Third order nonlinear optical polarisability induced by real electronic excitations in transition metal diimine and dithiolene complexes

The organo-transition metal complexes (Ni or Pt)bis(3,4-toluenediimine), Ni-bis(3,4-benzophenonediimine) and Ni-bis( 1-phenyl-2-bulyl-1,2-dithiolene), which are generally considered to be good candidates for third order nonlinear optical (NLO) applications, have been examined by sub-picosecond induced transparency measurements. The electronic decay times are around 2 ns for each of the compounds, while the orientational relaxation happens in the 100 ps range. High third order NLO parameters have been previously reported from degenerate four wave mixing experiments, but not confirmed by several other techniques. This disparity in results is clarified here in terms of noninstantaneous contributions from real electronic excitations.