Chinese Chemical Letters  2014, Vol.25 Issue (04):501-504   PDF    
Effect of the position of substitution on the electronic properties of nitrophenyl derivatives of fulleropyrrolidines:Fundamental understanding toward raising LUMO energy of fullerene electron-acceptor
Xuan Zhang , Xu-Dong Li    
Corresponding authors at:College of Chemistry, Chemical Engineering & Biotechnology, Donghua University, Shanghai 201620, China
Abstract: A series of substituted para-, meta- and ortho-nitrophenyl derivatives of fulleropyrrolidine were synthesized to investigate the effects of the position of substitution on electronic properties by using steady-state absorption and fluorescence spectra, combined with DFT calculations. The results confirmed that the position of substitution has little effect on absorption and fluorescence spectra, whereas a significant effect was observed on their LUMO energy levels. The theoretical calculations revealed that the LUMO energy of the ortho-nitrophenyl substituted derivative was increased 0.1 eV above those of para- and meta-substitution. The prominent effect of ortho-substitution was attributed to the through-space orbital interaction between spatially closed electron-withdrawing nitro group and fullerene cage. These findings could provide fundamental insights in raising LUMO levels of C60-based electron acceptor materials and an alternative strategy to increase open circuit voltage Voc in polymer solar cells.
Key words: Fullerene     Substituent effect     Orbital interaction     Organic photovoltaics    
1. Introduction

Fullerene (C60) has attracted much attention since its discovery in 1985,due to unique optoelectronic properties and potential applications in the material fields [1, 2, 3]. The excellent electron affinity of fullerene has made it a promising electron acceptor material for organic photovoltaics (OPVs),especially for the bulk heterojunction (BHJ) polymer solar cells [4, 5, 6]. For example,the typical fullerene derivative,[6, 6]-phenyl-C61-butyric acid methyl ester (PCBM),has been considered as a benchmark acceptor materialandwidelyusedintheactivelayerofphotovoltaicdevices [7]. Recently,the power conversion efficiency (PCE) of C60-based OPVs has been reported to break 10% [8],making it possible for commercial production. To further improve the device perfor- mance,the design of more efficient donor and acceptor materials, as well as the development of advanced manipulation processes become key issues. It has been established empirically that the open circuit voltage (Voc),a key factor determining the perfor- mance of OPVs,is proportional to 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 [9, 10]. Hence,raising the LUMO energy level of the acceptor is beneficial for increasing the Vocvalues. As a consequence,much effort has been expended in designing new fullerene acceptors with various functional groups to raise LUMO energylevels[11, 12, 13, 14, 15, 16, 17].However,theeffectsofsubstitutionposition of functional groups on the LUMO energy levels of fullerene derivatives have not been fully addressed. The general concept on the installation of organic electron-donating groups onto fullerene has been investigated [18]. Recently,Matsumoto et al. [19] reported a stabilization effect of LUMO energy of methanofuller- enes derivatives by placing methoxy donor groups in spatial proximity to the C60core.Tajima et al. [20] observed a considerable lowering of the reduction potentials in indolino- and benzofur- anol-fusedfullerenederivativeswithortho-methoxydonorgroups. Here we present the effect of position of substitution of para-, meta- and ortho-nitrophenyl derivatives of fulleropyrrolidine on their electronic properties by using steady-state absorption and fluorescencespectra,combinedwithDFTcalculations.Itwasfound that the substitution position of the electron withdrawing nitro group has little effect on both absorption and fluorescence spectra; whereas,a significant effect on the increase of the LUMO energy in ortho-nitrophenyl substituted fullerenes was revealed by the theoretical calculation,from which an increased open circuit voltage Voc was predicted. The prominent effect of ortho- substitution was explained by the through-space orbital interac- tion between the spatially close nitrophenyl functional group and the fullerene cage. The results of the present work could provide fundamental insights into the efficient up-shifting of LUMO levels of C60-based electron acceptors,and an alternative strategy to enhance the open circuit voltage Vocin organic photovoltaics.

2. Experimental

A series of fulleropyrrolidine derivatives with para-,meta- and ortho-nitrophenyl groups (Scheme 1,1-3) were similarly synthe- sized by Prato method as described previously [21],and identified by MALDI-TOF MS. The molecular structures are shown in Scheme 1. The absorption spectra were measured on a Persee TU-1901 spectrophotometer. Fluorescence spectra were recorded on a Hitachi F-7000 Fluorescence spectrometer. The geometries of fullerene derivatives1-3,C60andPCBMwere optimizedbydensity functional theory (DFT) at the B3LYP/6-31G(d) level,and the orbital energy and orbital analysis were calculated with higher basis set at the PBEPBE/6-311G(d,p) level,using the Gaussian 09 program package [22].

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Scheme 1.Structures of nitrophenyl fulleropyrrolidine derivatives 1–3.
3. Results and discussion

Fig. 1 shows the absorption and fluorescence spectra of nitrophenyl attached fulleropyrrolidine derivatives 1-3 in toluene, together with the spectra of pristine C60for comparison. All of the three fulleropyrrolidine derivatives exhibit similar broad absorp- tion bands from 400 nm to 750 nm besides two distinct peaks around 433 nm and 702 nm (Fig. 1a). As shown in Fig. 1b,these derivatives exhibit similar weak fluorescence emission around 712 nm. The pristine C60shows a broad absorption in the region of 400-750 nm,and is almost non-fluorescent. In comparison with pristine C60,the difference in photophysical properties of derivatives 1-3 can be rationalized by the symmetry breaking effect of substituent group,which results in a contraction of thep- system of C60and enhancement of forbidden transitions from the S1to S0state to some extent [23]. These observations indicate that the substitution position has little perturbation effect on their photophysical properties.

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Fig. 1. Absorption (a) and fluorescence spectra (b), excitation wavelength at 500 nm, of three nitrophenyl fulleropyrrolidine derivatives 1–3 and pristine C60in toluene.

To examine the effect of substitution position on the LUMO energy levels of fullerene acceptors,DFT calculations were performed by using the Gaussian 09 program package. Ground state geometrical structures of three fulleropyrrolidine derivatives 1-3 and C60were optimized at the B3LYP/6-31G(d) level,and the orbital energies were calculated at the PBEPBE/6-311G(d,p) level basedonaboveoptimizedgeometries.Forfullerenesystems,thePBE method has been verified to be more accurate in orbital energy calculationthanB3LYP[23].Foracomparison,severalparametersof C60and PCBM calculated by B3LYP and PBE methods are listed in Table 1,together with the corresponding experimental values. It is obvious that PBE yield more reliable results by comparison with experimental values,indicating the calculation method adopted here is suitable for molecular orbital energy estimation. The calculated results for three fulleropyrrolidine derivatives 1-3 and C60aresummarizedinTable2,andtheschematicdiagramsofHOMO and LUMO are shown in Fig. 2. It is evident that the HOMO-LUMO energy gap of fulleropyrrolidine derivatives is reduced from ca. 1.69 eV of C60to 1.50 eV,while their LUMO energies increase 0.05 (1),0.07 (2),and0.18 eV (3) incomparison to thatof C60(-4.18 eV), respectively.WhiletheHOMO-LUMOenergygapsalmostremained constant at 1.50 eV,among the three fulleropyrrolidine derivatives, the ortho-substitution drastically enhanced the LUMO energy level (up-shifting 0.18 eV). Based on a reported empirical equation [10], the open circuit voltages Vocare predicted for the potential BHJ polymer solar cells with classic poly(3-hexylthiophene-2,5-diyl) (P3HT,EHOMO= -4.9 eV [23]) as electron donor and nitrophenyl derivatives of fulleropyrrolidine as acceptors (Table 2). The predicted Voc value shows a sharp increase for 3 with ortho- substitution (Voc= 0.60 V),by comparison with 0.47 V and 0.49 V for 1 and 2 with para/meta-substitution,respectively.

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Fig. 2. Schematic diagrams of HOMO and LUMO energy levels of C60 and its derivatives 1–3 calculated at PBEPBE/6-311G(d,p) level.

Table 1
Comparison of HOMOenergy, LUMO energy, and HOMO–LUMO energy gap(Egap) of C60and PCBM calculated with B3LYP and PBE methods, as well as the values from experimental measurements.

Table 2
HOMO energy, LUMO energy, and HOMO–LUMO energy gap (Egap) of nitrophenyl fulleropyrrolidine derivatives 1–3 and C60calculated at PBEPBE/6-311G(d,p) level, along with the predicted open circuit voltage Voc.

To further understand the ortho-substitution effect on the effective rise of LUMO energy levels,orbital analysis was carried out for three nitrophenyl derivatives of fulleropyrrolidine 1-3,as well as C60,at the PBEPBE/6-311G(d,p) level. Fig. 3 displays the calculated HOMO and LUMO orbitals. The three fulleropyrrolidine derivatives 1-3 show very similar orbital distributions,with both the HOMO and the LUMO mainly located on the fullerene cage. Relative to C60,the electron delocalization for fulleropyrrolidine derivatives 1-3 are slightly weakened due to C55C double bond functionalization,which could cause a substantial up-shifting of the LUMO energy levels [23]. However,it should be noted that the dramatic rise of the LUMO level in the ortho-nitrophenyl derivative 3 should have exceptional explanation. By careful examination of orbital distribution of ortho-nitrophenyl substituted fulleropyrro- lidine derivative 3,a small amount of HOMO is found to be located ontheoxygenatombesidesthemajorityonthefullerenecage.This suggests that there are weak interactions between the lone pair electrons of oxygen inthe ortho-nitro group and thep-electrons on the fullerene cage,which may contribute to efficient rise of the LUMO energy level of 3 [19, 20]. A cofacial p-orbital interaction between the fullerene cage and the cyclobutene has been shown to decrease the electron affinity and thereby increases the LUMO energy level of C60significantly [25]. Thus,it can be concluded that the LUMO energy levels of three nitrophenyl substituted full- eropyrrolidine derivatives strongly depend on the substitution position,and the efficient rise of the LUMO level in ortho- nitrophenyl derivative 3 could be rationalized by the through- space orbital interaction between oxygen atom of side functional group and fullerene cage.

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Fig. 3.HOMO (top) and LUMO (bottom) contours for C60and its derivatives 1–3 calculated using PBEPBE/6-311G(d,p).
4. Conclusion

A series of para-,meta- and ortho-nitrophenyl derivatives of fulleropyrrolidine were synthesized to investigate the effect of position of substitution on their electronic properties by using steady-state absorption and fluorescence spectra,combined with DFT calculations. The results showed that the substitution position has little affect on absorption and fluorescence spectra; whereas, the theoretical calculation revealed a significant rise of the LUMO energy in ortho-nitrophenyl substituted fullerenes,from which an increased open circuit voltage Vocwas predicted. The through- space orbital interaction between the spatially close oxygen atom in electron-withdrawing nitro group and the fullerene cage could contribute to an efficient rise of the LUMO level in ortho- nitrophenyl substituted fulleropyrrolidine. These findings could providefundamental insightsin theeffectiverise ofLUMO levelsof C60-based electron acceptor materials and an alternative strategy to increase open circuit voltage Vocin BHJ polymer solar cells.

Acknowledgments

This work was financially supported by Shanghai Pujiang Program (No. 11PJ1400200),Innovation Program of Shanghai Municipal Education Commission (No. 12ZZ067),the Research Fund for the Doctoral Program of Higher Education of China,and the Fundamental Research Funds for the Central Universities.

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