Synthesis and properties of a covalently linked angular perylene imide dimer

Article abstract of DOI:10.1021/ol3029115

Utilizing the unexplored chemistry of a monocarbon analog to perylene bisimide, a covalently linked angular perylene dimer was synthesized. On the basis of measured optical properties and molecular modeling, the spectral changes relative to a monomeric reference perylene can be explained by an angle-dependent oblique exciton coupling model. With a roughly trigonal interchromophore arrangement, the dimer building block is promising for larger, cyclic assemblies to mimic naturally occurring light harvesting complexes.

Full text of DOI:10.1021/ol3029115

ORGANIC
LETTERS
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Vol. XX, No. XX
000–000
Synthesis and Properties of a Covalently
Linked Angular Perylene Imide Dimer
€
Karl J. Thorley and Frank Wurthner*
€
Universitat Wurzburg, Institut fur Organische Chemie, Am Hubland, 97074 Wurzburg,
€
€
€
Germany
wuerthner@chemie.uni-wuerzburg.de
Received October 22, 2012
ABSTRACT
Utilizing the unexplored chemistry of a monocarbon analog to perylene bisimide, a covalently linked angular perylene dimer was synthesized.
On the basis of measured optical properties and molecular modeling, the spectral changes relative to a monomeric reference perylene can be
explained by an angle-dependent oblique exciton coupling model. With a roughly trigonal interchromophore arrangement, the dimer building
block is promising for larger, cyclic assemblies to mimic naturally occurring light harvesting complexes.
Inspired by natural systems such as photosystem I and
II, there is currently a strong interest in the formation of
synthetic macrocyclic systems to replicate and understand
efficient light-harvesting processes. These range from su-
pramolecularself-assembled systems,1 aswellascovalently
linked persistent cycles,2 with examples of both isolated
and fully conjugated extended π-systems. Such macro-
cycles include cyclic oligomers of porphyrins,2,3 thiophenes,4
and phenylacetylenes.5 Until recently,6 perylene bisimides
(PBIs) had not been widely investigated in this respect,
despite their alluring electronic and optical properties.
Here, we report the synthesis of a covalently linked
angular perylene dimer 1, which might form the basis of
larger macrocyclic system 2 (Figure 1). Owing to the
trigonal arrangement of the perylene units in dimer 1
interchromophore interactions of the closely spaced per-
ylene chromophores can be derived. While the optical
properties of perylene dyes can be drastically changed by
chemical modification of the aromatic system,7 interac-
tions between adjacent molecules also strongly affect these
properties.8ꢀ10
Mostinterchromophore interactionsinperylenesystems
are of the H-aggregate type, where πꢀπ stacking of mole-
cules leads to hypsochromically shifted absorption.8 The
opposite effect has been observed in J-aggregate forma-
tion9 as well as covalently linked perylene oligomers.10
However, defined angle dependent perylene excitonic cou-
pling has yet to be explored, and only a few examples of
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2009, 131, 8952. (c) Kistler, K. A.; Pochas, C. M.; Yamagata, H.;
Matsika, S.; Spano, F. C. J. Phys. Chem. B 2012, 116, 77. (d) Langhals,
H.; Hofer, A.; Bernhard, S.; Siegel, J. S.; Mayer, P. J. Org. Chem. 2011,
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r
10.1021/ol3029115
XXXX American Chemical Society

Figure 2. Structures of literature known phenalenone derivative
3 and benzoperylenes 4 and 5.
of similar structures such as 5, utilizing the basic coupling
of two hydroxyphenalenone molecules.15 In contrast to
these two reports, we have targeted an unsubstituted
methine position to allow for further functionalization.
For the synthesis of target 1 (Scheme 1), the reaction
conditions of Wurm13 were applied to a perylene system 6
containing both imide and anhydride functionalities. Ad-
ditionally, p-tert-butylphenoxy bay substituents were cho-
sen to provide excellent solubility to the perylene dyes to
aid their isolation and characterization. The reaction of 6
with dialkylmalonate occurs only at the anhydride posi-
tion, yielding a monofunctionalized intermediate 7 along
with an alkylated side product 8. Unfortunately, the
desired product 7 showed low stability, so condensation
with benzaldehyde was performed without fully purifying
the intermediate. The resulting covalently linked perylene
dimer 9 was isolated in a 20% yield calculated over the two
steps from perylene 6. Formation of a central pyran ring
between the perylenes was achieved by heating with acetic
anhydride to give 1 in 72% yield.
Figure 1. Structure of the target molecule 1 of this work, and
general structure of possible macrocycle 2.
thisphenomenon withPBI dyes10 and otherchromophores
have been reported.11 Longer range energy transfer be-
tween perylene dyes at defined angles has, however, been
studied.12
The structure of target compound 1 was inspired by the
previously reported phenalenone dimer 3 (Figure 2), which
was synthesized over three steps from naphthalene an-
hydride.13 Using a similar reaction scheme with perylene
derivatives should lead to the target dimer 1. However,
only two reports of benzoperylenones, where an enol oxy-
gen is available to undergo further reaction, exist in the
literature. In 2008, Feng and co-workers reported the reac-
tion between a bay-substituted perylene bisanhydride with
2-methylquinone, leading to π-extended perylene dyes
such as 4.14 In 2010, Buffet et al. reported the synthesis
Perylene dimers 1 and 9 were characterized by NMR
spectroscopy (the spectra with assignments are shown in
the Supporting Information (SI)). Despite the relatively
1
broad H NMR peaks, presumably due to the restricted
rotation of the phenoxy substituents and perylene units, all
of the resonances of dimer 9 could be assigned. Notably,
the presence of hydrogen bonding16 at the center of
the molecule is observed. A sharp signal at 13 ppm with
an integral of 1 proton is seen at room temperature, while
an additional broader peak becomes observable at lower
temperature. The broad resonances of protons on the
phenoxy solubilizing groups also become more distinguish-
able at lower temperatures. The central “bridge” proton
appears at 6.2 ppm indicating the location on a saturated
carbon, but being affected by the ring current of the nearby
perylene π-cores. Upon formation of compound 1 the
(11) (a) Osuka, A.; Maruyama, K. J. Am. Chem. Soc. 1988, 110, 4454.
(b) Takahashi, K.; Gunji, A.; Guillaumont, D.; Pichierri, F.; Nakamura,
S. Angew. Chem., Int. Ed. 2000, 39, 2925. (c) Takita, R.; Song, C.;
Swager, T. M. Org. Lett. 2008, 10, 5003–5005.
(12) (a) Ahrens, M. J.; Sinks, L. E.; Rybtchinski, B.; Liu, W.; Jones,
B. A.; Giaimo, J. M.; Gusev, A. V.; Goshe, A. J.; Tiede, D. M.; Wasielewski,
M. R. J. Am. Chem. Soc. 2004, 126, 8284. (b) Hinze, G.; Haase, M.; Nolde,
€
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F.; Mullen, K.; Basche, Th. J. Phys. Chem. A 2005, 109, 6725.
(13) Wurm, G.; Wittmann, H. J. Liebigs Ann. Chem. 1973, 1973,
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(14) Feng, J.; Liang, B.; Wang, D.; Xue, L.; Li, X. Org. Lett. 2008, 10,
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(15) Buffet, N.; Grelet, E.; Bock, H. Chem.;Eur. J. 2010, 16, 5549.
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4437.
Oznobikhina, L. J. Mol. Struct. 2011, 1006, 234.
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Org. Lett., Vol. XX, No. XX, XXXX

Scheme 1. Synthesis of Covalently Linked Perylene Dimers
Figure 3. UV/vis absorption spectra of perylene derived com-
pounds measured in CH2Cl2 at 298 K at concentrations in the
range of 10ꢀ5 M.
absorption band between around 500ꢀ700 nm, reveals
smaller thanexpected valuesforboth9and 1(perperylene)
when compared to the monomer. The fluorescence spec-
trum of methoxybenzo[c,d]perylenone 8 (see SI) retains the
mirror image band shape (λmax = 633 nm) and high
quantum yield (Φfl = 0.87 in CH2Cl2) observed for
perylene bisimides. However, for each of the dimerized
compounds 9 and 1, no fluorescence was observed. This
lackof fluorescencemight be duetotheformationoftriplet
states which has previously been reported for covalently
linked perylene dimers.17
Table 1. Properties of the First Optical (S0ꢀS1) Transition of
Benzoperylene Compounds in CH2Cl2 at 298 K
hydrogen bonding is lost, while the bridge proton shifts
upfield to 5.5 ppm with minor shifts observed for the
aromatic signals.
λmax
νmax
(cmꢀ1
ε
fwhm
(cmꢀ1 a
μag
(D)b
(nm)
)
(Mꢀ1 cmꢀ1
)
)
Absorption and fluorescence spectroscopy reveal more
about the structural features of 1 and 9. The alkoxy-
benzoperylenone 8 was used as a reference compound
due to its high stability. It exhibits a small red shift in the
lowest energy absorption band relative to the anhydride
parent molecule 6, due to the slight reorganization of the
π-system. Relative to the monomeric precursor 8, dimerized
molecule 9 exhibits a bathochromic shift of 40 nm. After
formation of the pyran ring between the perylenes, dimer 1
shows the lowest energy absorption lying between that of
monomer 8 and dimer 9 (Figure 3). The absorption max-
ima might be expected to double in intensity due to the
presence of two chromophores. However, for dimer 9, the
extinction coefficient of the lowest energy transition is in
fact 19000 Mꢀ1 cmꢀ1 larger than the expected value.
In addition, the full width at half-maximum (fwhm,
measured from only the low energy side of the band)
decreases by 150 cmꢀ1 (Table 1). In contrast, the pyran
annelated dimer 1 has an extinction coefficient slightly
smaller than double that of monomer 8, although the
fwhm value is again decreased. The transition dipole
moment μag, measured for the whole of the S0ꢀS1
6
8
9
1
579
541
592
550
632
584
615
565
17 300
18 500
16 900
18 200
15 800
17 100
16 300
17 700
40 700
24 700
38 100
23 700
95 300
55 400
69 200
45 100
1210
1260
1110
1170
6.8
6.9
11.0
9.6
a Measured for the lowest energy transition only. b Measured be-
tween approximately 500 and 700 nm. See SI for more details.
The origin of the spectral changes can be explained using
an excitonic coupling model.18 For two excitonically
coupled chromophores, two excitonic states originate,
one located at higher and one at lower energy compared
tothe transition energyof the monomer. Ifthe two dyes are
parallel, only one transition is allowed leading to either a
(17) Veldman, D.; Chopin, S. M. A.; Meskers, S. C. J.; Groeneveld,
M. M.; Williams, R. M.; Janssen, R. A. J. J. Phys. Chem. A 2008, 112,
5846.
(18) (a) Kasha, M. Radiat. Res. 1963, 20, 55. (b) Kasha, M.; Rawls,
H. R.; Ashraf El-Bayoumi, M. Pure Appl. Phys. 1965, 11, 371.
Org. Lett., Vol. XX, No. XX, XXXX
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inhibits the rotational fluctuation of the perylene groups,
instead holding them at a defined angle from each other.
Assuming the transition dipoles of the perylenes lie along
the long axis of the perylene dyes, the angle between the
transition dipoles, R, was estimated to be R ≈ 120°. The
distancebetween the centers of the perylenes was measured
˚
to be 12.6 A for 9. Since R is larger than 90°, the excitation
to the lower energy excited state is stronger than to the
higher excited state (see the SI for more details). Therefore,
the red-shifted absorption band has a large intensity, while
a blue-shifted absorption band is not visible due to its weak
intensity and overlapping bands from higher energy tran-
sitions. Similar effects give rise to the absorption spectrum
observed for dimer 1.
In conclusion, we reported the synthesis of spatially
fixed, covalently linked perylene dimers 9 and 1. A carbon
analog of perylene bisimide allowed the construction of
such systems due to the availability of an enol oxygen,
which can either form stabilizing hydrogen bonding inter-
actions or undergo further reaction thus affecting the
molecular geometry. This is not possible with regular
perylene bismides. Angle dependent excitonic coupling
between adjacent perylene chromophores leads to bath-
ochromically shifted absorption. The trigonal arrange-
ment between the two perylene chromophores in dimer
1 suggests that further work might yield hexagonal
cyclic assemblies showing similar interchromophore
interactions.
Figure 4. (a) Splitting of energy levels in an oblique excitonic
coupling showing the excitonic coupling Δε and transition
0
strengths μag and μag00. (b) MMFF94 energy minimized struc-
ture of covalently linked perylene dimer 9, and parameters R, θ, r,
and μag (tBu-phenyl groups omitted for clarity).
blue or red shift in the absorption spectrum, depending on
the orientation of the chromophores. In an oblique cou-
pling case (Figure 4), transitions to both of the excited
states are partially allowed since the transition dipoles can
never fully cancel each other. Therefore, both a red- and a
blue-shifted band should be observed with differing inten-
sity. The parameters which describe the new system are the
exciton splitting (Δε) between the two new excited states,
and the transition dipole moments (μag0 and μag00) between
the ground state and each new excited state. Both are
dependent on μag (the transition dipole moment of the
individual perylene chromophores) and θ (the angle between
this transition dipole moment and the distance vector r).
In order to visualize the interaction between the chro-
mophores in 9, the structure was modeled using the
MMFF94 force field19 (Figure 4b). First, the modeled
structure confirmed the proximity of the protons in the
center of 9 for hydrogen bonding. Importantly, this
Acknowledgment. Financial support by the DFG for
the research unit “Light-induced dynamics in molecular
€
aggregates” at the University of Wurzburg is sincerely
acknowledged. K.J.T. thanks the Alexander von Humboldt
Foundation for a postdoctoral fellowship.
Note Added after ASAP Publication. After ASAP pub-
lication on November 19, 2012, references 10c,d were
added regarding bichromophoric perylene bisimides and
excitonic coupling within them; the new version reposted
November 30, 2012.
Supporting Information Available. Experimental de-
tails and characterization of new compounds. This ma-
terial is available free of charge via the Internet at http://
pubs.acs.org.
(19) Halgren, T. A. J. Comput. Chem. 1996, 17, 490.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX