PICOSECOND EXCITED-STATE DYNAMICS IN OCTAHEDRAL COBALT(III) COMPLEXES - INTERSYSTEM CROSSING VERSUS INTERNAL-CONVERSION

The results of transient absorption experiments are reported for aqueous solutions of the low-spin d6 complexes [Co(en)3](ClO4)3, [CO(tpen)](ClO4)3, and [Co(tppn)](ClO4)3, where en is ethylenediamine, tpen is tetrakis(2-pyridylmethyl)ethylenediamine and tppn is tetrakis(2-pyridylmethyl)-1,2-propylenediamine. Following excitation at 314 nm with a approximately 500-fs pulse, all three complexes exhibit biphasic relaxation kinetics. In the case of [Co(tpen)]3+ excited-state decay is biphasic with relaxation time constants of tau1 = 4 +/- 2 ps and tau2 = 44 +/- 5 ps. The data for [Co(tppn)]3+ give the similar values tau1 = 3 +/- 1 ps and tau2 = 51 +/- 3, whereas the data for [Co(en)3]3+ give tau1 = 2 +/- 1 ps and tau2 = 450 +/- 100 ps. On the basis of these results and the fact that no emission was observed for any of these complexes in the 350-900-nm range with a time-correlated single-photon-counting detection system, it is proposed that the lowest-energy excited state in these three Co(III) complexes is the 5T2 ligand-field state. Transient absorption spectra for the Co(III) complexes indicate that the slower process (tau2) is very likely a decay of the 5T2 ligand-field state back to the 1A1 ground state. Corresponding 5T2 --> 1A1 relaxation times for Fe(II) complexes have been reported to be in the approximately 1-1 20-ns range. The faster process (tau1) for each of the Co(III) complexes is tentatively assigned as excited-state decay from the lowest-energy singlet state, the 1T1 excited state. The results suggest that the 1LMCT state in both [Co(tpen)](ClO4)3 and [Co(tppn)](ClO4)3 has a lifetime of <1 ps.