Role of structural flexibility in the fluorescence and photochromism of salicylideneaniline: the general scheme of the phototransformations

The mechanism of light-induced transformation in the salicylideneaniline molecule was studied by semiempirical PM3 calculations. The structures and energies of the minima and saddle points (transition states) on the S₀, S₁ and T₁ potential energy hypersurfaces (PESs) were obtained, together with the gradient lines on the PESs. The structure-energy scheme was compared with the experimental findings. According to the results obtained, the following principle processes are observed: fast S₁ excited state intramolecular proton transfer (ESIPT), followed by typical ESIPT fluorescence; the formation of two S₁ twisted intramolecular charge transfer (TICT) structures which quench the ESIPT fluorescence; the diabatic formation of two ground state metastable coloured “post-TICT” structures responsible for photochromism.     

Direct preparation and structure determination of tertiary and secondary amine boranes from primary or secondary amine boranes

New secondary and tertiary amine borane derivatives were prepared in a one-pot reaction starting from primary amine boranes. The reaction involves treatment of an amine borane with 2 equivalents of s-BuLi at −78 °C. In general, mixtures of mono and di metallated products were obtained. Alkyl iodides and benzyl chloride reacted with the lithiated amine, but aldehydes and ketones were reduced. Conversion was high as determined by NMR, but moderate to low yields were obtained after chromatography, possibly due to decomposition on silica. Crystal structures were obtained for the compounds 3a, 3b and 3c.     

Negatively charged hyperbranched polyether-based polyelectrolytes

The preparation of carboxylated hyperbranched polyglycerols of narrow polydispersity was achieved by modification (78–90%) of the hydroxyl end groups via Michael addition of acrylonitrile, followed by hydrolysis. High conversion could only be achieved for low molecular weight starting materials (520 and 1,030 g mol⁻¹). The solution properties of the resulting materials were investigated by dynamic light scattering (DLS), showing the formation of large aggregates with size depending on the pH value. After deposition on a negatively charged mica surface, the structures observed by atomic force microscope (AFM) show the coexistence of aggregates and single macromolecules. Most interesting, in the case of the lower molecular weight sample (PG 520 g mol⁻¹), extended and ordered terrace structures were formed, which are unprecedented for hyperbranched polymers and are of interest for surface modification in general.     

Synthesis and structural characterization of [PhE(BNMe2)2]2 (E = P or As): Six-Membered P2B4 and As2B4 Rings with Unusual Structures

The reaction of either Li₂PPh or Li₂AsPh with the diborane(4) derivative B₂(NMe₂)₂Br₂ affords the compounds [PhP(BNMe₂)₂]₂ ( 1 ) or [PhAs(BNMe₂)₂]₂ ( 2 ) in good yield. Both 1 and 2 have cyclic structures featuring non-planar P₂B₄ or As₂B₄ six-membered rings which have chair configurations. Although all four borons in each ring have planar coordination, the two phosphorus or arsenic centers have different degrees of pyramidalization. Bond distances within the rings indicate that the B? B, B? P or B? As bonds are single, whereas the exo-B? N bond lengths are consistent with significant π-bonding. The ring structures of 1 and 2 are in sharp contrast to the related boron-nitrogen species (t-BuN)₂N₄Me₄ which has a nido-N₂B₄ framework. The attempted synthesis of the nitrogen analogue of 1 or 2 by using a similar approach did not result in the isolation of [PhN(BNMe₂)₂]₂, instead the tetramino diborane(4) species [B(NMe₂)NHPh]₂ ( 3 ), which has a structure similar to other tetramine diborane(4) compounds, was isolated.     

MENTHOR,a database system for the storage and retrieval of three-dimensional molecular structures and associated data searchable by substructural,biologic, physical,or geometric properties

Summary MENTHOR is a database system for the storage and retrieval of three-dimensional coordinate and charge information on molecules as well as of traditional biological and physical properties. Our molecular graphics system retrieves from MENTHOR structural information in individual molecules and receptor map/macromolecular binding site hypotheses. Substructural searches of MENTHOR are used to find starting coordinates for molecular modeling and traditional database searches of MENTHOR identify compounds for which modeling is needed. It also forms the data to be searched with ALLADDIN, our substructure/geometric search program. MENTHOR expedites molecular modeling by organizing previous work and facilitating transmission of information between individuals. Examples from modeling of D-2 receptor agonists are shown.     

Aldol condensations over reconstructed Mg-Al hydrotalcites: structure-activity relationships related to the rehydration method

Two different rehydration procedures in the liquid or gas phase have been applied to reconstruct mixed oxides derived from calcined hydrotalcite-like materials to be used as catalysts for aldol condensation reactions. The as-synthesized hydrotalcite, its decomposition product, as well as the reconstructed solids upon rehydration were characterized by XRD, N(2) adsorption, He pycnometry, FTIR, SEM, TEM, (27)Al MAS-NMR and CO(2)-TPD (TPD=temperature-programmed desorption). Compared to the Mg-Al mixed oxide rehydrated in the gas phase (HT-rg), that rehydrated in the liquid phase (HT-rl) exhibits a superior catalytic performance with respect to the aldol condensation of citral with ketones to yield pseudoionones and in the self-aldolization of acetone. The textural properties of HT-rl and HT-rg differ strongly and determine the catalytic behavior. A memory effect led to a higher degree of reconstruction of the lamellar structure when the mixed oxide was rehydrated in the gas phase rather than in the liquid phase, although liquid-phase rehydration under fast stirring produced a surface area that was 26 times greater. This contrasts to typical statements in the literature claiming a higher degree of reconstruction in the presence of large amounts of water in the medium. CO(2)-TPD shows that the number of OH(-) groups and their nature are very similar in HT-rg and HT-rl, and cannot explain the markedly different catalytic behavior. Accordingly, only a small fraction of the available basic sites in the rehydrated samples is active in liquid-phase aldol condensations. Our results support the model in which only basic sites near the edges of the hydrotalcite platelets are partaking in aldol reactions. Based on this, reconstructed materials with small crystallites (produced by exfoliation during mechanical stirring), that is, possessing a high external surface area, are beneficial in the reactions compared to larger crystals with a high degree of intraplatelet porosity.     

A facile synthesis of solid-emissive fluorescent dyes: dialkylbenzo[b]naphtho[2,1-d]furan-6-one-type fluorophores with strong blue and green fluorescence emission properties

A new type of organic fluorophores, dialkylbenzo[b]naphtho[2,1-d]furan-6-one-type fluorophores, exhibiting strong blue and green emission in the solid state has been easily synthesized by an one-step reaction. The X-ray crystal structure demonstrated that the structural form with a chair-shape with the sterical hindered dialkyl substituents and the 9-dibutylamino group prevents the fluorophores from forming short intermolecular contacts and produces intense solid-state fluorescence emission.     

Taking snapshots of photoexcited molecules in disordered media by using pulsed synchrotron X-rays

Photoexcited molecules are quintessential reactants in photochemistry. Structures of these photoexcited molecules in disordered media in which a majority of photochemical reactions take place remained elusive for decades owing to a lack of suitable X-ray sources, despite their importance in understanding fundamental aspects in photochemistry. As new pulsed X-ray sources become available, short-lived excited-state molecular structures in disordered media can now be captured by using laser-pulse pump, X-ray pulse-probe techniques of third-generation synchrotron sources with time resolutions of 30-100 ps, as demonstrated by examples in this review. These studies provide unprecedented information on structural origins of molecular properties in the excited states. By using other ultrafast X-ray facilities that will be completed in the near future, time-resolution for the excited-state structure measurements should reach the femtosecond time scales, which will make "molecular movies" of bond breaking or formation, and vibrational relaxation, a reality.     

Synthesis and solid-state structures of new cyclophane host molecules

Five new cyclophane host molecules (corrals) are prepared by linking together two α,α′-di(4-hydroxyphenyl)-1,4-diisopropylbenzene or α,α′-di(3,5-dimethyl-4-hydroxyphenyl)-1,4-diisopropylbenzene units with two permethylene spacers. Three small cyclophane hosts (boxes) are synthesized by cyclization of α,α′-di(4-hydroxyphenyl)-1,4-diisopropylbenzene with di(bromomethyl)benzene compounds. Solid-state structures of one corral and one box are reported.     

Homocoenzyme B12 and Bishomocoenzyme B12: Covalent Structural Mimics for Homolyzed,Enzyme‐Bound Coenzyme B12

Efficient electrochemical syntheses of “homocoenzyme B₁₂” ( 2 , Co_β‐(5′‐deoxy‐5′‐adenosyl‐methyl)‐cob(III )alamin) and “bishomocoenzyme B₁₂” ( 3 , Co_β‐[2‐(5′‐deoxy‐5′‐adenosyl)‐ethyl]‐cob(III )alamin) are reported here. These syntheses have provided crystalline samples of 2 and 3 in 94 and 77 % yield, respectively. In addition, in‐depth investigations of the structures of 2 and 3 in solution were carried out and a high‐resolution crystal structure of 2 was obtained. The two homologues of coenzyme B₁₂ ( 2 and 3 ) are suggested to function as covalent structural mimics of the hypothetical enzyme‐bound “activated” (that is, “stretched” or even homolytically cleaved) states of the B₁₂ cofactor. From crude molecular models, the crucial distances from the corrin‐bound cobalt center to the C5′ atom of the (homo)adenosine moieties in 2 and 3 were estimated to be about 3.0 and 4.4 Å, respectively. These values are roughly the same as those found in the two “activated” forms of coenzyme B₁₂ in the crystal structure of glutamate mutase. Indeed, in the crystal structure of 2 , the cobalt center was observed to be at a distance of 2.99 Å from the C5′ atom of the homoadenosine moiety and the latter was found to be present in the unusual syn conformation. In solution, the organometallic moieties of 2 and 3 were shown to be rather flexible and to be considerably more dynamic than the equivalent group in coenzyme B₁₂. The homoadenosine moiety of 2 was indicated to occur in both the syn and the anti conformations.