under vacuum at r.t. as well as in MeCy at 77 K, which is
attributable to the heavy atom effect of sulfur. Meanwhile,
p-tBuTxS exhibits a higher hole mobility compared with
that of m-tBuTxS in the evaluation of thin-film FET char-
acteristics. These results are discussed in terms of intrinsic
molecular structures and arrangements determined by X-ray
crystallography.
X
X
X
X
X
X
Keywords: Trioxatruxene
j
Trithiatuxene
j
Photoluminescence
4a (p-tBuTxO): X = O
5a
4b
(m-tBuTxO): X = O
(p-tBuTxS): X = S
5b (m-tBuTxS): X = S
1. Introduction
Chart 2. Structure of trioxatruxenes 4a and 4b and of
trithiatruxenes 5a and 5b.
Heteroatom-containing polycyclic arenes are currently rec-
ognized as an important class of compounds in the field of
organic electronics since they are relatively robust and often
exhibit invaluable charge-transporting ability and luminescent
property due to the presence of heteroatoms.1-3 In order to
enhance their processability upon increasing solubility, linear-
alkyl side chains are conventionally introduced to the mother
skeletons. The alkyl groups may also promote their charge-
transporting ability while furnishing smooth self-assembling
nature in the thin-film state. On the other hand, introduction of
bulky alkyl groups such as tert-butyl group may enhance not
only their solubility, but also molecular luminescence intensity
in the solid state while alleviating intermolecular π-π inter-
action. On the contrary, certain arranged crystal packing modes
that are suitable for effective charge transport are known to be
realized even with the substitution of tert-butyl groups in some
polycyclic heteroarenes as well as polyaromatic hydrocarbons.4
For instance, Geerts and Bao reported that 2,6-di(tert-butyl)-
substituted BTBT ([1]benzothieno[3,2-b]benzothiophene) 14a
shows a high hole mobility (Chart 1). Takimiya demonstrated
that its seleno analogue 24b also has a similar charge mobility.
We recently reported the synthesis and photoluminescent
properties of 2,8-di-tert-butylated BBFPy (bisbenzofuro[2,3-b:
2¤,3¤-e]pyrazine) 3a and its 3,9-di-tert-butyl isomer 3b, both of
which show intense photoluminescence in solution and solid
state.5 The 2,8-isomer 3a was demonstrated to specifically
exhibit a unique solid state mechanochromic fluorescence in
spite of its rigid central core structure, while the 3,9-isomer 3b
does not at al. The difference was interpreted in terms of
molecular arrangements determined by X-ray crystallography.
Besides linear heteroacenes such as BTBT and BBFPy,
polycyclic heteroarenes having C3h symmetry including benzo-
[1,2-b:3,4-b¤:5,6-b¤¤]trifuran1d,6 and benzo[1,2-b:3,4-b¤:5,6-b¤¤]-
trithiophene7 core π-systems have also attracted attention. We
recently communicated an effective synthetic method of benzo-
[1,2-b:3,4-b¤:5,6-b¤¤]trisbenzofuran,8 so-called trioxatruxene, of
which preparation in synthetically useful yield had been diffi-
cult to achieve.9 Ikeda described a different effective method
for constructing the trisbenzofuran.10 We also successfully
prepared its 3,8,13- and 2,7,12-tri-tert-butylated compounds,
4a (p-tBuTxO) and 4b (m-tBuTxO) (Chart 2). Preliminary
measurement of the solid state photoluminescence for 4a and
4b suggested that they might have rather different crystal
packing modes as anticipated. Consequently, we also have
prepared the corresponding trithia analogues 5a (p-tBuTxS)
and 5b (m-tBuTxS), i.e. 3,8,13- and 2,7,12-tri-tert-butylated
benzo[1,2-b:3,4-b¤:5,6-b¤¤]tris[1]benzothiophenes. Then, we
have investigated the effect of defference of the heteroatoms
and substitution pattern of the tert-butyl groups for the pairs of
4a and 4b and of 5a and 5b on the photophysical and charge-
transporting properties. The results are reported herein.
2. Results and Discussion
2.1 Synthesis.
The synthesis of 3,8,13-trisubstituted
trioxatruxene 4a was performed by means of the copper-
catalyzed Ullmann-type reaction of 1,3,5-tribromobenzene with
three equivalents of 4-tert-butylphenol to give 1,3,5-tris(4-tert-
butylphenoxy)benzene followed by palladium-catalyzed dehy-
drogenative triple cyclization using Pd(OCOCF3)2 and AgOAc
as catalyst and oxidant, respectively, as reported previously
(Scheme 1).8,11 Its 2,7,12-substituted isomer 4b was obtained
similarly.
The 3,8,13-tri-substituted thia-analogue 5a was synthesized
by the nucleophilic substitution of 1,3,5-tribromobenzene with
tBu
tBu
X
tBu
Br
X
tBu
i (X = O)
ii (X = O)
V (X = S)
X
X
iii or iv
(X = S)
Br
Br
X
X
tBu
4a, 4b: X = O
5a 5b
Se
S
tBu
,
: X = S
S
Se
2
Scheme 1. Synthesis of trioxatruxenes 4a and 4b and of
trithiatruxenes 5a and 5b. (i) ArOH, CuI, Cs2CO3, NMP,
160 °C, 24 h; p-tBu 59%, m-tBu 68%. (ii) Pd(OCOCF3)2,
AgOAc, PivOH, 130 °C, 10 h; 4a 70%, 4b 68%.
(iii) ArSH, NaH, DMF, 160 °C, 24 h; p-tBu 96%.
(iv) In(SAr)3, Pd(OAc)2, Xantphos, N(Et2)iPr, DMF,
120 °C, 2 h; m-tBu 92%. (v) Pd(OCOCF3)2, AgOAc,
K2CO3, PivOH, 130 °C, 48 h; 5a 22%, 5b 19%.
1
N
N
N
O
O
O
O
N
3a
3b
Chart 1. Structure of heteroacenes 1, 2, 3a, and 3b.
© 2020 The Chemical Society of Japan