Design of n-Channel Organic Semiconductors for Field-Effect Applications

Our main focus is on the synthesis and characterization of nitrogen-rich oligoacenes (anthracenes, tetracenes, pentacenes), and their application as n-channel semiconductors in organic field-effect transistors.

The primary targets are highly electron-deficient, self-complementary systems in which a large number of CH-N contacts serve as 'supramolecular synthons' to enforce p-stacking alignments in the solid state. By systematically increasing the size of the aromatic framework from naphthalene to pentacene, and by varying the number and positions of heteroatoms and/or substituents, general principles of crystal engineering within this class of compounds are derived. By comparison of closely related compounds, differing only slightly in the heteroatomic substitution pattern, a detailed understanding of the more subtle effects determining crystal structures is obtained.
The long-term goal of our research is to understand, how molecular properties and intermolecular orientation in the solid state correlate with the charge carrier mobility and stability of these semiconductors.

Our research is highly interdisciplinary and involves collaborations with leading experts in chemistry and physics departments. Some facets are the

• Development of novel synthetic routes to heterocycles with high electron affinity
• Investigation of optical, electrochemical, and spectroelectrochemical properties of heterocyclic compounds
• Preparation of ultra-pure organic materials by gradient sublimation and growth of single crystals by physical vapour growth techniques
• X-Ray crystal structure determination
• Determination of charge carrier mobilities from I-V characteristics of organic field effect transistors
• Computational investigations of molecular properties, aggregation, and charge transport

Reactive Intermediates and Reaction Mechanisms

We are also interested in reactive intermediates (e.g., aryl radicals, aryl cations, aryl anions, arynes) and their reactions that are relevant to the preparation of heterocyclic compounds. Typical reactions are cycloaromatizations involving radicals, anions, or biradicals or hetaryne cycloadditions. Understanding the properties and reactivities of key intermediates allows us to control selectivities, to optimize synthetic procedures, and to devise new pathways towards electron-deficient heterocycles. Density functional theory as well as high-level ab initio calculations are employed to study electronic structures and reaction mechanisms computationally. Spectroscopic properties (IR, UV, EPR) and elementary reactions of short-lived species are investigated in collaboration with the research group of Prof. Wolfram Sander (University of Bochum).

Another point of interest is the reactivity of acenes and heteroacenes in the ground as well as in excited and ionized states towards oxygen, water, dimerization etc. A high reactivity will ultimately lower the lifetime of electronic devices produced from these materials, or limit their applicability under various operating conditions. Investigating the decay mechanisms is thus crucial to the design of stable high-performance organic materials. We employ a combination of experimental and computational methods to study the reactivity of heterocyclic semiconductor compounds. Computational investigations involve collaborations with the research group of Prof. K. N. Houk (UCLA).