Multifunctional DyIII Enantiomeric Pairs Showing Enhanced Photoluminescences and Third-Harmonic Generation Responses through the Coordination Role of Homochiral Tridentate N,N,N-Pincer Ligands

A pair of ionic 1D Cu(II) chain enantiomers simultaneously displaying large second- and third-harmonic generation responses

By employing enantiomerically pure mono-bidentate N-donors (LR/LS) as chiral bridging ligands to react with Cu(ClO4)2(H2O)6 in CH3CN-DMF mixed solvent, respectively, a pair of ionic one-dimensional (1D) Cu(II) chain enantiomers formulated as {[CuLR(CH3CN)(DMF)H2O](ClO4)2}n/{[CuLS(CH3CN)(DMF)H2O](ClO4)2}n (D-1/L-1) were isolated and structurally characterized, where LR/LS = (-)/(+)-4,5-pinenepyridyl-2-pyrazine. They crystallize in the noncentrosymmetric (NCS) P212121 space group of an orthorhombic system due to the introduction of chiral LR/LS, and the ClO4- groups as counteranions reside in crystal lattices, thus leading to charge separation with large dipole moments in their molecular structures. Based on crystal samples, investigation on their nonlinear optical (NLO) behaviors showed that D-1 and L-1 display simultaneously much larger second- and third-harmonic generation (SHG and THG) responses than their analogues based on the same chiral N-donors (LR/LS) and Cu(NO3)2(H2O)3 with NO3- acting as the coordination group to bind Cu(II) ions. The SHG intensities of D-1/L-1 are 0.62/0.60 × KDP (KH2PO4), and THG intensities of D-1/L-1 are 238/228 × α-SiO2. Our finding indicates that coordination polymers (CPs) with charge separation and NCS structures, i.e., ionic CPs with NCS arrangements are the ideal NLO crystalline materials for the simultaneous observation of large SHG and THG responses, thus providing a new approach to obtain NLO-active CP crystalline materials with high-performance SHG and THG responses.

Influence of electron donor and acceptor substituents on tuning the nonlinear optical properties of zinc phthalocyanine derivatives

Four phthalocyanine derivatives with different electron donor and acceptor substituents (B1, B2, B3, and B4) were synthesized by a solid-phase melting method. The influence of substituent type on the nonlinear optical properties of the materials was investigated in detail. In the case of similar conjugated structures, B3, which has amino electron-donor groups, presents high intramolecular charge transfer, a low energy gap (2.05 eV), and good nonlinear optical properties. Compared with B3, B4 has a larger π-conjugated structure and its energy gap is 0.04 eV smaller. Moreover, B4 has stronger reverse saturation absorption (7 × 10-12 m/W) and optical limiting performance. The four phthalocyanine derivatives exhibit third harmonic generation (THG) characteristics. Furthermore, the third harmonic strengths of B1, B2, B3, and B4 are 4 times, 9 times, 11 times, and 21 times that of SiO2, respectively. So, B4 has the best application potential in laser protection and frequency conversion.

Two Chiral YbIII Enantiomeric Pairs with Distinct Enantiomerically Pure N-Donor Ligands Presenting Significant Differences in Photoluminescence, Circularly Polarized Luminescence, and Second-Harmonic Generation

Using enantiomerically pure bidentate and tridentate N-donor ligands (¹L_R/¹L_S and ²L_R/²L_S) to replace two coordinated H₂O molecules of Yb(tta)₃(H₂O)₂, respectively, two eight- and nine-coordinated Yb^III enantiomeric pairs, namely, Yb(tta)₃¹L_R/Yb(tta)₃¹L_S (Yb-R-1/Yb-S-1) and [Yb(tta)₃²L_R]·CH₃CN/[Yb(tta)₃²L_S]·CH₃CN (Yb-R-2/Yb-S-2), were isolated, in which Htta = 2-thenoyltrifluoroacetone, ¹L_R/¹L_S = (-)/(+)-4,5-pinene-2,2'-bipyridine, and ²L_R/²L_S = (-)/(+)-2,6-bis(4',5'-pinene-2'-pyridyl)pyridine. Interestingly, they not only present distinct degrees of chirality but also show large differences in near-infrared (NIR) photoluminescence (PL), circularly polarized luminescence (CPL), and second-harmonic generation (SHG). Eight-coordinated Yb-R-1 with an asymmetric bidentate ¹L_R ligand has a high NIR-PL quantum yield (1.26%) and a long decay lifetime (20 μs) at room temperature, being more than two times those (0.48%, 8 μs) of nine-coordinated Yb-R-2 with a C₂-symmetric tridentate ²L_R ligand. In addition, Yb-R-1 displays an efficient CPL with a luminescence dissymmetry factor g_lum = 0.077, being 4 × Yb-R-2 (0.018). In particular, Yb-R-1 presents a strong SHG response (0.8 × KDP), which is 8 × Yb-R-2 (0.1 × KDP). More remarkably, the precursor Yb(tta)₃(H₂O)₂ exhibits a strong third-harmonic generation (THG) response (41 × α-SiO₂), while the introduction of chiral N-donors results in the switching of THG to SHG. Our interesting findings provide new insights into both the functional regulation and switching in multifunctional lanthanide molecular materials.

Two temperature-induced 1D CuII chain enantiomeric pairs showing different magnetic properties and nonlinear optical responses

At different reaction temperatures, using Cu(NO₃)₂·3H₂O to react with enantiomerically pure N-donor ligands (L_S/L_R), respectively, two pairs of chiral one-dimensional (1D) Cu^II chain enantiomers formulated as [Cu(μ₂-NO₃)(NO₃)(L_S)]_n/[Cu(μ₂-NO₃)(NO₃)(L_R)]_n (S-1-Cu/R-1-Cu, formed at 40 °C with an NO₃^- group as a sole bridging ligand) and [Cu(μ₂-L_S)(NO₃)₂]_n/[Cu(μ₂-L_R)(NO₃)₂]_n (S-2-Cu/R-2-Cu, formed at 25 °C with L_S or L_R as a bridging ligand) were prepared, where L_S/L_R = (+)/(-)-4,5-pinenepyridyl-2-pyrazine. Interestingly, such a disparity in bridging ligands leads not only to their distinct structural features but also to their completely different magnetic couplings together with a large difference in their nonlinear optical responses. S-1-Cu with a 1D helical structure shows weak ferromagnetic coupling between Cu^II ions, while S-2-Cu with a 1D stairway-like structure presents weak antiferromagnetic coupling. In particular, they simultaneously possess both second- and third-harmonic generation (SHG and THG) responses in one molecule with large strength differences. More remarkably, S-1-Cu exhibits a very large THG response (162 × α-SiO₂), which is 22.5 times that of S-2-Cu, and the SHG strength of S-1-Cu is more than 3 times that of S-2-Cu. This work demonstrates that reaction temperature has a great impact on the self-assembled structures of coordination polymers and subsequently results in their large performance differences.

Two-dimensional dysprosium(III) coordination polymer: Structure, single-molecule magnetic behavior, proton conduction, and luminescence

A new dysprosium (III) coordination polymer [Dy(Hm-dobdc) (H₂O)₂]·H₂O (Dy-CP), was hydrothermal synthesized based on 4,6-dioxido-1,3-benzenedicarboxylate (H₄m-dobdc) ligand containing carboxyl and phenolic hydroxyl groups. The Dy(III) center adopts an octa-coordinated [DyO₈] geometry, which can be described as a twisted square antiprism (D_4d symmetry). Neighboring Dy(III) ions are interconnected by deprotonated Hm-dobdc³⁻ ligand to form the two-dimensional infinite layers, which are further linked to generate three-dimensional structure through abundant hydrogen bonds mediated primarily by coordinated and lattice H₂O molecules. Magnetic studies demonstrates that Dy-CP shows the field-induced slow relaxation of magnetization and the energy barrier U_eff/k_B and relaxation time τ₀ are 35.3 K and 1.31 × 10^–6 s, respectively. Following the vehicular mechanism, Dy-CP displays proton conductivity with σ equal to 7.77 × 10^–8 S cm⁻¹ at 353 K and 30%RH. Moreover, luminescence spectra reveal that H₄m-dobdc can sensitize characteristic luminescence of Dy(III) ion. Herein, good magnetism, proton conduction, and luminescence are simultaneously achieved, and thus, Dy-CP is a potential multifunctional coordination polymer material.

Modulating Two Pairs of Chiral DyIII Enantiomers by Distinct β-Diketone Ligands to Show Giant Differences in Single-Ion Magnet Performance and Nonlinear Optical Response

Using Dy(dbm)₃(H₂O) and Dy(btfa)₃(H₂O)₂ to react with enantiopure N-donors, (-)/(+)-4,5-pinenepyridyl-2-pyrazine (L_R/L_S), respectively, two pairs of chiral Dy^III enantiomers, Dy(dbm)₃L_R/Dy(dbm)₃L_S (R-1-Dy/S-1-Dy) and Dy(btfa)₃L_R/Dy(btfa)₃L_S (R-2-Dy/S-2-Dy) were obtained, wherein one of the benzene rings of dbm^- (dibenzoylmethanate) in R-1-Dy/S-1-Dy is displaced by the -CF₃ group of btfa^- (4,4,4-trifluoro-1-phenyl-1,3-butanedionate) in R-2-Dy/S-2-Dy. Interestingly, this substitution results not only in giant differences in their single-ion magnetic (SIM) performances but also in their completely different nonlinear optical (NLO) responses. R-1-Dy presents a large effective energy barrier (U_eff = 265.47 K) under zero applied field, being more than 4 × R-2-Dy (61.40 K). The discrepancy on their magnetic performances has been further elucidated by ab initio calculations. Meanwhile, R-1-Dy/S-1-Dy display the strongest third-harmonic generation responses (35/33 × α-SiO₂) among the known lanthanide NLO-active coordination compounds (CCs). On the contrary, R-2-Dy/S-2-Dy exhibit moderate second-harmonic generation responses (0.65/0.70 × KDP). These results not only give the first example of the CCs with both SMM/SIM behavior and a THG response but also provide an efficient strategy for achieving the function regulation and switch in multifunctional CCs.

Distinct nonlinear optical responses in three pairs of 2D homochiral Ag(i) enantiomers modulated by dicarboxylic acid ligands

Based on three pairs of 2D homochiral Ag(i) enantiomers, we demonstrate that both the symmetry and degree of π-conjugation of the ligands have a significant effect on the type and intensity of their NLO responses.