Multi-stimuli-responsive fluorescent switching properties of anthracene-substituted acylhydrazone derivative

Accepted Manuscript Title: Multi-stimuli-responsive fluorescent switching properties of anthracene-substituted acylhydrazone derivative Authors: Mingang Zhang, Jue Wei, Yina Zhang, Binglian Bai, Fangyi Chen, Haitao Wang, Min Li PII: DOI: Reference:

S0925-4005(18)31190-0 https://doi.org/10.1016/j.snb.2018.06.085 SNB 24922

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

8-1-2018 16-6-2018 19-6-2018

Please cite this article as: Zhang M, Wei J, Zhang Y, Bai B, Chen F, Wang H, Li M, Multi-stimuli-responsive fluorescent switching properties of anthracenesubstituted acylhydrazone derivative, Sensors and Actuators: B. Chemical (2018), https://doi.org/10.1016/j.snb.2018.06.085 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Multi-stimuli-responsive

fluorescent

switching

properties

of

anthracene-substituted acylhydrazone derivative

Mingang Zhanga,b, Jue Weia,*, Yina Zhangc, Binglian Baia,*, Fangyi Chenb, Haitao Wangb, Min Lib,*

College

of

Physics,

Jilin

University,

Changchun

130012,

[email protected], [email protected]

E-mail:

Laboratory for Automobile Materials (JLU), Ministry of Education. Jilin University,

Changchun 130012, PR China. E-mail: [email protected]

College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China.

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c

China.

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bKey

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a

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Graphical abstract

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The AIE-active AHP-T8 exhibits remarkable and reversible acid/base and

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grinding/heating stimulated fluorescence switching properties.

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Highlights 

Anthracene-substituted acylhydrazone derivative (AHP-T8) shows aggregationinduced emission behavior.



The AHP-T8 exhibits mechanofluorochromism in that grinding and heating/ fuming could change the emission colors.



The AHP-T8 exhibits remarkable and reversible acid/base stimulated fluorescence switching properties in organogel, solution and solid state.

Abstract

responsive

fluorescence

switching

behaviors.

The

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Anthracene-substituted acylhydrazone derivative (AHP-T8) exhibits multi-stimuliAHP-T8

exhibits

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mechanofluorochromism in that grinding and heating could change the emission colors. The transformation of crystalline features and amorphous state between the xerogel and

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ground states might be attributed to the mechanochromic luminescence properties.

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Protonation-deprotonation has a significant effect on the frontier molecular orbitals,

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resulting in AHP-T8 exhibiting remarkable and reversible acid/base stimulated fluorescence switching properties in both organogel and solid state. The multi-stimuli

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responsive fluorescence properties reveal that AHP-T8 may be a potential candidate

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for applications in sensing, security label and a fluorescence indicator for acids/bases. Keywords: Anthracene, Stimuli responsive, Aggregation induced emission (AIE),

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Mechanofluorochromism, Acidochromism.

1. Introduction

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Organic functional materials in response to external stimuli have attracted considerable attention due to their potential applications in sensors [1-9], displays, memory chips [10, 11], security inks [12, 13] and photoelectronic devices [14-16], etc. Several organic molecules with efficient and switchable solid state fluorescence have been developed, which respond to external stimuli such as light [17], mechanical force [18, 19],

acid/base [20, 21]. Although a number of mechanofluorochromic materials have been successfully developed [22-24], these materials currently still need to be explored in depth and an understanding of the structure-property relationships of solid-state emission behavior is mandatory in order to better design and develop the materials with intriguing properties. Acid/base responsive luminescent materials have also been

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developed based on the control of their molecular conformations or frontier molecular orbitals [25, 26]. For example, tetrahydro[5]helicene-based dye exhibited remarkable

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and reversible acid/base stimulated fluorescence switching properties in both solutions and solid state, in which the dye was changed from “D-A type” to “A-A type” in the

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presence of acid and the protonation has a significant effect on the frontier molecular orbitals [27]. However, reports on both mechanical force and acid/base responsive

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fluorescence switching of one chromophore are still limited [28-32]. Tian et al. reported that the reversible mechanochromism and protonation effect of a divinylanthracene

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derivative, protonation-deprotonation of the pyridine moieties has a significant effect

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on the frontier molecular orbitals, resulting in distinct green and red emissions under acid and base stimuli [33]. Wang et al. investigated the acidochromic and

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mechanochromic properties of a donor-acceptor structured compound and demonstrated that the fluorescence emission could be changed using different stimuli

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[34]. Recently, a high fluorescence emission organic four-coordinate difluoroboron complex with high yield has been developed and its colors and fluorescence emission

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can be switched by external stimuli including grinding and acids/bases [35]. In order to develop new stimuli-responsive luminescent materials and further understand the structure-property relationship of mechanofluorochromic materials, herein, we studied the mechanofluorochromic and acidofluorochromic behavior of anthracene-based acylhydrazone derivative AHP-T8 (Scheme 1), the fluorescence emission of AHP-T8 can be switched by grinding/heating and acids/bases.

2. Experimental section 2.1 Materials 3,4,5-methoxyl phenyl-9-anthracene acylhydrazone (AHP-T1) was synthesized by having 3,4,5-methoxylbenzhydrazide reacting with 9-anthraldehyde in ethanol under

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reflux condition for 4 hours. The crude products were purified by repeated recrystallization from tetrahydrofuran for further NMR, FT-IR. 1

H NMR (300 MHz, DMSO-d6): δ 12.00 (s, 1H), 9.66 (s, 1H), 8.75 (t, 3H), 8.19-8.17

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(d, 2H), 7.69-7.58 (m, 4H), 7.35 (s, 2H), 3.92-3.77 (d, 9H).

FT-IR (KBr, cm-1): 3189, 3057, 3038, 3005, 2978, 2962, 2935, 2834, 1641, 1585, 1561,

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1539, 1503, 1465, 1453, 1433, 1413, 1368, 1344, 1333, 1283, 1239, 1185.

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Single-Crystal XRD data of AHP-T1 crystal: Monoclinic, space group P21, a=

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4.9082(2) Å, b=13.7755(7) Å, c=15.3401(8) Å, α=90°, β=95.100(2)°, γ=90°,

reflections=4991,

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d=1.332g/cm3, V=1033.08(9) Å3, Z=2, T=303 K, total reflections=10726, unique R(int)=0.0334,

GOF=1.044,

Final

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indices

R1=0.0810,

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wR2=0.1688, R indices (all data) R1=0.0568, wR2=0.1459. Using the similar method, compound AHP-T8 was synthesized and characterized [36].

H NMR spectra were recorded on a Bruker Avance 300 MHz spectrometer, using

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1

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2.2 Characterization

dimethyl sulfoxide-d and chloroform-d as solvent and tetramethylsilane (TMS) as an internal standard (δ=0.00). X-ray diffraction (XRD) experiments were performed on a

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Bruker Avance D8 X-ray diffractometer. FT-IR spectra were recorded with a PerkinElmer spectrometer (Spectrum One B). The xerogels were obtained by freezing and pumping the organogel of AHP-T8 for 8 h, and then the xerogels were pressed into a tablet with KBr for FT-IR measurement. UV-vis absorption spectra were recorded on a Shimadzu UV-2550 spectrometer, and photoluminescence was measured on a Perkin-

Elmer LS 55 spectrometer. The room-temperature luminescence quantum yields in solutions were determined relative to quinine sulfate in sulfuric acid aqueous solution (0.546),

and

calculated

according

to

the

following

equation:

Φunk=Φstd(Iunk/Aunk)(Astd/Istd)(ηunk/ηstd)2, where Φunk is the radiative quantum yield of the sample; Φstd is the radiative quantum yield of the standard; Iunk and Istd are the integrated

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emission intensities of the sample and the standard, respectively; Aunk and Astd are the absorptions of the sample and the standard at the excitation wavelength, respectively;

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and ηunk and ηstd are the indexes of refraction of the sample and standard solutions (pure

solvents were assumed), respectively. The thermal properties of the compounds were investigated with a TA Q20 DSC instrument. The rate of heating and cooling was 10 ℃

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min-1. Field emission scanning electron microscopy (FE-SEM) images were taken with

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a SU8010 apparatus.

Scheme 1. The molecular structures of AHP-T1 and AHP-T8.

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3.1 Aggregation-induced emission behavior of AHP-T8 To

observe

the

aggregation-induced

emission

(AIE)

of

AHP-T8,

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photoluminescence (PL) emission behaviors in the mixture of THF-water were studied.

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The AHP-T8 is soluble in common organic solvents such as tetrahydrofuran (THF), dichloromethane, chloroform and toluene, but is insoluble in water. As shown in Fig. 1, when the water fraction is less than 50%, the PL intensity is very weak. However, when the water fraction reaches to 60%, the THF-water mixture’s luminescence significantly increased, and the emission peaks have a significantly red shift comparing

to that in the pure THF, shift from 408 nm to 492 nm. And the quantum yield increased fromΦ=0.06% to Φ=5.65%, which is approximately 94 times higher than that in the pure THF. Thus, the increase in PL intensity can be attributed to the AIE effect caused by the formation of molecular aggregates when the water is added into the solution. This result indicates that the compound AHP-T8 exhibits a strong AIE activity.

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Whereas when the water fraction is further increasing to 70%, the PL intensity decreases (Φ=0.27%). Then the PL intensity sharply enhances again with the water

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fraction keeping on increasing.

As can be seen from the Fig. 1b, the PL intensity for compound AHP-T8 in the THF-

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water mixtures shows no trend of monotonic increasing, but exhibits a zig-zag pattern

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after the formation of molecular aggregates. When the water fraction reaches to 60%,

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the macroscopic luminescent self-assembly aggregates are observed under UV light (Fig. 1c), and the entangled and dense fibrous aggregation morphology (Fig. S4a) like

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organogel forms, thus, we speculate the reason of the PL intensity reaching a maximum

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intensity at 60% water fraction (Φ=5.65%) may be the formation of the intermolecular hydrogen bonds between -C=O and H-N- groups reduce the bond rotation within AHP-

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T8 to prohibit the nonradiative transitions to some extent, which is similar to the gelation-induced enhanced fluorescence emission of compound AHP-T8 [36]. The

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larger aggregation forms with increasing water content at 70% (Fig. S4b), and the PL intensity decreases (Φ=0.27%). This may be due to the more contribution of the

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molecules on the surface of the aggregates in the emission intensity [37, 38]. It is probably due to the intermolecular hydrogen bonds between -C=O and H-N- groups weakened by water and then the entangled fibrous aggregation morphology destroyed with further increasing water content, the regular crystals particles forms (Fig. S4d), which afford higher quantum yield (90% water fraction, Φ=0.93%) [37].

In addition, the formation of various aggregates of the molecules can be also confirmed by the absorption spectra of AHP-T8 in the THF-water mixtures (Fig. S5). It can be seen that the absorption spectra of the water fraction is 60%, 70%, 80% and 90% exhibits completely differences in absorption band shapes and maxima, as well as the

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leveling-off tail in the visible region.

Fig. 1. (a) PL spectra of AHP-T8 in THF-water mixtures with different water fractions

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(λex= 365 nm); (b) a plot of emission intensity vs. water fractions; (c) fluorescence

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images of AHP-T8 in THF-water mixtures. 3.2 Mechanofluorochromic behavior of AHP-T8

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The AHP-T8 could form stable organogel in cyclohexane, and AHP-T8 exhibits gelation-induced enhanced fluorescence emission [36]. Interestingly, AHP-T8 displays a mechano-responsive fluorescence switching. AHP-T8 cyclohexane xerogel shows

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bright green luminescence at 501 nm (Φ=12.35%). Upon grinding directly with a pestle in a mortar, the bright green emissive transforms to a yellowish-green fluorescence (Fig. 2a) with an emission maximum at 515 nm (Φ=14.37%) (Fig. 2b). The bright green emission could be recovered by annealing the ground sample at 130 ℃ for 20 min or

fuming with dichloromethane (DCM) vapor, confirming that the mechanochromic

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property is reversible (Fig. S6).

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Fig. 2. (a) Fluorescence images and (b) PL spectra of AHP-T8 xerogel from

cyclohexane under external stimuli: grinding; annealing at 130 ℃ for 20 min; fuming

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with DCM (λex = 400 nm).

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To elucidate the mechanism of this mechanofluorochromism for AHP-T8, X-ray

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crystal structure analysis of AHP-T1 was firstly performed. Rod-like yellow crystals

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of AHP-T1 was successfully obtained by recrystallization from dimethylformamide solutions at room temperature. The molecular packing and the crystallographic data of

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AHP-T1 are summarized in Experimental Section. The compound AHP-T1 crystallizes in the monoclinic space group P21 with two molecules per unit cell. The

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anthracene ring and the benzene ring are almost vertical, as the dihedral angle (φ) (Fig.

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S8) is measured to be 87.2°, which implies that a highly twisted conformation exists in the single crystal. The anthracene moieties are overlapped in a J-aggregation manner with a separation of their planes equal to 3.424 Å, and the slip distance of the nearest

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neighbour anthracene ring along the long axis (Δy) and the short axis (Δx) of anthracene ring are 3.409 Å and 0.886 Å, respectively (Fig. 3a), suggesting π-π stacking interaction between anthracene groups. The hydrogen bonding length of -C=O···H-Nis 2.012 Å (Fig. 3b), which is in the range of moderate hydrogen bonds [39]. As shown

in Fig. 4, the crystal of AHP-T1 shows bright green luminescence at 507 nm (Φ

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=43.40%), which is similar to the emission of AHP-T8 cyclohexane xerogel.

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Fig. 3. (a) The top view of the nearest neighbour anthracene ring along the π-stacking

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direction in AHP-T1 crystals, (b) Molecular stacking and intermolecular hydrogen

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bonding interactions.

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Fig. 4. PL spectra of AHP-T1 crystals. The absorption spectrum (Fig. S11) of the AHP-T8 xerogel is obviously red-shifted compared with its solution, indicating the formation of the J-aggregation in xerogel. The maximum absorption undergoes blue shifts from 418 nm to 402 nm after grinding. Whereas compared to that of cyclohexane solutions (1×10-5 M), the maximum

absorptions after grinding are still red-shift, which shows that AHP-T8 still is Jaggregation in the ground state [40]. The blue-shift of absorption after grinding compared to xerogel may be due to the increase of the overlap of anthracene planes between the adjacent molecules, which is also consistent to the less overlap of neighboring anthracene rings leading to shorter wavelength emission, while longer

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wavelength emission was due to stronger interchromophore interaction of anthracene rings [41-43].

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To further gain an insight into the MFC behavior of AHP-T8, FT-IR, SEM, XRD and

DSC experiments were conducted. FT-IR spectra (Fig. S12) shows that the N-H

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stretching band of AHP-T8 shifts from 3180 cm-1 to 3201 cm-1 after grinding. This indicates that the grinding disrupts the intermolecular hydrogen bonds, which might

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lead to a variation in the molecular arrangements [44]. The intertwined fibrous network is observed for the xerogels (Fig. 5a). In contrast, the ground sample shows amorphous

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powder (Fig. 5b).

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The XRD pattern of the AHP-T8 cyclohexane xerogel consists of sharp strong firstorder diffraction and the dispersing second-order and fourth-order diffraction,

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suggesting a well-ordered layer structure (Fig. 6). Although the peak position of the ground sample are consistent with those of the xerogel sample, the diffraction peaks are

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attenuated obviously and some of the diffraction peaks even disappeared, indicating the amorphous nature after grinding. Annealing or fuming the ground sample, the sharp

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diffraction peaks reappear, suggesting that the mechanochromic property is reversible. The changes of aggregation state before and after grinding are also confirmed by DSC experiments. As shown in Fig. S14, there is clearly a new broad endothermic transition peak at 143.57 ℃ before the melting point (174.25 ℃) for ground sample.

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Fig. 5. SEM images of AHP-T8 xerogel from cyclohexane (a) before and (b) after

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grinding.

Fig. 6. XRD pattern of AHP-T8 xerogel from cyclohexane under external stimuli:

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grinding; annealing at 130 ℃ for 20 min; fuming with DCM.

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3.4 Acidochromism of AHP-T8 Another notable feature of compound AHP-T8 is that it shows a pronounced protonation effect. It is found that all AHP-T8 organogel, solution and cast films

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possess remarkable acidochromic behavior when they are stimulated by trifluoroacetic acid (TFA), hydrochloric acid and their vapors. As shown in Fig. 7, with the addition of TFA onto the top of the cyclohexane organogel, the yellow gel gradually vanishes and the gel transfers into brown red-colored solution, and the whole process finishes within 1 minute. Simultaneously, the green emission band at 493 nm of organogel

gradually quenches with the organogel turning into solution. The results indicate that AHP-T8 gel is sensitive to TFA, with naked eye sensing by gel-sol transition, obvious color and fluorescence changes. In addition, the AHP-T8 solution induced by TFA could be restored to the gel state by adding triethylamine (TEA). Additionally, the stimuli-responsive behavior of AHP-T8 in the solid state for

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acids/bases was also investigated (Fig. 8). The cast film was fumed with TFA vapors, the naked-eye-color of AHP-T8 films from faint yellow to deep yellow, while its

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fluorescent color markedly switched from light-green to orange yellow, with an emission maximum shifting from 502 nm to 555 nm. The above phenomenon could be

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quickly converted back to its original form as it was fumed with TEA vapor. This reversible emission could be easily switched by its treatment with TFA and TEA vapors.

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Very interestingly, the orange yellow emission can spontaneously convert back to its

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original light-green emission form within a few minutes at room temperature, this self-

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healing material will be an excellent candidate for acid sensing.

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Based on the color switching features for acid/base stimuli as mentioned above, AHPT8 chloroform solutions can serve as security ink under daylight. As shown in Fig. S15,

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when the characters ‘ZHMG’ were written on a yellowish green (as a covering color) printing paper by using cotton swabs with AHP-T8 chloroform solutions, it was unable

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to be recognized by the naked eye under daylight. However, the orange characters ‘ZHMG’ can be observed clearly after daubing TFA solution, and the orange characters

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can disappear within 30 seconds. So we speculated that AHP-T8 should have potential applications in data security protection.

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Fig. 7. PL spectra of AHP-T8 cyclohexane organogel between ‘off’ and ‘on’ by reversible treatment with TFA-TEA, the insets are photographs of AHP-T8 gel-sol

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transition by reversible treatment with TFA-TEA under daylight and UV light.

Fig. 8. PL spectra and photographs (under daylight and UV light) of AHP-T8 films by

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repeated treatment with TFA-TEA. To further understand the protonation effect, UV-vis absorption spectra of AHP-T8 and AHP-T8-TFA (AHP-T8-H+) in solutions and solid states have been studied. Upon

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titration of AHP-T8 solutions in chloroform with TFA, a new broad shoulder band at around 432 nm appears, and the color of solution changes from yellow to orange (Fig. S16). In the solid state, the absorption spectrum of AHP-T8 fumed by TFA vapors exhibits a 43 nm redshift compared to its original form (Fig. S17). Upon titration of AHP-T8 solutions in chloroform with TFA, the 1H NMR chemical shifts of the -CH=N-

and anthracene protons undergo obviously downfield shifts (Fig. S18), which reveal the changes in electron density on -CH=N- and anthracene hydrogen atoms. All these confirm the protonation of AHP-T8 when treated by TFA. In addition, the theory of frontier molecular orbits can further confirm the protonation of AHP-T8 (Fig. 9). VMD was employed for visualizing the molecular orbital and

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electron density variation [45]. The lowest unoccupied molecular orbital (LUMO) is mainly localized at anthracene rings and hydrazone groups, while the highest occupied

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molecular orbital (HOMO) is distributed over the whole molecule except alkoxy chains. In contrast, the LUMO of AHP-T8-H+ reduces from -2.05 eV to -6.04 eV, and the

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HOMO reduces from -5.12 eV to -8.03 eV. Therefore, its band gap is reduced by -1.99

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eV. Since the protonation enhances the electron withdrawing ability of imine, the

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LUMO should be more delocalized in the protonated molecules. As reported in the literature, the delocalization of the electron cloud is beneficial for stabilizing the

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molecular excited state, resulting in a decreased band gap [28]. Consequently, the

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delocalization of the LUMO stabilizes the excited molecule at a lower band gap, which is ultimately responsible for the red shift of the emission and absorption of the AHP-

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T8-H+ samples.

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Fig. 9. The molecular frontier orbital contributions and energy level diagrams of AHP-

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T8 (left) and AHP-T8-H+ (right) form calculated on the DFT a B3LYP/6-31G* level

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of theory.

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4. Conclusion

In conclusion, we demonstrated an AIE-active AHP-T8 with multi-stimuli-responsive

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features. AHP-T8 shows switched emission from blue to green in THF-water mixture

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solutions. The mechanochromic luminescent of AHP-T8 could be reversibly tuned by alternating stimulation of grinding, annealing (or fuming by DCM). XRD and DSC

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experiments reveal that the transformation between the xerogel with crystalline features and amorphous states is responsible for the mechanofluorochromism behavior. Especially, it is found that AHP-T8 exhibits remarkable and reversible acid/base

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stimulated fluorescence switching properties in organogel and solid state. The UV-vis absorption spectra, 1H NMR experiments and theoretical calculations can confirm that this switchable phenomenon attribute to the protonation/deprotonation of AHP-T8. Upon titration of AHP-T8 organogel with TFA, the protonation effect causes the dramatic phase transition from gel to sol with the concomitant color change visible to

the naked eyes and the on/off switching fluorescence effect. The AHP-T8 films with fumigation of TFA/TEA (or standing at room temperature) vapors exhibit their luminescence switching between green and orange yellow. The results indicate that AHP-T8 holds the potential applications into a solid-state fluorescence switching

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materials.

Acknowledgments

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This work was supported by the Natural Science Foundation of Jilin Province

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(20170101112JC) and Project 985-Automotive Engineering of Jilin University.

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Supplementary material

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Electronic supplementary information (ESI) available: Fig. S1-S19, Table S1, S2. CCDC-1815439 contains the supplementary crystallographic data in CIF for the AHP-

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T1.

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Biographies Mingang Zhang is a graduate student of the college of physics in Jilin University. Jue Wei received his PhD in 2004 from Jilin University. He is currently an associate professor of the college of physics in Jilin University. Yina Zhang is a graduate student of the college of chemistry and materials science in

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Anhui Normal University.

Binglian Bai was born in 1973 in China. She began her graduate research on

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supramolecular liquid crystals under the supervision of Prof. Min Li at the Jilin University in 2001. After she received her PhD in 2007, she joined the Jilin University. She is now a professor of Jilin University. Her research work is focused on synthesis

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and characterization of liquid crystals and organogels containing hydrazide,

N

azobenzene or oxadiazole group, and her current research mainly focused on the

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as aggregation-induced emission.

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organic materials responding to light, mechanical force, acid/base and/or cation as well

in Jilin University.

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Fangyi Chen is a graduate student of the college of materials science and engineering

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Haitao Wang received his B.S. in 2003 and his PhD in 2008 in materials science in Jilin University. He is currently an associate professor in Jilin University.

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Min Li is a professor of the college of materials science and engineering in Jilin University. She received her PhD in 1993. Her main research interest is focused on

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synthesis and characterization on the functional organic materials.