The Stability of Cyclodextrin Complexes in Solution.

Cyanines during the 1990s: A Review

The formation of semiconductor composites comprising multicomponent or multiphase heterojunctions is a very effective strategy to design highly active photocatalyst systems. This review summarizes the recent strategies to develop such composites, and highlights the most recent developments in the fi eld. After a general introduction into the different strategies to improve photocatalytic activity through formation of heterojunctions, the three different types of heterojunctions are introduced in detail, followed by a historical introduction to semiconductor heterojunction systems and a thorough literature overview. Special chapters describe the highly-investigated carbon nitride heterojunctions as well as very recent developments in terms of multiphase heterojunction formation, including the latest insights into the anatase-rutile system. When carefully designed, semiconductor composites comprising two or three different materials or phases very effectively facilitate charge separation and charge carrier transfer, substantially improving photocatalytic and photoelectrochemical effi ciency.

https://faculty.missouri.edu/~glaserr/3700s15/Marschall_Strategies_adfm201303214.pdf

Semiconductor Composites: Strategies for Enhancing Charge Carrier Separation to Improve Photocatalytic Activity

In semiconductor particles of nanometer size, a gradual transition from solid-state to molecular structure occurs as the particle size decreases. Consequently, a splitting of the energy bands into discrete, quantized levels occurs. Particles that exhibit these quantization effects are often called “Q-particles” or, generally, quantized material. The optical, electronic and catalytic properties of Q-particles drastically differ from those of the corresponding macrocrystalline substance. The band gap, a substance-specific quantity in macrocrystalline materials, increases by several electron volts in Q-particles with decreasing particle size. In Q-particles there are approximately as many molecules on the surface as in the interior of the particle. Therefore, the nature of the surface as well as the particle size is also largely responsible for the physico-chemical properties of the particle. Q-particles of many materials can be prepared in the form of colloidal solutions or embedded in porous matrices and are stable over a long period of time. In sandwich colloids, in which Q-particles of different materials are coupled, as well as in porous semiconductor electrodes containing Q-particles in the pores, very efficient primary charge separation is observed. As a result, sandwich colloids have greatly enhanced photocatalytic activity relative to the individual particles, while electrodes modified with Q-particles show high photocurrents. This article deals with the size quantization effect, the synthesis and characterization of Q-particles, as well as with the spectroscopic, electrochemical, and electron-microscopic investigation of these particles.

Colloidal Semiconductor Q‐Particles: Chemistry in the Transition Region Between Solid State and Molecules

Noncovalent, extrinsic fluorescent dyes are applied in various fields of protein analysis, e.g. to characterize folding intermediates, measure surface hydrophobicity, and detect aggregation or fibrillation. The main underlying mechanisms, which explain the fluorescence properties of many extrinsic dyes, are solvent relaxation processes and (twisted) intramolecular charge transfer reactions, which are affected by the environment and by interactions of the dyes with proteins. In recent time, the use of extrinsic fluorescent dyes such as ANS, Bis-ANS, Nile Red, Thioflavin T and others has increased, because of their versatility, sensitivity and suitability for high-throughput screening. The intention of this review is to give an overview of available extrinsic dyes, explain their spectral properties, and show illustrative examples of their various applications in protein characterization.

/pdf/extrinsic-fluorescent-dyes-as-tools-for-protein-nmhkb08wui.pdf

Extrinsic fluorescent dyes as tools for protein characterization.

An excited-state intramolecular proton transfer (ESIPT) process in 2-(2’-hydroxyphenyl) benzimidazole and -benzoxazole (HPBI and HBO, respectively) has been studied using steady-state and time-resolved emission spectroscopy at various temperatures and by semiempirical quantum chemical methods. For both of them two distinct ground-state rotamers I and I1 respectively responsible for the “normal” and the “tautomer” emission have been detected. In hydrocarbon solvents at room temperatureand at 77 K the tautomer emission predominates over the normal emission for both HPBI and HBO. This indicates that rotamer 11, responsible for the tautomer emission, is intrinsically stabler than rotamer I, which causes the normal emission. In alcoholic glass at 77 K for HPBI a dramatic enhancement of the normal emission is observed. It is suggested that due to the increased solvation, the more polar rotamer I becomes stabler than I1 for HPBI in alcohol and the substantial temperature variation is due to the change in the population of the two rotamers with temperature. From the detailed temperature variation in alcoholic medium the ground-state energy difference between rotamers I and I1 is determined. In dioxane-water mixtures it is observed that with the addition of water the quantum yield of the normal emission increases, which is ascribed to the inhibition of the ESIPT process due to the formation of an intermolecular hydrogen bond involving water. CNDO/S-CI calculations were performed optimizing thegroundstate geometry by the AM1 method. Details of the energy, dipole moment, and charge distribution of the rotamers in the ground state (SO) and the first excited singlet state (SI) and the barrier for the interconversion of I and I1 in SO, SI, and first excited triplet state are discussed. The calculation indicates that the barrier for the interconversion of the two rotamers is too high in the excited state (SI and TI) for free interconversion.

Excited-State Intramolecular Proton Transfer in 2-(2-Hydroxyphenyl)benzimidazole and -benzoxazole: Effect of Rotamerism and Hydrogen Bonding

Steady-state and time-resolved emission studies on Nile Red, a biological fluorescent probe, in homogeneous solutions and in zeolite 13X and LZY are reported. It is shown that at moderately high polarity the main nonradiative pathway of Nile Red is twisted intramolecular charge transfer (TICT) process, the rate of which increases exponentially with increase in polarity (ET(30)) of the medium. Due to this polarity-dependent TICT process Nile Red is a sensitive indicator of the micropolarity of biological environments. From the comparison of the emission properties of Nile Red in zeolite X and Y with those in homogeneous solutions the ET(30) values of zeolite 13X and LZY are estimated to be 55.5 ± 1. This indicates the polarity of the zeolites is similar to that of an aqueous methanol solution.

Twisted charge transfer processes of nile red in homogeneous solutions and in faujasite zeolite

Excited-state intramolecular proton transfer (ESIPT) of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) has been studied using steady-state and time-resolved emission spectroscopy in neutral (Triton X-100), anionic (sodium dodecyl sulfate, SDS) and cationic (cetyltrimethylammonium bromide, CTAB) micelles. It is found that the tautomer intensity and the lifetime of the tautomer emission are sensitive monitors to estimate the critical micellar concentration (cmc) for all three micelles. Above the cmc the intensity of the normal emission band decreases while there is a large enhancement of the intensity of the tautomer emission along with a red shift of the emission maxima by about 20 nm. The decay of the normal emission of HPBI remains similar below and above the cmc for all three micelles. However, the lifetime of tautomer emission of HPBI increases sharply above the cmc. The enhanced tautomer emission is ascribed to the HPBI molecules residing in the less polar and aprotic interior of the micelles.

Excited-State Intramolecular Proton Transfer of 2-(2'-Hydroxyphenyl)benzimidazole in Micelles

A donor-acceptor substituted aromatic system (E)-3-(4-Methylamino-phenyl)-acrylic acid methyl ester (MAPAME) has been synthesized, and its photophysical behavior obtained spectroscopically has been compared with the theoretical results. The observed dual fluorescence from MAPAME has been assigned to emission from locally excited and twisted intramolecular charge transfer states. The donor and acceptor angular dependency on the ground and excited states potential energy surfaces have been calculated both in vacuo and in acetonitrile solvent using time dependent density functional theory (TDDFT) and TDDFT polarized continuum model (TDDFT-PCM), respectively. Calculation predicts that a stabilized twisted excited state is responsible for red shifted charge transfer emission.

Photoinduced intramolecular charge transfer reaction in (E)-3-(4-Methylamino-phenyl)-acrylic acid methyl ester: A fluorescence study in combination with TDDFT calculation.

Debnarayan Nath

Papers

Excited-State Intramolecular Proton Transfer in 2-(2-Hydroxyphenyl)benzimidazole and -benzoxazole: Effect of Rotamerism and Hydrogen Bonding

Twisted charge transfer processes of nile red in homogeneous solutions and in faujasite zeolite

Excited-State Intramolecular Proton Transfer of 2-(2'-Hydroxyphenyl)benzimidazole in Micelles

Photoinduced intramolecular charge transfer reaction in (E)-3-(4-Methylamino-phenyl)-acrylic acid methyl ester: A fluorescence study in combination with TDDFT calculation.

Effect of environment on the magnetic field modulation of exciplex luminescence