Substitution Position and Vinylene Bond Geometry Modulating the

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Substitution Position and Vinylene Bond Geometry Modulating the Fluorescence Solvatochromism and Aggregation-Induced Emission of (9-Anthryl)vinyl(1-pyrenyl)vinylbenzene Isomers Yexin Li, Xiao-Feng Yang, Jin-Ling Miao, and Guoxin Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04392 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on September 2, 2016

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The Journal of Physical Chemistry

Substitution Position and Vinylene Bond Geometry Modulating the Fluorescence Solvatochromism and Aggregation-Induced Emission of (9-Anthryl)vinyl(1-pyrenyl)vinylbenzene Isomers Ye-Xin Li,* Xiao-Feng Yang, Jin-Ling Miao, and Guo-Xin Sun*

School of Chemistry and Chemical Engineering, University of Jinan, No. 336, West Road of Nan Xinzhuang, 250022, Jinan, China

ABSTRACT: Despite the lack of typical electron donor and acceptor in the molecule

structure,

a

pure

aromatic

hydrocarbon,

trans,trans-1-(9-anthryl)vinyl-4-(1-pyrenyl)vinylbenzene (trans,trans-1,4-AVPVB), shows unusual fluorescence solvatochromism with emission shift over 80 nm. In addition, it displays an aggregation-induced emission

(AIE)

activity.

For

comparison,

other

three

isomers,

trans,trans-1,2-AVPVB, trans,trans-1,3-AVPVB and trans,cis-1,4-AVPVB, were also synthesized. The effects of substitution position and vinylene bond geometry on solvatochromism, temperature-induced phase transition and AIE activity were systematically studied. Different from the other isomers, the as-prepared sample of trans,trans-1,2-AVPVB shows an endothermic peak and an exothermic peak before the melting point in the DSC curve. They correspond to a solid-to-gas transition and a solid-to-solid transition, 1

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respectively.

Theoretical

calculation

indicates

that

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the

fluorescence

solvatochromism may be related to the conformational change from the ground to the excited states. The solvatochromic degree is determined by conjugative effect. Trans,trans-1,2-AVPVB and trans,cis-1,4-AVPVB display moderate fluorescence solvatochromism. In contrast, the solvatochromic effect of trans,trans-1,3-AVPVB is weak due to meta-substitution. However, the conjugation interruption by meta-substitution is beneficial for solution emission, and trans,trans-1,3-AVPVB emits most efficiently in solution. Similar to trans,trans-1,4-AVPVB, trans,trans-1,2-AVPVB and trans,cis-1,4-AVPVB also display an AIE behavior.

INTRODUCTION Solvatochromism describes the pronounced change in the position of absorption and/or emission bands that accompanies a change in solvent polarity. Organic chromophores with solvatochromic property have been widely used to monitor microenvironmental polarity in chemical and biological systems.1-4 These materials usually have an electron donor (D) and an acceptor (A) connected by a conjugated bridge. The intramolecular charge transfer (ICT) from D to A endows them with a solvatochromic effect. This kind of materials also have unique optoelectronic properties.5,6 Recently, Tian et al. reported an anthranol derivative without typical D-π-A structural feature exhibited an apparent solvatochromic fluorescence with shift over 80 nm.7 The reason was attributed

2


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to the hole-electron separation in the excite state. As for pure aromatic hydrocarbons, they were usually used as the conjugated bridge in the D-π-A system. Because there is no typical electron donor, acceptor or heteroatom in the molecule structure, the charge-distribution difference between the ground state and the excited state is small.1 As a result, the solvatochromic effect of pure aromatic hydrocarbons was seldom reported and studied. Recently, the effects of substitution position on the optoelectronic properties of organic chromophores attracted much attention.8-16 Position isomers have the same atomic composition but different molecule configuration by varying atomic or substituent position on a parent structure. This minor change in molecule configuration could affect π-conjugation degree and intermolecular steric hindrance, which provides a feasible and efficient way to modulate the optoelectronic properties of organic chromophores. For example, Poriel et al. studied the structure–property relationship of ortho [2,1-c]-, meta [1,2-a]-, and para [1,2-b]dihydroindenofluorenes. The linkage pattern played an important role in singlet and triplet state energy levels. Meta-substitution induced a high ET value and displayed the best performance when they were used as the host in sky-blue phosphorescent organic light-emitting diodes.12 Several groups have demonstrated that the aggregation-induced emission (AIE) activity and the resulting electroluminescent performance of some tetraphenylethene (TPE) derivatives could be tuned by structural variation on the linkage mode or the substituent position on a parent structure.13-15 On the other hand, the minor

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modulation in the substitution position could affect the crystal morphology of organic materials. For example, 1,4-bis(4-methylstyryl)benzene molecules were prone to form elongated hexagonal microplates, while 1,4-bis(2-methylstyryl)benzene molecules were aggregated into rectangular microplates. Fluorescence resonance phenomenon was only observed in the latter microstructure.16 In prior studies, our group has explored the effects of linkage pattern on the AIE activity and optical properties of several (9-anthryl)vinylbenzene-based position isomers.17-19

Among

these

materials,

1-[(9-anthryl)vinyl]-4-[(9-phenanthryl)vinyl]benzene

compound

displayed

unexpected

fluorescence solvatochromism in spite of the lack of typical electron donor or acceptor in the molecule structure. However, the solvatochromic degree was minor and the emission shift was only 34 nm ongoing from hexane to DMSO as solvents.19 The mechanism of the solvatochromism is still unclear. If we use a polycyclic aromatic hydrocarbon with bigger π-conjugation to replace phenanthrene, the obtained pure aromatic hydrocarbon may have stronger fluorescence solvatochromism. With this in mind, by the substitution of pyrene for phenanthrene, we obtained a new AIE-active

material,

trans,trans-1-[(9-anthryl)vinyl]-4-[(pyrenyl)vinyl]benzene

(trans,trans-1,4-AVPVB). It displays unusual fluorescence solvatochromism with emission shift over 80 nm, which is the strongest emission solvatochromism among the

pure

aromatic

hydrocarbons

ever

reported.

For

comparison,

trans,trans-1,2-AVPVB, trans,trans-1,3-AVPVB and trans,cis-1,4-AVPVB were also

4


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synthesized (Figure 1). The effects of substitution position and vinylene bond geometry on solvatochromism, temperature-induced phase transition, AIE activity and optical property were systematically studied.

trans,trans-1,3-AVPVB trans,trans-1,4-AVPVB trans,cis-1,4-AVPVB

trans,trans-1,2-AVPVB

Figure 1. Molecule structures of four AVPVB isomers.

RESULT AND DISCUSSION Synthesis

and

thermal

trans,trans-1,3-AVPVB,

and

Horner-Wadsworth-Emmons

property.

Compounds

trans,trans-1,4-AVPVB reaction

from

the

trans,trans-1,2-AVPVB, were

prepared

corresponding

by

diethyl

[(9-anthyl)vinyl]benzylphosphate and pyrene-1-carbaldehyde (Scheme S1). This synthetic method provides for a selective formation of E-configured C=C bond.20 Trans,cis-1,4-AVPVB

was

synthesized

by

Wittig

reaction

from

anthracen-9-ylmethyltriphenylphosphonium bromide and pyrene-1-carbaldehyde. Although trans,trans-1,4-AVPVB were obtained simultaneously, it was difficult to separate this compound in a pure form. All of the materials were characterized by NMR, MS and elemental analyses. These data were in good agreement with the proposed structures. In the 1H NMR spectra of trans,trans-1,4-AVPVB, three doublets 5



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at 7.02, 7.44 and 8.30 ppm with the coupling constants of 16.0 and 16.4 Hz indicate that two C=C bonds should be both trans-configured. For trans,cis-1,4-AVPVB, one doublet at 7.01 ppm with a coupling constants of 12.2 Hz shows that the C=C bond connecting the phenyl and pyrenyl moieties would adopt a cis-configuration.21 All the solid samples are stable under ambient condition. Trans,cis-1,4-AVPVB is unstable in solution.

It

can

undergo

a

cis-to-trans

photoisomerization,

and

trans,trans-1,4-AVPVB could be detected by TLC test after storing the CH2Cl2 solution of trans,cis-1,4-AVPVB for 5 hours at ambient condition. In contrast, the other three isomers are stable in solution.

Figure 2. DSC curves of the as-prepared sample. The thermal properties of the as-prepared samples were investigated by differential scanning calorimetry (DSC) analyses (Figure 2). The melting point is 247 °C for trans,trans-1,2-AVPVB,

239

°C

for


274

°C

for

trans,trans-1,4-AVPVB and 237 °C for trans,cis-1,4-AVPVB. Interestingly, trans,trans-1,2-AVPVB shows an endothermic peak at 216 °C before the melting

6


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point, which is closely followed by an exothermic peak at 219 °C. The first peak is possibly related to a solid-to-gas phase transition. To verify this assumption, the thermogravimetric (TG) and DSC experiments of trans,trans-1,2-AVPVB as-prepared sample were simultaneously performed. A lid with a hole was placed on the crucible which was used to hold the test sample. As shown in Figure S1, a small loss in the sample weight could be clearly observed in the TG curve at a temperature corresponding to the endothermic peak in the DSC curve. This indicates that a sublimation process should take place at about 216 °C. Thus, trans,trans-1,2-AVPVB displays appreciable vapor pressure upon heating.22 The second exothermic peak is a solid-to-solid phase transition, which suggests that a thermodynamically stable polymorph of trans,trans-1,2-AVPVB should be produced at 219 °C. This is further proven by powder X-ray diffraction (XRD). As shown in Figure 3, after heating trans,trans-1,2-AVPVB as-prepared powder at about 230 °C, the XRD pattern of the obtained sample is different from that of the as-prepared sample. For clarification, the as-prepared

powder of


is

designated as

α phase

(trans,trans-1,2-AVPVB-α) and the new polymorph is designated as β phase (trans,trans-1,2-AVPVB-β). In contrast with trans,trans-1,2-AVPVB, the other three compounds have no solid-to-solid phase transition below the melting point.

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Figure 3. XRD patterns of trans,trans-1,2-AVPVB polymorphs: (a) as-prepared powder (trans,trans-1,2-AVPVB-α), (b) heating trans,trans-1,2-AVPVB powder at about 230 °C, and (c) calculated from the crystal structure of sublimated crystals (trans,trans-1,2-AVPVB-β). Crystal structure. The use of solution methods to grow crystals for single crystal X-ray diffraction failed. Fortunately, the crystals of trans,trans-1,2-AVPVB-β and trans,trans-1,4-AVPVB were successfully grown in a horizontal tube furnace by a physical vapour-transport method, and the crystal structures were clearly determined. However, the crystal structures of trans,trans-1,2-AVPVB-α, trans,trans-1,3-AVPVB and trans,cis-1,4-AVPVB were unsolved.

8


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Figure 4. (a) Stacking image of trans,trans-1,2-AVPVB-β molecules in the ac plane. (b) Stacking image along the b axis. Compound trans,trans-1,2-AVPVB-β crystallizes in the P21/c space group. There is one independent molecule per asymmetric unit. Due to the H···H steric hindrance between vinylic proton and the peripheral proton at anthryl ring, anthracene moiety deviates from the central benzene ring with a dihedral angle of 69.81º. In contrast, the H···H steric hindrance between pyrenyl proton and vinylic proton decreases drastically. Therefore, the dihedral angle between pyrenyl and phenyl is only 12.04º. The molecule packing is shown in Figure 4. In the ac plane, the adjacent aromatic rings take an edge-to-face or end-to-face arrangement with strong C-H⋅⋅⋅π interactions. Furthermore, H⋅⋅⋅H interactions (2.40 Å) also exist between adjacent parallel anthracene rings. Along the b axis, trans,trans-1,2-AVPVB molecules adopt a slipped face-to-face arrangement. Although the shortest C⋅⋅⋅C distance between adjacent parallel pyrene rings is 3.39 Å, the intermolecular π⋅⋅⋅π interaction would be very 9



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weak because the centroid⋅⋅⋅ centroid distance is 6.74 Å and there is few overlap between adjacent aromatic rings.

Figure 5. Stacking image of trans,trans-1,4-AVPVB molecules in the ac plane. Compound trans,trans-1,4-AVPVB crystallizes also in the P21/c space group. Similar to trans,trans-1,2-AVPVB-β, the anthryl moiety in trans,trans-1,4-AVPVB crystal structure also twists from the central benzene ring with a big dihedral angle of 74.44°, and the pyrenyl moiety has a small angle of 9.05° with the central benzene ring. The molecule packing is shown in Figure 5. In the ac plane, the adjacent aromatic rings adopt an edge-to-edge or end-to-edge arrangement with C-H⋅⋅⋅π interactions. Along the c axis, the adjacent molecules are antiparallel. Along the b axis, the adjacent aromatic rings also adopt a slipped face-to-face arrangement, and there is still no typical π⋅⋅⋅π interaction between adjacent aromatic rings. Photophysical property. UV-vis absorption and emission spectra of the four isomers in different solvents are shown in Figure S2 and Figure 6, and the

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corresponding photophysical data are summarized in Table 1. Usually, organic chromophores

with

a

typical

D-π-A

structural

feature

would

show

solvatochromism due to the ICT mechanism. Although there is no typical electron donor or acceptor group in the molecule structures of the four isomers, it is unexpected that, except trans,trans-1,3-AVPVB, all the other isomers display an apparent fluorescence solvatochromism. In solution, all the isomers show an absorption band in the range of 325-450 nm. With increasing solvent polarity, the absorption spectra show a weak bathochromic shift (Figure S2). In contrast, as shown in Figure 6, the fluorescence is more sensitive to solvent polarity. On going from hexane to DMSO as solvents, the emission maximum ranges from 488 to 536 nm for

trans,trans-1,2-AVPVB, from 470 to 481 nm for trans,trans-1,3-AVPVB, from 461 to 542 nm for trans,trans-1,4-AVPVB and from 455 to 494 nm for

trans,cis-1,4-AVPVB. The corresponding emission shift is 48, 11, 81 and 39 nm, respectively. Therefore, except trans,trans-1,3-AVPVB, the other three isomers all display an apparent fluorescence solvatochromism. Among them,

trans,trans-1,4-AVPVB shows the strongest solvatochromic effect. The more pronounced solvatochromism in the emission spectra than absorption is possibly due to the higher dipole moment in the excited state than in the ground state.19

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Figure

6.

Emission


spectra (b),

of

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(c)

(a), and

trans,cis-1,4-AVPVB (d) in different solvents. The concentration is 1×10-5 M for trans,trans-1,3-AVPVB and 5×10-5 M for other three isomers. λex = 375 nm. Table 1. Absorption and emission properties of four isomers in solution and solid states. solvent/solid trans,trans-1,2-AVPVB



trans,cis-1,4-AVPVB

a

λabs (nm) ε (M-1 cm-1) λema (nm) ΦFb

hexane

370

45000

488

4 ± 1.0%

CH2Cl2

375

53100

513

5 ± 1.0%

DMSO

379

52500

536

10 ± 1.4%

α phase

-

-

514

33 ± 3.8%

β phase

-

-

504

28 ± 0.8%

hexane

375

67600

470

53 ± 4.0%

CH2Cl2

379

68500

477

64± 1.0%

DMSO

383

70100

481

67 ± 3.4%

solid

-

-

502, 534

8 ± 1.4%

hexane

402

46000

461

3 ± 1.2%

CH2Cl2

411

45200

511

4 ± 1.2%

DMSO

418

38000

542

5 ± 1.0%

solid

-

-

534

35 ± 0.8%

hexane

367, 385

13400, 12300

455

3 ± 0.8%

CH2Cl2

370, 388

12100, 11300

468

3 ± 0.6%

DMSO

372, 390

12400, 12100

494

4 ± 0.6%

solid

-

-

515

38 ± 2.4%

λex = 375 nm. b Average ± standard deviation of four measurements.

To explain this intriguing fluorescence solvatochromism, theoretical calculation was conducted. The molecule structures in ground (S0) and first excited (S1) states 12


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were calculated in gas phase using DFT and TD-DFT frameworks at the CAM-B3LYP/6-31G level.23 The optimized geometries and structural parameters are shown in Table S1 and Figure S3, respectively. There are marked differences between the molecular geometries in the S0 and S1 states. The magnitude and the direction of the dipole moment of these isomers vary concomitantly with the conformational change. On the one hand, the direction of the dipole moment in the S1 state is opposite to that in the S0 state. On the other hand, for trans,trans-1,2-AVPVB, trans,trans-1,4-AVPVB and trans,cis-1,4-AVPVB, the magnitude of the dipole moment increases from S0 to S1 state, which is in agreement with the apparent fluorescence

solvatochromism.

On

the

contrary,

the

dipole

moment

of

trans,trans-1,3-AVPVB molecule decreases in the S1 state. This is in accordance with its weak fluorescence solvatochromism. Trans,trans-1,4-AVPVB

displays

the

strongest

fluorescence

solvatochromism among the four isomers. This is attributed to the conjugative effect.

For

the

three

position

isomers

of


trans,trans-1,3-AVPVB and trans,trans-1,4-AVPVB, the π-conjugation degree follows

a

para/ortho/meta

sequence.

The

π-conjugation

degree

in

trans,trans-1,4-AVPVB molecule is largest. Therefore, the electronic coupling between anthrylvinyl and pyrenylvinyl moieties induces a strong fluorescence solvatochromism. As for trans,trans-1,4-AVPVB and trans,cis-1,4-AVPVB, the cis/trans configuration of the C=C bond between pyrenyl and phenyl exerts

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a marked effect on the conjugation degree. As shown in Table S1 and Figure S3, cis configuration induces a high intramolecular H···H steric hindrance between

the

pyrenyl

proton

and

its

adjacent

phenyl

proton

in

trans,cis-1,4-AVPVB molecule. This results in a big dihedral angle of 53.98° between the pyrenyl and the central benzene ring in the S1 electronic state. In contrast, the corresponding H···H steric hindrance in trans,trans-1,4-AVPVB molecule is drastically decreased. The dihedral angle between the pyrenyl and the central phenyl in the optimized S1 geometry is 22.46°. Therefore, trans,trans-1,4-AVPVB molecule is more planar and then has a better

π-conjugation than cis,trans-1,4-AVPVB molecule. As a result, the former displays a much stronger fluorescence solvatochromism. Usually, the emission intensity of organic chromophores with a D-π-A structural feature would weaken in polar solvent owing to the formation of less-emissive ICT state.24 However, as for the four isomers, the emission intensity does not display a drastic decrease upon increasing the solvent polarity (Figure 6). Especially for trans,trans-1,2-AVPVB and trans,trans-1,4-AVPVB, the emission intensity even increases with the increased solvent polarity. As shown in Table 1, for trans,trans-1,2-AVPVB and trans,trans-1,3-AVPVB, the emission efficiency in DMSO solution is higher than that in hexane solution. This indicates that both trans,trans-1,2-AVPVB

and


display

a

negative

solvatokinetic effect. Negative solvatokinetic behavior means that the solution

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quantum yields increase with increasing solvent polarity. It was reported in some ICT chromophores with stilbenoid structure or with both n–π* and π–π* electronic configurations. The mechanism was usually explained by biradicaloid charge transfer, proximity effect and conformational changes.25-32 In all, the negative solvatokinetic effect could be generally attributed to the population of an emissive charge-transfer excited state.31,32 As for trans,trans-1,2-AVPVB and trans,trans-1,3-AVPVB, there is no typical electron donor and acceptor in the molecule structure. The negative solvatokinetic behavior may be attributed to the population of an emissive excited state. As shown in Table S2, with increasing the solvent polarity, the nonradiative (knr) rate of trans,trans-1,3-AVPVB is decreased. This may indicate that the nonradiative process could be effectively inhibited in polar solvent, which may play an important role in the increased emission efficiency.

Figure 7. Normalized solid emission spectra. λex = 375 nm. Although meta-substitution produces weak fluorescence solvatochromism, it is beneficial for the solution emission. As shown in Figure S4, the CH2Cl2 solution of trans,trans-1,3-AVPVB emits strongest among the four isomers. The corresponding solution emission efficiency is also highest (Table 1). In contrast, the other three 15



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isomers emit weakly in solution. There is also a marked difference in the fluorescence of these compounds in the solid phase. In the solid state, the emission maximum is 514 nm for trans,trans-1,2-AVPVB-α, 504 nm for trans,trans-1,2-AVPVB-β, 502 nm for trans,trans-1,3-AVPVB, 534 nm for trans,trans-1,4-AVPVB and 515 nm for trans,cis-1,4-AVPVB (Figure 7). Thus the emission color in the solid state could be tuned by varying the substitution position or the vinylene bond geometry. The solid emission quantum yield of trans,trans-1,3-AVPVB is only 10%, much lower than that of solution in either apolar or polar solvent. In contrast, for the other isomers, the emission efficiency in the solid state is all higher than that of the solution. Therefore, trans,trans-1,2-AVPVB, trans,trans-1,4-AVPVB and trans,cis-1,4-AVPVB all display an AIE effect.

Figure 8. Emission spectra of trans,trans-1,2-AVPVB (a), trans,trans-1,3-AVPVB (b), trans,trans-1,4-AVPVB (c) and trans,cis-1,4-AVPVB (d) in THF/water mixtures with different water volume fractions. The concentration was 3 × 10−5 M. 16


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To check the AIE activity, the emission spectra in THF/H2O mixtures were measured. The concentration was kept unchanged at 3 × 10−5 M. The compounds are insoluble in water. Thus the molecules would aggregate in the THF/H2O mixture with high water fraction. As shown in Figure 8, the emission of trans,trans-1,3-AVPVB is quenched at high water fractions. However, for the other three isomers, the fluorescence intensity all increases at high water fractions, showing an AIE behavior. A dramatic emission enhancement is observed when the water fraction is over 60% for trans,trans-1,2-AVPVB, 60% for trans,trans-1,4-AVPVB and 70% for trans,cis-1,4-AVPVB, which would be related to the aggregate formation.33,34 As for trans,trans-1,2-AVPVB and trans,trans-1,4-AVPVB, the emission color and intensity show some irregularity after the water content reaches the critical value. This is possibly due to the formation of different aggregation states, such as crystal particles and amorphous particles.35-39 It is difficult to control the formation of particles in the THF/H2O mixture with high water content. Thus the emission often becomes complicated because the solid-state emitting properties are dependent on both the molecule packing and the size of particles.22,35-39 In comparison with the THF solution, the emission spectra of trans,trans-1,4-AVPVB and trans,cis-1,4-AVPVB both display a pronounced bathochromic shift in the THF/H2O mixtures with high water content. The emission maximum of trans,trans-1,4-AVPVB shifts from 500 nm in THF solution to 540 nm in THF/H2O mixture with 90% water volume fraction. The corresponding emission shift of trans,cis-1,4-AVPVB is from 464 nm to 518 nm. This

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bathochromic shift may be related to the intermolecular interactions in the aggregate state. Anthrylvinylene is an important building block to obtain AIE-active materials. Prasad et al. reported the first 9,10-anthrylene-cored derivative with an aggregation-induced enhanced emission (AIEE) effect. The 9,10-anthrylene core was considered to play a crucial role in overcoming the fluorescence quenching in the aggregate.40

Through

the

analyses

of

the

crystal

structures

of

four

9,10-distyrylanthracene derivatives, Tian et al. considered that their AIE mechanism was attributed to the restricted intramolecular torsion between the 9,10-anthrylene core and the vinylene moieties.41 Although this kind of intramolecular torsion also exists in the (9-anthryl)vinyl-based derivatives, the aggregation-induced emission behaviors of such materials are more complicated. Our group has studied the AIE activity of some (9-anthryl)vinylbenzene-based position isomers.17-19,42 Based on the previous and present results, it could be concluded that, besides the intramolecular torsion between the 9-anthryl and the vinylene moiety, the substitution pattern is also an important factor to affect the AIE activity. As shown in Table 1, the substitution position exerts a much stronger effect on the solution emission efficiency than on the solid-state emission efficiency. For example, in DMSO solution, the fluorescence quantum yield ranges from 5% to 67% with the variation of the substitution position. In contrast, the solid-state quantum yield is only from 8% to 35%. In comparison with ortho- and para-substitution, meta-substitution is beneficial for the solution emission

18


Page 19 of 34

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because trans,trans-1,3-AVPVB emits most efficiently in solution. This is a common feature in the reported (9-anthryl)vinyl-based position isomers.17-19,42 The reason is possibly that meta-substitution could effectively decrease the nonradiative decay in solution.19 In the solid state, the freedom of torsional motion and the intermolecular fluorescence-quenching interaction are two important factors to affect the emission efficiency.40

As

for


the

intermolecular

fluorescence-quenching interaction possibly plays a dominant role in decreasing the emission efficiency in the solid state. Contrary to trans,trans-1,3-AVPVB, the other three isomers emit weakly in solution, indicating a high nonradiative decay rate in solution. This nonradiative process could be suppressed to some extent by the restricted torsional motion in the aggregate state, which induces the AIE effect.

CONCLUSION Four

(9-anthryl)vinyl(1-pyrenyl)vinylbenzene

isomers

were

synthesized

and

compared. Substitution position and vinylene bond geometry play important roles in solvatochromism, temperature-induced phase transition and optical properties. Different from the other three isomers, trans,trans-1,2-AVPVB as-prepared sample shows a solid-to-gas phase transition just before a solid-to-solid phase transition upon heating. Despite the lack of typical electron donor and acceptor in the molecule structure, all the isomers except trans,trans-1,3-AVPVB display an apparent fluorescence solvatochromism. Among them, trans,trans-1,4-AVPVB show the strongest solvatochromism with the emission shift over 80 nm upon increasing the

19



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Page 20 of 34

solvent polarity. The fluorescence solvatochromism may be related to the conformational change from the ground to the excited states, which leads to the change of the dipole moment. The solvatochromic degree is determined by conjugative

effect.

In

comparison

with


trans,trans-1,2-AVPVB and trans,cis-1,4-AVPVB display moderate fluorescence solvatochromism due to the low π-conjugation degree. Although meta-substitution induces weak solvatochromism, it is beneficial for solution emission and trans,trans-1,3-AVPVB emits strongest in solution. However, the fluorescence of trans,trans-1,3-AVPVB is quenched in the solid state. In contrast, the other three isomers all display an AIE behavior. This study would shed new light on the solvatochromism study pure aromatic hydrocarbons and the effect of linkage pattern on the optoelectronic properties of organic chromophores.

EXPERIMENTAL SECTION Materials and Instrumentations. All chemicals were purchased from J&K Scientific

and

used

as

received

without

further

purification.

1-diethoxyphosphorylmethyl-2-(9-anthryl)vinylbenzene, 1-diethoxyphosphorylmethyl-3-(9-anthryl)vinylbenzene, 1-diethoxyphosphorylmethyl-4-(9-anthryl)vinylbenzene

and

4-[(9-anthryl)vinyl]benzyltriphenylphosphonium bromide were prepared according to the published procedures.18,42,43

20


Page 21 of 34

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1

H and

13

C NMR spectra were recorded on a Bruker Avance 400 spectrometer.

Elemental analyses were performed using a Perkin Elmer 2400II elemental analyzer. High-resolution mass spectral analyses were carried out on a Bruker maXis UHR-TOF mass spectrometer using atmospheric pressure chemical ionization (APCI) mode. Absorption measurements were carried out on a TU-1800 spectrophotometer. Differential scanning calorimetry (DSC) analyses of the as-prepared samples were measured on a Netzsch DSC 200F3 instrument, in a flowing nitrogen atmosphere and with a 10 ºC min-1 to 400 ºC. To judge whether the endothermic peak at 216 °C is related to the sublimation, the thermogravimetric (TG) and DSC analyses of trans,trans-1,2-AVPVB-α sample were simultaneously measured on a METTLER TOLEDO TGA/DSC 1 instrument, in a flowing nitrogen atmosphere and with a 10 ºC min-1 to 400 ºC. Absolute photoluminescence quantum yields (ΦF) were determined on an Edinburgh Instruments FLS920 using an integrating sphere, and the error is 5%. Photoluminescence measurements and time-resolved emission decay behaviors were recorded on an Edinburgh Instruments FLS920. The photoluminescence experiments were performed for aerated solutions. The emission spectra were corrected for the instrumental spectral responses. X-ray diffraction investigations were carried out on a D8 Focus diffractometer equipped with Cu Kα radiation (λ = 1.5406 Å). Single-crystal X-ray diffraction measurements were conducted on an Oxford Diffraction Gemini E diffractometer. The structure was solved by direct method and refined by a full-matrix least-squares

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

technique

on

F2

using

SHELXL-97

Page 22 of 34

programs.44,45

CCDC-1470570

(trans,trans-1,2-AVPVB-β) and 1444490 (trans,trans-1,4-AVPVB) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge

from

the

Cambridge

Crystallographic

Data

Centre

via

www.ccdc.cam.ac.uk/data_request/cif. Trans,trans-1-(9-anthryl)vinyl-2-(1-pyrenyl)vinylbenzene

Synthesis.

(trans,trans-1,2-AVPVB). At room temperature, t-BuOK (0.20 g, 1.78 mmol) was added

in

batches

to

a

mixture

of

1-diethoxyphosphorylmethyl-2-(9-anthryl)vinylbenzene (0.34 g, 0.79 mmol) and pyrene-1-carbaldehyde (0.26 g, 1.13 mmol) in THF (20 mL) under nitrogen. After stirring overnight at room temperature, the mixture was poured in H2O (40 mL) and extracted by CH2Cl2. The organic phase was combined and dried over MgSO4. After evaporation of the solvent, the crude product was recrystallized from THF/hexane to afford a yellow solid (0.15 g, yield 37.5%). 1H NMR (CDCl3, 400 MHz) δ: 1H NMR (CDCl3, 400 MHz) δ: 7.43-7.51 (m, 7H), 7.72 (d, J = 16.0 Hz, 1H), 7.87-7.92 (m, 2H), 7.97-7.98 (m, 2H), 8.00-8.04 (m, 5H), 8.07 (d, J = 8.0 Hz, 1H), 8.12-8.16 (m, 3H), 8.25 (d, J = 8.0 Hz, 1H), 8.40-8.47 (m, 4H). 13C NMR (CDCl3, 100.6 MHz), δ: 123.08, 123.84, 125.19, 125.25, 125.37, 125.43, 125.78, 126.13, 126.76, 127.04, 127.30, 127.44, 127.59, 127.77, 127.98, 128.24, 128.40, 128.77, 128.87, 129.54, 129.88, 131.03, 131.07, 131.62, 131.65, 131.91, 133.02, 135.93, 136.56, 136.64. MS: m/z calcd for C40H26: 506.2035 [M]-; found: 506.2076. Anal. Calcd for C40H26: C,

22


Page 23 of 34

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94.83; H, 5.17. Found: C, 94.55; H, 5.23. Trans,trans-1-(9-anthryl)vinyl-3-(1-pyrenyl)vinylbenzene (trans,trans-1,3-AVPVB). This compound was prepared from t-BuOK (0.29 g, 2.59 mmol),

pyrene-1-carbaldehyde

(0.26

g,

1.13

mmol),

and

1-diethoxyphosphorylmethyl-3-(9-anthryl)vinylbenzene (0.37 g, 0.86 mmol) in THF (20 mL) using the similar procedure described for trans,trans-1,2-AVPVB. the crude product was recrystallized from THF to afford a yellow solid (0.17 g, yield 39.5%). 1

H NMR (CDCl3, 400 MHz) δ: 7.06 (d, J = 16.4 Hz, 1H), 7.45 (d, J = 16.0 Hz, 1H),

7.49-7.56 (m, 5H), 7.67-7.72 (m, 2H), 7.97-8.07 (m, 7H), 8.15 (d, J = 9.2 Hz, 1H), 8.18-8.22 (m, 3H), 8.32 (d, J = 16.0 Hz, 1H), 8.38 (d, J = 8.0 Hz, 1H), 8.41-8.44 (m, 3H), 8.55 (d, J = 9.2 Hz, 1H).

13

C NMR (CDCl3, 100.6 MHz), δ: 122.56, 123.20,

123.23, 123.83, 123.92, 125.14, 125.26, 125.33, 125.38, 125.50, 125.54, 125.71, 126.10, 126.20, 126.44, 126.45, 126.73, 127.52, 127.64, 127.86, 127.92, 128.04, 128.67, 128.89, 129.47, 129.95, 131.13, 131.15, 131.71, 131.80, 131.99, 132.85, 137.35, 138.03, 138.51, 138.59. MS: m/z calcd for C40H26: 506.2035 [M]-; found: 506.2061. Anal. Calcd for C40H26: C, 94.83; H, 5.17. Found: C, 94.10; H, 5.14. Trans,trans-1-(9-anthryl)vinyl-4-(1-pyrenyl)vinylbenzene (trans,trans-1,4-AVPVB). This compound was prepared from t-BuOK (0.31 g, 2.77 mmol),


(0.22

g,

0.96

mmol),

and

1-diethoxyphosphorylmethyl-4-(9-anthryl)vinylbenzene (0.40 g, 0.93 mmol) in THF (20 mL) using the similar procedure described for trans,trans-1,2-AVPVB. The crude

23



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Page 24 of 34

product was recrystallized THF/DMF to afford a yellow solid (0.28 g, yield 59.1%). 1

H NMR (CDCl3, 400 MHz) δ: 7.02 (d, J = 16.4 Hz, 1H), 7.44 (d, J = 16.4 Hz, 1H),

7.48-7.52 (m, 4H), 7.75-7.80 (m, 4H), 7.99-8.05 (m, 4H), 8.08 (s, 2H), 8.17-8.22 (m, 4H), 8.30 (d, J = 16.0 Hz, 1H), 8.38-8.44 (m, 4H), 8.56 (d, J = 9.2 Hz, 1H). 13C NMR (CDCl3, 100.6 MHz), δ: 123.19, 123.83, 125.10, 125.17, 125.26, 125.32, 125.38, 125.52, 125.70, 125.94, 126.19, 126.21, 126.70, 127.23, 127.35, 127.52, 127.66, 127.86, 128.66, 128.90, 129.94, 131.13, 131.15, 131.52, 131.72, 131.74, 132.03, 132.90, 137.03, 137.12, 137.71. MS: m/z calcd for C40H26: 506.2035 [M]-; found: 506.2056. Anal. Calcd for C40H26: C, 94.83; H, 5.17. Found: C, 95.04; H, 5.39. Trans,cis-1-(9-anthryl)vinyl-4-(1-pyrenyl)vinylbenzene This

compound

was


prepared

from

(0.22

t-BuOK g,

(trans,cis-1,4-AVPVB). (0.18

0.96

g,

1.61

mmol),

mmol), and

anthracen-9-ylmethyltriphenylphosphonium bromide (0.50 g, 0.79 mmol) in CH2Cl2 (20 mL) using the similar procedure described for trans,trans-1,2-AVPVB. The crude product was chromatographed on silica gel (petroleum ether) to produce two main compounds. The first is trans,cis-1,4-AVPVB, which can be purified by chromatography. The second is trans,trans-1,4-AVPVB, which is difficult to be separated in a pure form by chromatography. 1H NMR spectra demonstrate that minor trans,cis-1,4-AVPVB exists in this trans,trans-1,4-AVPVB sample (Figure S5). Trans,cis-1,4-AVPVB (0.075 g, yield 18.7%): 1H NMR (CDCl3, 400 MHz) δ: 6.81 (d, J = 16.4 Hz, 1H), 7.01 (d, J = 12.2 Hz, 1H), 7.17 (d, J = 8.0 Hz, 2H), 7.36-7.40 (m,

24


Page 25 of 34

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3H), 7.41-7.46 (m, 4H), 7.80 (d, J = 16.4 Hz, 1H), 7.97-8.06 (m, 4H), 8.08-8.13 (m, 4H), 8.20 (d, J = 7.7 Hz, 2H), 8.26-8.28 (m, 2H), 8.35-8.37 (m, 2H).

13

C NMR

(CDCl3, 100.6 MHz), δ: 124.61, 124.92, 125.03, 125.06, 125.19, 125.30, 125.37, 125.58, 126.12, 126.15, 126.48, 126.57, 127.26, 127.46, 127.62, 127.79, 128.80, 128.91, 129.20, 129.83, 130.87, 131.33, 131.53, 131.63, 132.01, 132.84, 133.06, 136.32, 136.77, 137.01. MS: m/z calcd for C40H26: 506.2035 [M]-; found: 506.2040. Anal. Calcd for C40H26: C, 94.83; H, 5.17. Found: C, 94.79; H, 5.28.

ASSOCIATED CONTENT

Supporting Information Computational detail, synthetic routes, absorption spectra in different solvents, optimized geometries and structural parameters, emission decay dynamics, thermal ellipsoid plot for the crystal structure, X-ray crystal structural data, Cartesian coordinates of optimized geometries and NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

25



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Page 26 of 34

This work was supported by the National Natural Science Foundation of China (21202061 and 21171069) and the Natural Science Foundation of Shandong Province of China (ZR2011EMQ007).

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Substitution Position and Vinylene Bond Geometry Modulating the

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