Influence of ring fusion stereochemistry on the stereochemical outcome in photo-induced Diels–Alder reaction of fused bicycloheptenone derivatives

Photo-isomerization of cis-cycloheptenone fused to a ring with a defined stereochemistry at the ring fusion to its trans-stereoisomer and in situ Diels–Alder reaction with cyclic and acyclic dienes have been investigated. The reaction was found to proceed through one of the two possible isomerized trans-enones that produce the thermodynamically more stable adduct with a trans-fused seven-membered ring system. This protocol has been employed to construct a tetracycle with trans-fused 6-7 ring system, the core structure present in several terpenes. The observed selectivity has been supported by computational method.     

Pump‐Pump‐Probe Spectroscopy of a Molecular Triad Monitoring Detrimental Processes for Photoinduced Charge Accumulation

Controlling light‐induced accumulation of electrons or holes is desirable in view of multi‐electron redox chemistry, for example for the formation of solar fuels or for photoredox catalysis in general. Excitation with multiple photons is usually required for electron or hole accumulation, and consequently pump‐pump‐probe spectroscopy becomes a valuable spectroscopic tool. In this work, we excited a triarylamine‐Ru(bpy)₃²⁺‐anthraquinone triad (bpy = 2,2′‐bipyridine) with two temporally delayed laser pulses of different color and monitored the resulting photoproducts. Absorption of the first photon by the Ru(bpy)₃²⁺ photosensitizer generated a triarylamine radical cation and an anthraquinone radical anion by intramolecular electron transfer. Subsequent selective excitation of either one of these two radical ion species then induced rapid reverse electron transfer to yield the triad in its initial (ground) state. This shows in direct manner that after absorption of a first photon and formation of the primary photoproducts, the absorption of a second photon can lead to unproductive electron transfer events that counteract further charge accumulation. In principle, this problem is avoidable by careful excitation wavelength selection in combination with good molecular design.     

Towards More Efficient,Greener Syntheses through Flow Chemistry

Technological advances have an important role in the design of greener synthetic processes. In this Personal Account, we describe a wide range of thermal, photochemical, catalytic, and biphasic chemical transformations examined by our group. Each of these demonstrate how the merits of a continuous flow synthesis platform can align with some of the goals put forth by the Twelve Principles of Green Chemistry. In particular, we illustrate the potential for improved reaction efficiency in terms of atom economy, product yield and reaction rates, the ability to design synthetic process with chemical and solvent waste reduction in mind as well as highlight the benefits of the real‐time monitoring capabilities in flow for highly controlled synthetic output.     

Surface-enhanced Raman spectroscopy toward application in plasmonic photocatalysis on metal nanostructures

Among photothermal, photovoltaic and photochemical techniques, photochemistry is superior in energy storage and transportation by converting photons into chemical fuels. Recently plasmonic photocatalysis, based on localized surface plasmon resonance (LSPR) generated from noble metal nanostructures, has attracted much attention. It promotes photochemical reaction efficiency by optimizing the solar spectrum absorption and the surface reaction kinetics. The deeper understanding is in urgent need for the development of novel plasmonic photocatalysts. Surface-enhanced Raman spectroscopy (SERS), which is also originated from the LSPR effect, provides an excellent opportunity to probe and monitor plasmonic photoreactions in situ and in real-time, with a very high surface sensitivity and energy resolution. Here, fundamentals of plasmonic photocatalysis and SERS are first presented based on their connections to the LSPR effect. Following by a validity analysis, latest studies of SERS applied for the plasmon mediated photochemical reaction are reviewed, focusing on the reaction kinetics and mechanism exploration. Finally, limitations of the present study, as well as the future research directions, are briefly analyzed and discussed.     

Review: Properties and chemical reactivity of metallo phthalocyanine and tetramethylbenzoannulene complexes grafted into a polymer backbone

The properties of coordination complexes grafted to a polymer backbone are reviewed in this article. The focus is on complexes of phthalocyanine and tetraazaannulene (5,7,12,14-tetramethyldibenzo[b,i][1,4,8,11]tetraazacyclotetradecine, also abbreviated tmdbztaa) with various transition metal ions and Al(III). In addition to the morphological characteristics of the materials, their thermal and photochemical reactions are reviewed. Processes such as catalysis of the lignin degrading process and the reduction of CO₂ are presented as examples of these materials potential applications.     

Photooxidation of Methane over TiO2

Fourier‐transform infrared spectroscopy has been employed to investigate the adsorption and photo‐oxidation of CH₄ over powdered TiO₂. The interaction between the CH₄ and TiO₂ surface is weak. It is found that no CH₄ molecules are adsorbed on the surface at 35 °C in a vacuum. Under UV irradiation, CH₄ decomposes to form CO_(a), CO_2(g), H20_(a), and HCOO_(a) in the presence of O₂. The photoreaction rate is retarded and only small amounts of CO_(a) and HCOO_(a) are formed in the absence of O₂. It is observed that the oxygen atoms of O₂ are incorporated into these photoproducts as ¹⁸O₂ is used. The major ¹⁸O‐containing products are C¹⁸O_(a), C¹⁸O_2(g), H₂ ¹⁸O_(a), HC¹⁶O¹⁸O_(a), and HC¹⁸O¹⁸O_(a) after 180 min UV irradiation. However, the extent of ¹⁸O incorporating into the adsorbed formate is dependent on UV irradiation time. In the early stage of UV irradiation HC¹⁶O¹⁶O_(a) is the major formate form indicating the involvement of TiO₂ lattice oxygens for its formation, but HC¹⁸O¹⁸O_(a) becomes the major one after 180 min indicating the involvement of ¹⁸O₂. Formate on TiO₂ further photodecomposes to CO_2(g), but not to CO_(a). CO_(a) formation is directly from CH₄ photodecomposition with the participation of TiO₂ lattice oxygens and O₂.     

Photodissociation Dynamics of Benzene at 193 nm

Photodissociation of benzene at 193 nm has been investigated using the photofragment translational spectroscopy (PTS) technique. H atom elimination channel for benzene at 193 nm is from a one‐photon dissociation process, while H₂ and CH₃ elimination channels come from a two‐photon excitation process.     

Real time chemical dynamics at surfaces

It is a major goal in surface science to make movies of molecules on surfaces, in which the reaction of the molecules on the surface can be followed on a femtosecond time scale, with sub-nanometer resolution. By moving the actors (the molecules) to precisely determined positions on the stage (the surface) at some well-defined moment in time, and subsequently making a space- and time-resolved documentary of what happens next, we would be able to understand the reactive interactions between molecules on surfaces in the greatest possible detail. This would enable us to set the stage and bring together the actors in such a way as to produce the chemical outcomes our society needs, by improving existing catalysts and designing novel catalysts, and by engineering novel reactions on surfaces. Any future director of such movies needs to know which techniques (i.e., which theoretical and experimental methods) hold promise for movie making, what has been done with these techniques, and what can be done with appropriate extensions. The methods we discuss are: (i) the time-dependent wave packet method, which is a theoretical method for simulating molecule–surface reactions with sub-nanometer resolution on a femtosecond time scale, (ii) molecular beam experiments, which allow detailed investigation of the molecule–surface interaction at a molecular level, and (iii) time-resolved laser pump–probe experiments, which allow reactions to be studied with femtosecond resolution. In particular, we discuss (i) theoretical studies of the dissociation reaction of hydrogen on metal surfaces, the reactive system presently understood at the greatest level of detail, (ii) the reactive and non-reactive scattering of heavy diatomics (NO,CO) from metal surfaces, and (iii) the competition between reaction of coadsorbed CO with O and desorption of CO, again on a metal surface. We examine possibilities to extend these methods to make movies at the desired level of detail. We also discuss which reactions are likely to provide good material for plots of movies that will be exciting for future generations of surface scientists.     

Photochemistry of cyclic vicinal tricarbonyl compounds Photolysis of alloxan in the presence of olefins containing allylic hydrogen

UV irradiation of an acetone solution of alloxan (1) in the presence of olefins containing allylic hydrogen leads to the formation of 5-hydroxy-5-alkenyl barbituric acids (2) resulting from an initial hydrogen abstraction process. On the other hand, photolysis of 1 with ethyl-1-propenyl ether yields [2+2] (oxetanes) and [4+2] (dioxenes) photocycloaddition products, beside the corresponding hydrogen abstraction products. When the irradiation was performed with olefins containing electron-acceptor substituents, product formation could not be observed.     

Re-examination of the photochemical reaction of octaisopropylcyclotetragermane

The photochemistry of octaisopropylcyclotetragermane was studied by laser flash photolysis and trapping experiments. Upon irradiation of the cyclotetragermane, the main reaction was a ring opening to form octaisopropyltetragermane-1,4-diyl biradical (λ_max = 310 and 550 nm), but generations of diisopropylgermylene and tetraisopropyldigermene are negligible.