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 .

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.

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.

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±0.02 and 2.689±0.008 eV, respectively).

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., one or two predominant isomers). They were treated with electrophiles E(+) to generate PFAF(CN)(E) or PFAF(CH3)2(E)2 derivatives, also with high regioselectivity (E(+)=CN(+), CH3(+), or H(+)). All of the predominant products, characterized by mass spectrometry and (19)F NMR spectroscopy, are new compounds. Some could be purified by HPLC to give single isomers. Two of them, C70(CF3)8(CN)2 and C70(CF3)10(CH3)2(CN)2, were characterized by single-crystal X-ray diffraction. DFT calculations were used to propose whether a particular reaction is under kinetic or thermodynamic control.

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 0.95 V, making it the strongest corannulene-based electron acceptor to date.

Solution-Phase Perfluoroalkylation of C60 Leads to Efficient and Selective Synthesis of Bis-Perfluoroalkylated Fullerenes.

A solution-phase perfluoroalkylation of C60 with a series of RFI 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 RFI reagents were investigated (CF3I, C2F5I, n-C3F7I, i-C3F7I, n-C4F9I, (CF3)(C2F5)CFI, n-C8F17I, C6F5CF2I, C6F5I, and 1,3-(CF3)2C6F3I) and eight of them (except for C6F5I and 1,3-(CF3)2C6F3I) were found to add the respective RF groups to C60 in solution. Efficient and selective synthesis of C60(RF)2 derivatives was developed.

In search of fullerene-based superacids: synthesis, X-ray structure, and DFT study of C60(C2F5)(5)H.

Simply super! The perfluoroalkylfullerene C(60)(C(2)F(5))(5) H is the first structurally characterized perfluoroalkylated fullerene-based acid and is also predicted to be the first gas-phase fullerene-based superacid.

Poly(perfluoroalkylation) of metallic nitride fullerenes reveals addition-pattern guidelines: synthesis and characterization of a family of Sc3N@C80(CF3)n (n = 2-16) and their radical anions.

A family of highly stable (poly)perfluoroalkylated metallic nitride cluster fullerenes was prepared in high-temperature reactions and characterized by spectroscopic (MS, (19)F NMR, UV-vis/NIR, ESR), structural and electrochemical methods. For two new compounds, Sc(3)N@C(80)(CF(3))(10) and Sc(3)N@C(80)(CF(3))(12,) single crystal X-ray structures are determined. Addition pattern guidelines for endohedral fullerene derivatives with bulky functional groups are formulated as a result of experimental ((19)F NMR spectroscopy and single crystal X-ray diffraction) studies and exhaustive quantum chemical calculations of the structures of Sc(3)N@C(80)(CF(3))(n) (n = 2-16). Electrochemical studies revealed that Sc(3)N@C(80)(CF(3))(n) derivatives are easier to reduce than Sc(3)N@C(80), the shift of E(1/2) potentials ranging from +0.11 V (n = 2) to +0.42 V (n = 10). Stable radical anions of Sc(3)N@C(80)(CF(3))(n) were generated in solution and characterized by ESR spectroscopy, revealing their (45)Sc hyperfine structure. Facile further functionalizations via cycloadditions or radical additions were achieved for trifluoromethylated Sc(3)N@C(80) making them attractive versatile platforms for the design of molecular and supramolecular materials of fundamental and practical importance.

Chemical tailoring of fullerene acceptors: synthesis, structures and electrochemical properties of perfluoroisopropylfullerenes.

High-temperature syntheses of the new C(60)(i-C(3)F(7))(2,4,6) and C(70)(i-C(3)F(7))(2,4) isomers and their characterization by spectroscopic methods, X-ray crystallography, cyclic voltammetry and density functional theory provide compelling evidence that they are superior electron acceptors than trifluoromethylfullerenes.

Dynamic HPLC study of C70 chlorination reveals a surprisingly selective synthesis of C70Cl8.

A dynamic HPLC study of C(70) chlorination led to the discovery, isolation, characterization, and development of the efficient preparatory procedures for two previously unknown soluble chlorofullerenes C(70)Cl(8) and C(70)Cl(6) and for insoluble [C(70)Cl(8)](n). A novel synthesis of 99% pure C(70)Cl(10) with a nearly quantitative yield was also developed. The first stability study of C(70)Cl(10,8,6) in solution showed that these compounds are very light-sensitive.

Soluble chlorofullerenes C(60)Cl(2,4,6,8,10). Synthesis, purification, compositional analysis, stability, and experimental/theoretical structure elucidation, including the X-ray structure of C(1)-C(60)Cl(10).

The efficacy of various analytical techniques for the characterization of products of C(60) chlorination reactions were evaluated by (i) using samples of C(60)Cl(6) of known purity and (ii) repeating a number of literature syntheses reported to yield pure C(60)Cl(n) compounds. The techniques were NMR, UV-vis, IR, and Raman spectroscopy, FAB, MALDI, LDI, ESI, and APCI mass spectrometry, HPLC, TGA, elemental analysis, and single-crystal X-ray diffraction. Most of these techniques are shown to give ambiguous or erroneous results, calling into question the composition and/or purity of nearly all C(60)Cl(n) compounds reported to date. The optimum analytical method for chlorofullerenes was found to be a combination of HPLC and either MALDI or APCI mass spectrometry. For the first time, the chlorination of C(60) by ICl, ICl(3), and Cl(2) was studied in detail using dynamic HPLC analysis and APCI mass spectrometry. Suitable conditions were found for the preparation of the new chlorofullerenes 1,7-C(60)Cl(2), 1,9-C(60)Cl(2), 1,6,9,18-C(60)Cl(4), and 1,2,7,10,14,24,25,28,29,31-C(60)Cl(10). The latter compound was also studied by (13)C NMR spectroscopy and X-ray diffraction, which led to the unambiguous determination of its asymmetric addition pattern. The unusual structure of C(60)Cl(10) was compared with other possible isomers using DFT-predicted relative energies. These results, along with additional experimental data and an analysis of the DFT-predicted frontier orbitals of likely intermediates, were used to rationalize the formation of the new compound C(60)Cl(10) from C(60)Cl(6) and excess ICl without the rearrangement of any C-Cl bonds. For the first time, the stability of C(60)Cl(n) compounds under a variety of conditions was studied in detail, leading to the discovery that they are, in general, very light-sensitive in solution. The X-ray structure of C(60)Cl(6) was also redetermined with higher precision.