Dresden 2009 – scientific programme

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O: Fachverband Oberflächenphysik

O 49: Ab-initio approaches to excitations in condensed matter II

O 49.9: Talk

Thursday, March 26, 2009, 12:30–12:45, SCH 251

Optical properties of molecular crystals: Periodicity meets molecular deformation patterns — •Reinhard Scholz and Linus Gisslen — Walter Schottky Institut, Technische Universität München, Am Coulombwall 3, 85748 Garching, Germany

In the calculation of the dielectric properties of molecular pigments, one has to reconcile the deformation of an excited molecule with the periodicity of the crystal. For a set of six perylene-based chromophores, we demonstrate that the excited geometry can be obtained most reliably when applying DFT to a modified occupation of the frontier orbitals, whereas the deformations obtained in TD-DFT remain systematically too small. In the crystalline state, electron and hole transfer mix neutral molecular excitations with charge transfer (CT) states, and as the largest transfer parameters occur along the stacking direction, they can easily be determined by ab initio calculations of a stacked dimer. The transition dipoles of molecular excitations and CT states can be derived from TD-DFT applied to a stacked dimer. These DFT-based calculations leave two essential parameters of the system undetermined: The transfer of a neutral excitation to its neighbours, and the energetic difference between neutral excitations and CT states. Albeit estimates for these quantities can be obtained from ab initio schemes, they are most precisely obtained from an exciton model accounting for the mixing of neutral molecular excitations and CT states, together with the deformation patterns of excited and charged molecular states. From the calculated lineshapes, one can derive quite precise energies of the CT states, a key parameter for device applications.