Fluorescent Rotary Switches: Four- vs Three-Substituted Phthalimide Boron Difluoride Schiff Base Complexes.

The influence of the substitution pattern in phthalimide boron difluoride Schiff base complexes as fluorescent molecular rotors has been investigated. Due to their ground-state zwitterionic structures, they have exhibited negative solvatochromism in absorption and blue-green emission with moderate to satisfactory photoluminescence quantum yields in solution. Ground-state and excited-state theoretical calculations and time-resolved emission spectroscopy revealed that the excited-state rotation is triggered by planar-induced charge transfer, resulting in switched emission toward the green region. Fluorescence lifetime measurements and species-associated emission spectra exhibited two emitting excited species in equilibrium via a planar transition-state barrier. The substitution pattern models showed different behavior in solid-state mechanochromic switching and were analyzed by subcell unit packing obtained from X-ray structure data. We have attempted to gain in-depth insight into the fluorescence mechanism and photoluminescence properties associated with the substitution pattern of the phthalimide motif in order to understand the structure-property-function relationship.

Spectroscopic behavior differences between lumazine and alloxazine in the DMSO-water mixture.

The absorption and emission spectra were investigated for lumazine, alloxazine and their cyanated or fluorinated derivatives, respectively. The spectroscopic properties were modulated by varying water concentration in dimethyl sulfoxide (DMSO). Some intriguing experimental results were found for the samples containing 65 % of water and 35% of DMSO. This finding is consistent with previously published molecular dynamics (MD) simulations confirming the concept of the 'local bulk' model. In this case, a notable decrease in absorption and emission intensities was registered, even larger than the water quenching observed in other cases. The changes in midrange DMSO concentrations could be explained by the formation of local solvents structures as predicted by MD, specifically the formation of DMSO·2H2O dimers. Experimentally, the cyano-substituted lumazine has shown a remarkable sensitivity to DMSO concentration. The spectroscopic measurements were interpreted using the density functional theory where the implicit DMSO solvent model was combined with explicit water molecules. Together with its enhanced water solubility, the cyanated lumazine derivate could be used for non-destructive DMSO detection in vitro for applications such as drug uptake monitoring, since DMSO is often used in pharmaceutical practice.

Colour-tuneable solid-state fluorescence of crystalline powders formed from push-pull substituted 2,5-diphenyl-stilbenes.

The solid-state fluorescence (SSF) of eight DPA-DPS-EWG derivatives (DPA = diphenylamino, DPS = 2,5-diphenyl-stilbene building block, EWG = electron withdrawing group) was studied. Varying the strength of the EWG enabled the tuning of the LUMO energy within a range broader than 1 eV, while the simultaneous changes of HOMO energy were less than 0.1 eV, according to cyclic voltammetry. The fluorescence maxima in dichloromethane laid between 483 and 752 nm and exhibited monoexponential decay and a photoluminescence quantum yield (PLQY) always higher than 35%. Six derivatives with a SSF PLQY higher than 10% in polycrystalline powder form continuously covered the range from 475 to 733 nm. Three components of SSF multiexponential decay, obtained by time-resolved fluorescence spectroscopy, were ascribed to exciton migration to nonfluorescent traps, and monomer-like and aggregate fluorescence. The character of the emitting aggregates was evaluated by quantum chemical modelling based on time-dependent density functional theory computations, carried out on the dimer arrangements obtained by X-ray diffractometry of the single crystals.

Complete Set of Diketopyrrolopyrrole Centrosymmetrical Cofacial Stacked Pairs.

Stacked centrosymmetrical dimers and simultaneously H-bonded and stacked hexamers of thiophene-substituted diketopyrrolopyrrole (ThDPP) were studied using DFT as models for crystals with slipped-stacked molecules in 1D columns. Eight stacked dimer arrangements were found, six of which are driven by the minimisation of electron repulsion and realised by placing the partially negatively charged atoms of the diketopyrrolopyrrole rings below the centre of an adjacent thiophene ring. Four of these stacks are related to N,N'-dialkylated derivatives of ThDPP found in the literature, while a further one is related to an N,N'-diacylated derivative. An analogous set of eight stacks was discovered computationally for phenyl-substituted DPP (PhDPP), four of which are known among H-bonded DPP pigments, and one more among N,N'-dialkylated PhDPP derivatives. The results shed more light on the mechanisms that drive the formation of stacks between nonaromatic (DPP) and aromatic (Th, Ph) rings. The excitation energies of the lowest four singlet states computed by TD DFT enabled excitonic coupling and energy separation between Frenkel-resonance-type and charge-transfer states to be established, depending on the equilibrium stack geometry.

Electrode Spacing as a Determinant of the Output Performance of Planar-Type Photodetectors Based on Methylammonium Lead Bromide Perovskite Single Crystals.

Methylammonium lead bromide is a very perspective hybrid semiconductor material, suitable for high-sensitive, filter-free photodetection of electromagnetic radiation. Herein, we studied the effect of electrode spacing on the output performance and stability of planar-type photodetectors based on high-quality MAPbBr3 single crystals. Such crystals, as large as 4.5×4.5×1.2 mm3 were synthesized via the inverse temperature crystallization method and were further used for the fabrication of planar Au/MAPbBr3/Au photodetectors with variable electrode spacing (in the range between 125 and 25 µm). We report that the electrode spacing has a profound impact on photocurrent densities and key detector parameters (responsivity R, external quantum efficiency EQE, and specific detectivity D*). In the studied fivefold electrode spacing, the photocurrent density increased over 4 times, with decreasing active area of the devices. This effect is attributed to intrinsic photocurrent amplification. Based on the transient photocurrent measurements and calculated key parameters, we determined the device sample with the best output performance. The champion sample with an electrode spacing of 50 µm exhibited great detection ability, especially for a low light intensity of 200 nWcm-2, for which we calculated the R of 19.55 A W-1, EQE of 4253%, and D* of 3.42 × 1012 Jones (cm Hz1/2 W-1). Moreover, the functional stability of this device showed a minimal reduction of photodetection ability after 2000 cycles, which makes it very promising for the next generation of optoelectronic devices.

Conductive Polymer PEDOT:PSS-Based Platform for Embryonic Stem-Cell Differentiation.

Organic semiconductors are constantly gaining interest in regenerative medicine. Their tunable physico-chemical properties, including electrical conductivity, are very promising for the control of stem-cell differentiation. However, their use for combined material-based and electrical stimulation remains largely underexplored. Therefore, we carried out a study on whether a platform based on the conductive polymer poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) can be beneficial to the differentiation of mouse embryonic stem cells (mESCs). The platform was prepared using the layout of a standard 24-well cell-culture plate. Polyethylene naphthalate foil served as the substrate for the preparation of interdigitated gold electrodes by physical vapor deposition. The PEDOT:PSS pattern was fabricated by precise screen printing over the gold electrodes. The PEDOT:PSS platform was able to produce higher electrical current with the pulsed-direct-current (DC) electrostimulation mode (1 Hz, 200 mV/mm, 100 ms pulse duration) compared to plain gold electrodes. There was a dominant capacitive component. In proof-of-concept experiments, mESCs were able to respond to such electrostimulation by membrane depolarization and elevation of cytosolic calcium. Further, the PEDOT:PSS platform was able to upregulate cardiomyogenesis and potentially inhibit early neurogenesis per se with minor contribution of electrostimulation. Hence, the present work highlights the large potential of PEDOT:PSS in regenerative medicine.

Green-, Red-, and Infrared-Emitting Polymorphs of Sterically Hindered Push-Pull Substituted Stilbenes.

The synthesis, XRD single-crystal structure, powder XRD, and solid-state fluorescence of two new DPA-DPS-EWG derivatives (DPA=diphenylamino, DPS=2,5-diphenyl-stilbene, EWG=electron-withdrawing group, that is, carbaldehyde or dicyanovinylene, DCV) are described. Absorption and fluorescence maxima in solvents of various polarity show bathochromic shifts with respect to the parent DPA-stilbene-EWGs. The electronic coupling in dimers and potential twist elasticity of monomers were studied by density functional theory. Both polymorphs of the CHO derivative emit green fluorescence (527 and 550 nm) of moderate intensity (10 % and 5 %) in polycrystalline powder form. Moderate (5 %) red (672 nm) monomer-like emission was also observed for the first polymorph of the DCV derivative, whereas more intense (32 %) infrared (733 nm) emission of the second polymorph was ascribed to the excimer fluorescence.

Singlet Fission in Thin Solid Films of Bis(thienyl)diketopyrrolopyrroles.

The singlet fission (SF) process discovered in bis(thienyl)diketopyrrolopyrroles (TDPPs) can boost their potential for photovoltaics (PV). The crystal structures of TDPP analogs carrying n-hexyl, n-butyl, or 2-(adamant-1-yl)ethyl substituents are similar, but contain increasingly slipped stacked neighbor molecules. The observed SF rate constants, kSF , (7±4), (9±3) and (5.6±1.9) ns-1 for thin films of the three compounds, respectively, are roughly equal, but the triplet quantum yields vary strongly: (120±40), (160±40) and (70±16), respectively. The recent molecular pair model reproduces the near equality of all three kSF at the crystal geometries and identifies all possible pair arrangements in which SF is predicted to be faster, by up to two orders of magnitude. However, it is also clear that the presently non-existent ability to predict the rates of processes competing with SF is pivotal for providing a guide for efforts to optimize the materials for PV.

Tunable Properties of Nature-Inspired N,N'-Alkylated Riboflavin Semiconductors.

A series of novel soluble nature-inspired flavin derivatives substituted with short butyl and bulky ethyl-adamantyl alkyl groups was prepared via simple and straightforward synthetic approach with moderate to good yields. The comprehensive characterization of the materials, to assess their application potential, has demonstrated that the modification of the conjugated flavin core enables delicate tuning of the absorption and emission properties, optical bandgap, frontier molecular orbital energies, melting points, and thermal stability. Moreover, the thin films prepared thereof exhibit smooth and homogeneous morphology with generally high stability over time.

Synthesis conditions influencing formation of MAPbBr3 perovskite nanoparticles prepared by the ligand-assisted precipitation method.

This work reports on an optimized procedure to synthesize methylammonium bromide perovskite nanoparticles. The ligand-assisted precipitation synthetic pathway for preparing nanoparticles is a cost-effective and promising method due to its ease of scalability, affordable equipment requirements and convenient operational temperatures. Nevertheless, there are several parameters that influence the resulting optical properties of the final nanomaterials. Here, the influence of the choice of solvent system, capping agents, temperature during precipitation and ratios of precursor chemicals is described, among other factors. Moreover, the colloidal stability and stability of the precursor solution is studied. All of the above-mentioned parameters were observed to strongly affect the resulting optical properties of the colloidal solutions. Various solvents, dispersion media, and selection of capping agents affected the formation of the perovskite structure, and thus qualitative and quantitative optimization of the synthetic procedure conditions resulted in nanoparticles of different dimensions and optical properties. The emission maxima of the nanoparticles were in the 508-519 nm range due to quantum confinement, as confirmed by transmission electron microscopy. This detailed study allows the selection of the best optimal conditions when using the ligand-assisted precipitation method as a powerful tool to fine-tune nanostructured perovskite features targeted for specific applications.

Surface Property Tuning of Methylammonium Lead Iodide by Plasma for Use in Planar Perovskite Solar Cells.

The demand for cheap and green energy as a replacement for fossil fuels has never been greater, and perovskite solar cells (PSCs) are among the leading means of meeting it. The surface properties of metal halide perovskite layers play crucial roles in the performance and durability of such cells. Consequently, a wide range of engineering processes for surface modification of perovskite layers has been investigated and among them is atmospheric pressure plasma (APP). Nevertheless, knowledge of the interaction between plasma and perovskite layers is still far from complete. In this work, CH3NH3PbI3 films were subjected to APP generated by a portable plasma source. A detailed understanding of band energy after plasma treatment is crucial to the investigation of the behavior of the perovskite layer. This study demonstrates a remarkable shift in the valence and conduction bands of a perovskite layer after plasma treatment, while band gap energy remains relatively constant. We found that short plasma treatment of perovskite layers resulted in higher performance and stability of PSCs.

Microwave-Assisted Preparation of Organo-Lead Halide Perovskite Single Crystals.

The efficiency of organo-lead halide perovskite-based optoelectronic devices is dramatically lower for amorphous materials compared to highly crystalline ones. Therefore, it is challenging to optimize and scale up the production of large-sized single crystals of perovskite materials. Here, we describe a novel and original approach to preparing lead halide perovskite single crystals by applying microwave radiation during the crystallization. The microwave radiation primarily causes precise heating control in the whole volume and avoids temperature fluctuations. Moreover, this facile microwave-assisted method of preparation is highly reproducible and fully automated, it and can be applied for various different perovskite structures. In addition, this cost-effective method is expected to be easily scalable because of its versatility and low energy consumption. The crystallization process has low heat losses; therefore, only a low microwave reactor power of 8-15 W during the temperature changes and of less than 1 W during the temperature holding is needed.

Cyclic Peptide Stabilized Lead Halide Perovskite Nanoparticles.

Combining the unique properties of peptides as versatile tools for nano- and biotechnology with lead halide perovskite nanoparticles can bring exceptional opportunities for the development of optoelectronics, photonics, and bioelectronics. As a first step towards this challenge sub 10 nm methylammonium lead bromide perovskite colloidal nanoparticles have been synthetizes using commercial cyclic peptide Cyclo(RGDFK), containing 5 amino acids, as a surface stabilizer. Perovskite nanoparticles passivated with Cyclo(RGDFK) possess charge transfer from the perovskite core to the peptide shell, resulting in lower photoluminescence quantum yields, which however opens a path for the application where charge transfer is favorable.

Density of bulk trap states of hybrid lead halide perovskite single crystals: temperature modulated space-charge-limited-currents.

Temperature-modulated space-charge-limited-current spectroscopy (TMSCLC) is applied to quantitatively evaluate the density of trap states in the band-gap with high energy resolution of semiconducting hybrid lead halide perovskite single crystals. Interestingly multicomponent deep trap states were observed in the pure perovskite crystals, which assumingly caused by the formation of nanodomains due to the presence of the mobile species in the perovskites.

Glycolated Thiophene-Tetrafluorophenylene Copolymers for Bioelectronic Applications: Synthesis by Direct Heteroarylation Polymerisation.

A series of copolymers containing a glycolated 1,4-dithienyl-2,3,5,6-tetrafluorophenylene unit copolymerized with thiophene, bithiophene, thienothiophene and 1,2,4,5-tetrafluorobenzene comonomer units were designed and synthesised by direct heteroarylation polymerisation. The optical, electrochemical, electrochromic and solid-state structural properties of the copolymers were investigated. The copolymers exhibit stable redox properties in organic solvents and promising redox properties in thin film configuration with an aqueous electrolyte. Finally, the potential of the copolymers as active materials in organic electrochemical transistors (OECTs) was assessed, and promising performance was shown as an accumulation-mode OECT material with a peak transconductance of 0.17 mS and a good on/off ratio of 105 for the thiophene copolymer.

Synthesis and photochromic properties of some N-phthalimide azo-azomethine dyes. A DFT quantum mechanical calculations on imine-enamine tautomerism and trans-cis photoisomerization.

This paper presents synthesis, photophysical characterization and quantum mechanical calculations of some N-phthalimide azo-azomethine dyes. The dyes were synthesized via azo coupling reaction between 2,4-substituted aromatic anilines and salicylic aldehyde followed by condensation reactions between azo dyes and N-aminophtalimide. Quantum chemical calculations to optimise the molecular geometry and to determine the electron densities of the trans (E) imine ⇌ enamine and the cis (Z) imine ⇌ enamine isomers and their vibrational frequencies have been computed by using DFT at B3LYP/6-31 + G(d,p) level of theory in vacuo. The effect of the used DMF solvent on the molecular structure and bond energies has been determined by using the IEFPCM model. Thermodynamic parameters such as total electronic energy E(RB3LYP), enthalpy H298 (sum of electronic and thermal enthalpies), free Gibbs energy G298 (sum of electronic and thermal free Gibbs energies) and dipole moment µ were computed in order to estimate the ΔE, Δµ, ΔH, ΔG and ΔS values. The NBO analysis was performed in order to understand the intramolecular charge transfer and the energy of resonance stabilization. After molecular geometry optimization, the electronic spectra were obtained by TD-DFT calculations at the above mentioned basis set using the IEFPCM model of DMF as a solvent. The solvatochromic effect of the dyes in four solvents with different polarity has been studied by UV-VIS spectroscopy and compared with the theoretically predicted. The coincidence between measured and calculated spectra is satisfactory. The dynamic photoisomerization experiments were performed in DMF under irradiation with UV light at λ = 365 nm (mostly E → Z) and with VIS light at λ = 400-800 nm (mostly Z → E). The spectra were recorded in the spectral region from 300 to 800 nm at identical sample concentrations of the three dyes and illumination times in order to investigate the photodynamical E → Z → E conversion of the NN chromophore group of the dyes as well as the imine ⇌ enamine tautomerization.

Novel Riboflavin-Inspired Conjugated Bio-Organic Semiconductors.

Flavins are known to be extremely versatile, thus enabling routes to innumerable modifications in order to obtain desired properties. Thus, in the present paper, the group of bio-inspired conjugated materials based on the alloxazine core is synthetized using two efficient novel synthetic approaches providing relatively high reaction yields. The comprehensive characterization of the materials, in order to evaluate the properties and application potential, has shown that the modification of the initial alloxazine core with aromatic substituents allows fine tuning of the optical bandgap, position of electronic orbitals, absorption and emission properties. Interestingly, the compounds possess multichromophoric behavior, which is assumed to be the results of an intramolecular proton transfer.

Atmospheric Dry Hydrogen Plasma Reduction of Inkjet-Printed Flexible Graphene Oxide Electrodes.

This study concerns a low-temperature method for dry hydrogen plasma reduction of inkjet-printed flexible graphene oxide (GO) electrodes, an approach compatible with processes envisaged for the manufacture of flexible electronics. The processing of GO to reduced graphene oxide (rGO) was performed in 1-64 s, and sp2 /sp2 +sp3 carbon concentration increased from approximately 20 % to 90 %. Since the plasma reduction was associated with an etching effect, the optimal reduction time occurred between 8 and 16 s. The surface showed good mechanical stability when deposited on polyethylene terephthalate flexible foils and significantly lower sheet resistance after plasma reduction. This method for dry plasma reduction could be important for large-area hydrogenation and reduction of GO flexible surfaces, with present and potential applications in a wide variety of emerging technologies.

Printing inks of electroactive polymer PEDOT:PSS: The study of biocompatibility, stability, and electrical properties.

Biocompatibility tests and a study of the electrical properties of thin films prepared from six electroactive polymer ink formulations based on poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) were performed. The aim was to find a suitable formulation of PEDOT:PSS and conditions for preparing thin films in order to construct printed bioelectronic devices for biomedical applications. The stability and electrical properties of such films were tested on organic electrochemical transistor (OECT)-based sensor platforms and their biocompatibility was evaluated in assays with 3T3 fibroblasts and murine cardiomyocytes. It was found that the thin films prepared from inks without an additive or any thin film post-treatment provide limited conductivity and stability for use in biomedical applications. These properties were greatly improved by using ethylene glycol and thermal annealing. Addition or post-treatment by ethylene glycol in combination with thermal annealing provided thin films with electrical resistance and a stability sufficient to be used in sensing of animal cell physiology. These films coated with collagen IV showed good biocompatibility in the assay with 3T3 fibroblasts when compared to standard cell culture plastics. Selected films were then used in assays with murine cardiomyocytes. We observed that these cells were able to attach to the PEDOT:PSS films and form an active sensor element. Spontaneously beating clusters were formed, indicating a good physiological status for the cardiomyocyte cells. These results open the door to construction of cheap printed electronic devices for biointerfacing in biomedical applications. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 1121-1128, 2018.