Expression of anti-Kasha's emission from amino benzothiadiazole and its utilization for fluorescent chemosensors and organic light emitting materials

Article abstract of DOI:10.1039/c8tc02416b

Various amino benzothiadiazoles (Am-BTDs) are synthesized and their photophysical properties in different states (solution, film, crystal, and powder) are investigated. Unusual anti-Kasha's emission is observed for those compounds with a strong electronic

Full text of DOI:10.1039/c8tc02416b

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Fine-tuning of gold nanorod dimensions and plasmonic properties using
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DOI: 10.1039/C8TC02416B
Journal Name
ARTICLE
Expression of Anti-Kasha’s emission from amino benzothiadiazole
and its utilizations for fluorescent chemosensors and organic light
emitting materials
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Zhixing Peng, Zaibin Wang, Zongwei Huang, Shaojie Liu, Ping Lu*, Yanguang Wang*
www.rsc.org/
Various amino benzothiadiazoles (Am-BTDs) are synthesized and their photophysical properties in different states
(solution, film, crystal, and powder) are investigated. The unusual anti-Kasha’s emission is observed for those compounds
with strong electronic coupling effect between donor and acceptor which directly results in the white emission of those
compounds in dilute solutions. Among these Am-BTDs, some of them can be utilized as fluorescent chemosensors in
sensing gaseous ammonia, and some of them can be developed for detecting fluoride anion with high selectivity and high
sensitivity. Moreover, aggregation causing quenching is not observed for these compounds in solids. They are emissive in
films, crystals and powders with high color purity and moderate efficiencies. The emission color could be tuned from
yellow to red according to the varied amino groups. Furthermore, they can be used as emissive layer in organic light
emitting diodes.
pure green emitter was approached.12c On the other hand,
modification on the BTD core by installing electron-
withdrawing fluoro or cyano on 5, or 6-position of BTD could
Introduction
2
,1,3-Benzothiadiazole (BTD) is a unique electron deficient
further lower its lowest unoccupied molecular orbital level
heterocycle which has been widely applied as a building block
in the synthesis of optoelectronic compounds for organic light
(
LUMO) and thereafter increase the electron affinity for red-
1
3-14
shifted absorption.
Instead of fluorination and cyanation,
1
emitting diodes (OLEDs), organic field effect transistors
borylation could also lower the LUMO level and enhance the
2
(
OFETs), solution-processable bulk-heterojunction organic
1
5
electron affinity. On the other hand, substitution on 5, or 6-
position of BTD with electron-donating alkoxy group changes
them into transmitters by improving the electron mobility.16
However, direct substitution on 4,7-positions with amino
group is seldom reported although this kind of modification
would dramatically change the energy level to a great extent
and have been demonstrated for some particular purposes
3
4
solar cells (BHJ-OSCs), organic flow battery, solar-hydrogen
5
6
7
conversion, bioprobes, and fluoride anion chemsensors.
Connection of BTD with AIE-fluogen (aggregation induced
emission active fluogen) induces these BTD-based compounds
emitting light in aggregates for long-term two-photon cell
8
imaging. General speaking, the π-extended BTD derivatives
facilitate planarization and polarization which enhance the
electronic communication between electron donor and
electron acceptor and simultaneously increase intermolecular
interactions and afford well-ordered crystal structures for
better device performance.
compounds are stable in both ground state and excited state
even after long period of irradiation. Therefore, discovery the
new BTD-based compounds in order to fit the requirement of
the steady development of the material science and life
science has always been the focus of research. By surfing the
literature, the conjugation normally extends from 4,7-positions
(Scheme 1). For instance, by covalently bonding 2-
aminopyridine to 4-position of BTD, BTD-AP possesses dual
functions of excited state intramolecular proton transfer
(ESIPT) and intramolecular charge transfer (ICT) and has been
2
a, 9
Meanwhile, most of BTD-based
applied as a mitochondria fluorescent marker with high
6b, 17
selectivity and stability.
By attaching p-nitrophenylamino
6a
group on the 4-postion of BTD, CBT-NH shows selectivity and
sensitivity to fluoride anion and works as a fluoride fluorescent
7
chemosensor. By incorporating with diarylamino, MR-3 was
designed for dye-sensitized solar cells with an apparently
12c
increment of the power conversion efficiency.
BTZ-DMAC as well as others, with two diarylamino groups on
,7-positions of BTD, have been designed for red TADF-
Recently,
10
11
1, 12
of BTD by covalently bonding alkenyl, alkynyl, and aryl.
For example, insertion of BTD into polyfluorene backbone, a
4
emitters (thermally activated delay fluorescence emitters) with
electroluminescence at 636 nm and a maximum external
quantum efficiency of 8.8%. Here, we would like to report
Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China.
*
E-mail: pinglu@zju.edu.cn, orgwyg@zju.edu.cn.
1
8
Electronic Supplementary Information (ESI) available: structural characterization
data, TGA, crystallographic data, photophysical spectra, DFT, CV. See the synthesis of a series of amino benzothiadiazole (Am-BTDs
,
DOI: 10.1039/x0xx00000x
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1
-
7
), the investigation of the amino effect on their benzothiadiazole as illustrated in Figure 1. The chemical shift
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DOI: 10.1039/C8TC02416B
a
photophysical properties in different states, and the of H in compounds 1-7 is found to be 6.49, 6.87, 7.27, 6.97,
utilizations of these compounds in fluorescent sensory and 7.55, 7.28, and 6.99, respectively. The smaller the chemical
material sciences.
shift is, the greater shielding of this nucleus is. With aliphatic
amine substituted, Ha in compounds and are highly
shielded in comparison with the ones in aromatic amine
substituted compounds . The best p-π conjugation between
lone pair electron of nitrogen and BTD core is seen in
compound . On the other hand, the chemical shift of H could
be used to refer to the relative acidity of N-H in compounds
1
2
3-7
1
c
1
-
5
. The larger chemical shift is, the stronger acidity of N-H
presents. Thus, compound
5
presents the strongest acidity
due to the electron withdrawing
nature of the cyano group. Acidities of N-H in compounds
are much stronger than those in compounds because of
the better delocalization of these conjugated bases. Moreover,
it is clear that chemical shifts of H and H in compounds
7.27 and 7.68 ppm) and (7.28 and 7.76 ppm) are almost
identical. Similar situation is observed for the pair of
compounds and . It indicates that the electronic effect by
among these compounds 1-5
3-5
1-2
a
b
3
(
6
Scheme 1. Representatives of amino substituted BTDs for versatile purposes.
4
7
the replacement of monoarylamino with diarylamino is almost
the same no matter how many aryl groups are tethered.
Crystal data of Am-BTDs (1-7)
Results and discussion
Synthesis of Am-BTDs (1-7)
Synthetic route to Am-BTDs
sequence of Suzuki coupling and Buckwald-Hartwig cross- after slow evaporation of their corresponding dilute solutions.
coupling was applied to prepare Am-BTDs
). The Crystallographic data is summarized in Table S1. The selected
(1-7
) is shown in Scheme 2. A Fortunately, single crystals of compounds 1-7 were obtained
(1-7
synthesized compounds could be classified into three dihedral angles and bond distances were indicated in Figure 2.
1
categories according to the amino groups, including aliphatic In a good accordance with H NMR findings, C-N bond length in
primary amine (
and aromatic secondary amine (
compounds
1 and 2), aromatic primary amine (3, 4, and 5
), compounds 1 and 2 is relatively shorter in comparison with
6
and
7
). Structures of those of others. It is found to be 1.366 Å and 1.369 Å for
1
13
1
-
7
were characterized by H NMR, C NMR compounds
1
and
2
respectively. This implies that the C-N
and has partial double bond character
according to the because of the strong electron donating nature of the nitrogen
thermogravimetric analysis (Figures S15). Decomposition in aliphatic amines. Meanwhile, the conjugation between
(
Figures S1-S14), IR, HRMS, and their single crystal analysis. All bond in compounds
1
2
o
compounds are stable to 276
C
o
o
temperatures vary from 276 C to 355 C, relying on the amino para-methoxyphenyl (PMP) and BTD is better in compounds
1
group (Table 1).
and 2 in comparison with those in others, indicated by the
dihedral angles between PMP and BTD as well as the C-C bond
lengths between PMP and BTD. Thus, combining with those
findings from 1H NMR, it could be concluded that
intramolecular charge transfer (ICT) apparently exist in
compounds
NMR analysis) or solid (X-ray analysis). However, to the series
of compounds , and , relatively longer C-N bond lengths
are observed, especially in the case of compound because of
the electron-withdrawing nature of cyano group. Moreover, a
more twisted PMP is seen in compounds , and on the
1 and 2 no matter whether the state is solution
(
3
,
4
5
5
3
,
4
5
basis of the dihedral angles and C-C bond lengths between
PMP and BTD. It implies that the electronic communication
between donor and acceptor in these D-A-D molecules (
and ) is weakened which makes these compounds having
multipul conformers possible. To the series of compounds
and , the three arenes swing around the nitrogen and exist in
3, 4,
5
6
7
Scheme 2. Synthetic route to compounds 1-7.
propeller structure. C-N bond length is determined to be 1.413
Å and 1.410 Å, while the distance between PMP and BTD is
NMR spectra of Am-BTDs (1-7) in CDCl
3
determined to 1.474 Å and 1.477 Å for compounds
respectively.
6 and 7,
The electron donating ability of the various amino groups
could be described by the chemical shift of H in 2,1,3-
a
2
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DOI: 10.1039/C8TC02416B
1
Figure 1. Comparative analysis of H NMR spectra of compounds 1-7.
Figure 2. Selected dihedral angles and bond lengths, based on the single crystal analysis.
Absorption of Am-BTDs (1-7) in dilute solutions
spectra were measured in polar solvent, such as acetonitrile,
all compounds presented blue-shifted absorptions with
decreased molar absorptivities, respectively (Figure 3b,
Figures S16d-f, Table 1).
At the beginning, we measured the absorption spectra of
compounds 1-7 in non-polar toluene solutions in a
-
5
concentration of 2 x 10 M (Figure S16, Table 1). With
In compounds 1-7, BTD works as the electron acceptor, while
aliphatic amine substituted, compounds
1
and
2
absorb light
3
p-methoxyphenyl and amino group function as the electron
donors. When these D-A-D compounds dissolved in polar
solvents, the HOMO/LUMO energy levels in ground states
decreased simultaneously because of the solvation. In more
polar solvent, the larger transition energy gap is observed
because of the more stabilization of the HOMO level.19
Emissions of Am-BTDs (1-7) in dilute solutions
at 471 nm and 477 nm with molar absorptovities of 6.38 x 10
3
and 6.43 x 10 , respectively (Figures 3a and S16a). With
aniline substituted, compound
3
absorbs light at 475 nm with
3
an increased molar absorptivity of 9.61 x 10 . Addition of
methoxy or cyano on the para-position of aniline tunes the
energy level as we expected. Thus, red-shifted absorption is
observed for compound
seen for compound (Figure S16b). With one more phenyl or
p-methoxyphenyl added, the further red-shifted absorption is
observed. Thus, compounds and absorb light at 479 nm
4, while blue-shifted absorption is
5
Emission spectra of compounds 1-7 in dilute toluene
-5
solutions are recorded in a concentration of 2 x 10 M,
excited at the corresponding maximum absorption
6
7
and 502 nm, respectively (Figure S16c). When the absorption
wavelengths (Table 1). Compound
1
emits light at 595 nm
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Table 1. Absorption and emission of 1-7 in solutions and in solids
λmaxabs ( nm )
.
DOI: 10.1039/C8TC02416B
λmaxem ( nm)
Tola)
ε×104)
MeCNa)
(ε×104)
Tola,b)
(Φ %)
MeCNa,b)
(Φ %)
Film
(Φ %)
Tde)
compound
Film
492
493
488
501
468
490
513
Stokes
124
122
107
117
108
96
Stokes
Powderd)
Crystald)
624
(
4
71
0.638)
77
0.643)
75
0.961)
83
0.889)
61
1.364)
79
0.906)
02
0.819)
469
(0.614)
595
(2.5)
623
(<0.1)
575
(10.8)
1
2
3
4
5
6
7
154
147
136
150
133
143
-- c)
601
614
579
612
566
575
622
276
328
305
327
351
299
355
(
4
473
(0.587)
599
(2.6)
620
(<0.1)
--c)
--c)
--c)
638
(
(
(
(
(
(
4
468
(0.904)
582
(12.7)
611
(1.0)
608
4
475
(0.865)
600
(10.6)
625
(<0.1)
637
4
449
(1.197)
569
(30.0)
582
(4.2)
559
(7.0)
578
4
468
(0.7)
575
(61.0)
611
(10.8)
602
(43.6)
591
5
490
(0.76)
630
(4.1)
128
--c)
-- c)
629
a)
measured at 2 x 10-5 mol/L, excited at its maximum absorption wavelength, b) quantum yield was obtained by using integrating sphere, c) emission is too weak to be
detected, d) excited at its the maximum absorption wavelength in toluene, e)
T
was determined at 5% weight lose.
d
with Stokes shift of 124 nm and a quantum yield of 2.5%
Figure 3a). Emission of compound in toluene is almost
identical to that of compound (Figure S16a). In these cases,
As an extreme example, compound
acetonitrile.
Excitation dependant emission (EDE)
7 is non-fluorescent in
(
2
1
intramolecular charge transfer (ICT) from aliphatic amine to
BTD core occurs in excited state and makes the large Stokes
shift possible. With aniline substituted, that eventually
retards the electron-donating ability of the nitrogen via p-π
conjugation between the lone pair electrons of nitrogen and
Excitation dependant emissions are observed (Figure 3,
Figure S17). To compound , two distinguishable emissive
1
peaks are recorded in toluene as well as in acetonitrile as the
excitation wavelength changes. In toluene, as the excitation
wavelength changes from 380 nm to 440 nm, the emission
intensity at shorter wavelength (483 nm) gradually decreases
while the emission intensity at longer wavelength (595 nm)
increases (Figure 3c). The emissive color change could be
seen by naked eye (Figure 3h). It is worthwhile to note that a
bright white emission is recorded when this dilute toluene
solution is excited at 400 nm. In acetonitrile, these two
emissive bands (547 nm and 623 nm) still exist as the
excitation wavelength changes from 380 nm to 440 nm
benzene, compound
of 107 nm and a quantum yield of 12.7%. With p-
methoxyphenyl (PMP) substituted, emission of compound
3 emits light at 582 nm with Stokes shift
4
is red-shifted with a larger Stokes shift (117 nm) and a lower
quantum yield (10.6%) in comparison with those of
compound
compound
3
5
. On the contrary, in case of cyano substituted,
emits light at 569 nm with a quantum yield of 30%
(Figure S16b). Introduction of one more phenyl group,
compound
6
emits light at 575 nm with the smallest Stokes
(Figure 3d). Similar situation is observed for compound
2
shift (96 nm) and the largest quantum yield (61.0%) in this
family. It might be ascribed to the propeller shape of
triphenylamine (TPA) and thus partially inhibited ICT.
(
Figures S17a and S17A). To compound , only one emissive
3
band at lower energy gap (582 nm) is seen in toluene as the
excitation wavelength changes from 360 nm to 420 nm. In
this case, the emission peak is not shifted, but with a
gradually increment of emission intensity (Figure S17b). In
acetonitrile, the two emissive bands (545 nm and 611 nm)
come back (Figure S17B). This phenomenon is reproducible in
Compound
7 emits light at the farthest (630 nm), but with a
much lower quantum yield (4.1%) (Figure S16c). In general,
the larger Stokes shift is, the lower quantum yield is
20
obtained. Emissions of compounds 1-6 in dilute acetonitrile
are all red-shifted to those of emissions in toluene (Figure 3b,
Figures S16d-f). In the excited state, the polar solvent tends
to stabilize the LUMO level and decreases the energy gap. As
a result, these compounds present large Stokes shifts in
acetonitrile (133 nm ~ 154 nm) but with significantly
decreased quantum yields. Some of them present weak
luminescence in polar solvent and their emission efficiencies
case of compound 4 (Figures S17c and S17C). To compound 5,
one emissive peak (569 nm) is recorded with tunable
emission intensities as the excitation wavelength changes
(Figure S17d). However, a gradually red-shifted emission,
accompanied by a decrement of emission intensity, is
observed as the excitation wavelength changes from 380 nm
to 440 nm (Figure S17D). When compound
6 is examined,
are too weak to be detected, such as compounds 1, 2, and 4.
only one emissive peak is recorded either in toluene or in
acetonitrile (Figures S17e and S17E). Emission spectrum of
4
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Figure 3. Absorption and emission (excitation at maximum absorption wavelength) of 1 in toluene (a) and in acetonitrile (b); excitation dependent emission of 1 in toluene (c) and
in acetonitrile (d); excitation spectra of 1 (monitored at two emissive bands as indicated in parenthesis) in toluene (e) and in acetonitrile(f); fluorescent decay of 1 in toluene with
the emission wavelength at 483nm or 595nm; (h) photo of 1 in toluene with different excitation wavelength.
compound
7
in toluene depends upon the excitation
547 nm (Figure 3f). Similar situation is recorded for
compound (Figures S19a and S19A). In case of compound 5
wavelength with the appearance of two changeable emissive
peaks (Figure S17f). However, emission in acetonitrile is too
weak to be detectable. Moreover, emission quantum yields
2
in acetonitrile, a much broad excitation peak is recorded
which is accordance with the excitation-dependent red-
shifted emission (Figure S19D and Figure S17D). On the basis
of the measurement of the excitation dependent emission
and their corresponding excitation spectra, it could be
preliminary concluded that the two emissive bands could be
respectively excited and the lower energy state is not
populated from the higher energy state.
of compounds
closely related to their excitation wavelengths. For instance,
when compound is excited at 380 nm, 400 nm, 420 nm, and
40 nm, emission quantum yield is determined to be 9.0%,
.5%, 1.2%, and 0.6%, respectively. Similar results are
and (Table S2).
1, 2, and 7 in dilute toluene solutions are
1
4
2
observed for compounds
2
7
These dual emissions are quite unusual and intriguing in
comparison with the normally observed two emissions bands
from D-A compounds. To a common D-A compound, dual
Then we move to the measurement of the lifetimes of these
dual emissions (Figure 3g, Figure S20, and Table S2). Figure 3g
presents the fluorescence decay traces of compound
1 in
emission bands originate from the locally excited emission
toluene recorded in emission at 483 nm and 595 nm,
respectively, corresponding to the dual emissions in Figure 3c.
Both decay traces are bi-exponential with lifetime
components of 1.32 ns (66.32%) and 10.96 ns (33.36%) for
emission band at 483 nm, 1.65 ns (90.83%) and 4.85 ns
LE
(
(
S
S
1
emission) and intramolecular charge transfer emission
emission) due to the conformational change in the
ICT
1
excited state which should be independent with the
21
excitation wavelength. In order to understand the origin of
these dual emissions, we firstly measured the purity of
sample in order to avoid the incorrect assessment which may
(9.17%) for emission band at 595 nm. To compound
acetonitrile, emission is too weak for the life time
measurement. To compound in toluene, one emission band
1 in
22
result from the impurity. To all of compounds we
investigated, the purity is guaranteed good enough for
photophysical study on the basis of HPLC analysis (Figures
S18). Secondly, we recorded excitation spectra for each of
3
at 582 nm (Figure S17b) fits single exponential with
fluorescent lifetime of 4.01 ns. Meanwhile, the two emission
bands (Figure S17B) fit bi-exponential function with lifetimes
of 1.11 ns (14.81%) and 12.67 ns (85.19%) for emission at 545
nm, 0.94 ns (93.11%) and 5.50 ns (6.89%) for emission at 611
two emission bands (Figure 3 and Figure S19). To a typical D-
ICT
A compound, the S
1
state could not be directly excited, and
state. Thus, their excitation spectra
LE
it is populated via S
1
nm, respectively. To compound 5 in toluene, emission at 569
monitored at two emission bands should be similar and
match with its absorption. However, to compound 1, both
excitation spectra show a distinguishable band when
monitored either at longer (595 nm) or at shorter wavelength
nm fits single exponential with lifetime of 6.84 ns, while two
components are recorded for compound 5 in acetonitrile
with 2.11 ns (52.98%) and 12.38 ns (47.02%), respectively
23
(Figure S17D). To compound 6, single emission peak is seen in
(
483 nm) in toluene (Figure 3e). When the excitation spectra
toluene (575 nm) and in acetonitrile (611 nm) (Figures S17e
and S17E). Both of them fit single-exponential function with
lifetimes of 15.26 ns and 6.80 ns, respectively. Finally, it could
are recorded in acetonitrile, two bands are observed in
excitation spectrum when monitored at 623 nm, while only
one band exists in excitation spectrum when monitored at
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be concluded that the emission at higher energy gap presents
longer lifetime than the emission at lower energy gap.
On the basis of above observations, it is clear that
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compounds 1-7 present excitation dependant emission (EDE)
and could be classified into three categories: (a) dual
emissions with variable emission intensities (1-4, 7), (b) single
emission with variable emission intensity (
6), and (c)
gradually red-shifted emission ). These phenomena
(
5
eventually break the Kasha-Vavilov rule, which states that
polyatomic molecules generally luminescence with
appreciable yield only from the lowest excited state of a
given multiplicity, irrespective of the initial photoexcited
Scheme 3. Illustration of the appearance of emission at larger energy gap.
actually a D-A-D-A system. Addition of cyano group makes
the conformation in excited states more complicated. The
resulting multiple conformers bring about red-shifted
emissions in acetonitrile as the excitation wavelength
23-24
state.
Emission at longer wavelength could be ascribed to
decay as usual D-A molecules present ICT emission,
while emission at shorter wavelength could be ascribed to S
1 0
S →S
2
25
→
S
0
decay. The variable quantum yield as a function of
changes. In comparison with compound
3, compound 6
excitation wavelength further proves the anti-Kasha behavior
presents single emission in either toluene or acetonitrile
because of the propeller structure of triarylamine and the
disappearance of the rigid structure. In another word, to
these compounds, the appreance of the anti-Kasha emission
is depending on the conformation. Rigid conformation tends
to present the anti-Kasha emission because of the inhibitted
vibrational relaxation (VR) and the internal conversion (IC).
Emissions of Am-BTDs (1-7) in solids as well as in frozen states
2
6
(Table S2).
To a typical fluorescent molecule, internal conversion (IC) and
vibration relaxation (VR) are faster than radiative and other
non-radiative decay (Scheme 3). Thus, population in S
occurs which finally results in the emission from S state. In
cases of compounds and , dual emissions are seen in both
1
state
1
1
2
toluene and in acetonitrile because of the strongest electron-
donating ability of aliphatic amines which leads to the most
charge transfer and the most planarization of molecule
among these compounds. As a result, the vibrational
Compounds
1-7 are all emissive in powders and in crystals
2
7
(Table 1, Figure S21). Emissions in crystals are all red-shifted
in comparison with the emissions in powders as we can see
from the emission spectra (Figure S21) and the photos under
UV light (Figure 4). The largest red-shifted emission (29 nm)
relaxation (VR) and the internal conversion (IC) from S
are effectively inhibited in these rigid molecules and the
emissions from S state come into being. To compounds
and in toluene, emissions from S state are effectively
2 1
→S
2
3
from powder to crystal is observed for compound
Emissions of compound are red-shifted in comparison with
those of compound in powder and in crystal. It could be
explained by the different molecular packing mode in single
crystal (Figure 5). To compound , two molecules vertically
3.
4
2
2
inhibited because of the extend conjugation of aryl amine
which retards the electron donating ability of nitrogen to BTD
core. However, in acetonitrile, the intramolecular charge
transfer is enhanced via the acetonitrile solvation. Thus,
1
1
align in anti-parallel, driven by the π-π stacking (3.390 Å)
between two individual BTD cores (Figure 5a). Meanwhile,
2
emission from S state comes back. To compound 5, it is
Figure 4. Chemical structures of compounds 1-7 (first row); pristine powders of compounds 1-7 under UV lamp (365 nm) (second row); crystalline states of compounds 1-7 under
UV lamp (365 nm) (third row). The numbers indicate their corresponding maximum emission wavelengths.
6
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Figure 5. Molecular packing in single crystals of 1 (a), 2 (b), 3 (c), 4 (d), 5 (e), 6 (f), and 7 (g).
-
3
two molecules horizontally align in anti-parallel, driven by the
short contact between the oxygen of methoxy and the sulfur
of BTD (3.125 Å) and multiple C-H…π interactions (2.353 Å,
determined to be 1.387, 1.416, and 1.435 g cm , respectively
(Table S1). It implies that compound is of the largest
tendency to change the emissive color in solid states among
3
28
2
.873 Å, 2.832 Å, and 2.886 Å). To compound
2
, because of
these compounds by applying external stimuli. Molecular
packing mode is essentially the same in single crystals of
the ball structure of amantadine, the π-π interaction between
the adjacent two BTD rings is effectively inhibited, with a
parallel but translational alignment of two molecules (Figure
compound
6 (Figure 5f) and compound 7 (Figure 5g). The
double short-contact between sulfur and nitrogen from the
two adjacent BTD cores results in a dimer with anti-parallel
alignment and further assembles to contruct a layer-by-layer
structure through CH…π interaction in compound
CH...N interaction in compound
Films are formed by spinning the dilute chlorobenzene
solution of compounds on quartz plates and then being
vaporized under atmosphere. Absorptions of compounds 1-7
5
b). The main intramolecular interaction is the electrostatic
force arising from C-H of benzene in PMP and π electron
circulation of BTD. The distance of this CH…π interaction is
determined to be 2.960 Å. Other interactions are CH…N
interaction (2.654 Å) between methoxy and BTD, CH…S
interaction (2.931 Å) between amantadine and BTD. The
6 and
7
.
1-7
parallel alignment of compound
2
induces the red-shifted
emission, while the anti-parallel alignment of compound
results in the blue-shifted emission.
1
in films are all red-shifted about 20 nm to those absorptions
in dilute acetonitrile solutions with larger full widths at half
maximum intensity (FWHMs) (Table 1, Figure S22). Three
In the single crystals of compound
conformers are seen. Four conformers exist in the single
crystal of compound , while two conformers for and
respectively. The four conformers in compound exist in two
3, 4, and 5, multiple
films, prepared from 1, 5, and 6, are fluorescent while others
3
4
5
,
are not. These three films emit light at 575 nm, 559 nm, and
602 nm with quantum yields of 10.8%, 7.0%, and 43.6%,
respectively. By doping these compounds in PMMA (9 x 10-5
3
layers, each of which possesses an anti-parallel dimer
because of the double short-contact between sulfur and
nitrogen from the adjacent two BTD cores. Further
intermolecular CH…O and CH…N interactions integrate these
dimers in layer by layer (Figure 5c). The molecular packing
mol 1-7 / 1 g PMMA), the films, doped with compounds
, are fluorescent and emit light at 563 nm and 571 nm,
respectively. Others are non-fluorescent (Figure S23).
As the excitation wavelength changes from 360 nm to 420nm,
all powders ( ) show a single emission (Figures S24). This is
5 and
6
mode in compound
4
is similar to what we have seen in
1-7
compound (Figure 5d). However, in the case of compound
the double short-contact between sulfur and nitrogen is
absent because of the insertion of the cyano group (Figure
3
5
,
a good indicative of the expression of anti-Kasha’s emission in
the solution. In the condensed phase, the energy released
from VR and IC processes could be relieved as heat through
intermolecular interactions which directly results in the
5
e). Crystal densities of compounds 3, 4, and 5 are
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Journal Name
depopulation of S
2
states.23 In another word, only one
) could be seen in the condensed phase.
(Silica GF254) into dilute dichloromethane solution of
View Article Online
DOI: 10.1039/C8TC02416B
emission band (S
1
→S
0
compound
2 and evaporating the solvent, a pristine plate
Similar situation is observed for their emissions in frozen
state. By freezing the dilute toluene solutions of compounds
with the doping of compound
2 is prepared. The pristine
plate is blue under UV light. Putting the pristine plate on the
top of the bottle of the concentrated HCl and saturating with
HCl vapor for 180 seconds, the plate becomes yellow under
UV light. The color could be turned back to blue after the
plate is re-saturated with ammonia vapor for 30 seconds. As
1
1 0
-7 in liquid nitrogen, only one emission band (S →S ) is
observed and the intensity changes as the excitation
wavelength changes (Figure S25).
Application as gaseous ammonia sensor
a practical usage, the powder of compound
2 could be
Absorption and emission spectra of compounds 1-6 rely on
directly used to detect ammonia and hydrogen chloride vapor
in a reusable way (Figures 6b and 6c). The color of the
powder changes between yellow and red under UV light
when the powder is alternatively exposure to hydrogen
chloride and ammonia vapor for 180 seconds and 30 seconds,
respectively.
the environmental acidity (Figure S26). When the absorption
spectra of these compounds are recorded in acetonitrile in
the presence of 0.1 M HCl, blue-shifted absorption is
observed for compounds
1 and 2. As we can see from the
photo (Figure 6a, inset photo), the yellow color fades in the
presence of HCl. Nevertheless, the emission turns on. Bright
yellow emission is seen when HCl is added to the acetonitrile
Application as fluoride anion sensor
solution of compound
be 22.2% for in acidic acetonitrile. The distinguishable
emission color change of compound could be used to
reversibly probe gaseous hydrogen chloride and gaseous
2
. The quantum yield is determined to
These compounds are versatile. Compound 5 might be
2
developed as a potential fluoride fluorescent chemosensor
due to the enhanced acidity of the NH group affected by the
electron withdrawing cyano group which is absent in other
compounds (Figure 7). Absorption spectrum changes as
2
2
9
ammonia (Figure S27). Dipping the fluorescent silica plate
Figure 6. (a) Acidity effect on absorption and emission of 2 in MeCN. Inset: photo before and after adding acid under day light and UV light (365nm); (b) Emission of powder 2,
exposed to HCl and NH vapor; (c) Reversible color change of powder 2 under alternative exposure to HCl and NH vapor for 180 and 30 s, respectively (ex 365nm).
3
3
Figure 7. Absorption (a), emission (b), and photo (c) of 5 (c = 5 x 10-5 M, DMSO) in the presence of varied amounts of TBAF; Absorption (d), emission (e), and photo (f) of 5 (5 x 10-5
M, DMSO) in the presence of 20 equivalents of various tetrabutylammonium salts in DMSO.
8
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tetrabutylammonium fluoride gradually approaches to 20
equivalents to compound (Figure 7a). Three isobestic
Kasha’s emission is observed for some Am-BTDs in
5
solutions which could be ascribed to the strong electronic
points appear in absorption spectra with values of 347, 430,
and 536nm, respectively. Solution color stays yellow as the
fluoride anion reaches to 10 equivalents. A dramatic change
is observed when the fluoride anion reaches to 12
equivalents. The color changes to green and keeps the
same beyond this point (Figure 7c). Meanwhile, a turn-off
emission is observed as fluoride anion adds to the dilute
solution of compound
5. When the fluoride anion reaches
1
0 equivalents, the emission intensity decreases to half of
Figure 8. (a) EL spectra of 6-based device operating at voltage of 7V. Inset: Image
of device; (b) Current density-Luminance–Voltage (L–J–V) characteristics.
its initial. As the amount of fluoride anion further increases,
the emission intensity decreases and finally turns off at 20
equivalents (Figure 7b).
Subsequently, we tested the anion selectivity. Thus, various
anions are respectively added to the dilute solution of
communication between nitrogen and BTD core. The
stronger the electronic communication is, the more
expression of anti-Kasha’s emission. Applying this abnormal
property, a white solution emission is approached.
Moreover, some Am-BTDs could be developed as
fluorescent pH sensors for detecting environmental acidity,
not only for probing the solution acidity but also for probing
the gaseous ammonia. It is also noteworthy that compound
-
compound 5. Absorption spectra keep unchanged when Cl ,
Br , I , NO , AcO , ClO , PF , H PO , HSO are respectively
3 4 6 2 4 4
-
-
-
-
-
-
-
-
tested except the hydroxide anion (Figures 7d and 7f). As
the gradual addition of the hydroxide anion, the absorption
spectrum alters and the alternation is almost identical to
the addition of fluoride anion (Figure 7d). Fortunately, the
discrimination between fluoride and hydroxide anions
could be realized by the measurement of their emission
spectra (Figures 7e and 7f). Addition of hydroxide leads to a
blue-shifted emission, while addition of fluoride turns
emission off.
5
could be developed as a fluorescent fluoride sensor. The
discrimination between fluoride and hydroxide could be
easily achieved by the fluorescent detection. Finally, Am-
BTDs are not only emissive in solutions but also emissive in
solids. Emissive color in solid is tunable and varies from
yellow to red depending on the different amino groups as
well as the different states. In film state, a maximum
quantum yield of 43.6% is approached which opens a
window for them to become the promising emitting
materials in OLEDs. Although the OLED result is not good
enough at this stage, the facile preparation, the tunable
color, the abnormal anti-Kasha’s emission and the thermal
stability of these new compounds are valuable and
worthwhile to note in the future.
Electrochemical studies of Am-BTDs (1-7)
Oxidation and reduction potentials obtaining from cyclic
voltammetry are applied to estimate the frontier orbital
levels and the results are summarized in Figure S29 and
Table S4. The energy gaps between HOMO and LUMO
obtaining from electrochemical study are in good
accordance with the energy gaps resulting from UV
measurements except for the cases of compound
1 and 4.
Meanwhile, the results obtaining from the theoretical
calculation (Figure S28,S30) are in good accordance with
those from UV measurements in tendency, but with much
higher absolute values.
Conflicts of interest
The authors declare no conflict of interest.
Electroluminescence of compound 6
To further extend the utility of these compounds, an
organic light emitting diode with a structure of ITO /
Acknowledgements
The authors thank Prof. Yizheng Jin for providing the
convenience in the fabrication of OLED device and thank
the national natural science foundation of China
PEDOT:PSS (40 nm) / compound
6 (65 nm) / TBPI (40 nm) /
LiF (1.5 nm) / Al (50 nm) is fabricated. The device emits light
at 625 nm (CIE: 0.63, 0.37) with HWHM of 107 nm at
voltage of 7 V and exhibits a turn-on voltage of 3.0 V at a
(21472173).
-2
brightness of 1 cd m , external quantum efficiency (EQE) of
-1
0
.91 %, luminous efficiency of 1.06 cd A and power
Notes and references
-1
efficiency of 0.81 lm W (Figure 8).
1
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In conclusion, by directly covalent bonding amino group to
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interesting and applicable fluorescent properties. Anti-
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Graphic abstract