  The formation of excited dimer states, so called excimers, is an important
phenomenon in many organic molecular semiconductors. In contrast to Frenkel
exciton-polaron excited states, an excimer is long-lived and energetically
low-lying due to stabilization resulting from a substantial reorganization of
the inter-molecular geometry. In this letter, 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 $\alpha$-phase as a model excimeric system. We show, that the
excimer forms in a two-step process starting with a fast dimerization
($\lesssim 0.4\, \text{ps}$) followed by a subsequent slow shear-twist motion
($14\, \text{ps}$) leading to an alignment of the $\pi$-systems of the involved
monomers. Furthermore, we show that while the same excimer geomtry is present
in partially fluorinated derivatives of ZnPc, the formation kinematics slow
down with increasing level of fluorination.
