Nagwa Ghoneim

Photophysics of Nile red in solution: Steady state spectroscopy

Spectroscopic properties of Nile red (NR) in organic solvents, binary solvent mixtures have been studied. Remarkable shifts in the emission band positions have been observed as a function of the polarity of the medium. In solvent mixtures, these shifts can be explained by the process of specific solvation known as dielectric enrichment. The displacement of the fluorescence band was also measured as a function of temperature to obtain the thermochromic shifts (15 cm(-1) K(-1) in methyltetrahydrofuran and 13.8 cm(-1) K(-1) in butanol). Excited state dipole moments were calculated from these shifts.

Solvatochromic shifts of non-dipolar molecules in polar solvents

Abstract Centro-symmetric molecules such as anthracene and its 9,10-disubstituted derivatives often show solvatochromic shifts of their absorption and fluorescence spectra in a series of solvents of different static dielectric constants. These shifts have been explained by the existence of small dipole moments of such molecules in excited states or by the “solvent Stark effect” which results from fluctuations of the electric field produced by the polar solvent. It is suggested that the origin of these solvatochromic shifts is the solute quadrupole-solvent dipole interaction which is not considered in the simple electrostatic models.

Study of the preferential solvation of some betaine dyes in binary solvent mixtures

The preferential solvation (PS) in mixed binary solvents, e.g. dichloromethane/acetonitrile, dichloromethane/methanol, acetonitrile/water has been studied by monitoring the charge transfer band of some betaine dyes. Since PS accounts for dielectric as well as specific interactions while dielectric enrichment only for the former, it was shown that in the case of dichloromethane/methanol, hydrogen bonding precedes dielectric enrichment whereas in the case of acetonitrile/water the inverse is true.

Comparative electron-transfer quenching rates of 9,10-dicyanoanthracene by various donors in solvents of different polarities

Multiple Rehm–Weller plots of the rate constants of quenching of 9,10-dicyanoanthracene fluorescence by electron transfer with a variety of donors of different adiabatic ionization potentials have been observed in polar solvents (1-fluoropentane and acetonitrile). The donors fall into two classes which can be described as ‘n’(localised positive charge in the cation) and ‘π’(delocalised charge), the former being the more efficient quenchers. Possible reasons for this difference are discussed: electrostatic and solvation energies, internal reorganisation energy. Current evidence favours the former explanation.

Dual luminescence, structure and excimers of 9-anthracene carboxylic acid

Abstract 9-Anthracene carboxylic acid (9ACA) shows two fluorescence spectra which have been variously attributed to twisted and planar molecular forms or to excimers and excited tetramers. The study of the concentration dependence shows that both emissions are of molecular origin. In alkaline solution the fluorescence is similar to that of anthracene, and this is explained by the wide energy gap between the antibonding orbitals of the aromatic ring and of the carbonyl group linked to the very strong donor O - .

Solvatochromic and thermochromic effects in low-temperature rigid matrices

The comparison of electronic of absorption and emission spectra of solute molecules between various liquid and sold media requires a knowledge of the solvatochromic shifts; If spectra taken at different temperatures are to be compared, the thermochromic shifts must be considered as well. There is a considerable body of empirical and theoretical knowledge in this field for solutes in liquid solvents, whereas little systematic work has been performed so far for the case of rigid matrices. In this paper some experimental observation are presented to relate the electronic transition energies of some solute molecules in liquid and solid environments at different temperatures; these observations are correlated with the macroscopic properties (dielectric constant, refractive index) of the medium, as well as with the glassy or crystalline structure of the matrix. Hydrogen bonding between the solute and the solvent is considered, in addition to the general dielectric interactions, and the anomalous behaviour of 1,4-dioxane as a liquid solvent or a rigid matrix is described.

Permanent and induced dipole moments in electronically excited aromatic molecules. The polarizability in excited states

The solvatochromic shifts of absorption and fluorescence spectra of trifluoromethyl derivatives of benzene and naphthalene show that the induced dipole moments in the first excited singlet state are very close to that of the ground state. The change in the polarizability of such molecules on electronic excitation is therefore very small, of the order of 10% from the ground-state polarizability.

Photochemical reactivity of para-aminobenzophenone in polar and non-polar solvents

Abstract In non-polar cyclohexane solution, para -aminobenzophenone (PAB) has a triplet yield of 0.82 and a reaction quantum yield of 0.21 towards hydrogen abstraction. In polar N , N -dimethylformamide (DMF), the triplet yield is 0.1, but the reaction quantum yield is below 10 −5 ; other reactive benzophenones are photoreduced efficiently in the same solvent. This difference in photoreactivity is related to the nature of the lowest triplet excited state, which is n -π ∗ (reactive) in polar solvents but of “charge transfer” (CT) type in polar media. In polar, protic solvents, such as ethanol, the triplet yield is very low as a result of quenching of the CT singlet excited state by proton transfer.

Structure of exciplexes: solvent and temperature dependences of charge transfer character.

The influence of temperature on the efficiency of fluorescence of the encounter complex (A- ...D+)* in intermolecular electron transfer starting from neutral precursors has been studied. In contrast to what is generally assumed, fluorescence emission of the exciplexes is found to be thermally activated. The systems studied here consist of N,N-dimethylaniline (DMA), and tributylamine (TBA) as electron donors and anthracene (An) as electron acceptor. This study has proved that the two exciplexes behave differently with temperature.

Exciplex formation between aminophthalimides and aliphatic halocompounds

Abstract The fluorescence spectra of 3- and 4-aminophthalimides (AP) show unusually large thermochromic shifts in CHCl3 and CHBr3, as well as anomalous behaviour in solvent mixtures such as DMF CHCl 3 . This is interpreted in terms of a highly dipolar exciplex formed between excited APs and clusters of halogenated molecules.