Tandem Pd-catalyzed annulation/coupling of acetylenic enamines with aryl triflates.

A cascade of 5-exo-dig intramolecular nucleophilic addition of enamine to terminal alkyne followed by cross coupling has been demonstrated for the first time. Two new C-C bonds are stereoselectively forged by a single Pd-complex capable of catalyzing two mechanistically diverse transformations. Mechanistic investigations identified cyclization as the rate limiting step relying on the facile displacement of OTf, weakly bound to the Pd-center, by the alkyne.

Tautomerization of single asymmetric oxahemiporphycene molecules on Cu(111).

We have studied 22-oxahemiporphycene molecules by a combination of scanning tunneling microscopy at low temperatures and density functional theory calculations. In contrast to other molecular switches with typically two switching states, these molecules can in principle exist in three different tautomers, due to their asymmetry and three inequivalent binding positions of a hydrogen atom in their macrocycle. Different tautomers are identified from the typical appearance on the surface and tunneling electrons can be used to tautomerize single molecules in a controllable way with the highest rates if the STM tip is placed close to the hydrogen binding positions in the cavity. Characteristic switching processes are explained by the different energy pathways upon adsorption on the surface. Upon applying higher bias voltages, deprotonation occurs instead of tautomerization, which becomes evident in the molecular appearance.

Piezoelectric Response and Substrate Effect of ZnO Nanowires for Mechanical Energy Harvesting in Internet-of-Things Applications.

Recently, an unprecedented growth in the internet of things (IoT) is being observed, which is becoming the main driver for the entire semiconductor industry. Reliable maintenance and servicing of the IoT is becoming challenging, knowing that the IoT nodes outnumber the human population by a factor of seven. Energy harvesting (EH) can overcome those difficulties, delivering the energyautonomous IoT nodes to the market. EH converts natural or waste energies (vibrations, heat losses, air flows, light, etc.) into useful energy. This article explores the performance of ZnO nanowires under mechanical actuation to characterize their piezoelectric performance. ZnO nanowires were fabricated using ALD and a subsequent chemical bath growth. AISI 301 steel was used as a substrate of the EH device to better fit the mechanical requirements for the piezoelectric generator. We determined that a thin layer of another oxide below ZnO provides outstanding adhesion. The samples were submitted under repetitive mechanical stress in order to characterize the output piezovoltage for different conditions. They exhibited a piezoelectric signal which was stable after hundreds of actuations. This shows good promise for the use of our device based on ZnO, an Earth-abundant and non-toxic material, as an alternative to the conventional and popular but harmful and toxic PZT. The designed measurement setup demonstrated that a AISI 301 steel substrate coated with ZnO deposited by ALD and grown in a chemical bath has promising performance as a piezoelectric material. Characterized ZnO samples generate up to 80 nJ of energy during 55 s runs under matched load conditions, which is sufficient to supply a modern IoT node.

The atomic layer deposition (ALD) synthesis of copper-tin sulfide thin films using low-cost precursors.

In this work we demonstrated the process of co-deposition of copper-tin sulfide species by the atomic layer deposition (ALD) technique using all-low-cost precursors. For the deposition of tin species, the tin(IV) chloride SnCl4was used successfully for the first time in the ALD process. Moreover, we showed that the successful deposition of the tin sulfide component was conditioned by the pre-deposition of CuSxlayer. The co-deposition of copper and tin sulfides components at 150 °C resulted in the in-process formation of the film containing Cu2SnS3, Cu3SnS4andπ-SnS phases. The process involving only tin precursor and H2S did not produce the SnSxspecies. The spectroscopic characteristic of the obtained materials were confronted with the literature survey, allowing us to discuss the methodology of the determination of ternary and quaternary sulfides purity by Raman spectroscopy. Moreover, the material characterisation with respect to the morphology (SEM), phase composition (XRD), surface chemical states (XPS), optical properties (UV-vis-NIR spectroscopy) and electric (Hall measurements) properties were provided. Finally, the obtained material was used for the formation of the p-n junction revealing the rectifyingI-Vcharacteristics.

Towards More Photostable, Brighter, and Less Phototoxic Chromophores: Synthesis and Properties of Porphyrins Functionalized with Cyclooctatetraene.

Free base and zinc porphyrins functionalized with cyclooctatetraene (COT), a molecule known as a good triplet-state quencher, have been obtained and characterized in detail by structural, spectral, and photophysical techniques. Substitution with COT leads to a dramatic decrease of the intrinsic lifetime of the porphyrin triplet. As a result, photostability in oxygen-free solution increases by two to three orders of magnitude. In non-degassed solutions, improvement of photostability is about tenfold for zinc porphyrins, but the free bases become less photostable. Similar quantum yields of photodegradation in free base and zinc porphyrins containing the COT moiety indicate a common mechanism of photochemical decomposition. The new porphyrins are expected to be much less phototoxic, since the quantum yield of singlet oxygen formation strongly decreases because of the shorter triplet lifetime. The reduction of triplet lifetime should also enhance the brightness and reduce blinking in porphyrin chromophores emitting in single-molecule regime, since the duration of dark OFF states will be shorter.

Photoeradication of bacteria with porphycenes: Substituent effects on the efficiency.

Considering the world-wide problem of growing antibiotic resistance of bacteria, photodynamic inactivation (PDI) has a potential to become the treatment approach against some infectious diseases. In our study, four differently substituted porphycenes were compared in terms of their bactericidal activity against E. faecalis. All tested compounds had a similar photophysical characteristics, i.e., there were no significant differences in the location of absorption bands or molar absorption coefficients. Also, singlet oxygen generation quantum yields were very similar. Surprisingly, differently substituted porphycenes caused very diverse PDI effects. Special attention was drawn to the tert-butyl moieties. Our studies demonstrated that the presence of these substituents lowers the bactericidal potential significantly and can completely block the activity when more than one moiety is introduced to the molecule. The porphycenes lacking tert-butyl groups exhibited much higher PDI potential and we assign this effect to different interactions of the differently substituted porphycenes with the bacterial cells. Most likely, the presence of tert-butyls impairs cell penetration by the photosensitizer. These results remind that the favorable photophysical characteristics do not ensure that the compound considered as a potential PDI agent can reach the microbial cells.

Unusual effects in single molecule tautomerization: hemiporphycene.

Parent hemiporphycene, a recently obtained constitutional isomer of porphyrin, exists in room temperature solutions and polymer matrices in the form of two trans tautomers interconverting via double hydrogen transfer. Using confocal fluorescence microscopy, it was possible to monitor tautomerization in single hemiporphycene molecules embedded in a PMMA film by monitoring the spectral and temporal evolution of their fluorescence spectra. The emission spectra of the two tautomeric forms are similar to those obtained from ensemble studies. However, the analysis of temporal spectral evolution reveals effects not detected in the bulk. For some single molecules, a large decrease of tautomerization rate was observed. This is interpreted as an indication of multidimensional character of the tautomerization coordinate and coupling of the reaction with the polymer relaxation processes. In addition, fluorescence lifetimes obtained for single molecules are significantly shorter than those measured for the bulk. It is proposed that the shortening is caused by environment-induced distortion of the molecule, which enhances the S0← S1 internal conversion rate by lowering the barrier to excited state single hydrogen transfer. This effect seems to reflect the specific morphology of thin (30 nm) polymer samples, because it is not observed in ensemble studies carried out using thick (tens of micrometers or more) PMMA films.

Supersonic jet spectroscopy of parent hemiporphycene: Structural assignment and vibrational analysis for S0 and S1 electronic states.

Hemiporphycene (HPc), a constitutional isomer of porphyrin, is studied under supersonic expansion conditions by means of laser-induced fluorescence, visible-visible hole-burning experiments, single vibronic level fluorescence techniques, and quantum chemical calculations. Only one trans form of jet-cooled HPc is observed, in contrast to solution studies that evidence a mixture of two trans tautomeric forms separated in energy by ∼1 kcal/mol. Reliable structural assignment is provided by simulating absorption and emission patterns at the density functional theory and time-dependent density functional theory levels of theory. The vibronic spectra are nicely reproduced for both electronic ground and lowest excited singlet states for the most stable trans form. In contrast to another porphyrin isomer, porphycene (Pc), no tunneling or photo-induced hydrogen transfer is detected. The lower symmetry of HPc compared with Pc and the concomitant non-equivalent positions of the inner-cavity nitrogen atoms result in a non-symmetric double minimum potential for tautomerization, larger energy barrier, and a longer tunneling distance, with the average intramolecular hydrogen bond length larger in HPc than in Pc. HPc readily forms hydrates that show red-shifted absorption relative to the bare molecule.

Two Macrocycles in One Shot: Synthesis, Spectroscopy, Photophysics, and Tautomerism of 23-Oxahemiporphycene and 21-Oxacorrole-5-carbaldehyde.

The synthesis of 23-oxahemiporphycene, the first monooxa analogue of hemiporphycene, a structural isomer of porphyrin, is reported. Its generation under McMurry reaction conditions is surprisingly accompanied by the appearance of a formyl derivative of oxacorrole, 21-oxacorrole-5-carbaldehyde. A mechanism for the formation of the latter is proposed, relying on pinacol rearrangement of titanium pinacolate. The structures of the most stable tautomeric forms are established for both compounds based on IR and NMR spectra combined with DFT calculations. Spectral and photophysical characteristics are compared with those of structurally similar macrocycles. Replacement of one nitrogen by oxygen in hemiporphycene has only a minor impact. In contrast, for corrole it leads to the enhancement of stability and to strongly reduced rates of nonradiative deactivation of the lowest excited singlet state. This is explained by the planarity of oxacorroles, achieved by removing one of the inner hydrogen atoms from the inner cavity. Unusual crystal packing is observed for the protonated form of 23-oxahemiporphycene, which exhibits a π-π stacked columnar alignment of positively charged macrocycle units.

Parent, Unsubstituted Hemiporphycene: Synthesis and Properties.

Among seven possible nitrogen-in constitutional isomers of porphyrin only one, porphycene, has been obtained so far in the free, unsubstituted form. Herein, the synthesis of another isomer, parent hemiporphycene (HPc), and its thorough structural, spectral, photophysical, electrochemical, and theoretical characterization are reported. Most of the properties of HPc are intermediate between those of porphyrin and porphycene, as evidenced by the values of inner-cavity dimensions, orbital-energy splittings, absorption coefficients, magnetic circular dichroism parameters, NH-stretching frequencies, fluorescence quantum yields, tautomerization rates, and redox potentials. The largest differences arise with respect to tautomerism, due to the low symmetry of HPc and inequivalence of the four nitrogen atoms that define the inner cavity. Two trans tautomers are observed, separated in energy by about 1 kcal mol-1 . Tautomerization from the higher- to the lower-energy form is detected in the lowest-excited singlet state and occurs at a rate that is about four orders of magnitude lower than that observed for porphycene. Hemiporphycene is a very good model for the investigation of inequivalent intramolecular H-bonds present in one molecule; two such bonds in HPc reveal unusual characteristics, and the bond strength results from the interplay between the N⋅⋅⋅N distance and the N-H-N angle.