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Fluorene and dihydroindenofluorene Rings for Optoelectronic Devices Outcomes

Periodic Reporting for period 1 - FRODO (Fluorene and dihydroindenofluorene Rings for Optoelectronic Devices Outcomes)

Reporting period: 2016-07-01 to 2018-06-30

Based on the recent developments of cyclo-para-phenylenes (CPPs), the project aimed to synthesize and study a new class of cyclic organic semi-conductors (OSCs) based on bridged oligophenylenes building blocks, such as cyclofluorene or cyclodihydroindenofluorene, for Organic Electronic (OE) applications. After synthesis, the main goal of this proposal was to define a precise structure properties relationship study on cyclic bridged oligophenylenes. Defining these general rules through such structure-properties relationship studies should allow a perfect control of the properties of cyclic bridged oligophenylenes, which is the foundation of the materials design for OE.
Another objectives for the applicant was to learn many new skills and knowledges in cyclic OSCs, physico-chemistry and in electronic devices fabrication and characterization and thus acquire a rare and multidisciplinary scientific profile, which is a key feature for an academic career. Indeed, at the beginning of this project, the applicant wished to become a permanent CNRS researcher at the University of Rennes 1 in the host group.
At the end of the action, seven new cyclic OSCs based on bridged oligophenylenes building blocks, such as fluorene and carbazole, have been synthesized, isolated and studied. They are varying by their size (4 or 5 units) and by the length of the alkyl chains linked to the bridge (to the carbone atom for fluorene and nitrogen atom for carbazole). Thus, a precise structure properties relationship study has been done, with a study of the influence of the number of units in the cyclofluorene on its properties and the influence of the presence of the bridge on the properties of cyclofluorenes and cyclocarbazoles (compared to CPP) and its nature (carbon atom or nitrogen atom), including the length of the alkyl chains. This last parameter allows us to tune the electronic properties of the cyclic bridged oligophenylenes, which was totally unexpected. Furthermore, one cyclocarbazole has been used as active layer in a p-type channel Organic Field Effect Transistor (OFET). This work shows not only that nanorings can be used as an active layer in an OFET but also provides a first benchmark for nanorings since this work is the first incorporation of a nanoring in an OFET and hence the first example of a transfer characteristic and a FE mobility.
Concerning the professional objectives of the applicant, they have been perfectly achieved since the applicant was successful to the CNRS competition thanks to her multidisciplinary scientific profile. Thus the applicant is a permanent CNRS researcher at the University of Rennes 1 in the host group since October 2018, 1st.
Concerning the cyclic bridged oligophenylenes, the conditions of synthesis and purification have been optimized. Thus, seven cyclic bridged oligophenylenes, based on fluorene or carbazole, having different size (4 or 5 units) and various alkyl chain length (from ethyl to hexyl), have been synthesized and isolated. Some linear analogues were also obtained, for comparison purpose.
Their physico-chemical properties have been determined by electrochemical studies (determination of the redox properties, electrochemical gap, HOMO/LUMO energy levels), NMR studies (determination of the conformation and the dynamic properties in solution). Their structural properties at the solid state have been determined by X-Ray diffraction (supramolecular arrangement) and STM. The photophysical properties have been determined through UV-visible absorption, fluorescence and phosphorescence measurements (stationary and time-resolved studies). Theoretical calculations were also carried out and correlated to the experimental data. For one example (obtained with the highest yield), the mobility of the charge carriers was determined using OFET.
Finally a precise structure properties relationship study has been done, the principal elements of which are as follows:
- Compared to their linear analogues, cyclic bridged oligophenylenes display a better control of the emission of the ring in the solid state, which augurs well for the future development of material,
- Compared to CPPs, the presence of a bridge induced different geometries both at the fundamental and excited states and hence different structural and electronic properties,
- Compared to one another, the nature of the bridge (carbon atom for fluorene vs nitrogen atom for carbazole) influence both the structural and the electrochemical properties and only slightly the spectroscopic properties,
- Compared to one another, the length of the alkyl chain linked to the bridge has an unexpected influence on the structural and electronic properties of the 4-cyclofluorenes, whereas the 5-cyclofluorenes and the 4-cyclocarboles are not influenced by this parameter, showing thus the specificity of the 4-cyclofluorenes family,
- Compared to one another, the nanoring size (4 or 5 units) influence both the dynamic behavior in solution and the structural arrangement at the solid state since the largest nanorings display a smaller strain energy than the smallest. Also, the emission properties are totally different.
The synthesis of the 4-cyclofluorene bearing ethyl chains and the specificity of the properties of this molecule compared to both linear analogue and 4-cyclofluorene bearing other alkyl chains have been published (L. Sicard, O. Jeannin, J. Rault-Berthelot, C. Quinton, C. Poriel, ChemPlusChem 2018, 83, 874) and presented during a congress (The Third International Symposium on the Synthesis and Application of Curved Organic π-Molecules & Materials, Oxford UK, 5-7 September 2018). The other results will be the object of future publications and presentations during future congresses (for example, the synthesis and the properties of cyclocarbazoles will be presented during the 9th International Conference on Molecular Electronics, Paris, 17-20th December 2018).
With this project, a precise structure properties relationship study on cyclic bridged oligophenylenes has been done. It allows us and others to design new nanorings for OE applications. Furthermore, the first incorporation of a nanoring in an OFET has been done, showing the potential of these molecules in OE. Therefore, the general impact, while being clearly scientific, can go far beyond fundamental science as the field of OSCs for OE will have strong consequences for the future of energy and lighting.