Organoimido-Polyoxometalate Nonlinear Optical Chromophores: A Structural, Spectroscopic, and Computational Study
Ten organoimido polyoxometalate (POM)-based chromophores have been synthesized and studied by hyper-Rayleigh scattering (HRS), Stark and Resonance Raman spectroscopies, and density functional theory (DFT) calculations. HRS β0 values for chromophores with resonance electron donors are significant (up to 139 × 10^(–30) esu, ∼5 times greater than that of the DAS+cation), but systems with no donor, or the –NO_2 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/N^(3/2) (where N is the number of bridge π-electrons) thus appear to break empirical performance limits (β0/N^(3/2) vs λ_(max)) 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 the 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 the extension of charge transfer (CT) onto the POM increases changes in the excited-state dipole moment. With extended π-systems, this effect does not increase CT distances, relative to a –NO_2 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 second-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.
© 2017 American Chemical Society. ACS AuthorChoice - This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. Received: March 22, 2017; Published: August 15, 2017. We thank the UK EPSRC National Crystallography Service in Southampton for X-ray data for 5, 7, and 10, the UK EPSRC National Mass Spectrometry Facility (Swansea) for MS, and the UK National Chemical Computing Service for access to ADF on Slater. This work was supported by the Iraqi Government (HCED scholarship to A.A.Y.), Royal Society of Chemistry, EPSRC (EP/M00452X/1), EU FP7 (Marie Curie IOF POMHYDCAT Contract No. 254339 to J.F.) and Fonds voor Wetenschappelijk Onderzoek-Vlaanderen (FWO-V, Ph.D. fellowship for N.V.S.). K.F. and H.K. would like to thank the BBSRC (No. BB/M018652/1) for funding. B.S.B. acknowledges support from the Beckman Institute of the California Institute of Technology. Stark data were collected and analyzed at the Molecular Materials Resource Center of the Beckman Institute of the California Institute of Technology. Accession Codes: CCDC 1553805–1553810 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by E-mailing firstname.lastname@example.org, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033. The authors declare no competing financial interest.
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