Stable Single‐Molecule Lines of Terrylene in Polycrystalline para‐Dichlorobenzene at 1.5 K

The spectroscopic properties of single terrylene (Tr) molecules are studied in a polycrystalline matrix of para‐dichlorobenzene (p‐DCB) at 1.5 K. Samples grown in a glass capillary show a very strong site at 597 nm, which is redshifted by more than 700 cm^?1 from the observed transition energy for Tr in p‐DCB prepared as a film on a coverslip (572 nm). Each of these two sites is characterized by measuring their single‐molecule spectroscopic parameters at 1.5 K. Lifetime‐limited linewidths of 45±5 MHz are found for both sites. Fluorescence detection rates reach 8×10⁴ count s^?1 at saturation. The spectral trails of the majority of single molecules show no spectral jumps, indicating an absence of interacting two‐level systems; however, the small distribution of linewidths may indicate weak interactions with low‐frequency modes. Frequency jumps are observed for 10 % of the molecules. The complete emission spectra from two different single molecules at the center of each of the two sites is presented. Debye–Waller factors of α_DW=0.33±0.05 for the normal site (572 nm) and α_DW=0.30±0.05 for the red site (597 nm) are reported. This new host–guest system provides a quick and easy way to obtain lifetime‐limited single‐molecule lines.     

A comparative study of two polymorphs of bis(1‐hydroxy‐2‐methylpropan‐2‐aminium) carbonate

Alkanolamines have been known for their high CO₂ absorption for over 60 years and are used widely in the natural gas industry for reversible CO₂ capture. In an attempt to crystallize a salt of (RS)‐2‐(3‐benzoylphenyl)propionic acid with 2‐amino‐2‐methylpropan‐1‐ol, we obtained instead a polymorph (denoted polymorph II) of bis(1‐hydroxy‐2‐methylpropan‐2‐aminium) carbonate, 2C₄H₁₂NO⁺·CO₃²⁻, (I), suggesting that the amine group of the former compound captured CO₂ from the atmosphere forming the aminium carbonate salt. This new polymorph was characterized by single‐crystal X‐ray diffraction analysis at low temperature (100 K). The salt crystallizes in the monoclinic system (space group C2/c, Z = 4), while a previously reported form of the same salt (denoted polymorph I) crystallizes in the triclinic system (space group P, Z = 2) [Barzagli et al. (2012). ChemSusChem, 5 , 1724–1731]. The asymmetric unit of polymorph II contains one 1‐hydroxy‐2‐methylpropan‐2‐aminium cation and half a carbonate anion, located on a twofold axis, while the asymmetric unit of polymorph I contains two cations and one anion. These polymorphs exhibit similar structural features in their three‐dimensional packing. Indeed, similar layers of an alternating cation–anion–cation neutral structure are observed in their molecular arrangements. Within each layer, carbonate anions and 1‐hydroxy‐2‐methylpropan‐2‐aminium cations form planes bound to each other through N—H…O and O—H…O hydrogen bonds. In both polymorphs, the layers are linked to each other via van der Waals interactions and C—H…O contacts. In polymorph II, a highly directional C—H…O contact (C—H…O = 156°) shows as a hydrogen‐bonding interaction. Periodic theoretical density functional theory (DFT) calculations indicate that both polymorphs present very similar stabilities.     

A low‐temperature modification of hexa‐tert‐butyldisilane and a new polymorph of 1,1,2,2‐tetra‐tert‐butyl‐1,2‐diphenyldisilane

Crystals of hexa‐tert‐butyldisilane, C₂₄H₅₄Si₂, undergo a reversible phase transition at 179 (2) K. The space group changes from Ibca (high temperature) to Pbca (low temperature), but the lattice constants a, b and c do not change significantly during the phase transition. The crystallographic twofold axis of the molecule in the high‐temperature phase is replaced by a noncrystallographic twofold axis in the low‐temperature phase. The angle between the two axes is 2.36 (4)°. The centre of the molecule undergoes a translation of 0.123 (1) Å during the phase transition, but the conformation angles of the molecule remain unchanged. Between the two tri‐tert‐butylsilyl subunits there are six short repulsive intramolecular C—H...H—C contacts, with H...H distances between 2.02 and 2.04 Å, resulting in a significant lengthening of the Si—Si and Si—C bonds. The Si—Si bond length is 2.6863 (5) Å and the Si—C bond lengths are between 1.9860 (14) and 1.9933 (14) Å. Torsion angles about the Si—Si and Si—C bonds deviate by approximately 15° from the values expected for staggered conformations due to intramolecular steric H...H repulsions. A new polymorph is reported for the crystal structure of 1,1,2,2‐tetra‐tert‐butyl‐1,2‐diphenyldisilane, C₂₈H₄₆Si₂. It has two independent molecules with rather similar conformations. The Si—Si bond lengths are 2.4869 (8) and 2.4944 (8) Å. The C—Si—Si—C torsion angles deviate by between −3.4 (1) and −18.5 (1)° from the values expected for a staggered conformation. These deviations result from steric interactions. Four Si—C(t‐Bu) bonds are almost staggered, while the other four Si—C(t‐Bu) bonds are intermediate between a staggered and an eclipsed conformation. The latter Si—C(t‐Bu) bonds are about 0.019 (2) Å longer than the staggered Si—C(t‐Bu) bonds.     

Synthesis,optical and photoconducting properties of platinum poly‐yne polymers functionalized with electron‐donating and electron‐withdrawing bithiazole units

By virtue of the electron‐donating and electron‐withdrawing properties of the thiazole ring, a new soluble rigid‐rod organometallic polymer containing electron‐donating and electron‐withdrawing trans‐[‐Pt‐(PBu₃)₂‐C≡C—R—C≡C—]_n (R = bithiazolediyl) groups is prepared by CuI‐catalyzed dehydrohalogenation. The thermal properties and the optical absorption and photoluminescence spectra of the polymer are reported. The polymer is luminescent with a singlet emission peak at 539 nm and photoconducting in a single‐layer sandwich structure photocell. The optical gap of the polymer is reduced compared to that for the oligothienyl analogue.     

Thermal characterization and solid‐state 13C‐NMR investigation of blends of poly(N‐phenyl‐2‐hydroxytrimethylene amine) and poly(N‐vinyl pyrrolidone)

The miscibility and thermal properties of poly(N‐phenyl‐2‐hydroxytrimethylene amine)/poly(N‐vinyl pyrrolidone) (PHA/PVP) blends were examined by using differential scanning calorimetry (DSC), high‐resolution solid‐state nuclear magnetic resonance (NMR) techniques, and thermogravimetric analysis (TGA). It was found that PHA is miscible with PVP, as shown by the existence of a single composition‐dependent glass transition temperature (T_g) in the whole composition range. The DSC results, together with the ¹³C crosspolarization (CP)/magic angle spinning (MAS)/high‐power dipolar decoupling (DD) spectra of the blends, revealed that there exist rather strong intermolecular interactions between PHA and PVP. The increase in hydrogen bonding and in T_g of the blends was found to broaden the line width of CH—OH carbon resonance of PHA. The measurement of the relaxation time showed that the PHA/PVP blends are homogeneous at least on the scale of 1–2 nm. The proton spin‐lattice relaxation in both the laboratory frame and the rotating frame were studied as a function of the blend composition, and it was found that blending did not appreciably affect the spectral densities of motion (sub‐T_g relaxation) in the mid‐MHz and mid‐KHz frequency ranges. Thermogravimetric analysis showed that PHA has rather good thermal stability, and the thermal stability of the blend can be further improved with increasing PVP content. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 237–245, 1999     

Two‐Photon Absorption and Time‐Resolved Stimulated Emission Depletion Spectroscopy of a New Fluorenyl Derivative

The synthesis, comprehensive linear photophysical characterization, two‐photon absorption (2PA), steady‐state and time‐resolved stimulated emission depletion properties of a new fluorene derivative, (E)‐1‐(2‐(di‐p‐tolylamino)‐9,9‐diethyl‐9H‐fluoren‐7‐yl)‐3‐(thiophen‐2‐yl)prop‐2‐en‐1‐one ( 1 ), are reported. The primary linear spectral properties, including excitation anisotropy, fluorescence lifetimes, and photostability, were investigated in a number of aprotic solvents at room temperature. The degenerate 2PA spectra of 1 were obtained with open‐aperture Z‐scan and two‐photon induced fluorescence methods, using a 1 kHz femtosecond laser system, and maximum 2PA cross‐sections of ～400–600 GM were obtained. The nature of the electronic absorption processes in 1 was investigated by DFT‐based quantum chemical methods implemented in the Gaussian 09 program. The one‐ and two‐photon stimulated emission spectra of 1 were measured over a broad spectral range using a femtosecond pump–probe‐based fluorescence quenching technique, while a new methodology for time‐resolved fluorescence emission spectroscopy is proposed. An effective application of 1 in fluorescence bioimaging was demonstrated by means of one‐ and two‐photon fluorescence microscopy images of HCT 116 cells containing dye encapsulated micelles.     

Exo conformers of N‐(pyridin‐2‐yl)‐ and N‐(pyridin‐3‐yl)norbornene‐5,6‐dicarboximide crystals

Two isomeric pyridine‐substituted norbornenedicarboximide derivatives, namely N‐(pyridin‐2‐yl)‐exo‐norbornene‐5,6‐dicarboximide, (I), and N‐(pyridin‐3‐yl)‐exo‐norbornene‐5,6‐dicarboximide, (II), both C₁₄H₁₂N₂O₄, have been crystallized and their structures unequivocally determined by single‐crystal X‐ray diffraction. The molecules consist of norbornene moieties fused to a dicarboximide ring substituted at the N atom by either pyridin‐2‐yl or pyridin‐3‐yl in an anti configuration with respect to the double bond, thus affording exo isomers. In both compounds, the asymmetric unit consists of two independent molecules (Z′ = 2). In compound (I), the pyridine rings of the two independent molecules adopt different conformations, i.e. syn and anti, with respect to the methylene bridge. The intermolecular contacts of (I) are dominated by C—H...O interactions. In contrast, in compound (II), the pyridine rings of both molecules have an anti conformation and the two independent molecules are linked by carbonyl–carbonyl interactions, as well as by C—H...O and C—H...N contacts.     

A new two‐dimensional ZnII coordination polymer based on 1,3‐bis(2‐methyl‐1H‐imidazol‐1‐yl)benzene and 5‐nitrobenzene‐1,3‐dicarboxylic acid: synthesis,crystal structure and physical properties

A novel two‐dimensional (2D) Zn^II coordination framework, poly[[μ‐1,3‐bis(2‐methyl‐1H‐imidazol‐1‐yl)benzene](μ‐5‐nitrobenzene‐1,3‐dicarboxylato)zinc(II)], [Zn(C₈H₃NO₆)(C₁₄H₁₄N₄)]_n or [Zn(NO₂‐BDC)(1,3‐BMIB)]_n [1,3‐BMIB is 1,3‐bis(2‐methyl‐1H‐imidazol‐1‐yl)benzene and NO₂‐H₂BDC is 5‐nitrobenzene‐1,3‐dicarboxylic acid], has been prepared and characterized by IR, elemental analysis, thermal analysis and single‐crystal X‐ray diffraction. Single‐crystal X‐ray diffraction analysis revealed that the compound is a new 2D polymer with a 6³ topology parallel to the (10) crystal planes based on left‐handed helices, right‐handed helical NO₂‐BDC–Zn chains and [Zn₂(1,3‐BMIB)₂]_n clusters. In the crystal, adjacent layers are further connected by C—H…O hydrogen bonds, C—H…π interactions, C—O…π interactions and N—O…π interactions to form a three‐dimensional structure in the solid state. In addition, the compound exhibits strong fluorescence emissions in the solid state at room temperature.     

Straightforward Access to Water‐Soluble Unsymmetrical Sulfoxanthene Dyes: Application to the Preparation of Far‐Red Fluorescent Dyes with Large Stokes’ Shifts

An efficient synthesis of water‐soluble unsymmetrical sulforhodamine/sulforhodol fluorophores containing a single julolidine fragment is presented. Owing to their valuable spectral properties in aqueous buffers, these dyes, especially those bearing a free aniline or phenol moiety, are valuable components of fluorogenic probes for a variety of biosensing applications. A further extension of this synthetic methodology to unusual phenols, namely 7‐N,N‐dialkylamino‐4‐hydroxy coumarins has enabled us to provide a new family water‐soluble dyes of large Stokes’ shift with far‐red spectral features.     

7‐Iodo‐5‐aza‐7‐deazaguanine ribonucleoside: crystal structure,physical properties,base‐pair stability and functionalization

The positional change of nitrogen‐7 of the RNA constituent guanosine to the bridgehead position‐5 leads to the base‐modified nucleoside 5‐aza‐7‐deazaguanosine. Contrary to guanosine, this molecule cannot form Hoogsteen base pairs and the Watson–Crick proton donor site N3—H becomes a proton‐acceptor site. This causes changes in nucleobase recognition in nucleic acids and has been used to construct stable `all‐purine' DNA and DNA with silver‐mediated base pairs. The present work reports the single‐crystal X‐ray structure of 7‐iodo‐5‐aza‐7‐deazaguanosine, C<sub>10</sub>H<sub>12</sub>IN<sub>5</sub>O<sub>5</sub> ( 1 ). The iodinated nucleoside shows an anti conformation at the glycosylic bond and an N conformation (O4′‐endo) for the ribose moiety, with an antiperiplanar orientation of the 5′‐hydroxy group. Crystal packing is controlled by interactions between nucleobase and sugar moieties. The 7‐iodo substituent forms a contact to oxygen‐2′ of the ribose moiety. Self‐pairing of the nucleobases does not take place. A Hirshfeld surface analysis of 1 highlights the contacts of the nucleobase and sugar moiety (O—H…O and N—H…O). The concept of pK‐value differences to evaluate base‐pair stability was applied to purine–purine base pairing and stable base pairs were predicted for the construction of `all‐purine' RNA. Furthermore, the 7‐iodo substituent of 1 was functionalized with benzofuran to detect motional constraints by fluorescence spectroscopy.     

Size‐based proteins separation using polymer‐entrapped colloidal self‐assembled nanoparticles on‐chip

We report on a facile method to stabilize colloidal self‐assembled (CSA) nanoparticles packed in microchannels for high speed size‐based separation of proteins. Silica nanoparticles, self‐assembled in a network of microfluidic channels, were stabilized with a methacrylate polymer prepared in situ through photopolymerization. The entrapment conditions were investigated to minimize the effect of the polymer matrix on the structure of the packing and the separation properties of the CSA beds. SEM shows that the methacrylate matrix links the nanoparticles at specific sphere–sphere contact points, improving the stability of the CSA structure at high electric fields (up to at least 1800 V/cm), allowing fast and efficient separation. The %RSD of the protein migration times varied between 0.3 and 0.5% (n = 4, in 1 day) and <0.83% over a period of 7 days (n = 28 runs) in a single device, at high field strength. The overall %RSD of protein migration times from chip‐to‐chip across a single fabrication run was 4.3% (n = 3) and between fabrication runs was 11% (n = 35), with 87% fabrication yield, demonstrating reproducible packing and entrapment behavior. The optimized entrapped CSA beds demonstrated better separation performance (plate height, H ～ 200 nm) than similarly prepared on‐chip CSA beds without the polymer entrapment. Polymer‐entrapped CSA beds also exhibited superior protein resolving power: the minimum resolvable molecular weight difference of proteins in the polymer‐entrapped CSA bed is 0.6 kDa versus ～9 kDa for the native silica CSA bed (i.e. without polymer entrapment).     

Effect of Medium Acidity and Photostability of 3‐(4‐Dimethylamino‐phenyl)‐1‐(2,5‐dimethyl‐thiophen‐3‐yl)‐propenone (DDTP): A New Green Emitting Laser Dye

On the line of a previous work on the spectral properties of some of heteroaryl chalcone, the effect of medium acidity and photoreactivity of 3‐(4‐dimethylamino‐phenyl)‐1‐(2,5‐dimethyl‐thiophen‐3‐yl)‐propenone (DDTP) has been investigated in dimethylformamide and in chloromethane solvents such as methylenechloride, chloroform and carbon tetrachloride. The dye solution (ca. 5×10⁻⁴ mol·L⁻¹ in DMF) gives a good laser emission in the range 470–560 nm with emission maximum at 515 nm upon pumping by nitrogen laser (λ_ex=337.1 nm). The laser parameters such as gain coefficient (α), emission cross section (δ_e) and half life energy (E_1/2) at maximum laser emission are also determined.     

X‐ray and DFT‐calculated structures of bis[N‐(quinolin‐8‐yl)benzamidato‐κ2N,N′]copper(II)

The application of transition metal chelates as chemotherapeutic agents has the advantage that they can be used as a scaffold around which ligands with DNA recognition elements can be anchored. The facile substitution of these components allows for the DNA recognition and binding properties of the metal chelates to be tuned. Copper is a particularly interesting choice for the development of novel metallodrugs as it is an endogenous metal and is therefore less toxic than other transition metals. The title compound, [Cu(C₁₆H₁₁N₂O)₂], was synthesized by reacting N‐(quinolin‐8‐yl)benzamide and the metal in a 2:1 ratio. Ligand coordination required deprotonation of the amide N—H group and the isolated complex is therefore neutral. The metal ion adopts a flattened tetrahedral coordination geometry with the ligands in a pseudo‐trans configuration. The free rotation afforded by the formal single bond between the amide group and phenyl ring allows the phenyl rings to rotate out‐of‐plane, thus alleviating nonbonded repulsion between the phenyl rings and the quinolyl groups within the complex. Weak C—H…O interactions stabilize a dimer in the solid state. Density functional theory (DFT) simulations at the PBE/6‐311G(dp) level of theory show that the solid‐state structure (C1 symmetry) is 79.33 kJ mol⁻¹ higher in energy than the lowest energy gas‐phase structure (C2 symmetry). Natural bond orbital (NBO) analysis offers an explanation for the formation of the C—H…O interactions in electrostatic terms, but the stabilizing effect is insufficient to support the dimer in the gas phase.     

Three different fluoro‐ or chloro‐substituted 1′‐deoxy‐1′‐phenyl‐β‐D‐ribofuranoses

Crystal structures are reported for three fluoro‐ or chloro‐substituted 1′‐deoxy‐1′‐phenyl‐β‐D‐ribofuranoses, namely 1′‐deoxy‐1′‐(2,4,5‐trifluorophenyl)‐β‐D‐ribofuranose, C₁₁H₁₁F₃O₄, (I), 1′‐deoxy‐1′‐(2,4,6‐trifluorophenyl)‐β‐D‐ribofuranose, C₁₁H₁₁F₃O₄, (II), and 1′‐(4‐chlorophenyl)‐1′‐deoxy‐β‐D‐ribofuranose, C₁₁H₁₃ClO₄, (III). The five‐membered furanose ring of the three compounds has a conformation between a C2′‐endo,C3′‐exo twist and a C2′‐endo envelope. The ribofuranose groups of (I) and (III) are connected by intermolecular O—H...O hydrogen bonds to six symmetry‐related molecules to form double layers, while the ribofuranose group of (II) is connected by O—H...O hydrogen bonds to four symmetry‐related molecules to form single layers. The O...O contact distance of the O—H...O hydrogen bonds ranges from 2.7172 (15) to 2.8895 (19) Å. Neighbouring double layers of (I) are connected by a very weak intermolecular C—F...π contact. The layers of (II) are connected by one C—H...O and two C—H...F contacts, while the double layers of (III) are connected by a C—H...Cl contact. The conformations of the molecules are compared with those of seven related molecules. The orientation of the benzene ring is coplanar with the H—C1′ bond or bisecting the H—C1′—C2′ angle, or intermediate between these positions. The orientation of the benzene ring is independent of the substitution pattern of the ring and depends mainly on crystal‐packing effects.     

Blue‐to‐Orange Color‐Tunable Laser Emission from Tailored Boron‐Dipyrromethene Dyes

A series of meso‐substituted boron‐bipyrromethene (BODIPY) dyes are synthesized and their laser and photophysical properties systematically studied. Laser emission covering a wide visible spectral region (from blue to orange) is obtained by just changing the electron donor character of the heteroatom at position 8. The additional presence of methyl groups at positions 3 and 5 results in dyes with a photostability similar to that of the unsubstituted dye but with much improved efficiency. Correlation of the lasing properties of the different dyes to their photophysical properties provides inklings to define synthetic strategies of new BODIPY dyes with enhanced efficiency and modulated wavelength emission over the visible spectral region.     

Synthesis and crystal structures of C24‐epimeric 20(R)‐ocotillol‐type saponins

Ocotillol‐type saponins have a wide spectrum of biological activities. Previous studies indicated that the configuration at the C24 position may be responsible for their stereoselectivity in pharmacological action and pharmacokinetics. Natural ocotillol‐type saponins share a 20(S)‐form but it has been found that the 20(R)‐stereoisomers have different pharmacological effects. The semisynthesis of 20(R)‐ocotillol‐type saponins has not been reported and it is therefore worthwhile clarifying their crystal structures. Two C24 epimeric 20(R)‐ocotillol‐type saponins, namely (20R,24S)‐20,24‐epoxydammarane‐3β,12β,25‐triol, C₃₀H₅₂O₄, (III), and (20R,24R)‐20,24‐epoxydammarane‐3β,12β,25‐triol monohydrate, C₃₀H₅₂O₄·H₂O, (IV), were synthesized, and their structures were elucidated by spectral studies and finally confirmed by single‐crystal X‐ray diffraction. The (Me)C—O—C—C(OH) torsion angle of (III) is 146.41 (14)°, whereas the corresponding torsion angle of (IV) is −146.4 (7)°, indicating a different conformation at the C24 position. The crystal stacking in (III) generates an R₄⁴(8) motif, through which the molecules are linked into a one‐dimensional double chain. The chains are linked via nonclassical C—H…O hydrogen bonds into a two‐dimensional network, and further stacked into a three‐dimensional structure. In contrast to (III), epimer (IV) crystallizes as a hydrate, in which the water molecules act as hydrogen‐bond donors linking one‐dimensional chains into a two‐dimensional network through intermolecular O—H…O hydrogen bonds. The hydrogen‐bonded chains extend helically along the crystallographic a axis and generate a C₄⁴(8) motif.     

Single‐Molecule Spectroscopy on a Ladder‐Type Conjugated Polymer: Electron–Phonon Coupling and Spectral Diffusion

We employ low‐temperature single‐molecule spectroscopy combined with pattern recognition techniques for data analysis on a methyl‐substituted ladder‐type poly(para‐phenylene) (MeLPPP) to investigate the electron–phonon coupling to low‐energy vibrational modes as well as the origin of the strong spectral diffusion processes observed for this conjugated polymer. The results indicate weak electron–phonon coupling to low‐frequency vibrations of the surrounding matrix of the chromophores, and that low‐energy intrachain vibrations of the conjugated backbone do not couple to the electronic transitions of MeLPPP at low temperatures. Furthermore, these findings suggest that the main line‐broadening mechanism of the zero‐phonon lines of MeLPPP is fast, unresolved spectral diffusion, which arises from conformational fluctuations of the side groups attached to the MeLPPP backbone as well as of the surrounding host material.     

High‐ and low‐temperature phases in isostructural 4‐chloro‐3‐nitroaniline and 4‐iodo‐3‐nitroaniline

The structures of 4‐chloro‐3‐nitroaniline, C₆H₅ClN₂O₂, (I), and 4‐iodo‐3‐nitroaniline, C₆H₅IN₂O₂, (II), are isomorphs and both undergo continuous (second order) phase transitions at 237 and 200 K, respectively. The structures, as well as their phase transitions, have been studied by single‐crystal X‐ray diffraction, Raman spectroscopy and difference scanning calorimetry experiments. Both high‐temperature phases (293 K) show disorder of the nitro substituents, which are inclined towards the benzene‐ring planes at two different orientations. In the low‐temperature phases (120 K), both inclination angles are well maintained, while the disorder is removed. Concomitantly, the b axis doubles with respect to the room‐temperature cell. Each of the low‐temperature phases of (I) and (II) contains two pairs of independent molecules, where the molecules in each pair are related by noncrystallographic inversion centres. The molecules within each pair have the same absolute value of the inclination angle. The Flack parameter of the low‐temperature phases is very close to 0.5, indicating inversion twinning. This can be envisaged as stacking faults in the low‐temperature phases. It seems that competition between the primary amine–nitro N—H...O hydrogen bonds which form three‐centred hydrogen bonds is the reason for the disorder of the nitro groups, as well as for the phase transition in both (I) and (II). The backbones of the structures are formed by N—H...N hydrogen bonding of moderate strength which results in the graph‐set motif C(3). This graph‐set motif forms a zigzag chain parallel to the monoclinic b axis and is maintained in both the high‐ and the low‐temperature structures. The primary amine groups are pyramidal, with similar geometric values in all four determinations. The high‐temperature phase of (II) has been described previously [Garden et al. (2004). Acta Cryst. C 60 , o328–o330].     

Heterocyclic tautomerism: reassignment of two crystal structures of 2‐amino‐1,3‐thiazolidin‐4‐one derivatives

The structures of 5‐(2‐hydroxyethyl)‐2‐[(pyridin‐2‐yl)amino]‐1,3‐thiazolidin‐4‐one, C₁₀H₁₁N₃O₂S, (I), and ethyl 4‐[(4‐oxo‐1,3‐thiazolidin‐2‐yl)amino]benzoate, C₁₂H₁₂N₂O₃S, (II), which are identical to the entries with refcodes GACXOZ [Váňa et al. (2009). J. Heterocycl. Chem. 46 , 635–639] and HEGLUC [Behbehani & Ibrahim (2012). Molecules, 17 , 6362–6385], respectively, in the Cambridge Structural Database [Allen (2002). Acta Cryst. B 58 , 380–388], have been redetermined at 130 K. This structural study shows that both investigated compounds exist in their crystal structures as the tautomer with the carbonyl–imine group in the five‐membered heterocyclic ring and an exocyclic amine N atom, rather than the previously reported tautomer with a secondary amide group and an exocyclic imine N atom. The physicochemical and spectroscopic data of the two investigated compounds are the same as those of GACXOZ and HEGLUC, respectively. In the thiazolidin‐4‐one system of (I), the S and chiral C atoms, along with the hydroxyethyl group, are disordered. The thiazolidin‐4‐one fragment takes up two alternative locations in the crystal structure, which allows the molecule to adopt R and S configurations. The occupancy factors of the disordered atoms are 0.883 (2) (for the R configuration) and 0.117 (2) (for the S configuration). In (I), the main factor that determines the crystal packing is a system of hydrogen bonds, involving both strong N—H...N and O—H...O and weak C—H...O hydrogen bonds, linking the molecules into a three‐dimensional hydrogen‐bond network. On the other hand, in (II), the molecules are linked via N—H...O hydrogen bonds into chains.     

ss‐NMR and single‐crystal X‐ray diffraction in the elucidation of a new polymorph of bischalcone (1E,4E)‐1,5‐bis(4‐fluorophenyl)penta‐1,4‐dien‐3‐one

We report a new polymorph of (1E,4E)‐1,5‐bis(4‐fluorophenyl)penta‐1,4‐dien‐3‐one, C₁₇H₁₂F₂O. Contrary to the precedent literature polymorph with Z′ = 3, our polymorph has one half molecule in the asymmetric unit disordered over two 50% occupancy sites. Each site corresponds to one conformation around the single bond vicinal to the carbonyl group (so‐called anti or syn). The other half of the bischalcone is generated by twofold rotation symmetry, giving rise to two half‐occupied and overlapping molecules presenting both anti and syn conformations in their open chain. Such a disorder allows for distinct patterns of intermolecular C—H…O contacts involving the carbonyl and anti‐oriented β‐C—H groups, which is reflected in three ¹³C NMR chemical shifts for the carbonyl C atom. Here, we have also assessed the cytotoxicity of three symmetric bischalcones through their in vitro antitumour potential against three cancer cell lines. Cytotoxicity assays revealed that this biological property increases as halogen electronegativity increases.