Tarek A. Fayed

Protonation dependent photoinduced intramolecular charge transfer in 2-(p-dimethylaminostyryl)benzazoles

The absorption and fluorescence spectra of 2‐(p‐dimethylaminostyryl)benzoxazole, DMASBO, and its benzothiazole analogue, DMASBT, have been studied in different solvents and at various acid concentrations. Two types of monocation are formed simultaneously in all solvents at relatively lower acid concentrations. The neutral DMASBO and DMASBT as well as the monocations formed by protonation of the –N = atom exhibit highly solvatochromic intramolecular charge transfer (ICT) emission. This indicated an increase in the dipole moment of the S1 state (μe). The μe values, which were calculated from the solvatochromic shifts and the ground state dipole moments, came out to be 14.3 and 12.6 D for DMASBT and DMASBO, respectively. For their azolium cations μe are 9.0 and 8.6 D. on the other hand, the monocations formed by protonation of the –NMe2 group as well as the dications formed at relatively higher acid concentration, showed insignificant spectral shift on changing the solvent polarity. Also, the effect of the heteroatom and the protonation site on the ICT process is discussed.

Photochemistry of N, Ń-ditridecyl-3,4:9,10-perylenetetracarboxylic diimide in chloromethane solvents

Abstract The spectral behaviour, lifetime and fluorescence quantum yield of N,N-ditridecyl-3,4:9,10-perylenetetracarboxylic diimide (DTPD) have been measured in chloromethane solvents, such as methylene chloride, chloroform and carbon tetrachloride. The excitation energy transfer to DTPD has been studied to improve the laser emission of DTPD. The value of energy transfer rate constant (KET) and critical transfer distance (R0) indicate a Forster type energy transfer mechanism. The photodecomposition of DTPD in chloromethane solvents has been also studied.

Preferential solvation of solvatochromic benzothiazolinic merocyanines in mixed binary solvents

Abstract The preferential solvation of α- and γ-oxobenzothiazolinic merocyanines was studied using UV-visible spectroscopy in binary solvent mixtures containing either water or methanol and cosolvents (formamide, ethanol, 2-propanol, DMF, CH3CN and 1,4-dioxane). In binary mixtures containing water, the dyes are preferentially solvated by the organic cosolvent over the whole composition range, only in the case of the α-oxo derivative in H2O–CH3CN mixtures the solvatochromic plot showed a biphasic behavior. In binary solvent mixtures comprising MeOH, the dyes showed no preference for either solvents in MeOH–EtOH mixtures, while in MeOH–CH3CN and 1,4-dioxane–MeOH mixtures the former solvent was the preferred one. Also, the results showed that γ-oxo derivative is preferentially solvated by the cosolvent in both MeOH-2-propanol and MeOH–DMF mixtures. Although the α-oxo analogue showed an ideal behavior in MeOH-2-propanol mixtures as well as in the MeOH rich region in MeOH–DMF mixtures, it is solvated preferentially by MeOH in the DMF rich side. Also, the role of solute-solvent and solvent-solvent interactions in determining the preferential solvation was discussed.

Photophysical properties and laser activity of 1,4-bis[β(4-quinolyl)vinyl]benzene

Abstract The spectral characteristics of 1,4-bis[β-(4-quinolyl)vinyl]benzene (B4QVB) have been investigated in different media. Both fluorescence and photoisomerization quantum yields have been determined in different organic solvents. The photoisomerization quantum yield is very small compared to the fluorescence yield, and both are sensitive to solvent polarity. B4QVB undergoes solubilization in both cationic and anionic micellar media. The protonation constants of B4QVB in ground and singlet-excited state have been evaluated. B4QVB is an efficient blue laser dye, where its solutions in dimethylfromamide and dimethyl sulphoxide give laser emission in the range 430–465 nm (λmax=450 nm) upon pumping with a nitrogen laser (λexc=337.1 nm). B4QVB acts as a good energy donor for N, N − -ditridecyl-3,4,9,10-perylenetetra-carboxylic diimide. The energy transfer obeys Forster type mechanism.

Fluorescence quenching of aromatic hydrocarbons by nitroxide radicals: a mechanismatic study

The fluorescence quenching of phenanthrene (Phen), 9-cyanophenanthrene (CPhen), 9-cyanoanthracene (CA), perylene (Per), 9,10-dicyanoanthracene (DCA), and 9,10-diphenylanthracene (DPA) using stable nitroxide radicals as quenchers has been studied by steady state and flash photolysis measurements. Both linearity and deviation from linearity in the Stern-Volmer plots have been observed. The active sphere model was used to discuss the upward curvature of the Stern-Volmer plots in case of Per, DCA, and DPA. The bimolecular quenching rate constant (kq) of Phen, CPhen and CA was found to be diffusion controlled while in other cases it is lower than the diffusion limit. On the basis of flash photolysis measurements as well as the overlap between the emission spectra of hydrocarbons and the absorption spectra of radicals, a resonance energy transfer mechanism is taken place in case of Per, DPA, DCA, and CA. For Phen and CPhen where the energy gap between the first excited singlet and the nearest lower triplet state is small, an induced intersystem crossing was suggested. Finally, the quenching process was discussed in terms of the free energy dependence (ΔG) of the electron transfer from nitroxide radicals to the excited hydrocarbons.

Excited state properties of an aza-analogue of distyrylbenzene. Solvent polarity and hydrogen-bonding effects

The steady-state and time-resolved fluorescence of phenylene-1,4-ethylene bis(4-quinoline), PhQ, have been measured in some organic solvents. The fluorescence quantum yield and lifetime are highly sensitive to the solvent nature—they decrease markedly with increasing the hydrogen bond donating ability and polarity of the solvent. The radiative rate constant is less sensitive to the solvent polarity while the nonradiative decay rate constant of the excited PhQ increases significantly from 4.9 × 108 to 26.4 × 108 s−1 on going from 1,4-dioxane to ethanol. The fluorescence quenching of PhQ by some alcohols, water, trifluoroacetic acid, phenol and anisole has been studied in 1,4-dioxane. The quenching efficiency is related to the hydrogen bond donating ability of the quencher. From the results of the quenching experiments and the large increase in the basicity of excited PhQ (ΔpKa = 6.65), excited state proton transfer, from the quencher to excited PhQ molecules, was suggested as a mechanism for fluorescence quenching of PhQ in protic solvents and acid solutions.

New Coordination Complexes of Cd(II) and Co(II) with Ethyl Isonicotinate Used for Catalytic Degradation of Acid Blue 92 Dye

Colorless crystals of the coordination polymer (CP) [Cd(EIN)2(SCN)2], 1, and pink crystals of the coordination complex [Co(EIN)4(SCN)2], 2 were prepared by the reaction of metal nitrate salt with ethyl isonicotinate (EIN) and thiocyanate anion as bridged ligand. The structures of 1 and 2 were characterized by elemental analyses, electronic, FT-IR, 1H-NMR spectroscopy, thermal analysis and X-ray diffraction measurements. The structure of the CP 1 consists of cyclic (CdSCN)n building blocks which exhibit chair conformation creating 1D-chain decorated by the coordinated EIN on both sides. The network structure of 2 contains discrete units of [Co(SCN)2(EIN)4] connected by extensive H-bonds to form 1D-chain. The free ethyl carboxylate groups play the essential role in the formation of the H-bonds extending the structures of 1 and 2 to 3D-network. The catalytic behavior of 1 and 2 was utilized for degradation of acid blue 92 dye (AB-92). The kinetic data indicated that 1 and 2 are effective catalysts for degradation of AB-92 while irradiation enhances significantly the rate of degradation. Disodium salt of terephthalic acid photoluminescence probing technology was carried out to identify the reactive oxygen species. Moreover, the efficiency of recycled the catalysts and the mechanism of degradation of AB-92 dye were also investigated.

Molecular aggregation, solvato- and acidochromic properties of symmetrical ketocyanines

AbstractThe absorption and fluorescence spectral characteristics of some symmetrical ketocyanine dyes have been investigated in different solvents of various polarities using steady-state absorption and emission spectroscopic techniques. The absorption spectral features are less sensitive to the solvent characteristics than the corresponding fluorescence spectra. A large excited state dipole moment parallel to the smaller ground state dipole moment was calculated. The large dipole moment change with the strongly solvatofluorochromic behaviour, as the solvent polarity increases, demonstrates the formation of an intramolecular charge-transfer (ICT) excited state, which is confirmed by theoretical calculation using semiempirical PM3 method. Also, the effect of dye concentration on the electronic absorption spectra for the investigated dyes has been studied in ethanol. In addition, the acidochromic behaviour of these dyes has been studied using sulphuric acid at different Ho values. A progressive red-shift in the absorption and emission spectra has been observed upon increasing the concentration of sulphuric acid with dramatic changes in colour. This suggests the design of potential optical sensors for probing acidity of the medium or water content in sulphuric acid. The calculated excited state protonation constants

New Supramolecular Coordination Polymers Based on Cd(II) and Co(II) with Ethyl Nicotinate and Thiocyanate Ligands as Effective Catalysts for Removal of Organic Dyes

\left( {\mbox{p}K_{\rm a}^\ast } \right)

Fluorescence Characteristics and Inclusion of ICT Fluorescent Probe in Organized Assemblies

are lower than p