Twisted pyrene-fused azaacenes

Article abstract of DOI:10.1039/c3cc48742c

An approach for introducing twists in pyrene-fused azaacenes is reported. Depending on the volume and the rigidity of the silyl groups, different-sized twist angles, which oscillate between 4°and 24°, are induced along the longitudinal conjugated backbone

Full text of DOI:10.1039/c3cc48742c

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Twisted pyrene-fused azaacenes†
Sandeep More,abc Sunil Choudhary,bc Alexander Higelin,d Ingo Krossing,bd
Cite this: Chem. Commun., 2014,
50, 1976
Manuel Melle-Francoe and Aurelio Mateo-Alonso*af
Received 15th November 2013,
Accepted 5th December 2013
DOI: 10.1039/c3cc48742c
www.rsc.org/chemcomm
An approach for introducing twists in pyrene-fused azaacenes is angles, which oscillate between 41 and 241, are induced along the
reported. Depending on the volume and the rigidity of the silyl longitudinal conjugated backbone. Additionally, DFT calculations
groups, different-sized twist angles, which oscillate between 48 and show that the LUMO and LUMO+1 are mostly degenerate and
248, are induced along the longitudinal conjugated backbone.
interchange for larger twists.
Pyrene-fused hexacene 1a8e with six confronting triisopropylsilyl
Significant advances in electronics1 and plasmonics2 are expected to (TIPS) substituents was obtained by cyclocondensation of 2a8e and
arise from customised polycyclic aromatic hydrocarbons (PAHs). 3a10 in good yields (78%). Although 1a is known,8e no crystallographic
The planar structure of PAHs can be constrained into exotic twisted structure has been reported. After many attempts, it was possible to
conformations by means of congested substitution patterns,3 which obtain crystals suitable for X-ray crystallography by slow evaporation
has provided exceptional optoelectronic properties,4 enhanced of CHCl3/MeOH solutions of 1a (Fig. 1). X-ray diffraction studies
solubility,5 and unique intermolecular p-contacts.6 Among these, evidence an alternated twisted structure for hexacene 1a, in which the
twisted pyrene-fused acenes4b,7 have emerged not only as challen- two quinoxaline units are in the same plane and the pyrene is tilted.
ging compounds from the synthetic point of view, but also as A quinoxaline–pyrene twist angle of 61 and an end-to-center twist
promising p-type materials for OLED applications.4b Their parent angle of 51 were observed.11,12 Although the TIPS groups are very large,
pyrene-fused azaacenes are electronically complementary (n-type) the offset between the quinoxaline and pyrene moieties limits the
and easier to synthesise by means of pyrazine-type cyclocondensa- spatial overlap among TIPS groups.
tions, which has allowed the preparation of higher pyrene-fused
oligomers8 and polymers.9 However, no strategies to deliberately this approach to obtain pyrene-fused azaacenes with a higher
twist pyrene-fused azaacenes have been described. twist. Much bulkier triisobutylsilyl (TIBS) substituents were
The observed deviations encouraged us to further develop
In this communication, we report a synthetic approach for introduced in order to overcome the offset of the central and
introducing twists in pyrene-fused azaacenes, which is based lateral substituents. In this case, 2b was synthesised from
on the introduction of acetylene substituents with bulky silyl compound 48e by Sonogashira coupling with (triisobutylsilyl)-
groups in confronting positions (Scheme 1). Depending on the acetylene13 followed by acid hydrolysis (Scheme 1). Diamine 3b
volume and the rigidity of the silyl groups, different-sized twist was prepared by Sonogashira cross-coupling between commer-
cially available 5 and (triisobutylsilyl)acetylene13 followed by
reduction with LiAlH4 (Scheme 1). Pyrene-fused hexacene 1b
a POLYMAT, University of the Basque Country UPV/EHU, Avenida de Tolosa 72,
E-20018 Donostia-San Sebastian, Spain. E-mail: amateo@polymat.eu
b School of Soft Matter Research, Freiburg Institute for Advanced Studies (FRIAS),
¨
Albert-Ludwigs-Universitat Freiburg, Albertstraße 19, 79104 Freiburg, Germany
c
¨
¨
Institut fur Organische Chemie und Biochemie, Albert-Ludwigs-Universitat
Freiburg, Albertstraße 21, 79104 Freiburg, Germany
d
e
¨
¨
Institut fur Anorganische und Analytische Chemie, Albert-Ludwigs-Universitat
Freiburg, Albertstraße 21, 79104 Freiburg, Germany
ˆ
˜
Centro de Ciencias e Tecnologias de Computaçao, CCTC Universidade do Minho,
4710-057 Braga, Portugal
f Ikerbasque, Basque Foundation for Science, Bilbao, Spain
† Electronic supplementary information (ESI) available: Experimental details for
the synthesis, characterisation and DFT calculations. CCDC 972121–972123. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3cc48742c
Scheme 1 Synthesis of twisted pyrene-fused azaacenes 1a–c.
1976 | Chem. Commun., 2014, 50, 1976--1979
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molecular structure (Fig. 2) and with previous observations.8e Almost
identical absorption features were observed on 1a–c (lmax = 435, 432,
and 436 nm, respectively) while some differences were discernible in
the UV region for 1b. Emission bands centred at 489, 487 and
485 nm were observed respectively for 1a–c (lex = 350 nm). The
electron-deficient nature of the pyrene-fused azaacene core is
reflected by cyclic voltammetry. Three electrochemical reduction
waves were observed, the first two reversible and the third one
irreversible. Shifts towards more cathodic potentials are observed
when comparing the reduction waves of 1b with 1a. In contrast, the
reduction waves of 1c are anodically shifted, which is consistent with
the more electron-withdrawing nature of TPS groups.
The HOMO–LUMO gaps were estimated from the absorption
onsets and are in the range of 2.7 eV (see Table 1). The LUMO levels
were estimated from the potential onsets of the first reduction waves,
which reflects the small influence of the alkyl nature of the TIPS and
TIBS substituents at the acetylene ends for 1a and 1b while the ELUMO
for 1c is substantially lowered as a consequence of the more electron-
withdrawing nature of the TPS groups as described above.
Fig. 1 Front and side views of the X-ray structure (ellipsoids at a 50%
probability level) of compounds 1a, 1b and 1c.
was obtained by cyclocondensation of 2b and 3b in 46% yields.
A theoretical estimation of the HOMO and LUMO levels in vacuo
Crystals suitable for X-ray crystallography were obtained by slow (B3LYP/6-31G*) follows the same trends of the measured optical
evaporation of CH2Cl2–MeOH solutions of 1b (Fig. 1). To our HOMO–LUMO gaps and electrochemical LUMO levels (Table 1),
surprise the crystal structure of 1b displayed a shorter quinoxa- with a good agreement between measured and calculated HOMO–
line–pyrene (41) and end-to-center twist angles (0.51). Even if this LUMO gaps. The overestimation of the energy levels is reasonable
result is contradictory, since TIBS groups are more voluminous due to the lack of solvent in the simulations. As expected, both
than TIPS, this can be easily rationalised in terms of rigidity. The the HOMO and the LUMO orbitals span along the pyrene-fused
additional methylene group increases the volume of TIBS; how- hexacene core (Fig. 3). The geometrical shapes of the HOMO (B2g
ever, it also provides an additional degree of conformational symmetrical) remain invariable with respect to the size of the twist
freedom. Therefore, the enhanced flexibility allows the accom- angle. Higher orbital coefficients are observed in the HOMO levels of
modation of all the substituents in the same plane to a greater 1a–c on most of the peripheral rings (A and B) of the hexacene core
extent, which results in smaller twist angles.
and also on the external rings (G and H) of the pyrene moiety (the
In order to overcome the offset between pyrene and quinoxaline assignments of the rings corresponds to the lettering shown on
substituents, we decided to introduce triphenylsilyl (TPS) groups, since Scheme 1). This is consistent with Clar rules14 that predict four
they combine the bulkiness and rigidity of three phenyl rings. Pyrene- sextets on the same rings. In the case of the LUMO, the shape of the
fused hexacene 3c was obtained by cyclocondensation of tetraketone 1c orbitals (B1g symmetrical) remains virtually unchanged for the less
and diamine 2c, which were synthesised following the same route set twisted pyrene-fused hexacenes 1a and 1b, which appeared to be
out in Scheme 1 but with (triphenylsilyl)acetylene. Slow evaporation of a more equally spread over the longitudinal axis of the pyrene-fused
CH2Cl2 solution of 3c provided crystals suitable for X-ray diffraction. tetraazahexacene core in comparison to the HOMO. Nevertheless,
The crystal structure showed a clear alternate twist of the aromatic core the LUMO coefficients for 1a and 1b are larger on rings A, B, E and F
out of the plane (Fig. 1). The angle between the quinoxaline and the and there is an almost negligible contribution from rings C and D.
pyrene moieties was found to be 241 with end-to-center angles of 211, Notably, the shape of the LUMO orbitals in the more twisted 1c
which largely surpassed the angles observed for 1a and 1b.
(B3u symmetrical) differs substantially from that observed in 1a
The absorption spectra of 1a–c displayed well-resolved vibronic and 1b. As a matter of fact, in 1c, a new and larger contribution
bands along the UV and the visible region consistent with the to the LUMO emerges from rings C and D with large orbital
Fig. 2 Left: absorption of 1a–c. Center: emission (lex = 350 nm) of 1a–c in THF. Right: cyclic voltammograms of 1a–c in THF (0.1 M nBu4PF6).
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Table 1 Selected photophysical and electrochemical data and experimental energy levels of 1a–c
E1I/2
E
E
Eonset
Egap(opt)
ELUMO(CV)
Egap(calc)
ELUMO(calc)
a
a
a
b
II
1/2
b
III b
pc
b
c
d
e
e
Compound
lmax
lonset
lem
1a
1b
1c
435
432
436
460
460
455
489
487
485
À0.95
À0.98
À0.87
À1.12
À1.15
À1.05
À1.85
À1.92
À1.71
À0.82
À0.84
À0.70
2.69
2.69
2.72
À3.49
À3.46
À3.54
3.0
3.1
3.1
À2.7
À2.7
À2.8
a
b
SCE
c
Measured in THF (nm). Measured from 0.1 M THF/nBu4PF6 vs. SCE (V) using ferrocene (Fc) as internal reference (E
= +0.48 V). Estimated
1/2Fc
d
from the absorption onset (eV). Estimated from CV EONSET according to ELUMO = À4.8 À e(EONSET À E1/2Fc) where E1/2Fc was measured in situ (eV).
e
Calculated B3LYP/6-31G* using Spartan 10 (eV).
fused azaacenes in fields in which electron-deficient organic materials
are currently used, such as electronics and plasmonics.
This work was carried out with the support of POLYMAT, the
Basque Science Foundation for Science (Ikerbasque), the Freiburg
Institute for Advanced Studies (Junior Research Fellowship),
Deutsche Forschungsgemeinschaft (AU 373/3-1 and MA 5215/4-1),
˜
Gobierno de Espana (Ministerio de Economia y Competitividad,
MAT2012-35826), Diputacion Foral de Guipuzcoa, the Portuguese
˜
ˆ
‘‘Fundaçao para a Ciencia e a Tecnologia’’ (PEst-OE/EEI/UI0752/2011
and CONC-REEQ/443/2005).
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12 End-to-center twist angles are given instead of the standard end-to-
end twist angles because of the alternate twists observed, otherwise
the sum of two equal alternate end-to-center angles with opposite
signs would be 01.
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