Intramolecular excimer formation between 3,6-di(thiophen-2-yl)pyrrolo[3,4- c]pyrrole-1,4(2H,5H)-dione chromophoric groups linked by a flexible alkyl spacer

Article abstract of DOI:10.1021/jp312315k

Bichromophoric molecules containing two 3,6-di(thiophen-2-yl)pyrrolo[3,4-c] pyrrole-1,4(2H,5H)-dione (DPP) moieties linked via aliphatic spacers of different length are synthesized. Optical absorption spectroscopy indicates that the molecules adopt an extended conformation in solution. Fluorescence spectroscopy shows that photons are emitted from the locally excited singlet state in an extended conformation. In sufficiently polar solvents, quenching of fluorescence occurs and fluorescence quantum yield (Φ_F) and fluorescence lifetime (τ_F) measurements indicate formation of an intramolecular excimer as the quenching mechanism. The redox potentials of the molecules and the solvent polarity dependence of the quenching are consistent with significant charge-transfer character of the excimer state. Photoinduced absorption measurements show enhanced intersystem crossing to the triplet state in polar solvents. Results indicate that in donor-acceptor π-conjugated materials involving the DPP moiety, excimer-like interchain polaron pair excited states could play an important role in the photophysics because of their close proximity in energy to the lowest singlet excited state.

Full text of DOI:10.1021/jp312315k

Article
pubs.acs.org/JPCA
Intramolecular Excimer Formation between 3,6-Di(thiophen-2-
yl)pyrrolo[3,4‑c]pyrrole-1,4(2H,5H)‑dione Chromophoric Groups
Linked by a Flexible Alkyl Spacer
́
Mindaugas Kirkus, Rene A. J. Janssen, and Stefan C. J. Meskers*
Molecular Materials and Nanosystems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
*
S Supporting Information
ABSTRACT: Bichromophoric molecules containing two 3,6-
di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione
(
DPP) moieties linked via aliphatic spacers of different length
are synthesized. Optical absorption spectroscopy indicates that
the molecules adopt an extended conformation in solution.
Fluorescence spectroscopy shows that photons are emitted from
the locally excited singlet state in an extended conformation. In
sufficiently polar solvents, quenching of fluorescence occurs and
fluorescence quantum yield (Φ ) and fluorescence lifetime (τ )
F
F
measurements indicate formation of an intramolecular excimer
as the quenching mechanism. The redox potentials of the
molecules and the solvent polarity dependence of the quenching
are consistent with significant charge-transfer character of the
excimer state. Photoinduced absorption measurements show enhanced intersystem crossing to the triplet state in polar solvents.
Results indicate that in donor−acceptor π-conjugated materials involving the DPP moiety, excimer-like interchain polaron pair
excited states could play an important role in the photophysics because of their close proximity in energy to the lowest singlet
excited state.
INTRODUCTION
character of the excitation. Experimental evidence for charge-
■
transfer character of photoexcitations in films of π-conjugated
π-Conjugated molecular materials are applied in a variety of
17
1
−5
polymer has come from photoinduced absorption, photo-
optoelectronic devices such as solar cells,
light-emitting
18
6
,7
8−12
conductivity, and luminescence measurements. When the
luminescence from dissolved chains and that from a solid film
of π-conjugated polymer are compared, the luminescence from
the films often shows an anomalous red shift that is usually
accompanied by reduction in luminescence quantum yield. In
the films, the excited state responsible for the luminescence
often has a much longer lifetime and can display a high
sensitivity toward applied electric fields expected for a state
diodes,
detectors,
light-emitting field effect transistors,
photo-
1
3−15
and so on. Many studies have been undertaken
to elucidate the nature of optical excitations in molecular solids.
In inorganic semiconductors, an optical excitation, or exciton, is
pictured as a loosely bound electron−hole pair (Wannier
exciton) in a hydrogen-like state with a large radius that can be
orders of magnitude larger than the Bohr radius a for atomic
hydrogen. The energy needed to separate the electron and hole
forming the exciton in Si and GaAs is small compared to the
2
0
19−21
with a high dipole moment.
Detailed spectroscopic analysis and theoretical calculations
on dimers of small aromatic molecules, for example, pyrene,
also indicate a significant contribution of charge-transfer
thermal energy k T. In contrast, for organic molecular materials,
b
an exciton is often viewed as a tightly bound electron−hole pair
in which the electron and hole are confined to the same
molecule (Frenkel exciton).
22,23
16
character to the lowest relaxed excited singlet states.
After
absorption of a single photon, a pair of pyrene molecules can
In the last decades, experimental and theoretical inves-
tigations on π-conjugated polymers and molecules have yielded
evidence that the Frenkel exciton description for organic
semiconductors needs to be extended, allowing also for
intermolecular charge-transfer character in the excited state.
This yields a picture in which an exciton is modeled as a
coherent superposition of electron−hole pair states with the
electron and hole on the same polymer combined with states
where the electron and hole are on neighboring molecular sites.
The contributions of pair states with electron and hole on
neighboring molecules contribute to the charge-transfer
form an excimer, a dimer that owes much of its stabilization to
2
4,25
the presence of the excitation energy.
The stabilization
contains contributions from delocalization of the singlet excited
state (exciton resonance) and from delocalization of electrons
over both molecules in the pair (charge resonance). The
exciton resonance describes contributions from electron−hole
Received: December 14, 2012
Revised: May 11, 2013
©
XXXX American Chemical Society
A
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Article
a
Scheme 1. Synthesis of the DPP-Based Dimers D1, D2, and D3 and Monomer M0
a
(
a) K CO , C H −I, DMSO, RT, overnight; (b) K CO , Br-(CH ) -Br, 18-crown-6, DMF, 120 °C, overnight; (c) K CO , C H −Br, 18-crown-6,
2
3
6
13
2
3
2
n
2
3
6
13
DMF, 120 °C, overnight.
pair states localized on one molecule, whereas the charge
performance of opto-electronic devices made of π-conjugated
56
resonance describes contributions from electron−hole pairs
polymers such as solar cells and light-emitting diodes.
26−29
with hole and electron on different molecules.
In recent years, π-conjugated polymers with a small band gap
(∼1.5 eV) have been developed for application in polymer solar
cells. A particular synthetic strategy to reach a small band gap is
to make an alternating copolymer of donor and acceptor
An elegant experimental approach to study excimer
3
0
formation, pioneered by Hirayama, is to link two π-
conjugated molecules via a flexible spacer. In the bichromo-
phoric molecules with flexible spacer, excimer formation is an
intramolecular folding process. Hirayama et al. studied
diphenyl, triphenyl, and di-o-tolyl aliphatic dimers and found
a strong dependence of the efficiency of excimer formation on
the conformational properties of the linker. In their case, an
excimer is possible only for aliphatic linkers with a length of n =
5
7,58
units.
One particular type of donor−acceptor polymer
involves the pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP)
moiety as acceptor and oligothiophene units as donor.
59
Because of the donor−acceptor structure, one may expect
that charge resonance is important in the lower excited state
and may also stabilize interchain, exciplex-like excited states
between the donor unit of one chain and an acceptor unit of
another neighboring chain. Spectroscopic studies of the
microcrystalline solid of DPP-based oligomers have indeed
shown red-shifted, exciplex-like emission, especially in cases
where solubilizing side chains on the molecules were
unbranched, allowing the molecules to come very close to
each other in the excited state.
3
carbon atoms, indicating that stabilization of the excimer by
charge resonance is possible only in very compact conforma-
tions. Later studies involving dimeric molecules with
chromophoric groups with a more extended π-system
3
1−33
34,35
36,37
(
naphthalene,
anthracene,
pyrene,
phenan-
3
8,39
40−44
45
threne,
phene,
carbazole,
dibenzofuran, dibenzothio-
4
6,47
48
and naphthalimide ) confirmed the importance
In this study, we further investigate excimer formation in
molecules containing two identical DPP chromophoric
moieties connected via a flexible linker. The flexible linker
allows us to investigate the interconversion of Frenkel-type
localized excited states into excimers stabilized by charge
resonance.
of the conformational dynamics and stability of the linker on
the excimer formation but also showed that with these
extended π-systems the conformation of the linker is less
restrictive. In many systems, excimer formation occurs even in
highly apolar solvents, indicating that the excimer has a small
49
dipole moment and is not highly polarized. Compounds with
rigid spacers that preclude face-to-face packing of the two π-
conjugated moieties indicate that without orbital overlap
RESULTS AND DISCUSSION
■
50
excimer formation is suppressed.
Synthesis. The structure and synthesis of DPP-based target
materials M0, D1, D2, and D3 are shown in Scheme 1.
Compound M0 was synthesized in a one-step alkylation
reaction of 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-
Apart from the homodimeric excited-state complexation
involved in excimer formation, bichromophoric systems with
flexible linker can also be used to study heterodimeric complex
formation in the excited state. Such intramolecular heterodimer
or exciplex formation can occur in systems containing an
electron-donating and an electron-accepting unit. In contrast to
excimer formation, exciplex formation is often highly sensitive
to solvent polarity. It has been argued that in systems with
sufficient driving force for electron transfer, exciplex formation
in a donor−linker−acceptor system with an extended
conformation can be initiated by photoinduced long-range
electron transfer across the linker followed by folding of the
system driven by the electrostatic interaction between ionized
(
2H,5H)-dione using 1-bromohexane, K CO as a base, and a
2 3
catalytic amount of 18-crown-6 in dimethylformamide (DMF)
at 120 °C. It was purified by several crystallizations from cold
DMF. Usually, DPPs with the linear alkyl chains can be
recrystallized from the various organic solvents, resulting in
analytically pure samples in high yield. The monoalkylation of
the DPP with 1 equiv of alkyl halide using the same reaction
conditions as mentioned above did not give the desired product
1. For the synthesis of the intermediate compound 1, we have
60
used a slightly modified procedure as reported before,
51−53
donor and acceptor unit.
This “harpooning” mechanism
resulting in an analytically pure intermediate compound 1 in
58% yield.
For the last step of the synthesis of the target dimers D1, D2,
and D3, the same reaction conditions were used as for M0. All
three dimers were purified by column chromatography on SiO2
and subsequently recrystallized from cold DMF. In the case of
can explain the polarity dependence and the fast decay of the
initial local excited state. Incidentally, the highly polarized and
strong charge-transfer character of the exciplex state can result
5
4,55
in fast intersystem crossing to the triplet excited state.
This
latter process could have a negative influence on the
B
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Article
dimer D1, recycling gel permeation chromatography was used,
followed by recrystallization from DMF. The structures and
molecular weights of the final compounds M0, D1, D2, and D3
were confirmed through NMR spectroscopy and matrix-
assisted laser desorption ionization time-of-flight mass spec-
trometry analyses.
D1, D2, and D3 show a remarkable difference in intensity. For
the shortest spacer, we find the lowest intensity at 2.19 eV. The
dimers D1, D2, and D3 show fluorescence with a band shape
that is virtually the same as that for for M0 without any
detectable additional red-shifted emission bands that could be
assigned to an excimer. In contrast, red-shifted emission is
common for dimeric molecules containing, for example, two
anthracene, naphthalene, pyrene, or carbazole chromophoric
Optical Properties. Figure 1 shows the absorption and
fluorescence emission spectra of D1, D2, D3, and M0 in dilute
3
4,40,63−65
groups.
We conclude that the fluorescence emitted by
D1, D2, and D3 in o-DCB originates from locally excited states
in an extended conformation of the dimeric molecule without
any appreciable interaction between the chromophoric groups.
This also holds for other solvents investigated (toluene,
benzonitrile, and 2-methyltetrahydrofuran (2-MeTHF)). Quan-
tum yields for fluorescence from the locally excited states were
determined and are listed in Table 1.
In an apolar solvent (toluene), the fluorescence quantum
yields for the dimeric DPP molecules are approximately the
same as that for the monomer M0. In more polar solvents, the
dimers show reduced quantum yields for fluorescence. The
lowest photoluminescence (PL) quantum yield was observed
for the dimer D1 with the shortest aliphatic spacer between
DPP chromophores in the most polar solvent investigated
(benzonitrile). This observation suggests that although in the
ground state the two DPP moieties in the dimeric molecule
hardly interact, in the excited state a photophysical pathway
involving charged, intermediate states is opened in polar
solvents, leading to deactivation of the locally excited singlet
state.
Figure 1. Absorption and emission spectra of the DPP-based dimers
D1, D2, and D3 and the reference compound M0 in dilute o-DCB
solutions at room temperature (298 K). Fluorescence spectra recorded
for samples with the same optical density at the photon energy of
excitation 2.36 eV.
The band shapes of the fluorescence spectra for M0 and
D1−D3 are virtually the same, indicating that only the
extended conformation of the dimers is emissive. Therefore,
the PL spectra alone do not reveal much about the origin of the
fluorescence quenching in the dimeric molecules. An additional
experimental technique was needed, so time-resolved PL
measurements were performed in organic solvents with
different polarities (Figure 2). The fluorescence lifetimes and
the corresponding fluorescence quantum yields in the different
organic solvents are summarized in Table 1. The reference
molecule M0 shows monoexponential fluorescence decay in all
solvents investigated with an excited-state lifetime in the range
of 5.5−6.8 ns depending on the solvent chosen. These values
are in agreement with values reported earlier for some
ortho-dichlorobenzene (o-DCB) solutions at room temperature.
The transitions observed can be assigned to the allowed optical
transition between the singlet ground state (S ) and the lowest
0
excited singlet state (S ). The absorption spectra of the dimers
1
are nearly identical to the absorption spectrum of the monomer
M0 with the same absorption maxima around 2.39 and 2.23 eV.
The absorption of dimers D1, D2, and D3 is not influenced by
the nature of the spacer in these molecules. This shows that
there is hardly any interaction between DPP chromophores in
the ground state. Absence of significant ground-state interaction
between chromophoric units is not uncommon in dimeric
6
1
systems, but systems with short spacers are known in which
62
intramolecular dimerization in the ground state is significant.
The fluorescence spectra for M0, D1, D2, and D3 in o-DCB
all show the same band shape with a maximum in the
fluorescence intensity at 2.19 eV, indicating emission from the
singlet excited state of the DPP moiety. When recorded under
identical excitation conditions, the fluorescence spectra for M0,
66
dialkylated DPP derivatives.
The dimer D1 shows a significant reduction in excited-state
lifetime upon going from apolar to polar solvents (Figure 2, left
panel). A monoexponential decay of the luminescence was
Table 1. Fluorescence Quantum Yields (Φ ) and (Amplitude Weighted Average) Fluorescence Lifetimes (τ ⟨τ ⟩ ) of M0, D1,
F
F,
F a
D2, and D3 in Different Organic Solvents and at Different Temperatures; Oxidation (E ) and Reduction (E ) Onsets and the
ox
red
CV
OPT
Electrochemical (Eg ) and Optical (Eg ) Band Gaps and Corresponding Energies of HOMO and LUMO Levels
toluene
o-DCB
2-MeTHF
benzonitrile
CV
b
b
b
c
b
d
d
e
e
EgCV
(eV)
EgOPT
(eV)
τF
⟨τF⟩a
⟨τF⟩a
τF
⟨τF⟩a
Eox
Ered
EHOMO
(eV)
ELUMO
(eV)
a
a
a
compd ΦF
(ns)
ΦF
(ns)
(ns)
(ns)
ΦF
(ns)
(V)
(V)
M0
D1
D2
D3
0.40
0.44
0.44
0.40
5.50
4.36
4.59
4.50
0.65
0.14
0.28
0.43
6.15
2.57
3.68
4.15
6.83
2.37
3.40
3.80
7.46
6.94
7.27
6.88
0.42
0.02
0.05
0.12
5.93
1.01
1.39
2.03
0.80
0.68
0.69
0.63
−1.43
−1.47
−1.46
−1.56
−5.60
−5.48
−5.49
−5.44
−3.38
−3.33
−3.34
−3.24
2.22
2.15
2.15
2.20
2.15
2.15
2.15
2.15
a
b
Quantum yields were determined relative to the fluorescence of a dilute M0 solution with OD < 0.1 at room temperature. Recorded in dilute
c
d
+
solutions at room temperature (298 K). Recorded in dilute 2-MeTHF solutions at 80 K. Half-potentials (V) vs Ag/AgCl calibrated against Fc/Fc
from reduction and oxidation. E
e
= [−(Eox/red vs Ag/AgCl − 0.4)] − 4.8 eV measured in o-DCB solutions at room temperature.
HOMO/LUMO
C
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Figure 2. Time-resolved PL lifetimes in different organic solvents of D1, D2, and D3 in different solvents measured at 298 K.
observed in apolar solvent (toluene) with a lifetime of about
denoted knrad. For the time-integrated relative quantum yield
for fluorescence for a single emitting species we expect
4
2
.36 ns. With an increase in the polarity of the solvent (o-DCB,
-MeTHF, benzonitrile), the lifetime of the luminescence is
krad
strongly reduced and the decay becomes nonexponential. The
multiexponential shape of the fluorescence may be due to
different conformers of these molecules. The decay can be
fitted with a biexponential function. For D3, the influence of
solvent polarity on the decay of fluorescence is different from
the behavior displayed by D1. For D3, a significant shortening
of the decay is observed only in a highly polar solvent
Φ =
= k τ
rad
krad + knrad
(1)
In the case of multiple emitting species, this may be generalized
67
to
Itot
IAbs
∑ Iτ
IAbs
I
i i
Φ =
=
=
∑
aiτi = k⟨τ⟩
a
IAbs
(2)
(benzonitrile, Figure 2, right panel); solvents of medium
polarity hardly affect the decay. The influence of solvent
polarity on the decay of fluorescence of D2 is intermediate
between the behavior of D1 and D3.
The results from the fluorescence quantum yield and lifetime
measurements at room temperature are summarized graphically
in Figure 3. The graph shows a correlation between the
For the locally excited state, we expect krad and k to be
independent of the length of the aliphatic linker. Hence a linear
correlation between Φ and ⟨τ⟩ is predicted (see the dashed
a
line in Figure 3). Although the experimental (Φ, τ) data points
qualitatively follow the predicted correlation, Figure 3 also
shows that, especially for the dimer with the smallest linker
(
D1), there is a deviation from the predicted values. The
measured quantum yields for D1 in polar solvents are
considerably smaller than those predicted from measured
decay times. The deviations from the simple dynamics
quenching could be explained by the presence of different
conformers that do not equilibrate on the time scale of the
decay of the excited state.
To find further experimental support for a dynamic
quenching process, we have investigated the fluorescence
properties of the dimers as a function of temperature using the
glass-forming solvent 2-MeTHF. In the right panel of Figure 4,
the absorption spectrum for D1 is shown at various
temperatures. At lower temperatures, the spectrum sharpens
and the wavelength of the maximum absorption intensity shifts
slightly toward lower energy. The absorbance of the monomer
M0 in 2-MeTHF solution changes with temperature in a
manner very similar to that shown by the dimer D1. At low
71
temperature (80 K), the absorption spectra for M0 and the
dimers are virtually identical. This indicates that at low
temperature the dimers adopt an extended conformation.
When a 2-MeTHF solution of D1 is cooled, the changes
observed in the steady-state fluorescence spectrum are very
Figure 3. Fluorescence quantum yield for M0, D1, D2, and D3 in
various solvents plotted versus amplitude weighted average excited-
state lifetime. The dashed line indicates behavior expected for purely
dynamic quenching.
71
similar to those observed for the monomeric unit. The
bandwidth for the fluorescence narrows (Figure 4, left panel).
The fluorescence intensity first decreases but then rises steeply
for temperatures approaching 80 K. At low temperature, the
Stokes shift is small (∼0.01 eV) and peak positions for the
dimer fluorescence are virtually the same as those for the
monomeric DPP chromophore. This confirms that the
fluorescence occurs from a locally excited state in the dimer
in an extended conformation. The fluorescence spectra show a
rather abrupt change in band shape between 170 and 80 K.
quantum yield (Φ) for fluorescence from the locally excited
state of the dimers and the amplitude weighted average
6
7
lifetime of the excited state ⟨τ⟩ . Such a correlation is
a
expected for a purely dynamical quenching process. The locally
excited state that is populated initially upon illumination in the
dimer can emit a photon rate constant krad but in addition may
also undergo dynamic processes leading to nonradiative decay.
The combined rate constant for all nonradiative processes is
D
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Figure 4. Fluorescence (left) and absorption (right) for a dilute solution of D1 in 2-MeTHF as a function of temperature. Excitation photon energy
for fluorescence: 2.36 eV.
6
9,70
Temperature-dependent fluorescence studies on π-conjugated
oligomers have indicated that solidification of the solvent can
cause discontinuous changes through modification of the
solute−solvent interaction. In our case, a discontinuous
change seems to occur near the glass transition of 2-MeTHF.
We note that the integrated intensity of the fluorescence
upon excitation with 2.36 eV photons decreases upon cooling
from 220 to 170 K and then increases sharply below 170 K
D1 using the MM2 force field (Figure 6)
indicate that low-
energy conformations exist with the alkane linker in the trans
conformation and DPP chromophoric groups oriented
obliquely. The oblique orientation may be favorable in the
ground state because it minimizes the quadrupolar electrostatic
interactions between the DPP units. The importance of the
extended conformation of D1 shown in Figure 6 can be further
supported by considering the relative orientation of the
transition dipole moments associated with the allowed
transition between the S and S states. For the DPP units
68
(Figure 4, left panel). Because the absorbance at 2.36 eV hardly
changes with temperature (Figure 4, right panel), we attribute
the rise in fluorescence intensity at liquid nitrogen temperature
to an increase of the quantum yield for fluorescence.
0
1
under study, the transition dipole is oriented parallel to the line
7
1
joining the α-protons on the thiophene rings. In the extended
conformation proposed, the transition dipoles of the DPP units
make an angle close to 90° with each other and are also
approximately perpendicular to the line joining the centers of
the DPP moieties. In such an orientation, the transition
dipole−dipole interaction between the DPP units is minimal,
resulting in a small Davydov splitting in the excited state. This
may then explain why the absorption and fluorescence spectra
of D1 differ so little from those of the monomer M0. For D2,
which has an even number of saturated carbon atoms in the
linker, the mutual orientation of the DPP moieties in extended
conformation will differ from those shown in the left panel of
Figure 6. Because of the larger distance possible between the
DPP units, the absence of experimental indications for
electronic coupling between the DPP moieties in the ground
state is less surprising.
The variable-temperature absorption and fluorescence
measurements confirm that in the dimer, the initially populated
excited state and the fluorescent state are very similar to the S1
excited state in the monomer and involve an extended
conformation of the dimer. Variable-temperature excited-state
lifetime measurements reveal differences in the nonradiative
decay pathways of dimer and monomer. While the fluorescence
lifetime of the monomer shows only a modest increase upon
going to liquid nitrogen temperatures (see Table 1), the
excited-state lifetime for D1 increases 3-fold upon lowering the
temperature of the solution. At 80 K, the fluorescence lifetime
for the dimer D1 becomes almost identical to the lifetime of the
M0 (Figure 5). The convergence of the lifetime of the locally
excited state in D1 toward the excited-state lifetime of M0
shows that the dynamic quenching process is inhibited at low
temperature.
The quenching of the fluorescence we ascribe to formation of
7
2
To explain the spectroscopic data, we propose that in
solution the dimers adopt predominantly an extended
conformation. Molecular mechanics energy minimizations on
a nonemissive, excimer-like excited state with a folded
conformation. In the right panel of Figure 6, a folded
conformation of D1 is illustrated. In this conformation, the
linker can retain its staggered low-energy conformation. π-
Stacking between the DPP groups is possible, and the
diketopyrrolopyrrole core of one DPP moiety is in close
contact with the thiophene ring of the other. In the excited
state, this relative orientation of the DPPs can be stabilized by
charge-transfer interaction. Temperature-dependent absorption
and fluorescence measurements indicate that, in the electronic
ground state, the folded conformation is not populated at any
temperature. This shows that the energy of the folded
conformation is higher than the energy of the extended
structure. The excess energy provided by the photoexcitation
makes the folded conformation energetically accessible after
photoexcitation. Yet, at low temperature, formation of the
folded state is kinetically impaired.
Figure 5. Fluorescence decay curves of D1 at the different
temperatures and M0 at 80 K (gray line, offset by multiplication by
a factor of 2) in dilute 2-MeTHF solutions using Eexct = 3.06 eV and
Eem = 1.94 eV.
Electrochemistry. In order to estimate the energetic
position of the charge-transfer states in the dimers, the
E
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Figure 6. Low-energy extended (left) and folded (right) conformations for D1 obtained from molecular mechanics minimizations using the MM2
force field. For each conformer, three views of the molecule in the x, y, and z directions are shown. The blue double-headed arrows in the schematic
representations indicate the transition dipoles associated with the transition from the ground state (S ) to the lowest excited singlet state (S ). For
0
1
simplicity, the terminal hexyl side chains have been replaced by methyl groups.
electrochemical properties of the dimers D1−D3 and the
reference compound M0 were studied by cyclic voltamperom-
etry (CV) (Figure 7). For the monomer M0 and the dimers
is induced in the dimers, which we attribute to dimerization of
73
the DPP moieties. This reaction can be suppressed by
limiting the scan range to −1.5 V. Using oxidation and
reduction potentials from the CV measurements, we calculate
the energy of the solvated, charge-separated state (CSS), that is,
+
−
74,75
DPP-DPP , using the Weller equation (eq 3).
qe2
ECT = q (E (D) − E (A)) −
ox
red
e
4
πε ε R
0
s cc
qe2
⎛
⎞
⎛
⎜
1
r
1 ⎞ 1
1
ε ⎠
− 8
+
⎟
⎠
⎜
−
⎟
+
−
⎝
πε0
r
⎝ε
ref
s
(3)
In this equation, E (D) and E (A) are the redox potentials
ox
red
of the donor and the acceptor, respectively. R is the center-to-
cc
center distance between the positive and negative charges in the
+
−
CSS. r and r are the radii of the positive and negative ions,
respectively. εref and ε are the relative permittivities of the
s
reference solvent used during CV measurements (in this case o-
DCB) and the solvent for which the energy of the solvated
charge-transfer states is being calculated (in this case toluene, 2-
MeTHF, o-DCB, benzonitrile), respectively. Because the time
scale for excimer formation is long compared to the dielectric
relaxation time of the solvents, the use of the static dielectric
7
6−78
constant seems justified.
For the calculation of the radii of
+
−
the positive (r ) and negative ions (r ), the density of ρ = 1.36
g/cm was used from the reported crystallographic data of 2,5-
diallyl-3,6-diphenylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione.
The radii r and r (4.75 Å) were calculated using eq 4:
3
7
9
Figure 7. Cyclic voltammograms of M0 and D1, D2, and D3 in o-
DCB measured at room temperature (298 K) under an N2
atmosphere. Potentials relative to Ag/AgCl calibrated against Fc/Fc .
+
−
+
2
4
3
3
× 10 M
+
−
r = r =
4
πρNA
(4)
D1−D3, CV was performed in o-dichlorobenzene (o-DCB)
solutions containing tetra-n-butylammonium hexafluorophos-
In this formula, M is the molecular weight of the model
phate (TBAPF , 0.1 M) at room temperature under a nitrogen
compound, ρ the density, and N Avogadro’s number. It is
known that electron transfer is favored in more polar solvents
because the solvent can stabilize the CSS.
In polar solvents, the intramolecular electron transfer from
one DPP unit to the other DPP unit of the same dimer
molecule can occur, resulting in positive and negative ions. The
energies of the CSSs of the dimers were estimated for solvents
of different polarity using Weller’s equation and the redox
potentials determined by CV measurements in dilute o-DCB
solutions (Figure 7). For the extended conformation of the
6
A
atmosphere (Figure 7). The corresponding redox potentials are
summarized in Table 1. All compounds show reversible
oxidation at potentials slightly below 1 V versus Ag/AgCl.
For molecules D1 and D2, the oxidation wave is broadened,
and this can be ascribed to the influence of the first oxidized
DPP moiety on the oxidation potential of the second DPP
moiety. All molecules also show reversible reduction waves near
−
1.3 V. Incidentally, we note that upon scanning to voltages
more negative than 1.5 V, a reversible electrochemical reaction
F
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The Journal of Physical Chemistry A
Article
Figure 8. (left) Schematic energy levels of M0, D1, D2, and D3 in different organic solvents of different polarity calculated according to the Weller
equation (eq 3) using Rcc = 11.25, 12.50, and 13.75 Å for D1, D2, and D3, respectively, corresponding to the fully extended conformation of the
molecules. (right) Energy of the CSS as a function the reaction coordinate for folding. For the folded dimer, Rcc = 5 Å was used to calculate the
energy.
dimers in toluene, the energies of the CSSs (■) are higher than
Photoinduced Absorption. Finally we discuss the fate of
the folded charge-transfer state of the dimers. Using near
steady-state photoinduced absorption spectroscopy (PIA), the
nature of long-lived excited states can be probed. In Figure 9,
those of the emissive S states for all dimers (Figure 8). For
1
solvents with higher polarity, for example, 2-MeTHF, o-DCB,
and benzonitrile, the energy of the CSS is lower than that of the
S1 state.
The energy calculated for the folded charge-transfer (CT)
state with Rcc = 5 Å is the same for all three dimers and is
almost independent of solvent polarity for this particular value
of R . The predicted energy of the CSS in folded conformation
cc
(
or CT state, 1.9 eV) is lower than the energy of the S state. In
1
the right panel of Figure 8, the evolution of the energy of the
CSS during the folding reaction is depicted. In toluene,
population of the extended CSS by long-range electron transfer
80
is energetically unfavorable. For the polar solvents (o-DCB
and benzonitrile), the extended CSS is thermodynamically
accessible. For D1 with the short alkyl spacer, long-range
electron transfer may be expected to be relatively fast so that
efficient formation of the folded conformation with the
extended CSS as an intermediate may be expected in a polar
solvent. In toluene, formation of the excimer-like state could in
principle occur via a different pathway, namely folding of the
Figure 9. Photoinduced absorption spectra for D1 recorded in dilute
toluene, o-DCB, and benzonitrile solutions at room temperature upon
excitation with 2.54 eV photons. The intensities have been corrected
for small differences in the optical density of the solutions at the
excitation wavelength.
dimer in the locally excited S state followed by charge transfer
1
occurring after the cofacial stacking of the DPP moieties. This
pathway requires that the time needed for folding in the
electrostatically neutral, locally excited state is shorter than or
comparable to the excited-state lifetime of the DPP unit (5.5 ns,
Table 1). For 1,3-bis(2-pyrenyl)propane, the rate of folding in
we show the PIA spectrum for D1 in various solvents at room
temperature induced by excitation with photon energy of 2.54
eV via the S ← S transition. The spectrum obtained shows
1
0
bands at 1.43 and 1.6 eV, which we interpret as the
−1
81
excimer formation is 0.13 ns in methylcyclohexane. Given
the fact that the DPP chromophore including its N-hexyl
substituent (383 Da) is considerably larger than the pyrenyl
substituent (201 Da), we argue that the rate of folding in the
photoinduced T ← T absorption of the DPP moiety under
n
1
83
study. This indicates that in polar solvents efficient
intersystem crossing (ISC) to the lowest triplet state T occurs.
1
The energy of T can be estimated to be 1.1 eV based on triplet
1
locally excited S state of the DPP dimers is lower than the
1
energy data for the structurally closely related 2,5-bis(2-
decay rate of the excited state, so excimer formation via folding
in the charge neutral, locally excited state is kinetically
disfavored. Folding in the CSS is driven by the electrostatic
attraction between oxidized and reduced DPP subunits and can
therefore be much faster than the diffusive folding in the
neutral, locally excited state. We note that the dynamics in the
dimeric compounds with a flexible linker could be interesting
for benchmarking the performance of modern DFT-based
ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-
83
(
2H,5H)-dione. This energy is considerably lower than the
energies estimated for the charge-separated and charge-transfer
84
states for the dimers (see Figure 8). We propose that in a
polar solvent efficient ISC occurs in the folded charge-transfer
state. Efficient ISC is known to occur in donor−acceptor dyads
with a flexible linker via the intramolecular charge-transfer
5
4,55
state.
In charge-transfer and charge-separated states, which
82
computational methods for the excited state.
have a radical pair character, various mechanisms contribute to
G
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The Journal of Physical Chemistry A
Article
the fast intersystem crossing, including hyperfine interactions
between the electron and nuclear spin degrees of freedom and
spin−orbit coupling in the electron-transfer steps leading to
and from the charge-transfer state. In toluene, where the
charge-transfer state does not form, the radical pair mechanisms
for ISC are not active; hence, the quantum yield for triplet
formation is expected to be much lower. In addition, in systems
with pronounced charge-transfer character, the energy gap
ACKNOWLEDGMENTS
■
This research has been supported by The Netherlands
Organization for Scientific Research (NWO) through a grant
in the VIDI scheme.
85
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3
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J
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