Maged El-Kemary

Spectral and photophysical studies of inclusion complexes of 2-amino-4,6-dimethyl pyrimidine with β-cyclodextrin

The interaction of 2-amino-4,6-dimethyl pyrimidine (ADMP) with beta-cyclodextrin (beta-CD) has been studied by means of UV absorption, steady state and time resolved fluorescence techniques. Spectral characteristics, bandwidths and photophysical parameters indicating that ADMP experience two different environments in aqueous solutions: bulk water and 1:1 (ADMP:beta-CD) inclusion complexation. The size restriction of the upper rim of beta-CD partially include ADMP and prevent the possibility of formation of 1:2 complex. The effective polarity of the cyclodextrin cavity experienced by the induced ADMP is equivalent with the polarity of an 80:20 methanol-water mixture.

Multiple emission in coumarins with heterocyclic substituents

The photophysical behavior of three coumarin dyes with heterocyclic substitution in 3-position and an electron-withdrawing cyano group in 4-position, Scheme 1b, have been studied in solvents of different polarity. The results are compared with those for rotation-inhibited coumarin 153 and coumarin 6, Scheme 1a. It is concluded that the dyes undergo an excited-state equilibrium reaction from an emissive intramolecular charge transfer (ICT) state to a twisted intramolecular charge transfer (TICT) state with forbidden emission in strongly polar solvents. In coumarins I and II this reaction acts as nonradiative decay involving the rotation about the bond joining the coumarin and the heterocyclic substituent in the 3-position because the TICT state reached is nonemissive. In contrast, coumarin III shows emission from two species with the longer wavelength emitting species being preferred in polar solvents. This is characteristic for an emissive TICT state. The donor strength of the heterocyclic substituent in 3-position plays a major role in increasing the CT character of coumarins I and II when compared to C153. Coumarin III possesses significantly larger Stokes shifts and a correspondingly higher excited state dipole moment than the comparable coumarin 6 lacking the electron-withdrawing cyano group in 4-position.

Inclusion of Paracetamol into β-cyclodextrin nanocavities in solution and in the solid state

We report on steady-state UV-visible absorption and emission characteristics of Paracetamol, drug used as antipyretic agent, in water and within cyclodextrins (CDs): β-CD, 2-hydroxypropyl-β-CD (HP-β-CD) and 2,6-dimethyl-β-CD (Me-β-CD). The results reveal that Paracetamol forms a 1:1 inclusion complex with CD. Upon encapsulation, the emission intensity enhances, indicating a confinement effect of the nanocages on the photophysical behavior of the drug. Due to its methyl groups, the Me-β-CD shows the largest effect for the drug. The observed binding constant showing the following trend: Me-β-CD>HP-β-CD>β-CD. The less complexing effectiveness of HP-β-CD is due to the steric effect of the hydroxypropyl-substituents, which can hamper the inclusion of the guest molecules. The solid state inclusion complex was prepared by co-precipitation method and its characterization was investigated by Fourier transform infrared spectroscopy, 1H NMR and X-ray diffractometry. These approaches indicated that Paracetamol was able to form an inclusion complex with CDs, and the inclusion compounds exhibited different spectroscopic features and properties from Paracetamol.

Fluorescence decay of singlet excited-state of safranine T and its interaction with ground-state of pyridinthiones in micelles and homogeneous media.

The fluorescence decays of safranine T were studied in different homogeneous solvents and heterogeneous micellar solutions. It has been found that micellization leads to an increase in the lifetime. The lifetime distributions were studied in micelles and homogeneous media. It was found that the different half-width distributions of the dye in different micelles are related to the different orientation of the dye in different micelles. Also, the fluorescence quenching of safranine T by 4,6-disubstituted-3-cyanopyridin-2(1H)-thiones was studied in chloroform, methanol and acetonitrile as well as in different micelles. In heterogeneous media the kq values for quenching of safranine T by thiones in various micelles increase on the following order: kq(CTAB) < kq(TX-100) < kq(SDS). This is due to the electrostatic interactions between the anionic SDS and the cationic moiety of safranine T and therefore the quenching process will be less significant.

Excited-state double-proton transfer of pyrimidines mediated by hydrogen-bonded complexes

Abstract The spectroscopy and dynamics of the excited-state double-proton transfer (ESDPT) in 2-amino-4,6-dimethyl pyrimidine (ADMP) and 2-amino-4-methoxy-6-methyl pyrimidine (AMMP) have been studied by means of steady-state and time-resolved measurements. The thermodynamic data indicating that dual hydrogen-bonding formation for ADMP/acid and AMMP/acid complexes are stronger than those obtained from ADMP and AMMP self-association. The fluorescence from the ADMP dimer in cyclohexane decays with rate ( k f ) of (1.1±0.1)×10 9 s −1 (0.9 ns), where the fluorescence from its tautomeric excited state formed by the proton transfer reaction decays with rate of (8.26±0.2)×10 8 s −1 (1.21 ns). However, the obtained k f (1.7±0.1)×10 9 s −1 for ADMP/acid tautomer of (0.58 ns) in cyclohexane is higher than that of the dimer. The results show that ∼89% molecules form dimers in the ground state and ∼86% of the excited molecules are present as dimers while the rest are present as monomers in 1×10 −2 M cyclohexane solution.

Halogen vs hydrogen bonding in thiazoline-2-thione stabilization with σ- and π-electron acceptors adducts: theoretical and experimental study.

Molecular charge-transfer complexes (CT) between thiazoline-2-thione (THZ) and different σ- (I2) and π-acceptors (Tetracyanoethylene (TCNE), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), and 2,3,5,6-tetrachloro-1,4-benzoquinone (CHL)) were investigated. UV-Vis absorption spectroscopy and theoretical calculations using both MP2/aug-cc-pVDZ-PP and B3LYP/6-311++G(d,p) level of theory were corroborated to study the nature of the stabilizing forces for THZ-I2, THZ-DDQ, THZ-TCNE, and THZ-CHL. Halogen bonding (XB) was the stabilizing attractive force in THZ-I2 and THZ-CHL whereas; hydrogen bonding (HB) was dominated in both THZ-TCNE, and THZ-DDQ complexes. Formation constant (K), extinction coefficient (ɛ), thermodynamic parameters such as enthalpy change (ΔH), entropy (ΔS), and Gibbs free energy (ΔG) were measured in different solvents.

Global and distribution analysis of fluorescence decays and spectrofluorimetric determination of stoichiometry and association constant of the inclusion complex of 2-amino-5,6-dimethyl-benzimidazole with β-cyclodextrin

The steady state and time-resolved fluorescence study of 2-amino-5,6-dimethyl-benzimidazole (ADBI) have been studied in aqueous solution of beta-cyclodextrin (beta-CD). The fluorescence decays were analyzed by global analysis and distribution analysis in order to get insight about the inclusion process. The fluorescence lifetime of ADBI is increased in beta-CD and an enhancement of the emission, is observed, together with negligible changes in the energy of ADBI in beta-CD. The experimental data show that beta-CD reacts with ADBI to form a 1:1 host-guest complex with association constant was determined to be 2074+/-77M(-1). Both global analysis and distribution analysis of the fluorescence decays support the formation of only 1:1 inclusion complex. AM1 calculation shows that the size of ADBI was appropriate for good insertion within the beta-CD cavity and the inclusion of ADBI inside the beta-CD cavity should takes place from the side of the amino group substituent.

Photoinduced electron transfer between fullerenes (C60/C70) and disubstituted naphthalenes using laser flash photolysis

Abstract Photoinduced electron-transfer processes between fullerenes C60/C70 and disubstituted-naphthalenes have been studied by laser flash photolysis. Quantum yields (ΦetT) and rate constants (ket) of electron transfer from naphthalenes to the excited triplet states of C60 ( 3 C 60 ∗ ) and C70 ( 3 C 70 ∗ ) have been determined by observing the transient absorption bands in the wide wavelength VIS/NIR regions. The observed ΦetT and ket values depend significantly on the nature and position of the substituents. The ket values are seen to correlate well with the free energy changes (ΔGet) for the electron transfer reactions on the basis of Rehm–Weller relation. Addition of pyridine (Py) enhanced significantly the rates and efficiencies of photoinduced electron transfer process of dihydroxy-naphthalenes (DHN). However, the effect of Py in the dimethoxy-naphthalenes (DMN) is far less than that of DHN. In the presence of octylviologen (OV2+), electron-mediation occurs from C60 − to OV2+ to generate OV +.

Orthogonal hydrogen/halogen bonding in 1-(2-methoxyphenyl)-1H-imidazole-2(3H)-thione-I2 adduct: An experimental and theoretical study

The molecular complex between 1-(2-methoxyphenyl)-1H-imidazole-2(3H)-thione (Hmim(OMe)) and iodine (I2) was investigated. Single crystal of [(Hmim(OMe))I2] adduct was grown by slow evaporation technique from chloroform at room temperature. Spectroscopic techniques such as FT-IR and Raman techniques, as well as elemental and thermal analysis were used to characterize the complex. The crystal structure shows that the formed adduct stabilized by two noncovalent interactions, namely, hydrogen bond (HB) and halogen bond (XB). Orthogonal HB/XB associated with iodine atom (I) was observed and fully characterized. The ability of iodine to behave as hydrogen bond acceptor and halogen bond donor was held responsible for the orthogonal HB/XB presence. In addition, the structure of Hmim(OMe)I2 was investigated theoretically using MP2/aug-cc-pVDZ level of theory. Natural bond orbital analysis (NBO) was used to investigate the molecular orbitals interactions and orbitals stabilization energies.

Spectral study and global analysis of fluorescence decays of the inclusion complexes of 2-amino-4,6-dimethyl pyrimidine with α- and γ-cyclodextrins

Abstract The steady state and time-resolved fluorescence study of 2-amino-4,6-dimethyl pyrimidine (ADMP) have been studied in aqueous solution of α- and γ-cyclodextrins (CDs). In the presence of CDs, the fluorescence spectra of ADMP point out an enhancement of the emission without significant shift. The experimental data show the presence of only one kind of complex for ADMP in α- and γ-CDs. Although, ADMP partially included within the interior CD cavities, it encapsulated inside the γ-CD cavity more deeply, compared with the α-CD. The results also revealed that the polar amino group along with the endocyclic nitrogen remains located near the aqueous environment.