The molecular host cucurbit [7]uril forms an extremely stable inclusion complex with the dicationic ferrocene derivative bis(trimethylammoniomethyl)ferrocene in aqueous solution. The equilibrium association constant for this host-guest pair is 3 ؋ 10 15 M ؊1 (Kd ‫؍‬ 3 ؋ 10 ؊16 M), equivalent to that exhibited by the avidinbiotin pair. Although purely synthetic systems with larger association constants have been reported, the present one is unique because it does not rely on polyvalency. Instead, it achieves its extreme affinity by overcoming the compensatory enthalpyentropy relationship usually observed in supramolecular complexes. Its disproportionately low entropic cost is traced to extensive host desolvation and to the rigidity of both the host and the guest.cucurbituril ͉ entropy control ͉ ferrocene derivatives ͉ host-guest complexation ͉ thermodynamics

by the recent rapid growth of interest in circularly polarized luminescence (CPL) of organic molecules, we have collected all the reliable CPL, as well as the corresponding circular dichroism (CD), data measured in fluid solutions. To analyze the correlation between CPL and CD, we employed the absorption and luminescence dissymmetry factors (g abs and g lum ) of the p-p* transition reported for chiral organic molecules of various categories, including planar chiral cyclophanes and helicenes, axially chiral biaryls and spiro compounds, and pointand axially chiral BODIPY derivatives. In rigid p-systems, the absorption and fluorescence spectra are often mirror images of each other with a small Stokes shift, reflecting the minimal conformational relaxation in the emissive excited state, which should also affect the chiroptical properties in the excited state and be better sensed by CPL. However, no comprehensive efforts have hitherto been made to correlate the two relevant chiroptical properties, i. e. CPL versus CD, and also to quantitatively elucidate the effects of conformational relaxation in the excited state on the CPL behavior. The global linear regression analysis of all the reported g abs and g lum values, though fairly scattered (see TOC), led us to a quantitative relationship: j g lum j = 0.81 j g abs j (r 2 = 0.60), which demonstrates that the CPL dissymmetry factor is proportional to, and smaller than, the CD dissymmetry factor. A closer look revealed that the slope of the plot, or the proportional coefficient, is a critical function of the class of compounds, varying from 0.99 for cyclophanes to 0.93 for biaryls, to 0.77 for BODIPYs, and then to 0.61 for helicenes/ helicenoids. The scattered g lum -g abs plot and the general trend g lum g abs appear to be inherent to the CPL of organic molecules in their isolated states, originating from the conformational flexibility, vibrational contribution, and Stokes shift that differ in each category.The absorption and emission intensities are proportional to the relevant dipole strength D, which is defined by Equation (7):In isotropic solutions, the electric quadrupole moment is cancelled out in R and generally small and negligible for D. The dissymmetric factors g abs and g lum are thus given by [Eq. (8)]:

Over the past decade, major progress in supramolecular polymerization has had a substantial effect on the design of functional soft materials. However, despite recent advances, most studies are still based on a preconceived notion that supramolecular polymerization follows a step-growth mechanism, which precludes control over chain length, sequence, and stereochemical structure. Here we report the realization of chain-growth polymerization by designing metastable monomers with a shape-promoted intramolecular hydrogen-bonding network. The monomers are conformationally restricted from spontaneous polymerization at ambient temperatures but begin to polymerize with characteristics typical of a living mechanism upon mixing with tailored initiators. The chain growth occurs stereoselectively and therefore enables optical resolution of a racemic monomer.

The chiroptical properties of a series of carbo[n]helicenes (n = 4-10) were investigated by the state-of-the-art approximate coupled cluster and density functional theory calculations. The theoretical calculation at the RI-CC2/TZVPP//DFT-D2-B97-D/TZVP level nicely reproduced the experimental CD spectra in both excitation energy and rotational strength without any shift or scaling. These calculations afforded the electric and the magnetic transition dipole moment vectors in [n]helicenes, allowing us to discuss the observed rotational strengths as a function of the number of benzene rings. Although the observed CD intensity was not immediately correlated to any of the calculated parameters, the anisotropy (g) factor of the (1)B(b) band and the specific rotation were found inversely proportional to n and nicely correlated with the helical pitch, but discontinuous at n = 6, where the aromatic rings start to overlap. In contrast, the g factor at the (1)B(a) band was rather insensitive to n. It was also revealed that the excitation energies of the (1)B(b) and (1)B(a) bands are inversely proportional to n over the entire range of n examined. The theoretical predictions also enabled us to rectify the erroneous experimental CD spectra of [5]- and [6]helicenes reported earlier, by using the enantiopure samples resolved by chiral HPLC.

Helicenes have attracted considerable attention due to their inherent helical chirality and extended π-conjugation. Recently, rapid progress has been witnessed in the preparation of double, triple, quadruple, quintuple, and sextuple helicenes, where plural helicene moieties are symmetrically arranged in a single molecule. While synthetic efforts and X-ray crystallographic analyses devoted to these multiple helicenes and theoretical investigations on their isomerization and racemization behaviors have been relatively well documented and reviewed in the literature, the chiroptical properties of the multiple helicities have been somewhat overlooked. This review discourses the cumulative and systematic investigations on the chiroptical properties such as the circular dichroism (CD) and circularly polarized luminescence (CPL) of multiple helicenes. Although the number and structural variations of multiple helicenes reported to date have been fairly limited, this review overviews the current status of the chemistry of multiple helicenes from the viewpoint of chiroptical properties and provides insights into the design principle for advanced chiroptical materials through the proper arrangement of multiple helices, highlighting the impact of the molecular symmetry on the chiroptical responses.