pH-Responsive Switchable Aggregation Phenomena of Xanthene Dyes Adsorbed on Tungsten(VI) Oxide Colloid Surface

The surface interactions between xanthene (XN) dyes, including rhodamine B, rhodamine 3B, rhodamine 19, rhodamine 6G, rhodamine 110, and rhodamine 123, and tungsten­(VI) oxide (WO₃) colloid particles were investigated. These XN dyes were strongly adsorbed as a monolayer onto the WO₃ surface via the electrostatic interaction between their peripheral cationic amino substituents and negatively surface-charged WO₃ colloid particles, and most of the ones adsorbed eventually formed stable π-stacked dimers. The geometry of dimers formed on the WO₃ colloid surface is discussed on the basis of the molecular exciton theory framework. Cationic XN dyes formed the approximately ideal face-to-face <i>H</i>-dimers on the WO₃ colloid surface, whereas zwitterionic ones had a higher tendency to form the oblique <i>J</i>-dimers. Additionally, we have experimentally demonstrated the possibility of pH-induced switching between <i>H</i>- and <i>J</i>-aggregation modes of the XN dye’s dimers formed on the WO₃ colloid surface. The findings lead to a better understanding of organic dye’s adsorption/aggregation behaviors on the metal oxide surface

Mechanism of Peripheral Substituent Effects on Adsorption–Aggregation Behaviors of Cationic Porphyrin Dyes on Tungsten(VI) Oxide Nanocolloid Particles

The adsorption and aggregation behaviors of the cationic porphyrin derivatives such as 5,10,15,20-tetrakis­(4-pyridyl)­porphyrin [TPyP], 5,10,15,20-tetrakis­(<i>N</i>-methyl-4-pyridyl)­porphyrin [TMPyP], 5,10,15,20-tetrakis­(<i>N</i>-ethyl-4-pyridyl)­porphyrin [TEPyP], and 5,10,15,20-tetrakis­(<i>N</i>-<i>n</i>-propyl-4-pyridyl)­porphyrin [TPPyP] (hereafter called “TPyP derivatives”) in the tungsten­(VI) oxide (WO₃) colloid aqueous solution at weak acidic pH were studied by UV–vis spectroscopy. The TPyP derivatives were strongly adsorbed as monolayer onto the WO₃ surface <i>via</i> the electrostatic interaction between their peripheral cationic substituents and negatively surface-charged WO₃ colloid particles, and most of the ones adsorbed eventually formed <i>J</i>-type dimers aligned in the head-to-tail fashion. These different dimerization states were effectively analyzed by the change of ratios among the intensities of exciton split Soret bands (<i>H</i>- and <i>J</i>-bands). Judging from the exciton coupling theory and adsorption measurements, we concluded that the <i>J</i>-dimer geometry of the TPyP derivatives adsorbed on the WO₃ colloid particle surface is strongly dependent on the presence and difference of peripheral substituents. The results described here indicate a new and promising way of designing surface supramolecular structures combination of two principles, the self-association of organic dyes, and the steric repulsive interaction between the peripheral substituents and the inorganic semiconductor surfaces

Photochromic Properties of Tungsten Oxide/Methylcellulose Composite Film Containing Dispersing Agents

Tungsten oxide-based photochromic films which changed reversibly in air between colorless– transparent in the dark and dark blue under UV irradiation were prepared by using methylcellulose as a film matrix and polyols such as ethylene glycol (EG), propylene glycol (PG), and glycerin (Gly) as dispersing agents. Influence of the dispersing agents and water in the films on the photochromic behavior was systematically studied. Under UV irradiation, absorption bands around 640 and 980 nm increased and the coloring rate was the following order: Gly > EG > PG. An increase in the amounts of dispersing agents or water accelerated the coloring rate. By increasing the water content of the film, a new absorption peak appeared at ca. 775 nm and the Raman spectra indicated a shift of W–O–W stretching vibration to lower wavenumber which was due to the formation of hydrogen bonding. All absorption spectra were fit by three Lorentz functions, whose bands were ascribed to various packing of WO₆ octahedra. After the light was turned off, the formation of W⁵⁺ was stopped and bleaching occurred by the reaction with O₂ in air to recover its original transparent state. We anticipate that the biodegradable photochromic films developed in this study can be applied in recyclable display medium and especially in detachable films for glass windows whose light transmission properties are changed by sunlight, i.e., for usage as an alternative of smart windows without applying voltage

Cyclodextrin-Assisted Surface-Enhanced Photochromic Phenomena of Tungsten(VI) Oxide Nanoparticles for Label-Free Colorimetric Detection of Phenylalanine

Herein are presented the results of experiments designed to evaluate the effectiveness of host–guest interactions in improving the sensitivity of colorimetric detection based on surface-enhanced photochromic phenomena of tungsten(VI) oxide (WO3) nanocolloid particles. The UV-induced photochromic coloration of WO3 nanocolloid particles in the presence of aromatic α-amino acid (AA), l-phenylalanine (Phe) or l-2-phenylglycine (Phg), and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TMβCDx) in an aqueous system was investigated using UV–vis absorption spectrometry. The characteristics of the adsorption modes and configurations of AAs on the WO3 surface have also been identified by using a combination of adsorption isotherm analysis and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). A distinct linear relationship was observed between the concentration of AAs adsorbed on the WO3 nanocolloid particles and the initial photochromic coloration rate in the corresponding UV-irradiated colloidal WO3 in aqueous media, indicating that a simple and sensitive quantification of AAs can be achieved from UV-induced WO3 photochromic coloration without any complicated preprocessing. The proposed colorimetric assay in the Phe/TMβCDx/WO3 ternary aqueous system had a linear range of 1 × 10–8 to 1 × 10–4 mol dm–3 for Phe detection, with a limit of detection of 8.3 × 10–9 mol dm–3. The combined results from UV–vis absorption, ATR-FTIR, and adsorption isotherm experiments conclusively indicated that the TMβCDx-complexed Phe molecules in the Phe/TMβCDx/WO3 ternary aqueous system are preferentially and strongly inner-sphere adsorbed on the WO3 surface, resulting in a more significant surface-enhanced photochromic phenomenon. The findings in this study provided intriguing insights into the design and development of the “label-free” colorimetric assay system based on the surface-enhanced photochromic phenomenon of the WO3 nanocolloid probe