Substitution Position Directing the Molecular Packing, Electronic

Article pubs.acs.org/JPCC

Substitution Position Directing the Molecular Packing, Electronic Structure, and Aggregate Emission Property of Bis[2-(9anthracenyl)vinyl]benzene System Ye-Xin Li,* Zhi Chen, Yu Cui, Guang-Ming Xia, and Xiao-Feng Yang School of Chemistry and Chemical Engineering, University of Jinan, 250022 Jinan, China ABSTRACT: Because of the difference in substitution position, compounds 1,2bis[2-(9-anthracenyl)vinyl]benzene (1,2-BAVB), 1,3-bis[2-(9-anthracenyl)vinyl]benzene (1,3-BAVB), and 1,4-bis[2-(9-anthracenyl)vinyl]benzene (1,4-BAVB) display different crystal packing and optical property. 1,4-BAVB molecules pack into zigzag structure. The π-electrons are averagely distributed on the whole backbone though the molecule adopts a twisted structure. Compounds 1,2-BAVB and 1,3-BAVB have a column-like structure, and the π-electrons are mainly confined on the anthracene units. Obvious π···π interactions exist in the aggregates of 1,3BAVB and 1,4-BAVB. The crystal packing and electronic structure exert dramatic influence on the photophysical property. Compound 1,4-BAVB is hardly emissive. Compound 1,3-BAVB is highly emissive in solution but quenched in the solid state. However, compound 1,2-BAVB displays an aggregation-induced emission behavior and an excimer-related fluorescence in solution. The relationship between the aggregate packing, electronic structure, and photophysical property was studied.

1. INTRODUCTION It is well-known that most organic chromophores are highly emissive in the solution but become weakly luminescent in the solid state.1 This aggregation-caused quenching (ACQ) phenomenon is mainly due to the intermolecular vibronic interactions which result into the nonradiative process such as excitonic coupling, excimer formation, and excitation energy migration to the impurity traps.2 At the very beginning of this new century, Tang and co-workers3 and Park and co-workers4 reported abnormal material systems that have aggregationinduced emission (AIE) property. Since their pioneering work, exploring new organic materials with AIE property and understanding the corresponding mechanism have attracted more and more attention. Some other systems bearing an AIE or aggregation-induced enhanced emission (AIEE) character have been researched (e.g., NPAFN,5 DPDSB,6 DCM derivatives,7 DTFO,8 carborane derivatives,9 DMDPS,10 tetraphenylgermole11). To understand this intriguing behavior, a few mechanisms have been proposed. Generally, AIE behaviors were attributed to the restriction of intramolecular rotation,3 formation of J-aggregates and intramolecular planarization,4 inhibition of photoisomerization and photocyclization,6 and restricted twisted intramolecular charge transfer state.12 However, it seems that some of these mechanisms are conflicting and none of them can be used universally.13,14 As the emission enhancement is caused by aggregation, studying the aggregate structure and corresponding electronic structure may be more important and reasonable. In the past few years, people have turn their eyes to understanding the relationship between the molecular packing and the AIE properties.6−8,15−19 Some new light was shed on © 2012 American Chemical Society

this question which, to some extent, changes our conventional concepts about the photoluminescence. So far, there is still much work to do to clarify the AIE behavior from the perspective of the molecular packing and electronic structure. As compared to inorganic materials, one of the prominent characteristics of organic materials is their structure flexibility. Subtle change in molecule structure can exert dramatic influence on the configuration, crystal stacking, electronic structure, and, furthermore, the optoelectronic property. Thus, the chromophores, which have similar molecule formula but display different optoelectronic properties, would be great helpful to research the flexibility and the structure−property relationship. In this paper, we focus on bis[2-(9-anthracenyl)vinyl]benzene system (Chart 1). The results show that the substitution position exerts dramatic influence on the molecular packing, electronic structure, and aggregate emission property. 1,4-Bis[2-(9-anthracenyl)vinyl]benzene (1,4-BAVB) is hardly emissive. 1,3-Bis[2-(9-anthracenyl)vinyl]benzene (1,3-BAVB) displays ACQ character. However, 1,2-bis[2-(9-anthracenyl)vinyl]benzene (1,2-BAVB) is AIE-active and displays an excimer-related emission behavior in solution. Thus, the variations in the substitution position present a good chance to understand the relationship between the crystal packing, electronic structure, and the AIE or ACQ behavior. Received: November 25, 2011 Revised: February 21, 2012 Published: February 22, 2012 6401

dx.doi.org/10.1021/jp211364m | J. Phys. Chem. C 2012, 116, 6401−6408

The Journal of Physical Chemistry C

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compounds adopt trans configuration. Compounds 1,2-BAVB and 1,3-BAVB are readily soluble in common solvents, allowing them to be easily purified by column chromatography. On the contrary, compound 1,4-BAVB has poor solubility in ordinary solvents. Both the solutions of 1,2-BAVB and 1,4-BAVB are much more unstable under ambient condition. Photochemical reactions were found to occur after storing their THF solutions (about 10−5 M) for 1 or 2 h, which may be related to the complicated photo-oxidation and isomerization reactions.25 1,3BAVB solution has a better environmental stability. However, obvious photochemical reactions still took place after the THF solution (about 10−5 M) were stored for 1 day. Compared to the solution, all the solids are stable under the same condition. Photophysical Property. As shown in Figure 1, the change of substitution position leads to different optical

Chart 1. Chemical Structures of 1,2-BAVB, 1,3-BAVB, and 1,4-BAVB

2. EXPERIMENTAL SECTION Materials and Instrumentation. All chemicals were purchased from Aldrich and Acros and used as received without further purification. Anthracen-9-ylmethyltriphenylphosphonium bromide, 1,4-BAVB, and 1,3-BAVB were prepared according to the published procedures.20−22 1H NMR and 13C NMR spectra were recorded on a Bruker Avance 300 spectrometer. Mass spectra were determined with an AXIMA-CFR plus MALDI-TOF mass spectrograph. Absorption measurements were carried out on a TU-1800 spectrophotometer. Photoluminescence (PL) measurements were recorded using a Hitachi F-4500 fluorescence spectrophotometer with a 150 W Xe lamp. Time-resolved emission decay behaviors were recorded on an Endinburgh Instruments FLS920. The absolute photoluminescence quantum yield (ΦF) values of the solid films and CH2Cl2 solution (5 × 10−5 M) were also determined on an Endinburgh Instruments FLS920 using an integrating sphere. Single-crystal X-ray diffraction measurements were conducted on an Oxford Diffraction Gemini E diffractometer. The structures were solved by direct methods and refined by a full-matrix leastsquares technique on F2 using SHELXL-97 programs.23,24 CCDC-848984 (1,2-BAVB), CCDC-848985 (1,3-BAVB), and 848986 (1,4-BAVB) 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. Synthesis. 1,2-Bis[2-(9-anthracenyl)vinyl]benzene (1,2BAVB). At 0 °C, t-BuOK (1.0 g, 8.9 mmol) was added in batch into a mixture of anthracen-9-ylmethyltriphenylphosphonium bromide (1.5 g, 2.8 mmol) and 1,2-benzene-dicarboxaldehyde (0.16 g, 1.2 mmol) in CH2Cl2 (20 mL) under nitrogen. After stirring overnight at room temperature, the mixture was poured into 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 chromatographed on silica gel (petroleum ether) to afford a yellow solid (0.060 g, yield 10.4%). 1H NMR (CDCl3, 300 MHz) δ: 7.28−7.29 (m, 2H), 7.31−7.32 (m, 2H), 7.37−7.42 (m, 6H) 7.52−7.56 (m, 2H), 7.86 (d, J = 16.5 Hz, 2H), 7.94−8.00 (m, 6H), 8.32−8.35 (m, 6H). 13C NMR (CDCl3, 75.47 MHz) δ: 124.58, 125.02, 125.32, 126.03, 126.45, 127.41, 127.76, 128.12, 129.16, 130.94, 132.16, 134.91, 135.80. MALDI-TOF, m/z: calcd, 482.2; found, 482.1.

Figure 1. Fluorescence image of the CH2Cl2 solutions (upper) and solids (lower) of compounds 1,2-BAVB, 1,3-BAVB, and 1,4-BAVB under illumination with a 365.0 nm UV lamp.

properties. Under illumination with a 365 nm UV lamp, the CH2Cl2 solution of compound 1,2-BAVB is hardly emissive, but the solid emits bright blue light, which is consistent with what the AIE concept described. Compound 1,3-BAVB displays an opposite behavior. The emission of the CH2Cl2 solution is much stronger than that of its solid, displaying an ACQ character. As for compound 1,4-BAVB, both the solid and solution emit very weakly. The optical absorption and fluorescence emission spectra of the three compounds in CH2Cl2 solution are shown in Figure 2. The absorption spectra of 1,4-BAVB solution show a red shift compared to those of the other two materials, which suggests that 1,4-BAVB molecule would be better conjugated. The solution of 1,2-BAVB has the similar absorption behavior as that of 1,3-BAVB. As for the fluorescence emission, the intensity of 1,3-BAVB solution is much stronger than that of 1,2-BAVB or 1,4-BAVB solution under the same measurement condition, which is consistent with the observation under the UV lamp. As 1,4-BAVB has poor solubility and is hardly emissive either in solution or solid form, the following optical property studies are focused on 1,2BAVB and 1,3-BAVB. To further study the aggregation-induced optical properties of compounds 1,2-BAVB and 1,3-BAVB, a solvent−nonsolvent fluorescence test, which is commonly used in the study of the AIE behavior, was performed.3 Water was used as the nonsolvent. EtOH and THF were used as the solvent for

3. RESULTS AND DISCUSSION Synthesis and Stability. Compound 1,2-BAVB was prepared by a similar method as compounds 1,3-BAVB and 1,4-BAVB. The study of the crystal structures shows that all the 6402



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fluorescence peak intensities of compound 1,2-BAVB are gradually intensified with the growing water fraction. A dramatic enhancement of luminescence emission is observed when the water fraction is over 20%, which would be related to the aggregate formation. The fluorescence intensity of compound 1,2-BAVB in aggregate state is apparently stronger than that of the EtOH solution. Therefore, compound 1,2BAVB is AIE-active. When compound 1,3-BAVB was done, the same test as 1,2-BAVB (Figure 3b). The emission intensities are decreased with the growing water fraction, displaying ACQ behavior. The changes of the fluorescence quantum yield (ΦF) from solution to film also confirm the opposite aggregation-induce fluorescence behaviors of 1,2-BAVB and 1,3-BAVB. The ΦF value of 1,2-BAVB solid is 12%, which is higher than that of the solution (ΦF < 1%). On the contrary, the ΦF value of 1,3-BAVB solid is 15%, which is much lower than that of its CH2Cl2 solution (ΦF = 58%). The substitution variation from 1,2- to 1,3-position produces a major increase in the solution fluorescence efficiency but a minor change in the solid fluorescence efficiency. Therefore, the opposite aggregationinduced fluorescence behaviors of 1,2-BAVB and 1,3-BAVB may be mainly attributed to the difference in the solution emission. The PL decay dynamics of 1,2-BAVB and 1,3-BAVB were measured, and the results are shown in Table 1. The decay behavior of the 1,2-BAVB film is in a double-exponential manner. However, the excited molecules in 1,3-BAVB film decay through three relaxation pathways, and the weighted mean lifetime is much longer than that of 1,2-BAVB film. The different decay dynamics of the two materials may be closely related to their aggregate microstructures as shown in the following section. The decay behavior of 1,3-BAVB in CH2Cl2 solution follows a single-exponential behavior with a lifetime of 3.17 ns, longer than that of 1,3-BAVB film. Another interesting difference is that the solution fluorescence of 1,2-BAVB is more sensitive to the solvent than that of 1,3-BAVB. As shown in Figure 4b, the solvent nature has little influence on the emission spectra of 1,3-BAVB. However, for 1,2-BAVB, dramatic changes in the spectrum shape and intensity were observed in different solvents (Figure 4a). What is important to point out is that there is not a strong dependence of emission spectra on the solvent polarity. Among the used solvent, benzene solution emits more intensely with an apparent red shift. To elucidate this phenomenon, the emission spectra of benzene solutions with different concentrations were measured. As shown in Figure 4c, the solution fluorescence displays a concentration-dependent behavior. There is mainly one sharp peak at 445 nm when the concentration is below 10−8 M. Above 10−7 M, a broad peak around 480 nm can be observed. At 10−5 M, this broad peak becomes dominant and the emission intensity is intensified. Up to 10−4 M, only emission at the longer wavelength was observed. Therefore, 1,2-BAVB solution displays a typical excimer-related fluores-

Figure 2. Optical absorption (UV/vis, normalized) and emission spectra in the CH2Cl2 solution (10−5 M) of compounds 1,2-BAVB, 1,3-BAVB, and 1,4-BAVB. The PL spectra were obtained by exciting at 389 nm. The emission spectra of compounds 1,2-BAVB and 1,4-BAVB were magnified 10 times for clarity.

compounds 1,2-BAVB and 1,3-BAVB, respectively. Concentration was kept at 10 μM. As shown in Figure 3a, the

Figure 3. PL spectra of 1,2-BAVB in EtOH/water mixtures (a) and 1,3-BAVB in THF/water mixtures (b) with different water volume fractions. The concentration is 1 × 10−5 M. Inset depicts the change of emission peak intensity versus water volume fraction.

Table 1. Fluorescence Decay Parameters of 1,2-BAVB as Drop-Casting Film and 1,3-BAVB in CH2Cl2 Solution and Film Form a

1,2-BAVB film 1,3-BAVB filmb 1,3-BAVB solutionb a

A1

τ1 (ns)

A2

τ2 (ns)

A3

τ3 (ns)

⟨τ⟩ (ns)

χ2

0.98 0.52 1.0

0.26 0.54 3.17

0.02 0.34

2.72 2.51

0.14

7.81

0.31 2.23 3.17

1.03 1.14 1.10

λex = 389 nm, λem = 497 nm. bλex = 389 nm, λem = 495 nm. 6403



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Figure 4. Emission spectra of 1,2-BAVB (a) and 1,3-BAVB (b) in solvents with different polarity (10−5 M) and as solid film. Emission spectra of 1,2BAVB (c) and 1,3-BAVB (d) in benzene solutions with different concentrations.

cence character as excimer emission is known to be concentration-dependent.26 The former sharp peak is assigned to the monomer emission which may be related to anthracene moiety, and the latter broad one could be related to the anthracene excimer emission.26,27 1,2-BAVB also displays a similar emission behavior using CH2Cl2 as the solvent. 1,2BAVB solid film has only one emission peak, which is close to the position of excimer emission in the concentrated solution (Figure 4a). This may imply that the emitting species in the solid state should be similar to those in concentrated solution. As for 1,3-BAVB solution, the emission shape does not show any obvious change with progressively increasing the concentration (Figure 4d). The solution emission of 1,3BAVB is possibly related to the isolated molecule. The much higher fluorescence efficiency of 1,3-BAVB solution may be related to the totally different emitting species from 1,2-BAVB, which originate from the change of substituent position. As shown in Figure 4b, the PL spectrum of 1,3-BAVB solid film shows an obvious red shift compared with that of solution. This behavior is also observed in the solvent−nonsolvent fluorescence test when the water fraction reaches 90% (Figure 3b). This red shift in the solid emission may be attributed to the molecular interaction within the aggregate. Crystal Structure. Crystal structure is most important to understand the relationship between the photophysics property and the molecular packing. The crystals of compounds 1,2BAVB and 1,4-BAVB were obtained by the physical vapor transport method through a two-zone horizontal tube furnace. 1,3-BAVB crystals were grown from an HCCl3/EtOH mixed solution. Their crystal data and collection parameters are summarized in Table 2. Compound 1,2-BAVB crystallizes in the P21/c space group. There are two symmetry-independent molecules per asymmetric unit. The torsion angles between the anthracenyl ring and the vinylene moiety are 53.88° and 73.55° for one molecule and 53.20° and 66.22° for the other one (Figure 5a).

Table 2. Crystal Data and Details of Collection and Refinement for the Three Compounds compound

1,2-BAVB

chemical formula formula mass crystal system a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] unit cell volume [Å3] temperature [K] space group Z radiation type F(000) reflections collected/ unique parameters R1, wR2 [I > 2σ(I))] R1, wR2 (all data) goodness of fit on F2 largest diff peak and hole [e Å−3]

C38H26 482.59 monoclinic 30.863(4) 6.4202(8) 26.959(4) 90.00 105.943(14) 90.00 5136.3(12) 291(2) P21/c 8 Mo Kα 2032.0 36762/10039

C38H26 482.59 monoclinic 6.35972(18) 13.7676(3) 28.6749(7) 90.00 93.600(2) 90.00 2505.76(11) 119.95(10) P21/c 4 Mo Kα 1016.0 13252/5094

1,3-BAVB

C38H26 482.59 monoclinic 8.1741(18) 16.331(3) 10.045(2) 90.00 111.00(3) 90.00 1251.8(5) 293(2) P21/c 2 Mo Kα 508.0 7013/2547

1,4-BAVB

685 0.0832, 0.1619 0.2116, 0.2234 1.006 0.19/−0.22

343 0.0403, 0.0969 0.0511, 0.1035 1.040 0.18/−0.25

172 0.0407, 0.1025 0.0659, 0.1096 1.054 0.17/−0.15

As shown in Figure 5b, 1,2-BAVB molecules pack into a column structure in the ac plane, and there exist strong C− H···π and H···H (2.351 Å) interactions in the aggregate structure. Within one column along b-axis, the twisted molecules adopt slipped face-to-face packing (Figure 5c). All the adjacent anthracene rings slip along a direction which is between the long and the short axes of the anthracene backbone. The shortest C···C distance between adjacent 6404



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Figure 5. (a) Two symmetry-independent molecule conformations of compound 1,2-BAVB. (b) Stacking image of 1,2-BAVB molecules in the ac plane. (c) Molecule packing in the ab plane. The hydrogen atoms have been omitted for clarity.

Figure 6. (a) Molecule structure of compound 1,3-BAVB. (b) Stacking image of 1,3-BAVB molecules in the bc plane. (c) Molecule packing in the ac plane. The hydrogen atoms have been omitted for clarity.

Figure 7. (a) Molecule structure of compound 1,4-BAVB. (b) Stacking image of 1,4-BAVB molecules in the ab plane. The hydrogen atoms have been omitted for clarity. (c) Molecule packing in the bc plane.

structure in the bc plane and C−H···π interactions exist in the aggregate structure. Within one column along a-axis, the twisted molecules also adopt slipped face-to-face packing. It is interesting that the two groups of anthracenyl units slip along different directions. One group with torsion angle of 67.32° slips along the short axis of the anthracene backbone, and the other with torsion angle of 58.73° is along the long axis (Figure 6c). Contrary to compound 1,2-BAVB, there exist typical π···π interactions within the crystal structure of compound 1,3-

anthracenyl units is 3.671 Å (the van der Waals radii of C is 170 pm). Thus, there is no typical π···π interaction. Compound 1,3-BAVB crystallizes in the same space group as compound 1,2-BAVB. There is one symmetry-independent molecule per asymmetric unit. The torsion angles between the anthracenyl ring and the vinylene moiety are 58.73° and 67.32°, respectively (Figure 6a). The packing motif of 1,3-BAVB molecules is similar to that of 1,2-BAVB molecules. As shown in Figure 6b, 1,3-BAVB molecules also pack into a column 6405



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Figure 8. LUMOs (upper) and HOMOs (lower) of compounds 1,2-BAVB, 1,3-BAVB, and 1,4-BAVB. The numbers in parentheses are the torsion angles.

BAVB. The π···π interaction is mainly situated on the anthracenyl group which slips along the long axis. The overlapped area is about one benzene ring (about 33.3% of the anthracene plane), and the vertical distance between the adjacent anthracene rings is 3.604 Å. As for the anthracenyl rings slipped along the short axis, there is no overlapped carbon atom and the shortest C···C distance is 4.229 Å, which is too long to form effective π···π interaction. However, this group of anthracene rings interacts with the phenyl unit in the adjacent column by the offset π···π interaction, and the shortest C···C distance is 3.386 Å. Compound 1,4-BAVB also crystallizes in the P21/c space group. Because of the centrosymmetric character, there is only one symmetry-independent half molecule per asymmetric unit. 1,4-BAVB molecule also adopts a twisted structure, and the torsion angle between the anthracene ring and the vinylene moiety is 51.51° (Figure 7a). Contrary to the column packing motif of compounds 1,2-BAVB and 1,3-BAVB, 1,4-BAVB molecules pack into a zigzag structure in the ab plane shown in Figure 7b. The adjacent anthracene rings are situated in an overlapping mode along the c-axis, and the overlapped area is about one benzene ring (about 33.3% of the anthracene plane). The centroid···centroid distance between the overlapped anthracene units is 5.465 Å, and the dihedral angle is 21.86°. Because of the small dihedral angle and the short centroid···centroid distance, there exists C···C interactions (3.30 Å) between adjacent anthracene rings (Figure 7c). Thus, this kind of interaction possesses some character of π···π interaction. Beside C···C interaction, there also exist C−H···π (2.853 Å) and H···H (2.40 Å) interactions within the aggregate structure. Electronic Structure. To understand the relationship between the optical property and electronic structure, the HOMOs and LUMOs of the three compounds were calculated by DFT/B3LYP/6-31G(d) based on the molecule conformation in the crystal structure (Figure 8). For compounds 1,2BAVB and 1,3-BAVB, the electrons are mainly located on the anthracence units. Both molecules have similar HOMO and LUMO energy levels, which is consistent with the similar optical absorption behavior as shown in Figure 2. For the HOMO and LUMO of 1,3-BAVB molecule, the π-electrons are confined mainly on the anthracene unit which has a lower torsion angle with the vinylene moiety. As for compound 1,2BAVB, the π-electron distributions of the two symmetryindependent molecules are different. For one molecule with the torsion angles of 53.88° and 73.55°, both the HOMO and LUMO electrons are distributed mainly on the anthracene unit with smaller torsion angle. For the other molecule with the

torsion angles of 53.20° and 66.22°, the LUMO electrons are distributed mainly on the anthracene unit with a smaller torsion angle, but the HOMO electrons are mainly on the anthracene unit with a larger torsion angle. Compound 1,4-BAVB has obvious different electronic structure from the other two materials. The electrons are averagely distributed on the whole molecule backbone though there exists obvious torsion between the anthracene ring and the vinylene moiety. Such an electron distribution imparts this molecules better conjugation character and thus shows a red shift of its absorption spectra compared to those of 1,2-BAVB and 1,3-BAVB. Therefore, on the basis of theoretical calculations, it is clear that the different electron distributions of the compounds could lead to their different optical properties. Mechanism Discussion. The molecule packing in the solid state would produce dual roles on the emission.28 On the one hand, intermolecular interaction could possibly promote the formation of species such as excimers and exciplexes, detrimental to the fluorescence. On the other hand, aggregation can restrict the intramolecular motion,3 intersystem crossing,29 intramolecular photochemical reactions,6 etc., which could block the nonradiative decay channels and enhance the emission. The competition between these two opposing factors determines the aggregation-induce fluorescence. For the bis[2(9-anthracenyl)vinyl]benzene system, the variations in substitution position bring dramatic changes in the intermolecular interaction mode and molecular packing to affect the aggregate emission. 1,4-BAVB has an apparently different packing structure from 1,2-BAVB and 1,3-BAVB. The molecules pack into zigzag structure and there exist partial π···π interactions between adjacent anthracene rings. The HOMO and LUMO electrons are averagely distributed on the whole molecule backbone, which may be helpful for the electron coupling between adjacent molecules. These aspects may mainly result into the weak emission character of 1,4-BAVB solid. Both 1,2-BAVB and 1,3-BAVB molecules adopt a more twisted configuration and pack into a column structure. The more distorted configuration could effectively weaken the interdipole interaction by increasing intermolecular distance. The shortest centroid···centroid distance between adjacent anthracenyls in 1,2-BAVB, 1,3-BAVB, and 1,4-BAVB aggregates is 6.239, 6.360, and 5.446 Å, respectively. The shortest C···C distance between adjacent anthracenyls is 3.671, 3.571, and 3.30 Å, respectively. Thus, the π···π interaction between adjacent anthracene rings in 1,2-BAVB and 1,3-BAVB aggregates is much weaker than that in 1,4-BAVB aggregate. Furthermore, the more distorted configuration could destroy 6406



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Notes

the molecule conjugation as the HOMO and LUMO electrons are mainly confined on one anthracene unit in 1,2-BAVB and 1,3-BAVB. These factors may result into the emission character of 1,2-BAVB and 1,3-BAVB solids. For 1,2-BAVB solid has a similar fluorescence efficiency to 1,3-BAVB, their opposite aggregation-induced emission behaviors are mainly attributed to the differences in the solution fluorescence. The change of substitution from 1,2- to 1,3position produces a drastic difference in the solution emitting species. 1,2-BAVB solution displays an excimer-related fluorescence character as the emission is concentrationdependent. As the fluorescence of 1,2-BAVB solid film is close to that in the concentrated solution, the emitting species in the solid state should be similar to those in concentrated solution. However, compared with the aggregate, the formation of excimer in solution is detrimental to the emission.8 The excited molecule and the ground molecule should first combine together to form an excimer and then a repulsive process to decay to the ground state. These actions would consume much excited state energy and subsequently effectively quench the fluorescence. In the solid state, such motions are resticted, which shut down the nonradiative decay and enhance the emission. To further prove this, the 10−4 M benzene solution of 1,2-BAVB was frozen by liquid nitrogen, and the PL spectrum was measured instantly. The PL shows a dramatic increase in intensity and a negligible shift in peak wavelength (λem = 473 nm). Therefore, the restriction of internal excimer motions may be the main reason to induce the AIE behavior. The short PL lifetime of 1,2-BAVB solid (0.31 ns) may be related to the unstable character of the excimer state in crystalline solid at room temperature.30,31 As for the dual roles of molecule packing on the emission, it is obvious that for 1,2-BAVB the advantageous side outweighs the disadvantageous one. As for 1,3-BAVB, the solution emission is related to the isolated molecule, not the excimer. The solution emission efficiency is much higher than that of 1,2-BAVB solution. However, the aggregation process of 1,3-BAVB molecules, in a whole, brings a disadvantageous effect on the emission.1,2

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Shandong Province of China (SZR1116), the National Natural Science Foundation of China (21077044), and the Doctoral Foundation of University of Jinan (XBS1048).

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4. CONCLUSION The crystal packing, electronic structure, and optical property of bis[2-(9-anthracenyl)vinyl]benzene system are sensitive to the substitution position. 1,4-BAVB molecules pack into a zigzag structure, and the π-electrons are averagely distributed on the whole backbone. Compounds 1,2-BAVB and 1,3-BAVB have a column-like structure, and the π-electrons are mainly confined on the anthracene units. Obvious π···π interactions exist in the aggregates of 1,3-BAVB and 1,4-BAVB. The crystal and electronic structures exert dramatic influence on the optical property. Compound 1,4-BAVB is hardly emissive. Compound 1,3-BAVB is highly emissive in the solution but quenched in the solid state. An excimer-related fluorescence behavior was observed on 1,2-BAVB solution. 1,2-BAVB displays an AIE behavior, which is mainly due to the restriction of the internal excimer motions. This study of structure-similar molecules with different aggregate emission properties would be helpful to further understand the relationship between the aggregate packing, electronic structure, and the photophysical property.

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(26) Chen, K.-H.; Yang, J.-S.; Hwang, C.-Y.; Fang, J.-M. Org. Lett. 2008, 10, 4401−4404. (27) Du, H.; Fuh, R. A.; Li, J.; Corkan, A.; Lindsey, J. S. Photochem. Photobiol. 1998, 68, 141−142. (28) Zhao, Z.; Chen, S.; Lam, J. W. Y.; Wang, Z.; Lu, P.; Mahtab, F.; Sung, H. H. Y.; Williams, I. D.; Ma, Y.; Kwok, H. S.; Tang, B. Z. J. Mater. Chem. 2011, 21, 7210−7216. (29) Katoh, R.; Suzuki, K.; Furube, A.; Kotani, M.; Tokumaru, K. J. Phys. Chem. C 2009, 113, 2961−2965. (30) Matsui, A.; Mizuno, K.; Kobayashi, M. J. Phys. (Paris) 1985, 46, C7. (31) Horiguchi, R.; Iwasaki, N.; Maruyama, Y. J. Phys. Chem. 1987, 91, 5135−5139.

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Substitution Position Directing the Molecular Packing, Electronic

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