Effect of Surface Site on the Spin State for the Interaction of NO with Pd2, Rh2 and PdRh Nanoparticles Supported at Regular and Defective MgO(001) Surfaces

An attempt has been made to analyze the effect of surface site on the spin state for the interaction of NO with Pd2, Rh2 and PdRh nanoparticles that supported at regular and defective MgO(001) surfaces. The adsorption properties of NO on homonuclear, Pd2, Rh2, and heteronuclear transition metal dimers, PdRh, that deposited on MgO(001) surface have been studied by means of hybrid density functional theory calculations and embedded cluster model. The most stable NO chemisorption geometry is in a bridge position on Pd2 and a top configuration of Rh2 and PdRh with N-down oriented. NO prefers binding to Rh site when both Rh and Pd atoms co-exist in the PdRh. The natural bond orbital analysis (NBO) reveals that the electronic structure of the adsorbed metal represents a qualitative change with respect to that of the free metal. The adsorption properties of NO have been analyzed with reference to the NBO, charge transfer, band gaps, pairwise and non-pairwise additivity. The binding of NO precursor is dominated by the E(i)Mx-NO pairwise additive components and the role of the support was not restricted to supporting the metal. The adsorbed dimers on the MgO surface lose most of the metal-metal interaction due to the relatively strong bond with the substrate. Spin polarized calculations were performed and the results concern the systems in their more stable spin states. Spin quenching occurs for Rh atom, Pd2, Rh2 and PdRh complexes at the terrace and defective surfaces. The adsorption energies of the low spin states of spin quenched complexes are always greater than those of the high spin states. The metal-support and dimer-support interactions stabilize the low spin states of the adsorbed metals with respect to the isolated metals and dimers. Although the interaction of Pd, Rh, Pd2, Rh2 and PdRh particles with Fs sites is much stronger than the regular sites O2-, the adsorption of NO is stronger when the particular dimers are supported on an anionic site than on an Fs site of the MgO(001). The encountered variations in magnetic properties of the adsorbed species at MgO(001) surface are correlated with the energy gaps of the frontier orbitals. The results show that the spin state of adsorbed metal atoms on oxide supports and the role of precursor molecules on the magnetic and binding properties of complexes need to be explicitly taken into account.

Efficient and Fast X-Ray Luminescence in Organic Phosphors Through High-Level Triplet-Singlet Reverse Intersystem Crossing

Organic scintillators that efficiently generate bright triplet excitons are of critical importance for highperformance X-ray-excited luminescence in radiation detection.However,the nature of triplet-singlet spinforbidden transitions in these materials often result in long-lived phosphorescence,which is undesirable for ultrafast X-ray detection and imaging.Here we demonstrate that the effect of hybridized local and charge-transfer(HLCT)excited states enables organic scintillators to exhibit highly efficient and fast radioluminescence(RL)in response to X-ray irradiation.Our experimental and theoretical investigation shows that the oxidized 1,8-naphthalimide-phenothiazine dyad(OMNI-PTZ 2)with HLCT-excited states has an enhanced overlap integral of the highest occupied molecular orbital(HOMO)and lowest unoccupied molecular orbital(LUMO)on MNIπ-orbitals,and moderate donor–acceptor electron interactions.As a result,the RL of these crystals exhibits a 61-fold increase and its monoexponential decay lifetime is three orders of magnitude faster compared to its corresponding thermally activated delayed fluorescence(TADF)molecule MNI-PTZ 1.We further demonstrate the practical utility of the OMNI-PTZ 2(G)in high-performance X-ray detection and imaging,achieving an X-ray dose sensitivity of 97 nGy s−1 and an exceptional spatial resolution of 20 lp/mm.Our study provides a promising molecular design principle for utilizing triplet excitons to develop high-efficiency and fast X-ray scintillators for the development of next-generation flexible and stretchable X-ray imaging detectors.

Multiscale description of molecular packing and electronic processes in small-molecule organic solar cells

This paper summarizes our recent works on theoretical modelling of molecular packing and electronic processes in small-molecule organic solar cells.Firstly,we used quantum-chemical calculations to illustrate exciton-dissociation and charge-recombination processes at the DTDCTB/C_（60） interface and particularly emphasized the major role of hot charge-transfer states in the exciton-dissociation processes.Then,we systematically analyzed the influence of DTDCTB surfaces with different features on the vacuum vapor deposition growth and packing morphologies of C_（60） via atomistic molecular dynamics simulations,and found that the formation of crystalline fullerene is the result of an integrated impact of stability,landscape,and molecular orientation of the substrate surfaces.Also,we investigated the impact of different film-processing conditions,such as solvent evaporation rates and thermal annealing,on molecular packing configurations in a neat small-molecule donor material,DPP（TBFu）_2,and discussed the correlation between charge mobility and molecular packing via atomistic simulations in combination with electronic-structure calculations and kinetic Monte Carlo simulations.

Dynamic Mechanism of Relaxation Paths Occurring in TPA-DCPP： Roles of Solvent and Temperature

The relaxation paths for triphenylamine（TPA）-2,3-dicyanopyrazino phenanthrene（DCPP）, which has a pull-push structure, were investigated via steady-state, time-resolved spectroscopy involving transient absorption and time-correlated single photon counting. By changing the solvent polarity we found that an intramolecular charge transfer（ICT） state acting as a ＂bright＂ state was responsible for the fluorescence character of TPA-DCPP. Meanwhile, a ＂dark＂ state gradually appeared and competed with the ICT state. This was likely to be responsible for the polarity-dependent evolution of fluorescence intensity and fluorescence lifetime. The temperature-dependent fluorescence character of the TPA-DCPP in toluene exhibited ICT processes at high temperatures prior to the relaxation path from the initial excited state to the ground state. Our results provide useful insight into the optoelectronic properties of these kinds of molecules.