“Chemistry on the Window Ledge”

 

The eco-sustainable "chemistry on the window ledge" of Prof. M. Fagnoni

          Cheap reagents, no external energy (no artificial irradiation, no cooling, no stirring), a catalyst with high turnover, high yield, single peak in GC, low environmental impact.  The Holy Grail?  Perhaps, but this is exactly what Prof. M. Fagnoni and coworkers at the University of Pavia (Italy) report for the functionalization of a series of electrophilic olefines.

            The strategy comprises the use of tetrabutylammonium decatungstate (TBADT), a cluster with formula (n-Bu₄N)₄W₁₀O₃₂, as photocatalyst.  TBADT efficiently absorbs in the UV-A region and, in the excited state, abstracts an H atom from alkanes, aldehydes, ethers and amides.  The radical left behind adds to electrophilic alkenes for alkylation or acylation (see picture).  At the end of the cycle, TBADT-H·gives back the H atom yielding the product and regenerating the catalyst.  The reactions, carried out on a window ledge under a few days of sunlight irradiation, have yields ranging from 50 to 90%, a value that in most cases is larger than that obtained using lamps.

            But most importantly, compared with traditional photochemical syntheses, “chemistry on the window ledge” under optimized conditions is considerably more eco-friendly, according to environmental indices.  And because reactions remains clean at higher concentrations (up to 0.5 M), production costs are significantly abated.

            As migration to eco-sustainable chemical productions becomes more and more pressing, photochemistry is projected to play a pivotal role.  After all, photons are “the ideal green reagent” as the authors observe, and this study demonstrates.

[“Solar Light-driven Photocatalyzed Alkylations. Chemistry on the Window Ledge”, S. Protti, D. Ravelli, M. Fagnoni, A. Albini, Chem. Commun. 2009, 7351-7353.]

An Oil Droplet Guided by Light

The mechanism of the remotely light-induced oil droplet motion

          In this video, a beam of light guides an oil droplet to describe the shape of the letter “N”.

            Manipulation of liquid droplets by light is but a strategy to achieve remote control over the flow of liquids without using mechanical parts, like valves or pumps.  Thus far, scientists have been able to light-induce translational motion of liquids at speeds only as large as 30 μm s^-1.  But now, researchers at the Normal School at Paris and the University of Rennes 1 (France), and Kyoto University (Japan) have introduced a new mechanism that can induce controlled motion of an oil droplet at the water/air interface at speeds 10 times as large.

            The water phase is a solution of an azobenzene derivative cationic surfactant named azoTAB (see picture).  With no irradiation, the less polar trans-azoTAB is the most stable form.  Irradiation at 365 nm induces trans-cis isomerization of azoTAB increasing the polarity of the irradiated region.  As a result, the surface tension, γ, at the water/oil interface increases.  Because the oil droplet prefers to lay on the lower surface tension interface, it moves away from the irradiate region, thus generating a net translational motion – the chromocapillary effect.

            Playing with this principle and taking advantage of the full cis-trans reversibility of azoTAB, the authors trapped the oil droplet in a potential energy well by concentric irradiatiation of the surrounding area.  Moving the well at reasonable speed causes the oil drop to follow and accurately describe any shape, as the movie shows.

[“Photomanipulation of a Droplet by the Chromocapillary Effect”, A. Diguet, R.-M. Guillermic, N. Mogome, A. Saint-Jalmes, Y. Chen, K. Yoshikawa, D. Baigl, Angew. Chem. Int. Ed. 2009, 48, 9281-9284.]

A Wire of Hydrogen Bonds

The excited state proton transfer in GFP studied by Prof. R. Mathies

          In this fine communication a sophisticated spectroscopic technique reveals the details of an elegant photochemical process in Green Fluorescent Protein (GFP), the widely used biomarker worth the Nobel Prize to its discoverers in 2008.

            The chromophore of GFP is composed of a phenol and an imidazolinone ring ends.  The two are connected through an intermolecular hydrogen bond “wire” made of terminations of different aminoacids and a water molecule (see figure).  Upon blue irradiation, an excited state proton transfer (ESPT) from the phenolic to the carboxylate adjacent to the imidazolinone occurs.  The new tautomeric form is long-lived green-light emitter.

            Using 50-fs resolution Raman spectroscopy, the group of Prof. R. Mathies at the University of California, Berkeley (USA) reveals the fine details of ESPT in GFP.  Initially laying in different planes, the phenolic proton and the water molecule are in an unfavorable configuration for hydrogen bonding.  However, in the excited state some electron density is shifted from the phenol to the imidazolinone ring, making the phenolic H more acidic.  Moreover, due to the activation of a wagging vibrational motion, the phenol ring swings upwards to reach out for the water molecule.

            As the GFP slides down the excited state potential energy surface (τ = 700 fs), it gets to an energetically favorable atomic configuration for proton transfer to occur through the hydrogen bond wire.

            The study illustrates that femtosecond Raman spectroscopy is an exciting and powerful technique to decipher ultrafast nuclear dynamics in complicated systems such as proteins.

[“Mapping GFP Structure Evolution During Proton Transfer with Femtosecond Raman Spectroscopy”, C. Fang, R. R. Frontiera, R. Tran, R. A. Mathies, Nature 2009, 462, 200-204.]

The Polymer That Does It All

PolyTBT: The polymer with outstanding potentials for optoelectronics

We like simplicity and we are attracted towards those objects that make our life easier. No wonders then that a Swiss army knife is a worldwide must-have: one object, many purposes.  But what about polymers for optoelectronics?

Well, there are polymers for red color, polymers for green, some suitable for near-IR applications, other completely transparent, or black; essentially, everything you like.  In recent years, however, scientists have started to look if some of these characteristics could be incorporated in one multichromic polymer, and some advances have actually been reported.

But the group of Prof. L. Toppare at the Middle East Technical University (Turkey), in collaboration with researchers at the University of California, Davis (USA), have taken the field to a higher level, for they reported a multichromic polymer capable of reproducing all three primary colors (red, green and blue), as well as black and transparent states, upon changing the applied potential (see picture).

The material, obtained from polymerizing 2-dodecyl-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole (TBT), contains both an electron-acceptor unit (benzotriazole) and the electron-rich thiophene. Therefore it is prone to undergo both oxidation and reduction with subsequent wide range color change.  Other pros are: solubility and easy processability, fluorescence emission, superior optical contrast in the near infrared region, and relatively fast color-switching time.

The ball passes now to chemical engineers and chemical industry to test this multi-purpose electrochromic material in applications like solar cells, color displays, smart windows, and optical communications.

Time will tell whether polyTBT is the Swiss knife of polymers in optoelectronics.

[“One Polymer for All: Benzotriazole Containing Donor-acceptor Type Polymer as a Multi-purpose Material”, A. Balan, D. Baran, G. Gunbas, A. Durmus, F. Ozyurt, L. Toppare, Chem. Comm. 2009, 6768-6770].