Kadem Meral

From Dark to Light to Fluorescence Resonance Energy Transfer (FRET): Polarity‐Sensitive Aggregation‐Induced Emission (AIE)‐Active Tetraphenylethene‐Fused BODIPY Dyes with a Very Large Pseudo‐Stokes Shift

The work presented herein is devoted to the fabrication of large Stokes shift dyes in both organic and aqueous media by combining dark resonance energy transfer (DRET) and fluorescence resonance energy transfer (FRET) in one donor-acceptor system. In this respect, a series of donor-acceptor architectures of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes substituted by one, two, or three tetraphenylethene (TPE) luminogens were designed and synthesised. The photophysical properties of these three chromophore systems were studied to provide insight into the nature of donor-acceptor interactions in both THF and aqueous media. Because the generation of emissive TPE donor(s) is strongly polarity dependent, due to its aggregation-induced emission (AIE) feature, one might expect the formation of appreciable fluorescence emission intensity with a very large pseudo-Stokes shift in aqueous media when considering FRET process. Interestingly, similar results were also recorded in THF for the chromophore systems, although the TPE fragment(s) of the dyes are non-emissive. The explanation for this photophysical behaviour lies in the DRET. This is the first report on combining two energy-transfer processes, namely, FRET and DRET, in one polarity-sensitive donor-acceptor pair system. The accuracy of the dark-emissive donor property of the TPE luminogen is also presented for the first time as a new feature for AIE phenomena.

Promotion by phosphate of Fe(III)- and Cu(II)-catalyzed autoxidation of fructose.

Although the oxidative destruction of glucose and fructose has been studied by several investigators over the past century, the mechanism by which phosphate promotes these oxidation reactions is not known. A wide range of oxidation products have been used to monitor the oxidation of sugars and free radicals have been shown to be involved. The influence of phosphate concentration on the rate of production of free radicals and several sugar oxidation products has been studied. It was found that fructose is much more susceptible to autoxidation than glucose, galactose, or sucrose. The promotion of sugar oxidation by phosphate was found to be iron dependent. Addition of the iron chelators, diethylenetriaminepentaacetic acid (DTPA) and desferrioxamine completely suppressed the oxidation reactions, even at high concentrations of phosphate. Formaldehyde was positively identified as a product of fructose oxidation by HPLC analysis of its acetylacetone adduct. A mechanism is proposed in which phosphate cleaves the oxo bridges of the iron(III)-fructose complex, based on UV spectral analysis and magnetic susceptibility measurements, and thereby catalyzes the autoxidation of fructose.

Novel phenomena for aggregation induced emission enhancement: highly fluorescent hydrophobic TPE-BODIPY couples in both organic and aqueous media

A series of easily prepared BODIPY (BOD) derivatives bearing tetraphenylethene (TPE) groups at the 8-position (TPE-BOD), 2,6-positions ((TPE)2-BOD), and 2,6,8-positions ((TPE)3-BOD) are presented. Although these fluorophores are highly emissive in THF, they are weakly fluorescent in the 60% water–THF solution. (TPE)2-BOD and (TPE)3-BOD compounds are able to become highly emissive in solutions with higher water content due to the amorphous aggregate formation, which is associated with dynamic crystallization. In contrast, highly intense emission of (TPE)BOD in THF is smoothly quenched on decreasing the solvation. The role of TPE units on AIEE activity has been investigated with fluorescence spectroscopic studies by means of comparison experiments performed on a BODIPY derivative carrying only phenyl substituents at the 2 and 6 positions. Aggregate formation in these derivatives was also demonstrated by LB-film studies and theoretical calculations supported the AIEE effect in longer wavelengths due to an increased conjugation. SEM was used to examine the morphological structure of organized microcrystals in THF–water mixtures of variable composition. Aggregate dimensions, determined from DLS analyses, are also consistent with results reported for many well-known AIEE active chromophores. To imagine the nature of the amorphous aggregates in solutions confocal fluorescence microscopy was used. In the light of these results, (TPE)2BOD and (TPE)3BOD are presented as novel long-wavelength AIEE active compounds being highly emissive in both polar and nonpolar solvents, functioning according to transition from their highly soluble states to amorphous aggregate states.

The effect of poly(vinyl alcohol) on the photophysical properties of pyronin dyes in aqueous solution: A spectroscopic study

The photophysical properties of pyronin B (PyB) and pyronin Y (PyY) in water and poly(vinyl alcohol) (PVA) aqueous solutions were studied by using absorption, steady-state fluorescence and time-resolved fluorescence spectroscopy techniques at room temperature. The spectroscopic and photophysical properties of pyronin dyes in the concentrated PVA aqueous solution were different than those found in water. The aggregation of the pyronin dyes in the concentrated PVA aqueous solution was prevented with ease while the dye aggregation was generally formed in water with high dye concentration. The decrease in the aggregation tendency of pyronin dyes in the concentrated PVA aqueous solution caused an increase in radiative transitions. The addition of PVA into the aqueous solution induced the enhancement in the fluorescence intensity of the dyes compared to those in water. As a result, the quantum yields of the dyes were improved by the addition of PVA at high loading. The time-resolved fluorescence study revealed that the fluorescence decay of dyes in all solutions were found to be single-exponential and the fluorescence lifetime of pyronin dyes in the concentrated PVA aqueous solution were also higher than those found in water.

Fabrication of morphology controlled graphene oxide-dye composite films at the air–water interface

This study presents a facile route for preparing two-dimensional (2D) graphene oxide-based composite films using the Langmuir–Blodgett (LB) method. Graphene oxide-dye composites are formed with the incorporation of GO sheets and dye molecules (pyronin Y) at the air–water interface, since GO sheets are perfectly capable of forming a floating-layer at the air–water interface. Graphene oxide–pyronin Y (PyY@GO) composites are successfully assembled as highly ordered thin films over a glass substrate using the LB method. The surface pressure–area (π–A) isotherm studies reveal that pristine GO sheets and PyY@GO composites are capable of forming stable Langmuir films at the air–water interface. The limiting areas of pristine GO sheets and the composites are determined, and the area of pristine GO sheets are found to be larger than that found on PyY@GO composites. This observation clearly indicates the interaction of GO sheets with PyY molecules at the air–water interface, resulting in the formation of PyY@GO composites. Next, the floating layers of GO sheets and PyY@GO composites are successfully transferred to the glass substrate, using the LB method, as mono- and multi-layer films. Their LB films are characterized by SEM and UV-vis spectroscopy. SEM images reveal that the density of the GO sheets and PyY@GO composites on the substrate are tuned by changing the film deposition pressure and the number of layers.

Polyelectrolytes-assisted layer-by-layer assemblies of graphene oxide and dye on glass substrate

Pyronin Y (PyY) and graphene oxide (GO) were assembled on a glass substrate by the electrostatic layer-by-layer (LbL) method with the assistance of polyelectrolytes. Firstly, the glass surface was modified with the successive adsorption of positively charged PEI (polyethylenimine) and negatively charged PSS (poly(sodium-p-styrenesulfonate)) to enhance the loading of PyY molecules and GO sheets. Secondly, PyY molecules and GO sheets were loaded on the glass substrate depending on their charges after the successive adsorption of polyelectrolyte layers via a procedure of {PEI/GO/PEI/PSS/PyY/PSS}n. Thus, the LbL thin films containing PyY molecules and GO sheets, which remained stable for months, were homogeneously coated on the glass substrate. The photophysical and morphological features of the LbL films were examined. The spectroscopic data of the LbL films based on PyY revealed that H-aggregate and monomers structures of the dye molecules in the LbL films were formed by a function of the number of layers. Furthermore, the effect of immersion time on the molecular interaction of PyY molecules with the other components were studied in detail. SEM (scanning electron microscope) was used to examine the morphologies of as-prepared LbL thin films. SEM images revealed that the morphologies of LbL thin films were changed by the immersion time and the number of layers.

The investigation of the electrical properties of Fe3O4/n-Si heterojunctions in a wide temperature range

Monodisperse 8nm Fe3O4 nanoparticles (NPs) were synthesized by the thermal decomposition of iron(III) acetylacetonate in oleylamine and then were deposited onto n-type silicon wafer having the Al ohmic contact. Next, the morphology of the Fe3O4 NPs were characterized by using TEM and XRD. The optical properties of Fe3O4 NPs film was studied by UV-Vis spectroscopoy and its band gap was calculated to be 2.16eV. Au circle contacts with 7.85×10(-3)cm(2) area were provided on the Fe3O4 film via evaporation at 10(-5)Torr and the Au/Fe3O4 NPs/n-Si/Al heterojunction device were fabricated. The temperature-dependent junction parameters of Au/Fe3O4/n-Si/Al device including ideality factor, barrier height and series resistance were calculated by using the I-V characteristics in a wide temperature range of 40-300K. The results revealed that the ideality factor and series resistance increased by the decreasing temperature while the barrier height decreases. The Richardson constant of Au/Fe3O4/n-Si/Al device was calculated to be 2.17A/K(2)cm(2) from the I-V characteristics. The temperature dependence of Au/Fe3O4/n-Si/Al heterojunction device showed a double Gaussian distribution, which is caused by the inhomogeneities characteristics of Fe3O4/n-Si heterojunction.

Synthesis, characterization and diode application of poly(4-(1-(2-phenylhydrazono)ethyl)phenol)

Poly(phenoxy-ketimine)s, which have a conjugated bond system, are a family of polyphenols that can be prepared by oxidative polycondensation reaction from a monomer containing both hydroxyl (–OH) and ketimine side groups. In this context, a novel poly(phenoxy-ketimine), poly(4-(1-(2-phenylhydrazono)ethyl)phenol), abbreviated as poly(4-PHEP), which includes a system of conjugated bonds and active hydroxyl groups, was synthesized and spectroscopically characterized by elemental analysis, FTIR (Fourier transform infrared) spectroscopy, NMR (nuclear magnetic resonance) absorption and fluorescence spectroscopy. The optical band gap of the polymer is determined to be 3.05 eV. In addition, electrical conductivity, solubility and thermal properties of poly(4-PHEP) were determined. Its electrical conductivity was found to be ∼8.55 × 10−2 S cm−1, which is the typical level for semiconductors. Afterwards, a polymer/p-type Si junction device was fabricated, and its rectifying behaviours, depending on some parameters including ideality factor, barrier height and series resistance values at room temperature, were examined by current–voltage (I–V) and capacitance–voltage (C–V) measurements. Consequently, these interesting properties of the polymer reveal that it has potentially beneficial applications in various fields of electronics as semiconducting materials.

Synthesis of a Novel m-Substituted Poly(phenoxy-imine) and Investigation of its Fluorescence and Some Properties

Oxidative polycondensation of 3-((2-phenylhydrazono)methyl)phenol (3-PHMP), a new m-substituted poly(phenoxy-imine), was studied using oxidants such as sodium hypochlorite, air (O2) and hydrogen peroxide in an aqueous alkaline medium under various polymerization conditions. The macromolecular structure and optical properties of the polymer were characterized with elemental analysis, Size Exclusion Chromatography (SEC), Fourier Transform Infrared (FTIR), Nuclear Magnetic Resonance (NMR), absorption and fluorescence spectroscopy techniques. As a result of fluorescence measurement, the fluorescence lifetime of poly(3-PHMP) in DMF was calculated as 2.88 ns (χ2= 1.12). An electrochemical property the monomer and polymer were also studied using Cyclic Voltammetry (CV) technology. According to the CV measurements, the electrochemical band gaps (Eg′) of 3-PHMP and poly(3-PHMP) were found to be 2.64 and 1.94 eV, respectively. Electrical conductivity of the polymer was measured by the four-point probe technique. The electrical conductivity of poly(3-PHMP) was found to be ∼3.2 × 10−2S/cm. Thermo Gravimetric Analysis (TGA) revealed poly(3-PHMP) to be stable against thermo-oxidative decomposition. In addition, the in vitro antimicrobial activities of the synthesized compounds were tested on various microorganisms.

Enhancement of photoluminescence lifetime of ZnO nanorods making use of thiourea

We have investigated correlation of photoluminescence lifetime between zinc oxide (ZnO) nanorods and thiourea-doped ZnO nanorods (tu: CH4N2S). Aqueous solutions of ZnO nanorods were deposited on glass substrate by using pneumatic spray pyrolysis technique. The as-prepared specimens were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and time-resolved photoluminescence spectroscopy (TRPL). The photoluminescence lifetime of ZnO nanorods and ZnO nanorods containing thiourea was determined as τ = 1.56±0.05 ns (χ2 = 0.9) and τ = 2.12±0.03 ns (χ2 = 1.0), respectively. The calculated lifetime values of ZnO nanorods revealed that the presence of thiourea in ZnO nanorods resulted in increasing the exciton lifetime. In addition to the optical quality of ZnO nanorods, their exciton lifetime is comparable to the longest lifetimes reported for ZnO nanorods. The structural improvement of ZnO nanorods, containing thiourea, was also elucidated by taking their SEM images which show the thinner and longer ZnO nanorods compared to those without thiourea.