Chemical Modification toward Long Spin Lifetimes in Organic Conjugated Radicals

Article abstract of DOI:10.1002/cphc.201800742

Organic semiconductors for spin-based devices require long spin relaxation times. Understanding their spin relaxation mechanisms is critical to organic spintronic devices and applications for quantum information processing. However, reports on the spin re

Full text of DOI:10.1002/cphc.201800742

Accepted Article
Title: Chemical Modification toward Long Spin Lifetimes in Organic
Conjugated Radicals
Authors: Ya-Zhong Dai, Bo-Wei Dong, Yi Kao, Zi-Yuan Wang, Hio-
Ieng Un, Zheng Liu, Zhi-Jun Liu, Liang Li, Fang-Bai Xie, Yang
Lu, Mei-Xing Xu, Ting Lei, Yu-Jie Sun, Song Gao, Shang-Da
Jiang, and Jian Pei
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To be cited as: ChemPhysChem 10.1002/cphc.201800742
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10.1002/cphc.201800742
ChemPhysChem
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Chemical Modification toward Long Spin Lifetimes in Organic
Conjugated Radicals
Ya-Zhong Dai,[a] Bo-Wei Dong,[b] Yi Kao,[b] Zi-Yuan Wang,[a] Hio-Ieng Un,[a] Zheng Liu,[b] Zhi-Jun Lin,[c]
Liang Li,[b] Fang-Bai Xie,[a] Yang Lu,[a] Mei-Xing Xu,[b] Ting Lei,[d] Yu-Jie Sun,[c] Jie-Yu Wang,[a] Song
Gao,*[b] Shang-Da Jiang,*[b] and Jian Pei*[a]
Abstract: Organic semiconductors for spin-based devices require Introduction
long spin relaxation times. Understanding their spin relaxation
mechanisms is critical to organic spintronic devices and applications
for quantum information processing (QIP). However, reports on the
spin relaxation mechanisms of organic conjugated molecules are rare
and the research methods are also limited. Herein, we study the
molecular design and spin relaxation mechanisms by systematically
varying the structure of a conjugated radical. We found that solid-state
relaxation times of organic materials are largely different from that in
π-Conjugated organic semiconductors (OSCs) have attracted
great interests in low-cost, flexible, and large-area optoelectronic
applications, such as field-effect transistors,[1] light-emitting
diodes[2] and solar cells,[3] due to their good mechanical properties
and readily tunable electronic structures.[4] Although the optical
and electronic properties of OSCs have been intensively studied,
much less efforts have been devoted to their spin-related
properties.[5,6] Compared to inorganic counterparts, OSCs have a
long electron spin diffusion length due to their weak spin-orbit
interaction and weak hyperfine interaction.[7] These features
endow OSCs promising materials for spin-related information
transmission, processing, and storage.[8-10]
Understanding the mechanism for spin relaxation is a key
factor to realize spin-based applications using OSCs.[11,12] The
electron spin relaxation can be assigned into two aspects: the
exchange of energy with the spin’s environment (the ‘lattice’)
which is characterized by the spin-lattice relaxation time (T1) and
spin dephasing which is characterized by the coherence time (Tm).
A long T1 is crucial to maintain a spin polarization along a certain
axis, while a long Tm enables the coherent manipulation of spins.
Recently, Takeya et al. reported the coexistence of ultra-long
spin lifetimes and coherent charge transport in organic single-
crystal semiconductor.[13] Sirringhaus et al. systematically studied
the spin-orbit coupling effect and spin relaxation mechanisms in
OSCs.[14]. Their work represent significant advances toward the
understanding of the spin relaxation mechanisms in OSCs.
However, these studies are limited to either gating induced
radicals or chemical doping induced radicals in solution. These
studies did not exclude electron-electron interaction and
environmental solvent effect, and cannot be directly related to the
intrinsic spin relaxation properties of OSCs. In addition, previous
studies for spin relaxation times in organic materials typically use
an indirect method via estimation linewidths by continuous wave
(cw) electron spin resonance (ESR) spectroscopy, which
introduces several approximations.[15]. The pulsed ESR method
is regarded as a direct way to obtain the spin relaxation time but
scarce applied in OSCs.[16,17]
solution state. We demonstrate that substitution of
a lower
gyromagnetic ratio nucleus (e.g. D, Cl) on the para-position of the aryl
rings in triphenylmethyl (TM) radical can significantly improve their
coherence times (Tm). Flexible thin films based on such radicals
exhibit ultra-long spin-lattice relaxation times (T1) up to 35.6(6) μs and
Tm up to 1.08(4) μs under ambient conditions, which are among the
longest values in film. More importantly, using the TM radical
derivative (5CM), we observed room-temperature quantum
coherence and Rabi cycles in thin film for the first time, suggesting
that organic conjugated radicals have great potentials for spin-based
information processing.
[a]
Title(s), Initial(s), Surname(s) of Author(s) including Corresponding
Author(s) Dr. Y.-Z. Dai, Z.-Y. Wang, H.-I. Un, F.-B. Xie, Y. Lu, Prof.
J.-Y. Wang, Prof. J. Pei*
Beijing National Laboratory for Molecular Sciences (BNLMS), Key
Laboratory of Bioorganic Chemistry and Molecular Engineering of
Ministry of Education, Key Laboratory of Polymer Chemistry and
Physics of Ministry of Education, Center of Soft Matter Science and
Engineering
College of Chemistry and Molecular Engineering, Peking University
Beijing 100871, China
E-mail: jianpei@pku.edu.cn
[b]
B.-W. Dong, Y. Kao, Z. Liu, L. Li, M.-X. Xu, Prof. S. Gao,* Prof. S.-
D. Jiang,*
Beijing National Laboratory for Molecular Sciences (BNLMS),
Beijing Key Laboratory for Magnetoelectric Materials and Devices,
College of Chemistry and Molecular Engineering
Peking University, Beijing 100871, China
E-mail: jiangsd@pku.edu.cn, gaosong@pku.edu.cn
Z.-J. Lin, Prof. Y.-J. Sun
State Key Laboratory of Membrane Biology, Biodynamic Optical
Imaging Center (BIOPIC),
School of Life Sciences
[c]
[d]
The triphenylmethyl (TM) radical and its derivatives have
already been used as building blocks for conductor in
spintronics[18,19] and semiconducting materials for organic light-
emitting diodes.[20] Herein, we investigate a series of D, Cl
substituted TM radical derivatives using pulsed ESR (Figure 1a).
We found that the substitution of a lower gyromagnetic ratio
nucleus (e.g. D, Cl) on the para-position can significantly improve
the spin coherence times. Previous spin relaxation studies in
organic semiconductors mainly focus on their solution state. We
Peking University
Beijing 100871, People’s Republic of China
Prof. T. Lei
Department of Materials Science and Engineering, College of
Engineering
Peking University
Beijing 100871, People’s Republic of China
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cphc.201800742
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show that the molecular packing in solid state would show a
significant influence on the spin relaxation time in organic
materials. Furthermore, the good process ability of the TM
radicals allow us to fabricate device-like thin-films based on TM
radicals that show ultra-long T1 up to 35.6(6) μs and Tm up to
1.08(4) μs. We observed that the TM systems demonstrated
quantum coherence and Rabi cycle in thin-film at room-
temperature. The long Tm and Rabi cycle are essential properties
for QIP. To our knowledge, only one group have reported such
behavior in thin-film using organic semiconductor (i.e. copper
phthalocyanine (CuPc)) but with temperature below 80 K.[21]
recovery method (π-T-π/2-τ-π-τ-echo) and Hahn-echo sequence
(π/2-τ-π-τ-echo) with varying the T and τ, respectively. Compared
with cw-ESR methods that estimate the T1 and Tm by the linewidth
of spectra, the pulsed ESR methods, such as inversion recovery
for T1 and Hahn echo decay for Tm, are deemed as direct ways to
obtain the more accurate values of spin relaxation times and
rarely applied in OSCs. To reduce the influence of intermolecular
electron-electron interactions on spin relaxation,[27], the radicals
were diluted in d8-toluene solution with a concentration of 0.1
mmol/L or dispersed in their corresponding diamagnetic
hydrogenated products with a mole fraction of 0.1%.
First, we obtained the g-factors of TM radicals at 5 K by the
echo-detected field-swept (EDFS) spectra (Figure 1c). The EDFS
spectrum is an integrated absorption spectrum, avoided the
modulation process of derivative cw-ESR spectrum. Different
values for the g-factors of TM radicals were measured with
Results and Discussion
The TM radicals were synthesized using a concise method
reported in literature[22] (Figure 1a). A Friedel-Crafts condensation
between polychlorobenzenes and chloroform gave the
corresponding triarylmethanes. Then, tetrabutyl ammonium
hydroxide was added to remove the central hydrogen, followed by
the DDQ oxidation to form the target radical molecules. The TM
radicals shows high stability after introducing ortho-chlorinated
phenyl rings,[23] and adopts a propeller-like conformation (Figure
1b). Further chlorination at para-position gives 3CM. Chlorination
at the meta-positions gives 4CM. Their deuterated analogues are
D3CM and D4CM. The perchlorinated TM derivative is 5CM. Their
corresponding diamagnetic hydrogenated products, where
hydrogen atoms are substituted at the central carbon, are named
as 3CM-H, 4CM-H, D3CM-H, D4CM-H, and 5CM-H. Compared
with deuteration modification,[24], chlorination is more easily
accessible and has been widely used for tuning the electronic
properties of OSCs.[25] But the influence of chlorination on spin
lifetimes has not been studied yet in OSCs.[26]
varying substituents (g3CM = 2.0049, gD3CM = 2.0053, g4CM
=
2.0033, gD4CM = 2.0030, g5CM = 2.0058 in solid dispersion state)
(Figure 2a). In conformity to previous reports, chemical
modification could shift the g-factors of radicals, but we found that
this effect reinforces in solid state. The difference of g-factors
paves the way to discern certain spins in the multi-spin solid
system.[28]
Figure 2 | Spin lifetimes data of TM radicals. a, The spin-lattice relaxation
times (T1) measured by inversion recovery method for TM radicals in the d8-
toluene solution and solid dispersion state. The error bars denote the standard
errors in T1, which are within the size of the symbols. The five T1 curves of TM
radicals keeps consistent in solution state, suggesting that the interaction with
environmental molecules dominates the spin-lattice relaxation. b, Temperature
dependence of the quantum coherence times (Tm) measured by Hahn echo
decay method for TM radicals in the d8-toluene solution and solid dispersion
state. The error bars denote the standard errors in Tm, which are within the size
of the symbols. Similarly to T1, the Tm curves shows a large difference between
the solid phase and the solution phase. We note that chlorination is a low cost
and easily-accessible method to improve the Tm in TM radicals, especially in
para- position. 5CM in the solid dispersion state exhibits an ultra-long Tm of
1.17(1) μs at 298 K, which is beneficial for QIP.
Figure 2a shows the T1 of TM radicals in the d8-toluene
solution (5 K - 100 K) and solid dispersion state (5 K - 298 K)
measured by inversion recovery method, exhibiting
a
monotonically decreasing function of temperature. We found that
T1 curves shows a large difference between the solid phase and
the solution phase. The values of T1 in the d8-toluenesolution
keeps consistent in the temperature range studied here,
suggesting that the interaction with solvent molecules dominates
the spin lattice relaxation. We observed that the T1 curves of 3CM
and D3CM in the solid dispersion state are distinct different from
4CM, D4CM and 5CM, which may be attributed to the influence
of molecular packing. In the view of the differences in both g-factor
and T1 in solid states and solutions, the studies on solid facilitate
their practical application in the future.
Figure 1 | TM radical structure. a, Synthesis route of TM radicals. b, The
molecule structure of TM radicals (green atoms: Cl; grey atoms: C; white atoms:
H). D and Cl atoms are further introduced at meta- or para- positions to study
their influence. The EDFS spectra for TM radicals are measured c, in d8-toluene
solution at 5 K; d, in solid dispersion state at 5 K, confirming the radical
structures. We found that chemical modification could shift the g-factors of
radicals, and such effect reinforces in solid state. The difference of g-factors
paves the way to discern certain spins in the multi-spin solid system.
The spin relaxation times of TM radicals by X-band pulsed
ESR experiments were investigated. The spin-lattice relaxation
time (T1) and coherence time (Tm) were measured by inversion
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Figure 2b shows the Tm of radicals in the d8-toluene solution
and the solid dispersion state. There are three factors that mainly
affect decoherence and satisfy the following formula:
coherence in solids.[29] DD could integrated with many desirable
functionalities, such as quantum gate,[29] but has not be applied to
organic conjugated systems. With DD to reduce the hyperfine
interaction, the coherence times achieved 37.6(2) μs at 10 K for
3CM in d8-toluene solution and 5.1(1) μs at ambient temperature
for D3CM in the solid dispersion state (Figure S5.7). The DD
experiment further indicates that the hyperfine interactions are the
main reason for the decoherence process in the TM radicals.[29]
1
1
1
1
=
+
+
푇푚 2푇
푇
푒푒
푇
푒푛
1
the molecular system decoherence usually has three ways: 1.
Spin-phonon interaction (this effect can be ignored in TM radicals,
since the T1 is much larger than Tm); 2. Electron-electron spin
coupling, which could be weakened by dispersion in the
isostructural diamagnetic host sample and the deuterated
toluene27; 3. Electron spin-nuclear spin hyperfine interaction. The
magnetic moments of the nuclear spins around the electron
furnish a local Overhauser field, leading to a fluctuation of
magnetic field applied on electron spin.[29] This effect could
accelerate the decoherence process of the electrons, reducing
the Tm. The degree of decoherence depends on two vital factors:
gyromagnetic ratio of nuclear spin and intensity of hyperfine
interaction. Substitution would tune such factors, but are few
studied in OSCs.
Figure 3 | Hyperfine effect in TM radicals. a, The spin density distribution of
TM radicals calculated at ub3lyp/6-31g*. The DFT result revealed that the
meta-positions of TM radicals have a less spin density population than the
para-positions. b, X-band HYSCORE spectra for 3CM, showing the 1H region
with Aiso= 3.1(1) MHz. c, X-band HYSCORE spectra for 4CM, showing the 1
H
region with Aiso= 6.8(6) MHz. For TM radicals, both of DFT and HYSCORE
Bearing substitution atoms with different gyromagnetic ratios
(훾1 : 42.5 MHz/T, 훾2 : 6.5 MHz/T, 훾35Cl: 4.2 MHz/T, and 훾37Cl: 3.5
results indicated substitution at the para-position has a larger influence on the
spin decoherence process than the meta-position.
H
D
MHz/T) at the para-positions, the Tm order (Tm (5CM) > Tm
(D4CM) > Tm (4CM)) was maintained during the entire measured
temperature range. Changing the substitutions at the meta-
positions gave the Tm order (Tm (5CM) > Tm (D3CM) > Tm (3CM)).
The substitution of a lower gyromagnetic ratio nucleus (i.e. D, Cl)
clearly improve the coherence time of TM radicals. While
deuteration is a generally accepted design strategy to enhance
Tm, the high cost of deuterated organic compounds limits its
applications. We note that chlorination is a low-cost and easily-
accessible method to improve the Tm of organic conjugated
molecules for QIP such as molecular quantum bit (qubits).
Practical spin-based information processing not only needs
long coherence times at room temperature, but also should
integrate with traditional devices.[32] The high stability and solution
processing ability of TM radicals enable the hunt for large-scale
and flexible devices, which has not been successfully achieved in
previous reports of NV center or metal ion-based paramagnetic
molecules. 5CM possessed the longest Tm among our TM
radicals with the coherence times up to 13.3(8) μs at 60 K in d8-
toluene solution and 1.17(1) μs under ambient temperature
conditions in the solid dispersion state, which are among the
highest values reported in OSCs.[13] In an effort to further develop
this area of research, we fabricated device-like films by vapor
decomposition and spin-coating of 5CM on polyethylene
terephthalate (PET) and quartz substrates. The transparent PET
and quartz substrates allow for optical techniques to detect or
manipulate the spins, while the flexible PET substrates are also
useful for constructing wearable devices. By simply implementing
a cubic molecular shape model with a mole ratio of 5CM : 5CM-H
= 1 : 1000, each spin center in 5CM was separated by ten
diamagnetic 5CM-H molecules. It is understood that the large
distance between the electron spins may help weaken electron-
electron interactions and enhance the Tm of the film. A 200-nm
5CM : 5CM-H = 1 : 1000 film was grew by vapor decomposition
(Figure 4 and method part). The EDFS of the 5CM-device-like
film revealed a g-factor of 2.0060. These values are consistent
with those of the 5CM-solid (g = 2.0058), suggesting that the
radicals preserve their open-shell character on the substrate
surface. Moreover, compared with the 5CM-solid (T1 = 35.1 (7)
and Tm = 1.17 (1) μs at 298 K), the 5CM-device-like film exhibits
a similar T1 of 35.6 (6) μs and Tm of 1.08(4) μs at 298 K,
suggesting that the interaction between the radicals and
substrates have a slight influence on the longitudinal and
transverse relaxation of the radicals. The Tm keeps nearly a
constant down to 80 K but far smaller than T1 in the entire
temperature range studied here as indicated in Figure 4a.
In addition, the relations of Tm (3CM) > Tm (4CM) are appeared
1
both in solution and solid state, despite that 3CM have more H
atoms than 4CM. Even the Hahn echo of 4CM vanishes above
150 K in solid dispersion state. It is interesting that the far protons
(at para- position) accelerate the decoherence process faster
than the closed protons (at meta- position), showing
a
contradiction with dipole-dipole interaction model, which the
intensity of hyperfine interaction is dominant by distance.[30]
Hyperfine sublevel correlation spectroscopy (HYSCORE) is a
two-dimensional four-pulse stimulated-echo experiment to
measure small hyperfine interactions unresolved in the cw-EPR
spectrum.[31] In order to further scrutinize the isotropic hyperfine
intensity (Aiso) of protons at meta- and para- positions, HYSCORE
experiments were conducted on 3CM and 4CM in a d8-toluene
solution at 60 K. By fast Fourier transformation (Figure 3b and 3c),
the Aiso of protons in 3CM is 3.1(1) MHz, which is conspicuously
smaller than Aiso of protons in 4CM (6.8(6) MHz). The density
functional theory (DFT) calculations demonstrate that the spin
density populated more in para- position than meta- position.
Thus, protons at para-positions provide a large isotropic hyperfine
interaction through Fermi contact effect,[30] which accorded with
the HYSCORE results.
Dynamical decoupling (DD), which uses stroboscopic spin flips
to decouple the electron spin from its weak interaction with the
environment, is an important strategy to preserve electron spin
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Rabi cycles, that is, the coherent driving of electrons between
the Zeeman energy levels, verify the ability to manipulate electron
spins for spintronics and quantum information processing. To
exemplify that the electron spins of the 5CM film can be placed
into any arbitrary superstition state, the echo-detected nutation
experiments were performed at different microwave powers for
the 5CM film at 298 K and noticeable Rabi cycles were detected
at 298 K with the frequencies (ΩR) of oscillations scaled linearly
with the magnetic field of the microwave power (B1),
demonstrating its great potential for qubit applications. Only
Aeppli’s group reported such behavior in thin-film state based-on
CuPc, but their observation was below 80 K.[16] To the best of our
knowledge, this is the first report of a molecular qubit film device
that undergoes Rabi cycles at room temperature.
fluorescence under ambient condition. These fluorescence
spectra strongly suggest the potential optical addressability of
5CM.
The results of our work have taken a considerable step forward
in designing conjugated radical thin-film device for spin-based
application. Figure 5 shows the T1 and Tm of the spin-based qubits
of the current molecular system above the liquid nitrogen
temperature range (77 K-298 K). Our work (5CM device-like thin-
film) is superior to other molecules in the temperature range of
77-298 K. The endohedral metallofullerene, N@C60, is illustrious
of its T1 and Tm and has been widely studied for spin-based
applications. But the expensive cost of production and instability
of N@C60 make it hard to be applied in practical situation.[33] The
high stability and solution processing ability of TM radicals make
it possible to realize large-scale and flexible film devices. In the
whole temperature range, 5CM device-like thin-film has a
comparable T1 to N@C60 and is much higher than other
complexes.[21,34-36] In addition, to date, only electron spin-based
quantum coherence of CuPc were observed in thin-film state,
offering great promise for quantum information processing. But
CuPc only exhibits quantum coherence below 80 K. 5CM thin-film
possesses Rabi cycles and a remarkable coherence time of
beyond 1 microsecond under ambient condition, which is far
surpasses the time scale for coherent manipulation of electron
spin (10 ns).[27] Thus, TM radicals have a greater prospect of QIP
than previous molecular systems.
Figure 4 |Device-like film of 5CM. a, The temperature dependence of T1 (wine)
and Tm (cyan) for 5CM-device-like film. The error bars denote the standard
errors in T1 and Tm, which are within the size of the symbols. The Tm of 5CM-
device-like film under ambient condition is 1.08(4) μs, which is the longest value
in molecular qubit film devices. Inset: the photograph of 5CM flexible thin-film.
b, Rabi cycles of 5CM device-like film at 298 K for different microwave powers.
The linear functions of the oscillation frequencies with respect to the B1 field are
plotted in the insets, showing a slope of 2.8 MHz/G. This is the first report of a
molecular qubit film device that undergoes Rabi oscillations under ambient
condition. c, AFM image of 5CM device-like film with a RMS of 3.5 nm. d, The
fluorescence map of 5CM spin-coating film on a SiO2 substrate. The yellow
circles indicate the 5CM clusters, demonstrating its optical addressability.
Figure 5. Temperature dependence of coherence and relaxation times. T1
(solid legends and solid lines) and Tm (open legends and dash lines) vs.
temperature measured by X-band ESR (340 mT) for 200 nm 5CM thin-film, and
other representative published molecular and solid state electronic spin qubit
systems. Data are from ref. 31 (N@C60, Wine)33, ref.21 (CuPc, Orange)21, ref.
32 ([V(C8S8)3]2-, Olive)34, ref. 33 (VO(dmit)2, Cyan)35 and ref. 34 (Cu(S2C4N2)2,
Violet)36. The 5CM thin-film exhibits T1 of 441-35.6 µs and Tm of 1.14-1.08 µs
from 80 K to 298 K which shows the T1 value is nearly 2 orders bigger than Tm
value. Comparing to the other qubits, the 5CM thin-film has a long spin-lattice
relaxation time and quantum coherence time under ambient conditions and it
has a potential to realize complex quantum computing duo to the better solution
process ability, flexibility, and optical addressability.
The optical technology is widely used to address and
manipulate the spins for spin-based QIP but quite rarely realized
in film state. The emission of 5CM comes from the radiative decay
of doublet excitons, which are spin-allowed. The onset absorption
of the compound is 420 nm, corresponding with an optical gap of
2.7 eV. From the photoluminescence (PL) spectrum, it can be
seen that 5CM exhibits an emission band at about 600 nm with a
weak shoulder at a longer wavelength. The PL quantum yield (Φf)
in dichloromethane is measured to be 1.6% at the excitation
wavelength of 370 nm. To explore the fluorescence of 5CM on the
film, we formed a cluster of 5CM of 100 nm size on the SiO2
surface by spin-coating and measured the fluorescence map
(Figure 4d). The cluster could clearly be discerned by
Conclusion
In summary, we studied the relationships between the structures
and spin relaxation mechanisms in an intrinsic conjugated radical
system by pulsed ESR methods. Based on careful structural
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design, the radical, 5CM, by substituting triphenylmethane
radicals with low gyromagnetic ratio Cl atoms, showed a ultra-long
T1 of 35.6 (6) μs and Tm of 1.08(4) μs in flexible thin-film under
ambient conditions, which is among the highest value ever
reported. We observed room-temperature quantum coherence
and Rabi cycles in thin film for the first time, suggesting that
organic conjugated radicals have great potentials for spin-based
information processing. Moreover, the luminescent nature of our
systems offering us the possibility of initialization of the spin states
by laser, rather than the low-temperature-high-field approach for
the normal semiconductor quantum dots. The thin film process
ability of our materials may help to construct complex devices
using current nanofabrication technologies.
Fluorescence imaging methods
Glass coverslips were purchased from Fisher Brand. We used piranha
solution to clean coverslips at temperature 90 °C in a water bath for about
30 minutes. The Piranha solution is a mixture of 3 parts of concentrated
sulfuric acid (H2SO4) and 1 part of 30% hydrogen peroxide (H2O2) solution.
Piranha solution should be freshly prepared. The cleaned coverslips were
coated by APTES (3-aminopropyl-triethoxysilane, Sigma-Aldrich). In detail,
2% APTES solution (93% methanol, 5% acetic acid, 2% APTES) was
added to cleaned coverslips and allowed to react overnight. Then a large
amount of deionized water was added to finish the reaction by quenching
free APTES, followed by careful wash by pieces. We then functionalized
the coverslips by PEG (polyethylene glycol). In detail, 100 mg m-PEG-SVA
and 10 mg SVA-PEG-SVA (m-PEG-SVA-5000, MW = 4,400; SVA-PEG-
SVA-3400, MW = 3,200; Laysan Bio. Inc) were dissolved in 1mL 100 mM
NaHCO₃ (pH 8.3), and the solution was centrifuged at 14800 rpm for 1
minute. The supernatant was transferred onto the coverslips and allowed
to react in a humid box for 2 hours. After reaction, the PEG-functionalized
coverslips were washed by deionized water and dried by N2 gas. The
Experimental Section
Methods:
radical solution were spin-coating on the glass with 2000 rpm.
ESR measurements
A Nikon N-STORM system equipped with a 100× oil objective (Nikon, NA
1.49) and a back-illuminated electron multiplying charge coupled device
camera (EMCCD, DU-897, Andor Technology) was used for fluorescence
imaging. The pixel size was 160 nm. A 1.5 × telescope was used to reduce
the pixel size to 106 nm. A 405 nm laser (Coherent) was used to excite the
5CM thin film coated coverslip in an objective-type total internal reflection
(TIR) illumination mode. The illumination power was 25 mW. The
exposure time was 100 millisecond.
The radical molecules were dissolved in a d8-toluene solution with the
concentration of 10-4 mol/L and crystalline dispersed in the isostructural
diamagnetic host in 1:1000 stoichiometric ratios for ESR experiments.
Films of 0.1% 5CM of area ~5 cm2 were sliced and packed into ESR tubes.
Both cw- and pulsed-ESR data were measured on a Bruker Elexsys
E580 spectrometer with MS-3 cavity (ω = 9.26 GHz). The low-temperature
environment was achieved by Oxford Instruments produced liquid helium
cryostats (CF935). The signal of the pulsed-ESR experiments was
collected by integrating the Hahn echo (π/2-τ-π-τ-echo) with τ = 200 ns.
The T1 values were measured by the inversion recovery method (π-T-π/2-
τ-π-τ-echo) with 4-step phase cycling. The Tm values were obtained by
using Hahn echo sequence with 16-step phase cycling. The dynamic
decoupling measurements were carried out by CPMG sequence (πx/2-τ-
(πy-2τ)n-πy-τ-echo) with 4-step phase cycling. The π/2 and π pulse lengths
in EDFS, T1 and Tm measurements were 20 and 40 ns with 6 dB
attenuation of the microwave power 300 W, respectively. The HYSCORE
was conducted by a 4-pulse sequence (π/2-τ-π/2-t1-π-t2-π/2-echo) with τ
= 34 ns, t1 = t2 = 150 ns and π/2 and π pulse lengths were 10 and 20 ns
with 0 dB attenuation and 16-step phase cycling. After fast Fourier
transform to get the two-dimensional spectrum of the frequency domain.
The nutation experiments were carried out with standard sequence (tp-T-
π/2-τ-π-τ-echo), where T > 5 Tm. The π/2 pulse lengths were adjusted to
10, 20 and 40 ns by 0 dB, 6 dB and 12 dB attenuation. The selective pulsed
Tm data were obtained by the Hahn echo (π/2-τ-π-τ-echo) with π/2 pulse
lengths were adjusted to 10, 20, 40, 80, 160, 320 and 640 ns by 0 dB, 6
dB, 12 dB, 18 dB, 24 dB, 30 dB and 36 dB attenuation.
Acknowledgements
This work was supported by the Major State Basic Research
Development Program (2015CB856505) from the MOST, the
National Key R&D Program of China (2017YFA0204701,
2017YFA0204903 and 2017YFA0505300), the National Natural
Science Foundation of China (grant nos. 21621061, 21601005,
21420102005, 21790360, 21573013, 21390412 and 31327901),
and the Young Elite Scientist Sponsorship Program of the China
Association of Science and Technology (grant no.
YESS20150011 and 31327901).
#Y.-Z. D., B.-W. D., and Y. K. contributed equally to this work.
Keywords: Organic semiconductors • organic spintronic devices
• triphenylmethyl radical • coherence times • spin relaxation
mechanisms
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Text for Table of Contents
Ya-Zhong Dai, Bo-Wei Dong, Yi Kao, Zi-
Yuan Wang, Hio-Ieng Un, Zheng Liu,
Zhi-Jun Lin, Liang Li, Fang-Bai Xie,
Yang Lu, Mei-Xing Xu, Ting Lei, Yu-Jie
Sun, Jie-Yu Wang, Song Gao*, Shang-
Da Jiang*, and Jian Pei*
Page No. – Page No.
Chemical Modification toward Long
Spin Lifetimes in Organic Conjugated
Radicals
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