We address the binding energy of charge-transfer excitons at organic semiconductor heterojunctions by investigating a polymer blend where the energy of the intramolecular singlet exciton is just sufficient to create separated charge pairs, placing the system at the threshold for photovoltaic operation. At 10 K, we report long-lived photoluminescence arising from charge recombination and triplet-exciton bimolecular annihilation. Both mechanisms regenerate singlet excitons in the electron acceptor, but we demonstrate that charge recombination dominates singlet regeneration dynamics on e300 ns time scales. This occurs by tunnelling of separated electron and holes across heterojunctions. The separated charge pairs are therefore degenerate with repopulated singlet states. From the difference of the charge-transfer and intrachain exciton emission energies, we determine that the binding energy of charge-transfer excitons with respect to bulk charge separation is g250 meV. Directed charge flow away from the heterojunction would avoid formation of strongly bound charge-transfer excitons, that act as traps and limit current generation in organic solar cells.

The advent of ultrafast highly brilliant coherent X-ray free-electron laser sources has driven the development of novel structure-determination approaches for proteins, and promises visualization of protein dynamics on sub-picosecond timescales with full atomic resolution. Significant efforts are being applied to the development of sample-delivery systems that allow these unique sources to be most efficiently exploited for high-throughput serial femtosecond crystallography. Here, the next iteration of a fixed-target crystallography chip designed for rapid and reliable delivery of up to 11 259 protein crystals with high spatial precision is presented. An experimental scheme for predetermining the positions of crystals in the chip by means of in situ spectroscopy using a fiducial system for rapid, precise alignment and registration of the crystal positions is presented. This delivers unprecedented performance in serial crystallography experiments at room temperature under atmospheric pressure, giving a raw hit rate approaching 100% with an effective indexing rate of approximately 50%, increasing the efficiency of beam usage and allowing the method to be applied to systems where the number of crystals is limited.

A portable sample viewing and alignment system is described which provides fast and reliable motion positioning for fixed target arrays at synchrotrons and free-electron laser sources.

How does chemistry scale in complexity to unerringly direct biological functions? Nass Kovacs et al. have shown that bacteriorhodopsin undergoes structural changes tantalizingly similar to the expected pathway even under excessive excitation. Is the protein structure so highly evolved that it directs all deposited energy into the designed function? It is difficult to overstate the importance of having atomic structures to help shape our thinking and understanding of matter. Structural information constrains the number of possible solutions in trying to piece together a puzzle in how matter undergoes transformation from one structure to another and the associated changes in material properties 1,2. In terms of understanding biological processes, this question always reduces to how the protein structure surrounding an active site has evolved to direct chemical processes into biological functions, typically with efficiencies well beyond our current capabilities to exploit chemistry. In this respect, bacteriorhodopsin (bR) serves as a model system for understanding structurefunction relationships for membrane proteins 3-5. This system functions as a light-driven, outward proton pump, which can be triggered by light to use time resolved optical methods to watch it function in real time. Its structure is composed of seven transmembrane α-helices that are covalently bound to a photoactive retinal molecule via a lysine residue through a Schiff base linkage (Fig. 1b). Upon absorbing a photon, the retinal chromophore undergoes rapid isomerization from an all-trans to 13-cis form passing through the I 460 (charge separated), J 625 (highly twisted) and K 590 (isomerized) intermediates. The retinal isomerization acts like a push in changing the electrostatic and structural environment around the active site. These changes in turn lead to a series of cascaded protein conformational changes to facilitate the transport of a proton from the retinal Schiff base to the extracellular side of the membrane via L 550 and M 410 intermediates. The retinal then undergoes reprotonation and thermal re-isomerization through the N 560 and O 630 intermediates, respectively, where it can then return to the bR 568 ground state. These processes have been well characterized spectroscopically and many of the long-lived