Multi-response quinoxaline-based fluorophores: Solvatochromism, mechanochromism, and water sensoring

Article abstract of DOI:10.1246/cl.190732

Three new D-π-A type quinoxaline-cored fluorophores were elaborately designed and synthesized. It was found that P-1, P-2, and P-3 (Ps) all presented bathochromic-shifted mechanochromic (MFC) nature upon grinding, accompanied with 30 nm, 8 nm, and 37 nm r

Full text of DOI:10.1246/cl.190732

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Multi-Response Quinoxaline-Based Fluorophores: Solvatochromism, Mechanochromism, and
Water Sensoring
Jia Yu, Zhifang Liu, Bowei Wang,* Yuqi Cao, Dongqi Liu, Yixian Wang, and Xilong Yan*
Advance Publication on the web November 12, 2019
doi:10.1246/cl.190732
© 2019 The Chemical Society of Japan
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1
Multi-Response Quinoxaline-Based Fluorophores: Solvatochromism, Mechanochromism, and
Water Sensoring
Jia Yu,a1 Zhifang Liu,a1 Bowei Wang,*1,2,3 Yuqi Cao,1 Dongqi Liu,1 Yixian Wang,1 and Xilong Yan*1,2.3
1School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300350, P. R. China.
2Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin, 300072, P. R. China.
3Tianjin Engineering Research Center of Functional Fine Chemicals, Tianjin, P. R. China
aThese authors contributed equally to this work.
E-mail: yan@tju.edu.cn (Xilong Yan), bwwang@tju.edu.cn (Bowei Wang)
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Three new D-π-A type quinoxaline-cored fluorophores
49 These compounds displayed significant red-shifted
50 solvatochromism, in which P-1, P-2, P-3 exhibited spectra
51 shift from 457 nm, 447 nm, 497 nm in n-hexane to 557 nm,
52 505 nm, 537 nm in DMF, respectively, along with 100 nm,
53 58 nm, 40 nm red-shifts. (Figure S4) In general, the polar
54 solvents have a better stabilization effect on the excited state
55 during π-π* transition process. As a result, the energy gap
56 gets narrower with the increase of solvent polarity,
57 accompanied with red-shifted PL spectra.13,14
were elaborately designed and synthesized. It was found that
P-1, P-2, and P-3 (Ps) all presented bathochromic-shifted
mechanochromic
(MFC)
natures
upon
grinding,
accompanied with 30 nm, 8 nm, and 37 nm red-shift,
respectively. The DSC and PXRD were recorded to reveal
the mechanism of the MFC process. Moreover, P-1 is
sensitive to water in tetrahydrofuran (THF), acetonitrile
(MeCN) and acetone with a detection limit of 0.12%,
10 0.0078%, and 0.0110%, respectively. This work provided a
11 rational strategy for developing multi-responsive
12 fluorescence materials.
13 Keywords: Multi-stimuli-responsive, Water detection,
14 Structure-function relationship
15
Organic fluorophores response to external force stimuli
16 have received extensive attention for their potential
17 applications as security ink,1 fluorescent chemosensors,2,3
18 and optical materials,4,5 and tremendous efforts have been
19 devoted to stimuli-responsive fluorophores, such as
20 mechanochromic materials and water probes.6-11 There
21 already have been some reports concerned with organic
22 water sensors.12 However, few organic molecules possess
23 the MFC, solvatochromic, and water sensoring properties at
24 the same time. In addition, most reported organic water
25 sensors are able to detect water for H-bond acceptor effect
26 or twisted intramolecular charge transfer (TICT) mechanism,
27 and almost none of them acted as a H-bond donor in the
28 water sensoring process. Moreover, the reported water
29 detection mechanism is mostly based on density functional
30 theory (DFT) calculation and few direct evidences have
31 been provided.
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Scheme 1. Chemical structures of Ps.
Moreover, the Lippert-Mataga equation, which relates
62 the Stokes shift () with solvent orientation polarizability,
63 was employed to evaluate the solvatochromism of Ps. It’s
64 obvious that P-1 shows distinct solvatochromic property
65 with the largest slope of fitting line. (Figure S5) Usually,
66 molecules with such outstanding solvatochromic red-shift
67 and excellent linearity in different solvents can be applied as
68 a water sensor in organic solvents.15
32
In this work, we introduced the P-1 based on 2, 3-
33 dimethylquinoxaline template, which is a mechano- and
34 solvatochromic material as well as a water-sensitive
35 material. Moreover, we developed another two quinoxaline-
36 cored fluorophores for comparison with P-1, and named as
37 P-2 and P-3 (Scheme 1). Furthermore, DFT calculation and
38 crystal data were provided for detailed investigation of
39 internal influence factors of water sensing and MFC
40 properties.
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70 Figure 1. Molecular orbital amplitude of P-1 in n-hexane
71 and DMF solution
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The synthesis routes and the characterization of these
42 Ps are illustrated in supporting information. As illustrated in
43 Figure S4, the absorption spectra of Ps displayed two
44 absorption peaks. The absorption peaks in the shorter and
45 longer wavelength region are attributed to π-π* transition at
46 aromatic rings and the whole molecules, respectively.
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As to gain a further understanding of the distinct ICT
74 effect of P-1, DFT calculation was carried out and
75 illustrated in Figure 1. The HOMO orbitals of P-1 were
76 distributed over the whole molecule, while the LUMO
77 orbitals were mainly concentrated on the quinoxaline and
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Considering their D-π-A structures, the PL spectra
48 were characterized in five solvents with different polarity.

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neighboring benzene ring, indicating well charge separated
D-π-A structure. In addition, the energy gap was 3.27 eV in
n-hexane solution and decreased to 3.19 eV in DMF,
attributing to the more stabilized excited state in polar
solvents. Consequently, the emission peaks shifted toward
the long wavelength area in polar solvents.15
Furthermore, the calculation of P-1-water hydrogen
bonding association was also employed to clarify the water
sensing ability of P-1. As illustrated in Figure S6, when P-1
33 the slope of PL intensity vs water fraction) The calculated
34 detection limits were 0.12%, 0.0078% and 0.0110% for P-1
35 in THF, MeCN, and acetone respectively. Compared with
36 the previously reported fluorescence water sensors, the
37 detection limit of P-1 in MeCN is standout among them.
38 (Table S1) Consequently, it is reasonable that P-1 can be
39 employed as an ultrasensitive water sensor in some types of
40 organic solvents.
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According to Lu et al,13 nonplanar D-A type π
10 was bonded with water molecule (P-1-H2O), the energy
11 level of both HOMO (−5.07 eV) and LUMO (−1.88 eV)
12 was obviously driven down, contributing to a narrower
13 energy gap (3.19 eV) compared to the isolated molecule
14 (3.33 eV), which led to an enhanced ICT effect. As a result,
15 the PL intensity experienced a dramatically decrease along
16 the addition of water.
42 conjunction molecules can exhibit MFC behavior and their
43 mechanochromism is anticipated. As illustrated in Figure 3,
44 the emission color of pristine P-1, P-2, and P-3 switched
45 from green, blue, and yellow to yellow, blueish-green, and
46 pale-orange by grinding, accompanied with 30 nm, 8 nm,
47 and 37 nm red-shift, respectively. And they show reversible
48 mechanochromic process by annealing. After grinding, the
49 fluorescence quantum yield (ΦF) of P-1 is decreased from
50 50% to 37% and the lifetime (τ) of P-1 is prolonged from
51 5.27 μs to 6.23 μs. (Table S2) Both P-2 and P-3 show
52 similar phenomenon. Before and after grinding, there are
53 obvious changes in the radiative decay rates (kr) and non-
54 radiative decay rates (knr) for Ps. (kr = ΦF/τ, knr = (1-ΦF)/τ).17
55 Therefore, grinding might alters the fluorescence emission
56 channels. The ΦF of Ps are decreased after grinding, which
57 is attributed to the increased knr or reduced kr by grinding.1
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On the basis of the above discussion, it is possible for
18 P-1 to present water sensitive behavior in organic solvents.
19 Thus, further study was carried out. The PL spectra of P-1 in
20 THF, MeCN, and acetone with water fraction vary from 0%
21 to 10% were illustrated in Figure 2. Typically, the emission
22 intensity of P-1 experienced a dramatically decrease
23 tendency along with slight bathochromic shift with the
24 addition of water. When water fraction (fw) reached up to
25 1.0%, the emission intensity of P-1 descended to 30% in
26 MeCN and a drastic decrease to 2% at fw = 10%. Meanwhile,
27 the PL intensity correlated well with fw, exhibiting perfect
28 linearity. Unsurprisingly, water sensor experiments
29 presented the same downtrend in THF and acetone.
30 Furthermore, the detection limits of P-1 in these organic
31 solvents were calculated with 3σ/k method.14,16 (σ refers to
32 the standard deviation of the blank signal, and k represents
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As we know, the MFC behavior always has an inherent
59 relationship with their staking modes transformation during
60 the grinding process.18 To determine the MFC mechanism,
61 PXRD of these molecules were studied. As demonstrated in
62 Figure 4, pristine samples exhibited sharp and strong curves,
63 indicating well-ordered structure.
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Figure 2. PL spectra of P-1 in (a) MeCN, (b) THF and (c) acetone with different amounts of water (from 0% to 10%, v/v). Plots of
the PL intensity of P-1 with different water contents in (d) MeCN, (e) THF and (f) acetone.
.
68

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21 displayed one endothermic peak in the high temperature-
22 zone, originated from the isotropic melt transition (Tm).
23 (Figure S7) As for the ground P-2, it exhibited an extra
o
24 endothermic peak at a relatively low temperature at 50 C,
25 which might refer to as the solid-solid phase transition.
26 Different from P-2, an extremely weak endothermic peak
27 was observed at 100 oC in P-1 and P-3.
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In order to further understand the correlation between
29 structures and MFC functions, we prepared crystals by slow
30 diffusion of MeCN into a saturated CH2Cl2 solution of
31 complexes. Finally, we got the single crystals of P-2, P-3,
32 and co-crystal with MeCN of P-1 (crystal model has been
33 squeezed).1 The crystal data were illustrated in Table S3,
34 Table S4, and Table S5. All these crystals belong to the
35 monoclinic system and adopted a loose J-type packing
36 modes, leading to structural defects and low lattice energy.
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Figure 3. Normalized PL spectra in solid states of the
original, ground and annealed samples. (a) P-1; (b) P-2; (c)
P-3.
Upon grinding, some sharp peaks of P-1 and P-3 got
broader or even disappeared while the spectra of P-2
experienced minor change. Moreover, some new peaks
appeared in the curves of P-1. The disappeared or broader
peaks of P-3 indicated a morphological transition from well-
10 order crystalline phases to disordered amorphous
11 structures.9 These new peaks of P-1 also suggested a
12 packing mode transformation from one crystal state to
13 another. As for P-2, we supposed its’ crystal structure
14 experienced little packing mode change.13 In general,
15 grinding of the sample sometimes cause solid-solid phase
16 transitions, and the rupture or the formation of new
17 intermolecular interactions, maintaining the crystalline order.
18 To understand the thermo properties of these three
19 compounds, the DSC and TGA results were recorded and
20 illustrated in Figure S7. The original samples of P-2
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Figure 4. XRD spectra of the original, ground and
39 annealed samples of (a) P-1, (b) P-2 and (c) P-3.
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42 interaction with water and other hydrogen bond acceptor
43 solvents. Meanwhile, the torsion angles of the linked
The co-crystal of P-1 indicated the hydrogen bond

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48 References and Notes
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benzene ring between quinoxaline and the carbazole moiety
were 18.19o and 29.92o, respectively. As for P-2 and P-3,
the dihedral angle and torsion angle was 44.91o and 82.28o,
respectively (torsion angle between the linked benzene ring
and quinoxaline was small enough to be negligible). (Figure
5)
Moreover, it was hard to find π-π interaction in the
crystal of P-1 and P-2. However, in the crystal of P-3, one-
dimensional π-π interaction occurred, with a distance of
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14 certain degree for the absence of the π-π interaction.19,20 As
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Figure 5. The configurations of (a) P-2 and (b) P-3.
77 15
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In this work, three D-π-A type multi-responsive
79 16
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20 materials were designed and synthesized. These compounds
21 exhibited distinct solvatochromism, in which P-1 exhibited
22 the largest spectra shift (λs = 100 nm). In addition, the
23 outstanding ICT effect and hydrogen bond effect with water
24 enabled P-1 sensitive to water in THF, MeCN and acetone
25 with a detection limit of 0.12%, 0.0078% and 0.0110%,
26 respectively. Upon addition of water in the organic solvents
27 of P-1, hydro-bond formed, which led to an enhanced ICT
28 process. Thus, the emission intensity decreased as a result,
29 enabling water detecting probability. In addition, the
30 hydrogen bond interaction was further proved by the co-
31 crystal of P-1 with MeCN. Moreover, Ps also presented vary
32 degrees of mechanochromism and explained by single
33 crystals as well as XRD and DSC results. Among them, P-3
34 exhibited the most distinct mechanochromism for its most
35 twisted structure and crystal-amorphous transformation up
36 grinding. As for P-2, the minor MFC red-shift might be
37 attributed to little phase transformation during the grounding
38 process. Notably, our work reported an organic water sensor
39 molecule for H-bond donor mechanism and provided a
40 rational strategy for the design of multi-response organic
41 materials.
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43 This work was financially supported by the National Natural
44 Science Foundation of China (Grant No. 21576194)
45
46 Supporting
Information
is
available
on
47 http://dx.doi.org/10.1246/cl.******.