Surface doping of rubrene single crystals by molecular electron donors and acceptors

The surface molecular doping of organic semiconductors can play an important role in the development of organic electronic or optoelectronic devices. Single-crystal rubrene remains a leading molecular candidate for applications in electronics due to its high hole mobility. In parallel, intensive research into the fabrication of flexible organic electronics requires the careful design of functional interfaces to enable optimal device characteristics. To this end, the present work seeks to understand the effect of surface molecular doping on the electronic band structure of rubrene single crystals. Our angle-resolved photoemission measurements reveal that the Fermi level moves in the band gap of rubrene depending on the direction of surface electron-transfer reactions with the molecular dopants, yet the valence band dispersion remains essentially unperturbed. This indicates that surface electron-transfer doping of a molecular single crystal can effectively modify the near-surface charge density, while retaining good charge-carrier mobility.

Association between constitution, axiography, orthodontic cast analysis, and upper body posture in women aged 31 to 40 years

OBJECTIVE

Whether it is primarily the spine that reacts with pain to the negative consequences of everyday stress and possibly the temporomandibular system as a result (ascending chain), or whether incorrect stress in the dental area has an influence on body geometry (descending chain), is still a controversially discussed topic. The aim of this study is to investigate possible relationships between constitutional, axiographic, and dental parameters with upper body posture.

MATERIAL AND METHODS

A total of 106 subjectively healthy women between 31 and 40 years of age voluntarily participated in this study. Data collection was done by filling out a questionnaire with constitutional and anamnestic parameters and by evaluating orthodontic casts, axiographic measurements, and video raster stereographic measurements. These data were analyzed using correlations and group comparisons, with the significance level set at p ≤ 0.05.

RESULTS

Positive correlations were shown between the constitutional factors of body weight and BMI and the lumbar bending angle (p = 0.01), the kyphosis angle (p = 0.001), and lordosis angle (weight p = 0.05; BMI p = 0.03). In the cast analysis, regardless of the direction of the midline shift (left/right/none), a left lateral tilt can be seen which is greatest at 2.12° with a left midline shift. In addition, the elevated pelvic side correlates with the side of the displacement of the jaw, with the stronger manifestation being on the left side. With a vertical anterior bite in the normal range, the kyphosis angle is 48.09°, while with a deep bite, it is 60.92°, and with an open bite, it is 62.47°; thus, the group in the normal range differs significantly (p = 0.01) from the other two. The greater the protrusion, the smaller the sagittal plane angles (kyphosis angle, lumbar bending angle, each p = 0.03), and the more dorsal the posture (p = 0.04). The lordosis angle differs significantly (p = 0.001) between the group of subjects with a protrusion in the normal range (52.34°) and the group with an increased advancement of the mandible (41.79°).

CONCLUSION

There is a correlation between body weight, BMI, midline shift, and protrusion, as well as the vertical anterior step and upper body posture in women between 31 and 40 years of age. Interdisciplinary functional examinations of the temporomandibular musculature, and also sustained orthodontic treatment, can contribute to an improvement in upper body posture.

Orbital-resolved observation of singlet fission

Singlet fission1-13 may boost photovoltaic efficiency14-16 by transforming a singlet exciton into two triplet excitons and thereby doubling the number of excited charge carriers. The primary step of singlet fission is the ultrafast creation of the correlated triplet pair17. Whereas several mechanisms have been proposed to explain this step, none has emerged as a consensus. The challenge lies in tracking the transient excitonic states. Here we use time- and angle-resolved photoemission spectroscopy to observe the primary step of singlet fission in crystalline pentacene. Our results indicate a charge-transfer mediated mechanism with a hybridization of Frenkel and charge-transfer states in the lowest bright singlet exciton. We gained intimate knowledge about the localization and the orbital character of the exciton wave functions recorded in momentum maps. This allowed us to directly compare the localization of singlet and bitriplet excitons and decompose energetically overlapping states on the basis of their orbital character. Orbital- and localization-resolved many-body dynamics promise deep insights into the mechanics governing molecular systems18-20 and topological materials21-23.

Site-selective functionalization of in-plane nanoelectrode-antennas

Stacked organic optoelectronic devices make use of electrode materials with different work functions, leading to efficient large area light emission. In contrast, lateral electrode arrangements offer the possibility to be shaped as resonant optical antennas, radiating light from subwavelength volumes. However, tailoring electronic interface properties of laterally arranged electrodes with nanoscale gaps - to e.g. optimize charge-carrier injection - is rather challenging, yet crucial for further development of highly efficient nanolight sources. Here, we demonstrate site-selective functionalization of laterally arranged micro- and nanoelectrodes by means of different self-assembled monolayers. Upon applying an electric potential across nanoscale gaps, surface-bound molecules are removed selectively from specific electrodes by oxidative desorption. Kelvin-probe force microscopy as well as photoluminescence measurements are employed to verify the success of our approach. Moreover, we obtain asymmetric current-voltage characteristics for metal-organic devices in which just one of the electrodes is coated with 1-octadecanethiol; further demonstrating the potential to tune interface properties of nanoscale objects. Our technique paves the way for laterally arranged optoelectronic devices based on selectively engineered nanoscale interfaces and in principle enables molecular assembly with defined orientation in metallic nano-gaps.

Mechano-Stimulus and Environment-Dependent Circularly Polarized TADF in Chiral Copper(I) Complexes and Their Application in OLEDs

Molecular emitters that combine circularly polarized luminescence (CPL) and high radiative rate constants of the triplet exciton decay are highly attractive for electroluminescent devices (OLEDs) or next-generation photonic applications, such as spintronics, quantum computing, cryptography, or sensors. However, the design of such emitters is a major challenge because the criteria for enhancing these two properties are mutually exclusive. In this contribution, we show that enantiomerically pure {Cu(Cbz^R)[(S/R)-BINAP]} [R = H (1), 3,6-tBu (2)] are efficient thermally activated delayed fluorescence (TADF) emitters with high radiative rate constants of k_TADF up to 3.1 × 10⁵ s^-1 from ^1/3LLCT states according to our temperature-dependent time-resolved luminescence studies. The efficiency of the TADF process and emission wavelengths are highly sensitive to environmental hydrogen bonding of the ligands, which can be disrupted by grinding of the crystalline materials. The origin of this pronounced mechano-stimulus photophysical behavior is a thermal equilibrium between the ^1/3LLCT states and a ³LC state of the BINAP ligand, which depends on the relative energetic order of the excited states and is prone to inter-ligand C-H···π interactions. The copper(I) complexes are also efficient CPL emitters displaying exceptional dissymmetry values g_lum of up to ±0.6 × 10^-2 in THF solution and ±2.1 × 10^-2 in the solid state. Importantly for application in electroluminescence devices, the C-H···π interactions can also be disrupted by employing sterically bulky matrices. Accordingly, we have investigated various matrix materials for successful implementation of the chiral copper(I) TADF emitters in proof-of-concept CP-OLEDs.

Standard values of the upper body posture in healthy adults with special regard to age, sex and BMI

In order to classify and analyze the parameters of upper body posture in clinical or physiotherapeutic settings, a baseline in the form of standard values with special regard to age, sex and BMI is required. Thus, subjectively healthy men and women aged 21-60 years were measured in this project. The postural parameters of 800 symptom-free male (n = 397) and female (n = 407) volunteers aged 21-60 years (Ø♀: 39.7 ± 11.6, Ø ♂: 40.7 ± 11.5 y) were studied. The mean height of the men was 1.8 ± 0.07 m, with a mean body weight of 84.8 ± 13.1 kg and an average BMI of 26.0 ± 3.534 kg/m². In contrast, the mean height of the women was 1.67 ± 0.06 m, with a mean body weight of 66.5 ± 12.7 kg and an average BMI of 23.9 ± 4.6 kg/m². By means of video rasterstereography, a 3-dimensional scan of the upper back surface was measured when in a habitual standing position. The means or medians, confidence intervals, tolerance ranges, the minimum, 2.5, 25, 50, 75, 97.5 percentiles and the maximum, plus the kurtosis and skewness of the distribution, were calculated for all parameters. Additionally, ANOVA and a factor analyses (sex, BMI, age) were conducted. In both sexes across all age groups, balanced, symmetrical upper body statics were evident. Most strikingly, the females showed greater thoracic kyphosis and lumbar lordosis angles (kyphosis: Ø ♀ 56°, Ø♂ 51°; lordosis: Ø ♀ 49°, Ø♂ 32°) and lumbar bending angles (Ø ♀ 14°, Ø♂ 11°) than the males. The distance between the scapulae was more pronounced in men. These parameters also show an increase with age and BMI, respectively. Pelvic parameters were independent of age and sex. The upper body postures of women and men between the ages of 21 and 60 years were found to be almost symmetrical and axis-conforming with a positive correlation for BMI or age. Consequently, the present body posture parameters allow for comparisons with other studies, as well as for the evaluation of clinical (interim) diagnostics and applications.

Spectroscopic analysis of vibrational coupling in multi-molecular excited states

Multi-molecular excited states accompanied by intra- and inter-molecular geometric relaxation are commonly encountered in optical and electrooptical studies and applications of organic semiconductors as, for example, excimers or charge transfer states. Understanding the dynamics of these states is crucial to improve organic devices such as light emitting diodes and solar cells. Their full microscopic description, however, demands sophisticated tools such as ab initio quantum chemical calculations which come at the expense of high computational costs and are prone to errors by assumptions as well as iterative algorithmic procedures. Hence, the analysis of spectroscopic data is often conducted at a phenomenological level only. Here, we present a toolkit to analyze temperature dependent luminescence data and gain first insights into the relevant microscopic parameters of the molecular system at hand. By means of a Franck-Condon based approach considering a single effective inter-molecular vibrational mode and different potentials for the ground and excited state we are able to explain the luminescence spectra of such multi-molecular states. We demonstrate that by applying certain reasonable simplifications the luminescence of charge transfer states as well as excimers can be satisfactorily reproduced for temperatures ranging from cryogenics to above room temperature. We present a semi-classical and a quantum-mechanical description of our model and, for both cases, demonstrate its applicability by analyzing the temperature dependent luminescence of the amorphous donor-acceptor heterojunction tetraphenyldibenzoperiflanthene:C₆₀ as well as polycrystalline zinc-phthalocyanine to reproduce the luminescence spectra and extract relevant system parameters such as the excimer binding energy.

Trigonal Copper(I) Complexes with Cyclic (Alkyl)(amino)carbene Ligands for Single-Photon Near-IR Triplet Emission

Molecular near-IR (NIR) triplet-state emitters are of importance for the development of new, organic-electronics-based telecommunication technologies as optical fibers operating in the corresponding spectral bands allow for data transfer over much longer distances due to the significantly lower attenuation. However, achieving such low-energy triplet excited states with good radiative rate constants is very challenging, and studies regarding the single-photon emission of organometallics in this energy range are scarce. We have prepared a series of trigonal Cu^I CAAC complexes bearing chelating ligands with O, N, S, and Se donor atoms and studied their photophysical properties in this context. The compounds show weak low-energy absorption in solution between 400 and 500 nm due to mixed Cu → CAAC ¹MLCT/LLCT states, resulting in yellow-green to orange appearance, which we have also correlated to the ¹⁵N NMR resonances of the π-accepting carbene ligand. In the solid state, phosphorescence from dominant ³(Cu → CAAC) CT states is observed at room temperature. The emission of the complexes is bathochromically shifted in comparison to structurally related linearly coordinated copper(I) CAAC complexes due to structural reorganization in the excited state to a T-shape. For [Cu(dbm)(CAAC^Me)], the broad phosphorescence with outstanding λ_max = 760 nm tailors out to ca. 1100 nm and leads to its proof-of-concept application as a nonclassical single-photon light source, constituting key functional units for the implementation of tap-proof data transfer.

Thermodynamic equilibrium between locally excited and charge-transfer states through thermally activated charge transfer in 1-(pyren-2'-yl)-o-carborane

Reversible conversion between excited-states plays an important role in many photophysical phenomena. Using 1-(pyren-2'-yl)-o-carborane as a model, we studied the photoinduced reversible charge-transfer (CT) process and the thermodynamic equilibrium between the locally-excited (LE) state and CT state, by combining steady state, time-resolved, and temperature-dependent fluorescence spectroscopy, fs- and ns-transient absorption, and DFT and LR-TDDFT calculations. Our results show that the energy gaps and energy barriers between the LE, CT, and a non-emissive 'mixed' state of 1-(pyren-2'-yl)-o-carborane are very small, and all three excited states are accessible at room temperature. The internal-conversion and reverse internal-conversion between LE and CT states are significantly faster than the radiative decay, and the two states have the same lifetimes and are in thermodynamic equilibrium.

Color-Switchable Subwavelength Organic Light-Emitting Antennas

Future photonic devices require efficient, multifunctional, electrically driven light sources with directional emission properties and subwavelength dimensions. Electrically driven plasmonic nanoantennas have been demonstrated as enabling technology. Here, we present the concept of a nanoscale organic light-emitting antenna (OLEA) as a color- and directionality-switchable point source. The device consists of laterally arranged electrically contacted gold nanoantennas with their gap filled by the organic semiconductor zinc phthalocyanine (ZnPc). Since ZnPc shows preferred hole conduction in combination with gold, the recombination zone relocates depending on the polarity of the applied voltage and couples selectively to either of the two antennas. Thereby, the emission characteristics of the device also depend on polarity. Contrary to large-area OLEDs where recombination at metal contacts significantly contributes to losses, our ultracompact OLEA structures facilitate efficient radiation into the far-field rendering transparent electrodes obsolete. We envision OLEA structures to serve as wavelength-scale pixels with tunable color and directionality for advanced display applications.

Nuclear dynamics of singlet exciton fission in pentacene single crystals

Singlet exciton fission (SEF) is a key process for developing efficient optoelectronic devices. An aspect rarely probed directly, yet with tremendous impact on SEF properties, is the nuclear structure and dynamics involved in this process. Here, we directly observe the nuclear dynamics accompanying the SEF process in single crystal pentacene using femtosecond electron diffraction. The data reveal coherent atomic motions at 1 THz, incoherent motions, and an anisotropic lattice distortion representing the polaronic character of the triplet excitons. Combining molecular dynamics simulations, time-dependent density-functional theory, and experimental structure factor analysis, the coherent motions are identified as collective sliding motions of the pentacene molecules along their long axis. Such motions modify the excitonic coupling between adjacent molecules. Our findings reveal that long-range motions play a decisive part in the electronic decoupling of the electronically correlated triplet pairs and shed light on why SEF occurs on ultrafast time scales.

Symmetrical dental occlusion blocking - changes of body sway and weight distribution in healthy subjects across 4 age decades

Objectives

Symmetrical dental occlusion blocking is used in dentistry as a quick diagnostic tool to test for potential influences of the craniomandibular system on body sway and weight distribution. This study presents the changes of body sway and pressure distribution in healthy subjects, free of a temporomandibular dysfunction (TMD). Immediate effects between occlusal blocking and rest position on body sway and body weight distribution in general, as well as for both genders and for four age decades will be evaluated.

Materials and methods

725 (396f/329 m) subjects (neither subjective signs of TMD nor acute/chronic complaints in the musculoskeletal system) volunteered (21 to 60 years) while both genders were divided into four age groups according to decades. A pressure measuring platform was used. Body sway and weight distribution were recorded in two dental occlusion conditions (a) in rest position and (b) symmetrical blocking (bicuspid region) by cotton rolls.

Results

Both, the frontal sway and the sagittal sway reduced by 0.67 mm (t(724) = − 3.9 (p <  0.001)) and by 0.33 mm (t(724) = − 3.4 (p <  0.001)). The relative pressure under the left forefoot increased by 0.33% (t(724) = 2.88 (p <  0.001)) and the relative pressure overall under the forefoot increased by 0.67% (t(724) = − 3.4 (p <  0.001)). Gender-specific, age-specific and BMI-specific reactions could not be identified.

Conclusions

Subjects, free of any TMD and with no complaints of the musculoskeletal system, show small changes of the body sway and weight distribution when biting symmetrically on a cotton roll. These changes are independent of age, gender or body mass index (BMI). Due to the relative large sample size, the presented results can also be seen as norm values when body sway is used as an additional assessment of a TMD.

Spatial Anisotropy of Charge Transfer at Perfluoropentacene-Pentacene (001) Single-Crystal Interfaces and its Relevance for Thin Film Devices

Archetypal donor-acceptor (D-A) interfaces composed of perfluoropentacene (PFP) and pentacene (PEN) are examined for charge transfer (CT) state formation and energetics as a function of their respective molecular configuration. To exclude morphological interference, our structural as well as highly sensitive differential reflectance spectroscopy studies were carried out on PFP thin films epitaxially grown on PEN(001) single-crystal facets. Whereas the experimental data supported by complementary theoretical calculations confirm the formation of a strong CT state in the case of a cofacial PFP-PEN stacking, CT formation is energetically less favorable and thus absent for the corresponding head-to-tail configuration as disclosed for the first time. In view of technological implementations, the knowledge gained on the single-crystal references is transferred to thin-film diodes composed of either stacked PFP/PEN bilayers or mixed PFP:PEN heterojunction interfaces. As demonstrated, their electronic and electroluminescent behavior can be consistently described by the absence or presence of interfacial CT states. Thus, our results hint at the thorough design of D-A interfaces to achieve the highest device performances.

HOMO-HOMO Electron Transfer: An Elegant Strategy for p-Type Doping of Polymer Semiconductors toward Thermoelectric Applications

Unlike the conventional p-doping of organic semiconductors (OSCs) using acceptors, here, an efficient doping concept for diketopyrrolopyrrole-based polymer PDPP[T]₂ -EDOT (OSC-1) is presented using an oxidized p-type semiconductor, Spiro-OMeTAD(TFSI)₂ (OSC-2), exploiting electron transfer from HOMO_OSC-1 to HOMO_OSC-2 . A shift of work function toward the HOMO_OSC-1 upon doping is confirmed by ultraviolet photoelectron spectroscopy (UPS). Detailed X-ray photoelectron spectroscopy (XPS) and UV-vis-NIR absorption studies confirm HOMO_OSC-1 to HOMO_OSC-2 electron transfer. The reduction products of Spiro-OMeTAD(TFSI)₂ to Spiro-OMeTAD(TFSI) and Spiro-OMeTAD is also confirmed and their relative amounts in doped samples is determined. Mott-Schottky analysis shows two orders of magnitude increase in free charge carrier density and one order of magnitude increase in the charge carrier mobility. The conductivity increases considerably by four orders of magnitude to a maximum of 10 S m^-1 for a very low doping ratio of 8 mol%. The doped polymer films exhibit high thermal and ambient stability resulting in a maximum power factor of 0.07 µW m^-1  K^-2 at a Seebeck coefficient of 140 µV K^-1 for a very low doping ratio of 4 mol%. Also, the concept of HOMO_OSC-1 to HOMO_OSC-2 electron transfer is a highly efficient, stable and generic way to p-dope other conjugated polymers.

Systematic changes of the static upper body posture with a symmetric occlusion condition

Background

Temporary occlusal changes and their influence on the upper body statics are still controversially discussed. Furthermore, concrete statements on whether age- or gender-specific differences in neurophysiological reactions exist are missing. Therefore, it is the aim of this study to evaluate the immediate effects of a symmetrical occlusion blocking on the upper body posture. These effects shall be investigated for both genders and for a larger age range.

Methods

In this study, 800 (407f/393 m) subjects volunteered aged from 21 to 60 years. Both genders were divided into four age groups according to decades. The three-dimensional upper body posture was measured by using the rasterstereography (ABW-Bodymapper). The habitual static posture was measured in two dental occlusion conditions (a) in rest position and (b) symmetrical blocking in the bicuspid region by cotton rolls.

Results

A significant reduction of the trunk length (0.72 mm; p <  0.001), an increase of the lumbar (0.30°; p <  0.001) and the thoracic bending angle (0.14°; p = 0.001), a reduction of the spinal forward decline (0.16°; p <  0.001) and a reduction of the scapular distance (0.36 mm; p = 0.001) was found. Gender-specific reactions can only be recorded in scapular distance, in that regard men reduce this distance while over all age groups women did not show a significant change.

Discussion

Slight gender- and age-independent reactions due to a symmetric occlusion blockade are shown: A gender independent reaction of the spinal related variables in the sagittal plane (thoracic and lumbar flexion angle, trunk length, spinal forward decline). In addition, a gender specific change of the shoulder blade distance could be observed, where men reduced the distance while female did not show a change. However, since these reactions are of a minimum amount, it can be concluded that neurophysiological compensation mechanisms work equally well regardless of age and sex, and the upper body posture of healthy people changes only very slightly due to a temporarily symmetrical altered bite position.

Correlating Nanoscale Optical Coherence Length and Microscale Topography in Organic Materials by Coherent Two-Dimensional Microspectroscopy

Many nanotechnology materials rely on a hierarchical structure ranging from the nanometer scale to the micrometer scale. Their interplay determines the nanoscale optical coherence length, which plays a key role in energy transport and radiative decay and, thus, the optoelectronic applications. However, it is challenging to detect optical coherence length in multiscale structures with existing methods. Techniques such as atomic force microscopy and transmission electron microscopy are not sensitive to optical coherence length. Linear absorption and fluorescence spectroscopy methods, on the other hand, were generally limited by inhomogeneous broadening, which often obstructs the determination of nanoscale coherence length. Here, we carry out coherent two-dimensional microspectroscopy to obtain a map of the local optical coherence length within a hierarchically structured molecular film. Interestingly, the nanoscale coherence length is found to correlate with microscale topography, suggesting a perspective for controlling structural coherence on molecular length scales by appropriate microscopic growth conditions.

Cyclic (Amino)(aryl)carbenes Enter the Field of Chromophore Ligands: Expanded π System Leads to Unusually Deep Red Emitting CuI Compounds

A series of copper(I) complexes bearing a cyclic (amino)(aryl)carbene (CAArC) ligand with various complex geometries have been investigated in great detail with regard to their structural, electronic, and photophysical properties. Comparison of [CuX(CAArC)] (X = Br (1), Cbz (2), acac (3), Ph₂acac (4), Cp (5), and Cp* (6)) with known Cu^I complexes bearing cyclic (amino)(alkyl), monoamido, or diamido carbenes (CAAC, MAC, or DAC, respectively) as chromophore ligands reveals that the expanded π-system of the CAArC leads to relatively low energy absorption maxima between 350 and 550 nm in THF with high absorption coefficients of 5-15 × 10³ M^-1 cm^-1 for 1-6. Furthermore, 1-5 show intense deep red to near-IR emission involving their triplet excited states in the solid state and in PMMA films with λ^em_max = 621-784 nm. Linear [Cu(Cbz)(^DippCAArC)] (2) has been found to be an exceptional deep red (λ_max = 621 nm, ϕ = 0.32, τ_av = 366 ns) thermally activated delayed fluorescence (TADF) emitter with a radiative rate constant k_r of ca. 9 × 10⁵ s^-1, exceeding those of commercially employed Ir^III- or Pt^II-based emitters. Time-resolved transient absorption and fluorescence upconversion experiments complemented by quantum chemical calculations employing Kohn-Sham density functional theory and multireference configuration interaction methods as well as temperature-dependent steady-state and time-resolved luminescence studies provide a detailed picture of the excited-state dynamics of 2. To demonstrate the potential applicability of this new class of low-energy emitters in future photonic applications, such as nonclassical light sources for quantum communication or quantum cryptography, we have successfully conducted single-molecule photon-correlation experiments of 2, showing distinct antibunching as required for single-photon emitters.

Luminescent Mono-, Di-, and Triradicals: Bridging Polychlorinated Triarylmethyl Radicals by Triarylamines and Triarylboranes

Up to three polychlorinated pyridyldiphenylmethyl radicals bridged by a triphenylamine carrying electron withdrawing (CN), neutral (Me), or donating (OMe) groups were synthesized and analogous radicals bridged by tris(2,6‐dimethylphenyl)borane were prepared for comparison. All compounds were as stable as common closed‐shell organic compounds and showed significant fluorescence upon excitation. Electronic, magnetic, absorption, and emission properties were examined in detail, and experimental results were interpreted using DFT calculations. Oxidation potentials, absorption and emission energies could be tuned depending on the electron density of the bridges. The triphenylamine bridges mediated intramolecular weak antiferromagnetic interactions between the radical spins, and the energy difference between the high spin and low spin states was determined by temperature dependent ESR spectroscopy and DFT calculations. The fluorescent properties of all radicals were examined in detail and revealed no difference for high and low spin states which facilitates application of these dyes in two‐photon absorption spectroscopy and OLED devices.

Ultrafast Pathways of the Photoinduced Insulator-Metal Transition in a Low-Dimensional Organic Conductor

Among functional organic materials, low-dimensional molecular crystals represent an intriguing class of solids due to their tunable electronic, magnetic, and structural ground states. This work investigates Cu(Me,Br-dicyanoquinonediimine)₂ single crystals, a charge transfer radical ion salt which exhibits a Peierls insulator-to-metal transition at low temperatures. The ultrafast electron diffraction experiments observe collective atomic motions at the photoinduced phase transition with a temporal resolution of 1 ps. These measurements reveal the photoinduced lifting of the insulating phase to happen within 2 ps in the entire crystal volume with an external quantum efficiency of conduction band electrons per absorbed photon of larger than 20. This huge cooperativity of the system, directly monitored during the phase transition, is accompanied by specific intramolecular motions. However, only an additional internal volume expansion, corresponding to a pressure relief, allows the metallic state for long times to be optically locked. The identification of the microscopic molecular pathways that optically drive the structural Peierls transition in Cu(DCNQI)₂ highlights the tailored response to external stimuli available in these complex functional materials, a feature enabling high-speed optical sensing and switching with outstanding signal responsivity.

Spatially resolved coherent 2D fluorescence spectroscopy within a high-NA microscope

We have developed coherent two-dimensional (2D) fluorescence micro-spectroscopy which probes the nonlinear optical response at surfaces via fluorescence detection with sub-micron spatial resolution. This enables the investigation of microscopic variations in laterally heterogeneous film samples which are of interests for sub-wavelength opto-electronic devices.

Coherent two-dimensional fluorescence micro-spectroscopy

We have developed coherent two-dimensional (2D) fluorescence micro-spectroscopy which probes the nonlinear optical response at surfaces via fluorescence detection with sub-micron spatial resolution. This enables the investigation of microscopic variations in heterogeneous systems. An LCD-based pulse shaper in 4f geometry is used to create collinear trains of 12-fs visible/NIR laser pulses in the focus of an NA = 1.4 immersion-oil microscope objective. We demonstrate the capabilities of the new method by presenting 2D spectra, analyzed via phase cycling, as a function of position of selected sub-micron regions from a laterally nanostructured polycrystalline thin film of fluorinated zinc phthalocyanine (F₁₆ZnPc).

The electronic character of PTCDA thin films in comparison to other perylene-based organic semi-conductors: ab initio-, TD-DFT and semi-empirical computations of the opto-electronic properties of large aggregates

Perylene-based compounds are promising materials for opto-electronic thin film devices but despite intense investigations, important details of their electronic structure are still under debate. For perylene-3,4,9,10-tetracarboxylic dianhydrid (PTCDA), the theoretical models predict a different relative energetic order of Frenkel and Charge Transfer (CT) states. Additionally, while one model indicates strong differences between PTCDA on one hand and other perylene-based compounds on the other, recent ab initio computations indicate electronic properties of all perylene-based compounds to resemble each other. Finally, the models disagree about the amount of mixing between CT and Frenkel states. Definitive answers to these questions are difficult because the approaches use various approximations. Up to date, the ab initio based methods employ rather small model systems and neglect environmental effects. In the present work, we improve our former approach by analyzing the effects of the various simplifications. In more detail, we increase the size of the model systems, include environmental effects and investigate the influence of exciton-phonon couplings on the absorption spectrum. The computations for larger aggregates were performed with the ZINDO/S approach, because benchmark computations show that it provides accurate vertical excitation energies for Frenkel, as well as CT states.

Low-Cost and Sustainable Organic Thermoelectrics Based on Low-Dimensional Molecular Metals

Thermoelectric generator composed of crystalline radical ion salts: The unipolar charge transport along the molecular stacks facilitates complementary p- and n-type organic thermoelectric materials of high electrical conductivity and of 1D electronic structure. The specific power output of 5 mW cm^-2 and the zT > 0.15 below 40 K demonstrate a new field of low-temperature thermoelectric applications unlocked by organic metals.

Hybrid metal-organic nanocavity arrays for efficient light out-coupling

The spatially and spectrally resolved photoluminescence (PL) of the archetypical molecular dye ZnPc in periodically ordered organic-silver nanocavities (NC) is investigated by confocal microscopy. The presented approach of long-range ordered pillar structures prepared by nanosphere lithography not only combines the advantages of nanopatterning and localized surface plasmon resonances (LSPR) to improve the light out-coupling efficiency in metal-organic hybrid assemblies, but allows for distinction between geometrical and plasmonic contributions to the PL enhancement, the latter supported by complementary finite-difference-time-domain (FDTD) simulations. Supplementary time-resolved optical measurements indicate exciton lifetime reduction by at least one order of magnitude to be the main mechanism for PL increase amongst the improvement of geometrical out-coupling.

A Cyclic Alkyl(amino)carbene as Two-Atom π-Chromophore Leading to the First Phosphorescent Linear CuI Complexes

The members of a series of linear and trigonal copper(I) complexes bearing a cyclic alkyl(amino)carbene (CAAC) ligand show surprising photophysical properties compared to those of the corresponding N-heterocyclic carbene (NHC) complexes. Whereas the linear NHC complexes [CuX(NHC)] are almost non-emissive, [CuX(CAAC)] (X=Cl, Br, I) and [Cu(CAAC)₂ ]PF₆ show very bright emissions from their triplet excited states in the blue to green region, displaying quantum yields of up to 65 % in the solid state, even though the π-acceptor comprises only the carbene C and N atoms with no other π conjugation. [Cu(CAAC)₂ ]PF₆ is the fastest Cu^I -based triplet state emitter characterized to date, not displaying thermally activated delayed fluorescence (TADF), with an intrinsic lifetime of only 10.6 μs, that is, k_r =9.4×10⁴  s^-1 , competitive with many Pt^II - and Ir^III -based emitters. In order to test the stability of such linear copper CAAC complexes in devices, some of our compounds have been applied in proof-of-principle organic light-emitting diodes (OLEDs). This case study thus demonstrates for the first time the use of CAACs as suitable π-chromophores for Cu^I -based phosphorescent emitters, and their implementation in OLEDs underlines the general applicability of this class of ligands in materials science.

Engineering near-infrared single-photon emitters with optically active spins in ultrapure silicon carbide

Vacancy-related centres in silicon carbide are attracting growing attention because of their appealing optical and spin properties. These atomic-scale defects can be created using electron or neutron irradiation; however, their precise engineering has not been demonstrated yet. Here, silicon vacancies are generated in a nuclear reactor and their density is controlled over eight orders of magnitude within an accuracy down to a single vacancy level. An isolated silicon vacancy serves as a near-infrared photostable single-photon emitter, operating even at room temperature. The vacancy spins can be manipulated using an optically detected magnetic resonance technique, and we determine the transition rates and absorption cross-section, describing the intensity-dependent photophysics of these emitters. The on-demand engineering of optically active spins in technologically friendly materials is a crucial step toward implementation of both maser amplifiers, requiring high-density spin ensembles, and qubits based on single spins.

Electroless preparation and ASAXS microstructural analysis of pseudocapacitive carbon manganese oxide supercapacitor electrodes

Anomalous small angle X-ray scattering (ASAXS) has been utilized as a noninvasive, integral tool to access the structural properties of carbon xerogel-manganese oxide electrodes with nanometer resolution. As these electrodes constitute the elementary functional units in supercapacitors and as their microstructure governs the macroscopic electrical performance, it is essential to gain a detailed morphological understanding of the underlying carbon particle scaffold coated with manganese oxide. We demonstrate that, in this regard, ASAXS provides a powerful technique and in combination with a theoretical core-shell model enables a quantitative estimation of the relevant structural parameters. As a result, we determined the thicknesses of the solution deposited MnO2 shells to range between 3 and 26 nm depending on the carbon particle size and thus on their effective surface area. By our core-shell modeling we conclude the revealed manganese oxide coatings on the carbon support to be rather thick, but nevertheless to show a high uniformity in thickness. At 1.8 ± 0.2 to 2.2 ± 0.1 g/cm(3) the related effective MnO2 densities of the shells are about 30% lower than the corresponding bulk density of 3.0 g/cm(3). This mainly originates from a substructure within the shell, whose growth is controlled by a pronounced reduction of the manganese precursor during layer formation. Finally, the presented ASAXS data are complemented by SEM and N2 sorption measurements, proving not only qualitatively the proposed flake-like MnO2 surface morphology but also confirming quantitatively the manganese shell thickness, complementary, on a local scale.

Ultrafast dynamics of excitons in tetracene single crystals

Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states S(n) on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S1 on a 40 ps timescale. The high energy Davydov component of the S1 exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale.

Optoelectronic processes in squaraine dye-doped OLEDs for emission in the near-infrared

A novel all-organic host-guest system for emission in the NIR is introduced and investigated with respect to its opto-electronic processes. The good agreement between theoretical and experimental results highlights the model character of this system and its potential for electroluminescent application. Comparative measurements provide access to the recombination mechanisms on molecular length scale and show that the emission behavior of the device under operation is controlled by charge carrier dynamics.

Triphenylene silanes for direct surface anchoring in binary mixed self-assembled monolayers

New triphenylene-based silanes 2-(ω-(chlorodimethylsilyl)-n-alkyl)-3,6,7,10,11-penta-m-alkoxytriphenylene 4 (Tm-Cn) with n = 8 or 9 and m = 7, 8, 9, 10, or 11 were synthesized, and their self-assembly behavior in the liquid state and at glass and silicon oxide surfaces was investigated. The mesomorphic properties of triphenylene silanes 4 (Tm-Cn) and their precursors 3 (Tm-Cn) were determined by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction. From the small-angle X-ray scattering (SAXS) regime, a preferential discotic lamellar mesophase can be deduced, and wide-angle X-ray scattering (WAXS) highlights the liquid-like characteristics of the alkyl side chains. To transfer these bulk structural properties to thin films, self-assembled monolayers (SAMs) were obtained by adsorption from solution and characterized by water contact angle measurements, null ellipsometry, and atomic force microscopy (AFM). Employing the concentration as an additional degree of freedom, binary SAMs of 2-(ω-(chlorodimethylsilyl)-undecyl)-3,6,7,10,11-penta-decyloxytriphenylene 4 (T10-C11) were coassembled with chlorodecyldimethylsilane or chlorodimethyloctadecylsilane, and their capability as model systems for organic templating was evaluated. The structure of the resulting binary mixed SAMs was analyzed by water contact angle measurements, null ellipsometry, and X-ray reflectivity (XRR) in combination with theoretical modeling by a multidimensional Parratt algorithm and AFM. The composition dependence of film thickness and roughness can be explained by a microscopic model including the steric hindrance of the respective molecular constituents.

Comparison of the electronic structure of different perylene-based dye-aggregates

Aggregates of functionalized polycyclic aromatic molecules like perylene derivatives differ in important optoelectronic properties such as absorption and emission spectra or exciton diffusion lengths. Although those differences are well known, it is not fully understood if they are caused by variations in the geometrical orientation of the molecules within the aggregates, variations in the electronic structures of the dye aggregates or interplay of both. As this knowledge is of interest for the development of materials with optimized functionalities, we investigate this question by comparing the electronic structures of dimer systems of representative perylene-based chromophores. The study comprises dimers of perylene, 3,4,9,10-perylene tetracarboxylic acid bisimide (PBI), 3,4,9,10-perylene tetracarboxylic acid dianhydride (PTCDA), and diindeno perylene (DIP). Potential energy curves (PECs) and characters of those electronic states are investigated which determine the optoelectronic properties. The computations use the spin-component-scaled approximate coupled-cluster second-order method (SCS-CC2), which describes electronic states of predominately neutral excited (NE) and charge transfer (CT) character equally well. Our results show that the characters of the excited states change significantly with the intermolecular orientation and often represent significant mixtures of NE and CT characters. However, PECs and electronic structures of the investigated perylene derivatives are almost independent of the substitution patterns of the perylene core indicating that the observed differences in the optoelectronic properties mainly result from the geometrical structure of the dye aggregate. It also hints at the fact that optical properties can be computed from less-substituted model compounds if a proper aggregate geometry is chosen.

Optical sensing of current dynamics in organic light-emitting devices at the nanometer scale

Photoluminescence quenching of single dibenzoterrylene (DBT) dye molecules in a polymeric organic light-emitting diode was utilized to analyze the current dynamics at nanometer resolution. The quenching mechanism of single DBT molecules results from an increase in the triplet-state population induced by charge carrier recombination on individual guest molecules. As a consequence of the long triplet-state relaxation time, its population results in a reduced photoluminescence of the dispersed fluorescent dyes. From the decrease in photoluminescence together with photon correlation measurements, we could quantify the local current density and its time-dependent evolution in the vicinity of the single-molecule probe. This optical technique establishes a non-invasive approach to map the time-resolved current density in organic light-emitting diodes on the nanometer scale.

One dimensional molecular dipole chain arrays on graphite via nanoscale phase separation

Molecular dipole chain arrays of chloroaluminium phthalocyanine (ClAlPc) on the graphite surface have been investigated by scanning tunneling microscopy. The inter-chain spacing can be tuned by the co-adsorption of di-indenoperylene (DIP) via nanoscale phase separation.

Tunable two-dimensional binary molecular networks

A novel approach to constructing tunable and robust 2D binary molecular nanostructures on an inert graphite surface is presented. The guest molecules are embedded into a host molecular matrix and constrained via the formation of multiple intermolecular hydrogen bonds. By varying the binary molecular ratio and the molecular geometry, various molecular arrays with tunable intermolecular distances are fabricated. The results suggest a promising route for the fabrication of ordered and stable molecular nanostructure arrays for molecular sensors, molecular spintronic devices, and molecular p-n nanojunctions.

Tetracene film morphology: comparative atomic force microscopy, X-ray diffraction and ellipsometry investigations

X-ray diffraction, atomic force microscopy and spectroscopic ellipsometry were used to study tetracene thin films as a function of deposition rate. A comparative analysis of the thickness and roughness values allows for detailed modelling of the film morphology. An interdigitated growth mode is established for the coexisting thin film and bulk phases. By comparison with the respective quinone-derivative of tetracene, we were additionally able to identify reaction products by their optical response.

Kramers-Kronig-consistent optical functions of anisotropic crystals: generalized spectroscopic ellipsometry on pentacene

The Kramers-Kronig relations between the real and imaginary parts of a response function are widely used in solid-state physics to evaluate the corresponding quantity if only one component is measured. They are among the most fundamental statements since only based on the analytical behavior and causal nature of the material response [Phys. Rev. 104, 1760-1770 (1956)]. Optical losses, for instance, can be obtained from the dispersion of the dielectric constant at all wavelengths, and vice versa [Handbook of optical constants of solids, Vol. 1, p. 35]. Although the general validity was never casted into doubt, it is a longstanding problem that Kramers-Kronig relations cannot simply be applied to anisotropic crystalline materials because contributions from different directions mix in a frequency-dependent way. Here we present a general method to identify frequency-independent principal polarizability directions for which the Kramers-Kronig relations are obeyed even in materials with lowest symmetry. Using generalized spectroscopic ellipsometry on a single crystal surface of triclinic pentacene, as an example, enables us to evaluate the complex dielectric constant and to compare it with band-structure calculations along the crystallographic directions. A general recipe is provided how to proceed from a macroscopic measurement on a low symmetry crystal plane to the microscopic dielectric properties of the unit cell, along whose axes the Kramers-Kronig relations hold.

Nanometer-thick Au-films as antireflection coating for infrared light

The optical properties of ultrathin Au films on silicon have been studied in the infrared over a wide frequency range from 200 to 10,000 cm(-1). Thick films show a Drude behavior; i.e., with increasing frequency the transmission increases; for films below the percolation threshold at about 5 nm a negative slope for the frequency-dependent transmission is observed. When the thickness is further reduced, between 1 and 3 nm an anomaly occurs: the relative transmission reaches maximum values above 100% compared with the bare substrate, indicating an antireflection coating of nanometer thickness for light of 5 microm wavelength. This anomaly can be explained in the framework of effective-medium theories.

Ultralow-power organic complementary circuits

The prospect of using low-temperature processable organic semiconductors to implement transistors, circuits, displays and sensors on arbitrary substrates, such as glass or plastics, offers enormous potential for a wide range of electronic products. Of particular interest are portable devices that can be powered by small batteries or by near-field radio-frequency coupling. The main problem with existing approaches is the large power consumption of conventional organic circuits, which makes battery-powered applications problematic, if not impossible. Here we demonstrate an organic circuit with very low power consumption that uses a self-assembled monolayer gate dielectric and two different air-stable molecular semiconductors (pentacene and hexadecafluorocopperphthalocyanine, F16CuPc). The monolayer dielectric is grown on patterned metal gates at room temperature and is optimized to provide a large gate capacitance and low gate leakage currents. By combining low-voltage p-channel and n-channel organic thin-film transistors in a complementary circuit design, the static currents are reduced to below 100 pA per logic gate. We have fabricated complementary inverters, NAND gates, and ring oscillators that operate with supply voltages between 1.5 and 3 V and have a static power consumption of less than 1 nW per logic gate. These organic circuits are thus well suited for battery-powered systems such as portable display devices and large-surface sensor networks as well as for radio-frequency identification tags with extended operating range.

Anomalous roughness evolution of rubrene thin films observed in real time during growth

We study the growth and structure of thin films of the organic semiconductor rubrene during organic molecular beam deposition (OMBD) on silicon oxide in situ and in real time using X-ray scattering. Using in situ grazing incidence diffraction (GID) we find a small degree of local order but an otherwise largely disordered structure, consistent with out of plane scans. Monitoring the surface morphology in real time during growth, we find relatively smooth films (surface roughness sigma below approximately 15 A for thicknesses up to at least 600 A) and a significant delay before the onset of roughening. This anomalous roughening in the beginning and crossover to normal roughening later during growth may be related to conformational changes of rubrene in the early stages of growth.

The effect of oxygen exposure on pentacene electronic structure

We use ultraviolet photoelectron spectroscopy to investigate the effect of oxygen and air exposure on the electronic structure of pentacene single crystals and thin films. It is found that O(2) and water do not react noticeably with pentacene, whereas singlet oxygen/ozone readily oxidize the organic compound. Also, we obtain no evidence for considerable p-type doping of pentacene by O(2) at low pressure. However, oxygen exposure lowers the hole injection barrier at the interface between Au and pentacene by 0.25 eV, presumably due to a modification of the Au surface properties.

LEED, STM, and TDS studies of ordered thin films of the rhombus-shaped polycondensed aromatic hydrocarbon C54H22, on MoS2, GeS, and graphite

Low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and thermal desorption spectroscopy (TDS) are used to study vacuum vapor-deposited molecular thin films of the rhombus-shaped polycondensed aromatic hydrocarbon "rhombus-C54", C54H22, on MoS2 and graphite (0001) and on GeS (010) substrates. It is found that this compound forms well-ordered incommensurate superstructures of the closest packed flat-lying molecules in well-defined azimuthal orientations to the substrate. These films are thermally remarkably stable. By TDS, a monolayer binding energy on graphite of 2.3 eV was derived, whereas the molecules in the second layer were found to be less strongly bound (1.9 eV). This difference allows the preparation of monolayers by desorbing multilayers at the appropriate temperature. Apparently, this molecule is a promising candidate for further studies aiming at applications in organic electronics such as organic field effect transistors or light emitting displays.

Health promotion partnerships: service and education addressing the health needs of vulnerable groups

This paper describes partnerships between service and education that can assist in meeting the health care needs of vulnerable population groups. Baccalaureate nursing students learn about population-based nursing practice as a means of addressing health needs. Each semester, groups of 8-10 senior students work with a community agency serving a population at risk. Students assess health needs and plan, implement, and evaluate a health promotion intervention with the population and the agency. Emphasis is placed on designing culturally appropriate interventions that are accomplished in partnership with the agency and population. Projects which illustrate the generalizability of this approach will be discussed. Such experiences reduce barriers that separate education from practice. Community agencies benefit as health needs that might not otherwise be met are addressed.