Pairwise Packing of Anthracene Fluorophore: Hydrogen-Bonding

Article pubs.acs.org/crystal

Pairwise Packing of Anthracene Fluorophore: Hydrogen-BondingAssisted Dimer Emission in Solid State Shugo Hisamatsu,† Hyuma Masu,‡,§ Masahiro Takahashi,‡ Keiki Kishikawa,‡ and Shigeo Kohmoto*,‡ †

Actinide Coordination Chemistry Group, Quantum Beam Science Directorate, Japan Atomic Energy Agency, 2-4 Shirakatashirone, Tohkai-mura, Ibaraki 319-1195, Japan ‡ Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan § Center for Analytical Instrumentation, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan S Supporting Information *

ABSTRACT: Anthracene derivatives possessing a carbamate group and an ester group at their 9- and 10-positions, respectively, were prepared to furnish pairwise packing of anthracene fluorophores in their crystal structures. They were nonluminescent in ethanol solution and showed AIE (aggregation-induced emission) in aqueous ethanol solution and in solid state. Crystal structure analysis of them showed that the H-bonding networks involved in their crystal structures could be classified into four patterns, H-bonding between the carbamate and the ester carbonyl (motif A), H-bonding between the carbamate and the ester oxygen atom (motif B), H-bonded cyclic dimer of carbamate moieties (motif C), and H-bonded chain among carbamate moieties (motif D). Compounds with pairwisely packed anthracene fluorophores showed dimer emission with the longer fluorescence wavelength than others without the pair formation. Fluorescence maximum wavelengths and lifetimes become longer proportional to the degree of overlapping of two facing anthracene π-planes.

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fluorophore of simple chemical structure is highly demanded. In the course of our search for anthracene fluorophores, we found that their carbamate derivatives showed dimer emission in solid state. Moreover, they were nonemissive in organic solvents but became emissive in aqueous solution. Usually, excimer or dimer emission is very weak in solid state. However, in some cases excimer emission became predominant over monomer emission in AIE.66,67 We use the term “dimer emission” for fluorescence we observed in the present results instead of excimer emission based on the experimental results which are described in the latter section of the paper. Our approach to AIE of the anthracene dimer is as follows: (1) Facile generation of restricted rotation of the substituent at its 9- or 10-position, which results in nonemissive state by the RIR effect. (2) Construction of a pairwise packing of anthracene moieties in the crystalline state by introducing two H-bonding sites, H-bond donor and acceptor sites, at its 9and 10-possitions, respectively. This pairwise packing interrupts RIR, which brings about AIE in the crystalline state. Moreover, the pairwise packing can promote the dimer emission. Although anthracene derivatives are well-known fluorophores and numerous studies have been issued on their luminescence, their excimer emissions are rare in solid state at room temperature except a few examples.68−75 It is generally

INTRODUCTION Recently, much attention has been paid to solid-state luminescence of organic compounds for its potential applications in sensors, fluorescent probes, and photoelectronic devices.1−12 It is well-known that solid-state fluorescence is largely affected by the packing arrangement of organic fluorophores.13−17 Moreover, solid-state fluorescence is often encountered the fact that many effective organic fluorophores in solution become nonemissive in solid state because of selfquenching, nonradiative processes in aggregates or excimer formation.7,18 However, in a few polymers efficient emission has been discovered in the aggregated state.19−21 This totally opposite way of emission in which nonemissive or weak emissive fluorophores in solution became emissive in solid state was also found in low-molecular-weight molecules. Since the discovery, this new concept of emission, AIE (aggregationinduced emission)22 or AIEE (aggregation-induced enhanced emission),23 is rapidly developed.2,24−37 They are applied to chemo-38,39 and biosensors,40−45 logic gates,46,47 mechanofluorochromic materials,48−52 and electronic devices such as organic light-emitting diodes (OLEDs),53 organic field-effect transistors (OFETs), and organic light-emitting field-effect transistors (OLETs).54 The reasons of disruption of emission in solution can be classified into several types, RIR (restriction of intramolecular rotation),55−58 CT59 or TICT (twisted intramolecular charge transfer),60 ESIPT (excited state intramolecular proton transfer),61−64 and the formation of ion pairs.65 To develop this new type of emission further, a © XXXX American Chemical Society

Received: January 19, 2015 Revised: April 3, 2015

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DOI: 10.1021/acs.cgd.5b00081 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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mmol), and the resulting mixture was stirred at 90 °C for 15 h. After the reaction, the resulting solution was added ethyl acetate (40 mL) and washed with water (20 mL × 4) and subsequently with saturated aqueous solution of NH4Cl (10 mL × 4). Organic layer was washed with saturated aqueous solution of NaHCO3 (40 mL) and dried over anhydrous magnesium sulfate. After evaporation of solvent, the residue was purified by chromatography on silica gel (hexane/ethyl acetate, 5:1) to afford 1 (60 mg, 48%) as yellow solid. After chromatography, 1 was recrystallized from ethyl acetate/n-hexane to afford yellow crystals. mp 154−156 °C; IR (KBr) 3354 (br), 3064 (w), 2975 (m), 2929 (m), 1714 (s), 1702 (s), 1294 (s), 1230 (s), 1214 (s) cm−1; 1H NMR (300 MHz, (CDCl3)) δ 8.11 (br, 2H), 7.98 (m, 2H), 7.50 (m, 4H), 6.86 (br, 1H), 4.89 (br, 1H), 4.70 (q, J = 7.2 Hz, 2H), 1.73−0.546 (br, 8H), 1.54 (t, J = 7.2 Hz, 3H); HRMS (ESI) calcd for C22H24NO4 [M + H]+ 366.1700, found 366.1693. In a similar manner to the synthesis of 1, analogous carbamate derivatives 2−15 were prepared. Ethyl 10-((Ethoxycarbonyl)amino)anthracene-9-carboxylate (2): mp 163−165 °C (ethyl acetate/hexane); IR (KBr) 3247 (br), 3051(w), 2984 (m), 1725 (s), 1685 (s), 1298 (s), 1209 (s) cm−1; 1 H NMR (500 MHz, (CDCl3)) δ 8.05 (br, 2H), 7.94 (br. d, 8.5 Hz, 2H), 7.45 (m, 4H), 7.04 (br, 1H), 4.70 (q, J = 7.0 Hz, 2H), 4.08 (br, 2H), 1.81 (br, 5H), 1.81−0.522 (br, 5H), 1.58 (t, J = 7.0 Hz, 3H); HRMS (ESI) calcd for C20H20O4N [M + H]+ 338.1387, found 338.1382. Ethyl 10-((tert-Butoxycarbonyl)amino)anthracene-9-carboxylate (3): mp 176−182 °C (ethyl acetate/hexane); IR (KBr) 3235 (br), 3121 (w), 2977 (m), 1724 (s), 1692 (s), 1362 (s), 1297 (s), 1210 (s) cm−1; 1H NMR (500 MHz, (CDCl3)) δ 8.06 (br, 2H), 7.93 (br. d, 8.5 Hz, 2H), 7.45 (br. dd, J = 6.5 Hz, 1.0 Hz, 2H), 7.43 (br. dd, J = 6.5 Hz, 1.5 Hz, 2H), 6.93 (br, 1H), 4.68 (q, J = 7.0 Hz, 2H), 1.58−1.21 (br, 9H), 1.53 (t, J = 7.0 Hz, 3H); HRMS (ESI) calcd for C22H24O4N [M + H]+ 366.1700, found 366.1692. Ethyl 10-(((((1R,2S,5R)-2-Isopropyl-5-methylcyclohexyl)oxy)carbonyl)amino)- anthracene-9-carboxylate (4): mp 153−155 °C (ethyl acetate/hexane); IR (KBr) 3351 (br), 3121 (w), 2955 (m), 2928 (m), 1723 (s), 1694 (s), 1240 (s), 1209 (s) cm−1; 1H NMR (500 MHz, (CDCl3)) δ 8.23 (br, 2H), 8.09 (br. d, J = 9 Hz, 2H), 7.61 (br, 4H), 7.01 (br, 1H), 4.81 (br, 2H), 4.69 (br, 1H), 2.33−0.578 (br, 21H); HRMS (ESI) calcd for C28H34O4N [M + H]+ 448.2482, found 448.2477. Ethyl 10-((Butoxycarbonyl)amino)anthracene-9-carboxylate (5): mp 130−132 °C (ethyl acetate/hexane); IR (KBr) 3279 (br), 2982 (w), 2959 (w), 1723 (s), 1688 (s), 1511 (s), 1253 (s), 1211 (s) cm−1; 1 H NMR (300 MHz, (CDCl3)) δ 8.17 (br, 2H), 8.01 (m, 2H), 7.54 (br. dd, J = 6.8 Hz, 3.0 Hz, 4H), 6.80 (br, 1H), 4.70 (q, J = 7.2 Hz, 2H), 4.00 (br, 2H), 1.87−0.478 (br, 7H), 1.54 (t, J = 7.2 Hz, 3H); HRMS (ESI) calcd for C22H24O4N [M + H]+ 366.1700, found 366.1692. Ethyl 10-((Propoxycarbonyl)amino)anthracene-9-carboxylate (6): mp 141−144 °C (ethyl acetate/hexane); IR (KBr) 3247 (br), 3051(w), 2984 (m), 1725 (s), 1685 (s), 1298 (s), 1209 (s) cm−1; 1H NMR (500 MHz, (CDCl3)) δ 8.05 (br, 2H), 7.94 (br. d, 8.5 Hz, 2H), 7.45 (m, 4H), 7.04 (br, 1H), 4.70 q, J = 7.0 Hz, 2H), 4.08 (br, 2H), 1.81 (br, 5H), 1.58 (t, J = 7.0 Hz, 3H); HRMS (ESI) calcd for C21H22O4N [M + H]+ 352.1543, found 352.1539. Ethyl 10-((iso-Propoxycarbonyl)amino)anthracene-9-carboxylate (7): mp 163−165 °C (ethyl acetate/hexane); IR (KBr) 3274 (br), 3070 (w), 2980 (m), 1724 (s), 1693 (s), 1254 (s), 1213 (s) cm−1; 1H NMR (500 MHz, (CDCl3)) δ 8.17 (br, 2H), 8.01 (m, 2H), 7.54 (br. dd, J = 7.0 Hz, 3.0 Hz, 4H), 6.71 (br, 1H), 5.09 (br, 1H), 4.68 (q, J = 7.0 Hz, 2H), 1.42−0.970 (br, 6H), 1.54 (t, J = 7.0 Hz, 3H); HRMS (ESI) calcd for C21H22O4N [M + H]+ 352.1543, found 352.1536. (S)-Ethyl 10-(((2-Methylbutoxy)carbonyl)amino)anthracene-9carboxylate (8): mp 79−81 °C (ethyl acetate/hexane); IR (KBr) 3278 (br), 3069 (w), 2958 (m), 2938 (m), 1735 (s), 1696 (s), 1300 (s), 1222 (s) cm−1; 1H NMR (300 MHz, (CDCl3)) δ 8.17 (br, 2H), 8.01 (m, 2H), 7.54 (br. dd, J = 6.8 Hz, 3.0 Hz, 4H), 6.81 (br, 1H), 4.70 (q, J = 7.2 Hz, 2H), 3.99 (br, 2H), 1.87−0.478 (br, 9H), 1.54 (t, J =

recognized that the quenching by adjacent anthracene molecules is the reason for the absence of their excimer emissions. The pairwisely packed dimers of anthracene fluorophores in an isolated manner can avoid quenching and possibly enhance their dimer emissions. Figure 1 illustrates our

Figure 1. Schematic representation of the pairwise packing of an anthracene derivative possessing H-bonding donor and acceptor sites and its dimer emission.

idea of the pairwise packing of anthracene moieties and the generation of dimer emission. Two-point H-bonding together with the π−π stacking of anthracene moieties afford a stacked anthracene dimer. If the H-bonding sites have the substituents bulky enough to cover the stacked anthracene dimer, the anthracene moieties can be segregated from other packed anthracene pairs. Thus, the pairwise packing of anthracene moieties can be achieved. Owing to this packing arrangement the quenching of anthracene dimer by neighboring anthracene moieties is greatly suppressed. In addition to this, an interruption of the restricted rotation of the substituents induces the AIE activity. Excimer or dimer emission is attractive from the viewpoint of the tuning of the color of fluorescence14,76−79 especially for white organic light-emitting devices.80−85 The selective generation of monomer or excimer (dimer) emission and also the mixing of these two emissions can afford an effective tool for this purpose.

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EXPERIMENTAL SECTION

Materials and Methods. All the reagents employed were commercially available and used as received without further purification. Melting points were determined on a Yanaco MP-S3 apparatus and were uncorrected. IR spectra were recorded on a PerkinElmer Spectrum Two by a KBr pellet method. 1H NMR spectra were recorded on Bruker DPX300NMR and JEOL JNM-LA500 spectrometers in CDCl3 with Me4Si as an internal standard. Absorption spectra were recorded on JASCO V-550 spectrometer in ethanol (concentration: 1.1 × 10−4 M) for solution sample and an integrating sphere was used for the measurements in solid state. Fluorescence spectra were recorded on JASCO FP-750 spectrometer in ethanol (concentration: 1.1 × 10−6 M) for solution sample. Highresolution ESI mass spectra were measured with a Thermo Fisher Exactive mass spectrometer. X-ray diffraction (XRD) experiments were performed with CuKα radiation by using a Rigaku RINT 2200 diffractometer. Differential scanning calorimetry (DSC) measurements were performed on a MAC Science DSC 3100S differential scanning calorimeter with heating rate of 3 °C min−1. Measurements of absolute quantum yields of fluorescence of solid samples were performed on a Hamamatsu Photonics Quantaurus-QY C11347−01. Fluorescence lifetimes were measured on time-resolved fluorescence spectrometer IBH 5000U. Preparation of Carbamates. Ethyl 10-((sec-Butoxycarbonyl)amino)-anthracene-9-carboxylate (1). A solution of 10-(ethoxycarbonyl)-anthracene-9-carboxylic acid (100 mg, 0.34 mmol), triethylamine (0.23 mL, 1.7 mmol) in dry toluene (1.0 mL), and dry secbutanol (1.0 mL) was added diphenylphosphoryl azide (0.11 mL, 0.51 B



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7.2 Hz, 3H); HRMS (ESI) calcd for C23H26O4N [M + H]+ 380.1856, found 380.1851. Ethyl 10-(((Cyclohexyloxy)carbonyl)amino)anthracene-9-carboxylate (9): mp 168−171 °C (ethyl acetate/hexane); IR (KBr) 3224 (br), 3117 (w), 2935 (m), 2855 (w), 1728 (s), 1700 (s), 1212 (s) cm−1; 1H NMR (500 MHz, (CDCl3)) δ 8.10 (br, 2H), 7.96 (br. d, 8.5 Hz, 2H), 7.49 (br, dd, J = 6.0 Hz, 1.0 Hz, 2H), 7.47 (br. dd, J = 6.5 Hz, 1.5 Hz, 2H), 6.97 (br, 1H), 4.69 q, J = 7.0 Hz, 2H), 2.04−0.986 (br, 9H), 1.54 (t, J = 7.0 Hz, 3H); HRMS (ESI) calcd for C24H26O4N [M + H]+ 392.1856, found 392.1851. Methyl 10-((tert-Butoxycarbonyl)amino)anthracene-9-carboxylate (10): mp 204−209 °C (ethyl acetate/hexane); IR (KBr) 3247 (br), 3117 (w), 2981 (w), 2847 (w), 1724 (s), 1687 (s), 1368 (s), 1214 (s) cm−1; 1H NMR (300 MHz, (CDCl3)) δ 8.21 (br, 2H), 8.01− 7.98 (m, 2H), 7.61−7.53 (m, 4H), 6.80−6.37 (br, 1H), 4.18 (s, 3H), 1.72−1.12 (br, 9H); HRMS (ESI) calcd for C21H20O4N [M-H]− 350.1398, found 350.1409. Propyl 10-((tert-Butoxycarbonyl)amino)anthracene-9-carboxylate (11): mp 160−162 °C (ethyl acetate/hexane); IR (KBr) 3378 (s), 3069 (w), 2967 (m), 2938 (w), 1725 (s), 1239 (s), 1204 (s) cm−1; 1 H NMR (300 MHz, (CDCl3)) δ 8.20 (br, 2H), 8.03−7.99 (m, 2H), 7.58−7.51 (m, 4H), 6.84−6.40 (br, 1H), 4.58 (t, 6.7 Hz, 3H), 1.91 (sex, 7.2 Hz, 2H), 1.74−1.13 (br, 9H), (t, 7.4 Hz, 3H); HRMS (ESI) calcd for C23H24O4N [M − H]− 378.1711, found 378.1728. iso-Propyl 10-((tert-Butoxycarbonyl)amino)anthracene-9-carboxylate (12): mp 192−195 °C (ethyl acetate/hexane); IR (KBr) 3242 (br), 3120 (w), 2980 (s), 2937 (w), 1717 (s), 1695 (s), 1212 (s) cm−1; 1H NMR (300 MHz, (CDCl3)) δ 8.16 (br, 2H), 8.00−7.96 (m, 2H), 7.55−7.49 (m, 4H), 6.89−6.50 (br, 1H), 5.62 (quint, 6.2 Hz, 1H), 1.75−1.06 (br, 9H), 1.53 (d, 6.3 Hz, 6H); HRMS (ESI) calcd for C23H24O4N [M − H]− 378.1711, found 378.1729. Benzyl 10-((tert-Butoxycarbonyl)amino)anthracene-9-carboxylate (13): mp 172−180 °C (ethyl acetate/hexane) for both crystals; IR (KBr) for blocks 3325 (br), 3065 (w), 2985 (m), 2937 (w), 1722 (s), 1704 (s), 1224 (s) cm−1; IR (KBr) for needles 3218 (br), 3112 (m), 3069 (m), 2965 (m), 1708 (s), 1366 (s), 1212 (s) cm−1; 1H NMR (500 MHz, (CDCl3)) δ 8.17 (br. d, 7.9 Hz, 2H), 7.92 (d, 7.9 Hz, 2H), 7.54−7.37 (m, 13H), 6.88−6.38 (br, 1H), 5.65 (s, 2H), 1.81−1.00 (br, 9H); HRMS (ESI) calcd for C27H24O4N [M − H]− 426.1711, found 426.1729. tert-Butyl Anthracene-9-ylcarbamate (14): mp 153−155 °C (ethyl acetate/hexane); IR (KBr) 3223 (br), 3100 (s), 3062 (s), 2973 (s), 2931 (m), 1687 (s), 1360 (s), 1164 (s) cm−1; 1H NMR (300 MHz, (CDCl3)) δ 8.43 (s, 1H), 8.17 (br. d, 8.0 Hz, 2H), 8.01 (d, 8.2 Hz, 2H), 7.57−7.45 (m, 4H), 6.80−6.27 (br, 1H), 1.76−1.03 (br, 9H); HRMS (ESI) calcd for C19H18O2N [M − H]− 292.1343, found 292.1350. Di-tert-butyl Anthracene-9,10-diyldicarbamate (15): mp 203− 206 °C (ethyl acetate/hexane); IR (KBr) 3270 (br), 3069 (m), 3011 (m), 2983 (s), 2935 (w), 1689 (s), 1364 (s), 1168 (s) cm−1; 1H NMR (300 MHz, (CDCl3)) δ 8.19 (br, 4H), 8.00−7.96 (dd, 6.8 Hz, 3.2 Hz, 4H), 6.83−6.32 (br, 2H), 1.75−1.03 (br, 18H); HRMS (ESI) calcd for C24H27O4N [M-H]− 407.1965, found 407.1987. Recrystallization. Preparation of single crystals for X-ray structural analysis was carried out by recrystallization at room temperature using a vapor diffusion method. A vessel containing 3−10 mg of sample dissolved in 3−6 mL of ethyl acetate was placed in a jar in which solvent to be permeated (hexane) was added. After they were left standing for several days, single crystals were obtained. X-ray Crystallography. X-ray diffraction data for the crystals were measured on Bruker APEX II CCD diffractometer. Data collections were carried out at low temperature by using liquid nitrogen. All structures were solved by direct methods SHELXS-9786 and the nonhydrogen atoms were refined anisotropically against F2, with fullmatrix least-squares methods SHELXL-97.86 All hydrogen atoms were positioned geometrically and refined as riding. Other details of refinements of the crystal structures are described in Supporting Information. Crystallographic data of the structural analyses have been deposited with the Cambridge Crystallographic Data Center, CCDC 1042494, 1042495, 1042496, 1042497, 1042498, 1042499, 1042500,

1042501, 969587, 969588, 1042502, and 1042503 for compounds 1, 2, 3, 4, 5, 10, 11, 12, 13G, 13B, 14, and 15, respectively.

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RESULTS AND DISCUSSION Crystal Structures. For pairwise packing, we employed carbamate and ester groups as the H-bond donor and acceptor, respectively. They were placed at 9- and 10-positions of anthracene. There are a few reports on polymorphs of carbamate derivatives.87−90 Cocrystals of phenyl carbamate with crown ether is known to afford several polymorphs.91 Carbamate moieties can possibly be assembled in two ways, Hbonded cyclic dimer (R22(8) H-bond motif) and chain (C(4) H-bond motif).92 The compounds possessing both carbamate and ester groups as H-bonding sites can possibly create another two types of H-bonding between the two groups. These four possible patterns of H-bonding to be created in the combination of carbamate and ester groups are presented as motifs A−D in Figure 2 together with an H-bonding cyclic

Figure 2. Schematic representation of four possible H-bonding motifs to be generated by anthracene carbamate derivatives possessing an ester group and an H-bonding cyclic motif of arenedicarboxamides for comparison: (a) a pairwisely stacked cyclic motif including H-bonding with an ester carbonyl (motif A), (b) a pairwisely stacked cyclic motif including H-bonding with an ester oxygen atom (motif B), (c) a dimeric motif of carbamates (motif C), (d) a linear array of carbamates (motif D), and (e) a one-dimensional tape of arenedicarboxamides.

motif of arenedicarboxamides.93,94 Motifs A and B are the Hbonding pattern to furnish the stacked anthracene pair in which carbamate and ester groups function as an H-bonding donor and an acceptor, respectively. Motif A type H-bonding is more favorable than that of motif B, because the more favorable Hbonding acceptor, the carbonyl oxygen atom, is involved in the former one. Motifs C and D correspond to the aforementioned H-bonding motifs of carbamates, H-bonded cyclic dimer and chain, respectively. Face-to-face overlap of arenes generated by C



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is segregated by alkyl substituents located on and beneath of the pair. These two parts, the stacked anthracene pair and the alkyl zone, are piled up alternately to create a column of pairwise packing. The structural difference between the two is in the orientation of the carbamate groups. The carbonyl of the carbamate group of 1 directs outward from the anthracene ring while that of 2 directs inward. This difference in the orientation of the carbamate groups and also the difference in the volume of the substituents result in a differing layer distance between the stacked pairs, 7.88 and 5.33 Å for 1 and 2, respectively. The CH/O interaction between the carbonyl oxygen atom and the hydrogen atom of the anthracene moiety in close proximity takes place as intercolumnar interaction. The well overlapped anthracene π-plane in the stacked pair is presented in Figure 4c and d for 1 and 2, respectively. Another type of pairwisely stacked anthracene fluorophore with motif B was found in the crystal structure of 11, the Boc (t-butoxycarbonyl) derivative with propyl ester. Its single crystals were obtained with space group P21/c. In this case, H-bonding is created between the NH hydrogen atom of the carbamate and the oxygen atom of the ester with the N···O atomic distances of 3.12 Å to afford a stacked dimer of 11 (Figure 5a). Stacked anthracene pairs are arranged orthogonally in an alternating way to give a herringbone structure. A top view of the well stacked anthracene pair is presented in Figure 5b. The distance between the two anthracene rings is 3.40 Å. Motif C (R22(8) H-bond motif), a cyclic dimer of carbamates, was found in the crystal structures of 3, 10, 12, 13B, 14, and 15. Figure 6 shows their packing diagrams and top views in which the degrees of overlapping of two adjacent anthracene rings are presented. Packing patterns of 3, 10, and 12 are almost identical (Figure 6a−c). All of them possess Boc groups in carbamate moieties. The carbamate and ester groups are placed in syn-positions. Even though the H-bonded dimer itself in motif C is not the stacked anthracene pair, the packing of the H-bonded dimers brings about stacked anthracene pairs. Two neighboring anthracene moieties of different dimers stacked together. The distances between the two stacked anthracene rings are 3.53, 3.46, and 3.32 Å for 3, 10, and 12, respectively. Stacked anthracene pairs are segregated from others. Alkyl substituents play an important role in this segregation. Similar to the cases of motifs A and B, these alkyl substituents are located on and beneath the anthracene pairs to segregate the pairs. In contrast to these carbamates with an ester group, monocarbamate 14 and dicarbamate 15 showed different packing structures with motif C. The packing of 14 is very simple. Owing to a monosubstitution of the anthracene moiety, it can create continuous array of H-bonded dimer via stacking of adjacent anthracene moieties (Figure 6d). The distance between the two stacked anthracene rings is 3.43 Å. Two tert-butyl groups occupy the space between the two stacked anthracene pairs. Substitution of the anthracene ring with two carbamate groups changes the way of packing completely. The molecule has the two same H-bonding sites. The motif C type H-bonding affords continuous H-bonding network. Because of steric hindrance originates in an anthracene ring, the molecules are H-bonded in an orthogonal way. Oblique columns of stacked anthracene moieties are created with the layer distance of 3.34 Å. These columns are arranged orthogonally in an alternating way. The overlapping of anthracene rings is not so efficient compared to others. Moreover, pairwise packing of anthracene ring could not be achieved in this substitution pattern.

H-bonding was examined by Lewis and co-workers by utilizing secondary arenedicarboxamieds.93,94 Out of three H-bonding motifs they found, one of them showed face-to-face overlap of arenes (Figure 2e). In this case, the layer distance between the two adjacent arene planes was ∼5 Å. In our case, the layer distance between the stacked two anthracene moieties is expected to be ∼3.5 Å, which is a typical π−π stacking distance of arenes. This difference originates in the difference in the number of atoms involved in H-bonding between the two arene planes. Three atoms are involved between two π-planes in the case of the arenedicarboxamides, while two atoms are involved in the present case. The arenedicarboxamides afford a onedimensional tape. In contrast, the carbamates give a stacked pair in an isolated manner. Figure 3 shows the chemical structures of anthracene derivatives 1−13 possessing carbamate and ester moieties we

Figure 3. Chemical structures of anthracene carbamates examined in this study.

examined in this study together with mono and dicarbamates, 14 and 15, respectively, for a comparative study. Out of 15 compounds examined, 1−5 and 10−15 gave single crystals suitable for X-ray analysis by recrystallization from ethyl acetate/hexane. Their crystal data are summarized in Table 1. The rest of the compounds, 6−9, gave crystals but they were not appropriate for single crystal X-ray analysis. All four types of H-bonding motifs were observed. Pairwise-packing motifs A and B are essential in the crystal structures of 1 and 2 and 11, respectively. Motif C is involved in the crystal structures of compounds 3, 10, 12, 13B, 14, and 15. Interestingly, these compounds except 15 afforded pairwisely stacked anthracene moieties via π−π stacking even though H-bonding between carbamate groups did not furnish the pair directly. Linear array of H-bonded carbamate groups (motif D) takes place in the crystal structures of 4 and 5. Motif D is inappropriate for pairwise packing. Figure 4 shows the crystal structures of 1 and 2. Both of them exhibit a similar structure with P1̅ space group. The carbamate and ester groups are in syn-relation to establish the motif A type H-bonding between them although the orientation of the carbamate moieties are different. Their conformations are cis,trans and trans,trans for 1 and 2, respectively. Two anthracene moieties were stacked with the distances of 3.55 and 3.43 Å for 1 and 2, respectively, assisted by H-bonding between the carbamate and ester groups with the N···O atomic distances of 2.92 and 3.05 Å for 1 and 2, respectively (Figure 4a and 4b). The H-bonded anthracene pair D



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Table 1. Crystallographic Data for Anthracene Carbamates formula crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Dc (Mg m−3) Z T (K) R1, [I > 2σ(I)] R2 [I > 2σ(I)] H-bonding motif

1

2

3

4

C22H23NO4 triclinic P1̅ 9.5065(8) 9.5969(8) 11.3468(9) 101.9170(10) 108.1790(10) 95.8050(10) 946.95(13) 1.282 2 173 0.0592 0.1528 A

C20H19NO4 triclinic P1̅ 9.3609(10) 10.1496(11) 10.7978(12) 70.3590(10) 81.1010(10) 63.0850(10) 861.54(16) 1.300 2 173 0.0536 0.1601 A

C22H23NO4 triclinic P1̅ 9.9458(14) 10.0520(14) 10.5202(15) 78.222(2) 68.714(2) 81.881(2) 956.8(2) 1.268 2 173 0.0450 0.1163 C

C28H33NO4 orthorhombic P212121 9.1569(10) 9.3909(10) 28.384(3) 90 90 90 2440.8(5) 1.218 4 173 0.0456 0.0987 D 13G

5 formula crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Dc (Mg m−3) Z T (K) R1, [I > 2σ(I)] R2 [I > 2σ(I)] H-bonding motif formula crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Dc (Mg m−3) Z T (K) R1, [I > 2σ(I)] R2 [I > 2σ(I)] H-bonding motif

C22H23NO4 orthorhombic P212121 8.3291(7) 9.2616(8) 24.507(2) 90 90 90 1890.5(3) 1.284 4 173 0.0387 0.0966 D

10 C21H21NO4 triclinic P1̅ 9.8643(6) 10.0722(6) 10.5475(6) 68.9190(10) 68.6080(10) 72.8500(10) 894.14(9) 1.305 2 173 0.0416 0.1139 C 13B C27H25NO4 triclinic P1̅ 6.3637(5) 12.4181(9) 14.4825(10) 82.3650(10) 86.1360(10) 81.0290(10) 1119.18(14) 1.269 2 173 0.0413 0.1163 C

11

12

C23H25NO4 monoclinic P21/c 9.4778(8) 9.5824(8) 21.3616(17) 90 95.2250(10) 90 1932.0(3) 1.305 4 173 0.415 0.1035 B 14



The motif D type H-bonding pattern was found in the crystal structures of 4 and 5. Recrystallization of 4 and 5 afforded single crystals both with P212121 space group. The X-ray structure of 4 is shown in Figure 7a. Similar to motif B, the carbamate and ester groups are placed in anti-position to establish the motif D type H-bonding between them. A chaintype H-bonding between carbamate moieties with C(4) Hbond motif is involved in the crystal structure of 4 with the N··· O atomic distance of 2.89 Å as shown in Figure 7a. The

C27H25NO4 monoclinic P21/n 18.3980(11) 13.5344(8) 18.9760(11) 90 108.0070(10) 90 4493.7(5) 1.264 8 173 0.0406 0.1005 A 15 C24H28N2O4 monoclinic C2/c 25.558(2) 5.7536(6) 18,2559(18) 90 125.0430(10) 90 2197.9(4) 1.234 4 173 0.0363 0.0971 C

anthracene moieties and the menthyl groups are aligned in an anti-parallel manner. In this packing, it is difficult to achieve stacking of anthracene rings. A similar packing was observed for 5 which is shown in Figure 7b. In this case, chiral crystals were obtained from an achiral molecule by recrystallization. The Hbonding pattern is the same as that of 4. The H-bonding between the carbamate moieties is furnished with the N···O atomic distance of 2.83 Å (Figure 7b). Their anthracene moieties are arranged to create helical columnar array in a chiral E



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distance of 3.03 Å. Two anthracene moieties are stacked with a distance of 3.51 Å (Figure 8a). The top view of the pair (Figure 8b) shows the well overlapped anthracene moieties. Its packing diagram shows that each pair is isolated by bulky tert-butyl group (see Figure S11 in Supporting Information). Conformational change of the carbamate moiety from anti to syn altered the way of H-bonding. The cyclic dimer of 13B is afforded by H-bonding with N···O atomic distance of 2.85 Å (Figure 8c). There is a slight overlap between the two neighboring anthracene moieties (Figure 8d). But the degree of overlapping is far less to compare with that of 13G. The distance between the two planes where the anthracene rings are located is 3.4 Å. The large difference in the degree of overlapping of anthracene moieties between polymorphs 13G and 13B brings about the difference in the color of fluorescence.2d Fluorescence Study. Monocarbamates possessing ester groups 1−13 are nonluminescent in ethanol solution. Monocarbamate 14 and dicarbamate 15 show very faint fluorescence in ethanol solution. However, all of them become luminescent in solid state. Figure 9a shows the photographs of the fluorescence of 3 irradiated by UV light (365 nm) as a representative example of the AIE phenomenon. Crystals of 3 exhibited green fluorescence at room temperature (Figure 9a, A), while it was nonfluorescent in solution. For comparison with the fluorescence in a frozen glass, an EPA (diethyl ether/ isopentane/ethyl alcohol = 5:5:2) solution of 3 was subjected for irradiation. The nonluminescent EPA solution at room temperature (Figure 9a, B) became luminescent with blue fluorescence when it was frozen by liquid nitrogen (Figure 9a, C). The color of the fluorescence was a typical of anthracene monomer emission. Because of its frozen conformation in a glassy EPA, quenching of the excited state of 3 was suppressed by the restricted rotation of the carbamoyl group.95−100 As a result, monomer emission was observed. It is interesting that three kinds of states can be generated regarding its fluorescence, excimer or dimer emission, monomer emission, and a nonemissive state. To investigate how the packing pattern of anthracene moieties effects on their luminescence, we examined absorption and fluorescence spectra of the carbamates. Figures 9b−d show the absorption and fluorescence spectra of 2−4 in ethanol and in solid state, respectively, as representative examples of the crystals exhibiting monomer and dimer emission. Carbamates 2 and 3 showed green fluorescence corresponding to their dimer emission while blue fluorescence from its monomer was observed for 4. Absorption and emission spectra in ethanol are indicated by blue dotted and solid lines, respectively and those in solid state are indicated by green dotted and solid lines, respectively. Obviously, no fluorescence was observed in ethanol. The absorption spectra in solid state were measured in a reflective mode. As shown in Figure 9b−d, absorption spectra of these carbamates in ethanol are typical of the anthracene fluorophore showing vibronic bands. In contrast, their absorption spectra in solid state show a broad band. Their end absorption wavelength is longer than that in ethanol because of the overlapping of anthracene moieties. The results indicate that excimer-like emission can be observed in their solid state. Interestingly, they were nonluminescent in ethanol. We presume that the RIR effect caused by the carbamate group is the possible reason for their nonluminescent properties in solution. In contrast to the nonluminescent nature of them in solution, they became luminescent in solid state and gave featureless and broad fluorescence spectra. The occurrence of

Figure 4. Pairwise packing of 1 and 2 showing H-bonding motif A: (a and b) Packing diagrams of 1 and 2, respectively, and (c and d) top views of stacked pair of 1 and 2, respectively, to represent the degree of overlapping of two anthracene rings. Blue dotted lines indicate Hbonding.

Figure 5. Pairwisely stacked crystal structures of 11 showing Hbonding motif B: (a) a packing diagram and (b) a top view. Blue dotted lines indicate H-bonding.

manner. Definitely, motif D is not suitable for the pairwise packing. In the course of the study, we found an example of the formation of polymorphic crystals based on the way of Hbonding. Two polymorphic crystals 13G and 13B were obtained by recrystallization of 13 from ethyl acetate/hexane by slow evaporation of solvent at ambient temperature. They exhibited different color of fluorescence by uv (365 nm) irradiation, green and blue for the former and the latter crystals, respectively. On the basis of the color of fluorescence these two types of crystals were collected separately. Crystal 13G was obtained as a major form. Single-crystal X-ray analysis of them showed that they were polymorphs and had different types of H-bonding. Type A and C motifs were observed in 13G and 13B, respectively. Figure 8 shows their crystal structures. A pairwise packing of two anthracene moieties is observed in 13G. This pair is created by H-bonding with N···O atomic F



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Figure 6. Crystal structures of 3, 10, 12, 14, and 15 with motif C together with top views of stacked anthracene pairs for 3, 10, 12, and 14, and a continuous stacking of anthracene moieties for 15: (a) 3, (b) 10, (c) 12, (d) 14, and (e) 15. Blue dotted lines indicate H-bonding.

are dimer emission. As described in the section of crystal structures, carbamates 1 and 3 took a pairwise packing. This well-overlapped packing of the anthracene pair contributes to the observed dimer emission. H-bonding is playing an important role in this packing. In the case of 4, which involves motif D, overlapping of anthracene moieties is inefficient confirmed from its single crystal X-ray analysis. No pairwise packing was observed. This reflects largely on its solid-state fluorescence. Its fluorescence maximum wavelength in solid state (459 nm) is fairly shorter than those of 1 and 3. Whether the observed long and short wavelengths of fluorescence were derived from dimer and monomer emission, respectively, was attested by the measurement of lifetimes of excited species. Table 2 shows emission maximum wavelengths, fluorescence lifetimes, and quantum yields of fluorescence of all the compounds examined including the ones which did not afford single crystals suitable for X-ray analysis. It is clear that carbamates 1−3, 10, 11, and 13G with longer fluorescence wavelength maxima (479−521 nm) possess longer fluorescence lifetimes (59−135 ns) indicating that their fluorescence should be derived from the dimer. In contrast, carbamates 4, 5, 9, and 13B with shorter fluorescence maximum wavelengths (458−

Figure 7. Packing diagrams of carbamates 4 and 5 with motif D. The H-bonding networks of 4 (a) and 5 (b) in which blue dotted lines indicate H-bonding between the carbamate moieties associated with the N−O atomic distances.

fluorescence originates in the disruption of RIR in solid state. The fluorescence maxima at 494 nm for 1 and 3 are rather longer than that of anthracene itself, which indicates that they G



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Table 2. H-Bonding Motif and the Solid-State Photoluminescent Properties

Figure 8. Two modes of H-bonding in crystal structures of polymorphic 13G and 13B. A side view (a) and a top view (b) of pairwisely stacked 13G, a side view of H-bonded pair of 13B (c), and a top view of two neighboring molecules in different pairs of 13B (d) to show the degree of overlapping of two anthracene moieties. Blue dotted lines indicate H-bonding associated with the N−O atomic distances.

compound

motif

λem (nm)b

lifetime (ns, (χ2))

ΦPLc

1 2 3 4 5 6 7 8 9 10 11 12 13Gd 13Bd 14 15

A A C D D a a a a C B C A C C C

494 501 494 459 458 471 466 491 462 500 479 463 521 466 475 468

59 (1.12) 84 (1.04) 90 (1.21) 3.7 (1.22) 4.4 (1.10) 29, 3.6 (1.23) 17, 4.3 (1.23) 20, 3.9 (1.24) 6.7 (1.24) 76 (1.19) 70 (1.06) 36 (1.25) 135 (1.09) 3.1 (1.23) 31, 3.1 (1.14) 11, 3.5 (1.06)

0.13 0.10 0.25 0.056 0.13 0.080 0.027 0.14 0.014 0.44 0.32 0.34 0.38 0.077 0.033 0.14

a

Not determined because of the lack of their single crystals suitable for X-ray crystallographic analysis. bFluorescence emission maximum wavelength. cAbsolute quantum yield of photoluminescence. dPolymorphic crystals.

carbamates were examined. Figure 10 shows excitation and fluorescence spectra of 13G and 13B as a representative

Figure 10. Normalized excitation and emission spectra of (a) polymorphic crystals 13G (solid line) and 13B (dotted line) and (b) PMMA polymer films containing 1 wt % (dotted line) and 50 wt % (solid line) of 13. Insets in the figure show the photographs of the crystals and the polymer films under UV (365 nm) irradiation.

example. Excitation spectra of 13G and 13B are not identical (Figure 10a). Excitation spectrum of 13G shows longer wavelength than that of 13B. The results indicate that fluorescence derived from 13G is dimer emission rather than excimer emission and that from 13B is monomer emission. If the emission is derived from its excimer, the excitation spectra should be identical to that of monomer emission. From the fluorescence lifetime of 13B, it is also obvious that its fluorescence is derived from the monomer. This difference can easily be understood from their crystal structures. Dimer emission can be observed preferentially from 13G in which two anthracene moieties are well stacked pairwisely. However, the degree of overlapping is not so efficient in the crystal structure of 13B resulting in the observation of monomer emission. There is a significant difference in the quantum yields of their fluorescence (Φ) in solid state. A relatively large value of Φ (0.38) was observed for 13G, while that of 13B was 0.077. Since 13 is nonemissive in solution, we do not have its

Figure 9. AIE behavior of 3. (a) Fluorescence of powdery crystals at room temperature (A), an EPA solution at room temperature (B), and an EPA glass frozen with liquid nitrogen (C) irradiated by UV light (365 nm), and absorption and fluorescence spectra and fluorescence microscope photographs (insets) of crystals 2 (b), 3 (c), and 4 (d). Blue and green dotted lines indicate the absorption spectra in ethanol (concentration = 1.1 × 10−4 M) and in solid state, respectively, and blue and green solid lines indicate the fluorescence spectra in ethanol (concentration = 1.1 × 10−6 M) and in solid state, respectively.

466 nm) showed shorter fluorescence lifetimes (3.1−6.7 ns) corresponding to their monomer emission. Emissions of 6−8 contained two types of excited species originated in monomer and dimer. To differentiate whether the long wavelength emission was derived from excimer or dimer, excitation spectra of the H



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Figure 11. Phase transition from 13B to 13G. (a) Photographs of fluorescence color change of crystalline 13B during 4 min of heating at 160 °C. Excitation wavelength: 365 nm. (b) The CIE xy chromaticity diagram of 13B and 13G. (c) The powder XRD pattern of 13B before (blue line) and after heating for 15 min at 160 °C (green line). The diffraction patterns indicated with black and red lines correspond to those simulated from single crystal X-ray diffraction analysis data of 13B and 13G, respectively. (d) DSC charts of 13B and 13G (heating rate = 3 °C/min).

From the fluorescence study of the carbamates, it is obvious that the degree of overlapping of anthracene moieties in the pair affects the fluorescence wavelengths and lifetimes. This is recognized that they are proportional to the degree of overlapping of π-planes of fluorophores.106 We attempted to figure out such relations based on the overlapping areas of the two facing anthracene π-planes estimated from their crystal structures. Figure 12a shows the relation between the degree of

authentic monomer emission in a diluted condition. Therefore, we examined the emission of 13 dispersed in a polymer film. Inhibition of the intramolecular rotation and stacking of anthracene moieties can be achieved in a polymer film with sparsely dispersed 13. A film of PMMA containing 1 wt % of 13 was prepared by evaporation of solvent from THF solution of PMMA and then it was subjected for photoluminescence study. The film showed blue fluorescence with maximum wavelength at 463 nm at ambient temperature with a quantum yield and lifetime of 0.44 and 8.6 ns, respectively. The results indicate that its fluorescence is derived from its monomer. Its fluorescence maximum wavelength and excitation spectrum are similar to those of 13B. Therefore, we can further confirm that the emission from 13B is monomer emission. In contrast, a polymer film containing 50 wt % of 13 showed green fluorescence with a maximum wavelength at 481 nm (Figure 10b). The film showed two excited species with lifetime of 58 and 6.7 ns indicating that both dimer and monomer emission are involved. Its fluorescence quantum yield is 0.18 which is smaller than that of the film dispersed with 1 wt % of 13. We are curious about whether phase transition exists between the two polymorphs. The phase transition from 13B to 13G was examined by heating a single crystal of 13B under fluorescence microscope observation. The color of fluorescence of 13B changed from blue to green at 160 °C. Figure 11a shows this fluorescence color change. It took about 4 min to change the color of the crystal completely from blue to green. The change did not occur all at once. It started from the end of the crystal. In the intermediate state, a part of the crystal emits green fluorescence. In the beginning of the change, a fluidic phase appeared first on the surface of the crystal and immediately, it was converted to the phase with green fluorescence. Reasonable interpretation of the phenomenon is that 13B melts to its isotropic phase and then the subsequent rapid phase transition occurs to give 13G. Figure 11b shows the CIE xy chromaticity diagram of 13B and 13G. The XRD pattern of powdered 13B after heating (15 min at 160 °C) was different from that of 13G (Figure 11c). The results indicate that the phase transition is not single-crystal-to-single-crystal transformation. The examples of fluorescence color change of crystals upon heating because of polymorphic phase transfer are very limited.101−105 DSC charts of 13G and 13B are presented in Figure 11d. When 13B was heated, a small endothermic peak (1.5 cal/g) was appeared at 146−147 °C, which corresponds to the phase transition from 13B to 13G. Both of them showed the peaks corresponding to their melting points at 178−180 °C.

Figure 12. Correlations between the degree of π-overlapping in the anthracene pair and (a) fluorescence maximum wavelengths, (b) fluorescence lifetimes, and (c) percentages of dimer emission, respectively.

overlapping of two anthracene planes and fluorescence maximum wavelengths. Even though the colinearity between them is not so good, still we can find meaningful correlation between them. Similar correlation is also observed between the degree of π-overlapping and their fluorescence lifetimes. Another interesting correlation is between the degree of πoverlapping and the percentage of dimer emission. Figure 12c indicates the existence of the threshold to generate dimer emission. Possibly, carbamates, 6−8, which involve monomer and dimer emissions, have the degree of π-overlapping below the threshold. AIE Phenomenon in Solution. Since our carbamates are nonluminescent in various organic solvents, such as ethanol, ethyl acetate, chloroform, THF, and DMSO, at room temperature and become luminescent in a frozen glassy state, we examined its AIE nature in solution. It is known that the aggregation of 9,10-diarylvinylanthracene derivatives is effected by the alkyl chain length and linking position of their endo groups.107−110 As a result, the molecular backbone conformation and intermolecular stacking structure should be significantly affected, which brings about changes of their optical properties. In our case, the bulky carbamate substituent showed restricted rotation as indicated by the broad proton signals in their 1H NMR spectra at room temperature (Supporting Information, Figure S16). This restricted rotation should be the I



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Author Contributions

cause of nonluminescent properties of the antracene-based carbamates. Their excited energies could be compensated with this restricted rotation. Figure 13 shows the fluorescence

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported in part by a Grant-in-Aid for Scientific Research from Japan Society for the Promotion of Science (JSPS) (No. 2541008). S.H. is grateful for JSPS Research Fellowships for Young Scientists.

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Figure 13. AIE of 13. (a) Fluorescence spectra and (b) photographs of fluorescence color change (excitation wavelength = 365 nm) in an aqueous ethanol solution (1.0 × 10−4 M) with varying amounts of water content.

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spectra of 13 in an aqueous ethanol solution with varying ratio of water content as a representative example. Nonfluorescent solution of 13 in ethanol became fluorescent by the addition of water exceeding 70 vol %. Its intensity increased with increasing amount of water content. The color of this AIE is similar to that of the fluorescence of 13G, which indicates that the overlapping of anthracene moieties is efficient and conformational mobility of the molecule is restricted in the aggregates. Its excitation spectrum indicates that its fluorescence is dimer emission similar to that observed in 13G.

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CONCLUSIONS We have demonstrated that anthracene derivatives possessing a carbamate and an ester group as an H-bonding donor and an acceptor, respectively, can generate pairwise packing of anthracene fluorophores in their crystal structures. These compounds are nonluminescent in organic solvents. However, they become luminescent with dimer emission in solid state owing to the pairwise packing in which an anthracene pair is segregated spatially from other pairs. They also show the AIE phenomenon in aqueous solution. The degree of π-overlapping in the anthracene pair affects fluorescence maximum wavelengths, fluorescence lifetimes, and percentages of dimer emission. The efficient overlapping can bring about longer fluorescence wavelengths and longer lifetimes, and the efficient generation of dimer emission. Our designing of organic crystal structures with pairwise-packing is useful for photoluminescence, especially for dimer emission in solid state. This concept can be applied to other fluorophoric compounds.

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ASSOCIATED CONTENT

S Supporting Information *

Tables of distances and angles of intermolecular hydrogen bonds, ORTEP diagrams, X-ray crystallographic information files (CIF) of 1−5, 10−12, 13G, 13B, 14, and 15, IR spectra (νNH and νCO region) of 1−9 in solid state (KBr pellet) and in solution (CHCl3). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-43-290-3420. J



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