Synthesis and Photo/Radiation Chemical Characterization of a New Redox-Stable Pyridine-Triazole Ligand.

Photocatalysis using transition-metal complexes is widely considered the future of effective and affordable clean-air technology. In particular, redox-stable, easily accessible ligands are decisive. Here, we report a straightforward and facile synthesis of a new highly stable 2,6-bis(triazolyl)pyridine ligand, containing a nitrile moiety as a masked anchoring group, using copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) click reaction. The reported structure mimics the binding motif of uneasy to synthesize ligands. Pulse radiolysis under oxidizing and reducing conditions provided evidence for the high stability of the formed radical cation and radical anion 2,6-di(1,2,3-triazol-1-yl)-pyridine compound, thus indicating the feasibility of utilizing this as a ligand for redox active metal complexes and the sensitization of metal-oxide semiconductors (e.g., TiO2 nanoparticles or nanotubes).

Autofluorescent antimalarials by hybridization of artemisinin and coumarin: in vitro/in vivo studies and live-cell imaging.

Malaria is one of our planet's most widespread and deadliest diseases, and there is an ever-consistent need for new and improved pharmaceuticals. Natural products have been an essential source of hit and lead compounds for drug discovery. Antimalarial drug artemisinin (ART), a highly effective natural product, is an enantiopure sesquiterpene lactone and occurs in Artemisia annua L. The development of improved antimalarial drugs, which are highly potent and at the same time inherently fluorescent is particularly favorable and highly desirable since they can be used for live-cell imaging, avoiding the requirement of the drug's linkage to an external fluorescent label. Herein, we present the first antimalarial autofluorescent artemisinin-coumarin hybrids with high fluorescence quantum yields of up to 0.94 and exhibiting excellent activity in vitro against CQ-resistant and multidrug-resistant P. falciparum strains (IC50 (Dd2) down to 0.5 nM; IC50 (K1) down to 0.3 nM) compared to reference drugs CQ (IC50 (Dd2) 165.3 nM; IC50 (K1) 302.8 nM) and artemisinin (IC50 (Dd2) 11.3 nM; IC50 (K1) 5.4 nM). Furthermore, a clear correlation between in vitro potency and in vivo efficacy of antimalarial autofluorescent hybrids was demonstrated. Moreover, deliberately designed autofluorescent artemisinin-coumarin hybrids, were not only able to overcome drug resistance, they were also of high value in investigating their mode of action via time-dependent imaging resolution in living P. falciparum-infected red blood cells.

Efficient charge-transfer from diketopyrrolopyrroles to single-walled carbon nanotubes.

In this contribution, the excited state charge-transfer interactions between single-walled carbon nanotubes (SWCNTs) and a variety of phenyl, 4-bromophenyl, and thiophene substituted diketopyrrolopyrroles (DPPs), is described. Atomic force microscopy (AFM) and aberration corrected high resolution transmission electron microscopy (AC-HRTEM) corroborated the successful formation of DPP/SWCNTs. Steady-state absorption, fluorescence, and Raman spectroscopies all gave insights into the impact on their ground and excited states as well as on the nature of their electronic communication/interaction. Of great value was time-resolved transient absorption spectroscopy on the femto- and nanosecond time-scales; it assisted in deciphering the charge-transfer mechanism from the DPPs to the SWCNT and in analyzing the dynamics thereof with transfer efficiencies of up to 81%. Important confirmation for the one-electron oxidized DPPs came from pulse radiolysis assays with focus on establishing their spectral fingerprints. Our full-fledged work demonstrates that the successful preparation of stable DPP/SWCNTs represents an important step towards establishing them as a viable alternative to porphyrin-based systems in emerging applications such as solar energy conversion.

Controlling and Fine-Tuning Charge-Transfer Emission in 2,6-Dicyanoaniline Multichromophores Prepared through Domino Reactions: Entry to a Potentially New Class of OLEDs.

Substituted 2,6-dicyanoanilines are versatile electron donor-acceptor compounds, which have recently received considerable attention, since they exhibit strong fluorescence and may have utility in the synthesis of fluorescent materials, non-natural photosynthetic systems, and materials with nonlinear optical properties. The majority of known synthetic procedures are, however, "stop-and-go" reaction processes involving time-consuming and waste-producing isolation and purification of product intermediates. Here, we present the synthesis of substituted 2,6-dicyanoanilines via atom-economical and eco-friendly one-pot processes, involving metal-free domino reactions, and their subsequent photochemical and photophysical measurements and theoretical calculations. These studies exhibit the existence of an easily tunable radical ion pair-based charge-transfer (CT) emission in the synthesized 2,6-dicyanoaniline-based electron donor-acceptor systems. The charge-transfer processes were explored by photochemical and radiation chemical measurements, in particular, based on femtosecond laser photolysis transient absorption spectroscopy and time-resolved emission spectroscopy, accompanied by pulse radiolysis and complemented by quantum chemical investigations employing time-dependent density-functional theory. This chromophore class exhibits a broad-wavelength-range fine-tunable charge recombination emission with high photoluminescence quantum yields up to 0.98. Together with its rather simple and cost-effective synthesis (using easily available starting materials) and customizable properties, it renders this class of compounds feasible candidates as potential dyes for future optoelectronic devices like organic light-emitting diodes (OLEDs).

Effect of the triazole ring in zinc porphyrin-fullerene dyads on the charge transfer processes in NiO-based devices.

Herein, the synthesis of three covalently linked donor-acceptor zinc porphyrin-fullerene (ZnP-C60) dyads (C60trZnPCOOH, C60trZnPtrCOOH and C60ZnPCOOH) is described, and their application as sensitizers in NiO-based dye-sensitized solar cells (DSCs) is discussed. To the best of our knowledge, this is the first example where covalently linked ZnP-C60 dyads have been used as chromophores in NiO-based DSCs. In an effort to examine whether the distance of the chromophore from the electron acceptor entity and/or the NiO surface affects the performance of the cells, a triazole ring was introduced as a spacer between ZnP and the two peripheral units C60 and -COOH. The triazole ring was inserted between ZnP and C60 in dyad C60trZnPCOOH, whereas both the anchoring group and C60 were connected to ZnP through triazole spacers in C60trZnPtrCOOH, and dyad C60ZnPCOOH did not contain any triazole linker. Photophysical investigation performed by ultrafast transient absorption spectroscopy in solution and on the NiO surface demonstrated that all the porphyrin-fullerene dyads exhibited long-lived charge-separated states due to electron shifts from the reduced porphyrin core to C60. The transient experiments performed in solution showed that the presence of triazole ring influenced the photophysical properties of the dyads C60trZnPCOOH and C60trZnPtrCOOH and in particular, increased the lifetime of the charge-separated states compared to that of the C60ZnPCOOH dyad. On the other hand, the corresponding studies on the NiO surface proved that the triazole spacer has a rather moderate impact on the charge separation (NiO-ZnP˙+-C60˙-) and charge recombination (NiO-3*ZnP-C60) rate constants. All three dyads exhibited enhanced performance in terms of photovoltaic measurements with more than threefold increase compared to the reference compound PhtrZnPCOOH in which the C60 acceptor is absent. Two different electrolytes were examined (I3-/I- and CoIII/II) and in most cases, the presence of the triazole ring enhanced their photovoltaic performance. The best performing dyad in I3-/I- was C60trZnPCOOH (PCE = 0.076%); in CoIII/II, the best performing dyad was C60trZnPtrCOOH (PCE = 0.074%).

Interfacing tetrapyridyl-C60 with porphyrin dimers via π-conjugated bridges: artificial photosynthetic systems with ultrafast charge separation.

We report on the synthesis, characterization and photophysical properties of a donor-bridge-acceptor supramolecular hybrid system, consisting of a tetrapyridyl fullerene derivative (C60-tpyr) as electron acceptor, with the four pyridyl groups as part of oligophenyleneethynylene/phenylenevinylene bridges, and zinc porphyrin dimers (ZnP)2 as electron donor species. Based on the metal-to-ligand coordination between the zinc metal centers of (ZnP)2 and the four pyridyl entities of C60-tpyr, a strong binding constant (5 × 105 M-1) for the formation of C60-tpyr·[(ZnP)2]2 was evidenced. Insights into the electronic interactions between the photoactive (ZnP)2 units and C60-tpyr emanated from complementary physicochemical assays, which were further supported by theoretical calculations. Notably, the absorption and emission titration assays revealed strong interactions between the electron donor and acceptor species within C60-tpyr·[(ZnP)2]2, both in the ground and excited state. Moreover, femtosecond and nanosecond laser photolysis transient absorption measurements were performed and provided solid evidence for intramolecular electron transfer processes derived from the singlet excited state of (ZnP)2 to C60-tpyr. Comparison with systems in which either four monomeric zinc porphyrins (ZnP) were complexed with C60-tpyr or a (ZnP)2 was coordinated with a dipyridylfullerene revealed the beneficial role of C60-tpyr in increasing the lifetime of charge-separation.

Synthesis and photophysical properties of a Sc3N@C80 -corrole electron donor-acceptor conjugate.

Embedding endohdedral metallofullerenes (EMFs) into electron donor-acceptor systems is still a challenging task owing to their limited quantities and their still largely unexplored chemical properties. In this study, we have performed a 1,3-dipolar cycloaddition reaction of a corrole-based precursor with Sc3 N@C80 to regioselectively form a [5,6]-adduct (1). The successful attachment of the corrole moiety was confirmed by mass spectrometry. In the electronic ground state, absorption spectra suggest sizeable electronic communications between the electron acceptor and the electron donor. Moreover, the addition pattern occurring at a [5,6]-bond junction is firmly proven by NMR spectroscopy and electrochemical investigations performed with 1. In the electronically excited state, which is probed in photophysical assays with 1, a fast electron-transfer yields the radical ion pair state consisting of the one-electron-reduced Sc3 N@C80 and of the one-electron-oxidized corrole upon its exclusive photoexcitation. As such, our results shed new light on the practical work utilizing EMFs as building blocks in photovoltaics.