Influence of the π‑Bridge-Fused Ring and Acceptor Unit Extension in D−π–A-Structured Organic Dyes for Highly Efficient Dye-Sensitized Solar Cells

Three new D−π–A-structured organic dyes, coded as SGT-138, SGT-150, and SGT-151, with the expansion of π-conjugation in the π-bridge and acceptor parts have been developed to adjust HOMO/LUMO levels and to expand the light absorption range of organic dyes. Referring to the SGT-137 dye, the π-bridge group was extended from the 4-hexyl-4H-thieno[3,2-b]indole (TI) to the 9-hexyl-9H-thieno[2′,3′:4,5]thieno[3,2-b]indole (TII), and the acceptor group was extended from (E)-3-(4-(benzo[c][1,2,5]thiadiazol-4-yl)phenyl)-2-cyanoacrylic acid (BTCA) to (E)-3-(4-(benzo[c][1,2,5]thiadiazol-4-ylethynyl)phenyl)-2-cyanoacrylic acid (BTECA), where TII was introduced as a π-bridging unit for the first time. It was determined that both extensions are promising strategies to enhance the light-harvesting ability. They present several features, such as (i) efficiently intensifying the extinction coefficient and expanding the absorption bands; (ii) exhibiting enhanced intramolecular charge transfer in comparison with the SGT-137; and (iii) being favorable to photoelectric current generation of dye-sensitized solar cells (DSSCs) with cobalt electrolytes. In particular, the π-spacer extension from TI to TII was useful for modulating the HOMO energy levels, while the acceptor extension from BTCA to BTECA was useful for modulating the LUMO energy levels. These phenomena could be explained with the aid of density functional theory calculations. Finally, the DSSCs based on new SGT-dyes with an HC-A1 co-adsorbent presented good power conversion efficiencies as high as 11.23, 11.30, 11.05, and 10.80% for SGT-137, SGT-138, SGT-150, and SGT-151, respectively. Furthermore, it was determined that the use of the bulky co-adsorbent, HC-A1, can effectively suppress the structural relaxation of dyes in the excited state, thereby enhancing the charge injection rate of SGT-dyes. The observations in time-resolved photoluminescence were indeed consistent with the variation in the PCE, quantitatively

A Eu^III Tetrakis(β-diketonate) Dimeric Complex: Photophysical Properties, Structural Elucidation by Sparkle/AM1 Calculations, and Doping into PMMA Films and Nanowires

Reaction of Ln^III with a tetrakis­(diketone) ligand H₄L [1,1′-(4,4′-(2,2-bis­((4-(4,4,4-trifluoro-3-oxobutanoyl) phenoxy)­methyl)­propane-1,3-diyl)­bis­(oxy)­bis­(4,1-phenylene))­bis­(4,4,4-trifluorobutane-1,3-dione)] gives new podates which, according to mass spectral data and Sparkle/AM1 calculations, can be described as dimers, (NBu₄[LnL])₂ (Ln = Eu, Tb, Gd:Eu), in both solid-state and dimethylformamide (DMF) solution. The photophysical properties of the Eu^III podate are compared with those of the mononuclear diketonate (NBu₄[Eu­(BTFA)₄], BTFA = benzoyltrifluoroacetonate), the crystal structure of which is also reported. The new Eu^III dimeric complex displays bright red luminescence upon irradiation at the ligand-centered band in the range of 250–400 nm, irrespective of the medium. The emission quantum yields and the luminescence lifetimes of (NBu₄[EuL])₂ (solid state: 51% ± 8% and 710 ± 2 μs; DMF: 31% ± 5% and 717 ± 1 μs) at room temperature are comparable to those obtained for NBu₄[Eu­(BTFA)₄] (solid state: 60 ± 9% and 730 ± 5 μs; DMF: 30 ± 5% and 636 ± 1 μs). Sparkle/AM1 calculations were utilized for predicting the ground-state geometries of the Eu^III dimer. Theoretical Judd–Ofelt and photoluminescence parameters, including quantum yields, predicted from this model are in good agreement with the experimental values, proving the efficiency of this theoretical approach implemented in the LUMPAC software (http://lumpac.pro.br). The kinetic scheme for modeling energy transfer processes show that the main donor state is the ligand triplet state and that energy transfer occurs on both the ⁵D₁ (44.2%) and ⁵D₀ (55.8%) levels. Furthermore, the newly obtained Eu^III complex was doped into a PMMA matrix to form highly luminescent films and one-dimensional nanowires having emission quantum yield as high as 67%–69% (doping concentration = 4% by weight); these materials display bright red luminescence even under sunlight, so that interesting photonic applications can be foreseen