Josemon Jacob

Upconversion photoluminescence in poly(ladder-type-pentaphenylene) doped with metal (II)-octaethyl porphyrins

We report on the optimization of the upconversion photoluminescence in films of conjugated polymers doped with platinum porphyrin. The upconversion emission was observed to take place at pump intensities as low as 0.5kW∕cm2. Comparison between the photoluminescence integral intensity of polyfluorene and ladder-type pentaphenylene polymer shows a five-fold increase of the efficiency of the upconversion process for the latter. The higher upconversion efficiency can be partially attributed to the reduced reabsorption of the photoluminescence in the case of the ladder-type pentaphenylene matrix.

Swapping field-effect transistor characteristics in polymeric diketopyrrolopyrrole semiconductors: debut of an electron dominant transporting polymer

A fact-finding study on thiophenyl diketopyrrolopyrrole (TDPP)-containing polymers for electronically convertible transport characteristics in organic field effect transistors (OFETs) is presented. In the subject of this consideration, a TDPP-based polymer with bis-benzothiadiazole (BisBT) units that serve as powerful electron-deficient building blocks, namely PDTDPP–BisBT, is prepared in order to achieve an n-channel transistor. The resulting polymer shows n-channel dominant ambipolar OFET characteristics and its electron mobility (1.3 × 10−3 cm2 V−1 s−1) is found to be one order of magnitude higher than the hole mobility. Besides, the PDTDPP–BisBTOFET performance is independent of film-deposition conditions due to its completely amorphous microstructure, supported by the atomic force microscopy (AFM) and X-ray diffraction (XRD) analyses. Herein, we report an intriguing discovery in sync with our previous studies that TDPP-based polymers can function as a p-type, n-type, or ambipolar organic semiconductor in accordance with the degree of electron affinity of the comonomers.

Superexchange-mediated electronic energy transfer in a model dyad

On the basis of time-dependent density functional theory (TD-DFT) calculations coupled to the polarizable continuum model (PCM) and single molecule spectroscopic studies, we provide a detailed investigation of excitation energy transfer within a model bi-chromophoric system where a perylene monoimide (PMI) donor is bridged to a terrylene diimide (TDI) acceptor through a ladder-type pentaphenylene (pPh) spacer. We find that the electronic excitation on the PMI donor significantly extends over the bridge giving rise to a partial charge transfer character and inducing a approximately 3-fold increase in the electronic interaction between the chromophores, which explains the failure of the Förster model in reproducing the observed energy migration rates when treating PMI as the donor. However, despite an increased charge transfer contribution in the effective donor state, the increase in solvent polarity is not accompanied by an enhancement in the electronic coupling between the subunits, which is rationalized from a detailed analysis of the excited-state wavefunctions.

Selective C-H bond activation of arenes catalyzed by methylrhenium trioxide

Abstract Arenes, in glacial acetic acid, are oxidized to para -benzoquinones by hydrogen peroxide when methylrhenium trioxide (CH 3 ReO 3 or MTO) is used as a catalyst. In some cases an intermediate hydroquinone was also obtained in lower yield. Oxidation of the methyl side chains of various methylbenzenes did not occur. The active catalyst species are the previously-characterized η 2 -peroxorhenium complexes, CH 3 Re(O) 2 ( η 2 -O 2 ) and CH 3 Re(O)( η 2 -O 2 ) 2 H 2 O). Separate tests showed that hydroquinones and phenols are oxidized by H 2 O 2 -MTO more rapidly than the simple arenes; in the proposed mechanism they are intermediate products. Higher conversions were found for the more highly-substituted arches, consistent with their being the most reactive species toward the electrophillically-active peroxide bound to rhenium. High conversions of the less substituted members of the series were not achieved, reflecting concurrent deactivation of MTO-peroxide, a process of greater import for the more slowly-reacting substrates.

Low-threshold amplified spontaneous emission in thin films of poly(tetraarylindenofluorene)

We report on amplification of blue light (469 nm) via amplified spontaneous emission in asymmetric planar waveguides of a fully arylated polyindenofluorene derivative. A very low threshold value of about 3μJ∕cm2(∼375W∕cm2) for amplification of light has been demonstrated by pumping the waveguides at 3.0 eV (410 nm) with a stripe shape excitation beam. Moderate gain coefficients up to 21cm−1 and loss coefficients around 6cm−1 have been measured. All measurements were performed under ambient conditions. No sample degradation could be observed which testifies to the excellent stability of the material against photodegradation.

Cobalt-catalyzed selective conversion of diallylanilines and arylimines to quinolines

A variety of diallylanilines are shown to undergo cobalt-carbonyl catalyzed rearrangement to quinolines. Diallylanilines are also used as allyl transfer reagents to convert benzaldimines into quinolines. Substitution in the 2- and 3-positions of the quinoline is featured in all transformations.

Improving the layer morphology of solution-processed perylene diimide organic solar cells with the use of a polymeric interlayer

Abstract Herein we demonstrate a method to improve the power conversion efficiency (PCE) parameter of organic photovoltaic (OPV) devices based on the electron acceptor N,N’-bis(1- ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (PDI) blended with the electron donor poly(indenofluorene)-aryloctyl (PIF-Aryl). The device parameters of the short-circuit current, open-circuit voltage and fill factor are found increased after the insertion of a thin poly [9, 9-dioctylfluorene-co-N- [4-(3-methylpropyl)]-diphenylamine] (TFB) photoactive interlayer between the hole-collecting electrode and the photoactive layer of the device. Unlike to most of the cases where interlayers serve as charge extractors, in our system the polymeric interlayer serves as a morphology modifying agent that drives the PDI component to segregate better at the interface with the device cathode; that is at the carrier-collecting electrode interface, which is not in physical contact with the interlayer. The processes of energy/charge transfer of the TFB excitons to/with the PIF-Aryl:PDI top-layer are also addressed. Charge transfer reactions dominate at the TFB/PIF-Aryl:PDI interface but no significant contribution in the photocurrent generation is seen in the photoaction spectra of the bilayer device.

Interplay of α,α‐ versus α,β‐Conjugation in the Excited States and Charged Defects of Branched Oligothiophenes as Models for Dendrimeric Materials

This article investigates the excited and charged states of three branched oligothiophenes with methyl-thienyl side groups as models to promote 3D arrangements. A comparison with the properties of the parent systems, linear all-α,α-oligothiophenes, is proposed. A wide variety of spectroscopic methods (i.e., absorption, emission, triplet-triplet transient absorption, and spectroelectrochemistry) in combination with DFT calculations have been used for this purpose. Whereas the absorption spectra are slightly blueshifted upon branching, both the emission spectra and triplet-triplet absorption spectra are moderately redshifted; this indicates a larger contribution of the β-linked thienyl groups in the delocalization of the S1 and T1 states rather than into the S0 state. The delocalization through the α,β-conjugated path was found to be crucial for the stabilization of the trication species in the larger branched systems, whereas the linear sexithiophene homologue can only be stabilized up to the dication species.

Nonisothermal crystallization and microstructural behavior of poly(ε-caprolactone) and granular tapioca starch-based biocomposites

ABSTRACT The nonisothermal crystallization behavior of poly(ε-caprolactone) (PCL) in the presence of varying concentrations of granular tapioca starch (GTS) was studied. Various crystallization parameters were studied by differential scanning calorimeter at four different cooling rates and these parameters were analyzed using Jeziorny, Ozawa, and Liu models. Kissinger method was used to estimate the activation energy (ΔE) of the PCL/GTS composites. The ΔE results suggested that the speed of crystallization was inhibited by GTS particles. Polarized light optical microscopy suggests formation of spherulite structure in PCL and PCL/GTS composites while no evidence of nucleation by GTS particles was observed.

Polymer-Based LEDs and Solar Cells

The efficiency of LEDs based on conjugated polymers have been enhanced so that they are now commercially viable. This has occurred in large part by better synthetic design of emissive materials, based on an improved understanding of reaction mechanisms and how these permit defect formation through side reactions, thus enabling their prevention by modification of synthetic route or monomer structure. The performance of polymer-based solar cells has also been improved so that they are now close to commercialization. Again this has been due to synthetic design based on an enhanced knowledge of the underlying processes affecting efficiency, enabling a better understanding of structure-property relationships. The prospects for the future of both classes of device are assessed.