Highly efficient visible-light driven photochromism: developments towards a solid-state molecular switch operating through a triplet-sensitised pathway

We introduce a new highly efficient photochromic organometallic dithienylethene (DTE) complex, the first instance of a DTE core symmetrically modified by two Pt(II) chromophores [Pt(PEt(3))(2)(C≡C)(DTE)(C≡C)Pt(PEt(3))(2)Ph] (1), which undergoes ring-closure when activated by visible light in solvents of different polarity, in thin films and even in the solid state. Complex 1 has been synthesised and fully photophysically characterised by (resonance) Raman and transient absorption spectroscopy complemented by calculations. The ring-closing photoconversion in a single crystal of 1 has been followed by X-ray crystallography. This process occurs with the extremely high yield of 80%--considerably outperforming the other DTE derivatives. Remarkably, the photocyclisation of 1 occurs even under visible light (>400 nm), which is not absorbed by the non-metallated DTE core HC≡C(DTE)C≡CH (2) itself. This unusual behaviour and the high photocyclisation yields in solution are attributed to the presence of a heavy atom in 1 that enables a triplet-sensitised photocyclisation pathway, elucidated by transient absorption spectroscopy and DFT calculations. The results of resonance Raman investigation confirm the involvement of the alkynyl unit in the frontier orbitals of both closed and open forms of 1 in the photocyclisation process. The changes in the Raman spectra upon cyclisation have permitted the identification of Raman marker bands, which include the acetylide stretching vibration. Importantly, these bands occur in the spectral region unobstructed by other vibrations and can be used for non-destructive monitoring of photocyclisation/photoreversion processes and for optical readout in this type of efficiently photochromic thermally stable systems. This study indicates a strategy for generating efficient solid-state photoswitches in which modification of the Pt(II) units has the potential to tune absorption properties and hence operational wavelength across the visible range.     

Sodium congener of the classical lithium methylchromate dimer: synthetic, X-ray crystallographic, and magnetic studies of Me8Cr2[Na(OEt2)]4

One of the milestone structures in the development of transition-metal complexes with metal-metal bonds of multiple bond order was the lithium methylchromate dimer Me(8)Cr(2)[Li(donor)](4) (donor = THF or Et(2)O). Using a simple salt metathesis reaction mixing this compound with sodium tert-butoxide, the sodium congener Me(8)Cr(2)[Na(OEt(2))](4) has been synthesized as a green crystalline compound and isolated in 51% yield. Its solid-state structure was determined by single-crystal X-ray diffraction. Exhibiting exact crystallographic C(4h) symmetry, this heavier alkali-metal chromate structure is also dimeric, formally comprising a (Me(8)Cr(4))(4-) tetranionic core with four peripheral Na(+) cations carrying supporting ether ligands. Its salient feature is the long Cr···Cr distance of 3.263(2) ?, which is remarkably elongated compared to that in the lithium THF-solvated congener [1.968(2) ?]. With respect to the methyl C atoms, the Cr coordination is distorted-square-planar. Each Na(+) interacts with four methyl C atoms, and there are also some short Na···H(C) contacts. Unlike for lithium chromate, no NMR spectroscopic data could be obtained for sodium chromate. The paramagnetic character of sodium chromate was confirmed by variable-temperature magnetization measurements, which indicated antiferromagnetic behavior.     

Reactivity of N-nitrosoamides in confined spaces

The thermal rearrangement of several N-nitrosoamides was studied by ¹H NMR in the context of reversible encapsulation. The N-nitrosoamide guests were isolated from the bulk solvent in a hydrogen-bonded dimeric host capsule which prevented their rearrangement. The guests appear to be preserved in their ground state conformations by the pressure exerted by the host. The conformations of the free and bound N-nitrosoamides are of comparable relative energies as determined by DFT calculations.     

Freezing points,osmotic coefficients,and activity coefficients of some salts in ethylene carbonate: A high dielectric constant solvent without hydrogen bonding

The freezing points, conductivities, and densities of NaI, KI, CsI, Bu₄NCl, Bu₄NBr, Bu₄NI, Et₄NBr, and Pr₄NBr (where Et = ethyl, Pr = propyl, and Bu =n-butyl) in ethylene carbonate have been measured. Osmotic and activity coefficients were calculated from the results. All of the salts studied are strong electrolytes. The trends in the osmotic coefficients of the alkali metal iodides are NaI>KI>CsI, showing that Na⁺ is more solvated by ethylene carbonate than Cs⁺. For the tetraalkylammonium halides, the order of osmotic coefficients are Et₄NBrPr₄NBrBu₄NCl>Bu₄NBr>Bu₄NI. This is the same order as observed in two other high-dielectric-constant solvents, water andN-methylacetamide. The results indicate that the smaller anions are more solvated than the larger anions in ethylene carbonate in contrast to the usual behavior of dipolar aprotic (basic) solvents, such as dimethyl sulfoxide.     

Differentiation of electrophilic and ambiphilic carbenes

Comparative selectivities toward alkenes of CH₃CCl, CCl₂, and CH₃OCCl establish the former two carbenes as electrophiles; the latter is an ambiphile.     

Synthesis of 3-arylsulfonylmethyl-1,2,4-oxadiazole-5-carboxylic acid derivatives

Several new 3-arylsulfonylmethyl-1,2,4-oxadiazole-5-carboxylic acid derivatives have been synthesized. A typical example, 3-[(4-chlorophenylsulfonyl)methyl]-1,2,4-oxadiazole-5-carboxylic acid ethyl ester ( 2c ), was prepared from the reaction of 2-(4-chlorophenylsulfonyl)acetamide oxime ( 1c ) with ethyl oxalyl chloride. The hydrazide derivative ( 3f ) showed antihypertensive activity in rats. Various structural modifications to improve activity are discussed.     

Multiply charged ions in the mass spectra of aromatics

The prominence of multiply charged molecular and fragment ions upon electron-impact in the mass spectrometer is proposed as an experimental, empirical indication of aromatic character. The effects of electron withdrawing and donating substituents on the production of multiply charged ions are considered and appearance potentials are noted for several species.