Non-conjugated anthracene derivatives and their mechanofluorochromic properties

Article abstract of DOI:10.1007/s11164-018-3331-2

A series of non-conjugated methylene–anthracene Schiff base derivatives (DNCAs) were designed and synthesized. Photoluminescence emission spectra indicated that DNCA-4 and DNCA-12 showed obviously mechanofluorochromic properties, and distinctive 26- and 3

Full text of DOI:10.1007/s11164-018-3331-2

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3331-2
Non-conjugated anthracene derivatives and their
mechanofluorochromic properties
Shuai Liu1 Lixia Liu2 Cheng Liang3 Ying Peng3
•
•
•
•
Shuangping Huang3 Xiaotong Li3 Meng Sun1 Xiaoji Wang2
•
•
•
Received: 4 December 2017 / Accepted: 13 February 2018
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract A series of non-conjugated methylene–anthracene Schiff base derivatives
(DNCAs) were designed and synthesized. Photoluminescence emission spectra
indicated that DNCA-4 and DNCA-12 showed obviously mechanofluorochromic
properties, and distinctive 26- and 37-nm hypochromic shifts were observed,
respectively. The experiment results revealed that there is no specific relationship
between alkoxy chain lengths and their stimuli–response behavior. The PXRD
profiles demonstrated a transformation from crystalline to amorphous state upon
grinding.
Keywords Anthracene Á Luminescence Á Stimuli–response
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11164-
018-3331-2) contains supplementary material, which is available to authorized users.
&
&
1
Meng Sun
sunm1127@163.com
Xiaoji Wang
2012207455@tju.edu.cn
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal
University, Nanchang 330013, China
2
School of Life Science, Jiangxi Science and Technology Normal University, Nanchang 330013,
China
3
School of Pharmacy, Jiangxi Science and Technology Normal University, Nanchang 330013,
China
123

S. Liu et al.
Introduction
Mechanofluorochromic (MFC) materials are a kind of smart materials which exhibit
fluorescence color changes in response to external pressure [1–6]. In 2001, Tang
reported the first kind of solid-state MFC materials [7]. Since then, a number of
MFC materials have been successfully developed [8–12], among which 9,10-
bisvinylanthracene derivatives (DSA) have become the focus of organic MFC
materials [13–19]. These materials emit weak fluorescence in solution but intense
fluorescence in the aggregated state. Based on current research, the MFC nature of
the above templates could be significantly affected by the peripheral aryl units
connected to the 9,10-anthylene core. In addition, the alkyl lengths can significantly
influence their MFC properties [20]. However, while many researchers have focused
on the MFC properties of conjugated 9,10-bisvinylanthracene derivatives
[5, 21–25], research on non-conjugated derivatives has seldom been reported.
Therefore, based on the structure of DSA, we modified the molecular structure and
further explored the fluorescent and MFC properties of non-conjugated 9,10-
bisvinylanthracene derivatives.
In this work, three non-conjugated methylene–anthracene Schiff base molecules
(DNCAs) were designed and synthesized. Furthermore, the relationships between
the lengths of alkoxy chains and MFC properties were discussed. The optical and
MFC properties were determined by UV–Vis absorption spectra and liquid/solid-
state photoluminescence (PL) emission spectra. The MFC mechanism was
investigated by powder wide-angle X-ray diffraction (PXRD). This work has laid
the preliminary foundation for research on solid-state fluorescence materials.
Experimental
Materials and characterization
All the raw materials were obtained commercially and used without further
purification. 1H NMR were recorded by using a Bruker Avance 400 MHz
spectrometer with CDCl3 as solvent. High-resolution mass spectra were measured
on a Bruker Paltonicsmicro TOF-Q II instrument. UV–Vis spectra were performed
on an Evolution 300 UV–Vis spectrophotometer. Photoluminescence spectra of
solids were obtained with a Horiba Jobin–Yvon Fluorolog-3 spectrophotometer.
PWXD measurements were recorded on a Miniflex 600 Powder X-ray Diffrac-
tometer of Rigaku, with the following instrument parameters: 35 V, 15 A, and
5°min-1
.
Synthesis of compound 1 [26]
To a round-bottom flask containing anthracene (1.00 g, 5.60 mmol), paraformalde-
hyde (0.17 g, 5.6 mmol), CTAB (22.4 mg, 0.06 mmol) and acetic acid (1.4 mL),
4.12 mL HBr aqueous solution (concentration 40%) were added dropwise. The flask
123

Non-conjugated anthracene derivatives and their…
was placed under N2 at 80 °C for 5 h. The mixture was cooled to room temperature
and filtered, washed with 100 mL distilled water, dried under vacuum and
recrystallized from toluene to give yellow needle-shaped crystals, 0.43 g, 21%
1
yield. H NMR (400 MHz, CDCl3, ppm): 7.57–7.59 (m, 4H), 7.36–7.33 (m, 4H),
4.46 (s, 4H).
Synthesis of compound 2 [27]
A mixture of compound 1 (0.24 g, 0.66 mmol) and methenamine (0.28 g,
2.00 mmol) was refluxed in 35 mL chloroform for 24 h. The mixture was cooled
to room temperature and the yellow solid was isolated by filtration and washed with
30 mL chloroform. The solid was refluxed in 30 mL ethanol and 10 mL
hydrochloric acid (37.5%) for 48 h and then cooled to 0 °C. After filtration, the
crude product was washed with 50 mL cold ethanol to give the hydrochloride of
compound 2. The above compound was dissolved in distilled water. After the pH
was adjusted to 8, the solution was extracted three times with chloroform
(3 9 30 mL). The combined organic phases were dried over anhydrous MgSO4 and
concentrated to give a yellow solid, 0.11 g, 73% yield.
General procedures for the synthesis of compounds 3 [28]
To a flask containing of 2-hydroxybenzaldehyde (3.00 g, 24 mmol), K2CO3 (3.18 g,
30 mmol), 30 mL N,N-dimethylformamide and KI (catalytic amount), 1-bromobu-
tane (2.40 mL, 27 mmol) were added dropwise. After stirring at 80 °C for 8 h, the
mixture was cooled to room temperature and 50 mL of distilled water was added.
The mixture was extracted three times with dichloromethane (3 9 30 mL). The
organic phase was washed with 10% NaOH solution (3 9 30 mL) and distilled
water (3 9 30 mL), respectively, dried over anhydrous MgSO4 and concentrated to
give a brown liquid, 3.30 g, 73% yield. Compounds 3–8 and 3–12 were prepared in
the same manner.
General procedures for the synthesis of compounds DNCAs
To a flask containing compound 2 (1.27 g, 5.38 mmol) and 3–4 (2.84 g, 16 mmol),
anhydrous MgSO4 (2.02 g, 21 mmol) and 20 mL anhydrous dichloromethane were
added. After stirring at 35 °C under N2 for 12 h, the mixture was concentrated,
filtered, recrystallized from dichloromethane/methanol (1/2, v/v), and filtered to
give a pale yellow solid, 1.27 g, 42% yield. M.p.: 160.1–160.2 °C. 1H NMR
(400 MHz, CDCl3, ppm): 8.72 (s, 2H), 8.45–8.42 (m, 4H), 7.95–7.93 (m, 2H),
7.56–7.54 (m, 4H), 6.92 (t, J = 7.60 Hz, 2H), 6.80 (d, J = 8.40 Hz, 2H), 3.82 (t,
J = 6.40 Hz, 4H), 1.51–1.44 (m, 4H), 1.15–1.05 (m, 4H), 0.77 (t, J = 7.60 Hz,
6H); HRMS (ESI): calcd. for C38H40N2O2 556.3084 [M ? H]?; found 557.3165.
DNCA-8: white solid, 1.44 g, 40% yield. M.p.: 110.2–110.3 °C. 1H NMR
(400 MHz, CDCl3, ppm): 8.77 (s, 2H), 8.46–8.44 (m, 4H), 7.93–7.92 (m, 2H),
7.56–7.54 (m, 4H), 6.91 (t, J = 7.60 Hz, 2H), 6.81 (d, J = 8.40 Hz, 2H), 3.83 (t,
123

S. Liu et al.
J = 6.40 Hz, 4H), 1.54–1.51 (m, 4H), 1.35–1.19 (m, 21H), 0.93 (t, J = 7.60 Hz,
6H); HRMS (ESI): calcd. for C46H56N2O2 668.4436 [M ? H]?; found 669.4415.
DNCA-12: yellow solid, 1.38 g, 33% yield. M.p.: 81.9–82 °C. 1H NMR
(400 MHz, CDCl3, ppm): 8.78 (s, 2H), 8.46–8.44 (m, 4H), 7.94–7.91 (m, 2H),
7.56–7.53 (m, 4H), 6.91 (t, J = 7.60 Hz, 2H), 6.81 (d, J = 8.40 Hz, 2H), 3.83 (t,
J = 6.40 Hz, 4H), 1.54–1.51 (m, 4H), 1.30–1.19 (m, 40H), 0.91 (t, J = 7.60 Hz,
6H); HRMS (ESI): calcd. for C54H72N2O2 780.5588 [M ? H]?; found 781.5665
(Scheme 1).
Results and discussion
UV–Vis spectra
UV–Vis absorption spectra of three DNCAs (DNCA-4, DNCA-8 and DNCA-12)
were examined. As shown in Fig. 1, the UV–Vis absorption spectra of all three
compounds are quite similar (kmax = 254 nm). The peaks centered at 250 nm can
be attributed to p–p* transitions, and the absorption bands located at 380 and
400 nm can be assigned to the weak ICT transition [29].
Mechanofluorochromic properties
Mechanofluorochromic properties were examined to explore the optical properties
of DNCAs. As shown in Fig. 2, it can be seen that DNCA-4, DNCA-8 and DNCA-
12 presented pale yellow, light white and yellow under natural light, respectively.
Upon mechanical stimuli, all of them displayed obvious color changes, into faint
Scheme 1 Synthesis and structures of DNCAs
123

Non-conjugated anthracene derivatives and their…
Fig. 1 UV–Vis absorption
spectra of DNCAs in THF
solution; concentration: 10 lM
Fig. 2 Emission images of DNCAs before and after grinding under natural light and a 365-nm UV lamp.
a DNCA-4, b DNCA-8 and c DNCA-12
yellow, milk white and white, respectively, under natural light. The fluorescence of
the ground DNCA-4 and DNCA-12 were changed from cyan to purple, while
DNCA-8 still emitted cyan fluorescence under a 365-nm UV lamp, indicating the
obvious MFC properties of DNCA-4 and DNCA-12. Furthermore, DNCA-12 was
found to be rather special as its fluorescent color could be changed by fuming with
DCM. All these samples returned to their original states on recrystallization from
DCM/methanol (1/2, v/v). This process indicated the unchanged chemical structures
of these compounds despite the changed fluorescence upon grinding or fuming.
123

S. Liu et al.
To quantify the optical properties of the DNCAs under pressure and fumigation,
fluorescence emission spectra of each sample were investigated: (1) the original
samples (original); (2) the mixed samples of original samples and KBr under
pressure of 2400 psi for 3 min (pressed); (3) the samples annealed under certain
temperature for 5 min (annealed); (4) the samples fumigated with dichloromethane
(fumed); (5) the original crystals fumigated with dichloromethane (original-fumed);
(6) the ground samples (ground); and (7) the ground samples fumigated with
dichloromethane (ground-fumed). All these solid fluorescence emission spectra are
shown in Fig. 3, and their fluorescence spectra data are listed in Table 1.
It can be seen from Table 1 and Fig. 3 that the DNCAs had different
performances in response to grinding and fuming stimulation. The original sample
of DNCA-4 exhibited a maximum fluorescence emission peak at 460 nm with two
shoulder peaks at 434 and 487 nm. After grinding, the maximum fluorescence
emission wavelength shifted to 434 nm along with a shoulder peak at 460 nm, and a
26-nm PL spectrum hypsochromic shift could be seen. For the original sample of
DNCA-8, there was a fluorescence emission peak at 442 nm and a shoulder at
464 nm. Both of the ground samples before and after fuming had exhibited a
maximum fluorescence emission wavelength at 440 nm with almost no wavelength
change. For DNCA-12, the maximum fluorescence emission wavelength exhibited a
37-nm blue-shift after grinding. The fluorescence emission peak of the ground
Fig. 3 The PL spectra of DNCAs under original, pressed, pressed-annealed, pressed-DCM fumed,
original-DCM fumed, ground, and ground-DCM fumed states
123

Non-conjugated anthracene derivatives and their…
Table 1 Peak emission wavelengths (k/nm) of DNCAs under various external stimuli
Samples
kOriginal kGround kOriginal-fumed kGround-fumed kPressed kAnnealed kFumed kRecrystal
DNCA-4
DNCA-8
460
442
434
440
434
460
442
434
460
440
434
434
439
460
434
439
432
434
439
432
459
442
466
DNCA-12 471
Dk
DNCA-4
DNCA-8
DNCA-12
Dk1
Dk2
Dk3
26
26
0
2
3
0
37
11
37
Ground-induced spectral shift, Dk1 = kOriginal - kGround
Pressing-induced spectral shift, Dk2 = kOriginal - kPressed
Fuming-induced spectral shift, Dk3 = kOriginal - kOriginal fumed
sample showed no significant change after fumigating. In conclusion, the maximum
fluorescence emission peak of the DNCAs showed blue-shifted mechanofluo-
rochromic properties. However, there was no general regularity between the MFC
properties and the changes in alkoxy chain lengths.
As we can see from Fig. 3, the fluorescence spectra of DNCA-4 and DNCA-8
show no difference after fumigation (Dk = 0 nm), while the maximum fluorescence
emission wavelength of DNCA-12 blue-shifted to 434 nm (Dk = 37 nm). We
speculate that the original states of the DNCAs are microcrystalline structures. After
grinding, the crystalline structures of these compounds were changed or destroyed,
which led to blue-shifts of their fluorescence spectra. When the ground samples
were fumigated, these compounds did not recover their original crystalline states.
On account of the solvatochromic properties of the DNCAs, we consider that only
the original sample of DNCA-12 underwent microcrystalline change during solvent
fumigation.
On the other hand, for DNCA-4 and DNCA-8, no fluorescence change upon
grinding, annealing or fuming was observed. The PL spectra of DNCA-12 exhibited
a blue-shifted tendency upon external pressure. After annealing and fuming, the
fluorescence exhibited a further blue-shifted PL spectrum. According to the above
results, we believe that the crystals of DNCA-12 could not be effectively changed or
damaged upon mechanical stimuli, but could be changed upon annealing or fuming.
PXRD experiments
In order to explore the mechanochromism of DNCAs, PXRD was performed. As
shown in Fig. 4, the diffraction patterns of the three original DNCAs exhibited
multiple sharp and intense reflections, indicating regular micro-crystalline struc-
tures. Upon grinding, the intensity of the diffraction peaks of DNCA-4 decreased
and even disappeared, which can be ascribed to the transformation from the
crystalline phase to the disorder amorphous state. However, the ground DNCA-12
123

S. Liu et al.
Fig. 4 Powder X-ray diffraction patterns of DNCAs solids
exhibited stronger reflections accompanied by some new peaks, which can be
attributed to the transformation between two different crystalline states [30, 31]. As
a result, a 26-nm spectral shift (Dk1) was obtained. No change could be seen when
the ground DNCA-4 compound was annealed. This result illustrated that the
disorder amorphous state was not able to return to the crystalline state by annealing.
Moreover, for the DCM fumigated form, no spectral shift can be seen from DNCA-
4, although the intensity of diffraction patterns distinctly declined. As shown in
Fig. 4, DNCA-8 displayed almost no diffraction pattern change. Therefore, there is
little relationship between the alkoxy lengths and fluorescence emissions, but
molecular stacking plays a leading role in the chromism of the DNCAs.
Conclusion
Three non-conjugated methylene–anthracene Schiff molecules (DNCAs) were
designed and synthesized. All these compounds exhibited MFC properties, but there
was no specific relationship between the changes of the alkoxy chain lengths and the
MFC properties. Among them, DNCA-4 and DNCA-12 obviously showed
mechanofluorochromic properties with Dk1 (koriginal - kground) of 26 and 37 nm,
respectively. However, only a shift of 2 nm was obtained from DNCA-8. PXRD
123

Non-conjugated anthracene derivatives and their…
profiles indicated a transformation between two different crystalline states, which
might account for the mechanochromism of the DNCAs. We hope our work provide
a new way to develop novel MFC materials.
Acknowledgement We thank the National Natural Science Foundation of China (Nos. 21562020;
21362012), Scientific Research Fund of Jiangxi Provincial Education Department (No. KJLD12036).
References
1. Y. Sagara, T. Kato, Nat. Chem. 1, 605 (2009)
2. Q.K. Qi, J.Y. Qian, X. Tan et al., Adv. Funct. Mater. 25, 4005 (2015)
3. Z.H. Guo, Z.X. Jin, J.Y. Wang et al., Chem. Commun. 50, 6088 (2014)
4. J. Wang, J. Mei, R. Hu et al., J. Am. Chem. Soc. 134, 9956 (2012)
5. B. Xu, M. Xie, J. He et al., Chem. Commun. 49, 273 (2013)
6. A. Pucciand, G. Riggeri, J. Mater. Chem. 21, 8282 (2011)
7. B. Tang, X. Zhan, G. Yu et al., J. Mater. Chem. 11, 2974 (2001)
8. R. Misra, T. Jadhav, B. Dhokale et al., Chem. Commun. 50, 9076 (2014)
9. L.Q. Yan, Z.N. Kong, Y. Xia et al., New J. Chem. 40, 7061 (2014)
10. Q. Zhao, L. Li, F.Y. Li et al., Chem. Commun. 6, 685 (2008)
11. F. Wang, X. Li, S. Wang et al., Chin. Chem. Lett. 27, 1592 (2016)
12. K. Duraimurugan, J. Sivamani, M. Sathiyaraj et al., J. Fluoresc. 26, 1211 (2016)
13. L. Bu, M. Sun, D. Zhang et al., J. Mater. Chem. C. 1, 2028 (2013)
14. X. Zhang, Z. Chi, J. Zhang et al., J. Phys. Chem. B. 115, 7606 (2011)
15. W. Liu, J. Wang, Y.Y. Gao et al., J. Mater. Chem. C. 2, 9028 (2014)
16. M. Zheng, D.T. Zhang, M.X. Sun et al., J. Mater. Chem. C. 2, 1913 (2014)
17. X. Zhang, Z. Chi, X. Zhou et al., J. Phys. Chem. C 116, 23629 (2012)
18. Y.J. Dong, B. Xu, J.B. Zhang et al., Angew. Chem. Int. Ed. 51, 10782 (2012)
19. H. Li, X. Zhang, Z. Chi et al., Org. Lett. 13, 556 (2011)
20. Y. Xiong, X.L. Yan, Y.W. Ma et al., Chem. Commun. 51, 3403 (2015)
21. H. Zhang, E. Guo, Y. Zhang et al., Chem. Mater. 21, 5125 (2009)
22. M. Sun, D. Zhang, Y. Li et al., J. Lumin. 148, 55 (2014)
23. M. Zheng, M. Sun, Y. Li et al., Dyes Pigments 102, 29 (2014)
24. J. Li, T. Liu, M. Zheng et al., J. Phys. Chem. C 117, 8404 (2013)
25. M. Zheng, M. Sun, D. Zhang, Dyes Pigments 101, 109 (2014)
26. D.E. Wu, M.N. Wang, Y.H. Luo et al., Cryst. Eng. Commun. 17, 9228 (2015)
27. H.Y. Chen, M. Zhao, Y. Li et al., Tetrahedron Lett. 55, 1802 (2014)
28. Y.Q. Cao, Y. Xi, X.Y. Teng et al., Dyes Pigments 137, 75 (2017)
29. Y.Q. Cao, L.G. Chen, Y. Xi et al., Mater. Lett. 212, 225 (2018)
30. C.P. Ma, X.Q. Zhang, L.T. Yang et al., Dyes Pigments 136, 85 (2017)
31. F. Qi, J.J. Lin, X.Q. Wang et al., Dalton Trans. 45, 7278 (2016)
123