UC Santa Barbara

Ligand field excitation of hexaamminerhodium(III), Rh(NH3)63+, in aqueous solution results in the photoaquation of one coordinated NH3. The measured quantum yields at 23° are 0.075 mol/einstein for 313-nm excitation of the 1A1g → 1T1g transition and 0.07 for 254-nm excitation of the 1A1g → 1T2g transition, suggesting efficient interconversion of the higher energy state to a common reactive excited state. Under similar conditions, the quantum yield for the perdeuterated complex Rh(ND3)63+ is about twice as large (0.14 in H2O, 0.15 in D2O, 313-nm excitation). However, the temperature dependence of the photoaquation quantum yields is measurably greater for the perprotio complex than for the perdeuterio complex. To explain these results, it is proposed that temperature-independent weak coupling and temperature-dependent strong coupling mechanisms are competitive in the nonradiative deactivation of the reactive excited state of Rh(NH3)63+.