Chi-Lun Mai

Molecular Engineering of Push–Pull Porphyrin Dyes for Highly Efficient Dye‐Sensitized Solar Cells: The Role of Benzene Spacers

Porphyrins have drawn much attention as sensitizers owing to the large absorption coefficients of their Soret and Q bands in the visible region. In a donor and acceptor zinc porphyrin we applied a new strategy of introducing 2,1,3-benzothiadiazole (BTD) as a π-conjugated linker between the anchoring group and the porphyrin chromophore to broaden the absorption spectra to fill the valley between the Soret and Q bands. With this novel approach, we observed 12.75% power-conversion efficiency under simulated one-sun illumination (AM1.5G, 100 mW cm(-2)). In this study, we showed the importance of introducing the phenyl group as a spacer between the BTD and the zinc porphyrin in achieving high power-conversion efficiencies. Time-resolved fluorescence, transient-photocurrent-decay, and transient-photovoltage-decay measurements were employed to determine the electron-injection dynamics and the lifetime of the photogenerated charge carriers.

Synthesis and characterization of porphyrin sensitizers with various electron-donating substituents for highly efficient dye-sensitized solar cells

A series of porphyrin dyes with an electron-donating group (EDG) attached at a meso-position (YD1–YD8) have been designed and synthesized for use as sensitizers in dye-sensitized solar cells (DSSC). The nature of the EDG exerts a significant influence on the spectral, electrochemical and photovoltaic properties of these sensitizers. Absorption spectra of porphyrins having an amino group show broadened Soret band and red-shifted Q bands with respect to those of reference porphyrin YD0. This phenomenon is more pronounced for porphyrins YD7 and YD8 that have a π-conjugated triphenylamine at the meso-position opposite the anchoring group. Upon introduction of an EDG at the meso-position, the potential for the first oxidation alters significantly to the negative whereas that for the first reduction changes inappreciably, indicating a decreased HOMO-LUMO gap. Results of density-functional theory (DFT) calculations support the spectroelectrochemical data for a delocalization of charge between the porphyrin ring and the amino group in the first oxidative state of diarylamino-substituted porphyrins YD1–YD4, which exhibit superior photovoltaic performance among all porphyrins under investigation. With long-chain alkyl groups on the diarylamino substituent, YD2 shows the best cell performance with JSC = 13.4 mA cm−2, VOC = 0.71 V, and FF = 0.69, giving an overall efficiency 6.6% of power conversion under simulated one-sun AM1.5 illumination.

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

Three novel efficient donor-acceptor porphyrins, MH1-MH3, with a pyridine-type acceptor and anchoring group were synthesized and their optical, electrochemical, and photovoltaic properties investigated. Replacing the commonly used 4-carboxyphenyl anchoring group with 2-carboxypyridine, 2-pyridone, and pyridine did not significantly change the absorption and electrochemical properties of the porphyrin dyes. These new porphyrin dyes MH show power conversion efficiencies of 8.3%, 8.5%, and 8.2%, which are comparable to that of the benchmark YD2-o-C8 (η=8.25%) under similar conditions. It was demonstrated that 2-carboxypyridine is an efficient and stable anchoring group as MH1 and showed better cell performance and long-term stability than YD2-o-C8 under light soaking conditions.

Acetylene-bridged dyes with high open circuit potential for dye-sensitized solar cells

A series of simple metal-free organic dyes MS1–MS3 have been designed and synthesized, and their optical, electrochemical, and photovoltaic properties were investigated. The molecular structures are based on a push–pull framework with a triphenylamine, naphthyldiphenylamine, or anthracenyl diphenylamine as the donor connected to a carboxyphenyl acceptor via a CC bond. The speciality of this new dye design is to obtain higher device open-circuit potentials. The dye-sensitized solar cells using this new sensitizer in combination with I−/I3− exhibited exceptionally high VOC values of 810, 844, and 866 mV for MS1, MS2, and MS3, respectively. The absorption bands of the anthracene-bridged dye MS3 showed remarkable peak broadening and red shifts due to the strong electronic coupling between the donor and bridge facilitated by the C–C triple bond, significantly promoting the light-harvesting capability. The resulting devices of MS3 showed large IPCE values of 75–80% in the region 370–590 nm, giving JSC/mA cm−2 = 8.16, VOC/mV = 866, FF = 0.76, and η = 5.44% under standard AM 1.5G one sun irradiation.

A Waterwheel-Shaped meso-meso-Linked Porphyrin Pentamer

Wheels on water? A waterwheel-shaped porphyrin pentamer has been synthesized by palladium-catalyzed cross-coupling reactions. The key intermediate is a boronate porphyrin, in which four boronic ester groups are directly attached to the meso-positions.

Porphyrin Dimers Bridged by an Electrochemically Switchable Unit

In the past decade much effort has been devoted to the synthesis and studies of molecular systems comprising porphyrin units bridged by well-defined p-conjugated spacers at the mesoor b-positions. These conjugated oligoporphyrin systems are expected to have potential applications in molecular wires and electronic devices due to their unique optical, physical, and chemical properties. Photophysical studies on electron transfer of a series of ferrocene–oligoporphyrin–fullerene triads show that a meso–meso butadiyne-bridged oligoporphyrin acts as an efficient molecular wire for long-range charge transfer over 65 &. More recently, the conductivity measurements on butadiyne-bridged porphyrin polymers indicate that the formation of doublestrand 4,4’-bipyridyl ladder complex by addition of bipyridine to the single-strand porphyrin polymer leads to an increase in conductivity by an order of magnitude. EPR studies performed by Therien and co-workers on meso– meso ethyne-linked porphyrin oligomers show that this type of conjugated molecules is able to mediate charge migration over a distance of about 75 &. The synthesis of meso– meso, b–b, b–b triply-linked porphyrin tapes reported by Osuka represents a milestone in the area of porphyrin molecular wires. The fully conjugated porphyrin tapes exhibit remarkable properties including very low optical HOMO– LUMO gap, low oxidation potential and rigid structure, and they may find use in a variety of molecular electronics. Despite extensive studies on the conjugated porphyrin arrays, oligoporphyrin molecular wires that show switchable conductance states by chemical or electrochemical means are rare. One example is the porphyrin arrays in which the individual macrocyles are laterally-linked by a quinonoid unit at the b,b’-positions of the porphyrin rings. In this system, the electronic communication between porphyrin units can be modulated by quinonoid/benzenoid conversion using chemical or electrochemical means. Another example of switchable porphyrin wires is the systems where proquinoidal units bridge two porphyrins. It is well known that the reversible conversion between quinone and hydroquinone can be achieved by chemical or electrochemical methods. Incorporation of a quinone unit into the central part of a porphyrin dimer via acetylene linkers would allow the interporphyrin interaction to be switched off and on in a controllable fashion and this type of molecules may have the potential for the application in switchable molecular wires. Therefore, we set out to design and synthesize a series of porphyrin dimers Ni22, Ni23, and Ni24 to explore their potential as redox-controllable switching molecules. The synthetic routes to porphyrin dimers Ni22, Ni23, and Ni24 were outlined in Scheme 1. [11] The coupling reaction of porphyrin Ni1 with the diiodide gave porphyrin dimer Ni22. [12] Demethylation of Ni22 to give Ni23 employing excess BBr3 was unsuccessful. However, the demethylation can be achieved by the reaction of excess BBr3 with the free base of the zinc analogue Zn22, obtained by a procedure similar to that for Ni22. [13] Subsequent nickel insertion afforded porphyrin dimer Ni23. The oxidation reaction of dimer Ni23 with PbO2 gave porphyrin Ni24. [14] For comparison purposes, porphyrin Ni5 was synthesized by employing a procedure similar to that for Ni22. The molecular structures of these porphyrins were confirmed by various spectroscopic methods such as NMR, UV/Vis, IR spectroscopy, and mass spectrometry. Figure 1 shows the crystal structure of compound Ni5. The bond lengths and angles fall in the normal range. The porphyrin ring adopts an essentially planar conformation [a] C.-L. Mai, Y.-L. Huang, Prof. C.-Y. Yeh Department of Chemistry National Chung Hsing University Taichung 402 (Taiwan) Fax: (+886) 4-2286-2547 E-mail : cyyeh@dragon.nchu.edu.tw [b] Dr. G.-H. Lee, Prof. S.-M. Peng Department of Chemistry National Taiwan University Taipei 106 (Taiwan) Supporting information for this article is available on the WWW under http://www.chemistry.org or from the author.

Synthesis of Octafluoroporphyrin

Despite the long list of known fluoroporphyrinoids, the most fundamental 2,3,7,8,12,13,17,18-octafluoroporphyrin (OFP) has not been synthesized until now. It is achieved by condensation of two molecules of tetrafluoro-dipyrrylmethane-2-carboxaldehyde in the presence of magnesium(II) salts. The fluorinated dipyrrylmethane also gives 5,15-bis(pentafluorophenyl)-OFP (F18P) with a reasonable yield. Both Mg/OFP and Zn/F18P in the solid-state reveal an essentially flat structure. The fluoro groups impart as much as a 0.5 V anodic shift for porphyrin ring oxidation/reduction, as well as hypsochromic shifts in the Uv-vis spectra.

Electroabsorption spectra of push–pull porphyrins in solution and in solid films

Polarized electroabsorption (E-A) spectra of highly efficient porphyrin sensitizers (YD2 and YD2-oC8) have been measured in benzene solution. Polarized E-A spectra of these push-pull porphyrins embedded in poly(methyl methacrylate) films or sensitized on TiO 2 films are also observed. Based on the analysis of the E-A spectra, the magnitude of the electric dipole moment both in the ground state and in the lowest excited state have been evaluated in solution and in solid films. The electric dipole moment in the excited state of these compounds is very large on TiO2 films, suggesting the interfacial charge transfer on TiO2 surface following photoexcitation of porphyrin dyes. The electric dipole moment in the excited state evaluated from the E-A spectra is very different from the one evaluated from the electrophotoluminescence spectra on TiO2, suggesting that the strong local field of TiO 2 films is applied to the fluorescing dyes attached to TiO 2 films.