Bromo induced reversible distinct color switching of a structurally simple donor-acceptor molecule by vapo, piezo and thermal stimuli

Article abstract of DOI:10.1039/c5tc00109a

Altering the luminescence properties of a material through external factors is an attractive feature that has the potential for various luminescence-related applications. Here, we report the synthesis and luminescence properties of two anthracene-based do

Full text of DOI:10.1039/c5tc00109a

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Bromo induced reversible distinct color switching
of a structurally simple donor–acceptor molecule
by vapo, piezo and thermal stimuli†
Cite this:DOI: 10.1039/c5tc00109a
Pachaiyappan Rajamalli, Parthasarathy Gandeepan, Min-Jie Huang and
Chien-Hong Cheng*
Altering the luminescence properties of a material through external factors is an attractive feature
that has the potential for various luminescence-related applications. Here, we report the synthesis
and luminescence properties of two anthracene-based donor acceptor compounds, N,N-di-p-
tolylanthracen-9-amine (TAA) and 10-bromo-N,N-di-p-tolylanthracen-9-amine (TAAB). In the solid
state, the bromo-substituted compound TAAB shows reversible visible switching of the emission by
external stimuli such as solvent, mechanical grinding and temperature. Single crystal X-ray studies,
powder-XRD analysis and theoretical calculations reveal that switchable emission originated from the
different stacking modes of TAAB. Furthermore, we have found that the bromo group interaction in the
solid state plays a crucial role in this tunable emission. The other anthracene-based compound TAA
does not show such switching of the emission by external stimuli. The observed changes in the
luminescence of TAAB by external stimuli suggest potential applications in rewritable optical media,
sensors, and optoelectronic devices.
Received 13th January 2015,
Accepted 9th February 2015
DOI: 10.1039/c5tc00109a
www.rsc.org/MaterialsC
as light,4 mechanical force,5 vapour6 and temperature,7 have
been reported.
1. Introduction
Stimuli-responsive organic luminescent materials are of great
In this context, the stimuli-responsive luminescent materials,
importance due to diverse applications such as sensors, security donor (D)–acceptor (A) based systems, are highly desirable,
inks, memories, displays, switches and organic light emitting if it can produce high-contrast fluorescence.8a–c To date,
diodes.1 Great efforts have been devoted to the molecular anthracene-based stimuli-responsive D–A systems are rarely
design and modification to achieve color tuning and lumines- reported,8d–g although anthracene derivatives are well-known
cence properties.2 Reversible switching of the luminescence luminescent materials, with wide applications in light-
properties of organic materials by controlling the molecular emitting devices.9a,b Hydrogen bonding, p–p stacking and
packing instead of chemical alteration is highly innovative and their collective interactions are important intermolecular
suitable for practical applications. Since the luminescence interactions for the construction of stimuli supramolecular
properties of organic solids greatly depend on the molecular systems.9b–d To explore a new system and a new mechanism
packing and intermolecular interaction,3 control of the molecular for tunable luminescence by various external factors with
orientation and stacking mode of fluorophores by external reversibility is still a great challenge for practical applications.
stimuli is an effective way to tune the luminescence properties Our continued interest to find out new non-covalent inter-
of organic materials. In this regard, luminescent materials action in the molecular packing and emission properties
which exhibit dynamically switchable solid state emission promotes us to undertake the current research.1 f,h Herein,
properties, when they are subjected to external stimuli such we report the synthesis of a multi-stimuli responsive lumi-
nescent material, 10-bromo-N,N-di-p-tolylanthracen-9-amine
(TAAB), its emission properties and crystal packing. This
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan.
E-mail: chcheng@mx.nthu.edu.tw; Fax: +886-3-572469; Tel: +886-3-5721454
† Electronic supplementary information (ESI) available: 1H and 13C spectra of
10-bromo-N,N-di-p-tolylanthracen-9-amine, the single crystal structure, the photo-
luminescence spectrum and powder-XRD. CCDC 1017742 and 1017743. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5tc00109a
luminescent material responses sensitively to vapo, piezo
and temperature stimuli reversibly with drastic color changes.
More interestingly, intermolecular interactions of C and H
with Br play a crucial role in the tunable emission properties.
These non-covalent interactions are confirmed by single
crystal-XRD.
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2. Results and discussion
2.1 Photophysical properties and solvatochromic effect
The structures of the molecules utilized in this study are shown
in Fig. 1. N,N-di-p-tolylanthracen-9-amine (TAA) was synthe-
sised in good yields from 9-bromoanthracene and ditolylamine
using a palladium tri-tert-butylphosphine complex as the catalyst.
Bromination of TAA by N-bromosuccinimide (NBS) gave the final
product TAAB in good yield (Fig. 1).10
The detailed synthetic procedures and characterization data are
given in the experimental section and ESI.† As depicted in Fig. S1
(ESI†), the absorptions of TAA and TAAB in different solvents are
nearly the same (lmax B 440 nm and 450 nm), but the emission of
TAA shifts from 474 nm in n-hexane to 537 nm in DCM (Fig. 2a).
Similarly the emission of TAAB shifts from 499 nm in n-hexane to
563 nm in DCM (Fig. 2b). The photographs of TAA and TAAB in
various solvents under UV illumination are shown in Fig. 2c and d,
respectively. The emission wavelengths strongly depend on the
solvent used, revealing a bathochromic shift as the solvent polarity
increases. Additionally, the fluorescence intensity is clearly
reduced in highly polar solvents. These results indicate that
the TAAB emission is likely a charge transfer type.
2.2 Theoretical calculations
To gain insight into the electronic states of these two com-
pounds, density functionalized theory (DFT) calculations were
performed using Gaussian 03 program. Fig. 3a shows the electron
cloud of TAA and the result reveals that the highest occupied
molecular orbital (HOMO) is mainly distributed on the electron
donating tolylamine units and to a less extent on the anthracene
group. On the other hand, the lowest unoccupied molecular
orbital (LUMO) mostly spreads over the electron-accepting
anthracene group. These results showed that TAA is a D–A dipole
molecule with strong charge transfer properties from HOMO to
LUMO in agreement with the observed solvent effect of the
emission spectra. The HOMO and LUMO (Fig. 3b) of TAAB are
similar to those of TAA. The HOMO is mainly distributed over
the electron donating tolylamine unit and to a less extent on the
bromoanthracence group and the LUMO mostly spreads over the
electron-accepting bromoanthracene group.
2.3 Molecular packing controlled emission
As shown in Fig. 4b, TAAB, upon crystallization from n-hexane,
Fig. 2 Emission spectra of TAA (a) and TAAB (b) in various solvents at RT
(10À5 M), and photographs of TAA (c) and TAAB (d) in various solvents
under UV irradiation.
appears as red material under ambient light with a yellow
orange luminescence at 577 nm under UV radiation (Fig. 4d)
in contrast to the emission at 499 nm in the same solution. On
the other hand, when the compound was crystallized from
DCM or exposed to DCM vapour, the crystals show a yellow
color, giving green luminescence at 535 nm. A 42 nm hypso-
chromic shift from the yellow orange emission was observed
(Fig. 4). Interestingly, the initial emission was recovered by
heating the same at 70 1C. Such conversion between yellow
orange and green emissions can be repeated many times
Fig. 1 Structures of TAA and TAAB.
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Fig. 3 Calculated electron contour plots (a) HOMO and LUMO molecular
orbitals of TAA, and (b) HOMO and LUMO molecular orbitals of TAAB.
without decreasing the intensity plausibly due to the non-
destructive nature of the solvent stimuli. It is worth noting that
the solid-state stimuli properties were not observed for TAA
suggesting that the Br group plays a crucial role in the tunable
emission properties of TAAB. The luminescence properties of
molecules in the solid state are known to be influenced by
intermolecular interactions.11 To understand the molecular
packing and the intermolecular interactions of TAAB, single
crystals grown from DCM and n-hexane were obtained by
solvent evaporation. The crystalline structures were then deter-
mined by single-crystal X-ray diffraction analysis.
Fig. 4 Photographs of TAAB crystals: (a) crystalized from DCM and
(b) from n-hexane under ambient light; (c) from DCM and (d) from
n-hexane under UV-light; (e) fluorescence spectra of TAAB crystals from
n-hexane (red line) and dichloromethane (green line).
In n-hexane, TAAB was crystallized as monoclinic with each
unit cell containing four TAAB molecules (Fig. S2, ESI†). On the
other hand, in DCM, TAAB was crystallized as orthorhombic
with each unit cell containing eight TAAB and 8 DCM mole-
cules (Fig. S2, ESI†). The packings of TAAB crystals grown from
different solvents are shown in Fig. 5. The crystals from
n-hexane show that the molecules are lined up in a head-to-tail
manner. As revealed in Fig. 5A, the Br group (head) interacts with
the hydrogens and carbons on the two tolyl groups (tail) of
the other molecule with interaction distances of 3.2–3.4 Å and
3.4–3.6 Å, respectively.
Since the Br group plays a crucial role in the emission
properties, we performed the DFT calculations of two TAAB
molecules with and without such interaction to see how strong
this head to tail interaction is. The result shows that the
interaction gives an energy decrease (stabilization) of 1.91 kcal
molÀ1 compared with the total energy of the two molecules
without such interaction. The crystal packing further suggests
Fig. 5 Crystal packing of TAAB: (A) crystalized from n-hexane; (B) crystalized
there is no anthracene–anthracene interaction. However, the from DCM; (C) the corresponding PXRD patterns of TAAB: (a) crystalized from
n-hexane and (b) crystalized from DCM.
anthracene moieties show p–p and CH–p interactions with the
tolyl groups of other molecules (Fig. 6). For the TAAB–DCM
crystal, the anthracene groups are packed in a head–head area of about 1/4 (partial overlap) of an anthracene moiety and a
manner. Each anthracene group overlaps with two other inter- distance of B3.5 Å between the two overlapping anthracene
molecular anthracene groups via p–p stacking with the p-overlap planes.
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Fig. 7 (a) Normalized emission spectra of TAAB before and after grinding,
(b) photographs of TAAB under ambient light before and after grinding.
Fig. 6 Single crystal structure of TAAB crystalized from n-hexane (anthracene
Fig. 8 Photoluminescence images of TAAB precipitated from n-hexane
and ground in a mortar under UV irradiation at 365 nm. (a) A ground
sample; (b) a sample after writing letters ‘C’ and ‘H’ with DCM solvent and
(c) the same sample after erasing the letters by heating at 70 1C for 10 min
and grinding.
tolyl and Br–tolyl interaction).
We also performed the DFT calculations of two TAAB
molecules with an anthracene–anthracene p–p interaction
shown in Fig. 5B. A stabilization energy of 0.95 kcal molÀ1
was found for the interaction of two anthracene groups. The
stabilization energies due to the intermolecular interactions
and the resultant HOMO and LUMO levels in the TAAB crystals
likely account for the distinct colors and emission wavelengths,
although the details need more studies. To further evaluate the
relationship between the vapochromism and the structures, we
measured the powder X-ray diffraction (PXRD) patterns of
the TAAB crystallized from n-hexane and from DCM. As shown
in Fig. 5C, different PXRD patterns were obtained for these
two samples. The observed patterns are consistent with the
simulated powder patterns from the single crystal X-ray analysis
of TAAB and TAAB–DCM (Fig. S3, ESI†).
The intensity decrease indicates that the grinding process
converted the crystalline TAAB to an amorphous state.13 The
observed small peaks in the PXRD pattern indicated that a
small part of the crystalline sample remained after grinding.
When the ground sample was heated at 140 1C for 10 min, the
peaks in the PXRD pattern grew stronger. This result suggested
that the amorphous ground sample could revert to the original
crystalline state by heating. To demonstrate their utility for the
practical application, a TAAB sample was taken into the mortar
and was ground. The sample exhibited greenish-yellow emission
under UV irradiation as shown in Fig. 8a. Letters ‘C’ and ‘H’ were
written on the powder using DCM solvent to give green emission.
The letters were erased upon heating the sample at 70 1C for
10 min and then grinding.
2.4 Tunable emission upon external pressure
It is interesting to mention that TAAB exhibits piezochromism
of the crystalline sample. When the material was ground in a
mortar by a pestle, drastic color change from orange red to
2.5 Temperature dependent fluorescence emission
yellow orange was found (Fig. 7). This property appears revers- Thermochromic photoluminescent materials in the solid state
ible as indicated by the heat treatment at 140 1C for 10 min of have received great attention due to the fact that photo-
the ground sample. Its emission color reverts back to that of the luminescence is one of the most sensitive and easily detectable
unground sample (Fig. S4, ESI†). As indicated in Fig. 7, the signals.7,14 However, in these cases, the thermochromism
emission maximum of TAAB at 577 nm was blue shifted to processes are sensitive to concentration and generally irrever-
543 nm after grinding. This is likely due to the fact that the sible. Additionally, the fluorescent materials need to be doped
intermolecular interactions (Fig. 5A and 6) were partially into polymers along with a reference fluorescent material. To
destroyed by the external pressure. It is noteworthy that grinding avoid these complications, a non-doped and highly lumines-
of crystals in many cases leads to red shift of the emission.12 cent material for the naked eye temperature indicator is highly
Conversely, TAA does not show a significant luminance change desired. Here, we demonstrate that TAAB shows a reversible
under external pressure and this result further confirms the high contrast thermoresponsive luminescence. When TAAB
importance of bromo interaction in piezochromism.
was cooled to 77 K, the initial orange red material turns to a
As shown in Fig. 5C-a, the PXRD pattern of TAAB shows yellow solid under ambient light and the yellow-orange emis-
intense and sharp peaks. In contrast, the ground sample of sion at 577 nm shifted to a bright greenish-yellow band at
TAAB afforded very weak diffraction intensity (Fig. S5, ESI†). 542 nm under UV irradiation. As shown in Fig. 9, the color
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3. Conclusion
We have synthesized an anthracene-based D–A system TAAB
which exhibits multi-stimuli responsive luminescence including
solvent, piezo and thermochromism with a distinct color change.
The crystalline sample of TAAB obtained from n-hexane exhibited
interesting vapochromism behaviors with emission colors
changing from yellow orange to green, upon exposure to
DCM solvents. Single crystal analysis and theoretical calculations
of the TAAB–DCM crystal indicated that the hypsochromic
emission likely originated from the change in the molecular
interaction of the TAAB molecules, change in Br-based inter-
action to anthracene-based one, leading to a larger energy gap.
Furthermore, the emission color change was also observed upon
grinding as well as cooling. It is noteworthy that this is the first
example of a D–A compound which exhibits reversible multi-
stimuli luminescence. The observed vapochromic, piezofluoro-
chromic and thermochromic properties of this compound suggest
potential applications in rewritable optical media, sensors,
and optoelectronic devices. Comprehensive investigation in
this direction is in progress in our laboratory.
Fig. 9 (a) PL spectra of TAAB at RT and 77 K (insets: the corresponding
photographs under UV light), (b) photographs at room temperature and
77 K under ambient light.
change is visible to the naked eye. The color change is rever-
sible, and as the sample was warmed to room temperature, its
color reverted back to orange red. This distinct reversible
luminescence switching is very rarely observed in pure organic
compounds in the solid state at low temperature.15 Further, to
understand the sensitivity of the thermochromism, the time
dependent fluorescence spectra were recorded at various time
intervals at 77 K and room temperature and are shown in
Fig. 10. The results suggest that fluorescence switching is fast
when the crystals were cooled to 77 K or warmed to RT.
4. Experimental section
4.1 General information
Reagents and solvents were used as purchased without further
1
purification unless otherwise stated. H and 13C NMR spectra
were recorded using a Varian Mercury 400 spectrometer. The
HRMS spectra were recorded on a Finnigan MAT-95XL mass
spectrometer. UV-vis spectra were recorded on a Hitachi U-3300
spectrophotometer and PL spectra were measured using a
Hitachi F-4500 fluorescence spectrophotometer. The molecular
geometry optimizations and electronic properties were computed
by carrying out the Gaussian 03 program with density functional
theory (DFT) and time-dependent DFT (TDDFT) calculations,
in which Becke’s three-parameter functional combined with
Lee, Yang, and Parr’s correlation functional (B3LYP) and hybrid
exchange–correlation functional with the 6-31G* basis set were
used. The molecular orbitals were visualized on the Gaussview
4.1 software.
4.2 Synthesis of N,N-di-p-tolylanthracen-9-amine (TAA)
A mixture of 9-bromoanthracene (5.00 g, 19.6 mmol), Pd(OAc)2
(43.0 mg, 0.19 mmol), tri-tert-butylphosphine (158.0 mg,
0.78 mmol), di-p-tolylamine (4.24 g, 21.6 mmol) and sodium
tert-butoxide (3.10 g, 32 mmol) in dry o-xylene (40 mL) was
placed in a sealed tube under a nitrogen atmosphere and was
stirred at 120 1C for 15 h. After cooling, water was added to the
reaction mixture and the mixture was then extracted with ethyl
acetate. The organic layer was dried over anhydrous MgSO4 and
evaporated under vacuum. The crude product was purified by
silica gel column chromatography eluted with n-hexane to give
the desired yellow solid in 89% yield. 1H NMR (400 MHz,
CDCl3): d 8.45 (s, 1H), 8.09 (d, 2H, J = 8.8 Hz), 8.02 (d, 2H, J =
8.4 Hz), 7.33–7.43 (m, 4H), 6.92 (s, 8H), 2.20 (s, 6H); 13C NMR
(100 MHz, CDCl3): d 145.8, 137.8, 133.1, 131.04, 130.4, 129.9,
Fig. 10 Fluorescence spectra of TAAB crystals after (a) the crystals were
cooled to 77 K; (b) the crystals at 77 K were warmed in the air to room
temperature at various time intervals.
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129.2, 126.8, 125.7, 124.8, 124.7, 120.3, 20.9; HRMS (EI+) calcd
for C28H23N 373.1830, found 373.1833; IR (KBr): 3025, 2919,
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4.3 Synthesis of 10-bromo-N,N-di-p-tolylanthracen-9-amine
(TAAB)
TAA (5.00 g, 134 mmol) and N-bromosuccinimide (NBS) (2.86 g,
161 mmol) were dissolved in DMF : CHCl3 (1 : 3, 60 mL) and the
solution was stirred at room temperature for 6 h. After comple-
tion of reaction, water was added to the reaction mixture and
the mixture was extracted with dichloromethane. The organic
layer was dried over anhydrous MgSO4 and evaporated under
vacuum. The product was purified by silica gel column chromato-
graphy eluted with n-hexane. Red crystals of TAAB were obtained
in 86% yield. 1H NMR (400 MHz, CDCl3): d 8.57 (d, 2H, J = 8.4 Hz),
8.13 (d, 2H, J = 8.4 Hz), 7.37–7.56 (m, 4H), 6.89–6.94 (m, 8H), 2.21
(s, 6H); 13C NMR (100 MHz, CDCl3): d 145.2, 138.1, 131.6, 131.5,
130.5, 129.7, 128.4, 127.2, 126.9, 124.9, 122.3, 120.0, 20.6; HRMS
(EI+) calcd for C28H22BrN 451.0936, found 451.0935; IR (KBr):
3025, 2919, 2854, 1608, 1504, 1436, 1405, 1353, 1315, 1292, 809,
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We thank Ministry of Science and Technology of Republic of
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J. Mater. Chem. C