Alessandro Varotto

Self-Organized Porphyrinic Materials

The self-assembly and self-organization of porphyrins and related macrocycles enables the bottom-up fabrication of photonic materials for fundamental studies of the photophysics of these materials and for diverse applications. This rapidly developing field encompasses a broad range of disciplines including molecular design and synthesis, materials formation and characterization, and the design and evaluation of devices. Since the self-assembly of porphyrins by electrostatic interactions in the late 1980s to the present, there has been an ever increasing degree of sophistication in the design of porphyrins that self-assemble into discrete arrays or self-organize into polymeric systems. These strategies exploit ionic interactions, hydrogen bonding, coordination chemistry, and dispersion forces to form supramolecular systems with varying degrees of hierarchical order. This review concentrates on the methods to form supramolecular porphyrinic systems by intermolecular interactions other than coordination chemistry, the characterization and properties of these photonic materials, and the prospects for using these in devices. The review is heuristically organized by the predominant intermolecular interactions used and emphasizes how the organization affects properties and potential performance in devices.

1,4-Fullerene Derivatives: Tuning the Properties of the Electron Transporting Layer in Bulk-Heterojunction Solar Cells**

Light and flexible photovoltaic devices based on organic materials are extensively studied as an alternative to expensive and fragile silicon-based solar cells. The efficiency of these devices is rapidly increasing with the most recent power conversion efficiency (PCE) of greater than 8% bringing them closer to commercial viability. Further improvements are needed and can be achieved by optimizing the ratio between donor and acceptor, modifying the electronic properties of the materials, and optimizing the morphology of the resulting bulk heterojunction (BHJ). A direct way to increase the efficiency is to lower the band gap of the donor material in order to absorb a greater fraction of the solar spectrum. This concept has been explored through the synthesis of a series of new low band gap polymers, which exhibit a decreased band gap as a result of lowering the lowest unoccupied molecular orbital (LUMO). An emerging challenge is the need for electronically compatible acceptors with sufficiently low LUMO levels so that charge separation is efficiently promoted. Optimum miscibility of a specific polymer and fullerene combination to create the optimum degree of phase separation is also a feature to take into account. Typically, the approach used to test new donor materials is to fabricate devices using PC61BM ([6,6]-phenyl-C61-butyric acid methyl ester), a well-studied benchmark acceptor. Only a limited number of other fullerene derivatives have been successfully employed. Although these fullerene derivatives bear different functional groups, they are related by the positioning of the substituents on carbons 1 and 2 of a six-membered ring. From literature studies it is apparent that even subtle modification of the nature and especially position of substituents can drastically alter the electronic properties of fullerene derivatives. For example, PC71BM has reduced symmetry that increases visible light absorption and has a positive effect on current generation in polymer BHJ cells. Here we report the synthesis of a series of novel fullerene derivatives functionalized through the “1,4” position and their use in organic photovoltaics (OPVs). Features that distinguish this class of fullerene derivatives are: 1) straightforward synthesis which includes versatility of functionalization with different substrates starting from the same material (fullerenol, Scheme 1); 2) tunable LUMO energy by appending electron-donating or electron-withdrawing groups; 3) lower symmetry which decreases the optical gap and produces an increased absorption in the visible (the extinction coefficient at 480 nm of a 1,4-adduct is approximately 8 times larger than that of PCBM) ; 4) tunable solubility which influences the morphology of the BHJ. Like PC71BM, 1,4addends have increased light absorption at ca. 500 nm (Figure 1) with the advantage that all the C60 derivatives

Phthalocyanine Blends Improve Bulk Heterojunction Solar Cells

A core phthalocyanine platform allows engineering of the solubility properties the band gap, shifting the maximum absorption toward the red. A simple method for increasing the efficiency of heterojunction solar cells uses a self-organized blend of phthalocyanine chromophores fabricated by solution processing.

Commercially viable porphyrinoid dyes for solar cells

Multifunctional molecules bearing different dyes, such as donor–acceptor systems, synthesized by covalent chemistry have provided a wealth of information on the fundamental nature of electron and energy transfer in organic systems and there is a growing literature on the materials properties of dyes on surfaces. However, in the vast majority of cases the synthetic costs of producing these covalently bound systems prohibit them from deployment in commercially viable devices. Thus, to achieve both the needed multifunctionality and to bring the synthetic costs in line with potential commercialization, supramolecular approaches to the formation of photonic materials can be exploited. This perspective focuses on porphyrinoids as exemplary dyes, but the concepts and design principles extend to other chromophores.

Fabrication of Metal Nanoparticles Using Toroidal Plasmid DNA as a Sacrificial Mold

A new method for synthesizing gold, nickel, and cobalt metal nanoparticles at room temperature from metal salts employing plasmid DNA in a toroidal topology as a sacrificial mold is presented. The diameter of the toroidal DNA drives the formation and size of the nanoparticle, and UV light initiates the oxidation of the DNA and concomitant reduction of the DNA bound metal ions. The nanoparticles were characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), and electron diffraction (ED).

Self-organization of a new fluorous porphyrin and C60 films on indium-tin-oxide electrode

The highly fluorinated alkyl moieties of a new porphyrin drive the self-organization of thin films with C(60) on ITO electrodes.

Self-organized nanostructured materials of alkylated phthalocyanines and underivatized C60 on ITO

Clicking thiols onto a core fluorous phthalocyanine (Pc) platform yields alkyl and fluoroalkyl derivatives. The Zn(II) Pc with 16 thioalkanes self-organizes with fullerene C60 into nearly monodispersed nanoparticles when cast on ITO surfaces. Particle size and surface density is controlled by varying the Pc/fullerene ratio and concentration. Fluorescence quenching indicates electronic interaction between the component molecules in the films of nanoparticles. The strong supramolecular interactions between the Pc with 8 or 16 fluorous alkanes inhibit incorporation of the fullerene.