A cost effective and eco-friendly one-pot process for PC61BM synthesis under aerobic conditions

Abstract

Here we demonstrate a cost effective and eco-friendly process for one-pot synthesis of PC61BM under aerobic conditions where, the key step of diazomethane intermediate preparation is modified. Instead of using pyridine and sodium methoxide under inert atmosphere, we used triethyl amine and dichloromethane under aerobic conditions. This process is envisaged as a green chemistry and will open channels for the large scale synthesis of PC61BM and its derivatives for solar cells applications without bothering for controlled environment conditions.

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[6,6]-Phenyl-C₆₁-butyric acid methyl ester (PC61BM) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC71BM) are the most conventional acceptor materials used in organic solar cells (OSCs) in combination with high as well as low band gap polymers such as P3HT and PTB7.¹ Inter-miscibility, phase separation and nano-morphology of donor–acceptor bulk heterojunction are controlled prominently by PC61BM, its process of synthesis as well as modifications.² Herein, we report one-pot synthesis of PC61BM under aerobic conditions which not only results in to PC61BM of similar quality (as with other process) but it also proves to be a cost effective approach with large scale synthesis possibilities.³

Synthesis of PC61BM was first introduced by Hummelen and Wudl in year 1995 via diazo alkane addition to fullerene[60] to prepare fulleroid and methanofullerene derivatives for their versatile applications. Also, in the same year its use as acceptor in OSC as soluble fullerene derivative was reported.⁴ Since then, the dipolar cycloaddition of diazoalkanes attracted particular attention where, diazoalkanes were generated in situ via base induced decomposition of tosylhydrazones and this method was found applicable even to unstable diazo compounds. Such 1,3 dipolar cycloadditon of diazo compound on fullerene[60] has been found to be very convenient route for synthesis of methanofullerenes using different types of metal catalysts, bases and solvent mediums for controlled additions.⁵ The primary [3 + 2] addition preferentially occurs at the [6,6] ring conjunct double bond of fullerene[60] to yield [6,6]-closed pyrazoline adduct.⁶ Subsequent release of nitrogen results in [5,6]-open fulleroid, which converts into stable [6,6]-closed methanofullerene through valence isomerisation.⁷

In terms of synthesis of PC61BM on large scale for practical use in organic solar cells, we do need a convenient and less hazardous synthesis route for cheaper technology and is the motivation for the present work.⁸ So far, for the synthesis of PC61BM and its derivatives, diazo intermediate is prepared by dissolving tosylhydrazone and sodium methoxide in pyridine under inert atmosphere. In the present communication, we have simplified this key step and PC61BM is prepared under air via one-pot procedure to reduce the cost of material for large scale production and also to make it more eco friendly by avoiding use of pyridine.⁹ The same reaction route is being exploited in our group for synthesis of several other new PC61BM derivatives for solar cells applications.

The synthesis route for PC61BM is shown in Scheme 1 (and ESI†). Ester was synthesized in presence of acid catalyst followed by p-tosyl hydrazone preparation in methanol. The key step is the conversion of hydrazone into diazomethane. Tosylhydrazone was dissolved in dichloromethane and cooled down to 0 °C followed by the addition of triethyl amine.¹⁰ No inert atmosphere was required and reaction mixture was stirred for ∼three hour at this temperature followed by addition of fullerene solution in o-dichlorobenzene (o-DCB). Reaction mixture was stirred at ∼75 °C and the progress of the reaction was monitored by TLC. After 18 hour, product was collected by precipitation with methanol followed by purification by column chromatography with toluene. [5,6]Isomer was collected as purple solution after fullerene in 95% purity (remaining being the traces of 1 or other isomer), which was further refluxed in o-DCB for 5 hour to convert into [6,6]PC61BM. The yield was 38–40% and ∼70% on the basis of converted fullerene for both small and large batches (see ESI†) with good reproducibility. A small fraction of bis/multi adduct was also isolated. To further prove the merit of given methodology over conventional one, PC61BM synthesis was performed following the conventional method but under aerobic conditions, i.e., using pyridine as solvent and sodium methoxide as base. The progress of the reaction was monitored by TLC and most of the fullerene was found unreacted even after 48 hours of heating resulting in only 2–3% yield of PC61BM.

Scheme 1 (i) MeOH, CH₃COOH, reflux; (ii) p-toluene sulfonyl hydrazide, MeOH, reflux; (iii) triethyl amine, dichloromethane, 0 °C; (iv) C₆₀ in o-DCB followed by column purification to isolate [5,6]adduct; (v) refluxed in o-DCB.

FTIR spectrum of 1 shows the characteristic peaks at 1737, 1445, 1428, 1187, 758, 698, 568, 547 and 527 cm⁻¹. UV-vis absorption analysis is one of the best methods to ascertain the [6,6]-closed methanofullerene structure with diagnostic absorption bands at 434 and 697 nm. As expected, the absorption spectrum of 1 recorded in dichloromethane shows the bands at 328, 430, 492 and 698 nm.^{3 1}H and ¹³C NMR clearly show the formation and conversion of [5,6]fulleroid into [6,6]methanofullerene on refluxing in o-DCB. Fig. 1 shows the ¹H NMR of 1 in CDCl₃.^{11 13}C NMR also shows the quaternary carbon at 50.7 ppm and bridgehead carbons at 78.8 ppm in accordance with the methanofullerene structure. The electrochemical behaviour of 1 was studied by cyclic voltammetry in o-DCB using a platinum disc working electrode and 0.1 M solution of tetra(n-butyl)ammonium hexafluorophosphate as supporting electrolyte. As shown in Fig. 2, [6,6]PC61BM exhibits four well defined quasireversible waves with half-cell potentials for the reduction at 1096, 1554, 2069, 2543 mV relative to Fc/Fc⁺.³

Fig. 1 ¹H NMR of 1 (a) full scale (solvent and water impurities between 0.5 and 1.5 ppm), (b) expanded aromatic region and (c) expanded aliphatic region.

Fig. 2 Cyclic voltammogram of 1 (10⁻³ M) at 100 mV s⁻¹, o-DCB/0.1 M TBAPF₆ vs. Ag wire.

Conclusions

In conclusion we have come up with a procedure for PC61BM and its derivatives synthesis in one-pot by in situ generating diazo intermediate via modified route where, use of inert atmosphere and pyridine is totally excluded to make PC61BM preparation more cost effective and eco friendly eventually lowering the ultimate organic solar cell cost.

Acknowledgements

The authors thank the financial support from CSIR, TAPSUN project and Dr Asit Patra for useful scientific discussions.

Notes and references

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11. [6,6]PC61BM ¹H NMR (δ, CDCl₃): 7.85 (d, 2H,o-H Ph), 7.48 (t, 2H, m-H Ph), 7.37 (m, 1H, p-H Ph), 3.66 (s, 3H, OCH₃), 2.84 (t, 2H, PhCCH₂), 2.45 (t, 2H, CH₂COOR), 2.11 (q, 2H, CH₂CH₂COOR). ¹³C NMR (δ, CDCl₃): 172.5 (CO₂Me), 32 peaks between 150–127, 78.8, 50.7, 32.9, 32.6, 21.3 ppm.

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Footnote

† Electronic supplementary information (ESI) available: The detailed experimental procedures and characterization data. See DOI: 10.1039/c4ra00321g

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