Enrico Marchi

Designing light harvesting antennas by luminescent dendrimers

Dendrimers are well defined, tree-like macromolecules, with a high degree of order and the possibility to contain selected chemical units in predetermined sites of their structure. Nowadays it is possible to design and synthesize dendrimers containing a variety of chromophoric groups organized in the dimensions of time, energy, and space so as to obtain efficient light-harvesting devices that can be useful for solar energy conversion, as well as for other purposes. In order to design the pattern and maximize the efficiency of light harvesting it is necessary to take into account not only the absorption and emission spectra of the various chromophoric units, but also their redox properties in the ground and excited states since dissipative photoinduced electron transfer processes can compete with energy transfer. Luminescence intensity measurements, both steady state and time dependent, fluorescence anisotropy, and transient absorption spectra are the techniques commonly used in this field. In this paper we discuss some paradigmatic cases including: (i) light harvesting within dendritic structures; (ii) light harvesting by molecules or ions hosted inside the dendritic structure; (iii) light harvesting in systems resulting from association of dendrimers with other large molecules.

Easy Separation of Δ and Λ Isomers of Highly Luminescent [IrIII]‐Cyclometalated Complexes Based on Chiral Phenol‐Oxazoline Ancillary Ligands

A new class of neutral cyclometalated iridium(III) complexes with enantiomerically pure C(1)-symmetric phenol-oxazolines (3a,b) have been synthetized in high yields and fully characterized. Resolution of the corresponding Δ(R) and Λ(R) or Δ(S) and Λ(S) isomers was easily achieved by conventional flash chromatography. The corresponding Δ and Λ helicities have been confirmed by CD spectroscopy and X-ray crystallography. Regarding the absorption and luminescence properties with unpolarized light, no significant difference between Δ and Λ isomers has been observed. A strong blue luminescence is observed for deaerated solutions of complexes 5a and 5b in CH(3)CN.

Anion Sensing in Aqueous Media by Photo‐active Transition‐Metal Bipyridyl Rotaxanes

A chloride anion templation methodology is utilized in the construction of novel transition-metal rhenium(I) and ruthenium(II) bipyridyl appended [2]rotaxanes. (1)H NMR spectroscopic titrations reveal the ability of the rotaxanes to selectively bind chloride over the more basic oxoanions, with the ruthenium(II) bipyridyl appended rotaxane strongly binding chloride in 30% water. Photophysical investigations demonstrate the ability of the rotaxanes to sense anions in aqueous media, with chloride being selectively complexed, in general agreement with NMR spectroscopy determined anion binding data.

Evaluation of phototoxicity of dendritic porphyrin-based phosphorescent oxygen probes: an in vitro study

Biological oxygen measurements by phosphorescence quenching make use of exogenous phosphorescent probes, which are introduced directly into the medium of interest (e.g. blood or interstitial fluid) where they serve as molecular sensors for oxygen. The byproduct of the quenching reaction is singlet oxygen, a highly reactive species capable of damaging biological tissue. Consequently, potential probe phototoxicity is a concern for biological applications. Herein, we compared the ability of polyethyleneglycol (PEG)-coated Pd tetrabenzoporphyrin (PdTBP)-based dendritic nanoprobes of three successive generations to sensitize singlet oxygen. It was found that the size of the dendrimer has practically no effect on the singlet oxygen sensitization efficiency in spite of the strong attenuation of the triplet quenching rate with an increase in the dendrimer generation. This unexpected result is due to the fact that the lifetime of the PdTBP triplet state in the absence of oxygen increases with dendritic generation, thus compensating for the concomitant decrease in the rate of quenching. Nevertheless, in spite of their ability to sensitize singlet oxygen, the phosphorescent probes were found to be non-phototoxic when compared with the commonly used photodynamic drug Photofrin in a standard cell-survival assay. The lack of phototoxicity is presumably due to the inability of PEGylated probes to associate with cell surfaces and/or penetrate cellular membranes. In contrast, conventional photosensitizers bind to cell components and act by generating singlet oxygen inside or in the immediate vicinity of cellular organelles. Therefore, PEGylated dendritic probes are safe to use for tissue oxygen measurements as long as the light doses are less than or equal to those commonly employed in photodynamic therapy.

Improving the layer morphology of solution-processed perylene diimide organic solar cells with the use of a polymeric interlayer

Abstract Herein we demonstrate a method to improve the power conversion efficiency (PCE) parameter of organic photovoltaic (OPV) devices based on the electron acceptor N,N’-bis(1- ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (PDI) blended with the electron donor poly(indenofluorene)-aryloctyl (PIF-Aryl). The device parameters of the short-circuit current, open-circuit voltage and fill factor are found increased after the insertion of a thin poly [9, 9-dioctylfluorene-co-N- [4-(3-methylpropyl)]-diphenylamine] (TFB) photoactive interlayer between the hole-collecting electrode and the photoactive layer of the device. Unlike to most of the cases where interlayers serve as charge extractors, in our system the polymeric interlayer serves as a morphology modifying agent that drives the PDI component to segregate better at the interface with the device cathode; that is at the carrier-collecting electrode interface, which is not in physical contact with the interlayer. The processes of energy/charge transfer of the TFB excitons to/with the PIF-Aryl:PDI top-layer are also addressed. Charge transfer reactions dominate at the TFB/PIF-Aryl:PDI interface but no significant contribution in the photocurrent generation is seen in the photoaction spectra of the bilayer device.

The Beauty of Chemistry in the Words of Writers and in the Hands of Scientists

Chemistry is a central science because all the processes that sustain life are based on chemical reactions, and all things that we use in everyday life are natural or artificial chemical compounds. Chemistry is also a fantastic world populated by an unbelievable number of nanometric objects called molecules, the smallest entities that have distinct shapes, sizes, and properties. Molecules are the words of matter. Indeed, most of the other sciences have been permeated by the concepts of chemistry and the language of molecules. Like words, molecules contain specific pieces of information that are revealed when they interact with one another or when they are stimulated by photons or electrons. In the hands of chemists, molecules, particularly when they are suitably combined or assembled to create supramolecular systems, can play a variety of functions, even more complex and more clever than those invented by nature. The wonderful world of chemistry has inspired scientists not only to prepare new molecules or investigate new chemical processes, but also to create masterpieces. Some nice stories based on chemical concepts (1) show that there cannot be borders on the Earth, (2) underline that there is a tight connection among all forms of matter, and (3) emphasize the irreplaceable role of sunlight.

Synthesis and solid-state fluorescence properties of pentacyclic 7-substituted-indeno[1′,2′:4,5]pyrido[2,1-a]isoindol-5-ones

With the aim to design fluorescent solids, a series of indeno[1′,2′:4,5]pyrido[2,1-a]isoindol-5-ones with various substituents was prepared. In these π-extended pentacyclic derivatives, the presence of a methyl group in the 7-position was found to have a critical influence on the fluorescence properties in the solid state. Crystal packing of the non-substituted derivatives shows strong π–π interactions causing quenching of the fluorescence. In contrast, by introducing a methyl substituent in the 7-position we obtained compounds with fluorescence quantum yield up to 32% in the solid state.

Pseudopeptide Foldamers designed for photoinduced intramolecular electron transfer

We have designed and prepared three pseudopeptide foldamers, called dyads 1, 2 and 3, equipped with a donor and an acceptor unit to promote intramolecular electron transfer after light excitation. All the three dyads contain the same donor and acceptor, which are a derivative of 1,5-dihydroxynaphthalene and a derivative of pyromellitic diimide, respectively. The donor and acceptor units are separated by hybrid foldamers of different length in order to vary both their distance and relative orientation. Specifically, one, two or three L-Ala-D-Oxd (Ala = alanine, Oxd = 4-carboxy-5-methyl-oxazolidin-2-one) units are contained in dyads 1, 2, and 3, respectively. Dyad 1 folds in a bent conformation in which the donor and acceptor units lie one close to the other, while dyads 2 and 3 preferentially assume an extended conformation. In all the three dyads both the donor and acceptor emissions are efficiently quenched via intramolecular electron transfer, as suggested by photophysical and electrochemical investigations. Because of its bent conformation dyad 1 exhibits a charge-transfer (CT) band at 410 nm in CH2Cl2 solution and a photoinduced electron transfer that occurs more efficiently than in dyads 2 and 3. Upon dissolving dyad 1 in DMSO, a competitive solvent for hydrogen bonds that establish in the pseudopeptide linker, the CT band disappears and the efficiency of electron transfer slightly decreases, in agreement with an unfolded conformation in which donor and acceptor units are no longer in close contact.

Blue and highly emitting [Ir(IV)] complexes by an efficient photoreaction of yellow luminescent [Ir(III)] complexes

A family of cyclometalated [Ir(III)] complexes containing two F2ppy ligands and a bis-oxazoline (box) ancillary ligand, either chiral (1) or achiral (2) has been synthesized. They exhibit photophysical and electrochemical properties quite similar to the isoelectronic and well known [Ir(F2ppy)2(acac)] complex. However, an unprecedented photoreactivity has been observed. Upon UV irradiation of air-equilibrated solutions of both complexes, stable photoproducts containing an [Ir(IV)] metal center are obtained. The photoproducts exhibit highly efficient blue luminescence with good OLED performances. The same [Ir(IV)] complexes are obtained also by electrochemical oxidation.

Luminescent Dendrimers as Ligands and Sensors of Metal Ions

Suitably designed luminescent dendrimers can play the role of ligands for luminescent and nonluminescent metal ions. This combination leads to species capable of exhibiting interesting and unusual properties, including (1) shielding excited states from quenching processes, (2) light harvesting, (3) conversion of incident UV light into visible or infra red emission, and (4) metal ions sensing with signal amplification.