A slight bluish-white fluorescence from: E, E-2,6-bis(4-cyanostyryl)pyridine pristine crystals

Article abstract of DOI:10.1039/c9ra09576d

Among various light emission phenomena, multicolour light emission, such as white light emission, is an interesting phenomenon. Bis-styrylbenzene derivatives show unique and excellent emission behaviours. In this study, three types of bis-styrene derivati

Full text of DOI:10.1039/c9ra09576d

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A slight bluish-white fluorescence from E,E-2,6-
bis(4-cyanostyryl)pyridine pristine crystals†
Cite this: RSC Adv., 2020, 10, 2727
*
Hiroyuki Mochizuki
and Hitoshi Kusama
Among various light emission phenomena, multicolour light emission, such as white light emission, is an
interesting phenomenon. Bis-styrylbenzene derivatives show unique and excellent emission behaviours.
In this study, three types of bis-styrene derivatives, with thiophene, pyridine, and benzene rings at the
central position, were synthesised, and their photophysical properties were studied in detail. Among
Received 17th November 2019
Accepted 6th January 2020
them, pristine crystals of E,E-2,6-bis(4-cyanostyryl)pyridine (Py4CN) showed
a slight bluish-white
DOI: 10.1039/c9ra09576d
emission because the Py4CN pristine crystals were mixtures of two separate crystals emitting blue and
rsc.li/rsc-advances
yellow-green fluorescence.
absorption and uorescence wavelength regions,26 and thus, is
widely used as a substituent for organic solar cell materials.28
Introduction
Additionally, it is predicted that the properties of bis-styrene
derivatives will depend on the aromatic ring introduced at the
central position; hence, it is essential to determine the changes
introduced by each aromatic ring.
Materials emitting multicolour light, such as white-light-
emitting materials, are of great interest for displays and illu-
mination devices.1–5 Multicolour illumination usually requires
multiple light sources; however, a singular light source would
be efficient from the viewpoint of device structure. In particular,
if a single organic light emitter compound can emit white light,
such a compound will be in great demand.
For three decades, organic p-conjugated materials,6–13 espe-
cially p-conjugated oligomers, have attracted considerable
attention due to their utility in tuning optical and electronic
properties.9–13 Among p-conjugated oligomers, the compounds
consisting of a bis-styrene skeleton have found application in
organic light-emitting devices and organic solar cells due to
their high photoelectronic performance based on their large
conjugated area.14–16 Bis-styrylbenzene derivatives (BSDs) show
unique photoemission behaviours. There have been many
reports on aggregation-induced emission (AIE), the phenom-
enon where the derivative does not emit light in solution but
emits light in aggregated state.17–22 (2Z,20Z)-2,20-(1,4-phenylene)
bis(3-phenylacrylonitrile) (BDCS) is a typical material for which
AIE was observed.23 BSD crystals have a very low threshold for
the amplied spontaneous emission (ASE) phenomenon, which
has garnered signicant attention in recent years for the
development of current injection-type organic lasers.24
Three kinds of bis-styrene derivatives bearing CN groups at
both terminals and with thiophene, pyridine, and benzene rings
at the central position were synthesised in this study. To the best
of our knowledge, no prior systematic evaluation has been con-
ducted for a series of bis-cyanostyrene derivatives substituted
with various aromatic rings at the central position. Furthermore,
we detail the changes in the properties of these compounds
depending on the central aromatic rings, and especially, report
that the crystals of the compound introduced with the pyridine
ring emitted a slightly bluish-white uorescence.
Experimental
The three compounds, E,E-2,5-bis(4-cyanostyryl)thiophene
(Th4CN), E,E-2,6-bis(4-cyanostyryl)pyridine (Py4CN), and E,E-
1,3-bis(4-cyanostyryl)benzene (3Be4CN), were synthesised by
the Horner–Wadsworth–Emmons reaction. The process for
synthesis and their melting points are detailed below. The
chemical structures of these three compounds are shown in
Fig. 1. E,E-1,4-bis(4-cyanostyryl)benzene (4Be4CN) was known26
and also synthesised for comparison. All these three
compounds are V-shaped; therefore, the linear structured
compound such as 4Be4CN was necessary as a good comparison
object, because the thiophene ring cannot produce a linear
derivative because of its structure.
Recently, these compounds with various terminal substitu-
ents have been reported; their properties depend on these
terminal substituents.11,25–27 Among the substituents, the
introduction of a cyano (CN) group induces widening of the
Research Center for Photovoltaics, National Institute of Advanced Industrial Science
and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra09576d
Synthesis
A 28% methanol solution of sodium methoxide (8.1 mmol; from
Wako Pure Chemical Industries, Ltd.) was added dropwise to
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The uorescence spectrum of the crystal was obtained with
a bre spectrometer.
The X-ray diffraction analysis at room temperature was per-
formed by following a simple wide-angle focusing method using
an X-ray diffractometer (SmartLab, RIGAKU) operated at 45 kV
and 200 mA (Cu Ka).
Fig. 1 Chemical structures of the compounds used in this study.
Computational details
The density functional theory (DFT) calculations were per-
formed by using Gaussian 16 soware at the Research Center
for Computational Science, Okazaki, Japan.29 The ground-state
geometry was optimised at the hybrid DFT level by using the
B3LYP functional, which combines Becke's three-parameter
exchange functional (B3)30,31 with the correlation functional of
Lee, Yang, and Parr (LYP).32 The 6-31G basis set33 was adopted in
all the systems. Frequency calculations were performed to
conrm the optimised structures with no imaginary frequen-
cies, corresponding to the real minima on the entire potential
energy surface. The solvent effects of dichloromethane (dielec-
tric constant ¼ 8.93) were modelled by using a conductor-like
polarizable continuum model (C-PCM)34 within the self-
consistent reaction eld theory.
a
stirred mixture of diethyl(4-cyanobenzyl)phosphonate
(8.0 mmol; from Tokyo Chemical Industry Co., Ltd.) and the
necessary dicarboxaldehyde (3.9 mmol) in 10 mL N,N-dime-
thylformamide (DMF) at room temperature. The dicarbox-
aldehydes used were 2,5-thiophenedicarboxaldehyde, 2,6-
pyridinedicarboxaldehyde, and 1,3-benzenedicarboxaldehyde
(Tokyo Chemical Industry Co., Ltd). This mixture was heated to
80–120 ꢀC for 2–4 h. Aerwards, the reaction was then
quenched with 2 : 1 DMF/H2O. The precipitate was then ltered
and re-crystallised twice from toluene or chlorobenzene, giving
the desired compounds.
The melting points of Th4CN, Py4CN, and 3Be4CN were 279–
281 ꢀC, 175–178 ꢀC, and 215–218 ꢀC, respectively. The 1H NMR
and 13C NMR spectra of Th4CN, Py4CN, and 3Be4CN are shown
in ESI.†
Results and discussion
Basic optical properties in the solution
Th4CN. 1H NMR 400 MHz (CDCl3), 6.89 (d, J ¼ 16.0 Hz, 2H),
7.06 (s, 2H), 7.29 (d, J ¼ 16.0 Hz, 2H), 7.53 (d, J ¼ 8.4 Hz, 4H),
7.63 (d, J ¼ 8.4 Hz, 4H). 13C NMR 100 MHz (CDCl3), 110.8, 118.9,
120.1, 126.7, 127.0, 128.8, 132.6, 141.2, 142.1. Elemental anal.:
calcd for C22H14N2S: C, 78.08; H, 4.17; N, 8.28; found, C, 78.18;
H, 4.10; N, 8.18%.
Fig. 2 shows the absorption spectra of the three compounds in
dilute dichloromethane solution (10ꢁ5 M). The main absorption
spectrum of Th4CN from 350 to 450 nm possessed one peak (at
410 nm) and two shoulders (at ꢂ385 and 440 nm). Similarly, the
Py4CN absorption spectrum exhibited two peaks (at 303 and 350
nm) and one shoulder (at ꢂ320 nm). The 3Be4CN absorption
spectrum showed one peak (at 327 nm) and two shoulders (at
Py4CN. 1H NMR 400 MHz (acetonitrile-d3), 7.39–7.43 (m,
4H), 7.74–7.82 (m, 9H), 7.87 (d, J ¼ 16.0 Hz, 2H). 13C NMR 100
MHz (CDCl3), 111.5, 118.9, 122.0, 127.5, 131.2, 131.4, 132.6,
137.4, 141.1, 154.6. Elemental anal.: calcd for C23H15N3: C,
82.86; H, 4.54; N, 12.60; found: C, 82.75; H, 4.62; N, 12.49%.
1
3Be4CN. H NMR 400 MHz (DMSO-d6), 7.43 (d, J ¼ 16.5 Hz,
2H), 7.47 (d, J ¼ 7.7 Hz, 2H), 7.51 (d, J ¼ 16.4 Hz, 2H), 7.61 (dd, J
¼ 7.7 Hz, 2H), 7.82 (d, J ¼ 8.6 Hz, 2H), 7.86 (d, J ¼ 8.6 Hz, 2H),
7.95 (s, 1H).13C NMR 100 MHz (CDCl3), 110.9, 118.9, 125.9,
126.9, 127.0, 127.4, 129.4, 131.9, 132.6, 137.0, 141.6. Elemental
anal.: calcd for C23H15N3: C, 86.72; H, 4.85; N, 8.43; found: C,
86.61; H, 4.92; N, 8.52%.
Basic properties measurements
The thermal properties of the products were evaluated by using
differential scanning calorimetry. The optical properties were
measured by using a uorescence spectrometer (F8600, Jasco)
equipped with an integrating sphere and a UV spectrophotom-
eter (UV-3600, SHIMADZU). The lifetime in the excited state and
ionisation potentials were measured by using a uorescence
lifetime spectrometer (HAMAMATSU, C11367) and a photoelec-
tron spectrophotometer in air (RIKEN KEIKI, AC-3), respectively.
The samples for determining optical and photoelectrical
properties were prepared by vapour deposition at a rate of
0.02 nm sꢁ1 to a total thickness of 100 nm for optical
Fig. 2 Solution-state absorption spectra of Th4CN (red line), Py4CN
measurement and 50 nm for the photoelectrical measurement. (blue line), 3Be4CN (green line), and 4Be4CN (black line).
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Table 1 The photophysical properties of four compounds. Upper part:
dilute dichloromethane solutions (10ꢁ5 M); lower part: as-evaporated
films
ꢂ310 and 340 nm). First, the peak absorbance of the three
compounds were in the order of Th4CN < Py4CN < 3Be4CN. The
full width at half maximums (FWHMs) of the absorption spectra
were in the order of 3Be4CN (55 nm) < Th4CN (77 nm) < Py4CN
(90 nm). Furthermore, the absorption edges of these
compounds were in the order of 3Be4CN < Py4CN < Th4CN. The
bandgap energies of Th4CN, Py4CN, and 3Be4CN, estimated
from their absorption edges, were 2.71, 3.16, and 3.43 eV,
respectively. It was observed that the introduction of a thio-
phene ring was effective for reducing the bandgap energy.
Compared with 4Be4CN, the absorption spectrum of 3Be4CN
covered the shorter wavelength region and showed larger
absorbance.
Fig. 3 shows the uorescence spectra of Th4CN, Py4CN, and
3Be4CN in dilute dichloromethane solution (10ꢁ5 M). The
Th4CN uorescence spectrum, with two peaks at 464 and
491 nm, covered the longest wavelength region among the three
compounds. The Py4CN spectrum also showed two peaks at 394
and 413 nm. 3Be4CN showed a top-hat-like spectrum of two
peaks at 383 and 397 nm, with the same intensities. In
comparison with the solution-state uorescence spectrum of
4Be4CN, the 3Be4CN spectrum was located in the shorter
wavelength region, as was the case in the absorption spectrum.
The FWHMs of the three compounds increased slightly in the
order of Py4CN (51 nm) < 3Be4CN (59 nm) < Th4CN (64 nm).
Since Th4CN did not show monomodal and wide uorescence
spectrum in dilute solution, it can be inferred that introduction
of the thiophene ring at the central position shied the uo-
rescence spectrum to a longer wavelength without any intra-
molecular charge transfer interaction.
Th4CN
Py4CN
3Be4CN
4Be4CN
Ff
0.08
0.80
0.12
0.61
sf/ns
kr/sꢁ1
Ff
0.3
2.3
1.8
0.7
2.6 ꢃ 108
3.5 ꢃ 108
0.7 ꢃ 108
8.7 ꢃ 108
0.06
0.26
0.10
0.45
sf/ns
kr/sꢁ1
3.1
1.0
3.2
15.0
0.2 ꢃ 108
2.6 ꢃ 108
0.3 ꢃ 108
0.3 ꢃ 108
3Be4CN were 0.08 and 0.12, respectively. This reveals that the
introduction of a thiophene ring lowered the Ff value. The Ff
value of 4Be4CN (0.61) was larger than that of 3Be4CN, sug-
gesting that the V-shaped structure could reduce the Ff value.
The uorescence lifetimes (sf) of the three compounds were in
the order of Py4CN (2.3 ns) > 3Be4CN (1.8 ns) > Th4CN (0.3 ns).
The kr values were calculated as kr ¼ Ff/sf; Py4CN showed
a higher kr value (3.5 ꢃ 108 sꢁ1) than 2.6 ꢃ 108 sꢁ1 and 0.7 ꢃ 108
sꢁ1 of Th4CN and 3Be4CN, respectively.
Basic optical properties in the solid
Fig. 4 shows the uorescence spectra of the as-evaporated lms of
the three compounds. The uorescence spectrum of the as-
evaporated Th4CN lm showed one peak (at 553 nm) and one
shoulder (at ꢂ530 nm). Similarly, the uorescence spectra of
Py4CN and 3Be4CN were bimodal (with peaks at 404 and 421 nm)
and monomodal (with a peak at 392 nm), respectively. Compared
with the solution-state uorescence spectrum of Th4CN, the
spectrum of the as-evaporated Th4CN lm shied by ꢂ100 nm.
The FWHM of the as-evaporated lm spectrum was 110 nm,
wider than that of the solution-state spectrum, suggesting that
Table 1 summarises the photophysical properties of the
three compounds. Py4CN showed the highest uorescence
quantum efficiency (Ff) of 0.80, while those of Th4CN and
Fig. 3 Solution-state fluorescence spectra of Th4CN (red line), Py4CN Fig. 4 Fluorescence spectra of the as-evaporated films of Th4CN (red
(blue line), 3Be4CN (green line), and 4Be4CN (black line).
line), Py4CN (blue line), 3Be4CN (green line), and 4Be4CN (black line).
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that of the solution-state spectrum, and its FWHM was ꢂ1.7
times larger than that of the solution-state spectrum.35 We
concluded that this was due to the intermolecular charge transfer
interaction.
The uorescence spectra of the as-evaporated Py4CN and
3Be4CN lms were located at almost the same wavelength as
their solution-state spectra. Furthermore, the FWHMs of Py4CN
and 3Be4CN were almost the same as those of the solution-state
spectra. The as-evaporated 4Be4CN lm showed a monomodal
and wide uorescence spectrum due to its intermolecular
charge transfer interaction.26 However, the uorescence spec-
trum of the as-evaporated 3Be4CN lm was similar to that ob-
tained in solution, suggesting that 3Be4CN shows no
intermolecular charge transfer interaction. The absorption
edges of the as-evaporated Th4CN, Py4CN and 3Be4CN lms,
determined from the excitation spectra, were 517, 418, and
374 nm, respectively, and these were shied to longer wave-
lengths by 61, 26, and 14 nm, respectively, from the absorption
edges in their solutions. The longer red shi of the absorption
edge for the as-evaporated Th4CN lm than those of the other
two lms was attributed to the intermolecular charge transfer
interaction.
Table 1 also summarises the photophysical properties of
the as-evaporated lms of the three compounds. The Ff value
of Py4CN was the highest (0.26) among the three compounds.
However, the Ff value of the as-evaporated Py4CN lms
became lower than that in the dilute solution (0.80). The Ff
values of Th4CN and 3Be4CN were 0.06 and 0.10, respectively,
slightly less than the corresponding values in their solutions.
The sf values of Th4CN, Py4CN, and 3Be4CN were 3.1, 1.0, and
3.2 ns, respectively. Generally, sf values in the solid states are
larger than those in the solution state. The sf of the as-
evaporated Py4CN lm was shorter than that in the solution
state. Only Py4CN showed kr > 1.0 ꢃ 108 sꢁ1, because sf of
Py4CN was 1.0 ns, even in the as-evaporated lms. The Ff
values of Th4CN and 3Be4CN were ꢂ0.1, and their sf values
were more than 3 ns, causing the kr values of Th4CN and
3Be4CN to be less than 1.0 ꢃ 108 sꢁ1
.
Slightly bluish-white emission from Py4CN pristine crystals
Fig. 5 shows the photomicrographs of the Py4CN pristine crys-
tals under irradiation at 365 nm. These crystals were obtained
from re-crystallisation, and emitted a slightly bluish-white
colour. Aer the Py4CN crystals were dissolved in chloroben-
Fig.
5 Photomicrographs of Py4CN crystal under irradiation at
zene, this solution was added dropwise on
a
poly-
365 nm. (Top) Pristine crystal. (Middle) The crystals obtained with
relatively high drying rate. (Bottom) The crystals with relatively low
drying rate.
dimethylsiloxane (PDMS) substrate, followed by distilling off
the chlorobenzene. If the chlorobenzene was dried relatively
ꢀ
quickly (with an open system at 26–27 C), needle-like crystals
were produced, emitting a blue uorescent glow (see the middle
image in Fig. 5). On the other hand, if the chlorobenzene was
dried relatively slowly by distilling off the solvent while under
cover with a Petri dish, the crystals emitted a yellow-green
uorescent glow (the bottom image in Fig. 5). From these
results, it can be concluded that the slightly bluish-white
luminescent crystals are a mixture of blue and yellow-green
luminescent crystals.
the as-evaporated Th4CN lm shows an intermolecular charge
transfer interaction effect, which is not seen in solution. This
phenomenon was also seen in other BSD systems. The emission
spectrum of CN-substituted BSD solid, (2Z,20Z)-3,30-(2,5-
dimethoxy-1,4-phenylene)bis(2-(4-butoxyphenyl)acrylonitrile) (b-
MODBDCS), also shied to longer wavelengths by ꢂ100 nm than
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absorption of blue light emission by the yellow-green uores-
cence crystal.
Fig. 7 shows the DSC thermogram of the rst heating scan of
the Py4CN pristine crystal. This thermogram had two endo-
thermic peaks at 173 and 182 ꢀC. Observation under a polar-
ising microscope revealed that the melting points of the crystals
emitting blue and yellow-green uorescence were 170 and
178 ꢀC, respectively. These suggest that the two peaks of the DSC
thermogram for the pristine crystal correspond to melting
points of the two crystals emitting blue and yellow-green uo-
rescence. Considering the peak intensity ratio of the DSC curve,
it was found that in the pristine crystals, the crystals emitting
yellow-green uorescence were signicantly more abundant
than those emitting blue uorescence.
Fig. 6 Fluorescence spectra of the Py4CN crystal; red line: pristine
crystal; blue line: the crystals obtained with relatively high drying rate;
green line: the crystals obtained with relatively low drying rate.
Fig. 6 shows the emission spectra of the Py4CN pristine
crystals under 365 nm irradiation. The emission spectrum was
spread from near 420 nm to around 700 nm, covering almost
the entire visible light range. Fig. 6 also shows the blue emission
spectrum of the needle-like crystals, with a peak at 429 nm. The
spectrum of the crystals for yellow-green uorescence was
monomodal with a peak of 523 nm. In Fig. 6, the rising edge on
the short-wavelength side for the pristine crystal was more
towards the longer-wavelength side than that of the blue-light-
emitting crystal. The reason for this is believed to be the
Fig. 8 XRD profiles. (Upper) The powder crystals emitting blue (blue
Fig. 7 DSC thermogram of the Py4CN pristine crystal at first heating line) and yellow-green (red line) fluorescence; (Lower) the pristine
scan.
crystal.
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Table 3 The calculated HOMO and LUMO energies of the four
compounds
Fig. 8 shows XRD proles of the powder crystals for blue and
yellow-green emissions. A lot of peaks were observed in both
crystals. There were signicant differences in the 2q range from
10ꢀ to 15ꢀ between the two crystals. In the range described
above, the crystals emitting blue uorescence exhibited two
peaks (13.7 and 14.8ꢀ), while the crystals emitting yellow-green
uorescence showed ve peaks (at 2q ¼ 10.7ꢀ, 11.8ꢀ, 12.6ꢀ,
13.3ꢀ, and 13.6ꢀ), and the positions of the peaks for both the
crystals were different. The XRD prole of the pristine crystals
obtained from re-crystallisation is also shown in Fig. 8.
Th4CN
Py4CN
3Be4CN
4Be4CN
HOMO (eV)
Bandgap (eV)
LUMO (eV)
ꢁ5.44
2.83
ꢁ2.62
ꢁ6.00
3.69
ꢁ2.31
ꢁ5.86
3.63
ꢁ2.22
ꢁ5.63
3.19
ꢁ2.44
meaning that the thiophene ring is more electronically positive
and electron-donating compared to the benzene ring. Table 2
also shows the bandgap energies and the LUMO levels, esti-
mated from the absorption edges of the as-evaporated lms.
Table 3 exhibits the calculated HOMO and LUMO values of
the four compounds. The calculated HOMO values were
generally higher than the measured IP values. The trend of the
calculated values was similar to that of the measured ones; the
calculated HOMO values also decreased in the same order of
Th4CN (ꢁ5.44 eV) > 4Be4CN (ꢁ5.63 eV) > 3Be4CN (ꢁ5.86 eV) >
Py4CN (ꢁ6.00 eV) as the measured IP values. On the other hand,
the trend of the measured LUMO values (Py4CN (ꢁ3.65 eV) <
Th4CN (ꢁ3.54 eV) < 4Be4CN (ꢁ3.30 eV) < 3Be4CN (ꢁ3.21 eV))
was different from that of the calculated ones (Th4CN (ꢁ2.62
eV) < 4Be4CN (ꢁ2.44 eV) < Py4CN (ꢁ2.31 eV) < 3Be4CN (ꢁ2.22
eV)). The measured bandgap energies of the four compounds
increased in the order of Th4CN (2.40 eV) < 4Be4CN (2.79 eV) <
Py4CN (2.97 eV) < 3Be4CN (3.32 eV), and the calculated LUMO
bandgap energies increased in the order of Th4CN (2.83 eV) <
4Be4CN (3.19 eV) < 3Be4CN (3.63 eV) < Py4CN (3.69 eV). The
difference in this tendency was due to the difference in the
bandgap energy of Py4CN. The calculated bandgap energy of
Py4CN (3.69 eV) was signicantly larger than the measured one
(2.97 eV): the difference between the calculated and measured
bandgap energies for Py4CN was 0.72 eV, much larger than
those for Th4CN (0.43 eV), 3Be4CN (0.31 eV), and 4Be4CN (0.40
eV). If the difference in the bandgap energy of Py4CN is about
the same as those of the other compounds, the trends of the
measured and calculated LUMO values would be similar.
Localizations of the HOMO and LUMO and optimised
structures for the four compounds are shown in ESI.†
In the XRD patterns of the pristine crystals, the peaks which
could be attributed to the yellow-green uorescence-emitting
crystals had high intensities, reecting the high composition
ratio of the crystals, as suggested by the DSC measurement
result. Five peaks were observed in the 2q range 10–15ꢀ in the
XRD patterns of the pristine crystals, and although these peak
intensities were different from those of the crystal emitting
yellow-green uorescence, the peak positions were almost the
same. On the other hand, the peaks assigned to the crystal
emitting blue uorescence were not as sharp as those of the
yellow-green uorescence crystals. This is probably because of
the lower composition ratio of the blue-uorescence crystals
than that of the yellow-green-uorescence crystals.
A commercial white LED is composed of a diode that emits
blue light (around 455 nm) and a phosphor that absorbs the
blue light emission and emits yellowish-green light (around 560
nm). If the emission wavelengths of the two crystals can be
shied to the longer wavelength side by 30 nm, it can function
as a singular source of white light.
Photoelectronic properties
These three novel compounds can be used as additives in
organic solar cells, organic light-emitting diode, etc. Thus,
photophysical properties, especially the ionisation potential (IP)
values of the three compounds were considered necessary for
study.
Table 2 shows that the IP values of the three compounds
studied decreased in the order of Th4CN (ꢁ5.94 eV) > 3Be4CN
(ꢁ6.53 eV) > Py4CN (ꢁ6.62 eV). Introduction of the CN group
lowered the IP value, and thus, the IP value of 4Be4CN was
ꢁ6.09 eV,26 suggesting that the V-shaped structure of 3Be4CN is
more susceptible to the effect of the CN group than the linear
4Be4CN molecule. The IP value of Py4CN was ꢁ6.62 eV, higher
than that of 3Be4CN. If the benzene ring was electronically
neutral, the pyridine ring was slightly electronically negative or
electron-withdrawing, compared to benzene. The IP value of
Th4CN was ꢁ5.94 eV, much higher than that of 3Be4CN,
Conclusions
Three novel BSDs, with thiophene, pyridine, and benzene rings
in central positions, were synthesised. Th4CN was located at the
longer-wavelength side compared to 3Be4CN and Py4CN in both
their solution-state absorption and emission spectra. Intermo-
lecular charge transfer interaction was observed in the Th4CN
solid lm. The IP values of Th4CN were higher than those of
Py4CN and 3Be4CN, suggesting that thiophene rings have more
electron-donating ability than pyridine and benzene. The IP
value of Py4CN was the lowest among the three compounds.
The trends of the measured and calculated values of IP (HOMO)
for the four compounds were similar. Furthermore, Py4CN
showed high emission ability, and the Py4CN pristine crystals
emitted a slightly bluish-white uorescence. Based on the DSC
and XRD measurements, as well as the emission spectrum of
Table 2 The measured HOMO and LUMO energies of the four
compounds
Th4CN
Py4CN
3Be4CN
4Be4CN
IP value (HOMO) (eV)
Bandgap (eV)
ꢁ5.94
2.40
ꢁ6.62
2.97
ꢁ6.53
3.32
ꢁ6.09
2.79
LUMO (eV)
ꢁ3.54
ꢁ3.65
ꢁ3.21
ꢁ3.30
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Conflicts of interest
There are no conicts to declare.
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Acknowledgements
The authors thank Dr Takashi Funaki for his helpful discussion
and valuable support. The theoretical calculations were per-
formed at the Research Center for Computational Science,
Okazaki, Japan.
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