M. Ashraf El-Bayoumi

EXCITED STATE INTERACTIONS OF 7-AZAINDOLE WITH ALCOHOL AND WATER

Abstract— The fluorescence spectrum of 7‐azaindole in alcohol is composed of two fluorescence bands. Effects of pH, temperature and solvent deuteration on the fluorescence spectra and quantum yields of 7‐azaindole and other model compounds in ethanol and in water are reported. The long wavelength band arises from a tautomeric species formed in an adiabatic photoreaction involving double proton transfer between one molecule of 7‐azaindole and one molecule of alcohol.

TIME-RESOLVED FLUORESCENCE STUDY OF INTRAMOLECULAR EXCIMER INTERACTION IN DINAPHTHYLPROPANE

Abstract Time development in the nanosecond range of the intramolecular excimer fluorescence of 1,3-dinaphthylpropane is studied using a single-photon counting technique. The rate constants for the association and dissociation process are obtained at different viscosities. The radiative lifetime of the excimer is discussed in terms of the excimer geometry.

Excited-state double proton transfer in 1-azacarbazole hydrogen-bonded dimers

Abstract 1-azacarbazole hydrogen-bonded dimers undergo photoinduced double proton transfer reaction in their lowest excited singlet state. A second emission band with a maximum at 510 nm arises from a tautomer formed in the excited singlet state as a result of the double proton transfer process.

Fluorescence quenching in phenylalanine and model compounds.

Abstract— The effects of the carboxylic and amino groups in their neutral and charged forms on the absorption and emission properties of phenylalanine are analyzed with the help of model compounds containing either substituent. Fluorescence yields of phenylalanine and model compounds were measured as a function of pH. Fluorescence quenching occurs in going from COO‐ to COOH and from NH3+ to NH2. Lowering the ionization potential of the phenyl group by substitution increases the quenching effect. This provides more evidence that an intramolecular charge‐transfer mechanism may be responsible for intersystem‐crossing rate enhancement and fluorescence quenching in phenylalanine and model compounds.

Intramolecular Charge‐Transfer Interactions in Benzyl Derivatives

Benzyl derivatives exhibit an enhancement of phosphorescence yields, a quenching of fluorescence intensities, and a shortening in their natural phosphorescence lifetimes relative to toluene. These results reflect an increase in intersystem crossing process, S1→ T with rate constant kisc. Evidence for an intramolecular charge‐transfer mechanism which enhances spin‐orbit coupling between states of different multiplicities is provided. This mechanism requires some electronic overlap and hence, a spatial approach between the benzene ring and the substituent. Viscosity as well as steric effects that influence such approach are also discussed.

Solvent-induced intramolecular exciplex fluorescence of β-naphthylmethyl amine

Abstract Intramolecular exciplex fluorescence of β-naphthylmethyl amine has been observed in polar media. The absence of exciplex fluorescence and the quenching of naphthalene-type fluorescence in hydrocarbon solvents is explained in terms of enhanced intersystem crossing. The crucial role the solvent can play in determining the energy and intensity of exciplex fluorescence is clearly demonstrated.

Phosphorescence Enhancement in Phenyl‐Substituted Methanes

Experimental evidence is presented which shows that the natural phosphorescence lifetimes, τP°, decrease with increased phenylation in the molecular series: toluene, diphenylmethane, triphenylmethane. This indicates that a spin—orbit enhancement mechanism is operative. Natural fluorescence lifetime data support the exciton molecular model for diphenylmethane and triphenylmethane and show that the phenyl groups have little electronic overlap. The experimentally observed natural fluorescence lifetimes, τ F°, at 77°K suggest that the exciton levels have similar transition moments with small forbidden character for the low‐energy component. The dramatic enhancement of phosphorescence in the series is due to several factors, namely, an increase in intersystem crossing rate constant kisc, an increase in phosphorescence rate constant kP, and a decrease in radiationless triplet decay constant kP′. Although the electronic overlap is small enough for the exciton model to be a valid approximation to explain singlet ...

A unique excitation wavelength dependence of excited-state proton transfer in para-N,N-dimethylaminosalicylic acid

Abstract Para-N,N-dimethylaminosalicylic acid (PDAS) in hydrocarbon solution at room temperature exhibits excited-state proton transfer. It is found that no proton transfer occurs upon excitation at 330 nm corresponding to the S 1 state (assigned to hydroxy-O charge-transfer origin). Rapid proton transfer (order of ps) occurs upon excitation at 300 nm corresponding to the S 2 state (assigned to amino-N charge-transfer origin), competing with S 2 to S 1 internal conversion. The observed dual fluorescence thus results from the excitation of electronic states of the same conformer.

LCI Pariser-Parr-Pople calculations of π-electron systems in nitrogen-heterocyclic molecules. Correlation with spectral data

Abstract Limited configuration interaction Pariser-Parr-Pople calculations were performed for 19 nitrogen heterocyclic aromatic molecules. After optimizing the semiempirical parameters for six “test” molecules, calculations were performed for the remaining thirteen molecules. Strong correlations were made with experimentally observed ionization potentials, singlet-singlet transition energies, polarizations and oscillator strengths as well as ground state molecular geometries for cases where these observations have been made. These correlations indicate spectral features which should be carefully examined experimentally to determine the effect of nitrogen substitution in aromatic hydrocarbon molecules.

Intramolecular triplet excimer formation in 1,3-diphenylpropane

Abstract Evidence is presented for the formation of an intramolecular triplet excimer in the case of 1,3-diphenylpropane during the softening of the isopentane degassed glass matrix. The red shifted band observed at 420 nm is viscosity dependent and is interpreted as excimer phosphorescence.