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Establishment of a linear correlation between the LUMO levels of fullerenes and the Hammett constants of substituents installed: An experimental and theoretical study
G Model
ARTICLE IN PRESS
SYNMET-14695; No. of Pages 4
Synthetic Metals xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Letter to the Editor Establishment of a linear correlation between the LUMO levels of fullerenes and the Hammett constants of substituents installed: An experimental and theoretical study
a r t i c l e
i n f o
Keywords: Fullerene LUMO level Electron acceptor Substituent effect
a b s t r a c t The LUMO levels of series fulleropyrrolidines equipped with electron-withdrawing and electron-donating groups were determined by electrochemical study and DFT calculation. A linear correlation between the LUMO levels of fullerenes and the Hammett constants of substituents installed was established for the first time. This may allow to predicting and finely tuning the LUMO level of fullerene electron-acceptor. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The excellent electron affinity of fullerene has made it good electron-acceptor material for the bulk heterojunction (BHJ) polymer solar cells [1–5]. Based on a tandem concept, the power conversion efficiency (PCE) of polymer solar cells has recently been broken 10% record [6–8], making it more promising for a practical commercial production. Among the developed polymer solar cells, poly-(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 -butyric acid methyl ester (PCBM) are the most popular polymer electron-donors and fullerene electron-acceptors, respectively [1–5]. It has been established that the difference between the lowest unoccupied molecular orbital (LUMO) energy level of the acceptor and the highest occupied molecular orbital (HOMO) energy level of the donor, governs the performance such as the open circuit voltage (Voc ) value of polymer solar cells [9,10]. Thus, to improve the device performance, the design of more efficient donor and acceptor materials to make the HOMO energy level of the donor lower and the LUMO energy level of the acceptor higher is desirable. In this regard, much effort has been devoted to tuning the HOMO energy levels of polymer electron-donors [11–15]. On the other hand, the fine-tuning of the LUMO level of fullerene electron-acceptor is crucial to improve the performance of polymer solar cells. Recently, raising the LUMO energy level of the fullerene electron-acceptor is also attracted much current interest [16–20]. Several concepts have been introduced to raise the LUMO energy level of fullerenes [21], for example, the installation of organic electron-donating groups onto fullerene [22], the introduction of multi-substitution to decrease of the size of the fullerene -electron-conjugated system [23–25], and the placement of ortho-substitution spatial proximity to the fullerene core [26,27]. In this letter, we reported a series of five fulleropyrrolidine derivatives (1–5, Scheme 1) varying only in the electronwithdrawing group (EWG) and electron-donating group (EDG) of phenyl ring attached on the periphery of fullerene core. We investigated the effects of substituent groups added to fullerene on
its LUMO energy level by the cyclic voltammetry (CV) electrochemistry and density functional theory (DFT) calculation. The LUMO energy levels of fulleropyrrolidine derivatives were raised 0.06–0.12 eV by varying the substituent from EWG to EDG in comparison with that of the parent fullerene C60 , in which a linear correlation between the LUMO energy levels and the Hammett constants of substituents was established for the first time. The LUMO energy levels of these fulleropyrrolidines calculated by the DFT calculation are also linearly correlated well with the experimental values. These findings may allow to predicting and finely tuning the LUMO energy level of fullerene electron-acceptor and provide fundamental insights in improving the performance of polymer solar cells. 2. Methods and materials Cyclic voltammetry (CV) curves were measured using a CHI 660D electrochemical workstation (Chenhua, Shanghai). The CV curves were collected at room temperature under N2 using a conventional three-electrode system (glassy carbon working electrode, Pt wire counter-electrode and Ag/Ag+ quasi-reference electrode) in 0.1 M tetrabutylammonium tetrafluoroborate (n-Bu4 BF4 ) solution in 1,2-dichlorobenzene (o-DCB) at a potential scan rate of 100 mV/s, where the reduction potentials were calibrated using ferrocene/ferrocenium (Fc/Fc+ ) redox couple as an internal standard. The geometries of fulleropyrrolidine derivatives were firstly optimized by density functional theory (DFT) at the B3LYP/631G(d) level, and the orbital energy and orbital analysis were then calculated with higher basis set at the PBEPBE/6-311G(d,p) level, using the Gaussian 09 program package [29]. Fulleropyrrolidine derivatives 1–5 were synthesized by refluxing the C60 , sarcosine and the corresponding benzaldehyde in toluene according to the reported procedures [31–33], and identified by MALDI-TOF-MS (AB Sciex 4700). 1, m/z calcd., C70 H10 N2 878.08; found, 878.17; 2, m/z calcd., C71 H13 NO2 911.09; found, 911.25; 3, m/z calcd., C69 H11 N 853.09; found, 853.17; 4, m/z calcd.,
http://dx.doi.org/10.1016/j.synthmet.2014.09.030 0379-6779/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: X. Zhang, et al., Establishment of a linear correlation between the LUMO levels of fullerenes and the Hammett constants of substituents installed: An experimental and theoretical study, Synthetic Met. (2014), http://dx.doi.org/10.1016/j.synthmet.2014.09.030
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Scheme 1. Structures of fulleropyrrolidine derivatives 1–5.
C70 H13 N 867.10; found, 867.18; 5, m/z calcd., C71 H16 N2 896.13; found, 896.92. 3. Results and discussions Fig. 1 showed the CV curves of 1–5 along with that of C60 for comparison and their electrochemistry properties were summarized in Table 1. It can be seen that fulleropyrrolidines 1–5 exhibit three well-defined and quasi-reversible redox peaks range from 0 to −2.6 V versus Ag/Ag+ . The first (E1 ), second (E2 ), and third (E3 ) reduction potentials of fulleropyrrolidines 1–5 are all negatively shifted, compared to the parent C60 (Fig. 1 and Table 1). The decrease of the first reduction potentials becomes larger when the substituent varying from the EWG to EDG. For example, 0.06 V and 0.12 V negative shift of E1 was observed in 1 and 5 with a strongest electron-acceptor and electron-donor, respectively. The LUMO energy levels of the fullerene derivatives were then estimated from their onset reduction potentials (Eonset red ), according to the equation LUMO = −e (Eonset red + 4.60) [18]. The Eonset red of 1–5 were −0.82, −0.83, −0.85, −0.86 and −0.88 V, corresponding to the LUMO energy levels of −3.78, −3.77, −3.75, −3.74 and −3.72 eV, respectively (Table 1). The LUMO energy levels of 1–5 are 0.06–0.12 eV higher than that of the parent C60 , depending on the nature of EWG and EDG of substituent, where the highest value of −3.72 eV was observed in 5 with a strongest EDG of −N(Me)2 . The higher LUMO energy levels are desirable for application as acceptors in polymer solar cells to get higher open-circuit voltage. To correlate the LUMO energy level with substituent effect, the Hammett constant () of substituent was employed [28]. As shown in Fig. 2, a good linear relationship was well established between the LUMO energy level and with an equation LUMO (eV) = −0.044 − 3.75 (R2 = 0.9798). The LUMO energy levels increased with increasing the electron-donating character and decreased with increasing electron-withdrawing character of
Fig. 1. Cyclic voltammograms of fulleropyrrolidine derivatives 1–5 and the pristine C60 in 1,2-dichlorobenzene versus Ag/Ag+ .
Fig. 2. Plots of the Hammett constants of substituents versus the experimental LUMO energy levels.
substituent. The linear correlation demonstrates that the finetuning of LUMO energy level of fullerene electron-acceptor can be achieved by installation of EWG and EDG, which represents an alternative strategy to gain the desirable LUMO energy levels of electron-acceptor to match the HOMO level of electron-donor in polymer solar cells. To further examine the substituent effect on the LUMO energy level of fullerene electron-acceptor, DFT calculation was performed by using the Gaussian 09 program package [29]. Ground state geometrical structures of five fulleropyrrolidine derivatives 1–5 and C60 were firstly optimized at the B3LYP/6-31G(d) level, and the orbital energies were then calculated at the PBEPBE/6-311G(d,p) level that has been verified to be more accurate for fullerene [27,30]. The calculated LUMO energy levels were listed in Table 1, and schematic diagram of the HOMO and LUMO of fullerenes were shown in Fig. 3. A similar substituent-dependent trend can be seen that the LUMO energy levels raised 0.06 (1), 0.16 (2), 0.21 eV (3), 0.23 eV (4), and 0.29 eV (5) in comparison with that of the parent C60 (−4.18 eV), respectively. Thus the calculated LUMO energy levels were plotted against the corresponding experimental values, and as shown in Fig. 4, a linear correlation was obtained with R2 = 0.9298, verifying the reliability of the selected theoretical calculation method. The discrepancy between the values of experimental and theoretical could result from the neglected solvation effect in the calculation [26]. Orbital analysis of LUMO and HOMO of fulleropyrrolidine derivatives 1–5 as well as C60 are displayed in Fig. 5. It can be seen that both the LUMO and HOMO of all fullerenes are mainly located on the C60 cage except 5, where the HOMO of 5 is mainly located on dimethylaniline moiety. In comparison with the parent fullerene C60 , the extent of electron delocalization for fulleropyrrolidine derivatives 1–5 is slightly weakened due to C C double bond functionalization, which could cause a substantial up-shifting of the LUMO energy levels [27,30]. Among fulleropyrrolidine derivatives
Please cite this article in press as: X. Zhang, et al., Establishment of a linear correlation between the LUMO levels of fullerenes and the Hammett constants of substituents installed: An experimental and theoretical study, Synthetic Met. (2014), http://dx.doi.org/10.1016/j.synthmet.2014.09.030
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Table 1 Electrochemical properties and calculated LUMO energy levels of fulleropyrrolidine derivatives 1–5 and the pristine C60 as well as the Hammett constant of substituent. Fullerenes C60 C60 1 2 3 4 5 a b c d e f
f
E1 [V]a
E2 [V]a
E3 [V]a
Eonset red [V]b
LUMO [eV]
−0.84 −0.83 −0.90 −0.91 −0.92 −0.93 −0.95
−1.24 −1.23 −1.30 −1.29 −1.30 −1.31 −1.34
−1.75 −1.72 −1.86 −1.84 −1.85 −1.86 −1.89
−0.75 −0.76 −0.82 −0.83 −0.85 −0.86 −0.88
−3.85 −3.84c −3.78c −3.77c −3.75c −3.74c −3.72c c
e – −4.18d −4.12d −4.02d −3.97d −3.95d −3.89d
– – 0.66 0.45 0 −0.17 −0.83
Half-wave potential (versus Ag/Ag+ ). Onset reduction potential. Calculated from LUMO = −e (Eonset red + 4.60). Values from the DFT calculation at PBEPBE/6-311G(d,p) level. Hammett constants of substituents were taken from Ref. [28]. Data were taken from Ref. [18].
Fig. 3. Schematic diagrams of HOMO and LUMO energy levels of C60 and fulleropyrrolidine derivatives 1–5 obtained from the DFT calculation with PBEPBE/6-311G(d,p).
Fig. 4. Plots of the experimental LUMO energy levels versus theoretical values.
1–5, it can be seen that the LUMO energy level of 3 is upper-shifted when an EDG was installed (4 and 5) and lower-shifted with an EWG (1 and 2). The substituent-dependent of the LUMO energy level of fullerene electron-acceptor would provide fundamental insights in designing novel fullerene electron-acceptor materials. The substituent effect on redox properties of fullerene derivatives has been mainly attributed to the inductive effect of attached addend [34]. Therefore, the introduction of EWG/EDG in 1–5 will substantially modulate the inductive effect of addend installed on fullerene cage, which depends the electron accepting/donating ability of substituent, namely, Hammett constant . This indicates that the electronic communication between addend and fullerene cage could be modulated by the EWG/EDG, and explains the reason of linear dependent of the LUMO energy level of fullerene on Hammett constant of substituent attached. However, the LUMO energy level of 3 only changes 30 meV with the introduction of the strongest electron donor (5) and acceptor (1), respectively (Table 1). This relatively small change can be attributed to the fact that the saturated pyrrolidine connecting group in fulleropyrrolidines 1–5
Fig. 5. LUMO (top) and HOMO (bottom) orbital contours for C60 and fulleropyrrolidine derivatives 1–5 obtained from the DFT calculation using PBEPBE/6-311G(d,p).
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may significantly prevent electronic communication between the addend and fullerene units, which could be further enhanced by using cyclopropane connecting groups [35]. 4. Conclusions In conclusion, a series of five fulleropyrrolidine derivatives, bearing an EWG or EDG on the phenyl ring attached on the periphery of fullerene core, were used to investigate the effects of substituent groups added to fullerene on its LUMO energy level by the CV electrochemistry and the DFT calculation. The LUMO energy levels of fulleropyrrolidine derivatives were raised 0.06–0.12 eV from the EWG to EDG substitution in comparison with that of the parent fullerene C60 , in which a linear correlation between the LUMO energy levels and the Hammett constants of substituents was well established. The LUMO energy levels of these fulleropyrrolidines calculated by the DFT calculation are also linearly correlated well with the experimental values. These findings may allow to predicting and finely tuning the LUMO energy level of fullerene electronacceptor and provide fundamental insights in designing more efficient fullerene electron-acceptor materials in polymer solar cells. Acknowledgments This work was financially supported by Innovation Program of Shanghai Municipal Education Commission (12ZZ067), Shanghai Pujiang Program (11PJ1400200), the Research Fund for the Doctoral Program of Higher Education of China (20120075120018), and the Fundamental Research Funds for the Central Universities (2232014D3-11 and 2232014G1-18). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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Xuan Zhang ∗ Li-Xia Ma Xu-Dong Li College of Chemistry, Chemical Engineering & Biotechnology, Donghua University, Shanghai 201620, China ∗ Corresponding
author. Tel.: +86 21 6779 2619. E-mail address: [email protected] (X. Zhang) 29 June 2014 15 September 2014 24 September 2014 Available online xxx
Please cite this article in press as: X. Zhang, et al., Establishment of a linear correlation between the LUMO levels of fullerenes and the Hammett constants of substituents installed: An experimental and theoretical study, Synthetic Met. (2014), http://dx.doi.org/10.1016/j.synthmet.2014.09.030
ARTICLE IN PRESS
SYNMET-14695; No. of Pages 4
Synthetic Metals xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Letter to the Editor Establishment of a linear correlation between the LUMO levels of fullerenes and the Hammett constants of substituents installed: An experimental and theoretical study
a r t i c l e
i n f o
Keywords: Fullerene LUMO level Electron acceptor Substituent effect
a b s t r a c t The LUMO levels of series fulleropyrrolidines equipped with electron-withdrawing and electron-donating groups were determined by electrochemical study and DFT calculation. A linear correlation between the LUMO levels of fullerenes and the Hammett constants of substituents installed was established for the first time. This may allow to predicting and finely tuning the LUMO level of fullerene electron-acceptor. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The excellent electron affinity of fullerene has made it good electron-acceptor material for the bulk heterojunction (BHJ) polymer solar cells [1–5]. Based on a tandem concept, the power conversion efficiency (PCE) of polymer solar cells has recently been broken 10% record [6–8], making it more promising for a practical commercial production. Among the developed polymer solar cells, poly-(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 -butyric acid methyl ester (PCBM) are the most popular polymer electron-donors and fullerene electron-acceptors, respectively [1–5]. It has been established that the difference between the lowest unoccupied molecular orbital (LUMO) energy level of the acceptor and the highest occupied molecular orbital (HOMO) energy level of the donor, governs the performance such as the open circuit voltage (Voc ) value of polymer solar cells [9,10]. Thus, to improve the device performance, the design of more efficient donor and acceptor materials to make the HOMO energy level of the donor lower and the LUMO energy level of the acceptor higher is desirable. In this regard, much effort has been devoted to tuning the HOMO energy levels of polymer electron-donors [11–15]. On the other hand, the fine-tuning of the LUMO level of fullerene electron-acceptor is crucial to improve the performance of polymer solar cells. Recently, raising the LUMO energy level of the fullerene electron-acceptor is also attracted much current interest [16–20]. Several concepts have been introduced to raise the LUMO energy level of fullerenes [21], for example, the installation of organic electron-donating groups onto fullerene [22], the introduction of multi-substitution to decrease of the size of the fullerene -electron-conjugated system [23–25], and the placement of ortho-substitution spatial proximity to the fullerene core [26,27]. In this letter, we reported a series of five fulleropyrrolidine derivatives (1–5, Scheme 1) varying only in the electronwithdrawing group (EWG) and electron-donating group (EDG) of phenyl ring attached on the periphery of fullerene core. We investigated the effects of substituent groups added to fullerene on
its LUMO energy level by the cyclic voltammetry (CV) electrochemistry and density functional theory (DFT) calculation. The LUMO energy levels of fulleropyrrolidine derivatives were raised 0.06–0.12 eV by varying the substituent from EWG to EDG in comparison with that of the parent fullerene C60 , in which a linear correlation between the LUMO energy levels and the Hammett constants of substituents was established for the first time. The LUMO energy levels of these fulleropyrrolidines calculated by the DFT calculation are also linearly correlated well with the experimental values. These findings may allow to predicting and finely tuning the LUMO energy level of fullerene electron-acceptor and provide fundamental insights in improving the performance of polymer solar cells. 2. Methods and materials Cyclic voltammetry (CV) curves were measured using a CHI 660D electrochemical workstation (Chenhua, Shanghai). The CV curves were collected at room temperature under N2 using a conventional three-electrode system (glassy carbon working electrode, Pt wire counter-electrode and Ag/Ag+ quasi-reference electrode) in 0.1 M tetrabutylammonium tetrafluoroborate (n-Bu4 BF4 ) solution in 1,2-dichlorobenzene (o-DCB) at a potential scan rate of 100 mV/s, where the reduction potentials were calibrated using ferrocene/ferrocenium (Fc/Fc+ ) redox couple as an internal standard. The geometries of fulleropyrrolidine derivatives were firstly optimized by density functional theory (DFT) at the B3LYP/631G(d) level, and the orbital energy and orbital analysis were then calculated with higher basis set at the PBEPBE/6-311G(d,p) level, using the Gaussian 09 program package [29]. Fulleropyrrolidine derivatives 1–5 were synthesized by refluxing the C60 , sarcosine and the corresponding benzaldehyde in toluene according to the reported procedures [31–33], and identified by MALDI-TOF-MS (AB Sciex 4700). 1, m/z calcd., C70 H10 N2 878.08; found, 878.17; 2, m/z calcd., C71 H13 NO2 911.09; found, 911.25; 3, m/z calcd., C69 H11 N 853.09; found, 853.17; 4, m/z calcd.,
http://dx.doi.org/10.1016/j.synthmet.2014.09.030 0379-6779/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: X. Zhang, et al., Establishment of a linear correlation between the LUMO levels of fullerenes and the Hammett constants of substituents installed: An experimental and theoretical study, Synthetic Met. (2014), http://dx.doi.org/10.1016/j.synthmet.2014.09.030
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2
Scheme 1. Structures of fulleropyrrolidine derivatives 1–5.
C70 H13 N 867.10; found, 867.18; 5, m/z calcd., C71 H16 N2 896.13; found, 896.92. 3. Results and discussions Fig. 1 showed the CV curves of 1–5 along with that of C60 for comparison and their electrochemistry properties were summarized in Table 1. It can be seen that fulleropyrrolidines 1–5 exhibit three well-defined and quasi-reversible redox peaks range from 0 to −2.6 V versus Ag/Ag+ . The first (E1 ), second (E2 ), and third (E3 ) reduction potentials of fulleropyrrolidines 1–5 are all negatively shifted, compared to the parent C60 (Fig. 1 and Table 1). The decrease of the first reduction potentials becomes larger when the substituent varying from the EWG to EDG. For example, 0.06 V and 0.12 V negative shift of E1 was observed in 1 and 5 with a strongest electron-acceptor and electron-donor, respectively. The LUMO energy levels of the fullerene derivatives were then estimated from their onset reduction potentials (Eonset red ), according to the equation LUMO = −e (Eonset red + 4.60) [18]. The Eonset red of 1–5 were −0.82, −0.83, −0.85, −0.86 and −0.88 V, corresponding to the LUMO energy levels of −3.78, −3.77, −3.75, −3.74 and −3.72 eV, respectively (Table 1). The LUMO energy levels of 1–5 are 0.06–0.12 eV higher than that of the parent C60 , depending on the nature of EWG and EDG of substituent, where the highest value of −3.72 eV was observed in 5 with a strongest EDG of −N(Me)2 . The higher LUMO energy levels are desirable for application as acceptors in polymer solar cells to get higher open-circuit voltage. To correlate the LUMO energy level with substituent effect, the Hammett constant () of substituent was employed [28]. As shown in Fig. 2, a good linear relationship was well established between the LUMO energy level and with an equation LUMO (eV) = −0.044 − 3.75 (R2 = 0.9798). The LUMO energy levels increased with increasing the electron-donating character and decreased with increasing electron-withdrawing character of
Fig. 1. Cyclic voltammograms of fulleropyrrolidine derivatives 1–5 and the pristine C60 in 1,2-dichlorobenzene versus Ag/Ag+ .
Fig. 2. Plots of the Hammett constants of substituents versus the experimental LUMO energy levels.
substituent. The linear correlation demonstrates that the finetuning of LUMO energy level of fullerene electron-acceptor can be achieved by installation of EWG and EDG, which represents an alternative strategy to gain the desirable LUMO energy levels of electron-acceptor to match the HOMO level of electron-donor in polymer solar cells. To further examine the substituent effect on the LUMO energy level of fullerene electron-acceptor, DFT calculation was performed by using the Gaussian 09 program package [29]. Ground state geometrical structures of five fulleropyrrolidine derivatives 1–5 and C60 were firstly optimized at the B3LYP/6-31G(d) level, and the orbital energies were then calculated at the PBEPBE/6-311G(d,p) level that has been verified to be more accurate for fullerene [27,30]. The calculated LUMO energy levels were listed in Table 1, and schematic diagram of the HOMO and LUMO of fullerenes were shown in Fig. 3. A similar substituent-dependent trend can be seen that the LUMO energy levels raised 0.06 (1), 0.16 (2), 0.21 eV (3), 0.23 eV (4), and 0.29 eV (5) in comparison with that of the parent C60 (−4.18 eV), respectively. Thus the calculated LUMO energy levels were plotted against the corresponding experimental values, and as shown in Fig. 4, a linear correlation was obtained with R2 = 0.9298, verifying the reliability of the selected theoretical calculation method. The discrepancy between the values of experimental and theoretical could result from the neglected solvation effect in the calculation [26]. Orbital analysis of LUMO and HOMO of fulleropyrrolidine derivatives 1–5 as well as C60 are displayed in Fig. 5. It can be seen that both the LUMO and HOMO of all fullerenes are mainly located on the C60 cage except 5, where the HOMO of 5 is mainly located on dimethylaniline moiety. In comparison with the parent fullerene C60 , the extent of electron delocalization for fulleropyrrolidine derivatives 1–5 is slightly weakened due to C C double bond functionalization, which could cause a substantial up-shifting of the LUMO energy levels [27,30]. Among fulleropyrrolidine derivatives
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Table 1 Electrochemical properties and calculated LUMO energy levels of fulleropyrrolidine derivatives 1–5 and the pristine C60 as well as the Hammett constant of substituent. Fullerenes C60 C60 1 2 3 4 5 a b c d e f
f
E1 [V]a
E2 [V]a
E3 [V]a
Eonset red [V]b
LUMO [eV]
−0.84 −0.83 −0.90 −0.91 −0.92 −0.93 −0.95
−1.24 −1.23 −1.30 −1.29 −1.30 −1.31 −1.34
−1.75 −1.72 −1.86 −1.84 −1.85 −1.86 −1.89
−0.75 −0.76 −0.82 −0.83 −0.85 −0.86 −0.88
−3.85 −3.84c −3.78c −3.77c −3.75c −3.74c −3.72c c
e – −4.18d −4.12d −4.02d −3.97d −3.95d −3.89d
– – 0.66 0.45 0 −0.17 −0.83
Half-wave potential (versus Ag/Ag+ ). Onset reduction potential. Calculated from LUMO = −e (Eonset red + 4.60). Values from the DFT calculation at PBEPBE/6-311G(d,p) level. Hammett constants of substituents were taken from Ref. [28]. Data were taken from Ref. [18].
Fig. 3. Schematic diagrams of HOMO and LUMO energy levels of C60 and fulleropyrrolidine derivatives 1–5 obtained from the DFT calculation with PBEPBE/6-311G(d,p).
Fig. 4. Plots of the experimental LUMO energy levels versus theoretical values.
1–5, it can be seen that the LUMO energy level of 3 is upper-shifted when an EDG was installed (4 and 5) and lower-shifted with an EWG (1 and 2). The substituent-dependent of the LUMO energy level of fullerene electron-acceptor would provide fundamental insights in designing novel fullerene electron-acceptor materials. The substituent effect on redox properties of fullerene derivatives has been mainly attributed to the inductive effect of attached addend [34]. Therefore, the introduction of EWG/EDG in 1–5 will substantially modulate the inductive effect of addend installed on fullerene cage, which depends the electron accepting/donating ability of substituent, namely, Hammett constant . This indicates that the electronic communication between addend and fullerene cage could be modulated by the EWG/EDG, and explains the reason of linear dependent of the LUMO energy level of fullerene on Hammett constant of substituent attached. However, the LUMO energy level of 3 only changes 30 meV with the introduction of the strongest electron donor (5) and acceptor (1), respectively (Table 1). This relatively small change can be attributed to the fact that the saturated pyrrolidine connecting group in fulleropyrrolidines 1–5
Fig. 5. LUMO (top) and HOMO (bottom) orbital contours for C60 and fulleropyrrolidine derivatives 1–5 obtained from the DFT calculation using PBEPBE/6-311G(d,p).
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may significantly prevent electronic communication between the addend and fullerene units, which could be further enhanced by using cyclopropane connecting groups [35]. 4. Conclusions In conclusion, a series of five fulleropyrrolidine derivatives, bearing an EWG or EDG on the phenyl ring attached on the periphery of fullerene core, were used to investigate the effects of substituent groups added to fullerene on its LUMO energy level by the CV electrochemistry and the DFT calculation. The LUMO energy levels of fulleropyrrolidine derivatives were raised 0.06–0.12 eV from the EWG to EDG substitution in comparison with that of the parent fullerene C60 , in which a linear correlation between the LUMO energy levels and the Hammett constants of substituents was well established. The LUMO energy levels of these fulleropyrrolidines calculated by the DFT calculation are also linearly correlated well with the experimental values. These findings may allow to predicting and finely tuning the LUMO energy level of fullerene electronacceptor and provide fundamental insights in designing more efficient fullerene electron-acceptor materials in polymer solar cells. Acknowledgments This work was financially supported by Innovation Program of Shanghai Municipal Education Commission (12ZZ067), Shanghai Pujiang Program (11PJ1400200), the Research Fund for the Doctoral Program of Higher Education of China (20120075120018), and the Fundamental Research Funds for the Central Universities (2232014D3-11 and 2232014G1-18). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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Xuan Zhang ∗ Li-Xia Ma Xu-Dong Li College of Chemistry, Chemical Engineering & Biotechnology, Donghua University, Shanghai 201620, China ∗ Corresponding
author. Tel.: +86 21 6779 2619. E-mail address: [email protected] (X. Zhang) 29 June 2014 15 September 2014 24 September 2014 Available online xxx
Please cite this article in press as: X. Zhang, et al., Establishment of a linear correlation between the LUMO levels of fullerenes and the Hammett constants of substituents installed: An experimental and theoretical study, Synthetic Met. (2014), http://dx.doi.org/10.1016/j.synthmet.2014.09.030