Triarylamine-substituted imidazole- and quinoxaline-fused push-pull porphyrins for dye-sensitized solar cells

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.

Naphthodithiophene-Fused Porphyrins: Synthesis, Characterization, and Impact of Extended Conjugation on Aromaticity

The fusion of tetrapyrroles with aromatic heterocycles constitutes a useful tool for manipulating their opto-electronic properties. In this work, the synthesis of naphthodithiophene-fused porphyrins was achieved through a Heck reaction-based cascade of steps followed by the Scholl reaction. The naphthodithiophene-fused porphyrins display a unique set of optical and electronic properties. Fusion of the naphtho[2,1-b:3,4-b']dithiophene to porphyrin (F2VTP) leads to a ~20% increase in the fluorescence lifetime, which is accompanied, unexpectedly, by a more than two-fold drop in the emission quantum yield (ϕ=0.018). In contrast, fusion of the isomeric naphtho[1,2-b:4,3-b']dithiophene to porphyrin (F3VPT) results in a ~1.5-fold increase in the fluorescence quantum yield (ϕ=0.13) with a concomitant ~30 % increase in the fluorescence lifetime. This behavior suggests that fusion of the porphyrin with the naphthodithiopheno-system mainly affects the radiative rate constant in the Q-state deactivation pathway, where the effects of the isomeric naphtho[2,1-b:3,4-b']dithiophene- versus naphtho[1,2-b:4,3-b']dithiophene-fusion are essentially the opposite. Interestingly, nucleus-independent chemical shifts analysis revealed a considerable difference between the aromaticities of these two isomeric systems. Our results demonstrate that subtle structural differences in the fused components of the porphyrin can be reflected in rather significant differences between the photophysical properties of the resulting systems.

β-Pyrrole Functionalized Push or Pull Porphyrins: Excited Charge Transfer Promoted Singlet Oxygen Generation

Singlet oxygen (1O2) producing photosensitizers are highly sought for developing new photodynamic therapy agents and facilitating 1O2-involved chemical reactions. Often singlet oxygen is produced by the reaction of triplet-excited photosensitizers with dioxygen via an energy transfer mechanism. In the present study, we demonstrate a charge transfer mechanism to produce singlet oxygen involving push or pull functionalized porphyrins. For this, 20 β-pyrrole functionalized porphyrins carrying either an electron-rich push or electron-deficient pull group have been newly synthesized. Photoexcitation of these push-pull porphyrins has been shown to produce high-energy MPδ+-Aδ- or MPδ--Dδ+ charge transfer states. Subsequent charge recombination results in populating the triplet excited states of extended lifetimes in the case of the push group containing porphyrins that eventually react with dioxygen to produce the reactive singlet oxygen of relatively higher quantum yields. The effect of the push and pull groups on the porphyrin periphery in governing initial charge transfer, the population of triplet excited states and their lifetimes, and resulting in improved singlet oxygen quantum yields are systematically probed. The improved performance of 1O2 generation by porphyrins carrying push groups is borne out from this study.

Small Molecules Containing Amphoteric Imidazole Motifs as Sensitizers for Dye-Sensitized Solar Cells: An Overview

Organic dyes, porphyrins and inorganic complexes containing imidazole (IM) motifs have been demonstrated as a new class of sensitizers in dye-sensitized solar cells (DSSCs). Particularly, the amphoteric nature of IM-based motifs allows them to be used as donors (D), auxiliary donors (D_A), linker/branch (π), or acceptors (A) in D-π-A-based organic dyes and porphyrins and also employed as cyclometalated heteroleptic and ancillary ligands in the Ru(II) and Ir(III) complexes for DSSCs. It is noteworthy that the introduction of IM chromophores in the dyes of D-π-A configuration can improve the light-harvesting properties and prohibit the charge recombination reactions due to the extension of the π-conjugated structures and hydrophobic nature. Similarly, in the case of inorganic complexes, the presence of IM motifs as ligands can improve the light-harvesting ability, give facilely tuned HOMO and LUMO energy levels, increase the charge recombination resistance and photostability. This results in enhanced photocurrent (J_SC) and photovoltage (V_OC) and consequently solar-to-power conversion efficiency (η) of DSSC devices based on Ru(II) and Ir(III) complexes. Considering the interesting DSSC applications of IM-derived molecules, in this review, we therefore comprehensively discuss their photophysical, electrochemical and photovoltaic properties reported so far and establish their structure-activity relationship to further advance the η of DSSCs. To the best of our knowledge, there is no such a review interpreting the importance of molecules possessing IM-motifs for DSSC applications to date.

Ultrasound-assisted transition-metal-free catalysis: a sustainable route towards the synthesis of bioactive heterocycles

Heterocycles of synthetic and natural origin are a well-established class of compounds representing a broad range of organic molecules that constitute over 60% of drugs and agrochemicals in the market or research pipeline. Considering the vast abundance of these structural motifs, the development of chemical processes providing easy access to novel complex target molecules by introducing environmentally benign conditions with the main focus on improving the cost-effectiveness of the chemical transformation is highly demanding and challenging. Accordingly, sonochemistry appears to be an excellent alternative and a highly feasible environmentally benign energy input that has recently received considerable and steadily increasing interest in organic synthesis. However, the involvement of transition-metal-catalyst(s) in a chemical process often triggers an unintended impact on the greenness or sustainability of the transformation. Consequently, enormous efforts have been devoted to developing metal-free routes for assembling various heterocycles of medicinal interest, particularly under ultrasound irradiation. The present review article aims to demonstrate a brief overview of the current progress accomplished in the ultrasound-assisted synthesis of pharmaceutically relevant diverse heterocycles using transition-metal-free catalysis.

Aromatic heterobicycle-fused porphyrins: impact on aromaticity and excited state electron transfer leading to long-lived charge separation

A new synthetic method to fuse benzo[4,5]imidazo[2,1-a]isoindole to the porphyrin periphery at the β,β-positions has been developed, and its impact on the aromaticity and electronic structures is investigated. Reactivity investigation of the fused benzoimidazo-isoindole component reveals fluorescence quenching of a zinc porphyrin (AMIm-2) upon treatment with a Brønsted acid. The reaction of the zinc porphyrin (AMIm-2) with methyl iodide initiated a new organic transformation, resulting in the ring-opening of isoindole with the formation of an aldehyde and dimethylation of the benzoimidazo component. The fused benzoimidazo-isoindole component acted as a good ligand to bind platinum(ii), forming novel homobimetallic and heterobimetallic porphyrin complexes. The fusion of benzoimidazo-isoindole on the porphyrin ring resulted in bathochromically shifted absorptions and emissions, reflecting the extended conjugation of the porphyrin π-system. Time-resolved emission and transient absorption spectroscopy revealed stable excited state species of the benzoimidazo-isoindole fused porphyrins. Zinc porphyrin AMIm-2 promoted excited state electron transfer upon coordinating with an electron acceptor, C₆₀, generating a long-lived charge-separated state, in the order of 37.4 μs. The formation of the exceptionally long-lived charge-separated state is attributed to the involvement of both singlet and triplet excited states of AMIm-2, which is rarely reported in porphyrins.

π-Extended porphyrins fused with benzo[4,5]imidazo[2,1-a]isoindole, showing unique electronic and photophysical properties, were newly synthesized. A long-lived charge-separated state was revealed upon coordination of C₆₀ to zinc porphyrin AMIm-2.

Antimony(+5) ion induced tunable intramolecular charge transfer in hypervalent antimony(v) porphyrins

Antimony(+5) insertion induces both electron-rich and electron-poor parts within the porphyrin structure resulting in a push–pull style intramolecular charge transfer.

Recent advances in the transition-metal-free synthesis of quinoxalines

Quinoxalines, also known as benzo[a]pyrazines, constitute an important class of nitrogen-containing heterocyclic compounds as a result of their widespread prevalence in natural products, biologically active synthetic drug candidates, and optoelectronic materials. Owing to their importance and chemists' ever-increasing imagination of new transformations of these products, tremendous efforts have been dedicated to finding more efficient approaches toward the synthesis of quinoxaline rings. The last decades have witnessed a marvellous outburst in modifying organic synthetic methods to create them sustainable for the betterment of our environment. The exploitation of transition-metal-free catalysis in organic synthesis leads to a new frontier to access biologically active heterocycles and provides an alternative method from the perspective of green and sustainable chemistry. Despite notable developments achieved in transition-metal catalyzed synthesis, the high cost involved in the preparation of the catalyst, toxicity, and difficulty in removing it from the final products constitute disadvantageous effects on the atom economy and eco-friendly nature of the transformation. In this review article, we have summarized the recent progress achieved in the synthesis of quinoxalines under transition-metal-free conditions and cover the reports from 2015 to date. This aspect is presented alongside the mechanistic rationalization and limitations of the reaction methodologies. The scopes of future developments are also highlighted.

Fluorenyl Indoline as an Efficient Electron Donor for Concerted Companion Dyes: Enhanced Light-Harvesting and Photocurrent

Concerted companion dyes (CC dyes) like XW61 have been demonstrated to be an effective platform for developing efficient DSSCs. However, the moderated phenothiazine-based electron donor in XW61 results in unsatisfactory J_sc. To address this problem, a stronger fluorenyl indoline-based electron donor has been used to construct porphyrin dye XW68 and organic dyes Y1-Y2. The stronger electron-donating character of the fluorenyl indoline unit leads to an enhanced J_sc value (20.48 mA·cm^-2) for the individual dye XW68. On this basis, CC dyes XW69-XW70-C8 have been designed and synthesized by combining the frameworks of Y1 and Y2 with XW68. The complementary absorption characters of the porphyrin and the organic dye moieties lead to panchromatic absorption with a strong light-harvesting capability from 350 to 700 nm and the onset wavelength extended to ca. 840 nm in the IPCE curves. As a result, excellent J_sc values have been achieved (>22 mA·cm^-2). In addition to the advantages of high J_sc, bulky octyl groups have been introduced into the donor of XW70-C8 to reduce dye aggregation and suppress charge recombination. Finally, a highest PCE of 11.1% with a satisfactory J_sc (22.25 mA·cm^-2) and an enhanced V_oc (750 mV) has been achieved upon coadsorption of XW70-C8 with CDCA. In addition, the CC dye XW70-C8-based solar cells exhibit excellent long-term photostability. These results provide an effective method for rationally improving the photovoltaic behavior, especially the J_sc of CC dyes, by introducing strong electron donor moieties with suitable substituents.

Unique Role of Heterole-Fused Structures in Aromaticity and Physicochemical Properties of 7,8-Dehydropurpurins

Porphyrins with a fused five-membered ring, such as 7,8-dehydropurpurins, have appeared as an emerging class of unique porphyrinoids. Their altered absorption spectra, relatively short lifetimes of excited states, and small HOMO-LUMO gaps arise from the harmony of the antiaromatic 20π-circuit and the aromatic 18π-circuit. In this regard, the electronic properties of 7,8-dehydropurpurins are expected to be controlled by modulating the contribution of the antiaromatic π-circuit to the whole aromaticity. Here the comparison of pyrrole- and phosphole-fused 7,8-dehydropurpurins is reported in terms of their aromaticity and physicochemical properties. The spectroscopic investigation revealed the larger contribution of the antiaromatic 24π-circuit in pyrrole-fused 7,8-dehyrdopurpurins than in phosphole-fused 7,8-dehydropurpurins. The DFT calculations also supported the feasibility of tuning the aromaticity of 7,8-dehydropurpurins by heterole-fused structures. Thus, the introduction of heterole-fused structures into porphyrinoids is a universal strategy to get new insight into aromaticity and their intrinsic properties in cyclic π-conjugated molecules.

Renaissance of Fused Porphyrins: Substituted Methylene-Bridged Thiophene-Fused Strategy for High-Performance Dye-Sensitized Solar Cells

Over the last decades, porphyrin sensitizers have made a remarkable contribution to performance improvement in dye-sensitized solar cells (DSSCs). In particular, versatile push-pull-type porphyrin sensitizers have achieved power conversion efficiencies (η) over 10% as a result of their improved light-harvesting abilities. Meanwhile, aromatic ring fusion to a porphyrin core is an attractive option for highly efficient DSSCs because of its expanded π-conjugation and resultant red-shifted absorption. Nevertheless, aromatic-fused porphyrin sensitizers have suffered rather low cell performances due to their mismatch of HOMO-LUMO levels, high aggregation tendency, and short lifetime of the excited states. Bearing these in mind, we envisioned that the fusion of substituted methylene-bridged small aromatic ring to a porphyrin core would overcome these drawbacks, boosting the cell performance. Herein, we report a series of substituted methylene-bridged thiophene-fused porphyrins, AfZnP, DfZnP, and DfZnP- iPr. After optimization, DSSC with the donor-side thiophene-fused DfZnP- iPr achieved an η-value of 10.1%, which is comparable to that of DSSC with GY50 (10.0%), a representative high-performance push-pull-type porphyrin sensitizer. More importantly, cosensitization of DfZnP- iPr with a complementary sensitizer LEG4 further led to an η-value of 10.7%, which is the highest value ever reported for DSSCs with fused porphyrin sensitizers. Therefore, our strategy will reboot the exploration of aromatic-fused porphyrin sensitizers for high-performance DSSCs.

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.

Role of hexyloxy groups in zinc phthalocyanines bearing sulfonic acid anchoring groups for dye-sensitized solar cells

Zinc phthalocyanine dyes bearing four sulfonic acid anchoring groups with (A-ZnPc) and without (H-ZnPc) four chloro and eight hexyloxy groups were used as sensitizers for dye-sensitized solar cells (DSSCs). The dyes were investigated in terms of their optical, electrochemical and photovoltaic properties. The presence of these groups in dye A-ZnPc resulted in both red-shifted absorption and decreased dye aggregation, which are beneficial for the improvement of device performance. In the presence of chenodeoxycholic acid (CDCA) as a coadsorbent, the DSSC based on H-ZnPc shows a power conversion efficiency (PCE) of 0.96%, which is improved by [Formula: see text]40% as compared to the device without CDCA. However, the PCE of an A-ZnPc-based device with CDCA slightly enhances from 1.15% (without CDCA) to 1.22%, indicating that the bulky hexyloxy groups with large steric hindrance can effectively suppress aggregation of the adsorbed dye. The results showed that the zinc phthalocyanine dye bearing bulky hexyloxy groups is a promising candidate to construct efficient coadsorbent-free DSSCs.

Imidazoporphyrins as supramolecular tectons: synthesis and self-assembly of zinc 2-(4-pyridyl)-1H-imidazo[4,5-b]porphyrinate

First evidence of self-assembling of imidazoporphyrins is revealed with the example of zinc(ii) 5,10,15,20-tetramesityl-2-(4-pyridyl)-1H-imidazo[4,5-b]porphyrin in the solid state and in solution.

Synthesis and Characterization of Novel β-Bis( N, N-diarylamino)-Substituted Porphyrin for Dye-Sensitized Solar Cells under 1 sun and Dim Light Conditions

In this work, we have synthesized a novel porphyrin dye named SK7, which contains two N, N-diarylamino moieties at two β-positions as electron-donating units and one carboxy phenylethynyl moiety at the meso-position as an electron-withdrawing, anchoring group. This novel dye was tested for the application in dye-sensitized solar cells. The light-harvesting behavior of SK7 and YD2 was investigated using UV-vis absorption and density functional calculation. The electron transport properties at the TiO₂/dye/electrolyte interface for SK7- and YD2-based devices were evaluated by electrochemical impedance spectroscopy. X-ray crystallographic characterization was conducted to understand the influence of two N, N-diarylamino units at two β-positions. The power conversion efficiencies of ca. 6.54% under 1 sun illumination (AM 1.5G) and ca. 19.72% under a T5 light source were noted for the SK7 dye. The performance of SK7 is comparable to that of dye YD2, which contains only one N, N-diarylamino moiety at the meso-position (ca. 7.78 and 20.00% under 1 sun and T5 light, respectively).

Post-synthetic methods for functionalization of imidazole-fused porphyrins

Several methods for the post-synthetic modification of imidazo[4,5-[Formula: see text]]porphyrins are reported. First, a synthetic approach to the isomeric difunctionalized porphyrins, containing two [Formula: see text]-fused 2-aryl-1[Formula: see text]-imidazole cycles at adjacent or opposite pyrrole rings of the macrocycle is developed. The core chemistry of this synthetic route is the transformation of 2-aryl-1[Formula: see text]-imidazo[4,5-[Formula: see text]]porphyrins into corresponding imidazodioxochlorins followed by Debus–Radziszewski condensation with aromatic aldehyde. Next, 2-(4-bromophenyl)-1[Formula: see text]-imidazo[4,5-[Formula: see text]]-5,10,15,20-tetramesitylporphyrin was transformed into useful carboxy- and phosphonato-substituted precursors for material chemistry according to palladium-catalyzed C–C and C–P bond forming reactions.

Three-Component Activation/Alkynylation/Cyclocondensation (AACC) Synthesis of Enhanced Emission Solvatochromic 3-Ethynylquinoxalines

2-Substituted 3-ethynylquinoxaline chromophores can be readily synthesized by a consecutive activation-alkynylation-cyclocondensation (AACC) one-pot sequence in a three-component manner. In comparison with the previously published four-component glyoxylation starting from electron-rich π-nucleophiles, the direct activation of (hetero)aryl glyoxylic acids allows the introduction of substituents that cannot be directly accessed by glyoxylation. By introducing N,N-dimethylaniline as a strong donor in the 2-position, the emission solvatochromicity of 3-ethynylquinoxalines can be considerably enhanced to cover the spectral range from blue-green to deep red-orange with a single chromophore in a relatively narrow polarity window. The diversity-oriented nature of the synthetic multicomponent reaction concept enables comprehensive investigations of structure-property relationships by Hammett correlations and Lippert-Mataga analysis, as well as the elucidation of the electronic structure of the emission solvatochromic π-conjugated donor-acceptor systems by DFT and time-dependent DFT calculations with the PBEh1PBE functional for a better reproduction of the dominant charge-transfer character of the longest wavelength absorption band.

Embedding heteroatoms: an effective approach to create porphyrin-based functional materials

Incorporation of planarized heteroatom(s) onto the porphyrin periphery is an effective approach to create porphyrin-based functional materials. In the last three decades, such an "embedding heteroatom" strategy has been actively explored in order to realize attractive electronic, optical, and electrochemical properties. This review aims to cover a variety of synthetic methodologies that have been developed for the construction of heteroatom-embedded porphyrins. Moreover, we also summarize their structure-property relationships as well as possible applications in various research fields including artificial photosynthesis, molecular engineering, organic electronics, and bioimaging.

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

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

Monobenzoporphyrins as Sensitizers for Dye-Sensitized Solar Cells: Observation of Significant Spacer-Group Effect

A series of monobenzoporphyrins (WH1-WH4) bearing different conjugated spacer groups were designed and synthesized as sensitizers for dye-sensitized solar cells. Although a phenyl spacer only has a minimal impact on the absorption bands of the monobenzoporphyrin, an ethynylphenyl (WH3) or a vinyl (WH4) spacer redshifts and broadens the absorption bands of the dyes to result in much enhanced light-harvesting ability. Dye-sensitized solar cells based on these monobenzoporphyrin dyes displayed remarkable differences in power conversion efficiencies (PCEs). The monobenzoporphyrin bearing no spacer (WH1) resulted in a PCE of only 0.5 %; in contrast, the monobenzoporphyrin bearing vinyl spacers (WH4) achieved a PCE of 5.2 %. The high efficiency of the WH4 cell is attributed to the higher light-harvesting ability, the lesser extent of aggregation on the TiO2 surface, and the more favorable electron-density distributions of the HOMO and LUMO for electron injection and collection. This work demonstrates the exceptional tunability of benzoporphyrins as sensitizers for dye-sensitized solar cells.

Effects of Bulky Substituents of Push-Pull Porphyrins on Photovoltaic Properties of Dye-Sensitized Solar Cells

To evaluate the effects of substituent bulkiness around a porphyrin core on the photovoltaic properties of porphyrin-sensitized solar cells, long alkoxy groups were introduced at the meso-phenyl group (ZnPBAT-o-C8) and the anchoring group (ZnPBAT-o-C8Cn, n = 4, 8) of an asymmetrically substituted push-pull porphyrin with double electron-donating diarylamino groups and a single electron-withdrawing carboxyphenylethynyl anchoring group. The spectroscopic and electrochemical properties of ZnPBAT-o-C8 and ZnPBAT-o-C8Cn were found to be superior to those of a push-pull porphyrin reference (YD2-o-C8), demonstrating their excellent light-harvesting and redox properties for dye-sensitized solar cells. A power conversion efficiency (η) of the ZnPBAT-o-C8-sensitized solar cell (η = 9.1%) is higher than that of the YD2-o-C8-sensitized solar cell (η = 8.6%) using iodine-based electrolyte due to the enhanced light-harvesting ability of ZnPBAT-o-C8. In contrast, the solar cells based on ZnPBAT-o-C8Cn, possessing the additional alkoxy chains in the anchoring group, revealed the lower η values of 7.3% (n = 4) and 7.0% (n = 8). Although ZnPBAT-o-C8Cn exhibited higher resistance at the TiO2-dye-electrolyte interface by virtue of the extra alkoxy chains, the reduced amount of the porphyrins on TiO2 by excessive addition of coadsorbent chenodeoxycholic acid (CDCA) for mitigating the aggregation on TiO2 resulted in the low η values. Meanwhile, the ZnPBAT-o-C8-sensitized solar cell showed the lower η value of 8.1% than the YD2-o-C8-sensitized solar cell (η = 9.8%) using cobalt-based electrolyte. The smaller η value of the ZnPBAT-o-C8-sensitized solar cell may be attributed to the insufficient blocking effect of the bulky substituents of ZnPBAT-o-C8 under the cobalt-based electrolyte conditions. Overall, the alkoxy chain length and substitution position around the porphyrin core are important factors to affect the cell performance.

Benzoporphyrins bearing pyridine or pyridine-N-oxide anchoring groups as sensitizers for dye-sensitized solar cell

Novel benzoporphyrins bearing pyridine or pyridine-[Formula: see text]-oxide groups were prepared through a concise method based on a Pd0 catalyzed cascade reaction. These benzoporphyrins were examined as sensitizers for dye-sensitized solar cells. Vicinal pyridine and vicinal pyridine-[Formula: see text]-oxide groups were introduced as new types of anchoring/acceptor groups for dye-sensitized solar cells for the first time. While all the porphyrins showed solar to electricity conversion, benzoporphyrins bearing pyridine-[Formula: see text]-oxide anchoring groups displayed higher conversion efficiency than benzoporphyrins bearing pyridine-anchoring groups.Opp-dibenzoporphyrins display broadened and red-shifted UV-vis absorption and emission bands as compared with those of the monobenzoporphyrins, which arises from the fusion of one more benzene ring and the attachment of two more electron-withdrawing groups to the porphyrin [Formula: see text]-positions. Cyclic voltammetry (CV) data and DFT calculation data obtained for these porphyrins agree well with their UV-vis absorption and fluorescence spectroscopic data. The HOMO energy level derived from the first oxidation potentials indicate that regeneration of the resulting porphyrin radical cation by the redox mediator (I[Formula: see text]/I[Formula: see text] is thermodynamically feasible for all these benzoporphyrin sensitizers (3, 5, 8 and 10). On the other hand, excited state energy levels of these benzoporphyrins calculated from the CV data, the UV-vis and fluorescence spectroscopic data are all slightly lower than the energy level of the conduction band of TiO2, suggesting insufficient driving force for efficient electron injection from the porphyrin excited singlet state to the conduction band of TiO2.

A triazine di(carboxy)porphyrin dyad versus a triazine di(carboxy)porphyrin triad for sensitizers in DSSCs

Two porphyrin-chromophores, i.e. triad PorZn-(PorCOOH)(2)-(piper)2 (GZ-T1) and dyad (PorZn)(2)-NMe2 (GZ-T1), have been synthesized and their photophysical and electrochemical properties have been investigated. The optical properties together with the appropriate electronic energy levels, i.e. the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels, revealed that both porphyrin assemblies can function as sensitizers for dye sensitized solar cells (DSSCs). The and -based DSSCs have been prepared and studied using 20 mM CDCA as coadsorbent and were found to exhibit an overall power conversion efficiency (PCE) of 5.88% and 4.56%, respectively (under an illumination intensity of 100 mW cm(-2) with TiO(2) films of 12 μm). The higher PCE of the -sensitized DSSC, as revealed from the current-voltage characteristic under illumination and the incident photon to current conversion efficiency (IPCE) spectra of the two DSSCs, is mainly attributed to its enhanced short circuit current (J(sc)), although both the open circuit voltage (V(oc)) and the fill factor are improved too. The electrochemical impedance spectra (EIS) demonstrated a shorter electron transport time, longer electron lifetime and higher charge recombination resistance for the DSSC sensitized with the dye as well as a larger dye loading onto the TiO(2) surface.

Phenothiazine-functionalized push–pull Zn porphyrin photosensitizers for efficient dye-sensitized solar cells

A series of zinc porphyrin dyes (JY24–27) featured phenothiazine moieties have been synthesized and applied as photosensitizers in dye-sensitized solar cells.

π-Extended diketopyrrolopyrrole–porphyrin arrays: one- and two-photon photophysical investigations and theoretical studies

Diketopyrrolopyrrole–porphyrin conjugates show remarkable NIR emission properties, high two-photon absorption cross-sections and significant singlet oxygen production efficiency.

On the synthesis of functionalized porphyrins and porphyrin conjugates via β-aminoporphyrins

A two-step methodology to prepare a series ofmeso-tetraarylporphyrin conjugates bearing water-soluble moieties, anchoring groups and receptor subunits.

Effects of Immersion Solvent on Photovoltaic and Photophysical Properties of Porphyrin-Sensitized Solar Cells

Memory effects in self-assembled monolayers (SAMs) of zinc porphyrin carboxylic acid on TiO2 electrodes have been demonstrated for the first time by evaluating the photovoltaic and electron transfer properties of porphyrin-sensitized solar cells prepared by using different immersion solvents sequentially. The structure of the SAM of the porphyrin on the TiO2 was maintained even after treating the porphyrin monolayer with different neat immersion solvents (memory effect), whereas it was altered by treatment with solutions containing different porphyrins (inverse memory effect). Infrared spectroscopy shows that the porphyrins in the SAM on the TiO2 could be exchanged with the same or analogous porphyrin, leading to a change in the structure of the porphyrin SAM. The memory and inverse memory effects are well correlated with a change in porphyrin geometry, mainly the tilt angle of the porphyrin along the long molecular axis from the surface normal on the TiO2, as well as with kinetics of electron transfer between the porphyrin and TiO2. Such a new structure-function relationship for DSSCs will be very useful for the rational design and optimization of photoelectrochemical and photovoltaic properties of molecular assemblies on semiconductor surfaces.

Synthesis of push–pull porphyrin with two electron-donating and two electron-withdrawing groups and its application to dye-sensitized solar cell

We synthesized for the first time a push–pull porphyrin dye bearing two diarylamino groups and two carboxyphenylethynyl groups as electron-donating and electron-withdrawing anchoring groups, respectively. The absorption spectrum displayed broad and red-shifted absorption, achieving panchromic light-harvesting in visible and NIR regions. Introduction of multiple push–pull groups into meso-positions is a promising strategy for the rational design of porphyrin sensitizers for light-harvesting applications. The preliminary photovoltaic performance is moderate (3.0%), but the extensive photocurrent generation matches with the excellent light-harvesting ability. Further modulation of the photovoltaic properties of porphyrin DSSCs will be possible by suitable selection of electron-donating and electron-withdrawing groups as well as introduction of the substituents into the porphyrin core.

Ambient temperature synthesis of β,β′-fused nickel(ii) pyrrolo[1,2-a]pyrazinoporphyrins via a DBSA-catalyzed Pictet–Spengler approach

Synthesis of novel β,β′-fused nickel(ii) pyrrolo[1,2-a]pyrazinoporphyrins is accomplished for the first time at ambient temperature via a DBSA-catalyzed Pictet–Spengler approach.

Porphyrins as excellent dyes for dye-sensitized solar cells: recent developments and insights

Porphyrin sensitizers have exhibited power conversion efficiencies that are comparable to or even higher than those of well-established highly efficient DSSCs based on ruthenium complexes.

Dye-sensitized solar cells with improved performance using cone-calix[4]arene based dyes

Three cone-calix[4]arene-based sensitizers (Calix-1-Calix-3) with multiple donor-π-acceptor (D-π-A) moieties are designed, synthesized, and applied in dye-sensitized solar cells (DSSCs). Their photophysical and electrochemical properties are characterized by measuring UV/Vis absorption and emission spectra, cyclic voltammetry, and density functional theory (DFT) calculations. Calix-3 has excellent thermo- and photostability, as illustrated by thermogravimetric analysis (TGA) and dye-aging tests, respectively. Importantly, a DSSC using the Calix-3 dye displays a conversion efficiency of 5.48 % in under standard AM 1.5 Global solar illumination conditions, much better than corresponding DSSCs that use the rod-shaped dye M-3 with a single D-π-A chain (3.56 %). The dyes offer advantages in terms of higher molar extinction coefficients, longer electron lifetimes, better stability, and stronger binding ability to TiO2 film. This is the first example of calixarene-based sensitizers for efficient dye-sensitized solar cells.

New organic donor-acceptor-π-acceptor sensitizers for efficient dye-sensitized solar cells and photocatalytic hydrogen evolution under visible-light irradiation

Two organic donor-acceptor-π-acceptor (D-A-π-A) sensitizers (AQ and AP), containing quinoxaline/pyrido[3,4-b]pyrazine as the auxiliary acceptor, have been. Through fine-tuning of the auxiliary acceptor, a higher designed and synthesized photoelectric conversion efficiency of 6.02% for the AQ-based dye-sensitized solar cells under standard global AM1.5 solar conditions was achieved. Also, it was found that AQ-Pt/TiO2 photocatalysts displayed a better rate of H2 evolution under visible-light irradiation (420 nm<λ<780 nm) because of the stability of the oxidized states and the lower rates of electron recombination. Importantly, sensitizers AQ and AP-Pt/TiO2 showed strong photocatalytic activity during continuous light soaking for 10 h with methanol as the sacrificial electron donor. Additionally, the processes of their intermolecular electron transfer were further investigated theoretically by using time-dependent DFT. The calculated results indicate that the auxiliary acceptor plays the role of an electron trap and results in broad spectral responses.

Synthesis, optical and electrochemical properties of new ferrocenyl substituted triphenylamine based donor–acceptor dyes for dye sensitized solar cells

Two new ferrocenyl substituted triphenylamine based donor–acceptor dyes D1 and D2 were synthesized and used as sensitizers for dye sensitized solar cells (DSSCs).

The importance of various anchoring groups attached on porphyrins as potential dyes for DSSC applications

The influence of different anchoring groups in the efficiency of porphyrin dye sensitized solar cells.