Structures and Chromogenic Properties of Bisimidazole Derivatives
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 7 1653-1662
N. Fridman, S. Speiser, and M. Kaftory* Department of Chemistry, TechnionsIsrael Institute of Technology, Haifa 32000, Israel ReceiVed February 28, 2006; ReVised Manuscript ReceiVed April 26, 2006
ABSTRACT: Two imidazole derivatives, 2,2′-(2,5-thiophenediyl)bis[4,5-diphenyl-1H-imidazole] (1) and 2,2′-(1,4-phenylene)bis[4,5-diphenyl-1H-imidazole] (3), were synthesized and crystallized from different solvents. The crystal solvates have different colors, and some of these compounds show piezochromism, photochromism, and thermochromism in the solid state and show solvatochromism, halochromism, and photochromism in solution. The X-ray crystal structures of bisimidazole derivatives of lophine and the spectroscopic properties and photochemistry of these compounds in solutions are reported. The thermal behavior of the crystalline compounds was studied by a calorimetric method, and it was shown that upon heating the crystal solvates lose the solvent molecules, and the color is changed to the original color of the nonsolvate parent compound. The conclusion is that hydrogen bonding is responsible for the different colors. 1. Introduction One of the most beautiful and exciting aspects of chemistry is the wide range of colored compounds that one might have. Even more unusual still are the changes in color brought about by chemical reactions and alteration of the physical parameters of the environments. Many substances exhibit reversible variation of color under different physical or chemical conditions such as temperature, pressure, light, pH, and polarity of the solvent. Such reversible color changes are collectively termed “chromotropism”. The design of novel chromogenic compounds that exhibit significant color changes upon complexation with different guest molecules has attracted increasing attention because of their potential applications in analytical chemistry and in material science.1 These compounds undergo color change due to induced structural transformations of the molecules. These color change phenomena are classified according to the stimulus that causes the change. For example, thermochromism and photochromism are the phenomena of reversible color change stimulated by heat and by light, respectively. These materials have been receiving very wide attention, because they can be used to improve display and in optical storage technologies2-4 and can be employed as signal transducers between microscopic and macroscopic worlds.5 There are thousands of switchable molecular systems, but it is quite rare to observe more than one kind of chromogenism in the same compound. The applications of chromogenic materials are very wide, ranging from glazing in buildings, automobiles, and planes to certain types of electronic display, use in ophthalmic products, and use as temperature indicators. Recently, heterocyclic imidazole derivatives have attracted considerable attention because of their unique linear and nonlinear optical properties.6,7 The chemistry of lophine derivatives has a long history in relation to important physicochemical phenomena such as chemiluminescence through its oxidation.8-13 Hayashi and Maeda have reported that oxidation of 2,4,5-triphenylimidazole affords dimeric products, which show photo-, piezo-, and thermochromic properties due to the reversible formation to the colored 2,4,5-triarylimidazolyl free radical.8-13 Only a few imidazole analogues, nitro derivatives of lophine,14-17 are known. It was reported that host/guest binding events were
detected by color changes resulting from associated conformational changes and thus shifts in energy of electronic transitions. Recently, we have studied the crystal structure and thermochromic properties of some lophine derivatives that form inclusion compounds with hydrogen donating or accepting guest molecules. Our purpose in the present work is to combine solidstate studies of bisimidazole lophines with solution spectroscopic investigations to study the influence of hydrogen bonding interactions on the observed properties. In the present paper, we describe a comprehensive study of the crystallography of 2,2′-(2,5-thiophenediyl)bis[4,5-diphenyl1H-imidazole] (1), 2,2′-(2,5-thiophene-diyl)bis[1-methyl-4,5diphenyl-1H-imidazole] (2), 2,2′-(1,4-phenylene)bis[4,5-diphenyl1H-imidazole](3),2,2′-(1,4-phenylene)bis[4,5-bis(4-methoxyphenyl)1H-imidazole] (4), 2,2′-(2,5-thiophene-diyl)bis[4,5-bis(4methoxyphenyl)-1H-imidazole] (5), and 2,2′-(1,4-phenylene)bis[1methyl-4,5-diphenyl-1H-imidazole] (6). Packing patterns of four
different solvates of 1 and 2 and two different solvates of 3, 4, 5, and 6 are reported. Compounds 2, 5, and 6 are novel bisimidazoles synthesized for this study. In addition, spectroscopy, photophysics, and photochemistry of the same imidazole derivatives were studied, and a summary of these results is presented here to establish the structure-determining aspects of their optical properties. 2. Experimental Section
* To whom correspondence should be addressed. E-mail: kaftory@ tx.technion.ac.il.
Preparation of the Materials. Commercially available reagents were purchased from Aldrich and used without further purification. Tet-
10.1021/cg060107j CCC: $33.50 © 2006 American Chemical Society Published on Web 06/01/2006
1654 Crystal Growth & Design, Vol. 6, No. 7, 2006 rahydrofuran (THF) was distilled from sodium/benzophenone under nitrogen immediately prior to use. Thermal analyses were carried out using a Thermal Analysis DSC Q10. Mass spectroscopy was performed for the analysis of compounds 1 and 3 (before and after irradiation). Low-resolution chemical ionization (CI) mass spectrometric analysis was carried out on a Finnigan TSQ-700 mass spectrometer with isobutane as carrier gas. High-resolution CI or electron impact (EI) mass spectrometry was investigated on a Autospec Premier instrument with isobutane as carrier gas. MH+ ion (m/z 521) for 1 and MH+ ion (m/z 515) for 3 were formed before irradiation. 1H NMR spectra were recorded on a Bruker AC400 or AC500 spectrometer at 298 K. Absorption spectra were recorded on a Cary 50 UV-vis spectrophotometer. Emission and excitation spectra were measured using a Perkin-Elmer LS 50-B spectrofluorimeter. For emission and excitation measurements, the sample concentration was maintained at ∼10-5 M. Spectroscopic grade solvents were used for spectral measurement without further purification. Excitations at the 320-400 nm region were isolated from an Osram XBO 150 W high-pressured xenon lamp by using a combination of Corning 7-54 and 0-53 filters. Fluorescence lifetimes were determined by using time-correlated single-photon counting (TCSPC) methods. We have used the TCSPC experimental setup in the laboratory of Professor Dan Huppert at Tel-Aviv University, as previously described.18 (E)-R-(2,5-Dimethyl-3-furyl-ethylidene)(isopropylidene) succinic anhydride, or more briefly 3-fulgide, actinometer19 was used to measure fluorescence quantum yield of the photochemical reactions. (1) (a) Synthesis. All inclusion compounds 1 and 3-5 were synthesized, in 70-80% yield, according to the Davidson method.20 Benzil or 4,4′-dimethoxybenzil (1 mmol), 2,5-thiophenedicarboxyaldehyde or terephthaldicarboxyaldehyde (0.5 mmol), and ammonium acetate (1.6 g) were dissolved in boiling glacial acetic acid (16 mL) and refluxed for ca. 5 h monitored by TLC. The reaction mixture was poured into ice-water and collected on a filter, washed with cold water, dried, and recrystallized from the suitable solvent. 2,2′-(2,5-Thiophenediyl)bis[1-methyl-4,5-diphenyl-1H-imidazole] (2) and 2,2′-(1,4-phenylene)-bis[1-methyl-4,5-diphenyl-1H-imidazole] (6) were synthesized in 70% yield by N-methylation of 1 or 3 according to the Tanino et al. method.21 To a solution of 1 or 3 (0.2 mmol, 0.1 g) and dimethyl sulfate (0.72 mmol, 0.066 mL) in 10 mL of acetone was added powdered anhydrous potassium carbonate (0.72 mmol, 0.1 g), and the mixture was refluxed for 4.5 h. After cooling, the mixture was poured into water, and the yellow crude was collected by filtration and dried in a desiccator. (b) Crystal Growth. Crystallization of 1 from dry MeCN yields large solvated green prisms of 1a with the host-guest ratio of (1:2). Crystallization of 1 from MeCN/acetone (10:1) yields large solvated yellow needles of 1b with the host-guest ratio of (1:2MeCN:2H2O). It turned out that water molecules are very important in building the three-dimensional network. Crystallization of 1 from AcOH yields large solvated yellow plates of 1c with the host-guest ratio of (1:2.5AcOH). Crystallization of 1 from a 3:1 mixture of benzene and DMF yields large solvated light yellow needles of 1d with the host-guest ratio of (1:2DMF:2H2O). Crystallization of 2 from a 1:1 mixture of MeCN and CHCl3 yields yellow plates of 2. Crystallization of 3 from AcOH yields solvated yellow plates of 3a with the host-guest ratio of (1:4AcOH). Crystallization of 3 from a 2:1 mixture of DMF and benzene yields colorless prisms of 3b with the host-guest ratio of (1:2DMF). Crystallization of 4 from a 2:1 mixture of DMF and benzene yields light yellow prisms of 4 with the host-guest ratio of (1:1DMF:5H2O), where the water molecules were found to have major role in building the three-dimensional network. Crystallization of 5 from a 1:1 mixture of acetone and n-heptane yields large solvated yellow needles of 5 with the host-guest ratio of (1:2acetone:2H2O), where the water molecules play the role of constructing the three-dimensional structure. Crystallization of 6 from a 1:1 mixture of MeOH and CHCl3 yields colorless needles of 7 with the host-guest ratio of (1:2MeOH). (c) Characterization. 1: mp 327 °C; 1H NMR (500 MHz, DMSOd6; δ, ppm) 13.52 (s, 2H), 8.06 (s, 2H), 7.52 (d, 4H), 7.44 (d, 4H), 7.34 (t, 4H), 7.28 (q, 6H), 7.26 (t, 2H). 2: mp 295 °C; 1H NMR (400 MHz, CDCl3; δ, ppm) 7.54 (d, 2H), 7.53-7.36 (m, 16H), 7.16 (t, 8H), 3.66 (s, 6H). 3: mp 442 °C. 4: mp 380 °C; 1H NMR (400 MHz, CDCl3; δ, ppm) 7.52 (8H, d), 7.33 (8H, d), 7.29 (2H, d), 3.63 (12H, s). 5: mp 415 °C; 1H NMR (400 MHz, CDCl3; δ, ppm) 7.91 (8H, d), 7.43 (4H, d), 6.92 (8H, d), 3.83 (12H, s). 6: mp 312 °C; 1H NMR (400 MHz,
Fridman et al. CDCl3; δ, ppm) 7.54 (d, 4H), 7.48 (d, 4H), 7.42 (d, 4H), 7.22 (t, 8H), 7.14 (t, 4H), 3.54 (s, 6H). The melting points of the solvates of each of the parent molecules are identical to the melting points of the free parent compounds. (2) Crystal Structure Determination. Single-crystal X-ray diffraction data was collected at 298 K with a Nonius Kappa CCD diffractometer using Mo KR radiation (λ ) 0.710 73 Å). Details of crystallographic data collection and crystal structure determination and refinement for 1a-1d, 2, 3a, 3b, 4, 5, and 6 are given in Tables 1 and 2. All the non-hydrogen atoms of the inclusion compounds were refined with anisotropic displacement parameters. The software programs used for data collection and reduction were KappaCCD22 and DENZO SMN,23 for structure solution and refinement SHELXS-97 and SHELXL97.24 The crystal structure data of compounds 1a-1d, 2, 3a, 3b, 4, 5, and 6 are provided in the Supporting Information and are deposited in the Cambridge Crystallographic Data Centre.
3. Results and Discussion Photochromism, thermochromism, and piezochromism in the solid state are observed. The color of the solvates of 1 changes from yellow to green, and the color change occurs irreversibly by heating, irradiation, or trituration. Compound 1 is recovered by recrystallization from the various organic solvents. 3.1 Photochromism in the Solid State. Photochromism is defined as “a reversible transformation of a chemical species in one or both directions by absorption of electromagnetic radiation between two forms, A and B, having different absorption spectra”. The solvates of 1 showed a marked phototropy when they were irradiated with light of a mercury lamp in the solid state and in a solution of organic solvent. Upon irradiation of orange plates of 1b at room temperature, the color of the crystals turned green immediately, while on irradiation of yellow plates of 1c at room temperature, the color of the crystals remain unchanged for about 3 h and then the crystals turned green. A pale green solution of 1 in acetonitrile turned to dark orange on exposure to sunlight at room temperature. Different species were obtained in basic and neutral acetonitrile solutions. 3.2. Piezochromism in the Solid State. Piezochromism is the phenomenon when crystals undergo a major change of color due to mechanical grinding. The induced color reVerts to the original color when the fractured crystals are kept in the dark or dissolved in an organic solvent. When the crystals of 1b were manually crushed for 2 min in an agate mortar, the yellow color turned to green irreversibly. This kind of perturbation, which is combined with a local heating effect due to friction, could be called “dynamic” piezochromism to distinguish it from that produced using a hydraulic press, which can be called “static” piezochromism.25 When green powder of 1b recrystallizes from organic solvent, for example, acetone, a yellow powder is obtained. Upon grinding 1b, it loses the solvent molecules, and the color turns back to green. 3.3. Thermochromism in the Solid State. Thermal Properties and Crystal Structure. Thermochromism is a thermally reversible induced transformation of a molecular structure or of a system (e.g., a solution) that produces a spectral change, typically, but not necessarily, of visible color. To investigate the relationship between the color of the crystals and the molecular packing in the crystals of 2,2′-(2,5thiophenediyl)bis[4,5-diphenyl-1H-imidazole] (1), the crystal structures of the four solvate compounds with various solvent molecules (MeCN, H2O, CH3COOH, DMF) were determined by X-ray analysis. The solvent molecules play a crucial role in building up the three-dimensional structure. The solvent mol-
Bisimidazole Derivatives
Crystal Growth & Design, Vol. 6, No. 7, 2006 1655 Table 1. Crystal Data for 1 and 2
parameter formula Mr crystal color, habit crystal system crystal size, mm3 space group a [Å] b [Å] c [Å] R [deg] β [deg] γ [deg] V [Å3] Z Dcalcd [g‚cm-3] µ(Mo KR) [cm-1] F(000) 2θmax [deg] reflns collected independent reflns largest difference peak [e‚Å-3] largest difference hole [e‚Å-3] no. of params Ra wRa GOFb
1a
1b
1c
1d
2
C34H24N4S: 2C2H3N 602.74 green, prism monoclinic 0.33 × 0.18 × 0.15 P21/c 11.448(2) 27.508(5) 11.328(2) 90 115.64(2) 90 3216.0(10) 4 1.264 0.137 928 50.1 5620 5620 0.246
C34H24N4S: 2C2H3N;2H2O 638.77 yellow, needle monoclinic 0.54 × 0.15 × 0.09 C2/c 20.254(4) 11.561(2) 30.395(6) 90 99.89(2) 90 7011(2) 8 1.210 0.134 2688 50.1 6184 6184 0.227
C34H24N4S: C2H4O2;1.5C2H3O2 668.80 yellow, plate triclinic 0.48 × 0.33 × 0.09 P1h 11.057(2) 12.024(2) 13.433(3) 74.34(2) 83.28(2) 84.33(2) 1703.6(3) 2 1.304 0.146 700 50.1 9937 6003 0.408
C34H24N4S: 2C3H9NO2;2H2O 702.86 yellow, needle monoclinic 0.54 × 0.15 × 0.06 P21/c 17.143(3) 19.147(4) 11.536(2) 90 96.79(2) 90 3760.6(6) 4 1.242 0.135 1488 50.1 11230 6518 0.357
548.68 yellow, plate monoclinic 0.30 × 0.24 × 0.12 C2/c 23.728(5) 12.387(2) 104.43(2) 90 104.43(2) 90 2835.6(9) 4 1.285 0.147 1152 50.1 3407 2486 0.143
-0.174
-0.289
-0.325
-0.406
-0.182
436 0.0488 0.1058 0.862
468 0.0528 0.1241 0.984
488 0.0673 0.1591 0.911
505 0.0876 0.2167 1.062
201 0.0439 0.0080 0.896
C36H28N4S
a R ) ∑|F | - |F |/∑|F |; wR ) [∑w(|F | - |F |)2/∑w|F |2]1/2. b GOF ) [∑w(|F | - |F |)2/(NO - NV)]1/2, where NO is the number of observations and o c o o c o o c NV is the number of variables.
Table 2. Crystal Data for 3-6 parameter formula Mr crystal color, habit crystal system crystal size, mm3 space group a [Å] b [Å] c [Å] R [deg] β [deg] γ [deg] V [Å3] Z Dcalcd [g‚cm-3] µ(Mo KR) [cm-1] F(000) 2θmax [deg] reflns collected independent reflns largest difference peak [e‚Å-3] largest difference hole [e‚Å-3] no. of params Ra wRa GOFb
3a
3b
4
5
6
C36H26N4: 4C2H4O2 754.82 yellow, plate triclinic 0.54 × 0.42 × 0.06 P1h 10.121(2) 14.163(3) 15.828(3) 66.47(1) 82.71(2) 69.57(1) 1949.1(7) 2 1.286 0.089 796 50.1 12136 6867 0.166
C36H26N4: 2C3H7NO 660.80 colorless, prism monoclinic 0.54 × 0.18 × 0.15 P21/c 6.216(2) 17.198(3) 16.983(3) 90 93.35(2) 90 1812.4(7) 2 1.211 0.076 700 50.0 5274 3154 0.215
C40H34N4O4: C3H7NO;5H2O 786.80 yellow, prism triclinic 0.54 × 0.30 × 0.21 P1h 10.671(2) 12.175(2) 18.115(4) 70.58(1) 87.17(2) 68.83(1) 2063.1 2 1.267 0.091 826 50.1 12357 7207 0.539
C38H30N4: 2CH4O 606.74 colorless, needle monoclinic 0.54 × 0.15 × 0.12 P21/c 12.762(3) 12.001(2) 12.175(2) 90 117.98(2) 90 1646.7(5) 2 1.224 0.076 644 50.1 4740 2773 0.175
C38H32N4O4: 2C3H6O;2H2O 792.92 yellow, needle monoclinic 0.54 × 0.21 × 0.18 P21/c 17.535(3) 25.094(5) 20.924(4) 90 113.45(2) 90 8447.3(2) 8 1.247 0.133 3360 50.1 14768 14768 0.160
-0.215
-0.155
-0.402
-0.150
-0.225
538 0.0515 0.1267 0.904
268 0.0566 0.1667 0.903
552 0.0698 0.1996 0929
268 0.0507 0.0934 0.729
1139 0.0423 0.0741 0.720
a R ) ∑|F | _ |F |/∑|F |; wR ) [∑w(|F | _ |F |)2/∑w|F |2]1/2. b GOF ) [∑w(|F | _ |F |)2/(NO _ NV)]1/2, where NO is the number of observations and NV o c o o c o o c is the number of variables.
ecules are linked to the 2,2′-(2,5-thiophenediyl)bis[4,5-diphenyl1H-imidazole] molecules (1) through hydrogen bonds. The thermochromic properties are determined by the removal of the solvent molecules. We describe here the thermal behavior and the crystal structure of the solvate compounds. 3.3.1. Thermal Properties of 2,2′-(2,5-Thiophenediyl)bis[4,5-diphenyl-1H-imidazole] (1) Solvates. The thermal behav-
ior of 1a is shown by the differential scanning calorimeter (DSC) thermograph given in Figure 1. The two endotherms between 350 and 400 K are assigned to the removal of the two acetonitrile molecules (∆H ) 115.2 kJ mol-1). We could not assign the endotherm at 414 K. The endotherm at 587 K is assigned to the melting of 1 (∆H ) 42.2 kJ mol-1). The thermal behavior of 1b is shown by the differential scanning calorimeter
1656 Crystal Growth & Design, Vol. 6, No. 7, 2006
Fridman et al.
Figure 1. DSC thermographs of 1a-1d.
(DSC) thermograph given in Figure 1. The endotherm between 370 and 420 K is assigned to the removal of the water and acetonitrile molecules (∆H ) 63.2 kJ mol-1). Upon removal of the solvents, the yellow crystals turn completely green. All our attempts to identify the nature of the exothermic peak observed between 450 and 455 K (∆H ) 15.03 kJ mol-1) failed. The X-ray powder diffraction (XRD) patterns of the powder sample before and after the exothermic peak are identical and show amorphous states. The endotherm at 601 K is assigned to the melting of 1 (∆H ) 38.0 kJ mol-1). The crystals 1b are unstable in air and turn green in a few days but revert to their original color when the sample is exposed to acetonitrile. When yellow needles of 1b were irradiated with light of a xenon lamp for few minutes, the crystals turned green immediately. The thermal behavior of 1c is shown by the DSC thermograph in Figure 1. Between 380 and 450 K, the molecules of acetic acid are removed, and the crystals become green (∆H ) 121.7 kJ mol-1). The second sharp endotherm at 601 K is assigned to the melting temperature of 1 (∆H ) 36.0 kJ mol-1). The thermal behavior of 1d is shown by the DSC thermograph in Figure 1. The first endotherm between 340 and 370 K is assigned to the removal of the water molecules (∆H ) 160.6 kJ mol-1), and the second endotherm between 420 and 460 K is assigned to the removal of the DMF molecules (∆H ) 46.6 kJ mol-1). Upon removal of the solvents, the yellow crystals turn completely green. The endotherm at 602 K is assigned to the melting of 1 (∆H ) 34.0 kJ mol-1). The differences in enthalpy of melting of the four solvates are due to the loss of material as a result of the solvent removal during the DSC measurements. 3.3.2. Crystal Structure of 2,2′-(2,5-Thiophenediyl)bis[4,5diphenyl-1H-imidazole] (1) Solvates. Molecular structure of 1 is presented in Figure 2. The numbering of the 2,2′-(2,5-
thiophenediyl)bis[4,5-diphenyl-1H-imidazole] molecule (1) is the same in all crystals. The differences in enthalpy of melting of the four solvates are due to the loss of material as a result of the solvent removal during the DSC measurements. The crystal structures of four solvates were determined by X-ray diffraction analysis to identify the structural effects on the different colors. The hydrogen bond patterns and the molecular packing of
Figure 2. Molecular structure of 1 (top) and 3 (bottom). The ellipsoids are drawn at the 50% probability level.
Bisimidazole Derivatives
Crystal Growth & Design, Vol. 6, No. 7, 2006 1657
Figure 5. Crystal structures of the solvate compound 1c.
Figure 3. Crystal structures of the solvate compound 1a.
Figure 6. Crystal structure of the solvate compound 1d.
Figure 4. Crystal structures of the solvate compound 1b.
bisimidazole derivatives (1-6) are shown in Figures 3-13. Dihedral angles between the rings of bisimidazole derivatives (1-6) are presented in Table 3. Hydrogen-bonding geometry for the four solvates is given in Table 4. The two acetonitrile molecules in 1a are hydrogen bonded to the NH groups of the two imidazole rings (Figure 3 and Table 4). Two independent molecules (designated A and B) were found in 1b, and they differ slightly in the conformation of the molecules, as shown in Table 3 by the dihedral angles between the imidazole rings and the thiophene ring and between the imidazole ring and two phenyl rings. Two water molecules in
1b serve as the binding glue between two NH groups of two crystallographically independent diimidazole molecules by the formation of hydrogen bonding. They are important in the construction of the three-dimensional packing arrangement. The water molecules are also hydrogen bonded to acetonitrile molecules, which serve to fill space (Figure 4 and Table 4). The acetic acid in the crystal structure of 1c is disordered. Two acetate molecules form the glue between two molecules of bisimidazole derivative through hydrogen bonding, thus forming a supramolecular arrangement (Figure 5 and Table 4). The role of the disordered acetate molecules is to link the pairs described above through hydrogen bonds. They also form hydrogen bonds with the ordered acetate molecules. The disordered molecules are responsible for the building of the three-dimensional structure. Unfortunately, the exact role cannot be determined due to the disorder. The water molecule in 1d links through hydrogen bonds two molecules of bisimidazole and therefore is responsible for the three-dimensional packing. The disordered DMF molecules are hydrogen bonded to the water molecules and have the role of space filling (Figure 6 and Table 4). Hydrogen-bonding geometry and structure description of compounds (2, 3a, 3b, 4, 5, and 6) given in Table 5 and Figures 7-13. Compound 2 crystallizes without solvent (Figure 7). There are three acetic acid molecules and one acetate molecule in 3a (Figure 8). A proton from the acetic acid was transferred to one of the imidazole rings. Therefore one nitrogen of the imidazole ring is positively charged. In 3b (Figure 9), the solvent molecules are disordered, hydrogen bonded to the imidazoles.
1658 Crystal Growth & Design, Vol. 6, No. 7, 2006
Fridman et al.
Figure 9. Crystal structure of the solvate compound 3b.
Figure 7. Crystal structure of compound 2.
Figure 10. Crystal structure of the solvate compound 4.
Figure 11. Crystal structure of compound 5 down the c axis. Figure 8. Crystal structure of the solvate compound 3a.
In 4 (Figure 10), disordered water molecules form the connection between imidazole rings via hydrogen bonds and are responsible for the three-dimensional network. As in 1d, the DMF molecules are also hydrogen bonded to the water molecules and have no significant role except filling space. In 5 (Figure 11), there are two crystallographically independent molecules of bisimidazole, four molecules of acetone, and four molecules of water in the asymmetric unit. The main difference between the two bisimi-
dazole molecules is the orientation of the methoxy group relative to the phenyl ring plane. In one of them, the methoxy groups are all aligned in the same direction in a propeller-like fashion, and in the second, one of the methoxy groups is pointing in the opposite direction of the propeller. Each of the four molecules of water is hydrogen bonded to three other molecules, two bisimidazole molecules and acetone (Figure 12). Each molecule of water forms a link between two bisimidazole molecules and the remaining HO is hydrogen bonded to the acetone molecule.
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Crystal Growth & Design, Vol. 6, No. 7, 2006 1659
Figure 14. Absorption spectra of 1-7 in C ≈ 10-5 M acetonitrile solution. Figure 12. Crystal structure of compound 5 down the b axis.
Figure 13. Crystal structure of compound 6. Table 3. Angles (deg) between Planes of the Compounds 1a-1d, 2, 3a, 3b, 4, 5, and 6a 1b planes
1a
molecule A
molecule B
1c
1d
2
A-B A-E B-C E-F B-D E-G
11.72 9.69 36.82 36.04 30.94 39.85
14.83 14.83 37.45 37.45 35.40 35.40
15.98 15.98 38.51 38.51 29.39 29.39
8.17 6.40 18.51 49.31 55.91 33.34
8.08 26.79 38.55 41.12 39.29 29.56
18.90 18.90 62.69 62.69 12.36 12.36
5 planes
3a
3b
4
molecule A
molecule B
6
A-B A-E B-C E-F B-D E-G
12.48 14.83 30.16 30.95 49.31 28.76
25.55 15.24 43.02 25.16 45.03 35.29
25.55 15.24 43.02 25.16 45.03 35.29
14.31 10.32 32.33 36.32 34.00 32.68
9.85 11.95 25.76 28.46 37.10 35.22
29.78 29.78 53.43 53.43 37.46 37.46
a
Notation for molecules 1-6 is shown in Figure 2.
Molecules of the bisimidazole are hydrogen bonded to a methanol molecule in 6 (Figure 13). What are the reasons for the differences in the crystal color? In the solid, the packing of molecules, the hydrogen bonding, and the molecular conformation may affect the energy related to electronic transitions, thus having an effect on the crystal color. The colors of the solvent-free 1 and of the solvate with acetonitrile (1a) are green, while the colors of the crystals of
the solvates of 1 either with acetonitrile and water (1b) or acetic acid (1c) or with DMF and water (1d) are yellow. The conformations of all molecules of 1 in the different solvates are similar, and therefore we rule out the effect of conformation on the crystal color (see Table 3). In all the solvates of 1, there is hydrogen bonding between the imidazole ring and the solvent molecules. However there is a major difference between the participation of the imidazole rings in 1a (unfortunately we have not been able to grow crystals of the solvent-free 1 and therefore its crystal structure is not known) in the hydrogen bond scheme compared with that in the other solvates. The imidazole hydrogen donor group (NH) is participating in hydrogen bonding with the hydrogen accepting group in each of the solvates. However, in 1a, the imidazole hydrogen accepting group (N) is not involved in hydrogen bonding while this group is hydrogen bonded to hydrogen donor groups in the other solvates. This is why the hydrogen bonding in 1b-1d determines the three-dimensional structure of the crystals. We therefore suspect that the difference in the hydrogen bonding affects the photophysical properties of the crystalline materials. Similarly we see the same differences in the two different solvates of 3. Crystals of 3a are yellow, while crystals of 3b are colorless. In 3b, only the imidazole hydrogen donor group (NH) participates in hydrogen bonding, while in 3b both hydrogen donor and hydrogen acceptor groups take part in the hydrogen bonding. 3.4. Spectroscopy and Photophysical Studies of 1-7 in MeCN Solution. We have conducted a thorough investigation of the spectroscopy, photophysics, and photochemical stability in MeCN solution. The results are summarized here, and a complete account is given elsewhere for the bisimidazole derivatives (1-6)26 and for lophine 7 as a reference compound.27 The absorption spectra of lophine derivatives 1-7 have been studied in MeCN as shown in Figure 14. These spectra were investigated in the concentration range of 10-5 to 10-3 M, Beer’s law was kept at this concentration range, thus indicating lack of aggregation of lophine derivatives. The lack of aggregation conforms with the fact that the compounds form crystals only when solvent is involved. In addition, the excitation and emission spectra of lophine derivatives 1-7 in MeCN solutions have been studied as shown in Figure 15, where the OD was kept at 0.192 for all solutions. Absorption and fluorescence properties of 1-6 and lophine (7) are presented in Table 6, together with the corresponding fluorescence quantum yields, Φf. The fluorescence intensity for benzene bisimidazole derivatives increased with addition of a MeO- substituent in 4,5-phenyl rings (compounds 3 and 4) and decreased for the thiophene bisimidazole derivative by
1660 Crystal Growth & Design, Vol. 6, No. 7, 2006
Fridman et al.
Table 4. Comparison of Hydrogen Bond Geometry of 1 and 3 D-H‚‚‚A
d(D-H), Å
d(H‚‚‚A), Å
N2-H2N2‚‚‚N5 N4-H4N4‚‚‚N6
0.860 0.860
2.267 2.294
N2A-H2NA‚‚‚O1 N2B-H2NB‚‚‚O2 O1-H1C‚‚‚N1 O1-H1D‚‚‚N1B O2-H2C‚‚‚N2 O2-H2D‚‚‚N1A
0.860 0.860 0.836 0.862 0.835 0.842
1.969 1.951 2.152 2.010 2.120 2.032
N2-H2N2‚‚‚O1 N3-H3N3‚‚‚O5A N4-H4N4‚‚‚O4 O2-H2O2‚‚‚N1
0.860 0.860 0.860 0.820
2.051 1.875 1.907 1.849
N2-H2N2‚‚‚O1 N4-H4N4‚‚‚O2
0.860 0.860
2.025 1.910
O1-H1O1‚‚‚O5 O3-H3O3‚‚‚N3 O7-H7O7‚‚‚O6 N1-H1N1‚‚‚O6 N1-H1N1‚‚‚O5 N2-H2N2‚‚‚O8 N4-H4N4‚‚‚O2
0.820 0.820 0.820 0.860 0.860 0.860 0.860
1.753 1.873 1.754 2.117 2.445 1.885 1.934
N1-H1N1‚‚‚O1
0.860
2.055
d(D‚‚‚A), Å
∠D-H‚‚‚A, deg
symmetry
1a 3.103 3.154
163.98 177.61
[x + 1, y, z + 1] [x + 1, y, z]
2.827 2.810 2.980 2.870 2.939 2.870
174.71 175.72 171.06 174.35 166.72 173.24
[-x + 1, -y + 1, -z + 1]
2.883 2.678 2.765 2.660
162.49 154.89 175.02 170.22
[-x + 1, -y, -z + 1]
2.868 2.770
166.15 178.12
[-x + 1, y - 1/2, -z + 1/2]
2.567 2.676 2.561 2.972 3.071 2.734 2.790
171.71 166.01 167.36 172.88 130.14 168.85 173.65
[x - 1, y, z] [x - 1, y, z] [x - 1, y, z] [-x + 2, -y, -z + 1] [-x + 2, -y, -z + 1]
2.912
174.60
[x + 1, y, z]
1b [x - 1/2, -y + 1/2, z - 1/2] [-x + 1, -y, -z + 1] [x + 1, -y + 1, z + 1/2]
1c
1d
3a
3b Table 5. Hydrogen Bond Geometry of 4-6 D-H‚‚‚A
d(D-H), Å
d(H‚‚‚A), Å
N2-H2N2‚‚‚O5A N2-H2N2‚‚‚O5B N3-H3N3‚‚‚O8A N3-H3N3‚‚‚O8A
0.860 0.860 0.860 0.860
2.143 2.194 1.996 2.363
N2A-H2G‚‚‚O6 N4A-H4G‚‚‚O8 N2B-H2H‚‚‚O5 N4B-H4H‚‚‚O7 O5-H5G‚‚‚O1 O5-H5H‚‚‚N1A O6-H6G‚‚‚O4 O6-H6H‚‚‚N3B O7-H7G‚‚‚N3A O7-H7H‚‚‚O2 O8-H8G‚‚‚O3 O8-H8H‚‚‚N1B
0.860 0.860 0.860 0.860 0.817 0.829 0.818 0.818 0.836 0.839 0.819 0.843
1.962 2.008 2.047 1.951 2.074 2.086 2.067 2.036 2.019 1.995 2.123 2.088
O1-H1‚‚‚N1
0.820
2.188
∠D-H‚‚‚A, deg
symmetry
2.985 2.986 2.848 3.130
166.25 153.17 170.75 148.67
[-x + 1, -y + 1, -z + 1] [-x + 1, -y + 1, -z + 1] [x, y - 1, z] [x, y - 1, z]
2.819 2.854 2.898 2.804 2.846 2.905 2.867 2.828 2.852 2.804 2.887 2.922
174.90 167.40 170.49 170.76 157.52 169.37 165.87 162.66 175.11 161.99 155.19 170.04
2.872
141.01
d(D‚‚‚A), Å 4
5 [-x, y + 1/2, -z + 1/2] [-x, y + 1/2, -z + 1/2]
6 Table 6. Absorption and Fluorescence Data for 1-7 compd
λmax, nm
λ(ex)max, nm
λ(em)max, nm
∆E, cm-1
, M-1 cm-1
Φf
τf, ns
1 2 3 4 5 6 7
385 365 355 365 391 330 311.5
384.5 367.5 360.5 366 392.5 337 311.5
458 451.5 423.5 440 470 416.5 386.5
4173.7 5062.5 4126.5 4595.1 4201.1 5664.0 6229.5
34 870 20 350 14 860 43 910 47 820 15 470 20 330
0.55 0.62 0.75 0.90 0.52 0.66 0.27
1.03 1.42 1.11 1.24 1.24 1.10 1.98
changing phenyl rings to MeO- substituents in 4,5-phenyl rings (compounds 1 and 5). Both 1 and 3 show red-shifted absorption and emission as compared to the MeO- substituted compounds 4 and 5, respectively. Comparison of 1 and 2 indicates that the
[x, -y + 1/2, z - 1/2]
1-methyl substituted thiophene bisimidazole derivative 2 shows blue-shifted absorption and emission spectra, as compared to the nonsubstituted thiophene bisimidazole derivative 1. The fluorescence intensity for 2 is increased compared to 1 by adding the MeO- substituent in the 4,5-phenyl rings, and it is decreased for thiophene bisimidazole derivative 1 by changing phenyl rings to the MeO- substituted compound 5. Absorption band maxima, λmax, the excitation spectra band maxima, λ(ex)max, the emission band maxima, λ(em)max, the peak molar absorptivity, , and the associated Stokes shifts, ∆E, together with the fluorescence quantum yields, Φf, are listed in Table 6. Comparison of the fluorescence properties for 1-7 with Φf ranging between 0.24 and 0.90 is shown in Table 6. Lophine 7 was used as a reference for the determination of
Bisimidazole Derivatives
Crystal Growth & Design, Vol. 6, No. 7, 2006 1661
Figure 15. Excitation (left) and emission spectra (right) of 1-7 in acetonitrile solution.
Figure 16. Excitation (left) and emission spectra (right) of 1 in various solvents: violet, acetic acid; yellow, acetone; light blue, chloroform; blue, ether; red, ethyl acetate; green, acetonitrile; brown, methanol.
Figure 17. Absorption spectra of 1 at different apparent pH meter readings in C ≈ 10-5 M acetonitrile solution.
Figure 18. Excitation and corrected emission spectra of 1 at different apparent pH meter readings in C ≈ 10-5 M acetonitrile solution.
Table 7. Fluorescence Properties of 1 in Various Solvents solvent
λ(ex)max, nm
λ(em)max, nm
∆E, cm-1
methanol acetic acid acetonitrile acetone chloroform ethyl acetate ether
382 392 384 384 395 385 384
457 503 460 460 485 457 454
4296.2 5629.2 4302.5 4302.5 4697.9 4092.2 4015.2
quantum yields. It has a Φf of 0.48 in hexane.27 The Φf of 1-methyl substituted thiophene bisimidazole derivative 2 (0.62) is larger than that of 1 (0.55). The fluorescence quantum yield of 1-methyl lophine comparing to lophine 7 and 1-methyl thiophene imidazole comparing to thiophene imidazole reduces from 0.48 to 0.29 in hexane and from 0.86 to 0.62, respectively.27 The fluorescence quantum yield of 1-methyl substituted benzene bisimidazole derivative 6 (0.66) is smaller than of 3 (0.75). Comparison of 1 with 3 shows that replacing the thiophene ring by a benzene ring increased the fluorescence by more than 24%. The fluorescence quantum yield of lophine 7, compared to thiophene imidazole, increases almost twice.27 Comparison of 1 and 5 shows that the substitution from H to p-MeO of phenyl rings almost does not change the fluorescence quantum yield. The fluorescence quantum yield for nonsubstituted ring derivatives for thiophene imidazole was 0.72 times that of p-MeO substituted phenyl rings.27 Comparison of 3 and 5 shows that the substitution from H to p-MeO at the phenyl rings increases the fluorescence quantum yield from 0.75 to 0.90.
Figure 19. Transition energies of absorption (νa) and emission (νf) versus apparent pH meter readings for 1 in C ≈ 10-5 M acetonitrile solution.
The fluorescence lifetimes, τf, were also determined and are shown in Table 6. The spectroscopy and photophysics of 1 are almost the same in neutral solutions in many solvents. For example Φf of 1 is the same in MeCN and MeOH. Figure 16 shows excitation and emission spectra of 1 in various solvents. The observed solvent effect is minimal, as manifested in the shifts of the peak excitation and emission wavelengths, λ(ex)max and λ(em)max, and the associated Stokes shifts, ∆E, as presented in Table 7. Compound 1 showed also halochromic behavior with changing pH in acetonitrile solution. The absorption, excitation, and fluorescence spectra of 1 at different pH in acetonitrile solution are presented in Figures 17 and 18. It was shown that λmax
1662 Crystal Growth & Design, Vol. 6, No. 7, 2006
Fridman et al. Scheme 1
increases with increasing pH, while the fluorescence intensity decreases with increasing pH. In case of a very acidic acetonitrile solution (pH ) 0.91), the absorbance band observed at λ(ex)max ) 373 nm, whereas in case of very basic acetonitrile (pH ) 12.5) solution, the absorbance band is red shifted to λ(ex)max ) 388 nm. The fluorescence intensity gradually reduces with increasing pH. In very basic acetonitrile solution, the product almost does not exhibit any fluorescence. The fluorescence band maximum (Figure 18) is observed at λ(em)max ) 478 nm in very acidic solution, whereas it is observed at λ(em)max ) 516 nm in very basic solution. Two clear isosbestic points observed at 329 and 382 nm and two unclear isosbestic points observed at 282 and 344 nm depending on pH suggest the presence of four different possible species of 1 (see Scheme 1). Figure 19 represents the energies of absorption (νa) and emission (νf) transitions of 1 in acetonitrile as a function of pH (pKa1 ) 11.39 ( 0.5; pK2 ) 3.69 ( 0.16).28 The observations show that these systems undergo deprotonation in basic pH. The results in solution indicate that the main factors controlling the spectroscopy and photophysics in these compounds are the properties of the NH hydrogen, similar to its role in the solid state. Conclusions We have found 10 new crystal structures for bisimidazole molecules. Some of them show chromotropic behavior. The photochemical properties of 1 and 3 and their derivatives were studied by irradiating basic and neutral degassed acetonitrile solutions with a medium-pressure xenon lamp and their photochemical quantum yields, ranging from 0.0011 to 0.0024, together with the corresponding fluorescence quantum yields, ranging from 0.52 to 0.90, and lifetimes, ranging from 1.03 to 1.42 ns, were determined. 2,2′-(2,5-Thiophenediyl)bis[4,5-diphenyl-1H-imidazole] was shown to exhibit unusual chromogenic properties that have promising applications. This compound is sensitive to different external stimulations causing color change such as UV irradiation, heat, increasing pressure, and changing the environmental pH. Thus, it can be used as a “multiway” optically switchable material.
Supporting Information Available: X-ray crystallographic information files (CIF) for compounds 1a-1d, 2, 3a, 3b, 4, 5, and 6. This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
(14) (15) (16) (17) (18) (19) (20) (21) (22) (23)
(24) (25) (26) (27)
Acknowledgment. This research was supported by the Fund for the Promotion of Research at the Technion and the Technion VPR Fund. We are grateful to Professor Yoav Eichen for many stimulating discussions.
(28)
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CG060107J
