Photochemical intermediates matrix isolation

Photochemical reactions in organic solids are important in practical fields as diverse as photography, biology, photoresist technology, polymerization, and the decomposition and stabilization of dyes, energetic materials, pharmaceuticals, and polymers [1], They have been equally important in basic research, particularly for preparing matrix-isolated reactive intermediates for spectroscopic investigation [2]. 

Either UV-VIS or IR spectroscopy can be combined with the technique of matrix isolation to detect and identify highly unstable intermediates. In this method, the intomediate is trapped in a solid inert matrix, usually one of the inert gases, at very low temperatures. Because each molecule is surrounded by inert gas atoms, there is no possiblity for intermolecular reactions and the rates of intramolecular reactions are slowed by the low temperature. Matrix isolation is a very useful method for characterizing intermediates in photochemical reactions. The method can also be used for gas-phase reactions which can be conducted in such a way that the intermediates can be rapidly condensed into the matrix. 

The results of low-temperature matrix-isolation studies with 6 [41a] are quite consistent with the photochemical formation of cyclo-Cif, via 1,2-diketene intermediates [59] and subsequent loss of six CO molecules. When 6 was irradiated at A > 338 nm in a glass of 1,2-dichloroethane at 15 K, the strong cyclobut-3-ene-1,2-dione C=0 band at 1792 cm in the FT-IR spectrum disappeared quickly and a strong new band at 2115 cm appeared, which was assigned to 1,2-diketene substructures. Irradiation at A > 280 nm led to a gradual decrease in the intensity of the ketene absorption at 2115 cm and to the appearance of a weak new band at 2138 cm which was assigned to the CO molecules extruded photo-chemically from the 1,2-diketene intermediates. Attempts to isolate cyclo-Cig preparatively by flash vacuum pyrolysis of 6 or low-temperature photolysis of 6 in 2-methyltetrahydrofuran in NMR tubes at liquid-nitrogen temperature have not been successful. 

Clearly, mechanistic investigations can provide circumstantial evidence for the participation of particular intermediates in a reaction but, here, we are concerned with the definitive observation of these species. If the intermediates are relatively stable then direct spectroscopic observation of the species during a room-temperature reaction may be possible As a rather extreme example of this, the zero-valent manganese radicals, Mn(CO>3L2 (L phosphine) can be photochemically generated from Mh2(CO)gL2, and, in the absence of O2 or other radical scavengers, are stable in hydrocarbon solution for several weeks (2, 3) However, we are usually more anxious to probe reactions in which unstable intermediates are postulated. There are, broadly speaking, three approaches - continuous generation, instantaneous methods and matrix isolation. 

The photochemical dissociation of Me2Ge from 7,7-dimethyl-l,4,5,6-tetraphenyl-2,3-benzo-7-germanorbomadiene (14) has been studied by flash photolysis, low-temperature matrix isolation and CIDNP 3H NMR techniques30. The results suggest that a biradical (15) is formed as an intermediate species in the photoreaction. The biradical is initially formed in the singlet state, which undergoes conversion to the triplet state before irreversible decomposition to form Me2Ge and tetraphenylnaphthalene (TPN) (reaction 19). 

Earlier, Dunkin and Thomson had observed that matrix-isolated triplet 10a did not undergo photochemical ring expansion.83 However, Morawietz and Sander have recently provided evidence for photochemical conversion of 310a and 310b to the corresponding fluorinated azirines (Scheme 19).48d This represents a rare instance where an azabicyclo[4.1.0]heptatriene, the putative intermediate in the ring expansion of a phenylnitrene, has actually been observed. 

Most chemical reactions can be slowed down by lowering the temperature. With low-temperature studies it is possible to prolong the lifetimes of the reactive intermediates so that they can be characterised by normal techniques. Matrix isolation allows experiments to be carried out at temperatures as low as 4K, in order to study species, such as radicals, that are produced photochemically at very low temperatures. The initial photoproduct is trapped within a rigid matrix that inhibits diffusion of the reactive species. The matrix material must be ... 

Firth S, Klotzbuecher WE, Poliakoff M, Turner JJ. Generation of Re2 (CO)9 (N2) from Re2(CO)10 identification of photochemical intermediates by matrix isolation and liquid-noble-gas techniques. Inorg Chem 1987 26(20) 3370-3375. 

The photochemical properties of [Ru(CO)3(dmpe)j (dmpe = l,2-bis(dimethylphosphino)ethane) have been studied by matrix isolation at 12 K and laser flash photolysis with UV-vis and IR detection at ambient temperatures. UV photolysis (A,ex = 234-376mn) in a matrix resulted in the formation of [Ru(CO)2(dmpe) S] (S = matrix host). Laser flash photolysis in heptane solution (A,ex = 266 or 308 nm) revealed that [Ru(CO)2(dmpe) (heptane)] was a short-lived intermediate fragment that reacted rapidly with [Ru(CO)3(dmpe)]. 

Summary A number of isomers of composition CsHiSi and C2H4Si2 have been generated by pulsed flash pyrolysis of appropriate precursors and isolated in an argon matrix. Their photochemical interconversions were studied. Quantum chemical calculations have been performed at the BLYP/6-31G level of theory. They play a decisive role in the identification of the highly reactive intermediates. Although silacyclobutadienes were shown to be minima on their respective energy hypersurfaces, no experimental evidence for the existence of such compounds was found. Instead, silylenes were detected, which undergo a variety of mutual interconversions. 

All the experiments described in this chapter consist of several steps. The first step is the matrix isolation of a precursor molecule in a large excess of argon (typical matrix ratios are >1000 1). The next step is the photochemical generation and spectroscopic characterization of the reactive intermediate. To... 

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