Browsing by Subject "Oligomers"

Now showing 1 - 15 of 15

Open Access
Air-stable n-channel diketopyrrolopyrrole-diketopyrrolopyrrole oligomers for high performance ambipolar organic transistors
(American Chemical Society, 2016) Mukhopadhyay, T.; Puttaraju, B.; Senanayak, S. P.; Sadhanala, A.; Friend, R.; Faber, H. A.; Anthopoulos, T. D.; Salzner, U.; Meyer A.; Patil, S.
n-channel organic semiconductors are prone to oxidation upon exposed to ambient conditions. Herein, we report design and synthesis of diketopyrrolopyrrole (DPP)-based oligomers for ambipolar organic thin-film transistors (OFETs) with excellent air and bias stability at ambient conditions. The cyclic voltammetry measurements reveal exceptional electrochemical stability during the redox cycle of oligomers. Structural properties including aggregation, crystallinity, and morphology in thin film were investigated by UV-visible spectroscopy, atomic force microscopy (AFM), thin-film X-ray diffraction (XRD), and grazing incidence small-angle X-ray scattering (GISAXS) measurements. AFM reveals morphological changes induced by different processing conditions whereas GISAXS measurements show an increase in the population of face-on oriented crystallites in films subjected to a combination of solvent and thermal treatments. These measurements also highlight the significance of chalcogen atom from sulfur to selenium on the photophysical, optical, electronic, and solid-state properties of DPP-DPP oligomers. Charge carrier mobilities of the oligomers were investigated by fabricating top-gate bottom-contact (TG-BC) thin-film transistors by annealing the thin films under various conditions. Combined solvent and thermal annealing of DPP-DPP oligomer thin films results in consistent electron mobilities as high as ∼0.2 cm2 V-1 s-1 with an on/off ratio exceeding 104. Field-effect behavior was retained for up to ∼4 weeks, which illustrates remarkable air and bias stability. This work paves the way toward the development of n-channel DPP-DPP-based oligomers exhibiting retention of field-effect behavior with superior stability at ambient conditions.
Open Access
Does the donor-acceptor concept work for designing synthetic metals? 2. theoretical investigation of copolymers of 4-(dicyanomethylene)-4H-cyclopenta[2, 1-b: 3, 4-b′]dithiophene and 3, 4-(ethylenedioxy)thiophene
(American Chemical Society, 2002) Salzner, U.; Köse, M. E.
Density functional theory (DFT) calculations were performed on oligomers of 3,4-(ethylenedioxy)thiophene (EDOT), 4-(dicyanomethylene)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CDM), and co-oligomers (CDM/ EDOT). Oligomer data were extrapolated to polymer values. Theoretical band gaps reproduce λmax from UV spectroscopy for PEDOT and are about 1 eV larger than electrochemical band gaps. λmax of PCDM/EDOT is predicted to be 0.42 eV smaller than that of PEDOT and 0.15 eV smaller than that of PCDM. PCDM/EDOT has a wide valence and an extremely narrow conduction "band". It is probably better not to refer to these localized states as a band at all. This rationalizes the mobility ratio of 500 between p-type and n-type charge carriers and the low n-type conductivity of PCDM/EDOT. The lack of dispersion of the conduction band is due to the very different EAs of EDOT and CDM.
Open Access
Effects of perfluorination on thiophene and pyrrole oligomers
(2010) Salzner, U.
The effect of perfluorination on thiophene and pyrrole oligomers in neutral, cationic, and anionic states was investigated with density functional theory at the (TD)B3P86-30%/6-31G* level. For the title compounds fluorination leads to planarization. For pyrroles a band gap reduction of 0.58 eV results, as unsubstituted pyrroles are nonplanar and disordered in the solid state. For thiophene the band gap is slightly increased as long thiophene oligomers are almost planar. Ionization energies and electron affinities increase upon fluorination by 0.65 and 0.60 eV for polythiophene and by 0.45 and 0.90 eV for polypyrrole. Conduction band widths increase by 0.5 for polythiophene and by 0.7 eV for polypyrrole. Spectra of charged (doped) forms are almost identical to those of the parent systems. Like parent systems, fluorinated oligomers with chain lengths of more than six rings develop a third UV absorption that increases in strength and decreases in energy upon chain length increase.
Open Access
Electronic structure of conducting organic polymers: insights from time-dependent density functional theory
(John Wiley & Sons Ltd., 2014) Salzner, U.
Conducting organic polymers (COPs) became an active field of research after it was discovered how thin films rather than insoluble infusible powders can be produced. The combination of the properties of plastics with those of semiconductors opened the research field of organic electronics. COPs share many electronic properties with inorganic semiconductors, but there are also major differences, e.g., the nature of the charge carriers and the amount of the exciton binding energy. Theoretical analysis has been used to interpret experimental observations early on. The polaron model that was developed from one-electron theories is still the most widely used concept. In the 1990s, time-dependent density functional theory (TDDFT) became available for routine calculations. Using TDDFT, electronic states of long oligomers can be calculated. Now UV spectra of neutral and oxidized or reduced species can be compared with in situ UV spectra recorded during doping. Likewise states of cations can be used to model photoelectron spectra. Analysis of states has resolved several puzzles which cannot be understood with the polaron model, e.g., the origin of the dual absorption band of green polymers and the origin of a 'vestigial neutral band' upon doping of long oligomers. DFT calculations also established that defect localization is not crucial for spectral changes observed during doping and that there are no bound bipolarons in COPs.
Open Access
High-stability, high-efficiency organic monoliths made of oligomer nanoparticles wrapped in organic matrix
(American Chemical Society, 2016) Soran-Erdem Z.; Erdem, T.; Gungor K.; Pennakalathil, J.; Tuncel, D.; Demir, Hilmi Volkan
Oligomer nanoparticles (OL NPs) have been considered unsuitable for solid-state lighting due to their low quantum yields and low temperature stability of their emission. Here, we address these problems by forming highly emissive and stable OL NPs solids to make them applicable in lighting. For this purpose, we incorporated OL NPs into sucrose matrix and then prepared their all-organic monoliths. We show that wrapping the OL NPs in sucrose significantly increases their quantum yield up to 44%, while the efficiency of their dispersion and direct solid-film remain only at ∼6%. We further showed ∼3-fold improved temperature stability of OL NP emission within these monoliths. Our experiments revealed that a physical passivation mechanism is responsible from these improvements. As a proof-of-concept demonstration, we successfully employed these high-stability, high-efficiency monoliths as color converters on a blue LED chip. Considering the improved optical features, low cost, and simplicity of the presented methodology, we believe that this study holds great promise for a ubiquitous use of organic OL NPs in lighting and possibly in other photonic applications.
Open Access
Investigation of charge carriers in doped thiophene oligomers through theoretical modeling of their UV/Vis spectra
(2008) Salzner, U.
The nature of the charge carriers in conducting organic polymers (COPs) is a long standing problem. Polythiophene is one of the prototypes of COPs and intensively studied. Because doping leads to changes in UV/vis spectra that are characteristic of the absorbing species, UV/vis spectra of charged thiophene oligomers with up to 25 rings were calculated with time-dependent density functional theory. The credibility of the method was established by comparing the results with a variety of theoretical levels and with experiment. Effects due to counterions (Cl3-) and solvent (CH 2Cl2) were examined. It was found that TDDFT employing hybrid functionals is accurate enough to distinguish the absorbing species. The findings offer an explanation for the experimentally observed difference in UV-spectra of medium-sized and long oligomers upon doping. As chain lengths of the oligomers increase and energy levels get closer, configuration interaction leads to additional absorption peaks in the high energy sub-band region (at around 1.5-2.5 eV). Thus, long oligomers do not behave differently from medium-sized ones upon doping, only their spectra are different. At low doping levels radical cations (polarons) are produced. At higher doping levels, dications that harbor weakly interacting polaron pairs are formed. Bipolarons are predicted only on very short chains or at high doping levels. There is no bipolaron binding energy and disproportionation of monocations into dications and neutral species is energetically unfavorable. © 2008 American Chemical Society.
Open Access
Ion specificity from small molecules to oligomers and beyond amide-based macromolecules
(2024-01) Farooq, Sobia
Presence of ions in aqueous solution regulate the properties of molecules in the same aqueous environment. Such alteration processes are mainly dependent on the concentration and the identity of ions. In this thesis, two parts of ion specific effects were aimed to be explored. First the synthesis and characterization of PNIPAM oligomers by using both reversible addition-fragmentation chain transfer (RAFT) and radical polymerization methods will be shown. Both of these methods give the control over molecular size of the polymer. Oligomers with charged and neutral end group were synthesized to comparatively investigate ion specific effect. These oligomers were also systematically characterized by using various analytical techniques such as phase transition temperature measurement, 1H-NMR and Gel permeation chromatography (GPC). Such oligomers were employed to investigate the specific ion effects via the salt influence on the Lower Critical solution temperature (LCST). By employing two sodium salts; NaCl (strongly hydrated) and NaSCN (weakly hydrated), it was found that strongly hydrated anions salt-out both charged and neutral oligomers, whereas weakly hydrated anions increase the phase transition temperature with a salting in mechanism. By empirical modeling with a Langmuir-type binding isotherm, a weak binding with a dissociation constant KD = 0.57 M for charged and KD = 1.13 M for neutral oligomers were demonstrated. The second part of this thesis focused on the specific ion effects beyond amide-based macromolecules i.e. hydroxypropyl cellulose (HPC) as a model for sugar-based macromolecules. Eight sodium salts were employed to demonstrate the entire Hofmeister series. Namely; NaSCN, NaI, NaNO3, NaClO4 NaCl, Na2SO4, Na2CO3, NaH2PO4 were measured on the phase transition temperature and 1H-NMR measurements. Salts of weakly hydrated anions; NaSCN, NaI, NaClO4 and NaNO3 showed a salting in mechanism and demonstrate a non-monotonic phase transition behavior. In contrast, salts of strongly hydrated anions; NaCl, Na2SO4, NaH2PO4 and Na2CO3 showed salting out mechanism with a monotonic decrease in the phase transition temperature. Additionally, the site-specific ion-macromolecule interaction was studied by 1H-NMR, and Correlation Spectroscopy (2D-COSY) NMR measurements. Although, the exact binding site cannot be specified, it was concluded that the ion binding site is at the side-chain hydroxypropyl groups and that yields the salting-in effect that was observed for the weakly-hydrated anions.
Open Access
Modeling photoelectron spectra of conjugated oligomers with time-dependent density functional theory
(American Chemical Society, 2010) Salzner, U.
With the aim of producing accurate band structures of conjugated systems by employing the states of cations, TDDFT calculations on conjugated oligomer radical cations of thiophene, furan, and pyrrole with one to eight rings were carried out. Benchmarking of density functional theory and ab initio methods on the thiophene monomer shows that the ΔSCF ionization potential (IP) is most accurate at the B3LYP/6-311G* level. Improvement of the basis set beyond 6-311G* leads to no further changes. The IP is closer to experiment at B3LYP/6-311G* than at CCSD(T)/CCPVQZ. For longer oligomers the ΔSCF IPs decrease too fast with increasing chain length with all density functionals. CCSD/6-311G* performs well if the geometries are optimized at the CCSD level. With MP2 geometries IPs decrease too fast. Peak positions in photoelectron spectra were determined by adding appropriate TDDFT excitation energies of radical cations to the ΔSCF IPs. The agreement with experiment and with Green function calculations shows that TDDFT excited states of radical cations at the B3LYP/6-311G* level are very accurate and that absorption energies can be employed to predict photoelectron spectra.
Open Access
Oligomerization of ethylene in a slurry reactor using a nickel/sulfated alumina catalyst
(American Chemical Society, 1997) Zhang, Q.; Kantcheva, M.; Dalla Lana, I. G.
During the oligomerization of ethylene over heterogeneous catalysts, the production of α-olefins may be lowered because of an accompanying deactivation of catalyst resulting from strong adsorption of products, by isomerization or by a tendency to copolymerize into branched products. The oligomerization of ethylene was studied using Ni(II)/sulfated alumina catalysts prepared with a nonporous fumed alumina (ALON) support. The influences of methods of catalyst preparation and activation upon oligomerization activity were screened using a gas - solid microreactor. On the basis of the test results obtained in the microreactor, a modified form of the superior catalyst was prepared and its performance was examined in more detail using a well-agitated gas - liquid - solid slurry reactor. This catalyst exhibited very good oligomerization activity with no apparent deactivation in the slurry reactor at temperatures at or below 298 K and at near-atmospheric pressure. Complete conversion of the ethylene with the production of mainly two oligomers, 1-butene and 1-hexene, was attained. After 34 h in the slurry, formation of a significant amount of two branched C6 isomers was observed.
Open Access
On the possibility of grafting conducting polymers into insulating ones
(Elsevier, 1996) Bahçeci, S.; Toppare, L.; Yurtsever, E.
The 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.
Open Access
Polyhedral oligomeric silsesquioxane-based hybrid networks obtained via thiol-epoxy click chemistry
(Springer, 2017) Bekin Acar, S.; Ozcelik, M.; Uyar, Tamer; Tasdelen, M. A.
A series of hybrid networks based on polyhedral oligomeric silsesquioxane (POSS) were prepared by thiol-epoxy click reaction using commercially available octakis-glycidyl-POSS (G-POSS), trimethylolpropane triglycidyl ether, and trimethylolpropane tris(3-mercaptopropionate) as monomers. The click reaction was simply catalyzed by lithium hydroxide which proceeded readily at ambient conditions in very good yields. The incorporation of G-POSS into the network was clearly determined by transmission electron microscopy, FTIR, and 1H-NMR spectroscopy techniques performed with a model study using 1-butane thiol and G-POSS molecules. The homogeneous distribution of G-POSS up to 5 wt% in the hybrid network was apparently confirmed by morphological investigations. By increasing G-POSS content higher than 5 wt%, the heterogeneous dispersion of G-POSS was determined from the tensile strength measurements. The significant decrease in tensile strength was possible due to the agglomeration of G-POSS. On the other hand, thermal properties of hybrid networks were compared together by thermogravimetric analyses, where all samples exhibited one-step degradation in the range of 220–500 °C. The thermal decomposition of hybrid network led to complete degradation of the organic part and favored the formation of stable carbonaceous and inorganic residues as char. Thus, the char yields of hybrid networks were increased to 6.2, 7.8, 10.1, 12.7, and 15.1% by G-POSS loadings from 0 to 15 wt%. This improvement was also a proof of the incorporation of G-POSS into the hybrid networks that resulted in high heat-resistant POSS-based hybrid networks compared to a sample without G-POSS.
Open Access
Theoretical analysis of poly(difluoroacetylene)
(American Chemical Society, 2003) Salzner, U.
The electronic structure of poly(difluoroacetylene) was investigated with the aim of determining whether it is a good candidate for a conducting polymer with high n-type conductivity. Positions of valence and conduction bands and bandwidths indicate that planar all-trans poly(difluoroacetylene) is p- and n-dopable and that on-chain mobility of electrons and holes is high. Various geometries of oligomers with eight carbon atoms were optimized and compared to those of decacyanooctatetraene and octatetraene. Decafluorooctatetraene has a tendency to adopt nonplanar structures, but the planar trans form lies only 5.44 kcal/mol above the helical minimum. Since the energy for planarization is small, poly(difluoroacetylene) might be planar in the solid state. This is in contrast to the cyano analogue for which the planar trans structure lies 23.26 kcal/mol above the helical minimum. Alternating acetylene and difluoroacetylene units give rise to planar polymers with reduced band gap. Bandwidths, ionization potential, and electron affinity are average between those of the homopolymers.
Open Access
Theoretical modeling of the doping process in polypyrrole by calculating UV/Vis absorption spectra of neutral and charged oligomers
(2008) Okur, S.; Salzner, U.
Changes in absorption spectra during doping of oligopyrroles were investigated with time-dependent density functional theory on optimized structures of neutral, singly, and doubly charged pyrrole oligomers with up to 24 rings. In the absence of counterions, defects are delocalized. Counterions induce localization. For dications two polarons on the same chain are preferred over a bipolaren. Intragap absorptions arise in charged species, no matter whether defects are localized or delocalized. Cations and dications give rise to two sub-band transitions. The cation peaks have lower energies than those of dications. The first excitations of cations have lower oscillator strengths than the second; for dications the second peak is weaker than the first. For very long oligomers, the second sub-band absorption vanishes and a third one appears at higher energy. The behavior of pyrrole oligomers is analogous to that of thiophene oligomers. Theoretical UV spectra for cations and dications of short oligomers (six to eight rings) match experimental spectra of polypyrrole at low and at high doping levels, respectively. The error in the theoretical calculations is about 0.4 eV, slightly larger than for thiophene oligomers at the same level of theory. © 2008 American Chemical Society.
Open Access
Tuned range-separated hybrids in density functional theory
(2010) Baer, R.; Livshits, E.; Salzner, U.
We review density functional theory (DFT) within the Kohn-Sham (KS) and the generalized KS (GKS) frameworks from a theoretical perspective for both time-independent and time-dependent problems. We focus on the use of range-separated hybrids within a GKS approach as a practical remedy for dealing with the deleterious long-range self-repulsion plaguing many approximate implementations of DFT. This technique enables DFT to be widely relevant in new realms such as charge transfer, radical cation dimers, and Rydberg excitations. Emphasis is put on a new concept of system-specific range-parameter tuning, which introduces predictive power in applications considered until recently too difficult for DFT.
Open Access
Ultralow-threshold up-converted lasing in oligofluorenes with tailored strong nonlinear absorption
(Royal Society of Chemistry, 2015) Guzelturk, B.; Kanibolotsky, A.L.; Orofino-Pena, C.; Laurand, N.; Dawson, M.D.; Skabara P.J.; Demir, Hilmi Volkan
Nonlinear optical response in organic semiconductors has been an attractive property for many practical applications. For frequency up-converted lasers, to date, conjugated polymers, fluorescent dyes and small organic molecules have been proposed but their performances have been severely limited due to the difficulty in simultaneously achieving strong nonlinear optical response and high performance optical gain. In this work, we show that structurally designed truxene-based star-shaped oligofluorenes exhibit strong structure-property relationships enabling enhanced nonlinear optical response with favorable optical gain performance. As the number of fluorene repeat units in each arm is increased from 3 to 6, these molecules demonstrate a two-photon absorption cross-section as high as 2200 GM, which is comparable to that of linear conjugated polymers. Tailored truxene oligomers with six fluorene units in each arm (T6) show two-photon absorption pumped amplified spontaneous emission with a threshold as low as 2.43 mJ cm-2, which is better than that of the lowest reported threshold in organic semiconductors. Furthermore, we show a frequency up-converted laser using the newly designed and synthesized star-shaped oligomer T6 with a threshold as low as 3.1 mJ cm-2, which is more than an order of magnitude lower than that of any conjugated polymer. Thus, these oligomers with enhanced nonlinear optical properties are highly attractive for bio-integrated applications such as photodynamic therapy and in vivo bio-sensing. © The Royal Society of Chemistry 2015.