Knowledge Bank

Photoexcitation of hexabromoplatinate and hexabromoosmate dianions in CT and ligand field states results in remarkably fast formation (sub 100 fs) of coherently excited photoproducts assigned to $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}PtBr5-$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}OsBr5-$. This is explained by the fact that one of the lowest triplet excited states in both $PtBr62-$ and $OsBr62-$ is repulsive. Surprisingly, the damping time and amplitude of the observed coherent oscillations in the pentabromoosmate (209 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$) and pentabromoplatinate (112 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$) species is quite different. The shorter damping time (0.4 ps) of $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}PtBr5-$ in comparison with the damping time (1 ps) of $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}OsBr5-$ in aqueous solution is related to faster dephasing in $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}PtBr5-$. The large-amplitude bending oscillations in pentabromoplatinate are tentatively assigned to the passage of the wavepacket through the Janh-Teller conical intersection(s), whereas in pentabromoosmate, the umbrella type of oscillations appears to be due to the coupling to the Os-Br symmetric stretching mode in the parent dianion. The coherent oscillations in both photoproducts show no dependence on solvent. The results are supported by DFT, TD-DFT and CASSCF/CASPT2 calculations of electronic structures, vertical electronic transitions and harmonic frequencies of initial dianions and photoproducts.