Optical Absorption Properties in Pentacene/Tetracene Solid Solutions.

Modifying the optical and electronic properties of crystalline organic thin films is of great interest for improving the performance of modern organic semiconductor devices. Therein, the statistical mixing of molecules to form a solid solution provides an opportunity to fine-tune optical and electronic properties. Unfortunately, the diversity of intermolecular interactions renders mixed organic crystals highly complex, and a holistic picture is still lacking. Here, we report a study of the optical absorption properties in solid solutions of pentacene and tetracene, two prototypical organic semiconductors. In the mixtures, the optical properties can be continuously modified by statistical mixing at the molecular level. Comparison with time-dependent density functional theory calculations on occupationally disordered clusters unravels the electronic origin of the low energy optical transitions. The disorder partially relaxes the selection rules, leading to additional optical transitions that manifest as optical broadening. Furthermore, the contribution of diabatic charge-transfer states is modified in the mixtures, reducing the observed splitting in the 0-0 vibronic transition. Additional comparisons with other blended systems generalize our results and indicate that changes in the polarizability of the molecular environment in organic thin-film blends induce shifts in the absorption spectrum.

Implementation of a spontaneous awakening/spontaneous breathing trial protocol in a surgical intensive care unit: a before and after study.

BACKGROUND: Prolonged invasive mechanical ventilation (IMV) influences patient outcome in multiple ways. In this regard the early weaning from IMV is a major goal to be achieved in the treatment of ICU patients. Adopting a weaning protocol that incorporates a Spontaneous Awakening Trial (SAT) and a Spontaneous Breathing Trial (SBT) seems to be essential to reach this goal. Most studies investigating the effectiveness of SAT/SBT protocols in ICU patients' outcomes have focused mainly on medical or mixed (medical and surgical), but not on exclusively surgical patient populations. Surgical patients usually experience more complications and often undergo revision surgeries, therefore needing longer sedation periods and adequate analgo-sedation therapy. Moreover, the longer IMV times make the weaning process more arduous. METHODS: Our retrospective data analysis therefore investigates the effectiveness of a SAT/SBT protocol implementation in the surgical ICU of Heidelberg University Hospital, focusing exclusively on surgical patients and their outcome related to the weaning process. The SAT/SBT protocol was adopted in Heidelberg ICU starting from 05/2019. We therefore analyzed the time period before and after the implementation between 03/2018 and 08/2020. Adult patients who required invasive ventilation for at least 48 hours were screened for study entry. Demographic data, clinical data and SOFA Score on admission, were collected to define the baseline characteristics of the two groups. Only patients with full adherence to the protocol were included. The primary outcome was defined as the successful extubation, intended as an extubation not followed by successive re-intubations until discharge from the ICU. We performed an univariate analysis to evaluate the rate of successful extubations between the two groups. RESULTS: In total, 199 patients were included in the analysis, 98 of which before the SAT/SBT protocol implementation (control group) and 101 after the SAT/SBT protocol implementation (intervention group). The successful extubation rate in the intervention group resulted in 82% (83/101 patients) compared to 64% (63/98 patients) in the control group (P<0.004). CONCLUSIONS: We conclude that even for an exclusively surgical patient population, the implementation of a SAT/SBT protocol could result in a higher rate of successful extubation.

Vacancy control in acene blends links exothermic singlet fission to coherence.

The fission of singlet excitons into triplet pairs in organic materials holds great technological promise, but the rational application of this phenomenon is hampered by a lack of understanding of its complex photophysics. Here, we use the controlled introduction of vacancies by means of spacer molecules in tetracene and pentacene thin films as a tuning parameter complementing experimental observables to identify the operating principles of different singlet fission pathways. Time-resolved spectroscopic measurements in combination with microscopic modelling enables us to demonstrate distinct scenarios, resulting from different singlet-to-triplet pair energy alignments. For pentacene, where fission is exothermic, coherent mixing between the photoexcited singlet and triplet-pair states is promoted by vibronic resonances, which drives the fission process with little sensitivity to the vacancy concentration. Such vibronic resonances do not occur for endothermic materials such as tetracene, for which we find fission to be fully incoherent; a process that is shown to slow down with increasing vacancy concentration.

Template-Free Orientation Selection of Rod-Like Molecular Semiconductors in Polycrystalline Films.

Many organic semiconductors (OSCs) feature strong optical anisotropy that can be exploited to increase the efficiency of optoelectronic devices. We demonstrate that for the technologically relevant, rod-like OSCs diindenoperylene (DIP), pentacene (PEN), and α-sexithiophene (6T) deposited on silicon oxide surfaces it is possible to prepare polycrystalline thin films in which the long molecular axis is oriented parallel to the substrate plane in a template-free fashion. In films grown by organic molecular beam deposition at room temperature or higher, the molecules are oriented upright standing (σ-orientation). Instead, the here-presented growth at low temperatures followed by slow annealing up to a temperature near molecular desorption has the effect of "freezing" the molecules with their long axis oriented parallel to the substrate plane (λ-orientation) while conferring them crystalline long-range order. We discuss the huge impact on the optical anisotropy of the films observed as a consequence of the orientation transition. Finally, we propose a mechanism for explaining the achieved λ-orientation, which is stable under environmental conditions.

Kinetics of Ion-Exchange Reactions in Hybrid Organic-Inorganic Perovskite Thin Films Studied by In Situ Real-Time X-ray Scattering.

The exchange of ions in hybrid organic-inorganic perovskites with the general formula APbX3 (A = MA, FA; X = I, Cl, Br) is studied in five different systems using in situ real-time grazing incident X-ray diffraction (GIXD). In systems where the organic cation is exchanged, we find a continuous shift of the lattice parameter. The relative shift compared to the pure materials is used to quantify the exchange. Whether or not a conversion is possible, as well as the amount of exchanged cations, depends on the halide used. In the case of the interconversion of MAPbI3 and MAPbCl3, we observe a decay of the diffraction peaks of the original perovskite and the emergence of new peaks corresponding to the structure with the alternative halide. Moreover, we determined the relevant time scales of the growth and decay of the perovskite structures.

Monitoring Self-Assembly and Ligand Exchange of PbS Nanocrystal Superlattices at the Liquid/Air Interface in Real Time.

We investigate in situ the structural changes during self-assembly of PbS nanocrystals from colloidal solution into superlattices, solvent evaporation, and ligand exchange at the acetonitrile/air interface by grazing incidence small-angle X-ray scattering (GISAXS). We simulate and fit the diffraction peaks under the distorted wave Born approximation (DWBA) to determine the lattice parameters. We observe a continuous isotropic contraction of the superlattice in two different assembly steps, preserving the body-centered cubic lattice with an overall decrease in the lattice constants of 11%. We argue that the first contraction period is due to a combination of solvent evaporation/annealing and capillary forces acting on the superlattice, whereas the second period is dominated by the effect of replacing oleic acid on the nanocrystal surface with the short and rigid cross-linker tetrathiafulvalene dicarboxylate. This work provides guidelines for optimized ligand exchange conditions and highlights the structural particularities arising from assembling NCs on liquid surfaces.