Precise control of intramolecular charge-transport: The interplay of distance and conformational effects

Article abstract of DOI:10.1002/chem.201204055

A new series of donor-bridge-acceptor (D-B-A) compounds consisting of π-conjugated oligofluorene (oFL) bridges between a ferrocene (Fc) electron-donor and a fullerene (C₆₀) electron-acceptor have been synthesized. In addition to varying the length of the bridge (i.e., mono- and bi-fluorene derivatives), four different ways of linking ferrocene to the bridge have been examined. The Fc moiety is linked to oFL: 1) directly without any spacer, 2) by an ethynyl linkage, 3) by a vinylene linkage, and 4) by a p-phenylene unit. The electronic interactions between the electroactive species have been characterized by cyclic voltammetry, absorption, fluorescence, and transient absorption spectroscopy in combination with quantum chemical calculations. The calculations reveal exceptionally close energy-matching between the Fc and the oFL units, which results in strong electronic-coupling. Hence, intramolecular charge-transfer may easily occur upon exciting either the oFLs or Fcs. Photoexcitation of Fc-oFL-C₆₀ conjugates results in transient radical-ion-pair states. The mode of linkage of the Fc and FL bridge has a profound effect on the photophysical properties. Whereas intramolecular charge-separation is found to occur rather independently of the distance, the linker between Fc and oFL acts (at least in oFL) as a bottleneck and significantly impacts the intramolecular charge-separation rates, resulting in beta values between β_CS 0.08 and 0.19 A^-1. In contrast, charge recombination depends strongly on the electron-donor-acceptor distance, but not at all on the linker. A value of β_CR (0.35±0.01 A^-1) was found for all the systems studied. Oligofluorenes prove, therefore, to be excellent bridges for probing how small structural variations affect charge transport in D-B-A systems. Finely tuned transfer: Chemical modification of the oligo-fluorene linkers between Fc and C₆₀ units enables fine-tuning of photoinduced charge-transfer processes in new donor-bridge-acceptor conjugates (see illustration). Copyright

Full text of DOI:10.1002/chem.201204055

FULL PAPER
DOI: 10.1002/chem.201204055
Precise Control of Intramolecular Charge-Transport:
The Interplay of Distance and Conformational Effects
Christina Schubert,[a] Mateusz Wielopolski,[b] Lars-Hendrik Mewes,[a]
Gustavo de Miguel Rojas,[a] Cornelia van der Pol,[c] Kathryn C. Moss,[c]
Martin R. Bryce,*[c] Jacques E. Moser,[b] Timothy Clark,[d] and Dirk M. Guldi*[a]
Abstract:
A
new series of donor–
sient absorption spectroscopy in combi-
nation with quantum chemical calcula-
tions. The calculations reveal excep-
tionally close energy-matching between
the Fc and the oFL units, which results
in strong electronic-coupling. Hence,
intramolecular charge-transfer may
easily occur upon exciting either the
oFLs or Fcs. Photoexcitation of Fc–
oFL–C60 conjugates results in transient
radical-ion-pair states. The mode of
linkage of the Fc and FL bridge has a
profound effect on the photophysical
properties. Whereas intramolecular
charge-separation is found to occur
rather independently of the distance,
the linker between Fc and oFL acts (at
least in oFL) as a bottleneck and sig-
nificantly impacts the intramolecular
charge-separation rates, resulting in
beta values between bCS 0.08 and
0.19 ꢀꢀ1. In contrast, charge recombi-
nation depends strongly on the elec-
tron-donor–acceptor distance, but not
at all on the linker. A value of bCR
(0.35ꢁ0.01 ꢀꢀ1) was found for all the
systems studied. Oligofluorenes prove,
therefore, to be excellent bridges for
probing how small structural variations
affect charge transport in D–B–A sys-
tems.
bridge–acceptor (D–B–A) compounds
consisting of p-conjugated oligofluor-
ene (oFL) bridges between a ferrocene
(Fc) electron-donor and a fullerene
(C60) electron-acceptor have been syn-
thesized. In addition to varying the
length of the bridge (i.e., mono- and
bi-fluorene derivatives), four different
ways of linking ferrocene to the bridge
have been examined. The Fc moiety is
linked to oFL: 1) directly without any
spacer, 2) by an ethynyl linkage, 3) by
a vinylene linkage, and 4) by a p-phe-
nylene unit. The electronic interactions
between the electroactive species have
been characterized by cyclic voltamme-
try, absorption, fluorescence, and tran-
Keywords: charge transfer · cyclo-
addition · donor–acceptor systems ·
fullerenes · molecular wires
Introduction
tures such as the photosynthetic reaction center for many
years.[1] Investigations of D–B–A compounds have also pro-
vided a thorough knowledge base and a huge portfolio of
molecular structures suitable for optoelectronics, solar-
energy conversion, and emerging nanoscale applications.[2]
All of these processes have one crucial aspect in common,
that is, a rapid and stepwise electron-transfer.
Donor–bridge–acceptor (D–B–A) compounds have now
served as model systems for understanding the charge trans-
port between molecular entities in more complex architec-
[a] C. Schubert, L.-H. Mewes, Dr. G. de Miguel Rojas, Prof. D. M. Guldi
Friedrich-Alexander-Universitꢁt Erlangen-Nꢂrnberg
Department of Chemistry and Pharmacy
For this reason, it is of special interest to study the nature
of charge transport in D–B–A systems, in which the length
and chemical structure of the bridge are varied systematical-
ly. In particular, the bridge plays the role of a molecular
wire that mediates the electronic coupling between donors
and acceptors and, therefore, renders charge-transfer reac-
tions possible over large distances,[2a,5a,3] comparable to
those in natural photosystems.[4] A common strategy for the
design of molecules that can function as molecular wires is
to incorporate p-conjugated oligomers in electron donor–ac-
ceptor conjugates. Many different oligomers have proved to
be suitable wire-like linkers between donors and acceptors.
These include, for example, oligo-phenylenevinylenes
(oPVs),[5] oligo-phenyleneethynylenes (oPEs),[6] oligo-fluo-
renes (oFLs),[7] oligo-acetylenes (oligoynes),[8] oligo-thio-
phenes,[9] oligo-vinylthiophenes,[10] oligo(ethynylene-10,20-
Institute for Physical Chemistry
Egerlandstr. 3, 91058 Erlangen (Germany)
E-mail: dirk.guldi@chemie.uni-erlangen.de
[b] Dr. M. Wielopolski, Prof. J. E. Moser
ꢃcolꢄ Polytechnique Fꢄdꢄrale de Lausanne
Institute of Chemical Sciences and Engineering
Photochemical Dynamics Group
Station 6, 1015 Lausanne (Switzerland)
[c] Dr. C. van der Pol, Dr. K. C. Moss, Prof. M. R. Bryce
Durham University, Department of Chemistry
Durham, DH1 3LE (UK)
E-mail: m.r.bryce@durham.ac.uk
[d] Prof. T. Clark
Friedrich-Alexander-Universitꢁt Erlangen-Nꢂrnberg
Department of Chemistry and Pharmacy and
Interdisciplinary Center for Molecular Materials (ICMM)
Computer Chemistry Center (CCC)
Nꢁgelsbacherstrasse 25, 91052 Erlangen (Germany)
porphyrindiyl-ethynylene)s,[11]
oligophenylenes,[12]
and
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201204055.
others.[13,14] It has been well-established that the electronic
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coupling between the donors and acceptors as mediated by
the p-conjugated bridge decays exponentially with the dis-
tance between them.[15] Thus, the rate of charge transfer,
both charge separation and charge recombination, is also
determined by an exponential expression:
homogenous p-conjugation by inserting a unit with slightly
different electronic properties leads to an increase of b for
both charge separation and charge recombination. In anoth-
er study,[22] we reported systems in which C60 and exTTF are
connected through oligo-vinylfluorenes (oVFL). The objec-
tive of this work was to probe the influence of a vinyl spacer
in fluorene-based molecular wires. It turned out that com-
bining the structural features of two types of molecular
wires with extremely low b values helped to improve the
charge-transfer features of oligofluorenes. Thus, photoin-
duced electron-transfer investigations have shed light on the
dependence of charge transport on the molecular structure
of the bridge, the nature of the redox-active moiety, and its
coupling to the linker.
We now report investigations of D–B–A systems, in which
ferrocene (Fc) donors and fullerene (C60) acceptors are con-
nected by oligo-fluorenes (oFLs) of various lengths. We
have varied both the length of the bridge and the nature of
the linkage between Fc and oFL by using four different
ways of linking ferrocene to the bridge, namely 1) directly
without any spacer, 2) by an ethynyl linkage, 3) by a vinyl-
ene linkage, and 4) by a p-phenylene unit (Figure 1). The
thrust of these studies is to examine the influence of the dif-
ferent linking patterns on the intramolecular charge-transfer
dynamics in solution. oFL bridges were chosen for their
electronic properties. They provide p-conjugative electron-
transport pathways with attenuation factors intermediate be-
tween oPVs and oPEs and are, therefore, ideally suited to
be fine-tuned by structural changes. These studies also com-
plement previous reports on D–B–A systems with oFL
bridges and exTTF[22,23] and Zn-porphyrin[7a] donors al-
though they may not be compatible with oPV- and oPE-
based D–B–A systems.
kCT ¼ k0eꢀbr
DA
in which k0 is a kinetic pre-factor, rDA the separation dis-
tance between the electron donor and electron acceptor,
and b is the attenuation factor. The latter is often used to
describe the capability of a p-conjugated wire-like oligomer
to transport electrical charges. In accordance with the ex-
pression above, the value of b should be as low as possible
to promote rapid and efficient charge-transfer reactions. A
low attenuation factor also means that the rate with which
the charge is transferred does not depend on the distance
between the donor and the acceptor. In such cases, the
transfer of charges is comparable to tunneling. Generally,
the b value varies with the degree of conjugation. This not
only includes the conjugation in the molecular bridge itself
but also throughout the entire pathway that electrons tra-
verse. In other words, a homogeneously conjugated bridge
guarantees a low b value only in the case of effective elec-
tronic-coupling to both the donor and the acceptor. The
coupling depends strongly on the relative energy levels and
energy gaps between donor and bridge as well as between
bridge and acceptor. Several groups have therefore studied
different oligomers bridging the same donor and acceptor
components in comparable D–B–A systems. Wasielewski
et al., for instance, have incorporated oligomers such as p-
phenylenevinylene,[16] p-phenylene,[17] and fluorene[18] to
bridge phenothiazine and perylene-3,4:9,10-bis(carboximide)
as donors and acceptors, respectively. Here, the oligomeric
p-conjugated bridges show a switch in the charge-transfer
mechanism from super-exchange to hopping at longer
bridge lengths. A more recent study[19] compares D–B–A
molecules, in which the same donor and acceptor are con-
nected through fluorenone, p-phenyleneethynylene, and the
previously reported p-phenylene.[20]
Results and Discussion
Synthesis: A series of Fc–spacer–oFL compounds was syn-
thesized to probe the relationship between ferrocene (Fc)
and an oligofluorene (oFL) bridge. Ethynyl, vinyl, and
phenyl spacers (2–4) were compared with reference 1, in
which Fc is directly linked to the oFL bridge. Both mono-
(n=1) and bi-fluorene (n=2) bridges were synthesized to
yield a set of eight compounds. A terminal-formyl group
was added in all cases in preparation for the final step to the
target electron-donor–acceptor conjugates 1–4, that is, the
Prato 1,3-dipolar cycloaddition reaction[24] of C60 with azo-
methine ylides formed in situ by treatment of the formyl
group with sarcosine in chlorobenzene heated at reflux. An
optimized ratio of C60/sarcosine/formyl reagent of 4:5:1 was
used. This gave the isolated adduct yields in the range of
48–68% with the practical benefit that the less-available
formyl component is the limiting reagent.
Another way to tune the donor–bridge/bridge–acceptor
gaps and vary the coupling into the redox-active moieties is
to modify the link between the bridge and the other compo-
nents. To this end, differences in b values are found for the
same oligomer connected either to different donors and ac-
ceptors, or to the same donors and acceptors through differ-
ent linking units.[21] In both cases the matches between the
donor-bridge and acceptor-bridge energy levels are refined.
This affects the charge injection rates from the donor to the
bridge and from the bridge into the acceptor. It is thus ap-
pealing not only to study the distance dependence of charge
transfer, but also the impact of subtle variations of the link-
ers connecting the bridge to the donors and acceptors.
Previous work[7] showed that inserting a phenyl moiety
between the donor and the bridge significantly alters the
charge-transport properties of the bridge by modifying the
p-conjugation pathway. In this particular case, hindering the
Scheme 1 shows the syntheses of 1a and 1b. Fluorene bor-
onic esters 10a[25] and 10b[25] were treated with bromoferro-
cene by using palladium-catalyzed Suzuki–Miyaura cross-
coupling conditions to give the intermediates 5a and 5b
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Control of Intramolecular Charge-Transport
FULL PAPER
The synthesis of the phenyl-
ene-linked systems 4a and 4b is
shown in Scheme 3. The fluo-
renyl units were functionalized
with phenyl linkers to give
14a,b before coupling with bro-
moferrocene to give precursors
8a,b. The structures of all the
new compounds were assigned
unambiguously based on their
1H and 13C NMR spectra and
mass spectrometry.
Solution electrochemical stud-
ies: The redox properties of the
D–B–A conjugates and the ref-
erences were studied by cyclic
voltammetry (CV). The meas-
urements were performed at
room temperature in a solvent
mixture of o-dichlorobenzene/
acetonitrile (4:1 v/v) with
Bu4NClO4 as the supporting
electrolyte. Table 1 summarizes
the relevant electrochemical
data. Figure 2 shows the CVs of
the monofluorene derivatives
1a–4a on both oxidative and
reductive scans.
Each of the D–B–A com-
pounds shows both oxidation
Figure 1. Structures of Fc–oFLn–C60 conjugates 1–4 and their references 5–9.
Scheme 2. Reagents and conditions: a) ethynylferrocene, Pd
PPh3, CuI, iPr2NH, 808C, 78 (6a); 23 (6b) 62 (7a), 67% (7b); b) vinylfer-
rocene, K2CO3, Bu4NBr, Pd(OAc)2 DMF, 958C, 62 (7a); 67% (7b);
c) C60, sarcosine, chlorobenzene, 1308C, 64 (2a); 56 (2b), 55 (3a), 48%
(3b).
ACHTUNGTRENNUNG(OAc)2,
Scheme 1. Reagents and conditions: a) bromoferrocene, PdACTHNUTRGNEUNG(OAc)2,
K2CO3, EtOH/H2O, 20 8C, 49 (5a); 29% (5b); b) C60, sarcosine, chloro-
benzene, 1308C, 52 (1a); 67% (1b).
AHCTUNGTRENNUNG
and, hence, compounds 1a and 1b. Scheme 2 shows the syn-
thesis of the analogous ethynyl-linked compounds 2a and
2b through the Sonogashira coupling of bromofluorene de-
rivatives 11a and 11b to yield the intermediates 6a and 6b.
The vinyl-linked compounds 3a and 3b were synthesized
similarly starting from 11a and 11b by using Heck condi-
tions to give the intermediates 7a and 7b.
and reduction waves in the CV (Figure 2). The bifluorene
analogues 1b–4b all behave in the same way. Oxidative
scans (Figure 2a) show quasi-reversible first oxidation po-
tentials that can be assigned to the one-electron oxidation of
Fc. Scanning further to 2000 mV reveals an additional irre-
versible process ascribed to fluorene oxidation in the region
between 1500 and 2000 mV. Some trends are apparent
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M. R. Bryce, D. M. Guldi et al.
Incorporating a vinyl spacer be-
tween the exTTF unit and the
FL bridge decreased the elec-
tronic repulsion between the
two electron-rich systems, al-
lowing the bridge to adopt a
more planar structure and, thus,
providing a continuous p-elec-
tron pathway from the donor
across the vinylfluorene bridge
to the C60 acceptor.
Conversely, compounds 2a
and 2b, which have an alkynyl
spacer, show an increase in oxi-
dation potential of 117 and
111 mV, respectively, compared
Scheme 3. Reagents and conditions: a) trimethyl[4-(4,4,5,5,-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl], Pd-
(PPh3)4, K2CO3, 1,4-dioxane, 858C, 84 % (12a); b) (4-bromophenyl)trimethylsilane, Pd(PPh3)4, K2CO3, toluene,
1108C, 87% (12b); c) ICl, CH2Cl2, 08C to 208C, 90 (13a); 96% (13b); d) B2pin2, KOAc, Pd(OAc)2, DMF,
908C, 60 (14a); 66% (14b); e) bromoferrocene, Pd(OAc)2, K2CO3, EtOH/H2O, 20 8C, 37 (8a); 28% (8b);
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
f) C60, sarcosine, chlorobenzene, 1308C, 68 (4a); 56% (4b).
with both Fc and all the other
compounds, implying signifi-
cantly reduced donor abilities
Table 1. Electrochemical data.[a]
of 2a and 2b, which can be ascribed to the elec-
tron-accepting alkyne unit. This is a similar shift to
that observed when an alkyne group is linked to the
electron-donating TTF.[8c]
2 [b]
E
ox
Compound
E11=2, red
E12=2, red
E13=2, red
E14=2, red
E11=2, ox
C60
9
ꢀ803
ꢀ920
ꢀ1207
ꢀ1673
2138
–
–
–
–
–
ꢀ1322
ꢀ1873
1a (Fc–FL–C60)
ꢀ915
ꢀ1335
ꢀ1886
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
209
203
314
308
178
173
233
218
197
227
219
320
315
189
181
230
216
1550
1556
1456
1462
1466
1462
1641
1585
–
In the reduction scans, each of the D–B–A com-
pounds shows three consecutive reversible one-elec-
tron reduction waves that are assigned clearly to
the fulleropyrrolidine moiety by comparison with
C60 and 9. Their potentials are very similar to those
of 9, indicating that electronic interactions in the
ground state are small. Predictably, these reduction
waves are absent in the formyl precursor com-
pounds (Table 1), for which a broad irreversible re-
duction wave arising from the substituted fluorenyl
moiety is observed at approximately ꢀ2.0 V.
1b (Fc–FL2–C60)
2a (Fc-yne–FL–C60)
2b (Fc-yne–FL2–C60)
3a (Fc-C=C–FL-C60)
3b (Fc-C=C–FL2-C60)
4a (Fc-Ph–FL–C60)
4b (Fc-Ph-FL2-C60)
Fc
5a (Fc–FL–CHO)
5b (Fc–FL2–CHO)
6a (Fc-yne–FL–CHO)
6b (Fc-yne–FL2–CHO)
7a (Fc-C=C–FL–CHO)
7b (Fc-C=C-FL2–CHO)
8a (Fc-Ph–FL–CHO)
8b (Fc-Ph–FL2–CHO)
ꢀ918
ꢀ1335
ꢀ1887
ꢀ909
ꢀ1325
ꢀ1881
ꢀ885
ꢀ1297
ꢀ1844
ꢀ913
ꢀ1330
ꢀ1879
ꢀ910
ꢀ1330
ꢀ1878
ꢀ915
ꢀ1333
ꢀ1888
ꢀ918
ꢀ1332
ꢀ1887
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
ꢀ2101[c]
ꢀ2078[c]
ꢀ1960[c]
ꢀ1950[c]
ꢀ2010[c]
ꢀ2011[c]
ꢀ2072[c]
ꢀ2071[c]
–
–
–
–
–
–
–
–
Molecular modeling: Geometry optimizations of 1–
4 (with methyl groups replacing hexyl) were per-
formed at the density functional theory (DFT) level
by using the unrestricted Hartree–Fock method
with the M062X hybrid meta functional[26] and the
LANL2DZ[27] valence basis set with Stuttgart/Dres-
den effective core potentials[28] for iron combined
–
[a] Potentials in mV (half-wave potentials: E1/2); scan rate 100 mVsꢀ1; glassy carbon
working electrode; Ag/AgNO3 reference electrode; Pt-wire counter electrode; 0.1m
Bu4NClO4 in o-DCB/CH3CN (4:1 v/v); 208C. [b] Broad, irreversible oxidation wave.
[c] Broad, irreversible reduction wave.
within the series. For compounds, in which Fc is either
linked directly to FL or through a phenylene spacer (1a, 1b,
4a, and 4b), the first oxidation step shows small cathodic
shifts of 12, 6, 36, and 21 mV, respectively, relative to Fc.
This slight increase in the oxidation potentials is caused by
the permanent dipole moments of the molecules, which
makes the molecular ensemble more difficult to oxidize.
Compounds 3a and 3b, which have a vinyl spacer, show a
small decrease in oxidation potential by 9 and 14 mV com-
pared with Fc. This suggests that the bridge in 3a and 3b is
able to donate electrons in addition to the Fc. Hence, the
communication between the donor and acceptor in these
systems is enhanced by the bridge. This result agrees with
the work of Wielopolski et al., in which an exTTF donor
was linked to C60 by using an oligo-vinylfluorene bridge.[22]
with 6-31G*[29] for carbon, nitrogen, and hydrogen. All com-
putations were performed with the Gaussian 09 suite of pro-
grams.[30]
The optimized geometries are shown in Figure 3 including
the dihedral angles between oFL planes among and between
oFL and the cyclopentadienyl rings of the Fc moieties. Com-
paring the structures shows that the dihedral angle between
the two FLs is (40ꢁ1)8 in all bifluorene-containing conju-
gates (1b, 2b, 3b, and 4b). This conformation represents a
compromise between the steric hindrance of the ortho-hy-
drogen atoms and the maximum overlap of p-orbitals.
Figure 3 shows that the linkage between Fc and the oFLs is
responsible for the largest structural variations within the
four different systems. This is particularly evident in the
monofluorene-based conjugates (1a, 2a, 3a, and 4a). This
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Control of Intramolecular Charge-Transport
FULL PAPER
bridge and shows significant overlap with the Fc units. As
expected, the cyclopentadienyl p-system provides the elec-
tronic communication between Fc and the oFL bridges. In
this regard, the overlap between the p-orbitals of the two
moieties will be crucial for charge injection into the oFL
spacer. Furthermore, the p-orbitals of the oFLs strongly
contribute to the HOMOs. This results from the close
energy-matching between the Fcs and the oFLs and allows
strong electronic-coupling. Hence, charge transfer may
easily occur upon exciting either the oFLs or the Fcs. In-
spection of the lower-energy molecular orbitals indicates
two additional aspects. Firstly, the lower MOs show signifi-
cant contributions from C60, which could result in charge
transfer. However, there is a significant energy gap between
HOMO and HOMO-1 in all systems (Table 2). On this
basis, we rule out the occurrence of charge transfer in solu-
tion, in which solvent stabilization effects will lead to a fur-
ther increase of the energy gap. Moreover, HOMO-2 and
HOMO-4 nicely corroborate the effects of the elongation of
the oFLs. As seen from the distribution of the electron den-
sities, upon elongation the HOMO-2 and HOMO-4 show in-
creased contributions to the electronic coupling between the
Fcs and C60s.
Photophysics: The electronic interactions in the excited
state were studied by means of photophysical methods. A
comparison of the ground state absorption spectra of the
references (5a–8a and 9) and the electron-donor–acceptor
conjugates (1–4) in toluene and THF revealed absorptions
of the individual building blocks (Figure 4). No spectroscop-
ic evidence for any aggregation was noted for the investigat-
ed references/conjugates in the concentration range of the
photophyscial experiments, that is, 10ꢀ6 m. In the UV spec-
tra, C60 transitions are discernable at 280 and 329 nm. Fc ab-
sorptions (338 nm) are masked by the broad features of the
oFLs. References 5a–8a show only slight dependence on
the linkage between Fc and oFL, except for 7a. Apparently,
the vinylene unit in 7a contributes for the most part to the
extension of the p-system of the oFLs. This situation
changes when going to 1–4. For example, upon increasing
the bridge length, the maxima shift in THF from 328 nm for
1a to 350 nm for 3b. At 450 nm, the Fc absorption is dis-
cernable in the form of a broad shoulder. Notably, a com-
parison between 1a,b and 4a,b reveals the same positions of
the absorption maxima. This is in line with the molecular
modeling studies, in which the phenyl units and the adjacent
FLs confine a dihedral angle of 31.7 and 35.28 for 4a and
4b, respectively. Hence, this out-of-plane torsion hampers
the p-conjugation and, in turn, the extension of the p-
system. On the other hand, the insertion of an ethyne
linker, which promotes a more planar configuration between
the Fc cyclopentadienyl and the adjacent oFLs, leads to a
slight extension of the p-conjugation. As seen for references
5a–8a, the most significant extension arises from the vinyl-
ene linkage in 3.
Figure 2. CVs of monofluorene derivatives 1a–4a in o-DCB/CH3CN (4:1
v/v): a) oxidative scans between 0 and +700 mV; b) reductive scans be-
tween 0 and ꢀ2400 mV (for conditions see Table 1).
must affect the overlap between the p-orbitals of Fc and
oFL and, as such, impact the overall p-conjugated pathway
between Fc, oFL, and C60. Importantly, the insertion of an
ethynyl or vinylene spacer, as in 2a and 3a, reduces the
electrostatic repulsion between the cyclopentadienyl and the
oFLs and favors planarity between the adjacent FLs and Fc.
On the contrary, the phenyl spacer in 4a induces a torsion
of more than 308 between the cyclopentadienyl ring and the
adjacent FL. In the difluorene systems (1b, 2b, 3b, and 4b)
the electrostatic repulsion between the FLs dominates the
configuration. Nevertheless, an almost planar structure is
found for 2b due to the ethyne linker.
The HOMO and LUMO energies are summarized in
Table 2. The LUMO energies are constant within a tenth of
a volt and the HOMO energies only vary in a narrow range
(from ꢀ6.4 to ꢀ6.7 eV). These results are consistent with
little mutual influence of the donor and acceptor moieties
through the bridge. In particular, the LUMO energies are
unperturbed compared with C60.
Examining the orbitals (Figure S1, the Supporting Infor-
mation) reveals the donor–acceptor chemical architecture of
the systems. The LUMO is always located on C60, whereas
the HOMO is homogenously distributed over the entire
The fluorescence assays were performed at 375 nm excita-
tion. This wavelength was chosen to excite mainly the oFL
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M. R. Bryce, D. M. Guldi et al.
unity. Importantly, the oFL flu-
orescence is quenched in THF
by a factor of 10 when compar-
ing oFLs in the absence (5a–
8a) and in the presence of C60
(1–4; Figure S4 and Table S2,
the Supporting Information),
which points to augmented oFL
excited-state deactivations in
the latter cases. Here, the
energy transfer from singlet ex-
cited oFLs to ground state C60
is operative (see below). The
C60 fluorescence is located in
the 680 to 800 nm region. In-
specting the C60 fluorescence
features in the electron-donor–
acceptor conjugates 1–4 in rela-
tion to 9 provides the first in-
sights into the excited-state de-
activation (lower part of
Figure 5). In 1–4, the C60 fluo-
rescence
is
considerably
quenched. At first glance, the
quenching depends on the dis-
tance between the donors and
the acceptors. Furthermore, the
dependence on the p-conjuga-
tion length of the bridge resem-
bles the trends of the absorp-
tion spectra. In other words, in-
creasing the length of the
bridge without changing the p-
conjugation leads to weaker
quenching of the C60 fluores-
Figure 3. M062X/(6-31G*+LANL2DZ) geometries with the corresponding dihedral angles depicted and stated
below in parenthesis. In 1a, 2a, and 3a, the angles are determined between the Fc cyclopentadienyl plane and
the FL plane (34.5, 18.2, and 20.28, respectively) and in 4a between the Fc cyclopentadienyl plane and the
phenyl ring (31.78) and the phenyl ring and the FL plane (39.68). In 1b, 2b, and 3b, the angles are determined
between the Fc cyclopentadienyl plane and the adjacent FL plane (35.2, 9.4, and 37.88, respectively) and be-
tween the two FL planes (40.3, 39.2, and 39.18, respectively). In 4b, the angles are determined between the Fc
cyclopentadienyl plane and the phenyl ring (35.28), the phenyl ring and the adjacent FL plane (38.28) and be-
tween two FL planes (38.88).
Table 2. M062X/(6-31G*+LANL2DZ) calculated energy levels [eV] of the frontier
cence. A closer look reveals that the quenching de-
pends also on the solvent polarity, that is, increasing
the polarity from toluene to THF and to benzoni-
trile increases the fluorescence quenching.
Complementary fluorescence measurements, in
which 1–4 were excited in the range of dominant
C60 absorption, that is, at 550 nm, led to similar
trends with quantum yields that ranged in THF
from 1.14ꢆ10ꢀ5 (1a) to 3.75ꢆ10ꢀ4 (2b; Figure S2
and Table S1, the Supporting Information). Since
orbitals for 1–4.
Orbital
1a
1b
2a
2b
3a
3b
4a
4b
LUMO
HOMO
HOMO-1
HOMO-2
HOMO-3
HOMO-4
ꢀ2.6
ꢀ6.7
ꢀ6.8
ꢀ7.0
ꢀ7.0
ꢀ7.1
ꢀ2.6
ꢀ6.5
ꢀ6.8
ꢀ7.0
ꢀ7.0
ꢀ7.1
ꢀ2.6
ꢀ6.6
ꢀ6.9
ꢀ7.0
ꢀ7.1
ꢀ7.1
ꢀ2.6
ꢀ6.5
ꢀ6.9
ꢀ7.0
ꢀ7.1
ꢀ7.1
ꢀ2.6
ꢀ6.5
ꢀ6.9
ꢀ7.0
ꢀ7.0
ꢀ7.1
ꢀ2.6
ꢀ6.4
ꢀ6.8
ꢀ7.0
ꢀ7.0
ꢀ7.1
ꢀ2.6
ꢀ6.7
ꢀ6.8
ꢀ7.0
ꢀ7.0
ꢀ7.1
ꢀ2.6
ꢀ6.5
ꢀ6.8
ꢀ7.0
ꢀ7.0
ꢀ7.1
units. The fluorescence spectra are divided into two regions:
the region between 410 and 600 nm and the region between
680 and 800 nm. As seen in the fluorescence spectra record-
ed with references 5a–8a, the oFLs emit strongly in the 410
to 600 nm region (the upper part of Figure 5). In terms of
fluorescence maxima the trend resembles that seen in ab-
sorption spectra, with the maxima shifting from 445 for 5 to
456 nm for 7. As seen from Figure S4 (in the Supporting In-
formation), the oFL fluorescence is strongly quenched in
the presence of C60 and Fc, which points to a rapid excited-
state energy-transfer from the photoexcited oFLs to C60. No-
tably, the fluorescence quantum yields in oFLs are close to
solvents of higher polarity tend to assist in stabilizing radi-
cal-ion pairs more than solvents of lower polarity, and the
quenching increases with decreasing electron-donor–accept-
or separation, a rapid charge-transfer process as deactivation
of the singlet-excited state is most probable (see below).
The radical-ion-pair stabilization was corroborated by
means of calculations of their respective energies (Table 3).
In contrast, a deactivation by means of energy transfer to
yield a Fc-centered excited state or by accelerated intersys-
tem crossing is unlikely to play a dominant role. Both of the
latter pathways should be solvent-polarity independent. The
quantum yields for all systems in toluene, THF, and benzo-
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Control of Intramolecular Charge-Transport
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Figure 4. Upper part: absorption spectra of references 5a and 8a. Central
part: absorption spectra of electron-donor–acceptor conjugates 1a, 1b,
2a, and 2b in THF at room temperature. Lower part: absorption spectra
of electron-donor–acceptor conjugates 3a, 3b, 4a, and 4b in THF at
room temperature.
Figure 5. Upper part: fluorescence spectra of references 5a and 8a in
THF, with matching absorption of 0.05 at the 375 nm excitation wave-
length. Central part: fluorescence spectra of reference 9 and conjugates
1a, 1b, 2a, 2b in THF, with matching absorption of 0.05 at the 375 nm
excitation wavelength. Lower part: fluorescence spectra of reference 9
and conjugates 3a, 3b, 4a, 4b in THF, with matching absorption of 0.05
at the 375 nm excitation wavelength.
nitrile are listed in Table 3. In corroboration of the oFLs to
C60 energy-transfer postulate, we recorded excitation spectra
that feature contributions from oFLs and C60 (Figure S3, the
Supporting Information).
Conclusive evidence for a successful charge-transfer reac-
tion comes from transient absorption measurements. An ex-
citation wavelength of 387 nm was chosen to excite the
oFLs in the references (5a–8a) and in the electron-donor–
acceptor conjugates (1–4). In all of the references (5a–8a)
the formation of the oFL singlet excited states is seen fea-
turing broad maxima in the 500 to 750 nm and in the 800 to
1100 nm regions. An example for 8a in THF is shown in
Figure 6. A closer look at the oFL singlet-excited-state for-
mation suggests that a short-lived intermediate (a higher-
lying singlet-excited-state) is involved. In particular, maxima
at 490 nm for 6a, at 560 nm for 7a, and at 530 nm for 8a
evolve and reflect the trend in p-conjugation. These higher-
lying singlet excited features transform on a timescale of
less than 1 ps into the corresponding oFL singlet signatures.
The maxima redshift in accordance with the trends estab-
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M. R. Bryce, D. M. Guldi et al.
Table 3. Photophysical data of Fc–oFL–C60 electron-donor–acceptor con-
jugates in solution at room temperature.[a]
lished in the absorption spectra, for example, from 560 for
5a to 660 nm for 7a. The oFL singlet excited-state features
decay, due the presence of the deactivating ferrocene, quick-
ly within tens of picoseconds (Table 3). As far as the identity
of the products is concerned, please note the lack of spectro-
C60ꢀFFL with lexc =375 nm
toluene THF PhCN
C60ꢀtsinglet [ps]
tCR [ps]
THF PhCN
THF
1a 1.2ꢆ10ꢀ5 1.9ꢆ10ꢀ6 9.1ꢆ10ꢀ7 <1
PhCN
<1
30
46
28
485
154
34
572
134
1b 1.6ꢆ10ꢀ4 3.6ꢆ10ꢀ5 2.0ꢆ10ꢀ5
2a 1.6ꢆ10ꢀ4 4.3ꢆ10ꢀ5 1.5ꢆ10ꢀ5
2b 2.9ꢆ10ꢀ4 2.0ꢆ10ꢀ4 1.0ꢆ10ꢀ4
3a 1.5ꢆ10ꢀ5 5.8ꢆ10ꢀ6 8.8ꢆ10ꢀ7
3b 9.9ꢆ10ꢀ5 1.6ꢆ10ꢀ5 9.8ꢆ10ꢀ6
4a 1.3ꢆ10ꢀ4 3.3ꢆ10ꢀ5 1.5ꢆ10ꢀ5
4b 1.9ꢆ10ꢀ4 8.7ꢆ10ꢀ5 6.3ꢆ10ꢀ5
34
42
31
scopic fingerprints for either the one-electron oxidized fer-
+
C
Cꢀ
rocene (Fc ) or the one-electron reduced oFLs (oFLs ). In
light of the aforementioned, we conclude that either an
energy transfer to an energetically lower-lying and spectro-
scopically weakly absorbing Fc triplet excited-state or, alter-
natively, a simple heavy atom affect (see below) dictates the
oFL singlet excited-state deactivation.
38
1382 2114
2.1
21
9.5
<1
31
27
728
164
33
823
189
31
[b]
[b]
–
–
2077 2278
oFLꢀFFL with lexc =350 nm
1a
3a
5a
6a
6b
7a
7b
8a
0.0025
0.0022
0.013
The situation clearly changes when inspecting the elec-
tron-donor–acceptor conjugates 1–4 upon 387 nm photoexci-
tation. Immediately upon excitation two bands arise in the
differential absorption spectra. In particular, one band
covers the 400 to 500 nm range, whereas another band dom-
inates the 800 to 1100 nm range. Notably, the transient re-
sembles the characteristics of the C60 singlet excited-state. In
principle, two different scenarios might be anticipated
(Scheme 4). Firstly, direct excitation into the oFLs to yield
their singlet excited-states followed by a rapid transduction
of the singlet-excited-state energy to the lower-lying C60 sin-
glet excited state. The actual energy-transfer occurs on a
faster timescale than the resolution of our experimental set-
up. This further confirms the stronger oFL fluorescence
quenching when C60 is present. Secondly, laser excitation di-
rectly powers the C60 singlet excited-state population. In
fact, the light participation of 1–4 at the 387 nm excitation
wavelength suggests relative distributions of 1.2 to 1.0 be-
tween the first and second scenario. The deactivation of the
C60 singlet excited-states occurs within a time frame of up to
30 ps. Such a singlet deactivation is in line with the distinct
quenching seen in the fluorescence experiments (upper part
of Figure 5). Table 3 lists all of the singlet-excited-state life-
times.
3.4ꢆ10ꢀ4
0.042
0.038
0.13
0.0097
[a] FFL: C60 and oFL fluorescence quantum yields; tsinglet: C60 singlet-ex-
cited-state lifetimes in picoseconds as determined by femtosecond transi-
ent absorption studies; tCR: C60 /Fc radical-ion-pair-state lifetimes as
determined by femtosecond transient absorption studies. [b] Cannot be
fit.
+
Cꢀ
C
The singlet-excited-state deactivation is kinetically and
spectroscopically linked to another process. In all of the
electron-donor–acceptor conjugates, the formation of the
Cꢀ
one-electron reduced C60 (C60 ) with its spectral signature
around 1010 nm evolves in the near-infrared part of the
spectrum (Figure 7, the Supporting Information, S5–S11). In
parallel, the visible part of the spectrum gives rise to the de-
velopment of maxima at around 500 and 680 nm. The
former resembles an additional feature of the one-electron
reduced C60, whereas the latter is probably a mixture of Fc+
and oFL+ according to the HOMO distribution (the Sup-
porting Information, Figure S1). As such, we infer that the
+
Cꢀ
C
charge-transfer yields C60 /Fc radical-ion-pair states in 1–
4. Further support for this notion was obtained from a ther-
modynamic consideration of the underlying driving forces to
+
Cꢀ
C
form C60 /Fc
(1.15ꢁ0.07 eV). Please note the sound
agreement between the values listed in Table 3 and the dif-
ferent C60 singlet-excited-state lifetimes, on one hand, and
the C60 fluorescence quantum yields, on the other hand.
In principle two competing deactivation processes, that is,
energy transfer to Fc or accelerated intersystem-crossing
Figure 6. Upper part: differential absorption spectra (visible and near-in-
frared) obtained upon femtosecond flash photolysis (387 nm, 100 nJ) of
8a in argon-saturated THF with time delays between 0.9 and 12.6 ps at
room temperature. Lower part: absorption–time profile at 515 nm moni-
toring the excited-state decay.
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Control of Intramolecular Charge-Transport
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Scheme 4. Various deactivation patterns for the singlet-excited-state deactivation in oFL (top, left hand side), oFL–Fc (center), and C60–oFL–Fc (lower,
right hand side). Solid arrows indicate initial processes, dashed arrows indicate subsequent processes, thick lines indicate participating states, and thin
lines indicate non-participating states.
due to iron, are energetically feasible.[31,32] With respect to
the earlier pathway, only formation of the Fc triplet excited-
state at as low as 1.12 eV is within reach, since the Fc singlet
excited-state lies unreachable at around 2.5 eV.[33] Under
similar experimental conditions, very weak absorption
changes in, for example, the 500 to 760 nm range[34] and a
lifetime of 600 ps were noted for the Fc triplet excited state
in a Fc reference.[35] Of importance is the spectral mismatch
between what is illustrated in Figure 7 and the Fc triplet ex-
cited-state. As far as the latter pathway is concerned, the C60
triplet excited-state (1.5 eV) in the form of the 690 nm fea-
ture should be seen. However, the lack of it leads us to rule
out any meaningful contribution from this deactivation
channel. The above experiments, considered in concert, indi-
cate that these deactivation pathways are at best minor con-
tributions in the overall deactivation of the C60 singlet excit-
donor–acceptor distance is responsible for this trend, where-
as in 2 the acceleration originates from the improved over-
lap of the p-orbitals due to the triple bond and largest b val-
ues. In the bifluorenyl structures 1b, 2b, 3b, and 4b, the dif-
ferences in the charge-separation dynamics are modulated,
owing to the fact that the distance between donor and ac-
ceptor governs the charge transfer.
The lifetimes of the radical-ion-pair states are relatively
short with values of up to 2.3 ns in 4b. As shown, not only a
significant distance dependence, but also the geometrical
properties, govern the charge recombination. Hence, the
long charge-separation lifetime in 4b is due to the longest
electron-donor–acceptor distance and the larger dihedral
angles, which block the charge recombination. Furthermore,
the distance dependence becomes crucial when going from
only one FL to two FLs: more than a ten-fold increase of
the radical-ion-pair lifetime is observed in all the conjugates
1–4. In accordance with the geometries and orbital represen-
tations from the molecular modeling, the second FL unit
adopts a dihedral angle of approximately 408 relative to the
first FL. Hence, the bridge between C60 and Fc loses its pla-
narity leading to a deceleration of the charge recombination.
Furthermore, the HOMO-2 and HOMO-4 (Supporting In-
formation, Figure S1) become involved in the charge-trans-
fer process, which underlines the augmented role of the
ed-state.
+
Cꢀ
C
From the first-order fitting of the C60 and Fc radical-
ion-pair state formation kinetics, the corresponding charge-
separation rate constants were derived (Table 3). Important-
ly, the charge-separation dynamics depend not only on the
overall electron-donor–acceptor distance, but also on the re-
spective linkages that connect Fc and the oFL part of the
bridge. For example, compounds 1 and 2 show accelerated
dynamics of charge separation. In 1, the short electron-
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M. R. Bryce, D. M. Guldi et al.
Figure 8. Representation of the charge-separation (lnkCS) and charge-re-
combination (lnkCR) rate constants on the center-to-center distances
(RCC) in nitrogen-saturated THF at room temperature for 4. The slopes
represent the different b values for charge separation (black) and charge
recombination (gray).
Table 4. The b values [ꢀꢀ1] for charge separation (CS) and charge re-
combination (CR) as resulted from the plots of the charge-separation
(lnkCS) and charge-recombination (lnkCR) rate constants on the center-
to-center distances (RCC) in nitrogen-saturated THF (Figure 8).
bCS
bCR
1a,b
2a,b
3a,b
4a,b
0.10
0.19
0.08
0.11
0.35
0.36
0.34
0.36
Figure 7. Upper part: differential absorption spectra (visible and near-in-
frared) obtained upon femtosecond flash photolysis (387 nm, 100 nJ) of
4a in argon-saturated THF with time delays between 16.0 and 700 ps at
room temperature. Lower part: absorption–time profiles at 495 and
1010 nm monitoring the charge separation and charge recombination.
depends strongly on the donor–acceptor distance, but not at
all on the linkage. For all systems a value of bCR (0.35ꢁ
0.01) ꢀꢀ1 was established. As aforementioned, the second
FL slows down the charge recombination by a factor of ten
due to the 408 dihedral angle between the two FLs. This
breaks the conjugation between the donor and acceptor and
leads to an energetic barrier. The difference in b values be-
tween charge separation and charge recombination results
from the fact that both occur through different sets of orbi-
tals.
bridge upon increasing the length. In the mono-FL systems
1a–4a, the linkage between Fc and FL impacts the charge
recombination due to the imposed structural features that
are seen to dominate the charge separation, with 1a and 3a
showing an accelerated charge-recombination.
Summarizing the charge separation and charge recombi-
nation rates in THF and analyzing their dependence on the
electron-donor–acceptor separation established the attenua-
tion factors. Figure 8 exemplifies the dependence of the
charge-transfer rates on the electron-donor–acceptor distan-
ces for 4. The different b values listed in Table 4 were deter-
mined from the slopes of the linear fits for all the systems.
With the exception of 2, the established b values for charge
separation are in the range of those formerly established for
oFLs,[23] namely 0.09 ꢀꢀ1. Importantly, whereas the charge
separation seems to occur rather independently of the dis-
tance, the linker between Fc and oFL significantly impacts
the charge-separation rates. In particular, the above men-
tioned structural and electronic features with regard to the
extension of the p-system and the orbital overlap between
Fc and oFL are reflected in the different b values for charge
separation. In fact, 3 exhibits the lowest value for b and 2
the highest value. Interestingly, the charge recombination
Conclusion
A series of new Fc–linker–oFLn-C60 (n=1, 2) conjugates, in
which oFL bridges are bound through various linkers to
electron-donating ferrocene and electron-accepting C60, has
been synthesized and characterized. In these systems the
linker is either a single bond, ethyne, vinylene or a phenyl-
ene unit. Summarizing, it has been shown that structural al-
terations of these linkers readily change the dihedral angles
between the Fcs and the oFLs and, hence, impact the orbital
overlap. In this way, it was possible to tune, with precision,
the intramolecular charge-transfer characteristics by varia-
tions of the linkers without interfering with the electronic
communication between the two electroactive units, namely
Fc and C60. In fact, charge injection into the bridge is con-
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Control of Intramolecular Charge-Transport
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trolled by structural arrangements imposed by the linkages,
which can be viewed as a bottleneck to this process.
Acknowledgements
This work was supported by the EC FP7 ITN “FUNMOLS” project no.
212942. M.W. and J.E.M. acknowledge financial support by the NCCR
MUST, a research instrument of the Swiss National Science Foundation.
Photoexcitation of either Fc or oFL moieties in THF and
benzonitrile results in intramolecular charge-transfer over
distances beyond 26 ꢀ from the electron-donating Fcs to the
electron-accepting C60. Competing processes such as energy
transfer to Fc or accelerated intersystem-crossing play no
significant role. The photophysical and quantum chemical
characterization of these Fc–linker–oFLn-C60 systems quali-
tatively and quantitatively corroborates the influence of the
structural parameters on the intramolecular charge-transfer
processes. Correlating the dynamics of intramolecular
charge-separation/charge-recombination with the electron-
donor–acceptor distances resulted in different attenuation
factors for charge separation and charge recombination.
Specifically, intramolecular charge-separation depends
strongly on the linker and only weakly on the donor–accept-
or distance. Intramolecular charge-recombination, on the
other hand, depends strongly on the D–A distance and is in-
dependent of the linker. These findings imply that the intra-
molecular charge-separation and charge-recombination must
occur through different sets of orbitals. The experimental re-
sults have been corroborated by using molecular modeling
studies. This work establishes that oFLs are excellent bridg-
ing sub-units for fine-tuning intramolecular charge-trans-
port, not only in electron-donor–bridge–acceptor conjugates
but very likely also between photoexcited chromophores
and wide band-gap semiconductors in, for example, dye-sen-
sitized solar cells.[36,37] Still, future experiments need to con-
firm the applicability of the current results to oPV- and
oPE-based D–B–A systems.
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Experimental Section
UV/Vis spectroscopy: Lambda 2 UV/Vis spectrophotometer (Perkin–
Elmer); absorption maxima lmax given in nm.
Steady state fluorescence spectroscopy: Horiba Jobin Yvon Fluoromax 3
spectrophotometer; at room temperature (298 K) in a 1 to 1 cm quartz
cuvette. All spectra were corrected for the instrument response. For exci-
tation wavelength below 450 nm a cut off filter (435 nm) was inserted.
For the determination of fluorescence quantum yields NMPC60 and oFL2
were used as reference compounds regarding C60 and oFL emission, re-
spectively.
Fs-transient absorption spectroscopy: Femtosecond transient absorption
studies were performed with 387 nm laser pulses (1 kHz, 150 fs pulse
width) from an amplified Ti/sapphire laser system (Model CPA 2101,
Clark-MXR Inc.; output 775 nm). For an excitation wavelength of
420 nm, a nonlinear optical parametric converter (NOPA) was used to
generate ultra-short tunable visible pulses out of the pump pulses. The
transient absorption pump probe spectrometer (TAPPS) is referred to as
a two-beam setup, in which the pump pulse is used as excitation source
for transient species and the delay of the probe pulse is exactly controlled
by an optical delay rail. As the probe (white-light continuum), a small
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Received: November 13, 2012
Revised: March 7, 2013
Published online: && &&, 2013
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Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Control of Intramolecular Charge-Transport
FULL PAPER
Finely tuned transfer: Chemical modi-
fication of the oligo-fluorene linkers
between Fc and C60 units enables fine-
tuning of photoinduced charge-transfer
processes in new donor-bridge-
Charge Transfer
C. Schubert, M. Wielopolski,
L.-H. Mewes, G. de Miguel Rojas,
C. van der Pol, K. C. Moss,
M. R. Bryce,* J. E. Moser, T. Clark,
D. M. Guldi* .................. &&&& — &&&&
acceptor conjugates (see illustration).
Precise Control of Intramolecular
Charge-Transport: The Interplay of
Distance and Conformational Effects
Chem. Eur. J. 2013, 00, 0 – 0
ꢅ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!