[60]Fullerene-based molecular triads with expanded absorptions in the visible region: Synthesis and photovoltaic properties

Article abstract of DOI:10.1021/jp0478413

Photoactive fulleropyrrolidine-perylenetetracarboxylic diimide-porphyrin triad (FPP) and its zinc analogue (ZnFPP) with expanded absorptions in the visible spectral region were synthesized. Steady absorption spectra and cyclic voltammetry (CV) measurement showed rather weak electronic interactions among the component chromophores in the ground state within the triads. Preliminary fluorescence measurements indicated the occurrence of a photoinduced electron-transfer process within the triads. These processes can be elicited either in the excited state of the perylenetetracarboxylic diimide chromophore or in the excited state of the porphyrin chromophore. The existence of photoinduced charge-separated states within triads in solution was confirmed by steady-state photolysis in the presence of methyl viologen (MV²⁺) as a sacrificed electron acceptor and 1-benzyl-1,4-dihydronicotinamide (BNAH) as a sacrificed electron donor. The photovoltaic devices have been fabricated on the basis of the triads and have shown clear photovoltaic behavior under illumination of white light. Though the power conversion efficiencies under simulated solar illumination of 68 mW·cm^-2 were found to be moderate (0.028% for FPP and 0.035% for ZnFPP), it is an encouraging result for application of such molecular donor-acceptor ensembles to photovoltaics.

Full text of DOI:10.1021/jp0478413

J. Phys. Chem. B 2004, 108, 16677-16685
16677
[60]Fullerene-Based Molecular Triads with Expanded Absorptions in the Visible Region:
Synthesis and Photovoltaic Properties
Shengqiang Xiao,† Yuliang Li,*,† Yongjun Li,† Junpeng Zhuang,† Ning Wang,† Huibiao Liu,†
Bin Ning,‡ Yang Liu,† Fushen Lu,† Louzhen Fan,‡ Chunhe Yang,† Yongfang Li,† and
Daoben Zhu*,†
CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100080, P. R. China, and Department of Chemistry, Beijing Normal UniVersity,
Beijing 100080, P. R. China
ReceiVed: May 19, 2004; In Final Form: August 25, 2004
Photoactive fulleropyrrolidine-perylenetetracarboxylic diimide-porphyrin triad (FPP) and its zinc analogue
(ZnFPP) with expanded absorptions in the visible spectral region were synthesized. Steady absorption spectra
and cyclic voltammetry (CV) measurement showed rather weak electronic interactions among the component
chromophores in the ground state within the triads. Preliminary fluorescence measurements indicated the
occurrence of a photoinduced electron-transfer process within the triads. These processes can be elicited
either in the excited state of the perylenetetracarboxylic diimide chromophore or in the excited state of the
porphyrin chromophore. The existence of photoinduced charge-separated states within triads in solution was
confirmed by steady-state photolysis in the presence of methyl viologen (MV2+) as a sacrificed electron
acceptor and 1-benzyl-1,4-dihydronicotinamide (BNAH) as a sacrificed electron donor. The photovoltaic devices
have been fabricated on the basis of the triads and have shown clear photovoltaic behavior under illumination
of white light. Though the power conversion efficiencies under simulated solar illumination of 68 mW‚cm-2
were found to be moderate (0.028% for FPP and 0.035% for ZnFPP), it is an encouraging result for application
of such molecular donor-acceptor ensembles to photovoltaics.
Introduction
photovoltaic materials, we are aiming to incorporate a fullerene
cage into such molecular dyads or triads which should offer an
optimal balance between synthetic accessibility, processability,
and efficient photoresponsibility expected to result in applica-
tions in photovoltaic devices. To date, many electro- and
photoactive chromophores were incorporated in donor-acceptor
molecular arrays on the basis of fullerene derivatives.8 Among
them, porphyrin analogues are mostly envisaged as components
of artificial molecular devices to mimic light energy conversion
due to their versatile optical, redox, and photochemical
properties.8e,9 However, light absorption is still limited due to
the narrow range of the sharp porphyrin Soret band (410-430
nm) and blank absorption between the Soret and Q-bands.
Recently, red dyes based on perylenetetracarboxylic diimide
(PTDI) have attracted considerable attention and have been
employed in various applications due to their exclusive chemical,
thermal, and photochemical stability.10 They have intensive
absorptions (ꢀ ≈ 105 M-1 cm-1 at 490 nm in CHCl3) between
porphyrin’s Soret and Q-bands and emit fluorescene with
quantum yields near unity.11 Perylenetetracarboxylic diimides
can also function as charge-transfer participants10e and charge
carriers.12 Both the optical and redox characteristics of these
chromophores can be manipulated by varying the substituents
on the imide functions.10b,13 Substitutions in the bay regions of
the perylene core can make these materials even excellent
electron donors.14 Thus, suitable chemical modification can
make perylenetetracarboxylic diimide an ideal accessory pig-
ment acting as an electron acceptor to porphyrin chromo-
phores10e,15 and electron donor to fullerenes. From the light-
harvesting standpoint, attractive photophysical properties make
bay region substituted perylenetetracarboxylic bis(imide)s good
Recently, developing inexpensive renewable energy sources
has stimulated tremendous interest in construction of light energy
conversion systems.1 It has been well established that fullerenes
have the ability of remarkable acceleration of photoinduced
charge separation and deceleration of charge recombination in
donor-fullerene systems.2 Parallel to blending functionalized
fullerenes with a π-conjugated polymer, incorporating fullerence
cages as pendants to a π-conjugated polymer backbone has been
anticipated toward the realization of organic solar cells with
high efficiency.3 Devices made from these material systems
reached power conversion efficiencies (η) up to 3.3% under
AM1.5 standard test conditions.4 Other viable approaches made
the use of molecular dyads with a functionalized fullerene
covalently linked to π-conjugated oligomers, which showed η
values around 10-2%.5 A power conversion of 0.37% was
achieved by utilizing a fullerene-azothiophene dyad as the
photovoltaic active layer under 80 mW‚cm-2 white light
illumation.6 The monochromatic power conversion efficiency
of the photovoltaic devices incorporating the oligothiophenes-
fullerene dyad (16 T/C60) as a single-component photoactive
material was evaluated as 0.4% recently.7
For the construction of efficient and low-cost photovoltaic
devices, enhanced light-harvesting efficiency of chromophore
molecules throughout the solar spectrum together with a highly
efficient conversion of the harvested light into electrical energy
is very essential. As part of our research to develop new
* To whom correspondence should be addressed. E-mail: ylli@iccas.ac.cn.
† Chinese Academy of Sciences.
‡ Beijing Normal University.
10.1021/jp0478413 CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/05/2004

16678 J. Phys. Chem. B, Vol. 108, No. 43, 2004
Xiao et al.
SCHEME 1: Synthesis of H2P and ZnP
SCHEME 2: Synthesis of N-(n-Dodecyl)-2-
(p-hydroxyphenyl)[60]fulleropyrrolidine (4)a
candidates as accessory pigments with porphyrins and fullerenes
for devising integrated systems to transmit and process solar
energy.
We sought to integrate the advantages of porphyrins, perylene-
tetracarboxylic diimide, and fullerenes to construct the light
energy conversion system with their absorption expanded in the
visible spectral region for a better matching of the solar
spectrum. In this contribution, we describe the synthesis of a
fullerene-perylenetetracarboxylic diimide-porphyrin triad (FPP)
and its zinc analogue (ZnFPP), which showed experimental
evidence for photoinduced electron transfer in a preliminary
study. The suitability of such molecules for photovoltaic
applications has been tested by the construction and character-
ization of photovoltaic devices based on the triads.
Results and Discussion
Synthesis. As shown in Scheme 1, 5-(4-hydroxyphenyl)-
10,15,20-tris(3′,4′,5′-trimethoxyphenyl)porphyrin (H2P) contain-
ing nine methoxy units to improve its solubility and electron
donative property was conveniently synthesized in 10% yield
from the acid-catalyzed condensation of 3,4,5-trimethoxyben-
zaldehyde and 4-hydroxybenzaldehyde with pyrrole in appropri-
ate portions. To overcome the intrinsic weak solubility of
fullerene and perylenetetracarboxylic diimides units for mean-
ingful spectroscopic characterization, both fulleropyrrolidine and
perylenetetracarboxylic diimide moieties were functionalized
with solubilizing dodecyl and octyl chains, respectively. Di-
bromination derivative N,N′-dioctyl-1,7-dibromoperylene-3,4:
9,10-tetracarboxylic bis(imide) (1) was synthesized according
to the literature.16 The preparation of fulleropyrrolidine 4
containing a phenol hydroxyl group was launched from p-
hydroxybenzaldehyde by following the synthetic route sum-
marized in Scheme 2. In our strategy, the protection of the
phenol hydroxyl and the subsequent N-dodecylation of com-
pound 5 through amination of aldhyde using sodium triacetoxy-
borohydride after Prato reaction17 makes the reaction conditions
very temperate with higher overall yield. Compound 3 is easily
deprotected to obtain fulleropyrrolidine 4 in 1:1 CH2Cl2/CH3-
OH with the catalysis of micro p-toluenesulfonic acid following
the N-dodecylation reaction. This method avoids the tedious
route to obtain the corresponding N-substituted R-amino acid
before the Prato reaction.18 Then, fulleropyrrolidine 4 reacted
with 2 equiv of 1 in toluene in the presence of anhydrous
potassium carbonate and 18-crown-6 affords PNA-PE with the
a Conditions: (i) CH2Cl2, dihydropyran, micro p-toluenesulfonic acid
monohydrate, rt (room temperature), 25 min (60%); (ii) PhCl, glycine,
reflux, 12 h; (iii) CH2Cl2/PhMe (6:1), 10 equiv of NaBH(AcO)3, 10
equv of HAc, 10 equiv of dodecanal, rt, 2 h; (iv) CH2Cl2/CH3OH (1:
1), micro p-toluenesulfonic acid monohydrate, rt, 5 h (95%).
yield of 53% as shown in Scheme 3, which is obviously different
from the usual preparation processes to obtain bay-region-
substituted perylenetetracarboxlic diimde with phenoxy units
in the solvent of N-methylpyrrole.19 The desired fullerene-
perylenetetracarboxylic diimide-porphyrin triad FPP was then
obtained by condensation of PNA-PE with H2P mediated by
the anhydrous potassium carbonate. Preparations of Zn(II)-
porphyrined samples ZnP and ZnFPP were carried out by the
treatment of H2P and FPP with Zn(OAc)2 in chloroform,
respectively. Compound 5 depicted in Chart 1 obtained from
p-methylphenol and 1 by the same method as described for
PNA-PE can be used as reference compounds together with
compounds 4, H2P, and ZnP. All compounds were fully
characterized on the basis of analytical results and spectral data
1
including H NMR, 13C NMR, MALDI-TOF MS, and FT-IR
(see Experimental Section).
Absorption Spectra. Figure 1 depictes the absorption spectra
of FPP and ZnFPP in chloroform at room temperature. The
absorption spectra of H2P, ZnP, and reference compounds 4

[60]Fullerene-Based Molecular Triads
J. Phys. Chem. B, Vol. 108, No. 43, 2004 16679
SCHEME 3: Synthesis of FPP and ZnFPP
CHART 1: Structure of PTDI Reference Compound 5
almost the same for the PTDI moiety in FPP and ZnFPP. Due
to the weak absorption in the region of 350-800 nm for
fulleropyrrolidine (432 nm, ꢀ ) 4400 M-1 cm-1; 705 nm, ꢀ )
540 M-1 cm-1 for reference compound 4),20 it is reasonable
that the absorption spectra of FPP and ZnFPP show superposi-
tion features of the fulleropyrrolidine, perylenetetracarboxylic
diimide, and porphyrin components. The virtually superimposed
absorption features of the component chromophores in FPP and
ZnFPP indicate no evidence for strong electronic interactions
among the three component chromophores in the ground state.
Electrochemical Properties. The electrochemical properties
of the triads and reference compounds were investigated by
cyclic voltammetry (CV) in o-dichlorobenzene and Bu4NPF6
(0.1 M) on a glassy carbon electrode. The results are summarized
in Table 1. Interestingly, the first reduction potential of the
PTDA chromophore in FPP and ZnFPP happened to be
overlapping with that of the fulleropyrrolidine chromophore,
which may result in a more complicated photophysical process
within the triads upon excitation. Closer inspection of the redox
features of the triads reveals a good correspondence with the
redox potential of the reference compounds. These results are
indicative of relatively weak ground-state electronic interactions
between the component chromophores within the triads of FPP
and ZnFPP, in agreement with the conclusion drawn from the
UV/vis spectra.
and 5 are also included. An obvious feature can be observed
that the perylenetetracarboxylic diimide unit absorbs strongly
in the region between the porphyrin Soret and Q-bands. The
free-base porhyrin Soret band of FPP is almost unchanged at
the position of 423 nm compared to that of H2P. As for the
weak free-base porphyrin Q(0,0), Q(0,1), Q(0,2), and Q(0,3)
bands at the positions of 648, 591, 554, and 518 nm, they were
also unchanged. Similar phenomena are also found for ZnFPP
with the zinc porphyrin Soret band at the position of 426 nm
and the weak porphyrin Q(0,0) and Q(0,1) bands at 596 and
553 nm, respectively. As for two phenoxy-substituted perylene-
tetracarboxylic diimide reference 5, it shows characteristic
aborption at the positions of 546, 512, 472, and 402 nm, which
are denoted as (0,0), (0,1), (0,2), and (0,3) bands of the
perylenetetracarboxylic diimide unit, respectively, which is
Fluorescence Spectra. Preliminary fluorescence measure-
ments of FPP, ZnFPP, and reference compounds 4, 5, H2P,
and ZnP were carried out in chloroform at the fixed concentra-
tion of 1 µM at room temperature. The fluorescence spectrum

16680 J. Phys. Chem. B, Vol. 108, No. 43, 2004
Xiao et al.
Figure 1. Absorption spectra (solid line) of compounds H2P, FPP, ZnP, ZnFPP, 4, and 5 in chloroform at room temperature at the fixed concentration
of 1.0 × 10-6 M. The porphyrin absorptions in the region of 450-800 nm have been multiplied by the factors as shown. The extinction coefficients
(ꢀ) of the porphyrin and zinc porphyrin Soret bands at 423 and 426 nm are 5.36 × 105 and 6.15 × 105 M-1 cm-1, respectively. The ꢀ value is 3.3
× 104 M-1 cm-1 for perylenetetracarboxylic diimide at 490 nm. The dotted lines are fluorescence spectra of H2P, ZnP, 4, and 5 in chloroform at
room temperature obtained by exciting the porphyrin unit at the Soret band and exciting the fullerene cage and perylenetetracarboxylic diimide at
490 nm, respectively. All spectra are normalized to the same amplitude in their corresponding panels.
TABLE 1: Reduction and Oxidation Potentials Ea (V vs SCE) for the Compounds 4, H2P, ZnP, FPP, and ZnFPP by CV on a
-
Glassy Carbon Electrode in o-Dichlorobenzene Solutions (n-Bu4N+PF6 as Supporting Electrolyte) at Room Temperature
reducn
oxidn: porphyrin
porphyrin
PTDI
C60
E(2)
E(1)
E(2)
E(1)
E(2)
E(1)
E(2)
E(1)
E(3)
4
-0.68
-1.08
-1.66
H2P
ZnP
FPP
ZnFPP
5
+1.06b
+0.85
+1.07b
+0.86
-1.31
-1.62
-1.30
-1.64d
-1.62
-1.90
+1.08
+1.09
-0.67c
-0.67c
-0.68
-0.87
-0.87
-0.86
-0.67c
-0.67c
-1.07
-1.07
-1.65
-1.90
-1.65d
a E ) (Epa + Epc)/2 at a scan rate of 50 mV/s. b Irreversible peaks. c,d Peaks are overlapping.
of H2P as shown in Figure 1 consists principally of the Q(0,0)
and Q(1,0) bands at 651 and 713 nm respectively. So does that
of ZnP at 602 and 647 nm. The fluorescence spectrum of 5
consists of a strong emission at the (0,0) band near 575 nm and
the (1,0) band near 625 nm. The emissions of FPP and ZnFPP
were monitored using excitation at wavelengths where either
the porphyrin unit or the perylenetetracarboxylic diimide unit
primarily absorbs (for the fluorescence spectra of FPP, ZnFPP,
and 4, see the Supporting Information). Upon excitation of FPP
and ZnFPP at 490 nm where porphyrin, PTDI, and C60 were
excited in a relative absorption ratio (%) of around 12:85:3,
almost complete quenching of the fluorescence corresponding
to the perylenetetracarboxylic diimide unit was observed
compared with that of reference 5 and no other chromorphore
fluorescence was detected. Likewise, upon excitation of FPP
and ZnFPP at the Soret band (423 nm for free-base porphyrin,
426 nm for zinc porphyrin), where porphyrin, PTDI, and C60
were excited in a relative absorption ratio (%) of 97:2.5:1.5, no
obvious emissions could be observed. In either situation, no
detectable emission from the fulleropyrrolidine unit (710 nm)
was found. However, for reference fulleropyrrolidine 4, the
emission from the C60 moiety was clearly seen when excited at

[60]Fullerene-Based Molecular Triads
J. Phys. Chem. B, Vol. 108, No. 43, 2004 16681
SCHEME 4: Photoreduction of MV2+ by the Triads of
FPP or ZnFPP in the Presence of BNAHa
the photophysical behaviors of FPP and ZnFPP in deoxygen-
ated solutions under irradiation upon adding methyl viologen
(MV2+) as a sacrificed electron acceptor and 1-benzyl-1,4-
dihydronicotinamide (BNAH) as a sacrificed electron donor.21
Considering the first oxidation potential of the porphyrin and
the first reduction potential of the C60 and PTDA moiety in
FPP, there is high capacity (1.74 eV) to thermodynamically
form a photoinduced charge-separated state. Similarly, the
photoinduced charge-separated state of ZnFPP with the energy
of 1.53 eV can be expected upon excitation as shown in Scheme
4. A comparison between the one-electron reduction potential
of C60 or PTDA (-0.67 eV) and the MV2+ (E(MV
) )
2+/MV•+
-0.45 V vs SCE)22 indicated that an electron-transfer form C60
or PTDA•- to MV2+ is exothermic by 0.22 eV (Scheme 4).
Also, when an external electron donor such as BNAH with the
one-electron oxidation potential of 0.57 V vs SCE23 is added
to the systems, an electron transfer from BNAH to P•+ (1.07V)
or ZnP•+ (0.86V) is expected to be exothermic by 0.5 and 0.29
eV, respectively. Thus, once the charge separation states are
obtained via photoirradiation of FPP or ZnFPP, the oxidation
of BNAH and the reduction of MV2+ should be achieved by a
•-
a
P and ZnP represent porphyrin and zinc porphyrin, respectively.
490 nm (see Figure 1). Therefore, there is no clear evidence
for the existence of singlet-singlet energy transfer from
porphyrin or perylenetetracarboxylic diimide to fulleropyrroli-
dine within the triads of FPP and ZnFPP. Such strong
quenchings of porphyrin and perylenetetracarboxylic diimide
components suggest the occurrence of intramolecular photoin-
duced charge-transfer processes within the triads. These pro-
cesses can be elicited either through the excited state of
perylenetetracarboxylic diimide chromophore or through that
of porphyrin chromophore.
reaction with P•+ (or ZnP•+) and C60 (or PTDA•-), respec-
tively. These processes were experimentally confirmed by
•-
24
photolysis of the FPP (P-PTDA-C60)/BNAH/MV2+ and
ZnFPP (ZnP-PTDA-C60)/BNAH/MV2+ systems with mono-
chromatized light of 423 nm in deoxygenated DMF/H2O (95:
5, v/v). Figure 2 depicts the steady-state photolysis of FPP and
Steady-State Photolysis in the Presence of a Sacrificed
Electron Acceptor and an Electron Donor. Motivated by its
steady-state fluorescence measurements, we further investigated
Figure 2. (a) Spectral changes observed in the steady-state photolysis of an Ar-saturated DMF/H2O (95:5, v/v) solution of BNAH (4.0 × 10-4 M),
MV2+ (8.0 × 10-4 M), and FPP (1.0 × 10-6 M) under irradiation with monochromatized light of λ ) 423 nm. (b) Spectral changes obtained for
the spectra at different irradiation times minus that at the beginning of irradiation according to (a). (The inset shows the absorption band at 605 nm
with a multiple of 5.) (c) Spectral changes observed in the steady-state photolysis of an Ar-saturated DMF/H2O (95:5, v/v) solution of BNAH (4.0
× 10-4 M), MV2+ (8.0 × 10-4 M), and ZnFPP (1.0 × 10-6 M) under irradiation with monochromatized light of λ ) 423 nm. (d) Spectral changes
obtained for the spectra at different irradiation times minus that right at the beginning of the irradiation according to (c). (The inset shows the
absorption band at 605 nm with a multiple of 5.)

16682 J. Phys. Chem. B, Vol. 108, No. 43, 2004
Xiao et al.
Figure 3. I/V curves of photovoltaic devices produced from FPP and ZnFPP under illumination with 68 mW‚cm2 white light (O) and dark (b)
for an active area of 0.12 cm2.
TABLE 2: Cell Photovoltaic Parameters under 68 mW/cm2
White Light Irradiation at Room Temperature
ZnFPP, in which a progressive decrease in the absorption band
of BNAH at 347 nm and obvious increase in the characteristic
MV•+ absorption band (λmax ) 398 and 605 nm)25 can be
observed with the time of the irradiations. By contrast, no
reaction occurs in the dark or in the absence of FPP or ZnFPP
under photoirradiation. It is worthwhile to note that an increase
tendency in the porphyrin Soret band (423 nm for FPP and
429 nm for ZnFPP) can also be observed from the spectral
change as shown in Figure 2b,d which was obtained by the
spectrum at the different irradiation time minus that right at
the beginning of the irradiation according to Figure 2a,c,
respectively, whereas, in the similar steady-state photolysis
experiments of the dyads only consisting of porphyrin and
PTDA chromophores, the porphyrin Soret band remained almost
unchanged.26 It may suggest that the C60 moiety has electronic
interactions with the porphyrin chromophore in FPP and ZnFPP
in the excited state.
Photovoltaic Devices. FPP and ZnFPP have been measured
as active materials in photovoltaic devices with the configura-
tion as ITO/PEDOT/SAMPLE/Al. Either of the pristine triads
was sandwiched between poly(3,4-ethylenedioxythiophene)
(PEDOT)-covered indium tin oxide (ITO) and aluminum
electrodes. The organic active layers were prepared by spin
coating from chlorobenzene solutions. As compared to devices
built by vacuum vapor deposition techniques using mono- and/
or multilayers of small molecules,27 the triads of FPP and
ZnFPP have the advantage of possible solution deposition and
the use of a single layer. In the bulk heterojunction approach,
which has been successfully realized for conjugated polymer/
fullerene solar cells,2a,28 problems with macroscopic phase
separation of the components, giving rise to inefficient charge
separation and transport, may occur.5b,29 In the triads of FPP
and ZnFPP, where the donor and acceptor components are
covalently linked, problems with phase separation are avoided.
Moreover, an important issue for improving the efficiency of
photovoltaic devices is the selection and development of new
molecules having good overlap of their absorption spectrum with
the terrestrial solar emission spectrum and high absorption
coefficients.30 These requirements are quite satisfied by the triads
of FPP and ZnFPP as can be drawn from the absorption spectra
of thin films (Figure S2 in the Supporting Information).
Figure 3 shows the current-voltage (I/V) characteristics for
the photovoltaic devices produced of FPP and ZnFPP in dark
and under illumination with 68 mW‚cm2 white light for an active
area of 0.12 cm2. The corresponding results concerning the
photovoltaic measurements are summarized in Table 2. Under
sample thickness/nm Isc/104A‚cm-2 Voc/V
FF
102η/%
FPP
ZnFPP
∼12
∼12
1.4
1.7
0.54
0.54
0.25
0.26
2.8
3.5
illumination, the devices show clear photovoltaic behaviors. The
I/V curves are completely reversible. The device diode behaviors
were found with a rectification ratio of ∼200 for FPP and ∼10
for ZnFPP in the dark between -1.5 and 1.5 V, respectively.
In the dark, a small kink is discernible in the semilogarithmic
plot of the I/V curve between 0 and 1.0 V representing the small
ohmic contribution from the shunts and the contacts of C60 with
Al and ITO electrodes.5b,31 From the results in Table 2, we can
see that the photovoltaic performances of ZnFPP is slightly
better than that of FPP. It can be explained that the energy level
of the charge-separated states resulted from the photoinduced
electron transfer decreased in comparison with that of FPP
containing free-base porphyrin moiety, due to the increased
donating ability of zinc porphyrin in ZnFPP as can be drawn
from the CV results. However, the power conversion efficiencies
in these tests are found to be still moderate and are in the same
amplitude as that reported previously.5,9a Further investigation
will be necessary to optimize the device structure and fully
clarify the photophysical features within the triads.
Conclusions
In conclusion, we have reported here the photoactive triad
consisting of fulleropyrrolidine, porphyrin, and perylenetetra-
carboxylic diimide with expanded absorptions in the visible
spectral region. Its zinc analogue was also investigated. Photo-
induced electron-transfer processes within the triad were
confirmed through preliminary fluorescence measurements and
steady-state photolysis. The fabrication of photovoltaic devices
using the triad was also realized. Though the power conversion
efficiencies under simulated solar illumination of 68 mW‚cm-2
were found to be moderate (0.028% for FPP and 0.035% for
ZnFPP), it is an encouraging result for application of such
molecular donor-acceptor ensembles to photovoltaics. Further
improvements on photovoltaic performances could be expected
by the use of related fullerene derivatives with a stronger
aborption in the visible range matching the solar spectrum.
Gaining insight into the photophysical features of the triads and
further structure optimization of the devices are currently
underway.

[60]Fullerene-Based Molecular Triads
J. Phys. Chem. B, Vol. 108, No. 43, 2004 16683
Experimental Section
as the eluent affords the pure product as purple crystals (Rf )
0.65, CHCl3/CH3COOC2H5, 10:1, yield 10%). 1H NMR
(CDCl3): 8.96 (s, 4H), 8.95 (d, 2H, J ) 4.7 Hz), 8.89 (d, 2H,
J ) 4.7 Hz), 8.05 (d, 2H, J ) 8.4 Hz), 7.48 (s, 6H), 7.17 (d,
2H, J ) 8.4 Hz), 4.18 (s, 9H), 3.98 (s, 18H). 13C NMR
(CDCl3): 155.7, 151.4, 137.9, 137.7, 135.6, 134.4, 130.9, 120.3,
120.0, 119.8, 113.8, 112.9, 61.3, 56.4. MALDI-TOF MS: m/e
900.7. FT-IR (KBr, ν (cm-1)): 3320.0, 2935.0, 2832.3, 1580.9,
1501.4, 1466.8, 1408.5, 1356.5, 1234.4, 1126.7, 1004.5, 976.6,
925.1, 803.2, 730.6.
Reagents were purchased as reagent grade from Acros or
Aldrich Corp. and were utilized as received unless indicated
otherwise. BNAH was prepared according to the literature.23
All solvents were purified using standard procedures. Evapora-
tion and concentration in vacuo were done at water aspirator
pressure, and compounds were dried at 10-2 Torr. Column
chromatography (CC): SiO2 (160-200 meshes). TLC glass
plates coated with SiO2 F254 were visualized by UV light. UV/
vis spectra were measured on a Hitachi U-3010 spectrometer.
FT-IR spectra were recorded as KBr pellets on a Perkin-Elmer
System 2000 spectrometer. The 400 MHz 1H NMR spectra and
13C NMR spectra (ppm) were recorded on a Bruker ARX400
spectrometer. The solvent signal was used as an internal
reference for both 1H and 13C NMR spectra. MALDI-TOF mass
spectrometric measurements were performed on Bruker Biflex
III MALDI-TOF.
Cyclic voltammetry (CV) was performed on CHI voltam-
metric analyzer (CHI 660b) at room temperature with a three-
electrode configuration in o-dichlorobenzene solution containing
a supporting electrolyte. A glassy carbon (i.d. ) 3 mm) disk
served as the working electrode, a platinum foil and a SCE being
the counter and the reference electrodes, respectively. Both the
counter and the reference electrodes were directly immersed in
the electrolyte solution. The surface of the working electrode
was polished with commercial R-alumina powder (CH Instru-
ments, Inc.; particle size 1.0 µm) prior to use. Tetra-n-
butylammonium hexafluorophosphate (>99%, Fluka) was re-
crystallized twice from ethanol and dried in a vacuum at 100
°C overnight prior to use and was employed as the supporting
electrolyte (0.1 M). Solutions were stirred and deaerated by
bubbling nitrogen for about 10 min prior to each voltammetric
measurement. The scan rate was 50 mV‚s-1 unless otherwise
specified.
4-(Tetrahydropyran-2-yloxy)benzaldehyde (2). Dihydro-
pyran (1.1 equiv) was added in a dropwise manner to micro
p-toluenesulfonic acid (5 mg) and p-hydroxybenzaldehyde (1
equiv) in CH2Cl2 bathed in cool water. After 0.5 h, the reaction
mixture was washed with saturated aqueous NaHCO3 solution
and then dried over anhydrous sodium sulfate. The solvent was
removed in vacuo, and the colorless liquid product was obtained
by column chromatography (SiO2 (160-200 mesh), 3:2 ethyl
acetate/petroleum ether, yield 60%). 1H NMR (CDCl3): 10.04
(s, 1H), 7.98 (d, 2H, J ) 8.8 Hz), 7.31 (d, 2H, J ) 8.7 Hz),
5.10 (m, 1H), 4.02 (m, 2H), 3.66 (m, 2H), 2.04 (m, 2H), 1.81
(m, 2H). 13C NMR (CDCl3): 190.9, 162.1, 131.8, 130.5, 116.5,
96.1, 62.9, 30.7, 25.4, 19.7. EI-MS: m/e 206.
2-(4-(Tetrahydropyran-2-yloxy)phenyl)[60]fullero-
pyrrolidine (3). A solution of C60 (1 equiv), glycine (2 equiv),
and 4 (5 equiv) was heated at reflux in chlorobenzene for 12 h.
The solvent was removed in vacuo, and the product was purified
by column chromatography (SiO2 (160-200 mesh), eluted by
toluene, yield 42%). 1H NMR (6:1 CS2/CDCl3): 7.80 (d, 2H, J
) 8.5 Hz), 7.20 (d, 2H, J ) 8.6 Hz), 5.88 (s, 1H), 5.52 (s, 1H),
5.23 (d, 1H, J ) 10.1), 5.02 (d, 1H, J ) 10.1), 4.00 (m, 2H),
3.73 (m, 2H), 3.24 (bs, 1H), 2.01 (m, 2H), 1.86 (m, 2H). 13C
NMR (6:1 CS2/CDCl3): 157.4, 154.0, 153.8, 153.2, 147.2,
146.9, 146.6, 146.5, 146.4, 146.3, 146.2, 146.1, 146.0, 145.8,
145.6, 145.4, 145.3, 144.7, 144.6, 144.4, 144.4, 143.2, 143.1,
142.8, 142.7, 142.4, 142.3, 142.2, 142.1, 142.0, 141.8, 141.6,
136.7, 136.3, 136.0, 135.9, 130.3, 129.2, 128.4, 125.5, 116.7,
115.9, 96.1, 77.0, 72.6, 61.8, 61.6, 30.8, 26.0, 19.3. MALDI-
TOF MS: m/e 940.3. FT-IR (KBr, ν (cm-1)): 3447 (br), 2933,
2848, 1806, 1509, 1430, 1237, 1178, 1035, 962, 827, 527.
Steady-state photolysis was carried out as follows: A square
quartz cuvette (10 mm i.d.) containing a deaerated N,N′-
dimethylformamide (DMF)-water (95:5, v/v) solution of PPP
or ZnPPP (1.0 × 10-6 M), MV2+ (8.0 × 10-4 M), and BNAH
(4.0 × 10-4 M) was irradiated with monochromatized light of
λ ) 423 nm from a 150 W metal halogen lamp through a filter.
The light intensity was determined as 2.3 mW/cm2. The
photochemical reactions were monitored by measuring the
absorption spectra at different irradiation times.
N-(n-Dodecyl)-2-(p-hydroxyphenyl)[60]fulleropyrroli-
dine (4). A mixture of 1 equiv of 5, 10 equiv of dodecanal, 10
equiv of sodium triacetoxyborohydride, and 10 equiv of acetic
acid was stirred in toluene/dichloromethane (1:6 in volume) at
room temperature for 2 h. The remaining borohydride was
destroyed by the addition of 20 mL of saturated aqueous sodium
hydrogen carbonate solution. After the mixture was dried with
anhydrous sodium sulfate, the solvent of the organic layer was
removed under reduced pressure. The resulted solid was
subsequently redissolved in 30 mL of CH2Cl2. After the addition
of ca. 5 mg of p-toluenesulfonic acid monohydrate to the
solution, 30 mL of methanol was then added in drops with
vigorous stirring for about 5 h. The mixture was washed with
saturated aqueous sodium hydrogen carbonate solution, dried
over anhydrous sodium sulfate, and evaporated to give crude
product, which was further purified by column chromatography
(SiO2 (160-200 mesh), eluent 1:1 petroleum ether/toluene) and
For fabrication of the photovoltaic device, poly(ethylene-
dioxythiophene) (PEDOT) used for a hole injection/transport
layer resulting in approximate thickness of 50 nm, as determined
from a surface profiler, was spin coated from an aqueous
dispersion under ambient conditions on precleaned (washing,
UV ozone treatment) ITO-covered glass substrates, and the layer
was dried by annealing the substrate on a hot plate. The pristine
dyads and triads were spin cast from chlorobenzene to give an
active layer of nearly 12 nm thickness as determined with a
surface profiler. Finally, a Al(100 nm) back-electrode was
deposited by thermal evaporation under vacuum (5 × 10-6
mbar). The active area of the device is 0.12 cm2. Illumination
was performed with a XQ-500 Xe lamp, set to 68 mW‚cm-2
.
H2P. A solution of 3,4,5-trimethoxybenzaldehyde (5.65 g,
33.9 mmol), 4-hydroxybenzaldehyde (1.42 g, 11.6 mmol), and
pyrrole (3.2 mL, 46.2 mmol) in dry CHCl3 (1200 mL) was
purged with nitrogen for 30 min, and then 0.1 mL of boron
trifluoride diethyl etherate was added. The solution was stirred
for 2 h at room temperature, and chloranil (12.3 g) was added.
The mixture was stirred overnight, and then the solvent was
removed. Column chromatography on silica gel with chloroform
1
subsequent washed with acetone (yield 95%). H NMR (6:1
CS2/CDCl3): 7.82 (bs, s, 2H), 7.02 (d, 2H, J ) 8.0 Hz), 5.23
(d, 1H, J ) 9.2 Hz), 5.15 (s, 1H), 4.25 (d, 1H, J ) 9.2 Hz),
3.36 (m, 1H), 2.68 (m, 1H), 2.00-2.19 (m, 2H), 1.40-1.80
(m, 18H), 1.05 (t, 3H, J ) 7.2 Hz). 13C NMR (6:1 CS2/
CDCl3): 156.7, 155.5, 154.3, 153.8, 147.3, 146.8, 146.6, 146.4,
146.3, 146.3, 146.2, 146.2, 146.1, 146.1, 145.9, 145.8, 145.6,

16684 J. Phys. Chem. B, Vol. 108, No. 43, 2004
Xiao et al.
145.5, 145.3, 145.3, 145.2, 145.2, 145.1, 144.7, 144.7, 144.4,
143.2, 143.0, 142.7, 142.6, 142.6, 142.4, 142.3, 142.1, 142.0,
141.8, 141.7, 141.5, 130.8, 129.6, 115.5, 82.1, 68.9, 66.9, 53.1,
32.0, 29.8, 29.8, 29.5, 28.4, 27.7, 22.8, 14.2. MALDI-TOF
MS: m/e 1023.8. FT-IR (KBr, ν (cm-1)): 3433 (br), 2922, 2850,
1612, 1512, 1460, 1170, 835, 527.
porphyrin-containing samples in chloroform and refluxed for
about 30 min. After cooling, the reaction mixture was washed
with saturated sodium bicarbonate aqueous solution and water
successively and dried over anhydrous sodium sulfate, and then
the solvent was removed under reduced pressure. Tedious
column chromatography on silica gel with chloroform as the
eluent afforded the corresponding Zn(II)-porphyrin-containing
targets.
ZnP. Rf: 0.24 (10:1 CHCl3/CH3COOC2H5). 1H NMR
(CDCl3): 9.23 (s, 4H), 9.21 (d, 2H, J ) 4.7 Hz), 9.15 (d, 2H,
J ) 4.7 Hz), 8.20 (d, 2H, J ) 8.1 Hz), 7.64 (s, 6H), 7.31 (d,
2H, J ) 8.4 Hz), 4.32 (s, 9H), 4.11 (s, 18H). MALDI-TOF
MS: m/e 962.5. FT-IR (KBr, ν (cm-1)): 3340.0, 2934.2, 2833.5,
1581.2, 1494.1, 1460.4, 1408.2, 1235.4, 1165.8, 1126.4, 1071.1,
1000.7, 940.9, 801.0, 724.1.
ZnFPP. Rf: 0.79 (10:1 CHCl3/CH3COOC2H5). 1H NMR
(CDCl3): 9.45 (1H), 9.30 (1H), 9.13 (2H), 9.04 (6H), 8.07-
8.50 (6H), 7.51-7.56 (8H), 7.20-7.26 (4H), 4.75 (1H), 4.60
(1H), 4.23 (10H), 4.12 (2H), 4.00 (20H), 3.75 (1H), 3.25 (1H),
1.20-2.00 (m, 44H), 0.84-0.91 (m, 9H). MALDI-TOF MS:
m/e 2597.2. FT-IR (KBr, ν (cm-1)): 2923.7, 2851.4, 1698.1,
1660.0, 1593.3, 1499.3, 1499.0, 1459.5, 1406.5, 1346.7, 1237.2,
1214.4, 1165.0, 1126.6, 1002.3, 941.3, 858.2, 809.9, 721.2,
527.2.
5. 5 was prepared with a yield of 80% according to the general
method described for the synthesis of compound PNA-PE
except that p-methylphenol was used as the starting material
instead of N-(n-dodecyl)-2-(p-hydroxyphenyl)[60]fulleropyr-
rolidine (4). 1H NMR (CDCl3): 9.50 (d, 2H, J ) 8.2 Hz), 8.55
(d, 2H, J ) 8.2 Hz), 8.27 (s, 2H), 7.25 (d, 4H, J ) 7.7 Hz),
7.06 (d, 4H, J ) 7.7 Hz), 4.12 (m, 4H), 2.40 (s, 6H), 1.70 (m,
4H), 1.26-1.50 (m, 20H), 0.85 (t, 6H, J ) 7.2 Hz). MALDI-
TOF MS: m/e 826.8. FT-IR (KBr, ν (cm-1)): 2925, 2855, 1697
(s), 1659 (s), 1596 (s), 1503 (s), 1408, 1336 (s), 1264 (s), 1199,
811, 750.
PNA-PE. A 102 mg amount of N-(n-dodecyl)-2-(p-hydroxy-
phenyl)[60]fulleropyrrolidine (4) was added into 60 mL of
toluene solution containing compound 3 (180 mg, 2 equiv), K2-
CO3 (2 equiv), and 18-crown-6 (2 equiv) under nitrogen
atmosphere at room temperature. Then the reaction mixture was
heated under reflux and detected through TCL. When the
starting fullerene material was consumed, the solvent was
evaporated under reduced pressure. The resulted mixture was
loaded for column chromatography to afford 90 mg of pure
target product PNA-PE (Rf ) 0.50, toluene, yield 53%) eluted
by toluene. 1H NMR (CDCl3): 9.57 (d, 1H, J ) 10.9 Hz), 9.32
(d, 1H, J ) 10.9 Hz), 8.87 (s, 1H), 8.60 (d, 1H, J ) 10.9 Hz),
8.57 (d, 1H, J ) 10.9 Hz), 8.16 (s, 1H), 7.90 (br, 2H), 7.18 (d,
2H, J ) 10.8 Hz), 5.12 (m, 2H), 4.08-4.24 (m, 5H), 1.19-
1.37 (m, 40H), 0.87-0.97 (m, 9H). 13C NMR (CDCl3): 162.9,
162.8, 162.3, 162.2, 155.5, 154.8, 154.0, 153.1, 147.2, 146.5,
146.3, 146.2, 146.2, 146.1, 146.0, 145.9, 145.8, 145.6, 145.5,
145.4, 145.3, 145.3, 145.2, 145.1, 144.6, 144.3, 143.1, 142.9,
142.6, 142.5, 142.3, 142.1, 142.0, 142.0, 141.8, 141.6, 141.4,
140.1, 140.1, 139.9, 139.9, 138.0, 136.7, 136.5, 135.9, 135.6,
133.5, 132.9, 132.3, 131.9, 131.1, 129.1, 129.0, 128.5, 127.1,
124.6, 124.2, 123.8, 123.6, 123.2, 122.5, 122.3, 122.1, 124.6,
124.2, 123.8, 123.6, 123.2, 122.5, 122.3, 122.1, 120.0, 81.7,
68.8, 53.3, 40.8, 31.9, 31.8, 29.7, 29.4, 29.3, 28.1, 27.3, 27.1,
22.7, 22.6, 14.2. MALDI-TOF: m/e 1712.5, 992.4. FT-IR (KBr,
ν (cm-1)): 2920.9, 2850.1, 1699.3, 1660.8, 1591.2, 1500.3,
1459.6, 1434.1, 1399.3, 1332.9, 1258.3, 1198.8, 1169.6, 808.2,
526.3.
FPP. A solution of 80 mg of PNA-PE in 40 mL of dry
toluene containing K2CO3 (2 equiv) and 18-crown-6 (2 equiv)
was stirred under N2 for 20 min, and subsequently H2P (50
mg, 1.2 equiv) was added. The mixture was heated to 100 °C
and continuously stirred until the reaction finished as detected
by TLC. The solvent was then evaporated under reduced
pressure, and the resulting mixture was loaded for column
chromatography to afford 60 mg of pure target product FPP
(Rf ) 0.83, CHCl3/CH3COOC2H5, 10:1, yield 60%) eluted by
chloroform. 1H NMR (CDCl3): 9.34-9.37 (m, 2H), 9.02 (2H),
8.93 (6H), 8.06-8.15 (6H), 7.45-7.65 (8H), 7.16-7.26 (d*d,
4H), 4.75 (1H), 4.25 (1H), 4.23 (10H), 4.10 (m, 4H), 4.01 (s,
18), 3.75 (br, 1H), 3.30 (br, 1H), 2.35 (br, 2H), 1.6-2.0 (m,
42H), 0.82-0.92 (m, 9H), -2.93 (s, 2H). 13C NMR (CDCl3):
162.6, 162.4, 155.1, 154.8, 154.3, 151.4, 146.0, 145.2, 144.9,
144.7, 144.5, 144.4, 144.2, 144.0, 143.7, 143.5, 143.1, 142.9,
142.7, 142.0, 141.5, 141.2, 141.0, 140.6, 140.4, 140.1, 139.9,
139.7, 139.3, 138.9, 138.3, 137.9, 137.6, 136.2, 135.7, 135.1,
134.7, 134.3, 134.0, 133.2, 132.8, 131.5, 130.6, 129.7, 129.1,
128.9, 128.6, 128.5, 128.0, 127.3, 126.9, 125.2, 124.7, 124.1,
124.0, 123.8, 123.6, 122.9, 122.4, 122.1, 121.9, 121.1, 120.3,
120.3, 118.5, 118.1, 117.8, 113.6, 113.0, 81.4, 66.5, 61.3, 56.5,
56.4, 53.2, 40.8, 31.9, 31.8, 29.8, 29.7, 29.4, 28.6, 28.1, 27.6,
27.3, 22.7, 14.1. MALDI-TOF MS: m/e 2535.2. FT-IR (KBr,
ν (cm-1)): 2923.8, 2851.6, 1698.1, 1660.0, 1499.7, 1461.3,
1406.8, 1336.2, 1235.1, 1167.1, 1126.9, 1008.5, 975.6, 924.7,
803.3, 729.8, 527.3.
Acknowledgment. This work has been supported by the
Major State Basic Research Development Program and the
National Natural Science Foundation of China (Grants 20151002
and 50372070).
Supporting Information Available: Fluorescence spectra
of FPP, ZnFPP, H2P, ZnP, and 4 and data for absorption of
thin films of FPP and ZnFPP at room temperature. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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