General Synthetic Approach toward Geminal-Substituted

Article pubs.acs.org/cm

General Synthetic Approach toward Geminal-Substituted Tetraarylethene Fluorophores with Tunable Emission Properties: X‑ray Crystallography, Aggregation-Induced Emission and Piezofluorochromism Guo-Feng Zhang,† Hongfeng Wang,† Matthew P. Aldred,*,§ Tao Chen,† Ze-Qiang Chen,† Xianggao Meng,*,‡ and Ming-Qiang Zhu*,† †

Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China § Department of Chemistry, Durham University, South Road, Durham DH1 3LE, United Kingdom ‡ Key Laboratory of Pesticide and Chemical Biology of the Ministry of Education, College of Chemistry, Central China Normal University, Wuhan, Hubei 430079, China S Supporting Information *

ABSTRACT: A general approach to the design and synthesis of a new series of geminal-substituted tetraarylethene (g-TAE) chromophores with aggregation-induced emission (AIE) properties has been developed via Corey−Fuchs reaction and subsequent Suzuki/Stille coupling reactions. To demonstrate the synthetic simplicity and versatility of this approach, fluorophores with diverse electronic characteristics have been synthesized. The validation of the geminal structure and bulky steric effect on the bond lengths and dihedral angles is further confirmed by the X-ray crystallography of three chromophores. With respect to single crystals of 2,2′-(2,2diphenylethene-1,1-diyl)-dinaphthalene (DPDN2), 1,1′-(2,2-diphenylethene-1,1-diyl)dinaphthalene (DPDN1 ), and 9,9′-(2,2-diphenylethene-1,1-diyl)dianthracene (DPDAn), intensive cofacial π−π stacking is absent in their molecular packing. The pseudopolymorphism of DPDAn crystals associated with different photoluminescence properties is investigated. In comparison to the solvent-free DPDAn crystal, the solvated DPDAn with embedded methanol or dichloromethane leads to some non-negligible conformational and packing alterations, which accounts for its distinct fluorescence properties. The AIE phenomena and optical properties of g-TAEs are probed with respect to steric and electronic effects. Along with their electrochemical characteristics and piezofluorochromism, our work has elucidated that this general approach can be utilized to develop a promising class of TAE materials for systematic investigations of its optoelectronic properties and crystal engineering.

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INTRODUCTION The fundamental understanding of the processes involved in fluorescence, at the molecular level, is one of the most intriguing topics in chemistry.1 Owing to the variability and flexibility of chemical modification, organic fluorescent materials, either in solution or solid state, have been developed to tune their physical properties. Particular attention has been focused on their potential applications in the fields of organic optoelectronic devices,2 sensors3 and bioimaging,4 in which organic fluorophores with efficient fluorescence quantum yields (ΦF) both in solution and/or solid state act as vital elements. In general, fluorophores, such as perylene diimides (PDIs), exhibit excellent ΦF even up to unity in organic solutions. 5 Unfortunately, most fluorophores suffer from aggregationcaused quenching (ACQ) in the solid state or in aqueous media, probably resulting from self-absorption or intrinsic intermolecular π−π stacking.6 Consequently, this leads to a drastic decrease or even a complete loss in ΦF.7 Although many endeavors have been devoted to alleviating the ACQ effect, © 2014 American Chemical Society

until now the achievements have been unsatisfactory, which is due to the aforementioned natural and intrinsic process of aggregate formation in the solid state.8 Therefore, the potential applications of many fluorophores are somewhat limited.9 Contrary to the notorious ACQ behavior, an opposite phenomenon, named aggregation-induced emission (AIE), was observed by Tang’s group in 2001.10 Fluorophores that exhibit AIE show quite poor fluorescence in solution, compared to their strongly enhanced emission in the “aggregate” or condensed state, demonstrating huge differences in the ΦF between their solution and condensed states. The underlying origin of the AIE phenomenon has been probed by previous experimental observations and theoretical calculations. Accordingly, the restriction of intramolecular rotation has been proposed to account for the intriguing AIE phenomenon.11 Received: April 18, 2014 Revised: July 2, 2014 Published: July 3, 2014 4433

dx.doi.org/10.1021/cm501414b | Chem. Mater. 2014, 26, 4433−4446

Chemistry of Materials

Article

Scheme 1. Synthetic Route of the TAE Compounds

skeletons can be tremendously expanded by replacement with other aryl ketones or other diverse boronic acids/esters.

With respect to the archetypal AIE-active tetraphenylethene (TPE) in solution, the peripheral phenyl rings that serve as the blades in this propeller-like structure rotate frequently in a relaxation channel for the excited state to decay, which results in fluorescence quenching. On the contrary, in the aggregate state, the frequency of the intramolecular rotations of these peripheral rings is dramatically suppressed. As a result, nonradioactive decay channels are restricted or even blocked, which enhances the photoluminescence of these compounds even up to quantitative ΦF.12 Owing to this unique process, AIE fluorophores used as novel bioprobes have become promising candidates for tumor imaging, long-term cell tracing, cancer cell detection, and so on.2e,13 The foremost materials reported in the literature that exhibit AIE-active characteristics are silole- and TPE-based materials. Silole-based materials are difficult to modify because of the presence of the active intermediate organometallic reagent.14 Compared with siloles, TPE-based materials are much more accessible and easier to modify. The primary synthetic methodologies for producing TPE-based materials are the McMurry reaction15 and the synthesis reported by Rathore and co-workers.16 However, these procedures are not suitable for many substrates due to the inability or difficulty to access the requisite diaryl ketones. Therefore, an alternative and facile synthetic methodology that allows a variety of possibilities for chemical modification to synthesize AIE-active materials is urgently desirable. To satisfy this demand, an alternative tetraarylethene (TAE) family is herein highlighted. In this work, we present a novel and facile synthesis to prepare AIE fluorophores via Corey− Fuchs reaction and subsequent Suzuki or Stille coupling, which has been successfully applied in the synthesis of polycyclic aromatic hydrocarbon (PAH) materials.17 The obtained geminal-substituted TAE (g-TAE) compounds exhibit tunable optical properties that depend on the geminal-substituted aryl groups. The similarities and disparities of the AIE characteristics compared with those of the well-studied TPE are investigated. Our results illustrate that this versatile methodology for the synthesis of AIE-active compounds with olefin

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EXPERIMENTAL METHODS

Synthesis. The synthesis of g-TAE fluorophores is a two-step procedure using conventional synthetic protocols outlined in Scheme 1a. 1,1-Dibromo-2,2-diphenylethene was prepared according to a previously reported method via Corey−Fuchs reaction of benzophenone.18 Subsequent treatment with the appropriate aryl boronic acid, boronic acid ester, or 2-(tributylstannyl)thiophene under Suzuki or Stille coupling conditions19 afforded the various g-TAE compounds with decent yields. Nine compounds have been synthesized, illustrating the ease and versatility of the synthetic route, i.e., 2,2′(2,2-diphenylethene-1,1-diyl)dithiophene (DPDT), 1,1′-(2,2-diphenylethene-1,1-diyl)-dinaphthalene (DPDN1), 2,2′-(2,2-diphenylethene1,1-diyl)dinaphthalene (DPDN2), 9,9′-(2,2-diphenylethene-1,1-diyl)dianthracene (DPDAn), 3,3′-(2,2-diphenylethene-1,1-diyl)di(9-methyl-9H-carbazole) (DPDCz), 2,2′-((2,2-diphenylethene-1,1-diyl)di(4,1phenylene))di(3,3-diphenylacrylo-nitrile) (DPDTPAN), 4,4′-(2,2-diphenylethene-1,1-diyl)di((1,2,2-triphenylvinyl)-benzene) (DPDTPE), and 6,6′-(2,2-diphenylethene-1,1-diyl)di(2-(2,6-diisopropylphenyl)1H-benzo[de]-isoquinoline-1,3(2H)-dione) (DPDNI). To further demonstrate the generality of this synthetic approach, an unsymmetrical TAE, i.e., 9-(1,2,2-triphenylvinyl)-anthracene (TPMAn), was obtained in a one-pot Suzuki reaction of 1,1-dibromo-2,2-diphenylethene with phenyl boronic acid and anthracene boronic acid (Scheme 1b). The detailed synthesis procedures and characterizations of the nine compounds are described from Schemes S1−S14 and Figures S1−S14 in the Supporting Information. Electrochemical Properties. Cyclic voltammetry was undertaken using a CHI Instruments 750D electrochemical workstation to investigate the redox properties of the g-TAE materials. All measurements were performed in dry dichloromethane (DCM), and tetra-n-butylammonium hexafluorophosphate (Bu4NPF6) (0.1 M in DCM) was selected as the supporting electrolyte. The solutions were purged with nitrogen to remove oxygen before the measurements. Measurements were accomplished using a 3.0 mm diameter glassy carbon working electrode, a platinum wire counter electrode, and a Ag/Ag+ wire reference electrode. Cyclic voltammetry was recorded at a scanning rate of 50 mV/s. Optical Properties Measurements. UV−vis absorption spectra were collected on a Shimadzu 3600 spectrophotometer, and fluorescence measurements were performed on an Edinburgh FLS 4434



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920 spectrofluorometer. Fluorescence lifetimes were measured using the time-correlated single photon counting (TCSPC) technique and were recorded on the Edinburgh FLS 920 instrument. The relative fluorescence quantum yields were obtained in comparison to diphenylanthracene (DPA) in cyclohexane as the standard (ΦF = 0.9).20 1 H and 13C NMR Spectroscopy. All experiments were performed on a 400 MHz Bruker AV400 NMR spectrometer at room temperature using CDCl3 or CD2Cl2 as the solvent. X-ray Crystallographic Studies. Diffraction data were collected at room temperature or low temperature using an Apex Duo X-ray CCD diffractometer working with graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å). Data collection, frame integration, data reduction using the multiscan method, and structure determination were carried out using APEX2 software. Empirical absorption corrections were applied to the data using the SADABS program. Structural refinements were performed by the full-matrix least-squares method on F2 with the SHELXTL-97 program. Anisotropic displacement parameters were refined for all non-hydrogen atoms. Hydrogens bonds to carbon atoms were placed at calculated positions with the appropriate AFIX instructions and refined using a riding model. Related parameters of some short contacts (such as π···π and C− H···π) in the crystal structures are measured using PLATON,21 Mercury, and Diamond software. Dihedral angles between the neighboring planes were calculated using Mercury software with all of the atoms on the same aryl ring selected as a plane. X-ray Powder Diffraction. Powder XRD patterns were obtained using a X’Pert PRO diffractometer from 2θ = 5−80° with Cu Kα radiation (λ = 1.5406 Å) under room temperature. Thermal Property. Thermal analyses of the synthesized compounds were carried out on a Pyris1 TGA (PerkinElmer Instruments) thermogravimetric analyzer under a nitrogen atmosphere with the heating rate set at 10 °C/min.

Scheme 2. Well-Developed Synthetic Methodologies Utilized To Synthesize TPE Derivativesa

a

(a) The synthesis reported by Rathore;16 (b) the McMurry reaction.15

structural modifications, owing to their difficultly in affording the requisite diaryl ketones for many substrates. Moreover, the McMurry coupling of different benzophenone derivatives for the synthesis of unsymmetrical TPEs, for example, in the synthesis of TPE-OH (Scheme S2, Supporting Information), using benzophenone and 4-hydroxybenzophenone, will produce a mixture of three components, with the yield of the product being sacrificed. In the homocoupling of monosubstituted benzophenone, the separation of the resultant TPEs with a statistical mixture of Z and E conformations still remains a big challenge. With respect to Rathore’s procedure, the functional groups present on the diphenylmethane and benzophenone moieties are restricted due to the utilization of an organolithium reagent (n-BuLi) and the formation of a lithium diphenylmethine salt that can both react with a carbonyl or other sensitive groups. In contrast, our proposed methodology effectively avoids the above issues encountered by both McMurry’s and Rathore’s methods, which provides a significant improvement with respect to structural modifications. Previously, Tang and co-workers designed and synthesized TAE derivatives by McMurry coupling of phenyl(pyren-1-yl)methanone23 and naphthalen-2-yl(phenyl)methanone,24 resulting in the formation of E and Z isomers in a nearly statistical mixture. From a one-pot reaction initially developed by Larock and co-workers,25 Tang designed and prepared Z-rich 1,2diphenyl-1,2-di(p-tolyl)ethylene by exploiting the substrates tolane, p-iodotoluene, and tri(p-tolyl)boroxin with Pd(PhCN)2Cl2 as the catalyst, leading to a higher ee value of 86% compared with that from the McMurry coupling.26 Rathore and co-workers reported a procedure for the synthesis of Z-TPE derivatives via Grignard reaction using Edibromoalkenes and the relevant Grignard reagent.27 However, without the steric hindrance of the ortho methyl group on the aryl Grignard reagent, the success of the Grignard coupling in producing pure isomers might be hampered. This E and Z isomer formation significantly prohibits the development of TAE-type materials regardless of the emerging requisite for a scalable and imperative methodology due to the fact that TAE motifs are potentially important candidates for storage devices, optomechanical switching, and energy/charge transport.16,28 Utilizing our proposed novel and facile strategy through an initial Corey−Fuchs reaction and a subsequent Suzuki/Stille

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RESULTS AND DISCUSSION Synthesis of the TAE Compounds. Our novel and facile method toward the synthesis of TAEs (Scheme 1) is suitable for many substrates on account of the wide range of functional groups tolerated by the Pd(0)-catalyzed Suzuki or Stille crosscoupling reactions, which furnishes the final materials in decent yields. Nine compounds have been designed and synthesized to illustrate the wide range of synthetic possibilities of this method. Thiophene, carbazole, and naphthalimide motifs are widely utilized in organic optoelectronic materials, owing to their electron-donating or -accepting ability.22 With this in mind, DPDT, DPDCz, and DPDNI were synthesized to investigate electronic effects on the AIE properties. Steric factors and their effects on AIE properties were also evaluated by increasing the rigidity and volume of the aryl rings from αnaphthalene and β-naphthalene to anthracene in DPDN1, DPDN2, and DPDAn, respectively. Incorporating AIE-active TPE and triphenylacrylonitrile (TPAN) units at the geminal positions afforded DPDTPE and DPDTPAN, in which DPDTPE exhibits piezofluorochromic properties and DPTPAN displays the capability of forming well-ordered onedimensional (1D) microstructures. Meanwhile, 9-(1,2,2-triphenylvinyl)-anthracene (TPMAn) was obtained by a one-pot Suzuki coupling reaction of phenyl boronic acid and anthracene-9-boronic acid with 1,1-dibromo-2,2-diphenylethene, which indicates that asymmetrical TAEs can also be achieved by this general approach. Currently, the majority of the materials possessing AIE properties that have been reported in the literature belong to two material families: silole- and TPE-based. Even though the latter species is more accessible via McMurry reaction15 and Rathore’s protocol (Scheme 2),16 they can hardly meet the increasing demand for diverse 4435



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Figure 1. ORTEP molecular structures of (a) DPDN2, (b) DPDN1, and (c) DPDAn shown as 50% thermal ellipsoid probability plots.

Figure 2. SEM images of micron-sized crystals of DPDN2, DPDN1, and DPDAn obtained in chloroform and methanol solvent mixtures by solvent layering procedures.

Figure 3. Part of the molecular packing showing the C−H···π interactions (the yellow dotted lines) in the crystals of (a) DPDN2, (b) DPDN1, and (c) DPDAn. The H···Cg distances are labeled.

X-ray Crystallography: Geminal Structures of DPDN2, DPDN1, and DPDAn. The single crystals of DPDN2, DPDN1, and DPDAn were investigated by X-ray crystallography (Figure 1). Solvent layering methods were employed for crystallization, i.e., polycrystalline powder samples were dissolved in chloroform, and then methanol was added to the solution to produce

coupling reaction, a wide range of TAE compounds with an olefin skeleton and peripheral blades can be obtained, which might be employed for systematic photophysical investigations and probing crystal engineering to construct the relationship between molecular packing and photoluminescence properties. 4436



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Table 1. Geometric Parameters of C−H···π Interactions in the Crystals of DPDN2, DPDN1, DPDAn (298 and 100 K), DPDAn· 0.5CH2Cl2, and DPDAn·CH3OH compd DPDN2 DPDN1 DPDAn (298 K) DPDAn (100 K) DPDAn·0.5CH2Cl2

DPDAn·CH3OH

C−H···Cga

C−H (Å)

H···Cg (Å)

C···Cg (Å)

∠C−H···Cg (deg)

C14−H14···Cg(1)i C32−H32···Cg(2)ii C7−H7···Cg(1)i C31−H31···Cg(2)ii C20−H20···Cg(1)i C33−H33···Cg(2)ii C10−H10···Cg(1)i C41−H41···Cg(2)ii C10−H10···Cg(1)i C12−H12···Cg(2)ii C25−H125···Cg(3)iii C27−H27···Cg(4)iv C43−H43···Cg(5)v O1−H1···Cg(1) C4−H4···Cg(2)i C6−H6···Cg(3)ii C20−H20···Cg(4)iii C22−H22···Cg(5)iv

0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.84 0.93 0.93 0.93 0.93

2.91 2.78 2.90 2.90 2.97 3.00 2.92 2.86 2.75 2.70 2.83 2.68 2.78 3.00 2.69 2.86 2.71 2.76

3.5709(19) 3.685(2) 3.609(4) 3.802(3) 3.762(2) 3.811(2) 3.6971(13) 3.6896(13) 3.622(3) 3.581(3) 3.666(3) 3.576(3) 3.605(19) 3.419(4) 3.594(2) 3.691(2) 3.592(2) 3.630(2)

129 163 135 165 140 151 142 150 154 155 147 158 141 113 158 146 154 153

a Notes: For DPDN2, Cg(1) and Cg(2) are the centroids of C3−C8 and C13−C18, respectively. Symmetry codes: (i) 1/4 + y, 1/4 − x, 1/4 + z; (ii) −1/4 + y, 1/4 − x, 1/4 − z. For DPDN1, Cg(1) and Cg(2) are the centroids of C12−C17 and C23−C28, respectively. Symmetry codes: (i) −x, −1/2 + y, 1/2 − z; (ii) 1 − x, 1 − y, −z. For DPDAn (298 K), Cg(1) and Cg(2) are the centroids of C9−C14 and C16−C21, respectively. Symmetry codes: (i) −1 + x, y, z; (ii) 1 + x, y, z. For DPDAn (100 K), Cg(1) and Cg(2) are the centroids of C16−C21 and C9−C14, respectively. Symmetry codes: (i) −1 + x, y, z; (ii) 1 + x, y, z. For DPDAn·0.5CH2Cl2, Cg(1) is the centroid of C31−C36, Cg(2) is the centroid of C17−C22, Cg(3) is the centroid of C37−C42, Cg(4) is the centroid of C2−C7, and Cg(5) is the centroid of C16−C17−C22−C23−C24−C29. Symmetry codes: (i) −x, 2 − y, 1 − z; (ii) 1 − x, 2 − y, 1 − z; (iii) 1 − x, 2 − y, −z; (iv) −x, 2 − y, −z; (v) 1 − x, 1 − y, −z. For DPDAn·CH3OH, Cg(1) is the centroid of C16−C17−C25−C24−C23-C18, Cg(2) is the centroid of C17−C25−C26−C27−C28−C29, Cg(3) is the centroid of C37−C42, Cg(4) is the centroid of C9−C14, and Cg(5) is the centroid of C31−C42. Symmetry codes: (i) 1 − x, 1 − y, −z; (ii) 2 − x, 1 − y, −z; (iii) 2 − x, 1 − y, 1 − z; (iv) 1 − x, 1 − y, 1 − z.

the benzene C−H and benzene ring. The C···Cg distances are 3.609(4) Å (C7···CgC12−C17) and 3.802(3) Å (C31···CgC23−C28) (Figure 3b and Table 1). Meanwhile, the 2D layer structure resulting from these two C−H···π interactions runs parallel to the (102) plane, which is spatially different from the (100) plane in DPDN2. By replacing the two acene groups from αnaphthalene in DPDN1 to anthracene in DPDAn, these defined interactions vary slightly, as indicated by the C···Cg distances of 3.811(2) Å for C33···CgC16−C21 and 3.762(2) Å for C20··· CgC9−C14 (Figure 3c and Table 1). Interestingly, in the DPDAn crystal, a 1D chain is formed by a combination of two C−H···π interactions parallel to the [100] axis, which is different from the 2D layer structure present in both DPDN1 and DPDN2. By comparison of the C···Cg distances in these three crystals (Table 1), when the substituents attached to the geminal carbon atom are changed from β-naphthalene to α-naphthalene and anthracene, the C···Cg distances are regularly prolonged, with the trend being in the following order: anthracene > αnaphthalene > β-naphthalene. The short contacts between the acene rings slip from the face-to-face positions, resulting in nonorthogonal edge-to-face C−H···π interactions. In other words, their intermolecular interactions are becoming weaker and weaker in the three compounds with the introduction of the varied rigid and bulky aryl rings. Although rich aromatic systems exist in all of them, they all lack cofacial π−π stacking interactions, which may account for the fluorescence quenching in many solid-state systems.2e Therefore, such alignments of aryl rings in the three compounds indicate that these TAE molecules should be beneficial for efficient photoluminescence in the solid state due to the absence of cofacial π−π stacking.

a biphasic system. Figure 2 shows the SEM images of the obtained crystal microstructures with uniform microrod or microbelt shapes. DPDN2, DPDN1, and DPDAn crystallize in the tetragonal I41/a, monoclinic P21/c, and triclinic P−1 space groups, respectively, in which the crystal structures were all determined under ambient conditions. The details of the crystallographic data for these crystals are summarized in Table S1 (see the Supporting Information for structures in CIF format). The coplanarity of the olefin skeleton and the peripheral aryl rings is prevented because of steric congestion between the associated aryl rings. As demonstrated by crystallography, the validation of our synthetic methodology for g-TAEs is unambiguously confirmed. From the crystal packing of DPDN2, DPDN1, and DPDAn measured at room temperature, the observed C−H···π interactions are mainly responsible for the intermolecular interactions. With regards to DPDN2, two kinds of C−H···π interactions are present (Figure 3a and Table 1): (1) those occurring between the β-naphthalene C−H and the adjacent βnaphthalene rings and (2) those between the benzene C−H and β-naphthalene ring, in which the measured C···Cg (Cg is a centroid defined by some specified atoms) distances are 3.5709(19) Å (C14···CgC3−C8) and 3.685(2) Å (C32··· CgC13−C18) (Table 1), respectively. A two-dimensional (2D) layer parallel to the (100) plane is thus formed by the intermolecular interactions of the adjacent molecules (Figure 3a). Whereas for DPDN1, in which the linkage site is changed from β-naphthalene in DPDN2 to α-naphthalene moieties, there are also two kinds of C−H···π interactions present in DPDN1: (1) those occurring between the α-naphthalene C−H and the adjacent α-naphthalene rings and (2) those between 4437



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Solvent Dependence in DPDAn Crystals. DPDAn crystals with different fluorescence properties were obtained by solvent layering or by solvent/mixed-solvent evaporation methods. Needle-like crystals containing solvent molecules, i.e., DPDAn·0.5CH2Cl2 and DPDAn·CH3OH, were acquired by solvent evaporation from a DCM solution (DPDAn· 0.5CH2Cl2) or by mixed-solvent evaporation from a methanol/chloroform binary solution (DPDAn·CH3OH). Chunkform crystals of DPDAn without solvent inclusion were obtained by solvent layering using methanol and chloroform (vide supra). For DPDAn·0.5CH2Cl2 and DPDAn·CH3OH, single-crystal diffraction experiments were performed at 100 K in order to avoid the potential disorder of the embedded solvent molecules. As solvent molecules can be easily introduced into DPDAn crystals, solvent-free DPDAn was also probed at 100 K to ascertain whether any structural phase transformations can be induced with temperature regulation. However, no phase transition was observed in it except for a normal thermal constriction change of the unit cell that occurred when the detection temperature was changed from 298 to 100 K. All the three types of DPDAn crystals belong to the triclinic P−1 space group, with different unit cell parameters. The DPDAn·CH3OH and DPDAn·0.5CH2Cl2 crystals (Figure 4a) exhibit cyan fluorescence with maximum

Figure 5. (a) Part of the molecular packing in the nonsolvated DPDAn crystals (100 K), with C−H···π interactions shown as yellow dotted lines. (b) Part of the molecular packing in the methanolsolvated DPDAn crystals, with π···π interactions shown as purple dotted lines, O−H···π interactions as blue dotted lines, and C−H···O interactions as cyan dotted lines. (c) Methanol molecule inserted into the cage composed of three DPDAn molecules. (d) DCM molecule inserted into the cage composed of two DPDAn molecules, with π···π interactions shown as purple dotted lines and C−Cl···π interactions as blue dotted lines. Only half of the disordered DCM molecule is shown for clarity.

Figure 4. (a) Three different kinds of DPDAn crystals with different λem. (b) Dihedral angles of the aryl rings.

emission wavelengths (λem) at 479 and 486 nm, respectively. With regard to the nonsolvated DPDAn crystals, bright chartreuse fluorescence was observed with λem bathochromically shifted to 522 nm. Additionally, full width at halfmaximum (fwhm) of the PL spectra for the DPDAn crystals with CH2Cl2, CH3OH, or no solvent is 48, 50, and 74 nm, respectively. These differences in the fluorescence properties are reflected in the molecular conformations and packing. For DPDAn·0.5CH2Cl2 and DPDAn·CH3OH, the dihedral angles are 92.53° and 92.76° for the anthracene rings and 71.18° and 71.56° for the phenyl rings, respectively. With respect to DPDAn, the dihedral angles of the anthracene rings and phenyl rings are 85.81° and 75.29°, respectively (Figure 4b). The alterations of the photoluminescence for DPDAn, DPDAn· CH3OH, and DPDAn·0.5CH2Cl2 may be partially derived from the disparities of the dihedral angles in these crystals as well as other factors (vide supra). In the crystal packing, significant differences are observed in the short contacts between DPDAn molecules and the surrounding solvent molecules. For DPDAn measured at low temperature, the C−H···π interactions are dominant, and the C41···CgC9−C14 and C10···CgC16−C21 distances (Figure 5a) are 3.6896(13) Å and 3.6971(13) Å (Table 1), respectively. In addition to the multiple C−H···π interactions that can be observed for DPDAn·CH3OH (Figure 5b and Table 1), with the C···Cg distances varied from 3.592(2) Å to 3.691(2) Å

between the adjacent DPDAn molecules, C−H···O, O−H···π, and π···π interactions also occur. The centroid-to-centroid (Cg···Cg) distances of the π···π interactions are 3.8078(16) Å and 3.8343(16) Å, with the anthracene rings parallel to each other. It is of significant interest that the methanol molecules are stabilized by three DPDAn molecules with C−H···O and O−H···π interactions (Figure 5c). Two of the DPDAn molecules are composed of a cage and enclose a methanol molecule by O−H···π interactions, with an O··· CgC16−C17−C25−C24−C23−C18 distance of 3.00 Å. Another DPDAn molecule is connected to the cage-like railing by π···π interactions, with a Cg···Cg distance of 3.808 Å, which further fastened the inside methanol (Figure 5c). For DPDAn· 0.5CH2Cl2, although DCM is inversion-symmetry-disordered over two positions, it can be easily distinguished that the solvent molecule is noncovalently bounded by four anthracene blades in the two adjacent DPDAn molecules (Figure 5d). Similar to that of DPDAn·CH3OH, the DCM molecule embedded in the DPDAn crystal structure is stabilized and confined by a cage composed of anthracene rings with C−H···π interactions (C43···CgC16−C17−C22−C23−C24−C29, 3.609(4) Å) and multiple C−Cl···π interactions.29 The distances between the chlorine atoms and the centroids are 3.426(3) Å (Cl1··· CgC2−C7) and 3.624(9) Å (Cl2···CgC17−C22), respectively. The corresponding C−Cl···Cg angles are 112.8(7)o and 102.1(8)°, 4438



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Figure 6. 1H NMR spectra of (a) DPDN2, (b) DPDN1, and (c) DPDAn in CD2Cl2 and (d) DPDAn in CD2Cl2 and D2O.

and the Cg···Cg distances of the π···π interactions between two inversion-related anthracene rings are 3.838(2) Å and 3.7584(19) Å. Recently, it was reported that solvent molecules involving DCM or chloroform as the guest are weakly noncovalently bonded with the fluorescent host molecules of biindenylboradiazaindacene (BDY-IN), thus resulting in different emission properties.30 Whereas in our case, the DCM solvent molecules are present in the channels of DPDAn stacks in the crystal phase with multiple short contacts (Figure 5d and Figure S20). It has been demonstrated that the σ-hole, an area of positive molecular electrostatic potential on the outermost van der Waals surface of halogen atoms in the continuation of the carbon−halogen bond, is responsible for attractive interactions between the regions of high electron density and halogen atoms.29b In the case of DPDAn·0.5CH2Cl2, the anthracene rings act as electron-rich fragments of σ-hole bonds. The multiple intermolecular interactions between the DCM and DPDAn molecules are dependent on the attractive and repulsive components between chlorine atoms and the πsystem of the anthracene rings, i.e., via directional σ-hole bonding and the diffuse electrostatic repulsion between the negative molecular electrostatic potential regions around the chlorine σ-holes and the anthracene rings, respectively. The major distances of Cl···centroid in DPDAn·0.5CH2Cl2 are 3.426 and 3.624 Å, indicating that the intermolecular interactions between DCM and DPDAn molecules are quite strong in comparison to the majority of Cl···centroid distances that are varied from 3.4 to 3.8 Å.29a The disordered DCM

molecules grab the neighboring DPDAn with C−Cl···π and C− H···π interactions, by which the whole complex of DPDAn· 0.5CH2Cl2 constructs a 2D network (Figure S20). With regard to DPDAn·CH3OH, classical intermolecular hydrogen bonding is absent. However, O−H···π and C−H···O interactions are observed, which stabilizes the methanol molecules in the channels of DPDAn molecules. Furthermore, the introduced solvent molecules probably compact the adjacent anthracene rings in the DPDAn crystal. As a consequence, slipped π···π interactions occur between the contiguous DPDAn molecules. On the basis of these observations, the presence of solvent molecules embedded into the channels of DPDAn in the crystal packing is presumably influential on the following factors: (i) molecular cohesion, which may have contributions from the formation of the π−π stacks between the peripheral anthracene rings in the neighboring host DPDAn molecules, and (ii) noncovalent interactions, suggesting multiple interactions will be emerged in the solvated crystals. These factors, induced by the embedded solvent molecules, will probably trigger the alteration of the molecular conformation and further prompt a change in photoluminescence properties. Through systematic comparisons of the molecular conformations in these three X-ray determined DPDAn crystals, the torsion angles induced by the anthracene rings are slightly increased ca. 2° from DPDAn to DPDAn·0.5CH2Cl2 or DPDAn·CH3OH (Table S2). The torsion angles induced by the benzene rings show a 5-fold increase compared with that induced by the anthracene rings. The solvomorphic structures are more easily twisted than the solvent-free DPDAn structure. 4439



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Figure 7. Molecular structures of (a) DPDN2, (b) DPDN1, and (c) DPDAn molecules with the labeled C−C bond lengths and the associated carbon atoms shown as 50% thermal ellipsoid probability.

hydrogens on the naphthalene moieties at the 4- and 5positions, respectively, and these hydrogens are marked with the same color both in the molecular structure and in the 1H NMR spectra (Figure 6b). Other hydrogens overlapping with the diphenyl ring moieties are difficult to resolve. However, for compound DPDN2, the hydrogens are much easier to classify, and the distributions of hydrogens are depicted in detail (Figure 6a) (highlighted by colored circles that are the same color as the spheres labeled in the molecular structure). The different chemical shift distributions of hydrogens between DPDN1 and DPDN2 might be ascribed to the adjacent distortion of the dinaphthalene rings. Additionally, the different chemical shifts of the two hydrogens located at the same 8position of the naphthalene rings in DPDN2 and DPDN1 reflect the unsymmetrical nature of the molecule and are probably due to steric effects. With regard to DPDAn (Figure 6c), in the range of 8.36−9.14 ppm, there is a very broad peak (see red circle, Figure 6c) without defined splitting that is overlapped with a defined single peak. The integration of this broad peak and the single defined peak are 4 and 2, respectively, and the single defined peak is assigned to the hydrogens at the 10positions on the anthracene rings, specified by the green color in Figure 6c. Generally, broad peaks in 1H NMR spectra indicate that there are some labile hydrogens in the compound capable of hydrogen bonding, such as amine, carboxyl, or hydroxyl groups. With this in mind, several drops of D2O were added to the same NMR tube to exchange the active hydrogens in the system. As shown in Figure 6d, the exchange of active hydrogens assigned to water indeed occurs. The intensity of the water peak in Figure 6c is dramatically suppressed and almost disappears in Figure 6d, and a new peak emerges at the position of 4.83 ppm, highlighted by a yellow arrow, which is assigned to the solvent peak of D2O (DHO) in CD2Cl2. However, the broad peak highlighted by red circles in the range of 8.36−9.14 ppm still remains, which demonstrates that these four hydrogens should be assigned to hydrogens on DPDAn (Figure 6c,d). Further demonstrated by 2D-NMR of DPDAn (Figures S15 and S16, Supporting Information) is that the hydrogens located at 8.36 and 7.91 ppm are related and observable in the NOESY and COSY spectra; these two types of hydrogen are labeled with the same color depicted in Figure 6c,d. With regard to TPMAn, the singlet and doublet hydrogen peaks at 8.32 and 7.92 ppm are ascribed to hydrogens at the 10position of the anthracene ring and hydrogens at the 4- and 5-

Meanwhile, for compounds DPDAn·0.5CH2Cl2 and DPDAn· CH3OH, π···π interactions exist between adjacent DPDAn molecules. Interestingly, fluorescence experiments indicate that the solid-state maximum emission wavelengths (λem) of DPDAn·0.5CH2Cl2 and DPDAn·CH3OH are hypsochromically shifted by 36 and 43 nm, respectively, compared to the λem of DPDAn, which should be largely attributed to the effect of solvent molecules, leading to the presence of host−guest interactions. Similar phenomena was recently observed by Zhang et al.,30 in which BDY-IN crystals show an intense orange (BDY-O) or red (BDY-R) emission when different solvent molecules were introduced (chloroform for the former and DCM for the latter). They observed that the existence of π···π interactions in the BDY-R crystal leads to a bathochromic shift of about 79 nm in comparison with BDY-O. However, the changes in the emission behaviors in our studies are probably related to the gradual increase in the dihedral angles between the anthracene rings and the progressive decrease in the dihedral angles between the phenyl rings (Figure 4b). The involvement of the solvent molecules probably accounts for the gradual variations in the dihedral angles of the anthracene and benzene rings. Consequently, the significant alterations of the dihedral angles also lead to the differences in the molecular conformations and packing, which results in different degrees of π conjugation. This should be a synergistic effect resulting from the following factors: (i) torsion angles, (ii) dihedral angles, (iii) π−π interactions, and (iv) solvent molecules. Additionally, such structure-dependent emission properties may occur from changes in the molecular bonding and antibonding orbitals and subsequent alterations of the spatial electron distributions in the contiguous DPDAn molecules, owing to the intermolecular interactions and the embedded solvent molecules. Probing the Steric Effect. The constitutional isomers DPDN1 and DPDN2 are easily distinguishable from the analysis of their 1H NMR spectra. The chemical shift (δ) of the protons in DPDN2 (Figure 6a) and DPDN1 (Figure 6b) is in the range of 7.12 to 7.78 ppm and 7.00 to 8.52 ppm, respectively. Compared with those of DPDN2, the chemical shifts of the proton signals for DPDN1 are located over a broader range. For the 1H NMR spectrum of DPDN1, the signals located at 8.25 and 8.50 ppm are attributed to the hydrogens on two different naphthalene moieties at the 8-position and are shifted more downfield compared to that of DPDN2 due to the peri-effect.31 The signals located at 7.69 and 7.78 ppm are assigned to the 4440



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positions of the anthracene ring, respectively (Figure S19). The doublet peak at 8.40 ppm, labeled with red color (Figure S19), is attributed to hydrogens at the 1- and 8-positions of the anthracene ring. In comparison to that of TPMAn and 2DNMR spectra (COSY, NOESY) of DPDAn (Figures S17− S19), the peak integrated with four hydrogens is ascribed to the hydrogens at the 1- and 8-positions on the anthracene rings that are labeled with red spheres in Figure 6c,d. From the above NMR results, the steric congestion between the geminal aryl rings in these g-TAE compounds may result in the different chemical shifts of the hydrogens, and these steric effects can lead to interesting structural and optical properties in the DPDN1, DPDN2, and DPDAn crystals. The lengths of the C−C bonds between the vinyl group and phenyl rings in all the three compounds are almost identical, ranging from 1.489(2) to 1.494(3) Å (Figure 7). However, the lengths of the single carbon−carbon bonds between the vinyl group and naphthalene rings or anthracene rings show more obvious variations, namely, 1.492(2)Å (C1−C21)/1.493(2) Å (C11−C21) for DPDN2, 1.501(3) Å (C11−C21)/1.502(2) Å (C1−C21) for DPDN1 and 1.510(2) Å (C15−C29)/1.511(2) Å (C1−C29) for DPDAn. Additionally, these trends in the C− C bond lengths (DPDN2 < DPDN1 < DPDAn) are perfectly consistent with data obtained from density functional theory (DFT) calculations using the B3LYP/6-31G(d) basis set (Figure S21). The gradual increase in these C−C bond lengths located at the geminal carbon atom is most likely induced by the increasing steric repulsion that follows the order βnaphthalene < α-naphthalene < anthracene. It is well-known that the phenyl rings in a biphenyl unit are located in the same plane and that the conjugation degree is greater in comparison with benzene; however, the plane and conjugation will dramatically decrease by the introduction of bulky groups at the ortho positions of the interannular ring junctions. For example, with respect to 2,2′,6,6′-tetramethylbiphenyl, in which bulky methyl groups are substituted at the ortho positions of the biphenyl unit, the plane of the two phenyl rings become close to perpendicular. As a result, the π−π conjugation between two phenyl rings will decrease, and the length of the relative carbon−carbon bond connecting the two phenyl rings will change to a suitable distance due to steric repulsion from the adjacent substituents.32 In a similar manner, the molecular cohesion of the hexaphenylbenzenes (HPBs) can be adjusted by the introduction of ortho substituents to the peripheral phenyl rings, and the C−C bond lengths between the central benzene and the ortho-substituted peripheral phenyl rings alter progressively.33 With regard to DPDN1 and DPDN2, these naphthalene moieties will rotate in order to arrive at the lowest-energy state to form a certain conformation that is able to reduce the repulsion. Because of the peri-effect (the steric repulsion between H8 and the α-substituted group on the αnaphthalene ring31), the steric congestion on the two αnaphthalene units in DPDN1 is stronger than that of the βnaphthalene units in DPDN2. Therefore, the C11−C21 and C1−C21 bond lengths in DPDN1 are larger than the C11−C21 and C1−C21 bond lengths in DPDN2. With regard to DPDAn, the C15−C29 and C1−C29 bond lengths on the geminal carbon atoms are longer compared with those in DPDN1 and DPDN2, presumably owing to the enlarged volume and enhanced rigidity of the anthracene ring. The dihedral angles of the two phenyl rings, which are placed perpendicular to the paper in Figure 8a−c, undergo a gradual shrinkage from 86.91° for DPDN2 and 80.94° for DPDN1 to

Figure 8. Dihedral angles between the phenyl rings in crystals of (a) DPDN2, (b) DPDN1, and (c) DPDAn in wireframe drawing; dihedral angles between the acene rings in crystals of (d) DPDN2, (e) DPDN1, and (f) DPDAn in wireframe drawing.

75.39° for DPDAn. This illustrates that dihedral angles decrease with the enlarged volume and enhanced rigidity of the geminal aryl substituents. Unusually, the dihedral angles of the acene rings in the crystals of DPDN2, DPDN1, and DPDAn are 76.79°, 99.48°, and 86.63° (Figure 8d−f), respectively. The volumes or bulkiness of the α-naphthalene and anthracene rings manifest some disparities in the dihedral angles of the acene rings in DPDN1 and DPDAn, which is probably associated with their dissimilar molecular packing. However, the inconsistencies of the data for the dihedral angles obtained from DFT calculations for these three compounds (Table S3) are possibly ascribed to the lack of intermolecular interactions in the DFT calculations with only a vacuum state considered. Therefore, it is not only the lengths of the δ bonds connecting the vinyl group and the geminal aryl substituents but also the dihedral angles between the phenyl rings that change with certain variations in these contorted structures of DPDN2, DPDN1, and DPDAn. These dihedral angle variations among the three compounds demonstrate that their configuration can be adjusted by introducing bulky polycyclic aromatic hydrocarbons and/or by the usage of the same substituents attached to different linkage positions (i.e., α- and β-naphthalene). Optical Properties. The analogous materials DPDN1, DPDN2, and DPDAn were initially designed and synthesized to investigate the alteration of optical properties by replacing the diphenyl rings in TPE with dinaphthalene or dianthracene units. The maximum UV−vis absorption peak (λabs) is dependent on the substituted groups connected with the vinyl skeleton. λabs of DPDN2 (Figure 9a and Table 2) in THF solution is located at 330 nm, which is bathochromically shifted by 22 nm with respect to λabs of DPDN1, which is located at 308 nm. Meanwhile, the λem of DPDN1 and DPDN2 in the solid state are located at 429 and 435 nm (Figure 9b and Table 2), respectively. With regard to the anthracene derivatives, the λabs of TPMAn in THF solution is present at 395 nm, and the λabs of DPDAn is located at 409 nm (Figure 9a). These defined vibronic peaks within the region from 330 to 440 nm are ascribed to the well-known π−π* transitions of anthracene.34 DPDAn in THF solution (10−5 M) emits yellow fluorescence, with λem at 559 nm under UV irradiation (Figure S22). The λem of TPMAn and DPDAn in the solid state is 437 and 512 nm, respectively (Figure 9b and Table 2). To further investigate the electronic characteristics on the optical properties of the g-TAE materials, electron-donating units, such as thiophene, carbazole, 4441



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Figure 9. (a) UV−vis absorption spectra of the g-TAE compounds in 10−5 M THF solution. (b) Normalized solid-state PL emission spectra of the g-TAE compounds (λabs of each g-TAE compounds was chosen as the excitation wavelength). (c) Fluorescence lifetimes of the g-TAE compounds in the solid state.

Table 2. Optical and Electrochemical Data of the TAE Materials λem (PL) (nm) λabsa (nm) DPDT DPDN1 DPDN2 DPDAn TPMAn DPDCz DPDTPAN DPDTPE DPDNI

338 308 330 409 395 343 348 332 378

solutiona

559

DFTb (eV)

ΦF (%)f

CV (eV)

solid

HOMO

LUMO

HOMOc

LUMOd

Ege

solution

aggregateg

αAIE

τ (ns)

478 429 435 512 437 479 524 450 523

−5.14 −5.26 −5.22 −4.94 −5.05 −4.77 −6.00 −5.00 −5.35

−1.48 −1.36 −1.42 −1.78 −1.66 −0.95 −2.57 −1.48 −2.01

−5.52 −5.69 −5.62 −5.48 −5.38 −5.22 −5.74 −5.51 −6.09

−2.40 −2.30 −2.35 −2.61 −2.34 −2.01 −2.69 −2.39 −3.15

3.12 3.39 3.27 2.87 2.98 3.21 3.03 3.12 2.94

0.2 1.7 0. 4 9.7 1.6 0.4 0.3 0.4 0.3

0.67 12.5 4.0 17.8 28.8 13.4 12.8 45.7 29.3

3.3 7.3 10.0 1.8 18 33.5 42.6 114 97.6

0.75 0.90 1.53 1.17/4.03h 2.84 3.45 2.69 2.54 1.97

a Measured for 10−5 M THF solution. bSimulated in vacuo based on Gaussian program. cHOMO (highest occupied molecular orbital) = −(Eonset‑ox + 4.8 − Eferrocene), where Eferrocene = 0.062 V, estimated using cyclic voltammetry using Bu4NPF6 in dry DCM. dLUMO (lowest unoccupied molecular orbital) = Eg + HOMO. eEg = optical band gap estimated from the onset wavelength (λonset) of the absorption spectra. f9,10-Diphenylanthracene (ΦF = 0.9 in cyclohexane) was used as a standard. gAggregates formed in 99% water in THF mixture solvents. hThe observed fluorescence decay was fitted for two-exponential decay with lifetimes of 1.17 ns in 30% and 4.03 ns in 70%, respectively.

materials (Figure 9c and Table 2) were investigated using timecorrelated single photon counting (TCSPC). The changes in fluorescence lifetimes from 0.7 to 3.5 ns reveal that the fluorescence decay of the TAEs is associated with electronic characteristics of the substituent groups at the geminal positions. Previous investigations on AIE-active materials, such as TPE-based materials, have demonstrated the importance of tunable electronic states for future applications.2e,35 Through our newly designed synthetic procedure, a wide variety of novel g-TAE materials with fluorescence tunability can be obtained, due to the ease of synthesis, with a wide range of commercially available substrates, which should promote the development of geminal-structured AIE-active fluorophores. Aggregation-Induced Emission Properties. The newly synthesized g-TAE materials exhibit AIE or aggregationinduced enhanced emission (AIEE) properties (Figure 10), which is similar to the archetypal AIE-active molecule TPE. When these g-TAE materials are well-dissolved in THF solution at low concentration, the rotation of their blades exhausts the energy in a relaxation channel and leads to fluorescence quenching (DPDAn is an exception). However, this pathway is blocked when the aryl rotors are fixed and restricted, either by aggregate formation, freezing, or somehow physically hindering their internal motions or by chemically “locking” the phenyl rings in place.11d,36 For example, when nanoaggregates are prepared by the reprecipitation method from THF solution into water, the AIE-active g-TAE materials

and TPE, were incorporated at the geminal positions. Also, electron-accepting units, such as naphthalimide and triphenylacrylonitrile (TPAN), were also incorporated at the geminal positions. Compared to the optical properties of TPE with λabs at 308 nm, the λabs in THF solution of DPDTPE, DPDT, and DPDCz (Figure 9a and Table 2) is bathochromically shifted to 332, 338, and 343 nm, respectively. The λem of the as-prepared sample of DPDT is located at 478 nm, which is bathochromically shifted compared with the as-prepared DPDTPE that displays λem at 450 nm (Figure 9b and Table 2). The λem of DPDCz, containing two carbazole units, is present at 479 nm. With regard to g-TAEs with electron-accepting units, the λabs in THF solution of DPDTPAN and DPDNI is at 332 and 378 nm (Figure 9a and Table 2), respectively. The λem of DPDTPAN and DPDNI in the solid state is 524 and 523 nm, respectively (Figure 9b and Table 2). Compared with the absorption properties of the TAE compounds in THF solution, their associated UV−vis absorption properties in a 99% water−THF solvent mixture and thin films were also recorded. The absorption “tails” in each absorption spectra in 99% water− THF are due to the Mie effect of the nanoparticles (Figure S23). In comparison to the λabs in THF solution, the λabs of each TAE in the associated thin-film state (Figure S24 and Table S4) is slightly red-shifted. A comparison of these analogues demonstrates that the emission properties are gradually tuned by incorporating electron-donating or -accepting groups into the geminal TAE structure. Also, the fluorescence decay lifetimes of these g-TAE 4442



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Figure 10. PL spectra of the g-TAEs (10−5 M) in THF solution and in the nanoaggregate state using a 99% water−THF solvent mixture. The λabs of each g-TAE compound was chosen as the excitation wavelength.

show enhanced fluorescence compared with that in their solution state, i.e., THF solution.37 The AIE-effect, which is defined as αAIE = ΦF,a/ΦF,s, can be used to evaluate the fluorescence contrast ratio between the condensed and solution states. The detailed αAIE data of these nine compounds were calculated and are shown in Table 2. As envisaged, DPDAn composed of bisanthracene rings exhibits the lowest AIE effect, with an αAIE value of 1.8, and DPDTPE displays the highest AIE effect, with an αAIE value up to 114. By replacing one anthracene ring in DPDAn with a phenyl ring, the steric effects in the TPMAn analogue should be significantly reduced. In comparison to DPDAn, the αAIE value of TPMAn is indeed up to 18, which is 10-fold higher than that of DPDAn and indicates that the steric repulsion of the aryl rings has a significant influence on the AIE effect. The other fluorophores show AIE phenomena with αAIE values varying from 3.3 to 97.6. The αAIE of DPDN2, DPDN1, and DPDAn gradually decreases from 10 to 7.3 to 1.8, respectively, which is consistent with the increase in steric repulsion. As envisaged, the molecular architecture of these three fluorophores can be regarded as being composed of two components: a central olefin skeleton and peripheral aryl rings serving as the rotors. The naphthalene and anthracene rings are associated with the central vinyl core via the linkage of δ bonds. If we picture the central vinyl core as the body of a butterfly and the aryl rings as the wings, with regard to DPDAn, the vinyl group is probably too small to handle the sheer size and rigidity of the anthracene wings. Consequently, the intramolecular

rotations are restricted even in solution, which gives rise to a relatively high ΦF,s and, therefore, a poor AIE effect value. DPDN1 and DPDN2 consist of naphthalene rings with a smaller volume that allows more freedom to rotate, and, as a result, higher αAIE values are obtained in comparison to that of DPDAn. Likely because of the peri-effect, the steric congestion between the two α-naphthalene rings attached to the geminal carbon atoms in DPDN1 is stronger than that of the βnaphthalene rings in DPDN2, which leads to the difference in the αAIE values between DPDN1 and DPDN2. Furthermore, the ΦF in the aggregate state of DPDT is remarkably lower than TPE,12b which is most likely due to the heavy atom effect caused by the sulfur atom in the thiophene unit. In this regard, the rigidity and volume of the peripheral aryl rings as well as the heavy atom effect greatly influence the AIE effect. Electrochemical and Thermal Properties. The electrochemical properties were evaluated by cyclic voltammetry in DCM at room temperature. The highest occupied molecular orbital (HOMO) level of g-TAE materials was obtained indirectly based on the onset potential of the first oxidation wave (vs ferrocene), +4.8 eV. The energy band gaps (Eg) were estimated from the edge of the low-energy UV−vis absorption.38 DPDCz displays excellent redox reversibility with its HOMO level at −5.2 eV (Figure S25), and it is perfectly matched with ITO, which is used as the substrate for OLEDs. On the basis of this unique property, DPDCz may be a promising hole-transporting and/or emitting material in OLEDs. The decomposition temperatures of these materials 4443



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vary from 196.5 to 388 °C (Figure S26 and Table S4). With regard to DPDAn·0.5CH2Cl2 and DPDAn·CH3OH, a stepwise decomposition procedure is observed. Likely because of the lower boiling point of DCM molecules, the TGA scan of DPDAn·0.5CH2Cl2 shows an initial weight loss at lower temperature compared to that of DPDAn·CH3OH (Figure S26b). Additionally, DFT calculations were carried out using in vacuo and PCM models, and the results demonstrate that the Eg of these g-TAE materials (Tables S5 and S6 and Figure S27) can be tuned by varying the geminal substituents and that the trends of the relative compounds are consistent with the data estimated from the onset wavelength (λonset) of the UV−vis absorption spectra. Piezofluorochromism of DPDTPE. In recent years, the development of novel materials exhibiting piezofluorochromic properties has been given increasing attention within academia. Generally, there are three main types of materials that exhibit piezofluorochromism: (1) small organic molecules, (2) polymers, and (3) organometallics. In the small organic molecules category, different material types, i.e., stilbenebased, anthracene-based, pyrene-based, and perylene-based, have been shown to exhibit piezofluorochromism.39 These systems are of particular interest owing to their ability to tune the optical properties of the fluorophores under external stimuli, i.e., stress, pressure, and tensile deformation, with demonstrable reversibility by solvent and/or thermal annealing. Research investigations of piezofluorochromism via pressure or stress stimuli are adequately emerging thanks to a better understanding of the mechanism of piezofluorochromism in organic molecules and, likewise, better molecular design. There are three main features that account for the piezofluorochromic behavior of various luminogens: (i) conformational changes of the crystal structures, (ii) morphology alterations between crystal and amorphous states, and (iii) liquid crystalline transitions. Pristine DPDTPE exhibits blue fluorescence, and ground DPDTPE displays cyan fluorescence (Figure 11a). Subsequently, after treating the ground DPDTPE sample with several droplets of DCM, the fluorescence immediately reverts from cyan to blue. Further investigation shows that this reversible fluorescence change can be cycled many times via alternating

the grinding and solvent treatment processes, with associated changes in PL emission between the blue fluorescence (455 ± 5 nm) and cyan fluorescence (480 ± 5 nm) (Figure 11b,c). The XRD patterns of ground DPDTPE are less defined than the patterns of pristine DPDTPE, which suggests that the piezofluorochromic mechanism of DPDTPE is probably due to the structural conformation alterations between its crystalline and amorphous states (Figure 11d). From the XRD patterns of the as-prepared samples of the gTAE materials (Figure S28), we can see that DPDCz and DPDNI display several broad peaks that indicate an amorphous morphology. In contrast, DPDT, DPDTPE, and DPDTPAN exhibit excellent crystalline features in their XRD patterns. DPDTPE did not show any signs of self-assembly on a silicon wafer substrate. However, by replacing a phenyl ring in TPE with a cyano group, the triphenylacrylonitrile (TPAN)containing molecule, DPDPAN, shows a stronger propensity to self-assemble.40 The defined microbelt structures of DPDTPAN can be constructed as shown by the SEM images (Figure S29). This unique self-organization ability possibly arises from the cyano groups, which probably not only reduce the repulsions but also enforce the intermolecular interactions among the adjacent molecules. Among the luminogens exhibiting AIE properties, TPE, or adducts involving TPE units, is the most promising material for applications in bioimaging and OLEDs. However, because of the intrinsic optical properties of TPE, it is difficult to synthesize deep blue fluorescent materials based on the TPE motif. Well-known TPE-based materials, such as triphenylamine-tetraphenylethenes (TPA-TPEs),12a oligofluorene-tetraphenylethenes (Fn-TPEs, n = 1−5),37 and pyrene-tetraphenylethenes (Py-TPEs)41 exhibit λem in the range of 450−520 nm, which is slightly red-shifted in comparison with that of TPE owing to the extended π−π conjugation.36b In DPDN1 and DPDN2, λem is 429 and 436 nm, indicating that they may be promising candidates for application in deep blue OLEDs. The enhanced steric repulsion on the geminal carbon atom will presumably have significant influence on the optical properties, such as photoluminescence properties and the AIE effect. Furthermore, by varying the substrates in the Corey−Fuchs and Suzuki coupling reactions, this novel and facile procedure provides a new opportunity for the synthesis of symmetrical and unsymmetrical TAEs (Figure 12) via a one-pot reaction

Figure 11. (a) Different fluorescence properties of DPDTPE after grinding and solvent fuming processes. (b) PL emission properties of pristine and ground DPDTPE. (c) Cycling behavior of the emission showing the reversibility of the process. (d) XRD patterns of pristine and ground DPDTPE.

Figure 12. Synthesis methodology of TAEs via Corey−Fuchs reaction and subsequent Suzuki or Stille coupling. 4444



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from 1,1-dibromo-2,2-diphenylethene, which will facilitate research focusing on the mechanism of AIE properties. Moreover, as was partly classified by X-ray crystallography, all of these geminal structures are quite twisted, and the relationship between the crystal structures and photoluminescence can be systematically investigated. From this viewpoint, we believe that this work will encourage the design and synthesis of more AIE-active TAE-based materials via this general approach.

CONCLUSIONS The dibromo olefination of diaryl ketones via Corey−Fuchs reaction and subsequent Suzuki or Stille coupling has been carried out to synthesize novel TAE fluorophores with tunable fluorescence. Because of the wide commercial availability of functionalized aryl ketones or boronic acids/boronic esters and tin reagents, this general approach will bring about tremendous variability and flexibility for the synthesis of TAE compounds and their analogues with AIE or AIEE properties. It is noteworthy that steric repulsion of the aryl rings on the geminal carbon atoms in DPDN2, DPDN1, and DPDAn compounds can affect not only the structural features (i.e., bond lengths and dihedral angles) but also their optical properties, especially the AIE effect. Furthermore, solvent molecules embedded in the crystal of DPDAn are associated with adjacent DPDAn molecules by multiple noncovalent interactions, which leads to the rearrangement of the molecular packing and the modification of the emission properties. The interesting crystal structures and optical, AIE, electrochemical, and piezofluorochromic properties of these g-TAE compounds indicate that they have potential applications in optoelectronic devices, sensors, and cellular tracking. Meanwhile, they might also be utilized as the necessary precursors for the synthesis of PAH materials. ASSOCIATED CONTENT

S Supporting Information *

Experiments details, crystallographic data, NMR figures, molecular simulations, SEM figure of DPDTPAN, electrochemical cyclic voltammetry. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*(M.-Q.Z.) E-mail: [email protected]. *(M.P.A.) E-mail: [email protected]. *(X.M.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (nos. 20874025 and 21174045) and the National Basic Research Program of China (grant no. 2013CB922104). M.P.A. acknowledges an NSFC Research Fellowship for International Young Scientists (21150110141) and the Key Fellowship of China Postdoctoral Science Foundation. M.-Q.Z. acknowledges the Open Program for Beijing National Laboratory for Molecular Sciences (BNLMS). We also thank the Analytical and Testing Center of Huazhong University of Science and Technology and the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for use of their facilities. 4445



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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on July 15, 2014, with an error in Figure 1. The corrected version was published ASAP on July 16, 2014.

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General Synthetic Approach toward Geminal-Substituted

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