Polymeric Frameworks as Organic Semiconductors with Controlled Electronic Properties

The rational assembly of monomers, in principle, enables the design of a specific periodicity of polymeric frameworks, leading to a tailored set of electronic structure properties in these solid-state materials. The further development of these emerging systems requires a combination of both experimental and theoretical studies. Here, we investigated the electronic structures of two-dimensional polymeric frameworks based on triazine and benzene rings by means of electrochemical techniques. The experimental density of states was obtained from quasi-open-circuit voltage measurements through a galvanostatic intermittent titration technique, which we show to be in excellent agreement with first-principles calculations performed for twoand three-dimensional structures of these polymeric frameworks. These findings suggest that the electronic properties depend not only on the number of stacked layers but also on the ratio of the different aromatic rings. SECTION: Energy Conversion and Storage; Energy and Charge Transport S the discovery of conductive polymers, semiconducting conjugated polymers have been of great interest in organic electronics applications, such as organic electroluminescent diodes, photovoltaic cells, photocatalyst polymers, and organic batteries. One of the most interesting properties pursued by organic electronics is bipolarity, existence of both pand n-type semiconducting behavior in the same material. This feature is not only important from a fundamental scientific viewpoint but also for its possible applications; bipolar organic compounds are promising candidates to promote further development of the field of organic electronics. The discovery of bipolarity in a new class of organic materials can foster future developments of the above-mentioned technologies. Indeed, recent works on π-conjugated microporous polymers show that polymeric frameworks are promising materials for organic electronics and even for organic spintronics. The experimental control of structural periodicities by choosing a monomer as the building block of the framework can lead to semiconducting systems with unique electronic properties, such as two-dimensional (2D) atomic crystals. The possibility of controlling bipolar organic semiconductors’ electronic properties can give rise to new electronic system-level design based on artificial semiconducting polymeric frameworks, which would represent a giant leap forward in the development of organic electronic devices. Here, we studied the electronic properties of covalent triazine-based frameworks (CTFs) by performing electrochemical measurements and comparing with first-principles electronic structure calculations. The CTFs are porous polymeric frameworks formed by cyclotrimerization of nitrile monomers, which have been applied in the implementation of catalyst materials and most recently for lithiumand sodium-based energy storage devices. They have a conjugated structure, consisting of benzene rings as electron donors and triazine rings as electron acceptors (Figure 1), with controllable photoluminescent properties. Despite the abovementioned properties, surprisingly, little work has been done in the study of CTFs toward an efficient implementation in organic electronics. In a previous study, we have carried out several electrochemical measurements to test the electrochemical properties of electrode materials, such as CTFs. From these experiments, important information about the electronic structure of materials can be obtained. Thus, we applied these electrochemical techniques in the present work to investigate the tunability of the electronic properties of porous polymeric frameworks, testing their properties as organic Received: June 26, 2013 Accepted: August 20, 2013 Published: August 20, 2013 Letter