Porphyrin Sensitizers Bearing a Pyridine-Type Anchoring Group for Dye-Sensitized Solar Cells

New 2-pyridone-based donor-acceptor dyes: the effect of the donor group position, type of π-linker and acid-base characteristics of the medium on the photophysical properties

A series of novel donor-acceptor pyrid-2-ones was synthesized. The influence of the donor group on the photophysical properties of chromophores in solution was shown by varying the methoxy group position in the electron-rich aromatic ring. The effect of the π-linker was also demonstrated by the comparison of the D-π-A pyridone with its spacer-free analogue (D-A) and with the chromophore bearing an additional thiophene bridge (D-π-π-A). It was found that the presence of a π-linker plays a crucial role in the implementation of an intramolecular charge transfer (ICT) from the donor aryl moiety to the pyridone acceptor. Thus, for the synthesized D-A, D-π-A and D-π-π-A derivatives, the photoluminescence quantum yield in DMSO solution decreases from 56.8 to 4.3 and 3.5%, respectively, along with a strong bathochromic shift both for absorption (66 nm) and emission (162 nm) bands. Also, the possibility of shifting the equilibrium in DMSO solution towards either the pyridone form or the anionic form by the addition of a strong acid (TFA) and base (DBN) respectively was shown. Drastic changes in the photoluminescence of the solutions showed prospects for the application of the synthesized donor-acceptor pyridone derivatives as naked-eye acid-base indicators in organic media.

Unraveling Structure-Performance Relationships in Porphyrin-Sensitized TiO2 Photocatalysts

Over the years, porphyrins have arisen as exceptional photosensitizers given their ability to act as chlorophyll-mimicking dyes, thus, transferring energy from the light-collecting areas to the reaction centers, as it happens in natural photosynthesis. For this reason, porphyrin-sensitized TiO2-based nanocomposites have been widely exploited in the field of photovoltaics and photocatalysis in order to overcome the well-known limitations of these semiconductors. However, even though both areas of application share some common working principles, the development of solar cells has led the way in what is referred to the continuous improvement of these architectures, particularly regarding the molecular design of these photosynthetic pigments. Yet, those innovations have not been efficiently translated to the field of dye-sensitized photocatalysis. This review aims at filling this gap by performing an in-depth exploration of the most recent advances in the understanding of the role played by the different structural motifs of porphyrins as sensitizers in light-driven TiO2-mediated catalysis. With this goal in mind, the chemical transformations, as well as the reaction conditions under which these dyes must operate, are taken in consideration. The conclusions drawn from this comprehensive analysis offer valuable hints for the implementation of novel porphyrin-TiO2 composites, which may pave the way toward the fabrication of more efficient photocatalysts.

DFT Studies on a Metal Oxide@Graphene-Decorated D-π1-π2-A Novel Multi-Junction Light-Harvesting System for Efficient Dye-Sensitized Solar Cell Applications

Graphene nanocomposites have emerged as potential photoanode materials for increased performance of the dye-sensitized solar cells (DSSCs) via charge transfer. Various metal-oxide-decorated graphene nanocomposites have widespread applications in energy devices, such as solar cells, fuel cells, batteries, sensors, electrocatalysis, and photocatalysis. However, the possible role of these composites in DSSC applications has largely remained unexplored. Herein, we studied a Sb₂O₃-decorated graphene-D-π₁-π₂-A sensitized TiO₂ nanocomposite (dye-(TiO₂)₉/Sb₂O₃@GO) as a model multi-junction light-harvesting system and examined the impact of various π-bridges on the optical and photovoltaic properties of the push-pull dye system employed in this light-harvesting system. We have shown that by changing the spacer unit, the light sensitivity of nanocomposites can be varied from visible to near-infrared wavelengths. Furthermore, with the integration of metal-oxide-decorated graphene nanocomposites on D-π₁-π₂-A systems and D-π-A systems, composite photoelectrodes displayed better optical and photovoltaic characteristics with an enhanced absorption spectrum in the wavelength range of 800-1000 nm. The performance of the D-π₁-π₂-A system has been evaluated in terms of various photovoltaic parameters such as the highest occupied molecular orbital-lowest unoccupied molecular orbital energy gaps, excited-state oxidation potential (E _dye ^*), free energy of electron injection (G _inject), total reorganization energy (λ_total), and open-circuit voltage (V _oc). This work throws light on the current trends and the future opportunities in graphene-metal oxide nanocomposite-based DSSCs for better harvesting of the solar spectrum and better performance of solar devices.

Ligands modification strategies for mononuclear water splitting catalysts

Artificial photosynthesis (AP) has been proved to be a promising way of alleviating global climate change and energy crisis. Among various materials for AP, molecular complexes play an important role due to their favorable efficiency, stability, and activity. As a result of its importance, the topic has been extensively reviewed, however, most of them paid attention to the designs and preparations of complexes and their water splitting mechanisms. In fact, ligands design and preparation also play an important role in metal complexes’ properties and catalysis performance. In this review, we focus on the ligands that are suitable for designing mononuclear catalysts for water splitting, providing a coherent discussion at the strategic level because of the availability of various activity studies for the selected complexes. Two main designing strategies for ligands in molecular catalysts, substituents modification and backbone construction, are discussed in detail in terms of their potentials for water splitting catalysts.

Tailoring benzo[α]phenoxazine moiety for efficient photosensitizers in dye sensitized solar cells via the DFT/TD-DFT method

Three benzo[α]phenoxazine-based dyes were designed by tailoring donor (D) and anchoring (A) moiety to benzo[α]phenoxazinetemplate via DFT and TD-DFT method for dye-sensitized solar cell (DSSC) applications.

Pyridyl/hydroxyphenyl versus carboxyphenyl anchoring moieties in Zn - Thienyl porphyrins for dye sensitized solar cells

For developing efficient DSCs, anchoring moieties need to be understood in detail with respect to a particular dye. Herein, we have synthesized new Zn-thienyl porphyrin dyes based on pyridyl and hydroxyphenyl anchors and compared with traditional carboxyphenyl anchoring group. The effect of such variation in anchoring groups was investigated with UV-vis and fluorescence spectroscopy as well as electrochemical studies. The hydroxyphenyl appended dye showed higher molar extinction coefficient and red shifted emission bands. Both pyridyl and hydroxyphenyl dyes showed adsorption over TiO₂ semiconductor. The dye loading amount was found to be greater for carboxyphenyl dye due to stronger bidendate coordination of the carboxy group, as revealed by FT-IR studies. The electrochemical potentials were found to be strongly influenced by the anchoring functionality and all the dyes showed positive driving force for electron injection to the conduction band of TiO₂ and regeneration of oxidized dye by the redox electrolyte. DSC devices were fabricated for each of the dyes and characterized well. Hydroxyphenyl dye showed higher V_oc as a result of lower back reactions. Transient photovoltage decay measurments indicated lower recombination rate in hydroxyphenyl and pyridyl than the carboxyphenyl anchored thienyl porphyrins offering potential for further dye optimization.

Push-Pull Zinc Porphyrins as Light-Harvesters for Efficient Dye-Sensitized Solar Cells

Dye-sensitized solar cell (DSSC) has been attractive to scientific community due to its eco-friendliness, ease of fabrication, and vivid colorful property etc. Among various kinds of sensitizers, such as metal-free organic molecules, metal-complex, natural dyes etc., porphyrin is one of the most promising sensitizers for DSSC. The first application of porphyrin for sensitization of nanocrystaline TiO2 can be traced back to 1993 by using [tetrakis(4-carboxyphenyl) porphyrinato] zinc(II) with an overall conversion efficiency of 2.6%. After 10 years efforts, Officer and Grätzel improved this value to 7.1%. Later in 2009, by constructing porphyrin sensitizer with an arylamine as donor and a benzoic acid as acceptor, Diau and Yeh demonstrated that this donor-acceptor framwork porphyrins could attain remarkable photovoltaic performance. Now the highest efficiencies of DSSC are dominated by donor-acceptor porphyrins, reaching remarkable values around 13.0% with cobalt-based electrolytes. This achievement is largely contributed by the structural development of donor and acceptor groups within push-pull framwork. In this review, we summarized and discussed the developement of donor-acceptor porphyrin sensitizers and their applications in DSSC. A dicussion of the correlation between molecular structure and the spectral and photovoltaic properties is the major target of this review. Deeply dicussion of the substitution group, especially on porphyrin's meso-position were presented. Furthermore, the limitations of DSSC for commercialization, such as the long-term stability, sophisticated synthesis procedures for high efficiency dye etc., have also been discussed.

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.

Development of type-I/type-II hybrid dye sensitizer with both pyridyl group and catechol unit as anchoring group for type-I/type-II dye-sensitized solar cell

A type-I/type-II hybrid dye sensitizer with a pyridyl group and a catechol unit as the anchoring group has been developed and its photovoltaic performance in dye-sensitized solar cells (DSSCs) is investigated. The sensitizer has the ability to adsorb on a TiO₂ electrode through both the coordination bond at Lewis acid sites and the bidentate binuclear bridging linkage at Brønsted acid sites on the TiO₂ surface, which makes it possible to inject an electron into the conduction band of the TiO₂ electrode by the intramolecular charge-transfer (ICT) excitation (type-I pathway) and by the photoexcitation of the dye-to-TiO₂ charge transfer (DTCT) band (type-II pathway). It was found that the type-I/type-II hybrid dye sensitizer adsorbed on TiO₂ film exhibits a broad photoabsorption band originating from ICT and DTCT characteristics. Here we reveal the photophysical and electrochemical properties of the type-I/type-II hybrid dye sensitizer bearing a pyridyl group and a catechol unit, along with its adsorption modes onto TiO₂ film, and its photovoltaic performance in type-I/type-II DSSC, based on optical (photoabsorption and fluorescence spectroscopy) and electrochemical measurements (cyclic voltammetry), density functional theory (DFT) calculation, FT-IR spectroscopy of the dyes adsorbed on TiO₂ film, photocurrent-voltage (I-V) curves, incident photon-to-current conversion efficiency (IPCE) spectra, and electrochemical impedance spectroscopy (EIS) for DSSC.

Design and synthesis of organic sensitizers with enhanced anchoring stability in dye-sensitized solar cells

D-π-A dyes have received a special attention in the field of dye-sensitized solar cells (DSSCs). In this kind of molecules, the acceptor group (A) generally acts as an anchor, enabling the adsorption of the dye onto the metal oxide substrate (TiO2) and providing a good electron injection. The search for new anchors represents a critical factor for the development of improved DSSCs and in recent years has been a very active research field. This mini-review focuses especially on our work on pyridine-derived anchoring groups for D-π-A dyes, with a special regard on the preparation and characterization of three different families of dyes and a critical evaluation of their stability and efficiency.

Synthesis and spectral evaluation of 5,10,15,20-tetrakis(3,4-dibenzyloxyphenyl)porphyrin

Porphyrins are of interest in many applications that involve electron transfer and absorption of light, such as solar energy and photodynamic cancer therapy. The newly synthesized 5,10,15,20-tetrakis(3,4-dibenzyloxyphenyl)porphyrin, TDBOPP, was characterized using 1H NMR, 13C NMR, UV-vis and fluorescence spectroscopy and MALDI-TOF/TOF high resolution mass spectrometry. Standard 1H NMR and 13C NMR experiments coupled with nuclear Overhauser effect (NOE) experiments confirmed the structure of the compound. The expected M+ and [M+H]+ ions are observed in the MALDI-TOF/TOF mass spectrum. The UV-vis absorption spectrum of TDBOPP shows a Soret band at 424 nm and three Q bands at 519 nm, 556 nm, and 650 nm with molar absorptivity 3.6×105 cm−1m−1, 1.6×104 cm−1m−1, 1.0×104 cm−1m−1 and 5.3×103 cm−1m−1, respectively. Excitation at 424 nm gives emission at 650 nm. The quantum yield of the porphyrin is 0.11.

Synthesis, optical and electrochemical properties, and photovoltaic performance of a panchromatic and near-infrared (D)2–π–A type BODIPY dye with pyridyl group or cyanoacrylic acid

(D)₂–π–A type BODIPY dyes bearing a pyridyl group or cyanoacrylic acid group and two diphenylamine–thienylcarbazole moieties which possess near-infrared adsorption ability as well as panchromatic adsorption ability, have been developed.

Ruthenium(ii) complexes incorporating carbazole–diazafluorene based bipolar ligands for dye sensitized solar cell applications

A new type of ruthenium sensitizers, S3 and S4, consisting of carbazole–diazafluorene based bipolar ancillary ligands have been synthesized and characterized for dye sensitized solar cells.

Impact of the molecular structure and adsorption mode of D-π-A dye sensitizers with a pyridyl group in dye-sensitized solar cells on the adsorption equilibrium constant for dye-adsorption on TiO2 surface

D-π-A dyes NI-4 bearing a pyridyl group, YNI-1 bearing two pyridyl groups and YNI-2 bearing two thienylpyridyl groups as the anchoring group on the TiO₂ surface have been developed as dye sensitizers for dye-sensitized solar cells (DSSCs), where NI-4 and YNI-2 can adsorb onto the TiO₂ electrode through the formation of the coordinate bond between the pyridyl group of the dye and the Lewis acid site (exposed Tiⁿ⁺ cations) on the TiO₂ surface, but YNI-1 is predominantly adsorbed on the TiO₂ electrode through the formation of the hydrogen bond between the pyridyl group of the dye and the Brønsted acid sites (surface-bound hydroxyl groups, Ti-OH) on the TiO₂ surface. The difference in the dye-adsorption mode among the three dyes on the TiO₂ surface has been investigated from the adsorption equilibrium constant (K_ad) based on the Langmuir adsorption isotherms. It was found that the K_ad values of YNI-1 and YNI-2 are higher than that of NI-4, and more interestingly, the K_ad value of YNI-2 is higher than that of YNI-1. This work demonstrates that that for the D-π-A dye sensitizers with the pyridyl group as the anchoring group to the TiO₂ surface the number of pyridyl groups and the dye-adsorption mode on the TiO₂ electrode as well as the molecular structure of the dye sensitizer affect the K_ad value for the adsorption of the dye to the TiO₂ electrode, that is, resulting in a difference in the K_ad value among the D-π-A dye sensitizers NI-4, YNI-1 and YNI-2.

Electrochemical fabrication of one-dimensional porphyrinic wires on electrodes

[5,15-Di(4-aminophenyl)-10,20-diphenylporphyrinato]zinc(ii) was found to electropolymerize on electrodes such as glassy carbon (GC), indium tin oxide (ITO), and tin oxide, to form a redox-active, stable, and reproducible π-conjugated polymer.

Synthesis of porphyrin sensitizers with a thiazole group as an efficient π-spacer: potential application in dye-sensitized solar cells

CNU-OC8 exhibits better photovoltaic performance than the benchmark YD2-OC8 sensitizer in a liquid I⁻/I₃⁻ redox electrolyte.

A novel self-assembly with horizontal anchor porphyrin for supramolecular solar cells

Novel horizontal anchor zinc porphyrin assemblies via zinc-to-ligand axial coordination approach immobilized on TiO₂ electrode surface have been impressive increased photoelectric conversion efficiency in supramolecular solar cell.

Near-infrared unsymmetrical blue and green squaraine sensitizers

Two novel panchromatic asymmetrical squaraine sensitizers (SPSQ1 and SPSQ2) have been synthesized, characterized and effectively used for TiO₂-based dye sensitized solar cells.

Pyridyl vs. bipyridyl anchoring groups of porphyrin sensitizers for dye sensitized solar cells

Two D–π–A porphyrins (A = pyridyl, bipyridyl) are synthesized and used as sensitizers in DSSCs. The enhanced photovoltaic performance of the bipyridyl-sensitized device is attributed to its faster electron injection and reduced charge recombination.

Bilayer structured supramolecular light harvesting arrays based on zinc porphyrin coordination polymers for enhanced photocurrent generation in dye sensitized solar cells

Coordination polymers based on an acylhydrazone zinc porphyrin were synthesized and applied in dye sensitized solar cells for enhanced photocurrent generation.