Closely stacked oligo(phenylene ethynylene)s: Effect of π-stacking on the electronic properties of conjugated chromophores

Article abstract of DOI:10.1021/ja3019065

In this work, a bicyclo[4.4.1]undecane scaffold is used to hold oligo(phenylene ethynylene) units in a cofacially stacked arrangement along the entire length of the conjugated units. We study the impact that the resulting strong interchain interactions ha

Full text of DOI:10.1021/ja3019065

Article
pubs.acs.org/JACS
Closely Stacked Oligo(phenylene ethynylene)s: Effect of π-Stacking
on the Electronic Properties of Conjugated Chromophores
Subodh P. Jagtap, Sukrit Mukhopadhyay, Veaceslav Coropceanu, Glen L. Brizius, Jean-Luc Bredas,
́
and David M. Collard*
School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology Atlanta,
Georgia 30332-0400, United States
S
* Supporting Information
ABSTRACT: In this work, a bicyclo[4.4.1]undecane scaffold
is used to hold oligo(phenylene ethynylene) units in a
cofacially stacked arrangement along the entire length of the
conjugated units. We study the impact that the resulting strong
interchain interactions have on the photophysical properties.
The length of the individual oligomer branches was varied from three to five rings to investigate the effect of conjugation on the
electronic properties of the stacked segments. Absorption and fluorescence spectra were recorded and compared to those of the
corresponding unstacked analogues. Time-dependent density functional theory calculations were carried out and helped to
rationalize the low-energy features present in the fluorescence spectra of the stacked systems. The calculations indicate that the
low-energy emissions are due to the presence of excimer-like states. The stronger intensity of the low-energy fluorescence band
observed in the five-ring stacked system compared to the three-ring analogue is attributed to the smaller activation barrier that
separates the local intrachain state and the excimer-like state in the former compound.
phores. This is ascribed to absorption by a single chromophoric
moiety, followed by relaxation to a lower energy “phane” state
that results from through-space interactions between the
stacked π-systems within the cyclophane core. However, in
these instances only the terminal phenyl rings of each
conjugated moieties are held in a stacked arrangement (i.e.,
those of the cyclophane core). This situation is in contrast to
the close-packed interaction along the entire length of
conjugated oligomers or between extended segments of
conjugated polymers, as is the case in thin films of many
organic semiconductors.33
In a recent study of the effect of stacking of conjugated
oligomers on the formation of cationic species as models for
bipolaron-like charge carriers, we used the bicyclo[4.4.1]-
undecane scaffold to hold pairs of terthiophene or pentathio-
phene units in a stacked arrangement, e.g., st-[Th3]2 in Figure
1.26 Here, we append linear oligo(phenylene ethynylene) units
to this scaffold to form compounds with a well-defined π-
stacked arrangement and use these to explore the effect of
stacking on the photophysical properties of conjugated systems.
The use of this scaffold results in the possibility of strong
interactions along the entire length of the conjugated segments.
As an initial step toward a better understanding of the
properties of the stacked compounds and of the impact of such
extended interchain interactions on the photophysics, we
describe here simple absorption and fluorescence measure-
ments complemented by detailed quantum-mechanical calcu-
lations.
INTRODUCTION
■
It is well established that π-conjugated organic molecules and
polymers present a strong relationship between their geometric
and electronic structures.1−6 This relationship manifests clearly
when it comes to a description of the optoelectronic
properties,7 since an electronic excitation can lead to significant
modifications of the intra- and intermolecular geometries8,9 that
in turn modify the electronic structure. Thus, the design of new
materials with optimal properties for organic electronics
applications such as organic field-effect transistors,10−12 light-
emitting diodes,13−15 or solar cells16−18 requires a fundamental
understanding of this relationship. In this respect, model
compounds with well-defined geometries containing only a few
interacting conjugated segments prove to be very useful, since
they allow for a comprehensive study that combines
spectroscopic techniques with high-level theoretical methods.
Various scaffolds have been used in order to hold pairs of
conjugated units atop one another. These include, for instance,
[2,2]paracyclophane,19−22 calix[4]arene,23 arene-annelated
bicyclo[4.4.1]undecane,24−27 4,5-disubstituted xanthene,28,29
m-terphenyl oxacyclophane,30 and macrocyclic oligothio-
phenes.31 Stacked assemblies of conjugated systems appended
to these scaffolds allow for the detailed exploration of the effect
of π−π interactions on the optoelectronic properties of π-
conjugated chains. For example, Bazan and co-workers have
explored the through-space interactions between pairs of
stilbenes32 or phenylene ethynylenes19 by using a pseudopara
disubstituted [2.2]paracyclophane scaffold to hold the
chromophores in a stacked arrangement, e.g., CP[PE3]2 in
Figure 1.19 Such stacked compounds display considerably larger
Stokes shifts than the analogous linear unstacked chromo-
Received: February 27, 2012
Published: March 28, 2012
© 2012 American Chemical Society
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coupling of 2 and phenylene ethynylene arms (see the
Supporting Information) followed by ketalization afforded the
stacked analogs st-[PE3]2 and st-[PE5]2; ketalization of 1
affords 3 (st-[PE1]2). Ketalization forces the two seven-
membered rings of the bicyclo[4.4.1]undecane scaffold into a
pseudo chair/pseudo chair conformation, thus stacking the
fused oligomers atop one another. Unstacked analogues of the
covalently stacked conjugated oligomers were prepared by
analogous Sonogashira coupling of phenylene ethynylene arms
to commercially available 2,3,5,6-tetramethyl-1,4-diiodoben-
zene.
1
NMR Spectral Analysis. The H NMR spectrum of 3 (st-
[PE1]2) confirms that the seven-membered rings of the
bicyclo[4.4.1]undecane core are in a pseudo chair conformation
such that the fused benzene rings are stacked atop one another.
The diastereotopic hydrogen atoms of the benzylic methylene
groups of st-[PE1]2 give rise to a pair of doublets of doublets at
δ 3.34 (Jgem = 15 Hz, Jvic = 2 Hz) and δ 2.58 (Jgem = 15, Jvic = 6
Hz) for the equatorial and axial positions, respectively. The
stacked nature of the arenes causes an upfield shift of the singlet
for the aromatic hydrogen atoms of 3 (δ 6.39) relative the
corresponding singlet of the unstacked linear analog 1 (δ 6.85).
Accordingly, the methyl groups on the bicycloundecane core do
not interfere with the stacking noted by Mataka for the
unsubstituted benzo-fused bicycle.24
Figure 1. Stacked conjugated compounds: Pseudopara [2.2]-
paracyclophane stacked oligo(phenylene ethynylene) CP[PE3]2 (R =
n-C12H25), in which only the terminal phenylene unit of the
conjugated oligomers are held in a stacked arrangement (ref 7);
dithieno-fused bicyclo[4.4.1]undecane stacked terthiophene, st-[Th3]2
(ref 10); and benzo-annelated bicyclo[4.4.1]undecane based stacked
oligo(phenylene ethynylene), st-[PE3]2.
The phenylene ethynylene arms of st-[PE3]2 and st-[PE5]2
do not impede the adoption of a stacked conformation by the
1
bicyclic scaffold. The H NMR spectrum of ketone [PE3]2
RESULTS AND DISCUSSION
exhibits a series of broad peaks at 2.8−3.5 ppm due to the time-
averaged signal of the benzylic protons in the rapidly
interconverting pseudo chair/chair, chair/boat, and boat/boat
conformations of the bicyclic core. Ketalization of ketone
[PE3]2 provides the stacked trimer st-[PE3]2, which exhibits of
a pair of doublets of doublets at δ 3.79 (J = 15, 6 Hz) and δ
3.30 (J = 15, 2 Hz) for the diastereotopic benzylic hydrogen
atoms. As with the single ring analog 3, there is an upfield shift
in the signals corresponding to the aromatic protons from δ
7.20−7.70 for the linear analog PE3 to δ 7.10−7.40 for the
stacked ketal st-[PE3]2. A similar set of peaks is observed for
the benzylic and aromatic protons of the analogous stacked
■
Synthesis of Stacked and Unstacked Model Com-
pounds. Tetramethyl-substituted benzo-annelated
bicyclo[4.4.1]undecanone 1 was prepared by treating dimethyl
1,3-acetonedicarboxylate with 1,2-bis(bromomethyl)-4,5-dime-
thylbenzene followed by saponification and decarboxylation
according to the methods described by Mataka, Figure 2.24 We
incorporated the four methyl groups on the fused arenes so as
to direct the subsequent placement of the conjugated arms to
the ortho positions and thereby provide the linear conjugated
oligomeric segments. Ketone 1 was iodinated (I2, Hg(OTf)2)
to provide the tetraiodo compound 2. Sonogashira cross-
Figure 2. Synthesis of stacked compounds 3 (st-[PE1]2), st-[PE3]2, and st-[PE5]2 and of the simple linear analogs PE3 and PE5.
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pentamer, st-[PE5]2: The benzylic hydrogen atoms give rise to
a doublets of doublets at δ 3.84 (J = 15, 6 Hz) and δ 3.30 (J =
15, 2 Hz), and the multiplet for the hydrogen atoms on the
phenylene ethynylene arms is shifted slightly upfield compared
to the unstacked analog, PE5 (see the Supporting Information),
suggesting that it also adopts a stacked architecture in solution.
X-ray Crystal Structures. The X-ray crystal structure of
ketal st-[PE1]2 confirms that the bicycloundecanone core
adopts a pseudo chair/chair conformation with a cofacially
stacked arrangement of the fused benzene rings. The distance
between the centers of the stacked benzene rings (d1) is 3.42 Å,
Figure 3 (top) and Table 1. The benzene rings are tilted at an
angle (θ1) of 16.8° with respect to one another: The inner pairs
of aromatic carbon atoms (i.e., those fused to the bicyclic
structure) are 3.03 Å apart, whereas the outer pairs (i.e., those
bearing methyl substituents) are held at a distance of 3.74 Å.
Thus, this conformation is similar to that reported by Mataka
for the compound lacking the methyl substituents (d1 = 3.56 Å;
θ1 = 25°).
Similarly, the X-ray crystal structure of st-[PE3]2 confirms a
pseudo chair/chair conformation for the bicyclic scaffold in the
presence of the phenylene ethynylene arms, Figure 3 (middle
and bottom). The central rings of the stacked conjugated trimer
moieties are separated by an intercentroid distance (d1) of 3.42
Å, with a tilt angle (θ1) of 17.9° between them. The dihedral
angles between the bridgehead and benzylic hydrogens inferred
from the crystallographic data are consistent with the coupling
1
constants obtained from H NMR analysis of solutions of the
ketals: The 6 Hz coupling constant between the bridgehead and
equatorial benzylic hydrogen atom is consistent with dihedral
angle of 52° using the Karplus equation,34 and the 47° dihedral
angle between the bridgehead and axial position results in a 2
Hz coupling. Thus, comparison of the conformation of the
cores of st-[PE1]2 and st-[PE3]2 indicates that the incorpo-
ration of the four arms does not cause repulsion between the
conjugated moieties, and that the stacked chair/chair
conformation observed in the solid state is preserved in
solution.
The X-ray crystal structure of st-[PE3]2 also indicates that the
peripheral rings are in a stacked arrangement, although there
occur slight distortions from planarity and linearity in the
stacked trimeric segments. The intercentroid distances between
pairs of rings on the periphery of the molecule are 3.84 Å (d2)
and 4.01 Å (d3), Figure 3 (middle). The larger intercentroid
distance between these pairs (referred to hereafter as “stack 2”
and “stack 3”) compared to the central pair (“stack 1”,
separated by d1) results from a small in-plane bending of one
conjugated segment toward, and the other away from, the
bicyclic scaffold, Figure 3 (bottom). This results in an offset
between the pairs of peripheral rings that we characterize by a
slip distance (ds), Figure 3 (bottom). The peripheral rings in st-
[PE3]2 are tilted out of the plane of the central aromatic rings
by an average of only 5°, thereby retaining conjugation along
the entire length of each segment in the stacked arrangement.
Being unconstrained by fusion to the scaffold itself, the pairs of
peripheral rings adopt a more coplanar arrangement (θ2 = 1.7°;
Figure 3. X-ray crystal structure of st-[PE1]2 (side view), and st-[PE3]2
(front and top views); di represents the intercentroid distance, θi the
interplane angle between the stacked phenyl rings, and ds the slip
distance between the stacked rings.
Table 1. Comparison of Selected Structural Parameters (Distances, di, and Angles, θi) of the Stacked Compounds st-[PEx]2 (x =
1, 3, 5) from X-ray Diffraction and DFT Optimization of Ground-State (gs) and Excited-State (ex) Geometries
intercentroid distance, di (Å)
interplane angle, θi (°)
geometry
d1
d2
d3
d4
d5
θ1
θ2
θ3
θ4
θ5
X-ray Crystallography
st-[PE1]2
st-[PE3]2
3.42
3.42
16.8
17.9
3.81
4.00
1.7
11.9
ωB97X-D/6-31g*
st-[PE3]2
gs
3.39
3.39
3.15
3.76
3.78
3.54
3.76
3.78
3.54
15.5
12.8
10.8
1.4
1.6
4.9
1.5
1.7
4.6
gs (CHCl3)
ex
st-[PE5]2
gs
3.39
3.40
3.17
3.69
3.68
3.47
3.69
3.68
3.47
3.79
3.82
3.71
3.80
3.81
3.71
13.6
12.8
11.0
0.6
0.7
0.2
1.4
1.6
1.4
2.0
1.2
1.3
2.2
1.8
1.8
gs (CHCl3)
ex
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θ3 = 11.9°) than the central rings (θ1 = 17.9°). We ascribe the
small differences in the torsion angle between the pairs of
phenylene ethynylenes on each side of the scaffold to crystal
packing forces since these differences are absent in our DFT-
optimized geometries. We note that the optimized geometries
lead to θ1 and θ2 values between 1°−2°. The stacked pentamer,
st-[PE5]2, does not provide diffraction-quality crystals to allow a
similar crystallographic analysis.
DFT-Optimized Ground-State Geometries. The opti-
mized geometries of the ground state of st-[PE3]2 and of st-
[PE5]2 were obtained using the ωB97XD functional with the 6-
31 g* basis set. We considered both the isolated molecules and
the molecules embedded in a dielectric continuum taking
implicit account of the solvent (here, CHCl3 that corresponds
to ε = 4.7). The calculations reveal that the solvent has only a
marginal effect on the optimized ground-state geometry in both
systems (Table 1). The DFT-derived geometry of st-[PE3]2
compares well with that obtained from the crystal structure,
Table 1. In particular, the calculations reproduce well the slight
nonlinearity and twisting of the two conjugated tiers. The slip
displacement of the peripheral benzene rings apparent in the
top view of the calculated geometry (Figure 4, top) closely
matches that observed in the X-ray crystal structure, Figure 3
(bottom). In the optimized geometry of the ground state, the
slip distance within both of the peripheral stacks (2 and 3) is
1.65 Å; in the X-ray crystal structure, the slip distance between
the benzene rings in stack 2 and stack 3 of st-[PE3]2 is ca. 1.55
Å.
For the stacked pentamer st-[PE5]2, the intercentroid
distances, torsional angles, and slip distances between the
inner pairs of stacked rings (stacks 2 and 3, ds = 1.60 Å) are
comparable to those in the stacked trimer, st-[PE3]2. The slip
distances between the pairs of external rings of st-[PE5]2
(stacks 4 and 5) are only slightly larger (1.70 Å) than for the
inner pairs. Thus, this result reinforces the contention that the
conjugated pentamer units are held in close proximity over
their entire length.
UV−Visible and Fluorescence Spectroscopies. The
stacked oligomers and unstacked linear analogues were
characterized by UV−vis and fluorescence spectroscopies to
explore the effect of stacking on the electronic structure of the
conjugated moieties. The absorption maximum of the stacked
trimer, st-[PE3]2, (330 nm) is blue-shifted by 60 meV relative
to that of the unstacked linear analog PE3 (325 nm). As seen
from Figure 5, the absorption edge of st-[PE3]2 contains a low-
energy tail, which is absent in the unstacked analog. A priori,
such a tail could be due to either a weak optically allowed
electronic state that contributes to the absorption in the stacked
trimer or to broadening of the monomeric state due to coupling
between the two moieties. The stacked compound shows
Figure 4. Sketch of the DFT-optimized geometries of the isolated st-
[PE3]2 and st-[PE5]2 molecules in the ground state and in the lowest
excited state; for the sake of clarity, the hydrogen atoms are not shown.
The innermost pair of stacked rings is marked as stack 1; stacks 2 and
3 are next in both st-[PE3]2 and st-[PE5]2; the latter compound has
additional stacks (4 and 5) at the periphery of the molecule.
Figure 5. UV−vis and fluorescence spectra: A, st-[PE3]2 (solid) (c =
1.4 × 10−6 M) and PE3 (dotted) (c = 2.9 × 10−6 M) in CHCl3, T = 23
°C; the inset depicts the emission spectra of stacked trimer on an
energy scale. B, st-[PE5]2 (solid) (c = 0.9 × 10−6 M) and PE5 (dotted)
(c = 1.8 × 10−6 M) in CHCl3, T = 23 °C; the inset depicts the
emission spectra of st-[PE5]2 on an energy scale.
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emission maxima at 382 and 397 nm, which are red-shifted
from the maxima displayed by the unstacked analog (360 and
375 nm), Figure 5 and Table 2. In addition, the stacked system
Table 3. DFT Estimates of the Energies in the Ground- and
Excited-State Geometries, Oscillator Strength (o.s.), and
Relaxation Energy λ
energy
Table 2. Absorption and Emission Maxima of Stacked and
Unstacked PE Oligomers
molecule
state
ground state (eV) [o.s.] excited state (eV) λ (eV)
PE3
S1
4.02 [1.72]
3.70 [0.01]
4.06 [2.61]
3.61 [3.76]
3.41 [0.01]
3.66 [6.71]
3.53
2.61
0.15
0.56
absorption (nm35
330
)
emission (nm35
)
st-[PE3]2
S1(E)
S2(L)
S1
a
[PE3]
360, 375
c
c
[3.76]
[3.44, 3.31]
PE5
3.21
2.52
0.06
0.42
a
b
st-[PE3]2
[PE5]
325
382, 397 , 490
st-[PE5]2
S1(E)
S2(L)
c
c
[3.81]
[3.30, 3.13, 2.52]
a
365
399, 418
c
c
[3.40]
[3.11, 2.97]
calculations on isolated molecules (see the Supporting
Information). The relaxation energy (λ) of the S1 state of
PE5 is estimated to be substantially smaller than that of PE3
(0.06 vs 0.15 eV), which is consistent with a more delocalized
nature of the excited state in the more extended system. This
result is in good agreement with the observed emission spectra:
with the increase in oligomer length from the trimer to the
pentamer, there occurs a decrease in the ratio of the 0−1 and
0−0 vibronic peaks that are well-resolved in the emission
spectra of both oligomers (Figure 5).
st-[PE5]2
355
398 (weak), 495 (br)
c
c
[3.49]
[3.09, 2.40]
a
b
c
Vibronic band. Shoulder. Energies for transitions are computed
from deconvoluted spectra.
shows a weak emission that appears as a broad shoulder in the
low-energy part of the spectrum at approximately 490 nm (∼
2.5 eV, see inset of Figure 5A).
More pronounced differences are observed between the
spectra of the pentameric homologues, st-[PE5]2 and PE5. The
absorption and emission maxima of both stacked and unstacked
pentamers are red-shifted from those of the corresponding
trimers, as expected from the greater extent of conjugation. The
absorption maximum of the stacked system is again slightly
blue-shifted (∼90 meV) relative to the unstacked oligomer
(355 nm versus 365 nm, Table 2). As in the case of stacked
trimer, there is a tail at the low-energy edge in the absorption
spectrum of st-[PE5]2. The most interesting feature, however, is
that the emission spectrum of the stacked pentamer, Figure 5B,
is significantly different from that of the stacked trimer. The
spectrum of the stacked pentamer is dominated by a broad
transition at low-energy with a maximum at ca. 495 nm. A low-
intensity high-energy band is observed at 398 nm (∼3.1 eV).
This matches the emission maximum of the unstacked linear
analog PE5.
The TD-DFT calculations reveal that the first-excited state of
both st-[PE3]2 and st-[PE5]2 is dominated by HOMO →
LUMO and HOMO-1 → LUMO+1 excitations, i.e., ψ(S1) = a|
H → L⟩ + b|H − 1 → L + 1⟩. Furthermore, by performing a
transformation to the oligomer MO basis set (see the
Supporting Information for details), ψ(S1) can be shown to
correspond to a linear combination of two oligomer-localized |
AB*⟩ and |A*B⟩ excitations and two interoligomer charge-
transfer (CT) |A+B−⟩ and |A−B+⟩ excitations:36
ψ(S1) = a|H → L⟩ + b|H − 1 → L + 1⟩
⎛
⎞
⎛
⎞
⎟
a + b
a − b
2
⎜
⎟
⎜
=
(|A*B⟩ − |AB*⟩) +
⎝
⎠
⎝
⎠
2
(|A−B+⟩ − |A+B−⟩)
(1)
TD-DFT calculations carried out on the ground-state
geometry indicate that the S1 state of both systems possesses
a significant CT character, ca. 15% and 10% for st-[PE3]2 and
st-[PE5]2, respectively. Importantly, the CT contribution
increases as the stacked systems relax to the equilibrium
geometry of the S1 state. In this case, we find that the intrachain
and CT excitations contribute approximately equally to ψ(S1).
We note that similar results are obtained at the semiempirical
intermediate neglect of differential overlap (INDO) level of
theory (see the Supporting Information). Based on these
findings, we ascribe the S1 states of the stacked systems to
excimer-like states (E). As a result of their CT character, the S1
states undergo large structural relaxations upon excitation.
Thus, in the fully relaxed S1 geometry of the stacked systems,
the intercentroid distance (di) between the individual oligomer
arms is reduced by ca. 0.20 Å compared to the ground-state
geometry. This is accompanied by a decrease in the angle
between the innermost benzene rings (θ1) of 3−5° (Table 1).
Such significant changes in geometry lead to relaxation energies
as large as 0.5 eV. These results are consistent with the fact that
low-energy emission bands are broad and structureless.
Analysis of the emission spectra points to the fact that at least
two electronic states appear to contribute to the fluorescence of
both stacked systems. In order to obtain a firm assessment of
the nature of these excited states, we turn next to a discussion
of the results of TD-DFT calculations of the excited states of
the unstacked and stacked systems.
TD-DFT Characterization of the Excited-State Proper-
ties. To gain further insight into the effect of π-stacking on the
photophysical properties of conjugated oligomers, we have
optimized the geometries of the lowest excited states of the
stacked systems and the unstacked model oligomers by means
of TD-DFT calculations. The calculated excited-state energies
are listed in Table 3. The TD-DFT S0 → S1 transition energies
of the unstacked oligomers are somewhat overestimated (∼0.2
eV) compared to the experimental absorption maxima. In
agreement with experiment, a ∼0.3−0.4 eV decrease in S1
energy is calculated for PE5 compared to PE3. In both
unstacked oligomers, the S1 state can be described essentially by
a single one-electron excitation from the highest occupied
molecular orbital (HOMO) to the lowest unoccupied
molecular orbital (LUMO). The implicit inclusion of the
solvent (geometry optimizations followed by the excited-state
calculations using TD-DFT) predicts a ca. 0.1 eV decrease in
optical gaps, but the trends remain the same as for the
In addition, the TD-DFT calculations reveal that the S2 states
in both st-[PE3]2 and st-[PE5]2 can be described as a linear
combination of the S1 states of the individual arms. The S2
states are thus excitonic in nature and can be assigned as local
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(L) states (a detailed description of the S2 state is provided in
the Supporting Information). The similarity between the
energies of the S2 states in the stacked systems and the
energies of the S1 states in the unstacked oligomers suggests
that upon stacking the states of a single conjugated system are
only weakly affected by interchain interactions.
The calculations also show that the difference between the
energies of the excimer-like state of st-[PE3]2 and st-[PE5]2 in
their relaxed geometries is very small, ca. 0.09 eV. These
theoretical results agree well with the fact that the low-energy
emission peaks of st-[PE3]2 and st-[PE5]2 appear at nearly the
same energy (∼2.4 eV, 490 nm). Inspection of the frontier
orbitals contributing to the excimer-like states indicates that in
the case of the stacked trimer, st-[PE3]2, the molecular orbitals
are delocalized over the entire length of the oligomer segment.
In contrast, for the stacked pentamer st-[PE5]2, the frontier
molecular orbitals are restricted to the central three-ring section
of the pentamer segments (Figure 6). This explains the
similarity between the E states of st-[PE3]2 and st-[PE5]2.
Figure 7. Schematic energy diagram showing the lowest local-excited
state (L), excimer-like state (E), and ground state (S0). The
absorption, emissions from the L (kr(L)) and E states (kr(E)), and
the forward (kr(L → E)) and backward (kr(E → L)) exchange
between the L and the E states are also depicted.
from the relevant excited states can be analyzed following
Zachariasse and co-workers by using the rate equation
described by eq 2.37 The ratio of emission intensities from
the E and L states (i.e., I(E)/I(L)) can then be written as
kr(E) kr(L → E)τ0(E)
kr(L) kr(E → L)τ0(E) + 1
I(E)
I(L)
=
(2)
where kr(E) and kr(L) are the radiative rate constants of the E
and L states, kr(L→E) and kr(E→L) are the forward and
backward reaction rates between the local and excimer-like
states, and τ0(E) is the fluorescence lifetime of the E state.
While a detailed investigation of these kinetic processes is
beyond the scope of this work, the main differences between
the emission patterns in st-[PE3]2 and st-[PE5]2 can be
analyzed on the basis of the spectroscopic and theoretical data
presented above. The TD-DFT calculations indicate that kr(E)/
kr(L) is of the order of 10−2 for both stacked systems. This
suggests that the difference between the emission intensities of
st-[PE3]2 and st-[PE5]2 is related to the second factor in eq 2
Figure 6. Frontier molecular orbitals of st-[PE3]2 (A and B) and st-
[PE5]2 (C and D) of the S1 state (in optimized geometries).
To summarize at this stage, the electronic-structure
calculations indicate that (i) the absorption band and high-
energy emission band in both st-[PE3]2 and st-[PE5]2 are
related to the second excited state, which corresponds to the
lowest excited state (L-state) of the unstacked oligomers, and
(ii) the low-energy broad emission band arises from the lowest
excited state, which is an excimer-like state; as a result of the
large geometry relaxation, this band is significantly red-shifted
in comparison to the lowest energy emission in the unstacked
oligomers.
The next point is to rationalize the differences in the overall
shapes of the emission spectra of st-[PE3]2 and st-[PE5]2. In
general, the emission features are expected to depend on the
relative populations and emission characteristics of the L and E
states. Here, in the absence of temperature- and time-resolved
measurements, we use a simple three-state model that is
outlined in Figure 7. In this framework, the emission intensities
kr(L → E)τ0(E)
= B(L → E)
kr(E → L)τ0(E) + 1
(3)
when kr(E→L)τ0(E) ≫ 1, one obtains B(L → E) = kr(L → E)/
kr(E → L) and parameter B is then proportional to the free
energy difference (driving force), ΔG0, related to the L → E
transition. This approximation turns out to work well for st-
[PE5]2. Indeed, using the experimental energies and the
calculated relaxation energies, we estimate for this system that
ΔG0 ≈ 0.40 eV and kr(L → E)/kr(E → L) ≈ 104. The ratio of
the forward and backward rates, along with the value of 10−2
derived for kr(E)/kr(L), is consistent with the much stronger
emission from the excimer-like state than from the local state
by approximately 2 orders of magnitude. The lower bound for
kr(L → E) in this case is given by the condition kr(L → E)τ0(E)
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Figure 8. Schematic representation of the photophysical behavior of benzo-fused bicyclo[4.4.1]undecane stacked oligo(phenylene ethynylene)s, st-
[PE3]2 (A) and st-[PE5]2 (B), and pseudopara [2.2]paracyclophane based stacked compound, CP[PE3]2 (C).
> 104. We note that in related cofacially stacked perylene-
diimide dimers, τ0 was found to be about 10−7 s.38 Assuming a
similar value for τ0 in the present systems leads to kr(L→E) >
1011 s−1 for st-[PE5]2. In the case of st-[PE3]2, we estimate ΔG0
≈ 0.24 eV and kr(L → E)/kr(E → L) ≈ 107. These results
suggest that for st-[PE3]2 the product kr(E → L)τ0(E) is less
than one and that the parameter B is kr(L → E)τ0(E).
According to eq 1 and recalling that kr(E)/kr(L) ≈ 10−2, the
product kr(L → E)τ0(E) should then be smaller than 100 in
order to reproduce a stronger emission from the local state than
from the excimer-like state, as is experimentally observed. This
result sets an upper bound for the forward reaction rate in st-
[PE3]2 with kr(L → E) < 109 s−1 . Based on the constraints that
this analysis places on the forward rate constant kr(L → E), the
activation barrier for the transition from the local excited state
to the excimer-like state in st-[PE3]2 is at least 0.1 eV larger
than in st-[PE5]2.
population from the L-state to the E-state, leading to a
significant red-shift in emission, see Figure 8B.
Interestingly, cyclophane-type stacked compounds, e.g.,
CP[PE3]2 in Figure 1, which provide for spatially limited
interchain interactions (i.e., only between the phenyl rings
within the cycolphane core), do not present contributions from
excimer-like states in their photophysics. Thus, it is important
to note that the design of the stacked molecules achieved in the
present work significantly modifies the low-energy photo-
physics compared to that of the pseudopara substituted stacked
cyclophane. In the latter case, the excimer-like state (“phane-
state”) remains higher in energy compared to the L-state,
particularly for longer oligomers. As a consequence, the
emission of CP[PE3]2 is dominated by the local L-state, with
no contribution from the E-state, as shown in Figure 8C, in
contrast to the emission from the longer bicycloundecanone-
stacked oligo(phenylene ethynylene).
Thus, this simple three-level model provides a reasonable
description of the difference in the emission spectra of st-
[PE3]2 and st-[PE5]2 by correlating the relative populations in
the local (L) and excimer-like (E) states. For both stacked
molecules, absorption leads to population of the L-state. This is
followed by a substantial geometric relaxation. By comparing
the experimental absorption and emission spectra of the
stacked and unstacked molecules, such a relaxation is found to
be on the order of 0.4 eV. The TD-DFT results are in good
agreement with the experimental observations. For st-[PE3]2,
the emission spectra is dominated by a radiative transition from
the L-state, as depicted in Figure 8A. In addition to this process,
a small fraction of the locally excited population decays into the
low-lying excimer-like state. However, a relatively high
activation barrier restricts the excited-state population such
that emission from the higher-energy L-state predominates.
Thus, the presence of the low-lying E-state results only in a
broad tail in the emission spectra, which is absent in the
corresponding unstacked analogue, PE3. In contrast, for st-
[PE5]2 the activation barrier appears to be lower than for st-
[PE3]2. As a consequence, there is a significant transfer of
CONCLUSIONS
■
A combined experimental and theoretical study of well-defined
stacked oligo(phenylene ethynylene)s provides very useful
insights into the effect of π−π interactions on the electronic
structure of closely packed conjugated chains. Our synthetic
strategy results in molecular architectures in which conjugated
units are stacked atop one another in such a way that strong
electronic interactions occur over the entire length of the
chromophoric moieties.
The absorption and emission spectra of the stacked
compounds were compared to those of the individual
unstacked oligomers. These optical studies, when analyzed
together with the results of TD-DFT calculations, clearly
demonstrate the effect of interchain interactions on the low-
energy photophysics of stacked compounds. A simple three-
state model provides an explanation for the large red shift
observed in st-[PE5]2 compared to its shorter analogue st-
[PE3]2. The origin of the shift lies in the competition between
emission from a local excited state and from an excimer-like
state.
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μmol), and PPh3 (21 mg, 80 μmol) in a 1:1 v/v mixture of DIPA and
THF (5 mL) was degassed using three cycles of freeze−pump−thaw
and back-purged with argon. Phenylacetylene (257 mg, 2.52 mmol)
was added dropwise and the mixture was heated at reflux for 48 h.
CH2Cl2 (50 mL) was added and the solution was washed sequentially
with saturated aqueous NH4Cl (50 mL) and H2O (50 mL). The
organic layer was dried over MgSO4 and the solvent was removed
under reduced pressure. The residue was dissolved in CH2Cl2 (5 mL)
and flushed through a silica gel column with CH2Cl2. The solvent was
removed under reduced pressure and the residue was recrystallized
from THF to afford the title compound as a yellow solid (150 mg,
Interestingly, cyclophane-type stacked compounds, which
provide for spatially limited interchain interactions, do not
present contributions from excimer-like states in their photo-
physics. Thus, stacked molecules such as st-[PE3]2 and st-
[PE5]2 serve as better-suited platforms to develop an
understanding of the interactions between cofacially stacked
conjugated chains, since they more closely resemble the
arrangement of the π-systems of semiconducting organic
oligomers and polymers in thin-film organic electronic devices.
1
49%). MP = 309−310 °C. H NMR (300 Hz, CDCl3): δ 7.20−7.80
EXPERIMENTAL SECTION
■
(m, 20H, Ar−H), 2.82−4.30 (br m, 10H, benzylic and bridgehead),
2.40−2.70 (br s, 12H, Ar−CH3). The low solubility of the product
precluded analysis by 13C NMR spectroscopy. IR (ATR): 2997, 2954,
2877, 1736, 1465, 1110, 943 cm−1. MS (MALDI), m/z (%) = 718.4
(M+, 80). HRMS (EI), m/z = Calcd. For C55H42O, 718.3235; Found,
718.3192, Δ = 5.9 ppm.
General Synthetic Methods. All starting materials were
purchased from commercial sources and used without further
purification unless there otherwise stated. THF was dried over sodium
benzophenone ketyl prior to distillation under argon. Thin-layer
chromatography (TLC) and column chromatography were performed
on flash grade silica (32−60 Å, Sorbent Technologies, Atlanta, GA).
NMR analysis was performed on a Bruker DSX 300 instrument using
CDCl3 as the solvent. Chemical shifts are referenced to internal
tetramethylsilane. IR analyses were performed on a Nicolet 4700 FTIR
with an ATIR attachment from Smart-Orbit Thermoelectronic
Corporation. Ultraviolet−visible analysis was performed on a
Shimadzu UV-2401PC spectrometer, and fluorescence spectroscopy
was performed on a Shimadzu RF-5301PC spectrofluorometer. Mass
spectra were determined on a VG-70SE instrument.
Stacked Trimer, st-[PE3]2. [PE3]2 (100 mg, 140 μmol) was
subjected to ketalization with ethylene glycol (200 mg, 3.23 mmol) in
the presence of p-toluenesufonic acid (1 mg) in benzene (50 mL)
according to the procedure described above for the synthesis of 3. The
product obtained from column chromatography was triturated with
hexanes to give the title compound as a yellow solid (90 mg, 85%).
1
MP = 295−296 °C. H NMR (300 MHz, CD2Cl2): δ 7.28−7.38 (m,
8H, Ar−H), 7.08−7.24 (m, 12H, Ar−H), 4.10 (s, 4H,
−OCH2CH2O−) 3.79 (dd, 4H, J = 16, 6 Hz, equatorial benzylic),
3.30 (dd, 4H, J = 15, 2 Hz, axial benzylic), 2.45−2.58 (m, 2H,
bridgehead), 2.38 (s, 12 H, Ar−CH3). 13C NMR (75 MHz, CDCl3): δ
218.1 (CO), 154.2, 152.1, 139.3, 138.6, 134.2, 133.2, 131.5
(aromatic), 71.1, 70.5 (−OCH2−), 52.2 (bridgehead), 39.4, 35.0
(benzylic), 19.2 (methyl). IR (ATR): 2988, 2938, 2877, 1608, 1502,
1105, 1077, 987 cm−1. MS (MALDI), m/z (%) = 762.3 (M+, 80).
HRMS (EI), m/z = Calcd. For C57H46O2, 762.3497; Found, 762.3339,
Δ = 20.8 ppm.
Ketone 1 was prepared using similar synthetic procedures to those
reported by Mataka24 for the preparation of analogous compound
lacking the four methyl groups (see the Supporting Information).
Synthetic procedures and characterization data for the compounds 2, 3
[PE3]2, st-[PE3]2, [PE5]2, and st-[PE5]2 are described below.
Ethylene Acetal of Tetramethyl Dibenzo[3,4-c:8,9-c']-
bicyclo[4.4.1]undecan-11-one, 3. A solution of 1 (100 mg, 310
μmol), ethylene glycol (200 mg, 3.23 mmol), and p-toluenesulfonic
acid (1 mg) in benzene (50 mL) was heated at reflux for 48 h with
removal of water via a Dean−Stark trap. The solvent was removed
under reduced pressure and the residue was subjected to column
chromatography (CH2Cl2) followed by recrystallization from hexanes
to give the title compound (95 mg, 84%) as a yellow crystalline solid.
MP = 217−218 °C. 1H NMR (300 MHz, CDCl3): δ 6.40 (s, 4H, Ar−
H), 4.05 (s, 4H, OCH2CH2O), 3.34 (dd, J = 15, 2 Hz, 4H, axial
benzylic), 2.58 (dd, J = 15, 6 Hz, 4H, equatorial benzylic), 2.24 (m,
2H, bridgehead), 1.98 (s, 12H, Ar−CH3). 13C NMR (75 MHz,
CDCl3): δ 135.4 (O−C−C-O), 133.8, 132.7, 127.3 (aromatic), 64.8
(−OCH2−), 45.8 (bridgehead), 31.1 (benzylic), 19.2 (ArCH3). IR
(ATR): 2997, 2930, 2877, 1439, 1383, 1110, 1040, 891 cm−1. MS
(MALDI), m/z (%) = 362.2 (M+, 70), 229.0 (100), 104.9 (75).
HRMS (EI), m/z = Calcd. For C25H30O2, 362.2246; Found, 362.2249,
Δ = 0.8 ppm.
Pentamer-Fused Bicycloundecanone, [PE5]2. (4-
(Phenylethynyl)phenyl)acetylene (430 mg, 2.20 mmol) was added
to a solution of 2 (300 mg, 0.36 mmol), Pd(PPh3)2Cl2 (56 mg, 80
μmol), CuI (16 mg, 80 μmol) and PPh3 (21 mg, 80 μmol) in a 1:1 v/v
mixture of DIPA/THF (10 mL) according the procedure described
above for the preparation of [PE3]2. The reaction mixture was cooled
to room temperature and poured into MeOH (200 mL). The
precipitated solid was removed by filtration and recrystallized from
THF to afford the title product as a green solid (120 mg, 30%). MP =
1
380 °C (decomposes). H NMR (300 Hz, C2D2Cl4, 80 °C): δ 7.20−
7.70 (m, 36H, Ar−H), 2.82−4.20 (br m, 10H, benzylic and
bridgehead), 2.40−2.70 (br s, 12 H, Ar−CH3). The low solubility of
the product precluded analysis by 13C NMR spectroscopy. IR (ATR):
2992, 2958, 2862, 1720, 1475, 1103, 932 cm−1. MS (MALDI), m/z
(%) = 1118.4 (M+, 80). HRMS (EI), m/z = Calcd. For C87H58O,
1118.449; Found, 1118.439, Δ = 9.0 ppm.
Tetraiodide, 2. Iodine (0.62 g, 5.0 mmol) was added to a solution
of 1 (200 mg, 620 μmol) and mercury(II) trifluoromethanesulfonate
(1.89 g, 3.72 mmol) in CH2Cl2 (10 mL) in an oven-dried Shlenk flask.
The mixture was stirred for 16 h, and CH2Cl2 (50 mL) was added.
The mixture was filtered and the organic layer was washed with a
saturated solution of Na2S2O3 (50 mL) followed by a saturated
solution of KI (50 mL). The organic layer was dried over MgSO4 and
the solvent was removed under reduced pressure to give a yellow
Stacked pentamer, st-[PE5]2. [PE5]2 (100 mg, 90 μmol) was
subjected to ketalization with ethylene glycol (200 mg, 3.23 mmol) in
the presence of p-toluenesulfonic acid (1 mg) in benzene (50 mL)
according to the procedure provided above for the preparation of st-
[PE3]2 to give the title compound as a green solid (85 mg, 82%). MP
1
= 354 °C (decomposes). H NMR (300 MHz, CDCl3): δ 7.40−7.47
1
(m, 8H, Ar−H), 7.16−7.34 (m, 28H, Ar−H), 4.18 (s, 4H,
−OCH2CH2O−) 3.84 (dd, 4H, J = 15, 6 Hz, equatorial benzylic),
3.30 (dd, 4H, J = 15, 2 Hz, axial benzylic), 2.45−2.58 (m, 2 H,
bridgehead), 2.38 (s, 12 H, Ar−CH3). The low solubility of the
product precluded analysis by 13C NMR spectroscopy. IR (ATR):
2987, 2934, 2867, 1615, 1510, 1109, 1065, 988 cm−1. MS (MALDI),
m/z (%) = 1162.4 (M+, 80). HRMS (EI), m/z = Calcd. For C89H62O2,
1162.47498; Found, 1162.4546, Δ = 17.5 ppm.
residue. H NMR analysis showed the presence of residual hydrogen
atoms on aromatic rings. Accordingly, the residue was resubjected to
iodination. Following workup, the residue was triturated with boiling
hexane and filtered to give the title compound as a white solid (0.4 g,
74%). MP = 216−217 °C. 1H NMR (300 MHz, CDCl3): δ 2.40−3.60
(br m, 10H, benzylic and bridgehead), 2.10−2.40 (br s, 12H, Ar−
CH3). Low solubility of the product precluded analysis by 13C NMR
spectroscopy. IR (ATR): 2914, 2854, 1711, 1493, 1163, 1007, 881,
738 cm−1. MS (MALDI), m/z (%) = 821.8 (M+, 100). HRMS (EI),
m/z = Calcd. For C23H24OI4, 821.7802; Found, 821.7810, Δ = 0.9
ppm.
Linear Unstacked Trimer, PE3. A solution of 2,3,5,6-tetramethyl-
1,4-diiodobenzene (500 mg, 1.30 mmol), Pd(Ph3P)Cl2 (50 mg, 70
μmol), CuI (15 mg, 70 μmol) and phenylacetylene (450 mg, 4.00
mmol) in a 1:1 v/v mixture of THF and piperidine (10 mL) was
stirred for 36 h under Ar. CH2Cl2 (50 mL) was added and the solution
Trimer-Fused Bicycloundecanone, [PE3]2. A solution of 2 (350
mg, 420 μmol), Pd(PPh3)2Cl2 (56 mg, 80 μmol), CuI (16 mg, 80
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was washed with sat. NH4Cl (100 mL) and H2O (100 mL). The
solvent was removed under reduced pressure and the residue was
subjected to column chromatography (CH2Cl2) followed by
recrystallization from hexane to afford the title compound as a yellow
ACKNOWLEDGMENTS
■
The research program on stacked conjugated oligomers in the
Collard group was initiated with the support of the National
Science Foundation (Award ECCS-437925). We thank the
Georgia Tech Center for Organic Photonics and Electronics
(COPE) for scholarship support provided to S.P.J. The work in
the Bredas group is supported by the STC program of the
́
National Science Foundation under Award DMR-0120967.
1
solid (400 mg, 93% yield). MP = 218−219 °C. H NMR (CDCl3): δ
7.53−7.59 (m, 4H, Ar−H), 7.33−7.38 (m, 6H, Ar−H), 2.50 (s, 12H,
Ar−CH3). 13C NMR (CDCl3): δ 135.7, 131.4, 128.4, 128.1, 123.9,
123.3 (aromatic), 98.1, 88.6 (−CC−), 18.4 (Ar−CH3). IR (ATR):
3066, 3036, 2927, 1598, 1495, 1017, 752 cm−1. MS (EI): m/z (%)
334.1 (M+, 100), 167.1 (20). HRMS (EI), m/z = Calcd. For C26H22,
334.1722; Found 334.1753, Δ = 9.3 ppm.
REFERENCES
■
Linear Unstacked Pentamer PE5. A solution of 2,3,5,6-
tetramethyl-1,4-diiodobenzene (250 mg, 650 μmol), Pd(Ph3P)Cl2
(23 mg, 30 μmol), CuI (6.0 mg, 30 μmol) and (4-(phenylethynyl)-
phenyl)acetylene (404 mg, 2.00 mmol) was treated according to the
procedure provided above for the preparation of PE3. . The product
obtained from column chromatography was triturated with hexanes to
afford the title compound as a yellow solid (330 mg, 96% yield). MP =
254−255 °C. 1H NMR (CDCl3): δ 7.50−7.59 (m, 12H, Ar−H),
7.31−7.40 (m, 6H, Ar−H), 2.51 (s, 12H, Ar−CH3). The low solubility
of the product precluded analysis by 13C NMR spectroscopy. IR
(ATR, neat): 3072, 3024, 2936, 1585, 1487, 1024, 751 cm−1. MS (EI):
m/z (%) 534.2 (M+, 100), 267.2 (10). HRMS (EI), m/z = Calcd. For
C42H30, 534.2348; Found 534.2340, Δ = 1.5 ppm.
Computational Studies. The geometry optimizations of the
unstacked molecules (PE3 and PE5) were performed using both
B3LYP and ωB97X/6-31g* functionals39 and the 6-31g* basis set. To
account for the dispersion interactions between the oligomer moieties
of the stacked molecules st-[PE3]2 and st-[PE5]2, the geometries were
obtained at the ωB97X-D/6-31g*40 level of theory. The geometry
optimizations were performed for the isolated molecules and by using
the continuum solvent model to implicitly account for the effect of
CHCl3 solvent. All geometries were confirmed to be minima by
additional vibrational frequency calculations. The low-lying excited
states were derived using time-dependent density functional theory
(TD-DFT) using the 6-31g* basis set and the long-range corrected
functionals ωB97X-D and ωB97X for the stacked and unstacked
molecules, respectively. Recent investigations have indicated that long-
range corrected functionals largely mitigate the issue related to the
spurious electron self-interaction characteristic of standard generalized
gradient functionals and provide a reliable description of charge-
transfer excitations.39−42 We note that high-level wave function
methods based on multireference perturbation theory36 have recently
been successfully applied to investigate the excimer states of various
oligoacene dimers. However, these approaches quickly become
prohibitively and time-consuming when geometry optimizations of
the excited states are required, as is the case for the present
investigations. Accordingly, we conducted TD-DFT calculations at the
same level of theory as that used for the ground-state calculations to
obtain the optimal geometries of the lowest excited states of the
stacked and unstacked systems. All DFT calculations were performed
using the Gaussian 09 package.43
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ASSOCIATED CONTENT
* Supporting Information
■
S
Synthetic procedures and structural characterization of stacked
compounds and unstacked linear models; TD-DFT results
when considering a dielectric medium corresponding to
CHCl3; and coordinates of the DFT-optimized isolated stacked
(st-[PE3]2 and st-[PE5]2) and unstacked (PE3 and PE5)
oligomers in the ground and excited states. This material is
available free of charge via the Internet at http://pubs.acs.org.
(28) Morisaki, Y.; Sawamura, T.; Murakami, T.; Chujo, Y. Org. Lett.
2010, 12, 3188.
AUTHOR INFORMATION
Corresponding Author
david.collard@chemistry.gatech.edu
■
(29) Giaimo, J. M.; Lockard, J. V.; Sinks, L. E.; Scott, A. M.; Wilson,
T. M.; Wasielewski, M. R. J. Phys. Chem. A 2008, 112, 2322.
(30) Mangalum, A.; Morgan, B. P.; Hanley, J. M.; Jecen, K. M.;
McGill, C. J.; Robertson, G. A.; Smith, R. C. Chem. Commun. 2010, 46,
5136.
Notes
The authors declare no competing financial interest.
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