The "solvation" of a H3+ molecule-ion by H2 molecule(s) and/or a He atom was examined in ab initio electronic structure calculations. Binding energies and geometrical parameters were determined for the ground electronic states of H"+ ( = 3,5,7,9,11) and HeH"+ ( = 3, 5,7,9) cluster ions. Floating spherical Slater orbitals (FSSO) were employed with full single and double configuration interaction (Cl). All of the clusters studied were found to be weakly bound (with respect to removal of "solvent" species). Inclusion of configuration interaction had a major effect; for > 5, it increased the energy required to remove an H2 molecule from H"+ by ~35% and that to remove the He atom from HeH"+ by ~300-400%. The net solvation energy per H2 (~3.5 kcal mol"1 for the first three solute molecules, ~1.5 kcal mol'1 for the fourth) was found to be about threefold larger than for He.

The anthracene cyclophane bis-anthracene (BA) can undergo a [4 + 4] photocycloaddition reaction that results in a photodimer with two cyclobutane rings. We find that the subsequent dissociation of the dimer, which involves the rupture of two carbon-carbon bonds, is strongly accelerated by the application of mild pressures. The reaction kinetics of the dimer dissociation in a Zeonex (polycycloolefin) polymer matrix were measured at various pressures and temperatures. Biexponential reaction kinetics were observed for all pressures, consistent with the presence of two different isomers of bis(anthracene). One of the rates showed a strong dependence on pressure, yielding a negative activation volume for the dissociation reaction of ΔV(++) = -16 Å(3). The 93 kJ/mol activation energy for the dissociation reaction at ambient pressure is lowered by more than an order of magnitude from 93 to 7 kJ/mol with the application of modest pressure (0.9 GPa). Both observations are consistent with a transition state that is stabilized at higher pressures, and a mechanism for this is proposed in terms of a two-step process where a flattening of the anthracene rings precedes rupture of the cyclobutane rings. The ability to catalyze covalent bond breakage in isolated small molecules using compressive forces may present opportunities for the development of materials that can be activated by acoustic shock or stress.

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Conjugated polymers blended with graphene represent a possible approach for making organic bulk heterojunction solar cells. In this paper, the time-resolved fluorescence dynamics of poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) blended with graphene microsheets derived from chemically reduced graphitic oxide are studied. Both polymers exhibit strong quenching and shortened fluorescence lifetimes when mixed with graphene. The fluorescence quenching function takes the form of e −k Q t 1/2 , where k Q is linearly proportional to the weight fraction of graphene in the blend. We consider two physical models to explain the origin of the fluorescence quenching. The first assumes that energy transfer occurs within a three-dimensional space to molecular scale defects within the graphene according to the standard Forster model with an energy transfer rate proportional to the donor−acceptor separation R −6. The second model assumes a quasi-two-dimensional environment where the energy transfer rate between the donor and graphene sheets is proportional to R −4. Using the second model, an estimate of ∼5 nm is obtained for the critical energy transfer radius for energy transfer between P3HT chains and graphene sheets. This value is in reasonable agreement with theory. Differences between the quenching behavior of graphene in MEH-PPV and P3HT blends are also discussed.

Experimental and theoretical results in support of nonlinear dynamic behavior of photosynthetic reaction centers under light-activated conditions are presented. Different conditions of light adaptation allow for preparation of reaction centers in either of two different conformational states. These states were detected both by short actinic flashes and by the switching of the actinic illumination level between different stationary state values. In the second method, the equilibration kinetics of reaction centers isolated from Rhodobacter sphaeroides were shown to be inherently biphasic. The fast and slow equilibration kinetics are shown to correspond to electron transfer (charge separation) at a fixed structure and to combined electron-conformational transitions governed by the bounded diffusion along the potential surface, respectively. The primary donor recovery kinetics after an actinic flash revealed a pronounced dependence on the time interval (deltat) between cessation of a lengthy preillumination of a sample and the actinic flash. A pronounced slow relaxation component with a decay half time of more than 50 s was measured for deltat > 10 s. This component corresponds to charge recombination in reaction centers for which light-induced structural changes have not relaxed completely before the flash. The amplitude of this component depended on the conditions of the sample preparation, specifically on the type of detergent used in the preparation. The redox potential parameters as well as the structural diffusion constants were estimated for samples prepared in different ways.