Don’t Know Which Direction to Go? Go Both, You Are in a Quantum World.

Dissociation of diazirine showing Fano resonance

Being able to know the pathway of a chemical reaction has enabled chemists to exercise a superior control over the process and suggest ways to increase yields. Spectroscopic techniques along with theoretical calculations have allowed us to detect reactive intermediate, study their properties and follow the events along the reaction coordinate.

In a communication in Angewandte, a team of South Korean researchers reported the fine details of the photoinduced dissociation of diazirine (CH₂N₂) around the transition state.

Observing the emission spectrum of the photofragment :CH₂, they noticed an asymmetric lineshape of several vibronic bands, a signature of the so-called Fano resonance.

In essence, upon excitation to S₁, the diazirine proceeds through a very shallow dissociative potential landscape. During the slide that leads to ring opening, an alternative pathway is available and some vibrational wavepackets split and travel both of them at the same time. The consequent wavefunction interference results in the characteristic Fano lineshape in the photofragment excitation spectrum. Only the vibrations affected by the double pathway show Fano resonance.

This observation, extremely unusual in polyatomic systems, is remarkably helpful to provide a clearer picture of the complicate chemical dynamics of diazirine.

[“Mode-Dependent Fano Resonance Observed in the Predissociation of Diazirine in the S1 State”, D.-S. Ahn, S.-Y. Kim, G.-I. Lim, S. Lee, Y. S. Choi, S. K. Kim, Angew. Chem. Int. Ed. 2010, 49, 1244-1247.]

Protecting against UV

UV-light sink mechanism of benzil 1

          The detrimental effect of UV exposure spans from skin cells to materials.  Irradiated, the molecules on the surface become excited and more reactive. As a result, materials may decompose and skin cells may be attacked by harmful agents. 

          The ozone layer in the stratosphere already blocks the most harmful UV rays coming from the Sun, but that is not enough especially for very sensitive materials and skin complexions.  Chemical research has developed molecules, named photostabilizers, that are capable of UV protection. Here is what they do: i. competitively, they absorb most of the incident UV light; ii. they are inert in the excited state; iii. they quickly dissipate the energy absorbed to return to the ground state. 

          The need for ever more efficient photostabilizers has brought researchers at the University of Valencia (Spain) to discover a benzil molecule, 1, that has all the right features. When excited by UV light, 1 undergoes fast and quantitative intersystem crossing to a triplet state – this is a typical behavior of ketones. However, unlike most ketones, the triplet state of 1 is remarkably low-lying. This not only prevents triplet sensitization to other potentially harmful compounds but also allows for a quick dissipation of energy to the ground state.  Triplet lifetime of 1 is so short that it could not be accurately measured using conventional techniques. 

          Compared to other similar photostabilizers, benzil 1 achieves better performances under intense irradiation and longer times, truly a sink of UV light. 

[“Efficient Ultra-Violet Energy Dissipation by an Aromatic Ketone”, A. El Moncef, M. C. Cuquerella, E. Zaballos, C. Ramírez de Arellano, A. Ben-Tama, S.-E. Stiriba, J. Pérez-Prieto, Chem. Commun. 2010, 46, 800-802]. 

How Do They Know?

Z to E photoisomerization in phytochromes

Do you ever wonder how plants know that it is time to form flowers or let their leaves fall? Not incidentally, this is determined a great deal by the amount and color of ambient light. Light-induced transformations are mediated by a photoreceptor called phytochrome, a protein that has buried inside a tetrapyrrole chromophore called bilin. Bilin is a photochromic molecule that switches back and forth between two configurations: one is inactive (Pr), the other (Pfr) causes a cascade of conformational changes that initiates signal transmission to the cell.

Researchers at the Universities of Kansas and Winsconsin-Madison (USA) report a study intended to elucidate the photochromism of bilin.

In its Pr, inactive configuration, bilin has three planar pyrrole rings and one that is almost perpendicular to the others. But when light is absorbed a Z to E isomerization at the double bond in red occurs; in the new Pfr configuration bilin appears to be planar. The thioester bond that links the bilin to a cystein aminoacid becomes more contort. In order to release the strain, the phytochrome undergoes a series of deep, but reversible, modifications that are passed on to an adjacent domain linked to a kinase protein that initiates signaling.

The study, based on NMR data and structural calculations, challenges a bit the accepted notion that rotation at the double bond in green is the result of light absorption by bilin, offering a new perspective in the fascinating field of the biological perception of light.

[“Structural Basis for the Photoconversion of a Phytochrome to the Activated Pfr Form”, A. T. Ulijasz, G. Cornilescu, C. C. Cornilescu, J. Zhang, M. Rivera, J. L. Markley, R. D. Vierstra, Nature 2010, 463, 250-254].

The Remarkable Pot

The double 3+2 photocycloaddition of acetal 1

          The chemical world is never stingy of surprises.  Perhaps because we have not achieved that level of mathematical description like we have in physics, that every chemical system may hide unexpected treasures.

          Cycloaddition reactions, for instance, where two unsaturated moieties react to form a cyclic compound, have been known for many decades and are widely used to increase the complexity of a structure, as new bonds and new stereocenters are formed.  Dr. C. S. Penkett and coworkers at the University of Sussex (UK) discovered an intramolecular [3+2] photocycloaddition that in one pot increases the complexity of acetal 1 by five new rings, four new C–C σ bonds, and seven new stereocenters, a truly remarkable feat.

          The final compound, 3, which is named fenestrane (from latin fenestra meaning window) has the central carbon atom shared by four five-member rings. It assumes an almost planar geometry, a wide distortion from the standard tetrahedron, and as such of great interest also to theoretical chemists.

          The reaction proceeds through two subsequent [3+2] photocycloadditions steps, though they both occur in the same pot.  UV irradiation up to completion of 1 leads to formation of adduct 2 as major product.  This transforms into 3 by a second irradiation.

          Although the chemical utility of fenestranes is still at its dawn, the discovery of an intramolecular double [3+2] photocycloaddition reaction provides synthetic chemists with an impressively efficient tool to create molecular complexity in a one-pot process.

[“The Double [3+2] Photoaddition Reaction”, C. S. Penkett, J. A. Woolford, I. J. Day, M. P. Coles, J. Am. Chem. Soc. 2010, 132, 4-5.]