Photoinduced Electron Transfer and Hole Migration in Nanosized Helical Aromatic Oligoamide Foldamers

Article abstract of DOI:10.1021/jacs.6b05668

A series of photoactive triads have been synthesized and investigated in order to elucidate photoinduced electron transfer and hole migration mechanism across nanosized, rigid helical foldamers. The triads are comprised of a central helical oligoamide foldamer bridge with 9, 14, 18, 19, or 34 8-amino-2-quinolinecarboxylic acid repeat units, and of two chromophores, an N-terminal oligo(para-phenylenevinylene) electron donor and a C-terminal perylene bis-imide electron acceptor. Time-resolved fluorescence and transient absorption spectroscopic studies showed that, following photoexcitation of the electron acceptor, fast electron transfer occurs initially from the oligoquinoline bridge to the acceptor chromophore on the picosecond time scale. The oligo(para-phenylenevinylene) electron donor is oxidized after a time delay during which the hole migrates across the foldamer from the acceptor to the donor. The charge separated state that is finally generated was found to be remarkably long-lived (>80 μs). While the initial charge injection rate is largely invariant for all foldamer lengths (ca. 60 ps), the subsequent hole transfer to the donor varies from 1 × 10⁹ s^-1 for the longest sequence to 17 × 10⁹ s^-1 for the shortest. In all cases, charge transfer is very fast considering the foldamer length. Detailed analysis of the process in different media and at varying temperatures is consistent with a hopping mechanism of hole transport through the foldamer helix, with individual hops occurring on the subpicosecond time scale (k_ET = 2.5 × 10¹² s^-1 in CH₂Cl₂). This work demonstrates the possibility of fast long-range hole transfer over 300 ? (through bonds) across a synthetic modular bridge, an achievement that had been previously observed principally with DNA structures.

Full text of DOI:10.1021/jacs.6b05668

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Article
Photoinduced electron transfer and hole migration
in nanosized helical aromatic oligoamide foldamers
Xuesong Li, Nagula Markandeya, Gediminas Jonusauskas, Nathan
D. McClenaghan, Victor Maurizot, Sergey A. Denisov, and Ivan Huc
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b05668 • Publication Date (Web): 21 Sep 2016
Downloaded from http://pubs.acs.org on September 21, 2016
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Photoinduced electron transfer and hole migration in nanosized hel-
ical aromatic oligoamide foldamers
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ǁ
ǁ
Xuesong Li,†, ,‡, Nagula Markandeya,†, ,‡Gediminas Jonusauskas,§ Nathan D. McClenaghan,¶ Vic-
tor Maurizot,‖Sergey A. Denisov,*,¶ Ivan Huc*,‖
† Univ. Bordeaux, CBMN (UMR 5248), Institut Européen de Chimie et Biologie, 2 rue Robert Escarpit, 33600 Pessac,
France
‖CNRS, CBMN (UMR 5248), Institut Européen de Chimie et Biologie, 2 rue Robert Escarpit, 33600 Pessac, France
§ Univ. Bordeaux, Laboratoire Ondes et Matières d'Aquitaine (UMR5798), 351 cours de la Libération 33405 Talence
cedex France
¶ Univ. Bordeaux, Institut des Sciences Moléculaires (UMR5255), 351 cours de la Libération, 33405 Talence cedex,
France
KEYWORDS: photoinduced electron transfer, hopping mechanism, long-range charge transfer, foldamer, charge separation.
ABSTRACT: A series of photoactive triads have been synthesized and investigated in order to elucidate photo-induced
electron transfer and hole migration mechanism across nanosized, rigid helical foldamers. The triads are comprised of a
central helical oligoamide foldamer bridge with 9, 14, 18, 19, or 34 8-amino-2-quinolinecarboxylic acid repeat units, and of
two chromophores, an N-terminal oligo(para-phenylenevinylene) electron donor and a C-terminal perylenebis-imide
electron acceptor. Time-resolved fluorescence and transient absorption spectroscopic studies showed that, following
photoexcitation of the electron acceptor, fast electron transfer occurs initially from the oligoquinoline bridge to the ac-
ceptor chromophore on the picosecond timescale. The oligo(para-phenylenevinylene) electron donor is oxidized after a
time delay during which the hole migrates across the foldamer from the acceptor to the donor. The charge separated state
that is finally generated was found to be remarkably long-lived (>80 µs). While the initial charge injection rate is largely
invariant for all foldamer lengths (ca. 60 ps), the subsequent hole transfer to the donor varies from 1×109 s–1 for the longest
sequence to 17×109 s–1 for the shortest. In all cases, charge transfer is very fast considering the foldamer length. Detailed
analysis of the process in different media and at varying temperatures is consistent with a hopping mechanism of hole
transport through the foldamer helix, with individual hops occurring on the subpicosecond timescale (kET = 2.5 × 1012 s–1 in
CH2Cl2). This work demonstrates the possibility of fast long range hole transfer over 300 Å (through bonds) across a syn-
thetic modular bridge, an achievement that had been previously observed principally with DNA structures.
Introduction
tron donor and electron acceptor units located at the
termini of, often rigid, oligomeric bridges. The oligomeric
bridges can be of various types, for example based on:
phenylenes,10–15 porphyrins,16 thiophenes17, fluorenone and
p-phenylethynylene18,19as well as organometallic molecu-
lar wires.12,17,20–22 These systems typically allow to assess
the effect of distance between donor and acceptor, as well
as the chemical composition of the bridge, on charge
separation and charge recombination rates.
Electron transport through nucleic acids and pro-
teins is central to a range of essential biomolecular pro-
cesses, including photosynthesis,1,2 enzyme catalysis3–5
and DNA repair.6–9It can proceed between chromophores
or other redox active chemical functions located at pre-
cise positions in space as a result of folding of the biopol-
ymer backbones. These chromophores or redox centers
may be in direct contact or, on the contrary, separated by
long (nanometric) distances, in which case electron
transport will greatly depend on the way charges may
transfer between functionalities. In order to elucidate
which parameters are critical to these biological func-
tions, and with the perspective to engineer related bioin-
spired artificial molecular devices, chemists have pro-
duced a variety of small and well-defined model molecu-
lar systems that are more amenable to study than bio-
macromolecules. For photo-induced charge transfer in-
vestigations, typical architectures are composed of elec-
Aside from rigid bridges, more flexible structures but
with an ability to adopt stable folded conformations, have
been used to control the relative positioning of donor and
acceptor units, and even of multiple chromophores. Ex-
amples include the use of peptides23–29 and nucleo-
tides23,30–41 and, in recent years, of non-natural backbones,
i.e. foldamers.42–46 Folding gives access to diverse and
well-defined relative orientations, the most common
being the cofacial orientation of stacked aromatic chro-
mophores. More specifically aromatic amide folda-
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mers47,48have emerged as a new class of tool for the fabri-
Page 2 of 13
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cation of well-defined, organic compatible, molecular and
supramolecular nanometric architectures with atomic
precision
including
sheets,49multi-stranded
heli-
ces,50,51multi-helical structures52,53 and host-guest receptor
systems54the size of which sometimes matches the size of
small proteins. The investigation of electron transport
within these objects thus bears relevance to their poten-
tial incorporation into devices, including sensors.
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Along this line, we previously reported the photo-
induced electron transfer between a perylenebis-imide
(PB) acceptor and an oligophenyene-vinylene (OPV)
donor separated by a foldamer bridge consisting of a heli-
cally-folded oligoamide of 8-amino-2-quinolinecarboxylic
acid (Q) comprised of 2, 4, 5 or 9 monomers (see molecu-
lar structures in Figure 1).55 These OPV-Qn-PB helical
oligomers have an exceptional conformational stability.
For example, even short sequences do not denaturate at
120°C in DMSO; helix handedness inversion occurs on the
time scale of hundreds of milliseconds for a tetramer,
hours for an octamer, and weeks for a hexadecamer.56,57
Taking advantage of this stability, the initial intention of
our earlier study was to evaluate the effect of varying both
the orientation and the distance between the donor and
acceptor in a controlled manner (i.e. at precise positions
with negligible helix dynamics when two turns are
reached, n ≥ 5) in order to decipher the respective effects
of these two parameters on charge transfer rates. Ulti-
mately, some dependence on both these parameters was
demonstrated. However, the main result of the study was
the serendipitous discovery that the quinoline-
carboxamide backbone facilitates charge transfer which
was shown to occur very rapidly (tens of picoseconds
timescale) and with a small dependence on distance (at-
tenuation factor was measured to be βCS ~0.05 Å-1). Charge
transport across these relatively short foldamer bridges
was thought to proceed via a superexchange mechanism,
i.e. by a direct electron coupling between the donor and
acceptor unitswavefunctions facilitated by the bridge.20,58
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Figure 1. Formulas of studied foldamers OPV-Qn-PB and Qn-
PB. Schematic spatial representations of the helical struc-
tures of OPV-Qn-PB with n = 9, 14, 19 and 34. The number of
helix turns in the cartoon corresponds to the actual number
expected in the molecular helix.The bottom right structure
shows a color coded energy minimized (Merck molecular
force field in Macromodel)model of OPV-Q34-PB. In this
model, alkyl and alkoxy chains have been removed or short-
ened for clarity.
As shown below, photo-induced electron transfer
was observed to occur very fast despite the length of the
structures. For these long oligomers, the temperature,
solvent and length dependence of the charge separation
rate are consistent with a hopping mechanism, consisting
of several steps, in one of which a hole migrates randomly
through the bridge, contrary to the initially proposed
superexchange mechanism. Moreover, we report the ob-
servation of very long-lived charge separated states
(>80 µs at 15 µM in CH2Cl2), a featureof relevance for pho-
tovoltaic applications.
The finding that charge transport is fast in the 18-
81 Å range (through bonds) naturally raised the question
of whether aromatic amide foldamers could mediate
charge transport over much longer distances and ulti-
mately be of use in photoactive molecular devices. An-
swering this question was for many years hampered by
the considerable challenge of synthesizing long foldamer
sequences equipped with complex functional groups. In
recent reports,59,60 we described optimized methods to
produce long quinoline carboxamide oligomers in high
yield, high purity and on multigram scales for oligomers
as long as a 32mer that paved the way to the work report-
ed here. We nowpresent the synthesis of oligoamide
foldamers bearing PB and OPV functional terminal func-
tions of unprecedented length. We investigated OPV-Qn-
PB with n = 9, 14, 18, 19, 34, corresponding to interchro-
mophore distances in the range of 14-51 Å through space
and 81-306 Å through bonds. In addition, Boc-Qn-PB dy-
ads were studied to gain insight into Qn bridge-PB inter-
actions.
Results
Molecular design. In this work, five different rigid
foldamer sequences OPV-Qn-PB with n = 9, 14, 18, 19, 34
with the same electron donor (OPV) and electron accep-
tor (PB) chromophores linked to the oligo-
quinolinecarboxamide foldamer termini were successfully
synthesized (Figure 1). The molecules were designed to
control both the distance between, and the relative orien-
tation of, donor and acceptor chromophores that are
expected to protrude from the helix in planes perpendicu-
lar to the helix axis. In all compounds except OPV-Q18-PB,
the number of quinolines in the bridge is such that OPV
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the availability of Q4, Q8, and Q16 that can be produced in
and PB adopt asimilarorientation in which the dihedral
gram quantities via a segment doubling approach60 with a
C-terminus protected as a methyl ester and an N-
terminus having a nitro group or a tert-butoxycarbonyl-
amino (Boc-amino) group as amine precursors. Key in-
termediates included a Q5 segment prepared by elonga-
tion of Q4; H2N-Q8-PB and H2N-Q17-PB; and an OPV-Q-
CO2H monocarboxylic acid (Figure 2) which was intro-
duced because of its reliable coupling at the amine termi-
nus of Qn segments, as opposed to the direct coupling of
the carboxylic acid of OPV which was less reproducible.
Thus, OPV-Q9-PB was readily obtained from H2N-Q8-PB.
This latter molecule could be elongated by five units to
H2N-Q13-PB, which gave access to OPV-Q14-PB. Similarly,
H2N-Q13-PB was again elongated by five units to give ac-
cess to H2N-Q18-PB and then OPV-Q19-PB. For the prepa-
ration of OPV-Q18-PB, Q16 was first elongated to produce
Q17 and then H2N-Q17-PB. Finally, Q16 was also converted
into OPV-Q17-CO2H which was coupled to H2N-Q17-PB to
yield OPV-Q34-PB. A new scheme to prepare OPV-Q4-PB
with the new OPV side chains is also reported but the
photophysical studies for this compound were not repeat-
ed (see Supporting Information). In addition, analogous
molecular dyad systems lacking the electron donor OPV
were prepared as well: Boc-Qn-PB where n = 1, 2, 4, 8, 17.
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angle (αDA) between the chromophores is kept at compa-
rable values(Figure 1 and Table 1). Indeed, solution and
solid state studies have demonstrated that Qn helices span
almost exactly two turns, i.e. a vertical rise of about 7 Å
along the helix axis, every five units.53,60Thus, OPV-Qn-PB
with n = 9, 14, 19, and 34 were expected to have chromo-
phores at a fixed orientation but at a distance that in-
creases by increments of 7 Å from n = 9 to n= 14 and n =
14 to n= 19, and of 21 Å between n = 19 and n = 34. In con-
trast, OPV-Q18-PB was expected to differ from OPV-Q19-
PB by a minimal change in interchromophore distance,
and by a significant change in the relative chromophore
orientation of about 360/2.5 = 144° (Figure S1). In order to
determine accurate distances through space (RDA) or
through bridge (Rbridge) and dihedral angles (αDA) between
the chromophores, the energy-minimized structures of all
oligomers were calculated(Figure S1, Table 1)
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Table 1. Distance through space (RDA) or through bridge
(Rbridge) and dihedral angle (αDA) between the OPV and PB
chromophores.
RDA (Å)a
Rbridge(Å)a,b
α
DA (°)c
OPV-Q9-PB
OPV-Q14-PB
OPV-Q18-PB
OPV-Q19-PB
OPV-Q34-PB
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171
-58.1
-66.6
-289.5
-77.7
306
-102.3
(a)
calculated from the first atom of PB to the first atom of
(b)
OPV beyond amide functions;
measured along the helix
molecular backbone as the sum of all bonds lengths through
the helix inner rim; (c) dihedral angle between OPV and PB
long axes.
Figure 2. Crystal structure of OPV-Q-CO2H. Included sol-
vent molecules and hydrogen atoms other than NH and OH
have been removed for clarity.
Synthesis. The synthesis of OPV-Qn-PB triads with
n = 9, 14, 18, 19, 34 is presented in detail in the supporting
information (schemes S1 to S4). The shortest sequence in
this series matches in length with the longest sequence
previously described but differs from it by its OPV
group.55 Indeed, we report the multigram scale synthesis
of a new OPV unit functionalized with isobutoxy solubil-
izing groups. Instead of isobutoxy groups, the previously
described OPV-Qn-PB oligomers incorporated a chiral
alkoxy side chain in their OPV units. The presence of the
stereogenic centers made NMR analysis of the triads more
complicated as the right-handed (P) and left-handed (M)
helical conformations were then diastereomeric and ap-
Spectroscopic
studies.Photoinduced
electron
transfer (PET), charge (electron-hole) separation (CS) and
charge recombination (CR) processes in OPV-Qn-PB sys-
tems in solution were studied using sub-pico and nano-
second transient absorption spectroscopy (TRABS) and
complementary time-resolved fluorescence spectroscopy
in the temperature range 180-298 K. Along with time-
resolved techniques, steady-state electronic absorption
and photoluminescence spectroscopy were used to obtain
further information (see Supporting Information). Spec-
troscopic studies were performed in chlorinated solvents
in which all compounds mentioned above are highly sol-
uble and for the purpose of comparing new results with
previous ones.55The solvents were thoroughly deoxygen-
ated by multiple freeze-pump-thaw cycles, since it was
found that, in the presence of oxygen, OPV photodamage
occurs for OPV-Qn-PB samples with n>9. Dichloro-
methane (CH2Cl2, spectroscopic grade, dielectric constant
ε = 8.9363) and chloroform (CHCl3, spectroscopic grade, ε
= 4.8163) were used to investigate the effect of the solvent
dielectric properties on photoinduced electron transfer
1
peared as distinct sets of anisochronous signals in the H
NMR spectra. In contrast, all P- and M-helical conformers
described here are enantiomers and their NMR spectro-
scopic features are identical. The PB unit was synthesized
as previously described61,62 with a free aromatic phenyl-
amine to be coupled to the C-terminus of Qn segments.
The synthetic schemes of these new OPV-Qn-PB
compounds differ substantially from those reported pre-
viously, and have been much improved. They make use of
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and charge separation. Some measurements recorded in lows estimation of the rate (kCT) and efficiency of electron
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toluene showed more complex behavior such as double
exponential PB fluorescence decays for longer variants. In
addition, charge separation from donor to acceptor is
disfavored in least polar solvents and does not occur in
toluene for bridges as short as OPV-Q19-PB. Studies in
toluene would thus not allow to compare the full range of
bridge lengths and were not pursued. Nevertheless, sever-
al experiments were carried out in toluene for compari-
son,to assess possible external heavy atom effects (EHE)
induced by chlorine atoms of the solvents.64 Our results
demonstrate no significant influence of EHE on intersys-
tem crossing in triads/dyads (vide infra).
transfer according to Eq. 1.
ꢄ
ꢀꢁꢂ = ꢀ − ꢀꢃ = ꢄ
−
Eq. 1
ꢅ
ꢅ
ꢆ
in which k is emission rate of the quenched fluoro-
phore in triad and dyad systems and k0corresponds to the
fluorescence rate of unquenched Boc-PB, τand τ0arethe
lifetime of fluorescence respectively. Table 2 lists the
fluorescence quantum yields and emission lifetime values
of the studied dyads and triads. For n ≥ 8, the triads and
dyads showed strong emission quenching which resulted
in shortening of emission lifetime down to 60 ps, corre-
sponding to electron transfer rates of 17 × 109 s–1. This
strong lifetime decrease correlates well with the extent of
fluorescence quenching (ca. 150–fold with respect to the
Boc-PB) observed in steady-state measurements of fluo-
rescence quantum yield in both CH2Cl2 and CHCl3 (Table
2, Figure S8). Shorter dyads (Boc-Qn-PB with n = 1, 2, 4)
showed lower PB fluorescence quenching rates, in the
range 2.7 × 109 to 8.2 × 109 s–1in CH2Cl2(τ = 118-335 ps, Ta-
ble 2, Figure S9-10).
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Electronic absorption spectroscopy. The absorp-
tion spectra of the individual components of the triads,
i.e. OPV, PB and Qn taken as separate entities, show
bands in distinct regions which allows to envisage chro-
mophore-specific excitation: 350–450 nm for OPV and
450–550 nm for PB, while Qn oligomers absorb in the 250–
400 nm range. The UV/vis absorption spectra of the stud-
ied triads (Figure S2) are qualitatively similar to the sum
of the spectra of the individual components (Figure S3)
and could be divided into the three regions ascribed to
Qn, OPV and PB. The OPV and PB absorption bands show
only small fluctuations in molar absorption coefficients
that are likely caused by changes in electronic structure
and environment. The structure and spectral position of
the vibronic bands of PB are similar for all triads and
dyads. At the concentration at which the experiments
were conducted (1.5×10–5 M for TRABS and ~10–6 M for
fluorescence), the absorption spectra display characteris-
tic vibronic progressions, indicating the absence of any
aggregation phenomena.65,66 Absorption bands of individ-
ual PB, OPV and quinoline units do not vary significantly
upon incorporation into triads or dyads. This indicates
the absence of strong conjugation between the chromo-
phores and adjacent quinoline units, and between quino-
line units themselves, in the ground state. Nevertheless,
the molar absorption coefficient of Qn does not increase
linearly with the number of bridge units, neither in Boc-
Qn-PB dyads nor in OPV-Qn-PB triads. Some hypo-
chromic effect is observed and the molar absorption per
quinoline unit decreases with length. Structural elements
of the bridge lying on the main axis of the triad appear to
screen each other causing reduction of absorption (Figure
S4-S5).
Excitation spectra recorded on the weak residual
emission from the quenched PB electron acceptor
(λ = 576 nm, Figure 3) revealed efficient electronic energy
transfer from the OPV-Qn chromophore to PB. Depend-
ing on the length of the Qn bridge, the time constant (τ)
of energy transfer ranges from subnanosecond to nano-
second, e.g.τ = 10 ps for OPV-Q9-PB (see Figure S12a) and
τ > 2000 ps for OPV-Q34-PB. Interchromophore electronic
energy transfer (EET) processes, which occur before elec-
tron transfer processes of interest if the OPV is excited,
will not be discussed further here. Indeed, selective exci-
tation of the PB, thereby effectively avoiding EET, was
performed throughout.
Table 2. Fluorescence quantum yields (Φfl) and emission
lifetimes (τ) of dyads and triads at 295 K in CH2Cl2 and
CHCl3.
Φfl in
Φfl in
τ, ps in
CH2Cl2
τ, ps in
CHCl3
Samples
CH2Cl2
CHCl3
Boc-Q1-PB
Boc-Q2-PB
Boc-Q4-PB
Boc-Q8-PB
Boc-Q17-PB
OPV-Q9-PB
OPV-Q14-PB
OPV-Q18-PB
OPV-Q19-PB
OPV-Q34-PB
Boc-PB
0.037
0.022
0.015
0.006
0.006
0.006
0.006
0.007
0.006
0.006
0.88
0.07
0.04
335
230
118
60
670
420
150
61
0.017
0.006
0.006
0.005
0.005
0.006
0.006
0.007
0.81
Photoluminescence spectroscopy. Earlier studies
of shorter OPV-Qn-PB triads showed that excitation of the
PB moiety resulted in a ca. 150 fold decrease of fluores-
cence. This was attributed to efficient quenching via an
electron transfer process towards the excited PB chromo-
phore.55Fluorescence quenching thus offers a method to
quantify electron transfer rates and efficiency in OPV-Qn-
PB triads and Boc-Qn-PB dyads. Measurements of the
emission quantum yield (Φfl) and changes in lumines-
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60
4000
3850
cence lifetime (
tive to the reference Boc-PB fluorophore (Φfl = 0.88,
= 4.00 ns in CH2Cl2; Φfl = 0.81, = 3.85 ns in CHCl3) al-
τ) in these multicomponent systems rela-
τ
τ
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Figure 3. Excitation (λobs = 576 nm) and emission
(λexc = 490 nm) spectra of OPV-Q19-PB in CH2Cl2.
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Transient absorption spectroscopy (TRABS). De-
tails of the sequence of processes following photoexcita-
tion of these multicomponent architectures, including
electron transfer, could be determined using TRABS stud-
ies. Measurements were performed simultaneously at a
number of wavelengths and at a certain time delay after
the excitation pulse (time-gated spectra). A collection of
spectra at different time delays constitutes the time-
resolved spectrum (Figure 4). A high dynamic range
streak-camera with broad time range detection (50 ps-
1 ms) was used to register transient signals, allowing stud-
ies of the evolution of transient signals with time con-
stants ranging from 60 ps to 100 µs. Additionally, ultrafast
TRABS experiments were performed (Figure S6) in order
to better resolve fast electron transfer processes.
Figure 4. TRABS maps of A) Boc-Q8-PB (λexc = 510 nm); B)
OPV-Q9-PB λexc = 510 nm); C) OPV-Q19-PB (λexc = 532 nm);
D) OPV-Q34-PB (λexc = 532 nm), at 298 K in CHCl3. Positive
absorption is shown in yellow and red, whereas ground state
bleaching (GSB) is shown in blue. Circles indicate the pres-
ence of Q•+ absorption band. In D), the vertical scale is ex-
panded to 10 ns to accommodate the slower hole transfer
through the long Q34 bridgeA better view with higher con-
trast of the quinoline-radical band is presented in Figure S18.
To determine the rate of overall charge separation in
triad systems, i.e. the electron transfer between PB and
OPV, we monitored the rate of OPV GSB (Figure S13),
that is the rate at which the HOMO of OPV is oxidized to
give OPV•+. OPV GSB kinetics were fitted to an exponen-
tial function and the corresponding time constant param-
eter was attributed to charge separation.Good fits were
obtained following this this time convention (see Figure
S13). Nevertheless, here, mono-exponential analysis is
purely phenomenological.83Similarly, time-resolved PB
fluorescence decay was fitted to an exponential function
to determine the rate of electron transfer from the Qn
bridge to PB that causes emission quenching.
TRABS maps of Boc-Q8-PB recorded at 298 K in
CHCl3 (Figure 4A) show that, after excitation of PB, posi-
tive absorption signatures corresponding to both PB
ꢇꢈ ← ꢇꢄ and PB anion radical (PB•–, centered at 700 nm)67
and to quinoline cation radical (Q•+, centered at 390 nm,
Figure S11a)68 appear simultaneously with a time constant
of ~60 ps. The same observation was madevia femto-
second TRABS measurements (Figure S11b). In contrast,
maps of OPV-Q9-PB (Figure 4B, Figure S12b) do not reveal
an absorption band which could be attributed to Q•+,
following excitation. Instead, OPV ground state bleaching
(GSB) is observed, concomitant with the appearance of a
PB•– band ( ≈ 60 ps) and of an OPV•+ band. However, for
τ
Table 3. Activation energy ∆G of initial charge injection
and overall charge separation.
longer Qn bridges, starting from OPV-Q14-PB, OPV GSB
and OPV•+ bands appearance clearly occurs with some
time delay with respect to the appearance of the PB•–
absorption band (Figure 4C, D). Moreover, for these long-
er bridges (n≥14), the presence of quinoline cation radical
Q•+ absorption band at 390 nm is present in TRABS maps
at early stages after excitation pulse (indicated by circles).
∆G(eV)
∆G(eV)
charge injection
charge separation
Samples
CH2Cl2
CHCl3
CH2Cl2
CHCl3
Boc-Q8-PB
0.105
0.098
0.105
0.095
0.100
0.088
–
–
–
OPV-Q9-PB
OPV-Q14-PB
OPV-Q18-PB
OPV-Q19-PB
OPV-Q34-PB
0.090
0.080
0.072
0.071
0.080
0.105
0.087
0.078
0.065
0.045
0.090
0.082
0.075
0.062
–
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ized by a slightly lower activation energy in CHCl3 than in
CH2Cl2 but the difference is minimal.
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Figure 5. Temperature dependence of charge separation rate
between OPV and PB in OPV-Qn-PB in CH2Cl2 and CHCl3,
calculated from the rate of OPV GSB appearance
(λexc = 532 nm). Charge separation does not occur for OPV-
Q34-PB in CHCl3 at temperatures lower than 273 K (see below
for explanation). Solid and dotted lines represent linear fits
of data recorded in CH2Cl2 and CHCl3, respectively.
Figure 6. Temperature dependence of the charge injection
rate from the quinoline bridge into excited PB low-lying
SOMO, calculated from emission rates (λexc = 532 nm,
λobs = 575 nm) of PB (see SI, Correction of rate of electron
injection transfer).
In order to elucidate the mechanism of electron
transfer, time-resolved fluorescence and TRABS were
conducted at different temperatures in the range of 180–
298 K in CH2Cl2 and 210-298 K in CHCl3 (see Supporting
Information). These experiments allowed the determina-
tion of electron transfer activation energies (Table 3, Fig-
ure 5-6) based on an Arrhenius-type dependence(Eq. 2),
where kCT– rate of charge transfer; ꢀG –electron transfer
activation energy; kB– Boltzmann constant; A– pre-
exponential factor; T– temperature.
Charge recombination. The charge separated state
(CSS) of the studied triads OPV•+-Qn-PB•– is formed in the
sub-nanosecond to nanosecond range. We observed that
charge recombination – i.e. the return to the ground state
– is much slower. The TRABS kinetics of the shortest triad
OPV-Q9-PB in CHCl3 shows that the absorption bands
corresponding to PB•– (centered at 720 nm) and OPV•+
(centered at 625 nm) relax with the same time constants,
τ
τ
CR = 150 ns (Figure S14). In CH2Cl2, a more polar solvent,
CR = 450 ns at a concentration of~ 15 µM. The disappear-
ꢀꢁꢂ = ꢉ ∙ exp ꢊꢋꢌꢐꢍꢑ
Eq. 2
ance of PB•– is accompanied by the formation of the PB
charge recombinedtriplet state, which is characterized by
intense absorption, centered at 502 nm (Figure 7), shifted
from 510 nm observed previously.69The triplet state only
appeared in this context. As shown in Figure S17a, the
absorption band of OPV•+ and PB•– for longer triads
OPV•+-Q14-PB•–, OPV•+-Q19-PB–, and also OPV•+-Q34-PB•–
are still observed more than 80 µs after the excitation
pulse. This demonstrates an extremely long CSS lifetime.
Charge recombination occurs in a similar manner in tolu-
ene, i.e.from CSS singlet to a charged recombined triplet
state. In the case of OPV-Q9-PB, charge recombination is
faster in toluene (time constant ~15 ns, Figure S14c) than
CH2Cl2 (time constant 450 ns, Figure S14b) and in chloro-
ꢎ
ꢏ
The temperature dependence of charge separation
and fluorescence quenching rates are presented in Figures
5 and 6, respectively, as plots of 1/T versus the logarithm
of ꢀꢁꢂ. These plots reveal an intriguing phenomenon: the
activation energy of electron transfer, ∆G, determined by
monitoring by OPV ground state bleaching, shows a
monotonic decrease upon increasing the number of quin-
olines in the bridge (see Table 3, charge separation).
Meanwhile activation energies determined from PB lumi-
nescence quenching are almost identical for all studied
samples (see Table 3, charge injection). An influence of
solvent dielectric permeability was observed, character-
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and PB emission lifetimes would both be expected to
form (time constant >150 ns, Figure S14a). This suggests
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no heavy atom effect in chlorinated solvents that would
favor intersystem crossing to triplet states. Furthermore,
we find that the slow charge recombination to the triplet
state is concentration dependent (e.g.for OPV-Q19-PB,
~60 µs at 15 µM and ~34 µs at 130 µM) suggesting the
involvement of bimolecular processes.
The relaxation of the CSS of dyads Q•+n-PB•–also oc-
curs with the concomitant formation of a PB’ triplet
statewith a time-constant at 298 K of 750 psin CH2Cl2and
400 ps in toluene (Figure S15a,b). This similarity again
suggests no strong role of the solvents in these processes.
depend on foldamer bridge length.
PB emission quenching is due to the formation of
the long-lived PB•–. TRABS data (see Boc-Q8-PB in Figure
4A and S11a) show that the absorption bands of Q•+ (cen-
tered at 390 nm) and PB•– appear at the same rate. This
leads to the conclusion that, after excitation of PB, elec-
tron transfer occurs from the quinoline bridge HOMO
towards the PB lowest-lying SOMO with a rate of
17 × 109 s–1 (τ = ~60 ps for n > 8). At 298 K the OPV GSB
appears with a time delay (Figure 4C, D, Figure S13, Table
2) relative to the PB•– band for triads with n≥14 (the case
n=9 will be considered below). This is consistent with a
two-step electron transfer in which the reduction of ex-
cited PB is followed by (and not synchronous with) the
oxidation of OPV. Thus, a global picture emerges in
which the reduction of excited PB results in the injection
of a hole in the Qn bridge through which it migrates to-
wards the OPV electron donor that it ultimately oxidizes
(Figure 8).
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The relaxation of the charged recombined PB triplet
state occurs on a much longer timescale than charge re-
combination – hundreds of microseconds (Figure S16-17).
Time constants were measured to be >340 µs for OPV-Q9-
PB and >420 µs for the longer OPV-Qn-PB triads (n=14, 18,
19, 34) in CHCl3. This process as well was found to be little
dependent on solvent (Figure S17b) and shows some con-
centration dependence: for OPV-Q19-PB in CH2Cl2, relaxa-
tion of triplet state occurs at >360 µs at 15 µM and ~86 at
130 µM).
In the case of OPV-Q9-PB, the rates of electron
transfer from the quinoline unit to PB and further hole
transfer towards OPV are equal to each other at 298 K
(17 × 109 s–1). For this shorter triad, a two-step hopping
mechanism for electron transfer may not be unambigu-
ously discriminated from a one-step mechanism as in, for
example, a superexchange. However, low temperature
studies allowed to demonstrate the prevalence of a two-
step mechanism for OPV-Q9-PB as well. Already at 273 K,
a difference between the rates of PB emission quenching
(τ = 85 ps) and OPV GSB (τ = 100 ps) can be observed
(Figure 5, 6). This difference increases upon lowering
temperature further. In addition, the temperature de-
pendence of electron transfer rate between PB and OPV
shows that this process has a positive activation energy,
whereas a negative activation energy is expected in the
superexchange mechanism.70–72 In other words, charge
separation via a superexchange may be anticipated to
become faster upon cooling because cooling results in an
increase of electron coupling between donor and acceptor
due to the decrease of thermally-induced disorder. Even
though the lack of negative activation energy alone does
not allow to completely rule out a superexchange mecha-
nism,73 it remains a strong indication. Previous studies on
short OPV-Qn-PB triads (n=2, 4, 5, 9) led to the conclu-
sion that charge transfer between PB and OPV is gov-
erned by a superexchange mechanism.55 The present in-
vestigation shows that a hopping mechanism contributes
when n ≥ 9.
Figure 7. TRABS spectrum of OPV-Q9-PB in CHCl3
(λexc = 532 nm, timescale 100-200 ns) and O.D. change at
502 nm.
Discussion
Evidence for the contribution of a hopping
charge transfer mechanism. Depending on the process,
electron transfer rates were found in three distinct time-
scales: initial PB emission quenching occurs in the sub-
nanosecond timescale, OPV GSB and the concomitant
appearance of OPV•+ absorption occur in the nanosecond
timescale, and charge separated state lifetimes are found
in the microsecond timescale. The rate of PB emission
quenching is independent of Qn bridge length and equal
to 60 ps for values of n > 8 (Table 2), while it varies with n
for shorter variants (see below). In contrast, OPV GSB is
increasingly slow as the Qn bridge is lengthened. OPV
GSB occurs almost 20 times faster in the shortest triad
OPV-Q9-PB (
Q34-PB (
GSB = 0.096 × 1010 s–1). This alone suggests a mul-
τ
GSB = 1.75 × 1010 s–1) than in the longest OPV-
τ
tistep charge separation pathway in these triads: if charge
transfer occurred in a single step, OPV GSB appearance
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mational freedom with respect to the bridge and thus a
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diminished electron coupling between bridge and PB. As
explained in the introduction, earlier studies have shown
that these helices are so stable that conformational dy-
namics are extremely slow. Yet the helix-PB linkage might
be influenced by the size and rigidity of the neighbor
helix. Here, we assume that electron transfer occurs from
the quinoline covalently attached to PB. Meanwhile, elec-
tron transfer might also occur from subsequent quino-
lines that come in close proximity to PB, albeit less effi-
ciently.
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Length dependence of hole transport rate
through the Qn bridge. We have shown that electron
transfer between PB and OPV mediated by a foldamer
bridge consists of two steps: electron transfer from the
bridge to the acceptor (PB)i.e. charge injection into the
bridge, and propagation of a hole through the bridge to
the donor (OPV). The propagation of the hole through
the bridge itself may be mediated by either a superex-
change or a hopping mechanism. An analysis of the
bridge-length dependence of the rate of hole transport
allows to discriminate between these two mechanisms.
Indeed, a characteristic of a pure superexchange process
is an exponential dependence of charge transfer rate (kCT)
on the length (l) of the bridge:
Figure 8. Simplified frontier orbital diagram of electron
transfer between PB and OPV through the Qn foldamer
bridge. The small red circles represent holes.
Efficiency of charge injection into the foldamer
bridge.We note that PB quenching rates in dyads Boc-
Qn-PB (n = 1, 2, 4, 8, Figure S9-10) increase almost expo-
nentially from n = 1 (3 × 109 s–1, τ = 330 ps) to n = 8
(17 × 109 s–1, τ = 60 ps) while they remain constant for
larger n values (Table 2). A possible explanation may be
drawn from the effects of geometry on electronic coupling
between PB and subsequent quinoline units, which define
electron transfer rates. Applying Marcus theory of elec-
tron transfer in the non-adiabatic regime, considering
that the quinoline-PB electronic coupling is weak, ꢀꢁꢂmay
be expressed as in Eq. 3:74
ꢞ
ꢡ
ꢀꢁꢂ = ꢉexp −ꢟꢠ
Eq. 4
where lmay correspond to the product of thenumber
of quinoline units multiplied by the length of an individu-
al quinoline unit, β– attenuation factor. This pattern is
not observed (Figure 9). Further evidence against a pure
superexchange comes from the positive activation energy
of electron transfer (see above). Instead, charge transfer
rate dependence on the number of quinoline units (i.e.
potential steps in a hopping mechanism) shows a hyper-
bolic behavior (Figure 9, Eq. 5) which, according to refer-
ences 6,22, constitutes evidence in favor of a hopping-like
mechanism. Equation 5 represents a simplified depend-
ence of charge transfer rate viaa hopping mechanism:
ꢄ
ꢀꢁꢂ
=
ꢒꢓ ꢕꢒ
ꢐ exp ꢊꢋꢎꢌꢛꢐ‡ꢑ
Eq. 3
ꢖꢗꢗꢘꢙꢓꢚꢎ
ꢔ
ꢏ
ꢏ
where ∆G‡ = (λ+∆GRP)2/4λ, with ∆GRP as the free en-
ergy change of radical ion pair formation, λ as the reor-
ganization energy, and ꢕꢜꢒꢝꢝas electronic coupling be-
tween initial and final states, T – temperature, kB – Boltz-
mann constant. Experiments were performed in low po-
larity solvents and, due to the large sizes of the acceptor
and donor, λ is assumed to be dominated by inner-sphere
reorganization.73 Note that the activation energy, ∆G, of
electron transfer from the quinoline bridge to PB varies
slightly between different samples and equals to ~0.1 eV
in CH2Cl2 (Table 3). This is also true for shorter dyads
BOC-Q1-PB and BOC-Q2-PB (Figure S19). The values for
∆G‡are slightly different (see Table S1) from ∆G, as the
ꢀꢁꢂ~ꢢꢋꢣ
Eq. 5
where N is the number of hops and η a phenomeno-
logical power parameter. In principle, knowledge of all
rates (charge trapping/detrapping, charge hopping, etc.),
and of energetic parameters (activation barriers, poten-
tials of oxidation and reduction)might allow modelingof
charge transfer dynamics in the system, as was previously
described for DNA (Thermally Induced Hopping mod-
el).73,75–79
influence of√(1/T)isto be taken into account.The activa-
tion energy of electron transfer rate does not depend on
length of the bridge, and we can conclude that theener-
getic term of Eq. 3 does not play important role in the
bridge length dependence of charge transfer rate. This
According to the random-walk theory which is fre-
quently applied to DNA hole transfer studies,6 the phe-
nomenological power parameter η in Eq. 5 equals to: a) 2
in the case of unbiased transfer; b) 1 ≤ η ≤ 2 for acceptor
direction-biased random walk; c) η > 2 for donor direc-
tion-biased random walk. Figure 9 shows that η is re-
duced from 2.25 to 1.1 upon reducing temperature from
298 K to 180 K in CH2Cl2. This result demonstrates quali-
tative changes in the electronic properties of the bridge,
and consequently on hole transport, with temperature.
Significant reduction of η is a result of the fact that longer
points to electron coupling ꢕꢒ as the origin of this de-
ꢖꢗꢗ
pendence.
The Boc-Q8-PB structure spans three full helix turns;
it has a rigid geometry with a well-defined relative orien-
tation of the quinoline units and the PB chromophore. At
lower values of n, the helix spans a lower number of
turns. This may result in a less well-defined geometry at
the helix-PB-linkage, providing PB with a larger confor-
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To account for the above observations, we propose a
bridges show smaller observed activation energies (Table
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3) of charge separation. At very low temperatures, the
rates ratio for long and short bridges becomes less pro-
nounced than at higher temperatures, leading to η dimi-
nution.
According to Jortner,6 it is possible to relate the cu-
mulative rate of hole transport and the rate of individual
hop step with number of steps (N):
simplified model based on classical understanding of
charge transfer through modular bridges. This model
takes into account that through-bridge hole transfer is
hindered by barriers between quinoline units as well as by
an attractive electrostatic interaction between its own
positive charge and the negatively charged PB•–. The elec-
trostatic interaction is diminished upon increasing sol-
vent polarity and may also be influenced by the polar
nature of the aromatic amide helix backbone which might
generate a macrodipole.6 For example, the dielectric con-
stant of CHCl3 is smaller than that of CH2Cl2, leading to a
weaker dielectric screening of the charges and thus to a
decrease of charge transfer rates, that is in agreement
with experiment (Figure 5, 9). In the extreme case of
OPV-Q34-PB, the hole never reaches OPV moieties in
CHCl3 at temperatures lower than 273 K (Figure 5), be-
cause the rate of hole transfer becomes slower than the
rate of charge recombination between PB•–anion radical
and the Q•+ quinoline cation-radical (Figure S15a).
ꢀꢁꢂ~ꢀꢤꢥꢦ
ꢢ
Eq. 6
9
giving the rate of a single hop (kHOP) at 298 K of
2.5 × 1012 s–1 (τ = 400 fs) in CH2Cl2 and 1.8 × 1012 s-1
(τ = 560 fs) in CHCl3.The foldamer bridges are rigid helical
structures and, therefore, one may wonder whether hole
transfer occurs between quinoline units that are directly
adjacent in the oligomeric sequence, or between a quino-
line and another appropriately oriented in an adjacent
turn (above or below) in the helix, but not adjacent in the
sequence. In the latter case, the number of steps for the
hole to hop through an entire bridge from the PB to the
OPV will be reduced since there are only 3 full-turns in
OPV-Q9-PB, and 6, 8, and 14 turns in OPV-Q14-PB, OPV-
Q19-PB and OPV-Q34-PB, respectively. Nevertheless, ac-
cording to the random-walk theory, the qualitative de-
scription of hole transport will be identical in both sce-
narios: if holes hop across the helix and skip some quino-
line steps, the number of hops is reduced but the rate of
single hops is commensurately reduced as well.
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The observed reduction of charge separation activa-
tion energy upon increasing bridge length is a key for
complete understanding of through-bridge charge trans-
fer. We must pay attention to the fact that experimental-
ly-determined activation energies of charge separation
(Table 3) take in account the overall process of charge
separation in triads, including through-bridge charge
transfer and quinoline-to-excited PB charge injection. The
activation energiescharacterize the overall energy barriers
that the charge must overcome between the initial and
final states of the charge separation process. For example,
charge injection and charge separation for OPV-Q9-PB are
not distinguishable at room temperature, suggesting that
charge injection is rate limiting in this case. The observed
reduction of charge separation activation energies with
increasing quinoline bridge length, while charge injection
activation energies remains constant,implies that charge
transport through longer bridges becomes faster at low
temperatures. This may be explained by the presence of
two mechanisms of charge transfer through the bridge:
hopping (a predominant process) and tunneling (a minor
process) between units whose respective contributions
depend on bridge length. Marcus77 demonstrated that
non-exponential temperature dependence of electrical
conductivity through DNA76 could be explained by as-
suming that there are two mechanisms of charge transfer
in long DNA: a hopping via localized states and a transfer
through partly delocalized states.A similar model could
explain observed activation energy variations in OPV-Qn-
PB triads. However,an open question remains: why does
the hopping mechanism have a higher relative weight
than tunneling for shorter triads? One could argue that
the Coulombic force between the negative charge on PB
and hole could cause hole localization in shorter triads.
With bridge length increase,influence of such forces
would be relatively diminished leading to hole delocaliza-
tion. Yet, in the absence of irrefutable evidence of hole
delocalization,the question remains open.
Figure 9. The dependence of the rates of electron transfer
between PB and OPV on the number of quinoline units in
the foldamer bridge at different temperatures. The dashed
line corresponds to an exponential fit, the solid lines corre-
spond to a hyperbolic fit.
OPV-Q18-PB differs from other triads by the relative
orientation of its OPV and PB(Figure 1). This difference
does not affect the charge separation rate between PB and
OPV. As shown in Figure 9, values for OPV-Q18-PB lie on
the theoretically-generated curves, showing that only
distance matters in determininghole transport rate
through long triads, in contrast to findings for very short
triads.55
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In general, it should be understood that as foldamer be understood in the light of different mechanisms: radi-
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length increases, the rate of individual hopping steps
appears to change. This suggests the coexistence of differ-
ent mechanisms whose relative contributions would vary
as the charge is further away from PB•–. For example, pure
hopping between adjacent sites may be mixed with a
longer range charge transfer, for example superexchange
or a variable length hopping including hopping between
consecutive turns of the helical bridge.76,77Deciphering the
contributions of these different mechanisms will account
for the differentΔꢧvalues for short and long bridges.
cal pair (RP) intersystem crossing and spin-orbit (SO)
intersystem crossing (ISC).80–82 The radical pair intersys-
tem crossing mechanism requires the coupled spins of the
radical pair in the charge-separated state to dephase,
leading to the formation of a triplet charge-separated
state, that evolves to a local charge recombined triplet
state (in this case that of PB). In spin-orbit intersystem
crossing, while electronsmove between orbitals of differ-
ent symmetry,a change of spin angular momentum is
required to fulfil total system angular momentum conser-
vation.81 Taking into account the extremely long lifetimes
of the CSS statesof long triads (n≥14),intermolecular in-
teractions may take place due to the diffusion of mole-
cules. This would increase the likelihood that a SO-ISC
mechanism contributes to the generation of the PB triplet
state afterCR. The concentration dependence of the CR
rate is consistent with this hypothesis.Significant distanc-
es between anion and cation radicals and screening ef-
fects due to the solvent decrease the probability of spin-
spin interaction in long triads and thus lower chance of
RP-ISC. Meanwhile, the concentration dependence did
not show a steep effect, thus we cannot exclude the con-
tribution of other mechanisms.A charge recombination
process in dyad Q8•+-PB•- also ends in PB triplet state for-
mation. It occurs in the subnanosecond range scale, as for
short triads, n<9. Charge recombination in OPV-Q9-PB
occurs also relatively fast. In these cases, intermolecular
processes may not play any significant role and intramo-
lecular CR processes must be considered. Other investiga-
tions such as time-resolved EPR would be needed to dif-
ferentiate RP-ISC and SO-ISC.81
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In previous work55on OPV-Qn-PB triads with n = 2,
4, 5, 9, fast electron transfer was attributed to a superex-
change process. We observed that PB emission was
quenched and that the quenching time constant for all
studied samples with bridges n≤9 is smaller than 50 ps.
This latter fact is important, as we have now showed that
electron transfer from the Qn bridge to excited PB in short
Qn-PB dyads (n≤4) in CHCl3 or CH2Cl2 is slower than that
(see Table 2). We thus infer that the initial electron injec-
tion from the bridge to PB does not take place for the
shorter triads and that a true superexchange must take
place to explain rapid quenching of PB emission. For
longer Qn segments, electron injection from the bridge to
PB is faster than overall charge separation, which implies
a hopping mechanism. We had previously noted that the
rate of electron transfer depending on the length of
foldamer bridge is not monotonously changing and has a
point of inflectionfor the triad where n = 5.55Combined
with current experimental results, the borderline between
pure superexchange and mixed mechanismsis seen to lie
in the range 5<n<9. For OPV-Q9-PB at ambient tempera-
ture, the growth of PB anion radical band and the appear-
ance of OPV ground state bleaching occur at the same
rates, which, at first sight, suggests that a superexchange
mechanism is at play. However, low temperature studies
clearly revealed a multi-step process implying hopping.
Only for this compound do our new conclusions differ
from those of our earlier study. For OPV-Qn-PB triads, a
transition from a pure superexchange (delocalized) elec-
tron transfer mechanism to a mixed electron transfer
through the bridge behavior (localized + delocalized)
occurs at n <9. The transition from a pure superexchange
to what we call a dual mechanism, could be understood in
terms of donor–acceptor conjugation in the excited state.
This is clearly observed for short foldamers and is absent
for longer foldamer-based triads, according to experi-
mental observation on electron transfer rates. Direct
donor to acceptor charge transfer is thus impossible in
longer foldamers and a multistep charge transfer takes
over.
Conclusion
We investigated time-resolved photo-induced
charge separation between an OPV electron donor and a
PB
acceptorthrough
a
helically-
foldedoligoquinolinebridge of increasinglength (9, 14, 18,
19, 34 units) in solvents with different polarity, CHCl3 and
CH2Cl2, and at different temperatures. The synthetic
availability of these large functional oligomers constitutes
an important advance in the field of foldamers. Charge
separation was found to occur very fast, in the sub-nano
and nanosecond timescales, even across the longest
bridges which span more than 306Å along the backbone,
an achievement that had previously been observed prin-
cipally with DNA. The general mechanism of charge sepa-
ration following excitationcan be described in foursteps:
1) PB electron acceptor photo-excitation; 2) electron injec-
tion from the HOMO of a quinoline unit of the bridge to
the electron acceptor SOMO with a rate of 17 × 109 s–1; 3)
hole migration along the helical oligoquinoline bridge
from the acceptor to the donor site by a random, reversi-
ble hopping between units, with single hop event rate at
298 K of 2.5 × 1012 s–1 (τ = 400 fs) in CH2Cl2 and 1.8 × 1012 s–1
(τ = 560 fs) in CHCl3; 4) hole trapping by OPV electron
donor finalizing the formation of the charge separated
state, which was shown to have an extremely long lifetime
of τ > 80 µs in CH2Cl2 at 15 µM. These conclusions com-
Mechanism of charge recombination. Studies on OPV-
Qn-PB triads on long timescales, hundreds of microsec-
onds, showed that recombination (OPV GSB band recov-
ers, and PB•- and OPV•+ bands decay simultaneously) leads
to the slow (tens of microseconds depending on concen-
tration) formation of the charge recombined PB triplet
state, that slowly relaxes to the ground state. The for-
mation of PB triplet state during intramolecular CR could
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hypothesis that electron transfer occurred via a superex-
change mechanism for very short bridges (n ≤5); the in-
terpretation for n = 9 was revised. The charge recombina-
tion of long foldamers probably occurs through intermo-
lecular interactions, leading to formation of long-lived
charged recombined triplet states of PB (τ > 360 µs in
CH2Cl2 at 15 µM). Variable temperature studies revealed
that the activation energy of charge transfer ∆G‡decreases
upon increasing bridge length. The origin of this phe-
nomenon is not yet certain but it could result from the
coexistence of two distinct mechanisms of hole transfer
along the bridge. Also, whether holes only hop between
quinolines adjacent in the sequence or whether they may
also directly hop between contiguous turns of the helix
remains to be elucidated. Progress along these lines is
currently being made in our laboratories and will be re-
ported in due course.
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Solution studies such as those presented above allow
to decipher complex intramolecular processes. Ultimate-
ly, incorporation into devices will require integration in
the solid state where numerous intermolecular processes
may occur. For example, the observation of long-lived
charge separated sates in solution, a desirable property
for applications in photovoltaics, may not be simply ex-
trapolated to the solid state. The successful integration of
aromatic foldamers into metal-organic-metal junctions
and their electron transport properties in the absence of
solvent will be reported elsewhere.
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ASSOCIATED CONTENT
Supporting Information. Experimental details for synthetic
procedures and spectroscopic measurements, spectroscopic
data and CIF for OPV-Q-CO2H. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
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* i.huc@iecb.u-bordeaux.fr
* serguei.denisov@u-bordeaux.fr
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Author Contributions
‡ These authors contributed equally to this work.
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Funding Sources
This work was supported by an ANR Grant (FOSET project
no. ANR-12-BS08-0007-01, post-doctoral fellowships to M. N.
and S. D.) and by the European Union under the Erasmus-
Mundus program (predoctoral fellowship to X. L.).
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ACKNOWLEDGMENT
We thank the facilities of IECB for expert assistance in mass
spectrometry, and Dr. S. Massip for refining the crystal struc-
ture of OPV-Q-CO2H.
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