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The known xenon-binding (±)-cryptophane-111 (1) has been functionalized with six [(η(5)-C(5)Me(5))Ru(II)](+) ([Cp*Ru](+)) moieties to give, in 89% yield, the first water-soluble cryptophane-111 derivative, namely [(Cp*Ru)(6)1]Cl(6) ([2]Cl(6)). [2]Cl(6) exhibits a very high affinity for xenon in water, with a binding constant of 2.9(2) × 10(4) M(-1) as measured by hyperpolarized (129)Xe NMR spectroscopy. The (129)Xe NMR chemical shift of the aqueous Xe@[2](6+) species (308 ppm) resonates over 275 ppm downfield of the parent Xe@1 species in (CDCl(2))(2) and greatly broadens the practical (129)Xe NMR chemical shift range made available by xenon-binding molecular hosts. Single crystal structures of [2][CF(3)SO(3)](6)·xsolvent and 0.75H(2)O@1·2CHCl(3) reveal the ability of the cryptophane-111 core to adapt its conformation to guests.

The pi contribution to the electron localization function (ELF) is used to compare 4npi- and (4n+2)pi-electron annulenes, with particular focus on the aromaticity of 4npi-electron annulenes in their lowest triplet state. The analysis is performed on the electron density obtained at the level of OLYP density functional theory, as well as at the CCSD and CASSCF ab initio levels. Two criteria for aromaticity of all-carbon annulenes are set up: the span in the bifurcation values DeltaBV(ELF(pi)) should be small, ideally zero, and the bifurcation value for ring closure of the pi basin RCBV(ELF(pi)) should be high (> or = 0.7). On the basis of these criteria, nearly all 4npi-electron annulenes are aromatic in their lowest triplet states, similar to (4n+2)pi-electron annulenes in their singlet ground states. For singlet biradical cyclobutadiene and cyclooctatetraene constrained to D4h and D8h symmetry, respectively, the RCBV(ELF(pi)) at the CASSCF level is lower (0.531 and 0.745) than for benzene (0.853), even though they have equal proportions of alpha- and beta-electrons.

The making and breaking of σ bonds is an integral part of almost all photochemical reactions. Yet, the electronic states of σ electrons are not nearly as well understood as the states of π-electron systems. Efforts in our laboratory to enhance the current state of their understanding are described, using the specific example of oligosilanes. We address the intrinsically cyclic nature of σ delocalization and its dependence on chain length and conformation, both in terms of theory and spectroscopic experiments, from the simplest disilane chromophore to the spectral properties of the individual conformers of permethylated heptasilane. We also describe a new low-energy luminescence from certain conformers of permethylated oligosilanes.

The interaction of xenon with cryptophane derivatives is analyzed by NMR by using either thermal or hyperpolarized noble gas. Twelve hosts differing by their stereochemistry, cavity size, and the nature and the number of the substituents on the aromatic rings have been included in the study, in the aim of extracting some clues for the optimization of (129)Xe-NMR based biosensors derived from these cage molecules. Four important properties have been examined: xenon-host binding constant, in-out exchange rate of the noble gas, chemical shift, and relaxation of caged xenon. This work aims at understanding the main characteristics of the host-guest interaction in order to choose the best candidate for the biosensing approach. Moreover, rationalizing xenon chemical shift as a function of structural parameters would also help for setting up multiplexing applications. Xenon exhibits the highest affinity for the smallest cryptophane, namely cryptophane-111, and a long relaxation time inside it, convenient for conservation of its hyperpolarization. However, very slow in-out xenon exchange could represent a limitation for its future applicability for the biosensing approach, because the replenishment of the cage in laser-polarized xenon, enabling a further gain in sensitivity, cannot be fully exploited.

The reversible trapping of small hydrocarbons and other gases by cryptophane-111 (1) in organic solution was characterized with variable-temperature (1)H NMR spectroscopy. Characteristic spectral changes observed upon guest binding allowed kinetic and thermodynamic data to be readily extracted, permitting quantification and comparison of different host-guest interactions. Previous work (J. Am. Chem. Soc. 2007, 129, 10332) demonstrated that 1, the smallest cryptophane to date, forms a complex with xenon with remarkably high affinity. Presently, it is shown that 1 also exhibits slow exchange dynamics with methane at reduced temperatures (delta(bound) = -5.2 ppm) with an association constant K(a) = 148 M(-1) at 298 K. In contrast, ethane and ethylene are poorly recognized by 1 with K(a) values of only 2 M(-1) and 22 M(-1), respectively; moreover, chloromethane (whose molecular volume is similar to that of xenon, approximately 42 A(3)) is not observed to bind to 1. Separately, molecular hydrogen (H(2)) gas is observed to bind 1, but in contrast to other ligands presently studied, H(2) complexation is spectrally manifested by fast exchange throughout virtually the entire range of available conditions, as well as by a complex dependence of the guest (1)H resonance frequency upon temperature and host concentration. Taken together, these results establish 1 as a selective host for small gases, with implications for the design of size- and geometry-selective sensors targeted for various gas molecules.

Photoelectron spectra of conformationally constrained methylated tetrasilanes 1−5 were measured, extending to the full 0−180 ° range the previously available 0−80° region of SiSiSiSi dihedral angles ω provided by the monocyclic analogues 6−9. The resulting complete picture of the conformational dependence of photoionization of the n-tetrasilane moiety is unfortunately somewhat distorted by differential polarizability effects of the alkyl chains used for conformational constraint. The lowest three ionization potentials are attributed to σSiSi bonding orbitals. The first and the third of these decrease a little as ω goes from 0 to 180°, whereas the second one increases distinctly. Within the framework of the Koopmans' approximation, these trends in backbone MO energies agree with those computed in the HF/TZ//MP2/TZ approximation and those obtained from the simple Hückel ladder C model.

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