New Aspects on the Origin of Color in the Solid State. Coherently Shifting of the Protons in Violurate Crystals T. Kolev,† B. B. Koleva,*,‡ R. W. Seidel,‡ M. Spiteller,† and W. S. Sheldrick‡ Institut fu¨r Umweltforschung, UniVersita¨t Dortmund, Otto-Hahn-Strasse 6, 44221 Dortmund, Germany, and Lehrstuhl fu¨r Analytische Chemie, Ruhr-UniVersita¨t Bochum, UniVersita¨tsstrasse 150, 44780 Bochum, Germany
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 8 3348–3352
ReceiVed February 14, 2009; ReVised Manuscript ReceiVed May 26, 2009
ABSTRACT: The self-assembly and properties of six novel violurates with organic counterions, that is, cyclohexylammonium violurate (1), piperidinium violurate 2.5 hydrate (2), tetrahydroisoquinolinium violurate monohydrate (3), quinuclidinium violurate tetrahydrate (4), pyridinium violurate (5), and 3-aminopyridinium violurate (6) are reported. Compounds 1-6 were synthesized and characterized in detail by means of single crystal X-ray diffraction, solution and solid-state UV/vis spectroscopy, linear-polarized infrared (IR-LD) spectroscopy of oriented colloids in a nematic host, mass spectrometry, and thermal methods. Quantum chemical ab initio and density functional theory (DFT) calculations were performed with a view to understanding the optical properties of the studied species. New phenomenon of the origin of the color in the solid state is observed and elucidated. Hydrogen bonding can have an important role in the design of organized structures and supramolecular devices and in tuning the physical properties of crystals.1-10 The design of new materials tailored by controlling the assembly of individual molecules in solids, using hydrogen bonding as a powerful noncovalent force for organizing organic molecules, is one of the most widely used strategies in crystal engineering.10 Considerable progress has been made in recent years by using squaric acid as an attractive template for generating tightly hydrogen-bonded self-assemblies from polarizable cations.9,10 During the course of our investigations on this topic, we observed a new phenomenon, namely, the adoption of a red color in the solid state by organic salts (Scheme S1, Supporting Information) in the presence of colorless counterions.9,11 The phenomenon cannot be described in items of classical crystallochromy, based on π-π stacking as used to explain the color of perylene pigments.12-15 The color is a result of the formation of infinite chains involving the participation of two hydrogensquarate anions, linked by a disordered proton and one solvent water molecule.9 A second independent confirmation of this phenomenon was obtained by studying 3-nitropyridinium-hydrogensquarate monohydrate,11 which also exhibits the same structural motif and a red color in the solid state.11 A central question is whether this phenomenon is specific for systems containing hydrogensquarates and polarized pyridinium cations in the presence of a solvent water molecule or whether it represents a more general concept for the origin of color in crystals. In an attempt to address these questions we have focused on a systematic study of nucleic acids such as uracil, thymine, and cytosine. These are interesting species for the purpose of controlling self-assembly because, in addition to their known affinity for complementary bases, they can also form complexes with transition metals,16 thereby allowing the generation of metal-organic selfassembled structures. One attractive template appeared to be violuric acid and especially violurate salts. Violuric acid is a weaker polyfunctional acid than squaric acid.17 Its remarkable ability to form complexes of differing colors with alkali and alkaline earth metals ions has been underlined in the last 20 years.18,19 However, crystallographic investigations of violurates with organic counterions are very rare. Only one structure of 8-dimethylaminonaphthalene1-dimethylammonium violurate methanol solvate has previously been reported.20 These compounds are characterized by their violet * Corresponding author. Tel.: +49 234 32 24190. E-mail:
[email protected]. † Universita¨t Dortmund. ‡ Ruhr-Universita¨t Bochum.
color in solution and in the solid state with a λmax value within the 575-585 nm range and an e value of about 1000 L mol-1 cm-1, which can be assigned to the n f π* transition of the violurate anion. We now report six new salts of violuric acid, namely, cyclohexylammonium violurate (1), piperidinium violurate 2.5 hydrate (2), tetrahydroisoquinolinium violurate monohydrate (3), quinuclidinium violurate tetrahydrate (4), pyridinium violurate (5), and 3-aminopyridinium violurate (6) (Scheme S2, Supporting Information). Four of these novel compounds, 2-4 and 6, were characterized structurally by single crystal X-ray diffraction. Routine spectroscopic and structural elucidation methods such as positive and negative fast atom bombardment (FAB) mass spectrometry, solution and solid state UV-vis spectroscopy, conventional and linear polarized IR-spectroscopy, and thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques have been applied. Quantum chemical calculations at the DFT, MP2, and CIS level of theory and 6-31++G** basis set are employed for predicting and supporting experimentally observed optical properties of the compounds. The crystal structures of 2-4 and 6 were established by X-ray diffraction. ORTEP diagrams of the cations and anions are presented in Figures S1-S4, respectively. Bond lengths and angles exhibit typical values for this class of compounds. The asymmetric unit of 2-4 contains a violurate anion and the respective cation. In contrast, the asymmetric unit of 6 comprises a nitrogen base cation and formally a violurate anion and a violuric acid molecule. Compound 2 crystallizes in the orthorhombic space group Pccn with Z ) 8. Violurate anions form a so-called hydrogen bonded crinkled type of stable chain by means of N-H · · · O hydrogen bonding interactions (2.776(3), 2.829(3) Å) along the crystallographic a axis (Figure 1). The anionic chromophore (CNO) interacts with the piperidinium ion by multicenter hydrogen bonding with N-H · · · N and N-H · · · O distances of 3.086(4) and 2.804(3) Å, respectively. The chains of violurate anions are joined to one other by a chain formed by the solvent water molecules running along the crystallographic c axis direction. π-π stacking interactions between adjacent symmetryrelated violurate anions occur in the c axis direction with a distance of the mean planes between the violurate rings of ca. 3.33 Å and a centroid-centroid separation of about 3.54 Å (Figure 1). Compound 3 crystallizes in the monoclinic space group P21/c with Z ) 4 and is structurally related to 2. The hydrogen bonding pattern between the violurate anions is the same as in the structure of 2 with N-H · · · O distances of 2.814(2) and 2.815(2) Å, resulting in chains of anions running along the b axis. Like the piperidinium ion in 2, the tetrahydroisoquinolinium cation interacts with the
10.1021/cg900188k CCC: $40.75 2009 American Chemical Society Published on Web 06/19/2009
Communications
Crystal Growth & Design, Vol. 9, No. 8, 2009 3349
Figure 1. Crystal structures of 2-4 and 6, respectively.
violurate anion via multicenter hydrogen bonding with N-H · · · N and N-H · · · O distances of 3.110(3) and 2.731(3) Å, respectively. The solvent water molecule participates in hydrogen bonding interactions with two adjacent violurate anions and a tetrahydroisoquinolinium cation. In contrast to 2, the chains of anions are not interconnected via hydrogen bonding interactions of solvent water molecules. As depicted in Figure 1, the π-π stacking features of 3 are essentially the same as observed in 2. The distance between the mean planes of violurate rings is about 3.48 Å and the corresponding centroid-centroid separation is ca. 3.64 Å. Compound 4 crystallizes triclinic in space group P1j with Z ) 2. As shown in Figure 1, the violurate anions in 4 form the same chain motif as observed in 2 and 3 with N-H · · · O distances of 2.894(5) and 2.921(5) Å. Like the secondary amine derived cationic hydrogen bonding donors in 2 and 3 the protonated tertiary nitrogen atom of the quinuclinidium ion in 3 is directed to a violurate ion to form a multicenter hydrogen bond with N-H · · · N and N-H · · · O distances of 3.008(6) and 2.959(6) Å, respectively. The solvent water molecules are involved in hydrogen bonding interactions leading to supramolecular chains which interconnect the chains of violurate anions via hydrogen bonding. The π-π stacking interactions between the violurate chains are depicted in Figure 1. The distance between the mean planes of the rings of adjacent violurate ions is ca. 3.36 Å, but the offset of the rings is apparently larger. The separation between the centroids defined by the violurate rings is about 4.10 Å. Compound 6 crystallizes also triclinic in P1j but with Z ) 1. A different assembly motif of the violurate anions was found. Two symmetry related violurate ions form a dimer via hydrogen bonding between the hydroxyimino groups. This dimer lies on a crystallographic center of inversion. The proton is disordered by symmetry between the hydroxyimino groups of the two molecules. The O5-H5 · · · H5′-O5a separation of 2.496(2) Å indicates a relatively strong hydrogen bond. The partial deprotonation is supported by the observed N5-O5 distance of 1.316(1) Å. In 2-4 the corresponding bond lengths are in the range of 1.244(2)-1.280(5) Å. For violuric acid monohydrate a value of 1.349(3) Å was reported.21 The hydrogen bonded dimers of violurate anions are interconnected via N-H · · · O hydrogen bonds of the same type as observed in 2-4 (2.881(1), 2.893(1) Å). This leads to the formation of twodimensional sheets of violurate anions (Figure 1). The interconnection of the sheets in the third dimension is achieved by hydrogen bonding interactions of the cations. The 3-aminopyridinium cation
is flat and shows positional disorder about a crystallographic center of inversion. The pyridinium moiety provides a moderately strong hydrogen bond to a violurate carbonyl group with a O-H · · · O distance of 2.689(2) Å. The amino group is directed to a carbonyl and a hydroxyimino group of two violurate anions via weaker O-H · · · O hydrogen bonds with donor-acceptor distances of 3.27(2) and 3.17(1) Å, respectively. The hydrogen bonded sheets of violurate anions are separated with an interlayer distance of about 3.04 Å, defined by the mean planes of violurate rings. The centroid-centroid distance of adjacent symmetry related violurate ions is ca. 4.28 Å, indicating a larger offset of the rings as observed in 2-4. The distance between the mean planes of adjacent symmetry related 3-aminopyridinium ions is ca. 3.29 Å. The corresponding centroid-centroid separation is about 4.73 Å. The angle between the mean planes of the violurate ring and the pyridinium ring is ca. 39.6°. For a detailed elucidation of the influence of the type of hydrogen bonding on the color of the violurates, we will make the following assumptions. First we must underline that the effect of deprotonation of the OH group in the violuric acid leads to a formation of the CdN-O- species, first proposed during studies on a series of inorganic violurate salts where the authors then concluded that the n f π* transition is responsible for the color.19 For this reason, our theoretical calculations and experimental elucidation of the dimers of violurate anions, and the corresponding mono-, di-, tri-, and tetra hydrate solvates, were performed in accordance with these preliminary assumptions. In the second part, we will model the experimentally observed type of interaction in compounds 2-4 and 6 with a view to obtain the electronic spectra of these systems and compare these with the experimental results. The data are also compared with that of neutral violuric acid. In contrast to neutral violuric acid, which exhibits an absorption band at λmax ) 380 nm with e ) 1021 L mol-1 cm-1 (Figure S5, Scheme S3, Supporting Information) leading to the yellow color of the compound in solid state and in solution, all violurate salts are characterized by violet solutions in water with λmax between 583 and 589 nm and e values of about 1000 L mol-1 cm-1. This provides direct confirmation of the proposed n f π* transition in the formulated NdO bond/s in the anion. Now, if we propose only solute-solvent interactions and combine these observations with the theoretical data, the band at about 580 nm is observed only in the cases of HOH · · · NO and/or HOH · · · ON interactions with the fragment. In the case of isolated violurate anions, and systems with only HOH · · · ON or NOH · · · ON interactions between the violurate anions, a hypsochromic effect
3350
Crystal Growth & Design, Vol. 9, No. 8, 2009
Communications
Figure 2. Solid-state UV-vis spectra of compounds 4-6.
is obtained. We can now make proposals concerning changes in the color in the solid state with regard to the color of the aqueous solution of the anion. In system 4, the solid-state UV/vis-spectrum is characterized by a band at 582 nm as a result of the participation of the CNO chromophore in interactions with the cationic moieties (Figure 1). As far as in the systems that crystal structures show an absence of the distributed proton systems, these compounds are characterized with the bands about 580 nm and their color is violet in the solid state. In contrast, compounds 5 and 6 are characterized by bands at 610 and 615 nm. If we compare the types of the interaction in the solid state, the new self-assembly type in the latter case, characterized by the half-positioned proton between both violurate species, apparently leads to a red color in solid state (Figure 2). This means that in the systems with the infinite layered structures of the anions with participation of the solvent water molecules, a coherently shifting of the protons at a speed on the order of the speed of light could be proposed, resulting in an observation of the band about 610 nm, leading to the red color of these compounds in the crystals. The possibility of forming infinite substructures with chromophores causing an infinite proton transfer
system was first reported for the hydrogensquarate salt of 4-cyanopyridine.9 With the present example containing different cationic and anionic fragments, we have now observed same phenomena. We, therefore, conclude that this is a new explanation for the possible origin of color in the solid state which differs from the previous classical crystallochromy type. The vibrational properties of the novel violurates were elucidated by means of the linear-polarized IR-spectroscopy. The interpretation of the data, shown in Figure 1, were interpreted by application of the reducing-difference procedure22-25 and by a comparison with the data of neutral violuric acid.19 We have focused only on the spectroscopic characteristics of the violurate anions in agreement with the main aim of our investigation. The other characteristics of the counterions are only mentioned as IR-spectroscopic regions or in the case of compound 6, the method allows us to investigate the out-of-plane vibrations of 3-aminopyridine in the salt (see below). The IR-spectra of compounds 1, 2, 4, and 5 are depicted in Figures 3 and 4. The gray rectangles depict the corresponding IRspectroscopic regions of the self-absorption of the nematic host, used as an orientation matrix for the suspended particles. For precise
Communications
Figure 3. Solid-state IR-spectra of compounds 1, 2, 4, and 5 in nematic host.
Figure 4. Solid-state IR-spectra of compounds 3 and 6 in KBr pellets.
Crystal Growth & Design, Vol. 9, No. 8, 2009 3351
3352
Crystal Growth & Design, Vol. 9, No. 8, 2009
interpretation of the data, the theoretical elucidation of the vibrational properties of neutral violuric acid and its anion were also performed. Figure S6 depicts the theoretical IR-spectra of latter compounds. In the neutral compound three combination bands were observed within the range 1800-1700 cm-1 with the bands at respectively 1778 cm-1, 1757 cm-1, and 1729 cm-1 belonging to νCdO + δN-H vibrations (see figure and ref 19). The bands at 841 cm-1 and 808 cm-1 could be attributed to characteristic out-ofplane modes belonging to δNH + δCNOH + νCCC and FCCC + FNOH + FNCN, respectively. In contrast to neutral violuric acid, there are three characteristic IR bands of the anion. The bands at 1720 cm-1, 1686 cm-1, and 756 cm-1 also exhibit combination character (Scheme S4, Supporting Information). The first two maxima corresponding to a combination of νCdO and δNH vibrations are similar to those of the squaric acid and its hydrogensquarate anion.9,11 In all of the experimental IR-spectra (Figures 3 and 4) the first two characteristics of the violurate anions are shifted to higher or lower frequency as a result of the participation of the anions in different types of self-associated substructures. In contrast, the third out-of-plane vibration is shifted about 80 cm-1 to higher frequency, which is also typical for the bending vibrations, when the groups participate in the intermolecular interactions in solid state. The bands about 3500 cm-1 correspond to the νOH stretching vibrations of the solvate water molecules, while those within 3450-3100 cm-1 range belong to νNH stretching vibrations of the violurate anion. In the case of compounds 5 and 6, the characteristic IR-bands of violurate are overlapped with those of the differently substituted aromatic pyridine fragments (see Figure 4). In the last case, the broadband within the whole 3100-2600 cm-1 region belongs to the νN+H stretching vibration,9 as is typical for protonated pyridines. The assignment of characteristic bands is confirmed by the application of the reducing-difference procedure which leads to a simultaneous elimination of the bands at 841 cm-1, 725 cm-1, and 503 cm-1 (FCCC + FNOH + FNCN) at equal dichroic ratio thus proving their out-of-plane bending vibration character (Figure S7, Supporting Information). The latter result is the same for all of the six violurates studied (Figure 1). In conclusion, we have reported a new phenomenon responsible for color of solid-state violurates which differs from classical crystallochromy, where the main origin of the different optical properties of the pigments in the crystal state is the π-stacking effect. In the systems with hydrogensquarates and/or violurates in the presence of polarized pyridinium cations and two solvent water molecules, where the disordered H-proton is equally disposed between anionic fragment leading to formation of infinite chains, the crystals are characterized by a red color, different from the color of the compounds in solution or without solvent water. That is why we can propose a coherent shifting of the protons in the system at a speed on the order of the speed of light. The observed new phenomenon and its further investigation with respect to tuning the optical properties of organic crystals could be of interest not only to the field of crystal engineering but also to physics and fundamental science in general. Experimental Section. The violurates 1-6 were synthesized by a common scheme, by mixing equimolar amounts of the corresponding amine and violuric acid in 20 mL of water, under continuous stirring at a temperature within 40-100 °C for 24 h. Precipitates and crystalline samples with different colors were obtained after leaving the resulting solutions to stand at 25 °C for about a week. Cyclohexylammonium violurate (1): Found: C, 46.85; H, 6.27; N, 21.86; [C10H16N4O4] calcd.: C, 46.87; H, 6.29; N, 21.86%, yield 56%; piperidinium violurate 2.5 hydrate (2): Found: C, 36.52; H, 6.88; N, 18.92; [C9H20N4O7] calcd.: C, 36.49; H, 6.80; N, 18.91%, yield 88%; tetrahydroisoquinolinium violurate monohydrate (3): Found: C, 50.66; H, 5.21; N, 18.17; [C13H16N4O5] calcd.: C, 50.65; H, 5.23; N, 18.17%, yield 91%; quinuclidinium violurate tetrahydrate (4): Found: C, 38.81; H, 7.10; N, 16.44; [C11H24N4O8] calcd.: C, 38.82; H, 7.11; N, 16.46%, yield 77%; pyridinium violurate (5): Found: C, 45.79; H, 3.44; N, 23.78; [C9H8N4O4] calcd.: C, 45.77; H, 3.41; N, 23.72%, yield 61% and 3-aminopyridinium violurate (6): Found: C, 43.02; H, 3.60; N, 27.88; [C9H9N5O4] calcd.: C, 43.03; H, 3.61; N, 27.88%, yield 90%.
Communications The most intensive signals in the positive FAB mass spectra of the compounds are those of the peaks at the m/z values, corresponding to the singly charged cations. The TGA and DSC data in the temperature range of 300-500 K confirm the presence of solvent molecule/s in the novel salts 2-4 as determined by elemental analysis. Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC 717925-717928 Copies of this information may be obtained from the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: +44 1223 336 033; e-mail:
[email protected] or http:// www.ccdc.cam.ac.uk).
Acknowledgment. B.K. wishes to thank the Alexander von Humboldt Foundation for a Fellowship and T.K. thanks the DAAD for a grant within the priority program “Stability Pact South-Eastern Europe” and the Alexander von Humboldt Foundation. The authors thank Heike Mayer-Figge for technical assistance. Supporting Information Available: Crystal structures, chemical diagrams of compounds 1-6; ORTEP diagrams of compounds 2-4, 6; UV-vis spectra of violuric acid and 1; HOHO and LUMO MOs; selected transition moments in violurate anion; IR spectra; experimental section; table of crystallographic and refinement data and violurate geometric parameters for compounds 2-4, 6. This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) Bosshard, Ch. Hulliger, J. Florsheimer, M. Gunter, P. In Organic Nonlinear Optical Materials; AdVances in Nonlinear Optics; Gordon and Breach Science Publishers SA: Postfach, Basel, 2001. (2) Chemla, D. Zyss, J. Nonlinear Optical Properties of Organic Molecules and Crystals; Chemla, D. Zyss, J., Eds.; Academic Press: New York, 1987; Vol. 1, pp 23-187. (3) Rusch, U.; Yao, S.; Wortmann, R.; Wurthner, F. Angew. Chem. 2006, 118, 7184–7186. (4) Wurthner, F.; Yao, S. Angew Chem., Int. Ed. 2000, 39, 1978–1980. (5) Yao, S.; Beginn, U.; Gress, T.; Lysetska, M.; Wurthner, F. J. Am. Chem. Soc. 2004, 126, 8336–8340. (6) Wu¨rthner, F.; Yao, S.; Heise, B.; Tschierskec, C. Chem. Commun. 2001, 2260–2263. (7) Wurthner, F.; Yao, S.; Beginn, U. Angew. Chem., Int. Ed. 2003, 42, 3247–3250. (8) Kolev, T.; Yancheva, D.; Stoyanov, S. AdV. Funct. Mater. 2004, 14, 799–804. (9) Koleva, B. B.; Kolev, T.; Seidel, R. W.; Mayer-Figge, H.; Spiteller, M.; Sheldrick, W. S. J. Phys. Chem. A 2008, 112, 2899–2905. (10) Kolev, T.; Mayer-Figge, H.; Seidel, R. W.; Sheldrick, W. S.; Spiteller, M.; Koleva, B. B. J. Mol. Struct. 2008, in press. (11) Koleva, B. B.; Kolev, T.; Seidel, R. W.; Spiteller, M.; Mayer-Figge, H.; Sheldrick, W. S. J. Phys. Chem. A 2009, 113 (13), pp 3088–3095. (12) Wu¨rthner, F.; Thalacker, Ch.; Sautter, A. AdV. Mater. 1999, 11, 754–758. (13) Wu¨rthner, F.; Bauer, Ch.; Stepanenko, V.; Yagai, Sh. AdV. Mater. 2008, 20, 1695–1698. (14) Kazmaier, P.; Hoffmann, R. J. Am. Chem. Soc. 1994, 116, 9684. (15) Aakeroy, C. Acta Crystallogr. 1997, B53, 569. (16) Salas-Peregrin, J. M.; Romero-Molina, M. A.; Ferro-Garcia, M. A.; Moreno-Carr, M. N. J. Therm. Anal. 1985, 30, 921. (17) de Oliveira, L. F. C.; Santos, P. S.; Rubim, J. C. J. Raman Spectrosc. 1991, 22, 197. (18) Bonacin, J.; Formiga, A. L. B.; de Melo, V.; Toma, H. E. Vibr. Spectrosc. 2007, 44, 133–141. (19) Awadallah, R.; Belal, A. M.; Issa, R.; Peacock, R. Spectrochim. Acta 1991, 47A, 1541–1546. (20) Nichol, G. S.; Clegg, W. Cryst. Growth Des. 2006, 6, 451. (21) Nichol, G. S.; Clegg, W. Acta Crystallogr. 2005, E61, o3788–o3790. (22) Ivanova, B. B.; Arnaudov, M. G.; Bontchev, P. R. Spectrochim. Acta 2004, 60A, 855–864. (23) Ivanova, B. B.; Tsalev, D. L.; Arnaudov, M. G. Talanta 2006, 69, 822–830. (24) Ivanova, B. B.; Simeonov, V.; Arnaudov, M.; Tsalev, D. Spectrochim. Acta 2007, 67A, 66–75. (25) Koleva, B. B.; Kolev, T. M.; Simeonov, V.; Spassov, T.; Spiteller, M. J. Inclus. Phenomenon 2008, 61, 319–333.
CG900188K
