Sensitive and Repeatable Photoinduced Luminescent Radicals from A Simple Organic Crystal

Article abstract of DOI:10.1002/anie.202014720

Photoinduced organic radicals are important for chemical and physical processes of organic materials, which are extensively investigated and applied in organic synthesis, photoelectronic devices and biotechnology. However, there are rare reports of the luminescence for these photoinduced radicals, especially in the condensed state. Herein, an unexpected and interesting luminescent radical is described, which can be rapidly and reversibly generated from a simple organic crystal by gentle light irradiation in air. It was revealed that the twist and asymmetric conformation of isolated molecule in its crystal with only weak C?H???π intermolecular interactions, which led to the generation of such photoinduced luminescent radicals. In addition, dual-channel photosensitive device with rapid response and well repeatability can be obtained based on the thin film of this organic crystal, showing both photoswitching on luminescence and conducting.

Full text of DOI:10.1002/anie.202014720

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Accepted Article
Title: Sensitive and Repeatable Photoinduced Luminescent Radicals
from A Simple Organic Crystal
Authors: Yingxiao Mu, Yanyan Liu, Haiyan Tian, Depei Ou, Li Gong,
Juan Zhao, Yanping Huo, Yi Zhang, Zhiyong Yang, and
Zhenguo Chi
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202014720
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Sensitive and Repeatable Photoinduced Luminescent Radicals
from A Simple Organic Crystal
Yingxiao Mu,[a][b] Yanyan Liu, Haiyan Tian, Depei Ou, Li Gong, Juan Zhao, Yi Zhang, Yanping
[a]
[a]
[a]
[d]
[c]
[a]
[b]
[a]
[a]
Huo, Zhiyong Yang* and Zhenguo Chi*
[a]
Y. Mu, Y. Liu, H. Tian, D. Ou, Prof. Y. Zhang, Prof. Z. Yang, Prof. Z. Chi
PCFM Lab, GDHPPC Lab, Guangdong Engineering Technology Research Center for High-performance Organic and Polymer Photoelectric Functional
Films, State Key Laboratory of OEMT, School of Chemistry, Sun Yat-sen University, Guangzhou, 510275 (China)
E-mail: yangzhy29@mail.sysu.edu.cn
chizhg@mail.sysu.edu.cn
[b]
[c]
[d]
Y. Mu, Prof. Y. Huo
School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, 510006 (China)
Prof. J. Zhao
School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou, 510275 (China)
Mr. L. Gong
Instrumental Analysis and Research Center, Sun Yat-sen University, Guangzhou, 510275 (China)
Supporting information for this article is given via a link at the end of the document.
Abstract: Photoinduced organic radicals are important for chemical
poly(dimethylsiloxane) hybrid system, which came from the
integrated emissions of the molecules, radicals and cations of
and physical processes of organic materials, which are extensively
investigated and applied in organic synthesis, photoelectronic devices, phenothiazine under air atmosphere.[9] But the radical species
and biotechnology. However, there are rare reports of the
luminescence for these photoinduced radicals, especially in the
condensed state. Herein, an unexpected and interesting luminescent
radical is described, which can be rapidly and reversibly generated
from a simple organic crystal by gentle light irradiation in air. It was
revealed that the twist and asymmetric conformation of isolated
molecule in its crystal with only weak C-H···π intermolecular
interactions, which led to the generation of such photoinduced
luminescent radicals. In addition, dual-channel photosensitive device
with rapid response and well repeatability can be obtained based on
the thin film of this organic crystal, showing both photoswitching on
luminescence and conducting.
were generated slowly over hours by continuously irradiated with
an UV lamp under air.
Triarylamine derivatives have been recognized as efficiently
electroactive and photoactive materials, which readily produced
radicals by electrochemical reaction/UV radiation and were
intensively used in optoelectronic applications.[ However, the
luminescence of their photoinduced radicals have been rarely
reported, especially in the condensed state.[11] Very surprisingly,
obvious photoinduced luminescent radicals were unexpectedly
observed from an organic crystal of substituted triphenylamine
(tri-p-tolylamine, T3TA), when shone under UV light. This simple
crystal exhibited interesting photoinduced luminescent color
switching from blue to pinkish-purple, with a clear new emission
peak at 580 nm, accompanied by the generation of triphenylamine
radicals under light radiation in air. In addition, it is sensitive and
repeatable of the generation and degradation of radicals to the
UV radiation in the T3TA crystal. The radicals would immediately
be created under light radiation, while they would degrade very
fast after removal of the radiative light. Therefore, this T3TA
10]
Photoinduced radicals (PIR) materials have been widely studied
and obtained brilliant applications in the fields of catalysis,
polymerization, photoswitching and photodynamic therapy.[
Extensive researches have been done on the photophysical
properties of these highly reactive radicals in different states.[2]
For example, there were many reports for photochromism of
nitrogen-containing compounds, such as hexaarylbiimidazole
derivatives in solution state,[ and 2,4,6-tri(4-pyridyl)-1,3,5-
triazine in crystalline state.[4] On the other hand, these
photoinduced radicals are generally non-luminescent, in spite of
their totally spin allowed radiative transition with doublet-spin
excitons for high fluorescence.[5] This is because of the sufficiently
stable of most isolated organic radicals under ambient conditions.
Another important reason is that organic radicals suffered from
aggregation-caused quenching of their luminescence in
condensed state, by electron transfer or spin-exchange
1]
crystal becomes
a
promising candidate for sensitive
photoswitching with highly response speed based on
luminescence on/off change. Moreover, high on/off gain of
3]
2
conductivity around 10 times can be obtained at a low applied
voltage of 10 V, based on the photoconductive device fabricated
by the T3TA polycrystalline thin film. These encouraging results
provide new insights of the photoinduced radicals in condensed
state as well as photosensitive materials with dual-channel
switching for advanced optoelectronic devices applications.
When
a polycrystalline solid of compound T3TA was
continuously irradiated with an extra LED UV lamp (365 nm, lamp
power: 5 W) at room temperature in air, the emission color of
T3TA crystal would change from blue to pinkish-purple (Figure 1b
and Video S1). At first glance, it looked like a photochromism for
the T3TA crystal, which have been extensively reported for
triarylamine derivatives before. When we checked the changes of
its absorbance spectrum after light irradiation, there was no any
[6]
interaction. As a result, there are few reports of photoinduced
radicals with luminescence.[
the
2a, 7]
Recently, Tang et al. reported
phosphorus-containing compound, tris(4-
chlorophenyl)phosphine, which can generate stable solid-state
emissive radicals under UV irradiation.[8] Other examples were
usually doped in polymer matrix with low concentration. Choi et.
al. recently reported the white luminescence of phenothiazine-
1
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obvious new peaks but only a red-shifted absorbance onset (~490
nm) in the spectrum (Figure S4). However, an obvious pinkish-
purple light would appear when this experiment was carried out in
dark. When observed in a fluorescence microscope, a clear red
emission of the T3TA crystal can be seen from the red channel
the irradiation light, as its EPR signals cannot be detected
anymore. Furthermore, the intensity of luminescence and EPR of
T3TA crystal would increase simultaneously when the UV
irradiation intensity increased from 1% to 100%, as shown in the
Figure S7. These results revealed that the unusual photoinduced
color switching of T3TA crystal actually came from the
photoinduced luminescent T3TA radicals in its crystal.
(
580 nm, Figure S5). These results indicated that the color
changes of the T3TA crystal under UV irradiation actually a
photoinduced luminescence switching, which was unusual and
further confirmed by the new emission peak arose at about 580
nm in its luminescent spectrum. As shown in Figure 1a, the
intensity of this photoinduced luminescent peak significantly
increased, as the irradiative time extended from 0.8 to 2.4 s.
Additionally, this new peak cannot be detected after the T3TA
crystal destroyed into amorphous powder, revealing the
crystalline state an important assistance for this unusual
phenomenon. Before continuously investigate such luminescence
switching, the structure of T3TA crystal has been identified
through analysis of nuclear magnetic resonance (NMR) and high-
resolution mass spectrometer (HRMS) measurements (Figures
S1-S3).
Figure 2. a) The electron paramagnetic resonance (EPR) spectra of T3TA
crystal before and under UV light continuous irradiation; b) The conformation of
T3TA in crystal, the angles between the three peripheral phenyl rings (red,
green and purple ones) and the central tertiary amine plane (orange one) were
inset.
In order to obtain insights of such sensitive photoinduced
luminescent radical in crystalline state, the single-crystal of T3TA
compound has been grown via solvent evaporation from its n-
hexane solution. The X-ray diffraction (XRD) pattern of T3TA
single-crystal was carefully analyzed, which revealed it
a
symmetry crystal with the space group Ci of the triclinic system,
as shown in Figure 2b. Notably, the central tertiary amine moiety
exhibited a perfect plane (the light orange plane in Figure 2b),
containing central N atom and the three C-N single bonds. This
indicated the lone pair electrons of central N atom possessed
good conjugation with these delocalized π electrons in the three
peripheral phenyl rings. Similar to its analogues reported,[14] T3TA
Figure 1. a) Photoluminescence spectra of T3TA crystal under UV light
irradiation with different irradiation time: 0, 0.8, 1.6 and 2.4 s, respectively. The
chemical structure of compound T3TA was inset; b) Photos of T3TA crystal
taken under UV lamp irradiation without (left) and under (right) continuous
irradiation; c) Luminescence intensity decay curve of T3TA crystal at 580 nm
peak after it irradiated for 5 s.
compound adopted
a twist conformation in crystal, which
peripheral phenyl rings kept certain directions out of the central
tertiary amine plane. Besides, the dihedral angles between these
phenyl rings and the amine plane are different. Especially, the
angle of one phenyl ring (63.4°) was much bigger than those (28.8°
and 31.1°) of the other two phenyl rings. As a result, the
delocalized N lone pair electrons were partially limited in the
central tertiary amine moiety and protected by these peripheral
phenyl groups, which facilitated the generation of radicals under
UV irradiation. Additionally, with such twist conformation, no any
π-π stacking, neither face-to-face nor edge-to-edge packing,
between T3TA molecules in the crystal can be found, as shown
in Figures 3a-3c. On the other hand, the phenyl groups in T3TA
molecule were restricted by lots of intermolecular C-H···π
interactions. In Figure 3d, several weak C-H···π interactions were
shown within distances of 3.082 to 3.368 Å, from the phenyl ring
center to the hydrogen atom. Therefore, the photoinduced T3TA
radicals were separated within one T3TA molecule, without
transitions between T3TA molecules inside the crystal. The T3TA
radical thus rapidly released its energy from its excited state to
ground state via. light emission with peak at 580 nm of short
lifetime (4.2 ns). Then the ground-state radicals continuous
deformed back to neutral T3TA molecules.
Notably, this photoinduced new emissive peak is different from
the phosphorescence peaks of T3TA crystal, which located at 490
and 520 nm at low temperature (Figure S6). The lifetime of this
new peak at 580 nm was also measured and calculated as 4.2 ns,
confirming its fluorescence characteristic (Figure 1c). So, we
measured the electron paramagnetic resonance (EPR) spectrum
of the irradiated T3TA crystal to check whether such new
luminescence associated with the generation of radicals in T3TA
crystal after irradiation. As shown in Figure 2a, prominent EPR
signals with a g value[12] of 2.009 for radicals were observed in the
irradiated crystal, displaying no hyperfine splitting. These EPR
signals and the g value are similar to the reported ones[13] for
radicals of triarylamine derivatives in solid state, which further
confirmed the production of T3TA radicals in its crystal under light
irradiation. In addition, the radicals were generated immediately
when the T3TA crystal exposure to UV light. And these
photoinduced radicals also disappeared rapidly after removing
2
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Figure 3. The molecular stacking and the intermolecular interactions inside
T3TA single-crystal: a) view from a-axis; b) view from b-axis; c) view from c-axis;
d) the weak C-H···π intermolecular interactions (green dashed line) and their
distances. The distances (Å) of C-H···π interaction have been inset in d).
To further understand the radical generation mechanism of
T3TA crystal, the nature of electronic states of T3TA molecules
have been interpreted by density function theory (DFT) and time-
dependent DFT (TD-DFT) calculations at the level of B3LYP/6-
3
1G(d) based on the crystalline structure and an optimized
Figure 4. The emission spectra and electronic states of T3TA molecules
interpreted by DFT calculations: a) molecular orbitals of T3TA molecule in
crystalline and gas states at the level of B3LYP/6-31G(d); b) UV absorbance
and photoluminescence spectra; c) the calculated energy level diagram and
molecular orbitals of T3TA radical in crystalline state at the level of M06-2X/6-
31G(d). The half-arrows indicate the state occupancy and electron spin
orientation.
conformation in gas state. The conformation of T3TA molecules
were different in the two states, especially the structural symmetry.
The T3TA molecule adopted an optimized conformation similar to
C
3V symmetry in gas phase, with the similar angles between the
phenyl moieties and the central amine plane (~41°, Figure 4a).
Without typical donor-acceptor structure, the calculated results
showed no obvious charge separation within the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) in gas-phase state. However, one of the peripheral
phenyl rings exhibited more twisted than the others in T3TA
molecule in the crystalline state, which broke down the structural
symmetry (the red phenyl ring in Figure 2b). This symmetry
breaking actually resulted in a small charge separation within the
HOMO and LUMO of T3TA molecule in crystalline state. As
shown in Figure 4a, the electron cloud located on the more twisted
phenyl ring (blue dashed circle) is slightly less than that on the
other two phenyl rings within HOMO, while the contrary is the
case within LUMO. It is reported that charge separation was in
favor of the photo-induced radical generation. Thus the
asymmetric conformation of T3TA molecule in crystalline state
should promote radical generation under UV irradiation. The
absorption and emission spectra of isolated T3TA molecule in
crystalline state were also simulated by TD-DFT calculations
based on its asymmetric structure, which were plotted in Figure
Additionally, an emission peak centered at around 580 nm for
isolated T3TA cation could be found by the TD-DFT calculated
results at the level of M06-2X/6-31G(d) in Figure 4b, which has
an oscillator strength (f) of up to 0.1995 (β-HOMO → singly
unoccupied molecular orbital (SUMO), ~2.52 eV, Figure 4c). Very
interestingly, this peak located at the exact position of the
photoinduced emission peak for the T3TA crystal in Figure 1a,
which indicated such photo-induced luminescent species should
be the T3TA radical cations. Furthermore, the detailed energy
level diagrams and wave functions of the HOMOs and LUMOs of
T3TA radical cation were in-depth analyzed (Figure 4c and S8).
The electron cloud in MOs (HOMOs, LUMOs, SOMOs and
SUMOs) spread over the whole T3TA molecule, without obvious
separation. And the optimized conformation of T3TA radical
cation in excited state also exhibited structural symmetry of C3V
similar to that of T3TA molecule in free state (Figure 4a). The
three phenyl rings kept similar torsional angles of ~37° to the
central amine plane, which are little smaller than those of free
state T3TA molecule. As a result, T3TA radical cation would
exhibit a conformation variation after excitation. As discussed
above, there are no any obvious stacking but only weak
intermolecular interactions between T3TA molecules in the crystal,
such as weak C-H···π interactions. This conformational change
4b. The predicted spectra possessed absorption peak at ~290 nm
and emission peak at ~380 nm, which exhibited good consistency
with the experimental data (Figure 1a and S4).
3
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thus became possible through phenyl rings rotations to a certain
extent in the crystal by light irradiation. On the other hand, the
excited-state T3TA radical cation also rapidly released its energy
via. emission and came back to the original conformation in
crystal, with those weak intermolecular C-H···π interactions inside
the crystal. It matched well the short lifetime of new luminescence
with peak at 580 nm in T3TA crystal. Therefore, the photoinduced
luminescence switching of T3TA crystal originated from its
radicals.
photoinduced electronic process as well. And the increasement of
photoconductivity reduced along the UV light intensity decreased
(Figure S12). It can be deduced that this changes on the
conductivity was relative to the photoinduced radicals in T3TA
crystal.
In conclusion, unexpected photoinduced luminescent radicals
from an organic crystal of simple substituted triphenylamine
compound (T3TA) has been carefully studied. Interestingly, the
luminescent radicals can be generated in a rapid and repeatable
way by exposed to gentle UV irradiation, which led to a dual-
channel photoswitching of both luminescence and conducting. It
was found that the T3TA molecules were separated in the crystal,
with no any π-π stacking and only weak C-H···π interactions
inside. And the twist and asymmetric conformation of the isolated
T3TA molecule in this organic crystal facilitated the generations
of radicals, based on the experimental and theoretical results.
Therefore,
photosensitive
device
with
dual-channel
photoswitching have been fabricated by T3TA polycrystalline thin
film, showing both luminescence and conducting switching when
triggered by UV light irradiation. It is notable that the
photoconducting behavior was sensitive, with a high on/off gain
2
around 10 times of photocurrent at a low voltage (10 V) and a
-2
low light intensity (10 mW cm ). Additionally, the dual-channel
photoswitching can be well repeated with a rapid responsibility,
as proved by the measurements of photoconducting and
conductive AFM of the device. These results provide opportunity
to design new optical and sensing systems for potential
applications in light-operated switching and encryption based on
photoinduced luminescent radicals. To explore more organic
crystals with such interesting photoinduced response, lots of
organic compounds will be tried in our lab in future.
Figure 5. The photoconducting property of T3TA polycrystalline thin film by light
irradiation: a) the schematic diagram of the photosensitive device based on
T3TA crystal; b) the current switching of T3TA crystal along the light irradiation
on and off; c) AFM and d) conductive AFM images of T3TA crystal along the UV
irradiation on and off. The scale bars have been inset in c) and d).
Acknowledgements
This interesting photoinduced luminescence switching along
radical generation in crystalline state make it suitable for dual-
channel photosensitive device, as radical generation can also
increase the material’s conductivity. A simple photosensitive
device with T3TA polycrystalline thin film was fabricated on glass
with Au electrodes, as shown in Figure 5a. Upon UV light
irradiation, the T3TA polycrystalline film showed not only
luminescence switching (Figure 1b) but also clear
photoconducting behavior (Figure 5b and S9-10), resulting in a
dual-channel switching to irradiation in ambient conditions. As
shown in Figure 5b, the conductivity jumped up and down along
with the light irradiation switched on/off, with both a rapid
responsibility and a well repeatability. The conductivity increased
This work was financially supported by the NSFC (51973239,
51733010, 51603232, 51873237, 51903254 and 52073315),
Guangdong Natural Science Funds for Distinguished Young
Scholar (2017B030306012), Tip-top Scientific and Technical
Innovative Youth Talents of Guangdong Special Support Program
(
2017TQ04C782), Science and Technology Program of
Guangzhou (201804010173) and Fundamental Research Funds
for the Central Universities. We thank Dr. Fujie Yang for help with
photosensitive device and Dr. Qianxi Dang for help with
theoretical calculation.
2
about 10 times at a low voltage of 10 V and a low light intensity
Keywords: photoinduced luminescence switching • radicals •
−
2
of 10 mW cm , suggesting photoexcitation of charge carrier in
the crystal and sensitive photoresponse of the device. In addition,
atomic force microscope (AFM) and conductive AFM images
have been collected under a bias voltage of 2V (Figures 5c, 5d
and S11), to further studied the changes on topography and
surficial current of T3TA crystal with photoirradiation. The
conductive AFM image showed a clear brighter contrast on the
surface of T3TA crystal during the light irradiation, indicating an
obvious current increase (Figure 5d). And this photocurrent
rapidly and significantly switched as the UV light turned on and
off, further conforming the sensitivity and repeatability of such
photoconducting behavior. On the other hand, there were no
changes on the topography of T3TA crystal, suggesting it a
photoconducting • organic crystal • rapid response
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Entry for the Table of Contents
Simultaneously dual-channel switching of luminescence and conducting can be achieved from a simple organic crystal by UV
irradiation, which is because of the sensitive and repeatable generation of photoinduced luminescent radicals inside.
6
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