Hexaazatrinaphthylenes with different twists

Article abstract of DOI:10.1002/chem.201304071

A synthetic strategy that allows the induction of twist angles of different sizes in 5,6,11,12,17,18-hexaazatrinaphthylene (HATNA) chromophores is reported. The different twist angles are accompanied by measurable changes in the emission and electrochemic

Full text of DOI:10.1002/chem.201304071

DOI: 10.1002/chem.201304071
Communication
&
Twisted Aromatics
Hexaazatrinaphthylenes with Different Twists
Sunil Choudhary,[c, d] Cristian Gozalvez,[a] Alexander Higelin,[e] Ingo Krossing,[c, e]
Manuel Melle-Franco,[f] and Aurelio Mateo-Alonso*[a, b]
Dedicated to Prof. Maurizio Prato on the occasion of his 60th birthday
thesis and study of twisted PAHs,[4] because these compounds
Abstract: A synthetic strategy that allows the induction of
twist angles of different sizes in 5,6,11,12,17,18-hexaazatri-
naphthylene (HATNA) chromophores is reported. The dif-
ferent twist angles are accompanied by measurable
changes in the emission and electrochemical characteris-
tics of HATNA.
have enhanced stabilities and unique electronic and chiroptical
properties, deriving from their distorted molecular structure
and packing properties. Some of the effects of linear twists on
the optoelectronic properties of PAHs have been suggested by
Wudl et al.[4f] and confirmed theoretically by Houk et al.[5] How-
ever, to the best of our knowledge, no experimental correla-
tion between twist size and optoelectronic properties has
been reported. This is because the strategies used to longitudi-
nally twist the aromatic core utilise substituents that have
a direct electronic influence on the PAH core that makes the
assignment of any effects derived from the twists difficult.[4b–l]
We describe herein a synthetic strategy that allows the in-
duction of twist angles of different size in HATNA chromo-
phores. Our twisting approach is based on the introduction of
rigid acetylenes with large silyl substituents in positions that
force the aromatic core to deviate from planarity, leading to
twisted-HATNA 1 (Scheme 1). Increasing the size and rigidity of
the terminal silyl groups (triethylsilyl<triisobutylsilyl<triiso-
Polycyclic aromatic hydrocarbons (PAHs) have received a great
deal of attention because they are being developed into in-
creasingly better performing semiconductors.[1] Both carbona-
ceous and nitrogenated PAHs with a C3 symmetry, such as star-
phenes[2] and cloverphenes,[3] are particularly interesting be-
cause they have shown high stabilities, even for derivatives
with a high degree of conjugation. Among these PAHs,
5,6,11,12,17,18-hexaazatrinaphthylene (HATNA) derivatives
have arisen as promising n-type semiconductors with charge
mobilities as high as 0.9 cm2 VÀ1 sÀ1.[2b,d–f]
In some cases, C3-symmetrical PAHs can adopt twisted con-
formations as the result of the steric strain induced by over-
crowding or congestion of the aromatic core by substitu-
ents.[2c,3a–d,f,g] Significant effort has been dedicated to the syn-
[a] C. Gozalvez, Prof. A. Mateo-Alonso
POLYMAT, University of the Basque Country UPV/EHU
Avenida de Tolosa 72, 20018 Donostia-San Sebastian (Spain)
Fax: (+34)943 50 6062
E-mail: amateo@polymat.eu
[b] Prof. A. Mateo-Alonso
Ikerbasque, Basque Foundation for Science
48011 Bilbao (Spain)
[c] S. Choudhary, Prof. I. Krossing
School of Soft Matter Research
Freiburg Institute for Advanced Studies (FRIAS)
Albert-Ludwigs-Universitꢀt Freiburg
Albertstrasse 19, 79104 Freiburg (Germany)
Scheme 1. Synthesis of twisted-HATNA 1a–c.
[d] S. Choudhary
Institut fꢁr Organische Chemie und Biochemie
Albert-Ludwigs-Universitꢀt Freiburg
Albertstrasse 21, 79104 Freiburg (Germany)
propylsilyl) results in a substantial increase in the size of the
twist angle of the HATNA core. Most importantly, the influence
of these substituents on the optoelectronic properties is
almost negligible because of their aliphatic nature and their
distance to the aromatic core, as illustrated by parallel studies
carried out on reference compounds. Therefore, any change
observed on the properties is undoubtedly owed to the size of
the twist angle and not related to the substituents. The differ-
ent twist angles are accompanied by measurable changes in
[e] A. Higelin, Prof. I. Krossing
Institut fꢁr Anorganische und Analytische Chemie
Albert-Ludwigs-Universitꢀt Freiburg
Albertstrasse 21, 79104 Freiburg (Germany)
[f] Prof. M. Melle-Franco
Centro de CiÞncias e Tecnologias de Computażo, CCTC
Universidade do Minho, 4710-057 Braga (Portugal)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304071.
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Communication
the emission and electrochemical characteristics of twisted-
HATNA, confirming that longitudinal twists provide an addi-
tional dimension to modulate the properties of organic semi-
conductors.
angles between blades and 16, 17 and 188 end-to-end twist
angles for each blade (Figure 1). The DFT-calculated structure
of 1b predicts an average twist angle of 548, which is in agree-
ment with the X-ray diffraction data.
The synthesis of twisted-HATNA 1a–c was carried out follow-
ing the route depicted in Scheme 1. Diamines 2a and 2c were
prepared following a route developed by Bunz and co-work-
ers,[6] whereas 2b was synthesised by a modification of this
procedure. Commercially available 3 is functionalised by Sono-
gashira coupling with (triethylsilyl)acetylene, (triisobutylsilyl)-
acetylene[7] or (triisopropylsilyl)acetylene to obtain 4a–c, which
after reduction with LiAlH4 yield 2a–c. Remarkably, the tricyclo-
condensation of diamines 2a–c, independently, with the com-
mercially available hexaketocyclohexane in acetic acid heated
to reflux, proceeded smoothly; the size of the substituents had
no apparent influence on the reaction, which yielded 1a–
c with moderate yields (25–36%).
The DFT-calculated structure of 1a[8] reveals that the triethyl-
silyl substituents force the HATNA core to deviate substantially
from planarity, to adopt an asymmetrical propeller-like struc-
ture (Figure 1). An average twist angle of 518 between blades
(48, 49 and 558 for each blade) and an average end-to-end
twist angle of 158 (12, 16 and 178 for each blade) are ob-
Similarly, slow evaporation of CHCl3/MeOH solutions of 1c
yielded single crystals suitable for X-ray diffraction. The crystal
structure of 1c[11] shows a much more significant distortion of
the HATNA core, consistent with the higher rigidity of triiso-
propyl substituents. For 1c a 678 torsion angle between blades
and a 208 end-to-end twist angle for each blade was observed.
The DFT model is consistent with the crystallographic structure
and a twist angle of 638 between blades was predicted. The
underestimation of the twist angle is reasonable, because the
simulations are carried out in vacuo.
The absorption features of 1a–c are similar to those ob-
served for HATNA derivatives, but appear bathochromically
shifted (ca. 45 nm) because of extended conjugation with six
acetylenes. When comparing the absorption spectra of 1a–c,
almost invariable absorption features were observed. Increas-
ing bathochromic shifts, of approximately 1 nm, with increas-
ing twist angles were discernible in the transitions between
300 and 500 nm (Figure 2 and Table 1). However, the same var-
iations were observed in the absorption spectra of acenothia-
diazoles 4a–c (Table 1 and Figure S7 in the Supporting Infor-
mation), which were used as a reference, illustrating that the
twist angles have no effect on the absorption properties.
On the contrary, the different twist angles do have an influ-
ence on the emission characteristics of twisted-HATNA. On
Figure 1. Top and side views of the DFT-optimised structure of compound
1a and of the X-ray structures of compounds 1b and 1c (ellipsoids are
shown at a 50% probability level, hydrogen atoms are omitted for clarity).
Twist angles between blades are given.
served.[9] This model is built using the same level of calcula-
tions (LDA 6-31g*) that reproduce the X-ray crystallographic
structures of 1b and 1c given below, which confirms its suit-
ability to estimate the structural distortions of 1a.
Figure 2. Normalised absorption spectra of 1a–c in CHCl3.
Single crystals of 1b were obtained by slow evaporation
from CHCl3/MeOH solutions. The crystal structure for 1b[10] re-
veals a complex structure, in which two unsymmetrical and
enantiomeric propeller-shaped structures are observed in the
unit cell with an average twist angle of 558 between blades
and an average end-to-end twist angle of 168. One of the
structures of 1b is less distorted and shows 48, 52 and 578 tor-
sion angles between blades and 9, 13 and 248 end-to-end
twist angles for each blade. On the other hand, the second
structure is more distorted, displaying 54, 57 and 628 torsion
Table 1. Selected photophysical and electrochemical data for 1a–c and
4a–c.
1a
1b
1c
4a
4b
4c
[a]
lmax
350
523
À0.84
À1.12
351
514
À0.95
À1.19
352
504
À0.93
À1.15
387
458
À1.20
–
388
458
À1.22
–
388
457
À1.21
–
[a]
lem
I[b]
1
E =
2
II[b]
1
E =
2
[a] CHCl3 [nm]. [b] E versus SCE [V] using ferrocene (Fc) as internal refer-
ence (E1/2Fc(SCE) = +0.48 V).
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1
1
tentials of 1b (DE = =À110 mV) and 1c (DE = =À90 mV) with
2
2
the less twisted 1a. The same trend is observed in the second
reduction wave on the comparison of 1b and 1c with the less
1
twisted 1a (DE = =À90 and À50 mV, respectively). The oppo-
2
1
site trend is observed on the comparison of the E = value of
2
the two reduction processes of 1b with those of 1c, which are
more cathodic. Again, studies carried out on compounds 4a–
c (Table 1 and Figure S7 in the Supporting Information), as ref-
erence compounds, show an almost negligible effect of the
substituents on the reduction potentials, corroborating that
the changes observed in the reduction potentials of 1a–c are
indeed an effect of the size of the twist angle.
In conclusion, we have reported a synthetic strategy that
provides access to a new family of HATNA derivatives with dif-
ferent twist angles by introducing silyl groups with different
size and rigidity. This has been evidenced by a combination of
theoretical calculations and X-ray crystallography. In addition,
this work provides direct experimental correlation between
twist size and properties, since the influence of the carefully
selected substituents with the electronic properties of the aro-
matic core is almost negligible. Therefore, the changes ob-
served on the properties of twisted-HATNA are a direct effect
of the size of the different twist angles and not of the substitu-
ents that can be considered electronically equivalent. In partic-
ular, increasing the size of the twist angles results in increasing
hypsochromic shifts, in the emission characteristics of twisted-
HATNA. Also, angle-dependent changes on the redox poten-
Figure 3. Normalised emission spectra (lex =350 nm) of 1a–c in CHCl3.
comparison of the emission spectra of 1a–c (Figure 3 and
Table 1), increasing hypsochromic shifts, as large as 19 nm, are
clearly observed as the twist angles increase. In principle, it
could be argued that such differences are an effect of the dif-
ferent substituents and not of the twist angles. However, paral-
lel emission studies carried out on 4a–c show an almost negli-
gible effect of the substituents on the emission spectra
(Table 1 and Figure S7 in the Supporting Information). There-
fore, the changes observed in the emission properties of 1a–
c are undoubtedly due to the size of the twist angle and are
not related to the substituents.
1
tials of twisted-HATNA have been observed with DE = as large
2
as À110 mV. Further studies that combine synthesis and calcu-
Similarly, the size of the twist angle also has an influence on
the redox chemistry of the HATNA core. Cyclic voltammetry re-
veals a set of two reversible reductions for 1a–c (Figure 4 and
lations will attempt to correlate solid-state and solution molec-
ular conformations. These will provide a more detailed picture
of the influence of the twist angles on the properties, in which
symmetry might play a crucial role, and will allow the exploita-
tion of the effects of non-planarity in the design of new organ-
ic semiconductors for specific applications.
1
Table 1). Half-wave potential shifts (DE = ) to more negative po-
2
tentials are observed on comparison of the first reduction po-
Acknowledgements
This work was carried out with support of the Freiburg Insti-
tute for Advanced Studies (Junior Research Fellowship), POLY-
MAT, the Basque Science Foundation for Science (Ikerbasque),
Gobierno Vasco (SAIOTEK), Diputacion de Guipuzkoa, and the
Portuguese “Fundażo para a CiÞncia e a Tecnologia” (CiÞncia
2008 and contracts PEst-OE/EEI/UI0752/2011 and CONC-REEQ/
443/2005).
Keywords: electrochemistry
·
fluorescence
·
hexaazatrinaphthylenes · polycyclic aromatic hydrocarbons ·
twisted aromatics
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[10] CCDC-953876 contains the supplementary crystallographic data for 1b.
These data can be obtained free of charge from The Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif:
¯
C
216H336N12Si12, Mw =3438.72, triclinic, space group P1, a=18.6464(3),
b=21.2919(3), c=29.5274(4) ꢁ; a=80.5574(8), b=83.6978(8), g=
73.0153(8)8; V=11035.9(3) ꢁ3; Z=2; 1calcd =1.035 MgmÀ3
F(000)=
3796; l=0.71073 ꢁ; T=100(2) K; absorption coefficient=0.120 mmÀ1
absorption correction: multiscan, Tmin =0.6900, Tmax =0.7461, 2qmax
;
;
=
47.068; number of (independent) reflections collected=137731
(32717); largest peak/hole [eꢁÀ3]=1.69/À0.66; GooF=1.040; R1=7.47
and wR2=20.72 for reflections I>2s(I), R1=10.52 and wR2=23.98 for
all reflections.
[11] CCDC-953875 contains the supplementary crystallographic data for 1c.
These data can be obtained free of charge from The Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif:
¯
C90H132N6OSi6, Mw =1482.82, trigonal, space group R3, a=31.1383(3),
b=31.1383(3), c=16.9549(3) ꢁ; a=90, b=90, g=1208; V=
14236.9(3) ꢁ3; Z=6; 1calcd =1.038 MgmÀ3; F(000)=4836; l=0.71073 ꢁ;
T=100(2) K; absorption coefficient=0.132 mmÀ1
; absorption correc-
tion: multiscan, Tmin =0.6761, Tmax =0.7461, 2qmax =57.618, number of
(independent) reflections collected=73589 (7272); largest peak/hole
[eꢁÀ3]=1.32/À0.35; GooF=1.073; R1=4.78 and wR2=13.61 for reflec-
tions I>2s(I), R1=5.24 and wR2=14.01 for all reflections.
Received: October 17, 2013
Published online on December 9, 2013
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