Connecting two phenazines with a four-membered ring: The synthesis, properties and applications of cyclobuta[1,2-: B:3,4- b ′]diphenazines

Article abstract of DOI:10.1039/c7tc04092j

Herein we report cyclobuta[1,2-b:3,4-b′]diphenazine (CBDP), a new π-electron molecular scaffold containing two phenazine moieties connected by a four-membered ring. With properly positioned silylethynyl substituting groups, CBDP offers a chromophore with

Full text of DOI:10.1039/c7tc04092j

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ISSN 2050-7526
PAPER
~
Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Connecting Two Phenazines with a Four-Membered Ring:
Synthesis, Properties and Applications of Cyclobuta[1,2-b:3,4-
b']diphenazines
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
Shuaijun Yang,a Ming Chua and Qian Miao＊
DOI: 10.1039/x0xx00000x
Herein we report cyclobuta[1,2-b:3,4-b']diphenazine (CBDP), a new π-electron molecular scaffold containing two
phenazine moieties connected by a four-membered ring. With properly positioned silylethynyl substituting groups, CBDP
offers a chromophore with large molar extinction coefficient, a luminophore with good quantum yield, and an n-type
organic semiconductor with field effect mobility as high as 0.30 cm2/Vs. These properties are not available with the
phenazine reference compounds, but can be tuned by adjusting the substituting positions of silylethynyl groups. On the
basis of single crystal structures, UV-vis absorption and DFT calculation, it is concluded that the two phenazine subunits in
CBDP are poorly conjugated in the ground state but strongly conjugated in the excited state, shedding light on the role of
the four-membered ring in conjugation.
www.rsc.org/
corresponding N-heteroacenes containing the same number of
six-membered rings because insertion of formally antiaromatic
Introduction
Combining benzenoid and cyclobutadienoid rings in one
polycyclic framework leads to interesting π-electron molecular
scaffolds, whose electronic structures are governed by both
,
,
,
3 4
2
aromaticity and antiaromaticity 1
Herein we report
cyclobuta[1,2-b:3,4-b']diphenazine (CBDP), a new π-electron
molecular scaffold having two phenazine moieties connected
by a four-membered ring. The linear π-backbone of CBDP is
closely related to N-heteroacenes, which have recently been
revivified by the development of new synthetic methodology
and the discovery of high-mobility n-type organic
,
semiconductors 5 based on N-heteroacenes.6 7 Having N atoms
replacing the CH units in the linear framework of acenes, N-
heteroacenes are similar to acenes becoming less stable as
their length increases. Such instability is an obstacle to
development of larger N-heteroacenes for organic
semiconductors or molecular wires with enhanced spatial
electronic delocalization. To meet this challenge, N-
heteroacenes were very recently extended through four-
membered rings, which increase the length of N-heteroacenes
without compromising their stability or changing their linear
,
,
shape.8 9 10 The resultant π-extended N-heteroacenes, such as
1a 9 and 1b 10 as shown in Fig. 1a, are more stable than the
aDepartment of Chemistry The Chinese University of Hong Kong, Shatin, New
Territories, Hong Kong, China. E-mail: miaoqian@cuhk.edu.hk.
bInstitute of Molecular Functional Materials (Areas of Excellence Scheme, University
Grants Committee), Hong Kong, China.
†
contributions to the field of organic semiconductors.
Electronic Supplementary Information (ESI) available: Details of synthesis and
characterization, DFT calculation, fabrication and characterization of organic thin
film transistors, NMR spectra, See DOI: 10.1039/x0xx00000x
We dedicate this paper to Prof. Fred Wudl in celebrating 50 years of his
Fig.
membered ring and the corresponding N-heteroacenes; (b) structures of CBDPs and
the related phenazines
1 (a) Structures of reported π-extended N-heteroacenes containing a four-
.
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scanning calorimetry (DSC), 3a and 4a decomposed at 422 °C
four-membered rings in fact increases the number of Clar's and 394 °C, respectively, while heating the solids of 3a and
–
b
aromatic sextets. However, 1a and 1b both met problems in 4a–b in air at 200 °C for four hours did not lead to detectable
becoming high-mobility n-type semiconductors. 1a is a poorer
electron acceptor than the corresponding N-heteroacene 2a
with its lowest unoccupied molecular orbital (LUMO) energy
level higher than that of 2a by 0.33 eV. Although 1b has a
LUMO energy level lower than that of 2b by 0.20 eV, the thin
films of 1b exhibited electron mobility (0.015 cm2/Vs) lower
than that of 2b by one order of magnitude presumably due to
the poor crystallinity of the films as a result of the non-central
symmetry of 1b. To solve these problems, we became
interested in linearly fused N-hetero polyarenes that
change in 1H NMR spectra.
differentiate themselves from 1a–b by having two identical N-
heteroacene units connected by a four membered ring.
Detailed below are synthesis, properties and applications of
silylethynylated CBDPs 3a–b and 4a–b, whose heptacyclic
backbone, as shown in Fig. 1b, can be regarded as a unique
dimer of phenazine with a four-membered carbocycle bridging
the two monomers. In order to better understand the effect of
the four-membered bridge in π-extension of N-heteroacenes,
Scheme 1. Synthesis of 3a–b and 4a–b.
3a–b and 4a–b were experimentally and computationally
studied in comparison to the corresponding phenazine
monomers, 5a and 6a (Fig. 1b), respectively. As found from the
study detailed below, CBDP offers interesting optical and
electronic properties that can be tuned by adjusting the
substituting positions of silylethynyl groups but are not offered
by the phenazine reference compounds, leading to solution-
processed n-type semiconductors with field effect mobility of
up to 0.30 cm2/Vs.
Crystal structures
Single crystals of 3b
, 4a and 6a suitable for X-ray
crystallographic analysis were obtained in this study from their
solutions by slow evaporation of solvents. 18 The crystal
structure of 5a, as reported by Bunz et. al, 19 contains two
crystallographically independent molecules with slightly
different bond lengths. The following discussion about bond
lengths of 5a refers to only one crystallographically
independent molecule. Fig. 2a shows the π-backbones of 3b
and 4a in comparison to those of 5a and 6a. In the crystals, the
heptacyclic framework of 3b is essentially flat, while that of 4a
is slightly bent along the long molecular axis as shown in Fig.
2b. As summarized in Table 1, the central four-membered
rings in 3b and 4a have similar bond lengths: 1.49 Å for C7−C8'
and C7'−C8, and 1.43−1.44 Å for C7−C8 and C7'−C8'. The
C7−C8' and C7'−C8 bonds in 3b and 4a are slightly shorter than
the corresponding bonds in biphenylene (1.51 Å),20 but still
Results and discussion
Synthesis
As shown in Scheme 1, CBDPs 3a
Buchwald–Hartwig cross coupling
phenylenediamine 7a
b12 with 2,3,6,7-tetraiodobiphenylene
), which was prepared from biphenylene by modifying the
reported procedures.13 In the same way, compounds 4a
were synthesized from and the corresponding
–b were synthesized by the
11
of the corresponding
–
(
8
–
b
slightly longer than typical nonconjugated single bonds
between two sp2-hybridized
8
21
C atoms (1.47−1.48 Å),
phenylenediamines 9a
–
b
, which were prepared from 1,2-
14
suggesting absence of π character in these two bonds. The
four-membered rings in 3b and 4a are bonded to four C atoms
(C6, C9, C6' and C9') with C−C bond length (1.34−1.35 Å) very
close to the typical bond length of C−C double bonds in
alkenes (1.31−1.34 Å),21 suggesting a radialene-like structure.
These bond lengths are in agreement with the earlier
conclusion that the π bonds in [N]phenylenes tend to localize
within the six-membered rings so that the 4π antiaromatic
diiodo-4,5-dinitrobenzene
in two steps following the
reported procedures with minor modification.15 Unlike the
reported synthesis of 1b 10
the above coupling reactions did
,
not yield N,N'-dihydro derivatives, which are expected as the
products of the Buchwald–Hartwig cross coupling. This can be
attributed to the spontaneous oxidation of the N,N'-dihydro
derivatives to the final products in agreement with the fact
that N,N'-dihydrophenazine is highly sensitive toward
oxidation.16 Phenazine 5a is a known compound as prepared
2, 22
character of the cyclobutadiene linkage can be minimized.
The C6−C7 and C8−C9 bonds in 3b and 4a are slightly shorter
than the corresponding bonds in 5a and 6a, while the C5a−C6,
C7−C8, C9−C9a and C5a−C9a bonds in 3b and 4a are slightly
longer than the corresponding bonds in 5a and 6a. As a result,
3b and 4a exhibit a larger degree of bond length alternation in
ring C than 5a and 6a. In connection with the greater bond
17
by following the reported procedures, while 6a was
synthesized by the condensation reaction of o-benzoquinone
and 4,5-dibromo-1,2-benzenediamine and the subsequent
Sonogashira coupling as detailed in ESI. CBDPs 3a–b and 4a–b
all exhibited good stability toward heating as well as ambient
air and light (Fig. S1–S3 in ESI†). As found from differential
2 | J. Name., 2012, 00, 1-3
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length alternation in ring C, the local aromaticity in 3b and 4a more than four rings overlapped, while those of 4a have only
was evaluated quantitatively with the harmonic oscillator two rings overlapped. In comparison to 3b and 4a, phenazines
model of aromaticity (HOMA),23 which is an aromaticity index 5a and 6a exhibit one-dimensional π-stacks with a head-to-tail
of individual rings based on bond lengths.24 A typical aromatic arrangement of the silylethynyl substituents as shown in Fig.
ring has a HOMA value of 1, and a smaller HOMA value S5 in ESI†, likely as a result of subsꢀtution on only one side of
indicates poorer aromaticity. As shown in Table 1, the HOMA these molecules. The π-to-π distances of 5a and 6a are 3.32 to
values for ring C decrease from 0.773 in 5a to 0.397 in 3b and 3.41 Å and 3.54 to 3.57 Å, respectively.
from 0.714 in 6a to 0.339 in 4a, respectively, indicating that
Table 1 Selected bond lengths and HOMA values of 3b, 4a, 5a and 6a from
the aromaticity of ring C in 5a and 6a is largely reduced upon
the crystal structures.
fusing to the four-membered ring. This is also in agreement
3b
4a
5a
6a
with calculated NICS values (Table S5 in ESI†).
C7−C8'
C7'−C8
C6−C7
1.490(7)
1.490(7)
1.345(6)
1.433(5)
1.345(6)
1.443(6)
1.448(5)
1.429(6)
0.647
1.494(5)
1.494(5)
1.342(5)
1.443(3)
1.339(5)
1.437(5)
1.445(3)
1.435(5)
0.781
—
—
—
—
1.372
1.409
1.366
1.408
1.430
1.432
0.557
1.341(2)
1.381(2)
1.352(2)
1.415(2)
1.424(2)
1.421(2)
0.683
C7−C8
C8−C9
C9−C9a
C5a−C9a
C5a−C6
A
B
C
HOMA
0.795
0.397
0.821
0.339
0.802
0.773
0.909
0.714
Table 2 Reduction potentials, absorption, emission, and frontier molecular
orbital energy levels of 3a–6a.
Experimental
Calculated
ꢀꢁꢄꢂꢃ ꢀꢁꢅꢂꢃ
LUMO
(eV)[b]
ꢆ
ꢆ
Фf
HOMO LUMO
ꢊꢋꢌ
ꢍꢇ
ꢇꢈꢉ
ꢇꢈꢉ
(V)[a] (V)[a]
(nm)[c] (nm)[d] (%)[e]
(eV)[f]
-6.04
-6.29
-6.03
-6.33
(eV)[f]
-3.49
-3.50
-3.06
-3.04
3a -1.21 -1.54
4a -1.15 -1.49
5a -1.53 -2.09
6a -1.48 -2.10
-3.89
-3.95
-3.57
-3.62
484
513
442
419
547
2.9
527 47.4
492
478
1.8
0.2
[a] Half-wave potential versus ferrocenium/ferrocene. [b] Estimated from LUMO =
−5.10 − ꢀꢁꢄꢂꢃ (eV). 25 [c] The longest-wavelength absorption maximum as measured
from a 1×10−5 M solution in CH2Cl2. [d] The shortest-wavelength emission maximum as
measured from a 1×10−5 M solution in CH2Cl2. [e] Fluorescence quantum yield. [f]
Calculated at the B3LYP level of DFT with 6-311++G(d,p)//6-31G(d,p) basis sets using
simplified model molecules (3a’, 4a', 5a’, and 6a’).
Photophysical properties and electronic structures
The most interesting aspect of CBDPs 3a–b and 4a–b is their
Fig. 2 (a) Molecular structures of 3b, 4a, 5a and 6a in single crystals with the
trialkylsilyl substituents removed; (b) π-stacking of 3b and 4a in the crystals.
(C and N atoms in (a) and in the polycyclic backbone in (b) are shown as
ellipsoids at the 50% probability level; the trialkylsilylethynyl substituents in
(b) are shown as sticks; hydrogen atoms are removed for clarity.)
electronic structures, which, in comparison to the
corresponding phenazines, 5a and 6a, can shed light on the
role of the four-membered ring in conjugation. To compare
the electronic structures of CBDP and phenazine, 3a–6a were
studied with cyclic voltammetry (CV), UV-vis absorption
spectroscopy, fluorescence spectroscopy, and density
functional theory (DFT) calculations. In the test window of CV,
3a and 4a both exhibited two reversible reduction waves but
did not exhibit any oxidation waves, while 5a and 6a both
exhibited one reversible reduction wave and one
quasireversible reduction wave with more negative reduction
potentials than those of 3a and 4a (Fig. S6 in ESI†). The half-
As shown in Fig. 2b, molecules of 3b in the crystals are
arranged in one-dimensional π-stacks with a π-to-π distance of
3.43 Å. With a slightly bent π-backbone, molecules of 4a form
similar one-dimensional π-stacks with π-to-π distances of 3.41
Å (Fig. 2b) in the crystals. Despite similar one-dimensional π-
stacks, 3b exhibits a larger degree of π-overlap than 4a. As
shown in Fig. S4 (ESI†), two π-stacked molecules of 3b have
wave reduction potentials of 3a
–
6a are shown in Table 2, and
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on the basis of the first reduction potentials versus those of 5a and 6a, respectively, in agreement with the UV-vis
ferrocenium/ferrocene, the LUMO energy levels are estimated absorptions of 3a and 4a red shifted relative to those of 5a
as −3.89 eV for 3a, −3.95 eV for 4a, −3.57 eV for 5a, and −3.62 and 6a, respectively. Fig. 5 shows the selected molecular
eV for 6a 25
As shown in Fig. 3, the UV-vis absorption spectra of 3a and 4a combining the corresponding molecular orbitals of 5a' and 6a'
exhibit apparent bathochromic shift relative to those of 5a and respectively, on the basis of the qualitative molecular orbital
6a, respectively, with much larger molar extinction coefficients theory.28 By comparing the molecular orbitals of 3a' and 5a'
(ε). The longest-wavelength absorption maximum of 3a occurs particularly, in respect of the symmetry, it is found that the
at 484 nm (ε = 1.22×105 L∙mol−1∙cm−1), which is red-shifted by HOMO and HOMO−1 of 3a' originate from the HOMO of 5a'
.
orbitals for 3a' and 4a', which can be regarded as a result of
,
,
,
42 nm relative to that of 5a. The bathochromic shift of 4a and the LUMO and LUMO+1 of 3a' originate from the LUMO of
relative to 6a is even larger, with the longest-wavelength 5a'. The LUMO and LUMO+1 of 3a' exhibit larger splitting than the
absorption maximum of 4a (513 nm, ε = 3.13×105 L∙mol−1∙cm−1) HOMO and HOMO−1 of 3a', suggesting that the two LUMOs of
red-shifted by 94 nm relative to that of 6a. The red shifts 5a' are combined with stronger interactions than the two
suggest considerable conjugation between the two phenazine HOMOs of 5a' in forming the corresponding molecular orbitals
subunits in 3a and 4a, and thus seem to contradict the bond of 3a'
lengths of the C7−C8' and C7'−C8 bonds in the crystal
structures of 3b and 4a, which suggest very poor conjugation
between the phenazine subunits in CBDPs. Such seeming
contradiction can be reconciled by recognizing that the ground
state and the electronically excited state may have different
degree of conjugation.26 The bond lengths in the crystal
structures correspond to the ground-state geometry reflecting
the conjugation in the ground state, while the UV-vis
absorption, as a result of electronic transition from the ground
state to the excited state, depends on the conjugation in both
the ground and excited states. Considering the crystal
structures are already indicative of very poor conjugation
between the phenazine subunits in 3b and 4a, we conclude
that the UV-vis absorption spectra of 3a and 4a are an
indication of effective conjugation between the phenazine
subunits in the excited state of CBDP. This conclusion is in
agreement with the excited-state aromaticity of 1,3-
cyclobutadiene and biphenylene, which is unfortunately not as
well-known as the ground-state antiaromaticity of these
compounds.26
.
29 Furthermore, the LUMOs of 3a and 4a exhibit large
Upon irradiation with UV light, solution of 3a in CH2Cl2
exhibited weak yellow fluorescence while solution of 4a in
CH2Cl2 exhibited strong green fluorescence as shown in Fig. 4.
Fig. 3 UV-vis absorption spectra of 3a versus 5a (top) and 4a versus 6a
(bottom) in CH2Cl2 at the same concentration (1×10−5 mol/L).
In comparison to 3a and 4a, phenazine 5a in solution exhibited
very weak blue fluorescence while 6a in solution did not show
visible fluorescence upon irradiation with UV light, in
agreement with the well-known nonfluorescent property of
27
nonsubstituted phenazine.
As detailed in ESI†, the
fluorescence quantum yield (Фf ) was measured as 0.03 for 3a
0.47 for 4a, 0.02 for 5a and 0.002 for 6a
To better understand the different photophysical properties of
3a and 4a, the frontier molecular orbitals of 3a 6a were
calculated using simplified model molecules 3a' 6a', which
,
.
–
–
have smaller trimethylsilyl (TMS) groups replacing the TIPS
groups to reduce computation cost. The geometries of these
simplified molecules were optimized at the B3LYP level of DFT
with the 6-31G(d,p) basis set, and the molecular orbitals were
then calculated with the 6-311++G(d, p) basis set. As shown in
Table 2, the calculated LUMO energy levels of 3a and 4a are
lower than those of 5a and 6a by about 0.4 eV, respectively,
while the calculated highest occupied molecular orbital
(HOMO) energy levels of 3a and 4a are essentially the same as
Fig. 4 Fluorescence spectra from solutions of 3a and 4a in CH2Cl2 (1×10−5
mol/L) when excited at 484 nm and 513 nm, respectively, and measured
under the same condition. (Inset: photograph for luminescence of the above
solutions of 3a and 4a upon irradiation with UV light at 365 nm.)
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Fig. 5 Energy diagrams and the pictorial representations of the selected molecular orbitals for 3a'–6a'.
coefficients on the four-membered ring with bonding interactions LUMO. The above results demonstrate that the electronic
between C7 and C8' as well as C7' and C8. 30 These structure of CBDP can be largely tuned by adjusting the
observations are in agreement with the conclusion that the substituting positions of silylethynyl groups.
two phenazine subunits in CBDP are poorly conjugated in the
Thin film transistors
ground state but strongly conjugated in the excited state.
Similarly, the LUMO and LUMO+1 of 4a' originate from the To test semiconductor properties of silylethynylated CBDPs,
LUMO of 6a' with large splitting. In contrast, the HOMO and top-contact bottom-gate thin film transistors were fabricated by
HOMO−1 of 4a' with very close energy levels originate from solution-based processes as detailed in ESI†. The solution-
different molecular orbitals of 6a'. The HOMO and HOMO−3 of processed films of 3b (Fig. 6a) and 4a consisted of crystalline
4a' originate from the HOMO−1 of 6a', while HOMO−1 and ribbons and fibers as found with a polarized-light microscope.
HOMO−2 of 4a' originate from the HOMO of 6a'. The HOMO−1 The X-ray diffraction (XRD) from the film of 3b exhibited a
and HOMO−2 of 4a' exhibit larger splitting than the HOMO and diffraction peak at 2θ = 6.95° (d spacing = 12.7 Å), which is in
HOMO−1 of 3a', suggesting the two HOMOs of 6a' interact accordance with the (010) diffraction as derived from the
more strongly than the two HOMOs of 5a' in forming the crystal structure of 3b. This indicates that molecules of 3b
corresponding molecular orbitals of 4a' and 3a', respectively. adopt an edge-on orientation on the dielectric surface with an
Such different interactions may be attributed to the fact that angle of 51.9° between the π-plane and the surface (Fig. S17 in
the HOMO of 6a' has higher density on C7 and C8 than that of ESI†). The XRD from the film of 4a exhibited a diffraction peak
5a'
HOMO of 4a' has a symmetry (b2g) different from that of 3a' any diffractions as derived from the crystal structure of 4a
.
31 Because of originating from the HOMO−1 of 6a', the at 2θ = 5.28° (d spacing = 16.7 Å), which do not correspond to
,
(au). As a result, the S1 to S0 transition of 4a' (B3u→Ag) is indicative of a thin film phase different from the single crystal
symmetry-allowed according to the Laporte rule, in agreement phase. In contrast, dip coating or drop casting solutions of 3a
with the high fluorescence quantum yield of 4a (Фf = 0.47). In and 4b under similar conditions resulted in tiny crystals that
contrast, the S1 to S0 transition of 3a' (B1g→Ag) is symmetry- were not suitable for fabrication of thin film transistors likely
forbidden in agreement with the low fluorescence quantum related to their molecular packing in the solid state, which,
yield of 3a (Фf = 0.03). Moreover, the symmetry-forbidden S0 however, remained unknown from crystal structures. As
to S1 transition of 3a is not responsible to the longest- measured from at least 30 channels in vacuum, 3b functioned
wavelength absorption of 3a (Figure 4), which in fact can be as an n-type semiconductor with field effect mobility of 0.13 ±
attributed to the symmetry-allowed S0 to S2 transition 0.05 cm2/Vs, which is higher than that of 1b by one order of
(Ag→B2u) involving electronic excitation from HOMO−1 to magnitude. The highest mobility of 3b is 0.30 cm2/Vs, which is
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extracted from the transfer I-V curve in the saturation regime a four-membered ring. With properly positioned silylethynyl
as shown in Fig. 6b. Compound 4a in the dip-coated films also substituting groups, CBDP offers a chromophore with large
functioned as an n-type semiconductor with field effect molar extinction coefficients,
a luminophore with good
mobility of 0.019 ± 0.007 cm2/Vs, which is lower than that of quantum yield, and an n-type organic semiconductor with field
3b by about one order of magnitude likely in relation to poorer effect mobility as high as 0.30 cm2/Vs. In contrast, these
π-overlap of 4a in the solid state and the deep grain properties are not available with the phenazine reference
boundaries as found from the atomic force microscope (AFM) compounds (5a and 6a). This suggests that the four-membered
image (Fig. S13 in ESI†). In control experiments, thin films of 5a ring is a versatile building block for designing novel π-electron
and 6a were also prepared by solution based process under systems for functional materials. On the basis of single crystal
similar conditions. Although both 5a and 6a were able to form structures, UV-vis absorption and DFT calculation, we conclude
continuous films containing crystalline fibers, deposition of that the two phenazine subunits in cyclobuta[1,2-b:3,4-
top-contact electrodes by thermal evaporation of gold b']diphenazine are poorly conjugated in the ground state but
appeared problematic. The films of 5a melt during deposition strongly conjugated in the excited state, shedding light on the
of gold because of the low melting point of 5a (96–98 °C), role of the four-membered ring in conjugation.
while the crystalline fibers of 6a cracked during this process. In
the resulting transistors, 6a functioned as an n-type
Acknowledgements
semiconductor with low field effect mobility in the range of
10−4 cm2/Vs, which can be attributed to its high LUMO energy
We thank Ms. Hoi Shan Chan (the Chinese University of Hong
level, the limited π-overlap in the solid state and the cracks in
Kong) for the single crystal crystallography. This work was
the films. On the other hand, thin films of 5a and 6a as dip
supported by the Research Grants Council of Hong Kong (GRF
coated on pre-fabricated bottom-contact gold electrodes did
14300217) and the University Grants Committee of Hong Kong
not exhibit any field effect presumably because of the poor
(project number: AoE/P-03/08).
contact between organic crystallites and the electrodes in
these devices. The performance of 3b and 4a in comparison to
that of 5a and 6a suggests that the four-membered ring is a
Conflicts of interest
useful linker to connect π-units for designing new n-type
There are no conflicts to declare.
semiconductors. The field effect mobility of 3b as reported
here is still lower than that of the state-of-the-art solution-
processed n-type organic semiconductors (i.e. > 1 cm2/Vs) by
one order of magnitude.5 As suggested by the low-lying and
Notes and references
fully delocalized LUMO and the large π-overlap in the solid
state, 3b may achieve higher electron mobility in thin film
transistors if the quality of films could be improved by
optimizing the conditions for device fabrication and the
interface structures.
1
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(b) drain current (IDS) versus gate voltage (VGS) with drain voltage (VDS) at 50
V for an OTFT of 3b with an active channel of W = 1 mm and L = 50 µm as
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Conclusions
In summary, this study puts forth a new π-electron molecular
scaffold, CBDP, by connecting two phenazine moieties through
6 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry C
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DOI: 10.1039/C7TC04092J
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24 HOMA is a quantitative measure of aromaticity based on bond
length as defined by
ꢏꢏ
ꢕꢕ
∑
HOMA = 1- ꢎ
ꢑRꢒꢓꢔ-Rꢕꢖ ꢕ
ꢗ
ꢅ for a carbocycle, or
ꢐ
ꢅ
ꢅ
HOMA = 1 − ꢘꢄ ꢙꢚꢛꢛ ꢑꢜꢝꢞꢟ − ꢜꢛꢛꢗ + ꢚꢛꢡ ꢑꢜꢝꢞꢟ − ꢜꢛꢡꢗ ꢢ
ꢛꢛ
ꢛꢡ
∑
∑
ꢠ
ꢠ
for a N-heterocycle, where n is the number of bonds taken into
the summation, αCC = 257.7 and αCN = 93.52 are empirical
normalization constants chosen to give HOMA = 0 for the
hypothetical Kekulé structures of the typical aromatic systems
with alternation of single and double bonds and HOMA = 1 for
the system with all bond lengths equal to the optimal value Ropt
(1.388 Å for C-C bonds and 1.334 Å for C-N bonds), and Ri is the
individual bond length in the ring. See: T. M. Krygowski and M.
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25 The commonly used formal potential of the redox couple of
ferrocenium/ferrocene (Fc+/Fc) in the Fermi scale is −5.1 eV,
which is obtained on the basis of an approximation neglecting
solvent effects with a formal potential of −4.46 eV for the
normal hydrogen electrode (NHE) in the vacuum scale and an
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Journal of Materials Chemistry C
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DOI: 10.1039/C7TC04092J
This study puts forth cyclobuta[1,2-b:3,4-b']diphenazine (CBDP), a new π-electron molecular scaffold containing two phenazine
moieties connected by a four-membered ring. CBDP exhibits interesting optical and electronic properties that can be tuned by
adjusting the substituting positions of silylethynyl groups but are not offered by the phenazine reference compounds, leading
to solution-processed n-type semiconductors with field effect mobility of up to 0.30 cm2/Vs.