Investigation of Application Potential of Diazapentacene Derivatives in Organic Field-Effect Transistors

Diazapentacene derivatives were synthesized and investigated for their potential application in organic field-effect transistors, with one derivative showing a rare n-type behavior.

The blog post written by Wudi Yang and Shuang Zhang and edited by Pavlo Dral.

Organic field-effect transistors (OFETs) are often built with pentacene, which is a p-type semiconductor. The introduction of nitrogen atoms into pentacene scaffold influences the chemical as well as electronic properties, and alleviates its major disadvantages, such as low chemical stability and low solubility. Swapping two middle-ring C–H groups with nitrogen atom or N–H group results in such compounds as 6,13–diazapentacene (DAP, isoelectronic to pentacene) or its reduced derivative 6,13-dihydro-6,13-diazapentacene (DHDAP). Both compounds were studied earlier for their application potential in OFETs. 

In our group’s recent publication, we further studied the characteristics of symmetrically substituted tetraphenyl derivatives of DHDAP and DAP — compounds 2 and 3, respectively. We explored the effects of arylation and introduction of nitrogen atoms on the solid-state structure and electronic properties as well as their semiconductor properties in OFETs.

Our experimental colleagues in Europe synthesized both compounds and characterized them by cyclic voltammetry, X-ray crystallography, and UV–vis spectroscopy. We further complemented the experimental results by theoretical calculations.

The chosen density functional theory (DFT) level of theory (ωB97X-D/def2-TZVP) reproduced well the X-ray geometry, with only a slight deviation observed for the spatial orientation of the lateral phenyls, while no significant differences for the diazapentacene core are discernible.

We explain the electronic influence of phenylation on the DAP core by theoretical calculations:

•  The HOMO to LUMO transition is mainly responsible for the lowest energy band. The largest contribution to the mid-energy band of compound 3 is from the HOMO–2 to LUMO transition. The small redshift of the absorption spectrum of 2 in comparison to the parent DHDAP reveals that the highly twisted conformation of the phenyl moieties limited electronic communication between the phenyl substituents and the diazapentacene core. 
•  Compared to compound 2, the fully aromatic derivative 3 exhibit a small redshift of only 5 nm. It originates from narrowing of the HOMO–LUMO gap. Interestingly, absorption band of 3is significantly redshifted compared to tetraphenylpentacene (TPP), i.e. nitrogen substitution leads to a significant change in electronic properties.

Our theoretical calculations also supplement the electrochemical results: according to the DFT-calculated, reduction potential the radical cation 2^.+  is in good agreement with the experimental value, the first reduction potential of 3 is well reproduced as well. 3 has rather low LUMO energy supported by a significant adiabatic electron affinity of 2.2 eV calculated at DFT in vacuum. In addition, reductions are reversible as observed by cyclic voltammetry (CV). This indicates the potential of 3 in OFET applications as an unusual n-type semiconductor.

• Miriam Hauschild, Michal Borkowski, Pavlo O. Dral, Tomasz Marszalek, Frank Hampel, Gaozhan Xie, Jan Freudenberg, Uwe H. F. Bunz*, Milan Kivala*, 5,7,12,14-Tetraphenyl-Substituted 6,13-Diazapentacenes as Versatile Organic Semiconductors: Characterization in Field Effect Transistors. Org. Mater. 2020, 3, 204–213. DOI: 10.1055/s-0040-1713856.