Coupled photocatalytic reactions for dehalogenation of polychlorinated hydrocarbons and generation of molecular hydrogen

The quantum yields of photocatalytic generation of hydrogen ( ) from alcohol-water mixtures containing a suspension of Pd/CdS (or CdS and Pd/SiO₂) increase in the presence of CCl₄ or C₆Cl₆ additives, which undergo dehalogenation. The reasons for the increase in are the increase in acidity during the process, and also suppression of electron-hole recombination as a result of acceptance of electrons from the conduction band by the halogenated hydrocarbons, which leads to generation of additional amounts of ethoxy radicals taking part in reduction of water. L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31 Prospekt Nauki, Kiev 252039, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 35, No. 3, pp. 162–166, May–June, 1999.     

A computational study to elucidate the extraordinary reactivity of three-membered heterocycles in nucleophilic substitution reactions

The accelerated rates of small-membered heterocycles relative to acyclic analogues are typically rationalized solely in terms of relief of ring strain. The relative rates of attack of ammonia on oxirane, oxetane, thiirane, and thietane were determined computationally in the gas phase at the MP2(Full)/6-31+G(d) level with respect to the model acyclic compounds methoxyethane and thiomethylethane. Because the cyclic ether and thioether pairs have very similar strain energies, they should react at similar rates by the S(N)2 mechanism if the degree of strain energy release in the transition state is approximately equal. The reactivity of the four-membered rings could be explained almost entirely by relief of strain. The three-membered rings reacted at rates at least 10(6) times faster than calculated from ring strain considerations alone. The electronic distribution of the transition states was determined using AIM methodology and found to indicate that bond cleavage was virtually complete, while bond formation was incomplete. Calculation of atomic charges by the Mulliken, AIM, CHELPG, and NBO methods indicated that positive charge at the reaction center was significantly lower for the three-membered rings than other members of the series. A simple electrostatic model identified differences in energy sufficient to account for the observed rate acceleration. The unique topological features of a three-membered ring make it possible for the partially negatively charged oxygen or sulfur to reduce the positive charge on the reaction center.     

A chronological review of photochemical reactions of ferrioxalate at the molecular level: New insights into an old story

Owing to its outstanding photoactivity, ferrioxalate is originally used as an actinometer and subsequent work has discovered that photochemistry of ferrioxalate is also fundamentally or technically important in atmospheric chemistry and water treatment. While the overall products generated from photolysis of ferrioxalate are known to include Fe(II), a series of oxidizing (e.g., ^?OH, O₂^??/HO₂^??) or reducing (C₂O₄^??/CO₂^??) radicals and H₂O₂, however, at the molecular level, the primary step of the photoreaction of ferrioxalate remains as an unsolved mystery due to the difficulty in examining such ultrafast processes. Benefiting from the development of time-resolved spectroscopy, this old question has been studied with increasing vigor recently, by means of such ever-more-sophisticated techniques (e.g., flash photolysis, time-resolved X-ray absorption spectroscopy (XAS), femtosecond infrared (IR) absorption spectroscopy, ultrafast photoelectron spectroscopy (PES)). There are two contrary views on the primary reaction mechanism: (1) Intramolecular electron transfer (ET) precedes the cleavage of the metal-ligand bond; (2) The dissociation of C–C or Fe–O bond occurs before intramolecular ET. Thus, this review presents a comprehensive summary about the overall reaction mechanism and molecular level mechanism of ferrioxalates. In chronological order, we have elaborated two predominant but controversial views from the perspectives of different experimental approaches. Some challenges and research opportunities in this active field are also briefly discussed.     

Using a pharmacophore representation concept to elucidate molecular similarity of dopamine antagonists

The pharmacophoric concept plays an important role in ligand-based drug design methods to describe the similarity and diversity of molecules, and could also be exploited as a molecular representation scheme. A three-point pharmacophore method was used as a molecular representation perception. This procedure was implemented for dopamine antagonists of the D(2) receptor subtype. The molecular structures of the antagonists included in this analysis were categorized into two structurally distinct classes. Using structural superposition with internal energy minimization, two pharmacophore models were deduced. Based on these two models other D(2) antagonists that fulfil them were derived and studied. This procedure aided the identification of the common 3D patterns present in diverse molecules that act at the same biological target and the extraction of a common molecular framework for the two structural classes. The pharmacophoric information was found to be suitable for guiding superposition of structurally diverse molecules, using a more biologically meaningful selection of the targeting points.     

Hydration of mineral surfaces probed at the molecular level

By employing the nonlinear optical, interface selective experiment of sum frequency spectroscopy together with independent ab initio and density functional theory calculations, we determine the functional species of a corundum (001) surface: doubly coordinated OH groups which differ in their bond tilt angles. The interaction of the functional species with the adjacent water molecules is also observed. In a large pH range around the point of zero charge, the interaction is not controlled electrostatically but by hydrogen bonding. The functional species' tilt angles are crucial parameters, determining whether the species act as hydrogen bond donors or acceptors.     

Mapping blood biochemistry by Raman spectroscopy at the cellular level

We report how Raman difference imaging provides insight on cellular biochemistry in vivo as a function of sub-cellular dimensions and the cellular environment. We show that this approach offers a sensitive diagnostic to address blood biochemistry at the cellular level. We examine Raman microscopic images of the distribution of the different hemoglobins in both healthy (discocyte) and unhealthy (echinocyte) blood cells and interpret these images using pre-calculated, accurate pre-resonant Raman tensors for scattering intensities specific to hemoglobins. These tensors are developed from theoretical calculations of models of the oxy, deoxy and met forms of heme benchmarked against the experimental visible spectra of the corresponding hemoglobins. The calculations also enable assignments of the electronic transitions responsible for the colour of blood: these are mainly ligand to metal charge transfer transitions.

We assign the electronic transitions responsible for the colour of blood and present a Raman imaging diagnostic approach for individual blood cells.     

Boolean and fuzzy logic implemented at the molecular level

In this work, it is shown how to implement both hard and soft computing by means of two structurally related heterocyclic compounds: flindersine (FL) and 6(5H)-phenanthridinone (PH). Since FL and PH have a carbonyl group in their molecular skeletons, they exhibit Proximity Effects in their photophysics. In other words, they have an emission power that can be modulated through external inputs such as temperature (T) and hydrogen-bonding donation (HBD) ability of solvents. This phenomenology can be exploited to implement both crisp and fuzzy logic. Fuzzy Logic Systems (FLSs) wherein the antecedents of the rules are connected through the AND operator, are built by both the Mamdani’s and Sugeno’s models. Finally, they are adopted as approximators of the proximity effect phenomenon and tested for their prediction capabilities. Moreover, FL as photochromic compound is also a multiply configurable crisp logic molecular element.     

High-throughput characterisation of materials by photoluminescence spectroscopy

An automatic system for high-throughput (HT) characterisation of large libraries of solid materials by photoluminescence spectroscopy is described. The system provides time-resolved transient emission spectra in the microsecond scale and can be employed for characterisation of materials of interest in the fields of catalysis and electroluminescence, amongst others. Here, we present its application to the optimisation of the ship-in-a-bottle synthesis of a novel electroluminescent polymer (PPV) and a photocatalyst (TP+), both encapsulated in large-pore zeolites.     

Application of artificial intelligence systems in molecular spectroscopy

The merits and demerits of data retrieval and artificial intelligence systems for identifying polyatomic molecules from their molecular spectra are considered. It is concluded that the creation of artificial intelligence systems provides the most promising developments for the future of analytical molecular spectroscopy. In these systems, experimental spectra are compared with computer-generated data in the course of solution of the analytical problem and not with data already stored in the data bank.     

Biomimetically constructing a hypoxia-activated programmable phototheranostics at the molecular level

The hypoxic microenvironment is considered the preponderant initiator to trigger a cascade of progression and metastasis of tumors, also being the major obstacle for oxygen consumption therapeutics, including photodynamic therapy (PDT). In this work, we report a programmable strategy at the molecular level to modulate the reciprocal interplay between tumor hypoxia, angiogenesis, and PDT outcomes by reinforcing synergistic action between a H₂O₂ scavenger, O₂ generator and photosensitizer. The modular combination of a catalase biomimetic (tri-manganese cryptand, 1) and a photosensitizer (Ce6) allowed the rational design of a cascade reaction beginning with dismutation of H₂O₂ to O₂ under hypoxic conditions to enhance photosensitization and finally photooxidation. Concurrently, this led to the decreased expression of the vascular endothelial growth factor (VEGF) and effectively reduced unwanted growth of blood vessels observed in the chick chorioallantois membrane (CAM). Notably, the proof-of-principle experiments using the tumor-bearing models proved successful in enhancing PDT efficacy, prolonging their life cycles, and improving immunity, which could be monitored by magnetic resonance imaging (MRI).

A programmable strategy at the molecular level to modulate the ratio of a catalyst and photosensitizer to maximize the collaborative efficiency of anti-angiogenesis and PDT.     

Monitoring molecular beacon DNA probe hybridization at the single-molecule level

We have monitored the reaction dynamics of the DNA hybridization process on a liquid/solid interface at the single-molecule level by using a hairpin-type molecular beacon DNA probe. Fluorescence images of single DNA probes were recorded by using total internal reflection fluorescence microscopy. The fluorescence signal of single DNA probes during the hybridization to individual complementary DNA probes was monitored over time. Among 400 molecular beacon DNA probes that we tracked, 349 molecular beacons (87.5 %) were hybridized quickly and showed an abrupt fluorescence increase, while 51 probes (12.5 %) reacted slowly, resulting in a gradual fluorescence increase. This ratio stayed about the same when varying the concentrations of cDNA in MB hybridization on the liquid/surface interface. Statistical data of the 51 single-molecule hybridization images showed that there was a multistep hybridization process. Our results also showed that photostability for the dye molecules associated with the double-stranded hybrids was better than that for those with the single-stranded molecular beacon DNA probes. Our results demonstrate the ability to obtain a better understanding of DNA hybridization processes using single-molecule techniques, which will improve biosensor and biochip development where surface-immobilized molecular beacon DNA probes provide unique advantages in signal transduction.     

Probing the organic-mineral interface at the molecular level in model biominerals

It is widely known that macromolecules, such as proteins, can control the nucleation and growth of inorganic solids in biomineralizing organisms. However, what is not known are the complementary molecular interactions, organization, and rearrangements that occur when proteins interact with inorganic solids during the formation of biominerals. The organic-mineral interface (OMI) is expected to be the site for these phenomena, and is therefore extraordinarily interesting to investigate. In this report, we employ X-ray absorption near edge (XANES) spectromicroscopy to investigate the electronic structure of both calcium carbonate mineral crystals and polypeptides, and detect changing bonds at the OMI during crystal growth in the presence of polypeptides. We acquired XANES spectra from calcium carbonate crystals grown in the presence of three mollusk nacre-associated polypeptides (AP7N, AP24N, n16N) and in the presence of a sea urchin spicule matrix protein, LSM34. All these model biominerals gave similar results, including the disruption of CO bonds in calcite and enhancement of the peaks associated with C-H bonds and C-O bonds in peptides, indicating ordering of the amino acid side chains in the mineral-associated polypeptides and carboxylate binding. This is the first evidence of the mutual effect of calcite on peptide chain and peptide chain on calcite during biomineralization. We also show that these changes do not occur when Asp and Glu are replaced in the n16N sequence with Asn and Gln, respectively, demonstrating that carboxyl groups in Asp and Glu do participate in polypeptide-mineral molecular associations.     

Multifunctional transition metal complexes: Information transfer at the molecular level

Recent investigations from our laboratory have described compelling experimental evidence to the effect that polyacetylenes operate as extremely effective molecular-scale wires for conducting electronic charge between redox-active terminals. The unusually low electronic resistivity of polyacetylenic bridges is derived from their relatively accessible HOMOs and LUMOs, which facilitate electron and hole tunnelling over long distances, and because of the excellent electronic coupling that occurs between adjacent carbon atoms, these being in very close proximity. In order to prevent direct participation of the acetylenic bridge in triplet energy-transfer processes or in light-induced electron-transfer reactions, it is prudent to restrict the conjugation length of the bridge to less than five ethynylene groups. We now consider various synthetic strategies for the engineering of such molecular systems that retain the favorable electronic properties of a polyacetylenic bridge but that include a relay or insulator in the bridging moiety. A convenient way to construct such systems is to use a Pt^II bis-acetylide as the spacer that separates terminal metal oligopyridine complexes. In this case, the central Pt^II complex becomes an insulator. By careful design of the system, this insulatory behavior can be exploited as a means by which to introduce directionality and selectivity into the system, and we demonstrate such effects by using polycyclic hydrocarbons and metalloporphyrins as the photoactive terminals. Similar effects can be obtained with polycyclic hydrocarbons built into the acetylenic wire and, in such cases, the energetics of the bridge can be tuned over an inordinately wide range by varying the extent of conjugation inherent to the aromatic nucleus. A special case is identified in which the polycycle itself possesses vacant coordination sites since the energy of the bridge can be further tuned by external complexation of adventitious cations. In order to provide for an energy gradient along the molecular axis, we have devised a versatile synthetic strategy for attaching different types of ligand to the terminals. This approach also facilitates both extension of the molecular axis and alteration of the molecular shape. The photophysical and electrochemical properties have been recorded for all the molecular systems reported herein and used as a simple experimental means by which to quantify the extent of electronic communication along the molecular axis. For mixed-metal or mixed-ligand systems, rates of intramolecular energy or electron transfer have been measured. In most cases, these rates are extremely fast and testify to the remarkable electronic coupling properties of this family of compounds. Finally, some consideration is given to the preparation of third-generation systems.     

Self-modelling analysis applied to nanosecond transient absorption spectroscopy of flavone: an aid to elucidate and characterise reaction intermediates

A nanosecond transient absorption spectroscopy study of flavone performed with a 248 nm pump radiation has been investigated with the support of a chemometric treatment: SIMPLISMA. The experimental spectra obtained in various solvent with a pump-probe delay lower than about 2 micros are in quite good concordance with those already presented in the literature. Nevertheless after about 10 micros, the spectrum pattern significantly evolves as a function of time particularly for the methanolic solution. A qualitative analysis together with a SIMPLISMA chemometric treatment of the experimental data allowed to elucidate and characterise two interdependent transient species in the alcoholic medium: the lowest T1 triplet state of flavone and the ketyl radical forming by H-abstraction reaction from the solvent. In cyclohexane and acetonitrile, the same species seem to be produced in the studied time-scale but the radical form is generated with variable quantum yield depending on the solvent polarity. The pure spectrum and the photochemical kinetics of each reaction intermediate could have been determined with the help of the second derivative SIMPLISMA calculation procedure.