Thomas Bura

High-Performance Solution-Processed Solar Cells and Ambipolar Behavior in Organic Field-Effect Transistors with Thienyl-BODIPY Scaffoldings

Green-absorbing dipyrromethene dyes engineered from bis-vinyl-thienyl modules are planar molecules, exhibiting strong absorption in the 713-724 nm range and displaying comparable electron and hole mobilities in thin films (maximum value 1 × 10(-3) cm(2)/(V·s)). Bulk heterojunction solar cells assembled with these dyes and a fullerene derivative (PC(61)BM) at a low ratio give a power conversion efficiency as high as 4.7%, with short-circuit current values of 14.2 mA/cm(2), open-circuit voltage of 0.7 V, and a broad external quantum efficiency ranging from 350 to 920 nm with a maximum value of 60%.

Color Tuning in New Metal‐Free Organic Sensitizers (Bodipys) for Dye‐Sensitized Solar Cells

Fun in the sun! A strategy has been devised for functionalizing and solubilizing boron dipyrromethene (Bodipy) dyes at the central boron atom and changing the color by increasing delocalization on the central core. This approach leads to the formation of stable B-C[triple bond]C and pyrrole--C=C linkages suitable for use in TiO(2)-sensitized devices (see figure).

Multi-donor molecular bulk heterojunction solar cells: improving conversion efficiency by synergistic dye combinations

Molecular bulk heterojunction solar cells with a power conversion efficiency of 1.70% have been fabricated using as donor BODIPY dyes with complementary light-harvesting properties.

Artificial light-harvesting arrays: electronic energy migration and trapping on a sphere and between spheres.

A sophisticated model of the natural light-harvesting antenna has been devised by decorating a C(60) hexa-adduct with ten yellow and two blue boron dipyrromethene (Bodipy) dyes in such a way that the dyes retain their individuality and assist solubility of the fullerene. Unusually, the fullerene core is a poor electron acceptor and does not enter into light-induced electron-transfer reactions with the appended dyes, but ineffective electronic energy transfer from the excited-state dye to the C(60) residue competes with fluorescence from the yellow dye. Intraparticle electronic energy transfer from yellow to blue dyes can be followed by steady-state and time-resolved fluorescence spectroscopy and by excitation spectra for isolated C(60) nanoparticles dissolved in dioxane at 293 K and at 77 K. The decorated particles can be loaded into polymer films by spin coating from solution. In the dried film, efficient energy transfer occurs such that photons absorbed by the yellow dye are emitted by the blue dye. Films can also be prepared to contain C(60) nanoparticles loaded with the yellow Bodipy dye but lacking the blue dye and, under these circumstances, electronic energy migration occurs between yellow dyes appended to the same nanoparticle and, at higher loading, to dye molecules on nearby particles. Doping these latter polymer films with the mixed-dye nanoparticle coalesces these multifarious processes in a single system. Thus, long-range energy migration occurs among yellow dyes attached to different particles before trapping at a blue dye. In this respect, the film resembles the natural photosynthetic light-harvesting complexes, albeit at much reduced efficacy. The decorated nanoparticles sensitize amorphous silicon photocells.

Highly substituted Bodipy dyes with spectroscopic features sensitive to the environment.

A general method for the synthesis of butterfly-shaped difluoroboradiaza-s-indacenes with different substituents has been developed. The mixed AB dye was produced in two successive Knoevenagel reactions involving first 4-dimethylaminobenzaldehyde and second 4-hydroxybenzaldehyde. Two pathways to A(2)B(2) and ABCD derivatives were investigated starting from a tetramethyl-substituted Bodipy. The pivotal compound is a bis-styryl phenol derivative also produced by a Knoevenagel reaction without the need to protect the phenol functions. Methylation of the phenol functions provides mono- and bis-derivatives which have been used to construct tetrastyryl A(2)B(2) and ABCD derivatives, respectively. Stepwise alkylation of the monomethoxyphenol dye provided a mixed dye that was converted to a tris-substituted compound, isolated as a mixture of regioisomers. The fourth styryl function was introduced by using 4-dimethylaminobenzaldehyde and forcing conditions. These highly colored dyes display outstanding optical properties with absorption wavelengths spanning from 573 to 718 nm and emission wavelengths from 585 to 800 nm. The high quantum yields, nanosecond excited state lifetimes, and weak Stokes shifts are typical of singlet emitters. The presence of both dimethylamino and phenol residues in some of these dyes makes them sensitive to acids and bases, allowing the tuning of the optical properties over a large wavelength range as a function of pH. A unique three-color ratiometric pH sensor based on both absorption and fluorescence has been characterized and studied in detail. The tris- and tetra-substitution of the methyl groups by vinyl residues induces a weaker bathochromic shift than that due to mono- and disubstitution. Consequently, replacing an alkoxy function by a dimethylamino moiety in the tetra-substituted derivatives has little effect on the spectroscopic features.

Direct (Hetero)arylation Polymerization: Trends and Perspectives

Conjugated polymers have attracted much attention in recent years, as they can combine the best features of metals or inorganic semiconducting materials (excellent electrical and optical properties) with those of synthetic polymers (mechanical flexibility, simple processing, and low-cost production), thereby creating altogether new scientific synergies and technological opportunities. In the search for more efficient synthetic methods for the preparation of conjugated polymers, this Perspective reports advances in the field of direct (hetero)arylation polymerization. This recently developed polymerization method encompasses the formation of carbon-carbon bonds between simple (hetero)arenes and (hetero)aryl halides, reducing both the number of synthetic steps and the production of organometallic byproducts. Along these lines, we describe the most general and adaptable reaction conditions for the preparation of high-molecular-weight, defect-free conjugated polymers. We also discuss the bottleneck presented by the utilization of certain brominated thiophene units and propose some potential solutions. It is, however, firmly believed that this polymerization method will become a versatile tool in the field of conjugated polymers by providing a desirable atom-economical alternative to standard cross-coupling polymerization reactions.

Absorption tuning of monosubstituted triazatruxenes for bulk heterojunction solar cells.

A series of triazatruxene (TAT)-functionalized Bodipy dyes were prepared by a sequence of reactions involving either cross-coupling reactions promoted by Pd complexes or a Knoevenagel reaction leading to a vinyl linker. The new dyes show large absorption coefficients and fluorescence quantum yields as well as interesting electrochemical properties. The blue dyes of this series exhibit interesting photovoltaic effects (V(OC) = 0.83 V, J(SC) = 3.6 mA/cm(2), efficiency 0.9%) in bulk heterojunction solar cells, due to the good hole mobility imported by the TAT entity.

Realizing the full potential of conjugated polymers: innovation in polymer synthesis

Plastic electronics is closely linked to advances in polymer synthesis. Based on conjugated polymers, this technology aims to exploit the features of metals and inorganic semi-conductors while preserving mechanical properties unique to polymers. In the first part of this review, we present a retrospective study of the development of different polymerization protocols with their respective key polymers. We report different methods, starting with Ziegler–Natta followed by electro-polymerization, metathesis, Kumada, Negishi, GRIM, Stille, and Suzuki. In the second part of this review, we outline the recent advances made in direct (hetero)arylation polymerization (DHAP) which is particularly promising for the future development of efficient, greener and low-cost electronic devices.

Water-soluble phosphonate-substituted BODIPY derivatives with tunable emission channels.

Water-soluble BODIPY dyes have been readily obtained by introduction of phosphonate fragments either on the boron for the green and yellow emitting dyes or on the side chain for the red emitting dyes. Hydrolysis of the phosphonate is realized at the end of the reaction sequence and allows isolation of the targets by precipitation. All these novel dyes are soluble and fluorescent in water with quantum yields in the 23-59% range and emission wavelength spanning from 667 to 509 nm.

Vectorial Photoinduced Energy Transfer Between Boron–Dipyrromethene (Bodipy) Chromophores Across a Fluorene Bridge

A series of novel multichromophoric, luminescent compounds has been prepared, and their absorption spectra, luminescence properties (both at 77 K in rigid matrix and at 298 K in fluid solution), and photoinduced intercomponent energy-transfer processes have been studied. The series contains two new multichromophoric systems 1 and 2, each one containing two different boron-dipyrromethene (Bodipy) subunits and one bridging fluorene species, and two fluorene-Bodipy bichromophoric species, 6 and 7. Three monochromophoric compounds, 3, 4, and 5, used as precursors in the synthetic process, were also fully characterized. The absorption spectra of the multichromophoric compounds are roughly the summation of the absorption spectra of their individual components, thus demonstrating the supramolecular nature of the assemblies. Luminescence studies show that quantitative energy transfer occurs in 6 and 7 from the fluorene chromophore to the Bodipy dyes. Luminescence studies, complemented by transient-absorption spectroscopy studies, also indicate that efficient inter-Bodipy energy transfer across the rigid fluorene spacer takes place in 1 and 2, with rate constants, evaluated by several experimental methods, between 2.0 and 7.0 x 10(9) s(-1). Such an inter-Bodipy energy transfer appears to be governed by the Förster mechanism. By taking advantage of the presence of various protonable sites in the substituents of the lower-energy Bodipy subunit of 1 and 2, the effect of protonation on the energy-transfer rates has also been investigated. The results suggest that control of energy-transfer rate and efficiency of inter-Bodipy energy transfer in this type of systems can be achieved by an external, reversible input.