Ludovic Favereau

Heteroleptic copper(I)–polypyridine complexes as efficient sensitizers for dye sensitized solar cells

The synthesis and the physico-chemical characterization of HETPHEN based heteroleptic copper(I)–bis(diimine) complexes are reported. For TiO2 based dye sensitized solar cells (DSCs), the latter display impressive photoconversion efficiencies (PCEs), unprecedented for first row transition metal coordination complexes.

An Efficient Synthesis of Imidazolinium Salts Using Vinyl Sulfonium Salts

The synthesis of imidazolinium salts from the reaction of formamidines and (2-bromoethyl)diphenylsulfonium triflate is described. A variety of symmetrical and unsymmetrical imidazolinium triflate salts were synthesized in high yield in short reaction times under mild conditions. Aromatic and aliphatic N-substituents work well. The reaction is proposed to proceed via generation of a vinyl sulfonium salt intermediate from the bromoethylsulfonium triflate.

Diketopyrrolopyrrole-zinc porphyrin, a tuned panchromatic association for dye-sensitized solar cells

Aiming at collecting a maximum amount of energy with one panchromatic sensitizer, three novel dyes based on zinc porphyrin and diketopyrrolopyrrole units have been synthesized. Suitable dye functionalization resulted in a very broad coverage of the solar spectrum and an efficient photovoltaic energy conversion reaching 7.74% in a DSC device.

Six-Coordinate Zinc Porphyrins for Template-Directed Synthesis of Spiro-Fused Nanorings.

Five-coordinate geometry is the standard binding mode of zinc porphyrins with pyridine ligands. Here we show that pseudo-octahedral six-coordinate zinc porphyrin complexes can also be formed in solution, by taking advantage of the chelate effect. UV–vis–NIR titrations indicate that the strength of this second coordination is ca. 6–8 kJ mol–1. We have used the formation of six-coordinate zinc porphyrin complexes to achieve the template-directed synthesis of a 3D π-conjugated spiro-fused array of 11 porphyrin units, covalently connected in a nontrivial topology. Time-resolved fluorescence anisotropy experiments show that electronic excitation delocalizes between the two perpendicular nanorings of this spiro-system within the experimental time-resolution of 270 fs.

Enantiopure versus Racemic Naphthalimide End-Capped Helicenic Non-fullerene Electron Acceptors: Impact on Organic Photovoltaics Performance

Impact of the enantiopurity on organic photovoltaics (OPV) performance was investigated through the synthesis of racemic and enantiomerically pure naphthalimide end-capped helicenes and their application as non-fullerene molecular electron acceptors in OPV devices. A very strong increase of the device performance was observed by simply switching from the racemic to the enantiopure forms of these π-helical non-fullerene acceptors with power conversion efficiencies jumping from 0.4 to about 2.0 % in air-processed poly(3-hexylthiophene)-based devices, thus highlighting the key role of enantiopurity in the photovoltaic properties.

Enantiopure cycloiridiated complexes bearing a pentahelicenic N-heterocyclic carbene and displaying long-lived circularly-polarized phosphorescence

A fused π-helical N-heterocyclic carbene (NHC) system was prepared and examined through its diastereoisomerically pure cycloiridiated complexes. The latter display light-green phosphorescence with unusually long lifetimes and circular polarization that depends on both the helical NHC P/M stereochemistry and the iridium Δ/Λ stereochemistry. These unprecedented features are attributed to extended π conjugation within the helical carbenic ligand and efficient helicene-NHC-Ir interaction.

Molecular-structure control of electron transfer dynamics of push–pull porphyrins as sensitizers for NiO based dye sensitized solar cells

Porphyrin dyes were synthesized for use in p-type (NiO) dye sensitized solar cells based on different design principles. One porphyrin was designed with a significant charge transfer character in the excited state because of push–pull effects of the substituents. Another porphyrin had instead an appended NDI acceptor group (NDI = naphthalene diimide). The dyes were characterized by spectroscopic, electrochemical and DFT methods. Solar cells based on sensitized, meso-porous NiO showed rather poor performance compared to other organic dyes, but with a clear improvement for the dye with the NDI acceptor. Ultrafast transient absorption spectroscopy and nanosecond laser photolysis showed that hole injection into NiO was followed by unusually rapid charge recombination, predominantly on a 50–100 ps time scale, which is likely the main reason for the poor photovoltaic performance. Again the porphyrin with the NDI group showed a more long-lived charge separation that should lead to better dye regeneration in a solar cell, which can explain its better photovoltaic performance.

Breaking the Symmetry in Molecular Nanorings.

Because of their unique electronic properties, cyclic molecular structures ranging from benzene to natural light-harvesting complexes have received much attention. Rigid π-conjugated templated porphyrin nanorings serve as excellent model systems here because they possess well-defined structures that can readily be controlled and because they support highly delocalized excitations. In this study, we have deliberately modified a series of six-porphyrin nanorings to examine the impact of lowering the rotational symmetry on their photophysical properties. We reveal that as symmetry distortions increase in severity along the series of structures, spectral changes and an enhancement of radiative emission strength occur, which derive from a transfer of oscillator strength into the lowest (k = 0) state. We find that concomitantly, the degeneracy of the dipole-allowed first excited (k = ±1) state is lifted, leading to an ultrafast polarization switching effect in the emission from strongly symmetry-broken nanorings.

Synthesis, Spectroelectrochemical Behavior, and Chiroptical Switching of Tris(β-diketonato) Complexes of Ruthenium(III), Chromium(III), and Cobalt(III)

Five tris(β-diketonato) complexes of ruthenium(III), chromium(III), and cobalt(III) [Ru(Buacac)3 (1), Ru(Oacac)3 (2), Cr(Buacac)3 (3), Cr(Oacac)3 (4), and Co(Buacac)3 (5), where Buacac = 3-butylpentane-2,4-dionato and Oacac = 3-octylpentane-2,4-dionato] with a chiral propeller-like structure have been prepared. Ligands and complexes syntheses are presented together with characterization of the compounds by 1H and 13C NMR spectroscopy, mass spectrometry, IR, UV-vis, electronic circular dichroism (ECD) spectroscopy, electrochemistry studies, and first-principles calculations. The crystal structures of 1 and 5 have also been obtained and analyzed. A comparison of the 1H NMR spectra of diamagnetic (ligands and 5) and paramagnetic (1 and 2) species is presented. Optical resolution of the five complexes has been achieved for the first time by supercritical fluid chromatography using a chiral column, giving rise to very high purity grades in all cases. ECD measurements and calculations have led to the assignment of the absolute configuration, Δ or Λ, of each enantiomer for 1-5. Spectroelectrochemical UV-vis and ECD studies have been performed on ruthenium Λ-2 and chromium Λ-4 complexes, revealing their redox-triggered chiroptical switching confirming the noninnocence character of the β-diketonate ligands.

Correction: Synthesis and properties of push–pull porphyrins as sensitizers for NiO based dye-sensitized solar cells

Zinc porphyrin derivatives were prepared and investigated in NiO-based p-type dye sensitized solar cells. The first series consist of trans disubstituted push–pull porphyrins containing a nitrophenyl unit as an electron withdrawing group and a benzoic acid unit as an anchoring group. An ethynyl spacer was introduced between the porphyrin meso positions and the nitrophenyl or/and the benzoic acid. In a second series a secondary electron acceptor such as benzyl-viologen or a naphthalene diimide was appended to the porphyrin. All the compounds were characterized by absorption and emission spectroscopies, theoretical calculations and by electrochemistry. The fast charge recombination certainly limits the photovoltaic performances of the first series, which remain modest (Jsc ≈ 0.5 mA cm−2, Voc≈ 110 mV, ff ≈ 36% and PCE ≈ 0.02%) compared to already reported highly performing sensitizers for NiO. However, promising performances (Jsc = 1.38 mA cm−2, Voc = 127 mV, ff = 32%, and PCE = 0.038%) were recorded for the dyad with naphthalene diimide.