Metal/molecule/metal transport junctions can transport charge in the elastic scattering (Landauer) regime if the injection gap is large and the molecule is relatively short. Stochastic switching and broad conduction peak distributions have been observed in such junctions. We examine the effect of altering interface geometry on transport, using density functional calculations. For most structures, variations in conductance of order 0-300% are found, but when an atomic wire of Au binds to the molecule, symmetry changes can modify currents by a factor of 10(3).

The widespread and long-term use of TNT has led to extensive study of its thermal and explosive properties. Although much research on the thermolysis of TNT and polynitro organic compounds has been undertaken, the kinetics and mechanism of the initiation and propagation reactions and their dependence on the temperature and pressure are unclear. Here, we report a comprehensive computational DFT investigation of the unimolecular adiabatic (thermal) decomposition of TNT. On the basis of previous experimental observations, we have postulated three possible pathways for TNT decomposition, keeping the aromatic ring intact, and calculated them at room temperature (298 K), 800, 900, 1500, 1700, and 2000 K and at the detonation temperature of 3500 K. Our calculations suggest that at relatively low temperatures, reaction of the methyl substituent on the ring (C-H alpha attack), leading to the formation of 2,4-dinitro-anthranil, is both kinetically and thermodynamically the most favorable pathway, while homolysis of the C-NO(2) bond is endergonic and kinetically less favorable. At approximately 1250-1500 K, the situation changes, and the C-NO(2) homolysis pathway dominates TNT decomposition. Rearrangement of the NO(2) moiety to ONO followed by O-NO homolysis is a thermodynamically more favorable pathway than the C-NO(2) homolysis pathway at room temperature and is the most exergonic pathway at high temperatures; however, at all temperatures, the C-NO(2) --> C-ONO rearrangement-homolysis pathway is kinetically unfavorable as compared to the other two pathways. The computational temperature analysis we have performed sheds light on the pathway that might lead to a TNT explosion and on the temperature in which it becomes exergonic. The results appear to correlate closely with the experimentally derived shock wave detonation time (100-200 fs) for which only the C-NO(2) homolysis pathway is kinetically accessible.

A novel method for the bromination of perylene diimides, PDI (1), under mild conditions is reported. Variation of the reaction conditions allows mono- and dibromination of PDIs to afford 2 and 3 (these can be separated through standard procedures) or exclusive dibromination to afford 3. Pure 1,7 regioisomers are obtained through repetitive crystallization. The structure of 1,7-3b was elucidated by a single-crystal X-ray analysis. The facility of the bromination reaction, which decreases in the order 1a > 1b > 1c, depends on PDI aggregation propensities. Monobrominated PDIs were utilized for the syntheses of novel unsymmetrical piperidinyl (4a and 4b) and trimethylsilylethynyl derivatives (5a and 5b). Computational studies (DFT) on imide substituent rotation in PDIs reveal that in the case of bulky groups there is a restricted rotation leading to isomers, in agreement with our experimental results. An aromatic core twist in PDIs bearing one and two bromine substituents was also investigated by DFT.

Perylene diimide (PDI) bearing polyethylene glycol substituents at the imide positions was reduced in water with sodium dithionite to produce an aromatic dianion. The latter is stable for months in deoxygenated aqueous solutions, in contrast to all known aromatic dianions which readily react with water. Such stability is due to extensive electron delocalization and the aromatic character of the dianion, as evidenced by spectroscopic and theoretical studies. The dianion reacts with oxygen to restore the parent neutral compound, which can be reduced again in an inert atmosphere with sodium dithionite to give the dianion. Such reversible charging renders PDIs useful for controlled electron storage and release in aqueous media. Simple preparation of the dianion, reversible charging, high photoredox power, and stability in water can lead to development of new photofunctional and electron transfer systems in the aqueous phase.