Synthesis and charge transport properties of new methanofullerenes

Article abstract of DOI:10.1039/c6nj03445d

New methanofullerene derivatives have been synthesized and characterized for their electron transport properties. Two new methanofullerenes (PC61BP & HC61NP) have been synthesized by using our reported amine assisted cycloaddition reaction (AACA) method i

Full text of DOI:10.1039/c6nj03445d

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DOI: 10.1039/C6NJ03445D
Synthesis And Charge Transport Properties Of New
Methanofullerenes
Samya Naqvi,‡ Neha Gupta,‡ Neelam Kumari, Jyoti Garg and Rachana Kumar*
‡ Authors contributed equally.
*Corresponding Author
Advanced Materials and Devices Division
Flexible Organic Energy Devices Group,
National Physical Laboratory
Dr. K. S. Krishnan Marg, New Delhi
Indiaꢀ110012
Email: rachanak.npl@nic.in
Phone Number: +91ꢀ11ꢀ4560ꢀ8577
Fax: +91ꢀ11ꢀ4560ꢀ9310

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Abstract
New methanofullerene derivatives have been synthesized and characterized for their electron
transport properties. Two new methanofullerenes (PC61BP & HC61NP) have been
synthesized by our reported amine assisted cycloaddition reaction (AACA) method in good
yield. PC61BM is also synthesized by AACA method for comparison of properties. PC61BP
possess phenyl group and butyric acid pentyl ester substituents on C(61) bridging carbon of
cyclopropane ring on fullerene. HC61NP possess completely different structure where C(61)
bridging carbon is bonded to a proton and 2ꢀnitro cinnamyl group. Both the products have
been well characterized by FTIR, UVꢀvis absorption spectroscopy and NMR for their
structure. Electrochemical properties have been evaluated by cyclic voltammetry. Finally the
electron mobility of the materials have been calculated at room temperature by preparing
electronꢀonly devices. PC61BP shows the highest mobility compared to HC61NP and
PC61BM and proved to be a promising acceptor material for organic solar cells.

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Introduction
Organic solar cells contribute immensely towards the renewable source of energy generation
at low cost.1 So far the intensive research efforts for the synthesis of new materials and
device developments have produced the solar cells with high efficiency.2 However, the most
needful aspect of their application on large scale which is the impact on environment due to
disposals during material synthesis as well as the fabrication processes has been neglected.
Burke et al., reported the unfavourable effects on environment by the fabrication of organic
solar cells.3 They also identified the green chemical routes for the synthesis of semiconductor
polymers which are helpful in extenuating the negative environmental externalities of
manufacturing organic solar cells especially on large scale to cover the huge land areas.
Several green or less environment detrimental methods are reported for the synthesis of donor
semiconductor polymers4 but there was no report for acceptors until we first reported the
cost effective and ecoꢀfriendly triethylamine catalyzed synthesis of [6,6]ꢀphenylꢀC61ꢀbutyric
acid methyl ester, [6,6]PC61BM monoadduct under total aerobic conditions.5 We also
introduced a novel amine assisted cycloaddition reactions (AACA approach) for synthesis of
diazoalkanes intermediate from aromatic and aliphatic hydrazone under very mild
homogeneous condition using amine as catalyst and further cycloaddition reaction on
6
fullerene surface to synthesize novel methanofullerenes. There is continuous need of new
methanofullerenes with improved properties and costꢀeffective, less environment detrimental
high yielding synthesis process for large area applications.
Fullerene possess outstanding electron acceptor properties due to its high electron affinity,
low reorganization energy and triply degenerated LUMO (lowest unoccupied molecular
orbitals) to accept up to 6 electrons. This makes it best material to be used in organic solar
cells with variety of donor molecules. PCBM is till date irreplaceable material in high
7
performance organic solar cells . As an alternate material to PCBM with better properties, we

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have come up with a process of synthesis of higher ester derivative of PCBM, i.e., [6,6]ꢀ
phenylꢀC61ꢀbutyric acid pentyl ester (PC61BP) and a novel methanofullerene with completely
different types of addends attached exohedrally on fullerene. In the present paper we are
introducing two methanofullerene synthesized by our AACA approach. Molecule 1 is higher
ester derivative of PC61BM, i.e., PC61BP and molecule 2 (HC61NP) is synthesized from 2ꢀ
nitrocinnamaldehyde (Figure 1). PC61BP possess similar structure to PC61BM with phenyl
group and butyric acid pentyl ester substituents on C(61) bridging carbon of cyclopropane
ring on fullerene. On the other hand HC61NP has completely different structure where the
C(61) bridging carbon of cylcopropane ring on fullerene is bonded to a proton and 2ꢀnitro
cinnamyl group. As reported earlier the substitution on fullerene for the formation of
methanofullerenes raises the LUMO levels and by selecting type of substituents (electron
8
donating or withdrawing) we can finely tune the LUMO level. It will be interesting to study
the effect of different types groups attached to cyclopropane ring (ꢀH vs ꢀPh) on electron
transport properties and power conversion efficiency. Materials have been well characterized
for their structure and electrochemical properties. The transport properties are also evaluated
at room temperature by preparing electronꢀonly devices and compared with PCBM
synthesized by AACA process. Both the materials show promising electron mobility however
PC61BP shows the highest of them all.
H
NO2
O
O
OCH3
OC5H11
PC61BM
HC61NP
PC61BP
Figure 1. Structures of PC61BP, HC61NP and PC61BM synthesized by AACA process.
Experimental Section
Materials and Characterization techniques:

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Pentyl 4ꢀbenzoylbutyrate is prepared from reaction of 4ꢀbenzoyl butyric acid with 1ꢀ
pentanol in acidic medium. Pentyl 4ꢀbenzoylbutyrate pꢀtosylhydrazone was prepared
as reported method from pentyl ester of 4ꢀbenzoyl butyric acid followed by reaction
9
with pꢀtoluene sulfonyl hydrazide. 4ꢀbenzoyl butyric acid, pꢀtoluene sulfonyl
hydrazide and all the amines were purchased from SigmaAldrich and used without
further purification. [60]Fullerene was purchased from 1ꢀMaterial. All solvents were
purchased from AlfaAesar and used after purification and drying.
All the products were characterized by Fourier Transform Infrared spectroscopy
(FTIR) using KBr pallets on Perkin Elmer FTIR Spectrum 2. FTIR spectra were
collected over a range from 3500 to 500 cmꢀ1. A background in air was done before
scanning the samples. UVꢀvis spectroscopy measurement was performed on a
Shimadzu UVꢀvis spectrophotometer (UVꢀ1800) using 0.3 mM solution in
chloroform. 1H and 13C NMR spectra were recorded on Jeol 400 MHz spectrometer in
CDCl3 using tetramethylsilane as internal standard. Molecular weights of the products
were confirmed from MALDIꢀTOFꢀTOF mass spectrometry on AB SCIEX using αꢀ
cyano 4ꢀhydroxy cinnamic acid matrix. Cyclic voltammetry measurements were
performed using a three electrode standard configuration with a platinum wire as
counter electrode and Ag wire as reference electrode and Ptꢀdisc as working electrode
in
a
0.1
M
TBAPF6 (tetraꢀnꢀbutylammoniumhexafluorophosphate) in oꢀ
Dichlorobenzene solution as electrolyte at 20 mV/s scan rate. Current vs Voltage was
measured on an Autolabpotentiostate. Atomic force microscopy (AFM) was
performed on Vecco Nanoscope. Samples were prepared in 0.25 mg/mL
concentration, filtered through 0.5 micron filters and spin coated on glass substrates.

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(a)
NNHTs
O
O
O
O
O
(ii)
(i)
OC5H11
OH
OC5H11
(iii)
N2
O
(iv)
(v)
OC5H11
O
O
OC5H11
OC5H11
(i) 1-Pentanol , acetic acid, 50 oC, 6 hour; (ii) p-toluene sulfonyl hydrazide (1.2 eq), methanol, reflux 24 hours; (iii)
o
ethyl acetate (20 mL), Triethyl amine 0oC, 3 hours; (iv) fullerene in toluene, 80 C, 18 hours; (v) Toluene, reflux, 5
hours.
(b)
NO2
NNHTs
H
NO2
O
NO2
N2
(i)
(ii)
H
H
(iii)
H
NO2
(i) p-toluene sulfonyl hydrazide (1.2 eq), methanol, reflux 24 hours; (ii) ethyl acetate (20 mL), Triethyl amine 0oC, 3
hours; (iii) fullerene in toluene, 80 oC, 18 hours.
Scheme 1. Synthesis of (a) PC61BP and (b) HC61NP by AACA process.
Synthesis
General method of Ester synthesis (Scheme 1a): 4ꢀbenzoyl butyric acid (5.2 mM) is
dissolved in 50 mL 1ꢀpentanol and ~2 mL acetic acid with a few drops of conc. Sulfuric acid
is added. The temperature of the reaction mixture is raised to refluxing and progress of the
reaction is monitored by running TLC in dichloromethane. After ~ 6 hr, reaction mixture is
concentrated and product is purified by column chromatography using dichloromethane as
solvent. Ester comes out as first fraction. Yield ~90%. FTIR (KBr) 1735, 1686 cmꢀ1.
General method of hydrazone synthesis (Scheme 1a&b): Ester (5.2 mM) and pꢀtoluene
sulfonyl hydrazide (1.2 eq) are dissolved in methanol (50 mL) and refluxed with stirring for
~6 hr followed by stirring at room temperature over night. Crystals of hydrazone come out on

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cooling and collected after washing with cold methanol. Yield ~99 %. FTIR (ν, cmꢀ1): 3111,
1712, 1371, 1228, 1172 cmꢀ1.
General method of synthesis of PC61BP and HC61NP (Scheme 1): Pentyl4ꢀ
benzoylbutyrate/2ꢀnitrocinnamaldehyde (5.2 mM) and pꢀtoluene sulfonyl hydrazide
(1.2 eq, 1.162 g, 6.24 mM) are dissolved in methanol (50 mL) and refluxed with
stirring for ~6 hr followed by stirring at room temperature over night. Crystals of
hydrazone come out on cooling and collected after washing with cold methanol. This
pꢀtosylhydrazone (0.14 mM) is dissolved in ethyl acetate (10 mL) and cooled down to
ꢀ10oC. Catalytic amount of triethylamine (0.7 mL) is added and stirred for three hours
at this temperature. A solution of C60 (0.3 eq, 33.6 mg, 0.046 mM) in toluene (10 mL)
o
is added and temperature is raised to 80 C. Stirred at this temperaure for 18 hour
followed by precipitation with methanol. Solid is collected by cerifugation and loaded
on silica gel column (200 mm x 18 mm) for purification with touene. Unreacted
fullerene comes as first fraction followed by monoadduct ([5,6]PC61BP, 42 % yield;
[5,6]HC61NP 45 % yield). [5,6]isomer is converted to [6,6] isomer by refluxing in
toluene (5mg/mL) for 5 hour (yield 100 %).
PC61BP: Yield:~42 %; FTIR (ν, cmꢀ1): 2957, 2923, 2855, 1727, 1456, 1429, 1260, 1120,
1
1071; UVꢀvis (DCM) (nm): 404, 430, 493 and 697. H NMR (δ, CDCl3): 7.80 (d, 2H,oꢀH
Ph), 7.48 (t, 2H, mꢀH Ph), 7.40 (m, 1H, pꢀH Ph), 4.0 (t, 2H, OCH2), 2.84 (m, 4H, PhCCH2,
COOCH2CH2), 2.45 (t, 2H, CH2COOR), 2.11 (q, 2H, CH2CH2COOR), 1.53 (m, 2H,
COOCH2CH2CH2), 1.18 (m, 2H, COOCH2CH2CH2CH2), 0.80 (t, 3H,
COOCH2CH2CH2CH3). 13C NMR (δ, CDCl3): 172.5 (CO2Me), 32 peaks between 150ꢀ127,
78.8, 63.7, 50.7, 33.1, 32.7, 30.9, 28.6, 27.0, 21.3, 12.9 ppm; HRMS (MALDIꢀTOF) for
formula C76O2H22 calculated mass 966.1619 experimental mass 966.1625 (M+)

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HC61NP: Yield:~45 %; FTIR (ν
, cmꢀ1) 2964, 1638, 1520, 1424, 1346, 1064; λmax
1
(nm): 405, 434, 498, 697; H NMR (CDCl3, 400 MHz), δ (ppm) 4.9 (d, J = 7.6 Hz,
1H), 6.5 (d, J = 7.6 Hz, 1H), 7.0 (m, 1H), 7.4 (t, J = 7.68 Hz, 1H), 7.6 (t, J = 7.4 Hz,
1H), 7.8 (br d, J = 6.0 Hz, 1H), 8.0 (d, J = 7.56 Hz, 1H); 13C NMR (CDCl3, 100
MHz), δ (ppm) 67.0(C61), 86.0(C(sp3) of C60), 86.2(C(sp3) of C60), 115, 123.5,
123.6, 123.8, 124.2, 124.6, 124.8, 125.9, 126.1, 126.5, 127.5, 127.6, 127.8, 128.5,
128.6, 128.7, 128.8, 129.5, 131, 131.8, 135.2 ppm; HRMS (MALDIꢀTOF) for formula
C69O2NH7 calculated mass 881.0474 experimental mass (m/z) 881.0470 (M+).
Fabrication of Device
Preparation of zinc oxide (ZnO) precursor solution: ZnO precursor solution was prepared
by dissolving 1g (4.5mM) zinc acetate dihydrate (Zn(CH3CO2)2.2H2O) in 10 mL
methoxyethanol with addition of 0.254 mL monoethanolamine (MEA) as chelating agent
(stabilizer). The mixture solution was then stirred on a hot plate at 60 °C for about 2 hours
until the solution changed to a clear transparent solution. This solution was then left for a day
before use in device fabrication.
Current–Voltage Characteristics: Devices with the architecture ITO/ZnO/PC61BM/Al
were constructed and investigated under ambient conditions. Glass plate coated with
patterned transparent ITO electrode are ultrasonically cleaned with detergent, distilled water,
acetone and isopropyle alcohol, dried and used as anode. ZnO precursor solution was coated
on to the substrates by using spin coater at 3000 rpm. The substrates were annealed at 250° C
for 30 minutes. Acceptor solution (40 mg/mL) prepared in chlorobenzene was spin coated on
top of the ZnO layer and then dried at 120 °C for 15 min. Finally, the substrates were loaded
into a thermal evaporator for the deposition of the cathode material i.e., 100 nm Al layer was
deposited on top of the acceptor material under high vacuum. IꢀV was measure using
computer controlled Keithley 236 source measure unit.

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Results and Discussion
So far the best approach for the cyclopropanation addition to C60 is through the 1,3 dipolar
cycloaddition of diazo compound generated from their tosyl hydrazone precursors in pyridine
in presence of sodium methoxide under inert atmosphere.9 In the present work, we have
followed our more promising AACA approach for the cyclopropanation reaction which is
more ecoꢀfriendly and occurs under aerobic conditions with excellent yield and purity.5,6
AACA approach is widely applicable for generation of different types of diazoalkanes
intermediates and further cycloaddition reaction on fullerene ball in presence of amine under
completely aerobic conditions.
AACA approach opens up new energy efficient and less hazardous synthesis process
for methanofullerenes which are irreplaceable materials in various applications and
especially in organic solar cells which is going to be huge market in near future
requiring bulk synthesis of these materials at cheapest cost.10 Dichloromethane is
preferred solvent over any other chlorinated solvent and amines are green substitute
for pyridine making this methodology much more ecoꢀfriendly with least
environmental impact.11,12 To make it completely green process in the present study
we have performed the reactions in ethyl acetate in place of dichloromethane for
diazoalkanes generation.
Diazomethane intermediate 1b is generated from ester 1a insitu in presence of tert.
amine in ethyl acetate at 0oC under aerobic condition. Toluene solution of C60 is added
and heated at 80 oC for 18 hours. Reaction mixture was concentrated and subjected to
column chromatography to collect the second band after unreacted fullerene giving
single spot in TLC. This is the kinetically favourable [5,6] fulleroid (in 13C NMR
bridgehead carbon appears at 64 ppm). Refluxing this fulleroid in toluene converts
totally to [6,6]PC61BP by thermal isomerisation.

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The structure of the PC61BP is confirmed by spectroscopic techniques. FTIR clearly shows
the presence of alkyl CꢀH stretching at 2957, 2923, 2855 cmꢀ1, C=O bond stretching at 1727
cmꢀ1. 1H NMR and 13C NMR further confirm the formation and purity of [6,6]
methanofullerene (1) where the bridging carbon and bridgehead carbons appear at 64 and 89
1
ppm respectively in 13C NMR. H NMR clearly shows all the alkyl and aromatic proton
peaks.
Similar to above, tosylhydrazones 2b was prepared by refluxing 2ꢀnitrocinnamldehyde with
pꢀtoluene sulfonyl hydrazide in methanol. On cooling down the reaction mixture, the crystals
of hydrazone separate out and washed with cooled methanol, dried and used for next step
reaction with fullerene[60]. Similar to phenylꢀdiazoenone reaction, aerobic reaction
conditions were used for reaction in presence of triethyl amine base in cooled ethyl acetate.
After 3 hours, toluene solution of fullerene[60] was added and temperature was raised to 80
oC. The progress of the reaction was monitored by TLC. Reaction mixture was evaporated in
rotaꢀvapor and put on silica gel column for purification. Product HC61NP was collected after
small amount of unreacted fullerene using toluene as eluent. NMR data completely supports
the formation and high purity of monoꢀadduct with [6,6]methanofullerene geometry as
observed by us in our previous report for such structures.6 1H NMR spectrum of HC61NP
shows the typical chemical shift at 4.9 ppm for the bridging C(61) proton. Similarly, the 13C
NMR chemical shift of the bridging and bridgehead carbons appeared at 67.7 and 86.0, 86.2
ppm respectively.
Figure 2 shows the absorption spectra of PC61BP & HC61NP and compared with PCBM in
chloroform solution (25 ꢀM) and film. PC61BP and HC61NP show completely overlapping
peaks at 328, 404, 435 and 697 nm confirming the [6,6] addition on fullerene core akin to
13
[6,6]PC61BM (2a). The optical absorption spectra of PC61BP, HC61NP and PCBM were

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also recorded in thin film (2b). The absorption spectra show extended absorption in whole
visible region. Red shifted absorption band appear at 347 nm with broad absorption between
14
400 to 800 nm due to aggregation.
Figure 2. Normalized absorption spectra of PC61BP, HC61NP and PC61BM in (a) chloroform solution and (b)
films.
Cyclicꢀvoltammetry (CV) was performed to evaluated the electrochemical properties
of the products and compared with PC61BM synthesized by AACA approach (Figure
3). CV was done in dry oꢀdichlorobenzene using 0.1 M supporting electrolyte (nꢀ
TBAPF6) and three electrode system where, a platinum disc was used as working

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electrode, silverꢀwire and a platinumꢀwire as reference and counter electrode
respectively. Ferrocene/ferricenium couple was an internal reference to calibrate the
redox potentials. PC61BP and HC61NP show similar behaviour to PC61BM with first
reduction potential located at ~ ꢀ1.33 and ꢀ1.38 V respectively. Both the products show
two reversible reduction potentials and the LUMO energy level, calculated from the
onset reduction potential, was found to be ~ꢀ3.6 eV.
Figure 3. CV curves of PC61BP, PC61BM and HC61NP in dry oꢀdichlorobenzene using 0.1 M supporting electrolyte (nꢀ
TBAPF6).
The materials were then tested for their electron transport at room temperature by
making electron only devices. The electron mobility of materials were investigated by
simplest and most versatile space charge limited current (SCLC) model for organic
15
semiconductors with device structure of ITO/ZnO/methanofullerene/Al. SCLC
measures charge carrier mobility perpendicular to the film plane. The current shows
quadratic dependency on voltage at room temperature which is the characteristic of
space charge limited current. The energy band diagram of the device is shown in
figure inset. The electron mobility can be calculated by the following MottꢀGurney
16
equation

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9
ꢉꢊ ꢋ ꢊꢌꢍꢎꢏ
ꢐꢑ
ꢀꢁꢂꢃꢂ ꢄ ꢅꢆꢅꢇꢈꢆ
8
Herein, JSCLC stands for current density, L is the thickness of the device, V is the
applied potential, Vbi is the builtꢀinꢀpotential, 0 is vacuum permittivity and its value is
ε
8.85x10ꢀ12 F/m, εr is the permittivity of the active layer in the device and its value is
17
taken as 3.9, µ0 is the electron mobility of fullerene materials.
Charge carrier transport property is an important parameter for solar cell characterization.
The cells are fabricated in such a way that the carriers of opposite sign have been selectively
blocked by choosing the relevant contacts of different work function. For electronꢀonly (EO)
devices, suppression of hole injection can be done by using various metal oxide barrier, such
as aluminum oxide (Al2O3), ZnO, TiO2 etc. The single carrier device structure has been
shown in the scheme 2.
Scheme 2. Schematic of holeꢀonly and electronꢀonly devices.
The mechanism of injection of charge carrier depends on the bandꢀedge mismatch due to
which potential barrier at the interface is created. The work function of ZnO (7.4 eV) does
not match with ITO (4.7 eV) which blocks the hole injection from ITO into acceptor. So that
only electrons are expected to flow while on the other hand, due to the mismatch between the
work function of cathode and acceptor material, the holes are not allowed to flow. The dark Iꢀ
V data are plotted as log(JSCLC) vs. log(V) for both PC61BP and HC61NP for 200 nm active

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layer thickness (see ESI). The calculated electron mobility is 1.5 x10ꢀ3 cm2/V.s for PC61BP,
0.6x10ꢀ5 cm2/Vs for HC61NP and 0.1x10ꢀ3 cm2/Vs for reference PC61BM. The calculated
electron mobility of reference PC61BM is in accordance to the previous reported data for
18
electron only devices. PC61BP shows better electron mobility than PC61BM. HC61NP
shows lower mobility but still much higher or equivalent to previously reported novel
,
19 20
fullerene acceptors.
We also investigated the electron mobility of PC61BP and HC61NP at variable thickness via
electron only devices geometry (Scheme 2). The solution in chlorobenzene was prepared with
different concentrations of 20, 30 and 40 mg/mL and spin coated on the top of the ITO/ZnO
substrates. and charge carrier mobility was determined from the SCLC model. Generally the
charge carrier mobility in organic materials is low due to structural disorder in the films and
decreases with decreasing film thickness. In our case also charge carrier mobility was found
21
to decrease with decreasing active layer thickness. The calculated electron mobility for
PC61BP with film thickness of 200, 90, 75, 60 nm was 1.5 x 10ꢀ3 cm2/V.s, 1.5 x 10ꢀ4
cm2/V.s, 4.45 x 10ꢀ5 cm2/V.s and 1.7 x 10ꢀ5 cm2/V.s respectively (Figure 4a). Similarly for
HC61NP the electron mobility was calculated to be 0.6 x 10ꢀ5 cm2/V.s, 0.6 x 10ꢀ6 cm2/V.s,
0.18 x 10ꢀ6 cm2/V.s and 0.68 x 10ꢀ7 cm2/V.s for 200, 90, 75, 60 nm film thickness respectively
(Figure 4b). Therefore, thickness of the organic film is an essential parameter for determining
the charge carrier mobility. The decreased film thickness mainly limits the charge carrier
injection in the bulk of the material which leads to reduced carrier collection rate resulting in
decreased charge carrier mobility.

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(a)
(b)
Figure 4. Plot of electron mobility of (a) PC61BP and (b) HC61NP for different film thickness.
Conclusions
In conclusion, we have synthesized two new methanofullerenes i.e., PC61BP and
HC61NP, via our ecoꢀfriendly AACA methodology for cycloaddition reaction on
fullerene. Both the materials have been synthesized in good yield and purity. Materials
have been characterized for their structure using different spectroscopic techniques
and finally for their electron transport property. HC61NP shows quite promising
electron mobility but the high mobility of PC61BP make it a better candidate to be

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used as acceptor in organic solar cells in place of PC61BM. High yielding, ecoꢀ
friendly and cost effective AACA approach for high performing methanofullerene
synthesis open up low cost methodology which will eventually lower the ultimate
organic solar cell cost.
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Al
Cathode
ITO/ZnO
Anode
+
New methanofullerene derivatives were synthesized via cost effective and eco-friendly AACA
approach to obtain better mobility than PC61BM.