Interaction-induced negative differential resistance in asymmetric molecular junctions.

Combining insights from quantum chemistry calculations with master equations, we discuss a mechanism for negative differential resistance (NDR) in molecular junctions, operated in the regime of weak tunnel coupling. The NDR originates from an interplay of orbital spatial asymmetry and strong electron-electron interaction, which causes the molecule to become trapped in a nonconducting state above a voltage threshold. We show how the desired asymmetry can be selectively introduced in individual orbitals in, e.g., oligo(phenyleneethynylene)-type molecules by functionalization with a suitable side group, which is in linear conjugation to one end of the molecule and cross-conjugated to the other end.

Photoabsorption studies of neutral green fluorescent protein model chromophores in vacuo.

We report on gas-phase experimental and theoretical studies on the neutral form of the green-fluorescent protein (GFP) chromophore using six different models, each carrying a spectator positive charge. Theoretical studies were carried out to quantify the effect of the spectator charge on the absorption maximum of the true neutral. The study also includes models having the possibility of forming intra-molecular hydrogen bonds, and their effect on the absorption profile is analyzed. The charge redistribution caused by a strong intra-molecular hydrogen bond was found to give rise to a red shift in going from non-hydrogen bonded to hydrogen bonded models. For the non-hydrogen bonded models, the length of the side chain as well as the group carrying the spectator charge, was varied to explore the possibility of shifts in absorption maximum due to these variations. No shifts were observed. The implications of these results in tuning the absorption maximum of the neutral form of the GFP chromophores are discussed.

The gas-phase absorption spectrum of a neutral GFP model chromophore.

We have studied the gas-phase absorption properties of the green fluorescent protein (GFP) chromophore in its neutral (protonated) charge state in a heavy-ion storage ring. To accomplish this we synthesized a new molecular chromophore with a charged NH(3) group attached to a neutral model chromophore of GFP. The gas-phase absorption cross section of this chromophore molecule as a function of the wavelength is compared to the well-known absorption profile of GFP. The chromophore has a maximum absorption at 415 +/- 5 nm. When corrected for the presence of the charged group attached to the GFP model chromophore, the unperturbed neutral chromophore is predicted to have an absorption maximum at 399 nm in vacuum. This is very close to the corresponding absorption peak of the protein at 397 nm. Together with previous data obtained with an anionic GFP model chromophore, the present data show that the absorption of GFP is primarily determined by intrinsic chromophore properties. In other words, there is strong experimental evidence that, in terms of absorption, the conditions in the hydrophobic interior of this protein are very close to those in vacuum.