ChemInform Abstract: Pulsed-Laser Photochemical Study of Tris(1,10-phenanthroline)chromium(III) Ion in Acidic and Alkaline Aqueous Media

The photochemistry and photophysics of tris(1,10-phenanthroline)chromium(III) ion, Cr(phen)/sub 3//sup 2 +/, following pulsed-laser excitation at 347 nm, have been studied in deaerated acidic and alkaline aqueous media (pH 3.0-11.1) at room temperature by using conductivity and visible optical detection methods. The lifetimes of the lowest doublet excited state(s) from which is derived the major portion of the photochemistry are little affected by changes in pH as monitored by emission and excited-state absorption nor are the initial intensity levels of these phenomena noticeably altered by pH changes. Alterations in pH do have significant effects on the conductivity signals, which correlate to photoaquation. Conductivity studies for natural pH (ca. pH 7.1) provide evidence for a ground-state intermediate with a pK/sub a/ value around 7. In basic media, an apparent single stage of conductivity occurs, and the values of the observed rate constant and the resulting overall change in conductance are dependent upon the concentration of hydroxide ion. The quantum yield for aquation at the highest pH of 11.1 is determined as 0.08 (+/-20%). By contrast, photochemical reaction in acidic media is found to be very much reduced relative to that occurring in alkaline solutions. A mechanism, incorporating a six-coordinate, monodentate phenathroline intermediate,more » is proposed, and it is used to discuss the occurrence of photoracemization and to compare the behavior of Cr(phen)/sub 3//sup 3 +/, where bpy designates the 2,2'-bipyridine ligand.« less