MaCSE Research - Molecular materials for electronics

The emission of stable and efficient blue light is a major issue in organic technologies.
Our works have made it possible not only to obtain new highly efficient organic semiconductors (Angew. Chem. 2015, Adv. Funct. Mater 2018) but also to shed new light on fundamental concepts such as the impact of molecular arrangement on the evolution of the energy of frontier orbital and singlet and triplet states (Acc. Chem. Res. 2018, Chem. Comm. 2019-Feature article). Thus, by modifying the intensity of the electronic coupling between two π-conjugated fragments by steric and / or electronic effects, we have shown that it was possible to selectively modulate the energies of the frontier orbitals while keeping a very high level of energy of the triplet state, essential property for an application as a host matrix in a PhOLED (ACS Appl. Mater. Interfaces, 2017).

These works now allow the creation of molecular systems efficient for organic semiconductors. The group is now a leading group in the design of host materials for PhOLEDs and works in collaboration with recognized teams in the development of electronic devices (University of Shoochow-China, LPICM / École Polytechnique-France).
These fundamental approaches made it possible to take a significant step forward in 2019. Indeed, new generations of semiconductors, only constituted of carbon and hydrogen atoms have been synthesized and integrated as host matrix in PhOLEDs emitting blue light. The performance obtained made it possible to establish a new record in the field - an external quantum efficiency > 23% (Angew. Chem. 2019) and open up new promising perspectives.

Another application in Organic Electronics relates to n-type field effect transistors (OFETs).
Since 2013, we have been developing new families of electron acceptors built on various organic compounds skeletons : oligophenylenes and indacenothiophenes (ACS Appl. Mater. Interfaces, 2017) and bithiazolidinylidene-tetrathione and birhodanines (J. Mater. Chem. C 2015). In particular, in the case of bithiazolidinylidene-tetrathione. Mobilities of up to 0.29 cm².V^-1.s^-1 could be obtained and the OFET performance remains unchanged after several months of exposure to air (J. Mater. Chem. C 2015).
The various structural modifications made to this family of acceptors and their studies within devices have made it possible to identify the importance of chalcogen ∙∙∙ chalcogen interactions on the great stability of these devices in air (J. Mater. Chem . C 2017).
These acceptors can also generate charge transfer complexes with different donors such as pyrene, perylene and coronene. Within devices, these complexes exhibit ambipolar properties depending on interchain interactions (CrystEngCom. 2019).
This work is the subject of a sustained collaboration with Prof. T. Mori of the Tokyo Institute of Technology (TITech).

The Molecular conductors axis is based on a manipulation of overlapping interactions between radical species in crystalline solids to promote, control and optimize the conductivity of molecular materials of three kinds :

• single component conductors
• charge transfer salts
• ion salts radicals.

In these systems, subtle changes in chemical structure, the application of pressure or electric field can induce drastic changes in crystal structures (phase transitions) and electronics (metallic, semiconductor, ferroelectric behavior, charge or spin order, etc) and the emergence of remarkable physical phenomena (Mott transition, ferroelectricity) with potential applications (memories, sensors).
In this dynamic, our research activity has mainly focused on the control of the molecular symmetry of tetrathiafulvalenes (TTF) derivatives and on original chemistry of dithiole complexes.

Unlike classic TTF derivatives with D_2h symmetry (TMTTF, BEDT-TTF), our group focused on the use of TTF derivatives with reduced symmetries such as C_2v (Z isomers) and C_2h (E isomers and orthodisubstituted TTFs), whose preparation and crystallization are more difficult.
Their use in salts with mixed valence led to the discovery of original structural and electronic properties as in the (o-Me₂TTF) Cl / Br (Phys. Rev. B 2019) or (o-Me₂TTF)₂NO₃ (IUCrJ 2018) salts.
Moreover, the mixture (Z, E) - (SMe)₂Me₂TTF in its charge transfer salt with TCNQ crystallizes with only the E isomer in a structure (PCCP 2019) which strongly differs from the prototype compound TTF • TCNQ.
Control of the redox properties by the Me and SMe substituents leads to a quasi-commensurable partial charge transfer in segregated stacks and a metallic behavior with a metal-insulator transition pushed back to lower temperatures under pressure, thanks to the S ∙∙∙ S side contacts between regular stacking of TTFs.

We have also developed a family of stable radicals, such as the dithiolene complexes of neutral free radical gold [Au (dithiolene)₂] ^• (ρ = 1) obtained by oxidation of anionic complexes d⁸ with closed layer [Au (dithiolene)₂]^- . These dithiolene complexes form unidimensional one-dimensional chains with high conductivities (Chem. Eur. J. 2017) and behave like Mott insulators with very small gap (Phys. Rev. B 2018). We have shown that the use of physical pressure or short electrical pulses makes it possible to generate a resistive switching between two states, an initial insulating state (state '0') and a metallic state (state '1') (J. Phys. Chem. C 2015), a property that can be valued in electronic memories of the "Resistive Random Access Memories" (RRAM) type. Different structural modifications have been developed in order to better understand the structure / organization / property relationships. The first radical molecular dithiolene complex which behaves like a metal at ambient pressure has thus been obtained ([Au (Me-thiazdt) 2] •, (J. Am. Chem. Soc. 2018).