Gold(III) Porphyrins Containing Two, Three, or Four β,β′-Fused Quinoxalines. Synthesis, Electrochemistry, and Effect of Structure and Acidity on Electroreduction Mechanism

Gold­(III) porphyrins containing two, three, or four β,β′-fused quinoxalines were synthesized and examined as to their electrochemical properties in tetrahydrofuran (THF), pyridine, CH₂Cl₂, and CH₂Cl₂ containing added acid in the form of trifluoroacetic acid (TFA). The investigated porphyrins are represented as Au­(PQ₂)­PF₆, Au­(PQ₃)­PF₆, and Au­(PQ₄)­PF₆, where P is the dianion of the 5,10,15,20-tetrakis­(3,5-di-<i>tert</i>-butylphenyl)­porphyrin and Q is a quinoxaline group fused to a β,β′-pyrrolic position of the porphyrin macrocycle. In the absence of added acid, all three gold­(III) porphyrins undergo a reversible one-electron oxidation and several reductions. The first reduction is characterized as a Au^III/Au^II process which is followed by additional porphyrin- and quinoxaline-centered redox reactions at more negative potentials. However, when 3–5 equivalents of acid are added to the CH₂Cl₂ solution, the initial Au^III/Au^II process is followed by a series of internal electron transfers and protonations, leading ultimately to triply reduced and doubly protonated Au^II(PQ₂H₂) in the case of Au^III(PQ₂)⁺, quadruply reduced and triply protonated Au^II(PQ₃H₃) in the case of Au^III(PQ₃)⁺, and Au^II(PQ₄H₄) after addition of five electrons and four protons in the case of Au^III(PQ₄)⁺. Under these solution conditions, the initial Au­(PQ₂)­PF₆ compound is shown to undergo a total of three Au^III/Au^II processes while Au­(PQ₃)­PF₆ and Au­(PQ₄)­PF₆ exhibit four and five metal-centered one-electron reductions, respectively, prior to the occurrence of additional reductions at the conjugated macrocycle and fused quinoxaline rings. Each redox reaction was monitored by cyclic voltammetry and thin-layer spectroelectrochemistry, and an overall mechanism for reduction in nonaqueous media with and without added acid is proposed. The effect of the number of Q groups on half-wave potentials for reduction and UV–visible spectra of the electroreduced species are analyzed using linear free energy relationships

Kinetic Analysis of Photochemical Upconversion by Triplet−Triplet Annihilation: Beyond Any Spin Statistical Limit

Upconversion (UC) via triplet−triplet annihilation (TTA) is a promising concept to improve the energy conversion efficiency of solar cells by harvesting photons below the energy threshold. Here, we present a kinetic study of the delayed fluorescence induced by TTA to explore the maximum efficiency of this process. In our model system we find that more than 60% of the triplet molecules that decay by TTA produce emitters in their first excited singlet state, so that the observed TTA effiency exceeds 40% at the point of the highest triplet emitter concentration. This result thoroughly disproves any spin-statistical limitation for the annihilation efficiency and thus has crucial consequences for the applicability of an upconvertor based on TTA, which are discussed

Dye-Sensitized Solar Cell with Integrated Triplet–Triplet Annihilation Upconversion System

Photon upconversion (UC) by triplet–triplet annihilation (TTA-UC) is employed in order to enhance the response of solar cells to sub-bandgap light. Here, we present the first report of an integrated photovoltaic device, combining a dye-sensitized solar cell (DSC) and TTA-UC system. The integrated device displays enhanced current under sub-bandgap illumination, resulting in a figure of merit (FoM) under low concentration (3 suns), which is competitive with the best values recorded to date for nonintegrated systems. Thus, we demonstrate both the compatibility of DSC and TTA-UC and a viable method for device integration