The first steps of the light-induced biradical and zwitterion formation from the clusters [Os3(CO)10(α-diimine)] studied with ultrafast time-resolved absorption spectroscopy

The (sub)picosecond time-resolved transient absorption spectra of two triangular [Os-3(CO)(10)(alpha-diimine)] clusters have been studied to establish the primary photoprocesses responsible for the formation of biradicals and zwitterions. The TA spectra of [Os-3(CO)(10)(Pr-AcPy)] obtained by excitation into its visible absorption band, show a bleach due to the disappearance of the parent cluster and a new absorption with a maximum at 630 nm. In a non-coordinating solvent the bleach and absorption decay with a lifetime of 25 +/- 2 ps but do not disappear completely. The bleach decays to approximately 30% of the initial signal and the transient absorption changes into a much broader absorption without a distinct maximum. The initial transient absorption is assigned to the excited state of the cluster having predominant sigma(Os-Os) pi*(Pr-i-AcPy) character. From the relaxed excited state the cluster partially decays to the ground state and partially produces biradicals. The lifetime of the excited state does not depend on the solvent as long as it is non-coordinating, but it depends on the energy of this (3)sigmapi* excited state, as observed for [Os-3(CO)1(0))(dmb)]. This effect is attributed to a lowering of the barrier for the reaction from the (3)sigmapi* state. In coordinating acetonitrile (MeCN) the excited state of [Os-3(CO)(10)(Pr-i-AcPy)] decays double-exponentially. The longer lifetime (tau = 21.4 ps) matches that observed in non-coordinating solvents and is assigned to biradical formation. In agreement with previous observations that zwitterion formation in coordinating solvents must occur in the picosecond time domain, the second and faster process (tau = 2.9 ps) is assigned for zwitterion formation. These zwitterions are formed by heterolytic splitting of an Os-Os bond induced by coordination of MeCN to the Os(CO)(2)(Pr-i-AcPy) moiety in the excited state of the cluster. Time-resolved absorption studies in the microsecond time domain showed that the MeCN-coordinated biradicals convert with a lifetime of 13.7 mus into zwitterions. The unique result of this study is that coordinating solvents such as MeCN may induce both homolytic and heterolytic cleavage of a metal-metal bond in such clusters. (C) 2002 Elsevier Science B.V. All rights reserved.