Excimer formation in zinc-phthalocyanine revealed using ultrafast electron diffraction

The formation of excited dimer states, so called excimers, is an important phenomenon in many organic molecular semiconductor solid state aggregates. In contrast to Frenkel exciton-polarons, an excimer is long-lived and energetically low-lying due to stabilization resulting from a substantial reorganization of the intermolecular geometry. Here, we show that ultrafast electron diffraction can follow the dynamics of solid-state excimer formation in polycrystalline thin films of a molecular semiconductor, revealing both the key reaction modes and the eventual structure of the emitting state. We study the prototypical organic semiconductor zinc-phthalocyanine (ZnPc) in its crystallographic α-phase as a model excimeric system. We show that the excimer forms in a two-step process starting with a fast dimerization (approx. 0.4 ps) followed by a subsequent slow shear-twist motion (14 ps) leading to an alignment of the π-systems of the involved monomers. This structural distortion persists well beyond 300 ps. Furthermore, we show that while the same excimer geometry is present in partially fluorinated derivatives of ZnPc, the formation kinematics slow down with increasing level of fluorination.