Unusual Photochemistry Research Articles

Unusual Photochemistry in Aromatic Dithioimides: Quantitative Thione Reduction Promoted by Ether Solvents.

We report a mechanistic investigation of an aromatic dithioimide (2SS) displaying puzzling yet efficient photochemistry in ether solvents. Perplexingly, 2SS dissolved in ether solvents in a sealed and degassed vial was photochemically converted to the corresponding diimide (2OO), as determined by 1H NMR following product extraction. With no external sources of oxygen in the sample, could the oxygen in 2OO be from the ether itself? To study this unprecedented proposition, we attempt to uncover the ether's involvement in this reaction. As seen by laser-flash photolysis, 2SS appears to first react with the solvent from its singlet excited state. Following the reaction by NMR under rigorously oxygen- and water-free conditions led to the identification of a photoreductive pathway that quantitatively transformed one thione into a methylene to yield 2SH2. Subsequent oxidation of 2SH2 or irradiation of 2SS under air proved that molecular oxygen was indeed necessary to observe an oxidative pathway leading to 2OO, ruling out the initially proposed involvement of an ether oxygen. An explanation of 2SS desulfurization was further revealed through the study of solvent by-products by GC-MS analysis. Supported by DFT calculations, a mechanism is proposed to involve a chain reaction initiated by photochemically generated ether radical.

Vitamin B12 photoreceptors.

Photoreceptor proteins enable living organisms to sense light and transduce this signal into biochemical outputs to elicit appropriate cellular responses. Their light sensing is typically mediated by covalently or noncovalently bound molecules called chromophores, which absorb light of specific wavelengths and modulate protein structure and biological activity. Known photoreceptors have been classified into about ten families based on the chromophore and its associated photosensory domain in the protein. One widespread photoreceptor family uses coenzyme B12 or 5'-deoxyadenosylcobalamin, a biological form of vitamin B12, to sense ultraviolet, blue, or green light, and its discovery revealed both a new type of photoreceptor and a novel functional facet of this vitamin, best known as an enzyme cofactor. Large strides have been made in our understanding of how these B12-based photoreceptors function, high-resolution structural descriptions of their functional states are available, as are details of their unusual photochemistry. Additionally, they have inspired notable applications in optogenetics/optobiochemistry and synthetic biology. Here, we provide an overview of what is currently known about these B12-based photoreceptors, their discovery, distribution, molecular mechanism of action, and the structural and photochemical basis of how they orchestrate signal transduction and gene regulation, and how they have been used to engineer optogenetic control of protein activities in living cells.

A hydrogen-bonded CHF⋯HF complex: IR spectra and unusual photochemistry.

A hydrogen-bonded CHF⋯HF complex was characterized by FTIR matrix isolation spectroscopy and ab initio calculations. Three possible structures of this complex were found at the coupled-cluster with single, double, and perturbative triple excitations [CCSD(T)/L3a_3] level of theory. The comparison between the experiment and theory reveals that the most stable structure with the binding energy of 6.48 kcal/mol is formed upon x-ray irradiation of isolated CH2F2 molecules in noble gas matrices (Ne, Ar, Xe). This species appears to be the first known intermolecular complex of monofluorocarbene, and its identification was unambiguously proved by IR absorptions corresponding to HF deformation (libration), CF stretching, H-C-F bending, and CH and HF stretching modes. It is worth noting that the corresponding spectral features in an argon matrix were previously tentatively ascribed to CH2F2 +· and HF⋯CHF-· [L. Andrews and F. T. Prochaska, J. Chem. Phys. 70, 4714 (1979)], but the calculations performed in the present study definitely support the re-assignment. The observed CHF⋯HF complex can be converted to the parent CH2F2 under the action of light with λ < 525 nm. The plausible mechanism of this conversion using the conical intersection concept is discussed.

Synthesis and unusual photochemistry of a highly reactive pyrimidinedione

The herein obtained 3-methyl-5,5-diphenylpyrimidine-2,4-(3H,5H)-dione forms upon photolysis an unusual bicyclic product, namely, 6-methyl-3,3,8,8-tetraphenyl-8,8a-dihydroimidazo[1.5-c]pyrimidine-5,7(3H,6H)-dione in 21% yield. The transformation starts with elimination of carbon monoxide and methyl isocyanate from the molecule of substrate via a ‘Type A’ photochemical process giving 3,3-diphenylazirine. Then 1,3-dipole species generated from the latter is trapped by the starting compound via a [3 + 2] cycloaddition.

Water-Assisted Self-Photoredox of 3-(Hydroxymethyl)benzophenone: An Unusual Photochemistry Reaction in Aqueous Solution

An unusual photochemistry of water-assisted self-photoredox of 3-(hydroxymethyl) benzophenone 1 has been investigated by CASPT2//CASSCF computations. The water-assisted self-photoredox is found to proceed via three sequential reactions: an excited-state intermolecular proton transfer (ESIPT), a photoinduced deprotonation, and a self-redox reaction. Upon photoexcitation at 243 nm, the system of 1 is taken to the Franck-Condon region of a short-distance charge transfer (SCT) state of S(SCT)((1)ππ*) and then undergoes ESIPT with a small barrier of ∼3.4 kcal/mol producing the intermediate 2. Subsequently, the singlet-triplet crossing (STC) of STC ((1)ππ*/(3)ππ*) relays 2 by intersystem crossing to the T(SCT)((3)ππ*) state followed by a deprotonation reaction overcoming a moderate barrier of ∼8.0 kcal/mol and finally produces the triplet biradical intermediate 3. Another moderate barrier (∼5.8 kcal/mol) in the T(SCT)((3)ππ*) state has to be overcome so as to relax to a second singlet-triplet crossing STC(T/S0) that allows an efficient spin-forbidden decay to the ground state. The self-redox reaction aided by water molecules occurs with tiny barriers in the S0 state via two steps, protonation of the benzhydrol carbon to produce intermediate 4 and then deprotonation from the benzylic oxygen to yield the final product 3-formylbenzhydrol 5.

Structural and sequence analyses of an unusual bacteriophytochrome from R. palustris

Bacteriophytochromes (Bphs) are bacterial red‐light photoreceptors found in photosynthetic and non‐photosynthetic bacteria. Bphs undergo reversible photoconversion between two distinct red (Pr) and far‐red (Pfr) light absorbing states. An organic cofactor, biliverdin (BV) is required for Pr/Pfr photoconversion. Bphs are composed of a photosensory core (PC) including PAS, GAF and PHY domains and an effector domain, usually a histidine kinase. We report on the preliminary crystal structure of the unusual Bph from Rhodopseudomonas palustris, RpBphP3 (P3) PC bound to BV in the Pr state. Unlike classical Bphs, P3 photoconverts between Pr and near‐red (Pnr) states. P3 crystals (30–60 □m in all dimensions) yielded diffraction at 100 K to a maximum resolution of 2.8 Å. P3 crystals are in P212121 space group with unit cell dimensions of a=155.92 Å, b=185.32 Å, and c=199.53 Å. The initial P3 structure was determined with the molecular replacement (MR) method implemented in HKL3000 using the crystal structure of P3 lacking PHY domain (PDB ID code 2OOL) as search model. P3 crystallized as a dimer, which is physiologically relevant oligomeric state of the protein. Informed by structural and sequence analyses, we identify amino acids in the BV‐binding pocket of P3 and at the dimer interface that play a role in the unusual photochemistry and fluorescence of this protein.

The Unusual Photochemistry of Dendrimers with an Anthracene Core

The Unusual Photochemistry of Dendrimers with an Anthracene Core

The Unusual Photochemistry of Dendrimers with an Anthracene Core

A remarkable antenna effect observed in dendrimers with an anthracene core and dendrons of the Fréchet type (see picture) results in strongly increased fluorescence quantum yields relative to that of anthracene. The UV-induced [4π+4π] cycloaddition, in which the anthracene core reacts with a benzene ring of the dendrons, leads to novel photodimers.

The spectroscopy of thioformaldehyde. MR-CI studies on the triplet states of H2CS

Abstract Using multireference CI methods, the CS stretching (C2v) and out-of-plane (Cs) potentials were calculated for triplet states of H2CS. From the CS stretch potentials the adiabatic excitation energies, CS equilibrium distances, vibrational frequencies and Franck-Condon (FC) factors were evaluated. Both the 3 ( σ, π ∗ ) and 3 ( π, π ∗ ) ← X systems are predicted to be FC active, leading to long progressions. The 3 ( σ, π ∗ ) state is found in the same energy region as S2, and may therefore play a role in the unusual photo-chemistry and photo-physics assocaited with S2 ← S0. It is shown that 3 ( π, π ∗ ) is nonplanar, with a stabilization of ∼ 0.05 eV over the planar configuration, having RCS = 3.48 a0 and an out-of-plane angle of ∼ 28°. Also, 3 ( σ, π ∗ ) is predicted to be nonplanar, with RCS = 3.55 a0, θ ∼ 56° and a stabilization energy of ∼ 0.17 eV. The 3 ( π, π ∗ ) ← X system, while predicted to be weak, lies in an energy region free from other states. Electron impact spectroscopy offers the best chance of observing this system. Vertical excitation energies and dipole moments will also be given.

Biphoton holographic storage in Methyl Orange and Ethyl Orange dyes

Biphoton holograms are recorded by a He-Ne laser in Methyl Orange and Ethyl Orange dyes on exposure to a 514.5-nm argon-ion laser, and the dynamic of holographic recording is reported. The recording utilizes an unusual photochemistry that is due to either to a triplet-triplet transition from a long-lived triplet lower state of the cis configuration or a transition back to the trans state.

Novel biphotonic holographic storage in a side-chain liquid crystalline polyester

We report novel biphotonic holographic storage of text and gratings on unoriented films of a side-chain liquid crystalline polyester capable of high density storage and complete erasure. The holograms have a typical size of 1 mm. The recording utilizes unusual photochemistry involving azo dye molecules. We believe that this technique would have a great potential in the recording of thousands of holograms in a two-dimensional plane, as for instance in text retrieval systems and in the fabrication of high density interconnects in optical neural networks.

Unusual photochemistry of 4-chlorobiphenyl in water

Unusual photochemistry of 4-chlorobiphenyl in water

Unusual photochemistry of cyclobutanone near its predissociation threshold

Unusual photochemistry of cyclobutanone near its predissociation threshold