Excimer Formation in Zinc-phthalocyanine Revealed using Ultrafast Electron Diffraction

Abstract

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.

References in corpus (5)

• Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
• Advanced capabilities for materials modelling with Quantum ESPRESSO
• Coherent and incoherent electron-phonon coupling in graphite observed with radio-frequency compressed ultrafast electron diffraction
• Solving the Jitter Problem in Microwave Compressed Ultrafast Electron Diffraction Instruments: Robust Sub-50 fs Cavity-Laser Phase Stabilization
• Excimer Formation in Zinc-phthalocyanine Revealed using Ultrafast Electron Diffraction

Cited by in corpus (2)

• Excimer Formation in Zinc-phthalocyanine Revealed using Ultrafast Electron Diffraction
• Prospects for Direct Electron Detectors in Ultrafast Electron Diffraction and Scattering Experiments