10 Search Results

Singlet fission (SF) has the potential to bypass the Shockley–Queisser limit for solar cell efficiency through the production of two electron–hole pairs per photon. However, in polycrystalline pentacene this goal is hindered by slow charge transfer from triplets (=10⁷ s^–1) after SF. In this paper we demonstrate that slow charge transfer is an intrinsic property of triplet states in this material and most likely not connected with the triplet pair states that may result from singlet fission. We compare two perylene diimide/pentacene charge transfer systems that differ only by triplet generation mechanism: SF versus intersystem crossing (ISC), sensitized by usingmore » a soluble lead phthalocyanine derivative. We use time-resolved microwave conductivity (TRMC) to measure the charge yield in each system and transient absorption (TA) to follow the triplet population dynamics. These experiments are described by a single global kinetic model, with most of its parameters fixed via control experiments. While we observe modest differences in the charge-transfer rate constants between each sample, 4–10× as a function of triplet generation mechanism, all samples remain far below the predicted diffusion-limited rate constant of 10⁷–10⁸ s^–1.« less

In this report, the photostability of certain organic photovoltaic (OPV) active layers was demonstrated to improve by as much as a factor of five under white light illumination in air with the use of 1,7-bis-trifluoromethylfullerene (C₆₀(CF₃)₂) as the acceptor in place of PC₆₀BM. However, the results were highly dependent on the structure and functionality within the donor material. Twelve combinations of active layer blends were studied, comprised of six different high-performance donor polymers (two fluorinated and four non-fluorinated donors) and two fullerene acceptors (PC₆₀BM and C₆₀(CF₃)₂). The relative rates of irreversible photobleaching of the active layer blends were found tomore » correlate well with the electron affinity of the fullerene when the polymer and fullerene were well blended, but a full rationalization of the photobleaching data requires consideration of both the electron affinity of the fullerene as well as the relative miscibility of the polymer–fullerene components in the blend. Miscibility of those components was probed using a combination of time-resolved photoluminescence (TRPL) measurements and scanning tunneling microscopy (STM) imaging. The presence of fluorinated aromatic units in the donor materials tend to promote more intimate mixing with C₆₀(CF₃)₂ as compared to PC₆₀BM. The full results of these photobleaching studies and measurements of donor–acceptor miscibility, considered alongside additional photoconductance measurements and preliminary device work, provide new molecular optimization insights for improving the long-term stability of OPV active layers.« less

Herein, we report the synthesis and excited-state dynamics for a series of homoleptic copper(I) trifluoromethylated phenanthroline complexes with two, three, and four trifluoromethyl functional groups. Our analysis of the steady-state absorbance and emission, transient-absorption spectroscopy, and electronic-structure-theory calculations results enable in-depth analysis of the pseudo-Jahn–Teller distortion inhibition from increased steric hindrance of the trifluoromethyl functional group relative to the prototypical dimethyl phenanthroline complex. Surprisingly, our results demonstrate that the greatest degree of pseudo-Jahn–Teller distortion inhibition is achieved with trifluoromethylation of only the 2 and 9 positions by an unusual combination of steric hindrance and stabilization of a nondistorted ¹MLCT manifoldmore » observed by transient kinetic lifetimes and optimized excited-state structures. The intersystem-crossing (ISC) lifetime for the 2,9-bis(trifluoromethyl)-1,10-phenanthroline Cu(I) complex is 69 ps, while the triplet excited-state lifetime and emission quantum yield are 106 ns and 4 × 10^–3, respectively. Further trifluoromethylation of the phenanthroline yields a greater σ bond inductive withdrawing force on the phenanthroline nitrogens, ultimately resulting in weaker coordination to the copper. Last, the surprising success of the 2,9-bis(trifluoromethyl)-1,10-phenanthroline Cu(I) complex by adjusting both ligand sterics and electronic properties outlines a new strategy for developing long-lived Cu(I) charge-transfer complexes.« less

2 ABSTRACT We report a solution, solid-state, and computational study of PAH/PAH(CF3)n donor/acceptor (D/A) charge-transfer complexes that involve six PAH(CF3)n acceptors with known gas-phase electron affinities that range from 2.11(2) to 2.805(15) eV and four PAH donors, including seven CT co-crystal X-ray structures that exhibit hexagonal arrays of mixed p stacks with 1/1, 1/2, or 2/1 D/A stoichiometries (PAH = anthracene, azulene, perylene, pyrene, triphenylene; n = 4, 5). These are the first D/A CT complexes with PAH(CF3)n acceptors to be studied in detail. The nine D/A combinations were chosen to allow several structural and electronic comparisons to be made,more » providing new insights about controlling D/A interactions and the structures of CT co-crystals. They include, among others, CT complexes of the same PAH(CF3)n acceptor with four PAH donors and CT complexes of the same donor with four PAH(CF3)n acceptors. All nine CT complexes exhibit charge-transfer bands in solution with ?max between 466 and 600 nm. A plot of E(?max) vs. [IE(donor) - EA(acceptor)] for the nine CT complexes studied is linear with a slope of 0.72 ± 0.03 eV/eV. This plot is the first of its kind for CT complexes with structurally related donors and acceptors for which precise experimental gas-phase IEs and EAs are known. It demonstrates that conclusions based on the common assumption that the slope of a CT E(?max) vs. [IE - EA] plot is unity may be incorrect in at least some cases and should be reconsidered.« less

New insights into the process of high‐temperature trifluoromethylation of polycyclic aromatic hydrocarbons (PAHs) were gained in a detailed study of the reactions between corannulene (C ₂₀ H ₁₀ ) and CF ₃ I gas. Ten new poly(trifluoromethyl)corannulenes were made in these reactions and were fully characterized. A detailed computational study explains the observed trends in the product distribution and why some products cannot be prepared even with an excess of the reagent gas. The approach utilized here may have good predictive value for designing new effective and selective synthetic methodologies for various thermally stable electron‐deficient PAHs.

Two organic photovoltaic (OPV) donor materials (one polymer and one small molecule) are synthesized from the same constituent building blocks, namely thiophene units, cyclopentathiophene dione (CTD), and cyclopentadithiophene (CPDT). Photobleaching dynamics of these donor materials are then studied under white light illumination in air with blends of PC₇₀BM and the bistrifluoromethylfullerene 1,7-C₆₀(CF₃)₂. For both the polymer and small molecule blends, C₆₀(CF₃)₂ stabilizes the initial rate of photobleaching by a factor of 15 relative to PC70BM. However, once the small molecule:C₆₀(CF₃)₂ blend bleaches to ~80% of its initial optical density, the rate of photobleaching dramatically accelerates, which is not observed inmore » the analagous polymer blend. We probe that phenomenon using time-resovled photoluminescence (TRPL) to measure PL quenching efficiencies at defined intervals during the photobleaching experiments. The data indicates the small molecule donor and C₆₀(CF₃)₂ acceptor significantly de-mix with time, after which the blend begins to bleach at approximately the same rate as the neat donor sample. The work suggests that perfluoroalkylfullerenes have great potential to stabilize certain OPV active layers toward photodegradation, provided their morphology is stable.« less

Two series of aromatic compounds with perfluoroalkyl (RF) groups of increasing length, 1,3,5,7-naphthalene(RF)4 and 1,3,5,7,9-corannulene(RF)5, have been prepared and their electronic properties studied by low-temperature PES (i.e., gas-phase electron affinity (EA) measurements). These and many related compounds were also studied by DFT calculations. The data demonstrate unambiguously that the electron-withdrawing ability of RF substituents increases significantly and uniformly from CF3 to C2F5 to n-C3F7 to n-C4F9.