Bromine-Substituted Fluorene: Molecular Structure, Br–Br Interactions, Room-Temperature Phosphorescence, and Tricolor Triboluminescence

Article abstract of DOI:10.1002/anie.201811660

Organic tribophosphorescence materials are rarely reported and the introduction of Br atoms may be a practical way to design such materials. Here four bromine-substituted fluorene-based derivatives are presented and BrFlu?CBr, having fluorescence-phosphorescence dual-emission induced not only by UV light but also by mechanical stimulus, manifests the highest phosphorescence efficiency of 4.56 % upon photoirradiation. During the grinding process, three different triboluminescent spectra were identified. Upon introduction of a mechanical stimulus, the triboluminescence emission is cyan, whereas after an extended period it changed to blue. After removing the mechanical stimulus, green-white phosphorescent emission was observed. Careful research on single-crystal structures and theoretical calculations demonstrate that strong Br???Br interactions are vital to facilitate spin-orbit coupling and promote intersystem crossing, thus generating the unique properties.

Full text of DOI:10.1002/anie.201811660

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Accepted Article
Title: Bromine-Substituted Fluorene: Simple Molecular Structure,
Strong Br···Br Interactions, and the Resultant RTP and Unique
Tri-Color Triboluminescent Properties
Authors: Zhen Li
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201811660
Angew. Chem. 10.1002/ange.201811660
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10.1002/anie.201811660
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COMMUNICATION
Bromine-Substituted Fluorene: Simple Molecular Structure,
Strong Br--Br Interactions, and the Resultant RTP and Unique Tri-
Color Triboluminescent Properties
Jiaqiang Wang,[a] Can Wang,[a] Yanbin Gong,[a] Qiuyan Liao,[a] Mengmeng Han,[a] Tianjiao Jiang,[a]
Qianxi Dang,[a] Yaqin Li,[a] Qianqian Li[a] and Zhen Li*[a,b]
Abstract: Organic tribophosphorescence materials are rarely
reported and the introduction of Br atoms may be a practicable way
to design such materials. Here four bromine-substituted fluorene-
based derivatives are presented and BrFlu-CBr with fluorescence-
phosphorescence dual-emission induced not only by UV light but
also by mechanical stimulus manifests the highest phosphorescence
efficiency of 4.56% upon photoirradiation. During grinding process,
three different triboluminescent spectra were classified, namely tri-
color triboluminescence switching. At the very beginning, upon
mechanical stimulus, the triboluminescence emission is cyan; while
after a long time, it changed to blue. Excitedly, after stopping
mechanical stimulus, green-white phosphorescent emission was
observed. Careful research on single crystal structures and
theoretical calculations demonstrate that strong Br···Br interactions
are vital to facilitate spin-orbit coupling and promote intersystem
crossing, thus generating the unique property.
luminogens to increase the ISC efficiency.
As a matter of fact, many effective approaches, for instance,
crystallization, enhancing π-π interactions, adopting compact
face to face packing, expanding conjugation and self-assembly
in supramolecular frameworks, have been introduced to
organics with phosphorescence at room temperature, to figure
out the problem of the highly forbidden triplet-singlet
transitions.[6] Besides, heavy atom effect (HAE) is also known as
an impactful method to achieve the strong spin-orbit coupling
(SOC) between excited singlet and triplet states and increase
the intersystem crossing (ISC) rate constant.[7] On the other
hand, intermolecular halogen-bonding interactions, especially C-
Br···Br-C interactions, are not often investigated on metal-free
room temperature phosphorescence (RTP) system.[8] Thus, we
wonder whether the introduction of HAE with Br···Br interaction
could help to explore compounds possessing high
phosphorescence efficiency and ML property.
Thus, in consideration of the HAE and Br···Br interaction
mentioned above for the possibly enhanced ISC, we designed
four fluorene derivatives, namely Flu-CBr, BrFlu-CBr, 2BrFlu-
CBr and BrFlu-C (Figure 1), with simple molecular structure and
different number of bromine atoms on the fluorene rings and/or
alkyl chains, since fluorene and its derivatives are often used as
building blocks for π-conjugated polymers and oligomers in
opto-electronic materials, sensing and bioimaging agents and so
forth.[9] And actually, a bromo-substituted fluorene derivative has
been reported by Takeuchi et al. to display RTP with a
phosphorescence yield of 5.9% in solution.[10] Excitedly, BrFlu-
CBr crystals demonstrate distinct ML emissions upon grinding
and very high phosphorescence quantum yield at room
temperature (up to 4.56%) , in comparison with other three
crystals and some other RTP materials with simple molecular
structures as well, like naphthalimide and aromatic aldehyde.[4c]
More importantly, when those crystals were scraped at the very
beginning, cyan triboluminescence was observed. After stopping
mechanical stimulus, green-white tribophosphorescence emitted
out. Additionally, mechanical agitation for a long time produced
blue triboluminescence. Thus, tri-color ML emissions switching
was successfully realized. The PL and ML characterization,
crystal structures and theoretical calculations are presented in
detail and help to understand that strong Br···Br interactions
contribute to the resultant RTP and ML properties.
Mechanoluminescence (ML), including piezoluminescence,
triboluminescence and sonoluminescence, is luminescence
induced by mechanical stimulus on a solid.[1] With potential
applications in area such as lighting source, pressure sensor
and display device, mechanoluminescence is still attracting
widespread interest despite its long-standing history, which
could trace back to the first report by Francis Bacon in 1605.[1,2]
However, the number of purely organic ML luminogens remains
limited and many organic ML compounds only have UV
emission band, thus leading to hindrance to explore the ML
process and achieve practical application.[1b,3] So there is an
urgent need to develop much more ML materials to learn about
the relationship between mechanoluminescence phenomena
and molecular structures. As necessary process in
photoluminescence or electroluminescence for thermally
activated delayed fluorescence (TADF) materials and
phosphorescence materials, the intersystem crossing (ISC) has
been well investigated.[4] However, the ISC transition upon
mechanical stimulus is rarely mentioned. Recently, our group
presented the first report of unusual mechanophosphorescence
luminogen with AIE property and successful marriage of ML and
ISC.[5] In consequence, it is possible to rationally design ML
[a]
[b]
J. Wang, C. Wang, Y. Gong, Q. Liao, M. Han, T. Jiang, Q. Dang, Y.
Li, Prof. Q. Li, Prof. Z. Li
Department of Chemistry
Wuhan University, Wuhan 430072, China. Fax: (86)27-68756757
E-mail: lizhen@whu.edu.cn or lichemlab@163.com
Prof. Z. Li
Flu-CBr, BrFlu-CBr, 2BrFlu-CBr and BrFlu-C were
conveniently synthesized by the reaction of fluorene or bromosu-
bstituted fluorene and alkyl bromides (Supporting Information,
Scheme S1). The molecular structures were fully characterized
1
by H NMR, 13C NMR, elemental analysis and single-crystal X-
Institute of Molecular Aggregation Science, Tianjin University,
Tianjin
ray diffraction. The UV-Vis absorption spectra and
300072, China
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201811660
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COMMUNICATION
light produced
luminophore
bromine atom
Sn
S1
kISC
T1
S0
higher Φp
heavy atom effect
How about the
introduction of
Br atoms?
Φp
TL
1.22%
4.56%
0.79%
0.39%
mechanical stimulus
mechanical stimulus
Figure 1. The influence of HAE, the combination of fluorene derivatives and HAE and multi-color switching of triboluminescence for BrFlu-CBr upon mechanical
stimulus. E: excitation; IC: internal conversion; F: fluorescence; P: phosphorescence; Φp: the quantum yield of phosphorescence emission for crystals of the four
compounds; TL: triboluminescence.
photoluminescence (PL) spectra of these four compounds were
measured in tetrahydrofuran (THF) solution. They all showed
similar shapes (Supporting Information, Figure S1), indicating
that there are minor differences for the electronic structures of
these molecules, which could be reflected in density functional
theory (DFT) results.
The emission patterns of crystals for Flu-CBr, BrFlu-CBr,
2BrFlu-CBr and BrFlu-C all consist of fluorescence and
phosphorescence segments with different luminescence
efficiencies (Supporting Information, Figure S2). The
peaksmainly locate in the ultraviolet region and their lifetimes
are similar. The peaks of RTP emissions exhibit microsecond-
order delays with tiny discrepancies, whereas BrFlu-CBr has
much higher phosphorescence efficiency (4.56%). Accordingly,
more than 20 ms after UV excitation off, only the
phosphorescence of BrFlu-CBr could be detected (Supporting
Information, Figure S3). The photophysical data of crystals of
these fluorene derivatives at room temperature are summarized
in Table S1 (Supporting Information). It is worth mentioning that
the transition of excitons from singlet to triplet with efficient
ISC.[13] Furthermore, after continuous grinding, the crystalline
sample was converted into powder with weak blue emission
under the mechanical stimulus (Figure 2d, inset). From the ML
spectrum at this state (Figure 2d), we found the relative intensity
of the band in long-wavelength region decreased. Different ML
emissions with obviously different spectra in various states
suggest that the molecular packing is of great importance to the
unique ML property. Mechanical grinding or scraping will lead to
the loss of crystalline structure, during which, BrFlu-CBr sample
undergoes tricolor ML emission switching.
RTP
UV on
f
a
e
crystal
crystal
ground lightly
ground heavily
FL
stop scraping
ground
UV off
Ⅱ
Ⅰ
Ⅲ
scraping
ML-at the very beginning
b
c
5
10 15 20 25 30 35 40 45 50
2/degree
105
104
103
102
101
100
g
crystal
ground lightly
ground heavily
the rate constants of intersystem crossing (kISC
)
and
phosphorescence (kP) of BrFlu-CBr are 1.008×108 s-1 and 71.40
s-1, which are much higher than those of other three luminogens,
indicating the most efficient ISC in this process.[11] Therefore,
further investigations were performed to find out if BrFlu-CBr
was ML-active similar to other ML luminogens[12] with such
efficient ISC.
0
5
10
15
20
25
30
35
40
45
50
Time (ns)
ML-after stopping
mechanical stimulus
ML-after mechanical stimulus
for a long time
d
300
350
400
450
500
550
600
650
700
750
0
2
4
6
8
10
Wavelength (nm)
Time (ms)
By scraping or grinding the crystalline samples at room
temperature, there was cyan triboluminescence produced on
BrFlu-CBr as expected, demonstrating its unique ML
characteristic. As shown in Figure 2b, BrFlu-CBr crystals exhibit
Figure 2. a) PL spectra of crystals (top) and powder after ground lightly
(bottom) for BrFlu-CBr. Inset: The luminescent photos of crystals for BrFlu-CBr
before and after UV excitation (365 nm) off. b) ML spectrum of BrFlu-CBr in
the beginning. Inset: The corresponding ML image. c) ML spectrum of BrFlu-
CBr after stopping mechanical stimulus. Inset: The corresponding ML image.
d) ML spectrum of BrFlu-CBr after mechanical stimulus for a long time. Inset:
The corresponding ML images. e) The calculated CIE coordinates in CIE 1931
color space chromaticity diagram based on the ML spectra at different time: Ⅰ.
at the very beginning; Ⅱ. after stopping mechanical stimulus; Ⅲ. After
mechanical stimulus for a long time. f) XRD patterns of BrFlu-CBr in different
states. g) Phosphorescence decay curves of BrFlu-CBr in different states.
Inset: Fluorescence decay curves of BrFlu-CBr in different states.
a
fluorescence-phosphorescence dual-emission, which is
coincident with its PL spectrum with similar proportion of
fluorescence and phosphorescence. It is mechanoluminescence
at the very beginning. Further serious study helps to make out
that after stopping grinding the ML spectrum we recorded
(Figure 2c) is different from that at first. Actually, the light is
green-white and the difference is apparent under soft light
(Figure 2e). The transient ML spectrum corresponds well with
the phosphorescence spectrum of BrFlu-CBr crystals, powerfully
confirming that triplet excitons could be generated in the ML
process. In other words, the mechanical stimulus also induces
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10.1002/anie.201811660
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The mechanochromism (MC) effect of BrFlu-CBr was also
investigated. Accompanying with grinding, dropping of the
phosphorescence band could be observed (Figure 2a, bottom).
Powder X-ray diffraction (PXRD) patterns were also carried out
to evaluate the changes in crystals (Figure 2f). BrFlu-CBr
crystals exhibit intensely sharp peaks, illustrating the good
crystallinity. After light grinding, these peaks decrease with
diminished intensity, which convert to broader peaks upon heavy
grinding. This means the crystals were destroyed to some extent
under the mechanical stimulus, and some strong intermolecular
interactions should disappear or largely weaken. In addition,
Figure 2g shows an apparent decrease of phosphorescence
lifetime upon grinding, while fluorescence lifetime does not have
evident change in different states. It proves that intersystem
hindrance of bromine atoms could not be balanced with the
Br···Br interaction. Consequently, it could only adopt stable
conformation with long distances between bromine atoms. Thus,
there are many bromine atoms but no strong Br···Br interactions
and low phosphorescence efficiency for 2BrFlu-CBr crystals.
Furthermore, some theoretical calculations were carried out.
The configurations of the excited state and possible ISC of
monomers and dimers were calculated by the time-dependent
density functional theory (TD-DFT). When triplet state (Tn)
contains the same transition configuration components as that in
singlet state (S1) with its energy level difference determined to
be within ±0.3 eV, the transition from S1 to Tn is commonly
considered to be the ISC channel.[5,13] For isolated BrFlu-CBr,
although there are two main channels with two minor channels,
crossing is weakened after grounding, which facilitates
a
quantitative analysis of the mechanical process. It comes to a
preliminary estimate that crystallization is important to the
photophysics properties of the compounds.[6a,14]
a
b
To gain deeper insight into the origin of the above
phenomena, single crystal structures and molecular packing of
Flu-CBr, BrFlu-CBr, 2BrFlu-CBr and BrFlu-C were examined
carefully. It is determined to be the intermolecular interaction
when the distance is within the critical value of 4 Å. Thus, it is
significant to note that there are one kind of Br···Br interaction
(−C−Br···Br−C− interaction with a distance of 3.577 Å) in Flu-
CBr crystals (Figure 3a) and two kinds of Br···Br interaction
(=C−Br···Br−C− interaction with a distance of 3.125 Å and
−C−Br···Br−C− interaction with a distance of 3.528 Å) in BrFlu-
CBr crystals (Figure 3b), while there are no Br···Br interaction in
2BrFlu-CBr and BrFlu-C crystals (Figure 3c and 3d), which give
rise to different intersystem crossing efficiency among these four
compounds. Indeed, each molecule shares two Br···Br
interactions with neighboring molecules in the single crystal
structure of Flu-CBr (Figure 3a, inset), while the number for
BrFlu-CBr is four (Figure 3b, inset). Wayne White et al.[15] noted
that the addition of sodium iodide to 2-naphthalene sulfonate
solution at room temperature dramatically increases the
phosphorescence intensity, which is known as the external
heavy atom effect. Here, Br···Br interactions play a pivotal role
like the external heavy atom effect to increase the SOC and
promote the ISC between singlet and triplet states. Moreover,
with the aid of strong Br···Br interactions, they form rigid
structures, through which, the vibrational loss of triplets and
molecular slippage under a stimulus of mechanical force are
largely suppressed. The synergy between efficient ISC and the
stabilization of triplet excitons is accountable for the high
phosphorescence efficiency of dual-emissive BrFlu-CBr.
Molecular slippage may cause energy loss through non-radiative
relaxation channels upon mechanical stimulus, as described in
previous literatures.[16] Once being ground, the ordered crystal
lattice collapses and transforms into microcrystals. Excited
states produced during this process including stable triplet states
mentioned above could return to the ground state via radiative
c
d
Figure 3. Molecular stacking of four fluorene-based derivatives: a) Flu-CBr,
b)BrFlu-CBr, c)2BrFlu-CBr, d)BrFlu-C. The green dot lines represent Br···Br
interactions. The pictures inset show the examples of molecules with Br···Br
intermolecular interactions.
a smallest energy gap of 0.1052 eV can be only found in a minor
channel between S1 and T5 (Figure 4). When it comes to dimers
of the compound, increased number of ISC channels and
decreased gap of energy levels are both obtained. In BrFlu-CBr
crystals, T11 and T12 in dimer1 and T9 and T11 in dimer2 are close
to S1, and the transitions from S1 to them are main ISC channels.
They show small energy gaps from 0.0087 to 0.0302 eV, which
are reduced by a large margin compared to those in monomers.
Additionally, three minor channels exist in dimer1. As to Flu-CBr,
the main channels in dimers have quite low value of ∆ (S1-Tn),
but they are also the single channel in each dimer (Supporting
Information, Figure S17). The energy levels of Tn of main
channels for 2BrFlu-CBr and BrFlu-C lie more far from S1
(Supporting Information, Figure S18 and S19). Thus, it is
obvious that strong Br···Br interactions help to develop effective
interactions in dimers, such as C−H···Br−C and C−H···π
interactions. Strong SOC and enhanced ISC are realized while
energy loss through non-radiative relaxation channels is
restricted, thus achieving high phosphorescence efficiency upon
photo or mechanical force excitation.
pathway,
as
the
result,
mechanofluorescence
and
mechanophosphorescence appear. Once crystals are crushed,
the restrictions on non-radiative transition of triplets diminish and
the phosphorescence emission efficiency decreases evidently.
And the destabilization of triplet states for BrFlu-CBr could be
reflected in its ML spectrum. As to 2BrFlu-CBr, the steric
The important role of Br···Br interactions in crystalline state at
room temperature also could be indirectly proved at low
temperature.[17] At 77 K, the lifetime of delayed component of
BrFlu-CBr solution is not very different from others. Relatively,
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the phosphorescence lifetime of BrFlu-CBr crystals has a huge
increase (Supporting Information, Figure S4d). After UV
excitation off, BrFlu-CBr crystals showed more intense
phosphorescence
Keywords: triboluminescence • room-temperature
phosphorescence• heavy atom effect • multi-color switching
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of phosphorescence for BrFlu-CBr crystals at low temperature
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BrFlu-CBr
crystals
exhibit
fluorescence-
phosphorescence dual-emission under UV light or mechanical
stimulus. Upon mechanical stimulus for a long time, the ML
emission of BrFlu-CBr could change from cyan to blue because
of the destruction of the crystalline structure, while after stopping
mechanical stimulus, green-white phosphorescence could be
observed. This is the first example of multi-color switching of
triboluminescence tuning by phosphorescence intensity in purely
organic compound. On the basis of careful investigation of single
crystal structure and theoretical calculation, it is concluded that
strong Br···Br interactions are vital to facilitate SOC and promote
ISC, resulting in unique properties for BrFlu-CBr. This validates
that adopting strong halogen interactions with external HAE may
be feasible to develop more RTP and tribophosphorescence
materials, opening a new avenue for the further development of
ML luminogens with RTP property.
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Acknowledgements
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We are grateful to the National Science Foundation of China
(no.21734007, 51573140) and Hubei Province (2017CFA002)
for financial support.
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This article is protected by copyright. All rights reserved.

10.1002/anie.201811660
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Jiaqiang Wang, Can Wang, Yanbin
Gong, Qiuyan Liao, Mengmeng Han,
Tianjiao Jiang, Qianxi Dang, Yaqin Li,
Qianqian Li and Zhen Li*
Bromine-Substituted Fluorene:
Simple Molecular Structure, Strong
Br···Br Interactions, and the
Resultant RTP and Unique Tri-Color
Triboluminescent Properties
BrFlu-CBr crystals exhibit fluorescence-phosphorescence dual-emission under mechanical stimulus wihout excitation by a UV
lamp. Upon mechanical stimulus for a long time, the TL emission of BrFlu-CBr could change from cyan to weak blue, while
after stopping mechanical stimulus, green-white phosphorescence could be observed, which is multi-color switching of
triboluminescence tuning by phosphorescence intensity.
This article is protected by copyright. All rights reserved.