Electric quadrupole moments of neutron-rich nuclei 32Al and 31Al

The electric quadrupole moments for the ground states of ³²Al and ³¹Al have been measured by the β ray-detected nuclear quadrupole resonance method. Spin-polarized ³²Al and ³¹Al nuclei were obtained from the fragmentation of ⁴⁰Ar projectiles at E/A?=?95 MeV/nucleon, and were implanted in a single crystal α-Al₂O₃ stopper. The measured Q moment of ³²Al, |Q(³²Al)|?=?24(2) mb, is in good agreement with a conventional shell-model calculation with a full sd model space and empirical effective charges, while that of ³¹Al is considerably smaller than the sd calculations.     

Structure‐Dependent Electronic Nature of Star‐Shaped Oligothiophenes,Probed by Ensemble and Single‐Molecule Spectroscopy

We have investigated the photophysical properties of star‐shaped oligothiophenes with three terthiophene arms (meta to each other, S3 ) or six terthiophene arms (ortho‐, meta‐, and para‐arranged, S6 ) connected to an ethynylbenzene core to elucidate the relationship between their molecular structure and electronic properties by using a combination of ensemble and single‐molecule spectroscopic techniques. We postulate two different conformations for molecules S3 and S6 on the basis of the X‐ray structure of hexakis(5‐hexyl‐2‐thienlyethynyl)benzene and suggest the coexistence of these conformers by using spectroscopic methods. From the steady‐state spectroscopic data of compound S6 , we show that the exciton is delocalized over the core structure, but that the meta‐linkage in compound S3 prevents the electronic communication between the arms. However, in single‐molecule spectroscopic measurements, we observed that some molecules of compound S3 showed long fluorescence lifetimes (about 1.4 ns) in the fluorescence‐intensity trajectories, which indicated that π electrons were delocalized along the meta linker. Based on these observations, we suggest that the delocalized exciton is intensely sensitive towards the dihedral angle between the core and the adjacent thiophene ring, as well as to the substituted position of the terthiophene arms. Our results highlight that the fluorescence lifetimes of compounds S3 and S6 are strongly correlated with the spatial location of their excitons, which is mainly affected by their conformation, that is, whether the innermost thiophene rings are facing each other or not. More interestingly, we observed that the difference between the degrees of ring‐torsional flexibility of compounds S3 and S6 results in their sharply contrasting fluorescence properties, such as a change in fluorescence intensity as a function of temperature.     

Steric Control in the π‐Dimerization of Oligothiophene Radical Cations Annelated with Bicyclo[2.2.2]octene Units

A Two series of oligothiophenes 2 (nT) (n=4,5), annelated with bicyclo[2.2.2]octene (BCO) units at both ends, and quaterthiophenes 3 a – c , annelated with various numbers of BCO units at different positions, were newly synthesized to investigate the driving forces of π‐dimerization and the structure–property relationships of the π‐dimers of oligothiophene radical cations. Their radical‐cation salts were prepared through chemical one‐electron oxidation by using nitrosonium hexafluoroantimonate. From variable‐temperature electron spin resonance and electronic absorption measurements, the π‐dimerization capability was found to vary among the members of the 2 (nT)^{+ .} SbF₆^? series and 3 ^{+ .} SbF₆^? series of compounds. To examine these results, density functional theory (DFT) calculations at the M06‐2X/6‐31G(d) level were conducted for the π‐dimers. This level of theory was found to successfully reproduce the previously reported X‐ray structure of ( 2 (3T))₂²⁺ having a bent π‐dimer structure with cis–cis conformations. The absorption bands obtained by time‐dependent DFT calculations for the π‐dimers were in reasonable agreement with the experimental spectra. The attractive and repulsive forces for the π‐dimerization were divided into four factors: 1) SOMO–SOMO interactions, 2) van der Waals forces, 3) solvation, and 4) Coulomb repulsion, and the effects of each factor on the structural differences and chain‐length dependence are discussed in detail.     

The importance of a single methyl group in determining the reaction chemistry of pentamethylcyclopentadienyl cyclooctatetraenyl uranium metallocenes

The steric factors that allow trivalent [(C(5)Me(5))(3)U] (1) to function as a three-electron reductant with C(8)H(8) to form tetravalent [{(C(5)Me(5))(C(8)H(8))U}(2)(μ-C(8)H(8))] (2) have been explored by examining the synthesis and reactivity of the intermediate, "[(C(5)Me(5))(2)(C(8)H(8))U]" (3), and the slightly less crowded analogues, [(C(5)Me(5))(C(5)Me(4)H)(C(8)H(8))U] and [(C(5)Me(4)H)(2)(C(8)H(8))U], that have, successively one less methyl group. The reaction of [{(C(5)Me(5))(C(8)H(8))U(μ-OTf)}(2)] (4; OTf=OSO(2) CF(3)) with two equivalents of KC(5)Me(5) in THF gave ring-opening to "[(C(5)Me(5))(C(8)H(8))U{O(CH(2))(4)(C(5) Me(5))}]" consistent with in situ formation of 3. Reaction of 4 with two and four equivalents of KC(5)Me(4)H generates two equivalents of [(C(5)Me(5))(C(5)Me(4)H)(C(8)H(8))U] (5) and [(C(5)Me(4)H)(2)(C(8)H(8))U] (6), respectively, which in contrast to 3 were isolable. Tetravalent 5 reduces phenazine and PhEEPh (E=S, Se, and Te) to form the tetravalent uranium reduction products, [{(C(5)Me(5))(C(8)H(8))U}(2)(μ-C(12)H(8)N(2))] (7), [{(C(5)Me(5))(C(8)H(8))U}(2)(μ-SPh)(2)] (8), [{(C(5)Me(5))(C(8)H(8))U}(2)(μ-SePh)(2)] (9), and [{(C(5)Me(5))(C(8)H(8))U}(2)(μ-TePh)(2)] (10), consistent with sterically induced reduction. In contrast, the less sterically crowded 6 does not react with these substrates.