Photo-induced phosphorescence and mechanoluminescence switching in a simple purely organic molecule

Article abstract of DOI:10.1039/c8tc06202a

A simple organic molecule, 2-(9H-carbazol-9-yl)ethanol (CZEO), displays both mechanoluminescence (ML) and photo-induced room-temperature phosphorescence (RTP). The D-A type dimer formed in the crystals was found to play a crucial role in the light-stimulated luminescent response, responsible for the turn-on RTP and turn-off ML switching of CZEO.

Full text of DOI:10.1039/c8tc06202a

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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DOI: 10.1039/C8TC06202A
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Photo-induced both phosphorescence and mechanoluminescence
switch from a simple purely organic molecule
Received 00th January 20xx,
Accepted 00th January 20xx
Qiuqin Huang,a Xiaofei Mei,a Zongliang Xie,b Duobin Wu,a Shuming Yang,a Wenjing Gong,a Zhenguo
Chi,b Zhenghuan Lin*a and Qidan Linga
DOI: 10.1039/x0xx00000x
www.rsc.org/
A simple organic molecule, 2-(9H-carbazol-9-yl)ethanol (CZEO), by UV irradiation,31 and that the ML emission wavelengths could be
display both mechanoluminescence (ML) and photo-induced manipulated by weak intermolecular interactions in the crystalline
room-temperature phosphorescence (RTP). D-A type dimer state.10 However, to the best of our knowledge, the regulation on
formed in crystals play
a crucial role in light-stimulated both persistent RTP and ML through external stimulus has yet to be
luminescent response, and is responsible for turn-on RTP switch realized in a molecular system.
and turn-off ML switch of CZEO.
Some carbazole (CZ)-based crystals can exhibit orange-yellow
persistent RTP ranging from 520 to 600 nm at air, due to the triplet
excitons stabilized by intermolecular interactions and molecular H-
aggregation caused by nearly planar aromatic heterocyclic
configuration.31-38 Furthermore, several CZ derivatives have been
found to be ML active for forming crystalline structure with the
piezoelectric space group nature.21, 39, 40 It inspires us to concentrate
on CZ derivatives to seek simple purely organic materials with both
persistent RTP and ML. Herein, 2-(9H-carbazol-9-yl)ethanol (CZEO,
Fig. 1a) was found to be ML-active and photo-induced RTP. D-A type
dimer formed in crystals plays a crucial role in light-stimulated
luminescent response, and is responsible for turn-on RTP switch
and turn-off ML switch of CZEO. The unique nature of CZEO can
make it be successfully applied in the field of anti-counterfeiting.
CZEO could be easily obtained by an alkylation of carbazole (CZ)
with 2-bromoethanol (Scheme S1). 2-(3,6-dibromo-9H-carbazol-9-
yl)ethanol (BCZEO) was also be synthesized through bromination
and alkylation from CZ for a comparison purpose. Photophysical
properties of CZEO and BCZEO in DCM solution and solid were fully
investigated. In DCM, CZEO shows two main absorption bands at
294 nm and 324 nm with a shoulder peak at 340 nm (Fig. S1).
Absorption bands of BCZEO exhibit a 10 nm of red shift relative to
that of CZEO, due to the p-π conjugative effect of Br. Upon
excitation at maximum absorption peak, CZEO gives a strong
emission at 349 nm with a quantum yield (Φ) of 26%, while BCZEO
is basically non-emissive in DCM for heavy atom effect of Br.
Persistent room temperature phosphorescence (RTP) and
mechanoluminescence (ML) have attracted much attention for the
potential applications in the fields of photonic devices,1, 2 display,3,4
bioimaging,5, anti-counterfeiting,7 and pressure sensing.8, It is
desirable to develop purely organic materials with RTP and ML, due
to their advantages of low cost, structural diversity, and flexible
modification and processing.10-17 However, organic triplet state
excitons easily decay through non-radiative channel (such as
molecular movement, thermal and collisional processes, and
oxygen quenching),18-20 while the stress-caused excited state is
mainly attributed to the electrostatic excitation induced by crystal
fracture.21 As a result, organic materials with both persistent RTP
and ML are very rare, but important for understanding complex
luminescence processes, and relationship between structure and
optical property.13, 21
6
9
On the other hand, smart organic materials with changeable
luminescence induced by external stimuli are popular and useful for
probes and sensors, optical devices, data storage and processing.22-
30
Although incorporating a persistent phosphorescent guest and a
photochromic guest into anamorphous steroidal host obtains
photoreversible on–off recording of red persistent RTP,17 it is
difficult to directly regulate the RTP or ML properties through a
simple external stimulus, like light, heat, and electricity. It is
reported that the phosphorescent lifetime of molecular rotors
containing a triazine core, could be prolonged from 1.8 to 1330 ms
Compared with the emission in solution, both of CZEO and
BCZEO in solid display a red-shifted emission for intermolecular
interactions, and improved luminescent intensity for suppressing
molecular motion (Fig. 1b). For example, CZEO emits blue light with
wavelength in the range from 350 nm to 550 nm and Φ of 42%,
excited at 340 nm. Under excitation at the same wavelength, BCZEO
shows a dual emission at 395 nm and 583 nm with a 4% of total
a.Fujian Provincial Key Laboratory of Polymer Materials, College of Chemistry and
Materials Science, Fujian Normal University, Fuzhou 350007, China.
b.State Key Laboratory of Optoelectronic Material and Technologies, School of
Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, China
*E-mail: zhlin@fjnu.edu.cn
Electronic Supplementary Information (ESI) available: Synthesis and additional
characterization and analysis of carbazole derivatives. CCDC 1880922 and 1880923.
See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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induced phosphorescence turn-on switch based on CZEO has been
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obtained (Fig. 1c, insert). However, photo-aDctOivI:a1t0io.1n03d9o/eCs8nTCo0t6m20a2kAe
the RTP intensity and lifetime of BCZEO increase substantially (Fig.
S6 and S7), like CZEO.
More interestingly, CZEO crystals exhibit mechanoluminescence
properties. When squeezed CZEO by a sharp glass rod or a steel
spatula, a blue luminescence was observed by naked eyes. The ML
photograph and spectrum are shown in Fig. 2b. An emissive peak at
412 nm with two shoulder peaks at 375 nm and 436 nm is found in
the ML spectrum of CEZO, which is consistent with its
photoluminescence spectrum (Fig. 1b). It demonstrates that the ML
is originated from the singlet state of CZEO. However, the ML
Fig. 1 a) Structure of CZEO and BCZEO. b) Emission spectra of CZEO activity of CZEO turns invalid after it is activated by UV light for 2
and BCZEO in solid. c) Phosphorescence spectrum of photo- min. It causes the formation of a turn-off ML switch triggered by UV
activated CZEO excited at 364 nm. Insert: photograph of CZEO light (Fig. 2b, insert). The video documented ML process of CZEO
without and with activation after turned off UV light. d) Photograph without and with photo-activation is shown in Supplementary
of photo-activated CZEO before and after turned off UV light.
Movie 2 and 3, respectively.
To make clear the origin of turn-on RTP switch and turn-off ML
switch of CZEO, the single-crystal structure of CZEO before and after
activated for 5 min was analyzed by single-crystal x-ray diffraction
(XRD) with molecular structure showed in Fig. 2c and 2d, and
crystallographic data summarized in Table S2. CZEO crystallizes in
the tetragonal system and the space group I 41 with two molecules
formed dimer through weak C-H…π (2.876, 2.954 and 3.177Å)
interaction. The included angle of two carbazole rings is measured
to be 62.2°. There are 16 molecules in one unit cell with each four
molecules fixed by hydrogen bonds C-H…O (2.643 or 2.656 Å, Table
S3) to get a cluster. Each molecule can form a dimer with another
molecule in the other cluster through the above mentioned C-H…π
interaction. After photo-activation, the bond length of C-H…O in
cluster increases to 2.667 and 2.672Å, while the distance of C-H…π
interaction in dimer decreases to 2.869, 2.938 and 3.172 Å,
accompanied by the included angle of two carbazole rings reduced
to 61.6° (Fig. S8). These data indicate that photo-activation makes
the dimer in crystal turn tighter, which should be contributed to
both RTP and ML switch.
quantum yield which is more than 200 times higher than that of
3,6-dibromo-9H-carbazole (BCZ, Φ=0.02%). Photoluminescent
spectra of CZEO and BCZEO are agreed with that of CZ and BCZ (Fig.
S2), respectively. It indicates that 2-hydroxylethyl group has little
effect on electronic transition of ground and excited states.
To get an insight into the excited state relaxation processes of
CZEO and BCZEO, their transient luminescence properties were
investigated with the time-resolved transient decay in solid plotted
in Fig. S3 and S4.As expected, the emission at both 374 nm and 410
nm of CZEO in solid presents fast decay with about a 7.48 ns and
15.91 ns lifetimes (τ), respectively. However, the emission at 395
nm and 583 nm of BCZEO gives a fast decay (τ= 0.16 ns) and a slow
decay (τ= 4.37 ms), respectively. Consequently, two emissive bands
at 395 nm and 583 nm of BCZEO in solid should be assigned to
fluorescence and phosphorescence, respectively. The relevant
experimental data of CZEO and BCZEO are summarized in Table S1.
It is known that CZ shows a persistent RTP at 555 nm with 910
ms of lifetime.33 However, the luminescent lifetime of CZEO at 555
nm measured to be 15.6 ns indicates that it still belongs to
fluorescence, like the emission at 374 nm and 410 nm. Interestingly,
CZEO displays a persistent RTP at 555 nm with a lifetime of 739.6
ms after irradiated for 2 min under a 365 nm of UV light (i.e. photo-
activation). The time-resolved transient luminescence decays of
CZEO in solid without and with activation were shown in Fig. S5.
Phosphorescence spectrum of UV-activated CZEO excited at 364 nm
exhibits two main emissive peaks at 410 nm and 551 nm (Fig. 1c).
The former should be originated from the delayed fluorescence,
while the latter is ascribed to phosphorescence.33 The
photoactivated phosphorescence process was documented in Fig.
2a. Lifetime of CZEO in solid is improved from 15.6 ns to 6.6 ms
after 30 s of activation, and reaches 739.6 ms when activated for
120 s. After that, the increase of lifetime with activation time is no
longer prominent. Furthermore, the activated CZEO can deactivate
spontaneously when stopped irradiation of UV light. For instance,
the lifetime of CZEO activated for 2 min decreases from 739.6 ms to
0.03 ms after stopped irradiation for 560 s. It indicates that photo
activation makes CZEO molecules form a metastable state which
can yield RTP which lasts for several seconds after turned off the UV
light (Fig. 1d, Supplementary Movie 1). Consequently, a photo
Fig. 2 a) Lifetime change of CZEO in photoactivated and deactivated
process. b) Mechanoluminescence (ML) spectrum of photo-
activated CZEO. Insert: ML photograph of CZEO without and with
activation. Dimer structure (c) and packing structure (d) of pristine
CZEO in one unit cell: Unit of distance and angle is Å and degree (°),
respectively.
2 | J. Name., 2012, 00, 1-3
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To investigate the role of dimer in the luminescence of CZEO, equilibrium position. As a result, RTP of CZEO is invalid, because
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DOI: 10.1039/C8TC06202A
time-dependent density functional theory(TD-DFT) calculations non-radiation decay of triplet excitons is dominant. After activation,
were performed on the dimer derived from its single crystal tight dimer is formed and hard to be separated by external force,
structures by using B3LYP and 6-31G (d) as the function and the consequently causing ML invalid. On the other hand, strong
basis set, respectively. The computed vertical absorption properties interactions between two molecules of dimer suppress their
(Table S4) indicate that the first excited state (S1) transition occurs movement, which makes radiation decay of triplet excitons turn
predominantly between the HOMO and LUMO, and that S2 and S3 dominant, thus generating RTP. powder XRD curve (Fig. S11)
(degenerate states) are mainly corresponding to the transition from indicates that grinding impose little effect on crystal structure of
the HOMO to LUMO+1, and from HOMO-1 to LUMO, respectively. CZEO. it indirectly confirms that the formation and fracture of loose
The relative molecular orbital plots are shown in Fig. 3a. It is found molecular dimer of CEZO is responsible for the turn-off ML switch.
that the electron density in the HOMO and LUMO populates on two
Effect of photo stimulus on luminescence of CZ and a known
separate molecules in dimer, respectively. The electron transfer ML-active carbazole derivative, 9-isopropyl-9H-carbazole (CZIP),
from HOMO to LUMO occurs from one donor (D) molecule to was also investigated in the same air atmosphere. CZ and CZIP
another acceptor (A) molecule, and results in a charge transfer (CT) display phosphorescent emission at 555 nm and 550 nm,
state, which causes a large red-shift in the excited spectrum of corresponding to 612.0 ms and 82.1 ms of lifetime, respectively
crystal, relative to solution (Fig. S9).The formed CT states would (Fig. S12).The measured lifetime of CZ is different from the reported
bridge between the excited singlet and triplet states, which is one,33 which is should be attributed to the difference of crystal
helpful to create efficient ISC transitions.13, 41 It can be confirmed by quality. After activated for 2 min, the RTP lifetime of CZ at 555 nm
the calculated transition configurations of CZEO dimer in the singlet changes a little. However, significant improvement (about 10 times)
and triplet excited state (Table S5, Fig. S10). There are four triplet of lifetime has been found for CZIP. From the reported crystal
states (T2, T3, T4 and T5) with the same transition orbital structure of CZ and CZIP (Fig. S13 and S14),33, 41 we found that every
compositions as S1 and less than 0.3 eV of energy level difference CZ molecules can form four pairs of dimer with neighboring
between S1 and Tn. It means that four effective ISC channels molecules through C-H…π interaction. The compact packing of CZ
obtained from D-A dimer of CZEO.
can effectively stabilize the triplet state and result in persistent RTP
Both of single-crystal structure analysis and calculation results without photo-activation. It explains why UV irradiation imposes a
indicate that the dimer structure plays an important role in the little effect on the RTP. In the case of CZIP, four molecules form
luminescent properties of CZEO. Then, based on the dimer similar cluster as CZEO in packing structure through weak short
structure, the possible mechanism of turn-on RTP switch and turn- contact. However, the intermolecular interaction in cluster is
off ML switch of CZEO crystal is presented (Fig. 3b). Before obvious weaker than hydrogen bonding (C-H…O) in CZEO, which
activation, hydrogen bonding (O-H…O) fixes four CZEO molecules, makes interaction between dimer of CZIP relatively stronger than
which makes them form loose dimer severally with other four that of CZEO. Consequently, CZIP exhibits a longer lifetime of RTP
molecules. The loose D-A dimer with CT nature is easily separated (τ=82.1 ms) than CZEO without photo-activation. The lifetime of
by external mechanic force, consequently producing electronic field activated CZIP can increase to 833.9 ms for tighter dimer formed by
excited ML. Meanwhile, the loose D-A dimer can induce singlet photoinduction. Based on the result, we deduced that the ML
excitons turn into triplet excitons through ISC. However, due to activity of CZIP would turn invalid after photo-activation as CZEO.
flexibility of side chain, the loose dimer can vibrate and move at the The decuction was confirmed by experimental results that the
activated CZIP did not show ML.
The different light-stimulated responses of CZ, CZEO and CZIP
on their RTP can be employed to design two anti-counterfeiting
experiments. A Chinese word pattern was assembled in the first
experiment by three rectangular frames made from three kinds of
crystal powder, respectively (Fig. 4a). All of rectangular frames
show blue fluorescence under UV light. When turned off UV light,
only the frame of CZ emits orange light for persistent RTP. After
photo-activation of 10 s and 120 s, the pattern displays two orange
frames for RTP of CZ and CZIP, and three orange frames for RTP of
three compounds. In the second experiment (Fig. 4b), a benzene
ring pattern was obtained by assembling crystal powder of CZ, CZEO
and BCZEO. Under UV light, the pattern displays blue fluorescence
of CZ and CZEO, and orange RTP of BCZEO. When turned off UV
light, only persistent RTP of CZ can be observed. After photo-
activation of 120 s, orange RTP of both CZ and CZEO was found.
In summary, a simple and known purely organic molecule,
carbazole (CZEO) with 2-hydroxyethyl side chain, exhibit unique
light-stimulated responsive RTP and ML. From single crystal of
Fig. 3 a) Calculated frontier orbitals of CZEO dimer. b) Possible
mechanism of turn on RTP switch and turn off ML switch of CZEO
crystal: F, kP and knr presents fluorescence, rate constant of
phosphorescent emission and non-radiation decay, respectively.
CZEO, it is found that strong hydrogen bonding interactions
between side chain fix four molecules to form a cluster, and make
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10 Z. Xie, T. Yu, J. Chen, E. Ubba, L. Wang, Z. Mao, T. Su, Y.
each molecule form a loose D-A type dimer with another molecule
of the other cluster. Consequently, CZEO displays effective blue ML
for the fracture of dimer induced by external force, and invalid RTP
for the
View Article Online
Zhang, M. P. Aldred and Z. Chi, Chem. Sci., 2018, 9, 5787-
DOI: 10.1039/C8TC06202A
5794.
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and W. Yang, Chem. Commun., 2018, 54, 94-97.
12 Z. Cheng, H. Shi, H. Ma, L. Bian, Q. Wu, L. Gu, S. Cai, X. Wang,
W.-w. Xiong, Z. An and W. Huang, Angew. Chem. Int. Ed.,
2018, 57, 678-682.
13 J. Yang, X. Gao, Z. Xie, Y. Gong, M. Fang, Q. Peng, Z. Chi and
Z. Li, Angew. Chem. Int. Ed., 2017, 56, 15299-15303.
14 K. K. Neena, P. Sudhakar, K. Dipak and P. Thilagar, Chem.
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3
Fig. 4 Anti-counterfeiting application of Chinese word pattern (a)
and benzene ring pattern (b) employing CZ, CZEO, CZIP and BCZEO
as emitting materials.
6
5
, 1700116.
movement of dimer molecules which consuming the triplet state
energy. After photo-activation, the hydrogen bonding interactions
turn weak, while the D-A type dimer become compact. The tight
dimer inhibits the movement of molecules and makes CZEO
produce efficient orange and persistent RTP, and invalid ML. As a
result, photo-induced RTP switch (turn-on type) and ML switch
(turn-off type) have been realized successfully in a purely organic
material. It also provides a simple method to regulate RTP and ML
simultaneously, and a useful material applicable to the field of
security and anti-counterfeiting.
23
H. Yao, J. Wang, H. Chen, X. Mei, Z. Su, J. Wu, Z. Lin and
Q. Ling, RSC Adv., 2017, , 12161-12169.
24 X. Mei, J. Wang, Z. Zhou, S. Wu, L. Huang, Z. Lin and Q. Ling,
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and Takashi Kato, Adv. Mater., 2016, 28, 1073-1095.
26 X. Mei, K. Wei, G. Wen, Z. Liu, Z. Lin, Z. Zhou, L. Huang, E.
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This work was supported by the National Natural Science
Foundation of China (21374017 and 21574021), the Natural
Science Foundation of Fujian Province (2017J01684).
27 R. Zheng, X. Mei, Z. Lin, Y. Zhao, H. Yao, W. Lv and Q. Ling, J.
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3
29 Z. Lin, Y. Ma, X. Zheng, L. Huang, E. Yang, C.-Y. Wu, T. J. Chow
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Conflicts of interest
There are no conflicts to declare.
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DOI: 10.1039/C8TC06202A
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Journal of Materials Chemistry C
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DOI: 10.1039/C8TC06202A
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Photo-induced both phosphorescence and mechanoluminescence switch from a simple
purely organic molecule
Qiuqin Huang, Xiaofei Mei, Zongliang Xie, Duobin Wu, Shuming Yang, Wenjing Gong, Zhenguo
Chi, Zhenghuan Lin and Qidan Ling
A simple organic molecule, 2-(9H-carbazol-9-yl)ethanol (CZEO), displays both
mechanoluminescence (ML) and photo-induced room-temperature phosphorescence
(RTP). D-A type dimer formed in crystals play a crucial role in light-stimulated
luminescent response, and is responsible for turn-on RTP switch and turn-off ML
switch of CZEO.