Methane Activation by MH⁺ (M = Os, Ir, and Pt) and Comparisons to the Congeners of MH⁺ (M = Fe, Co, Ni and Ru, Rh, Pd)

The mechanism of ligated-transition-metal- [MH⁺ (M = Os, Ir, and Pt)] catalyzed methane activation has been computed at the B3LYP level of density functional theory. The B3LYP energies of important species on the potential energy surfaces were compared to CCSD­(T) single-point energy calculations. Newer kinetic and dispersion-corrected methods such as M05-2X provide significantly better descriptions of the bonding interactions. The reactions take place more easily along the low-spin potential energy surface. The minimum-energy pathway proceeds as MH⁺ + CH₄ → M­(H)₂(CH₃)⁺ → TS → MH­(CH₂)­(H₂)⁺ → MH­(CH₂)⁺ + H₂. The ground states are ⁵Π, ⁴Σ^–, and ¹Σ⁺ for OsH⁺, IrH⁺, and PtH⁺, respectively. The energy level differences of the reactants between the high- and low-spin states gradually become smaller from OsH⁺ to PtH⁺, being 30.66, 9.17, and 0.09 kcal/mol, respectively. The CH bond can be readily activated by MH⁺ (M = Os, Ir, and Pt) with a negligible barrier in the low-spin state; thus, OsH⁺, IrH⁺, and PtH⁺ are likely to be excellent mediators for the activition of the CH bond of methane. H₂ elimination is quite facile without barriers in the presence of excess reactants. The products of the reactions of MH⁺ (M = Os, Ir, and Pt) + methane are all carbene complexes MH­(CH₂)⁺. The exothermicities of the reactions are 3.99, 15.66, and 12.14 kcal/mol, respectively. The results for MH⁺ (M = Os, Ir, and Pt) are compared with those for the first- and second-row congeners, and the differences in behavior and mechanism are discussed

Cosensitization of D‑A-π‑A Quinoxaline Organic Dye: Efficiently Filling the Absorption Valley with High Photovoltaic Efficiency

In the efficient cosensitization, the pure organic sensitizers with high molecular extinction coefficients and long wavelength response are highly preferable since the dye loading amount for each dye in cosensitization is decreased with respect to single dye sensitization. A D-A-π-A featured quinoxaline organic sensitizer <b>IQ21</b> is specifically designed. The high conjugation building block of 4<i>H</i>-cyclopenta­[2,1-<i>b</i>:3,4-<i>b</i>′]­dithiophene (CPDT) is introduced as the π bridge, instead of the traditional thiophene unit, especially in realizing high molecular extinction coefficients (up to 66 600 M^–1 cm^–1) and extending the light response wavelength. With respect to the reference dye <b>IQ4</b>, the slightly lower efficiency of <b>IQ21</b> (9.03%) arises from the decrease of <i>V</i>_OC, which offsets the gain in <i>J</i>_SC. While cosensitized with a smaller D-π-A dye <b>S2</b>, the efficiency in <b>IQ21</b> is further improved to 10.41% (<i>J</i>_SC = 19.8 mA cm^–2, <i>V</i>_OC = 731 mV, FF = 0.72). The large improvement in efficiency is attributed to the well-matched molecular structures and loading amounts of both dyes in the cosensitization system. We also demonstrated that coabsorbent dye <b>S2</b> can distinctly compensate the inherent drawbacks of <b>IQ21</b>, not only enhancing the response intensity of IPCE, making up the absorption defects around low wavelength region of IPCE, but also repressing the charge recombination rate to some extent

Effect of a Long Alkyl Group on Cyclopentadithiophene as a Conjugated Bridge for D–A−π–A Organic Sensitizers: IPCE, Electron Diffusion Length, and Charge Recombination

The option of using conjugated π-linkers is critical for rational molecular design toward an energy-level strategy for organic sensitizers. To further optimize photovoltaic performance, methyl- and octyl-substituted 4<i>H</i>-cyclopenta­[2,1-<i>b</i>:3,4-<i>b</i>′]­dithiophene (CPDT) are introduced into D–A−π–A featured sensitizers. Along with CPDT, instead of thiophene as conjugated bridge, <b>WS-39</b> and <b>WS-43</b> exhibit an extended spectral response due to the excellent conjugation and coplanarity of CPDT. Specifically, we focused on the critical effect of length of the alkyl group linked to the bridging carbon atoms of CPDT on the photovoltaic performances. Octyl-substituted <b>WS-39</b> shows a broader IPCE onset with an enhanced photovoltage relative to the analogue <b>WS-5</b>. In contrast, <b>WS-43</b>, with methyl substituted on the CPDT moiety, presents a relatively low quantum conversion efficiency within the whole spectral response region, along with low photocurrent density. <b>WS-43</b> displays a distinctly low IPCE platform, predominately arising from the short electron diffusion length with significant electron loss during the electron transport. The relative movement of the conduction band edge (<i>E</i>_CB) and charge transfer resistance as well as lifetime of injected electrons are studied in detail. Under standard AM 1.5 conditions, <b>WS-39</b>-based solar cells show a promising photovoltaic efficiency of 9.07% (<i>J</i>_SC = 16.61 mA cm^–2, <i>V</i>_OC = 770 mV, FF = 0.71). The octyl chains attached on CPDT can provide <i>dual protection</i> and exhibit a high propensity to prevent binding of the iodide–triiodide redox couple, producing an efficient shielding effect to retard the charge recombination and resulting in improvement of <i>V</i>_OC. Our research paves the way to explore more efficient sensitizers through ingenious molecular engineering