Alessio Orbelli Biroli

Tetraaryl ZnIIPorphyrinates Substituted at β-Pyrrolic Positions as Sensitizers in Dye-Sensitized Solar Cells: A Comparison withmeso-Disubstituted Push-Pull ZnIIPorphyrinates

A facile and fast approach, based on microwave-enhanced Sonogashira coupling, has been employed to obtain in good yields both mono- and, for the first time, disubstituted push-pull Zn(II) porphyrinates bearing a variety of ethynylphenyl moieties at the β-pyrrolic position(s). Furthermore, a comparative experimental, electrochemical, and theoretical investigation has been carried out on these β-mono- or disubstituted Zn(II) porphyrinates and meso-disubstituted push-pull Zn(II) porphyrinates. We have obtained evidence that, although the HOMO-LUMO energy gap of the meso-substituted push-pull dyes is lower, so that charge transfer along the push-pull system therein is easier, the β-mono- or disubstituted push-pull porphyrinic dyes show comparable or better efficiencies when acting as sensitizers in DSSCs. This behavior is apparently not attributable to more intense B and Q bands, but rather to more facile charge injection. This is suggested by the DFT electron distribution in a model of a β-monosubstituted porphyrinic dye interacting with a TiO2 surface and by the positive effect of the β substitution on the incident photon-to-current conversion efficiency (IPCE) spectra, which show a significant intensity over a broad wavelength range (350-650 nm). In contrast, meso-substitution produces IPCE spectra with two less intense and well-separated peaks. The positive effect exerted by a cyanoacrylic acid group attached to the ethynylphenyl substituent has been analyzed by a photophysical and theoretical approach. This provided supporting evidence of a contribution from charge-transfer transitions to both the B and Q bands, thus producing, through conjugation, excited electrons close to the carboxylic anchoring group. Finally, the straightforward and effective synthetic procedures developed, as well as the efficiencies observed by photoelectrochemical measurements, make the described β-monosubstituted Zn(II) porphyrinates extremely promising sensitizers for use in DSSCs.

Influence of Porphyrinic Structure on Electron Transfer Processes at the Electrolyte/Dye/TiO2 Interface in PSSCs: a Comparison between meso Push–Pull and β-Pyrrolic Architectures

Time-resolved photophysical and photoelectrochemical investigations have been carried out to compare the electron transfer dynamics of a 2-β-substituted tetraarylporphyrinic dye (ZnB) and a 5,15-meso-disubstituted diarylporphyrinic one (ZnM) at the electrolyte/dye/TiO2 interface in PSSCs. Although the meso push-pull structural arrangement has shown, up to now, to have the best performing architecture for solar cell applications, we have obtained superior energy conversion efficiencies for ZnB (6.1%) rather than for ZnM (3.9%), by using the I(-)/I3(-)-based electrolyte. To gain deeper insights about these unexpected results, we have investigated whether the intrinsic structural features of the two different porphyrinic dyes can play a key role on electron transfer processes occurring at the dye-sensitized TiO2 interface. We have found that charge injection yields into TiO2 are quite similar for both dyes and that the regeneration efficiencies by I(-), are also comparable and in the range of 75-85%. Moreover, besides injection quantum yields above 80%, identical dye loading, for both ZnB and ZnM, has been evidenced by spectrophotometric measurements on transparent thin TiO2 layers after the same adsorption period. Conversely, major differences have emerged by DC and AC (electrochemical impedance spectroscopy) photoelectrochemical investigations, pointing out a slower charge recombination rate when ZnB is adsorbed on TiO2. This may result from its more sterically hindered macrocyclic core which, besides guaranteeing a decrease of π-staking aggregation of the dye, promotes a superior shielding of the TiO2 surface against charge recombination involving oxidized species of the electrolyte.

Oligothiophenes Nano-organized on a Cyclotetrasiloxane Scaffold as a Model of a Silica-Bound Monolayer: Evidence for Intramolecular Excimer Formation

Excimer formation in a new class of terthiophene-based fluorophores covalently bonded to a cyclotetrasiloxane scaffold has been demonstrated and the photophysical process ruling it has been investigated in detail and modeled theoretically. In contrast to the conventional systems in which long-living fluorophores such as pyrene are linked in the same molecule, an excimer is formed only when two terthiophene-based branches nano-organized on the same cyclotetrasiloxane scaffold are close enough together when excitation takes place. In such a case, excimer formation is extremely efficient, and the new bound excited states are quite stable.

Light-Induced Regiospecific Bromination of meso-Tetra(3,5-di-tert-butylphenyl)Porphyrin on 2,12 β-Pyrrolic Positions

The antipodal introduction of two bromine atoms on the 2,12 β-pyrrolic position of 5,10,15,20-tetra(3,5-di-tert-butylphenyl)porphyrin was successfully achieved by a light-induced reaction of the substrate with excess NBS. Complexation with Ni(II) of the major regioisomer led to good quality crystals, suitable for X-ray structure determination with unprecedented probability levels. The regiospecific character of the synthetic procedure and the exactness of the bromine atom position assignment were thus confirmed, suggesting an unexpected electrophilic aromatic substitution pathway rather than a free-radical halogenation process. A QTAIM topological analysis on the DFT-optimized wave function of the monosubstituted free-base porphyrin intermediate carrying a bromine atom in C2 β-pyrrolic position confirmed the largest negative charge for the C12 carbon atom in antipodal position, in agreement with the proposed electrophilic aromatic substitution mechanism.

Coupling of Zinc Porphyrin Dyes and Copper Electrolytes: A Springboard for Novel Sustainable Dye-Sensitized Solar Cells

The combination of β-substituted Zn2+ porphyrin dyes and copper-based electrolytes represents a sustainable route for economic and environmentally friendly dye-sensitized solar cells. Remarkably, a new copper electrolyte, [Cu(2-mesityl-1,10-phenanthroline)2]+/2+, exceeds the performance reached by Co2+/3+ and I-/I3- reference electrolytes.

Intriguing Influence of −COOH-Driven Intermolecular Aggregation and Acid–Base Interactions with N,N-Dimethylformamide on the Second-Order Nonlinear-Optical Response of 5,15 Push–Pull Diarylzinc(II) Porphyrinates

A series of 5,15 push-pull meso-diarylzinc(II) porphyrinates, carrying one or two -COOH or -COOCH3 acceptor groups and a -OCH3 or a -N(CH3)2 donor group, show in N,N-dimethylformamide and CHCl3 solutions a negative and solvent-dependent second-order nonlinear-optical (NLO) response measured by the electric-field-induced second-harmonic generation (EFISH) technique, different from the structurally related zinc(II) porphyrinate carrying a -N(CH3)2 donor group and a -NO2 acceptor group, where a still solvent-dependent but positive EFISH second-order response was previously reported. Moreover, when a -N(CH3)2 donor group and a -COOH acceptor group are part of a sterically hindered 2,12 push-pull β-pyrrolic-substituted tetraarylzinc(II) porphyrinate, the EFISH response is positive and solvent-independent. In order to rationalize these rather intriguing series of observations, EFISH measurements have been integrated by electronic absorption and IR spectroscopic investigations and by density functional theory (DFT) and coupled-perturbed DFT theoretical and 1H pulsed-gradient spin-echo NMR investigations, which prompt that the significant concentration effects and the strong influence of the solvent nature on the NLO response are originated by a complex whole of different aggregation processes induced by the -COOH group.