Natalia B. Shustova

Turn-On Fluorescence in Tetraphenylethylene-Based Metal–Organic Frameworks: An Alternative to Aggregation-Induced Emission

Coordinative immobilization of functionalized tetraphenylethylene within rigid porous metal-organic frameworks (MOFs) turns on fluorescence in the typically non-emissive tetraphenylethylene core. The matrix coordination-induced emission effect (MCIE) is complementary to aggregation-induced emission. Despite the large interchromophore distances imposed by coordination to metal ions, a carboxylate analogue of tetraphenylethylene anchored by Zn(2+) and Cd(2+) ions inside MOFs shows fluorescence lifetimes in line with those of close-packed molecular aggregates. Turn-on fluorescence by coordinative ligation in a porous matrix is a powerful approach that may lead to new materials made from chromophores with molecular rotors. The potential utility of MCIE toward building new sensing materials is demonstrated by tuning the fluorescence response of the porous MOFs as a function of adsorbed small analytes.

Selective Turn-On Ammonia Sensing Enabled by High-Temperature Fluorescence in Metal–Organic Frameworks with Open Metal Sites

We show that fluorescent molecules incorporated as ligands in rigid, porous metal-organic frameworks (MOFs) maintain their fluorescence response to a much higher temperature than in molecular crystals. The remarkable high-temperature ligand-based fluorescence, demonstrated here with tetraphenylethylene- and dihydroxyterephthalate-based linkers, is essential for enabling selective and rapid detection of analytes in the gas phase. Both Zn2(TCPE) (TCPE = tetrakis(4-carboxyphenyl)ethylene) and Mg(H2DHBDC) (H2DHBDC(2-) = 2,5-dihydroxybenzene-1,4-dicarboxylate) function as selective sensors for ammonia at 100 °C, although neither shows NH3 selectivity at room temperature. Variable-temperature diffuse-reflectance infrared spectroscopy, fluorescence spectroscopy, and X-ray crystallography are coupled with density-functional calculations to interrogate the temperature-dependent guest-framework interactions and the preferential analyte binding in each material. These results describe a heretofore unrecognized, yet potentially general property of many rigid, fluorescent MOFs and portend new applications for these materials in selective sensors, with selectivity profiles that can be tuned as a function of temperature.

Phenyl Ring Dynamics in a Tetraphenylethylene-Bridged Metal-Organic Framework: Implications for the Mechanism of Aggregation-Induced Emission

Molecules that exhibit emission in the solid state, especially those known as aggregation-induced emission (AIE) chromophores, have found applications in areas as varied as light-emitting diodes and biological sensors. Despite numerous studies, the mechanism of fluorescence quenching in AIE chromophores is still not completely understood. To this end, much interest has focused on understanding the low-frequency vibrational dynamics of prototypical systems, such as tetraphenylethylene (TPE), in the hope that such studies would provide more general principles toward the design of new sensors and electronic materials. We hereby show that a perdeuterated TPE-based metal-organic framework (MOF) serves as an excellent platform for studying the low-energy vibrational modes of AIE-type chromophores. In particular, we use solid-state (2)H and (13)C NMR experiments to investigate the phenyl ring dynamics of TPE cores that are coordinatively trapped inside a MOF and find a phenyl ring flipping energy barrier of 43(6) kJ/mol. DFT calculations are then used to deconvolute the electronic and steric contributions to this flipping barrier. Finally, we couple the NMR and DFT studies with variable-temperature X-ray diffraction experiments to propose that both the ethylenic C═C bond twist and the torsion of the phenyl rings are important for quenching emission in TPE, but that the former may gate the latter. To conclude, we use these findings to propose a set of design criteria for the development of tunable turn-on porous sensors constructed from AIE-type molecules, particularly as applied to the design of new multifunctional MOFs.

Conformational locking by design: relating strain energy with luminescence and stability in rigid metal-organic frameworks.

Minimization of the torsional barrier for phenyl ring flipping in a metal-organic framework (MOF) based on the new ethynyl-extended octacarboxylate ligand H(8)TDPEPE leads to a fluorescent material with a near-dark state. Immobilization of the ligand in the rigid structure also unexpectedly causes significant strain. We used DFT calculations to estimate the ligand strain energies in our and all other topologically related materials and correlated these with empirical structural descriptors to derive general rules for trapping molecules in high-energy conformations within MOFs. These studies portend possible applications of MOFs for studying fundamental concepts related to conformational locking and its effects on molecular reactivity and chromophore photophysics.

Energy Transfer on Demand: Photoswitch-Directed Behavior of Metal–Porphyrin Frameworks

In this paper, a photochromic diarylethene-based derivative that is coordinatively immobilized within an extended porphyrin framework is shown to maintain its photoswitchable behavior and to direct the photophysical properties of the host. In particular, emission of a framework composed of bis(5-pyridyl-2-methyl-3-thienyl)cyclopentene (BPMTC) and tetrakis(4-carboxyphenyl)porphyrin (H4TCPP) ligands anchored by Zn(2+) ions can be altered as a function of incident light. We attribute the observed cyclic fluorescence behavior of the synthesized porphyrin-BPMTC array to activation of energy transfer (ET) pathways through BPMTC photoisomerization. Time-resolved photoluminescence measurements show a decrease in average porphyrin emission lifetime upon BPMTC insertion, consistent with an ET-based mechanism. These studies portend the possible utilization of photochromic ligands to direct chromophore behavior in large light-harvesting ensembles.

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.

Redox-Tuning Endohedral Fullerene Spin States: From the Dication to the Trianion Radical of Sc3N@C80(CF3)2 in Five Reversible Single-Electron Steps

Synthetic/separation procedures for endohedral fullerenes, especially nitride cluster fullerenes M3N@C2n (NCFs), have progressed to the point in which sufficient amounts are now available for the exploration of their electronic and chemical properties. NCFs were recently found to be superior to C60 in stabilizing charge-separated states in donor–acceptor dyads. In contrast to the wealth of information about oxidized (cationic) and reduced (anionic) C60 derivatives, much less is known about NCF cations and anions, and even less is known about their derivatives. NCFs exhibit electrochemically irreversible reductions, at slow scan rates, that are chemically reversible. On the other hand, some M3N@C80(X)n derivatives have reversible electron-transfer steps even at low scan rates (hereinafter C80 shall specifically denote the C80-Ih(7) cage). Spectroscopic data for NCF ions are limited to 1) ex situ ESR spectra of Sc3N@C80 [3] and the monoanion of a pyrrolidino cycloadduct of Y3N@C80 [4] and 2) in situ ESR/Vis/NIR spectra of Sc3N@C68 + and Sc3N@C68 . The trifluoromethylation of NCFs was recently developed and several Sc3N@C80ACHTUNGTRENNUNG(CF3)n derivatives were isolated and characterized by mass spectrometry, UV/Vis and NMR spectroscopy, and single-crystal X-ray diffraction. We now report the existence of the monoand dications and the mono-, di-, and trianions of the simplest derivative, Sc3N@C80ACHTUNGTRENNUNG(CF3)2 (I), and an in-depth study of three of them by ESR and Vis/NIR spectroelectrochemistry. The cyclic voltammetry of I, shown in Figure 1, exhibits three reversACHTUNGTRENNUNGible one-electron reductions at 1.16, 1.65, and 2.04 V versus Fe(Cp)2 + /0 and two reversible one-electron oxidations at 0.47 and 0.60 V versus Fe(Cp)2 + . The dotted lines show that I has a smaller + / electrochemical gap than Sc3N@C80 (II); the 0/ and + /0 E1/2 values for I are shifted, respec[a] Dr. A. A. Popov, A. L. Svitova, Prof. Dr. L. Dunsch Department of Electrochemistry and Conducting Polymers Leibniz Institute for Solid State and Materials Research Dresden 01069 (Germany) Fax: (+49) 351-4659-745 E-mail : a.popov@ifw-dresden.de l.dunsch@ifw-dresden.de [b] N. B. Shustova, Prof. S. H. Strauss, Dr. O. V. Boltalina Department of Chemistry, Colorado State University Fort Collins, CO 80523 (USA) Fax: (+1) 970-491-1801 E-mail : olga.boltalina@colostate.edu steven.strauss@colostate.edu [c] M. A. Mackey, C. E. Coumbe, Prof. J. P. Phillips, Prof. S. Stevenson Department of Chemistry and Biochemistry University of Southern Mississippi, Hattiesburg, MS 39406 (USA) Fax: (+1) 601-266-6075 E-mail : janice.phillips@usm.edu steven.stevenson@usm.edu Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000205. Figure 1. Cyclic voltammetry of Sc3N@C80 ACHTUNGTRENNUNG(CF3)2 (1,2-C6H4Cl2 (0.1 m), TBABF4, 25 8C, 20 mV s ). The dotted lines show the E1/2 values of Sc3N@C80 at fast scan rates. [3b, 7] The inset shows Vis/NIR absorption spectra of a) Sc3N@C80ACHTUNGTRENNUNG(CF3)2 and b) Sc3N@C80 in toluene.

Nitrogen Directs Multiple Radical Additions to the 9,9′‐Bi‐1‐aza(C60‐Ih)[5,6]fullerene: X‐ray Structure of 6,9,12,15,18‐C59N(CF3)5

Azafullerenes, in which a cage carbon atom is replaced by a nitrogen atom, are the only type of heterofullerenes that can be made in practical amounts, and this makes it possible to probe the effects of cage-atom substitution on the physical and chemical properties. 4] Herein, we report a unique type of isomerism in azafullerenes bearing trifluoromethyl groups, which is attributed to the directing role of the nitrogen atom, and the first X-ray structure of the C59N derivative prepared directly from (C59N)2. Figure 1 shows the negative-ion atmospheric-pressure chemical ionization (NI APCI) mass spectra of the products of the high-temperature trifluoromethylation of (C59N)2 with CF3I obtained either in a sealed ampoule (top) or in a flow tube (bottom). The thermolysis of (C59N)2 yields a radical monomer C59NC which readily adds up to 15 (bottom spectrum) or even 19 CF3 groups per cage (top spectrum) to form closed-shell species C59N(CF3)n, where n is an odd number. In these reactions, a nonsoluble dark-brown solid dimer (C59N)2 is quantitatively converted into a volatile, thermally stable orange crystalline material, which is highly soluble in many organic and fluoroorganic solvents. When trifluoromethyl groups were added to C60 under similar conditions, the closed-shell species with even n values up to 22 were formed (see the Supporting Information). The product in the flow tube was separated by chromatography into four main fractions: I) C59N(CF3)11-15, II) C59N(CF3)9, III) C59N(CF3)7, and IV) C59N(CF3)5 (inset in Figure 1). This one-step HPLC process yielded a 98 % compositionally pure sample of C59N(CF3)5 as proven by NI APCI mass spectrometry, which detects also a single isomer with Cs symmetry (see the F NMR spectrum in Figure 2, top). The absence of terminal CF3 groups (seen as quartets in the F NMR spectra) implies that five CF3 groups should be arranged in a para 5 (or p) loop rather than as a ribbon of edge-sharing p-C6(CF3)2 hexagons as most commonly observed in the C60(CF3)n compounds. [5] The absorption spectrum (see the Supporting Information) is similar to that of Cs-C60(CF3)6. [6] The most probable addition pattern that agrees with the spectroscopic data and was previously observed for azafullerenes features an isolated pyrrole moiety on the fullerene core (Figure 1). Interestingly, C60X6 compounds with a similar addition pattern of skew-pentagonal pyramid (SPP) are formed abundantly, if not regioselectively, in various room-temperature reactions (see references in the Supporting Information). However, SPP-Cs-C60(CF3)6 was found only as a minor isomer in the high-temperature synthesis, whereas C1-C60(CF3)6 with a ribbon addition pattern (para -meta-para ; pmp) was at least ten times more abundant. 6] The Cs isomer is also 14.4 kJ mol 1 less stable than pmp-C1-C60(CF3)6 at the DFT Figure 1. Negative-ion APCI mass spectra of the C59N(CF3)n products obtained in a sealed glass ampoule at 530 8C for 24 h (top) and a hot flow tube at 500 8C for 3 h (bottom). The HPLC trace (inset) of the crude product (100% toluene eluent).

Substituent effects in a series of 1,7-C60(RF)2 compounds (RF = CF3, C2F5, n-C3F7, i-C3F7, n-C4F9, s-C4F9, n-C8F17): electron affinities, reduction potentials and E(LUMO) values are not always correlated

A series of seven structurally-similar compounds with different pairs of RF groups were prepared, characterized spectroscopically, and studied by electrochemical methods (cyclic and square-wave voltammetry), low-temperature anion photoelectron spectroscopy, and DFT calculations (five of the compounds are reported here for the first time). This is the first time that a set of seven RF groups have been compared with respect to their relative effects on E1/2(0/−), electron affinity (EA), and the DFT-calculated LUMO energy. The compounds, 1,7-C60(RF)2 (RF = CF3, C2F5, i-C3F7, n-C3F7, s-C4F9, n-C4F9 and n-C8F21), were found to have statistically different electron affinities (EA), at the ±10 meV level of uncertainty, but virtually identical first reduction potentials, at the ±10 mV level of uncertainty. The lack of a correlation between EA and E1/2(0/−), and between E(LUMO) and E1/2(0/−), for such similar compounds is unprecedented and suggests that explanations for differences in figures of merit for materials and/or devices that are based on equating easily measurable E1/2(0/−) values with EAs or E(LUMO) values should be viewed with caution. The solubilities of the seven compounds in toluene varied by nearly a factor of six, but in an unpredictable way, with the C2F5 and s-C4F9 compounds being the most soluble and the i-C3F7 compound being the least soluble. The effects of the different RF groups on EAs, E(LUMO) values, and solubilities should help fluorine chemists choose the right RF group to design new materials with improved morphological, electronic, optical, and/or magnetic properties.

X-ray structure and DFT study of C1-C60(CF3)12. A high-energy, kinetically-stable isomer prepared at 500 °C

The title compound, prepared from C60 and CF3I at 500 °C, exhibits an unusual fullerene(X)12 addition pattern that is 40 kJ mol−1 less stable than the previously reported C60(CF3)12 isomer.