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Publication Metadata only A quantum mechanical study of the electrochemical polymerization of pyrrole(Elsevier Science Sa, 2001) Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Mechanism for the electrochemical polymerization of pyrrole is studied using accurate density functional theory; (DFT) calculations. The primary emphasis is on the structures and stability of intermediates generated during various mechanisms. Structures of the radical cations, which play role in reactions, an optimized to elucidate radical-radical and radical-neutral pathways. The competing probabilities of reactions between various size oligomers are discussed in terms of their thermodynamical stability.Publication Metadata only A theoretical study of structural defects in conjugated polymers(Elsevier Science Sa, 1999) Yurtsever, Mine; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Accurate ab-initio calculations are performed for pyrrole and thiophene oligomers bonded through alpha and beta carbons. The thermodynamical stabilitiy of all possible binding types including the branched forms of tetramers and pentamers are compared. Employing the probabilities obtained from these calculations, a Monte Carlo type growth scheme is applied to predict branching as functions of the chain length and temperature. A high degree of branching for polypyrrole is reported whereas the linear chains dominate the structure of polythiophene.Publication Metadata only AuPt alloy nanoparticles supported on graphitic carbon nitride: in situ synthesis and superb catalytic performance in the light-assisted hydrolytic dehydrogenation of ammonia borane(Elsevier, 2022) N/A; Department of Chemistry; Department of Chemistry; Aksoy, Merve; Korkut, Sibel Eken; Metin, Önder; PhD Student; Researcher; Faculty Member; Department of Chemistry; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); N/A; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM) / Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; N/A; N/A; 46962Addressed herein is the enhancement of catalytic activity of Pt-based nanocatalysts in the hydrolysis of ammonia borane (AB) via in-situ synthesis of bimetallic AuPt alloy nanoparticles (NPs) supported on graphitic carbon nitride (gCN). The presented in-situ synthesis protocol yielded gCN/AuxPt100-x (x = 0, 8, 15, 33) nanocatalysts with highly dispersed AuxPt100-x NPs having the average particle sizes varied in the range of 1.6-2.6 nm over the gCN nanosheets. The generated gCN/Pt92Au8 (600.3 mol H-2 mol Pt-(1) min(-1)) and gCN/Pt85Au15 (587.1 mol H2 mol Pt-1 min(-1)) nanocatalysts showed higher catalytic activity compared to gCN/Pt-100 (525.7 mol H-2 mol Pt-1 min(-1)) under white-light irradiation, attributed to the synergistic effects aroused in the AuPt alloy NPs and heterojunctions formed between gCN and AuPt alloy NPs. The detailed characterization of photophysical properties of gCN/AuxPt100-x nanocatalysts revealed that their boosted catalytic activity is attributed to the improved charge kinetics, higher light absorption, and effective electron transfer channels from gCN to the bimetallic AuPt alloy NPs. The role of photogenerated carriers in the photocatalytic AB dehydrogenation was also elucidated via scavenger studies. This study shows that gCN/AuxPt100-x nanocatalysts can be prepared in situ during the hydrolysis of AB at room temperature and the yielded nanocatalysts have a significant role in boosting the hydrogen production from the light-assisted hydrolysis of AB.Publication Metadata only Coupled cavity DQW semiconductor lasers(Elsevier Science Sa, 2000) Sağol, B.E.; Avrutin, Eugene A.; De la Rue, R.M.; Laybourn, P.J.R.; Stanley, C.R.; Department of Physics; Serpengüzel, Ali; Faculty Member; Department of Physics; College of Sciences; 27855Coupled cavity effects has been observed in the electroluminescence spectra of monolithic GaAs/GaAlAs double quantum well graded index separate confinement heterostructure semiconductor diode lasers. A time domain analysis has been performed in order to simulate the experimentally observed results. The theoretically calculated spectra are in good agreement with the experimentally observed spectra.Publication Metadata only Electronic excitations in stacked oligothiophenes(Elsevier, 2001) Yurtsever, Mine; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Electronic spectroscopy of oligothiophenes is studied using the semi-empirical ZINDO methodology. Geometry of the neutral, and doubly charged bipolaronic forms of oligomers from 3T to 8T are optimized. Their coordinates are then used to generate various stacked structures up to tetramers. The effects of the length of the chains, number of layers in stacks, stacking distance between chains, deviation from planarity and number and relative positions of bipolaronic forms on the shapes of the electronic excitations are investigated.Publication Metadata only Multiple bipolaron formation in oligothiophenes(Elsevier Science Sa, 1999) Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Semi-empirical calculations on large oligomers of thiophene containing up to 24 monomer units are carried out. The geometry of neutral as well as highly oxidized structures with charges of +2, +4, and +6 are optimized in order to understand the charge localization in the absence of counter ions. The bond length alternation in the polymer backbone is shown to provide a useful tool for studying bipolaron formation. The effects of defects such as branches, alpha-beta bondings or deviations from planarity on the structures of oxidized forms are investigated. The bipolarons are found to spread over six monomer units. Defects disturb the bipolaron formation and possibly impede the mobility of them.Publication Metadata only On the possibility of grafting conducting polymers into insulating ones(Elsevier, 1996) Bahçeci, Salim; Toppare, Levent; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129The possibility of grafting between conducting polymers, like polypyrrole (PPy) and polyaniline (PAn), and insulating polymers, such as polybisphenol A carbonate (PC) and polyamide (PA), is studied via semi-empirical methods using the AM1 parametrization. Several experimental studies on the issue have previously revealed that a chemical interaction exists between the couples (PAn-PC, PPy-PC and PPy-PA) during the electrochemical synthesis of PAn and PPy in the insulating host matrices. Here we present additional theoretical evidence indicating that such grafting is possible, at least for small oligomers.Publication Metadata only Optimization of argon-air DBD plasma-assisted grafting of polyacrylic acid on electrospun POSS-PCUU(Pergamon-Elsevier Science Ltd, 2023) Salehi, Roya; Mahkam, Mehrdad; Siahpoush, Vahid; Rahbarghazi, Reza; l; Abbasi, Farhang; Gargari, Ziba Zakeri; Sokullu, Emel; Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); School of MedicineIt is accepted that significant interfacial reactions take place in engineered tissues between biomaterial surfaces and the host's tissue in the body. The lack of appropriate functional groups limits long-term biocompatibility and successful biological response of biomaterials. Therefore, the cell-biomaterial affinity should be increased by functional groups grafting to the surface of biomaterials which provide the basic properties of the desired tissue. For the first time in this study, PAAc grafting was performed using two-step argon-air DBD plasma at atmospheric pressure in a few seconds of exposure time, to modify the surface of POSS-PCUU nanofibers to selectively in-crease their superficial properties while maintaining the required mechanical properties. The Response Surface Methodology was used for experimental design to optimize the operating conditions of carboxylic acid grafting at the electrospun POSS-PCUU surface. Nanofiber surface modification was confirmed using ATR-FTIR, FE-SEM, AFM, WCA, and tensile test. The grafting of PAAc to the nanofiber surface was proved by the presence of a broad hydroxyl band in ATR-FTIR spectrum, the morphological changes observed in the SEM and AFM images, and the reduction of the water contact angle. The stress-strain behavior at the optimum point also showed an acceptable reduction in tensile strength. Furthermore, the effects of two variables, plasma processing time and plasma copolymerization time were optimized and investigated using the CCD method at five levels of carboxylic acid grafting density. The grafting of PAAc onto the nanofiber surface (73.69 +/- 2.1 mu g/cm2) produced at reaction conditions displayed great agreement with the predicted results by the model. Results showed that the modified PAAc-POSS-PCUU nanofibers will be a desirable surface for the immobilization of various ECM proteins with high potential in small-diameter vascular graft applications.Publication Metadata only Rotational potential functions for oligomethylthiophenes(Elsevier Science Sa, 2003) Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129The bulk structure of polythiophenes is mostly determined by the rotational degrees of freedom as tyre are various packing possibilities within the bulk structure. The optical and electronic properties are also closely associated with the rotational motion around monomer-monomer bonds. Here we present accurate Density-Functional-Theory (DFT) calculations for head-to-tail methyl substituted oligothiophenes for generating a rotational force field The rotational potential function of the dieter is computed on a very fine grid and expanded in terms of the cosines of the torsional angle. The torsional space of the regular thiophene trimers and tetramers is fully spanned These energy values are compared to those obtained from additive potentials. The errors coming from this approach and several coupling possibilities to reduce the errors are discussed. It is noted that as the chain grows, the near-planar structures become more stable than the dieter calculations suggest.Publication Metadata only Structural studies of polypyrroles - I. an ab-initio evaluation of bonding through alpha and beta carbons(Elsevier, 1999) Yurtsever, Mine; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Detailed ab-initio calculations of a number of pyrrole oligomers are carried out in order to find out relative stability of structures bonded through alpha and beta carbons. Energetics of dimers, trimers and tetramers with all possible linkage types and several pentamers are obtained from fully optimized geometry. The linear as well as the branched forms of tetramers and pentamers are compared so that probabilities of branching can be estimated. The energy differences of different types of structures suggest that a great deal of branching is probable. It is shown that relative energies of 65 oligomers can be fairly reproduced by a simple partitioning in terms of monomer types. The approximate probability function generated in this manner is used in the following paper in a statistical mechanical approach to estimate the extent of bonding involving beta carbons as well as branching in polypyrroles.