Photochemical sensitised

A powerful explosion which occurred dining distillation of a 10-year-old sample of the alcohol was attributed to presence of peroxy compounds formed by autoxidation, possibly involving 2-butanone as an effective photochemical sensitiser [1], After a later explosion, it was found that the sample being distilled contained 12% of peroxide [2], A further incident involved a 12-year old sample which exploded at the end of distillation, and which also contained a high level of peroxide. Several other stock alcohols were found to contain much lower levels of peroxide than the 2-butanol, and recommendations on clean-up or disposal, depending on the level of peroxide, are made [3], A further report of an explosion at the end of laboratory distillation confirms the potential for peroxide formation on prolonged storage of 2-butanol [4],... 

Curri ML, Petrella A, Striccoli M et al. Photochemical sensitisation process at photosynthetic pig-ments/Q-sized colloidal semiconductor hetero-junctions. Synth Met 2003 139 593-596. 

MarinkovicS, HofFmannN Efficient radical addition of tertiary amines to electron-deficient alkenes using semiconductors as photochemical sensitisers. Chem Commun (Camb) 2001, (17) 1576—1577. 

Let us discuss some specific examples showing photochemical sensitisation. Direct irradiation of 1, 3-butadiene in solution gives cyclobutene and bicyclobutane with minor amount of dimers. 

Isol. from essential oils. Has soporific props. Solvent, resin intermed. Photochemical sensitiser. Used in extraction separation of Se from Te. Plates (freq. obt. as liq.). Sol. EtOH, Et20 insol. H.O. df 1.029. Mp 20°. Bp 202°,... 

These polymers need to be made photosensitive for use as photoresists and this is achieved by the incorporation of bisazide sensitisers. On exposure to light the photochemical reaction induced by the bisazide results in rapid crosslinking of the polymer rendering it insoluble in the developing solvent. 

Distillation to small volume of a small sample of a 4-year-old mixture of the alcohol with 0.5% of the ketone led to a violent explosion, and the presence of peroxides was subsequently confirmed [1]. Pure alcohols which can form stable radicals (secondary branched structures) may slowly peroxidise to a limited extent under normal storage conditions (isopropanol to 0.0015 M in brown bottle, subdued light during 6 months to 0.0009 M in dark during 5 years) [2], The presence of ketones markedly increases the possibility of peroxidation by sensitising photochemical oxidation of the alcohol. Acetone (produced during autoxidation of isopropanol) is not a good sensitiser, but the presence of even traces of 2-butanone in isopropanol would be expected to accelerate markedly peroxidation of the latter. Treatment of any mixture or old sample of a secondary alcohol with tin(II) chloride and then lime before distillation is recommended [3], The product of photosensitised oxidation is 2-hydroperoxy-2-propanol [4]. 

The evaporated residue from sensitised photochemical oxidation of the polyene ignited spontaneously, on several occasions explosions occurring. 

Explain the photochemical principles of the dye-sensitised solar cell and understand the importance of various physicochemical parameters to the overall performance of such a cell. 

Photochemical addition of ammonia and primary amines to aryl olefins (equation 42) can be effected by irradiation in the presence of an electron acceptor such as dicyanoben-zene (DCNB)103-106. The proposed mechanism for the sensitised addition to the stilbene system is shown in Scheme 7. Electron transfer quenching of DCNB by t-S (or vice versa) yields the t-S cation radical (t-S)+ Nucleophilic addition of ammonia or the primary amine to (t-S)+ followed by proton and electron transfer steps yields the adduct and regenerates the electron transfer sensitizer. The reaction is a variation of the electron-transfer sensitized addition of nucleophiles to terminal arylolefins107,108. 

Figure 4.14 Sensitisers used for singlet oxygen production in photochemical synthesis.

The photochemical cis trans isomerization is effected by direct irradiation or in the presence of a sensitiser or a catalyst. 

Prior to a study of the possible photochemical reaction of a compound, its spectrum in the ultraviolet or visible region must be determined in order that a light source emitting the appropriate wavelength of radiant energy may be selected. In the case of a sensitised photochemical reaction the spectrum of the sensitiser should be determined. 

Mills, A. Morris, S. Davies, R. Photomineralisation of 4-chlorophenol sensitised by titanium dioxide A study of the intermediates, J. Photochem Photobiol. 1993, A70, 183. 

Poulain L, Mailhot G, Wong-Wah-Chung P, Bolte M. Photodegradation of chlortoluron sensitised by iron(III) aquacomplexes. J Photochem Photobiol A Chem 2003 159 81-8. 

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