Excimer Formation in Zinc-phthalocyanine Revealed using Ultrafast Electron Diffraction

TL;DR

This study uses ultrafast electron diffraction to observe the formation and structural dynamics of excimers in zinc-phthalocyanine thin films, revealing a two-step process and effects of fluorination on kinetics.

Contribution

It demonstrates the capability of ultrafast electron diffraction to track excimer formation in organic semiconductors and details the two-step structural evolution process.

Findings

Excimer forms via a fast dimerization within 0.4 ps.

A slow shear-twist motion occurs over 14 ps.

Fluorination slows down excimer formation kinetics.

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.