Cyclic emitter with tetraphenylsilane and tetraphenylethene units exhibiting tunable color emissions

Article abstract of DOI:10.1246/cl.170636

A novel cyclic emitter composed of tetraphenylsilane and tetraphenylethene backbone was successfully synthesized through a convenient homocoupling procedure. The optical and thermal properties of the compound were revealed. Intriguingly, the compound showed different color emissions in solid film, THF solution, and THF/water mixtures. It is assumed that the violet emission was from an isolated component of the emitter, whereas the sky blue and the green emissions were from a crystalline state, and an amorphous state, respectively. The red-shifted emissions were caused by the change of nature of the excitonic coupling.

Full text of DOI:10.1246/cl.170636

CL-170636
Received: June 29, 2017 | Accepted: August 8, 2017 | Web Released: August 19, 2017
Cyclic Emitter with Tetraphenylsilane and Tetraphenylethene Units
Exhibiting Tunable Color Emissions
Hiroaki Itoi,1 Taehee Jang,1 Shinji Kanehashi,2 Takeshi Shimomura,2 and Kenji Ogino*1
1Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology,
2-24-16 Nakacho, Koganei, Tokyo 184-8588
2Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588
(E-mail: kogino@cc.tuat.ac.jp)
A novel cyclic emitter composed of tetraphenylsilane and
tetraphenylethene backbone was successfully synthesized
through a convenient homocoupling procedure. The optical
and thermal properties of the compound were revealed.
Intriguingly, the compound showed different color emissions
in solid film, THF solution, and THF/water mixtures. It is
assumed that the violet emission was from an isolated
component of the emitter, whereas the sky blue and the green
emissions were from a crystalline state, and an amorphous state,
respectively. The red-shifted emissions were caused by the
change of nature of the excitonic coupling.
ical stability. Their σ*-π* conjugation endows them with low-
lying lowest unoccupied molecular orbitals (LUMO)s and
hence high electron affinity and high electron mobility.14 The
combination of TPE with silane affords AIE-active blue
materials with high ΦF values (55-64%) in the solid state.
Blue-emitting OLEDs fabricated utilizing these luminogens as
emitters showed good performances.15
Our research group reported the synthesis of cyclic
oligomers composed of triphenylamine via Pd-catalyzed C-N
coupling reaction of A-B type monomer.16 It has been found that
thermal and morphological stabilities are improved due to the
absence of end groups in the applications to hole transporting
materials. The rigid nature of cyclic molecules is also advanta-
geous for fluorescent materials since it prevents nonradiative
transition by hampering intramolecular motion.
Keywords: McMurry reaction
| Cyclic emitter |
Mechanochromism
Intensive studies have been conducted on organic light-
emitting diodes (OLEDs), as they have a great potential to be
applied to large full-color displays.1-3 The range of applications
of organic functional compounds has recently expanded rapidly,
with especially innovative uses in optoelectronics.4 Most
conjugated organic molecules have been reported to display
red, green, and blue electroluminescence (EL).5 Emitters with
high efficiency and long lifetime of red and green light have
recently been developed. However, it seems very difficult to
develop materials that emit violet or blue light with high
efficiency since a wide band gap, with a large difference between
the energy levels of adjacent hole and electron transporting
layers, is an intrinsic property of blue-light emitters. Moreover,
if the single emitter exhibits tunable color emission dependent
on the aggregation state, it is possible to fabricate a multicolor
device with the emitter.6
More than 80 years ago, unusual photophysical behaviors
induced by the aggregation of pseudoisocyanine chloride (PIC
chloride) were independently reported by Scheibe,7 and Jelley.8
The aggregates of PIC chloride (J-aggregate) in aqueous
solutions exhibit a sharp, red-shifted absorption band, as well
as a strong fluorescence. In 2001, Tang and co-workers found a
similar phenomenon, designated aggregation-induced emission
(AIE),9 which is a contrasting phenomenon of the aggregation-
caused quenching (ACQ)10 effect. Aggregation enhances the
intensity of fluorescence from the compound without quench-
ing.11 Among various fluorophores, molecules with tetraphenyl-
ethene (TPE) unit exhibit a strong AIE effect.12 Nowadays,
many researchers have developed a lot of efficient blue emitters
using TPE as the key building block because TPE derivatives are
easily synthesized and afford strong AIE effect.
In this report, we designed a novel cyclic emitter with
tetraphenylsilane and alkylated TPE backbone which is appro-
priate for wet processes in device fabrication. Characteristic
photophysical and thermal properties are presented. In addition,
preliminary results for the mechanochromic nature of the
synthesized emitter are also demonstrated.
Scheme 1 shows the synthetic route of a targeted cyclic
compound. A ketone intermediate 1 was synthesized from
4-bromobenzoyl chloride and amylbenzene using aluminum
chloride. A pinacol ester 2 was synthesized from bis(4-
bromophenyl)diphenylsilane and bis(pinacolato)diboron using
a palladium catalyst. The compound 1 was reacted with
compound 2 to afford a diketone 3. The cyclic compound 4
was synthesized from the homocoupling of 3 via McMurry
reaction in 1,4-dioxane in diluted conditions to prevent for-
mation of linear polymeric by-products.17 It is considered that
the final reaction, the homocoupling of the dog-leg shaped
ketone precursor 3 facilitates the formation of cyclic compound
4 without the contamination by larger cyclic by-products.
Actually, 4 was isolated only by column chromatography and
recrystallization (yield of the final step; 10.4%). The synthetic
procedure for the cyclic compound 4 is as follows; To a solution
of tetrachlorotitanium (1.60 mL, 14.6 mmol) in 1,4-dioxane
(900 mL), zinc powder (1.91 g, 29.2 mmol) was added slowly
with stirring at ¹10 °C under a nitrogen atmosphere, and then
the mixture was heated at 100 °C for 2 h. To the suspension,
a solution of compound 3 (1.12 g, 1.46 mmol) in 1,4-dioxane
(100 mL) was added, and then the mixture was heated at reflux
for 15 h. A solution of 10% K2CO3 in water was carefully added
and the mixture was extracted with CH2Cl2. The organic layer
was washed with brine and dried over anhydrous MgSO4, and
the solvents were evaporated to give a crude product, which was
purified by silica gel column chromatography (eluent: CH2Cl2
and hexane) and recrystallized from toluene and EtOH to give
Recently, aryl-substituted silanes have been considered as
attractive OLED materials for blue electrophorescence devices.13
Current studies focus on the improvement of their morpholog-
1546 | Chem. Lett. 2017, 46, 1546–1549 | doi:10.1246/cl.170636
© 2017 The Chemical Society of Japan

Table 1. Photophysical and thermal characteristics of compound 4
em/nm
-
-
abs/nm
-
abs/nm
-
em/nm
-em/nm
c
Compound
in THF/water
(10/90)a
Tg/°C
Td/°C
Eg /eV
in THFa
in filmb
in THFa
in filmb
4
352
343
417
470
509
167
458
2.96
b
c
a1 © 10¹4 M. Film drop-casted on a quartz plate. Estimated from the onset of the absorption spectrum in film: 1240/-onest
.
Figure 1. UV-vis absorption spectrum of compound 4 in pure
THF, and PL spectra in pure THF, THF/water mixture (fw =
90%), and solid film. Excitation wavelength: 365 nm.
smaller than those of other OLED emitters partially ascribed
to its conformationally rigid cyclic structure. As shown in
Figure 1, in THF/water (10/90), the emission maximum was
red-shifted to 470 nm affording a relatively broad peak with
shoulder around 440 and 510 nm. In a film state, the emission
around 510 nm became predominate, and appeared to be
greenish to the naked eye. Since compound 4 includes TPE
moieties, it is reasonably presumed that the increased intensity
of red-shift emission in THF/water is related to the formation
of nanoparticles similar to the observation in AIE luminogens.
In order to confirm this assumption, its emission behaviors in
THF and THF/water mixtures with varying water fractions ( fws)
were monitored.
Scheme 1. Synthetic route of compound 4.
Figure 2 shows the change of PL spectra (a), and the PL
intensity, and fluorescent images (b) for the solution in THF and
THF/water mixtures. When the water contents were less than
50%, compound 4 showed violet emission with the peak at
417 nm, and the intensity of the emission decreased with the
increase of water content indicating that the weak aggregation
of the fluorophore occurred. The mixture containing 60% water
exhibited an emission peak at 470 nm with shoulders around
440 and 510 nm. Over water content of 60%, the intensities
at 470 nm slightly decreased. Judging from the dependence
of water content, it is suggested that the violet emission
was from a single component of 4, and the sky blue emission
arose from partially crystalline nanoparticles resulting from the
precipitation. In the solution state, the intramolecular rotation of
TPE units, which is known as the low-frequency modes of the
reorganization energy, serves as a relaxation channel for the
excited state to decay, while in the aggregated state, the rotation
is strongly restricted to block the nonradiative decay pathway
and produce the radiative excitons. The red-shifted emission in
the nanoparticles is considered to be caused by the dipole-dipole
interactions between neighboring molecules leading to the
excitonic coupling. In the case of compound 4, even in THF
the compound 4, which was characterized by 1H- and 13C NMR.
These spectra are presented in Supporting Information (SI)
(Figures S1 and S2). The thermal properties of compound 4 was
investigated by differential scanning calorimetry (DSC) and
thermogravimetric (TGA) analyses as shown in Figures S3 and
S4. As listed in Table 1, compound 4 shows high thermal
decomposition temperature (Td; 458 °C), and a glass transition at
167 °C. Due to the cyclic rigid structure, compound 4 has higher
glass-transition temperature (Tg) than other TPE substituted
silane applicable to electroluminescent devices.15
Figure 1 shows photoluminescence (PL) spectra for 4 in
THF, THF/water (10/90) solutions, and a drop-cast film
together with UV-vis absorption spectrum in THF. Table 1 lists
the optical properties for 4. The band gap was determined from
the absorption edge in the absorption spectrum with the value of
2.96 eV for 4, which is close to other blue emissive layer
materials.18,19 For the THF solution, a violet emission peak was
observed at 417 nm with the shoulders around 440 and 470 nm.
The Stokes shift was 65 nm, which was determined with PL and
UV-vis absorption spectra. Width at half maximum of the PL
spectrum of the THF solution was 42 nm, which appears to be
Chem. Lett. 2017, 46, 1546–1549 | doi:10.1246/cl.170636
© 2017 The Chemical Society of Japan | 1547

(a)
(b)
Figure 2. (a) PL spectra of compound 4 in THF/water mixtures with different water fractions (fw). Excitation wavelength: 365 nm.
(b) Emission peak intensity (Ipeak) of compound 4 in THF and THF/water at varying fws at 417 and 470 nm. Concentration =
1.0 © 10¹4 M. Inset: photographs of the solutions of compound 4 in THF/water mixtures (fw = 0-90%) under UV irradiation.
by the fact that the thermogram was recorded on the second scan
(a)
(b)
experiment, and the stable amorphous nature of compound 4 due
to the conformational versatility.
For the ground sample, the sharp diffractions observed in
as-prepared sample vanished, and only amorphous hollow was
observed indicative of amorphous nature of ground sample.
Drop-cast solution afforded highly transparent film, suggesting
that the obtained film was also amorphous. Consequently, the
origin of mechanochromism is related to the crystalline and
amorphous transition, and it is reasonable that the emissions at
470 and 510 nm originate from the crystalline and amorphous
solids, respectively.
Figure 3. Photographs of solids for (a) as-prepared and (b)
ground compound 4 taken under UV light (-ex = 365 nm).
solution the rigid conformation of the cyclic structure, immo-
bilizing the two phenyl groups in each of the TPE units, has
partially hampered the rotation of the phenyl rings, resulting in
the relatively intense violet emission.
Red-shifted emissions from amorphous TPE-containing
luminogens have been often reported.20 It is generally recog-
nized that in the crystalline state of TPE derivatives, the phenyl
rings in TPE moiety are twisted to fit into crystalline lattices, and
in the amorphous state, more planar conformation changing the
nature of the excitonic coupling from the crystalline state, is
adopted to afford the red-shifted emission. In order to validate
our speculation, crystalline parameters should be elucidated.
Fluorescent quantum yields (ΦF’s) of as-prepared and film
samples were determined to be 51 and 53%, respectively
(excitation at 365 nm). These values are comparable to that of
the solution (57%).
In conclusion, a cyclic emitter composed of tetraphenyl-
silane and TPE moieties was successfully synthesized. The key
step for the macrocyclization is McMurry reaction in dilute
conditions. The compound in THF solution, THF/water mix-
tures ( fw = 90%), and the solid film showed violet, sky blue,
and green emissions, respectively. Mechanochromism based
on the crystalline-amorphous transition was also observed. The
detailed emission mechanism and the OLED evaluation of
compound 4 will be revealed in the near future. The tunable
nature of the emission color with relatively high fluorescent
quantum yields makes it possible to fabricate a multicolor device
utilizing compound 4 as a single emitter.
Remarkably, in the film state, the emission peak of
compound 4 was located at 510 nm which is red-shifted to
93 nm compared with the THF solution, although the wave-
length at the absorption maximum was 343 nm similar to that
of the solution as shown in Figure S5. Figure 3 shows the
photographs of as-prepared (a) and ground (b) compound 4
under UV-irradiation suggesting that compound 4 possesses
mechanochromic nature as reported for other TPE-containing
luminogens.12b,20 Judging from the colors, the natures of
aggregates in THF/water (10/90), and the film are similar to
that of as-prepared and that of ground samples, respectively.
Indeed, PL spectrum for as-prepared sample (Figure S6)
measured with an integrating sphere showed almost the same
wavelength at absorption maximum (480 nm) as that in THF/
water (10/90). In order to understand the origin of mechano-
chromism, X-ray diffraction (XRD) powder patterns were
obtained as shown in Figure S7. Since XRD pattern exhibits a
number of sharp diffraction peaks and amorphous hallow around
2ª = 20°, as-prepared sample (recovered after recrystallization)
consisted of both crystalline and amorphous domains. In DSC
thermogram (Figure S3), no thermal transition such as cold
crystallization and melting was observed except glass transition.
Inconsistency between XRD and DSC results can be explained
Supporting Information is available on http://dx.doi.org/
10.1246/cl.170636.
1548 | Chem. Lett. 2017, 46, 1546–1549 | doi:10.1246/cl.170636
© 2017 The Chemical Society of Japan

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Chem. Lett. 2017, 46, 1546–1549 | doi:10.1246/cl.170636
© 2017 The Chemical Society of Japan | 1549