Shakir T. Abdel-Halim

Photophysical properties and semiempirical calculations of perylene-3,4,9,10-tetracarboxylic tetramethylester (PTME)

The spectral behavior and fluorescence quantum yield of perylene-3,4,9,10-tetracarboxylic tetramethylester (PTME) have been measured in different solvents. Both electronic absorption and fluorescence spectra are not sensitive to medium polarity. The dye exhibits high fluorescence quantum yield and high photostable. Crystalline solid of PTME gives excimer-like emission at 530 nm. The laser activity of PTME has been investigated. The dye solution in N,N-dimethylformamide (DMF) gives laser emission around 480 nm upon excitation by 337.1 nm nitrogen laser pulse. The excitation energy transfer from 7-dimethylamino-4-methylcoumarine (DMC) to PTME has also has been studied and the value of energy transfer rate constant, k(ET), and critical transfer distance, R(0) indicate a Förster-type mechanism. The photodecomposition of PTME in chloromethane solvents has been also studied. We applied semiempirical MO calculations using (PM3 and ZINDO-CI) calculations to explain the geometric and electronic behaviors of the PTME molecule in both ground and excited states and make a correlation with the experimental observations.

Effect of solvent on absorption and fluorescence spectra of a typical fluorinated azo dye for its acidic and basic structures.

The effect of 15 polar solvents on absorption and fluorescence energies of a typical fluorinated azo dye, 4-(2,3,5,6-tetrafluoro-pyridin-4-yl azo)-phenol, was reported for its acidic, MH, and basic, M, structures. For MH, the absorption energy is described on the basis of multi-linear equation with Tafts π* (solvent polarity) and β (hydrogen bond acceptor) parameters while the fluorescence energy varies rectilinearly with free energy of transferring the proton to the surrounding solvent, ΔG(t)°. For M, the hydrogen bonding donor ability of protic solvent, α, is a predominant factor which affects the absorption energy while in aprotic solvents, the absorption energy correlates linearly with Kirkwood function. As the ability of the solvent for hydrogen bonding increases, the absorption band width will increase in parallel with the transition energy.

Spectroscopic characteristics of an amphiphilic merocyanine dye: A probe of polarity in micelles

Abstract The amphiphilic merocyanine dye C16M was synthesized and its absorption spectra were investigated in organic solvents of varying polarities and solutions of different pH. The spectral behaviour was also studied in aqueous and in reversed micelles of two differently charged detergents, CDBA (cetyldimethylbenzylammonium chloride) and AOT (sodium bis 2-ethylhexyl sulfosuccinate). The results show that the pKa value of C16M is more acidic by four pKa units in aqueous micelles than that in methanolic buffer solutions. The spectral changes observed in methanol/H2O mixtures are indicative of self-aggregation of C16M forming micelles. It was found that this molecule exhibits an extreme negative solvatochromic effect, which permits its use as a chemical probe to monitor the polarity of aqueous micellar systems as well as the polarity of the water pools in reversed micelles.

Protolytic equilibria of 2-hydroxytropolone (4,5-d) thiophene in homogeneous and micellar solutions

Abstract The present study comprises an investigation of the optical absorption and fluorescence spectra of the title compound (HT) in homogeneous solutions of ethanol, cyclohexane, and sulphuric acid, and in aqueous micellar systems of anionic (NaLS), cationic (CTABr) and non-ionic (Triton X-100) surfactants. This compound behaves as monoprotic acid in buffer solutions of pH = 1–13 containing 3% v/v ethanol. It has p K = 8.2, but in the first excited singlet state the p K * drops to 3.9. However, another protolytic equilibrium is established with p K 2 = 0.45 and p K * 2 = 2.15 in concentrated H 2 SO 4 solutions. Contrary to all other media studied, the dissociated form of HT was observed in CTABr micellar solution with an apparent red shift indicating that the HT molecules are incorporated into the detergent layer of the micelles and at the interface of the aggregates. The influence of micellar solutions on the acid—base equilibrium of HT reveals that the effect of the charge distribution of the counter ions in the double layer is much larger than the effective dielectric constant at the site of solubilization.

PHOTOCHEMICAL cis/trans ISOMERIZATION OF AN AMPHIPHILIC MEROCYANINE DYE IN HOMOGENEOUS SOLUTION AND REVERSED MICELLES

Abstract— In the present study, the photochemical cis⇌trans isomerization of the amphiphilic merocy‐anine dye (4‐hydroxy‐l‐cetyl stilbazolium betaine C16M) is reported. The structure of each isomer is established from high resolution FT 1H‐NMR analysis. The photochemical quantum yields φt→c, and φc→t are determined in methanol and in reversed micelles of the system (CDBA, benzene, water). The quantum yield φc→t increases in reversed micelles compared to that in methanol. The reverse results are obtained for φt→c. The micellar effect on the photochemical trans→cis isomerization will be discussed.