Organoimido-polyoxometalate nonlinear optical chromophores: A structural, spectroscopic, and computational study

Ten organoimido polyoxometalate (POM) based chromophores have been synthesized and studied by hyperRayleigh Scattering (HRS), Stark and Resonance Raman spectroscopies and DFT calculations. HRS β0 values for chromophores with resonance electron donors are significant (up to 139 × 10-30 esu, ca. 5 × that of the DAS+ cation), but systems with no donor, or the –NO2 acceptor show no activity, in some cases despite large DFT-predicted β-values. In active systems with short (phenyl) π-bridges, β0-values comfortably exceed that of the purely organic structural analogue N,N-dimethyl-4-nitroaniline (DMPNA), and intrinsic β-values, β0/N3/2 (N = number of bridge π-electrons) thus appear to break empirical performance limits (β0/N3/2 vs λmax) for planar organic systems. However, β0-values obtained for extended systems with a diphenylacetylene bridge are comparable to or lower than that of their nitro analogue N,N-dimethyl-4-[(4-nitrophenyl)ethynyl]-aniline (DMNPEA). Resonance Raman spectroscopy confirms involvement of the POM in the electronic transitions whether donor groups are present or not, but Stark spectroscopy indicates that in their absence the transitions have little dipolar character (hence NLO inactive), consistent with DFT-calculated frontier orbitals which extend over both POM and organic group. Stark and DFT also suggest that β is enhanced in the short compounds because extension of charge transfer (CT) onto the POM increases excited state dipole moment changes. With extended π-systems this effect does not increase CT distances relative to a –NO2 acceptor, so β0-values do not exceed that of DMNPEA. Overall, our results show that: (i) the organoimido–POM unit is an efficient acceptor for 2nd order NLO, but an ineffective donor; (ii) the nature of electronic transitions in arylimido-POMs is strongly influenced by the substituents of the aryl group; and (iii) organoimido-POMs outperform organic acceptors with short π-bridges, but lose their advantage with extended π-conjugation.