Donor and Geometry Optimization: Fresh Perspectives for the Design of Polyoxometalate Charge Transfer Chromophores

Three linear, dipolar arylimido-polyoxometalate (POM) and one 2-dimensional bis-functionalized arylimido-polyoxometalate charge transfer chromophore, with diphenylacetylene bridges, have been synthesized and studied by spectroelectrochemistry, hyper-Rayleigh scattering (HRS), and DFT/TD-DFT calculations. The linear systems show that with julolidinyl (Jd) and -NTol₂ donor groups, the alkyne bridge yields high second-order nonlinear optical (NLO) coefficients β (Jd, β_0, = 318 × 10^-30 esu; -NTol₂, β_0, = 222 × 10^-30 esu), indeed the Jd compound gives the highest NLO activity of any organoimido-POM to date with minimal decrease in transparency. The bis-functionalized 2D (C_2v) POM derivative showed increased activity over its monofunctionalized analogue with no decrease in transparency, although the NLO response was only minimally two dimensional. Spectroelectrochemistry and TD-DFT calculations showed switchable linear optical responses for the monofunctionalized derivatives due to the weakened charge transfer character of the electronic transitions in the reduced state, while TD-DFT also indicated potential for switched NLO responses. These have been demonstrated by electrochemistry-HRS for the Jd compound, but cyclability is limited by relatively poor stability in the reduced state. IR and CV studies for these sterically protected arylimido polyoxometalates indicate that decomposition proceeds via a breakdown of the {Mo₆} cluster in the reduced state, rather than simple solvolysis of the Mo≡N bond.