Luminescent Molecules/LDHS Composite Ultrathin Films:Desiagn, Fabrication And Their VOCS Sensing Properties

Layered double hydroxides (LDHs) are an important type of inorganic layered materials and its unique two-dimensional confined environment can improve the stability of the interlayer guest molecules, numerous composite functional film materials researches based on LDHs are increasing up to now. Light-emitting organic materials are widely used in display and lighting fields because of their outstanding luminescence properties, but they are prone to aggregate and cause serious quenching effect. In our work, LDHs nanosheets were selected as the host layer, neutral organic molecules were selected as the interlayer guest, and a series of neutral organic molecules/LDHs nanocomposite luminescent films were prepared successfully by the layer-by-layer (LBL) assembly method. Neutral molecules included blue emitting polymer poly(vinyl carbazole) (PVK), green emitting metal complexes tris-(8-hydroxy-quinoline) aluminum (Alq3), tris[2-(4,6-difluorophenyl) pyridinato-C2, N] iridium (III) (Ir(F2ppy)3) and red laser dye 4-(dicyanomethylene)-2-methyl-6-(4-dimethylamino)-4H-pyran (DCM). Phosphorescence and fluorescence films were obtained by optimizing the preparation conditions. Furthermore, these composite ultrathin films showed fluorescent response performance to the volatile organic compounds (VOCs) and exhibited good reversibility and stability, due to the unique luminescent properties and two-dimensional FRET process of the ultrathin films. Further studies of the possible response mechanism were explored. Our research work was a beneficial exploration for the preparation and application of organic-inorganic composite materials. The main contents of this paper are as follows:1. Phosphorescent films Ir(F2ppy)3@PVK/LDH were fabricated by using the LBL assembly method. Small-angle XRD, SEM, AFM results demonstrated that the composite films were uniform and long-range ordered due to the flexible polymer chains and hydrogen-bonding interaction between the polymer and LDHs. Under the PVK maximal excitation (293 nm), the photoluminescence spectra of the composite ultrathin films showed the emission of Ir(F2ppy)3 while PVK was almost quenched, demonstrated the FRET process occurred within the interlayers of LDHs nanosheets with high efficiency (0.892). Moreover, it was found that the phosphorescent films showed a reversible two-state photoemission switching (ON for cyan emission of Ir(F2ppy)3 vs. OFF for blue emission of PVK) in the absence or presence of VOC vapors, which meaned they can be employed as a phosphorescent sensor for common VOCs.2. PS-PAA@DCM/LDH luminous ultrathin films with solvatochromism were fabricated by the LBL assembly method based on block copolymer micelle poly(styrene-b-acrylic acid) (PS-b-PAA) incorporated neutral molecules DCM and LDHs nanosheets. The DCM molecules encapsulated in the micelle hydrophobic core and 2D interlayer of LDHs exhibited non-aggregating fluorescence behavior, and acquired prolonged fluorescence lifetime and increased luminescence anisotropy value. The UTFs maintained the photoluminescence properties of DCM with well-defined orange fluorescence originated from the locally excited (LE) state.’Furthermore, these ultrathin films demonstrated typical solvatochromism fluorescent properties of DCM, with the PL peak strongly dependent on solvent polarity and can be regulated from orange to red (560-600 nm) by exposing in polar VOCs, resulting from the DCM twisted intramolecular charge transfer (TICT) character. And these ultrathin films showed fast and sensitive fluorescence solvatochromic response to polar VOCs. Therefore, this work opens up the avenue for potential applications of DCM in the VOCs polarity sensors.3. Three kinds of FRET luminescent hybrid films (PVK@DCM/LDH, PVK@Alq3@DCM/LDH, PVK@Ir(F2ppy)3@DCM/ LDH) were fabricated based on LDHs and several neutral materials by hydrogen-bonding interaction LBL assembly method. Neutral molecules included blue emitting PVK, green emitting Alq3 and Ir(F2ppy)3, and red emitting DCM. Compared to the mixed solution of organic molecules, the assembled ultrathin film significantly inhibited the aggregation and quenching of organic molecule, which can be ascribed to the effectively dispersing in the confined interlayer of LDHs, thus improved the luminous efficiency of the composite film. A highly fluorescence film with large Stokes shifts were obtained with the help of cascade FRET process. The phosphorescent Ir(F2ppy)3 doped ultrathin film showed extended DCM fluorescence lifetime (-4-fold enhancement compared with DCM solution state) due to the triplet-singlet exciton excitated energy transfer. Moreover, the highly photoluminescent PVK@Ir(F2ppy)3@DCM/LDH films showed selective and reversible fluorescence signal towards VOCs based on interfering the 2D cascade FRET process, implying its potential application to detect the leakage of VOCs selectively, such as methanol, toluene, DMSO and nitrobenzene.