Emission lifetimes for Rh(NH3)5Cl2+, A, and Rh(NH3)5Br2+, B, are 30.1 ± 0.7 nsec and 24.1 ± 1.2 nsec in aqueous acidified solution at 5 °C, respectively, with corresponding apparent activation energies of 5.1 and 5.4 kcal mol−1, again respectively. In both cases the emission maximum is at about 14 kK. There is excited absorption, which decays with the corresponding emission lifetime, with maxima at 490 nm and 620 nm, for A and B respectively. This second excited state is also photoreactive. The emissions are quenched by hydroxide ion, the bimolecular quenching rate constants being 2.1 × 1010 M−1 sec−1 and 2.7 × 1010 M−1 sec−1 at 5 °C, again respectively. Studies with A showed that the other bases such as carbonate and cyanide ions also quench. A undergoes only chloride photoaquation, which is 87% quenched on quenching emission. B is known to show both bromide and ammonia photoaquation; the latter is fully quenched on quenching emission, while the former reaction mode is unaffected. Possible excited state schemes are discussed, one including a reactive quintet state.
Die Photoaquatisierung des Komplexions (I) entspricht einem Cl ‐ ‐H 2 O Austausch, während bei (III) sowohl ein Br ‐ ‐H 2 O als auch ein NH 3 ‐H 2 O‐Austausch stattfindet.
Photophysical kinetic results have played an important role in assessing excited state relaxation pathways in transition metal complexes. The applicability of a kinetic analysis is critically dependent on the quality of the individual decay rates, the temperature range examined, and the model used to extract the activation parameters. The extensive literature describing the temperature dependence of excited state depopulation in d3 and d6 complexes permits an evaluation of both the power and limitations of kinetic arguments in assessing the mechanism of excited state relaxation.
AbstractAus der Temp‐Abhängigkeit der Emission und der Absorption wäßriger Rh(NH3 ätLösungen, die beide dem angeregten T′1′‐Zustand zuzuschreiben sind, ergibt sich die gleiche Aktivierungsenergie für beide Prozesse.
