Mohamed E. El-Khouly

Photosynthetic Antenna–Reaction Center Mimicry by Using Boron Dipyrromethene Sensitizers

Various molecular and supramolecular systems have been synthesized and characterized recently to mimic the functions of photosynthesis, in which solar energy conversion is achieved. Artificial photosynthesis consists of light-harvesting and charge-separation processes together with catalytic units of water oxidation and reduction. Among the organic molecules, derivatives of BF2-chelated dipyrromethene (BODIPY), porphyrins little sister, have been widely used in constructing these artificial photosynthetic models due to their unique properties. In these photosynthetic models, BODIPYs act as not only excellent antenna molecules, but also as electron-donor and -acceptor molecules in both the covalently linked molecular and supramolecular systems formed by axial coordination, hydrogen bonding, or crown ether complexation. The relationships between the structures and photochemical reactivities of these novel molecular and supramolecular systems are discussed in relation to the efficiency of charge separation and charge recombination. Femto- and nanosecond transient absorption and photoelectrochemical techniques have been employed in these studies to give clear evidence for the occurrence of energy- and electron-transfer reactions and to determine their rates and efficiencies.

Light harvesting a gold porphyrin–zinc phthalocyanine supramolecular donor–acceptor dyad

We reported herein the spectroscopic, electrochemical and laser photolysis studies for the newly constructed light harvesting supramolecular dyad composed of gold porphyrin (AuPpy), as an electron acceptor, and zinc phthalocyanine (ZnPc), as an electron donor, to mimic the reaction centre in the photosynthetic system. For this, gold porphyrin has been functionalized by pyridine units, which axially coordinated with zinc phthalocyanine to form the stable supramolecular AuPpy:ZnPc with a rate of 2.94 × 104 M-1. Steady-state fluorescence measurements showed significant quenching of the singlet excited ZnPc emission with addition of AuPpy, suggesting an electron transfer from the singlet excited ZnPc to AuPpy. The electron transfer character was confirmed by recoding the characteristic absorption band of the zinc phthalocyanine radical cation in the NIR region by a femtosecond laser photolysis technique. The findings that the AuPpy:ZnPc supramolecular dyad exhibits relatively long-lived radical-ion pairs and absorbs light in a wide range of the solar spectrum suggest that it would be useful as a photosynthetic reaction centre.

Spectroscopic and thermodynamic studies of light harvesting perylenediimide derivative - zinc porphyrin complex in aqueous media

Self-assembly of perylene derivative such as N,N-bis(2(trimethylammonium iodide) ethylene)perylene-3,4,9,10-tetracarboxyldiimide (TAIPDI) can produce one-dimensional form (1D) in an aqueous media. The ability of one-dimensional TAIPDI to form light harvesting complex with water-soluble zinc porphyrin (ZnTPPS4) via the π-π and electrostatic interactions has been described. Owing to electronic interactions between the π-systems, the complex formation is accompanied by pronounced absorption spectral changes in the UV/Vis absorption bands. The formation constant of the ZnTPPS4-TAIPDI complex has been determined as 2.60×104M-1 suggests a moderately stable complex. The steady-state fluorescence measurements exhibited fluorescence quenching of both the singlet TAIPDI and ZnTPPS4 because of the electron transfer process from the electron-donating ZnTPPS4 to the electron-accepting TAIPDI. Based on the picosecond time-resolved fluorescence, the rate and quantum yield of the electron transfer were found to be 2.47×1010s-1 and 0.99, respectively, indicating fast and efficient electron transfer. The thermodynamic parameters of the complex formation have been determined from the stopped-flow measurements. The interaction between ZnTPPS4 and TAIPDI occurs in two steps, a fast and reversible step followed by a slow and irreversible one. The activation parameters for the complex formation (ΔH#=22±5kJmol-1 and ΔS#=-123±18JK-1mol-1), (ΔH#=133±4kJmol-1 and ΔS#=167±13Jmol-1K-1) were determined from variable temperature studies for the on and the off of the first step and ΔS#=246±37.89Jmol-1K-1 and ΔH#=130±11kJmol-1 for the second step. The negative and positive ΔS# values found for the interaction reactions are consistent with an associative interaction for the first step followed by dissociative mechanism for both the off and the second step.

Light-Harvesting Phthalocyanine-Diketopyrrolopyrrole Derivatives: Synthesis, Spectroscopic, Electrochemical, and Photochemical Studies.

A new family of light-harvesting zinc phthalocyanine (ZnPc)-diketopyrrolopyrrole (DPP) hybrids have been synthesized and characterized. The absorption spectral measurements showed that the major absorptions of DPP (450-600u2005nm) are complementary to those of zinc phthalocyanine (300-400 and 600-700u2005nm). Therefore, the designed hybrids absorb over a broad range in the visible region. The geometric and electronic structures of the dyads were probed by initio B3LYP/6-311G methods. The majority of the HOMOs were found to be located on the ZnPc, while the majority of the LUMOs were on the DPP units. The DPP units serve as the antenna, which upon excitation undergo efficient singlet-singlet energy transfer to the attached ZnPc units. The formed singlet ZnPc, in turn, donates its electron to the electron-deficient DPP forming the low-lying radical ion pairs ZnPc.+ -DPP.- (energy=1.44-1.56u2005eV as calculated from the electrochemical measurements). The excited-state events were confirmed by using a transient absorption technique in the picosecond-microsecond time range, as well as a time-resolved emission technique. The rates of energy transfer from the singlet DPP to ZnPc were found to be extremely fast >1010 u2005s-1 , while the rates of electron transfer from the singlet excited state of ZnPc to DPP were found to be 3.7-6.6×109 u2005s-1 .

Energy transfer between two light harvesting phthalocyanine derivatives as model for artificial photosynthetic antenna: Laser photolysis studies

The singlet-singlet and triplet-triplet energy transfer studies between two light harvesting phthalocyanine derivatives namely, zinc tert-butyl phthalocyanine ZnTBPc, and zinc octabutyloxy phthalocyanine ZnOBuPc, have been investigated to mimic the efficient energy-transfer process among the chlorophyll units in the natural photosynthesis. Optical absorption and emission studies showed the strong absorption and fluorescence bands for ZnOBuPc in the NIR region, which are largely red-shifted compared to that of the ZnTBPc. The strong overlap between the emission spectrum of ZnTBPc and the absorption spectrum of ZnOBuPc in toluene suggests the occurrence of the energy transfer from the singlet ZnTBPc (1.80u202feV) to the singlet ZnOBuPc (1.59u202feV). From the time-resolved fluorescence technique, the rate of energy transfer process was found to be 6.79u202f×u202f107u202fs-1. From the nanosecond laser photolysis measurements, the energy transfer from the triplet ZnTBPc to the lower lying triplet ZnOBuPc was clearly observed with a rate of 5.77u202f×u202f105u202fs-1. These observations suggest the usefulness of the examined combination (ZnOBuPc/ZnTBPc) as a simple model for the artificial antenna in the photosynthetic systems.

Decontamination of radioactive cesium ions using ordered mesoporous monetite

We report herein the fabrication of an environmentally friendly, low-cost and efficient nanostructured mesoporous monetite plate-like mineral (CaHPO4) as an adsorbent for removal of radioactive cesium ions from aqueous solutions. The phase and textural features of the synthesized mesoporous monetite were well characterized by XRD, FTIR, SEM, HRTEM, DLS, TGA/TDA, and N2 adsorption/desorption techniques. The results indicate that the cesium ions were effectively adsorbed by the mesoporous monetite ion-exchanger (MMT-IEX) above pH 9.0. Different kinetic and isotherm models were applied to characterize the cesium adsorption process. The fabricated monetite exhibited a monolayer adsorption capacity up to 60.33xa0mg g−1 at pH of 9.5. The collected data revealed the higher ability of CaHPO4 for the removal of Cs(I) from aqueous media in an efficient way.

Graphene oxide–metal oxide nanocomposites: fabrication, characterization and removal of cationic rhodamine B dye

The fabrication and characterization of graphene oxide (GO) nanosheets and their reaction with Fe3O4 and ZrO2 metal oxides to form two nanocomposites, namely graphene oxide–iron oxide (GO–Fe3O4) and graphene oxide–iron oxide–zirconium oxide (GO–Fe3O4@ZrO2), have been examined. The fabricated nanocomposites were examined using different techniques, e.g.transmission electron microscopy, X-ray diffraction, zeta potential measurement and Fourier transform infrared spectroscopy. Compared to GO, the newly fabricated GO–Fe3O4 and GO–Fe3O4@ZrO2 nanocomposites have the advantage of smaller band gaps, which result in increased adsorption capacity and photocatalytic effects. The results also showed the great effect of the examined GO–metal oxide nanocomposites on the decomposition of cationic rhodamine B dye, as indicated by steady-state absorption and fluorescence, time correlated single photon counting and nanosecond laser photolysis techniques. The antibacterial activity of the fabricated GO and GO–metal oxides has been studied against Gram-positive and Gram-negative bacteria.