Photo-excitation of a light-harvesting supra-molecular triad: a Time-Dependent DFT study

We present the first time-dependent density-functional theory (TDDFT) calculation on a light harvesting triad carotenoid-diaryl-porphyrin-C60. Besides the numerical challenge that the ab initio study of the electronic structure of such a large system presents, we show that TDDFT is able to provide an accurate description of the excited state properties of the system. In particular we calculate the photo-absorption spectrum of the supra-molecular assembly, and we provide an interpretation of the photo-excitation mechanism in terms of the properties of the component moieties. The spectrum is in good agreement with experimental data, and provides useful insight on the photo-induced charge transfer mechanism which characterizes the system.Comment: Accepted for publication on JPC, March 09th 200

Synthese und Charakterisierung von amphiphilen Porphyrin-Perylen Dyaden: Die Grundlagen für optoelektronische Membranen in Wasser

In this dissertation, two unsymmetrically substituted chromophores (porphyrin and perylene) were synthesized and coupled via a linear and rigid phenyl-ethynyl-phenyl linker to a dyad. By introduction of a Newkome dendron with three or nine negative charges, four novel and water-soluble porphyrin–perylene assemblies were produced for the first time. The 18–21 step synthesis procedure was planned, realized and constantly optimized. Using an improved perylene synthesis cascade with novel purification methods, complex perylene imides were obtained in 80–90% yield. A multistep porphyrin synthesis provided unsymmetrical AB2C porphyrins without scrambling in up to 30% yield. The four synthesized dyads were systematically analyzed via NMR, IR, MS, UV-Vis and emission spectroscopy. The absorption and emission behavior in THF and water indicated a photoinduced excited-state interaction between both chromophores and the formation of a porphyrin(+)–linker–perylene(–) charge-separated state. Energy and electron transfer between porphyrin and perylene were calculated, simulated and measured by spectroscopic experiments. The aggregation behavior could be adjusted by addition of base or by amine/nitrogen compounds (such as pyridine) which were found to coordinate axially to the zinc porphyrin in the dyad.Im Rahmen dieser Dissertation wurden zwei unsymmetrisch substituierte Chromophore (Porphyrin und Perylen) synthetisiert und mittels einer linearen und starren Phenyl-ethinyl-phenyl-Brücke zu einer Dyade verknüpft. Durch Anfügen eines Newkome-Dendrons mit drei bzw. neun negativen Ladungen wurden erstmalig vier neuartige und wasserlösliche Porphyrin–Perylen-Systeme hergestellt. Der 18–21-stufige Syntheseprozess wurde geplant, durchgeführt und durchgehend optimiert. Durch eine verbesserte Perylen-Synthese-Kaskade mit neuentwickelten Aufreinigungs-methoden wurden komplexe Perylenimide in 80–90% Ausbeute erhalten. Eine mehrstufige Porphyrinsynthese ermöglichte es, unsymmetrische AB2C-Porphyrine ohne Scrambling in bis zu 30% Ausbeute zu erhalten. Die vier synthetisierten Dyaden wurden intensiv mittels NMR, IR, MS, UV-Vis und Emissionsspektroskopie untersucht. Das Absorptions- und Emissionsverhalten in THF und Wasser wies auf eine photoinduzierte Interaktion zwischen beiden Chromophoren und auf die Entstehung eines ladungsgetrennten Porphyrin(+)–Linker–Perylene(–) Zustandes hin. Der Energie- und Elektronentransfer zwischen Porphyrin und Perylen wurde berechnet, simuliert und in spektroskopischen Experimenten nachgewiesen. Das Aggregationsverhalten konnte reguliert werden durch Zugabe von Base oder durch Amin- bzw. Stickstoffverbindungen (z.B. Pyridin), welche axial an das Zink-Porphyrin der Dyade koordinierten

Comparison of rhenium–porphyrin dyads for CO₂ photoreduction: photocatalytic studies and charge separation dynamics studied by time-resolved IR spectroscopy

We report a study of the photocatalytic reduction of CO₂ to CO by zinc porphyrins covalently linked to [ReI(2,2′-bipyridine)(CO)₃L]⁺/⁰ moieties with visible light of wavelength >520 nm. Dyad 1 contains an amide C₆H₄NHC(O) link from porphyrin to bipyridine (Bpy), Dyad 2 contains an additional methoxybenzamide within the bridge C₆H₄NHC(O)C₆H₃(OMe)NHC(O), while Dyad 3 has a saturated bridge C₆H₄NHC(O)CH₂; each dyad is studied with either L = Br or 3-picoline. The syntheses, spectroscopic characterisation and cyclic voltammetry of Dyad 3 Br and [Dyad 3 pic]OTf are described. The photocatalytic performance of [Dyad 3 pic]OTf in DMF/triethanolamine (5 : 1) is approximately an order of magnitude better than [Dyad 1 pic]PF₆ or [Dyad 2 pic]OTf in turnover frequency and turnover number, reaching a turnover number of 360. The performance of the dyads with Re–Br units is very similar to that of the dyads with [Re–pic]⁺ units in spite of the adverse free energy of electron transfer. The dyads undergo reactions during photocatalysis: hydrogenation of the porphyrin to form chlorin and isobacteriochlorin units is detected by visible absorption spectroscopy, while IR spectroscopy reveals replacement of the axial ligand by a triethanolaminato group and insertion of CO₂into the latter to form a carbonate. Time-resolved IR spectra of [Dyad 2 pic]OTf and [Dyad 3 pic]OTf (560 nm excitation in CH₂Cl₂) demonstrated electron transfer from porphyrin to Re(Bpy) units resulting in a shift of ν(CO) bands to low wavenumbers. The rise time of the charge-separated species for [Dyad 3 pic]OTf is longest at 8 (±1) ps and its lifetime is also the longest at 320 (±15) ps. The TRIR spectra of Dyad 1 Br and Dyad 2 Br are quite different showing a mixture of 3MLCT, IL and charge-separated excited states. In the case of Dyad 3 Br, the charge-separated state is absent altogether. The TRIR spectra emphasize the very different excited states of the bromide complexes and the picoline complexes. Thus, the similarity of the photocatalytic data for bromide and picoline dyads suggests that they share common intermediates. Most likely, these involve hydrogenation of the porphyrin and substitution of the axial ligand at rhenium

Design and synthesis of benzimidazole phenol-porphyrin dyads for the study of bioinspired photoinduced proton-coupled electron transfer

Benzimidazole phenol-porphyrin dyads have been synthesized to study proton-coupled electron transfer (PCET) reactions induced by photoexcitation. High-potential porphyrins have been chosen to model P680, the photoactive chlorophyll cluster of photosynthetic photosystem II (PSII). They have either two or three pentafluorophenyl groups at the meso positions to impart the high redox potential. The benzimidazole phenol (BIP) moiety models the Tyrz-His190 pair of PSII, which is a redox mediator that shuttles electrons from the water oxidation catalyst to P680•+. The dyads consisting of a porphyrin and an unsubstituted BIP are designed to study one-electron one-proton transfer (E1PT) processes upon excitation of the porphyrin. When the BIP moiety is substituted with proton-accepting groups such as imines, one-electron two-proton transfer (E2PT) processes are expected to take place upon oxidation of the phenol by the excited state of the porphyrin. The bis-pentafluorophenyl porphyrins linked to BIPs provide platforms for introducing a variety of electron-accepting moieties and/or anchoring groups to attach semiconductor nanoparticles to the macrocycle. The triads thus formed will serve to study the PCET process involving the BIPs when the oxidation of the phenol is achieved by the photochemically produced radical cation of the porphyrin.Fil: Mora, Sabrina Jimena. Arizona State University; Estados Unidos. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Departamento de Química Orgánica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Heredia, Daniel Alejandro. Universidad Nacional de Río Cuarto. Instituto para el Desarrollo Agroindustrial y de la Salud. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto para el Desarrollo Agroindustrial y de la Salud; ArgentinaFil: Odella, Emmanuel. Arizona State University; Estados Unidos. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales. Departamento de Química; ArgentinaFil: Vrudhula, Uma. Arizona State University; Estados UnidosFil: Gust, Devens. Arizona State University; Estados UnidosFil: Moore, Thomas A.. Arizona State University; Estados UnidosFil: Moore, Ana L.. Arizona State University; Estados Unido

Development and Application of Porphyrin-Maquette Complexes: Towards Artificial Photosynthesis

This study looks at developing an artificial reaction centre with photogenerated electron transfer. Naturally occurring photosynthetic reaction centres utilise proteins to house photoactive material, which is coupled with redox enzymes for catalytic reactions to produce chemical fuels. Photosynthetic systems provide inspiration for designing new means to produce renewable energy, whereby sunlight can produce fuels in the form of chemical energy. Artificially designed proteins with synthetically produced porphyrins can potentially be employed to mimic the natural photosynthetic reaction centre for light-induced charge separation. This charge separation is fundamental for conversion of light energy to other forms of energy