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, β0,zzz = 318 ×
10–30 esu; −NTol2, β0,zzz = 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 (C2v)
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.