Spectroscopic and electrical characterization of α,γ-bisdiphenylene-β-phenylallyl radical as an organic semiconductor

Article abstract of DOI:10.1007/s11164-018-3282-7

The semiconductor properties of the earliest known stable radical, α,γ-bisdiphenylene-β-phenylallyl radical (Koelsch radical, 1^?) were assessed using spectroscopic and electrical techniques. This radical undergoes reversible redox processes, an

Full text of DOI:10.1007/s11164-018-3282-7

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3282-7
Spectroscopic and electrical characterization
of α,γ‑bisdiphenylene‑β‑phenylallyl radical
as an organic semiconductor
Yasunori Matsui1,2 · Minoru Shigemori1 · Toshiyuki Endo3 · Takuya Ogaki1,2
·
Eisuke Ohta1,2 · Kazuhiko Mizuno1 · Hiroyoshi Naito2,3 · Hiroshi Ikeda1,2
Received: 3 April 2017 / Accepted: 5 September 2017
© Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract The semiconductor properties of the earliest known stable radical, α,γ-
bisdiphenylene-β-phenylallyl radical (Koelsch radical, 1•) were assessed using spec-
troscopic and electrical techniques. This radical undergoes reversible redox pro-
cesses, and it has low redox potentials. In addition, 1• possesses long wavelength
absorption bands, owing to the existence of a singly-occupied molecular orbital
whose energy level lies between those of the HOMO and LUMO. A spin-coated thin-
flm of 1• displays photocurrent and an electron mobility of 6.3 × 10−7 cm2 V−1 s−1
on a trially-fabricated organic feld efect transistor.
Keywords Stable radical · Koelsch radical · BDPA radical · Organic feld efect
transistor · Organic semiconductor
Introduction
Organic radicals, which are open-shell species possessing unpaired electrons, are
well known as short-lived intermediates in organic chemistry [1–3]. As a result of
Electronic supplementary material The online version of this article (https://doi.org/10.1007/
s11164-018-3282-7) contains supplementary material, which is available to authorized users.
*
Hiroshi Ikeda
ikeda@chem.osakafu-u.ac.jp
1
2
3
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture
University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
The Research Institute for Molecular Electronic Devices (RIMED), Osaka Prefecture University,
1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
Department of Physics and Electronics, Graduate School of Engineering, Osaka Prefecture
University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
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Y. Matsui et al.
Fig. 1 Typical orbital diagrams
of closed- and open-shell spe-
cies
Fig. 2 Structure of 1•–C6H6
the existence of a singly-occupied molecular orbital (SOMO, Fig. 1), these radicals
have an electronic structure that difers from those of closed-shell species. Because
the odd-electron center does not satisfy the octet rule, radicals have a much higher
chemical reactivity than their closed-shell analogs. Many approaches have been
devised to design “stable radicals” [4–6], which mainly rely on the use of electronic
and/or kinetic methods to block potential chemical reaction pathways. Prototypical
examples of stable radicals include nitroxide, phenalenyl and triarylmethyl radicals.
Perhaps the earliest example of species of this type is the α,γ-bisdiphenylene-β-
phenylallyl (BDPA) radical (1•, the so-called “Koelsch radical”, Fig. 2), which was
synthesized by Koelsch [7–9].
Carbon-centered radicals have also attracted great attention recently in the feld
of organic electronic materials. Owing to the existence of narrow SOMO–LUMO or
HOMO–SOMO energy gaps, radicals absorb and emit long-wavelength light even
when they are not part of highly π-conjugated systems [10–13]. Moreover, radicals
are suitable for use as core functional materials in highly reversible redox systems,
such as a secondary battery [14–16], because their reduced and oxidized forms are
the corresponding closed-shell anions and cations. Much attention has been given
recently to organic radicals in the solid state where the unpaired electrons cause
highly-attractive intermolecular interactions [17, 18]. This is an especially interest-
ing phenomenon in the context of organic semiconductors, in which strong intermo-
lecular interactions are required to promote smooth carrier transport between neigh-
boring molecules.
In the study described below, we explored the potential of using BDPA radical
1• as an organic semiconductor. In addition to exploring the properties of 1• by
using various spectroscopic and electrochemical techniques, we evaluated its use
1 3

Spectroscopic and electrical characterization of…
Scheme 1 Synthetic route for preparation of 1•–C6H6 [7–9, 20]
as a semiconductor material. For this purpose, the photocurrent [19] and organic
feld efect transistor (OFET) characteristics of a spin-coated thin-flm of 1• were
determined.
Results and discussion
Synthesis
Radical 1• was synthesized starting with bromobenzalfuorene (2), utilizing the pro-
cedure reported earlier (Scheme 1) [7–9, 20]. The route begins with reaction of the
Grignard reagent prepared from 2 with fuorenone to form alcohol 3. Conversion
of 3 to the corresponding chloride 4 followed by reduction using silver produces
the target radical 1•. Recrystallization of 1• from benzene gives a 1:1 co-crystal of
1•–C6H6 as dark green plates with a metallic luster.
Photophysical properties
A benzene solution of 1•–C6H6 (4 × 10−5 M) is brown, and it has respective weak
and intense visible absorption bands at 863 and 489 nm (Fig. 3, red). These bands
are assigned to the respective HOMO→SOMO and SOMO→LUMO transitions
based on the results of time-dependent density functional theory (TD-DFT) calcu-
lations (B3LYP/6-31G**, Fig. 4) [21, 22].1,2 No remarkable solvent efects on the
wavelength maxima of these absorption bands were observed, which is in accord
1
For the details, see the Supplementary Material.
As an example of a DFT study of Radicals, see Ref. [22].
2
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Y. Matsui et al.
Fig. 3 UV–Vis absorption
spectra of (red) a benzene solu-
tion of 1•–C6H6 (4 × 10−5 M)
and (blue) a spin-coated thin-
flm of 1•. (Color fgure online)
Fig. 4 TD-DFT-calculated
electronic transitions of 1• and
their assignments [UB3LYP/6-
31G(d,p)]. The letters “α” and
“β” added to the molecular
orbital number mean the α and
β-spins, respectively
with the expectation that the ground and excited states of radical 1• have no signif-
cant localization of the formal charges.
Next, we prepared a thin-flm of 1• (~ 100 nm) by employing the spin-coating
method (800 rpm) using a CHCl3 solution of 1•–C6H6 (saturated, ca. 1 wt%). The
thin-flm is also brown and has absorption bands at 870 and 489 nm (Fig. 3, blue),
which are slightly redshifted as compared with those in the spectrum of a benzene
solution of 1•–C6H6. The redshifts suggest some electronic intermolecular interac-
tion, which may lead to the formation of J-aggregates of 1•, in the thin-flm.
Redox properties of 1•
The redox properties of the radical 1• were determined by using cyclic voltammetry
(CV) [23]. The voltammogram of 1•–C6H6 in CH3CN contains highly reversible
one-electron oxidation and reduction waves at EOX = +0.77 and ERED = –0.36 V
1∕2
1∕2
versus SCE, respectively (Fig. 5). The oxidation and reduction events correspond to
a loss and gain of one electron from and to the SOMO, respectively (Fig. 1).
1 3

Spectroscopic and electrical characterization of…
Fig. 5 Cyclic voltammogram of
1•–C6H6 in CH3CN containing
nBu4N+ClO4− (0.1 M) vs. SCE.
Working and counter electrodes:
Pt
Scheme 2 Redox reactions of 1• to form 1− and 1+
Therefore, unlike typical closed-shell species, the optical band gap of 1•
(HOMO–SOMO, 863 nm, 1.44 eV) does not correspond to the diference between
the reduction and oxidation potentials (1.13 eV). Note that the shape of the voltam-
mogram does not change over repetitive scans because the oxidized and reduced
species 1+ and 1− (Scheme 2) are closed-shell molecules that possess high stabilities
under the employed conditions. The results of the electrochemical measurements
suggest that 1• has the potential of serving as a semiconductor material with a high
electron-donating and electron-accepting nature.
Photocurrent measurements
To evaluate the semiconductor properties of 1•, its photocurrent in a thin-flm was
assessed [19] (See footnote 1). For this purpose, interdigital Al electrodes were
vacuum-deposited (5 × 10−3 Pa, 90 min) on the spin-coated thin-flm of 1•. Pho-
tocurrent measurements were carried out by applying a 5 kV cm−1 electric feld to
the flm while subjecting it to photoirradiation using a Xe lamp. The photocurrent
spectrum of the flm contains a maximum at 498 nm (Fig. 6, red), which matches
the absorption spectrum of the thin flm of 1• (Fig. 6, blue). This observation sug-
gests that a mobile carrier is generated by excitation of 1• in the thin-flm. Accord-
ing to the results of TD-DFT calculations (Fig. 4), the carrier of the photocurrent is
an electron in the conduction band. Note that this thin-flm has an amorphous solid
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Y. Matsui et al.
Fig. 6 (Red) Photocurrent and (blue) absorption spectra of the spin-coated thin-flm of 1•. (Color fgure
online)
(a)
(b)
Fig. 7 a Structure and b transfer characteristics of the bottom-gate top-contact OFET of 1•
character, judging from the halo pattern obtained from X-ray difraction analysis
(See footnote 1).
Mobility measurements
To evaluate the carrier mobility of the spin-coated thin-flm of 1•, a bottom-gate
top-contact OFET device (Fig. 7, left) was fabricated using the spin-coating method
(flm thickness: ~ 100 nm) (See footnote 1). Under a nitrogen atmosphere, the OFET
device has an electron mobility (μFET) of 6.3 × 10−7 cm2 V−1 s−1, a threshold voltage
(Vth) of 5.84 V (Fig. 7, right) at a drain voltage (VD) of 80 V.
To gain further insights into its properties, the OFET device fabricated from
the thin-flm of 1• was subjected to optical microscope analysis. The micrographs
obtained using this technique show that both brown and green regions exist in
the channel (Fig. 8). Interestingly, the OFET exhibits n-channel characteristics in
the green region but not in the brown region. Based on the fact that crystals of 1•
obtained from a benzene solution are dark green with a metallic luster, the green and
1 3

Spectroscopic and electrical characterization of…
(a)
(b)
(c)
electrode
electrode
electrode
channel
channel
channel
electrode
electrode
electrode
Fig. 8 Optical micrographs of a active and b, c inactive areas of the OFET
brown regions in the channel likely correspond to crystalline and amorphous phases,
respectively. On the other hand, both regions in the channel display photocurrent
activity. Inspection of large crystals (Fig. 8c) shows that locations in the channel,
where green and brown regions are intermingled, do not display any OFET charac-
teristics. Unfortunately, a reproducible method to form a continuous green thin-flm
has not been uncovered in our eforts thus far.
Conclusion
In the study described above, we explored the organic semiconductor properties of
BDPA radical 1• by spectroscopic and electrical methods in the solution phase and
on a spin-coated thin-flm. Radical 1• was found to have long-wavelength absorp-
tion bands and low redox potentials owing to the existence of the SOMO. The
results of photocurrent measurements show that a spin-coated thin-flm of 1• has
n-channel semiconductor characteristics. The solution-processed OFET of 1• has
electron mobility only in crystalline phase regions. It is believed that modifcations
of the structure of the BDPA radical will lead to improvements in OFET perfor-
mance by increasing the homogeneity of the thin-flm.3 Although our measurements
explored the n-channel characteristics of 1•, we recognize that organic radicals can
also behave as p-channel and/or ambipolar semiconductors. Therefore, organic radi-
cals have the potential of playing an important role in organic semiconductors in the
future [24–28].
3
The BDPA radical has been commercially available from Aldrich since February 2015. See: www.
sigmaaldrich.com/catalog/product/aldrich/152560.
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Y. Matsui et al.
Experimental section
Preparation of the BDPA radical 1•
α,γ‑Bisdiphenylene‑β‑phenylallyl alcohol (3)
Bromobenzalfuorene (2, 6.66 g, 20 mmol) and THF (5 mL) were added to a dry
round-bottom fask containing magnesium turnings (488 mg, 20 mmol) and a
small amount of iodine under an argon atmosphere. After evolution of heat, addi-
tional THF (60 mL) was added, and the mixture was stirred at 45 °C for 1.5 h. Flu-
orenone (3.61 g, 20 mmol) in THF (20 mL) was added dropwise, and the resulting
mixture was stirred at refux for 14 h. After quenching by sat. aq. NH4Cl (50 mL),
the mixture was extracted with diethyl ether (30 mL × 3). The organic layer was
washed with aq. NaCl and concentrated in vacuo. Toluene (30 mL) was added to
the orange residue to give a solid. Filtration gave (5.12 g, 59%) as yellow crystals.
mp = 195–196 °C (mp = 192–193.5°C [8]); 1H NMR (300 MHz, CDCl3) δppm 2.92
(brs, 1H, OH), 5.47 (d-like, J = 8.1 Hz, 1H), 6.55 (d-like, J = 6.9 Hz, 2H), 6.66 (m,
1H), 6.91 (m, 2H), 7.00 (d-like, J = 7.5 Hz, 1H), 7.12 (m, 1H), 7.16–7.79 (m, 12H),
9.46 (d-like, J = 8.1 Hz, 1H); IR (KBr) ν = 3448, 3057, 1443 cm−1.
α,γ‑Bisdiphenylene‑β‑phenylallyl Chloride (4)
A solution of 3 (4.35 g, 10 mmol) and CaCl2 (1.1 g, 10 mmol) in benzene (30 mL)
was stirred at 5–15 °C. HCl gas, generated in another fask by slow addition of
H2SO4 (50 mL) to NaCl (50 g), was added to the benzene solution. The resulting
mixture was stirred for 2 h, fltrated, and extracted with benzene (50 mL × 3). The
organic layer was washed with brine (50 mL) and concentrated in vacuo to give a
solid that was subjected to recrystallization from hexane–benzene to give (3.42 g,
76%) as bright yellow prisms. dp > 123 °C (dp = 125°C [9]); 1H NMR (300 MHz,
CDCl3) δppm 5.49 (d-like, J = 7.5 Hz, 1H), 6.48 (brs, 2H), 6.76 (brs, 1H), 6.97
(brs, 1H), 7.09–7.21 (m, 3H), 7.34–7.37 (m, 3H), 7.47–7.78 (m, 10H); IR (KBr)
ν = 3057, 1443, 780, 743, 725, 703, 615 cm−1.
Complex of α,γ‑bisdiphenylene‑β‑phenylallyl radical and benzene (1•–C6H6) [19]
A THF solution of (4.54 g, 10 mmol) and silver powder (10.2 g, 95 mmol) under
an argon was stirred for 5 h at room temperature. Filtration followed by concentra-
tion in vacuo gave a solid that was subjected to recrystallization from benzene to
give 1•–C6H6 (2.72 g, 55%) as dark green plates with a metallic luster. dp > 126 °C
[obtained by DSC, probably corresponding to the loss of C6H6 (See footnote 1).
1
mp = 222–223°C [9]]; H NMR (300 MHz, CDCl3) δppm 7.36 (s, 6H); 13C NMR
(75 MHz, CDCl3) δppm 128.4 (6C); IR (KBr) ν = 3855–3630, 1560 cm−1; MS (EI):
m/z = 417 (M+, 100), 253 (29), 339 (19). Anal. Calcd. for C39H27: C, 94.51; H, 5.49.
Found: C, 94.34; H, 5.39.
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Spectroscopic and electrical characterization of…
OFET device fabrication
On a pre-cleaned SiO2/Si substrate, a THF solution of 1•–C6H6 (2.4 wt%) was spin-
coated (500 rpm, 10 s to 800 rpm, 30 s, flm thickness: ~ 100 nm) [29–32]. After
drying under vacuum, Al electrodes were vacuum-deposited on the coated sub-
strate (Channel length: 100 μm). The conduction band of the thin-flm of 1• was
low enough to inject a carrier from Al. OFET characteristics were measured under a
nitrogen atmosphere.
Acknowledgements The authors acknowledge valuable discussions with Mr. Yu Suenaga at OPU.
This study was partially supported by JSPS KAKENHI grant numbers JP24109009 (Innovative Area
Stimuli-responsive Chemical Species), JP17H01265, JP23350023, JP23108718 (pi-Space), JP21108520,
JP21655016, and JP19350025. YM also acknowledges the Program to Disseminate Tenure Tracking Sys-
tem, MEXT, Japan.
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