New synthetic routes to Z-shape functionalized perylenes

Article abstract of DOI:10.1021/ol3014667

Z-shape (1,2,7,8-tetrasubstituted) perylene derivatives are novel chromophores with great potential in various applications. Yet, the synthetic entry into this class of molecules is hitherto quite limited. In this communication, the synthesis of a series

Full text of DOI:10.1021/ol3014667

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
New Synthetic Routes to Z-Shape
Functionalized Perylenes
Billa Bhargava Rao, Jun-Ru Wei, and Chih-Hsiu Lin*
Institute of Chemistry, Academia Sinica, 128 Academia Road, Nankang, Taipei, Taiwan
chl@chem.sinica.edu.tw
Received May 29, 2012
ABSTRACT
Z-shape (1,2,7,8-tetrasubstituted) perylene derivatives are novel chromophores with great potential in various applications. Yet, the synthetic
entry into this class of molecules is hitherto quite limited. In this communication, the synthesis of a series of Z-shape perylene derivatives via a
double WittigÀKnoevenagel benzannulation protocol is reported. Preliminary photophysical and electrochemical studies indicate that the frontier
orbital energy levels of these new perylene systems are modulated by electronic, regiochemical, and conformational effects.
Perylene is an important class of organic chromophores.
Although first developed as dyes and pigments,1 their
recent applications have reached far beyond their original
role. These molecules are especially versatile in the field of
organic electronics where perylene derivatives are frequently
employed in devices including field effect transistors,2 light
emitting diodes,3 photovoltaic cells,4 optical switches,5 and
molecular wires.6
imide and ester derivatives have been synthesized.7 This
substitution pattern is referred to as linear or peri as
depicted in Scheme 1. Recently, researchers have also
developed protocols to further functionalize these linear
derivatives at the bay (1,6,7,12) positions via halogenation8
or at the ortho (2,5,8,11) positions via ruthenium complex
catalyzed CÀH activation.9
On the other hand, the 1,2,7,8-substituted perylene de-
rivative (generally referred to as a Z-shape or ortho-bay
derivative) is quite rare. The only example was published
by Meador and co-workers10 in 2006. The key step in their
synthesis is the DielsÀAlder trapping of o-xylylenols
(generated via Norrish type II hydrogen abstraction) by
maleimides to result in Z-shape perylene diimides (PDIs).
Since the photochemistry requires an aryl ketone as its
substrate, the Z-shape perylene thus produced inevitably
carries phenyl substitutions at its peri positions. To broaden
Because of the commercial availability of 3,4,9,10-
perylenetetracarboxylic dianhydride, numerous perylene
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Shinokubo, H.; Osuka, A. Org. Lett. 2009, 11, 5426. (e) Jiao, C.; Huang,
K.-W.; Guan, Z.; Xu, Q.-H.; Wu, J. Org. Lett. 2010, 12, 4046.
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10.1021/ol3014667
XXXX American Chemical Society

our understanding of this new class of perylenes and to
explore their potential applictions, we developed a new
synthetic strategy to Z-shape perylene with a much wider
substrate scope. The preliminary results are reported herein.
after Heck coupling13 (with styrene) and ozonolysis. The
dicarbaldehyde 5 was then subjected to the WittigÀ
Knoevenagel benzannulation as described previously to
furnish the Z-shape perylene diimide (1a), tetranitrile (1b),
and tetraester (1c). It should be noted that the benzannula-
tions proceed in respectable yields despite mixtures of E/Z
isomers being produced in Wittig reactions. Such synthetic
efficiency is acceptable considering four CÀC bonds are
formed in one reaction. Successive additions of different
Wittig reagents during the benzannulation reactions can
lead to hybrid perylenes with dissimilar substitutions at
two sides of the molecules (1d, 1e, and 1f). Another class of
hybrid perylenes (1g and 1h) is centrally symmetric yet
carries different substitutions at the 1 and 2 positions.
Their synthesis can be accomplished by utilizing Wittig
reagents generated by Michael addtion to (E)-alkyl
3-cyanoacrylate or alkylation of ethyl (triphenylphos-
phoranylidene)acetate (see Supporting Information). With
a similar retrosynthesis and the identical starting com-
pound, we can also synthesize dialkyl perylene-1,7-dicar-
boxylate2 from precursor 6 (synthesizedviaHeckcoupling
as a mixture of olefin regioisomers) via a double Knoeve-
nagel condensation.
Scheme 1. Retrosynthetic Analysis
Scheme 2. Synthesis of 1aÀh and 2a
We recently reported a WittigÀKnoevenagel benzannu-
lation strategy to synthesize acene diesters, dinitriles, and
diimides.11 Starting from aromatic ortho-dialdehydes,
we performed Wittig olefination with 2-(triethyl-λ5-phos-
phanylidene)-succinyl derivatives such as diesters, dini-
triles, or imides. (The Wittig reagents were generated from
the Michael addition of triethyl phosphine to electron-
deficient alkenes.) With DBU as a basic catalyst, the inter-
mediates can then undergo intramolecular Knoevenagel
condensation to produce acene diester, dinitrile, or imide
(Scheme 1). Because the perylene skeleton is composed of
two naphthalene units, we can adopt this benzannulation
reaction to make Z-shape perylenes. As depicted in
Scheme 1, 9,10-anthraquinone-1,5-dicarbaldehyde should
be the proper starting material for this purpose. After a
double benzannulation reaction, Z-shape perylenes with
electron-withdrawing substitutions at ortho-bay positions
should be selectively produced.
The synthetic route to the target Z-shape tetrasubsti-
tuted perylenes (1aÀh) is outlined in Scheme 2. The key
building block 9,10-anthraquinone-1,5-dicarbaldehyde 5
was synthesized from 1,5-diiodo-9,10-anthraquinone 412
a Reagents and conditions: (a) Styrene, (MeCN)2PdCl2, Bu4NBr, Et3N,
toluene, 100 °C, 24 h, 64%; (b) Ozone, CH2Cl2, (CH3)2S, À78 °C, 10 min,
then to rt, 2 h, 78%; (c) PEt3, A or B or C substrate (A = 1-Dodecyl-1H-
pyrrole-2,5-dione; B = Fumaronitrile; C = Diethylmaleate), CH2Cl2, 0 °C
to rt, 2 h, DBU, rt, 2 h; (d) Octylbut-3-enoate, (MeCN)2PdCl2, Bu4NBr,
Et3N, 100 °C, 24 h, 59%; (e) NaOtBu, tert-butanol, rt, 2 h, 26%.
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(c) Lin, Y.-C.; Lin, C.-H.; Chen, C.-Y.; Sun, S.-S.; Pal, B. Org. Biomol.
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B
Org. Lett., Vol. XX, No. XX, XXXX

In Scheme 3, we describe a similar strategy to synthesize
Z-shape perylene compounds with extra alkoxy substitutions
at the bay and peri positions. Since the appropriately sub-
stituted diformyl anthraquinones are not readily accessible as
5, we altered the synthetic sequence by performing the Wittig
(or WittigÀHorner) reaction on dialkoxyl diformyl anthra-
cene 7 and 10 (synthesized via Vilsmeier formylation)14 to
yield 8 and 11 respectively. The anthracene core was then
oxidized with chromium oxide15 to yield anthraquinone
derivatives 9 and 12. These two compounds were then
subjected to Knoevenagel condensation in the presence of
DBU to produce the desired perylene derivatives 3a and 3b.
A single crystal of 3a suitable for crystallography ana-
lysis was grown from a saturated chloroform solution. As
expected, the perylene skeleton of 3a shows a significant
twist near the bay area (Figure 1). The dihedral angle
between two naphthyl units (25.16°) is evidently smaller
than that of 1,6,7,12-tetrachloro PDI (36.7°),16a yet com-
parable to those of bay disubstituted derivatives such as
Meador’s Z-shape PDI (25°)10 and 1,7-bis-perfluorobutyl
PDI (28.9°).16b Such twisting effectively prevents intermo-
lecular πÀπ interactions. The closest intermolecular con-
tact in the unit cell is between the perylene plane and
decyloxy chain along the b axis (Supporting Information).
Figure 1. X-ray crystal structure of 3a.
The frontier orbital energy levels of these Z-shape
perylenes are probedbycyclicvoltammetry, and the results
are listed in Table 1. The first reductive wave of diimide 1a
(À0.99 eV) is very similar to that of its linear counterpart
PDI (Table 1),17 indicating little positional effect. How-
ever, a positional effect can manifest via steric interactions.
Forexample, all ester groups in3,4,9,10-perylene tetraester
(PTE) mustbeorthogonal tothe perylene plane toalleviate
the 1,3-interaction. The resonance effect of the electron-
withdrawing ester groups isweakenedin this conformation;
hence the reductive potential of PTE18 is noticeably more
negative than that of 1c. The steric factor is also responsible
for the discrepancy between 1g’s and 1h’s reductive poten-
tials; when the bulkier ester group is placed at the bay
position, the perylene skeleton is more twisted and there-
fore harder to reduce. Likewise, the dramatic elevation of
the reductive potential from À0.65 eV for 1b to À0.97 eV
for 3a should also be attributed to a similar skeletal twist.
Predictably, electron-donating alkoxyl substituents lower
the oxidative potential considerably (1.52 eV in 1c to 1.07 eV
in 3b). Among the three types of functional groups, nitrile
clearly exerts the strongest stabilization on the LUMO.
The first reduction wave of tetranitrile 1b is À0.64 eV,
which is on par with many electron acceptors or N-type
organic materials.
Scheme 3. Synthesis of 3a and 3ba
The photophysical data are also listed in Table 1.
Selected absorption and emission spectra (1aÀc, 2, and
3a) are shown in Figure 2. Both absorption and emission
spectra of 1a and 1c are blue-shifted compared to linear
counterparts like PDI and PTE. Because the LUMO levels
of 1a and 1c are comparable or even lower than those of
PDI and PTE, the hypsochromic shifts must be attributed
to the suppression of HOMO levels by the ortho and bay
substitutions. More surprisingly, absorption maxima of 1c
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3 h, 55%; (b) CrO3, AcOHÀH2O, acetone, rt, 24 h, 52%; (c) DBU,
CH2Cl2, rt, 3 h, 46%; (d) NaH, bis(2,2,2-trifluoroethyl) phosphite,
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Org. Lett., Vol. XX, No. XX, XXXX
C

Table 1. Optical and Electrochemical Data
λ
max/nm
λ
max/nm
compd
(log ε)a
(Φfl)b
Ered/V
Eox/V
1a
1b
1c
482 (4.56)
457 (4.48)
429 (4.56)
468 (4.49)
463 (4.55)
451 (4.47)
433 (4.54)
459 (4.52)
440 (4.43)
516 (4.48)
459 (4.50)
523
500 (0.36)
475 (0.38)
486 (0.07)
487 (0.55)
497 (0.18)
504 (0.35)
486 (0.17)
480 (0.67)
503 (0.24)
552 (0.83)
550 (0.54)
536
À0.998
À0.648
À1.319
À0.905
À1.143
À1.043
À1.088
À0.987
À1.335
À0.974
À1.487
À1.06
1.625
À
1.520
À
1d
1e
1.570
À
1f
1g
1h
2
À
À
1.326
1.472
1.067
À
3a
3b
PDI
PTE
472
493
À1.50
À
a Measured in dichloromethane with a concentration of 1 Â 10À5 M.
b Determined using the integrating sphere method as described by de
Mello et al.19
and 1g exhibit slight hypsochromic shifts even to unsub-
stituted perylene (436 nm) and 1,7-diester 2 (440 nm). These
results clearly indicate functionalization at the ortho position
leads to hypsochromic shifts by suppressing the HOMO
levels. This conclusion is well corroborated by other recently
reported ortho substituted PDI derivatives.9 Compounds with
ester groups at bay positions (1c, 1e, 1f, 1g, 2) generally exhibit
less structured emission spectra, lower fluorescence quantum
yields, and larger Stokes shifts, all indicating rather flexible
equilibrium structures.
In conclusion, we have synthesized Z-shape pery-
lene systems via a WittigÀKnoevenagel benzannula-
tion strategy. The photophysical and electrochemical
properties of these compounds were studied. Some
compounds with low LUMO energies are promising
candidates for n-channel semiconductors. Since the
reactive peri sites are unsubstituted in these perylenes,
a wide range of further functionalization can be car-
ried out, including the synthesis of pushÀpull pery-
lenes, perylene-based electron acceptors, and higher
Figure 2. Normalized absorption and emission spectra of com-
pounds 1aÀc, 2, and 3a.
rylenes.20 All these challenges and opportunities are
currently being exploited in our group.
Acknowledgment. The study was partly funded by the
TIGP program at Academia Sinica. We wish to express
gratitude to the instrumental facility (mass, X-ray) at the
Institute of Chemistry for their service.
Supporting Information Available. Experimental details,
spectral characterizations, X-ray, UV, CV graphs are in-
cluded in the Supporting Information. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
(20) (a) Jiao, C.; Huang, K.-W.; Luo, J.; Zhang, K.; Chi, C.; Wu, J.
Org. Lett. 2009, 11, 4508. (b) Pschirer, N. G.; Kohl, C.; Nolde, F.; Qu, J.;
€
Mullen, K. Angew. Chem., Int. Ed. 2006, 45, 1401.
The authors declare no competing financial interest.
D
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