Aggregation-induced emission (AIE) has been harnessed in many systems through the principle of restriction of intramolecular rotations (RIR) based on mechanistic understanding from archetypal AIE molecules such as tetraphenylethene (TPE). However, as the family of AIE-active molecules grows, the RIR model cannot fully explain some AIE phenomena. Here, we report a broadening of the AIE mechanism through analysis of 10,10',11,11'-tetrahydro-5,5'-bidibenzo[a,d][7]annulenylidene (THBDBA), and 5,5'-bidibenzo[a,d][7]annulenylidene (BDBA). Analyses of the computational QM/MM model reveal that the novel mechanism behind the AIE of THBDBA and BDBA is the restriction of intramolecular vibration (RIV). A more generalized mechanistic understanding of AIE results by combining RIR and RIV into the principle of restriction of intramolecular motions (RIM).

In general, optical emission in the solid-state is red-shifted with respect to the solution phase. A series of recently synthesized compounds exhibits aggregation induced blue-shifted emission (AIBSE) phenomena. By employing a polarizable continuum model (PCM) and a hybrid quantum mechanics/molecular mechanics (QM/MM) approach, we investigate the excited-state electronic structures of some typical AIE-active molecules both in solvents and in aggregates at the time-dependent density functional theory (TD-DFT) level. It is found that the AIBSE phenomena originate from the smaller reorganization energy in aggregates than in the solution phase, as evidenced through the restricted structural relaxation, planarization in the excited state, and freezing of low-frequency out-of-plane twists in the transition state.

Electrospray ionization was used to generate doubly charged complex ions composed of the uranyl ion and nitrile ligands. The complexes, with general formula [UO2(RCN)n]2+, n = 0-5 (where R=CH3-, CH3CH2-, or C6H5-), were isolated in an ion-trap mass spectrometer to probe intrinsic reactions with H2O. For these complexes, two general reaction pathways were observed: (a) the direct addition of one or more H2O ligands to the doubly charged complexes and (b) charge-reduction reactions. For the latter, the reactions produced uranyl hydroxide, [UO2OH], complexes via collisions with gas-phase H2O molecules and the elimination of protonated nitrile ligands.

With the fast development of nuclear energy, the issue related to spent nuclear fuel reprocessing has been regarded as an imperative task, especially for the separation of minor actinides. In fact, it still remains a worldwide challenge to separate trivalent An(iii) from Ln(iii) because of their similar chemical properties. Therefore, understanding the origin of extractant selectivity for the separation of An(iii)/Ln(iii) by using theoretical methods is quite necessary. In this work, three ligands with similar structures but different bridging frameworks, Et-Tol-DAPhen (La), Et-Tol-BPyDA (Lb) and Et-Tol-PyDA (Lc), have been investigated and compared using relativistic density functional theory. The electrostatic potential and molecular orbitals of the ligands indicate that ligand La is a better electron donor compared to ligands Lb and Lc. The results of QTAIM, NOCV and NBO suggest that the Am-N bonds in the studied complexes have more covalent character compared to the Eu-N bonds. Based on the thermodynamic analysis, [M(NO)(HO)] + L + 2NO = [ML(NO)] + 8HO should be the most probable reaction in the solvent extraction system. Our results clearly verify that the relatively harder oxygen atoms offer these ligands higher coordination affinities toward both of the An(iii) and Ln(iii) ions compared to the relatively softer nitrogen atoms. However, the latter possess stronger affinities toward An(iii) over Ln(iii), which partly results in the selectivity of these ligands. This work can afford useful information on achieving efficient An(iii)/Ln(iii) separation through tuning the structural rigidity and hardness or softness of the functional moieties of the ligands.

A series of actinide (An) species of L-An-N compounds [An = Pa-Pu, L = [N(CH2CH2NSiPr(i)3)3](3-), Pr(i) = CH(CH3)2] have been investigated using scalar relativistic density functional theory (DFT) without considering spin-orbit coupling effects. The ground state geometric and electronic structures and natural bond orbital (NBO) analysis of actinide compounds were studied systematically in neutral and anionic forms. It was found that with increasing actinide atomic number, the bond length of terminal multiple An-N1 bond decreases, in accordance with the actinide contraction. The Mayer bond order of An-N1 decreases gradually from An = Pa to Pu, which indicates a decrease in bond strength. The terminal multiple bond for L-An-N compounds contains one σ and two π molecular orbitals, and the contributions of the 6d orbital to covalency are larger in magnitude than the 5f orbital based on NBO analysis and topological analysis of electron density. This work may help in understanding of the bonding nature of An-N multiple bonds and elucidating the trends and electronic structure changes across the actinide series. It can also shed light on the construction of novel An-N multiple bonds.

Recovery of uranium from seawater is extremely challenging but important for the persistent development of nuclear energy, and thus exploring the coordination structures and bonding nature of uranyl complexes becomes essential for designing highly efficient uranium adsorbents. In this work, the interactions of uranium and a series of adsorbents with various well-known functional groups including amidoximate (AO(-)), carboxyl (Ac(-)), glutarimidedioximate (HA(-)), and bifunctional AO(-)/Ac(-), HA(-)/Ac(-) on different alkyl chains (R'═CH3, R″═C13H26) were systematically studied by quantum chemical calculations. For all the uranyl complexes, the monodentate and η(2) coordination are the main binding modes for the AO(-) groups, while Ac(-) groups act as monodentate and bidentate ligands. Amidoximes can also form cyclic imide dioximes (H2A), which coordinate to UO2(2+) as tridentate ligands. Kinetic analysis of the model displacement reaction confirms the rate-determining step in the extraction process, that is, the complexing of uranyl by amidoxime group coupled with the dissociation of the carbonate group from the uranyl tricarbonate complex [UO2(CO3)3](4-). Complexing species with AO(-) groups show higher binding energies than the analogues with Ac(-) groups. However, the obtained uranyl complexes with Ac(-) seem to be more favorable according to reactions with [UO2(CO3)3](4-) as reactant, which may be due to the higher stability of HAO compared to HAc. This is also the reason that species with mixed functional group AO(-)/Ac(-) are more stable than those with monoligand. Thus, as reported in the literature, the adsorbability of uranium can be improved by the synergistic effects of amidoxime and carboxyl groups.

Self-defect clusters in bulk matrix might affect the thermodynamic behavior of fission gases in nuclear fuel such as uranium dioxide. With first-principles local spin-density approximation plus U calculations and taking xenon as a prototype, we find that the influence of oxygen defect clusters on the thermodynamics of gas atoms is prominent, which increases the solution energy of xenon by a magnitude of 0.5 eV, about 43% of the energy difference between the two lowest lying states at 700 K. Calculation also reveals a thermodynamic competition between the uranium vacancy and trivacancy sites to incorporate xenon in hyperstoichiometric regime at high temperatures. The results show that in hypostoichiometric regime neutral trivacancy sites are the most favored position for diluted xenon gas, whereas in hyperstoichiometric condition they prefer to uranium vacancies even after taking oxygen self-defect clusters into account at low temperatures, which not only confirms previous studies but also extends the conclusion to more realistic fuel operating conditions. The observation that gas atoms are ionized to a charge state of Xe + when at a uranium vacancy site due to strong Madelung potential implies that one can control temperature to tune the preferred site of gas atoms and then the bubble growth rate. A solution to the notorious metastable states difficulty that frequently encountered in density functional theory plus U applications, namely, the quasiannealing procedure, is also discussed.

We investigate the excited-state decay processes for the 3-(2-cyano-2- phenylethenyl-Z)-NH-indole (CPEI) in the solid phase through combined quantum mechanics and molecular mechanics (QM/MM) and vibration correlation formalisms for radiative and nonradiative decay rates, coupled with time-dependent density functional theory (TDDFT). By comparing the isolated CPEI molecule and the molecule-in-cluster, we show that the molecular packing through intermolecular hydrogen-bonding interactions can hinder the excited-state nonradiative decay and thus enhance the fluorescence efficiency in the solid phase. Aggregation effect is shown to block the nonradiative decay process through hindering the low-frequency vibration motions. The fluorescence quantum yields for both isolated molecule and aggregation are predicted to be insensitive to temperature due to the hydrogen-bonding nature, and their values at room temperature are consistent with the experiment.