Electrochemical, EPR and Quantum Chemical Study of Pyrene-Cyclobutene Conjugates
Authors: Kolacna Lucie , Klíma Jiří , Liška Alan , Ludvik Jiri 
Published: July 2021

Various biodegradable organic semiconductors are currently searched and developed for e.g. thin film transistors (OTFTs), organic light-emitting diodes (OLEDs), photovoltaic cells, sensors, radiofrequency identification tags etc. This work is focused on precursors for organic field-effect transistors – OFETs which would be based on a tetrasubstituted double bond serving as the central organic "core" (without heavy metals) and bearing photo- or redox active centers interconnected by π-conjugated bridges. These variably delocalized systems exhibit intramolecular electronic communication (charge transfer, polarizability) depending on the oxidation or excitation state.
In the first part of the project, the tetrasubstituted double bond as the core was substituted by cyclobutene and cyclohexene. These "central" molecules bear various aromatic and/or unsaturated pendants, where dominating redox centre(s) are pyrene derivatives.
Pyrene represents here an important structural unit which could be involved in π-delocalized systems. It can be electrochemically (and reversibly) oxidized and reduced, therefore it represents important redox center which can be used as a redox probe for more detailed investigations of electron distribution. In addition to this, pyrene exhibit fluorescence properties.
For this aim, an extended series of pyrene conjugates based on the cyclobutene and cyclohexene as model precursors was synthesized and their redox properties as well as the relationship between their structure and delocalization (electron distribution) were electrochemically investigated using rotation disk voltammetry, polarography and cyclic voltammetry and their in-situ combination with simultaneously recorded EPR spectra. The experiments were performed in non-aqueous media using AN and DMF as solvents and glassy carbon and mercury (DME, HMDE) as working electrodes in order to obtain thermodynamically relevant data not influenced by solvent or electrode material. The experiments were accompanied by comparison – correlation with quantum chemical calculations.
For the fundamental redox characterization, the first oxidation (Eox1) and the first reduction (Ered1) potentials are the most important. In addition to this, their difference correlates with HOMO-LUMO gap and reflects also the spectroscopic - photophysical properties. Nevertheless, we tried to analyze and interpret even the other redox steps in order to outline the redox mechanism and to understand the fate of the molecule after the primary electron transfer, that means follow-up reactions and the geometry changes connected with the changing extent of electron delocalization. Detailed analysis of electrochemical data gives information about intramolecular electron communication and reveals relationship between structure and redox properties, which is important for their tuning and further application.
Although pyrene and cyclobutene derivatives seem to be totally explored, their redox properties and ET-initiated reaction mechanisms are not clear and the respective literature is missing. It is generally accepted that pyrene being reduced forms reversibly a stable radical anion and at more negative potentials unstable dianion. However, even the primary radical anion undergoes follow-up reaction yielding the same product like the dianion exhibiting thus most probably disproportionation. As the final product, we expect the 4,5-dihydro derivative, but its proof was not still successful. Gradual reduction of two directly interconnected pyrenes (1,1-dimer) involves partial irreversibility due to the slow intramolecular electron transfer caused by mutual twisting and thus limited electron communication.
Similarly, electrochemical reduction of cyclobutene ring was not performed, up to now. Theoretically (based on the analogy) the ring should be reductively open giving rise the cis-disubstituted double bond, our experiments, however, point to the saturation of the double bond preserving the cyclobutane ring. In any case, it was proved that the cyclobutene bridging unit represents extension of delocalized system showing partial reversibility of its first reduction step.
Whereas electrochemical oxidation of pyrenes and other aromatic substituents results in anodic polymerization and blocking of the electrode surface, the detailed reduction mechanisms are still under investigation.
Acknowledgements
This work is supported by grant GAČR 18-12150 S. The authors are grateful to Tomáš Tobrman and Peter Polák (University of Chemistry and Technology Prague) for organic synthesis of the compounds.

Figure 1