Technologies which convert light into energy, and vice versa, rely on complex, microscopic transport processes in the condensed phase, which obey the laws of quantum mechanics, but hitherto lack systematic analysis and modeling. Given our much improved understanding of multicomponent, disordered, highly structured, open quantum systems, this 'focus on' collection collects cuttingedge research on theoretical and experimental aspects of quantum transport in truly complex systems as defined, e.g., by the macromolecular functional complexes at the heart of photosynthesis, by organic quantum wires, or even photovoltaic devices. To what extent microscopic quantum coherence effects can (be made to) impact on macroscopic transport behavior is an equally challenging and controversial question, and this 'focus on' collection provides a setting for the present state of affairs, as well as for the 'quantum opportunities' on the horizon.

opened a discussion of the introductory lecture by Majed Chergui: My question pertains to the theoretical interpretation of the interesting time-resolved circular dichroism (CD) experiments on DNA that you presented. The determination of nucleic acid base conformation from CD data is a fairly complex and challenging problem, which has been studied for many years. In an oligonucleotide with many bases, the CD spectrum depends on the numerous non-degenerate couplings between the many transition dipole moments of the bases, which in turn depends on their relative distances and orientations (i.e. base conformation). In general, there is no unique mapping between a given CD spectrum and a specic oligonucleotide conformation. How might this issue be addressed for the structural interpretation of time-resolved CD experiments on oligonucleotides?Majed Chergui responded: This is a very good question and I agree with you that the modelling of CD spectra is far from trivial for systems like nucleic acid bases or even peptides. What I showed in my talk aimed at starting to resolve this problem, at least in the case of peptide chains, using thioamide substitution. We investigated a dipeptide whose two carbonyl oxygen atoms were replaced by sulfur, so that the strong lowest-lying pi-pi* transitions are shied to the red. This not only brings them in the range of our time-resolved CD set-up but more importantly, it offers a site specic labelling of the peptidic chains.Similar substitutions are possible with DNA strands and they are already used in either uorescence or CD studies, e.g. 2-aminopurine dinucleotides, which we are planning to use.From a more general perspective, there are still huge challenges to be overcome in order to make time-resolved CD a reliable and easy to implement method: