Modular Synthesis of Polybenzo[b]silole Compounds for Hole‐Blocking Material in Phosphorescent Organic Light Emitting Diodes

A diversity‐oriented synthetic strategy allowed us to design a series of conjugated molecules containing multiple benzosilole units that can be utilized as efficient hole‐blocking materials for phosphorescent organic light emitting diodes (OLEDs). Some of these compounds showed a performance surpassing that of the current standard, bathocuproine. The new compounds were easily synthesized in a modular fashion from a previously reported 3‐stannyl benzosilole building unit. Studies on the properties of these compounds in solution and in the solid state indicate that they possess high electron affinity, high ionization potential, and form stable amorphous films that show high electron‐drift mobility. The correlation between their molecular properties and the efficiency of the OLED device performance is also investigated.     

The structure of the complex of cellulose I with ethylenediamine by X-ray crystallography and cross-polarization/magic angle spinning 13C nuclear magnetic resonance

X-ray crystallographic and cross-polarization/magic angle spinning ¹³C nuclear magnetic resonance techniques have been used to study an ethylenediamine (EDA)-cellulose I complex, a transient structure in the cellulose I to cellulose III_I conversion. The crystal structure (space group P2 ₁ ; a = 4.546 Å, b = 11.330 Å, c = 10.368 Å and γ = 94.017°) corresponds to a one-chain unit cell with one glucosyl residue in the asymmetric unit, a gt conformation for the hydroxymethyl group, and one EDA molecule per glucosyl residue. Unusually, there are no O–H···O hydrogen bonds between the cellulose chains; the chains are arranged in hydrophobic stacks, stabilized by hydrogen bonds to the amine groups of bridging EDA molecules. This new structure is an example of a complex in which the cellulose chains are isolated from each other, and provides a number of insights into the structural pathway followed during the conversion of cellulose I to cellulose III_I through EDA treatment.     

Synthesis and properties of high‐molecular‐weight stereo di‐block polylactides with nonequivalent D/L ratios

Di‐stereoblock polylactides (di‐sb‐PLA: PLLA‐b‐PDLA) having high molecular weight (M_n > 100 kDa) were successfully synthesized by two‐step ring‐opening polymerization (ROP) of L ‐ and D ‐lactides using tin(2‐ethylhexanoate) as a catalyst. By optimizing the polymerization conditions, the block sequences were well regulated at non‐equivalent feed ratios of PLLA and PDLA. This synthetic method consisted of three stages: (1) polymerization of either L ‐ or D ‐lactide to obtain a PLLA or PDLA prepolymer with a molecular weight less than 50 kDa, (2) purification of the obtained prepolymer to remove residual lactide, and (3) polymerization of the enantiomeric lactide in the presence of the purified prepolymer. Their ¹³C and ³¹P NMR spectra of the resultant di‐sb‐PLAs strongly supported their di‐stereo block structure. These di‐sb‐PLAs, having weight‐average molecular weights higher than 150 kDa, were fabricated into polymer films by solution casting and showed exclusive stereocomplexation. The thermomechanical analysis of the films revealed that their heat deformation temperature was limited probably because of their low crystallinity owing to the non‐equivalent PLLA/PDLA ratio. The blend systems of the di‐sb‐PLAs having complementary stereo‐sequences (the one with a long PLLA block and the other with long PDLA block) were also prepared and characterized to enhance the sc crystallinity. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 794–801, 2010     

Synthesis and Structure–Activity Relationship of Vicenistatin,a Cytotoxic 20‐Membered Macrolactam Glycoside

We have developed two syntheses of vicenistatin and its analogues. Our first‐generation strategy included the rapid and sequential assembly of the macrocyclic lactam by using an intermolecular Horner–Wadsworth–Emmons reaction between the C3–C13 fragment and the C1–C2, C14–C19 fragment, followed by an intramolecular Stille coupling reaction. The second‐generation strategy utilized a ring‐closing metathesis of a hexaene intermediate to generate the desired 20‐membered macrolactam. This second‐generation strategy made it possible to prepare synthetic analogues of vicenistatin, including the C20‐ and/or C23‐demethyl analogues. Evaluation of the cytotoxic effect of these analogues indicated the importance of the fixed conformation of aglycon for determining the biological activity of the vicenistatins.     

Study on δ‐Form Complex in a Syndiotactic Polystyrene/Organic Molecules System, 1. Preferential Complexing Behavior of Xylene Isomers

To investigate the preferential complexing behavior of isomeric xylenes, syndiotactic polystyrene (sPS) membranes are prepared using varying compositions of m‐ and p‐xylene. Complex formation between sPS and the xylenes was studied by means of thermogravimetric and FT‐IR analyses to determine the exact amounts of solvent molecules present per styrene repeating unit. A preferential complexing ability of p‐xylene was revealed due to its favorable interaction with sPS.     

Effective synthesis of diiodinated picene and dibenzo[a,h]anthracene by AuCl-catalyzed double cyclization

6,13-Diiododibenzo[a,h]anthracene and 5,8-diiodopicene were synthesized by AuCl-catalyzed double cyclization. The highly selective reaction yielded a new class of peri-halogenated fused aromatics.     

Diversity of potential hydrogen bonds in cellulose I revealed by molecular dynamics simulation

We have performed molecular dynamics calculations using a revised version of the Gromos56A_carbo force field to understand the consequences of the different potential hydrogen bonding patterns on the structural stability and thermal behavior of the I_α and I_β forms of native cellulose. For each allomorph, we considered three patterns of hydrogen bonds: two patterns obtained from neutron diffraction data refinement and a regular mixture of the two. Upon annealing, the hydrogen bonding schemes of cellulose I_β, irrespective of the starting structure, re-arranged into the main hydrogen bond pattern experimentally observed (pattern A). On the other hand, the I_α structures, irrespective of the starting hydrogen bonding pattern, converged to a non-experimental structure where the adjacent chains are shifted along the chain direction by 0.12 nm in the hydrogen-bonded plane, and the hydroxymethyl group conformation alternates between gt and tg along the chain. The exotic structure in I_α might be a consequence of a deficiency in force field parameters and/or potential molecular arrangement in less crystalline cellulose.     

Morphological changes in the cellulose and lignin components of biomass occur at different stages during steam pretreatment

Morphological changes to the different components of lignocellulosic biomass were observed as they occurred during steam pretreatment by placing a pressure reaction cell in a neutron beam and collecting time-resolved neutron scattering data. Changes to cellulose morphology occurred mainly in the heating phase, whereas changes in lignin morphology occurred mainly in the holding and cooling phases. During the heating stage, water is irreversibly expelled from cellulose microfibrils as the elementary fibrils coalesce. During the holding phase lignin aggregates begin to appear and they increase in size most noticeably during the cooling phase. This experiment demonstrates the unique information that in situ small angle neutron scattering studies of pretreatment can provide. This approach could be useful in optimizing the heating, holding and cooling stages of pretreatments to allow the exact size and nature of lignin aggregates to be controlled in order to enhance enzyme accessibility to cellulose and therefore the efficiency of biomass conversion.