Synthesis, photophysical properties, and photovoltaic devices of oligo(p-phenylene vinylene)-fullerene dyads

10174

J. Phys. Chem. B 2000, 104, 10174-10190

Synthesis, Photophysical Properties, and Photovoltaic Devices of Oligo(p-phenylene

vinylene)-fullerene Dyads^|

Emiel Peeters,^†Paul A. van Hal,^†Joop Knol,^‡Christoph J. Brabec,^§N. Serdar Sariciftci,^§

J. C. Hummelen,^‡and Rene´ A. J. Janssen*^,†

Laboratory of Macromolecular and Organic Chemistry, EindhoVen UniVersity of Technology, P.O. Box 513,

5600 MB EindhoVen, The Netherlands, Stratingh Institute and MSC, UniVersity of Groningen, Nijenborgh 4,

9747 AG Groningen, The Netherlands, and Christian Doppler Laboratory for Plastic Solar Cells, Johannes

Kepler UniVersity Linz, Altenbergerstrasse 69, A-4040 Linz, Austria

ReceiVed: May 8, 2000; In Final Form: August 31, 2000

The synthesis of a homologous series of oligo(p-phenylene vinylene)-fulleropyrrolidines (OPVn-C₆₀, n )

1-4, where n is the number of phenyl rings) is described. The photophysical properties of these donor-

acceptor dyads and the corresponding model compounds, R,ω-dimethyl-2,5-bis(2-(S)-methylbutoxy)-1,4-

phenylene vinylene oligomers (OPVn, n ) 2-4) and N-methylfulleropyrrolidine (MP-C₆₀), are studied as a

function of the conjugation length in solvents of different polarity and as thin films. Fast singlet energy

transfer occurs after photoexcitation of the OPVn moiety of the dyads toward the fullerene moiety in an

apolar solvent. Photoexcitation of the dyads in a polar solvent results in electron transfer for OPV3-C₆₀and

OPV4-C₆₀, and to some extent for OPV2-C₆₀, but not for OPV1-C₆₀. These results are compared to the results

obtained for mixtures of OPVn and MP-C₆₀in the same solvents. The solvent-dependent change in free

energy for charge separation of the donor-acceptor systems is calculated from the Weller equation, and the

rate constants for energy and electron transfer are derived from the fluorescence lifetime and quenching. The

results show that in a polar solvent electron transfer in these dyads is likely to occur via a two-step process,

that is, a very fast singlet energy transfer prior to charge separation. In thin solid films of OPV3-C₆₀and

OPV4-C₆₀, a long-lived charge-separated state is formed after photoexcitation. The long lifetime in the film

is attributed to the migration of charges to different molecules. A flexible photovoltaic device is prepared

from OPV4-C₆₀.

Introduction

conversion efficiency of these photovoltaic cells is restricted

by the carrier collection efficiency and the limited charge carrier

mobility in disordered homogeneous blends. Therefore, studying

photovoltaic efficiency in relation to morphology and meso-

scopic ordering in the active layer is of profound interest.⁸

Photoinduced charge transfer between π-conjugated polymers

and fullerene C₆₀is of considerable interest for photovoltaic

energy conversion using organic and polymer materials as an

active layer. Time-resolved spectroscopy has revealed that

photoinduced electron transfer in polymer-fullerene blends

occurs in the femtosecond time domain, several orders of

magnitude faster than the radiative or other nonradiative decay

processes.^1-3Therefore, the quantum efficiency of photoinduced

charge separation across the donor-acceptor (D-A) interface

in these mixtures is assumed to be close to unity. The efficient

charge separation in these mixtures is assisted by the large

interfacial area between donor and acceptor phases, which is

formed spontaneously during spin coating.⁴Surprisingly, the

back electron transfer is orders of magnitude slower than the

photoinduced forward electron transfer and the charge-separated

state persists even in the microsecond and millisecond time

domains.^1,5The large asymmetry between forward and backward

electron-transfer rates in these bulk heterojunctions enables the

collection of photoinduced charges.^6,7At present, the energy

The rapid advancement in fullerene chemistry allows the

covalent functionalization of C₆₀with electron donors.⁹In recent

years, many C₆₀-based donor-acceptor dyads have been

synthesized and investigated to gain insight into the intramo-

lecular photophysical processes, such as energy and electron

transfer.¹⁰Although these dyads can serve as model compounds

for the π-conjugated polymer-fullerene photovoltaic cells, only

a few examples have been reported in which π-conjugated

oligomers are covalently attached to C₆₀.^11-15Apart from being

well-defined model systems for photophysical characterization,

the covalent linkage between donor and acceptor in these

molecular dyads provides a simple method to achieve dimen-

sional control over the phase separation in D-A networks.

Here, we report the synthesis of a homologous series of well-

defined donor-C₆₀dyad molecules with a π-conjugated oligo-

(p-phenylene vinylene) as the donor moiety (OPVn-C₆₀, n )

1-4, where n is the number of phenyl rings; see Figure 1).¹⁶

We have studied the photophysical properties of these oligo-

(p-phenylene vinylene)-fulleropyrrolidines in detail, both as

single molecules in solution and as assemblies in thin films,

and the results are compared to those of mixtures of N-

methylfulleropyrrolidine (MP-C₆₀) and a series of R,ω-dimethyl-

* Corresponding

author.

Phone:

(+)31.40.2473597.

Fax:

(+)31.40.2451036. E-mail: r.a.j.janssen@tue.nl.

^|Dedicated to Professor Fred Wudl on the occasion of his 60th birthday

in acknowledgment of his seminal contributions to the chemistry and physics

of conjugated polymers and fullerenes.

^†Eindhoven University of Technology.

^‡University of Groningen.

^§Johannes Kepler University Linz.

Published on Web 10/19/2000

Oligo(p-phenylene vinylene)-fullerene Dyads

J. Phys. Chem. B, Vol. 104, No. 44, 2000 10175

Figure 1. Structure of OPVn-C₆₀dyads and OPVn and MP-C₆₀model compounds.

SCHEME 1

oligo(p-phenylene vinylene)s (OPVn’s, n ) 2-4; see Figure

1). We demonstrate that both energy and electron transfer occur

in solution on short time scales. The differentiation between

the two processes depends on the conjugation length of the

oligomer and the polarity of the solvent. In apolar solvents, we

invariably observe energy transfer, whereas in more polar

solvents photoinduced electron transfer occurs for n ) 3 and 4

and to some extent for n ) 2. From fluorescence spectroscopy,

we infer that the electron transfer in polar solvents is likely to

be preceded by energy transfer and, hence, occurs in a two-

step process. The discrimination between energy and electron

transfer is semiquantitatively accounted for by the Weller

equation for photoinduced charge separation.¹⁷In thin solid

films, we observe photoinduced electron transfer for the two

longest systems, that is, OPV3-C₆₀and OPV4-C₆₀. Using the

latter as the single photoactive material, a working photovoltaic

cell has been demonstrated.

Results and Discussion

Synthesis. The oligo(p-phenylene vinylene)s, 6, 8, and 10,

containing an aldehyde functionality, were synthesized by

applying the Wittig-Horner coupling reaction (Scheme 1).¹⁸

A Williamson etherification of methylhydroquinone with enan-

tiomerically pure (S)-2-methylbutyl-p-toluenesulfonate gave 1,4-

bis[(S)-2-methylbutoxy]-2-methylbenzene, 1, in 94% yield.

Subsequent formylation of 1 using phosphorus oxychloride and

N,N-dimethylformamide (DMF) in refluxing chloroform gave

aldehyde 2 in 86% yield after column chromatography and

recrystallization. Compound 3 was isolated in 53% yield by

radical bromination of 1 with 1.2 equiv of N-bromosuccinimide

(NBS) and 2,2′-azobis(2-methylpropionitrile) (AIBN) in dry

refluxing carbon tetrachloride followed by ionic bromination

of the intermediate with 1.3 equiv of NBS in dry refluxing

tetrahydrofuran. Phosphonate 4 was obtained from 3 by a

10176 J. Phys. Chem. B, Vol. 104, No. 44, 2000

Peeters et al.

SCHEME 2

SCHEME 3

Michaelis-Arbuzov reaction. The Wittig-Horner coupling

reaction of 4 with 2 gave compound 5 in a yield of 73% after

recrystallization. Bromide 5 was transformed into aldehyde 6

by treatment with n-butyllithium and DMF according to the

Bouveault method. Pure 6 was isolated in 88% yield after

recrystallization. The formylated PPV oligomers 8 and 10 were

obtained by repetition of the Wittig-Horner and Bouveault

reactions. The relatively low yield of 10 is due to the low

solubility of 9 in diethyl ether.

The donor-acceptor dyads OPVn-C₆₀were prepared using

the well-established Prato reaction¹⁸(Scheme 2). A chloro-

benzene solution of the aldehyde (2, 6, 8, or 10), N-methyl-

glycine, and C₆₀was stirred for 16 h in the dark at reflux

temperature to yield a mixture of C₆₀, the desired monoadducts,

and higher adducts. The OPVn-C₆₀dyads (n ) 1-4) were

isolated (as mixtures of two diastereomers) after column

chromatography in 44, 47, 48, and 40% yields, respectively.

Reference compound MP-C₆₀was prepared according to the

reported procedure.¹⁹

Figure 2. UV/vis spectra of donor-acceptor OPVn-C₆₀dyads (1.8 ×

10^-5M) and MP-C₆₀in chloroform. The inset shows the spectra in the

600-800 nm range, where the absorption at 703 nm, characteristic of

methylfulleropyrrolidines, is observed.

The R,ω-dimethyl-oligo(p-phenylene vinylene)s with an even

number of phenyl rings (OPV2 and OPV4) were prepared by a

McMurry homocoupling of the aldehydes 2 and 6 using a

reductive mixture of titanium tetrachloride and zinc in tetrahy-

drofuran (Scheme 3). Pure oligomers were isolated by crystal-

lization in yields of 31 and 41%, respectively. To obtain OPV3,

the bisphosphonate compound 13 was synthesized (Scheme 3).

Etherification of hydroquinone with enantiomerically pure (S)-

2-methylbutyl-p-toluenesulfonate gave 1,4-bis[(S)-2-methylbu-

toxy]benzene, 11, in 75% yield. Two-fold chloromethylation

of 11 yielded compound 12, which was subsequently converted

to the corresponding bisphosphonate 13. The double Wittig-

Horner reaction of 13 with aldehyde 2 yielded OPV3 in 29%

yield after column chromatography.

All compounds used in the photophysical investigations were

fully characterized using ¹H and ¹³C NMR spectroscopy, mass

spectrometry, FT-IR, and elemental analysis.

Ground-State Absorption Spectra. The linear absorption

spectra of the OPVn-C₆₀dyads in dilute chloroform solution

(Figure 2) closely correspond to a superposition of the spectra

of the individual donor and acceptor. For a direct comparison,

Figure 2 also shows the absorption spectrum of MP-C₆₀, which

is almost identical to that of OPV1-C₆₀in the range of 300-

800 nm. Very similar spectra were recorded in other solvents,

such as toluene and o-dichlorobenzene (ODCB). Hence, the

covalently linked fullerene and OPV moieties retain the

electronic properties of the separate molecules and charge

transfer in the ground state does not occur. For all dyads, the

Oligo(p-phenylene vinylene)-fullerene Dyads

J. Phys. Chem. B, Vol. 104, No. 44, 2000 10177

TABLE 1: Redox Potentials of OPVn-C₆₀, OPVn, and

MP-C₆₀in V vs SCE, As Determined in Dichloromethane

compound E⁰_red(V) E⁰_ox1(V) compound E⁰_red(V) E⁰_ox1(V)

OPV1-C₆₀

OPV2-C₆₀

OPV3-C₆₀

OPV4-C₆₀

-0.70

1.19

0.97

0.85

0.77

OPV2

OPV3

OPV4

MP-C₆₀

0.90

0.80

0.75

-0.70

and reduction waves of the OPVn-C₆₀dyads, OPVn’s, and MP-

C₆₀are collected in Table 1. The oxidation potentials of the

OPVn-C₆₀dyads are shifted to slightly higher potentials in

comparison with the dimethyl-substituted OPVn oligomers. This

shift is tentatively ascribed to the different influence of an

electron-withdrawing fullerene moiety compared to an electron-

donating methyl group.

Photophysical Studies. Before describing the photoexcita-

tions of the OPVn-C₆₀dyads and the OPVn/MP-C₆₀mixtures

in solution, we briefly discuss the photophysical properties of

the individual reference compounds (OPVn and MP-C₆₀).

Figure 3. Temperature dependence of the UV/vis absorption of OPV4-

C₆₀dissolved in 2-methyltetrahydrofuran recorded in the range from

290 to 80 K by cooling in 15 K steps.

Photoexcitation of OPVn in Solution. Recently, we have

reported on the singlet and triplet photoexcitations of oligo(p-

phenylene vinylene)s (OPVn’s, n ) 2-7).²¹The singlet excited

state, OPVn(S₁), decays radiatively or nonradiatively to the

ground state and via intersystem crossing to the OPVn(T₁) triplet

state. The singlet excited state lifetimes (τ) have been determined

for OPV2 (τ ) 1.80 ns), OPV3 (τ ) 1.70 ns), and OPV4 (τ )

1.32 ns) in toluene solution at room temperature, and there is

no significant dependence of τ on the nature of the solvent.²¹

The CW-modulated photoinduced absorption (PIA) spectra of

OPV3 and OPV4 under matrix-isolated conditions in 2-meth-

yltetrahydrofuran at 100 K exhibit a T_nr T₁transition at 2.00

and 2.27 eV for OPV3 and at 1.80 eV for OPV4 with triplet

excited state lifetimes of 7.9 and 3.6 ms, respectively.²¹The

intensity of the T_nr T₁absorptions increases linearly with the

pump intensity, consistent with a monomolecular decay mech-

anism.²¹

spectra exhibit strong absorptions between 200 and 350 nm and

a weak absorption at ∼703 nm (inset Figure 2), characteristic

of fulleropyrrolidines. For OPV2-C₆₀, OPV3-C₆₀, and OPV4-

C₆₀, distinct absorptions are observed at 360, 412, and 438 nm,

respectively. These positions are similar to the absorption

maxima found in dilute chloroform solutions of OPV2 (357 nm),

OPV3 (406 nm), and OPV4 (436 nm), respectively, and are

ascribed to the π-π* transition of the OPVn moieties. The UV/

vis spectra demonstrate that it is possible to excite the fullerene

moiety selectively at 528 nm (one of the lines available from

the Ar-ion laser), because the OPVn moieties do not absorb at

this wavelength. As a result of the low absorption coefficient

of the fulleropyrrolidine in the 440-470 nm region, the OPV4

moiety, and to some extent the OPV3 moiety, can be excited

selectively with light of 458 nm. It is not possible to selectively

excite either the OPV2 or OPV1 segments without concomitant

excitation of the fullerene moiety.

Photoexcitation of MP-C₆₀in Solution. Photoexcitation of

MP-C₆₀in toluene or ODCB results in weak fluorescence at

1.74 eV and a long-lived triplet excited state. The fluorescence

Low-Temperature Absorption Spectra. Figure 3 shows the

UV/vis absorption spectra of OPV4-C₆₀in a 2-methyltetrahy-

drofuran solution between 290 and 80 K. At 290 K, the π-π*

absorption band of the OPV4 moiety is inhomogeneously

broadened with λ_max) 439 nm. With decreasing temperature,

the spectra change significantly as the absorption maximum

continuously shifts to higher wavelengths. At about 155 K,

vibronic fine structure appears in the π-π* absorption band.

Further cooling results in a significant narrowing of the vibronic

features, and at 80 K three to four vibronic bands can be

quantum yield in toluene is known to be 6 × 10^{-4 21}

.

Singlet

excited state lifetimes of 1.45¹⁵and 1.28 ns²²have been reported

for toluene solutions. The quantum yield for intersystem crossing

from MP-C₆₀(S₁) to MP-C₆₀(T₁) is near unity,²²and the lifetime

of this triplet state is about 200 µs at room temperature.¹⁵The

triplet-state PIA spectrum of MP-C₆₀exhibits a T_nr T₁

absorption at 1.78 eV with a characteristic shoulder at 1.54 eV.¹⁵

The energy level of the MP-C₆₀(T₁) triplet state has been

determined from phosphorescence to be at 1.50 eV above the

ground-state level.²²

distinguished. At this temperature, the 0-0 transition (λ_max

)

485 nm) has gained the highest intensity and the spacing

between the vibronic bands is about 1300-1400 cm^-1, consis-

tent with a CdC stretch vibration mode.¹⁹These changes with

temperature are interpreted as a gradual loss of conformational

disorder because of a planarization of the OPV4 moiety resulting

in a larger number of fully planar oligomer chains. In accordance

with this geometrical relaxation, we find that the onset of the

π-π* transition band shifts only slightly with temperature and

that the absorption bands characteristic of the rigid fullerene

moiety undergo only minor changes with temperature.

Intramolecular Singlet Energy Transfer in OPVn-C₆₀

Dyads in Toluene. The fluorescence spectra of the OPVn-C₆₀

(n ) 1, 2, 3, and 4) dyads, dissolved in toluene, are shown in

Figure 4. The spectra were corrected for the Raman scattering

of toluene.²³Although fluorescence from the OPVn moieties

of the dyads can be observed for n > 1, it is quenched by more

than 3 orders of magnitude compared to that of the pristine

OPVn oligomers (Table 2). Apart from the strongly quenched

OPVn emission, the spectra show a weak fullerene fluorescence

of MP-C₆₀(S₁) at 715 nm (Figure 4). The excitation spectra of

the fullerene fluorescence coincide with the absorption spectra

of the OPVn-C₆₀dyads (Figure 5). Surprisingly, the fullerene

fluorescence quantum yield in toluene is equal for all four dyads,

nearly identical to that of MP-C₆₀, and does not alter with the

excitation wavelength. Hence, the fluorescence spectra of OPVn-

Electrochemistry. The cyclic voltammograms of OPVn-C₆₀

(n ) 1-4) exhibit one to three quasi-reversible one-electron

oxidation waves from the OPVn moiety and one reduction wave

from the pyrrolidine-bridged C₆₀moiety. Although the first

oxidation potential decreases with increasing n, the reduction

potential remains constant at -0.70 V versus saturated calomel

electrode (SCE). The half-wave potentials for the first oxidation

10178 J. Phys. Chem. B, Vol. 104, No. 44, 2000

Peeters et al.

TABLE 2: Quenching Factors Q of the OPVn and MP-C₆₀Fluorescence of the OPVn-C₆₀Dyads in Toluene and ODCB^a

Q_OPVntoluene

k_ET(s^-1

)

Q_OPVnODCB

Q_MP-C₆₀ODCB

k_CSⁱ(s^-1

)

k_CS^d(s^-1

)

OPV2-C₆₀

OPV3-C₆₀

OPV4-C₆₀

5200

3500

1500

2.9 × 10¹²

2.1 × 10¹²

1.1 × 10¹²

8100

3500

1900

5

26

>50

3.4 × 10⁹

1.7 × 10¹⁰

>3.4 × 10¹⁰

1.8 × 10¹³

>7.2 × 10¹³

6.3 × 10¹³

^aRate constants for energy transfer (k_ET) and charge separation (k_CSand k_CS^d) as calculated from (1), (3), and (4).

i

Figure 5. Normalized UV/vis absorption (solid lines) and fluorescence

excitation spectra of the fullerene emission at 715 nm (dashed lines)

of the OPVn-C₆₀dyads in toluene at 295 K: (a) n ) 1, (b) n ) 2, (c)

n ) 3, and (d) n ) 4. In each case, the fluorescence excitation spectrum

shows a close correspondence to the absorption spectrum.

Figure 4. Fluorescence spectra of the OPVn-C₆₀dyads in toluene and

ODCB, recorded at 295 K: (a) n ) 1, λ_ex) 330 nm; (b) n ) 2, λ_ex

)

366 nm; (c) n ) 3, λ_ex) 415 nm; (d) n ) 4, λ_ex) 443 nm. The

fluorescence spectra in toluene (solid lines) were normalized to the

fullerene emission at 715 nm. The residual OPVn emission for n )

2-4 can be seen in the 400-600 nm range. The emission of the OPVn-

C₆₀dyads in ODCB (dash-dotted lines) is normalized with the same

constant as that used for the emission in toluene. A nearly complete

quenching of the fullerene emission is observed for n ) 2-4, whereas

the OPVn emission decreases only slightly.

intensity (-∆T

I^p, p ) 0.80-1.00) consistent with a

monomolecular decay mechanism, but it should be noted that

bimolecular mechanisms such as the quenching of the MP-C₆₀-

(T₁) state by molecular oxygen would also give the same

behavior. The lifetime of the triplet state is in the range of 140-

280 µs.

C₆₀provide clear evidence for an efficient intramolecular singlet

energy transfer from the OPVn(S₁) state to the fullerene moiety

for n > 1.²³

An estimate for the rate constants of the intramolecular singlet

energy transfer (k_ET) can be obtained from the extent of

quenching of the OPVn fluorescence in the dyads and the singlet

excited-state lifetime of the OPVn oligomers¹⁵via eq 1.

The observations from fluorescence and PIA spectra provide

strong evidence that in toluene the OPVn moieties of the OPVn-

C₆₀dyads with n > 1 serve as an antenna system to funnel the

excitation energy to the fullerene moiety.²⁴

Intermolecular Triplet Energy Transfer in OPVn/MP-C₆₀

Mixtures in Toluene. The photophysical processes change

dramatically when mixtures of MP-C₆₀and OPVn in toluene

are excited instead of the covalently bound OPVn-C₆₀dyads.

Although intermolecular energy transfer from singlet excited

oligo(p-phenylene vinylene)s to MP-C₆₀is energetically possible

in OPVn/MP-C₆₀mixtures, it is less likely to occur because

such transfer is limited by diffusion and the nanosecond lifetime

of the singlet excited state. Hence, the OPVn(S₁) state in these

mixtures will decay via fluorescence and intersystem crossing

to the OPVn(T₁) state. The PIA spectrum of MP-C₆₀and OPV4

(1:1 molar ratio) in toluene, recorded upon selective excitation

of OPV4 at 458 nm, exhibits a band at 1.80 eV with a weak

shoulder at 1.52 eV (Figure 7). The PIA signal in the high-

energy region is obscured by the extremely intense OPV4

fluorescence in the region from 1.85 to 2.5 eV, which could

only be partially corrected. The negative -∆T/T above 2 eV is

an artifact caused by this correction. A monomolecular decay

mechanism is inferred from the intensity dependence of the 1.80

eV PIA band (-∆T I^p, p ) 0.89-0.92). A lifetime of ∼200

Q_OPVn- 1

k_ET

)

(1)

τ_OPVn

Here, τ_OPVnis the lifetime of the singlet excited state of the

pristine OPVn molecules and Q_OPVnis the quenching ratio of

the OPVn fluorescence of the OPVn-C₆₀dyad in comparison

with the OPVn molecule. The rate constants are collected in

Table 2 and indicate that an extremely fast (ca. <1 ps) singlet

energy transfer occurs in OPV2-C₆₀, OPV3-C₆₀, and OPV4-

C₆₀.

The MP-C₆₀(S₁) state formed via the intramolecular singlet

energy transfer in the OPVn-C₆₀dyads is expected to decay

predominantly via intersystem crossing to the MP-C₆₀(T₁) state,

apart from some radiative decay. Consistent with this expecta-

tion, the PIA spectrum recorded for all four dyads in toluene

solution shows the characteristic MP-C₆₀T_nr T₁absorption

at 1.78 eV with a shoulder at 1.54 eV (Figure 6a). The PIA

bands increase in a nearly linear fashion with the excitation

Oligo(p-phenylene vinylene)-fullerene Dyads

J. Phys. Chem. B, Vol. 104, No. 44, 2000 10179

Figure 8. PIA spectra of OPVn (4 × 10^-4M) codissolved with MP-

C₆₀(4 × 10^-4M) in ODCB at 295 K recorded with excitation at 528

nm: (a) OPV2/MP-C₆₀, (b) OPV3/MP-C₆₀, and (c) OPV4/MP-C₆₀.

Figure 6. (a) PIA spectra of OPVn-C₆₀dyads (4 × 10^-4M) in toluene

at 295 K, recorded with excitation at 351.3 and 363.8 nm for n ) 1

and 2 and at 457.9 nm for n ) 3 and 4. (b) PIA spectra of OPVn (4 ×

10^-4M) codissolved with MP-C₆₀(4 × 10^-4M) in toluene at 295 K

with excitation at 528 nm.

6b), exhibit an absorption at 1.78 eV with an associated shoulder

at 1.54 eV, characteristic of MP-C₆₀(T₁).¹⁵The monomolecular

decay (-∆T I^p, p ) 0.89-0.96) and lifetime of 150-260 µs

associated with these PIA bands support this assignment.

Furthermore, weak fullerene fluorescence at 1.73 eV (715 nm)

is observed under these conditions for all three mixtures. No

characteristic PIA bands of OPVn^+•radical cations or MP-C₆₀

-•

radical anions are discernible under these conditions. From these

observations, we conclude that electron transfer from the ground

state of the OPVn molecules to the singlet or triplet excited

state of MP-C₆₀does not occur in toluene solution.

Intermolecular Electron Transfer in OPVn/MP-C₆₀Mix-

tures in o-Dichlorobenzene. The dielectric constant (permit-

tivity) of ODCB (ꢀ ) 9.93) is significantly higher than that of

toluene (ꢀ ) 2.38). Photoinduced electron transfer will be more

favored in this more polar solvent because the Coulombic

attraction between the resulting opposite charges is more

screened and the charged ions are better solvated than in toluene.

Indeed, the PIA spectra of 1:1 molar mixtures of MP-C₆₀and

OPV4 or OPV3 in ODCB give direct spectral evidence for

intermolecular photoinduced electron transfer. For both mixtures,

an intense PIA spectrum of the charge-separated state was

observed after selective excitation of MP-C₆₀at 528 nm (Figure

8b,c). Strong absorptions for the OPV4^+•(0.66, 1.52, and 1.73

eV) and OPV3^+•(0.77, 1.70, and 1.97 eV) radical cations were

observed, together with the characteristic absorption band of

the MP-C₆₀^-•radical anion at 1.24 eV. For each PIA band, the

change in transmission as a function of the modulation frequency

was recorded and fitted to the expression for bimolecular decay,

resulting in estimates for the charge-separated state lifetime of

2.5-3.1 ms for OPV4^+•/MP-C₆₀^-•and 4.1-4.7 ms for OPV3^+•/

MP-C₆₀^-•, respectively. Increasing the pump beam intensity

resulted in a nonlinear increase of the absorption intensities

(-∆T I^p, p ) 0.62-0.71). Although for bimolecular decay a

square-root intensity dependence of the PIA signal is often

observed, it is noticeable that the pump intensity dependence

of the PIA signal becomes linear when the modulation frequency

Figure 7. PIA spectra of OPV4 (4 × 10^-4M) codissolved with MP-

C

₆₀(4 × 10^-4M) in toluene at 295 K recorded with excitation at 457.9

nm.

µs was determined by varying the modulation frequency

between 30 and 3800 Hz. The spectral features and the lifetime

are characteristic for the MP-C₆₀(T₁) state. They do not coincide

with the PIA spectrum recorded for OPV4, which shows a

maximum at 1.80 eV, without any shoulder to lower energy.²¹

The observation of the MP-C₆₀(T₁) spectrum and, hence, the

quenching of the OPV4(T₁) state indicate that after intersystem

crossing an efficient intermolecular triplet energy transfer occurs

from the OPV4(T₁) state to MP-C₆₀, generating the MP-C₆₀-

(T₁) state. Because the photogenerated OPV4(T₁) state is

quenched by the presence of MP-C₆₀, we conclude that the triplet

state energy of OPV4 is higher than 1.50 eV, the triplet state

energy of MP-C₆₀. Because shorter oligo(p-phenylene vinylene)s

are expected to have an even higher triplet level, we can

conclude that the fullerene triplet level corresponds to the lowest

excited state in toluene for all dyads studied here.

Consistently, the PIA spectra of toluene solutions containing

MP-C₆₀and OPVn (n ) 2, 3, or 4) in a 1:1 molar ratio, recorded

using selective photoexcitation of MP-C₆₀at 528 nm (Figure

10180 J. Phys. Chem. B, Vol. 104, No. 44, 2000

Peeters et al.

is much larger than the inverse bimolecular lifetime.²⁵In the

present case with ω ) 275 Hz and τ ≈ 2-5 ms, both are of the

same magnitude and an intermediate value can be expected.

Therefore, we conclude that the results are consistent with

bimolecular decay and, hence, with intermolecular recombina-

tion of positive and negative charges. Because MP-C₆₀is initially

excited, we attribute the formation of radical ions to an

intermolecular electron transfer between ground-state OPV3 or

OPV4 as a donor and the triplet state of MP-C₆₀as an acceptor.²⁶

The PIA spectra of the mixture of OPV2 and MP-C₆₀in

ODCB, recorded with selective excitation of MP-C₆₀at 528 nm

(Figure 8a), exhibits the transitions of the MP-C₆₀(T₁) state at

1.78 and 1.54 eV. These bands increase linearly with the

excitation intensity (-∆T I^p, p ) 0.99-1.00) and correspond

to a lifetime of 170-290 µs. Together with the concurrent

absence of polaron absorptions, we infer that intermolecular

charge transfer between OPV2 and MP-C₆₀does not occur in

ODCB.²⁷

Intramolecular Electron Transfer in OPVn-C₆₀Dyads in

o-Dichlorobenzene. Figure 4 shows the Raman corrected

emission spectra of the OPVn-C₆₀dyads in ODCB obtained

upon (near) selective photoexcitation of the OPV moiety. Similar

to solutions in toluene, the fluorescence of the OPVn moiety

of the dyads is strongly quenched compared to the fluorescence

Figure 9. PIA spectra of the OPVn-C₆₀dyads in ODCB (4 × 10^-4

M) at 295 K for (a) n ) 1, (b) n ) 2, (c) n ) 3, and (d) n ) 4. The

spectra were recorded with excitation at 351.1 and 363.8 nm for n )

1 and 2 and at 457.9 nm for n ) 3 and 4.

of the pristine OPVn in ODCB. The quenching ratios, Q_OPVn

,

collected in Table 2, are somewhat larger than those in toluene.

The most dramatic difference between the fluorescence spectra

of the OPVn-C₆₀dyads recorded in the two different solvents,

however, is the strong quenching of the fullerene emission at

715 nm for solutions of OPV2-C₆₀, OPV3-C₆₀, and OPV4-C₆₀

in ODCB but not for OPV1-C₆₀. The fullerene quenching ratio,

9c,d). Although the intensities of the OPVn^+•radical cation and

-•

MP-C₆₀radical anion absorptions are significantly lower in

comparison with the PIA signals observed for mixtures of OPVn

and MP-C₆₀in ODCB, the characteristic features are evident.

Remarkably, lifetimes up to 20 ms can be observed for these

charge-separated states. In view of the expected (sub)nanosecond

lifetime,¹⁰we consider this extremely long lifetime to be

incompatible with an intramolecular charge-separated state and

attribute the signals to an intermolecular charge-separated state.

This intermolecular charge-separated state is formed either by

direct charge transfer between singlet excited OPVn-C₆₀(S₁) and

a second dyad in the ground state or by charge transfer from

Q_C, calculated with respect to the fluorescence of MP-C₆₀in

60

ODCB, increases with conjugation length of the OPVn moiety

from approximately 5 for OPV2-C₆₀to 26 for OPV3-C₆₀to more

than 50 for OPV4-C₆₀(Table 2). The quenching of the fullerene

emission in the dyads may result from either an ultrafast process

that quenches the initially formed OPVn(S₁) state or a rapid

relaxation of the MP-C₆₀(S₁) state, once this is formed via energy

transfer. The experimental observation that the residual OPVn

fluorescence of the OPVn-C₆₀dyads in ODCB is comparable

to that in toluene gives support to the latter explanation.

the short-lived intramolecular charge-separated OPVn^+•-C₆₀

-•

state to a neutral OPVn-C₆₀dyad, resulting in separate OPVn^+•-

C₆₀and OPVn-C₆₀radical ions. The longer lifetime as

Intramolecular charge separation is energetically favored over

intermolecular charge separation because the spatial separation

of charges, and hence the Coulombic term, is limited by the

fixed distance between donor and acceptor within the covalent

donor-acceptor dyad. Because intermolecular photoinduced

electron transfer occurs in mixtures of MP-C₆₀and OPV4 or

OPV3 in ODCB, we may expect that intramolecular photoin-

duced electron transfer occurs in the corresponding OPV3-C₆₀

and OPV4-C₆₀dyads in ODCB as well. The lifetime of an

intramolecularly charge-separated state in molecular donor-

fullerene dyads, however, usually does not extend into the

microsecond regime but is limited to the low nanosecond time

domain.¹⁰For cases in which the donor is separated from the

fullerene by a long rigid bridge^28,29or the donor has a low

oxidation potential (e.g., tetrathiafulvalene or ferrocene),^30-32

the lifetime of the charge-separated state was found to be in

the microsecond range. In general, excited states with a lifetime

τ e ∼10 µs cannot be detected with the CW-modulated PIA

technique because the steady-state concentration that is achieved

in the modulated experiment is below the detection limit (∆T/T

≈ 10^-6). Therefore, it is surprising to see that the PIA spectra

of OPV3-C₆₀and OPV4-C₆₀dissolved in ODCB exhibit the

characteristic absorptions of a charge-separated state (Figure

-•

compared to OPVn/MP-C₆₀(n ) 3 or 4) mixtures is due to the

lower concentration of the OPVn^+•-C₆₀and OPVn-C₆₀^-•radical

ions and, hence, the reduced bimolecular decay rate.

Photoexcitation of OPV2-C₆₀in ODCB leads to seemingly

contradictory observations: although the fullerene emission is

partly quenched (Figure 4, Table 2), consistent with electron

transfer, the MP-C₆₀(T₁) state is observed in the PIA spectrum

(Figure 9b), indicating a combination of energy transfer and

intersystem crossing. The result can be rationalized as follows;

if the energy level of the lowest lying neutral excited state,

OPV2-C₆₀(T₁), is close to the energy level of the intramolecular

charge-separated state, energy and electron transfer will occur

simultaneously. The likelihood of this degeneracy will be

demonstrated in the section on energetics of charge transfer.

The PIA spectrum of OPV1-C₆₀in ODCB shows the

absorption of the fullerene triplet state (Figure 9a), and no

significant quenching of the fullerene emission is observed

(Figure 4a). Both observations are consistent with the absence

of an intramolecular photoinduced electron transfer in OPV1-

C₆₀, which can be rationalized by the high oxidation potential

of the OPV1 moiety.

Oligo(p-phenylene vinylene)-fullerene Dyads

J. Phys. Chem. B, Vol. 104, No. 44, 2000 10181

long lifetime in the films is in strong contrast with the short

lifetime of the intramolecular charge-separated state as inferred

from the experiments in ODCB. Typically, the lifetime of an

intramolecular charge-separated state in fullerene-containing

dyads and triads is in the (sub)nanosecond time scale,¹⁰although

a few examples are known with a lifetime in the microsecond

domain.^28-32Therefore, we propose that the long lifetimes in

the film are due to migration of the hole and/or the electron to

other molecules in the film, after the photoinduced electron

transfer.³³The resulting intermolecular charge-separated state

can no longer decay via the fast geminate intramolecular back

electron transfer and will therefore have an increased lifetime.

At present, we have no indication that an initially formed

intramolecular charge transfer facilitates the formation of the

long-lived intermolecular charge-separated state.

Photovoltaic Devices of OPV4-C₆₀. The increased lifetime

of the charge-separated state in the solid state, resulting from

the migration of opposite charges to different molecules, extends

into the millisecond time domain similar to that of polymer-

fullerene composite films.^1,5This opens the possibility of

utilizing the OPVn-C₆₀dyads as the active material in a

photovoltaic device similar to that using the blends. As an

important difference with previous bulkheterojunction cells,^6,7

the covalent linkage between donor and acceptor in these

molecular dyads restricts the dimensions of the phase separation

between the oligomer and the fullerene that could freely occur

in blends of the individual components. Through a link of the

donor and acceptor via a covalent bond, a predefined phase

segregation of nanoscopic dimension could possibly be created

that would give rise to a well-ordered bicontinuous interpen-

etrating network of both donor and acceptor. The nanoscopic

dimension of the phase separation is advantageous because the

exciton diffusion length in conjugated polymers is limited to

that length scale. Molecular ordering will enhance the charge

carrier mobility and hence the separation of photogenerated

electrons and holes. Finally, the bicontinuous network will

ensure the unrestricted transport of electrons and holes to the

electrodes. Each of these three factors, generation, stabilization,

and transport, may improve the efficiency of photovoltaic cells.

The OPVn-C₆₀dyads can be considered as a primitive attempt

to obtain more ordered and well-defined phase-separated D-A

networks.

Figure 10. PIA spectra of OPVn-C₆₀thin films on quartz: (a) n ) 1,

(b) n ) 2, (c) n ) 3, and (d) n ) 4. The spectra were recorded at 80

K by exciting at 351.1 and 363.8 nm for n ) 1 and 2 and at 457.9 nm

for n ) 3 and 4 with 25 mW and a modulation frequency of 275 Hz.

Photoinduced Electron Transfer of OPVn-C₆₀in Thin

Films. Thin films of OPVn-C₆₀(n ) 1-4) were prepared by

casting from solution onto quartz substrates. For OPV1-C₆₀and

OPV2-C₆₀, the PIA spectra of the thin films recorded at 80 K

with excitation at 351 and 363 nm are similar (Figure 10a,b).

In both spectra, a broad low-intensity absorption is discernible

between 1.0 and 2.2 eV, with a maximum at about 1.72 eV.

The exact origin of the broad band is presently not known. The

spectrum shows some similarity to that of the MP-C₆₀(T₁) state

in solution, and the two characteristic absorption bands of oligo-

(p-phenylene vinylene) radical ions are absent. Therefore, we

tentatively assign the spectra (at least in part) to triplet-triplet

absorptions of the fullerene moiety. The lifetime of the MP-

C₆₀(T₁) state, as derived from modulation frequency dependency

measurements, is ∼230 µs for OPV1-C₆₀and ∼275 µs for

OPV2-C₆₀. These spectral characteristics suggest that photoin-

duced electron transfer does not occur (or occurs only to a small

extent) in thin films of OPV1-C₆₀and OPV2-C₆₀.

To test the applicability of the molecular dyads for light

energy conversion, we have prepared photovoltaic devices in

which OPV4-C₆₀is sandwiched between aluminum and poly-

ethylenedioxythiophene polystyrenesulfonate (PEDOT-PSS)-

covered indium-tin oxide (ITO) electrodes. Figure 11 shows

the semilogarithmic plot of the I/V curves of a typical device

in the dark and under white-light illumination of a halogen lamp

at ∼65 mW cm^-2. The I/V curves are completely reversible,

and the device shows diode behavior with a rectification ratio

between -2 and +2 V of approximately 100, which shows that

the device has few or no shunts. In the dark, a small kink is

discernible in the semilogarithmic plot of the I/V curve between

0 and 1 V, representing the small ohmic contribution from the

shunt resistance. Under ∼65 mW cm^-2white-light illumination,

a short-circuit current (I_sc) of 235 µA cm^-2and an open-circuit

voltage (V_oc) of 650 mV are observed for this device. The fill

factor (FF), defined as (I_max×V_max)/(I_sc×V_oc) with I_maxand V_max

corresponding to the point of maximum power output, is 0.25.

The relatively low FF can be explained by recombination of

charge carriers either in the bulk or at the electrode interfaces.

Because the photocurrent was found to be linear with the light

intensity, we rule out bimolecular recombination and suggest

The PIA spectra of thin films of OPV3-C₆₀and OPV4-C₆₀

recorded with excitation at 458 nm differ dramatically from

those of OPV1-C₆₀and OPV2-C₆₀(Figure 10c,d). For OPV3-

C₆₀and OPV4-C₆₀, the characteristic absorption band of the

-•

MP-C₆₀radical anion is observed at 1.25 eV. Furthermore,

the spectra show absorptions for the OPV3^+•and OPV4^+•

radical cations at 0.82 and 1.66 eV (OPV3^+•) and at 0.64, 1.44,

and 1.70 eV (OPV4^+•). These observations give direct spectral

evidence for photoinduced electron transfer. The intensity of

the PIA bands increases approximately with the square-root of

the pump intensity (-∆T I^p, p ) 0.46-0.57 for OPV3-C₆₀

and p ) 0.36-0.50 for OPV4-C₆₀). This square-root intensity

dependency indicates a bimolecular decay mechanism, consistent

with the recombination of positive and negative charges. Varying

the modulation frequency from 30 to 3800 Hz results in a

continuous decrease of the intensity of the PIA bands of OPV3-

C₆₀and OPV4-C₆₀, indicating a distribution of lifetimes. The

average lifetime of the charge-separated states in thin films of

OPV3-C₆₀and OPV4-C₆₀is on the order of 0.5-1.5 ms. This

10182 J. Phys. Chem. B, Vol. 104, No. 44, 2000

Peeters et al.

TABLE 3: Center-to-Center Distance (R_cc) of Donor and

Acceptor in OPVn-C₆₀Dyads, Radius (r⁺) of OPVn Radical

Cation, Energy of OPVn(S₁) State, and Free Energies of

∞

Intramolecular (G_cs) and Intermolecular (G_cs

)

Charge-Separated States in OPVn-C₆₀Dyads and OPVn/

MP-C₆₀Mixtures Calculated from (2)

E_OPVn(S1)

(eV)

R_cc(Å) r⁺(Å)

solvent G_cs(eV) G_cs(eV)

OPV1-C₆₀

OPV2-C₆₀

7.0

3.3

4.0

4.6

5.1

4.77

toluene

ODCB

toluene

ODCB

toluene

ODCB

toluene

ODCB

2.10

1.64

1.99

1.48

1.94

1.40

1.91

1.34

9.5

3.03

2.64

2.44

2.56

1.56

2.38

1.47

2.29

1.42

OPV3-C₆₀12.3

OPV4-C₆₀15.3

For OPVn-C₆₀and mixtures of OPVn and MP-C₆₀in solution,

E_oxand E_redwere determined via cyclic voltammetry in

dichloromethane (ꢀ ) 8.93) (Table 1). The R_ccdistances (Table

3) in the dyads were determined by molecular modeling,

assuming that the charges are located at the centers of the OPV

and fullerene moieties. For intermolecular charge transfer, the

value of R_ccwas set to infinity. The radius of the negative ion

of C₆₀was set to r^-) 5.6 Å, based on the density of C₆₀.²⁸To

estimate the radii for the positive ions (a radius being a

substantial simplification for the one-dimensionally extended

conjugated OPV moieties!), we used the van der Waals volume

of OPV molecules, ignoring the 2-methylbutoxy side chains

because the positive charge will be confined to the conjugated

segment. For stilbene, an experimental value of r⁺) 3.96 Å

can be obtained from the density (F ) 1.159 g cm^-3), derived

from the X-ray crystallographic data,³⁷via r⁺) [3M/

(4πFN_A)]^1/3. For the other oligomers, no crystallographic data

are available and we calculated the van der Waals volumes of

benzene, stilbene, 1,4-distyrylbenzene, and 4,4′-distyrylstilbene

using Macromodel and a MM2 force field. After correction for

26% free-volume in a close packing of spheres, the values

collected in Table 3 were obtained. The experimental and

theoretical values for stilbene are in close agreement. With these

Figure 11. Semilogarithmic plot of the current/voltage curves of a

PET/ITO/PEDOT-PSS/OPV4-C₆₀/Al photovoltaic cell in which OPV4-

C₆₀is the active material. Open squares represent the dark curve, and

solid squares represent data recorded under ∼65 mW cm^-2white-light

illumination.

surface recombination as a possible mechanism for the low FF.

Because aluminum is known to give rather good ohmic contacts

in bulk heterojunction cells,³⁴we suggest surface recombination

at the ITO electrode as the likely cause of the low FF. The

present values of I_scand V_ocare significantly enhanced in

comparison with the device characteristics of a related oligo-

(p-phenylene vinylene)-C₆₀dyad¹⁴and are quite similar to those

previously reported for π-conjugated polymer/fullerene solar

cells,³⁵although there has been considerable progress in energy

conversion efficiencies of these devices recently.³⁶The results

indicate that indeed a bicontinuous network of the donor and

acceptor moieties is formed in a film of OPV4-C₆₀. Two factors

limit the intrinsic efficiency of the OPV4-C₆₀-based device,

compared to that of the corresponding poly(p-phenylene vi-

nylene)/fullerene blends. First, the absorption spectrum of the

OPV4 oligomer does not cover the wavelength range of the

corresponding polymer because of the reduced conjugation

length. Second, the intrinsic fullerene/OPV weight ratio of 0.71

in OPV4-C₆₀is significantly lower than the optimized fullerene/

polymer weight ratio of 4, presently used in the most efficient

polymer solar cells.³

approximations, the free energies of the intramolecular (G_cs

)

∆G_cs+ E₀₀) and intermolecular (G_cs^∞) ∆G_cs^∞+ E₀₀) charge-

separated states have been calculated (Table 3), and the relative

ordering of the states is depicted in Figure 12.

From Figure 12b and Table 3, it is clear that the intramo-

lecular charge-separated state is energetically located below the

OPVn(S₁) state in both solvents and for each n. However, in

toluene all charge-separated states are higher in energy than the

MP-C₆₀(S₁) and MP-C₆₀(T₁) states, which are located at 1.74

and 1.50 eV, respectively. In ODCB, the situation changes

dramatically; the energy of the intramolecular charge-separated

state drops below that of the MP-C₆₀(S₁) state for each n and

even drops below that of the corresponding MP-C₆₀(T₁) state

except for n ) 1, indicating that electron transfer will result in

a gain of free energy for n > 1. In fact, the predictions based

on eq 2 (Table 3) are in excellent agreement with the quenching

of the MP-C₆₀(S₁) fluorescence, as shown in Figure 4, which

occurs for n > 1. For the intermolecular charge-separated states,

the relevant state for comparison is the MP-C₆₀(T₁) state only,

because these charge-separated states are formed via the triplet

manifold. Table 3 and Figure 12a show that intermolecular

electron transfer is energetically favored in ODCB for OPV3

and OPV4 but not for OPV2,³⁸again in full agreement with

the experimental results inferred from PIA spectroscopy (Figure

8).

Energetic Considerations for Energy and Electron Trans-

fer. To rationalize the observed differentiation between energy

and electron transfer by the photoexcited OPVn-C₆₀dyads in

apolar and polar solvents, we calculated the change in free

energy for charge separation (∆G_cs) using the Weller equation:¹⁷

e²

∆G_cs) e(E_ox(D) - E_red(A)) - E₀₀

-

4πꢀ₀ꢀR_cc

e²

8πꢀ_{0 r}

1

+

1

r^-

1

+

-

(2)

(

)(

)

ꢀ_refꢀ_ref

In this equation, E_ox(D) and E_red(A) are the oxidation and

reduction potentials of the donor and acceptor molecules or

moieties measured in a solvent with relative permittivity ꢀ_ref,

E₀₀is the energy of the excited state from which the electron

transfer occurs, and R_ccis the center-to-center distance of the

positive and negative charges in the charge-separated state. The

radii of the positive and negative ions are given by r⁺and r^-,

and ꢀ_sis the relative permittivity of the solvent; -e is the

electron charge, and ꢀ₀is the vacuum permittivity.

The close correspondence of experimental results with the

relative ordering of the various excited states as derived from

Oligo(p-phenylene vinylene)-fullerene Dyads

J. Phys. Chem. B, Vol. 104, No. 44, 2000 10183

Figure 12. Excited-state energy levels. The singlet (S₁) energy levels of OPVn and MP-C₆₀(solid bars) were determined from fluorescence data.

The MP-C₆₀(T₁) level (solid bar and dashed line) was taken from literature phosphorescence data.²²The levels of the charge-separated states for (a)

intermolecular charge transfer in OPV n/MP-C₆₀mixtures and (b) intramolecular charge transfer in OPVn-C₆₀dyads were determined using eq 2

(see text and Table 3). The open squares are for toluene, and the solid squares are for ODCB.

eq 2 shows the strength of this approach in explaining the

discrimination between photoinduced energy and charge transfer

in conjugated oligomer-fullerene dyads.

Kinetics of Energy and Electron Transfer. A semiquanti-

tative estimate for the rate constants of the various photophysical

processes can be obtained from the fluorescence quenching. On

the basis of the quenching ratios of the OPV fluorescence and

the OPVn singlet excited state lifetimes, the rate constants for

energy transfer reactions in toluene solutions were estimated

to be 2.9 × 10¹², 2.1 × 10¹², and 1.1 × 10¹²s^-1for OPV2-C₆₀,

OPV3-C₆₀, and OPV4-C₆₀, respectively (Table 2). We assume

that similar rate constants for energy transfer will apply to

ODCB solutions because the energy level and the lifetime of

the singlet excited states are not strongly affected by the polarity

of the solvent.²¹For OPV2-C₆₀, OPV3-C₆₀, and OPV4-C₆₀,

intramolecular photoinduced electron transfer is observed in

ODCB, as evidenced by the quenching of the MP-C₆₀(S₁)

Figure 13. Schematic diagram describing energy levels of singlet (S₀

fluorescence. In principle, electron transfer can take place either

directly from the initially formed OPVn(S₁) state or indirectly,

and S₁), triplet (T₁and T_n), and charge-separated (CS) states of OPVn-

C₆₀dyads. The energy transfer (k_ET) and the indirect (k_CSⁱ) and direct

in a two-step process, via the MP-C₆₀(S₁) state (Figure 13). For

indirect photoinduced electron transfer, that is, after singlet

(k_CS^d) charge-separation processes are indicated with the curved dotted

arrows. The solid arrow describes the initial excitation of the OPVn

moiety. Other symbols are k_rand k′_rfor the radiative rate constants, k_nr

and k′_nrfor the nonradiative decay constants, k_iscand k′_iscfor the

intersystem crossing rate constants, and k_Tand k′_TTfor the rate constants

for triplet-energy transfer, each for OPVn and MP-C₆₀, respectively.

i

energy transfer to MP-C₆₀, the rate constant (k_cs) is given by

eq 3.

Q_C- 1

60

i

k_cs

)

(3)

τ

transfer reaction. In this case, the rate constant (k_cs^d) for electron

C₆₀

transfer can be derived from

Here, Q_Cis the quenching ratio of the fullerene emission

of the OPVn-C₆₀dyads in ODCB in comparison with MP-C₆₀,

60

(Q_OPVn- 1)(Q_C- 1)

60

d

k_cs

)

(4)

and τ is the lifetime of the singlet excited state of MP-C₆₀

C₆₀

τ_OPVn

(1.45 ns).¹⁵The values for k_csⁱin Table 2 show that the electron

transfer from the MP-C₆₀(S₁) state occurs in the time scale of

∼60 ps for OPV3-C₆₀and ∼30 ps for OPV4-C₆₀but is much

slower for OPV2-C₆₀(∼290 ps). If a direct electron transfer

from the OPVn(S₁) state occurred, the decrease in fullerene

emission would necessarily result from a quenching of the

OPVn(S₁) state, because it would be faster than the energy

d

The calculated values of k_cs(Table 2) indicate that direct

intramolecular electron transfer would have to be extremely fast,

especially for OPV3 and OVP4 (14-20 fs). Moreover, if direct

electron transfer occurred, an additional quenching of the

residual OPVn emission must be expected. However, Figure 4

shows that there is no significant additional quenching of the

10184 J. Phys. Chem. B, Vol. 104, No. 44, 2000

Peeters et al.

TABLE 4: Reorganization Energy (λ), Free-Energy Change

(∆G_cs), and Barrier (∆G^q_cs) for Intramolecular Electron

Transfer in OPVn-C₆₀Dyads in Different Solvents Relative

to the Energies of the OPVn(S₁) and MP-C₆₀(S₁) States, As

Calculated from (2), (5), and (6)

indirect charge-transfer mechanisms as listed in Tables 3 and

4, it is possible to estimate the electronic coupling. For the

indirect mechanism, that is, charge separation after singlet

i

energy transfer, the values calculated for V using eq 7 and k_cs

are Vⁱ) 30 ( 3 cm^-1for OPV3-C₆₀and OPV4-C₆₀in ODCB.

For the direct mechanism, eq 7 indicates that V should be on

the order of V^d) 1300-2600 cm^-1to explain the very high

rate constants k_cs^d. Such a strong coupling would probably cause

differences in the absorption spectrum of the OPVn-C₆₀dyads

in comparison with the linear superposition of the spectra of

OPVn’s and MP-C₆₀, which is not observed experimentally.

Moreover, such coupling is much larger than the interaction

expected between two chromophores separated by a bridge of

three σ bonds.^28,42

OPVn(S₁)

MP-C₆₀(S₁)

∆G_cs

(eV)

∆G^q

∆G_cs

(eV)

∆G^q

cs

compound solvent λ (eV)

(eV)

OPV2-C₆₀toluene

ODCB

OPV3-C₆₀toluene

ODCB

OPV4-C₆₀toluene

ODCB

0.34

0.80

0.34

0.83

0.35

0.86

-1.04

-1.55

-0.71

-1.24

-0.53

-1.10

0.36

0.18

0.09

0.05

0.02

0.23

-0.28

0.18

-0.36

0.15

0.24

0.09

0.20

0.07

0.18

0.06

-0.42

OPVn emission in ODCB in comparison with the quenching

already achieved by energy transfer in toluene. Hence, the

fluorescence quenching experiments strongly suggest that pho-

toinduced electron transfer in the OPVn-C₆₀dyads in ODCB

solution is a two-step process when the OPVn is excited initially,

involving singlet energy transfer prior to charge separation.²⁶

The Activation Barrier for Charge Separation. Further

insight into the kinetics of charge separation can be obtained

from the activation barrier of charge separation. The Marcus

equation provides an estimate for the barrier for photoinduced

electron transfer from the change in free energy for charge

separation (∆G_cs) and the reorganization energy (λ):³⁹

We conclude that the kinetic analysis supports the conclusion

inferred from the residual OPVn fluorescence of the OPVn-

C

₆₀dyads in ODCB, that charge separation in these systems is

preceded by energy transfer when the OPVn moiety is initially

excited.

Conclusions

We have synthesized a novel series of oligo(p-phenylene

vinylene)-fulleropyrrolidines OPVn-C₆₀and the corresponding

model compounds OPVn and MP-C₆₀. Upon photoexcitation

of the donor-acceptor dyads and OPVn/MP-C₆₀mixtures in

solution, fast singlet energy transfer occurs. Depending on the

conjugation length of the OPVn moiety/molecule and the relative

permittivity of the medium, a charge-separated state may form

in a subsequent reaction, in which an electron is transferred from

the OPVn donor to the fullerene acceptor as the net result.

In an apolar solvent, toluene, the fluorescence and PIA spectra

of the OPVn-C₆₀dyads provide clear evidence for efficient

intramolecular photoinduced singlet energy transfer from the

photoexcited OPVn moiety to the ground-state fullerene moiety,

irrespective of the conjugation length. The fast energy transfer

is followed by a nearly quantitative intersystem crossing to the

fullerene triplet state. For mixtures of OPVn and MP-C₆₀in

toluene, an intermolecular triplet energy transfer from the

OPVn(T₁) state to the ground state of MP-C₆₀occurs, generating

the corresponding MP-C₆₀(T₁) triplet state.

In the more polar solvent ODCB, photoexcitation of MP-C₆₀

codissolved with OPV3 or OPV4 results in the formation of a

charge-separated state, because of intermolecular electron

transfer from ground-state OPV3 or OPV4 to the MP-C₆₀(T₁)

state. For a cosolution of OPV2 and the fullerene derivative in

ODCB, we do not observe photoinduced polaron absorptions.

Intramolecular charge-separated states are formed upon

excitation of the OPVn moiety in solutions of OPV2-C₆₀, OPV3-

C₆₀, or OPV4-C₆₀in ODCB. The quenching of the fullerene

fluorescence in this more polar solvent indicates that the forward

intramolecular electron transfer occurs within about 60 and 30

ps after the initial singlet energy transfer for OPV3-C₆₀and

OPV4-C₆₀, respectively. Because the initial singlet energy

transfer occurs within only 1 ps, these numbers also apply for

the overall process from light absorption to charge separation.

Charge separation is not observed for OPV1-C₆₀in ODCB. The

observed discrimination between intra- and intermolecular

energy and electron-transfer processes as a function of conjuga-

tion length and polarity of the solvent is in full agreement with

the estimated free energies of the charge-separated state as

derived from eq 2. The experimental result that the residual

OPVn fluorescence of the OPVn-C₆₀dyads in ODCB is similar

to that in toluene suggests that intramolecular photoinduced

charge-separation occurs by an indirect mechanism, that is, after

∆G^q_cs) (∆G_cs+ λ)²/4λ

(5)

The reorganization energy consists of internal (λ_i) and solvent

(λ_s) contributions. The latter can be calculated in the Born-

22,28,40

Hush approach via

e²1 1

1

r^-

1

R_cc

1

ꢀ_s

λ_s)

+

-

2

(6)

+

(

)

(

]

)

[

4πꢀ₀2

r

n

in which n is the refractive index of the solvent. For the internal

reorganization energy, we estimate λ_i) 0.3 eV.^10c,22,28The

values for λ ) λ_i+ λ_sobtained in this way are compiled in

Table 4 for n > 1, together with the free-energy change (∆G_cs)

and barrier (∆G^q_cs) for intramolecular electron transfer in the

OPVn-C₆₀dyads relative to the OPVn(S₁) and MP-C₆₀(S₁)

excited states. Table 4 shows that the forward photoinduced

charge separation in the OPVn-C₆₀dyads in ODCB originating

from the MP-C₆₀(S₁) state is in the “normal” Marcus region (λ

> -∆G_cs) and that the barrier for electron transfer in OPVn-

C₆₀is less than ∼0.1 eV for n > 1. Direct forward photoinduced

charge separation originating from the OPVn(S₁) state would

occur in the Marcus “inverted” region (λ < -∆G_cs), irrespective

of the solvent or conjugation length of the OPVn donor (Table

4). The latter is a consequence of the higher energy of the singlet

excited state. The estimates for ∆G^qchange only little when

cs

different values are used for λ_i; when λ_iis varied from 0.2 to

0.4 eV, ∆G^q_csremains <0.1 eV for both OPV3-C₆₀and OPV4-

C₆₀in ODCB.

The nonadiabatic charge separation rate constant is a function

of the energy barrier ∆G^q_cs, the reorganization energy, and the

electronic coupling (V) between donor and acceptor in the

excited state:⁴¹

-∆G^q

1/2

4π³

cs

k_cs)

V²exp

(7)

(

)

h²λk_BT

[

]

k_BT

Using the values for λ and for k_csand ∆G^q_csfor the direct and

Oligo(p-phenylene vinylene)-fullerene Dyads

J. Phys. Chem. B, Vol. 104, No. 44, 2000 10185

singlet energy transfer from the OPVn(S₁) state to the MP-C₆₀

moiety. This is supported by the strong electronic coupling (V^d),

which would be necessary to explain the quenching of the

fullerene emission by the direct mechanism. The conclusion of

a two-step mechanism, however, contrasts with the experimental

observation of subpicosecond electron transfer in conjugated

polymer-fullerene blends.^2,3Future experiments using ultrafast

spectroscopic methods will be performed to investigate the

temporal evolution of the photoexcitations in these dyads in

more detail and to enhance our insight into the energy versus

electron-transfer processes. Such studies will also elucidate the

possible differences between electron transfer in these dyads in

solution and in composite films of conjugated polymers and

fullerenes.

In the solid phase (i.e., in thin films), long-lived charge-

separated states are observed for OPV3-C₆₀and OPV4-C₆₀but

not for OPV1-C₆₀and OPV2-C₆₀. The long lifetimes are

attributed to intermolecular charge-separated states resulting

from the migration of holes and/or electrons to other molecules

in the films. Therefore, we conclude that the long lifetime of

the charge-separated state is a material rather than a molecular

property. From the longest dyad, OPV4-C₆₀, we have prepared

photovoltaic devices. Although the device efficiency is not

optimized, we find encouraging short-circuit currents (I_sc) 235

µA cm^-2) and open-circuit voltages (V_oc) 650 mV) under ∼65

mW cm^-2white-light illumination. These findings indicate that

the donor-acceptor dyads form a bicontinuous D-A network.^6,7

As an important clue for the future design of more efficient

molecular bulkheterojunction materials, the action of the dyad

device indicates clearly that concurrent electron and hole

transport through a bicontinuous donor-acceptor network on

even the finest molecular scale does not necessarily suffer from

a massive charge recombination problem.

chemical ionization mass spectrometry (APCI-MS) was per-

formed on a PE-Sciex API 300 MS/MS mass spectrometer with

a mass range of 3000.

Cyclic Voltammetry. Cyclic voltammograms were recorded

with 0.1 M tetrabutylammonium hexafluorophosphate (TBAH)

as the supporting electrolyte using a Potentioscan Wenking

POS73 potentiostat. The substrate concentration was typically

10^-3M. The working electrode was a platinum disk (0.2 cm²),

and the counter electrode was a platinum plate (0.5 cm²). A

saturated calomel electrode was used as reference electrode,

calibrated against the Fc/Fc⁺couple (+0.470 V vs SCE).

Photoinduced Absorption. Photoinduced absorption (PIA)

spectra were recorded between 0.25 and 3 eV by exciting with

a mechanically modulated continuous wave Ar-ion laser pump

beam and monitoring the resulting change in transmission (∆T)

after dispersion by a triple-grating monochromator, using Si,

InGaAs, and InSb (cooled) detectors. The PIA spectra were

recorded with the pump beam in an almost parallel direction to

the probe beam. Oxygen-free solutions were studied at room

temperature, whereas thin films were held at 80 ( 0.1 K, using

an Oxford Optistat continuous flow cryostat. The changes in

∆T as a function of the pump beam intensity (I) were fitted to

a power law equation -∆T I^p. For values of p close to unity,

we assumed monomolecular decay of the excited species; for p

≈ 0.5 a bimolecular decay mechanism was supposed. Excited-

state lifetimes were determined by fitting the changes in

transmission as a function of the modulation frequency (ω) to

the expressions for either monomolecular decay (eq 8) or

bimolecular decay (eq 9).²⁵

Igτ_m

-∆T

(8)

2

1 + ω²τ

x

m

R tanh R

R + tanh R

x

-∆T

Ig/â

(9)

(

)

Experimental Section

General. UV/vis absorption spectra were recorded on a

Perkin-Elmer Lambda 900 or a Perkin-Elmer Lambda 40

spectrophotometer. Low-temperature spectra were recorded

using an Oxford Optistat continuous flow cryostat; during

measurements the temperature was kept constant within 0.1 K.

Spectra at different temperatures were not corrected for volume

changes of the solvents. Fluorescence spectra were recorded on

a Perkin-Elmer LS 50B spectrometer, using a 10 nm bandwidth

and optical densities of the solutions of 0.1 at the excitation

wavelength.

Here, τ_mis the lifetime for monomolecular decay and g is

the efficiency of generation of the photoinduced species. The

bimolecular decay constant â determines the intensity of the

PIA signal via eq 9, where R ) π/(ωτ_b) and τ_b) (gIâ)^-0.5, the

bimolecular lifetime under steady-state conditions. It is important

to note that the bimolecular lifetime, τ_b, depends on the

experimental conditions such as concentration and pump beam

intensity.

Photovoltaic Devices. The devices were prepared on poly-

(ethylene terephthalate) (PET) substrates covered with patterned

indium-tin oxide (ITO) as the transparent electrode. First, the

substrates were covered with polyethylenedioxythiophene poly-

styrenesulfonate (PEDOT-PSS, Baytron-P) to prevent shunting;

subsequently a thin layer of the donor-acceptor dyad was

deposited from a 2 wt % solution in toluene at 80 °C, using the

doctor blade technique. Thin stripes of aluminum were deposited

as counter electrodes by vacuum evaporation to form active

areas of 6 mm².

Materials. C₆₀(99.5%) was purchased from Southern Chemi-

cal Group, LCC. All other reagents were purchased from Acros

and Aldrich and used without further purification. For the PIA

measurements, toluene was distilled over potassium/sodium and

ODCB was dried over activated alumina and subsequently

distilled at reduced pressure. For synthetic purposes, DMF was

stored over BaO or 4.0 Å molecular sieves and THF and diethyl

ether were distilled over potassium/sodium. For column chro-

matography, Merck silica gel 60 (particle size of 0.063-0.200

mm) or Merck aluminum oxide 90 (neutral; activity I, particle

size of 0.063-0.200 mm) was used.

NMR spectra were recorded on a Varian Unity Plus spec-

1

trometer at frequencies of 500 and 125 MHz for H and ¹³C

nuclei, respectively, on a Bruker AM-400 spectrometer at

frequencies of 400 and 100 MHz for ¹H and ¹³C nuclei,

respectively, or on a Varian Gemini spectrometer at frequencies

of 300 and 75 MHz for ¹H and ¹³C nuclei, respectively.

Tetramethylsilane (TMS) was used as an internal standard for

¹H NMR, and CDCl₃or CS₂was used as an internal standard

for ¹³C NMR. Infrared (FT-IR) spectra were recorded on a

Perkin-Elmer 1605 FT-IR spectrophotometer or a Perkin-Elmer

Spectrum One UATR FT-IR spectrophotometer. Elemental

analyses were performed on a Perkin-Elmer 2400 series II CHN

analyzer. Gas chromatography/mass spectrometry (GC-MS)

analyses were performed on a Shimadzu GCMS-QP5000

equipped with a WCOT fused silica column (length ) 15 m,

i.d. ) 0.25 mm). Matrix-assisted laser desorption ionization

time-of-flight mass spectrometry (MALDI-TOF MS) was

performed on a Perseptive DE PRO Voyager MALDI-TOF mass

spectrometer using a dithranol matrix. Atmospheric pressure

10186 J. Phys. Chem. B, Vol. 104, No. 44, 2000

Peeters et al.

Abbreviations. AIBN, 2,2′-azobis(2-methylpropionitrile);

t-BuO, tert-butoxide; DMF, N,N-dimethylformamide; NBS,

N-bromosuccinimide; THF, tetrahydrofuran; R_f, thin-layer chro-

matography retention factor.

2H), 3.81 (m, 2H), 3.74 (m, 2H), 1.88 (m, 2H), 1.57 (m, 2H),

1.21 (m, 2H), 1.04 (2d, 6H), 0.94 (t, 6H); ¹³C NMR (CDCl₃) δ

151.18, 149.49, 125.90, 117.10, 115.71, 113.13, 74.78, 73.66,

34.80, 28.43, 26.09, 26.05, 16.65, 16.54, 11.33; GC-MS (MW

) 420.0) m/z ) 420.1 [M]⁺. Anal. Calcd for C₁₇H₂₆Br₂O₂: C,

48.36; H, 6.21. Found: C, 48.59; H, 6.13.

1,4-Bis[(S)-2-methylbutoxy]-2-methylbenzene (1). Meth-

ylhydroquinone (49.6 g, 0.40 mol), (S)-2-methylbutyl-p-tolu-

enesulfonate (203.52 g, 0.84 mol), and tetrabutylammonium

chloride (13.34 g, 0.048 mol) were added to a suspension of

K₂CO₃(332 g, 2.4 mol) in dry 2-butanone (400 mL) under an

atmosphere of dry argon. The reaction mixture was stirred for

16 h at reflux temperature. After the mixture was cooled to room

temperature, the suspension was filtered and the solvent was

removed in vacuo. The resulting crude product was purified by

column chromatography (silica gel, hexane/CHCl₃2:1, R_f)

0.8). Evaporation of the solvent yielded 99.3 g (94%) of 1 as a

colorless oil: IR (UATR) 2961, 2918, 2876, 1501, 1465, 1215,

1045, 1011, 789 cm^-1; ¹H NMR (CDCl₃) δ 6.72 (dd, 1H), 6.68

(s, 1H), 6.63 (dd, 1H), 3.70 (m, 4H), 2.20 (s, 3H), 1.83 (m,

2H), 1.54 (m, 2H), 1.25 (m, 2H), 0.98 (m, 6H), 0.92 (m, 6H);

¹³C NMR (CDCl₃) δ 152.95, 151.48, 127.97, 117.64, 111.85,

111.47, 73.40, 34.98, 34.81, 26.22, 26.16, 16.66, 16.52, 16.34,

11.36, 11.29; GC-MS (MW ) 264.2) m/z ) 264.2 [M]⁺.

Diethyl{2,5-bis[(S)-2-methylbutoxy]-4-bromo-benzyl}-

phosphonate (4). Triethyl phosphite (15.6 g, 0.094 mol) and 3

(32.93 g, 0.078 mol) were stirred at 160 °C for 1.5 h while the

liberated ethyl bromide was distilled off. The reaction mixture

was cooled to 75 °C, and the excess triethyl phosphite was

removed by distillation under reduced pressure to leave 4 (37.2

g, 100%) as a light yellow oil: ¹H NMR (CDCl₃) δ 7.02 (s,

1H), 6.96 (s, 1H), 4.02 (m, 4H), 3.74 (m, 4H), 3.18 (d, 2H),

1.86 (m, 2H), 1.56 (m, 2H), 1.30 (m, 2H), 1.25 (t, 6H), 1.02

(2d, 6H), 0.93 (t, 6H); ¹³C NMR (CDCl₃) δ 150.81 (d), 149.18

(d), 119.99 (d), 116.52, 116.12 (d), 110.38 (d), 74.42, 73.66,

61.69 (d), 34.64, 34.60, 26.05 (d), 25.89, 25.84, 16.42, 16.31,

16.17, 16.11, 11.10.

(E)-4-{4-Methyl-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-

bis[(S)-2-methylbutoxy]bromobenzene (5). A solution of

aldehyde 2 (46.78 g, 0.16 mol) in dry DMF (300 mL) was added

dropwise to a solution of 4 (76.72 g, 0.16 mol) and t-BuOK

(20.6 g, 0.184 mol) in dry DMF (200 mL) under an atmosphere

of dry argon. The resulting viscous reaction mixture was stirred

for 8 h at room temperature and subsequently poured on crushed

ice (1000 g). Aqueous HCl (5 M, 240 mL) was added, and the

aqueous phase was extracted with CHCl₃(3 × 300 mL). The

combined organic layers were washed with 3 M HCl and dried

over MgSO₄. The solvent was evaporated, and 5 (72.33 g, 73%)

was obtained as a yellow solid after recrystallization from

ethanol: IR (UATR) 2960, 2929, 2873, 1508, 1458, 1385, 1202,

2,5-Bis[(S)-2-methylbutoxy]-4-methylbenzaldehyde (2).

POCl₃(96 mL, 1.05 mol) was added to a mixture of dry DMF

(68 mL, 0.88 mol) and dry CHCl₃(120 mL) under an

atmosphere of dry argon. After the mixture was stirred for 1 h,

1 (63.36 g, 0.24 mol) was added and the reaction mixture was

stirred for 48 h at reflux temperature. After the mixture was

cooled to room temperature, the mixture was poured on ice-

water (1500 mL), stirred for 1 h, and extracted with diethyl

ether (500 mL); during the extraction NaCl was added to

promote phase separation. The organic phase was washed with

1 M HCl (3 × 300 mL), water (3 × 300 mL), and a saturated

NaHCO₃solution (300 mL). After the organic phase was dried

over MgSO₄and the solvent was evaporated in vacuo, the crude

product was purified by column chromatography (silica gel,

hexane/CHCl₃2:1, R_f) 0.8). Recrystallization from methanol

yielded 48.8 g (86%) of 2 as white crystals: IR (UATR) 2963,

2922, 2876, 2850, 1672, 1610, 1503, 1466, 1411, 1391, 1262,

1

1039, 970, 848 cm^-1; H NMR (CDCl₃) δ 7.46 (d, 1H), 7.34

(d, 1H), 7.15 (s, 1H), 7.05 (2s, 2H), 6.71 (s, 1H), 3.80 (m, 8H),

2.22 (s, 3H), 1.89 (m, 4H), 1.60 (m, 4H), 1.31 (m, 4H), 1.04

(m, 12H), 0.96 (m, 12H); ¹³C NMR (CDCl₃) δ 151.63, 150.80,

150.43, 149.93, 127.81, 127.49, 124.77, 123.73, 121.23, 117.99,

116.19, 111.03, 110.54, 108.39, 74.68, 74.56, 74.45, 73.35,

35.07, 34.95, 34.83, 26.31, 26.27, 26.24, 26.11, 16.78, 16.69,

16.58, 16.41, 11.45, 11.36, 11.34; APCI-MS (MW ) 616.3)

m/z ) 617.5 [M + H]⁺. Anal. Calcd for C₃₅H₅₃BrO₄: C, 68.06;

H, 8.65. Found: C, 68.13; H, 8.59.

1

1212, 1048, 1032, 1007, 877, 860, 760, 707 cm^-1; H NMR

(CDCl₃) δ 10.45 (s, 1H), 7.20 (s, 1H), 6.78 (s, 1H), 3.80 (m,

4H), 2.26 (s, 3H), 1.87 (m, 2H), 1.54 (m, 2H), 1.27 (m, 2H),

1.03 (t, 6H), 0.93 (m, 6H); ¹³C NMR (CDCl₃) δ 189.20, 156.24,

136.74, 122.94, 122.94, 115.48, 108.03, 73.75, 73.12, 34.80,

34.74, 26.12, 26.06, 17.15, 16.56, 16.51, 11.25, 11.23; GC-

MS (MW ) 292.2) m/z ) 292.3 [M]⁺. Anal. Calcd for

C₁₈H₂₈O₃: C, 73.93; H, 9.65. Found: C, 73.86; H, 9.54.

(E)-4-{4-Methyl-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-

bis[(S)-2-methylbutoxy]benzaldehyde (6). Bromide 5 (37.24

g, 60.23 mmol) was dissolved in dry diethyl ether (400 mL).

The solution was cooled to -10 °C, and 1.6 M n-butyllithium

in hexane (60 mL) was added slowly. After the mixture was

stirred for 5 min, the cooling bath was removed and dry DMF

(21.3 mL) was added dropwise. The mixture was stirred for

another hour at room temperature. After addition of 6 M HCl

(100 mL), the organic layer was washed with water (2 × 250

mL), with a saturated NaHCO₃solution (300 mL), and again

with water (300 mL). The organic layer was dried over MgSO₄,

and the solvent was evaporated. Recrystallization from methanol/

diethyl ether (5:1) yielded 30.1 g (88%) of 6 as a yellow solid:

IR (UATR) 2958, 2922, 2874, 1672, 1594, 1508, 1463, 1424,

1-Bromo-2,5-bis[(S)-2-methylbutoxy]-4-bromomethylben-

zene (3). NBS (53.4 g, 0.30 mol) and AIBN (15.0 g, 0.09 mol)

were added to a solution of 1 (66.0 g, 0.25 mol) in dry CCl₄

(250 mL) under an atmosphere of dry argon. After the reaction

mixture was stirred for 1 h under reflux, it was subsequently

allowed to cool to room temperature and filtered. After

evaporation of the solvent, hexane (100 mL) was added to the

residue and the resulting suspension was filtered and evaporated

to dryness. The remaining residue was dissolved in dry THF

(250 mL); NBS (57.9 g, 0.325 mol) was added, and the reaction

mixture was stirred at reflux temperature for 1 h. After

evaporation of the solvent, hexane (100 mL) was added. The

solution was filtered, and the solvent was removed in vacuo.

Crystallization of the residue from ethanol yielded 55.8 g (53%)

of 3 as a white crystalline solid: IR (UATR) 2961, 2922, 2875,

1495, 1462, 1388, 1296, 1220, 1205, 1044, 882, 862, 839, 745

1

1387, 1203, 1041, 966, 872, 727 cm^-1; H NMR (CDCl₃) δ

10.45 (s, 1H), 7.61 (d, 1H), 7.42 (d, 1H), 7.31 (s, 1H), 7.21 (s,

1H), 7.07 (s, 1H), 6.72 (s, 1H), 3.81 (m, 8H), 2.23 (s, 3H),

1.91 (m, 4H), 1.61 (m, 4H), 1.31 (m, 4H), 1.05 (m, 12H), 0.95

(m, 12H); ¹³C NMR (CDCl₃) δ 188.97, 156.42, 151.63, 150.78,

150.50, 135.42, 128.76, 126.79, 124.23, 123.79, 120.93, 116.09,

110.12, 109.46, 108.45, 74.44, 73.85, 73.63, 73.32, 35.05, 34.91,

34.69, 34.61, 26.22, 26.15, 16.77, 16.70, 16.60, 16.47, 11.44,

1

cm^-1; H NMR (CDCl₃) δ 7.03 (s, 1H), 6.86 (s, 1H), 4.49 (s,

Oligo(p-phenylene vinylene)-fullerene Dyads

J. Phys. Chem. B, Vol. 104, No. 44, 2000 10187

11.33, 11.28; GC-MS (MW ) 566.4) m/z ) 566.6 [M]⁺. Anal.

Calcd for C₃₆H₅₄O₅: C, 76.28; H, 9.60. Found: C, 76.23; H,

9.63.

m/z ) 1165.8 [M + H]⁺. Anal. Calcd for C₇₁H₁₀₅BrO₈: C,

73.11; H, 9.07. Found: C, 73.40; H, 9.16.

(E,E,E)-4-[4-{4-(4-Methyl-2,5-bis[(S)-2-methylbutoxy]styryl)-

2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]-

styryl]-2,5-bis[(S)-2-methylbutoxy]benzaldehyde (10). n-Bu-

tyllithium (1.6 M, 1.5 mL) was added slowly at -10 °C to a

stirred solution of compound 9 (1.17 g, 1.0 mmol) in a mixture

of dry THF (20 mL) and dry diethyl ether (30 mL) under an

atmosphere of dry argon. After 30 min, the cooling bath was

removed, dry DMF (1 mL) was added, and the mixture was

stirred for 1 h at room temperature. After addition of 6 M HCl

(4 mL), the aqueous layer was extracted with CH₂Cl₂. The

organic fractions were collected, and the solvent was evaporated.

The residue was dissolved in CHCl₃and then washed with water

and dried over Na₂SO₄. The solvent was evaporated, and the

residue was precipitated from CHCl₃into methanol and further

purified by column chromatography (aluminum oxide, gradient;

hexane/diethyl ether 3:1 f diethyl ether) to yield 0.30 g (27%)

of 10: IR (UATR) 2958, 2921, 2873, 2858, 1678, 1593, 1507,

(E,E)-4-{4-(4-Methyl-2,5-bis[(S)-2-methylbutoxy]styryl)-

2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]-

bromobenzene (7). According to the procedure for the synthesis

of 5, aldehyde 6 (3.40 g, 6.0 mmol, 20 mL of DMF) was added

to a solution of 4 (2.88 g, 6.0 mmol) and t-BuOK (0.81 g, 7.2

mmol) in DMF (10 mL). Recrystallization from methanol

yielded 4.28 g (80%) of 7 as a yellow-orange solid: IR (UATR)

2959, 2917, 2873, 1509, 1462, 1433, 1387, 1347, 1252, 1203,

1047, 964, 851 cm^-1; ¹H NMR (CDCl₃) δ 7.54-7.38 (4d, 4H),

7.19 (s, 1H), 7.17 (s, 1H), 7.15 (s, 1H), 7.10 (s, 1H), 7.08 (s,

1H), 6.72 (s, 1H), 3.82 (m, 12H), 2.23 (s, 3H), 1.92 (m, 6H),

1.63 (m, 6H), 1.22 (m, 6H), 1.06 (m, 18H), 0.97 (m, 18H); ¹³

C

NMR (CDCl₃) δ 151.68, 151.13, 150.93, 150.46, 149.97,

127.92, 127.62, 127.38, 126.69, 125.15, 123.49, 123.19, 122.03,

121.63, 118.01, 116.28, 111.32, 110.65, 110.21, 109.86, 108.37,

74.71, 74.65, 74.47, 74.30, 74.17, 73.37, 35.11, 35.05, 34.96,

34.64, 26.36, 26.29, 26.26, 26.12, 16.80, 16.73, 16.70, 16.59,

16.40, 11.45, 11.36, 11.33; APCI-MS (MW ) 890.5) m/z )

891.8 [M + H]⁺. Anal. Calcd for C₅₃H₇₉BrO₆: C, 71.36; H,

8.93. Found: C, 71.12; H, 9.01.

1464, 1420, 1386, 1346, 1251, 1201, 1044, 966, 852, 725 cm^-1

;

¹H NMR (CDCl₃) δ 10.45 (s, 1H), 7.66 (d, 1H), 7.60-7.42

(5d, 5H), 7.34 (s, 1H), 7.25 (s, 1H), 7.21 (s, 1H), 7.20 (s, 1H),

7.19 (s, 1H), 7.18 (s, 1H), 7.11 (s, 1H), 6.74 (s, 1H), 3.86 (m,

16H), 2.23 (s, 3H), 1.94 (m, 8H), 1.62 (m, 8H), 1.34 (m, 8H),

1.06 (m, 24H), 0.97 (m, 24H); ¹³C NMR (CDCl₃) δ 188.96,

156.44, 151.72, 151.49, 151.24, 151.07, 151.1, 150.68, 150.52,

135.27, 128.65, 127.96, 127.64, 126.96, 126.53, 126.36, 125.23,

124.04, 123.41, 123.21, 122.33, 121.86, 121.70, 116.33, 110.36,

110.26, 110.20, 109.95, 109.70, 108.45, 74.70, 74.33, 74.25,

74.17, 73.95, 73.75, 73.43, 35.14, 35.06, 34.99, 34.92, 34.87,

26.39, 26.28, 26.20, 16.81, 16.75, 16.70, 16.63, 16.37, 11.45,

11.35, 11.29; APCI-MS (MW ) 1114.8) m/z ) 1116.0 [M +

H]⁺.

(E,E)-4-{4-(4-Methyl-2,5-bis[(S)-2-methylbutoxy]styryl)-

2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]-

benzaldehyde (8). According to the procedure for the synthesis

of 6, compound 7 (5.0 g, 5.6 mmol, 75 mL diethyl ether) was

treated subsequently with n-butyllithium (1.6 M in hexane, 5.6

mL) and dry DMF (2.5 mL). Recrystallization from hexane

yielded 4.08 g (86%) of 8 as an orange solid: IR (UATR) 2958,

2917, 2874, 1675, 1508, 1463, 1422, 1388, 1347, 1261, 1201,

1

1120, 1043, 1008, 965, 849, 725 cm^-1; H NMR (CDCl₃) δ

10.47 (s, 1H), 7.64 (d, 1H), 7.53 (d, 1H), 7.52 (d, 1H), 7.46 (d,

1H), 7.32 (s, 1H), 7.24 (s, 1H), 7.20 (s, 1H), 7.17 (s, 1H), 7.10

(s, 1H), 6.72 (s, 1H), 3.83 (m, 12H), 2.24 (s, 3H), 1.93 (m,

6H), 1.62 (m, 6H), 1.33 (m, 6H), 1.07 (m, 18H), 0.96 (m, 18H);

¹³C NMR (CDCl₃) δ 188.94, 156.36, 151.61, 151.39, 150.84,

150.54, 150.44, 135.19, 128.67, 127.74, 126.45, 126.00, 124.91,

123.64, 123.56, 121.61, 121.40, 116.17, 110.23, 110.06, 109.67,

109.48, 108.24, 74.53, 74.19, 74.02, 73.82, 73.58, 73.28, 35.05,

34.98, 34.90, 34.84, 34.78, 26.30, 26.21, 26.13, 16.76, 16.70,

16.66, 16.58, 16.39, 11.44, 11.34, 11.28; APCI-MS (MW )

840.6) m/z ) 842.0 [M + H]⁺.

(E,E,E)-4-[4-{4-(4-Methyl-2,5-bis[(S)-2-methylbutoxy]styryl)-

2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]-

styryl]-2,5-bis[(S)-2-methylbutoxy]bromobenzene (9). Ac-

cording to the procedure for the synthesis of 5, aldehyde 8 (1.35

g, 1.6 mmol) in DMF (25 mL, 40 °C) was added to a solution

of 4 (0.79 g, 1.65 mmol) and t-BuOK (0.24 g, 2.15 mmol) in

dry DMF (5 mL). Precipitation from CHCl₃into methanol and

recrystallization from 2-propanol afforded 1.30 g (70%) of 9

as an orange solid: IR (UATR) 2958, 2918, 2873, 2859, 1507,

1463, 1420, 1386, 1347, 1251, 1201, 1045, 965, 852 cm^-1; ¹H

NMR (CDCl₃) δ 7.56-7.36 (6d, 6H), 7.19 (3s, 3H), 7.17 (2s,

2H), 7.11 (s, 1H), 7.08 (s, 1H), 6.73 (s, 1H), 3.83 (m, 16H),

2.23 (s, 3H), 1.94 (m, 8H), 1.64 (m, 8H), 1.34 (m, 8H), 1.08

(m, 24H), 0.98 (m, 24H); ¹³C NMR (CDCl₃) δ 151.63, 151.08,

151.01, 150.89, 150.40, 149.92, 127.70, 127.67, 127.54, 127.27,

127.00, 126.87, 125.14, 123.39, 123.01, 122.85, 122.38, 122.10,

121.63, 117.94, 116.25, 111.33, 110.56, 110.07, 110.03, 109.86,

109.84, 108.27, 74.62, 74.41, 74.18, 74.13, 73.31, 35.08, 35.02,

34.93, 34.81, 26.35, 26.27, 26.24, 26.10, 16.79, 16.73, 16.69,

16.58, 16.40, 11.46, 11.36, 11.34; APCI-MS (MW ) 1164.7)

N-Methyl-2-{4-methyl-2,5-bis[(S)-2-methylbutoxy]phenyl}-

3,4-fulleropyrrolidine (OPV1-C₆₀). N-Methylglycine (287 mg,

3.0 mmol) and 2 (292 mg, 1.0 mmol) were added to a solution

of C₆₀(0.72 g, 1.0 mmol) in chlorobenzene (200 mL) under an

atmosphere of dry argon. The reaction mixture was stirred at

reflux temperature for 16 h in the dark. The solvent was

evaporated, and the residue was purified by column chroma-

tography (silica gel). Elution with CS₂afforded unreacted C₆₀,

and further elution with cyclohexane/toluene (3:1) afforded the

desired product fraction (R_f) 0.3). The product fraction was

concentrated in vacuo to a volume of ∼10 mL and precipitated

by addition of methanol. The precipitate was washed three times

with methanol to afford 452 mg (44%) of OPV1-C₆₀(mixture

of two diastereomers) as a brown powder: IR (KBr) 2956, 2910,

2870, 2778, 1503, 1462, 1200, 1032, 770 cm^-1; ¹H NMR (CS₂,

D₂O insert) δ 7.49 (s, 0.5H), 7.48 (s, 0.5H), 6.78 (s, 1H), 5.59

(s, 1H), 5.06 (d, 1H), 4.41 (d, 1H), 4.02-3.66 (m, 4H), 2.92 (s,

1.5H), 2.91 (s, 1.5H), 2.32 (s, 3H), 1.98 (m, 1H), 1.88-1.55

(m, 3H), 1.51-1.27 (m, 2H), 1.24-1.00 (m, 12H); ¹³C NMR

(CS₂, D₂O insert) δ 156.55, 156.54, 155.00, 154.94, 154.34,

154.31, 153.61, 153.60, 151.28, 151.19, 150.97, 147.02, 147.01,

146.61, 146.59, 146.51, 146.18, 146.17, 146.03, 145.99, 145.96,

145.89, 145.84, 145.81, 145.71, 145.70, 144.46, 144.35, 144.26,

144.12, 142.88, 142.81, 142.46, 142.43, 142.39, 142.36, 142.09,

142.08, 142.06, 142.03, 141.95, 141.89, 141.77, 141.54, 141.53,

141.51, 141.43, 141.42, 140.01, 139.93, 139.54, 139.40, 136.36,

136.35, 135.95, 135.85, 135.83, 134.41, 134.36, 127.20, 127.18,

122.41, 122.34, 114.53, 114.44, 112.42, 112.30, 76.47, 75.37,

75.34, 73.13, 73.08, 73.04, 72.98, 69.65, 68.93, 39.97, 35.21,

35.19, 35.13, 26.75, 26.50, 17.00, 16.96, 16.95, 16.80, 16.76,

12.03, 11.99, 11.97, 11.93; MALDI-TOF MS (MW ) 1039.3)

10188 J. Phys. Chem. B, Vol. 104, No. 44, 2000

Peeters et al.

m/z ) 1039.6 [M + H]⁺. Anal. Calcd for C₈₀H₃₃NO₂: C, 92.38;

39.98, 39.97, 35.36, 35.30, 35.27, 35.23, 35.18, 35.16, 26.80,

26.76, 26.46, 17.04, 17.01, 16.99, 16.96, 16.76, 16.70, 16.69,

12.01, 11.99, 11.97, 11.93, 11.90, 11.85; MALDI-TOF MS

(MW ) 1587.6) m/z ) 1588.7 [M + H]⁺. Anal. Calcd for

C₁₁₆H₈₅NO₆: C, 87.69; H, 5.39; N, 0.88. Found: C, 87.70; H,

5.30; N, 0.84.

H, 3.20; N, 1.35. Found: C, 92.15; H, 3.15; N, 1.33.

N-Methyl-2-{4-(4-methyl-2,5-bis[(S)-2-methylbutoxy]styryl)-

2,5-bis[(S)-2-methylbutoxy]phenyl}-3,4-fulleropyrrolidine

(OPV2-C₆₀). According to the synthetic procedure for OPV1-

C₆₀, C₆₀(720 mg, 1.0 mmol) was mixed with N-methylglycine

(287 mg, 3.0 mmol) and 6 (566 mg, 1.0 mmol) in chlorobenzene

(200 mL). Chromatography (silica gel, CS₂then cyclohexane/

toluene 2:1, R_f) 0.3), precipitation, and washing yielded 614

mg (47%) of OPV2-C₆₀(mixture of two diastereomers) as a

brown powder: IR (KBr) 2956, 2944, 2871, 2776, 1504, 1461,

N-Methyl-2-(4-{4-[4-(4-methyl-2,5-bis[(S)-2-methylbutoxy]-

styryl)-2,5-bis[(S)-2-methylbutoxy]styryl]-2,5-bis[(S)-2-meth-

ylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]phenyl)-3,4-ful-

leropyrrolidine (OPV4-C₆₀). Similar to the procedure for the

synthesis of OPV1-C₆₀, C₆₀(216 mg, 0.3 mmol) was mixed

with N-methylglycine (80 mg, 0.9 mmol) and 8 (335 mg, 0.3

mmol) in chlorobenzene (70 mL). After column chromatography

(silica gel, CS₂then cyclohexane/toluene 1:1, R_f) 0.25), a

fluorescent contamination was still present. Further purification

by column chromatography (silica gel, pentane/CH₂Cl₂3:2, R_f

) 0.12), precipitation, and washing afforded 224 mg (40%) of

OPV4-C₆₀(mixture of two diastereomers) as a yellow-brown

powder: IR (KBr) 2958, 2912, 2872, 1504, 1462, 1419, 1384,

1

1412, 1195, 1040, 968, 852, 768 cm^-1; H NMR (CS₂, D₂O

insert) δ 7.59 (s, 0.5H), 7.58 (s, 0.5H), 7.42 (d, 1H), 7.24 (d,

1H), 7.17 (s, 1H), 6.99 (s, 1H), 6.69 (s, 1H), 5.63 (s, 1H), 5.07

(d, 1H), 4.43 (d, 1H), 4.08-3.73 (m, 8H), 2.95 (s, 3H), 2.31 (s,

3H), 2.02 (m, 3H), 1.93-1.56 (m, 5H), 1.54-1.28 (m, 4H),

1.27-1.02 (m, 24H); ¹³C NMR (CS₂, D₂O insert) δ 156.48,

156.47, 154.90, 154.84, 154.18, 154.15, 153.51, 151.56, 151.19,

150.59, 150.50, 150.11, 147.03, 147.01, 146.57, 146.55, 146.49,

146.12, 146.11, 146.04, 145.98, 145.96, 145.89, 145.85, 145.81,

145.71, 145.47, 145.43, 145.34, 145.23, 145.15, 145.03, 145.01,

144.99, 144.97, 144.93, 144.87, 144.47, 144.33, 144.27, 144.11,

142.87, 142.81, 142.46, 142.43, 142.38, 142.36, 142.08, 142.06,

142.03, 141.96, 141.95, 141.92, 141.88, 141.76, 141.59, 141.52,

141.49, 141.48, 141.47, 140.02, 139.94, 139.64, 139.47, 139.46,

136.33, 135.94, 135.86, 135.84, 134.43, 134.38, 127.85, 128.83,

127.02, 124.70, 124.44, 124.38, 123.63, 123.61, 121.28, 115.64,

114.25, 114.13, 108.61, 108.51, 108.00, 76.50, 75.39, 75.36,

73.95, 73.89, 73.82, 72.90, 72.85, 72.73, 69.67, 68.95, 39.96,

35.34, 35.25, 35.17, 35.16, 35.14, 26.82, 26.79, 26.75, 26.46,

17.02, 16.99, 16.97, 16.76, 16.69, 16.68, 12.01, 11.99, 11.97,

11.94, 11.85; MALDI-TOF MS (MW ) 1313.4) m/z ) 1314.2

[M + H]⁺. Anal. Calcd for C₉₈H₅₉NO₄: C, 89.54; H, 4.52; N,

1.07. Found: C, 89.35; H, 4.45; N, 0.99.

1

1254, 1200, 1043, 968, 853, 770 cm^-1; H NMR (CS₂, D₂O

insert) δ 7.62 (s, 0.5H), 7.61 (s, 0.5H), 7.49 (d, 1H), 7.48-

7.43 (3d, 3H), 7.73 (d, 1H), 7.38 (d, 1H), 7.21 (s, 1H), 7.13 (s,

1H), 7.12 (s, 1H), 7.11 (2s, 2H), 7.03 (s, 1H), 6.72 (s, 1H),

5.66 (s, 1H), 5.10 (d, 1H), 4.46 (d, 1H), 4.11-3.77 (m, 16H),

2.98 (s, 1.5H), 2.97 (s, 1.5H), 2.34 (s, 3H), 2.07 (m, 7H), 1.93-

1.60 (m, 9H), 1.59-1.31 (m, 8H), 1.30-1.02 (m, 48H); ¹³C

NMR (CS₂, D₂O insert) δ 156.47, 156.45, 154.88, 154.81,

154.15, 154.12, 153.51, 151.56, 151.22, 150.72, 150.71, 150.63,

150.56, 150.09, 147.03, 147.02, 146.56, 146.54, 146.49, 146.11,

146.04, 145.99, 145.97, 145.90, 145.86, 145.82, 145.72, 145.47,

145.43, 145.34, 145.23, 145.16, 145.04, 145.02, 145.00, 144.98,

144.94, 144.88, 144.48, 144.33, 144.28, 144.12, 142.82, 142.47,

142.44, 142.39, 142.37, 142.07, 142.03, 141.97, 141.95, 141.93,

141.89, 141.76, 141.60, 141.52, 141.51, 140.03, 139.95, 139.66,

139.48, 136.33, 135.95, 135.86, 135.84, 134.44, 134.39, 127.69,

127.67, 127.40, 127.38, 126.82, 126.71, 126.67, 125.00, 124.74,

124.68, 123.30, 123.28, 122.83, 122.61, 122.12, 121.45, 115.70,

114.23, 114.11, 109.58, 109.45, 109.35, 108.66, 108.56, 107.87,

76.51, 75.40, 75.37, 73.93, 73.89, 73.57, 73.51, 72.92, 72.90,

72.88, 72.72, 69.68, 68.95, 39.97, 35.37, 35.29, 35.27, 35.23,

35.18, 35.16, 26.81, 26.76, 26.46, 17.03, 16.99, 16.96, 16.76,

16.70, 12.01, 11.99, 11.97, 11.94, 11.90, 11.85; MALDI-TOF

MS (MW ) 1861.8) m/z ) 1861.4 [M + H]⁺. Anal. Calcd for

N-Methyl-2-{4-[4-(4-methyl-2,5-bis[(S)-2-methylbutoxy]-

styryl)-2,5-bis[(S)-2-methylbutoxy]styryl]-2,5-bis[(S)-2-meth-

ylbutoxy]phenyl}-3,4-fulleropyrrolidine (OPV3-C₆₀). Analo-

gous to the synthesis of OPV1-C₆₀, C₆₀(720 mg, 1.0 mmol)

was mixed with N-methylglycine (287 mg, 3.0 mmol) and 8

(841 mg, 1.0 mmol) in chlorobenzene (200 mL). After chro-

matography (silica gel, CS₂then cyclohexane/toluene 3:2, R_f

) 0.25), precipitation, and washing, 762 mg (48%) of OPV3-

C₆₀(mixture of two diastereomers) was obtained as a brown

powder: IR (KBr) 2957, 2912, 2871, 1504, 1461, 1414, 1381,

C₁₃₄H₁₁₁NO₈: C, 86.38; H, 6.00; N, 0.75. Found: C, 86.30; H,

1

1335, 1245, 1197, 1041, 968, 855, 769 cm^-1; H NMR (CS₂,

5.91; N, 0.78.

D₂O insert) δ 7.62 (s, 0.5H), 7.61 (s, 0.5H), 7.48 (d, 1H), 7.43

(2d, 1H), 7.42 (d, 1H), 7.36 (d, 1H), 7.21 (s, 1H), 7.11 (s, 1H),

7.10 (s, 1H), 7.02 (s, 1H), 6.71 (s, 1H), 5.65 (s, 1H), 5.09 (d,

1H), 4.45 (d, 1H), 4.11-3.77 (m, 12H), 2.97 (2s, 3H), 2.34 (s,

3H), 2.06 (m, 5H), 1.96-1.59 (m, 7H), 1.58-1.33 (m, 6H),

1.32-1.00 (m, 36H); ¹³C NMR (CS₂, D₂O insert) δ 156.47,

156.46, 154.88, 154.82, 154.16, 154.13, 153.51, 153.50, 151.56,

151.21, 150.71, 150.69, 150.60, 150.52, 150.09, 147.03, 147.02,

146.56, 146.54, 146.49, 146.11, 146.10, 146.04, 145.99, 145.97,

145.90, 145.86, 145.82, 145.72, 145.47, 145.43, 145.34, 145.23,

145.16, 145.04, 145.02, 145.00, 144.98, 144.94, 144.88, 144.48,

144.33, 144.28, 144.12, 142.88, 142.82, 142.47, 142.44, 142.39,

142.37, 142.07, 142.04, 141.97, 141.95, 141.93, 141.89, 141.76,

141.60, 141.52, 141.51, 141.49, 141.48, 140.03, 139.95, 139.66,

139.48, 136.33, 136.32, 135.95, 135.86, 135.84, 134.43, 134.39,

127.72, 127.70, 127.53, 126.86, 126.44, 124.94, 124.68, 124.63,

123.33, 123.31, 122.94, 121.99, 121.39, 115.68, 114.23, 114.10,

109.62, 109.32, 108.65, 108.55, 107.87, 76.51, 75.40, 75.36,

73.94, 73.87, 73.57, 73.48, 72.91, 72.86, 72.71, 69.67, 68.95,

(E)-1,2-Bis{4-methyl-2,5-bis[(S)-2-methylbutoxy]phenyl}-

ethane (OPV2). Zinc dust (1.18 g, 18.0 mmol) was suspended

in dry THF (50 mL) under an atmosphere of dry argon. TiCl₄

(1.0 mL, 9.0 mmol) was added dropwise at -10 °C, and then

the mixture was stirred for 1 h at reflux temperature. Aldehyde

2 (0.88 g, 3.0 mmol) in dry THF (5 mL) was added, and the

solution was stirred for another 16 h at reflux temperature. After

the solution was cooled to room temperature, 10% aqueous K₂-

CO₃(75 mL) was added under vigorous stirring. The stirring

was continued for 30 min, and the dark blue solid that formed

was filtered. The solid was thoroughly washed with CHCl₃. The

chloroform extracts were combined, and the solvent was

evaporated. The residue was dissolved in hexane and filtered,

and the solvent was removed in vacuo. Recrystallization from

2-propanol/ethanol (9:1) yielded 0.26 g (31%) of OPV2: mp

73.5 °C; IR (KBr) 2960, 2929, 2874, 1512, 1464, 1411, 1391,

1

1249, 1208, 1047, 972, 855, 708 cm^-1; H NMR (CDCl₃) δ

7.41 (s, 2H), 7.08 (s, 2H), 6.70 (s, 2H), 3.87-3.70 (m, 8H),

2.31 (s, 6H), 1.89 (m, 4H), 1.60 (m, 4H), 1.21 (m, 4H), 1.05

Oligo(p-phenylene vinylene)-fullerene Dyads

J. Phys. Chem. B, Vol. 104, No. 44, 2000 10189

(m, 12H), 0.95 (m, 12H); ¹³C NMR (CDCl₃) δ 151.66, 150.25,

127.16, 125.27, 121.98, 116.35, 108.22, 74.70, 73.31, 35.11,

34.95, 26.31, 26.24, 16.77, 16.69, 16.36, 11.47, 11.37; GC-

MS (MW ) 552.4) m/z ) 552.6 [M]⁺. Anal. Calcd for

C₃₆H₅₆O₄: C, 78.21; H, 10.21. Found: C, 77.92; H, 10.72.

4 h, and the liberated ethyl chloride was distilled off. Column

chromatography (silica gel, ethyl acetate) afforded pure 13 (2.50

g, 63%) as a white solid: IR (UATR) 2966, 2930, 2878, 1514,

1463, 1418, 1390, 1240, 1218, 1030, 954, 896, 830, 777, 722

1

cm^-1; H NMR (CDCl₃) δ 6.85 (d, 2H), 3.94 (m, 8H), 3.74-

3.60 (m, 4H), 3.15 (d, 4H), 1.77 (m, 2H), 1.49 (m, 2H), 1.22

(m, 2H), 1.15 (dt, 12H), 0.95 (d, 6H), 0.87 (t, 6H); ¹³C NMR

(CDCl₃) δ 150.55 (d), 119.52 (d), 114.88 (d), 73.83, 61.96 (d),

35.13, 27.06, 26.33, 25.67 (d), 16.83, 16.53, 16.47, 11.50; GC-

MS (MW ) 550.3) m/z ) 550.4 [M]⁺.

(E,E,E)-1,2-Bis(4-{4-methyl-2,5-bis[(S)-2-methylbutoxy]-

styryl}-2,5-bis[(S)-2-methylbutoxy]phenyl)ethene (OPV4).

Analogous to the synthesis of OPV2, aldehyde 6 (1.70 g, 3.0

mmol) in dry DMF (20 mL) was mixed with a reductive mixture

of zinc (1.18 g, 18.0 mmol) and TiCl₄(1.0 mL, 9.0 mmol) in

dry THF (50 mL). Recrystallization from hexane yielded 0.69

g (41%) of OPV4: mp 204 °C; IR (KBr) 2960, 2917, 2874,

1509, 1566, 1421, 1389, 1350, 1252, 1204, 1047, 968, 853, 711

(E,E)-1,4-Bis{4-methyl-2,5-bis[(S)-2-methylbutoxy]styryl}-

2,5-bis[(S)-2-methylbutoxy]benzene (OPV3). Bisphosphonate

13 (0.28 g, 0.5 mmol) was dissolved in dry DMF (5 mL) under

an argon atmosphere, and t-BuOK (0.17 g, 1.5 mmol) was

added. After 15 min, aldehyde 2 (0.29 g, 1.0 mmol), dissolved

in dry DMF (10 mL), was added dropwise, and the reaction

mixture was stirred for 4 h. The solution was poured on crushed

ice, and 6 M HCl (6 mL) was added. The aqueous phase was

extracted with CHCl₃(2 × 20 mL), and the combined organic

layers were subsequently washed with 3 M HCl and water. The

organic layer was dried over Na₂SO₄; the solvent was removed

in vacuo, and the residue was purified by column chromatog-

raphy (aluminum oxide, hexane/diethyl ether 2:1). Evaporation

of the solvent afforded 120 mg (29%) of OPV3 as a yellow

solid: mp 138 °C; IR (KBr) 2960, 2916, 2874, 1511, 1468,

1

cm^-1; H NMR (CDCl₃) δ 7.63 (s, 2H), 7.51 (d, 2H), 7.46 (d,

2H), 7.20 (2s, 4H), 7.11 (s, 2H), 6.73 (s, 2H), 3.96-3.70 (m,

16H), 2.23 (s, 6H), 1.93 (m, 8H), 1.64 (m, 8H), 1.35 (m, 8H),

1.16-0.90 (m, 48H); ¹³C NMR (CDCl₃) δ 151.63, 151.07,

150.93, 150.40, 127.55, 127.49, 127.14, 125.17, 122.93, 122.51,

121.65, 116.25, 110.00, 109.85, 108.25, 74.62, 74.22, 73.30,

35.08, 35.04, 35.93, 26.36, 26.32, 26.24, 16.79, 16.69, 16.39,

11.47, 11.36. Anal. Calcd for C₇₂H₁₀₈O₈: C, 78.50; H, 9.88.

Found: C, 78.61; H, 10.27.

1,4-Bis[(S)-2-methylbutoxy]benzene (11). Ground KOH

(16.00 g, 285 mmol) was added to EtOH (250 mL), and the

mixture was stirred for 20 min. Upon addition of hydroquinone

(12.10 g, 110 mmol), the solution turned dark brown. After the

solution was stirred for another 30 min, the reaction mixture

was heated to reflux temperature and after 15 min (S)-2-

methylbutyl-p-toluenesulfonate (58.64 g, 242 mmol) was added

dropwise. The resulting suspension was stirred for another 16

h, cooled to room temperature, and filtered. The solvent was

evaporated, and purification by column chromatography (silica

gel, hexane/CH₂Cl₂3:2, R_f) 0.55) yielded 20.76 g (75%) of

11 as a white solid: IR (UATR) 2961, 2914, 2876, 1506, 1466,

1389, 1225, 1042, 822, 780 cm^-1; ¹H NMR (CDCl₃) δ 6.80 (s,

4H), 3.80-3.60 (m, 4H), 1.82 (m, 2H), 1.55 (m, 2H), 1.22 (m,

2H), 1.03-0.86 (m, 12H); ¹³C NMR (CDCl₃) δ 153.34, 115.34,

73.54, 34.77, 26.13, 16.51, 11.29; GC-MS (MW ) 250.2) m/z

) 250.2 [M]⁺.

1

1422, 1390, 1347, 1252, 1206, 1049, 972, 854, 711 cm^-1; H

NMR (CDCl₃) δ 7.49 (d, 2H), 7.44 (d, 2H), 7.17 (s, 2H), 7.09

(s, 2H), 6.71 (s, 2H), 3.94-3.68 (m, 12H), 2.23 (s, 6H), 1.90

(m, 6H), 1.62 (m, 6H), 1.33 (m, 6H), 1.06 (m, 18H), 0.96 (m,

18H); ¹³C NMR (CDCl₃) δ 151.65, 150.95, 150.39, 127.46,

127.32, 125.23, 122.82, 121.73, 116.28, 109.94, 108.29, 74.66,

74.24, 73.33, 35.10, 35.05, 34.95, 26.36, 26.33, 26.24, 16.79,

16.70, 16.40, 11.47, 11.38; APCI-MS (MW ) 826.6) m/z )

828 [M + H]⁺. Anal. Calcd for C₅₄H₈₂O₆: C, 78.40; H, 9.99.

Found: C, 78.35; H, 10.40.

Acknowledgment. We thank Mr. J. L. J. van Dongen for

recording the mass spectra, Ms. J. J. Apperloo for recording

the electrochemical data, and Dr. R. P. Sijbesma and Dr. L.

Sanchez for assistance in computational chemistry. These

investigations were supported by the Council for Chemical

Sciences (CW) with financial aid from The Netherlands

Technology Foundation (STW, 349-3562). Further support was

obtained from The Netherlands Organization for Energy and

Environment (NOVEM) in the NOZ-PV program (146.120-

008.1 and 146.120.008.3), the Dutch Ministry of Economic

Affairs, the Ministry of Education, Culture and Science, the

Ministry of Housing, Spatial Planning and the Environment

through the E.E.T. program (EETK97115), the Council for

Chemical Sciences of The Netherlands Organization for Sci-

entific Research (CW-NWO), and the Eindhoven University of

Technology in the PIONIER program (98400).

1,4-Bischloromethyl-2,5-bis[(S)-2-methylbutoxy]benzene

(12). A solution of 11 (10.04 g, 40.1 mmol) in 1,4-dioxane (60

mL) was cooled to 0 °C, and 37% aqueous HCl (35 mL) was

added. HCl gas was bubbled through the solution while 36%

aqueous formaldehyde (20 mL) was added dropwise. Under

continuous purging with HCl gas, the reaction mixture was

stirred at room temperature for 1.5 h and cooled to 0 °C. Another

portion of 36% aqueous formaldehyde (15 mL) was added

dropwise; the mixture was subsequently stirred at room tem-

perature for 1 h and cooled to 0 °C, and aqueous formaldehyde

(10 mL) was added once more. The reaction mixture was heated

to 60 °C and stirred for 16 h; after 2 h the purging with HCl

gas was stopped. The reaction mixture was concentrated to about

15 mL; methanol (150 mL) was added, and the suspension was

filtered. The solid was recrystallized from hexane to afford 10.15

g (73%) of 12 as transparent crystals: IR (UATR) 2959, 2923,

2876, 1515, 1465, 1442, 1416, 1395, 1313, 1226, 1137, 1039,

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1

918, 872, 739, 706 cm^-1; H NMR (CDCl₃) δ 6.89 (s, 2H),

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the spectra of the OPVn-C₆₀solutions, after correction for the absorbed

light intensity by measuring the second-order diffraction of the excitation

light from the grating of the monochromator.

(24) For n ) 1, the question of energy transfer is less relevant because

the OPV1 moiety cannot be selectively excited.

Article Doi

Synthesis, photophysical properties, and photovoltaic devices of oligo(p-phenylene vinylene)-fullerene dyads

DOI: 10.1021/jp001717b

Source and publish data:

Authors:

Article abstract of DOI:10.1021/jp001717b

Full text of DOI:10.1021/jp001717b

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