Organoimido-Polyoxometalate Nonlinear Optical Chromophores: A Structural, Spectroscopic, and Computational Study

Abstract

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 β values for chromophores with resonance electron donors are significant (up to 139 × 10 esu, ∼5 times greater than that of the DAS cation), but systems with no donor, or the -NO acceptor show no activity, in some cases, despite large DFT-predicted β-values. In active systems with short (phenyl) π-bridges, β values comfortably exceed that of the purely organic structural analogue N,N-dimethyl-4-nitroaniline (DMPNA), and intrinsic β-values, β/N (where N is the number of bridge π-electrons) thus appear to break empirical performance limits (β/N vs λ) for planar organic systems. However, β 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 acceptor, so β 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.

Crystal structure and photophysical properties of a novel polyoxomolybdate porphyrin

“…photoluminescence diagram of complex I, with the excitation spectrum shown with dashed lines and the emission spectra shown with solid lines.was found previously(Hermosilla-Ibá ñ ez et al, 2019;Lv et al, 2012;Al-Yasari et al, 2017).…”

“…There is no evidence that the [Mo 6 O 19 ] 2Àanions have any effect on the structure of the TCPP unit. To our knowledge, generally, a saddle structure is observed if no metal ions coordinate to TCPP(Diskin-Posner et al, 2000;Al- Yasari et al, 2017). If coordination occurs, a planar structure is observed…”

Crystalline Porous Materials for Nonlinear Optics

“…Al-Yasari et al designed a series of organoimido POM-based dipolar chromophores and the relationship between structure and NLO properties was clarified through calculations and experiments. [193] It was revealed that although imido-POM is an unusually efficient acceptor there is no NLO response in the absence of a resonant electron donors. Compared with organic analogs, however, in the presence of a resonance donor and a short π-bridge, the imino-POM acceptor extends the charge transfer onto the POM, thereby increasing the dipole moment and thus the NLO response.…”

Substitution Effect on the Charge Transfer Processes in Organo-Imido Lindqvist-Polyoxomolybdate

“…The functionalization of this compound with organo-imido fragments has a general formula [Mo 6 O (19−x) (N-R) x ] 2− , where x = 1–6 and N-R is an organic unit derived from a isocyanate or a primary amine molecule bonded to a molybdenum centre [7,8,9]. This functionalization permits to obtain and design new organic-inorganic hybrid systems where the electronic properties can be modulated depending on the nature of the incorporated organic fragment [10].…”

“…Five aliphatically-functionalized hexamolybdates were taken from the literature [21]. Seven aromatic systems with different electron donating and withdrawing abilities were also taken from the literature [10,22,23]. For the spectroscopic study, some molecules being the most representative systems were used.…”