Azide light-initiated decomposition

The efficient light-initiated decomposition of azides has been the basis for commercially important photoresist formulations for the semiconductor industry. A common approach is to mix a diazide, such as diazadibenzylidenecyclohexanone (I), with an unsaturated hydrocarbon polymer. Excitation of the difunction-al sensitizer produces highly reactive nitrenes which crosslink the polymer by a variety of paths including insertion into both carbon-carbon double bonds and carbon-hydrogen bonds, and by generation of radicals. The polymer component in the most widely used resists is polyisoprene which has been partially eye Iized by reaction with p-toluenesulfonic acid G). Other polymers used include polycyclopentadiene and the copolymer of cyclopentadiene and a-methyI styrene ( ). 

Investigation of the photochemistry of phenyl azide has been underway for nearly as long as the study of its thermal chemistry. In his 1959 review of carbenes and nitrenes, Kirmse [14] tells of earlier, unpublished work on the photolysis of phenyl azide carried out in Homer s laboratory. At first, photolysis was viewed simply as an additional approach to formation of nitrenes [12, 15, 16]. However, it was quickly realized that light-initiated decomposition of azides provides access to an important array of chemical and spectroscopic tools that permit detailed examination of important questions. In particular, photolysis of aryl azides permits examinations at room temperature or, specially, at low temperature in rigid media where normally reactive intermediates can be stable indefinitely. Furthermore, the use of fast, pulsed lasers as light sources allows the direct detection of shortlived intermediates and enables the detailed study of their reactions. In recent years, most inquiries into the chemistry of aryl azides have focused on application of the tools photolysis makes possible for characterization of the nature and role of reactive intermediates in their chemical transformations. 

The endothermic nitride is susceptible to explosive decomposition on friction, shock or heating above 100°C [1], Explosion is violent if initiated by a detonator [2], Sensitivity toward heat and shock increases with purity. Preparative precautions have been detailed [3], and further improvements in safety procedures and handling described [4], An improved plasma pyrolysis procedure to produce poly (sulfur nitride) films has been described [5], Light crushing of a small sample of impure material (m.p. below 160°C, supposedly of relatively low sensitivity) prior to purification by sublimation led to a violent explosion [6] and a restatement of the need [4] for adequate precautions. Explosive sensitivity tests have shown it to be more sensitive to impact and friction than is lead azide, used in detonators. Spark-sensitivity is, however, relatively low [7],... 

Since the active ester end of the molecule is subject to hydrolysis (half-life of about 20 minutes in phosphate buffer at room temperature conditions), it should be coupled to an amine-containing protein or other molecule before the photolysis reaction is done. During the initial coupling procedure, the solutions should be protected from light to avoid decomposition of the phenyl azide group. The degree of derivatization should be limited to no more than a 5- to 20-fold molar excess of sulfo-SBED over the quantity of protein present to prevent possible precipitation of the modified molecules. For a particular protein, studies may have to be done to determine the optimal level of modification. 

Relationships existing between structure, stability and thermal, photochemical and explosive decomposition (sometimes spontaneous) of the inorganic azides has been extensively investigated and reviewed [1,2]. The ignition characterisitcs of explosive inorganic azides, with or without added impurities under initiation by heat or light have been discussed [3],... 

D.J. Moore, Thermal Decomposition of Barium Azide , Nature 203, 860—61 (1964) 132) J. Roth, Initiation of Lead Azide by High Intensity Light , JChemPhys 41, 1929—36 (1964) 133) G. Odian et al, Radiation-... 

Before the photolyzing step is initiated, the reagent should be handled in the dark or protected from light to avoid decomposition of the phenyl azide group. 

A considerable amount of work has been canied out on the fast decomposition of azides. This was initiated by heat, friction, impact, shock, light, electric field and atomic particles. 

Thus, with respect to the initiation of reaction, early work demonstrated the usefulness of a macroscopic, thermal model of the process and enabled the response of the more sensitive azides to be rationalized in a qualitative and sometimes semiquantitative way. The more difficult task of understanding the phenomena on an electronic or a molecular basis began to bear fruit, and gross, quantitative predictions of slow decomposition by heat or light became possible. However, unless their sensitivities had been first empirically established by statistical experiments, it remained impossible to predict the response of samples to different stimuli or to induce reaction with finesse or precision. Spontaneous initiation and explosion, such as encountered with crystals growing in solution, could not be explained by any mechanism, thermal, photochemical, or tribochemical. 

Photodecomposition studies indicate that the initial step in the decomposition reaction is the absorption of light at 254 nm, which creates excited states of the azide ion whose lifetime is probably impurity controlled. Intensity data suggest that the decomposition reaction occurs by a bimolecular reaction between (N3) states, although details concerning energy transport and the nature of the decomposition site (lattice distortions, impurity centers, crystal surfaces, etc.) remain unanswered. Conjectures concerning probable reaction paths have been based on information about the intermediate products, identified by the optical and ESR measurements previously discussed. 

Roth verified the accuracy of the model by using different light intensities. Because the lead azide was partially decomposed, it is possible that any lead formed by decomposition could be important. It would be interesting to determine if pure, freshly grown lead azide gives similar results, since this would provide evidence on whether or not photochemical decomposition is the initial step. 

Brish and coworkers considered the following mechanisms for initiation (1) The impact pressure of light. Calculation showed, however, that this only amounted to 10" bar and could be disregarded. (2) Electrical breakdown. The breakdown strength of compressed lead azide is 10 V/cm. Since at the critical intensity for initiation the laser light produces an average electric field of 0.7 X 10 V/cm, and self-focusing effects could increase this a further 3-5 times, this is clearly a possible mechanism. (3) Photochemical decomposition... 

Light, mechanical shock, heat, and certain catalysts can be initiators of explosive reactions. Hydrogen and chlorine react explosively in the presence of light. Examples of shock-sensitive materials include acetylides, azides, organic nitrates, nitro compounds, perchlorates, and many peroxides. Acids, bases, and other substances can catalyze the explosive polymerizations. The catalytic effect of metallic contamination can lead to explosive situations. Many metal ions can catalyze the violent decomposition of hydrogen peroxide. 

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