Exciton-Vibrational Couplings in Homo- and Heterodimer Stacks of Perylene Bisimide Dyes within Cyclophanes: Studies on Absorption Properties and Theoretical Analysis

Article abstract of DOI:10.1002/chem.201603205

The optical properties of a series of three cyclophanes comprising either identical or different perylene bisimide (PBI) chromophores were studied by UV/Vis absorption spectroscopy and their distinctive spectral features were analyzed. All the investigated cyclophanes show significantly different absorption features with respect to the corresponding constituent PBI monomers indicating strong coupling interactions between the PBI units within the cyclophanes. DFT calculations suggest a π-stacked arrangement of the PBI units at close van der Waals distance in the cyclophanes with rotational displacement. Simulations of the absorption spectra based on time-dependent quantum mechanics properly reproduced the experimental spectra, revealing exciton-vibrational coupling between the chromophores both in homo- and heterodimer stacks. The PBI cyclophane comprising two different PBI chromophores represents the first example of a PBI heterodimer stack for which the exciton coupling has been investigated. The quantum dynamics analysis reveals that exciton coupling in heteroaggregates is indeed of similar strength as for homoaggregates.

Full text of DOI:10.1002/chem.201603205

DOI: 10.1002/chem.201603205
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Supramolecular Chemistry |Hot Paper|
Exciton-Vibrational Couplings in Homo- and Heterodimer Stacks
of Perylene Bisimide Dyes within Cyclophanes: Studies on
Absorption Properties and Theoretical Analysis
David Bialas,[a] Christoph Brꢀning,[b] Felix Schlosser,[a] Benjamin Fimmel,[a] Johannes Thein,[a]
Volker Engel,*[b] and Frank Wꢀrthner*[a]
Dedicated to Professor Gerhard Bringmann on the occasion of his 65th birthday
Abstract: The optical properties of a series of three cyclo-
phanes comprising either identical or different perylene bis-
imide (PBI) chromophores were studied by UV/Vis absorp-
tion spectroscopy and their distinctive spectral features were
analyzed. All the investigated cyclophanes show significantly
different absorption features with respect to the correspond-
ing constituent PBI monomers indicating strong coupling in-
teractions between the PBI units within the cyclophanes.
DFT calculations suggest a p-stacked arrangement of the PBI
units at close van der Waals distance in the cyclophanes
with rotational displacement. Simulations of the absorption
spectra based on time-dependent quantum mechanics prop-
erly reproduced the experimental spectra, revealing exciton-
vibrational coupling between the chromophores both in
homo- and heterodimer stacks. The PBI cyclophane compris-
ing two different PBI chromophores represents the first ex-
ample of a PBI heterodimer stack for which the exciton cou-
pling has been investigated. The quantum dynamics analysis
reveals that exciton coupling in heteroaggregates is indeed
of similar strength as for homoaggregates.
Introduction
eroaggregates as present in, for example, bulk heterojunction
solar cells, exciton coupling between dyes with differing ab-
sorption energies is considered to be negligible. This view was
also expressed in the still quite limited number of structurally
defined synthetic heteroaggregate systems bearing p-stacks of
different dyes,[5] albeit a close look on some of the absorption
spectra reveal deviations with regard to the simple addition of
the spectra of the monomeric building blocks. These devia-
tions were related to conformational distortions, solvent ef-
fects, or charge-transfer interactions. In this regard, our recent
study on a merocyanine foldamer bearing two different mero-
cyanine dyes with distinct absorption maxima at ~540 and
~650 nm provided a so-far unique showcase example, reveal-
ing strong exciton coupling in the closely p-stacked conforma-
tion despite an energetic offset of >3000 cmꢀ1 between the
absorption energies of the two dyes. As a result, significantly
different absorption spectra were recorded for this heterodi-
mer system compared to the addition of the spectra of the
two monomeric dyes.[6]
Exciton coupling is among the most vividly discussed proper-
ties of molecular materials because of its fundamental impor-
tance for the understanding of coherent energy transport in
natural light-harvesting systems[1] as well as its relevance for
various functional properties of organic dyes and semiconduc-
tors with relevance for (opto-)electronic and photovoltaic ap-
plications.[2] Moreover, the molecular exciton theory as intro-
duced by Davydov and Kasha[3] fifty years ago constitutes the
basic concept to derive structural insights into supramolecular
arrangements in dye aggregates, for example, H- and J-aggre-
gates.[4] Accordingly, plenty of research has been carried out
on exciton coupling to obtain structural information as well as
insight into functional properties of dye aggregates. Interest-
ingly, however, all this research has focused for half of a century
on molecular aggregates obtained from identical dye or organ-
ic semiconductor molecules, that is, homoaggregates. For het-
[a] D. Bialas, Dr. F. Schlosser, Dr. B. Fimmel, J. Thein, Prof. Dr. F. Wꢀrthner
Universitꢁt Wꢀrzburg
This, so far, unique example constituted the starting point
for our current study on the exciton coupling in perylene bis-
imide based homo- and heteroaggregates. Perylene-3,4:9,10-
bis(dicarboximides) (in short perylene bisimides; PBIs) repre-
sent a privileged class of chromophores for various applica-
tions owing to their outstanding optical and electronic fea-
tures. They exhibit high photostability,[7] easily tunable absorp-
tion, fluorescence, and redox properties,[8] and thus comply
with the basic requirements for applications in organic elec-
tronics[9] and photovoltaics.[10] In addition, their unique self-as-
Institut fꢀr Organische Chemie and Center for Nanosystems Chemistry
Am Hubland, 97074 Wꢀrzburg (Germany)
E-mail: wuerthner@chemie.uni-wuerzburg.de
[b] C. Brꢀning, Prof. Dr. V. Engel
Universitꢁt Wꢀrzburg
Institut fꢀr Physikalische und Theoretische Chemie
Emil-Fischer-Str. 42, 97074 Wꢀrzburg (Germany)
E-mail: volker.engel@uni-wuerzburg.de
Supporting information for this article can be found under http://
dx.doi.org/10.1002/chem.201603205.
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sembly behavior based on p–p-stacking and hydrogen-bond-
ing interactions makes them highly versatile building blocks
for the construction of functional supramolecular assemblies.[11]
The optical properties of PBI assemblies not only depend on
the structural features of the constituent monomeric chromo-
phores, but also very decisively on the molecular arrangement
in the assembly, leading to exciton coupling regimes that
cover the whole range from H- to J-type.[11,12] For PBIs the sit-
uation is, however, more challenging than for the above-men-
tioned merocyanine dyes because the optical excitation of PBIs
is accompanied by a strong coupling to aromatic carbon–
carbon stretching vibrations. The resultant complex exciton–vi-
brational coupling in PBI aggregates requires accordingly
a more elaborate quantum dynamical treatment that we and
others have pursued in work on PBI p-stacks of various size
and the other one without any bay substituents (“orange PBI”,
oPBI). For comparison, cyclophane o2-CP containing two bay-
unsubstituted PBI chromophores with orange color (“orange
PBI”) has also been investigated.
Herein we report that the hetero-PBI cyclophane or-CP ex-
hibits quite different UV/Vis absorption spectral features than
those calculated from the absorption spectra of the individual
dyes or those of the homo-PBI cyclophanes o2-CP and r2-CP.
Our theoretical studies based on time-dependent quantum
mechanics reveals exciton-vibrational coupling between the
chromophore units not only in the homo-PBI cyclophanes but
also in the hetero-PBI cyclophane, explaining the distinctive
absorption features of these p-stacked dimers.
but always comprised of the same type of PBI chromo- Results and Discussion
phores.[13,14]
Synthesis
Since supramolecular aggregates consisting of different
types of chromophores can provide functionalities that cannot
be realized with homoaggregates,[5] gaining insight into the
exciton coupling of PBI hetero-p-stacks should be a rewarding
task. Therefore, we elucidate here the absorption properties of
a first example of a structurally well-defined PBI heterodimer
p-stack by quantum dynamical analysis, along with two homo-
dimer p-stacks for comparison.
Cyclophane o2-CP comprising two orange PBIs and cyclo-
phane or-CP bearing one orange and one red PBI chromo-
phore were synthesized according to the routes depicted in
Scheme 1. Starting from commercially available perylene-
3,4:9,10-tetracarboxylic bisanhydride 1, cyclophane o2-CP was
synthesized in four steps (Scheme 1, route a). The imidization
of 1 with 4-iodoaniline, followed by Sonogashira coupling reac-
tion with triisopropylsilyl (TIPS)-functionalized phenylene acety-
lene derivative 3[17] and subsequent deprotection of the silyl
group with tetrabutylammonium fluoride afforded the PBI pre-
cursor 5 for the final step in good overall yield. Pd-catalyzed
homocoupling of 5 with [PdCl2(PPh3)2] as the catalyst and cop-
per(I) iodide as co-catalyst afforded the desired cyclophane o2-
CP bearing two identical PBI chromophores (Scheme 1,
route a).
For the detailed study of interchromophore interactions,
model systems with distinct orientations of the chromophores
are required.[15] For this purpose, PBI cyclophanes[16] bearing
two PBI chromophores positioned by a spacer in a fixed geom-
etry are properly suited. Recently, we have reported the syn-
thesis of cyclophane r2-CP (in the notation of the cyclophanes
The target cyclophane or-CP comprising two different PBI
chromophores was synthesized in two-steps, one-pot reaction
by deprotection of the TIPS groups in literature-known PBI de-
rivative 6[17] and newly synthesized PBI 4 (Scheme 1, route a)
with tetrabutylammonium fluoride and a subsequent Pd-cata-
lyzed heterocoupling reaction of the in situ generated termi-
nally acetylene-substituted orange and red PBI derivatives in
3.4% isolated yield (Scheme 1, route b). The low yield of this
hetero PBI cyclophane is caused by the formation of undesired
polymeric byproducts and homocoupled cyclophanes o2-CP
and r2-CP. The latter PBI cyclophane has been previously re-
ported.[17] For the detailed synthetic procedures and characteri-
zation data of new compounds, see the Supporting Informa-
tion.
(CP) “o” and “r” indicate the parent orange-colored and the
tetraphenoxy-substituted red-colored PBI chromophores, re-
spectively) which comprises two identical PBI chromophores
bearing tert-butylphenoxy substituents in the bay positions
(“red PBI”, rPBI), leading to intense emission in the red spectral
range.[17] This cyclophane affords co-facially stacked PBI chro-
mophores, thus the phenylene butadiynylene spacer unit of
r2-CP allows a perfect preorganization of the PBI chromo-
phores in a p-stacked arrangement. To investigate PBI hetero-
dimer stacks in terms of exciton coupling, we have newly syn-
thesized, by employing the same spacer, the cyclophane or-CP
consisting of two different PBI chromophores, one bearing
tert-butylphenoxy groups in the bay positions (“red PBI”, rPBI)
UV/Vis absorption spectroscopy
Exciton-vibrational couplings manifest in the UV/Vis absorption
bands of the respective optical transitions. Accordingly, the PBI
cyclophanes o2-CP, r2-CP, and or-CP were studied in compari-
son with the respective monochromophoric PBI references by
UV/Vis spectroscopy in high permittivity (er =9.0) solvent di-
chloromethane for dilute solutions to ensure the absence of
any intermolecular aggregates formed by self-association. The
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Scheme 1. Synthetic route to a) cyclophane o2-CP bearing two bay-unsubstituted orange PBI chromophores and b) cyclophane or-CP consisting of one
orange and one red PBI chromophore.
monochromophoric reference PBI 4 shows the typical absorp-
tion bands of a core-unsubstituted PBI chromophore with pro-
nounced vibronic fine structure (Figure 1a, dash-dotted line).[11]
The 0–0 transition with the highest absorption is located at
528 nm and exhibits a vibronic progression with less intense
absorption bands at 491, 459, and 431 nm. In contrast, the
spectrum of cyclophane o2-CP containing two identical core-
unsubstituted oPBI chromophores shows a reversal of the
band intensities compared to the reference PBI 4 at around
530 and 490 nm (Figure 1a, solid line). This phenomenon is
characteristic for helically p-stacked oPBI chromophores and
arises from exciton-vibrational coupling.[13] The spectra indeed
resemble those of self-assembled closely stacked oPBI dyes,[18]
for which our analysis yielded a value of 524 cmꢀ1 for the exci-
ton coupling strength, that is, a situation called the intermedi-
ate coupling regime.[13] A similar situation is given for the red
cyclophane r2-CP (Figure 1b, solid line). Here the monochro-
mophoric rPBI reference 6 bearing four tert-butylphenoxy sub-
stituents in bay positions shows a bathochromic shift to
583 nm compared to the core-unsubstituted oPBI reference 4
with broader absorption bands and a loss of vibronic fine
structure (Figure 1b, dash-dotted line). These less defined vi-
bronic progresses are caused by the bulky tert-butylphenoxy
substituents, which induce a rotational twist of the two naph-
thalene-imide subunits thereby reducing the rigidity of the
scaffold and enabling a broader conformational distribu-
tion.[19,20] The absorption band at 541 nm belongs to the vi-
bronic progression of the S0–S1 absorption band, while the
weak absorption band at 454 nm arises from the S0–S2 transi-
tion.[21] As in the case of cyclophane o2-CP, the absorption
spectrum of cyclophane r2-CP bearing two identical core-tetra-
phenoxy-substituted rPBI chromophores shows a reversal of
the band intensities at around 580 and 540 nm with respect to
the corresponding reference PBI derivative 6, indicating a simi-
lar p-stacked arrangement with exciton-vibrational coupling of
the rPBI chromophores in this cyclophane as well (Figure 1b,
solid line).[17]
As immediately apparent from Figure 1c, the situation for
cyclophane or-CP bearing one core-unsubstituted and one
core-tetraphenoxy-substituted PBI chromophore is more com-
plex. Thus, the absorption spectrum of or-CP (solid line) does
neither equal the sum of the spectra of the respective refer-
ence compounds 4 and 6 (situation without exciton coupling,
dash-dotted line) nor resemble the spectra of o2-CP and r2-CP.
However, a clearly visible redistribution of oscillator strength
among the dyes is indicative for an exciton coupling interac-
tion between the different types of chromophores within this
cyclophane. The shift of the 0–0 absorption band of the con-
stituent PBIs is, however, very modest for cyclophane or-CP
with a small bathochromic shift of the absorption band of the
core-tetraphenoxy-substituted PBI chromophore from 583 to
586 nm and a smaller modulation of the absorption of the
core-unsubstituted PBI compared to o2-CP. As for the homo-
dimers o2-CP and r2-CP, additional bands at 500 and 469 nm
are observed, which are of higher intensity and show a batho-
chromic shift compared to the absorption bands of the vibron-
ic progression of reference PBI 4. Taken together, these spec-
troscopic observations clearly indicate an exciton coupling be-
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utilized PBI cores—oPBI being planar and rPBI having a twisted
p-core (rotational twist of 25-308 between the two naphtha-
lene imide subunits)[19]—and the sterical demands of the tetra-
phenoxy substituents, a deeper insight into the specific molec-
ular arrangement of the PBI chromophores within the cyclo-
phanes is required. Therefore, DFT calculations were performed
by employing B97D3[23] as a functional and def2-SVP[24] as
a basis set. The functional includes a dispersion correction that
is required to describe p–p-interactions between the PBI chro-
mophores in an adequate way.[25] The geometry-optimized
structures of the PBI cyclophanes are shown in Figure 2. All
Figure 1. UV/Vis absorption spectra of a) reference 4 (d) and cyclophane
o2-CP (c), b) reference 6 (d) and cyclophane r2-CP (c), and c) sum
of the spectra of 4 and 6 (d) as well as the spectrum of cyclophane or-CP
(c) in dichloromethane (cꢁ10ꢀ5 m) at room temperature.
tween the two different PBI chromophores within cyclophane
or-CP leading to distinctive changes in the optical absorption
properties.
The absorption spectra of all three cyclophanes in the less
polar solvent toluene are very similar to the ones measured in
dichloromethane (Figure S1, Supporting Information). Besides
a hypsochromic shift for cyclophanes r2-CP and or-CP, which is
known as positive solvatochromism of PBI chromophores,[22]
the shape of the spectra is not affected by the solvent polarity.
This indicates similar coupling strengths in toluene and di-
chloromethane for all three cyclophanes revealing the same ar-
rangements of the chromophores in both solvents.
Figure 2. Geometry-optimized structures of PBI cyclophanes a) o2-CP, b) r2-
CP, and c) or-CP (B97D3, def2-SVP, hexyl groups were replaced by methyl
groups). Hydrogen atoms were omitted for clarity. a represents the rotation-
al displacement and r the distance between the chromophores in the re-
spective cyclophanes as depicted in Figure 2a.
three cyclophanes show p–p-stacking between the PBI chro-
mophores with distances (r) of 3.24–3.84 ꢂ and rotational dis-
placements (a) of 10.6–28.68 between the chromophores. The
molecular arrangement of the chromophores in cyclophane
o2-CP (r=3.24 ꢂ, a=28.68) represents the ideal ground-state
geometry of two co-facially stacked core-unsubstituted oPBI
chromophores,[25] corroborating our molecular design for
a spacer unit that supports the desired preorganization of
these chromophores in the cyclophanes. While the core-unsub-
stituted oPBI chromophores in o2-CP are almost perfectly
planar, the naphthalene imide units of the core-tetraphenoxy-
substituted red PBI in r2-CP are twisted by 278 due to the ster-
ical constraints in the bay area imposed by the bulky tert-bu-
The subsequent theoretical studies are devoted to establish
a proper understanding of this exciton coupling in heterodim-
ers in the presence of strong vibronic coupling as given for PBI
dyes and the majority of other common chromophores.
Quantum chemical studies
According to CPK models, the p-conjugated phenylene buta-
diynylene tethers are supposed to preorganize the PBI units in
a rather stiff or even compressed co-facial p-stacking arrange-
ment. Nevertheless, due to the different geometries of the two
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tylphenoxy substituents.[19,26] Notably, also this value is in ac-
cordance with crystallographic data for monomeric tetraphe-
noxy-substituted PBIs.[19,27] The phenoxy substituents prevent,
however, a close contact of the PBI chromophores in twisted
conformation, resulting in a larger distance of 3.84 ꢂ between
the p-scaffolds in r2-CP. As our calculation shows, the phenyl-
ene butadiynylene tether enforced helical displacement of
24.58 enables all eight phenoxy substituents to accommodate
without sterical congestions as a homochiral dye pair (both
dyes exhibit the same plane chirality, that is, PP or MM). For or-
CP, the situation is obviously more demanding because
a planar and a nonplanar dye have to interact by co-facial p–
p-stacking. According to our calculations, the interchromo-
phore distance in or-CP is smaller compared to that of r2-CP
and indeed close to o2-CP despite the fact that one PBI con-
tains bulky substituents and a twisted p-core. Remarkably, to
enable such a close contact, the planarity of the core-unsubsti-
tuted oPBI chromophore is lost in cyclophane or-CP leading to
a rotational twist of approximately 118 caused by a templating
effect of the core-twisted (268) at bay positions tetrasubstitut-
ed rPBI chromophore. Accordingly, the utilized phenylene bu-
tadiynylene bridging unit indeed acts as a bracket that com-
presses the two dyes on top of each other.
Quantum dynamics studies
The absorption properties of dye aggregates consisting of
identical chromophores can be easily rationalized with Kasha’s
exciton theory assuming an interaction between the oscillating
transition dipole moments of the constituent chromophores.[3b]
Exciton coupling between two chromophores results in a Davy-
dov splitting[3a] of the excited states with a splitting energy of
2J, in which J is the exciton coupling energy, as illustrated in
the exciton state diagram for a dimer stack of two identical PBI
chromophores in a hypothetical perfectly collinear arrange-
ment in Figure 3a. The two nondegenerate excited states S1’
and S1’’ differ in the phase relation between the oscillating
transition dipole moments. The excitation to the highest excit-
ed state (S1’’) is allowed due to the in-phase oscillation of the
transition dipole moments, while the transition to the lower
excited state is forbidden since the transition dipole moments
are oscillating out of phase. Hence, the absorption spectrum of
a H-dimer exhibits a hypsochromically shifted absorption band
with respect to the monomer absorption band (Figure 3a). As
shown in our previous work on a merocyanine heterodimer
composed of two chromophores with different excited state
energies, a splitting of the excited states occurs as well in this
case.[6] In a heterodimer, the splitting energy amounts to
Figure 3. Schematic exciton state diagram for H-type PBI dimer stack com-
posed of a) identical and b) different chromophores in a hypothetical collin-
ear arrangement and neglecting vibronic coupling. The orange and red
arrows illustrate the relationship between the oscillating transition dipole
moments. S1,o and S1,r denote the excited states of orange and red PBI, re-
spectively. DEo,r is the energy difference between the excited states S1,o and
S1,r, and J represents the exciton coupling energy.
sights, we can expect two absorption bands in the UV/Vis
spectrum with a hypsochromic and a bathochromic shift with
regard to the absorption bands of the involved monomeric
chromophores and a redistribution of oscillator strength, that
is, an increase of absorbance for the higher energy and a de-
crease of absorbance for the lower energy band.
For the given PBI cyclophanes, the situation is, however,
more complex due to the strong coupling of the PBI S0!S1
transition to aromatic CC-stretching modes and a rotational
displacement of the chromophores. In the case of a rotational
displacement of the chromophores, the transition to the lower
excited states becomes partially allowed,[3b] resulting in the
presence of a second less intense absorption band with a bath-
ochromic shift (“J-band”) even for identical chromophores. Due
to the fact that for the PBI S0!S1 transition the vibrational
coupling with about 1400 cmꢀ1 is stronger than the exciton
coupling with about 500 cmꢀ1 (so-called intermediate coupling
regime) the main signature of the exciton coupling is indeed
not a pronounced shift of the absorption band but the reversal
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
DEo;r þ 4J2 with DEo;r as the energy difference between the
excited states of the different PBI chromophores S1,o and S1,r,
respectively (Figure 3b).[28] Importantly, in contrast to the
homodimer, the transition to the lowest excited state becomes
partially allowed in such a heterodimer, even for a perfect colli-
nearly stacked H-type dimer. This effect may be attributed to
different magnitudes of the resp. transition dipole moments of
the chromophores or simply to differences in the excited-state
energies of the two coupled chromophores. Based on these in-
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of the intensity of the two lowest energy bands[13,14] as ob-
served for o2-CP and r2-CP homodimers in Figure 1a,b. Ac-
cordingly, the simple model described above has to be extend-
ed by considering rotational displacements as well as vibra-
tional coupling between the chromophores. Following our ear-
lier work on homoaggregates of PBIs[13] we can be confident
to obtain a conclusive understanding of the absorption bands
of o2-CP and r2-CP homodimers and novel insights into the
hitherto unexplored impact of exciton-vibrational couplings on
the spectra of hetero-PBI cyclophane or-CP.
Towards this goal we have performed time-dependent quan-
tum mechanical simulations. We solve the time-dependent
Schrçdinger equation for a Hamiltonian of Holstein type,[29,30]
which in many cases gives an excellent description of vibronic
spectra[4a,31] and has been applied to self-assembled PBI aggre-
gates before.[13] The absorption spectra are then obtained by
a Fourier transform of the calculated time-dependent autocor-
relation function.[32] A more detailed description of the proce-
dure and the model Hamiltonian can be found in the Support-
ing Information.
lar values for the vibrational frequency and the Huang–Rhys
factor indicate that the substituents attached to the red PBI
monomer do not influence the vibrational motion significantly.
Turning to the homodimers, we fix the angle a between the
transition dipole moments to 28.68 for o2-CP and 24.58 for r2-
CP, respectively (see Figure 2). The absolute values of the tran-
sition dipole moments result, at least in the case of homodim-
ers, only in a global prefactor in the spectrum, and is therefore
set equal to one. We find best agreement for the simulated
spectra with experiment for an exciton coupling energy of J=
613 cmꢀ1 for o2-CP and 500 cmꢀ1 for r2-CP. These values are
comparable to the coupling strength of 524 cmꢀ1 found for
self-assembled homodimers of core-unsubstituted PBI mole-
cules.[13]
The two exciton states illustrated in Figure 3a overlap, be-
cause the coupling strength is small compared to the vibra-
tional frequency. Since the S0!S1‘ transition is only weakly al-
lowed for angles of roughly 258, we attribute the line-shape
mostly to vibrational levels in the S1’’ exciton state. There, the
intensities of the peaks marked with an asterisk in the figure
are strongly dependent on the exciton coupling energy.[31,33]
We note that the Huang–Rhys factors are increased to x=0.88
(orange) and 0.83 (red) to achieve a better agreement with the
experiment. This may indicate that in the dimers a strain acts
on the PBI scaffold, and thus changes the excited-state equilib-
rium geometry of the monomer units. Such possible geometry
deformations are not the focus of this work, and more extend-
ed electronic structure calculations involving excited-state ge-
ometry optimizations would have to be conducted in the
future.
In Figure 4, we compare calculated and measured spectra
for the monochromophoric reference PBIs 4 and 6 (panels a,c)
and the o2-CP and r2-CP homodimers (panels b,d). We use the
monomer spectra to fix the parameters that enter the model
Hamiltonian. These are the frequencies w, Huang–Rhys-factors
x, and excitation energies DE. Best agreement with experiment
is achieved for the sets: w=1411 cmꢀ1, x=0.79, DE=
18954 cmꢀ1 for the orange monomer and w=1331 cmꢀ1, x=
0.78 and DE=17164 cmꢀ1 for the red monomer. The very simi-
We now apply the same model to the or-CP heterodimer.
The calculated spectra for the sum of the monomers and the
heterodimer are shown in Figure 5. In contrast to the homo-
dimers, the relative strength of the transition dipole moments
is important here. Their absolute values will, however, still
result in a global factor. We will therefore only give the
ꢀ
ratio m=m(r)/m(o) of the individual absolute values. The sum of
the monomer spectra is best matched by a model spectrum
for an uncoupled dimer (J=0) when we set the dipole
ꢀ
moment ratio m=0.69. This, however, most probably underes-
timates the value of m(r) because the oscillator strength corre-
sponds to the total area of the spectral band and the spectrum
for the red monomer is broader for reasons discussed above.
All other parameters have been kept at the values determined
from the individual monomer spectra. For the spectrum of the
or-CP heterodimer, we fix the angle a between the transition
dipole moments to a value of 10.68 as indicated in Figure 2c
and the Huang–Rhys factors at the values determined from
the homodimer spectra. From the comparison of calculated
and measured spectra, we arrive at a dipole moment ratio
ꢀ1
ꢀ
of m=0.81 and an exciton coupling energy of J=427 cm . It
Figure 4. Comparison of experimental (d, measured in dichloromethane)
and calculated spectra (c): a) reference PBI 4 (oPBI), b) o2-CP homodimer,
c) reference PBI 6 (rPBI), and d) r2-CP homodimer. The calculated spectra are
normalized to match the measured peak of highest intensity in each case.
The value of the exciton coupling energy J is given for the two dimers. It es-
sentially influences the intensity of the low energy peaks (*), which are
marked in panels b and d. The model does not account for the S0!S2 transi-
tion seen around 22000 cmꢀ1 in panels c and d.
is somewhat smaller than that found for the homodimers, but
still comparable in its numerical value. Again, the coupling
strength has a large effect on the intensity of the 0,0 transition
peak of the oPBI chromophore (marked with an asterisk in Fig-
ure 5b). The ratio of the dipole moments further determines
the intensity of the low-energy band originating mainly from
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ent PBI chromophores represents the first example of PBI
hetero-p-stacks for which exciton–vibrational coupling has
been explored. The new insights derived from our work justify
further efforts toward an understanding of optical properties
of larger PBI hetero-p-stacks. Based on those studies tailored
dye aggregate systems should become accessible for function-
al materials with desired properties.
Experimental Section
Detailed information can be found in the Supporting Information.
The document includes experimental procedures, details of the
synthetic procedures, characterization of all new compounds
(NMR, mass spectrometry), details of the quantum chemical calcu-
lations, and graphical accounts of the NMR and mass spectra.
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (DFG) for
support of this project in the framework of the research group
FOR 1809 on “Light-induced dynamics in molecular aggre-
gates” at the Universitꢃt Wꢀrzburg. D. B. thanks the Fonds der
Chemische Industrie for a PhD fellowship.
Figure 5. Comparison of experimental (d, measured in dichloromethane)
and calculated spectra (c): a) Sum of measured spectra of reference PBI
dyes 4 (oPBI) and 6 (rPBI) and calculated uncoupled or-CP heterodimer. Also
shown is the theoretical spectrum of reference PBI 6. b) Coupled hetero-
dimer. Values for the exciton coupling energy J and the ratio of the magni-
tudes of the dipole moments are indicated.
Keywords: cyclophanes · dyes · exciton coupling · vibronic
coupling · p–p stacking
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Received: July 5, 2016
Published online on && &&, 0000
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FULL PAPER
Dyes/pigments: The absorption proper-
ties of a series of three cyclophanes
comprising either the same or different
perylene bisimide (PBI) dyes have been
studied. All cyclophanes exhibit differ-
ent absorption features with respect to
the corresponding constituent mono-
meric dyes (see figure).
&
Supramolecular Chemistry
D. Bialas, C. Brꢀning, F. Schlosser,
B. Fimmel, J. Thein, V. Engel,*
F. Wꢀrthner*
&& – &&
Exciton-Vibrational Couplings in
Homo- and Heterodimer Stacks of
Perylene Bisimide Dyes within
Cyclophanes: Studies on Absorption
Properties and Theoretical Analysis
Chem. Eur. J. 2016, 22, 1 – 9
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