Elucidation of Anthracene Arrangement for Excimer Emission at

Article pubs.acs.org/crystal

Elucidation of Anthracene Arrangement for Excimer Emission at Ambient Conditions Misa Sugino,† Yusuke Araki,† Keisuke Hatanaka,† Ichiro Hisaki,† Mikiji Miyata,† and Norimitsu Tohnai*,†,‡ †

Department of Material and Life Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan ‡ PRESTO Japan Science and Technology Agency, Gobancho, Chiyodaku, Tokyo 102-0076, Japan S Supporting Information *

ABSTRACT: Solid-state excimer emission of anthracene and its derivatives is a rare case at ambient conditions. We have designed organic salts composed of 9,10-bis(4-aminophenyl)anthracene (BAPA) and mineral acids in order to regulate the anthracene arrangement for investigation of the plausible geometry. From fluorescence measurement of three BAPA salts (nitrate: salt 1, chloride: salt 2, phosphate: salt 3), emission colors were changed by the effect of the mineral acids on the crystal structure. Notably, salt 3 crystal exhibited a bluish-green color derived from excimer emission at ambient conditions. On the basis of X-ray crystallographic analysis, the excimer emission of the salt 3 crystal was attributed to a tilt−slide type of anthracene geometry. In the geometry, π-planes of the anthracene moieties partially overlapped next to each other, an angle between the πplanes is 44°, and the nearest C−C distance is 3.7 Å. Such molecular geometry of partial overlapping of the anthracene rings and slightly longer C−C distance than that of common active π−π interaction (3.4−3.5 Å) was constructed of OH···O hydrogen bonds among mineral acid ions. These results suggest that the hydrogen bonds among mineral acid ions lead to the proximity of BAPA, following the excimer emission.

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INTRODUCTION Excimer emission of fluorophores is a highly useful phenomenon with great potential in the development of functional optical materials for applications such as bioimaging and sensing.1,2 High-resolution imaging is made possible by excimer emission with a large Stokes shift, with fluorescence spectra of the excimer characterized by long wavelength emission in comparison to the monomer emission.3 To date, there have been many studies involving the excimer emission of aromatic hydrocarbons.1−6 However, anthracene and its derivatives have the inherent problem that their photodimerization is more favorable than their excimer emission.7 Therefore, various sophisticated approaches to achieving such emission have been developed, including incorporation into nanocapsules,8 aggregation control of anthracene-substituted dendrimers,9 and attachment of anthracene to zirconium phosphonates.10 In the crystalline state, further approaches have been evaluated based on suppression of photodimerization at low temperature11 or changes in lattice type under highpressure conditions.12 Judging from the reported results, the excimer emission of anthracene and its derivatives in their crystalline states under ambient conditions still remains a significant challenge. © XXXX American Chemical Society

Herein, we propose a plausible molecular arrangement of an anthracene derivative in the crystalline state that exhibits solidstate excimer emission under ambient conditions. In addition, we demonstrate a supramolecular methodology for construction of this specific anthracene arrangement. Organic salts of 9,10-bis(4-aminophenyl)anthracene (BAPA) with mineral acids were designed according to the knowledge gained in our previous experiments: that the solid-state fluorescence of organic salts of anthracene-2,6-disulfonic acid can be modulated by alkylammonium cations and that salt formation is useful for modulating the molecular arrangement of anthracene moieties.13 Unlike the excimer emission, an arrangement for photodimerization have been well-known (e.g., parallel overlapping between anthracene planes and the interplanar spacing of about 3.5 Å).7a,c Additionally, we have reported that face-to-face πstacked arrangements of anthracene result in dimer emission following molecular interaction in the ground state.14 Therefore, we assumed that preventing an active interaction between Received: August 1, 2013 Revised: September 21, 2013

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dx.doi.org/10.1021/cg401166v | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Scheme 1

Figure 1. Photographs of salts 1−3 in (a) DMSO solution and (b) crystalline state under UV irradiation (λ = 365 nm). Fluorescence spectra and excitation spectra of salts 1−3 in DMSO solution (c and e) and crystalline state (d and f). The concentration was maintained at 1 × 10−5 M. Fluorescence excitation spectra was monitored at λem,max.

the ground-state fluorophores should be the key approach of anthracene arrangement for excimer emission at ambient conditions. BAPA is a 9,10-diphenylanthracene derivative, belonging to a class of compounds that do not form a closely packed molecular arrangement. However, the addition of mineral acids enables the formation of charge-assisted hydrogen bonds at the amino sites of BAPA. In the crystalline state, these strong hydrogen bonds allow the normally repellent molecules to be situated close to one another, which is highly

advantageous for regulating the geometry of the anthracene moieties (Scheme 1).15,16 Furthermore, the anthracene arrangement can be conveniently modulated by the particular mineral acid used, and it is possible to determine a plausible arrangement that allows excimer emission in the crystalline state. The investigation reported herein, elucidates important factors involved in the modulation of the emission process. Moreover, this knowledge introduces a new strategy that enables maximization of the potential of fluorophores and B



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vibrational structures, and the spectra decreased at the fast decay (Figures S1 and S2 of the Supporting Information). It is indicated that the emission of salts 1 and 2 crystals were attributed to monomer species. However, the TRES results shown in Figure 3 demonstrate the presence of a fast decay

provides new insight into the design and development of organic fluorescent materials.

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RESULTS AND DISCUSSION Fluorescence Properties in Solution. Organic salts (nitrate: salt 1, chloride: salt 2, and phosphate: salt 3) were formed by mixing the corresponding mineral acid with BAPA. The resulting compounds were recrystallized from methanol, forming well-refined crystals suitable for effective evaluation of their photophysical properties and crystal structures. Salts 1, 2, and 3 were dissolved in dimethylsulfoxide (DMSO) solution at 1 × 10−5 M and used to measure the fluorescence properties under sufficiently dilute conditions. These solutions were observed to emit yellow fluorescence around at 554 nm, and their emission and excitation spectra completely overlapped (Figure 1, panels a, c, and e). This suggests that changing the mineral acid had a negligible effect on the luminescence properties of BAPA. Fluorescence Properties in the Solid State. The crystal of salt 3 showed green fluorescence, whereas those of salts 1 and 2 were blue (Figure 1b). Figure 1d shows the solid-state emission spectra of salts 1, 2, and 3. The spectra of salts 1 and 2 exhibit vibrational structures with emission maximum wavelengths (λem,max) at 423 nm and 430 nm, respectively. These features can be attributed to monomer emission by making comparisons with published data.3,17 Although the spectrum obtained for the crystal of salt 3 also showed an emission maximum at 432 nm with a vibrational band, the emission intensity significantly increased in the longer wavelength region. The excitation spectrum at λem,max of salt 3 had slightly broader features than those of salts 1 and 2, but the red edge of the excitation spectrum was highly similar, suggesting absorption of the same species in all of the crystals (Figure 1f). This suggests that the crystal of salt 3 absorbs as a monomer but emits from the lower-energy state, possibly an excimer. As excimer species arise from a combination of an excited and an unexcited molecule, the red edges of the excitation spectra of the monomer and excimer species should be similar. The excitation spectra of salt 3 were reviewed in more detail at short, medium, and long wavelength regions, in order to examine the possibility that additional species such as dimers were formed in the ground state. It can be seen in Figure 2 that the excitation spectra agree with each other, eliminating the possibility that there were additional species present in the ground state. Time-Resolved Emission Spectroscopy (TRES) and Fluorescence Lifetimes. Time-resolved emission spectroscopy (TRES) revealed the existence of excimer emission in the crystal of salt 3. The TRES of salts 1 and 2 crystals showed only

Figure 3. Time-resolved emission spectra of a crystal of salt 3.

component at around 432 nm and a slow decay component at around 490 nm. The TRES results shown in Figure 3 demonstrate the presence of a fast decay component at around 432 nm and a slow decay component at around 490 nm. The emission spectra of the short-lived species can be assigned to the monomer because the spectral shape is similar to those of salts 1 and 2 in the crystalline state (Figure 1d). While vibrational structure is evident in the emission spectra of the short-lived species at later time points, the spectra are broader and shifted to longer wavelengths. This behavior indicates that the excimer species was formed in addition to the monomer. The fluorescence lifetimes (τF) for the crystals of salts 1, 2, and 3 are summarized in Table 1. Comparison of these values Table 1. Solid-State Fluorescence Properties of Salts 1−3 salt

λema (nm)

ΦFb

τFc (ns)

1 2 3

423 430 432

0.014 0.018 0.027

0.77 (37.8%), 2.94 (41.6%), 10.90 (20.6%) 0.60 (14.6%), 3.84 (37.2%), 14.19 (48.2%) 0.55 (9.14%), 2.95 (18.6%), 9.33 (42.9%), 29.20 (29.3%)

a Maximum wavelength of fluorescence emission spectra. bFluorescence quantum efficiency. cFluorescence lifetimes for emission in the long wavelength region (λem = 490 nm).

indicate that the crystal of salt 3 had an additional component at a longer τF of 29.20 ns (29.3%). Taken together, the obtained experimental data on the photophysical properties demonstrate a long-lived emissive process attributed to excimer formation in the crystal of salt 3. In addition, it was observed that the excimer emission was stable, with no dimerization occurring under ambient conditions. Elucidation of Anthracene Arrangement for Excimer Emission. To elucidate the reason behind the excimer formation in the crystal of salt 3, X-ray crystallographic studies were carried out (Table 2). In the crystalline state, all three salts formed lamellar structures composed of BAPA layers and mineral acid layers (Figure 4). BAPA molecules in the crystals of salts 1 and 2 were almost the same packing mode, whereas it is different from that in the salt 3 crystal (Figures S3−S5 of the Supporting Information). As shown in the bottom panels of Figure 4, each ionized amino group of BAPA and the mineral

Figure 2. Fluorescence excitation spectra of salt 3 monitored at λem = 432 nm, 460 nm, and 489 nm. C



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Table 2. Crystallographic Parameters of Salts 1−3 formula Fw crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z dcalcd (g m−3) T (K) reflections observed reflections unique R1 [I > 2.0σ(I)] Rw (all data) CCDC no.a a

salt 1

salt 2

salt 3

C26H22N4O6 486.48 orthorhombic Pbca 8.61350(16) 10.56801(19) 25.8505(5) 90 90 90 2353.11(7) 4 1.373 153 2165 1659 0.0439 0.1128 938213

C26H22Cl2N2 433.38 monoclinic P21 10.8562(2) 8.0322(1) 14.1325(3) 90 102.1459(8) 90 1204.76(4) 2 1.195 153 2974 2842 0.1173 0.3859 938214

C26H26N2O8P2 556.45 monoclinic C2/c 32.1445(13) 9.5074(4) 8.5267(4) 90 104.161(2) 90 2526.67(19) 4 1.463 153 2682 2077 0.0681 0.2050 938215

Figure 5. Dihedral angles of BAPA in the crystalline state of (a) salt 1, (b) salt 2, and (c) salt 3.

dihedral angles were the same in salt 1 and salt 3 (88° and 84°, respectively), whereas salt 2 featured two different angles: 81° and 89°. As a result, this small conformational difference leads to only a slight shift of the solid-state emission spectra in Figure 1. DFT calculations [B31YP/6-31G(d)] also show that electron density distrubution of 9,10-BAPA were extended by decrease of the dihedral angles (Figure S3 of the Supporting Information). Moreover, such difference also has no influence on the increase of the emission at a longer wavelength region. These results suggested that the excimer emission was derived from intermolecular geometry of the anthracene moieties. Figure 6 shows the anthracene arrangements and top view of neighboring anthracene pairs in the crystal structures. Because of the steric hindrance due to the amino phenyl groups, no active π−π interactions were present for any of the salts, as in the case of face-to-face stacking. In salts 1 and 2, the anthracene moieties of BAPA were arranged as herringbone structures, although these were slightly different to those known to occur for unmodified anthracene (Figure 6, panels a and b). The anthracene molecules were found to be oriented almost perpendicularly to one another (θAnt = 81° and 75° for the crystals of salts 1 and 2, respectively), as shown in Figure 6 (panels d and e). The solid-state emission spectra of salts 1 and 2 showed sharp bands derived from the monomer species, and these molecular packing modes of BAPA molecules are very

See ref 18.

acid ions were held together by charge-assisted hydrogen bonds. In addition, we also considered a difference of BAPA conformation in the crystalline state. The conformation between anthracene moieties and two aniline moieties were nearly perpendicular in the three salt crystals (Figure 5). Significant conformational differences may have an effect on the length of π-conjugation, which become one of the critical factors for the change of fluorescence properties.19 Both

Figure 4. Crystal structures of the organic salts comprising BAPA and mineral acid. Light blue boxes represent the hydrogen-bonding network. a, b, and c represent salts 1, 2, and 3, respectively. The blue solid lines within the black circle represent hydrogen bonds between dihydrogenphosphate ions. Gray: carbon, red: oxygen, blue: nitrogen, green: chlorine, and yellow: phosphorus. D



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Figure 6. (a−c) Anthracene arrangement of the organic salts comprising BAPA and mineral acid. (d−f) Top view of a neighboring anthracene pair.

Figure 7. (a) Hydrogen bonds of the dihydrogenphosphate ion with BAPA and neighboring dihydrogenphosphate ions in salt 3 crystal. (b) Construction of an anthracene arrangement for excimer emission at ambient conditions.

emission in the crystalline state that the anthracene moieties arrange not only with partial and tilted overlapping of the πplane but also with a slightly longer distance between anthracene planes compared to that of active π−π interaction. Construction of Anthracene Arrangement for Excimer Emission. X-ray crystallographic studies also revealed details of the hydrogen bond networks. As shown in the bottom panels of Figure 4c, hydrogen bonds linked the mineral acid ion to the amino sites of the BAPA molecules in each of the three crystals. Interestingly, for salt 3, hydrogen bonds were formed between the neighboring dihydrogenphosphate ions themselves, which did not occur for the nitrate or chloride ions of salts 1 and 2, respectively. Figures S8−S10 of the Supporting Information shows a part of the hydrogen bond networks between BAPA molecules and mineral acid ions in the crystals of salt 1−3. In the case of salts 1 and 2, one of amino groups of BAPA molecules is linked by hydrogen bonds with mineral acid ions, whereas these molecules dispersed in the crystal structures. On the other hand, the linked BAPA molecules are arranged close together in the salt 3 crystal. Moreover, the hydrogen bonds formed between dihydrogen phosphate ions effectively clamped all of the amino groups of BAPA molecules. Figure 7a shows hydrogen bonds between dihydrogenphosphate ions. Each of these ions formed a total of seven hydrogen bonds: three NH··· O bonds with ionized NH groups of BAPA and four OH···O bonds with neighboring dihydrogenphosphate ions. Owing to this complex hydrogen bond network, the proximity of the anthracene moieties was significantly enhanced in the crystal of

similar. (Figure 1d and Figures S3 and S4 of the Supporting Information). However, the emission spectra of the salt 2 crystal shows little decrease of the vibrational structure and small red shift compared to that of the salt 1 crystal. Hirshfeld surfaces of BAPA molecules in the crystalline state showed that BAPA molecules in the salt 2 crystal have closer CH−π interaction than that of the salt 1 crystal (Figure S7 of the Supporting Information). Therefore, the small differences of the emission spectra are probably caused by CH−π interaction as well as the difference of the dihedral angles of BAPA. Although the anthracene moieties in salt 3 were also arranged in a herringbone-like structure, the angle between the anthracene moieties was found to be the smallest out of the three crystals (θAnt = 44°), and the arrangement allowed for partial overlapping of the π-planes (Figure 6, panels c and f). In addition, in salt 3, the nearest C−C distance was 3.66 Å, which is significantly longer than the usual π−π interaction (3.4−3.5 Å).20 Such molecular geometry is clearly different from that frequently reported in investigations concerning excimer emission. Pyrene is one molecule that has been investigated in detail. In its crystalline state, the molecules arrange in parallel pairs with an interplanar distance of 3.5 Å.4,21 In contrast, in the case of anthracene, the geometry of the closely packed moieties gives mainly photodimers or dimer emission associated with the interaction in the ground state.14,22 Alternatively, the tilt− slide type of geometry provides a preference for the excimer emission as opposed to photodimerization under ambient conditions. Therefore, it is essential for achieving excimer E



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salt 3. These results suggest that the BAPA molecules were allowed to approach each other by the hydrogen bonds between mineral acid ions. On the other hand, the steric hindrance of the amino phenyl groups prevented face-to-face stacking, thus preventing dimer formation. Consequently, both the molecular design and the modulation of the molecular arrangement by the hydrogen bonds had an effect on the anthracene geometry and the resulting excimer emission in the crystalline state (Figure 7b).

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures for the crystals and X-ray crystallographic files in cif format for the organic salts (CCDC: 938213− 938215). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +81-6-68797404.

CONCLUSION

Author Contributions

In summary, organic salts comprised of anthracene derivatives and mineral acids have been used to establish a plausible molecular arrangement for excimer emission in the crystalline state under ambient conditions. The crystal of BAPA phosphate (salt 3) emitted stable excimer fluorescence (green) without the occurrence of dimerization. All photophysical properties show that the tilt−slide anthracene geometry gives preference for excimer emission. Crystal structure analysis revealed that the geometry of neighboring anthracene pairs consisted of a partial π-plane overlap (θAnt = 44°) and a longer C−C distance (dc−c = 3.66 Å) than is observed for common π−π interactions. Such an arrangement was facilitated by the formation of hydrogen bonds not only between the protonated amino groups of the BAPA but also between the phosphate ions themselves, which allowed for enhanced proximity between the neighboring BAPA molecules. In addition, the conformation of BAPA was found to be important for ensuring that there was a large enough gap between the anthracene moieties to prevent dimerization. These findings provide new strategies for the development of functional optical materials with bioimaging and sensing applications.

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Article

These authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by KAKENHI (Grants 22685014, 24108723, and 24655124) from MEXT (Japan). M. Sugino thanks the GCOE Program of Osaka University. The synchrotron radiation experiments were performed at BL38B1 in the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal no. 2012A1592 and 2013A1617).

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REFERENCES

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EXPERIMENTAL SECTION

Preparation of the Organic Salt of 9,10-BAPA and Mineral Acids. 9,10-BAPA and commercially available mineral acids were mixed in chloroform in a 1:2 molar ratio to yield an organic salt. The solution was evaporated under reduced pressure, and the precipitate corresponding mineral acids of the salts was obtained. These colors were white green, white yellow green, and white blue for nitrate (salt 1), chloride (salt 2), and phosphate (salt 3), respectively. Preparation of Single Crystals. The organic salt comprised of 9,10-BAPA and mineral acids was dissolved in methanol. Slow evaporation of the solvent at room temperature gave the single crystals. The single crystals were picked from the solution by filtration and used for the measurements. Crystallographic Analysis of Single Crystals. X-ray diffraction data of the crystal of salt 1 was measured on a Rigaku R-AXIS RAPID diffractometer with a two-dimensional area detector using graphitemonochromatized Cu Kα radiation (λ = 1.54187 Å). X-ray diffraction data of other crystals were collected by using the synchrotron radiation (λ = 0.8000 Å) the BL18B1 in the SPring-8 with approval of JASRI (Proposal no. 2012A1592 and 2013A1617). The cell refinements were performed by HKL2000.23 Direct methods (SIR-2004 and SHELXS97) were used for the structure solution.24,25 All calculations were performed with the observed reflections [I > 2σ(I)] by the program CrystalStructure crystallographic software26 except for refinement, which was performed using SHELXL-97. All nonhydrogen atoms were refined with anisotropic displacement parameters and hydrogen atoms were placed in idealized positions and refined as rigid atoms with the relative isotropic displacement parameters. F



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Elucidation of Anthracene Arrangement for Excimer Emission at

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