Discrete Dimeric Anthracene Stackings in Solids with Enhanced

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Discrete Dimeric Anthracene Stackings in Solids with Enhanced Excimer Fluorescence Haichao Liu,†,⊥ Dengli Cong,‡,⊥ Bao Li,† Ling Ye,† Yunpeng Ge,† Xiaohui Tang,† Yue Shen,† Yating Wen,† Jun Wang,† Changjiang Zhou,† and Bing Yang*,† †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China ‡ College of Pharmacy, Jilin University, Changchun 130021, P. R. China S Supporting Information *

ABSTRACT: Polyaromatic compounds are significant members of leading candidates for organic semiconductors and optical materials. However, a thorny problem of polyaromatic materials is that their good emissive abilities in solutions are seriously weakened in solids due to strong π−π interactions between aromatics. As a typical case, the intermolecular π−π interaction tends to form excimers for polyaromatic system, which were always considered to quench fluorescence and decrease luminous efficiency in the past decades. Herein, anthracene is modified by meta-substituted bromobenzene to facilitate the formation of discrete dimeric stack in solids, leading to the enhanced anthracene excimer fluorescence. Particularly, instead of excimer quenching fluorescence, the more anthracene dimers in solids, the higher fluorescence efficiency, namely, excimer-induced enhanced solid-state emission. This work not only provides a meta-substituted strategy for molecular design to form excimer in solids but also demonstrates that high-efficiency solid-state emission can be achieved by excimer species. fficient organic light-emitting materials have shown great potential for the applications in organic light-emitting diodes (OLEDs),1−3 organic light-emitting field-effect transistors (OLEFETs),4−6 organic lasers,7 and chemo- and biosensors.8−10 Polyaromatic compounds have been always significant members of leading candidates for organic semiconductors and optical materials. However, a thorny problem of polyaromatic materials is that their good emissive abilities in solutions are seriously weakened in solids, due to strong π−π interactions between aromatics that activate nonradiative pathways of excited states, causing fluorescence quenching.11−14 Still in most cases these organic materials are employed in solid-state form. To solve this problem, the usual strategies of achieving high-efficiency solids mainly include chemical (such as modification of polyaromatic molecules with bulky substituents) and physical (such as polyaromatic materials as guests doped in hosts) approaches to suppress π−π stacking.11−14 As a typical case, the intermolecular π−π interaction tends to form excimers in polyaromatic system, which were always considered to quench fluorescence and decrease luminous efficiency in the past decades.11−18 The reason for excimers quenching fluorescence is generally thought to be that radiative transitions of excimers themselves are forbidden together with simultaneously activated nonradiative transitions.15−24 Even so, excimers still play significant roles in white OLEDs25−28 and chemo- and bio- sensors,29−34 taking advantage of their characteristics such as largely red-shifted

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emission spectra or long lifetimes relative to their monomers.19−24,35−38 Different from the traditional cognition on excimer, some very high-efficiency excimer fluorescence has recently aroused great interest of scientific researchers.39−42 Kohmoto group designed a series of anthracene derivatives possessing a carbamate group and an ester group at their 9- and 10positions, which exhibited aggregation-induced emission (AIE) characteristics possibly due to the isolated anthracene dimers in crystal enhancing the excimer emission by the assistance of Hbonding networks.39 Our group has also reported one anthracene derivative crystal, 2-(anthracen-9-yl)thianthrene (2-TA-AN), which has spatially isolated anthracene pairs in antiparallel stacking, and this crystal demonstrated the excimer fluorescence with a very high photoluminescence (PL) quantum yield (η PL ) of 80% and a long lifetime. 40 Coincidentally, Yoshizawa group designed a V-shaped bisanthracene derivative with discrete dimeric anthracene stackings in crystalline powder, which also exhibited anthracene-excimer fluorescence with high ηPL = 72% in solid mono-1.41 Importantly, the strategy of molecular design to construct this discrete dimeric stack between adjacent anthracene units can be revealed from these crystal structures as mentioned Received: March 30, 2017 Revised: May 2, 2017 Published: May 9, 2017 A

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(DMSO), demonstrating no charge transfer (CT) responsible for emission of ANP-m-Br (Figure S2). Rapid precipitation of ANP-m-Br from the mixture of dichloromethane/methanol resulted in light yellow crystalline powder being collected, which emitted bright green under UV irradiation (365 nm) with the fluorescent maximum of 520 nm (Figure 1a). This emissive behavior of ANP-m-Br in powder sharply contrasts with that in THF solution, which aroused our interest. Then, we tried to collect crystals by slow evaporation of ANP-m-Br in the mixture of dichloromethane/methanol, and two kinds of crystals with different fluorescence colors were obtained through hand separation. Under UV irradiation (Figure 1a,c), one bulk-shaped crystal B showed bluish-green color (λmax = 479 nm), while the other fusiform-shaped crystal G exhibited green color (λmax = 521 nm). Moreover, crystal G could be individually prepared from high-concentration mixed dichloromethane/methanol solutions (V/V = 2:1). Time-resolved fluorescence measurements (Figure 1b) revealed a gradually elongated lifetimes from THF solution to crystals, and to powder. ANP-m-Br in THF solution showed a monoexponential decay with one short lifetime of 1.81 ns. Both crystal B and G were characteristic by double-exponential decays with longlived components of 54.69 ns (45.05%) and 95.47 ns (61.65%), respectively. In particular, ANP-m-Br powder presented a monoexponential decay with only one lifetime as long as 94.10 ns, which was almost the same as long-lived component (95.47 ns) of crystal G. Surely, this result also demonstrated certain similarity of emissive species between crystal G and powder, together with their similar PL spectra. Preliminarily, from monomer to different aggregations, ANP-m-Br exhibited the marked distinction in both PL spectra and lifetimes, which should be originated from different anthracene stacking modes based on previous reports.39,43−45 Essentially, an apparent difference between crystal B and G can be analyzed through X-ray diffraction (XRD) experiments. ANP-m-Br crystallized in triclinic P1̅ symmetry for whether crystal B or crystal G. In single unit cell of crystal B or crystal G, there were four molecules with no particular intermolecular interactions between anthracene units. Increasing the number of unit cell to eight (2a*2b*2c), a specific dimeric stacking were observed for anthracene moiety in crystal B and crystal G. To conveniently inspect the dimeric anthracene stacks in eight unit cells, the peripheral molecules irrelevant to dimers were removed. As a result in crystal B (Figure 2a), there was only one dimer to be found in eight unit cells, presenting an interplanar distance (side view) of 3.518 Å and overlap ratio (top view) of approximate 23%. Each of dimeric anthracene units interacted in the form of C−H···π with nondimeric anthracene units (Figure 2a) or benzene rings (Figure S4) of adjacent molecules. Besides dimeric anthracene units, other anthracene units showed C−H···π interactions with each other (Figure S5). Different from crystal B, there was four dimers existing in eight unit cells of crystal G (Figure 2b), presenting an interplanar distance (side view) of 3.545 Å and overlap ratio (top view) of approximate 26%. Each of dimeric anthracene units interacted in the form of C−H···π with nondimeric anthracene units of adjacent molecules. Moreover, there were Br···Br interactions with the distance of 3.823 Å (Figure S6), which were absent in crystal B. One side of Br···Br interactions occurred to anthracene dimer, which may be significant for the formation of more dimers in crystal G. To more sufficiently demonstrate the role of Br substituent, we chose 9-phenylanthracene (ANP) as a comparison. Rapid precipitation and

above. On the whole, the rational candidate should be composed of two moieties: planar aromatics (π-moiety) and side substituents. On the one hand, the π-moiety with only onesided substitution is appropriate for dimer formation with antiparallel overlap of π−π stacking as large and close as possible; on the other hand, the one-sided substituent can serve as a spacer to separate dimers from each other, guaranteeing discrete and uniform dimeric stack. Conceivably, the size and orientation of side substituent should play a crucial role in the spatially isolated dimeric stack formation to realize enhanced excimer emission. Based on our previous report on excimer, anthracene formed discrete dimers in solids, being modified by 2-position thianthrene instead of 1-position thianthrene.40 To achieve this goal, meta-substitution in anthracene was first carried out to see if it works well, together with parasubstitution for the purpose of comparison. As an example, we designed and synthesized a simple molecule 9-(3bromophenyl)anthracene (ANP-m-Br) as shown in Figure 1

Figure 1. (a) PL spectra of ANP-m-Br in THF solution (λmax = 398, 418, 438 nm), crystal B (λmax = 479 nm), crystal G (λmax = 521 nm), and powder (λmax = 520 nm). (b) Time-resolved fluorescence spectra of ANP-m-Br in THF solution (τ = 1.81 ns), crystal B (τ1 = 18.53 (54.95%), τ2 = 54.69 (45.05%)), crystal G (τ1 = 51.02 (38.35%), τ2 = 95.47 (61.65%)), and powder (τ = 94.10 ns). (c) Molecular structure of ANP-m-Br and corresponding photographs in THF solution, crystals, and powder under 365 nm irradiation.

that anthracene unit was connected on the meta position of bromobenzene. Interestingly, a large color change was observed from deep blue (λmax = 418 nm in tetrahydrofuran (THF) solution) to bluish green (λmax = 479 nm in crystal B), and finally to green (λmax = 521 nm in crystal G or λmax = 520 nm in crystalline powder). Moreover, this obvious change of color was accompanied by a significant increase of ηPL, which is ascribed to a growing dimeric anthracene stacks in solids through experimental evidence. In detail, ANP-m-Br showed the structured absorption band from 300 to 405 nm in THF solution, assigned to π−π* transition of anthracene moiety, and deep-blue emission with structured PL spectrum at 398, 418, and 438 nm (Figure S1 and Figure 1a). Moreover, ANP-m-Br showed no change of PL spectra in different solvents from hexane to dimethyl sulfoxide B

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concentration of anthracene dimer in crystal structure, which probably affect PL efficiency significantly. As a matter of fact, a larger proportion of anthracene dimer in crystal G induces a higher ηPL than that of crystal B. Meanwhile, it needs to be mentioned again that crystal G and powder of ANP-m-Br presented almost the same PL spectra and long-lived components. This indicates the identical emissive species of anthracene dimer between crystal G and powder of ANP-m-Br. Moreover, ANP-m-Br powder possessed the higher ηPL than that of crystal G, which can be ascribed to the purer excimer state in powder than that in crystal G. This point can be well supported by a monoexponential fluorescence decay of powder instead of double-exponential fluorescence decay of crystal G. In other words, according to PL spectra, lifetimes and luminous efficiencies of different phase states, it was clear that ANP-m-Br could form anthracene dimer and excimer emission in solids. Because what fluorescence behaviors of dimer present are ascribed to excited state, it is reasonable to consider that it is excimer-induced (excimer is the abbreviation of excited dimer) enhanced emission (EIEE) in solids instead of traditionally quenching fluorescence. As a comparison, 9-(4-bromophenyl)anthracene (ANP-p-Br) was designed and synthesized, in which anthracene unit was connected on the para position of bromobenzene. ANP-p-Br showed the same absorption and PL spectra in THF solution as those of ANP-m-Br (Figures S8 and 3c). Once rapid precipitation and slow recrystallization of ANP-p-Br from the mixture of dichloromethane/methanol, white powder and colorless crystal were collected with deep-blue fluorescence under UV irradiation (365 nm). It can be inferred from the apparent colors of ANP-p-Br solids that powder or crystal is not characteristic by excimer fluorescence (Figure 3a). XRD

Figure 2. Stacking structures of ANP-m-Br in (a) crystal B and (b) crystal G (①: 3.346 Å, 2.739 Å, 2.925 Å. ②: 3.201 Å, 2.747 Å, 3.036 Å. ③: 3.098 Å, 2.596 Å, 2.922 Å. ④: 3.569 Å, 2.797 Å, 2.934 Å).

slow recrystallization of ANP from the mixture of dichloromethane/methanol, white powder and colorless crystal were respectively collected.46 As shown in Figure S7, ANP showed deep-blue emission in THF solution (λmax = 416 nm), crystal (λmax = 438 nm), and powder (λmax = 432 nm). XRD measurements revealed a long-range π−π stacking (3.706 Å) of anthracene units in crystal. In addition, photophysical measurements demonstrated small redshifts of PL spectra and still short lifetimes in solids relative to those in THF solution, which indicates the absence of excimer formation in ANP solids (crystal and powder). This result further confirms the important role of Br substituent in the formation of discrete anthracene stacks. The Br substituent essential can be tentatively assigned to the proper size and steric hindrance of Br meta-substituent for the construction of discrete dimeric stacking. It is noteworthy that ANP-m-Br presented a gradual increase of ηPL as follows: 16% (THF solution), 22% (crystal B), 34% (crystal G), and final 49% (powder). Simultaneously, ηPLs in different solvents were also investigated. As a result, ηPLs of ANP-m-Br showed the fluctuant trend from hexane to DMSO (Table S1), probably due to different viscosity effects of solvents on molecular nonradiative vibrations. Curiously, what factors that greatly affect the luminous efficiencies need to be clarified for ANP-m-Br in these solid states. As a comparison, between crystal B and crystal G, one difference is the overlapped area of anthracene dimer although their interplanar distances are almost the same. It is well-known that solid-state fluorescence could be modulated by changing the arrangement of anthracene units in previous reports.47−49 Analogously, overlapped area between two anthracene rings mainly affects the emission wavelength.39,43−45 Both crystals B and G demonstrated excimer characteristics with largely red-shifted emission spectra and elongated lifetimes relative to those of monomer (ANP-m-Br in dilute THF solution). Due to the larger overlap between two anthracene rings, crystal G showed obviously red-shifted PL spectrum (42 nm) relative to crystal B. The other difference between crystal B and crystal G is the

Figure 3. (a) Molecular structure of ANP-p-Br and corresponding photographs in THF solution, crystal, and powder under 365 nm irradiation. (b) Stacking structures of ANP-p-Br in crystal (①: π−π, 3.690 Å. ②: C−H···π, 2.923 Å). (c) PL spectra of ANP-p-Br in THF solution (λmax = 418 nm), crystal (λmax = 444 nm), and powder (λmax = 438 nm). (d) Time-resolved fluorescence spectra of ANP-p-Br in THF solution (4.65 ns), crystal (τ1 = 2.37 ns (32.14%), τ2 = 10.34 ns (67.86%)), and powder (τ1 = 3.70 ns (32.94%), τ2 = 8.13 ns (67.51%)). C

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measurements revealed a long-range π−π stacking with displacement along the long axis of anthracene units in crystal of ANP-p-Br (Figure 3b). In addition, photophysical measurements demonstrated small redshifts of PL spectra and still short lifetimes (Figure 3d) in solids relative to those in THF solution, which indicates the absence of excimer formation in ANP-p-Br solids. In terms of ηPL, there was almost no change for ANP-pBr in THF solution (56%), crystal (55%), and powder (56%), indicating the approximate monomer emission. As a result, the one-sided modification in anthracene with meta-linked bromobenzene indeed facilitates discrete dimeric stacking and the formation of excimer of anthracene, which was further confirmed by the comparison between ANP-m-Br and ANP-pBr. In summary, we verified a molecular design strategy to construct a discrete dimeric stack of anthracene in solids and to form the excimer-induced enhanced emission. The chemical modification was performed on anthracene with meta-linked bromobenzene to obtain ANP-m-Br, which showed a large redshift of PL spectra in solids (crystal B, crystal G, and powder) relative to that in THF solution. The combination of photophysical and XRD measurements revealed the structural nature of these different states: monomer in THF solution, small proportion anthracene dimers in crystal B, large proportion anthracene dimers in crystal G, and finally pure anthracene dimers in powder. In ANP-m-Br solids (crystals and powder), excimer emission characteristic was observed. What is more, the larger proportion of anthracene dimers in solids, the higher ηPL for ANP-m-Br solids. As an essence, the excimer formation can also induce the enhanced emission and highefficiency fluorescence, which is exactly contrary to the traditional view of excimer quenching fluorescence. Excimer formation becomes one of strategies to achieve high efficiency solid-state luminescence, and more efforts need be made to explore the foundation for excimer formation and enhanced emission. This work not only provides a molecular design for excimer formation in solids but also exactly demonstrates excimer-induced enhancement of solid-state fluorescence.

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ORCID

Bao Li: 0000-0003-0727-9764 Bing Yang: 0000-0003-4827-0926 Author Contributions ⊥

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (51673083 and 51473063), the Ministry of Science and Technology of China (2013CB834801 and 2015CB655003), the Graduate Innovation Fund of Jilin University (Project 2016011), and the Open Project Foundation of the State Key Laboratory of Luminescence and Applications (Changchun Institute of Optics, Fine Mechanics and Physics of the Chinese Academy of Science, SKLA-2016-04).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00460. General information on measurement, synthetic routes to ANP-m-Br and ANP-p-Br, single crystal X-ray crystallography, absorption spectra of ANP-m-Br and ANP-p-Br in THF solutions, solvatochromic PL spectra and ηPL values of ANP-m-Br in different solvents, photographs and spectra of ANP (in THF solution, crystal, and powder), and intermolecular interaction in ANP-m-Br and ANP-p-Br crystals (PDF) Accession Codes

CCDC 1528729−1528731 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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H.L. and D.C. contributed equally.

Notes

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. D

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