We study nucleation and multilayer growth of the perylene derivative PTCDI-C8 and find a persistent layer-by-layer growth, transformation of island shapes, and an enhancement of molecular diffusivity in upper monolayers (MLs). These findings result from the evaluation of the ML-dependent island densities, obtained by in situ real-time grazing incidence small angle X-ray scattering measurements and simultaneous X-ray growth oscillations. Complementary ex situ atomic force microscopy snapshots of different growth stages agree quantitatively with both X-ray techniques. The rate and temperature-dependent island density is analyzed using different mean-field nucleation models. Both a diffusion limited aggregation and an attachment limited aggregation model yield in the first two MLs the same critical nucleus size i, similar surface diffusion attempt frequencies in the 1019-1020 s-1 range, and a decrease of the diffusion barrier Ed in the 2nd ML by 140 meV.

Next-generation molecular devices and machines demand the integration of molecular switches into hierarchical assemblies to amplify the response of the system from the molecular level to the meso- or macro-scale. Here, we demonstrate that multi-azobenzene oligomers can assemble to form robust supramolecular nanofibers in which they can be switched repeatedly between the E- and Z-configuration. While in isolated oligomers the azobenzene units undergo reversible photoisomerization independently, in the nanofibers they are coupled via intermolecular interactions and switch cooperatively as evidenced by unusual thermal and kinetic behavior. We find that the photoisomerization rate from the Z-isomer to the E-isomer depends on the fraction of Z-azobenzene in the nanofibers, and is increased by more than a factor of 4 in Z-rich fibers when compared to E-rich fibers. This demonstrates the great potential of coupling individual photochromic units for increasing their quantum efficiency in the solid state with potential relevance for actuation and sensing.

Organic semiconductors have been widely studied as interfacial layers to improve the performance of photovoltaics based on emerging absorber materials, such as organic bulk heterojunctions or methylammonium lead halide perovskites. Despite the intense practical interest, fundamental insight into how such interfacial layers affect the interfacial charge-transfer processes underlying selectivity and recombination is more limited. We use the silicon interdigitated back contact (IBC) solar cell to determine well-defined measures of the selectivity and recombination for organic semiconductor contacts to silicon and to separate the contributions of electron/hole charge-transfer velocities and barrier heights in determining the selectivity. Two primary parameters are measured: R = (J(on)/J(on)*)(J(op)/J(op)*) and S-h = J(op)/J(on), where J(on) and J(op) are the exchange current densities for electrons and holes at the interface and the starred quantities are maximum reference values given by ideal thermionic emission theory. The parameter Sh is a measure of hole selectivity (inverse of electron selectivity), and R is a measure of recombination. We used the IBC solar cell to demonstrate that a cationic (anionic) polyfluorene interfacial layer modifies the PEDOT-PSS contact to silicon by lowering R four (five) orders of magnitude and the hole selectivity Sh by 13 (five) orders magnitude consistent with a 0.29 V (0.12 V) change in barrier height dominating any charge-transfer velocity asymmetry in determining the selectivity. These results show that both selectivity and recombination must be considered in determining the action of interfacial layers. In the case of the ionically functionalized polyfluorenes studied, the sign of the ionic functionality determines selectivity, while the polyfluorene backbone introduces a tunneling barrier that reduces the overall recombination rate.

We have synthesized a series of aza[8]cycloparaphenylenes containing one, two, and three nitrogens to probe the impact of nitrogen doping on optoelectronic properties and solid state packing. Alkylation of these azananohoops afforded the first donor-acceptor nanohoops where the phenylene backbone acts as the donor and the pyridinium units act as the acceptor. The impact on the optoelectronic properties was then studied experimentally and computationally to provide new insight into the effect of functionalization on nanohoops properties.

We investigate the influence of light on the growth process and resulting phase coexistence of the organic semiconductor α-sexithiophene (6T). We demonstrate that 6T thin films deposited on potassium chloride (KCl) in dark environments exhibit a bimodal growth, with phase coexistence of both low-temperature (LT) and high-temperature (HT) polymorphs. In contrast, films grown under illumination with 532 nm light at 1.5 W/cm2 exhibit an increased purity of the LT phase, while the HT phase growth is slowed down by about a factor of 4. To understand the mechanism behind this optical control, we use in situ X-ray diffraction, atomic force microscopy, optical absorption measurements, as well as first-principles calculations for the optical absorption spectra of the HT and LT phase. We deduce that the phase purification is due to optical heating of the molecular film and lower cohesive energy of the HT phase compared to the LT phase, so that nucleation and growth of the HT phase are significantly reduced by light...

Single strands of azobenzene main chain polymers exhibiting alkyl side chains can be largely and reversibly contracted and extended with light; We show that upon self-assembly in a thin layered film they act as "molecular zippers" that can be opened and closed with UV- and blue light, respectively. Simultaneously in situ recorded time-resolved X-ray diffraction and optical spectroscopy measurements, together with scanning force microscopy show that upon the light-induced E -> Z isomerization of the main chain azobenzenes the layered film morphology remains, while the initially highly ordered alkyl side chains become disordered. Already the E Z isomerization of about 20% of all azobenzene chromophores triggers a complete disorder of the alkyl chains. The kinetics of this partial amorphization of the film is about 18 times slower than the ensemble kinetics of the initial azobenzene photoisomerization. This is the first demonstration of a rigid main chain polymer film with reversibly photoswitchable side chain crystallinity.

We use thermal annealing to improve smoothness and to increase the lateral size of crystalline islands of n-tetratetracontane (TTC, C44H90) films. With in situ x-ray diffraction, we find an optimum temperature range leading to improved texture and crystallinity while avoiding an irreversible phase transition that reduces crystallinity again. We employ real-time optical phase contrast microscopy with sub-nm height resolution to track the diffusion of TTC across monomolecular step edges which causes the unusual smoothing of a molecular thin film during annealing. We show that the lateral island sizes increase by more than one order of magnitude from 0.5 μm to 10 μm. This desirable behavior of 2d-Ostwald ripening and smoothing is in contrast to many other organic molecular films where annealing leads to dewetting, roughening, and a pronounced 3d morphology. We rationalize the smoothing behavior with the highly anisotropic attachment energies and low surface energies for TTC. The results are technically relevant for the use of TTC as passivation layer and as gate dielectric in organic field effect transistors.

The incorporation of a cationically functionalized fullerene interfacial layer (NMFP-Br) into an inverted poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction photovoltaic cell results in a significant power conversion efficiency (PCE) improvement of 67%, from 2.1% to 3.5%, relative to cells without an interfacial layer. The incorporation of NMFP-Br as an ITO modifying interfacial layer results in a 190 mV increase in the open-circuit voltage, 13% increase in fill factor, and 250% reduction in series resistance. Cell efficiencies are greater than or comparable to other reported cells based on organic electron injection layers with the same active layer and electrode configuration. The orthogonal solubility afforded by ionic functionality allows for sequential solution phase deposition without the necessity of chemical cross-linking of the fullerene based interfacial layer. Inverted devices incorporating a single phase, ionically functionalized fullerene interfacial layer have not been previously demonstrated. The unusually high conductivity of NMFP-Br films is investigated and shown to be ~3 orders of magnitude higher than PCBM and contributes to the substantial reduction in series resistance.

The solution phase n-doping of C-60 and PCBM with tetrabutylammonium fluoride is shown to occur via an initial chemical reaction followed by electron transfer to a second fullerene molecule. The formation of ionic and radical intermediate species has significant implications for the use of ionically functionalized materials as electron-selective interface layers in OPVs.

We have prepared high-quality, densely packed, self-assembled monolayers (SAMs) of carboxy-terminated alkyl chains on Si(111). The samples were made by thermal grafting of methyl undec-10-enoate under an inert atmosphere and subsequent cleavage of the ester functionality to disclose the carboxylic acid end-group. X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray diffraction (GIXD) indicate a surface coverage of about 50% of the initially H-terminated sites. In agreement, GIXD implies a rectangular unit mesh of 6.65 and 7.68 Å side lengths, containing two molecules in a regular zigzag-like substitution pattern for the ester- and carboxy-terminated monolayer. Hydrolysis of the remaining H-Si(111) bonds at the surface furnished HO-Si(111) groups according to XPS and attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) studies. The amide-terminated alkyl SAM on Si(111) assembled in a 2-(6-chloro-1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HCTU)-mediated one-pot coupling reaction under an inert atmosphere, whereby the active ester forms in situ prior to the reaction with an amino-functionalized photoswitchable fulgimide. ATR-FTIR and XPS studies of the fulgimide samples revealed closely covered amide-terminated SAMs. Reversible photoswitching of the headgroup was read out by applying XPS, ATR-FTIR, and difference absorption spectra in the mid-IR. In XPS, we observed a reversible breathing of the amide/imide C1s and N1s signals of the fulgimide. The results demonstrate the general suitability of HCTU as a reagent for amide couplings to carboxy-terminated alkyl SAMs and the on-chip functionalization toward photoswitchable Si(111) surfaces.

We present a comprehensive electronic structure analysis of structurally simple BN heterocycles using a combined UV-photoelectron spectroscopy (UV-PES)/computational chemistry approach. Gas-phase He I photoelectron spectra of 1,2-dihydro-1,2-azaborine 1, N-Me-1,2-BN-toluene 2, and N-Me-1,3-BN-toluene 3 have been recorded, assessed by density functional theory calculations, and compared with their corresponding carbonaceous analogues benzene and toluene. The first ionization energies of these BN heterocycles are in the order N-Me-1,3-BN-toluene 3 (8.0 eV) < N-Me-1,2-BN-toluene 2 (8.45 eV) < 1,2-dihydro-1,2-azaborine 1 (8.6 eV) < toluene (8.83 eV) < benzene (9.25 eV). The computationally determined molecular dipole moments are in the order 3 (4.577 D) > 2 (2.209 D) > 1 (2.154 D) > toluene (0.349 D) > benzene (0 D) and are consistent with experimental observations. The λ(max) in the UV-vis absorption spectra are in the order 3 (297 nm) > 2 (278 nm) > 1 (269 nm) > toluene (262 nm) > benzene (255 nm). We also establish that the measured anodic peak potentials and electrophilic aromatic substitution (EAS) reactivity of BN heterocycles 1-3 are consistent with the electronic structure description determined by the combined UV-PES/computational chemistry approach.

A substantial reduction in the rate of irreversible polymer photo-oxidation was observed through the ionic stabilization of the polymer-O2 charge-transfer complex (CTC) in amorphous polythiophene thin films. Through the incorporation of anionic functionality containing mobile cations, it was found that CTC stability increases with increasing cation charge density. This results in an increased rate of electron transfer to molecular oxygen relative to photosensitization and reaction of 1O2, leading to a reduction in the overall rate of polymer degradation. UV-vis and FTIR spectroscopy were utilized to determine the identity of intermediate and irreversible photodegradation products as well as the effect of ion identity on the dominant photo-oxidation mechanism. As polymer-O2 CTCs are common in the photo-oxidation of many conjugated polymers, these results have significant implications for the ongoing effort to produce commercially viable organic electronic and photonic devices.

Herein we report the synthesis and characterization of a series of 6,12-diarylindeno[1,2-b]fluorenes (IFs). Functionalization with electron donor and acceptor groups influences the ability of the IF scaffold to undergo two-electron oxidation and reduction to yield the corresponding 18- and 22-π-electron species, respectively. A single crystal of the pentafluorophenyl-substituted IF can serve as an active layer in an organic field-effect transistor (OFET). The important finding is that the single-crystal OFET yields an ambipolar device that is able to transport holes and electrons.

Für mehr Akzeptanz! 2,8-Disubstituierte Indeno[1,2-b]fluorene (siehe Strukturen; TIPS=Triisopropylsilyl) wurden synthetisiert und charakterisiert. Elektrochemische, optische und theoretische Daten zeigen, dass diese Elektronenakzeptor-Kohlenwasserstoffe niedrige HOMO- und LUMO-Energien aufweisen und ihre Bandlücken mit denen gebräuchlicher organischer n-Halbleitermaterialien vergleichbar sind.

A series of 6,12-bis[(trialkylsilyl)ethynyl]indeno[1,2-b]fluorene-5,11-diones has been synthesized. X-ray crystallographic analysis of these compounds reveals that triisopropylsilyl (TIPS) substitution on the alkyne terminus affords the largest number of intermolecular π-π interactions in the solid state. Conversely, use of trialkylsilyl groups smaller or larger than TIPS furnishes a variety of crystal-packing motifs that contain fewer π-π interactions. Electrochemical and photophysical data suggest that these molecules are excellent electron-accepting materials.

ADVERTISEMENT RETURN TO ISSUEPREVCommunication to the...Communication to the EditorNEXTIonic Functionality and the Polyacetylene–Oxygen Charge-Transfer ComplexChristopher D. Weber, Stephen G. Robinson, and Mark C. Lonergan*View Author Information Department of Chemistry and The Materials Science Institute, University of Oregon, Eugene, Oregon 97403, United StatesE-mail: [email protected]Cite this: Macromolecules 2011, 44, 12, 4600–4604Publication Date (Web):June 3, 2011Publication History Received22 April 2011Published online3 June 2011Published inissue 28 June 2011https://doi.org/10.1021/ma200932yCopyright © 2011 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views471Altmetric-Citations4LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (2 MB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Absorption,Organic polymers,Oxygen,Polymers,Thin films Get e-Alerts

The diffusion of dopant counterions has made the formation of conjugated polymer p-n junctions challenging, We demonstrate polyelectrolyte mediated electrochemistry (PMEC) its a three-electrode technique for separately introducing n- and p-type regions in an ion-functionalized polyacetylene structure to form it p-n junction. PMEC uses a polyelectrolyte-based supporting electrolyte and ion-functionalized conjugated polymers to control the ions available for exchange between a solid sample and supporting electrolyte, and in this way, select for oxidative vs reductive electrochemical processes. A bilayer consisting of solid layers of anionically and cationically functionalized poly-acetylenes was driven first to -1.5 V vs SCE and then to 0.6 V vs SCE using tetrabutylammonium polystyrene sulfonate/acetonitrile as it supporting electrolyte. The negative potential step n-doped the entire Structure, and the positive potential step selectively p-doped the anionically functionalized layer to form a p-n junction its followed by spectroelectrochemistry and supported by current-voltage characterization. The polyacetylene p-n junctions were observed to exhibit diode behavior with ideality factors in the range of 3-3.5. Rectification ratios of greater than 500 at 1 V were achieved, and the junctions exhibited a small photovoltaic effect.