ACH - Models in Chemistry 136. (1999)
4. szám - RESEARCH ARTICLES - Šima, J.: Origin of the wavelength dependence of the photoredox quantum yield for iron(III) complexes
ц - Models in chemistry 136 (4), pp 375-384 (1999) Origin of the wavelength dependence of the photoredox quantum yield for iron(III) complexes Jozef Sima Department of Inorganic Chemistry, Slovak Technical University, Radlinského 9, SI2 37 Bratislava, Slovakia Received August 11, 1998 The paper rationalizes an origin of the different experimental wavelength dependences of the Fe11 photoformation quantum yield as a consequence of five main factors, namely the different rate constant of intersystem crossing and/or internal conversion and of rovibrational relaxation, the possible existence of multi-channel reactivity pathways, the accessibility of several photoredox reactive states differing in lifetime, multiplicity and reactivity, the redox reversibility of oxidized ligand, and the efficiency of solvent cage recombination of primary photoredox products. The issue is exemplified on the photoredox behaviour of iron(III) chelates with anionic EDTA, allomaltolato and oxalato ligands in aqueous media. Introduction In the photoredox chemistry of transition metal complexes three kinds of wavelength dependence of the overall quantum yield have been observed. The most frequent case is a gradual decrease of the quantum yield with increasing incident radiation wavelength. As for iron complexes, a substantial amount of experimental data is gathered in the literature (1 —3]. An extensive application of |Ре(С204)з)3~ in actinometry is based predominantly on the fact that its photoredox quantum yield is nearly wavelength independent in a wide spectral region [4J. There are few complexes with a peculiar wavelength dependence of the quantum yield, exhibiting a maximum at a certain wavelength and a gradual decrease in both the lower and higher energy directions. The complex [Fe(HEDTA)| can serve as an example [5]. Various explanations of the different efficiency of photoredox reactions have been suggested in the chemical literature. Some of them have, however, been formulated as tentative proposals since the development of the methods permitting their verification has been still only in progress and researchers could not operate with a satisfactory amount of quantitative data (it relates mainly to results obtained by techniques working in the subpicosecond time domain). An additional problem relating 1217-8969/99/$ 5.00 © 1999 Akadémiai Kiadó, Buda/ml