Organic pi-conjugated molecules are intriguing building blocks for opto-electronic applications, offering a playground for achieving tailored properties and functions through molecular size, functionalization, dimensionality, and molecule-substrate interaction. Owing to the weak inter-molecular binding, the single-molecule features naturally pre-determine the properties of condensed phases. Nevertheless, molecular assemblies, thin films, molecular crystals, and organic/(in)organic interfaces exhibit excitation spectra that substantially deviate from those of the gas phase. Here, electronic-structure theory faces a significant challenges since, going from single molecules to crystalline phases, requires a bridge between the concepts of quantum chemistry and solid-state physics. A methodology that is able to consistently capture the features of molecular materials, from single molecules to their condensed-phases, including organic crystals, adsorbate systems, hybrid materials [1], and nanostructures, is an essential prerequisite to predict their excited-state properties. I will show how a synergy of density-functional theory (DFT), time-dependent DFT, and many-body perturbation theory can lead to insight into the puzzling interactions behind the optical spectra of molecular materials, and what methodology can (not) be trusted in a particular situation [2]. Selected examples will show how we explore, control, and predict hybrid excitons [3,4], the light-matter interaction in optical switches and at organic-inorganic interfaces, or manipulation of growth by light [5].
[1] C. Draxl, D. Nabok, and K. Hannewald, Organic/inorganic hybrid materials: Challenges for ab initio methodology, Accounts of Chemical Research 47, 3225 (2014). [2] Caterina Cocchi and Claudia Draxl, Optical Spectra from Molecules to Crystals: Insight from Many-Body Perturbation Theory, preprint. [3] P. Puschnig, P. Amiri, and C. Draxl, Band renormalization of a polymer physisorbed on graphene investigated by many-body perturbation theory, Phys. Rev. B 86, 085107 (2012). [4] M. Milko, P. Puschnig, E. Menna, J. Gao, M. A. Loi, and C. Draxl, Evidence of hybrid excitons in weakly interacting nano-peapods, J. Chem. Phys. Lett. 4, 2664 (2013). [5] L. Pithan, C. Cocchi, H. Zschiesche, C. Weber, A. Zykov, S. Bommel, S. J. Leake, P. Schäfer, C. Draxl, and S. Kowarik, Light controls polymorphism in thin films of sexithiophene, Crystal Growth & Design (2015); DOI: 10.1021/cg501734w.