Blending through-space and through-bond π-π-coupling in [2,2′]-paracyclophane-oligophenylenevinylene molecular wires

Article abstract of DOI:10.1021/ja401239r

A series of ZnP-pCp-oPPV-C₆₀ conjugates covalently connected through [2,2′]-paracyclophane-oligophenylenevinylene (pCp-oPPV) bridges containing one, two, and three [2,2′]-paracyclophanes (pCps) has been prepared in multistep synthetic procedure

Full text of DOI:10.1021/ja401239r

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Article
Blending Through-Space and Through-Bond #-#-
Coupling in [2,2´]-Paracyclophane-oPPV Molecular Wires
Mateusz Wielopolski, Agustin Molina-Ontoria, Christina Schubert, Johannes T Margraf, Evangelos
Krokos, Johannes Kirschner, Andreas Gouloumis, Timothy Clark, Dirk M. Guldi, and Nazario Martín
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja401239r • Publication Date (Web): 16 May 2013
Downloaded from http://pubs.acs.org on May 21, 2013
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Blending ThroughꢀSpace and ThroughꢀBond πꢀπꢀCoupling
in [2,2´]ꢀParacyclophaneꢀoPPV Molecular Wires
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Mateusz Wielopolski,† Agustín Molina-Ontoria,‡ Christina Schubert,† Johannes Margraf,†
Evangelos Krokos,† Johannes Kirschner,† Andreas Gouloumis,‡ Timothy Clark,◊ Dirk M. Guldi*†
*‡
I
and Nazario Martín .
†Friedrich-Alexander-Universität Erlangen-Nürnberg, Department of Chemistry and Pharmacy & Interdisciplinary
Center for Molecular Materials (ICMM), Egerlandstrasse 3, 91058 Erlangen, Germany
‡Departamento de Química Orgánica, Facultad de Química, Universidad Complutense, E-28040 Madrid, Spain
◊Computer-Chemie-Centrum, Cluster of Excellence Engineering of Advanced Materials, University of Erlangen,
Nägelsbachstr. 25, 91052 Erlangen, Germany
IIMDEA-Nanociencia, Campus de Cantoblanco, E-28049 Madrid, Spain
e-mail: guldi@chemie.uni-erlangen.de; nazmar@quim.ucm.es
RECEIVED DATE
ABSTRACT: A series of ZnP-pCp-oPPV-C60 conjugates
covalently connected through [2,2’]-paracyclophane-
oligophenylenevinylene (pCp-oPPV) bridges containing
one, two, and three [2,2’]-paracyclophanes (pCps) have
been prepared in multistep synthetic procedures
involving Horner-Wadsworth-Emmons olefination
reactions and/or Heck type Pd-catalyzed reactions.
Molecular modeling suggests that charge transfer is
INTRODUCTION
Natural photosynthetic processes exhibit state-of-
the-art efficiencies in terms of charge transfer to create
electrochemical potentials and utilization thereof to
drive chemical reactions.1,2 Organic photovoltaic
applications require precise control over charge-
transfer rates to achieve maximum quantum yields.
Hence, the natural photosynthetic system serves as a
unique model for artificial solar energy conversion
systems and for probing charge-transfer processes on
the molecular scale. In this regard, a plethora of
artificial photosynthetic mimics have been developed,
in which the rates of light-induced charge transfer, that
is, charge separation (kCS) and charge recombination
(kCR), have been fine-tuned.3,4,5,6 For long-lived charge
separation to occur, the subtle interplay between
different parameters is the key challenge, especially
with increasing complexity of the molecular electron
donor-acceptor conjugate/hybrid. The case of
interfacing electron donors and electron acceptors with
effectively mediated by the pCp-oPPVs through
a
predominant hole-transfer mechanism. Photophysical
investigation supports molecular modeling and reveals
two major trends. On the one hand, C60 excitation of 1,
2, and 3 leads exclusively to charge transfer between
•-
pCp and C60 to afford a ZnP-(pCp -oPPV)•+-C60 radical
ion pair state without giving rise to a subsequent
•-
charge shift to yield the ZnP•+-pCp-oPPV-C60 radical
ion pair state. On the other hand, ZnP excitation of 1, 2,
and 3 results in a rather slow charge transfer between
•-
ZnP and C60, after which the ZnP•+-pCp-oPPV-C60
radical ion pair state evolves. In temperature-
dependent ZnP fluorescence experiments, which were
performed in the temperature range from 273 to 338
K, two domains are discernible; low and high
temperature behaviors. In the low temperature range
(i.e., below 30°C) the rate constants do not change,
suggesting that a superexchange mechanism is the
modus operandi. In the high temperature range (i.e., >
30°C) the rate constants increase. Moreover, we find
rather strong distance dependence for 1 and 2 and
weak distance dependence for 2 and 3. A damping
factor of 0.145 Å-1 is derived for the former pair, and
0.012 Å-1 for the latter.
molecular wires creates
a
unique class of
photosynthetic model systems for which different
synthetic methodologies provide a structural tool for
fine tuning the charge-transfer rates and to study the
photoinduced charge transfer on a rather simplified
level.
Porphyrins, on one hand, often constitute the
electron donors in these electron donor-acceptor
conjugates/hybrids due to the relative ease of
synthesis, chemical stability, and excellent light
absorption properties.7 The electron acceptor, on the
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other hand, is in many photosynthetic model systems
distinguishing between superexchange and hopping
charge transfer mechanisms. As a complement to the
C60, which has been shown to feature ideal electron
acceptor properties.8 Finally, the molecular wire is
meant to assure the electronic coupling between the
electroactive termini, namely donors and acceptors.9 It
has been shown that by varying the chemical structure
of the molecular wire that links donor and acceptor it is
possible to gain control over charge transfer rates and
the charge transfer mechanism, that is, superexchange
versus hopping.10,11 A key factor is hereby the π-
conjugation, not only within the molecular wire itself
but also between the molecular wire and the
electroactive termini. Notably, the ultimate goal in all of
these electron donor-acceptor systems is to transfer
experimental
approach,
molecular
modeling
calculations were carried out to assess the electronic
perturbations imposed on the oPPV electronic
structure upon insertion of one, two, and three pCps.
Oct
tBu
tBu
N
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tBu
tBu
N
N
N
N
Zn
1
tBu
tBu
Oct
N
HexO
tBu
tBu
OHex
electrons from the donors to the acceptors at
a
tBu
tBu
N
N
N
N
Zn
maximum rate, while slowing down the charge
Oct
12
N
recombination as much as possible – kCS >> kCR
.
tBu
tBu
2
HexO
Recently, we have documented that inserting single
[2,2´]-paracyclophanes (pCp) with distinct through-
space π-π-interactions into highly conjugated oligo-p-
phenylenevinylenes (oPPV) affects the electron-
mediating properties of oPPVs.13 In pCp, the close
proximity between the arenes results in strong
electronic and structural interactions. As a matter of
fact, styryl-[2,2´]-paracyclophanes exhibit highly
conductive behavior and efficient through-space π-π
coupling.14 In pCp-oPPV conjugates, the pCps impose a
OHex
HexO
tBu
tBu
OHex
3
tBu
tBu
N
N
N
N
Zn
tBu
tBu
Chart 1. ZnP-pCp-oPPV-C60 1-3
molecular junction behavior, which accelerates kCS and
RESULTS AND DISCUSSION
13
slows down kCR
.
Synthesis:
synthetic procedures following
2
and
3
were prepared in multistep
well established
On the basis of our earlier work, which describes the
charge transport in ZnP-C60 electron donor-acceptor
a
procedure involving [2,2']-paracyclophane (pCp), zinc
porphyrin (ZnP), and fullerene (C60) – see Supporting
Information for details. The precursor dialdehydes 12
and 19 bearing two and three pCp units, respectively,
were prepared in turn by using Horner-Wadsworth-
Emmons olefination reactions and/or Heck type Pd-
catalyzed reactions. In a subsequent synthetic step,
ZnPs were covalently connected to the dialdehydes
(12, 19) by applying similar synthetic methodologies
affording 13 and 20, respectively. Finally, 2 and 3 were
obtained by 1,3-dipolar cycloaddition reaction from the
corresponding azomethine ylides, which were
generated in situ from N-octylglycine and aldehydes 13
conjugates
bridged
by
[2,2]-paracyclophane-
oligophenylenevinylene (pCp-oPPV) of different oPPV
lengths, we present here the charge transport
properties through pCp-oPPV wires comprising
multiple pCps, that is, one, two, and three. By
implementing a second and a third pCp we expected to
introduce additional junctions into the oPPVs and favor
a charge hopping mechanism over a superexchange
mechanism. Importantly, such a change in mechanism
is thought to impact the charge transfer rates,
especially the charge recombination, and, in turn, to
increase the charge separated state lifetimes. Apart
from investigations by means of time-resolved
spectroscopies on the femto- and nanosecond
and 20, respectively, to C60
, following Prato’s
protocol.15 1 was prepared by following the method
previously reported in our group – Scheme 1.16
timescales,
temperature-dependent
steady-state
experimental techniques were employed to survey the
rate constants for intramolecular charge transfer
occurring after photoexcitation. The latter are keys in
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C8H17
N
a
CHO
CHO
CHO
Br
b
e
Br
tBu
tBu
9
10
22
21
pCpꢀC60
tBu
tBu
PO(OMe)2
N
N
N
N
Zn
b
tBu
tBu
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7
CHO
tBu
tBu
tBu
tBu
a
c
CHO
Br
e
N
N
N
N
Zn
1
OHC
Br
d
5
4
6
8
tBu
tBu
CHO
C6H13
O
tBu
tBu
CHO
OC6H13
I
OC6H13
C6H13
O
f
e
tBu
tBu
d
N
N
2
Zn
I
N
N
OC6H13
12
OC6H13
g
OHC
11
13
tBu
tBu
C6H13
O
C6H13
O
CHO
R
I
CHO
OC6H13
CHO
OC6H13
h
OC6H13
C6H13
O
17
C6H13
O
R
I
15 R = ꢀCHO
j
18
OHC
OC6H13
i
OC6H13
14
OC6H13
16 R = ꢀCH=CH2
CHO
k
C6H13
O
CHO
C6H13
O
OC6H13
C6H13
O
tBu
tBu
OC6H13
e
C6H13
O
OC6H13
3
tBu
tBu
d
N
N
N
OC6H13
Zn
N
OHC
19
20
tBu
tBu
Scheme 1 Synthesis of 1-3. Reagents and conditions: a) Pd(PPh3)4, tributylvinyltin, toluene, 100 °C, 12 h, y = 89-92 %; b)
n-BuLi (1.5 eq), DMF (1.5 eq) THF, −78 °C, to RT., 3 h, y = 75 %; c) sec-BuLi (2.2 eq), ether/THF, −78 °C, DMF (2.2 eq) to
RT., 16 h, y = 55 %; d) 7 (0.5 eq), tBuOK, dry THF, reflux, 2h, y = 41-49 %; e) C60, N-octylglycine, ClPh, reflux, 7h, y = 39-
40 %; f) 10 (2 eq), Pd(OAc)2, Bu4NBr, K2CO3, DMF, 100 °C, 4h, y = 79 %; g) n-BuLi (1.1 eq), DMF (1.1 eq) THF, 0 °C, 2h, y =
75 %; h) 5 (0.5 eq), Pd(OAc)2, Bu4NBr, K2CO3, DMF, 100 °C, 6h, y = 81 %; i) Tebbe reagent (2.2 eq), pyridine (cat.), THF, 0
°C, 2h, y = 90 %; j) 17 (2 eq), Pd(OAc)2, Bu4NBr, K2CO3, DMF, 100 °C, 8h, y = 75 %; k) 9 (2.2 eq.), Pd(OAc)2, Bu4NBr, K2CO3,
DMF, 100 °C, 4h, y = 63 %.
Electrochemistry: The electrochemical properties of
1, 2, and 3 and the references ZnP and pCp-C60 have
been investigated by cyclic voltammetry (CV) at room
temperature in THF and in ortho-dichlorobenzene, as
shown in Figure S1 and Tables S1 and S2 (Supporting
Information). In the cathodic scan sweep, 1, 2, and 3
exhibit four reduction waves at -0.84, -1.41, -1.73, -2.03
V. The first, second, and fourth reversible one-electron
reductions agree with the C60 reduction in the C60-pCp
reference, as a consequence of the saturation of a
double bond of the fullerene. The third quasi-reversible
one electron reduction wave corresponds to the ZnP
reduction, which is in excellent agreement with the
reference ZnP. In the anodic scan, 1, 2, and 3 show the
quasi-reversible one-electron oxidation wave of ZnP
(0.54 V). Additional experiments were performed in
ortho-dichlorobenzene to determine the second ZnP-
based oxidation potential and oxidations derived from
the molecular wire (Table S1
–
Supporting
Information). Upon careful examination of all of the
redox data we conclude that no significant electronic
interactions exist between the different redox- and
photoactive constituents in 1, 2, and 3 in the ground
state.
Molecular Modeling: Density functional theory (DFT)
and semiempirical molecular orbital (MO) theory were
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used to characterize the geometrical and electronic
localizes the HOMO on the bridge with some small
coefficients on ZnP, whereas in 1 and 2 the main
contribution to the HOMO and lower orbitals (HOMO-1
to HOMO-3) still resides on the ZnP donor.
Consequently, localization of the bridge states is most
pronounced in 3. The lowest unoccupied molecular
orbitals (LUMO) are localized on C60. Interestingly,
strong through-space π-interactions in the pCps allow
π-conjugation to continue through the Cp system. This
was further corroborated by analyzing the HOMO and
LUMO of 18 – Figure 2 – where a complete conjugation
throughout the entire structure is observed.
properties of 1, 2, and 3 and their impact on their
charge-transfer behavior. All geometry optimizations
were performed using the hybrid B3LYP17 hybrid
exchange-correlation functional with the 6-31G(d)18
basis set as implemented in the Gaussian0919 program
package. In order to complete the study, references 12,
20
15, 18, 19, 19-C60
,
20 and a pCp-C60 analogue were
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also investigated at the same level of theory.
The resulting molecular geometries of 1, 2, and 3
revealed dihedral angles between the pCps and the
phenylenevinylene (oPPV) phenyl rings in the bridge
moieties of 31 to 45 degrees – Figure S2 (Supporting
Information). The ZnP donors are nearly perpendicular
(~70°) to the plane of the π-spacers, which implies only
34% of the effective planar overlap. Therefore, the
ZnPs are essentially electronically decoupled. The
dihedral angles between the pCps and the oPPVs
decrease with increasing bridge length from 1 to 3,
because the length of the π-conjugated system
increases and the gain in enthalpy due to the
planarization is greater than the steric hindrance.
The effect of increasing π-conjugation upon
increasing length of the spacer is clearly visible in the
energies of the frontier orbitals – Table S3 (Supporting
Information). Whereas in 1 the HOMO to HOMO-2
orbitals are separated by gaps of 0.14 to 0.43 eV,
respectively, these values decrease for 2 and 3 to the
range of 0.03 to 0.09 eV, indicating strong π-
interactions.
Reference compounds pCp-C60, 18, 19-C60, and 20
were used to analyze the electron donating and
accepting features of the different building blocks ZnP,
C60, oPPV, and pCp. As mentioned above, when neither
ZnP nor C60 is present (18), the electron density is
evenly distributed throughout the entire molecule and
the through-space conjugation across the pCps allows
sufficient electronic communication to guarantee π-
overlap throughout the whole molecular structure.
pCp-C60, 19-C60 and 20 allow the electron donating and
electron accepting features of the building blocks to be
evaluated. As seen in Figure 2, the LUMOs of both pCp-
C60 and 19-C60 are localized on C60, consistent with its
The absence of total planarity between the pCps and
oPPVs and ZnP and the bridge structure strongly
impacts the electronic properties and the
communication between the ZnP electron donor and
the C60 electron acceptor, which varies with the length
of the π-conjugated spacer. The frontier orbitals shown
in Figure 1 illustrate the impact of structure on the
electronic features of the systems.
electron accepting features.
A comparison of the
HOMOs between pCp-C60 and 19-C60 shows that
elongating the bridge increases the spatial separation
of the HOMO and LUMO. In pCp-C60, the HOMO-1
exhibits electron density contributions on the Cp but
significant electronic coupling to C60 remains because
of the short distance between the fullerene and Cp. The
HOMO in 19-C60, on the other hand, is entirely
decoupled from the LUMOs, indicating that a long
bridge moiety can act as a more efficient electron
donor than a single Cp unit. Indeed, the evidently
smaller HOMO to LUMO energy gap of 1.72 eV in 19-C60
may render an intramolecular electron transfer from
the bridge to C60 easier than in pCp-C60 with its energy
gap of 2.63 eV. The electron donating properties of the
Cp-oPPVs are further corroborated by considering the
HOMOs and LUMOs of 20, where the ZnP takes the role
of the electron acceptor, with the HOMO remaining
localized on the bridge. The energies of the HOMOs are
equal (-4.76 eV) in 19-C60 and 20. Furthermore, the
results show that at large donor to acceptor distances,
the bridge retains the electronic communication due to
superexchange interactions mediated by low-energy
orbitals.
Figure 1. Representative frontier orbital schemes of ZnP-
oPPV-pCp-C60 1, 2, and 3 (B3LYP/6-31G(d)).
Hence, the highest occupied molecular orbital
(HOMO) in 1 is quite localized on the ZnP donor due to
the dihedral angle of nearly 70°. However, the HOMO
orbital coefficients extend into the π-spacer, suggesting
significant interactions with the bridge orbitals. As the
length of the spacer increases, the coupling of the
HOMO into the bridge is enhanced due to a lowering of
the energies of the bridge orbitals upon increasing π-
interactions. As a consequence, the HOMO in 2 is
delocalized throughout the entire bridge. In 3, on the
other hand, the large distance between ZnP and C60
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C60) and localized on the ZnP, is energetically less
favorable than in 19-C60. Hence, a less pronounced shift
of electron density onto vacant orbitals of ZnP is
observed and the corresponding change of dipole
moment is reduced by
a factor of nearly three.
Summarizing, in addition to confirming the roles of ZnP
and C60 as the electron donor and acceptor,
respectively, these results show the significant ability
of the pCp-oPPVs to accept holes and donate electrons
in the presence of energetically well-matched donors
and acceptors.
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Considering the ZnP-pCp-oPPV-C60s (1, 2 and 3) the
charge-transfer states obtained correspond well with
the molecular orbitals shown in Figure 1. As seen from
these orbital representations, charge transfer will
occur from the ZnP to C60 in 1, whereas in 2 and 3 an
increasing contribution from the bridge states is
evident. Consequently, for 1 the first four charge-
transfer states obtained show the positive charge
localized on the ZnP moiety with high changes of dipole
moment in the range of 100 Debye. However,
energetically separated by 0.3 eV from the other states
a bridge charge transfer state with a Δꢀ value of 28
Debye was also obtained (Figure S3 Supporting
Information). In 2, on the other hand, only the first two
states are pure HOMO to LUMO charge-transfer
excitations with Δꢀ of 166 Debye due to the increased
distance between the positive and negative charges.
Following, two bridge charge-transfer excitations (Δꢀ =
61 Debye) mix with the HOMO to LUMO transitions as a
consequence of the mixing of the orbitals of ZnP and
pCp-oPPV. The energy separation between the ZnP/C60
and pCp-oPPV/C60 charge-transfer states drops to 0.01
and 0.02 eV. Finally, in 3 the lowest energy charge-
transfer transition stems from an intramolecular
electron transfer between the bridge and C60 and
exhibits a Δꢀ value of 60 Debye. Within the first five
charge-transfer excitations, three correspond to pCp-
oPPV to C60 charge-transfer processes and two to ZnP
to C60 excitations with high changes of dipole moment
Figure 2. Representative frontier orbital schemes of pCp-
C60, 18, 19-C60, and 20 (B3LYP/6-31G(d)).
However, even though such ground-state orbital
interpretations are appealing, simple arguments based
on the Kohn-Sham orbitals may not be reflected exactly
in the nature of the excited states. DFT does not
reproduce charge separation in molecules well, so that
we might also expect the above Kohn-Sham orbitals to
be more deleocalized than would be found with other
techniques. However, semiempirical configuration
interaction (CI) calculations have proven to be very
effective for such calculations. Therefore, singles-only
CI (CIS) calculations with an active window of 20
occupied and 20 virtual orbitals and the AM1
Hamiltonian21 as implemented in the VAMP 10.022
package were used to obtain a full description of the
electronic states of the systems investigated. In the
current context, we only address the charge-transfer
states in 1, 2, 3, pCp-C60, 19-C60, and 20. Note that,
although these calculations were performed "in vacuo",
the nature of the excited states (i.e. their degree of
localization) does not usually change when solvent
effects are simulated by a continuum solvent model, in
contrast to some very polarizable ground-state radical
ions. Only the relative energies of the states are
affected. Beginning with the two fullerene references,
pCp-C60 and 19-C60, charge transfer states with dipole
moment changes that depend on the distance between
the positive and the negative charges were obtained. In
all of these (see Figure S4, Supporting Information) the
positive charge is localized on the bridge part adjacent
to C60. These intramolecular charge-transfer states
correspond well with the orbital representations
(Figure 2) and corroborate the hole-accepting features
of the pCp-oPPVs, which make them reasonable
electron donors. Furthermore, as predicted by the
orbital discussion above, the charge transfer states in
pCp-C60 occur at higher energies due to the higher
HOMO-LUMO gap, although the dipole moment changes
are lower due to better electronic coupling between
HOMO and LUMO. Importantly, this once more
confirms the improvement of electron donating
properties of the Cp moieties in longer bridges.
Populating the LUMO in 20, which is relatively high in
energy (-1.99 eV) as compared with C60 (-3.04 eV in 19-
(Δꢀ
= 226 Debye). Importantly, all these charge-
transfer states are found in 3 within an energy range of
only 0.13 eV.
Remarkably, the electrostatic potential mappings of
the bridge states reveal that the positive charge
invariably resides on the pCp-oPPV adjacent to C60
–
Figure 3. This is due to the weaker donor character of
the pCp-oPPVs as compared with ZnP. Hence, the
charge separation remains stabilized only within a
short distance between the donor and acceptor and
represents an intermediate product within the ZnP to
C60 electron-transfer process. Obviously, the
stabilization of such intermediate charge-transfer
states increases with increasing bridge length. This, in
turn may increase their lifetimes and make it feasible
to detect them by time-resolved spectroscopic
methods.
Most importantly, the pCp-oPPVs act mainly as hole
transporters because of their energetically high-lying
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occupied orbitals. The role of the bridge is restricted to
However, these are effectively present due to the
efficient electronic communication, even over a
distance of more than 48 Å, as found for 3.
mediating the electronic coupling between the ZnP and
C60 (vide infra). However, as seen from the CI
calculations, increasing the length of the bridge
stabilizes these intermediate states more and more and
makes them energetically accessible. Hence, in 2 and 3,
the bridge excitations mix with the main HOMO to
LUMO charge transfer. Therefore, a change of charge-
transfer mechanism may be expected upon increasing
temperature or length of the linker. In 1, where charge
delocalization is less pronounced than in 2 and 3, a
coherent tunneling process presumably governs the
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charge transfer, whereas going from
2 to 3 the
probability of observing a subsequent charge hopping
from the ZnP to the bridge and from the bridge to C60
increases.
Figure 4. Upper part – room temperature absorption
spectra of 19 (black), 20 (grey), ZnP (red), NMPC60 (dark
yellow), and pCp-C60 (green) in THF. Lower part – room
temperature absorption spectra of 1 (black), 2 (grey), and
3 (red) in THF.
Figure 3. Molecular electrostatic potential projected onto
the 0.017 e− Å−3 isodensity surfaces of the ground and
selected excited states of 2, mapped from -0.3 (blue) to 0.1
(red) Ha e-1.
Figure 3 shows the molecular electrostatic potential
of the ground and selected singlet excited states of 2,
mapped onto electron density isosurfaces. These states
are representative for the different photophysical
processes that may occur. The local excitation of the
ZnP (5S) features high oscillator strength and
practically no shift in charge density. At higher
energies, several states display a charge shift from the
Absorption and fluorescence spectroscopy: In the
absorption spectra, the Soret and Q-Bands of ZnP at
430 nm and 550/604 nm, respectively, of 1, 2, and 3
dominate the visible part – Figure 4. The characteristic
absorptions of C60 and pCp-oPPV, on the other hand,
evolve in the ultraviolet part. In particular, C60 absorbs
between 250 and 300 nm, whereas the pCp-oPPVs
exhibit broad maxima between 300 and 450 nm. A
closer look at the position of the Soret and Q bands
reveals a red shift of about 20 nm for 1, 2, and 3
relative to ZnP.
pCp bridge to C60, with relatively low oscillator
strengths (9S, 10S, and 16S). The direct excitation into a
•-
ZnP•+-pCp-oPPV-C60 radical ion pair state (11S) is
impossible owing to the weak orbital overlap, which
leads to zero oscillator strength. Therefore, this state
may only be populated from another excited state.
Further insights into the origin of these absorptions
came from complementary investigations with
references 12, 18, and 19 – Figure S5 (Supporting
Information). Important is the fact that the overall
absorption characteristics depend on the number and
the positioning of the pCps. In particular, the
absorption spectra of all references reveal two main
features, that is, a rather broad band between 300 and
360 nm and a rather sharp band in the 370 to 450 nm
region. The 300 to 360 nm bands correlate with oPPV
centered transitions, whereas the 370 to 450 nm bands
In summary, the calculations clearly suggest that the
charge transfer is effectively mediated by the pCp-
oPPVs with a predominant hole-transfer mechanism.
Delocalization and trapping of the holes may occur only
upon strong electronic interactions between the
electron donating and electron accepting orbitals.
These interactions are mainly governed by the distance
between electron donor and electron acceptor.
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relate to transitions that are centered on pCp.
wires. Thus, inserting several through-space
conjugated pCps into 12 and 19, and into 2 and 3
lowers, on one hand, the oPPV π-conjugation but
maintains, on the other hand, the electronic
communication through the molecular wires.
Interestingly, when comparing 18 with 19 blue-shifted
oPPV absorptions evolve in the presence of more than a
single pCp. In particular, blue-shifts evolve from 340 to
325 nm for 18 versus 19. The blue shift seen in the
oPPV transitions infers that an increasing number of
pCps interrupts the oPPV π-conjugation. In other
words, the through-bond oPPV conjugation is broken
and replaced by the through-space pCp conjugation.
Despite the aforementioned, the red-shift from 406 to
411 nm of the pCp absorption in 12 and 19 as the
overall length increases corroborates that the π-
conjugation is intact but depends on the oPPV
conjugation length. As such, it is safe to assume that the
electronic coupling between oPPVs and pCps is rather
strong.
In order to explore the aspect of through-space
versus through-bond conjugation, steady-state and
time-resolved emission as well as transient absorption
measurements were performed.
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With this information in hands, we are able to dissect
the absorption features in the electron donor-acceptor
conjugates 1, 2, and 3. oPPV absorptions are
discernable between 300 and 350 nm, whereas the pCp
absorptions are masked by the much stronger
absorbing Soret band of ZnP. In fact, they appear as
features that are superimposed on the Soret band.
Variation of the solvent from toluene to THF and
benzonitrile, hardly affects the positions of the oPPV
and pCp maxima. Equally important is that altering the
length of the molecular wires affords appreciable
changes. Taken the aforementioned into concert, the
spectral results suggest the lack of extended
conjugation throughout the corresponding molecular
Figure 5. Room temperature fluorescence spectra of ZnP
(dark yellow), 1 (grey), 2, and 3 (red) in THF solutions
that reveal the same absorption of 0.2 at the 420 nm
excitation.
Table 1. Quantum yields (Фf), charge separation rate (kCS) rate constants, charge recombination (kCR) rate
constants, and fluorescence lifetimes (τTCSPC and τSS) of 1, 2, 3, and their references in various solvents.
solvent
Фf
kCS [s-1]
kCR [s-1]
τSSa [ns]
τTCSPCb [ns]
ZnPc
1c
0.04
-
-
2.4
2.4
toluene
THF
0.004
5.2 x 109
6.2 x 108
1.0 x 106
2.6 x 107
2.7 x 107
0.20
0.22
PhCN
toluene
THF
2c
0.028
0.027
0.025
0.030
0.031
0.024
0.71
1.65
1.65
1.51
1.81
1.84
1.44
1.68
1.44
1.48
1.90
1.79
1.70
PhCN
toluene
THF
3c
5.2 x 108
PhCN
THF
12d
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15d
18d
19d
20d
THF
THF
THF
THF
0.20
0.57
0.70
0.024
6 x 10-4
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e
NMPC60
1.2
1.2
a
Lifetimes calculated from steady-state emission via equation kcs = [(Ф (reference) - (Ф (conjugate)]/[τ (reference) (Ф
(conjugate)] (1); b Lifetimes determined by TCSPC experiments; c excited at 420 nm; d excited at 350 nm; e excited at 350 nm
All three of the redox-active constituents, that is, ZnP,
pCp-oPPV, and C60, emit singlet excited state energy in
different spectral regions of the spectrum. For example,
bleaching at 400 and 530 nm and broad transient
maxima at 640 and 860 nm – Figure 6.
420 nm excitation leads to
a rather strong ZnP
fluorescence (Ф = 4.0 x 10-2) between 570 and 700 nm
– Figure 5 – whereas the pCp-oPPV fluorescence (Ф =
1.1 x 10-1) maximizes around 466 nm – Figure S6
(Supporting Information) – upon 350 nm excitation.
The characteristic but weak C60 singlet excited state
fluorescence (Ф = 6.0 x 10-4) evolves around 710 nm
upon 350 nm excitation. In 1, 2, and 3, the ZnP
emission is quenched upon ZnP excitation at, for
example, 420 nm. The quantum yields – in THF: 4.3 x
10-3 for 1, 2.7 x 10-2 for 2 and 3.1 x 10-2 for 3 – imply a
rather weak but evident quenching. Additionally, a
comparison of the quantum yields in solvents of
different polarities (Table 1) reveals an even weaker
dependence on solvent polarity – vide infra.
Hence, it seems that the excited state interactions
between the strongly fluorescent and electron donating
ZnP and the electron accepting C60 are rather weak. As a
matter of fact, an energy transfer from the
photoexcited ZnP to C60 as the sole deactivation
pathway is ruled out on the basis of complementary
TCSPC fluorescence lifetime measurements. The
corresponding fluorescence lifetimes upon 403 nm
excitation with values for 2 of 1.68 x 10-9 s in toluene,
1.44 x 10-9 s in THF, and 1.44 x 10-9 s in benzonitrile as
well as for 3 of 1.90 x 10-9 s in toluene, 1.79 x 10-9 s in
THF, and 1.70 x 10-9 s in benzonitrile reflect the steady-
state fluorescence experiments. In particular, the
dependence on solvent polarity and on the length of the
pCp-oPPVs is reconfirmed. In line with recent
Figure 6. Upper part – differential absorption spectra
(visible and near-infrared) obtained upon femtosecond
flash photolysis (387 nm, 110 nJ) of 19 in argon saturated
THF with several time delays between 0 and 6750 ps at
room temperature. Lower part – differential absorption
spectra (visible and near-infrared) obtained upon
femtosecond flash photolysis (387 nm, 110 nJ) of 20 in
argon saturated THF with several time delays between 0
and 6750 ps at room temperature.
experiments,13 this indicates
deactivation pathway. Finally, comparing the pCp-oPPV
fluorescence upon 350 nm excitation in 19 and 20
reveals a significant fluorescence quenching, and a
sensitized porphyrin fluorescence
(Supporting Information)– upon 350 nm excitation.
a charge transfer as
–
Figure S6
Transient absorption spectroscopy: Thus, to work
out the contributions from energy and/or charge
transfer to the overall deactivation processes we
employed complementary transient absorption
measurements. To this end, excitation at 387 nm of
argon-saturated THF solutions of the references 12 and
19 leads to an instantaneous formation of their singlet-
singlet absorptions characterized by transient
Representative differential absorption spectra for
ZnP, taken after laser pulse at 387 or 420 nm in toluene
solution, are displayed in Figure S7 of the Supporting
Information. The differential spectrum recorded
immediately after the laser pulse is characterized by
bleaching of the porphyrin Q-band absorption at 560
and appearance of transients at 480 nm and between
580 and 750 nm. These are spectral attributes of the
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ZnP singlet excited state (2.04 eV), which decays slowly
(4.0 × 108 s-1) in toluene to the energetically lower-
lying ZnP triplet excited state (1.53 eV) via intersystem
crossing. Now, considering the transient spectra of 20 –
Figure 6 – the observation of an instantaneous growth
of a broad absorption between 570 and 750 nm affirms
selective excitation of the ZnP. In addition, the singlet
excited state features of pCp-oPPV are discernable. The
relative ratio depends on the excitation wavelength
with 387 nm that favors pCp-oPPV excitation and 420
nm that leads predominantly to ZnP excitation. In 20,
the pCp-oPPV centered singlet-singlet absorptions
decay, however, with accelerated dynamics of less than
2 ps. The latter transform rapidly to the corresponding
ZnP singlet excited state. Owing to the fact that the ZnP
singlet excited state is not appreciably impacted by the
presence of pCp-oPPV, the pCp-oPPV to ZnP energy
transfer is followed by intersystem crossing to the
corresponding ZnP triplet excited state, namely broad
transients that maximize around 840 nm. Notable, the
overall broadening of the bands results from an overlap
between ZnP and pCp-oPPV absorptions.
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In pCp-C60, at early times, the differential absorption
spectra upon 387 nm excitation are practically
identical to those seen with C60, that is, a broad
transient at 900 nm, attesting to the formation of the
C60 singlet excited state – Figure 7. In the C60 reference,
intersystem crossing (5.0 × 108 s-1) to the energetically
lower lying C60 triplet dominates the deactivation of the
singlet excited state of C60. In stark contrast to C60, after
a delay time of ca. 25 ps, a new transition around 510
nm starts to grow in for pCp-C60. This is accompanied
by appearance of an absorption in the near-infrared
region around 1000 nm, whose formation is completed
within 30 ps. Based on spectral comparisons, the
former is ascribed to the pCp π-radical cation (pCp•+),
while the latter is due to the C60 π-radical anion (C60•-).
Charge recombination dynamics were analyzed by
following the absorption decay of the reduced form of
C60 and of the oxidized form of pCp yielding a lifetime of
1300 ps in THF.
Figure 7. Upper part – differential absorption spectra
(visible and near-infrared) obtained upon femtosecond
flash photolysis (387 nm, 200 nJ) of pCp-C60 in argon
saturated THF with several time delays between 0.1 and
4250 ps at room temperature. Lower part
– time
absorption profiles at 1020 nm monitoring the charge
separation and charge recombination dynamics.
Finally, turning to the ZnP-pCp-oPPV-C60s (1, 2, and
3) a differentiation between 420 and 387 nm excitation
should be made. Considering the light partition in 1, 2,
and 3 at 420 and 387 nm, ZnP absorbs exclusively at
the earlier wavelength, while at the latter the ZnP to C60
absorption ratio is 1 to 1. Upon 420 nm excitation of 1,
2, and 3, the instantaneous formation of transient
spectra are discernable that resemble those of the ZnP
singlet excited state despite the presence of C60
–
Figures 7 and 8. Here, the ZnP singlet excited state
features decay on the time scale of up to 8 ns with
lifetimes that range, for example, in THF from 0.2 for 1
and 1.65 for 2 to 1.84 ns for 3. These values are
comparable to those determined in the time-resolved
fluorescence measurements. Interesting is that at the
end of the 8 ns time scale the ZnP triplet excited state
features dominate the differential absorption
spectrum. From the latter we infer that the competition
between ZnP intersystem crossing and ZnP•+-pCp-
•-
oPPV-C60 charge transfer must be heavily on the
earlier side. Still, spectroscopic evidence for ZnP•+-pCp-
•-
oPPV-C60
– vide infra – suggests that electron
tunneling is still operative in 2 and 3.
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of the ZnP singlet excited state evolves that developed
during the 420 nm excitation experiments – vide supra
– affords the ZnP•+-pCp-oPPV-C60•- radical ion pair state.
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Figure 8. Upper part – differential absorption spectra
(visible and near-infrared) obtained upon femtosecond
flash photolysis (420 nm, 100 nJ) of 2 in argon saturated
toluene with several time delays between 0.1 and 7500 ps
at room temperature. Lower part
– time absorption
Figure 9. Upper part – differential absorption spectra
(visible and near-infrared) obtained upon nanosecond
flash photolysis (532 nm, 10 mJ) of 2 in argon saturated
THF with a time delay of 275 ns at room temperature.
profiles at 555 nm monitoring the charge separation
dynamics.
In sharp contrast, exciting 1, 2, and 3 at 387 nm leads
besides ZnP centered singlet excited state
characteristics (vide supra) to C60 centered singlet
excited-state features. In terms of C60, the presence of
ZnP and pCp-oPPV suppresses the C60 centered
intersystem crossing and seems to favor a decay with,
for example, 6.2 x 108 s-1 for 2 and 5.2 x 108 s-1 for 3 in
THF. It is the radical ion pair state that is formed.
Evidence for the radical ion pair state formation comes
from maxima in the visible (i.e., 510 nm) and in the
near-infrared regions (i.e., 1000 nm). The features in
the visible correspond, however, to the pCp π-radical
cation (pCp•+) rather than to the ZnP π-radical cation
(ZnP•+), whereas in the near-infrared region the 1000
nm maximum agrees well with those of the C60 π-
radical anion (C60•-). Important criteria are the kinetic
resemblance between the decay of the C60 singlet
Lower part
– time absorption profile at 1000 nm
monitoring the charge recombination dynamics. Please
note that the oscillations may be artifact due to the lamp
pulser.
•-
To gather evidence for the ZnP•+-pCp-oPPV-C60
radical ion pair state formation and its decay we turned
to nanosecond flash photolysis experiments – Figure 9.
Thereby, 532 nm excitation rather than 355 nm
excitation was used to ensure mainly ZnP excitation of
2 and 3. Immediately after the laser excitation a weak
but appreciable absorption, especially in the near-
infrared region, is registered. In argon saturated THF
the decay is biexponential. On the early time scale it is
monomolecular, while on the longer time scale it is
bimolecular. Turning to experiments that were
performed in oxygen saturated THF, the earlier decay
is marginally affected, while the latter decay is heavily
quenched. From this we conclude that the long-lived
species must be the ZnP triplet excited state.
Spectroscopic support for this postulate comes from
the 840 nm marker. In sharp contrast, the C60 π-radical
anion is the dominating species with its 1000 nm
marker during the early times. Notably, the
competition between ZnP intersystem crossing/ZnP
•-
excited state and the growth of ZnP-(pCp-oPPV)•+-C60
radical ion pair state. Evidently, the deactivation of the
singlet excited state in 2 and 3 leads exclusively to a
charge transfer between pCp and C60 without giving
rise to a subsequent charge shift to yield the ZnP•+-pCp-
•-
oPPV-C60 radical ion pair state. Such a charge shift
would be endergonic due to the unfavorable ZnP
oxidation. In terms of ZnP, the same slow deactivation
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•-
triplet excited state growth and ZnP•+-pCp-oPPV-C60
radical ion pair state formation is heavily on the earlier
side. Still, a small fraction of the latter is formed. In
other words, a moderately efficient, but long-lived
ZnP•+-pCp-oPPV-C60 radical ion pair state is formed.
•-
This is in sound agreement with the slightly quenched
ZnP centered fluorescence in 2 and 3. In terms of ZnP•+-
•-
pCp-oPPV-C60 radical ion pair state lifetime, values of
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~ 260 ± 40 ns were reached for 2 and 3.
Mechanistic aspects: Relating the charge separation
dynamics upon 420 nm ZnP excitation in THF, to the
electron donor-acceptor separation (i.e., center-to-
center distance, RCC) enables us to evaluate the
damping factor of the pCp-oPPV bridges. The overall
relationship reveals a nonlinear dependence for the
charge separation for the three ZnP-pCp-oPPV-C60s (1,
2, and 3). In particular, rather strong distance
dependence for 1 and 2 is followed by weak distance
dependence for 2 and 3. The former give a damping
factor of 0.145 Å-1, and the latter a value of 0.012 Å-1.
Notably, 0.012 Å-1 is in perfect agreement with what we
have established earlier for oPPV bridges (i.e., 0.01 ±
0.005 Å-1).10c 0.145 Å-1, on the other hand, is nearly a
magnitude larger.
Figure 10. Temperature dependence of the charge
separation rate constants for 1, 2, and 3 in the 273 to 338
K
regime based on the temperature-dependent
fluorescence experiments in benzonitrile.
To shed light onto the charge transfer mechanisms in
1, 2, and 3 yielding ZnP•+-pCp-oPPV-C60•-, we probed
the temperature dependence of the ZnP fluorescence
quenching in ortho-dichlorobenzene and benzonitrile
in the range from 273 to 338 K – Figure 10. Here, we
used the fluorescence quantum yields to extrapolate
the rate constants for charge separation at each
temperature and treated them in the Arrhenius
formalism, that is, plotting (ln(kCS) versus T-1. Overall,
the rate constants do not vary in ortho-
Figure 11. Temperature dependence of the charge
separation rate constants for 1, 2, and 3 in the 280 and
dichlorobenzene and benzonitrile. Obviously,
a
334
femtosecond transient absorption experiments in
benzonitrile
K regime based on the temperature-dependent
superexchange mechanism is the modus operandi. In
the high temperature range (i.e., > 30°C) the rate
constants increase. Therefore, we can conclude that a
superexchange mechanism dominates the charge
separation in all our experiments as they were carried
out at room temperature. Independent confirmation
for the aforementioned came from an analysis of ZnP
singlet excited features in benzonitrile in the range
from 280 to 334 K (Figure 11). Overall, activation
barriers of 1.0, 4.4, and 6.1 KJ/mol for 1, 2, and 3,
.
CONCLUSIONS
We have carried out the synthesis of a series of ZnP-
pCp-oPPV-C60 conjugates covalently connected through
[2,2’]-paracyclophane-oligophenylenevinylene (pCp-
oPPV) bridges containing one, two, and three pCps.
respectively,
corroborate
the
superexchange
mechanism.
Molecular modeling studies have shown that due to
the strong electron-accepting properties of C60, a
charge transfer occurs between C60 and the adjacent
pCp. As already shown in preliminary work,13 these
states may act as intermediate steps in the charge
transfer process. Their stability originates from the fact
that the changes of the dipole moment are small, which
is simultaneously the reason because only the pCp unit
closest to C60 is oxidized in 2 and 3. Importantly, the
occurrence and stability of these intermediate low-
energy charge transfer states ratifies their extremely
long lifetimes. They mix electronically with the ZnP•+-
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pCp-oPPV-C60•- radical ion pair states, which leads to an
overall increase in stability of the charge separated
state. This nicely complies with the results from recent
work with pCp-oPPV bridges, where we have shown
that, when assuming that hole transport dominates
charge transport in π-conjugated molecular wires, a
lack of delocalization across the pCp linkers will
certainly preclude strong electronic communication
between the electron donor and acceptor.
(6) Kuciauskas, D.; Liddell, P. A.; Lin, S.; Johnson, T. E.;
Weghorn, S. J.; Lindsey, J. S.; Moore, A. L.; Moore, T. A.; Gust, D.
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(c) Guldi, D. M. Chem. Commun. 2000, 5, 321. (d) Prato, M.;
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ZnP excitation of 1, 2, and 3 results in a rather slow
charge transfer between ZnP and C60, at which end the
•-
ZnP•+-pCp-oPPV-C60 radical ion pair state evolves.
Notably, C60 excitation of 1, 2, and 3 leads exclusively to
a charge transfer between pCp and C60 without giving
rise to a subsequent charge shift to yield the ZnP•+-pCp-
•-
oPPV-C60
radical ion pair state. Temperature
dependent ZnP singlet excited state decays, that is,
fluorescence and transient absorption experiments,
corroborate that in the low temperature range (i.e., <
30°C) the rate constants are invariable. Here,
a
superexchange mechanism is the modus operandi.
Moreover, relating the charge separation dynamics to
the electron donor acceptor separation enabled us to
evaluate the damping factor of the pCp-oPPV bridges.
To this end, rather strong distance dependence for 1
and 2 featuring a damping factor of 0.145 Å. is followed
by weak distance dependence for 2 and 3 with a value
of 0.012 Å-1.
Acknowledgments. Financial support by MINECO of
Spain (CTQ2011-24652, PIB2010JP-00196, 2010C-07-
25200,
and
Consolider-Ingenio
CSD2007-00010),
FUNMOLS (FP7-212942-1), and CAM (MADRISOLAR-2
S2009/PPQ-1533) is acknowledged. The Deutsche
Forschungsgemeinschaft (SFB and Clusters of Excellence
Engineering of Advanced Materials), A. M. O. and A. G.,
thank the CAM, and the MEC of Spain for a research grant
and a Ramón y Cajal contract, respectively.
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Gouloumis, A.; Martín, N.; Echegoyen, L.; Guldi, D. M. Chem.
Commun. 2011, 47, 2270.
(17) Stephens, P. J; Devlin, F. J; Chablowski, C. F; Frisch, M,
J. Phys. Chem. 1994, 98, 11623.
Supporting Information: Experimental procedures
with complete spectroscopic and structural analysis,
including Supporting Figures and Tables. This material is
available free of charge via the Internet at
http://pubs.acs.org.
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calculations:
Oct
N
HexO
HexO
OHex
OHex
O
19ꢀC60
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