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

Tuning Intramolecular Charge Transfer in Antimony(V) Porphyrin through Axial Fluorination

Modulation of intramolecular charge transfer (ICT) has been tested in two antimony(V) porphyrins, SbT(DMP)P(OMe)2·PF6 and SbT(DMP)P(OTFE)2·PF6, where the meso-positions are occupied by 3,5-dimethoxyphenyl (DMP), and the axial positions are linked with either methoxy (OMe) or trifluoroethoxy (OTFE) units, respectively. The presence of the Sb(+5) ion makes the porphyrin center electron poor. Under this situation, placing electron-rich units in the meso-position creates a condition for push-pull type ICT in the SbT(DMP)P(OMe)2·PF6. Remarkably, it is shown that the ICT character can be further enhanced in SbT(DMP)P(OTFE)2·PF6 with the help of electron-withdrawing TFE units in the axial position, which makes the porphyrin center even more electron scarce. The steady-state and transient studies as well as solvatochromism studies establish the ICT in SbT(DMP)P(OMe)2·PF6 and SbT(DMP)P(OTFE)2·PF6, and the strength of the ICT can be modulated by exploiting the structural properties of antimony(V) porphyrin. The existence of ICT is further supported by density functional theory calculations. The transient studies show that upon excitation of these porphyrin, their charge-transfer states convert to a full charger-separated states with appreciable lifetimes.

Advances and opportunities in Group 15 porphyrin chemistry

The chemistry of Group 15 porphyrins has been established relatively well among the main-group porphyrins. Thus far phosphorus(III), phosphorus(V), arsenic(III), arsenic(V), antimony(III), antimony(V), and bismuth(III) porphyrins have been reported. Their unique axial-bonding ability, rich redox, and optical properties offer an advantage over other main-group or transition metal porphyrins. They could be excellent candidates for a variety of applications such as solar energy harvesting, molecular electronics, molecular catalysis, and biomedical applications. Despite these unique properties, the Group 15 porphyrins are not exploited at their fullest capacity. Recently, there has been some interest, where the richness of Group 15 porphyrin chemistry was explored for some of the above applications. In this context, this article summarizes recent advances in Group 15 porphyrin chemistry and attempts to unravel the tremendous opportunities of these remarkable porphyrins.

Visible Light Reductive Photocatalysis of Azo-Dyes with n-n Junctions Based on Chemically Deposited CdS

New composite photocatalysts have been obtained by chemical bath deposition of CdS on top of either nanostructured crystalline ZrO₂ or TiO₂ films previously deposited on conductive glass FTO. Their morphological, photoelectrochemical and photochemical properties have been investigated and compared. Time resolved spectroscopic, techniques show that in FTO/TiO₂/CdS films the radiative recombination of charges, separated by visible illumination of CdS, is faster than in FTO/ZrO₂/CdS, evidencing that carrier dynamics in the two systems is different. Photoelectrochemical investigation evidence a suppression of electron collection in ZrO₂/CdS network, whereas electron injection from CdS to TiO₂ is very efficient since trap states of TiO₂ act as a reservoir for long lived electrons storage. This ability of FTO/TiO₂/CdS films is used in the reductive cleavage of N=N bonds of some azo-dyes by visible light irradiation, with formation and accumulation of reduced aminic intermediates, identified by ESI-MS analysis. Needed protons are provided by sodium formate, a good hole scavenger that leaves no residue upon oxidation. FTO/TiO₂/CdS has an approximately 100 meV driving force larger than FTO/ZrO₂/CdS under illumination for azo-dye reduction and it is always about 10% more active than the seconds. The films showed very high stability and recyclability, ease of handling and recovering.

New nitroindazole-porphyrin conjugates: Synthesis, characterization and antibacterial properties

The synthesis of new porphyrin-indazole hybrids by a Knoevenagel condensation of 2-formyl-5,10,15,20-tetraphenylporphyrin and N-methyl-nitroindazolylacetonitrile derivatives is reported. The target compounds were isolated in moderate to good yields (32-57%) and some of the isolated porphyrin-indazole conjugates showed good performance in the generation of singlet oxygen when irradiated with visible light. Their efficiency as photosensitizers in the photoinactivation of methicillin resistant Staphylococcus aureus-MRSA was evaluated. All derivatives showed to be able to photoinactivate the MRSA bacteria. Compound 3a appears to be the most promising photosensitiser (PS) in the photoinactivation of these bacteria, despite being the least efficient in singlet oxygen generation. The addition of potassium iodide (KI) significantly potentiated the antimicrobial Photodynamic Therapy (aPDT) process mediated by all the analysed porphyrin-indazole conjugates. The combined action of nitroindazole-porphyrins with potassium iodide (KI) action appears to be promising in the photoinactivation of MRSA.

Fluorinated ZnII Porphyrins for Dye-Sensitized Aqueous Photoelectrosynthetic Cells

Three perfluorinated Zn^II porphyrins were evaluated as n-type sensitizers in photoelectrosynthetic cells for HBr and water splitting. All the dyes are featured by the presence of pentafluorophenyl electron-withdrawing groups to increase the ground-state oxidation potential and differ for the nature and position of the π-conjugate linker between the core and anchoring group tasked to bind the metal oxide, in order to assess the best way of coupling with the semiconductor. A phenyl-triazole moiety was used to link the carboxylic anchoring group onto the meso position, while an ethynyl-phenyl linker was chosen to bridge carboxylic and cyanoacrylic groups onto the β-pyrrolic position. A combination of electrochemical, computational, and spectroscopic investigations confirmed the strong electron-withdrawing effect of the perfluorinated porphyrin core, which assures all the investigated dyes of the high oxidation potential required to the coupling with water oxidation catalysts (WOC). Such an electron-poor core, however, affects the charge separation character of the dyes, as demonstrated by the spatial distribution of the excited states, leading to a nonquantitative charge injection, although tilting of the molecules on the semiconductor surface could bring the porphyrin ring closer to the semiconductor, offering additional charge-transfer pathways. Indeed, all the dyes demonstrated successful in the splitting of both aqueous HBr and water, with the best results found for the SnO₂/TiO₂ photoanode sensitized with the β-substituted porphyrin equipped with a cyanoacrylic terminal group, achieving 0.4 and 0.1 mA/cm² photoanodic currents in HBr and water under visible light, respectively. The faradaic yield for oxygen evolution in the presence of an Ir^IV catalyst was over 95%, and the photoanode operation was stable for more than 1000 s. Thus, the perfluorinated porphyrins with a cyanoacrylic anchoring group at the β-position should be considered for further development to improve the charge-transfer character.

Efficient Sunlight Harvesting by A4 β-Pyrrolic Substituted ZnII Porphyrins: A Mini-Review

Dye-Sensitized Solar Cells (DSSCs) are a highly promising alternative to conventional photovoltaic silicon-based devices, due to the potential low cost and the interesting conversion efficiencies. A key-role is played by the dye, and porphyrin sensitizers have drawn great interest because of their excellent light harvesting properties mimicking photosynthesis. Indeed, porphyrins are characterized by strong electronic absorption bands in the visible region up to the near infrared and by long-lived π^* singlet excited states. Moreover, the presence of four meso and eight β-pyrrolic positions allows a fine tuning of their photoelectrochemical properties through structural modification. Trans-A₂BC push-pull Zn^II porphyrins, characterized by a strong and directional electron excitation process along the push-pull system, have been extensively investigated. On the other hand, A₄ β-pyrrolic substituted tetraaryl Zn^II porphyrins, which incorporate a tetraaryl porphyrinic core as a starting material, have received lower attention, even if they are synthetically more attractive and show several advantages such as a more sterically hindered architecture and enhanced solubility in most common organic solvents. The present contribution intends to review the most prominent A₄ β-substituted Zn^II porphyrins reported in the literature so far for application in DSSCs, focusing on the strategies employed to enhance the light harvesting capability of the dye and on a comparison with meso-substituted analogs.

Investigation of the push–pull effects on β-functionalized zinc porphyrin coordinated to C60 donor–acceptor conjugates

Two novel β-functionalized push–pull zinc porphyrins with amine or phenyl push-groups and cyclic imide or carboxylic esters pull-groups have been newly synthesized for light energy harvesting applications. The ethynylphenyl spacers extended the conjugation of the porphyrin π-systems, as reflected by their red-shifted absorbance and fluorescence spectra. Computational studies performed at the B3LYP/6-31G* level indicated no steric hindrance between the porphyrin π-system and the substituents. The calculated HOMO and LUMO orbitals displayed significant delocalization in 1, where the electron density in the HOMO was localized over the push groups, whereas the LUMO was extended over the porphyrin π- and pull-groups. On the other hand, 2 did not display significant orbital segregation, which is attributed to its weaker push–pull character. Electrochemical studies revealed smaller HOMO–LUMO gaps for the push–pull zinc porphyrins. As a consequence of the push–pull effects, reduction in fluorescence intensity and lifetime was observed. Femtosecond transient absorption spectral studies revealed successful formation of singlet excited state in both of the push–pull porphyrins. Donor–acceptor conjugates were subsequently built and characterized by coordinating an electron acceptor, C60Im, via metal–ligand axial coordination. Efficient photo-induced charge separation in both donor–acceptor conjugates was witnessed wherein the charge-separated states persisted tens of nanoseconds prior returning to the ground state.

Porphyrins for Second Order Nonlinear Optics (NLO): An Intriguing History

This short review outlines the main results obtained by our research group over the last 15 years in the field of porphyrins and metal porphyrins for second order nonlinear optics (NLO). This overview aims to provide a general framework of the key factors which affect the second order NLO response of porphyrin chromophores. The pivotal role of the porphyrin ring as π-conjugated linker, the nature of the metal center, the substitution pattern which features the geometrical arrangement of donor and acceptor substituents in the different classes of porphyrin NLO-phores, as well as the aggregation phenomena and the role of solvents are addressed in detail.

Designing Push-Pull Porphyrins for Efficient Dye-Sensitized Solar Cells

Over the past decade, tremendous effort has been made to improve the light-harvesting ability of push-pull porphyrin dyes. Despite notable success achieved in this direction, push-pull porphyrin dyes still suffer from a poor light-harvesting efficiency owing to the lack of absorption between the Soret and Q-bands. To tackle this issue, here we design a series of push-pull porphyrin dyes with anchoring groups either at meso- or β-position using calculations based on first-principles time-dependent density functional theory. In contrast to the common perception, we find that porphyrin dyes bearing an electron-donor at the meso-position and an electron-acceptor at the β-position produce an additional extended band between the Soret and Q-bands appearing at around 500 nm due to S₀ → S₃ excitation, leading to a much higher light-harvesting performances compared to meso- and β-disubstituted ones. In addition, changing the π-conjugated linker at the acceptor site from ethylene linker (C═C) to acetylene linker (C≡C) further improves the light-harvesting ability of meso-β-porphyrin dyes, making them promising candidates for dye-sensitized solar cell application.

Porphyrin-sensitized solar cells: systematic molecular optimization, coadsorption and cosensitization

As a promising low-cost solar energy conversion technique, dye-sensitized solar cells have undergone spectacular development since 1991. For practical applications, improvement of power conversion efficiency has always been one of the major research topics. Porphyrins are outstanding sensitizers endowed with strong sunlight harvesting ability in the visible region and multiple reaction sites available for functionalization. However, judicious molecular design in consideration of light-harvest, energy levels, operational dynamics, adsorption geometry and suppression of back reactions is specifically required for achieving excellent photovoltaic performance. This feature article highlights some of the recently developed porphyrin sensitizers, especially focusing on the systematic dye structure optimization approach in combination with coadsorption and cosensitization methods in pursuing higher efficiencies. Herein, we expect to provide more insights into the structure-performance correlation and molecular engineering strategies in a stepwise manner.

Zn-Porphyrin propped with hydantoin anchor: synthesis, photophysics and electron injection/recombination dynamics

In this work, Zn-porphyrin with a hydantoin anchor (ZnPHy) was designed and synthesized for dye-sensitized solar cell (DSC) applications. The synthesized ZnPHy was well characterized using IR, NMR and mass spectral techniques, and satisfactory results were obtained. Cyclic voltammetry, UV-visible absorption, steady-state fluorescence, time-resolved fluorescence and transient absorption spectroscopic techniques were employed to elucidate the electrochemical and photophysical properties of ZnPHy. The obtained properties revealed that the synthesized ZnPHy can be used as a photosensitizer for DSC applications. The nature of ZnPHy binding onto the TiO₂ surface was investigated using ATR-FTIR and UV-Vis absorption measurements. The amount of adsorbed ZnPHy on TiO₂ surface was reasonably fit using the Langmuir adsorption isotherm, with a binding constant value of 1.03 × 10⁵ M^-1. Time-resolved measurements were used to elucidate the rate of electron injection and the regeneration and recombination kinetics for ZnPHy/TiO₂ film. The ZnPHy showed a high electron injection rate with a ϕ_inj of 99%. Intriguingly, the rate of electron recombination is much slower than the rates reported for carboxyl-based Zn-porphyrins. Such a high electron injection and slow electron recombination rate are beneficial to produce long-lived electrical current in a photovoltaic device. Thus, the ZnPHy-sensitized TiO₂ electrode showed the best photovoltaic performance, with the short-circuit photocurrent density (J_SC), open-circuit voltage (V_OC) and fill factor (ff) of 3.49 mA cm^-2, 0.6 V and 0.52, respectively, yielding an overall conversion efficiency (η) of 1.01%. For comparison, the ZnCOOH-sensitized electrode was also fabricated under the same conditions and yielded the η value of 0.84%. Hence, the hydantoin moiety could be a potential alternative anchoring group for DSC applications.

4D-π-1A type β-substituted ZnII-porphyrins: ideal green sensitizers for building-integrated photovoltaics

Two novel green β-substituted Zn^II-porphyrins, G1 and G2, based on a 4D-π-1A type substitution pattern have been synthesized. Their enhanced push-pull character, by reduction of H-L energy gaps, promotes broadening and red-shifting of absorption bands. The effective synthetic pathway and the remarkable spectroscopic properties make G2 ideal for BIPV application.

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 Co^2+/3+ and I^-/I₃^- reference electrolytes.

Investigation of the push–pull effects on β-functionalized benzoporphyrins bearing an ethynylphenyl bridge

β-Functionalized opp-dibenzoporphyrins show significant push–pull effects.