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Triboluminescence of Centrosymmetric Lanthanide ?-Diketonate Complexes with Aggregation-Induced Emission.

ABSTRACT: Triboluminescence (TL) is a form of light emission induced upon mechanical forces on the material. However, our understanding of this phenomenon is still unclear and more examples are therefore needed in order to elucidate its mechanism. In this work, two types of TL complexes, [Eu(pp-dbm-Cl?)?phen] and [Eu(mm-dbm-Cl?)?phen], which also displays aggregation-induced emission (AIE) were synthesized and investigated for its photo-physical and crystal structural properties. These complexes were crystallized in a centro-symmetric space group P2?/n, and remarkably, displayed TL upon grinding that may be due to the presence of extensive ?···?, C-H···? and C-H···Cl-C interactions in the close molecular packing of its structure. This rare example deviates from the widely accepted mechanism of TL, hence widening the scope of our understanding in the area.

SUBMITTER: Wong HY

PROVIDER: S-EPMC6412301 | biostudies-literature | 2019 Feb

REPOSITORIES: biostudies-literature

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Triboluminescence of Centrosymmetric Lanthanide β-Diketonate Complexes with Aggregation-Induced Emission.

Wong Ho-Yin HY Chan Wesley Ting Kwok WTK Law Ga-Lai GL

Molecules (Basel, Switzerland) 20190213 4

Triboluminescence (TL) is a form of light emission induced upon mechanical forces on the material. However, our understanding of this phenomenon is still unclear and more examples are therefore needed in order to elucidate its mechanism. In this work, two types of TL complexes, [Eu(<i>pp</i>-dbm-Cl₂)₃phen] and [Eu(<i>mm</i>-dbm-Cl₂)₃phen], which also displays aggregation-induced emission (AIE) were synthesized and investigated for its photo-physical and crystal structural properties. These compl ...[more]

PMID: 30781832

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Project description:Positional isomers of zinc-nitrobenzoate complexes possessing pyridine -3-(or-4-) carboxamide are used for a comparative theoretical and experimental study to understand their utility as model complexes to understand the role of metal-to-ligand charge transfer in aggregation-induced emission (AIE). Among the five different model zinc complexes, four of them are non-ionic, and one is an ionic complex. The frontier molecular energy levels of different combinations of the positional isomeric complexes and the absorption maximum were ascertained by density functional theory calculations. The PolyQ value obtained from solid samples of each complex is different. Shifts in the emissions to higher wavelengths than the expected emission for the S1 to S0 transition were observed due to aggregations. The highest value of PolyQ among the complexes was 13.56% observed for emission at 439 nm (λex = 350) of the non-ionic complex, namely, (di-aqua)bis(pyridine-3-carboxamide)di(2-nitrobenzoato)zinc(II) monohydrate. Close resemblance in emission lifetime decay profiles of the solid samples of those complexes and the respective solutions of those complexes in dimethyl sulfoxide with or without water showed a common trend, suggesting aggregation-induced emission in each case. Aggregation-induced emission caused by adding water in dimethyl sulfoxide solution of each complex showed an initial increase without a shift in the emission wavelength followed by a quenching with a shift of the respective emission peak to a short wavelength. Dynamic light scattering studies showed an increase in the average particle sizes upon an increase in the concentration of water. This indicated initial participation of water molecules to form aggregates with the complexes, favoring an increase in the AIE intensity. Aggregation of each complex changes with the concentration of water, and an increase in the concentration of water beyond a characteristic limit causes lowering of the emission intensity to the short wavelength.

Project description:Five lanthanide complexes constructed from a stilbene derivative, (E)-N',N'-bis(pyridin-2-ylmethyl)-4-styrylbenzoyl hydrazide (HL), and two β-diketonates (2-thenoyltrifluoroacetonate, tta), with or without a trifluoroacetate anion (CF3CO2 -), namely, [Ln(tta)2(HL) (CF3CO2)] [LnC45H32F9N4O7S2, Ln = La (1), Nd (2), Eu (3), or Gd (4)] and [Yb(tta)2(L)] (YbC43H31F6N4O5S2 (5), L = deprotonated HL), were synthesized and characterized. Crystals of these five complexes were obtained and analyzed by single-crystal X-ray diffraction. These complexes all belonged to the monoclinic P21/c space group. For La3+, Nd3+, Eu3+, and Gd3+, the central lanthanide ion was nine-coordinate with a monocapped twisted square antiprism polyhedron geometry. The central Yb3+ ion of complex 5 was eight-coordinate with a distorted double-capped triangular prism polyhedron geometry. Among the five complexes, trans-to-cis photoisomerization of the stilbene group in gadolinium complex 4 showed the largest quantum yield. Complexes 2, 3, and 4 showed dual luminescence and photoisomerization functions. The luminescence change of complex 3 was reversible upon the trans-to-cis photoisomerization process. The sensitization efficiencies of luminescent europium complex 3 in acetonitrile solutions and in the solid state were 49.9 and 42.6%, respectively. These medium sensitization efficiencies led to the observation of simultaneous photoisomerization and luminescence, which further confirmed our previous report that photoisomerization of the stilbene group within complexes was related to the lanthanide ion energy level and whether a ligand-to-metal center or ligand-to-ligand charge-transfer process was present. In complexes 1-5, in addition to the intramolecular absorption transition of the ligand itself (IL, πHL-πHL * and πtta-πtta*), the presence of a ligand-to-ligand charge-transfer transition between tta and HL (LLCT, πtta-πHL * or πHL-πtta *) indicated whether the triplet-state energy of HL was able to transfer to the excited energy level of the lanthanide ions, leading to different extents of HL photoisomerization. These results provide an important route for the design of new dual-function lanthanide-based optical switching materials.

Dataset Information

Triboluminescence of Centrosymmetric Lanthanide ?-Diketonate Complexes with Aggregation-Induced Emission.

Publications

Triboluminescence of Centrosymmetric Lanthanide β-Diketonate Complexes with Aggregation-Induced Emission.

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