K.M. Nalin de Silva

Non-linear optical properties of novel fluorenyl derivatives—ab initio quantum chemical calculations

Abstract We report accurate ab initio studies of the first static hyperpolarizabilities ( β ) of fluorenyl derivatives in which electron donating (D) and electron accepting (A) groups were introduced either side of the fluorenyl ring system. Geometries of all molecules were optimized at the Hartree–Fock level in a series of steps, first with the STO-3G minimal basis set, then with the 3-21G split valence basis set and finally with the 6-31G basis set. The first static hyperpolarizabilities of these molecules were calculated using Hartree–Fock level using 6-31G basis set using gaussian 98W. The calculated hyperpolarizabilities of these molecules were compared with biphenyl derivatives and other available data in the literature. To understand this phenomenon in the context of molecular orbital picture, we examined the molecular HOMOs and molecular LUMOs generated via gaussian 98W. The study reveals that the fluorenyl derivatives have large β values hence in general may have potential applications in the development of non-linear optical materials.

Push–pull porphyrins as non-linear optical materials: ab initio quantum chemical calculations

Abstract We report accurate ab initio quantum chemical calculations of the first static hyperpolarizability ( β ) of porphyrin systems containing porphyrin moiety as the electron donor and quinone and pyromellitimide as the electron acceptors separated by a spacer which influence the electronic communication between donor and acceptor. Geometries of all porphyrin molecules were optimised at the Hartree–Fock level with STO-3G minimal basis set and with the 3-21G split valence basis set using gaussian 98W. In addition to non-linear properties, polarizabilities ( α ) of these molecules are also reported. The calculated values of β were compared with β of tertraphenyl porphyrin derivatives reported in the literature. We examine the HOMO and LUMO generated via gaussian 98W in order to explain the variation of β in these highly conjugated molecular systems. The study suggested that these porphyrin molecules in general may have potential applications in the development of non-linear optical materials.

Self-Assembled Monolayer Initiated Electropolymerization: A Route to Thin-Film Materials with Enhanced Photovoltaic Performance

Continuing progress in the field of organic polymer photovoltaic (PV) devices requires the development of new materials with better charge-transport efficiency. To improve this parameter, we have investigated surface-attached bilayer polymer PV thin films prepared starting from a covalently attached monolayer of an electroactive initiator using sequential electropolymerization of dithiophene and its derivatives. These systems were found to show significantly increased photocurrent generation quantum yields as compared to systems made through conventional approaches. In addition, the described PV thin films possess remarkable mechanical, air, and photostability. These properties likely arise from the more uniform and better ordered bulk layer morphologies as well as tighter covalently bonded contacts at the interfacial junctions, contributing to improved charge transport. While more studies on the fundamental reasons behind the discovered phenomenon are currently underway, this information can be readily applied to build more efficient organic polymer photovoltaics.

Nano-MgO reinforced chitosan nanocomposites for high performance packaging applications with improved mechanical, thermal and barrier properties

Chitosan nanocomposite thin films were fabricated by incorporating MgO nanoparticles to significantly improve its physical properties for potential packaging applications. A novel in-situ method was developed to synthesise spherical shaped MgO nanoparticles by heat-treating magnesium carbonate/poly(methyl methacrylate) (PMMA) composite precursor. Optimum mechanical properties of chitosan composites were yielded at 5 (w/w%) of MgO concentration, where tensile stress and elastic modulus significantly improved by 86% and 38%, respectively, compared to those of pure chitosan films. These improvements are due to the interaction of hydroxyl and amine groups of chitosan with MgO as confirmed by FTIR spectroscopy. Fracture surface morphology indicated the interplay between MgO dispersion and aggregation on the mechanical properties at different MgO concentrations. Furthermore, the chitosan/MgO nanocomposites displayed remarkable thermal stability, flame retardant properties (satisfied V0 rating according to the UL-94 standards), UV shielding and moisture barrier properties, which could certainly add value to the packaging material.

A method for top down preparation of chitosan nanoparticles and nanofibers

A method of top down preparation of chitosan nanoparticles and nanofibers is proposed. Chitin nanofibrils (chitin NFs) were prepared using ultrasonic assisted method from crab shells with an average diameter of 5 nm and the length less than 3 μm as analyzed by atomic force microscopy and transmission electron microscopy. These chitin nanofibers were used as the precursor material for the preparation of chitosan nanoparticles and nanofibers. The degree of deacetylation of these prepared chitosan nanostructures were found to be approximately 98%. In addition these chitosan nanostructures showed amorphous crystallinity. Transmission electron microscopic studies revealed that chitosan nanoparticles were roughly spherical in nature and had diameters less than 300 nm. These larger particles formed through self-assembly of much smaller 25 nm particles as evidenced by the TEM imaging. The diameter and the length of the chitosan nanofibers were found to be less than 100 nm and 3 μm respectively. It is envisaged that due to the cavitation effect, the deacetylated chitin nanofibers were broken down to small pieces to form seed particles. These seed particles can then be self-assembled to form larger chitosan nanoparticles.

What Is the Smallest Saturated Acyclic Alkane that Cannot Be Made

Saturated acyclic alkanes may show a high degree of strain if they have many branches close to each other. We report calculations which indicate how strained a molecule may become before it falls apart rapidly at room temperature and so allow us to identify the simplest alkane which cannot be made.

Ultrasound energy to accelerate dye uptake and dye–fiber interaction of reactive dye on knitted cotton fabric at low temperatures

Acoustic cavitation formed due to propagation of ultrasound wave inside a dye bath was successfully used to dye cotton fabric with a reactive dye at lower temperatures. The energy input to the system during sonication was 0.7 W/cm(2). This was within the energy range that contributes towards forming cavitation during ultra-sonication. The influence of ultrasound treatment on dye particle size and fiber morphology is discussed. Particle size analysis of the dye bath revealed ultra-sonication energy was capable of de-agglomeration of hydrolyzed dye molecules during dyeing. SEM micrograph and AFM topographical image of the fiber surface revealed fiber morphology remains unchanged after the sonication. The study was extended in understanding the contribution of ultrasound method of dyeing towards achieving good color strength on the fabric, compared to the normal heating method of dyeing. Study showed color strength obtained using ultra sound method of dyeing is higher compared to normal heating dyeing. Ultrasound energy was able to achieve the good color strength on cotton fabric at very low temperature such as 30 °C, which was approximately 230% more than the color strength achieved in normal heating method of dyeing. This indicates that energy input to the system using ultrasound was capable of acting as an effective alternative method of dyeing knitted cotton fabrics with reactive dye.

A new role for surfactants in the formation of cobalt nanoparticles

During wet-chemical synthesis of nanoparticles it is believed that surfactants interact with the particles after their nucleation thereby controlling their crystal structure, shape and size. Contrary to this, our investigations presented in this paper reveal a new role for surfactants: influencing the reaction pathways, prior to nucleation, leading to the formation of cobalt nanoparticles. The results from the mechanistic investigation of the influence of various surfactants on the formation of cobalt nanoparticles carried out using time dependent FT-IR spectroscopy support this observation. The utilization of different surfactants such as oleic acid (OA), trioctylphosphine oxide (TOPO), octadecylamine (ODA), and trioctylphosphine (TOP) led to differences in reaction pathways and reaction intermediates, prior to the nucleation, leading to the formation of Co nanoparticles with very different properties. The particle size and size distribution were obtained from transmission electron microscopy (TEM). The electronic and geometric properties of the cobalt nanoparticles obtained were determined using synchrotron radiation based X-ray absorption spectroscopy (XAS) and the magnetic properties were measured using SQUID magnetometry.

A curcumin activated carboxymethyl cellulose–montmorillonite clay nanocomposite having enhanced curcumin release in aqueous media

A novel curcumin activated carboxymethylcellulose-montmorillonite nanocomposite is reported. A superabsorbent biopolymer; carboxymethyl cellulose (CMC) was used as an emulsifier for curcumin which is a turmeric derived water insoluble polyphenolic compound with antibacterial/anti-cancer properties. Montmorillonite (MMT) nanoclay was incorporated in the formulation as a matrix material which also plays a role in release kinetics. It was observed that water solubility of curcumin in the nanocomposite has significantly increased (60% release within 2h and 30 min in distilled water at pH 5.4) compared to pure curcumin. The prepared curcumin activated carboxymethylcellulose-montmorillonite nanocomposite is suitable as a curcumin carrier having enhanced release and structural properties.

Long-Chain 3,4-Ethylenedioxythiophene/Thiophene Oligomers and Semiconducting Thin Films Prepared by Their Electropolymerization

A series of soluble H-terminated conjugated oligomers incorporating 3,4-ethylenedioxythiophene (EDOT) combined with a small number of thiophene units and ranging in length from four to eight EDOT/thiophene groups was prepared with the ultimate goal to investigate if facile formation of a reactive trication radical species would enable electrochemical polymerization of such long-chain oligomers. Spectroscopic and electrochemical studies of the oligomers revealed some general dependencies of their electronic properties on the total number and position of EDOT groups. It was the number of consecutive EDOT units rather than total number of these units which was found to have the most profound effect on electronic energy gap and conjugation length. This influence originates from the especially strong planarization induced in the conjugated backbone by the incorporation of EDOT units. In contrast, incorporation of thiophene units was found to result in loss of the conformational stabilization. This phenomenon was analyzed using the natural bond orbital computational approach, which revealed the predominantly hyperconjugative nature of the EDOT-induced conformational stabilization. Whereas shorter oligomers, in agreement with the general consensus, were found to be inert toward electrochemical polymerization due to low reactivity of electrochemically generated cation radical and dication species, the longest oligomer showed an unprecedentedly efficient electropolymerization to yield a stable thin film of an electroactive polymer. The efficient electropolymerization of the long-chain oligomer was found to be in agreement with the formation of a reactive trication radical species. The electronic and spectral properties of the resulting semiconducting polymer film were studied by various electrochemical and spectroelectrochemical methods, as well as conductive probe AFM technique, and revealed a number of unusual features (such as electrical rectifying switching behavior) consistent with the possibility of increased molecular order in this material.