Room-temperature phosphorescence-to-phosphorescence mechanochromism of a metal-free organic 1,2-diketone

Article abstract of DOI:10.1039/c9tc04176a

An organic 1,2-diketone, which exhibits color change of room-temperature phosphorescence (RTP) upon mechanical stimulation, is reported. Experimental and theoretical investigations revealed that this mechanochromism is based on a molecular rotational isomerization associated with a crystalline-to-amorphous transition. In general, amorphization weakens intermolecular interactions, resulting in RTP quenching. In contrast, the present diketone shows an even higher phosphorescence quantum yield after amorphization, representing the first RTP-to-RTP mechanochromism of a metal-free organic molecule. We have disclosed that a planar conformer of the diketone, which is responsible for the RTP in the amorphous state, involves intramolecular chalcogen bonding. Having this noncovalent interaction, the diketone would be a promising platform for the design of mechanochromic and RTP-active organic materials; flexible enough for a large conformational change, while rigid enough to exhibit efficient RTP in the amorphous state.

Full text of DOI:10.1039/c9tc04176a

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DOI: 10.1039/C9TC04176A
Journal Name
ARTICLE
Received 00th January
Room-Temperature Phosphorescence-to-Phosphorescence
Mechanochromism of a Metal-Free Organic 1,2-Diketone
2
0xx,
Yosuke Tani,* Morihisa Terasaki, Mao Komura and Takuji Ogawa*a
An organic 1,2-diketone, which exhibits color change of room-temperature phosphorescence (RTP) upon mechanical
stimulation, is reported. Experimental and theoretical investigation revealed that this mechanochromism is based on a
molecular rotational isomerization associated with crystalline-to-amorphous transition. In general, amorphization weakens
intermolecular interactions, resulting in RTP quenching. By contrast, the present diketone shows even higher
phosphorescence quantum yield after amorphization, representing the first RTP-to-RTP mechanochromism of a metal-free
organic molecule. We have disclosed that a planar conformer of the diketone, which is responsible for the RTP in the
amorphous, involves intramolecular chalcogen bonding. Having this noncovalent interaction, the diketone would be a
promising platform for the desing of mechanochromic and RTP-active organic materials; flexible enough for a large
conformational change, while rigid enough to exhibit efficient RTP in the amorphous state.
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
dione), many aromatic 1,2-diketones are known to undergo
photorotamerization around the vicinal dicarbonyl, from a skew
Introduction
Mechanochromic luminescence (MCL), a phenomenon where conformation in the ground state (and the Franck–Condon state)
luminescence color changes upon application of a mechanical to a trans-planar (TP) conformation at the S
and T minima
1
1
stimulus, has gained significant attention in recent years. In (Figure 1a).5 These compounds show faint but discernible
particular, MCL involving room-temperature phosphorescence phosphorescence from the relaxed TP conformers in deaerated
1
(
RTP) is of interest owing to its long lifetime; potential fluid media. By contrast, in low-temperature matrices,5-6 in host-
applications include sensors and security inks that can visualize guest complexes,7 or in crystalline form,8 rotational
a history of mechanical force in a time-resolved manner, without isomerization is disturbed so that a distinct color is emitted from
2
background fluorescence and scattered light. However, most of the skew conformation. However, to the best of our knowledge,
such materials utilize heavy metals to attain their RTP such environment-dependent color switching of organic
characteristics. Development of metal-free organic counterparts diketones has never been utilized for mechanochromic materials.
will expand the variation of material design and the field of their This is in part due to the lack of bistability as the TP conformer
applications, but is challenging because of their particularly slow is not metastable in the ground state.
spin-flip processes. Until now, reported metal-free organic
materials are mainly based on a “turn-off” of RTP, wherein RTP
3
is quenched upon mechanical stimuli. All of these materials
exhibit RTP only in a rigid crystal lattice, while RTP of
4
amorphous organic molecules is rarely achieved. Therefore,
when the mechanical stimulus breaks the lattice to induce
crystal-to-amorphous phase transition, their RTP will be
quenched.
The utilization of crystal-to-amorphous phase transitions is
expected to be the most accessible design of MCL materials, if
one can associate the amorphization with color change. In this
context, aromatic 1,2-diketones are interesting because they emit
distinct colors depending on the rigidity of their environment.5
As has been well-studied for benzil (1,2-diphenylethane-1,2-
a.Department of Chemistry, Graduate School of Science, Osaka University,
Machikaneyama 1-1, Toyonaka, Osaka 560-0043, Japan.
E-mail: y-tani@chem.sci.osaka-u.ac.jp; ogawa@chem.sci.osaka-u.ac.jp
Fig. 1 Schematic representation of molecular potential energy curves for
(a) benzil and (b) thenil, illustrating the skew and trans-planar (TP)
†
Electronic Supplementary Information (ESI) available: Synthetic procedures, single
conformations and relative energies. Note that the rigidity of the
environment will affect the height of the barrier to rotamerization.
crystal X-ray diffraction data, NMR spectra, physicochemical properties and
theoretical calculation. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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a) Photoluminescence (PL) Spectra of 1 and 2,2’-Dibromobenzilin Cyclohexane (TIPS = triisopropylsilyl)
DOI: 10.1039/C9TC04176A
phos
phos
under N2
under air
under N2
under air
fluor
phos 0.1%
phos = 13%
4
70
520
570
620
Wavelength /nm
670
720
450
500
550
600
650
700
750
800
850
Wavelength /nm
b) Solid-State PL Spectra of 1G and 1Y
c) Solid-StatePL Decay Curves of 1G and 1Y
1
0000
10000
1000
100
10
5
22
569
1G: <t> = 101 ms
1Y: <t> = 51 ms
t
t
1
= 39 ms (12%)
= 109 ms (88%)
t
t
1
= 32 ms (60%)
= 78 ms (40%)
1
G
Y

phos = 4%
1000
2
2
1
phos = 10%
100
1
0
1
1
0
200
400
600
800
0
200
400
600
800
4
70
520
570
620
Wavelength /nm
670
720
Time /ms
Time /ms
d) Powder X-Ray Diffraction Patterns
002)
single crystal
simulated)
e) Single-Crystal X-Ray Diffraction Structure of 1
(
(002)
(
b
c
as obtained: 1G
after grinding: 1Y
a
after exposing to
CHCl vapor
3
5
15
25
q /deg
35
45
2
Fig. 2 (a) Molecular structure, photoluminescence (PL) spectra, and phosphorescence quantum yields (phos) of 1 (left) and 2,2′-dibromobenzil (right) in
cyclohexane (2 × 10–6 and 4 × 10–5 M, respectively) at room temperature. (b) Solid-state PL spectra with phos and (c) PL decay curves of 1G and 1Y under
ambient conditions. (d) PXRD profiles of (from top to bottom) the single crystal of 1 (simulated), 1G, 1Y, and after exposure of 1Y to CHCl vapor, with
3
corresponding photographs taken under irradiation at 365 nm. (e) Left: single crystal X-ray structure of 1 with two out of four contacting molecules,
wherein short contacts are shown as blue–green dotted lines. Right: packing structure seen along the b axis. Hydrogen atoms are omitted for clarity. The
(002) plane is drawn as a white dotted line.
We envisioned that 2,2’-thenil, a thiophene analog of benzil, first example of RTP-to-RTP mechanochromism in a metal-free
can be a promising platform for the development of RTP organic compound.
luminophores as well as MCL materials, because its TP
conformer should become metastable thanks to intramolecular
chalcogen bonding (Figure 1b). Chalcogen bonding is a highly Results and discussion
directional interaction between chalcogenides and Lewis basic or
Thenil
1 (Figure 2a, inset) was readily synthesized from 2-
negatively charged functionalities, such as sulfur and carbonyl
oxygen, with a strength that is in some cases comparable to that
bromothiophene in three steps.11 To assess the effect of
chalcogen bonding, we also synthesized 2,2′-dibromobenzil and
compared their photophysical properties in cyclohexane (Figure
9
of hydrogen bonding. Similar noncovalent interactions such as
halogen bonding have been exploited in an intermolecular
manner to obtain rigid surrounding environment and facilitate
RTP.10 In contrast, intramolecular noncovalent interactions will
play vital roles in realizing RTP mechanochromism, by allowing
large conformational changes while stabilizing the TP conformer
and suppressing nonradiative decay. Herein, we successfully
utilize intramolecular chalcogen bonding to achieve efficient
RTP from a conformationally flexible molecule, and report the
2
a). The photoluminescence (PL) spectrum of 2,2′-
dibromobenzil was similar to the well-known spectrum of
benzil, consisting of fluorescence and phosphorescence, and its
phosphorescence quantum yield phos was 0.1% or below in
degassed cyclohexane. In contrast, the PL spectrum of
exhibited only structured phosphorescence with an emission
maximum (em) at 569 and 624 nm.12 Moreover, phos of
was
5
1
1
2
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as high as 13% at room temperature in degassed cyclohexane. As S17). Thus, the RTP of
we will discuss later, the RTP of
from the TP conformer, in which conformer the presence of
chalcogen bonding is experimentally and theoretically that the TP conformer constitutes only a minor portion of 1Y
1
in solution, and hence that of 1Y, are
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DOI: 10.1039/C9TC04176A
1
in solution is plausibly emitted plausibly emitted from TP conformer.
Second, absorption and excitation spectra of 1Y indicated
,
demonstrated. Therefore, this considerable enhancement of RTP however it is a principal component that leads to the yellow RTP
can be a representative of the contribution from chalcogen (Figures 3). The absorption spectrum of solid 1Y was similar to
bonding: making the TP conformer rigid to suppress the that of
1 in cyclohexane (Figure 3a). This result implies that: (1)
nonradiative decay, and/or activating the directed heavy atom 1Y is an ill-ordered state without any significant intermolecular
effect10a to facilitate intersystem crossing and radiative decay electronic interactions, and (2) the distribution of conformers in
from T
Encouraged by the intense RTP in solution, we investigated spectrum of 1Y (em = 570 nm, Figure 3a) was much different
the solid-state emission properties of . Recrystallization of from its absorption spectrum, exhibiting a distinct peak with abs
from CHCl
/methanol afforded a crystalline powder, 1G, which = 372 nm.13 According to TD-DFT calculation (B3LYP-D3/6-
1
.
1Y is similar to that in solution. Interestingly, the excitation
1
1
3
showed an unstructured emission with em = 522 nm at room 311Gd), this peak can be assigned to the TP conformer (Figure
temperature in air (Figure 2b). Scratching the solid using a 3b, orange line). On the other hand, the absorption spectra of
spatula induced an emission color change from green to yellow, solid 1Y and
1 in cyclohexane are similar to the simulated
accompanied by the emergence of a new peak at em = 569 nm. spectrum of skew conformer (Figure 3b, green line), indicating
By uniformly grinding the solid, we obtained the state hereafter that the major conformer in solution as well as in 1Y would be
denoted as 1Y, for which the peak at 522 nm was negligible. The the skew. These results suggest that a small amount of the TP
area-weighted mean PL lifetimes <t> of 1G and 1Y were 101 conformer exists in 1Y and is a principal component that
and 51 ms, respectively, confirming that both emissions are produces the yellow RTP. The presence of both the skew and TP
phosphorescence (Figure 2c). Here, the mechanical stimulation conformer was also supported by the PL spectrum of 1Y at 77 K,
did not quench RTP, but rather enhanced phos from 4% to 10%. which exhibited a dual emission from the skew and TP
Powder X-ray diffraction (PXRD) analysis of 1G revealed sharp conformer, with em = 524 and 570 nm, respectively (Figure
diffraction peaks, while that of 1Y showed broadened weak S11).
peaks, indicating that amorphization occurred upon grinding
(
Figure 2d). The green RTP and distinct PXRD pattern were
restored either by exposing 1Y to CHCl vapor or by heating it
Figures 2d and S1–S4). These results clearly demonstrated that
3
(
1.2
.0
0.8
a)
1
exhibits RTP-to-RTP mechanochromism based on a
1
1 1Y YA Ab bs so or pr pt itoi on n
1Y YE Ex xc ci ti at at i to i on n
crystalline-to-amorphous phase transition.
To further understand the mechanism of the observed
mechanochromism, structures in both phases were investigated.
1
i nA bc s- H. oe fx 1
in c-hexane
0.6
0.4
0.2
0.0
We obtained a single crystal of
3
1 from CHCl /hexane, whose X-
ray structure demonstrated a skew conformation with a dihedral
angle of the vicinal dicarbonyl to be 109.5(2)° (Figure 2e). The
PXRD pattern simulated from the crystal structure was in good
agreement with that of 1G (Figure 2d), confirming that
the skew conformation in 1G. Note that the diffraction peak at
q = 6.2° corresponds to the (002) plane, which sits between the
triisopropylsilyl (TIPS) groups of neighboring molecules (Figure
e, right). Considerable decrease of this peak upon grinding, and
1 also has
3
00
350
400
450
500
Wavelength /nm
2
1.2
0.8
0.6
0.4
0.2
0.0
b)
2
1.0
.8
0.6
T TP P
Skew
moderate recovery upon fumigation or heating, imply that the
atoms around this plane readily loose thier periodicity and are
disordered after recovery. For 1Y, on the other hand, following
three key results provide a plausible picture of the conformation
and dynamics.
SK
0
0.4
0.2
0.0
First, the PL spectrum of 1Y was almost identical to that of
1
in solution, suggesting the conformational similarity of the
emitting species (Figures 2a left and 2b). Similar to benzil, which
is known to emit from a TP conformer in solution, showed
280
330
380
430
480
Wavelength /nm
1
negligible solvatochromic shifts in the PL spectra (em <2 nm,
Figure S10 and Table S2).5b This is consistent with the transition
from the TP conformer of C2h symmetry, whose dipole moments
are cancelled out. In addition, time-dependent density functional
theory (TD-DFT) calculations indicated that the TP conformer is
Fig. 3 (a) UV-vis absorption (orange dotted line) and excitation spectra
(orange solid line, em = 570 nm) of the solid 1Y and absorption spectrum
of 1 in cyclohexane (blue). (b) Simulated absorption spectra of the TP
(orange) and skew (green) conformers of 1 with oscillator strength,
calculated at the TD-B3LYP-D3/6-311G(d) level of theory. Note that the x-
axes range 300–500 nm for the top while 280–480 nm for the bottom.
1
far more stable than the skew conformer in the T states (Figure
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bonding interaction in the crystal structure was evidVeienwtAfrtrioclme O:n(li1ne)
DOI: 10.1039/C9TC04176A
the S1–O1a distance of 2.707(2) Å, which is much shorter than
3.5
the sum of their van der Waals radii (3.32 Å), and (2) the C1–
S1–O1a angle of 167.4°, which suggests the alignment of an
oxygen lone pair (lp) with the “-hole” or the * orbital of the
S1–C1 bond. These structural features are corroborated by a
S1
3.0
natural bonding orbital (NBO) analysis of
describes the S–O interaction as lp(O)→*(S–C) donor–
acceptor interaction of 7.74 kcal/mol for the entire molecule
2. This analysis
T1
2.5
(
Figures 5b and S19). We expect that such reasonable chalcogen
bonding is also present in the TP conformer of and makes
1
important contribution to its MCL and RTP behavior.14
2
.0
.5
Taking these results into account, a plausible MCL
0
mechanism of
stimulus breaks the lattice and weakens the intermolecular
interactions, a minor portion of becomes TP conformer. The
1 could be as follows. When a mechanical
Int
Skew
TP
S0
1
yellow RTP occurs efficiently in the amorphous, because the TP
conformer enjoys intramolecular chalcogen bonding and is
inherently emissive without the aid of intermolecular
interactions. On the other hand, the remaining skew conformer
does not exhibit green RTP in such loose environment. Instead,
0
it decays to S through non-radiative channels, which may
include energy transfer to the TP conformers. In the PL spectrum
of 1Y at 77 K (Figure S11), the phosphorescence from skew
conformer emerged, indicating that the absence of the green RTP
in 1Y is (at least in part) due to thermally-activated non-radiative
decay pathway. Energy transfer should also contribute to some
0
.0
1
60
80
Around the Blue Bond
Reaction Coordinate: Dihedral Angle Relative to Int (degree)
0
80
160
Around the Green Bond
4
Fig. 4 Potential energy curve of isolated 2 along the dihedral angles of the
indicated bonds calculated at the B3LYP-D3/6-311G(d) level with a step of
0°. Energies for S
calculated for vertical excitation.
2
1
(black triangle) and T (orange diamond) were
1
Third, a potential energy curve along the rotational extent, since the excitation spectrum of 1Y has a non-negligible
isomerization coordinate was calculated in gas phase using 3,3’- intensity in the region 300–340 nm. Thus, in the present system,
dibromo-2,2’-thenil
2 as a model compound (Figure 4). The the mechanical stimulation switches the emitting species,
calculation revealed a triple-well potential, with the skew, TP, although not the whole but only a minor portion of molecules
and an intermediate less-stable conformer. The rotational changes the conformation.
–
1
barriers are estimated to be less than 10 kcal·mol . Although the
To provide clues for future molecular design, we would like
relative energy of these conformers varied depending on the to summarize substituent effects on the present RTP
9
calculation method and solvent effects (Table S4), TP is clearly mechanochromism (Figure 6). Thenil
one of the stable conformers in S . We also calculated and plotted TIPS groups, exhibited intense RTP in degassed cyclohexane
the energies for vertical excitations (S and T ), which further (phos = 22%). Solution-phase UV-vis absorption and emission
2, which has H in place of
0
1
1
supports that the RTP in solution is emitted from the TP
conformer.
To gain insight into the origin of the stability and unusual
RTP efficiency of the TP conformer, we examined the presence
of chalcogen bonding. Gratifyingly, we found that thenil
2 had a
TP conformation in its single crystal, although has the same
2
central structure as
1
(Figure 5a). The presence of a chalcogen
under ambient light
1
2
3
2
.707(2) Å
Br1a
C1
O1a
C5 C5a
crystal ground
under 365 nm
crystal ground
crystal ground
S1
C2
C4
C3
Br1
O1
3.056(2) Å
crystal ground
crystal ground
crystal ground
Fig. 5 Left) ORTEP structure from single crystal X-ray diffraction of 2.
Thermal ellipsoids are set at the 50% probability level. Right) Principal
lp(O)→*(S–C) NBO donor–acceptor interactions. Isovalue = 0.05.
Fig. 6 Molecular structures of thenil derivatives 1–3 and their photographs
before and after grinding. TIPS = triisopropylsilyl.
4
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emission. These results indicate that thenil is VaiewpArrtoicmle iOsnilninge
DOI: 10.1039/C9TC04176A
platform for designable mechano-responsive phosphorescent
materials. Studies aimed at developing novel MCL materials, as
well as the utilization of efficient RTP of thenil in amorphous
and even in solution, are now ongoing.
Before
Scratched
After 24 h
Conflicts of interest
Fig. 7 Slow self-recovering behavior of 1 at room temperature.
There are no conflicts to declare.
Photographs were taken under irradiation at 365 nm.
Acknowledgements
spectra were similar to those of
has little effect on the molecular photophysical properties
Figure S9). On the other hand, TIPS group affects the molecular
arrangement in solid-state. Single crystal of consists of TP
conformer arranged in a slipped 1D columnar packing with an
interplanar distance of 3.47 Å (Figures 5 and S6). Introduction
of TIPS group would prevent such columnar packing, weaken
intermolecular interactions that stabilize TP conformer, and
1, indicating that the silyl group
This work was supported by the JSPS KAKENHI (Grant Numbers
JP25110002, JP17K17863, and JP19K15542). Y.T. is grateful to
the Kyoto Technoscience Center for financial support. We are
very grateful for the assistance of Dr. Soichiro Kawamorita and
Prof. Dr. Takeshi Naota (Osaka University) with the low-
(
2
temperature
photoluminescence
measurement.
The
theoretical calculations were partly performed using the
Research Center for Computational Science, Okazaki, Japan.
make the molecules of
close packing, the emission of
1
in the crystal skewed. As a result of this
is quenched in the solid-state
2
despite its TP conformation (Figure 6), as is often the case in
planar aromatic molecules.15 It would be important to avoid
unfavorable electronic interactions in solid-state, for which the
introduction of a bulky substituent such as TIPS group is
effective. We also synthesized 5,5’-bis(triisopropylsilyl)-2,2’-
Notes and references
1
. (a) Y. Sagara, S. Yamane, M. Mitani, C. Weder and T. Kato, Adv.
Mater., 2016, 28, 1073-1095; (b) Z. Ma, Z. Wang, M. Teng, Z. Xu
and X. Jia, ChemPhysChem, 2015, 16, 1811-1828.
2. P. Xue, J. Ding, P. Wang and R. Lu, J. Mater. Chem. C, 2016, 4,
thenil
MCL behavior (Figure 5). Single-crystal X-ray structure analysis
revealed that has a more planar conformation than does, with
a dihedral angle of the vicinal dicarbonyl to be 152.9(1)° for
while 109.5(2)° for (Figures S7 and 2e). The bromo
substituents probably destabilize a planar conformation,16 so that
3, an analog of 1 that lacks Br. Thenil 3 showed PL but no
6
688-6706.
3
. (a) Z. Mao, Z. Yang, Y. Mu, Y. Zhang, Y. F. Wang, Z. Chi, C. C. Lo, S.
3
1
Liu, A. Lien and J. Xu, Angew. Chem. Int. Ed., 2015, 54, 6270-6273;
3
(
b) C. Li, X. Tang, L. Zhang, C. Li, Z. Liu, Z. Bo, Y. Q. Dong, Y.-H. Tian,
1
Y. Dong and B. Z. Tang, Adv. Opt. Mater., 2015, 3, 1184-1190; (c)
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Lu, Chem. Commun., 2015, 51, 10381-10384; (d) Y. Gong, G.
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Lu, A. Lien and M. R. Bryce, Angew. Chem. Int. Ed., 2016, 55,
1
forms crystal of the largely twisted conformer. Substituents on
3,3’-position of thiophene ring must be one of the controlling
factors for the molecular-level bistability of thenil, and they
would also affect its MCL behavior.
In addition to the mechano-responsive behavior, thenil
exhibits “slow self-recovering” characteristics (Figure 7). When
was gently scratched and left for 24 h at room temperature,
1
2
181-2185; (f) B. Xu, H. Wu, J. Chen, Z. Yang, Z. Yang, Y. C. Wu, Y.
Zhang, C. Jin, P. Y. Lu, Z. Chi, S. Liu, J. Xu and M. Aldred, Chem.
Sci., 2017, 8, 1909-1914; (g) H. Wu, P. Zhao, X. Li, W. Chen, H.
Ågren, Q. Zhang and L. Zhu, ACS Appl. Mater. Interfaces, 2017, 9,
1
G
the yellow RTP color almost completely returned to the original
green. Although the details are currently under investigation, it
could be caused by recrystallization promoted by the remaining
crystal domains,17 because thoroughly ground powder 1Y hardly 4. (a) Kenry, C. Chen and B. Liu, Nat. Commun., 2019, 10, 2111; (b)
3
865-3872; (h) G. He, L. Du, Y. Gong, Y. Liu, C. Yu, C. Wei and W.
Z. Yuan, ACS Omega, 2019, 4, 344-351.
showed self-recovering. Such properties may bring threshold
characteristics regarding time and force when visualizing the
mechanical force history.
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Conclusions
In summary, we have demonstrated that thenil
free organic compound exhibiting
mechanochromism. Mechanistic studies revealed that the bulk
MCL behavior of reflects a molecular-level conformational
bistability. Thus, the mechanical stimulation weakens
intermolecular interactions and transforms the conformation of a
minor portion of the molecules, leading to distinct colors of
1 is the first metal-
RTP-to-RTP 6. (a) T.-S. Fang and L. A. Singer, Chem. Phys. Lett., 1978, 60, 117-
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Vieira Ferreira, J. P. Da Silva and J. C. Moreira, J. Phys. Chem. B,
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J. Name., 2013, 00, 1-3 | 5
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J Po lue ran sael od fo Mn aot te rai ad l jsu Cs ht emm ai rs gt ri yn sC
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ARTICLE
002, 106, 12584-12593; (b) M. R. Ams, D. Ajami, S. L. Craig, J.-S.
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1
1
1
1. See Supporting Information (SI) for detail.
2. The lifetime was 37 ms in degassed cyclohexane (Figure S13).
3. In line with this, phos of 1Y depends on the excitation
wavelength, reaching the highest value of 0.10 with ex = 370 nm.
4. According to DFT calculation, optimized structure of TP
conformer of 1 has S–O distance of 2.71 Å and C–S–O angle of
1
1
69°, which are quite similar to those of 2 in the single crystal
(
Figure S16).
1
1
1
5. (a) J. B. Birks, Photophysics of Aromatic Molecules, Wiley,
London, 1970; (b) M. Shimizu and T. Hiyama, Chem. Asian. J.,
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010, 5, 1516-1531.
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structure of 2 (Figure 5a), while there are no donor-acceptor
interaction found in NBOs.
7. (a) S. Ito, T. Taguchi, T. Yamada, T. Ubukata, Y. Yamaguchi and M.
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6
| J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry C
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DOI: 10.1039/C9TC04176A
Table of Contents Entry:
An organic 1,2-diketone with intramolecular chalcogen bonding overcomes the room-
temperature phosphorescence (RTP) quenching problem on amorphization, representing
the first RTP-to-RTP mechanochromism of a metal-free organic molecule.
Phos-to-Phos Mechanochromism
Br
O
O
S
Br
H
R
R
S
S
R
O
Br
S
R
O
H
Br
Crystalline
Amorphous
Heat or Vapor


Metal-free phosphor
Rotational isomerism
 Amorphous RTP
 Chalcogen bonding