A. Suwaiyan

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.

Room temperature fluorescence of 5-chlorouracil tautomers

Abstract Fluorescence and absorption spectra of 10 −4 M 5-chlorouracil in aqueous solutions at different pH values are reported at room temperature. The shape of the fluorescence spectrum is found to change when the excitation wavelength is varied. In addition to the predominant diketo tautomer ( I ), the keto-enol tautomer, namely 5-chloro-2-hydroxy-4(3H)-pyrimidinone ( II ), is believed to contribute to the absorption spectrum of 5-chlorouracil in the spectral range 240–330 nm. Using picosecond laser pulses, lifetimes and time-resolved spectra have also been recorded. Both experiments support the explanation of obtained results through acid-base equilibrium and the tautomeric contribution in the ground state.

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.

AB INITIO CALCULATIONS AND VIBRATIONAL SPECTRA OF 5-METHOXY INDOLE

Abstract Polarized Fourier transformed infrared (FT-IR) spectra of partially oriented 5-methoxy indole crystals and the Raman spectra of the melt and polycrystalline 5-methoxy indole are reported. Solvent effect study on the IR spectrum of 5-methoxy indole is also reported. Observed frequencies for normal modes are compared with those calculated from normal mode coordinate analysis carried out on the basis of ab initio force fields using the 3–21G basis set theoretical optimized geometry. The manifestations of the NH- π interaction in the vibrational spectra are discussed.

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.

Absorption spectra of substituted indoles in stretched polyethylene films

Abstract The stretched film technique has been used to resolve the two overlapping 1 L a and 1 L b transitions in the absorption spectra of some substituted indoles. The substituent effects on the absorption spectrum of indole, N -ethyl indole, 3-methyl indole and 7-methyl indole are discussed. The directions of the transition moments of these indoles with respect to the long molecular axis have been determined and the differences were attributed to the changes in the molecular geometry on excitation. The vibronic structure in the absorption spectra of these indoles is also considered.

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.

The polarized absorption spectrum of 5-methyl indole in polyethylene and polyvinylalcohol films

The stretched film technique has been used to resolve the two overlapping 1La and 1Lb transitions of indole. 5-Methyl indole was studied as a model compound in two polymer matrices, polyethylene (PE) and polyvinylalcohol (PVA). The directions of the transition moments with respect to the long molecular axis are found to be in PE − 10° and +22° for 1La and 1Lb, respectively. These angles are found to be −25° and +29° in PVA. 1La has two prominent vibrations in PE at 2040 and 2895 cm−1 from the origin. 1Lb has four prominent vibrations in PE at 761, 1308, 1937 and 3196 cm−1 from the origin. In PVA, 1La has two prominent vibrations at 2149 and 2874 cm−1 from the origin and 1Lb has four prominent vibrations at 1049, 1352, 2229 and 3196 cm−1 from the origin.