Enhancement of the third-order susceptibility of cis-Mo(CO)₄(PPh₃)₂ solution by oxygen in different solvents

Detailed degenerate four wave mixing (DFWM) studies of one molybdenum complex, cis-Mo(CO)₄(PPh₃)₂ in dichloromethane (CH₂Cl₂) and tetrahydrofuran (THF) are reported in this paper. Upon exposure to air/oxygen, the χ⁽³⁾ values of CH₂Cl₂ and THF solutions of this complex increase dramatically with time. However, when excess free ligand is added to CH₂Cl₂ solution of the complex, the increase in χ⁽³⁾ value is stopped. Hence, it is possible to obtain a CH₂Cl₂ solution with high χ⁽³⁾ value by first oxidizing the solution and then adding excess ligand to stabilize the solution. For THF solution, probably due to the different mechanism causing the increase of χ⁽³⁾ values, the increase in χ⁽³⁾ values doesn't stop even though excess free ligand is added. Further, the increase in the χ⁽³⁾ values of both the CH₂Cl₂ and THF solutions parallels the increase in the linear absorption but it is unclear whether the increased linear absorption plays any role in the increase in the χ⁽³⁾ value of the solution. This study also shows how degenerate four wave mixing can be used as a very sensitive technique to detect the chemical change in a solution when the third order optical nonlinearities of the reactant and product are very different.