Effect of extra liganding on spectral and kinetic characteristics and on quenching of excited states of multiporphyrin complexes by molecular oxygen

Based on studies of spectral and kinetic parameters of dimers of Zn porphyrins and of multiporphyrin self-assembling complexes formed on their basis, we find that extra liganding of dimers by pyridine inappreciably lowers the energy of the triplet level E(T ₁). We show that, in this case, the nonradiative deactivation T ₁ ↝ S ₀ of the electronic excitation energy of dimers Zn porphyrins increases not only due to increasing Franck-Condon factor. We discuss mechanisms of the quenching action of an extra ligand related to an accepting role played by high-frequency overtones of vibrations of extra ligand molecules, to an enhancement of the spin-orbit interaction due to energy lowering of σπ* states, and to out-of-plane distortion of dimers caused by the displacement of the Zn²⁺ ion out of the plane of the tetrapyrrole macrocycle. Quenching of triplet states of extra liganded dimers of Zn porphyrins by molecular oxygen in liquid solutions at 295 K depends on the character of donor-acceptor interactions with pyridine and rigidity of a linking molecular fragment. We find that the rate constants of oxygen quenching of the excited electronic states S ₁ and T ₁ of multiporphyrin complexes depend on their structure and composition, as well as steric hindrances, created by dimers (screening effects) for contact interactions of a π-conjugated system of the free base (extra ligand) with molecular oxygen. Screening effects of extra ligands by dimer molecules of Zn porphyrins, which reduce the oxygen quenching rate constants k _S and k _T , are found to barely affect the singlet-oxygen generation quantum yield γΔ.     

Spectrum of gases liberated upon the stepwise heating of single-walled carbon nanotubes deuterated under pressure

JETP Letters - The mass spectra of gases liberated from single-walled carbon nanotubes saturated with deuterium under a pressure of 5 GPa at temperatures up to 50°C (10.8 wt % D) have been...     

The Action of High-Power Pulsed Beams of Nanosecond Duration on Cadmium–Mercury Telluride Semiconductors

Calculations have been performed for the processes occurring in cadmium–mercury tellurides exposed to high-power pulsed beams in the region of high power densities. In particular, the depth distributions of the target temperature and temperature gradients, the quasi-static and dynamic stresses, and the effect of these factors on the redistribution of the interstitial impurity atoms have been calculated.     

Analysis of the conformation composition of ethane-bisporphyrin solutions from absorption spectra in the region of the soret band

It is found that in absorption spectra of porphyrin dimers linked by a flexible bridge, the shape of the Soret band strongly depends on the solvent, which is explained by differences in the conformation composition of the dimers studied. The conformation composition was analyzed with the deconvolution method, which made it possible to resolve and sharpen all components of the complex contour, of the Soret band. Using different solvents, we studied changes in the behavior of every elementary band with a model of exciton (dipole-dipole) interaction and found that in all the solutions this dimer occurred in the form of face-to-face and side-to-side conformers. The relative content of face-to-face conformation markedly increased with the polarity of the solvent. Institute of Molecular and Atomic Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 716–723, November–December, 1997.     

Synthesis of novel bridged dinitrogen heterocycles and their evaluation as potential fragments for the design of biologically active compounds

A library of saturated bridged heterocycles based on 3,6-diazabicyclo[3.2.1]octane-2,4-dione and bispidine scaffolds (mean compound molecular weight is approximately 300 Da) with up to three stereocenters and four diversity points has been synthesized. Synthetic scaffold modifications leading to an increase in molecular complexity were studied. Well-defined stereochemical structures of both compound sets was confirmed by X-ray studies and halogenoaryl substituents were inserted appropriately for the design of novel non-basic serine protease inhibitors. Comprehensive molecular modeling has been performed for all synthesized compounds giving rationales of ligand–enzyme interactions with thrombin and trypsin. Biological testing confirmed moderate inhibitory activity of halogen-substituted saturated diazabicyclic small molecules towards thrombin.     

Experimental setup for determining the carrier mobility using a transit time technique

A new scheme of the experimental setup for measuring the carrier mobility in thin films of organic semiconductor materials was proposed. Major factors controlling the time of exponential relaxation of the measuring system were analyzed. On this basis, conclusions were made on operating parameters of the measuring equipment required to reduce as much as possible the relaxation time. According to this, a new setup was developed. A qualitative improvement of current kinetic curves for a control sample was experimentally shown using the new setup. This allowed determination of transit times and carrier mobilities with high reliability.     

Off-axis pumping of a photonic crystal fiber laser

We present a new method for free space pumping of double-clad photonic crystal fiber lasers and amplifiers. The method is based on off-axis injection of the pump beam, exploiting the relatively high numerical aperture of the photonic crystal fiber cladding. It prevents feedback from the laser into the pump source, does not require any special treatment to the fiber, and could lead to a more compact resonator. Experimental results with the new pumping method show improved laser performance.     

A new calcium-sensor based on ion-selective conductometric microsensors - membranes and features

Based on the concept of ion-selective conductometric microsensors (ISCOM) a new calcium sensor was developed and characterized. ISCOM have a single probe, all-solid-state construction and do not need a reference electrode. These sensors are amenable to miniaturization and integration in the true sense of integrated circuit and microsystem technologies. The detection is accomplished by measurement of the bulk conductance Gm of a thin polymeric membrane containing an ion-complexing agent, where the magnitude of Gm can be related to the content of the primary ion in the analyzed solution. Thin-film platinum electrodes forming an interdigitated electrode are used as the transducer to detect the conductivity of the polymeric membrane. Optimization of the membrane composition was carried out by testing different types of calcium-ionophores, polymers, and plasticizers. The sensor characteristics have been investigated. The limit of detection is about 10(-7) mol L(-1). The dynamic range is 10(-6)-10(-1) mol L(-1) with a response time of less than 5 s. These parameters are comparable to those of corresponding potentiometric calcium selective electrodes (ISE). The Ca(2+)-ISCOM demonstrates good practical relevant selectivities against typical interfering ions for biomedical and environmental applications.