Trifluoromethylated Phenanthroline Ligands Reduce Excited-State Distortion in Homoleptic Copper(I) Complexes.

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 MLCT manifold observed by transient kinetic lifetimes and optimized excited-state structures.

Fluorination-Induced Evolution of Columnar Packing in Fluorous Triphenylenes and Benzotriphenylenes.

Use of D -symmetrical triphenylene (TRPH) as a substrate for high-temperature radical reactions with C F I under varying conditions resulted in the introduction of four types of fluorinated substituents: ω-C F H, c-C F , c-C F , and c-C HF . In contrast to the previous work on direct (poly)substitutions with R groups in polycyclic aromatic hydrocarbons (PAHs), in this work regiospecificity, selectivity, and high yield were achieved for TRPH(C F ) and TRPH(C F ) . New single-crystal structural data for seven compounds combined with literature crystallographic data allowed for the first detailed and precise analysis of the effects of fluorous substituent types, their number, and their position(s) on the TRPH core on the solid-state packing, and more specifically, the degree of π-π overlap between neighboring molecules, which is linked to charge transport properties.

Fluorination-Induced Evolution of Columnar Packing in Fluorous Triphenylenes and Benzotriphenylenes.

Invited for this month's cover are the groups of Steven Strauss/Olga Boltalina at Colorado State University, and Yu-Sheng Chen at the NSF's ChemMatCARS. The cover picture shows the evolution of fluorous triphenylene crystal packing motifs against the backdrop of the Very Large Array near Magdalena in New Mexico, where scientists study the evolution of the universe. The stepwise structural evolution in columnar packing observed was possible because of the development of a one-step selective synthesis for triphenylene(C F ) derivatives (n = 1-3).

Slow charge transfer from pentacene triplet states at the Marcus optimum.

Singlet fission promises to surpass the Shockley-Queisser limit for single-junction solar cell efficiency through the production of two electron-hole pairs per incident photon. However, this promise has not been fulfilled because singlet fission produces two low-energy triplet excitons that have been unexpectedly difficult to dissociate into free charges. To understand this phenomenon, we study charge separation from triplet excitons in polycrystalline pentacene using an electrochemical series of 12 different guest electron-acceptor molecules with varied reduction potentials.

PAH/PAH(CF ) Donor/Acceptor Charge-Transfer Complexes in Solution and in Solid-State Co-Crystals.

A solution, solid-state, and computational study is reported of polycyclic aromatic hydrocarbon PAH/PAH(CF ) donor/acceptor (D/A) charge-transfer complexes that involve six PAH(CF ) 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 π-stacks with 1/1, 1/2, or 2/1 D/A stoichiometries (PAH=anthracene, azulene, coronene, perylene, pyrene, triphenylene; n=5, 6).

Experimental and DFT Studies of the Electron-Withdrawing Ability of Perfluoroalkyl (R ) Groups: Electron Affinities of PAH(R ) Increase Significantly with Increasing R Chain Length.

Two series of aromatic compounds with perfluoroalkyl (R ) groups of increasing length, 1,3,5,7-naphthalene(R ) and 1,3,5,7,9-corannulene(R ) , have been prepared and their electronic properties studied by low-temperature photoelectron spectroscopy (PES) (for gas-phase electron affinity measurements). These and many related compounds were also studied by DFT calculations. The data demonstrate unambiguously that the electron-withdrawing ability of R substituents increases significantly and uniformly from CF to C F to n-C F to n-C F .

Variation of excited-state dynamics in trifluoromethyl functionalized C60 fullerenes.

We report on electronically excited-state dynamics of three different trifluoromethyl C60 fullerenes (TMFs, C60(CF3)n: C60/4-1, C60/6-2, and C60/10-1, featuring four, six, and ten trifluoromethyl groups, respectively) using steady-state and time-resolved optical spectroscopy as well as ultrafast pump/probe transient absorption spectroscopy. C60/4-1 and C60/6-2 dissolved in toluene solvent show near-unity S1 → T1 intersystem crossing quantum yield (ΦISC), ca. 1 ns S1-state lifetimes, and microsecond-timescale T1-state lifetimes, which are typical of the fullerene class.

Tuning the driving force for exciton dissociation in single-walled carbon nanotube heterojunctions.

Understanding the kinetics and energetics of interfacial electron transfer in molecular systems is crucial for the development of a broad array of technologies, including photovoltaics, solar fuel systems and energy storage. The Marcus formulation for electron transfer relates the thermodynamic driving force and reorganization energy for charge transfer between a given donor/acceptor pair to the kinetics and yield of electron transfer. Here we investigated the influence of the thermodynamic driving force for photoinduced electron transfer (PET) between single-walled carbon nanotubes (SWCNTs) and fullerene derivatives by employing time-resolved microwave conductivity as a sensitive probe of interfacial exciton dissociation.

Incremental Tuning Up of Fluorous Phenazine Acceptors.

In a simple, one-step direct trifluoromethylation of phenazine with CF3 I we prepared and characterized nine (poly)trifluoromethyl derivatives with up to six CF3 groups. The electrochemical reduction potentials and gas-phase electron affinities show a direct, strict linear relation to the number of CF3 groups, with phenazine(CF3)6 reaching a record-high electron affinity of 3.24 eV among perfluoroalkylated polyaromatics.

Copper Causes Regiospecific Formation of C4 F8 -Containing Six-Membered Rings and their Defluorination/Aromatization to C4 F4 -Containing Rings in Triphenylene/1,4-C4 F8 I2 Reactions.

The presence of Cu in reactions of triphenylene (TRPH) and 1,4-C4 F8 I2 at 360 °C led to regiospecific substitution of TRPH ortho C(β) atoms to form C4 F8 -containing rings, completely suppressing substitution on C(α) atoms. In addition, Cu caused selective reductive-defluorination/aromatization (RD/A) to form C4 F4 -containing aromatic rings. Without Cu, the reactions of TRPH and 1,4-C4 F8 I2 were not regiospecific and no RD/A was observed.

(19)F NMR-, ESR-, and vis-NIR-spectroelectrochemical study of the unconventional reduction behaviour of a perfluoroalkylated fullerene: dimerization of the C70(CF3)10(-) radical anion.

The most abundant isomer of C70(CF3)10 (70-10-1) is a rare example of a perfluoroalkylated fullerene exhibiting electrochemically irreversible reduction. We show that electrochemical reversibility at the first reduction step is achieved at scan rates higher than 500 V s(-1). Applying ESR-, vis-NIR-, and (19)F NMR-spectroelectrochemistry, as well as mass spectrometry and DFT calculations, we show that the (70-10-1)(-) radical monoanion is in equilibrium with a singly-bonded diamagnetic dimeric dianion.

Energy-dependent gas-phase fragmentation of fluorofullerene multiply charged anions (MCAs).

Long-lived di- and trianions have been formed from fluorofullerenes in the gas phase by electrospray ionization. Fragmentation of multiply charged anions has been induced by multiple low-energy collisions. Two complementary dissociation experiments have been conducted.

Corannulene Molecular Rotor with Flexible Perfluorobenzyl Blades: Synthesis, Structure and Properties.

Two members of a new class of organic-acceptor perfluorobenzyl corannulenes were prepared by gas-phase and highly-selective solution-phase reactions at elevated temperatures. The peculiar single-crystal X-ray structure of C5-C20H5(CF2C6F5)5 revealed two high-energy conformers with drastically different bowl depths and orientations of perfluorobenzyl blades; the conformers are alternating in columnar packing arrangements and every pair is sandwiched by toluene molecules.

A faux hawk fullerene with PCBM-like properties.

Reaction of C, CFCFI, and SnH(-Bu) produced, among other unidentified fullerene derivatives, the two new compounds 1,9-C(CFCF)H () and 1,9-C(-CF(2-CF)) (). The highest isolated yield of was 35% based on C. Depending on the reaction conditions, the relative amounts of and generated were as high as 85% and 71%, respectively, based on HPLC peak integration and summing over all fullerene species present other than unreacted C.

Fullerene cyanation does not always increase electron affinity: an experimental and theoretical study.

The electron affinities of C70 derivatives with trifluoromethyl, methyl and cyano groups were studied experimentally and theoretically using low-temperature photoelectron spectroscopy (LT PES) and density functional theory (DFT). The electronic effects of these functional groups were determined and found to be highly dependent on the addition patterns. Substitution of CF3 for CN for the same addition pattern increases the experimental electron affinity by 70 meV per substitution.

Poly(trifluoromethyl)azulenes: structures and acceptor properties.

Six new poly(trifluoromethyl)azulenes prepared in a single high-temperature reaction exhibit strong electron accepting properties in the gas phase and in solution and demonstrate the propensity to form regular π-stacked columns in donor-acceptor crystals when mixed with pyrene as a donor.

Single-step gas-phase polyperfluoroalkylation of naphthalene leads to thermodynamic products.

High-temperature gas-phase, solvent- and catalyst-free reaction of naphthalene with an excess of RF I reagent (RF CF3 , C2 F5 , n-C3 F7 , and n-C4 F9 ) was used for the first time to produce a series of highly perfluoroalkylated naphthalene products NAPH(RF )n with n=2-5. Four 95+ % pure 1,3,5,7-NAPH(RF )4 with RF CF3 , C2 F5 , n-C3 F7 , and n-C4 F9 were isolated using a simple chromatography-free procedure. These new compounds were fully characterized by (19) F and (1) H NMR spectroscopy, X-ray crystallography (for RF CF3 and C2 F5 ), atmospheric-pressure chemical ionization mass spectrometry, and cyclic and square-wave voltammetry.

An elusive fulvene 1,7,11,24-C60(CF3)4 and its unusual reactivity.

The X-ray crystal structure of a trifluoromethylfullerene (TMF), 1,7,11,24-C60(CF3)4, is reported for the first time. This elusive intermediate, while highly air stable as a solid, exhibits highly regioselective reactivity towards molecular oxygen in polar solvents, and only when exposed to light.

Fullerene "superhalogen" radicals: the substituent effect on electronic properties of 1,7,11,24,27-C60X5.

Hexasubstituted fullerenes with the skew pentagonal pyramid (SPP) addition pattern are predominantly formed in many types of reactions and represent important and versatile building blocks for supramolecular chemistry, biomedical and optoelectronic applications. Regioselective synthesis and characterization of the new SPP derivative, C60(CF3)4(CN)H, in this work led to the experimental identification of the new family of "superhalogen fullerene radicals", species with the gas-phase electron affinity higher than that of the most electronegative halogens, F and Cl. Low-temperature photoelectron spectroscopy and DFT studies of different C60X5 radicals reveal a profound effect of X groups on their electron affinities (EA), which vary from 2.

C20H4(C4F8)3: a fluorine-containing annulated corannulene that is a better electron acceptor than C60.

At sixes and sevens: The reaction of corannulene with 35 equivalents of 1,4-C4F8I2 is an efficient and a relatively selective process that yields two main products in which six H atoms are substituted with three C4F8 moieties that form six- and seven-membered rings. Low-temperature photoelectron spectroscopy showed the electron affinity of the major isomer (shown) exceeds that of C60 (2.74±0.

Regioselective sequential additions of nucleophiles and electrophiles to perfluoroalkylfullerenes: which cage C atoms are the most reactive and why?

The sequential addition of CN(-) or CH3(-) and electrophiles to three perfluoroalkylfullerenes (PFAFs), C(s)-C70(CF3)8, C1-C70(CF3)10, and C(s)-p-C60(CF3)2, was carried out to determine the most reactive individual fullerene C atoms (as opposed to the most reactive C=C bonds, which has previously been studied). Each PFAF reacted with CH3(-) or CN(-) to generate metastable PFAF(CN)(-) or PFAF(CH3)2(2-) species with high regioselectivity (i.e.

Solution-Phase Perfluoroalkylation of C Leads to Efficient and Selective Synthesis of Bis-Perfluoroalkylated Fullerenes.

A solution-phase perfluoroalkylation of C with a series of RI reagents was studied. The effects of molar ratio of the reagents, reaction time, and presence of copper metal promoter on fullerene conversion and product composition were evaluated. Ten aliphatic and aromatic RI reagents were investigated (CFI, CFI, -CFI, -CFI, -CFI, (CF)(CF)CFI, -CFI, CFCFI, CFI, and 1,3-(CF)CFI) and eight of them (except for CFI and 1,3-(CF)CFI) were found to add the respective R groups to C in solution.

A buckybowl with a lot of potential: C5-C20H5(CF3)5.

Lots of potential: a trifluoromethylated corannulene, C(5)-C(20)H(5)(CF(3))(5), has been prepared and characterized spectroscopically and by X-ray crystallography. The structure exhibits a highly ordered bowl stacking that is unusual for corannulenes with acyclic substituents. The first reduction of C(5)-C(20)H(5)(CF(3))(5) is anodically shifted by 0.