Structures of Bifunctional Molecules Containing Two Very Different

ARTICLE pubs.acs.org/crystal

Structures of Bifunctional Molecules Containing Two Very Different Supramolecular Synthons: Carboxylic Acid and Strong π 3 3 3 π Stacking 1,8-Naphthalimide Ring Daniel L. Reger,* Agota Debreczeni, Jacob J. Horger, and Mark D. Smith Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States

bS Supporting Information ABSTRACT: A series of molecules containing a carboxylic acid and a 1,8-naphthalimide group joined by different linkers (HLC1 = CH2; HLC2 = CH2CH2; HLC3 = CH2CH2CH2; HLophen = ortho-C6H4; HLC4 = para-C6H4; HLala = S-CHCH3) have been prepared and structurally characterized. The structures of HLC1, HLC3, and HLala are similar, with alternating hydrogen bonding of the carboxylic acids and π 3 3 3 π stacking interactions of the naphthalimide groups assembling the molecules into parallel chains that are linked into sheets by a second set of π 3 3 3 π stacking interactions. Hydrogen bonding and π 3 3 3 π stacking interactions of the naphthalimide groups also assemble HLC2 into chains, but the chains are alternately oriented at nearly right angles causing the interchain π 3 3 3 π stacking interaction to organize the chains in an open three-dimensional structure. Three of these open structural units interpenetrate forming a unique three-dimensional network. The rigid ortho-arene linker in HLophen directs the orientation of the π 3 3 3 π stacking interaction of the naphthalimide rings to be at 60°; when combined with the hydrogen bonding interactions helical one-dimensional chains form that pack into a unique rhombohedral architecture. In the structure of HLC4 3 DMF, each acid group is hydrogen bonded with the dimethylformamide (DMF) molecule; the structure contains one-dimensional ribbons supported only by the π 3 3 3 π stacking interactions from the 1,8-naphthalimide groups. All six molecules show fluorescence in the 432449 nm region. Overall these structural studies show that the 1,8-naphthalimide supramolecular synthon is extremely versatile because it can simultaneously enter into multiple noncovalent interactions.

’ INTRODUCTION The use of molecules that contain supramolecular synthons as building blocks to assemble functional materials and network solids through noncovalent interactions has became an important synthetic strategy in chemistry and materials science.1 By incorporating different functionalities into the building units, interesting physical or chemical properties of the constructed networks can be achieved. Hydrogen bonding is the most predominant organizational tool in the construction of supramolecular network solids. It has been described as “the masterkey interaction in supramolecular chemistry” due to its clearly defined, reproducible, and transferable directionality properties.2 Another interaction that is a recurrent motif in crystalline solids is π 3 3 3 π stacking between aromatic rings. Although this interaction is not as directional as hydrogen bonding, an order of stability in the interaction of two π systems has been well established: π-deficient π-deficient > π-deficient π-rich > π-rich π-rich, where the first type of interaction can approach the importance of hydrogen bonding in a properly chosen aromatic system.3 In the past few years, we have focused on exploiting the π 3 3 3 π stacking capabilities of the strongly π-deficient 1,8-naphthalimide group (see Scheme 1). In our initial studies, it was shown that this naphthalimide moiety incorporated into bis(pyrazolyl)methane and 2,20 -bipyridine ligand systems leads to association r 2011 American Chemical Society

into dimers of metal complexes of the ligands. Importantly, we were able to show that these interactions were substantial, as they are observed not only in the solid state but also in solution.4 In addition, the silver(I) complexes of the bis(pyrazolyl)methane ligands form interesting coordination polymers supported solely by the π 3 3 3 π stacking.4b Recently, we have prepared bifunctional molecules that contained both a carboxylic acid and a 1,8-naphthalimide π-stacking synthon. One initial goal of these studies was to utilize these molecules, after deprotonation of the acid group, as ligands in the construction of metalorganic framework (MOF) type architectures.4d,e Initially, two carboxylate ligands (LC2, LC3, see Scheme 1) were prepared and used to form complexes with the paddlewheel Cu2(O2CR)4 type secondary building unit (SBU) core.4d Structural investigations of these complexes demonstrated the formation of complex supramolecular structures dominated by the π 3 3 3 π stacking of the 1,8naphthalimide, indicating the importance and versatility of this supramolecular synthon. In addition to the structural investigations of these metal complexes, we have now examined the extended network solids that can be built directly from these new bifunctional molecules Received: May 19, 2011 Revised: July 18, 2011 Published: August 03, 2011 4068

dx.doi.org/10.1021/cg200636k | Cryst. Growth Des. 2011, 11, 4068–4079

Crystal Growth & Design

ARTICLE

by self-assembly through hydrogen bonding interactions of the carboxylic acid groups combined with the π 3 3 3 π stacking interactions of the 1,8-naphthalimide rings. The unique feature of these architectures is that they are assembled by two very different but relatively strong noncovalent forces. Further, we wanted to investigate how the 1,8-naphthalimide rings would enter into different types of π 3 3 3 π stacking interactions in the crystalline structures of these bifunctional materials. In the construction of the copper architectures, we observed that modification of the length of the link between the carboxylate donor group and the 1,8-naphthalimide ring greatly influenced the supramolecular architecture of the copper compounds. How these types of structural changes would influence the solid state structures of the carboxylic acids was of interest. To fully develop these investigations, in addition to the two acids listed above, we have prepared and investigated the solid state architecture of four additional carboxylic acid/1,8-naphthalimide containing molecules, which contain additional types of organic linkers between the two functional groups (Scheme 1). The enantiopure HLala was studied because we were interested in how the chiral center would impact structure, an addition that makes it trifunctional. Thus, we report the structure analysis of five bifunctional and one trifunctional molecule in order to determine how the two very different supramolecular synthons in these molecules will direct

the self-assembly of highly organized network solids as the linkers between the two are changed.

’ EXPERIMENTAL SECTION General Considerations. All reactants were used as purchased from Aldrich. The 1H NMR spectra were recorded on a Varian Mercury VX 300 spectrometer; the chemical shifts are reported in ppm and are referenced to the protonated solvent residual. The 13C NMR spectra were recorded on a Bruker Avance DRX 400 spectrometer, and the chemical shifts in ppm were referenced to residual deuterated solvent signal. The attached proton test (APT), gradient heteronuclear single quantum correlation (HSQC), and heteronuclear multiple bond correlation (HMBC) experiments were performed on the same Bruker Avance DRX 400 spectrometer. Mass spectrometric measurements were obtained on a VG 70S instrument. The fluorescence spectra were recorded on a Perkin-Elmer LS 55 fluorescence spectrometer. Ground solid samples were used in a 4 nm cell. Elemental analyses were performed by Robertson Microlit Laboratories (Ledgewood, NJ). HLC3 and HLC2 were synthesized as previously reported.4d Synthesis of 2-(1,8-Naphthalimido)ethanoic Acid, (HLC1). A dimethylformamide (DMF) (100 mL) solution of 1,8-naphthalic anhydride (1.98 g, 10 mmol) and 2-aminoethanoic acid (0.75 g, 10 mmol) was heated at reflux overnight. Upon addition of the hot reaction mixture to ice and cold water, the product, HLC1, precipitated. The resulting white precipitate was filtered, washed with diethylether (50 mL), and air-dried to yield 2.10 g (8.2 mmol, 82%). Anal. Calcd. (Found) for C14H9NO4: C 65.88 (65.99), H 3.55 (3.57), N 5.49 (5.40). HRMS: Calcd. for C14H9NO4 256.0610, found 256.0617. 1H NMR (CDCl3, 300 MHz) δ 8.64 (d, J = 7.5 Hz, 2H, napht), 8.25 (d, J = 8.1 Hz, 2H, napht), 7.78 (t, J = 7.8 Hz, 2H, napht), 5.01 (s, 2H, NCH2COOH). 13C NMR (DMSO-d6, 100.6 MHz): δ 169.4 (COOH), 163.1 (CdO), 134.9 (CHCCdO napht), 131.4 (CCHCH napht), 131.1 (CHCHCH napht), 127.4 (CHCHCH napht; CCCdO napht), 121.5 (CCdO napht), 41.2 (CH2). Gradient HSQC and HMBC experiments were used to make the assignment of the carbon resonances in the 13C NMR spectrum. (S)-2-(1,8-Naphthalimido)propanoic Acid, (HLala). Solid potassium hydroxide (1.12 g, 20 mmol) was added to a stirred solution of (S)-2-aminopropanoic acid (L-alanine, 1.96 g, 22 mmol) in water (25 mL). This solution was allowed to stir for 20 min, and then an ethanol solution (75 mL) of 1,8-naphthalic anhydride (3.96 g, 20 mmol)

Scheme 1. Molecules Coupling a Carboxylic Acid Group with the 1,8-Naphthalimide Ring

Table 1. Selected Crystal and Refinement Data for HLC1, HLC3, HLala, HLC2, HLophen, HLC4 3 DMF HLC1

HLC3

HLala

HLC2

HLophen

HLC4 3 DMF

formula

C14H9NO4

C16 H13NO4

C15H11NO4

C15H11NO4

C19H11NO4

C22H18N2O5

fw, g mol1

255.22

283.27

269.25

269.25

317.29

390.38

cryst syst

monoclinic

monoclinic

monoclinic

monoclinic

trigonal

monoclinic

space group

P21/c

P21/n

P21

C2/c

R3c

C2/c

T (K) a, Å

100(2) 8.3327(3)

150(2) 7.2902(4)

150(2) 7.0532(3)

294(2) 19.6828(19)

150(2) 23.9850(8)

150(2) 12.7242(6)

b, Å

18.8237(7)

11.9959(7)

14.0806(6)

9.6776(9)

23.9850(8)

20.4744(10)

c, Å

7.0193(3)

14.9283(9)

12.0234(6)

14.1849(14)

26.9081(18)

7.5919(4)

R, deg

90

90

90

90

90

90

β, deg

91.799(1)

102.195(1)

90.999(1)

114.061(2)

90

114.921(1)

γ, deg

90

90

90

90

120

90

V, Å3

1100.45(7)

1276.06(13)

1193.90(9)

2467.2(4)

13405.8(11)

1793.69(15)

Z R1, I > 2σ(I)

4 0.0408

4 0.0359

4 0.0293

8 0.0431

36 0.0351

4 0.0416

wR2, I > 2σ(I)

0.1084

0.0984

0.0756

0.1145

0.0628

0.1135

4069

dx.doi.org/10.1021/cg200636k |Cryst. Growth Des. 2011, 11, 4068–4079


ARTICLE

Scheme 2. Synthesis of Bifunctional and Trifunctional Molecules

was added. The solution was heated at reflux for 68 h, during which time the cream-colored suspension of 1,8-naphthalic anhydride dissolved. The heat was removed, and a solution of 1 M aqueous HCl (20 mL, 20 mmol) was added. The stirring was stopped, and the reaction mixture was allowed to stand undisturbed overnight. The solid product that formed was filtered and washed with 4 50 mL portions of water followed by 50 mL ice-cold anhydrous ethanol. The solid was dried under a vacuum overnight to afford analytically pure HLala, yield: 4.59 g (17.0 mmol, 85%). Anal. Calcd. (Found) for C15H11NO4: C 72.5 (72.63), H 3.95 (3.81), N 4.23 (3.48). HRMS: Calcd. for C15H11NO4 269.0688, found 269.0684. 1H NMR (CDCl3, 300 MHz) δ 8.62 (d, J = 7.3 Hz, 2 H, napht), 8.24 (d, J = 8.2 Hz, 2 H, napht), 7.77 (t, J = 7.6 Hz, 2 H, napht), 5.84 (q, J = 7.0 Hz, 1 H, R-CH), 1.71 (d, J = 7.1 Hz, 3 H, β-CH3). 13C NMR (DMSO-d6, 100.6 MHz): δ 172.5 (COOH), 164.0 (CdO), 135.8 (CHCCdO napht), 132.4 (CCHCH napht), 132.2 (CHCHCH napht), 128.5 (CHCHCH napht), 122.8 (C-CdO napht), 49.6 (R-CH), 15.7 (β-CH3). The CCCdO napht resonance was identified at 128.2 ppm in the 13C NMR spectrum in CD2Cl2.

Synthesis of 2-(1,8-Naphthalimido)benzoic Acid, (HLophen). 1,8-Naphthalic anhydride (1.98 g, 10 mmol) and 2-aminobenzoic acid (1.65 g, 12 mmol) were heated at reflux in DMF (120 mL) overnight. Upon addition of the hot reaction mixture to ice and cold water the product, HLophen, precipitated. The resulting white precipitate was filtered, washed with diethylether (150 mL), and air-dried to yield 2.88 g (9.1 mmol, 91%). Anal. Calcd. (Found) for C19H11O4N: C 71.92 (71.90); H 3.49 (3.40); N 4.41 (4.37). HRMS: Calcd. for C19H11O4N 317.0688, found 317.0687. 1H NMR (DMSO-d6, 300 MHz) δ 8.52 (2d, 4H, napht), 8.12 (dd, J = 7.8 Hz, J = 1.5 Hz, 1H, phen), 7.91 (t, J = 7.8 Hz, 2H, napht), 7.77 (td, J = 7.6 Hz, J = 1.6 Hz, 1H, phen), 7.62 (td, J = 7.6 Hz, J = 1.1 Hz, 1H, phen), 7.52 (d, J = 7.5 Hz, 1H, phen). 13C NMR (DMSO-d6, 100.6 MHz): δ 165.8 (COOH), 163.7 (CdO), 136.3 (NC phen), 134.6 (CHCHCH napht), 133.2 (CHCHCH CCOOH phen), 131.5 (CCHCH napht), 131.2 (CHCCOOH phen), 131.0 (NCCH phen), 130.8 (CHCCdO napht), 129.0 (CCOOH phen), 128.8 (CHCHCCOOH phen), 127.9 (CC CdO napht), 127.3 (CHCHCH napht), 122.4 (CCdO napht). APT, gradient HSQC, and HMBC experiments were used to make the assignment of the carbon resonances in the 13C NMR spectrum.

Synthesis of 4-(1,8-Naphthalimido)benzoic Acid, (HLC4). A DMF (120 mL) solution of 1,8-naphthalic anhydride (1.98 g, 10 mmol) and p-aminobenzoic acid (1.65 g, 12 mmol) was heated at reflux overnight. Upon addition of the hot reaction mixture to ice and cold water, the product, HLC4, precipitated. The resulting white precipitate was filtered, washed with 50 mL diethylether, and dried under a vacuum,

Figure 1. Pairs of hydrogen bonded dimers associated through π 3 3 3 π stacking into a two-dimensional sheet in the structure of HLC1 (view down crystallographic axis a direction).

yield: 2.29 g (7.2 mmol, 72%). Anal. Calcd. (Found) for C19H11O4N: C 71.92 (71.67); H 3.49 (3.42); N 4.41 (4.32). HRMS: Calcd. for C19H11O4N 317.0688, found 317.0679. 1H NMR (DMSO-d6, 300 MHz) δ 8.52 (2d, 4H, napht), 8.09 (d, J = 8.4 Hz, 2H, phen), 7.91 (t, J = 7.8 Hz, 2H, napht), 7.55 (d, J = 8.4 Hz, 2H, phen). 13C NMR (DMSO-d6, 100.6 MHz): δ 167.4 (COOH), 164.0 (CdO), 140.7 (NC phen), 135.0 (CHCCdO napht), 131.9 (CCHCH napht), 131.3 (CHCHCH napht), 131.2 (CCOOH phen), 130.4 (CHCCOOH phen), 130.0 (NCCH phen), 128.4 (CC CdO napht), 127.7 (CHCHCH napht), 123.0 (CCdO napht). Gradient HSQC and HMBC experiments were used to make the assignment of the carbon resonances in the 13C NMR spectrum. Crystallographic Studies. X-ray diffraction intensity data for each compound were measured on a Bruker SMART APEX CCD-based diffractometer system (Mo KR radiation, λ = 0.71073 Å).5 Raw area detector data reduction was performed with SAINT+ and SADABS programs.5 Direct methods structure solution, difference Fourier calculations, and full-matrix least-squares refinement against F2 were performed with SHELXTL.6 Details of the data collection are given in Table 1, while further information regarding the structure solution and refinement for each structure follows below. 4070



Figure 2. View of the same nine dimers as in Figure 1 after a 90° rotation, showing the corrugated sheet. For HLC1 final unit cell parameters were determined by least-squares refinement of 8508 reflections from the data set. The compound crystallizes in the space group P21/c as determined by the pattern of systematic absences in the intensity data. The asymmetric unit consists of one molecule. Non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms bonded to carbon were placed in geometrically idealized positions and included as riding atoms. The carboxylic hydrogen was located in a difference map and refined freely. For HLC3 final unit cell parameters were determined by least-squares refinement of 5911 reflections from the data set. The monoclinic space group P21/n was determined uniquely by the pattern of systematic absences in the intensity data. The asymmetric unit consists of one molecule. All nonhydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms bonded to carbon were placed in geometrically idealized positions and included as riding atoms. The carboxylic hydrogen is disordered over both oxygen atoms (O3 and O4) in the proportion H3/H4 = 0.40(4)/0.60(4). These occupancies were successfully refined with isotropic displacement parameters. The occupancies were constrained to sum to unity, and OH distances were restrained to be similar. For HLala final unit cell parameters were determined by least-squares refinement of 9993 reflections from the data set. The compound crystallizes in the monoclinic system. The space group P21 was determined by examination of the pattern of systematic absences in the intensity data and by the successful solution and refinement of the structure. A check for missed symmetry elements was performed with the ADDSYM program implemented in PLATON,7 which verified the space group choice. The asymmetric unit consists of two crystallographically independent but chemically identical molecules. All nonhydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms bonded to carbon were located in difference maps before being placed in geometrically idealized positions and included as riding atoms. The carboxylic hydrogen atoms were located and refined freely. Because there are no atoms heavier than oxygen in the crystal, Friedel opposites were merged during refinement and the absolute structure could not be determined from the X-ray data. Crystals of HLC2 were initially examined as low temperature (T = 150 K) but were found to be poorly crystalline. Eventually, it was established that the crystals lose crystallinity upon cooling from room temperature, resulting in visible cracking accompanied by broadening of the diffraction peaks. Reported unit cell parameters were determined by least-squares refinement of 3177 reflections from the data set collected at 294 K. The compound crystallizes in the space group C2/c as determined by the pattern of systematic absences in the intensity data and by the successful solution and refinement of the structure. The asymmetric unit consists of one molecule. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were located in difference maps before placed in geometrically idealized positions and included as riding atoms.

ARTICLE

Figure 3. Two-dimensional structure of HLC3; view down crystallographic axis b direction (60% occupancy structure). For HLophen because of the weak diffracting power of all available crystals, data were truncated at 2θ = 45°. Final unit cell parameters were determined by least-squares refinement of 2483 reflections from the data set. The compound crystallizes in the space group R3c (#167) as determined by the pattern of systematic absences in the intensity data and by the successful solution and refinement of the structure. The asymmetric unit consists of one molecule. All atoms of the molecule refined normally with the exception of the carboxylic acid group. The COOH group is rotationally disordered, causing both CO distances to appear approximately equal in the average structure. This disorder is reflected in a large prolate displacement ellipsoid for O3. Likewise a single hydrogen atom position could not be determined; two halfoccupied hydrogen atoms were located in reasonable positions and refined subject to d(OH) = 0.90(1) Å distance restraints and U(iso, H) = 1.5U(eq, O). The largest residual electron density peaks are located in the vicinity of the disordered COOH group. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms bonded to carbon were placed in geometrically idealized positions and included as riding atoms. Trial refinements in the space group R3c (#161) showed the same disorder. Final unit cell parameters for HLC4 were determined by least-squares refinement of 3605 reflections from the data set. The pattern of systematic absences in the intensity data was consistent with the space groups Cc and C2/c. C2/c was eventually confirmed. The asymmetric unit consists of half of one C19H11NO4 molecule and half of a DMF molecule, both located on a 2-fold axis of rotation. Both species are disordered around the 2-fold axis, but only the COOH group and the CHO part of the DMF molecule are affected (atoms O3 and H13). The disorder scrambles the carboxylic acid group, such that both CO distances are equal. An identical disorder was observed in the space group Cc. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms bonded to carbon were placed in geometrically idealized positions and included as riding atoms. The carboxylic hydrogen was located in a difference map and refined freely. Atoms affected by the 2-fold rotational disorder (H2, O3, H13) were refined with half-occupancy.

’ RESULTS Synthesis. The bifunctional carboxylate compounds HLC2 and HLC3 were prepared from commercially available amino acids in one-pot reactions, as previously reported.4d The compounds HLC1, HLophen, and HLC4 were obtained similarly by the reaction of 1,8-naphthalic anhydride with the corresponding amino acid in DMF (Scheme 2). These compounds have been previously synthesized for use in other types of studies but have not been investigated for the benefit of crystal engineering.8 Our modified preparations all have straightforward work-ups and 4071



ARTICLE

Figure 4. Sheet structure of HLC3 rotated 90° in the horizontal direction from Figure 3.

Figure 5. (a) Asymmetric unit in the structure of HLala showing two crystallographically independent molecules; (b) illustration of the two independent molecules superimposed. Displacement ellipsoid drawn at the 50% probability level.

proceed in high yield, with full characterization of the products presented. The trifunctional HLala was prepared by reaction of the ethanolic potassium hydroxide solution of the naturally occurring (S)-propanoic acid (L-alanine) with 1,8-naphthalic anhydride, followed by the addition of aqueous hydrochloric acid. Other research groups prepared HLala using the conditions of refluxing 1,8-naphthalic anhydride in DMSO, DMF, or pyridine.9 These reactions in DMSO or DMF, in our hands, could not reliably ensure the retention of configuration of the chiral center. The reaction conditions described here both prevent racemization of the amino acid chiral center and eliminate the need for chromatographic purification. Solid-State Structure Analysis. Crystals of HLC1 and HLC4 suitable for X-ray diffraction were obtained by slow diffusion of diethylether into a methanol or dimethylformamide solution of the compound, respectively. HLC2 was crystallized by slow evaporation from a methanol solution, and HLC3 and HLophen were crystallized by slow evaporation from a dichloromethane solution. Crystals of HLala were obtained by layering hexane onto the dichloromethane solution of the compound. In the crystalline architecture of HLC1, the carboxylic acid groups participate in strong hydrogen bonding interactions generating dimers, Figure 1, with an OH 3 3 3 O distance of 1.77(2) Å (O 3 3 3 O = 2.675(1) Å) and OHO angle of 176(2)°. These dimers are organized into chains by the π 3 3 3 π stacking of the electrondeficient 1,8-naphthalimide groups; three such chains (green, blue

Figure 6. Hydrogen bonding and naphthalimide π 3 3 3 π stacking interactions in the sheet structure of HLala (view down crystallographic axis c direction).

and orange) are pictured in Figure 1. The naphthalimide rings of the adjacent chains are involved in additional π 3 3 3 π stacking interactions generating a two-dimensional sheet structure (Figure 1). In both of these interactions, the dipole vectors of the naphthalimide groups (which run through the center of the fused aromatic rings and point toward the nitrogen atom) are oriented at 104°. This orientation coupled with the arrangement of the linker leads to corrugated sheets, Figure 2. The perpendicular distance between the aromatic rings is short at 3.40 Å and the rings are nearly parallel. The rings also have substantial overlap, as measured by the “slippage” parameter χ, which is the third side of the right triangle formed with the average perpendicular distance between the rings and the line joining the central carbon atoms of the two rings.4d In this case, χ is 1.44 Å, which is in the range of strong interactions (0.622.4 Å) as we have shown previously.4 There are no important intermolecular interactions in the third dimension, the crystallographic a direction. The overall solid state structure of HLC3 is analogous to HLC1 containing hydrogen-bonded dimers organized into a twodimensional sheet by the π 3 3 3 π stacking of the 1,8-naphthalimide groups, Figure 3. The carboxylic hydrogen involved in hydrogen bonding is disordered over both oxygen atoms in the proportion 0.40/0.60. The average OH 3 3 3 O distance is 1.80 Å (O 3 3 3 O = 2.634(1) Å) with an average OHO angle of 170°. In this case, there are two different types of π 3 3 3 π stacking interactions with similar parameters. In the chain direction, the perpendicular distance between the parallel aromatic rings is 3.42 Å with a slippage value of 1.68 Å. In the other interaction, 4072



ARTICLE

Figure 7. View of the sheet structure of HLala after a 90° rotation in the horizontal direction relative to Figure 6.

Figure 8. (a) Pairs of molecules associated through hydrogen bonds (red dots) in the crystal structure of HLC2. The dimers are organized into chains by π 3 3 3 π stacking of the naphthalimide groups oriented at 180° (green and red pair); (b) view of the same dimers after 90° rotation, showing that the two chains are oriented in alternate directions.

which assembles the chains into a two-dimensional sheet, the naphthalimide rings are 3.46 Å apart and the value of χ is 1.12 Å. In comparing the two-dimensional structure of HLC3 and HLC1, it is worth nothing that in the case of HLC3 the more flexible propylene arms allow the naphthalimide groups to be in positions where the dipole vectors are oriented at 180°; an orientation that is slightly favored.4d This orientation coupled with the arrangement of the linker leads to flat sheets (Figure 4), as opposed to the corrugated sheets found in HLC1, but again there are no important intermolecular interactions in the third dimension. In addition to the carboxylic acid and the 1,8-naphthalimide groups, the trifunctional HLala molecule contains a chiral center introduced from an enantiopure amino acid. The asymmetric unit consists of two crystallographically independent, chemically nearly identical molecules as illustrated in Figure 5a,b. Stereochemistry cannot be determined from the X-ray data because of the lack of heavy atoms; stereoconfigurations were set as “S” for both C13 and C28 because of the known stereochemistry of the starting amino acid and the structures of metal complexes of the

deprotonated form of this molecule, where this stereochemistry was determined by X-ray crystallography. Again, the HLala molecules are organized into dimers through strong hydrogen bonding interactions, with an average OH 3 3 3 O distance of 1.89 Å (O 3 3 3 O average = 2.71 Å) and OHO angle of 176°. The pairs of hydrogen-bonded dimers are assembled into two-dimensional sheets by two different types of π 3 3 3 π stacking interactions of the naphthalimide rings as shown in Figure 6. In the chain direction, the naphthalimide rings are 3.52 Å apart vertically with a χ value of 1.14 Å, indicating a substantial overlap between the rings. A second type of π-stacking extends the structure in the second dimension by assembling the onedimensional chains into sheets. In this interaction the perpendicular distance between the rings is 3.33 Å and the slippage value is 2.00 Å. In both interactions the dipole vectors of the naphthalimide rings are oriented at 129° and when coupled with the arrangement of the linker lead to flat sheets, Figure 7. The supramolecular architecture of HLC2 is more complex than the structures previously described. In this case, the hydrogen-bonded carboxylate dimers are organized into two sets of 4073



ARTICLE

Figure 9. (a) Three-dimensional network structure of HLC2 ; (b) view of the same network after a 90° rotation. The red dots represent the hydrogen bonds.

Figure 10. Complete crystal packing of HLC2, showing the three interpenetrated networks color-coded with green, blue, and orange.

4074



ARTICLE

Figure 11. ORTEP view of HLophen. Displacement ellipsoid drawn at the 50% probability level. H3 and H4 are each half-occupied.

equivalent chains by π 3 3 3 π stacking of the 1,8-naphthalimide groups. In these chains, the dipole vectors of the naphthalimide rings are oriented at 180°. Two units of each of these chains (green and red colored) are pictured in Figure 8a, where the perpendicular distance between the parallel aromatic rings is 3.40 Å with a χ value of 1.77 Å. A second type of π 3 3 3 π stacking of the 1,8-naphthalimide groups (between green and red colored rings) holds the chains together, but in contrast to the structures described above the two chains are oriented in alternate directions, as shown in Figure 8b, which pictures the chains rotated 90° from Figure 8a. In this interaction, the dipole vectors are oriented at 79.8°. The perpendicular distance is 3.65 Å with a χ value of 0.44 Å, and the tilt between the stacked rings is 3.5°. The combined effects of the hydrogen bonding of the carboxylic acid groups and the two types of π 3 3 3 π stacking of the naphthalimide moieties create an open three-dimensional network. Figure 9a shows seven units of the stacked naphthalimide rings linked into a network by the hydrogen bonds. Figure 9b shows the network rotated 90° from Figure 9a, and is the same orientation shown in Figure 8b. The hydrogen bonded dimers and the π 3 3 3 π stacked naphthalimide rings with the 180° orientation of the dipole vectors create one-dimensional chain structures. These chains are oriented at nearly a right angle and are connected into a three-dimensional structure by the π 3 3 3 π stacking interaction of the naphthalimide moieties with the dipole vectors oriented at 79.8°, assembling the chains along the c crystallographic axis. In the overall structure, there are three of these symmetry equivalent networks, which interpenetrate generating a threedimensional, 3-fold interpenetrated network structure. Figure 10 shows two dimensions of the interpenetrated network. Each of the three networks is color-coded, and the units in each network are made up by six stacked naphthalimide rings. There are no important interactions between the three networks. The structure of HLophen also shows unique features, specific to the ortho-arene linker between the naphthalimide moiety and the carboxylic acid group. An ORTEP view of the molecular structure is shown in Figure 11. The plane of the ortho-arene linker is perpendicular to the plane of the naphthalimide ring; the C1N1C13C18 dihedral angle is 91°. The carboxylic acid group is disordered with similar C19O3/O4 distances and is nearly coplanar with the arene linker (the C13C18C19O4 torsion angle is 9.8°). The HLophen molecules are self-assembled into dimers through hydrogen bonding interactions of the carboxylic acid groups. In Figure 12a, three pairs of such hydrogen-bonded dimers are illustrated. In these interactions, the average OH 3 3 3 O

Figure 12. (a) Pairs of dimers associated into helical chains by the π 3 3 3 π stacking interactions of the naphthalimide rings; (b) view of the chains down crystallographic c axis. Both disordered orientations of the COOH group are pictured.

distance is 1.71 Å (O 3 3 3 O = 2.60 Å) with an average OHO angle of 174°. The pairs of dimers are further connected into helical chains by the π 3 3 3 π stacking interactions of the naphthalimide rings. Each naphthalimide ring of the dimeric unit π 3 3 3 π stacks with naphthalimide moieties of two adjacent dimers, Figure 12a. Figure 12b pictures the same chain as 12a but rotated approximately 90° in the horizontal direction (down crystallographic axis c direction). In all of these interactions, the dipole vectors of the naphthalimide rings are oriented at 60°, and the perpendicular distance between the rings is 3.58 Å with a χ value of 0.25 Å. These helical chains are close packed generating a huge rhombohedral structure with the unit cell volume of 13406 Å3 (see Figure 13). Neighboring chains do not have any substantial π 3 3 3 π stacking interactions, but there are weaker CH 3 3 3 O interactions between a carbonyl oxygen atom of the naphthalimide rings and a CH group of the arene rings. In these interactions, the H 3 3 3 O distances are 2.44 and 2.46 Å, respectively, with CHO angles of 141° and 152°. Both right- and left-handed chains are present in the structure as pictured in Figure 14, which shows two adjacent chains (the green and light blue) from Figure 13 along the crystallographic c axis. Empty channel-like pores are formed by this close-packed arrangement of the helical chains (see Figure 13). The pores are 4.3 3.2 Å wide. HLC4 crystallizes with one DMF solvent molecule per acid. Both species are disordered; the disorder affects the ligand COOH group and CHO part of DMF. Figure 15 pictures the molecular structure of one of the two disordered forms of the compound. The naphthalimide moiety and the arene group adopt a twisted arrangement, where the C1N1C8C9 dihedral angle is 62°, as shown in Figure 15b. The carboxylic acid group is coplanar with the arene linker (the O2C12C11C10 dihedral angle is 2.2°) and is involved in hydrogen bonding interaction with the DMF solvent molecules, where the OH 3 3 3 O distance is 1.73(3) Å (O 3 3 3 O = 2.514(2) Å) with an OHO angle of 178(4)°. 4075



ARTICLE

Figure 13. Close packed structure of HLophen; view down crystallographic c axis direction.

Although the hydrogen bonding interaction of the acid groups with the DMF solvent molecules prevents dimer formation, as observed in the other five molecules above, the naphthalimide rings are involved in π 3 3 3 π stacking interactions very similar to the previous compounds, connecting the solvated HLC4 3 DMF units into one-dimensional ribbons along crystallographic axis c direction. Figure 16 pictures two such interdigitated ribbons. All the naphthalimide rings are parallel and 3.36 Å apart with the dipole vectors oriented at 180°. The slippage value is 1.80 Å, which is indicative of strong interactions. There are no other substantial noncovalent forces in the structure of HLC4 3 DMF. Weaker CH 3 3 3 π interactions between the dimethylformamide solvent molecule and the arene π system hold the ribbons together. The CH 3 3 3 arene ring centroid distance is 2.80 Å, and the CH 3 3 3 arene ring centroid angle is 139°. Fluorescence Spectral Properties of the Compounds. The solid state fluorescence excitation spectra of the six compounds are shown in Figure 17 and 18. For HLC1 the λmax,ex was at 389 nm, while for all the other compounds λmax,ex was in the range of 378380 nm. All compounds show a second λmax,ex band at 263 nm (264 nm for HLC1), and a third band at 248249 nm (243 nm for HLC4), respectively. Figurs 19 and 20 shows the fluorescence emission profiles of the compounds. HLC3 has a blue-purple fluorescence with a λmax,em at 432 nm. For the other compounds, the λmax,em was between 441449 nm, showing blue fluorescence.

’ DISCUSSION The solid state structures of six bifunctional molecules containing both a strong π 3 3 3 π stacking naphthalimide group and a hydrogen bonding carboxylic acid have been

studied in which the two functional groups are systematically separated by different linkers. The overall supramolecular structures of HLC1, HLC3, and HLala are surprisingly similar. Alternating hydrogen bonding interactions and π 3 3 3 π stacking interactions of the naphthalimide groups assemble the molecules into chains. These chains are parallel and organized into two-dimensional sheets by π 3 3 3 π stacking interactions of the 1,8-naphthalimide groups between different chains. Subtle but important differences arise in the structural parameters due to the presence of different alkyl linkers between the two functional groups. In the structure of HLC1, the short methylene (CH2) linker places the naphthalimide rings in positions where the dipole vectors in the π 3 3 3 π stacking interactions are oriented at 104°, forming corrugated sheets, as shown in Figure 2. In the case of HLC3, the two-dimensional sheets are flat because the more flexible propylene linker allows a 180° orientation of the naphthalimide dipole vectors (see Figure 4). In the structure of the enantiopure HLala, where the linker is an ethylidene (CH3CH) group, the 129° orientation of the dipole vectors of the naphthalimide rings in the two types of π-stacking interactions builds up a less corrugated sheet than in the analogous achiral HLC1. In this case, the structure is influenced by steric interactions introduced by the extra CH3 group in the chiral linker of HLala. The enantiopure center in HLala does not introduce interesting chiral features into the supramolecular structure. In contrast, metal complexes of analogous ligands have structures that contain a “chiral pocket” arrangement of the naphthalimide groups10 or have a complicated homochiral helical structure.11 In the solid state structure of HLC2, the π-stacking of the 1, 8-naphthalimide moieties along with the hydrogen bonds of the carboxylic groups generate a unique 3-fold interpenetrated threedimensional network. As in the three above structures, the π-stacking 4076



ARTICLE

Figure 15. (a) The structure of DMF solvated HLC4. The red dots represent the hydrogen bonds. (b) View of the twisted arrangement of the naphthalimide and arene rings in the molecular structure of HLC4 3 DMF. Displacement ellipsoid drawn at the 50% probability level. One disorder component shown.

Figure 14. Two complementary helical chains in the close packed structure of HLophen (view along crystallographic c axis). The purple dots represent the weaker CH 3 3 3 O interactions between two adjacent chains. Both disordered orientations of the COOH group are pictured.

of the 1,8-naphthalimide rings (with a 180° orientation of the dipole vectors) along with the hydrogen bonding of the carboxylic groups generate one-dimensional chains, but these chains are alternately oriented at nearly right angles. The chains are assembled into an open three-dimensional network by a second π 3 3 3 π stacking interaction, where the naphthalimide dipole vectors are oriented at 79.8°. Because of this open structure of one network, a complex interpenetrated structure is formed by two additional symmetry equivalent networks. This type of three-dimensional interpenetrated structure is only observed with HLC2, indicating the substantial impact of the systematic variations in the linker as investigated in this work. We note that the structure of the alcohol analogue of HLC2, N-(2-hydroxyethyl)-1,8-naphthalimide, has been briefly communicated.12 In the case of HLophen, the rigid ortho-arene linker directs the formation of a unique supramolecular architecture. The naphthalimide rings of the neighboring hydrogen bonded dimers are involved in a π 3 3 3 π stacking interaction, where the dipole vectors are oriented at 60°. This type of π 3 3 3 π stacking interaction of the naphthalimide rings, combined with the hydrogen bonding of the carboxylic acid groups, facilitates the formation of helical onedimensional chains along the crystallographic axis c direction, as can be seen in Figure 12. These individual chains are packed into a unique

rhombohedral architecture with no additional supramolecular interactions. The orientation of the naphthalimide rings prevents additional π 3 3 3 π stacking interactions, again demonstrating the importance of the stereochemical influence of the linker. In HLC4 the linear directional orientation of the para-arene arms places the naphthalimide rings of the adjacent units in positions in which the dipole vectors of the rings can form π 3 3 3 π stacking interactions at 180° angles, leading to the formation of one-dimensional ribbons. Since the carboxylic acid group of HLC4 is involved in hydrogen bonding interaction with the DMF solvent molecules, there are no other strong noncovalent interactions in the crystalline structure of this compound, and overall the supramolecular structure is reduced to the formation of onedimensional ribbons. An important result of these structural investigations is that the π 3 3 3 π stacking naphthalimide rings appear to be a more “versatile” supramolecular synthon than the carboxylic acids. While the latter certainly form strong hydrogen bonding interactions in all of the structures, that interaction occurs only once per acid group, forming dimers in five of the compounds. In a similar structural manner, the naphthalimide group links these dimers into chains. Importantly, the π 3 3 3 π stacking interaction could be viewed as forming dimers and the hydrogen bonding the chains. What is clear is that, in addition to the π 3 3 3 π stacking interaction that supports these chains, if the linker allows the proper orientation of the naphthalimide groups they can interact a second time with suitably oriented naphthalimide rings in other chains to form sheet structures, as observed in compounds HLC1, HLC3, and HLala. In HLC2, because of the alternating orientation of the chains, a complex three-dimensional structure is formed by the interchain π 3 3 3 π stacking interactions. These interchain π 3 3 3 π stacking interactions can be prevented by introducing a sterically hindered linker as in HLophen that orients the rings such as to prevent this type of interaction from taking place. In all cases reported here, the metric parameters indicate the π 3 3 3 π stacking interactions are substantial.3 The rings are parallel or nearly parallel and the ringring separations are short. In addition, the rings have substantial overlap, as indicated by the “slippage” parameter, χ, that ranges from 0.25 to 2.0, values that all are in the range of strong interactions as we have shown previously.4 In addition, the organizational ability of the acid group can be diminished as demonstrated by the structure of HLC4 3 DMF, where it is “capped” by a DMF solvent molecule, limiting its contribution to the supramolecular structure and leaving the naphthalimide group as the dominant organizing structural 4077



ARTICLE

Figure 16. Two ribbons (green and orange) in the solid state structure of HLC4 3 DMF. Both disordered orientations of HLC4 and the DMF molecule are pictured.

Figure 17. Fluorescence excitation spectra of HLC1, HLC3, and HLala in the solid state.

Figure 19. Fluorescence emission spectra of HLC3, HLC1, and HLala in the solid state with excitation at 263 nm.

Figure 18. Fluorescence excitation spectra of HLC4, HLC2, and HLophen in the solid state.

Figure 20. Fluorescence emission spectra of HLC2, HLophen, and HLC4 in the solid state with excitation at 263 nm.

motif. Structures showing “capping” of hydrogen bonding functional groups bonded to 1,8-naphthalimide rings in other bifunctional molecules have been described in recent publications by Baruah et al., including the structure of HLophen cocrystallized with pyridine.13

It is also interesting to compare these results with metal complexes of these compounds, where the carboxylic acid is deprotonated and bonded to a metal. In the case of complexes with the paddlewheel Cu2(O2CR)4 type SBU we have studied previously, the “square” architecture of the SBU in conjunction 4078


Crystal Growth & Design with the π 3 3 3 π stacking interactions of the naphthalimide rings generally leads to complex three-dimensional structures. The SBU unit, as it is designed to be, is a multidimensional “node”, whereas the acid group in the compounds reported here is only a one-dimensional link. As expected from previous solution studies of analogous compounds,14 all of these compounds show fluorescence in the 432449 nm region. Only HLC3 is shifted from the very narrow range of 441449 nm. These latter compounds are good candidates for use as fluorescent brighteners.15 It was anticipated that changes in the supramolecular structures of the compounds might impact on the fluorescence,16 but given that the types of π 3 3 3 π stacking interactions that are observed in the structures are fairly constant, these results are not unexpected. The shifting of HLC3 is likely a result of this compound being the only one of those with an alkyl substituted naphthalimide that has the rings oriented exactly head to tail in the π 3 3 3 π stacking interaction, the favored arrangement.4d It will be interesting in future publications to compare these values with values measured on metal complexes of the deprotonated acids.

’ ASSOCIATED CONTENT Supporting Information. X-ray crystallographic files in CIF format for the structural determinations. This information is available free of charge via the Internet at http://pubs.acs.org.

bS

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors acknowledge with thanks the financial support of the National Science Foundation through Grant CHE-1011736. ’ REFERENCES (1) (a) Gonzalez-Rodriguez, D.; Schenning, A. P. H. J. Chem. Mater. 2011, 23, 310. (b) Yang, W.; Greenaway, A.; Lin, X.; Matsuda, R.; Blake, A. J.; Wilson, C.; Lewis, W.; Hubberstey, P.; Kitagawa, S.; Champness, N. R.; Schroder, M. J. Am. Chem. Soc. 2010, 132, 14457. (c) Klaus, M.; Vasylyeva, V. Cryst. Eng. Comm. 2010, 12, 3989. (d) Anthonya, S. P.; Varughesea, S.; Drapera, S. M. J. Phys. Org. Chem. 2010, 23, 1074. (e) Gamez, P.; Reedijk, J Eur. J. Inorg. Chem. 2006, 29. (f) Wang, Y.; Xu, H.; Zhang, X. Adv. Mater. 2009, 21, 2849. (g) Amabilino, D. B.; Veciana, J. Top. Curr. Chem. 2006, 265, 253. (h) Aoki, K.; Nakagawa, M.; Seki, T.; Ichimura, K. Bull. Chem. Soc. Jpn. 2002, 75, 2533. (i) Russell, V. A.; Evans, C. C.; Li, W.; Ward, M. D. Science 1997, 276, 575. (j) Hathwar, V. R.; Thakur, T. S.; Guru Row, T. N.; Desiraju, G. R. Cryst. Growth. Des. 2011, 11, 616. (k) Desiraju, G. R. Chem. Commun. 1997, 1475. (l) Reddy, D. S.; Craig, D. C.; Desiraju, G. R. J. Am. Chem. Soc. 1996, 118, 4090. (m) Thalladi, V. R.; Satish Goud, B.; Hoy, V. J.; Allen, F. H.; Howard, J. A. K.; Desiraju, G. R. Chem. Commun. 1996, 401. (n) Desiraju, G. R. Angew. Chem., Int. Ed. 1995, 34, 2311. (o) Varshney, D. B.; Papaefstathiou, G. S.; MacGillivray, L. R. Chem. Commun. 2002, 1964. (2) (a) Steed, J. W.; Atwood, J. L. Supramolecular Chemistry, 2nd ed.; John Wiley & Sons, Ltd.: UK, 2009; pp 2832; (b) Braga, D.; Brammer, L.; Champness, N. R. CrystEngComm 2005, 7, 1. (c) Moulton, B.; Zaworotko, M. J. Chem. Rev. 2001, 101, 1629. (3) (a) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885. (b) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525.(c) Steed, J. W.;

ARTICLE

Atwood, J. L. Supramolecular Chemistry, 2nd ed.; John Wiley & Sons, Ltd.: UK, 2009; pp 3335. (4) (a) Reger, D. L.; Elgin, J. D.; Semeniuc, R. F.; Pellechia, P. J.; Smith, M. D. Chem. Commun. 2005, 4068. (b) Reger, D. L.; Semeniuc, R. F.; Elgin, J. D.; Rassolov, V.; Smith, M. D. Cryst. Growth Des. 2006, 6, 2758. (c) Reger, D. L.; Elgin, J. D.; Smith, M. D; Simpson, B. K. Polyhedron 2009, 28, 1469. (d) Reger, D. L.; Debreczeni, A.; Reinecke, B.; Rassolov, V.; Smith, M. D.; Semeniuc, R. F. Inorg. Chem. 2009, 48, 8911. (e) Reger, D. L.; Debreczeni, A.; Smith, M. D. Inorg. Chim. Acta 2010, 364, 10. (5) SMART Version 5.630, SAINT+ Version 6.45 and SADABS Version 2.05; Bruker Analytical X-ray Systems, Inc.: Madison, Wisconsin, USA, 2003. (6) Sheldrick, G. M. SHELXTL, Version 6.14; Bruker Analytical X-ray Systems, Inc.: Madison, Wisconsin, USA, 2000. (7) (a) LePage, Y. J. Appl. Crystallogr. 1987, 20, 264. (b) Spek, A. L. J. Appl. Crystallogr. 1988, 21, 578.(c) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht University: Utrecht, The Netherlands, 1998. (8) (a) Li, H.; Jiang, Z.; Wang, X.; Zheng, C. Synth. Commun. 2006, 36, 1933. (b) Li, H.-T.; Jiang, Z.-Q.; Zheng, J.; Wang, X.; Pan, Y.; Wang, F.; Yu, S.-Q. Res. Chem. Intermed. 2006, 32, 43. (c) Gawronski, J.; Kwit, M.; Gawronska, K. Org. Lett. 2002, 4, 4185. (d) Gawronski, J.; Grajewski, J. Tetrahedron: Assymetry 2004, 15, 1524. (9) (a) Jursic, B. S.; Patel, P. K. Tetrahedron 2005, 61, 919. (b) Liang, C.; Collet, F.; Robert-Peillard, F.; M€uller, P.; Dodd, R. H.; Dauban, P. J. Am. Chem. Soc. 2008, 130, 343. (c) Collet, F.; Dodd, R. H.; Dauban, P. Org. Lett. 2008, 10, 5473. (d) Davies, H. M. L.; Hedley, S. J.; Bohall, B. R. J. Org. Chem. 2005, 70, 10737. (e) M€uller, P.; Bernardinelli, G.; Allenbach, Y. F.; Ferri, M.; Flack, H. D. Org. Lett. 2004, 6, 1725. (10) (a) Reger, D. L.; Horger, J. J.; Smith, M. D. Chem. Commun. 2011, 47, 2805. (b) DeAngelis, A.; Dmitrenko, O; Yap, G. P. A.; Fox, J. M. J. Am. Chem. Soc. 2009, 131, 7230. (c) Lindsay, V. N. G.; Lin, W.; Charette, A. B. J. Am. Chem. Soc. 2009, 131, 16383. (d) DeAngelis, A.; Boruta, D. T.; Lubin, J.-B.; Plampin, J. N., III; Yap, G. P. A.; Fox, J. M. Chem. Commun. 2010, 46, 4541. (11) (a) Reger, D. L.; Horger, J.; Smith, M. D.; Long, G. J. Chem. Commun. 2009, 6219. (b) Reger, D. L.; Horger, J. J.; Smith, M. D.; Long, G. J.; Fernande, G. Inorg. Chem. 2011, 50, 686. (12) Sun, J.; Yuan, A.; Wang, H.-B.; Sun, J. Acta Crystallogr. 2009, E65, o1210. (13) (a) Singh, D.; Bhattacharyya, P. K.; Baruah, J. B. Cryst. Growth Des . 2010, 10, 348. (b) Sarma, R. J.; Tamuly, C.; Barooah, N.; Baruah, J. B. J. Mol. Struct. 2007, 829, 29. (14) Alexiou, M. S.; Tychopoulos, V.; Ghorbanian, S.; Tyman, J. H. P.; Brown, R. G.; Brittain, P. I. J. Chem. Soc. Perkin Trans. 2 1990, 837. (15) (a) Yang, J.-X.; Wang, X.-L.; Wang, X.-M.; Xu, L.-H. Dyes Pigments 2005, 66, 83. (b) Qian, X.; Tang, J.; Zhang, J.; Zhang, Y. Dyes Pigments 1994, 25, 109. (16) (a) Fu, X.-F.; Yue, Y.-F.; Guo, R.; Li, L.-L.; Sun, W.; Fang, C.-J.; Xu, C.-H.; Yan, C.-H. CrystEngComm 2009, 11, 2268. (b) Mizobe, Y.; Hinoue, T.; Yamamoto, A.; Hisaki, I.; Miyata, M.; Hasegawa, Y.; Tohnai, N. Chem.—Eur. J. 2009, 15, 8157and references therein. (c) Bindu, A.; Kelly, L. A. J. Phys. Chem. B 2003, 107, 12534.

4079


Structures of Bifunctional Molecules Containing Two Very Different

Recommend Documents