Advancing Inorganic Photochemistry

Dyotropic-like photorearrangement in a phosphazane cage

          Supramolecular chemistry, the subfield that studies intermolecular recognition, was brought to general attention in 1987 by the Nobel committee.  Since then, new types of crown ethers, cryptands, and ever more elegant host-guest complexes have been reported in the literature.

          Recently, inorganic chemists have developed macrocyclic ligands whose chemistry is akin to that of the organic counterpart, providing to the field new avenues to explore.  At present, most of these inorganic complexes are frameworks made of P-N bonds.  Adding to the field, the groups of Drs. D. S. Wright and A. Steiner respectively at the University of Cambridge and Liverpool (UK) discovered that phosphazane cage [PN^tBu]₄ (1α in the picture) rearranges into the highly strained isomer 1β, upon UV light irradiation.

          According to calculations, photoisomerization starts with a P-N bond rupture and proceeds through a triplet intermediate, a singlet zwitterion, and a subsequent intramolecular cycloaddition.  Overall, however, the reaction is reminiscent of the dyotropic rearrangement observed in organic chemistry, where two σ-bonds migrate (see picture) in regioselective fashion.

          The discovery, unique on its own, demonstrates the feasibility to reproduce the regioselectivity of organic photochemistry in P-N macrocycles, opening up exciting possibilities in novel cage systems design, and perhaps generalize the approach to other main group frameworks, such as Si-N.

[“The Regioselective Photochemical Rearrangement of a-[PN^tBu]₄”, H. Bladt, S. Gonzalez Calera, J. M. Goodman, R. J. Less, V. Naseri, A. Steiner, D. S. Wright, Chem. Comm. 2009, 6637-6639.]

Ruthenium Complex to Compress and Decompress Data

2-To-4 Decoder based on a Ru complex

          Extensively studied for the last thirty years, ruthenium complex [Ru(bpy)₃]²⁺ has virtually no secrets for photochemists.  Nevertheless, scientists at the University of Bologna (Italy) lead by Prof. Balzani proposed a new conceptual interpretation for it: as molecular 4-to-2 encoder and 2-to-4 decoder.

            In the same way that electronic circuits encode large files into smaller ones and viceversa, so [Ru(bpy)₃]²⁺ can compress 4-bit data and decompress them back.

            2-To-4 decoding is achieved by electrochemically oxidizing (In₀) or reducing (In₁) [Ru(bpy)₃]²⁺ and then measuring the absorbance at three different wavelengths (450 nm: Out₀, 310 nm: Out₁, and 530 nm: Out₂), as well as the emission intensity at 620 nm (Out₃).  For example, when the complex is oxidized to [Ru(bpy)₃]³⁺ (In₀=1, In₁=0), the absorbance at 310 nm is above a chosen threshold (thus, Out₁=1), but the absorbances at 450 and 530 nm are both below threshold; since the oxidized complex does not emit, we obtain Out₀=Out₂=Out₃=0.  On the other hand, if the complex is reduced to [Ru(bpy)₃]⁺ (In₀=0, In₁=1), the output is Out₀=Out₁=Out₃=0, Out₂=1, because of the above-threshold absorbance at 530 nm.

          [Ru(bpy)₃]²⁺ itself (In₀=In₁=0) and a suitable combination of oxidized and reduced forms (In₀=In₁=1) complete the spectrum of possibilities, (see truth table).  Once the output is read, the system is quickly reconfigured applying apposite voltage.  (4-To-2 encoding is analogously achieved.)

            The use of molecules as logic gates, envisioning molecular computers, dates back in the early 90s, but the beauty of this approach resides in its simplicity and creativity.

[“Old Moelcules, New Concepts: [Ru(bpy)₃]²⁺ as a Molecular Encoder-Decoder” P. Ceroni, G. Bergamini, V. Balzani, Angew. Chem. Int. Ed. 2009, 48, 8516-8518].

The Sun of Wisconsin

The acyclic enone cycloaddition developed by Prof. Yoon

          Photoinduced [2+2] cycloaddition of enones was first reported in 1909.  Since then, it has become a common route to produce cyclobutane-containing structures.  However, its synthetic usefulness has been limited to cyclic enones, because, upon excitation, they cannot undergo cis-trans isomerisation, a common side photoreaction of alkenes in the excited state.

            Now, the group of Prof. T. P. Yoon at the University of Wisconsin-Madison (USA) has developed a synthetic approach that extends the [2+2] cycloaddition to acyclic enones, a “long-standing unsolved problem in synthetic chemistry”.

            Since the reaction is thought to go through a radical anion intermediate, they chose to start with an aryl enone (1), as this stabilizes the negatively charged intermediate.  Then, in order to avoid enone dimerization (reaction of 1 with itself), they selected compound 2, which, thanks to the lack of β-substituents, is more of an electrophile than 1 and thus more prone to combine with the anionic intermediate.  The novel approach, however, is the use of a Ru complex to harvest visible light, serving as the photocatalyst for the cycloaddition.  The result is the formation of the reaction intermediate without direct photoexcitation of 1, diverting the mechanism from the unwanted cis-trans isomerization.  After four hour of irradiation on the roof of the Chemistry Department – in a sunny Wisconsin day, one would imagine – the reaction produces compound 3 with 84% yield and high diastereoselectivity.

            By varying a great deal of substituent, Prof. Yoon has thus developed a highly efficient and general photochemical route towards 4-member ring containing structures.

[“Crossed Intermolecular [2+2] Cycloadditions of Acyclic Enones via Visible Light Photocatalysis”, J. Du, T. P. Yoon, J. Am. Chem. Soc. 2009, 131, 14604-14605].

Protecting Conjugation

Boron chromophores protection strategy of Prof. S. Wang

          Light can damage all sort of things: paintings, textiles, our skin, and also the π-conjugation of N,C–chelate boryl compounds.  As we shield our valuables, also these molecules need to be protected from photodegradation.

            N,C–Chelate boryl compounds have a four-coordinate B atom (see picture) that provides an excellent electron transport character and high emission yield.  Possible applications in molecular electronics (OLED, photovoltaics, organic transistors) can be easily devised, but despite promising potential uses, they are rare in literature, mostly because of their instability in air.

            Now, Prof. S. Wang at Queen’s University (Canada) has found a clever way to overcome this problem, which promises to stimulate innovative research on these, and related, compounds.

            An olefinic group is attached to the π-system, which, upon excitation, undergoes trans-cis isomerization, leaving essentially unperturbed the characteristics of the B-based chromophore (top scheme).  Irradiation in the absence of protecting olefine, instead, produces B–C bond rupture to form a strained and highly reactive rearrangement product (bottom scheme), which has prevented advances in N,C-chelate boryl compounds as conjugated organic materials.

            In addition, according to the authors, this strategy can be useful to protect a wider range of chromophores from photodegradation.

[“Enhancing the Photochemical Stability of N,C–Chelate Boryl Compounds: C–C Bond Formation versus C═C Bond cis,trans-Isomerization”, C. Baik, Z. M. Hudson, H. Amarne, S. Wang, J. Am. Chem. Soc. 2009, 131, 14549-14559].