Secondary electron emission is the most important stage in the mechanism of radiation damage to DNA biopolymers induced by primary ionizing radiation. These secondary electrons ejected by the primary electron impacts can produce further ionizations, initiating an avalanche effect, leading to genome damage through the energy transfer from the primary objects to sensitive biomolecular targets, such as nitrogenous bases, saccharides, and other DNA and peptide components. In this work, the formation of positive and negative ions of purine bases of nucleic acids (adenine and guanine molecules) under the impact of slow electrons (from 0.1 till 200 eV) is studied by the crossed electron and molecular beams technique. The method used makes it possible to measure the molecular beam intensity and determine the total cross-sections for the formation of positive and negative ions of the studied molecules, their energy dependences, and absolute values. It is found that the maximum cross section for formation of the adenine and guanine positive ions is reached at about 90 eV energy of the electron beam and their absolute values are equal to 2.8 × 10(-15) and 3.2 × 10(-15) cm(2), respectively. The total cross section for formation of the negative ions is 6.1 × 10(-18) and 7.6 × 10(-18) cm(2) at the energy of 1.1 eV for adenine and guanine, respectively. The absolute cross-section values for the molecular ions are measured and the cross-sections of dissociative ionization are determined. Quantum chemical calculations are performed for the studied molecules, ions and fragments for interpretation of the crossed beams experiments.

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Time-dependent density functional theory with linear and quadratic response technology is used to calculate electronic structure, spectra, and spin-orbit coupling effects for analysis of the main mechanism for phosphorescence of the recently synthesized iridium complex [bis(2-phenylpyridine)(2-carboxy-4-dimethylaminopyridine)iridium(III)]. This compound exhibits strong green phosphorescence which is used in solution processable organic light-emitting diode devices (OLEDs) to overcome the efficiency limit imposed by the formation of triplet excitons. Attempting to foresee new structure-property relations that can guide an improved design of OLED devices based on phosphorescence of the lowest triplet state, we have conducted a theoretical analysis of the photophysical properties of a series of iridium cyclometalated complexes.

Unsaturated hydrocarbons, such as acetylene and ethylene, show strong geometrical distortions when coordinated to transition metals or to surfaces; the bonding is normally analysed in terms of a π-donation—π*-backdonation process. In the present work we use chemisorption of the unsaturated hydrocarbons (ethylene, acetylene, and benzene) on cluster models of the copper (100), (110), and (111) surfaces to demonstrate the importance of considering the available excited states of the free molecule in analyzing the bonding scheme of the adsorbate at the surface. By comparison to the structures of the triplet excited states in the gas phase we demonstrate that these must be considered as the states actually involved in the bonding. This implies a spin-uncoupling in both adsorbate and substrate as part of the chemisorption process or bond formation. In particular, for benzene we identify the quinoid gas phase triplet state as the specific state that is most strongly bound to the Cu(110) substrate; the structure is an inverted boat form. The gas phase antiquinoid triplet state leads to a planar, less strongly bound, chemisorbed state. By explicitly considering the excited state of the adsorbate that corresponds to the bonding state—the ground state for the chemisorbed system—barriers in the chemisorption path are analyzed in terms of avoided crossings between the initial closed-shell singlet state and the bond-prepared excited triplet state, which, together with the substrate, forms an overall singlet. It is argued that this picture with bond-preparation through spin-uncoupling can be very useful to understand and predict reaction paths in heterogeneous catalysis.

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Single-wall nanotube based on tetraoxa[8]circulene monomer.

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Ultralong organic phosphorescence strongly depends on the formation of aggregation, while it is difficult to obtain in dilute environments on account of excessive internal and external molecular motions. Herein, ultralong single-molecule phosphorescence (USMP) at room temperature was achieved in the monomer state by coassembling biphenyl and naphthalene derivatives at low density with poly(vinyl alcohol) (PVA), where PVA provides a confined environment to stabilize the triplet state. Various factors that affect the USMP were studied, including aggregation, conformation, temperature, and moisture. In these systems, the formation of aggregates through intermolecular stacking and hydrogen bonding interactions in the film or crystal phases completely suppresses the USMP. However, the fluorescence is enhanced when coassembling these compounds at high concentration with PVA and becomes stronger in their powder state, indicating that the intersystem crossing process is blocked by the aggregation. Theoretical calculations suggest that the aggregation depresses spin−orbit coupling between the excited singlet and triplet states and enhances the nonradiative quenching process. Moreover, a relatively twisted conformation is more conducive to the occurrence of intersystem crossing than planar conformation. The USMP shows delicate and reversible sensitivity to the changes of temperature and moisture, rendering them with the applicability as smart organic optoelectronic materials.