Péter Baranyai

Triplet state spectroscopic studies on some 5,10,15,20-tetrakis(methoxyphenyl)porphyrins

Abstract Extensive triplet state spectroscopic investigations were carried out with a series of 5,10,15,20-tetrakis(methoxyphenyl)porphyrins. Triplet absorption spectra, triplet lifetime, triplet quantum yield and quantum yield for singlet oxygen production were determined with different absorption and emission techniques, using the frequency-doubled beam of a Nd:YAG laser. It has been found that these synthetic porphyrins are effective photosensitizers which can be used as model compounds to investigate the theoretical and instrumental aspects of PDT.

Anion‐, Solvent‐, Temperature‐, and Mechano‐Responsive Photoluminescence in Gold(I) Diphosphine‐Based Dimers

A series of [Au2 (nixantphos)2](X)2 (nixantphos=4,6-bis(diphenylphosphino)-phenoxazine; X=NO3, 1; CF3 COO, 2; CF3 SO3, 3; [Au(CN)2], 4; and BF4, 5) complexes that exhibit intriguing anion-switchable and stimuli-responsive luminescent photophysical properties have been synthesized and characterized. Depending on their anions, these complexes display yellow (3), orange (4 and 5), and red (1 and 2) emission colors. They exhibit reversible thermo-, mechano-, and vapochromic luminescence changes readily perceivable by the naked eye. Single-crystal X-ray studies show that the [Au2 (nixantphos)2](2+) cations with short intramolecular Au⋅⋅⋅Au interactions are involved as donors in an infinite N-H⋅⋅⋅X (X=O and N) hydrogen-bonded chain formation with CF3 COO(-) (2 C) and aurophilically linked [Au(CN)2](-) counterions (4 C). Both crystals show thermochromic luminescence; their room temperature red (2 C) and orange (4 C) emission turns into yellow upon cooling to 77 K. They also exhibit reversible mechanochromic luminescence by changing their emission color from red to dark (2 C), and orange to red (4 C). Compounds 1-5 also display reversible mechanochromic luminescence, altering their emission colors between orange (1) or red (2) to dark, as well as between yellow (3) or orange (4 and 5) to red. Detailed photophysical investigations and correlation with solid-state structural data established the significant role of NH⋅⋅⋅X interactions in the stimuli-responsive luminescent behavior.

MONTE-CARLO MODEL FOR THE HYDROGENATION OF ALKENES ON METAL CATALYST

A Monte‐Carlo model for the simulation of alkene hydrogenation on metallic catalysts has been developed and implemented in Fortran language. We describe the model employed for ethylene hydrogenation on platinum and show the flow chart of the program. Computational characteristics such as number of necessary calculations to reach steady state, running times on different platforms, and effect of the size of the catalyst matrix, are presented. Good correlation between simulated and experimental data was observed. A subroutine allows for visual observation of the reaction. This approach is very useful for obtaining a personal impression of the important factors governing the reaction. By using this example the advantages of Monte‐Carlo simulation to test the level of understanding of catalytic phenomena are discussed. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 396–403, 1998

A stimuli-responsive double-stranded digold(I) helicate

We report herein a stimuli-responsive dinuclear double stranded [Au2L2]2+ helicate assembled from gold(I) atoms and phenyl-substituted diphosphine ligands derived from a xanthene-type backbone (L). The conformational flexibility of the dinuclear [Au2L2]2+ helicate allows a diversity of molecular conformations and packing arrangements that lead to different solid-state emission colours. Blue (IB), bluish green (IIG) and yellow (IIIY) emitting crystalline and red emitting (IVR) amorphous forms of this double stranded [Au2L2]2+ helicate have been obtained by slight modification of the crystallization conditions. Different molecular conformations and packing arrangements of dinuclear double stranded [Au2L2]2+ helicates that result in different non-covalent interactions played the most significant role in tailoring the solid-state luminescence properties. On the basis of the single crystal structural data and photophysical studies, we found that an increasing number of intra- and mainly intermolecular noncovalent interactions locked and rigidified the twisted conformation of the double stranded [Au2L2]2+ helicate, and enhanced π-stacking between its aromatic units induced the red-shift in solid-state luminescence emission. The solid-state luminescence colour of this double stranded [Au2L2]2+ helicate can be switched reversibly from blue to red by external (mechanical and chemical) stimuli.

Photodynamic effects of novel 5,15-diaryl-tetrapyrrole derivatives on human colon carcinoma cells.

Preliminary in vitro cytotoxicity studies on a panel of meso diaryl-substituted tetrapyrrole derivatives newly synthesized in our laboratory have shown that these compounds are photodynamically active on the human colon carcinoma cell line HCT116. In the present study, we investigate some mechanistic aspects of the photodynamic action of the most active compounds in the series, namely the 5-phenyl-15-(3-methoxyphenyl)porphyrin (1), the 5-phenyl-15-(3-hydroxyphenyl)porphyrin (2) and the 5,15-diphenylporphyrin (3). The results of the cytotoxicity studies indicate that the novel photosensitisers (PSs) are more potent in vitro than m-THPC (Foscan), a powerful PS already approved for clinical use in photodynamic therapy (PDT). A series of experiments were performed to elucidate a number of aspects in the mechanism of PS-induced phototoxicity, including, intracellular accumulation and subcellular localization of the PSs, induction of apoptosis, and generation of reactive oxygen species (ROS) and NO*. All the compounds tested exhibit similar singlet oxygen quantum yields; differential intracellular accumulation can contribute to the observed differences in phototoxicity. Flow cytometric studies indicate that all the tested compounds induce apoptosis; however, their cytotoxic effect does not seem to rely solely on this process. Generation of significant amounts of reactive oxygen species (ROS) and NO* were also observed; however, the contribution of this latter effect to the overall phototoxicity is unclear. Taken together, our observations suggest that the diaryl derivatives included in the present study could represent promising leads for the development of novel photosensitizing agents.

On the selection and design of proteins and peptide derivatives for the production of photoluminescent, red-emitting gold quantum clusters

Novel pathways of the synthesis of photoluminescent gold quantum clusters (AuQCs) using biomolecules as reactants provide biocompatible products for biological imaging techniques. In order to rationalize the rules for the preparation of red-emitting AuQCs in aqueous phase using proteins or peptides, the role of different organic structural units was investigated. Three systems were studied: proteins, peptides, and amino acid mixtures, respectively. We have found that cysteine and tyrosine are indispensable residues. The SH/S-S ratio in a single molecule is not a critical factor in the synthesis, but on the other hand, the stoichiometry of cysteine residues and the gold precursor is crucial. These observations indicate the importance of proper chemical behavior of all species in a wide size range extending from the atomic distances (in the AuI-S semi ring) to nanometer distances covering the larger sizes of proteins assuring the hierarchical structure of the whole self-assembled system.

Quenching of Porphyrin Triplet and Singlet Oxygen by Stable Nitroxide Radicals: Importance of Steric Hindrance

To study the nature of quenching, we used stable nitroxide radicals (most of which contain piperidine or pyrrolidine rings) as quenchers of triplet hematoporphyrin as well as of singlet molecular oxygen. A characteristic feature of quenching triplet porphyrin is a near-diffusion-limited rate constant, whereas the rate constant for quenching singlet oxygen is about three orders of magnitude lower. Accessibility of the nitroxide moiety in radicals was characterized quantitatively based on semi-empirical calculations (at AM1 level), with the use of van der Waals radii of the species. While variation in the rate constant values for quenching triplet porphyrin can be fully explained by the steric hindrance of the neighbouring groups of the nitroxide radical, no such effect can be observed in quenching singlet oxygen.

Novel gold(I) diphosphine-based dimers with aurophilicity triggered multistimuli light-emitting properties

We report a design strategy for the preparation of stimuli-responsive materials with multicolour emission that is based on a single type of luminophore molecule comprising gold(I) and a flexible diphosphine ligand. Multistimuli-responsive luminescent dinuclear [Au2(dpephos)2](X)2 (dpephos = bis(2-diphenylphosphino)-phenyl ether; X = NO3, 1; CF3SO3, 2; BF4, 3; PF6, 4; SbF6, 5) complexes were obtained by rapid mechanochemical syntheses. The single crystal X-ray diffraction analysis of complexes 1–5 revealed that the [Au2(dpephos)2]2+ cations do not exhibit short intramolecular aurophilic interactions. Supported by the structural flexibility of the dpephos ligand, these intramolecular Au⋯Au contacts are highly sensitive to external stimuli, such as light, temperature, mechanical grinding and exposure to solvent vapours. The ensuing materials, which comprise only gold(I)–diphosphine luminophores, can emit green, yellow and red colours, and the colour of the emitted light can be switched by light, heat or pressure. A change from green to yellow in the emission was observed upon heating to room temperature under the 365 nm excitation, whereas a change from yellow to red was achieved by mechanical grinding. Moreover, the green emission can be switched to red by changing the excitation wavelength to 312 nm. All these reversible luminescence colour changes are readily perceivable by the naked eye, and they are attributed to small structural modifications induced by external stimuli that also modulate the intramolecular Au⋯Au interaction. The red emission from these gold(I) diphosphine-based dimers can be attributed to the presence of intramolecular aurophilic interaction.

Synthesis and Enantiomeric Recognition Studies of Optically Active Pyridino-Crown Ethers Containing an Anthracene Fluorophore Unit

Novel enantiopure pyridino-18-crown-6 ether-based sensor molecules containing an anthracene fluorophore unit were synthesized. Their enantiomeric recognition abilities toward the enantiomers of 1-phenylethylamine hydrogen perchlorate (PhEt), 1-(1-naphthyl)ethylamine hydrogen perchlorate (NapEt), phenylglycine methyl ester hydrogen perchlorate (PhgOMe), and phenylalanine methyl ester hydrogen perchlorate (PheOMe) were examined in acetonitrile using fluorescence spectroscopy. The sensor molecules showed appreciable enantiomeric recognition toward the enantiomers of NapEt, PhEt, and PhgOMe. The highest enantioselectivity was found in the case of crown ether containing isobutyl groups in the macroring and the enantiomers of NapEt. Chirality 28:562-568, 2016.

A H-shaped heterometallic Sn4Au4 system with guest-tuneable multicolour and selective luminescence sensing properties

A H-shaped heterometallic Sn4Au4 system that displays tuneable guest-dependent optical and photoluminescent properties has been synthesized. It shows selective dye sorption, while its stimuli-responsive methylred-rich solid is sensitive towards pH variations. This non-emissive Sn4Au4 system exhibits multicolour luminescence in the presence of aniline derivatives.