Enabling technologies for ice giant exploration.

Future missions to an ice giant planet, especially orbital missions, are technologically challenging. But with one exception, radioisotope power sources (RPSs), the technologies that would enable such missions are currently available. RPSs are not a new technology, but devices used in the past that are appropriate to an ice giant mission are no longer available without engineering development work (currently unfunded), and it is uncertain whether the new NASA unit under development will be available for flight in time to take advantage of the best transfer trajectories of the next 15 years. This paper describes technologies already in hand that enable an ice giant mission, but for them to be useful they must be maintained. If an enabling technology is lost a replacement must be developed, potentially impacting the cost and schedule of a mission. In addition to the enabling technologies, there are a number of technologies that, while not enabling, could greatly enhance the science return and science value of a mission, making the programmatic aspects of approval an easier task and the funding of those development tasks a high priority. This article is part of a discussion meeting issue 'Future exploration of ice giant systems'.

Photophysical studies of metal to ligand charge transfer involving quadruply bonded complexes of molybdenum and tungsten.

Photoinduced metal-to-ligand charge transfer transitions afford numerous applications in terms of photon energy harvesting. The majority of metal complexes studied to date involve diamagnetic systems of d(6), d(8), and d(10) transition metals. These typically have very short-lived, ∼100 fs, singlet metal to ligand charge transfer ((1)MLCT) states that undergo intersystem crossing to triplet metal to ligand charge transfer ((3)MLCT) states that are longer lived and are responsible for much of the photophysical studies. In contrast, the metal-metal quadruply bonded complexes of molybdenum and tungsten supported by carboxylate, O2CR, and related amidinate ligands (RN)2C(R') have relatively long-lived (1)MLCT states arising from M2δ to Lπ* transitions. These have lifetimes in the range 1-20 ps prior to intersystem crossing to T1 states that may be (3)MLCT or (3)MMδδ* with lifetimes of 1-100 ns and 1-100 µs, respectively. The M2 quadruply bonded complexes take the form M2L4 or M2L4-nL'n where n = 1-3. Thus, in their photoexcited MLCT states, these compounds pose the question of how the charge resides on the ligands. This Account reviews the current knowledge of how charge is positioned with time in S1 and T1 states with the aid of active IR reported groups located on the ligands, for example, C≡X multiple bonds (X = C, N, or O). Several examples of localized and delocalized charge distributions are noted along with kinetic barriers to the interconversion of MLCT and δδ* states. On the 50th anniversary of the recognition of the MM quadruple bond, these complexes are revealing some remarkable features in the study of the photophysical properties of metal-ligand charge transfer states.

Synthesis, Structure, and Photophysical Properties of Mo2(NN)4 and Mo2(NN)2(T(i)PB)2, Where NN = N,N'-Diphenylphenylpropiolamidinate and T(i)PB = 2,4,6-Triisopropylbenzoate.

Two dimolybdenum compounds featuring amidinate ligands with a C≡C bond, Mo2(NN)4 (I), where NN = N,N'-diphenylphenylpropiolamidinate, and trans-Mo2(NN)2(T(i)PB)2 (II), where T(i)PB = 2,4,6-triisopropylbenzoate, have been prepared and structurally characterized by single-crystal X-ray crystallography. Together with Mo2(DAniF)4 (III), where DAniF = N,N'-bis(p-anisyl)formamidinate, all three compounds have been studied with steady-state UV-vis, IR, and time-resolved spectroscopy methods. I and II display intense metal to ligand charge transfer (MLCT). Singlet state (S1) lifetimes of I-III are determined to be 0.7, 19.1, and 2.0 ps, respectively. All three compounds have long-lived triplet state (T1) lifetimes around 100 µs. In femtosecond time-resolved infrared (fs-TRIR) experiments, one ν(C≡C) band is observed at the S1 state for I but two for II, which indicate different patterns of charge distribution. The electron would have to be localized on one NN ligand in I and partially delocalized over two NN ligands in II to account for the observations. The result is a standard showcase of excited-state mixed valence in coordination compounds.

Electronic and Spectroscopic Properties of Avobenzone Derivatives Attached to Mo2 Quadruple Bonds: Suppression of the Photochemical Enol-to-Keto Transformation.

From the reactions between Mo2(T(i)PB)4, where T(i)PB is 2,4,6-triisopropylbenzoate, and 2 equiv of the acids 4-formylbenzoic acid, HBzald; 4-(3-oxo-3-phenylpropanoyl)benzoic acid, HAvo; and 4-(2,2-difluoro-6-phenyl-2H-1λ(3),3,2λ(4)-dioxaborinin-4-yl)benzoic acid, HAvoBF2, the compounds Mo2(T(i)PB)2(Bzald)2, I; Mo2(T(i)PB)2(Avo)2, II; and Mo2(T(i)PB)2(AvoBF2)2, III, have been isolated. Compounds I and II are red, and compound III is blue. The new compounds have been characterized by (1)H NMR, MALDI-TOF MS, steady-state absorption and emission spectroscopies, and femtosecond and nanosecond time-resolved transient absorption and infrared spectroscopies. Electronic structure calculations employing density functional theory and time-dependent density functional theory have been carried out to aid in the interpretation of these data. These compounds have strong metal-to-ligand charge transfer, MLCT, and transitions in the visible region of their spectra, and these comprise the S1 states having lifetimes ∼5-15 ps. The triplet states are Mo2δδ* with lifetimes in the microseconds. The spectroscopic properties of I and II are similar, whereas the planarity of the ligand in III greatly lowers the energy of the MLCT and enhances the intensity of the time-resolved spectra. The Mo2 unit shifts the ground state equilibrium entirely to the enol form and quenches the degradation pathways of the avobenzone moiety.

Electronic and spectroscopic properties of avobenzone derivatives attached to ditungsten quadruple bonds.

From the reactions between W2(T(i)PB)4, where T(i)PB is 2,4,6-triisopropylbenzoate, and 2 equiv of acid, 4-formylbenzoic acid, HBzald, 4-(3-oxo-3-phenylpropanoyl)benzoic acid, HAvo, or 4-(2,2-difluoro-6-phenyl-2H-1λ(3),3,2λ(4)-dioxaborinin-4-yl)benzoic acid, HAvoBF2, three new compounds W2(T(i)PB)2(Bzald)2, I, W2(T(i)PB)2(Avo)2, II, and W2(T(i)PB)2(AvoBF2)2, III, have been prepared. As solid compounds I and II are blue while compound III is green. Characterization of these compounds has been carried out by means of (1)H NMR, MALDI-TOF MS, steady-state absorption and emission spectroscopies, and femtosecond and nanosecond transient absorption and time-resolved infrared spectroscopies. Compounds I and II have strong metal to ligand charge transfer, MLCT, transitions in the visible region of their spectra while compound III exhibits MLCT absorption in the near-infrared (λmax = 1017 nm). All three have S1 states that have corresponding lifetimes of ∼3-30 ps and are (1)MLCT in character. The triplet states are (3)MLCT with lifetimes in the range 3-10 ns. Density functional theory and time-dependent density functional theory were employed to perform electronic structure calculations in order to aid in the interpretation of these data. The spectroscopic properties of I and II are similar while the planarity of the ligand in III greatly lowers the energy of the MLCT state. The W2 unit enables direct observation of intersystem crossing from the (1)MLCT state to (3)MLCT state via the use of ultrafast spectroscopy.

Photophysical properties of cis-Mo2 quadruply bonded complexes and observation of photoinduced electron transfer to titanium dioxide.

The compounds cis-Mo2(DAniF)2(L)2 have been prepared, where DAniF = (N,N')-p-dianisyl formamidinate and L = thienyl-2-carboxylate (Th), 2,2'-bithienyl-5-carboxylate (BTh), and 2,2':5',5″-terthienyl-5-carboxylate (TTh). The compounds have been characterized by proton nuclear magnetic resonance ((1)H NMR), ultraviolet-visible (UV-vis) absorption and emission, differential pulse voltammetry, and time-resolved transient absorption and infrared (IR) spectroscopy. An X-ray crystal structure was obtained for the thienyl complex. The related salt [(n)Bu4N]2[Mo2(DAniF)2(TTh-CO2)2], where TTh-CO2 = 2,2':5',2″-terthienyl-5,5″-dicarboxylate, has also been prepared and employed in the attachment of the complex to TiO2 nanoparticles. The latter have been characterized by ground-state Fourier transform infrared spectroscopy (FTIR) and femtosecond time-resolved IR spectroscopy. The time-resolved data provide evidence for sub-picosecond charge injection from the Mo2 center to the semiconducting oxide particle.

Mo2 paddlewheel complexes functionalized with a single MLCT, S1 infrared-active carboxylate reporter ligand: preparation and studies of ground and photoexcited states.

From the reactions between Mo2(DAniF)3pivalate (DAniF = N,N'-di(p-anisyl)formamidinate) and the carboxylic acids LH, the title compounds Mo2(DAniF)3L have been prepared and characterized: compounds I (L = O2CC≡CPh), II (L = O2CC4H2SC≡CH), and III (L = O2CC6H4-p-CN). The new compounds have been characterized in their ground states by spectroscopy ((1)H NMR, ultraviolet-visible absorption, near-infrared absorption, and steady state emission), cyclic voltammetry, and density functional theory calculations. The compounds show strong metal Mo2 to ligand L δ-π* transitions in their visible spectra. The nature of the S1 (1)MLCT and T1 states has been probed by time-resolved (femtosecond and nanosecond) transient absorption and infrared spectroscopy. The observed shifts of the C≡C and C≡N vibrational modes are found to be consistent with the negative charge being localized on the single L in the S1 states, while the T1 states are (3)Mo2 δδ*. The present results are compared to earlier studies of the photoexcited states of trans-Mo2(2,4,6-triisopropylbenzoate)2L2 compounds that have been assigned as either localized or delocalized.

Metal-metal quadruple bonds supported by 5-ethynylthiophene-2-carboxylato ligands: preparation, molecular and electronic structures, photoexcited state dynamics, and application as molecular synthons.

From the reaction between M2(T(i)PB)4 and 2 equiv of 5-ethynylthiophene-2-carboxylic acid (H-ThCCH) in toluene, the complexes trans-M2(T(i)PB)2(ThCCH)2, where M = Mo (I) or W (II) and T(i)PB = 2,4,6-triisopropyl benzoate, have been isolated and characterized by (1)H NMR, IR, MALDI-TOF MS, UV-vis, steady-state emission, transient absorption, and time-resolved infrared (TRIR) spectroscopies and single-crystal X-ray crystallography for I. The molecular structure of I confirms the trans-substitution pattern and the extended conjugation of the ethynylthienyl ligands via interaction with the Mo2δ orbital. The HOMO of both I and II is the M2δ orbital, and the intense color of the compounds (I is red and II is blue) is due to the M2δ-to-ThCCH (1)MLCT transition. The S1 states for I and II are (1)MLCT. The T1 state is (3)MLCT for II, but (3)MoMoδδ* for I. The TRIR spectra of the ν(C≡C) stretch in the MLCT states are consistent with the delocalization of the electron over both ThCCH ligands. Compound I is shown to be a synthon for the preparation of trans-Mo2(T(i)PB)2(ThCCPh)2 (III) and trans-Mo2(T(i)PB)2(ThCCAuPPh3)2 (IV). Both III and IV have been characterized spectroscopically and by single-crystal X-ray diffraction. The structure of III indicates the extended π-conjugation of the trans-ethynyl-thienyl units extends to the added phenyl rings.

Coordination of N,N-chelated Re(CO)3Cl units across a Mo2 quadruple bond: synthesis, characterization, and photophysical properties of a Re-Mo2-Re triad and its component pieces.

2-(2-Pyridyl)-4-methylthiazole carboxylic acid (PMT-H) and rhenium tricarbonyl chloride react to form the red crystalline compound fac-Re(PMT-H)(CO)3Cl, I, which is an analog of the well-known Re(bpy)(CO)3Cl molecule, where bpy is 2,2'-bipyridine. The acids PMT-H (2 equiv) and Re(PMT-H)(CO)3Cl (2 equiv) also react with Mo2(T(i)PB)4 (T(i)PB = 2,4,6-triisopropylbenzoate) in toluene to give the red compound trans-Mo2(T(i)PB)2(PMT)2, II, and the royal blue compound trans-Mo2(T(i)PB)2[(PMT)Re(CO)3Cl]2, III, respectively. The X-ray and spectroscopic characterization of I confirms its close relationship with Re(bpy)(CO)3Cl, as does the spectroscopic characterization of compounds II and III as analogs of other compounds of the form trans-M2(TiPB)2L2, where L is a π-acceptor ligand. Electronic structure calculations on model compounds II' and III', where formate ligands substitute for T(i)PB, show that the highest occupied molecular orbital (HOMO) in II is Mo2δ. When the Re(CO)3Cl unit is attached to the PMT ligand to form III, this orbital is stabilized significantly and now becomes associated with a close in energy band of Re d(6), t2g type orbitals. Oxidation of III is shown to be Mo2-based, as evident by EPR spectroscopy, and the lowest-energy electronic absorption corresponds to a Mo2δ-to-PMT π* transition. The S1 states in both II and III are metal-to-ligand charge-transfer (MLCT), and the lowest-energy triplet sate, T1 is (3)MoMoδδ*, as evidenced by its steady state emission spectral features. The excited states of compounds I (T1) and III (S1 and T1) have been investigated by time-resolved infrared spectroscopy (TRIR). The spectral features of I parallel those for Re(bpy)(CO)3Cl, with the lowest-energy T1 state corresponding to Re dπ to PMT-H π* charge transfer, producing higher-energy CO stretching vibrations relative to the ground state. For III, the CO vibrations are shifted to lower energy, consistent with charge being located on the PMT ligand, which enhances Re-to-CO backbonding. In the MoMoδδ* T1 state, however, the backbonding is reduced to the PMT ligand, and the CO stretches are at slightly higher energy relative to the ground state.

Concerning the ground state and S1 and T1 photoexcited states of the homoleptic quadruply bonded complexes Mo2(O2CC6H4-p-X)4, where X = C≡C-H or C≡N.

The preparation of the homoleptic MM quadruply bonded complexes Mo2(O2CC6H4-p-X)4, where X = C≡C-H (I) or C≡N (II), is reported along with the solution characterization data and electronic structure calculations employing density functional theory. The compounds are colored orange (I) and red (II) due to the metal-to-ligand charge transfer involving the HOMO, Mo2δ, and LUMO, which is a ligand-based π* combination. Studies of the S1 state, (1)MLCT, by femtosecond time-resolved infrared spectroscopy indicate that the negative charge is distributed principally over two trans ligands. The T1 states are (3)MoMoδδ* as determined by NIR emission and nanosecond transient absorption.

Electronic structure and excited-state dynamics of the molecular triads: trans-M2(T(i)PB)2[O2CC6H5-η6-Cr(CO)3]2, where M = Mo or W, and T(i)PB = 2,4,6-triisopropylbenzoate.

From the reactions between M(2)(T(i)PB)(4) and HO(2)CC(6)H(5)-η(6)-Cr(CO)(3) (2 equiv), the title compounds trans-M(2)(T(i)PB)(2)[O(2)CC(6)H(5)-η(6)-Cr(CO)(3)](2), where M = Mo or W, and T(i)PB = 2,4,6-triisopropylbenzoate have been prepared and characterized. Compound I (M = Mo) was characterized by a single crystal X-ray structural determination which revealed a centrosymmetric MoMo quadruply bonded molecule. Compound I is red and the tungsten complex II is blue as a result of intense metal-to-ligand charge transfer (MLCT), which is principally M(2)δ to benzoate π* with some chromium t(2g) participation, according to calculations employing density functional theory. Compound I shows dual emission from S(1) and T(1) states that are assigned (1)MLCT and (3) MoMoδδ*, respectively. Both complexes have been studied by time-resolved infrared spectroscopy (TRIR) in the region of the carbonyl stretching frequency. Compound II displays a shift of ν(CO) to lower energy in both the (1)MLCT and (3)MLCT states in THF, while I in CH(2)Cl(2) shows ν(CO) bands shifted to both higher and lower energy. We attribute the shift to higher energy seen for I to a Cr t(2g) to benzoate π* transition which mixes with the Mo(2)δ to benzoate charge transfer upon excitation at 514 nm. In THF compound I undergoes a reversible photodissociation, potentially due to CO loss. Based on the TRIR of the carbonyl vibrations, it is proposed that the MLCT states are delocalized over both benzoate Cr(CO)(3) groups, as supported by calculations.

MM Quadruply Bonded Complexes Supported by Vinylbenzoate Ligands: Synthesis, Characterization, Photophysical Properties and Application as Synthons.

From the reactions between M2(T i PB)4 compounds and meta and para - vinylbenzoic acids (2 equiv) in toluene at room temperature the compounds trans-M2(T i PB)2L2, where L = m-vinylbenzoate 1A (M = Mo) and 1B (M = W) and T i PB = 2,4,6-triisopropylbenzoate, and where L = p-vinylbenzoate 2A (M = Mo) and 2B (M = W) have been isolated. Compounds 1A and 2A have been shown to undergo Heck carbon-carbon coupling reactions with phenyliodide to produce trans-Mo2(T i PB)2(O2CC6H4-m-CH=CH-C6H5)2,3A and trans-Mo2(T i PB)2(O2CC6H4-p-CH=CH-C6H5)2, 4A. The molybdenum compounds 1A and 2A have been structurally characterized by single crystal X-ray crystallography. All the new compounds have been characterized by 1H NMR, IR, UV-Visible absorption and emission spectroscopy, high resolution MALDITOF MS, fs- and ns- transient absorption spectroscopy and fs- time-resolved IR spectroscopy. Electronic structure calculations employing density functional theory, DFT, and time-dependent DFT have been employed to aid in the interpretation of spectral data. All compounds show intense absorptions in the visible region corresponding to M2δ to Lπ* charge transfer transitions. The lifetimes of the 1MLCT state fall in the range of 1 - 10 ps and for the molybdenum complexes the T1 states are 3δδ* with lifetimes ~50 µs while for the tungsten complexes the T1 are 3 MLCT with lifetimes in the range of 3 - 10 ns.

Cometary science. Subsurface properties and early activity of comet 67P/Churyumov-Gerasimenko.

Heat transport and ice sublimation in comets are interrelated processes reflecting properties acquired at the time of formation and during subsequent evolution. The Microwave Instrument on the Rosetta Orbiter (MIRO) acquired maps of the subsurface temperature of comet 67P/Churyumov-Gerasimenko, at 1.6 mm and 0.5 mm wavelengths, and spectra of water vapor. The total H2O production rate varied from 0.3 kg s(-1) in early June 2014 to 1.2 kg s(-1) in late August and showed periodic variations related to nucleus rotation and shape. Water outgassing was localized to the "neck" region of the comet. Subsurface temperatures showed seasonal and diurnal variations, which indicated that the submillimeter radiation originated at depths comparable to the diurnal thermal skin depth. A low thermal inertia (~10 to 50 J K(-1) m(-2) s(-0.5)), consistent with a thermally insulating powdered surface, is inferred.

4-Nitrophenyl- and 4'-nitro-1,1'-biphenyl-4-carboxylates attached to Mo2 quadruple bonds: ground versus excited state M2δ-ligand conjugation.

From the reactions between Mo2(T(i)PB)4, where T(i)PB = 2,4,6-triisopropylbenzoate and two equivalents of the carboxylic acid LH (LH = 4-nitrobenzoic acid and 4'-nitro[1,1'-biphenyl]-4-carboxylic acid) the compounds trans-M2(T(i)PB)2L2 have been prepared: I (L = 4-nitrobenzoate and M = Mo), II (L = 4'-nitro-1,1'-biphenylcarboxylate and M = Mo) and III (L = 4-nitrobenzoate and M2 = MoW). The compounds have been characterized by (1)H NMR, UV-Vis and steady state emission spectroscopy, ns and fs transient absorption spectroscopy and cyclic voltammetry. These data are compared with predictions based on electronic structure calculations on model compounds where T(i)PB is substituted for formate. Together these data indicate stronger ground-state coupling of the Mo2δ and ligand π* systems in I relative to II but this order is reversed in the photo excited S1(1)MLCT state. Attempts to prepare the W2 containing analogs were unsuccessful.

Modulating the M2δ-to-ligand charge transfer transition by the use of diarylboron substituents.

From the reactions between the quadruply bonded complexes M2(T(i)PB)4, where M = Mo or W and T(i)PB = 2,4,6-triisopropylbenzoate, and the carboxylic acids HOOC-C6H4-4-B(mesityl)2, LH (2 equivalents) the complexes trans-M2(T(i)PB)2L2 have been prepared. The new compounds have been characterized by (1)H NMR, MALDI-TOF MS, UV-Vis-NIR and steady-state emission spectroscopy, time-resolved transient absorption spectroscopy and cyclic voltammetry. These results are compared with the related properties of the benzoates, M2(T(i)PB)2(O2CPh)2 (prepared similarly) and with DFT calculations on model compounds where formate substitutes for T(i)PB. The new compounds M2(T(i)PB)2L2 are intensely colored in toluene or THF solutions: red (M = Mo) and green (M = W) and the introduction of the p-B(mesityl)2 group notably shifts these metal to ligand charge transfer transitions to lower energy in comparison to the benzoate complexes M2(T(i)PB)2(O2C-C6H5)2. Upon the addition of fluoride ions these intense absorptions are shifted to much higher energy in a reversible manner for M = Mo.