Rational design and synthesis of yellow-light emitting triazole fluorophores with AIE and mechanochromic properties

Article abstract of DOI:10.1039/C9CC00262F

Previously, we reported that N-2-aryl triazoles (NATs) exhibited good fluorescence activity in the UV/blue light range. In an effort to achieve biocompetitive NAT fluorophores with green/yellow emission, a new class of 4-keto-2-(4′-N,N-diphenyl)-phenyl triazoles were designed and synthesized. Herein, we present our study on these novel fluorophores which demonstrated excellent luminescence emission both in solution (Φ up to 96%) and in the solid state (Φ up to 43%). Furthermore, these new compounds showed aggregation-induced emission (AIE) properties and reversible mechanochromic luminescence properties, which suggested their potential applications in chemical and materials science.

Full text of DOI:10.1039/C9CC00262F

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Rational design and synthesis of yellow-light emitting triazole
fluorophores with AIE and mechanochromic properties
Qi Lai,a Qing Liu,a Kai Zhao,a Chuan Shan,b Lukasz Wojtas,b Qingchuan Zheng,c Xiaodong Shi,*a,b
Zhiguang Song*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Previously, we reported N-2-aryl triazoles (NAT) exhibited good position and electron-deficient substitutions on the C-4
fluorocent activity in UV/blue light range. In the effort to achieve position, a series of yellow emitting NAT fluorophores (lem up
biocompetitive NAT fluorofores with green/yellow emission, a new to 550 nm) with high quantum yields both in solution and in
class of 4-keto-2-(4’-N,N-dipheyl)-phenyl triazoles were designed solid state were obtained. Moreover, these compounds
and synthesized. Herein, we present our study of these novel showed aggregation induced emission (AIE) properties and
fluorophores which demonstrated excellent luminencent emission interesting mechanochromic luminance, suggesting a promising
both in solution (
up to 96%) and in solid state ( up to 43%). strategy to construct solid fluorophores from simple 1,2,3-
F F
Furthermore, these new compounds showed aggregation-induced triazole derivatives (Scheme 1B).
emission (AIE) properties as well as reversible mechanochromic
luminescence properties, which suggested their potential
applications in chemical and material science.
A)
N-2 aryl triazole (NAT)
Progress:
Ph
N
N
N
N
stable small molecule;
easy preparation;
R’
N
N
good quantum efficiency
R
Ph
Confine:
UV/blue emission and hard
to tune red shift
Fluorescence active small organic molecules are important class
of compounds in chemical1, biological2 and material3 research.
In general, many factors including high luminescence efficiency,
high thermal stability, good accessibility and easy modification
are assessed for organic phosphor’s applications.4
no photo emission
λem between 365-415 nm
B) This work: Yellow emitting NAT with unexpected mechanochromic luminescence
N
Achieving red-shift emission by combination of two groups
NPh2
λem = 419 nm
N
Ph
N
Ph
Ph
N
N
In the past decade, our efforts on triazole derivative
synthesis5 have led to the discovery of N-2-aryl-1,2,3-triazoles
as good fluorescence emission compounds, while the N-1
isomers (1,4-disubstituted-triazole) showed almost no
emission.6 As shown in Scheme 1A, changing different aryl
substitutions on both N-2 and C-4 position can only cause a
small emission wavelength shift. Although fluorophores with
UV/blue light emission are important for certain applications7,
those with emission at lower energy (green/yellow/red light
region) is preferred for applications in biological systems.8
Herein, we have devoted many efforts to developing NAT-
derivatives with longer emission wavelength. By simultaneously
incorporating electron-rich substitutions on the N-2 phenyl
O
N
N
Ph
Ph
Ph
N
N
O
λem = 524 nm
N
Ph
Ph
22 analogues; 9 X-ray single crystal structures
λem = 366 nm
THF H2O
3h
grinding
fuming
99%
3h
2c
2c
2c
solid
mechanochromic luminescence
Scheme 1. Developing yellow emitting NAT fluorophores.
As a general design principle, extending conjugation system
of the fluorophore will lead to red-shift of the emission
wavelength due to the reduced energy gap between its highest
occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO).9 Considering that 1,2,3-triazole is a
highly electron deficient functional group, incorporation of
electron-donating groups (EDG) should bring the emission to
red-shift. However, even with strong EDG like N, N-dimethyl
(
1c), limited red-shift was observed with lem at 427 nm. (Figure
1A, see detailed syntheses of all compounds and their optical
dates in SI). Notably, 1,2,3-triazole is a very stable aromatic
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Journal Name
[a]
structure. Thus, it often forms poor conjugation between N-2
aryl due to energy penalty for breaking aromaticity. As a result,
EDG substituted NAT at best could only form resonance
structure A, bearing triazole (TA) stabilized anion, which leads
to the limited emission change.
Table 1. Optical properties of EDG/EWG modified NAT compounds inVTieHwF sAortluictlieonOnline
DOI: 10.1039/C9CC00262F
Ph
R
Ph(O)C
N
N
N
Ar
N
N
N
N
O
O
N
R
N
NPh2
R
Ph
3a: R=H;
3b: R=OMe;
3c: R=NMe2;
Ph
Ar=4-NPh2-C6H4
3e: R=4-F-C6H4; 3f: R=4-Cl-C6H4;
3g: R=4-Br-C6H4; 3h: R=C6F5
3i: R=2-naphthenyl; 3m: R=2-F-C6H4
3n: R=2,6-2F-C6H3;
3j: R=TMS; 3k: R=H;
3l: R=4-(N,N-Ph2)C6H4
(A) Limited impact of EDG on N-2-aryl gorup
;
N
3d: R=
λexcitation
(nm)
λemission
(nm)
Φ
(%)
;
Stokes shift
(nm)
N
N
R
N
1a
1b
1c
1d
295
305
316
348
351
374
427
419
65
69
98
96
93
78
Life time
Life time
lex
lem
F
(%)
lex lem
(nm) (nm)
F
(%)
Comp
Comp
(nm) (nm)
t
avg(ns)
3.099 0.37
t
avg(ns)
3.199 0.18
111
71
1a, R=H; 1b, R=OMe;
1c, R=NMe2; 1d, R=NPh2
3a
3b
3c
3d
3e
3f
330
297
335
341
365
357
365
366
377
550
473
524
540
542
3h
3i
3j
3k
3l
368 470
364 537
369 533
368 530
362 537
369 520
370 505
-
0.62
4.1
1.3
25
12
11
2.116
1.464
2.556
1.918
3.494
1.467
18
52
36
4.3
5.7
7.8
6.309
1.707
3.182
2.224
1.576
(B) Hypothesis: extending electron conjugation with the combination EDG and EWG
N
N
N
N
N
N
3m
3n
N
N
N
N
N
O
N
N
3g
[a]Fluorescence emission of compound 3a-3l. Concentration: 20 µmol/L in THF.
B
representative triazole
resonance structrues
N
N
EWG
N
N
O
N
N
N
N
N
N
N
First, for all the C-4 phenylcarbonyl substituted triazole (3b
-
3d), EDGs at N-2 aryl gave rise to the obvious redshift.
Interestingly, structurally rigid carbozole substituted NAT 3d
gave less red-shift, which was likely caused by the poor
conjugation between nitrogen and N-2 phenyl due to their
steric repulsion. Evaluation of the arylcarbonyl group on C-4
position confirmed that electron deficient aryl substitutcents
could enhance the overall emission. NATs with 4-F, 4-Br and 4-
Cl benzene and naphthenyl substitution all resulted in red-shifts
with strong emission. In contrast, the penta-fluoro benzene
extended conjugation
A
C
Figure 1. General strategy to achieve NAT red-shift
.
To access extended conjugation, an accessible TA
resonance structure is necessary, which requires the overall
electron density delocalized from the three adjacent nitrogens
to the carbon. With this consideration in mind, we postulated
that introducing electron withdrawing group (EWG) on triazole
carbon with help the electron delocalization, favoring the
Thus, we proposed that the
combination of EDG on N-2 aryl and EWG on carbon will build
the extended conjugation, giving the desired red-shift NAT as
B
substituted NAT 3h gave less red-shift (lmax=470 nm) and very
resonance structure
B
.
low quantum yield in solution. This can be explained by the
strong electronic repulsion between F and triazole N, which
were confirmed by the crystal structures showing longer
distance between F-N (2.908 Å) in 3h over the H-N distance
(2.760 Å) in 3e. (see crystal structures in Figure S85). Similarly,
the 2-fluoro phenyl and 2,6-difluoro phenyl substitution NAT
3m and 3n also exhibited low fluorescence quantum yield in
solution, which further supported the above conclusion.
shown in intermediate
C (Figure 1B). To verify this hypothesis,
we prepared triazole analogues 2a-2d with varied EWGs on
triazole C-4 position. Their PL data are summarized in Figure 2
.
R
N
N
NPh2
2d
1d
2c
2b
2a
In addition, different substituents on triazole C-5 position
were evaluated. Besides phenyl (2c), NATs with other
functional groups were prepared, including trimethylsilyl (3j),
hydrogen (3k) and electron-donating 4-(N,N-diphenyl)phenyl
N
Ph
1d: R = H; 2a: R = CN; 2b: R = C(O)CH3;
2c: R = C(O)Ph; 2d: R = COOC2H5;
λexcitation λemission red-shift
Φ
Gap energy
(nm) (%)
(eV)
(nm)
(nm)
(
3l). Although red-shift caused by C-5 substitution with either
-
1d
2a
2b
348
364
358
419
471
464
78
87
93
-
52
45
6.14
6.09
trimethylsilyl or hydrogen was trivial (about 10 nm), the overall
emissions remained in the green-yellow light region.
Importantly, significant enhancement of emission quantum
yields was achieved, which was likely caused by the improved
conjugation with less steric hindrance on C-5 position.
Interestingly, incorporation of another EDG on the C-5 position
3l did not result in efficient emission, which might be ascribed
for the lack of effective electronic conjugations. Therefore, by
screening NAT of various substitutions, we developed an
effective strategy to achieve NAT green/yellow fluorophore via
tuning the combination of EDG/EWG on NAT.
2c
2d
365
277
524
449
105
17
96
5.83
-
350
400
450
500
550
600
650
700
30
Figure 2. Significant red-shift with EDG/EWG substituted TPA-NAT
.
As proposed, introducing EWG (cyan or carbonyl group)
on C-4 position gave the expected red-shift while maintaining
excellent fluorescence emission.
phenylcarbonyl substitution (2c), a very dramatic red-shift (105
nm) was observed. Compared with blue-emitting compounds
2a and 2b, quantum yield for 2c was lower, which is common
for long wavelength emission fluorophore. Density functional
theory (DFT) calculations based on single crystal structures were
Notably, with 4-
As motioned above, while electron deficient arylcarbonyl
NATs (such as 3f-3g) generally gave red-shift emissions with
high efficiency, it is worth noticing that the pentafluorophenyl
substituted NAT 3h resulted in poor emission with quantum
also conducted (Table S13). Compared with 2a/2b, 2c possesses
a smaller energy gap (5.83 eV), thus giving a longer wavelength yield <1%. To understand the cause of these results, single
crystal structures of nine NATs have been successfully obtained
(see details in Figure S78-86). Structures of compounds 2c and
3h are shown in Figure 3. For compound 2c, the dihedral angle
between triazole and carbonyl (C=O) is 37.2 o and TA vs Ph is
of fluorescence emission. Encouraged by this result, a series of
EDG/EWG modified NAT compounds were prepared and their
optical properties in THF solution are summarized in Table 1
.
2 | J. Name., 2012, 00, 1-3
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54.1o, not even close to coplanar conformation. Moreover, kr=6.392x107 s-1 and knr=1.579x108 s-1. Thus, it is clear that for
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DOI: 10.1039/C9CC00262F
while compounds 2c and 3h clearly gave different PL property compound 3h, the nonradiative decay is the key factors for the
low FPL in THF solution.12 This also lead to the observed 160
in solution, the solid-state X-ray crystals showed very similar
geometry. Their twisted conformation aroused our interest to
know whether this type of NAT would show different
luminescence properties in solid state and exhibit aggregation-
induced emission (AIE).
times increase of quantum yield in solid state.
By comparing the fluorescence quantum yields of these
compounds in solution and solid state, we estimated that many
of these new NATs might show effective AIE property, especially
for those compounds with FPL(solid)
/FPL(solution)>1, including 2c,
3b 3c 3e 3h 3l and 3m. In fact, the fluorescence intensity of
,
,
-
,
dihedral angle
37.3o
many of these compounds decreased gradually with the
increase of water content up to 70% (Figure 4A). This may be
due to the influence of the polarity of solvent on the TICT
state.13 Further increase of the water fraction led to the
significant increase of fluorescence intensity, reflecting typical
AIE phenomenon. The titration spectra of compound 3h are
shown in Figure 4. (see others in Fig. S26-46). Changing the NAT
54.1o
R5 = H/F
59.8o
30.2o
N
N
O
N
NPh2
Ph
λem=524 nm, Φ=18%
λem=473 nm, Φ<0.2%
rotation at C-4-keto-aryl
2c
3h
Figure 3. Crystal structures revealing large dihedral angle.
o
In 2001, Tang’s group first introduced the concept of
aggregation-induced emission (AIE) into the community.10
Since then, a great number of organic molecules have been
reported with AIE properties and applied in a wide range of
material research and industrial applications.11 To explore the
potential aggregation induced emission of these new type of
fluorophores, we first measured their solid-state luminescence
property. The data of some representative compounds are
summarized in Table 2 (see detailed data in Figure S19-24).
sample measuring temperature from 0 to 50 C gives a slight
blue-shift in most cases, suggesting temperature has a small
influence on the TICT state (Figure S13-18). Interestingly, in
most of cases, addition of water (AIE test) gave unusual
hypochromic shift. This may be ascribed for different emission
modes of these compounds (eg. local emission vs ICT emission).
This was supported by the case of 3c where two emission bands
were observed (Fig. S35). One band at short wavelength may
come from the local emission while the another band is related
to ICT emission. The observed hypochromic shift may be
ascribed to one of these specific emission modes.
Table 2. Solid state Optical properties of EDG/EWG modified NATs
A
B
0
90
Figure 4. (A) PL spectra of 3h (10 μM) in acetone and THF/water mixtures with
different water fractions (fw). (B) Plots of emission intensity of 3h versus the water
mixtures.
During the exploration of NAT solid-state emissions, we
found that some the NATs showed the mechanochromic
property. Upon grinding of solid powder, some NATs showed
clear red-shift emission.
The emission shifts of these
compounds are summarized in Table 3
.
Table 3. NAT mechanochromic emission [a]
Most of the EDG/EWG modified NATs exhibited solid-state
emission with different wavelengths. Detailed spectroscopic
data along with CIE 1931 chromaticity diagram are provided in
supporting information (Figure S25). Importantly, N, N-
diphenyl substitutions on N-2 aryl position is crucial for high
l
em-solution (nm)
l
em-solid (nm) lem grinding (nm) Dlem (nm)
2c
3c
3e
3g
3m
524
550
524
542
520
457
504
464
478
461
492
514
487
499
494
35
10
23
21
33
[a]Here we discuss compounds whose fluorescence emission peaks change more
than 10 nm before and after grinding.
solid-state luminance emission (such as 1d, 2a-2c, 3e-3n) with
quantum yields up to 40%. To understand this phenomenon,
fluorescence lifetime (τ) of these NATs were measured both in
solution and in the solid state. Based on those dates, radiative
transition rate constants kr and nonradiative transition rate
constants knr were calculated. For compound 3h, in THF
solution, transition rate constants are kr=5.627x105 s-1 and
knr=3.128x108 s-1. In solid state, these rate constants are
It is known that grinding could cause the collapse of crystal
lattice, converting crystalline material into amorphous
material.14 This has been confirmed by the power diffraction
XRD (Figure S68-72). Breaking the packing can disrupt the
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intermolecular interaction, which would force the NATs to
adopt a more planar conformation and contribute to the
observed mechanochromic luminescence.15 This emission shift
is fully reversible: heating the solid sample to 170 oC for 1
minute gave the initial emission profile with no change. This
2
3
(a) J. Qi, C. Sun, A. Zebibula, H. Zhang, R. T. K. Kwok, X. Zhao,
W. Xi, J. W. Y. Lam, J. Qian and B. Z. Tang, Adv. Mater., 2018,
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process has been repeated multiple cycles as shown in Figure 5
.
Compounds with twisted conformation (2c 3c 3e and 3g)
,
,
demonstrated reasonable emission change after grinding. For
3j which have a coplanar conformation, emission in solid state
did not change after grinding, suggesting that the twisted
conformation between triazole (TA) and 4-arylcarbonyl may be
vital to this mechanochromic luminescence. Compound 3f and
3h, both having twisted conformation in single crystal form (see
Figure S78-86), did not show mechanochromic property. This is
likely associated with the stronger steric hindrance, causing the
higher energy barrier to adopt coplanar conformation. The
absorption spectra of these NATs under heating and ground
conditions were also measured, and there was no significant
changes observed (Figure S73-77). Further mechanism studies
on those phenomena are currently undergoing in our lab.
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Figure 5. (A) PL spectra of unground and ground 2c. (B) Reversible switching of the
emission of 2c through repeated grinding/heating cycles.
Chem. Sci., 2017, 8, 2047-2055; (b) Z. Li, Y. F. Wang, C. Zeng,
In summary, we reported the successful development of
EDG/EWG modified NATs as a new class of organic solid
fluorophores with tunable emission from blue to yellow light
region. With the conformations confirmed by single crystal
structures, we systematically investigated the relationship
between their optical properties and their structures. Notably,
the use of C-4-phenylcarbonyl group and C-5-Phenyl in triazole
ring successfully led to twisted conformation which is crucial for
good emission efficiency of those in solid state. What's more,
some NATs revealed AIE properties and reversible
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properties, suggesting
promising future for employing this strategy to prepare new
organic solid fluorescent materials.
We are grateful to the NSF (CHE-1619590), NIH
(1R01GM120240-01), NSFC (21228204), Jilin Province
(20170307024YY, 20190201080JC) for financial support.
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Conflicts of interest
There are no conflicts to declare.
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