Synthesis, Structure, and 19F NMR Spectra of 1,3,7,10,14,17,23,28,31,40-C60(CF3)10

Isomer Selective Thermosalience and Luminescence Switching in Organic Crystals

Organic crystals that respond to external stimuli are interesting for the design of smart materials. Here, we show that molecular engineering can transform simple naphthalidenimine-boron complexes─known for their exciting photophysical properties─into functional materials that exhibit thermosalience and thermal-luminescence switching. Detailed crystallographic and spectroscopic investigations revealed the role of subtle molecular parameters in deciphering charge-transfer interactions, which in turn imparted dynamic properties to the crystals. The simultaneous observation of thermally induced jumping and luminescence switching makes these crystals ideal for optoelectronic applications.

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.

Isolation and structure determination of missing fullerenes Gd@C74(CF3) n through in situ trifluoromethylation

Our trifluoromethyl functionalization method enables the dissolution and isolation of missing metallofullerenes of Gd@C74(CF3) n . After multi-stage high-performance liquid chromatography purification, Gd@C74(CF3)3 and two regioisomers of Gd@C74(CF3) are isolated. X-ray crystallographic analysis reveals that all of the isolated metallofullerenes react with CF3 groups on pentagons of the D 3 h-symmetry C74 cages. Highest occupied molecular orbital-lowest unoccupied molecular orbital gaps of these trifluoromethylated derivatives, estimated by absorption spectra, are in the range 0.71-1.06 eV, consistent with density functional calculations.

Crystalline functionalized endohedral C60 metallofullerides

Endohedral metallofullerenes have been extensively studied since the first experimental observation of La@C60 in a laser-vaporized supersonic beam in 1985. However, most of these studies have focused on metallofullerenes larger than C60 such as (metal)@C82, and there are no reported purified C60-based monomeric metallofullerenes, except for [Li@C60]+(SbCl6)- salt. Pure (metal)@C60 compounds have not been obtained because of their extremely high chemical reactivity. One route to their stabilization is through chemical functionalization. Here we report the isolation, structural determination and electromagnetic properties of functionalized crystalline C60-based metallofullerenes Gd@C60(CF3)5 and La@C60(CF3)5. Synchrotron X-ray single-crystal diffraction reveals that La and Gd atoms are indeed encapsulated in the Ih-C60 fullerene. The HOMO-LUMO gaps of Gd@C60 and La@C60 are significantly widened by an order of magnitude with addition of CF3 groups. Magnetic measurements show the presence of a weak antiferromagnetic coupling in Gd@C60(CF3)3 crystals at low temperatures.

Facile Separation, Spectroscopic Identification, and Electrochemical Properties of Higher Trifluoromethylated Derivatives of [70]Fullerene

We survey the structure and electronic properties of the family of higher trifluoromethylated C70 (CF3 )n molecules with n=14, 16, 18, and 20. Twenty-two available compounds, of which thirteen are newly obtained and characterized, demonstrate the broad diversity of π-system topologies, which enabled us to study the interplay between the CF3 addition pattern and the electronic properties. UV/Vis spectroscopic and cyclic voltammetric studies demonstrate the importance of the exact addition pattern rather than the plain number of addends. Of particular interest is the skew pentagonal pyramid (SPP) addition pattern, which enables formation of closed-shell cyclopentadienyl anions C70 (CF3 )n-1- through CF3 detachment upon electron transfer. A detailed study of the process is presented for a SPP-C70 (CF3 )16 where potentiostatic electrolysis at the second reduction potential gives C70 (CF3 )15- oxidizable to a persistent C70 (CF3 )15. radical. Together with the literature data for the lower C70 (CF3 )n compounds with n=2-12, the present results show good correlation between the experimental boundary level positions and the DFT predictions. The compounds turn out to be electron acceptor molecular semiconductors with experimental LUMO energies and HOMO-LUMO gaps within the ranges of -4.3 to -3.7 eV and 1.6 to 3.3 eV, respectively, depending on the shape of the conjugated fragments. The HOMO levels fall within the range of -5.6 to -6.9 eV and show linear correlation with the number of addends.

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.

Crystallographic Evidence for Direct Metal-Metal Bonding in a Stable Open-Shell La2 @Ih -C80 Derivative

Endohedral metallofullerenes (EMFs) have novel structures and properties that are closely associated with the internal metallic species. Benzyl radical additions have been previously shown to form closed-shell adducts by attaching an odd number of addends to open-shell EMFs (such as Sc3 C2 @Ih -C80 ) whereas an even number of groups are added to closed-shell EMFs (for example Sc3 N@Ih -C80 ). Herein we report that benzyl radical addition to the closed-shell La2 @Ih -C80 forms a stable, open-shell monoadduct instead of the anticipated closed-shell bisadduct. Single-crystal X-ray diffraction results show the formation of a stable radical species. In this species, the La-La distance is comparable to the theoretical value of a La-La covalent bond and is shorter than reported values for other La2 @Ih -C80 derivatives, providing unambiguous evidence for the formation of direct La-La bond.

(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

¹⁹F NMR spectroelectrochemistry is applied to study the reduction mechanism of perfluoroalkylated fullerene C₇₀(CF₃)₁₀.

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 ¹⁹F 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 ¹⁹F 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.

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 (⊙···⊙) distances of ≤10.34 Å is significantly greater, and the average ⊙···⊙ distance is shorter, for 2 (10 nearest neighbors; ave. ⊙···⊙ distance = 10.09 Å) than for solvent-free crystals of PCBM (7 nearest neighbors; ave. ⊙···⊙ distance = 10.17 Å). Finally, the thermal stability of 2 was found to be far greater than that of PCBM.

Transalkylation of higher trifluoromethylated fullerenes with C70: a pathway to new addition patterns of C70(CF3)8

We report three new isomers of C70(CF3)8, structurally related to p(7)mp-C70(CF3)10, that are inaccessible by direct trifluoromethylation, but can be easily identified among the products of the transalkylation of higher trifluoromethylfullerenes with C70. The reported compounds are characterized by UV/Vis, 1 D and 2 D COSY (19)F NMR spectroscopy, and DFT calculations. A rather unusual addition pattern is observed in p(6),i-C70(CF3)8 in which one addend is attached remotely from the others; polarization of the adjacent unsaturated bonds by the addend makes the molecule readily oxidizable.

Synthesis of oxazolidinofullerenes/thiazolidinofullerenes: novel reaction of [60]fullerene with isocyanates/isothiocyanates promoted by ferric perchlorate

The facile one-step reaction of [60]fullerene with isocyanates/isothiocyanates in the presence of ferric perchlorate generates a series of novel oxazolidinofullerenes/thiazolidinofullerenes.

Current status and future developments of endohedral metallofullerenes

Endohedral metallofullerenes (EMFs), a new class of hybrid molecules formed by encapsulation of metallic species inside fullerene cages, exhibit unique properties that differ distinctly from those of empty fullerenes because of the presence of metals and their hybridization effects via electron transfer. This critical review provides a balanced but not an exhaustive summary regarding almost all aspects of EMFs, including the history, the classification, current progress in the synthesis, extraction, isolation, and characterization of EMFs, as well as their physiochemical properties and applications in fields such as electronics, photovoltaics, biomedicine, and materials science. Emphasis is assigned to experimentally obtained results, especially the X-ray crystallographic characterizations of EMFs and their derivatives, rather than theoretical calculations, although the latter has indeed enhanced our knowledge of metal-cage interactions. Finally, perspectives related to future developments and challenges in the research of EMFs are proposed. (381 references).

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 R_FI 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 R_FI 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 R_F groups to C₆₀ in solution. Efficient and selective synthesis of C₆₀(R_F)₂ derivatives was developed.

Comprehensive multiphase NMR spectroscopy: basic experimental approaches to differentiate phases in heterogeneous samples

Heterogeneous samples, such as soils, sediments, plants, tissues, foods and organisms, often contain liquid-, gel- and solid-like phases and it is the synergism between these phases that determine their environmental and biological properties. Studying each phase separately can perturb the sample, removing important structural information such as chemical interactions at the gel-solid interface, kinetics across boundaries and conformation in the natural state. In order to overcome these limitations a Comprehensive Multiphase-Nuclear Magnetic Resonance (CMP-NMR) probe has been developed, and is introduced here, that permits all bonds in all phases to be studied and differentiated in whole unaltered natural samples. The CMP-NMR probe is built with high power circuitry, Magic Angle Spinning (MAS), is fitted with a lock channel, pulse field gradients, and is fully susceptibility matched. Consequently, this novel NMR probe has to cover all HR-MAS aspects without compromising power handling to permit the full range of solution-, gel- and solid-state experiments available today. Using this technology, both structures and interactions can be studied independently in each phase as well as transfer/interactions between phases within a heterogeneous sample. This paper outlines some basic experimental approaches using a model heterogeneous multiphase sample containing liquid-, gel- and solid-like components in water, yielding separate (1)H and (13)C spectra for the different phases. In addition, (19)F performance is also addressed. To illustrate the capability of (19)F NMR soil samples, containing two different contaminants, are used, demonstrating a preliminary, but real-world application of this technology. This novel NMR approach possesses a great potential for the in situ study of natural samples in their native state.

Efficient regioselective [4+2] cycloaddition of o-quinodimethane to C(70)(CF(3))(10)

The Diels-Alder reaction of C(1)-C(70)(CF(3))(10) and 3,6-dimethoxy-1,2-quinodimethane leads regioselectively to the formation of a new cycloadduct that has been fully characterized by spectroscopic and electrochemical methods as well as by X-ray diffraction.

Acyl radical reactions in fullerene chemistry: direct acylation of [60]fullerene through an efficient decatungstate-photomediated approach

A versatile and highly efficient photochemical methodology for the direct acylation of C(60) has been developed. This approach utilizes a wide variety of acyl radicals derived from aldehydes through a hydrogen atom abstraction process mediated by tetrabutylammonium decatungstate [(n-Bu(4)N)(4)W(10)O(32)]. The single addition reaction of these acyl radicals to [60]fullerene proceeded selectively to afford a novel class of previously unexplored fullerene-based materials. Product analysis of this reaction showed that decarbonylation and acylation pathways compete when a tertiary or phenylacetyl aldehyde is the starting material. However, a decrease of the reaction temperature was found to be effective in overcoming the decarbonylation encountered in certain acyl radical additions to C(60); the carbonyl radical addition precedes decarbonylation even in the cases where the decarbonylation rate constant exceeds 10(6) s(-1) (i.e., phenylacetaldehyde). The regiochemistry of the t-butyl radical addition was also found to be thermally controlled. The present methodology is directly applicable even in the cases of the cyclopropyl-substituted aldehydes, where rapid rearrangement of the cyclopropyl acyl radical intermediate can potentially occur. A mechanistic approach for this new reactivity of C(60) has been also provided, based mainly on intra- and intermolecular deuterium isotope effect studies.

ESR-Vis/NIR spectroelectrochemical study of C70(CF3)2-(*) and C70(C2F5)2-(*) radical anions

A spectroelectrochemical study of the two isostructural asymmetric perfluoroalkyl derivatives C(1)-7,24-C(70)(CF(3))(2) and C(1)-7,24-C(70)(C(2)F(5))(2) is presented. Reversible formation of their stable monoanion radicals is monitored by cyclic voltammetry and by in situ ESR-Vis-NIR spectroelectrochemistry. The ESR spectrum of the C(70)(CF(3))(2) (-*) radical is a 1:3:3:1 quartet with a (19)F hyperfine coupling constant (a(F)) of 0.323(4) G, demonstrating that the unpaired spin is coupled to only one of the two CF(3) groups. The (13)C satellites are assigned to specific carbon atoms. The ESR spectrum of the C(70)(C(2)F(5))(2) (-*) radical is an apparent octet with an apparent a(F) value of 0.83(2) G. DFT calculations suggest that this pattern is due to the superposition of spectra for four nearly isoenergetic C(70)(C(2)F(5))(2) (-*) conformers. Time-dependent DFT calculations suggest that the NIR band at 1090 nm exhibited by both C(70)(R(f))(2) (-*) radical anions is assigned to the SOMO-->LUMO+3 transition. The analogous NIR band exhibited by the closed-shell C(70)(CF(3))(2) (2-) dianion was blue-shifted to 1000 nm.

1,3,7,10,14,17,21,28,31,42,52,55-Dodeca-kis(trifluoro-meth-yl)- 1,3,7,10,14,17,21,28,31,42,52,55-dodeca-hydro-(C(60)-I)[5,6]fullerene

The title compound, C(72)F(36), is one of four isomers of C(60)(CF(3))(12) for which crystal structures have been obtained. The fullerene mol-ecule has an idealized I(h) C(60) core with the 12 CF(3) groups arranged in an asymmetric fashion on two ribbons of edge-sharing C(6)(CF(3))(2) hexa-gons, a para-meta-para-para-para-meta-para ribbon and a para-meta-para ribbon, giving an overall pmp(3)mp,pmp structure. There are no cage Csp(3)-Csp(3) bonds. The F atoms of two CF(3) groups are disordered over two positions; the site occupancy factors are 0.85/0.15 and 0.73/0.27. There are intra-molecular F⋯F contacts between pairs of CF(3) groups on the same hexa-gon that range from 2.521 (3) to 2.738 (4) Å.