Bright NUV mechanofluorescence from a terpyridine-based pure organic crystal

Article abstract of DOI:10.1039/c7cc08064f

A bright NUV mechanofluorescent terpyridine-based material with a piezoelectric space group was obtained. The single crystal analysis and quantum chemical calculations indicated the existence of enhanced intermolecular and intramolecular interactions and

Full text of DOI:10.1039/c7cc08064f

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Bright NUV mechanofluorescence from a terpyridine-based pure
organic crystal
Qikun Sun,a Liangliang Tang,a Zhenzhen Zhang,a Kai Zhang,a Zongliang Xie,b Zhenguo Chi,b Haichang
Zhang,*a Wenjun Yang*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A bright NUV mechanofluorescent terpyridine-based material with recently, some aggregation-induced emission (AIE) molecules
piezoelectric space group was obtained. The single crystal analysis have been found to be ML-active materials emitting blue and
and quantum chemical calculation indicated the existence of green light.7-8 However, bright organic ML luminogens were
enhanced intermolecular and intramolecular interactions and still limited in number and kind, and the understanding on the
intersystem crossing, but which did not facilitate the room crystal structure–ML activity and the luminescence mechanism
temperature phosphorescence.
were not comprehensive and clear. Moreover, there have
been few reports on organic ML luminogens with bight NUV
emission and no AIE effect at present.8 We considered that
NUV organic ML emitters could not only help ones to further
understand ML process but also serve as the energy donor
(host) to generate various visible ML and expand the color
gamut. Therefore, to broaden the ML family and deepen the
understanding on ML mechanism, it was necessary to exploit
other kinds of bright ML organic materials with non-AIE effect
and NUV emission.
Mechanoluminescence (ML) was first pointed out by Francis
Bacon in 1605 based on his attentive observation on hard
sugar fracture.1 This type of luminescence could be excited by
various external mechanical actions, such as stretching,
cleaving, cutting, rubbing, grinding, shaking or scratching,
sonication, etc.2 It was predicted that nearly half of all solids
probably showed ML phenomenon.3 However, for centuries,
inorganic salts and metal-organic complex have been the
mainstay of ML materials, and only about 400 papers were
published before the 21st century.2–4 Recently, an increasing
interest in ML has emerged as evidenced by more 400 papers
published within the past 17 years, owing to the re-recognition
of its potential applications in display, lighting, bioimaging, and
stress sensing5 and its academic significance in understanding
fundamental photo-physical process.
Based on the above considerations, in the current work, we
have designed and synthesized a terpyridine derivative (BP-TPY
Scheme 1) to investigate its mechanoluminescence (ML),
photoluminescence (PL), and crystal structure. The details for
synthesis and characterization were depicted in the Supporting
Information. As BP-TPY has a short conjugation and a branched
configuration, the efficient NUV emission could be expected.
The polar boric acid ester and lone pair electron-containing
pyridyl units should render the crystal strong intermolecular
and intramolecular interactions. Moreover, this noncentro-
symmetric tapered molecule could be prone to stack in the
piezoelectric space group. Thus, a bright NUV emission could
be expected under light and mechanical stimuli.
Conjugated organic luminophores might be promising ML
candidates because of their structure diversity, intrinsic
luminescence, easy accessibility and modification. However,
Their ML properties started to draw serious research attention
only within the past few years,6 Generally, the crystals with
piezoelectric space groups (dipolar structures and noncentro-
symmetric molecular arrangements), strong intermolecular
and intramolecular interactions and crystalline fluorescence
were expected to be promising ML candidates. For example,
a.Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province
(QUST), School of Polymer Science & Engineering, Qingdao University of Science
& Technology, 53-Zhengzhou Road, Qingdao 266042, P. R. China.
haichangzhang@hotmail.com; ywjph2004@qust.edu.cn.
b.State Key Laboratory of Optoelectronic Material and Technologies, School of
Chemistry and Chemical Engineering, Sun Yat-sen University Guangzhou, 510275
P. R. China. chizhg@mail.sysu.edu.cn.
Scheme 1. Structure and synthetic route of BP-TPY
† Electronic Supplementary Informaꢀon (ESI) available: Details for synthesis and
characterization Figure S1–Figure S6, crystallographic data and CCDC code and
others. See DOI: 10.1039/x0xx00000x
Figure S1 showed the absorption and PL spectra of BP-TPY in
THF solution and solid states. The light absorption for both
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solution and solid film mainly appeared before 330 nm with 2), these light emissions should occur from the same excited
the peak at 300 nm. Under 300 nm light excitation, BP-TPY states, regardless of the mode of excitation.
emitted NUV light with the emission peaks at 358 and 368 nm
for the solution and the solid, respectively, and a good color
purity evidenced by the narrow full width at half-maximum of
35 nm. The corresponding fluorescence efficiency were 15%,
13%, respectively, which was commendable in view of its large
band gap,small size, and low conjugation degree. BP-TPY could
be regarded as a non-AIE but multi-state emission organic dye
although its fluorescence efficiency was decreased by 50% in
the aqueous media (Figure S2).
Figure 2. (a) PL and ML images of crystalline BP-TPY powder and grain at 298 K in dark
room. (b) ML spectrum of BP-TPY crystal grain upon grinding with a spatula in dark
room at room temperature and the comparison with its PL spectra recorded at 77 k
and 298 K under 300 nm excitation.
To gain an insight into the effect of crystal structure on ML
effect, the single crystal of BP-TPY was cultured and analyzed.
The result revealed that BP-TPY adopted two different twisted
conformations to pack each other and form a monoclinic
crystal system with the noncentrosymmetric and piezoelectric
Figure 1. PL images of BP-TPY in solid state and THF solution taken at different times
space group Pc (Figure S5, Table S1). The twisted backbone
before and after turn-off of 254 nm excitation light (upper) and the PL spectra (down)
and small-size conjugation without any electron donor moiety
contributed to the NUV emission. As expected, there were
indeed strong intermolecular and intramolecular interactions
in BP-TPY crystal (Figure 3), which should largely reduce the
possible energy loss through non-radiative relaxation under
the mechanical stimulus. These results could explain bright
mechanoluminescence of BP-TPY crystal. On the other hand, it
was known that the crystal fracture could separate electrical
charges and ionize air (such as nitrogen), and the discharge of
the ionized nitrogen could emit a multiple-peak spectrum in
the range of 310–430 nm with fine six-finger structure.[4a]
Since the strong absorption of BP-TPY was before 310 nm,
together with the spectral identity between light and
mechanical excitation, the structure-less ML should result from
the direct excitation of crystal fracture-induced piezoelectric
field, rather than the secondary excitation of nitrogen
discharge emission on BP-TPY crystal. It was noticed that the
under 300 nm excitation at 77K.
Under UV light excitation, BP-TPY crystal and solution all
exhibited the remarkable fluorescence–phosphorescence dual
emission at 77 K (Figure 1), but only fluorescence emission was
observed at room temperature (Figure S1). BP-TPY crystal had
a short (2.4 ns) and a long (1.53 s) luminescence lifetimes at 77
K but only a short fluorescence lifetime of 1.9 ns at room
temperature (Figure S3). When the crystalline powder of BP-
TPY was ground in a glass bottle using metal or plastic spade in
dark room, a NUV light emission could be observed by naked
eye and video recorded by digital camera (iPhone 7, Figure 2
and Figure S4). Unfortunately, the emission spectrum was not
able to analyzed by a CCD spectrometer, probably due to that
the light signal was not enough strong. In order to enhance the
brightness, the bulky crystal grain was prepared by slowly
evaporating its chloroform/methanol solution and ground as
described for the crystalline powder. As expected, a bright
deep blue light could be saw and well analyzed by a CCD
spectrometer, which afforded a UV emission spectrum with
the peak at 372 nm. Since ML spectrum matched very well
with the room-temperature photofluorescence spectra (Figure
intermediate
Br-TPY
crystal
exhibited
unique
photoluminescence but no ML activity (Figure S6), which was
not surprising since the present each identified ML compound
is still an isolated event and its ML activity was impossibly
predicted in advance.
2 | J. Name., 2012, 00, 1-3
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Recently, Li et al. have found an unique example of the from the energy loss possibly through non-radiative relaxation
fluorescence–phosphorescence dual mechanoluminescence at channels from T1 to S0 as well as the endothermicity of ISC
room temperature from a m-terphenyl boric acid ester (DPP- process from S1 to Tn, which could significantly decrease the
BO)6f. By comparing the optical properties of BP-TPY and DPP- phosphorescence emission probability. In other words, ML
BO, we could find that they had the same photoluminescence features and mechanisms still remained complicated and
behaviors, such as fluorescence–phosphorescence dual photo- ambiguous and were dependent on the studied compound
luminescence at low temperature and only photofluorescence structures. Enhancing the ISC transitions was only necessary
at room temperature. However, why BP-TPY only exhibited the but not sufficient conditions for the spin-forbidden triplet-to-
mechanofluorescence, which required further investigations.
singlet phosphorescence emission.
Figure 4. Energy level diagram of the two closely coupled BP-TPY molecules and the
possible channels for the intersystem crossing from singlet S1 to triplet Tn. There were 9
Tn states containing 19 possible channels for ISC based on the same transition orbital
compositions between Tn and S1 within ± 0.3 eV energy level (S0 = 0 eV).
In summary, a new terpyridine-based ML material (BP-TPY)
with bright NUV emission has been obtained. BP-TPY is a multi-
state emission and non-AIE-active luminophore, and the ML
spectrum is almost the same as its PL spectra. There are strong
intermolecular and intramolecular interactions and much
increased number of ISC transitions in BP-TPY crystals with
multiple heteroatoms, but room-temperature photo- and
mechano-phosphorescence are not observed. Therefore, for
spin-forbidden phosphorescence emission, enhancing the ISC
transitions was only necessary but not sufficient conditions,
and ML features and mechanisms still remain ambiguous and
seem to be dependent on the studied molecular structures.
NUV ML emissions are very meaningful because they could not
only help to understand the ML process but also serve as the
energy donor to generate various visible ML emissions by in-
situ exciting other fluorescent dopants. The investigation on
the “BP-TPY as host function” is another interesting subject
and is underway in our laboratory.
Figure 3. The intermolecular and intramolecular interactions in BP-TPY single crystal.
There were four strong intramolecular CH···N interactions for each molecule, and
intermolecular four CH···π, one CH···N, and one CH···O interactions between the two
closely coupled molecules.
Generally, luminophores containing multiple heteroatoms
like N, P, and O were prone to phosphorescence emission
since the lone pair electrons should favour the n–π* transition
and populate triplet excitons through facilitating the spin-
forbidden transition of singlet-to-triplet excited states (inter-
system crossing, ISC). Moreover, by analyzing the packing
structure of BP-TPY crystal, we found that the intermolecular
and intramolecular interactions in BP-TPY crystal were
significantly stronger than those in DPP-BO one, in which the
additional C-H···N interactions were observed besides of
shorter C-H···O interaction distance (2.518 Å versus 3.184 Å)
for former. These interactions were considered to be much
beneficial to the n–π* transition and the ISC enhancement. To
further understand the ISC feature, TD-DFT calculation was
carried out based on a pair of strong interacted BP-TPY derived
from the single crystal structure (Figure 4). It was found that
there were 9 Tn states containing 19 possible channels for ISC
based on the same transition orbital compositions between Tn
and S1 within ± 0.3 eV energy level9 (Table S2 in the Supporting
Information). The much increased number of ISC transitions
for BP-TPY than DPP-BO should be much more favourable for
populating triplet excitons. Thus, the absence of room-
temperature phosphorescence in BP-TPY crystal should result
Financial support for this research was provided by the
National Natural Science Foundation of China (No. 51673105,
51573081, 51473084). We thank Open Project of the State Key
Laboratory of Supramolecular Structure and Materials of Jilin
University (SKLSSM-201728).
Conflicts of interest
There are no conflicts to declare.
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DOI: 10.1039/C7CC08064F
Bright NUV mechanofluorescence from a terpyridine-based
pure organic crystal
Qikun Sun,a Liangliang Tang,a Zhenzhen Zhang,a Kai Zhang,a Zongliang Xie,b
Zhenguo Chi,b Haichang Zhang,*a Wenjun Yang*a
aKey Laboratory of Rubber-plastics of Ministry of Education/Shandong Province (QUST), School
of Polymer Science & Engineering, Qingdao University of Science &Technology, 53-Zhengzhou
Road, Qingdao, 266042, P. R. China. E-mail: haichangzhang@hotmail.com ；
ywjph2004@qust.edu.cn.
bState Key Laboratory of Optoelectronic Material and Technologies, School of Chemistry and
Chemical Engineering, Sun Yat-sen University Guangzhou, 510275 ， P. R. China.
chizhg@mail.sysu.edu.cn.
Bright NUV mechanofluorescence from a terpyridine-based pure organic crystal with
non-centrosymmtric packing and piezoelectric space group.