ic7b00708_si_001.pdf (4.75 MB)
Organoimido-Polyoxometalate Nonlinear Optical Chromophores: A Structural, Spectroscopic, and Computational Study
journal contribution
posted on 2017-08-14, 21:31 authored by Ahmed Al-Yasari, Nick Van Steerteghem, Hayleigh Kearns, Hani El Moll, Karen Faulds, Joseph A. Wright, Bruce S. Brunschwig, Koen Clays, John FieldenTen
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 −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 (where N is the 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 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 −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 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.
History
Usage metrics
Categories
Keywords
π- bridgesDMPNA2 acceptor showDFT-calculated frontier orbitalsDMNPEANLOHRS β 0 valuesOrganoimido-Polyoxometalate Nonlinear Optical Chromophorestransitionincrease CT distancesResonance Raman spectroscopyStarkβ 0 valuesDFT-predicted β- valuesresonance electron donorsbridge π- electronsResonance Raman spectroscopiesDASPOM increases changes
Licence
Exports
RefWorks
BibTeX
Ref. manager
Endnote
DataCite
NLM
DC