A new fullerene C60-didodecyloxy benzene dyad: An evidence for ground state electron transfer

Article abstract of DOI:10.1016/j.cplett.2005.08.043

Synthesis, complete spectroscopic characterization and cyclic voltammetric measurements of a new didodecyloxybenzene-C₆₀ dyad is described. The dyad showed distinct ground state intramolecular charge transfer as predicted from molecular orbital

Full text of DOI:10.1016/j.cplett.2005.08.043

Chemical Physics Letters 414 (2005) 198–203
www.elsevier.com/locate/cplett
A new fullerene C60–didodecyloxy benzene dyad:
An evidence for ground state electron transfer
1
*
S. Shankara Gayathri , Archita Patnaik
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India
Received 5 July 2005; in final form 28 July 2005
Available online 16 September 2005
Abstract
Synthesis, complete spectroscopic characterization and cyclic voltammetric measurements of a new didodecyloxybenzene–C60
dyad is described. The dyad showed distinct ground state intramolecular charge transfer as predicted from molecular orbital calcu-
lations and complimented by absorption and fluorescence spectroscopic behavior along with cyclic voltammetric studies, in solvents
of varying polarity. The ab initio B3LYP/3-21G(d) electronic structure calculations are indicative of a non-planar ground state
structure, 8 kJ/mol lower in energy than the planar structure. A ꢀ13° dihedral angle between the benzene ring and the oxygen
of the alkoxybenzene moieties of the dyad is attributed to the antibonding interactions associated with the HOMO components.
Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction
recombination (CR) in C60-based systems. C60 deriva-
tives fabricated as electrodes display photovoltaic prop-
erties and some are employed in self-assembled
monolayers and Langmuir–Blodgett films to realize high
solar energy conversion efficiency [11,12]. The donor
molecules employed to the C60-dyad component are aro-
matic amino compounds [13,14], tetrathiafulvalenes
(TTF) [6,15,16], ferrocene [17,18], porphyrins [19,20],
oligothiophenes [21], etc. Oxygen containing donors like
alkoxybenzenes [22,23], furans [24], pyrans [16] have
also been used to form charge transfer (CT) complexes
with C60. Alkoxy groups in benzene rings give rise to
low oxidation potentials due to their strong p-resonance
electron donating ability [22]. Functionalization of C60
as pyrazoline [23] and oxazole [24] derivatives or with
donors like TTF [15] gave rise to intramolecular ground
state CT complexes with a CT band at around 470 nm
shifting towards longer wavelengths upon increasing
the polarizability of the solvent. Further, C60 derivatized
as [6,6]-phenyl-C61-butyric acid-methyl ester (PCBM)
has been studied extensively as electron-acceptor frag-
ments in the bulk heterojunction photovoltaic cells
[25,26].
The possibility of replacing silicon with new organic
semiconductors in photovoltaic devices is a promising
task for large area applications. Thus, a variety of dyads
are synthesized with covalently linked electron donor
and acceptor fragments capable of undergoing light in-
duced electron transfer [1–4]. Molecular bridges are
built into these dyads in order to induce a long lived
charge-separated (CS) state for potential use in subse-
quent charge stabilization processes [5]. As electron
acceptors, quinones [6,7], radialenes [8] and fullerene[60]
[9] have been used, among which C60 is widely used; as
in such systems, C60 retains its redox properties as well
as photophysical properties. The low reorganization en-
ergy of C60 [10] affords various unique photoinduced
phenomena, such as, fast charge separation, charge
*
Corresponding author. Fax: +9144 2257 4202.
E-mail addresses: gaya3shank@yahoo.com (S.S. Gayathri),
archita59@yahoo.com (A. Patnaik).
1
Present address: Department of Chemistry, Indian Institute of
Technology Madras, Chennai 600036, India.
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.08.043

S.S. Gayathri, A. Patnaik / Chemical Physics Letters 414 (2005) 198–203
199
The above literature reports prompted us to synthe-
size a dyad with PCBM and didodecyloxy benzene as
acceptor and donor compounds. At the design stage a
hydrophobic–hydrophilic–hydrophobic network was
aimed to study its molecular self-organization. Herein,
we report the synthesis, electronic structure and the
ground state electronic properties of C60–didodecyloxy
benzene dyad.
added ferrocene in acetonitrile:toluene (1:4 v/v) and
CH2Cl2 solvent under argon atmosphere. The solvents
used for cyclic voltammetry were purchased from Merck
Limited and were further purified according to standard
procedures [27]. The measurements were done at argon
atmosphere. Absorption measurements were done on
Varian Cary 5E UV–vis spectrophotometer. The sol-
vents used were of UV grade and the donors aniline
and pyridine were purified prior to use. Ferrocene and
TTF were purchased from Aldrich Chemicals, USA
and were used without further purification. Steady-state
fluorescence spectra were measured on Jobin Yvon fluo-
rolog-3-11 spectrofluorimeter with a resolution of
0.2 nm.
2. Experimental section
2.1. General and materials
1H NMR and 13C NMR spectra were recorded on a
Bruker AdvanceTM 400 MHz Fourier transform NMR
spectrometer. The IR spectra were recorded on JASCO
FTIR 410 spectrometer. MALDITOF-MS spectra were
acquired on Voyager DE PRO Biospectrometry Work-
station (Applied Biosystems) with 2,5-dihydroxybenzoic
acid as matrix. Elemental analyses were performed on a
Perkin–Elmer 2400 series CHNS/O Analyser. Melting
points were measured with a Cintex melting point
apparatus and uncorrected. Cyclic voltammetric mea-
surements were carried out on an CHI 608B Electro-
chemical workstation (CH Instruments, USA) with
Glassy Carbon working and Platinum counter elec-
trodes, and Silver wire as the pseudoreference electrode,
n-Bu4NPF6 as the supporting electrolyte with internally
3. Results and discussion
3.1. Synthesis and characterization
The synthesis of the dyad 5 is shown in Scheme 1 and
the complete procedure is given in supporting informa-
tion. Compound 6 was prepared as previously reported
[28]. The alkylation of the 3,4-dihydroxy benzoate fol-
lowed by deprotection of the ethyl ester gave the corre-
sponding acid that served as the donor addend for the
subsequent tether-functionalisation with the PCBM.
Further the presence of this donor in the dyad made it
well soluble in most of the common organic solvents
OH
O
O
OH
O
O
a
3
b
O
O
O
OH
O
O
2
c
1
O
O
O
OH
O
d
4
O
O
O
O
O
O
OH
O
5
6
Scheme 1. (a) C12H25Br, K2CO3, 18-crown-6, butanone, 24 h, reflux; (b) NaOH, H2O:C2H5OH (1:4), 8 h, reflux; (c) SOCl2, 3 h, reflux;
hydroquinone, CH2Cl2, pyridine, 6 h, reflux; (d) DCC, o-DCB, 6, 12 h, 0–25 °C.

200
S.S. Gayathri, A. Patnaik / Chemical Physics Letters 414 (2005) 198–203
such as CH2Cl2, CHCl3, tetrahydrofuran, toluene, o-
weak charge transfer interaction between the donor
and the C60 moiety in the ground state. The alkoxy
groups of the donor have significant amplitude in the
HOMO, contributed by the 2p orbitals on the oxygen
atoms of didodecyloxy groups situated perpendicular
to the plane of benzene ring. These oxygen 2p orbitals
appear with phases opposite to the adjacent orbitals on
the benzene ring resulting in antibonding interactions
that destabilize the HOMO and produce radical ion
pair in the dyad. Further, a deviation from planarity,
i.e., a dihedral angle of ꢀ13° was noted between the
benzene and the oxygen of the alkoxybenzene moiety
leading to a distorted configuration. To understand
this, a single point energy calculation was performed
with the dihedral angle set to zero, thus enforcing a ri-
gid rotation on the molecule to adopt a planar config-
uration. The energy of this planar structure lies only
8 kJ/mol higher in energy than the optimized structure
and is viewed as the energy barrier for the molecule to
attain planarity.
dichlorobenzene, benzonitrile, dioxan, etc. The dyad
1
was fully characterized by H and 13C NMR, FTIR,
MALDI-MS, and UV–vis spectroscopy.
3.2. Molecular orbital calculations
Density functional theory (DFT) calculations at a
moderate level with B3LYP/3-21G(d) basis set often
yielded correct geometry and electronic structures,
agreeing with experimental results [18,20]. In the present
investigation, structure optimization was done using
GAUSSIAN 03 suite of programs [29], in order to probe
the equilibrium geometry and the electronic structure
of the dyad. Fig. 1a shows the geometry optimized struc-
ture on Born–Oppenheimer potential energy surface
˚
leading to a dyad length of 34.56 A, a bridge length of
˚
11.22 A and a total dipole moment of of 8.9 D.
The molecular orbital energy levels of the individual
donor and acceptor molecules as compared to those of
the dyad in Fig. 1d shows the lowest occupied molecular
orbital (LUMO) energy levels of the acceptor compare
well with LUMO of the dyad and the highest occupied
molecular orbital (HOMO) energy levels of the donor
are closer in energy with the HOMO energy level of
the dyad implying the localization of the HOMO and
LUMO energy levels on the donor and acceptor parts
of the dyad.
The molecular orbital isosurfaces in Fig. 1 shows the
majority of the electron distribution of the LUMO to
be located on the C60 spheroid and of the HOMO is
located on the alkoxybenzene donor with a small orbi-
tal coefficient spread on to the bridge, suggesting a
3.3. Solution phase redox behavior and intramolecular
electron transfer
Determination of redox potentials in the formed
molecular donor–acceptor type systems is important to
probe the existence of charge transfer (CT) interactions
between the donor and the acceptor in the ground state
and also to evaluate its energetics. The electrochemical
behaviour of the compounds 4, 5 and C60 were studied
using cyclic voltammetry (CV) at room temperature on
glassy carbon working electrode in toluene/acetonitrile
(TL/ACN, 4:1 v/v) solvent mixture with tetrabutyl
Fig. 1. Ab initio B3LYP/3-21G(d) calculated (a) geometry optimized structure, (b) frontier HOMO and (c) frontier LUMO of the didodecyloxy
benzene–fullerene C60 dyad. (d) Comparison of the HOMO and LUMO energy levels for the donor, the dyad and the acceptor.

S.S. Gayathri, A. Patnaik / Chemical Physics Letters 414 (2005) 198–203
201
Table 1
0.8
0.7
0.6
0.5
0.4
0.3
0.2
1.0
0.8
0.6
0.4
0.2
0.0
Half wave potentials [E1/2 = (Epa + Epc)/2] of the dyad 5a in volts
measured vs. the potential for the redox couple of internally added
Fc/Fc+
Compound
Eo1x
Er2ed
Er3ed
Er4ed
Er5ed
C60
5
ꢁ0.94
ꢁ1.06
ꢁ1.35
ꢁ1.39
ꢁ1.50
ꢁ1.76
ꢁ2.14
ꢁ2.08
ꢁ2.28
+0.80b
+0.50b
+0.60b
ꢁ2.61
5 in CH2Cl2
2
410
420
430
440
450
460
470
benzene
toluene
chlorobenzene
o- dichlorobenzene
dichloromethane
carbontetrachloride
tetrahydrofuran
carbon disulphide
benzonitrile
a
Working electrode: glassy carbon; counter electrode: platinum;
reference: Ag|Ag+ as pseudo reference electrode; supporting electro-
lyte: 0.1 M TBAPF6; solvent: toluene/acetonitrile (4:1 v/v) unless
specified.
b
Irreversible according to CV.
ammonium hexafluorophosphate (TBAPF6) as the sup-
porting electrolyte. The CVs were acquired under argon
atmosphere and are summarized in Table 1 (see Fig. 2).
The dyad exhibited three pairs of reversible
400
450
500
550
600
650
700
750
800
wavelength (nm)
Fig. 3. Absorption spectra of the dyad 5 in solvents of varying
polarity. Inset: expanded view of the 431 nm peak showing the red
shifts with increasing solvent polarity.
(Er2ed; Er3ed; E4red) and one pair of quasi reversible (Er5ed
)
redox waves corresponding to the fullerene core
(Fig. 2). In general, these reduction potential values
are shifted to more negative values in respect to C60.
This shift stems from the saturation of a double bond
in the C60 core that raises the LUMO energy of the
resulting organo-fullerene moiety. Further, these shifts
of the order of ꢀ100 mV suggest a partial ground state
CT from the donor to the C60 moiety.
On the anodic side, the dialkoxybenzene moiety pre-
sents good donor strength with one one-electron irre-
versible wave (Eo1x) at around +0.80. These values are
anodically shifted as compared with that of 2 by
ꢀ200 mV, indicating the existence of an appreciable de-
gree of ground state intramolecular CT supported by the
molecular orbital calculations. Further, the presence of
the ester groups in the dyadÕs bridge helps in facilitating
the electron transport. The influence of ester group on
the electronic properties of dyads has been widely dis-
cussed by Prato et al. [30,17].
Further to prove this CT behavior, CV experiments
were performed in a more polar solvent CH2Cl2 under
similar conditions and the results are shown in Table
1. The ground state CT behavior becomes more evident
from the larger shifts of order of 300 and 200 mV in
comparison with that of C60 and the dyad, respectively,
in TL/ACN solvent (Fig. 2).
3.4. Solvatochromism in solvents of varying dielectric
constants
Absorption spectrum of the dyad showed a character-
istic feature at 431 nm, characteristic of [6,6]-bridged
monoadducts. Further, a weak absorption at 697 nm is
assigned to 0–0 transition is also observed [23,24]. The
HOMO–LUMO gap estimated as 1.7 eV compares rea-
sonably with the overestimated calculated value of
2.3 eV from DFT B3LYP/3-21G(d). Another broad fea-
ture in 450–650 nm region in Fig. 3 represents the visible
transitions in the dyad which is indicative of the strong
electron donating capability of the present donor. This
band is weaker or almost absent in the case of other
dyads with weak electron-donating groups [23]. Similar
broad bands have been attributed to weak electronic
interactions between donor and acceptor moieties and
have been reported for intramolecular CT in other
C60-based dyads [15,17], or intermolecular CT transi-
tions with added donors [31]. With changing solvents
polarizability [32], the above absorption bands shifted
appreciably attributing to the formation of intramolecu-
lar ground state charge transfer complex.
100
50
TL/ACN
CH2Cl2
0
-50
-100
-150
-200
-250
2
1
0
-1
-2
-3
E (in Volts) vs Fc / Fc+
Fig. 2. Cyclic voltammograms of the dyad 5, 1mM toluene:acetonitrile
(4:1 v/v); and in CH2Cl2; supporting electrolyte: 0.1 M TBAPF6; scan
rate: 500 mV/s; working electrode: glassy carbon.

202
S.S. Gayathri, A. Patnaik / Chemical Physics Letters 414 (2005) 198–203
Addition of trifluoroacetic acid (TFA) to the dyad in
4. Conclusion
various solvents did not show any observable change in
the absorption intensity, peak position and shape indi-
cating absence of any proton acceptance by the dyad
from TFA in these solvents.
The present investigation has successfully dealt with a
molecular design, synthesis and structure characteriza-
tion of a new PCBM acceptor-based dyad, the didode-
cyloxy benzene–C60 with alkoxy substituents in the
benzene ring, contributing a stronger electron donating
capability to the donor. Sufficient antibonding interac-
tions between the alkoxy group and the benzene ring re-
vealed the destabilization of the HOMO which results in
the formation of radical ion pair. Pronounced intramo-
lecular charge transfer as revealed by absorption and
electrochemical investigations justify the dyad to be de-
picted as Cd60–donordþ in the ground state.
3.5. Emission behavior of the dyad from steady-state
fluorescence spectra
The fluorescence spectra of the dyad were acquired
at room temperature in solvents of varying polarity
upon excitation at 431 nm and are shown in Fig. 4.
Each spectrum with a peak maximum at ꢀ706 nm
and a shoulder above 760 nm are typical of mono-
adduct derivatives of C60. Hence, emission from the
singlet excited state of C60 is thus inferred. Further,
the dyad shows solvatochromic red shifts from
766 nm in CCl4 (e = 2.24) to 789, 785 and 792 nm in
polar CH2Cl2 (e = 8.93), benzonitrile (e = 26) and
CS2 (e = 2.24, highly polarizable), respectively. A
similar CT character has been assigned in isoxazolo-
fullerene–donor (donor = dimethylaniline, ferrocene,
furan) dyads to CT character [24,33]. It is also noted
that the fluorescence quenching is more marked on
moving from a non-polar solvent to the more polar
benzonitrile. Thus, the intramolecular CT interaction
of the dyad in the ground state leads to quenching
of fluorescence in the excited state, probably because
of the high degree of CT in excited state in more
polar solvents. Further, an attempt made to correlate
the stokes shifts with the orientational polarizability
as given by Lippert–Mataga equation did not yield
linear variation. This may be due to the fact that
the equation fails for elongated molecules like the
present dyad where ellipsoid form will be more
appropriate [34].
Acknowledgement
This work is supported by Department of Science and
Technology (DST), Government of India under Grant
No. SP/SI/H-37/2001.
Appendix A. Supplementary data
The synthetic procedure for the dyad 5 preparation is
available at www.sciencedirect.com. Supplementary
data associated with this article can be found, in the on-
line version at doi:10.1016/j.cplett.2005.08.043.
References
[1] F. Fungo, L. Otero, C.D. Borsarelli, E.N. Durantini, J.J. Silber,
L. Sereno, J. Phys. Chem. B 106 (2002) 4070.
[2] M.A. Loi, P. Denk, H. Hoppe, H. Neugebauer, C. Winder, D.
Meissner, C. Brabec, N.S. Sariciftci, A. Gouloumis, P. Vazquez,
T. Torres, J. Mater. Chem. 13 (2003) 700.
[3] A.S. Lukas, P.J. Bushard, M.R. Wasilewski, J. Phys. Chem. A 106
(2002) 2074.
16
[4] M.R. Wasilewski, Chem. Rev. 92 (1992) 435.
[5] J.G. Vos, R.J. Forster, T.E. Keyes, in: Interfacial Supramolecular
Assemblies, Wiley, West Sussex, England, 2003.
[6] Y. Yamashita, M. Tomura, J. Mater. Chem. 8 (1998) 1933.
[7] T.W. Kim, H. Lee, D. Byun, D. Kang, Thin Solid Films 331
(1998) 8.
[8] K. Takahashi, Y. Tarutani, Chem. Commun. (1994) 519.
[9] N. Martin, L. Sanchez, B. Illescas, I. Perez, Chem. Rev. 98 (1998)
2527.
benzene
toluene
chlorobenzene
o- dichlorobenzene
dichloromethane
carbontetrachloride
tetrahydrofuran
carbon disulphide
14
12
10
benzonitrile
8
[10] H. Imahori, K. Hagiwara, T. Akiyama, M. Aoki, S. Taniguchi, T.
Okada, M. Shirakawa, Y. Sakata, Chem. Phys. Lett. 263 (1996)
545.
[11] M. Maggini, G. Possamai, E. Menna, G. Soccrano, N. Camioni,
G. Ridolfi, G. Casalbore-Miceli, L. Franco, M. Ruzzi, C. Corvaja,
Chem. Commun. (2002) 2028.
[12] H. Imahori, H. Norieda, H. Yamada, Y. Nishimura, I. Yamazaki,
Y. Sakata, S. Fukuzumi, J. Am. Chem. Soc. 123 (2001) 100.
[13] S. Komamine, M. Fujitsuka, O. Ito, K. Morikawi, T. Miyata, T.
Ohno, J. Phys. Chem. 104 (2004) 11497.
6
4
2
0
650
675
700
725
750
775
800
wavelength (nm)
Fig. 4. Fluorescence spectra (normalized at 650 nm) of the dyad 5 in
solvents of varying polarity.
[14] K.G. Thomas, V. Biju, D.M. Guldi, P.V. Kamat, M.V. George, J.
Phys. Chem. 103 (1999) 8864.

S.S. Gayathri, A. Patnaik / Chemical Physics Letters 414 (2005) 198–203
203
[15] H. Nishikawa, S. Kojima, T. Kodama, I. Ikemoto, S. Suzuki, K.
Kikuchi, M. Fujitsuka, H. Luo, Y. Araki, O. Ito, J. Phys. Chem.
A 108 (2004) 1881.
[16] D.V. Konarev, R.N. Lyubovskaya, N.V. Drichko, E.I. Yuda-
nova, Y.M. Shulga, A.L. Litvinov, V.N. Semkin, B.P. Tarasov, J.
Mater. Chem. 10 (2000) 803.
[24] F. Langa, P. de la Cruz, E. Espildora, A. GonzaÕlez-CorteÕs, A. de
la Hoz, V. Lopez-Arza, J. Org. Chem. 65 (2000) 8675.
[25] A. Dhanabalan, J.K.J. van Duren, P.A. van Hal, L.J. van
Dongen, R.A. Jannsen, J. Adv. Funct. Mater. 11 (2001) 255.
[26] L. Chen, D. Godovsky, O. Inganas, J.C. Hummelen, R.A.
Jannsen, M. Svensson, M.R. Adersson, Adv. Mater. 12 (2000)
1367.
[27] W.L.F. Armarego, D.D. Perrin, fourth edn., Purification of
Laboratory Chemicals, Butterworth–Heinemann, Elsevier Science,
Woburn, MA, 1996, p. 67.
[28] J.C. Hummelen, B.W. Knight, F. LePeq, F. Wudl, J. Yao, C.L.
Wilkins, J. Org. Chem. 60 (1995) 532.
[29] M.J. Frisch et al., GAUSSIAN 03, Revision C.02, Gaussian, Inc.,
Wallingford, CT, 2004.
[30] M. Prato, M. Maggini, C. Giacometti, G. Soccrano, G. Sandona,
G. Farnia, Tetrahedron 52 (1996) 5221.
[31] M.C. Rath, H. Pal, T. Mukherjee, J. Phys. Chem. A 103 (1999)
4993.
[17] D.M. Guldi, M. Maggini, G. Soccrano, M. Prato, J. Am. Chem.
Soc. 119 (1997) 974.
[18] F. DÕSouza, M.E. Zandler, P.M. Smith, G.R. Deviprasad, K.
Arkady, M. Fujitsuka, O. Ito, J. Phys. Chem. A 106 (2002) 649.
[19] P.A. Liddel, D. Kuciauka, J.P. Sumida, B. Nash, D. Nguyen, A.L.
Moore, T.A. Moore, D. Gust, J. Am. Chem. Soc. 119 (1997) 1400.
[20] F. DÕSouza, G.R. Deviprasad, M.E. Zandler, V.T. Hoang, A.
Klykov, M. VanStipdonk, A. Perera, M.E. El-Khouly, M.
Fujitsuka, O. Ito, J. Phys. Chem. A 106 (2002) 3243.
[21] N. Armaroli, G. Accorsi, J. Gisselbrechdt, M. Gross, V. Krasnikov,
D. Tsamouras, G. Hadziioannou, M.J. Gomez-Escalonilla, F.
Langa, J. Eckert, J. Nirengarten, J. Mater. Chem. 12 (2002) 2077.
[22] T. Khan, J.J.W. McDouall, E.J.L. McInnes, P.J. Skabara, P.
Frere, S.J. Coles, M.B. Hursthouse, J. Mater. Chem. 13 (2003)
2490.
[32] C. Reichardt, second edn., Solvents and Solvent Effects in Organic
Chemistry, VCH, Weinheim, 1988.
[33] K.B. Simonsen, V.V. Konovalova, T. Kawai, M.P. Cava, L.D.
Kispert, R.M. Metzger, J. Becher, Perkin Trans. 2 (1999) 657.
[34] B. Valeur, in: Molecular Fluorescence, Principles and Applica-
tions, Wiley-VCH, Weinheim, 2002.
[23] F. Langa, P. de la Cruz, E. Espildora, A. De la Hoz, J.L.
Bourdelande, L. Sanchez, N. Martin, J. Org. Chem. 66 (2001)
5033.