Mechano ﬂ uorochromic and thermochromic properties of simple tetraphenylethylene derivatives with fused ﬂ uorine containing 1,4-dioxocane rings

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Mechano ﬂ uorochromic and thermochromic properties of simple tetraphenylethylene derivatives with fused ﬂ uorine containing 1,4-dioxocane rings

Hongxiang Wu

^a

, Yue Jiang

^b^,^*

, Yang Ding

^b

, Yuying Meng

^e

, Zhuo Zeng

^a^,^**

,

Cl ement Cabanetos

^f

, Guofu Zhou

^c

, Jinwei Gao

^b^,^***

, Junming Liu

^b^,^d

, Jean Roncali

^f

aCollege of Chemistry&Environment, South China Normal University, Guangzhou, 510006, PR China

bInstitute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China Normal University, Guangzhou 510006, PR China

cElectronic Paper Displays Institute, South China Normal University, Guangzhou 510006, PR China

dLaboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, PR China

eSun Yat-Sen University, School of Chemistry, Guangzhou, 510275, PR China

fGroup Linear Conjugated Systems, CNRS Moltech-Anjou, University of Angers, 2 Bd Lavoisier, 49045 Angers, France

a r t i c l e i n f o

Article history:

Received 19 April 2017 Received in revised form 6 July 2017

Accepted 6 July 2017 Available online 13 July 2017

Keywords:

TPE

Tetraﬂuorobutylenedioxy loops Mechanoﬂuorochromism Thermalchromism

a b s t r a c t

A series of tetraphenylethylenes (TPE) derivatives in which one or more of the phenyl rings have been replaced by a fusedfluorine containing 2,3,4,5-tetrahydrobenzo[b][1,4]dioxocane unit have been synthesized. The analysis of the photoluminescence emission under application of mechanical grinding and thermal treatment show that in addition to the expected aggregation induced emission behavior observed in solution, these simple modifications of the TPE system confer mechanofluorochromic properties to the corresponding materials. The results of X-ray diffraction and theoretical calculations suggest that the introduction of tetrafluorobutylenedioxy“loops”control the balance between weak intermolecular interactions and thus the interconversion between “macro” and “micro-aggregates”

which is proposed as the basic mechanism for the observed MFC properties.

1. Introduction

Organic materials endowed with mechanochromic (MC) or mechanofluorochromic (MFC) properties namely properties un- dergo changes in their absorption orfluorescence emission spectrum in response to the external mechanical stimuli such as hydrostatic pressure, grinding or stretching are subject to very significant current interest[1e4].

Over the past few years, various molecular structures with MFC properties have been reported including stilbene[5], tetraphenylethylene (TPE) [6e8], triphenylamine (TPA) [9e11], anthracene [12,13]and metal-organic orb-diketone boron complexes[14,15].

Besides potential applications in variousﬁelds, like light emitting

diodes[16], memory devices[17], or sensorss[18,19], the peculiar properties of these materials also pose some interesting fundamental problems related to the packing mode of the molecular unit and especially the kinetics of the mechanically/thermally induced transitions.

Hence, intermolecular interactions (Inter-I) represent a key factor that controls the molecular self-assembly and packing ar- rangements and therefore the chemo-physical and thermal properties in the solid state. Moreover, previous work suggested that an

“appropriate crystallization” capability is a prerequisite for MC materials [20]. Consequently, modulating/tuning the Inter-I through pi-stacking[21], hydrogen bonding[22]or dipole-dipole interaction[23]turns out to be an efﬁcient strategy. Complemen- tarily, we have recently demonstrated that the simultaneous presence of lipophilic and hydrophilic parts in push-pull molecules contributes in obtaining metastable materials with MFC behavior coupled to non-linear optical properties[24].

Herein, we report the synthesis of a series of molecules based on tetraphenylethylene (TPE) in which butylenedioxy- and

*Corresponding author.

**Corresponding author.

***Corresponding author.

E-mail addresses: jiangyue871116@gmail.com (Y. Jiang), zhuoz@scnu.edu.cn (Z. Zeng),gaojw@scnu.edu.cn(J. Gao).

Contents lists available atScienceDirect

Dyes and Pigments

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / d y e p i g

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tetrafluorobutylenedioxy-loops have been introduced at one or two phenyl groups of the TPE core. The electronic properties of the molecules have been characterized by UV-Vis and fluorescence spectroscopy as well as cyclic voltammetry. Preliminary evaluations clearly demonstrated that the materials with tetrafluorobutylenedioxy-loops (C1andC2) belong to the class of MFC luminogens. The thorough investigations of mechano- and thermal- fluorochromic kinetics for thefirst time has revealed the effect of

“micro-”and“macro-”aggregates on the MFC process.

2. Results and discussion 2.1. Synthesis

Compounds C1-C4 were synthesized according to the route depicted in Scheme 1. The TPE block ofC1 andC3 was built by Suzuki cross-coupling reaction between (2-bromoethene-1,1,2- triyl)tribenzene (S1) and 3,4-dimethoxyphenylboronic acid to give (2-(3,4-dimethoxyphenyl)ethene-1,1,2-triyl)tribenzene (S2) in 77% yield. Demethylation of the latter with boron tribromide gave 4-(1,2,2-triphenylvinyl)benzene-1,2-diol (S3) in 81% yield. Then, the target compoundsC1andC3 were prepared in 93% and 13%

yield respectively via Williamson reaction betweenS3and 2,2,3,3- tetrafluorobutane-1,4-diyl bis(trifluoromethanesulfonate) (X) and butane-1,4-diyl bis(4-methylbenzenesulfonate) (Y). It is noteworthy that the moderated yield ofC3 can be attributed to the 10,000 times lower reactivity of OTs than OTf [25]. In parallel, Williamson reaction of (3,4-dihydroxyphenyl)(phenyl)methanone (D1) withXandYgave compoundsC2andC4in 79% and 10% yield respectively. Finally, the isomers ofC4are inseparable and¹H NMR and X-ray diffraction on single crystal confirmed the conformation oftrans-isomer forC2.

2.2. Thermal properties

While compoundsC1andC2were isolated as solids, oily compounds were recovered after puriﬁcation for C3 andC4. Conse- quently, the thermal properties of the latter were not further investigated. As shown inFig. 1, the DSC curve of the as-obtainedC1 crystals exhibits aﬁrst endothermic peak at 117C followed by a small peak at 134C.

RegardingC2, two peaks were recorded at 141C and 223 C respectively. The absence of birefringence under polarized optical microscopy (POM) suggests that the splitting of the peak cannot be

Scheme 1.Synthesis of the target compoundsC1-C4. i: 5% mol Pd(PPh3)2Cl2, K3PO4, toluene, 110C 12 h; ii: BBr3, DCM, 25C, 24 h; iii: CH3CN, K2CO3, 90C, 12 h; iv: CH3CN, K2CO3, 90C, 72 h; v: Zn, TiCl4, THF, reﬂux, 12 h.

Temperature [oC]

50 100 150 200 250 300

]g m/ W m[ C S D

-1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

C1 C2

od n E

Fig. 1.DSC curve ofC1andC2with 10 K/min heating rate under nitrogen protection.

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attributed to the formation of a liquid crystal phase.A contrario, inverted ﬂuorescence microscopy conﬁrmed that C1 crystals partially melt atca.110C leading to the breaking of large crystals into smaller ones that totally melt atca.140C (Fig. 8b). From this point onwards, large crystals will be referenced as“macro aggregates”and the smaller one as“micro aggregates”.

2.3. Cyclic voltammetry

Cyclic voltammetry was performed in methylene chloride in the presence of 0.1 M tetrabutylammonium hexaﬂuorophosphate as the supporting electrolyte (Fig. 2). The cyclic voltammograms ofC1 andC2present an irreversible oxidation wave with an anodic peak potential (E_pa) at 1.36 and 1.35 V and an irreversible reduction wave peaking atEpc¼ 1.35 and1.38 V respectively. Comparison the conventionalTPE(Epaat 1.40 V) shows that the electron donating effect of the butylenedioxy loops onC1and C2leads to a small negative shift ofca50 mV in the anodic region.

2.4. UV-Vis absorption spectroscopy

The UV-Vis absorption spectra ofC1,C2andTPEwere recorded in THF solution and as thinfilms spin-cast on quartz substrates (Table 1andFig. 3). The spectra recorded in solution show afirst band in the 230e270 nm region ascribed to the absorption of the phenyl rings, followed by a less intense band atlmax309 nm corresponding to the conjugated TPE system. The introduction of butylenedioxy loops leads to the red shift of thefirst absorption band from 238 nm for TPE to 240 and 244 nm forC1 and C2 respectively. Furthermore, the molar extinction coefficient (ε) ofC1 andC2is significantly higher than that of TPE, possibly due to the

electron donating effect of the alkoxy groups. As generally observed for conjugated molecules, the spectra of the solidfilms present a broadening of the absorption band due to Inter-I. For both solution and solid-state spectra the tetrafluorobutylenedioxy-loops produce a small red shift of thefirst absorption band and have practically no effect on the band at longer wavelength.

2.5. Aggregation-induced emission (AIE)

Introduced by Tang and co-workers in 2001[26]TPE is probably the most widely employed building block for the synthesis of molecules endowed with aggregation-induced emission (AIE) properties[27,28]. To investigate the effects of the substitution of TPE by the tetraﬂuorobutylenedioxy loops, UV-Vis absorption and ﬂuorescence emission spectra of TPE,C1 and C2 were recorded upon addition of increasing fractions of water in THF solutions.

Fig. 4depicts the UV-Vis proﬁles recorded in a 10:90 THF/water mixture. It turns out that the high fraction of water induces for all compounds a bathochromic shift oflmaxand a broadening of the spectra. However, it is worth noting that the reference TPE is Fig. 2.Cyclic voltammograms of compoundsTPE,C1andC2. 1 mM in 0.1 M Bu4NPF6/

CH2Cl2, scan rate 50 mVs^-1, Pt working electrodes.

Table 1

UV-Vis absorption and cyclic voltammetry data forC1,C2andTPE.

lS max (nm)

lF max (nm)

εmax

(M¹cm¹) Epa

(V vs SCE) Epc

(V vs SCE)

TPE 238, 309 244, 325 23000, 14000 1.40 1.33

C1 240, 309 244, 317 50000, 28000 1.36 1.35

C2 244, 309 248, 322 44000, 23000 1.35 1.38

Wavelength (nm)

300 400 500 600 700

ecnabrosbA

0.0 0.1 0.2 0.3 0.4 0.5

C1 C2 TPE

Wavelength [nm]

300 400 500 600 700

].u.a[ecnabrosbA

0.00 0.05 0.10 0.15 0.20 0.25 0.30

C1 C2 TPE

Fig. 3.UV-Vis absorption spectra of compoundTPE,C1, C2: in THF solution (10⁵M) (up); as thinﬁlm spin-cast on quartz from THF solution (down).

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characterized by a discernible new shoulder atca420 nm which is absent in the spectra of C1andC2. This difference suggests that bare TPE has a stronger propensity to aggregate than its substituted versions.

Photoluminescence responses of C1, C2 and TPE were then analyzed byﬂuorescence spectroscopy. As depicted inFig. 5, no ﬂuorescence emission was observed in pure THF for all compounds.

However, increasing the volume fraction of water beyond 80% leads to the triggering ofﬂuorescence emission with emission maximum

at 460 nm forTPE, shifting to 471 nm for bothC1andC2. These results thus conﬁrm the similarly toTPEsinceC1andC2can also exhibit prominent AIE process.

Wavelength [nm]

300 400 500 600 700

ecnabrosbAdezilamroN

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Wavelength [nm]

300 400 500 600 700

ecnabrosbAdezilamroN

0.0 0.2 0.4 0.6 0.8 1.0

Wavelength [nm]

300 400 500 600 700

ecnabrosbAdezilamroN

0.0 0.2 0.4 0.6 0.8 1.0

Fig. 4.UV-Vis Absorption spectra ofTPE(top),C1(middle) andC2(bottom). In THF (black) and in 1:9 THF/water (red). (For interpretation of the references to colour in thisﬁgure legend, the reader is referred to the web version of this article.)

Wavelength [nm]

350 400 450 500 550

ecnecseroulF

80%

82%

84%

86%

88%

90%

Wavelength [nm]

400 450 500 550

ecnecseroulF

80%

82%

84%

86%

88%

90%

Wavelength [nm]

400 450 500 550

ecnecseroulF

80%

82%

84%

86%

88%

90%

Fig. 5.Fluorescence emission spectra ofTPE(top),C1(middle) andC2(bottom) in THF-water mixtures. Concentration: 0.5 mM. Excitation wavelength: 300 nm.

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2.6. Calculations

Molecular optimal geometries ofC1to C4were calculated by DFT based B3LYP functional with the 6-31G(d,p) basis set using the Gaussian09 software package[29]. The computed structures show no imaginary frequency, thus ensuring energetic minima. In the neutral state,C1toC4molecules show the expected representative propeller-like shape of the common TPE block. Furthermore, it was also shown that the butylenedioxy loops adopt a chair conformation mainly induced by the number of atoms involved in the cycles (Fig. S1)[30]. Then, to gain further insight into the stacking patterns, the Materials Studio[31]polymorph predictor module was used. Through the calculation of the total energy between two adjacent molecules, packing styles with the top 10 minimal energy are listed inTable S1for both compounds. First regardingC2, top 10 lowest total energy ranging from 102.7 to 103.0 eV display minor difference (0.3eV) and each of their corresponding packing styles presents weak F…F interaction (2.850 Å to 2.866 Å) of butylenedioxy loops at both sides. For instance, the most stable packing style is illustrated inFig. 6(c). In the case ofC1, a signiﬁcantly larger energy difference of 1.3eV was estimated from 77.9eV of lowest total energy jump to next 79.2 eV.

The calculated most stable packing mode of C1 shows interactions between the electron-rich TPE and the electron-deficient tetrafluoroethyl without reasonable CH…F interactions (Fig. 6a and Table S1C1 P-1-1) while the second stable packing results from the CH…F interaction characterized by a distance of 2.659 Å (Table S1 C1 P-1-2 andFig. 6b). Thus regardingC3and C4, the absence of the intermolecular interactions associated with fluorine atoms, provides a further support to understand their oily state at room temperature. The results from these calculations are consistent with the more prominent two-step thermal behavior ofC1vsC2 due to the larger Inter-I associated with the presence of two tetrafluorobutylenedioxy loops inC2.

2.7. Single crystallography

Used as the reference structure, the basicTPEpresents two sets of benzene rings around the central C¼C bond, with dihedral angles of 55 and 85 between phenyl rings fixed on different alkene carbons and 79and 74 between phenyl rings attached on the same alkene carbon [32] (Fig. 7). Regarding the butylenedioxy functionalized molecules, only single crystals ofC2were obtained by slow evaporation of dichloromethane solutions. It turns out that the dihedral angles between phenyl rings borne by the same alkene carbons are ofca.89while those on different alkene carbons are of ca.49and 68respectively (Fig. 7). As might be expected, introduction of fluorine substituted butylenedioxy loops clearly increases the intramolecular propelling level. Moreover, the crystal structure also reveals that the butylenedioxy loops adopt atrans- chair conformation with C12-O2-C17 bond angle of 115.22 that does not inhibit the packing of the TPE core. More interestingly, two kinds of H…F interactions,ie, F3…H2 and F4…H17B with distance of 2.624 Å and 2.589 Å respectively were monitored (Fig. 6eef). It is noteworthy that these two distances are shorter than the sum of the Van der Waals Radius of hydrogen and fluorine (ie 2.67 Å).

These weak Inter-Is are consistent with the much higher melting point ofC1andC2compared toC3andC4.

Fig. 6.(a)e(c) Calculation result: (a)C1packing with the minimum total energy. (b)C1packing with the second minimum total energy. (c)C2packing with the minimum total energy. (d)e(f) Photograph ofC2single crystals: (d) structure ofC2. (e) Vertical CH…F interaction ofC2. (f) In-plane CH…F interaction ofC2.

Fig. 7.Crystal structures ofTPE(Left) andC2(Right).

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2.8. MFC

It has been shown that due to its high propensity to crystallize, bareTPEdoes not exhibit MFC behavior[33,34]. On the other hand, previous work has shown that the modiﬁcation of the TPE core by partial rigidiﬁcation[33]or by introduction of various substituents such as dimethylamino [35], methoxy[36] led to materials pre- senting MFC properties.

Samples of C1 and C2, as polycrystalline powders are re- crystallized from DCM solution. Pristine and ground crystals were compared to each other and exposed to thermal treatment at different temperatures. Images recorded under monochromatic illumination at 365 nm by invertedﬂuorescence microscopy are depicted inFigs. 8 and 9.

Upon grinding, theﬂuorescence ofC1changes from purple blue (Fig. 8a) to sky blue (Fig. 8c) which corresponds to a red shift from 415 nm to 448 nm of theﬂuorescence emission maximum (lem).

Thermal annealing of the pristine crystals at 110C produces partial melting, in agreement with DSC data (Fig. 1), while the color of ﬂuorescence turns to sky blue (lem¼453 nm) with an emission spectrum quite close to that of the ground crystals (Fig. 8b and c).

On the other hand, heating the ground pristine crystal at 110C produces the reverse effect with a hypsochromic shift oflem to 435 nm (Fig. 8d). Further heating, up to 140 C, induces a total melting of both the pristine and the ground samples.

ForC2, the maximum of the ﬂuorescence emissionlem shifts bathochromically from purple blue (419 nm) to sky blue (433 nm) (Fig. 9a and d) under mechanical stimulation. Upon slow heating the pristine crystals begin to melt at 120 C whilelem shifts to 458 nm (Fig. 9b). On the other hand, a 5 min thermal treatment of the ground sample at 110C leads to a hypsochromic shift oflem

from 433 nm back to 419 nm (Fig. 9d) and the sample exhibits again the purple blueishﬂuorescence characteristic of the pristine sample at room temperature (Fig. 9a). However, keeping the sample at 110C for a longer time (30 min) leads to a stabilized emission spectrum similar to that of the ground samples recorded at room temperature withlem¼435 nm, (Figs. 9c and 8e).

2.9. Powder X-ray diffraction

These various mechanically and thermally induced trans- formations have been followed using powder X-ray diffraction Fig. 8.Left: Pics ofC1powder recorded at various temperatures under invertedﬂuorescence microscopy (excitation wavelength 365 nm). Right:ﬂuorescence emission spectra ofC1 (excitation wavelength 300 nm)“P”stands for“pristine crystals”and“G”for“ground powders”.

Fig. 9.Left: Pics ofC2powder recorded at various temperatures under invertedﬂuorescence microscopy (excitation wavelength 365 nm). Right:ﬂuorescence emission spectra ofC1 (excitation wavelength 300 nm)“P”stands for“pristine crystals”and“G”for“ground powders”.

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(XRD) measurements. The pristine crystals ofC1present multiple peaks in the range of 10e30 (Fig. 10). Grinding of the crystals produces the disappearance of these peaks whereas a subsequently TA partially restores the initial signals. The XRD curve of the pris- tineC2also presents multiple peaks between 0 and 40, but in the diagram of groundC2, the intensity of the peak at 17increases while that of the other peaks decreases. A 5 min TA at 110C restores some signals (e.g.10.7and 14.7) common to the pristine sample while after 30 min TA at 110C only the strong peak at 17 observed on the ground samples remains, which suggests that a prolonged TA induces a modiﬁcation of the crystalline state ofC2.

2.10. Discussion

The foregoing results show that introducing of tetra- ﬂuorobutylenedioxy loops on one or two of the phenyl rings of TPE can effectively confer MFC properties on the resulting materials.

Based on the thermal and luminescent properties of the materials it can be proposed that the application of external mechanical force to the pristine“macro aggregates”ofC1leads to their conversion into“micro aggregates”associated with a change of the ﬂuores- cence emission from purple blue to sky blue. Application of a subsequent TA at 110C to the ground samples rebuilt the purple blue“macro aggregates”. Although a similar behavior is observed forC2, a prolonged TA at 110leads to a change of the purple blue state to sky blueﬂuorescent crystals. These various results show that 110C represents a key temperature for the metastableC1and C2.

As suggested by single crystal XRD and calculations, intermolecular H…F and F…F interactions play a fundamental role in both the molecular packing and the sensitivity of the materials to mechanical and thermal stimulations. Thus, it can be proposed that mechanical forces or heat essentially destroy the“macro order” controlled by weak H…F interactions whereas the“micro order” controlled by relatively strong interactions, such as attraction between electron-rich TPE block and electron-deﬁcient tetraﬂuoro- ethylene block forC1and the stronger H…F interaction forC2are less affected by these external stimulations.

3. Conclusion

To summarize tetraphenylethylenes derivatized with one or two tetraﬂuorobutylene loops have been synthesized. The analysis of the photoluminescence emission under application of mechanical

grinding and thermal treatment show that in addition to the expected AIE behavior observed in solution, these simple modiﬁca- tions of the TPE system confer MFC properties to the corresponding materials. The results of X-ray diffraction and theoretical calculations suggest that the introduction of tetraﬂuorobutylenedioxy loops controls the balance between weak intermolecular interactions and thus the interconversion between “macro” and

“micro-aggregates”which is proposed as the basic mechanism for the observed MFC properties.

Supporting information

CCDC 1524482 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.

uk/data_request/cif.

Acknowledgements

We thank theﬁnancial support from National Natural Science Foundation of China (21272080, 51571094), China Postdoctoral Science Foundation (2016M590795), National Key Research Pro- gram of China (2016YFA0201002), Guangdong Province Foundation (2016KCXTD009, 2014B090915005, and 2016A010101023), and Guangdong Innovative Research Team Program (2011D039). We also thank the support from the Joint International Research Lab- oratory of Optical Information. Hongxiang Wu and Yue Jiang contributed equally to this work.

Appendix A. Supplementary data

Supplementary data related to this article can be found athttp://

dx.doi.org/10.1016/j.dyepig.2017.07.026.

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