Effect of Donor Groups on the Performance of Cyclometalated Ruthenium Sensitizers in Dye-Sensitized Solar Cells

Recent Investigations on Thiocyanate-Free Ruthenium(II) 2,2'-Bipyridyl Complexes for Dye-Sensitized Solar Cells

Three decades ago, dye-sensitized solar cells (DSSCs) emerged as a method for harnessing the energy of the sun and for converting it into electricity. Since then, a lot of work has been devoted to create better global photovoltaic efficiencies and long term stability. Among photosensitizers for DSSCs, thiocyanate-free ruthenium(II) complexes have gained increasing interest due to their better stability compared to conventional thiocyanate-based complexes, such as benchmark dyes N719 and Z907. In this mini-review, two classes of thiocyanate-free Ru(II) complexes are presented: (a) bis-bipyridyl compounds bearing an ancillary cyclometalating bidentate ligand; (b) bipyridyl compounds bearing non-cyclometalating ancillary ligands. The coverage, mainly from 2014 up to now, is not exhaustive, but illustrates the most recent design strategies and photovoltaic properties of these two families of ruthenium(II) dyes.

Dye-sensitized solar cells strike back

Dye-sensitized solar cells (DSCs) are celebrating their 30th birthday and they are attracting a wealth of research efforts aimed at unleashing their full potential. In recent years, DSCs and dye-sensitized photoelectrochemical cells (DSPECs) have experienced a renaissance as the best technology for several niche applications that take advantage of DSCs' unique combination of properties: at low cost, they are composed of non-toxic materials, are colorful, transparent, and very efficient in low light conditions. This review summarizes the advancements in the field over the last decade, encompassing all aspects of the DSC technology: theoretical studies, characterization techniques, materials, applications as solar cells and as drivers for the synthesis of solar fuels, and commercialization efforts from various companies.

The function of Chalcogenophene in the Cyclomatelated Ring of the Cycloruthenated Dyes applied in Dye-Sensitized Solar Cell

Three thiocyanate-free cycloruthenated complexes, DUY24-O, DUY24, and DUY24-Se containing furan, thiophene, and selenophene, respectively, as a part of the cycloruthenated ring, were designed to reveal the function of the chalcogen atom on the physicochemical and photovoltaic performance of the cycloruthenated sensitizers applied in dye-sensitized solar cells (DSCs). The three sensitizers have a similar molecular size; therefore, the effect of molecular dimensions on their photovoltaic performance can be negligible. NMR data, electron-donating resonance effects, optical properties, and the energy levels of the frontier orbitals reveal that the physical/photovoltaic properties of the three sensitizers were affected significantly by the chalcogen atom on the cyclometalated chalcogenophene ring. The λ_max (both in ethanol and adsorbed on TiO₂), frontier orbital level, and dye loading of thiophene- and selenophene-containing dyes are very close. Nevertheless, DUY24-Se has a higher molar absorption coefficient compared to DUY24; therefore, the DSC based on DUY24-Se has higher efficiency (8.4% under AM1.5 G one-sun and 26% under T5-light at ca. 6000 lux) than that sensitized with the DUY24 dye. These efficiencies are also higher than those (7.9 and 21.6%, respectively) of the cell dyed with N719, fabricated using the same conditions. The better performance of the device sensitized with DUY24-Se compared to DUY24-based cells suggests that selenophene is as good as (or even better than) thiophene to be a part of the cyclometalated ring for thiocyanate-free cycloruthenated sensitizers applied in DSCs. Furan-containing DUY24-O has much worse photovoltaic performance compared to the other two dyes. This is not only because DUY24-O has the shortest λ_max, the lowest molar absorption coefficient, and the highest HOMO level but also the lowest dye loading (because of the strong interaction between the oxygen in furan and TiO₂, the array of DUY24-O occupies more surface when adsorbed on TiO₂) and the fastest charge recombination. The physicochemical and photovoltaic properties as well as the adsorption behavior of the dye on the TiO₂ anode for the cycloruthenated sensitizers affected significantly by the chalcogen atom of the chalcogenophene on the cyclometalated ring provide a new strategy to design high-efficiency NCS-free cyclometalated sensitizers for DSCs.

cis-Locked Ru(II)-DMSO Precursors for the Microwave-Assisted Synthesis of Bis-Heteroleptic Polypyridyl Compounds

We describe a synthetic strategy for the preparation of bis-heteroleptic polypyridyl Ru(II) complexes of the type [Ru(L1)₂(L2)]²⁺ (L1 and L2 = diimine ligands) from well-defined Ru(II) precursors. For this purpose, a series of six neutral, anionic, and cationic cis-locked Ru(II)-DMSO complexes (2–7) of the general formula [Y] fac-[RuX(DMSO–S)₃(O–O)]ⁿ (where O–O is a symmetrical chelating anion: oxalate (ox), malonate (mal), acetylacetonate (acac); X = DMSO–O or Cl^–; n = −1/0/+1 depending on the nature and charge of X and O–O; when present, Y = K⁺ or PF₆^–) were efficiently prepared from the well-known cis-[RuCl₂(DMSO)₄] (1). When treated with diimine chelating ligands (L1 = bpy, phen, dpphen), the compounds 2–7 afforded the target [Ru(L1)₂(O–O)]^0/+ complex together with the undesired (and unexpected) [Ru(L1)₃]²⁺ species. Nevertheless, we found that the formation of [Ru(L1)₃]²⁺can be minimized by carefully adjusting the reaction conditions: in particular, high selectivity toward [Ru(L1)₂(O–O)]^0/+ and almost complete conversion of the precursor was obtained within minutes, also on a 100–200 mg scale, when the reactions were performed in absolute ethanol at 150 °C in a microwave reactor. Depending on the nature of L1 and concentration, with the oxalate and malonate precursors, the neutral product [Ru(L1)₂(O–O)] can precipitate spontaneously from the final mixture, in pure form and acceptable-to-good yields. When spontaneous precipitation of the disubstituted product does not occur, purification from [Ru(L1)₃]²⁺ can be rather easily accomplished by column chromatography or solvent extraction. By comparison, under the same conditions, compound 1 is much less selective, thus demonstrating that locking the geometry of the precursor through the introduction of O–O in the coordination sphere of Ru is a valid strategic approach. By virtue of its proton-sensitive nature, facile and quantitative replacement of O–O in [Ru(L1)₂(O–O)]^0/+ by L2, selectively affording [Ru(L1)₂(L2)]²⁺, was accomplished in refluxing ethanol in the presence of a slight excess of trifluoroacetic acid or HPF₆.

cis-Locked Ru(II)-DMSO complexes bearing a symmetrical chelating anion, such as [K] fac-[RuCl(DMSO−S)₃(η²-mal)] (2), are suitable precursors for the two-step selective preparation of bis-heteroleptic polypyridyl compounds of the type [Ru(L1)₂(L2)]²⁺ (L1 and L2 = diimine ligands).

Cyclometalated Ru(ii) complexes with tunable redox and optical properties for dye-sensitized solar cells

Cyclometalated Ru(ii) complexes with 2-arylbenzimidazoles demonstrate enhanced light-harvesting properties and good characteristics in dye-sensitized solar cells.

Unusually Slow Internal Conversion in N-Heterocyclic Carbene/Carbanion Cyclometallated Ru(II) Complexes: A Hammett Relationship

A series of six [Ru(bpy)₂(NHC-R)]⁺ complexes were synthesized and characterized, where bpy = 2,2'-bipyridine and NHC-R is an N-heterocyclic carbene covalently linked to a carbanion with a number of substituents, R = -OMe (1), -Me (2), -H (3), -Cl (4), -CO₂Et (5), and -NO₂ (6). The effects of these strongly σ-donating NHC-R ligands on the ground-state electronic structure and on the excited-state character and dynamics were probed using electrochemistry, TD-DFT calculations, and steady-state absorption and emission spectroscopies, along with ultrafast transient absorption and time-resolved IR measurements. The excitation of 1-5 with a 400 nm pulse (irf = 85 fs) results in the population of a high energy singlet state, S_n, that rapidly intersystem crosses into a high-lying triplet state, T_n. Over the course of 7-22 ps, T_n relaxes to the lowest lying triplet state, T₁, which is metal/ligand-to-ligand charge transfer, ³Ru(d)/NHC(π) → bpy(π*) in character. These ³ML-LCT states decay to regenerate the ground state with lifetimes, τ, that range from <8 to 15 ns at 298 K and from 10 to 23 ns at 77 K in CH₃CN. Both the excited-state lifetime at 77 K and the T_n → T₁ rate of internal conversion of 1-5 are dependent on the substituent R, and the latter correlates with the Hammett parameter (σ⁺_p) of the NHC-R ligand. Excitation of 1-5 with low energy light, 550-670 nm, does not result in the population of T_n, as only T₁ is observed. In the case of 6, excitation is expected to populate a ¹Ru(d)/NHC(π) → NHC(π*) state localized on the NHC-NO₂ ligand, which decays to a higher energy ³Ru(d)/NHC(π) → NHC(π*) state followed by internal conversion to the ³Ru(d)/NHC(π) → bpy(π*) T₁ state with τ = 250 ps; the population of both states is independent of excitation wavelength in 6. This work demonstrates that the introduction of one NHC-R ligand in these complexes permits the population of a higher energy triplet state that decays to T₁ in the picosecond time range. The relatively slow T_n → T₁ internal conversion in these complexes makes  the population of the higher-energy state potentially useful for more efficient charge injection into semiconductors for solar energy conversion or to aid in accessing dissociative metal-centered states for drug delivery. Overall, this work shows the ability to synthetically access valuable excited-state dynamics using the two different Ru-C bonds of the asymmetric NHC-R ligands.