-CBA and P3HT:S-P3HT:R-APM-CBA (1:1 in wt%) blends are studied by atomic force microscope (AFM) and showed in the Figure 10. The surface roughness of P3HT:R-APM-CBA and
P3HT:S-APM-CBA thin films are 14.7 and 12.6 nm,
respectively. The difference in roughness is probably due to the different composition of the regioisomers of APM-CBA.
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CONCLUSIONSFullerene bis-adducts with intrinsically high-lying LUMO energy levels are superior N-type materials for PSCs to
improve open-circuit voltage. However, double functionaliza-tion reacfunctionaliza-tion of fullerene with addend moieties inevitably creates a mixture of regioisomers. The fullerene bis-adduct
isomers having different LUMO energy levels and steric
geometries could greatly influence molecular and
morpho-logical properties in PSCs. To investigate the isomer effect of a
C60 bisadduct, random (4-acetatephenyl)-4-methylphenyl
methano C60 bis-adduct (R-APM-CBA) was synthesized by
the traditional reaction, whereas regio-selective S-APM-CBA was achieved by a “tether-directed remote functionalization”
strategy. The significant reduction in the number of
regioisomers in S-APM-CBA is manifested by the 1H NMR,
HPLC measurements and theoretical calculation. Surprisingly, the PSC using S-APM-CBA turned out to yield much lower Jsc
of 1.48 mA/cm2and FF of 32.16% compared to the
R-APM-CBA-based device with Jscof 6.63 mA/cm2and FF of 44.3%. Consistently, the electron-only device of S-AMP-CBA showed much lower electron mobility than that of R-AMP-CBA by 4-folds. These results imply that the electronic shallow traps as a result of the LUMO energy variations are insignificant in the AMP-CBA system. The structural geometry of trans-4-III, the most probable isomer in S-AMP-CBA, might prevent the intermolecular facial contact, hindering the electron trans-porting. This research demonstrated that the steric effect of regioisomers in a given C60bis-adduct is more crucial than the electronic shallow-trap effect. Reducing isomers of C60 bis-adduct can be achievable by TDRF; however, targeting a specific bis-adduct isomer with an optimal structural geometry for superior charge-transporting property remains a challenging task. It is envisioned that this model study will pave the way for further research on controlling the properties of the bis-adduct fullerene materials, which plays a key role in making breakthrough of organic solar cells.
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EXPERIMENTAL SECTIONSynthesis of Compound 1. A solution of aluminum chloride (2 g, 15 mmol) in dry toluene was added slowly p-methoxybenzoyl chloride (2 g, 10 mmol) at 0°C. The mixture was stirred at room temperature for 10 h. The reaction was quenched by pouring the mixture into ice/ water and then extracted with ethyl acetate/H2O. The crude product
was purified by silica column to give the product (hexane:ethyl acetate = 5:1, v/v, 4.2 g, 93%).1H NMR (300 MHz, CDCl
3)δ = 2.44 (s, 3H),
3.89 (s, 3H), 6.96 (d, J = 9.0 Hz, 2H), 7.27 (d, J = 7.8 Hz, 2H), 7.68 (d, J = 7.8 Hz, 2H), 7.81 (d, J = 9.0 Hz, 2H).
Figure 8. J−V curves of the PSCs based on (ITO/PEDOT:PSS/ P3HT:S-APM-CBA or R-APM-CBA (1: 1 wt %)/Ca/Al) under illumination of AM 1.5G (100 mW/cm2).
Table 3. Device Characteristics of (ITO/PEDOT:PSS/ P3HT:S-APM-CBA or P3HT:R-APM-CBA (1:1 wt %)/Ca/ Al)
acceptor Voc(V) Jsc(mA/cm2) FF (%) PCE (%)
S-APM-CBA 0.80 1.48 32.16 0.38
R-APM-CBA 0.84 6.63 44.30 2.46
Figure 9.J−V curves of the electron-only devices based on the ITO/ ZnO/P3HT:S-APM-CBA or P3HT:R-APM-CBA (1: 1 wt %)/Al configuration.
Synthesis of Compound 2. A solution of compound 1 (10 g, 44 mmol) in dry dichloromethane was slowly added boron tribromide (1M, 44.2 mL, 44.2 mmol) at 0°C. The mixture was stirred at room temperature for 24 h. The resulting solution was poured into ice/water and then extracted with CH2Cl2/H2O. The organic phase was dried by
anhydrous MgSO4, and then concentrated in vacuo. The crude
product was purified by silica column to give the product (hexane:ethyl acetate = 1: 1, v/v, 8.4 g, 90%).1H NMR (300 MHz,
CDCl3)δ = 2.43 (s, 3H), 6.63 (s, 1H), 6.92 (d, J = 8.4 Hz, 2H), 7.27
(d, J = 8.1 Hz, 2H), 7.68 (d, J = 8.1 Hz, 2H), 7.76 (d, J = 8.4 Hz, 2H). EI-MS (m/z): 212.
Synthesis of Compound 3a. A mixture of compound 2 (1 g, 4.7 mmol), 1,4-bis(bromomethyl)benzene (0.62 g, 2.36 mmol), and NaH (0.34 g, 14 mmol) was dissolved in anhydrous DMF (20 mL) and stirred under a N2 atmosphere at room temperature for 24 h. The
resulting mixture was extracted with CH2Cl2/H2O. The organic layer
was dried by anhydrous MgSO4 and concentrated in vacuo. The
residue was washed by hexane for several times to afford a white solid (0.99 g, 40%).1H NMR (300 MHz, CDCl
3)δ = 2.44 (s, 6H), 5.17 (s,
4H), 7.03 (d, J = 8.7 Hz, 4H), 7.28 (d, J = 7.8 Hz, 4H), 7.49 (s, 4H), 7.68 (d, J = 8.1 Hz, 4H), 7.81 (d, J = 9 Hz, 4H). FAB-MS (m/z): 526. Synthesis of Compound 3b. In a similar manner for compound 3a, compound 3b was obtained in a 48% yield.1H NMR (300 MHz, CDCl3)δ = 1.55−1.60 (m, 4H), 1.85−1.89 (m, 4H), 2.43 (s, 6H),
4.06 (t, J = 6.5 Hz, 6H), 6.94 (d, J = 9 Hz, 4H), 7.27 (d, J = 7.2 Hz, 4H), 7.67 (d, J = 8.1 Hz, 4H), 7.80 (d, J = 8.7 Hz, 4H).
Synthesis of compound 4a. A mixture of compound 3a (3 g, 5.7 mmol) and tosylhydrazide (2.34 g, 0.125 mmol) was dissolved in toluene (100 mL). The solution was refluxed for 12 h. The mixture was concentrated in vacuo and purified by column chromatography on silica gel (hexane: EA = 1:1, v/v). The residue was dissolved in THF (10 mL) and precipitated into hexane (150 mL). The solid compound wasfiltered off and dried by vacuum (3.98 g, 81%).1H NMR (300 MHz, CDCl3)δ = 2.31−2.43 (m, 12H), 5.05−5.16 (m, 4H), 6.84−
7.86 (m, 30H). FAB-MS (m/z): 864.
Synthesis of Compound 4b. In a similar manner, compound 4b was obtained in a yield of 70%.1H NMR (300 MHz, CDCl
3)δ = 1.50−
1.58 (m, 4H), 1.79−1.85 (m, 4H), 2.31−2.42 (m, 12H), 3.92−4.07 (m, 4H), 6.76−7.86 (m, 26H). FAB-MS (m/z): 844.
Synthesis of Compound 5a. Compound 4a (0.36 g, 0.42 mmol), C60(0.3 g, 0.42 mmol), and NaH (0.06g, 2.5 mmol) were dissolved in
dry toluene (500 mL) under nitrogen. The resulting mixture was refluxed and stirred for 12 h. The mixture was concentrated in vacuo and then purified by column chromatography (SiO2, toluene: hexane =
1: 1, v/v). The solid residue was dissolved in the CS2 and then
precipitated into methanol. The solid wasfiltered off and then washed by methanol for several times. The black compound was dried in vacuum (25 mg, 5%).1H NMR (300 MHz, CDCl3)δ = 2.11−2.42 (m,
6H), 4.95−5.45 (m, 4H), 6.55−7.99 (m, 28H).FAB-MS (m/z): 1215. Synthesis of Compound 5b. In a similar manner for compound 5a, compound 5b was obtained in a yield of 13%.1H NMR (300 MHz,
CDCl3)δ = 1.25−1.62 (m, 8H), 2.32−2.41 (m, 4H), 3.99−4.16 (m,
4H), 6.71−7.99 (m, 16H). FAB-MS (m/z): 1196.
Synthesis of Compound 6. To a solution of compound 5a (100 mg, 0.08 mmol) in dry dichloromethane was slowly added boron
tribromide (1M, 0.32 mL, 0.32 mmol) at 0 °C. The mixture was stirred at room temperature for 6 h. A further quantity of boron tribromide (0.32 mL, 0.32 mmol) was added and the mixture was stirred for extra 12 h. The resulting solution was poured into ice/water and then extracted with CH2Cl2/H2O. The organic phase was dried by
anhydrous MgSO4, and then concentrated in vacuo. The crude
product was purified by silica column to give the product (toluene: ethyl acetate =1: 2, v/v, 78 mg, 85%).1H NMR (300 MHz, CDCl
3)δ
= 2.30−2.44 (m, 6H), 5.07 (s, 2H), 6.76−7.91 (m, 16H). FAB-MS (m/z): 1113.
Synthesis of S-APM-CBA. To a solution of compound 6 (80 mg, 0.072 mmol) in the dry dichloromethane with three drops Et3N was
added acetyl chloride (16.8 mg, 0.216 mmol) at 0°C. The resulting mixture stirred at room temperature for 12 h. The resulting solution extracted with CH2Cl2/H2O and the organic layer was concentrated in
vacuo. The crude product was purified by silica column to give the compound (toluene: ethyl acetate = 10: 1, v/v, 77 mg, 90%).1H NMR
(400 MHz, CDCl3)δ = 2.26−2.40 (m, 12H), 7.08−7.28 (m, 8H), 7.81−8.06 (m, 8H).13C NMR (100 MHz, CDCl 3)δ = 21.2, 21.3, 25.6, 29.7, 54.8, 68.0, 130.7, 131.0, 131.2, 131.3, 131.5, 131.8, 135.0, 135.7, 136.1, 136.2, 137.0, 137.1, 137.6, 137.8, 137.9, 138.0, 139.7, 140.8, 141.3, 141.9, 142.1, 142.2, 142.7, 143.0, 143.8, 144.6, 144.7, 144.9, 145.1, 145.3, 145.4, 145.6, 145.7, 146.2, 146.6, 146.9, 147.8, 147.9, 150.0, 150.1, 169.1, 169.2. FAB-MS (m/z): 1197.
Synthesis of Compound 7. In a similar manner for compound 4a, compound 7 was obtained in a yield of 75%.1H NMR (300 MHz, CDCl3)δ = 2.32 and 2.42 (s, 6H), 3.78 and 3.87 (s, 3H), 6.79 (d, J =
6.6 Hz, 2H), 6.96−7.09 (m, 3H), 7.25−7.53 (m, 6H), 7.83 (d, J = 6.3 Hz, 2H). FAB-MS (m/z): 395.
Synthesis of Compound 8. In a similar manner for compound 5a, compound 8 was obtained in a yield of 35%.1H NMR (300 MHz,
CDCl3)δ = 2.17−2.46 (m, 6H), 3.68−3.90 (m, 6H), 6.61−7.32 (m,
10H), 7.37−8.14 (m, 6H). FAB-MS (m/z): 1140.
Synthesis of Compound 9. In a similar manner for compound 6, compound 9 was obtained in a yield of 93%.1H NMR (300 MHz,
CDCl3)δ = 2.04−2.46 (m, 6H), 3.75−5.08 (m, 2H), 6.76−8.20 (m,
16H). FAB-MS (m/z): 1113.
Synthesis of R-APM-CBA. In a similar manner for S-APM-CBA, R-APM-CBA was obtained in a yield of 87%.1H NMR (400 MHz, CDCl3)δ = 2.08−2.47 (m, 12H), 7.10−8.31 (m, 16H).13C NMR (100 MHz, CDCl3)δ = 21.3, 27.6, 30.0, 51.7, 54.5, 54.8, 56.3, 121.7, 126.0, 127.7, 128.1, 129.5, 129.9, 130.2, 130.7, 131.0, 131.3, 131.8, 132.1, 134.1, 135.0, 135.6, 136.2, 136.5, 137.1, 137.8, 138.7, 139.7, 140.0, 140.6, 140.9, 141.5, 141.8, 142.2, 142.5, 143.0, 143.5, 144.3, 145.3, 145.8, 146.2, 146.5, 147.1, 147.9, 148.8, 149.4, 150.0, 151.0, 169.1. FAB-MS (m/z): 1197.
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ASSOCIATED CONTENT*
S Supporting InformationDevice fabrication and characterization, NMR spectra, all the possible structures of stereoisomers for a C60 bis-adduct, theoretical calculations, hole-only devices, and open circuit light intensity measurement. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATIONCorresponding Author
*E-mail: yjcheng@mail.nctu.edu.tw.
Notes
The authors declare no competingfinancial interest.
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ACKNOWLEDGMENTSWe thank the National Science Council and the Ministry of Education, and Center for Interdisciplinary Science (CIS) of
the National Chiao Tung University, Taiwan, for financial
support. We thank the National Center of High-Performance Computing (NCHC) in Taiwan for computer time and facilities.
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