Efficient synthesis of 5-oxatruxene and the unusual influence of oxygen heteroatom on its physico-chemical properties

Article abstract of DOI:10.1039/c7nj04729k

In this paper, we describe the effective synthesis of oxatruxene and its photophysical, electrochemical and structural features that transpire when a methylene unit in a truxene core is replaced by an oxygen atom to yield 5-oxatruxene. This modification broke the C_3h symmetry and at the same time introduced a more electronegative structure as revealed by a four-fold increase in the quantum yield as compared to truxene. The derivatives of this material have great potential in optoelectronic devices and in organic photovoltaics. This proposed new synthetic method can be used to introduce other heteroatoms into the truxene system.

Full text of DOI:10.1039/c7nj04729k

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Efficient Synthesis of 5-Oxatruxene and Unusual Influence of Oxygen
Heteroatom on its Physico-Chemical Properties
a
a
a
a
Krzysztof Górski* , Justyna Mech-Piskorz* , Krzysztof Noworyta , Barbara Leśniewska , Marek
Pietraszkiewicza
a
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland. E-mail:
kgorski@ichf.edu.pl, jmech@ichf.edu.pl
Electronic supplementary information ꢀESIꢁ available including synthetic procedure, H, 13C NMR spectra, additional
1
†
spectroscopic, thermal, electrochemical and calculation data.

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ABSTRACT: This paper describes effective synthesis of oxatruxene and its photophysical, electrochemical and structural
features that transpire when a methylene unit in a truxene core is replaced by oxygen to yield 5-oxatruxene. This modifica-
tion broke C3h symmetry and at the same time introduced more electronegative structure, as revealed by a four-fold increase
in quantum yield, while compared to truxene. Derivatives of this material have great potential in optoelectronic devices and
in organic photovoltaics. The proposed new synthetic method can be used for introducing other heteroatoms into the trux-
ene
system.
minescence. These compounds were successfully used in
OLEDs and as lasing material in organic lasers.25,26 It seems
that all possibilities of peripheral modification have been
explored.34-36 However, it turned out that complicated and
robust structure is not the key to obtain desirable com-
pounds. Simple modification of the truxene unit by incor-
poration of heteroatom in the cyclic scaffold gives surpris-
ing results. First report about synthesis of heterocyclic
1
.
Introduction
Organic advanced materials gain tremendous impact on
manufacturing of OLEDs ꢀorganic light emitting diodesꢁ,
OFETs ꢀorganic field effect transistorsꢁ and OPVs ꢀorganic
photovoltaicsꢁ. The assumptions are not always fulfilled,
but the effort towards simpler and lower cost production
was enormous in the recent years and the stake is high.1
Nowadays organic devices can compete with inorganic
analogues in many areas like illumination, displays, radio
frequency identification ꢀRFIDꢁ tags, or solar cells. Despite
many optimistic reports about organic electronics, the
performance of organic materials still has to be improved.
The research is continued in many directions: optimization
of the position of energy levels and carrier mobility, en-
hancement of a quantum yield, improvement of thermal,
chemical and mechanical stability and facilitating the pro-
duction process.
3
7
truxenes was published in 1978. Acidic trimerization of
three identical subunits results in trioxa-,37,38 trithia-39-41
and triazatruxenes.19,40,42-47 The
C
3
symmetry of trihe-
h
trotruxene is preserved, and the most visible change in
new compounds is the difference in the intermolecular
interaction. These compounds found their application
mostly as cores of discotic liquid crystals, however they
seem to be promising in organic electronic, non-linear
optics and even as supercapacitors.48,49,50 It is much harder
to introduce one, or two heteroatoms in the truxene skele-
ton, an example of which is 5-oxatruxene. Breaking the
symmetry of the truxene basic moiety leads to expected
spectral changes, very promising in OLEDs and OPV appli-
cations. However, synthetic methods used in obtaining
triheterotruxenes usually fail in the case of monohetero-
and diheterotruxenes. 5-Thiatruxene, recently obtained by
Maciejczyk et. al51 was synthetized by modification of
commonly used approach in truxene chemistry. Unfortu-
nately, this method fails when oxygen atom is introduced
into the truxene core. This paper presents new, ingenious
Up to now, modified truxene structures occur in photo-
voltaics as: sensitizers1-6 or hole transporting materials, in
7
8
9-15
as fluorescence probes,16-18 anticancer
OFETs, OLEDs,
drugs19 and finally in non-linear optics for two photon
absorption14,20-22 or organic lasers.23-26
In many cases truxenes stand as stable and compact scaf-
folds, with convenient spots for modification, Chart 1 aꢁ.
Optimization of the position of energy levels seems to be
the fundamental step in obtaining desired material. One
possible way to tune the energy levels in truxenes is to
incorporate different conjugated units such as oligothio-
phenes,27 oligofluorenes,25,28-30 oligoꢀthienylethynyleneꢁ31,
or oligoꢀp-phenyleneꢁ.32 Other way for band-gap tuning
can achieved by the ring-fusing process in naphthalene
modified truxenes.33 Some of the modifications result in
solid-state materials with pure and stable blue electrolu-
approach
for
obtaining
10,10,15,15-tetraethyl-5-
oxatruxene, which can be successfully applied in demand-
ing organic electronics.
2

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Platinum spiral and silver wire were used as counter and
pseudo-reference electrode, respectively. Glassy carbon
working electrode, 2 mm in diameter, has been used
throughout all experiments. Decamethylferrocene has been
used as an internal potential reference and all potentials
were recalculated to its half-wave potential first, and then
referred to ferrocene/ferrocenium redox couple for easier
comparison with literature data. All voltammograms were
2
3
a)
b)
1
4
1
5
5
14
6
1
3
7
O
1
0
12
9
8
11
Chart 1. Structure of purely carbon truxene aꢁ ꢀred spots
mark modification placesꢁ and 5-oxatruxene bꢁ.
recorded in 0.1 M ꢀTBAꢁPF , ACN solutions deaerated prior
6
to measurements by passing through it stream of argon
N5.0.
2
.
Experimental
Absorption spectroscopy measurements were per-
formed on UV-Vis spectrophotometers: UV-2401 and UV-
700 equipped with an integrating sphere ꢀShimadzu,
3
.
Results and Discussion
2
Truxene is an aromatic compound consisting of fluorene
Japanꢁ. Emission spectra were collected using a Fluorolog
FL3-22 instrument with an integrating sphere ꢀHoriba
Jobin Yvon, Franceꢁ. Measured samples were dissolved in
dichloromethane ꢀdistilled prior to the experimentꢁ, in
methylcyclohexane ꢀspectroscopic grade, Sigma-Aldrichꢁ.
Solid state measurements were performed in barium sul-
phate ꢀspectroscopic grade, delivered by Shimadzuꢁ. Calcu-
lations were made by using DFT theory with B3LYP/6-31
gꢀd,pꢁ functional and basis set for molecules in vacuum.
For modelling Gaussian 09 Rev. E.01 software was used.
The X-ray diffraction data were collected at X-ray diffrac-
tometer SuperNova ꢀAgilent, UKꢁ equipped with a CuKα
micro-focus X-ray source and an Oxford Cryosystems open-
flow nitrogen cryostat. Measurements were carried out at
fused with two indene moieties, Chart
1
aꢁ.
5
,5,10,10,15,15-hexaethyltruxene, CCC has the same core
with additional six out of plane ethyl groups presented at
Chart 2 aꢁ. The presence of alkyl substituents significantly
increases the solubility of this compound. 5-Oxatruxene is
identical as truxene with one exception, fluorene is re-
placed by dibenzo[b,d]furan, Chart 1 bꢁ. Analogous to the
previous, 10,10,15,15-tetraethyl-5-oxatruxene, OCC has
additional four ethyl groups, Chart 2 bꢁ.
a)
b)
O
1
00K. Crystal structures are deposited in the Cambridge
Crystallographic Data Centre as CCDC 1526309-1526310.
Thermogravimetry analysis, TGA and differential scanning
calorimetry, DSC were performed with a TG-DSC 111 in-
strument ꢀSetaram Instrumentation, Franceꢁ under nitro-
gen. All electrochemical experiments were performed in a
standard three-electrode electrochemical cell connected to
SP-300 potentiostat/galvanostat ꢀBio-Logic, Franceꢁ driven
by EC-Lab software. Tetra-n-butylammonium hexafluoro-
Chart 2. Structure of CCC aꢁ and OCC bꢁ.
The role of heteroatom in the aromatic structures was
examined in the case of fluorene and dibenzofuran as the
basic truxene subunits. This experiments were done as a
proof of concept, how the heteroatom influences quantum
yield of the compound. Value of the fluorescence quantum
yield of fluorene and dibenzofuran is practically the same
ꢀ
Φ = 4%ꢁ. The difference in quantum yield is insignificant
phosphate, ꢀTBAꢁPF , electrochemical grade, decame-
6
in the case of subunits but is extreme in CCC and OCC com-
pounds.
thylferrocene, and anhydrous acetonitrile ꢀACNꢁ, all of
Sigma-Aldrich, were used without further purification.
3

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3
.1. Synthesis
Heteroaromatic compounds which contain endocyclic
oxygen atom attract significant interest, due to their poten-
tial applications in organic electronics52,53 or as bioactive
substances,54-56 thus multiple synthesis methods of such
compounds have been developed. However, all they fail
when it comes to the synthesis of 5-oxatruxene 9. Due to
this fact, we were forced to develop a new, effective syn-
thetic method leading to the formation of 9, which is essen-
tial for the development of heterotruxene-based electroe-
mitters. A key step of proposed synthetic path was photo-
cyclization of the corresponding precursor 7. Quantum-
mechanical calculations and visualization of LUMO orbital
suggest that photocyclization may occur, Figure 1.
Figure 1. Structure of 7 aꢁ and its LUMO orbital bꢁ. Arrows
mark pair of corresponding electron densities participat-
ing in photocyclization.
In Scheme 1 synthetic path of OCC is presented. The ad-
dition of organolithium compound 2 to the corresponding
ketone 5 in tetrahydrofuran did not lead to the formation
of desired product 6 because of two reasons. Firstly, in-
complete generation of organolithium compound 2, sec-
ondly keto-enol tautomerism, which strongly shifts equi-
librium to enol form 5’, Scheme 2. In such conditions effi-
cient deprotonation of compound 5’ to 5” occurs. To favour
the formation of ketone form 5, tetrahydrofuran was re-
placed by toluene, but in this condition organolithium
compound 2 cannot be generated efficiently by direct lithi-
ation of benzo[b]furan 1. To omit this problem, first 2-
iodobenzo[b]furan 3 was synthesized, which after isolation
was
lithiated
in
toluene.
nBuLi
Li
O
O
O
2
3
1
1
nBuLi
1) nBuLi, 2) I2
I
O
CH SO H
3 3
100%
O
HO
O
MgSO4
H SO
2
4
4
4%
6
7
O
4
O
5
5
0%
H
H
h
ν
, I2
KOH
C H Br
O
O
O
hexane
O
2
5
7
8
9
OCC
5
1%
Scheme 1. OCC synthesis path.
4

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tetrahydrofuran
toluene
O
HO
5
5'
2
2
-
-
O
O
O
6
'
5''
Scheme 2. Keto-enol equilibrium of 5.
The obtained alcohol 6, without purification, was dehy-
drated to form precursor of 5-oxatruxene, 7. During pho-
tocyclization of 7 we observed the formation of polar by-
products ꢀconfirmed by TLCꢁ, which probably possessed
in their structure hydroxyl or ketone groups in 10 and 15
positions of 5-oxatruxene core 9. The last step of synthe-
sis of soluble derivative OCC was four-fold ethylation of
obtained system 9. Standard condition used in alkylation
of fluorene, or carbazole did not give the expected result.
Due to the presence of numerous by-products, OCC ob-
tained with these methods is difficult to isolate, which is
directly reflected in low reaction yield. Multiple experi-
ments confirm that the limiting factor of alkylation reac-
tion yield is presence of oxygen in the solvents used.
Truxene anions which were generated during alkylation
are more prone to oxidation than corresponding fluorene
or carbazole anions. The use of deaerated solvents solves
this problem, allowing to obtain OCC with high yield
Figure 2. Normalized absorption ꢀredꢁ and emission
blueꢁ spectra of CCC measured in dichloromethane aꢁ
ꢀ
ꢀ
51%ꢁ.
and in the solid state bꢁ. Excitation in the solution: 307
nm, in the solid state: 330 nm.
3
.2. Photophysical properties and quantum chemi-
cal calculations
Absorption and emission spectra of CCC are presented
at Figure 2. Absorption curve in dichloromethane, Figure
aꢁ consists of three major peaks, with dominant at the
absorption wavelength abs = 306 nm and molar absorp-
tion coefficient = 76000 dm ·mol ·cm .
The fluorescence spectrum of CCC in dichloromethane,
shows also three well separated peaks at λflu = 353, 370
and 387 nm. Absorption and emission spectra in di-
chloromethane at first seems to have very large Stokes
shift. However, the absorption spectrum shows also very
low intensity transitions in the low energy range, where
three weak bands at 348, 333 and 318 nm are observed,
2
λ
3
-1
-1
ε
5

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erage
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3
-1
-1
Figure 2 aꢁ insert. These three transitions have the same
energy, but relatively higher intensity in the solid state,
Figure 2 bꢁ. What is more, emission shape in the solid
state compared to the emission profile in dichloro-
methane is different. This can be result of reabsorption
process of high energy peaks, which is much more effi-
cient in dense-packed solid state structure than in solu-
tion.57 The lack of close π-π interaction in the solid state is
confirmed by no spectral shift observed between solution
and solid state. Introduction of oxygen atom into the
aromatic truxene system changes its photophysical prop-
erties by appearance of low energy transitions. First of all,
ε
around 30000 dm ·mol ·cm , Figure 3 aꢁ. OCC
emission spectrum in the solution has similar pattern to
-1
CCC but at the same time is red-shifted of about 330 cm .
Absorption spectrum of OCC in the solid loses its nearly
constant ε, observed in the solution. The lowest energy
transitions have relatively higher intensity. The entire
portion of the absorption in the solid state is clearly red-
shifted in comparison to spectrum in solution by about
-1
1
500 cm , as a result of significant π-stacking interaction
of this compound in the solid state. Emission line in the
solid state is distinctly different from emission in the
solution because of reabsorption, which importantly
decreases the intensity of the first emission peak ꢀclear in
the solutionꢁ. Measured fluorescence quantum yield in
the solid state is 22% and it is slightly lower than value
obtained in the solution, 37%.
breaking the
C
3
h
symmetry in OCC is manifested by ap-
pearing of low energy transitions with significant intensi-
ty, Figure 3, where three well resolved peaks at 352, 337
and 322 nm are visible.
Table 1. Calculated vs experimental transitions of CCC
and OCC.
aꢁ
λ
cal
λ
obs
Transition
f
osc
[
nm]
[nm]
348
318
306
S
0
S
0
S
0
→
→
→
S
1
S
2
S
3
0.0000
0.0000
0.4512
319
285
281
CCC
S
0
S
0
S
0
→
→
→
S
1
S
2
S
3
0.0540
0.0683
0.4507
327
294
283
352
322
303
OCC
aꢁ
measured in dichloromethane
Neglecting very low intensity bands in CCC, the dis-
tance between absorption and emission spectra is very
-1
-1
large ꢀ4400 cm ꢁ, as compared with OCC ꢀ550 cm ꢁ. The
explanation of such a behaviour is given by selection
rules based on symmetry of molecules. CCC has
C
3
h
sym-
metry, whereas OCC only , what implies that in CCC
C
s
Figure 3. Normalized absorption ꢀredꢁ and emission
some transitions are forbidden. Quantum-mechanical
calculations supply information that in truxene first two
ꢀ
blueꢁ spectra of OCC measured in dichloromethane aꢁ
and in the solid state bꢁ. Excitation in the solution: 312
nm, in the solid state: 320 nm.
electron transitions ꢀ
S
0
→
S
1
and
S
0
→
S ꢁ, are forbidden,
2
due to orbitals symmetry ꢀoscillator strength,
fosc for
Stokes shift of OCC spectrum in the solvent has the val-
these transitions equals zeroꢁ, Table 1. As was mentioned
before, absorption bands of corresponding transitions
can be observed in the absorption spectrum of CCC with
6
-
1
ue 550 cm . Interestingly, absorption spectrum has near-
ly constant intensity in the region 268-303 nm, with av-

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very low intensities because of vibronic coupling. Break-
ing symmetry of the molecule due to introduction of oxy-
gen atom in OCC allows transitions to and . As a con-
sequence, additional bands in absorption spectrum ꢀ322,
37, 352 nmꢁ can be observed. Moreover, heteroatom
strongly influences radiative constant, , which in di-
chloromethane for CCC is 5.00·10 s , whereas in OCC is
unchanged, which suggests the lack of strong intermolec-
ular interactions. On the other hand, nonradiative con-
stant is two times lower than in dichloromethane. The
explanation of such behaviour is more rigid surrounding
of every molecule in the solid state, limiting the oscilla-
tions and rotations, which as a result, increases the fluo-
S
1
S
2
3
k
r
6
-1
rescence quantum yield twice ꢀ
Φ
flu = 19%ꢁ. In the case of
= 1.29·108
7
-1
ten times higher, 6.17·10 s . At the same time nonradia-
OCC the radiative constant increases twice,
k
r
7
-1
s-1, whereas
8
-1
tive constant for CCC is 5.06·10 s , whereas for OCC is
knr is fourfold higher 4.59·10 s . Such in-
8
-1
two times higher, 1.05·10 s . As a result fourfold in-
crease of OCC fluorescence quantum yield ꢀ
relative to CCC ꢀ flu = 9%ꢁ is observed.
crease of nr can be explained by the existence of an addi-
k
Φ
flu = 37%ꢁ
tional nonradiative channel. Crystallographic structure of
OCC suggest the formation of excimers during irradiation
which are responsible for dissipating the energy of excit-
Φ
In the solid state, photophysical analysis of additional
nonradiative relaxation mechanisms has to be consid-
ered. For CCC the radiative constant practically remains
ed
state.
Table 2. Photophysical properties of CCC and OCC.
aꢁ
aꢁ
τaꢁ
aꢁ
aꢁ
bꢁ
bꢁ
τ bꢁ
bꢁ
-1
bꢁ
cꢁ
λ
flu
Φ
flu
k
r
k
nr
λ
flu
Φ
flu
k
r
k
nr
λ
phos
-
1
-1
-1
[
nm] [%]
53
370
87
60
377
93
[ns]
[s ]
[s ]
[nm]
-
[%]
[ns]
[s ]
[s ]
[nm]
3
4
4
54
CCC
OCC
9
18
5.00·106 5.06·107
6.17·107 1.05·108
370
19 34.7
5.48·106 2.33·107
1.29·108 4.59·108
90
3
385
-
3
4
4
60
97
37
6
403
420
22
1.7
3
aꢁ measured in dichloromethane, bꢁ measured in solid state, cꢁ measured in methylcyclohexane at 77 K.
Changes of spectroscopic properties are also noticeable
in emission from triplet state
1
T , Figure 4.
7

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Figure 4. Phosphorescence of CCC and OCC in methyl-
cyclohexane at 77 K. Excitation for CCC: 307 nm, for OCC:
are presented in Figure 5. Introduction of oxygen atom in
5-oxatruxene breaks the molecular symmetry from to
, which removes the degeneracy of the orbitals. One
C
3h
3
12 nm.
C
s
essential feature is a decrease in the HOMO-LUMO gap in
OCC what is in agreement with measured UV-Vis spectra.
In CCC the lack of spectral shift between absorption in
the solution and the solid state described above is con-
firmed by X-ray analysis. Six ethyl groups in CCC molecule
effectively separates molecules from each other, which
significantly hinders aggregation. Crystallographic struc-
ture gives an evidence for difficulty of stacking of CCC
molecules, Figure 6. CCC crystallizes in orthorhombic,
In CCC phosphorescence spectrum, single, narrow, high
intensity peak at 490 nm is present. This probably corre-
sponds to transition from ν’ = 0 level to higher vibra-
tional level of state. In OCC phosphorescence, two of
high intensity peaks are observed, at 460 nm and 497 nm,
which may correspond to transitions between ν’ = 0 →
ν’’ = 0 and ν’ = 0 → higher vibrational level, re-
spectively.58
T
1
S
0
T
1
S
0
T
1
S
0
Theoretical energy diagram for truxene and 5-
oxatruxene, along with corresponding molecular orbitals
space group Pna21.
Figure 5. Calculated energy diagram of the highest occupied ꢀredꢁ and the lowest unoccupied ꢀblueꢁ orbitals of truxene and
5
-oxatruxene.
Molecular
orbitals
shapes
are
presented
next
to
the
energy
levels.
OCC has only four ethyl groups, thus separation of mol-
ecules is more difficult, Figure 7. What is more, replace-
ment of one diethylmethylene moiety ꢀEt
2
Cꢁ by oxygen
atom creates place prone to π-stacking. OCC crystallizes
8

Page 9 of 14
in monoclinic, space group
New Journal of Chemistry
/n. Crystal structure of
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DOI: 10.1039/C7NJ04729K
P
2
1
OCC consists of dimers arranged in columns from one
directions, Figure 7 and herringbone pattern from anoth-
er direction, Figure 8.
Figure 6. Crystal structure of CCC molecules.
Figure 7. Crystal structure of OCC molecules ꢀintermolecular distances are given in Åꢁ.
them, we performed TGA and DSC analyses for CCC and
OCC. Measurements were carried out in the temperature
range 20-240 °C and 20-230 °C for CCC and OCC respec-
-
1
tively, with a scan rate of 5 °C·min , in nitrogen atmos-
phere. Both substances do not decompose in the exam-
ined temperatures, which confirms their high thermal
stability. CCC has relatively broad melting peak
m
T with
maximum at 217.2 °C, indicating the presence of different
crystalline forms ꢀFigure S2ꢁ. X-ray structure clearly
shows that CCC crystallize with some disorder in the
structure ꢀFigure S1ꢁ. Occurrence of polymorphism in
truxenes is known from previous work.59 Liquid phase of
CCC overcools what was observed as a crystallization
Figure 8. Herringbone orientation of OCC molecules.
peak at temperature
T
cry = 98.7 °C significantly lower
= 217.2 °C, Table 3. No
than melting temperature,
T
m
3
.3. Thermal properties
glass transition was observed in the measured system.
Subsequent heating and cooling cycles reflect the same
The presence of oxygen atom in the truxene core affects
the thermal properties of the truxene. To investigate
9

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situation. In the case of OCC the melting peak is much
sharper, with maximum at 214.7 °C what can be inter-
preted as the presence of only one crystalline form ꢀFig-
values a third, broad, oxidation peak at 1.59 V appears. It
indicates irreversible process leading to formation of
electrochemically inactive species as corroborated by
disappearance of reduction peaks at 0.42 V and 0.93 V in
the reverse scan. Differences in redox properties of CCC
and OCC in positive potential range indicate strong influ-
ence of oxygen atom on formation of cation radical during
oxidation. In the potential window from -0.5 to -3.4 V
only one reduction peak, characteristic for irreversible
process, is visible for CCC at -3.25 V and for OCC at -
3.04 V, ꢀFigure S5 a,bꢁ. Cyclic voltammograms recorded
at various scan rates for CCC ꢀFigure S6 aꢁꢁ indicate grad-
ual appearance of corresponding oxidation peak at -
ure S3ꢁ. During cooling OCC undergoes glassification at
T
g
=
107.8 °C. Subsequent heating, surprisingly, reveals
crystallization peak at 125.0 °C and the shift of the melt-
ing point which this time is observed at 222.4 °C. At the
following cooling cycle glass transition is observed like
for the first cycle at 107.8 °C. Subsequent heating-cooling
cycle do not bring any changes. Increase of the OCC melt-
ing point in the second cycle for about 7.7 °C may suggest
formation of different crystalline form. The difference in
enthalpy of fusion ΔHfusion in CCC and OCC of value 3.8
-
1
-1
kJ·mol is caused by greater amount of energy necessary
to overcome π-stacking between OCC molecules.
Table 3. Thermal properties of CCC and OCC.
3.07 V at scan rates higher than 0.2 V·s . This behaviour
indicates relatively slow decomposition of the reduction
product, and therefore, formation of relatively stable
anion radical. Similar measurements performed for OCC
-
1
T
m
[°C]
217.2
214.7 107.8
T
g
[°C]
T
cry [°C]
Hfusion [J·g ]
ꢀ
Figure S6 bꢁꢁ do not show appearance of any reverse
98.7c
25.1
28.9
-1
CCC
OCC
-
oxidation peak even at the scan rate of 5 V·s , indicating
much faster decomposition of anion radical in this case.
This observation is supported by quantum mechanical
calculations which show that in the case of anion radical
of CCC negative charge is located rather in the central
part of the molecule and partially on the one of peripheral
benzene ring ꢀFigure 9 aꢁꢁ.
125.0h
c - during cooling cycle, h – during heat cycle
3
.4. Electrochemistry
To investigate the redox properties of CCC and OCC cy-
clic voltammetry was performed. During oxidation of CCC
one quasi-reversible redox process is observed with for-
mal potential at 1.06 V and corresponding reduction peak
appears at 0.97 V ꢀFigure S4 aꢁꢁ. When potential scan is
extended to more positive potentials, additional irre-
versible oxidation process at 1.81 V occurs. The lack of
corresponding reduction peak in reverse scan indicates
chemical decomposition of the second oxidation product.
OCC measured in the same potential range shows differ-
ent behaviour, ꢀFigure S4 bꢁꢁ. Firstly, irreversible oxida-
tion process at 0.76 V is observed, followed by the ap-
pearance of quasi-reversible redox process at 0.96 V. In
the reverse scan a small peak at 0.42 V corresponding to
reduction of the first oxidation product is also observed.
When reverse potential is extended to more positive
Figure 9. CCC aꢁ and OCC bꢁ anion radicals.
In the case of OCC the negative charge is additionally
delocalised over benzo[b]furan moiety, ꢀFigure 9 bꢁꢁ. Due
to presence of the six ethyl groups, the charge is more
shielded leading to the formation of more stable anion
radical in CCC than in the case of OCC.
The absolute HOMO and LUMO energies can be calcu-
lated from electrochemical data according to equations 1
and 2.
10

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DOI: 10.1039/C7NJ04729K
this material in optoelectronic devices is possible, pro-
vided it will be modified.52 Implementation of proper
donor, or acceptor moiety at the periphery of OCC mole-
cule leads to donor-acceptor systems which - in general -
are widely applied as emitters in OLED devices. Breaking
ꢂꢃꢄꢅꢄꢀeVꢁ = −eꢀE ꢀVꢁ + 4.71 Vꢁ
(1)
ꢆꢇ
EꢈꢉꢅꢄꢀeVꢁ = −eꢀEꢊꢋꢌꢀVꢁ + 4.71 Vꢁ60
ꢀ2ꢁ
Value of ELUMO calculated for OCC is only 0.02 eV more
positive than the value for CCC, while value of EHOMO is
.33 eV more positive for OCC than for CCC. Therefore,
the
C
3
h
symmetry, creates non-equivalent places suscep-
0
tible to selective modification. Moreover the derivatiza-
tion yield is higher in the case of OCC than CCC, trioxa-,
triaza- and trithatruxene. In the solution OCC absorption
spectrum, becomes nearly constant in the region 268-303
nm, with an average molar absorption coefficient around
calculated electrochemical bandgap is slightly higher for
CCC than for OCC ꢀ3.74 eV vs. 3.43 eVꢁ, which is con-
sistent with results obtained from spectroscopic meas-
urements, Table 5.
3
-1
-1
Table 4. Summarized redox potentials of truxene CCC
30000 dm ·mol .cm , Figure 3 aꢁ. Regarding thermal
properties, OCC has tendency for glass formation. This
feature is indispensable in OLED devices fabrication. OCC
creates a good skeleton for further modification and OCC
derivatives have great potential in optoelectronic applica-
tions.
and OCC in 0.1 M ꢀTBAꢁPF , ACN solution.
6
Eoxꢀ1ꢁ
Eoxꢀ2ꢁ
E
oxꢀ3ꢁ
E
red
[
V]*
[V]*
1.81
[V]*
-
[V]*
-3.25
CCC
OCC
1.06
0.76
0.96
1.59
-3.04
Replacement of diethylmethylene moiety by oxygen at-
om is responsible for π-stacking. In the solid state OCC
dimers creates columns, what makes this material poten-
tially useful as organic conductors in OFETs. OCC opens a
new class of monoheterotruxenes which can be treated as
a basic units for further modifications. Current organic
optoelectronics is facing a problem with low durability of
devices, thus new compounds, other than carbazole, or
dibenzotiophene are urgently needed. The proposed
synthetic plan should be applicable towards the synthesis
of other heteroatom containing truxene analogues, lead-
ing to the development of a new class of truxene com-
pounds with modulated physical properties.
*
vs. Fc/Fc+
Table 5. Onset potentials and calculated HOMO, LUMO
energies and band gap for truxene CCC and CCO in 0.1 M
ꢀ
TBAꢁPF , ACN solution.
6
ꢆꢍꢎꢋꢏ
ꢆꢍꢎꢋꢏ
ꢂ
E
HOM
ꢂ
E
LUMO
ΔEelc
ΔEopt
ꢆꢇ
ꢊꢋꢌ
1
2
[
V]* [eV]
[V]*
[eV]
[eV]
[eV]
CCC 0.99 -5.70 -2.75 -1.96
OCC 0.66 -5.37 -2.77 -1.94
3.74
3.43
3.55
3.49
*
vs. Fc/Fc+
1
ΔEelc = ELUMO - EHOMO
2
ΔEopt determined from intersection of absorption and
emission bands
4
.
Conclusion
CONFLICTS OF INTEREST
Presented data reveal the changes in physical proper-
ties of the molecule as a result of the introduction of oxy-
gen atom into truxene system. The most significant
changes are seen in the photophysical properties i.e. in-
crease of the quantum yield from 9 to 37% and appear-
ance of low energy bands. Although the quantum yield of
pure OCC is not record-breaking, and the redox processes
are not fully reversible, thus the potential application of
There are no conflicts to declare.
ACKNOWLEDGMENT
This research was supported in part by PL-Grid Infra-
structure and the Institute of Physical Chemistry of the
Polish Academy of Sciences. The Authors cordially thank
11

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Professor Jacek Waluk, of the Institute of Physical Chem-
ꢀ15ꢁ
ꢀ16ꢁ
J. Huang, B. Xu, J.-H. Su, C. H. Chen, H. Tian,
istry of the Polish Academy of Sciences, for his sugges-
tions and comments concerning spectroscopic part and
HOMO-LUMO calculations, and Oksana Pietraszkiewicz
for preparation of crystals.
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5
-oxatruxene presents new quality among truxenes. It is promising compound for the new class of
optoelectronic materials.