Petr Klán

Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy

The review covers the knowledge on photoremovable protecting groups and includes all relevant chromophores studied in the time period of 2000–2012; the most relevant earlier works are also discussed.

Ice (photo)chemistry. Ice as a medium for long-term (photo)chemical transformations--environmental implications.

This review accounts for the current knowledge about the distribution, accumulation, and chemical/photochemical transformations of persistent, bioaccumulative, and toxic compounds (PBTs) in water ice, especially in the connection with polar regions and atmospheric cloud particles. (Photo)reactions on/in ice are discussed in terms of photochemistry, photobiology, paleochemistry, as well as astrophysics. Authors propose a model, in which a significant amount of some PBTs are generated by (photo)chemistry of primary pollutants in ice, which may subsequently be released to the environment. It is argued that ice photochemistry might play an important role in the chemical transformations in cold ecosystems and in the upper atmosphere, particularly now when the ozone layer is partially depleted.

The electrodeless discharge lamp: a prospective tool for photochemistry

This work expands our previous research on an original photochemical reactor consisting of the electrodeless discharge lamp (EDL) inside a reaction mixture that generates ultraviolet radiation in the microwave (MW) field. The effects of temperature MW output power of the reactor, and EDL envelope material, were already published [P. Muller, P. Klan, V. Cirkva, J. Photochem. Photobiol. A: Chem. 158 (2003) 1]. Now we report on the emission characteristics (250650 nm) of various EDLs containing different fill materials (Hg, HgI2, Cd, I2, KI, P, Se, and S). While distinct line emission peaks were found for mercury, cadmium and phosphorus fills, iodine, selenium and sulfur-containing EDLs emitted continuous bands. Sulfur-containing EDLs are proposed for phototransformations that are of environmental interest due to the emission flux comparable to solar terrestrial radiation. It is concluded that the right choice of EDL envelope fill material can be very useful in planning an efficient course of photochemical process without necessity of filtering off the undesirable part of the UV radiation by other tools, such as glass or solution filters or monochromators. Quantum efficiency measurements of the Norrish type II reaction were accomplished to compare usefulness of the EDLs in photochemical applications. Sulfur and phosphorus-containing EDLs were found as the most efficient sources of UV radiation.

Anion-Free Bambus[6]uril and Its Supramolecular Properties

Methods for the preparation of anion-free bambus[6]uril (BU6) are presented. They are based on the oxidation of iodide anion, which is bound inside the macrocycle, utilizing dark oxidation by hydrogen peroxide or photooxidation in the presence of titanium dioxide. Anion-free BU6 was found to be insoluble in any of the investigated solvents; however, it dissolves in methanol/chloroform (1:1) or acetonitrile/water (1:1) mixtures in the presence of the tetrabutylammonium salt of a suitable anion. The association constants with halide ions, BF(4)(-), NO(3)(-), and CN(-), were measured by (1)H NMR spectroscopy. The highest association constant (8.9×10(5) M(-1)) was found for the 1:1 complex of BU6 with I(-) in acetonitrile/water mixture. A number of crystal structures of BU6 complexes with various anions were obtained. The influence of the anion size on the macrocycle diameter is discussed together with an unusual arrangement of the macrocycles into separate layers.

The electrodeless discharge lamp: a prospective tool for photochemistry Part 4. Temperature- and envelope material-dependent emission characteristics

This work extends our previous research on an original photochemical reactor—the electrodeless discharge lamp (EDL) inside a reaction mixture that generates ultraviolet radiation in the microwave (MW) field. This arrangement was found to be a straightforward solution for homogeneous as well as heterogeneous photochemical experiments that need to be carried out at higher temperatures. Here, we report the emission characteristics (250–600 nm) of EDL as a function of temperature, MW output power of the reactor, EDL envelope material, and properties of solvents used in photochemical reactions. Relative intensities of the individual emission peaks were found to be largely dependent on temperature (in the region of 35–174 °C): the short-wavelength bands (particularly the 254 nm peak) were suppressed with increasing temperature. Solvents absorbing MW significantly reduced the EDL emission intensity. It is concluded that the right choice of EDL envelope material and reaction conditions is essential for an efficient course of a photochemical process in this experimental arrangement.

The electrodeless discharge lamp: a prospective tool for photochemistry Part 3. The microwave photochemistry reactor

A simple and original microwave photochemical reactor is described in detail. It consists of an electrodeless discharge lamp placed into the reaction vessel in a modified domestic microwave oven. The microwave field generates ultraviolet radiation by the lamp at the same time when it interacts with the reaction mixture. The construction, experimental set-ups, applications, and safety precautions are discussed.

Fluorescein analogues as photoremovable protecting groups absorbing at ∼520 nm.

A new photoremovable protecting group, (6-hydroxy-3-oxo-3H-xanthen-9-yl)methyl (1), with a molar absorption coefficient ε of ∼4 × 10(4) m(-1) cm(-1) at ∼520 nm for the release of carboxylates or phosphates is reported. Three derivatives of 1 (diethyl phosphate, acetate, and bromide) were isolated as complexes with DDQ and shown to release the ligands with quantum yields ≤2.4% in aqueous solution.

Fluorescein analogue xanthene-9-carboxylic acid: a transition-metal-free CO releasing molecule activated by green light.

6-Hydroxy-3-oxo-3H-xanthene-9-carboxylic acid is introduced as the first transition-metal-free carbon monoxide releasing molecule activated by visible light (photoCORM). This water-soluble fluorescein analogue releases carbon monoxide in both water and methanol upon irradiation at 500 nm. When selectively irradiated in the presence of hemoglobin (Hb) under physiological conditions, released CO is quantitatively trapped to form carboxyhemoglobin (COHb). The reaction progress can be accurately monitored by characteristic absorption and emission properties of the reactants and products.

Photochemical activity of organic compounds in ice induced by sunlight irradiation: The Svalbard project

Field experiments in Ny-Alesund, Spitsbergen Island (Svalbard) demonstrated that water ice can be a suitable reaction medium for photochemical transformations of organic pollutants. Several aromatic carbonyl, chloro, nitro or hydroxy compounds, frozen in the ice-matrix samples, underwent very efficient sunlight-induced chemical changes. The photoproducts, in many cases completely different from those obtained from liquid solution photolysis, might pose a high toxicological risk to biota when they enter the environment. It is concluded that the results could be applicable to natural snow and ice, and that possible photochemical transformations should be considered in the ice-core record studies.

The electrodeless discharge lamp: a prospective tool for photochemistry: Part 2. Scope and limitation

We have previously reported results from the study of an original photochemical reactor consisting of an electrodeless discharge lamp (MWL) placed into the reactor vessel of a microwave oven [P. Klan, J. Literak, M. Hajek, J. Photochem. Photobiol. A: Chemistry 128 (1999) 145]. The microwave field generates ultraviolet irradiation in the lamp at the same time as it interacts with the studied sample, which is therefore affected by a simultaneous UV/VIS and MW irradiation. Here, five different common photoreactions have been investigated in the reactor: Norrish type II reaction of valerophenone and its p-methyl derivative, photo-Fries reaction, photoreduction of acetophenone, photolysis of a phenacyl ester, and a radical nucleophilic substitution of chlorobenzene in methanol. The reaction conversions and product distributions have been studied in terms of the MWL quality and the scale of the experiment. In addition, MW experiments were compared to those using a conventional UV lamp. The electrodeless lamp is presented as a very simple, economic, and efficient tool for photochemistry.