Aromatic inorganic acid radical

Article abstract of DOI:10.1007/s11426-019-9641-2

Stable radicals are challenging to prepare due to their intrinsic high reactivity. Herein, three trisphenolamine radicals were readily synthesized and exhibited unexpected thermal/electrochemical stability and semiconductor property. These three nitroxide

Full text of DOI:10.1007/s11426-019-9641-2

SCIENCE CHINA
Chemistry
•ARTICLES•
https://doi.org/10.1007/s11426-019-9641-2
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Aromatic inorganic acid radical
Jiawen Zhou1†, Weiya Zhu1†, Miao Zeng1, Qingqing Yang2, Ping Li2, Linfeng Lan1,
Junbiao Peng1, Yuan Li1*, Fei Huang1* & Yong Cao1
1State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China
University of Technology, Guangzhou 510640, China;
2Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials, Jiangsu National Synergetic Innovation
Center for Advanced Materials, Nanjing University of Posts & Telecommunications, Nanjing 210023, China
Received August 19, 2019; accepted October 14, 2019; published online November 26, 2019
Stable radicals are challenging to prepare due to their intrinsic high reactivity. Herein, three trisphenolamine radicals were readily
synthesized and exhibited unexpected thermal/electrochemical stability and semiconductor property. These three nitroxide
radicals could be considered as a class of aromatized nitro groups or HNO3 derivatives. The closed-shell nitro-like and open-shell
nitroxide resonance structure contribute to their outstanding stability. Furthermore, the tunable ground states, extremely low band
gap and p-type charge transport properties were systematically investigated. More importantly, the work presents the concept of
aromatic inorganic acid radical (AIAR) and aggregation-induced radical (AIR) mechanism to understand the intrinsic structure-
property relationship of these radicals. In addition, we provide a novel strategy for the design of stable and low bandgap radicals
for organic electronics, magnetics, spintronics, etc.
organic electronics, aromatic nitric acid radical, aggregation-induced radical, polycyclic aromatic hydrocarbons,
spintronics
Citation:
Zhou J, Zhu W, Zeng M, Yang Q, Li P, Lan L, Peng J, Li Y, Huang F, Cao Y. Aromatic inorganic acid radical. Sci China Chem, 2019, 62, https://doi.
org/10.1007/s11426-019-9641-2
1 Introduction
lower the reactivity of radical species and enhance stability
[14–16]. The localized radical molecules can greatly reduce
the possibility of reactions in chemical kinetics and thus
exhibit good stability due to steric hindrance effect [17].
Polycyclic aromatic hydrocarbons (PAHs) with singlet
ground states are relatively stable to oxygen, light or heat due
to kinetic blocking [18–20]. The stable zethrenes can even be
stored in the air for a month [21]. Moreover, the typical
nitroxide radical group 2,2,6,6-tetramethylpiperidin-1-oxyl
(TEMPO) exhibits excellent air-stability and achieves ex-
tensive applications under the protection of four methyl
groups [22–24]. The extension of the π conjugation is an-
other promising strategy for isolating unstable radical mo-
lecules [25,26]. The Kekule-type delocalized diradicals
could be less reactive to molecular oxygen due to the use of a
smaller aromatic stabilization energy and thermodynamic
stabilization of the radical moiety [27,28].
Organic radical molecules have recently aroused a growing
interest due to their unique electronic [1], nonlinear optical
[2] and magnetic properties [3], and promising applications
in luminescent materials [4], organic spintronics [5], ther-
moelectric materials [6], and energy storage devices [7].
However, these materials often exhibit molecular instability
due to their intrinsic high reactivity, which will limit their
practical applications [8]. Since the Thiele’s and Chichiba-
bin’s hydrocarbon were reported [9,10], much efforts have
been devoted to exploring stable organic radicals [11–13].
Numerous localized diradicals and delocalized diradicals
were successfully prepared based on creative designs to
†These authors contributed equally to this work.
*Corresponding authors (email: celiy@scut.edu.cn; msfhuang@scut.edu.cn)
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019
chem.scichina.com link.springer.com

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Zhou et al. Sci China Chem
In the previous work, TEMPO radicals show high stability
TPA-O2 and BT-TPA-O4 were successfully synthesized and
exhibited excellent thermal/electrochemical stability, com-
pared to the traditional water-oxygen sensitive radical ca-
tions of triarylamines [34–36]. The proposed closed-shell
quinoid structure of conjugated nitroxides is stable, and is
accompanied by a variety of resonance structures containing
unpaired electrons, as indicated in Scheme 1. Moreover, we
discovered that the closed-shell resonance structure is mar-
velously similar to the structure of nitro group, which ex-
plains the stability of these radical molecules. Considering
the large challenge to obtain air-stable radicals with triplet
ground states [37], the TPA-O2, NO2-TPA-O2 and BTBr-
TPA-O2 might exhibit triplet ground states due to the odd
number (1-/11-) positions of their two radicals in these mo-
lecules, respectively. Interestingly, we found NO2-TPA-O2
and BTBr-TPA-O2 also exhibited singlet ground states
with the resonance structures in which the diradicals were
distributed on even positions. The novel mechanism of aro-
matic inorganic acid radicals (AIAR) was proposed to un-
derstand their chemical/physical properties in the following
discussion.
due to the steric hindrance and reversible redox property
based on their electron transfer process between open-shell
TEMPO radical and TEMPOnium (Scheme 1(a)) [29].
Phenoxy radical shows relative stability due to the radical
delocalization on the quinoid structure. In order to enhance
the stability of phenoxy radical, the galvinoxyl radicals with
tert-butyl groups were successfully prepared and exhibited
excellent sterical stability, along with the emergence of
quinoid structure and spin delocalization of unpaired elec-
tron over the whole of the conjugated skeleton [30,31].
Another galvinoxyl derivative Yang’s biradical (Scheme 1
(b)) with three phenoxy radicals exhibited biradical property
because of the formation of covalent bond between the un-
paired p electron of the methine carbon atom and any radical
spin [32]. Sakamaki et al. [33] prepared a novel triphenyla-
mine derivative having two phenoxy radicals by substituting
nitrogen atom for central carbon atom (Scheme 1(b)). The
triphenylamine phenoxyl radical showed the unexpected
stability due to the closed-shell electronic structure and two
unusual C–N bonds with multiple-bond character.
In order to further stabilize the triphenylamine phenoxyl
radicals, we proposed to introduce the conjugated nitroxides
without any steric protection to prepare the semiconductive
radicals with good stability by delocalizing unpaired elec-
trons and introducing electron-withdrawing groups (Scheme
1). These neutral conjugated nitroxides NO2-TPA-O2, BTBr-
2 Experimental
2.1 Materials
All the materials were purchased from commercial sources
Scheme 1 Resonance structures of radical motivated from aromatic HNO3. (a) The redox behavior of Tempo radical and unstable phenoxyl radical; (b)
structures of Yang’s biradical [32] and triphenylamine phenoxyl radical [33]; the resonance structures of (c) TPA-O2 radical, (d) NO2-TPA-O2 radical, (e)
BTBr-TPA-O2 radical and (f) BT-TPA-O4 radical motivated from aromatic HNO3. The resonance structures of singlet and triplet states are driven by the AIR
mechanism and the thermal excitation, respectively (color online).

3
Zhou et al. Sci China Chem
and used as received. The raw materials used in the experi-
ment were as follows: 4-methoxy-N-(4-methoxyphenyl)-N-
(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)ani-
line, 4,7-dibromobenzo[c][1,2,5] thiadiazole, 4-bromo-1-
nitrobenzene and 4,4′-dimethoxytriphenylamine. The de-
tailed synthesis of compounds NO2-TPA-O2, BTBr-TPA-O2
and BT-TPA-O4 were presented in Scheme 2.
spectrometer (Germany). ESR spectra of samples were re-
corded at a signal attenuation of 16 dB. Dewars used to
support thermocouples were different for low temperature
and high temperature tests. Quartz EPR standard quality
tubes with an outer diameter of 3 mm were purchased from
Beijing Synthware Glass Inc. (China). A Quantum Design 7
Tesla SQUID-VSM system was available for the magnetic
measurement in this work. Powder sample was sealed in a
plastic capsule. Magnetic moment was measured in the
temperature range of 2 to 400 K, with an applied field of
500 Oe. The empty plastic capsule exhibited diamagnetic
and its magnetic moment was measured for correction. The
modified Bleaney-Bowers equation that was used for the
curve fitting as follow:
2.2 Measurements
The 1H NMR spectra of samples were recorded by DRX-400
or 600 spectrometer (400 and 500 MHz 1H NMR frequency,
Bruker Co., Germany) in CDCl3 or DMSO-d6 at room tem-
perature. The ultraviolet visible near-infrared (UV-Vis-NIR)
spectra were obtained by a UV-3600 spectrophotometer
(SHIMADZU Co., Japan). Cyclic voltammetry (CV) was
performed in CH2Cl2 solution with 0.1 M tetrabutylam-
moniun hexafluorophosphate (Bu4NPF6) as the supporting
electrolyte at a scan rate of 100 mV/s, a Hg/HgCl2 (Saturated
KCl solution) electrode as the reference electrode, a carbon-
glass electrode as the working electrode, a Pt line electrode
as the counter electrode and ferrocene/ferrocenium (Fc/Fc+)
as an internal reference on electrochemistry workstation
(CHI660E, China). Electron spin resonance (ESR) spectra
were measured on a Bruker ELEXSYS-II E500 CW-EPR
2
2
Ng2µ
kT
Ng2µ
2kT
2
=
(1 ) +
+ TIP(1
)
3+ e 2J
kT
where ρ is the fraction of s=1/2 impurity, TIP is the tem-
perature independent paramagnetism due to a small energy
gap between ground singlet state and excited triplet state.
Density functional theory (DFT) calculations were per-
formed with the Gaussian 09 suite of programs. The geo-
metry optimizations of methoxy and hydroxyl substituted
precursors were carried out at B3LYP/6-31G* level of theory
and the optimized geometries of radical counterparts were
performed with UB3LYP/6-31G* level of theory on the
singlet ground state [38,39]. The single energy and spin
density distribution of three kinds of neutral conjugated
nitroxide radicals in the closed-shell, open-shell singlet and
open-shell triplet states were performed with B3LYP/6-311+
+G** level of theory.
2.3 The fabrication of the hole-only devices
The hole-only devices were fabricated with the structure of
indium tin oxides (ITO)/HTM/Ag and ITO/PEDOT:PSS/
HTM/MoO3/Ag. BT-TPA-OMe or BT-TPA-O4 solutions
were prepared in toluene or ethanol (20 mg/mL), respec-
tively, stirred at 50 °C for 10 h and filtered it with a 0.22 μm
filter element. The ITO-coated glass substrates were cleaned
by sonication in detergent, deionized water, acetone, and
isopropyl alcohol and dried in an oven at 75 °C for 10 h.
PEDOT:PSS aqueous solution was spin-coated at 3,500
r/min for 30 s (thickness of 30 nm) and thermally annealed at
145 °C on a hotplate for 15 min in the air. The layer of BT-
TPA-OMe or BT-TPA-O4 was spin-coated on PEDOT:PSS
film at 800–2,000 r/min. A 10 nm MoO3 layer and a 100 nm
Ag layer were subsequently evaporated through a shadow
mask to define the active area of the devices (0.0516 cm2)
and form a top anode. All of the fabrication processes were
performed inside a controlled atmosphere of nitrogen glove
box (Vacuum Atmosphere, USA) that contained less than
Scheme 2 Synthetic routes for NO2-TPA-O2, BTBr-TPA-O2 and BT-
TPA-O4.

4
Zhou et al. Sci China Chem
10 ppm oxygen and moisture. The current density-voltage
(J-V) characteristics of hole-only devices were measured
with a Keithley 236 sourcemeter under dark. The experi-
mental J-V characteristics of hole-only devices are given by
(NO2-TPA-O2) and 4-(7-bromo-2,1,3-benzothiadiazol-4-yl)-
N,N-bis(4-hydroxyphenyl) benzenamine (BTBr-TPA-O2)
was shown in Scheme 2: BT-TPA-OMe (1 eq.), NO2-TPA-
OMe (1 eq.) or BTBr-TPA-OMe (1 eq.) in dry CH2Cl2
(15 mL) was cooled to −70 °C by ethanol in cold trap, and
then BBr3 (10 eq. or 5 eq.) was added dropwise under N2
over 5 min. The mixture was then stirred at room tempera-
ture for 12 h under nitrogen atmosphere with precipitation of
solid product. Methanol (15 mL) was added dropwise into
the reacting solution to quench BBr3. After dropping the
solution into deionized water, the solids can be separated by
filtration and the crude product was purified by washing with
H2O and CH2Cl2 successively. The crude compounds were
purified by silica gel column chromatography (eluted:etha-
nol/petroleum ether=3:1, v/v) to obtain target products. The
final powder products were sent to vacuum drying at 60 °C
for 24 h. The oxidation products BT-TPA-O4, NO2-TPA-O2 and
BTBr-TPA-O2 could be obtained by exposing powder to air.
9
8
V 2
d3
.
JSCLC
=
r µp
0
where ε0εr, μp and d are the permittivity, hole mobility at low
voltage and thickness of the hole transport layer, respec-
tively. Using a relative dielectric constant εr of 3 and bulit-in
voltage Vbi=1.5 V, it is observed from the slope of the log(J)
versus log(V) plot [40] that the current density depends
quadratically on the voltage. The applied voltage should be
corrected for the voltage drop across the ITO series re-
sistance VRs, which amounts to 10 Ω in these substrates [41].
2.4 General procedures for the preparation of
NO2-TPA-OMe, BTBr-TPA-OMe and BT-TPA-OMe
4,4′-(2,1,3-Benzothiadiazole-4,7-diyl)bis[N,N-bis(4-meth-
oxyphenyl)benzenamine (BT-TPA-OMe), N,N-bis(4-meth-
oxyphenyl)-4′-nitro[1,1′-biphenyl]-4-amine (NO2-TPA-OMe)
were simply prepared by Suzuki coupling in 87.8% and
90.1% yield, respectively, which were the tetrakis(triphe-
nylphosphine) palladium Pd(PPh3)4 (0.05 eq.)-catalyzed
cross coupling between 4-methoxy-N-(4-methoxyph-enyl)-
N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)
aniline (2.5 eq.) and 4,7-dibromobenzo[c][1,2,5]thiadiazole
(1 eq.) as well as 4-methoxy-N-(4-methoxyphenyl)-N-(4-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline
(1 eq.) and 4-bromo-1-nitrobenzene (1 eq.). These raw ma-
terials were dissolved in a mixture of degassed toluene
(15 mL), ethanol (5 mL) or tetrahydrofuran (THF, 10 mL)
and Na2CO3 (2 M, 2 mL) in a 100 mL two-necked round
bottomed flask under nitrogen atmosphere. The mixture was
refluxed for 24 h under nitrogen atmosphere. Next, the sol-
vent was removed under vacuum and the organic phase was
extracted three times with water and dichloromethane. After
concentration, the crude compound was purified by silica gel
column chromatography (eluted:dichloromethane/petroleum
ether=1:2, v/v) to give BT-TPA-OMe and NO2-TPA-OMe.
Besides, 4-(7-bromo-2,1,3-benzothiadiazol-4-yl)-N,N-bis(4-
methoxyphenyl)benzenamine (BTBr-TPA-OMe) was ob-
tained from by-product recovery during the preparation of
BT-TPA-OMe. The final powder product was sent to vacuum
drying at 55 °C for 24 h.
3 Results and discussion
3.1 The design of aromatic nitric acid radicals
Initially viewing the chemical structure of HNO3, we pro-
posed to insert two phenyl groups into HNO3 and thus ob-
tained the stable diradical TPA-O2 with triplet ground state as
the aromatic group produced the electron delocalization ef-
fect of free electrons of TPA-O2 (Scheme 1(c)). TPA-O2
would process two resonance structures including closed-
shell nitro-like and open-shell diradical. We prepared TPA-
OH2 and tried to obtain TPA-O2 diradical by oxidation of
TPA-OH2. Finally, we found the TPA-O2 was not stable and
the single crystal of TPA-OH2 was collected as presented in
Figure 1 [42]. It can be observed that the hydrogen bond
length of TPA-OH2 is 1.935 Å, illustrating hydrogenation of
unstable triplet ground state. Besides, strong electron-with-
drawing groups such as nitro and benzothiadiazole (BT)
were introduced to further stabilize unpaired electrons. Both
NO2-TPA-O2 (Scheme 1(d)) and BTBr-TPA-O2 (Scheme 1
(e)) will process several resonance structures including the
nitro-like closed-shell structure, as well as open-shell struc-
tures with potential singlet and triplet states due to the co-
existence of even/odd positions of diradicals. This is distinct
from canonical resonance structures (Scheme 1(b)) proposed
in previous work [33]. This interesting and reasonable hy-
pothesis makes us successfully achieve the tunable ground
states of this type of diradicals, which is of great importance
for this field [21]. In order to further enhance the radical
delocalization, the symmetrical bisphenolamine radicals are
introduced to increase the conjugation length and BT-TPA-
O4 is designed and expected to show stable diradical property
due to its more diverse structural resonances comparing with
previous ones.
2.5 General procedures for the preparation of
NO2-TPA-O2, BTBr-TPA-O2 and BT-TPA-O4
Synthesis of 4,4′,4″,4‴-[2,1,3-benzothiadiazole-4,7-diylbis
(4,1-phenylenenitrilo)] tetrakis-phenol (BT-TPA-O4), N,N-
bis(4-hydroxyphenyl)-4′-nitro[1,1′-biphenyl]-4-amine

5
Zhou et al. Sci China Chem
Figure 1 X-ray crystallographic structure and crystal packing of TPA-
OH2 at 100 K (color online).
NO2-TPA-O2 and BTBr-TPA-O2 were successfully syn-
thesized to investigate the effect of conjugation and electron-
withdrawing effect on formation of nitroxide radicals. NO2-
TPA-O2 exhibits unique performance that the disappearance
1
of H NMR spectrum signal (Figure S4, Supporting In-
formation online) and the absorption at 600–800 nm in UV-
Vis-NIR absorbance spectrum in solution (Figure 2(a)).
These results clearly demonstrate the existence of radicals
for NO2-TPA-O2 in solution. The absorption characteristics
of BTBr-TPA-OH2 are consistent with that of BTBr-TPA-
OMe in solution, indicating the dominant hydroxyl groups
rather than phenoxy radical. The highly conjugated backbone
boosts to new visible or NIR absorption of NO2-TPA-O2 and
BTBr-TPA-O2 at 527 nm in Figure 2(a) and 660 nm in Figure
2(b) in film, respectively. The black powder of NO2-TPA-O2
or BTBr-TPA-O2 is also in good agreement with the ab-
sorption of radicals in the visible region.
The typical peaks of UV-Vis absorption appeared at 480
and 485 nm for BT-TPA-OMe and BT-TPA-OH4 in solution,
respectively. It was in good agreement with expected beha-
vior that demethylation had ultra-weak effect on absorption
in the visible region [43]. However, the situation is quite
different in film, as depicted in Figure 2(c). BT-TPA-O4
showed a typical absorption peak in the range of
450–1,000 nm, which showed great red shift comparing with
that of BT-TPA-OMe. It can be explained that the transfor-
mation of OH groups of BT-TPA-OH4 to phenoxy radicals is
occurring and the resonance structural characteristics of
tetra-radicals are dominated after being exposed to air. Be-
sides, we noticed that the color of BT-TPA-O4 in powder was
black but dilute solution was dull red, compared to the red
BT-TPA-OMe in both solid and solution state. It is re-
cognized that the intrinsic BT-TPA-OH4 is red like BT-TPA-
OMe if the OH groups are still in the backbone of BT-TPA-
OH4 but BT-TPA-O4 is black due to radical formation [43].
Accordingly, a small amount of BT-TPA-OH4 is easily oxi-
dized in air and regenerated under interaction with proton in
water. The distinct absorption and black powders of NO2-
TPA-O2, BTBr-TPA-O2 and BT-TPA-O4 in film demonstrate
the existence of radicals. In addition, we did not detect the
Figure 2 UV-Vis-NIR spectra of (a) NO2-TPA-OMe and NO2-TPA-O2,
(b) BTBr-TPA-OMe and BTBr-TPA-O2, (c) BT-TPA-OMe and BT-TPA-O4
in solution as well as film, respectively (color online).
generation of fluorescence from these radical molecules,
which indicated efficient photothermal conversion and po-
tential application in photothermal therapy field.
3.2 The stability and ground states of aromatic nitric
acid radicals
These conjugated nitroxide radicals exhibited typical organic
semiconductor properties and strong electro-chemical sta-
bility (Figure 3). The highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)

6
Zhou et al. Sci China Chem
Figure 3 Electrochemical stability results of (a) NO2-TPA-OMe, (b) BTBr-TPA-OMe and (c) BT-TPA-OMe as well as demethylated counterparts (d) NO2-
TPA-O2, (e) BTBr-TPA-O2 and (f) BT-TPA-O4, respectively. All of CV curves were measured 15 cycles in dichloromethane solution containing 0.1 M
nBu4NPF6 as a supporting electrolyte and the scan rate was 0.1 V/s. The methoxy substituted precursors were dissolved in dichloromethane at a concentration
of 5 mg/mL and target radical compounds coated on carbon-glass electrode to form film during measurements. Potential values are reported with saturated
calomel electrode as the reference electrode using the Fc+/Fc couple (0.27 V) as an internal standard (color online).
energy levels were listed in Table S1 (Supporting Informa-
tion online). The HOMO energy levels of demethylated
counterparts NO2-TPA-O2, BTBr-TPA-O2 and BT-TPA-O4
are recorded from CV measurements as −5.06, −5.08 and
−5.02 eV, respectively. The HOMO energy levels have been
improved compared with the corresponding value −5.16,
−5.13 and −5.10 eV of methoxy substituted precursors, re-
spectively. The symmetrical bisphenolamine radical BT-
TPA-O4 serves as a conjugated extension of nitroxides such
as TEMPO radical, equipped with analogous redox behavior
and strongest stability in Figure 3(f) [29]. The redox beha-
vior of BT-TPA-O4 showed the excellent reversibility of
conjugated nitroxides structure. Besides, this conjugated
nitroxide structure showed good thermal stability, exhibiting
equivalent spin concentrate for BT-TPA-O4 exposed to air at
room temperature after high temperature measurements in
Figure 4. Furthermore, considering the contribution of the
existence of rich resonance structures and the push-pull ef-
fect between the bisphenolamine donor and BT acceptor, the
extended conjugation effects in aromatic nitroxides also re-
sponse to the excellent electrochemical and thermal stability
of BT-TPA-O4 [44]. This result provides us a ready route to
obtain stable low band gap and stable radical semi-
conductors.
even number positions of free electrons in these molecules
[37]. ESR measurements were applied to study the ground
state of these stable radicals. It can be observed from Figures
S7–S9 that there has been significant increase in the relative
intensity of electron spin resonance after demethylation, due
to the large amounts of radical generation with oxidation of
phenolic hydroxyl in air. In addition, the g values of NO2-
TPA-O2, BTBr-TPA-O2 and BT-TPA-O4 are subtle different
indicating diverse delocalization of radicals (Scheme 1(f)).
This also explains the existence of shoulder side peak in the
typical ESR spectrum (Figure S9) of BT-TPA-O4. Besides,
the ESR signal intensities from high to low are BTBr-TPA-
O2, NO2-TPA-O2 and BT-TPA-O4 under the same tested
amount.
To further investigate the electronic structures and radical
properties, we carried out variable temperature ESR (VT-
ESR) measurement for BT-TPA-O4 under argon atmosphere
at low temperature and high temperature zone, respectively.
The ESR signal intensity increased with increasing tem-
perature from room temperature (RT) to 500 K in Figure 5
(b). It is reasonable the enhanced ESR signal stems from the
growing excited triplet state at the elevated temperature.
Importantly, this result excludes the statement that the gen-
eration of free radicals originates from the doping of oxygen
or other impurities. The singlet-triplet energy gap (ΔES-T) of
BT-TPA-O4 was estimated by superconducting quantum in-
It is known that the ground state of these molecules are
important issues to understand due to the different odd and

7
Zhou et al. Sci China Chem
Figure 4 ESR spectra of BT-TPA-O4 in powder (28.5 mg, 0.0415 mmol,
Attenuation=14 dB) measured at room temperature after experiencing high
temperature measurements (color online).
Figure 6 χT-T plot for the solid BT-TPA-O4. The measured data from 2 to
250 K were fitted by modified Bleaney-Bowers equation (color online).
Figure 7 ESR spectra of BTBr-TPA-O2 (9.8 mg, 0.02 mmol, Attenua-
tion=20 dB) in powder at room temperature. Inset: the forbidden ΔMS=±2
resonance (Attenuation=4 dB) at room temperature (color online).
O2 Curie-like behavior and an excited triplet state of BTBr-
TPA-O2 [12,13].
3.3 The proposed mechanism of AIAR
In previous work, there are reports on bis(triarylamine) di-
cations isolated by using the weakly coordinating anions
[Al(ORF)4]− as the counterions [45–47]. It is completely
different to explain the origin of radical in term of pheno-
lamine radicals. The singlet ground states of NO2-TPA-O2
and BTBr-TPA-O2 can be understood by our proposal on the
formation and stabilization of diradicals in Scheme 1(d, e)
considering the even (1-/16-) positions of two radicals in
NO2-TPA-O2 and BTBr-TPA-O2. The stability of NO2-TPA-
O2 and BTBr-TPA-O2 comes from the highly efficient de-
localization of diradicals in them. The inherent mechanism is
different from the stability from the steric hindrance effect of
phenoxy and TEMPO radicals [33]. Therefore, the in-
troduction of strong electron-withdrawing groups and ex-
tended conjugation contribute to stabilize radicals in
thermodynamic and achieve tunable ground states. Our re-
Figure 5 (a) ESR spectra of BT-TPA-O4 in powder (28.5 mg,
0.0415 mmol, Attenuation=14 dB) measured at 150, 200, 242 and 295 K,
respectively; (b) ESR spectra of BT-TPA-O4 in powder (28.5 mg,
0.0415 mmol, Attenuation=14 dB) measured at 296, 345, 400, 450 and
498 K, respectively. The paramagnetic tube was filled with argon during
the measurement. The different dewars were used in low temperature and
high temperature tests (color online).
terfering device (SQUID) measurements for a powder sam-
ple at the range of 2–400 K. Data from 2 to 250 K were fitted
by modified Bleaney-Bowers equation and gave a ΔES-T
=
−0.749 kcal/mol (Figure 6), indicating that BT-TPA-O4 had a
singlet open-shell ground state. Besides, the spin-triplet of
BTBr-TPA-O2 was detected with the forbidden resonance
resulting from ΔMS=±2 (Figure 7), suggesting BTBr-TPA-

8
Zhou et al. Sci China Chem
sults are different from the previous work [33], and our
proposal is more reasonable to explain the singlet ground
states as the two electrons can be localized on the 1-/16-
positions of NO2-TPA-O2 and BTBr-TPA-O2 molecules
(Scheme 1(c, d)).
have multiple-bond character [33]. However, it is hard to
explain that phenolamine radicals in this work exhibit re-
markable paramagnetic properties and singlet ground state in
ESR and SQUID measurements according to the previous
mechanism. It will be more reasonable that the AIAR me-
chanism is proposed to explain the stability and para-
magnetism of these phenolamine radicals, compared with the
previous work [33].
For all methoxy and hydroxy substituted precursors, their
ground-state geometries are optimized at B3LYP/6-31G*
level of theory. There is almost no significant change in the
optimized structure with the substitution of hydroxyl to
methoxy for NO2-TPA-OMe (Figures S10, S11), BTBr-TPA-
OMe (Figures S13, S14) and BT-TPA-OMe (Figures S16,
S17). The optimized result is consistent with similar UV-Vis
absorption characteristics of methoxy and hydroxy sub-
stituted compounds. The geometry optimization of BT-TPA-
O4 (Figure 8) is investigated with UB3LYP/6-31G* level of
theory, suggesting that all phenoxy groups are quinoidal
structures with remarkable reduced dihedral angle (C20-
C19-N2-C13: 32.08°), contributing to strong absorption in
NIR region because of better molecular planarity and elec-
tron delocalization. The situation is mirrored for NO2-TPA-
O2 (Figure S12) and BTBr-TPA-O2 (Figure S15) that the
C–N bonds and C–O bonds change from single bond to nitro-
like closed-shell structure, contributing to expected thermal
and electrochemical stability without steric hindrance pro-
tection of tertiary butyl. This result is similar to the previous
report that triphenylamine phenoxy radical (Scheme 1(b))
possesses closed-shell electron structure and C–N bonds
Phenolamine radicals could be generated due to the
transformation from the nitro-like closed-shell structure to
open-shell aromatic nitroxides, driven by the aromaticity and
torsion angle of phenoxy groups. Besides, the aromatic nitric
acid could be regarded as a class of conjugated nitroxide
radicals, which will process two types of resonance struc-
tures including closed-shell nitro-like and open-shell dir-
adical. To investigate the stability of different geometries, the
single energy and spin density distribution of three kinds of
neutral conjugated nitroxide radicals in the closed-shell,
open-shell singlet and open-shell triplet states are performed
with B3LYP/6-311++G** level of theory (Figure 9). The
calculated results (Table S2) show that the singlet ground
states of NO2-TPA-O2, BTBr-TPA-O2 and BT-TPA-O4 are at
a similar energy with the closed-shell structures, which
boosts to stabilize the radical without steric hinderance
protection. Besides, the triplet states with slightly higher
energy explain the extremely strong ESR intensity at room
temperature and are in accordance with the measurement
results of ESR and SQUID. Therefore, the singlet ground
state diradicals indicate that diradicals will be induced with
the formation of the quinoid structure of backbone in the
aggregation state, i.e. aggregation-induced radical (AIR).
The formation of the quinoid-diradical structure is similar to
our previous work on the low bandgap semiconductors [44].
Figure 8 The geometry optimizations of BT-TPA-OMe (B3LYP/6-
31G*), BT-TPA-OH4 (B3LYP/6-31G*) and BT-TPA-O4 (UB3LYP/6-31G*)
were performed. Inset: the selected bond lengths and dihedral angles.
Addition information can be found in Supporting Information online
(Figures S16–S18) (color online).
Figure 9 The spin density distribution of (a) NO2-TPA-O2, (b) BTBr-
TPA-O2 and (c) BT-TPA-O4 in the open-shell triplet states, respectively.
Blue and green surfaces represent α and β spin density, respectively (color
online).

9
Zhou et al. Sci China Chem
hole mobilities of BT-TPA-OMe and BT-TPA-O4 were cal-
culated to be 7.1×10−5 and 2.3×10−4 cm2/(V s), respectively,
by fitting the data using the space charge limited current
(SCLC) model with the classical device structure [48]. The
enhancement of the hole mobility of BT-TPA-O4 is ascribed
to the quinoid resonance structure, good electron delocali-
zation and lowered band gap, contributing to highly efficient
hole injection and enhanced hole mobility. Besides, con-
sidering inherent high spin concentrate, AIARs will probably
exhibit a highly enhanced conductivity in its non-doped
state. This provides us a new concept for the design of or-
ganic semiconductors with high charge mobility. Further-
more, considering the high spin density of these stable
radicals, the work to introduce highly planar and conjugated
polyradicals is under progress in our lab. These materials
might show different electron transport behaviors from tra-
ditional organic semiconductors and half-metallic or metallic
characteristics as exact synthetic metals.
4 Conclusions
In summary, we discovered and reported three new pheno-
lamine radicals that showed unexpected stability and low
band gap. The good thermal stability at elevated temperature
as well as electrochemical stability comes from their dis-
tinctive aromatic nitro-like resonance structure. The in-
troduction of strong electron-withdrawing groups, extended
conjugation and enhanced electron delocalization contribute
to the stability of radicals and tunable ground states. Fur-
thermore, the conjugated nitroxide radicals could be obtained
by a simple synthetic route and exhibited absorption in the
NIR region as well as obvious enhancement of hole mobility.
Meanwhile, it is worthy to mention that we, for the first time,
proposed a novel viewpoint to understand the formation of
radicals motivated from aromatization of HNO3, which are
different from previous radical materials. This strategy can
also be applied to the aromatization of other inorganic acids
including H2CO3, H2SO4 and H3PO4, thus developing stable
high-spin AIAR materials with planar and highly conjugated
structure for application in electronic, spintronic and mag-
netic devices.
Figure 10 Experimental J-V characteristics of different devices with
space charge limited current fitting for BT-TPA-OMe and BT-TPA-O4,
respectively. (a) J-V characteristics of ITO/BT-TPA-OMe or BT-TPA-O4/
Ag devices; (b) J-V characteristics of ITO/PEDOT:PSS/BT-TPA-OMe or
BT-TPA-O4/MoO3/Ag hole-only devices (color online).
The excited triplet state with the configuration of multiple
aromatic phenyl group will be observed by ESR via thermal
excitation, as shown in Scheme 1. The existence of rich re-
sonance structures supports their unexpected strong thermal
and electrochemical stability, interesting paramagnetism and
tunable ground states. This strategy can also be applied to the
aromatization of other inorganic acids including H2CO3,
H2SO4 and H3PO4 to obtain AIARs and prepare potential
diradicals with triplet ground states.
3.4 The enhanced hole mobility of aromatic nitric acid
radical
Acknowledgements This work was supported by the Pearl River S&T
Nova Program of Guangzhou (201710010194), the National Natural Sci-
ence Foundation of China (61574061, 21520102006, 21634004, 51521002,
91633301), and the Foundation of Guangzhou Science and Technology
Project (201707020019).
The hole mobilities of BT-TPA-OMe and BT-TPA-O4 were
evaluated with the hole-only devices fabricated with differ-
ent device structure as the inset of Figure 10. The hole mo-
bility of pristine BT-TPA-O4 film was obviously enhanced
compared to BT-TPA-OMe (Figure 10(a)). The hole mobility
of BT-TPA-O4 can be significantly enhanced by three orders
of magnitude comparing with BT-TPA-OMe in this device
structure due to the suitable HOMO level of −4.81 eV, which
matches well with ITO. In the device of Figure 10(b), the
Conflict of interest The authors declare that they have no conflict of
interest.
Supporting information The supporting information is available online at
http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors.

10
Zhou et al. Sci China Chem
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