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 1MLCT manifold 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.

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. We observe separate optima in the charge yield as a function of driving force for singlet and triplet excitons, including inverted regimes for the dissociation of both states. Molecular acceptors can thus provide a strategic advantage to singlet fission solar cells by suppressing singlet dissociation at optimal driving forces for triplet dissociation. However, even at the optimal driving force, the rate constant for charge transfer from the triplet state is surprisingly small, ~107 s-1, presenting a previously unidentified obstacle to the design of efficient singlet fission solar cells.

PAH/PAH(CF3 )n 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(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 π-stacks with 1/1, 1/2, or 2/1 D/A stoichiometries (PAH=anthracene, azulene, coronene, perylene, pyrene, triphenylene; n=5, 6). 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, providing new insights about controlling D/A interactions and the structures of CT co-crystals. The comparisons 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 λmax between 467 and 600 nm. A plot of E(λmax ) versus [IE(donor)-EA(acceptor)] for the nine CT complexes studied is linear with a slope of 0.72±0.03 eV eV-1 . 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(λmax ) versus [IE-EA] plot is unity may be incorrect in at least some cases and should be reconsidered.

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(C4 F8 )n derivatives (n = 1-3). These compounds are expected to find application in liquid crystal displays and organic electronics. Read the full text of the article at 10.1002/cplu.201800451.

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

Use of D3h -symmetrical triphenylene (TRPH) as a substrate for high-temperature radical reactions with C4 F8 I2 under varying conditions resulted in the introduction of four types of fluorinated substituents: ω-C4 F8 H, c-C4 F8 , c-C4 F4 , and c-C4 HF3 . In contrast to the previous work on direct (poly)substitutions with RF groups in polycyclic aromatic hydrocarbons (PAHs), in this work regiospecificity, selectivity, and high yield were achieved for TRPH(C4 F8 ) and TRPH(C4 F8 )3 . 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. Comparison of isostructural partially fluorinated benzotriphenylenes, 2,3-TRPH(C4 F4 ) and 2,3-TRPH(C4 HF3 ), revealed an unexpectedly large (30 %) drop of π-π overlap, when only one fluorine atom was replaced with the hydrogen atom in a C4 F4 moiety. Theoretical and potentially practical implications of this work may include further testing and elaboration of computational methods describing solid-state interactions and predictions of transport properties of organic semiconductors, and further advances in the molecular design of high-performing TRPH-based organic materials and supramolecular architectures for organic optoelectronics.

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

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 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 RF substituents increases significantly and uniformly from CF3 to C2 F5 to n-C3 F7 to n-C4 F9 .

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. On the other hand, C60/10-1 exhibits a dominant sub-nanosecond nonradiative S1 → S0 relaxation mechanism and negligible ΦISC, therefore decreasing the average excited-state lifetime (τavg) by about 5 orders of magnitude compared to that of C60/4-1 and C60/6-2 (τavg ≈ 17 µs and 54 µs for C60/4-1 and C60/6-2, respectively, whereas τavg ≈ 100 ps for C60/10-1). These excited-state characteristics of C60/4-1 and C60/6-2 are preserved in polymer matrix, suggesting that fullerene/polymer interactions do not modulate intrinsic photophysics of trifluoromethyl-substituted fullerenes. The contrasting excited-state study results of C60/4-1 and C60/6-2 to that of C60/10-1 infer that intrinsic optical properties and excited-state dynamics can be affected by the substitution on the fullerene.

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. For the first time, we observed the Marcus inverted region (in which driving force exceeds reorganization energy) and quantified the reorganization energy for PET for a model SWCNT/acceptor system. The small reorganization energies (about 130 meV, most of which probably arises from the fullerene acceptors) are beneficial in minimizing energy loss in photoconversion schemes.

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(ß) 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. These results, supported by DFT calculations, are the first examples of Cu-promoted 1) regiospecific perfluoroannulation, 2) preparative C-F activation, and 3) RD/A. HPLC-purified products were characterized by X-ray diffraction, low-temperature PES, and (1) H/(19) F NMR.

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.

(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. This study is the first example of (19)F NMR spectroelectrochemistry, which promises to be an important method for the elucidation of redox mechanisms of fluoroorganic compounds. Additionally, we demonstrate the importance of combining different spectroelectrochemical methods and quantitative analysis of the transferred charge and spin numbers in the determination of the redox mechanism.

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. Firstly, unequivocal connectivity has been established between the precursor ion and the first and second generation of its product ions. Secondly, dianions have been studied in energy-resolved collisions, allowing the elucidation of the energetic demands of the dissociations. It was possible to study dianions with odd and even F numbers, possessing even and odd electron configurations, respectively. The fragmentation behavior is governed by the electronic stability of the anionic protagonists, in that ions with the less stable odd-electron configuration dissociate into species with the more stable even-electron configuration. Dianions with an odd electron count release an F˙ atom to turn into an even electron system while retaining the charge state. Dianions with an even electron count undergo a more energy demanding charge separation reaction (Coulomb explosion) into an F(-) anion and an even electron monoanion. The studied trianions behave accordingly. F2 loss is prominent only with monoanions within an even-to-even electron fragmentation cascade. The trianions are long-lived with lifetimes of at least 0.1 s.

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 C60, C6F5CF2I, and SnH(n-Bu)3 produced, among other unidentified fullerene derivatives, the two new compounds 1,9-C60(CF2C6F5)H (1) and 1,9-C60(cyclo-CF2(2-C6F4)) (2). The highest isolated yield of 1 was 35% based on C60. Depending on the reaction conditions, the relative amounts of 1 and 2 generated in situ were as high as 85% and 71%, respectively, based on HPLC peak integration and summing over all fullerene species present other than unreacted C60. Compound 1 is thermally stable in 1,2-dichlorobenzene (oDCB) at 160 °C but was rapidly converted to 2 upon addition of Sn2(n-Bu)6 at this temperature. In contrast, complete conversion of 1 to 2 occurred within minutes, or hours, at 25 °C in 90/10 (v/v) PhCN/C6D6 by addition of stoichiometric, or sub-stoichiometric, amounts of proton sponge (PS) or cobaltocene (CoCp2). DFT calculations indicate that when 1 is deprotonated, the anion C60(CF2C6F5)- can undergo facile intramolecular SNAr annulation to form 2 with concomitant loss of F-. To our knowledge this is the first observation of a fullerene-cage carbanion acting as an SNAr nucleophile towards an aromatic C-F bond. The gas-phase electron affinity (EA) of 2 was determined to be 2.805(10) eV by low-temperature PES, higher by 0.12(1) eV than the EA of C60 and higher by 0.18(1) eV than the EA of phenyl-C61-butyric acid methyl ester (PCBM). In contrast, the relative E1/2(0/-) values of 2 and C60, -0.01(1) and 0.00(1) V, respectively, are virtually the same (on this scale, and under the same conditions, the E1/2(0/-) of PCBM is -0.09 V). Time-resolved microwave conductivity charge-carrier yield × mobility values for organic photovoltaic active-layer-type blends of 2 and poly-3-hexylthiophene (P3HT) were comparable to those for equimolar blends of PCBM and P3HT. The structure of solvent-free crystals of 2 was determined by single-crystal X-ray diffraction. The number of nearest-neighbor fullerene-fullerene interactions with centroid···centroid (⊙···âŠ™) distances of ≤10.34 Å is significantly greater, and the average ⊙···âŠ™ distance is shorter, for 2 (10 nearest neighbors; ave. ⊙···âŠ™ distance = 10.09 Å) than for solvent-free crystals of PCBM (7 nearest neighbors; ave. ⊙···âŠ™ distance = 10.17 Å). Finally, the thermal stability of 2 was found to be far greater than that of PCBM.

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. The synthesis of a new fullerene derivative, C70(CF3)10(CN)2, is reported for the first time.

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. DFT calculations confirm that the proposed synthesis yields the most stable isomers that have not been accessed by alternative preparation techniques.

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 2.76 eV (X=CH3) to 4.47 eV (X=CN). The measured gas-phase EA of the newly synthesized C60(CF3)4CN equals 4.28(1) eV, which is about 1 eV higher than the EA of Cl atom. An observed remarkable stability of C60(CF3)4CN(-) in solution under ambient conditions opens new venues for design of air-stable molecular complexes and salts for supramolecular structures of electroactive functional materials.