Zinc Porphyrin–Re(I) Bipyridyl–Fullerene Triad: Synthesis, Characterization, and Kinetics of the Stepwise Electron-Transfer Processes Initiated by Visible Excitation

A new triad system featuring one zinc porphyrin and one fullerene moieties attached to a central redox-active Re­(I) connector was obtained in remarkable yield by cleverly exploiting a facile two-step synthesis. Detailed description and discussion on the characterization of this multicomponent system and of its parent free-base analogue are presented, along with a kinetic study of the stepwise electron-transfer processes occurring upon visible excitation

Photoinduced Energy Transfer Processes within Dyads of Metallophthalocyanines Compactly Fused to a Ruthenium(II) Polypyridine Chromophore

An unsymmetric, peripherally octasubstituted phthalocyanine (Pc) 1, which contains a combination of dipyrido[3,2-f:2‘,3‘-h] quinoxaline and 3,5-di-tert-butylphenoxy substituents, has been obtained via a statistical condensation reaction of two corresponding phthalonitriles. Synthetic procedures for the selective metalation of the macrocyclic cavity and the periphery of 1 were developed, leading to the preparation of the key precursor metallophthalocyanines 3−5 in good yields. Two different strategies were applied to the synthesis of compact dyads MPc−Ru(II) 6−8 (M = Mg(II), Co(II), Zn(II)). Intramolecular electronic interactions in these dyads were studied by absorption, emission, and transient absorption spectroscopy. Upon photoexcitation, these dyads exhibit efficient intramolecular energy transfer from the Ru(II) chromophore to the MPc moiety

Kinetic Studies of the Reduction of [Co(dmgH)₂(py)(Cl)] Revisited: Mechanisms, Products, and Implications

We report on a mechanistic investigation regarding the reduction of [Co^III(dmgH)₂­(py)­(Cl)] (dmg = dimethylglyoxime) by several complementary techniques. The reduction of [Co^III(dmgH)₂­(py)­(Cl)] was initiated by either electrochemical, photochemical, or pulse radiolytical techniques, and the corresponding products were analyzed by ESI mass spectrometry. In addition, all of the rate constants for each step were determined. We have found solid experimental as well as theoretical evidence for the appearance of a dinuclear complex [Co^IICo^III­(dmgH)₄­(py)₂­(H₂O)₂]⁺ to be the final product of reduction, implying the initially reduced form of [Co^III(dmgH)₂­(py)­(Cl)] undergoes a dimerization with the starting material in solution

Supramolecular Assembly of Multicomponent Photoactive Systems via Cooperatively Coupled Equilibria

Here, we show that the synergistic interplay between two binding equilibria, acting at different sites of a (Zn)­phthalocyanine-amidine molecule (<b>Pc1</b>), enables the dissociation of the photoinactive phthalocyanine dimer (<b>Pc1</b>)₂ into a three-component system, in which a sequence of light harvesting, charge separation, and charge shift is successfully proven. The aforementioned dimer is assembled by dual amidine-Zn­(II) coordination between neighboring <b>Pc1</b> molecules and gives rise to high association constants (<i>K</i>_D ≈ 10¹¹ M^–1). Such extraordinary stability hampers the individual binding of either carboxylic acid ligands through the amidine group or pyridine-type ligands through the Zn­(II) metal atom to (<b>Pc1</b>)₂. However, the combined addition of both ligands, which cooperatively bind to different sites of <b>Pc1</b> through distinct noncovalent interactions, efficiently shifts the overall equilibrium toward a photoactive tricomponent species. In particular, when a fullerene-carboxylic acid (<b>C</b>_<b>60</b><b>A</b>) and either a dimethylamino-pyridine (<b>DMAP</b>) or a phenothiazine-pyridine ligand (<b>PTZP)</b> are simultaneously present, the photoactivity is turned on and evidence is given for an electron transfer from photoexcited <b>Pc1</b> to the electron-accepting <b>C</b>_<b>60</b><b>A</b> that affords the <b>DMAP-Pc1</b>^•+-<b>C</b>_<b>60</b><b>A</b>^•– or <b>PTZP-Pc1</b>^•+-<b>C</b>_<b>60</b><b>A</b>^•– radical ion pair states. Only in the latter case does a cascade of photoinduced electron transfer processes afford the <b>PTZP</b>^•+<b>-Pc1-C</b>_<b>60</b><b>A</b>^•– radical ion pair state. The latter is formed via a thermodynamically driven charge shift evolving from <b>PTZP-Pc1</b>^•+-<b>C</b>_<b>60</b><b>A</b>^•– and exhibits lifetimes that are notably longer than those of <b>DMAP-Pc1</b>^•+-<b>C</b>_<b>60</b><b>A</b>^•–

Chemical Modification of a Tetrapyrrole-Type Photosensitizer: Tuning Application and Photochemical Action beyond the Singlet Oxygen Channel

Reactive oxygen species (ROS) formed by light activated photosensitizers (PSs) are the hallmark of photodynamic therapy (PDT). It is generally accepted that commonly used PSs generate singlet oxygen (¹O₂) as the cell-toxic species via type II photosensitization. We explored here the consequences of chemical modification and the influence of the net charge of a cationic tetrahydroporphyrin derivative (THPTS) relative to the basic molecular structure on the red-shift of absorption, solubility, mechanistic features, and photochemical as well as cell-toxic activity. In order to shed light into the interplay between chemical modification driven intra- and intermolecular photochemistry, intermolecular interaction, and function, a number of different spectroscopic techniques were employed and our experimental studies were accompanied by quantum chemical calculations. Here we show that for THPTS neither ¹O₂ nor other toxic ROS (superoxide and hydroxyl radicals) are produced directly in significant quantities in aqueous solution (although the formation of singlet oxygen is energetically feasible and as such observed in acetonitrile). Nevertheless, the chemically modified tetrapyrrole photosensitizer displays efficient cell toxicity after photoexcitation. The distribution and action of THPTS in rat bladder caricinoma AY27 cells measured with fluorescence lifetime imaging microscopy shows accumulation of the THPTS in lysosomes and efficient cell death after irradiation. We found evidence that THPTS in water works mainly via the type I mechanism involving the reduction rather than oxidation of the excited triplet state THPTS­(T₁) via efficient electron donors in the biosystem environment and subsequent electron transfer to produce ROS indirectly. These intriguing structure–activity relationships may indeed open new strategies and avenues in developing PSs and PDT in general

Case Study for Artificial Photosynthesis: Noncovalent Interactions between C₆₀-Dipyridyl and Zinc Porphyrin Dimer

In this study, a new modified C₆₀ derivative with an oPE/oPPV conjugated bridge bearing two pyridyl groups has been used in combination with a flexible porphyrin dimer (<b>ZnP</b>_<b>2</b>) to construct an electron donor/acceptor hybrid (<b>C</b>_<b>60</b><b>-dipyr·ZnP</b>_<b>2</b>). This hybrid is based on metal to ligand coordination between the zinc centers of the porphyrin dimer and the two pyridyl groups that oPE/oPPV linker bears. In order to investigate the interactions between the electron donor and acceptor entities, both in the ground state and in the excited states, comprehensive photophysical assays have been carried out. In particular, both absorption and fluorescence titrations provided evidence for strong interactions between the electron donor and the electron acceptor within the hybrid. A binding constant (<i>K</i>_ass) in the order of 5.0 × 10⁵ M^–1 has been derived. Furthermore, transient absorption measurements revealed intramolecular electron-transfer from the photoexcited porphyrin dimer (<b>ZnP</b>_<b>2</b>) to the fullerene derivative (<b>C</b>_<b>60</b><b>-dipyr</b>), leading to a long-lived charge-separated state with a lifetime of up to 1525 ps

Antenna Effect in BODIPY-(Zn)Porphyrin Entities Promotes H₂ Evolution in Dye-Sensitized Photocatalytic Systems

In this study, we report the utilization of BODIPY-(Zn)­Porphyrin hybrids in photocatalytic H2 production from water. These entities were applied as photosensitizers upon their chemisorption onto the surface of platinum-doped titanium dioxide nanoparticles (Pt-TiO2), which acted as photocatalysts. To evaluate the impact of the different connectivity between the chromophores in photocatalytic in H2 evolution, we employed two diverse BODIPY-(Zn)­Porphyrin entities, in which the BODIPY moiety is either covalently attached (BDP-Por) or axially coordinated (BDP­(Im)-Por) with the (Zn)­Porphyrin. The covalently connected dyad (BDP-Por) presented higher catalytic activity (17 500 TONs) compared to the axial coordinated (BDP­(Im)-Por, 13 700 TONs). In BDP-Por dyad, an additional BDP­(Im) moiety was introduced and the formed hybrid (BDP-Por-BDP­(Im)) outperformed the aforementioned systems, due to the enhanced light harvesting ability. Overall, we developed highly efficient dye-sensitized photocatalytic systems (DSPs) based on noble-metal-free photosensitizers reaching 18 600 turnover numbers (TONs) and 225 mmol­(H2) g­(cat)−1 h–1

Synthesis and Photophysics of Coaxial Threaded Molecular Wires: Polyrotaxanes with Triarylamine Jackets

Conjugated polyrotaxanes jacketed with hole-transport groups have been synthesized from water-soluble polyrotaxanes consisting of a polyfluorene-alt-biphenylene (PFBP) conjugated polymer threaded through β-cyclodextrin macrocycles. The hydroxyl groups of the oligosaccharides were efficiently functionalized with triphenylamine (TPA) so that every polyrotaxane molecule carries a coat of about 200 TPA units, forming a supramolecular coaxial structure. This architecture was characterized using a range of techniques, including small-angle X-ray scattering. Absorption of light by the TPA units results in excitation energy transfer (EET) and photoinduced electron transfer (ET) to the inner conjugated polymer core. These energy- and charge-transfer processes were explored by steady-state and time-resolved fluorescence spectroscopy, femtosecond transient absorption spectroscopy, and molecular modeling. The time-resolved measurements yielded insights into the heterogeneity of the TPA coat: those TPA units which are close to the central polymer core tend to undergo ET, whereas those on the outer surface of the polyrotaxane, far from the core, undergo EET. Sections of the backbone that are excited indirectly via EET tend to be more remote from the TPA units and thus are less susceptible to electron-transfer quenching. The rate of EET from the TPA units to the PFBP core was effectively modeled by taking account of the heterogeneity in the TPA–PFBP distance, using a distributed monopole approach. This work represents a new strategy for building and studying well-defined arrays of >100 covalently linked chromophores

Surface Functionalization and Electronic Interactions of ZnO Nanorods with a Porphyrin Derivative

To optimize electron transfer and optoelectronic properties in nanoparticulate thin films for electronics we show the surface functionalization of ZnO nanorods by means of replacing surface active 2-[2-(2-methoxyethoxy)­ethoxy]­acetic acid (TODA) by a redoxactive organic component, that is, 5,10,15,20-(phenoxyacetat)-porphyrin bearing four carboxylic acids as possible ZnO anchors. Microscopy–transmission electron microscopy–and spectroscopy–optical spectroscopy–verifies the successful and homogenous integration of the porphyrin onto the surface of ZnO nanorods, a process that is facilitated by the four anchoring groups. Photophysical investigations based on emission and absorption spectroscopy prompt to distinct electronic interactions between ZnO nanorods and the porphyrins. Consequently, we performed further photophysical studies flanked by pulse radiolysis assays to corroborate the nature of the electronic interactions