Heinz Dürr

Preparation and application of new ruthenium(II) polypyridyl complexes as sensitizers for nanocrystalline TiO2

Abstract A series of Ru-(II) polypyridyl complexes, containing carboxylic- and sulphonic ‘anchor’ groups, were synthesized as sensitizers for nanocrystalline TiO 2 . Depending on number and type of ligands the sensitizers are divided into three classes: tris-homoleptic complexes 1a – d , tris-heteroleptic complexes 2a – d and bis-complexes 3a–f . The compounds were characterized and the photophysical and electrochemical properties were examined. These studies proved the suitability of 1a–3f as sensitizers for TiO 2 -based photoelectrochemical ‘Graetzel’-type solar cells. A comparison of the performance of the complexes 1a – 3f in the cell relative to Ru(dcbpy) 2 (NCS) 2 3d as internal standard for mean efficiency η is presented, whereby Ru(dcphen) 2 (NCS) 2 3f is shown to have an efficiency of η rel. =62.3% relative to 3d . A possible reason for the particular efficiency of 3d as sensitizer is discussed.

A New Photochromic System Based on a Pyridazinopyrrolo[1,2-b]pyridazine with Ultrafast Thermal Decoloration

The nucleophilic addition of the substituted pyridazines 2a−e to diacetylspirocyclopropene (1) in dry ether solution afforded the dihydroindolizines 3a−e, which undergo ring opening to the betaine 3′a−e after irradation with UV light. Condensation of the diacetyl groups of the dihydroindolizines (DHIs) 3a−e with hydrazine hydrate in a diethyl ether/ethanol mixture gave the new pyridazinopyrrolo[1,2-b]pyridazines 4a−e. Compounds 4a−e showed no photochromism at room temperature or after cooling with liquid nitrogen. However laser flash spectroscopy was successfully used for the determination of both the half-life and absorption maxima of the betaines 4′a−e.

Biomimetic approaches to the construction of supramolecular catalysts: titanium dioxide—platinum antenna catalysts to reduce water using visible light

Abstract Thermally and photochemically stable catalysts in systems that convert the energy of visible light into the chemical energy of dihydrogen gas have yet to be fully developed and all proposed systems suffer, to varying extents, from loss of activity with use. We have attempted to model natures photochemical reaction center through the use of TiO 2 antenna catalysts and very small amounts of Pt that form active centers on the TiO 2 semiconductor surface. This system is relatively inexpensive, reproducible, extremely stable, efficient in conversion of light to dihydrogen in aqueous solutions and displays higher linearity in dihydrogen evolution with greater efficiency in recycling than do other previously reported catalysts. Here we report the self-hydrogenation properties of our TiO 2 Pt catalysts in comparison with well known Pt, Ru and Os catalysts as well as their catalytic efficiency in systems for the sacrificial photoreduction of water. The sensitizers employed in this study were a recently synthesized new class of bis-heteroleptic complexes [Ru(bpy) 2 (PP)] 2+ , [Ru(bpy) 2 (PPB)] 2+ , [Ru(bpy) 2 (PPB-pCl)] 2+ containing one pyridyl—pyrimidine ligand and [Ru(bpy) 3 ] 2+ and [Ru(bpz) 3 ] 2+ as well known standard sensitizers. In order to obtain maximum information about the TiO 2 Pt catalysts we varied the components and conditions in our sacrificial systems for water reduction: MV 2+ , PVS MPVS were used as electron relays, EDTA and TEOA as sacrificial electron donors. Furthermore, the dependence of the dihydrogen evolution rates on the pH of the photolysis solution and its ionic strength were investigated. The techniques used in this study are cyclic voltammetry, steady state and time resolved luminescence spectroscopy, Stern—Volmer quenching, measurements of the quantum yields for PVS reduction, and volumetric measurements of dihydrogen photoproduction. A linear dependence was found between the dihydrogen production rate of the TiO 2 Pt catalysts and the quantum yield for the MV 2+ and PVS photoreduction at low ionic strength, showing that the new catalysts are reduced by the electron relays and convert protons into dihydrogen in a highly efficient manner

Steric substituent effects of new photochromic tetrahydroindolizines leading to tunable photophysical behavior of the colored betaines

Abstract Various substituted photochromic tetrahydroindolizine (THIs) derivatives 4a–r have been synthesized by nucleophilic addition of 1-phenyl-3,4-dihydroisoquinolines 2 to spirocyclopropene 1. After irradiation of the THIs 4a–r with UV light, a blue to green colored zwitterionic betaines 3a–r was formed which undergoes a thermal electrocyclization leading to colorless THIs 4a–r. Both the half-life of the thermal back reaction and the absorption spectra of the colored forms are found to be strongly dependent on the position and size of the substituent in the 10′b-phenyl ring. A strong pronounced effect was observed by comparison of each ortho-substituents to the corresponding para-substituents which showed increase in the half-life by a factor of 4 for o-, p-fluoro-substitution 3h–j and a factor of 2000 for o-, p-iodo-substitution 3q,r. Meta- and para-substitution showed no pronounced influence on the half-life. The steric effect of different groups was well proved by CHARMm calculation. The computational results provided a new insight into the reaction pathway signifying the steric effect of substituents on THIs 4. The energies of the unsubstituted THI 4a, the ortho-substituted THIs 4b,e,k and the meta-derivatives 4c,f,l as well as their betaines for the cisoid 3″ and transoid 3‴ were studied. The structures and energies for 3b,e,k,c,f,l were monitored. In conclusion, the formation of the twisted open form 3′ is responsible for value of the rate constant of the thermal cyclization.

Photoinduced electron transfer in supramolecular assemblies of transition metal complexes

Abstract Photoinduced electron transfer in supramolecular assemblies consisting of π -donor dialkoxyarene-functionalized photosensitizers and bipyridinium electron acceptors is examined. The photosensitizers include Ru(II)-tris-bipyridine complexetethered by multi-branch one-shell and two-shell dialkoxybenzene π -donor sites or a Zn(II)-porphyrin capped by a dialkoxybenzene receptor site. The photosensitizer/electron-acceptor supramolecular complexes behave as non-covalent diads and polyads. Effective internal electron transfer quenching within the supramolecular assemblies proceeds. A quantitative model that accounts for the photoinduced electron transfer in the systems is formulated.

Photochromic Spirotetrahydroazafluorenes: Part V. Why Photochromic Molecules with Rigid Region B Exhibiting Extremely Fast Bleaching Process?

ABSTRACT Spirotetrahydroindolizines (THIs) 1a-g and Spirotetrahydroazafluorenes (THF) 2a-g were previously synthesized. The high reaction rate of the thermal 1,5-electrocyclization results in the half-lives of the colored betaines 2′a-g range from 3.9 to 5.4 ns, thus showing the existence of the shortest lived colored form observed so far for pyrazoline based photochromes. However, laser flash spectroscopy was successfully used for the determination of both the half-life and absorption maxima of the colored betaines 2′a-g. Time resolved transient spectroscopy shows the geometry of the betaines 2′a-g to be related to one of the two detectable intermediates 1′a-g or 1′′a-g.

Emission spectra of photochromic spiro[1,8-a]dihydroindolizines and mechanism of the electrocyclic ring opening reaction

Abstract The photochemical pericyclic ring opening of the new photochromic system of spiro[1,8-a]dihydroindolizines (DHIs) was studied. From absorption and emission spectra the singlet energy E(S1) of the DHIs was calculated to be of the order of 60 – 70 kcal mol−1. Measurements of the fluorescence quantum yields φF and the singlet lifetimes τs of the DHIs (at room temperature and 77 K) as well as quenching experiments gave radiative lifetimes τrs which are two orders of magnitude larger than the τrs values calculated by the Strickler—Berg equation, suggesting that the two excited singlets involved in absorption and emission are not the same. From negative reaction quenching and negative chemiexcitation by dioxetane the excited state of the DHIs was concluded to be a singlet. The pericyclic ring opening may therefore be regarded as a fast singlet reaction.

A Novel 2,2′-Bipyridine[2]catenane and Its Ruthenium Complex: Synthesis, Structure, and Intramolecular Electron Transfer— A Model for the Photosynthetic Reaction Center

The complexation of a novel [2]catenane incorporating 2,2′-bipyridine and cyclobis(paraquat-p-phenylene) within a ruthenium complex afforded a catenane-type artificial-photosynthesis assembly. Studies by X-ray analysis, molecular modeling (MM2), NMR, and cyclic voltammetry revealed the catenane structure and indicated conformers with different sensitizer–acceptor distances. Emission studies and laser flash photolysis confirmed the occurrence of efficient photoinduced electron transfer between the noncovalently linked sensitizer (Ru2+ center) and the acceptor (bisviologen), as depicted. Biexponential decay with long-lived charge-separated states was observed.

Photochromic nucleic base units suitable for nucleic acid labelling

New molecules incorporating a uracil nucleic base and a dihydroindolizine (DHI) unit linked via spacer arms, i.e., uracil-spyrodihydroindolizine (4), were synthesised as models for light sensitive systems for nucleic acid labelling. The uracil-DHI (4) undergoes easy photocoloration to the uracil-betaine (5) generating the UV-detectable species. Preliminary results show rather weak binding of uracil-DHI (4) to calf thymus-DNA.

A new photochromic system - potential limitations and perspectives

A definition and a survey of property changes of photochromic systems is given. The known photochromic molecules are treated. A new class of photochromics is introduced, property changes are discussed and a short general overview of applications for classical and new systems is presented. Chemistry, especially photochemistry is developing rapidly in new directions. Whereas in the past molecules and their properties have been in the center of interest this aspect is only the basis for further problems or questions. Functions of molecules are an additional requirement which have to be investigated. Functions open then a way to applications either in the field of material science or life sciences. Photochromism is an exemplary field in this respect (ref. 1). This paper is treating these aspects in the context mentioned. Photochromism can be defined in simple terms as a light induced (at last in one direction) conversion of a species A to an isomer B, where A and B have different absorption spectra and energy content.