Léo Bucher

Synthesis, Electrochemistry, and Photophysics of Aza-BODIPY Porphyrin Dyes.

The synthesis of dyad and triad aza-BODIPY-porphyrin systems in two steps starting from an aryl-substituted aza-BODIPY chromophore is described. The properties of the resulting aza-BODIPY-porphyrin conjugates have been extensively investigated by means of electrochemistry, spectroelectrochemistry, and absorption/emission spectroscopy. Fluorescence measurements have revealed a dramatic loss of luminescence intensity, mainly due to competitive energy transfer and photoinduced electron transfer involving charge separation followed by recombination.

BODIPY–diketopyrrolopyrrole–porphyrin conjugate small molecules for use in bulk heterojunction solar cells

Two small molecules denoted as BD-pPor and BD-tPor composed of a central BODIPY core surrounded with two DPP and two porphyrin units have been designed and synthesized. In BD-pPor and BD-tPor, porphyrins are linked to the central BODIPY by phenyl and thiophene bridges, respectively. The optical and electrochemical properties were systematically investigated in order to employ them as donors along with PC71BM as an acceptor for solution processed bulk heterojunction organic solar cells. After the optimization of the active layer, the organic solar cells based on BD-pPor and BD-tPor exhibit overall power conversion efficiencies of 6.67% and 8.98% with an energy loss of 0.63 eV and 0.50 eV. The low value of energy loss for BD-tPor may be related to the low LUMO offset between the BD-tPor and PC71BM (0.31 eV) as compared to that between BD-pPor and PC71BM (0.36 eV). The low energy loss also leads to a higher value of open-circuit voltage for the BD-tPor based OSC than its BD-pPor counterpart, despite the slightly deeper HOMO energy level of BD-pPor. The enhanced values of Jsc and FF of the BD-tPor based OSCs may be related to the better exciton dissociation and charge transport, as confirmed from the PL spectra and charge carrier mobility. These results indicate that the combination of BODIPY, DPP and porphyrin in the same conjugate is very promising for small molecule organic solar cells.

Porphyrin Antenna-Enriched BODIPY–Thiophene Copolymer for Efficient Solar Cells

Low bandgap A-π-D copolymer, P(BdP-DEHT), consisting of alternating BOronDIPYrromethene (BODIPY) and thiophene units bridged by ethynyl linkers, and its porphyrin-enriched analogue, P(BdP/Por-DEHT), were prepared, and their optical and electrochemical properties were studied. P(BdP-DEHT) exhibits strong absorption in the 500-800 nm range with an optical bandgap of 1.74 eV. On the basis of cyclic voltammetry, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are evaluated to be -5.40 and -3.66 eV, respectively. After the anchoring of zinc(II) porphyrin on the BODIPY unit, P(BdP/Por-DEHT) displays broadened absorption, thanks to porphyrins, and the optical bandgap decreases to 1.59 eV because of extension of BODIPY conjugation. The resulting estimated HOMO and LUMO energy levels, respectively, move to -5.32 and -3.73 eV. After optimization of the P(BdP-DEHT) or P(BdP/Por-DEHT) to PC71BM weight ratio to 1:2 in dichlorobenzene solution, the bulk heterojunction polymer solar cells show overall power conversion efficiencies (PCEs) of 3.03 and 3.86%, respectively. After solvent vapor annealing (SVA) treatment in CH2Cl2 for 40 s, the PCEs increased to 7.40% [Voc of 0.95 V, Jsc of 12.77 mA/cm2, and fill factor (FF) of 0.61 with energy loss of 0.79 eV] and 8.79% (Voc of 0.92 V, Jsc of 14.48 mA/cm2, and FF of 0.66 with energy loss of 0.67 eV). The increase in the PCE for P(BdP/Por-DEHT)-based devices is mainly attributed to the enhancement in Jsc and FF, which may be related to the broader absorption spectra, lower band gap, and better charge transport of P(BdP/Por-DEHT) compared to P(BdP-DEHT). This could also be related to the optimized nanoscale morphology of the active layer for both efficient exciton dissociation and charge transport toward the electrodes and a balanced charge transport in the device, induced by the SVA treatment of the active layer.

Synthesis and characterization of zinc carboxy–porphyrin complexes for dye sensitized solar cells

Two zinc porphyrins, 2 and 8, have been synthesized. Porphyrin 8 displays better electronic communication between the dye and the TiO2 electrode. Photophysical measurements and electrochemistry experiments suggest that both porphyrins are very promising sensitizers for dye-sensitized solar cells (DSSCs). It was found that their molecular orbital energy levels favor electron injection and dye regeneration in DSSCs. Solar cells sensitized by 2 and 8 were fabricated, and it was found that they show power conversion efficiencies (PCEs) of 5.27% and 7.13%, respectively. Photovoltaic measurements (J–V curves) together with the incident photon-to-electron conversion efficiency spectra of the two cells reveal that the higher PCE value of the DSSC based on 8 is ascribed to the higher short-circuit current (Jsc), open-circuit voltage (Voc), and dye loading values. Moreover, the larger charge recombination resistance, longer electron lifetime and shorter electron transport time for 8 also confirm the higher value of the Voc and the Jsc and the FF for the DSSC based on 8.

Synthesis and Characterization of Carbazole‐Linked Porphyrin Tweezers

Herein the synthesis, spectroscopic characterization, two-photon absorption and electrochemical properties of 3,6-disubstituted carbazole tweezers is reported. A dimer resulting from a Glaser homocoupling was isolated during a Sonogashira coupling reaction between a diethynyl-carbazole spacer and a 5-bromo-triarylporphyrin and the properties of this original compound were compared with the 3,6-disubstituted carbazole bisporphyrin tweezers. The dyads reported herein present a two-photon absorption maximum at 920 nm with two-photon absorption cross-section in the 1200 GM range. Despite a strong linear absorption in the Soret region and moderate fluorescence quantum yield, they both lead to a high brightness reaching 30 000 M(-1)  cm(-1) .

Cyclotriveratrylene-Containing Porphyrins

The C3-symmetric cyclotriveratrylene (CTV) was covalently bonded via click chemistry to 1, 2, 3, and 6 zinc(II) porphyrin units to various host for C60. The binding constants, Ka, were measured from the quenching of the porphyrin fluorescence by C60. These constants vary between 400 and 4000 M(-1) and are considered weak. Computer modeling demonstrated that the zinc(II) porphyrin units, [Zn], exhibit a strong tendency to occupy the CTV cavity, hence blocking the access for C60 to land on this site. Instead, the pincer of the type [Zn]----[Zn] and in one case [Zn]----CTV, were found to be the most probable geometry to promote host-guest associations in these systems.