Uwe K.A. Klein

Normal and twisted intramolecular charge-transfer fluorescence of 4-dimethylaminobenzonitrile in α-cyclodextrine cavities

Dual fluorescence of 4-dimethylaminobenzonitrile (DMABN) has been used to study the behavior of DMABN in α-cyclodextrine (α-CD). Results obtained indicate that DMABN in an aqueous α-CD solution probably exists in three different environmental conditions; polar, slightly polar and hydrophobic. DMABN surrounded by water molecules represents the most polar and least rigid environment. DMABN molecules have two different possible positions inside the α-CD cavity. The first position could arise when the dimethylamino group of the DMABN molecule is headed towards the larger rim of the α-CD cavity representing the slightly polar and slightly rigid environment. The second position could arise when the dimethylamino group of the DMABN molecule is headed towards the smaller rim of the α-CD cavity representing the least polar and most rigid environment. Therefore, it is suggested that the relative positions and contributions of the two fluorescence bands of DMABN in α-CD are determined by restricted molecular motion (controlled by the polarity and free volume of the α-CD cavity) rather than by the polarity alone.

Laser Doppler velocimeter for remote measurement of polluted water and aerosols discharges

A continuous wave Doppler velocimeter has been developed for remote estimation of the volume of polluted water and gaseous discharges using an actively stabilized ring dye laser. Doppler shifted backscattered signals induced by fluids in motion using heterodyne detection technique has been applied for measurement of Doppler shifts. Polluted water and gaseous discharge flow rates have been measured remotely for simulation purposes. The system is capable of measuring Doppler shifts as low as 0.25 MHz corresponding to 0.1 m/s velocities of the fluids.

Proton transfer reaction of 4-hydroxy-1-naphthalenesulphonate in methanol-water and ethanol-water mixtures

The excited state proton transfer rate of 4-hydroxy-1-naphthalenesulphonate has been studied in methanol-water and ethanol-water mixtures. The lifetimes of the probe are measured at six different temperatures between 5 and 60°C. The decay is single exponential in the long time regime and proton transfer rates have been found to increase with the square of the concentration of water in alcohol-water mixtures. The decay has been explained by a new model proposing a water dimer to be the effective proton acceptor in the proton transfer process.

Picosecond study of solute-solvent interaction of the excited state of indole

Abstract The lifetime of indole is shortened when benzene is added to a cyclohexane solution. A proposed mechanism is the formation of an exciplex through hydrogen bonding. The finding from the picosecond study presented here shows that the anomalous behavior of indole is not totally determined by the solvent polarity. The temperature quenching of indoles fluorescence in various solvents has also been investigated and results explained by the dissociation of the exciplex.

Acidity of Zeolite Y—Probed by Adsorption of 1-Naphthylamine and Studied by Laser-Induced Fluorescence Spectroscopy

Proton transfer reactions between zeolite Y surface and 1-naphthylamine (NA) in the ground and excited states have been studied by laser-induced picosecond spectroscopy. The acidic form of zeolite Y readily protonates NA in the ground state. At low acid strength of the zeolite the excited state of the protonated NA transfers back the proton to the zeolite surface as indicated by the fluorescence spectra. At high acid strength of the zeolite the fluorescence comes from the protonated form of NA and from an adduct “X” previously found in highly concentrated HClO4 solutions. The concentrations of the protonated NA and X increase with the reduction in the unit cell size. The presence of these species is discussed in terms of the next nearest neighbor “NNN” theory of zeolite Y acidity and the role of the non-framework aluminum. The acidity of the zeolite is estimated, based on the fluorescence lifetimes of X, to vary from 3.7 to 17 M HClO4 depending on the unit cell size. Low loading levels of NA in the zeolite pores are best in studying the proton transfer reaction and for the estimation of the surface acidity of zeolite Y.

Acidity of All-Silica MCM-41—Studied by Laser Spectroscopy of Adsorbed Fluorescent Probe Compounds

The polarity within the channel environment of all-silica MCM-41 and the surface acidity were probed by the adsorption of rhodamine-B lactone (RhB-L) and 1-naphthylamine (NA) respectively. The photochemical properties of these probe compounds were studied by laser-induced fluorescence spectroscopy and compared with their properties in solution. The environment within the channels was found to be as polar as methanol. The fluorescence wavelength and decay rate have shown that when adsorbed on all-silica MCM-41, the lactone ring in RhB-L was opened to form the zwitterion (RhB-Z). It was not possible to assess whether RhB-L was protonated by the surface of MCM-41 because the fluorescence of both the RhB-Z and the cationic form RhB-C are almost identical. NA was protonated at the ground state on the surface of all-silica MCM-41. The proton, however, was donated back to the surface upon excitation with laser. From the pKa of the conjugate acid of NA and the fluorescence decay time of the excited state of NA (NA*), the acidity of the surface of the all-silica MCM-41 was estimated to be equivalent to an aqueous perchloric acid with pH 1.8–2.5, depending on the level of loading of NA. The origin of the acid surface was concluded to be the silanol groups known to be present on the surface. The nature of these groups and the influence of the polarity and the solvation effect of the framework on the acidity were discussed.

Time-resolved spectroscopy of 4-hydroxy-1-naphthalenesulphonate in alcohol—water mixtures

Abstract The initial proton transfer dynamics of 4-hydroxy-1-naphthalenesulphonate has been studied in alcohol—water (7:3) mixtures at room temperature by using picosecond time-resolved emission spectroscopy. Consecutive processes of dissociation, recombination, re-dissociation have been observed in methanol—water and ethanol—water mixtures. However, no recombination has been found in the 1-propanol—water mixture. The need for a particular solvent structure for the proton hydration is discussed.

Dynamics of an excited acid in 1-propanol—water mixtures

Abstract The excited state proton transfer reaction of 4-hydroxy-1-naphthalenesulphonate in 1-propanol—water mixtures has been studied at six different temperatures ranging from 5 to 60°C. Because of the hydrophobic interaction in these mixtures, the dissociation of the probe cannot be explained by a simple water dimer model which we found to be applicable to methanol—water and ethanol—water mixtures. In 1-propanol—water mixtures the proton acceptor concentration, however, shows a linear dependence on the water concentration above a certain critical concentration of water. This behavior is explained by a new model assuming a water dimer within a water cluster to be the effective proton acceptor.

Absorption, fluorescence and saturation of 127I2 in the region 16999.100–17001.300 cm−1

Abstract A highly resolved absorption spectrum of 127 I 2 in the region from 16999.100 to 17001.300 cm −1 is measured using a ring dye laser. The observed spectral lines, which show distinct fluorescence with the dominant colors red, orange, yellow and green have been assigned. Red lines are assigned to v ″=0, orange lines to v ″=1, yellow lines to v ″=2, and green lines to v ″=4. The assignments are also supported by the saturation behavior of the different lines. It is found that red lines are not saturable using laser powers upto 400 mW while lines having the other colors are saturable. Results are explained and a mathematical formula relating the saturation power P s to the incident laser power and the absorbance is presented.

Ground and Excited States Proton Transfer Reactions of 1,8-Diaminonaphthalene in Perchloric Acid Solutions

The proton-transfer reaction of 1,8-diaminonaphthalene (1,8-DAN) in acidic medium was studied by means of fluorescence and picosecond spectroscopic techniques. It has been found that there are three different forms of 1,8-DAN in the ground state, but only two different forms in the excited state. The absorption of the mono-cation form of 1,8-DAN is found to be a mixture of the neutral form and the di-cation form. However, the emission is found to be the same as the neutral form, due to the fast dissociation of the mono-cation form once it is excited. The fluorescence of the mono-cation form of 1,8-DAN shows a small shift under different excitation wavelengths. The di-cation form only fluoresces if no free water cluster is available as a proton acceptor. The reaction in the excited state is shown to be a diabatic quenching reaction. With the help of quantum yields and fluorescence lifetime measurements these results are interpreted in terms of a new photochemical scheme. All dissociation and quenching rate constants, pKa and kq, have been determined.