123.光化学に関するデモンストレーション

This report describes the following five experiments which can be easily performed as demonstration to show the nature of the excited states participating in photochemical phenomena. 1. Cleavage of molecules induced by irradiation with sun light or a lamp: A dilute solution of bromine in toluene is exposed to sun light or an OHP lamp. The orange color of bromine is faded and fumes are produced due to production of benzyl bromide and hydrogen bromide by the way of radical chain reactions initiated by photoinduced cleavage of molecular bromine into bromine atoms. 2. Absorption and emission: Irradiation of dilute ethanol solution of anthracene with a black light lamp gives characteristic violet fluorescence from an excited state of anthracence. Other dyes are available to show fluorescence of different color. 3. Photoinduced electron transfer: A dilute thionine solution in dilute aqueous sulfuric acid containing a Fe(II) salt is placed on an OHP projector. Illumination of the solution with the OHP lamp leads to fading of violet color giving colorless solution by the way of electron transfer from Fe(II) ion to the excited state of thionine. On stopping the illumination, the resulted colorless solution is reverted to the violet solution due to reverse electron transfer occuring in the dark. 4. Flame reaction: Each of metallic chlorides such as NaCl, KCl, LiCl, RbCl, CsCl, CaCl_2, SrCl_2 and BaCl_2 etc. is placed in the oxidizing flame of a Bunsen burner or an alcoholic lamp. The flame shows color characteristic to each metal as emission from the excited state of the metallic atoms produced by reactions in flame. 5. Chemiluminescence: Mixing of aqueous solutions of luminol/NaOH and of hydrogen peroxide/potassium ferricyanide immediately affords strong violet emission as fluorescence from the excited state of an oxidized product of luminol. Demonstration of these experiments provides visual material for understanding of basic principles of photochemistry to students