Intermolecular Slipped

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Multicolor Emissions by the Synergism of Intra/Intermolecular Slipped #-# Stackings of Tetraphenylethylene-DiBODIPY conjugate Hai-Tao Feng, Jia-Bin Xiong, Yan-Song Zheng, Biao Pan, Chun Zhang, Lei Wang, and Yongshu Xie Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03765 • Publication Date (Web): 23 Oct 2015 Downloaded from http://pubs.acs.org on October 30, 2015

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Chemistry of Materials

Multicolor Emissions by the Synergism of Intra/Intermolecular Slipped π-π π Stackings of Tetraphenylethylene-DiBODIPY conjugate Hai-Tao Feng,† Jia-Bin Xiong,† Yan-Song Zheng,*,† Biao Pan,§ Chun Zhang,*,‡ Lei Wang,§ and Yongshu Xie⊥. †

Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. ‡

College of Life Science and Technology, Huazhong University of Science and Technology, Wuhan, China. §

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, ⊥ China. School of Chemistry, Institute of Fine Chemicals, East China University of Science & Technology, Shanghai, China. ABSTRACT: Luminescent materials with tunable optical properties are of great importance for their potential application in the fields of optical-recording systems, sensors, security materials, informational displays, and molecular machines. To date, tuning double-color luminescence is easy to be realized in many stimuli-responsive organic materials, while multicolor luminescence is very scarce. Herein, three new TPE-BODIPY conjugates, composed of a tetraphenylethylene (TPE) core bearing BODIPY units, were synthesized. The TPE-BODIPY 1 conjugate bearing two neighboring BODIPY units could emit strong fluorescence up to 100% quantum yield and multicolor light from green, yellow, to red (GYR) both in solutions and in poly(methyl methacrylate) films with aggregation of it. Moreover, the GYR multicolor emissions in solution could interconvert by adding and removing THF. The crystal structure of the compound disclosed that two BODIPY units were completely perpendicular to the phenyl ring of TPE core but parallel with each other. This extraordinary conformation resulted in intra/intermolecular slipped π-π stackings and even the synergism of these two slipped π-π stackings, rendering the compound the multicolor fluorescence. In contrast, other two TPE-BODIPY 2 and 3 had no intramolecular π-π stacking and only displayed green and yellow emission.

INTRODUCTION The difluoroboradiazaindacene (BODIPY)derivatives, a synthetically versatile class of fluorophores,1,2 have attracted particular interest in sensing, imaging and photoelectric devices because of their notably high fluorescence quantum yields, narrow and high-intensity emission peaks.3,4 Although most BODIPYs are brightly fluorescent in dilute solution, their emission is often weakened or even totally annihilated with increase of concentration or aggregation because of the effect of “aggregation-caused quenching” (ACQ), which limits their further applications.5-10 Therefore, search for novel BODIPYs with high emission characteristics both in solution and in solid state is necessary. For this purpose, it has been recognized as a promising protocol to introduce units with aggregation-induced emission (AIE) effect into BODIPY system, such as tetraphenylethylene (TPE) and its derivatives. So far, several TPE-BODIPY conjugates have been developed. For example, Tang and coworkers reported TPE-containing BODIPYs with high absolute fluorescence quantum yield in solid state (Φf =27%) but with low one in solution (Φf =0.3%).11 Atılgan’s

BODIPYs, bearing multiple TPE units at a BODIPY core, displayed a relative fluorescence quantum yield of 60% in solution, and a solid-state Φf value of 15%.12 Fu’s group reported BODIPYs containing TPE with a relative fluorescence quantum yield of 56% in CH2Cl2, and the solid state quantum yields was up to 15%.13 Rational molecular design to control the intra- or inter-molecular movement and aggregation of BODIPYs might afford novel luminescent materials not only with high fluorescence quantum yield both in solution and in solid state, but also with tunable emission properties such as multicolor emission. However, no such TPE-BODIPY molecules have hitherto been reported. Luminescent materials with tunable optical properties is of great importance for their potential application in the fields of optical-recording systems, sensors, security inks, informational displays, and molecular machines.14-24 To date, tuning or switching of double-color luminescence has been realized in many stimuli-responsive organic materials, while multicolor luminescence is very scarce. Several strategies have been used to achieve multicolor emission, such as mechanically

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Herein, we designed and synthesized a novel TPE-BODIPY conjugate 1 (Scheme 1) that was composed of a TPE moiety and two BODIPY units, which displayed triple-color luminescence from green to yellow to red based on intra/intermolecular slipped π-π stacking without changing the excitation energy. In contrast, two new conjugates 2 and 3 with no intramolecular π-π stacking only displayed green and yellow emission. Scheme 1. Structure of TPE-BODIPY conjugate 1-3. OCH3

absorption profile of BODIPY chromophores with a sharp band at 507 nm and a shoulder peak at 480 nm, which were attributed to the 0-0 and 0-1 vibronic band of S0-S1 transition, respectively (Figure 1).41,42 In contrary, 1 had an additional long wavelength band at 515 nm besides the above two bands. This result indicated that there was an interaction between the two neighboring BODIPY groups of 1 but no any interaction between the two trans-BODIPY groups of 2. Similarly, 2 and 3 emitted fluorescence light at the same wavelength of 514 nm in cyclohexane, which was due to the isolated BODIPY unit. However, 1 showed a dual emission at 516 nm and 550 nm. Obviously, the 516 nm emission resulted from the BODIPY monomer but the 550 nm emission should be ascribed to the π-π stacking of the two neighboring BODIPY groups. The fluorescence quantum yields measured using fluorescein as standard were 106%, 102% and 96% for 1, 2 and 3, respectively, in very diluted solution.

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induced multicolor switching,25-30 thermally induced multicolor change,31 mixing various pure colors from different emitting materials32-34. However, few examples of multicolor tuning for one ﬂuorophore without changing the excitation energy are reported.35,36 Recently, two excellent examples about the multicolor luminescence are reported by Saito35 and Tian36 under the same excitation wavelength. The researchers in these two papers ingeniously exploited the change of elongated π-π conjugation in different environments, such as in polymer matrix, in solution, and in crystals or in different polar solvents to carry out the multicolor emission. However, little work has been done on the control of their optical properties based on aggregation-induced multicolor emissions.37-40 Taking these into account, to develop multicolored and AIE-active luminescent materials without changing the excitation energy is much more attractive.

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RESULTS AND DISCUSSION TPE-BODIPY 1 bearing two neighboring BODIPY groups was synthesized using corresponding TPE dialdehyde as starting material according to standard method for preparation of BODIPY compounds. For comparison, compound 2 bearing two trans-BODIPY groups and 3 bearing only one BODIPY group were also synthesized. 1H NMR, 13C NMR, HRMS spectra and X-ray crystallography confirmed the structure and purity of all these three TPE-BODIPY conjugates. TPE-BODIPY 1-3 are all soluble in convenient organic solvents. In hexane, 2 and 3 displayed a standard

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Figure 1 (A) Absorption spectra of 1, 2 and 3 (2.0 × 10 M) -6 and (B) fluorescence spectra of 1, 2 and 3 (1.0 × 10 M) in cyclohexane.

In some polar solvents such as THF, dioxane, ethyl acetate, isopropanol, and chloroform, 1 emitted strong green light (Figure S17 − S18 and Table S1). Just like the dual emission in cyclohexane, 1 also displayed a dual emission in THF at low concentration while 2 and 3


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showed a single emission in THF (Figure S19 − S21). With increasing concentration of 1, the dual emission gradually turned to be a single emission and had an obvious bathochromic shift. For 2 and 3, they also showed a bathochromic shift but it was less than that of 1. In UV-Vis spectrum measurements, compounds 1, 2 and 3 all had not obvious shift of the maximum absorption wavelength at low concentration (Figure S22A, S23 and S24) but a bathochromic shift was observed at high concentration (larger than 2.5 × 10−5 M) (Figure S22B). In solid state, the UV-Vis spectra of 1, 2 and 3 in KBr film even showed an additional absorption band at longer wavelength of 547 nm, 542 nm and 541 nm, respectively (Figure S25), which should result from aggregation in solid state. By comparing the absorption wavelength (507 nm, Figure 1A) of the TPE-BODIPY conjugates in solution, the absorption wavelength of the solid had a bathochromic shift of 40 nm, 35 nm and 34 nm for 1, 2 and 3, respectively. As shown in Figure 2, the emission maximum wavelength λmax had a 15 nm of bathochromic shift from 546 nm to 561 nm as the concentration of 1 in THF was increased from 1.0 × 10−6 M to 1.0 × 10−3 M, thus double-color change from green to yellow could be seen. Upon evaporating up the solvent, the resultant solid powder of 1 could even emit a red fluorescence, which had a 49 nm shift compared with the diluted solution, and the solid of 2 and 3 emitted orange light (Figure S26). The 49 nm shift in emission spectra corresponded to the 40 nm shift in absorption spectra from 507 nm in solution to 547 nm in the solid state. The absolute fluorescence quantum yield measured by integrate sphere was 100% for both green light-emitting solution and yellow one in THF, and that of the solid for 1, 2 and 3 were 47%, 8.0% and 2.0%, respectively.

water-THF mixed solvent instead of by changing the concentration. Kept the concentration at 1.0 × 10−4 M, the solution of 1 that emitted green light in THF showed a yellow fluorescence when the water fraction was raised to 50%. After 95% water was added, the solution became an apparent suspension and emitted a red fluorescence. Moreover, when THF solvent was continued to add into the red light emission suspension, the red light emission could be transferred into yellow and then green one. On the contrary, when the green light emission suspension was evaporated to remove some THF, the green light emission could become yellow and then red one. Therefore, the GYR emissions could interconvert by adding and removing the THF in the suspension (Figure 3). The solution of 1 exhibited a 19 nm of bathochromic shift from green light (λmax 548 nm) to yellow one (λmax 567 nm) and a 38 nm of bathochromic shift to red one (λmax 586 nm) compared with the green light. As observed by dynamic light scattering (DLS) diagram (Figure S27), the solution of 1 in 50% water was composed of very tiny and very uniform aggregates with a very narrow diameter range of 7.8 to 8.0 nm although the solution seemed to be very clear by the naked eyes. In contrast, the apparent suspension of 1 in 95% water mixtures was composed of bigger particles with a diameter in a range of 100 nm to 450 nm (Figure S28), which were more than ten fold bigger than the particles in 50% water. Using 2 and 3, only green and yellow emissions could be realized and the red emission could not be obtained by changing the water fraction. As a try on application of the TPE-BODIPY conjugate, the bioimaging ability of TPE–BODIPY 1 was explored with HeLa cells in vitro. The red light-emitting suspension of 1 could selectively stain the cytoplasm of living cells but not their nuclei through an endocytosis after incubating for 4 h (Figure S29).

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Figure 2 Fluorescence spectra of 1 in THF at 1.0 × 10 M -3 (green line), at 1.0 × 10 M (yellow line), and in crystal state (red line). λex = 350 nm. Inset: photos of RYG emissions of 1 in THF at different concentration and in crystal state under irradiation of a 365 nm lamp.

More interesting, the green-yellow-red (GYR) emissions could be realized by changing the water fraction which could adjust the dissolvable ability of a



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Figure 3 Top: Photos of 1 (1.0 × 10 M) in THF, 1:1 THF/water and 5/95 THF/water under 365 nm light. Middle: Photos of GYR interconversion of 1 by adding and evaporating THF in the suspension under 365 nm light, red emission: [1] = 2.5 × −4 10 M in 5/95 THF/water. Bottom: Fluorescence spectra of 1 −4 in THF, 1:1 THF/water and 5/95 THF/water, [1] = 1.0 × 10 M, λex = 350 nm.

Because the emitting materials are commonly used as solid thin films for their commercial applications in electronic and photonic devices, the modulation of solid-state optical properties of luminophors has technological implications. With this in mind, poly (methyl methacrylate) (PMMA) films containing 1 were developed for the solid-state fluorescence analysis. For this purpose, a 1-PMMA film (1.0 cm × 1.5 cm) was fabricated by dropping the solution of 1 in methyl methacrylate with different concentrations (1.0 × 10−5 M, 1.0 × 10−3 M, and 1.0 × 10−2 M, total of 100 μL) onto the surface of a quartz slide. As a result, three 1-PMMA films with different weight ratio of 1 to PMMA varied from 4.68 × 10−6, 4.68 × 10−4 to 4.68 × 10−3 were prepared after the solution that was dropped on the slide was heated at 80 o C for 4 h. As shown in Figure 4, the 4.68 × 10−6 1-PMMA film emitted green light, the 4.68 × 10−4 1-PMMA film emitted yellow light, and the 4.68 × 10−3 1-PMMA film emitted red light under irradiation of a 365 nm UV lamp, which exhibited a same phenomenon as the solutions with different water fraction. In addition, excited by 350 nm, 470 nm or 510 nm light, the PMMA films containing 1, just like its solutions, gave a same color emission. The absolute fluorescence quantum yields measured by integrate sphere were 8.4%, 9.4% and 2% for green, yellow and red film prepared by spin coating, respectively.

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TPE and its derivatives are the most studied AIE compounds,43-45 however, the mechanism is not entirely clear although the AIE feature is generally ascribed to the restriction of intramolecular rotations (RIR).46 Here, and the reason why 1 in aggregating state still emitted fluorescence and exhibited GYR emissions with aggregation was also fascinating. To understand this, the single crystal suitable for X-ray diffraction was obtained by slow evaporation of the solution of 1 in mixed CHCl3/CH3OH solvent (Figure 5).47 Just like TPE and all its derivatives, the four phenyl rings of the TPE core of 1 are in a propeller-like conformation due to their steric repulsions. Noticeably, each of two BODIPY units in the TPE-BODIPY molecule is almost perpendicular to the phenyl ring bearing BODIPY unit because of the steric inhibition of the neighboring methoxy group. It shows that the dihedral angles between the BODIPY plane and the connected phenyl ring plane are 89.76o and 85.60o. As a result, there is almost no conjugation between BODIPY unit and phenyl ring. It is impossible that the red emission of 1 in solid state is ascribed to the molecular planarization of 1. Interestingly, the vertical conformation of the BODIPY unit relative to the phenyl ring makes the two BODIPY units almost parallel each other. The dihedral angle between the two BODIPY planes is only 16.65o, and the shortest distance between them is 3.433 Å (C8 to C43). Therefore, there is a slipped π-π stacking between the two BODIPY units, which confirms that the additional absorption band and emission band of 1 in solution truly results from the intramolecular π-π stacking interaction. Although the overlap of the two BODIPY groups is not big, it is very effective in changing the photophysical property of 1.

Figure 4 Photos of 1-PMMA films with different weight −6 −4 ratios of 1 to PMMA from 4.68 × 10 (A), 4.68 × 10 (B) to −3 4.68 × 10 (C) under 365 nm light.


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Compared with solution, the 40 nm shift in absorption spectra of the solid 1 that was in accordance with the 49 nm shift in emission spectra of the solid 1 further confirmed the elongated intermolecular π-π stackings in solid state.

Figure 5 Crystal structure of 1. (A) Top view and (B) side view. (C) Packing diagrams of 1 viewed along with b axis. Solvent molecules and hydrogen atoms are omitted for clarity. (D) The intra/intermolecular slipped π-π stackings of 1. The TPE units and hydrogen atoms are omitted for clarity.

Moreover, between two molecules of 1, there are two almost parallel BODIPY units that belong to different molecule (Figure 5C and 5D). The dihedral angle between the planes of two BODIPY units is only 21.13o, and the shortest distance between them is 3.427 Å (C9-C49’). Obviously, there is another slipped π-π stacking between molecules. As a result, all BODIPY units of 1 in aggregation state are in a slipped π-π stacking mode through both intramolecular π-π stacking interaction and intermolecular one. Meanwhile, it is also interestedly noted that the ground dipole moments of all neighboring BODIPY units are almost orthogonal while the planes of the BODIPY units are parallel. By the intra/intermolecular slipped π-π stacking, the fluorescence change of 1 with concentration as well as with aggregation could be more clearly explained. At low concentration, both BODIPY monomer emission and π-π stacking emission could be observed because the rotation of the two neighboring phenyl rings bearing BODIPY group could bring the two BODIPY groups to be close or to be away. The intensity of both the BODIPY emission and the π-π stacking one increased with concentration. However, when the concentration was continued to increase until the molecules started to aggregate, the motion of the phenyl rings was limited and the two BODIPY groups tended to stay close rather than away because the conformation of 1 with two BODIPY groups in close position was stable (also see the calculation result in Figure 8). Therefore, the monomeric BODIPY emission decreased and the π-π stacking emission increased. Meanwhile, not only the intramolecular π-π stacking but also the involvement of the intermolecular π-π stacking interactions, especially the synergism of them, gave rise to the obvious bathochromic shift. As the concentration was further increased and the aggregates grew bigger, more intermolecular π-π stackings were involved and larger bathochromic shift up to a red emission occurred.

It is reported that parallel ground dipole moments of two overlapping BODIPY groups could emit excimer-like emission48,49 but the antiparallel ground dipole moments49 of them aroused fluorescence quenching.48-50 In the solid state of 1, while the planes of any two neighboring BODIPY groups were near to be parallel, the ground dipole moments of them were almost orthogonal. Therefore, both the intramolecular slipped π-π stacking and the intermolecular ones could give rise to emission and bathochromic shift, which is similar to the alternated stacks of oPE -BODIPY systems.51 Fortunately, the single crystal of 2 suitable for X-ray diffraction was also obtained by slow evaporation of the solution in mixed CHCl3/CH3OH solvent (Figure 6).47 Like the crystal structure of 1, the TPE unit was twisted but with bigger dihedral angles between phenyl rings and the double bond plane. Each of two BODIPY units in the TPE-BODIPY molecule of 2 is almost perpendicular to the phenyl ring bearing BODIPY and had a big dihedral angle of 78o between the BODIPY plane and the connected phenyl ring plane. Although the two BODIPY planes were parallel each other (dihedral angle of 0o), there were not intramolecular π-π stacking due to their inverse direction that led to depart far away each other. However, between different molecules, the closing two BODIPY units were parallel each other (dihedral angle of 0o) and had a contact distance of 3.746 Å. Therefore, there exited an intermolecular slipped π-π stackings in crystal state of 2. This should be the reason why the emission showed a bathochromic shift at high concentration or in solid state. Powder XRD pattern of compound 1 and 2 rapidly precipitated in a mixed solvent of chloroform and methanol were similar with the simulated XRD pattern from crystal structure of them, which demonstrated the intermolecular slipped π-π stackings in powder solid of these TPE-BODIPY compounds (Figure S30−S32). This was also in accordance with the result of emission and absorption spectra of solid of 1, 2 and 3.



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To further verify this conclusion, variable-temperature fluorescence experiment was carried out (Figure S33). When the solution of 1 in THF at 2.0 × 10−4 M was gradually heated to 60 oC from 25 oC, it exhibited a hypsochromic shift from yellow to green and an increase of the fluorescence intensity. While the solution was cooled to 25 oC again, the yellow emission was recovered from the green one. This demonstrated that the aggregation (cooling) and deaggregation (heating) involving slipped π-π stackings truly played a key role on the color change of the emission. Time-resolved emission decay showed that the fluorescence lifetimes (τ) had a prolonging change from 5.3 ns, 6.2 ns, 7.2 ns to 8.7 ns when the concentration of 1 in THF increased from 1.0 × 10−6 M, 1.0 × 10−5 M, 1.0 × 10−4 M to solid state, respectively (Figure 7A). Similarly, the fluorescence lifetimes for green, yellow to red PMMA films also increased from 3.7 ns, 6.3 ns to 7.0 ns, respectively (Figure 7B). This indicated that longer fluorescence lifetime came from bigger aggregates involving more slipped π-π stackings, which was consistent with the literature reports.52,53 In our cases, the slipped π-π stacking emission is different from both the classical excimer emission and the typical J-aggregation emission. In excimer, chromophores interact only in excitation state but not in ground state.54,55 For J-aggregation,56,57 generally speaking, it has a comparatively narrow emission besides bathochromic shift but the emission of 1, 2 and 3 are wide.

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Figure 6 Crystal structure (A) of 2 and intermolecular slipped π-π stackings of it (B). Solvent molecules and hydrogen atoms are omitted for clarity.

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Figure 7 (A) Time-resolved fluorescence decay curves of 1 at -6 -5 -4 1.0 × 10 M (τ = 5.3 ns), 1.0 × 10 M (τ = 6.2 ns), 1.0 × 10 M (τ = 7.2 ns) in THF, and in solid film (τ = 8.7 ns) prepared by evaporating the solution in THF. (B) Time-resolved fluorescence decay curves of 1 in PMMA films emitting green light (τ = 3.7 ns), yellow light (τ = 6.3 ns) and red light (τ = 7.0 ns). The PMMA films containing 1 was prepared by spin coating of solution of 1 and PMMA in THF.

To further demonstrate the unique photophysical property, density functional theory (DFT) analysis was carried out for these TPE-BODIPY compounds. As expected, both HOMO and LUMO orbitals of 1, 2 and 3 mainly resided on BODIPY chromophores rather than TPE unit due to mutual perpendicular relationship of BODIPY and TPE groups. Therefore, the HOMO and LUMO electron density of 2 bearing two trans-BODIPY groups only localized on the individual chromophores and no electron exchange occurred between the two trans-BODIPY groups. In contrary, the HOMO orbital of 1 only localized on one BODIPY skeleton but the LUMO orbital resided solely on another BODIPY skeleton (Figure 8). It meant that electron of 1 could transit from one BODIPY to another BODIPY group when it was excited. This confirmed the intramolecular π-π stacking interaction of 1. Meanwhile, the energy difference of LUMO and HOMO orbitals of 1 was 2.76 eV, which was smaller than that of 2 (2.98 eV) and 3 (2.99 eV) (Figure


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S34). This calculation result is also comparable to the absorption spectrum data.

AUTHOR INFORMATION Corresponding Author Yan-Song Zheng*, E-mail: [email protected]; Chun Zhang*, [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

LUMO (-2.33 eV)

HOMO (-5.09 eV)

Figure 8 Calculated frontier molecular orbitals for 1 by DFT.

CONCLUSION In conclusion, novel TPE-BODIPY conjugates 1-3, composed of a TPE core bearing BODIPY units, were synthesized. It was found that the TPE-BODIPY 1 bearing two neighboring BODIPY units could emit strong fluorescence up to 100% quantum yield and give GYR multicolor fluorescence both in solution and in PMMA film. More interestingly, the GYR multicolor emissions in solution could interconvert by adding and removing THF. Crystal structure of this compound disclosed that two BODIPY units were completely perpendicular to the connected phenyl ring of TPE unit, but they were parallel each other due to the steric inhibition of the neighboring methoxy groups. This kind of extraordinary conformation resulted in intramolecular slipped π-π stacking, intermolecular slipped π-π stacking, the synergism of these two slipped π-π stacking, and orthogonal ground dipole moments of neighboring BODIPY units, which rendered the aggregate a GYR multicolor emission. The red light-emitting aggregates of this TPE-BODIPY conjugates 1 could selectively stain the cytoplasm of living cells but not their nuclei. This is the first finding for the most studied TPE and its derivatives to exhibit aggregate emission in the presence of π-π stacking interactions. This finding gives us a hint that the connection of fluorophore units at meta-position of phenyl rings of a TPE core with bulky group(s) can afford intramolecular slipped π-π stacking and multicolor emitting fluorophore upon aggregation. Owing to its straightforward synthesis, TPE-BODIPY 1 has potential applications in optical devices. Compared with other fluorescent molecules, TPE-BODIPY 1 may be fabricated into multicolor light emitting devices that will benefit from a simple triple-color tunability.

ASSOCIATED CONTENT Supporting Information. Experimental details for the 1 13 synthesis, HNMR, CNMR, HRMS spectra, absorption and PL spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

The authors thank National Natural Science Foundation of China (21072067) for financial support and thank the Analytical and Testing Centre at Huazhong University of Science and Technology for measurement.

REFERENCES (1) Loudet, A.; Burgess, K., BODIPY dyes and their derivatives: syntheses and spectroscopic properties. Chem. Rev. 2007, 107, 4891-4932. (2) Nastasi, F.; Puntoriero, F.; Campagna, S.; Olivier, J.-H.; Ziessel, R., Hybrid complexes: Pt (II)-terpyridine linked to various acetylide-bodipy subunits. Phys. Chem. Chem. Phys. 2010, 12, 7392-7402. (3) Komatsu, T.; Oushiki, D.; Takeda, A.; Miyamura, M.; Ueno, T.; Terai, T.; Hanaoka, K.; Urano, Y.; Mineno, T.; Nagano, T., Rational design of boron dipyrromethene (BODIPY)-based photobleaching-resistant fluorophores applicable to a protein dynamics study. Chem. Commun. 2011, 47, 10055-10057. (4) Nepomnyashchii, A. B.; Bröring, M.; Ahrens, J.; Bard, A. J., Synthesis, photophysical, electrochemical, and electrogeneratedchemiluminescence studies. Multiple sequential electron transfers in BODIPY monomers, dimers, trimers, and polymer. J. Am. Chem. Soc. 2011, 133, 8633-8645. (5) Hepp, A.; Ulrich, G.; Schmechel, R.; Von Seggern, H.; Ziessel, R., Highly efficient energy transfer to a novel organic dye in OLED devices. Synth. Met. 2004, 146, 11-15. (6) Ozdemir, T.; Atilgan, S.; Kutuk, I.; Yildirim, L. T.; Tulek, A.; Bayindir, M.; Akkaya, E. U., Solid-state emissive BODIPY dyes with bulky substituents as spacers. Org. Lett. 2009, 11, 2105-2107. (7) Zhang, D.; Wen, Y.; Xiao, Y.; Yu, G.; Liu, Y.; Qian, X., Bulky 4-tritylphenylethynyl substituted boradiazaindacene: pure red emission, relatively large Stokes shift and inhibition of self-quenching. Chem. Commun. 2008, 4777-4779. (8) Lu, H.; Wang, Q.; Gai, L.; Li, Z.; Deng, Y.; Xiao, X.; Lai, G.; Shen, Z., Tuning the Solid-State Luminescence of BODIPY Derivatives with Bulky Arylsilyl Groups: Synthesis and Spectroscopic Properties. Chem.–Eur. J. 2012, 18, 7852-7861. (9) Gai, L.; Lu, H.; Zou, B.; Lai, G.; Shen, Z.; Li, Z., Synthesis and spectroscopic properties of bodipy dimers with effective solid-state emission. RSC Adv. 2012, 2, 8840-8846. (10) Liu, H.; Lu, H.; Xu, J.; Liu, Z.; Li, Z.; Mack, J.; Shen, Z., Boron-pyridyl-imino-isoindoline dyes: facile synthesis and photophysical properties. Chem. Commun. 2014, 50, 1074-1076. (11) Hu, R.; Gómez-Durán, C. A.; Lam, J. W.; Belmonte-Vázquez, J. L.; Deng, C.; Chen, S.; Ye, R.; Peña-Cabrera, E.; Zhong, Y.; Wong, K. S., Synthesis, solvatochromism, aggregation-induced emission and cell imaging of tetraphenylethene-containing BODIPY derivatives with large Stokes shifts. Chem. Commun. 2012, 48, 10099-10101. (12) Baglan, M.; Ozturk, S.; Gür, B.; Meral, K.; Bozkaya, U.; Bozdemir, O. A.; Atılgan, S., Novel phenomena for aggregation induced emission enhancement: highly fluorescent



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

hydrophobic TPE-BODIPY couples in both organic and aqueous media. RSC Adv. 2013, 3, 15866-15874. (13) Li, Z.; Chen, Y.; Lv, X.; Fu, W.-F., A tetraphenylethene-decorated BODIPY monomer/dimer with intense fluorescence in various matrices. New J. Chem. 2013, 37, 3755-3761. (14) Irie, M.; Fukaminato, T.; Sasaki, T.; Tamai, N.; Kawai, T., Organic chemistry: a digital fluorescent molecular photoswitch. Nature 2002, 420, 759-760. (15) Olson, C. E.; Previte, M. J.; Fourkas, J. T., Efficient and robust multiphoton data storage in molecular glasses and highly crosslinked polymers. Nat. Mater.2002, 1, 225-228. (16) Lim, S. J.; An, B. K.; Jung, S. D.; Chung, M.; Park, S. Y., Photoswitchable organic nanoparticles and a polymer film employing multifunctional molecules with enhanced fluorescence emission and bistablephotochromism. Angew. Chem. Int. Ed. 2004, 43, 6346-6350. (17) Hirata, S.; Watanabe, T., Reversible thermoresponsive recording of fluorescent images (TRF). Adv. Mate. 2006, 18, 2725-2729. (18) Toal, S. J.; Jones, K. A.; Magde, D.; Trogler, W. C., Luminescent silole nanoparticles as chemoselective sensors for Cr (VI). J. Am. Chem. Soc. 2005, 127, 11661-11665. (19) Jayanty, S.; Radhakrishnan, T., Enhanced fluorescence of remote functionalized diaminodicyanoquinodimethanes in the solid state and fluorescence switching in a doped polymer by solvent vapors. Chem.–Eur. J. 2004, 10, 791-797. (20) Ning, Z.; Chen, Z.; Zhang, Q.; Yan, Y.; Qian, S.; Cao, Y.; Tian, H., Aggregation-induced Emission (AIE)-active Starburst Triarylamine Fluorophores as Potential Non-doped Red Emitters for Organic Light-emitting Diodes and Cl2 Gas Chemodosimeter. Adv. Funct. Mater. 2007, 17, 3799-3807. (21) Kishimura, A.; Yamashita, T.; Yamaguchi, K.; Aida, T., Rewritable phosphorescent paper by the control of competing kinetic and thermodynamic self-assembling events. Nat. Mater. 2005, 4, 546-549. (22) Zhang, X.; Rehm, S.; Safont-Sempere, M. M.; Würthner, F., Vesicular perylene dye nanocapsules as supramolecular fluorescent pH sensor systems. Nat. Chem. 2009, 1, 623-629. (23) Feringa, B. L., Molecular switches. Wiley Online Library: 2001; Vol. 42. (24) Balzani, V.; Venturi, M.; Credi, A., Molecular devices and machines: a journey into the nanoworld. John Wiley & Sons: 2006. (25) Sagara, Y.; Kato, T., Brightly Tricolored Mechanochromic Luminescence from a Single-Luminophore Liquid Crystal: Reversible Writing and Erasing of Images. Angew. Chem. Int. Ed. 2011, 123, 9294-9298. (26) Sagara, Y.; Kato, T., Brightly Tricolored Mechanochromic Luminescence from a Single-Luminophore Liquid Crystal: Reversible Writing and Erasing of Images. Angew. Chem. Int. Ed. 2011, 50, 9128-9132. (27) Teng, M. J.; Jia, X. R.; Chen, X. F.; Wei, Y., A Dipeptide-Based Multicolored-Switching Luminescent Solid Material: When Molecular Assemblies Meet Mechanochemical Reaction. Angew. Chem. Int. Ed. 2012, 124, 6504-6507. (28) Ma, Z.; Teng, M.; Wang, Z.; Yang, S.; Jia, X., Mechanically Induced Multicolor Switching Based on a Single Organic Molecule. Angew. Chem. Int. Ed.2013,52, 12268-12272. (29) Yuan, W. Z.; Tan, Y.; Gong, Y.; Lu, P.; Lam, J. W.; Shen, X. Y.; Feng, C.; Sung, H. H. Y.; Lu, Y.; Williams, I. D., Synergy between twisted conformation and effective

Page 8 of 10

intermolecular interactions: strategy for efficient mechanochromicluminogens with high contrast. Adv. Mater. 2013, 25, 2837-2843. (30) Ma, C.; Xu, B.; Xie, G.; He, J.; Zhou, X.; Peng, B.; Jiang, L.; Xu, B.; Tian, W.; Chi, Z., An AIE-active luminophore with tunable and remarkable fluorescence switching based on the piezo and protonation–deprotonation control. Chem. Commun. 2014, 50, 7374-7377. (31) Zhao, Y.; Gao, H.; Fan, Y.; Zhou, T.; Su, Z.; Liu, Y.; Wang, Y., Thermally Induced Reversible Phase Transformations Accompanied by Emission Switching Between Different Colors of Two Aromatic-Amine Compounds. Adv. Mater. 2009, 21, 3165-3169. (32) Su, H.-C.; Chen, H.-F.; Fang, F.-C.; Liu, C.-C.; Wu, C.-C.; Wong, K.-T.; Liu, Y.-H.; Peng, S.-M., Solid-state white light-emitting electrochemical cells using iridium-based cationic transition metal complexes. J. Am. Chem. Soc. 2008, 130, 3413-3419. (33) Yu, X.-M.; Kwok, H.-S.; Wong, W.-Y.; Zhou, G.-J., High-efficiency white organic light-emitting devices based on a highly amorphous iridium (III) orange phosphor. Chem. Mater. 2006, 18, 5097-5103. (34) D'Andrade, B. W.; Holmes, R. J.; Forrest, S. R., Efficient Organic Electrophosphorescent White-Light-Emitting Device with a Triple Doped Emissive Layer. Adv. Mater. 2004, 16, 624-628. (35) Yuan, C.; Saito, S.; Camacho, C.; Irle, S.; Hisaki, I.; Yamaguchi, S., A π-Conjugated System with Flexibility and Rigidity That Shows Environment-Dependent RGB Luminescence. J. Am. Chem. Soc. 2013, 135, 8842-8845. (36) Zhang, Z.; Wu, Y.-S.; Tang, K.-C.; Chen, C.-L.; Ho, J.-W.; Su, J.; Tian, H.; Chou, P.-T., Excited-State Conformational/Electronic Responses of Saddle-Shaped N, N′-Disubstituted-Dihydrodibenzo [a, c] phenazines: Wide-Tuning Emission from Red to Deep Blue and White Light Combination. J. Am. Chem. Soc. 2015, 137, 8509-8520. (37) Tong, H.; Hong, Y.; Dong, Y.; Ren, Y.; Häussler, M.; Lam, J. W.; Wong, K. S.; Tang, B. Z., Color-tunable, aggregation-induced emission of a butterfly-shaped molecule comprising a pyran skeleton and two cholesteryl wings. J. Phys. Chem. B 2007, 111, 2000-2007. (38) Galer, P.; Korošec, R. C.; Vidmar, M.; Šket, B., Crystal Structures and Emission Properties of the BF2 Complex 1-Phenyl-3-(3, 5-dimethoxyphenyl)-propane-1, 3-dione: Multiple Chromisms, Aggregation-or Crystallization-Induced Emission, and the Self-Assembly Effect. J. Am. Chem. Soc. 2014, 136, 7383-7394. (39) Yoshii, R.; Hirose, A.; Tanaka, K.; Chujo, Y., Functionalization of Boron Diiminates with Unique Optical Properties: Multicolor Tuning of Crystallization-Induced Emission and Introduction into the Main Chain of Conjugated Polymers. J. Am. Chem. Soc. 2014, 136, 18131-18139. (40) Yan, X.; Cook, T. R.; Wang, P.; Huang, F.; Stang, P. J., Highly emissive platinum (II) metallacages. Nat.Chem. 2015, 7, 342-348. (41) Lu, H.; Mack, J.; Yang, Y.; Shen, Z., Structural modification strategies for the rational design of red/NIR region BODIPYs. Chem. Soc. Rev. 2014, 43, 4778-4823. (42) Qi, H.; Teesdale, J. J.; Pupillo, R. C.; Rosenthal, J.; Bard, A. J., Synthesis, Electrochemistry, and ElectrogeneratedChemiluminescence of Two BODIPY-Appended Bipyridine Homologues. J. Am. Chem. Soc. 2013, 135, 13558-13566.


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(43) Hong, Y.; Lam, J. W.; Tang, B. Z., Aggregation-induced emission. Chem. Soc. Rev. 2011, 40, 5361-5388. (44) Ding, D.; Li, K.; Liu, B.; Tang, B. Z., Bioprobes based on AIE fluorogens. Acc. Chem. Res. 2013, 46, 2441-2453. (45) Wang, M.; Zhang, G.; Zhang, D.; Zhu, D.; Tang, B. Z., Fluorescent bio/chemosensors based on silole and tetraphenyletheneluminogens with aggregation-induced emission feature. J. Mater. Chem. 2010, 20, 1858-1867. (46) Zhao, Z.; He, B.; Nie, H.; Chen, B.; Lu, P.; Qin, A.; Tang, B. Z., Stereoselective synthesis of folded luminogens with arene–arene stacking interactions and aggregation-enhanced emission. Chem. Commun. 2014, 50, 1131-1133. (47) The crystallographic data of compounds 1 and 2 have been deposited in the Cambridge Structural Database as CCDC 996694 and CCDC 1413460, respectively. (48) Alamiry, M. A.; Benniston, A. C.; Copley, G.; Harriman, A.; Howgego, D., IntramolecularExcimer Formation for Covalently Linked Boron Dipyrromethene Dyes. J. Phys. Chem. A 2011, 115, 12111-12119. (49) Mukherjee, S.; Thilagar, P., Fine-Tuning Dual Emission and Aggregation-Induced Emission Switching in NPI– BODIPY Dyads. Chem. Eur. J. 2014, 20, 9052-9062. (50) Bergström, F.; Mikhalyov, I.; Hägglöf, P.; Wortmann, R.; Ny, T.; Johansson, L. B.-Å., Dimers of dipyrrometheneborondifluoride (BODIPY) with light spectroscopic applications in chemistry and biology. J. Am. Chem. Soc. 2002, 124, 196-204. (51) Florian, A.; Mayoral, M. J.; Stepanenko, V.; Fernández, G., Alternated Stacks of Nonpolar Oligo

(p-phenyleneethynylene) -BODIPY Systems. Chem. Eur. J. 2012, 18, 14957-14961. (52) Yoo, H.; Yang, J.; Yousef, A.; Wasielewski, M. R.; Kim, D., Excimer formation dynamics of intramolecular π-stacked perylenediimides probed by single-molecule fluorescence spectroscopy. J. Am. Chem. Soc. 2010, 132, 3939-3944. (53) Sherwood, G. A.; Cheng, R.; Chacon-Madrid, K.; Smith, T. M.; Peteanu, L. A.; Wildeman, J., Chain Length and Substituent Effects on the Formation of Excimer-Like States in Nanoaggregates of CN-PPV Model Oligomers. J. Phys. Chem. C 2010, 114, 12078-12089. (54) Holzhauser, C.; Wagenknecht, H.-A., DNA and RNA “Traffic Lights”: Synthetic Wavelength-Shifting Fluorescent Probes Based on Nucleic Acid Base Substitutes for Molecular Imaging. J. Org. Chem. 2013, 78, 7373-7379. (55) Inouye, M.; Hayashi, K.; Yonenaga, Y.; Itou, T.; Fujimoto, K.; Uchida, T. a.; Iwamura, M.; Nozaki, K., A Doubly Alkynylpyrene-Threaded [4] Rotaxane That Exhibits Strong Circularly Polarized Luminescence from the Spatially Restricted Excimer. Angew. Chem. Int. Ed. 2014, 53, 14392-14396. (56) Würthner, F.; Kaiser, T. E.; Saha-Möller, C. R., JAggregates: From Serendipitous Discovery to Supramolecular Engineering of Functional Dye Materials. Angew. Chem. Int. Ed. 2011, 50, 3376-3410. (57) Fennel, F.; Wolter, S.; Xie, Z.; Plötz, P.-A.; Kühn, O.; Würthner, F.; Lochbrunner, S., Biphasic self-assembly pathways and size-dependent photophysical properties of perylenebisimide dye aggregates. J. Am. Chem. Soc. 2013, 135, 18722-18725.



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